THERMO-ECONOMIC ANALYSIS OF A MIXTURE OF RC-318 AND PENTANE AS WORKING FLUID IN A HIGH TEMPERATURE SOLAR ORC

asme-orc2015 Tracking Number 155

Organic Rankine Cycle (ORC) technology is well-documented to be economically viable when integrated with cheap heat sources such as geothermal or waste heat recovery. However, when coupled to a solar field, the same ORC is found to be expensive for each unit of electricity generated. This is because the solar collectors, generally parabolic troughs, used in these low temperature (< 150 °C) cycles are directly adopted from steam based solar Rankine cycles operating at ~ 300 °C. The solar field cost associated with ORCs for the same power generation can be brought down by increasing its maximum temperature and hence cycle efficiency. This motivates the thought to explore the possibilities of high temperature solar based ORCs which in theory have the potential to be more economically viable. Further, these high temperature ORCs can be compared to the steam based solar Rankine cycle where the operating boundaries (maximum and minimum cycle temperatures) are more likely to be identical. In literature so far, high temperature ORCs have not been explored in a great depth. Herein, we investigate working fluids suitable for high temperature ORCs for solar applications which yield higher cycle efficiencies and have the potential of realizing lower initial cost. This paper aims to compare the cost of the power block (measured in $) per unit of electricity generated (measured in We) for a high-temperature ORC with that of a steam based Rankine cycle operating under identical conditions. The analysis considers a 10,000 m2 solar field area that supplies heat to a power block for a location at the Tropic of Cancer which is roughly the mean of Indian extreme latitudes. Such a solar field ensures an averaged supply of 4 MWth for 12 solar hours on a Vernal equinox day. Hot heat transfer fluid (HTF) from this solar field is stored in a pebble based thermal storage tank. This ensures a steady heat input to the working fluid in the power cycle which is either a regenerative ORC or a steam Rankine cycle. The thermodynamic model developed herein is realistic since it accounts for the inevitable irreversibilities in various components. Design of heat exchangers and condenser is based on the standards followed in industry and is then coupled to the economic model wherein various components are modeled as per their respective capacities. NIST REFPROP database is exhaustively searched and 20 fluids are shortlisted based on the criteria of having positive condenser pressure (at 45 °C) and zero ozone depletion potential (ODP). The fluids are further scrutinized considering their thermal stabilities at 300 ºC. Those stable at 300 °C are either flammable (propane, butane) or have a very high GWP (RC-318). However, the mixture of propane and RC-318 (30 and 70 % on molar basis) is observed to overcome the demerits of the individual components and helps in achieving the possibility of having an organic fluid stable up to 350 °C. Subsequently, a thermodynamic analysis is performed which predicts that the efficiency of the steam cycle is ~ 2 % higher than that of the mixture based ORC. However, economic evaluation of both the cycles suggests that the mixture based cycle cost can be ~ 40 % lesser than the steam based cycle cost because of lower volumetric flow rates in the former resulting in a more compact power block and lower initial investment. Finally, both the cycles are compared on the parameter of $/We for solar applications which for the mixture based ORC is found to be 15 % lesser.