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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276 results found

Romanos P, Pantaleo A, Markides C, Energy management and enhanced flexibility of power stations via thermal energy storage and secondary power cycles, 11th International Conference on Applied Energy

The operation of power plants must meet a series of requirements in order to enable the increasing penetration of intermittent renewable energy and the consequent intensifying demand for flexible generation. It is proposed here that during off-peak demand, steam can be extracted from Rankine-cycle power stations for the charging of thermal storage tanks that contain suitable phase-change materials (PCMs); during peak demand time, these thermal energy storage (TES) tanks can act as the heat sources of secondary thermal power plants in order to generate power, for example as evaporators of organic Rankine cycle (ORC) plants that are suitable for power generation at reduced temperatures and smaller scales. This type of solution offers greater flexibility than TES-only solutions that store thermal energy and then release this back to the base power station, in that it allows both derating andover-generation compared to the base power-station. The approach is here applied to a case study of a 670-MW rated nuclear power station, since nuclear power stations are generally suitable for baseload generation and the proposed system configuration could increase the operational flexibility of such plants.

Conference paper

Herrando M, Pantaleo AM, Wang K, Markides CNet al., 2019, Solar combined cooling, heating and power systems based on hybrid PVT, PV or solar-thermal collectors for building applications, Renewable Energy, Vol: 143, Pages: 637-647, ISSN: 0960-1481

A modelling methodology is developed and used to investigate the technoeconomic performance of solar combined cooling, heating and power (S-CCHP) systems based on hybrid PVT collectors. The building energy demands are inputs to a transient system model, which couples PVT solar-collectors via thermal-store to commercial absorption chillers. The real energy demands of the University Campus of Bari, investment costs, relevant electricity and gas prices are used to estimate payback-times. The results are compared to: evacuated tube collectors (ETCs) for heating and cooling provision; and a PV-system for electricity provision. A 1.68-MWp S-CCHP system can cover 20.9%, 55.1% and 16.3% of the space-heating, cooling and electrical demands of the Campus, respectively, with roof-space availability being a major limiting factor. The payback-time is 16.7 years, 2.7-times higher than that of a PV-system. The lack of electricity generation by the ETC-based system limits its profitability, and leads to 2.3-times longer payback-time. The environmental benefits arising from the system’s operation are evaluated. The S-CCHP system can displace 911 tonsCO2/year (16% and 1.4× times more than the PV-system and the ETC-based system, respectively). The influence of utility prices on the systems’ economics is analysed. It is found that the sensitivity to these prices is significant.

Journal article

Russell AW, Kahouadji L, Mirpuri K, Quarmby A, Piccione PM, Matar OK, Luckham PF, Markides CNet al., 2019, Mixing viscoplastic fluids in stirred vessels over multiple scales: An experimental and CFD approach, Chemical Engineering Science, Vol: 208, ISSN: 1873-4405

Dye visualisation techniques and CFD are used to characterise the flow of viscoplastic CarbopolTM solutions in stirred vessel systems over multiple scales. Centrally-mounted, geometrically-similar Rushton turbine (RT) impellers are used to agitate various Carbopol 980 (C980) fluids. The dimensionless cavern diameters, Dc/D, are scaled against a combination of dimensionless parameters: Rem-0.3Rey0.6n-0.1ks-1, where Rem, Rey, n and ks are the modified power-law Reynolds number, yield stress Reynolds number, flow behaviour index and impeller geometry constant, respectively. Excellent collapse of the data is demonstrated for the fluids and flows investigated. Additional data are collected using a pitched-blade turbine (PBT) with cavern size similarity being shown between the RT and PBT datasets. These results are important in the context of scale-up/scale-down mixing processes in stirred vessels containing complex fluids and can be used to show that flow similarity can be achieved in these systems if the processes are scaled appropriately.

Journal article

Wang K, Herrando M, Pantaleo AM, Markides CNet al., 2019, Technoeconomic assessments of hybrid photovoltaic-thermal vs. conventional solar-energy systems: Case studies in heat and power provision to sports centres, Applied Energy, Vol: 254, Pages: 1-16, ISSN: 0306-2619

This paper presents a comprehensive analysis of the energetic, economic and environmental potentials of hybrid photovoltaic-thermal (PVT) and conventional solar energy systems for combined heat and power provision. A solar combined heat and power (S-CHP) system based on PVT collectors, a solar-power system based on PV panels, a solar-thermal system based on evacuated tube collectors (ETCs), and a S-CHP system based on a combination of side-by-side PV panels and ETCs (PV-ETC) are assessed and compared. A conventional CHP system based on a natural-gas-fired internal combustion engine (ICE) prime mover is also analysed as a competing fossil-fuel based solution. Annual simulations are conducted for the provision of electricity, along with space heating, swimming pool heating and hot water to the University Sports Centre of Bari, Italy. The results show that, based on a total installation area of 4000 m2 in all cases, the PVT S-CHP system outperforms the other systems in terms of total energy output, with annual electrical and thermal energy yields reaching 82.3% and 51.3% of the centre’s demands, respectively. The PV system is the most profitable solar solution, with the shortest payback time (9.4 years) and lowest levelised cost of energy (0.089 €/kWh). Conversely, the ETC solar-thermal system is not economically viable for the sports centre application, and increasing the ETC area share in the combined PV-ETC S-CHP system is unfavourable due to the low natural gas price. Although the PVT S-CHP system has the highest investment cost, the high annual revenue from the avoided energy bills elevates its economic performance to a level between those of the conventional PV and ETC-based S-CHP systems, with a payback time of 13.7 years and a levelised cost of energy of 0.109 €/kWh. However, at 445 tCO2/year, the CO2 emission reduction potential of the PVT S-CHP system is considerably higher (by 40–75%) than those of the all other solar systems (254&ndash

Journal article

Pantaleo A, Simpson M, Rotolo G, Distaso E, Oyewunmi O, Sapin P, Depalma P, Markides Cet al., 2019, Thermoeconomic optimisation of small-scale organic Rankine cycle systems based on screw vs. piston expander maps in waste heat recovery applications, Energy Conversion and Management, Vol: 200, ISSN: 0196-8904

The high cost of organic Rankine cycle (ORC) systems is a key barrier to their implementation in waste heat recovery (WHR) applications. In particular, the choice ofexpansion device has a significant influence on this cost, strongly affecting the economic viabilityof an installation. In this work, numerical simulations and optimisation strategies are used to compare the performance and profitability of small-scale ORC systems using reciprocating-piston orsingle/two-stage screw expanders whenre covering heat from the exhaust gases of a 185-kWinternal combustion engine operating in baseload mode. The study goes beyond previous work by directly comparingthese small-scaleexpanders fora broad range of working fluids, and by exploring the sensitivity of project viability to key parameters such as electricity price and onsite heat demand.For the piston expander, a lumped-massmodel and optimisation based on artificial neural networks are used to generate performance maps, while performance and cost correlations from the literature are used for the screw expanders. The thermodynamic analysisshows that two-stage screw expanders typically deliver more power than either single-stage screw or piston expanders due to their higher conversion efficiencyat the required pressure ratios. The best fluids areacetone and ethanol, as these provide a compromise between the exergy losses in the condenser and in the evaporatorin this application. The maximum net power output isfound to be 17.7kW, from an ORC engine operating withacetone anda two-stage screw expander. On the other hand, the thermoeconomic optimisation shows that reciprocating-piston expandersshow a potential for lowerspecific costs, and sincesuchan expander technology is not mature, especially at these scales, this finding motivates further consideration of this component. A minimum specific investment cost of 1630€/kW is observed for an ORC engine with a pisto

Journal article

Chatzopoulou MA, Lecompte S, De Paepe M, Markides CNet al., 2019, Off-design optimisation of organic Rankine cycle (ORC) engines with different heat exchangers and volumetric expanders in waste heat recovery applications, Applied Energy, Vol: 253, Pages: 1211-1236, ISSN: 0306-2619

Organic Rankine cycle (ORC) engines often operate under variable heat-source conditions, so maximising performance at both nominal and off-design operation is crucial for the wider adoption of this technology. In this work, an off-design optimisation tool is developed and used to predict the impact of varying heat-source conditions on ORC operation. Unlike previous efforts where the performance of ORC engine components is assumed fixed, here we consider explicitly the time-varying operational characteristics of these components. A bottoming ORC system is first optimised for maximum power output when recovering heat from the exhaust gases of an internal-combustion engine (ICE) running at full load. A double-pipe heat exchanger (HEX) model is used for sizing the ORC evaporator and condenser, and a piston-expander model for sizing the expander. The ICE is then run at part-load, thus varying the temperature and mass flow rate of the exhaust gases. The tool predicts the new off-design heat transfer coefficients in the heat exchangers, and the new optimum expander operating points. Results reveal that the ORC engine power output is underestimated by up to 17% when the off-design operational characteristics of these components are not considered. In particular, the piston-expander isentropic efficiency increases at off-design operation by 10–16%, due to the reduced pressure ratio and flow rate in the system, while the evaporator effectiveness improves by up to 15%, due to the higher temperature difference across the HEX and a higher proportion of heat transfer taking place in the two-phase evaporating zone. As the ICE operates further away from its nominal point, the off-design ORC engine power output reduces by a lesser extent than that of the ICE. At an ICE part-load operation of 60% (by electrical power), the optimised ORC engine with fluids such as R1233zd operates at 77% of its nominal capacity. ORC off-design performance maps are generated, for characterising a

Journal article

Simpson M, Chatzopoulou M, Oyewunmi O, Le Brun N, Sapin P, Markides Cet al., 2019, Technoeconomic analysis of internal combustion engine - organic Rankine cycle systems for combined heat and power in energy-intensive buildings, Applied Energy, Vol: 253, Pages: 1-13, ISSN: 0306-2619

For buildings with low heat-to-power demand ratios, installation of internal combustion engines (ICEs) for combined heat and power (CHP) results in large amounts of unused heat. In the UK, such installations risk being ineligible for the CHP Quality Assurance (CHPQA) programme and incurring additional levies. A technoeconomic optimisation of small-scale organic Rankine cycle (ORC) engines is performed, in which the ORC engines recover heat from the ICE exhaust gases to increase the total efficiency and meet CHPQA requirements. Two competing system configurations are assessed. In the first, the ORC engine also recovers heat from the CHP-ICE jacket water to generate additional power. In the second, the ORC engine operates at a higher condensing temperature, which prohibits jacket-water heat recovery but allows heat from the condenser to be delivered to the building. When optimised for minimum specific investment cost, the first configuration is initially found to deliver 20% more power (25.8 kW) at design conditions, and a minimum specific investment cost (1600 £/kW) that is 8% lower than the second configuration. However, the first configuration leads to less heat from the CHP-ICE being supplied to the building, increasing the cost of meeting the heat demand. By establishing part-load performance curves for both the CHP-ICE and ORC engines, the economic benefits from realistic operation can be evaluated. The present study goes beyond previous work by testing the configurations against a comprehensive database of real historical electricity and heating demand for thirty energy-intensive buildings at half-hour resolution. The discounted payback period for the second configuration is found to lie between 3.5 and 7.5 years for all of the buildings considered, while the first configuration is seen to recoup its costs for only 23% of the buildings. The broad applicability of the second configuration offers attractive opportunities to increase manufacturing volumes an

Journal article

Escriva EJS, Acha S, LeBrun N, Francés VS, Ojer JMP, Markides CN, Shah Net al., 2019, Modelling of a real CO2 booster installation and evaluation of control strategies for heat recovery applications in supermarkets, International Journal of Refrigeration, Vol: 107, Pages: 288-300, ISSN: 0140-7007

This paper compares and quantifies the energy, environmental and economic benefits of various control strategies recovering heat from a CO2 booster system in a supermarket for space heating with the purpose of understanding its potential for displacing natural gas fuelled boilers. A theoretical steady-state model that simulates the behaviour of the CO2 system is developed and validated against field measurements obtained from an existing refrigeration system in a food-retail building located in the United Kingdom. Five heat recovery strategies are analysed by modifying the mass flows and pressure levels in the condenser. The model shows that a reduction of 48% in natural-gas consumption is feasible by the installation of a de-superheater and without any advanced operating strategy. However, the CO2 system can fully supply the entire space-heating requirement by adopting alternative control strategies, albeit by penalising the coefficient of performance (COP) of the compressor. Results show that the best energy strategy can reduce total consumption by 32%, while the best economic strategy can reduce costs by 6%. Findings from this work suggest that heat recovery systems can bring substantial benefits to improve the overall efficiency of energy-intensive buildings; although trade-offs need to be carefully considered and further analysed before embarking on such initiatives.

Journal article

Li X, Song J, Yu G, Liang Y, Tian H, Shu G, Markides CNet al., 2019, Organic Rankine cycle systems for engine waste-heat recovery: Heat exchanger design in space-constrained applications, Energy Conversion and Management, Vol: 199, ISSN: 0196-8904

Organic Rankine cycle (ORC) systems are a promising solution for improving internal combustion engine efficiencies, however, conflicts between the pressure drops in the heat exchangers, overall thermodynamic performance and economic viability are acute in this space-constrained application. This paper focuses on the interaction of the heat exchanger pressure drop (HEPD) and the thermo-economic performance of ORC systems in engine waste-heat recovery applications. An iterative procedure is included in the thermo-economic analysis of such systems that quantifies the HEPD in each case, and uses this information to revise the cycle and to resize the components until convergence. The newly proposed approach is compared with conventional methods in which the heat exchangers are sized after thermodynamic cycle modelling and the pressure drops through them are ignored, in order to understand and quantify the effects of the HEPD on ORC system design and working fluid selection. Results demonstrate that neglecting the HEPD leads to significant overestimations of both the thermodynamic and the economic performance of ORC systems, which for some indicators can be as high as >80% in some cases, and that this can be effectively avoided with the improved approach that accounts for the HEPD. In such space-limited applications, the heat exchangers can be designed with a smaller cross-section in order to achieve a better compromise between packaging volume, heat transfer and ORC net power output. Furthermore, we identify differences in working fluid selection that arise from the fact that different working fluids give rise to different levels of HEPD. The optimized thermo-economic approach proposed here improves the accuracy and reliability of conventional early-stage engineering design and assessments, which can be extended to other similar thermal systems (i.e., CO2 cycle, Brayton cycle, etc.) that involve heat exchangers integration in similar applications.

Journal article

Charogiannis A, Markides CN, 2019, Spatiotemporally resolved heat transfer measurements in falling liquid-films by simultaneous application of planar laser-induced fluorescence (PLIF), particle tracking velocimetry (PTV) and infrared (IR) thermography, Experimental Thermal and Fluid Science, Vol: 107, Pages: 169-191, ISSN: 0894-1777

We present an optical technique that combines simultaneous planar laser-induced fluorescence (PLIF), particle tracking velocimetry (PTV) and infrared (IR) thermography for the space-and time-resolved measurement of the film-height, 2-D velocity and 2-D free-surface temperature in liquid films falling over an inclined, resistively-heated glass substrate. Using this information and knowledge of the wall temperature, local and instantaneous heat-transfer coefficients (HTCs) and Nusselt numbers, Nu, are also recovered along the waves of liquid films with Kapitza number, , and Prandtl number, . By employing this technique, falling-film flows are investigated with Reynolds numbers in the range , wave frequencies set to , 12 and 17 Hz, and a wall heat flux set to  W cm−2. Complementary data are also collected in equivalent (i.e., for the same mean-flow Re) flows with  W cm−2. Quality assurance experiments are performed that reveal deviations of up to 2-3% between PLIF/PTV-derived film heights, interfacial/bulk velocities and flow rates, and both analytical predictions and direct measurements of flat films over a range of conditions, while IR-based temperature measurements fall within 1 °C of thermocouple measurements. Highly localized film height, velocity, flow-rate and interface-temperature data are generated along the examined wave topologies by phase/wave locked averaging. The application of a heat flux ( W cm−2) results in a pronounced “thinning” of the investigated films (by 18%, on average), while the mean bulk velocities compensate by increasing by a similar extent to conserve the imposed flow rate. The axial-velocity profiles that are obtained in the heated cases are parabolic but “fuller” compared to equivalent isothermal flows, excluding any wave-regions where the interface slopes are high. As the Re is reduced, the heating applied at the wall penetrates through the film, resulting in a pronounced coupling between th

Journal article

van Kleef L, Oyewunmi O, Markides C, 2019, Multi-objective thermo-economic optimization of organic Rankine cycle (ORC) power systems in waste-heat recovery applications using computer-aided molecular design techniques, Applied Energy, Vol: 251, ISSN: 0306-2619

In this paper, we develop a framework for designing optimal organic Rankine cycle (ORC) power systems that simultaneously considers both thermodynamic and economic objectives. This methodology relies on computeraided molecular design (CAMD) techniques that allow the identification of an optimal working fluid during the thermo-economic optimization of the system. The SAFT-γ Mie equation of state is used to determine the necessary thermodynamic properties of the designed working fluids, with critical and transport properties estimated using empirical group-contribution methods. The framework is then applied to the design of sub-critical and non-recuperated ORC systems in different applications spanning a range of heat-source temperatures. When minimizing the specific investment cost (SIC) of these systems, it is found that the optimal molecular size of the working fluid is linked to the heat-source temperature, as expected, but also that the introduction of a minimum pinch point constraint that is commonly employed to account for inherent trade-offs between system performance and cost is not required. The optimal SICs of waste-heat ORC systems with heat-source temperatures of 150 °C, 250 °C and 350 °C are £10,120/kW, £4,040/kW and £2,910/kW, when employing propane, 2-butane and 2- heptene as the working fluids, respectively. During a set of MINLP optimizations of the ORC systems with heatsource temperatures of 150 °C and 250 °C, it is found that 1,3-butadiene and 4-methyl-2-pentene are the bestperforming working fluids, respectively, with SICs of £9,640/kW and £4,000/kW. These substances represent novel working fluids for ORC systems that cannot be determined a priori by specifying any working-fluid family or by following traditional methods of testing multiple fluids. Interestingly, the same molecules are identified in a multi-objective optimization considering both the total investment cost and net power output

Journal article

Cherdantsev AV, An JS, Charogiannis A, Markides CNet al., 2019, Simultaneous application of two laser-induced fluorescence approaches for film thickness measurements in annular gas-liquid flows, International Journal of Multiphase Flow, Vol: 119, Pages: 237-258, ISSN: 0301-9322

This paper is devoted to the simultaneous application of two spatiotemporally resolved optical techniques capable of liquid film thickness measurements, namely Planar Laser-Induced Fluorescence (PLIF) and Brightness-Based Laser-Induced Fluorescence (BBLIF), to co-current downward annular gas-liquid flows. A single laser sheet is used to excite the liquid film, which has been seeded with a fluorescent dye, along a longitudinal/vertical plane normal to the pipe wall. Two cameras, one for each technique, are placed at different angles to the plane of the laser sheet in order to recover, independently by the two techniques, the shape of the gas-liquid interface along this section. The effect of the angle between the laser sheet and the PLIF camera axis is also investigated. In film regions where the gas-liquid interface is smooth and flat, the conventional approach used for interpreting PLIF data is affected by total internal reflection of the fluorescent light at the free surface, or “mirror effect”, which leads to an overestimation of the film thickness that increases as the angle between the laser sheet and the camera axis is decreased. Nonetheless, local features such as light intensity maxima or minima are often located within the fluorescent signals that correctly identify the true interface, which in these conditions also coincides well with the BBLIF film-thickness measurement. When a correction for the mirror effect based on simple flat-film optical calculations is applied, this leads to PLIF results that correspond well to the true film thickness. Interestingly, it is further found that interfacial three-dimensionality, and in particular azimuthal/circumferential non-uniformity, can lead to underestimation of film thickness by PLIF that in some cases counteracts the overestimation due to the mirror effect. Smaller angles between the laser sheet and camera axis make PLIF less susceptible to this error. In regions where the film surface is rough, inc

Journal article

Li X, Tian H, Shu G, Zhao M, Markides C, Hu Cet al., 2019, Potential of carbon dioxide transcritical power cycle waste-heat recovery systems for heavy-duty truck engines, Applied Energy, Vol: 250, Pages: 1581-1599, ISSN: 0306-2619

Carbon dioxide transcritical power cycle (CTPC) systems are considered a new and particularly interesting technology for waste-heat recovery. In heavy-duty truck engine applications, challenges arise from the highly transient nature of the available heat sources. This paper presents an integrated model of CTPC systems recovering heat from a truck diesel engine, developed in GT-SUITE software and calibrated against experimental data, considers the likely fuel consumption improvements and identifies directions for further improvement. The transient performance of four different CTPC systems is predicted over a heavy-heavy duty driving cycle with a control structure comprising a mode switch module and two PID controllers implemented to realize stable, safe and optimal operation. Three operating modes are defined: startup mode, power mode, and stop mode. The results demonstrate that CTPC systems are robust and able to operate safely even when the heat sources are highly transient, indicating a promising potential for the deployment of this technology in such applications. Furthermore, a system layout with both a preheater and a recuperator appears as the most promising, allowing a 2.3% improvement in brake thermal efficiency over the whole driving cycle by utilizing 48.9% of the exhaust and 72.8% of the coolant energy, even when the pump and turbine efficiencies are as low as 50%. Finally, factor analysis suggests that important directions aimed at improving the performance and facilitating CTPC system integration with vehicle engines are: 1) ensuring long-duration operation in power mode, e.g., by employment in long-haul trucks; and 2) enhancing pump and turbine performance.

Journal article

Najjaran Kheirabadi A, Freeman J, Ramos Cabal A, Markides Cet al., Experimental Investigation of an Ammonia-Water-Hydrogen Diffusion Absorption Refrigerator, Applied Energy, ISSN: 0306-2619

Journal article

Najjaran Kheirabadi A, Freeman J, Ramos Cabal A, Markides Cet al., Experimental investigation of an ammonia-water-hydrogen diffusion absorption refrigerator, Applied Energy, ISSN: 0306-2619

Diffusion absorption refrigeration (DAR) is a small-scale cooling technology that can be driven purely by thermal energy without the need for electrical or mechanical inputs. In this work, a detailed experimental evaluation was undertaken of a newly-proposed DAR unit with a nominal cooling capacity of 100~W, aimed at solar-driven cooling applications in warm climates. Electrical cartridge heaters were used to provide the thermal input which was varied in the range 150-700 W, resulting in heat source temperatures of 175--215 C measured at the generator. The cooling output during steady-state operation was determined from the power consumed by an electric heater used to maintain constant air temperature in an insulated box constructed around the evaporator. Tests were performed with the DAR system configured with the default manufacturer's settings (22 bar charge pressure and 30 % ammonia concentration). The measured cooling output (to air) across the range of generator heat inputs was 24--108 W, while the coefficient of performance (COP) range was 0.11--0.26. The maximum COP was obtained at a generator heat input of 300 W. Results were compared to performance predictions from a steady-state thermodynamic model of the DAR cycle, showing a reasonable level of agreement at the nominal design point of system, but noteworthy deviations at part-load/off-design conditions. Temperature measurements from the experimental apparatus were used to evaluate assumptions used in the estimation of the model state point parameters and examine their influence on the predicted system performance.

Journal article

Charogiannis A, Sik An J, Voulgaropoulos V, Markides CNet al., 2019, Structured planar laser-induced fluorescence (S-PLIF) for the accurate identification of interfaces in multiphase flows, International Journal of Multiphase Flow, Vol: 118, Pages: 193-204, ISSN: 0301-9322

Annular flows are employed in numerous engineering and industrial processes relating to the chemical, oil and gas, solar and nuclear energy industries. Yet, the reliable time- and space-resolved measurement of film thickness in these flows still eludes us, as the moving and wavy interface renders the application of optical diagnostics, such as planar laser-induced fluorescence (PLIF), particularly challenging. In this research article, we present a novel adaptation of PLIF, which we refer to as structured PLIF (S-PLIF), and with which we seek to suppress the errors in PLIF-derived film thickness measurements due to total internal reflection (TIR) of the emitted fluorescence at the phase boundary. The proposed measurement approach relies on a periodic modulation of the laser-light intensity along the examined region of the flow in order to generate fluorescence images with alternating bright and dark regions. An image-processing methodology capable of recovering the location of the true gas-liquid interface from such images is presented, and the application of S-PLIF is demonstrated in liquid films in a vertical pipe over the Reynolds number range . The results from this technique are compared to simultaneously recovered, “conventional” (uncorrected) PLIF data, as well as data from other techniques over the same range of conditions, demonstrating the efficacy of S-PLIF. A comparison amongst S-PLIF data obtained with the observation angle between the laser-sheet plane and the camera’s observation axis set to and 90 ∘ is also performed, showing that the employment of is highly advantageous in avoiding distortions caused by reflections of the emitted fluorescence at the film free-surface. The instantaneous and average film-thickness uncertainties of S-PLIF are estimated to be below 10% and 5%, respectively, when measuring smooth films; an improvement over the other optical measurement techniques considered in this work. Finally, the application of S-

Journal article

Al Kindi A, Markides C, Wang K, Pantaleo Aet al., Thermodynamic assessment of steam-accumulation thermal energy storage in concentrating solar Power plants, International Conference on Applied Energy 2019

Concentrated Solar Power (CSP) plants are usually coupled with Thermal Energy Storage (TES) in order to increase the generation capacity and reduce energy output fluctuations and the levelized cost of the energy. In Direct Steam Generation (DSG) CSP plants, a popular TES option relies on steam accumulation. This conventional option, however, is constrained by temperature and pressure limits, and delivers saturated or slightly superheated steam at low pressure during discharge, which is undesirable for part-load turbine operation. However, steam accumulation can be integrated with sensible-heat storage in concrete to provide high-temperature superheated steam at higher pressures. The conventional steam accumulation option and the integrated concrete-steam option are presented, analysed and compared in this paper. The comparison shows that the integrated option provides more storage capacity by utilizing most of the available thermal power in the solar receiver. Further, the integrated option delivers higher power output with enhanced thermal efficiency for longer periods when the power plant is solely operating using the stored thermal energy. An application to the 50 MW Khi Solar One CSP plant, based on solar tower and in operation in South Africa, is proposed.

Conference paper

Song J, Simpson M, Wang K, Markides Cet al., 2019, Thermodynamic assessment of combined supercritical CO2 (SCO2) and organic Rankine cycle (ORC) systems for concentrated solar power, International Conference on Applied Energy 2019

Concentrated solar power (CSP) systems are acknowledged as a promising technology for solar energy utilisation. Supercritical CO2 (SCO2) cycle systems have emerged as an attractive option for power generation in CSP applications due to the favourable properties of CO2 as a working fluid. In order to further improve the overall performance of such systems, organic Rankine cycle (ORC) systems can be used in bottoming-cycle configuration to recover the residual heat. This paper presents a thermodynamic performance assessment of a combined SCO2/ORC system in a CSP application using parabolic-trough collectors. The parametric analysis indicates that the heat transfer fluid (HTF) temperature at the inlet of the cold tank, and the corresponding HTF mass flow rate, have a significant influence on the overall system performance. The results suggest that the combined system can offer significant thermodynamic advantages at progressively lower temperatures. Annual simulations for a case study in Seville (Spain) show that, based on an installation area of 10,000 m2, the proposed combined cycle system could deliver an annual net electricity output of 2,680 MWh when the HTF temperature at the cold tank inlet is set to 250 °C, which is 3% higher than that of a stand-alone CO2 cycle system under the same conditions. Taking the size of the thermal storage tanks into consideration, a lower HTF temperature at the cold tank inlet and a lower mass flow rate would be desirable, and the combined system offers up to 66% more power than the stand-alone version when the HTF inlet temperature is 100 °C.

Conference paper

Unamba CK, Sapin P, Li X, Song J, Wang K, Shu G, Tian H, Markides CNet al., 2019, Operational optimisation of a non-recuperative 1-kWe organic Rankine cycle engine prototype, Applied Sciences, Vol: 9, Pages: 3024-3024, ISSN: 2076-3417

Several heat-to-power conversion technologies are being proposed as suitable for waste-heat recovery (WHR) applications, including thermoelectric generators, hot-air (e.g., Ericsson or Stirling) engines and vapour-cycle engines such as steam or organic Rankine cycle (ORC) power systems. The latter technology has demonstrated the highest efficiencies at small and intermediate scales and low to medium heat-source temperatures and is considered a suitable option for WHR in relevant applications. However, ORC systems experience variations in performance at part-load or off-design conditions, which need to be predicted accurately by empirical or physics-based models if one is to assess accurately the techno-economic potential of such ORC-WHR solutions. This paper presents results from an experimental investigation of the part-load performance of a 1-kWe ORC engine, operated with R245fa as a working fluid, with the aim of producing high-fidelity steady-state and transient data relating to the operational performance of this system. The experimental apparatus is composed of a rotary-vane pump, brazed-plate evaporator and condenser units and a scroll expander magnetically coupled to a generator with an adjustable resistive load. An electric heater is used to provide a hot oil-stream to the evaporator, supplied at three different temperatures in the current study: 100, 120 and 140 ∘ C. The optimal operating conditions, that is, pump speed and expander load, are determined at various heat-source conditions, thus resulting in a total of 124 steady-state data points used to analyse the part-load performance of the engine. A maximum thermal efficiency of 4.2 ± 0.1% is reported for a heat-source temperature of 120 ∘ C, while a maximum net power output of 508 ± 2 W is obtained for a heat-source temperature at 140 ∘ C. For a 100- ∘ C heat source, a maximum exergy efficiency of 18.7 ± 0.3% is achieved. A detailed exergy analysis all

Journal article

Li X, Song J, Simpson M, Wang K, Sapin P, Shu G, Tian H, Markides Cet al., THERMO-ECONOMIC COMPARISON OF ORGANIC RANKINE AND CO2 CYCLE SYSTEMS FOR LOW-TO-MEDIUM TEMPERATURE APPLICATIONS, 5th International Seminar on ORC Power Systems

Conference paper

Le Brun N, Markides CN, 2019, A Galinstan-Filled Capillary Probe for Thermal Conductivity Measurements and Its Application to Molten Eutectic KNO3-NaNO3-NaNO2 (HTS) up to 700K (vol 36, 3222, 2015), INTERNATIONAL JOURNAL OF THERMOPHYSICS, Vol: 40, ISSN: 0195-928X

The successful measurement of the thermal conductivity of molten salts is a challenging undertaking due to the electrically conducting and possibly also aggressive nature of the materials, as well as the elevated temperatures at which these data are required. For accurate and reproducible measurements, it is important to develop a suitable experimental apparatus and methodology. In this study, we explore a modified version of the transient hot-wire method, which employs a molten-metal-filled capillary in order to circumvent some of the issues encountered in previous studies. Specifically, by using a novel flexible U-shaped quartz-capillary, filled with a eutectic mixture of gallium, indium and tin, commercially known as Galinstan, we proceed to measure the thermal conductivity of molten eutectic KNO3–NaNO3–NaNO2–NaNO2. The new probe is demonstrated as being able to measure the thermal conductivity of this molten salt, which is found to range from 0.48 W⋅m−1⋅K−1 at 500 K to 0.47 W⋅m−1⋅K−1 at close to 700 K, with an overall expanded uncertainty (95 % confidence) of 3.1 %. The quartz is found to retain its electrically insulating properties and no current leakage is detected in the sample over the investigated temperature range. The thermal conductivity data reported in the present study are also used to elucidate a partial disagreement found in the literature for this material.

Journal article

Olympios A, Pantaleo AM, Sapin P, Van Dam K, Markides Cet al., Centralised vs distributed energy systems options: District heating for the Isle of Dogs in London, ICAE2019: The 11th International Conference on Applied Energy

This work focuses on a multi-scale framework for the design and comparison of low-carbon heat generation solutions to serve the residential and commercial thermal energy demand of high energy density urban areas. The adopted methodology assesses the cost and performance of four configurations integrated in a district heating network: (i) centralised cogeneration with gas turbine and bottoming steam turbine with flexible heat-to-electricity ratio; (ii) centralised cogeneration with gas-fired internal combustion engine; (iii) distributed building-integrated ground-source heat pumps for domestic hot water only; and (iv) distributed building-integrated ground-source heat pumps for both domestic hot water and space heating. Cost and performance data were obtained by conducting relevant market research and developing a simplified heat pump thermodynamic model. The different configurations are evaluated utilizing whole-year space heating and hot water demand profiles for the Isle of Dogs area in East London, UK. Scale effects are included by considering various technology size scenarios and the results indicate that a 50 MW centralised internal combustion cogeneration system appears to be the most profitable option, while the competitiveness of building-integrated heat pumps is dependent on their size.

Conference paper

Chakrabarti A, Proeglhoef R, Bustos-Turu G, Lambert R, Mariaud A, Acha S, Markides CN, Shah Net al., 2019, Optimisation and analysis of system integration between electric vehicles and UK decentralised energy schemes, Energy, Vol: 176, Pages: 805-815, ISSN: 0360-5442

Although district heat network schemes provide a pragmatic solution for reducing the environmental impact of urban energy systems, there are additional benefits that could arise from servicing electric vehicles. Using the electricity generated on-site to power electric vehicles can make district heating networks more economically feasible, while also increasing environmental benefits. This paper explores the potential integration of electric vehicle charging into large-scale district heating networks with the aim of increasing the value of the generated electricity and thereby improving the financial feasibility of such systems. A modelling approach is presented composed of a diverse range of distributed technologies that considers residential and commercial electric vehicle charging demands via agent-based modelling. An existing district heating network system in London was taken as a case study. The energy system was modelled as a mixed integer linear program and optimised for either profit maximisation or carbon dioxide emissions minimisation. Commercial electric vehicles provided the best alternative to increase revenue streams by about 11% against the current system configuration with emissions effectively unchanged. The research indicates that district heating network systems need to carefully analyse opportunities for transport electrification in order to improve the integration, and sustainability, of urban energy systems.

Journal article

Unamba C, Li X, Song J, Wang K, Shu G, Tian H, Sapin P, Markides CNet al., Off-design performance of a 1-kWe organic Rankine cycle (ORC) system, 32nd International Conference on Efficiency, Costs, Optimization, Simulation and Environmental Impact of Energy Systems (ECOS 2019), Publisher: ECOS

Several heat-to-power conversion technologies are being proposed as suitable for waste heat recovery (WHR) applications, including thermoelectric generators, hot-air (e.g., Ericsson or Stirling) engines, and vapour-cycle engines such as steam or organic Rankine cycle (ORC) power systems. The latter has demonstrated the highest efficiencies at low and intermediate scales and heat-source temperatures. However, ORC systems suffer a deterioration in performance at part-load or off-design conditions, and the high global warming potential (GWP) or flammability of common working fluids is an increasing concern. This paper presents the experimental investigation of a 1-kWe ORC test facility under time-varying heat-source conditions. It aims to compare the part-load performance of various architectures with different working fluids, namely: (i) R245fa, which is widely used in ORC systems, and (ii) low-GWP HFOs. The experimental apparatus is composed of a rotary-vane pump, brazed-plate evaporators and condensers, and a scroll expander with an adjustable load. An electric heater is used to provide a hot oil stream at three different temperatures: 80, 100 and 120 °C. The optimal operating conditions, i.e., pump speed and expander load, are determined for each architecture at various heat-source conditions. A maximum thermal efficiency of 2.8% is reported for a heat-source temperature of 100 °C, while a maximum net power output of 430 W is obtained for a heat source at 120 °C. An exergy analysis allows us to quantify the contribution of each component to the overall exergy destruction. The share of the evaporator, condenser and expander units remain major for all three heat-source conditions, while the exergy destroyed in the pump is negligible in comparison (below 4%).

Conference paper

Voulgaropoulos V, Aguiar GM, Matar OK, Bucci M, Markides CNet al., 2019, Temperature and velocity field measurements of pool boiling using two-colour laser-induced fluorescence, infrared thermometry and particle image velocimetry, 10th International Conference on Multiphase Flow

We study nucleate pool boiling in water at saturation temperature and ambient pressure under low heat fluxes. A combinationof high-speed and spatially-resolved diagnostic tools are developed and applied to provide detailed insight into the flow andheat transfer mechanisms during bubble life cycle. Two fluorescent dyes with non-overlapping spectra are seeded into thewater and are excited by a Nd:YLF laser sheet at 527 nm. A two-colour laser-induced fluorescence method is employedto individually track the fluorescence of each dye by connecting two cameras, equipped with separate optical filters, to abeamsplitter and a lens. Tracer particles are also introduced in the water to perform simultaneous particle image velocimetrymeasurements. Finally, synchronised high-speed infrared thermometry is conducted to acquire the surface temperature fieldover the heater. The links between the interfacial/bubble dynamics, flow and heat transfer are investigated. Superheated liquidfrom the thermal boundary layer adjacent to the heater is displaced upwards, due to the growth and departure of the bubbles.Two counteracting vortices form on each side of the bubbles during their departure and rise, which contribute to the scavengingand mixing of the bulk water, resulting in a trail of superheated liquid below them.

Conference paper

Moran H, Magnini M, Markides C, Matar Oet al., 2019, Inertial and buoyancy effects on horizontal flow of elongated bubbles in circular channels, ICMF 2019, Publisher: ICMF

The effects of gravity and inertia on the liquid film thickness surrounding elongated bubble flow in a horizontal tube of circularcross-section are studied through numerical simulations. At low Reynolds and Bond numbers, the inertial and buoyancy effectsare negligible and the liquid film thickness at the tube wall is a function of the Capillary number only; if tube diameter isincreased to the millimetre scale, however, buoyancy forces become significant. Simulations are performed with OpenFOAM(version 1606) and the built in Volume-of-Fluid method for a range of Reynolds, Bond and Capillary numbers, namelyRe= 1−1000,Bo= 0.05−0.42andCa= 0.02−0.09respectively. Two-dimensional simulations capture asymmetry ofthe liquid film thickness due to gravitational effects, but do not capture bubble inclination relative to the channel centreline,as has been demonstrated experimentally in the literature. Three-dimensional simulations capture the transverse flow of thefilm as it drains from the top to the bottom of the tube, and are thus able to demonstrate bubble inclination. Further simulationsthat introduce phase change to the elongated bubble model are underway, aiming to investigate boiling flows, with experimentsbeing performed for comparison and validation.

Conference paper

Simpson M, Schuster S, Ibrahim D, Oyewunmi O, Sapin P, White A, Markides Cet al., Small-scale, low-temperature ORC systems intime-varying operation: Turbines orreciprocating-piston expanders?, 32ND INTERNATIONAL CONFERENCE ON EFFICIENCY, COST, OPTIMIZATION, SIMULATION AND ENVIRONMENTAL IMPACT OF ENERGY SYSTEMS

Conference paper

Ibrahim D, Oyewunmi O, Haslam A, Pantaleo A, Markides Cet al., Computer-aided working fluid design and optimisation of organic Rankine cycle (ORC) systems under varying heat-source conditions, 32ND INTERNATIONAL CONFERENCE ON EFFICIENCY, COST, OPTIMIZATION, SIMULATION AND ENVIRONMENTAL IMPACT OF ENERGY SYSTEMS

Conference paper

Voulgaropoulos V, Zadrazil I, Le Brun N, Bismarck A, Markides CNet al., 2019, On the link between experimentally‐measured turbulence quantities and polymer‐induced drag reduction in pipe flows, AIChE Journal, ISSN: 0001-1541

In this study, we investigate the hydrodynamics of polymer‐induced drag reduction in horizontal turbulent pipe flows. We provide spatiotemporally resolved information of velocity and its gradients obtained with particle image velocimetry (PIV) measurements in solutions of water with dissolved polyethylene oxide (PEO) of three different molecular weights, at various dilute concentrations and with flow Reynolds numbers from 35, 000 to 210, 000. We find that the local magnitudes of important turbulent flow variables correlate with the measured levels of drag reduction irrespective of the flow Reynolds number, polymer weight and concentration. Contour maps illustrate the spatial characteristics of this correlation. A relationship between the drag reduction and the turbulent flow variables is found. The effects of the polymer molecular weight, its concentration and the Reynolds number on the flow are further examined through joint probability distributions of the fluctuations of the streamwise and spanwise velocity components.

Journal article

Wang K, Markides C, 2019, Solar hybrid PV-thermal combined cooling, heating and power systems, The 5th International Conference on Polygeneration (ICP 2019), Publisher: ICP

We review hybrid photovoltaic-thermal (PV-T) technology for the combined provision of heating, coolingand power, present the state-of-the-art and outline recentprogress, including by researchers at the Clean Energy Processes (CEP) Laboratory,on aspects from component innovationto system integration,operational strategiesand assessmentsin key applications. Technologies appropriate for integration with PV-T collectors include thermal (hot and cold) and electrical storage, heat-driven heating/cooling (e.g., absorption,adsorption) and/orelectrically-driven heating/cooling (e.g., heat pump, air-conditioning)systems. Thermoeconomic assessments ofPV-Tcollectors integrated within wider solar-energy systems with such technologies inrepresentative applications have been conducted, including for energy provision to residential, commercial and public buildings, and industrial process heating applications. Studies have shown that PV-T technology has an excellent decarbonisation potential and can covera significant amount of the energy demandof end-users given reasonable areas. Further efforts relating to technology innovation and, primarily,cost reduction are required to improve its economiccompetitiveness over conventional fossil-fuel and other alternative solutions. Advanced heat-loss suppression techniques and spectral beam splitting concepts have emergedas promising directions for ground-breaking innovationin this area.

Conference paper

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