Page - 161 - in Water, Energy, and Environment - A Primer

shouldalsobenotedthat thesubstancemovedagainstgravity toa higher level (and therefore to a higher potential energy) doesn’t have to be water. Some companies today are revisiting a concept first proposed in the mid-19th century whereby a windmill would be employed to raise a quantity of iron balls, and these balls would then be allowed to fall into buckets on one side of awheel, causing thewheel to rotate and thus drive a machine. Modern versions of this concept substitute gravel for iron balls and the mechanical system drives a turbine and generates electricity. 9.2.7 Thermalstorage Thermal storage allows us to store energy in the formof heat or cold for use at another time. Power-generating examples include modern solar thermal power plants which use concentrated sunlight to produce all of their energy during daylight hours. Surplus energy produced during these hours can be stored thermally in the form of hot oil or molten salt, and other higher-temperature storage schemes are being explored. Another approach is to use off-peak electricity to cool water or create ice, which can be used in a building’s cooling system to lower air-conditioning electricity demand during the day. Both types of thermal storage are in use today. 9.3 APPLICATIONS Energy storage systems can be used to deliver a broad range of benefits to both the electrical grid and the grid’s customers. For customers these include backup power, increased self- consumption of PV-generated electricity, reduction of peak demand charges, and optimized management of time-of-use utility rates. For utilities, energy storage provides a range of important ancillary services such as frequency and voltage control, peak shaving, deferral of investments in distribution and transmission infrastructure, relief of transmission Energystorage 161