27 results found
Olympios AV, Pantaleo AM, Sapin P, et al., 2020, On the value of combined heat and power (CHP) systems and heat pumps in centralised and distributed heating systems: Lessons from multi-fidelity modelling approaches, Applied Energy, Vol: 274, Pages: 1-19, ISSN: 0306-2619
This paper presents a multi-scale framework for the design and comparison of centralised and distributed heat generation solutions. An extensive analysis of commercially available products on the UK market is conducted to gather information on the performance and cost of a range of gas-fired combined heat and power (CHP) systems, air-source heat pumps (ASHPs) and ground-source heat pumps (GSHPs). Data-driven models with associated uncertainty bounds are derived from the collected data, which capture cost and performance variations with scale (i.e., size and rating) and operating conditions. In addition, a comprehensive thermoeconomic (thermodynamic and component-costing) heat pump model, validated against manufacturer data, is developed to capture design-related performance and cost variations, thus reducing technology-related model uncertainties. The novelty of this paper lies in the use of multi-fidelity approaches for the comparison of the economic and environmental potential of important heat-generation solutions: (i) centralised gas-fired CHP systems associated with district heating network; (ii) gas-fired CHP systems or GSHPs providing heat to differentiated energy communities; and (iii) small-scale micro-CHP systems, ASHPs or GSHPs, installed at the household level. The pathways are evaluated for the case of the Isle of Dogs district in London, UK. A centralised CHP system appears as the most profitable option, achieving annual savings of £13 M compared to the use of decentralised boilers and a levelised cost of heat equal to 31 £/MWhth. However, if the carbon intensity of the electrical grid continues to reduce at current rates, CHP systems will only provide minimal carbon savings compared to boilers (<6%), with heat pumps achieving significant heat decarbonisation (55–62%). Differentiating between high- and low-performance and cost heat pump designs shows that the former, although 25% more expensive, have significantly lower annualised
Olympios A, Hoisenpoori P, Mersch M, et al., 2020, Optimal design of low-temperature heat-pumping technologies and implications to the whole energy system, The 33rd International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems.
This paper presents a methodology for identifying optimal designs for air-source heat pumps suitable for domestic heating applications from the whole-energy system perspective, accounting explicitly for a trade-off between cost and efficiency, as well as for the influence of the outside air temperature during off-design operation. The work combines dedicated brazed-plate and plate-fin heat-exchanger models with compressor efficiency maps, as well as equipment costing techniques, in order to develop a comprehensive technoeconomic model of a low-temperature air-source heat pump with a single-stage-compressor, based on the vapour-compression cycle. The cost and performance predictions are validated against manufacturer data and a non-linear thermodynamic optimisation model is developed to obtain optimal component sizes for a set of competing working fluids and design conditions. The cost and off-design performance of different configurations are integrated into a whole-energy system capacity-expansion and unit-dispatch model of the UK power and heat system. The aim is to assess the system value of proposed designs, as well as the implications of their deployment on the power generation mix and total transition cost of electrifying domestic heat in the UK as a pathway towards meeting a national net-zero emission target by 2050. Refrigerant R152a appears to have the best design and off-design performance, especially compared to the commonly used R410a. The size of the heat exchangers has a major effect on heat pump performance and cost. From a wholesystem perspective, high-performance heat pumps enable a ~20 GW (~10%) reduction in the required installed power generation capacity compared to smaller-heat-exchanger, low-performance heat pumps, which in turn requires lower and more realistic power-grid expansion rates. However, it is shown that the improved performance as a result of larger heat exchangers does not compensate overall for the increased technology cost, with
Sapin P, Simpson M, Olympios A, et al., 2020, Cost-benefit analysis of reversible reciprocating-piston engines with adjustable volume ratio in pumped thermal electricity storage, 33rd International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems (ECOS 2020), Publisher: ECOS
Decarbonisation of heating, cooling and/or power services through the utilisation of renewable en-ergy sources relies on the development of efficient and economically-viable energy storage technolo-gies, ideally without geographical constraints. Pumped thermal electricity storage (PTES) is a strongcandidate technology – along with reversible Rankine cycle, (advanced adiabatic) compressed airenergy storage (CAES), and liquid air energy storage (LAES). One of the leading PTES variants isthe reversible Joule-Brayton cycle engine, where energy is stored as sensible heat in hot and coldthermal stores, while the temperature difference is achieved through gas compression and expansionprocesses. For cost reasons, and to achieve high round-trip efficiencies, it is advantageous for thecompression and expansion machines used in PTES plants to be reversible. Positive-displacementdevices offer this possibility. In particular, recent developments in pneumatically or electromagneti-cally actuated intake and exhaust valves could pave the way for high-efficiency reversible reciprocat-ing compression-expansion devices based on variable-valve control in real time. Advanced variablevalve timing (VVT) is a promising feature that allows piston machines not only to be operated bothas reversible compression and expansion devices, but also to maintain high efficiencies over a widerange of operating conditions, thanks to the possibility of adjusting the built-in volume ratio of a par-ticular machine. With enhanced part-load performance, such disruptive piston machines offer greatpotential for round-trip efficiency enhancement and cost minimisation of PTES storage plants. In thiswork, a cost-benefit analysis of innovative VVT-fitted reciprocating-piston technology is performedusing: (i) comprehensive dynamic reduced-order models to predict the compressor-expander perfor-mance for design optimisation, and (ii) Schumann-style one-dimensional models for simulating heatand mass transf
Nemati H, Moghimi MA, Sapin P, et al., 2020, Shape optimisation of air-cooled finned-tube heat exchangers, International Journal of Thermal Sciences, Vol: 150, Pages: 1-14, ISSN: 1290-0729
The use of annular fins in air-cooled heat exchangers is a well-known solution, commonly used in air-conditioning and heat-recovery systems, for enhancing the air-side heat transfer. Although associated with additional material and manufacturing costs, custom-designed finned-tube heat exchangers can be cost-effective. In this article, the shape of the annular fins in a multi-row air heat exchanger is optimised in order to enhance performance without incurring a manufacturing cost penalty. The air-side heat transfer, pressure drop and entropy generation in a regular, four-row heat exchanger are predicted using a steady-state turbulent CFD model and validated against experimental data. The validated simulation tool is then used to perform model-based optimisation of the fin shapes. The originality of the proposed approach lies in optimising the shape of each fin row individually, resulting in a non-homogenous custom bundle of tubes. Evidence of this local-optimisation potential is first provided by a short preliminary study, followed by four distinct optimisation studies (with four distinct objective functions), aimed at addressing the major problems faced by designers. Response-surface methods – namely, NLPQL for single-objective and MOGA for multi-objective optimisations – are used to determine the optimum configuration for each optimisation strategy. It is shown that elliptical annular-shaped fins minimise the pressure drop and entropy generation, while circular-shaped fins at the entrance region (i.e., first row) can be employed to maximise heat transfer. The results also show that, for the scenario in which the total heat transfer rate is maximised and the pressure drop minimised, the pressure drop is reduced by up to 31%, the fin weight is reduced up to 23%, with as little as a 14% decrease in the total air-side heat transfer, relative to the case in which all the fins across the tube bundle are circular. Moreover, in all optimised cases, the entropy
Kadivar MR, Moghimi MA, Sapin P, et al., 2019, Annulus eccentricity optimisation of a phase-change material (PCM) horizontal double-pipe thermal energy store, Journal of Energy Storage, Vol: 26, ISSN: 2352-152X
The application of phase-change materials (PCMs) has received significant interest for use in thermal energy storage (TES) systems that can adjust the mismatch between the energy availability and demand. In the building sector, for example, PCMs can be used to reduce air-conditioning energy consumption by increasing the thermal capacity of the walls. However, as promising this technology may be, the poor thermal conductivity of PCMs has acted as a barrier to its commercialization, with many heat-transfer enhancement solutions proposed in the literature, such as microencapsulation or metal foam inserts, being either too costly and/or complex. The present study focuses on a low-cost and highly practical solution, in which natural-convective heat transfer is enhanced by placing the PCM in an eccentric annulus within a horizontal double-pipe TES heat exchanger. This paper presents an annulus-eccentricity optimisation study, whereby the optimal radial and tangential eccentricities are determined to minimize the charging time of a PCM thermal energy store. The storage performance of several geometrical configurations is predicted using a computational fluid dynamics (CFD) model based on the enthalpy-porosity formulation. The optimal geometrical configuration is then determined with response surface methods. The horizontal double-pipe heat exchanger studied considered here is an annulus filled with N-eicosane as the PCM for initial studies. In presence of N-eicosane, for the concentric configuration (which is the baseline case), the charging is completed at Fo = 0.64, while the charging of optimum eccentric geometries with the quickest and slowest charging is completed at Fo = 0.09 and Fo = 2.31, respectively. In addition, an investigation on the discharging performance of the studied configurations with N-eicosane shows the quickest discharge occurs with the concentric annulus case at Fo = 0.99, while the discharge time of the proposed optimum annuli is about three times
Pantaleo A, Simpson M, Rotolo G, et al., 2019, Thermoeconomic optimisation of small-scale organic Rankine cycle systems based on screw vs. piston expander maps in waste heat recovery applications, Energy Conversion and Management, Vol: 200, ISSN: 0196-8904
The high cost of organic Rankine cycle (ORC) systems is a key barrier to their implementation in waste heat recovery (WHR) applications. In particular, the choice ofexpansion device has a significant influence on this cost, strongly affecting the economic viabilityof an installation. In this work, numerical simulations and optimisation strategies are used to compare the performance and profitability of small-scale ORC systems using reciprocating-piston orsingle/two-stage screw expanders whenre covering heat from the exhaust gases of a 185-kWinternal combustion engine operating in baseload mode. The study goes beyond previous work by directly comparingthese small-scaleexpanders fora broad range of working fluids, and by exploring the sensitivity of project viability to key parameters such as electricity price and onsite heat demand.For the piston expander, a lumped-massmodel and optimisation based on artificial neural networks are used to generate performance maps, while performance and cost correlations from the literature are used for the screw expanders. The thermodynamic analysisshows that two-stage screw expanders typically deliver more power than either single-stage screw or piston expanders due to their higher conversion efficiencyat the required pressure ratios. The best fluids areacetone and ethanol, as these provide a compromise between the exergy losses in the condenser and in the evaporatorin this application. The maximum net power output isfound to be 17.7kW, from an ORC engine operating withacetone anda two-stage screw expander. On the other hand, the thermoeconomic optimisation shows that reciprocating-piston expandersshow a potential for lowerspecific costs, and sincesuchan expander technology is not mature, especially at these scales, this finding motivates further consideration of this component. A minimum specific investment cost of 1630€/kW is observed for an ORC engine with a pisto
Simpson M, Chatzopoulou M, Oyewunmi O, et al., 2019, Technoeconomic analysis of internal combustion engine - organic Rankine cycle systems for combined heat and power in energy-intensive buildings, Applied Energy, Vol: 253, Pages: 1-13, ISSN: 0306-2619
For buildings with low heat-to-power demand ratios, installation of internal combustion engines (ICEs) for combined heat and power (CHP) results in large amounts of unused heat. In the UK, such installations risk being ineligible for the CHP Quality Assurance (CHPQA) programme and incurring additional levies. A technoeconomic optimisation of small-scale organic Rankine cycle (ORC) engines is performed, in which the ORC engines recover heat from the ICE exhaust gases to increase the total efficiency and meet CHPQA requirements. Two competing system configurations are assessed. In the first, the ORC engine also recovers heat from the CHP-ICE jacket water to generate additional power. In the second, the ORC engine operates at a higher condensing temperature, which prohibits jacket-water heat recovery but allows heat from the condenser to be delivered to the building. When optimised for minimum specific investment cost, the first configuration is initially found to deliver 20% more power (25.8 kW) at design conditions, and a minimum specific investment cost (1600 £/kW) that is 8% lower than the second configuration. However, the first configuration leads to less heat from the CHP-ICE being supplied to the building, increasing the cost of meeting the heat demand. By establishing part-load performance curves for both the CHP-ICE and ORC engines, the economic benefits from realistic operation can be evaluated. The present study goes beyond previous work by testing the configurations against a comprehensive database of real historical electricity and heating demand for thirty energy-intensive buildings at half-hour resolution. The discounted payback period for the second configuration is found to lie between 3.5 and 7.5 years for all of the buildings considered, while the first configuration is seen to recoup its costs for only 23% of the buildings. The broad applicability of the second configuration offers attractive opportunities to increase manufacturing volumes an
Unamba CK, Sapin P, Li X, et al., 2019, Operational optimisation of a non-recuperative 1-kWe organic Rankine cycle engine prototype, Applied Sciences, Vol: 9, Pages: 3024-3024, ISSN: 2076-3417
Several heat-to-power conversion technologies are being proposed as suitable for waste-heat recovery (WHR) applications, including thermoelectric generators, hot-air (e.g., Ericsson or Stirling) engines and vapour-cycle engines such as steam or organic Rankine cycle (ORC) power systems. The latter technology has demonstrated the highest efficiencies at small and intermediate scales and low to medium heat-source temperatures and is considered a suitable option for WHR in relevant applications. However, ORC systems experience variations in performance at part-load or off-design conditions, which need to be predicted accurately by empirical or physics-based models if one is to assess accurately the techno-economic potential of such ORC-WHR solutions. This paper presents results from an experimental investigation of the part-load performance of a 1-kWe ORC engine, operated with R245fa as a working fluid, with the aim of producing high-fidelity steady-state and transient data relating to the operational performance of this system. The experimental apparatus is composed of a rotary-vane pump, brazed-plate evaporator and condenser units and a scroll expander magnetically coupled to a generator with an adjustable resistive load. An electric heater is used to provide a hot oil-stream to the evaporator, supplied at three different temperatures in the current study: 100, 120 and 140 ∘ C. The optimal operating conditions, that is, pump speed and expander load, are determined at various heat-source conditions, thus resulting in a total of 124 steady-state data points used to analyse the part-load performance of the engine. A maximum thermal efficiency of 4.2 ± 0.1% is reported for a heat-source temperature of 120 ∘ C, while a maximum net power output of 508 ± 2 W is obtained for a heat-source temperature at 140 ∘ C. For a 100- ∘ C heat source, a maximum exergy efficiency of 18.7 ± 0.3% is achieved. A detailed exergy analysis all
Li X, Song J, Simpson M, et al., 2019, THERMO-ECONOMIC COMPARISON OF ORGANIC RANKINE AND CO2 CYCLE SYSTEMS FOR LOW-TO-MEDIUM TEMPERATURE APPLICATIONS, 5th International Seminar on ORC Power Systems
Olympios A, Pantaleo AM, Sapin P, et al., 2019, Centralised vs distributed energy systems options: District heating for the Isle of Dogs in London, ICAE2019: The 11th International Conference on Applied Energy
This work focuses on a multi-scale framework for the design and comparison of low-carbon heat generation solutions to serve the residential and commercial thermal energy demand of high energy density urban areas. The adopted methodology assesses the cost and performance of four configurations integrated in a district heating network: (i) centralised cogeneration with gas turbine and bottoming steam turbine with flexible heat-to-electricity ratio; (ii) centralised cogeneration with gas-fired internal combustion engine; (iii) distributed building-integrated ground-source heat pumps for domestic hot water only; and (iv) distributed building-integrated ground-source heat pumps for both domestic hot water and space heating. Cost and performance data were obtained by conducting relevant market research and developing a simplified heat pump thermodynamic model. The different configurations are evaluated utilizing whole-year space heating and hot water demand profiles for the Isle of Dogs area in East London, UK. Scale effects are included by considering various technology size scenarios and the results indicate that a 50 MW centralised internal combustion cogeneration system appears to be the most profitable option, while the competitiveness of building-integrated heat pumps is dependent on their size.
Simpson M, Schuster S, Ibrahim D, et al., 2019, Small-scale, low-temperature ORC systems intime-varying operation: Turbines orreciprocating-piston expanders?, 32ND INTERNATIONAL CONFERENCE ON EFFICIENCY, COST, OPTIMIZATION, SIMULATION AND ENVIRONMENTAL IMPACT OF ENERGY SYSTEMS
Unamba C, Li X, Song J, et al., 2019, Off-design performance of a 1-kWe organic Rankine cycle (ORC) system, 32nd International Conference on Efficiency, Costs, Optimization, Simulation and Environmental Impact of Energy Systems (ECOS 2019), Publisher: ECOS
Several heat-to-power conversion technologies are being proposed as suitable for waste heat recovery (WHR) applications, including thermoelectric generators, hot-air (e.g., Ericsson or Stirling) engines, and vapour-cycle engines such as steam or organic Rankine cycle (ORC) power systems. The latter has demonstrated the highest efficiencies at low and intermediate scales and heat-source temperatures. However, ORC systems suffer a deterioration in performance at part-load or off-design conditions, and the high global warming potential (GWP) or flammability of common working fluids is an increasing concern. This paper presents the experimental investigation of a 1-kWe ORC test facility under time-varying heat-source conditions. It aims to compare the part-load performance of various architectures with different working fluids, namely: (i) R245fa, which is widely used in ORC systems, and (ii) low-GWP HFOs. The experimental apparatus is composed of a rotary-vane pump, brazed-plate evaporators and condensers, and a scroll expander with an adjustable load. An electric heater is used to provide a hot oil stream at three different temperatures: 80, 100 and 120 °C. The optimal operating conditions, i.e., pump speed and expander load, are determined for each architecture at various heat-source conditions. A maximum thermal efficiency of 2.8% is reported for a heat-source temperature of 100 °C, while a maximum net power output of 430 W is obtained for a heat source at 120 °C. An exergy analysis allows us to quantify the contribution of each component to the overall exergy destruction. The share of the evaporator, condenser and expander units remain major for all three heat-source conditions, while the exergy destroyed in the pump is negligible in comparison (below 4%).
Chatzopoulou MA, Simpson M, Sapin P, et al., 2019, Off-design optimisation of organic Rankine cycle (ORC) engines with pistonexpanders for medium-scale combined heat and power applications, Applied Energy, Vol: 238, Pages: 1211-1236, ISSN: 0306-2619
Organic Rankine cycle (ORC) engines often operate under variable heat-source conditions, so maximising performance at both nominal and off-design operation is crucial for the wider adoption of this technology. In this work, an off-design optimisation tool is developed and used to predict the impact of varying heat-source conditions on ORC operation. Unlike previous efforts where the performance of ORC engine components is assumed fixed, here we consider explicitly the time-varying operational characteristics of these components. A bottoming ORC system is first optimised for maximum power output when recovering heat from the exhaust gases of an internal-combustion engine (ICE) running at full load. A double-pipe heat exchanger (HEX) model is used for sizing the ORC evaporator and condenser, and a piston-expander model for sizing the expander. The ICE is then run at part-load, thus varying the temperature and mass flow rate of the exhaust gases. The tool predicts the new off-design heat transfer coefficients in the heat exchangers, and the new optimum expander operating points. Results reveal that the ORC engine power output is underestimated by up to 17% when the off-design operational characteristics of these components are not considered. In particular, the piston-expander isentropic efficiency increases at off-design operation by 10–16%, due to the reduced pressure ratio and flow rate in the system, while the evaporator effectiveness improves by up to 15%, due to the higher temperature difference across the HEX and a higher proportion of heat transfer taking place in the two-phase evaporating zone. As the ICE operates further away from its nominal point, the off-design ORC engine power output reduces by a lesser extent than that of the ICE. At an ICE part-load operation of 60% (by electrical power), the optimised ORC engine with fluids such as R1233zd operates at 77% of its nominal capacity. ORC off-design performance maps are generated, for characterising a
Chatzopoulou MA, Sapin P, Markides C, 2018, Optimisation of off-design internal combustion-organic Rankine engine combined cycles, ECOS 2018 - 31st International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, Publisher: ECOS
Organic Rankine cycle (ORC) enginesare an efficient means of convertinglow-to-medium renewable orwaste heat to useful power. In practicalapplications, ORC systemsexperience varying thermalinputprofile,due to the dynamic nature of realheat sources. Maximisingthe uptake of this technology requiresoptimisedORC designsand sizing tomaintain high efficiencyand power output,not only at full-load operation, but also under off-design conditions. Key for maintaining the efficient operationof the systemis the maximisation of heat extraction from the heat source, inthe ORC evaporator. In this paper, the off-design operation of an ICE-ORC combined heat and power (CHP) system is investigated, to optimise the ORC performance under varying ICE load conditions. First, the ORC enginethermodynamic design is optimised for the 100% load operation of the ICE. Alternative working fluids are investigated, including low ODP/GWP refrigerants and hydrocarbons. The ORC system is then sized using two different heat exchanger (HEX) architectures; tube-in-tube (DPHEX) and plate (PHEX) designs, at designconditions. The sizing results reveal that the PHEX area requirements are almost 50% lower than the respective ones for DPHEX, while recovering equivalent quantities of heat. Next, the ORC engine operation is optimised atpart-load ICE conditions, and the HEX heat transfer coefficients (HTCs) are predicted. Results indicate that: i) PHEX HTCs are up to 50% higher than DPHEX equivalents;ii)HTCsdecreaseat part load for both HEXs, but because the average temperaturedifferenceincreases, the overall HEX effectiveness improves; and iii) the ORC system with a PHEX evaporator has slightly higher power output thanthe DPHEX equivalent at off-design operation.Overall, the modelling tool developed here can predict ORC performance over an operating envelopeand allows the selection ofoptimal designsand si
Sapin P, Simpson M, Kirmse C, et al., 2018, Dynamic modeling of water-droplet spray injection in reciprocating-piston compressors, ECOS 2018 - 31st International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems
Sapin P, Simpson M, Kirmse C, et al., 2018, A lumped-mass analysis of water evaporation in reciprocating-piston compressors
Simpson M, Sapin P, Rotolo G, et al., 2017, Efficiency map of reciprocating-piston expanders for ORC applications, 4th Annual Engine Organic Rankine Cycle Consortium Workshop 2017
Unamba CK, White M, Sapin P, et al., 2017, Experimental investigation of the operating point of a 1-kW ORC system, 4th International Seminar on ORC Power Systems (ORC), Publisher: Elsevier Science BV, Pages: 875-882, ISSN: 1876-6102
The organic Rankine cycle (ORC) is a promising technology for the conversion of waste heat from industrial processes as well as heat from renewable sources. Many efforts have been channeled towards maximizing the thermodynamic potential of ORC systems through the selection of working fluids and the optimal choice of operating parameters with the aim of improving overall system designs, and the selection and further development of key components. Nevertheless, experimental work has typically lagged behind modelling efforts. In this paper, we present results from tests on a small-scale (1 kWel) ORC engine consisting of a rotary-vane pump, a brazed-plate evaporator and a brazed-plate condenser, a scroll expander with a built-in volume ratio of 3.5, and using R245fa as the working fluid. An electric oil-heater acted as the heat source, providing hot oil at temperatures in the range 120-140 °C. The frequency of the expander was not imposed by an inverter or the electricity grid but depended directly on the attached generator load; both the electrical load on the generator and the pump rotational speed were varied in order to investigate the performance of the system. Based on the generated data, this paper explores the relationship between the operating conditions of the ORC engine and changes in the heat-source temperature, pump and expander speeds leading to working fluid flow rates between 0.0088 kg/s and 0.0337 kg/s, from which performance maps are derived. The experimental data is, in turn, used to assess the performance of both the individual components and of the system, with the help of an exergy analysis. In particular, the exergy analysis indicates that the expander accounts for the second highest loss in the system. Analysis of the results suggests that increased heat-source temperatures, working-fluid flow rates, higher pressure ratios and larger generator loads improve the overall cycle efficiency. Specifically, a 46% increase in pressure ratio from 2.4
Simpson M, Rotolo G, Sapin P, et al., 2017, Thermodynamic performance maps of reciprocating-piston expanders for operation at off-design and part-load conditions, 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Publisher: ICHMT
Renewable energy sources, such as solar-thermal or geothermal heat, and low-/medium-grade industrial waste-heat can be converted into useful power and/or heating with a variety of technologies, including organic Rankine cycle (ORC) and vapour-compression heat-pump systems. The thermodynamic performance and cost of these technologies depends crucially on the efficiency of key components, including the compressor or expander used. Reciprocating-piston machines can be advantageous over turbomachines and other positive-displacement machines at intermediate scales (~10s-100s of kW) thanks to their ability to operate with relatively high isentropic efficiencies at large expansion ratios. However, modelling the thermodynamic losses in reciprocating-piston expanders, with a view towards designing high-performance machines, is a complex undertaking. The aim of this paper is to develop a spatially-lumped, yet dynamic model of a piston expander suitable for early-stage engineering design, that can provide simplification without sacrificing accuracy. The unsteady heat transfer between the gas and the cylinder walls, and the mass leakage are predicted independently with correlations available in the literature and simplified one-dimensional models, respectively. However, the turbulence induced by the mass intake through the piston rings can affect the gas-to-wall heat transfer. In order to address this dependency two complementary approaches are used. Compression and expansion processes are simulated in a gas spring configuration (i.e. without valve systems) using a computational fluid dynamics (CFD) model developed using the open-source code OpenFOAM, where the loss mechanisms are solved directly. The results are then compared with predictions from the heuristic lumped model based on heat tr
Sapin PMC, Simpson M, White AJ, et al., 2017, Lumped dynamic analysis and design of a high-performance reciprocating-piston expander, 30th International Conference on Efficiency, Cost, Optimisation, Simulation and Environmental Impact of Energy Systems., Publisher: ECOS
A spatially-lumped dynamic model of a reciprocating-piston expander is presented in this paper. The model accounts for the three main loss mechanisms in realistic piston machines, namely: pressure losses through the intake and exhaust valves, heat transfer between the gas and the surrounding cylinder walls, and the mass leakagebetween the compression/expansion chamber and the crankcasethrough the piston rings. The model also accounts for real-gas effects with the fluid properties calculated from the NIST database using REFPROP. The numerical calculations are first compared with experimental pressure-volume-temperature data obtained on a custom reciprocating-piston gas spring over a range of oscillation frequencies. The comparison between numerical and experimental results shows good agreement. It also allows the most accurate heat transfer correlationto be selectedfor calculating the gas-to-wallin-cylinderheat transfer. The semi-heuristic modelling tool is thenused to design an expander forspecific pressure ratiosand mass flowrate, and to predict the thermodynamic performance of the piston device over arange of part-load conditions.
Taleb A, Barfuss C, Sapin P, et al., 2017, CFD analysis of thermally induced thermodynamic losses in the reciprocating compression and expansion of real gases, 1st International Seminar on Non-Ideal Compressible-Fluid Dynamics for Propulsion & Power
Taleb AI, Sapin PMC, Barfuß C, et al., 2017, CFD Analysis of Thermally Induced Thermodynamic Loses in the Reciprocating Compression and Expansion of Real Gases, 1st International Seminar on Non-Ideal Compressible-Fluid Dynamics for Propulsion and Power, Publisher: IOP Publishing: Conference Series, ISSN: 1742-6588
The efficiency of expanders is of prime importance in determining the overallperformance of a variety of thermodynamic power systems, with reciprocating-piston expandersfavoured at intermediate-scales of application (typically 10–100 kW). Once the mechanical lossesin reciprocating machines are minimized (e.g. through careful valve design and operation), lossesdue to the unsteady thermal-energy exchange between the working fluid and the solid walls ofthe containing device can become the dominant loss mechanism. In this work, gas-spring devicesare investigated numerically in order to focus explicitly on the thermodynamic losses that arisedue to this unsteady heat transfer. The specific aim of the study is to investigate the behaviourof real gases in gas springs and to compare this to that of ideal gases in order to attain a betterunderstanding of the impact of real-gas effects on the thermally induced losses in reciprocatingexpanders and compressors. A CFD-model of a gas spring is developed in OpenFOAM. Threedifferent fluid models are compared: (1) an ideal-gas model with constant thermodynamicand transport properties; (2) an ideal-gas model with temperature-dependent properties; and(3) a real-gas model using the Peng-Robinson equation-of-state with temperature and pressure-dependent properties. Results indicate that, for simple, mono- and diatomic gases, like helium ornitrogen, there is a negligible difference in the pressure and temperature oscillations over a cyclebetween the ideal and real-gas models. However, when considering heavier (organic) molecules,such as propane, the ideal-gas model tends to overestimate the pressure compared to the real-gasmodel, especially if the temperature and pressure dependency of the thermodynamic propertiesis not taken into account. In fact, the ideal-gas model predicts higher pressures by as much as25% (compared to the real-gas model). Additionally, both ideal-gas models underestimate thethermall
Sapin P, Taleb A, White AJ, et al., 2016, EXPERIMENTAL ANALYSIS OF LOSS MECHANISMS IN A GAS SPRING, ASME Power and Energy Conference, Publisher: ASME, Pages: ES2016-59631-ES2016-59631
Reciprocating-piston compressors and expanders arepromising solutions to achieve higher overall efficiencies invarious energy storage solutions. This article presents anexperimental study of the exergetic losses in a gas spring. Consideringa valveless piston-cylinder system allows us to focuson the thermodynamic losses due to thermal-energy exchangeprocesses in reciprocating components. To differentiate this latterloss mechanism from mass leakages or frictional dissipation,three bulk parameters are measured. Pressure and volume arerespectively measured with a pressure transducer and a rotarysensor. The gas temperature is estimated by measuring theTime-Of-Flight (TOF) of an ultrasonic pulse signal across thegas chamber. This technique has the advantage of being fast andnon-invasive. The measurement of three bulk parameters allowsus to calculate the work as well as the heat losses throughouta cycle. The thermodynamic loss is also measured for differentrotational speeds. The results are in good agreement withprevious experimental studies and can be employed to validateCFD or analytical studies currently under development
Taleb A, Sapin P, Barfuß C, et al., 2016, Wall temperature and system mass effects in a reciprocating gas spring, INTERNATIONAL CONFERENCE ON EFFICIENCY, COST, OPTIMIZATION, SIMULATION AND ENVIRONMENTAL IMPACT OF ENERGY SYSTEMS
Sapin P, Taleb A, Barfuß C, et al., 2016, Thermodynamic Losses in a Gas Spring: Comparison of Experimental and Numerical Results, International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics
Reciprocating-piston devices can be used as high-efficiencycompressors and/or expanders. With an optimal valve design andby carefully adjusting valve timing, pressure losses during intakeand exhaust can be largely reduced. The main loss mechanismin reciprocating devices is then the thermal irreversibility dueto the unsteady heat transfer between the compressed/expandedgas and the surrounding cylinder walls. In this paper, pressure,volume and temperature measurements in a piston-cylindercrankshaft driven gas spring are compared to numerical results.The experimental apparatus experiences mass leakage while theCFD code predicts heat transfer in an ideal closed gas spring.Comparison of experimental and numerical results allows one tobetter understand the loss mechanisms in play. Heat and masslosses in the experiment are decoupled and the system lossesare calculated over a range of frequencies. As expected, compressionand expansion approach adiabatic processes for higherfrequencies, resulting in higher efficiency. The objective of thisstudy is to observe and explain the discrepancies obtained betweenthe computational and experimental results and to proposefurther steps to improve the analysis of the loss mechanisms.
Taleb A, Barfuß C, Sapin P, et al., 2016, The Influence of Real Gases Effects on Thermally Induced Losses in Reciprocating Piston-Cylinder Systems, International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics
The efficiency of expanders is of prime importance for variousclean energy technologies. Once mechanical losses (e.g. throughvalves) are minimized, losses due to unsteady heat exchange betweenthe working fluid and the solid walls of the containingdevice can become the dominant loss mechanism. In this device,gas spring devices are investigated numerically in order to focusexplicitly on the thermodynamic losses that arise due to thisunsteady heat transfer. The specific aim of this study is to investigatethe behaviour of real gases in gas springs and comparethis to that of ideal gases in order to attain a better understandingof the impact of real gas effects on the thermally losses inreciprocating piston expanders and compressors. A CFD-modelof a gas spring is developed in OpenFOAM. Three different gasmodels are compared: an ideal gas model with constant thermodynamicand transport properties; an ideal gas model withtemperature-dependent properties; and a real gas model using thePeng-Robinson equation of state with temperature and pressuredependentproperties. Results indicate that, for simple, monoanddiatomic gases like helium or nitrogen, there is a negligibledifference in the pressure and temperature oscillations over a cyclebetween the ideal and real gas models. However, when lookingat a heavier (organic) molecule such as propane, the ideal gasmodel tends to overestimate the temperature and pressure comparedto the real gas model, especially if no temperature dependencyof thermodynamic properties is taken into account. Additionally,the ideal gas model (both alternatives) underestimatesthe thermally induced loss compared to the real gas model forheavier gases. Real gas effects must be taken into account in orderto predict accurately the thermally induced loss when usingheavy molecules in such devices.
Taleb AI, Sapin P, Barfuss C, et al., 2016, Wall temperature and system mass effects in a reciprocating gas spring
© 2016 University of Ljubljana. Reciprocating-piston devices can be used as high-efficiency compressors or expanders in small-scale Rankine cycle engines for power generation or in energy storage systems. The thermodynamic performance of piston-cylinder devices is adversely affected by the unsteady heat transfer between the compressed/expanded gas and the surrounding cylinder walls. Gas springs are an excellent model for the study of these losses because they exhibit the same complex heat transfer due to periodic pressure oscillations while avoiding the complexities of gas intake or exhaust. In this paper, results from CFD simulations of gas springs are compared to experimental data obtained in a piston-cylinder crankshaft-driven gas spring that experiences mass leakage. The temperature of the walls of the gas spring and the system mass are not known precisely in the experiments and are important parameters that determine the operation and performance of the system. The aim of this paper is to use complementary experimental and computational data in order to study the effects of these two parameters. Initial (mass) and boundary (wall temperature) conditions of the CFD are varied to match experimental measurements. It is found that the mass of the system has little influence on the temperature while an increase leads to a higher mean cyclic pressure without affecting the pressure ratio. In other words, the mass in a perfectly sealed gas spring only influences the operational pressure but not the performance of the system.
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