Publications
482 results found
McTigue JD, Farres-Antunez P, Markides CN, et al., 2022, Pumped Thermal Energy Storage With Liquid Storage, Encyclopedia of Energy Storage: Volume 1-4, Pages: 19-28, ISBN: 9780128197301
Pumped Thermal Energy Storage (PTES) uses electricity to power a heat pump; transferring heat from a cold space to a hot space forms a hot and a cold thermal reservoir, thereby storing energy. To discharge, the temperature difference between the two stores is used to drive a heat engine which generates electricity. In this chapter, PTES systems which store energy in liquids will be described. The chapter will concentrate on Joule-Brayton power cycles with molten salt storage. Part-load operation of Joule-Brayton PTES with liquid storage can be managed with “inventory control,” and it is demonstrated that this control method leads to relatively good performance over a range of operating points.
McTigue JD, Farres-Antunez P, Markides CN, et al., 2022, Integration of Heat Pumps With Solar Thermal Systems for Energy Storage, Encyclopedia of Energy Storage: Volume 1-4, Pages: 46-58, ISBN: 9780128197301
This chapter considers the combination of solar thermal systems with an energy storage device known as a Carnot Battery which charges thermal storage with a heat pump or electric heater. Integrating these systems can provide a variety of advantages, such as dispatchable renewable power generation and electricity storage services. In this chapter a variety of methods for hybridizing these systems are described, and ideal cycle analysis is used to assess their relative merits.
Van den Bergh WJ, Dirker J, Markides CN, et al., 2022, Influence of non-steady transient heat flux on flow boiling heat transfer and pressure drop in horizontal pipes, International Journal of Heat and Mass Transfer, Vol: 182, Pages: 1-15, ISSN: 0017-9310
Transient heat fluxes imposed on solid surfaces can significantly affect the heat transfer and pressure drop of flow boiling processes in diverse fields ranging from microprocessor electronics cooling to waste-heat recovery and large-scale direct steam generation in concentrated solar applications. A series of simulated transients were applied in this experimental study to investigate the saturated flow boiling of R-245fa in a horizontal pipe. The test section consisted of an 8.31 mm inner diameter, 800 mm long heated pipe. The imposed transient conditions involved spatially uniform but temporally varying heat fluxes imposed on the pipe. A baseline condition with a saturation temperature of 35°C, a heat flux of 7.5 kW/m2 and a mass flux of 200 kg/m2s was considered over a vapour quality range from 0.10 to 0.85. Motivated by direct steam generation application case studies and based on actual solar direct normal irradiation data, reductions with an amplitude of 75% of the baseline heat flux were imposed over a period of 30 s. The waveform types were step, triangular and sinusoidal pulses, and were applied in a controlled fashion. It was found that during the step perturbation, the heat transfer coefficient was approximately 30% lower than the steady state condition. The triangular and sinusoidal perturbations resulted in heat transfer coefficients that were 8% lower than that of the steady state. The pressure gradient through the test section was unaffected by the imposed perturbations.
Li W, Luo X, Yang P, et al., 2022, Solar-thermal energy conversion prediction of building envelope using thermochemical sorbent based on established reaction kinetics, Energy Conversion and Management, Vol: 252, Pages: 1-17, ISSN: 0196-8904
In this paper, the dehydration (heat charge) and hydration (discharge) reaction kinetics of thermochemical sorbents synthesised in previous work by the author is established by using the isothermal method, with the aim of understanding their thermochemical conversion behaviour and developing reaction models for numerical simulations. The effects of temperature, reaction advancement, and vapour pressure are fully considered and employed in a thermochemical energy storage model. The derived dehydration reaction activation energies of the LiOH/LiCl@ expanded graphite (LiO2C1@EG and LiO3C1@EG) sorbents are 54.7 and 52.2 kJ/mol, respectively, which are lower than that of pure LiOH·H2O. To achieve the dual-function of space heating and air purification in an efficient manner, a novel solar building envelope combining thermochemical energy storage and photocatalysis is proposed and studied numerically based on the established reaction kinetics. Fresh air can be produced during solar harvesting. The porous wall, which is made of a composite sorbent, absorbs thermal energy to heat air near the wall and thus creates a chimney effect in the channel for continuous space heating. During discharge, the desorbed heat storage wall adsorbs the moist air and the hydration reaction enthalpy can be used again for air heating. The total efficiency including the equivalent formaldehyde degradation efficiency and the thermal efficiency is around 81% when the solar radiation is 600 W/m2. Results indicate that this passive building envelope can achieve a higher heat harvesting and utilisation efficiency in a more compact space compared to previous studies. Moreover, the influence of radiation intensity on air purification and thermal performance is investigated. The present work provides new insights and promotes the integration of passive solar building envelopes and thermochemical energy storage.
Fang S, Xu Z, Zhang H, et al., 2021, High-performance multi-stage internally-cooled liquid desiccant dehumidifier for high gas-liquid flow ratios, Energy Conversion and Management, Vol: 250, Pages: 1-14, ISSN: 0196-8904
Liquid desiccant dehumidification provides a pathway to high-flow air pretreatment of air compressors for en-ergy savings. However, high air-to-solution flow ratios (i.e., over 4.0) may result in an unacceptable decrease in dehumidification effectiveness, and few studies have managed to overcome this challenge. This study aims to experimentally demonstrate that the multi-stage internally-cooled liquid desiccant dehumidifier (MILDD) is capable of improving the effectiveness at extremely high air-to-solution flow ratios over 10.0. A laboratory bench of the MILDD is designed and tested in various operational conditions. Based on the finite difference model, the experimental results of dehumidification effectiveness are analyzed in terms of the heat and mass transfer process such as irreversible loss and driving forces. The specific cooling capacity associated with the energy efficiency is further studied by considering different desiccant regeneration efficiency. In addition, the experimental latent effectiveness from present and previous work is compared and correlated. Results show that the measured latent effectiveness of the MILDD exceeds 0.42 and goes even up to 1.02 at high air-to-solution flow ratios, i.e., 8.6–20.1, while existing liquid desiccant dehumidifiers maintain a comparable effectiveness only at much lower flow ratios, i.e., below 4.0. The proposed model and correlation also have been validated with a considerable accuracy for predicting the performance of internally-cooled dehumidifiers. This work has experimentally demonstrated the ability of the multi-stage internally-cooled liquid desiccant dehumidifier to overcome the low effectiveness at high gas–liquid flow ratios, which advances the potential application of liquid desiccant dehu-midification in the air compression process.
Lebedev A, Dobroselsky K, Safonov A, et al., 2021, Control of the turbulent wake flow behind a circular cylinder by asymmetric sectoral hydrophobic coatings, Physics of Fluids, Vol: 33, Pages: 1-7, ISSN: 1070-6631
We demonstrate that sectoral coating by a hydrophobic fluoropolymer is an effective method for controlling flow separation and the turbulent wake behind a cylinder in high Reynolds number flows (Re = 2.2 × 105). Time-resolved particle image velocimetry measurements show that the shape of the wake and trajectory of large-scale vortex structures are inclined due to delayed flow separation on one side of the cylinder. Near-wall, high-resolution visualization reveals that this effect is related to micro-bubbles traveling along the coated surface. The properties of the coatings and bubble presence did not deteriorate, even after many hours of continuous facility operation.
Moran H, Pervunin K, Matar O, et al., 2021, Laser-based diagnostics of slug flow boiling in a horizontal pipe, Interfacial Phenomena and Heat Transfer, Vol: 9, Pages: 27-41, ISSN: 2169-2785
We present results from an experimental investigation on flow boiling, in the slug flow regime, of refrigerant R245fa through a 12.6-mm inner diameter horizontal plain pipe using particle image velocimetry (PIV) and an interface detection method. The study is supplemented by an overview of the state-of-the-art in experimental research of two-phase dispersed pipe flows and the development of modern optical and laser-based full-field non-intrusive measurement techniques as applied to these flows. We consider different flow conditions, with heat fluxes over the range 5.3 to 7.9 kW/m2 and mass fluxes from 300 to 460 kg/m2•s. Significant disturbances in the instantaneous velocity fields are revealed in both the noses and tails of slugs, with their values being two times higher behind vapour bubbles. The slug passage frequency is determined based on the results of the interface detection method. The vapour bubble velocity is found to increase linearly with the interfacial velocity of the two-phase mixture, while its gradient grows with the heat flux. Moreover, at increased heat fluxes the bubbles may move even faster than the mixture itself, which implies that they must significantly enhance local turbulence, thereby additionally intensifying heat transfer. In addition to the conclusions, we provide practical recommendations for possible future research in this particular field of fluid mechanics and the further development of sophisticated laser-based measurement techniques for boiling, and similar, flows.
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
McTigue JD, Farres-Antunez P, J KS, et al., 2021, Techno-economic analysis of recuperated Joule-Brayton pumped thermal energy storage, Energy Conversion and Management, Vol: 252, ISSN: 0196-8904
This article describes a techno-economic model for pumped thermal energy storage systems based on recuperated Joule-Brayton cycles and two-tank liquid storage. Models have been developed for each component, with particular emphasis on the heat exchangers. Economic metrics such as the power and energy capital costs (i.e., per-kW and per-kWh capacity) and levelized cost of storage are evaluated by gathering numerous cost correlations from the literature, thereby enabling estimates of uncertainty. It is found that the use of heat exchangers with effectivenesses up to 0.95 is economically worthwhile, but higher values lead to rapidly escalating component size and system cost. Several hot storage fluids are considered; those operating at the highest temperatures (chloride salts) improve the round-trip efficiency but the benefit is marginal and may not warrant the additional material costs and risk when compared to lower-temperature nitrate salts. Cost-efficiency trade-offs are explored using a multi-objective optimization algorithm, yielding optimal designs with round-trip efficiencies in the range 59–72% and corresponding levelized storage costs of 0.12 0.03 and 0.38 0.10 $/kWhe. Lifetime costs are competitive with lithium-ion batteries for discharging durations greater than 6 h under current scenarios.
Anvari-Moghaddam A, Besagni G, Markides CN, 2021, Multi-energy islands: Advances in local district heating, cooling and power systems, APPLIED THERMAL ENGINEERING, Vol: 199, ISSN: 1359-4311
Wang Y, Song J, Chatzopoulou MA, et al., 2021, A holistic thermoeconomic assessment of small-scale, distributed solar organic Rankine cycle (ΟRC) systems: Comprehensive comparison of configurations, component and working fluid selection, Energy Conversion and Management, Vol: 248, Pages: 1-19, ISSN: 0196-8904
In this paper, results from comprehensive thermoeconomic assessments of small-scale solar organic Rankine cycle (ORC) systems are presented based on weather data in London, UK, which is taken as representative of a temperate climate with modest temperature changes, mild winters and moderate summers. The assessments consider a range of: (i) solar collector types (flat-plate, evacuated-tube, and evacuated flat-plate collectors); (ii) power cycle configurations (basic/recuperative, partial/full evaporating, and subcritical/transcritical cycles); (iii) expander types (scroll, screw, and piston) and designs; and (iv) a set of suitable working fluids. All possible solar-ORC system designs are optimised by considering simultaneously key parameters in the solar field and in the power cycle in order to obtain the highest electricity generation, from which the best-performing systems are identified. Selected designs are then subjected to detailed, annual simulations considering the systems’ operation, explicitly considering off-design performance under actual varying weather conditions. The results indicate that, among all investigated designs, solar-ORC systems based on the subcritical recuperative ORC (SRORC), evacuated flat-plate collectors (EFPCs), a piston expander, and isobutane as the working fluid outperforms all the other system designs on thermodynamic performance, whilst having the highest annual electricity generation of 1,100 kW·h/year (73 kW·h/year/m2) and an overall thermal efficiency of 5.5%. This system also leads to the best economic performance with a levelised cost of energy (LCOE) of ~1 $/kW·h. Apart from the specific weather data used for these detailed system simulations, this study also proceeds to consider a wider range of climates associated with other global regions by varying the solar resource available to the system. Interestingly, it is found that the optimal solar-ORC system design remains unchanged for different cond
Huang G, Markides CN, 2021, Spectral-splitting hybrid PV-thermal (PV-T) solar collectors employing semi-transparent solar cells as optical filters, Energy Conversion and Management, Vol: 248, Pages: 1-15, ISSN: 0196-8904
Spectral splitting is a promising design methodology that can significantly improve the performance of hybrid photovoltaic-thermal (PV-T) collectors. However, conventional spectral-splitting PVT (SSPVT) collectors require additional optical components, which significantly increases the complexity and cost of the collector. This study proposes SSPVT collector designs that employ semi-transparent photovoltaic (PV) solar cells, which act as both the electricity generator as well as the spectral-splitting optical filter. In these designs, a part of the solar spectrum is absorbed by the semi-transparent solar cells for electricity generation, while the rest (especially the near-infrared region of the solar spectrum) is transmitted to an absorber where it generates a high-temperature thermal energy output. Three types of emerging semi-transparent solar cells, i.e., cadmium telluride (CdTe), perovskite solar cells (PVSCs) and polymer solar cells (PSCs), are selected for investigation in this context. A comprehensive two-dimensional model of such SSPVT collectors is developed and used to investigate their electrical and thermal performance. The results show that the proposed designs are effective at thermally decoupling the PV cells from the solar thermal absorber, thereby promoting a higher electrical efficiency and enabling the simultaneous generation of low-temperature thermal energy (<60 °C), along with high-temperature thermal energy (100–200 °C) under one sun. For example, a PVSC-based SSPVT collector is shown to be capable of simultaneously generating: electricity with an efficiency of 13.8%, high-temperature heat (150 °C) with a thermal efficiency of 21.1%, and low-temperature heat (50 °C) with a thermal efficiency of 22.5%. The relative performance between the CdTe-, PVSC- and PSC-based collectors depend on the relative value of the high-temperature thermal energy to that of electricity. It is concluded that semi-transparent solar cells are
Al Kindi A, Aunedi M, Pantaleo A, et al., 2021, Thermo-economic assessment of flexible nuclear power plants in the UK’s future low-carbon electricity system: role of thermal energy storage, 16th Conference on Sustainable Development of Energy, Water and Environment Systems, Publisher: SDEWES
Nuclear power plants are commonly operated as baseload units due to their low variable costs, high investment costs and limited ability to modulate their output. The increasing penetration of intermittent renewable power will require additional flexibility from conventional generation units, in order to follow the fluctuating renewable output while guaranteeing security of energy supply. In this context, coupling nuclear reactors with thermal energy storage could ensure a more continuous and efficient operation of nuclear power plants, while at other times allowing their operation to become more flexible and cost-effective. This study considers options for upgrading a 1610-MWel nuclear power plant with the addition of a thermal energy storage system and secondary power generators. The analysed configuration allows the plant to generate up to 2130 MWel during peak load, representing an increase of 32% in nominal rated power. The gross whole-system benefits of operating the proposed configuration are quantified over several scenarios for the UK’s low-carbon electricity system. Replacing conventional with flexible nuclear plant configuration is found to generate system cost savings that are between £24.3m/yr and £88.9m/yr, with the highest benefit achieved when stored heat is fully discharged in 0.5 hours (the default case is 1 hour). At an estimated cost of added flexibility of £42.7m/yr, the proposed flexibility upgrade to a nuclear power plant appears to be economically justified for a wide range of low-carbon scenarios, provided that the number of flexible nuclear units in the system is small.
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.
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.
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.
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.
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.
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
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
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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.
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.
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.
Song J, Olympios A, Mersch M, et al., 2021, Integrated organic Rankine cycle (ORC) and heat pump (HP) systems for domestic heating, ECOS 2021 - The 34rth International Conference On Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, Publisher: ECOS
Space and water heating represent a significant share of the overall energy consumption in the domestic sector. Decarbonising heat, though challenging, is acknowledged as having a key role to play(as exemplifiedby the Domestic Renewable Heat Incentive launched in 2014 in the UK, amongst other)in achievingemissionsreduction targets andalleviatingproblems related to energy shortage and environmental deterioration. Novel, highly efficientheating technologies have attracted increasing interest in this context, in particular in regions with colderclimatesand higherheating demands. Specifically, thermally-driven heat-pumping technologies are a promising solution to meetingenergy-efficiency targets by increasing the effectiveheat-to-fuelratio(HFR)of heatingsystems. In this paper,thermally-driven integrated organic Rankine cycle (ORC) and heat pump (HP) systems are proposed for domestic heating applications, in which the ORC system is driven by heat from fuel (e.g., gas) combustion and generates power to drive an air-source vapour-compression HP system. A heat-transfer fluid is heatedin the condensers of the two sub-systems to the required temperature for heat provision. Two system configurations with reversed heat-transfer fluidflow directions are presented and compared. Suitable, lowglobal-warming-potential (GWP) working fluids for both the ORC and HP systems are considered and parametric optimisation is performed to determine optimal thermodynamic performanceand system layouts. In aconfiguration in whichthe heat-transfer fluidflows firstthroughthe HP condenser andthen through the ORC condenser in series,the HFRreaches values of 1.26-2.04 forair-source temperaturesranging from -15 to 15 °C and for heat provision temperaturesfrom 35 °C to 60 °C.Aperformance enhancement up to 8-19% relative to theconfiguration withthe heat-transfer fluidflowingin thereversedirection, i.e., through the ORC condenser and then theHP condenser in serie
Mersch M, Olympios A, Sapin P, et al., 2021, Solar-thermal heating potential in the UK: A techno-economic whole-energy system analysis, ECOS 2021 - The 34rth International Conference On Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, Publisher: ECOS
We investigate the potential of solar-thermal collectorsas a sustainable heat-generation technology in the UK. The costs and performance of commercially-available collectors are surveyed and four representative collectors are investigated using a techno-economic model of solar heating for households. A parametric study of different collectorsand storage tank sizes is conducted to assess the potential and economics of different system layouts. It is shown that moderately-sized systems with a collector area of 4m2 and a tank size of 150L can provide up to 70% of the domestic hot water demand of a typical household in the UK. Based on the data from the solar-thermal heating model at household scale, performance maps are developed to estimate the heat output from different systems under varying operating conditions. These are then used to assess solar-thermal systems in a heating-sector decarbonisation model.The model is a mixed-integer linear programming model that optimises the capacity expansion of the UK domestic heating sector until 2050 as well as the annual operating schedules of the different technologies. It is found that solar-thermal heating requires incentives in order to be competitive with hydrogen boilers or electric heat pumps. However, if solar thermal collectors are deployed, they provide significant system value by reducing the demand for carbon-neutral hydrogen or electricity. An investment incentive of £3,000per solar-thermal system leads to a deployment of over150GW of solar-thermal capacity by 2050, which reduces the annual hydrogen demand by 240 TWh compared to the baseline without solar-thermal heating, while the electricity demand increases by 90 TWh due to heat pumps and electric resistive heatersbeing used as backup heatingtechnologies.
Tripanagnostopoulos Y, Huang G, Wang K, et al., 2021, Photovoltaic/Thermal Solar Collectors, Reference Module in Earth Systems and Environmental Sciences
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.
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