IDENTIFICATION OF ORC PARAMETERS FOR OPTIMIZATION OF THERMAL STORAGE MEDIUM COST IN SOLAR ORC

asme-orc2015 Tracking Number 153

Organic Rankine cycle (ORC) provides the means to convert low grade energy to useful work in an efficient manner. The ORC system can be integrated with renewable energy source such as a concentrated solar thermal system using parabolic trough collectors. However, in case of solar ORCs, issues like fluctuating nature of solar insolation are reported to adversely affect the ORC performance. Thermal storage in this regard can help in suppressing the fluctuations and also provide an option for power generation during non-solar hours. Among the various possible thermal storage technologies, pebble bed system is found to be economical and hence, is studied in this paper in conjunction to ORC with two different working fluids in it. This paper analyzes the effect of ORC working fluid on the cost associated with the pebble bed based thermal energy storage (PB-TES) tank. The physical layout considered here consists of two closed loops, a) heat transfer fluid (HTF) and b) working fluid loop. Cold HTF from a PB-TES is pumped to a parabolic trough collector where it gets heated and is then sent for the storage in a PB-TES tank. Working fluid loop is a regenerative ORC where the expander exhaust heat is recovered to heat up the pump outlet fluid. The two loops interact via a boiler where the HTF coming from a PB-TES tank is cooled and working fluid in turn is heated up to the expander inlet temperature. A case study of the above mentioned physical system is carried out using R-245fa and R-134a as working fluids for a 100 kWe ORC, with ethylene glycol (for temperatures up to 180 °C) or Therminol VP-1 (beyond 180 °C) as an HTF. DNI calculations are performed for the latitude corresponding to the tropic of Cancer (approximately mean of extreme latitudes of India) and the day of vernal equinox using ASHRAE clear sky model while using single axis tracking scheme for parabolic trough. Control strategy followed in the solar field is has been adopted to regulate HTF mass flow rate to achieve steady HTF temperature (also equal to HTF maximum temperature) throughout the day. A procedure is developed to calculate the mass of HTF required in a PB-TES tank to suppress inlet temperature fluctuations in the expander. For a given energy storage, the temperature difference of HTF across the boiler and the mass of HTF stored in a PB-TES tank are found to be inversely co-related. This temperature difference is further found to be dependent on ORC working fluid. In case of R-245fa cycle, the temperature difference is~15 °C, whereas it can be as high as 75 °C for an R-134a transcritical cycle, with a pinch temperature of 5 °C in the boiler for the both cases. This affects the mass of HTF required in a PB-TES tank which is a major cost factor. In case of R-134a, cost of HTF is found to be ~5 times lower than that for an R-245fa system.