NREL scientists exploring how to make silicon and CdTe solar cells greener...

06 Jul 2022

...and a “smart window” developed by TNO and partners in project Sunovate has performed successfully in real world conditions.

How to reduce the carbon impact of an already green technology? This is the question researchers Hope Wikoff, Samantha Reese, and Matthew Reese, at NREL, the U.S. National Renewable Energy Laboratory, address in their paper in Joule, Embodied Energy and Carbon from the Manufacture of Cadmium Telluride and Silicon Photovoltaics.

In the paper, the team focuses on the two dominant deployed photovoltaic technologies: silicon and cadmium telluride (CdTe) PV. These “green” technologies can help reduce carbon emissions — but their manufacturing processes can themselves result in greenhouse gas emissions.

“Green technologies are awesome, but as we are working to scale them up to an incredible magnitude, it makes sense to take a close look to see what can be done to minimize the impact,” said Samantha Reese, a senior engineer and analyst in NREL’s Strategic Energy Analysis Center.

To understand the overall impact of these green technologies on global decarbonization goals, the NREL team looked beyond traditional metrics like cost, performance, and reliability. They evaluated “embodied” energy and carbon — the sunk energy and carbon emissions involved in manufacturing a PV module — as well as the energy payback time.

‘Bigger picture’

“Most advances have been driven by cost and efficiency because those metrics are easy to evaluate,” said Matthew Reese, a physics researcher at NREL. “But if part of our goal is to decarbonize, then it makes sense to look at the bigger picture. There is certainly a benefit to trying to push efficiencies, but other factors are also influential when it comes to decarbonization efforts.”

Samantha Reeses added, “One of the unique things that was done in this paper is that the manufacturing and science perspectives were brought together. We combined life-cycle analysis with materials science to explain the emission results for each technology and to examine effects of future advances.”

The manufacturing location and the technology type both have a major impact on embodied carbon and represent two key knobs that can be turned to influence decarbonization. By looking at present-day grid mixes in countries that manufacture solar, the authors found that manufacturing with a cleaner energy mix — compared to using a coal-rich mix — can reduce emissions by a factor of two.

Furthermore, although Si PV presently dominates the market, thin-film PV technologies like CdTe and perovskites provide another path to reducing carbon intensity by an additional factor of two. This insight matters because of the limited carbon budget available to support the expected scale of PV manufacturing in the coming decades.

“If we want to hit the decarbonization goals set by the Intergovernmental Panel on Climate Change, as much as one sixth of the remaining carbon budget could be used to manufacture PV modules,” said Matthew Reese. “That is the scale of the problem — it’s a massive amount of manufacturing that has to be done in order to replace the energy sources being used today.”

Nancy Haegel, center director of NREL’s Materials Science Center, said, “One of the big strengths of PV is that it has this positive feedback loop. As we clean up the grid—in part by adding more PV to the grid—PV manufacturing will become cleaner, in turn making PV an even better product.”

TNO develops promising ‘smart window’

A “smart window” developed by the Netherlands-based research center TNO and partners in the Interreg project Sunovate, performs successfully in “real world” conditions according to preliminary pilot results.

The window is designed to automatically switch between blocking heat from the sun and letting it pass. It is optimized to reduce energy consumption in moderate climates with cold winters and warm summers, such as in the Netherlands.

TNO says that its preliminary results show that the smart window transitions from an infrared transparent to blocking state as soon as direct sunlight hits the window and ambient temperatures are above 20°C. The transition back to the infrared transparent state usually happens over night when the glass surface cools down.

This change ensures an optimized use of solar heat leading to reduced energy demand for heating and cooling simultaneously. This can lead to additional energy and cost savings of up to 8% and 23.70 €/m2 glass per year when compared to state of the art HR++ windows.

TNO and partners started pilot testing this new SunSmart technology in January, 2022, to gather information on the adaptive properties and performance of the innovative energy efficient window in real environment. Two 1 m²-sized smart window demonstrators were recently produced and implemented at the SolarBEAT test facilities in Eindhoven.

This is the first time that the adaptive thermochromic effect of the new smart windows has been demonstrated in real world conditions and so far the results are promising. TNO will further monitor the window demonstrators until the end of this year, to obtain information on the switching performance during all four seasons.

The active material in the smart window is thermochromic; it changes its optical properties at a specific temperature. TNO and its partners have designed the material for high visible transparency, a switching temperature around 20°C and to only change transparency in the infrared.

Thus, the window is optimized for highest energy savings, whilst staying completely transparent to the human eye. The switch happens autonomously and is intrinsic to the laminated glass, so that the window can be installed in regular frames without special installation requirements.