SUN STORAGE OPTIONS

Thermal storage: How the options stack up; More thermal storage options are opening up, but how do they measure up against molten salt storage technologies on price and efficiency?
Andrew Williams and Rikki Stancich, 10 December 2010 (CSP Today)

"Thermal energy storage systems are useful for CSP because they allow direct energy storage…Several alternative CSP storage options…are currently under development…[Germany’s] Kraftanlagen München (KAM) has been developing ceramic storage technology for several years and has successfully tested it up to a scale of 10 MWh…Storage occurs at very high temperatures (approximately 700°C)…When discharging…the heat-storage module releases its energy…US technology developer Wilson Solarpower Corporation…has also developed a…[ceramic] solar battery…[S]torage can be designed for between 2 and 16 hours…

"…Compressed Air Energy Storage (CAES) uses off-peak energy to run air compressors that pump air into either storage tanks or underground caverns. The air is stored under pressure and expanded via turbines to generate energy when electricity is needed…Brayton Energy is jointly developing a hybrid dish-Brayton (turbine) CAES system..[with] SolarCAT and Southwest Solar…next to an existing salt cavern…[on land] suitable for installing a 100MWe (peak) generating system…[the] salt caverns can provide roughly 1 GWh energy storage capacity…"

Schematic of the Brayton CAES concept (click to enlarge)

"…[Australia’s Lloyd Energy Systems] graphite-based solar energy-storage system…will make Cloncurry the first Australian town powered solely by CSP…[7200 mirrors] will guide the sun's rays into holes in the bottoms of 54 elevated graphite cubes, heating them to 1800C. The stored heat is then used to generate steam for turbines on demand…[US firm UC Supply has proposed] combining graphite composites, such as graphene, with ceramic…UK firm Mansion Ecopartners has developed a stone storage solution, designed to be used either with its proprietary compact linear fresnel technology (with a unique receiver that uses nitrogen as the heat transfer fluid); or for conventional power generation…[S]olar-heated air…is passed through [a drum filled with basalt stones]…to form a thermocline…a hot zone at the top, a transition zone and a cool zone at the bottom…[Reversing] the flow…[withdraws] the heat…Stone-based systems have the advantage of being very cheap…[because basalt] is very common and widely available…

"Phase change materials, or PCM, uses less storage material and smaller containment vessels through turning solid material into liquid (thereby changing its phase). The approach is far more efficient than raising the temperature of a material, and…can significantly drive down the costs of concentrated solar power (CSP)…[C]onstruction began in May on the world’s largest PCM heat storage project in Carboneras, Spain, a 700 kwh development under the aegis of ENDESA [and partners]…[T]he US Department of Energy recently earmarked up to US$21.5m for eight contracts…[With a] PCM-TES system…the cost of thermal storage can be reduced by over 30% and the levelized cost of electricity by 5-8%…"

Schematic of the molten salts concept, the most proven of the cutting edge options (click to enlarge)

"…[T]he most advanced thermal storage technology is the two-tank system using molten salt as the storage medium…[It] is being deployed in the Andasol 1-3 50MW parabolic trough plants in southern Spain, and is planned for Abengoa Solar’s 280MW Solana plant in Arizona…[M]olten salts are stored in two tanks; one hot, the other cold. Salts on the way to the hot tank for storage are heated through a heat exchanger…[T]o recover the thermal energy, salts pass back through the same exchanger transferring the heat to the oil, which reaches temperatures of just under 400 degrees Celsius…[A] single-tank thermocline storage system…can significantly lower the cost of storage by replacing some of the salt with a low-cost quartzite rock and sand filler. SunLab reports that thermocline with quartzite costs in the region of US$20/kWh…Bell Independent Power Corp., is working on a single tank molten salt storage system that withdraws heat until the salt is solid, and in this way makes use of the phase change ability of the molten salt…

"…[M]olten salt is generally used at lower temperatures meaning it is not always fully comparable with alternatives…[It] can probably be scaled-up further than ceramic storage. Storage costs should be comparable…Phase-change seems to be less cost-effective…One advantage of using graphite and stone is that they are extremely common…Graphite has the added attraction of increasing its heat storage capacity as storage temperature rises…[CAES] is generally effective, but must be located at suitable sites and often requires the installation of a relatively complex infrastructure…Generally speaking, thermal storage is likely to need a further period of development…[and] need to prove that such alternatives are efficient in terms of energy loss and temperature drops, are cost-competitive with existing systems and have a long service-life."