Researchers have unveiled a new solar collector that can generate both electricity and high-temperature heat from sunlight, utilizing a dielectric Bragg mirror that splits the spectrum and boosts efficiency.
Image Credit: Gergitek/Shutterstock.com
Splitting Sunlight for Maximum Power and Heat
Spectral-splitting PVT collectors overcome the limitations of conventional PVT systems by employing an optical filter to separate the solar spectrum based on wavelength.
Photons most effective for electricity generation (typically high-energy, shorter wavelengths) are directed to a photovoltaic cell, while the remaining photons (low-energy, longer wavelengths) are transmitted to a solar thermal absorber to produce heat. The key to this technology is the spectral-splitting optical filter.
This study focuses on a dielectric Bragg mirror, designed with a specific cutoff wavelength (λb) to match the bandgap of the selected photovoltaic cell.
The system implemented here uses a Fresnel lens to concentrate sunlight onto the Bragg mirror. The mirror is engineered to reflect wavelengths shorter than 870 nm (the GaAs bandgap) toward a concentrated gallium arsenide (GaAs) photovoltaic cell for electricity generation, while transmitting wavelengths longer than 870 nm to a solar thermal absorber for heat generation.
This spectral separation effectively creates two distinct energy streams: one optimized for electricity and the other for heat, allowing the GaAs cell to operate with reduced thermal load while the thermal absorber achieves elevated temperatures.
Putting the System to the Test
Optical characterization of the Bragg mirror and other components was performed using a UV–Vis–NIR spectrophotometer.
An integrating sphere was used to capture both diffuse and specular components of light, and a universal measurement module was employed to resolve the angular dependence of the dielectric Bragg mirror. The measured reflectance and transmittance spectra were multiplied by the standard AM1.5D solar spectrum to quantify spectrally resolved absorption and the optical power delivered to the photovoltaic and thermal receivers under concentrated sunlight.
The electrical performance of the GaAs PV cell was tested under concentrated solar illumination using a continuous dual-lamp solar simulator. The electrical measurements were conducted using a high-precision source meter. Given the intense optical flux and dynamic thermal behavior, an initial dual-scale approach was employed: I-V measurements were taken frequently every 15 seconds to capture transient responses, followed by steady-state measurements at 5-minute intervals with finer voltage steps across the operational range to ensure accurate characterization of the stabilized electrical output.
Thermal performance was evaluated by simultaneously monitoring temperatures. The solar thermal absorber housing features a polymethylmethacrylate vacuum enclosure capped with fused silica glazing and mirrored inner walls to minimize conductive and radiative heat losses.
The solar thermal absorber was supported by thin silica frames to further reduce heat losses. The GaAs PV cell was equipped with active gas cooling (nitrogen gas) and an infrared temperature sensor for accurate temperature measurement. The system's design incorporates a thermal storage block and gas cooling for the PV cell to manage concentrated energy.
Spectral Splitting Works - With Room to Improve
The experimental investigation validated the spectral-splitting capability of the dielectric Bragg mirror. The mirror successfully separated the solar spectrum, reflecting part of the spectrum (<870 nm) to the GaAs PV cell and transmitting the rest (>870 nm) to the thermal absorber. This resulted in an achieved reflectance of 91.4 % above the GaAs bandgap and a transmittance of 79.7 % below it. The Bragg mirror exhibited negligible absorption and self-heating, confirming its suitability for spectral splitting under concentrated illumination.
Under concentrated illumination (a geometric factor of 100), the system's performance, based on the total incident solar energy (full-spectrum performance), showed an electrical efficiency of 6.5 % for the GaAs cell, which operated at 36 oC. The thermal capture efficiency of the solar thermal absorber was 8.8 % at an operating temperature of 80 oC. The peak stagnation temperature of the thermal absorber, measured without any heat extraction, reached a high of 209.1 oC.
Despite the successful decoupling, both the electrical and thermal efficiencies were limited primarily by optical losses. These losses stemmed mainly from misalignment issues and interference-related reflection losses within the prototype setup. Specifically, the electrical efficiency was preserved at 6.5 % despite an 8.6 % optical reflection loss below the bandgap, thanks to a measured 5.2 K reduction in the PV cell encapsulation temperature due to the thermal decoupling. The measured efficiencies remain lower than expected due to thermal and alignment-related losses under concentration, highlighting challenges that require further optimization.
A Clear Path to Smarter Hybrid Solar
This study successfully demonstrated the feasibility and performance potential of spectral-splitting PVT collectors utilizing a dielectric Bragg mirror to achieve thermal-electrical decoupling. The core strategy involves reflecting the high-energy ultraviolet and visible wavelengths to a GaAs PV cell and transmitting the lower-energy infrared photons to a thermal absorber.
The Bragg mirror's angularly weighted cutoff wavelength of 870 nm closely matched the GaAs bandgap, validating the effectiveness of this spectral-splitting approach.
Journal Reference
Lehmann B. and Huang G (2026) Hybrid solar collectors based on Bragg-mirror spectral splitting for simultaneous heat and electricity generation. Solar Energy. 306:114288. DOI: 10.1016/j.solener.2025.114288, https://www.sciencedirect.com/science/article/pii/S0038092X25010515