Imperial College London

ProfessorChristosMarkides

Faculty of EngineeringDepartment of Chemical Engineering

Professor of Clean Energy Technologies
 
 
 
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Contact

 

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

 
 
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Location

 

404ACE ExtensionSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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265 results found

Moran H, Gupta A, Voulgaropoulos V, Markides Cet al., Autoignition of a liquid n-heptane jet injected into a confined turbulent hot co-flow, 3rd SEE SDEWES 2018, Publisher: SDEWES

Alternatives to conventional combustion engines, such as gasoline direct injection engines, homogeneous charge compression injection engines and dual-fuel turbines, promise improved fuel efficiency and reduced emissions. The present study of liquid-fuel autoignition in turbulent flows explores the underlying phenomena in these applications towards next-generation combustors. Experiments have been performed on the autoignition of continuous liquid n-heptane jets injected axisymmetrically into confined turbulent coflows of preheated air. Jet atomisation was characterised using high-speed imaging, and autoignition locations and corresponding delay times were recorded for various bulk air temperatures and velocities. Two turbulence-generating plates with different perforation sizes were used to investigate the role of turbulence in affecting the phenomena under investigation. Smaller droplets formed in flows with lower turbulence intensities and larger integral lengthscales. The autoignition length increased and delay time decreased with increasing bulk air velocity, the latter being contrary to results from pre-vaporized n-heptane autoignition in an identical apparatus.

Conference paper

Lecompte S, Chatzopoulou MA, Markides C, De Paepe Met al., Off-design comparison of subcritical and partial evaporating ORCs in quasi-steady state annual simulations, ICAE2010 - 10th International Conference on Applied Energy, Publisher: Elsevier, ISSN: 1876-6102

The subcritical ORC (SCORC) is considered the industry standard due to its simple configuration, acceptable efficiency and lowcosts. However, it is known that alternative ORC configurations have the potential to increase efficiency. A cycle modification which closely resembles the SCORC is the partial evaporating ORC (PEORC), where a two-phase mixture of liquid-vapour enters the expander instead of superheated vapour. In theoretical studies at design conditions, higher power outputs are achieved for the PEORC compared to the SCORC. This work aims to go a step further by investigating the performance of the SCORC and PEORC under time-dependent operating conditions. A direct comparison between the SCORC and PEORC is made for identically sized systems using as input the waste heat stream of a waste incinerator plant and the changing ambient conditions. Performance maps of both cycle configurations are compiled and the benefit of an expander operating at variable speed is briefly discussed. The results indicate that for the specific case under investigation, the PEORC has an increased annually averaged net power output of 9.6% compared to the SCORC. Use of annually averaged input conditions results in an overestimation of the net power output for both the SCORC and PEORC, and furthermore, the relative improvement in power output for the PEORC is reduced to 6.8%. As such, the use of time-averaged conditions when comparing cycle architectures should preferably be avoided.

Conference paper

Mellor A, Guarracino I, Llin LF, Alonso-Alvarez D, Riverola A, Thoms S, Paul DJ, Markides CN, Chemisana D, Maier S, Ekins-Daukes Net al., 2018, Specially designed solar cells for hybrid photovoltaic-thermal generators, IEEE Photovoltaic Specialists Conference (PVSC), Publisher: IEEE

The performance of hybrid photovoltaic-thermal systems can be improved using PV cells that are specially designed to generate both electricity and useful heat with maximum efficiency. Present systems, however, use standard PV cells that are only optimized for electrical performance. In this work, we have developed two cell-level components that will improve the thermal efficiency of PV-T collectors, with minimal loss of electrical efficiency. These are a spectrally-selective low- emissivity coating to reduce radiative thermal losses, and a nanotextured rear reflector to improve absorption of the near- infrared part of the solar spectrum for heat generation.

Conference paper

Charogiannis A, Denner F, Van Wachem BGM, Kalliadasis S, Markides CNet al., Heat tranfer phenomena in falling liquid films: A synergistic experimental and computational study, International Heat Transfer Conference

We employ planar laser-induced fluorescence (PLIF), particle tracking velocimetry (PTV) and infrared thermography (IR) towards the detailed investigation of the flow and heat transfer phenomena underlying harmonically-excited, gravity-driven film flows falling over an inclined, electrically-heated substrate. PLIF is used to generate space and time-resolved film-height measurements, PTV to retrieve two-dimensional (2-D) velocity-field information, and IR to recover the temperature of the film free-surface. The experiments are complemented by direct numerical simulations (DNSs) that provide additional information on the liquid temperature, viscosity and velocity distributions between the flow inlet and the location along the axial direction of the flow where optical measurements are conducted. By adoption of this synergistic approach, we recover results on the spatiotemporal evolution of the flow and temperature fields, and link the variation of the gas-liquid interface temperature along the waves to the variation of the local film-height, flow-rate and streamwise and cross-stream velocity components. Despite the intermittent observation of localized hotspots in the experiments, which constitute precursors to the formation of thermal rivulets, the mean wall-temperature, bulk liquid-temperature and gas-liquid interface temperature display clear trends with respect to the mean film-thickness, which largely dictates the heat transfer performance of the examined film flows.

Conference paper

Sapin P, Simpson M, Kirmse C, Markides Cet al., Dynamic modeling of water-droplet spray injection in reciprocating-piston compressors, ECOS 2018 - 31st International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems

Conference paper

Ibarra R, Zadrazil I, Matar O, Markides CNet al., 2018, Dynamics of liquid-liquid flows in horizontal pipes using simultaneous two-line planar laser-induced fluorescence and particle velocimetry, International Journal of Multiphase Flow, Vol: 101, Pages: 47-63, ISSN: 0301-9322

Experimental investigations are reported of oil-water stratified and stratified-wavy flows in horizontal pipes using a simultaneous two-line (two-colour) technique based on combining planar laser-induced fluorescence with particle image/tracking velocimetry. This approach allows the study of fluid combinations with properties similar to those encountered in industrial field-applications in terms of density, viscosity, and interfacial tension, even though their refractive indices are not matched. The flow conditions studied span mixture velocities in the range 0.3 – 0.6 m/s and low water-cuts up to 20%, corresponding to in situ (local) Reynolds numbers of 1750 – 3350 in the oil phase and 2860 – 11650 in the water phase, and covering the laminar/transitional and transitional/turbulent flow regimes for the oil and water phases, respectively. Detailed, spatiotemporally-resolved in situ phase and velocity data in a vertical plane aligned with the pipe centreline and extending across the entire height of the channel through both phases are analysed to provide statistical information on the interface heights, mean axial and radial (vertical) velocity components, (rms) velocity fluctuations, Reynolds stresses, and mixing lengths. The mean liquid-liquid interface height is mainly determined by the flow water cut and is relatively insensitive (up to 20% the highest water cut) to changes in the mixture velocity, although as the mixture velocity increases the interfacial profile transitions gradually from being relatively flat to containing higher amplitude waves. The mean velocity profiles show characteristics of both laminar and turbulent flow, and interesting interactions between the two co-flowing phases. In general, mean axial velocity profiles in the water phase collapse to some extent for a given water cut when normalised by the mixture velocity; conversely, profiles in the oil phase do not. Strong vertical velocity components can modify the shape of th

Journal article

Juggurnath D, Elahee MK, Dauhoo MZ, Khoodaruth A, Olakoyejo OT, Adelaja AO, Meyer JP, Markides CNet al., Numerical modelling of turbulent condensing flows in a smooth horizontal tube, 10th International Conference on Boiling and Condensation Heat Transfer (ICBCHT2018)

Conference paper

Unamba C, Najjaran Kheirabadi A, Freeman J, Herrando M, Markides Cet al., 2018, High-Efficiency Hybrid PV and Solar-Thermal Combined Cooling and Power Technologies, 3rd Energy Future Conference (EF III)

Solar energy can be used to provide heat or to generate electricity (many land areas in the world have sufficient solar irradiance based on Figure 1). Most solar panels designed for one of these purposes, with electrical photovoltaic (PV) panels being typically less than 20% efficient. PV cells experience a deterioration in efficiency when operated at high temperatures, which occurs when the solar irradiance and generation from such systems are at their highest. Hybrid PV-thermal (PVT) solar collector technology combines PV modules with the contacting flow of a cooling fluid in a number of configurations, and offers advantages when space is at a premium and there is demand for both heat and power [1,2]. By far the most common use of the thermal-energy output from PVT systems is to provide hot water at 50-60 °C for households or commercial use, however, a much wider range of opportunities arises at higher temperatures (typically above 60 °C) where refrigeration cycles can be used.Meanwhile, non-concentrating solar thermal (ST) collectors, such as evacuated tube collectors (ETC), can be designed to operate with a high thermal efficiency in the range 80-200 °C, making them suitable for a wider range of thermodynamic power and cooling cycles, such as the organic Rankine cycle (ORC) and the diffusion absorption refrigeration cycle (DAR), which can be tailored to a particular solar heat source though careful selection of an appropriate working fluid [3,4].In this work, we investigate two alternative system configurations for the provision of solar combined cooling and power (S-CCP) in a distributed domestic application. Both systems use the same reference household energy demand for cooling and power and are constrained by the same total available solar collection area.

Poster

Sapin P, Simpson M, Kirmse C, Markides CNet al., 2018, A lumped-mass analysis of water evaporation in reciprocating-piston compressors

Poster

Denner F, Charogiannis A, Pradas M, Markides C, van Wachem B, Kalliadasis Set al., 2018, Solitary waves on falling liquid films in the inertia-dominated regime, Journal of Fluid Mechanics, Vol: 837, Pages: 491-519, ISSN: 0022-1120

We offer new insights and results on the hydrodynamics of solitary waves on inertiadominatedfalling liquid films using a combination of experimental measurements,direct numerical simulations (DNS) and low-dimensional (LD) modelling. The DNSare shown to be in very good agreement with experimental measurements in termsof the main wave characteristics and velocity profiles over the entire range ofinvestigated Reynolds numbers. And, surprisingly, the LD model is found to predictaccurately the film height even for inertia-dominated films with high Reynoldsnumbers. Based on a detailed analysis of the flow field within the liquid film, thehydrodynamic mechanism responsible for a constant, or even reducing, maximumfilm height when the Reynolds number increases above a critical value is identified,and reasons why no flow reversal is observed underneath the wave trough above acritical Reynolds number are proposed. The saturation of the maximum film heightis shown to be linked to a reduced effective inertia acting on the solitary waves asa result of flow recirculation in the main wave hump and in the moving frame ofreference. Nevertheless, the velocity profile at the crest of the solitary waves remainsparabolic and self-similar even after the onset of flow recirculation. The upper limitof the Reynolds number with respect to flow reversal is primarily the result ofsteeper solitary waves at high Reynolds numbers, which leads to larger streamwisepressure gradients that counter flow reversal. Our results should be of interest in theoptimisation of the heat and mass transport characteristics of falling liquid films andcan also serve as a benchmark for future model development.

Journal article

Chatzopoulou M, Markides CN, Thermodynamic assessment of organic Rankine cycle systems during off-design operation in combined heat and power (CHP) application., 3rd Thermal and Fluids Engineering Conference

Conference paper

Charogiannis A, Denner F, van Wachem BGM, Kalliadasis S, Markides CNet al., 2017, Statistical characteristics of falling-film flows: A synergistic approach at the crossroads of direct numerical simulations and experiments, Physical Review Fluids, Vol: 2, ISSN: 2469-990X

We scrutinize the statistical characteristics of liquid films flowing over an inclined planar surface based on film height and velocity measurements that are recovered simultaneously by application of planar laser-induced fluorescence (PLIF) and particle tracking velocimetry (PTV), respectively. Our experiments are complemented by direct numerical simulations (DNSs) of liquid films simulated for different conditions so as to expand the parameter space of our investigation. Our statistical analysis builds upon a Reynolds-like decomposition of the time-varying flow rate that was presented in our previous research effort on falling films in [Charogiannis et al., Phys. Rev. Fluids 2, 014002 (2017)], and which reveals that the dimensionless ratio of the unsteady term to the mean flow rate increases linearly with the product of the coefficients of variation of the film height and bulk velocity, as well as with the ratio of the Nusselt height to the mean film height, both at the same upstream PLIF/PTV measurement location. Based on relations that are derived to describe these results, a methodology for predicting the mass-transfer capability (through the mean and standard deviation of the bulk flow speed) of these flows is developed in terms of the mean and standard deviation of the film thickness and the mean flow rate, which are considerably easier to obtain experimentally than velocity profiles. The errors associated with these predictions are estimated at ≈1.5% and 8% respectively in the experiments and at <1% and <2% respectively in the DNSs. Beyond the generation of these relations for the prediction of important film flow characteristics based on simple flow information, the data provided can be used to design improved heat- and mass-transfer equipment reactors or other process operation units which exploit film flows, but also to develop and validate multiphase flow models in other physical and technological settings.

Journal article

Pantaleo AM, Fordham J, Oyewunmi OA, Markides CNet al., 2017, Intermittent waste heat recovery via ORC in coffee torrefaction, 9th International Conference on Applied Energy, ICAE2017, Publisher: Elsevier, Pages: 1714-1720, ISSN: 1876-6102

Coffee torrefaction is carried out by means of hot air at average temperature of 200-240°C and with intermittent cycles where a lot of heat is discharged from the stack. CHP systems have been investigated to provide heat to the process. However, much of the heat released in the process is from the afterburner that heats up the flue gas to higher temperatures to remove volatile organic compounds and other pollutants. In this paper, the techno-economic feasibility of utilising waste heat from a rotating drum coffee roasting with partial hot gas recycling is assessed. A cost analysis is adopted to compare the profitability of two systems configurations integrated into the process. The case study of a major coffee torrefaction firm with 500 kg/hr production capacity in the Italian energy framework is taken. The CHP options under investigation are: (i) regenerative topping micro gas turbine (MGT) coupled to the existing modulating gas burner to generate hot air for the roasting process; (ii) intermittent waste heat recovery from the hot flue gas through an organic Rankine cycle (ORC) coupled to a thermal storage buffer. The results show that the profitability of these investments is highly influenced by the natural gas/electricity cost ratio, by the coffee torrefaction production capacity and intermittency level of discharged heat. In this case study, MGT seems to be more profitable than waste heat recovery via ORC due to the intermittency of the heat source and the relatively high electricity/heat cost ratio.

Conference paper

Simpson M, Sapin P, Rotolo G, De Palma P, Pantaleo AM, Markides CNet al., Efficiency map of reciprocating-piston expanders for ORC applications, 4th Annual Engine Organic Rankine Cycle Consortium Workshop 2017

Conference paper

Pantaleo AM, Chatzopoulou MA, Oyewunmi O, de palma P, amirante R, rotolo G, Markides Cet al., 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 STATIONNARY ENGINE INDUSTRIES

Conference paper

Acha S, Mariaud A, Shah N, Markides CNet al., 2017, Optimal design and operation of low-carbon energy technologies in commercial buildings, 30th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems - ECOS 2017

© 2017 IMEKO Non-domestic buildings are large energy consumers and present many opportunities with which to enhance the way they produce and consume electricity, heating and cooling. If energy system integration is feasible, this can lead to significant reductions in energy use and emissions associated with building operations. Due to their diverse energy requirements, a broad range of technologies in flexible solutions need to be evaluated to identify the best alternative. This paper presents an integrated energy-systems model that optimizes the selection and operation of distributed technologies for commercial buildings. The framework consists of a comprehensive technology database, half-hourly electricity cost profiles, conventional fuel costs and building features. This data is applied to a mixed-integer linear programming model that optimizes the design and operation of installed technologies based on a range of financial and environmental criteria. The model aims to guide decision makers in making attractive investments that are technically feasible and environmentally sound. A case study of a food distribution centre in the UK is presented to illustrate the economic and environmental benefits the proposed integrated energy systems model could bring against a business as usual (BaU) approach. The technology portfolio considered in the case study includes combined heat and power (CHP) and organic Rankine cycle (ORC) engines, absorption chillers, photovoltaic (PV) panels, and battery systems. The results clearly illustrate the different outcomes and trade-offs that can emerge when stakeholders champion different technologies instead of making an exhaustive assessment. Overall, the model provides meaningful insights that can allow stakeholders to make well informed investment decisions when it comes to the optimal configuration and operation of energy technologies in commercial buildings.

Conference paper

Acha Izquierdo S, Mariaud A, Shah N, Markides Cet al., 2017, Optimal Design and Operation of Distributed Low-Carbon Energy Technologies in Commercial Buildings, Energy, Vol: 142, Pages: 578-591, ISSN: 0360-5442

Commercial buildings are large energy consumers and opportunities exist to improve the way they produce and consume electricity, heating and cooling. If energy system integration is feasible, this can lead to significant reductions in energy consumption and emissions. In this context, this work expands on an existing integrated Technology Selection and Operation (TSO) optimisation model for distributed energy systems (DES). The model considers combined heat and power (CHP) and organic Rankine cycle (ORC) engines, absorption chillers, photovoltaic panels and batteries with the aim of guiding decision makers in making attractive investments that are technically feasible and environmentally sound. A retrofit case study of a UK food distribution centre is presented to showcase the benefits and trade-offs that integrated energy systems present by contrasting outcomes when different technologies are considered. Results show that the preferred investment options select a CHP coupled either to an ORC unit or to an absorption chiller. These solutions provide appealing internal rates of return of 28–30% with paybacks within 3.5–3.7 years, while also decarbonising the building by 95–96% (if green gas is used to power the site). Overall, the TSO model provides valuable insights allowing stakeholders to make well-informed decisions when evaluating complex integrated energy systems.

Journal article

Freeman JP, Najjaran Kheirabadi A, Edwards R, Reid M, Hall R, Ramos A, Markides Cet al., Testing and simulation of a solar diffusion-absorption refrigeration system for low-cost solar cooling in India, ISES Solar World Congress 2017

Conference paper

Riverola A, Mellor AV, Alonso Alvarez D, Ferre LLin L, Guarracino I, Markides CN, Paul DJ, Ekins-Daukes Net al., 2017, Mid-infrared emissivity of crystalline silicon solar cells, Solar Energy Materials and Solar Cells, Vol: 174, Pages: 607-615, ISSN: 0927-0248

The thermal emissivity of crystalline silicon photovoltaic (PV) solar cells plays a role in determining the operating temperature of a solar cell. To elucidate the physical origin of thermal emissivity, we have made an experimental measurement of the full radiative spectrum of the crystalline silicon (c-Si) solar cell, which includes both absorption in the ultraviolet to near-infrared range and emission in the mid-infrared. Using optical modelling, we have identified the origin of radiative emissivity in both encapsulated and unencapsulated solar cells. We find that both encapsulated and unencapsulated c-Si solar cells are good radiative emitters but achieve this through different effects. The emissivity of an unencapsulated c-Si solar cell is determined to be 75% in the MIR range, and is dominated by free-carrier emission in the highly doped emitter and back surface field layers; both effects are greatly augmented through the enhanced optical outcoupling arising from the front surface texture. An encapsulated glass-covered cell has an average emissivity around 90% on the MIR, and dips to 70% at 10 µm and is dominated by the emissivity of the cover glass. These findings serve to illustrate the opportunity for optimising the emissivity of c-Si based collectors, either in conventional c-Si PV modules where high emissivity and low-temperature operation is desirable, or in hybrid PV-thermal collectors where low emissivity enables a higher thermal output to be achieved.

Journal article

Pantaleo AM, markides C, fordham J, Oyewunmi, Pantaleo AM, markides CN, Oyewunmi O, fordham Jet al., 2017, Intermittent waste heat recovery: Investment profitability of ORC cogeneration for batch, gas-fired coffee roasting, ICAE 2017, Publisher: Elsevier, Pages: 575-582, ISSN: 1876-6102

Coffee roasting is a highly energy intensive process with much of the energy being lost in intermittent cycles as discharged heatfrom the stack. In this work, combined heat and power (CHP) systems based on micro gas-turbines (MGT) are investigated forproviding heat to the roasting process. Much of the heat released in a coffee roaster is from the afterburner that heats up the fluegases to high temperatures in order to remove volatile organic compounds (VOCs) and other pollutants. An interesting solutionfor utilizing waste heat is assessed through energy and material balances of a rotating drum coffee roasting with partial hot gasrecycling. A cost assessment methodology is adopted to compare the profitability of three proposed system configurationsintegrated into the process. The case study of a major coffee torrefaction plant with 500 kg/h production capacity is assumed tocarry out the thermo-economic assessment, under the Italian energy framework. The CHP options under investigation are:(i) regenerative topping MGT coupled to the existing modulating gas burner to generate hot air for the roasting process;(ii) intermittent waste-heat recovery from the hot flue-gases through an organic Rankine cycle (ORC) engine coupled to athermal storage buffer; and (iii) non-regenerative topping MGT with direct recovery of turbine outlet air for the roasting processby means of an afterburner that modulates the heat demand of the roasting process. The results show that the profitability of theseinvestments is highly influenced by the natural gas/electricity cost ratio, by the coffee torrefaction production capacity and by theintermittency level of discharged heat. The MGT appears as a more profitable option than waste-heat recovery via the ORCengine due to the intermittency of the heat source and the relatively high electricity/heat cost ratio.

Conference paper

Pantaleo AM, Camporeale SM, Sorrentino A, Miliozzi A, Shah N, Markides CNet al., 2017, Solar/biomass hybrid cycles with thermal storage and bottoming ORC: System integration and economic analysis, 4th International Seminar on ORC Power Systems (ORC), Publisher: Elsevier, Pages: 724-731, ISSN: 1876-6102

This paper focuses on the thermodynamic modelling and thermo-economic assessment of a novel arrangement of a combined cycle composed of an externally fired gas turbine (EFGT) and a bottoming organic Rankine cycle (ORC). The main novelty is that the heat of the exhaust gas exiting from the gas turbine is recovered in a thermal energy storage from which heat is extracted to feed a bottoming ORC. The thermal storage can receive heat also from parabolic-trough concentrators (PTCs) with molten salts as heat-transfer fluid (HTF). The presence of the thermal storage between topping and bottoming cycle facilitates a flexible operation of the system, and in particular allows to compensate solar energy input fluctuations, increase capacity factor, increase the dispatchability of the renewable energy generated and potentially operate in load following mode. A thermal energy storage (TES) with two molten salt tanks (one cold and one hot) is chosen since it is able to operate in the temperature range useful to recover heat from the exhaust gas of the EFGT and supply heat to the ORC. The heat of the gas turbine exhaust gas that cannot be recovered in the TES can be delivered to thermal users for cogeneration.The selected bottoming ORC is a superheated recuperative cycle suitable to recover heat in the temperature range of the TES with good cycle efficiency. On the basis of the results of the thermodynamic simulations, upfront and operational costs assessments and subsidized energy framework (feed-in tariffs for renewable electricity), the global energy conversion efficiency and investment profitability are estimated.

Conference paper

Pantaleo AM, markides, Oyewunmi, Chatzopoulou, white M, haslamet al., 2017, Integrated computer-aided working-fluid design and thermoeconomic ORC system optimisation, ORC-2017, Publisher: Elsevier, Pages: 152-159, ISSN: 1876-6102

The successful commercialisation of organic Rankine cycle (ORC) systems across a range of power outputs and heat-source temperatures demands step-changes in both improved thermodynamic performance and reduced investment costs. The former can be achieved through high-performance components and optimised system architectures operating with novel working-fluids, whilst the latter requires careful component-technology selection, economies of scale, learning curves and a proper selection of materials and cycle configurations. In this context, thermoeconomic optimisation of the whole power-system should be completed aimed at maximising profitability. This paper couples the computer-aided molecular design (CAMD) of the working-fluid with ORC thermodynamic models, including recuperated and other alternative (e.g., partial evaporation or trilateral) cycles, and a thermoeconomic system assessment. The developed CAMD-ORC framework integrates an advanced molecular-based group-contribution equation of state, SAFT-γ Mie, with a thermodynamic description of the system, and is capable of simultaneously optimising the working-fluid structure, and the thermodynamic system. The advantage of the proposed CAMD-ORC methodology is that it removes subjective and pre-emptive screening criteria that would otherwise exist in conventional working-fluid selection studies. The framework is used to optimise hydrocarbon working-fluids for three different heat sources (150, 250 and 350 °C, each with mcp = 4.2 kW/K). In each case, the optimal combination of working-fluid and ORC system architecture is identified, and system investment costs are evaluated through component sizing models. It is observed that optimal working fluids that minimise the specific investment cost (SIC) are not the same as those that maximise power output. For the three heat sources the optimal working-fluids that minimise the SIC are isobutane, 2-pentene and 2-heptene, with SICs of 4.03, 2.22 and 1.84 £/W res

Conference paper

Oyewunmi OA, Lecompte S, De Paepe M, Markides CNet al., 2017, Thermoeconomic analysis of recuperative sub- and transcritical organic Rankine cycle systems, 4th International Seminar on ORC Power Systems, Publisher: Elsevier, Pages: 58-65, ISSN: 1876-6102

There is significant interest in the deployment of organic Rankine cycle (ORC) technology for waste-heat recovery and power generation in industrial settings. This study considers ORC systems optimized for maximum power generation using a case study of an exhaust flue-gas stream at a temperature of 380°C as the heat source, covering over 35 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, with optimal 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 is necessary for ORC systems with constraints imposed on their evaporation and condensation pressures. This includes, for example, limiting the minimum condensation pressure to atmospheric pressure to prevent sub-atmospheric operation of this component, as is the case when employing heavy hydrocarbon and siloxane working fluids. For scenarios where such operating constraints are relaxed, the optimal cycles do not feature a recuperator, with some systems showing more than three times the generated power than with this component, albeit at higher investment costs.

Conference paper

Georgiou S, Acha Izquierdo S, Shah N, Markides Cet al., 2017, Assessing, benchmarking and analyzing heating and cooling requirements for glasshouse food production: a design and operation modelling framework, 1st International Conference on Sustainable Energy and Resource Use in Food Chains, ICSEF 2017, Publisher: Elsevier, Pages: 164-172, ISSN: 1876-6102

Growing populations, increase in food demand, society’s expectations for out of season products and the dependency of the food system on fossil fuels stress resources due to the requirements for national production and from importation of products from remote origins. Quantifying the use of resources in food production and their environmental impacts is key to identifying distinctive measures which can develop pathways towards low carbon food systems. In this paper, a modelling approach is presented which can quantify the energy requirements of heated glasshouse food production. Based on the outputs from the model, benchmarking and comparison among different glasshouse types and growers is possible. Additionally, the effect of spatial and annual weather trends on the heating and cooling requirements of glasshouses are quantified. Case study results indicate that a reduction in heating requirements of about 50%, and therefore an equivalent carbon footprint reduction, can be achieved by replacing a single glass sealed cover with a double glass sealed cover.

Conference paper

Unamba CK, White M, Sapin P, Freeman J, Lecompte S, Oyewunmi OA, Markides CNet al., 2017, Experimental investigation of the operating point of a 1-kW ORC system, 4th International Seminar on ORC Power Systems (ORC), Publisher: Elsevier Science BV, Pages: 875-882, ISSN: 1876-6102

The organic Rankine cycle (ORC) is a promising technology for the conversion of waste heat from industrial processes as well as heat from renewable sources. Many efforts have been channeled towards maximizing the thermodynamic potential of ORC systems through the selection of working fluids and the optimal choice of operating parameters with the aim of improving overall system designs, and the selection and further development of key components. Nevertheless, experimental work has typically lagged behind modelling efforts. In this paper, we present results from tests on a small-scale (1 kWel) ORC engine consisting of a rotary-vane pump, a brazed-plate evaporator and a brazed-plate condenser, a scroll expander with a built-in volume ratio of 3.5, and using R245fa as the working fluid. An electric oil-heater acted as the heat source, providing hot oil at temperatures in the range 120-140 °C. The frequency of the expander was not imposed by an inverter or the electricity grid but depended directly on the attached generator load; both the electrical load on the generator and the pump rotational speed were varied in order to investigate the performance of the system. Based on the generated data, this paper explores the relationship between the operating conditions of the ORC engine and changes in the heat-source temperature, pump and expander speeds leading to working fluid flow rates between 0.0088 kg/s and 0.0337 kg/s, from which performance maps are derived. The experimental data is, in turn, used to assess the performance of both the individual components and of the system, with the help of an exergy analysis. In particular, the exergy analysis indicates that the expander accounts for the second highest loss in the system. Analysis of the results suggests that increased heat-source temperatures, working-fluid flow rates, higher pressure ratios and larger generator loads improve the overall cycle efficiency. Specifically, a 46% increase in pressure ratio from 2.4

Conference paper

Markides C, Charogiannis A, 2017, Application of planar laser-induced fluorescence for the investigation of interfacial waves and rivulet structures in liquid films flowing down inverted substrates, Interfacial phenomena and heat transfer, Vol: 4, Pages: 235-252, ISSN: 2169-2785

We investigate the interfacial topology of liquid-film flows falling under an inverted planarsubstrate by conducting space- and time-resolved film-height measurements. A planarlaser-induced fluorescence (PLIF) technique is employed for this purpose, with a twocameraarrangement that allows us to image a region of the flow extending ≈ 40 mm oneither side of the centre of the film span, at a distance 330 mm downstream of the flowinlet. The substrate inclination angle is set to β = −30 °, the working fluid comprises 82%glycerol and 18% water (by weight), and the flow Reynolds number, Re, is varied in therange Re = 0.6 − 8.2. The uncertainty associated with the instantaneous film-height measurementis estimated at less than 3%. Depending on the flow Re, we observe a range ofinteresting flow regimes typically characterised by pronounced rivulet formation and spatiotemporalcoherence, which deviate from expectations of liquid-films flows falling overplanar substrates. Over the range Re = 0.6 − 3.5, a series of regime transitions take place,followed by the generation of regular, in both space and time, 3-D solitary pulses ‘riding’over rivulet flow structures. These waves grow with increasing flow Re, as more liquidis drawn away from the rivulet troughs due to gravity. Finally, the wave frequencies andrivulet wavelengths are investigated by employment of power spectral density (PSD) andwavelet analyses. The application of PSD analysis offers superior resolution in the frequencydomain when performed on temporally varying film-height data, whereas waveletanalysis is preferred when considering the spatially varying film-height data due to thelimited spatial extent and low number of captured rivulets in the imaged region.

Journal article

Acha Izquierdo S, lambert R, Shah N, Markides C, delangle Aet al., 2017, Modelling and optimising the marginal expansion of an existing district heating network, Energy, Vol: 140, Pages: 209-223, ISSN: 0360-5442

Although district heating networks have a key role to play in tackling greenhouse gas emissions associated with urban energy systems, little work has been carried out on district heating networks expansion in the literature. The present article develops a methodology to find the best district heating network expansion strategy under a set of given constraints. Using a mixed-integer linear programming approach, the model developed optimises the future energy centre operation by selecting the best mix of technologies to achieve a given purpose (e.g. cost savings maximisation or greenhouse gas emissions minimisation). Spatial expansion features are also considered in the methodology.Applied to a case study, the model demonstrates that depending on the optimisation performed, some building connection strategies have to be prioritised. Outputs also prove that district heating schemes' financial viability may be affected by the connection scenario chosen, highlighting the necessity of planning strategies for district heating networks. The proposed approach is highly flexible as it can be adapted to other district heating network schemes and modified to integrate more aspects and constraints.

Journal article

Wright SF, Zadrazil I, Markides CN, 2017, A review of solid–fluid selection options for optical-based measurements in single-phase liquid, two-phase liquid–liquid and multiphase solid–liquid flows, Experiments in Fluids, Vol: 58, ISSN: 1432-1114

Experimental techniques based on optical measurement principles have experienced significant growth in recent decades. They are able to provide detailed information with high-spatiotemporal resolution on important scalar (e.g., temperature, concentration, and phase) and vector (e.g., velocity) fields in single-phase or multiphase flows, as well as interfacial characteristics in the latter, which has been instrumental to step-changes in our fundamental understanding of these flows, and the development and validation of advanced models with ever-improving predictive accuracy and reliability. Relevant techniques rely upon well-established optical methods such as direct photography, laser-induced fluorescence, laser Doppler velocimetry/phase Doppler anemometry, particle image/tracking velocimetry, and variants thereof. The accuracy of the resulting data depends on numerous factors including, importantly, the refractive indices of the solids and liquids used. The best results are obtained when the observational materials have closely matched refractive indices, including test-section walls, liquid phases, and any suspended particles. This paper reviews solid–liquid and solid–liquid–liquid refractive-index-matched systems employed in different fields, e.g., multiphase flows, turbomachinery, bio-fluid flows, with an emphasis on liquid–liquid systems. The refractive indices of various aqueous and organic phases found in the literature span the range 1.330–1.620 and 1.251–1.637, respectively, allowing the identification of appropriate combinations to match selected transparent or translucent plastics/polymers, glasses, or custom materials in single-phase liquid or multiphase liquid–liquid flow systems. In addition, the refractive indices of fluids can be further tuned with the use of additives, which also allows for the matching of important flow similarity parameters such as density and viscosity.

Journal article

Pantaleo, Fordham J, Oyewunmi OA, Markideset al., Optimal sizing and operation of on-site combined heat and power systems for intermittent waste-heat recovery, 9th International Conference on Applied Energy (ICAE2017), Publisher: Elsevier, ISSN: 1876-6102

Coffee roasting is a highly energy intensive process with much of the energy being lost in intermittent cycles as discharged heatfrom the stack. In this work, combined heat and power (CHP) systems based on micro gas-turbines (MGT) are investigated forproviding heat to the roasting process. Much of the heat released in a coffee roaster is from the afterburner that heats up the fluegases to high temperatures in order to remove volatile organic compounds (VOCs) and other pollutants. An interesting solutionfor utilizing waste heat is assessed through energy and material balances of a rotating drum coffee roasting with partial hot gasrecycling. A cost assessment methodology is adopted to compare the profitability of three proposed system configurationsintegrated into the process. The case study of a major coffee torrefaction plant with 500 kg/h production capacity is assumed tocarry out the thermo-economic assessment, under the Italian energy framework. The CHP options under investigation are:(i) regenerative topping MGT coupled to the existing modulating gas burner to generate hot air for the roasting process;(ii) intermittent waste-heat recovery from the hot flue-gases through an organic Rankine cycle (ORC) engine coupled to athermal storage buffer; and (iii) non-regenerative topping MGT with direct recovery of turbine outlet air for the roasting processby means of an afterburner that modulates the heat demand of the roasting process. The results show that the profitability of theseinvestments is highly influenced by the natural gas/electricity cost ratio, by the coffee torrefaction production capacity and by theintermittency level of discharged heat. The MGT appears as a more profitable option than waste-heat recovery via the ORCengine due to the intermittency of the heat source and the relatively high electricity/heat cost ratio.

Conference paper

Freeman J, Guarracino I, Kalogirou SA, Markides CN, Freeman JP, Guarracino I, Kalogirou SA, Markides CN, Freeman J, Guarracino I, Kalogirou SA, Markides CNet al., 2017, A small-scale solar organic Rankine cycle combined heat and power system with integrated thermal-energy storage, Heat Powered Cycles Conference 2016, Publisher: Elsevier, Pages: 1543-1554, ISSN: 1873-5606

In this paper, we examine integrated thermal energy storage (TES) solutions for a domestic-scale solar combined heat and power (S-CHP) system based on an organic Rankine cycle (ORC) engine and low-cost non-concentrating solar-thermal collectors. TES is a critical element of solar-thermal systems. It can allow, depending on how it is implemented, improved matching to the end-user demands, improved load factors, higher average efficiencies and overall performance, as well as reduced component and system sizes and costs, especially in climates with high solar-irradiance variabilities. The operating temperature range of the TES solution must be compatible with the solar-collector array and with the ORC engine operation in order to maximise the overall performance of the system. Various combinations of phase change materials (PCMs) and solar collectors are compared and the S-CHP system's electrical performance is simulated for selected months in the contrasting climates of Cyprus and the UK. The key performance indicator of the ORC engine (net-work output) and the required TES volume are compared and discussed. The PCM-TES solutions that enable the best summer performance from an ORC engine sized for a nominal ~1-kWe output in combination with a 15-m2 solar collector array result in diurnal volume requirements as low as ~100 L in Cyprus and 400 - 500 L in the UK. However, the required TES volume is strongly in influenced by the choice of operational strategy for the system in order to match the domestic load profiles. In a full-storage strategy in which electrical energy generation from the ORC engine is offset to match the week-day evening peak in demand, it is found that a ~20% higher total daily electrical output per unit storage volume can be achieved with a PCM compared to liquid water as a sensible storage medium. The isothermal operation of the PCM during phase-change allows for a smaller diurnal storage temperature swing and a higher energy conversion efficiency

Conference paper

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