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 (Exp_r) 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 Exp_r, 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.     

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.     

低温地热发电有机朗肯循环的优化

采用(火用)分析方法及PR状态方程,建立了低温地热发电有机朗肯循环的工质优选及主要参数优化热力学方法.比较计算了以10种干流体有机工质为循环工质的低温地热发电有机朗肯循环的输出功率、(火用)效率及其余主要热力性能.结果表明,低温地热发电有机朗肯循环的性能极大地受工质的物性及蒸发温度的影响.总体来看,随着工质临界温度的升...     

Second law analysis of a fossil-geothermal hybrid power plant with thermodynamic optimization of geothermal preheater

There is an urgent and compelling need to develop innovative and more effective ways to integrate sustainable renewable energy solutions into the already existing systems or, better yet, create new systems that all together make use of renewable energy. This study aims to establish the optimum working conditions of a geothermal preheater in a power plant that makes use of both renewable and nonrenewable energy resources, where renewable (geothermal) energy is used to boost the power output in an environmentally sustainable way. Hence, two models, one, a simplified model of a Rankine cycle with single reheat and regeneration, and another, with a geothermal preheater substituting the low-pressure feedwater heater (LPFWH), were compared. The Engineering Equations Solver software was used to perform an analysis of the thermodynamic performance of the two models designed. An analysis was done to evaluate the energetic and exergetic effects of replacing a LPFWH with a geothermal preheater sourcing heat from a low temperature geothermal resource (100°C-160°C). Results from the thermodynamic analysis reveal that the hybridization boosts the power output by approximately 4% and it is superior in terms of the second law. Entropy generation minimization analysis was then employed to establish optimal working conditions of the hybrid system (ie, the geothermal preheater modeled as a downhole coaxial heat exchanger).     

Thermodynamic analysis and performance optimization of organic rankine cycles for the conversion of low‐to‐moderate grade geothermal heat

The present study considers a thermodynamic analysis and performance optimization of geothermal power cycles. The proposed binary‐cycles operate with moderately low temperature and liquid‐dominated geothermal resources in the range of 110°C to 160°C, and cooling air at ambient conditions of 25°C and 101.3 kPa reference temperature and atmospheric pressure, respectively. A thermodynamic optimization process and an irreversibility analysis were performed to maximize the power output while minimizing the overall exergy destruction and improving the First‐law and Second‐law efficiencies of the cycle. Maximum net power output was observed to increase exponentially with the geothermal resource temperature to yield 16–49 kW per unit mass flow rate of the geothermal fluid for the non‐regenerative organic Rankine cycles (ORCs), as compared with 8–34 kW for the regenerative cycles. The cycle First‐law efficiency was determined in the range of 8–15% for the investigated geothermal binary power cycles. Maximum Second‐law efficiency of approximately 56% was achieved by the ORC with an internal heat exchanger. In addition, a performance analysis of selected pure organic fluids such as R123, R152a, isobutane and n‐pentane, with boiling points in the range of ?24°C to 36°C, was conducted under saturation temperature and subcritical pressure operating conditions of the turbine. Organic fluids with higher boiling point temperature, such as n‐pentane, were recommended for non‐regenerative cycles. The regenerative ORCs, however, require organic fluids with lower vapour specific heat capacity (i.e. isobutane) for an optimal operation of the binary‐cycle. Copyright © 2015 John Wiley & Sons, Ltd.     

不同有机工质下的燃气分布式能源光热补偿供能系统研究

为了提高能源利用率,实现节能减排,以总能系统概念为基础,采用燃气和太阳能为能源,水蒸气与有机工质为工作介质,建立燃气分布式供能系统的热力模型和能效模型,对其热经济性和有机工质进行了计算与筛选。以国内某机场分布式能源站机组为例,基于太阳能的能源补充作用以及有机工质气化潜热小的特性,建立燃气分布式能源光热补偿供能系统,经热经济性计算与分析,结果表明:在100%、75%、50%制冷(制热)工况时、新模型在维持原机组总热耗的情况下,机组冷源损失减少,热经济性提高;经过对3种有机工质的筛选,R245FA是较适合于新模型的有机工质。     

Challenges and opportunities of Rankine cycle for waste heat recovery from internal combustion engine

Internal combustion engines (ICEs) are the major consumers of crude oil, and thus improvements in the fuel consumption performance of ICEs would be significant for global energy conservation and emission reduction. Owing to the constraints in the engine structure and combustion efficiency, more than half of the fuel combustion heat in ICEs is wasted. Therefore, ICE waste heat recovery (ICE-WHR) shows huge potential. Rankine cycle (organic or inorganic) provides a promising solution for ICE-WHR, which could balance efficiency and practicality. In this review, recent advances in Rankine cycles for ICE-WHR are summarized and discussed. To evaluate results from various existing studies, a uniform evaluation standard, thermodynamic perfection, was proposed based on the benchmark of the heat source based ideal thermodynamic cycle(H-iCycle), which is determined by achieving ideal thermal matching to external boundary conditions. Based on this, the effects of three major factors (cycle configuration, working fluid, and key components) on the performance of Rankine cycle can be investigated. In addition, a discussion of several application concerns, including backpressure, weight, power output type, off-design performance dynamic response, and control, enables us to gain a comprehensive understanding and assessment of Rankine cycles in ICE-WHR. With respect to working fluids, C_xH_yO_z and siloxanes with high critical temperature (such as cyclohexane, benzene, toluene, and MM) have a satisfactory thermal matching with waste heat sources. Basic Rankine cycles using these working fluids could yield a high thermodynamic perfection of up to 54.1%. With respect to the cycle configuration, cascade Rankine cycles and dual-pressure Rankine cycles are expected to achieve the highest thermodynamic perfection of 62.3%. Finally, major challenges and perspectives for the future development of Rankine cycles in ICE-WHR are discussed. Four promising research directions suggested in this review include the active design of desirable working fluids, advanced cycle configuration design based on “energy utilization according to quality,” integrated scheme research at three levels (component, system, and energy management), and advanced coordinative control.     

Analysis of power and cooling cogeneration using ammonia-water mixture

Development of innovative thermodynamic cycles is important for the efficient utilization of low-temperature heat sources such as solar, geothermal and waste heat sources. This paper presents a parametric analysis of a combined power/cooling cycle, which combines the Rankine and absorption refrigeration cycles, uses ammonia-water mixture as the working fluid and produces power and cooling simultaneously. This cycle, also known as the Goswami Cycle, can be used as a bottoming cycle using waste heat from a conventional power cycle or as an independent cycle using solar or geothermal energy. A thermodynamic study of power and cooling cogeneration is presented. The performance of the cycle for a range of boiler pressures, ammonia concentrations and isentropic turbine efficiencies are studied to find out the sensitivities of net work, amount of cooling and effective efficiencies. The roles of rectifier and superheater on the cycle performance are investigated. The cycle heat source temperature is varied between 90-170 °C and the maximum effective first law and exergy efficiencies for an absorber temperature of 30 °C are calculated as 20% and 72%, respectively. The turbine exit quality of the cycle for different boiler exit scenarios shows that turbine exit quality decreases when the absorber temperature decreases.     

A two-phase flow model for geothermal wells in the presence of non-condensable gas

Quantitative information on the phenomena occuring during the upward flow of a geothermal fluid in water-dominated wells is a requisite for designing the wellhead system and optimizing resource exploitation. The geothermal fluid consists, for the most part, of a two-phase mixture of water containing dissolved salts, steam and non-condensable gases. Various, closely interrelated effects must therefore be taken into consideration: pressure drop of the rising fluid; heat and mass transfer between the phases (due to evaporation and desorption); heat exchange with rock formations. Simultaneous application of the mass, energy and momentum equations results in a rather complex model that can be solved by a numerical computer program. The model described here accounts for the effects of: the presence of salts, when computing all the thermodynamic properties of the fluids, especially enthalpy, density, vapour pressure of the brine and superheated steam enthalpy; the presence of non-condensable gases, considering their deviations from ideal behaviour and their contribution to density; the heat exchange with the surrounding rock formations; variation in salt concentration along the flow-path; possible variation in pipe diameter and surface roughness with height. The simplified hypotheses adopted are: fluid flow is stationary; thermodynamic equilibrium conditions exist between the phases in each point along the well; the non-condensable gases are assumed to be CO₂; Henry's law is assumed valid and the quantity dissolved chemically is assumed negligible; the salts are assumed to be NaCl; the activity coefficients are unitary; liquid surface tension and viscosity values are assumed equal to those of pure water. Comparison of the results of the computer program and the experimental pressure and temperature profiles shows that these are in satisfactory agreement within a rather wide range of operative conditions. The noncondensable gases, even in very low concentrations, were shown to be of importance to these calculations. Once the experimental temperature and pressure profiles are known, the model will also permit calculation of the concentration of non-condensable gases. The most efficient of the two correlations used to compute pressure drop in two-phase regimes seems to be that devised by CISE^‡, which is based on global parameters not correlated to the different flow regimes.     

Heat transfer problems for the production of hydrogen from geothermal energy

Electrolysis at low temperature is currently used to produce Hydrogen. From a thermodynamic point of view, it is possible to improve the performance of electrolysis while functioning at high temperature (high temperature electrolysis: HTE). That makes it possible to reduce energy consumption but requires a part of the energy necessary for the dissociation of water to be in the form of thermal energy.

A collaboration between France and Iceland aims at studying and then validating the possibilities of producing hydrogen with HTE coupled with a geothermal source. The influence of the exit temperature on the cost of energy consumption of the drilling well is detailed.

To vaporize the water to the electrolyser, it should be possible to use the same technology currently used in the Icelandic geothermal context for producing electricity by using a steam turbine cycle. For heating the steam up to the temperature needed at the entrance of the electrolyser three kinds of heat exchangers could be used, according to specific temperature intervals.     

Energy performance and numerical optimization of a screw expander–based solar thermal electricity system in a wide range of fluctuating operating conditions

This paper provides fundamental principles to study the thermodynamic performance of a new screw expander–based solar thermal electricity plant. While steam turbines are generally used in direct steam generation solar systems without admitting fluid in two-phase conditions, steam screw expanders, as volumetric machines, can convert thermal to mechanical energy also by expanding liquid-steam mixtures without a decline in efficiency. In effect, steam turbines are not as competitive as screw expanders when the net power is smaller than 2 MW and for low-grade heat sources. The solar electricity generation system proposed in this paper is based on the steam Rankine cycle: Water is used as both working fluid and storage, parabolic trough collectors are used as a thermal source, and screw expanders are used as power machines. Since screw expanders can operate at off-design working conditions in several situations when installed in direct steam generation solar plants, studying expander performance under fluctuating working situations is a crucial issue. The main aim of the present paper is to establish a thermodynamic model to study the energetic benefits of the proposed power system when off-design operating conditions and variable solar radiation occur. This entails, first and foremost, developing overexpansion and underexpansion numerical models to describe the polytropic expansion phase, which considers all the losses affecting performance of the screw expander under real operating conditions. To assess the best operating conditions and maximum efficiency of the whole power system at part-load working conditions under fluctuating solar radiations, parametric optimization is then improved in a wide range of variable working conditions, assuming condensation pressures of water increasing from 0.1 to 1 bar, under an evaporation temperature rising from 170°C to 300°C.     

Thermodynamic and economic assessments and multi-criteria optimization of a novel poly-generation plant using geothermal energy and multi heat recovery technique

Smart use of clean energy sources for achieving higher performance and designing cost-effective systems is recognized as an essential solution for reducing fossil fuel consumption. In this regard, this study supports a comprehensive evaluation and multi-criteria optimization of a novel poly-generation plant embracing geothermal energy from thermodynamic and thermoeconomic perspectives. Hence, the utilization of modified subsystems and smart use of multi heat recovery processes are projected and appraised. In this regard, the plant consists of a double-flash binary geothermal subsystem, an organic Rankine cycle in combination with an ejector refrigeration cycle considering a zeotropic working fluid (a mixture of pentane and R142b), a heating production heat exchanger, and a proton exchange membrane electrolyzer with the combined production of cooling, heating, power, and hydrogen. The crucial thermodynamic and thermoeconomic variables are investigated against key parameters and concluded that the sensitivity of outcomes is more evident with the variation in zeotropic working fluid composition and the vapor quality at the heating production heat exchanger's outlet. The attained results at the optimum mode demonstrated, the energy and exergy efficiencies of the plant as well as total unit costs of products are as being 44.5%, 35.8%, and 18.8 $/GJ, respectively.     

The Iceland Deep Drilling Project: a search for deep unconventional geothermal resources

The Iceland Deep Drilling Project (IDDP) is a long-term program to improve the economics of geothermal energy by producing supercritical hydrous fluids from drillable depths. Producing supercritical fluids will require the drilling of wells and the sampling of fluids and rocks to depths of 3.5–5 km, and at temperatures of 450–600 °C. The IDDP plans to drill and test a series of such deep boreholes in the Krafla, Nesjavellir and Reykjanes geothermal fields in Iceland. Beneath these three developed high-temperature systems frequent seismic activity continues below 5 km, indicating that, even at supercritical temperatures, the rocks are brittle and therefore likely to be permeable, even where the temperature is assumed to exceed 550–650 °C. Temperature gradients are greater and fluid salinities smaller at Nesjavellir and Krafla than at Reykjanes. However, an active drilling program is underway at Reykjanes to expand the existing generating capacity and the field operator has offered to make available one of a number of 2.5 km deep wells to be the first to be deepened to 5 km by the IDDP. In addition to its potential economic significance, drilling deep at this location, on the landward extension of the Mid-Atlantic Ridge, is of great interest to the international science community. This paper examines the prospect of producing geothermal fluids from deep wells drilled into a reservoir at supercritical temperatures and pressures. Since fluids drawn from a depth of 4000–5000 m may prove to be chemically hostile, the wellbore and casing must be protected while the fluid properties are being evaluated. This will be achieved by extracting the fluids through a narrow retrievable liner called the “pipe”. Modelling indicates that if the wellhead enthalpy is to exceed that of conventionally produced geothermal steam, the reservoir temperature must be higher than 450 °C. A deep well producing 0.67 m³/s steam (2400 m³/h) from a reservoir with a temperature significantly above 450 °C could, under favourable conditions, yield enough high-enthalpy steam to generate 40–50 MW of electric power. This exceeds by an order of magnitude the power typically obtained from a conventional geothermal well in Iceland. The aim of the IDDP is to determine whether utilization of heat from such an unconventional geothermal resource at supercritical conditions will lead to increased productivity of wells at a competitive cost. If the IDDP is an economic success, this same approach could be applied in other high-temperature volcanic geothermal systems elsewhere, an important step in enhancing the geothermal industry worldwide.     

An investigation of the technical and economical feasibility of using low temperature geothermal sources in Colorado

A preliminary feasibility study of utilizing low temperature geothermal sources in Colorado to heat buildings has been completed. It is concluded that the technology for using geothermal sources to heat building exists and that the cost of heat will be between $2 and 5 per million Btu delivered. Although geothermal heating is more expensive than heating with natural gas at current prices, it is considerably less expensive than heating by means of current solar thermal conversion methods. It is, therefore, recommended that an exploratory well be drilled in a place such as Glenwood Springs, Colorado, to define the thermal, chemical, and geological characteristics of a geothermal source in detail as a first step toward utilization of geothermal energy on a commercial scale in Colorado.     

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%.     

Parametric sensitivity study of operating and design variables in wellbore heat exchangers

A numerical study was conducted to evaluate the potential for using Wellbore Heat Exchangers (WBHX) to extract heat for use in electricity generation. Variables studied included operational parameters such as wellbore geometries, working fluid properties, circulation rates, and regional properties including basal heat flux and formation rock type. Energy extraction is strongly affected by fluid residence time, heat transfer contact area, and formation thermal properties. Water appears to be the most appropriate working fluid. The effects of tubing properties and casing lengths are of second-order.On the basis of a sensitivity study, a Best Case model was simulated, and results compared against the geothermal fluid requirements of existing power generation plants that use low-temperature geothermal fluids. Even assuming ideal work conversion to electricity, a WBHX cannot supply sufficient energy to generate 200 kWe at the onset of pseudo-steady-state (PSS) conditions. Using realistic conversion efficiencies it is unlikely that the system would be able to generate 50 kWe at the onset of PSS.     

Temperature matching method of selecting working fluids for geothermal heat pumps

The fact, that some areas need district heating through heat pumps and at the same time recover residual heat from geothermal water, presents a new working condition: feed water temperature of heat network 80 °C, return water temperature 65 °C; discarded geothermal water temperature 40 °C and its emission standard temperature below 30 °C. But none of known pure refrigerants and mixtures can meet this requirement. The paper introduces a novel approach named temperature-matching method, which provides a direction in selecting high-performance working fluids for further research. It is shown from the results that the mean COPs of binary and ternary mixtures are 4.85 and 4.74 respectively, but that of pure refrigerants is 4.12 under the same ambient condition. This point indicates that temperature matching contributes to energy saving. The novel approach to high-performance working fluids can be conveniently introduced into other working conditions.     

The feasibility of using geothermal energy in hydrogen production

A feasibility study exploring the use of geothermal energy in hydrogen production is presented. It is possible to use a thermal energy to supply heat for high temperature electrolysis and thereby substitute a part of the relatively expensive electricity needed. A newly developed HOT ELLY high temperature steam electrolysis process operates at 800 – 1000°C. Geothermal fluid is used to heat fresh water up to 200°C steam. The steam is further heated to 900°C by utilising heat produced within the electrolyser. The electrical power of this process is reduced from 4.6 kWh per normalised cubic meter of hydrogen (kWh/Nm³ H₂) for conventional process to 3.2 kWh/Nm³ H₂ for the HOT ELLY process implying electrical energy reduction of 29.5%. The geothermal energy needed in the process is 0.5 kWh/Nm³ H₂. Price of geothermal energy is approximately 8–10% of electrical energy and therefore a substantial reduction of production cost of hydrogen can be achieved this way. It will be shown that using HOT ELLY process with geothermal steam at 200°C reduces the production cost by approximately 19%.     

Analysis and assessment of a geothermal based cogeneration system and lithium extraction

This paper presents the thermodynamic analyses for a double flash-binary based integrated geothermal power plant which consists of two steam turbines and one expander in the organic Rankine cycle that uses ammonia as the working fluid and a lithium extraction sub system. The main useful outputs of the plant are electricity, heat for floor heating and lithium carbonate (Li₂CO₃). The aim of this study is to assess the overall system performance energetically and exergetically. Based on the results obtained from this study, the overall energy and exergy efficiencies are 58.41% and 66.63%, respectively. The present results also show that the Li₂CO₃ is produced at the rate of 9.52 × 10⁻³ kg/s. In addition, the effects of changing several important operating parameters and ambient conditions on the energy and exergy efficiencies and the performance of the subsystems are investigated.     

A review of thermodynamic cycles and working fluids for the conversion of low-grade heat

This paper presents a review of the organic Rankine cycle and supercritical Rankine cycle for the conversion of low-grade heat into electrical power, as well as selection criteria of potential working fluids, screening of 35 working fluids for the two cycles and analyses of the influence of fluid properties on cycle performance. The thermodynamic and physical properties, stability, environmental impacts, safety and compatibility, and availability and cost are among the important considerations when selecting a working fluid. The paper discusses the types of working fluids, influence of latent heat, density and specific heat, and the effectiveness of superheating. A discussion of the 35 screened working fluids is also presented.