Conference Paper

Cost and Performance Tradeoffs of Alternative Solar-Driven S-CO2 Brayton Cycle Configurations

Authors:
To read the full-text of this research, you can request a copy directly from the author.

Abstract

This paper evaluates cost and performance tradeoffs of alternative supercritical carbon dioxide (s-CO2) closed-loop Brayton cycle configurations with a concentrated solar heat source. Alternative s-CO2 power cycle configurations include simple, recompression, cascaded, and partial cooling cycles. Results show that the simple closed-loop Brayton cycle yielded the lowest power-block component costs while allowing variable temperature differentials across the s-CO2 heating source, depending on the level of recuperation. Lower temperature differentials led to higher sensible storage costs, but cycle configurations with lower temperature differentials (higher recuperation) yielded higher cycle efficiencies and lower solar collector and receiver costs. The cycles with higher efficiencies (simple recuperated, recompression, and partial cooling) yielded the lowest overall solar and power-block component costs for a prescribed power output.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the author.

... The supercritical CO 2 (sCO 2 ) cycle has been shown to provide significant advantages compared to traditional steam Rankine cycles [1][2][3][4][5]. One of the most important advantages is the ability to reach significantly higher efficiencies than steam Rankine cycles [5]. ...
... In order to achieve high efficiencies, the sCO 2 recuperators with effectiveness greater than 90%. The high pressures and temperatures of the cycle limit the materials that can be used to construct the recuperators, and the use of higher strength, exotic materials leads to more expensive components compared to Rankine cycle components. ...
Article
Supercritical CO2 cycles have the ability to reach high efficiency because of their high turbine inlet temperatures. However, high effectiveness recuperators (> 90%) are needed to achieve high efficiencies. Periodic flow re-generators have been proposed as an alternative to conventional heat exchanger designs such as Printed Circuit Heat Exchangers (PCHEs) or micro-tube heat exchangers. Regenerators use a packed bed of spheres to alter-natively store and release heat. Since the hot and cold fluid streams are not in direct contact, the design of the regenerator is much simpler and less expensive than a recuperator. A model of the regenerator has been created that can predict the effectiveness, pressure drop, and carryover when given the regenerator size and operating conditions. To verify this model, an experimental test facility has been constructed that is capable of testing regenerators at temperatures up to 550 °C and pressures up to 2400 psi at a heat transfer rate of approximately 10 kW. Experimental data was collected and compared to the model predictions. The model was able to predict effectiveness to within approximately 2% and pressure drop to within approximately 20%; both results are acceptable. However, it was necessary to develop a correction for carryover to account for the differences be-tween the model predictions and experimental data. The final result of this work is a verified model of the regenerator that can be used to quickly and accurately design and optimize a regenerator for a sCO2 Brayton cycle.
... In the simple 45 s-CO2 cycle, five phases (compression, regeneration, heating, expansion and cooling) are enough to achieve 46 high efficiencies at low cost. According to Ho et al (Ho et al., 2015), this configuration could be the 47 optimum option to achieve the lowest cost of electricity in CSP systems. However, the large internal 48 irreversibility in the regenerator (owing to the higher specific heat of the high-pressure side than that of the 49 low-pressure side) lowers the efficiency of simple cycles, and other layouts are proposed to mitigate this 50 problem. ...
... However, the compressor inlet 494 temperature is limited by the cold source in terms of heat transfer capacity and temperature. While the use 495 of water could achieve to reduce the compressor inlet temperature to Tc,in = 32 ºC, the use of air (used in 496 locations with scarce of water) could have more difficulties and typical compressor inlet temperatures are 497 between 40 ºC and 55 ºC (Ho et al., 2015;Neises and Turchi, 2019). In the reference case, with compressor 498 inlet temperature Tc,in = 40 ºC, the efficiency of the multi-heating system is 1.2% higher than the standard 499 system, and the additional thermal power is 35.9% of the total. ...
Article
Full-text available
This work analyses the features and performance of supercritical CO2 cycles with multi-heating (heat supplies at different temperature) in Concentrating Solar Power plants, including its internal coherent integration. The specific features of multi-heating cycles fit perfectly with the characteristics of concentrating solar energy. The integration requires the design of a new type of solar field to accommodate the solar radiation hitting the receiver to the multi-heating requirements. This new solar field has a similar configuration to the solar tower, but receiver and heliostats are divided into two sections, and each section meets different requirements in concentration ratio, fluid temperature, and absorbed heat flux. This new solar field, called multi-heating solar tower, achieves higher efficiencies than standard solar towers with softer thermal requirements. The flexibility of the multi-heating structure is very useful to solve efficiently the restrictions of supercritical CO2 cycles caused by the variations of specific heat close to the critical point. The resulting CSP plant with multi-heating achieves higher efficiency than plants with standard solar towers and supercritical CO2 cycle.
... The inlet, outlet and heat transfer rates are provided by our industry partner, Westinghouse, making the black box simplification viable. The energy balance for the black box assumption can be seen in Equation (9). ...
Article
Full-text available
Solar power has innate issues with weather, grid demand and time of day, which can be mitigated through use of thermal energy storage for concentrating solar power (CSP). Nuclear reactors, including lead-cooled fast reactors (LFRs), can adjust power output according to demand; but with high fixed costs and low operating costs, there may not be sufficient economic incentive to make this worthwhile. We investigate potential synergies through coupling CSP and LFR together in a single supercritical CO2 Brayton cycle and/or using the same thermal energy storage. Combining these cycles allows for the LFR to thermally charge the salt storage in the CSP cycle during low-demand periods to be dispatched when grid demand increases. The LFR/CSP coupling into one cycle is modeled to find the preferred location of the LFR heat exchanger, CSP heat exchanger, sCO2-to-salt heat exchanger (C2S), turbines, and recuperators within the supercritical CO2 Brayton cycle. Three cycle configurations have been studied: two-cycle configuration, which uses CSP and LFR heat for dedicated turbocompressors, has the highest efficiencies but with less component synergies; a combined cycle with CSP and LFR heat sources in parallel is the simplest with the lowest efficiencies; and a combined cycle with separate high-temperature recuperators for both the CSP and LFR is a compromise between efficiency and component synergies. Additionally, four thermal energy storage charging techniques are studied: the turbine positioned before C2S, requiring a high LFR outlet temperature for viability; the turbine after the C2S, reducing turbine inlet temperature and therefore power; the turbine parallel to the C2S producing moderate efficiency; and a dedicated circulator loop. While all configurations have pros and cons, use of a single cycle offers component synergies with limited efficiency penalty. Using a turbine in parallel with the C2S heat exchanger is feasible but results in a low charging efficiency, while a dedicated circulator loop offers flexibility and near-perfect heat storage efficiency but increasing cost with additional cycle components.
... Recently, the research community has also devoted efforts towards assessing the costs of the components required in large scale sCO 2 power systems. In Ho et al. (2015) first simplified cost functions for sCO 2 based heat exchangers, turbine and compressors were introduced. Later on, the database has been extended in Carlson et al. (2017) and data for a nominal power range from 1 to 100 MW e have been included in Weiland et al. (2019). ...
Article
Full-text available
This work presents an innovative indirect supercritical CO 2-air driven concentrated solar power plant with a packed bed thermal energy storage. High supercritical CO 2 turbine inlet temperature can be achieved, avoiding the temperature limitations set by the use of solar molten salts as primary heat transfer fluid. The packed bed thermal energy storage enables the decoupling between solar irradiation collection and electricity production, and it grants operational flexibility while enhancing the plant capacity factor. A quasi steady state thermo-economic model of the integrated concentrating solar power plant has been developed. The thermo-economic performance of the proposed plant design has been evaluated via multi-objective optimizations and sensitivity analyses. Results show that a Levelized Cost of Electricity of 100 $/MWh e and a capacity factor higher than 50% can be achieved already at a 10 MW e nominal size. Such limited plant size bounds the capital investment and leads to more bankable and easily installable plants. Results also show that larger plants benefit from economy of scale, with a 65 $/MWh e cost identified for a 50 MW e plant. The receiver efficiency is found to be the most influential assumption. A 20% decrease in receiver efficiency would lead to an increase of more than 15% of the Levelized Cost of Electricity. These results show the potential of indirect supercritical CO 2-air driven concentrated solar power plant and highlight the importance of further air receiver development. More validations and verification tests are needed to ensure the system operation during long lifetime.
... The capital investment has been calculated by adding the direct cost for all the main specific components (sCO2 turbomachinery, sCO2 heat exchangers, heliostat field and land, tower, receiver, TES, balance of plant and contingencies) and indirect cost (engineering, procurement and construction and taxes). The cost of the power block components have been calculated according to the scaling cost function in [32][33]. The heliostat and land cost has been evaluated based on the scaling function in [34]. ...
Conference Paper
The present work introduces an indirect supercritical CO2–air driven concentrated solar plant with a packed bed thermal energy storage. The proposed plant design enables a supercritical CO2 turbine inlet temperature of 800°C, overcoming the temperature limits imposed by the use of solar molten salts as primary heat transfer fluid. Furthermore, the packed bed thermal energy storage permits the decoupling between thermal power collection from the sun and electricity generation. Besides, the thermal energy storage unit grants operational flexibility and enlarges the plant capacity factor, making it as available as a conventional coal facility. A transient thermodynamic model of the integrated concentrating solar plant, including receiver, thermal energy storage, intermediate heat exchangers and supercritical CO2 power cycle has been developed. This same model has been used to evaluate the thermodynamic performance of the proposed plant design over a complete year. A similar model has been implemented to simulate a supercritical CO2 plant driven by a more traditional solar molten salt loop. A comparison of the thermodynamic performance of the two plant designs has been performed. A complete economic model has been developed in order to evaluate the economic viability of the proposed plant. Furthermore, a multi-objective optimization have been executed in order to assess the influence of the thermal energy storage size, supercritical CO2 turbine inlet temperature and plant solar multiple on the key performance indicators. Results show that the proposed indirect supercritical CO2–air driven with a packed bed thermal energy storage concentrated solar plant leads to improved thermo-economic performance with respect to the molten salts driven design. Enhancements in the power cycle efficiency and in the overall electricity production can be achieved, with a consequent reduction in the levelized cost of electricity. Particularly, for a design net electrical power production of 10MWe a minimum levelized cost of electricity has been calculated at 89.4 $/MWh for a thermal energy storage capacity of 13.9 hours at full load and a plant solar multiple of 2.47 corresponding to a capital investment of about 73.4 M$.
... Besides, the minimum attainable savings of electricity are about 30%, and correspond to the case of direct self-consumption of PV electricity, without storage. The results shown in Figure 3 [39], [69], [70] and medium (30 -300 kWel) [71] sizes. Unfortunately, current state of the art small-scale (< 10 kWel) closed-cycle engines, which have been developed for both terrestrial and space-power applications, have either low conversion efficiency (e.g. ...
Article
Full-text available
This article assesses whether it is profitable to store solar PV electricity in the form of heat and convert it back to electricity on demand. The impact of a number of technical and economic parameters on the profitability of a self-consumption residential system located in Madrid is assessed. The proposed solution comprises two kinds of heat stores: a low- or medium-grade heat store for domestic hot water and space heating, and a high-grade heat store for combined heat and power generation. Two cases are considered where the energy that is wasted during the conversion of heat into electricity is employed to satisfy either the heating demand, or both heating and cooling demands by using a thermally-driven heat pump. We compare these solutions against a reference case that relies on the consumption of grid electricity and natural gas and uses an electrically-driven heat pump for cooling. The results show that, under relatively favourable conditions, the proposed solution that uses an electrically-driven heat pump could provide electricity savings in the range of 70 – 90% with a payback period of 12 – 15 years, plus an additional 10 – 20% reduction in the fuel consumption. Shorter payback periods, lower than 10 years, could be attained by using a highly efficient thermally driven heat pump, at the expense of increasing the fuel consumption and the greenhouse gas emissions. Hybridising this solution with solar thermal heating could enable significant savings on the global emissions, whilst keeping a high amount of savings in grid electricity (>70%) and a reasonably short payback period (<12 years).
... Besides, the minimum attainable savings of electricity are about 30%, and correspond to the case of direct self-consumption of PV electricity, without storage. The results shown in Figure 3 [39], [69], [70] and medium (30 -300 kWel) [71] sizes. Unfortunately, current state of the art small-scale (< 10 kWel) closed-cycle engines, which have been developed for both terrestrial and space-power applications, have either low conversion efficiency (e.g. ...
Preprint
Full-text available
This article assesses whether it is profitable to store solar PV electricity in the form of heat and convert it back to electricity on demand. The impact of a number of technical and economic parameters on the profitability of a self-consumption residential system located in Madrid is assessed. The proposed solution comprises two kinds of heat stores: a low- or medium-grade heat store for domestic hot water and space heating, and a high-grade heat store for combined heat and power generation. Two cases are considered where the energy that is wasted during the conversion of heat into electricity is employed to satisfy either the heating demand, or both heating and cooling demands by using a thermally-driven heat pump. We compare these solutions against a reference case that relies on the consumption of grid electricity and natural gas and uses an electrically-driven heat pump for cooling. The results show that, under relatively favourable conditions, the proposed solution that uses an electrically-driven heat pump could provide electricity savings in the range of 70-90% with a payback period of 12-15 years, plus an additional 10-20% reduction in the fuel consumption. Shorter payback periods, lower than 10 years, could be attained by using a highly efficient thermally driven heat pump, at the expense of increasing the fuel consumption and the greenhouse gas emissions. Hybridising this solution with solar thermal heating could enable significant savings on the global emissions, whilst keeping a high amount of savings in grid electricity (> 70 %) and a reasonably short payback period (< 12 years).
... The main difference is that the latter can obtain higher efficiencies than the former at the expense of more complexity. Some authors analyze them from an economical point of view [4], but further work is needed to get to a final conclusion yet. ...
Conference Paper
Solar tower has become one of the most attractive CSP (Concentrating Solar Power) technologies during the last years, and its integration with supercritical Brayton cycles for next generation of CSP plants seems to be very promising. This study shows the benefits of applying different concentrations to different sections of a tower receiver and use these sections to heat supercritical CO2 at different enthalpies in order to take advantage of the supercritical Brayton cycles characteristics. This new solar thermal system, called Solar Heater, allows to reduce system concentration and temperatures in relation to a common solar tower system, which works at a unique enthalpy level. The results show that the system efficiency of the Solar Heater is a 9% higher than in a standard Solar Tower. In terms of thermal design, this new solar thermal engine uses the basis of thermal coherence to obtain higher efficiency with lower material requirements.
... The integration of sCO 2 receivers with storage is today a major, unsolved challenge; thermal [18] storage of supercritical fluids is not viable [22] and the use of a different TES medium does not seem to be an efficient option. In this context, the use of different HTMs as working fluid appears as the best option for integration of solar power towers with storage and sCO 2 cycles. ...
Chapter
Solar power-tower systems (also known as central receiver systems) can efficiently achieve high temperatures because of the high concentration ratios they can achieve using different configurations of the collector field and receiver. The combination of solar power towers with high-temperature cycles permits to increase in the global efficiency in the conversion of solar radiation to electricity with respect to concentrated solar power (CSP) plants based on the sub-critical Rankine cycle and could result in levelized cost of energy (LCOE) reduction, as far as the increase in efficiency outweighs the increased costs associated with the use of more expensive equipment and materials. Although operating temperatures higher than 1000°C can be achieved with solar power towers, there are still significant technological barriers that must be overcome before CSP plants operating at these elevated temperatures reach the market. The use of supercritical power cycles operating at temperatures in the range 600-800°C, only moderately higher than those of the current CSP plants, has been identified as a promising path to increase the efficiency and reduce the LCOE of the next generation of CSP plants that would require technological developments achievable in the short to medium term. Another option to increase the efficiency of concentrated solar thermal (CST) plants that requires only incremental technology developments is the concept known as decoupled solar combined cycles, based on a high-temperature topping cycle whose rejected heat is used to charge a thermal energy storage system which, in turn, feeds a bottoming cycle. The concepts presented in this chapter are excellent candidates to become the next generation(s) of commercial CST plants, although in some cases significant technology developments are required.
... Cayer, Galanis, and Nesreddine (2010) Multi-objective optimisation has been also proposed for the integration of thermodynamic performance and system cost of combined cycles (Koch, Cziesla, and Tsatsaronis (2007);Pouraghaie et al. (2010); Ahmadi and Dincer (2011)) and cogeneration power cycles (Sahoo (2008);Sayyaadi (2009)). For the case of S-CO 2 Brayton cycles, Ho et al. (2015) performed an thermo-economical analysis of four configurations (simple, recompression, cascaded, and partial cooling cycles), while Zhao et al. (2015) carried out a multi-objective optimisation of two different configurations (simple and recompression) by maximising the exergy efficiency and minimising the lowest cost per power ($/kW). The previous analysis included a cost analysis of S-CO 2 , however the costs associated with this technology still have some uncertainties, especially those with regards to turbomachinery, recuperators and additional costs (materials, manufacturing,etc.) ...
Article
Full-text available
In this paper, optimisation of the supercritical CO(Formula presented.) Brayton cycles integrated with a solar receiver, which provides heat input to the cycle, was performed. Four S-CO(Formula presented.) Brayton cycle configurations were analysed and optimum operating conditions were obtained by using a multi-objective thermodynamic optimisation. Four different sets, each including two objective parameters, were considered individually. The individual multi-objective optimisation was performed by using Non-dominated Sorting Genetic Algorithm. The effect of reheating, solar receiver pressure drop and cycle parameters on the overall exergy and cycle thermal efficiency was analysed. The results showed that, for all configurations, the overall exergy efficiency of the solarised systems achieved at maximum value between 700°C and 750°C and the optimum value is adversely affected by the solar receiver pressure drop. In addition, the optimum cycle high pressure was in the range of 24.2–25.9 MPa, depending on the configurations and reheat condition. © 2016 The Commonwealth Scientific and Industrial Research Organisation
Article
The objective of this paper is to understand the benefits that one can achieve for large-scale supercritical CO2 (S-CO2) coal-fired power plants. The aspects of energy environment and economy of 1000 MW S-CO2 coal-fired power generation system and 1000 MW ultra-supercritical (USC) water-steam Rankine cycle coal-fired power generation system are analyzed and compared at the similar main vapor parameters, by adopting the neural network genetic algorithm and life cycle assessment (LCA) methodology. Multi-objective optimization of the 1000 MW S-CO2 coal-fired power generation system is further carried out. The power generation efficiency, environmental impact load, and investment recovery period are adopted as the objective functions. The main vapor parameters of temperature and pressure are set as the decision variables. The results are concluded as follows. First, the total energy consumption of the S-CO2 coal-fired power generation system is 10.48 MJ/kWh and the energy payback ratio is 34.37%. The performance is superior to the USC coal-fired power generation system. Second, the resource depletion index of the S-CO2 coal-fired power generation system is 4.38 µPRchina,90, which is lower than that of the USC coal-fired power generation system, and the resource consumption is less. Third, the environmental impact load of the S-CO2 coal-fired power generation system is 0.742 mPEchina,90, which is less than that of the USC coal-fired power generation system, 0.783 mPEchina,90. Among all environmental impact types, human toxicity potential HTP and global warming potential GWP account for the most environmental impact. Finally, the investment cost of the S-CO2 coal-fired power generation system is generally less than that of the USC coal-fired power generation system because the cost of the S-CO2 turbine is only half of the cost of the steam turbine. The optimal turbine inlet temperature T5 becomes smaller, and the optimal turbine inlet pressure is unchanged at 622.082°C/30 MPa.
Article
Increasing demand of electricity and severer concerns to environment call for green energy sources as well as efficient energy conversion systems. SCO2 power cycles integrated with concentrating solar power (CSP) are capable of enhancing the competitiveness of thermal solar electricity. This article makes a comprehensive review of supercritical CO2 power cycles integrated with CSP. A detailed comparison of four typical CSP technologies is conducted, and the cost challenge of currently CSP technologies is pointed out. The thermophysical properties of sCO2 and the corresponding two real gas effects are analyzed elaborately to express the features of sCO2 power cycles. An extensive review of sCO2 layouts relevant for CSP including 12 single layouts and 1 combined layout is implemented logically. Strengths and weaknesses of sCO2 power cycles over traditional steam‐Rankine cycle generally adopted in current CSP plants are concluded, followed by metal material degration summary in CSP relevant temperature sCO2 environment, which shows that the nickel‐based alloy is a proper structural material candidate for sCO2‐CSP integration. Thermodynamic analyses of sCO2 power cycles when integrated with CSP are divided into three level of which design‐point analysis and off‐design modeling are conducted and compared, more researches into the off‐design point analysis, dynamic modeling, especially the transient behavior are suggested. Economic analysis of the integrated system is concluded and presents a considerable levelized cost of electricity reduction of 15.6% to 67.7% compared to that of state of art CSP. Taking the thermodynamic and eco-nomic analysis into consideration, target designs of sCO2 power cycles for CSP are summarized in three aspects. Finally, current theoretical and experimental researches of sCO2 power cycles integrated with CSP for market penetration are introduced. The strengths, weaknesses, and potential solutions to the gaps of three potential pathways (molten salt pathway, particle pathway, and gas phase pathway) to realize the integration of sCO2 power cycles in the next CSP generation plants up to 700°C are reviewed. In general, the integration of sCO2 power cycles with CSP technologies exhibits promising expectations for facilitating the competitiveness of thermal solar electricity.
Conference Paper
Full-text available
In this paper, two configurations of the S-CO2 Brayton cycles (i.e., the single-recuperated and recompression cycles) are thermodynamically modeled and optimized through a multi-objective approach. Two semi-conflicting objectives, i.e., cycle efficiency (ηc) and cycle specific power (Φsp) are maximized simultaneously to achieve Pareto optimal fronts. The objective of maximum cycle efficiency is to have a smaller and less expensive solar field, and a lower fuel cost in case of a hybrid scheme. On the other hand, the objective of maximum specific power provides a smaller power block, and a lower capital cost associated with recuperators and coolers. The multi-objective optimization is carried out by means of a genetic algorithm which is a robust method for multidimensional, nonlinear system optimization. The optimization process is comprehensive, i.e., all the decision variables including the inlet temperatures and pressures of turbines and compressors, the pinch point temperature differences, and the mass flow fraction of the main compressor are optimized simultaneously. The presented Pareto optimal fronts provide two optimum trade-off curves enabling decision makers to choose their desired compromise between the objectives, and to avoid naive solution points obtained from a single-objective optimization approach. Moreover, the comparison of the Pareto optimal fronts associated with the studied configurations reveals the optimum operational region of the recompression configuration where it presents superior performance over the single-recuperated cycle.
Article
Full-text available
Field test as well as laboratory measurements have shown that the performance of systems that combine heat pumps with combi- storages is often lower than expected. One reason that has been identified is a large gap between the temperature level provided by the heat pump condenser and the useful heat distributed for space heating and/or DHW. Within this work, different options for hydraulic integration and control of a heat pump connected to a solar combi-storage are investigated by means of annual system simulations. The results show that unfavorable hydraulic integration can lead to additional electric energy demand of the system of up to 45% (>1000 kWh electric energy per year) compared to a - well designed - reference solution with the same components. Based on the system performance analysis, recommendations are given for the hydraulic integration of heat pumps into systems with combi-storages.
Article
Full-text available
A heating system that combines a ground source heat pump and solar collectors with a combistore was measured in detail in the laboratory by means of “whole system testing”. The test rig thereby emulated a building with floor heating, hot water tappings, flat plate solar thermal collectors and a ground source heat exchanger. The tested heating system covered the heat demand for space heating and domestic hot water preparation in a reliable way with a good annual system performance factor of 4.5 that was determined after the physical test by means of test-calibrated simulation models. Nevertheless the measurements revealed that there is still room for improvements. It was found that the heat pump operates in space heating mode in an oscillating on/off behavior with short runtimes and respectively with a high number of starts and in addition with a supply temperature that is in average 8 K above the temperature that is needed in the heat distribution. Furthermore a significant transport of energy, from the upper part of the store that is reserved for domestic hot water preparation to the lower part, reserved for space heating, was detected.
Article
Full-text available
Sandia National Laboratories is investigating advanced Brayton cycles using supercritical working fluids for use with solar, nuclear or fossil heat sources. The focus of this work has been on the supercritical CO cycle (S-CO2) which has the potential for high efficiency in the temperature range of interest for these heat sources, and is also very compact, with the potential for lower capital costs. The first step in the development of these advanced cycles was the construction of a small scale Brayton cycle loop, funded by the Laboratory Directed Research & Development program, to study the key issue of compression near the critical point of CO. This document outlines the design of the small scale loop, describes the major components, presents models of system performance, including losses, leakage, windage, compressor performance, and flow map predictions, and finally describes the experimental results that have been generated.
Article
Full-text available
The optimum performance of a regenerative Brayton cycle was analyzed. The model includes external and internal irreversibilities coming from four main sources: coupling to external heat reservoirs, turbine and compressor nonisentropic processes, pressure losses in the heater and the cooler, and the regenerator. In terms of the parameters accounting for each type of irreversibility, explicit numerical results are presented for the maximum efficiency, maximum power output, efficiency at maximum power output, power output at maximum efficiency, as well as for the pressure ratios required for maximum efficiency and maximum power. This analysis could provide a general theoretical tool for the optimal design and operation of real regenerative gas turbine power plants. © 1997 American Institute of Physics.
Article
Full-text available
Using an improved Brayton cycle as a model, a general analysis accounting for the efficiency and net power output of a gas-turbine power plant with multiple reheating and intercooling stages is presented. This analysis provides a general theoretical tool for the selection of the optimal operating conditions of the heat engine in terms of the compressor and turbine isentropic efficiencies and of the heat exchanger efficiency. Explicit results for the efficiency, net power output, optimized pressure ratios, maximum efficiency, maximum power, efficiency at maximum power, and power at maximum efficiency are given. Among others, the familiar results of the Brayton cycle (one compressor and one turbine) and of the corresponding Ericsson cycle (infinite compressors and infinite turbines) are obtained as particular cases.
Article
Full-text available
The purpose of this work is to obtain a more precise calculation of the effective limits to the efficiency, of several cyclic heat engines. This calculation is based, first, on the equations describing the irreversible efficiency, and second, on a method which results from a general criterion to maximize this efficiency, applicable to several heat engines. With this method, we apply the criterion to maximize efficiencies; establish lower and upper bounds, corresponding to the efficiencies of Curzon–Ahlborn-like and Carnot-like heat engines; and, finally, find analytical or numerical expressions for the efficiencies ηme and ηmax. ηmax is the maximum irreversible efficiency; ηme is the efficiency in which the irreversible efficiency achieves its maximum, in a similar way to the Curzon–Ahlborn efficiency (maximum work or power). The method was applied to a Brayton cycle, presenting internal dissipations of the working fluid and irreversibilities due to the finite-rate heat transfer between the heat engine and its reservoirs. Also, we applied this method to a Carnot cycle including the irreversibilities of a finite-rate heat transfer between the heat engine and its reservoirs, heat leak between the reservoirs, and internal dissipations of the working fluid. The results obtained for the Brayton cycle are more general and useful than those in the relevant literature.
Article
Full-text available
Realistic upper bounds can be placed on the power and efficiency of real heat engines via a relatively simple analytic treatment of primary sources of irreversibility. Generalized curves for heat engine performance, their universal nature, and quantitative evaluation of upper bounds for power and efficiency are derived for several engine types, specifically: Brayton cycle (gas turbines), Rankine cycle (steam turbines), and cycles with sizable heat leaks, such as thermoelectric generators. The key irreversibility sources include fluid friction, the constraint of the equation of state of the engine’s working fluid, and heat leak. It is demonstrated that maximum power and maximum efficiency operating points are usually relatively close, with the associated implications for the selection of optimal heat engine operating conditions. The limitations of past analyses of endoreversible cycles as models for real heat engines will be discussed and the fortuitous nature of agreement between their predictions and actual heat engine performance will be explained.
Article
Full-text available
For a general class of heat engines operating at maximum power, in which the generic sources of irreversibility are finite‐rate heat transfer and friction only, we investigate (1) the time‐dependent driving functions that maximize power when heat input and heat rejection are constrained to be nonisothermal, as is the case in many conventional heat engines, and (2) the specific impact of friction on the nature of the engine cycle that maximizes power, and on the engine’s power‐efficiency characteristic. The extent to which maximum power is affected by the constraints on the driving function is evaluated, as well as the time divisions on the different branches of the optimal cycle. The fundamental differences in engine performance that arise from frictional losses being internally dissipative, as opposed to externally dissipative, are derived, and illustrative examples are presented.
Article
Full-text available
If hot water storage tanks are optimally integrated in heat or cold supply systems, they contribute to a reduction of required capacity, fuel and operation costs. Unfortunately, even today remarkable heat losses and internal losses occur in hot water storage tanks. The potential for cost and energy reductions is not completely utilized yet. Here, not only heat losses to the ambience, but also internal losses play a decisive role. Main focus of the presented work is the description of the single losses at hot water storage tanks and the determination of the correlation between the losses, the tank design and the mode of operation. Furthermore the effects of the losses in the tank on the efficiency of different types of heat generators and the input of primary energy into the system have been examined. The results are based on extended numerical modeling with the CFD-code Fluent as well as experimental test with three storage tanks. The quantitative comparison of the losses for selected examples shows the current shortcomings but even the potential for an optimized hot water storage tank design.
Article
The benefits of thermal stratification in sensible heat storage were investigated for several residential solar applications. The operation of space heating, air conditioning, and water heating systems with water storage was simulated on a computer. The performance of comparable systems with mixed and stratified storage was determined in terms of the fraction of the total load supplied by solar energy. The effects of design parameters such as collector efficiency, storage volume, flow rates, etc., on the relative advantage of stratified over well-mixed storage were assessed. The results show that significant improvements in system performance (5 - 15%) may be realized if stratification can be maintained in the storage tank. The magnitude of the improvement is greatest and the sensitivity to design variables smallest in the service hot water application. The results also show that the set of design parameters which describes the optimum system is likely to be substantially different for a system employing stratified storage than for a mixed storage system. In both the water heating and space heating applications, collector flow rates lower than currently suggested for mixed storage systems were found to yield optimum performance for a system with stratified storage.
Article
Of the mechanisms to improve efficiency for solar-thermal power plants, one of the most effective ways to improve overall efficiency is through power cycle improvements. As increases in operating temperature continue to be pursued, supercritical CO2 Brayton cycles begin to look more attractive despite the development costs of this technology. Further, supercritical CO2 Brayton has application in many areas of power generation beyond that for solar energy alone. One challenge particular to solar-thermal power generation is the transient nature of the solar resource. This work illustrates the behavior of developmental Brayton turbomachinery in response to a fluctuating thermal input, much like the short-term transients experienced in solar environments. Thermal input to the cycle was cut by 50% and 100% for short durations while the system power and conditions were monitored. It has been shown that despite these fluctuations, the thermal mass in the system effectively enables the Brayton cycle to continue to run for short periods until the thermal input can recover. For systems where significant thermal energy storage is included in the plant design, these transients can be mitigated by storage; a comparison of short- and long-term storage approaches on system efficiency is provided. Also, included in this work is a data set for stable supercritical CO2 Brayton cycle operation that is used to benchmark computer modeling. With a benchmarked model, specific improvements to the cycle are interrogated to identify the resulting impact on cycle efficiency and loss mechanisms. Status of key issues remaining to be addressed for adoption of supercritical CO2 Brayton cycles in solar-thermal systems is provided in an effort to expose areas of necessary research.
Article
A brief review of power generation thermodynamics. Reversibility and Availability. Basic gas turbine cycles. Cycle effeciency with turbine cooling. Full calculations of plant effeciency. Wet gas turbine plants. The combined cycle gas turbine (CCGT). Novel gas turbine cycles. The gas turbine as a cogeneration plant.
Article
A stable thermal stratification in solarthermal storage tanks increases the energy efficiency of these systems. Especially in charging and discharging cycles, mixing occurs due to jet flows. The reliable prediction of the influence of the storage and of the charging device geometry on the loading behaviour is essential for the layout and improvement of stratified storage systems. A model approach for the computational calculation of the time-dependent temperature distribution in stratified storage tanks based on the one-dimensional heat transport equation is described in the present study. The numerical solution was obtained by application of the first order Upwind-discretization scheme. This basic approach was further refined by the consideration of charging jet flows and local turbulences in the area of stratification according to the strategies of Jirka (2004) and Mott and Woods (2009) and implemented in MATLAB. Two simulation examples of different complexity have shown that the enhanced model could increase the calculation accuracy in comparison to similar CFD and experimental studies. The results of the MATLAB program were reached with much less calculation effort than the results of the CFD simulation.
Article
In this study, an elastohydrodynamic model was created for predicting the pressure field in a compliant thrust bearing assembly lubricated by high pressure CO2. This application is of significance due to ongoing research into the closed-cycle supercritical CO2 turbine as a high-efficiency alternative to steam turbines. Hardware development for this concept has been led by Sandia National Laboratories, where turbomachinery running on gas foil thrust and journal bearings is being tested. The model accounts for the fluid velocity field, hydrodynamic pressure, and frictional losses within the lubrication layer by evaluating the turbulent Reynolds equation coupled with an equation for structural deformation in the bearings, and the fluid properties database RefProp v9.0. The results of numerical simulations have been compared with empirical correlations, with reasonable agreement attained. Of particular interest is the contrast drawn between the performance of high pressure CO2 as a lubricant, and ambient pressure air. Parametric studies covering a range of fluid conditions, operating speeds, and thrust loads were carried out to illustrate the value of this model as a tool for improved understanding and further development of this nascent technology.
Conference Paper
The US Department of Energy is currently focused on the development of next-generation nuclear power reactors, with an eye towards improved efficiency and reduced capital cost. To this end, reactors using a closed-Brayton power conversion cycle have been proposed as an attractive alternative to steam turbines. The supercritical-CO2 recompression cycle has been identified as a leading candidate for this application as it can achieve high efficiency at relatively low operating temperatures with extremely compact turbomachinery. Sandia National Laboratories has been a leader in hardware and component development for the supercritical-CO2 cycle. With contractor Barber-Nichols Inc, Sandia has constructed a megawatt-class S-CO2 cycle test-loop to investigate the key areas of technological uncertainty for this power cycle, and to confirm model estimates of advantageous thermodynamic performance. Until recently, much of the work has centered on the simple S-CO2 cycle — a recuperated Brayton loop with a single turbine and compressor. However work has recently progressed to a recompression cycle with split-shaft turbo-alternator-compressors, unlocking the potential for much greater efficiency power conversion, but introducing greater complexity in control operations. The following sections use testing experience to frame control actions made by test loop operators in bringing the recompression cycle from cold startup conditions through transition to power generation on both turbines, to the desired test conditions, and finally to a safe shutdown. During this process, considerations regarding turbocompressor thrust state, CO2 thermodynamic state at the compressor inlet, compressor surge and stall, turbine u/c ratio, and numerous other factors must be taken into account. The development of these procedures on the Sandia test facility has greatly reduced the risk to industry in commercial development of the S-CO2 power cycle.
Conference Paper
Supercritical CO2 (S-CO2) power cycles offer the potential for better overall plant economics due to their high power conversion efficiency over a moderate range of heat source temperatures, compact size, and potential use of standard materials in construction [1,2,3,4]. Sandia National Labs (Albuquerque, NM, US) and the US Department of Energy (DOE-NE) are in the process of constructing and operating a megawatt-scale supercritical CO2 split-flow recompression Brayton cycle with contractor Barber-Nichols Inc. [5] (Arvada, CO, US). This facility can be counted among the first and only S-CO2 power producing Brayton cycles anywhere in the world. The Sandia-DOE test-loop has recently concluded a phase of construction that has substantially upgraded the facility by installing additional heaters, a second recuperating printed circuit heat exchanger (PCHE), more waste heat removal capability, higher capacity load banks, higher temperature piping, and more capable scavenging pumps to reduce windage within the turbomachinery. With these additions, the loop has greatly increased its potential for electrical power generation — according to models, as much as 80 kWe per generator depending on loop configuration — and its ability to reach higher temperatures. To date, the loop has been primarily operated as a simple recuperated Brayton cycle, meaning a single turbine, single compressor, and undivided flow paths. In this configuration, the test facility has begun to realize its upgraded capacity by achieving new records in turbine inlet temperature (650°F/615K), shaft speed (52,000 rpm), pressure ratio (1.65), flow rate (2.7 kg/s), and electrical power generated (20kWe). Operation at higher speeds, flow rates, pressures and temperatures has allowed a more revealing look at the performance of essential power cycle components in a supercritical CO2 working fluid, including recuperation and waste heat rejection heat exchangers (PCHEs), turbines and compressors, bearings and seals, as well as auxiliary equipment. In this report, performance of these components to date will be detailed, including a discussion of expected operational limits as higher speeds and temperatures are approached.
Article
The main theme of this paper is to study the flammability suppression of hydrocarbons by blending with carbon dioxide, and to evaluate these mixtures as possible working fluids in organic Rankine cycle for medium temperature concentrated solar power applications. The analysis takes into account inevitable irreversibilities in the turbine, the pump, and heat exchangers. While the isopentane + CO2 mixture suffers from high irreversibility mainly in the regenerator owing to a large temperature glide, the propane + CO2 mixture performs more or less the same as pure propane albeit with high cycle pressures. In general, large temperature glides at condensing pressures extend the heat recovery into the two-phase dome, which is an advantage. However, at the same time, the shift of the pinch point towards the warm end of the regenerator is found to be a major cause of irreversibility. In fact, as the number of carbon atoms in alkanes decreases, their blend with CO2 moves the pinch point to the colder end of the regenerator. This results in lower entropy generation in the regenerator and improved cycle efficiency of propane + CO2 mixtures. With this mixture, real cycle efficiencies of 15–18% are achievable at a moderate source temperature of 573 K. Applicability for a wide range of source temperatures is found to be an added advantage of this mixture.
Article
Low grade thermal energy from sources such as solar, geothermal and industrial waste heat in the temperature range of 380–425 K can be converted to electrical energy with reasonable efficiency using isopentane and R-245fa. While the former is flammable and the latter has considerable global warming potential, their mixture in 0.7/0.3 mole fraction is shown to obviate these disadvantages and yet retain dominant merits of each fluid. A realistic thermodynamic analysis is carried out wherein the possible sources of irreversibilities such as isentropic efficiencies of the expander and the pump and entropy generation in the regenerator, boiler and condenser are accounted for. The performance of the system in the chosen range of heat source temperatures is evaluated. A technique of identifying the required source temperature for a given output of the plant and the maximum operating temperature of the working fluid is developed. This is based on the pinch point occurrence in the boiler and entropy generation in the boiling and superheating regions of the boiler. It is shown that cycle efficiencies of 10–13% can be obtained in the range investigated at an optimal expansion ratio of 7–10.
Article
The purpose of this study is to figure out the thermal stratification mechanism of a storage tank and thereby to determine optimum design and operating conditions. To this end, a computer program is developed to investigate the fluid flow in a tank, using Patankars SIMPLE algorithm. The validation of program is made successfully by the comparison against experimental data measured with a bench scale facility. Further a systematic investigation has been made in terms of important design and operational parameters such as storage tank size (commercial-scale and bench-scale), loading time, shape of diffuser, turbulence model and inlet velocity or Fr No. Considering the thermal efficiency of storage tank is critically impaired by the effect of flow recirculation and mixing by turbulence, a novel model with minimum mixing between hot and cold water is proposed for the evaluation of the performance of storage system by the assumption of the uniform plug-type flow. This is made by solving only the following governing equation of temperature which has no convective mixing with constant axial velocity, that is
Article
Supercritical carbon dioxide based Brayton cycle for possible concentrated solar power applications is investigated and compared with trans- and sub-critical operations of the same fluid. Thermal efficiency, specific work output and magnitude of irreversibility generation are used as some of the performance indicators. While the thermal efficiency increases almost linearly with low side pressure in the sub- and trans-critical cycles, it attains a maximum in the supercritical regime at ∼85 bar after which there are diminishing returns on increasing the low side pressure. It is also found that supercritical cycle is capable of producing power with a thermal efficiency of >30% even at a lower source temperature (820 K) and accounting for foreseeable non-idealities albeit with a higher turbine inlet pressure (∼300 bar) which is not matched by a conventional sub-critical cycle even with a high source temperature of 978 K. The reasons for lower efficiency than in an ideal cycle are extracted from an irreversibility analysis of components, namely, compressor, regenerator, turbine and gas cooler. Low sensitivity to the source temperature and extremely small volumetric flow rates in the supercritical cycle could offset the drawback of high pressures through a compact system.
Article
The idea is to find out whether 2nd law efficiency optimization may be a suitable trade-off between maximum work output and maximum 1st law efficiency designs for a regenerative gas turbine engine operating on the basis of an open Brayton cycle. The primary emphasis is placed on analyzing the ideal cycle to determine the upper limit of the engine. Explicit relationships are established for work and entropy production of the ideal cycle. To examine whether a Brayton cycle may operate at the regime of fully reversible characterized by zero entropy generation condition, the cycle net work is computed. It is shown that an ideal Brayton-type engine with or without a regenerator cannot operate at fully reversible limit. Subsequently, the analysis is expanded to an irreversible cycle and the relevant relationships are obtained for net work, thermal efficiency, total entropy production, and second law efficiency defined as the thermal efficiency of the irreversible cycle divided by the thermal efficiency of the ideal cycle. The effects of the compressor and turbine efficiencies, regenerator effectiveness, pressure drop in the cycle and the ratio of maximum-to-minimum cycle temperature on optimum pressure ratios obtained by maximization of 1st and 2nd law efficiencies and work output are examined. The results indicate that for the regenerator effectiveness greater than 0.82, the 2nd law efficiency optimization may be considered as a trade-off between the maximum work output and the maximum 1st law efficiency.
Article
A theoretical and experimental analysis of water jets entering a solar storage tank is performed. CFD calculations of three inlet designs with different inlet flow rates were carried out to illustrate the varying behaviour of the thermal conditions in a solar store. The results showed the impact of the inlet design on the flow patterns in the tank and thus how the energy quality in a hot water tank is reduced with a poor inlet design. The numerical investigations were followed by experiments. A test solar store, similar to the store investigated by numerical modelling was constructed with cylindrical transparent walls so that the flow structures due to the inlet jets could be visualized. With the three inlets, nine draw-off tests with different inlet flow rates were carried out and the temperature stratification in the tank was measured during the draw-offs. The experimental results were used in an analysis using the first and second law of thermodynamics. The results showed how the entropy changes and the exergy changes in the storage during the draw-offs influenced by the Richardson number, the volume draw-off and the initial tank conditions.
Article
In this study, the influence of inlet geometry on the degree of stratification attainable in thermocline thermal energy storage is investigated. The turbulent mixing caused by different inlet geometries is quantified using a mixing index introduced in a one-dimensional flow model. The mixing index is correlated with the flow parameters for three different inlet configurations. Based on the obtained correlations it is concluded that the inlet geometry starts to influence thermal stratification in a thermocline thermal storage tank for Richardson numbers below 3.6.
Article
A theoretical analysis of differently designed solar combi systems is performed with weather data from the Danish Design Reference Year (55 deg N). Three solar combi system designs found on the market are investigated. The investigation focuses on the influence of stratification on the thermal performance under different operation conditions with different domestic hot water and space heating demands. The solar combi systems are initially equipped with heat exchanger spirals and direct inlets to the tank. A step-by-step investigation is performed demonstrating the influence on the thermal performance of using inlet stratification pipes at the different inlets. Also, how the design of the space heating system, the control system of the solar collectors, and the system size influence the thermal performance of solar combi systems are investigated. The work is carried out within the Solar Heating and Cooling Programme of the International Energy Agency (IEA SHC), Task 32.
Article
Many gas turbines simulation codes have been developed to estimate power plant performance both in design and off-design conditions in order to establish the adequate control criteria or the possible cycle improvements; estimation of pollutant emissions would be very important using these codes in order to determine the optimal performance satisfying legal emission restrictions. This paper present the description of a 1-D emission model to simulate different gas turbine combustor typologies, such as conventional diffusion flame combustors, Dry-Low NOx combustors (DLN) based on lean-premixed technology (LPC) or Rich Quench Lean scheme (RQL) and the new catalytic combustors. This code is based on chemical reactor analysis, using detailed kinetics mechanisms, and it is integrated with an existing power plant simulation code (ESMS Energy System Modular Simulator) to analyze the effects of power plant operations and configurations on emissions. The main goal of this job is the study of the interaction between engine control and combustion system. This is a critical issue for all DLN combustors and, in particular, when burning low-LHV fuel. The objective of this study is to evaluate the effectiveness of different control criteria with regard to pollutant emissions and engine performances. In this paper we present several simulations of actual engines comparing the obtained results with the experimental published data.
Article
A steady-flow approach for finite-time thermodynamics is used to calculate the maximum thermal efficiency, its corresponding power output, adiabatic temperature ratio, and thermal-conductance ratio of heat transfer equipment of a closed Brayton heat engine. The physical model considers three types of irreversibilities: finite thermal conductance between the working fluid and the reservoirs, heat leaks between the reservoirs, and internal irreversibility inside the closed Brayton heat engine. The effects of heat leaks, hot-cold reservoir temperature ratios, turbine and compressor isentropic efficiencies, and total conductances of heat exchangers on the maximum thermal efficiency and its corresponding parameters are studied. The optimum conductance ratio could be found to effectively use the heat transfer equipment, and this ratio is increased as the component efficiencies and total conductances of heat exchangers are increased, and always less than or equal to 0.5.
Article
A solar energy powered Rankine cycle using supercritical CO2 for combined production of electricity and thermal energy is proposed. The proposed system consists of evacuated solar collectors, power generating turbine, high-temperature heat recovery system, low-temperature heat recovery system, and feed pump. The system utilizes evacuated solar collectors to convert CO2 into high-temperature supercritical state, used to drive a turbine and thereby produce mechanical energy and hence electricity. The system also recovers heat (high-temperature heat and low-temperature heat), which could be used for refrigeration, air conditioning, hot water supply, etc. in domestic or commercial buildings. An experimental prototype has been designed and constructed. The prototype system has been tested under typical summer conditions in Kyoto, Japan; It was found that CO2 is efficiently converted into high-temperature supercritical state, of while electricity and hot water can be generated. The experimental results show that the solar energy powered Rankine cycle using CO2 works stably in a trans-critical region. The estimated power generation efficiency is 0.25 and heat recovery efficiency is 0.65. This study shows the potential of the application of the solar-powered Rankine cycle using supercritical CO2.
Article
It is well known that the choice of upper pressure limit in a transcritical CO2 refrigeration cycle is independent of the gas cooler approach temperature. This is in contrast to sub-critical cycles, where the condensing pressure is invariably governed by the ambient conditions. The criteria used for limiting the upper pressure limit in a transcritical cycle are the state of maximum COP for a given set of evaporating and gas cooler exit temperatures. The latter is governed by the local ambient conditions and the possible approach. This paper provides a thermodynamic basis for the evaluation of this pressure for the case of ideal compression and with some compressors available in the market. In addition, it also provides an additional criterion that minimizes the cycle irreversibility which is predominantly due to gas throttling. This paper evaluates the pressure limits for these two criteria for some typical evaporating temperatures and ambient conditions. The possible compressor discharge temperatures in each case are calculated and criteria for two-stage compression are identified.
Article
The efficiency of a Carnot engine is treated for the case where the power output is limited by the rates of heat transfer to and from the working substance. It is shown that the efficiency, ɛ, at maximum power output is given by the expression ɛ = 1 - (T2/T1)1/2 where T1 and T2 are the respective temperatures of the heat source and heat sink. It is also shown that the efficiency of existing engines is well described by the above result.
Article
This study is to systematically analyze the effect of various kinds of design factors on the stratification performance of a rectangular storage tank. Special interest is focused on the diffuser configuration, which crucially impacts the performance of a storage tank. Herein, a new diffuser shape is proposed, which exemplifies improved performance. Three-dimensional unsteady numerical experiments are conducted for four design parameters of a stratified thermal storage tank: Three design parameters with three levels (i.e., the Reynolds number=400, 800, and 1200; the Froude number=0.5, 1.0, and 2.0; and the area ratio of the diffuser to tank cross-section=0.0327, 0.0582, and 0.131) and one design parameter having two levels (i.e., diffuser type=radial plate type and radial adjusted plate type). Orthogonal array L18(2×37) is adopted for the analysis of variance. The result gives quantitative estimation of the various design parameters affecting the performance and helps to determine the main factors for the optimum design of a stratified thermal storage tank. In the range of parameters considered, the Reynolds number is found to be the most dominant parameter. Moreover, the diffuser shape plays a significant role on the performance of a stratified thermal storage tank.
Article
The flow and heat transfer characteristics in a cylindrical hot water store during the charging process under adiabatic thermal boundary conditions were studied numerically in the present paper. The charging efficiency was used to evaluate the thermal stratification. The emphasis was put on the effects of charging temperature differences, charging velocities, charging flow rates and length to diameter ratios on the charging efficiency. The results were summarized both in dimensional and dimensionless forms. They indicate that the charging efficiency depends mainly on the modified Richardson number RiH,f and Peclet number PeH,f, which present the combined effects of charging temperature difference and charging velocity on the charging efficiency. If RiH,f is larger than 0.25, the charging efficiency is above 97%. At a given Richardson number the increase of Peclet number leads to a higher charging efficiency. For H/D less than 4, the increase of the height to diameter ratio H/D can improve the charging efficiency as well. The effect of the Fourier number (or charging flow rate) on the charging efficiency, however, is relatively small. A correlation of the numerical results was obtained for the design of effective hot water stores.
Article
An irreversible cycle model of a regenerative-intercooled-reheat Brayton heat engine along with a detailed parametric study is presented in this paper. The power output and the efficiency are optimized with respect to the cycle temperatures for a typical set of operating conditions. It is found that there are optimal values of the turbine outlet temperature, intercooling, reheat and cycle pressure ratios at which the cycle attains the maximum power output and efficiency. But the optimal values of these parameters corresponding to the maximum power output are different from those corresponding to the maximum efficiency for the same set of operating condition. The maxima of the power output and efficiency again changes as any of the cycle parameters is changed. The maximum power point and the maximum efficiency point exist but the power output corresponding to the maximum efficiency is found to be lower than that can be attained. The optimum operating parameters, such as the turbine outlet temperature, intercooling, reheat and cycle pressure ratios etc. corresponding to the maximum power output and corresponding to the maximum efficiency are obtained and discussed in detail. This cycle model is general and some of the results obtained by earlier workers can be derived directly from the present cycle model as a special case.
Article
A simple model for predicting the performance of reciprocating chillers is developed. The basic irreversibilities that mitigate against fast and slow cooling rates are accounted for. The model proposed here is consistent with real performance data for 30 chillers that span a range of cooling rates from 30 to 1300 kW and, with adjustable parameters that characterize a particular chiller, is shown to be capable of reproducing actual performance data to within better than experimental uncertainty.
Article
In a theoretical and experimental investigation into the thermal performance of stratified hot water stores, a transient three-dimensional finite-volume based model was validated by comparison with measured temperatures from a series of thirty-two experiments in which the inlet velocities, temperatures and initial store stratification profiles were varied. A parametric analysis ascertained the effect of inlet and outlet port locations on store performance for a range of operating conditions. The effects of finite volume size on predicted levels of entrainment and diffusion in the inlet region are reported.
Article
A theoretical model is presented which predicts the effects of storage tank stratification on the instantaneous performance of a liquid-based solar heating system. The results are presented in terms of a stratification coefficient which is defined to be the ratio of the actual useful energy gain to the energy gain that would be achieved in a fully mixed tank. This stratification coefficient is shown to be a system constant which depends on only two dimensionless system parameters. The closed form model is compared with a detailed numerical simulation and also with experimental data taken with a solar water heater. Both the simulation data and the experimental data agree favorably with the theoretical model.
Article
Researchers have discovered that by using low collector flow rates (roughly one-seventh of those that have been generally used) and by taking measures to ensure the water in the storage tank remains stratified, the energy delivered by a forced-flow solar system can be increased substantially. In addition, the lower collector flow rates permit substantial savings in system cost, mainly through reduction in plumbing costs. This paper reviews the state of the art in this highly promising change in approach. Items discussed include physical reasons for the predicted performance improvement, savings in system cost, current knowledge in tank stratification, methods of analyzing and computer-simulating the systems, and finally, full-scale system experiments.
Article
In this paper, finite-time thermodynamics has been applied to ecologically optimize the power output of closed Brayton cycles for infinite thermal capacitance rates of the heat reservoirs. The optimum values of power, thermal efficiency and second-law efficiency of Brayton cycles are presented. The ratio of ecologically optimum power to maximum power is independent of the numbers of transfer units of the hot-side and the cold-side heat exchangers, and this ratio is much higher than the ratio of the entropy generation rate at maximum ecological function to that at maximum power. With the introduction of the ecological function, the improvement in second-law efficiency and in thermal efficiency is evident, especially for low hot-cold temperature ratios. Moreover, the thermal efficiency at maximum ecological function is about the average of the maximum-power efficiency and the reversible Carnot efficiency.
Article
It is acknowledged that the Curzon–Ahlborn efficiency ηCA determines the efficiency at maximum power production of heat engines only affected by the irreversibility of finite rate heat transfer (endoreversible engines), but ηCA is not the upper bound of the efficiencies of heat engines. This is conceptually different from the role of the Carnot efficiency ηC which is indeed the upper bound of the efficiencies of all heat engines. Some authors have erroneously criticized ηCA as if it were the upper bound of the efficiencies of endoreversible heat engines. Although the efficiencies of real heat engines cannot attain the Carnot efficiency, it is possible, and often desirable, for their efficiencies to be larger than their respective maximum power efficiencies. In fact, the maximum power efficiency is the allowable lower bound of the efficiency for a given class of heat engines. These important conclusions may be expounded clearly by the theory of finite time thermodynamics.
Article
Analysis of a stratified store thermocline entrainment process is done by a side-by-side comparison of experiments and a direct numerical simulation. The experimental and numerical analyses are shown to be complementary; where the experiment is a true realisation of the thermocline entrainment process, the numerical simulation provides the temporal and spatial resolution for detailed analysis. The agreement between both is good. It appears that during collision of the buckling jet with the thermocline inhomogeneous penetration and back flow occur. At the point where the jet flows back into the bottom layer, upper-layer fluid is dragged into the bottom layer. After detachment, the dragged-down fluid filament becomes unstable due to overturning motions, resulting in Kelvin–Helmholtz-like waves. Subsequently the fluid filament is completely mixed with bottom-layer fluid by the action of stretching- and-folding stimulated diffusion. From a comparison between 2D- and 3D-simulation results it appeared that for store optimisation 2D-numerical simulations provide sufficient accuracy.
Article
The diagnostic capability of a simple thermodynamic model for chiller performance is illustrated by a case study for a commercial, installed centrifugal chiller. Performance data were measured both prior to, and subsequent to, chiller maintenance that improved chiller efficiency. Using these experimental measurements, we show that the simple thermodynamic model, originally developed for reciprocating and absorption chillers, (1) succeeds in predicting the fundamental relation between coefficient of performance and cooling rate for the centrifugal chiller, and (2) permits a clear diagnostic analysis of heat exchanger fouling on chiller performance.RésuméOn démontre, par le biais d'une étude de cas d'un refroidisseur centrifuge commercial, déjà installé, les possibilités d'un modèle thermodynamique simple pour l'évaluation des performances des refroidisseurs. On a mesuré les données de performance avant et après une opération d'entretien qui a amélioré le rendement du refroidisseur. Avec l'appui de ces mesures expérimentales, nous montrons que le modèle thermodynamique simple, mis au point, à l'origine, pour les refroidisseurs à piston et à absorption, permet, d'une part, de prévoir la relation fondamentale entre le coefficient de performance et la puissance frigorifique pour le refroidisseur centrifuge et, d'autre part, d'effectuer un diagnostic clair de l'effet de l'encrassement des échangeurs de chaleur sur la performance du refroidisseur.
Article
The thermal performance of solar heating systems is strongly influenced by the thermal stratification in the heat storage. The higher the degree of thermal stratification is, the higher the thermal performance of the solar heating systems. Thermal stratification in water storage can for instance be achieved by use of inlet stratifiers combined with low flow operation in the solar collector loop. In this paper, investigation of a number of different fabric stratification pipes is presented and compared to a non-flexible inlet stratifier. Additional, detailed investigation of the flow structure close to two fabric stratification pipes is presented for one set of operating conditions by means of the optical PIV (Particle Image Velocimetry) method.
Break-even transients for two simple recuperated Brayton test configurations
  • S Wright
  • T Conboy
  • G Rochau
S. Wright, T. Conboy, G. Rochau, Break-even transients for two simple recuperated Brayton test configurations, in: Supercritical CO2 Power Cycle Symposium, May 24 and 25, Arvada, CO, 2011.
Initial split-flow operations of a supercritical CO2 recompression Brayton cycle
  • T Conboy
  • J Pasch
T. Conboy, J. Pasch, Initial split-flow operations of a supercritical CO2 recompression Brayton cycle, Transactions of American Nuclear Society 106 (2012) 593-596.