Imperial College London


Faculty of EngineeringDepartment of Chemical Engineering

Professor of Clean Energy Technologies



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




404ACE ExtensionSouth Kensington Campus





Publication Type

432 results found

Olympios AV, Aunedi M, Mersch M, Krishnaswamy A, Stollery C, Pantaleo AM, Sapin P, Strbac G, Markides CNet al., 2022, Delivering net-zero carbon heat: technoeconomic and whole-system comparisons of domestic electricity- and hydrogen-driven technologies in the UK, Energy Conversion and Management, Vol: 262, ISSN: 0196-8904

Proposed sustainable transition pathways for moving away from natural gas in domestic heating focus on two main energy vectors: electricity and hydrogen. Electrification would be implemented by using vapour-compression heat pumps, which are currently experiencing market growth in many countries. On the other hand, hydrogen could substitute natural gas in boilers or be used in thermally–driven absorption heat pumps. In this paper, a consistent thermodynamic and economic methodology is developed to assess the competitiveness of these options. The three technologies, along with the option of district heating, are for the first time compared for different weather/ambient conditions and fuel-price scenarios, first from a homeowner’s and then from a whole-energy system perspective. For the former, two-dimensional decision maps are generated to identify the most cost-effective technologies for different combinations of fuel prices. It is shown that, in the UK, hydrogen technologies are economically favourable if hydrogen is supplied to domestic end-users at a price below half of the electricity price. Otherwise, electrification and the use of conventional electric heat pumps will be preferred. From a whole-energy system perspective, the total system cost per household (which accounts for upstream generation and storage, as well as technology investment, installation and maintenance) associated with electric heat pumps varies between 790 and 880 £/year for different scenarios, making it the least-cost decarbonisation pathway. If hydrogen is produced by electrolysis, the total system cost associated with hydrogen technologies is notably higher, varying between 1410 and 1880 £/year. However, this total system cost drops to 1150 £/year with hydrogen produced cost-effectively by methane reforming and carbon capture and storage, thus reducing the gap between electricity- and hydrogen-driven technologies.

Journal article

Hull G, Markides CN, Reay D, Zhao T, Bansal P, Murthy SSet al., 2022, Applied Thermal Engineering celebrates its 25th anniversary, Applied Thermal Engineering, Vol: 208, Pages: 1-2, ISSN: 1359-4311

Journal article

Voulgaropoulos V, Aguiar GM, Markides CN, Bucci Met al., 2022, Simultaneous laser-induced fluorescence, particle image velocimetry and infrared thermography for the investigation of the flow and heat transfer characteristics of nucleating vapour bubbles, International Journal of Heat and Mass Transfer, Vol: 187, Pages: 1-14, ISSN: 0017-9310

Boiling is an effective heat removal process, used for heat exchange and thermal management purposes in many technological applications, from the scale of microelectronic devices to nuclear reactors. However, the physical mechanisms involved in this process are not fully understood yet due to the complexity that arises from the many interacting underlying sub-processes involved in the nucleation, growth and detachment of bubbles that occur during the process. Here, we present an advanced methodology based on combined, synchronized high-speed infrared (IR) thermometry, ratiometric two-colour laser-induced fluorescence (2cLIF) and particle image velocimetry (PIV), along with sample results of an experimental investigation conducted in deionized water, aimed at elucidating the mechanisms involved in the bubble lifecycle. IR thermometry is used to measure the time-dependent 2-D temperature and heat flux distributions over a boiling surface, and 2cLIF is used to measure the time-dependent temperature-field in a vertical plane, in the liquid phase around developing bubbles. Furthermore, PIV is used to measure the velocity fields around the bubbles, in the same plane as 2cLIF. The investigation reveals and allows us to quantify fundamental heat transfer aspects such as the contribution of triple contact line evaporation to the bubble growth process, the dynamics of the near-wall superheated liquid layer, the mixing effect produced by bubble growth and departure, convection effects around the bubble, and quenching heat transfer. Specifically, we observe that, in our experiment, with slowly growing bubbles, the microlayer does not form, and the evaporation at the solid-liquid-vapour contact line contributes to approximately one third of the total heat transferred to the bubble. We also observed that the fluid that rewets the dry spot at the bubble base, as the bubble departs from the boiling surface, comes from the near-wall superheated thermal boundary layer adjacent to the

Journal article

Al Kindi A, Aunedi M, Pantaleo A, Strbac G, Markides Cet al., 2022, Thermo-economic assessment of flexible nuclear power plants in future low-carbon electricity systems: Role of thermal energy storage, Energy Conversion and Management, Vol: 258, ISSN: 0196-8904

The increasing penetration of intermittent renewable power will require additional flexibility from conventional plants, in order to follow the fluctuating renewable output while guaranteeing security of energy supply. In this context, coupling nuclear reactors with thermal energy storage could ensure a more continuous and efficient operation of nuclear power plants, while at other times allowing their operation to become more flexible and cost-effective. This study proposes options for upgrading a 1610-MWel nuclear power plant with the addition of a thermal energy storage system and secondary power generators. The total whole-system benefits of operating the proposed configuration are quantified for several scenarios in the context of the UK’s national electricity system using a whole-system model that minimises the total system costs. The proposed configuration allows the plant to generate up to 2130 MWel during peak load, representing an increase of 32% in nominal rated power. This 520 MWel of additional power is generated by secondary steam Rankine cycle systems (i.e., with optimised cycle thermal efficiencies of 24% and 30%) and by utilising thermal energy storage tanks with a total heat storage capacity of 1950 MWhth. Replacing conventional with flexible nuclear power plants is found to generate whole-system cost savings between £24.3m/yr and £88.9m/yr, with the highest benefit achieved when stored heat is fully discharged in 0.5 h. At an estimated cost of added flexibility of £42.7m/yr, the proposed flexibility upgrades to such nuclear power plants appears to be economically justified with net system benefits ranging from £4.0m/yr to £31.6m/yr for the examined low-carbon scenarios, provided that the number of flexible nuclear plants in the system is small. This suggests that the value of this technology is system dependent, and that system characteristics should be adequately considered when evaluating the benefits of diffe

Journal article

Zhu S, Yu G, Jiang C, Wang T, Zhang L, Wu Z, Hu J, Markides CN, Luo Eet al., 2022, A novel thermoacoustically-driven liquid metal magnetohydrodynamic generator for future space power applications, Energy Conversion and Management, Vol: 258, Pages: 115503-115503, ISSN: 0196-8904

The generation of electricity in space is a major issue for space exploration, and among the viable alternatives, nuclear power systems appear to present a particularly suitable solution, especially for deep space exploration. Recent developments in thermoacoustic engine and liquid metal magnetohydrodynamic (LMMHD) generator technologies have shown that thermoacoustically-driven LMMHD generators are a promising thermal-to-electrical converter option for space nuclear reactors. In order to improve the power density and capacity of current thermoacoustically-driven LMMHD generators, a novel three-stage looped thermoacoustically-driven LMMHD generator is proposed and investigated in this work. A numerical model of the integrated system including a lumped parameter sub-model for the LMMHD generator is developed and validated. Using this model, the effect of key geometric and operating parameters on the operation and performance of the proposed system are investigated numerically, and acoustic field distributions are presented. The results indicate that when the heat source and sink temperatures are 900 K and 300 K, respectively, a thermal-to-electric efficiency of 27.7% with a total electric power of 4750 W can be obtained at a load factor of 0.92. This work provides guidance for the design of similar systems and contributes to the development of a new thermal-to-electrical conversion technology for space applications.

Journal article

Li G, Li M, Taylor R, Hao Y, Besagni G, Markides CNet al., 2022, Solar energy utilisation: Current status and roll-out potential, APPLIED THERMAL ENGINEERING, Vol: 209, ISSN: 1359-4311

Journal article

Zhao Y, Song J, Liu M, Zhao Y, Olympios AV, Sapin P, Yan J, Markides CNet al., 2022, Thermo-economic assessments of pumped-thermal electricity storage systems employing sensible heat storage materials, Renewable Energy, Vol: 186, Pages: 431-456, ISSN: 0960-1481

Three distinct pumped-thermal electricity storage (PTES) system variants based on currently available sensible heat storage materials are presented: (i) Joule-Brayton PTES systems with solid thermal reservoirs; (ii) Joule-Brayton PTES systems with liquid thermal stores; and (iii) transcritical Rankine PTES systems with liquid thermal stores. Parametric design optimisation is performed for each PTES system variant considering various system configurations, working fluids and storage media from a thermodynamic perspective. The results show that amongst the investigated systems, the recuperative transcritical Rankine PTES system with CO2 as the working fluid and Therminol VP-1 as the storage material achieves the highest roundtrip efficiency of 68%. Further to the optimal thermodynamic performance of these system, their corresponding capital costs are also evaluated. The economic performance comparisons of selected optimal PTES designs reveal that the recuperative transcritical Rankine PTES system with CO2 and Therminol VP-1 exhibits the lowest capital cost of 209 M$ for the given power capacity (50 MW) and discharge duration (6 h). The influences of the power capacity and discharge duration are also investigated, with results showing that the lowest power and energy capital costs are 3790 $/kW (discharge duration of 2 h) and 396 $/kWh (discharge duration of 12 h), respectively.

Journal article

Madurai Elavarasan R, Mudgal V, Selvamanohar L, Wang K, Huang G, Shafiullah GM, Markides CN, Reddy KS, Nadarajah Met al., 2022, Pathways toward high-efficiency solar photovoltaic thermal management for electrical, thermal and combined generation applications: A critical review, Energy Conversion and Management, Vol: 255, Pages: 1-31, ISSN: 0196-8904

Photovoltaic (PV) panels convert a portion of the incident solar radiation into electrical energy and the remaining energy (>70 %) is mostly converted into thermal energy. This thermal energy is trapped within the panel which, in turn, increases the panel temperature and deteriorates the power output as well as electrical efficiency. To obtain high-efficiency solar photovoltaics, effective thermal management systems is of utmost. This article presents a comprehensive review that explores recent research related to thermal management solutions as applied to photovoltaic technology. The study aims at presenting a wide range of proposed solutions and alternatives in terms of design approaches and concepts, operational methods and other techniques for performance enhancement, with commentary on their associated challenges and opportunities. Both active and passive thermal management solutions are presented, which are classified and discussed in detail, along with results from a breadth of experimental efforts into photovoltaic panel performance improvements. Approaches relying on radiative, as well as convective heat transfer principles using air, water, heat pipes, phase change materials and/or nanoparticle suspensions (nanofluids) as heat-exchange media, are discussed while including summaries of their unique features, advantages, disadvantages and possible applications. In particular, hybrid photovoltaic-thermal (PV-T) collectors that use a coolant to capture waste heat from the photovoltaic panels in order to deliver an additional useful thermal output are also reviewed, and it is noted that this technology has a promising potential in terms of delivering high-efficiency solar energy conversion. The article can act as a guide to the research community, developers, manufacturers, industrialists and policymakers in the design, manufacture, application and possible promotion of high-performance photovoltaic-based technologies and systems.

Journal article

Gkaniatsou E, Chen C, Cui FS, Zhu X, Sapin P, Nouar F, Boissière C, Markides CN, Hensen J, Serre Cet al., 2022, Producing cold from heat with aluminum carboxylate-based metal-organic frameworks, Cell Reports Physical Science, Vol: 3, Pages: 1-18, ISSN: 2666-3864

Worldwide cooling energy demands will increase by four times by 2050. Thermally driven cooling technology is an alternative solution to electric heat pumps in removing hazardous refrigerants and harnessing renewables and waste heat. We highlight the advantages of water-stable microporous aluminum-carboxylate-based metal-organic frameworks, or Al-MOFs, as sorbents in the application of producing cold from heat. Here, we synthesize the Al-MOFs with green and scalable processes, which are prerequisites for exploring various industrial and civil applications. A proof-of-concept full-scale adsorption chiller with different Al-MOFs is built up with optimized configurations derived from various characterization techniques. The tested Al-MOFs achieve thermal efficiency above 0.6 and specific cooling power over 1 kW/kg in typical cooling scenarios. Notably, when solar thermal energy is used as the heat source in an outdoor validation, Al-MOFs are weather-resilient solutions that exhibit a stable energy conversion efficiency under fluctuating operating conditions (ambient temperature and solar irradiation).

Journal article

Mameli M, Besagni G, Bansal PK, Markides CNet al., 2022, Innovations in pulsating heat pipes: From origins to future perspectives, Applied Thermal Engineering, Vol: 203, Pages: 1-9, ISSN: 1359-4311

Since the early 1990s, the pulsating heat pipe (PHP) has emerged as one of the most innovative, effective and potentially more convenient passive two-phase heat transfer systems, thanks to its good performance, versatility, and construction simplicity. On the other hand, the PHP is characterized by complex thermohydraulic behaviour that still presents a true challenge to designers, which has led to significant interest by a growing number of researchers.The technological readiness level (TLR) of this technology is quite broad depending on the application: for instance, the industrial community is starting to consider the PHP as a reliable solution for electronic cooling in ground conditions, while implementations in the cryogenic temperature range and in space environments is also being extensively explored.This vision paper aims at shedding light on the current knowledge and prediction capability of PHP numerical models, on unsolved phenomenological issues, on the current technological challenges and the future perspectives of this fascinating heat transfer device.Specifically, after a general introduction and a brief overview of the current knowledge and the open issues of PHPs, special focus is devoted to the following topics: flat-plate PHP assessments; advancements in PHP modelling and simulation; flow stabilization techniques; non-conventional fluids subdivided into fluid mixtures, self-rewetting fluids, nanofluids; cryogenic applications, space applications, and finally the newest frontiers of flexible PHPs.Each section is accompanied by a brief roadmap providing directions for future research based on key challenges, which are also gathered and summarized in the final outlook section.

Journal article

Romanos P, Al Kindi A, Pantaleo AM, Markides CNet al., 2022, Flexible nuclear plants with thermal energy storage and secondary power cycles: Virtual power plant integration in a UK energy system case study, e-Prime - Advances in Electrical Engineering, Electronics and Energy, Vol: 2, Pages: 1-24, ISSN: 2772-6711

Electricity markets are fast changing because of the increasing penetration of intermittent renewable generation, leading to a growing need for the flexible operation of power plants to provide regulation services to the grid. Previous studies have suggested that conventional power plants (e.g., nuclear) may benefit from the integration of thermal energy storage (TES), as this enables greater flexibility. In conventional Rankine-cycle power plants, steam can be extracted during off-peak periods to charge TES tanks filled with phase-change materials (PCMs); at a later time, when this is required and/or economically favourable, these tanks can feed secondary thermal power plants to generate power, for example, by acting as evaporators of organic Rankine cycle (ORC) plants. This solution offers greater flexibility than TES-only solutions that store thermal energy and then release this back to the base power plant, as it allows both derating and over-generation. The solution is applied here to a specific case study of a 670 MW el nuclear power plant in the UK, which is a typical baseload power plant not intended for flexible operation. It is found a maximum combined power of 822 MW el can be delivered during peak demand, which is 23% higher than the base plant’s (nominal) rated power, and a maximum derating of 40%, i.e., down to 406 MW el during off-peak demand. An operational energy management strategy (EMS) is then proposed for optimising the charging of the TES tanks during off-peak demand periods and for controlling the discharging of the tanks for electricity generation during peak-demand periods. An economic analysis is performed to evaluate the potential benefits of this EMS. Profitability in the case study considered here can result when the average peak and off-peak electricity price variations are at least double those that occurred in the UK market in 2019 (with recent data now close to this), and when TES charge/discharge cycles are performed more than

Journal article

Sarabia Escriva EJ, Hart M, Acha Izquierdo S, Soto Frances V, Shah N, Markides Cet al., 2022, Techno-economic evaluation of integrated energy systems for heat recovery applications in food retail buildings, Applied Energy, Vol: 305, ISSN: 0306-2619

Eliminating the use of natural gas for non-domestic heat supply is an imperative component of net-zero targets. Techno-economic analyses of competing options for low-carbon heat supply are essential for decision makers developing decarbonisation strategies. This paper investigates the impact various heat supply configurations can have in UK supermarkets by using heat recovery principles from refrigeration systems under different climatic conditions. The methodology builds upon a steady-state model that has been validated in previous studies. All refrigeration integrated heating and cooling (RIHC) systems employ CO2 booster refrigeration to recover heat and provide space heating alongside various technologies such as thermal storage, air-source heat pumps (ASHPs) and direct electric heaters. Seven cases evaluating various technology combinations are analysed and compared against a conventional scenario in which the building is heated with a natural gas boiler. The specific combinations of technologies analysed here contrasts trade-offs and is a first in the literature. The capital costs of these projects are considered, giving insights into their business case. Results indicate that electric heaters are not cost-competitive in supermarkets. Meanwhile, RIHC and ASHP configurations are the most attractive option, and if a thermal storage tank system with advanced controls is included, the benefits increase even further. Best solutions have a 6.3% ROI, a payback time of 16 years while reducing energy demand by 62% and CO2 emissions by 54%. Such investments will be difficult to justify unless policy steers decision makers through incentives or the business case changes by implementing internal carbon pricing.

Journal article

Tripanagnostopoulos Y, Huang G, Wang K, Markides CNet al., 2022, 3.08 - Photovoltaic/Thermal Solar Collectors, Comprehensive Renewable Energy, Second Edition: Volume 1-9, Pages: 294-345, ISBN: 9780128197271

Photovoltaic (PV) modules convert, depending on cell type, about 5–20% of the incoming solar radiation into electricity, with most of the remaining energy converted to heat that is ultimately rejected to the environment and lost, while also increasing the temperature of the PV cells and therefore decreasing their electrical efficiency. This undesirable effect can be partially avoided by implementing a suitable thermal management solution involving the circulation of a coolant fluid. Such solar collectors, which incorporate a circulating fluid that is at a lower temperature than that of PV cells with the aim of cooling the latter, and that is thereby heated through its interaction with the module, are referred to hybrid PV/thermal (PV/T or PVT) collectors. A prominent associated feature of PV/T collectors is that they provide dual (electrical and thermal) energy outputs, thus increasing the total useful energy delivered from a given area. Most PV/T collectors can be split into water-cooled (PV/T-water) and air-cooled (PV/T-air) types, although the coolant medium can be any other fluid phase. Commercial products exist and installations are available, however, this solar technology has not yet found the market penetration of PV or solar-thermal systems, and most PV/T applications have been for demonstration purposes. In addition to flat-type PV/T collector designs, which are the most common commercially, a number of alternative PV/T collector designs have been proposed, including flat-box collectors, designs based on spectral splitting concepts and concentrating PV/T collectors (CPVT) that employ reflectors or lenses and concentrating PV cells, in all cases aiming to deliver a cost-effective solution for solar energy conversion and the delivery of useful energy to different end-users in different applications. Hybrid PV/T solar collectors can be considered either as PV modules combined with a cooling component that can deliver a useful thermal output (hot water o

Book chapter

Van den Bergh WJ, Dirker J, Markides CN, Meyer JPet al., 2022, Influence of non-steady transient heat flux on flow boiling heat transfer and pressure drop in horizontal pipes, International Journal of Heat and Mass Transfer, Vol: 182, Pages: 1-15, ISSN: 0017-9310

Transient heat fluxes imposed on solid surfaces can significantly affect the heat transfer and pressure drop of flow boiling processes in diverse fields ranging from microprocessor electronics cooling to waste-heat recovery and large-scale direct steam generation in concentrated solar applications. A series of simulated transients were applied in this experimental study to investigate the saturated flow boiling of R-245fa in a horizontal pipe. The test section consisted of an 8.31 mm inner diameter, 800 mm long heated pipe. The imposed transient conditions involved spatially uniform but temporally varying heat fluxes imposed on the pipe. A baseline condition with a saturation temperature of 35°C, a heat flux of 7.5 kW/m2 and a mass flux of 200 kg/m2s was considered over a vapour quality range from 0.10 to 0.85. Motivated by direct steam generation application case studies and based on actual solar direct normal irradiation data, reductions with an amplitude of 75% of the baseline heat flux were imposed over a period of 30 s. The waveform types were step, triangular and sinusoidal pulses, and were applied in a controlled fashion. It was found that during the step perturbation, the heat transfer coefficient was approximately 30% lower than the steady state condition. The triangular and sinusoidal perturbations resulted in heat transfer coefficients that were 8% lower than that of the steady state. The pressure gradient through the test section was unaffected by the imposed perturbations.

Journal article

Li W, Luo X, Yang P, Wang Q, Zeng M, Markides CNet al., 2022, Solar-thermal energy conversion prediction of building envelope using thermochemical sorbent based on established reaction kinetics, Energy Conversion and Management, Vol: 252, Pages: 1-17, ISSN: 0196-8904

In this paper, the dehydration (heat charge) and hydration (discharge) reaction kinetics of thermochemical sorbents synthesised in previous work by the author is established by using the isothermal method, with the aim of understanding their thermochemical conversion behaviour and developing reaction models for numerical simulations. The effects of temperature, reaction advancement, and vapour pressure are fully considered and employed in a thermochemical energy storage model. The derived dehydration reaction activation energies of the LiOH/LiCl@ expanded graphite (LiO2C1@EG and LiO3C1@EG) sorbents are 54.7 and 52.2 kJ/mol, respectively, which are lower than that of pure LiOH·H2O. To achieve the dual-function of space heating and air purification in an efficient manner, a novel solar building envelope combining thermochemical energy storage and photocatalysis is proposed and studied numerically based on the established reaction kinetics. Fresh air can be produced during solar harvesting. The porous wall, which is made of a composite sorbent, absorbs thermal energy to heat air near the wall and thus creates a chimney effect in the channel for continuous space heating. During discharge, the desorbed heat storage wall adsorbs the moist air and the hydration reaction enthalpy can be used again for air heating. The total efficiency including the equivalent formaldehyde degradation efficiency and the thermal efficiency is around 81% when the solar radiation is 600 W/m2. Results indicate that this passive building envelope can achieve a higher heat harvesting and utilisation efficiency in a more compact space compared to previous studies. Moreover, the influence of radiation intensity on air purification and thermal performance is investigated. The present work provides new insights and promotes the integration of passive solar building envelopes and thermochemical energy storage.

Journal article

Fang S, Xu Z, Zhang H, Rong Y, Zhou X, Zhi X, Wang K, Markides CN, Qiu Let al., 2021, High-performance multi-stage internally-cooled liquid desiccant dehumidifier for high gas-liquid flow ratios, Energy Conversion and Management, Vol: 250, Pages: 1-14, ISSN: 0196-8904

Liquid desiccant dehumidification provides a pathway to high-flow air pretreatment of air compressors for en-ergy savings. However, high air-to-solution flow ratios (i.e., over 4.0) may result in an unacceptable decrease in dehumidification effectiveness, and few studies have managed to overcome this challenge. This study aims to experimentally demonstrate that the multi-stage internally-cooled liquid desiccant dehumidifier (MILDD) is capable of improving the effectiveness at extremely high air-to-solution flow ratios over 10.0. A laboratory bench of the MILDD is designed and tested in various operational conditions. Based on the finite difference model, the experimental results of dehumidification effectiveness are analyzed in terms of the heat and mass transfer process such as irreversible loss and driving forces. The specific cooling capacity associated with the energy efficiency is further studied by considering different desiccant regeneration efficiency. In addition, the experimental latent effectiveness from present and previous work is compared and correlated. Results show that the measured latent effectiveness of the MILDD exceeds 0.42 and goes even up to 1.02 at high air-to-solution flow ratios, i.e., 8.6–20.1, while existing liquid desiccant dehumidifiers maintain a comparable effectiveness only at much lower flow ratios, i.e., below 4.0. The proposed model and correlation also have been validated with a considerable accuracy for predicting the performance of internally-cooled dehumidifiers. This work has experimentally demonstrated the ability of the multi-stage internally-cooled liquid desiccant dehumidifier to overcome the low effectiveness at high gas–liquid flow ratios, which advances the potential application of liquid desiccant dehu-midification in the air compression process.

Journal article

Lebedev A, Dobroselsky K, Safonov A, Starinskiy S, Sulyaeva V, Lobasov A, Dulin V, Markides CNet al., 2021, Control of the turbulent wake flow behind a circular cylinder by asymmetric sectoral hydrophobic coatings, Physics of Fluids, Vol: 33, Pages: 1-7, ISSN: 1070-6631

We demonstrate that sectoral coating by a hydrophobic fluoropolymer is an effective method for controlling flow separation and the turbulent wake behind a cylinder in high Reynolds number flows (Re = 2.2 × 105). Time-resolved particle image velocimetry measurements show that the shape of the wake and trajectory of large-scale vortex structures are inclined due to delayed flow separation on one side of the cylinder. Near-wall, high-resolution visualization reveals that this effect is related to micro-bubbles traveling along the coated surface. The properties of the coatings and bubble presence did not deteriorate, even after many hours of continuous facility operation.

Journal article

Moran H, Pervunin K, Matar O, Markides Cet al., 2021, Laser-based diagnostics of slug flow boiling in a horizontal pipe, Interfacial Phenomena and Heat Transfer, Vol: 9, Pages: 27-41, ISSN: 2169-2785

We present results from an experimental investigation on flow boiling, in the slug flow regime, of refrigerant R245fa through a 12.6-mm inner diameter horizontal plain pipe using particle image velocimetry (PIV) and an interface detection method. The study is supplemented by an overview of the state-of-the-art in experimental research of two-phase dispersed pipe flows and the development of modern optical and laser-based full-field non-intrusive measurement techniques as applied to these flows. We consider different flow conditions, with heat fluxes over the range 5.3 to 7.9 kW/m2 and mass fluxes from 300 to 460 kg/m2•s. Significant disturbances in the instantaneous velocity fields are revealed in both the noses and tails of slugs, with their values being two times higher behind vapour bubbles. The slug passage frequency is determined based on the results of the interface detection method. The vapour bubble velocity is found to increase linearly with the interfacial velocity of the two-phase mixture, while its gradient grows with the heat flux. Moreover, at increased heat fluxes the bubbles may move even faster than the mixture itself, which implies that they must significantly enhance local turbulence, thereby additionally intensifying heat transfer. In addition to the conclusions, we provide practical recommendations for possible future research in this particular field of fluid mechanics and the further development of sophisticated laser-based measurement techniques for boiling, and similar, flows.

Journal article

McTigue JD, Farres-Antunez P, J KS, Markides CN, White AJet al., 2021, Techno-economic analysis of recuperated Joule-Brayton pumped thermal energy storage, Energy Conversion and Management, Vol: 252, ISSN: 0196-8904

This article describes a techno-economic model for pumped thermal energy storage systems based on recuperated Joule-Brayton cycles and two-tank liquid storage. Models have been developed for each component, with particular emphasis on the heat exchangers. Economic metrics such as the power and energy capital costs (i.e., per-kW and per-kWh capacity) and levelized cost of storage are evaluated by gathering numerous cost correlations from the literature, thereby enabling estimates of uncertainty. It is found that the use of heat exchangers with effectivenesses up to 0.95 is economically worthwhile, but higher values lead to rapidly escalating component size and system cost. Several hot storage fluids are considered; those operating at the highest temperatures (chloride salts) improve the round-trip efficiency but the benefit is marginal and may not warrant the additional material costs and risk when compared to lower-temperature nitrate salts. Cost-efficiency trade-offs are explored using a multi-objective optimization algorithm, yielding optimal designs with round-trip efficiencies in the range 59–72% and corresponding levelized storage costs of 0.12 0.03 and 0.38 0.10 $/kWhe. Lifetime costs are competitive with lithium-ion batteries for discharging durations greater than 6 h under current scenarios.

Journal article

Voulgaropoulos V, Kadivar M, Moghimi MA, Maher M, Alawadi H, Matar OK, Markides CNet al., 2021, A combined experimental and computational study of phase-change dynamics and flow inside a sessile water droplet freezing due to interfacial heat transfer, International Journal of Heat and Mass Transfer, Vol: 180, Pages: 1-15, ISSN: 0017-9310

This study experimentally and numerically investigates the freezing characteristics and fluid dynamics of millimetre-sized sessile water droplets submerged in silicone oil at sub-zero temperatures under free convection. Individual water droplets were cooled to sub-zero temperatures (260-270 K) via interfacial heat transfer between the two liquid phases, in an approach different to studies in the literature where the cooling is done either from the solid substrate or from a low-temperature gas phase (such as air) surrounding the droplets. Laser-induced fluorescence was employed to perform spatiotemporally-resolved measurements of the phase distribution (from which interface distributions, freezing fronts, and rates were extracted). The particle image velocimetry was used to generate information on the velocity fields inside the liquid droplets. The experimental data are complemented by computational fluid dynamics (CFD) simulations, which showed acceptable qualitative and quantitative agreement with the experimental results. The experimental and simulation results indicated that prior to the initiation of freezing, two counteracting recirculation zones are generated in the central plane of the droplets, one on either side of the centreline, leading to a net upward flow at the edges and a downward flow in the centre due to the natural convection driven by internal temperature gradients. The nucleation sites appear on the external regions of the recirculation structures (which are locations with higher shear). Once freezing starts, the natural circulation patterns are suppressed, and instead, a sole downwards flow dominates, which is the result of the freezing layer suppressing the water phase. CFD results demonstrated a relatively wide temperature and pressure distribution in the water droplet at the beginning of the freezing stage, which gradually diminishes as the freezing process proceeds. The effect of droplet size and oil temperature on the freezing rates were in

Journal article

Van den Bergh WJ, Moran HR, Dirker J, Markides CN, Meyer JPet al., 2021, Effect of low heat and mass fluxes on the boiling heat transfer coefficient of R-245fa, International Journal of Heat and Mass Transfer, Vol: 180, Pages: 1-19, ISSN: 0017-9310

In-tube flow boiling at low mass and heat fluxes is of increasing interest particularly for low-concentration solar power systems, refrigerators, heat pumps, and other thermal management components and systems. In this study, the flow boiling of R-245fa was investigated experimentally for vapour qualities ranging from 0.05 to 0.90, mass fluxes of 40, 60 and 80 kg/m2s, and heat fluxes of 2.5, 5.0 and 7.5 kW/m2. Tests were done at quasi-steady-state conditions in a horizontal smooth tube with an inner diameter of 8.31 mm and a heated length of 0.8 m, at a saturation temperature of 35°C. It was found that the heat transfer coefficient was influenced by both the mass and heat fluxes. At any given vapour quality and heat flux combination, an increase in the mass flux resulted in an increase in the heat transfer coefficient. However, the magnitude of the increase and sensitivity to the mass flux was not the same in all of the test cases. Higher vapour quality cases were more sensitive to the mass flux than lower vapour quality cases, except at low heat flux conditions. In the most peculiar case, at the lowest considered heat flux of 2.5 kW/m2, severe sensitivity to mass flux was observed at vapour qualities between 0.2 and 0.3. For all other heat fluxes, the heat transfer coefficient was found to be independent of the vapour quality except when the mass flux was high, where increased vapour quality resulted in improved heat transfer coefficients. Various correlations were investigated, but none of them captured the trends for the lowest heat flux.

Journal article

Wang Y, Song J, Chatzopoulou MA, Sunny N, Simpson MC, Wang J, Markides CNet al., 2021, A holistic thermoeconomic assessment of small-scale, distributed solar organic Rankine cycle (ΟRC) systems: Comprehensive comparison of configurations, component and working fluid selection, Energy Conversion and Management, Vol: 248, Pages: 1-19, ISSN: 0196-8904

In this paper, results from comprehensive thermoeconomic assessments of small-scale solar organic Rankine cycle (ORC) systems are presented based on weather data in London, UK, which is taken as representative of a temperate climate with modest temperature changes, mild winters and moderate summers. The assessments consider a range of: (i) solar collector types (flat-plate, evacuated-tube, and evacuated flat-plate collectors); (ii) power cycle configurations (basic/recuperative, partial/full evaporating, and subcritical/transcritical cycles); (iii) expander types (scroll, screw, and piston) and designs; and (iv) a set of suitable working fluids. All possible solar-ORC system designs are optimised by considering simultaneously key parameters in the solar field and in the power cycle in order to obtain the highest electricity generation, from which the best-performing systems are identified. Selected designs are then subjected to detailed, annual simulations considering the systems’ operation, explicitly considering off-design performance under actual varying weather conditions. The results indicate that, among all investigated designs, solar-ORC systems based on the subcritical recuperative ORC (SRORC), evacuated flat-plate collectors (EFPCs), a piston expander, and isobutane as the working fluid outperforms all the other system designs on thermodynamic performance, whilst having the highest annual electricity generation of 1,100 kW·h/year (73 kW·h/year/m2) and an overall thermal efficiency of 5.5%. This system also leads to the best economic performance with a levelised cost of energy (LCOE) of ~1 $/kW·h. Apart from the specific weather data used for these detailed system simulations, this study also proceeds to consider a wider range of climates associated with other global regions by varying the solar resource available to the system. Interestingly, it is found that the optimal solar-ORC system design remains unchanged for different cond

Journal article

Huang G, Markides CN, 2021, Spectral-splitting hybrid PV-thermal (PV-T) solar collectors employing semi-transparent solar cells as optical filters, Energy Conversion and Management, Vol: 248, Pages: 1-15, ISSN: 0196-8904

Spectral splitting is a promising design methodology that can significantly improve the performance of hybrid photovoltaic-thermal (PV-T) collectors. However, conventional spectral-splitting PVT (SSPVT) collectors require additional optical components, which significantly increases the complexity and cost of the collector. This study proposes SSPVT collector designs that employ semi-transparent photovoltaic (PV) solar cells, which act as both the electricity generator as well as the spectral-splitting optical filter. In these designs, a part of the solar spectrum is absorbed by the semi-transparent solar cells for electricity generation, while the rest (especially the near-infrared region of the solar spectrum) is transmitted to an absorber where it generates a high-temperature thermal energy output. Three types of emerging semi-transparent solar cells, i.e., cadmium telluride (CdTe), perovskite solar cells (PVSCs) and polymer solar cells (PSCs), are selected for investigation in this context. A comprehensive two-dimensional model of such SSPVT collectors is developed and used to investigate their electrical and thermal performance. The results show that the proposed designs are effective at thermally decoupling the PV cells from the solar thermal absorber, thereby promoting a higher electrical efficiency and enabling the simultaneous generation of low-temperature thermal energy (<60 °C), along with high-temperature thermal energy (100–200 °C) under one sun. For example, a PVSC-based SSPVT collector is shown to be capable of simultaneously generating: electricity with an efficiency of 13.8%, high-temperature heat (150 °C) with a thermal efficiency of 21.1%, and low-temperature heat (50 °C) with a thermal efficiency of 22.5%. The relative performance between the CdTe-, PVSC- and PSC-based collectors depend on the relative value of the high-temperature thermal energy to that of electricity. It is concluded that semi-transparent solar cells are

Journal article

Al Kindi A, Aunedi M, Pantaleo A, Strbac G, Markides Cet al., 2021, Thermo-economic assessment of flexible nuclear power plants in the UK’s future low-carbon electricity system: role of thermal energy storage, 16th Conference on Sustainable Development of Energy, Water and Environment Systems, Publisher: SDEWES

Nuclear power plants are commonly operated as baseload units due to their low variable costs, high investment costs and limited ability to modulate their output. The increasing penetration of intermittent renewable power will require additional flexibility from conventional generation units, in order to follow the fluctuating renewable output while guaranteeing security of energy supply. In this context, coupling nuclear reactors with thermal energy storage could ensure a more continuous and efficient operation of nuclear power plants, while at other times allowing their operation to become more flexible and cost-effective. This study considers options for upgrading a 1610-MWel nuclear power plant with the addition of a thermal energy storage system and secondary power generators. The analysed configuration allows the plant to generate up to 2130 MWel during peak load, representing an increase of 32% in nominal rated power. The gross whole-system benefits of operating the proposed configuration are quantified over several scenarios for the UK’s low-carbon electricity system. Replacing conventional with flexible nuclear plant configuration is found to generate system cost savings that are between £24.3m/yr and £88.9m/yr, with the highest benefit achieved when stored heat is fully discharged in 0.5 hours (the default case is 1 hour). At an estimated cost of added flexibility of £42.7m/yr, the proposed flexibility upgrade to a nuclear power plant appears to be economically justified for a wide range of low-carbon scenarios, provided that the number of flexible nuclear units in the system is small.

Conference paper

Li Y, Markides CN, Sunden B, Xie Get al., 2021, Heat transfer deterioration in upward and downward pipe flows of supercritical n-decane for actively regenerative cooling, International Journal of Thermal Sciences, Vol: 168, Pages: 1-13, ISSN: 1290-0729

In this paper, we consider the flow and heat transfer behaviour of turbulent upward and downward flows of supercritical n-decane, in order to reveal the features of heat transfer deterioration (HTD) that would be expected in relevant active regenerative cooling systems for scramjet engines. Specific focus is placed on key velocity-field features that appear in these flows. Following the validation of six turbulence models, the SST k-ω and RNG k-ϵ models are found to be suitable for simulating the upward and downward flow cases, respectively. “M” type velocity profiles (a non-monotonicity of the velocity along the radial direction) are observed, which arise due to a spatially-varying interplay between the inertial and viscous forces in the flow domain, while larger velocity gradients in the buffer layer are also observed that contribute to the phenomenon of HTD. Furthermore, it is found that the secondary flows as well as the different mass fluxes that arise due to the velocity increase from the wall to the flow core zone (i.e., the influencing range and intensity of cross-sectional kinetic energy), respectively, are observed in the HTD development region, as well as the HTD peak area and degradation regions. A zone of higher thermal diffusion appears in the near-wall region, which acts as a thermal barrier and contributes to HTD.

Journal article

Khaljani M, Harrison J, Surplus D, Murphy A, Sapin P, Markides CN, Mahmoudi Yet al., 2021, A combined experimental and modelling investigation of an overground compressed-air energy storage system with a reversible liquid-piston gas compressor/expander, Energy Conversion and Management, Vol: 245, Pages: 1-19, ISSN: 0196-8904

We consider a small-scale overground compressed-air energy storage (CAES) system intended for use in micro-grid power networks. This work goes beyond previous efforts in the literature by developing and showing results from a first-of-a-kind small-scale (20 kWh) near-isothermal CAES system employing a novel, reversible liquid-piston gas compressor and expander (LPGC/E). Additionally, we extend our study to assessments, for the first time, of the economic and environmental characteristics of these small-scale overground CAES systems through a combination of experimental, thermodynamic, technoeconomic and environmental analyses. Five system configurations are considered: (1) CAESbase, which is the base-case system; (2) CAESplate, in which parallel plates are inserted into the LPGC/E as a heat exchanger for achieving near-isothermal compression and expansion; (3) CAESPCM, in which a phase change material (PCM) is employed to store thermal energy from the compressed air during charging that is later recovered during discharge; (4) CAESPCM&plate, which is a combination of the CAESplate and CAESPCM arrangements; and (5) CAESheater, in which a heater is utilised instead of the PCM to preheat the compressed air during discharge. Data for the validation of a computational design tool based on which the assessments were performed were obtained from a prototype of the CAESbase system. Results show that the CAESPCM&plate system exhibits the highest roundtrip efficiency of 63% and the shortest payback period of 7 years; the latter with the inclusion of governmental incentives and an electricity smart export guarantee (SEG) support rate of 5.5 p/kWh (6.8 ¢/kWh). The CAESPCM&plate system is found to be cost-effective even without incentives, with a payback period of 10 years. This system is also associated with 71 tonnes of fuel consumption savings and reduced CO2 emissions amounting to 51 tonnes over a lifetime of 20 years.

Journal article

Loni R, Mahian O, Markides CN, Bellos E, le Roux WG, Kasaeian A, Najafi G, Rajaee Fet al., 2021, A review of solar-driven organic Rankine cycles: Recent challenges and future outlook, Renewable and Sustainable Energy Reviews, Vol: 150, ISSN: 1364-0321

The organic Rankine cycle (ORC) is an effective technology for power generation from temperatures of up to 400 °C and for capacities of up to 10 MWel. The use of solar irradiation for driving an ORC is a promising renewable energy-based technology due to the high compatibility between the operating temperatures of solar thermal collector technologies and the temperature needs of the cycle. The objective of this review paper is to present and discuss the operation principles of solar-ORC technology and the wide range of solar-ORC systems that have been studied in the literature. Various solar thermal technologies that can drive the ORC are investigated, such as the flat plate collector, evacuated tube collector, compound parabolic collector, parabolic trough collector, linear Fresnel reflectors, dish concentrators and solar towers. Both simulation studies and experimental investigations are included in the study. Hybrid systems and different thermal storage techniques are also examined in detail. Moreover, systems with ORC which produce many useful outputs such as cooling, heating and fresh water are studied because they present high sustainability indexes. The limitations of the technology are also highlighted, along with critical suggestions aimed at steering future research in this field. The final conclusions indicate that the development of trigeneration and polygeneration systems with ORC sub-systems is a promising avenue, not only for the future development of solar-ORC technology but also for the development of renewable and sustainable energy systems in a broader context.

Journal article

Anvari-Moghaddam A, Besagni G, Markides CN, 2021, Multi-energy islands: Advances in local district heating, cooling and power systems, APPLIED THERMAL ENGINEERING, Vol: 199, ISSN: 1359-4311

Journal article

Voulgaropoulos V, Patapas A, Lecompte S, Charogiannis A, Matar OK, De Paepe M, Markides CNet al., 2021, Simultaneous laser-induced fluorescence and capacitance probe measurement of downwards annular gas-liquid flows, International Journal of Multiphase Flow, Vol: 142, Pages: 103665-103665, ISSN: 0301-9322

This study focuses on the characterisation of downwards annular gas-liquid (air-water) flows, by employing a combi-nation of advanced laser-based and capacitance-based measurement methods. A variant of laser-induced fluorescence(LIF), referred to as structured-planar laser-induced fluorescence (S-PLIF), eliminates biases commonly encounteredduring film-thickness measurements of gas-liquid flows, due to refraction and reflection of the light at the interface. Abespoke capacitance probe is also assembled to enable temporally resolved film-thickness measurements with high tem-poral resolution along the circumferential perimeter of the pipe. We compare the film mean thickness, roughness, andprobability density functions obtained with each method. We find that both methods are able to measure time-averagedfilm thickness to within<20% deviations from each other and from results obtained from the available literature. Theresulting probe data suggest a biased (suppressed) standard deviation of the film thickness, which can be attributed toits working principle, i.e., measuring the film thickness averaged along the circumferential perimeter of the pipe. Theauto-correlation functions of the time-traces provide an insight into the characteristic time-scales of the flows, whichspan a range from∼10 ms for highly gas-sheared flows and increase to about 30 ms for the less turbulent falling films.The power spectral densities reveal modal frequencies that start from 2.5 Hz for falling films, and increase with the gasReynolds number by almost an order of magnitude. The turbulent wave activity (slope in the power spectrum) reduceswith a decrease in gas shear, and shows similarities to the decay of homogeneous and isotropic turbulence. The sizes ofthe bubbles entrained in the liquid film are measured from the S-PLIF images, and exhibit log-normal distribution thatbecome flatter with a decrease in the gas Reynolds number. The normalised location of the bubbl

Journal article

Huang G, Wang K, Riera Curt S, Franchetti B, Pesmazoglou I, Markides Cet al., 2021, On the performance of concentrating fluid-based spectral-splitting hybrid PV-thermal (PV-T) solar collectors, Renewable Energy, Vol: 174, Pages: 590-605, ISSN: 0960-1481

Concentrating fluid-based spectral-splitting hybrid PV-thermal (SSPVT) collectors are capable of high electrical and thermal efficiencies, as well as high-temperature thermal outputs. However, the optimal optical filter and the maximum potential of such collectors remain unclear. In this study, we develop a comprehensive two-dimensional model of a fluid-based SSPVT collector. The temperature distributions reveal that these designs are effective in thermally decoupling the PV module from the high-temperature filter flow-channel, improving the electrical performance of the module. For a Si solar cell-based SSPVT collector with optical filter #Si400-1100, the filter channel is able to produce high-temperature thermal energy (400 °C) with an efficiency of 19.5%, low-temperature thermal energy (70 °C) with an efficiency of 49.5%, and electricity with an efficiency 17.5%. Of note is that the relative fraction of high-temperature thermal energy, low-temperature thermal energy and electricity generated by such a SSPVT collector can be adjusted by shifting the upper- and lower-bound cut-off wavelengths of the optical filter, which are found to strongly affect the spectral and energy distributions through the collector. The optimal upper-bound cut-off always equals the bandgap wavelength of the solar cell material (e.g., 1100 nm for Si, and 850 nm for CdTe), while the optimal lower-bound cut-off follows more complex selection criteria. The SSPVT collector with the optimal filter has a significantly higher total effective efficiency than an equivalent conventional solar-thermal collector when the relative value of the high-temperature heat to that of electricity is lower than 0.5. Detailed guidance for selecting optimal filters and their role in controlling SSPVT collector performance under different conditions is provided.

Journal article

This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.

Request URL: Request URI: /respub/WEB-INF/jsp/search-html.jsp Query String: limit=30&id=00345500&person=true&page=2&respub-action=search.html