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

479 results found

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

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

Journal article

Handagama NB, White MT, Sapin P, Markides CNet al., 2023, Renewable and waste-heat utilisation technologies, Publisher: Cambridge University Press, ISBN: 9781108691093

Understand the science and engineering behind renewable and waste-heat utilisation techniques with this thorough reference. Provides you with the knowledge and tools necessary to assess the technical and economic potential of heat-upgrading, heat-to-power and thermally-driven heating and cooling technologies, as well as a variety of thermal energy storage solutions. In particular, design-agnostic thermodynamic performance indicators and technology-specific design and costing methods are provided, which can be used to select the most suitable technologies for a wide range of applications, typically waste-heat recovery opportunities that exist within various industries, or domestic heating and cooling supply using renewable heat sources. Essential reading for professionals across the energy sector, chemical, manufacturing and mechanical engineering who have an interest in energy generation, conversion, storage and efficient heat utilisation.


Handagama NB, White MT, Sapin P, Markides CNet al., 2023, Renewable and Waste-Heat Utilisation Technologies, Publisher: Cambridge University Press, ISBN: 9781108480772

<jats:p>Understand the science and engineering behind conventional and renewable heat loss recovery techniques with this thorough reference. Provides you with the knowledge and tools necessary to assess the potential waste-heat recovery opportunities that exist within various industries and select the most suitable technology. In particular, technologies that convert waste heat into electricity, cooling or high-temperature heating are discussed in detail, alongside more conventional technologies that directly or indirectly recirculate heat back into the production process. Essential reading for professionals in chemical, manufacturing, mechanical and processing engineering who have an interest in energy conservation and waste heat recovery.</jats:p>


Mersch M, Markides CN, Mac Dowell N, 2023, The impact of the energy crisis on the UK’s net-zero transition, iScience, Vol: 26, Pages: 1-24, ISSN: 2589-0042

Recent drastic increases in natural gas prices have brought into sharp focus the inherent tensions between net-zero transitions, energy security, and affordability. We investigate the impact of different fuel prices on the energy system transition, explicitly accounting for the increasingly coupled power and heating sectors, and also incorporate the emerging hydrogen sector. The aim is to identify low-regret decisions and optimal energy system transitions for different fuel prices. We observe that the evolution of the heating sector is highly sensitive to gas price, whereas the composition of the power sector is not qualitatively impacted by gas prices. We also observe that bioenergy plays an important role in the energy system transition, and the balance between gas prices and biomass prices determines the optimal technology portfolios. The future evolution of the prices of these two resources is highly uncertain, and future energy systems must be resilient to these uncertainties.

Journal article

Guo J, Song J, Markides C, 2023, Studies on intensification mechanism of supercritical CO2 flows during heating processes, the 17th International Heat Transfer Conference

The unique characteristics of supercritical CO2 (SCO2) make it have promising potential in chemical engineering, energy conversion and other fields, but the drastic variations in thermophysical properties also bring great challenges to the prediction of heat transportation processes. The thermal-hydraulic performance of supercritical CO2 (SCO2) flows in horizontal pipes during heating processes is investigated numerically from the viewpoints of the first and second laws of thermodynamics. Heated flows through pipes with a diameter of 4 mm and mass flux of 400 kg/(m2·s), at a pressure of 8.0 MPa, with three heat fluxes (50 kW/m2, 75 kW/m2 and 100 kW/m2) are simulated. The results showed that the heat transfer irreversibility is more than 4 times higher on average when the highest heat flux of 100 kW/m2 is applied relative to the lowest heat flux of 50 kW/m2, while the peak heat transfer coefficientincreases by ~1.4 times when the heat flux decreases from 100 kW/m2 to 50 kW/m2. The thermal acceleration effect is negligible, while the buoyancy effect leads to secondary flows and affects the heat transfer and flow characteristics significantly. A jet flow in the near-wall region at the bottom of the pipe improves the synergy between the temperature gradient and velocity fields, leading to a higher (more than 2 times) heat transfer coefficient in this region than in the near-wall region at the top of the pipe. The present work provides insights into the mechanisms and characteristics of SCO2 flow heat transfer as well as practical guidance on the design and optimisation of relevant components.

Conference paper

Yang M, Moghimi MA, Loillier R, Markides CN, Kadivar Met al., 2023, Design of a latent heat thermal energy storage system under simultaneous charging and discharging for solar domestic hot water applications, Applied Energy, Vol: 336, Pages: 1-18, ISSN: 0306-2619

Design of a latent heat thermal energy storage system under simultaneous charging and discharging for solar domestic hot water applications

Journal article

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

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

Journal article

Lotfi M, Mersch M, Markides CN, 2023, Experimental and numerical investigation of a solar-thermal humidification-dehumidification desalination plant for a coastal greenhouse, Cleaner Engineering and Technology, Vol: 13, Pages: 1-15, ISSN: 2666-7908

Meeting the water demand for agriculture is considered one of the most crucial challenges societies face worldwide. Seawater greenhouses are a promising irrigation water source for locations where it is necessary to source freshwater from saline groundwater or seawater. Coupled with hybrid humidification-dehumidification (HDH) plants, powered by solar-thermal collectors, they can provide valuable freshwater using a free and accessible energy resource. Compared to conventional desalination systems, they can reduce emissions associated with desalination, have lower operational and maintenance costs, and are more suitable for deployment in remote areas. In this work, an experimental and numerical study of a solar-powered HDH desalination system in a seawater greenhouse is presented, considering an open-air/open-water (OAOW) cycle. In addition to the development of a thermodynamic model of the plant, a pilot plant consisting of a full-scale greenhouse for crop production, a HDH unit, 15 solar-thermal collectors, a firebox hot water boiler and two spiral coil heat exchangers was constructed and field-tested in Bushehr, Iran, under real meteorological conditions. The freshwater production of the pilot plant approached 2000 L per day. Thanks to an optimised hydroponic irrigation system, which reduced irrigation water consumption to approximately 1000 L per day, the greenhouse was self-sufficient in terms of water consumption and generated a freshwater surplus. In the experimental work, where the hot water temperature at the humidifier inlet was limited to 70 °C to prevent fouling, a maximum gain output ratio (GOR) of 2.3 was achieved. The computational model was validated and found to be in good agreement with experimental data, with GOR predictions to within a maximum deviation of 25 %. The model was then used for parametric performance studies beyond the limits of the experimental arrangement. A maximum GOR of 3.2 was achieved at a humidifier inlet temperature of 80

Journal article

Melnik A, Bogoslovtseva A, Petrova A, Safonov A, Markides CNet al., 2023, Oil–water separation on hydrophobic and superhydrophobic membranes made of stainless steel meshes with fluoropolymer coatings, Water, Vol: 15, Pages: 1-13, ISSN: 2073-4441

In this work, membranes were synthesized by depositing fluoropolymer coatings onto metal meshes using the hot wire chemical vapor deposition (HW CVD) method. By changing the deposition parameters, membranes with different wetting angles were obtained, with water contact angles for different membranes over a range from 130° ± 5° to 170° ± 2° and a constant oil contact angle of about 80° ± 2°. These membranes were used for the separation of an oil–water emulsion in a simple filtration test. The main parameters affecting the separation efficiency and the optimal separation mode were determined. The results reveal the effectiveness of the use of the membranes for the separation of emulsions of water and commercial crude oil, with separation efficiency values that can reach over 99%. The membranes are most efficient when separating emulsions with a water concentration of less than 5%. The pore size of the membrane significantly affects the rate and efficiency of separation. Pore sizes in the range from 40 to 200 µm are investigated. The smaller the pore size of the membranes, the higher the separation efficiency. The work is of great economic and practical importance for improving the efficiency of the membrane separation of oil–water emulsions. It lays the foundation for future research on the use of hydrophobic membranes for the separation of various emulsions of water and oil products (diesel fuel, gasoline, kerosene, etc.).

Journal article

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

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

Journal article

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

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

Journal article

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

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

Journal article

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

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

Journal article


Flow boiling in miniaturised channels is one of the most promising solutions for the efficient heat removal from next-generation high-power-density electronic devices. The existing models in design tools used to predict boiling heat transfer rates are largely based on empirical correlations and are very specific for the regime of boiling observed. A unified model that would be entirely based on the underlying physics of boiling and vapour bubble formation is an elusive task primarily due to our limited understanding of flow boiling. In this paper, we present results from an experimental investigation of the fundamental physics of a single vapour bubble formation inside a vertically oriented minichannel with a square cross-section and a hydraulic diameter of 5 mm. The experiments were performed for a selected set of mass fluxes (33-100 kg/m2s) and inlet subcooling degrees (5-15 K) using HFE-7100 as the working fluid. The test section is optically accessible from all directions, with one side of the channel fitted with an indium tin oxide (ITO) coated sapphire substrate to locally heat the fluid and generate vapour bubbles. Using diffuse backlight and a high-speed camera, visual observations of the vapour bubble formation process from a natural nucleation site for heat fluxes in the range 1.8-2.2 kW/m2 was conducted. The recorded time-lapse images were post-processed to extract the dynamic variations of vapour bubble features including width, height, base diameter, equivalent diameter, growth rate and sliding velocity over multiple bubble cycles. Using the least square circle fit method, the dynamic variations of upstream and downstream contact angles were also quantified. The temporal evolution of the bubble dynamic characteristics was compared to the isothermal bubble growth model in order to assess the conformity of their results, and gain insight into changes in the superheat level of the liquid phase adjacent to the growing vapour bubble.

Conference paper

Chougule SS, Bolegave GG, Soni B, Pander C, Kamble V, Markides CNet al., 2023, An Investigation of the Synthesis and Optical Properties of a Novel Ag/ZnO Hybrid Nanofluid for Spectral Splitting in Photovoltaic-Thermal Systems, Pages: 1747-1754

The deficient utilisation of the solar spectrum in conventional hybrid concentrating photovoltaic-thermal (CPV-T) technologies leads to a detrimental decrease in PV efficiency due to elevated temperatures. Solar spectral beam splitting (SBS) is an advancement in PV-T system design" which aims to use the full solar spectrum with minimal optical losses. The implementation of fluid-based SBS designs is economically feasible, with optical features that can be tuned by selecting suitable nanofluids with a desired concentration. Fluid-based SBS filters are advantageous over other filters for PV-T systems due to their ability to operate simultaneously as thermal storage as well as heat transfer media in these systems. In the present study, we report on the optical and thermophysical properties of a novel water-based Ag-ZnO hybrid nanofluid. The filter is synthesised by adding Ag to ZnO nanoparticles by a wet chemical method for improved stability. Silver (Ag) allows visible light harvesting (down conversion of UV to the visible region of the solar spectrum) and good optical properties in the visible and near-IR regions. An Ag shell can be embedded into the core of zinc oxide (ZnO) nanoparticles for improved stability. The presence of ZnO enables excellent optical properties, including high visible transmittance and high UV absorption. The presence of structural defects in ZnO induces colour centres which are deep traps emitting in the visible. Ag-ZnO nanofluids with different nanoparticle concentrations were tested to measure absorbance and transmittance using UV spectroscopy. These nanofluid filters can be used for full spectrum utilisation (by SBS) which helps in achieving: (i) down conversion in the UV region, (ii) transmit visible and near IR (NIR) region (desired wavelength of Si PV cell optoelectronic efficiency) and (iii) absorb (filter) the IR region of the solar spectrum (for downstream thermal use/applications).

Conference paper

Aunedi M, Olympios AV, Pantaleo AM, Mersch M, Markides CNet al., 2023, Role of energy storage in residential energy demand decarbonization: system-level techno-economic comparison of low-carbon heating and cooling solutions, Pages: 2309-2321

This paper explores various combinations of electric heat pumps (EHPs), hydrogen boilers (HBs), electric boilers (EBs), hydrogen absorption heat pumps (AHPs) and energy storage technologies (electric and thermal) to assess their potential for matching heating and cooling demand at low cost and with low carbon footprint. Thermodynamic and component-costing models of various heating and cooling technologies are integrated into a whole-energy system cost optimisation model to determine cost-effective configurations of heating and cooling systems that minimise the overall investment and operation cost for both the system and the end-user. Case studies presented in the paper focus on two archetypal systems that differ in terms of heating and cooling demand and availability profiles of solar and wind generation. The proposed approach quantifies how the cost-efficient portfolios of low-carbon heating and cooling solutions are driven by the characteristics of the system such as share of variable renewables or heating and cooling demand. Modelling results suggest that capacity choices for heating and cooling technologies will vary significantly depending on system properties. More specifically, air-to-air EHPs, with their cost and efficiency advantages over air-to-water EHPs, could make a significant contribution to low-carbon heat supply as well as cooling, although their contribution may be constrained by the compatibility with existing heating systems. They are found to be a useful supplementary source of space heating that is able to displace between 20 and 33 GWth of capacity of other heating technologies compared to the case where they do not contribute to space heating.

Conference paper

Bakkaloglu S, Mersch M, Sunny N, Markides C, Shah N, Hawkes Aet al., 2023, ECOS 2023: How far should the UK go with negative emission technologies?, Pages: 2939-2949

Negative Emissions Technologies (NETs), such as Bioenergy with Carbon Capture and Storage (BECCS) and Direct Air Carbon Capture and Storage (DACCS), are potentially valuable to offset carbon emissions and therefore commonly deployed in global climate change mitigation scenarios. However, they are controversial and sometimes seen as a means of delaying or avoiding emissions reduction efforts. Nonetheless, the UK has set an ambitious target of engineering 57 Mt CO2 per year of removals by 2050 to achieve net zero emissions[1]. This study uses the UK TIMES, technology-rich bottom-up energy system model to investigate the nationwide deployment of NETs in the energy system, while varying model parameters to provide an overview of decarbonisation in line with the UK's net zero ambitions. We investigated DACCS and BECCS NETs technologies with regards to technological uncertainties and sensitivities. We revised the TIMES model structure for NETs implementation to ensure proper integration with industry. Our analysis estimates that the UK can remove 78.5 Mt CO2 by 2050 under the balanced Net Zero Scenario. However, by integrating an updated characterisation of removal technologies, and enabling tighter integration of DACCS into industrial clusters, we can achieve a removal capacity of up to 209 Mt CO2 by 2050 based on our preliminary results. Additionally, a 50% reduction in DACCS cost could further increase the removal capacity to 218 Mt CO2. This study provides valuable insights for policymakers and stakeholders in the UK and beyond, highlighting how NETs can be integrated in industrial strategy.

Conference paper

Lee JI, Song J, Markides CN, 2023, CO<inf>2</inf> cycles, Power Generation Technologies for Low-Temperature and Distributed Heat, Pages: 163-206, ISBN: 9780128182376

CO2-based (both transcritical and supercritical) cycle systems have emerged as a promising option for power generation thanks to their robust thermodynamic performance as well as advantages offered by CO2 as a working fluid, which is nontoxic, nonflammable, and robust to decomposition at high-temperature conditions. Good thermodynamic performance in these systems is promoted by the good thermal match that can be achieved between the cycle and heat source(s), again as a consequence of the thermodynamic properties of CO2. Heat from fossil-fuel combustion as well as solar, geothermal, biomass heat and waste-heat recovery are all potential application areas for CO2 cycle systems, covering heat-source temperatures over a wide range from 300°C to 1200°C, with a thermodynamic efficiency of 20%–65%. When the turbine inlet temperature is ~500°C the thermal efficiency of supercritical (s-CO2) cycle systems reaches ~30%, but a thermal efficiency of 60% can be achieved when the turbine inlet temperature reaches 1200°C. Moreover the high density of CO2 in the supercritical region allows compact component and system design, which is particularly advantageous in space-limited applications. Although the technology has not yet been deployed widely, economic performance projections of s-CO2 cycle systems have been performed. A variety of such assessments have predicted that (1) the specific investment cost of s-CO2 cycle systems will fall in the range 900–1650$/kW in waste-heat recovery applications, (2) the levelized cost of energy (LCOE) of coal-fired CO2 power plants can be as low as ~70–90$/MWh, (3) the unit cost of electricity of s-CO2 cycle systems in solar applications can reach 0.07–0.09$/kWh, and (4) a total cost saving of up to 30% can be achieved by CO2 cycle systems relative to steam Rankine cycle systems. Research on CO2 cycle systems is extensive and spans diverse areas from component (especially turbomachine and heat exchanger) d

Book chapter

Grosu L, Feidt M, Jaworski AJ, Ja’fari M, Markides CNet al., 2023, Oscillatory flow power cycles, Power Generation Technologies for Low-Temperature and Distributed Heat, Pages: 207-289, ISBN: 9780128182376

Book chapter

Wang K, Markides CN, 2023, Summary and future outlook, Power Generation Technologies for Low-Temperature and Distributed Heat, Pages: 473-480, ISBN: 9780128182376

A wide range of power generation technologies suitable for low-temperature and distributed heat recovery and conversion, along with relevant thermal energy storage options, have been presented in the chapters of this book. The power generation technologies have been primarily categorized into Rankine cycles and their variants (steam Rankine, organic Rankine, and Kalina cycles), CO2 cycles (transcritical and supercritical CO2 cycles), unsteady cycles (Stirling engines, thermoacoustic engines, and thermofluidic oscillators), solid-state devices (thermoelectric, thermomagnetic, and pyroelectric generators), and other emerging technologies (thermoelectrochemical cells, thermally regenerative electrochemical cycles, thermo-osmotic power generation, and membrane-based polygeneration systems). A brief summary of these technologies is presented in this chapter. The approximate power output, efficiency, operation (i.e., heat input), temperature, and cost ranges, along with the maturity level, appropriate applications, and future outlook of these technologies, are presented, discussed, and compared. The importance of thermal energy storage as part of the implementation of these solutions is then highlighted.

Book chapter

Markides CN, Wang K, 2023, Preface, Power Generation Technologies for Low-Temperature and Distributed Heat

Journal article

Wang K, Smith IK, Markides CN, 2023, Overview of low-temperature distributed heat and fundamentals, Power Generation Technologies for Low-Temperature and Distributed Heat, Pages: 1-48, ISBN: 9780128182376

This chapter presents an overview of low-temperature distributed heat from its definition, features, characteristics, and potential in various regions and sectors as well as the thermodynamic fundamentals of power cycles for heat recovery and conversion. Significant inconsistencies exist in the definition of the temperature boundaries that demark low-/medium-/high-temperature heat across the literature. A broader definition for low-temperature and distributed heat is proposed and adopted here, based on a comparison to the temperatures achieved by combustion and the scale of centralized power plants, which includes not only waste heat from various processes and sectors but also heat from renewable energy sources. Estimations of the waste heat potentials in the world and major regions showed that a significant share of energy consumptions is finally dissipated as waste heat, of which the majority is below 100°C. Thermodynamic fundamentals of power cycles for recovering waste heat are then presented, which highlights that matching the temperature changes of the heat source and the working fluid is important. Power cycles based on pressurizing, evaporating, expanding, and condensing volatile fluids, also referred to as two-phase cycles, can be superior in terms of conversion efficiency to those based on ideal or other single-phase gases that do not feature phase change thanks to their high work ratios and superior matching characteristics.

Book chapter

Markides CN, Wang K, 2023, Power Generation Technologies for Low-Temperature and Distributed Heat, ISBN: 9780128182376

Power Generation Technologies for Low-Temperature and Distributed Heat presents a systematic and detailed analysis of a wide range of power generation systems for low-temperature (lower than 700-800°C) and distributed heat recovery applications. Each technology presented is reviewed by a well-known specialist to provide the reader with an accurate, insightful and up-to-date understanding of the latest research and knowledge in the field. Technologies are introduced before the fundamental concepts and theoretical technical and economic aspects are discussed, as well as the practical performance expectations. Cutting-edge technical progress, key applications, markets, as well as emerging and future trends are also provided, presenting a multifaceted and complete view of the most suitable technologies. A chapter on various options for thermal and electrical energy storage is also included with practical examples, making this a valuable resource for engineers, researchers, policymakers and engineering students in the fields of thermal energy, distributed power generation systems and renewable and clean energy technology systems.


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

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

Journal article

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

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


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

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

Journal article

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

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

Journal article

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

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

Journal article

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

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

Journal article

Maghrabi A, Song J, Sapin P, Markides CNet al., 2022, Data-driven approaches for techno-economic assessment of waste heat recovery and utilisation in the industrial sector, 17th Conference on Sustainable Development of Energy, Water and Environment Systems (SDEWES 2022)

The industrial sector is a critical element in the sustainability transition as it is currently the largestconsumer of fossil fuels, and the consumption is forecasted to continue to increase. Approximatelyone-fifth of the total industrial primary energy consumption is wasted due to the lack of provenattractive schemes for effective recovery. When addressing the opportunities of industrial wasteheat recovery (WHR), it is found that the feasibility depends on multiple factors, including the formsand capacities of the heat sources, the potential heat sinks, and the effectiveness, technologicalmaturity, and economic impact of available technologies. Developing systematic approaches toidentify optimal WHR options for different applications is key to effectively reduce plant-scaleenergy consumption. In particular, power consumption accounts for more than half of the industrialenergy use, and its share is expected to grow with the expansion of electrification aspirations. Inthis paper, industrial WHR technologies are investigated, and tools are developed to understand thesustainability and techno-economic impact of integrating these technologies within industrialprocesses. We specifically propose a data-driven technology-agnostic approach to evaluate the useof heat engines, which can in practice be organic Rankine cycle (ORC) systems, and of thermally-driven (i.e., absorption) heat pumps in the context of industrial WHR for plant-scale power demandreduction. The scope of this work explores three pathways to achieving efficiency improvementsin bulk chemicals plants, represented by olefins production facilities, which are: (i) direct onsitepower generation; (ii) enhancement of existing power generation processes; and (iii) reduction inpower consumption by compressor efficiency improvements through waste-heat-driven cooling.The techno-economic performance of these technologies is assessed, with particular attention toindustrial facilities that reside in hot climates, using fi

Conference paper

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