As the grid is transitioned away from a heavy reliance on coal and gas it’s becoming clear that storage is the solution to so many of the National Electricity Market’s problems, which are defined by inadequate connection opportunities in parts of the country where the sun and wind are most reliable.

Developers of clean energy projects need storage to help with firming but not all investors are willing. The energy market is in a waiting game, holding out for those who are brave enough to sign off on grid-connected batteries or pumped hydro.

Another form of storage is standing at the sidelines, however, and preparing to prove itself with a 50MWh trial in Victoria – followed, it hopes, by a 1GWh monster.

“We have a new way of doing generation and a new way of doing storage,” says Will Mosley, head of business development at RayGen, which offers a “solar hydro” solution that utilizes Australia-made technology.

RayGen’s solution is best described in two stages: generation and storage. For generation, a field of parabolic mirrors, or heliostats, track the sun and focus its rays onto a PV module mounted on a tower. “We convert the concentrated light directly to electricity,” Mosley says. The multi-junction PV cells atop the tower are the type used in satellites, where different layers reap the energy from different parts of the spectrum: the top layer, or junction, responds to deep blue-violet light, the middle to yellow-green and the bottom to red-infrared. 

Solar up top

Concentrating solar energy onto focal points pushes PV cell technology to the limit. “We have a beam that’s strong enough to melt steel,” Mosley says. To extract heat from the panels so they stay within their operating temperature water is pumped through the system and the heated water that flows away from the tower heads is stored.

Almost 90% of energy that hits the panels is converted to electricity and heat: about a third as electricity and two-thirds as heat.

The cells have a 20-year design life and can take the punishment, Mosley says, citing a NASA trip to Mercury where a satellite collected data from the Sun-side of the planet. “A lot of these cells on satellites are lasting 20 or 30 years plus.”

The PV cell technology is expensive, but you need less of it. At peak, a cell used in RayGen’s tower will receive solar energy about 900 times greater than non-concentrated light that falls on a regular array; on average, the concentrated light is 750 times greater than unfocused.

One of the three 250kW concentrating units at work in a RayGen plant in Newbridge, Victoria.

Only four-square-metres of the panels are required to deliver 1MW of electricity, which can be deployed on one tower. The 250kW concentrating unit pictured with this story is about a quarter the size of those proposed for the project in Carwarp, Victoria. 

The modules are produced by RayGen in Nunawading, Melbourne. “That’s our proprietary solution,” he says. “These cells have to be protected, they have to manage thermal cycling over 20 years, they have to have the heat extraction at very, very high efficiencies – it’s very difficult to do, and that’s what we do.”

In April US-based PV company NREL released a six-junction cell capable of 47% efficiency, which Mosley concedes is “interesting” although he won’t be drawn on whether RayGen is looking at it.

Concentrating the solar energy that is blasted onto expensive, high-efficiency cells is of course a great way to bring down the cost of electricity produced. “No-one is trying to [deploy] these triple-junction cells over a wide area,” he says. “It’s too expensive.”

Putting it into physical metrics, it means the componentry for a 1GW solar project can fit into about 12 shipping containers, whereas a normal solar farm would need 24,000 shipping containers of solar panels per gigawatt, he says.

Storage down below

The next stage in the process is storage. The water cooling system is a closed-loop where it enters the modules as tepid and is heated to about 95°C. “Where you would normally see a 200MW solar farm we see a 200MW electricity solar farm plus a 400MW heat source,” Mosley says. “We extract that heat and capture it as hot water.”

A normal solar farm is 15-20% efficient, he says, where the remainder, up to 85%, is wasted as heat. “It degrades the panels and contributes to hotter surroundings.” 

The possibilities for RayGen’s technology as a form of storage are contained in the near 90°C difference in temperature between the water sent down from the towers and water cooled to near freezing using electricity from the grid or the onsite concentrated PV modules. To dispatch that stored energy the hot and cold water, stored in rubber-insulated ponds, is used to boil and condense a fluid within an Organic Rankine Cycle process to power a turbine. 

“Hot water comes in, boils the ammonia, the boiling spins a turbine, and then the cold water re-condenses the ammonia into a liquid so you can begin the cycle of the turbine again,” he says.

The turbine runs continually. 

The 50MWh Carwarp project, in the late stages of feasibility and development, is equivalent to 17 hours’ of storage. “So when the pits are fully charged [the hot and cold are at optimum temperatures] we can run the Rankine cycle for 17 hours.” The project will rely on 4MW of concentrated solar PV. Mosley says the Carwarp project will make up about one-seventh of the non-pumped hydro market for storage when it is completed in 2021. “At a price point [per kWh] that will be substantially lower cost than existing battery projects,” he says.

Central control

Mosley is describing an enormous work of plumbing, much like the human body, where clogged vessels and fluctuating pressure levels would bode ill health. “The storage side looks like a large industrial refrigeration plant,” he says. “It’s got a lot of the same stuff.”

RayGen has 3MW of solar cogeneration in operation, with three towers in Australia and a tower in China, and the company has been running a control system for its PV towers for years. It has since developed an optimization model for its storage solution thanks to an inhouse PhD. “We have a system that can model all of these different dynamics.” The model will be tested in the Carwarp project.

RayGen has also enlisted the expertise of global engineering consultant Niras, which has worked on vast district heating projects in Denmark. 

The storage solution can be scaled “much larger”, Mosley says, leading into RayGen’s vision for a 200MW solar plant with 100MW/1GWh storage, to be developed in partnership with Photon Energy. “If you want to go from 50MWh to 1GWh you simply add more towers, more water and more mirrors,” he says. 

The Carwarp project is located propitiously to help settle congestion troubles for renewables in western Victoria, but no decisions have been made about where in Australia, or the world, would be the best site for the 1GWh project. “We are looking at different applications,” Mosley says, potentially onsite with industry or as a large grid-connected plant.

“We’ve always known we have this opportunity to bring the economics of pumped hydro and flexibility of batteries and build these larger power plants in renewable energy zones,” he says. “These power plants can deliver much longer durations of storage and much greater utilization of the grid than existing storage solutions.”

RayGen has been manufacturing its solar receivers at Nunawading, in Melbourne, since 2015. “The modules we manufacture are twice the efficiency of normal solar panels and about 4,000 times the power density,” he says. “The economics are completely different. It means we can keep high-tech local manufacturing jobs in Australia, as this product scales.”

The company is looking to increase its manufacturing capacity to 100MW from 25MW. The 38% efficient wafers are sourced from Azur Space.

The 50MWh project is expected to cost $25 million. The 1GW project will target unit cost reductions “of 50% or more”, he says. 

Pumped hydro parallel

Mosley describes RayGen’s solution in similar terms to pumped hydro, and he can explain why. A height difference of a thousand metres for a pumped hydro application is similar to a 90°C water temperature difference, he says. “But we don’t need to locate ourselves in sensitive areas where there is a height difference of a thousand metres. Our solution looks like a small agricultural dam that’s covered, and we’re able to locate that right in the heart of grid-congested solar regions.”

The solution does away with the problem of building expensive transmission between an optimally located pumped hydro plant and the best solar and wind regions. “We can knock out that long-distance cable and place the storage right in the heart of where you need it,” he says. “A good solar resource is typically a flatland. We’re able to locate pumped hydro economics at a more manageable topography … delivering the benefits of pumped hydro: long life, no degradation with scaling, low toxicity. We can deliver all of those advantages but some of the trade-offs with pumped hydro we avoid.”

The Carwarp trial is backed by $3 million in ARENA funding and will be developed in partnership with AGL. It hopes to commission the plant in 2021.