Weather conditions and daily solar cycles create variability in energy production, and this results in a discrepancy between supply and demand.
A recent pilot project in south-eastern Australia, however, using an organic Rankine cycle (ORC) equipped with flexible turboexpanders in an energy storage system, has proved to be a promising step forward.
Turboexpanders have long played a central role within ORCs, a process used to transform low- to medium-grade heat into electricity. These machines, equipped with variable inlet guide vanes (IGVs), provide flexibility for use where ambient temperature fluctuations may alter operating pressures in an ORC. The ability to run efficiently with a wide range of operation (and weather patterns) allows for additional energy recovery.
Opened in August 2023, in Carwarp, Victoria, the RayGen Power Plant Carwarp (RPPC) pilot plant combines solar and thermal-hydro energy-storage facilities while utilising ORC technology with Atlas Copco Gas and Process turboexpanders.
How does RPPC work? A field of smart mirrors concentrates sunlight onto photovoltaic solar modules in a tower-mounted receiver. This produces 1 MW of electricity for every 2 MW of heat, with a combined energy conversion efficiency of around 90%. The heat is removed by cooling water, which is then stored in an insulated pit at near boiling temperature. At the same time, electricity sourced from the photovoltaic system (or the grid) powers a chiller to produce cold energy stored in a separate water pit. The stored energy can be then converted back to electricity using an ORC at a chosen time, producing stable electricity even when the sun is not shining.
In the ORC process, heat from the hot water pit is transferred to a pressurised working fluid, in this case, anhydrous ammonia. This fluid evaporates and then expands through a turboexpander, generating power via a generator. Before the cycle can repeat, the working fluid is condensed back into a liquid by rejecting heat to the cold-water pit, which is at a near freezing temperature. The fluid is then repressurised by a pump before starting the cycle again.
To match fluctuations in energy prices, RPPC’s medium-duration thermal-hydro energy-storage system is designed for multiple rapid start-ups (to manage multiple daily start-stop cycles). These changes in energy pricing are central to the benefits of employing medium-duration energy storage systems: if renewable-energy production is high, the energy supply will likely exceed demand, which would see energy prices drop. Such lower energy prices mean cold-energy storage via the chillers makes economic sense. At times of low renewable energy supply, however, prices increase, and the energy that has been stored can be sold to the grid.
RPPC’s success highlights the capabilities of turboexpanders and ORC technology in capturing and storing solar energy. Importantly, it also marks a significant step forward in sustainable power solutions. By incorporating proven technologies into medium-duration energy-storage systems, the industry can better handle the variability inherent in renewable energy sources. This successful demonstration opens the door to developing larger commercial scale plants in Australia and internationally.
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