371 results found
Van den Bergh WJ, Moran HR, Dirker J, et al., 2021, Effect of low heat and mass fluxes on the boiling heat transfer coefficient of R-245fa, International Journal of Heat and Mass Transfer, Vol: 180, Pages: 1-19, ISSN: 0017-9310
In-tube flow boiling at low mass and heat fluxes is of increasing interest particularly for low-concentration solar power systems, refrigerators, heat pumps, and other thermal management components and systems. In this study, the flow boiling of R-245fa was investigated experimentally for vapour qualities ranging from 0.05 to 0.90, mass fluxes of 40, 60 and 80 kg/m2s, and heat fluxes of 2.5, 5.0 and 7.5 kW/m2. Tests were done at quasi-steady-state conditions in a horizontal smooth tube with an inner diameter of 8.31 mm and a heated length of 0.8 m, at a saturation temperature of 35°C. It was found that the heat transfer coefficient was influenced by both the mass and heat fluxes. At any given vapour quality and heat flux combination, an increase in the mass flux resulted in an increase in the heat transfer coefficient. However, the magnitude of the increase and sensitivity to the mass flux was not the same in all of the test cases. Higher vapour quality cases were more sensitive to the mass flux than lower vapour quality cases, except at low heat flux conditions. In the most peculiar case, at the lowest considered heat flux of 2.5 kW/m2, severe sensitivity to mass flux was observed at vapour qualities between 0.2 and 0.3. For all other heat fluxes, the heat transfer coefficient was found to be independent of the vapour quality except when the mass flux was high, where increased vapour quality resulted in improved heat transfer coefficients. Various correlations were investigated, but none of them captured the trends for the lowest heat flux.
Voulgaropoulos V, Kadivar M, Moghimi MA, et al., 2021, A combined experimental and computational study of phase-change dynamics and flow inside a sessile water droplet freezing due to interfacial heat transfer, International Journal of Heat and Mass Transfer, Vol: 180, Pages: 1-15, ISSN: 0017-9310
This study experimentally and numerically investigates the freezing characteristics and fluid dynamics of millimetre-sized sessile water droplets submerged in silicone oil at sub-zero temperatures under free convection. Individual water droplets were cooled to sub-zero temperatures (260-270 K) via interfacial heat transfer between the two liquid phases, in an approach different to studies in the literature where the cooling is done either from the solid substrate or from a low-temperature gas phase (such as air) surrounding the droplets. Laser-induced fluorescence was employed to perform spatiotemporally-resolved measurements of the phase distribution (from which interface distributions, freezing fronts, and rates were extracted). The particle image velocimetry was used to generate information on the velocity fields inside the liquid droplets. The experimental data are complemented by computational fluid dynamics (CFD) simulations, which showed acceptable qualitative and quantitative agreement with the experimental results. The experimental and simulation results indicated that prior to the initiation of freezing, two counteracting recirculation zones are generated in the central plane of the droplets, one on either side of the centreline, leading to a net upward flow at the edges and a downward flow in the centre due to the natural convection driven by internal temperature gradients. The nucleation sites appear on the external regions of the recirculation structures (which are locations with higher shear). Once freezing starts, the natural circulation patterns are suppressed, and instead, a sole downwards flow dominates, which is the result of the freezing layer suppressing the water phase. CFD results demonstrated a relatively wide temperature and pressure distribution in the water droplet at the beginning of the freezing stage, which gradually diminishes as the freezing process proceeds. The effect of droplet size and oil temperature on the freezing rates were in
Khaljani M, Harrison J, Surplus D, et al., 2021, A combined experimental and modelling investigation of an overground compressed-air energy storage system with a reversible liquid-piston gas compressor/expander, Energy Conversion and Management, Vol: 245, Pages: 1-19, ISSN: 0196-8904
We consider a small-scale overground compressed-air energy storage (CAES) system intended for use in micro-grid power networks. This work goes beyond previous efforts in the literature by developing and showing results from a first-of-a-kind small-scale (20 kWh) near-isothermal CAES system employing a novel, reversible liquid-piston gas compressor and expander (LPGC/E). Additionally, we extend our study to assessments, for the first time, of the economic and environmental characteristics of these small-scale overground CAES systems through a combination of experimental, thermodynamic, technoeconomic and environmental analyses. Five system configurations are considered: (1) CAESbase, which is the base-case system; (2) CAESplate, in which parallel plates are inserted into the LPGC/E as a heat exchanger for achieving near-isothermal compression and expansion; (3) CAESPCM, in which a phase change material (PCM) is employed to store thermal energy from the compressed air during charging that is later recovered during discharge; (4) CAESPCM&plate, which is a combination of the CAESplate and CAESPCM arrangements; and (5) CAESheater, in which a heater is utilised instead of the PCM to preheat the compressed air during discharge. Data for the validation of a computational design tool based on which the assessments were performed were obtained from a prototype of the CAESbase system. Results show that the CAESPCM&plate system exhibits the highest roundtrip efficiency of 63% and the shortest payback period of 7 years; the latter with the inclusion of governmental incentives and an electricity smart export guarantee (SEG) support rate of 5.5 p/kWh (6.8 ¢/kWh). The CAESPCM&plate system is found to be cost-effective even without incentives, with a payback period of 10 years. This system is also associated with 71 tonnes of fuel consumption savings and reduced CO2 emissions amounting to 51 tonnes over a lifetime of 20 years.
Loni R, Mahian O, Markides CN, et al., 2021, A review of solar-driven organic Rankine cycles: Recent challenges and future outlook, Renewable and Sustainable Energy Reviews, Vol: 150, ISSN: 1364-0321
The organic Rankine cycle (ORC) is an effective technology for power generation from temperatures of up to 400 °C and for capacities of up to 10 MWel. The use of solar irradiation for driving an ORC is a promising renewable energy-based technology due to the high compatibility between the operating temperatures of solar thermal collector technologies and the temperature needs of the cycle. The objective of this review paper is to present and discuss the operation principles of solar-ORC technology and the wide range of solar-ORC systems that have been studied in the literature. Various solar thermal technologies that can drive the ORC are investigated, such as the flat plate collector, evacuated tube collector, compound parabolic collector, parabolic trough collector, linear Fresnel reflectors, dish concentrators and solar towers. Both simulation studies and experimental investigations are included in the study. Hybrid systems and different thermal storage techniques are also examined in detail. Moreover, systems with ORC which produce many useful outputs such as cooling, heating and fresh water are studied because they present high sustainability indexes. The limitations of the technology are also highlighted, along with critical suggestions aimed at steering future research in this field. The final conclusions indicate that the development of trigeneration and polygeneration systems with ORC sub-systems is a promising avenue, not only for the future development of solar-ORC technology but also for the development of renewable and sustainable energy systems in a broader context.
Li Y, Markides CN, Sunden B, et al., 2021, Heat transfer deterioration in upward and downward pipe flows of supercritical n-decane for actively regenerative cooling, International Journal of Thermal Sciences, Vol: 168, Pages: 1-13, ISSN: 1290-0729
In this paper, we consider the flow and heat transfer behaviour of turbulent upward and downward flows of supercritical n-decane, in order to reveal the features of heat transfer deterioration (HTD) that would be expected in relevant active regenerative cooling systems for scramjet engines. Specific focus is placed on key velocity-field features that appear in these flows. Following the validation of six turbulence models, the SST k-ω and RNG k-ϵ models are found to be suitable for simulating the upward and downward flow cases, respectively. “M” type velocity profiles (a non-monotonicity of the velocity along the radial direction) are observed, which arise due to a spatially-varying interplay between the inertial and viscous forces in the flow domain, while larger velocity gradients in the buffer layer are also observed that contribute to the phenomenon of HTD. Furthermore, it is found that the secondary flows as well as the different mass fluxes that arise due to the velocity increase from the wall to the flow core zone (i.e., the influencing range and intensity of cross-sectional kinetic energy), respectively, are observed in the HTD development region, as well as the HTD peak area and degradation regions. A zone of higher thermal diffusion appears in the near-wall region, which acts as a thermal barrier and contributes to HTD.
Sarabia Escriva EJ, Hart M, Acha Izquierdo S, et al., 2021, Techno-economic evaluation of integrated energy systems for heat recovery applications in food retail buildings, Applied Energy, ISSN: 0306-2619
Voulgaropoulos V, Patapas A, Lecompte S, et al., 2021, Simultaneous laser-induced fluorescence and capacitance probe measurement of downwards annular gas-liquid flows, International Journal of Multiphase Flow, Vol: 142, Pages: 103665-103665, ISSN: 0301-9322
This study focuses on the characterisation of downwards annular gas-liquid (air-water) flows, by employing a combi-nation of advanced laser-based and capacitance-based measurement methods. A variant of laser-induced fluorescence(LIF), referred to as structured-planar laser-induced fluorescence (S-PLIF), eliminates biases commonly encounteredduring film-thickness measurements of gas-liquid flows, due to refraction and reflection of the light at the interface. Abespoke capacitance probe is also assembled to enable temporally resolved film-thickness measurements with high tem-poral resolution along the circumferential perimeter of the pipe. We compare the film mean thickness, roughness, andprobability density functions obtained with each method. We find that both methods are able to measure time-averagedfilm thickness to within<20% deviations from each other and from results obtained from the available literature. Theresulting probe data suggest a biased (suppressed) standard deviation of the film thickness, which can be attributed toits working principle, i.e., measuring the film thickness averaged along the circumferential perimeter of the pipe. Theauto-correlation functions of the time-traces provide an insight into the characteristic time-scales of the flows, whichspan a range from∼10 ms for highly gas-sheared flows and increase to about 30 ms for the less turbulent falling films.The power spectral densities reveal modal frequencies that start from 2.5 Hz for falling films, and increase with the gasReynolds number by almost an order of magnitude. The turbulent wave activity (slope in the power spectrum) reduceswith a decrease in gas shear, and shows similarities to the decay of homogeneous and isotropic turbulence. The sizes ofthe bubbles entrained in the liquid film are measured from the S-PLIF images, and exhibit log-normal distribution thatbecome flatter with a decrease in the gas Reynolds number. The normalised location of the bubbl
Huang G, Wang K, Riera Curt S, et al., 2021, On the performance of concentrating fluid-based spectral-splitting hybrid PV-thermal (PV-T) solar collectors, Renewable Energy, Vol: 174, Pages: 590-605, ISSN: 0960-1481
Concentrating fluid-based spectral-splitting hybrid PV-thermal (SSPVT) collectors are capable of high electrical and thermal efficiencies, as well as high-temperature thermal outputs. However, the optimal optical filter and the maximum potential of such collectors remain unclear. In this study, we develop a comprehensive two-dimensional model of a fluid-based SSPVT collector. The temperature distributions reveal that these designs are effective in thermally decoupling the PV module from the high-temperature filter flow-channel, improving the electrical performance of the module. For a Si solar cell-based SSPVT collector with optical filter #Si400-1100, the filter channel is able to produce high-temperature thermal energy (400 °C) with an efficiency of 19.5%, low-temperature thermal energy (70 °C) with an efficiency of 49.5%, and electricity with an efficiency 17.5%. Of note is that the relative fraction of high-temperature thermal energy, low-temperature thermal energy and electricity generated by such a SSPVT collector can be adjusted by shifting the upper- and lower-bound cut-off wavelengths of the optical filter, which are found to strongly affect the spectral and energy distributions through the collector. The optimal upper-bound cut-off always equals the bandgap wavelength of the solar cell material (e.g., 1100 nm for Si, and 850 nm for CdTe), while the optimal lower-bound cut-off follows more complex selection criteria. The SSPVT collector with the optimal filter has a significantly higher total effective efficiency than an equivalent conventional solar-thermal collector when the relative value of the high-temperature heat to that of electricity is lower than 0.5. Detailed guidance for selecting optimal filters and their role in controlling SSPVT collector performance under different conditions is provided.
Bianchi G, Besagni G, Tassou SA, et al., 2021, Overview and outlook of research and innovation in energy systems with carbon dioxide as the working fluid, APPLIED THERMAL ENGINEERING, Vol: 195, ISSN: 1359-4311
Song J, Wang Y, Wang K, et al., 2021, Combined supercritical CO2 (SCO2) cycle and organic Rankine cycle (ORC) system for hybrid solar and geothermal power generation: Thermoeconomic assessment of various configurations, Renewable Energy, Vol: 174, Pages: 1020-1035, ISSN: 0960-1481
Hybrid solar and geothermal utilisation is a promising option for effective exploitation of renewable energy sources. Concentrated solar power (CSP) systems with geothermal preheating are acknowledged as an attractive solution, with supercritical CO2 (SCO2) cycle systems adopted for power generation thanks to the favourable properties offered by CO2 as a working fluid. In order to further improve the overall performance of such systems, organic Rankine cycle (ORC) systems can be added as bottoming cycles to recover the heat rejected from the topping SCO2 cycle system and also to utilise surplus geothermal heat available after the brine is used for preheating in the SCO2 system. This paper proposes four configurations of combined SCO2-ORC system for hybrid solar and geothermal power generation and performs detailed thermodynamic and economic assessments based on actual conditions in Seville, Spain. The results reveal that combined systems in which the geothermal-brine stream is split into two parallel flows and utilised separately by the topping SCO2 cycle and bottoming ORC systems are preferable. A split geothermal-stream combined system with the ORC working fluid first utilising geothermal heat followed in series by heat from the topping SCO2 cycle system delivers a net power output of 2940 kW, which is the maximum among all the proposed configurations and is 45% higher than that of a standalone SCO2 plant. A similar combined system with a reversed ORC flow direction such that the organic fluid is preheated first by utilising heat from the SCO2 cycle system and then by geothermal heat has a specific cost corresponding to the maximum net power output of 2880 $/kW, which is the lowest among all the configurations and is 22% lower than that of the standalone SCO2 plant. Annual performance evaluation shows that the combined systems can achieve significant improvements, ranging from 22% to 45%, over the total electricity generation of the standalone SCO2 plant, which de
Mahian O, Bellos E, Markides CN, et al., 2021, Recent advances in using nanofluids in renewable energy systems and the environmental implications of their uptake, Nano Energy, Vol: 86, Pages: 1-28, ISSN: 2211-2855
It has been more than two decades since the discovery of ‘nanofluids’ – mixtures of common liquids and solid nanoparticles with at least one dimension below 100 nm in size. While colloidal suspensions of particles (which include larger particles) have been studied for several decades, the term ‘nanofluids’ designates fluid systems that have enhanced thermal and optical properties. Although barriers to their commercial adoption remain, the field of nanofluids has continued to grow. Many studies have considered the effects of adding nanoparticles on the thermal efficiency and exergy efficiency of renewable energy systems particularly solar systems, however, few have investigated their potential for emission reductions. Critically, since renewable energy technologies aim to reduce the environmental impact of energy systems, this review focuses on whether nanofluids provide a net environmental benefit. Thus, in addition to providing a comprehensive overview of this body of literature from an environmental perspective, this review also highlights areas for future work that could help ensure that nanofluids have a net positive environmental impact in renewable energy systems going forward.
Moran HR, Zogg D, Voulgaropoulos V, et al., 2021, An experimental study of the thermohydraulic characteristics of flow boiling in horizontal pipes: Linking spatiotemporally resolved and integral measurements, Applied Thermal Engineering, Vol: 194, Pages: 1-17, ISSN: 1359-4311
Data are presented from experiments of flow boiling in a horizontal pipe. Specifically, refrigerant R245fa was evaporated in a 12.6 mm stainless steel pipe to which a uniform heat flux of up to 38 kW/m was applied. The bespoke facility operated at mass fluxes in the range 30–700 kg/m s and a saturation pressure of 1.7 bar. Flow patterns were identified through high-speed imaging and the resulting flow pattern map is compared to existing maps in the literature. Predictive methods for the pressure drop and heat transfer coefficient from common correlations are also compared to the present experimental data, acting as verification of the facility and methods used for the macroscale boiling flows investigated in this work. Laser-induced fluorescence (for the identification of the liquid phase) and particle image velocimetry (for the provision of velocity-field information) were also developed and successfully applied, providing detailed spatially- and temporally-resolved interfacial property, phase distribution and liquid-phase velocity-field data, alongside traditional integral pressure drop and overall heat transfer measurements. The laser-based methods provide new insight into the hydrodynamic and thermal characteristics of boiling flows at this scale, which are linked to the integral thermohydraulic data on flow regimes, pressure drops and heat transfer. This enhanced understanding can improve the design and operation of flow-boiling applications such as organic Rankine cycles and concentrating solar power facilities operating in the direct steam generation mode.
Sun F, Xie G, Song J, et al., 2021, Thermal characteristics of in-tube upward supercritical CO2 flows and a new heat transfer prediction model based on artificial neural networks (ANN), Applied Thermal Engineering, Vol: 194, Pages: 1-13, ISSN: 1359-4311
The potential employment of supercritical carbon dioxide (sCO2) flows in heated tubes in many applications requires accurate and reliable predictions of the thermal characteristics of these flows. However, the ability to predict such flows remains limited due to a lack of a complete fundamental understanding, with traditional prediction capabilities relying on either simple empirical correlations or highly complex and computationally demanding simulation methods both of which limit the design of next-generation systems. To overcome this challenge, a prediction model based on artificial neural network (ANN) is proposed and trained by 5780 sets of experimental wall temperature data from upward flows with a very satisfactory root mean square error (RMSE) and mean relative error that are less than 1.9 °C and 1.8%, respectively. The results confirm that the structured model can provide satisfactory prediction capabilities overall, as well specific performance with mean relative error under the normal, enhanced and deteriorated heat transfer (NHT, EHT and DHT) conditions of 1.8%, 1.6% and 1.7%, respectively. The proposed model’s ability to predict the heat transfer coefficient in these flows is also considered, and it is shown that the mean relative error is less than 2.8%. Thus, it is confirmed that it has a better prediction accuracy than traditional empirical correlations. This work indicates that such ANN methods can provide a real alternative for adoption in select thermal science and engineering applications, shedding a new light and giving added insight into the thermal characteristics of heated supercritical fluids.
Olympios AV, McTigue JD, Sapin P, et al., 2021, Pumped-thermal electricity storage based on Brayton cycles, Reference Module in Earth Systems and Environmental Sciences, ISBN: 9780124095489
Pumped-thermal electricity storage (PTES) based on a reversible (Joule-)Brayton cycle is a promising grid-scale energy storage technology, whose working principle is to store electricity in the form of high-grade thermal energy. This chapter provides an overview of the inner workings, operating principle and current development status of the many PTES variants, as proposed to date in the scientific literature or by manufacturers. The potential and competitiveness of the various candidate designs is quantified by – and discussed thanks to the definition of – specific techno-economic indicators. Investment cost and thermodynamic performance estimates are reported and used to assess the value of this technology as a potential large-scale, long-duration and long-lifetime energy storage option with unique sector-coupling features and low geographical constraints.
Zhao YL, Wang CY, Liu M, et al., 2021, Configuration Optimization of Carnot Battery Energy Storage System Based on Transcritical Cycles, Kung Cheng Je Wu Li Hsueh Pao/Journal of Engineering Thermophysics, Vol: 42, Pages: 1659-1666, ISSN: 0253-231X
Carnot battery is a novel and promising technology that can realize large-scale energy storage. This investigation focuses on a Carnot battery energy storage system based on the CO2 transcritical cycle, six system configurations employing different thermal energy storage materials and with or without recuperation are firstly designed and the thermodynamic parameters are optimized. Then, detailed comparison analyses from thermo-economic perspectives are conducted and the optimal system configuration is obtained. The main results reveal that the roundtrip efficiency of the recuperated Carnot battery system enhances when compared to the non-recuperated system; the recuperated system using rapeseed oil as the storage material achieves the highest roundtrip efficiency (75.28%) and energy density (36.61 kWh•m-3), which is respective 36.67% and 25.68 kWh•m-3 higher than the non-recuperated system. Among six different system configurations of the Carnot battery, the total capital costs range from 11.5 to 36.3 106USD and the energy capital costs are in the range of 954~3028 USD•kW-1•h-1; in the cost breakdown, the highest cost of heat exchangers can reach up to 54%, the highest cost of thermal stores can reach up to 14%, and the costs of expanders and a compressor change in the range of 18%~31% and 14%~26%, respectively; compared with other system configurations, the recuperated Carnot battery system using Therminol as the storage material has the best economic performance.
Enayatollahi H, Sapin P, Unamba CK, et al., 2021, A control-oriented ANFIS model of evaporator in a 1-kWe organic Rankine cycle prototype, Electronics, Vol: 10, Pages: 1-18, ISSN: 1450-5843
This paper presents a control-oriented neuro-fuzzy model of brazed-plate evaporators for use in organic Rankine cycle (ORC) engines for waste heat recovery from exhaust-gas streams of diesel engines, amongst other applications. Careful modelling of the evaporator is both crucial to assess the dynamic performance of the ORC system and challenging due to the high nonlinearity of its governing equations. The proposed adaptive neuro-fuzzy inference system (ANFIS) model consists of two separate neuro-fuzzy sub-models for predicting the evaporator output temperature and evaporating pressure. Experimental data are collected from a 1-kWe ORC prototype to train, and verify the accuracy of the ANFIS model, which benefits from the feed-forward output calculation and backpropagation capability of the neural network, while keeping the interpretability of fuzzy systems. The effect of training the models using gradient-descent least-square estimate (GD-LSE) and particle swarm optimisation (PSO) techniques is investigated, and the performance of both techniques are compared in terms of RMSEs and correlation coefficients. The simulation results indicate strong learning ability and high generalisation performance for both. Training the ANFIS models using the PSO algorithm improved the obtained test data RMSE values by 29% for the evaporator outlet temperature and by 18% for the evaporator outlet pressure. The accuracy and speed of the model illustrate its potential for real-time control purposes.
Zhao Y, Zhao C, Wen T, et al., 2021, High Temperature Sensible Storage—Industrial Applications, Encyclopedia of Energy Storage, Editors: Cabeza, Publisher: Elsevier, ISBN: 9780128197301
Thermal energy storage is a key technology for addressing the challenge of fluctuating renewable energy generation and waste heat availability, and for alleviating the mismatch between energy supply and demand. Thanks to their simple construction, operation and low costs, sensible heat storage solutions have been widely used in many applications. This chapter aims to introduce sensible heat storage and to summarize its industrial application at high temperatures (> 300 °C).
Soldado JC, Pesyridis A, Sphicas P, et al., 2021, Axial turbo-expander design for organic Rankine cycle waste-heat recovery with comparative heavy-duty diesel engine drive-cycle performance assessment, Frontiers in Mechanical Engineering, Vol: 7, Pages: 1-15, ISSN: 2297-3079
Despite the high thermal efficiency achieved by modern heavy-duty diesel engines, over 40% of the energy contained in the fuel is wasted as heat either in the cooling or the exhaust gases. By recovering part of the wasted energy, the overall thermal efficiency of the engine increases and the pollutant emissions are reduced. Organic Rankine cycle (ORC) systems are considered a favourable candidate technology to recover exhaust gas waste heat, because of their simplicity and small backpressure impact on the engine performance and fuel consumption. The recovered energy can be transformed into electricity or directly into mechanical power. In this study, an axial turbine expander for an ORC system was designed and optimized for a heavy-duty diesel engine for which real-world data were available. The impact of the ORC system on the fuel consumption under various operating points was investigated. Compared to an ORC system equipped with a radial turbine expander, the axial design improved fuel consumption by between 2 and 10% at low and high engine speeds. Finally, the benefits of utilising ORC systems for waste heat recovery in heavy-duty trucks is evaluated by performing various drive cycle tests, and it is found that the highest values of fuel consumption were found in the NEDC and the HDUDDS as these cycles generally involve more dynamic driving profiles. However, it was in these cycles that the ORC could recover more energy with an overall fuel consumption reduction of 5 and 4.8%, respectively.
Moran H, Voulgaropoulos V, Zogg D, et al., 2021, Experimental observations of flow boiling in horizontal tubes for direct steam generation in concentrating solar power plants, 16th UK Heat Transfer Conference (UKHTC2019), Publisher: Springer Singapore
Dirker J, van den Bergh WJ, Moran HR, et al., 2021, Influence of inlet vapour quality perturbations on the transient response of flow-boiling heat transfer, International Journal of Heat and Mass Transfer, Vol: 170, Pages: 1-12, ISSN: 0017-9310
The effect a transient heat flux has on in-tube boiling has not been studied extensively for some refrigerants commonly proposed for use in concentrated solar power organic Rankine cycle systems. In this study, the effect of abrupt step changes (upwards and downwards) in the inlet vapour quality to a flow-boiling test section on the heat transfer coefficient was considered. Tests were conducted with R-245fa at a saturation temperature of 35°C in an 800 mm horizontal smooth tube with an inner diameter of 8.31 mm and a constant test section heat flux of 7.5 kW/m2. Initial inlet vapour qualities ranged between base values of 0.15 and 0.40 with mass fluxes of 200 and 300 kg/m2s. Baseline heat transfer coefficients at steady-state conditions were determined, followed by a series of transient-state response investigations. For these, sharp upward and downward step perturbations of the inlet vapour quality were considered. It was found that for a step size magnitude of 0.13 in the vapour quality, the actual heat transfer coefficient differed from the expected quasi-steady-state heat transfer coefficients during the transient. During the downward step, it was 8.7 to 11.7% higher than the expected heat transfer coefficient, while during the upward step, it was 9.3 to 26.0% lower for a mass flux of 200 kg/m2s, depending on the initial inlet vapour quality. For a mass flux of 300 kg/m2s, it was 7.2% and 16.7% higher and 13.8 to 17.8% lower for the downward and upward step respectively.
Habibollahzade A, Mehrabadi ZK, Markides C, 2021, Comparative thermoeconomic analyses and multi-objective particle swarm optimization of geothermal combined cooling and power systems, Energy Conversion and Management, Vol: 234, ISSN: 0196-8904
Comparative parametric and multi-objective optimization analyses of three novel geothermal systems are performed for combined cooling and power generation. The first (Configuration (a)) consists of an absorption power cycle and an ejector refrigeration cycle, the second (Configuration (b)) of a modified Kalina cycle and an absorption refrigeration cycle, and the third (Configuration (c)) of a double-flash power cycle and an ejector refrigeration cycle, in all cases for power generation and cooling, respectively. Both thermodynamic (energy, exergy) and economic criteria are compared to gain an understanding of the characteristics and performance of these systems, and to ascertain the most appropriate system for different scenarios. Results from the parametric study show that Configuration (a) has the highest power output and exergy efficiency, but lowest cooling capacity and overall (power plus cooling) thermal efficiency, while Configuration (b) has the highest cooling capacity and thermal efficiency, but lowest power output and exergy efficiency. From an exergoeconomic perspective, Configuration (a) has the lowest and Configuration (b) the highest total specific cost. Configuration (c) maintains, generally, a thermoeconomic performance in-between those of the other two systems. The optimization results indicate that if the thermal efficiency and total specific cost are considered competing objectives over a range of well conditions, the optimal solutions obtained by the LINMAP method for Configurations (a) to (c) have thermal efficiencies of 19.1%, 43.0%, 42.4%, exergy efficiencies of 57.6%, 23.6%, 33.1%, total cost rates of 436 $/h, 558 $/h, 596 $/h, and total specific costs of 29.7 $/GJ, 66.9 $/GJ, 43.5 $/GJ. If the exergy efficiency and total cost rate are considered competing objectives, the corresponding values are 13.0%/29.1%/10.5%, 67.3%/30.5%/37.3%, 362/353/384 $/h, and 24.9/67.5/42.7$/GJ, respectively.
Li X, Lecompte S, Van Nieuwenhuyse J, et al., 2021, Experimental investigation of an organic Rankine cycle with liquid-flooded expansion and R1233zd(E) as working fluid, Energy Conversion and Management, Vol: 234, Pages: 1-20, ISSN: 0196-8904
A new concept of liquid-flooded expansion has been proposed as a performance increasing modification of the basic ORC targeted at low-temperature heat sources. However, little research demonstrates the potential of this technology especially experimentally. In this paper, an experimental test facility based on a conventional recuperative ORC system was constructed with an independent liquid flooding loop that enables testing the influence of liquid flooding on a modified single-screw expander as well as on the cycle itself. Experiments were performed at various pressure ratios (3.3–4.1) over the expander and flooding ratios (0–0.3) with R1233zd(E) as the working fluid and a standard lubricant oil as the flooding medium. The data reduction and uncertainty analysis were also discussed in depth. In total, 142 steady-state points were obtained. Compared with the baseline organic Rankine cycle, the maximum improvement of the liquid-flooded expansion on the expander power output can be 9.1%, although at slightly worse expander inlet conditions. The maximum enhancement of the isothermal efficiency of the expander was 9.5%. Results also showed that the expander power output, the net power output and the thermal efficiency were enhanced with the increase of the flooding liquid amount. The potential of an organic Rankine cycle system with liquid-flooded expansion can be further examined if over-expansion losses can be reduced and larger amount of oil can be injected, i.e., with higher pressure ratios and higher flooding ratios. Overall, this study provides insights into performance improvement by means of modifying the cycle thermodynamics itself.
Song J, Wang Y, Wang J, et al., 2021, Optimal design of supercritical CO2 (S-CO2) cycle systems for internal combustion engine (ICE) waste-heat recovery considering heat source fluctuations, The 4th European sCO2 Conference 2021, Pages: 205-211
Supercritical CO2 (S-CO2) cycle systems have emerged as an attractive alternative for internal combustion engine (ICE) waste heat recovery thanks to the advantages offered by CO2 as a working fluid , incl uding robust performance and system compactness. The engine exhaust gases are the main available heat source from ICEs with promising thermodynamic potential for further utilisation, and whose conditions, i.e., temperature and mass flow rate, vary based on the ICE operating strategy load. These heat source variations have a critical influence on the performance of a bottoming S-CO2 cycle system, which needs to be carefully considered in the design stage. This paper explore s the optimal design of S-CO2 cycle system s for ICE waste heat recovery considering heat source fluctuations as well as the probability of their occurrence as arising from actual ICE operation. A variety of heat source conditions are selected for separate design s of an S-CO2 cycle system and performance prediction under all possible scenarios is evaluated via detailed design and off design models, so as to select the optimal design that is able to match the heat source fluctuations and exhibit the best performance from thermodynamic and economic perspectives. The advantage of this approach relative the conventional ones that only consider one specific design condition is that it avoid s either over or under sizing of the S-CO2 cycle system, which also achieves comprehensive insight of the interplay between the bottoming heat recovery system and the ICE, and provides valuable guidance for further system optimisation.
Zhao Y, Liu M, Song J, et al., 2021, Advanced exergy analysis of a Joule-Brayton pumped thermal electricity storage system with liquid-phase storage, Energy Conversion and Management, Vol: 231, Pages: 1-19, ISSN: 0196-8904
Pumped thermal electricity storage is a thermo-mechanical energy storage technology that has emerged as a promising option for large-scale (grid) storage because of its lack of geographical restrictions and relatively low capital costs. This paper focuses on a 10 MW Joule-Brayton pumped thermal electricity storage system with liquid thermal stores and performs detailed conventional and advanced exergy analyses of this system. Results of the conventional exergy analysis on the recuperated system indicate that the expander during discharge is associated with the maximum exergy destruction rate (13%). The advanced exergy analysis further reveals that, amongst the system components studied, the cold heat exchanger during discharge is associated with the highest share (95%) of the avoidable exergy destruction rate, while during charge the same component is associated with the highest share (64%) of the endogenous exergy destruction rate. Thus, the cold heat exchanger offers the largest potential for improvement in the overall system exergetic efficiency. A quantitative analysis of the overall system performance improvement potential of the recuperated system demonstrates that increasing the isentropic efficiency of the compressor and turbine from 85% to 95% significantly increases the modified overall exergetic efficiency from 37% to 57%. Similarly, by increasing the effectiveness and decreasing the pressure loss factor of all heat exchangers, from 0.90 to 0.98 and from 2.5% to 0.5% respectively, the modified overall exergetic efficiency increases from 34% to 54%. The results of exergy analyses provide novel insight into the innovation, research and development of pumped thermal electricity storage technology.
Calise F, Cappiello FL, Vicidomini M, et al., 2021, Energy and economic assessment of energy efficiency options for energy districts: case studies in Italy and Egypt, Energies, Vol: 14, Pages: 1-24, ISSN: 1996-1073
In this research, a technoeconomic comparison of energy efficiency options for energy districts located in different climatic areas (Naples, Italy and Fayoum, Egypt) is presented. A dynamic simulation model based on TRNSYS is developed to evaluate the different energy efficiency options, which includes different buildings of conceived districts. The TRNSYS model is integrated with the plug-in Google SketchUp TRNSYS3d to estimate the thermal load of the buildings and the temporal variation. The model considers the unsteady state energy balance and includes all the features of the building’s envelope. For the considered climatic zones and for the different energy efficiency measures, primary energy savings, pay back periods and reduced CO2 emissions are evaluated. The proposed energy efficiency options include a district heating system for hot water supply, air-to-air conventional heat pumps for both cooling and space heating of the buildings and the integration of photovoltaic and solar thermal systems. The energy actions are compared to baseline scenarios, where the hot water and space heating demand is satisfied by conventional natural gas boilers, the cooling demand is met by conventional air-to-air vapor compression heat pumps and the electric energy demand is satisfied by the power grid. The simulation results provide valuable guidance for selecting the optimal designs and system configurations, as well as suggest guidelines to policymakers to define decarbonization targets in different scenarios. The scenario of Fayoum offers a savings of 67% in primary energy, but the associated payback period extends to 23 years due to the lower cost of energy in comparison to Naples.
Huang G, Wang K, Markides CN, 2021, Efficiency limits of concentrating spectral-splitting hybrid photovoltaic-thermal (PV-T) solar collectors and systems, Light: Science and Applications, Vol: 10, Pages: 1-1, ISSN: 2047-7538
Spectral splitting is an approach to the design of hybrid photovoltaic-thermal (PVT) collectors that promises significant performance benefits. However, the ultimate efficiency limits, optimal PV cell materials and optical filters of spectral-splitting PVT (SSPVT) collectors remain unclear, with a lack of consensus in the literature. We develop an idealized model of SSPVT collectors and use this to determine their electrical and thermal efficiency limits, and to uncover how these limits can be approached through the selection of optimal PV cell materials and spectral-splitting filters. Assuming that thermal losses can be minimized, the efficiency limit, optimal PV material and optimal filter all depend strongly on a coefficient w, which quantifies the value of the delivered thermal energy relative to that of the generated electricity. The total (electrical plus thermal) efficiency limit of SSPVT collectors increases at higher w and at higher optical concentrations. The optimal spectral-splitting filter is defined by sharp lower- and upper-bound energies; the former always coincides with the bandgap of the cell, whereas the latter decreases at higher w. The total effective efficiency limit of SSPVT collectors is over 20% higher than those of either standalone PV modules or standalone ST collectors when w is in the range from 0.35 to 0.50 and up to 30% higher at w ≈ 0.4. This study provides a method for identifying the efficiency limits of ideal SSPVT collectors and reports these limits, along with guidance for selecting optimal PV materials and spectral-splitting filters under different conditions and in different applications.
Ibarra R, Matar OK, Markides CN, 2021, Experimental investigations of upward-inclined stratified oil-water flows using simultaneous two-line planar laser-induced fluorescence and particle velocimetry, International Journal of Multiphase Flow, Vol: 135, Pages: 1-16, ISSN: 0301-9322
Experiments are performed in low-inclination (≤ 5°) upward stratified oil (Exxsol D140) and water flows. The flows are investigated using a novel two-line laser-based diagnostic measurement technique that combines planar laser-induced fluorescence and particle image/tracking velocimetry to obtain two-dimensional (2-D) space- and time-resolved phase and velocity information. The technique enables direct measurements in the non-refractive-index-matched fluids of interest, as opposed to substitute fluids which are matched optically but whose properties may be less representative of those in real field applications. Flow conditions span in situ Reynolds numbers in the range 1300-3630 in the oil phase and 1810-11540 in the water phase, and water cuts of 10% and 20%. Instantaneous velocity vector-fields reveal the presence of complex flow structures in the water phase at low mixture velocities, which become less coherent with increasing pipe inclinations. These structures contribute to the generation of interfacial waves, increase the unsteadiness of the flow and the rate of momentum transfer to the oil phase. Statistical information on the interface heights, mean axial and wall-normal velocity profiles and fluctuations, Reynolds stresses, and mixing lengths is obtained from the analysis of the spatiotemporally resolved phase and velocity data. The normalised mean and rms velocity characteristics (velocity fluctuations and Reynolds stress) are shown to be weakly-dependent on the pipe inclination as the mixture velocity increases. Finally, predictions from a linear mixing-length model agree reasonably well with measurements for the water layer and near-interface regions.
Moran HR, Magnini M, Markides CN, et al., 2021, Inertial and buoyancy effects on the flow of elongated bubbles in horizontal channels, International Journal of Multiphase Flow, Vol: 135, Pages: 1-13, ISSN: 0301-9322
When a long gas bubble travels in a horizontal liquid-filled channel of circular cross-section, a liquid film is formed between the bubble and the channel wall. At low Reynoldsand Bond numbers, inertial and buoyancy effects are negligible, and the liquid film thicknessis a function of the capillary number only. However, as the tube diameter is increased to themillimetre scale, both buoyancy and inertial forces may become significant. We present theresults of a systematic analysis of the bubble shape, inclination, and liquid film thicknessfor a wide range of capillary, Bond, and Reynolds numbers, namely 0.024≤Cal≤0.051,0.11≤Bo≤3.5, and 1≤Rel≤750. Three-dimensional numerical simulations of the floware performed by employing the Volume-Of-Fluid method implemented in OpenFOAM. Inagreement with previous studies, we observe that buoyancy lifts the bubble above the chan-nel axis, making the top liquid film thinner, and thickening the bottom film. As the Bondnumber approaches unity, the cross-sectional shape of the bubble deviates significantly froma circular shape, due to flattening of the bottom meniscus. The simulations demonstratethe existence of a cross-stream film flow that drains liquid out of the top film and drives ittowards the bottom film region. This drainage flow causes inclination of the bubble, witha larger inclination angle along the bottom plane of the bubble than the top. As buoyancybecomes even more significant, draining flows become less effective and the bubble inclina-tion reduces. A theoretical model for the liquid film thickness and bubble speed is proposedembedding dependencies on both capillary and Bond numbers, which shows good agreementwith the reported numerical results. Inertial forces tend to shrink the bubble cross-sectionand further lift the bubble above the channel centreline, so that the bottom film thicknessincreases significantly with the Reynolds number, whereas the top film thickness is less
Olympios AV, McTigue J, Farres Antunez P, et al., 2021, Progress and prospects of thermo-mechanical energy storage – A critical review, Progress in Energy, Vol: 3, Pages: 1-44, ISSN: 2516-1083
The share of electricity generated by intermittent renewable energy sources is increasing (now at 26% of global electricity generation) and the requirements of affordable, reliable and secure energy supply designate grid-scale storage as an imperative component of most energy transition pathways. The most widely deployed bulk energy storage solution is pumped-hydro energy storage (PHES), however, this technology is geographically constrained. Alternatively, flow batteries are location independent and have higher energy densities than PHES, but remain associated with high costs and low lifetimes, which highlights the importance of developing and utilizing additional larger-scale, longer-duration and long-lifetime energy storage alternatives. In this paper, we review a class of promising bulk energy storage technologies based on thermo-mechanical principles, which includes: compressed-air energy storage (CAES), liquid-air energy storage (LAES) and pumped-thermal electricity storage (PTES). The thermodynamic principles upon which these thermo-mechanical energy storage (TMES) technologies are based are discussed and a synopsis of recent progress in their development is presented, assessing their ability to provide reliable and cost-effective solutions. The current performance and future prospects of TMES systems are examined within a unified framework and a thermoeconomic analysis is conducted to explore their competitiveness relative to each other as well as when compared to PHES and flow battery systems. This includes carefully selected thermodynamic and economic methodologies for estimating the component costs of each configuration in order to provide a detailed and fair comparison at various system sizes. The analysis reveals that the technical and economic characteristics of TMES systems are such that, especially at higher discharge power ratings and longer discharge durations, they can offer promising performance (round-trip efficiencies higher than 60%) along wit
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