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Publication Title | A DUAL-SOURCE ORGANIC RANKINE CYCLE (DORC) FOR IMPROVED EFFICIENCY IN CONVERSION OF DUAL LOW- AND MID-GRADE HEAT SOURCES

Organic Rankine Cycle

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Proceedings of the ASME 2009 3rd International Conference oPf rEonceregdyinSgustoafinEaSb2i0li0ty9 Energy SustainabilEityS2009 JJuulyly1199-2-233, ,2200099, ,SSaannFFrarannccisisccoo, ,CCaalilfiofornrniaia, USA

ES2009-90220 ES2009-90220

A DUAL-SOURCE ORGANIC RANKINE CYCLE (DORC) FOR IMPROVED EFFICIENCY IN CONVERSION OF DUAL LOW- AND MID-GRADE HEAT SOURCES

F. David Doty and Siddarth Shevgoor

Doty Scientific, Inc., 700 Clemson Road, Columbia SC 29229 USA

ABSTRACT

Detailed thermodynamic and systems analyses show that a novel hybrid cycle, in which a low-grade (and low-cost) heat source (340 K to 460 K) provides the boiling enthalpy and some of the preheating while a mid-grade source (500 K to 800 K) provides the enthalpy for the final superheating, can achieve dramatic efficiency and cost advantages. Four of the more significant differences from prior bi-level cycles are that (1) only a single expander turbine (the most expensive component) is required, (2) condenser pressures are much higher, (3) the turbine inlet temperature (even with a low- grade geothermal source providing much of the energy) may be over 750 K, and (4) turbine size is reduced.

The latent heat of vaporization of the working fluid and the differences in specific heats between the liquid and vapor phases make full optimization (approaching second-law limits) impossible with a single heat source. When two heat sources are utilized, this problem may be effectively solved – by essentially eliminating the pinch point. The final superheater temperature must also be increased, and novel methods have been investigated for increasing the allowable temperature limit of the working fluid by 200 to 350 K. The usable temperature limit of light alkanes may be dramatically increased by (1) accommodating hydrogen evolution from significant dehydrogenation; (2) periodically or continually removing undesired reaction products from the fluid; (3) minimizing the fraction of time the fluid spends at high temperatures.

Detailed simulation results are presented for the case where (1) the low-grade heat source (such as geothermal) is 400 K and (2) the mid-grade Concentrated Solar Power (CSP) heat source is assumed to be 720 K. For an assumed condensing temperature of 305 K and working fluid flow rate of 100 kg/s, preliminary simulations give the following: (1) low-grade heat input is 25 MWT; (2) mid-grade heat input is 24 MWT; (3) the electrical output power is 13.5 MWE; and (4) the condenser rejection is only 35 MWT. For comparison, with a typical bi-level ORC generating similar power from this geothermal source alone, the low-grade heat requirement would be ~100 MWT.

1. INTRODUCTION

A large number of different Organic Rankine Cycles (ORCs) for the production of mechanical and then electrical power from a single thermal source (either low-grade or mid- grade) have been highly developed over the past century, but still they typically achieve only 35% to 55% of Carnot efficiency limits. (The debates on exactly how this limit should be defined come down largely to assumptions about the distributions of temperatures in the sources and sink.)

Our motivation for taking a fresh look at ORCs came from the need to do a better job with conversion of waste heat in a novel carbon-neutral-fuels process we are developing – a method for synthesizing renewable fuels of all types (ethanol, gasoline, jet fuel...) from waste CO2, H2O, and off-peak wind energy [1]. The process we’ve proposed, dubbed WindFuels, ends up with two large sources of waste heat – one at about 420 K (from the hot-water electrolyzer) and one at about 580 K (from the Fischer Tropsch reactor). It seemed from simple exergy considerations that it should be possible to design a single cycle that took much better advantage of the ‘availability’ of two separate (each essentially isothermal) heat sources than could be achieved with two heat engines, each operating from its own source. An obvious additional application for a cycle that achieves higher conversion efficiency when two heat sources are simultaneously available would be a geothermal-CSP hybrid, and that is the specific application we’ll address in this paper.

Most commercial power conversion cycles, even low- grade until quite recently, have ended up as conventional steam cycles. However, other – organic – working fluids have been used, in Organic Rankine Cycles (ORCs). The appeal of the ORC comes from the much lower heat of vaporization of organic fluids compared to steam and the possibility of much higher condenser pressure, which reduces the size of the expander turbines and condensers. Still, the latent heat of vaporization of the working fluid and the differences in specific heats between the liquid and vapor phases make full optimization (approaching second-law limits) impossible with a single heat source. (New fluids that eliminate these problems are not likely to be found.) Problems also arise from

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Copyright 2009 by ASME

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