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

329 results found

Oyewunmi OA, Pantaleo AM, markides CN, 2017, ORC cogeneration systems in waste-heat recovery applications, 9th International Conference on Applied Energy (ICAE2017), Publisher: Elsevier, ISSN: 1876-6102

The performance of organic Rankine cycle (ORC) systems operating in combined heat and power (CHP) mode is investigated. TheORC-CHP systems recover heat from selected industrial waste-heat fluid streams with temperatures in the range 150 °C – 330 °C. Anelectrical power output is provided by the expanding working fluid in the ORC turbine, while a thermal output is provided by the coolingwater exiting the ORC condenser and also by a second heat-exchanger that recovers additional thermal energy from the heat-sourcestream downstream of the evaporator. The electrical and thermal energy outputs emerge as competing objectives, with the latter favouredat higher hot-water outlet temperatures and vice versa. Pentane, hexane and R245fa result in ORC-CHP systems with the highest exergyefficiencies over the range of waste-heat temperatures considered in this work. When maximizing the exergy efficiency, the second heatexchangeris effective (and advantageous) only in cases with lower heat-source temperatures (< 250 °C) and high heat-delivery/demandtemperatures (> 60 °C) giving a fuel energy savings ratio (FESR) of over 40%. When maximizing the FESR, this heat exchanger isessential to the system, satisfying 100% of the heat demand in all cases, achieving FESRs between 46% and 86%.

Conference paper

Herrando M, Ramos A, Zabalza I, Markides CNet al., 2017, Structural characterization and energy performance of novel hybrid PVT solar-panels through 3-D FEM and CFD simulations, ECOS 2017

Hybrid Photovoltaic-Thermal (PVT) panels generate both power and heat from the same area with overall efficiencies up to 70%. This work assesses the performance of novel hybrid PVT solar panels considering alternative geometries and materials that maximize heat transfer while allowing weight and cost reductions. A three-dimensional (3-D) model previously developed and validated using 3-D Finite-Element and Computational Fluid-Dynamics (FEM and CFD) software is used for this purpose. The most promising configurations and materials for the absorber-exchanger unit of the proposed PVT panel are studied to analyse their energy performance and behaviour in terms of a thermal-stress assessment. Apart from an assessment of the steady-state performance, for the type of solar PVT panels considered, especially those made of polymeric materials, it is important to evaluate the thermal expansion that the collector suffers, so as to verify whether the associated thermal stresses and strains are within the limits that guarantee a proper performance during its lifetime. The most promising PVT panel is then integrated within a Solar Combined Heat and Power (S-CHP) system for power and heating provision to a single-family house located in Zaragoza (Spain), in order to assess its daily energy performance through transient simulations on half-hourly basis. The results show that these novel polymeric PVT panel configurations are a promising alternative to commercial PVT panel designs, achieving an improved thermal performance compared to a reference case (4% higher optical efficiency and 15% lower heat loss coefficient), while suffering lower strains in most of the PVT layers. Furthermore, the novel polycarbonate 3×2 mm flat-box configuration has the potential to cover, on average, around 50% of the total space heating and Domestic Hot Water (DHW) demand and around 87% of the total electricity demand (including lighting, cooling and home appliances).

Conference paper

Georgiou S, Dowell NM, Shah N, Markides CNet al., 2017, Thermo-economic comparison of liquid-air and pumped-thermal electricity storage, 30th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, ECOS 2017

© 2017 IMEKO An efficient and affordable electricity storage system can assist the increasing penetration of intermittent renewable-energy generation, while the difference in the demand and price of peak and off-peak electricity can make its storage of financial interest. Technical indicators (e.g., roundtrip efficiency, energy and power density) along with economic indicators (e.g., capital, operating and maintenance costs) are expected to have a substantial combined impact on the competitiveness of any electricity storage technology or system under consideration. In this paper we will present thermodynamic models of two newly proposed medium- to large-scale electricity storage systems, namely ‘Liquid-Air Energy Storage’ (LAES) and ‘Pumped-Thermal Electricity Storage’ (PTES). The LAES model is validated against data from a pilot plant in operation in the UK; no such equivalent PTES plant exists. As with most new technologies, the lack of cost information makes the economic analysis and comparison a significant challenge. A costing effort for the two systems based on the module costing technique is also presented with the overriding aim of performing a preliminary economic feasibility assessment of the two systems. Based on initial results, PTES achieves higher roundtrip efficiencies, although the performance of LAES is found to be significantly enhanced through the utilisation of waste heat (and cold) streams. In terms of costs, LAES is estimated to have lower capital costs by roughly £600/kW. The most expensive components in both systems are the compression and expansion devices.

Conference paper

Sapin PMC, Simpson M, White AJ, Markides Cet al., 2017, Lumped dynamic analysis and design of a high-performance reciprocating-piston expander, 30th International Conference on Efficiency, Cost, Optimisation, Simulation and Environmental Impact of Energy Systems., Publisher: ECOS

A spatially-lumped dynamic model of a reciprocating-piston expander is presented in this paper. The model accounts for the three main loss mechanisms in realistic piston machines, namely: pressure losses through the intake and exhaust valves, heat transfer between the gas and the surrounding cylinder walls, and the mass leakagebetween the compression/expansion chamber and the crankcasethrough the piston rings. The model also accounts for real-gas effects with the fluid properties calculated from the NIST database using REFPROP. The numerical calculations are first compared with experimental pressure-volume-temperature data obtained on a custom reciprocating-piston gas spring over a range of oscillation frequencies. The comparison between numerical and experimental results shows good agreement. It also allows the most accurate heat transfer correlationto be selectedfor calculating the gas-to-wallin-cylinderheat transfer. The semi-heuristic modelling tool is thenused to design an expander forspecific pressure ratiosand mass flowrate, and to predict the thermodynamic performance of the piston device over arange of part-load conditions.

Conference paper

Ramos Cabal Alba RA, Chatzopoulou M, Freeman James JF, Markides Christos CNMet al., 2017, Optimisation of a high-efficiency solar-driven organic Rankine cycle for applications in the built environment, The 30th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, Publisher: ECOS

Recent years have seen a strong increase in the uptake of solar technologies in the built environment. Incombined heat and power (CHP) or cogeneration systems, the thermodynamic and economic ‘value’ of theelectrical output is usually considered to be greater than that of (an equivalent) thermal output, and thereforethe prioritisation of the electrical output in terms of system-level optimisation has been driving much of theresearch, innovation and technology development in this area. In this work, the potential of a solar CHPtechnology based on an organic Rankine cycle (ORC) engine is investigated. We present thermodynamicmodels developed for different collectors, including flat-plate collectors (FPC) and evacuated-tube collectors(ETC) coupled with a non-recuperative sub-critical ORC architecture to deliver power and hot water by usingthermal energy rejected from the engine. Results from dynamic 3-D simulations of the solar collectors togetherwith a thermal energy storage (TES) tank are presented. TES offers an important buffering capability duringperiods of intermittent solar radiation, as well as the potential for demand-side management (DSM). Resultsare presented of an optimisation analysis to identify the most suitable working fluids for the ORC unit, in whichthe configuration and operational constraints of the collector array are taken into account. The most suitableworking fluids (R245fa and R1233zd) are then chosen for a whole-system optimisation performed in a southernEuropean climate. The system configuration with an ETC array is found to be best-suited for electricityprioritisation, delivering an electrical output of 3,605 kWh/yr from a 60 m2 array. In addition, the system supplies13,175 kWh/yr in the form of domestic hot water, which is equivalent to more than 6 times the average annualhousehold demand. A brief cost analysis and comparison with photovoltaic (PV) systems are also performed.

Conference paper

Chatzopoulou MA, Markides CN, 2017, Advancements in organic Rankine cycle system optimisation for combined heat and power applications: components sizing and thermoeconomic considerations, 30th International Conference on Efficiency, Cost, Optimisation, Simulation and Environmental Impact of Energy Systems, Publisher: ECOS

There is great interest in distributed combined heat and power (CHP) generation in the built environment due to the higher overall efficienciesattained in comparison to separate provision of these vectors. Organic Rankine cycle (ORC) systems are capable of generating additional electricity from the thermal outputs of CHP engines, improving the electrical conversion efficiency and power-to-heat ratio of suchsystems. Thermodynamic analysis and technical feasibility are at the core of the development of these systems, whilea critical factor for the wider adoption of ORC systems concerns their economic proposition. Obtainingcredible estimates of system costs requires correct sizing of individual components. This work focuses on the thermodynamic optimisation, sizing and costing of ORC units in CHP applications, over a range of heat-source temperatures. The working fluids examined include R245fa, R1233zd, Pentane and Hexane, due to their good performance and favourable environmental characteristics. The optimalcycles obtained can increase the power-to-heat ratio of the complete CHP-ORCsystem by up to 65%.Alternative equipment sizing methods are then applied for each fluid and the resultant component sizes are compared. The cost estimates obtained from the alternative methods are also compared to real ORC application. Based on this, a hybrid costing method is proposed andapplied to an ORC system design,in order to obtain the specific investment cost (SIC). The results indicate that as the heat source temperature increases, the power output increases, resultingin larger and more expensive components. Nevertheless, the SIC drops from 17GBP/W for low-power outputs to 1.1GBP/W for high-temperature/high-power outputs.

Conference paper

Handagama N, White M, Markides CN, 2017, Cooling with carbon dioxide, Energy World, Pages: 28-29, ISSN: 0307-7942

With a growing global demand for cooling and more restrictive legislation coming into force concerning the selection of refrigerants, the refrigeration industry is looking for new alternatives. Nareshkumar Handagama, Martin White and Christos Markides discuss the role that carbon dioxide could play.

Journal article

Efstratiadi M-A, Acha S, Shah N, Markides CNet al., 2017, Analysis of Closed Loop Water-Cooled Refrigeration Systems for the Food Retail Industry: A UK Case Study, 2017 ASHRAE ANNUAL CONFERENCE PAPERS, ISSN: 2578-5257

Journal article

Efstratiadi M, Acha Izquierdo S, Shah N, Markides Cet al., 2017, Analysis of a closed-loop water-cooled refrigeration system in the food retail industry: A UK case study, 2017 ASHRAE Annual Conference, Publisher: ASHRAE, ISSN: 2578-5257

The need for refrigeration in the food retail industry and specifically in supermarkets, currently accounts for about 30% to 60% of the total energy consumed in the UK stores. A key characteristic of this consumption, is the high amount of low-grade heat rejected by the condensation units to the ambient air. The aim of this study, which focuses on transcritical CO2 (R744) refrigeration cycles, is to assess whether the use of a water-cooled condenser rejecting heat to the soil via an intermediate closed-loop water-circuit, can improve the overall cooling performance, while also considering the economic implications of this modifications. In this work, a detailed model simulating the operation of an existing supermarket refrigeration system is presented and validated against field data measurements taken from a refrigeration system in a UK supermarket. The examined direct-expansion system comprises an air-cooled condenser coupled with two sets of compressors for the provision of intermediate and low-temperature cooling. This baseline model is then modified and used to evaluate the performance of a similar system, in which a water-cooled condenser is used instead of the existing air-cooled unit or in parallel to it. Preliminary results indicate that the use of water-cooled condensers has the potential to reduce the energy consumption of these refrigeration systems by up to a factor of 5 when the external temperature is high. However, in cold ambient conditions, the air-cooled condensers reject 10% less heat, resulting in a better system performance. Furthermore, a more thorough case study is developed in order to examine the yearly operation of the existing system, and to compare this to various water-cooled alternatives. The analysis indicates a reduction of approximately 3% in the energy consumed by the water-cooled system (compared to the reference benchmark air-cooled system), and a reduction of almost 6%, for a hybrid system with coupled air-cooled and water-coole

Conference paper

Charogiannis A, Markides C, 2017, An experimental study of unsteady heat-transfer in harmonically excited falling-films by application of advanced optical diagnostics, 9th World Conference on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics

Detailed space- and time-resolved flow field and heat transfer information was generated in harmonically ex-cited, gravity-driven films falling over an inclined, heated plane. Simultaneous planar laser-induced fluorescence imaging(PLIF), particle tracking velocimetry (PTV) and infrared thermography (IR) was employed in order to recover space- andtime- resolved film-height, two-dimensional (2-D) velocity, and gas-liquid interface temperature data respectively. Localand instantaneous heat-transfer coefficient measurements (HTC) were also generated, using this information and knowl-edge of the wall temperature at the location where optical measurements were conducted. Our results indicate a strongcoupling between the film height, bulk velocity and flow-rate, on one hand, and the HTC on the other, along the thinnerregions of the examined films. In contrast, the HTC is insensitive to film such variations in thicker film-regions.

Conference paper

Lecompte S, An JS, Charogiannis A, Paepe MD, Markides Cet al., 2017, Simultaneous capacitive probe and planar laser-induced fluorescence measurements in downwards gas-liquid annular flow, 9th World Conference on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics

Various experimental techniques are available to analyse two-phase flows. The measurement concept and theapplicability can however vary greatly. Prime examples from the opposite spectrum are planar laser-induced measurements (PLIF) versus capacitive probes. PLIF is an optical technique, it is non-intrusive but optical access is necessary. PLIF based measurements are known for their high temporal and spatial resolution but require a costly set-up. In contrast, the capacitive probe is another non-intrusive technique but doesn’t require optical access. It is fairly easy to set up, robust, and is cheap to construct. To rigorously compare both techniques, simultaneous PLIF and capacitive probe measurements are made in this work. As the void fraction is one of the key parameters to classify flow regimes, both techniques are compared on the determination of the void fraction. This is done for a limited set of six annular flows. The experiments were performed in a downward annular-flow facility with demineralized water - air as working medium. The first results indicate that both techniques give similar volume averaged void fractions. The mean absolute percentage error and the maximum relative error between both techniques are 0.30% and 0.54%, respectively.The PLIF measurements confirm however to have a better spatial resolution.

Conference paper

Ibarra R, Matar OK, Markides CN, 2017, Flow structures in low-inclination stratified oil-water pipe-flows using laser-based diagnostic techniques, International Conference on Multiphase Production, Publisher: BHR Group, Pages: 71-85

In this work, a novel two-line laser-based diagnostic measurement technique was developed and applied to obtain combined space- and time-resolved phase and velocity information in low-inclination upward (<+5°) stratified flows of oil (Exxsol D140) and water. The strength of this technique is in enabling direct measurements in the non-refractive-index-matched fluids of interest, as opposed to substitute (optically matched) fluids whose properties may be less representative of those in real field-applications. The experimental test-section consisted of a 32-mm internal diameter pipe with a total length of 8.5 m. Results reveal interesting interactions between the co-flowing liquid phases. The velocity gradients at the interface are enhanced at high pipe inclinations for upward flows as the oil and water velocities increase and decrease, respectively. This also has a direct effect on the velocity fluctuations (quantified through their rms) and on the interfacial instabilities, which in turn affect the local velocity distributions in both phases.

Conference paper

Pantaleo, Rotolo G, De Palma P, Amirante R, Oyewunmi OA, Chatzopoulou, Markides C, Simpson Met al., 2017, Thermo-economic optimization of small-scale ORC systems for heat recovery from natural gas internal combustion engines for stationary power generation, 4th Annual Engine ORC Consortium Workshop for the Automotive and Stationary Engine Industries

Conference paper

Pantaleo AM, Camporeale SM, Miliozzi A, Russo V, Mugnozza GS, Markides CN, Shah Net al., 2017, Thermo-economic assessment of an externally fired hybrid CSP/biomass gas turbine and organic Rankine combined cycle, 8th International Conference on Applied Energy (ICAE), Publisher: Elsevier, Pages: 174-181, ISSN: 1876-6102

This paper focuses on the thermo-economic analysis of a hybrid solar-biomass CHP combined cycle composed by a 1.3-MW externally fired gas-turbine (EFGT) and a bottoming organic Rankine cycle (ORC) plant. The primary thermal energy input is provided by a hybrid concentrating solar power (CSP) collector-array coupled to a biomass boiler. The CSP collector-array is based on parabolic-trough concentrators (PTCs) with molten salts as the heat transfer fluid (HTF) upstream of a fluidized-bed furnace for direct biomass combustion. Thermal-energy storage (TES) with two molten-salt tanks (one cold and one hot) is considered, as a means to reducing the variations in the plant's operating conditions and increasing the plant's capacity factor. On the basis of the results of the thermodynamic simulations, upfront and operational costs assessments, and considering an Italian energy policy scenario, the global energy conversion efficiency and investment profitability are estimated for 2 different sizes of CSP arrays and biomass furnaces. The results indicate the low economic profitability of CSP in comparison to only biomass CHP, because of the high investment costs, which are not compensated by higher electricity sales revenues.

Conference paper

Pantaleo AM, Camporeale SM, Markides CN, Mugnozza GS, Shah Net al., 2017, Energy performance and thermo-economic assessment of a microturbine-based dual-fuel gas-biomass trigeneration system, 8th International Conference on Applied Energy (ICAE), Publisher: Elsevier, Pages: 764-772, ISSN: 1876-6102

The focus of this paper is on the energy performance and thermo-economic assessment of a small scale (100 kWe) combined cooling, heat and power (CCHP) plant serving a tertiary/residential energy demand fired by natural gas and solid biomass. The plant is based on a modified regenerative micro gas-turbine (MGT), where compressed air exiting the recuperator is externally heated by the hot gases produced in a biomass furnace. The flue gases after the recuperator flow through a heat recovery system (HRS), producing domestic hot water (DHW) at 90 °C, space heating (SH), and also chilled water (CW) by means of an absorption chiller (AC). Different biomass/natural gas ratios and an aggregate of residential end-users in cold, average and mild climate conditions are compared in the thermo-economic assessment, in order to assess the trade-offs between: (i) the lower energy conversion efficiency and higher investment cost when increasing the biomass input rate; (ii) the higher primary energy savings and revenues from feed-in tariffs available for biomass electricity exported into the grid; and (iii) the improved energy performance, sales revenue and higher investment and operational costs of trigeneration. The results allow for a comparison of the energy performance and investment profitability of the selected system configuration, as a function of the heating/cooling demand intensity, and report a global energy efficiency in the range of 25-45%, and IRR in the range of 15-20% assuming the Italian subsidy framework.

Conference paper

Mariaud A, Acha S, Ekins-Daukes N, Shah N, Markides CNet al., 2017, Integrated optimisation of photovoltaic and battery storage systems for UK commercial buildings, Applied Energy, Vol: 199, Pages: 466-478, ISSN: 1872-9118

Decarbonising the built environment cost-effectively is a complex challenge public and private organisations are facing in their effort to tackle climate change. In this context, this work presents an integrated Technology Selection and Operation (TSO) optimisation model for distributed energy systems in commercial buildings. The purpose of the model is to simultaneously optimise the selection, capacity and operation of photovoltaic (PV) and battery systems; serving as a decision support framework for assessing technology investments. A steady-state mixed-integer linear programming (MILP) approach is employed to formulate the optimisation problem. The virtue of the TSO model comes from employing granular state-of-the-art datasets such as half-hourly electricity demands and prices, irradiance levels from weather stations, and technology databases; while also considering building specific attributes. Investment revenues are obtained from reducing grid electricity costs and providing fast-frequency response (FFR) ancillary services. A case study of a distribution centre in London, UK is showcased with the goal to identify which technologies can minimise total energy costs against a conventional system setup serving as a benchmark. Results indicate the best technology configuration is a combination of lithium-ion batteries and mono-crystalline silicon PVs worth a total investment of £1.72 M. Due to the available space in the facility, the preferred PV capacity is 1.76 MW, while the battery system has a 1.06 MW power capacity and a 1.56 MWh energy capacity. Although PV performance varies across seasons, the solution indicates almost 30% of the energy used on-site can be supplied by PVs while achieving a carbon reduction of 26%. Nonetheless, PV and battery systems seem to be a questionable investment as the proposed solution has an 8-year payback, despite a 5-year NPV savings of £300k, implying there is still a performance gap for such systems to be massively

Journal article

Pantaleo AM, Camporeale SM, Miliozzi A, Russo V, Shah N, Markides CMet al., 2017, Novel hybrid CSP-biomass CHP for flexible generation: thermo-economic analysis and profitability assessment, Applied Energy, Vol: 204, Pages: 994-1006, ISSN: 1872-9118

This paper focuses on the thermo-economic analysis of a 2.1-MWe and 960 kWt hybrid solar-biomass combined heat and power (CHP) system composed of a 1.4-MWe Externally Fired Gas-Turbine (EFGT) and a 0.7-MWe bottoming Organic Rankine Cycle (ORC) power plant. The primary thermal energy input is provided by a hybrid Concentrating Solar Power (CSP) collector array covering a total ground area of 22,000–32,000 m2, coupled to a biomass boiler. The CSP collector array is based on parabolic-trough concentrators (PTCs) with molten salts as the heat transfer fluid (HTF), upstream of a 4.5–9.1 MWt fluidized-bed furnace for direct biomass combustion. In addition, two molten-salt tanks are considered that provide 4.8–18 MWh (corresponding to 1.3–5.0 h) of Thermal Energy Storage (TES), as a means of reducing the variations in the plant’s operating conditions, increasing the plant’s capacity factor and total operating hours (from 5500–6000 to 8000 h per year). On the basis of the results of the thermodynamic simulations, upfront and operational costs assessments, and considering an Italian energy policy scenario (feed-in tariffs, or FiTs, for renewable electricity), the global energy conversion efficiency and investment profitability of this plant are estimated for different sizes of CSP and biomass furnaces, different operation strategies (baseload and modulating) and cogenerative vs. electricity-only system configurations. Upfront costs in the range 4.3–9.5 MEur are reported, with operating costs in the range 1.5–2.3 MEur annually. Levelized costs of energy from around 100 Eur/MWh to above 220 Eur/MWh are found, along with net present values (NPVs) from close to 13,000 to −3000 kEur and internal rates of return (IRRs) from 30% down to almost zero when prioritizing electrical power generation (i.e., not in cogenerative mode). In all cases the economic viability of the systems deteriorate for larger CSP section sizes. The

Journal article

Lecompte S, Oyewunmi OA, Markides C, Lazova M, Kaya A, van den Broek M, De Paepe Met al., 2017, Case study of an organic Rankine cycle (ORC) for waste heat recovery from an electric arc furnace (EAF), Energies, Vol: 10, ISSN: 1996-1073

The organic Rankine cycle (ORC) is a mature technology for the conversion of waste heat to electricity. Although many energy intensive industries could benefit significantly from the integration of ORC technology, its current adoption rate is limited. One important reason for this arises from the difficulty of prospective investors and end-users to recognize and, ultimately, realise the potential energy savings from such deployment. In recent years, electric arc furnaces (EAF) have been identified as particularly interesting candidates for the implementation of waste heat recovery projects. Therefore, in this work, the integration of an ORC system into a 100 MWe EAF is investigated. The effect of evaluations based on averaged heat profiles, a steam buffer and optimized ORC architectures is investigated. The results show that it is crucial to take into account the heat profile variations for the typical batch process of an EAF. An optimized subcritical ORC system is found capable of generating a net electrical output of 752 kWe with a steam buffer working at 25 bar. If combined heating is considered, the ORC system can be optimized to generate 521 kWe of electricity, while also delivering 4.52 MW of heat. Finally, an increased power output (by 26% with combined heating, and by 39% without combined heating) can be achieved by using high temperature thermal oil for buffering instead of a steam loop; however, the use of thermal oil in these applications has been until now typically discouraged due to flammability concerns.

Journal article

White MT, Oyewunmi OA, Haslam A, Markides Cet al., 2017, Exploring optimal working fluids and cycle architectures for organic Rankine cycle systems using advanced computer-aided molecular design methodologies, 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics (HEFAT2017), Publisher: ICHMT

The combination of computer-aided molecular design(CAMD) with an organic Rankine cycle (ORC) power-systemmodel presents a powerful methodology that facilitates an in-tegrated approach to simultaneous working-fluid design andpower-system thermodynamic or thermoeconomic optimisation.Existing CAMD-ORC models have been focussed on simplesubcritical, non-recuperated ORC systems. The current workintroduces partially evaporated or trilateral cycles, recuperatedcycles and working-fluid mixtures into the ORC power-systemmodel, which to the best knowledge of the authors has not beenpreviously attempted. A necessary feature of a CAMD-ORCmodel is the use of a mixed-integer non-linear programming(MINLP) optimiser to simultaneously optimise integer working-fluid variables and continuous thermodynamic cycle and eco-nomic variables. In this paper, this feature is exploited by in-troducing binary optimisation variables to describe the cycle lay-out, thus enabling the cycle architecture to be optimised along-side the working fluid and system conditions. After describingthe models for the alternative cycles, the optimisation problemis completed for a defined heat source, considering hydrocar-bon working fluids. Two specific case studies are considered,in which the power output from the ORC system is maximised.These differ in the treatment of the minimum heat-source outlettemperature, which is unconstrained in the first case study, butconstrained in the second. This is done to replicate scenariossuch as a combined heat and power (CHP) plant, or applicationswhere condensation of the waste-heat stream must be avoided.In both cases it is found that a working-fluid mixture can per-form better than a pure working fluid. Furthermore, it is foundthat partially-evaporated and recuperated cycles are optimal forthe unconstrained and constrained case studies respectively.

Conference paper

Lecompte S, Oyewunmi OA, Markides CN, Lazova M, Kaya A, Ameel B, van den Broek M, De Paepe Met al., 2017, Potential of organic Rankine Cycles (ORC) for waste heat recovery on an electric arc furnace (EAF), 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics (HEFAT2017), Publisher: ICHMT

The organic Rankine cycle (ORC) is a mature technology to convert low temperature waste heat to electricity. While several energy intensive industries could benefit from the integration of an ORC, their adoption rate is rather low. One important reason is that the prospective end-users find it difficult to recognize and realise the possible energy savings. In more recent years, the electric arc furnaces (EAF) are considered as a major candidate for waste heat recovery. Therefore, in this work, the integration of an ORC coupled to a100 MWe EAF is investigated. The effect of working with averaged heat profiles, a steam buffer and optimized ORC architectures is investigated. The results show that it is crucial to take into account the heat profile variations for the typical batch process of an EAF. An optimized subcritical ORC(SCORC) can generate an electricity output of 752 kWe with a steam buffer working at 25 bar. However, the use of a steam buffer also impacts the heat transfer to the ORC. A reduction up to 61.5% in net power output is possible due to the additional isothermal plateau of the steam

Conference paper

Ramos Cabal A, Guarracino I, Mellor A, Alonso Álvarez D, Childs P, Ekins Daukes N, Markides CNet al., 2017, Solar-Thermal and Hybrid Photovoltaic-Thermal Systems for Renewable Heating, Publisher: Grantham Institute, Imperial College London

Headlines Heat constitutes about half of total global energy demand. Solar heat offers key advantages over other renewable sources for meeting this demand through distributed, integrated systems. Solar heat is a mature sustainable energy technology capable of mass deployment. There is significant scope for increasing the installed solar heat capacity in Europe. Only a few European countries are close to reaching the EU target of 1 m2 of solar-thermal installations per person. One key challenge for the further development of the solar-thermal market arises from issues related to the intermittency of the solar resource, and the requirement for storage and/or backup systems. The former increases investment costs and limits adaptability. An analysis of EU countries with good market development, suggests that obligation schemes are the best policy option for maximising installations. These do not present a direct cost to the public budget, and determine the growth of the local industry in the long term. Solar-thermal collectors can be combined with photovoltaic (PV) modules to produce hybrid PV-thermal (PV-T) collectors. These can deliver both heat and electricity simultaneously from the same installed area and at a higher overall efficiency compared to individual solar-thermal and PV panels installed separately. Hybrid PV-T technology provides a particularly promising solution when roof space is limited or when heat and electricity are required at the same time.


Markides C, Tunnicliffe H, 2017, A concentrated effort, TCE The Chemical Engineer, Pages: 38-41, ISSN: 0302-0797

Concentrated solar power (CSP) has received less attention and funding compared to its more popular solar power rival, especially since photovoltaic (PV) technology is more viable in widespread smaller-scale applications. CSP has the ability to store thermal energy unlike conventional photovoltaics. CSP power stations that has storage facilities can utilize heat to generate power even when the sun is out. According to Christos Markides, reader in clean energy processes at the Department of Chemical Engineering at Imperial College London, UK, a silicon PV panel has an efficiency, in terms of solar energy transformed into electricity, or about 15%-20%, while CSP technologies will have efficiencies close to 25%. Markides and his team are researching on a slightly different CSP concept, in which H2O that drives the turbine generation cycle is also utilized as the heat transfer fluid that is sent to the parabolic collectors, rather than using an intermediate thermal oil, a process called direct steam generation. Markides is leading a five-year collaborative CSP research project with African universities - the University of Pretoria in South Africa, the University of Lagos in Nigeria, and the University of Mauritius. The project is a program grant funded by the UK Department for International Development.

Journal article

White MT, Oyewunmi OO, Haslam AJ, Markides CNet al., 2017, Industrial waste-heat recovery through integrated computer-aided working-fluid and ORC system optimisation using SAFT-γ Mie, Energy Conversion and Management, Vol: 150, Pages: 851-869, ISSN: 0196-8904

A mixed-integer non-linear programming optimisation framework is formulated and developed that combines a molecular-based, group-contribution equation of state, SAFT-γγ Mie, with a thermodynamic description of an organic Rankine cycle (ORC) power system. In this framework, a set of working fluids is described by its constituent functional groups (e.g., since we are focussing here on hydrocarbons: single bondCH3, single bondCH2single bond, etc. ), and integer optimisation variables are introduced in the description the working-fluid structure. Molecular feasibility constraints are then defined to ensure all feasible working-fluid candidates can be found. This optimisation framework facilitates combining the computer-aided molecular design of the working fluid with the power-system optimisation into a single framework, thus removing subjective and pre-emptive screening criteria, and simultaneously moving towards the next generation of tailored working fluids and optimised systems for waste-heat recovery applications. SAFT-γγ Mie has not been previously employed in such a framework. The optimisation framework, which is based here on hydrocarbon functional groups, is first validated against an alternative formulation that uses (pseudo-experimental) thermodynamic property predictions from REFPROP, and against an optimisation study taken from the literature. The framework is then applied to three industrial waste-heat recovery applications. It is found that simple molecules, such as propane and propene, are the optimal ORC working fluids for a low-grade (150 °C) heat source, whilst molecules with increasing molecular complexity are favoured at higher temperatures. Specifically, 2-alkenes emerge as the optimal working fluids for medium- and higher-grade heat-sources in the 250–350 °C temperature range. Ultimately, the results demonstrate the potential of this framework to drive the search for the next generation of ORC systems, and to

Journal article

Oyewunmi OA, white MT, Chatzopoulou M, Haslam A, Markideset al., 2017, Integrated Computer-Aided Working-Fluid Design and Power System Optimisation: Beyond Thermodynamic Modelling, 30th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems (ECOS 2017), Publisher: ECOS-2017

Improvements in the thermal and economic performance of organic Rankine cycle (ORC) systems are requiredbefore the technology can be successfully implemented across a range of applications. The integration ofcomputer-aided molecular design (CAMD) with a process model of the ORC facilitates the combinedoptimisation of the working-fluid and the power system in a single modelling framework, which should enablesignificant improvements in the thermodynamic performance of the system. However, to investigate theeconomic performance of ORC systems it is necessary to develop component sizing models. Currently, thegroup-contribution equations of state used within CAMD, which determine the thermodynamic properties of aworking-fluid based on the functional groups from which it is composed, only derive the thermodynamicproperties of the working-fluid. Therefore, these do not allow critical components such as the evaporator andcondenser to be sized. This paper extends existing CAMD-ORC thermodynamic models by implementinggroup-contribution methods for the transport properties of hydrocarbon working-fluids into the CAMD-ORCmethodology. Not only does this facilitate the sizing of the heat exchangers, but also allows estimates of systemcosts by using suitable cost correlations. After introducing the CAMD-ORC model, based on the SAFT-γ Mieequation of state, the group-contribution methods for determining transport properties are presented alongsidesuitable heat exchanger sizing models. Finally, the full CAMD-ORC model incorporating the componentmodels is applied to a relevant case study. Initially a thermodynamic optimisation is completed to optimise theworking-fluid and thermodynamic cycle, and then the component models provide meaningful insights into theeffect of the working-fluid on the system components.

Conference paper

Oyewunmi OA, Lecompte S, De Paepe M, Markideset al., 2017, Thermodynamic Optimization of Recuperative Sub- and Transcritical Organic Rankine Cycle Systems, 30th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems (ECOS 2017), Publisher: ECOS-2017

There is significant interest in the deployment of organic Rankine cycle (ORC) technology for waste-heatrecovery and power generation in industrial settings. This study considers ORC systems optimized formaximum power generation using a case study of an exhaust flue-gas stream at a temperature of 380 °C asthe heat source, covering over 30 working fluids and also considering the option of featuring a recuperator.Systems based on transcritical cycles are found to deliver higher power outputs than subcritical ones, withoptimal evaporation pressures that are 4-5 times the critical pressures of refrigerants and light hydrocarbons,and 1-2 times those of siloxanes and heavy hydrocarbons. For maximum power production, a recuperator isnecessary for ORC systems with constraints imposed on their evaporation and condensation pressures. Thisincludes, for example, limiting the minimum condensation pressure to atmospheric pressure to prevent subatmosphericoperation of this component, as is the case when employing heavy hydrocarbon and siloxaneworking fluids. For scenarios where such operating constraints are relaxed, the optimal cycles do not featurea recuperator, providing some capital cost savings, with some cycles showing more than three times thegenerated power than with this component, making investments in sub-atmospheric components worthwhile.

Conference paper

Ramos Cabal A, Chatzopoulou MA, Guarracino I, Freeman J, Markides CNet al., 2017, Hybrid photovoltaic-thermal solar systems for combined heating, coolingand power provision in the urban environment, Energy Conversion and Management, Vol: 150, Pages: 838-850, ISSN: 0196-8904

Solar energy can play a leading role in reducing the current reliance on fossil fuels and in increasing renewable energy integration in the built environment. Hybrid photovoltaic-thermal (PV-T) systems can reach overall efficiencies in excess of 70%, with electrical fficiencies in the range of 15-20% and thermal efficiencies of 50% or higher. In most applications, the electrical output of a hybrid PV-T system is the priority, hence the contacting fluid is used to cool the PV cells to maximise their electrical performance, which imposes a limit on the fluid's downstream use. When optimising the overall output of PV-T systems for combinedheating and cooling provision, this technology can cover more than 60% of the heating and about 50% of the cooling demands of households in the urban environment. To achieve this, PV-T systems can be coupledto heat pumps or absorption refrigeration systems as viable alternatives to vapour-compression systems. This work considers the techno-economic challenges of such systems, when aiming at a low cost per kWh of energy generation of PV-T systems for co- or tri-generation in the housing sector. First, the viability and afordability of the proposed systems are studied in ten European locations, with local weather pro files, using annually and monthly averaged solar-irradiance and energy-demand data. Based on annual simulations, Seville, Rome, Madrid and Bucharest emerge as the most promising locations from those examined, and the most efficient system confi guration involves coupling PV-T panels to water-to-water heat pumps that usethe PV-T thermal output to maximise the system's COP. Hourly resolved transient models are then defi ned in TRNSYS in order to provide detailed estimates of system performance, since it is found that the temporal resolution (e.g. hourly, daily, yearly) of the simulations strongly affects their predicted performance. The TRNSYS results indicate that PV-T systems have the potential to cover 60% of the heating

Journal article

Gupta A, Markides CN, 2017, An experimental study of the autoignition of polydispersed liquid-fuel droplets in a confined high-temperature turbulent coflow, 8th European Combustion Meeting, Publisher: Combustion Institute

We present experimental data on the autoignition of polydispersed droplets of liquid n-pentane injected axisymmet-rically from a circular nozzle into a confined turbulent coflow of hot air at atmospheric pressure, with the aim ofexamining the emergence of autoignition in the presence of flow, mixture and phase inhomogeneities. In the regime ofinterest, autoignition occurred in the form of random spots. At higher air temperatures and lower fuel injection veloc-ities, autoignition was observed closer to the injector; the corresponding delay (residence) times also decreased. Withincreasing air velocity and hence turbulent velocity fluctuations, autoignition moved downstream but the delay timesdecreased. The results are also compared to equivalent results obtained with n-heptane (from previous experiments inthe same apparatus). The data can be used for the development of advanced multiphase turbulent combustion models.

Conference paper

Guarracino I, Freeman J, Markides CN, 2017, Solar heat and power with thermal energy storage in the UK, SolaStor

Solar energy has the potential to cover a high fraction of thedemand for heat and electricity in residential buildings. Fig. 1 showsthe variation in incident solar irradiation received across Europe.In London the annual solar irradiation is ~1100 kWh/m2 per year,while the typical domestic energy consumption per household is~12000 kWh/year for heating and ~4000 kWh/year for electricity.Thus the solar energy received on a rooftop of ~15 m2is enoughpotentially enough to provide the entire annual demand fordomestic energy.Our research focuses on various aspects of two solar technologiesfor the combined provision of heating and power (CHP): solarorganic Rankine cycle systems with low-to-medium temperaturesolar-thermal collectors (Figs. 2-3) and hybrid photovoltaic/thermal(PVT) systems. (Fig.4).


White M, Sayma AI, Markides CN, 2017, Supersonic flow of non-ideal fluids in nozzles: An application of similitude theory and lessons for ORC turbine design and flexible use considering system performance, 1st International Seminar on Non-Ideal Compressible-Fluid Dynamics for Propulsion and Power (NICFD), Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588

Conference paper

Taleb AI, Sapin PMC, Barfuß C, Fabris D, Markides CNet al., 2017, CFD Analysis of Thermally Induced Thermodynamic Loses in the Reciprocating Compression and Expansion of Real Gases, 1st International Seminar on Non-Ideal Compressible-Fluid Dynamics for Propulsion and Power, Publisher: IOP Publishing: Conference Series, ISSN: 1742-6588

The efficiency of expanders is of prime importance in determining the overallperformance of a variety of thermodynamic power systems, with reciprocating-piston expandersfavoured at intermediate-scales of application (typically 10–100 kW). Once the mechanical lossesin reciprocating machines are minimized (e.g. through careful valve design and operation), lossesdue to the unsteady thermal-energy exchange between the working fluid and the solid walls ofthe containing device can become the dominant loss mechanism. In this work, gas-spring devicesare investigated numerically in order to focus explicitly on the thermodynamic losses that arisedue to this unsteady heat transfer. The specific aim of the study is to investigate the behaviourof real gases in gas springs and to compare this to that of ideal gases in order to attain a betterunderstanding of the impact of real-gas effects on the thermally induced losses in reciprocatingexpanders and compressors. A CFD-model of a gas spring is developed in OpenFOAM. Threedifferent fluid models are compared: (1) an ideal-gas model with constant thermodynamicand transport properties; (2) an ideal-gas model with temperature-dependent properties; and(3) a real-gas model using the Peng-Robinson equation-of-state with temperature and pressure-dependent properties. Results indicate that, for simple, mono- and diatomic gases, like helium ornitrogen, there is a negligible difference in the pressure and temperature oscillations over a cyclebetween the ideal and real-gas models. However, when considering heavier (organic) molecules,such as propane, the ideal-gas model tends to overestimate the pressure compared to the real-gasmodel, especially if the temperature and pressure dependency of the thermodynamic propertiesis not taken into account. In fact, the ideal-gas model predicts higher pressures by as much as25% (compared to the real-gas model). Additionally, both ideal-gas models underestimate thethermall

Conference paper

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