Science never stands still and even Australia’s “father of photovoltaics”, University of NSW’s Professor Martin Green, predicts efficiency records for the technology he has dedicated his career to will keep being broken long into the future.
“If we come back to this conference in 20 or 30 years’ time the exhibitors will be showing 40%-efficient panels,” Green told an audience at the Smart Energy Conference and Exhibition in Sydney in April.
Green was speaking on a panel hosted by LONGi Solar, an R&D partner of UNSW which entered the Australian market in 2016 and by 2018 had 10MW of capacity installed in Queensland, NSW and Victoria. The Chinese manufacturer hopes to reach 45GW worldwide by 2020.
The LONGi range of modules uses the PERC cell technology developed by researchers at the School of Photovoltaic and Renewable Energy Engineering at UNSW, with efficiencies up to 24.6%. PERC cells (for “passivated emitter rear contact”) include a layer on the back to reflect unabsorbed light for a second attempt to yield extra efficiency. That little bit extra makes a big difference, and the PERC cell has held the record for cell efficiency for 30 of the past 34 years.
Records made to be broken
It’s a big gap between 24.6% today and 40% in the future, but Green says the UNSW team is hoping to get there by using tandem stacks of cells. The university holds the record for the most efficient module – 40.6% efficiency – and it is looking at ways to reach similar performance outside the lab using affordable materials.
Green says UNSW has been working to produce a practical version of the high-efficiency “stack cells”, where by stacking cells of different materials the composite cell can convert different colours of sunlight more efficiently. “But it’s hard to find a material that has all the advantages of silicon; to find a material that is as inexpensive, as stable, as non-toxic, as abundant – all those features that make silicon such a strong candidate for photovoltaics.”
Any material would have to be tough enough to match long warranties (LONGi offers 30 years on its panels), and Green expects the industry standard for warranties on modules may one day stretch to 50 years. “Finding a material that will last that long, apart from silicon, is where the challenge lies.”
The PERC technology opened up the possibility for bifacial panels, he says, “but the industry has to push to higher efficiency, so we’re hoping we have our stack cells ready in time so we can transition straight from PERC to that type of technology.” Forty percent is within reach.
When UNSW started researching PV in the 1970s efficiencies were in the 16-17% range, set by a satellite laboratory in the US. By 1983 the university had won its first record for cell efficiency, where the team had success using silicon oxides to reduce the activity on the top surface of the cell. “Then we started thinking ahead a little bit … about the bottom of the cell,” says Green. “That’s when I drew my first diagram of a PERC cell.”
The team’s first 20% efficient cell came in 1985. It was a landmark at the lab. “People thought we’d get close to that efficiency but never over it.”
About that time Stanford University in the US formed Sunpower, whose powerful and robust cell technology set the cat among the pigeons in the world of solar. In response, UNSW turned to the PERC cell idea to find the secret of greater efficiency. It came in 1989, when the team set another efficiency record using PERC. “That provided the workhorse for our research over the coming 15 years,” Green says.
In 1999, a 25% efficiency PERC cell set the benchmark for silicon cells for the next 15 years.
“It’s been really gratifying to see the industry take up PERC. As the industry grows companies like LONGi are able to fabricate the cell more and more cheaply,” he says.
As marginal cost of manufacturing reaches zero, cell efficiency becomes the most important factor in driving down the price of modules. “We always believed efficiency was the key to reducing the cost of photovoltaics. If we come back to this conference in 20 or 30 years’ time the exhibitors will be showing 40% efficient panels.”
Cooler and more powerful
Another route to squeezing more energy out of sunlight is to find ways to keep temperatures down in PV modules, to slow the chemical reaction that takes place within them. Green’s team has calculated if module temperature is 5°C lower the lifetime energy production of the unit can be increased by 50%.
A solar module in bright sunshine will become 30°C hotter than the ambient temperature, he says. If that could be restricted to 20-25°C, power output and module life would be extended. “We’ve been looking at ways you might do that without changing the basic module design.”
Most of the sun’s energy that is not converted to electricity is retained in the module as heat, with some radiated out and some carried away by air flow. Also, silicon cells use sunlight up to near-infrared, so there is still a lot of sunlight that is not used. “If you could find a way of reflecting that heat before it comes to the module you could also reduce the temperature,” he says.
The team at UNSW hopes to demonstrate a module that can operate 10°C cooler than standard modules in bright sunshine, and to find cheap practical ways to produce modules that operate 5°C cooler than standard for 50% higher lifetime energy production. If funding is received from ARENA, Green hopes to launch the project later in 2019.
LONGi’s bifacial half-cell PERC module and all-black half-cell module products are big sellers in the Australian market, company president Li Wenxue told the conference. The Chinese firm has been a strategic research partner of UNSW since 2015, as part of a multimillion-dollar commitment to R&D that accounts for 6-7% of revenue.
“LONGi came to the Australian market in the hope of bringing solar technology products with higher value and clean energy with lower LCOE,” Li said.