Performance Analysis of a Rankine-Goswami Combined Cycle

Improving the efficiency of thermodynamic cycles plays a fundamental role in reducing the cost of solar power plants. These plants work normally with Rankine cycles which present some disadvantages due to the thermodynamic behavior of steam at low pressures. These disadvantages can be reduced by introducing alternatives such as combined cycles which combine the best features of each cycle. In this paper a combined Rankine-Goswami cycle (RGC) is proposed and a thermodynamic analysis is conducted. The Goswami cycle, used as a bottoming cycle, uses ammonia-water mixture as the working fluid and produces power and refrigeration while power is the primary goal. This bottoming cycle, reduces the energy losses in the traditional condenser and eliminates the high specific volume and poor vapor quality presented in the last stages of the lower pressure turbine in the Rankine cycle. In addition, the use of absorption condensation in the Goswami cycle, for regeneration of the strong solution, allows operating the low pressure side of the cycle above atmospheric pressure which eliminates the need for maintaining a vacuum pressure in the condenser. The performance of the proposed combined Rankine-Goswami cycle, under full load, was investigated for applications in parabolic trough solar thermal plants for a range from 40 to 50 MW sizes. A sensitivity analysis to study the effect of the ammonia concentration, condenser pressure and rectifier concentration on the cycle efficiency, network and cooling was performed. The results indicate that the proposed RGC provide a difference in net power output between 15.7 and 42.3% for condenser pressures between 1 to 9 bars. The maximum effective first law and exergy efficiencies for an ammonia mass fraction of 0.5 are calculated as 36.7% and 24.7% respectively for the base case (no superheater or rectifier process).