Imperial College London

ProfessorChristosMarkides

Faculty of EngineeringDepartment of Chemical Engineering

Professor of Clean Energy Technologies
 
 
 
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Contact

 

+44 (0)20 7594 1601c.markides Website

 
 
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Location

 

404ACE ExtensionSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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432 results found

Nemati H, Moghimi MA, Markides CN, 2023, Heat transfer characteristics of thermally and hydrodynamically developing flows in multi-layer mini-channel heat sinks, International Journal of Heat and Mass Transfer, Vol: 208, Pages: 124052-124052, ISSN: 0017-9310

Journal article

Wieland C, Schifflechner C, Braimakis K, Kaufmann F, Dawo F, Karellas S, Besagni G, Markides CNet al., 2023, Innovations for organic Rankine cycle power systems: Current trends and future perspectives, Applied Thermal Engineering, Vol: 225, ISSN: 1359-4311

Since the early 2000s, organic Rankine cycle (ORC) technology has experienced rapid development and market uptake. More than 4.5 GW of total ORC power plant capacity has been installed since then. Due to its flexibility, suitability for small- to medium-scale installations, its applicability to low- to medium-temperature heat sources, and compact design, ORC technology is already considered as state-of-the-art. However, to further increase the technical and economic potential of this technology, significant research is needed to further improve efficiency and other key performance indicators, as well as to address environmental and safety aspects. Therefore, it is important to research new technologies and to foster innovations for improving performance; but it is of similar importance to resolve operational challenges with applied research that focuses closer to the market, and the technology provider and user needs. This vision article outlines the current state-of-the-art and presents current research trends. Parts of this article summarize the research progress reported at the 6th International Seminar on ORC Power Systems (ORC2021) and accompanies the associated special issue published following this event. The article highlights research trends at the concept level, but also at the component and system levels and, therefore, provides a holistic overview for the interested reader regarding the current challenges and potential future research activities in this area. Beyond the variety of cycle concepts in Section 2.1, the different heat sources and applications in Section 2.2, rotating device and turbomachinery (pumps and expansion devices) options in Section 2.3, current R&D trends for working fluids are presented in Section 2.4. This editorial concludes with a more applied perspective on ORC systems, covering process control and a general perspective on the technology in Section 2.5. With an increasing number of ORC plants operating in the field, their in

Journal article

Li Z, Brun NL, Gasparrini C, Markides CNet al., 2023, A novel nonlinear radiative heat exchanger for molten-salt applications, Applied Thermal Engineering, Vol: 225, Pages: 1-11, ISSN: 1359-4311

One of the main difficulties associated with using molten salts as heat transfer fluids (HTFs) in practical applications such as in the solar and nuclear energy industries is the possibility of solidifying molten salts. Since the salts proposed for Generation-IV nuclear reactors (MSRs) have higher melting points, this is especially true for those reactors. The consequences of accidental freezing within the direct reactor auxiliary cooling system (DRACS) of MSRs could be catastrophic. DRACS is a passive safety system designed to remove decay heat from the reactor core during an emergency. The present study examines, as a case study, the freezing problem in the most vulnerable component of DRACS, namely the natural draft heat exchanger (NDHX). For this application, a unique and innovative heat exchanger design termed radiative heat exchanger (RHX) is proposed, with nonlinear heat transfer characteristics. Specifically, by suppressing convection and exploiting thermal radiation as the primary heat transfer mechanism, the RHX concept reduces the heat rejection/removal rate passively to allow safe operation when the molten salt coolant temperature drops and is near its melting point, while maintaining a high capacity for decay heat extraction in case the coolant temperature rises during an accident and the reactor becomes overheated; this moderation of the heat removal rate is accomplished without the requirement for external (manual or automatic) control. A thermohydraulic model is developed and applied to evaluate the proposed RHX with the existing conventional NDHX. Compared to the conventional design, the RHX has the potential to reduce heat removal by approximately 30 % at low temperatures and near-solidification conditions. In the case of the prototypical 20 MW reactor with a decay rate of 0.2 MW, RHX extends the freezing time of molten salt by three times, to approximately 18 h. During high-temperature events, such as the initial stages of a reactor scram, the RHX

Journal article

Yang M, Moghimi MA, Loillier R, Markides CN, Kadivar Met al., 2023, Design of a latent heat thermal energy storage system under simultaneous charging and discharging for solar domestic hot water applications, Applied Energy, Vol: 336, ISSN: 0306-2619

Latent heat thermal energy storage (LHTES) systems using phase change materials (PCMs) have appeared as promising solutions for energy storage when harnessing renewable energy sources in a wide range of engineering applications. The present study focuses on the design of horizontal shell-and-tube PCM-based LHTES systems capable of simultaneous charging and discharging in solar domestic hot water (SDHW) applications. Two scenarios are investigated: (i) initially fully charged, and (ii) initially fully discharged LHTES systems, in both cases with a 30-min charge/discharge time interval. Configurations with key geometrical design variations are considered to identify the best radial and tangential positions of the heat transfer fluid (HTF) tubes inside the shell that enhance storage performance against the following criteria: (i) gained and released thermal power, and (ii) total gained and released energy per unit mass of PCM. The distance between the hot and cold HTF tubes was maintained constant and an LHTES with horizontally aligned HTF tubes was selected as a baseline case. The findings showed that tangential displacement had a considerable impact on the performance of the system, while the effect of radial displacement was marginal. A design with displacements of ¼ tube diameter and 90° in the radial and tangential positions of the HTF tubes, respectively, had promising performance in both considered scenarios. In comparison to the baseline case, which had the hot and cold tubes positioned horizontally, and symmetrically on the shell's central plane, this configuration demonstrated a 103.02% enhancement in energy delivery in the fully discharged and a 2% enhancement in the fully charged scenario, respectively.

Journal article

He W, Huang G, Markides CN, 2023, Synergies and potential of hybrid solar photovoltaic-thermal desalination technologies, Desalination, Vol: 552, Pages: 1-18, ISSN: 0011-9164

Solar desalination has emerged as a sustainable solution for addressing global water scarcity in the energy-water nexus, particularly for remote areas in developing countries. How to use the light spectrum through solar devices can profoundly affect the solar energy utilization, desalination rates, off-grid applicability, and water affordability. Solar photovoltaic (PV) and solar thermal (ST) respectively have enabled a variety of interesting solar desalination technologies, but the resulting applications usually limit the integration between solar and desalination to be either electrically or thermally connected. Here this review paper explores smart co-uses of heat and electricity from the sun to improve the efficiency, productivity, and independence of various solar desalination processes. It is found that coupling solar photovoltaic-thermal (PVT) with desalination could be a practical and immediately deployable route for plausibly more sustainable solar desalination than current solutions, because the combined electrical and thermal energy outputs from PVT panels could be used synergistically to catalyze the improvement on the solar energy efficiency, specific energy consumption, and specific water production, as well as the operational independence for off-grid applications. Our preliminary analysis indicated an up to 20 % lower cost of PVT-desalination than current solar PV-desalination and ST-desalination but also with challenges discussed.

Journal article

Zhao Y, Song J, Liu M, Zhang K, Markides CN, Yan Jet al., 2023, Multi-objective thermo-economic optimisation of Joule-Brayton pumped thermal electricity storage systems: Role of working fluids and sensible heat storage materials, Applied Thermal Engineering, Vol: 223, Pages: 1-14, ISSN: 1359-4311

Pumped-thermal electricity storage (PTES), with the advantages of reduced geographical constraints, low capital costs, long lifetimes and flexible power ratings, is a promising large-scale energy storage technology for future power systems. In this work, thermo-economic models of Joule-Brayton PTES systems with solid thermal reservoirs (STRs) and liquid thermal stores (LTSs) were developed, and detailed parametric analyses of the two systems were performed. The results reveal that elevated maximum charging temperatures are beneficial for both thermodynamic and economic performance, and that there are optimal values for the packed-bed void fraction, heat-exchanger effectiveness and turbomachine polytropic pack from a thermo-economic perspective for the two PTES system variants. Multi-objective thermo-economic optimisation of PTES systems at a fixed power capacity (10 MW) and discharging duration (6 h) was also conducted. It is found that helium is the best working fluid candidate for both PTES systems, and that the best options for the storage material are magnetite for PTES systems with STRs, and the combination of Hitec XL + Therminol 66 + Butane for PTES systems with LTSs. In the investigated design space for both systems, PTES systems with STRs are more attractive as the total purchase cost is lower for the same roundtrip efficiency as PTES systems with LTSs. Using the technique for order of preference by similarity to the ideal solution decision-making method, and a selected weighted matrix (1:1), the optimal solutions amongst the Pareto front solutions were determined. The optimal roundtrip efficiency and total purchase cost are 71.8 % and 37.7 M$ for PTES systems with STRs, and are 56.0 % and 36.0 M$ for PTES systems with LTSs, respectively. The conclusions and proposed approach can provide useful guidance for the further development, design and optimisation of PTES technology.

Journal article

Arias DM, García-Valladares O, Besagni G, Markides CNet al., 2023, A vision of renewable thermal technologies for drying, biofuels production and industrial waste, gas or water recovery, Applied Thermal Engineering, Vol: 223, Pages: 1-7, ISSN: 1359-4311

Rapid population growth and the finite nature of fossil-fuel resources have given rise to an urgent interest in sustainable energy development. Thermal technologies include several devices and systems able to improve energy use, recycle resources and waste, and harness renewable energy sources. In particular, thermal technologies applied to renewable sources have been improving over the years in terms of performance costs have reduced, however, several challenges remain that need to be overcome. This vision article is concerned with the prevailing challenges for thermal technologies applied to renewable energy; specifically, solar drying technologies, focussing on medium and large-capacity solar drying systems, thermal devices for waste heat and gas treatment/recovery, and thermochemical technologies for the valorization of biomass to fuels. Firstly, a brief description of each technology is provided while remarking on their importance in energy transition and resource recovery scenarios. Subsequently, key challenges are identified and promising directions and areas for exploration and future research are suggested. The most common critical challenges for the further development and deployment of these technologies include improved designs and material use to decrease final costs and minimize environmental impact. In particular, managing this process within a circular economy perspective would be necessary for improved sustainability. Incorporating renewable energy sources with thermal technologies to reduce the need for fossil fuels is of major interest globally, and optimizing the combined use of mathematical, computational, and experimental tools is the most promising approach for accelerated understanding and development in this space.

Journal article

Collignon R, Caballina O, Lemoine F, Markides CN, Castanet Get al., 2023, Heat transfer enhancement in wavy falling films studied by laser-induced fluorescence, International Journal of Heat and Mass Transfer, Vol: 202, Pages: 1-15, ISSN: 0017-9310

The characteristics of thin liquid films flowing down a uniformly heated and inclined plane are investigated, with heat transfer across the wavy films quantified using up-to-date optical measurement techniques based on laser-induced fluorescence (LIF). A planar two-colour LIF technique provides the temperature distribution inside the films, but requires a high degree of wave regularity for the spatial reconstruction. A pointwise adaptation of the aforementioned technique, with much finer temporal sampling, provides simultaneous measurements of the average temperature over the film height and of the film thickness. Despite the loss of spatial resolution, the latter technique can be applied to diverse situations, especially when the waves lose their regularity and have large amplitudes. With these two approaches, the enhancement of heat transfer due to surface waves is traced along the film flow. A growing thermal boundary layer is found close to the inlet of the flow (i.e., first few cm), but its thickness remains small relative to the film thickness. Therefore, the heat transfer coefficient (HTC) is observed to be insensitive to the shape and amplitude of the waves at the free surface. A critical distance is necessary for the thermal boundary layer to be thick enough to interact with the flow structures associated with the waves, and the critical length scales with the Peclet number of the flow based on the specific flow rate. Several experiments are conducted to quantify the influence of the main flow parameters that control the HTC, such as the Reynolds number, the inclination angle and the wave frequency. For moderate wave amplitudes, the internal structure of the film is insensitive to the wave dynamics, and the temperature distribution is essentially dominated by thermal diffusion in the direction normal to the heated wall. Classical Nusselt theory is found to be applicable to the unperturbed (flat) film flows with some limited adjustments to predict the heat t

Journal article

Lotfi M, Mersch M, Markides CN, 2023, Experimental and numerical investigation of a solar-thermal humidification-dehumidification desalination plant for a coastal greenhouse, Cleaner Engineering and Technology, Pages: 100610-100610, ISSN: 2666-7908

Journal article

Guo J, Song J, Narayan S, Pervunin KS, Markides CNet al., 2023, Numerical investigation of the thermal-hydraulic performance of horizontal supercritical CO2 flows with half-wall heat-flux conditions, Energy, Vol: 264, Pages: 1-13, ISSN: 0360-5442

Thermo-hydraulic characteristics of supercritical CO2 (SCO2) flows in horizontal tubes with half-wall heat-flux conditions are investigated numerically, which is a common practice such as applications in solar parabolic trough collectors, while the heat transfer performance and the underlying mechanisms have not been fully understood. In heated flows, buoyancy acts to inhibit heat transfer when the top half of the tube wall is heated, however, when the bottom half of the tube wall is heated, this inhibition is alleviated, and the synergy between the temperature gradient and velocity fields improves thanks to the secondary flow in the near-wall region at the bottom wall. As a result, the heat transfer coefficient is ∼95% higher (on average) than in the case when the top half of the tube wall is heated. When the bottom half of the tube wall is cooled, buoyancy is expected to enhance heat transfer, while the synergy between the temperature gradient and velocity fields is supressed by the secondary flow in the near-wall region at the bottom of the tube. Conversely, when the top half of the tube wall is cooled, the buoyancy effect inhibits heat transfer, while the synergy between the temperature gradient and velocity fields is improved by the secondary flow in the near-wall region at the top of the tube, which eventually leads to an increase of ∼21% (on average) in the heat transfer coefficient relative to the case when the bottom half of the tube wall is cooled. Finally, the heat transfer discrepancy due to different heat flux conditions revealed in this study are employed in a heat exchanger model, indicating that the thermal performance of this device can be increased by ∼6% through an appropriate arrangement of the hot and cold flows without additional costs.

Journal article

Jalili M, Ghazanfari Holagh S, Chitsaz A, Song J, Markides CNet al., 2023, Electrolyzer cell-methanation/Sabatier reactors integration for power-to-gas energy storage: Thermo-economic analysis and multi-objective optimization, Applied Energy, Vol: 329, Pages: 1-17, ISSN: 0306-2619

The main objective of this study is to compare and optimize two power-to-gas energy storage systems from a thermo-economic perspective. The first system is based on a solid oxide electrolyzer cell (SOEC) combined with a methanation reactor, and the second is based on a polymer electrolyte membrane electrolyzer cell (PEMEC) integrated into a Sabatier reactor. The first system relies on the co-electrolysis of steam and carbon dioxide followed by methanation, whereas the basis of the second system is hydrogen production and conversion into methane via a Sabatier reaction. The systems are also analyzed for being applied in different countries and being fed by different renewable and non- renewable power sources. Simulation results of both systems were compared with similar studies from the literature; the errors were negligible, acknowledging the reliability and accuracy of the simulations. The results reveal that for the same carbon dioxide availability (i.e., flow rate), the SOEC-based system has higher exergy and power-to-gas efficiencies, and lower electricity consumption compared to the PEMEC-based system. However, the PEMEC-based system produces 1.2 % more methane, also with a lower heating value (LHV) of the generated gas mixture that is 7.6 % higher than that of the SOEC-based system. Additionally, the levelized cost of energy (based on the LHV) of the SOEC-based system is found to be 11 % lower. A lifecycle analysis indicates that the lowest lifecycle cost is attained when solar PV systems are employed as the electricity supply option. Eventually, the SOEC-based system is found to be more attractive for power-to-gas purposes from a thermo-economic standpoint.

Journal article

Baker W, Acha S, Jennings N, Markides C, Shah Net al., 2022, Decarbonisation of buildings: Insights from across Europe, Decarbonisation of buildings: Insights from across Europe, Publisher: The Grantham Institute

This report considers four key challenges facing the UK in reducing carbon emissions from its building stock, and shares insights from across Europe that have the potential to help the UK to decarbonise and increase the energy efficiency of its buildings.

Report

Vujanovic M, Besagni G, Duic N, Markides CNet al., 2022, Innovation and advancement of thermal processes for the production, storage, utilization and conservation of energy in sustainable engineering applications, Applied Thermal Engineering, Vol: 221, Pages: 1-7, ISSN: 1359-4311

This vision article accompanies a special issue of Applied Thermal Engineering dedicated to the 16th Conference on Sustainable Development of Energy, Water and Environment Systems (SDEWES) held in Dubrovnik in 2021, and summarizes a selection of papers presented at the conference. At the focal point are a range of topics related to thermal processes as these arise in energy production, storage, utilization and conservation, covering fundamental research, the development of technical solutions for diverse sustainable engineering applications, technoeconomic analyses, and issues relating to the potential and integration of technologies from higher-level approaches. Thermal processes are the basis of numerous sustainable engineering applications and their understanding and improvement are increasingly required in the context of improved resource use efficiency and reduced environmental impact. Applications of interest include thermal systems used in buildings, thermochemical processes, seawater treatment, thermal storage solutions and renewable energy resource use. Emerging challenges in this space have given an opportunity to scientists, researchers and engineers to actively contribute to the development of relevant technological solutions, which are covered briefly in the present article.

Journal article

Olympios AV, Sapin P, Freeman J, Olkis C, Markides CNet al., 2022, Operational optimisation of an air-source heat pump system with thermal energy storage for domestic applications, Energy Conversion and Management, Vol: 273, Pages: 1-23, ISSN: 0196-8904

Electricity-driven air-source heat pumps are a promising element of the transition to lower-carbon energy systems. In this work, operational optimisation is performed of an air-source heat pump system aimed at providing space heating and domestic hot water to a single-family dwelling. The novelty of this work lies in the development of comprehensive thermal network models of two different system configurations: (i) a standard configuration of a heat pump system coupled to a hot-water cylinder; and (ii) an advanced configuration of a heat pump system coupled to two phase-change material thermal stores. Three different objective functions (operational cost, coefficient of performance, and self-sufficiency from a locally installed solar-PV system) are investigated and the proposed mixed-integer, non-linear optimisation problems are solved by employing a genetic algorithm. Simulations are conducted at two carefully selected European locations with different climate characteristics (Oban in Scotland, UK, and Munich in Southern Germany) over four seasons represented by typical weather weeks. Comparison of key results against a conventional operating strategy reveals that the use of smart operational strategies for the operation of the heat pump and thermal stores can lead to considerable economic savings for consumers and significant performance improvements over the system lifetime. Optimising the operation of the standard configuration leads to average annual cost savings of up to 22% and 20% at the UK and German locations, respectively. The optimisation of the advanced configuration with the two PCM stores shows even higher potential for economic savings – up to 39% and 29% per year at the respective locations – as this configuration allows for greater operational flexibility, and high-electricity-price periods can be almost completely avoided. Depending on the objective function, configuration and location, the system seasonal coefficient of performance va

Journal article

Al Kindi A, Sapin PAUL, Pantaleo A, Wang KAI, Markides Cet al., 2022, Thermo-economic analysis of steam accumulation and solid thermal energy storage in direct steam generation concentrated solar power plants, Energy Conversion and Management, Vol: 274, Pages: 1-27, ISSN: 0196-8904

In direct steam generation (DSG) concentrated solar power (CSP) plants, a common thermal energy storage (TES) option relies on steam accumulation. This conventional option is constrained by temperature and pressure limits, and delivers saturated or slightly superheated steam at reduced pressure during discharge, which is undesirable for part-load turbine operation. However, steam accumulation can be integrated with sensible-heat storage in concrete to provide higher-temperature superheated steam at higher pressure. In this paper, this conventional steam accumulation option (existing) and an integrated concrete-steam TES option (extended) are described and analysed, and their thermo-economic performance are compared taking the 50-MW Khi Solar One DSG CSP plant in South Africa as a case study. The results show that the extended option with five 10-m long, square cross-section concrete blocks, each with 3600 equally spaced tubes, provides an additional TES capacity of 177 MWh compared to the existing configuration as a result of utilising most of the available thermal power in the solar receivers. Moreover, the extended option delivers 58 % more electricity with a 13 % enhancement in thermalefficiency during TES discharging mode. With an estimated additional investment of $4.2M, the levelised costs of storage and electricity for Khi Solar One with the extended TES option are, respectively, 29 % and 6 % lower than those obtained with the existing TES option. With the extended TES option, the projected net present value of Khi Solar One increases by 73 %, from $41M to $71M, at an average electricity price of 280 $/MWh.

Journal article

Guo J, Song J, Zhao Y, Pervunin KS, Markides CNet al., 2022, Thermo-hydraulic performance of heated vertical flows of supercritical CO2, International Journal of Heat and Mass Transfer, Vol: 199, Pages: 1-17, ISSN: 0017-9310

The thermo-hydraulic characteristics of heated supercritical CO2 (SCO2) flows are investigated numerically in a vertical pipe from first- and second-law perspectives, and the influence of the flow direction, mass flux and heat flux (both distribution and average value) are evaluated. Two mass flux (254 kg/(m2∙s) and 400 kg/(m2∙s)) and three average heat flux (30 kW/m2, 50 kW/m2 and 70 kW/m2) conditions are simulated at an inlet temperature of 288 K and a pressure of 8.0 MPa (corresponding pseudo-critical temperature of 308 K) in a 4-mm diameter pipe. The simulation results reveal that the heat transfer is enhanced and the irreversibility is reduced in downward flows relative to flows without gravity, whereas the heat transfer deteriorates and the irreversibility is increased in upward flows. Both higher heat fluxes and lower mass fluxes also further hinder heat transfer in the upward flows, and multiple peaks are observed in the axial wall temperature profile. Moreover, it is found that the heat-flux distribution has a significant effect on the heat transfer performance of upward flows; the heat transfer further deteriorates and the irreversibility is further increased when a linearly decreasing heat-flux distribution is applied to the wall, while the heat transfer deterioration is alleviated when a linearly increasing heat-flux distribution is used. An analysis of the heat transfer mechanism indicates that the turbulence production in the core region of the supercritical flow is suppressed, and the accumulation of gas-like fluid in the near-wall region is promoted by the buoyancy effect in upward flows, leading to severe heat transfer deterioration and a sharp increase in the wall temperature, which is similar to the critical heat-flux phenomenon in subcritical boiling. The present study provides insights into the heat transfer characteristics of SCO2 flows, as well as practical guidance on the design and optimisation of relevant components and equipment.

Journal article

Specklin M, Deligant M, Sapin P, Solis M, Wagner M, Markides CN, Bakir Fet al., 2022, Numerical study of a liquid-piston compressor system for hydrogen applications, APPLIED THERMAL ENGINEERING, Vol: 216, ISSN: 1359-4311

Journal article

Taleb AI, Barfuß C, Sapin P, White AJ, Fabris D, Markides CNet al., 2022, Simulation of thermally induced thermodynamic losses in reciprocating compressors and expanders: Influence of real-gas effects, Applied Thermal Engineering, Vol: 217, Pages: 118738-118738, ISSN: 1359-4311

The efficiency of positive-displacement components is of prime importance in determining the overall performance of a variety of thermodynamic systems. Losses due to the unsteady thermal-energy exchange between the working fluid and the solid walls of the device are an important loss mechanism. In this work, heat transfer in gas-spring devices is investigated numerically in order to focus explicitly on these thermodynamic losses. The specific aim of the study is to investigate the behaviour of real gases in gas springs and compare this to that of ideal gases in order to understand the impact of real-gas effects on the thermally induced losses in reciprocating expanders and compressors. This work relates these losses to the fluid properties and quantifies the influence of the thermophysical models applied. A CFD-model of a gas spring is developed in OpenFOAM. Four different fluid models are compared: (i) a perfect-gas model (i.e., an ideal-gas model with constant thermodynamic and transport properties); (ii) an ideal-gas model with temperature-dependent properties; (iii) a real-gas model using the Peng-Robinson equation-of-state with temperature and density-dependent properties; and (iv) a real-gas model using gas-property tables to interpolate values of thermodynamic and transport properties as functions of temperature and pressure. Results indicate that for simple, mono- and diatomic gases, like helium or nitrogen, there is a negligible difference in the pressure and temperature oscillations over a cycle between the ideal and real-gas models. However, when considering heavier (organic) molecules, such as propane, the ideal-gas model tends to overestimate the temperature and pressure (by as much as 20%) compared to the real-gas model. A real-gas model that uses the Peng-Robinson equation of state underestimates the pressure relative to the more accurate model based on look-up tables by as much as 10%. Furthermore, both ideal-gas and Peng-Robinson models underestimat

Journal article

Gupta A, Guo H, Zhai M, Markides CNet al., 2022, Primary atomization of liquid-fuel jets in confined turbulent pipe-flows of air at elevated temperatures, International Journal of Heat and Mass Transfer, Vol: 196, Pages: 1-12, ISSN: 0017-9310

The primary atomization of evaporating laminar liquid jets of either pure n-pentane or n-heptane injected continuously from a circular nozzle concentrically and axisymmetrically into high-temperature turbulent coflows of air in confined a pipe has been studied experimentally. Fuel-jet bulk velocities up to 2.7 m/s, air coflow bulk velocities up to 37 m/s and temperatures up to 1150 K were investigated within a 25 mm round pipe. In the near-field region of the injector nozzle, liquid fuel jets were formed whose length increased with the fuel injection velocity and decreased with the air velocity. Smaller droplets were formed at both higher fuel and air velocities. Droplet diameters within the range from 200 to 900 µm were measured, indicating the polydispersed nature of the fuel spray. The mean droplet diameter increased non-monotonically with increasing axial distance from the nozzle due to a complex competition between droplet coalescence and evaporation. No significant effect of the air inlet temperature was observed within the investigated range of conditions on the jet length and droplet size. However, smaller droplets were obtained in flows with lower turbulence intensities and longer longitudinal integral lengthscales. Analysis of the data reveals that the mean droplet diameters can be predicted by the empirical expression (dmean/djet) = 34 × 103 We∆1.29 Re∆-3, independent of the two perforated plates selected and used in the present work. The resulting data can contribute towards a better understanding of interfacial dynamics, and the development and validation of advanced multiphase-flow and reacting-flow models.

Journal article

Zhao Y, Song J, Zhao C, Zhao Y, Markides CNet al., 2022, Thermodynamic investigation of latent-heat stores for pumped-thermal energy storage, Journal of Energy Storage, Vol: 55, Pages: 1-24, ISSN: 2352-152X

As a large-scale energy storage technology, pumped-thermal energy storage uses thermodynamic cycles and thermal stores to achieve energy storage and release. In this paper, we explore the thermodynamic feasibility and potential of exploiting cascaded latent-heat stores in Joule-Brayton cycle-based pumped-thermal energy storage systems. A thermodynamic model of cascaded latent-heat stores is developed, and the effects of the heat store arrangement (i.e., total stage number and stage area) and fluid velocity in the thermal store tubes as key parameters that affect the heat storage and release rates, as well as the roundtrip efficiency, are evaluated. A pure electricity-storage mode and a combined heating and power mode are proposed and investigated, which allows such technologies to transform from a pure electricity storage system to an energy management system supplying power and multi-grade thermal and cold energy, and also to integrate with external waste heat and/or cold sources. Results show that the roundtrip efficiency of cascaded latent-heat stores is higher in the combined heating and power mode than in the pure electricity-storage mode, and that roundtrip efficiencies ranging from 62 % to 100 % can be achieved in the combined heating and power mode, accompanied by a corresponding pressure loss gradient ranging from 10 Pa/m to 2270 Pa/m. A comparison with packed-bed and liquid sensible-heat stores is also performed, and the results indicate that if these can be well designed, cascaded latent-heat stores can deliver comparable performance in terms of the total heat storage and release rates, roundtrip efficiency and flow resistance loss. Therefore, it is concluded that cascaded latent-heat stores can be considered for use in Joule-Brayton cycle-based pumped-thermal energy storage systems aimed at intelligent energy management for the provision of power and multi-grade heat and cold, if the costs can justify this decision.

Journal article

Peacock J, Huang G, Song J, Markides CNet al., 2022, Techno-economic assessment of integrated spectral-beam-splitting photovoltaic-thermal (PV-T) and organic Rankine cycle (ORC) systems, Energy Conversion and Management, Vol: 269, Pages: 1-18, ISSN: 0196-8904

Promising solar-based combined heating and power (CHP) systems are attracting increasing attention thanks to the favourable characteristics and flexible operation. For the first time, this study explores the potential of integrating a novel spectral-beam-splitting (SBS), hybrid photovoltaic-thermal (PVT) collector and organic Rankine cycle (ORC) technologies to maximise solar energy utilisation for electricity generation, while also providing hot water/space heating to buildings. In the proposed collector design, a parabolic trough concentrator (PTC) directs light to a SBS filter. The filter reflects long wavelengths to an evacuated tube absorber (ETA), which is thermally decoupled from the cells in the PVT tube, subsequently enabling a high-temperature fluid stream to be provided by the ETA to an ORC sub-system for secondary power generation. The SBS filter’s optical properties are a key determinant of the system’s performance, with maximum electricity generation attained when the filter transmits wavelengths between 485 and 860 nm onto the PVT tube, while the light outside this range is reflected onto the ETA. The effect of key design parameters and system capacity on techno-economic performance is investigated, considering Spain (Sevilla), the UK (London) and Oman (Muscat) as locations to capture climate and economic impacts. When operated for maximum electricity generation, the combined system achieves a ratio of heat to power of ∼1.3, which is comparable to conventional CHP systems. Of the total incident solar energy, 24% and 31% is respectively converted to useful electricity and heat, with 54% of the electricity being generated by the PV cells. In Spain, the UK and Oman, respective electricity generation of 1.8, 0.9 and 2.1 kWhel/day per m2 of PTC area is achieved. Energy prices are found to be pivotal for ensuring viable payback times, with attractive payback times as low as 4–5 years obtained in the case of Spain at system capacities o

Journal article

Guo J, Song J, Han Z, Pervunin KS, Markides CNet al., 2022, Investigation of the thermohydraulic characteristics of vertical supercritical CO2 flows at cooling conditions, Energy, Vol: 256, Pages: 1-15, ISSN: 0360-5442

The thermohydraulic characteristics of supercritical CO2 flows in a vertical tube at cooling conditions are numerically investigated, and the influence of the heat-flux condition and of the flow direction are evaluated. Constant (i.e., uniform), linearly increasing and linearly decreasing heat-flux conditions are considered as three typical heat-flux distributions over the pipe length. The simulation results show that there exists a maximum heat transfer coefficient at all heat-flux conditions when the fluid bulk temperature is slightly higher than the pseudo-critical temperature, but also that the heat-flux condition has little effect on the peak value of the heat transfer coefficient. From the viewpoint of the second law of thermodynamics, the influence of the heat-flux condition on the local entropy generation can be attributed to the distributed matching between the heat flux and the difference between the wall temperature and the fluid bulk temperature, as a better matching is associated with a higher uniformity of the local entropy generation and reduced overall irreversibilities. Upward and downward flows are considered, along with flows without gravity as a baseline case for comparison purposes, with the field synergy principle employed to explain the different phenomena in these flows. The buoyancy effect laminarises the downward flows and raises the temperature gradient; hence, the heat transfer deteriorates and the irreversibility increases. In the upward flows, the buoyancy effect augments the turbulence and alleviates the variations in temperature and velocity in the core region, consequently reducing the irreversible loss and enhancing heat transfer. The present study provides insights into the mechanisms of supercritical CO2 heat transfer characteristics as well as practical guidance on the design and optimisation of relevant components.

Journal article

Zhou X, Zhang H, Rong Y, Song J, Fang S, Xu Z, Zhi X, Wang K, Qiu L, Markides CNet al., 2022, Comparative study for air compression heat recovery based on organic Rankine cycle (ORC) in cryogenic air separation units, Energy, Vol: 255, Pages: 124514-124514, ISSN: 0360-5442

The annual energy consumption of the cryogenic air separation units (ASUs) reaches 205 TWh in China, over 80% of which is consumed in the compression processes while over 60% of the compression work is dissipated as waste heat. Efficient recovery and utilization of this amount of heat is expected to bring significant economic and environmental benefits. Organic Rankine cycle (ORC) based waste heat recovery systems for generating extra electricity or/and cooling the inlet air of the air compressors are proposed to achieve power saving and evaluated in terms of thermodynamic, economic and environmental metrics. These include an ORC-based electric generator (ORC-e) for extra electricity, an electrically coupled ORC and vapor compression refrigerator (ORC-e-VCR) and a mechanically coupled ORC and VCR (ORC-m-VCR) for extra electricity and compression power saving. A 60,000-Nm3/h scale cryogenic ASUs is selected for case studies and influence of the feed-air temperature and humidity is focused in the analyses. The results show that among these three systems, the ORC-m-VCR and ORC-e-VCR systems have similar performance when the expansion work-electricity conversion efficiency (ηe) is 90%, reaching the highest energy saving ratio of 11.7% and economic benefits with net present value achieving 154 million CNY. The ORC-m-VCR system outperforms the other two systems with ηe of 60% and 30%. This work presents comprehensive comparison of various heat recovery systems and provides practical guidance for configuration selection and design to achieve effective energy saving in air compression processes.

Journal article

Hoseinpoori P, Olympios AV, Markides CN, Woods J, Shah Net al., 2022, A whole-system approach for quantifying the value of smart electrification for decarbonising heating in buildings, Energy Conversion and Management, Vol: 268, Pages: 1-24, ISSN: 0196-8904

This paper uses a whole system approach to examine system design and planning strategies that enhance the system value of electrifying heating and identify trade-offs between consumers’ investment and infrastructure requirements for decarbonising heating in buildings. We present a novel integrated model of heat, electricity and gas systems, HEGIT, to investigate different heat electrification strategies using the UK as the case study from two perspectives: (i) a system planning perspective regarding the scope and timing of electrification; and (ii) a demand-side perspective regarding the operational and investment schemes on the consumer side. Our results indicate that complete electrification of heating increases peak electricity demand by 170%, resulting in a 160% increase in the required installed capacity in the electricity grid. However, this effect can be moderated by implementing smart demand-side schemes. Grid integration of heat pumps combined with thermal storage at the consumer-end was shown to unlock significant potential for diurnal load shifting, thereby reducing the electricity grid reinforcement requirements. For example, our results show that a 5 b£ investment in such demand-side flexibility schemes can reduce the total system transition cost by about 22 b£ compared to the case of relying solely on supply-side flexibility. In such a case, it is also possible to reduce consumer investment by lowering the output temperature of heat pumps from 55 °C to 45 °C and sharing the heating duty with electric resistance heaters. Furthermore, our results suggest that, when used at a domestic scale, ground-source heat pumps offer limited system value since their advantages (lower peak demand and reduced variations in electric heating loads) can instead be provided by grid-integration of air-source heat pumps and increased thermal storage capacity at a lower cost to consumers and with additional flexibility benefits for the electricity gr

Journal article

Jiangfeng G, Song J, Pervunin K, Markides Cet al., 2022, Heat exchanger arrangements in supercritical CO2 Brayton cycle systems: an analysis based on the distribution coordination principle, 16TH INTERNATIONAL CONFERENCE ON HEAT TRANSFER, FLUID MECHANICS AND THERMODYNAMICS AND EDITORIAL BOARD OF APPLIED THERMAL ENGINEERING

Supercritical CO2 Brayton cycle systems have emerged as apromising option for power generation, in particular at lagerscales, where it is necessary to adopt series or parallel heatexchanger arrangements in order to achieve large amounts ofheat exchange. In this work, a variety of heat exchangerarrangement schemes (series, parallel, and hybrid) are proposedand explored in the context of supercritical CO2 Brayton cyclesystems. The results show that the heat load depends not only onthe values of key parameters (thermal conductance, temperaturedifference, etc.), but also on their distribution coordination.Moreover, the whole coordination can be improved via suitablyadjusting the flow fraction among the heat exchangers,eventually improving the overall heat load. An appropriateadjustment of the flow fraction in heat exchangers that are inseries/parallel is preferable to improving the match between thehot and cold fluids, leading to a decrease in the thermodynamicirreversibility. Taking the generally recognised supercritical CO2recompression Brayton cycle systems as a focal point for ouranalysis, it is found that the optimal split ratio ranges from 0.3 to0.5, which is in line with results reported in literature. Theoptimal split ratio improves the distribution coordination of theparameters in the low-temperature recuperator, eventuallyreducing the irreversible loss. The present work providesvaluable guidance to the design and optimisation of heatexchanger arrangements for supercritical CO2 Brayton cyclesystems as well as other relevant systems.

Conference paper

Obalanlege MA, Xu J, Markides CN, Mahmoudi Yet al., 2022, Techno-economic analysis of a hybrid photovoltaic-thermal solar-assisted heat pump system for domestic hot water and power generation, Renewable Energy, Vol: 196, Pages: 720-736, ISSN: 0960-1481

This work investigates the techno-economic performance of a hybrid photovoltaic-thermal (PVT) solar-assisted heat-pump system for covering the electrical and hot-water demands of a three-bedroom terraced house in Belfast, United Kingdom with four occupants. This system combines a water-to-water heat pump with PVT panels to deliver both electricity and hot water for domestic consumption. The PVT array provides a source of low-temperature heat for the water-to-water heat pump, while cooling the PVT array and thus preventing the electrical efficiency degradation that occurs at higher operating temperatures. Analyses have been performed for PVT arrays of different size, including 12-panel, 20-panel and 24-panel systems. Results show that, thanks to its lower initial investment cost, the most economically viable system configuration for the household considered in this work is based on a 12-panel PVT array covering a total area of 16.3 m2. This system has the potential to produce 2.4 MWh of (gross) electricity and 2.0 MWh of hot water per year, which is equivalent to just over 30% of the electrical and 80% of the hot-water demands of the household under consideration, with the PVT array acting to reduce the electricity consumption of the heat pump in the heating system by a little over 60%. The system has lower recurring yearly costs relative to current household energy systems that use electricity from the grid and natural gas, despite having higher investment costs. It is also found that the system can reduce the investigated household's annual CO2 emissions by 910 kg per year (about 18 tonnes over a lifetime of 20 years) and that, with an electricity generation incentive rate of 5 p/kWh and a heat generation incentive rate of 21 p/kWh, the aforementioned system has a discounted payback period of 14 years.

Journal article

Alagumalai A, Yang L, Ding Y, Marshall JS, Mesgarpour M, Wongwises S, Rashidi MM, Taylor RA, Mahian O, Sheremet M, Wang L-P, Markides CNet al., 2022, Nano-engineered pathways for advanced thermal energy storage systems, Cell Reports Physical Science, Vol: 3, Pages: 101007-101007, ISSN: 2666-3864

Nearly half of the global energy consumption goes toward the heating and cooling of buildings and processes. This quantity could be considerably reduced through the addition of advanced thermal energy storage systems. One emerging pathway for thermal energy storage is through nano-engineered phase change materials, which have very high energy densities and enable several degrees of design freedom in selecting their composition and morphology. Although the literature has indicated that these advanced materials provide a clear thermodynamic boost for thermal energy storage, they are subject to much more complex multiscale governing phenomena (e.g., non-uniform temperatures across the medium). This review highlights the most promising configurations that have been proposed for improved heat transfer along with the critical future needs in this field. We conclude that significant effort is still required to move up the technological readiness scale and to create commercially viable novel nano-engineered phase change systems.

Journal article

Winchester B, Huang G, Sandwell P, Nelson J, Markides CNet al., 2022, Integrated simulation and optimisation of hybrid photovoltaic-thermal (PV-T) and photovoltaic systems for decentralised rural hot water provision and electrification, The 35th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, Publisher: Danmarks Tekniske Universitet (DTU)

Demands for electricity and hot water continue to rise worldwide, with many people in low-income countries, especially in rural areas, lacking access to these basic services. Decentralised minigrids, capable of powering small off-grid communities, are increasingly used in low-income countries as a means of providing power to the 13% of people globally without access to electricity. Hybrid solar photovoltaic-thermal (PV-T) collectors combine both photovoltaic (PV) cell and solar-thermal absorbers and, therefore, output both electricity and heat from a single collector with efficiency benefits over standalone PV panels and solar-thermal collectors. Despite this, no models have yet been developed capable of assessing the performance of PV-T collectors generalisable across a range of off-grid settings. We present an integrated model for simulating and optimising combined systems comprising PV panels and PV-T collectors, accurate to within +/- 5% rms error, connected to wider electrical and hot water systems, and employ this to evaluate their potential to meet both electrical and hot-water demands of rural communities. We provide a tool for simulating the lifetime output fromcombined PV and PV-T systems, assessing their economic and environmental impact, and for optimising the systems to meet the needs of specific communities. We carry out simulations for a case study of a combined PV and PV-T system in Uttar Pradesh, India, and find that the system is able to meet 59.3% and 33.5% of hot water demand for upper and lower bounds for installed capacity. We carry out optimisations for static high demand and growing low-demand scenarios and find that that 35 kWpel and 5 hot-water tanks and 75 kWpel and 15 hot-water tanks are needed to meet these demand scenarios respectively.

Conference paper

Alrowais R, Shahzad MW, Burhan M, Bashir MT, Chen Q, Xu BB, Kumja M, Markides CN, Ng KCet al., 2022, A thermally-driven seawater desalination system: proof of concept and vision for future sustainability, Case Studies in Thermal Engineering, Vol: 35, Pages: 102084-102084, ISSN: 2214-157X

Since the 1970s, commercial-scale thermally-driven seawater desalination plants have been powered by low-grade energy sources, drawn either with low-pressure bled-steam from steam turbines or the solar renewable energy harvested that are supplied at relatively low temperatures. Despite the increasing trend of seawater reverse osmosis plants, the role of thermal desalination methods (such as multi-stage flashing and multi-effect distillation) in GCC countries is still relevant in the Arabian Gulf, arising from higher salinity, the frequent algae blooms of seawater and their ability to utilize low temperature heat sources. Given the urgent need for lowering both the capital and operating costs of all processes within the desalination industry and better thermodynamic adaptation of low-grade heat input from renewable sources, the present paper addresses the abovementioned issues by investigating the direct contact spray evaporation and condensation (DCSEC) method. A DCSEC system comprises only hollow chambers (devoid of membranes or tubes, minimal use of chemical and maintenance) where vapor generation (flashing) utilizes the enthalpy difference between the sprayed feed seawater and the saturated vapor enthalpy of the vessels. Concomitantly, vapor is condensed with spray droplets of cooler water (potable) in adjacent condenser vessels, employing a simple design concept. We present detailed design and real seawater experiments data of a DCSEC system for the first time. The water production cost is calculated as $0.52/m3, which is one of the lowest figures reported compared to commercial processes presented by Global Water Intelligence.

Journal article

Sun F, Xie G, Song J, Markides CNet al., 2022, Proper orthogonal decomposition and physical field reconstruction with artificial neural networks (ANN) for supercritical flow problems, Engineering Analysis with Boundary Elements, Vol: 140, Pages: 282-299, ISSN: 0955-7997

The development of mathematical models, and the associated numerical simulations, are challenging in higher-dimensional systems featuring flows of supercritical fluids in various applications. In this paper, a data-driven methodology is presented to achieve system order reduction, and to identify important physical information within the principal flow features. Firstly, a new hybrid neural network based on radial basis function (RBF) and multi-layer perceptron (MLP) methods, namely RBF-MLP, is tested to achieve a highly nonlinear approximation. When provided with 1000 nonlinear test samples, this model provides an excellent prediction accuracy with a maximum regression coefficient (R) of 0.99 and a minimum root mean square error (RMSE) below 1%. Furthermore, the model is also proven to be flexible enough to capture accurately the turbulent fluctuation characteristics, even at significant nonlinear buoyancy conditions. Secondly, the high-dimensional buoyancy data is collected and integrated into a matrix database. Subsequently, a proper orthogonal decomposition (POD) approach is employed to reduce the high-dimensional database, and to obtain a set of low-dimensional POD basis-spanned space, which defines a reduced-order system with low-dimensional basis vectors. The results reveal that the first five order modes contain dominant flow features, accounting for 93% of the total mode energy, which can be selected to reconstruct the physical flow field. Thirdly, a new data-driven POD-ANN model is established to construct the nonlinear mapping between the full-field buoyancy data and decomposed basis vectors. It is also demonstrated that the POD-ANN model reconstructs the principal flow features accurately and reliably. This POD-ANN model can be used to provide new insights for reduced-order modelling and for reconstructing physical fields of higher-dimensional nonlinear flow cases.

Journal article

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