A German-British research team has developed a novel building-integrated hybrid PV-thermal window (PVTW) that can simultaneously produce electricity and domestic hot water (DHW).
“The installation of common solar panels and collectors in the built environment requires access to significant roof space, which is limited. This motivates the development of high-efficiency, building-integrated technologies that can maximize space utilization and energy provision,” said the group. “In this work, a building-integrated hybrid PVTW is fabricated and tested, composed of a semi-transparent PV layer and a selectively absorptive liquid-based thermal absorber.”
The main component of the PVTW is a glass layer with an array of spaced amorphous silicon (a-Si) micro-stripes. Only a part of the sunlight is absorbed by the PV layer for electricity generation, while the remaining portion is transmitted to the 4 mm-thick water layer below it. Two brass tubes are used as the inlet and outlet gates for the water, and two polycarbonate frames are employed to clamp one standard low-iron glass layer.
Testing occurred on a London roof between July 16 and 23, 2021, when the maximum ambient temperature was measured at 34 C and solar irradiance was about 1,100 W m−2 at midday. The setup was tested at inclination angles of 30°, 60°, and 90°, and its performance was compared to that of a solar thermal window (STW) reference module lacking the PV component. The latter was identical to the PVTW, except for substituting the PV layer with a glass layer, resulting in two glass layers.
Image: Imperial College London, Advanced Science, CC BY 4.0
The testing showed that the PVTW system can achieve an electric efficiency of 3.6% and a thermal efficiency of 17.6%.
“The PVTW achieved efficiencies of 3.6% in electricity and 10.7% in heat generation at a 30° inclination. The ability to produce around 50 C hot water makes it appropriate for domestic applications, while its electricity generation supports the building’s energy needs,” the team noted. “Adjusting the PVTW’s inclination angle from 30° to 90° demonstrates the importance of orientation in system performance, with changes in output temperature and thermal efficiency observed.”
Namely, at an inclination of 90°, the PVTW system recorded an electricity efficiency of 3.3%, a thermal efficiency of 17.6%, and a maximum outlet water temperature of about 42 C. “Compared to a standalone STW, the PVTW not only provides higher temperature hot water but also shows a 10% absolute increase in thermal efficiency, along with electricity generation,” they added.
“To understand the potential impact of the PVTW in meeting a building’s thermal energy needs, we can estimate the PVTW surface required to meet the hot-water demand of a typical three bedroom terraced house in London, UK, occupied by 2 adults and 2 children,” the academics said. “Using an energy balance, an overall PVTW surface of no more than around 1.2 m2 at a 30° inclination would be needed to meet this demand instantaneously. Assuming a wider system with a hot-water storage tank, we estimate an overall PVTW surface of ≈2.8 m2 at the same inclination would be needed to meet this entire daily demand without the need for a backup boiler.”
The proposed system was presented in “A Building-Integrated Hybrid Photovoltaic-Thermal (PV-T) Window for Synergistic Light Management, Electricity and Heat Generation,” published in Advanced Science. Researchers from Imperial College London in the UK and Karlsruhe Institute of Technology in Germany conducted the study.
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