Low-temperature Geothermal Sources Research Articles

Comparison of thermodynamic cycles for power production from low-temperature geothermal heat sources

The performance of different types of organic Rankine cycles (ORCs) and of the Kalina cycle is investigated and optimized for low-temperature (100–150°C) geothermal heat sources. A variety of configurations is worth considering. The ORCs can be subcritical or transcritical and can have one or more pressure levels. Each cycle can be a simple one, have recuperation or turbine bleeding and up to 80 different working fluids can be used. Comparison of these cycles to each other and to an optimized Kalina cycle concludes that the transcritical and multi-pressure subcritical cycles are the best ones. Exergetic plant efficiencies of above 50% can be achieved when the brine is allowed to cool down as much as possible. A constraint on the brine outlet temperature causes a strong decrease in the mechanical power output of the cycle. Due to the low heat source temperatures, a low condenser temperature and small temperature differences in the heat exchanger are very important.

Modeling and optimization of a binary geothermal power plant

A model is developed for an existing organic Rankine cycle (ORC) utilizing a low temperature geothermal source. The model is implemented in Aspen Plus® and used to simulate the performance of the existing ORC equipped with an air-cooled condensation system. The model includes all the actual characteristics of the components. The model is validated by approximately 5000 measured data in a wide range of ambient temperatures. The net power output of the system is maximized. The results suggest different optimal operation strategies based on the ambient temperature. Existing literature claims that no superheat is optimal for maximum performance of the system; this is confirmed only for low ambient temperatures. For moderate ambient temperatures (Tamb ≥ 1.7 °C) superheat maximizes net power output of the system. The value of the optimal superheat increases with increasing ambient temperature. The optimal operation boosts the total power produced in a year by 9%. In addition, a simpler and semi-analytic model is developed that enables very quick optimization of the operation of the cycle. Based on the pinch condition at the condenser, a simple explicit formula is derived that predicts the outlet pressure of the turbine as a function of mass flow rate of working fluid.

Trends and prospects of development of geothermal power engineering

Analysis of the state and prospects of geothermal energetics development in Russia and foreign countries is carried out. Trends of improving the geothermal energy technologies are considered. An estimation of the development of promising binary power units for electric energy production by using low-temperature geothermal and other heat sources is given. The geothermal projects being dominant and best suited to be implemented in Russia are estimated.

A comparative study of power optimization in low-temperature geothermal heat source driven R125 transcritical cycle and HFC organic Rankine cycles

In order to compare the power output of an R125 transcritical cycle with that of HFC subcritical ORCs for a low-temperature geothermal heat source of about 100 °C, an R125 transcritical cycle and subcritical ORCs using R134a, R245fa and R152a were optimized for power output by a simulation method. In contrast to previous studies, to fairly compare the power of different cycles, power optimizations were carried out for given heat source and sink inlet temperatures, and given flow rates based on the typical power plant thermal-capacitance-rate ratio. The total overall conductance (TOC) which implies heat exchanger cost was fixed, whereas the allocation of the overall conductance between the vapor generator and the condenser was optimized in the simulation. Results show that under simulation conditions considered in the present study, the power output of an R125 transcritical cycle was greater than that of subcritical ORCs when a TOC was higher than 35 kW/K. When a TOC was lower than that, transcritical cycle's power output was slightly less than that of subcritical cycles.

Power enhancement potential of a mixture transcritical cycle for a low-temperature geothermal power generation

The power enhancement potential of a mixture transcritical cycle was investigated by comparison between the power of an R134a subcritical cycle and that of binary HFC (hydrofluorocarbon) mixture transcritical cycles for a low-temperature geothermal heat source of about 100°C. To fairly compare the power of the cycles by using different working fluids, each cycle was optimized from the view point of the power by three (or four for mixture cycles) design parameters. The first two are the turbine inlet temperature and pressure and the third is the ratio of the length of the vapor generator to the combined length of the vapor generator and the condenser. For the mixture cycles, the mixture composition was also optimized. In contrast to previous studies, to achieve a more realistic comparison, the characteristics of the working fluid's heat transfer and pressure drop were considered. Results show that the optimized R125-R245fa mixture transcritical cycle yields 11% more power than does the optimized R134a subcritical cycle under the simulation conditions considered in the present study.

Exergoeconomic optimization of an Organic Rankine Cycle for low-temperature geothermal heat sources

Exergoeconomic optimization of an Organic Rankine Cycle for low-temperature geothermal heat sources

Exergoeconomic optimization of an Organic Rankine Cycle for low-temperature geothermal heat sources

An exergoeconomic analysis of a geothermal power generation for a low-temperature resource is performed. An Organic Rankine Cycle (ORC) with isobutane and isopentane as working fluids is considered as binary power plant. A systematic parameter variation is done for the minimum temperature difference in the evaporator and condenser. The most suitable design parameters are evaluated under exergetic, economic and exergoeconomic criteria. The specific costs of electricity generation are minimal for the use of isobutane as working fluid and minimal temperature difference of 3 K at evaporation and 7 K at condensation. The most suitable concept for isopentane leads to only 0.4 % higher specific costs although the second law efficiency is 4.75 % lower. The exergoeconomic analysis permits to consider important criteria, like design and operating parameters in the fluid selection for ORC applications.

Environmental and thermal benefits of linking Brayton, absorption chiller, and organic Rankine cycles with low-enthalpy geothermal source at an arid-zone area of Libya

A combined system which consists of Brayton, LiBr/H2O absorption chiller, and HFC 245fa organic Rankine cycles has been developed and linked to a high potential low-temperature geothermal source of energy. Situated in an arid-zone area at Waddan City Libya, the geothermal source could provide sufficient energy for the City and surrounding villages and satisfy their full electrical demand of 100 MW in an uninterrupted and stable way. The system could also supply district heating and hot water. The simulated results show that the output power and thermal efficiency of the combined cycles were improved by 5 per cent and 1.5 per cent, respectively, compared to a standalone gas turbine and it also reduced carbon emissions by 55.7 per cent (291 g/kWh instead of 649 g/kWh). Incorporating district heating to the electrical energy raised the energy utilization factor to 55.1 per cent.

Hybrid Fuel Cell and Geothermal Resources for Air-Conditioning Using an Absorption Chiller in Algeria

This Hybrid Energy System (HES) is mainly based on proton exchange membrane fuel cell (PEMFC) technology, which is supplied by a fuel reforming process for hydrogen production, starting from natural gas. The aim of this paper is to analyse the feasibility study of a process by combining a low-temperature geothermal source with a PEMFC subsystem (PEMFCS). This analysis allows the development of an onsite independent Hybrid Energy System used for air-conditioning, and especially the use of hot water for space cooling. PEMFC technology with an LiBr/H2O absorption system has been introduced to provide fresh air by a cooling process in a school canteen for 240 students, located in the western region of Algeria. The feasibility study shows that combining geothermal sources and PEMFC technology for air-conditioning production constitutes a promising solution.

An Evaluation of R134a and R245fa as the Working Fluid in an Organic Rankine Cycle Energized from a Low Temperature Geothermal Energy Source

The characteristics of an organic Rankin cycle designed to operate with a low temperature geothermal source and constant temperature cooling water supplied from freshwater ponds typical to those found near Waddan City in the Al Jufrah region of Libya were examined. Two working fluids were examined and it was concluded that the most suitable for this application was R-245fa. The off design performance of the organic Rankine cycle was examined and it was shown that the cycle is controlled by the performance of the condenser which is cooling water side temperature limited.

Comparative analysis of natural and conventional working fluids for use in transcritical Rankine cycle using low-temperature geothermal source

Theoretical analyses of natural and conventional working fluids-based transcritical Rankine power cycles driven by low-temperature geothermal sources have been carried out with the methodology of pinch point analysis using computer models. The regenerator has been introduced and analyzed with a modified methodology considering the considerable variation of specific heat with temperature near the critical state. The evaluations of transcritical Rankine cycles have been performed based on equal thermodynamic mean heat rejection temperature and optimized gas heater pressures at various geothermal source temperature levels ranging from 80 to 120°C. The performances of CO2, a natural working fluid most commonly used in a transcritical power cycle, have been indicated as baselines. The results obtained show: optimum thermodynamic mean heat injection temperatures of transcritical Rankine cycles are distributed in the range of 60 to 70% of given geothermal source temperature level; optimum gas heater pressures of working fluids considered are lower than baselines; thermal efficiencies and expansion ratios (Expr) are higher than baselines while net power output, volume flow rate at turbine inlet (V1) and heat transfer capacity curves are distributed at both sides of baselines. From thermodynamic and techno-economic point of view, R125 presents the best performances. It shows 10% higher net power output, 3% lower V1, 1.0 time higher Expr, and 22% reduction of total heat transfer areas compared with baselines given geothermal source temperature of 90°C. With the geothermal source temperature above 100°C, R32 and R143a also show better performances. R170 shows nearly the same performances with baselines except for the higher V1 value. It also shows that better temperature gliding match between fluids in the gas heater can lead to more net power output. Copyright © 2010 John Wiley & Sons, Ltd.

Fluids and parameters optimization for a novel cogeneration system driven by low-temperature geothermal sources

A novel cogeneration system was proposed and techno-economically investigated, consisting of a low-temperature geothermally-powered organic Rankine cycle (ORC) subsystem, an intermediate heat exchanger subsystem and a heat pump subsystem. The main purpose is to identify suitable working fluids (among 27 fluids with boiling point temperature ranging from −47.69 to 47.59 °C) and optimized cycle parameters for the ORC-based power generation subsystem. The screening criteria include net power output per unit mass flow rate of hot source ( P net), the ratio of total heat transfer area to net power output ( A/ W net), and electricity production cost (epc). Results show that there exists optimum evaporating temperatures maximizing the P net value and minimizing the A/ P net and epc values. The optimum temperatures vary with different screening criteria and fluids. Optimized fluids based on each screening criteria are not the same. E170, R600 and R141b show the lowest A/ W net and epc values with averagely 3.78% lower P net value than R236ea which presents the largest P net value.

Selection of working fluids for a novel low-temperature geothermally-powered ORC based cogeneration system

A novel cogeneration system driven by low-temperature geothermal sources was investigated in this study. This system consists of a low-temperature geothermally-powered organic Rankine cycle (ORC) subsystem, an intermediate heat exchanger and a commercial R134a-based heat pump subsystem. The main purpose is to identify appropriate fluids which may yield high PPR (the ratio of power produced by the power generation subsystem to power consumed by the heat pump subsystem) value and QQR (the ratio of heat supplied to the user to heat produced by the geothermal source) value. Performances of the novel cogeneration system under disturbance conditions have also been studied. Results indicate that fluids group presenting higher normal boiling point values shows averagely 7.7% higher PPR values and R236ea and R245ca outstand among the group. Δ T P (pinch temperature difference in heat exchangers) and η t (turbine efficiency) values play more important roles on the variation of PPR values. QQR values change slightly with various Δ T P , η t and η rp (refrigerant pump efficiency) values while the variation range is larger under various geothermal source and heating supply parameters. Smaller Δ T P value, higher η t value, higher geothermal source parameters and lower heating supply parameters lead to higher PPR values but lower QQR values.

Working fluids of a low-temperature geothermally-powered Rankine cycle for combined power and heat generation system

A novel combined power and heat generation system was investigated in this study. This system consists of a low-temperature geothermally-powered organic Rankine cycle (ORC) subsystem, an intermediate heat exchanger and a commercial R134a-based heat pump subsystem. The advantages of the novel combined power and heat generation system are free of using additional cooling water circling system for the power generation subsystem as well as maximizing the use of thermal energy in the low-temperature geothermal source. The main purpose is to identify suitable working fluids (wet, isentropic and dry fluids) which may yield high PPR (the ratio of power produced by the power generation subsystem to power consumed by the heat pump subsystem) value and QQR (the ratio of heat supplied to the user to heat produced by the geothermal source) value. Parameters under investigation were evaporating temperature, PPR value and QQR value. Results indicate that there exits an optimum evaporating temperature to maximize the PPR value and minimize the QQR value at the same time for individual fluid. And dry fluids show higher PPR values but lower QQR values. NH3 and R152a outstand among wet fluids. R134a outstands among isentropic fluids. R236ea, R245ca, R245fa, R600 and R600a outstand among dry fluids. R236ea shows the highest PPR value among the recommended fluids.

The Influence of Coolant Temperature on a Single-Effect Lithium Bromide—Water Absorption Chiller when Energized from a Low-Temperature Source

A simulation study of several schematic models of both water- and air-cooled single-effect LiBr—H2O absorption chillers energized by a high-potential, low-temperature (73 °C) geothermal source has been carried out using IPSEpro refrigeration software package. All the simulated models were validated against a lithium bromide—water pressure—temperature DÜhring chart. The results obtained show that when the coefficient of performance, heat input into the generator, and cooling mass flowrates are fixed, the cycle parameters are highly affected by variation of the coolant temperature. This information will be useful to designers of absorption refrigeration systems.

Comparative analysis of CO2-based transcritical Rankine cycle and HFC245fa-based subcritical organic Rankine cycle using low-temperature geothermal source

A detailed thermodynamic and techno-economic comparison is presented for a CO2-based transcritical Rankine cycle and a subcritical organic Rankine cycle (ORC) using HFC245fa (1,1,1,3,3-pentafluoro-propane) as the working fluid driven by the low-temperature geothermal source, in order to determine the configuration that presents the maximum net power output with a minimum investment. The evaluations of both Rankine cycles have been performed based on equal thermodynamic mean heat rejection temperature by varying certain system operating parameters to achieve each Rankine cycle’s optimum design at various geothermal source temperature levels ranging from 80°C to 120°C. The results obtained show that the optimum thermodynamic mean heat injection temperatures of both Rankine cycles are distributed in the scope of 55% to 65% of a given geothermal source temperature level, and that the CO2-based transcritical Rankine cycle presents 3% to 7% higher net power output, 84% reduction of turbine inlet volume flow rate, 47% reduction of expansion ratio and 1.68 times higher total heat transfer capacity compared with the HFC245fa-based subcritical ORC. It is also indicated that using the CO2-based transcritical system can reduce the dimension of turbine design. However, it requires larger heat transfer areas with higher strength heat exchanger materials because of the higher system pressure.

Hydrogeochemical and isotopic composition of a low-temperature geothermal source in northwest Turkey: case study of Kirkgecit geothermal area

Chemical and isotopic compositions of three hot springs and one cold spring in the Kirkgecit geothermal field, located 15 km southwest of Canakkale-Biga in the northwest of Turkey, were monitored five times during 2005 and 2007. The physico-chemical characteristics of the hot springs are average discharge 3–3.5 L/s, surface temperature 45–52°C, pH 8.9–9.3, and electrical conductivity (EC) 620–698 μS/cm. The cold spring has a temperature of 12–13°C, pH 7.5–8.3, and EC 653–675 μS/cm. The hot waters are Na-SO4 type, whereas the cold water is Ca-HCO3 type. Chemical geothermometers suggest that the reservoir temperature is around 80–100°C. The isotopic data (oxygen-18, deuterium and tritium) indicate that the thermal waters are formed by local recharge and deep circulation of meteoric waters.

Economy of exploiting heat from low-temperature geothermal sources using a heat pump

The article describes the economy of exploiting heat from low-temperature geothermal sources for high-temperature heating of buildings using a heat pump. For the exploitation of low-temperature geothermal sources, a two stage heat pump with a heat exchanger was planned. The pump consists of two single stage heat pumps which use different refrigerants at each stage. At stage 1, the calculation of the heat pump is conducted with refrigerant R407c; at stage 2 of the heat pump, the refrigerant R600a is used. The main operational characteristics of a two stage heat pump are presented in the form of diagrams. For the exploitation of heat from geothermal water with a temperature of 45 °C, a profitability evaluation of the investment in the heat pump was carried out, using the method of the net present value. In the research, also the coefficient of profitability and the period of time in which the investment is going to return itself were established.

The Performance of the Kalina Cycle System 11(KCS-11) With Low-Temperature Heat Sources

The possibility of exploiting low-temperature heat sources has been of great significance with ever increasing energy demand. Optimum and cost-effective design of the power cycles provide a means of utilization of low-temperature heat sources which might otherwise be discarded. In this analysis, the performance of the Kalina cycle system 11 (KCS11) is examined for low-temperature geothermal heat sources and is compared with an organic Rankine cycle. The effect of the ammonia fraction and turbine inlet pressure on the cycle performance is investigated in detail. Results show that for a given turbine inlet pressure, an optimum ammonia fraction can be found that yields the maximum cycle efficiency. Further, the maximum cycle efficiency does not necessarily yield the optimum operating conditions for the system. In addition, it is important to consider the utilization of the various circulating media (i.e., working fluid, cooling water, and heat resource) and heat exchanger area per unit power produced. For given conditions, an optimum range of operating pressure and ammonia fraction can be identified that result in optimum cycle performance. In general, the KCS11 has better overall performance at moderate pressures than that of the organic Rankine cycle.

Optimum design criteria for an Organic Rankine cycle using low-temperature geothermal heat sources

A cost-effective optimum design criterion for Organic Rankine power cycles utilizing low-temperature geothermal heat sources is presented. The ratio of the total heat exchanger area to net power output is used as the objective function and was optimized using the steepest descent method. Evaporation and condensation temperatures, geothermal and cooling water velocities are varied in the optimization method. The optimum cycle performance is evaluated and compared for working fluids that include ammonia, HCFC123, n-Pentane and PF5050. The optimization method converges to a unique solution for specific values of evaporation and condensation temperatures and geothermal and cooling water velocities. The choice of working fluid can be greatly affect the objective function which is a measure of power plant cost and in some instances the difference could be more than twice. Ammonia has minimum objective function and maximum geothermal water utilization, but not necessarily maximum cycle efficiency. Exergy analysis shows that efficiency of the ammonia cycle has been largely compromised in the optimization process than that of other working fluids. The fluids, HCFC 123 and n-Pentane, have better performance than PF 5050, although the latter has most preferable physical and chemical characteristics compared to other fluids considered.