Global Challenges of Current Building-Integrated Solar Water Heating Technologies and Its Prospects: A Comprehensive Review

Abstract

Among the renewable energy resources, solar thermal is one of the technologies that significantly contribute to energy supply and reduce global greenhouse gas (GHG) emissions. Solar water heating (SWH) contributes a large proportion of the global solar thermal capacity, with 63% installation for domestic hot water (DHW) systems, 28% for large DHW systems, and the remaining 9% for other applications such as swimming pools heating, solar district heating, and space heating. Still, now, there are many challenges and limitations of those SWH technologies. The present work reviews the current challenges faced in the domestic SWH industry globally. The integration of thermal energy storage (TES) systems for better collector’s radiation absorption and overall performance are also highlighted. Many countries worldwide seem to face similar challenges within the SWH technologies and industry. It is proven that these challenges and limitations can hinder the global capacity of solar thermal utilization.

1. Introduction

Renewable energy will be an essential source of energy production, since these resources can be utilized repeatedly to create usable energy [1]. Social, environmental, and economic issues can be avoided with the use of renewable energies, as these resources are considered environmentally sustainable, with little or no emission of exhaust and harmful gases, such as carbon dioxide (CO₂), carbon monoxide (CO), sulfur dioxide (SO₂), etc., [1]. In addition, renewable energy sources are considered ideal alternatives to mitigate emissions and increase the economy, energy stability, and employment opportunities. On top of that, poverty can also be minimized because most people in poor regions/countries depend on natural resources for general energy production [2].

Solar thermal is one of the most widely used renewable energy technologies around the globe for harnessing beneficial energies from the sun. Unlike solar photovoltaic technology, which produces electricity, solar thermal uses the high-temperature radiation from sunlight to provide heat for water and air heating in residential, commercial, and industrial buildings. The global market development and trends report from the Solar Heating and Cooling Technology Collaboration Programme (SHC TCP) of the International Energy Agency (IEA) stated that at the end of 2019, there were 479 GW_th of solar thermal capacity in operation worldwide [3]. This cumulative capacity amounted to 389 TWh of solar thermal energy generated, leading to the savings of 41.9 million tons of oil and 135.1 million tons of CO₂ [3]. There are three categories of solar thermal applications: low-temperature, medium-temperature, and high-temperature. The low-temperature solar thermal application involves swimming pool and space heating and drying, while medium- and high-temperature applications are generally for water/air heating and electric power production, respectively [4,5]. Solar energy can be of great value in countries where the sun appears to shine in most places for most of the year. Solar thermal collectors are utilized in solar thermal systems to absorb the direct solar radiation from the sun as thermal energy [6]. This thermal energy is converted into heat energy, which is then transferred to a working fluid (i.e., water, air, thermo oil, etc.). The heat carried by the working fluid is used to provide for water heating, steam, or space heating applications [7,8]. For unglazed water collectors, flat-plate collectors, evacuated tube collectors, and unglazed and glazed air collectors, the installed capacity of 483 GW_th, equivalent to 690 million m² overall collector area, was in operation in 68 countries by the end of 2018 [3]. Of the total capacity, 81.7% was accounted for in China and Europe alone, with an individual capacity of 337.6 GW_th and 56.8 GW_th, respectively [3].

For SWH systems, the two dominant solar collector designs are flat-plate and evacuated tube types. With a global share of about 70%, evacuated tube collectors are the prevailing solar thermal collector technology, superseded by flat-plate collectors with approximately 23%, unglazed water collectors, around 6%, and glazed and unglazed air collectors with only 0.2% [3]. China contributed 83% of all the newly installed evacuated tube collectors in 2018. In Europe, on the contrary, most solar collectors established in 2018 were of the flat-plate types with 71.9% capacity [3].

Table 1. Previously conducted studies and reviews on solar water heating technologies.

Title	Year	Description	Shortfall	Source
Residential Solar Water Heating: California Adopters and their Experiences	2021	Survey on the adoption of SWH systems in 227 single-family households in California. Challenges at each stage of adoption are lined up.	Challenges are portrayed based on the adopters’ point of view and specific to particular households.	[9]
Effect of Design and Operating Parameters on Thermal Performance of Low-temperature Direct Absorption Solar Collectors: A Review	2021	Highlights the current challenges for direct absorption solar collectors (DASC) and its future directions in domestic sectors.	Issues integrated towards DASC only and are not specific to SWH technologies.	[10]
A Review on Techno-Economic Assessment of Solar Water Heating Systems in the Middle East	2021	Recent studies on techno-economic assessment for SWH integration in the Middle East region are reviewed.	The current position, progress, barriers, and challenges of the broader SWH market are explored in the Middle East.	[11]
Hybrid solar water heating/nocturnal radiation cooling system I: A review of the progress, prospects, and challenges	2019	Combined heating and cooling functionality using a single collector/radiator is presented. The prospects, advancements, and a few barriers are discussed.	Discussions are only centralized on hybrid solar water heating and cooling systems.	[12]
Causes of Failure of the South African Solar Water Heating Programme and the Forgone Social Benefits	2019	Assessment of the failure of an SWH program in South Africa. Institutional, social, and technical challenges are addressed.	Challenges are only catered to the targeted population and sample in South African regions.	[13]
Addressing Climate Change Through Green Technology with Reference to Solar Water Heating System Scheme: A Review of State of Punjab (India)	2018	Investigation towards the subsidized SWH project under Punjab Energy Development Agency (PEDA). SWH installation process and functionality challenges are highlighted.	Issues focused on Ludhiana, Jalandhar, and Kapurthala districts in Punjab, India.	[14]
Building Integration of Solar Renewable Energy Systems towards Zero or Nearly Zero Energy Buildings	2015	Suggestions for better integration of solar thermal systems on building roofs and façades.	Issues are addressed aesthetically.	[15]
Analysis of the Opportunities and Challenges of Solar Water Heating System (SWHS) in India: Estimates from the Energy Audit Surveys & Review	2012	Survey of SWH system market and opportunities in India. The main emphasis on prospects and challenges of SWHs in India.	Challenges are only accentuated in terms of India’s financial and market sides.	[16]
The Challenges of Designing and Building a Net Zero Energy Home in a Cold High-Latitude Climate	2008	Solutions for technical and policy challenges for the implementation of net-zero technologies in Canadian residencies.	Challenges are non-specific to domestic SWH technologies (a few sectors are covered).	[17]

below shows some of the existing studies on the challenges of SWH technologies in a few regions of the world.

The scope of study for this review paper encompasses the global challenges and prospects of SWH technologies involving flat plates and evacuated tubes as solar collectors. The challenges are branched further into the technical and economic challenges for the specified SWH technologies. The authors also proposed several potential solutions to the stated challenges, covering the governmental, industrial, and local level support, the public awareness of SWH implementation, the costs, and the SWH system’s enhancement methods. The future research directions of the designated SWH technologies are also included. The literature searches are made globally concerning several countries from several continents. The current study is carried out to create a solid platform for prospective readers to find relevant information, particularly regarding any issues and limitations related to domestic SWH technologies, upon conducting their SWH works.

This paper aims to gather as many technical and economic challenges as possible regarding global domestic solar water heating technologies. To the best of the authors’ knowledge, such a study has not been published to date. Many existing studies and reviews usually focus on one aspect of the SWH technologies. They either emphasize one type of SWH technology, one region or market, or a specific type of challenge. Meanwhile, the current review paper comprehensively summarizes technical and economic challenges for implementing solar water heating technologies with flat-plate and evacuated tube collectors worldwide. The prospects of SWH technologies are first introduced, followed by the concepts and principles of SWH systems with flat-plate and evacuated tube collectors. Then, the challenges to the said SWH technologies are dedicated in a concise section. Additionally, the paper attempts to provide potential solutions to the issues highlighted. Finally, the future research direction of SWH technologies is also addressed.

The information discussed in this paper will be a notable reference for those keen on conducting research or projects utilizing SWH technologies, particularly regarding the potential complications they might encounter later in the process. Since this review paper summarizes numerous essential findings concerning the topic of discussion, it would help the readers to assimilate relevant information that suits their research. One can also use this paper as a guideline to dodge any potential faults to their SWH projects.

The key findings that also serve as the main strengths of this review paper are highlighted as follows:

• Clear overviews of multiple SWH system configurations, including direct and indirect, passive and active, and compact and split modes within both FPC- and ETC-SWH technologies;
• Concise comparisons of the performance of FPC- and ETC-SWH technologies that encompass several fundamental parameters;
• Technical challenges in FPC-SWH technology are thoroughly evaluated, particularly with regard to the collector’s mechanical dysfunctionality, pipe fouling due to the circulating fluids, pipe overheating, and heat losses;
• Technical challenges in different types of ETC-SWH technology are heavily assessed in terms of the malfunction hazards from the vacuumed space, collector overheating, potential glass tube burst, pipe corrosion, scaling formation in the glass tubes, and leakage problem;
• Economic barriers in both FPC- and ETC-SWH technologies are described based on a few countries from multiple continents with high initial costs and massive subsidies given to fossil fuels as the ultimate obstacles;
• Potential solutions to the stated challenges are proposed based on the support from governmental, industrial, and local players as well as on the application of thermal energy storage (TES) within the SWH systems;
• TES integration is extensively discussed in two separate components of the SWH system, i.e., the solar collector and the insulated water tank;
• The future research directions of SWH technologies are focused on the enhancement methods, TES materials, PVT systems, and the approaches to reducing solar thermal system pricing.

2. Methodology

This paper uses the semi-systematic or narrative review approach during the article selection process. As illustrated in Figure 1, the method comprises five steps—literature search, keywords, selection criteria, critical assessment, and text drafting [18]. The literature search step broadly covers the previous and recent studies on the global SWH technologies for domestic usage. A few research questions guard the search, including: (1) How are SWH technologies integrated into the domestic application? (2) What are the typical and atypical challenges that interfere with smooth SWH adaptations? (3) How are those challenges precisely countered? (4) How would this review paper’s attempt to accumulate multiple kinds of challenges of the said technology for domestic usage contribute to the effortless SWH utilization in real-life case scenarios? The keyword step promotes limitations to the literature search by narrowing down the relevant articles and eliminating irrelevant ones. The main keyword combinations include ‘energy’, ‘solar energy’, ‘water heater’, and ‘solar collector’.

Next, the selection criteria step is divided into two segments—inclusion and exclusion. Inclusion criteria consider the comprehended search materials involving all types of papers (i.e., journal articles, conference papers and proceedings, reports, book sections, manuscripts, and theses), articles published in recognized databases, and grey literature. The electronic search uses Scopus, Semantic Scholar, and Web of Science databases. On the other hand, exclusion criteria involve the articles published in languages other than English and those with inaccessible full texts. Next, the critical assessment is the final selection stage, where the authors evaluate the most fitted pieces to be incorporated into the review paper. The cohesiveness of the filtered articles is determined based on their abstracts, sub-topics, and conclusions. Finally, the text drafting step is where the authors outline the core findings from the selected articles. Figure 2 below shows the year-wise trend for the number of SWH-related publications in the current paper.

3. Prospects of Solar Water Heating Technology

Water heating has always been one of the essential systems in a household. It comes second after space heating and cooling as part of the most critical segment of the house [19]. It also accounts for the second-highest electricity consumption should electric heaters or geysers be installed. High energy use for water heating puts pressure on the national grid to overproduce electricity and harms electricity prices [19]. As electricity is most commonly produced by burning fossil fuels, the extensive use of electricity for water heating adds more to making harmful greenhouse gases.

Regarding this matter alone, it is important for cleaner and greener alternative technologies to step into the game and help save the environment. One such technology is solar water heating (SWH), which is becoming more popular due to the evolving technologies in recent years. SWH can be the most efficient alternative to conventional water heaters such as electric geysers and fuel-fed boilers [20]. Because this technology utilizes sunlight as the energy source, SWH systems can reduce the reliance on traditional water heating fuels such as fuel wood or propane fossil fuels to power electrical-based water heating systems [21,22]. Additionally, SWH technology has remarkable long-term environmental and economic benefits.

Therefore, it can be said that solar water heating technologies have achieved some of the Sustainable Development Goals (SDGs) proposed by the United Nations (UN). The SDGs include the following: (1) SDG 4—Quality Education; (2) SDG 7—Affordable and Clean Energy; (3) SDG 9—Industry, Innovation, and Infrastructure; (4) SDG 11—Sustainable Cities and Communities; (5) SDG 13—Climate Action; (6) SDG 17—Partnerships to Achieve the Goal. Figure 3 below displays the impacts of SWH technologies as they fit the criteria in the abovementioned SDGs.

SWH technology is widely used in many regions worldwide, either in urban or rural areas. Using an SWH unit in a household could considerably shrink the energy consumption and the electricity bill, because free solar energy has taken up the job of providing hot water to the occupants. The San Francisco Department of the Environment stated that solar water heaters could reduce the electricity bill by up to 60 to 80% [24]. Indirectly, a country’s energy security can be ensured as a significant chunk of the energy is lifted from using an SWH system. Additionally, substituting conventional (electricity- or fossil fuel-based) water heating units with those powered by solar energy will significantly reduce carbon emissions. Scholars in various studies have proven a correlation between lower usage of energy that reduces the pressure on the national grid to mitigate environmental change [25,26]. An estimated 1500 pounds of carbon dioxide per year (lbs. CO₂/year) will be diminished with the employment of one typical residential solar water heater [27].

From an economic perspective, SWH is an energy-efficient technology that benefits the users and communities in the long run. Users typically have to make an initial investment to purchase and install the SWH system, and the payback period is generally four to eight years [19]. Hence, after a few quick years, the users can start enjoying the hot water with minimal costs (after counting the maintenance costs that incur once every few years). A cost analysis of Bangladesh’s solar hot water system confirmed that solar water heaters are more cost-effective than conventional water heaters [28].

Solar water heaters in the current market usually come equipped with backup electric heating elements to aid water heating in scenarios where the sunlight is unavailable at night or during cloudy and rainy days when there is little solar radiation. However, the presence of the electric heating element seems to defeat the purpose of having a solar water heater in the first place because, at some point, users still need to use electricity to operate the system. That being said, running an SWH system using solar energy for 24 h is impossible. Thus, a full deployment of an SWH system using one hundred percent solar energy is only possible when an energy storage system is utilized [29]. A thermal energy storage (TES) system within the SWH system enables an uninterrupted supply of hot water from the solar water heater for household applications throughout the day.

Recent progress in SWH technologies involving phase change materials (PCMs) as energy storage has been widely reviewed and explored by scholars and researchers worldwide. The heat storage medium (i.e., PCM) extracts thermal energy from the sun’s radiation and stores it until it is needed to heat water later when the sunlight is no longer obtainable. Several merits acquired by integrating PCMs in domestic SWH systems include the high capacity and low volume for hot water storage and the isothermal operating during the stages of charging and discharging thermal energy [29]. Due to its isothermal behavior, the presence of a PCM in the TES system enables a high density of heat energy storage. The temperature inside the thermal storage tank is kept constant until the PCM is circulated to heat the household water [30]. For solar hot water production, the PCM temperature range usually falls between 40 and 80 °C [31]. There are various techniques to incorporate PCMs within SWH systems under current studies. The two modes to integrate PCMs into the system are on either the collector side or the water tank side. These modes are discussed further in the latter sections of this paper.

4. Solar Water Heating Technologies

The main components of a solar water heating (SWH) system include solar collectors, insulated storage tanks, a supporting stand, and connecting pipes and instrumentation [32]. There are two modes of working principle for an SWH system: (1) an active system that utilizes pumps and controls, and (2) a passive system that does not involve any external equipment. An active system can be further classified into two types, which are direct (open-loop) circulation systems and indirect (closed-loop) circulation systems [33]. Active systems are more efficient, but passive SWH systems are generally cheaper. Household water gets preheated by the collectors before circulating into the conventional backup water heater. These systems work best in mild freezing climates where temperatures do not go below freezing point [34]. In thermosyphon systems, water flows by gravity from the cold-water tank to the collectors to be heated by the radiation [35]. The thermosyphon principle creates a natural convection that allows the now heated water to circulate through the risers to the hot water tank (or to the upper part of the tank in a compact, built-in-tank solar water heater). For this purpose, the collectors must be mounted below the water tank so that heated water may rise to the tank [33].

There are many variations of solar water heater systems and collector combinations in the current market. Each system has a different working principle depending on the type of collector, tank positioning, and system configurations. Solar water heaters typically come in two modes: (1) split tank system and (2) compact (built-in) tank system. This section discusses the SWH systems (i.e., working principle, functioning parts, etc.) with flat-plate and evacuated tube solar collectors employing active or passive, direct or indirect, and compact or split systems.

4.1. Global Innovation of Solar Water Heating Technologies

Self-motivation needs mainly drove the brisk expansion of most European countries’ solar water heater industry. The production of SWH systems, especially for domestic hot water usage, has become an integral part of many countries, including China, Australia, Germany, Greece, and the United States, where SWH system production forms a considerable chunk of their industrial sector businesses [36]. Commercially available solar water heaters come in various packaged forms, including active or passive modes (with or without anti-freeze working fluid), solar-assisted heat pump systems (for active mode system), and active circulation systems driven by pumps or thermosyphon principle with particular ranges of flow rates [36]. Research for nanofluids skyrocketed when Choi and Eastman [37] first demonstrated that nanoparticle-engineered heat-transfer fluids have higher thermal conductivities than conventional HTFs.

Table 2. The global evolution of SWH technologies.

Year	Technology	Location	Configuration	References
1760s–early 1800s	Bare tank heater	USA	Two small boxes (filled with water) are fabricated inside a rectangular box. The inner side of the rectangular box is insulated, and a glass cover top is installed to trap heat.	[38]
1891	1st commercial solar water heater	USA	A new design is patented with an integration of a hot box for better heat accumulation.	[38]
Early 1900s	Split system solar water heater	USA	The same collector-storage configuration is used, with water being heated in a new way: a non-freezing liquid is used in the collector and flows through a coil in the storage tank to pass heat to the water. The now-cooled liquid returns to the collector, and the cycle repeats.	[39]
1960s–1970s	Cylindrical solar water heater	Japan	A simple design comprises a cylindrical-shaped metal water tank embedded in a box with a glass cover.	[38,40]
1970s–late 1980s	Integral collector-tank solar water heater	Australia	The water storage tank is fabricated on the collector’s upper edge. This configuration avoids unnecessary piping and a heavy water storage tank in the attics. Due to its easy installation on Australian pitched roof houses, the integral collector-tank design is preferable.	[38,39]
Late 1980s	Solar water heater with FPCs	Canada	The production line of composite copper-aluminum absorbers for FPCs is launched. A self-designed selective oxidation line is being developed.	[41]
1990s	Solar water heater with ETCs	China	Significant breakthroughs were achieved when China developed the all-glass vacuum tubes production line. Over the years, the invention of heat pipes within glass tubes emerged.	[41]
Recent years	Solar water heater with FPCs and ETCs	-	Many types with multiple configurations. Various enhancement methods have been introduced (e.g., nanofluids, TES, fins, heat pump, PVT, geometric pipe design, etc.)	-

and

Table 3. Several global innovations of SWH technologies in the recent years.

Year	Location	Innovation	Configuration	Results	References
2010	China	An FPC solar water heater with a PCM-TES	Paraffin (PCM) saturated in aluminum foams is integrated onto the solar collector. The PCM layer is embedded underneath the collector’s selective absorber plate.	A remarkable enhancement of the heat-transfer ability is achieved. The aluminum foams help with a better temperature distribution in the PCM layer.	[42]
2013	Taiwan	A prototype of a PVT-assisted heat pump water heating system (PVT-HPWH)	An array of PVT evaporators is connected with a 1 kW HPSW and installed on a rooftop. A water tank of 200 L capacity is utilized.	Steady outputs are obtained even under fluctuated weather conditions.	[43]
2014	Iran	Copper (II) oxide (CuO)-water nanofluid in FPC	CuO particle size is 40 nm. CuO-water nanofluid acts as the working fluid in an FPC with flow rates ranging from 1 to 3 kg/min.	The collector efficiency with the nanofluid increases by 21.8% compared to just water as the working fluid.	[44]
2015	Malaysia	A heat-pipe ETC solar water heater equipped with a finned PCM-TES tank	An array of heat-pipe ETC is connected to a manifold filled with PCM. A vertical finned heat exchanger is fabricated inside the TES tank.	The solar water heater with a TES unit has higher efficiency than a conventional system.	[45]
2018	Bangladesh	Serpentine-type thermosyphon FPC solar water heater	The serpentine pipes are placed on top of the FPC’s absorber plate. The solar water heater works on the thermosyphon principle.	The proposed passive SWH system yields an exergy efficiency of about 3.7%.	[46]
2020	India	Cerium dioxide (CeO2) nanoparticles incorporated into a PCM-TES of all-glass ETC solar water heater	The TES unit is in nano-embedded PCM (NEPCM) and standard PCM forms, assembled in a horizontal cylindrical tank in the middle of a 125 L solar water tank.	The energy and exergy efficiencies of the PCM-TES SWH system are 10.89% and 3.96% higher than those without PCM; 1.0 mass% of CeO2 is adequate for maximum efficiencies.	[47]
2020	Egypt	An ETC solar water heater with novel helical finned heat pipes using PCM-TES	A multi-step helix of 0.5 mm thick coper disks is welded onto the heat pipes of 5 evacuated tubes	PCM with helical fins achieved better temperature homogeneity than that of regular fins.	[48]
2021	Nigeria	Active SWH systems with serpentine and risers-head flat-plate configurations	A 1.5 m2 is divided into two sections to accommodate the two pipe configurations. The first section is the risers-head pattern, while the second section is the serpentine pattern.	The serpentine model has a 2.62% more usable thermal energy than the risers-head. It also has better efficiency on a cloudy day.	[49]

below display the evolution and several studies of the most recent advancement regarding SWH technologies, respectively.

4.2. SWH System with Flat-Plate Collector (FPC)

An indirect FPC solar water heater circulates a heat-transfer (transition) fluid inside the copper pipes in the collectors before transferring the absorbed heat to the cold household water in the storage tank. Because the water is not in direct contact with the collector’s copper pipes, a heat exchanger system is installed within the water tank to transfer the heat from the transition fluid. Note here that the transition fluid could also be water circulating inside the copper coil of the heat exchanger. Many researchers have experimented with nanofluids as the working fluid within the collector pipes [50,51,52]. Nanofluids are proven to have improved the overall efficiency of the FPC itself [53]. In an indirect, closed-loop flat-plate collector solar water heating (FPC-SWH) system, flat-panel collectors absorb solar radiation from the sun and convert it to heat before transferring it to the transition fluid inside the panel’s copper pipes.

In a passive, compact (built-in-tank) system, as the transition fluid is heated up and its temperature increases, it naturally rises through the collector by the thermosyphon principle. It then circulates into the heat exchanger jacket within the water tank [54]. Figure 4 illustrates the working principle of an FPC solar water heater with a thermosyphon system. Here, heat is transferred from the transition fluid to water through the heat exchanger. The cooled transition fluid flows down the pipes to the collector area to be heated up again and continues the cycle [54]. A mechanical pump is needed to circulate the transition fluid within the heat exchanger’s copper coil installed in the split water tank for an active system. This system usually connects the collectors to a well-insulated manifold fabricated above them. Since the active systems do not involve the thermosyphon principle, the water tank could be installed below the collectors (usually placed inside the house). An FPC solar water heater with an active system is demonstrated in Figure 5. During heating up with FPCs, both beam and diffuse solar radiations are implied [55].

Rather than a heat-transfer fluid, a direct FPC solar water heater circulates household water throughout the copper pipes in the collectors. Here, water directly contacts the copper pipes during the heat-transfer process from the collectors. The working principle is more straightforward in this type of solar water heater. The sun’s energy is absorbed directly by the collector to the cold water running inside the copper pipes. This heated water then naturally rises to the water storage tank, and the cycle continues as cold water from the tank continuously flows down the collector to get heated. This heated water rises again to the tank to be consumed immediately or stored for later uses.

The direct SWH system possesses one only drawback: it is possible there could be toxic contaminations from the copper pipes as the household water comes in direct contact with them. However, no chemicals can be added to the water for protection, since the water used around the house is the same water circulating through the collector. Direct SWH systems are mainstream in regions with warmer climates and minimum cold days. If used in the winter season, a drain pipe (to drain water) is installed to prevent water from freezing in the pipes. Again, water circulation occurs using the thermosyphon principle (passive system) for a compact design because the water tank is mounted on top of the collectors. For a split system, a mechanical pump (active system) is required to circulate water around the system, from the water tank to the collectors and to the tank again. Figure 6, below, clearly shows the different types of FPC-SWH systems.

4.3. SWH System with Evacuated Tube Collector (ETC)

Like FPC, ETC solar water heaters also comprise a few system combinations. There are three types of ETCs: (1) water-in-glass, (2) U-type, and (3) heat pipe. Here, only the heat-pipe ETCs are considered an indirect SWH system because they need working fluids to transfer the heat absorbed from the sunlight to the household water. The working fluid inside the heat pipes follows the thermosyphon principle during the heat-transfer process. The fluid rises up the pipes to the condenser bulb to release the absorbed heat to household water in the tank as it gets heated up. Four design modes are currently available in the market for indirect, heat-pipe ETC solar water heaters: (1) compact design with regular water tank, (2) compact design with copper coil-integrated water tank, and (3) split design with manifold and regular water tank, and (4) split design with manifold and copper coil-integrated water tank.

In a compact design with a regular water tank, the heat pipe’s condenser bulb directly heats the tank’s household water (Figure 7a). However, for a tank with a copper coil heat exchanger, the heated water in the tank heats the circulating household water in the coil instead (Figure 7b). Here, the water inside the tank acts as a heat-transfer fluid that passes the heat to the circulating household water inside the copper coil. Two variations for a split system tank are a regular tank and a copper coil-integrated tank (Figure 8). Insulated manifolds with copper pipes are fabricated above the collector tubes to aid the heat transfer. For an indirect, regular split system, a mechanical pump circulates household water within the copper pipes in the manifolds and then back to the tank. For an indirect, copper coil-split system, a mechanical pump circulates the heat-transfer fluid within the copper pipes in the manifolds. The pump then circulates the HTF back to the copper coil inside the water tank to allow the heat-transfer process to the household water (the heat-transfer process occurs inside the water tank).

On the other hand, the water-in-glass and U-type ETCs are considered direct systems, for they require water to flow through the collectors to be heated up directly. Similarly, they share the same working principle: heated water naturally rises by the thermosyphon effect in the water tank. In contrast, cold water from cold water storage flows down the collectors to get heated up, continuing the cycle. The only difference with the heat-pipe ETCs is that instead of circulating inside the copper pipes, household water in water-in-glass and U-type ETC solar water heaters circulate directly inside the evacuated glass tubes during the heat-transfer process. Water-in-glass ETCs absorb heat energy from the evacuated tubes’ walls, while in U-type, water flows inside the U-shaped copper tubes to allow for heat transfer. Figure 9 below shows the different evacuated tube collector solar water heating (ETC-SWH) systems.

4.4. Overall Performance of SWH Technologies

Regardless of the type of SWH technologies in use, they all hold the same purposes: to heat water in the tank solely using thermal energy from sunlight and to be able to retain the hot water at the desired temperature for as long as possible throughout the day. The integral device of an SWH system is the solar collector. Hence, different collectors have different performance and efficiency levels to the solar hot water production. To avoid unnecessary spending on an SWH system, one needs to consider a few aspects of today’s various SWH technologies. Depending on the location, weather conditions, and hot water usage of a household, one SWH technology may overthrow the other in terms of overall system performance.

Table 4. Performance comparison of ETC- and FPC-SWH technologies. Adapted from Sunflower Solar [59].

	Evacuated Tube Collector SWH System	Flat-Plate Collector SWH System
Thermal output	Fast	Moderate
Performance	Good performance at hotter temperature	Poor performance at hotter temperature
Heat dissipation	Insignificant during the day	High
Efficiency in frigid temperatures	Moderate	Low due to water freezing
Range of temperature (°C)	60–120	60–80
Insulation	Polyurethane insulation on hot water tank	Glass or rockwool insulation on collector and hot water tank
Tube scaling risk	Little to none—with periodic maintenance	Substantial on the metal tubes
Peak absorption period	All-day long whenever the sun is out	Optimum at noon
Yearly hot water output	350 days	≈ 300 days
Life span	>15 years	>15 years
Maintenance	Simple—one faulty tube would not affect the whole system’s operation	Complex and high cost—glass sheet is expensive, and there is the possibility of replacing the whole panel due to any faulty

shows the performance comparison of the two most commonly implemented SWH systems: the evacuated tube collector solar water heating (ETC-SWH) and flat-plate collector solar water heating (FPC-SWH) systems.

5. Challenges in Solar Water Heating Technologies and Industry

5.1. Technical Challenges

This section of the paper presents the global technical challenges the existing solar water heating technology faces, explicitly focusing on solar collectors. Regardless of the types used, all collectors seem to possess the same issues regarding the sun as the ultimate energy source. Since the Earth only receives sunlight during the day, there is a limited number of operating hours that can be contained by the collectors [60]. On top of that, there are always possibilities for weather uncertainties such as rainy and cloudy days. In countries or regions that experience cold temperatures, the sudden temperature drop during the winter season adds more to the intermittency problems of using the sun as the primary source of energy [61]. There are high chances that intermittent issues affect the overall performance of a solar collector because the full potential of energy efficiency for a solar thermal panel is dependent on the amount of solar radiation that can be absorbed and intercepted by the collectors [62]. The mismatch between the sun’s supply of energy and the user’s energy consumption [63,64,65] and the variability of solar radiation throughout the day leads to a breach between energy demand and energy supply [66].

In the case of a solar water heating system, this mismatch results in an atrocious capacity utilization factor, which further reduces the system’s overall efficiency [65]. Conventional solar water heaters are heavily dependent on solar irradiance; hence, over a specific period, they will be highly affected by the intermittent availability of solar energy [67]. Most conventional solar water heaters come equipped with additional electrical heating elements to compensate for the unattainability of solar thermal energy. The use of electrical heating elements contributes to more energy consumption and the emission of greenhouse gases. Apart from electrical heating elements, a heat pump is another piece of equipment employed for the same purpose. This energy-saving device can increase the grade of thermal energy for higher-level thermal applications [67]. However, because the heat pump’s heating capacity is also affected by solar radiation, the issue of reliance on solar radiation for the heat pump solar water heaters remains [67]. As for the solar air heaters, apart from the mismatch between the solar radiation availability in the daytime and the higher heat demand during nighttime, another issue that is recognized by Kabeel et al. [68] in finned-plate solar air heater is poor thermal efficiency due to the lack of heat transfer from the absorber to the moving air.

The two most prevailing solar collectors (FPC, ETC) possess issues and challenges to combat for the best solar absorption and overall system efficiencies. Since this paper is mainly focused on domestic solar water heaters, only the challenges faced by FPCs and ETCs in solar water heating systems are addressed in detail.

5.1.1. Issues and Challenges in FPC Solar Water Heaters

The solar collector of flat-plate type in SWH systems is famous in the market because they possess simple designs and only require minimum maintenance [69]. Apart from being inexpensive to manufacture, other advantages of FPCs are their ability to accumulate both beam and diffuse radiations, and they need no sun-tracking due to their fixed-in-position design structure [70]. However, to maintain a prolonged practical life, FPCs need to endure the unfavorable impacts of the ultraviolet radiation from the sun and the possibility of clogging due to the acidity, alkalinity, or hardness of the heat-transfer fluid circulating inside the pipes. There is also the risk of water freezing due to extremely low temperature, the risk of overheating due to extremely high temperature, the toppling of dust and moisture on the outer glazing, and even the potential breakage of the glazing from a hail fall or thermal expansion [70,71]. The condensation and moisture problems can cause early failure of the internal pipe materials and eventually reduce the system’s performance [72]. There are also the risks of scaling mineral deposits within the metal pipes, mainly when low-quality water is circulated inside them [59]. Too much scaling would affect the water heating efficiency and cause probable excessive usage of the electric auxiliary heater. The tube scaling also leads to blockages between the riser pipes and the overall piping network in the SWH unit. Moreover, heat dissipation performance in the metal tubes may also be affected [73].

In the case of scorching days with intense solar radiation, the collector could experience overheating and pipe fouling [71]. Overheating from heat accumulation could also happen on days of less heat usage [71]. On top of that, heat loss to the surrounding atmosphere is always likely, due to radiation and convection (e.g., heat loss through the glass cover), especially when a glass temperature increases after radiation absorption [5,69,70]. Not only that, FPCs only contribute to technical and economic effectiveness during warm, sunny days when solar radiation is abundant [69,74]. FPC without an automatic draining device causes water circulating inside the pipes to freeze in cold areas because of the low ambient air temperature. The frozen water’s strain could burst the pipes, and the collector could encounter total damage [75]. Due to their high thermal losses, FPC is also not an ideal option for high-operating-temperature applications [76].

Some practical considerations that need to be considered when maneuvering flat-plate collectors in SWH systems include the possibility of a non-uniform flow of fluid throughout the collector risers due to manufacturing errors. When this happens, the sections that experience a lower fluid flow rate experience a lower heat removal factor and truncate the system performance [70]. Next, the inevitable risk of cover material and absorber plate coating degradations over the years could impact the collector’s transmittance and indirectly affects the collector’s long-term performance [70]. Furthermore, a transparent cover plate, usually formed of glass, is used on most solar collectors. The glass material would cause a reflection of approximately 8% from the incident solar radiation, resulting in decreased collector heat output [77]. Finally, the collector’s mechanical design may also impact its performance, as in the case of water or moisture penetration into the collector, which could cause condensation on the underside of the glass and significantly reduce its functional features. The water penetration issue could lead to system failures [69,70]. As far as the manufacture of the collectors is concerned, it is critical to have a collector housing that can sustain handling and installation and encapsulate the entire collector elements and keep them clean of water and dust penetration throughout the collector’s lifespan [70].

Wang et al. [71] overcame the freezing and overheating problems in FPCs by integrating dual-phase change material within the collector’s structure. Two layers of low-melting and high-melting PCMs, each with 15 °C and 70 °C temperatures, respectively, were installed in the space under the absorber plate in the collector [71]. They found that the temperature rise in the absorber plate was prolonged by 1.6 h under high-temperature conditions. Under low-temperature conditions, the temperature drop was extended by at least 3.1 h [71]. Hussain and Harrison [78] prevented overheating in an FPC solar water heater by incorporating a passive air-cooling channel at the back of the collectors, while Gladen et al. [79] used thermotropic materials to provide overheat protection to the FPC polymer solar absorber. To overcome the freezing of FPC in cold regions, Zhou et al. [75] improved the antifreeze performance by employing a transparent insulation material honeycomb inside a prototype FPC.

In a recent study, a PCM was employed within an FPC alongside the antifreeze to prevent it from freezing damage during cold nights [75]. The temperature difference between the collector’s inlet and outlet was reduced with increased heat losses (conduction, convection, radiation). To improve the collector’s thermal efficiency and thus broaden the temperature difference, Bhowmik and Amin [80] employed a reflector to increase the reflectivity of the collector. They discovered that a reflector improved the collector’s efficiency by about 10% [80]. Chougule et al. [81] increased the efficiency of a thermosyphon FPC by implementing carbon nanotube (CNT) nanofluid concentration with water as the working fluid. Their experiment proved that the FPC with nanofluid was more efficient than water, and the efficiency increased when a higher concentration of CNT nanoparticles was added to water [81]. Aruna et al. [82] also obtained the same results in their experiment of a thermosyphon heat-pipe FPC with titanium dioxide (TiO₂) nanofluid and propanol as the working fluid. However, some drawbacks regarding the utilization of nanofluids in all heat-pipe solar collectors addressed by Hussein et al. include: (1) concern for the stability of the nanoparticle dispersion in the long run, (2) lower nanofluid specific heat when compared to the base fluid, (3) high toxicity of nanofluids, (4) high expenses for nanofluid preparation and testing, (5) risk of corrosion and erosion to the collector, and (6) high viscosity of nanofluids [72].

5.1.2. Issues and Challenges in ETC Solar Water Heaters

Solar water heater collectors of evacuated tube types overcome heat losses through the glass cover by the vacuum within the annual space between the two-layer glass tubes [69]. The structure of an ETC design has proven that the amalgamation of a selective surface and a convection suppressor leads to good system performance at higher temperatures [70]. However, the sealed vacuum between the tubes can get the collectors very hot, up to the point where it exceeds the boiling point of water during hot summer months [83]. The overly high temperature inside the collector tubes may lead to significant problems in the SWH system, as overheating and glass tube cracking may occur [83]. Moreover, evacuated tubes could get heated up too quickly when installed on flat roofs, when the water circulation in the system is too slow, or if the system is oversized [84].

The water-in-glass type of ETC has an exclusive advantage because they are made entirely out of glass, and they do not need to deal with the extra layer of the glass tube for heat transfer. This feature eliminates potential leakage losses between the tubes, making it cheaper than the other types of ETCs (i.e., heat pipe and U-type) [70]. However, this is a design only befitting low-pressure systems. There is a risk of losing all of the working fluid (i.e., water) even if there is one tube break (this type of tube cannot be drained because water is directly circulating within the collectors) [70]. As water-in-glass ETCs absorb heat from the sun through the circulating water inside the tubes, a few other downsides that can notably be seen are the need for extra cost and energy to flow water inside the tubes. There are also risks for heat-transfer impediment by the low specific heat water, possible corrosion of pipes, risk of water freezing during cold nights, and the potential of water circulating backward at night because of the lower temperature outside [85].

Kumbhar [86] has gathered and reported the common failures of water-in-glass ETCs. One of the most frequently occurring problems is manifold leakages due to the failure of sealing gaskets or washers during tube installation. This sealing failure could lead further to corrosion to the manifold and the formation of algae and scaling from the leaking water [86]. For the evacuated tubes themselves, overheating could also happen when underutilized heat from the collectors exerts more pressure and eventually causes breakage to the glass tubes [86]. When the collectors absorb heat, there could be increased pressure caused by the steam collected in the collectors. Here, excessive heating to the flowing water could happen, and when the accumulated pressure is high enough, the glass tube starts to break and burst or explode [86].

Another problem encountered is the salt formation within the collector tubes. Over time, salts from the circulating water will start accumulating on the inner walls of the tubes. Without proper and frequent checking and monitoring, layers of salt will keep forming and eventually block the water flow inside the tubes. Salt forming could also lead to glass tube bursting because the water trapped between the salt layers gets superheated by the intense solar radiation and forms high-pressure steam from within [86]. This particular issue enhances the importance of water quality used in the system.

The possibility of overheating happens in water-in-glass ETCs and all types of ETCs due to the employment of fragile glass tubes [76]. Currently, the low thermal efficiency of heat-pipe ETCs is one of the main challenges in the solar industry [76]. The thermal efficiency of the heat pipe is impaired and restricted by various factors, including entrainment, capillary, sonic, velocity, and boiling limitation [87,88,89,90,91]. Moreover, gravity may also impact the efficiency of the heat pipe. The collector’s thermal performance is also dependent on the inclination angle. Here, determining an optimum inclination angle is crucial because a higher inclination angle creates a smaller surface area for heat absorption by the collectors [90].

To increase the overall thermal efficiency of heat-pipe ETCs, many researchers and scholars have attempted the integration of nanofluids into the working fluid inside the heat pipes. It has been proven that the thermal performance of heat pipes increases with higher nanofluid volumetric concentrations and bigger nanoparticle sizes due to the decrease in the thermal resistance of the heat pipes [92]. A prototype solar collector with aluminum oxide (Al₂O₃)-water nanofluid designed by Putra et al. [90] produced the same results as Kang et al. [92] hypothesis, where their prototype’s thermal resistance was as low as 5.32 °C/W. The influence of nanofluid lowered the working fluid’s thermal resistance and provided higher thermal performance and heat transfer for their prototype collectors [90]. The U-type ETCs are not as flexible as the heat pipe ones because they have heat-transfer fluid (i.e., water) that flows into and out of every glass tube, making the whole system vulnerable even if one tube cracks or breaks [83]. Other than that, a comparison study by Redpath [91] shows that the ETC-SWH systems with internal condensers are 17% more efficient that the external ones. The internal and external heat-pipe condensers are illustrated in Figure 10 and Figure 11, respectively.

5.2. Economic Challenges

Unprecedented economic challenges are again driven by the higher costs of SWH system purchase, installation, and maintenance compared to conventional technologies. Despite the system’s benefits, consumers in many parts of the world find it challenging to implement one due to their financial restrictions [93,94]. There is also the possibility of some hidden costs that could incur with installing an SWH system. Some hidden costs include reinforcing the roof to support the weight of fully-filled solar tanks (for the compact, roof-mounted system). The plumbing modification cost on the roof to the piping system inside the household, the insulation cost, and the cost to supply cold water to the system are part of the hidden costs [95]. Another hidden cost that tends to be overlooked is the cost of replacing broken roof tiles should any mishaps occur during the SWH system installation.

With the SWH industry not adequately developed in South Africa, economic barriers have prevented the people from utilizing SWH systems to a vast extent. The country’s limited number of manufacturers and SWH products perpetuated the relatively high prices of the SWH systems in South Africa [96]. Even with many incentives offered to the consumers, an analysis conducted in India identified low natural gas prices as one of the main barriers that hinder the outspread implementation of the SWH system in some areas [16]. Here, the higher costs to manufacture an SWH system cannot compete with the cheaper and more affordable option of the electricity-based water heating system. The low tariffs for off-peak electricity and gas have also been a challenge in Australia [97] and some other parts of the world. A group of respondents to a survey carried out in Sydney divulged that gas water heaters always delivered guaranteed hot water, and they could save over AUD 200 for maintenance every five years [98]. In South East Mediterranean Countries (SEMCs), the market prospect for renewable technologies, including SWH technology, was inhibited by the enormous subsidies for fossil fuels and electricity [99].

6. Potential Solutions to the Challenges and Future Research Directions

6.1. Government, Industrial, and Local Level Support

Despite the challenges currently faced by the SWH technologies industry, it is no doubt that water heating is an essential part of a human’s daily life. Hence, SWH technologies will always be relevant to consumers in many parts of the world. To combat the increasing demands, all stakeholders in the SWH industry must dive deeper to compete with the conventional water heater markets. Policy support plays a crucial role in the government’s higher deployment of solar water heaters. Direct policies such as targets, programs, obligations, and mandates fall into this category [100]. Apart from that, to alleviate some of the loads from high initial costs, financial incentives such as subsidies and low-interest loans can be offered to prospective users [100,101,102,103]. However, government support should not only be limited to the end-users. Technical experts and know-how can be molded through various standard certificates, training, and retraining policy measures. This way, the maturing of the solar water heater sector can be further developed [100]. Policy implementation for other non-financial elements, such as marketing strategies and consumer trust, is also essential [104].

The other key factor that can considerably boost the SWH industry includes solid local support to set the seal on the thorough utilization of solar water heaters. China has achieved this even without much support from the Central Government, the strong support at the local level has led the SWH industry in the country to develop notably well in recent years [105]. China has also proven that vehement reinforcement at the grass-roots level to implement the SWH system across the country could contribute significantly to its low carbon transitions [105]. Strict law enforcement on the requirement to install solar water heaters on every new building in urban and rural areas brings significant growth to the industry. In this prospect, incentives and subsidies from the government promote further development of a country’s SWH industry [19]. A city bylaw has also enforced installing solar water heaters in many households in South Africa. Their solar water heater bylaw was strictly imposed on every new building that was being and will be built in the city, existing buildings that required extra water heating equipment, and all buildings that were going to be retrofitted [19]. The city bylaw is a highly effective mechanism to drive the implementation of SWH technologies and stimulate the SWH industry. It can be applied to other parts of the world with precise enforcement.

6.2. Public Awareness

The public is aware of the country’s resource scarcity in China. Because of this, the general end-users have positive attitudes toward contributing to the government’s plan for their low carbon transitions [19]. Apart from the efforts from the government, industrial, and local level stakeholders, the grassroots level awareness among the locals towards the benefits of solar water heaters should be regulated. A fascinating fact is that most people in China are very environmentally aware, contributing to the most significant breakthrough in the form of solar water heaters. This movement aligns with the value-belief-norm (VBN) theory proposed by a group of researchers in 1999. A few recent studies suggest that the social-psychological and behavioral determinants of individuals involved could directly impact the acceptability of the people towards any sustainable energy transition moves [104,106,107,108].

Economic incentives such as job opportunities tailored to growth and clean energy policy have played a key role in China’s massive adoption of SWH technologies [109]. In Georgia, local project partners raised awareness of solar systems among its people by creating information campaigns and spreading it across the country via press releases, radio shows, television broadcasts, and newspaper articles [19]. This move ensured adequate information reached the people in rural areas such as villages. It was demonstrated to be effective when the interest amongst the local population seemed to increase progressively [19]. Tunisia also undertook the awareness-raising campaign approach, but their targeted audiences, aside from the consumers, also included the commercial banks, financial institutions, and technology providers [19]. That approach was a brilliant move from Tunisia, as approaching multilevel individuals in the SWH industry structure could secure long-term knowledge and expertise within the SWH industry itself.

6.3. Low Initial Cost

Ensuring low initial solar water heater installation costs is vital for a rigorous penetration of the technology, especially in rural areas. A 2009 policy in China has provided subsidies of 13% reduction from the wholesale price for installing solar water heaters for the countryside households [105]. This move has undoubtedly improved the availability and use of SWH technology in rural areas of the country. India has also done the same where soft loans, capital subsidies, rebates, and accelerated depreciation are provided to residential, industrial, and commercial customers to install SWH systems [16].

Apart from mechanisms by the government, affordable and low-cost SWH systems within the rural areas can be achieved by training local men and women to maintain the systems using local materials. This approach is proven successful in Georgia, where they taught local workmen and women and adapted local materials for installing solar warm water heaters for low-income households within remote areas in the country [19]. Also, essential monitoring and maintenance education and training on the system should be imposed on providers and homeowners [19]. A similar appeal has been made in Brazil’s national project, the ‘1000 Roofs Programme’. They trained technicians in planning, installing, servicing, and maintaining the solar thermal system installation [110].

Another way to lower the initial cost of solar water heater installation is by reducing the manufacturing cost. A novel study conducted by researchers in Malaysia utilized recycled solid waste (i.e., crushed glass) as an alternative material for solar collector main heat absorbers. They found that a black-colored crushed glass absorber has a promising potential as its maximum efficiency was comparable to that of a regular aluminum absorber [111,112]. However, this modification is still thin on the ground, and thorough research is required in the coming years.

6.4. Thermal Energy Storage (TES) in the SWH System

Thermal energy storage (TES) integrated within the SWH system enables storing thermal (heat) energy from the sunlight for later uses when the sun is no longer available at night or when the sunlight is sparsely obtainable during cloudy, rainy days. Many researchers and scholars are recently exploring TES in SWH systems due to its inexpensiveness to installation, easy maintenance, and simple fabrication [30]. A TES system requires a heat storage medium to store the energy, and a phase change material (PCM) is usually adapted for this purpose [113,114]. The heat storage medium acts as a heat-transfer medium that stores the excess energy from sunlight during the day and releases the heat later on for use when the sunlight is no longer accessible. Implementing a TES inside the SWH system allows the continuous supply of hot water from the solar water heater for most of the day for domestic applications. A PCM in the TES system enables a high thermal energy storage density due to its isothermal nature. The temperature within the thermal storage tank remains unchanged until the PCM is needed to heat the household water [30]. A review paper by Douvi et al. [31] extensively studies various aspects of PCMs in solar domestic hot water systems. Some of the potential PCMs for TES in SWH systems are shown in

Table 5. A few potential PCMs for energy storage in SWH systems.

PCM	Chemical Composition	Melting Point (°C)	Heat of Fusion (kJ/kg)	Specific Heat (kJ/kg∙°C)	Reference
Paraffin wax	CnH2n+2	56	142.7	2.4 (s) 1.6 (l)	[115]
Urea	CO(NH2)2	132	250.66	1.34 (s) 2.99 (l)	[116]
Sodium acetate trihydrate (SAT)	NaC2H3O2∙3H2O	58	264.18	1.7 (s) 2.9 (l)	[116]
Lead acetate trihydrate	C2H3O2Pb∙3H2O	75	174	0.76 (s) 2.14 (l)	[116]
Tricosane	C23H48	45–47	210	2.89 (s) 2.89 (l)	[117]
Sodium acetate	NaCH3COO∙3H2O	57–58	226	1.7 (s) 2.79 (l)	[117]
Water	H2O	0	333	2.09 (s) 4.18 (l)	[117]
Barium hydroxide	Ba(OH)2·8H2O	78–82	193	1.17	[118]
Erythritol and graphite composite	-	119	312.2	-	[119]
Myristic acid	C14H28O2	51–52	196.2	1.60 (s)	[120]
Erythritol	C4H10O4	119	332–340	1.38 (s) 2.76 (l)	[121]
Pentaerythritol	C5H12O4	186–187	287–298	-	[121]
Azelaic acid	C9H16O4	98–108	174	-	[121]
Sebacic acid	C10H18O4	130–134	228	-	[121]
Dimethyl terephthalate	C10H10O4	142	170	-	[121]
P-toluic acid	C8H8O2	180	167	-	[121]
Octadecanamide	C18H37NO	106	211	-	[121]
Organic chemical-based HS29	-	29	190	1.51 (s) 2.62 (l)	[65]

below.

6.4.1. TES Integration on Collectors

One of the methods of TES integration into the SWH systems is placing the PCM at the collector side. Directly adding PCMs to the solar collectors often results in higher efficiencies [31]. Various designs of solar collector modification with a TES system have been conducted, and only some have made the way towards commercialization [60]. Among the standard placements of PCMs are underneath the absorber plate (in FPCs) and around the heat pipes (on ETCs) [31]. Feliński and Sekret [122] analyzed the effects of PCM-integrated ETC on the operating parameter characteristics of an SWH system. They used technical grade paraffin inside the glass tubes and found that the PCM was able to delay the release of heat during hot water peak loads. The water temperature also increased during the lowest solar radiation hours [122,123]. In China, Li and Zhai [119] integrated a PCM composited by Erythritol, expanded graphite within the aluminum pipes, and placed them inside the evacuated tubes. Their results revealed that the storage efficiency could reach 40.2% for mid-temperature applications [119]. Papadimitratos et al. [121] also used Erythritol and combined it with Tritriacontane (dual-PCM) inside an ETC. They immersed the heat pipes entirely in the dual-PCM layer (Figure 12) and figured that the collector efficiency improved by 26% to 66% [121].

The integration of paraffin wax PCM in the evacuated tubes of the collectors was performed by Mahfuz et al. [124] and Wang et al. [125]. They discovered that the PCM was indeed able to reduce the thermal losses inside the tubes and enhance the overall efficiency of the collectors—refer to Figure 13. In their work, Essa et al. [126] filled 2.3 kg of paraffin wax around the tubes of U-type ETC and studied the effects of heat-transfer fluid (HTF) flow rates on the thermal delivery from the PCM—refer to Figure 14. They found that the system was at its best efficiency when low flow rates were applied because the paraffin could go through a complete phase change under lower flow rates [126]. Khan et al. [60] reviewed recent progress on various designs of solar collectors integrated with latent heat TES systems. Xue [118] made a performance comparison between a regular and a PCM-integrated water-in-glass ETC and discovered that the system with the PCM performed better. Abokersh et al. [127] modified a finned and non-finned U-type ETC with the addition of paraffin wax inside the collector tubes. They discovered the existence of the fins and the PCM integration did improve the overall system efficiency.

Lin et al. [129] investigated the effects of a TES system on the solar water heater performance by integrating paraffin wax directly onto the lower surface of the absorber plate on an FPC. Their design again proved the effectiveness of PCM with the increased hot water temperature and overall system efficiency. Meanwhile, Carmona and Palacio [130] in Colombia evaluated the performance of an FPC by incorporating a PCM container between the absorber plate and the insulation layer. Zhou et al. [131] combined an FPC with antifreeze characteristics using a PCM to store thermal energy during the daytime and prevent freezing at night. As shown in Figure 15 and Figure 16, the PCM module is layered between the collector pipes and the insulation layer. The system revealed that it still worked on days with an average temperature between 0 °C and 5 °C [131]. Suffer et al. [132] placed a soft paraffin wax PCM at the back of the collector as a storage media. Their experiment revealed that the PCM could heat water during nighttime to supply hot water the following day. A few other studies on the integration of TES systems on solar collectors are displayed in

Table 6. A few studies on the integration of TES systems on solar collectors.

PCM	Melting Point (°C)	Collector Type	PCM Placement	Results	Reference
Paraffin wax	40	Heat-pipe FPC	PCM layer was placed at the bottom of the solar absorber plate and heat pipe.	The PCM layer improved the overall system performance.	[133]
Paraffin wax	46.7	FPC	A PCM container is placed at the back of the collector’s copper pipes.	Depending on the weather conditions, around 45% to 54% efficiency enhancement was achieved.	[134]
Urea–sodium acetate trihydrate (USAT)	30	FPC	PCM mixture was poured into the collector’s container.	The melting point of the storage material was increased by 12.5 °C.	[116]
Barium hydroxide	78–82	All glass ETC	A PCM storage unit was built-in inside each collector tube.	Efficiency was enhanced by 8% as compared to conventional SWH unit.	[118]
Erythritol and graphite composite	119	ETC	PCM was filled in aluminum pipes inside the tubes	Average storage efficiency of nearly 41% was achieved.	[119]
Triacontane and Erythritol	71, 118	Heat-pipe ETC	The heat pipes inside evacuated tubes are fully immersed in the dual-PCM.	There was 26% efficiency improvement for normal operation and 66% for stagnation mode.	[121]

.

6.4.2. TES Integration on Water Tanks

Another method to integrate a TES into the SWH systems is placing the PCMs directly inside or on the water tanks. Paraffin wax is one of the most extensively used PCMs in studies in many parts of the world. A group of researchers from Thrissur, India, modified a 60 L water tank with the integration of a PCM tank filled with Paraffin C-32 as the heat storage medium and found that the hot water availability inside the water tank was prolonged by over 3 h when compared to the conventional solar water heater [64]. Rathore [65] investigated the performance of a solar water heater integrated with a stainless steel PCM cylinder and discovered that the thermal performance of the water heater was indeed enhanced with the presence of the PCM storage. Another study in India also confirmed that for a solar water heater to have better thermal performance, it is a prerequisite that the water heater is equipped with the addition of a PCM with high latent heat and a large surface area for heat-transfer purposes [135].

Beih and Chakra [136] in Lebanon examined the performance of a domestic solar hot water storage tank with a sodium acetate trihydrate (SAT) PCM heating in the sphere and tube forms against an electrical resistance (ER). They revealed that the tank with a PCM storage could keep the water inside the tank warmer than 55 °C for 10 h and reduced the energy consumption and CO₂ emissions by about 6.5 MWh and 5.5 tons, respectively [136]. Deng et al. [137] also utilized SAT as a PCM in their domestic hot water heater prototype: 35 kg of the SAT PCM was placed inside the mantle of the water tank and was layered around to insulate the water tank from top to bottom—refer to Figure 17 and Figure 18. They revealed that the PCM layer worked well as a thermal battery and that their design had the potential to penetrate domestic heat storage for the low-temperature solar thermal market [137].

A group of researchers in Turkey had the same design as Deng et al. [137] to integrate PCMs as a layer of ‘insulation blanket’ for the water tank. Instead of SAT, they experimented with a few PCMs, including potassium fluoride, lithium metaborate dihydrate, strontium hydroxide octahydrate, barium hydroxide octahydrate, aluminum ammonium sulfate, and sodium hydrogen phosphates. Their findings proved better solar water heater efficiencies when PCMs were utilized [138]. A nanocomposite PCM consisting of paraffin wax and silica nanoparticles was tested using a compact, heat-pipe evacuated tube solar water heater [139]. The nanocomposite PCM tank was integrated directly inside the solar water tank, and a performance comparison was later made against the conventional system. The nanocomposite PCM system was the most practical, with an energy efficiency of 74.79%.

A simulation study conducted by Najafian et al. [140] concluded that the discharge time of a domestic solar water heater has a direct correlation with the amount of PCM placed inside the water tank, the diameter of the PCM container, and also the location of the PCM container inside the water tank—Figure 19. Here, a more significant amount of PCM placed inside the tank prolonged the duration of hot water availability. However, it is necessary to find the optimum amount of PCM required as any extra amount can contribute to more electricity consumption and increase the overall mass of the SWH system [140]. As for the location of the PCM in the water tank, placing PCM containers at the top of the tank would be the wisest choice to ensure that the PCM loses heat more slowly [140]. Finally, decreasing the diameter of PCM containers would reduce the surface area for the heat transfer between the PCM and cold water. Lower surface area enables the PCM to lose heat more slowly and indirectly; it will increase the discharge time [140]. A few other studies on the integration of TES systems on solar water tanks are listed in

Table 7. A few studies on the integration of TES systems on solar water tanks.

PCM	Melting Point (°C)	PCM Placement	Results	Reference
Paraffin wax C-32	66–69	A smaller tank of PCM was fabricated at the center of the solar water tank.	Energy discharge for a system with PCM is longer than the conventional one (hot water available for a more extended period).	[64]
HS29 Calcium chloride-based	29	PCM was placed in a stainless-steel cylinder outer tank.	The capacity for heat storage and thermal performance of the system was enhanced.	[65]
Sodium acetate trihydrate (SAT)	58	PCM was placed in aluminum cylinders and tubes within a solar water tank.	Primary energy consumption was reduced, and water was kept warmer than 55 °C for 10 h.	[136]
SAT composite	58	PCM was sealed in the mantle (22 mm thickness) in between the solar tank’s inner tank and insulation material.	The PCM layer within the water tank was proven to act well as a heat storage medium, with over 70% of the stored heat energy contributed to the hot water supply.	[137]
Aluminum ammonium sulfate and sodium hydrogen phosphate mixture	42–60	A heat storage tank with PCM was embedded underneath the water tank.	The PCM-integrated heat storage tank has higher efficiency with maximum exergy efficiency of 22%.	[138]
Commercial-grade paraffin wax	63.74	A PCM container was fabricated inside the solar tank.	Solar collector’s energy efficiency was increased by over 10%, exergy efficiency was improved by 5%, and there was a higher hot water temperature in the morning.	[139]

.

6.5. Thin-Film Material Coating on FPC

High-efficient FPCs operating at temperatures above 100 °C can be produced with improvised insulation of the transparent cover plates [141,142,143]. Thin-film material is a low emitting antireflection coating applied on the glass cover of an FPC. The purpose is to increase the transmitted energy into the collector’s absorber and pipes. The refractive index of the coating material must be low to minimize the incident solar radiation reflection [77]. Stacking multiple layers of thin-film materials enhances the coating’s performance [144]. Common materials include magnesium fluoride and lithium fluoride [145]. Nanomaterials can also boost the efficiency of thin-film coating materials [144]. Ajeena et al. [144] reviewed the fundamental frameworks, fabrication methods, and self-cleaning nanocoating for various nanocoating technology.

Khoukhi et al. [77] investigated multiple single-layered, homogenous, non-absorbing antireflection films coated on the outer layer of FPCs’ glass covers. Compared to the uncoated glass cover, the coating materials indicated an increment of 2% of the solar radiation transmissivity and 2% to 17% reflectance reduction. Ehrmann and Reineke-Koch [143] studied the efficiency of an FPC with a low emitting double-glazing coating applied on the inner side of the glass cover. The FPC’s efficiency improved with an 85% higher solar transmittance than a conventional FPC. To absorb most of the incident solar radiation and reduce radiative energy losses, Taylor and Viskanta [146] suggested that the cover plates be highly transparent for short-wavelength solar radiation. Coating the inner side of the cover plate with a semiconducting metallic oxide film is one way to do it.

6.6. Feedwater Treatment Process

High-quality feedwater to the collector is vital for SWH systems that use water as the heat-transfer fluid to avoid mineral deposit buildup or scaling. One way to remove the minerals from the water is by using water softeners [147]. Depending on the water quality, mild acidic solutions (e.g., vinegar, lye) can be circulated through the collector’s metal pipe once every few years [147,148]. However, in direct SWH systems, adding chemicals to circulating water is inconvenient because the water will be directly consumed by the users [149]. The non-chemical means for treating feedwater, specifically for SWH system application, are not found in the existing literature. Nevertheless, several physical water treatments (PWTs) prevent scale build-up in heat exchangers. Since they deal with the same mineral scaling issue, these treatments are also deemed applicable in the case of SWH feedwater.

One of the foremost mechanisms of PWT is bulk precipitation. PWT causes suspended particles of mineral ions to form in bulk water, essentially referred to as bulk precipitation. When settled in the water, the suspended particles are likely to enlarge, resulting in particulate fouling on the surface [150]. Instead of forming a hard and difficult to remove scale (i.e., precipitation fouling), particulate fouling develops a soft and easily removed sludge layer on the heat-transfer surface [151]. Hence, the mineral buildup issue can be mitigated by simply adding the tiny, suspended particles to the water [150]. Another PWT mechanism is the magnetic water treatment. Due to changes in solution properties, magnetically treated water produced several forms of scale formation [152,153]. Donaldson [154] implied that the magnetic field causes modifications to the particle charges and hence, alters the crystal growth in water. Apart from magnetic treatment, electric fields can also be utilized for treating feedwater. Each of these mechanisms is elaborated in detail by Cho and Kim [150].

6.7. Future Research Direction of SWH Technologies

Numerous researchers and scholars worldwide are racing for breakthroughs to make domestic solar water heaters more thermally efficient and affordable for all human nations across the globe. The studies generally focus on every fundamental component of a solar water heating system, including the solar collectors, solar tank, thermal storage system, solar absorber material, heat-transfer fluid, system flow rates, etc. This review paper focuses on some of the most thriving and arising branches of the whole SWH system development, including the thermal energy storage material, the utilization of nanoparticles for the enhancement of the TES materials, and the hybrid solar photovoltaic-thermal (PVT) system. This segment of the paper also discusses the potential strategies to reduce the overall production cost of the solar thermal systems as suggested and conducted by the International Energy Agency’s Solar Heating and Cooling Programme.

6.7.1. Solar Water Heater Performance Enhancement Methods

Researchers worldwide have come up with various innovative methods to enhance the performance of SWH systems. The heat-transfer process of a solar water heater can be improved by optimizing the collectors’ overall design and shape. Also, heat transfer is boosted between the solar absorber, and the heat-transfer fluid (HTF) is made possible by a variation of in-flow rates and vortex generators [155]. Some of the crucial elements from the overall design parameters for the optimum thermal performance of an SWH system include the total collector area, the amount/thickness of insulation, and the mass flow rate of the HTF [156]. Many studies and experiments are currently being conducted to improve the thermal efficiency of solar water heaters. In a case study in Saudi Arabia, the flat-plate solar collectors were modified by installing side reflectors on both (right and left) sides of the collector box (Figure 20). The side reflectors were added to concentrate both diffuse and direct radiations from the sun towards the collector surface. The modification resulted in a significant improvement to the thermal efficiency of the collectors with a 12% increment from the regular ones. The output temperature of the water was also increased by 12 °C [157]. Another design modification was made in Romania, where the flat-plate solar collector was developed with a triangular shape (Figure 21). The novel design was explored to minimize the distortion of the collector’s central body and optimize the flow distribution within the collector for a better overall system thermal efficiency [158].

A modification was made to the absorber of a solar collector by Kim et al. [159]. They placed a series of transparent heating tubes on top of the absorber plate of the collector. They had colored water flowing through the tubes. They discovered that the heating tube with black-colored water gave out the highest temperature rise compared to the pure water and red- and violet-colored ones. A group of researchers developed an inner surface treatment for micro-grooved aluminum flat-plate heat pipes via chemical corrosion. The targets were to enhance the thermal efficiency and reduce the overall costs of the collectors. The experimental study revealed that the treated heat pipes resulted in an 80% increase in the collector’s thermal performance and a 44% reduction in the thermal resistance [160]. Muthuraman et al. [161] in Tamil Nadu, India, experimented with three geometric configurations of the FPC’s flow tubes: straight, curved, and spiral-shaped. They concluded that the spiral tube configuration yielded the best efficiencies of 69% and 73% for water mass flow rates of 0.0045 kg/s and 0.006 kg/s, respectively. Mentioned in this paper are some of the most recent and fascinating modifications that have been made by researchers around the world towards the enhancement of SWH systems. New studies and discoveries towards the advancement of SWH systems performance are expected to progress as new technologies and ideas surface rapidly.

The literature above indicates that the performance enhancement of solar water heaters is indispensable in the new research direction of SWH systems. Therefore, it is recommended that more research be conducted on the novel design parameters of the solar collectors and how they affect the overall hot water production of the SWH unit. Another suggestion is for more theoretical and experimental studies of these novel collector designs to be conducted in the actual hot water consumption patterns in various locations and climatic conditions. Moreover, investigations on the effects of operational parameters such as the mass flow rates of the HTFs and the angle of inclination of the collectors are worth looking into as they certainly contribute towards the optimum operation of the entire SWH system.

6.7.2. Thermal Energy Storage (TES) Materials

A TES system must be designed from the storage materials with high density, high latent heat of fusion, high specific heat, relative melting point, minimal supercooling, high thermal conductivity, low vapor pressure, high thermal stability, high chemical stability, minimal volume change, and which are abundant and accessible, non-toxic, non-corrosive, non-flammable, and low-cost [162]. Due to their considerably high thermal storage capacity, phase change materials (PCMs) have always been the go-to alternative to be incorporated in any thermal storage system. The enhancement of PCMs using nanomaterials (nanoparticles and nanofluids) has always been a hot topic for thermal energy storage material advancement. Some of the most actively and extensively studied PCMs are paraffin waxes, fatty acids, salt hydrates, and eutectic mixtures (metal and metallic alloys) [140,163]. The research and experiments on these PCMs are expected to keep advancing and evolving for many years as new technical innovations emerge.

Besides PCMs, scientists and scholars started to turn their attention toward something new and state-of-the-art with renewable resources as storage materials. Hungary’s experimental study was conducted using new renewable raw materials for latent heat thermal energy storage (LHTES). Beeswax, a renewable natural resource constituted of a mixture of esters, was used as thermal storage material, and its thermal behavior was evaluated. The three scenarios under investigation included the characterization of beeswax methyl ester derivatives, the synthesis of beeswax with hydrogenated cooking oils, and the combination of beeswax with paraffin [164]. The transesterified beeswax gave 15% higher melting or crystallization enthalpies when compared to the regular beeswax [164]. The mixture of beeswax and hydrogenated waste cooking oil also showed promising results for its adaptation as a thermal storage material. Although this area of research is yet to be widely explored and developed, the results from this experiment show that there will be significant impacts on storing heat energy while simultaneously taking advantage of using completely renewable sources for the overall solar heating system.

Integrating a TES unit into a solar water heater involves an additional component in the SWH network (e.g., PCM tank or PCM layer). Hence, in the TES-integrated SWH system research direction, a few considerations have to be factored in. It is recommended that more research is conducted to address the complexity of the overall SWH system once a TES unit is added. Some aspects to look out for are the potential thermal losses, the installation and modification costs, and the added space required (in or out of the SWH system). In latent heat TES systems, the PCMs tend to solidify when the melting points are not reached. Hence, further investigations on PCM performance under demanding conditions (i.e., little radiation) are needed. Furthermore, more studies on the enhancement of PCM properties such as the mass density, thermal conductivity, specific heat, and heat of fusion are also recommended.

6.7.3. Hybrid Solar Photovoltaic-Thermal (PVT) System

Solar photovoltaic-thermal (PVT) systems are the hybridization of two systems—solar electric (PV) and solar thermal (T)—that allows for energy conversion in one implementation for more significant outputs per unit area covered [165]. They use the same physical features to generate power and collect heat [166]. Even though the PVT systems are relatively new within the solar water heating markets, their development and advancement are expected to skyrocket in the shortcoming years. An optimum cooling system is one of the keys to having the best overall PVT performance [167]. Also, nanofluids as the working fluids are proven to enhance heat-transfer rate within the PVT systems [168,169]. As suggested by the California Energy Commission, one novel method of utilizing PVT includes PV-to-hydronic water heating. The PV system could also directly power a heat pump water heater [170]. The feasibility of this kind of technology is proven in a laboratory study for saving delivered electricity to a certain extent compared to the readily available SWH system coupled with a heat pump water heater [171]. This part of the technology development is still rare and not yet commercialized, leaving room for future advancement in SWH systems. Considering the demand and lower costs of PV panels nowadays, there will likely be considerable optimism about this technology in the coming years.

One of the hurdles that need to be taken care of is finding an efficient way to discharge and transfer heat from the hybrid collector [172]. Researchers have addressed several methods to counter this issue, as reported in [166,172,173]. To enhance the heat-transfer performance, a research team developed a novel design to integrate PCMs and nanofluids into the PVT systems [174,175,176]. An experimental study in Kuala Lumpur, Malaysia, investigated the effects of cascading the two types of solar collectors (i.e., FPC and heat-pipe ETC) directly to a PVT system. The study reported that the PVT system cascaded with a heat-pipe ETC delivered better performance than conventional [177].

For PVT collectors to be as competent as the other collectors in the SWH market, a fluid outlet temperature higher than 60 °C is required [178]. Since PVT is considerably novice in the market, new research and strategies to reduce the convective, radiative, and electrical losses at high temperatures are recommended. It is also suggested that more in-depth studies on clear low-emissivity coatings for PV modules be made. As mentioned in the literature above, incorporating nanofluids in the PVT system to increase the heat-transfer rate should be explored further. In addition, coupling PV-powered heat pump within the SWH network seems like another alternative worth looking into. Hence, more theoretical and experimental studies, especially for long-term operations, are necessary for a more mainstream PVT-SWH system utilization in the forthcoming years.

6.7.4. Reducing the Price of Solar Thermal Systems

Some aspects that can be looked into for lower cost of the solar thermal system are exploring new materials and detailed analyses of the production processes [165]. IEA-SHC’s Task 54 3-year project (2015–2018) has researched new materials. They foraged for new and upgraded materials that could lead to up to 40% reduced solar thermal system purchase costs [165]. They focused on polymeric materials, polymeric compounds, continuous and discontinuous processes, and collector systems with all-polymeric materials. With a total reengineering of the solar thermal systems, more than 50% cost reductions were possible. Small-scale hot water systems had the most potential for the novel all-polymeric system, especially for warm and tropical climates [165].

The National Renewable Energy Laboratory (NREL) has previously taken this approach. They claimed that big chunks of solar water heater production’s weight and overall costs could be trimmed using polymeric materials instead of conventional metallic materials. However, they highlighted a few key issues, including overheating and freezing protection, glazing designs, material testing for optimum durability, and straightforward designs to ensure reliability, lower cost, and easy installation [179]. The highlighted issues indicate that the roadmap for realizing low-cost solar thermal systems is spun out, as many aspects still need to be addressed, and the room for future research is still huge.

One successfully developed and commercialized system was Borealis AG’s tailor-made absorber material for hot water collectors using a polypropylene block copolymer [180]. A recent study tested a novel and inexpensive flat-plate solar water heater design by employing black Plexiglas plates as the energy insulator and absorber. Their fabrication cost was reduced by 13.7%, and the efficiency was analogous compared to the standard metallic collectors. The lower price of black Plexiglas made from recycled plastic waste adds to the advantage of producing cheaper, comparable, if not better, performance of solar water heaters than conventional ones [181].

Investigations from the SHC Task 54 project also indicated that the SWH system price could be further reduced by implementing lighter materials for the solar collectors. They used new lightweight materials from high-performance polymers as absorber materials and engineered plastics as glazing materials [165]. These materials were believed to curtail the collector’s weight by roughly 8 kg/m². The weight reduction would further lower costs due to easier handling during the production, transport, and installation [165]. Other measures considered were pre-insulated plastic pipes and over-heating protection elements for the collectors and the drain-back systems [165]. This novel invention is yet to be widely developed, and only very few designs are currently available on the market. Thus, the possibility of this particular technology being deeply explored in the future is relatively high.

In the future research directions for reducing the solar thermal system pricing, further studies on the techno-economical and life cycle analyses of SWH technologies are strongly recommended. Apart from the effects of economic parameters on the cost competitiveness of the solar thermal systems, the investment cost and the reduced energy cost from the lesser use of electricity should be considered in the economic analysis. Additionally, the investigations for new lower-cost materials are also commended. In searching for new materials, more research similar to the SHC Task 54 project should also be performed in different parts of the world.

7. Conclusions

This paper presents the global technical and economic challenges domestic SWH technologies and industry face. Detailed discussions on SWH modes and working principles alongside the TES integration into the designs are also addressed to understand the system further. Some potential solutions and future research directions are also covered to give a better picture of how the industry could evolve for better and more advanced future development. Researchers and scholars around the globe have attempted many modifications and upgrades to ensure optimum operation of the domestic SWH systems. Although the challenges and limitations of SWH systems are rarely reported, that does not mean there are no issues. This review paper highlights various technical and economic issues regarding implementing several domestic SWH technologies. The potential solutions suggested should give readers a clear idea of how to tackle the possible problems encountered.

The economic challenges within the industry can be overcome when the government, industrial stakeholders, and public (users/consumers) play their respective roles to ensure the proper implementation and operation of the SWH system throughout the country. As for the technical difficulties, further investigation and thorough checking of the possible emerging failures should be considered to prevent total damage to the collectors. Any damage to these collectors should be avoided at all costs, as the expense to repair the damage could be as high as replacing them with the new ones. Regarding the SWH technology, the focus should be on the thermosyphon system, as it accounted for over 55% of global installation. As Global Market Insight predicts that the SWH market size will likely exceed USD 3 billion by 2025, it is worth the efforts and resources from all parties to enhance the said technologies.

Moreover, many countries are now shifting their attention towards the sustainability of SWH technologies, including two of the most significant contributors to the worldwide SWH installation. China aims to increase its clean energy targets from 20% to 35% by 2030. The United States also projects to expand over 10% of its solar water heater market by 2025. This would not only escalate the development and advancement within the SWH industry but also affect the global growth rate of solar thermal utilization capacity.