A slightly frustrating factor about solar energy is that generation efficiency falls as temperatures rise. The centre of Australia might be a global hot spot for irradiance levels, but a solar plant built there would be less efficient than an equivalent system in a much colder but similarly clear-skied part of the world.
Various manufacturers offer solutions where heat is drawn away from arrays and diverted for use somewhere else, but this sub-sector of PV is relatively unknown. In an effort to shed a little more light on PV-thermal (PVT), as it’s known, a paper from an International Energy Agency task force has highlighted what it believes is a growing market for this hybrid form of PV.
PV-thermal is a busy sector, says Sunovate co-founder Glen Ryan, with about 30-odd manufacturers around the world taking various approaches. In Sunovate’s solution air in a thermal panel that sits under a PV panel is drawn through by a fan. The heat is ducted and can be used directly as space heating or fed into a heat exchanger or heat pumps.
West Australia-based Sunovate has been running a demonstration system for about 18 months, where air is drawn up from a Perth residential property, heated beneath a five-panel rooftop PV array and used to heat a space down below or released outdoors. It can be used as a closed loop, open loop or hybrid, Ryan says.
At night, air circulated through the system warms the panels, which release that energy into space, and cooler air comes back down below.
Blowing hot and cold
The efficiency of a PVT system is dependent on the temperature of the heating application, with pool heating more efficient than space heating, which is more efficient that hot water heating.
The technology originated as a method of cooling modules so that they might produce more electricity, as PV efficiency falls when temperatures rise. “That’s where we started our journey,” Ryan says. The amount of quality heat that was being produced inspired the team to refocus its approach.
Every solar cell under constant irradiation will have a distinct stagnation temperature, Ryan says. Cells on rooftop arrays commonly reach about 85°C in summer, but he gives an example of a built-in PV system at the Sunovate laboratory where cells reach close to 100°C.
“We’ve been able to pull the temperature of that panel down within 10 degrees of ambient on hot days,” he says.
Have you heard of PVT?
It’s hard to provide a hard rule for the relationship between cell temperature and efficiency, but Ryan says for some PV technology you can expect a 10°C drop to result in a 5% increase in efficiency. Newer panels that are better tuned to manage heat may see a 4% increase in efficiency from a 10°C fall in cell temperature.
The International Energy Agency report identifies hurdles affecting the uptake of PVT: a lack of mandated renewable targets for heating/cooling and molecular fuels, compared with the electricity targets that drive PV sales; the combination of the two technologies makes return on investment modelling more complex, and; a lack of visibility of PVT among governments, architects, planners, educators and industry associations means a lack of awareness among solar retailers and installers.
The technology pictured above is from Spanish company Abora, another partner in the IEA’s Solar Heating and Cooling taskforce. The Australian Photovoltaic Institute supported Sunovate’s application to join the taskforce.
The IEA report found the potential for PVT is particularly strong for commercial businesses where heating requirements are elevated during the day, such as agro-industrial processes (greenhouses, dairies) and solar water desalination and stills.
There are a number of Australian PVT technology manufacturers but take-up has been very slow. At the end of 2019, Australia had only 547 square metres of installed PVT, compared with 485,000m2 in France, 281,000m2 in South Korea, 133,000m2 in China and 112,000m2 in Germany.