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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295 results found

Najjaran Kheirabadi A, Freeman J, Ramos Cabal A, Markides Cet al., Experimental Investigation of an Ammonia-Water-Hydrogen Diffusion Absorption Refrigerator, Applied Energy, ISSN: 0306-2619

Journal article

Najjaran Kheirabadi A, Freeman J, Ramos Cabal A, Markides Cet al., Experimental investigation of an ammonia-water-hydrogen diffusion absorption refrigerator, Applied Energy, ISSN: 0306-2619

Diffusion absorption refrigeration (DAR) is a small-scale cooling technology that can be driven purely by thermal energy without the need for electrical or mechanical inputs. In this work, a detailed experimental evaluation was undertaken of a newly-proposed DAR unit with a nominal cooling capacity of 100~W, aimed at solar-driven cooling applications in warm climates. Electrical cartridge heaters were used to provide the thermal input which was varied in the range 150-700 W, resulting in heat source temperatures of 175--215 C measured at the generator. The cooling output during steady-state operation was determined from the power consumed by an electric heater used to maintain constant air temperature in an insulated box constructed around the evaporator. Tests were performed with the DAR system configured with the default manufacturer's settings (22 bar charge pressure and 30 % ammonia concentration). The measured cooling output (to air) across the range of generator heat inputs was 24--108 W, while the coefficient of performance (COP) range was 0.11--0.26. The maximum COP was obtained at a generator heat input of 300 W. Results were compared to performance predictions from a steady-state thermodynamic model of the DAR cycle, showing a reasonable level of agreement at the nominal design point of system, but noteworthy deviations at part-load/off-design conditions. Temperature measurements from the experimental apparatus were used to evaluate assumptions used in the estimation of the model state point parameters and examine their influence on the predicted system performance.

Journal article

Voulgaropoulos V, Zadrazil I, Le Brun N, Bismarck A, Markides CNet al., 2019, On the link between experimentally‐measured turbulence quantities and polymer‐induced drag reduction in pipe flows, AIChE Journal, Vol: 65, Pages: 1-13, ISSN: 0001-1541

In this study, we investigate the hydrodynamics of polymer‐induced drag reduction in horizontal turbulent pipe flows. We provide spatiotemporally resolved information of velocity and its gradients obtained with particle image velocimetry (PIV) measurements in solutions of water with dissolved polyethylene oxide (PEO) of three different molecular weights, at various dilute concentrations and with flow Reynolds numbers from 35, 000 to 210, 000. We find that the local magnitudes of important turbulent flow variables correlate with the measured levels of drag reduction irrespective of the flow Reynolds number, polymer weight and concentration. Contour maps illustrate the spatial characteristics of this correlation. A relationship between the drag reduction and the turbulent flow variables is found. The effects of the polymer molecular weight, its concentration and the Reynolds number on the flow are further examined through joint probability distributions of the fluctuations of the streamwise and spanwise velocity components.

Journal article

Charogiannis A, Sik An J, Voulgaropoulos V, Markides CNet al., 2019, Structured planar laser-induced fluorescence (S-PLIF) for the accurate identification of interfaces in multiphase flows, International Journal of Multiphase Flow, Vol: 118, Pages: 193-204, ISSN: 0301-9322

Annular flows are employed in numerous engineering and industrial processes relating to the chemical, oil and gas, solar and nuclear energy industries. Yet, the reliable time- and space-resolved measurement of film thickness in these flows still eludes us, as the moving and wavy interface renders the application of optical diagnostics, such as planar laser-induced fluorescence (PLIF), particularly challenging. In this research article, we present a novel adaptation of PLIF, which we refer to as structured PLIF (S-PLIF), and with which we seek to suppress the errors in PLIF-derived film thickness measurements due to total internal reflection (TIR) of the emitted fluorescence at the phase boundary. The proposed measurement approach relies on a periodic modulation of the laser-light intensity along the examined region of the flow in order to generate fluorescence images with alternating bright and dark regions. An image-processing methodology capable of recovering the location of the true gas-liquid interface from such images is presented, and the application of S-PLIF is demonstrated in liquid films in a vertical pipe over the Reynolds number range . The results from this technique are compared to simultaneously recovered, “conventional” (uncorrected) PLIF data, as well as data from other techniques over the same range of conditions, demonstrating the efficacy of S-PLIF. A comparison amongst S-PLIF data obtained with the observation angle between the laser-sheet plane and the camera’s observation axis set to and 90 ∘ is also performed, showing that the employment of is highly advantageous in avoiding distortions caused by reflections of the emitted fluorescence at the film free-surface. The instantaneous and average film-thickness uncertainties of S-PLIF are estimated to be below 10% and 5%, respectively, when measuring smooth films; an improvement over the other optical measurement techniques considered in this work. Finally, the application of S-

Journal article

Al Kindi A, Markides C, Wang K, Pantaleo Aet al., Thermodynamic assessment of steam-accumulation thermal energy storage in concentrating solar Power plants, International Conference on Applied Energy 2019

Concentrated Solar Power (CSP) plants are usually coupled with Thermal Energy Storage (TES) in order to increase the generation capacity and reduce energy output fluctuations and the levelized cost of the energy. In Direct Steam Generation (DSG) CSP plants, a popular TES option relies on steam accumulation. This conventional option, however, is constrained by temperature and pressure limits, and delivers saturated or slightly superheated steam at low pressure during discharge, which is undesirable for part-load turbine operation. However, steam accumulation can be integrated with sensible-heat storage in concrete to provide high-temperature superheated steam at higher pressures. The conventional steam accumulation option and the integrated concrete-steam option are presented, analysed and compared in this paper. The comparison shows that the integrated option provides more storage capacity by utilizing most of the available thermal power in the solar receiver. Further, the integrated option delivers higher power output with enhanced thermal efficiency for longer periods when the power plant is solely operating using the stored thermal energy. An application to the 50 MW Khi Solar One CSP plant, based on solar tower and in operation in South Africa, is proposed.

Conference paper

Song J, Simpson M, Wang K, Markides Cet al., 2019, Thermodynamic assessment of combined supercritical CO2 (SCO2) and organic Rankine cycle (ORC) systems for concentrated solar power, International Conference on Applied Energy 2019

Concentrated solar power (CSP) systems are acknowledged as a promising technology for solar energy utilisation. Supercritical CO2 (SCO2) cycle systems have emerged as an attractive option for power generation in CSP applications due to the favourable properties of CO2 as a working fluid. In order to further improve the overall performance of such systems, organic Rankine cycle (ORC) systems can be used in bottoming-cycle configuration to recover the residual heat. This paper presents a thermodynamic performance assessment of a combined SCO2/ORC system in a CSP application using parabolic-trough collectors. The parametric analysis indicates that the heat transfer fluid (HTF) temperature at the inlet of the cold tank, and the corresponding HTF mass flow rate, have a significant influence on the overall system performance. The results suggest that the combined system can offer significant thermodynamic advantages at progressively lower temperatures. Annual simulations for a case study in Seville (Spain) show that, based on an installation area of 10,000 m2, the proposed combined cycle system could deliver an annual net electricity output of 2,680 MWh when the HTF temperature at the cold tank inlet is set to 250 °C, which is 3% higher than that of a stand-alone CO2 cycle system under the same conditions. Taking the size of the thermal storage tanks into consideration, a lower HTF temperature at the cold tank inlet and a lower mass flow rate would be desirable, and the combined system offers up to 66% more power than the stand-alone version when the HTF inlet temperature is 100 °C.

Conference paper

Unamba CK, Sapin P, Li X, Song J, Wang K, Shu G, Tian H, Markides CNet al., 2019, Operational optimisation of a non-recuperative 1-kWe organic Rankine cycle engine prototype, Applied Sciences, Vol: 9, Pages: 3024-3024, ISSN: 2076-3417

Several heat-to-power conversion technologies are being proposed as suitable for waste-heat recovery (WHR) applications, including thermoelectric generators, hot-air (e.g., Ericsson or Stirling) engines and vapour-cycle engines such as steam or organic Rankine cycle (ORC) power systems. The latter technology has demonstrated the highest efficiencies at small and intermediate scales and low to medium heat-source temperatures and is considered a suitable option for WHR in relevant applications. However, ORC systems experience variations in performance at part-load or off-design conditions, which need to be predicted accurately by empirical or physics-based models if one is to assess accurately the techno-economic potential of such ORC-WHR solutions. This paper presents results from an experimental investigation of the part-load performance of a 1-kWe ORC engine, operated with R245fa as a working fluid, with the aim of producing high-fidelity steady-state and transient data relating to the operational performance of this system. The experimental apparatus is composed of a rotary-vane pump, brazed-plate evaporator and condenser units and a scroll expander magnetically coupled to a generator with an adjustable resistive load. An electric heater is used to provide a hot oil-stream to the evaporator, supplied at three different temperatures in the current study: 100, 120 and 140 ∘ C. The optimal operating conditions, that is, pump speed and expander load, are determined at various heat-source conditions, thus resulting in a total of 124 steady-state data points used to analyse the part-load performance of the engine. A maximum thermal efficiency of 4.2 ± 0.1% is reported for a heat-source temperature of 120 ∘ C, while a maximum net power output of 508 ± 2 W is obtained for a heat-source temperature at 140 ∘ C. For a 100- ∘ C heat source, a maximum exergy efficiency of 18.7 ± 0.3% is achieved. A detailed exergy analysis all

Journal article

Li X, Song J, Simpson M, Wang K, Sapin P, Shu G, Tian H, Markides Cet al., THERMO-ECONOMIC COMPARISON OF ORGANIC RANKINE AND CO2 CYCLE SYSTEMS FOR LOW-TO-MEDIUM TEMPERATURE APPLICATIONS, 5th International Seminar on ORC Power Systems

Conference paper

Le Brun N, Markides CN, 2019, A Galinstan-Filled Capillary Probe for Thermal Conductivity Measurements and Its Application to Molten Eutectic KNO3-NaNO3-NaNO2 (HTS) up to 700K (vol 36, 3222, 2015), INTERNATIONAL JOURNAL OF THERMOPHYSICS, Vol: 40, ISSN: 0195-928X

The successful measurement of the thermal conductivity of molten salts is a challenging undertaking due to the electrically conducting and possibly also aggressive nature of the materials, as well as the elevated temperatures at which these data are required. For accurate and reproducible measurements, it is important to develop a suitable experimental apparatus and methodology. In this study, we explore a modified version of the transient hot-wire method, which employs a molten-metal-filled capillary in order to circumvent some of the issues encountered in previous studies. Specifically, by using a novel flexible U-shaped quartz-capillary, filled with a eutectic mixture of gallium, indium and tin, commercially known as Galinstan, we proceed to measure the thermal conductivity of molten eutectic KNO3–NaNO3–NaNO2–NaNO2. The new probe is demonstrated as being able to measure the thermal conductivity of this molten salt, which is found to range from 0.48 W⋅m−1⋅K−1 at 500 K to 0.47 W⋅m−1⋅K−1 at close to 700 K, with an overall expanded uncertainty (95 % confidence) of 3.1 %. The quartz is found to retain its electrically insulating properties and no current leakage is detected in the sample over the investigated temperature range. The thermal conductivity data reported in the present study are also used to elucidate a partial disagreement found in the literature for this material.

Journal article

Olympios A, Pantaleo AM, Sapin P, Van Dam K, Markides Cet al., Centralised vs distributed energy systems options: District heating for the Isle of Dogs in London, ICAE2019: The 11th International Conference on Applied Energy

This work focuses on a multi-scale framework for the design and comparison of low-carbon heat generation solutions to serve the residential and commercial thermal energy demand of high energy density urban areas. The adopted methodology assesses the cost and performance of four configurations integrated in a district heating network: (i) centralised cogeneration with gas turbine and bottoming steam turbine with flexible heat-to-electricity ratio; (ii) centralised cogeneration with gas-fired internal combustion engine; (iii) distributed building-integrated ground-source heat pumps for domestic hot water only; and (iv) distributed building-integrated ground-source heat pumps for both domestic hot water and space heating. Cost and performance data were obtained by conducting relevant market research and developing a simplified heat pump thermodynamic model. The different configurations are evaluated utilizing whole-year space heating and hot water demand profiles for the Isle of Dogs area in East London, UK. Scale effects are included by considering various technology size scenarios and the results indicate that a 50 MW centralised internal combustion cogeneration system appears to be the most profitable option, while the competitiveness of building-integrated heat pumps is dependent on their size.

Conference paper

Chakrabarti A, Proeglhoef R, Bustos-Turu G, Lambert R, Mariaud A, Acha S, Markides CN, Shah Net al., 2019, Optimisation and analysis of system integration between electric vehicles and UK decentralised energy schemes, Energy, Vol: 176, Pages: 805-815, ISSN: 0360-5442

Although district heat network schemes provide a pragmatic solution for reducing the environmental impact of urban energy systems, there are additional benefits that could arise from servicing electric vehicles. Using the electricity generated on-site to power electric vehicles can make district heating networks more economically feasible, while also increasing environmental benefits. This paper explores the potential integration of electric vehicle charging into large-scale district heating networks with the aim of increasing the value of the generated electricity and thereby improving the financial feasibility of such systems. A modelling approach is presented composed of a diverse range of distributed technologies that considers residential and commercial electric vehicle charging demands via agent-based modelling. An existing district heating network system in London was taken as a case study. The energy system was modelled as a mixed integer linear program and optimised for either profit maximisation or carbon dioxide emissions minimisation. Commercial electric vehicles provided the best alternative to increase revenue streams by about 11% against the current system configuration with emissions effectively unchanged. The research indicates that district heating network systems need to carefully analyse opportunities for transport electrification in order to improve the integration, and sustainability, of urban energy systems.

Journal article

Unamba C, Li X, Song J, Wang K, Shu G, Tian H, Sapin P, Markides CNet al., Off-design performance of a 1-kWe organic Rankine cycle (ORC) system, 32nd International Conference on Efficiency, Costs, Optimization, Simulation and Environmental Impact of Energy Systems (ECOS 2019), Publisher: ECOS

Several heat-to-power conversion technologies are being proposed as suitable for waste heat recovery (WHR) applications, including thermoelectric generators, hot-air (e.g., Ericsson or Stirling) engines, and vapour-cycle engines such as steam or organic Rankine cycle (ORC) power systems. The latter has demonstrated the highest efficiencies at low and intermediate scales and heat-source temperatures. However, ORC systems suffer a deterioration in performance at part-load or off-design conditions, and the high global warming potential (GWP) or flammability of common working fluids is an increasing concern. This paper presents the experimental investigation of a 1-kWe ORC test facility under time-varying heat-source conditions. It aims to compare the part-load performance of various architectures with different working fluids, namely: (i) R245fa, which is widely used in ORC systems, and (ii) low-GWP HFOs. The experimental apparatus is composed of a rotary-vane pump, brazed-plate evaporators and condensers, and a scroll expander with an adjustable load. An electric heater is used to provide a hot oil stream at three different temperatures: 80, 100 and 120 °C. The optimal operating conditions, i.e., pump speed and expander load, are determined for each architecture at various heat-source conditions. A maximum thermal efficiency of 2.8% is reported for a heat-source temperature of 100 °C, while a maximum net power output of 430 W is obtained for a heat source at 120 °C. An exergy analysis allows us to quantify the contribution of each component to the overall exergy destruction. The share of the evaporator, condenser and expander units remain major for all three heat-source conditions, while the exergy destroyed in the pump is negligible in comparison (below 4%).

Conference paper

Voulgaropoulos V, Aguiar GM, Matar OK, Bucci M, Markides CNet al., 2019, Temperature and velocity field measurements of pool boiling using two-colour laser-induced fluorescence, infrared thermometry and particle image velocimetry, 10th International Conference on Multiphase Flow

We study nucleate pool boiling in water at saturation temperature and ambient pressure under low heat fluxes. A combinationof high-speed and spatially-resolved diagnostic tools are developed and applied to provide detailed insight into the flow andheat transfer mechanisms during bubble life cycle. Two fluorescent dyes with non-overlapping spectra are seeded into thewater and are excited by a Nd:YLF laser sheet at 527 nm. A two-colour laser-induced fluorescence method is employedto individually track the fluorescence of each dye by connecting two cameras, equipped with separate optical filters, to abeamsplitter and a lens. Tracer particles are also introduced in the water to perform simultaneous particle image velocimetrymeasurements. Finally, synchronised high-speed infrared thermometry is conducted to acquire the surface temperature fieldover the heater. The links between the interfacial/bubble dynamics, flow and heat transfer are investigated. Superheated liquidfrom the thermal boundary layer adjacent to the heater is displaced upwards, due to the growth and departure of the bubbles.Two counteracting vortices form on each side of the bubbles during their departure and rise, which contribute to the scavengingand mixing of the bulk water, resulting in a trail of superheated liquid below them.

Conference paper

Moran H, Magnini M, Markides C, Matar Oet al., 2019, Inertial and buoyancy effects on horizontal flow of elongated bubbles in circular channels, ICMF 2019, Publisher: ICMF

The effects of gravity and inertia on the liquid film thickness surrounding elongated bubble flow in a horizontal tube of circularcross-section are studied through numerical simulations. At low Reynolds and Bond numbers, the inertial and buoyancy effectsare negligible and the liquid film thickness at the tube wall is a function of the Capillary number only; if tube diameter isincreased to the millimetre scale, however, buoyancy forces become significant. Simulations are performed with OpenFOAM(version 1606) and the built in Volume-of-Fluid method for a range of Reynolds, Bond and Capillary numbers, namelyRe= 1−1000,Bo= 0.05−0.42andCa= 0.02−0.09respectively. Two-dimensional simulations capture asymmetry ofthe liquid film thickness due to gravitational effects, but do not capture bubble inclination relative to the channel centreline,as has been demonstrated experimentally in the literature. Three-dimensional simulations capture the transverse flow of thefilm as it drains from the top to the bottom of the tube, and are thus able to demonstrate bubble inclination. Further simulationsthat introduce phase change to the elongated bubble model are underway, aiming to investigate boiling flows, with experimentsbeing performed for comparison and validation.

Conference paper

Simpson M, Schuster S, Ibrahim D, Oyewunmi O, Sapin P, White A, Markides Cet al., Small-scale, low-temperature ORC systems intime-varying operation: Turbines orreciprocating-piston expanders?, 32ND INTERNATIONAL CONFERENCE ON EFFICIENCY, COST, OPTIMIZATION, SIMULATION AND ENVIRONMENTAL IMPACT OF ENERGY SYSTEMS

Conference paper

Ibrahim D, Oyewunmi O, Haslam A, Pantaleo A, Markides Cet al., Computer-aided working fluid design and optimisation of organic Rankine cycle (ORC) systems under varying heat-source conditions, 32ND INTERNATIONAL CONFERENCE ON EFFICIENCY, COST, OPTIMIZATION, SIMULATION AND ENVIRONMENTAL IMPACT OF ENERGY SYSTEMS

Conference paper

Wang K, Markides C, 2019, Solar hybrid PV-thermal combined cooling, heating and power systems, The 5th International Conference on Polygeneration (ICP 2019), Publisher: ICP

We review hybrid photovoltaic-thermal (PV-T) technology for the combined provision of heating, coolingand power, present the state-of-the-art and outline recentprogress, including by researchers at the Clean Energy Processes (CEP) Laboratory,on aspects from component innovationto system integration,operational strategiesand assessmentsin key applications. Technologies appropriate for integration with PV-T collectors include thermal (hot and cold) and electrical storage, heat-driven heating/cooling (e.g., absorption,adsorption) and/orelectrically-driven heating/cooling (e.g., heat pump, air-conditioning)systems. Thermoeconomic assessments ofPV-Tcollectors integrated within wider solar-energy systems with such technologies inrepresentative applications have been conducted, including for energy provision to residential, commercial and public buildings, and industrial process heating applications. Studies have shown that PV-T technology has an excellent decarbonisation potential and can covera significant amount of the energy demandof end-users given reasonable areas. Further efforts relating to technology innovation and, primarily,cost reduction are required to improve its economiccompetitiveness over conventional fossil-fuel and other alternative solutions. Advanced heat-loss suppression techniques and spectral beam splitting concepts have emergedas promising directions for ground-breaking innovationin this area.

Conference paper

Aguiar GM, Voulgaropoulos V, Matar OK, Markides CN, Bucci Met al., Experimental investigation of bubble nucleation, growth and departure using synchornized IR thermometry, two-colour LIF and PIV, 18th International Topical Meeting on Nuclear Reactor Thermal Hydraulics - NURETH, Publisher: American Nuclear Society (ANS)

Boiling is a very effectiveheat removal process exploited in many applications, from electronic devicesto nuclear reactors. However, the physical mechanisms involved in this process are not fully understood yet, due toits complexity, whicharises from the many interacting sub-processes involved in the nucleation, growth, and detachment of isolated bubbles. Here, we present the methodology and initialresults from an experimental investigation aimed at elucidating and quantifying the mechanisms involved in a bubble life cycle (fromnucleation until departure). Towards this aim, we use synchronized high-speed infrared(IR)thermometry, ratiometric two-color laser-induced fluorescence (2cLIF) and particle image velocimetry (PIV). Infrared thermometry is used to measure the time-dependent temperature and heat flux distributions overthe boilingsurface, which are usefulto quantify the transfer of energy associated with the evaporation of the micro-layer. Two-color laser-induced fluorescence is used to measure the time-dependent temperature distribution in the liquid phase. Particle image velocimetry is employedto measure the velocity field around the bubble, necessary to elucidate the bubble growth and departure mechanisms. The investigation also revealsother fundamental heat transferaspects such as the dynamics of the near-wall superheated liquid layer, the mixing effect produced by bubble growth and departure, as well as convection effects around the bubble.

Conference paper

Guarracino I, Freeman J, Ramos A, Kalogirou SA, Ekins-Daukes NJ, Markides CNet al., 2019, Systematic testing of hybrid PV-thermal (PVT) solar collectors in steady-state and dynamic outdoor conditions, Applied Energy, Vol: 240, Pages: 1014-1030, ISSN: 0306-2619

Hybrid photovoltaic-thermal (PVT) collectors have been proposed for the combined generation of electricity and heat from the same area. In order to predict accurately the electrical and thermal energy generation from hybrid PVT systems, it is necessary that both the steady-state and dynamic performance of the collectors is considered. This work focuses on the performance characterisation of non-concentrating PVT collectors under outdoor conditions. A novel aspect concerns the application of existing methods, adapted from relevant international standards for flat plate and evacuated tube solar-thermal collectors, to PVT collectors for which there is no formally established testing methodology at present. Three different types of PVT collector are tested, with a focus on the design parameters that affect their electrical and thermal performance during operation. Among other results, we show that a PVT collector suffers a 10% decrease in thermal efficiency when the electricity conversion is close to the maximum power point compared to open-circuit mode, and that a poor thermal contact between the PV laminate and the copper absorber can lead to a significant deterioration in thermal performance. The addition of a glass cover improves the thermal efficiency, but causes electrical performance losses that vary with the glass transmittance and the solar incidence angle. The reduction in electrical efficiency at large incidence angles is more significant than that due to elevated temperatures representative of water-heating applications. Dynamic performance is characterised by imposing a step change in irradiance in order to quantify the collector time constant and effective heat capacity. This paper demonstrates that PVT collectors are characterised by a slow thermal response in comparison to ordinary flat plate solar-thermal collectors, due to the additional thermal mass of the PV layer. A time constant of ∼8 min is measured for a commercial PVT module, compared to <

Journal article

Olympios A, Le Brun N, Acha Izquierdo S, Lambert R, Shah N, Markides Cet al., Installation of a dynamic controller for the optimal operation of a CHP engine in a supermarket under uncertainty, ECOS2019: 32nd International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems

This work is concerned with the integration and coordination of decentralized combined heat and power (CHP) systems in commercial buildings. Although extensive research has been performed on theoretically optimizing the design, sizing and operation of CHP systems, less effort has been devoted to an understanding of the practical challenges and the effects of uncertainty in implementing advanced algorithms to real-world applications. This paper provides details of an undergoing field trial involving the installation of a dynamic controller for the optimal operation of an existing CHP engine, which provides electricity and heat to a supermarket. The challenges in developing and applying an optimization framework and the software architecture required to implement it are discussed. Deterministic approaches that involve no measure of uncertainty provide limited useful insight to decision makers. For this reason, the methodology here develops a stochastic programming technique, which performs Monte Carlo simulations that can consider the uncertainty related to the exporting electricity price. The method involves the formation of a bi-objective function that represents a compromise between maximizing the expected savings and minimizing the associated risk. The results reveal a risk-return trade-off, demonstrating that conservative operation choices emerging from the stochastic approach can reduce risk by about 15% at the expense of a noticeably smaller reduction of about 10% in expected savings.

Conference paper

Wang K, Pantaleo AM, Herrando M, Pesmazoglou I, Franchetti B, Markides Cet al., Thermoeconomic assessment of a spectral-splitting hybrid PVT system in dairy farms for combined heat and power, The 32nd International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems (ECOS 2019)

Conference paper

Chatzopoulou MA, Simpson M, Sapin P, Markides CNet al., 2019, Off-design optimisation of organic Rankine cycle (ORC) engines with pistonexpanders for medium-scale combined heat and power applications, Applied Energy, Vol: 238, Pages: 1211-1236, ISSN: 0306-2619

Organic Rankine cycle (ORC) engines often operate under variable heat-source conditions, so maximising performance at both nominal and off-design operation is crucial for the wider adoption of this technology. In this work, an off-design optimisation tool is developed and used to predict the impact of varying heat-source conditions on ORC operation. Unlike previous efforts where the performance of ORC engine components is assumed fixed, here we consider explicitly the time-varying operational characteristics of these components. A bottoming ORC system is first optimised for maximum power output when recovering heat from the exhaust gases of an internal-combustion engine (ICE) running at full load. A double-pipe heat exchanger (HEX) model is used for sizing the ORC evaporator and condenser, and a piston-expander model for sizing the expander. The ICE is then run at part-load, thus varying the temperature and mass flow rate of the exhaust gases. The tool predicts the new off-design heat transfer coefficients in the heat exchangers, and the new optimum expander operating points. Results reveal that the ORC engine power output is underestimated by up to 17% when the off-design operational characteristics of these components are not considered. In particular, the piston-expander isentropic efficiency increases at off-design operation by 10–16%, due to the reduced pressure ratio and flow rate in the system, while the evaporator effectiveness improves by up to 15%, due to the higher temperature difference across the HEX and a higher proportion of heat transfer taking place in the two-phase evaporating zone. As the ICE operates further away from its nominal point, the off-design ORC engine power output reduces by a lesser extent than that of the ICE. At an ICE part-load operation of 60% (by electrical power), the optimised ORC engine with fluids such as R1233zd operates at 77% of its nominal capacity. ORC off-design performance maps are generated, for characterising a

Journal article

Driker J, Juggurnath D, Kaya A, Osowade EA, Simpson M, Lecompte S, Rahim Abadi SMAN, Voulgaropoulos V, Adelaja AO, Dauhoo MZ, Khoodaruth A, Obayopo SO, Olakoyejo OT, Khalil MK, De Paepe M, Meyer JP, Markides CNet al., 2019, Thermal energy processes in direct steam generation solar systems: Boiling, condensation and energy storage, Frontiers in Energy Research, Vol: 6, ISSN: 2296-598X

Direct steam generation coupled with solar energy is a promising technology which can reduce the dependency on fossil fuels. It has the potential to impact the power-generation sector as well as industrial sectors where significant quantities of process steam are required. Compared to conventional concentrated solar power systems, which use synthetic oils or molten salts as the heat transfer fluid, direct steam generation offers an opportunity to achieve higher steam temperatures in the Rankine power cycle and to reduce parasitic losses, thereby enabling improved thermal efficiencies. However, this is associated with non-trivial challenges, which need to be addressed before such systems can become more economically competitive. Specifically, important thermal-energy processes take place during flow boiling, flow condensation and thermal-energy storage, which are highly complex, multi-scale and are multi-physics in nature that involve phase-change, unsteady and turbulent multiphase flows in the presence of conjugate heat transfer. This paper reviews our current understanding and ability to predict these processes, and knowledge that has been gained from experimental and computational efforts in the literature. In addition to Rankine cycles, organic Rankine cycle applications, which are relevant to lower operating temperature conditions, are also considered. This expands the focus to beyond water as the working fluid and includes refrigerants also. In general, significant progress has been achieved, yet there remain challenges in our capability to design and to operate effectively high-performance and low-cost systems with confidence. Of interest are the flow regimes, heat transfer coefficients and pressure drops during the thermal processes present in direct steam generation systems including those occurring in the solar collectors, condensers and relevant energy storage schemes during thermal charging and thermal discharging. A brief overview of some energy storage

Journal article

Efstratiadi M, Acha Izquierdo S, Shah N, Markides Cet al., 2019, Analysis of a closed-loop water-cooled refrigeration system in the food retail industry: A UK case study, Energy, ISSN: 0360-5442

Refrigeration in supermarkets accounts between 30% and 60% of total electricity demand in UK stores. The aim of this study is to conduct a pre-feasibility analysis of whether the use of a water-cooled configuration rejecting heat to the soil can improve the overall cooling performance of commercial refrigeration systems against air-cooled designs. In this work, a model simulating the operation of an existing refrigeration system is presented and validated against field data measurements taken from a supermarket. The examined system is used as a baseline and then modified to evaluate the impact of installing a water-cooled gas cooler. Results indicate that the use of water-cooled gas coolers has the potential to reduce electrical consumption of refrigeration systems by up to a factor of 5 when external temperatures are high. Overall, annual operation indicates the water-cooled alternative uses 3% less electricity than the air-cooled approach. A hybrid system is also considered consisting of coupled air-cooled and water-cooled units operating in parallel, for which an energy reduction of 6% is obtained compared against the baseline system. An economic evaluation of these systems shows promising results with a payback period of about 5 years for systems installed in new stores, although retrofits are costlier.

Journal article

Franchini S, Charogiannis A, Markides CN, Blunt MJ, Krevor Set al., 2019, Calibration of astigmatic particle tracking velocimetry based on generalized Gaussian feature extraction, Advances in Water Resources, Vol: 124, Pages: 1-8, ISSN: 0309-1708

Flow and transport in porous media are driven by pore scale processes. Particle tracking in transparent porous media allows for the observation of these processes at the time scale of ms. We demonstrate an application of defocusing particle tracking using brightfield illumination and a CMOS camera sensor. The resulting images have relatively high noise levels. To address this challenge, we propose a new calibration for locating particles in the out-of-plane direction. The methodology relies on extracting features of particle images by fitting generalized Gaussian distributions to particle images. The resulting fitting parameters are then linked to the out-of-plane coordinates of particles using flexible machine learning tools. A workflow is presented which shows how to generate a training dataset of fitting parameters paired to known out-of-plane locations. Several regression models are tested on the resulting training dataset, of which a boosted regression tree ensemble produced the lowest cross-validation error. The efficiacy of the proposed methodology is then examined in a laminar channel flow in a large measurement volume of 2048, 1152 and 3000 μm in length, width and depth respectively. The size of the test domain reflects the representative elementary volume of many fluid flow phenomena in porous media. Such large measurement depths require the collection of images at different focal levels. We acquired images at 21 focal levels 150 μm apart from each other. The error in predicting the out-of-plane location in a single slice of 240 μm thickness was found to be 7 μm, while in-plane locations were determined with sub-pixel resolution (below 0.8 μm). The mean relative error in the velocity measurement was obtained by comparing the experimental results to an analytic model of the flow. The estimated displacement errors in the axial direction of the flow were 0.21 pixel and 0.22 pixel at flows rates of 1.0 mL/h and 2.5 mL/h, respectively. These resu

Journal article

Herrando Zapater M, Ramos Cabal A, Zabalza I, Markides Cet al., 2019, A comprehensive assessment of alternative absorber-exchanger designs for hybrid PVT-water collectors, Applied Energy, Vol: 235, Pages: 1583-1602, ISSN: 0306-2619

In this paper, 26 alternative absorber-exchanger designs for hybrid PV-Thermal (PVT) solar collectors are proposed and compared against a reference-case, commercial sheet-and-tube PVT collector. The collectors involve different geometric design features based on the conventional sheet-and-tube configuration, and also on a newer flat-box structure constructed from alternative polymeric materials with the aim of maintaining or even improving heat transfer and overall (thermal and electrical) performance while achieving reductions in the overall weight and cost of the collectors. The main contributions of this research include: (i) the development and validation of a detailed 3-D computational finite-element model of the proposed PVT collector designs involving multi-physics processes (heat transfer, fluid dynamics and solid mechanics); (ii) results from comparative techno-economic analyses of the proposed PVT designs; and, (iii) further insights from thermal stress and structural deformation analyses of the proposed collectors, which are crucial for ensuring long lifetimes and especially important in the case of polymeric collectors. The results show that, in general, the flat-box designs (characterised by a thin absorber plate) are not sensitive to the flow-channel size or construction material, at least within the range of investigation. A PVT collector featuring a polycarbonate (PC) flat-box design with 3 × 2 mm rectangular channels appears to be a particularly promising alternative to commercial PVT collectors, achieving a slightly improved thermal performance compared to the reference case (with a 4% higher optical efficiency and 15% lower linear heat-loss coefficient), while also lowering the weight (by around 9%) and investment cost (by about 21%) of the collector. The structural analysis shows that the maximum von Mises stress experienced in the absorber-exchanger of the PC flat-box collector is considerably lower than that in the copper sheet-and-tube c

Journal article

Mazur C, Hoegerle Y, Brucoli M, Van Dam K, Guo M, Markides C, Shah Net al., 2019, A holistic resilience framework development for rural power systems in emerging economies, Applied Energy, Vol: 235, Pages: 219-232, ISSN: 0306-2619

Infrastructure and services within urban areas of developed countries have established reliable definitions of resilience and its dependence on various factors as an important pathway for achieving sustainability in these energy systems. However, the assessment, design, building and maintenance of power systems situated in rural areas in emerging economies present further difficulties because there is no a clear framework for such circumstances. Aiming to address this issue, this paper combines different visions of energy-related resilience both in general and under rural conditions in order to provide a robust practical framework for local and international stakeholders to derive the right actions in the rural context of emerging economies. An in-depth review is implemented to recompile information of resilience in general, in energy systems and in rural areas in particular, and a number of existing frameworks is also consulted. In order to acknowledge the particular circumstances and identify the important factors influencing the resilience of rural electrification in emerging economies, a holistic rural power system resilience framework is developed and presented. This consists of twenty-one indicators for technical resilience, eight indicators for social resilience, and thirteen indicators for economic resilience. This framework can be used by system owners and operators, policy makers, NGOs and communities to ensure the longevity of power systems. This work also paves the way for the creation of appropriate and effective resilience standards specifically targeted for application in these regions - aiming to achieve the delivery of global and local sustainability goals.

Journal article

Simpson M, Chatzopoulou MA, Oyewunmi O, Markides Cet al., 2019, Technoeconomic analysis of internal combustion engine – organic Rankine cycle cogeneration systems in energy-intensive buildings, 10th International Conference on Applied Energy, Publisher: Elsevier, Pages: 2354-2359, ISSN: 1876-6102

Organic Rankine cycle (ORC) systems are a promising technology for converting heat to useful power, especially in combined heat and power (CHP) applications with significant quantities of surplus heat that would otherwise be wasted. Beyond the technical performance of these systems, their economic feasibility is crucially important for their wider deployment. In this study, a technoeconomic optimisation of CHP systems is performed in which ORC engines convert heat recovered from internal combustion engines (ICEs), and specifically from both the ICE hot-water output and exhaust-gas stream. The overall aim is to evaluate the impact of the ORC power output and of the components’ design and capital cost on the financial viability of a relevant project, while evaluating a range of candidate working fluids. Results indicate that ORC designs optimised for maximum power output correspond to higher specific investment cost (SIC), with the best performing fluids achieving a SIC of £2100 per kW. In contrast, optimisation for minimum SIC returns values as low as £1700 per kW, or 20% lower. For systems designed and optimised for maximum power, a large fraction of jacket water heat is recovered, while for minimum SIC the utilisation drops to minimise the size and cost of the heat exchangers. The best-performing ORC designs for minimum SIC have discounted payback periods (DPPs) of 4 – 5 years, while those optimised for power output have DPPs of 6 – 7 years, however, the net present values (NPVs) of the latter designs are up to 27% higher than the former. Therefore, there is a trade-off to consider over the project life between high-capacity ORC engines with a high SIC and longer DPP, and designs with minimal SIC but lower power output, shorter DPP and lower NPV. The effect of increasing the amount of hot water required by the building is also analysed, and the ORC engine is shown to be sensitive to this factor for some work

Conference paper

Wang K, Herrando M, Pantaleo AM, Markides CNet al., 2019, Thermoeconomic assessment of a PV/T combined heating and power system for University Sport Centre of Bari, 10th International Conference on Applied Energy (ICAE2018), Publisher: Elsevier, Pages: 1229-1234, ISSN: 1876-6102

This paper presents a thermoeconomic analysis of a solar combined heating and power (S-CHP) system based on hybridphotovoltaic-thermal (PV/T) collectors for the University Sport Centre (USC) of Bari, Italy. Hourly demand data for space heating,swimming pool heating, hot water and electricity provision as well as the local weather data are used as inputs to a transient modeldeveloped in TRNSYS. Economic performance is evaluated by considering the investment costs and the cost savings due to thereduced electricity and natural gas consumptions. The results show that 38.2% of the electricity demand can be satisfied by thePV/T S-CHP system with an installation area of 4,000 m2. The coverage increases to 81.3% if the excess electricity is fed to thegrid. In addition, the system can cover 23.7% of the space heating demand and 53.8% of the demands for the swimming pool andhot water heating. A comparison with an equivalent gas-fired internal combustion engine (ICE) CHP system shows that the PV/Tsystem has a higher payback time, i.e., 11.6 years vs. 3 years, but outperforms the ICE solution in terms of CO2 emission reduction,i.e., 435 tons CO2/year vs. 164 tons CO2/year. This work suggests that the proposed PV/T S-CHP system has a good potential ofdecarbonisation, while the economic competitiveness should be further enhanced to boost its deployment.

Conference paper

Harraz AA, Freeman J, Wang K, Mac Dowell N, Markides CNet al., 2019, Diffusion-absorption refrigeration cycle simulations in gPROMS using SAFT-γ Mie, Energy Procedia, Vol: 158, Pages: 2360-2365, ISSN: 1876-6102

Diffusion-absorption refrigeration (DAR) is a clean thermally-powered refrigeration technology that can readily be activated by low- to medium-grade renewable heat. There is an ongoing interest in identifying or designing new working fluids for performance improvement, particularly in solar applications with non-concentrating solar collectors providing heat at temperatures < 150 °C. In this work, the state-of-the-art statistical associating fluid theory (SAFT) is adopted for predicting the thermodynamic properties of suitable DAR working fluids. A first-law thermodynamic analysis is performed in the software environment gPROMS for a DAR cycle using ammonia as the refrigerant, water as the absorbent and hydrogen as the auxiliary gas. The simulation results show good agreement with experimental data generated in a prototype DAR system with a nominal cooling capacity of 100 W. In particular, at a charge pressure of 17 bar and when delivering cooling at 5 °C, the model results agree with experimental COP data to within ± 7 % over a range of heat inputs from 150 to 500 W. The maximum coefficient of performance (COP) is estimated to be 0.24 at a heat input of 250 W. The group-contribution SAFT-γ Mie equation of state is of particular interest as it offers good agreement with experimental data and provides flexibility in extending the model to test different working fluids with a high degree of fidelity. A methodology is also presented that allows the DAR thermodynamic analysis and working-fluid modelling to be integrated into a more general technology optimisation framework.

Journal article

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