Modified exergoeconomic modeling of geothermal power plants

In this study, a modified exergoeconomic model is proposed for geothermal power plants using exergy and cost accounting analyses, and a case study is in this regard presented for the Tuzla geothermal power plant system (Tuzla GPPS) in Turkey to illustrate an application of the currently modified exergoeconomic model. Tuzla GPPS has a total installed capacity of 7.5 MW and was recently put into operation. Electricity is generated using a binary cycle. In the analysis, the actual system data are used to assess the power plant system performance through both energy and exergy efficiencies, exergy losses and loss cost rates. Exergy efficiency values vary between 35% and 49% with an average exergy efficiency of 45.2%. The relations between the capital costs and the exergetic loss/destruction for the system components are studied. Six new exergetic cost parameters, e.g., the component annualized cost rate, exergy balance cost, overall unavoidable system exergy destruction/loss cost rate, overall unavoidable system exergy destruction/loss cost rate, overall unavoidable system exergy production cost rate and the overall unavoidable system exergy production cost rate are studied to provide a more comprehensive evaluation of the system.     

Life cycle assessment of geothermal binary power plants using enhanced low-temperature reservoirs

Geothermal binary power plants that use low-temperature heat sources have gained increasing interest in the recent years due to political efforts to reduce greenhouse gas emissions and the consumption of finite energy resources. The construction of such plants requires large amounts of energy and material. Hence, the question arises if geothermal binary power plants are also environmentally promising from a cradle-to-grave point of view. In this context, a comprehensive Life Cycle Analysis (LCA) on geothermal power production from EGS (enhanced geothermal systems) low-temperature reservoirs is performed. The results of the analysis show that the environmental impacts are very much influenced by the geological conditions that can be obtained at a specific site. At sites with (above-) average geological conditions, geothermal binary power generation can significantly contribute to more sustainable power supply. At sites with less favorable conditions, only certain plant designs can make up for the energy and material input to lock up the geothermal reservoir by the provided energy. The main aspects of environmentally sound plants are enhancement of the reservoir productivity, reliable design of the deep wells and an efficient utilization of the geothermal fluid for net power and district heat production.     

Optimal design of binary cycle power plants for water-dominated,medium-temperature geothermal fields

Exploitation of lower temperature, water-dominated geothermal fields is analyzed, and a methodology for optimizing geothermal binary plants is discussed. The geothermal fluid inlet temperatures considered are in the 110–160 °C range, while the return temperature of the brine is assumed to be between 70 and 100 °C. The analysis shows that the brine specific consumption, ranging from 20 to 120 kg s⁻¹ for each net MW produced, and the efficiency of the plants, ranging from 20% to 45% in terms of Second Law efficiency, are dictated mainly by the combination of the brine inlet temperature, the brine rejection temperature and the energy conversion cycle being used. For given operating conditions and with correct matching between working fluid and energy conversion cycle, it is possible to obtain very similar performances in a number of different cases. It is shown that optimization of the plant can yield improvements of up to 30–40% in terms of reduction of brine specific consumption compared to conventional design.     

Performance and parametric investigation of a binary geothermal power plant by exergy

Exergy analysis of a binary geothermal power plant is performed using actual plant data to assess the plant performance and pinpoint sites of primary exergy destruction. Exergy destruction throughout the plant is quantified and illustrated using an exergy diagram, and compared to the energy diagram. The sites with greater exergy destructions include brine reinjection, heat exchanger and condenser losses. Exergetic efficiencies of major plant components are determined in an attempt to assess their individual performances. The energy and exergy efficiencies of the plant are 4.5% and 21.7%, respectively, based on the energy and exergy of geothermal water at the heat exchanger inlet. The energy and exergy efficiencies are 10.2% and 33.5%, respectively, based on the heat input and exergy input to the binary Rankine cycle. The effects of turbine inlet pressure and temperature and the condenser pressure on the exergy and energy efficiencies, the net power output and the brine reinjection temperature are investigated and the trends are explained.     

Controls on stibnite precipitation at two New Zealand geothermal power stations

Stibnite (Sb₂S₃) forms in the heat-exchanger units of several New Zealand binary geothermal power stations. By analysing aqueous samples collected from two representative plants, it was determined that stibnite forms at a rate of 8.7 and 15.8 kg/day at the Rotokawa and Ngawha power stations, respectively. These results were compared to theoretical predictions of stibnite solubility. It was shown that pH change is the principal cause of stibnite deposition at Rotokawa, while at Ngawha the effect of temperature decrease is more significant. Antimony was not detected in vapour-line samples, suggesting that transport is completely within the aqueous phase.     

Ideal thermal efficiency for geothermal binary plants

The Carnot cycle is reviewed as to its appropriateness to serve as the ideal model for geothermal binary power plants. It is shown that the Carnot cycle sets an unrealistically high upper limit on the thermal efficiency of these plants. A more useful model is the triangular (or trilateral) cycle because binary plants operating on geothermal hot water use a non-isothermal heat source. The triangular cycle imposes a lower upper bound on the thermal efficiency and serves as a more meaningful ideal cycle against which to measure the performance of real binary cycles. Carnot and triangular cycle efficiencies are contrasted and the thermal efficiencies of several actual binary cycles are weighed against those of the ideal triangular cycle to determine their relative efficiencies. It is found that actual binary plants can achieve relative efficiencies as high as 85%. The paper briefly discusses cycles using two-phase expanders that in principle come close to the ideal triangular cycle.     

Fifty years of geothermal power generation at Wairakei

The challenges and changes that have occurred over the last 50 years of remarkable service from the Wairakei Geothermal Power Project are reviewed. The project was initially constructed during the 1953–1963 period. Plant changes including the decommissioning of the high-pressure turbine generators, the installation of a 3.5-MW intermediate-low pressure steam turbine at the Wairakei Power Station in 1996, the commissioning of the 55 MW Poihipi Power Station in 1997, the 14 MW binary power plant at the Wairakei Power Station in 2005, and a proposed new station to be constructed in the Te Mihi area in 2011–2016 are briefly discussed. Also reviewed are steamfield aspects including steam separation processes, a pilot scheme that was designed to carry hot geothermal water some distance before flash steam generation by pressure reduction, steam production from vapor-dominated regions in the Wairakei reservoir, geothermal water injection, and cascade and direct heat uses. Finally, various aspects of the Wairakei development that have contributed to its success are described. It is anticipated that the geothermal resource will be producing beyond 2028 at generation levels 50% above the current (2008) level.     

Dry cooling towers as condensers for geothermal power plants

The aim of this paper is to present scaling laws for a dry natural draft cooling tower by modelling the heat exchanger and the tower supports as a porous medium. Porous medium modelling of the tube bundles that allows a vigorous theoretical analysis of the problem is adopted. Scale analysis is used as the theoretical tool to study the problem of turbulent free convection through the heat exchanger bundles and along the cooling tower chimney. Results are then compared with full numerical simulation of the problem to observe splendid agreement.     

Exergetic analysis of various types of geothermal power plants

Based on available surveys, it has been shown that Iran has substantial geothermal potential in the north and north-western provinces, where in some places the temperature reaches 240 °C. In order to better exploit these renewable resources, it is necessary to study this area. Thus, the aim of this paper is a comparative study of the different geothermal power plant concepts, based on the exergy analysis for high-temperature geothermal resources. The considered cycles for this study are a binary geothermal power plant using a simple organic Rankine cycle (ORC), a binary geothermal power plant using an ORC with an internal heat exchanger (IHE), a binary cycle with a regenerative ORC, a binary cycle with a regenerative ORC with an IHE, a single-flash geothermal power plant, a double-flash geothermal power plant and a combined flash-binary power plant. With respect to each cycle, a thermodynamic model had to be developed. Model validation was undertaken using available data from the literature. Based on the exergy analysis, a comparative study was done to clarify the best cycle configuration. The performance of each cycle has been discussed in terms of the second-law efficiency, exergy destruction rate, and first-law efficiency. Comparisons between the different geothermal power plant concepts as well as many approaches to define efficiencies have been presented. The maximum first-law efficiency was found to be related to the ORC with an IHE with R123 as the working fluid and was calculated to be 7.65%. In contrast, the first-law efficiency based on the energy input into the ORC revealed that the binary cycle with the regenerative ORC with an IHE and R123 as the working fluid has the highest efficiency (15.35%). Also, the maximum first-law efficiency was shown to be given by the flash-binary with R123 as the working fluid and was calculated to be 11.81%.     

Modelling of hydrogen production from hydrogen sulfide in geothermal power plants

Geothermal power plants emit high amount of hydrogen sulfide (H₂S). The presence of H₂S in the air, water, soils and vegetation is one of the main environmental concerns for geothermal fields. There is an increasing interest in developing suitable methods and technologies to produce hydrogen from H₂S as promising alternative solution for energy requirements. In the present study, the AMIS technology is the invention of a proprietary technology (AMIS^® - acronym for “Abatement of Mercury and Hydrogen Sulfide” in Italian language) for the abatement of hydrogen sulphide and mercury emission, is primarily employed to produce hydrogen from H₂S. A proton exchange membrane (PEM) electrolyzer operates at 150 °C with gaseous H₂S sulfur dimer in the anode compartment and hydrogen gas in the cathode compartment. Thermodynamic calculations of electrolysis process are made and parametric studies are undertaken by changing several parameters of the process. Also, energy and exergy efficiencies of the process are calculated as % 27.8 and % 57.1 at 150 °C inlet temperature of H₂S, respectively.     

Research on the generation of electricity from the geothermal resources in Simav region, Turkey

One of the greatest problems in using renewable energy sources is the great variability of energy level, both in the short and long term. Geothermal energy, by nature, has high availability because the source is not dependent on weather conditions, so it is among the most stable renewable energy sources. Geothermal energy has the potential to play an important role in the future energy supply of Turkey. Although Turkey has the second-highest geothermal energy potential in Europe, electricity generation from geothermal energy is rather low.This study examines the use of geothermal energy in electricity generation and investigates the applicability of the existent geothermal energy resources to electricity generation in the Kütahya–Simav region, Turkey. The binary cycle is used in the designed power plant for electricity generation from geothermal fluid in which the percentage of liquid is high and which is at lower temperature. In this power plant, R134a is chosen as the secondary fluid, whose boiling point temperature is lower than that of water, and is used instead of geothermal fluid in a second cycle. The thermal efficiency of the designed power plant is measured to be 12.93%.     

Investment prospects for geothermal power in El Salvador''s electricity market

A mixed-integer optimizing programming model was created to simulate capacity expansion for the electricity market in El Salvador. Various demand scenarios were constructed, under which capacity expansion alternatives were tested. Results showed that possible geothermal projects were able to meet the growing energy needs of El Salvador, while yielding relatively low prices for the end-user. A best case projection for 2020 showed an increased proportion of geothermal generation in the energy mix by 6% compared to the present mix.

Much of the current generating plants and planned capacity are distanced from the load center, San Salvador. In order to meet the country's increasing demand, it was found that generating capacity investment should be accompanied by transmission upgrades. Even when current conditions were simulated, transmission congestion appeared to be present. Results from some expansion scenarios showed that transmission congestion increased nodal prices despite the addition of further generating capacity.     

Application of metal foams in air-cooled condensers for geothermal power plants: An optimization study

Optimized design of metal foam heat exchangers, as replacements for finned-tubes in air-cooled condensers of a geothermal power plant, is presented here. Two different optimization techniques, based on first and second law (of thermodynamics) are reported. While the former aims at the highest heat transfer rate with as low pressure drop as possible, the latter minimizes the generated entropy in the thermodynamic system. Interestingly, the two methods lead to the same optimal design. The new design has been compared to the conventional air-cooled condenser designed and optimized by using the commercially available software ASPEN. It is shown that while the heat transfer rate increases significantly (by an order of magnitude) compared to the finned-tube for the same main flow obstruction height, the pressure drop increase is within an acceptable range. Further comparison between the two systems are carried out, making use of Mahjoob and Vafai's performance factor developed specifically for metal foam heat exchangers.     

Geophysics in geothermal prospecting

Geophysical prospecting of high temperature geothermal reservoirs aims at identifying either fluid trapping structures or anomalies related to the properties of the hydrothermal fluid and rock to fluid interactions. Two types of reservoir environments can be characterized: (i) sedimentary reservoirs when a carbonate reservoir is generally capped by a dominantly argillaceous, hydraulically impervious and thermally insulating cover, and (ii) volcanic and volcano-sedimentary reservoirs associated with hydrothermally altered areas. Based on the aforementioned exploration goals and reservoir settings, a wide spectrum of geophysical methods can be applied whose selection is largely commanded by local geological conditions and expected reservoir morphology. Major geophysical techniques are reviewed and their potential, as to geothermal reservoir prospecting issues, discussed.     

Exergoeconomic analysis and optimization of basic,dual-pressure and dual-fluid ORCs and Kalina geothermal power plants: A comparative study

In present work, the basic, dual-pressure and dual-fluid ORCs and Kalina cycle for power generation from the geothermal fluid reservoir are compared from energy, exergy and exergoeconomic viewpoints. To do so, first thermodynamic models are applied to the considered cycles; then by developing cost flow rate balance and auxiliary equations using SPECO method for all components, the cost flow rate and unit cost of exergy for each stream are calculated. The results show that the turbine in basic and Kalina cycles and low pressure turbine in dual-pressure and dual-fluid ORCs have the maximum value of sum of total cost rate associated with exergy destruction and total capital investment cost rate. Thus, more attention should be paid for these components from the exergoeconomic viewpoint. The cycles are optimized to obtain maximum produced electrical power in the cycles as well as minimum unit cost of produced power. The optimization results show that among the considered cycles, dual-pressure ORC has the maximum value of produced electrical power. This is 15.22%, 35.09% and 43.48% more than the corresponding values for the basic ORC, dual-fluid ORC and Kalina cycle, respectively in optimal condition. Also Kalina cycle has the minimum value of unit cost of power produced and its value in optimum state is 26.23%%, 52.09% and 66.74% less than the corresponding values for the basic ORC, dual-pressure ORC and dual-fluid ORC, respectively in optimal condition. Finally a parametric study is carried out to assess the effects on thermodynamic and exergoeconomic parameters of the considered cycles of operating pressures and ammonia mass concentration.     

Performance analysis and optimization of a hydrogen liquefaction system assisted by geothermal absorption precooling refrigeration cycle

The present paper deals with the hydrogen liquefaction with absorption precooling cycle assisted by geothermal water is modeled and analyzed. Uses geothermal heat in an absorption refrigeration process to precool the hydrogen gas is liquefied in a liquefaction cycle. High-temperature geothermal water using the absorption refrigeration cycle is used to decrease electricity work consumption in the gas liquefaction cycle. The thermoeconomic optimization procedure is applied using the genetic algorithm method to the hydrogen liquefaction system. The objective is to minimize the unit cost of hydrogen liquefaction of the composed system. Based on optimization calculations, hydrogen gas can be cooled down to ?30 °C in the precooling cycle. This allows the exergetic cost of hydrogen gas to be reduced to be 20.16 $/GJ (2.42 $/kg LH₂). The optimized exergetic cost of liquefied hydrogen is 4.905 $/GJ (1.349 $/kg LH₂), respectively.     

Possibilities of electricity generation in the Republic of Croatia by means of geothermal energy

In the Republic of Croatia there are some medium temperature geothermal sources by means of which it is possible to produce electricity. However, only recently concrete initiatives for the construction of geothermal power plants have been started. Consequently, the paper provides proposals of the possible cycles for the Republic of Croatia. On the example of the most prospective geothermal source in the Republic of Croatia detailed analysis for the proposed energy conversion cycles is performed: for Organic Rankine Cycle (ORC) and Kalina cycle. On the basis of analysis results both the most suitable cycle for selected and for other geothermal sources in the Republic of Croatia are proposed. It is ORC which in case of the most prospective geothermal source in the Republic of Croatia has better both the thermal efficiency (the First Law efficiency) and the exergetic efficiency (the Second Law efficiency): 14.1% vs. 10.6% and 52% vs. 44%. The ORC gives net power of 5270 kW with mass flow rate 80.13 kg/s, while the Kalina cycle gives net power of 3949 kW with mass flow rate 35.717 kg/s.     

Candidate radial-inflow turbines and high-density working fluids for geothermal power systems

Optimisation of Organic Rankine Cycle (ORCs) for binary-cycle geothermal applications could play a major role in determining the competitiveness of low to moderate temperature geothermal resources. Part of this optimisation process is matching cycles to a given resource such that power output can be maximised. Two major and largely interrelated components of the cycle are the working fluid and the turbine. Both components need careful consideration: the selection of working fluid and appropriate operating conditions as well as optimisation of the turbine design for those conditions will determine the amount of power that can be extracted from a resource. In this paper, we present the rationale for the use of radial-inflow turbines for ORC applications and the preliminary design of several radial-inflow machines based on a number of promising ORC systems that use five different working fluids: R134a, R143a, R236fa, R245fa and n-Pentane. Preliminary meanline analysis lead to the generation of turbine designs for the various cycles with similar efficiencies (77%) but large differences in dimensions (139-289 mm rotor diameter). The highest performing cycle, based on R134a, was found to produce 33% more net power from a 150 °C resource flowing at 10 kg/s than the lowest performing cycle, based on n-Pentane.     

Efficiency improvement for geothermal power generation to meet summer peak demand

Geothermal power is an important part of New Zealand's renewable electricity supply due to its attractive cost and reliability. Modular type binary cycle plants have been imported and installed in various geothermal fields in New Zealand, with plans for further expansion. Power output of these plants deteriorates in the summer because plant efficiency depends directly on the geothermal resource and the ambient temperature. As these plants normally use air-cooled condensers, incorporating a water-augmented air-cooled system could improve the power output in summer thereby matching the peak air-conditioning demand. In this work, power generation for the Rotokawa plant was characterized using a similar plant performance and local weather. The improved performance was modelled for retrofit with a wet-cooling system. Maximum generation increase on the hottest day could be 6.8%. The average gain in power over the summer, November–February, was 1.5%, and the average gain for the whole year was 1%. With current binary unit generation capacity at the Rotokawa plant of 35 MW, investment in a water-augmented air-cooled system could provide 2 MW of peak generation on the hottest days. This investment in efficiency is found to compare favourably to other supply options such as solar PV, wind or gas.     

Dual-fluid-hybrid power plant co-powered by low-temperature geothermal water

Three variants of power plants fuelled or co-fuelled by geothermal water have been assessed, with the aim of making the best use of the energy contained in a stream of 80–120 °C geothermal water. Heat-flow calculations for three power plant types, namely an Organic Rankine Cycle (ORC) power plant, a dual-fluid-hybrid power plant and a single-fluid hybrid-fuelled power plant, are presented. The analysis shows the thermodynamic benefits, in terms of the extent of using the thermal energy of low-temperature geothermal water, that arise from utilizing hybrid and dual-fluid-hybrid power plants rather than ORC power plants. The dual-fluid plant optimizes the use of the geothermal water, but the hybrid plant makes the best overall utilization of the energy compared to separate ORC and fuel-fired plants.