Another difference between central and distributed generation is the optimum ground cover ratio (GCR). The electric power collection system of the distributed generation approach is relatively insensitive to GCR and therefore system performance is weakly related to GCR. This is not the case for the central generation approach where high temperature fluid lines are used for energy transport. Based on earlier studies (Ref. 1), a GCR equal to 0.4 is used as near optimum for the various versions of the central generation system. Other issues of interest are the startup time from a cold start and associated difficulties, as well as overnight cooldown. This energy investment to achieve design operating condition in some cases is not a negligible amount of energy compared to the daily energy collected. The Rankine steam energy conversion plant can operate over a range of temperatures (250°C to 550°) and operating conditions (saturated steam to superheated steam). When neither saturated nor superheated steam is produced by the field of collectors, a steam generator (heat exchanger) is used between the energy transport fluid and the steam. Figure 5 illustrates the general layout of a central generation plant. The three major subsystems of this plant are the collectors, energy transport network and the Rankine energy conversion plant; these subsystems are discussed in the following sections. 2.2.1 Collectors The parabolic dish "point" focusing solar collector was described in a general manner in Section 2.1.1. All of that material is applicable to central generation systems. The differences are in two areas; these are the cavity receiver design and the optimum size of the dish. Different transport fluids can be considered such as water, steam NaK, He and gases such as CO and H2 (chemical transport). The design of the receiver will
RkJQdWJsaXNoZXIy MTU5NjU0Mg==