Photovoltaic (PV) panels account for a majority of the cost of photovoltaic thermal (PVT) panels. Bifacial silicon solar panels are attractive for PVT panels because of their potential to enhance electrical power generation from the same silicon wafer compared with conventional monofacial solar panels. This paper examines the performance of air-based bifacial PVT panels with regard to the first and second laws of thermodynamics. Four air-based bifacial PVT panels were designed. The maximum efficiencies of 45% to 63% were observed for the double-path-parallel bifacial PVT panel based on the first law of thermodynamics. Single-path bifacial PVT panel represents the highest exergy efficiency (10%). Double-path-parallel bifacial PVT panel is the second preferred design as it generates up to 20% additional total energy compared with the single-path panel. However, the daily average exergy efficiency of a double-path-parallel panel is 0.35% lower than that of a single-path panel.
Solar energy is one of the most environment friendly sources of renewable energies that can be utilized in thermal and electrical applications. Photovoltaic thermal (PVT) collectors are especially designed to generate both electrical and thermal energies simultaneously. Solar air-heater panels, also referred to as hybrid panels, have been widely studied in the last decade and have been found to be strongly suitable for applications that require both electrical and thermal energies, such as space heating and drying.
A PVT panel generally consists of PV cells, an absorber plate, and a heat removal system. In a bifacial PVT panel, monofacial PV cells are replaced by bifacial PV cells, and the absorber plate is replaced by a reflector. Reducing the number of solar cells will significantly reduce the cost of the module, given that solar cells are expensive [
Recent studies on PV solar cells have developed the bifacial PV solar cell [
Radiation absorptions of monofacial and bifacial PV solar cells.
Solar radiation absorption by the rear surface increases electrical energy generation [
Bifacial solar cells increase electrical efficiency of flat-plate PV panels at negligible cost increase [
Intensity of solar radiation on the rear surface affects the power generated by a bifacial PV panel [
Building-integrated applications of monofacial PV and PVT panels have been widely studied, whereas the performance of building-integrated bifacial solar panels has not. Bifacial PV panels could be integrated into residential and commercial buildings as window-integrated, wall-integrated, or parking lot-integrated panels [
Bifacial solar cells partially cover the panel area. In wall-integrated application, the panel is installed with an offset distance from the wall. Front surface of bifacial solar cells absorb a portion of solar radiation, whereas a portion of solar radiation penetrates through transparent vacant space between cells; the wall reflects light back to the rear surface of bifacial cells [
Window-integrated bifacial PV panels produce electrical energy while permitting penetration of faint solar radiation into the interior area for lighting of residential or commercial buildings [
Air-based PVT collectors are useful for both industrial and residential applications, such as drying and space heating. Air-based panel designs are classified according to the number or glazing and air-path. Single-glazing double-path (monofacial) panels have higher electrical and thermal efficiencies compared with single-path panel types [
Substituting monofacial PV cells with bifacial PV cells has led to the development of bifacial PVT panels. A bifacial PVT panel equipped with aluminum reflector generates 40% additional electrical and thermal energies [
Most of the existing PVT panel designs are inappropriate for bifacial solar cells because the absorber plate covers the rear surface of bifacial solar cells [
The current research aims to develop new designs of air-based PVT panels based on bifacial solar cells and to evaluate the performance of the panel with regard to the first and second laws of thermodynamics. This paper represents steady-state simulation and daily simulation based on the climate of Malaysia.
Electrical and thermal performances of PVT panel strongly depend on cell temperature, air flow rate, and packing factor of the solar panel. Energy balance method and exergy method are the two most well-known methods for performance evaluation of PVT panels. The former (energy balance method) is based on the first law of thermodynamics, whereas the latter (exergy method) is based on the second law of thermodynamics.
Varieties of air-based PVT panels have been widely studied in the last decade. Most PVT panel designs are incorporated with an opaque absorber plate on which PV cells are pasted. A few monofacial PV and PVT panel designs have the potential to be developed into a bifacial PVT panel [
In a bifacial PVT panel, the absorber plate should be substituted by an appropriate reflector installed beneath the bifacial PV cell to provide solar radiation on the rear surface of bifacial solar cells. The reflector is the key component of a bifacial PV panel. Both mirror-type and diffuse-type reflectors are suitable for bifacial panel design. Figure
Cross-section view of bifacial PVT panel: (a) Model
Configurations of four bifacial PVT panels are described as follows.
Single-path, air-based bifacial PVT panel with a reflector placed beneath the PV lamination. Air flows between the lamination and the reflector (Figure
Double-path, air-based PVT panel with additional layer of glazing above the PV lamination. Two parallel air streams exist. One air channel is above the PV lamination, and the other is beneath the lamination (Figure
This model has a similar configuration to Model
This panel was developed based on Model
All four models were simulated based on the first law of thermodynamics. The following assumptions were made. Both surfaces of the PV cell have the same electrical efficiencies. The reflector has 70% reflection efficiency. Natural convection is suppressed inside the panel. The only temperature gradient is along the panel. Thermal losses through insulation are negligible. Ambient airspeed is equal to 1 m s−1.
Energy balance equations were developed and solved for all models. Output energy of the PVT panel consists of electrical and thermal parts, and efficiency of the panel is a combination of thermal and electrical efficiencies.
Free convection heat transfer coefficients in ambient condition can be obtained from Duffie and Beckman [
Internal forced convection heat transfer coefficients is defined as [
The total efficiency of the PVT panel is a function of electrical and thermal efficacies.
Both surfaces of the bifacial PV cell contribute to electrical energy generation, where the total electrical energy generated by bifacial PV cell is defined as follows:
The actual electrical efficiency of the PV cell varies according to its temperature as [
The thermal efficiency of a PVT panel is defined as the ratio of thermal energy transferred to working fluid(s) over total solar radiation reaching the collector area:
Electrical energy has a higher value compared with thermal energy, which should be considered in calculating total efficiency [
Aforementioned studies evaluated the panels based on the first law of thermodynamics. However, exergy method evaluates panels based on the second law of thermodynamics. In exergy method, environment temperature is the reference temperature in calculating the theoretical maximum work done by the system in order to reach equilibrium with the reference environment [
Increasing the number of glazing is an advantage in PVT panel. Glazing reduces thermal losses and increases thermal efficiency, which is an advantage according to the first law of thermodynamics. However, solar radiation reflected back by the cover glass reduces solar radiation to solar cells, thereby decreasing electrical energy generation. Electrical energy has a higher value compared with thermal energy according to the first law of thermodynamics [
In accordance with the second law of thermodynamics, high-value electrical energy loss from reflected light caused by additional glazing should be compared with thermal energy saved from additional glazing.
In PVT panels, the maximum output exergy is observed at an inlet fluid temperature of 35°C, whereas the maximum output energy is observed at an inlet fluid temperature of 31°C [
Comparison of energy and exergy efficiencies.
Panel type | Energy efficiency | Exergy efficiency |
---|---|---|
Unglazed PVT-air [ |
45% | 10.75 |
Single glazed PVT-air [ |
55% | 13.5% |
Single glazed PVT-air [ |
55%–66% | 12%–15% |
Double glazed T-air [ |
— | 7.4% |
PVT-air [ |
33%–45% | 11.3%–16% |
PVT-water [ |
— | 3%–15% |
PVT: photovoltaic thermal; T: thermal.
The total output exergy of the PVT panel is higher than that of PV panel, and the thermal energy output of a PVT panel is significantly higher than its electrical energy output [
Output exergy of a PVT panel is the sum of electrical and thermal exergies [
Electrical energy could be efficiently converted to work. Thus, electrical exergy is assumed to be equal to electrical energy [
Thermal exergy is defined as [
Exergy of solar radiation reaching the PVT panel is defined as [
Exergy efficiency is defined as the output exergy (the sum of electrical and thermal output exergies) over the exergy of solar radiation [
The mathematical model used in simulating the four panels at steady-state and daily climate of Malaysia is based on the first and second laws of thermodynamics.
The total efficiency of each of the four model panels was calculated using (
Total efficiencies of the four models at different packing factors.
Increasing the air flow rate leads to increase of internal forced convection heat transfer coefficients according to (
Steady-state exergy was computed using (
Electrical exergy of the four panel designs.
Models
Thermal exergy of all four models was calculated using (see (
Thermal exergy efficiency of the four panel designs.
Increasing the air flow rate from 0.03 kg/s to 0.14 kg/s did not significantly affect electrical exergy, as shown in Figure
This observation is in accordance with the second law of thermodynamics and indicates that panels operate at low temperature that reduces the quality of output thermal power (exergy) with respect to the second law of thermodynamics (
The exergy output of a PVT panel is the sum of thermal and electrical exergies (see (
Models
Monthly average, hourly solar radiation, and dry bulb temperature of Malaysia [
As indicated by steady-state simulation, panel Models
Model
Total exergy efficiencies of the four models at different packing factors.
Air temperature increase in Models
Daily average energy and exergy efficiencies of Models
Daily average efficiencies of Models
The exergy efficiencies of the panels were improved by increasing the packing factor from 0.3 to 0.7. Meanwhile, the energy efficiencies showed less dependence on the packing factor. This contrast is because total exergy output is mainly dependent on electrical energy output (see (
Four air-based PVT panels were equipped with bifacial solar panel and were studied based on the first and second laws of thermodynamics. Double-path-parallel panel had the highest total energy efficiency (35% to 75%), followed by double-path-counter flow panel, double-path-returning flow panel, and single-path panel. However, the single-path panel showed the highest exergy efficiency (4.48% to 10.15%), followed by double-path-parallel panel, double-path-counter flow panel, and double-path-returning flow panel. Double-path panels have an additional glazing that improved thermal energy and exergy efficiencies but have a negative effect on the electrical energy and exergy outputs. Single-path panel is the best option if electrical energy is the desired energy output of the panel, whereas double-path-parallel panel is recommended for maximum thermal energy output. Daily energy and exergy simulations were performed under the tropical climate of Malaysia. Double-path-parallel panel represented a daily average energy efficiency that was 20% higher than that of single-path panel. However, daily average exergy efficiency of double-path-parallel panel was only 0.35% lower than that of single-path panel at the same flow rate. PVT panels are mostly used for drying applications at tropical climate of Malaysia; the double-path-parallel panel is mostly recommended to maximize thermal energy harvest. The harvested electrical energy could be used to run electrical components of drying system such as air blowers and control system. High packing factor is preferable with regard to the first and second laws of thermodynamics. Economic analysis would be able to recommend the optimum packing factor.