ReviewEfficiency of geothermal power plants: A worldwide review
Introduction
Geothermal power development is witnessing a rapid growth worldwide. The short-term forecast indicates an installed capacity of 18,500 MWe by the year 2015. This represents an increase of approximately 73% from 2010 (Bertani, 2010).
Geothermal power generation is characterised by high fixed (initial) cost and a relatively low variable (running) cost (Dickson and Fanelli. 2003). Therefore, geothermal power stations are normally used for base load with reported high capacity and availability factors (AGEA, 2010).
The conversion efficiency is of significant importance for resource estimation studies during the early pre-feasibility and feasibility stages of the development and when calculating the power potential of newly drilled geothermal wells. The conversion efficiency is the ratio of net electric power generated (MWe) to the geothermal heat produced/extracted from the reservoir (MWth).
Geothermal power plants have lower efficiency relative to other thermal power plants, such as coal, natural gas, oil, and nuclear power stations (Fig. 1).
It is commonly assumed that only 10% of the energy from the produced geothermal fluid can be converted to electricity (IEA, 2007). Barbier (2002) suggests that the power conversion efficiency from geothermal steam ranges from 10 to 17%. While Dickson and Fanelli (2003) gave a 18% efficiency for a single flash system with inlet pressure of 6.5 bar. However, each geothermal power plant has its own conversion efficiency, which depends on many factors. For example, Chena Hot Springs (Aneke et al., 2011, Holdmann and List, 2007) binary plant has an efficiency of only 1% due to an average fluid enthalpy of 306 and a temperature of 73 °C, while Darajat (Ibrahim et al., 2005, Kaya et al., 2011) in Indonesia reaches an efficiency of 20.7%.
For resource estimation, the AGEA (2010) gave preference to using a specified process/technology rather than using an efficiency of conversion based on the energy removed.
This study reviews the efficiencies of geothermal power plants based on the type of plant and the features of the geothermal fluid. The efficiency of a power station is evaluated as follows: net electricity produced/thermal energy input (Ibrahim et al., 2005). In geothermal power plants, the energy input can be defined as total mass of fluid (kg/s) multiplied by the average production enthalpy (kJ/kg) as shown below:where W is the running capacity (kWe), is the total mass flow rate (kg/s), and h is the reservoir enthalpy (kJ/kg).
Exergy analysis, which is the maximum power output that could theoretically be obtained from a geothermal system relative to the surrounding (ambient temperature) is not considered in this work. Exergy analysis is normally performed to optimise production from an “existing” energy conversion system once they reach their design operating conditions (DiPippo, 2012). Exergy analysis is used to identify those elements within a plant that are most in need of redesign to improve their efficiency (DiPippo, 2012).
This work provides a high-level assessment of the conversion efficiency of geothermal power plants based on available “published” data from the current worldwide experience.
Section snippets
Factors affecting efficiency
Geothermal fluid is extracted from a production well, it passes through many processes and/or different pieces of equipment on its way to the power station. During this time the geothermal fluid loses energy that is not used to produce power.
As geothermal fluid enters the well from the reservoir it is considered as a constant enthalpy (throttling) process. However, as the fluid start to travel up the well, there will be a loss in energy (enthalpy). This is because as the fluid travels against
Geothermal steam plant efficiency
The amount of energy that can be converted to electricity is limited by the second law of thermodynamics. It is also a function of and the optimum plant design and the efficiency of different components. Bodvarsson (1974), Nathenson (1975) and the AGEA (2010) gave a conversion efficiency based on geothermal fluid temperature only (Fig. 2).
However, Fig. 2 can only be used for liquid dominated reservoirs, which may not apply to systems with excess enthalpy and/or high enthalpy vapour dominated
Efficiency of binary plants
Binary geothermal power plants are closed cycles that converts heat from the geothermal fluid into electricity by transferring the heat to another low boiling point working fluid to generate electricity (DiPippo, 2012, Saleh et al., 2007, Bliem and Mines, 1991).
Fig. 8 shows a simple binary system commonly used.
Carnot and triangular are ideal closed power cycles of thermal efficiency. These ideal processes are reversible heat transfers. Therefore no temperature difference between the heat source
Summary
Fitting all the available data (Table 5, Table 6, Table 7, Table 8) with one curve (Fig. 13) produces a generic model for the conversion efficiency as a function of enthalpy:
The data in Fig. 13 also gives a worldwide geothermal power plant average conversion efficiency of 12%.
Summary of the conversion efficiencies for binary, single flash–dry steam and double flash is given in Fig. 14. This clearly shows that double flash plants have higher conversion efficiency than
Conclusions
Several factors affect the conversion efficiency of the geothermal power plants. This includes; system design, NCG content, heat loss from equipment, turbines and generators efficiencies, parasitic load, weather and other factors.
The following conclusions were based on total produced heat using published data from 94 geothermal power plants from around the world.
The average conversion efficiency of geothermal plants is 12%, which is lower that for all conventional thermal power plants.
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