Subcritical Rankine Cycle Research Articles

Dynamic Simulation of a Molten Salt-Steam Superheater for Load-Following Applications

Abstract The growing use of intermittent renewables in electrical grids increasingly motivates load-following operations as a crucial capability of dispatchable power plants. However, frequent load variations in steam generation equipment can cause premature heat exchanger failure. This paper simulates the dynamic behavior of a high pressure, U-tube/U-shell, salt-to-steam superheater typically found in tower-type concentrating solar power subcritical Rankine cycles. Results focus on responses during load-following and inlet temperature changes. The proposed model is a finite volume method, and thermodynamic and heat transfer properties of both fluids are allowed to vary spatially and temporally. Several flow ramping schemes are investigated, including proportionally equal ramps and proportionally dissimilar ramping, where one fluid reaches its mass flow setpoint faster than the other. Results indicate that salt outlet temperature overshoot can occur if ramp rates are of sufficiently high magnitude, and that U-bend metal temperature rate of change can be approximately 2.5× that observed at either outlet. If proportionally-matched ramping is not possible, ramping steam more slowly than the salt is preferred over the alternative, as cold side and U-bend temperature responses are better regulated. Additionally, mass flow rate and inlet temperature changes are shown to elicit unique responses in the tube bundle metal.

Techno-Economic Analysis of Using Reversible Turbomachinery for Pumped Thermal Energy Storage Systems

Abstract The objective of this paper is to assess the techno-economic performance of different cycle configurations for pumped thermal energy storage (PTES), including the effects of charging electricity costs. Reversible turbomachinery was employed to reduce the capital cost of the system. Brayton cycles with different working fluids and a subcritical Rankine cycle operating with ammonia were compared. Both liquid and packed bed thermal storages were investigated. A new cost correlation for turbomachines, initially established for the turbines of organic Rankine cycles, was developed for compressors and reversible machines. This correlation is based on the number of stages and physical size of the machine, which were estimated considering thermodynamic as well as mechanical limitations. The results indicate that for a plant size of 50 MW and a discharge duration of 8 h, the Brayton system with liquid storage and helium as a working fluid has the lowest levelized cost of storage at 0.138 $/kWh, mainly due to the high thermal conductivity of the fluid. Packed bed thermal energy storage systems were found to be more expensive than liquid storage systems due to the large cost of the pressure vessels, with cost parity reached at a discharge duration of 4 h. However, at this duration, lithium-ion batteries are likely to be cheaper. The results suggest that the levelized cost of storage for the Rankine cycle-based system is slightly higher at 0.151 $/kWh.

Process design methodology for rankine cycle based on heat matching

The Rankine cycle serves as a crucial technical tool for waste-heat recovery. Matching heat sources and working fluids presents a challenging problem among the extensive research on the process design and selection of working fluids for the Rankine cycle. This study introduces a process design methodology for the Rankine cycle based on heat matching. It can optimize the performance between heat sources and working fluids without the need to preset the configuration for the cycle. The objective is to ultimately derive the most efficient configuration and operating conditions for the Rankine cycle. This method relies on establishing a matching relationship between temperature and heat diagram curves of both heat sources and working fluids. The relationship can be described by a simple linear programming problem, serving as the fundamental model for the proposed method. Demonstration of the feasibility and accuracy of this method involved computing the performance of a subcritical Rankine cycle for waste heat recovery in an ammonia-diesel dual-fuel engine. Three different working fluids were considered: ammonia, R123, and R245fa. Ammonia exhibited superior performance with an overall system efficiency improvement of 5.18 %, surpassing R123 (5.01 %) and R245fa (5.00 %). The competitiveness of the ammonia Rankine cycle has been established, resulting in a nearly zero-carbon-emission power system potentially achieving an overall efficiency of 53.23 %. The process design method based on heat matching notably reduces the modeling efforts required for the Rankine cycle. This reduction provides substantial support for engineering design and facilitates wider applications of the Rankine cycle.

Techno-economic assessment of Carnot batteries for load-shifting of solar PV production of an office building

This study presents a techno-economic assessment of Carnot batteries for load-shifting of solar PV production of an office building considering variable electricity production, demand and pricing. The building has an annual electricity demand of 2600 MWh and a maximum power demand of 0.564 MW. The Carnot battery studied is based on a subcritical Rankine cycle with sensible thermal storage. A generic Carnot battery model and sizing methodology are presented and applied to the case study. It is found that implementing a Carnot battery is not cost effective for the studied case. Larger system sizes increase the integration of solar PV production, but decrease the net present value. Increasing the thermodynamic efficiency of the Carnot battery is beneficial to improve the financial feasibility and to reach higher levels of solar PV integration. The system investment cost should reduce minimally with a factor 9.4 and 4.1 to make the base and ideal system viable respectively. Alternatively, a minimum electricity tax or 326 €/MWh and 122 €/MWh could be implemented. The Carnot battery is not financially competitive with Li-ion batteries under the current scenario. The ideal Carnot battery becomes competitive at charging durations starting from 7 h.

Energy and Exergy-Based Screening of Various Refrigerants, Hydrocarbons and Siloxanes for the Optimization of Biomass Boiler–Organic Rankine Cycle (BB–ORC) Heat and Power Cogeneration Plants

The cogeneration of power and heat was investigated for Biomass Boiler–Organic Rankine Cycle (BB–ORC) plants with the characteristics of typical units, such as the 1 MWel Turboden ORC 10 CHP. The thermodynamic analysis of the ORC unit was undertaken considering forty-two (42) dry and isentropic candidate pure working fluids. Only subcritical Rankine cycles were considered, and the pinch point temperature differences for the evaporation and condensation heat exchangers were kept constant at 10 °C in all cases. The study provides an original and unique screening of almost all pure working fluids that are considered appropriate in the literature under the same operation and optimization conditions and compiles them into a single reference. In its conclusions, the study provides useful fluid selection and design guidelines, which may be easily followed depending on the optimization objective of the ORC designer or operator. In general, hydrocarbons are found to lie in the optimum middle range of the fluid spectrum, between the siloxanes that maximize the production of mechanical power and the refrigerants that maximize the production of heat. Specific hydrocarbon fluids, such as cyclopentane, heptane, hexane, benzene, and toluene, are found as rational options for maximum mechanical efficiency when operating with practically feasible condensation pressures between 10 and 200 kPa. At condensation pressures below 10 kPa, ethylbenzene, o-xylene, m-xylene, p-xylene, and nonane are also found to be feasible options. Finally, cyclopentane, hexane, and MM (hexamethyldisiloxane) are selected as the most appropriate options for cogeneration plants aiming simultaneously at high mechanical power and maximum temperature water production.

Development and Analysis of an Integrated Mild/Partial Gasification Combined Cycle

Abstract Around 50% of the world’s electrical power supply comes from the Rankine cycle, and the majority of existing Rankine cycle plants are driven by coal. Given how unattractive coal is as an energy resource in spite of its high energy content, it becomes necessary to find a way to utilize coal in a cleaner and more efficient manner. Designed as a potential retrofit option for existing Rankine cycle plants, the integrated mild/partial gasification combined (IMPGC) cycle is an attractive concept in cycle design that can greatly increase the efficiency of coal-based power plants, particularly for retrofitting an old Rankine cycle plant. Compared with the integrated gasification combined cycle (IGCC), IMPGC uses mild gasification to purposefully leave most of the volatile matters within the feedstock intact (hence, yielding more chemical energy) compared with full gasification and uses partial gasification to leave some of the remaining char un-gasified compared with complete gasification. The larger hydrocarbons leftover from the mild gasification process grant the resulting syngas a higher volumetric heating value, leading to a more efficient overall cycle performance. This is made possible due to the invention of a warm-gas cleanup process invented by the Research Triangle Institute (RTI), called the high-temperature desulfurization process (HTDP), which was recently commercialized. The leftover char can then be burned in a conventional boiler to boost the steam output of the bottom cycle, further increasing the efficiency of the plant up to 47.9% (based on lower heating value, or LHV). This paper will first analyze the individual concepts used to create the baseline IMPGC model, including the mild and partial gasification processes themselves, the warm-gas cleanup system, and the integration of the boiler with the heat recovery steam generator (HRSG). This baseline will then be compared with four other common types of power plants, including subcritical and ultra-supercritical (USC) Rankine cycles, IGCC, and natural gas. The results show that IMPGC consistently outperforms all other forms of coal-based power. IMPGC is more efficient than the standard subcritical Rankine cycle by nine percentage points, more than a USC Rankine cycle by nearly four points, and more than IGCC by seven points.

Optimization of solar-coal hybridization for low solar augmentation

This work presents a process to determine the preliminary optimal configuration of a concentrating solar power-coal hybrid power plant with low solar augmentation, and is demonstrated on a regenerative steam Rankine cycle coal power plant in Castle Dale, UT, USA (average DNI of 542 W m−2). A representative plant model is developed and validated against published data for a coal power plant. The simplifications that lead to the representative model from a coal power plant model include combining multiple feedwater heaters, combining turbines, and using a mass-average calculation for extraction steam properties. Comparing net power generation and boiler heating estimates from the representative model to the benchmark power plant, the representative model predictions are accurate to within ±2.5% of the accepted value. Methods for quantifying solar resource based on geography and simulating a concentrating solar power field arrangement are provided and the solar contribution to electrical power output is estimated using an exergy balance. A financial model is also included to estimate the solar marginal levelized cost of electricity and payback time using a cash-flow analysis. A multi-objective optimization routine is then employed to determine the optimal configuration using the models described in this study.It is shown that a solar augmentation of >3% of boiler heating is required for a hybrid design to be considered thermodynamically feasible. However, as the augment fraction is increased, the financial benefit from fuel savings is insufficient, without a carbon tax, to offset the higher capital cost. Optimization results, constrained to a maximum solar field size of 20 ac, are also provided assuming a common carbon tax value (16 USD sh.tn.−1). The resulting optimal design for the Castle Dale Plant with a carbon tax and no premium indicates the use of parabolic trough collector technology at an augment fraction of k=9% to bypass feedwater heater 6. The resulting marginal solar levelized cost of electricity is 9.5 x 10-4 USD kWh−1 with an estimated payback time of 25.2 years. With a green energy premium price of 0.018 USD kWh−1 and no carbon tax, the payback time reduces to 3.9 years. This process can be applied to a subcritical Rankine cycle coal power plant for which operating data and meteorological data are available to evaluate preliminary hybridization feasibility.

Retracted: “DEVELOPMENT AND ANALYSIS OF AN INTEGRATED MILD/PARTIAL GASIFICATION COMBINED (IMPGC) CYCLE”

Abstract Around 50% of the world's electrical power supply comes from the Rankine cycle, and the majority of existing Rankine cycle plants are driven by coal. Given how unattractive coal is as an energy resource in spite of its high energy content, it becomes necessary to find a way to utilize coal in a cleaner and more efficient manner. Designed as a potential retrofit option for existing Rankine cycle plants, the Integrated Mild/Partial Gasification Combined (IMPGC) Cycle is an attractive concept in cycle design that can greatly increase the efficiency of coal-based power plants, particularly for retrofitting an old Rankine cycle plant. Compared to the Integrated Gasification Combined Cycle (IGCC), IMPGC uses mild gasification to purposefully leave most of the volatile matters within the feedstock intact (hence, yielding more chemical energy) compared to full gasification and uses partial gasification to leave some of the remaining char un-gasified compared to complete gasification. The larger hydrocarbons left over from the mild gasification process grant the resulting syngas a higher volumetric heating value, leading to a more efficient overall cycle performance. This is made possible due to the invention of a warm gas cleanup process invented by Research Triangle Institute (RTI), called the High Temperature Desulfurization Process (HTDP), which was recently commercialized. The leftover char can then be burned in a conventional boiler to boost the steam output of the bottom cycle, further increasing the efficiency of the plant, capable of achieving a thermal efficiency of 47.9% (LHV). This paper will first analyze the individual concepts used to create the baseline IMPGC model, including the mild and partial gasification processes themselves, the warm gas cleanup system, and the integration of the boiler with the heat recovery steam generator (HRSG). This baseline will then be compared with four other common types of power plants, including subcritical and ultra-supercritical (USC) Rankine cycles, IGCC, and natural gas. The results show that IMPGC consistently outperforms all other forms of coal-based power. IMPGC is more efficient than the standard subcritical Rankine cycle by nine percentage points, more than a USC Rankine cycle by nearly four points, and more than IGCC by seven points.

Thermodynamic Efficiency Maximum of Simple Organic Rankine Cycles

The increase of the maximal cycle temperature is considered as one of the best tools to increase cycle efficiency for all thermodynamic cycles, including Organic Rankine Cycles (ORC). Technically, this can be done in various ways, but probably the best solution is the use of hybrid systems, i.e., using an added high-temperature heat source to the existing low-temperature heat source. Obviously, this kind of improvement has technical difficulties and added costs; therefore, the increase of efficiency by increasing the maximal temperature sometimes has technical and/or financial limits. In this paper, we would like to show that for an ideal, simple-layout ORC system, a thermodynamic efficiency-maximum can also exist. It means that for several working fluids, the thermodynamic efficiency vs. maximal cycle temperature function has a maximum, located in the sub-critical temperature range. A proof will be given by comparing ORC efficiencies with TFC (Trilateral Flash Cycle) efficiencies; for wet working fluids, further theoretical evidence can be given. The group of working fluids with this kind of maximum will be defined. Generalization for normal (steam) Rankine cycles and CO2 subcritical Rankine cycles will also be shown. Based on these results, one can conclude that the increase of the maximal cycle temperature is not always a useful tool for efficiency-increase; this result can be especially important for hybrid systems.

Comparative analysis and evaluation of different working fluids for the organic rankine cycle performance

The aim of this study is to find the best working fluid for Organic Rankine Cycles (ORCs) at specific temperature limits, with a focus on thermodynamic analysis, environmental impact, and protection. A subcritical Rankine cycle with organic working fluids was used to produce electricity using a low-temperature heat source. The six organic fluids (R600, R245fa, R290, R134a, R123 & R600a) were selected, studied, and compared for ORC applications to get high efficiency and COP. The study found that because of pressure factor, efficiency, expander, R600a, R134a & R290 are the most preferable choices. In terms of COP and pressure ratio, R600a is best among the six fluids. However, if we consider factors like mass flow rate, refrigerating capacity and COP, then R245fa & R123 are the suitable choices. However, the ODP of R123 is found to be 0.012, and given the targeted zero tolerance for ODP compounds, R123 working fluid would be phased out sooner than expected, especially given its significant GWP of 76. The GWP of R245fa is at an intolerable level of 820.

Green and efficient configuration of integrated waste heat and cold energy recovery for marine natural gas/diesel dual-fuel engine

Energy conservation and emission reduction form a dominant theme in our time, hence, researchers urgently need to develop green and efficient technologies to adapt to new rules. To achieve the cascade utilization of waste heat and cold energy as well as meet the daily energy and life requirements of ships, a green and efficient configuration is proposed, which include the analysis and matching of supercritical and subcritical Rankine cycles, comprehensive utilization of liquefied natural gas cold energy, carbon separation and capture, and hydrocarbon adsorption. Foremost, the simulation results are compared with the literature data to verify the accuracy of the proposed integrated system. Subsequently, a point-by-point analysis of the operation parameters that affect the system performance is performed. In addition, the method for the efficient utilization of cold energy, and the contribution of quantitative cold energy are proposed. After obtaining the optimal operation conditions, the economic and thermodynamic performance of the cold energy recovery system and integrated system is calculated. The results demonstrate that the integrated system is feasible in economy and could meet the daily needs of the ship. The output work, energy efficiency, and exergy efficiency reach 270.78 kW, 26.89% and 54.33%, respectively. Finally, after the installation of the energy-saving technology, the system shows excellent emission reduction characteristics. Therefore, the configuration is found to be an effective implementation of a novel idea for energy conservation and emission reduction.

Maximizing specific work output extracted from engine exhaust with novel inverted Brayton cycles over a large range of operating conditions

The heat contained in internal combustion engine exhaust gases can be converted into mechanical energy by using an Inverted Brayton Cycle (IBC). In this paper, five different IBC versions are numerically modeled and optimized to maximize their specific work output: (i) basic IBC, (ii) IBC with liquid water drainage (IBC/D), (iii) IBC with liquid water drainage and a steam turbine (IBC/D/S), (iv) IBC with liquid water drainage and a refrigeration cycle (IBC/D/R), and (v) IBC with liquid water drainage, a steam turbine and a refrigeration cycle (IBC/D/S/R). The three latter cycles are presented for the first time in literature. The optimization is performed for a wide range of inlet gases temperatures (600–1200 K) and heat sink temperatures (280–340 K). Among the five IBCs, the IBC/D/S/R has the highest specific work output for the whole range of operating temperatures. A comparison with the subcritical Rankine cycle and Organic Rankine Cycles using isobutane and benzene shows that an IBC system might be a better choice for specific operating temperatures. Liquid water addition in the IBC/D/S/R leads to optimized designs using only the steam turbine at high inlet gas temperatures, indicating that a Rankine cycle is better suited for these conditions.

Annual performance of subcritical Rankine cycle coupled to an innovative particle receiver solar power plant

Concentrated solar power plants using molten salts as heat transfer and storage fluid have emerged as the preferred commercial solution for solar thermal electricity in central receiver technology. Despite their ability to store large amounts of thermal energy and efficient receiver designs, further efficiency improvements are constrained by tight temperature restrictions when using molten salts (290 °C–565 °C). In this work, a novel heat transfer fluid based on a dense particle suspension (DPS) is used due to its excellent thermophysical properties that extend the operating temperature of solar receiver and allow its coupling with higher-efficiency power cycles. In this paper, the design of a DPS solar receiver working at 650 °C has been optimized for two commercial sizes (50 MWth and 290 MWth) coupled to an optimized subcritical Rankine cycle. The results showed that a five-extraction reheated Rankine cycle operating at 610 °C and 180 bar maximizes power plant efficiency when coupled with a DPS central receiver, giving 41% power block efficiency and 23% sun-to-electricity efficiency. For optimization purposes at design point conditions, in-house code programmed into MATLAB platform was used while TRNSYS software was employed for annual plant performance analysis.

Temperature Control of Evaporators in Automotive Waste Heat Recovery Systems

This paper presents a control strategy for the steam generation process in automotive waste heat recovery systems that are based on the subcritical Rankine cycle. The central question is how to regulate the flow of water into the evaporator such that dry steam is generated at its outlet, subject to large variations in the heat input. Tight control of this process increases the amount of recovered energy while ensuring safe system operation. The method consists of inversion-based feedforward combined with output feedback on the temperature of the evaporator, which is estimated using exhaust gas measurements. As this method does not require a high fidelity evaporator model, it is easy to implement. It is demonstrated on an experimental setup, where the exhaust flow is imitated by electrically heated air. On an automotive driving cycle, steam was generated reliably with a superheating temperature of 10-20 [K].

Solar Towers with Supercritical Steam Parameters - is the Efficiency Gain worth the Effort?

The concentrating solar power (CSP) industry is working intensively on cost reductions to increase the economic competitiveness of CSP plants. Efforts include new power plant concepts, optimised component manufacturing, as well ashigher steam parameter and these indeed lead to cost reductions over the last years. However, further improvements are required and this paper analyses the impact of supercritical steam parameters in CSP plants. A few decades ago the coal industry moved from subcritical Rankine cycle power plants, currently all CSP plants operate at subcritical conditions, to supercritical steam parameters to improve cycle efficiency. Many supercritical coal plants are now in commercial operation with significant engineering experience available in regards to plant design, construction and operation. The CSP industry can use this expertise to reproduce the benefits in their plants but some challenges also exist. Currently, supercritical CSP steam has only been shown at demonstration scale and the upscale to the smallest turbine size for supercritical parameters, being 250 MWe steam turbine capacity, is very significant. This paper compares three different 250 MWe (net) solar tower scenarios, one with subcritical and two with supercritical steam parameters. One supercritical power plant scenario is based on a novel high temperature stable molten salt that allows steam parameter of 620 °C at 280 bar and the second uses current molten salt and natural gas to reach supercritical conditions. The analysis shows that the net plant cycle efficiency can be raised from 41.3% in a subcritical to 44.2% in a supercritical concept, which translates into a levelised cost of electricity reduction of 4.3%.

Selecting working fluids in an organic Rankine cycle for power generation from low temperature heat sources

This paper presents a thermodynamic study carried out on the use of low-temperature heat sources for power generation through a subcritical Rankine cycle with organic working fluids. An analysis of the state of the art of this technology shows the selection of the working fluid as an open research line, since until now there is no fluid that can meet all environmental and technical aspects to be considered in these cycles. Hence, we have developed a series of simulations that allow us to study the behavior of the Rankine cycle with different configurations and fluids (wet, dry and isentropic) which has led us to observe the influence on the overall cycle efficiency when we change the type of fluids used (refrigerants, hydrocarbons and water) as well as the conditions of temperature, pressure, flow, etc. With the work realized, the viability of this type of processes is demonstrated for the recovery of heat in industry and/or the use of renewable sources of low and medium temperature for the production of electricity.

Study and optimization of a solar subcritical organic Rankine cycle

Abstract The theoretical and experimental studies presented in this paper show the potential of producing mechanical power by a system combining a solar thermal collector with an organic Rankine cycle (ORC). A theoretical model based on heat transfer equations was developed and predicted the thermal equilibrium state of a small-scale single glazed flat plate collector. Experiments conducted with a commercial collector validated the modeling predictions. The thermal performances of the commercial collector and improved double-glazed flat plate collector were then simulated for different operational conditions. A subcritical Rankine cycle was simulated for organic fluids R134a, R227ea and R365mfc operating between the solar collector and a fixed temperature sink. In order to establish the optimum configuration, a set of parametric studies was carried out. Special attention is given to the optimum mass flow rate and to the impact made by improving the thermal performance of the solar collector. The results show that the expected net mechanical power strongly depends on the fluid mass flow rate and the optimum flow is a linear function of the solar radiation. Reducing the collector heat losses appears as the most relevant solution before the choice of the working fluid to improve the overall installation performance. Finally, this configuration can produce net mechanical power with an optimum efficiency up to 11%.

Optimal concentration and temperatures of solar thermal power plants

Using simple, finite-time, thermodynamic models of solar thermal power plants, the existence of an optimal solar receiver temperature has previously been demonstrated in literature. Scant attention has been paid, however, to the presence of an optimal level of solar concentration at which solar-to-electric efficiency is maximised. This paper addresses that gap. A simple mathematical model is developed to assess the influence of key system temperatures, thermal conductances and the receiver irradiance (radiation striking unit receiver area) upon the optimal receiver temperature and solar-to-electric efficiency of Rankine-cycle, solar-trough and solar-tower, direct-steam and molten-salts, power plants. The analysis shows that, as the thermal resistance of the solar receiver and condenser increases, the optimal receiver temperature increases whilst the optimal receiver irradiance decreases. The optimal level of receiver irradiance, for solar thermal plants employing a service fluid of molten salts, is found to occur within a range of values achievable using current solar tower technologies. The efficiency trade-off involved in using molten salts rather than direct steam is investigated. The optimal receiver temperatures calculated suggest the use of sub-critical Rankine cycles for solar trough plants, but super-critical Rankine cycles for solar tower plants, if the objective is to maximise solar-to-electric efficiency.

Potential application of Rankine and He-Brayton cycles to sodium fast reactors

Traditionally all the demos and/or prototypes of the sodium fast reactor (SFR) technology with power output, have used a steam sub-critical Rankine cycle. Sustainability requirement of Gen. IV reactors recommends exploring alternate power cycle configurations capable of reaching high thermal efficiency. By adopting the anticipated working parameters of next SFRs, this paper investigates the potential of some Rankine and He-Brayton layouts to reach thermal efficiencies as high as feasible, so that they could become alternates for SFR reactor balance of plant. The assessment has encompassed from sub-critical to super-critical Rankine cycles and combined cycles based on He-Brayton gas cycles of different complexity coupled to Organic Rankine Cycles. The sub-critical Rankine configuration reached at thermal efficiency higher than 43%, which has been shown to be a superior performance than any of the He-Brayton configurations analyzed. By adopting a super-critical Rankine arrangement, thermal efficiency would increase less than 1.5%. In short, according to the present study a sub-critical layout seems to be the most promising configuration for all those upcoming prototypes to be operated in the short term (10–15 years). The potential of super-critical CO 2-Brayton cycles should be explored for future SFRs to be deployed in a longer run.

Thermodynamic analysis and optimization of power cycles using a finite low-temperature heat source

The analysis of a subcritical Rankine cycle with superheating, operating between a constant flowrate low-temperature heat source and a fixed temperature sink, according to the principles of classical and finite size thermodynamics, is presented. The results show the existence of two optimum evaporation pressures: one minimizes the total thermal conductance of the two heat exchangers, whereas the other maximizes the net power output. A comparison of such results for five working fluids leads to the selection of R141b for a system generating 10% of a reference power which depends on the specified source and sink characteristics; for the conditions under consideration this reference power is 6861 kW. The results for this particular system show that the minimum total thermal conductance of the two heat exchangers is 1581 kW K−1; the corresponding thermal efficiency is 12.6% and the total exergy losses are 13.8% of the source's exergy. Slightly more than 50% of the exergy destruction occurs in the vapor generator. Copyright © 2011 John Wiley & Sons, Ltd.