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## HYBRID SOLAR PHOTOVOLTAIC/THERMAL TECHNOLOGIES—AN OVERVIEW

Photovoltaic and solar thermal technologies are both well developed and promising ways for harvesting energy from the sun. Combining the two technologies into one system is an attractive way to leverage space and potentially improve the overall solar energy utilization. Unfortunately, photovoltaics suffer from degradation in efficiency when operating at elevated temperatures, making their integration into hybrid systems challenging. Here, we present an overview of hybrid photovoltaic/thermal technologies. The article first focuses on the key definitions for efficiency for both systems, and potential ways to consider calculating the combined the efficiency. The remainder focuses on complex system design approaches, with some of the challenges and benefits of each approach highlighted.

1. INTRODUCTION

Solar energy is typically divided into two different branches based on the mechanism employed in the conversion process of incoming photons. Photovoltaics (PV) focuses on the direct conversion of incoming solar energy to electrical energy. Solar thermal energy focuses on the conversion of photons into useful thermal energy or heat. PV devices work by absorbing and converting incoming photons with energy levels above the bandgap of the cell materials into electron–hole pairs; photons below and well above the bandgap end up generating significant wasted thermal energy in the PV material. Solar thermal systems work by absorbing incoming photons, typically across the entire solar spectrum, and directly converting them into thermal energy, where a heat transfer fluid gathers useful thermal energy.

Due to the amount of thermal energy generated in PV devices, and the desire to keep operating temperatures low, a compelling argument can be made for coupling a PV device with a solar thermal collector to form a hybrid system, typically referred to as a photovoltaic/thermal (PV/T) collector (Chow, 2010). A simple schematic illustration of a flat plate PV/T collector is shown in Fig. 1. In the arrangement shown in Fig. 1, the incoming solar flux is incident on the PV cells, which absorb the incident radiation and convert a portion directly to electrical energy while generating waste thermal energy in the PV cell materials. A working fluid is then used to remove the waste thermal energy from the PV cells, acting as a coolant for the PV cells, which can then be delivered to a load for thermal energy.

Figure 1. Conceptual schematic illustration of a hybrid PV/T collector with a thermal absorber coupled to a PV module

2. DEFINING EFFICIENCY

The efficiency of the PV collector is typically defined as follows (Markvart, 2009):

 $$\eta_{\text{PV}} =\dfrac{V_{\text{oc}} I_{\text{sc}} \text{FF}}{GA_{\text{PV}} }$$ (1)

where $$V_{\text{oc}}$$ is the open-circuit voltage; $$I_{\text{sc}}$$ is the short-circuit current; FF is the fill factor; $$G$$ is the solar irradiance; and $$A_{\text{PV}}$$ is the PV area. The efficiency of the thermal collector is typically defined as follows based on the useful thermal energy collected by the working fluid (Duffie and Beckman, 2013):

 $$\eta_{\text{thermal}} =\dfrac{\dot{m}c_{p} (T_{\text{out}} -T_{\text{in}} )}{GA_{\text{thermal}} }$$ (2)

where $$\dot{m}$$ is the mass flow rate; $$c_{p}$$ is the specific heat of the working fluid; $$T_{\text{in}}$$ is the inlet temperature; $$T_{\text{out}}$$ is the outlet temperature; and $$A_{\text{thermal}}$$ is the thermal collector area. In many cases, particularly PV/T systems without solar concentration, the $$A_{\text{PV}}$$ and $$A_{\text{thermal}}$$ values represent the same area. In PV/T systems with concentration, these areas can be the same, but are likely to be different. In those cases, extreme caution must be taken in reporting the efficiency.

An important consideration in a hybrid PV/T collector is the overall efficiency; therefore, PV/T systems using concentration (or with different areas between thermal and PV systems) should report efficiency based on the overall aperture area. The simplest approach to obtaining the overall efficiency is one in which the thermal and PV efficiencies are just added:

 $$\eta_{\text{overall}} =\eta_{\text{thermal}} +\eta_{\text{PV}}$$ (3)

While such an approach to assessing the overall efficiency is easily determined, it likely oversimplifies the value of a hybrid PV/T collector since it assumes that the thermal and electrical energy values are equal. Additionally, the thermal efficiency is typically much greater than the PV efficiency, such that it can often dominate the overall efficiency, making it difficulty to compare system performance across multiple designs. One highly useful approach is to utilize a weighting factor for the relative thermal and electrical energy values, as given by the following equation (Huang et al., 2021):

 $$\eta_{\text{overall, weighted}} =w\eta_{\text{thermal}} +\eta_{\text{PV}}$$ (4)

where $$w$$ is the weighting function that reflects the value of the thermal energy to the value of the electrical energy. This approach is highly useful since the $$w$$ value can be defined based on thermodynamics if consideration is given to converting thermal energy to working temperatures, cost, or environmental approaches that allow for a high degree of flexibility. However, if this approach is used it is important to note how the $$w$$ value is defined and determined.

Another useful approach is to utilize the overall exergetic efficiency, particularly if the thermal energy is going to be used for electrical energy production, which can be obtained by the following equation (Widyolar et al., 2018):

 $$\eta_{\text{exergy}} =\eta_{\text{Carnot}} \eta_{\text{thermal}} +\eta_{\text{PV}}$$ (5)

where $$\eta_{\text{Carnot}}$$ is the Carnot efficiency of a heat engine. This approach captures the potential of the thermal energy to produce useful mechanical work and includes the impact of high-temperature thermal energy collection. Regardless of which overall efficiency approach is utilized, it is also useful to report the total energy produced from the PV system and the thermal system employed in a given design.

3. COMPLEX DESIGNS

An important aspect of any PV system is the deleterious effect of temperature, where the role of increasing temperature can be shown to negatively impact the efficiency of the PV cell, as indicated by the following equation (Markvart, 2009):

 $$\eta_{\text{PV}} =\eta_{\text{ref}} \left[1+\beta_{\text{ref}} (T_{\text{PV}} -T_{\text{ref}} )\right]$$ (6)

where $$\eta_{\text{ref}}$$ is the cell efficiency at the reference temperature; $$\beta_{\text{ref}}$$ is the cell efficiency temperature coefficient; $$T_{\text{PV}}$$ is the PV cell temperature; and $$T_{\text{ref}}$$ is the reference temperature. As demonstrated by Eq. (6), the PV cell efficiency is highly dependent on temperature. Thus, the embodiment observed in Fig. 1 clearly indicates a challenge with PV/T systems since the PV cell temperature is highly dependent on the thermal collector. Capturing more thermal energy at higher working temperatures requires higher PV cell temperatures, resulting in lower PV efficiency. This leads to two approaches to design configuration. The first approach is to use the working fluid of the thermal receiver to cool the PV cell. As shown in Fig. 1, this can be done when a balance is struck between limiting the PV cell temperature and maximizing the useful temperature gain. An alternative approach is to use a high-velocity working fluid to minimize the PV cell temperature in order to actively cool the PV cell; however, the downside to this approach is that the useful temperature rise in the fluid is also minimized (Mittelman et al., 2007). This approach is typically employed in PV/T systems under concentrated solar irradiance.

The second design concept used to minimize PV cell temperature gain is to decouple the thermal receiver from the PV receiver. This allows for higher temperature operation of the thermal receiver and the potential for lower temperature operation of the PV receiver. A summary of thermal decoupling approaches, although not exhaustive, is shown in Fig. 2.

Figure 2. Conceptual schematic illustrations of thermally coupled hybrid PV/T receivers: (left) spectral splitting using a dichroic mirror with a fluid-cooled PV receiver; (right) spectrally selective fluid thermal receiver with a fluid-cooled PV receiver

In thermal decoupling, the primary goal is to separate the incoming sunlight either through some form of spectral splitting (Ju et al., 2017; Mojiri et al., 2013) or selective absorption/transmission (Goel et al., 2020). As shown in Fig. 2 (left-hand side), in this configuration a dichroic mirror is inserted into the beam path of the full spectrum irradiance and a portion of the light is reflected to the PV receiver (which may be actively cooled) and the rest is transmitted to the thermal receiver (Stanley et al., 2016; Wingert et al., 2020). Figure 2 (right-hand side) also shows a thermal receiver placed in the full spectrum beam path that is capable of spectrally transmitting/absorbing a portion of the spectrum and passing the rest to the PV receiver (Hassani et al., 2016; Otanicar et al., 2018).

Both concepts for thermal decoupling are highly dependent on the optical properties of the spectrum splitting technology since that is critical to determining the overall percentage of energy directed toward each technology. PV technology is highly dependent on the wavelength (bandgap energy) incident on the module for efficient absorption and conversion to electricity. Thermal technologies are largely agnostic to the wavelength as long as the absorbers are highly black. In addition to the overall performance being highly dependent on the wavelength of the spectrally selective component (Brekke et al., 2016), recent work has shown that the optimal spectral properties are highly dependent on the weighting factor ($$w$$), which optimizes the overall efficiency (Bierman et al., 2016; Huang et al., 2021).

4. CONCLUSIONS

Hybrid photovoltaic/thermal technologies are well positioned for increased market penetration as decarbonization efforts grow worldwide. Additionally, many countries are pursuing high degrees of electrification, which could see further growth as the waste heat from a typical PV system becomes more valuable in cases where carbon dioxide emissions are being reduced. One of the biggest commercial challenges is deciding on which of the different PV/T and output thermal temperature approaches to use. This creates a market where each customer segment likely needs a different working fluid temperature and has a different weighting function of the value of heat to electricity. Additionally, thinking critically about how to evaluate the overall efficiency and clearly reporting how that was determined will be critical for further advances and deployment.

#### REFERENCES

Bierman, D.M., Lenert, A., and Wang, E.N. (2016). Spectral Splitting Optimization for High-Efficiency Solar Photovoltaic and Thermal Power Generation, Appl. Phys. Lett., 109(24): 243904.

Brekke, N., Otanicar, T., DeJarnette, D., and Hari, P. (2016) A Parametric Investigation of a Concentrating Photovoltaic/Thermal System with Spectral Filtering Utilizing a Two-Dimensional Heat Transfer Model, J. Sol. Energy Eng., 138(2): 021007.

Chow, T.T. (2010) A Review on Photovoltaic/Thermal Hybrid Solar Technology, Appl. Energy, 87(2): 365–379.

Duffie, J.A. and Beckman, W.A. (2013) Solar Engineering of Thermal Processes, Hoboken, NJ: John Wiley and Sons.

Goel, N., Taylor, R.A., and Otanicar, T. (2020) A Review of Nanofluid-Based Direct Absorption Solar Collectors: Design Considerations and Experiments with Hybrid PV/Thermal and Direct Steam Generation Collectors, Renewable Energy, 145: 903–913.

Hassani, S., Taylor, R.A., Mekhilef, S., and Saidur, R. (2016) A Cascade Nanofluid-Based PV/T System with Optimized Optical and Thermal Properties, Energy, 112: 963–975.

Huang, G., Wang, K., and Markides, C.N. (2021) Efficiency Limits of Concentrating Spectral-Splitting Hybrid Photovoltaic-Thermal (PV-T) Solar Collectors and Systems, Light Sci. Appl., 10(1): 28.

Ju, X., Xu, C., Han, X., Du, X., Wei, G., and Yang, Y. (2017) A Review of the Concentrated Photovoltaic/Thermal (CPVT) Hybrid Solar Systems Based on the Spectral Beam Splitting Technology, Appl. Energy, 187: 534–563.

Markvart, T., Ed. (2009) Solar Electricity, Hoboken, NJ: John Wiley and Sons.

Mittelman, G., Kribus, A., and Dayan, A. (2007) Solar Cooling with Concentrating Photovoltaic/Thermal (CPVT) Systems, Energy Convers. Manage., 48(9): 2481–2490.

Mojiri, A., Taylor, R., Thomsen, E., and Rosengarten, G. (2013) Spectral Beam Splitting for Efficient Conversion of Solar Energy—A Review, Renew. Sustain. Energy Rev., 28: 654–663.

Otanicar, T., Dale, J., Orosz, M., Brekke, N., DeJarnette, D., Tunkara, E., Roberts, K., and Harikumar, P. (2018) Experimental Evaluation of a Prototype Hybrid CPV/T System Utilizing a Nanoparticle Fluid Absorber at Elevated Temperatures, Appl. Energy, 228: 1531–1539.

Stanley, C., Mojiri, A., Rahat, M., Blakers, A., and Rosengarten, G. (2016) Performance Testing of a Spectral Beam Splitting Hybrid PVT Solar Receiver for Linear Concentrators, Appl. Energy, 168: 303–313.

Widyolar, B., Jiang, L., and Winston, R. (2018) Spectral Beam Splitting in Hybrid PV/T Parabolic Trough Systems for Power Generation, Appl. Energy, 209: 236–250.

Wingert, R., O'Hern, H., Orosz, M., Harikumar, P., Roberts, K., and Otanicar, T. (2020) Spectral Beam Splitting Retrofit for Hybrid PV/T Using Existing Parabolic Trough Power Plants for Enhanced Power Output, Sol. Energy, 202(15): 1–9.

#### Referencias

1. Bierman, D.M., Lenert, A., and Wang, E.N. (2016). Spectral Splitting Optimization for High-Efficiency Solar Photovoltaic and Thermal Power Generation, Appl. Phys. Lett., 109(24): 243904.
2. Brekke, N., Otanicar, T., DeJarnette, D., and Hari, P. (2016) A Parametric Investigation of a Concentrating Photovoltaic/Thermal System with Spectral Filtering Utilizing a Two-Dimensional Heat Transfer Model, J. Sol. Energy Eng., 138(2): 021007.
3. Chow, T.T. (2010) A Review on Photovoltaic/Thermal Hybrid Solar Technology, Appl. Energy, 87(2): 365–379.
4. Duffie, J.A. and Beckman, W.A. (2013) Solar Engineering of Thermal Processes, Hoboken, NJ: John Wiley and Sons.
5. Goel, N., Taylor, R.A., and Otanicar, T. (2020) A Review of Nanofluid-Based Direct Absorption Solar Collectors: Design Considerations and Experiments with Hybrid PV/Thermal and Direct Steam Generation Collectors, Renewable Energy, 145: 903–913.
6. Hassani, S., Taylor, R.A., Mekhilef, S., and Saidur, R. (2016) A Cascade Nanofluid-Based PV/T System with Optimized Optical and Thermal Properties, Energy, 112: 963–975.
7. Huang, G., Wang, K., and Markides, C.N. (2021) Efficiency Limits of Concentrating Spectral-Splitting Hybrid Photovoltaic-Thermal (PV-T) Solar Collectors and Systems, Light Sci. Appl., 10(1): 28.
8. Ju, X., Xu, C., Han, X., Du, X., Wei, G., and Yang, Y. (2017) A Review of the Concentrated Photovoltaic/Thermal (CPVT) Hybrid Solar Systems Based on the Spectral Beam Splitting Technology, Appl. Energy, 187: 534–563.
9. Markvart, T., Ed. (2009) Solar Electricity, Hoboken, NJ: John Wiley and Sons.
10. Mittelman, G., Kribus, A., and Dayan, A. (2007) Solar Cooling with Concentrating Photovoltaic/Thermal (CPVT) Systems, Energy Convers. Manage., 48(9): 2481–2490.
11. Mojiri, A., Taylor, R., Thomsen, E., and Rosengarten, G. (2013) Spectral Beam Splitting for Efficient Conversion of Solar Energy—A Review, Renew. Sustain. Energy Rev., 28: 654–663.
12. Otanicar, T., Dale, J., Orosz, M., Brekke, N., DeJarnette, D., Tunkara, E., Roberts, K., and Harikumar, P. (2018) Experimental Evaluation of a Prototype Hybrid CPV/T System Utilizing a Nanoparticle Fluid Absorber at Elevated Temperatures, Appl. Energy, 228: 1531–1539.
13. Stanley, C., Mojiri, A., Rahat, M., Blakers, A., and Rosengarten, G. (2016) Performance Testing of a Spectral Beam Splitting Hybrid PVT Solar Receiver for Linear Concentrators, Appl. Energy, 168: 303–313.
14. Widyolar, B., Jiang, L., and Winston, R. (2018) Spectral Beam Splitting in Hybrid PV/T Parabolic Trough Systems for Power Generation, Appl. Energy, 209: 236–250.
15. Wingert, R., O'Hern, H., Orosz, M., Harikumar, P., Roberts, K., and Otanicar, T. (2020) Spectral Beam Splitting Retrofit for Hybrid PV/T Using Existing Parabolic Trough Power Plants for Enhanced Power Output, Sol. Energy, 202(15): 1–9.