Basic Flow on Thermally Driven Pump in Supercritical CO2 Solar Rankine Cycle System

Abstract

Supercritical CO₂ Rankine cycle system (SRSC) was originally designed and constructed in 2004 for combining the solar energy and CO₂ as working fluid to produce electric and thermal energy output. The ordinary supercritical CO₂ Rankine cycle system which combined of solar collector, turbine, heat exchanger and a mechanical feed pump, the mechanically feed pump itself has mechanically loss and also needs to use electric energy to operation that makes the overall efficiency of system decreases. To deal with these issues, the new invention of thermally driven pump has been purposed to use instead of mechanical feed pump as a key element of the system. The thermally driven pump is a refrigerant (liquid CO₂)-circulations pump composed of two expansion tanks. The hot water and cool water are used to heat and cooling the tanks of thermally driven pump for generate the pressure difference state between thermally driven pump and outside of itself. Because of no mechanical movement and also no electric consumption, thermally driven pump is designed to be utilized on power sourcing device compared to the mechanical feed pump. By the high pressure state of supercritical CO₂, it is expanded and flow out, conversely, the low pressure state will induce the CO₂ flow in to thermally driven pump, which CO₂ flow can be achieved in SRSC without electric energy usage. In estimation of the thermally driven pump, it can give higher pressure and temperature at the outlet of the pump compared with mechanically feed pump, it is easier to achieve supercritical state of CO₂. Moreover, the result of the system efficiency becomes higher of mechanical feed pump becomes higher, which usually gives about 3.5% higher than mechanical feed pump in practical condition. However, in results it was found that the thermally driven pump needs time lag to pump the fluid in action that causes non-continuous flow due to the time lagging in cooling and heating process. In order to achieve the time usage studied in the thermally driven pump, time usage in the heating process, which is the most important significant of the process in thermally driven pump is investigated. The experimental result of the thermally driven pump, flow rate at exit point, and also temperature and pressure, are verified and discuss in this study. In this study the experiment conditions for thermally driven pump are applied both to the system analysis and real experiment to reduce the time usage. The time usage of thermally driven pump operation in the purpose of understanding the working characteristic and suitable condition is verified. The time usage in this study can be faster as 36% when increasing the hot water temperature.