THERMO-ECONOMIC EVALUATION OF ORC'S FOR VARIOUS WORKING FLUIDS

asme-orc2015 Tracking Number 154

Disparity between electricity demand and its generation motivates to utilize every possible energy resource to bridge the gap to the extent of realizing the marginal potential of the low temperature heat (< 200 °C) such as geothermal, waste heat from industrial processes or low concentration (< 100) solar collector systems. Though available in plenty, these sources are distributed in space, thus making power generation inherently distributed by nature. In this regard, organic Rankine cycle (ORC) with its ability of scalability, promises reasonable thermodynamic efficiencies. A number of fluids have been proposed in the literature and optimized based on their thermodynamic performance. However, it is worth noting that a cycle with slightly higher efficiency need not necessarily result in lower capital cost.. This paper presents a detailed thermo-economic analysis for various fluids. The criterion is to select zero OPD working fluids having positive condenser pressures at 35 °C (condenser temperature). A total of 20 fluids are shortlisted based on above criteria. Parametric studies are performed by varying working fluid, source side temperature, pinch temperature in heat exchangers namely boiler, regenerator and condenser, condenser area and power generation capacity ranging from 5 to 500 kWe. Various components considered on working fluid side are a plunger pump, plate type of heat exchangers for both regenerator and boiler, a scroll expander and an air cooled condenser. A limit of 60 bar is imposed on high side pressure in view of the pressure rating of plate heat exchangers. Similarly, components considered on heat source side are gear pump and heat transfer fluid. Ethylene glycol is selected for temperatures below 180 °C and Therminol VP-1 for higher temperatures up to 250 °C. The thermo-economic model accounts for basic instrumentation required from a control perspective which also includes variable frequency drives for working fluid pump and HTF pump. Cost functions are generated for the above mentioned components for various power capacity ranges. Finally, a Matlab code is developed to evaluate the thermodynamic performance of a given working fluid for the various parameters considered above along with the associated economic implications to arrive at a $/We (Figure of Merit, FOM). A one to one comparison is performed among the various fluids to minimize the $/We associated with them. The following observations made: i. Condenser pinch point, its area and fan power are the most significant parameters driving the figure of merit. ii. $/We for a given fluid under the given operating conditions inversely scales with the power generation capacity approaching an asymptote around 100 kWe. iii. Condenser pressure and FOM are found to be inversely related. For a particular power capacity and operating conditions (source and sink temperature) increase in condenser pressure reduces $/We. However, it is not recommended to have condenser pressures beyond 15 bar which would result in lower expansion ratio in the expander due to upper pressure limit of 60 bar. In this aspect, R-134a and R-152 are found to have lower initial investment.