1. Introduction
Solar energy is commonly collected as heat and electricity through thermal and photovoltaic (PV) technologies, respectively. A hybrid photovoltaic-thermal (PVT) integrates a solar thermal absorber and a PV into one unit. Whereas the PV cells generate electricity, the integrated thermal system absorbs residual heat energy from the cells and thus reduces their temperature in the process and also enhances their performance [
1,
2,
3]. The two most cost-effective working fluids are water and air, with water type found to be more efficient [
4]. Hybrid PVT collectors can reach net (electrical plus thermal) efficiencies of 70% or higher, with electrical efficiencies up to 15–20% and thermal efficiencies exceeding 50%, depending on the conditions [
5]. The PVT technologies have the potential to reduce the use of materials, installation time, and the required space [
6]. The advantage of PVTs in generating both electricity and thermal energy simultaneously makes them handy for domestic applications. However, despite the immense potential, commercial PVT systems are still not as popular as stand-alone, and separately installed, PV and thermal systems [
7,
8].
PVT technologies have been studied since 1970s, including variation in designs, working fluids and other performance-influencing factors [
2,
3]. The thermal and electric energy outputs depend on many factors, some of which are irradiance, ambient temperature, wind speed, circulating fluid temperature and flowrate [
7,
9,
10]. It is therefore important that more experimental data from different environmental conditions are collected to enrich available data to cover the different places in the world. The precise projection of solar collector behavior is key for ensuring proper design and reduction in underperformance or system failure; while improved models of PVT systems are required for optimization of the design and operating parameters in order to achieve higher electrical and thermal energy yields and increased energy savings [
8].
Unlike indoor test conditions, climatic conditions could vary significantly affecting general performance and resilience of designs. For instance, the harsh harmattan weather conditions of sub-Saharan West Africa sets it apart from other places in the world. Although the efficiency of PVT collectors are affected by meteorological conditions, several of the studies to predict the performance of PVTs were carried out in Europe [
7,
8,
9,
11] and many parts of Asia [
12,
13,
14,
15,
16,
17], with minimal experimental investigation records on the subject in sub-Saharan Africa in the open literature. Nevertheless, Rejeb et al. numerically investigated a photovoltaic/thermal sheet and tube collector for the semi-arid climatic with hot summer and mild winter in North Africa [
10]; but again, their data correspond to simulations, not experimental work.
Africa is home to 17% of the world’s population, but, generates 4% of global power supply. As of 2018, the electrification rate in sub-Saharan Africa was 45% with frequent electricity disruptions and economic losses. This and many more have hampered industrial expansion on the continent. Meanwhile, the continent has the richest solar resources in the world, but accounts for less than 1% of global solar PV installed capacity [
18]. Solar resources provide the option of decentralized (and off-grid) solutions to remote settlements. The number of people who gained access to electricity through solar home systems in sub-Saharan Africa increased from two million in 2016 to approximately five million in 2018 [
18]. This shows that, with the right policies, solar could become one of the top resources in overcoming the energy deficits on the continent. Thus, more research into solar technologies, like PVT, on the continent is needed for informed decision-making by stakeholders.
The few studies on PVT technology in Africa in the literature were based on climates of North Africa sub-region [
10,
19,
20] and the country of South Africa [
20,
21,
22,
23]. In the case of West Africa, studies on solar technology were either separately conducted on solar photovoltaic systems [
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36] or solar thermal systems [
36,
37,
38,
39], with very little evidence of studies on performance of the combined technology (PVT) in the literature. The objective of this experimental study is therefore to assess the real-life performance of a water-based commercial PVT module against a PV system installed in a dynamic environment of Kumasi Ghana (in West Africa). The results from this study will provide valuable information about the viability of water PVT as an alternative source of energy for provision of warm water and electricity in (especially off-grid locations) Ghana. Furthermore, experimental data collected and presented in this study will serve as input and validation parameters for modelling and simulation of PVT systems. To the authors, this is the first study of a commercialized PVT in the dynamic tropical weather conditions of West Africa.
2. Materials and Methods
2.1. Experimental Set-up
The experimental set-up for the study was installed on the roof of the Department of Mechanical Engineering laboratory (6.68° N, 1.57° W), Kwame Nkrumah University of Science and Technology (KNUST), Kumasi-Ghana. The set-up consisted of a conventional solar PV and hybrid PVT installations (
Figure 1). For ease of comparison and commercial availability, both modules were made up of mono-crystalline Silicon (mc-Si) PV technology. The selected specifications for the modules are shown in
Table 1.
Special care was taken to ensure the PV and PVT modules were sourced from the same manufacturer for similarities in peripheral material composition and assembling techniques. The 200 W rated commercial PVT module had a layer of 72 mc-Si PV cells with a flat copper plate thermal system securely attached on its back. A thin adhesive layer, made up of ethylene-vinyl acetate (EVA) layer and the tedlar layer, was used to fix the PV module on to the thermal absorber plate. This compound adhesive layer also acted as a shock absorber to further strengthen the PV module. The heat conducted by the copper thermal plate from the PV cells was transferred by fluid (water in this case) flowing through 14 evenly distributed parallel copper pipes attached to the absorber plate and running from inlet to outlet manifolds. The thermal absorber was then covered with an insulator material and then finally with an aluminum foil to complete the thermal insulation on its back.
Both unshaded PVT and PV modules were oriented towards the south and inclined at a fixed angle of 8°. Tilt angle allowed natural cleaning of the modules during rainfalls, which reduced soiling by dust settlement on the installations, and ensured optimum capture of the solar irradiation for the location. In addition, the modules were manually cleaned on regular basis to reduce the effects of soiling. As shown in
Figure 2, beneath the mounting frame for the PVT installations were two separated compartments for housing the main electrical/logging circuitry and water circulation system.
2.2. Schematic of the Experimental Setup and Water Circulation
Figure 3 shows a schematic diagram of the experimental setup. The main heat transfer medium for the PVT was by force-circulated water in a closed-loop system. A flow jet direct current (DC) pump, with regulated input power, circulated the water at solar irradiance above 150 W/m
2 when the thermal absorber temperature was greater than its inlet water temperature. The solar irradiance was measured with a pyranometer mounted in-plane on the PVT module. Manual valves V1 and V2 were used to regulate the flow rate and monitored with a mechanical spring flow meter F (see
Figure 3). The relatively warm water from the PVT went through a copper-coiled heat exchanger submerged in 70 L of water contained in an insulated tank. For the purpose of the experiment, the circulating water returning into the PVT inlet was further cooled with a water-to-air heat exchanger. This also reduced the problem of potential heat recirculation from the water tank back to the PVT, making it a better stand-alone system.
2.3. Instrumentations, System Control and Data Collection
A programmable data logger (CR300, Campbell Scientific, Logan, UT, USA) was used to record both meteorological and the modules’ performance data. The measurements were sampled every 10s and then averaged over 15-min periods, from which hourly, daily and monthly data were determined. The meteorological measurements were the global solar irradiance in plane-of-array (W/m2), ambient wind velocity (m/s), ambient relative humidity (%) and ambient air temperature (°C). The electrical energy yields from the PVT and PV modules were measured by two dedicated maximum power point tracking (MPPT) battery chargers (BIM 205 Version 1.0, MicroStep-MIS, Bratislava, Slovak Republic). Batteries, radiator fans, data logger and circulation pump were powered from the battery charger, serving as external loads to the modules. Logged data from the battery chargers included output currents (A) and voltages (V) from modules.
Apart from ambient temperature, all other temperatures were separately measured with calibrated temperature sensors (PT100, Campbell Scientific, Logan, Utah, USA). As shown in
Figure 3, measured temperatures included the PVT inlet water temperature (T
1), PVT module back temperature (T
2), PVT outlet water temperature (T
3), PV module back temperature (T
4) and water storage tank temperature (T
5). In addition to recording data, the logger was programed to control the functionalities of the mechanical components in the PVT setup based on real-time in-plane global irradiation. Active water circulation through the PVT was kept at a constant flowrate per desired set value.
Table 2 shows the list and basic characteristics of the instrumentation used in this study.
2.4. Data Analysis
2.4.1. Module Temperature
The peak or rated power of a PV module is determined under standard test conditions (STC), which are solar irradiance of 1 kW/m
2, module temperature of 25 °C and air-mass ratio (AM) of 1.5 (AM = 1.5). However, in real life outdoor situations, the ambient conditions are different from these STC, and hence, the PV module power output will differ from the rated power. The cell temperature
(°C) at any ambient temperature
(°C) is given as:
where
NOCT is the nominal operating cell temperature (°C),
is the in-plane global irradiance (W/m
2),
is cell efficiency (%),
is the effective transmittance-absorptance,
is the loss coefficient (W/m
2 °C) and
means parameter Z at
NOCT. The loss coefficient can further be expressed as:
where
is wind speed (m/s). Additionally, at
NOCT,
is 20 °C,
is 800 W/m
2 and
is 1 m/s, at no load operation (i.e.,
= 0). Equation (1) can therefore be simplified as:
Thus the effect of solar irradiance, ambient temperature and wind speed on solar PV module can be quantified by their impact on the module temperature as given in Equation (3).
2.4.2. PV Performance Indices-Energy Yield, Performance Ratio and Efficiency
The performance of a PV system is usually examined using a number of selected performance indices, including energy yield, performance ratio and efficiency. The energy yield is defined as output normalized by the PV system’s rated capacity. It specifies how many hours in a day the PV system must operate at its rated power in order to produce the same amount of energy as was measured [
40,
41]. It is given as:
where
is the array yield in kWh/kW
p/day,
is the average daily module DC energy output (kWh/day) and
is the rated kilowatt peak electrical power (kW
p) of the PV module at STC.
The performance ratio (
PR) measures the overall effect of losses on the rated output of the system and indicates how close its performance is to the ideal performance during real life operation. The PR is useful for the comparison of modules that receive different amounts of irradiation, especially due to geographical location and or PV inclination. It is given as [
40,
41]:
where
Sh (h/day) is the plane-of-array average daily peak sun-hours, which is the same as the reference yield,
YR. The reference yield is the ratio of the total in-plane solar radiation to the array reference irradiance,
(usually taken as 1 kW/m
2). It is a measure of the theoretical energy available at a specific location over a specified time period [
41] given as:
The PV module efficiency is given as:
where
is the module total surface area (m
2) and
is the in-plane solar irradiance (kW/m
2).
is the DC power from the module in kW. Depending on the available data and desire level of resolution, the efficiencies can be determined on instantaneous, hourly, daily, monthly and annual bases [
41].
2.4.3. PVT Performance Indices—Heat Gained, Thermal Energy Yield and Efficiency
The overall performance of a PVT system is a combination of both PV (electricity) and its thermal (heat energy) components. The thermal gain of the system is given as:
where
is the water mass flow rate (kg/s),
is the specific heat of water (kJ/kg °C) and Δ
T (°C) is the temperature difference, expressed as:
where
and
are the inlet and outlet water temperature, respectively (see
Figure 3).
The mass flow rate
Mw can also be expressed in volumetric terms as:
where
Vw is the volumetric flow rate in m
3/s and
is the density of water (kg/m
3) at temperature
T. Both
and
were assumed to be constant (
= 4.18 J/kg °C,
= 1000 kg/m
3) throughout the analysis presented in the study.
The thermal efficiency of the PVT is given as:
Combining Equations (7) and (11), the overall efficiency of a PVT system is given as:
2.4.4. Clearness Index
The clearness index is the fraction of the solar radiation reaching the top of the atmosphere that makes it through the atmosphere to reach the Earth’s surface. It is normally calculated as a ratio of the monthly averaged daily global solar radiation on horizontal surface () to the monthly averaged daily extraterrestrial solar radiation () at a given site.
The monthly daily average clearness index (
KT):
The extraterrestrial irradiance
can be determined using the mathematical expression:
where
is the day of the year,
is the latitude of the site,
δ is the declination angle of the sun,
is the extraterrestrial solar constant 1.37 kW/m
2 and
is the sunrise hour given as:
can then be calculated as:
where
R is the number of days in the month. For this study, monthly daily average clearness indexes were generated for the site using HOMER Pro energy simulation software (Version 3.13.6, Homer Energy, Boulder, CO, USA) [
42] which employs existing global data sources and libraries in its predictions.
4. Conclusions
In this paper, a comparative performance valuation was conducted on water-based PVT and PV modules made of mc-Si cell technology in a dynamic environment for 2019.
The highest recorded instantaneous module temperatures were 70.6 °C and 60.5 °C for the PV module and PVT module, respectively, recorded in October. On the average, the PV module temperature remained relatively higher than that of PVT by 1.3% to 6.9%.
The annual total energy output for the PV module was 194.79 kWh/m2 while that of the PVT for electrical and thermal outputs was 149.92 kWh/m2 and 1087.79 kWh/m2, respectively.
The annual daily mean electrical energy yield for the PV and PVT were 3.21 kWh/kWp/day and 2.72 kWh/kWp/day, respectively.
The annual performance ratios based on only electrical energy for the PV and PVT were 79.2% and 51.6%, respectively, whereas their capacity factors were, respectively, 13.35% and 11.3%.
The monthly average electrical efficiency values for PV and PVT were 11.6–12.7% and 9.9–11.5% respectively. The thermal efficiency of the PVT had a wider variation from 29.44% to 44.84%. There is however the need to improve the thermal efficiency of commercial PVTs.
This study has shown that the flat plate water PVT application is feasible in environments with similar weather conditions to that of Kumasi. It could also be concluded that, based on the general performance of the two technologies, the PV is a better choice for very large-scale grid-connected systems, where the interest is mostly in electrical energy production. However, for domestic applications and small scale grid systems with provision for thermal energy use, the PVT is a better option. The study could not however cover the exergy analysis, economic evaluations and life cycle assessment of the current PVT/PV setup. These should be carried out so that the actual cost of PVT setup, the net cost of produced energy and their environmental impact could be determined. This information could be useful to stakeholders in Ghana in making informed decisions in energy systems.