Optimization of solar Rankine-cycle systems

Abstract

Solar energy can be converted into electricity very effectively through a Rankine-Cycle operating at high temperatures. The main parameters that affect the overall solar-to-electrical conversion efficiency are studied in detail and the optimum sets of conditions that provide the best efficiency are obtained for a small sized solar plant having a peak power capacity of 100 KW. The main factors studied in detail are: type of solar collector, Rankine-cycle working fluid and operating temperatures of the solar collector and the Rankine-cycle. The problem is set up such that the influence of these factors can be analyzed simultaneously to arrive at the optimum set of combinations. Temperatures range from 140 to 1100°F. It was envisaged that a lower temperatures, some of the organic fluids, rather than commonly used water, would generate higher Rankine-cycle efficiencies.

Seven organic fluids that are most desirable for use in a Rankine-cycle are considered. They are: R-11, R-12, R-113, Toluene, Chlorobenzene, Thiophene and Pyridine. Water is included as a reference fluid. Organic fluids, because they are generally found to be in superheated state after expansion through turbine are well suited to take advantage of regeneration which enhances cycle efficiency significantly. Reheat cycle improves the performance of water/steam power cycle. Thus, two types of cycles are considered: 1) Rankine-Cycle and 2) Rankine-Cycle with regeneration using organic fluids or Rankine-Cycle with reheat using water. Different types of solar collectors can be used to generate temperatures ranging from 150°F up to 1100°F. The total energy collected per year per unit area using four different generic types of collectors- flat plate, stationary compound parabolic concentrating (CPC), one axis tracking - parabolic trough and two axes tracking - paraboloid disc collector - at different operating temperatures is computed by the use of hourly solar insolation data. The results of the energy collected by solar collectors and the efficiencies obtained by Rankine-cycles operating at the corresponding temperatures are coupled to arrive at the net electrical energy conversion per year for three climatically different locations - Madison, Wisconsin, Miami, Florida, and Albuquerque, New Mexico. It was found that paraboloid disc collector using water as a Ranking-cycle fluid operating at a temperature of 1030°F provides the maximum energy per unity area and also has the least capital cost per unit power output for all three different locations. Analysis indicated that at optimum conditions, for a given collector and power cycle, the choice of Rankine fluid and the corresponding operating temperatures are practically same for all the locations. IN a Rankine-cycle, at low temperatures, water is found to be far inferior to organic fluids. However, when reheat cycle is used, water gives the efficiency results very comparable to that obtained by organic fluids in a Rankine-Cycles with regeneration or reheat cycles at optimum temperatures ranges from 2.5 to 30%.