Geothermal Energy Conversion Research Articles

Geothermo-mechanical energy conversion using shape memory alloy heat engine

AbstractThe shift towards renewable energy sources like geothermal energy has become desirable due to the recurrent energy crisis and global warming challenges influenced by fossil fuels. Geothermo-mechanical energy conversion using shape memory alloy (SMA) heat engines presents a novel and sustainable approach for harnessing geothermal energy. Shape memory alloys, known for their ability to undergo reversible phase transformations driven by temperature changes, are ideal for thermal-to-mechanical energy conversion. This paper explores the design and performance of an SMA heat engine that utilizes geothermal heat sources to drive mechanical work. The engine operates by cycling between the high-temperature geothermal environment and a cooler sink, exploiting the shape memory effect to generate mechanical motion. By integrating geothermal energy and SMA technology, this system offers a potential solution for renewable energy generation, with applications in remote or off-grid locations. The paper also investigates output power and the thermodynamic efficiency. A model is formulated and the engine behavior is simulated. A series of experiments are conducted for engine output power and efficiency. The model is compared to the experimental data for validation. The engine developed a maximum power of 3.5, 8.5, and 11.5 watts at 60, 80, and 90 °C respectively. The proposed SMA-based geothermo-mechanical energy conversion system offers a promising solution for efficient, reliable, and scalable geothermal energy harvesting. This research contributes to the development of innovative, efficient geothermal energy conversion technologies, supporting global renewable energy goals and reducing greenhouse gas emissions. This innovative energy conversion mechanism could play a key role in the future of sustainable power generation.

Self-superheated combined flash binary geothermal cycle using transcritical-CO2 power cycle with LNG heat sink as the secondary cycle

Combined flash binary geothermal cycle (CFBGC) is an efficient geothermal energy conversion technology. Natural gas (NG) is a preferred fuel in the current energy scenario. LNG gasification is a needed step for delivering NG among the end users. In the present study, a self-superheated single-flash geothermal steam cycle, a transcritical CO2 power cycle and an LNG gasification unit are integrated into a CFBGC. This study shows that the LNG gasification rate and power output can be increased simultaneously by increasing the steam turbine inlet pressure. At a higher steam turbine inlet pressure, desirable steam quality (i.e., 0.9) at the steam turbine exit is maintained by implementing self superheating of the steam. It is observed that 15°C DSH of steam enables the CFBGC to operate at a steam turbine inlet pressure that substantially enhances the output power without a noticeable increase in levelized electricity cost (LEC). The CFBGC operating at this condition yields 9.97% higher power output compared to that of the CFBGC operating at steam turbine inlet pressure requiring no DSH of steam. As a geothermal-based power plant emits very low CO2, the proposed energy system may emerge as a future sustainable energy option.

Optimum operation of an ejector-assisted combined flash-binary geothermal cycle using R245fa-isopentane mixture-based secondary working fluids

Combined flash binary geothermal cycle (CFBGC) is a traditional geothermal energy conversion technology. However, in a CFBGC, substantial exergy destruction occurs during the steam flashing. To address this issue, in the present study, the steam flashing device of the conventional CFBGC is replaced by an ejector. The ejector enhances steam cycle power output by ensuring a higher steam flow rate through the high-pressure steam turbine stage. The modified cycle is designated as the ejector assisted combined flash binary geothermal cycle (EACFBGC). To take advantage of temperature glide, various compositions of R245fa-isopentane mixture are considered as secondary working fluid. Optimum operating parameters corresponding to the maximum second law efficiencies of the proposed and conventional cycles are explored by applying an iterative method. Both considered cycles yield the highest second law efficiencies with the 0.7 mol fraction of isopentane in the working fluid mixture. For geo-fluid available at 170 °C, the second law efficiency of the optimized EACFBGC is about 8.08% higher than that of the optimized conventional CFBGC. The corresponding levelized cost of electricity (LCOE) is also 5.7% lower. In a word, the proposed EACFBGC can be considered as the upgraded CFBGC with higher energy conversion efficiency and a lower LCOE.

Effect of nanofluid on a Low-enthalpy geothermal plant

In Italy 40% of total energy consumption is destined for buildings. To reduce energy consumption and consequently achieve a reduction in carbon dioxide emissions, it is necessary to design well and build better. The technologies that make it possible to increase the energy saving of buildings for civil use are linked to three main aspects: envelope, insulation, and air conditioning; high efficiency systems, integrated building-plant design, use of appliances and lighting with low energy consumption, use of energy from renewable sources; building automation. In this work, the energy demand of a building located in Campania is supplied by a low-enthalpy geothermal plant where the working fluid is a nanofluid. Building thermal loads and performance of the geothermal plant are evaluated by means of TRNSYS®. Borehole heat exchangers design is carried out by means of the ASHRAE procedure. The geothermal system is designed considering the geological characteristics of the site, the characteristics of the geothermal plant and energy conversion system. The performance of the system is evaluated for different nanofluid as working fluid, water-Al2O3, water-CuO and water-SiO2, for several nanoparticle concentrations.

Modelling and control strategy of a distributed small‐scale low‐temperature geothermal power generation system

At present, commercial geothermal power stations are mainly high-temperature and medium-temperature geothermal energy, while the large number of low-temperature geothermal energy resources are rarely used for power generation applications. This paper proposes a new model for energy conversion in low-temperature geothermal power systems. The basic structure and working principle of this new model are analysed, in detail, a low-temperature geothermal model and a Stirling engine model for geothermal energy conversion are established, and the control strategies on the rotor and stator sides of the Stirling-driven doubly-fed generator are discussed. Finally, the simulation analysis is carried out, and its results show that the thermal conversion efficiency of Stirling’s engine can reach 38%, the output AC voltage of the power generation system reaches the set value 50Hz, 190V, and the DC bus voltage is constant at 300V, which verified the validity of the proposed power generation model and related control strategies. This system expands the range of available heat sources in geothermal power generation, makes geothermal power generation more flexible in terms of site selection, power generation capacity and control, and provides a new idea for geothermal power generation structure.

Thermodynamic analysis and comparative investigation of a novel total flow and Kalina cycle coupled system for fluctuating geothermal energy utilization

Geothermal energy is a sustainable energy source with no carbon footprint that has enormous potential for easing the climate change crisis. To improve geothermal energy conversion efficiency, a novel total flow and Kalina cycle (KC) coupled system that can overcome geothermal resource fluctuation to a certain extent is introduced. The total flow cycle is used to adapt to geothermal resource dryness fluctuations, and the KC can adapt to heat source temperature fluctuations by regulating ammonia concentrations. Mathematical models of this proposed system are established to compare its performance at both constant and variable ammonia concentrations. Simulation results show that a lower concentration is suitable for higher temperatures and leads to better system performance. The novel system adapts to a heat source with a certain dryness. When the geothermal water dryness is 0.1 and the temperature is 170 °C, the thermal and exergy efficiencies of the novel system can reach 12.84% and 38.91%, respectively, with the measure of regulating concentration. The comparative results indicate that the proposed system offers better performance than a basic KC system from both the first and second laws of thermodynamics, and the concentration regulation measure enhances the proposed system.

Thermodynamic performance comparison of various energy storage systems from source-to-electricity for renewable energy resources

This study discusses and thermodynamically analyzes several energy storage systems, namely; pumped-hydro, compressed air, hot water storage, molten salt thermal storage, hydrogen, ammonia, lithium-ion battery, Zn-air battery, redox flow battery, reversible fuel cells, supercapacitors, and superconducting magnetic storage through the first and second law of thermodynamics. By fixing an electrical output of 100 kW for all systems, the energy efficiencies obtained for the considered energy storage methods vary between 10.9% and 74.6% whereas, the exergy efficiencies range between 23.1% and 71.9%. The exergy destruction rates are also calculated for each system ranging from 1.640 kW to 356 kW. The highest destruction rate is obtained for the solar-driven molten salt thermal energy storage system since it includes thermal energy conversion via the heliostat field. Furthermore, the roundtrip efficiencies for the electrochemical and electromagnetic storage systems are compared with the analyzed systems, ranging from 58% to 94%. Renewable sources (solar, wind, ocean current, biomass, and geothermal) energy conversion efficiencies are also considered for the final round-trip performances. The molten salt and hot water systems are applicable to solar, geothermal, and biomass. The highest source-to-electricity efficiency is obtained for the super magnetic storage with 37.6% when using wind, ocean current, and biomass sources.

Energy, exergy, and cost comparison of Goswami cycle and cascade organic Rankine cycle/absorption chiller system for geothermal application

Combined cooling and power production can improve the overall efficiency of a geothermal energy conversion system. Most systems have complex structures, characterized by a large number of devices resulting in a greater heat loss, low energy conversion efficiency and higher costs. This paper presents a geothermal system based on a modified Goswami cycle, aimed to produce cooling and power simultaneously in one loop. The proposed system has a simple structure with few components, and the heat rejected to the environment by the rectifier is internally recovered to preheat the strong solution, involving lower heat losses and higher conversion efficiencies. In order to investigate the exergy destructions and losses, the overall performances, and the total absolute component cost, the modeling of the proposed system is established in detail. The Goswami system performance is evaluated for the conditions of an actual application, the geothermal reservoir of Torre Alfina (Italy), and the results compared with those of a cascade system based on organic Rankine cycle coupled with a water/lithium bromide absorption chiller. Results show that for the same power output produced in both systems, the best scenario is the Goswami cycle system with an exergy efficiency of 35.53% against 32.77% of the cascade system case. In addition, the component cost analysis showed that the highest total absolute component cost is obtained for the cascade system case with a total of 14.6 M€, while the Goswami case showed a lower value with a total of 11.8 M€.

Maiden coordinated load frequency control strategy for ST‐AWEC‐GEC‐BDDG‐based independent three‐area interconnected microgrid system with the combined effect of diverse energy storage and DC link using BOA‐optimised PFOID controller

This work presents a maiden approach of coordinated frequency control of novel solar tower (ST)-Archimedes wave energy conversion (AWEC)-geothermal energy conversion (GEC)-biodiesel driven generator (BDDG)-energy storage (ES) units and direct current (DC) links based independent three-area interconnected microgrid system (ImGS). A recent metaheuristic technique, named butterfly optimisation algorithm (BOA) is applied to obtain the optimal gains of the controllers employed with the ImGS and system participation factors. The dynamic performance of proportional–integral derivative (PID), PID with filter (PIDN), proportional–fractional-order integral derivative (PFOID) controllers with their gains tuned by different algorithms such as particle swarm optimisation (PSO), firefly algorithm (FA), whale optimisation algorithm (WOA), and BOA have been compared. Further, the effect of ES units and DC links in all the areas is analysed first time in ImGS. The results have established the superiority of the BOA-based PFOID controllers under different real-world scenarios in terms of frequency deviation, tie-line power, and objective functions. Finally, rigorous sensitivity analysis has been conducted to evaluate the superiority of BOA-optimised PFOID controller towards preserving system stability of ImGS with ±25% change in synchronising tie-line coefficients and bias values, and +20% change in loading condition without resetting the nominal condition gain values.

Thermodynamic Model of Geothermal Resources for Low-medium Temperatures Energy Conversion Process Optimization

Environmental and economic concerns are motivating manufacturers and public entities towards the use of renewable energy, which is continuously increasing its market penetration. Among the possible renewable energy resources, geothermal is particularly attractive compared to others such as solar and wind, mainly because of its continuity and dispatchability. Most studies in geothermal energy conversion systems model the resource as pure water or steam, while in reservoir simulations it is common practice to apply advanced geochemical modelling to estimate the long-term productivity. The presence of CO2 and saline equilibria may determine power plant optimisation conditions that differ from the ones which assume the resource as pure water or steam. The thermodynamic properties of the geothermal resource with high carbon dioxide (CO2) contents (1 to 8% in mass) within the 298 – 473 K temperature and 15 bar pressure are examined. The model applied focuses on the Equations of State (EoS) to be used in the calculation of the physical/thermodynamic properties of a mixture. The obtained results are validated through the comparison of different commercial software (UNISIM®, EES®, REFPROP®, TREND®). As an example, the Torre Alfina geothermal resource data (Middle Italy) are considered and the effects on the performance of a binary (ORC) geothermal power plant producing are examined in terms of energy and exergy efficiency. A sub-critical Iso-butane and a supercritical R134a power cycle are compared, and the advantages of the supercritical solution are demonstrated.

Feasibility study of a geothermal energy system for indoor swimming pool in Campi Flegrei area

Technologies based on a renewable energy source can be an optimal solution to the heating of a swimming pool, which can be very expensive in terms of energy demand. In this work, the energy demand of a swimming pool located in Campi Flegrei area (Naples surroundings, Italy) is supplied by a geothermal plant. Campi Flegrei are a very advantageous location for a geothermal plant installation at either low or medium temperature. Temperature gradient of the ground is usually equal to 0.03 K/m, but in the Campi Flegrei area it is significantly larger. With the exception of hottest areas, which are characterized by steaming ground, thermal springs and fumarolic emissions, a good value is equal to about 0.2 K/m. The swimming pool is 25.0 m long, 17.0 m wide and it has an average depth of 1.80 m. Building thermal loads are evaluated by means TRNSYS®. The energy demand for water heating is evaluated taking into account heat losses from pool surface and for water evaporation. The geothermal system is designed taking into account the geological and hydro-geological characteristics of the site, the characteristics of the geothermal plant and energy conversion system. A cost analysis is also presented which shows that the whole setup is economically satisfactory.

Comparative Analysis of Designing Solar and Geothermal Power Plants: A Case Study

Geothermal and solar thermal are renewable, clean energy sources with immense potential for electricity generation. Concentrating Solar Power can achieve very high temperatures and high efficiencies compared to geothermal power plants. However, it is intermittent and must be coupled with thermal storage or another source for continuous power generation. Geothermal resources exist in varying temperatures but are too small for economic power production; however, it is not intermittent. This paper briefly summarizes the location-specific design considerations for geothermal and solar thermal plants. The performance of both these types of power plants is analysed in terms of capacity factor, thermal energy storage hours, solar multiple, area requirement, and levelized cost of energy for a given set of environmental conditions at two separate locations, Las Cruces, US and Aydin, Turkey. Electricity consumption for an example airport at Aydin is provided by DHMI Airport authority administration, which was used as load profile for Aydin. This analysis is performed using the System Advisor Model. Simulation of parabolic trough, power tower, linear Fresnel, dish Stirling, and geothermal energy conversion systems are performed and the results are compared.

Recent trends in the development of heat exchangers for geothermal systems

The potential use of geothermal resources has been a remarkable driver for market players and companies operating in the field of geothermal energy conversion. For this reason, medium to low temperature geothermal resources have been the object of recent rise in consideration, with strong reference to the perspectives of development of Organic Rankine Cycle (ORC) technology.The main components of geothermal plants based on ORC cycle are surely the heat exchangers. A lot of different heat exchangers are required for the operation of ORC plants. Among those it is surely of major importance the Recovery Heat Exchanger (RHE, typically an evaporator), in which the operating fluid is evaporated. Also the Recuperator, in regenerative Organic Rankine Cycle, is of major interest in technology. Another important application of the heat exchangers is connected to the condensation, according to the possibility of liquid or air cooling media availability. The paper analyzes the importance of heat exchangers sizing and the connection with the operation of ORC power plants putting in evidence the real element of innovation: the consideration of the heat exchangers as central element for the optimum design of ORC systems.

A Hybrid Geothermal Energy Conversion Technology - A Potential Solution for Production of Electricity from Shallow Geothermal Resources

Geothermal energy has been successfully employed in Switzerland for more than a century for direct use but presently there is no electricity being produced from geothermal sources. After the nuclear power plant catastrophe in Fukushima, Japan, the Swiss Federal Assembly decided to gradually phase out the Swiss nuclear energy program. Deep geothermal energy is a potential resource for clean and nearly CO2-free electricity production that can supplant nuclear power in Switzerland and worldwide. Deep geothermal resources often require enhancement of the permeability of hot-dry rock at significant depths (4-6km), which can induce seismicity. The geothermal power projects in the Cities of Basel and St. Gallen, Switzerland, were suspended due to earthquakes that occurred during hydraulic stimulation and drilling, respectively. Here we present an alternative unconventional geothermal energy utilization approach that uses shallower, lower-temperature, naturally permeable regions, that drastically reduce drilling costs and induced seismicity. This approach uses geothermal heat to supplement a secondary energy source. Thus this hybrid approach may enable utilization of geothermal energy in many regions in Switzerland and elsewhere, that otherwise could not be used for geothermal electricity generation. In this work, we determine the net power output, energy conversion efficiencies, and economics of these hybrid power plants, where the geothermal power plant is actually a CO2-based plant. Parameters varied include geothermal reservoir depth (2.5-4.5 km) and turbine inlet temperature (100-220°C) after auxiliary heating. We find that hybrid power plants outperform two individual, i.e., stand-alone geothermal and waste-heat power plants, where moderate geothermal energy is available. Furthermore, such hybrid power plants are more economical than separate power plants.

Integration of deep geothermal energy and woody biomass conversion pathways in urban systems

Urban systems account for about two-thirds of global primary energy consumption and energy-related greenhouse gas emissions, with a projected increasing trend. Deep geothermal energy and woody biomass can be used for the production of heat, electricity and biofuels, thus constituting a renewable alternative to fossil fuels for all end-uses in cities: heating, cooling, electricity and mobility. This paper presents a methodology to assess the potential for integrating deep geothermal energy and woody biomass in an urban energy system. The city is modeled in its entirety as a multiperiod optimization problem with the total annual cost as an objective, assessing as well the environmental impact with a Life Cycle Assessment approach. For geothermal energy, deep aquifers and Enhanced Geothermal Systems are considered for stand-alone production of heat and electricity, and for cogeneration. For biomass, besides direct combustion and cogeneration, conversion to biofuels by a set of alternative processes (pyrolysis, Fischer-Tropsch synthesis and synthetic natural gas production) is studied. With a scenario-based approach, all pathways are first individually evaluated. Secondly, all possible combinations between geothermal and biomass options are systematically compared, taking into account the possibility of hybrid systems. Results show that integrating these two resources generates configurations featuring both lower costs and environmental impacts. In particular, synergies are found in innovative hybrid systems using excess geothermal heat to increase the efficiency of biomass conversion processes. The application to a case study demonstrates the advantages of using a system approach for the analysis over a stand-alone comparison between options.

Favorable Combination of Schottky Barrier and Junctionless Properties in Field-Effect Transistors for High Temperature Applications

High-temperature transistors are of great interest for industrial applications, for instance, oil&gas and geothermal exploration, avionic, space, automotive and energy conversion. Especially, energy conversion electronics using wide-bandgap power transistors, i.e. SiCFET, in combination with high-temperature silicon gate-driver circuits can significantly increase system efficiency and reduce costs [1]. Our improved planar virtually dopant-free silicon-based double gate SOI FET with Ni and WTiN front-gate electrodes demonstrates ultra-low drain leakage currents at high-temperatures opening a path to record operating temperatures for silicon-based FET devices. Device Structure The long-channel device fabrication relies on a standard CMOS-compatible prototyping process using SOITEC Smart-Cut™ SOI-substrates with the lowest commercially available boron background doping of 1E15cm-3 enabling high carrier mobilities. Mid-gap Schottky barrier (SB) contacts are fabricated via nickel silicidation. The metal front-gate (FG) electrode consist of reactively sputtered WTiN for NMOS or Nickel for PMOS operation on a 8nm thick SiO2 gate dielectric. Channel height (CH) is approximated to 15nm. The back-gate (BG) formed by the silicon handle wafer is insulated by 145nm buried SiO2 (BOX) (Fig.1). Device Operation The device is modeled as a combination of two interacting FETs, one fully ambipolar Schottky barrier backside FET and a second unipolar junctionless (JL) topside FET [2,3]. The BG influences the whole body layer including the S/D Schottky barriers forming an ambipolar enhancement mode SBFET. Therefore, the dominant charge carrier type, i.e. electrons (VBG»0V) or holes (VBG«0V) in the channel region is defined by the BG polarity. In this configuration, the channel is located close to the body silicon to BOX interface (Fig.3). The charge carriers originate from the mid-gap NiSi SB contacts and enter the body layer at room temperature mainly by thermionic field emission (TFE) and at elevated temperatures by thermionic emission (TE) (Fig.2). In contrast, the depletion mode JLFET formed by the FG electrode only locally affects the charge carrier density in the middle of the channel and controls the current flow between source and drain with excellent electrostatic control as illustrated by the carrier density simulation in Fig.3. The combination of the enhancement and depletion mode operation in one device can appositely be summarized as enhancement suppression or dehancement mode operation (DeFET). Results & Discussion P- and NMOS transistor behavior is realized by applying the appropriate BG polarity to the very same device enabling transistor level reconfigurability (Fig.4). Maximum on-state current and threshold voltage are proportional to the magnitude of VBG (Fig.4). The work function difference between Ni and WTiN metal front-gate devices as well as process variations result in a significant threshold shift (Fig.4). The channel height (Fig.1) is a crucial parameter in device fabrication as it directly affects the off-state leakage current and threshold voltage [7]. Regarding high-temperature performance, the use of the SOI substrate eliminates bulk drain leakage while the introduction of SB contacts minimizes the band-to-band tunneling effect in comparison to classic steep S/D pn-junctions. For rising operating temperatures up to 470K, the on-state current increases from 8 to 46µA as TFE and TE increase allowing an overcompensation of the temperature dependent carrier mobility degradation effect (Fig.5,right). At the same time the drain leakage current is only increasing from <100fA to 2nA from 300 to 470K due to the high potential barrier induced by the FG (Fig.5,left). Gate leakage currents are <6pA during all measurements. Furthermore, PMOS behavior with nickel metal front-gate follows the same trends. Device simulations predict that reducing the channel height and introducing mid-κ gate dielectrics, i.e. Al2O3, renders record operating temperatures possible even for scaled devices limited only by reliability. Comparing the here presented DeFET generation with previous generations and other long-channel Si-based high-temperature FET structures illustrates the clear advantage of our device concept in suppressing drain leakage currents at high temperatures (Fig.6) [4-7]. Conclusion In this contribution, we demonstrate by means of experimental measurement data and device simulations the significantly improved drain leakage suppression at high operating temperatures of the planar dehancement mode FET CMOS technology in comparison to our previously published results in [7]. The combination of SB and JLFET properties opens a possible path for silicon based FET operation up to temperatures of 700K for various industrial applications.

An Overview of Ocean Thermal and Geothermal Energy Conversion Technologies and Systems

The Earth and the oceans have a natural temperature gradient between the surface and given depths. The natural temperature gradient is used to operate renewable energy systems, such as power generation and space heating/cooling, which operate on similar thermodynamic concepts. For a given purpose, different technologies are used to extract heat from the two resources. Even though the concepts of the two systems are the same, their energy extracting performance is different. An overview of the similarities between the usages of the two thermal resources is presented in this paper. The energy balances at the surface of the Earth and the oceans, system performances, availabilities and economics are also presented.

Favorable Combination of Schottky Barrier and Junctionless Properties in Field-Effect Transistors for High Temperature Applications

High-temperature capable transistors are of great interest for industrial applications, for instance, oil & gas and geothermal exploration, avionic, space, automotive and energy conversion. Especially, energy conversion electronics using wide-bandgap power transistors, i.e. SiC- or GaN-based FET, in combination with high-temperature silicon gate-driver circuits can significantly increase system efficiency and reduce costs. Our improved planar virtually dopant-free silicon-based double gate SOI FET with reactively sputtered WTiNx front-gate electrodes demonstrates ultra-low drain leakage currents at high-temperatures opening a path to record operating temperatures for silicon-based FET devices.

Modeling the CO2-based enhanced geothermal system (EGS) paired with integrated gasification combined cycle (IGCC) for symbiotic integration of carbon dioxide sequestration with geothermal heat utilization

The global warming potential of carbon dioxide (CO2) emphasizes more the sequestration of CO2 otherwise emitted from coal-fired power plants in the future. This study is focused on pairing a coal-fired integrated gasification combined cycle (IGCC) plant with enhanced geothermal system (EGS) for simultaneous sequestration of CO2 and extraction of geothermal heat energy for subsequent electricity generation by an organic Rankine cycle (ORC) in enhanced geothermal systems (EGS). By assuming the reservoir characteristics for two different geothermal source temperatures 200°C and 300°C, heat transfer calculations show that larger reservoir volume (>1km3) is necessary for the sustained extraction of geothermal heat energy over a period of 25 years. The temperature and pressure profiles of CO2 in the injection well and the production well, the corresponding power output from the ORC for five different working fluids, are simulated by ASPEN Plus Version 7.3. The reservoir conditions and the type of working fluid selected determine the power output in the ORC. The temperature and the pressure of the CO2 at the outlet of the production well are greater than that at the injection well due to the heating of CO2 in the reservoir during the extraction of geothermal heat energy. Therefore, a combination of a high pressure turbine and an organic Rankine cycle is beneficial for the conversion of geothermal energy from CO2 into electricity before its recirculation into the injection well. Among the secondary working fluids used in the modeling of the ORC, the time-averaged net EGS power is highest for isobutane and lowest for isopentane over a period of 25 years. When isobutane is used as a secondary working fluid, the time-averaged power output over a period of 25 years for two geothermal reservoirs at an initial geothermal source temperature of 300°C and 200°C are 46MWe and 21MWe, respectively. When neopentane is used as a secondary working fluid, the time-averaged power output for a period of 25 years is 37MWe for an initial geothermal source temperature of 300°C and 17MWe for an initial geothermal source temperature of 200°C. Pairing IGCC with EGS can considerably recover some of the energy lost during the sequestration of CO2 (50MWe) from a 629MWe IGCC plant.

Geothermal power plants: Evolution and performance assessments

This paper traces the technical development of geothermal plants from the very beginning, focusing on the efficiency of the geothermal-to-electrical energy conversion. The plants at Larderello (Italy) and Wairakei (New Zealand) are examined in some detail. Included are discussions and analyses of little-known plants in Africa and on the Italian island of Ischia that pioneered the development of geothermal energy conversion systems beyond the dry steam plants in Tuscany. Presented are important milestones in geothermal power plant development that opened the way to worldwide expansion of the use of geothermal resources for electricity generation. It is shown that while there has been a slight trend toward higher efficiency over the years, the improvement is less than has occurred in conventional fossil-fueled power plants. The efficiency gains result mainly from more elaborate conversion systems whose extra capital costs may not be justified in all cases. Nevertheless, the data show that geothermal power plants can be designed and built to achieve high efficiencies, comparable to or even greater than conventional plants when appropriate economic conditions and incentives exist.