Solar Heating with Flat-Plate Collectors in Residential Buildings: A Review

Abstract

The depletion of fossil energy reserves has intensified the interest in renewable energy sources, such as solar energy. Solar water heating represents an environmentally clean technology, with an abundant, permanent, renewable energy source that does not pollute or harm the ecosystem. In this context, the objective of the work was to revisit the theme of residential solar heating in relation to the use of flat-plate solar collectors. This study combined bibliometrics techniques and a systematic literature review. The results indicated that by considering the period from 1993 to 2020, we could find several publications revealing that the interest in this subject still remains high and current. Themes related to water heating and ambient cooling showed consistency in the publications, while studies focused on integrating solar thermal energy with other chemical processes, such as distillation or desalination, indicated that significant research is required in this area.

1. Introduction

In search of alternatives, the researchers and governments seek the so-called renewable sources to increase the efficiency of use and lower the cost of electricity generation. The decision basis for the energy source used must be safe, economical, and environmentally correct. Due to the desirable impact, the decrease in dependence on fossil fuel use, and availability, it is widely believed that solar energy should be one of the most widely used sources of energy [1].

With technological advances and the trend toward sustainable energy through renewable sources, the development of technologies that use this type of energy has thus become a priority, whether for industrial purposes, domestic uses, or scientific research [2]. The use of solar energy, in the form of thermal energy (object of the present study) or photovoltaic energy, is one of the most promising energy alternatives to face the challenges of the new millennium guaranteeing sustainability.

Thirugnanasambandam et al. [3] claimed that solar energy is by far the most abundant of all available energy sources, including renewable and non-renewable, and can be explored directly and indirectly. However, 73.20% of the energy currently consumed in the world comes from non-renewable sources, mainly petroleum and natural gas [4].

According to Salamoni and Rüther [5], the least sunny region in Brazil has solar irradiation rates of around 1642 kWh/m²/year, which are above the values presented in the area with the highest solar incidence in Germany, which receives about 1300 kWh/m²/year. Despite having better weather conditions, in the market comparison, Brazil is behind this European country. Therefore, the use of solar energy to heat water in buildings in Brazil should be encouraged.

Yirfa [6] pointed out three major benefits of using solar energy. The socioeconomic benefits are reduced energy costs and increased job creation to manufacture the equipment that makes up a solar energy system. On the other hand, the benefits of energy saving reflect a decrease in oil imports for energy generation for a given country, and finally, the environmental benefits that are directly linked to the decrease in the burning of fossil fuels, causing the production of greenhouse gases to be minimized.

In order to obtain an overview of what has been researched within this theme, it is of fundamental importance that a systematic review be carried out. For Bereton et al. [7], a systematic review allows the researcher a rigorous and reliable assessment of research carried out within a specific theme. The systematic literature review (RLS) is an instrument to map published works on a specific research topic so that the researcher is able to elaborate a synthesis of existing knowledge on the subject [8].

Hepbasli and Kalinci [9] carried out a review of heat pump heating systems, taking into account the energetic and exergetic aspects through mathematical modeling. The authors concluded that systems with heat pumps and electrical back-up have higher operational efficiency when compared to conventional systems, reaching 65% and 70–90% efficiency, respectively. In addition, the systems have a useful life of 10 and 20 years, respectively, which enhances their use.

Chow [10] observed the development trends of photovoltaic and thermal technology (PVT). The author highlighted two main types of systems, the water-cooled, PVT/w, and the air-cooled, PVT/a. These systems can achieve efficiencies from 55% to 80% (PVT/w) and from 38% to 75% (PVT/a), respectively. However, the number of collectors and systems available were limiting and the main barriers to technological advancement were low reliability and high implementation cost.

Sadhishkumar and Balusamy [11] reviewed the solar board system to improve heat transfer considering the analysis of its design, selective coatings, thermal insulation, thermal system reflection angles, and working fluids in order to optimize the thermal insulation performance of solar collectors. The systems with thermal energy storage and the solar collector with twisted tape stood out as the technologies with the highest technical and economic efficiency among those evaluated. The other systems showed great potential; however, the authors highlighted the need for new studies and technological improvement.

Willem et al. [12] performed a review on the performance and energy efficiency of heat pump water heaters. Through experiments and laboratory measurements, it was concluded that although most systems operated with a power coefficient in the range of 1.8 to 2.5, there were some technology updates that operated with a power coefficient between 2.8 to 5.5.

Hohne, Kusakana, and Numbi [13] carried out a review of renewable and non-renewable water heating technologies in South Africa, which included electric water heaters, solar heaters, heat pump heaters, and geothermal heating. The authors concluded that hybrid systems have a shorter payback and a longer replacement time of parts, but have a high initial cost due to the value of the collectors, although this cost can be reimbursed in terms of energy savings.

Although this research is about residential use, it should be noted that solar heating systems can also be related to different applications such as the manufacture of materials and metallurgical processes. Fernadez-Gonzalez et al. [14], in their review, demonstrated that solar heating systems have costs that do not depend on the required temperature, and in this way, can compete with modern technologies. The thermal energy that can be captured using solar concentrators makes them a profitable option, particularly for processes taking place below 300 °C, an operating temperature achievable by most solar concentrator regimes. Other authors have indicated the feasibility of solar heating for lime production [15], the superplastics industry [16], and space applications [17].

In order to understand the theme of residential solar heating in the face of the use of flat-plate collectors, this article carried out a systematic literature review, examining articles written on this topic. For this, the main categories (clusters) were identified and analyzed, as well as the main authors and their productions.

2. Materials and Methods

This study used bibliometrics and content analysis, as the use of systematic procedures increases the reliability and accuracy of the study’s conclusions and results [18]. The systematic literature review (RSL) steps adopted were adapted from Tranfield et al. [19] and Bezerra et al. [20], which are: exploratory, development, and analysis of results. The exploratory stage was essential to delimit the study area, as well as to analyze the main terms adopted by the articles (strings) [19]. According to Bezerra et al. [20], the development stage represents the review itself, this stage being initiated by data collection. For this, the initial sample was obtained from the ISI Web of Knowledge (Web of Science) database. Figure 1 shows the description and refinement of the initial research regarding solar heating, as well as makes explicit what the inclusion and exclusion criteria were for this preliminary research.

The initial search (sample 1) was obtained by entering the inputs “solar energy” and “water heating” linked by the “OR” option. It is worth noting that the initial keywords were placed in parentheses so that the search base algorithm understood that the search should be carried out with the entire set of words and not only with isolated words. A second fact to be explained at this stage is the choice of the search mode, in this case, as it was the major theme of the research to be carried out, the terms entered were defined by “title”. Finally, the search operator selected was “OR” (or) for the reason that it lists titles that are composed of both inputs simultaneously and in isolation as well.

The inclusion and exclusion criteria that were adopted for the first stage of the research (in this case, type of document) were intended to refine the initial research, reducing the number of publications and increasing the accuracy of the sample, taking into account that articles and reviews have a greater impact than other types of production.

In the second stage of the search, two new filters (strings) were inserted, making the macro-thematic topic more related to the environment to be studied; in this case, the search mode was changed to “topic” so that the filters were executed within the universe previously observed. After the inclusion of the strings, two more filters were executed, those being the “type of file” and the “period of publications”, and with this, the final sample related to residential solar heating with flat-plate collectors was obtained.

After obtaining the sample, the most important step of the entire review began, the data analysis. In this stage, quantitative and qualitative analyses were included. Initially, the quantitative analyses addressed: (a) cumulative value of the number of publications and citations from 1970 to 2019; (b) division of publications by scientific areas according to the Web of Science (WOS); (c) geographic distribution of the countries where the sample surveys were developed; and (d) co-occurrence of the most cited keywords. In the qualitative analysis, the theme of residential solar heating with the use of flat-plate collectors was divided into nine groups (clusters), namely: C1—Efficiency (collector or system); C2—Cooling/Refrigeration; C3—Water heating; C4—Architecture and Design; C5—Hybrid systems; C6—Heating and Cooling; C7—Environment; C8—Innovations, and C9—Market analysis. After this division, an analysis of the study method adopted by these authors was also carried out, where within each cluster, it was measured which authors adopted the simulation by software and which adopted experimental models, with the exception of the last cluster because it is a marketing analysis.

The analysis of the results, the last stage of the search, is presented in the subsequent section, containing the graphic representations necessary for understanding the analyses described in this section. It is noteworthy that after defining the final sample, a dynamic reading of each article inserted in the sample was performed so that a refined treatment could be performed in order to obtain the publications specific to residential solar heating with the use of flat-plate collectors.

3. Results and Discussion

Solar Heating Overview

This section presents an analysis of the evolution of publications and citations of the sample obtained regarding residential solar heating with the use of flat-plate solar collectors. Figure 2 shows the cumulative growth specific to publications within the previously defined time frame. For a better understanding, this analysis was subdivided by document type, namely, articles and reviews. Although the beginning of the timeline was defined in the year 1970, the publications related to residential solar heating using flat-plate collectors began for the first time in 1993 [21].

As shown in Figure 2, until 2010, no review was published, occurring for the first time in 2011. In the years 2013, 2016, and 2019, there were only articles published. With regard to reviews, 2014 was the only year in which publications of this modality surpassed the articles with two reviews published, while for the same year, only one article was published. It is worth noting that 2019 was the year with the highest publication rate, a total of 10 publications. All these productions were published in 28 different journals. The most published journal was Energy and Buildings, with seven publications.

Continuing the quantitative analysis, Figure 3 presents the same study previously carried out; this time, the citations in residential solar heating with flat-plate solar collectors were the object of study.

It can be seen that the first citation was made three years after the first publication. As already presented, the pioneering publication took place in 1993 and Comakli et al. [21] were the first authors cited. This mention was made by the author of the second publication, Al-Homoud et al. [22].

In this analysis, it is noteworthy that the year 2011 marks the beginning of both the publication of reviews and their citations. Ibrahim et al. [23], in addition to being the authors of the first review, were also the most cited among the others, with a total of 210 citations.

It was also observed that between the years 2011 and 2017, the number of citations of reviews remained constantly growing, and from 2018 onwards, a drop in these citations could be seen. On the other hand, the citations of articles remained emerging, although in 2018, there was a small reduction that was resumed in the following year. All these citations added up to a total of 1222 citations.

Figure 4 shows the analysis carried out with regard to the areas of publication of articles related to residential solar heating with flat-plate solar collectors. In this analysis, it is worth noting that an article may fall into more than one academic category.

Figure 5 shows the geographic distribution of the countries where the surveys were carried out. Although the research universe was limited to 56 articles, there are studies that were developed through partnerships, whether academic or professional. Thus, more than one country could be assigned to a given article.

Finally, Figure 6 shows the co-occurrence of keywords over the years. A total of 24 words could be observed with a minimum amount occurrence of four times. It could also be observed that the most cited words were “performance”, “design”, “simulation”, “energy”, and “solar energy”, which had the highest occurrence between the years 2014 to 2016.

What could also be observed is that the level of internal relationship between the keywords was strong, forming some groups also known as “clusters”. In this case, there were four large groups, namely: Cluster 1, composed of the words collectors, efficiency, energy, renewable energy, solar collector, solar energy, storage, and system. Cluster 2, composed of buildings, collector, flat-plate collector, model, PCM, and systems. Cluster 3, with design, dynamic simulation, performance, simulation, and TRNSYS. Finally, Cluster 4, with experimental validation, flat-plate collector, performance evaluation, solar collectors, and solar cooling.

Starting the qualitative analyses, it was possible to observe nine clusters addressed by the authors of publications related to residential solar heating with flat-plate collectors, which are as presented: Efficiency (collector or system) (C1); Cooling/Refrigeration (C2); Water heating (C3); Architecture and Design (C4); Hybrid systems (C5); Heating and Cooling (C6); Environment (C7); Innovations (C8) and Market analysis (C9). Next, each of the clusters are defined.

Table 1. Overview of solar heating categories.

Category	References	Amount	Quotes
Efficiency (collector/system) (C1)	Comakli, et al. [21]; Kalogirou, et al. [24]; Argiriou et al. [25]; Gunerhan; Hepbasli [26]; Notton et al. [27]; Chargui; Sammouda [28]; Aydin et al. [29]; Pang et al. [30].	08	255
Cooling/Refrigeration (C2)	Al-Homoud et al. [22]; Syed et al. [31]; Sayegh [32]; Balghouthi et al. [33]; Lizarte et al. [34]; Lhendup et al. [35]; Hosseinzadeh; Taherian, [36]; Ferreira; Kim [37]; Guo et al. [38]; Ammari et al. [39]; Figaj et al. [40].	11	415
Water heating (C3)	Hassan; Beliveau [41]; Sutthivirode et al. [42]; Hugo; Zmeureanu [43]; Hang et al. [44]; Şerban et al. [45]; Hossain et al. [46]; Lenz et al. [47]; Bamisile et al. [48]; Zhou, Zhihua et al. [49]; Vegas; Cuevas [50]; Rey; Zmeureanu [51]; Rosato et al. [52]; Huang et al. [53]; Hashemi et al. [54].	15	220
Architecture and Design (C4)	Chung et al. [55]; Ibrahim et al. [23]; Buonomano et al. [56]; Wang et al. [57]; Buonomano et al. [58].	05	303
Hybrid systems (C5)	Hang et al. [44]; Li et al. [59]; Chargui; Awani [60].	03	109
Heating and Cooling (C6)	Li et al. [59]; Calise et al. [61]; Nord et al. [62]; Vall et al. [63]; Buonomano et al. [64]; Gao et al. [65].	06	97
Environment (C7)	Li et al. [66]; Rabbani; Hooshyar [67]; Anastaselos et al. [68].	03	36
Innovations (C8)	Reda [69]; Gaglia et al. [70]; Casano et al. [71]; Ghaebi et al. [72]; Kalogiro et al. [73]; Ghaebi et al. [74].	06	50
Market analysis (C9)	Huang et al. [75].	01	8

shows the allocation of authors within the clusters shown above. One can observe the large number of studies that were related to clusters C2 and C3, while clusters such as C5, C7, and C9 had a very small number of publications.

The first cluster (C1) deals with a group where the main objective of the publications was aimed at efficiency, for the system as a whole or just for the collector used, Gunerhan and Hepbasli [26] evaluated the SWH system consisting of a flat-plate solar collector, a heat exchanger and a circulation pump, based on experimental data from the province of Izmir, Turkey. The authors used exergy analysis (second law) for sizing and found product and fuel exergy efficiency values between 3.27% and 4.39% for the solar collector. Notton et al. [27] developed a mathematical model, based on the finite difference method in Matlab^®, where the efficiency of the system they analyzed increased from 41% to 76% through physical modifications in the equipment that comprise the system, such as new positioning of the cold-water pipe instead of insulating it and changing the thermal insulation and thickness of pipes; Chargui and Sammouda [28] studied a thermosyphon system (TYPE 45) that uses solar energy as an unlimited renewable energy to produce heat, changing the areas of capture of collectors. To analyzed the performance of a thermosyphon solar water heater, the computational system in TRNSYS was used. The results indicated a solar fraction; that is, use of the available solar radiation absorbed by the collectors, 85%, supplying the demand of a six-person residence.

All these previously mentioned authors analyzed the efficiency of the complete system through simulations. The other authors included in this cluster conducted their research for collector efficiency. Comakli et al. [21] obtained an efficiency of 70%; the solar collectors used in this system were constructed by modifying flat-plate water-cooled collectors, and the best results were obtained using the pump system in series.

Colored collectors are of interest to architecture for structuring more harmonious buildings. With this in mind, Kalogirou et al. [24] analyzed the difference in efficiency between colored collectors, simulating installations in three different locations at different latitudes, Nicosia, Cyprus (35° N), Athens, Greece (38° N), and Madison, Wisconsin (43° N), with the aid of TRNSYS. The authors concluded that the difference in efficiency between them and traditional black collectors was between 7 and 18%, which they still considered acceptable. Argiriou et al. [25] conducted numerical simulations, using TRNSYS software, to evaluate a solar-assisted absorption heat pump system. The results showed estimated energy savings in the range of 20 to 27% over a conventional cooling installation using a compression-type heat pump. Aydin et al. [29] analyzed the heat storage and the performance of the coupled solar collector based on the first and second laws of thermodynamics. Thus, they found for solar collectors an efficiency of 70.4% and an exergetic efficiency of 2.5%; and Pang et al. [30] investigated the electrical and thermal efficiency of the flat-plate photovoltaic and thermal (PV/T) collector in four configurations (air-type, water-type, nanofluid-type, and bi-fluid-type) and identified the water-type PV/T collector as the one with the highest thermal efficiency due to the high specific heat capacity of water, reaching values in the range of 33–70%, influenced by the absorber structures. The authors also evaluated different locations and thus inferred the impact of the radiation as variable.

The second cluster (C2) is the group intended for those authors who had research related to the use of solar heating for cooling or refrigeration, either with or without the aid of chillers by the adsorption or absorption methods. Authors such as Al-Homoud et al. [22], Syed et al. [31], Sayegh [32], Balghouthi et al. [33], Lizarte et al. [34], Ammari et al. [39], and Figaj et al. [40] used the absorption process to achieve the studied cooling. Only Ferreira; Kim [37] obtained the desired cooling through adsorption. One work that deserves to be highlighted among the others in this group is that of Lhendup et al. [35], who researched nocturnal cooling, which differs somewhat from the others, in view of the fact that this type of cooling takes into account the long waves of radiation. The authors used an unglazed collector combined with a borehole heat exchanger and heat pump. The simulations were performed for cities in Australia with TRNSYS.

Cluster C3 was the one that presented the largest number of studies. In this group were included authors who focused on residential water heating. Authors such as Hassan; Beliveau [41], Zhou et al. [49], Huang et al. [53], and Hashemi et al. [54], studied solar power systems for multifamily structures. Hugo and Zmeureanu [43], Hossain et al. [46], Serban et al. [45], Bamisile et al. [48], Vega and Cuevas [50], and Buonomano et al. [56] studied solar power systems for single-family buildings. Here, we highlight two papers: the first one is that of Hang et al. [44], which evaluated the performance of flat-plate and evacuated tube solar collector heating systems for typical US residential buildings, considering scenarios with natural gas or electricity support. The authors highlighted that although evacuated tube collectors have greater thermal efficiency, the high production costs result in collectors with a smaller area, which are not sufficient for the required heating. The results showed that flat-plate solar water heating systems with natural gas auxiliary heaters performed best among all types and in all locations studied.

The second one is that of Rosato et al. [52], who studied a small centralized and renewable hybrid district heating system based on the exploitation of solar energy and integrated with a seasonal well thermal energy storage. Eleven cases were simulated, varying the technology of solar thermal collectors, total number of SCs and PTVs, configuration of the solar circuit, and total opening area of the solar field, as well as the control logic simulations of energy consumption and costs.

The C4 cluster, Architecture and Design, brings together publications for studies with structures called building-integrated photovoltaic (BIPV) and building-integrated photovoltaic thermal (BIPVT), a nomenclature intended for civil construction structures that integrate panels into their composition that generate energy; in this case, photovoltaic and thermal. In Chung et al. [55], an investigation was carried out on the longitudinal mean pressure and amplitude distributions of solar collector models. With the aim of reducing possible damage resulting from winds, the effects of a horizontal cylinder or a guide plate were addressed. The results showed that the presence of a horizontal cylinder (or water storage tank) tended to decrease the magnitude of the mean differential pressure near the front edge of the flat panel (or solar collector). This indicated a decrease in the average lifting force and more safety to systems exposed to these weather conditions. The other authors of C4 focused their productions on the architectural part.

With a focus on hybrid systems, only three studies comprise group C5. Hang et al. [44], who was also in C3, worked on a solar and gas system, as presented above. On the other hand, Li et al. [59] studied a system where solar energy (flat-plate solar thermal) was responsible for satisfying the heat pump power demand for residential building hot water, heating, and cooling loads in Beijing, China. The authors identified the solar heating system as a bottleneck. The thermodynamic analyses showed that the solar collector exergy efficiency was within 0.35–3.60%. In this way, more optimized technologies can favor the integration of hybrid plants.

Finally, Chargui; Awani [60] coupled a flat-plate collector to a heat pump system, aiming to minimize the electrical energy consumed by the building. It was demonstrated that the COP of the heat pump was enhanced with the increase in solar radiation during the typical sunny day and was also strengthened in proportion to the solar collector area.

Cluster C6 was intended for authors who have studied solar heating systems with dual behavior; that is, systems that are able to heat and also cool either a working fluid or an environment. Noteworthy here is the previously mentioned system by Li et al. [59], which presented this behavior and was hybrid as well.

The seventh cluster, called Environment, is composed of authors who had their systems as part of some treatment systems or performed a life cycle assessment (LCA). For example, Rabbani; Hooshyar [67] used solar thermal energy to treat polluted effluents with the aid of platinum flat-plate solar collectors reaching temperatures of approximately 55 °C, and with that, the analyzed effluent was disinfected, keeping fecal coliform standards below those determined by the World Health Organization (1000 MPN/100 mL) and an average number of Nemathoda eggs of less than one per liter. Li et al. [66], who used the same energy for desalination as membrane distillation, concluded that a system with a solar absorption area of 1.6 m² can produce 4 L of drinking water and 4.5 kWh of thermal energy at 45 °C.

For LCA, Anastaselos et al. [68] developed a study involving the life cycle of the most used panels in order to show which one had the best cost-benefit ratio, from its acquisition to its disposal. Among the solar energy systems (solar and photovoltaic collectors), it was observed that the use of evacuated tube collectors, coupled to a natural gas boiler, had the lowest environmental impact to achieve the required heating, while the natural gas boiler, floor heating, and mono -Si PV system had the highest cost of emissions.

The C8 cluster contains research considered innovative in its content. Differing a little from the other clusters, this group is composed of publications such as Casano et al. [71], who developed a prototype of a parabolic concentrator, and Ghaebi et al. [72] and [74] who studied thermal energy systems in aquifers with the aid of heat exchangers and cooling towers in Iran. Last but not least, Kalogirou et al. [73] developed the new generation of the main software used for modeling and simulation of solar power systems, TRNSYS, which was used in several works presented in this review.

The last cluster, C9, was intended for research involving the economic sector. Huang et al. [75] studied the financial market for both the production and sale of collectors in the largest producer of solar collectors in the world, China. The authors identified that the Chinese market had a decrease in the last decade but that it preceded a recovery in the medium to long term, driven by the heating cleaning policy and industrial applications. These results may have an impact on global use, given that China is the world’s largest producer.

Still based on clusters presented, a second analysis could be performed. Within each group, there are those authors who consolidated their research based on mathematical simulations and climate analysis through the most diverse software (TRNSYS, MatLab, EES, and RetScreen), and there are also those who obtained their results through experimental data. Here, we highlight the absence of cluster 9 in this approach, which was due to the fact that this group deals with market analysis. At the end of this investigation, it was concluded that approximately 40.0% (22 articles) adopted simulation as the study method used, leaving 60.0% (33 articles) that developed their analyses in experimental systems.

Table 2. Study method adopted by the authors of the residential solar heating sample with flat-plate collectors.

Cluster	Simulation	Experimental
C1	4	4
C2	4	7
C3	6	8
C4	1	4
C5	1	2
C6	2	3
C7	1	2
C8	3	3

shows the division of publications by cluster.

4. Conclusions

In the present work, the most relevant contribution is the mapping and division of the main categories that have been addressed within research solar energy related to residential solar heating with flat-plate solar collectors. Analyzing the 56 articles, selected through bibliometric techniques and systematic literature review, it was observed that the publications are still recent, as shown, and from 2015 onwards, an increase in the number of publications began.

The group that presented the highest number of publications was the C3 cluster (water heating), with 25% of the analyzed productions. Articles dealing with water heating have been published since 2008, but it was from 2016 onwards that there was an evolution of publications in this cluster. Meanwhile, works on cooling and refrigeration (C2) have been written since 1996, being considered the cluster with the highest regularity in publications. This group was the second largest within this context, reaching approximately 19.6% of publications.

The clusters with the lowest publication rates and consequently, the lowest literary impacts were C5 (hybrid systems), C7 (environment), and C9 (market analysis), which together accounted for 12.5% of publications. The titles included in these groups are very recent, from the years 2011 to 2019, leading to the belief that these lines of research need a greater demand for studies.

Thus, the literature pointed out that works related to water heating and cooling and refrigeration of environments are already developed and developed constantly, but works such as those by Rabbani’s; Hooshyar [67] and Li et al. [66], which integrate solar energy with some other type of process, are scarce.

Given the above, considering the categories with the lowest number of publications, integrative studies of solar heating should be investigated, since the premise of solar energy as a whole is to be a clean and environmentally viable source of energy.