Page - 113 - in Water, Energy, and Environment - A Primer

steampowerplant that lacksstoragewithout increasingthesizeof that steam plant, enables generation to match the utility load profile, allows power generation after the Sun goes down and for up to 24 hours, and can be partnered with intermittent renewable energy sources to provide firmed-uppower. Disadvantages include high upfront capital costs for concentrators and storage, although these costs have been coming down, and the fact that CSTP requires unscattered ‘direct normal’ solar radiation, thus limitingwhereCSTPplants canbe located.This suggests desert locationswith limited cloud cover, which are often arid. CSTP also needs exhaust cooling, aswith any steampower plant, creating a requirement forwater or air cooling.Water limitations necessitate air cooling in some locations, with a penalty in generating efficiency, and capital and energy costs. CSTP power plants also require large surface areas for placement of concentrators, typically 5–10 acres per MWof capacity. 8.2.2.6 Thermal storage SEGSunitsusedorganicheat transfer fluid(HTF)as theirstorage medium.OrganicHTFscanonlybeusedbelow800°F.Parabolic troughs can operate at just over 1000°F, and thus use of HTF storage limits plant efficiency by up to 12%. PowerTowers can reach higher temperatures (greater than 2000°F) but have only been used to date with molten salt storage. Molten salts (mixtures of sodiumnitrate andpotassiumnitrate)melt at 430°F and thus must be kept heated when used to transfer and store heat. Theirmaximumstorage temperature is 950°F. An interestingquestionwith respect to thermal storage is: can we do better?Modern high-efficiency thermal power plants can be designed to use steam at 1300 to 1400°F. An ideal storage temperature for these plants would be 1500 to 1700°F. A heat transfer fluid and storage method that operates at temperatures above those of HTF and molten salt would lead to significant energy cost reductions (.30%). Renewableenergy 113