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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324 results found

Wang Y, Markides CN, Chachuat B, 2018, Optimization-based investigations of a thermofluidic engine for low-grade heat recovery, IFAC-PapersOnLine, Vol: 51, Pages: 690-695, ISSN: 2405-8963

This paper presents an analysis of the non-inertive-feedback thermofluidic engine (NIFTE) under cyclic steady-state conditions. The analysis is based on a nonlinear model of NIFTE that had previously been validated experimentally, and applies an optimization-based approach to detect the cyclic steady states (CSS). The stability of the CSS is furthermore determined by analyzing their monodromy matrix. It is found that NIFTE can exhibit multiple CSS for certain values of the design parameters, which may be either stable or unstable, a result that had not been reported before. Subsequently, a parametric study is conducted by varying key design parameters, revealing that higher efficiencies could be achieved by controlling the engine at different CSS, including unstable ones. Lastly, the paper investigates the trade-offs between efficiency and work output in NIFTE.

Journal article

McTigue JD, Markides C, White AJ, 2018, Performance response of packed-bed thermal storage to cycle duration perturbations, Journal of Energy Storage, Vol: 19, Pages: 379-392, ISSN: 2352-152X

Packed-bed thermal stores are integral components in numerous bulk electricity storage systems and may also be integrated into renewable generation and process heat systems. In such applications, the store may undergo charging and discharging periods of irregular durations. Previous work has typically concentrated on the initial charging cycles, or on steady-state cyclic operation. Understanding the impact of unpredictable charging periods on the storage behavior is necessary to improve design and operation. In this article, the influence of the cycle duration (or ‘partial-charge’ cycles) on the performance of such thermal stores is investigated. The response to perturbations is explained and provides a framework for understanding the response to realistic load cycles.The packed beds considered here have a rock filler material and air as the heat transfer fluid. The thermodynamic model is based on a modified form of the Schumann equations. Major sources of exergy loss are described, and the various irreversibility generating mechanisms are quantified.It is known that repeated charge-discharge cycles lead to steady-state behavior, which exhibits a trade-off between round-trip efficiency and stored exergy, and the underlying reasons for this are described. The steady state is then perturbed by cycles with a different duration. Short duration perturbations lead to a transient decrease in exergy losses, while longer perturbations increase it. The magnitude of the change in losses is related to the perturbation size and initial cycle period, but changes of 1–10 % are typical. The perturbations also affect the time to return to a steady-state, which may take up to 50 cycles. Segmenting the packed bed into layers reduces the effect of the perturbations, particularly short durations.Operational guidelines are developed, and it is found that packed beds are more resilient to changes in available energy if the store is not suddenly over-charged (i.e. longer

Journal article

Wang K, Herrando M, Pantaleo AM, Markides CNet al., 2018, Thermodynamic and economic assessments of a hybrid PVT-ORC combined heating and power system for swimming pools, Heat Powered Cycles Conference 2018

The thermodynamic and economicperformance of a solar combined heatand power(S-CHP) system based on an array of hybrid photovoltaic-thermal (PVT) collectorsandan organic Rankine cycle (ORC)engineis considered for the provision of heating and power to swimming poolfacilities. Priority is given to meeting the thermaldemand of the swimming pool,in order to ensure a comfortable condition for swimmers in colderweather conditions, while excessthermal output from the collectorsat highertemperatures is converted toelectricityby the ORC engine inwarmerweather conditions. The thermodynamic performance of this system and its dynamic characteristicsare analysed on the basis of a transient thermodynamic model. Various heat losses and gains are considered in accordance toenvironmental and user-relatedfactorsfor both indoor and outdoor swimming pools. A case study is then performed for the swimming pool atthe UniversitySportCentre (USC)of Bari, Italy. The results show thatemployinga zeotropic mixture of R245fa/R227ea (30/70%) as the ORC working fluidallows such an ORC systemto generate~50% more power than when usingpure R236eadue to the better temperature matchof the cycle tothe low-temperature hot-water heat sourcefrom the output of the PVT collectors.Apart from generatingelectricity, the ORC enginealso alleviatesPVT collectoroverheating,and reducesthe required size of the hot-water storage tank. With an installation of 2000 m2of PVT collectors, energetic analysesindicate that the proposedS-CHP systemcan cover 84-96% of the thermal demand of the swimming pool during the warm summer months and 61% of itsannually integratedtotal thermal demand. In addition, the system produces a combined (from thecollectors andORC engine) of 328 MWhofelectricityper year, corresponding to 36% of the total electricity demand of the USC, with ~4% coming from the ORC engine.Theanalysis suggestsa minimum payback time of 12.7yearswith anopt

Conference paper

Georgiou S, Aunedi M, Strbac G, Markides Cet al., 2018, Application of liquid-air and pumped-thermal electricity storage systems in low-carbon electricity systems, Heat Powered Cycles - HPC-2018

In this study, we considertwo medium-to large-scale electricity storage systems currently under development, namely ‘Liquid-Air Energy Storage’ (LAES) and ‘Pumped-Thermal Electricity Storage’ (PTES). Consistent thermodynamic models and costing methodologies for the twosystems are presented,with the objective of integrating the characteristics of these technologies intoa whole-electricity system assessment model,andassessingtheirsystem-levelvalue in different scenarios for power system decarbonisation.It is found that the value of storage variesgreatlydepending on the cumulative installed capacity of storage in the electrical system, withthe storage technologies providinggreater marginal benefits at low penetrations. Two carbon target scenarios showed similar results, with a limited effect of the carbon target on the system value of storage (althoughit is noted thatthis may change for even more ambitious carbon targets). On the other hand, the location and installed capacity of storage plants isfound to have a significantimpact on the system value and acceptable cost of thesetechnologies. The whole-system value of PTES was foundto be slightly higher than that of LAES, driven by a higher storage duration and efficiency,however, due to the higher power capital cost of PTES, this becomes less attractive for implementation at lower volumes than LAES.

Conference paper

Georgiou S, Shah N, Markides C, 2018, A thermo-economic analysis and comparison of pumped-thermal and liquid-air electricity storage systems, Applied Energy, Vol: 226, Pages: 1119-1133, ISSN: 0306-2619

Efficient and affordable electricitystorage systemshave a significant potential tosupport thegrowth and increasingpenetration of intermittent renewable-energy generationinto the gridfrom an energy system planning and management perspective,whiledifferencesin the demand and price ofpeak and off-peak electricity can make its storage of economicinterest. Technical (e.g.,roundtrip efficiency,energy andpower capacity)as well aseconomic (e.g.,capital, operating and maintenance costs)indicators are anticipatedto have a significantcombined impact on the competitiveness of anyelectricity storage technology or systemunder considerationand, ultimately, will cruciallydetermine their uptake and implementation.In this paper,we present thermo-economicmodels of two recentlyproposedmedium-to large-scale electricity storage systems, namely ‘Pumped-Thermal Electricity Storage’ (PTES) and ‘Liquid-Air Energy Storage’ (LAES), focusing on system efficiency and costs. The LAESthermodynamic model isvalidated against datafrom anoperationalpilot plant in the UK; no such equivalent PTES plant exists, although one is currently underconstruction. Ascommonwith most newly proposedtechnologies, the absenceof cost dataresults tothe economic analysis and comparisonbeinga significant challenge.Therefore, acosting effort for the two electricity storage systems that includes multiple costing approaches based on the module costing technique is presented,with the overriding aim of conducting a preliminary economic feasibility assessment and comparison of the two systems. Based on the results, it appears that PTES has the potential to achievehigher roundtrip efficiencies,althoughthis remains to be demonstrated. LAESperformance isfound to be significantly enhanced through the integration and utilisation of waste heat (and cold)streams.In terms of economicson the other hand,and at the

Journal article

Chatzopoulou MA, Markides C, 2018, Thermodynamic optimisation of a high-electrical efficiency integrated internal combustion engine – organic Rankine cycle combined heat and power system, Applied Energy, Vol: 226, Pages: 1229-1251, ISSN: 0306-2619

Organic Rankine cycle (ORC) engines are suitable for heat recovery from internal combustion engines (ICE) in combined heat and power (CHP) systems. However, trade-offs must be considered between ICE andORC engine performance in such integrated solutions. The ICE design and operational characteristics influence its own performance along withthe exhaust-gas conditions available as heat source to the ORC engine, impacting ORC design and performance, while the heat-recovery heat exchanger (ORC evaporator) will affect the ICE operation. In this paper, an integrated ICE-ORC CHP whole-system optimisation framework is presented. This differs from other efforts in that we develop and apply a fully-integrated ICE-ORC CHP optimisation framework, considering the design and operation of both the ICE and ORC enginessimultaneously within the combined system, to optimise the overall system performance. A dynamic ICE model is developed and validated, along with a steady-state model of subcritical recuperative ORC engines. Both naturally aspirated and turbocharged ICEs are considered, of two different sizes/capacities. Nine substances (covering low-GWP refrigerants and hydrocarbons) are investigated as potential ORC working fluids. The integrated ICE-ORC CHP system isoptimised for eithermaximum total power output, or minimum fuel consumption. Resultshighlight that by optimising the complete integrated ICE-ORC CHP system simultaneously, the total power output increases by up to 30% in comparison to a nominal system design. In the integrated CHP system,the ICE power output is slightly lower than that obtained for optimal standalone ICE application, as the exhaust-gas temperature increases to promote the bottoming ORC engine performance, whose power increasesby 7%. The ORC power output achieved accounts for up to 15% of the total power generated by the integrated system, increasing the system efficiency by up to 11%. When only power optimisation is performed, the

Journal article

Pantaleo AM, de palma P, Fordham J, Oyewumni O, Markides CNet al., 2018, Integrating cogeneration and intermittent waste-heat recovery in food processing: Microturbines vs. ORC systems in the coffee roasting industry, Applied Energy, Vol: 225, Pages: 782-796, ISSN: 0306-2619

Coffee roasting is a highly energy intensive process wherein a large quantity of heat is discharged from the stack at medium-to-high temperatures. Much of the heat is released from the afterburner, which is required to remove volatile organic compounds and other pollutants from the flue gases. In this work, intermittent waste-heat recovery via thermal energy storage (TES) and organic Rankine cycles (ORCs) is compared to combined heat and power (CHP) based on micro gas-turbines (MGTs) for a coffee roasting plant. With regard to the former, a promising solution is proposed that involves recovering waste heat from the flue gas stream by partial hot-gas recycling at the rotating drum coffee roaster, and coupling this to a thermal store and an ORC engine for power generation. The two solutions (CHP + MGT prime mover vs. waste-heat recovery + ORC engine) are investigated based on mass and energy balances, and a cost assessment methodology is adopted to compare the profitability of three system configurations integrated into the selected roasting process. The case study involves a major Italian roasting plant with a 500 kg per hour coffee production capacity. Three options are investigated: (i) intermittent waste-heat recovery from the hot flue-gases with an ORC engine coupled to a TES system; (ii) regenerative topping MGT coupled to the existing modulating gas burner to generate hot air for the roasting process; and (iii) non-regenerative topping MGT with direct recovery of the turbine outlet air for the roasting process. The results show that the profitability of these investments is highly influenced by the natural gas and electricity prices and by the coffee roasting production capacity. The CHP solution via an MGT appears as a more profitable option than waste-heat recovery via an ORC engine primarily due to the intermittency of the heat-source availability and the high electricity cost relative to the cost of natural gas.

Journal article

White MT, Markides CN, Sayma AI, 2018, Working-Fluid Replacement in Supersonic Organic Rankine Cycle Turbines, JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER-TRANSACTIONS OF THE ASME, Vol: 140, ISSN: 0742-4795

Journal article

Georgiou S, Acha S, Shah N, Markides Cet al., 2018, A generic tool for quantifying the energy requirements of glasshouse food production, Journal of Cleaner Production, Vol: 191, Pages: 384-399, ISSN: 0959-6526

Quantifying the use of resources in food production and its environmental impact is key to identifying distinctive measures which can be used to develop pathways towards low-carbon food systems. In this paper, a first-principle modelling approach is developed, referred to as gThermaR (Glasshouse-Thermal Requirements). gThermaR is a generic tool that focuses on the energy requirements of protected heated production, by integrating holistic energy, carbon, and cost modelling, food production, data analytics and visualization. The gThermaR tool employs historic data from weather stations, growing schedules and requirements specific to grower and product needs (e.g. set-point temperatures, heating periods, etc.) in order to quantify the heating and cooling requirements of glasshouse food production. In the present paper, a case study is reported that employs a database compiled for the UK. Another relevant feature of the tool is that it can quantify the effects that spatial and annual weather trends can have on these heating and cooling requirements. The main contribution of this work, therefore, concerns the development a tool that can provide a simple integrated approach for performing a wide range of analyses relevant to the thermal requirements of heated glasshouses. The tool is validated through collaborations with industrial partners and showcased in a case study of a heated glasshouse in the UK, offering the capacity to benchmark and compare different glasshouse types and food growth processes. Results from the case study indicate that a significant reduction in the heating requirement and, therefore, carbon footprint, of the facility can be achieved by improving key design and operational parameters. Results indicate savings in the peak daily and annual heating requirements of 44-50% and 51-57% respectively, depending on the region where the glasshouse is located. This improvement is also reflected in the carbon emissions and operating costs for the different en

Journal article

Cherdantsev A, An J, Charogiannis A, Markides Cet al., 2018, Cross-validation of PLIF and BBLIF towards the detailed study of gas-sheared liquid films in downward annular flows, 19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics

This paper is devoted to the development and application of two spatiotemporally resolved optical techniquescapable of liquid film thickness measurements in downward annular (co-current) gas-liquid flows, namely PlanarLaser-Induced Fluorescence (PLIF) and Brightness-Based Laser-Induced Fluorescence (BBLIF). A single laser sheet is used to excite the liquid film, which has been doped with a fluorescent dye, along a longitudinal/vertical planenormal to the pipe wall. Two cameras (one for each technique) are placed at different angles to the plane of the lasersheet in order to recover independently the shape of the gas-liquid interface along this section. The effect of theangle between the laser sheet and the PLIF camera axis is also investigated. It is found that, at film regions wherethe gas-liquid interfaceis smooth, the conventional approach used for interpreting PLIF data is vulnerable to totalinternal reflection of the fluorescent light at the free surface, which leads to an overestimation of the film thicknessthat increases as the angle between the laser sheet and the camera axis is decreased. Nonetheless, local features suchas light intensity maxima or minima can often be located within the fluorescent signal that correctly identify theinterface, which in these conditions also coincides with the BBLIF film-thickness measurement. The BBLIFmeasurement, on the other hand, can lead to localized overestimation of the film thickness at flow regions withsignificant wave activity, i.e. steep slopes or agitated films, and around gas bubbles entrained into the liquid film, or an underestimation inside the gas bubbles. Correction procedures are developed to compensate distortions causedby both methods that would make these techniques more accurate for standalone employment. The simultaneousapplication of both techniqu

Conference paper

Wright SF, Charogiannis A, Voulgaropoulos V, Matar OK, Markides CNet al., 2018, Laser-based measurements of stratified liquid-liquid pipe flows interacting with jets in cross-flow, 19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics

At low velocities, horizontal liquid-liquid flows uder go gravitation ally-induced stratification, which in many practical applications complicatessignificantly the direct measurement of the average properties of theflow. The extent of flow stratification, however, can be limited through in line mixing leading to the formation of liquid-liquid dispersions withmore homogenous properties. In this work, we focus on the use of‘active’ mixing methods using jets in cross flows (JICFs). In this paper,a dedicated experimental flow facility for the investigation of such flowsis presented, along with the accompanying laser-based optical measurement techniques and associated algorithms that have beendeveloped for this investigation. The facility allows simultaneous,space-and time-resolved phase and velocity information to be generatedvia plan ar laser-induced fluorescence (PLIF) and particle velocimetry(PIV/ PTV), with stereo-PIV used to provide information on the third (out-of-plane) velocity component. Preliminary experimental results arepresented which demonstrate the capabilities of this arrangementfor optically examining stratified liquid-liquid flows interacting withJICFs, leading to new insights into these complex flows. The key resultsinclude phenomena of jets interacting with the liquid-liquid inter face,recirculation zones that lead to further mixing, the presence of complexcompound droplets, droplet size distributions, and water concentrationprofiles.

Conference paper

Georgiou S, Shah N, Markides C, 2018, Potential for Carbon Savings from the Deployment of Liquid-Air and Pumped-Thermal Electricity Storage Systems, Offshore Energy and Storage 2018

Conference paper

Herrando M, Pantaleo AM, Wang K, Markides Cet al., 2018, Technoeconomic assessment of a PVT-based solar combined cooling heating and power (S-CCHP) system for the university campus of Bari, 13th Conference on Sustainable Development of Energy, Water and Environment Systems - SDEWES Conference, Publisher: SEDWES

In thiswork weanalyse the year-round technoeconomicperformance of a solar combined cooling, heating and power (S-CCHP)system that features polymeric flat-box PVT collectorscoupled via a thermal store to an absorptionchiller. The hourly space heating (SH), cooling and electricitydemands of the University Campus of Bari are used as inputs to amodeldeveloped in TRNSYS.Current electricity and gas prices are considered in order to estimate the annual cost savings which, together with the system’s investment cost, allow an estimation of itspayback time (PBT). The results are then compared to a PV systemthat matchesthe electricity demand of the Campus (including the electricity required to run the current HVAC system for air-conditioning).The results show that the main limiting factorfor the implementationof the S-CCHP systemis the roof-space availability in this application. Asystem with aninstalled power of 1.68MWpcan cover14% of the SH, 66% of the cooling and 17% oftheelectricaldemands of the Campus. The system’s PBTis estimated at 19.3years, which is 3 times higher than the PBTof a PV system of the same installed power, nevertheless,the proposed S-CCHPsystemhas the potential to displace 1,170tons CO2/year, or 50% more than theequivalentPV solution.

Conference paper

Acha Izquierdo S, Lambert R, Le Brun N, Markides C, Shah Net al., 2018, Optimal CHP investments applying sensitivity analyses and financial risk management indicators, 2018 ASHRAE Annual Conference, Publisher: ASHRAE

Evaluating combined heat and power (CHP) investments for commercial applications is not a straightforward task.This work assesses the impact multipletechno-economicuncertainties can have on CHP investments.Understanding the impact these uncertaintiescanhave on projectviabilityallows decision-makers to make informed decisions on capital intensive projects. In this work,amathematical model described previously (Cedillos et al.)wasused to select the optimal CHP size and calculate a reference solution on the financial viability of such investments in a set of buildings. After generating these reference CHP solutions, the impact of uncertainty is then assessed by applying Monte-Carlo based sensitivity analyses and financial and risk management key performance indicators (KPIs). Results suggest thefour most influential parameters in CHP priojects areheating demand, installation costs, electricity prices, and electricity demand. For attractive investments cost uncertainty made projects vary in their payback from2.6 to 6.9years, while for unattractive investments payback ranged from 4 to 11.3 years. Furthermore, Monte-Carlo results illustrate thedifferent distributions of each project, with significant variations in tails risks; indicating which sites are more suitable than others. Results presented show the economic impactuncertainties have onCHP projects, hence allowing decision-makers to make informed decisions before committing their resources to such capital-intensive projects.

Conference paper

Acha Izquierdo S, Shah N, Markides C, Le Brun N, Lambert Ret al., 2018, Fuel cells as CHP systems in commercial buildings: a case study for the food retail sector, 2018 ASHRAE Annual Conference, Publisher: ASHRAE

This study investigates fuel cells as combined heat and power systems (CHPs) for distributed applications in commercial buildings, specifically supermarkets. Up-to-date technical data from a specialized manufacturing company wasinvestigated and used to conduct a case study analysis on several food retail buildings using half-hourly historical data. A detail mathematical model, described in previous publications (Cedillos et al. 2016, Achaet al.2018), was used to simulate the performance of fuel cells through a year of operation in each supermarket. The simulations employ comprehensive energy market costing data and practical informationto evaluate project feasibility such as installation workcosts. The results of the simulations are discussed and a techno-economic assessment is conducted to evaluate the main factors affecting the economics of fuel cell projects.In addition, a comparative analysis with competing CHP technologies (internal combustion engines) is covered. Results show that fuel cells are becoming financially competitivealthough combustion engines are still amoreviableoption. For large-size supermarketsthe payback time forinstalling a fuel cell system is 4.7-5.6years versus 3.6-5.6years for internal combustion engines. The work alsodiscusses the prospects of fuel cells under different market and policy scenariosas well astechnologicalimprovements; thus,offering insights in what are the key aspects which can foster fuel cell installations

Conference paper

van Kleef LMT, Oyewunmi OA, Harraz AA, Haslam AJ, Markides CNet al., 2018, Case studies in computer-aided molecular design (CAMD) of low- and medium-grade waste-heat recovery ORC systems, ECOS 2018 - 31st International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, Publisher: ECOS

Organic Rankine cycle (ORC) engines are suitable for theconversion oflow-grade heat into useful power. While numerous substances are available asORC working-fluid candidates, computer-aided molecular design (CAMD) techniques allow the rigorous selection of an optimal working fluid during system optimisation. The aim of this present study is to extend an existing CAMD-ORC framework [1,2] by incorporating, in addition to thermodynamic performance objectives, economic objectives when determining the optimal systemdesign, while maintaining the facility of selecting optimal working fluids. The SAFT-γ Mie equation of state is used to predictthethermodynamic properties of theworking fluids(here, hydrocarbons)that are relevant to the systems’economic appraisalsand critical/transport properties are estimated using empirical group-contribution methods. System investment costs are estimated with equipment cost correlations for the key system components, andthe stochastic NSGA-II solver is used for system optimisation. From a set of NLP optimisations, it is concluded that the optimal molecular size of the working fluid is linked to the heat-source temperature. The optimal specific investment cost (SIC) values were £10,120/kW and£4,040/kW when using heat-source inlet temperatures of 150°Cand250°C (representative of low-and medium-gradeheat) respectively, andthe corresponding optimal working fluids were propane, 2-butane and 2-heptene.

Conference paper

Pantaleo AM, Camporeale S, Sorrentino A, Braccio G, Markides Cet al., 2018, Distributed heat and power generation: thermoeconomic analysis of Biomass-fired Rankine cycle systems with molten salts as heat transfer fluid, The 31st International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, Publisher: ECOS

Distributed cogeneration systems can be used to serve onsite energy demands in industrial and commercial buildings. In market segments with highly variable heat-demand patterns, the thermal plant is often composed of a boiler that is operated at part load in case of low thermal demands. To improve the plant flexibility and its overall energy efficiency, the biomass boiler can be coupled to a combined heat and power (CHP) generation system, as an alternative to a heat-only plant. In this work, three thermodynamic configurations are compared: (A) a biomass furnace that acts as a heat-source for a steam Rankine cycle (ST) plant coupled to an organic Rankine cycle (ORC) engine; (B) the same as Case A but without the bottoming ORC; and (C): the same as Case A but without the steam cycle. All configurations assume the cogeneration of heat and power to match onsite energy demands. The plant adopts a molten salt (MS) circuit to transfer heat from the biomass furnace to the power generation system. The energy analysis assumes a ternary MS mixture operating up to 450 °C and with minimum temperature of 200 °C. Two organic fluids (Pentafluoropropane R245fa and Toluene) are considered, based on the temperature of heat available to the ORC engine. In the combined cycle of Case A, R245fa is selected and the maximum cycle temperature is 130 °C, with a global electrical efficiency of 16.6%. In Case C, when only the ORC system is used with Toluene as the working fluid, the electrical efficiency is 18.8% at the higher turbine inlet temperature of 330 °C. Production of hot water for cogeneration at different temperature levels is also considered. Based on the results of the thermodynamic simulations, upfront and operational costs assessments, and feed-in tariffs for renewable electricity, energy efficiency and investment profitability are estimated.

Conference paper

Chatzopoulou MA, Sapin P, Markides C, 2018, Optimisation of off-design internal combustion-organic Rankine engine combined cycles, ECOS 2018 - 31st International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, Publisher: ECOS

Organic Rankine cycle (ORC) enginesare an efficient means of convertinglow-to-medium renewable orwaste heat to useful power. In practicalapplications, ORC systemsexperience varying thermalinputprofile,due to the dynamic nature of realheat sources. Maximisingthe uptake of this technology requiresoptimisedORC designsand sizing tomaintain high efficiencyand power output,not only at full-load operation, but also under off-design conditions. Key for maintaining the efficient operationof the systemis the maximisation of heat extraction from the heat source, inthe ORC evaporator. In this paper, the off-design operation of an ICE-ORC combined heat and power (CHP) system is investigated, to optimise the ORC performance under varying ICE load conditions. First, the ORC enginethermodynamic design is optimised for the 100% load operation of the ICE. Alternative working fluids are investigated, including low ODP/GWP refrigerants and hydrocarbons. The ORC system is then sized using two different heat exchanger (HEX) architectures; tube-in-tube (DPHEX) and plate (PHEX) designs, at designconditions. The sizing results reveal that the PHEX area requirements are almost 50% lower than the respective ones for DPHEX, while recovering equivalent quantities of heat. Next, the ORC engine operation is optimised atpart-load ICE conditions, and the HEX heat transfer coefficients (HTCs) are predicted. Results indicate that: i) PHEX HTCs are up to 50% higher than DPHEX equivalents;ii)HTCsdecreaseat part load for both HEXs, but because the average temperaturedifferenceincreases, the overall HEX effectiveness improves; and iii) the ORC system with a PHEX evaporator has slightly higher power output thanthe DPHEX equivalent at off-design operation.Overall, the modelling tool developed here can predict ORC performance over an operating envelopeand allows the selection ofoptimal designsand si

Conference paper

Simpson M, Pantaleo AM, De Palma P, Markides Cet al., 2018, Design and thermo-economic optimisation of small-scale bottoming ORC systems coupled to biomass CHP gasification cycles, ECOS 2018 - 31st International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems., Publisher: ECOS

Optimisation of a small-scale bottoming organic Rankine cycle (ORC) engine is carried out for a combined biomass-gasifier-CHP system, drawing heat from the syngas conditioning unit of the gasifier and the exhaust gas of the internal combustion(IC) engine. The optimisation considers different working fluids and selection of a positive-displacement expander. Single-and two-stage screw expanders and single-stage reciprocating-piston expanders are modelled in order to capture the variation in their performance at a range of design points.Double-pipe heat exchangers are employed for both evaporator and condenser, leading to a low-cost but bulky design.The system is optimised first for maximum electrical power output, and second for minimum specific investment cost(SIC). Cost correlations are used for each of the principal ORC components. The optimal design for minimum SIC is found to be a two-stage screw expander withethanol as the working fluid, which produces a 13.6% increase in the electrical power output relative to the system without an ORC.The investment attractiveness of the whole system with and without the bottoming ORC is assessed and the system is found tobe profitable for avoided electricity costs above 150 $/MWheland biomass costs of 50 $/t, with the ORC making the system more attractive in all cases studied.Discounted payback periods range from 12years at 150 $/MWhelto 3.5years at 250 $/MWhelforthe system with ORC.

Conference paper

Najjaran Kheirabadi A, Harraz AA, Freeman J, Mac Dowell N, Markides CNet al., 2018, Numerical and experimental investigations of diffusion absorption refrigeration systems for use with low temperature heat sources, ECOS 2018 - 31st International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, Publisher: ECOS

he diffusion absorption refrigeration (DAR) cycle is a technology of increasing interest thanks to its suitabilityfor providing cooling from a thermal energyinputin a range of applications. Itcan bedistinguished from other absorption refrigeration cycles by its employment of a thermally-driven bubble pump to circulate the working fluid, which gives it anability to operate entirely off-gridwithout an electricity input. In this work,we present results from an experimentalcampaign aimed atcharacterisingthe performance of aprototypeammonia-water-hydrogen DAR system with a nominal cooling capacity of 100 W,over a range of operating conditions, specifically with a view ofadaptingthe system for use in low-temperature applications. In the experiments, the heat input to the DAR generator is provided over a range of temperatures from175to215°Cby using electrical cartridge heaters. The system is charged to 22 bar, and the ammonia mass concentration of the working fluid mixture is 30%. The resulting coefficient of performance (COP) of the system is measured in the range 0.12to 0.26. A new methodology for the selection of optimal working-fluid mixtures using the state-of-the-art, statistical associating fluid theory (SAFT) approach implemented within the process modelling software gPROMS®is also presented. The experimental results will be used for futurevalidation of a thermodynamic model of the cycle. Finally,the performance of the system in a solar application is investigated, with a thermal inputprovided by an array of evacuated tube heat pipe solar collectors. The system pressure and condensation temperature are found to be key factors in determining the performance of solar-DAR systems.

Conference paper

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

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

Conference paper

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

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

Conference paper

Charogiannis A, Denner F, Van Wachem BGM, Kalliadasis S, Markides CNet al., 2018, Heat tranfer phenomena in falling liquid films: A synergistic experimental and computational study, International Heat Transfer Conference

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

Conference paper

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

Conference paper

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

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

Journal article

Juggurnath D, Elahee MK, Dauhoo MZ, Khoodaruth A, Olakoyejo OT, Adelaja AO, Meyer JP, Markides CNet al., 2018, Numerical modelling of turbulent condensing flows in a smooth horizontal tube, 10th International Conference on Boiling and Condensation Heat Transfer (ICBCHT2018)

Conference paper

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

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

Poster

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

Poster

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

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

Journal article

Chatzopoulou M, Markides CN, 2017, Thermodynamic assessment of organic Rankine cycle systems during off-design operation in combined heat and power (CHP) application., 3rd Thermal and Fluids Engineering Conference

Conference paper

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