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
to

323 results found

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., 2017, 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., 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 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

Le Brun N, Charogiannis A, Hewitt GF, Markides CNet al., 2017, Tackling coolant freezing in generation-IV molten salt reactors, 25th International Conference on Nuclear Engineering, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 1-7

In this study we describe an experimental system designed to simulate the conditions of transient freezing which can occur in abnormal behaviour of molten salt reactors (MSRs). Freezing of coolant is indeed one of the main technical challenges preventing the deployment of MSR. First a novel experimental technique is presented by which it is possible to accurately track the growth of the solidified layer of fluid near a cold surface in an internal flow of liquid. This scenario simulates the possible solidification of a molten salt coolant over a cold wall inside the piping system of the MSR. Specifically, we conducted measurements using water as a simulant for the molten salt, and liquid nitrogen to achieve high heat removal rate at the wall. Particle image velocimetry and planar induced fluorescence were used as diagnostic techniques to track the growth of the solid layer. In addition this study describes a thermo-hydraulic model which has been used to characterise transient freezing in internal flow and compares the said model with the experiments. The numerical simulations were shown to be able to capture qualitatively and quantitatively all the essential processes involved in internal flow transient freezing. Accurate numerical predictive tools such the one presented in this work are essential in simulating the behaviour of MSR under accident conditions.

Conference paper

Freeman JP, Najjaran Kheirabadi A, Edwards R, Reid M, Hall R, Ramos A, Markides Cet al., 2017, 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, 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

Pantaleo AM, markides C, fordham J, Oyewunmiet 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

Dong F, Le Brun N, Markides C, 2017, Heat transfer correlations for the generation-IV molten salt reactor, 15th UK Heat Transfer Conference 2017

Molten salts are highly suitable heat transfer fluids for high temperature applications, such as nextgeneration nuclear reactors but also concentrated solar power systems, owing to their stability and capacity to transfer heat safely at low pressures compared to, e.g. pressurised water. The design of relevant components and equipment, and the operation of these resulting plants depend crucially on reliable heat transfer descriptions, which have proven a challenge given the harsh conditions in which relevant data needs to be generated. Some experiments have been performed in the literature, nevertheless, the results have been associated with significant deviations from expectations (predictions) based on standard heat transfercorrelations. In this paper we re-interpret such data from the literature based on revised knowledge of the thermal properties of these salts, and show that if this is done appropriately, standard correlations are indeed applicable to molten-salt flows with errors similar to those expected from such empirical approaches.

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

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

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., 2017, 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 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

Oyewunmi OA, Kirmse CJW, Pantaleo AM, Markides CNet al., 2017, Performance of working-fluid mixtures in ORC-CHP systems for different heat-demand segments and heat-recovery temperature levels, Energy Conversion and Management, Vol: 148, Pages: 1508-1524, ISSN: 0196-8904

In this paper, we investigate the adoption of working-fluid mixtures in ORC systems operating in combined heat and power (CHP) mode, with a power output provided by the expanding working fluid in the ORC turbine and a thermal energy output provided by the cooling water exiting (as a hot-water supply) the ORC condenser. We present a methodology for selecting optimal working-fluids in ORC systems with optimal CHP heat-to-electricity ratio and heat-supply temperature settings to match the seasonal variation in heat demand (temperature and intermittency of the load) of different end-users. A number of representative industrial waste-heat sources are considered by varying the ORC heat-source temperature over the range 150–330 °C. It is found that, a higher hot-water outlet temperature increases the exergy of the heat-sink stream but decreases the power output of the expander. Conversely, a low outlet temperature (~30 °C) allows for a high power-output, but a low cooling-stream exergy and hence a low potential to heat buildings or to cover other industrial thermal-energy demands. The results demonstrate that the optimal ORC shaft-power outputs vary considerably, from 9 MW up to 26 MW, while up to 10 MW of heating exergy is provided, with fuel savings in excess of 10%. It also emerges that single-component working fluids such as n-pentane appear to be optimal for fulfilling low-temperature heat demands, while working-fluid mixtures become optimal at higher heat-demand temperatures. In particular, the working-fluid mixture of 70% n-octane + 30% n-pentane results in an ORC-CHP system with the highest ORC exergy efficiency of 63% when utilizing 330 °C waste heat and delivering 90 °C hot water. The results of this research indicate that, when optimizing the global performance of ORC-CHP systems fed by industrial waste-heat sources, the temperature and load pattern of the cogenerated heat demand are crucial factors affecting the selection of the working fl

Journal article

Charogiannis A, An J, Markides C, 2017, A novel optical technique for accurate planar measurements of film-thickness and velocity in annular flows, 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics 2017, Publisher: EDAS

Gas-liquid annular flow is one of many possible two-phase flow regimes that are encountered in the (e.g., parabolic collector) solar fields of direct-evaporation concentrated solar-power (CSP) plants. Conventional planar laser-induced fluorescence (PLIF) has been used previously to investigate the liquid film topology (i.e. film thickness) in annular flows, however, limitations have been found regarding the accurate identification of the gas-liquid interface with this technique, especially when the interface is smooth. Therefore, a novel variation of PLIF, which we refer to as structured planar laser-induced fluorescence (S-PLIF), has been developed to overcome these limitations. In this study, S-PLIF is used to investigate the topology of falling films in a vertical pipe over the range ReL≈ 150 – 1500. Comparison of S-PLIF at two different angles (70° and 90°) shows that the technique performs better with an observation angle of 70° as this minimizes the distortions caused by the radial liquid film structure. In addition, S-PLIF70 shows good agreement with data from other techniques that have shown reliability when studying smooth films over the same range of conditions.

Conference paper

Simpson M, Rotolo G, Sapin P, De Palma P, White AJ, Markides CNet al., 2017, Thermodynamic performance maps of reciprocating-piston expanders for operation at off-design and part-load conditions, 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Publisher: ICHMT

Renewable energy sources, such as solar-thermal or geothermal heat, and low-/medium-grade industrial waste-heat can be converted into useful power and/or heating with a variety of technologies, including organic Rankine cycle (ORC) and vapour-compression heat-pump systems. The thermodynamic performance and cost of these technologies depends crucially on the efficiency of key components, including the compressor or expander used. Reciprocating-piston machines can be advantageous over turbomachines and other positive-displacement machines at intermediate scales (~10s-100s of kW) thanks to their ability to operate with relatively high isentropic efficiencies at large expansion ratios. However, modelling the thermodynamic losses in reciprocating-piston expanders, with a view towards designing high-performance machines, is a complex undertaking. The aim of this paper is to develop a spatially-lumped, yet dynamic model of a piston expander suitable for early-stage engineering design, that can provide simplification without sacrificing accuracy. The unsteady heat transfer between the gas and the cylinder walls, and the mass leakage are predicted independently with correlations available in the literature and simplified one-dimensional models, respectively. However, the turbulence induced by the mass intake through the piston rings can affect the gas-to-wall heat transfer. In order to address this dependency two complementary approaches are used. Compression and expansion processes are simulated in a gas spring configuration (i.e. without valve systems) using a computational fluid dynamics (CFD) model developed using the open-source code OpenFOAM, where the loss mechanisms are solved directly. The results are then compared with predictions from the heuristic lumped model based on heat tr

Conference paper

Najjaran Kheirabadi A, Freeman J, Ramos Cabal A, Markides Cet al., 2017, EXPERIMENTAL PERFORMANCE ANALYSIS OF AN AMMONIA-WATER DIFFUSION ABSORPTION REFRIGERATION CYCLE, ASME 2017 Summer Heat Transfer Conference

Conference paper

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

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

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

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

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

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

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