Hybrid Perovskite-Photovoltaic and Solar-Thermal Harvesting

Abstract

Single-junction photovoltaics have inherent limitations as low-energy photons below bandgap cannot generate electrical power, wasting solar energy. Here, a hybrid solar energy harvesting concept is presented. The prototype's core component consists of a semi-transparent perovskite solar module that converts high-energy solar photons into electrical power at 14.3% efficiency, while directing ≈55% of the low-energy photons below bandgap to a solar-thermal collector. The prototype can achieve an overall exergy efficiency of ≈30.0%, with exergy referring to the usefulness of transformed solar energy and its potential to do work. This high efficiency comes from 1) photovoltaic conversion, 2) low-temperature heat generation (60 °C) from photovoltaic thermalization losses, and 3) high-temperature heat generation (900 °C) from the low-energy photons in a modeled large-scale high-concentration-ratio solar concentrator. The exergy efficiency can advance to >40% if using a record-efficiency perovskite photovoltaic. Overall, this study provides a high-efficiency solution for harvesting the full solar spectrum.

Keywords: PV/T; heating; perovskite; photovoltaics; solar.

Figure 1

Schematic illustration of the concept of hybrid perovskite‐photovoltaic and solar‐thermal (PVST) harvesting. a) Concept of hybrid PVST based on selectively‐transmissive semi‐transparent perovskite photovoltaics (SPV). Most of the ultraviolet (UV) and visible (VIS) spectrum is absorbed by the SPV to excite electron‐hole (e‐h) pairs for electricity generation, and unavoidably generating heat in the SPV. The waste heat in the SPV can be recovered by attaching a heat exchanger below the SPV with flowing coolants (e.g., gas). A part of the infrared (IR) solar spectrum passes through the SPV and is then absorbed by a solar‐thermal (ST) absorber for high‐temperature heat generation. The solar spectrum can be effectively split at the wavelength ≈780 nm. We refer to this type of concept or design as PVST‐1. b) Concept of hybrid PVST design based on a selectively‐reflective perovskite component, i.e., a combination an SPV with an IR reflector (IRR). The waste heat in the SPV can be recovered by attaching a heat exchanger below the SPV with flowing coolants (e.g., gas or liquid). The IR solar spectrum is reflected by the IRR and then absorbed by an ST absorber. Both the reflected sunlight at the top surface of the SPV and the reflected IR light by the IRR can be absorbed by the ST absorber. We refer to this type of concept or design as PVST‐2. Of note is that the figures presented herein illustrate the underlying concepts rather than the detailed designs. Based on the outlined concepts, more details on the design and geometry will be provided and discussed in subsequent sections of this study.

Figure 2

Optical properties of key components of SPV, IRR, and ST. a) Layer structure of SPV. b) Photographs of the front sides of SPV, ST, IRR, and SPV + IRR. c) Optical properties of SPV (absorptivity α _SPV and global transmittance τ _SPV), ST (absorptivity α _ST) and IRR (global reflectance γ _IRR). SPV shows good selectively‐transmissive performance. d) Specular transmittance of SPV (τ _{S_SPV}) and specular reflectance of SPV + IRR (γ _{S_SPV + IRR}) for various incident angles. e) Spectrally‐weighted average transmittance and absorptance of SPV, and average reflectance and absorptance of SPV + IRR at the wavelength range of 0.3–2.5 µm.

Figure 3

Photovoltaic performance of SPV. a) Photograph of the electrical testing platform. A nitrogen gas cooling channel is on the bottom of the SPV to control its operating temperature. b) Current‐voltage and power‐voltage curves of SPV at operating temperature 25 °C and under the normal incident angle. c) Photovoltaic power conversion efficiency of SPV as a function of operating temperature under the normal incident angle. d) Power conversion efficiency of SPV as a function of incident light angle.

Figure 4

Solar energy conversion process and exergy analysis of hybrid perovskite‐photovoltaic and solar‐thermal (PVST) harvesting process. a) Solar energy conversion process of PVST. b) Exergy analysis for the outputs of heat and electrical power. The exergy output is the maximum amount of work that can be produced by the PVST process, which consists of electrical power output and the exergy portion in heat. c) The percentages of solar energy converted to electrical power (ELE), heat in SPV (H_SPV) and heat in ST (H_ST) in various sunlight incident angles for PVST‐1 and PVST‐2, when the SPV and heat temperatures are at ambient temperature, and without solar concentration. d) The ratio of exergy and anergy in heat as a function of the heat temperature. The ratio was calculated based on a thermodynamic‐cycle method as detailed in the Method section. The ST heat temperature can be increased via concentrating the sunlight, while the SPV heat temperature is fixed at 60 °C due to the operating temperature limit of the SPV module. e) The exergy efficiency of PVST‐1 and PVST‐2 processes for various hypothetical ST heat temperatures (450 and 900 °C) as a function of sunlight incident angle. Achieving these temperatures is feasible using high‐concentration‐ratio solar collectors.^{[ 27 ]} The overall exergy efficiency can be further improved when using the record‐efficiency perovskite cell (η _ele = 27.0%) in the literature.^{[ 30 ]}

Figure 5

Design and performance of concentrated hybrid perovskite‐photovoltaic and solar‐thermal (PVST) collectors. a) A Fresnel‐lens (FL) solar concentrator. b) A hybrid PVST collector based on the PVST‐2 concept, which can be upgraded from the FL solar concentrator by attaching the SPV + IRR components above the mirrors of the FL. Regular heat exchangers with flowing coolants in the cooling channels can be attached below the SPV + IRR collector to recover waste heat from the PV modules. c) The percentages of solar energy converted to electrical power (ELE), heat in SPV (H_SPV) and heat in ST (H_ST) in various sunlight incident angles (φ) and fraction of diffuse light. d) Exergy efficiencies of hybrid PVST collectors based on FL and solar dish (SD) solar concentrators. FL solar concentrators can achieve 450 °C heat output, while SD solar concentrators can achieve a higher‐temperature heat output with 900 °C. The PCEs of the SPV in this study and the record‐efficiency SPV in the literature are 14.3% and 27.0%.^{[ 30 ]}