摘要： Solar and wind are time-dependent energy resources. Energy needs for a very wide variety of applications are also time dependent but in a different fashion than the energy supply. Consequently, the storage of energy or other product of the process is necessary if these renewable resources are to meet substantial portions of our energy needs. Energy storage can be accomplished using thermal, mechanical, or chemical processes. Thermal storage is accomplished by heating or cooling a substance and recovering the energy at a later time by reversing the process. Mechanical storage and recovery can be accomplished by raising and lowering a mass, typically water, from one level to another level or by changing the rotational speed of a spinning wheel. Chemical storage is commonly accomplished through batteries but other chemical reactions are possible. Sometimes it is convenient to convert one form of energy to another before storing. The optimum capacity of an energy storage system depends on the expected time dependence of the energy source, the nature of loads to be met, the degree of reliability needed for the process, the manner in which auxiliary energy is supplied, and an economic analysis that determines how much of the load should be carried by solar or wind and how much by the auxiliary energy source. Note that auxiliary energy is assumed to be part of the process. The needed auxiliary energy could be another form of renewable energy. Consider a very large renewable energy system (like a large utility) where wind might make up temporary lack of solar (or vice versa) and hydro power is always available. In this chapter we set forth the principles of several energy storage methods and show how their capacities and rates of energy input and output can be calculated. In the example problems, as in the collector examples, we arbitrarily assume temperatures or energy quantities. In reality, these must be found by simultaneous solutions of the equations representing all of the system components. These matters are taken up in Chapter 10.