Imperial College London

ProfessorChristosMarkides

Faculty of EngineeringDepartment of Chemical Engineering

Professor of Clean Energy Technologies
 
 
 
//

Contact

 

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

 
 
//

Location

 

404ACE ExtensionSouth Kensington Campus

//

Summary

 

Publications

Publication Type
Year
to

275 results found

Georgiou S, Acha S, Shah N, Markides C, 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

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, 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., 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

Chatzopoulou MA, Lecompte S, De Paepe M, Markides Cet al., Off-design operation of ORC engines with different heat exchanger architectures in waste heat recovery applications, ICAE2018 - 10th International Conference on Applied Energy, Publisher: Elsevier, ISSN: 1876-6102

Organic Rankine cycle(ORC)engines in waste-heat recovery applications experience variable heat-source conditions (i.e. temperature and mass flow ratevariations). Therefore maximisingthe ORC system performance under off-design conditions is of key importance, for the financial viability and wider adoption of these systems. In this paper,the off-design performance of an ORC engineis investigated, with screw expander andtwoheat exchanger (HEXs)architectures, while recovering heat from an internal combustion engine (ICE).Unlike previous studieswhere the ORC expanderand HEXsperformance is assumed fixed duringoff-design operation, in this work we consider the time-varying characteristics of the system components.Firstly, nominal system sizing results indicate that the screw expander isentropic efficiency exceeds 80%,whilethe plate HEXs (PHEXs) heat transferarea requirements are 50% lower,than the respective ones for double pipe (DPHEX) design. Next, the ORC engine operation is optimised at part-load (PL) ICE conditions.Although, the HEXs heat transfer coefficients decrease by 30% with part-load, the HEX effectiveness increasesby up to 20%, due to higher temperature difference across the working fluids.Findingsalso reveal that the PHEX performance is less sensitive to the off-design operation. Optimum off-design power output maps indicatethattheORC enginePL reduces to 72%, for ICE PL of 60%, while ORCengines with PHEXsgenerate slightlymore power,for the same heat source conditions. Overall, the modelling tool developed predictsthe ORC performance over an operating envelope and allows the selection of optimal designs and sizes of ORC HEXsand expanders.The findings can be used by ORC plant operators to optimise the ORC engine power output, given the varying heat source conditions observed on their site,and by ORC vendors to inform HEX andexpander design decisions.

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

Chatzopoulou MA, Sapin P, Markides C, Chatzopoulou MA, Sapin P, Markides Cet al., 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

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

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

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

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

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

Conference paper

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

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

Conference paper

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

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

Conference paper

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

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

Conference paper

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

Conference paper

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

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

Journal article

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

Conference paper

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

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

Poster

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

Poster

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

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

Journal article

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

Conference paper

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

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

Journal article

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

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

Conference paper

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

Conference paper

Pantaleo AM, Chatzopoulou MA, Oyewunmi O, de palma P, amirante R, rotolo G, Markides Cet al., THERMO-ECONOMIC OPTIMIZATION OF SMALL-SCALE ORC SYSTEMS FOR HEAT RECOVERY FROM NATURAL GAS INTERNAL COMBUSTION ENGINES FOR STATIONARY POWER GENERATION, 4TH ANNUAL ENGINE ORC CONSORTIUM WORKSHOP FOR THE AUTOMOTIVE AND STATIONNARY ENGINE INDUSTRIES

Conference paper

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

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

Conference paper

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

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

Journal article

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

Conference paper

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

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

Journal article

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

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

Conference paper

This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.

Request URL: http://wlsprd.imperial.ac.uk:80/respub/WEB-INF/jsp/search-html.jsp Request URI: /respub/WEB-INF/jsp/search-html.jsp Query String: id=00345500&limit=30&person=true&page=3&respub-action=search.html