Imperial College London

ProfessorChristosMarkides

Faculty of EngineeringDepartment of Chemical Engineering

Professor of Clean Energy Technologies
 
 
 
//

Contact

 

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

 
 
//

Location

 

404ACE ExtensionSouth Kensington Campus

//

Summary

 

Publications

Publication Type
Year
to

412 results found

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

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

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

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, 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

Alrowais R, Shahzad MW, Burhan M, Bashir MT, Chen Q, Xu BB, Kumja M, Markides CN, Ng KC, 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

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

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

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

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

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

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

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

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

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

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: http://wlsprd.imperial.ac.uk:80/respub/WEB-INF/jsp/search-html.jsp Request URI: /respub/WEB-INF/jsp/search-html.jsp Query String: respub-action=search.html&id=00345500&limit=30&person=true