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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283 results found

Charogiannis A, An JS, Markides CN, 2015, A Simultaneous Planar Laser-Induced Fluorescence, Particle Image Velocimetry and Particle Tracking Velocimetry Technique for the Investigation of Thin Liquid-Film Flows, Experimental Thermal and Fluid Science, Vol: 68, Pages: 516-536, ISSN: 0894-1777

Journal article

Ibarra R, Matar OK, Markides CN, Zadrazil Iet al., 2015, An experimental study of oil-water flows in horizontal pipes, Multiphase 2015, Publisher: BHR Group

This paper reports an effort to investigate the effect of flow velocities and inlet configurations on horizontal oil-water flows in a 32 mm ID acrylic pipe using water and an aliphatic oil (Exxsol D140) as test fluids. The flows of interest were analysed using pressure drop measurements and high-speed photography in an effort to obtain a flow pattern map, pressure gradient profiles and measures of the in situ phase fractions. The experiments reveal a particular effect of the inlet configuration on the observed flow pattern. A horizontal plate, installed at the inlet, generates a transition to stratified flow when the plate height closely matched the in situ water height at high input oil fractions.

Conference paper

Kirmse C, Taleb AJ, Oyewunmi OA, Haslam AJ, Markides CNet al., Performance comparison of a novel thermofluidic organic-fluid heat converter and an organic rankine cycle heat engine, 3rd International Seminar on ORC Power Systems (ASME ORC 2015)

The Up-THERM engine is a novel two-phase heat engine with a single moving part–a vertical solidpiston–that relies on the phase change of a suitable working fluid to produce a reciprocating displacementand sustained thermodynamic oscillations of pressure and flow rate that can be converted to useful work.A model of the Up-THERM engine is developed via lumped dynamic descriptions of the various enginesub-components and electrical analogies founded on previously developed thermoacoustic principles.These are extended here to include a description of phase change and non-linear descriptions of selectedprocesses. The predicted first and second law efficiencies and the power output of a particular Up-THERM engine design aimed for operation in a specified CHP application with heat source and sinktemperatures of 360 ○C and 10 ○C, are compared theoretically to those of equivalent sub-critical, nonregenerativeorganic Rankine cycle (ORC) engines. Five alkanes (from n-pentane to n-nonane) are beingconsidered as possible working fluids for the aforementioned Up-THERM application, and these arealso used for the accompanying ORC thermodynamic analyses. Owing to its mode of operation, lackof moving parts and dynamic seals, the Up-THERM engine promises a simpler and more cost-effectivesolution than an ORC engine, although the Up-THERM is expected to be less efficient than its ORCcounterpart. These expectations are confirmed in the present work, with the Up-THERM engine showinglower efficiencies and power outputs than equivalent ORC engines, but which actually approach ORCperformance at low temperatures. Therefore, it is suggested that the Up-THERM can be a competitivealternative in terms of cost per unit power in low-power/temperature applications, especially in remote,off-grid settings, such as in developing countries where minimising upfront costs is crucial.

Conference paper

Freeman J, Hellgardt K, Markides CN, 2015, An Assessment of Solar-Thermal Collector Designs for Small-Scale Combined Heating and Power Applications in the UK, Heat Transfer Engineering, Vol: 36, Pages: 1332-1347, ISSN: 1521-0537

Journal article

Freeman J, Hellgardt K, Markides CN, 2015, An assessment of solar-powered organic Rankine cycle systems for combined heating and power in UK domestic applications, Applied Energy, Vol: 138, Pages: 605-620, ISSN: 1872-9118

Journal article

Markides CN, Smith TCB, 2015, A Dynamic Model for the Optimization of Oscillatory Low Grade Heat Engines, International Conference of Computational Methods in Sciences and Engineering (ICCMSE 2010), Publisher: American Institute of Physics (AIP), Pages: 417-420, ISSN: 1551-7616

The efficiency of a thermodynamic system is a key quantity on which its usefulness and wider applicationrelies. This is especially true for a device that operates with marginal energy sources and close to ambient temperatures.Various definitions of efficiency are available, each of which reveals a certain performance characteristic of a device. Ofthese, some consider only the thermodynamic cycle undergone by the working fluid, whereas others contain additionalinformation, including relevant internal components of the device that are not part of the thermodynamic cycle. Yet othersattempt to factor out the conditions of the surroundings with which the device is interfacing thermally during operation. Inthis paper we present a simple approach for the modeling of complex oscillatory thermal-fluid systems capable ofconverting low grade heat into useful work. We apply the approach to the NIFTE, a novel low temperature difference heatutilization technology currently under development. We use the results from the model to calculate various efficienciesand comment on the usefulness of the different definitions in revealing performance characteristics. We show that theapproach can be applied to make design optimization decisions, and suggest features for optimal efficiency of the NIFTE.

Conference paper

Markides CN, Smith TCB, A dynamic model for the optimization of oscillatory low grade heat engines, AIP Conference Proceedings, Vol: 1642, ISSN: 1551-7616

The efficiency of a thermodynamic system is a key quantity on which its usefulness and wider application relies. This is especially true for a device that operates with marginal energy sources and close to ambient temperatures. Various definitions of efficiency are available, each of which reveals a certain performance characteristic of a device. Of these, some consider only the thermodynamic cycle undergone by the working fluid, whereas others contain additional information, including relevant internal components of the device that are not part of the thermodynamic cycle. Yet others attempt to factor out the conditions of the surroundings with which the device is interfacing thermally during operation. In this paper we present a simple approach for the modeling of complex oscillatory thermal-fluid systems capable of converting low grade heat into useful work. We apply the approach to the NIFTE, a novel low temperature difference heat utilization technology currently under development. We use the results from the model to calculate various efficiencies and comment on the usefulness of the different definitions in revealing performance characteristics. We show that the approach can be applied to make design optimization decisions, and suggest features for optimal efficiency of the NIFTE.

Journal article

Hussain T, Markides CN, Balachandran R, 2015, Flame dynamics in a micro-channeled combustor, Publisher: American Institute of Physics (AIP), ISSN: 1551-7616

The increasing use of Micro-Electro-Mechanical Systems (MEMS) has generated a significant interest in combustion-based power generation technologies, as a replacement of traditional electrochemical batteries which are plagued by low energy densities, short operational lives and low power-to-size and power-to-weight ratios. Moreover, the versatility of integrated combustion-based systems provides added scope for combined heat and power generation. This paper describes a study into the dynamics of premixed flames in a micro-channeled combustor. The details of the design and the geometry of the combustor are presented in the work by Kariuki and Balachandran [1]. This work showed that there were different modes of operation (periodic, a-periodic and stable), and that in the periodic mode the flame accelerated towards the injection manifold after entering the channels. The current study investigates these flames further. We will show that the flame enters the channel and propagates towards the injection manifold as a planar flame for a short distance, after which the flame shape and propagation is found to be chaotic in the middle section of the channel. Finally, the flame quenches when it reaches the injector slots. The glow plug position in the exhaust side ignites another flame, and the process repeats. It is found that an increase in air flow rate results in a considerable increase in the length (and associated time) over which the planar flame travels once it has entered a micro-channel, and a significant decrease in the time between its conversion into a chaotic flame and its extinction. It is well known from the literature that inside small channels the flame propagation is strongly influenced by the flow conditions and thermal management. An increase of the combustor block temperature at high flow rates has little effect on the flame lengths and times, whereas at low flow rates the time over which the planar flame front can be observed decreases and the time of e

Conference paper

Zadrazil I, Matar OK, Markides CN, Phase-locked measurements of gas-liquid horizontal flows, American Physical Society - Division of Fluid Dynamics

A flow of gas and liquid in a horizontal pipe can be described in terms of various flow regimes, e.g. wavy stratified, annular or slug flow. These flow regimes appear at characteristic gas and liquid Reynolds numbers and feature unique wave phenomena. Wavy stratified flow is populated by low amplitude waves whereas annular flow contains high amplitude and long lived waves, so called disturbance waves, that play a key role in a liquid entrainment into the gas phase (droplets). In a slug flow regime, liquid-continuous regions travel at high speeds through a pipe separated by regions of stratified flow. We use a refractive index matched dynamic shadowgraphy technique using a high-speed camera mounted on a moving robotic linear rail to track the formation and development of features characteristic for the aforementioned flow regimes. We show that the wave dynamics become progressively more complex with increasing liquid and gas Reynolds numbers. Based on the shadowgraphy measurements we present, over a range of conditions: (i) phenomenological observations of the formation, and (ii) statistical data on the downstream velocity distribution of different classes of waves.

Conference paper

Charogiannis A, Zadrazil I, Markides CN, Wave and flow field phenomena in planar falling films by simultaneous Laser-Induced Fluorescence and Particle Image/Tracking Velocimetry, American Physical Society - Division of Fluid Dynamics

Conference paper

Zadrazil I, Markides CN, Turbulence Scaling in Pipe Flows Exhibiting Polymer-Induced Drag Reduction, American Physical Society - Division of Fluid Dynamics

Non-intrusive laser based diagnostics technique, namely Particle Image Velocimetry, was used to in detail characterise polymer induced drag reduction in a turbulent pipe flow. The effect of polymer additives was investigated in a pneumatically-driven flow facility featuring a horizontal pipe test section of inner diameter 25.3 mm and length 8 m. Three high molecular weight polymers (2, 4 and 8 MDa) at concentrations of 5 -- 250 wppm were used at Reynolds numbers from 35000 to 210000. The PIV derived results show that the level of drag reduction scales with different normalised turbulence parameters, e.g. streamwise and spanwise velocity fluctuations, vorticity or Reynolds stresses. These scalings are dependent of the distance from the wall, however, are independent of the Reynolds numbers range investigated.

Conference paper

Gupta A, Mathie R, Markides CN, 2014, An experimental and computational investigation of a thermal storage system based on a phase change material: Heat transfer and performance characterization, Computational Thermal Sciences, Vol: 6, Pages: 341-359, ISSN: 1940-2554

The integration of latent heat storage solutions into modern heating and cooling systems has the potential to enhance overall system performance compared to standard hot water systems (radiators and tanks) due to an augmentation of the stored heat by the latent heat of a suitable material. This paper presents computational predictions complemented by experimental measurements of the dynamic behavior and performance of an active thermal storage system for domestic applications, based on the use of a hydrated salt phase change material (PCM) and a conventional cylindrical storage tank. The thermal storage (heating) and extraction (cooling) rates for this PCM-filled tank are compared to a water-filled tank. Flow and temperature fields are analyzed in a customized storage tank design for heat transfer and performance characterization. Experimental findings show good agreement with full 3-D simulation results. The heat removal characteristic is identified as being the main factor limiting the arrangement’s performance when compared to a water-based system, due to the solidification of the PCM onto the pipes, and a significant consequent decrease in heat flux. It is confirmed that the PCM thermal storage solution has the capability to store a large amount of heat effectively, but design improvements are required to eliminate the cooling-limited heat transfer process in the investigated arrangement.

Journal article

Oyewunmi OA, Taleb AI, Haslam AJ, Markides CNet al., 2014, An assessment of working-fluid mixtures using SAFT-VR Mie for use in organic Rankine cycle systems for waste-heat recovery, Computational Thermal Sciences, Vol: 6, Pages: 301-316, ISSN: 1940-2503

© 2014 by Begell House, Inc. Working-fluid mixtures offer an improved thermal match to heat source streams in organic Rankine cycles (ORCs) over pure (single) fluids. In the present work we investigate the selection of working-fluid mixtures and component mixing ratios for an ORC system from a thermodynamic and economic point of view. A mathematical model of a subcritical, nonregenerative ORC is constructed. We employ the SAFT-VR Mie equation of state, a state-of-the-art version of the statistical associating fluid theory (SAFT), to predict the thermodynamic state properties and phase behavior of the fluid mixtures. The effect of the working-fluid mixture selection on the efficiency and power output from the cycle is investigated, as is its effect on the sizes of the various components of the ORC engine. This is done in order to appreciate the role that the fluid mixtures have on the investment/capital costs attributed to the installation of such a unit, intended for waste-heat recovery and conversion to power. Results of an ORC using a binary decane–butane mixture as the working fluid demonstrate a significant improvement in the cost per unit power output compared to the two pure fluid components. Specifically, the added costs of the four main ORC system components (pump, expander, and two heat exchangers) were found to be as low as 120–130 £/kW, 20–30% lower compared to the pure fluids.

Journal article

McTigue JD, White AJ, Markides CN, 2014, Parametric studies and optimisation of pumped thermal electricity storage, Applied Energy, Vol: 137, Pages: 800-811, ISSN: 0306-2619

Journal article

Guarracino I, Freeman J, Markides CN, 2014, Dynamic testing and modelling of solar collectors, 14th UK Heat Transfer Conference 2015

Solar-thermal collectors operating under real conditions rarely reach steady-state operation due to temporal fluctuations inthe climate/environmental conditions and thermal loads. Figure 1 shows typical UK weather map. The incident irradiance,the ambient temperature and the wind speed can vary during the day as shown in Figures 2-4. These figures show the datacollected in London at a temporal resolution of 1-minute. As a consequence of the time-varying inputs, collector models thatdescribe dynamic behaviour are required for the accurate prediction of the thermal output and for optimising the controlstrategy of such systems. We develop detailed 3-D thermal sub-models that can be adapted to various geometries orcollector configurations, including vacuum-tube thermal collectors, sheet-and-tube thermal and PV/T collectors.

Poster

Zadrazil I, Markides CN, 2014, An experimental characterization of liquid films in downwards co-current gas-liquid annular flow by particle image and tracking velocimetry, International Journal of Multiphase Flow, Vol: 67, Pages: 42-53, ISSN: 0301-9322

Journal article

Neophytou MK-A, Markides CN, Fokaides PA, 2014, An experimental study of the flow through and over two dimensional rectangular roughness elements: Deductions for urban boundary layer parameterizations and exchange processes, Physics of Fluids, Vol: 26, Pages: 086603-1-086603-21, ISSN: 1089-7666

This paper investigates the flow through and over two-dimensional rectangular roughnesselements, arranged in a building-street canyon geometry through a series ofexperiments. Geometries of different packing densities of the roughness elements(λp) were examined and the packing density values ranged from λp = 0.30 to 0.67.The purpose of the work is: (i) to investigate the flow physics observed both at theboundary layer scale as well as at the scale within the roughness elements for a rangeof packing densities, (ii) to deduce parameterizations of the adjusted rough boundarylayer and their variation with a change in the packing density, and (iii) givena particular interest in and application to the urban atmosphere, a final aim at theroughness-element scale is to deduce the variation of the breathability with the packingdensity variation. Particle image velocimetery measurements of the velocity flowfield as well as the turbulent kinetic energy and the Reynolds Stress (within and upto well-above the street canyons) were conducted. The results reveal qualitative flowfeatures as well as features of the adjusted boundary layer structure—in particular theroughness and inertial sublayers, which can be associated with the surface roughnesslength, zero-plane displacement thickness, and the friction velocity. The lowest frictionvelocities are exhibited in the geometries with the highest- and lowest packingdensities while the maximum friction velocities are observed in the medium-packedgeometries. The exchange processes and breathability at the level of the roughnesselements top were characterized and quantified by a mean exchange velocity. Theresults show that unlike friction velocity, the normalized exchange velocity (overthe mean bulk velocity) for the most dense and sparse geometries differ by morethan 80%, with the denser-packed geometries exhibiting lower exchange velocities;this is shown to be related with the thickness of the developed roughness sublay

Journal article

Morgan R, Markides CN, Zadrazil I, Hewitt GFet al., Investigation into Liquid-Liquid Flow Instability Mechanisms using Laser-Based Optical Diagnostic Techniques, The Geoff Hewitt Celebration Conference – Multiphase Flow: Theory, Modelling, Simulation and Experimentation

Conference paper

Charogiannis A, Markides CN, Mathie R, 2014, Combined PLIF-IR thermal measurements of wavy film flows undergoing forced harmonic excitation, 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics (HEFAT2014)

A combined PLIF/IR thermography technique wasdeveloped and employed towards the measurement of unsteadyand conjugate heat transfer in thin, gravity-driven falling liquidfilm flows (with and without flow pulsation) over an inclinedheated metal foil. Simultaneous, local film thickness, film andsubstrate temperature, heat flux exchanged with a heated foiland heat transfer coefficient results are reported for a range ofelectrically applied heat input values, flow Reynolds (Re)numbers and flow pulsation frequencies. Moreover, interfacialwave velocities were calculated from cross-correlations acrosssuccessive thickness profiles. Results concerning theinstantaneous and local heat transfer coefficient variation andhow this is correlated with the instantaneous and local filmthickness variation (waves) suggest that the heat transfercoefficient experiences an enhancement in thinner films. Theparticular observation is most probably attributed to a numberof unsteady flow phenomena within the wavy fluid films thatare not captured by the steady analysis. At low flow Re numbervalues the mean Nusselt (Nu) was around 2.5, in agreementwith laminar flow theory, while at higher Re values, higher Nuwere observed. Finally, lower wave amplitude intensities wereassociated with higher heat transfer coefficient fluctuationintensities.

Conference paper

Morgan RG, Ibarra R, Zadrazil I, Markides CNet al., 2014, On the role of inlet flow instabilities on horizontal initially stratified liquid-liquid flow development, 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

For a given pair of fluid phases, liquid-liquid flows aregenerally described in terms of regimes (e.g. stratified, wavy ordispersed), which are a function of the Reynolds numbers of theindividual phases, the geometry of the flow, as well as the inletconditions and the distance from the inlet. Typically, injectingthe heavier phase at the bottom of the channel and the lighterphase at the top is the common inlet configuration whenestablishing a liquid-liquid flow for study in a laboratoryenvironment. This configuration corresponds to that expectedin a naturally separated flow orientation, on the assumption thatat long lengths the density difference between the two phaseswill lead to this arrangement of the two phases. In this study, aseries of experiments were designed to investigate the influenceof injecting the heavier phase at the top of the pipe rather thanat the bottom. This modification introduces the possibility ofphase breakup near the inlet by an additional instabilitymechanism (due to the density difference between the twoliquids), which would not appear had the phases beenintroduced in the conventional inlet flow arrangement. Weperform detailed flow measurements and observe that this flowarrangement gives rise to altered flow structures downstream.Moreover, our results suggest that the effects of this instabilitynear the inlet may persist along the pipe and influence theobserved flow behaviour even at long lengths.

Conference paper

Zadrazil I, Markides CN, A Mechanism of Polymer Induced Drag Reduction in Turbulent Pipe, 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

Polymer induced drag reduction in turbulent pipe flow wasinvestigated using a non-intrusive laser based diagnostictechnique, namely Particle Image Velocimetry (PIV). The dragreduction was measured in a pressure-driven flow facility, in ahorizontal pipe of inner diameter 25.3 mm at Reynoldsnumbers ranging from 35 000 to 210 000. Three high-molecular-weight polymers (polyethylene oxide 2x10^6 –8x10^6 Da) at concentrations in the range of 5 – 250 wppm wereused. The results, obtained from the PIV measurements, showthat the drag reduction scales with the magnitude of thenormalized streamwise and spanwise rms velocity fluctuationsin the flow. This scaling seems to universal, and is independentof the Reynolds number and in some cases also independent ofthe distance from the wall where the velocity fluctuations areconsidered. Furthermore, the instantaneous PIV observationsindicate that as the level of drag reduction increases, the flow inthe pipe is separated into a low-momentum flow region near thepipe wall and a high-momentum flow region in the turbulentcore. Based on these findings a new mechanism of polymericdrag reduction is proposed in this paper.

Conference paper

Charogiannis A, Markides CN, 2014, Experimental study of falling films by simultaneous laser-induced fluorescence, particle image velocimetry and particle tracking velocimetry, 17th International Symposium on Applications of Laser Techniques to Fluid Mechanics

measurement technique based on the simultaneous implementation of Laser-Induced Fluorescence (LIF),Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) has been applied to the study of wavyliquid falling film flows characterized by low Reynolds (Re) and Kapitza (Ka) numbers. The presently examined Renumber range was 2.2 – 8.2, while the Ka number range was 28.6 – 41.4. The experimental methodology wasdeveloped with the ultimate aim of allowing for the evaluation of the local and instantaneous film thickness, interfacialvelocity and velocity field from within the illuminated liquid volume underneath the wavy interface. The majorchallenges associated with the simultaneous implementation of the two optical diagnostic techniques were, firstly, thedevelopment of a refractive index correction approach allowing for liquids of different properties (surface tension andviscosity) to be tested, secondly, the identification of the location of the two liquid boundaries (solid-liquid and gasliquid)in the LIF images, and lastly, the isolation of out-of-plane reflections from primary scattering regions in the rawPIV images. Following a detailed account of the novel practices formulated and utilized in tackling the aforementionedchallenges, the efficacy of the proposed methodology is demonstrated through comparisons between laser-basedmeasurements conducted in flat films, film thickness measurements performed with a micrometer, and the solution tothe Navier-Stokes equation based on the assumptions of one-dimensional (1-D), steady and fully developed flow. Inaddition, sample film topology results are presented for a range of flow pulsation frequencies (1 – 8 Hz), while filmthickness and interfacial velocity time traces were reconstructed and are presented along with film thickness andinterfacial velocity statistical results for select flow conditions.

Conference paper

Zadrazil I, Markides CN, On Disturbance and Ripple Waves in Downwards Annular Flow: Observations by Simultaneous PLIF and PIV/PTV, 17th International Symposium on Applications of Laser Techniques to Fluid Mechanics

Conference paper

Markides CN, Solanki R, Galindo A, 2014, Working fluid selection for a two-phase thermofluidic oscillator: Effect of thermodynamic properties, APPLIED ENERGY, Vol: 124, Pages: 167-185, ISSN: 0306-2619

Journal article

Zadrazil I, Markides CN, On Disturbance and Ripple Waves in Downwards Annular Flow: Observations by Simultaneous PLIF and PIV/PTV, 52. European Two-Phase Flow Group Meeting

Conference paper

Zadrazil I, Matar OK, Markides CN, 2014, An experimental characterization of downwards gas-liquid annular flow by laser-induced fluorescence: Flow regimes and film statistics, INTERNATIONAL JOURNAL OF MULTIPHASE FLOW, Vol: 60, Pages: 87-102, ISSN: 0301-9322

Journal article

White A, McTigue J, Markides C, 2014, Wave propagation and thermodynamic losses in packed-bed thermal reservoirs for energy storage, Applied Energy, Vol: 130, Pages: 648-657, ISSN: 0306-2619

Journal article

Herrando M, Markides CN, Hellgardt K, 2014, A UK-based assessment of hybrid PV and solar-thermal systems for domestic heating and power: System performance, Applied Energy, Vol: 122, Pages: 288-309, ISSN: 1872-9118

The goal of this paper is to assess the suitability of hybrid PVT systems for the provision of electricity andhot water (space heating is not considered) in the UK domestic sector, with particular focus on a typicalterraced house in London. A model is developed to estimate the performance of such a system. The modelallows various design parameters of the PVT unit to be varied, so that their influence in the overall systemperformance can be studied. Two key parameters, specifically the covering factor of the solar collectorwith PV and the collector flow-rate, are considered. The emissions of the PVT system are compared withthose incurred by a household that utilises a conventional energy provision arrangement. The resultsshow that for the case of the UK (low solar irradiance and low ambient temperatures) a complete coverageof the solar collector with PV together with a low collector flow-rate are beneficial in allowing thesystem to achieve a high coverage of the total annual energy (heat and power) demand, while maximisingthe CO2 emissions savings. It is found that with a completely covered collector and a flow-rate of 20 L/h,51% of the total electricity demand and 36% of the total hot water demand over a year can be coveredby a hybrid PVT system. The electricity demand coverage value is slightly higher than the PV-only systemequivalent (49%). In addition, our emissions assessment indicates that a PVT system can save up to16.0 tonnes of CO2 over a lifetime of 20 years, which is significantly (36%) higher than the 11.8 tonnesof CO2 saved with a PV-only system. All investigated PVT configurations outperformed the PV-only systemin terms of emissions. Therefore, it is concluded that hybrid PVT systems offer a notably improvedproposition over PV-only systems.

Journal article

Mathie R, Markides CN, White AJ, 2014, A Framework for the Analysis of Thermal Losses in Reciprocating Compressors and Expanders, Heat Transfer Engineering, Vol: 35, Pages: 1435-1449, ISSN: 1521-0537

This article presents a framework that describes formally the underlying unsteady and conjugate heat transfer processes that are undergone in thermodynamic systems, along with results from its application to the characterisation of the reciprocating compression and expansion processes in a gas spring. Specifically, a heat transfer model is proposed that solves the one-dimensional unsteady heat conduction equation in the solid simultaneously with the first law in the gas phase, with an imposed heat transfer coefficient. Even at low compression ratios (of 2.5), notable effects of the solid walls are revealed, with thermodynamic cycle losses of up to 20% (relative to equivalent adiabatic and reversible processes) when unfavourable solid and gas materials are selected, and closer to 10-12% for more common material choices. The contribution of the solid towards these values, through the variations attributed to the thickness of the cylinder wall, is about 10% and 2-4%, respectively; showing a maximum at intermediate thicknesses. At higher compression ratios (of 6) a 19% worst case loss is reported for common materials. These results suggest strongly that, in designing high-efficiency reciprocating machines, the full conjugate and unsteady problem must be considered and that the role of the solid in determining performance cannot, in general, be neglected.

Journal article

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