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

Le Brun N, Hewitt GF, Markides CN, 2016, Transient freezing of molten salts in pipe-flow systems: application to the direct reactor auxiliary cooling system (DRACS), Applied Energy, Vol: 186, Pages: 56-67, ISSN: 0306-2619

The possibility of molten-salt freezing in pipe-flow systems is a key concern for the solar-energy industry and a safety issue in the new generation of molten-salt reactors, worthy of careful consideration. This paper tackles the problem of coolant solidification in complex pipe networks by developing a transient thermohydraulic model and applying it to the ‘Direct Reactor Auxiliary Cooling System’ (DRACS), the passive-safety system proposed for the Generation-IV molten-salt reactors. The results indicate that DRACS, as currently envisioned, is prone to failure due to freezing in the air/molten-salt heat exchanger, which can occur after approximately 20 minutes, leading to reactor temperatures above 900 °C within 4 hours. The occurrence of this scenario is related to an unstable behaviour mode of DRACS in which newly formed solid-salt deposit on the pipe walls acts to decrease the flow-rate in the secondary loop, facilitating additional solid-salt deposition. Conservative criteria are suggested to facilitate preliminary assessments of early-stage DRACS designs. The present study is, to the knowledge of the authors, the first of its kind in serving to illustrate possible safety concerns in molten-salt reactors, which are otherwise considered very safe in the literature. Furthermore, and from a broader prospective, the analytical tools developed in this study can also be applied to examine the freezing propensity of molten-salt flows in other complex piping systems where standard, finite element approaches are computationally too expensive.

Journal article

Zhang K, Chen X, Markides CN, Yang Y, Shen Set al., 2016, Evaluation of ejector performance for an organic Rankine cycle combined power and cooling system, Applied Energy, Vol: 184, Pages: 404-412, ISSN: 0306-2619

Power-generation systems based on organic Rankine cycles (ORCs) are well suited and increasingly employed in the conversion of thermal energy from low temperature heat sources to power. These systems can be driven by waste heat, for example from various industrial processes, as well as solar or geothermal energy. A useful extension of such systems involves a combined ORC and ejector-refrigeration cycle (EORC) that is capable, at low cost and complexity, of producing useful power while having a simultaneous capacity for cooling that is highly desirable in many applications. A significant thermodynamic loss in such a combined energy system takes place in the ejector due to unavoidable losses caused by irreversible mixing in this component. This paper focuses on the flow and transport processes in an ejector, in order to understand and quantify the underlying reasons for these losses, as well as their sensitivity to important design parameters and operational variables. Specifically, the study considers, beyond variations to the geometric design of the ejector, also the role of changing the external conditions across this component and how these affect its performance; this is not only important in helping develop ejector designs in the first instance, but also in evaluating how the performance may shift (in fact, deteriorate) quantitatively when the device (and wider energy system within which it functions) are operated at part load, away from their design/operating points. An appreciation of the loss mechanisms and how these vary can be harnessed to propose new and improved designs leading to more efficient EROC systems, which would greatly enhance this technology’s economic and environmental potential. It is found that some operating conditions, such as a high pressure of the secondary and discharge fluid, lead to higher energy losses inside the ejector and limit the performance of the entire system. Based on the ejector model, an optimal design featuring a sm

Journal article

Morgan RG, Ibarra R, Zadrazil I, Matar OK, Hewitt GF, Markides CNet al., 2016, On the role of buoyancy-driven instabilities in horizontal liquid–liquid flow, International Journal of Multiphase Flow, Vol: 89, Pages: 123-135, ISSN: 0301-9322

Horizontal flows of two initially stratified immiscible liquids with matched refractive indices, namely an aliphatic hydrocarbon oil (Exxsol D80) and an aqueous-glycerol solution, are investigated by combining two laser-based optical-diagnostic measurement techniques. Specifically, high-speed Planar Laser-Induced Fluorescence (PLIF) is used to provide spatiotemporally resolved phase information, while high-speed Particle Image and Tracking Velocimetry (PIV/PVT) are used to provide information on the velocity field in both phases. The two techniques are applied simultaneously in a vertical plane through the centreline of the investigated pipe flow, illuminated by a single laser-sheet in a time-resolved manner (at a frequency of 1–2 kHz depending on the flow condition). Optical distortions due to the curvature of the (transparent) circular tube test-section are corrected with the use of a graticule (target). The test section where the optical-diagnostic methods are applied is located 244 pipe-diameters downstream of the inlet section, in order to ensure a significant development length. The experimental campaign is explicitly designed to study the long-length development of immiscible liquid–liquid flows by introducing the heavier (aqueous) phase at the top of the channel and above the lighter (oil) phase that is introduced at the bottom, which corresponds to an unstably-stratified “inverted” inlet orientation in the opposite orientation to that in which the phases would naturally separate. The main focus is to evaluate the role of the subsequent interfacial instabilities on the resulting long-length flow patterns and characteristics, also by direct comparison to an existing liquid–liquid flow dataset generated in previous work, downstream of a “normal” inlet orientation in which the oil phase was introduced over the aqueous phase in a conventional stably-stratified inlet orientation. To the best knowledge of the authors this

Journal article

White AJ, McTigue JD, Markides CN, 2016, Analysis and optimisation of packed-bed thermal reservoirs for electricity storage applications, PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART A-JOURNAL OF POWER AND ENERGY, Vol: 230, Pages: 739-754, ISSN: 0957-6509

Journal article

Oyewunmi OA, Kirmse CJW, Markides CN, HEAT EXCHANGER ANALYSIS OF AZEOTROPE MIXTURES IN ORGANIC RANKINE CYCLES, UK Heat Transfer Conference

Conference paper

Delangle ACC, 2016, MODELLING AND OPTIMISATION OF A DISTRICT HEATING NETWORK’S MARGINAL EXTENSION

District heating networks have a key role to play in tackling greenhouse gas emissions associated with urban energy systems. In this context, renewed attention has recently been paid to them and there is a global trend towards the acceleration of district heating expansion. If several existing networks even plan to extend, little work has been carried out on district heating networks expansion in the literature. The following thesis develops a methodology to find the best district heating network expansion strategy under given constraints. After analysing the heat demand and establishing buildings connection scenarios, the model developed optimises the energy centre expansion over a twelve years’ time horizon. Spatial expansion aspects are also included. The optimisation approach was applied to the case of the Barkantine district heating network in the Isle of Dogs, London. The model demonstrated that depending on the optimisation performed (costs or greenhouse gas emissions), some connection strategies have to be privileged. It also proved that district heating scheme’s financial viability may be affected by the connection scenario chosen, highlighting the necessity of planning strategies for district heating networks. The proposed approach can be adapted to other district heating network schemes and modified to integrate more aspects and constraints.

Thesis dissertation

Georgiou S, Markides CN, Shah N, 2016, Decarbonisation of food supply chains from an energetic perspective through optimisation and technological modelling: A holistic approach, Perspectives on Environmental Change DTP Conference 2016

Poster

Freeman J, Ramos Cabal A, Mac Dowel N, Markides CNet al., An experimentally validated model of a solar-cooling system based on an ammonia-water diffusion-absorption cycle, The 8th International Conference on Applied Energy – ICAE2016

An experimentally validated thermodynamic model of a domestic-scale solar-cooling system based on an ammonia-water diffusion-absorption refrigeration (DAR) cycle is presented. The model combines sub-component descriptions of a DAR unit and a suitably sized (matched) solar-collector array, which are validated separately;outdoor tests are performed on an evacuated-tube (ET) collector over a range of solar-irradiance conditions, while a 150-W (nominal rating) DAR unit is tested in the laboratory with a thermal input provided by controlled electrical heaters. A COP of 0.2 is reported for the DARunit when operating with a generator temperature of 155 °C and a system charge pressure of 20.7 bar. Using the experimentally validated solar-cooling system model, it is found that the area of the collector array required to power the system depends strongly on the type of collector. Annual simulations are also performed in various geographical regions order to predict the system’s cooling output. It is found that a single DAR unit with a 3-m2 ET arrayhas the potential to provide 150-200 kWh per year of coolingin a southern European climate, which amounts approximately to the per capita demand for space cooling in residential dwellings in the same region.

Conference paper

Oyewunmi OA, Simó Ferré-Serres, Steven Lecompte, Martijn van den Broek, Michel De Paepe, Markides CNet al., An assessment of subcritical and trans-critical organic Rankine cycles for waste-heat recovery, The 8th International Conference on Applied Energy – ICAE2016, Publisher: Elsevier, ISSN: 1876-6102

Organic Rankine cycle (ORC) systems are increasingly being deployed for waste-heat recovery and conversion in industrial settings. Using a case study of an exhaust flue-gas streamat a temperature of 380 °C as the heat source, an ORC system power output in excess of 10MW is predicted at exergy efficiencies ranging between 20% and 35%. By comparison with available experimental data, the thermodynamic properties (including those in the supercritical region) of working fluids are shown to be reliably predicted by the SAFT-VR Mie equation of state; this verification is quite important as this is the first time that the SAFT-VR Mie equation of state is used forthermodynamic property predictionof working fluids in their supercriticalstateintrans-critical ORC systems.Various cycle configurations and the use of working-fluid mixtures are also investigated. ORC systems operating on trans-critical cycles and those incorporating an internal heat exchanger(IHE)are seen to be beneficial from a thermodynamic perspective, they are,however,more expensive than the simpleORC system considered (subcritical cycle with no IHE).Furthermore, ORC systems using pure working fluids are associated withslightly lower costs than those with fluid mixtures. It is concluded thatabasicORCsystem utilizingpure working fluidsshowsthe lowest specific investment cost(SIC)in the case study considered.

Conference paper

Lecompte S, Oyewunmi OA, Markides C, van den Broek M, De Paepe Met al., Preliminary experimental results of an 11 kWe organic Rankine cycle, The 8th International Conference on Applied Energy – ICAE2016, Publisher: Elsevier, ISSN: 1876-6102

The organic Rankine cycle(ORC)is considered a viable technology forconvertinglow-and medium-temperature heat to electricity. However,many of ORC systems in practical applications operate in off-design conditions. In order to characterize thisoperation, experimental data is needed. In this paper, the commissioning of an 11 kWe ORC is described with special attention to the processingof the data. A filtering algorithm is introduced to isolate steady-state working points. This filter is thenappliedtothe raw experimental data. In addition,the reliability of the experimental data is evaluated by investigating the heat balancesover the heat exchangers and error propagation of the measurementuncertainties. The result of this work is a test-setup which is fully ready for high-accuracyand reliablemeasurements,including the post-processingsteps. In the future, off-design models will be validatedwith the acquired experimental dataand especially two-phase expansion will be further investigated.

Conference paper

Cedillos D, Acha Izquierdo S, Shah N, Markides Cet al., 2016, A Technology Selection and Operation (TSO) optimisation model for distributed energy systems: Mathematical formulation and case study, Applied Energy, Vol: 180, Pages: 491-503, ISSN: 1872-9118

This paper presents a model which simultaneously optimises the selection and operation of technologies for distributed energy systems in buildings. The Technology Selection and Operation (TSO) model enables a new approach for the optimal selection and operation of energy system technologies that encompasses whole life costing, carbon emissions as well as real-time energy prices and demands; thus, providing a more comprehensive result than current methods. Utilizing historic metered energy demands, projected energy prices and a portfolio of available technologies, the mathematical model simultaneously solves for an optimal technology selection and operational strategy for a determined building based on a preferred objective: minimizing cost and/or minimizing carbon emissions. The TSO is a comprehensive and novel techno-economic model, capable of providing decision makers an optimal selection from a portfolio of available energy technologies. The current portfolio of available technologies is composed of various combined heat and power (CHP) and organic Rankine cycle (ORC) units. The TSO model framework is data-driven and therefore presents a high level of flexibility with respect to time granularity, period of analysis and the technology portfolio. A case study depicts the capabilities of the model; optimisation results under different temporal arrangements and technology options are showcased. Overall, the TSO model provides meaningful insights that allow stakeholders to make technology investment decisions with greater assurance.

Journal article

Le Brun N, Markides C, 2016, Framework for the energetic assessment of South and South-East Asia fixed chimney bull’s trench kiln, ICIEA 2016, Publisher: EDP Sciences

One of the major sources of fuel consumption and greenhouse gas emission in South and South-East Asia is brick manufacturing. One of the most commonly implemented technologies for brick manufacturing in this region is the fixed chimney Bull’s trench kiln (FCBTK). This type of technology largely depends on manual labour and is very inefficient when compared to more modern technologies. Because the adoption of more advanced technologies is hindered by the socio-economical background, the much needed innovations in the brick sector are necessarily related to improving/modifying the FCBTK already operational. However, few scientific studies have been conducted on FCBTK probably due to the basic level of technological development. Such studies are however important to systematically and methodologically assess the challenges and solutions in FCBTK. In this study we develop a thermo-energetic model to evaluate the importance of the parameters pertained to FCBTK construction and operation. The prospective of this study is to build an initial thermo-energetic framework that will serve as a basis to investigate possible energetic improvements.

Conference paper

Le Brun N, Markides C, 2016, A transient model for simulating the freezing process of molten-salt coolants, ICIEA 2016, Publisher: EDP Sciences

Even though molten salts have many useful characteristic, especially as coolants for nuclear reactors, they are prone to freezing due to their high melting point. The solidification of the salt inside the piping system could cause structural damage and stop the flow of coolant with possible serious problems. Modelling the freezing process is therefore of primary importance for nuclear safety. In this study a quasi-steady thermo-hydraulic model has been derived and implemented to describe the transient freezing of molten salts. The partial differential equations describing the solidification/melting of the salt are solved numerically using a combination of standard explicit and implicit methods. Validation of the model is presented based on previous experimental studies for two separate cases.

Conference paper

Freeman J, Guarracino I, Unamba CK, Oyewunmi OA, Le Brun N, White MT, White MT, Markides CNet al., Developing a test bed for small-scale ORC expanders in waste-heat recovery applications, 3rd Annual Engine ORC Consortium Workshop

Conference paper

White MT, Oyewunmi OA, Haslam AJ, Markides CNet al., Integrated working-fluid design and ORC system optimisation for waste-heat recovery using CAMD and the SAFT-γ Mie equation of state, 3rd Annual Engine ORC Consortium Workshop

Conference paper

Charogiannis A, Pradas M, Denner F, van Wachem BGM, Kalliadasis S, Markides CNet al., 2016, Hydrodynamic characteristics of harmonically excited thin-film flows: Experiments and computations, 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

Conference paper

Oyewunmi OA, Ferré-Serres S, Markides C, Supercritical organic Rankine cycles in waste-heat recovery using SAFT-VR Mie, 3rd International Meeting of Specialists on Heat Transfer and Fluid Dynamics at Supercritical Pressure (HFSCP2016)

Conference paper

Ibarra R, Morgan R, Zadrazil I, Matar O, Markides Cet al., 2016, Investigation of oil-water flow in horizontal pipes using simultaneous two-line planar laser-induced fluorescence and particle velocimetry, HEFAT, 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

The flow of oil and water in pipes represents a challenging configuration in multiphase flows due to complex hydrodynamics which are still not fully understood. This can be observed in the large number of flow regimes encountered, which extend from smooth stratified flows to complex dispersions such as droplets of oil-in-water and water-in-oil. These flow configurations are the result of the inherent properties of the liquid phases, e.g., their densities and viscosities, interfacial tension and contact angle, as well as of flow conditions and related phenomena, such as turbulence, which have a direct effect on the interface instabilities giving rise to flow regime transitions. In this paper, experimental data are reported that were acquired at low water cuts and low mixture velocities using an aliphatic oil (Exxsol D140) and water as the test fluids in an 8.5 m long and 32 mm internal diameter horizontal pipe. A copper-vapour laser, emitting two narrow-band laser beams, and two high-speed cameras were used to obtain quantitative simultaneous information of the flow (specifically, spatiotemporally resolved fluid-phase and velocity information in both phases) based on simultaneous two-line Planar Laser-Induced Fluorescence (PLIF) and Particle Image and Tracking Velocimetry (PIV/PTV). To the best knowledge of the authors this is the first such instance of the application of this combined technique to these flows. It is found that the rms of the fluctuating velocity show peaks in high shear regions, i.e. at the pipe wall and interface.

Conference paper

Charogiannis A, Zadrazil I, Markides CN, 2016, Thermographic Particle Velocimetry (TPV): An Experimental Technique forSimultaneous Interfacial Temperature and Velocity Measurements Using anInfrared Thermograph, 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics, Publisher: Lisbon Symposium

Conference paper

Sapin P, Taleb A, White AJ, Fabris D, Markides CNet al., 2016, EXPERIMENTAL ANALYSIS OF LOSS MECHANISMS IN A GAS SPRING, ASME Power and Energy Conference, Publisher: ASME, Pages: ES2016-59631-ES2016-59631

Reciprocating-piston compressors and expanders arepromising solutions to achieve higher overall efficiencies invarious energy storage solutions. This article presents anexperimental study of the exergetic losses in a gas spring. Consideringa valveless piston-cylinder system allows us to focuson the thermodynamic losses due to thermal-energy exchangeprocesses in reciprocating components. To differentiate this latterloss mechanism from mass leakages or frictional dissipation,three bulk parameters are measured. Pressure and volume arerespectively measured with a pressure transducer and a rotarysensor. The gas temperature is estimated by measuring theTime-Of-Flight (TOF) of an ultrasonic pulse signal across thegas chamber. This technique has the advantage of being fast andnon-invasive. The measurement of three bulk parameters allowsus to calculate the work as well as the heat losses throughouta cycle. The thermodynamic loss is also measured for differentrotational speeds. The results are in good agreement withprevious experimental studies and can be employed to validateCFD or analytical studies currently under development

Conference paper

Kirmse CJW, Oyewunmi, Taleb A, Haslam A, Markides Cet al., 2016, A two-phase single-reciprocating-piston heat conversion engine: Non-linear dynamic modelling, Applied Energy, Vol: 186, Pages: 359-375, ISSN: 0306-2619

A non-linear dynamic framework is presented for the modelling of a novel two-phase heat engine termed ‘Up-THERM’, which features a single solid moving-part (piston). When applied across the device, a constant temperature difference between an external (low- to medium-grade) heat source and an external heat sink is converted into sustained and persistent oscillations of pressure and volumetric fluid displacement. These oscillations are transformed in a load arrangement into a unidirectional flow from which power is extracted by a hydraulic motor. The Up-THERM engine is modelled using a system of first-order differential equations that describe the dominant thermal/fluid processes in each component of the device. For certain components where the deviations from a linear approximation are non-negligible (gas spring in the displacer cylinder, check valves and piston valve, and heat exchangers), a non-linear description is employed. A comparison between the linear and non-linear descriptions of the gas spring at the top of the displacer cylinder reveals that the non-linear description results in more realistic predictions of the oscillation frequency compared to experimental data from a similar device. Furthermore, the shape of the temperature profile over the heat-exchanger surfaces is modelled as following a hyperbolic tangent function, based on findings from an experimental investigation. Following the validation of these important device components, a parametric study is performed on the Up-THERM engine model with the aforementioned non-linear component descriptions, aimed at investigating the effects of important geometric parameters and of the heat-source temperature on key performance indicators, namely the oscillation frequency, power output and exergy efficiency of the engine. The results indicate that the geometric design of the displacer cylinder, including the height of the gas spring at the top of the cylinder, and the heat-source temperature hav

Journal article

Kirmse CJW, Oyewunmi OA, Haslam AJ, Markides Cet al., 2016, Comparison of a novel organic-fluid thermofluidic heat converter and an organic Rankine cycle heat engine, Energies, Vol: 9, ISSN: 1996-1073

The Up-THERM heat converter is an unsteady, two-phase thermofluidic oscillator that employs an organic working fluid, which is currently being considered as a prime-mover in small- to medium-scale combined heat and power (CHP) applications. In this paper, the Up-THERM heat converter is compared to a basic (sub-critical, non-regenerative) organic Rankine cycle (ORC) heat engine with respect to their power outputs, thermal efficiencies and exergy efficiencies, as well as their capital and specific costs. The study focuses on a pre-specified Up-THERM design in a selected application, a heat-source temperature range from 210 °C to 500 °C and five different working fluids (three n-alkanes and two refrigerants). A modeling methodology is developed that allows the above thermo-economic performance indicators to be estimated for the two power-generation systems. For the chosen applications, the power output of the ORC engine is generally higher than that of the Up-THERM heat converter. However, the capital costs of the Up-THERM heat converter are lower than those of the ORC engine. Although the specific costs (£/kW) of the ORC engine are lower than those of the Up-THERM converter at low heat-source temperatures, the two systems become progressively comparable at higher temperatures, with the Up-THERM heat converter attaining a considerably lower specific cost at the highest heat-source temperatures considered.

Journal article

Oyewunmi OA, Kirmse CJW, Haslam AJ, Muller EA, Markides CNet al., 2016, Working-fluid selection and performance investigation of a two-phase single-reciprocating-piston heat-conversion engine, Applied Energy, Vol: 186, Pages: 376-395, ISSN: 0306-2619

We employ a validated first-order lumped dynamic model of the Up-THERM converter, a two-phase unsteadyheat-engine that belongs to a class of innovative devices known as thermofluidic oscillators, which containfewer moving parts than conventional engines and represent an attractive alternative for remote or off-gridpower generation as well as waste-heat recovery. We investigate the performance the Up-THERM withrespect to working-fluid selection for its prospective applications. An examination of relevant working-fluidthermodynamic properties reveals that the saturation pressure and vapour-phase density of the fluid play importantroles in determining the performance of the Up-THERM – the device delivers a higher power outputat high saturation pressures and has higher exergy efficiencies at low vapour-phase densities. Furthermore,working fluids with low critical temperatures, high critical pressures and exhibiting high values of reducedpressures and temperatures result in designs with high power outputs. For a nominal Up-THERM designcorresponding to a target application with a heat-source temperature of 360 ◦C, water is compared withforty-five other pure working fluids. When maximizing the power output, R113 is identified as the optimalfluid, followed by i-hexane. Fluids such as siloxanes and heavier hydrocarbons are found to maximize theexergy and thermal efficiencies. The ability of the Up-THERM to convert heat over a range of heat-sourcetemperatures is also investigated, and it is found that the device can deliver in excess of 10 kW when utilizingthermal energy at temperatures above 200 ◦C. Of all the working fluids considered here, ammonia, R245ca,R32, propene and butane feature prominently as optimal and versatile fluids delivering high power over awide range of heat-source temperatures.

Journal article

Oyewunmi OA, Markides C, 2016, Thermo-Economic and Heat Transfer Optimization of Working-Fluid Mixtures in a Low-Temperature Organic Rankine Cycle System, Energies, Vol: 9, ISSN: 1996-1073

In the present paper, we consider the employment of working-fluid mixtures in organicRankine cycle (ORC) systems with respect to thermodynamic and heat-transfer performance,component sizing and capital costs. The selected working-fluid mixtures promise reduced exergylosses due to their non-isothermal phase-change behaviour, and thus improved cycle efficienciesand power outputs over their respective pure-fluid components. A multi-objective cost-poweroptimization of a specific low-temperature ORC system (operating with geothermal water at 98 ◦C)reveals that the use of working-fluid-mixtures does indeed show a thermodynamic improvementover the pure-fluids. At the same time, heat transfer and cost analyses, however, suggest that it alsorequires larger evaporators, condensers and expanders; thus, the resulting ORC systems are alsoassociated with higher costs. In particular, 50% n-pentane + 50% n-hexane and 60% R-245fa + 40%R-227ea mixtures lead to the thermodynamically optimal cycles, whereas pure n-pentane and pureR-245fa have lower plant costs, both estimated as having ∼14% lower costs per unit power outputcompared to the thermodynamically optimal mixtures. These conclusions highlight the importanceof using system cost minimization as a design objective for ORC plants.

Journal article

Kirmse CJW, Oyewunmi OA, Haslam AJ, Markides CNet al., A thermo-economic assessment and comparison of the Up-THERM heat converter and an organic Rankine cycle engine, Heat Powered Cycles Conference 2016

In this paper we present a thermodynamic and economic comparison of a recently proposed two-phasethermofluidic oscillator known as the Up-THERM heat converter and the more established organic Rankine cycle(ORC) engine, when converting heat at temperatures below 150 °C using the refrigerant R-227ea as the workingfluid. The Up-THERM heat converter is being considered as a possible prime mover for small- to medium-scalecombined heat and power (CHP) applications. Using suitable thermodynamic models of both systems, it is foundthat the power output and thermal efficiencies of a pre-specified Up-THERM design are generally lower thanthose of an equivalent ORC engine. The Up-THERM, however, also demonstrates higher exergy efficiencies andis associated with lower capital costs, as expected owing to its simple construction and use of fewer and morebasic components. Interestingly, the specific costs (per rated kW) of the ORC engine are lower than those of theUp-THERM converter at lower heat source temperatures, specifically below 130 °C, whereas the Up-THERMbecomes a more cost effective alternative (in terms of the specific cost) to the ORC engine at higher temperatures.

Conference paper

Freeman J, Hellgardt K, Markides CN, 2016, Working Fluid Selection and Electrical Performance Optimisation of a Domestic Solar-ORC Combined Heat and Power System for Year-Round Operation in the UK, Applied Energy, Vol: 186, Pages: 291-303, ISSN: 0306-2619

In this paper, we examine the electrical power-generation potential of adomestic-scale solar combined heating and power (S-CHP) system featuringan organic Rankine cycle (ORC) engine and a 15-m2solar-thermal collectorarray. The system is simulated with a range of organic working fluids andits performance is optimised for operation in the UK climate. The findingsare applicable to similar geographical locations with significant cloud coverage,a low solar resource and limited installation areas. A key feature of thesystem’s design is the implementation of fixed fluid flow-rates during operationin order to avoid penalties in the performance of components suffered atpart-load. Steady operation under varying solar irradiance conditions is providedby way of a working-fluid buffer vessel at the evaporator outlet, whichis maintained at the evaporation temperature and pressure of the ORC. Byincorporating a two-stage solar collector/evaporator configuration, a maximumnet annual electrical work output of 1070 kWh yr−1(continuous averagepower of 122 W) and a solar-to-electrical efficiency of 6.3% is reported withHFC-245ca as the working fluid at an optimal evaporation temperature of126 ◦C (corresponding to an evaporation pressure of 16.2 bar). This is equivalentto ∼ 32% of the electricity demand of a typical/average UK home, andrepresents an improvement of more than 50% over a recent effort by the sameauthors based on an earlier S-CHP system configuration and HFC-245fa asthe working fluid [1]. A performance and simple cost comparison with standalone,side-by-side PV and solar-thermal heating systems is presented.

Journal article

Guarracino I, Freeman J, Markides CN, Experimental evaluation of a 3-D dynamic solar-thermal collector model under time-varying environmental conditions, ECOS 2016, 29th International conference on Efficiency, Cost, Optimization, Simulation and Environmental impact of Energy Systems

Reliable dynamic models are required for the correct prediction of the performance of solar-thermal collectors under variable solar-irradiance conditions. In this paper we present a 3-dimensional (3-D) dynamic thermal model applied to three different collector geometries: a flat plate collector (FPC), an evacuated tube collector (ETC), and also a hybrid photovoltaic-thermal (PVT) collector. Results from the model are evaluated against real data from a series of dynamic and steady-state experiments performed in Limassol, Cyprus and London, UK. The 3-D model equations are summarised and the test apparatuses and procedures are described. In the transient response tests, the model is found to under-predict the time constant for the ETC and PVT collectors by 35-55%, while for the simpler FPC the time constant is under-predicted by 20-35%. The collector model is also implemented into a wider domestic hot-water system model that includes a hot-water storage tank, in order to assess performance predictions over a diurnal operating period on an intermittently cloudy day. The results are compared to a single-node quasi-steady state model that uses the collector steady-state efficiency coefficients and a single-node dynamic model that uses a lumped collector thermal capacity (determined using experimental and calculation-based methods in the European Standard for solar collector testing). The 3-D model is shown to provide promising results that are within the range predicted by the two single-node dynamic models. For the PVT collector simulated under intermittent conditions, the predicted net daily energy gain to the store is found to be within 2% of experimentally obtained results. By comparison, a quasi-steady state model based on the collector’s steady-state efficiency curve is found to over-predict the thermal energy gain to the store by 8% over the same operating period.

Conference paper

Kirmse CJW, Oyewunmi OA, Haslam A, Markides CNet al., A THERMO-ECONOMIC COMPARISON OF THE UP-THERM HEAT CONVERTER AND AN ORGANIC RANKINE CYCLE HEAT ENGINE, 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

In this paper we compare a recently proposed two-phase thermofluidicoscillator device termed ‘Up-THERM’ to a basic(sub-critical, non-regenerative) equivalent organic Rankine cycle(ORC) engine. In the Up-THERM heat converter, a constanttemperature difference imposed by an external heat source andsink leads to periodic evaporation and condensation of the workingfluid, which gives rise to sustained oscillations of pressureand volumetric displacement. These oscillations are convertedin a load arrangement into a unidirectional flow, which passesthrough a hydraulic motor that extracts useful work from the device.A pre-specified Up-THERM design is being considered in aselected application with two n-alkanes, n-hexane and n-heptane,as potential working fluids. One aim of this work is to evaluatethe potential of this proposed design. The thermodynamic comparisonshows that the ORC engine outperforms the Up-THERMheat converter in terms of power output and thermal efficiency,as expected. An economic comparison, however, reveals that thecapital costs of the Up-THERM are lower than those of the ORCengine. Nevertheless, the specific costs (per unit power) favourthe ORC engine due to its higher power output. Some aspects ofthe proposed Up-THERM design are identified for improvement

Conference paper

Oyewunmi OA, Kirmse CJW, Pantaleo AM, Markides Cet al., Performance of working-fluid mixtures in an ORC-CHP system for different heat demand segments, 29th international conference on Efficiency, Cost, Optimisation, Simulation and Environmental Impact of Energy Systems

Organic Rankine cycle (ORC) power systems are being increasingly deployed for waste heat recovery andconversion to power in several industrial settings. In the present paper, we investigate the use of working-fluidmixtures in ORC systems operating in combined heat and power mode (ORC-CHP) with shaft power providedby the expander/turbine and heating provided by the cooling-water exiting the condenser. The waste-heatsource is a flue gas stream from a refinery boiler with a mass flow rate of 560 kg/s and an inlet temperature of330 °C. When using working fluids comprising normal alkanes, refrigerants and their subsequent mixtures, theORC-CHP system is demonstrated as being capable of delivering over 20 MW of net shaft power and up to15 MW of heating, leading to a fuel energy savings ratio (FESR) in excess of 20%. Single-component workingfluids such as pentane appear optimal at low hot-water supply temperatures, and fluid mixtures becomeoptimal at higher temperatures, with the combination of octane and pentane giving an ORC-CHP systemdesign with the highest efficiency. The influence of heat demand intensity on the global system conversionefficiency and optimal working fluid selection is also explored.

Conference paper

Charogiannis A, Zadrazil I, Markides C, 2016, Development of a Thermographic Imaging Technique for Simultaneous Interfacial Temperature and Velocity Measurements, 3rd International Conference on Fluid Flow, Heat and Mass Transfer (FFHMT’16), Publisher: International ASET Inc., Pages: 116-116

An experimental technique, hereby referred to as ‘thermographic particle velocimetry’ (TPV) and capable of recovering twodimensional(2-D) surface temperature and velocity measurements at the interface of multiphase flows is presented. The proposedtechnique employs a single infrared (IR) imager and highly reflective, silver-coated particles, which when suspended near or at theinterface, can be distinguished from the surrounding fluid due to their different emissivity. The development of TPV builds upon ourprevious IR imaging studies of heated liquid-film flows; yet, the same measurement principle can be applied for the recovery of 2-Dtemperature- and velocity-field information at the interface of any flow with a significant density gradient between two fluid phases. Theimage processing steps used to recover the temperature and velocity distributions from raw IR frames are demonstrated by applicationof TPV in a heated and stirred flow in an open container, and include the decomposition of each raw frame into separate thermal andparticle frames, the application of perspective distortion corrections and spatial calibration, and the implementation of standard particleimage velocimetry algorithms. Validation experiments dedicated to the measurement of interfacial temperature and velocity were alsoconducted, with deviations between the results generated from TPV and those from accompanying conventional techniques not exceedingthe errors associated with the latter. Finally, the capabilities of the proposed technique are demonstrated by conducting temperature andvelocity measurements at the gas-liquid interface of a wavy film flow downstream of a localised heater.

Conference paper

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