Encyclopedia of Global Resources part 41 provides a wide variety of perspectives on both traditional and more recent views of Earth's resources. It serves as a bridge connecting the domains of resource exploitation, environmentalism, geology, and biology, and it explains their interrelationships in terms that students and other nonspecialists can understand. The articles in this set are extremely diverse, with articles covering soil, fisheries, forests, aluminum, the Industrial Revolution, the . Department of the Interior, the hydrologic cycle, glass, and placer mineral deposits. . | 370 Energy storage Global Resources Advantages of pumped hydro units include simple operation high reliability low maintenance long life quick start from a standstill and economic generation of peaking electrical energy. Several such systems are in operation in the United States. Power-generating capacities of these systems vary between 5 megawatts and 2 000 megawatts or higher. The overall efficiencies of these power plants vary between 65 and 90 percent these figures include the efficiencies of pumps hydraulic turbines and generators and losses from the upper reservoir . In spite of the technical and economic viability of pumped hydro the requirement of a specific type of topography and some environmental concerns limit its application. To overcome these problems underground pumped hydro storage can be used. In this case a large cavern or an aquifer can be used as the lower reser voir. Solar Heat Storage When converted into heat solar energy can be stored in the form of sensible heat and latent heat. Sensible heat is stored in a material by raising its temperature. The amount of sensible heat stored in a material is equal to the product of the mass specific heat and the temperature rise of the material. The most common sensible heat storage materials include water propylene glycol rocks and molten salts. Water has the highest specific heat value. The higher the temperature rise the greater the amount of heat stored. However the highest temperature is limited by the properties of the material. Thermal energy can also be stored as latent heat in a material when it changes phase as from solid to liquid or liquid to vapor. Some materials also change phase from solid to vapor directly or from one solid phase to another. The amount of latent heat stored in a material is equal to the product of the mass of the material and its latent heat. Because materials change phase at a constant temperature latent heat is stored and retrieved at a fixed temperature known as the .
