Organic Rankine Cycle
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How to Increase Geothermal Power Conversion Efficiencies
Queensland Geothermal Energy Centre, School of Mech. and Mining Engrg, The Uni. of Queensland firstname.lastname@example.org
Geothermal power plants in dry regions typically use air cooling and consequently suffer from reduced efficiencies during hot days. In this paper, it will be demonstrated it may be possible to use solar boosting to maintain the geothermal plant power generation in such instances. It will be shown that commercial feasibility of solar boosting is improved when the cycle fluid is heated over a variable temperature range instead of the steam or organic Rankine cycles with fixed evaporation points. Results with supercritical CO2 cycles will be compared against more conventional alternatives.
Keywords: power conversion; supercritical cycles; solar energy; hybrid plants; hot rock geothermal.
Two factors prevent widespread adoption of HFR (Hot Fractured Rock) geothermal power in Australia:
! Compared to other renewables, there is a higher technical risk in HFR projects, namely in locating, establishing and exploiting a reservoir. This risk will be constrained with progress but will never disappear since each new prospect will be unique.
! Compared to coal- or gas-fired plants, geothermal plants need to dump several times more waste heat per MW of electricity generated. Without easy access to water, this can be a problem.
Higher potential rewards make risks more affordable. The reward from HFR investment is the electricity generation. If this reward can be increased, then higher risks would be acceptable to the investors.
The electricity generation from a given geothermal fluid source depends on:
! the fraction of the heat extracted from the geothermal fluid; and
! the efficiency of converting the extracted heat to electricity.
Let us start with the second one, a.k.a. the thermal efficiency. The thermal efficiency for a power conversion cycle is defined as the net turbine shaft power divided by rate of heat input into the cycle working fluid. The theoretical limit on thermal efficiency is the efficiency of the Carnot cycle, which represents ideal power conversion conditions. The actual efficiencies are usually much lower than the theoretical efficiency due to unavoidable losses. A good indicator of
the maturity of a technology is what fraction of its Carnot efficiency it is able to deliver in actual operations.
In Figure 1, we plot the fraction of the corresponding Carnot efficiencies realised by operating geothermal, nuclear, coal, and combined-cycle gas turbine plants. The geothermal plant efficiencies are for binary plants calculated from data provided in Tester (2006) and the other plant data are from Willson (2007).
Figure 1: Fraction of the theoretical limit realised by different power generation technologies.
The important message from Figure 1 is that less than 40% of the ideal efficiency is realised in actual geothermal practice and the ratio is as low as 30% for low reservoir or high ambient temperatures. In contrast, any other modern power technology is able to enjoy around 70% of its ideal efficiency limit. Clearly, the geothermal energy practice has room to improve.
A significance source for efficiency loss in geothermal power plants is the irreversibility in the heat exchangers, e.g. see Demuth(1979), Larjola(1996), Vargas(2000), Chen(2006), Bronicki(2008).
Such irreversibilities can be significantly reduced by continuously matching the cycle fluid temperature against the temperature of the geothermal fluid during the heat exchange process. This is not possible in a conventional Rankine-cycle plant but easy in a supercritical cycle.
Case Study Definition
The analysis will be based on conditions that can be found in a typical Cooper basin HFR geothermal reservoir. We are assuming that hot brine is produced from a number of production wells at a temperature of 250oC at the rate of 500 l/s. The ambient air temperatures change from
Australian Geothermal Energy Conference 2009
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