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
to

329 results found

Olympios A, Pantaleo AM, Sapin P, Van Dam K, Markides Cet al., 2019, 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

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

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

Ibrahim D, Oyewunmi O, Haslam A, Pantaleo A, Markides Cet al., 2019, 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

Simpson M, Schuster S, Ibrahim D, Oyewunmi O, Sapin P, White A, Markides Cet al., 2019, 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

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

Aguiar GM, Voulgaropoulos V, Matar OK, Markides CN, Bucci Met al., 2019, Experimental investigation of bubble nucleation, growth and departure using synchornized IR thermometry, two-colour LIF and PIV, 18th International Topical Meeting on Nuclear Reactor Thermal Hydraulics - NURETH, Publisher: American Nuclear Society (ANS)

Boiling is a very effectiveheat removal process exploited in many applications, from electronic devicesto nuclear reactors. However, the physical mechanisms involved in this process are not fully understood yet, due toits complexity, whicharises from the many interacting sub-processes involved in the nucleation, growth, and detachment of isolated bubbles. Here, we present the methodology and initialresults from an experimental investigation aimed at elucidating and quantifying the mechanisms involved in a bubble life cycle (fromnucleation until departure). Towards this aim, we use synchronized high-speed infrared(IR)thermometry, ratiometric two-color laser-induced fluorescence (2cLIF) and particle image velocimetry (PIV). Infrared thermometry is used to measure the time-dependent temperature and heat flux distributions overthe boilingsurface, which are usefulto quantify the transfer of energy associated with the evaporation of the micro-layer. Two-color laser-induced fluorescence is used to measure the time-dependent temperature distribution in the liquid phase. Particle image velocimetry is employedto measure the velocity field around the bubble, necessary to elucidate the bubble growth and departure mechanisms. The investigation also revealsother fundamental heat transferaspects such as the dynamics of the near-wall superheated liquid layer, the mixing effect produced by bubble growth and departure, as well as convection effects around the bubble.

Conference paper

Guarracino I, Freeman J, Ramos A, Kalogirou SA, Ekins-Daukes NJ, Markides CNet al., 2019, Systematic testing of hybrid PV-thermal (PVT) solar collectors in steady-state and dynamic outdoor conditions, Applied Energy, Vol: 240, Pages: 1014-1030, ISSN: 0306-2619

Hybrid photovoltaic-thermal (PVT) collectors have been proposed for the combined generation of electricity and heat from the same area. In order to predict accurately the electrical and thermal energy generation from hybrid PVT systems, it is necessary that both the steady-state and dynamic performance of the collectors is considered. This work focuses on the performance characterisation of non-concentrating PVT collectors under outdoor conditions. A novel aspect concerns the application of existing methods, adapted from relevant international standards for flat plate and evacuated tube solar-thermal collectors, to PVT collectors for which there is no formally established testing methodology at present. Three different types of PVT collector are tested, with a focus on the design parameters that affect their electrical and thermal performance during operation. Among other results, we show that a PVT collector suffers a 10% decrease in thermal efficiency when the electricity conversion is close to the maximum power point compared to open-circuit mode, and that a poor thermal contact between the PV laminate and the copper absorber can lead to a significant deterioration in thermal performance. The addition of a glass cover improves the thermal efficiency, but causes electrical performance losses that vary with the glass transmittance and the solar incidence angle. The reduction in electrical efficiency at large incidence angles is more significant than that due to elevated temperatures representative of water-heating applications. Dynamic performance is characterised by imposing a step change in irradiance in order to quantify the collector time constant and effective heat capacity. This paper demonstrates that PVT collectors are characterised by a slow thermal response in comparison to ordinary flat plate solar-thermal collectors, due to the additional thermal mass of the PV layer. A time constant of ∼8 min is measured for a commercial PVT module, compared to <

Journal article

Unamba C, Li X, Song J, Wang K, Shu G, Tian H, Sapin P, Markides CNet al., 2019, 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

Olympios A, Le Brun N, Acha Izquierdo S, Lambert R, Shah N, Markides Cet al., 2019, Installation of a dynamic controller for the optimal operation of a CHP engine in a supermarket under uncertainty, ECOS2019: 32nd International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems

This work is concerned with the integration and coordination of decentralized combined heat and power (CHP) systems in commercial buildings. Although extensive research has been performed on theoretically optimizing the design, sizing and operation of CHP systems, less effort has been devoted to an understanding of the practical challenges and the effects of uncertainty in implementing advanced algorithms to real-world applications. This paper provides details of an undergoing field trial involving the installation of a dynamic controller for the optimal operation of an existing CHP engine, which provides electricity and heat to a supermarket. The challenges in developing and applying an optimization framework and the software architecture required to implement it are discussed. Deterministic approaches that involve no measure of uncertainty provide limited useful insight to decision makers. For this reason, the methodology here develops a stochastic programming technique, which performs Monte Carlo simulations that can consider the uncertainty related to the exporting electricity price. The method involves the formation of a bi-objective function that represents a compromise between maximizing the expected savings and minimizing the associated risk. The results reveal a risk-return trade-off, demonstrating that conservative operation choices emerging from the stochastic approach can reduce risk by about 15% at the expense of a noticeably smaller reduction of about 10% in expected savings.

Conference paper

Wang K, Pantaleo AM, Herrando M, Pesmazoglou I, Franchetti B, Markides Cet al., 2019, Thermoeconomic assessment of a spectral-splitting hybrid PVT system in dairy farms for combined heat and power, The 32nd International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems (ECOS 2019)

Conference paper

Chatzopoulou MA, Simpson M, Sapin P, Markides CNet al., 2019, Off-design optimisation of organic Rankine cycle (ORC) engines with pistonexpanders for medium-scale combined heat and power applications, Applied Energy, Vol: 238, 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

Lecompte S, Chatzopoulou MA, Markides C, De Paepe Met al., 2019, Off-design comparison of subcritical and partial evaporating ORCs in quasi-steady state annual simulations, ICAE2010 - 10th International Conference on Applied Energy, Publisher: Elsevier, Pages: 2064-2069, ISSN: 1876-6102

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

Conference paper

Driker J, Juggurnath D, Kaya A, Osowade EA, Simpson M, Lecompte S, Rahim Abadi SMAN, Voulgaropoulos V, Adelaja AO, Dauhoo MZ, Khoodaruth A, Obayopo SO, Olakoyejo OT, Khalil MK, De Paepe M, Meyer JP, Markides CNet al., 2019, Thermal energy processes in direct steam generation solar systems: Boiling, condensation and energy storage, Frontiers in Energy Research, Vol: 6, ISSN: 2296-598X

Direct steam generation coupled with solar energy is a promising technology which can reduce the dependency on fossil fuels. It has the potential to impact the power-generation sector as well as industrial sectors where significant quantities of process steam are required. Compared to conventional concentrated solar power systems, which use synthetic oils or molten salts as the heat transfer fluid, direct steam generation offers an opportunity to achieve higher steam temperatures in the Rankine power cycle and to reduce parasitic losses, thereby enabling improved thermal efficiencies. However, this is associated with non-trivial challenges, which need to be addressed before such systems can become more economically competitive. Specifically, important thermal-energy processes take place during flow boiling, flow condensation and thermal-energy storage, which are highly complex, multi-scale and are multi-physics in nature that involve phase-change, unsteady and turbulent multiphase flows in the presence of conjugate heat transfer. This paper reviews our current understanding and ability to predict these processes, and knowledge that has been gained from experimental and computational efforts in the literature. In addition to Rankine cycles, organic Rankine cycle applications, which are relevant to lower operating temperature conditions, are also considered. This expands the focus to beyond water as the working fluid and includes refrigerants also. In general, significant progress has been achieved, yet there remain challenges in our capability to design and to operate effectively high-performance and low-cost systems with confidence. Of interest are the flow regimes, heat transfer coefficients and pressure drops during the thermal processes present in direct steam generation systems including those occurring in the solar collectors, condensers and relevant energy storage schemes during thermal charging and thermal discharging. A brief overview of some energy storage

Journal article

Efstratiadi M, Acha Izquierdo S, Shah N, Markides Cet al., 2019, Analysis of a closed-loop water-cooled refrigeration system in the food retail industry: A UK case study, Energy, ISSN: 0360-5442

Refrigeration in supermarkets accounts between 30% and 60% of total electricity demand in UK stores. The aim of this study is to conduct a pre-feasibility analysis of whether the use of a water-cooled configuration rejecting heat to the soil can improve the overall cooling performance of commercial refrigeration systems against air-cooled designs. In this work, a model simulating the operation of an existing refrigeration system is presented and validated against field data measurements taken from a supermarket. The examined system is used as a baseline and then modified to evaluate the impact of installing a water-cooled gas cooler. Results indicate that the use of water-cooled gas coolers has the potential to reduce electrical consumption of refrigeration systems by up to a factor of 5 when external temperatures are high. Overall, annual operation indicates the water-cooled alternative uses 3% less electricity than the air-cooled approach. A hybrid system is also considered consisting of coupled air-cooled and water-cooled units operating in parallel, for which an energy reduction of 6% is obtained compared against the baseline system. An economic evaluation of these systems shows promising results with a payback period of about 5 years for systems installed in new stores, although retrofits are costlier.

Journal article

Mazur C, Hoegerle Y, Brucoli M, Van Dam K, Guo M, Markides C, Shah Net al., 2019, A holistic resilience framework development for rural power systems in emerging economies, Applied Energy, Vol: 235, Pages: 219-232, ISSN: 0306-2619

Infrastructure and services within urban areas of developed countries have established reliable definitions of resilience and its dependence on various factors as an important pathway for achieving sustainability in these energy systems. However, the assessment, design, building and maintenance of power systems situated in rural areas in emerging economies present further difficulties because there is no a clear framework for such circumstances. Aiming to address this issue, this paper combines different visions of energy-related resilience both in general and under rural conditions in order to provide a robust practical framework for local and international stakeholders to derive the right actions in the rural context of emerging economies. An in-depth review is implemented to recompile information of resilience in general, in energy systems and in rural areas in particular, and a number of existing frameworks is also consulted. In order to acknowledge the particular circumstances and identify the important factors influencing the resilience of rural electrification in emerging economies, a holistic rural power system resilience framework is developed and presented. This consists of twenty-one indicators for technical resilience, eight indicators for social resilience, and thirteen indicators for economic resilience. This framework can be used by system owners and operators, policy makers, NGOs and communities to ensure the longevity of power systems. This work also paves the way for the creation of appropriate and effective resilience standards specifically targeted for application in these regions - aiming to achieve the delivery of global and local sustainability goals.

Journal article

Herrando Zapater M, Ramos Cabal A, Zabalza I, Markides Cet al., 2019, A comprehensive assessment of alternative absorber-exchanger designs for hybrid PVT-water collectors, Applied Energy, Vol: 235, Pages: 1583-1602, ISSN: 0306-2619

In this paper, 26 alternative absorber-exchanger designs for hybrid PV-Thermal (PVT) solar collectors are proposed and compared against a reference-case, commercial sheet-and-tube PVT collector. The collectors involve different geometric design features based on the conventional sheet-and-tube configuration, and also on a newer flat-box structure constructed from alternative polymeric materials with the aim of maintaining or even improving heat transfer and overall (thermal and electrical) performance while achieving reductions in the overall weight and cost of the collectors. The main contributions of this research include: (i) the development and validation of a detailed 3-D computational finite-element model of the proposed PVT collector designs involving multi-physics processes (heat transfer, fluid dynamics and solid mechanics); (ii) results from comparative techno-economic analyses of the proposed PVT designs; and, (iii) further insights from thermal stress and structural deformation analyses of the proposed collectors, which are crucial for ensuring long lifetimes and especially important in the case of polymeric collectors. The results show that, in general, the flat-box designs (characterised by a thin absorber plate) are not sensitive to the flow-channel size or construction material, at least within the range of investigation. A PVT collector featuring a polycarbonate (PC) flat-box design with 3 × 2 mm rectangular channels appears to be a particularly promising alternative to commercial PVT collectors, achieving a slightly improved thermal performance compared to the reference case (with a 4% higher optical efficiency and 15% lower linear heat-loss coefficient), while also lowering the weight (by around 9%) and investment cost (by about 21%) of the collector. The structural analysis shows that the maximum von Mises stress experienced in the absorber-exchanger of the PC flat-box collector is considerably lower than that in the copper sheet-and-tube c

Journal article

Franchini S, Charogiannis A, Markides CN, Blunt MJ, Krevor Set al., 2019, Calibration of astigmatic particle tracking velocimetry based on generalized Gaussian feature extraction, Advances in Water Resources, Vol: 124, Pages: 1-8, ISSN: 0309-1708

Flow and transport in porous media are driven by pore scale processes. Particle tracking in transparent porous media allows for the observation of these processes at the time scale of ms. We demonstrate an application of defocusing particle tracking using brightfield illumination and a CMOS camera sensor. The resulting images have relatively high noise levels. To address this challenge, we propose a new calibration for locating particles in the out-of-plane direction. The methodology relies on extracting features of particle images by fitting generalized Gaussian distributions to particle images. The resulting fitting parameters are then linked to the out-of-plane coordinates of particles using flexible machine learning tools. A workflow is presented which shows how to generate a training dataset of fitting parameters paired to known out-of-plane locations. Several regression models are tested on the resulting training dataset, of which a boosted regression tree ensemble produced the lowest cross-validation error. The efficiacy of the proposed methodology is then examined in a laminar channel flow in a large measurement volume of 2048, 1152 and 3000 μm in length, width and depth respectively. The size of the test domain reflects the representative elementary volume of many fluid flow phenomena in porous media. Such large measurement depths require the collection of images at different focal levels. We acquired images at 21 focal levels 150 μm apart from each other. The error in predicting the out-of-plane location in a single slice of 240 μm thickness was found to be 7 μm, while in-plane locations were determined with sub-pixel resolution (below 0.8 μm). The mean relative error in the velocity measurement was obtained by comparing the experimental results to an analytic model of the flow. The estimated displacement errors in the axial direction of the flow were 0.21 pixel and 0.22 pixel at flows rates of 1.0 mL/h and 2.5 mL/h, respectively. These resu

Journal article

Simpson M, Chatzopoulou MA, Oyewunmi O, Markides Cet al., 2019, Technoeconomic analysis of internal combustion engine – organic Rankine cycle cogeneration systems in energy-intensive buildings, 10th International Conference on Applied Energy, Publisher: Elsevier, Pages: 2354-2359, ISSN: 1876-6102

Organic Rankine cycle (ORC) systems are a promising technology for converting heat to useful power, especially in combined heat and power (CHP) applications with significant quantities of surplus heat that would otherwise be wasted. Beyond the technical performance of these systems, their economic feasibility is crucially important for their wider deployment. In this study, a technoeconomic optimisation of CHP systems is performed in which ORC engines convert heat recovered from internal combustion engines (ICEs), and specifically from both the ICE hot-water output and exhaust-gas stream. The overall aim is to evaluate the impact of the ORC power output and of the components’ design and capital cost on the financial viability of a relevant project, while evaluating a range of candidate working fluids. Results indicate that ORC designs optimised for maximum power output correspond to higher specific investment cost (SIC), with the best performing fluids achieving a SIC of £2100 per kW. In contrast, optimisation for minimum SIC returns values as low as £1700 per kW, or 20% lower. For systems designed and optimised for maximum power, a large fraction of jacket water heat is recovered, while for minimum SIC the utilisation drops to minimise the size and cost of the heat exchangers. The best-performing ORC designs for minimum SIC have discounted payback periods (DPPs) of 4 – 5 years, while those optimised for power output have DPPs of 6 – 7 years, however, the net present values (NPVs) of the latter designs are up to 27% higher than the former. Therefore, there is a trade-off to consider over the project life between high-capacity ORC engines with a high SIC and longer DPP, and designs with minimal SIC but lower power output, shorter DPP and lower NPV. The effect of increasing the amount of hot water required by the building is also analysed, and the ORC engine is shown to be sensitive to this factor for some work

Conference paper

Wang K, Herrando M, Pantaleo AM, Markides CNet al., 2019, Thermoeconomic assessment of a PV/T combined heating and power system for University Sport Centre of Bari, 10th International Conference on Applied Energy (ICAE2018), Publisher: Elsevier, Pages: 1229-1234, ISSN: 1876-6102

This paper presents a thermoeconomic analysis of a solar combined heating and power (S-CHP) system based on hybridphotovoltaic-thermal (PV/T) collectors for the University Sport Centre (USC) of Bari, Italy. Hourly demand data for space heating,swimming pool heating, hot water and electricity provision as well as the local weather data are used as inputs to a transient modeldeveloped in TRNSYS. Economic performance is evaluated by considering the investment costs and the cost savings due to thereduced electricity and natural gas consumptions. The results show that 38.2% of the electricity demand can be satisfied by thePV/T S-CHP system with an installation area of 4,000 m2. The coverage increases to 81.3% if the excess electricity is fed to thegrid. In addition, the system can cover 23.7% of the space heating demand and 53.8% of the demands for the swimming pool andhot water heating. A comparison with an equivalent gas-fired internal combustion engine (ICE) CHP system shows that the PV/Tsystem has a higher payback time, i.e., 11.6 years vs. 3 years, but outperforms the ICE solution in terms of CO2 emission reduction,i.e., 435 tons CO2/year vs. 164 tons CO2/year. This work suggests that the proposed PV/T S-CHP system has a good potential ofdecarbonisation, while the economic competitiveness should be further enhanced to boost its deployment.

Conference paper

Harraz AA, Freeman J, Wang K, Mac Dowell N, Markides CNet al., 2019, Diffusion-absorption refrigeration cycle simulations in gPROMS using SAFT-γ Mie, Energy Procedia, Vol: 158, Pages: 2360-2365, ISSN: 1876-6102

Diffusion-absorption refrigeration (DAR) is a clean thermally-powered refrigeration technology that can readily be activated by low- to medium-grade renewable heat. There is an ongoing interest in identifying or designing new working fluids for performance improvement, particularly in solar applications with non-concentrating solar collectors providing heat at temperatures < 150 °C. In this work, the state-of-the-art statistical associating fluid theory (SAFT) is adopted for predicting the thermodynamic properties of suitable DAR working fluids. A first-law thermodynamic analysis is performed in the software environment gPROMS for a DAR cycle using ammonia as the refrigerant, water as the absorbent and hydrogen as the auxiliary gas. The simulation results show good agreement with experimental data generated in a prototype DAR system with a nominal cooling capacity of 100 W. In particular, at a charge pressure of 17 bar and when delivering cooling at 5 °C, the model results agree with experimental COP data to within ± 7 % over a range of heat inputs from 150 to 500 W. The maximum coefficient of performance (COP) is estimated to be 0.24 at a heat input of 250 W. The group-contribution SAFT-γ Mie equation of state is of particular interest as it offers good agreement with experimental data and provides flexibility in extending the model to test different working fluids with a high degree of fidelity. A methodology is also presented that allows the DAR thermodynamic analysis and working-fluid modelling to be integrated into a more general technology optimisation framework.

Journal article

Chatzopoulou MA, Lecompte S, De Paepe M, Markides Cet al., 2019, Off-design operation of ORC engines with different heat exchanger architectures in waste heat recovery applications, ICAE2018 - 10th International Conference on Applied Energy, Publisher: Elsevier, Pages: 2348-2353, ISSN: 1876-6102

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

Conference paper

Ibrahim D, Oyewunmi O, Haslam A, Markides Cet al., 2019, COMPUTER-AIDED WORKING FLUID DESIGN AND POWER SYSTEM OPTIMIZATION USING THE SAFT-γ MIE EQUATION OF STATE, 4th Thermal and Fluids Engineering Conference (TFEC)

Conference paper

Juggurnath D, Dauhoo MZ, Elahee MK, Khoodaruth A, Dirker J, Meyer JP, Markides CNet al., 2019, Numerical simulations of condensing R134a flows in horizontal pipes, 4th ASTFE Thermal and Fluids Engineering Conference (TFEC), Publisher: Begell House, Pages: 413-422

Three-dimensional numerical simulations were performed of condensing R134a flows in a smooth horizontal pipe with an inner diameter of 8.4 mm and a length of 1.5 m, and validated against experimental results. A constant mass flux of 100 kg m-2 s-1 was considered and the influence of vapour qualities (0.25 to 0.75) and saturation temperatures (30 ?C and 40 ?C) on the resulting flow regimes and heat transfer characteristics of these flows were investigated. The volume-of-fluid (VOF) method was employed in the numerical framework to track and reconstruct the interface between the liquid and vapour phases. The simulations, given the imposed flow conditions, produced stratified wavy flow which are in agreement with the expected flow pattern based on the El Hajal flow pattern map. The heat transfer coefficient in the numerically simulated flows were found to be in good agreement (within 1.3%) with corresponding experimentally-measured values. From the simulations, the liquid-phase height at the bottom of the pipe was observed to be smaller with increasing vapour quality, which results in an increase in the heat transfer coefficient. A thicker film thickness and lower heat transfer coefficient were noted at the higher saturation temperature.

Conference paper

Harraz AA, Najjaran A, Sacks R, Freeman J, Olympios AV, Mac Dowell N, Markides CNet al., 2019, Experimentally validated simulations of a diffusion absorption refrigeration system, Pages: 1045-1056

© ECOS 2019 - Proceedings of the 32nd International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems. All rights reserved. Diffusion absorption refrigeration (DAR) is a small-scale, thermally-driven cooling technology that operates passively without the need for mechanical or electrical inputs. Due to the lack of a compressor, DAR systems are charged with an auxiliary gas to enable single-pressure operation. Although DAR units have a simple construction and are easy to operate, their modelling presents challenges arising from the complexity of the physical processes that take place and govern operation. Few experimentally validated models offer a reliable prediction of the DAR system performance over a wide range of operating conditions. This paper combines results from experimental investigations on a laboratory-scale ammonia-water-hydrogen DAR system with a nominal cooling output of ∼100 W with model predictions of the performance characteristics of this system. In previous work, the DAR cycle was modelled using first-law thermodynamic analysis in the gPROMS environment for a single cooling delivery temperature and a single-charge pressure, using a group-contribution equation-of-state based on the statistical associating fluid theory (SAFT). Here, extended model validation is performed to investigate the effect of key operating parameters on system performance, including the generator heat input (varied from 150 W to 700 W), the cooling delivery temperature (set to two levels: 5 ◦C and 23 ◦C) and the system charge pressure (18 bar and 21 bar). The measured coefficient of performance (COP) was between 0.02 and 0.29. The present model predicts the maximum COP well over the heat-input range from 250 W to 550 W. Hence, the model shows good agreement with experiments, particularly when the heat-input rate is at or below the system’s design-point. Conclusions are drawn concerning the ability of models to

Conference paper

Charogiannis A, Denner F, Van Wachem B, Benoit S, Kalliadasis S, Markides Cet al., 2018, Experimental investigations of liquid falling films flowing under an inclined planar substrate, Physical Review Fluids, Vol: 3, ISSN: 2469-990X

We report on detailed and systematic experiments of thin liquid films flowing as a result of the action of gravity under an inverted planar substrate. A measurement technique based on planar laser-induced fluorescence (PLIF) was developed and applied to a range of such flows in order to provide detailed space- and time-resolved film-height information. Specifically, the experimental campaign spanned three inclination angles (β=−15∘, −30∘, and −45∘, in all cases negative with respect to the vertical), two water-glycerol solutions (with Kapitza numbers of Ka=13.1 and 330), and flow Reynolds numbers covering the range Re=0.6–193. The collection optics were arranged so as to interrogate a spanwise section of the flow extending about 40mm symmetrically on either side the centerline of the film span (80mm in total), at a distance 330 mm downstream of the flow inlet. A range of flow regimes, typically characterized by strong three dimensionality and pronounced rivulet formation, were observed depending on the imposed inlet flow conditions. In the lower liquid Kapitza number Ka(=13.1) flows and depending on the flow Re, the free surface of the film was populated by smooth rivulets or regular sequences of solitary pulses that traveled over the rivulets. In the higher liquid Ka(=330) flows, rivulets were observed typically above Re≈30, depending also on the inclination angle, and grew in amplitude until quasi-two-dimensional fronts developed intermittently that were associated with distinct thin-film regions of varying length and frequency. These regions are of particular interest as they are expected to affect strongly the heat and mass transfer capabilities of these flows. The occurrence of the fronts was more pronounced, with higher wave frequencies, in film flows at smaller negative inclinations for the same flow Re. The rivulet amplitude was found to increase at larger inclinations for the same Re and showed a nonmonotonic trend with in

Journal article

Riverola A, Mellor A, Alvarez DA, Llin LF, Guarracino I, Markides CN, Paul D, Chemisana D, Ekins-Daukes Net al., 2018, Experimental and theoretical study of the infrared emissivity of crystalline silicon solar cells, IEEE 44th Photovoltaic Specialist Conference (PVSC), Publisher: IEEE, Pages: 1339-1341, ISSN: 0160-8371

Conference paper

Sacks RE, Markides C, 2018, Compressed air energy storage – a new heat-integration, liquid-compression approach, Energy Learning

Journal article

Herrando M, Ramos A, Freeman J, Zabalza I, Markides CNet al., 2018, Technoeconomic modelling and optimisation of solar combined heat and power systems based on flat-box PVT collectors for domestic applications, Energy Conversion and Management, Vol: 175, Pages: 67-85, ISSN: 0196-8904

We investigate solar combined heat and power (S-CHP) systems based on hybrid photovoltaic-thermal (PVT) collectors for the simultaneous provision of domestic hot water (DHW), space heating (SH) and power to single- family homes. The systems include PVT collectors with a polycarbonate flat-box structure design, a water storage tank, an auxiliary heater and a battery storage subsystem. A methodology is developed for modelling the en- ergetic and economic performance of such PVT-based S-CHP systems, which is used to optimally size and operate systems for covering the energy demands of single-family reference households at three selected locations: Athens (Greece), London (UK) and Zaragoza (Spain). The results show that optimised systems are capable of covering ∼65% of the annual household electricity demands in Athens, London and Zaragoza when employing 14.0, 17.0 and 12.4 m2 collector array areas respectively, while also covering a significant fraction of the thermal energy demands in Athens (∼60%) and Zaragoza (∼45%); even in London, almost 30% of the reference household’s thermal demand is covered by such a system. A corresponding economic analysis reveals that, despite the suitability of Athens’ weather conditions for implementing such solar-energy systems, the payback time (PBT) of the optimised S-CHP system in Athens is 15.6 years in contrast to the 11.6 years predicted for Zaragoza, due to the lower electricity prices in Greece. On the other hand, the high carbon emission factor of the electricity grid in Greece makes these systems particularly promising at this location. Specifically, the in- vestigated systems have the potential to displace 3.87, 1.65 and 1.54 tons of CO2 per year in Athens, London and Zaragoza, when substituting the conventional means for household energy provision (i.e. grid electricity and gas- fired boilers). Furthermore, it is demonstrated that the optimised systems outperform benchmark equivalent systems comprisin

Journal article

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