Organic Rankine Cycle
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Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015
Options to Use Solar Heat to Enhance Geothermal Power Plant Performance
School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072, Australia email@example.com
Keywords: Power generation; solar hybrid; air-cooled condensers. ABSTRACT
Many future geothermal plant locations are located in dry regions of the world. This is especially true for future EGS (Enhanced Geothermal Systems) locations.
These are areas also blessed with high levels of solar incidence. The solar heat can be captured at different temperatures using different solar collector technologies. This makes it possible to consider a number of scenarios in which solar heat can be used to boost the geothermal plant performance.
The Queensland Geothermal Energy Centre of Excellence (QGECE) of the University of Queensland is investigating some of the most promising methods in which solar and geothermal heat can be combined. Most of these methods have been considered with a focus on future EGS plants but they may also be applicable to more conventional geothermal applications. The combination amounts more than the sums of the individual components in this instance.
The paper will describe how solar and geothermal heat can be combined to generate more value than what can be generated by each source individually.
The applications will include the following: boosting geothermal fluid temperatures to increase cycle conversion efficiencies; enhancing the performance of natural draft dry cooling towers by using solar heat; and solar chilling of future supercritical CO2 EGS plants to maintain higher efficiencies.
There are three main possibilities in which solar heat can be used to help the performance of a geothermal power plant: boost the temperature of the geothermal fluid before it enters the plant; further boost the binary cycle fluid temperature after it is heated by the geothermal fluid first; and help the air-cooled condenser performance to boost the cycle efficiency.
The first two options increase the power plant efficiency by increasing the turbine inlet temperature. The last option increases the power plant efficiency by reducing the turbine outlet pressure.
Mathur(1979) assessed the feasibility of solar geothermal hybrids for flash geothermal plants where solar energy was used to increase the enthalpy of the geothermal fluid. They found that the hybrid plant produced electricity cheaper than a solar-only plant but more expensive than a geothermal-only plant. There has not been much attention given to solar+geothermal hybrids since then. In more recent years, Lentz and Almanza[9, 10] proposed using solar power from parabolic trough concentrators to increase the steam flow from geothermal wells. The proposal was to produce steam by running the water from the cooling tower through the collector field and to inject the steam into the fluid coming from the well to increase its enthalpy.
There has not been serious consideration given to hybrids in binary plants. The vast majority of the binary geothermal plants are based on Rankine cycles using an organic fluid or steam depending on the resource temperature (Di Pippo, 2009).
It is difficult to make solar boosting economically feasible with a Rankine-cycle plant. This is because most of the heat added to the Rankine cycle is latent heat at the saturation temperature corresponding to the turbine design inlet pressure. It is not possible to significantly shift this temperature upwards without changing the turbine inlet pressure. Changing the pressure requires a separate turbine. Therefore, solar boosting of a geothermal plant based on a Rankine cycle requires two turbines: one for geothermal-only operation and the other for geothermal + solar. While such turbines exist, the optimum operating point would be elusive because of variable solar incidence and the inability of the plant to optimally track such variations.
Solar and geothermal heat can be combined without affecting the turbine inlet pressure if the heat transfer occurs on a gliding temperature line. A gliding-temperature cycle is one in which the heat addition and the heat removal occur over varying temperatures rather than at fixed evaporation and condensation points. An ammonia-water cycle is a gliding-temperature cycle (Kalina, 1982) and so are other mixed fluid cycles recently proposed, e.g. Angelino(1998), Colonna(2003), Chen(2011), This paper will consider solar-geothermal hybrids using supercritical and transcritical cycles.
Zhang and Lior (2006), Zhang et al(2006) and Cayer et al(2009) proposed the use of supercritical CO2 cycles with waste heat and similar resources at the same temperature range as a high-grade EGS resource, i.e. about 250 oC. The comparative benefits of these cycles are even more pronounced for geothermal applications, where a very important figure of merit is the power production from a given subsurface investment. Gurgenci et al(2006) and Atrens et al(2009,2010,2011) combined the concept of a supercritical CO2 cycle with a supercritical CO2 geothermal reservoir as proposed by Brown (2000). In a supercritical geothermal CO2 siphon plant, the supercritical fluid is sent into the reservoir, extracts the heat and rises to the surface to turn a turbine. There are significant
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