Effect of the ratio of specific heats on a small scale solar Brayton cycle

摘要

Concentrated solar energy can be considered as the Brayton cycle's heat source since unlike conventional solar-thermal plants, which concentrate the sun's energy to generate steam for driving a turbine, the Brayton cycle does not use water. Instead, the concentrated solar energy is used to heat the gas, which is the working medium and then expands through a gas turbine to generate power. The major advantage of Brayton cycle is its potential for low operation and maintenance cost, and these engines are therefore considered for both small-scale and large-scale power applications. When the pressure losses occurring in the regenerative Brayton cycle are accounted for, the cycle efficiency depends on the ratio of specific heats of the working fluid, pressure loss coefficient and the ratio of minimum to maximum gas temperature. By utilizing a numerical analytic method, the efficiency characteristics of a regenerative closed Brayton cycle under the condition of different ratio of specific heats of the working fluids were studied at different working temperatures. Three different working fluids, namely, Helium, tetrafluoromethane (CF4 or Refrigerant-14, non-toxic and non-flammable) and air were analysed and compared for a small scale closed regenerative Brayton cycle which operates at a lower temperature that can be achieved easier by solar energy. It has been found that there exist an optimum thermal efficiency and its corresponding optimum compressor pressure ratio which varies for different working fluids. The influence of the ratio of specific heats and temperature ratio of the cycle the thermal efficiency of the cycle were investigated.