295 results found
Voulgaropoulos V, Brun NL, Charogiannis A, et al., 2020, Transient freezing of water between two parallel plates: A combined experimental and modelling study, International Journal of Heat and Mass Transfer, Vol: 153, Pages: 1-13, ISSN: 0017-9310
The transient freezing/solidification of water subjected to shear flow inside a rectangular cell is investigated under laminar flow conditions. A flow of freezing water is established inside the cell by cooling the top surface of the conductive, copper plate that forms the cell’s top side by contact with boiling liquid nitrogen (C). This heat removal results in an ice layer that forms and grows gradually on the ceiling of the cell, which is subjected to shear from the flow below it inside the channel. The spatiotemporal characteristics of the ice layer are recorded with optical, laser-based measurements and are compared with predictions from a transient freezing model that is developed for this purpose. Furthermore, tracer particles are introduced into the flow to aid the tracking of the ice layer and to allow for measurements based on particle image velocimetry (PIV) of the velocity field inside the flow during the ice-layer evolution. After an initial time-lag/‘buffer’ period (of s) that depends on the flow conditions, a quasi-linear growth of the ice layer is observed; at longer times the thickness of the ice layer reaches a maximum and then decreases again. The increase in the thickness, and hence thermal resistance, of the ice layer is counter-balanced by a decrease in the temperature of the copper plate and, therefore, a decrease in the temperature difference across the ice layer. Furthermore, it is found that the flow is associated with symmetric velocity profiles, recorded along the vertical spanwise length between the ice layer at the top of the cell and the floor of the cell, while an increase of the velocity maxima is recorded as the ice layer gradually thickens and, consequently, the flow cross-section is reduced. A constant heat flux of 19.7 × 103 W m is measured on the top side of the channel, while the heat transfer coefficient on the top side of the channel is found to be in the range of 90–110 W m K depending on the wa
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
Romanos P, Voumvoulakis E, Markides CN, et al., 2020, Thermal energy storage contribution to the economic dispatch of island power systems, CSEE Journal of Power and Energy Systems, Vol: 6, Pages: 100-110, ISSN: 2096-0042
In this paper the provision of flexible generation is investigated by extracting steam from Rankine-cycle power stations during off-peak demand in order to charge thermal tanks that contain suitable phase-change materials (PCMs); at a later time when this is required and/or is economically effective, these thermal energy storage (TES) tanks can act as the heat sources of secondary thermal power plants in order to generate power, for example as evaporators of, e.g., organic Rankine cycle (ORC) plants that are suitable for power generation at reduced temperatures and smaller scales. This type of solution offers greater flexibility than TES-only technologies that store thermal energy and release it back to the base power station, since it allows both derating but also over-generation compared to the base power-station capacity. The solution is applied in a case study of a 50-MW rated oil-fired power station unit at the autonomous system of Crete. The optimal operation of the TES system is investigated, by solving a modified Unit Commitment – Economic Dispatch optimization problem, which includes the TES operating constraints. The results indicate that for most of the scenarios the discounted payback period is lower than 12 years, while in few cases the payback period is 5 years.
Schuster S, Markides CN, White AJ, 2020, Design and off-design optimisation of an organic Rankine cycle (ORC) system with an integrated radial turbine model, Applied Thermal Engineering, Pages: 115192-115192, ISSN: 1359-4311
This paper investigates the design and thermodynamic optimisation of both sub- and transcritical organic Rankine cycle (ORC) power systems featuring radial turbines via performance calculations using mean-line models. The emphasis is on rapid performance predictions for a given turbine geometry, as well as geometric optimisation for a given heat source. From three specified quantities, which are the turbine inlet temperature, inlet pressure and mass flow rate, the other flow properties (e.g., outlet pressure and temperature) are computed, together with derived quantities which are required for cycle- or system-level assessments, such as the isentropic efficiency of the turbine. Experimental investigations from the open literature suitable for validation purposes are summarised and analysed with respect to their strengths and weaknesses. Similar computational fluid dynamic (CFD) simulations are also used to complement the available experimental data. The main contributions of this paper are that it provides a comprehensive overview of radial turbine performance modelling, and that it proposes a detailed framework that can be used for the improved development of efficient thermodynamic power systems based on a unified mean-line model that is validated against experimental data and supported by CFD results. Specifically, predictions from the mean-line model show good accuracy over a wide range of operating conditions for different turbine designs and fluids with compressibility factors from 0.6 - 1.0. Finally, in order to demonstrate its efficacy, the integrated radial turbine and ORC system design framework is used in a case study of a nominally 400-kW power system with propane as the working fluid in low-grade waste-heat application, where the turbine inlet temperature is fixed at 150 ° C and the condenser temperature is fixed at 15 ° C. The novelty of this work arises from the optimisation of the turbine nozzle vane position at off-design conditions. This fe
Gupta A, Qadri UA, Koutita K, et al., 2020, Experimental investigation of the flow in a micro-channelled combustor and its relation to flame behaviour, Experimental Thermal and Fluid Science, Pages: 110105-110105, ISSN: 0894-1777
The dynamic behaviour of periodic laminar premixed acetylene-air flames in a micro-channelled combustor consisting of an array of five planar rectangular channels was found to be influenced by the equiv- alence ratio and flow-rate of the continuously and steadily injected premixed fuel charge. Three distinct flame stages were observed — planar, chaotic and trident, which were strongly correlated to the flow dynamics. The effect of the flow on the flame behaviour was investigated by characterizing the cold flow in a scaled-up model channel with the same aspect ratio as the combustion micro-channel. Direct flow visualization using flow tracers and quantitative velocity-field data from PIV measurements showed both an increase in the bottom recircula- tion zone reattachment length (along the floor of the channel) and a decrease in the lateral recirculation zone reattachment length (along the sides of the channel) with increasing flow Reynolds number. Comparison of the flow and flame transition locations downstream of the injection point suggested that the location of trident flame onset coincides with the flow bottom recirculation zone reattachment length. The planar-chaotic flame transition location was observed to be influenced by the homogeneity of the mixture downstream of the injection plane.
Gasparrini C, Rana D-S, Le Brun N, et al., On the stoichiometry of zirconium carbide, Scientific Reports, ISSN: 2045-2322
Georgios M, Emilio Jose S, Acha Izquierdo S, et al., CO2 refrigeration system heat recovery and thermal storage modelling for space heating provision in supermarkets: An integrated approach, Applied Energy, ISSN: 0306-2619
Pantaleo AM, Camporeale S, Sorrentino A, et al., 2020, Hybrid solar-biomass combined Brayton/organic Rankine-cycle plants integrated with thermal storage: Techno-economic feasibility in select Mediterranean areas, Renewable Energy, Vol: 147, Pages: 2913-2931, ISSN: 1879-0682
This paper presents a thermodynamic analysis and techno-economic assessment of a novel hybrid solar-biomass power-generation system configuration composed of an externally fired gas-turbine (EFGT) fuelled by biomass (wood chips) and a bottoming organic Rankine cycle (ORC) plant. The main novelty is related to the heat recovery from the exhaust gases of the EFGT via thermal energy storage (TES), and integration of heat from a parabolic-trough collectors (PTCs) field with molten salts as a heat-transfer fluid (HTF). The presence of a TES between the topping and bottoming cycles facilitates the flexible operation of the system, allows the system to compensate for solar energy input fluctuations, and increases capacity factor and dispatchability. A TES with two molten salt tanks (one cold at 200 °C and one hot at 370 °C) is chosen. The selected bottoming ORC is a superheated recuperative cycle suitable for heat conversion in the operating temperature range of the TES. The whole system is modelled by means of a Python-based software code, and three locations in the Mediterranean area are assumed in order to perform energy-yield analyses: Marseille in France, Priolo Gargallo in Italy and Rabat in Morocco. In each case, the thermal storage that minimizes the levelized cost of energy (LCE) is selected on the basis of the estimated solar radiation and CSP size. The results of the thermodynamic simulations, capital and operational costs assessments and subsidies (feed-in tariffs for biomass and solar electricity available in the Italian framework), allow estimating the global energy conversion efficiency and the investment profitability in the three locations. Sensitivity analyses of the biomass costs, size of PTCs, feed-in tariff and share of cogenerated heat delivered to the load are also performed. The results show that the high investment costs of the CSP section in the proposed size range and hybridization configuration allow investment profitability only in the
Georgiou S, Aunedi M, Strbac G, et al., 2020, On the value of liquid-air and Pumped-Thermal Electricity Storage systems in low-carbon electricity systems, Energy, Vol: 193, ISSN: 0360-5442
We consider two medium-to-large scale thermomechanical electricity storage technologies currently under development, namely ‘Liquid-Air Energy Storage’ (LAES) and ‘Pumped-Thermal Electricity Storage’ (PTES). Consistent thermodynamic models and costing methods based on a unified methodology for the two systems from previous work are presented and used with the objective of integrating the characteristics of the technologies into a whole-electricity system assessment model and assessing their system-level value in various scenarios for system decarbonization. It is found that the value of storage depends on the cumulative installed capacity of storage in the system, with storage technologies providing greater marginal benefits at low penetrations. The system value of PTES was found to be slightly higher than that of LAES, driven by a higher storage duration and efficiency, although these results must be seen in light of the uncertainty in the (as yet, not demonstrated) performance of key PTES components, namely the reciprocating-piston compressors and expanders. At the same time, PTES was also found to have a higher power capital cost. The results indicate that the complexity of the decarbonization challenge makes it difficult to identify clearly a ‘best’ technology and suggest that the uptake of either technology can provide significant system-level benefits.
Song J, Loo P, Teo J, et al., 2020, Thermo-Economic Optimization of Organic Rankine Cycle (ORC) Systems for Geothermal Power Generation: A Comparative Study of System Configurations, FRONTIERS IN ENERGY RESEARCH, Vol: 8, ISSN: 2296-598X
Oyewunmi O, Lozano Santamaria F, Markides C, et al., Modelling two-phase flows in renewable power generation systems, 5th Thermal and Fluids Engineering Conference (TFEC)
Song J, Li X, Ren X, et al., 2020, Thermodynamic and economic investigations of transcritical CO2-cycle systems with integrated radial-inflow turbine performance predictions, Applied Thermal Engineering, Vol: 165, ISSN: 1359-4311
Transcritical CO2 (TCO2) cycle systems have emerged as a promising power-generation technology in certain applications. In conventional TCO2-cycle system analyses reported in the literature, the turbine efficiency, which strongly determines the overall system performance, is generally assumed to be constant. This may lead to suboptimal designs and optimization results. In order to improve the accuracy and reliability of such system analyses and offer insight into how knowledge of these systems from earlier analyses can be interpreted, this paper presents a comprehensive model that couples TCO2-cycle calculations with preliminary turbine design based on the mean-line method. Turbine design parameters are optimized simultaneously to achieve the highest turbine efficiency, which replaces the constant turbine efficiency used in cycle calculations. A case study of heat recovery from an internal combustion engine (ICE) using a TCO2-cycle system with a radial-inflow turbine is then considered, with results revealing that the turbine efficiency is influenced by the system’s operating conditions, which in turn has a significant effect on system performance in both thermodynamic and economic terms. A more generalized heat source is then considered to explore more broadly the role of the turbine in determining TCO2-cycle power-system performance. The more detailed turbine-design modelling approach allows errors of the order of up to 10-20% in various predictions to be avoided for steady-state calculations, and potentially of an even greater magnitude at off-design operation. The model allows quick preliminary designs of radial-inflow turbines and reasonable turbine performance predictions under various operating conditions, and can be a useful tool for more accurate and reliable thermo-economic studies of TCO2-cycle systems.
Emadi MA, Chitgar N, Oyewunmi O, et al., Working-fluid selection and thermoeconomic optimisation of a combined cycle cogeneration dual-loop organic Rankine cycle (ORC) system for solid-oxide fuel cell (SOFC) waste heat recovery, Applied Energy, ISSN: 0306-2619
A novel combined-cycle system is proposed for the cogeneration ofelectricityand cooling, in which a dual-loop organic Rankine cycle (ORC)engine is used for waste-heat recovery from a solidoxide fuel cellsystem equipped witha gas turbine(SOFC-GT). Electricity is generated by the SOFC, its associated gas turbine, the two ORC turbines and a liquefied natural gas (LNG)turbine; the LNGsupply tothe fuel cell is also used as the heat sink to the ORC enginesandas a cooling medium for domestic applications. The performance of the system with 20 different combinationsof ORC working fluids isinvestigated by multi-objective optimisationof its capitalcostrateand exergy efficiency, using an integrationof a genetic algorithm and a neural network. The combination of R601(top cycle) and Ethane(bottom cycle)isproposed for the dual-loop ORC system, due to the satisfaction of the optimisationgoals, i.e., an optimal trade-off between efficiency and cost.With theseworking fluids, the overall system achieves an exergy efficiency of51.6%, a total electrical powergeneration of1040kW, with the ORC waste-heat recovery system supplying 20.7% of thispower,and a cooling capacityof 567kW. In addition, an economic analysisof theproposed SOFC-GT-ORCsystemshowsthat the cost of production of an electrical unit amounts to$33.2perMWh, which is 12.9%and 73.9%lowerthan the levelized cost of electricityofseparateSOFC-GT and SOFC systems,respectively. Exergy flow diagrams are usedto determine the flow rate of the exergy andthe value of exergy destructionin each component. In the waste heat recovery system,exergy destruction mainly occurs within theheat exchangers, the highestof which isin the LNG cooling unit followedby the LNG vaporiser and the evaporator ofthe bottom-cycleORCsystem, highlightingthe importance of these components’designin maximising the performance of the overall system.
Qiu L, Zhu N, Feng Y, et al., 2019, A review of recent advances in thermophysical properties at the nanoscale: from solid state to colloids, Physics Reports, ISSN: 0370-1573
Nanomaterials possess superior optical, electrical, magnetic, mechanical, and thermal properties, which have made them suitable for a multitude of applications. The present review paper deals with recent advances in the measurement and modeling of thermophysical properties at the nanoscale (from the solid state to colloids). For this purpose, first, various techniques for the measurement of the solid state properties, including thermal conductivity, thermal diffusivity, and specific heat capacity, are introduced. The main factors that affect the solid state properties are grain size, grain boundaries, surface interactions, doping, and temperature, which are discussed in detail. After that, methods for the measurement and modeling of thermophysical properties of colloids (nanofluids), including thermal conductivity, dynamic viscosity, specific heat capacity, and density, are presented. The main parameters affecting these properties, such as size, shape, and concentration of nanoparticles, aggregation, and sonication time are studied. Furthermore, the properties of not only simple nanofluids but also hybrid nanofluids (which are composed of more than one type of nanoparticles) are investigated. Finally, the main research gaps and challenges are listed.
Herrando M, Pantaleo AM, Wang K, et al., 2019, Solar combined cooling, heating and power systems based on hybrid PVT, PV or solar-thermal collectors for building applications, Renewable Energy, Vol: 143, Pages: 637-647, ISSN: 0960-1481
A modelling methodology is developed and used to investigate the technoeconomic performance of solar combined cooling, heating and power (S-CCHP) systems based on hybrid PVT collectors. The building energy demands are inputs to a transient system model, which couples PVT solar-collectors via thermal-store to commercial absorption chillers. The real energy demands of the University Campus of Bari, investment costs, relevant electricity and gas prices are used to estimate payback-times. The results are compared to: evacuated tube collectors (ETCs) for heating and cooling provision; and a PV-system for electricity provision. A 1.68-MWp S-CCHP system can cover 20.9%, 55.1% and 16.3% of the space-heating, cooling and electrical demands of the Campus, respectively, with roof-space availability being a major limiting factor. The payback-time is 16.7 years, 2.7-times higher than that of a PV-system. The lack of electricity generation by the ETC-based system limits its profitability, and leads to 2.3-times longer payback-time. The environmental benefits arising from the system’s operation are evaluated. The S-CCHP system can displace 911 tonsCO2/year (16% and 1.4× times more than the PV-system and the ETC-based system, respectively). The influence of utility prices on the systems’ economics is analysed. It is found that the sensitivity to these prices is significant.
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
Romanos P, Pantaleo A, Markides C, Energy management and enhanced flexibility of power stations via thermal energy storage and secondary power cycles, 11th International Conference on Applied Energy
The operation of power plants must meet a series of requirements in order to enable the increasing penetration of intermittent renewable energy and the consequent intensifying demand for flexible generation. It is proposed here that during off-peak demand, steam can be extracted from Rankine-cycle power stations for the charging of thermal storage tanks that contain suitable phase-change materials (PCMs); during peak demand time, these thermal energy storage (TES) tanks can act as the heat sources of secondary thermal power plants in order to generate power, for example as evaporators of organic Rankine cycle (ORC) plants that are suitable for power generation at reduced temperatures and smaller scales. This type of solution offers greater flexibility than TES-only solutions that store thermal energy and then release this back to the base power station, in that it allows both derating andover-generation compared to the base power-station. The approach is here applied to a case study of a 670-MW rated nuclear power station, since nuclear power stations are generally suitable for baseload generation and the proposed system configuration could increase the operational flexibility of such plants.
Russell AW, Kahouadji L, Mirpuri K, et al., 2019, Mixing viscoplastic fluids in stirred vessels over multiple scales: An experimental and CFD approach, Chemical Engineering Science, Vol: 208, ISSN: 1873-4405
Dye visualisation techniques and CFD are used to characterise the flow of viscoplastic CarbopolTM solutions in stirred vessel systems over multiple scales. Centrally-mounted, geometrically-similar Rushton turbine (RT) impellers are used to agitate various Carbopol 980 (C980) fluids. The dimensionless cavern diameters, Dc/D, are scaled against a combination of dimensionless parameters: Rem-0.3Rey0.6n-0.1ks-1, where Rem, Rey, n and ks are the modified power-law Reynolds number, yield stress Reynolds number, flow behaviour index and impeller geometry constant, respectively. Excellent collapse of the data is demonstrated for the fluids and flows investigated. Additional data are collected using a pitched-blade turbine (PBT) with cavern size similarity being shown between the RT and PBT datasets. These results are important in the context of scale-up/scale-down mixing processes in stirred vessels containing complex fluids and can be used to show that flow similarity can be achieved in these systems if the processes are scaled appropriately.
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
Wang K, Herrando M, Pantaleo AM, et al., 2019, Technoeconomic assessments of hybrid photovoltaic-thermal vs. conventional solar-energy systems: Case studies in heat and power provision to sports centres, Applied Energy, Vol: 254, Pages: 1-16, ISSN: 0306-2619
This paper presents a comprehensive analysis of the energetic, economic and environmental potentials of hybrid photovoltaic-thermal (PVT) and conventional solar energy systems for combined heat and power provision. A solar combined heat and power (S-CHP) system based on PVT collectors, a solar-power system based on PV panels, a solar-thermal system based on evacuated tube collectors (ETCs), and a S-CHP system based on a combination of side-by-side PV panels and ETCs (PV-ETC) are assessed and compared. A conventional CHP system based on a natural-gas-fired internal combustion engine (ICE) prime mover is also analysed as a competing fossil-fuel based solution. Annual simulations are conducted for the provision of electricity, along with space heating, swimming pool heating and hot water to the University Sports Centre of Bari, Italy. The results show that, based on a total installation area of 4000 m2 in all cases, the PVT S-CHP system outperforms the other systems in terms of total energy output, with annual electrical and thermal energy yields reaching 82.3% and 51.3% of the centre’s demands, respectively. The PV system is the most profitable solar solution, with the shortest payback time (9.4 years) and lowest levelised cost of energy (0.089 €/kWh). Conversely, the ETC solar-thermal system is not economically viable for the sports centre application, and increasing the ETC area share in the combined PV-ETC S-CHP system is unfavourable due to the low natural gas price. Although the PVT S-CHP system has the highest investment cost, the high annual revenue from the avoided energy bills elevates its economic performance to a level between those of the conventional PV and ETC-based S-CHP systems, with a payback time of 13.7 years and a levelised cost of energy of 0.109 €/kWh. However, at 445 tCO2/year, the CO2 emission reduction potential of the PVT S-CHP system is considerably higher (by 40–75%) than those of the all other solar systems (254&ndash
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
Chatzopoulou MA, Lecompte S, De Paepe M, et al., 2019, Off-design optimisation of organic Rankine cycle (ORC) engines with different heat exchangers and volumetric expanders in waste heat recovery applications, Applied Energy, Vol: 253, 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
Li X, Song J, Yu G, et al., 2019, Organic Rankine cycle systems for engine waste-heat recovery: Heat exchanger design in space-constrained applications, Energy Conversion and Management, Vol: 199, ISSN: 0196-8904
Organic Rankine cycle (ORC) systems are a promising solution for improving internal combustion engine efficiencies, however, conflicts between the pressure drops in the heat exchangers, overall thermodynamic performance and economic viability are acute in this space-constrained application. This paper focuses on the interaction of the heat exchanger pressure drop (HEPD) and the thermo-economic performance of ORC systems in engine waste-heat recovery applications. An iterative procedure is included in the thermo-economic analysis of such systems that quantifies the HEPD in each case, and uses this information to revise the cycle and to resize the components until convergence. The newly proposed approach is compared with conventional methods in which the heat exchangers are sized after thermodynamic cycle modelling and the pressure drops through them are ignored, in order to understand and quantify the effects of the HEPD on ORC system design and working fluid selection. Results demonstrate that neglecting the HEPD leads to significant overestimations of both the thermodynamic and the economic performance of ORC systems, which for some indicators can be as high as >80% in some cases, and that this can be effectively avoided with the improved approach that accounts for the HEPD. In such space-limited applications, the heat exchangers can be designed with a smaller cross-section in order to achieve a better compromise between packaging volume, heat transfer and ORC net power output. Furthermore, we identify differences in working fluid selection that arise from the fact that different working fluids give rise to different levels of HEPD. The optimized thermo-economic approach proposed here improves the accuracy and reliability of conventional early-stage engineering design and assessments, which can be extended to other similar thermal systems (i.e., CO2 cycle, Brayton cycle, etc.) that involve heat exchangers integration in similar applications.
Escriva EJS, Acha S, LeBrun N, et al., 2019, Modelling of a real CO2 booster installation and evaluation of control strategies for heat recovery applications in supermarkets, International Journal of Refrigeration, Vol: 107, Pages: 288-300, ISSN: 0140-7007
This paper compares and quantifies the energy, environmental and economic benefits of various control strategies recovering heat from a CO2 booster system in a supermarket for space heating with the purpose of understanding its potential for displacing natural gas fuelled boilers. A theoretical steady-state model that simulates the behaviour of the CO2 system is developed and validated against field measurements obtained from an existing refrigeration system in a food-retail building located in the United Kingdom. Five heat recovery strategies are analysed by modifying the mass flows and pressure levels in the condenser. The model shows that a reduction of 48% in natural-gas consumption is feasible by the installation of a de-superheater and without any advanced operating strategy. However, the CO2 system can fully supply the entire space-heating requirement by adopting alternative control strategies, albeit by penalising the coefficient of performance (COP) of the compressor. Results show that the best energy strategy can reduce total consumption by 32%, while the best economic strategy can reduce costs by 6%. Findings from this work suggest that heat recovery systems can bring substantial benefits to improve the overall efficiency of energy-intensive buildings; although trade-offs need to be carefully considered and further analysed before embarking on such initiatives.
Wang K, Pantaleo AM, Mugnozza GS, et al., 2019, Technoeconomic assessment of solar combined heat and power systems based on hybrid PVT collectors in greenhouse applications, ISSN: 1757-8981
© 2019 IOP Publishing Ltd. All rights reserved. This paper presents a technoeconomic analysis of a solar combined heat and power (S-CHP) system based on hybrid photovoltaic-thermal (PVT) collectors for distributed cogeneration in a greenhouse tomato-farm in Bari, Italy. The thermal and electrical demands of the greenhouse of interest are currently fulfilled by a gas-fired CHP system that features an internal combustion engine (ICE) prime mover, and partially by an auxiliary gas boiler and electricity from the grid. A PVT-water S-CHP system is designed and sized based on a transient model, with hourly weather data and measured demand data given as inputs. Annual simulations are performed to predict the transient behaviour of the S-CHP system and to assess the system's energy outputs. The economic profitability of such solution is also evaluated by considering the investment costs and cost savings due to the reduced on-site energy consumption. The results show that, with an installation area of 30,000 m2, the PVT S-CHP system is able to cover up to 73% of the annual thermal demand of the greenhouse, while delivering a net electrical output 2.6 times that of the annual electrical demand. This performance is similar to that achieved by the equivalent ICE-CHP system (92% and 2 times, respectively). Furthermore, the total annual cost saving of the PVT S-CHP system is more than 6 times higher than that of the ICE system, due to the much lower fuel cost of the PVT system. Similarly, the potential CO2 emission reduction associated with the PVT system is considerably higher, at 3010 tCO2/year saved (vs. 86 tCO2/year). The payback time of the PVT system is not significantly longer than that of the ICE system (10.4 years vs. 8.4 years), but its levelized cost of energy is much lower (0.076 /kWh vs. 0.132 /kWh) due to the higher annual cost savings. These results indicate that such PVT S-CHP systems have an excellent technoeconomic potential in the proposed greenhouse appli
van Kleef L, Oyewunmi O, Markides C, 2019, Multi-objective thermo-economic optimization of organic Rankine cycle (ORC) power systems in waste-heat recovery applications using computer-aided molecular design techniques, Applied Energy, Vol: 251, ISSN: 0306-2619
In this paper, we develop a framework for designing optimal organic Rankine cycle (ORC) power systems that simultaneously considers both thermodynamic and economic objectives. This methodology relies on computeraided molecular design (CAMD) techniques that allow the identification of an optimal working fluid during the thermo-economic optimization of the system. The SAFT-γ Mie equation of state is used to determine the necessary thermodynamic properties of the designed working fluids, with critical and transport properties estimated using empirical group-contribution methods. The framework is then applied to the design of sub-critical and non-recuperated ORC systems in different applications spanning a range of heat-source temperatures. When minimizing the specific investment cost (SIC) of these systems, it is found that the optimal molecular size of the working fluid is linked to the heat-source temperature, as expected, but also that the introduction of a minimum pinch point constraint that is commonly employed to account for inherent trade-offs between system performance and cost is not required. The optimal SICs of waste-heat ORC systems with heat-source temperatures of 150 °C, 250 °C and 350 °C are £10,120/kW, £4,040/kW and £2,910/kW, when employing propane, 2-butane and 2- heptene as the working fluids, respectively. During a set of MINLP optimizations of the ORC systems with heatsource temperatures of 150 °C and 250 °C, it is found that 1,3-butadiene and 4-methyl-2-pentene are the bestperforming working fluids, respectively, with SICs of £9,640/kW and £4,000/kW. These substances represent novel working fluids for ORC systems that cannot be determined a priori by specifying any working-fluid family or by following traditional methods of testing multiple fluids. Interestingly, the same molecules are identified in a multi-objective optimization considering both the total investment cost and net power output
Charogiannis A, Markides CN, 2019, Spatiotemporally resolved heat transfer measurements in falling liquid-films by simultaneous application of planar laser-induced fluorescence (PLIF), particle tracking velocimetry (PTV) and infrared (IR) thermography, Experimental Thermal and Fluid Science, Vol: 107, Pages: 169-191, ISSN: 0894-1777
We present an optical technique that combines simultaneous planar laser-induced fluorescence (PLIF), particle tracking velocimetry (PTV) and infrared (IR) thermography for the space-and time-resolved measurement of the film-height, 2-D velocity and 2-D free-surface temperature in liquid films falling over an inclined, resistively-heated glass substrate. Using this information and knowledge of the wall temperature, local and instantaneous heat-transfer coefficients (HTCs) and Nusselt numbers, Nu, are also recovered along the waves of liquid films with Kapitza number, , and Prandtl number, . By employing this technique, falling-film flows are investigated with Reynolds numbers in the range , wave frequencies set to , 12 and 17 Hz, and a wall heat flux set to W cm−2. Complementary data are also collected in equivalent (i.e., for the same mean-flow Re) flows with W cm−2. Quality assurance experiments are performed that reveal deviations of up to 2-3% between PLIF/PTV-derived film heights, interfacial/bulk velocities and flow rates, and both analytical predictions and direct measurements of flat films over a range of conditions, while IR-based temperature measurements fall within 1 °C of thermocouple measurements. Highly localized film height, velocity, flow-rate and interface-temperature data are generated along the examined wave topologies by phase/wave locked averaging. The application of a heat flux ( W cm−2) results in a pronounced “thinning” of the investigated films (by 18%, on average), while the mean bulk velocities compensate by increasing by a similar extent to conserve the imposed flow rate. The axial-velocity profiles that are obtained in the heated cases are parabolic but “fuller” compared to equivalent isothermal flows, excluding any wave-regions where the interface slopes are high. As the Re is reduced, the heating applied at the wall penetrates through the film, resulting in a pronounced coupling between th
Cherdantsev AV, An JS, Charogiannis A, et al., 2019, Simultaneous application of two laser-induced fluorescence approaches for film thickness measurements in annular gas-liquid flows, International Journal of Multiphase Flow, Vol: 119, Pages: 237-258, ISSN: 0301-9322
This paper is devoted to the simultaneous application of two spatiotemporally resolved optical techniques capable of liquid film thickness measurements, namely Planar Laser-Induced Fluorescence (PLIF) and Brightness-Based Laser-Induced Fluorescence (BBLIF), to co-current downward annular gas-liquid flows. A single laser sheet is used to excite the liquid film, which has been seeded with a fluorescent dye, along a longitudinal/vertical plane normal to the pipe wall. Two cameras, one for each technique, are placed at different angles to the plane of the laser sheet in order to recover, independently by the two techniques, the shape of the gas-liquid interface along this section. The effect of the angle between the laser sheet and the PLIF camera axis is also investigated. In film regions where the gas-liquid interface is smooth and flat, the conventional approach used for interpreting PLIF data is affected by total internal reflection of the fluorescent light at the free surface, or “mirror effect”, which leads to an overestimation of the film thickness that increases as the angle between the laser sheet and the camera axis is decreased. Nonetheless, local features such as light intensity maxima or minima are often located within the fluorescent signals that correctly identify the true interface, which in these conditions also coincides well with the BBLIF film-thickness measurement. When a correction for the mirror effect based on simple flat-film optical calculations is applied, this leads to PLIF results that correspond well to the true film thickness. Interestingly, it is further found that interfacial three-dimensionality, and in particular azimuthal/circumferential non-uniformity, can lead to underestimation of film thickness by PLIF that in some cases counteracts the overestimation due to the mirror effect. Smaller angles between the laser sheet and camera axis make PLIF less susceptible to this error. In regions where the film surface is rough, inc
Li X, Tian H, Shu G, et al., 2019, Potential of carbon dioxide transcritical power cycle waste-heat recovery systems for heavy-duty truck engines, Applied Energy, Vol: 250, Pages: 1581-1599, ISSN: 0306-2619
Carbon dioxide transcritical power cycle (CTPC) systems are considered a new and particularly interesting technology for waste-heat recovery. In heavy-duty truck engine applications, challenges arise from the highly transient nature of the available heat sources. This paper presents an integrated model of CTPC systems recovering heat from a truck diesel engine, developed in GT-SUITE software and calibrated against experimental data, considers the likely fuel consumption improvements and identifies directions for further improvement. The transient performance of four different CTPC systems is predicted over a heavy-heavy duty driving cycle with a control structure comprising a mode switch module and two PID controllers implemented to realize stable, safe and optimal operation. Three operating modes are defined: startup mode, power mode, and stop mode. The results demonstrate that CTPC systems are robust and able to operate safely even when the heat sources are highly transient, indicating a promising potential for the deployment of this technology in such applications. Furthermore, a system layout with both a preheater and a recuperator appears as the most promising, allowing a 2.3% improvement in brake thermal efficiency over the whole driving cycle by utilizing 48.9% of the exhaust and 72.8% of the coolant energy, even when the pump and turbine efficiencies are as low as 50%. Finally, factor analysis suggests that important directions aimed at improving the performance and facilitating CTPC system integration with vehicle engines are: 1) ensuring long-duration operation in power mode, e.g., by employment in long-haul trucks; and 2) enhancing pump and turbine performance.
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