Publications
483 results found
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).
Olympios A, Krishnaswamy A, Stollery C, et al., 2021, Techno-economic comparison of hydrogen- and electricity-driven technologies for the decarbonisation of domestic heating, 16th Conference on Sustainable Development of Energy, Water and Environment Systems (SDEWES 2021)
Sustainable transition pathways currently being proposed for moving away from the use of natural gas and oil in domestic heating focus on two main energy vectors: electricity and hydrogen. The former transition would most likely be implemented using electric vapour-compression heat pumps, which are currently experiencing market growth in many industrialised countries. Electric heat pumps have proven to be an efficient alternative to gas boilers under certain conditions, but their techno-economic potential is highly dependent on the local climate conditions. Hydrogen-based heating systems, which could potentially utilise existing natural gas infrastructure, are being proposed as providing an attractive opportunity to maximise the use of existing assets to facilitate the energy-system transition. In this case, hydrogen can substitute natural gas in boilers or in thermally driven absorption heat pumps. Both heating system transition pathways may involve either installing new technologies at the household level or producing heat in centralised hubs and distributing it via district-heating systems. Although the potential of hydrogen in the context of heating decarbonisation has been explored in the past, a comprehensive comparison of electricity- and hydrogen-driven domestic heating options is lacking in literature. In this paper, a thermodynamic and economic methodology is developed to assess the competitiveness of a domestic-scale ammonia-water absorption heat pump driven by heat from a hydrogen boiler compared to a standalone hydrogen boiler, a classic vapour-compression heat pump and district heating, all from a homeowner’s perspective. Using a previously developed electric heat pump model, the different systems are compared for various climate conditions and fuel-price scenarios under a unified framework. The coefficient of performance of the absorption heat pump system under design conditions and the total system cost are found to be 1.4 and £5400, resp
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, Pages: 175-178
Direct steam generation (DSG) for concentrating solar power (CSP) is an emerging technology that can unlock new avenues for efficient and affordable energy utilisation and expand the current capabilities of CSP. However, the direct evaporation of steam in parabolic trough solar collectors presents control and operational challenges due to the inherently complex two-phase nature and inherent unsteadiness of the boiling flow inside the tubes. Thus, a fundamental understanding of the hydrodynamic and heat transfer characteristics of this two-phase flow is required. A bespoke flow boiling facility has been constructed for the investigation of boiling flows using laser diagnostic techniques to gain accurate spatio-temporally resolved information on the flow characteristics. The facility has been validated and a proof of concept of the application of particle image velocimetry to boiling flows performed.
Voulgaropoulos V, Aguiar GM, Bucci M, et al., 2021, Simultaneous laser- and infrared-based measurements of the life cycle of a vapour bubble during pool boiling, 16th UK Heat Transfer Conference (UKHTC2019), Publisher: Springer Singapore, Pages: 169-173
Nucleate boiling is one of the most effective heat removal modes and has found use in a wide range of cooling applications, from the scale of state-of-the-art densely packed integrated circuits to the majority of current nuclear reactors. While a substantial amount of research has been performed over the years on both pool and flow boiling, this has predominantly focused on qualitative visualisation, often high-speed, aimed at observing the complex and multiphase transport phenomena involved in nucleate boiling, and the development of empirical methods to try to quantify global quantities of interest, such as heat transfer coefficients and pressure drops. In this work, simultaneous laser-based diagnostic and infrared techniques are developed to obtain detailed spatio-temporally-resolved measurements of temperature and velocity fields for single-bubble nucleate boiling. The results show the intrinsic coupled nature of the flow and thermal fields and provide insight into the interaction of these phenomena.
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
Ajaev V, Gambaryan-Roisman T, Davalos-Orozco LA, et al., 2021, Preface to special issue: heat transfer, waves, and vortext phenomena in two-phase flows, Interfacial phenomena and heat transfer, Vol: 9, Pages: V-VI, ISSN: 2169-2785
Liu Z, Romagnoli A, Sapin P, et al., 2021, Dynamic control strategies for a solar-ORC system using first-law dynamic and data-driven machine learning models, 6th International Seminar on ORC Power Systems, Pages: 1-14, ISSN: 2709-7609
In this study, we developed and assessed the potential of dynamic control strategies for a domestic scale 1-kW solar thermal power system based on a non-recuperated organic Rankine cycle (ORC) engine coupled to a solar energy system. Such solar-driven systems suffer from part-load performance deterioration due to diurnal and inter-seasonal fluctuations in solar irradiance and ambient temperature. Real-time control strategies for adjusting the operating parameters of these systems have shown great potential to optimise their transient response to time-varying conditions, thus allowing significant gains in the power output delivered by the system. Dynamic model predictive control strategies rely on the development of computationally efficient, fast-solving models. In contrast, traditional physics-based dynamic process models are often too complex to be used for real-time controls. Machine learning techniques (MLTs), especially deep learning artificial neural networks (ANN), have been applied successfully for controlling and optimising nonlinear dynamic systems. In this study, the solar system was controlled using a fuzzy logic controller with optimised decision parameters for maximum solar energy absorption. For the sake of obtaining the optimal ORC thermal efficiency at any instantaneous time, particularly during part-load operation, the first-law ORC model was first replaced by a fast-solving feedforward network model, which was then integrated with a multi-objective genetic algorithm, such that the optimal ORC operating parameters can be obtained. Despite the fact that the feedforward network model was trained using steady-state ORC performance data, it showed comparable results compared with the first-principle model in the dynamic context, with a mean absolute error of 3.3 percent for power prediction and 0.186 percentage points for efficiency prediction.
Liang Z, Wang K, Huang G, et al., 2021, Thermodynamic analyses of a solar-hydrogen energy system based on SBS PV-T and SOEC/SOFC technologies, Pages: 1323-1326
Spectral-beam splitting (SBS) hybrid photovoltaic-thermal (PV-T) collectors are able to generate, from the same aperture area, both electricity and thermal energy, at a temperature high enough to make this useful in a wide range of applications. This is a promising technology, especially in area-constrained environments, as it can achieve very high overall (electrical plus thermal) efficiencies. Combining SBS PV-T collectors with reversible solid oxide electronic cell/solid oxide fuel cell (SOEC/SOFC) systems can help address the intermittent nature of the solar resource, since the collected solar energy by the SBS PV-T collectors can be converted to and stored as hydrogen by the SOEC module. If and when needed, the hydrogen can later be converted back to electricity by the SOFC module. In this paper, we present numerical models that has been developed for the SBS PV-T collector and SOEC/SOFC system. Parametric analyses based on these models have been performed in order to identity operational characteristics and optimal designs, looking to integrated systems that maximize overall energy efficiency. It is found that the water vapor temperature and flow rate through the SOEC/SOFC module are crucial for the performance of this component, but that this leads to a reduced SBS PV-T collector thermal efficiency. Based on the results, we propose a novel hybrid solar-hydrogen system concept that involves combining SBS PV-T collectors, a Rankine cycle engine and a reversible SOEC/SOFC module.
Denbow C, Le Brun N, Dowell NM, et al., 2020, The potential impact of Molten Salt Reactors on the UK electricity grid, Journal of Cleaner Production, Vol: 276, Pages: 1-18, ISSN: 0959-6526
The UK electricity grid is expected to supply a growing electricity demand and also to cope with electricity generation variability as the country pursues a low-carbon future. Molten Salt Reactors (MSRs) could offer a solution to meet this demand thanks to their estimated low capital costs, low operational risk, and promise of reliably dispatchable low-carbon electricity. In the published literature, there is little emphasis placed on estimating or modelling the future impact of MSRs on electricity grids. Previous modelling efforts were limited to quantifying the value of renewable energy sources, energy storage and carbon capture technologies. To date, no study has assessed or modelled MSRs as a competing power generation source for meeting decarbonization targets. Given this gap, the main objective of this paper is to explore the cost benefits for policy makers, consumers, and investors when MSRs are deployed between 2020 and 2050 for electricity generation in the UK. This paper presents results from electricity systems optimization (ESO) modelling of the costs associated with the deployment of 1350 MWe MSRs, from 2025 onwards to 2050, and compares this against a UK grid with no MSR deployment. Results illustrate a minimum economic benefit of £1.25 billion for every reactor installed over this time period. Additionally, an investment benefit occurs for a fleet of these reactors which have a combined net present value (NPV) of £22 billion in 2050 with a payback period of 23 years if electricity is sold competitively to consumers at a price of £60/MWh.
Hart M, Austin W, Acha S, et al., 2020, A roadmap investment strategy to reduce carbon intensive refrigerants in the food retail industry, Journal of Cleaner Production, Vol: 275, Pages: 1-17, ISSN: 0959-6526
High global warming potential (GWP) refrigerant leakage is the second-highest source of carbon emissions across UK supermarket retailers and a major concern for commercial organizations. Recent stringent UN and EU regulations promoting lower GWP refrigerants have been ratified to tackle the high carbon footprint of current refrigerants. This paper introduces a data-driven modelling framework for optimal investment strategies supporting the food retail industry to transition from hydrofluorocarbon (HFC) refrigeration systems to lower GWP systems by 2030, in line with EU legislation. Representative data from a UK food retailer is applied in a mixed integer linear model, making simultaneous investment decisions across the property estate. The model considers refrigeration-system age, capacity, refrigerant type, leakage and past-performance relative to peer systems in the rest of the estate. This study proposes two possible actions for high GWP HFC refrigeration systems: a) complying with legislation by retrofitting with an HFO blend (e.g. R449-A) or b) installing a new natural refrigerant system (e.g. R744). Findings indicate that a standard (i.e. business-as-usual) investment level of £6 m/yr drives a retrofitting strategy enabling significant reduction in annual carbon emissions of 71% by the end of 2030 (against the 2018 baseline), along with meeting regulatory compliance. The strategy is also highly effective at reducing emissions in the short term as total emissions during the 12-year programme are 59% lower than would have been experienced if the HFC emissions continued unabated. However, this spending level leaves the business at significant risk of refrigeration system failures as necessary investments in new systems are delayed resulting in an ageing, poorly performing estate. The model is further tested under different budget and policy scenarios and the financial, environmental, and business-risk implications are analysed. For example, under a more agg
Zhao Y, Zhao CY, Markides CN, et al., 2020, Medium- and high-temperature latent and thermochemical heat storage using metals and metallic compounds as heat storage media: A technical review, Applied Energy, Vol: 280, Pages: 1-32, ISSN: 0306-2619
Latent and thermochemical heat storage technologies are receiving increased attention due to their important role in addressing the challenges of variable renewable energy generation and waste heat availability, as well as the mismatch between energy supply and demand in time and space. However, as the operating storage temperature increases, a series of challenging technical problems arise, such as complex heat transfer mechanisms, increased corrosion, material failure, reduced strength, and high-temperature measurement difficulties, especially for metals and metallic compounds as heat storage media. This paper reviews the latest research progress in medium- and high-temperature latent and thermochemical heat storage using metals and metallic compounds as storage media from a technical perspective and provides useful information for researchers and engineers in the field of energy storage. In this paper, the status and challenges of medium- and high-temperature latent and thermochemical heat storage are first introduced, followed by an assessment of metals and metallic compounds as heat storage media in latent and thermochemical heat storage applications. This is followed by a comprehensive review of three key issues associated with medium/high-temperature latent heat storage applications: heat transfer enhancement, stability and corrosion, as well as a discussion of four key issues associated with medium/high-temperature thermochemical heat storage: heat transfer, cycling stability, mechanical property and reactor/system design. Finally, the prospects of medium/high-temperature latent and thermochemical heat storage are summarized.
Bock BD, Bucci M, Markides CN, et al., 2020, Pool boiling of refrigerants over nanostructured and roughened tubes, International Journal of Heat and Mass Transfer, Vol: 162, Pages: 1-13, ISSN: 0017-9310
This study investigated the heat transfer performance of three nanostructured surfaces and two plain surfaces: one roughened and one polished during the saturated pool boiling of refrigerants R-134a at 5 and 25 °C and R-245fa at 20 °C. Nanocoatings were applied to polished copper tubes through a layer-by-layer (LbL) process that deposited silica nanoparticles, a chemical oxidation process where an intertwined mat of sharp copper oxide (CuO) structures were generated and a commercial nanocoating process (nanoFLUX). A polished copper tube and a roughened copper tube were tested as comparison cases. All tubes were tested in the horizontal position in pool boiling over heat fluxes of 20 to 100 kW/m2, followed by a further increase in heat flux in an attempt to reach critical heat flux. The tubes were internally water heated and Wilson plots were conducted to characterise the internal heat transfer characteristics. The nanoFLUX surface had the highest heat transfer coefficients, the LbL and polished surfaces had the lowest heat transfer coefficients, and the CuO and roughened surfaces had intermediate heat transfer coefficients. The nanoFLUX surface had between 40 and 200% higher heat transfer coefficients than those of the polished tube. Both roughened tubes and nanocoated tubes showed typical exponentially increased heat transfer coefficients as heat flux was increased. However, the nanoFLUX and CuO surfaces displayed more heat flux sensitivity compared with the other surfaces. The nanoFLUX surfaces outperformed the other nanostructured surfaces due to a higher nucleation site density and outperformed the roughened tube due to a unique heat transfer mechanism. The nanoFLUX and CuO surfaces also experienced reduced critical heat flux compared with plain surfaces, thought to be caused by the trapping of vapour in the fibrous nanostructures, resulting in reduced wetting in the Cassie-Baxter state.
Bock BD, Bucci M, Markides CN, et al., 2020, Falling film boiling of refrigerants over nanostructured and roughened tubes: Heat transfer, dryout and critical heat flux, International Journal of Heat and Mass Transfer, Vol: 163, Pages: 1-19, ISSN: 0017-9310
Falling film evaporators offer an attractive alternative to flooded evaporators as the lower fluid charge reduces the impact of leaks to the environment and associated safety concerns. A study was conducted of saturated falling film boiling of two refrigerants on one polished, one roughened and three nanostructured copper tubes in order to evaluate the potential of nanostructures in falling film refrigerant evaporators. Tubes were individually tested, placed horizontally within a test chamber and heated by an internal water flow with refrigerant distributed over the outside of the tubes. Wilson plots were used to characterise the internal water heat transfer coefficients (HTCs). A layer-by-layer (LbL) process was used to create the first nanostructured tube by coating the outside of a tube with silica nanoparticles. A chemical bath was used to create copper oxide (CuO) protrusions on the second nanostructured tube. The third tube was coated by following a commercial process referred to as nanoFLUX. R-245fa at a saturation temperature of 20 °C and R-134a at saturation temperatures of 5 °C and 25 °C were used as refrigerants. Tests were conducted over a range of heat fluxes from 20 to 100 kW/m and refrigerant mass film flow rates per unit length from 0 to 0.13 kg/m/s, which corresponds to a film Reynolds number range of 0 to approximately 1500 to 2500, depending on the refrigerant. Heat fluxes were increased further to test whether the critical heat flux (CHF) point due to a departure from nucleate boiling (DNB) could be reached. The CuO and nanoFLUX tubes had the lowest film Reynolds numbers at which critical dryout occurred at heat fluxes near 20 kW/m2, but as the heat fluxes were increased towards 100 kW/m2, critical dryout occurred at the highest film Reynolds numbers of the tubes tested. Furthermore, in some higher heat flux cases, CHF as a result of DNB for the CuO and nanoFLUX tubes was reached before critical dryout occurred, and DNB became the lim
Huang G, Markides C, 2020, On the potential of employing semi-transparent solar cells as optical filters for spectral-splitting hybrid PV-thermal (PV-T) solar collectors, International Conference on Applied Energy 2020
Fatigati F, Vittorini D, Wang Y, et al., 2020, Design and operational control strategy for optimum off-design performance of an ORC plant for low-grade waste heat recovery, Energies, Vol: 13, Pages: 5846-5846, ISSN: 1996-1073
The applicability of organic Rankine cycle (ORC) technology to waste heat recovery (WHR) is currently experiencing growing interest and accelerated technological development. The utilization of low-to-medium grade thermal energy sources, especially in the presence of heat source intermittency in applications where the thermal source is characterized by highly variable thermodynamic conditions, requires a control strategy for off-design operation to achieve optimal ORC power-unit performance. This paper presents a validated comprehensive model for off-design analysis of an ORC power-unit, with R236fa as the working fluid, a gear pump, and a 1.5 kW sliding vane rotary expander (SVRE) for WHR from the exhaust gases of a light-duty internal combustion engine. Model validation is performed using data from an extensive experimental campaign on both the rotary equipment (pump, expander) and the remainder components of the plant, namely the heat recovery vapor generator (HRVH), condenser, reservoirs, and piping. Based on the validated computational platform, the benefits on the ORC plant net power output and efficiency of either a variable permeability expander or of sliding vane rotary pump optimization are assessed. The novelty introduced by this optimization strategy is that the evaluations are conducted by a numerical model, which reproduces the real features of the ORC plant. This approach ensures an analysis of the whole system both from a plant and cycle point of view, catching some real aspects that are otherwise undetectable. These optimization strategies are considered as a baseline ORC plant that suffers low expander efficiency (30%) and a large parasitic pumping power, with a backwork ratio (BWR) of up to 60%. It is found that the benefits on the expander power arising from a lower permeability combined with a lower energy demand by the pump (20% of BWR) for circulation of the working fluid allows a better recovery performance for the ORC plant with respect to t
Gupta A, Markides CN, 2020, Autoignition of an n-heptane jet in a confined turbulent hot coflow of air, Experimental Thermal and Fluid Science, Vol: 119, Pages: 1-23, ISSN: 0894-1777
The autoignition of a continuous, single jet of pure liquid n-heptane injected concentrically and axisymmetrically from a water-cooled circular nozzle into a confined turbulent hot coflow (CTHC) of air at atmospheric pressure has been investigated experimentally at air temperatures up to 1150 K and velocities up to 40 m/s. The aim of this work was to examine the emergence of liquid-fuel autoignition in the presence of flow, mixture and phase inhomogeneities, to which end, the velocity, temperature and fuel-droplet fields inside the CTHC reactor were characterized in a series of dedicated measurement campaigns. Distinct phenomena were identified concerning the emergence of various regimes: no autoignition, random spots, and continuous flame. In the random spots regime, autoignition appeared in the form of well-defined, discrete localized spots occurring randomly within the reactor, similar to observations in a similar apparatus with gaseous fuels (Markides, 2005; Markides and Mastorakos, 2005, 2011; Markides et al., 2007). High-speed optical measurements of these random spots were made from which the autoignition locations/lengths were measured, and then used to infer average autoignition delay, or residence, times from injection based on the bulk air velocity. An increase in the air temperature moved the region of autoigniting spots closer to the injector nozzle, thus decreasing the autoignition length and also decreasing the autoignition delay time. Generally, autoignition moved downstream with increasing bulk air velocity, but the delay times decreased contrary to the aforementioned earlier work with pre-vaporized n-heptane in this geometry. Of interest is the finding that at the highest investigated air velocities, the autoignition length decreased as the air velocity increased, which again deviates from the same earlier work with vaporized n-heptane. Furthermore, higher liquid injection velocities also resulted in increased autoignition lengths and times. The re
Wang K, Pantaleo AM, Herrando M, et al., 2020, Spectral-splitting hybrid PV-thermal (PVT) systems for combined heat and power provision to dairy farms, Renewable Energy, Vol: 159, Pages: 1047-1065, ISSN: 0960-1481
Dairy farming is one of the most energy- and emission-intensive industrial sectors, and offers noteworthy opportunities for displacing conventional fossil-fuel consumption both in terms of cost saving and decarbonisation. In this paper, a solar-combined heat and power (S–CHP) system is proposed for dairy-farm applications based on spectral-splitting parabolic-trough hybrid photovoltaic-thermal (PVT) collectors, which is capable of providing simultaneous electricity, steam and hot water for processing milk products. A transient numerical model is developed and validated against experimental data to predict the dynamic thermal and electrical characteristics and to assess the thermoeconomic performance of the S–CHP system. A dairy farm in Bari (Italy), with annual thermal and electrical demands of 6000 MWh and 3500 MWh respectively, is considered as a case study for assessing the energetic and economic potential of the proposed S–CHP system. Hourly simulations are performed over a year using real-time local weather and measured demand-data inputs. The results show that the optical characteristic of the spectrum splitter has a significant influence on the system’s thermoeconomic performance. This is therefore optimised to reflect the solar region between 550 nm and 1000 nm to PV cells for electricity generation and (low-temperature) hot-water production, while directing the rest to solar receivers for (higher-temperature) steam generation. Based on a 10000-m2 installed area, it is found that 52% of the demand for steam generation and 40% of the hot water demand can be satisfied by the PVT S–CHP system, along with a net electrical output amounting to 14% of the farm’s demand. Economic analyses show that the proposed system is economically viable if the investment cost of the spectrum splitter is lower than 75% of the cost of the parabolic trough concentrator (i.e., <1950 €/m2 spectrum splitter) in this application. The influenc
Le Brun N, Simpson M, Acha S, et al., 2020, Techno-economic potential of low-temperature, jacket-water heat recovery from stationary internal combustion engines with organic Rankine cycles: A cross-sector food-retail study, Applied Energy, Vol: 274, Pages: 1-14, ISSN: 0306-2619
We examine the opportunities and challenges of deploying integrated organic Rankine cycle (ORC) engines to recover heat from low-temperature jacket-water cooling circuits of small-scale gas-fired internal combustion engines (ICEs), for the supply of combined heat and power (CHP) to supermarkets. Based on data for commercially-available ICE and ORC engines, a techno-economic model is developed and applied to simulate system performance in real buildings. Under current market trends and for the specific (low-temperature) ICE + ORC CHP configuration investigated here, results show that the ICE determines most economic savings, while the ORC engine does not significantly impact the integrated CHP system performance. The ORC engines have long payback times (4–9 years) in this application, because: (1) they do not displace high-value electricity, as the value of exporting electricity to the grid is low, and (2) it is more profitable to use the heat from the ICEs for space heating rather than for electricity conversion. Commercial ORC engines are most viable (payback ≈ 4 years) in buildings with high electrical demands and low heat-to-power ratios. The influence of factors such as the ORC engine efficiency, capital cost and energy prices is also evaluated, highlighting performance gaps and identifying promising areas for future research.
Olympios AV, Pantaleo AM, Sapin P, et al., 2020, On the value of combined heat and power (CHP) systems and heat pumps in centralised and distributed heating systems: Lessons from multi-fidelity modelling approaches, Applied Energy, Vol: 274, Pages: 1-19, ISSN: 0306-2619
This paper presents a multi-scale framework for the design and comparison of centralised and distributed heat generation solutions. An extensive analysis of commercially available products on the UK market is conducted to gather information on the performance and cost of a range of gas-fired combined heat and power (CHP) systems, air-source heat pumps (ASHPs) and ground-source heat pumps (GSHPs). Data-driven models with associated uncertainty bounds are derived from the collected data, which capture cost and performance variations with scale (i.e., size and rating) and operating conditions. In addition, a comprehensive thermoeconomic (thermodynamic and component-costing) heat pump model, validated against manufacturer data, is developed to capture design-related performance and cost variations, thus reducing technology-related model uncertainties. The novelty of this paper lies in the use of multi-fidelity approaches for the comparison of the economic and environmental potential of important heat-generation solutions: (i) centralised gas-fired CHP systems associated with district heating network; (ii) gas-fired CHP systems or GSHPs providing heat to differentiated energy communities; and (iii) small-scale micro-CHP systems, ASHPs or GSHPs, installed at the household level. The pathways are evaluated for the case of the Isle of Dogs district in London, UK. A centralised CHP system appears as the most profitable option, achieving annual savings of £13 M compared to the use of decentralised boilers and a levelised cost of heat equal to 31 £/MWhth. However, if the carbon intensity of the electrical grid continues to reduce at current rates, CHP systems will only provide minimal carbon savings compared to boilers (<6%), with heat pumps achieving significant heat decarbonisation (55–62%). Differentiating between high- and low-performance and cost heat pump designs shows that the former, although 25% more expensive, have significantly lower annualised
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