283 results found
Guarracino I, Freeman J, Ekins-Daukes N, et al., PERFORMANCE ASSESSMENT AND COMPARISON OF SOLAR ORC AND HYBRID PVT SYSTEMS FOR THE COMBINED DISTRIBUTED GENERATION OF DOMESTIC HEAT AND POWER, HEFAT, 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics
Solar-thermal collectors and photovoltaic panels are effectivesolutions for the decarbonisation of electricity and hot waterprovision in dwellings. In this work, we provide the first insightfulcomparison of these two competing solar-energy technologies forthe provision of combined heating and power (CHP) in domesticapplications. The first such system is based on an array of hybridPV-Thermal (PVT) modules, while the second is based on a solarthermalcollector array of the same area (based on a constrainedroof-space) that provides a thermal-energy input to an organicRankine cycle (ORC) engine for electricity generation. Simulationresults of the annual operation of these two systems are presentedin two geographical regions: Larnaca, Cyprus (as an example of ahot, high-irradiance southern-European climate) and London, UK(as an example of a cooler, lower-irradiance northern-Europeanclimate). Both systems have a total collector array area of 15 m2,equivalent to the roof area of a single residence, with the solarORCsystem being associated with a lower initial investment cost(capex) that is expected to play a role in the economic comparisonbetween the two systems. The electrical and thermal outputs of thetwo systems are found to be highly dependent on location. ThePVT system is found to provide an annual electricity output of2090 kWhe yr-1in the UK, which increases to 3620 kWhe yr-1inCyprus. This is equivalent to annual averages of 240 and 410 We,respectively, or between 60% and 110% of household demand.The corresponding additional thermal (hot water) output alsoincreases, from 860 kWhth yr-1in the UK, to 1870 kWhth yr-1inCyprus. It is found that the solar-ORC system performance ishighly sensitive to the system configuration chosen; the particularconfiguration studied here is found to be limited by the amount ofrejected thermal energy that can be reclaimed for water heating.The maximum electrical output from the solar-ORC configurationexplored in this study is 450 kWhe yr-1(
Taleb A, Sapin P, Barfuß C, et al., Wall temperature and system mass effects in a reciprocating gas spring, INTERNATIONAL CONFERENCE ON EFFICIENCY, COST, OPTIMIZATION, SIMULATION AND ENVIRONMENTAL IMPACT OF ENERGY SYSTEMS
Taleb AI, Timmer MAG, El-Shazly MY, et al., 2016, A single-reciprocating-piston two-phase thermofluidic prime-mover, Energy, Vol: 104, Pages: 250-265, ISSN: 0360-5442
We explore theoretically a thermodynamic heat-engine concept that has the potential of attaining a high efficiency and power density relative to competing solutions, while having a simple construction with few moving parts and dynamic seals, allowing low capital and operating costs, and long lifetimes. Specifically, an unsteady heat-engine device within which a working uid undergoes a power cycle featuring phase-change, termed the `Evaporative Reciprocating-Piston Engine' (ERPE), is considered as a potential prime mover foruse in combined heat and power (CHP) applications. Based on thermal/uid-electrical analogies, a theoretical ERPE device is conceptualized initially in the electrical-analogy domain as a linearized, closed-loop active electronic circuit model. The circuit-model representation is designed to potentially exhibit high efficiencies compared to similar, existing two-phase unsteady heat engines. From the simplified circuit model in the electrical domain, and using the thermal/uid-electrical analogies, one possible configuration of a correspondingphysical ERPE device is derived, based on an early prototype of a device currently under development that exhibits some similarities with the ERPE, and used as a physical manifestation of the proposed concept. The corresponding physical ERPE device relies on the alternating phase change of a suitable working-fluid (here, water) to drive a reciprocating displacement of a single vertical piston and to produce sustained oscillations of thermodynamic properties within an enclosed space. Four performance indicators are considered: the operational frequency, the power output, the exergy efficiency, and the heat input/temperature difference imposed externally on the device's heat exchangers that is necessary to sustain oscillations. The effects of liquidinertia, viscous drag, hydrostatic pressure, vapour compressibility and two-phase heat transfer in the various engine components/compartments a
Cherdantsev AV, An J, Zadrazil I, et al., An investigation of film wavy structure in annular flow using two simultaneous LIF approaches, 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics
The paper is devoted to development and validation of film thickness measurement techniques in interfacial gas-liquid flows. The specific flow investigated here is that of downwards (co-flowing) annular flow in a vertical pipe, however, many of the observations and findings are transferable to similar flow geometries. Two advanced spatially resolved techniques, namely planar laser-induced fluorescence and brightness-based laser-induced fluorescence , are used simultaneously in the same area of interrogation. A single laser sheet is used to excite fluorescence along one longitudinal section of the pipe, and two cameras (one for each method) are placed at different angles to the plane of the laser sheet in order to independently recover the shape of the interface along this section. This allows us to perform a cross-validation of the two techniques and to analyse their respective characteristics, advantages and shortcomings.
Acha Izquierdo S, Van Dam KH, Markides C, et al., 2016, Simulating residential electricity and heat demand in urban areas using an agent-based modelling approach, Energycon 2016, Publisher: IEEE
Cities account for around 75% of the global energy demand and are responsible for 60-70% of the global greenhouse gasses emissions. To reduce this environmental impact it is important to design efficient energy infrastructures able to deal with high level of renewable energy resources. A crucial element in this design is the quantitative understanding of the dynamics behind energy demands such as transport, electricity and heat. In this paper an agent-based simulation model is developed to generate residential energy demand profiles in urban areas, influenced by factors such as land use, energy infrastructure and user behaviour. Within this framework, impact assessment of low carbon technologies such as plug-in electric vehicles and heat pumps is performed using London as a case study. The results show that the model can generate important insights as a decision support tool for the design and planning of sustainable urban energy systems.
Barfuss C, 2016, Numerical Simulation of Heat Transfer Effects during Compression and Expansion in Gas Springs
Sapin P, Taleb A, Barfuß C, et al., Thermodynamic Losses in a Gas Spring: Comparison of Experimental and Numerical Results, International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics
Reciprocating-piston devices can be used as high-efficiencycompressors and/or expanders. With an optimal valve design andby carefully adjusting valve timing, pressure losses during intakeand exhaust can be largely reduced. The main loss mechanismin reciprocating devices is then the thermal irreversibility dueto the unsteady heat transfer between the compressed/expandedgas and the surrounding cylinder walls. In this paper, pressure,volume and temperature measurements in a piston-cylindercrankshaft driven gas spring are compared to numerical results.The experimental apparatus experiences mass leakage while theCFD code predicts heat transfer in an ideal closed gas spring.Comparison of experimental and numerical results allows one tobetter understand the loss mechanisms in play. Heat and masslosses in the experiment are decoupled and the system lossesare calculated over a range of frequencies. As expected, compressionand expansion approach adiabatic processes for higherfrequencies, resulting in higher efficiency. The objective of thisstudy is to observe and explain the discrepancies obtained betweenthe computational and experimental results and to proposefurther steps to improve the analysis of the loss mechanisms.
Taleb A, Barfuß C, Sapin P, et al., The Influence of Real Gases Effects on Thermally Induced Losses in Reciprocating Piston-Cylinder Systems, International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics
The efficiency of expanders is of prime importance for variousclean energy technologies. Once mechanical losses (e.g. throughvalves) are minimized, losses due to unsteady heat exchange betweenthe working fluid and the solid walls of the containingdevice can become the dominant loss mechanism. In this device,gas spring devices are investigated numerically in order to focusexplicitly on the thermodynamic losses that arise due to thisunsteady heat transfer. The specific aim of this study is to investigatethe behaviour of real gases in gas springs and comparethis to that of ideal gases in order to attain a better understandingof the impact of real gas effects on the thermally losses inreciprocating piston expanders and compressors. A CFD-modelof a gas spring is developed in OpenFOAM. Three different gasmodels are compared: an ideal gas model with constant thermodynamicand transport properties; an ideal gas model withtemperature-dependent properties; and a real gas model using thePeng-Robinson equation of state with temperature and pressuredependentproperties. Results indicate that, for simple, monoanddiatomic gases like helium or nitrogen, there is a negligibledifference in the pressure and temperature oscillations over a cyclebetween the ideal and real gas models. However, when lookingat a heavier (organic) molecule such as propane, the ideal gasmodel tends to overestimate the temperature and pressure comparedto the real gas model, especially if no temperature dependencyof thermodynamic properties is taken into account. Additionally,the ideal gas model (both alternatives) underestimatesthe thermally induced loss compared to the real gas model forheavier gases. Real gas effects must be taken into account in orderto predict accurately the thermally induced loss when usingheavy molecules in such devices.
Denner F, Pradas M, Charogiannis A, et al., 2016, Self-similarity of solitary waves on inertia-dominated falling liquid films, Physical Review E, Vol: 93, ISSN: 1539-3755
We propose consistent scaling of solitary waves on inertia-dominated falling liquid films, which accurately accounts for the driving physical mechanisms and leads to a self-similar characterization of solitary waves. Direct numerical simulations of the entire two-phase system are conducted using a state-of-the-art finite volume framework for interfacial flows in an open domain that was previously validated against experimental film-flow data with excellent agreement. We present a detailed analysis of the wave shape and the dispersion of solitary waves on 34 different water films with Reynolds numbers Re=20–120 and surface tension coefficients σ=0.0512–0.072Nm−1 on substrates with inclination angles β=19∘–90∘. Following a detailed analysis of these cases we formulate a consistent characterization of the shape and dispersion of solitary waves, based on a newly proposed scaling derived from the Nusselt flat film solution, that unveils a self-similarity as well as the driving mechanism of solitary waves on gravity-driven liquid films. Our results demonstrate that the shape of solitary waves, i.e., height and asymmetry of the wave, is predominantly influenced by the balance of inertia and surface tension. Furthermore, we find that the dispersion of solitary waves on the inertia-dominated falling liquid films considered in this study is governed by nonlinear effects and only driven by inertia, with surface tension and gravity having a negligible influence.
Guarracino I, Mellor A, Ekins-Daukes N, et al., 2016, Dynamic coupled thermal-and-electrical modelling of sheet-and-tube hybrid photovoltaic/thermal (PVT) collectors, Applied Thermal Engineering, Vol: 101, Pages: 778-795, ISSN: 1873-5606
In this paper we present a dynamic model of a hybrid photovoltaic/thermal (PVT) collector with a sheet-and-tube thermal absorber. The model is used in order to evaluate the annual generation of electrical energy along with the provision of domestic hot-water (DHW) from the thermal energy output, by using real climate-data at high temporal resolution. The model considers the effect of a non-uniform temperature distribution on the surface of the solar cell on its electrical power output. An unsteady 3-dimensional numerical model is developed to estimate the performance of such a collector. The model allows key design parameters of the PVT collector to vary so that the influence of each parameter on the system performance can be studied at steady state and at varying operating and atmospheric conditions. A key parameter considered in this paper is the number of glass covers used in the PVT collector. The results show that while the thermal efficiency increases with the additional glazing, the electrical efficiency deteriorates due to the higher temperature of the fluid and increased optical losses, as expected. This paper also shows that the use of a dynamic model and of real climate-data at high resolution is of fundamental importance when evaluating the yearly performance of the system. The results of the dynamic simulation with 1-min input data show that the thermal output of the system is highly dependent on the choice of the control parameters (pump operation, differential thermostat controller, choice of flow rate etc.) in response to the varying weather conditions. The effect of the control parameters on the system's annual performance can be captured and understood only if a dynamic modelling approach is used. The paper also discusses the use of solar cells with modified optical properties (reduced absorptivity/emissivity) in the infrared spectrum, which would reduce the thermal losses of the PVT collector at the cost of only a small loss in electrical output
Charogiannis A, Heiles B, Mathie R, et al., Spatiotemporally resolved heat transfer measurements in falling-film flows over an inclined heated foil, International Symposium and School of Young Scientists INTERFACIAL PHENOMENA AND HEAT TRANSFER
Charogiannis A, Markides C, Zadrazil I, 2016, Thermographic Particle Velocimetry (TPV) for Simultaneous Interfacial Temperature and Velocity Measurements, International Journal of Heat and Mass Transfer, Vol: 97, Pages: 589-595, ISSN: 0017-9310
We present an experimental technique, that we refer to as ‘thermographic particle velocimetry’ (TPV),which is capable of the simultaneous measurement of two-dimensional (2-D) surface temperature andvelocity at the interface of multiphase flows. The development of the technique has been motivated bythe need to study gravity-driven liquid-film flows over inclined heated substrates, however, the samemeasurement principle can be applied for the recovery of 2-D temperature- and velocity-field informationat the interface of any flow with a sufficient density gradient between two fluid phases. The proposedtechnique relies on a single infrared (IR) imager and is based on the employment of highly reflective(here, silver-coated) particles which, when suspended near or at the interface, can be distinguished fromthe surrounding fluid domain due to their different emissivity. Image processing steps used to recover thetemperature and velocity distributions include the decomposition of each original raw IR image into separatethermal and particle images, the application of perspective distortion corrections and spatial calibration,and finally the implementation of standard particle velocimetry algorithms. This procedure isdemonstrated by application of the technique to a heated and stirred flow in an open container. In addition,two validation experiments are presented, one dedicated to the measurement of interfacial temperatureand one to the measurement of interfacial velocity. The deviations between the results generatedfrom TPV and those from accompanying conventional techniques do not exceed the errors associatedwith the latter.
Chatzopoulou MA, Keirstead J, Fisk D, et al., Characterising the impact of HVAC design variables on buildings energy performance, using a Global Sensitivity Analysis framework, CLIMA 2016 - 12th REHVA World Congress
Chatzopoulou MA, Keirstead J, Fisk D, et al., Informing low carbon HVAC systems modelling and design, using a Global Sensitivity analysis framework, ASME 2016 Power and Energy
Le Brun N, Markides, Bismarck, et al., 2016, On the drag reduction effect and shear stability of improved acrylamide copolymers for enhanced hydraulic fracturing, Chemical Engineering Science, Vol: 146, Pages: 135-143, ISSN: 0009-2509
Polymeric drag reducers, such as partially hydrolysed polyacrylamide (PHPAAm), are important chemical additives in hydraulic fracturing fluids as they can significantly decrease the frictional pressure drop in the casing (by up to 80%),resulting in an increase of the injection rate that can be delivered to the fracturing point. The incorporation of sodium 2-acrylamido-2-methylpropane sulfonic acid (NaAMPS) moieties in to polyacrylamide (PAAm) can further improve the performance of fracturing fluids by addressing some compatibility issues related to the use of PHPA Am, e.g., the sensitivity to water salinity . In this study, three types of poly(acrylamide-co-NaAMPS) and pure PHPAAm were investigated with respect to polymer induced drag reduction and mechanical polymer degradationin turbulent pipe flow in a pressure-driven pipe flow facility. The test section comprised a horizontal 1” bore circular cross-section pipe. The facility was modified in order to allow, long time/length experiments by automatically recirculating the polymer solution in a closed-loop through the test section.The presence of NaAMPS groups in the copolymer backbone is found to increase the ability of PHPAAm to reduce frictional drag while the vulnerability to mechanical degradation remains unaffected. The drag reduction of NaAMPS copolymer solutions can be described by a modified version of Virk’s correlation (1967), extended to include the effect of Reynolds number. Polymer mechanical degradation is found to proceed until the friction reducer is almost ineffective in reducing drag. This phenomenon is in contrast with the most common correlationfor polymer degradation, which predicts the existence of a n asymptotic(but finite) limit to the reduced drag reduction.
Acha S, Le Brun N, Lambert R, et al., 2016, UK half-hourly regional electricity cost modelling for commercial end users
© 2018 University of Minho. All rights reserved. The rising prices of electricity in the UK risks rendering businesses uncompetitive if these costs are not controlled. This issue has created the need to properly comprehend the tariffs and costing framework that influence the total cost of electricity for non-domestic customers. This paper details an open source method to model UK electricity regional costs (MUKERC) for commercial end-users; allowing users to visualise and calculate the cost of the electricity they consume. The methodology consists in a bottom-up model that defines individually all the tariff components and then aggregates them to quantify the cost of a kWh across each half-hour of the day. The disaggregated structure of MUKERC allows users to conduct specific analysis of tariff components and to understand their rich temporal and spatial features. This granularity facilitates understanding which tariffs influence costs more during different time periods. Emphasis is given to showcasing commodity prices and network charges; known as Transmission Use of System and Distribution Use of System tariffs. ‘Representative day’ electricity price curves for different day types, voltage level connections, and across different UK regions for 2016-17 are presented. Outputs from MUKERC can better inform companies on their energy costs and therefore allows them to perform comprehensive and bespoke energy management and energy efficiency strategies as it is possible to understand when and where the cost of electricity is more expensive. Results show that commercial buildings connected at Low Voltage in North Wales and Merseyside and the South West face the highest average electricity prices, whereas consumers connected to High Voltage in London and the North West have the cheapest electricity in the UK. Other significant findings indicate sites connected at low-voltage pay 7.5% more than high-voltage sites, winter weekday costs are 18% higher than s
An JS, Morgan RG, Hale CP, et al., 2016, A three-phase slug flow investigation by tomographic dual-beam X-ray imaging: Slug frequency measurement and lessons for correlation development and application, Multiphase Science and Technology, Vol: 28, Pages: 71-98, ISSN: 0276-1459
© 2016 by Begell House, Inc. New measurement data on three-phase (air-oil-water) slug flows are reported in a long (37 m), largediameter (3 in nominal, 77.9 mm bore) pipe, generated by using a nonintrusive technique based on a dual-beam X-ray tomography system. Based on this measurement data, the frequency of hydrodynamic slugs is determined at a position 30.6 m (∼400 diameters) downstream of the inlet and over a range of inlet flow conditions with superficial velocities: 2-6 m/s (air), 0-0.5 m/s (oil), 0-0.5 m/s (water), from which slug frequency trends specific to this three-phase flow system are identified and reported in the literature for the first time. The slug frequency data are subsequently used to examine the feasibility and reliability of using slightly modified versions of many of the current two-phase gas-liquid slug frequency correlations in order to predict the measured three-phase slug frequencies observed in the experiments. It is found that, in general, these correlations provide poor slug frequency predictions in the investigated flows; nevertheless, the correlations that tend to perform best are those that include terms that attempt to account for variations in the fluid properties. The approach presented in this paper provides a method for reasonable three-phase slug frequency prediction as a first approximation, although the accuracy of this prediction can be improved if the apparent liquid-liquid mixture-viscosity can be determined more reliably in situ. The data made available in the present paper are to the best knowledge of the authors not presently available in the literature, and can be used to develop and validate advanced multiphase flow models, beyond acting as a benchmark database for correlation checks and improvement, as is done here.
Chatzopoulou M-A, Keirstead J, Fisk D, et al., 2016, INFORMING LOW CARBON HVAC SYSTEMS MODELLING AND DESIGN, USING A GLOBAL SENSITIVITY ANALYSIS FRAMEWORK, 10th ASME International Conference on Energy Sustainability, Publisher: AMER SOC MECHANICAL ENGINEERS
Oyewunmi OA, Markides CN, 2015, EFFECT OF WORKING-FLUID MIXTURES ON ORGANIC RANKINE CYCLE SYSTEMS: HEAT TRANSFER AND COST ANALYSIS, 3RD International Seminar on ORC Power Systems, Publisher: University of Liège and Ghent University
The present paper considers the employment of working-fluid mixtures in organic Rankine cycle (ORC)systems with respect to heat transfer performance, component sizing and costs, using two sets of fluidmixtures: n-pentane + n-hexane and R-245fa + R-227ea. Due to their non-isothermal phase-change behaviour,these zeotropic working-fluid mixtures promise reduced exergy losses, and thus improved cycleefficiencies and power outputs over their respective pure-fluid components. Although the fluid-mixturecycles do indeed show a thermodynamic improvement over the pure-fluid cycles, the heat transfer andcost analyses reveal that they require larger evaporators, condensers and expanders; thus, the resultingORC systems are also associated with higher costs, leading to possible compromises. In particular,70 mol% n-pentane + 30 mol% n-hexane and equimolar R-245fa + R-227ea mixtures lead to the thermodynamicallyoptimal cycles, whereas pure n-pentane and pure R-227ea have lower costs amounting to14% and 5% per unit power output over the thermodynamically optimal mixtures, respectively.
Markides CN, 2015, Low-Concentration Solar-Power Systems Based on Organic Rankine Cycles for Distributed-Scale Applications: Overview and Further Developments, Frontiers in Energy Research, Vol: 3, ISSN: 2296-598X
This paper is concerned with the emergence and development of low-to-medium-grade thermal-energy-conversion systems for distributed power generation based on thermo- dynamic vapor-phase heat-engine cycles undergone by organic working uids, namely organic Rankine cycles (ORCs). ORC power systems are, to some extent, a relatively established and mature technology that is well-suited to converting low/medium-grade heat (at temperatures up to ~300–400°C) to useful work, at an output power scale from a few kilowatts to 10s of megawatts. Thermal ef ciencies in excess of 25% are achievable at higher temperatures and larger scales, and efforts are currently in progress to improve the overall economic viability and thus uptake of ORC power systems, by focusing on advanced architectures, working- uid selection, heat exchangers and expansion machines. Solar-power systems based on ORC technology have a signi cant potential to be used for distributed power generation, by converting thermal energy from simple and low-cost non-concentrated or low-concentration collectors to mechanical, hydrau- lic, or electrical energy. Current elds of use include mainly geothermal and biomass/ biogas, as well as the recovery and conversion of waste heat, leading to improved energy ef ciency, primary energy (i.e., fuel) use and emission minimization, yet the technology is highly transferable to solar-power generation as an affordable alternative to small-to- medium-scale photovoltaic systems. Solar-ORC systems offer naturally the advantages of providing a simultaneous thermal-energy output for hot water provision and/or space heating, and the particularly interesting possibility of relatively straightforward onsite (thermal) energy storage. Key performance characteristics are presented, and important heat transfer effects that act to limit performance are identi ed as noteworthy directions of future research for the further development of this technology.
Oyewunmi OA, Taleb A, Haslam A, et al., 2015, On the use of SAFT-VR Mie for assessing large-glide fluorocarbon working-fluid mixtures in organic rankine cycles, Applied Energy, Vol: 163, Pages: 263-282, ISSN: 1872-9118
By employing the SAFT-VR Mie equation of state, molecular-based models are developed from which the thermodynamic properties of pure (i.e., single-component) organic fluids and their mixtures are calculated. This approach can enable the selection of optimal working fluids in organic Rankine cycle (ORC) applications, even in cases for which experimental data relating to mixture properties are not available. After developing models for perfluoroalkane (n-C4F10 + n-C10F22) mixtures, and validating these against available experimental data, SAFT-VR Mie is shown to predict accurately both the single-phase and saturation properties of these fluids. In particular, second-derivative properties (e.g., specific heat capacities), which are less reliably calculated by cubic equations of state (EoS), are accurately described using SAFT-VR Mie, thereby enabling an accurate prediction of important working-fluid properties such as the specific entropy. The property data are then used in thermodynamic cycle analyses for the evaluation of ORC performance and cost. The approach is applied to a specific case study in which a sub-critical, non-regenerative ORC system recovers and converts waste-heat from a refinery flue-gas stream with fixed, predefined conditions. Results are compared with those obtained when employing analogue alkane mixtures (n-C4H10 + n-C10H22) for which sufficient thermodynamic property data exist. When unlimited quantities of cooling water are utilized, pure perfluorobutane (and pure butane) cycles exhibit higher power outputs and higher thermal efficiencies compared to mixtures with perfluorodecane (or decane), respectively. The effect of the composition of a working-fluid mixture in the aforementioned performance indicators is non-trivial. Only at low evaporator pressures (< 10 bar) do the investigated mixtures perform better than the pure fluids. A basic cost analysis reveals that systems with pure perfluorobutane (and butane) fluids are associated with rela
Markides CN, Mathie R, Charogiannis A, 2015, An experimental study of spatiotemporally resolved heat transfer in thin liquid-film flows falling over an inclined heated foil, International Journal of Heat and Mass Transfer, Vol: 93, Pages: 872-888, ISSN: 0017-9310
This paper describes the development of an experimental technique that combines simultaneous planar laser-induced fluorescence (PLIF) and infrared (IR) thermography imaging, and its application to the measurement of unsteady and conjugate heat-transfer in harmonically forced, thin liquid-film flows falling under the action of gravity over an inclined electrically heated-foil substrate. Quantitative, spatiotemporally resolved and simultaneously conducted measurements are reported of the film thickness, film free-surface temperature, solid–liquid substrate interface temperature, and local/instantaneous heat flux exchanged with the heated substrate. Based on this information, local and instantaneous heat-transfer coefficients (HTCs) are recovered. Results concerning the local and instantaneous HTC and how this is correlated with the local and instantaneous film thickness suggest considerable heat-transfer enhancement relative to steady-flow predictions in the thinner film regions. This behaviour is attributed to a number of unsteady/mixing transport processes within the wavy films that are not captured by laminar, steady-flow analysis. The Nusselt number Nu increases with the Reynolds number Re; at low Re values the mean Nu number corresponds to 2.5, in agreement with the steady-flow theory, while at higher Re, both the Nu number and the HTC exhibit significantly enhanced values. Evidence that the HTC becomes decoupled from the film thickness for the upper range of observed film thicknesses is also presented. Finally, smaller film thickness fluctuation intensities were associated with higher HTC fluctuation intensities, while the amplitude of the wall temperature fluctuations was almost proportional to the amplitude of the HTC fluctuations.
Markides CN, Herrando M, 2015, Hybrid PV and solar-thermal systems for domestic heat and power provision in the UK: Techno-economic considerations, Applied Energy, Vol: 161, Pages: 512-532, ISSN: 0306-2619
A techno-economic analysis is undertaken to assess hybrid PV/solar-thermal (PVT) systems for distributedelectricity and hot-water provision in a typical house in London, UK. In earlier work (Herrando et al., 2014), asystem model based on a PVT collector with water as the cooling medium (PVT/w) was used to estimateaverage year-long system performance. The results showed that for low solar irradiance levels and lowambient temperatures, such as those associated with the UK climate, a higher coverage of total householdenergy demands and higher CO2 emission savings can be achieved by the complete coverage of the solar collectorwith PV and a relatively low collector cooling flow-rate. Such a PVT/w system demonstrated an annualelectricity generation of 2.3 MW h, or a 51% coverage of the household’s electrical demand (compared to anequivalent PV-only value of 49%), plus a significant annual water heating potential of to 1.0 MW h, or a 36%coverage of the hot-water demand. In addition, this system allowed for a reduction in CO2 emissionsamounting to 16.0 tonnes over a life-time of 20 years due to the reduction in electrical power drawn fromthe grid and gas taken from the mains for water heating, and a 14-tonne corresponding displacement of primaryfossil-fuel consumption. Both the emissions and fossil-fuel consumption reductions are significantlylarger (by 36% and 18%, respectively) than those achieved by an equivalent PV-only system with the samepeak rating/installed capacity. The present paper proceeds further, by considering the economic aspects ofPVT technology, based on which invaluable policy-related conclusions can be drawn concerning the incentivesthat would need to be in place to accelerate the widespread uptake of such systems. It is found that,with an electricity-only Feed-In Tariff (FIT) support rate at 43.3 p/kW h over 20 years, the system cost estimatesof optimised PVT/w systems have an 11.2-year discounted payback period (PV-only: 6.8 years). Therole and i
Le Brun N, Markides CNM, 2015, A Galinstan-Filled Capillary Probe for Thermal Conductivity Measurements and its Application to Molten Eutectic KNO3-NaNO3-NO2 (HTS) up to 700 K, International Journal of Thermophysics, Vol: 36, Pages: 3222-3238, ISSN: 1572-9567
The successful measurement of the thermal conductivity of molten salts isa challenging undertaking due to the electrically conducting and possibly alsoaggressive nature of the materials, as well as the elevated temperatures atwhich these data are required. For accurate and reproducible measurementsit is important to develop a suitable experimental apparatus and methodology.In this study we explore a modified version of the transient hot-wiremethod, which employs a molten-metal-filled capillary in order to circumventsome of the issues encountered in previous studies. Specifically, by using anovel flexible U-shaped quartz-capillary, filled with a eutectic mixture of gallium,indium and tin, commercially known as Galinstan, we proceed to measurethe thermal conductivity of molten eutectic KNO3−NaNO3−NaNO2.The new probe is demonstrated as being able to measure the thermal conductivityof this molten salt, which is found to range from 0.48 W/m K at500 K to 0.47 W/m K at close to 700 K, with an overall expanded uncertainty(95% confidence) of 3.1%. The quartz is found to retain its electricallyinsulating properties and no current leakage is detected in the sample overthe investigated temperature range. The thermal conductivity data reportedin the present study are also used to elucidate a partial disagreement foundin the literature for this material.
Palanisamy K, Taleb AI, Markides CN, 2015, Optimizing the Non-Inertive-Feedback Thermofluidic Engine for the Conversion of Low-Grade Heat to Pumping Work, HEAT TRANSFER ENGINEERING, Vol: 36, Pages: 1303-1320, ISSN: 0145-7632
Markides CN, Heyes AL, 2015, Selected Papers from the Thirteenth UK Heat Transfer Conference, HEAT TRANSFER ENGINEERING, Vol: 36, Pages: 1163-1164, ISSN: 0145-7632
Ibarra R, Zadrazil I, Markides CN, et al., Towards a Universal Dimensionless Map of Flow Regime Transitions in Horizontal Liquid-Liquid Flows, 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics
Charogiannis A, Markides CN, Denner F, et al., 2015, A simultaneous application of PLIF-PIV-PTV for the detailed experimental study of the hydrodynamic characteristics of thin film flows, 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics (HEFAT2015)
Kirmse C, Oyewunmi OA, Haslam AJ, et al., 2015, A two-phase single-reciprocating-piston heat conversion engine, 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics (HEFAT2015)
This paper considers an energy-conversion heat-engineconcept termed ‘Up-THERM’. This machine is capable ofconverting low- to medium-grade heat to useful positivedisplacementwork through the periodic evaporation andcondensation of a working fluid in an enclosed space. Thesealternating phase-change processes drive sustained oscillations ofthermodynamic properties (pressure, temperature, volume) as theworking fluid undergoes an unsteady thermodynamic heatenginecycle. The resulting oscillatory flow of the working fluidis converted into a unidirectional flow in a hydraulic loadarrangement where power can be extracted from the machine.The engine is described with lumped dynamic modelsconstructed using electrical analogies founded on previouslydeveloped thermoacoustic and thermofluidic principles, whichare extended here to include a description of the phase-changeheat-transfer processes. For some sub-components of the engine,such as the gas spring, valves and the temperature profile in theheat exchangers, deviations from the linear theory are nonnegligible.These are modelled using non-linear descriptions. Inparticular, the results of linear and non-linear descriptions of thegas spring are compared using three important performanceindicators — efficiency, power output and frequency.The non-linear description of the gas spring results in morerealisticpredictions of the oscillation frequency compared todirect measurements on an experimental prototype of a similarengine. Owing to its mode of operation and lack of moving parts,the Up-THERM engine does offer a much simpler and morecost-efficient solution than alternative engines for heat recoveryand solar applications. The results from this work suggest thatthis technology can be a competitive alternative in terms of costper unit power in low-power, small-scale applications, especiallyin remote, off-grid settings, for example in developing countrieswhere minimising upfront costs is crucial.
Oyewunmi OA, Haslam AJ, Markides CN, 2015, Towards the computer-aided molecular design of organic rankine cycle systems with advanced fluid theories, SusTEM 2015 International Conference, Pages: 180-189
Organic Rankine cycle (ORC) power-generation systems are increasingly being deployed for heat recovery and conversion from geothermal reservoirs and in several industrial settings. Using a case study of an exhaust flue-gas stream, an ORC power output in excess of 20 MW is predicted at thermal efficiencies ranging between 5% and 15%. The considerable influence on cycle performance of the choice of the working fluid is illustrated with alkane and perfluoroalkane systems modelled using the SAFT-VR Mie equation of state (EoS); in general, the more-volatile pure components (n-butane or n-perfluorobutane) are preferred although some mixtures perform better at restricted cycle conditions.The development of computer-aided molecular design (CAMD) platforms for ORC systems requires both cycle and working-fluid models to be incorporated into a single framework, for the purposes of whole-system design and optimization. Using pure alkanes and their mixtures as a case study, we test the suitability of the recent group-contribution SAFT- Mie EoS method for describing the thermodynamic properties of working fluids relevant to the analysis of ORC systems. The theory is shown to predict accurately the relevant properties of these fluids, thereby suggesting that this SAFT-based CAMD approach is a promising approach towards working-fluid design of ORC power systems.
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