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Second Thermal and Fluids Engineering  Conference

ISSN: 2379-1748
ISBN: 978-1-56700-430-4

HEAT PUMPS FOR MOLTEN SALT ENERGY STORAGE

M. Ramos Gonzalez
Mechanical Engineering Department University of Nevada, Las Vegas, 4505 S. Maryland Pkwy., Las Vegas, NV 89154, USA

William Culbreth
Mechanical Engineering Department University of Nevada, Las Vegas, 4505 S. Maryland Pkwy., Las Vegas, NV 89154, USA

Abstract

Molten nitrate salts are currently being used to store heat generated from concentrated solar facilities at several commercial sites in the United States and Europe. Solar energy collected during the day is used to heat solar salt, a eutectic mixture of sodium and potassium nitrate, which is stored in large "hot" tanks at a temperature of about 560°C. The molten salt is passed through a steam generator to power a conventional Rankine cycle to generate electricity. The cooled salt is stored in a "cold" tank at about about 290°C. These tanks lose around 1% of their stored heat during the night. Molten salt energy storage allows power plants to generate electricity at night using the stored heat, which eliminates problems caused by power variations when solar energy is affected by intermittent cloud cover. We have been studying the use of heat pumps to use molten salt energy storage as an efficient method to store electricity. The Rankine cycle efficiency in modern power plants is limited by Carnot efficiency to about 40%. Heat pumps can be used to transfer heat from the cold tank to the hot tank with a coefficient of performance (COP) greater than one, resulting in an overall storage efficiency approaching 100%; this could serve as an efficient way to store large quantities of electrical power produced from photovoltaics or other electrical sources for overnight or later use as electrical power and as an efficient electrical storage system for the nation's electrical grid.

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