Organic Rankine Cycle
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Search Completed | Title | SELECTION OF WORKING FLUID AND EXERGY ANALYSIS OF AN ORGANIC RANKINE CYCLE
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International Journal of Advance Research In Science And Engineering http://www.ijarse.com IJARSE, Vol. No.4, Special Issue (01), March 2015 ISSN-2319-8354(E)
SELECTION OF WORKING FLUID AND EXERGY
ANALYSIS OF AN ORGANIC RANKINE CYCLE
Aseem Desai1, Deepa Agrawal2, Sauradeep Datta3, Sanjay Rumde4
1,2,3,4 Mechanical Engineering Department, Maharashtra Institute of Technology ( India) ABSTRACT
There are two degrees of freedom available in designing an Organic Rankine Cycle (O.R.C) - the organic fluid and the evaporator pressure. The upper limit of the latter will be decided by the heat source inlet temperature. The performance of 12 organic fluids including some newly developed silicone oils - D4, MDM, have been analyzed on the basis of thermal efficiency, exergy destruction (neglecting pressure drop) and size of the system required. The optimum evaporator pressure was found for each fluid using power output as the objective function. The source was taken as a flue gas mixture at 150 ̊C. The exergy destruction due to two irreversibility –heat transfer across a finite temperature difference and pressure drop in a shell and tube heat exchanger was quantified only for R245fa and recalculated for varying source exit temperature. The average tube side heat transfer coefficient and that for liquid-only were obtained from HTRI software for a given geometry. The pressure drop in the shell was obtained using Bell-Delaware method and that in the tube was calculated separately for the single phase and two phase part.
Keywords: Evaporator, Exergy analysis, Organic Rankine Cycle
The need for better efficiencies in power plants has never been greater. With drastic climate change occurring all over the world, we need to reduce dangerous emissions into the atmosphere. More than 50% waste heat is unused; converting some of this heat into work can attenuate the problem by reducing the consumption of fossil fuels . Another solution to this problem is the use of renewable sources of energy, like solar and geothermal, which are low grade heat sources. The operation at lower temperatures requires an organic fluid in the power cycle. Organic fluids have much lower normal boiling point than water hence is suitable for low temperature applications. Also organic fluids have lower specific volume as compared to steam hence the corresponding size of the components of the power plant will be considerably smaller.
The importance of exergy is often ignored but as per the Carnot corollary, maximum work is developed only when the irreversibility's are eradicated. Increasing the exit temperature of the source increases the Log Mean Temperature Difference (LMTD) in the evaporator and hence reduces its size. The reduced size and mass flux should give lower pressure drop. Hence, while the exergy destruction due to the temperature difference increases with increase in source exit temperature, the pressure drop irreversibility reduces with the same .
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