A new concept in thermal engineering optimizationthe pericritical cycle with multi-heating and its application to concentrating solar power

Abstract

This work performs a detailed analysis of the integration of pericritical cycles (cycles with the compressor working in the surroundings of the critical point) in Concentrating Solar Power plants. A coherent integration between these elements results in a plant composed of a novel thermodynamic cycle layout, called multi-heating cycle, and a new type of solar field. The specific features of multi-heating cycles fit perfectly with the characteristics of concentrating solar energy, which results in a system with high potential to reduce the costs of electricity production in plants of this type of energy. The multi-heating cycle is presented as a solution to reduce the high irreversibilities of regeneration in pericritical cycles. While the heat in simple regenerative cycles is supplied by means of a single heat source, in multiple heating cycles part of that heat is replaced by others supplied at lower temperature by additional heat sources. The exergy efficiency of multi-heating cycles is greater than in simple regenerative cycles due to the lower exergetic cost of additional heat sources. Simple regenerative pericritical cycles and multi-heating cycles are studied by means of a comprehensive and systematic thermodynamic analysis. Reduced thermo-physical properties define these cycles with the purpose of generalizing its characterization setting aside the fluid. The analysis of the pericritical region shows the special relevance of the so-called discontinuity line, which separates the thermodynamic diagram into two regions: liquid and gaseous. The cycle performance is characterized according to the compressor position in the thermodynamic diagram with respect to the discontinuity line and to the cycle conditions. The greater exergy efficiency of multi-heating cycles with respect to simple cycles comes at the expense of bigger intermediate heat exchangers. The volume of the heat exchangers is calculated using printed circuit heat exchangers. The use of CO2 as working fluid shows that wet-cooled cycles achieve higher exergy efficiencies with smaller heat exchanger volumes than dry-cooled cycles. The integration of multi-heating cycles in Concentrating Solar Power plants requires the design of a new type of solar field to accommodate the solar radiation impinging on the receiver to the multi-heating requirements. This new solar field has a configuration similar to the configuration of a solar tower, but with the receiver and the heliostats divided into two sections. Each section meets different requirements in concentration ratio, fluid temperature, and absorbed heat flux. This new solar field achieves greater efficiencies than standard solar towers, but with lower thermal requirements. A techno-economic analysis shows that a system with multi-heating cycle and the new solar field not only achieves high performance, but also a great potential to reduce the costs of CSP plants.