459 results found
Aunedi M, Olympios AV, Pantaleo AM, et al., 2023, System-driven design and integration of low-carbon domestic heating technologies, Renewable and Sustainable Energy Reviews, Vol: 187, ISSN: 1364-0321
This research explores various combinations of electric heat pumps (EHPs), hydrogen boilers (HBs), electric boilers (EBs), hydrogen absorption heat pumps (AHPs) and thermal energy storage (TES) to assess their potential for delivering cost-efficient low-carbon heat supply. The proposed technology-to-systems approach is based on comprehensive thermodynamic and component-costing models of various heating technologies, which are integrated into a whole-energy system optimisation model to determine cost-effective configurations of heating systems that minimise the overall cost for both the system and the end-user. Case studies presented in the study focus on two archetypal systems: (i) the North system, which is characterised by colder climate conditions and abundant wind resource; and (ii) the South system, which is characterised by a milder climate and higher solar energy potential. The results indicate a preference for a portfolio of low-carbon heating technologies including EHPs, EBs and HBs, coupled with a sizable amount of TES, while AHPs are not chosen, since, for the investigated conditions, their efficiency does not outweigh the high investment cost. Capacities of heat technologies are found to vary significantly depending on system properties such as the volume and diversity of heat demand and the availability profiles of renewable generation. The bulk of heat (83–97%) is delivered through EHPs, while the remainder is supplied by a mix of EBs and HBs. The results also suggest a strong impact of heat demand diversity on the cost-efficient mix of heating technologies, with higher diversity penalizing EHP relatively more than other, less capital-intensive heating options.
Zhu S, Wang T, Jiang C, et al., 2023, Experimental and numerical study of a liquid metal magnetohydrodynamic generator for thermoacoustic power generation, Applied Energy, Vol: 348, ISSN: 0306-2619
Combining a thermoacoustic cycle engine with a liquid metal magnetohydrodynamic (LMMHD) generator will result in a thermal power generation system with no mechanical moving parts and high reliability. This disruptive technology has drawn much attention in space nuclear power generation, especially in recent years. It requires an LMMHD generator to work at a higher frequency than conventional LMMHD generators targeted for ocean wave energy conversion. However, the operating characteristics and loss mechanisms of LMMHD generators at high operating frequencies remain poorly understood, and experimental characterization of such a generator is lacking. In this work, a three-dimensional transient numerical analysis of a high-frequency LMMHD generator is performed based on multi-physics field simulation software COMSOL, to understand the operating characteristics of the generator, and the effects of inlet velocity, load resistance, and operating frequency on the generator’s performance. Furthermore, an LMMHD generator prototype was designed, constructed, and tested under different inlet velocities, load resistances, and frequencies by using a linear compressor for the first time. When the operating frequency and inlet velocity are 15 Hz and 4.3 m/s, the output voltage and current of the generator prototype reached 113 mV and 1720 A, with an output power of 68 W at a corresponding acoustic-to-electric efficiency of 24 %. A discrepancy between the numerical predictions and the experimental results was found, which gave insight into where further improvements can be made. This work reveals the operating characteristics and losses mechanism of LMMHD generators operating at higher frequencies and contributes to the development of high-efficiency generators for thermoacoustic power generation.
Maghrabi AM, Song J, Markides CN, 2023, How can industrial heat decarbonisation be accelerated through energy efficiency?, Applied Thermal Engineering, Vol: 233, ISSN: 1359-4311
The ongoing energy transition necessitates commitments from various sectors to utilise resources more efficiently. Amongst these, the industrial sector, which is associated with high energy and resource consumption and emissions, has been attracting attention specifically aimed at performance enhancements and continuous progress in energy utilisation. The continued evolution of industrial operations and performance requires energy efficiency measures to be developed and implemented. Diverse portfolios of products, wide-ranging types of equipment, processes and, subsequently, plants, are adopted in the industrial sector, such that energy efficiency measures vary widely, along with their effectiveness, technological maturity, technical and economic impact. It remains a challenge to select the optimal energy efficiency measure(s) for a specific industry, plant and process, given the specific asset requirements. In this context, the development of systematic approaches for identifying optimal energy efficiency measures is of great interest. In this vision paper, we present an assembly of available systematic tools for advancing the energy efficiency of plants and sites in the industrial sector. The contribution of this work to the field of industrial heat decarbonisation arises from developing and proposing the use of a new holistic framework as a guide for the continuous performance improvement of thermal-energy-intensive industries through a series of energy efficiency measures and actions. Specifically, the framework suggests initiating efforts from a proposed top-down peer benchmarking practice aimed at identifying gaps in energy-efficiency performance across products, plants, processes and equipment. In a second stage, recommendations are made in form of a list of steps to close these gaps, starting with conducting equipment gap closure analyses, followed by design improvement studies at the process, plant and site levels using tools such as pinch analysis, steam s
Maghrabi A, Song J, Sapin P, et al., 2023, Electricity demand reduction through waste heat recovery in olefins plants based on a technology-agnostic approach, Energy Conversion and Management: X, Vol: 20, Pages: 1-18, ISSN: 2590-1745
Developing systematic approaches for the identification of optimal WHR options in industrial applications is key to reducing plant-scale energy demands. In particular, electricity consumption accounts for more than half of industrial energy use, and its share is expected to grow with progressive electrification. In this paper, industrial WHR technologies including organic Rankine cycle (ORC) and absorption systems are investigated, and tools are developed to understand the sustainability and techno-economic impact of integrating these technologies within industrial processes and facilities. We specifically propose a data-driven technology-agnostic approach to evaluate the use of heat engines, which can in practice be ORC systems, and thermally-driven (i.e., absorption) heat pumps in the context of industrial WHR for plant-scale electricity demand reduction. The aim of this work is to explore three pathways for achieving efficiency improvements in bulk chemicals plants, represented here by olefins production facilities: (i) direct onsite power generation; (ii) enhancement of existing power generation processes; and (iii) reduction in power consumption by compressor efficiency improvements through waste-heat-driven cooling. The techno-economic performance of these technologies is assessed for five different countries representing a diverse portfolio of climates, technical and economic parameters (including utility prices), using fine-tuned thermodynamic and market-based costing models. The results reveal that the proposed approach has the potential to reduce emissions by between 5,000 tCO2(eq.)/year and 101,500 tCO2(eq.)/year depending on the scenario. The marginal abatement cost of the proposed solutions ranges from -1,200 $/tCO2(eq.) to -35 $/tCO2(eq.), with a payback time between 1.5 and 8 years depending on the scenario considered.
Jamil MA, Shahzad MW, Xu BB, et al., 2023, Energy-efficient indirect evaporative cooler design framework: An experimental and numerical study, Energy Conversion and Management, Vol: 292, ISSN: 0196-8904
A remarkable surge in cooling demand is observed in the last decades. Currently, the cooling market is dominated by mechanical vapor compression chillers which are energy intensive and use harmful chemical refrigerants. Therefore, the current focus of the current research in cooling is the development of unconventional, sustainable cooling systems. In this regard, indirect evaporative coolers have shown significant potential (particularly under hot-dry climates) with high energy efficiency, low cost, water-based sustainable operation, and benign emissions. However, these systems are in the development stage and have not yet been fully commercialized because of certain design challenges. An innovative indirect evaporative cooler is proposed, fabricated, and experimentally tested in this study. Particularly, the study is focused on the development of heat transfer coefficient correlation for the system for commercial-scale design and expansion. This is because the earlier available correlation is based on simple airflow between parallel plates assumption and does not incorporate the effect of the evaporative potential of the system resulting in under/over-estimation of the heat transfer characteristics. The results showed that the proposed system achieved a temperature drop of 20 °C, a cooling capacity of around 180 W, and an overall heat transfer coefficient of up to 30 W/m2K. Moreover, the study presents an experiment-regression-based heat transfer coefficient correlation that satisfactorily captures the effect of outdoor air temperature and airflow rate ratio which are critical in the design of evaporative coolers. The proposed correlation showed a high (±5%) with experimental data thus making it suitable for the future design of IEC systems over assorted operating scenarios.
Hoseinpoori P, Hanna R, Woods J, et al., 2023, Comparing alternative pathways for the future role of the gas grid in a low-carbon heating system, Energy Strategy Reviews, Vol: 49, Pages: 1-25, ISSN: 2211-467X
This paper uses a whole-system approach to examine different strategies related to the future role of the gas grid in alow-carbon heat system. A novel model of integrated gas, electricity and heat systems, HEGIT, is used to investigate fourkey sets of scenarios for the future of the gas grid using the UK as a case study: a) complete electrification of heating; b)conversion of the existing gas grid to deliver hydrogen; c) a hybrid heat pump system; and d) a greener gas grid. Ourresults indicate that although the infrastructure requirements, the fuel or resource mix, and the breakdown of costs varysignificantly over the complete electrification to complete conversion of the gas grid to hydrogen spectrum, the total systemtransition cost is relatively similar. This reduces the significance of total system cost as a guiding factor in policy decisionson the future of the gas grid. Furthermore, we show that determining the roles of low-carbon gases and electrification fordecarbonising heating is better guided by the trade-offs between short- and long-term energy security risks in the system,as well as trade-offs between consumer investment in fuel switching and infrastructure requirements for decarbonisingheating. Our analysis of these trade-offs indicates that although electrification of heating using heat pumps is not thecheapest option to decarbonise heat, it has clear co-benefits as it reduces fuel security risks and dependency on carboncapture and storage infrastructure. Combining different strategies, such as grid integration of heat pumps with increasedthermal storage capacity and installing hybrid heat pumps with gas boilers on the consumer side, are demonstrated toeffectively moderate the infrastructure requirements, consumer costs and reliability risks of widespread electrification.Further reducing demand on the electricity grid can be accomplished by complementary options at the system level, suchas partial carbon offsetting using negative emission technologies
Aunedi M, Al Kindi AA, Pantaleo AM, et al., 2023, System-driven design of flexible nuclear power plant configurations with thermal energy storage, Energy Conversion and Management, Vol: 291, Pages: 1-14, ISSN: 0196-8904
Nuclear power plants are expected to make an important contribution to the decarbonisation of electricity supply alongside variable renewable generation, especially if their operational flexibility is enhanced by coupling them with thermal energy storage. This paper presents a system modelling approach to identifying configurations of flexible nuclear plants that minimise the investment and operation costs in a decarbonised energy system, effectively proposing a system-driven design of flexible nuclear technology. Case studies presented in the paper explore the impact of system features on plant configuration choices. The results suggest that cost-efficient flexible nuclear configurations should adapt to the system they are located in. In the main low-carbon scenarios and assuming standard-size nuclear power plants (1,610 MWel), the lowest-cost system configuration included around 500 MWel of additional secondary generation capacity coupled to the nuclear power plants, with 4.5 GWhth of thermal storage capacity and a discharging duration of 2.2 h. Net system benefits per unit of flexible nuclear generation for the main scenarios were quantified at £29-33 m/yr for a wind-dominated system and £19-20 m/yr for a solar-dominated system.
Herrando M, Wang K, Huang G, et al., 2023, A review of solar hybrid photovoltaic-thermal (PV-T) collectors and systems, Progress in Energy and Combustion Science, Vol: 97, Pages: 1-74, ISSN: 0360-1285
In this paper, we provide a comprehensive overview of the state-of-the-art in hybrid PV-T collectors and the wider systems within which they can be implemented, and assess the worldwide energy and carbon mitigation potential of these systems. We cover both experimental and computational studies, identify opportunities for performance enhancement, pathways for collector innovation, and implications of their wider deployment at the solar-generation system level. First, we classify and review the main types of PV-T collectors, including air-based, liquid-based, dual air–water, heat-pipe, building integrated and concentrated PV-T collectors. This is followed by a presentation of performance enhancement opportunities and pathways for collector innovation. Here, we address state-of-the-art design modifications, next-generation PV cell technologies, selective coatings, spectral splitting and nanofluids. Beyond this, we address wider PV-T systems and their applications, comprising a thorough review of solar combined heat and power (S–CHP), solar cooling, solar combined cooling, heat and power (S–CCHP), solar desalination, solar drying and solar for hydrogen production systems. This includes a specific review of potential performance and cost improvements and opportunities at the solar-generation system level in thermal energy storage, control and demand-side management. Subsequently, a set of the most promising PV-T systems is assessed to analyse their carbon mitigation potential and how this technology might fit within pathways for global decarbonization. It is estimated that the REmap baseline emission curve can be reduced by more than 16% in 2030 if the uptake of solar PV-T technologies can be promoted. Finally, the review turns to a critical examination of key challenges for the adoption of PV-T technology and recommendations.
Tafuni A, Giannotta A, Mersch M, et al., 2023, Thermo-economic analysis of a low-cost greenhouse thermal solar plant with seasonal energy storage, Energy Conversion and Management, Vol: 288, Pages: 1-11, ISSN: 0196-8904
Reduction of greenhouse gas emissions is today mandatory to limit the increase of ambient temperature. This paper provides a numerical study of a thermal solar plant using a seasonal dual-media sensible heat thermal energy storage system for supplying the total energy demand of a greenhouse located in the South of Italy, avoiding the use of the gas boiler. The aim of the work is to assess the technical and economic performance of a low-cost pit storage system, made of gravel and water, placed under the greenhouse to save surface. The study provides an original analysis of the charging and discharging phases during one year of operation on the basis of the real hourly heating demand and on real weather data. A sensitivity analysis of the levelized cost of heat is carried on with respect to the solar-collector area and to the storage-pit volume. The analysis shows that a minimum-cost design solution exists to cover 100% of the heat demand with an estimated levelized cost of heat of 153.3 EUR/MWh. The results demonstrate that dual-media thermal energy storage systems with solar thermal collectors represent a viable solution for reducing the environmental impact of greenhouses.
Mersch M, Sapin P, Olympios AV, et al., 2023, A unified framework for the thermo-economic optimisation of compressed-air energy storage systems with solid and liquid thermal stores, Energy Conversion and Management, Vol: 287, Pages: 1-15, ISSN: 0196-8904
Compressed-air energy storage is an attractive option for satisfying the increasing storage demands of electricity grids with high shares of renewable generation. It is a proven technology that can store multiple gigawatt hours of electricity for hours, days and even weeks at a competitive cost and efficiency. However, compressed–air energy storage plants need to be designed carefully to deliver these benefits. In this work, a consistent thermo-economic optimisation framework is applied to assess the performance and costs of different compressed–air energy storage configurations across different scales. Special attention is paid to the thermal energy stores, with both solid packed-bed stores and liquid stores examined as viable options for advanced compressed–air energy storage plants and different storage materials proposed for both options. The comprehensive thermo-economic optimisation, considering different system layouts, thermal energy storage technologies and storage materials, and system scales is a key novelty of the presented work. A configuration with two packed–bed thermal energy stores using Basalt as the storage material is found to perform best, achieving an energy capital cost of 140 $/kWh, a power capital cost of 970 $/kW and a roundtrip efficiency of 76% at a nominal discharge power of 50 MW and a charging / discharging duration of 6 h. The best-performing liquid storage material is solar salt, which is associated with an energy capital cost of 170 $/kWh and a power capital cost of 1,230 $/kW. Systems with liquid thermal energy stores however are found generally to perform worse than systems with packed–bed thermal energy stores both in terms of cost and efficiency across all scales.
Nemati H, Moghimi MA, Markides CN, 2023, Heat transfer characteristics of thermally and hydrodynamically developing flows in multi-layer mini-channel heat sinks, International Journal of Heat and Mass Transfer, Vol: 208, Pages: 1-12, ISSN: 0017-9310
In this study, a novel type of air-cooled heat sink is proposed, which consists of several layers of mini-channels. In this design, the hydraulic diameter is smaller than in conventional types of heat sinks, such as plate-fin heat sinks, and consequently, the achievable heat transfer rates are higher. To predict the cooling performance of this heat sink, an innovative analytical method is proposed, results from which are complemented by an extensive number of numerical simulations of the simultaneously developing flows, thermally and hydrodynamically, inside a rectangular channel of the heat sink. The results of the analytical method are compared against two- and three-dimensional simulations and good agreement is found, while from the simulations, correlations are proposed for the Nusselt number in these flows. Finally, the performance of the proposed heat sink is compared to a plate-fin heat sink. This comparison reveals that entropy generation in the latter is around 27% higher than in the former, and suggests a promising advantage of the proposed heat sink design.
Huang G, Xu J, Markides C, 2023, High-efficiency bio-inspired hybrid multi-generation photovoltaic leaf, Nature Communications, Vol: 14, ISSN: 2041-1723
Most solar energy incident (>70%) upon commercial photovoltaic panelsis dissipated as heat, increasing their operating temperature, and leading to significant deterioration in electrical performance. The solar utilization efficiency of commercial photovoltaic panels is typically below 25%. Here, we demonstrate a hybrid multi-generation photovoltaic leaf concept that employs a biomimetic transpiration structure made of eco-friendly, low-cost and widely-available materials for effective passive thermal management and multi-generation. We demonstrate experimentally that bio-inspired transpiration can remove~590 W/m2 of heat from a photovoltaic cell, reducing the cell temperature by ~26 °C under an irradianceof 1000 W/m2, and resulting in a 13.6% increase in electrical efficiency. Furthermore, the photovoltaic leaf is capable of synergistically utilizing the recovered heat to co-generate additional thermal energy and freshwater simultaneously within the same component, significantly elevating the overall solarutilization efficiency from 13.2% to over 74.5%, along with over 1.1 L/h/m2 of clean water.
Kamel MA, Lobasov AS, Lakshmi Narayanan SN, et al., 2023, Hydrate growth over a sessile drop of water in cyclopentane, Crystal Growth and Design, Vol: 23, Pages: 4273-4284, ISSN: 1528-7483
Liquid cyclopentane is frequently used in hydrate formation studies as an analogue of natural gas because cyclopentane hydrates are stable above the ice melting point at ambient pressure. In this study, hydrate growth was established on a sessile water drop of 11 mm in diameter and 4.5 mm in height (volume of 0.25 mL) immersed in liquid cyclopentane. The hydrate formation mechanism and growth processes were observed optically over an extended range of subcooling temperatures from 5.1 to 15.2 °C, with the cyclopentane bulk temperature maintained in different experimental runs between 2.6 and −7.5 °C. Qualitative and quantitative comparisons were performed to confirm the absence of ice freezing during hydrate formation, and thus, the lack of contamination of the latter from the former in the experiments. Different transformations in the hydrate film morphology were registered from macroscopic observation over the considered range of subcooling temperatures, with the hydrate crystals composing the film taking the form of polyhedral, dendritic, or spherulitic structures. It was also found that the hydrate growth rate varied depending on the subcooling temperature, with the variation of the growth rate as a function of this temperature changing from a power to an (approximately) linear law with an increase in the degree of subcooling. We postulate that hydrate film growth can be governed by different mechanisms, whose roles change over the range of explored subcooling temperatures.
Iqbal Q, Fang S, Zhao Y, et al., 2023, Thermo-economic assessment of sub-ambient temperature pumped-thermal electricity storage integrated with external heat sources, Energy Conversion and Management, Vol: 285, Pages: 1-14, ISSN: 0196-8904
Thermally integrated pumped-thermal electricity storage (TI-PTES) offers the opportunity to store electricity as thermal exergy at a large scale, and existing studies are primarily focused on TI-PTES systems based on high-temperature thermal energy storage. This paper presents a thermo-economic analysis of a “cold TI-PTES” system which converts electricity into cold energy using a vapor compression refrigeration (VCR) unit and stores it at sub-ambient temperatures during the charging process, and generates electricity by using an organic Rankine cycle (ORC) working between the sub-ambient temperature and an external low-grade heat source during the discharging process. The effects of key parameters, i.e., mass flowrate and temperature of the storage medium, ORC evaporation temperature, component efficiencies, and pinch-point temperature differences, on the system performance are evaluated based on a whole-system thermo-economic model. The results reveal that the roundtrip efficiency and levelized cost of storage (LCOS) of the system increases while the electrical energy storage capacity decreases as the temperatures of the two cold storage tanks approach each other. When the temperature of the cold storage tank 1 rises from 1 °C to 8 °C while the cold storage tank 2 remains as 13 °C, there is an increase of 25% and 20% in the roundtrip efficiency and LCOS respectively while the energy storage capacity decreases by 69%. A roundtrip efficiency of 0.74 and LCOS of 0.32 $/kWh are achieved with a heat source temperature of 85 °C, using a mass flowrate and temperature of the cold storage medium of 50 kg/s and 1 °C. Furthermore, any change in cold storage medium mass flowrate changes both electrical energy storage capacity and power output by the same proportions. With a continuous high-flowrate external heat source, the LCOS can be as low as 0.17 $/kWh. By providing sufficient heat from an external heat source, the proposed system possesses
Zhu S, Wang K, González-Pino I, et al., 2023, Techno-economic analysis of a combined heat and power system integrating hybrid photovoltaic-thermal collectors, a Stirling engine and energy storage, Energy Conversion and Management, Vol: 284, ISSN: 0196-8904
This paper presents a comprehensive analysis of the energetic, economic and environmental performance of a micro-combined heat and power (CHP) system that comprises 29.5 m2 of hybrid photovoltaic-thermal (PVT) collectors, a 1-kWe Stirling engine (SE) and energy storage. First, a model for the solar micro-CHP system, which includes a validated transient model for the SE micro-CHP unit, is developed. Parametric analyses are performed throughout a year to evaluate the effects of key component sizes and operating parameters, including collector flow rate, storage tank size, SE micro-CHP flow rate, and battery capacity, on the energetic, economic and environmental performance of the proposed system using real hourly weather data, and thermal and electrical energy demand profiles of a detached house located in London (UK). The optimum component sizes and operating parameters are determined accordingly. The daily and monthly operating characteristics of the system are evaluated, and its annual performance is compared to those of a reference system (gas boiler plus grid electricity), as well as of other alternative solar-CHP systems including a PVT-assisted heat pump system and a standalone PVT system. The results indicate that the installation of such a system can achieve an annual electricity self-sufficiency of 87% and an annual thermal energy demand coverage of 99%, along with annual primary energy savings and carbon emission reduction rate of 35% and 37% relative to the reference system. Over 30 years of operation, the net present value (NPV) of the proposed system is £1990 and the discounted payback period is 28 years. The economics of the proposed system is very sensitive to utility prices, especially the electricity purchase price. Relative to the alternative solar systems, the proposed system offers greater environmental benefits but has a longer payback period. This implies that although the energy saving and emission reduction potential of the proposed syst
Wieland C, Schifflechner C, Braimakis K, et al., 2023, Innovations for organic Rankine cycle power systems: Current trends and future perspectives, Applied Thermal Engineering, Vol: 225, Pages: 1-17, ISSN: 1359-4311
Since the early 2000s, organic Rankine cycle (ORC) technology has experienced rapid development and market uptake. More than 4.5 GW of total ORC power plant capacity has been installed since then. Due to its flexibility, suitability for small- to medium-scale installations, its applicability to low- to medium-temperature heat sources, and compact design, ORC technology is already considered as state-of-the-art. However, to further increase the technical and economic potential of this technology, significant research is needed to further improve efficiency and other key performance indicators, as well as to address environmental and safety aspects. Therefore, it is important to research new technologies and to foster innovations for improving performance; but it is of similar importance to resolve operational challenges with applied research that focuses closer to the market, and the technology provider and user needs. This vision article outlines the current state-of-the-art and presents current research trends. Parts of this article summarize the research progress reported at the 6th International Seminar on ORC Power Systems (ORC2021) and accompanies the associated special issue published following this event. The article highlights research trends at the concept level, but also at the component and system levels and, therefore, provides a holistic overview for the interested reader regarding the current challenges and potential future research activities in this area. Beyond the variety of cycle concepts in Section 2.1, the different heat sources and applications in Section 2.2, rotating device and turbomachinery (pumps and expansion devices) options in Section 2.3, current R&D trends for working fluids are presented in Section 2.4. This editorial concludes with a more applied perspective on ORC systems, covering process control and a general perspective on the technology in Section 2.5. With an increasing number of ORC plants operating in the field, their in
Li Z, Brun NL, Gasparrini C, et al., 2023, A novel nonlinear radiative heat exchanger for molten-salt applications, Applied Thermal Engineering, Vol: 225, Pages: 1-11, ISSN: 1359-4311
One of the main difficulties associated with using molten salts as heat transfer fluids (HTFs) in practical applications such as in the solar and nuclear energy industries is the possibility of solidifying molten salts. Since the salts proposed for Generation-IV nuclear reactors (MSRs) have higher melting points, this is especially true for those reactors. The consequences of accidental freezing within the direct reactor auxiliary cooling system (DRACS) of MSRs could be catastrophic. DRACS is a passive safety system designed to remove decay heat from the reactor core during an emergency. The present study examines, as a case study, the freezing problem in the most vulnerable component of DRACS, namely the natural draft heat exchanger (NDHX). For this application, a unique and innovative heat exchanger design termed radiative heat exchanger (RHX) is proposed, with nonlinear heat transfer characteristics. Specifically, by suppressing convection and exploiting thermal radiation as the primary heat transfer mechanism, the RHX concept reduces the heat rejection/removal rate passively to allow safe operation when the molten salt coolant temperature drops and is near its melting point, while maintaining a high capacity for decay heat extraction in case the coolant temperature rises during an accident and the reactor becomes overheated; this moderation of the heat removal rate is accomplished without the requirement for external (manual or automatic) control. A thermohydraulic model is developed and applied to evaluate the proposed RHX with the existing conventional NDHX. Compared to the conventional design, the RHX has the potential to reduce heat removal by approximately 30 % at low temperatures and near-solidification conditions. In the case of the prototypical 20 MW reactor with a decay rate of 0.2 MW, RHX extends the freezing time of molten salt by three times, to approximately 18 h. During high-temperature events, such as the initial stages of a reactor scram, the RHX
Mersch M, Markides CN, Mac Dowell N, 2023, The impact of the energy crisis on the UK’s net-zero transition, iScience, Vol: 26, Pages: 1-24, ISSN: 2589-0042
Recent drastic increases in natural gas prices have brought into sharp focus the inherent tensions between net-zero transitions, energy security, and affordability. We investigate the impact of different fuel prices on the energy system transition, explicitly accounting for the increasingly coupled power and heating sectors, and also incorporate the emerging hydrogen sector. The aim is to identify low-regret decisions and optimal energy system transitions for different fuel prices. We observe that the evolution of the heating sector is highly sensitive to gas price, whereas the composition of the power sector is not qualitatively impacted by gas prices. We also observe that bioenergy plays an important role in the energy system transition, and the balance between gas prices and biomass prices determines the optimal technology portfolios. The future evolution of the prices of these two resources is highly uncertain, and future energy systems must be resilient to these uncertainties.
Guo J, Song J, Markides C, 2023, Studies on intensification mechanism of supercritical CO2 flows during heating processes, the 17th International Heat Transfer Conference
The unique characteristics of supercritical CO2 (SCO2) make it have promising potential in chemical engineering, energy conversion and other fields, but the drastic variations in thermophysical properties also bring great challenges to the prediction of heat transportation processes. The thermal-hydraulic performance of supercritical CO2 (SCO2) flows in horizontal pipes during heating processes is investigated numerically from the viewpoints of the first and second laws of thermodynamics. Heated flows through pipes with a diameter of 4 mm and mass flux of 400 kg/(m2·s), at a pressure of 8.0 MPa, with three heat fluxes (50 kW/m2, 75 kW/m2 and 100 kW/m2) are simulated. The results showed that the heat transfer irreversibility is more than 4 times higher on average when the highest heat flux of 100 kW/m2 is applied relative to the lowest heat flux of 50 kW/m2, while the peak heat transfer coefficientincreases by ~1.4 times when the heat flux decreases from 100 kW/m2 to 50 kW/m2. The thermal acceleration effect is negligible, while the buoyancy effect leads to secondary flows and affects the heat transfer and flow characteristics significantly. A jet flow in the near-wall region at the bottom of the pipe improves the synergy between the temperature gradient and velocity fields, leading to a higher (more than 2 times) heat transfer coefficient in this region than in the near-wall region at the top of the pipe. The present work provides insights into the mechanisms and characteristics of SCO2 flow heat transfer as well as practical guidance on the design and optimisation of relevant components.
He W, Huang G, Markides CN, 2023, Synergies and potential of hybrid solar photovoltaic-thermal desalination technologies, Desalination, Vol: 552, Pages: 1-18, ISSN: 0011-9164
Solar desalination has emerged as a sustainable solution for addressing global water scarcity in the energy-water nexus, particularly for remote areas in developing countries. How to use the light spectrum through solar devices can profoundly affect the solar energy utilization, desalination rates, off-grid applicability, and water affordability. Solar photovoltaic (PV) and solar thermal (ST) respectively have enabled a variety of interesting solar desalination technologies, but the resulting applications usually limit the integration between solar and desalination to be either electrically or thermally connected. Here this review paper explores smart co-uses of heat and electricity from the sun to improve the efficiency, productivity, and independence of various solar desalination processes. It is found that coupling solar photovoltaic-thermal (PVT) with desalination could be a practical and immediately deployable route for plausibly more sustainable solar desalination than current solutions, because the combined electrical and thermal energy outputs from PVT panels could be used synergistically to catalyze the improvement on the solar energy efficiency, specific energy consumption, and specific water production, as well as the operational independence for off-grid applications. Our preliminary analysis indicated an up to 20 % lower cost of PVT-desalination than current solar PV-desalination and ST-desalination but also with challenges discussed.
Yang M, Moghimi MA, Loillier R, et al., 2023, Design of a latent heat thermal energy storage system under simultaneous charging and discharging for solar domestic hot water applications, Applied Energy, Vol: 336, Pages: 1-18, ISSN: 0306-2619
Design of a latent heat thermal energy storage system under simultaneous charging and discharging for solar domestic hot water applications
Lotfi M, Mersch M, Markides CN, 2023, Experimental and numerical investigation of a solar-thermal humidification-dehumidification desalination plant for a coastal greenhouse, Cleaner Engineering and Technology, Vol: 13, Pages: 1-15, ISSN: 2666-7908
Meeting the water demand for agriculture is considered one of the most crucial challenges societies face worldwide. Seawater greenhouses are a promising irrigation water source for locations where it is necessary to source freshwater from saline groundwater or seawater. Coupled with hybrid humidification-dehumidification (HDH) plants, powered by solar-thermal collectors, they can provide valuable freshwater using a free and accessible energy resource. Compared to conventional desalination systems, they can reduce emissions associated with desalination, have lower operational and maintenance costs, and are more suitable for deployment in remote areas. In this work, an experimental and numerical study of a solar-powered HDH desalination system in a seawater greenhouse is presented, considering an open-air/open-water (OAOW) cycle. In addition to the development of a thermodynamic model of the plant, a pilot plant consisting of a full-scale greenhouse for crop production, a HDH unit, 15 solar-thermal collectors, a firebox hot water boiler and two spiral coil heat exchangers was constructed and field-tested in Bushehr, Iran, under real meteorological conditions. The freshwater production of the pilot plant approached 2000 L per day. Thanks to an optimised hydroponic irrigation system, which reduced irrigation water consumption to approximately 1000 L per day, the greenhouse was self-sufficient in terms of water consumption and generated a freshwater surplus. In the experimental work, where the hot water temperature at the humidifier inlet was limited to 70 °C to prevent fouling, a maximum gain output ratio (GOR) of 2.3 was achieved. The computational model was validated and found to be in good agreement with experimental data, with GOR predictions to within a maximum deviation of 25 %. The model was then used for parametric performance studies beyond the limits of the experimental arrangement. A maximum GOR of 3.2 was achieved at a humidifier inlet temperature of 80
Melnik A, Bogoslovtseva A, Petrova A, et al., 2023, Oil–water separation on hydrophobic and superhydrophobic membranes made of stainless steel meshes with fluoropolymer coatings, Water, Vol: 15, Pages: 1-13, ISSN: 2073-4441
In this work, membranes were synthesized by depositing fluoropolymer coatings onto metal meshes using the hot wire chemical vapor deposition (HW CVD) method. By changing the deposition parameters, membranes with different wetting angles were obtained, with water contact angles for different membranes over a range from 130° ± 5° to 170° ± 2° and a constant oil contact angle of about 80° ± 2°. These membranes were used for the separation of an oil–water emulsion in a simple filtration test. The main parameters affecting the separation efficiency and the optimal separation mode were determined. The results reveal the effectiveness of the use of the membranes for the separation of emulsions of water and commercial crude oil, with separation efficiency values that can reach over 99%. The membranes are most efficient when separating emulsions with a water concentration of less than 5%. The pore size of the membrane significantly affects the rate and efficiency of separation. Pore sizes in the range from 40 to 200 µm are investigated. The smaller the pore size of the membranes, the higher the separation efficiency. The work is of great economic and practical importance for improving the efficiency of the membrane separation of oil–water emulsions. It lays the foundation for future research on the use of hydrophobic membranes for the separation of various emulsions of water and oil products (diesel fuel, gasoline, kerosene, etc.).
Arias DM, García-Valladares O, Besagni G, et al., 2023, A vision of renewable thermal technologies for drying, biofuels production and industrial waste, gas or water recovery, Applied Thermal Engineering, Vol: 223, Pages: 1-7, ISSN: 1359-4311
Rapid population growth and the finite nature of fossil-fuel resources have given rise to an urgent interest in sustainable energy development. Thermal technologies include several devices and systems able to improve energy use, recycle resources and waste, and harness renewable energy sources. In particular, thermal technologies applied to renewable sources have been improving over the years in terms of performance costs have reduced, however, several challenges remain that need to be overcome. This vision article is concerned with the prevailing challenges for thermal technologies applied to renewable energy; specifically, solar drying technologies, focussing on medium and large-capacity solar drying systems, thermal devices for waste heat and gas treatment/recovery, and thermochemical technologies for the valorization of biomass to fuels. Firstly, a brief description of each technology is provided while remarking on their importance in energy transition and resource recovery scenarios. Subsequently, key challenges are identified and promising directions and areas for exploration and future research are suggested. The most common critical challenges for the further development and deployment of these technologies include improved designs and material use to decrease final costs and minimize environmental impact. In particular, managing this process within a circular economy perspective would be necessary for improved sustainability. Incorporating renewable energy sources with thermal technologies to reduce the need for fossil fuels is of major interest globally, and optimizing the combined use of mathematical, computational, and experimental tools is the most promising approach for accelerated understanding and development in this space.
Zhao Y, Song J, Liu M, et al., 2023, Multi-objective thermo-economic optimisation of Joule-Brayton pumped thermal electricity storage systems: Role of working fluids and sensible heat storage materials, Applied Thermal Engineering, Vol: 223, Pages: 1-14, ISSN: 1359-4311
Pumped-thermal electricity storage (PTES), with the advantages of reduced geographical constraints, low capital costs, long lifetimes and flexible power ratings, is a promising large-scale energy storage technology for future power systems. In this work, thermo-economic models of Joule-Brayton PTES systems with solid thermal reservoirs (STRs) and liquid thermal stores (LTSs) were developed, and detailed parametric analyses of the two systems were performed. The results reveal that elevated maximum charging temperatures are beneficial for both thermodynamic and economic performance, and that there are optimal values for the packed-bed void fraction, heat-exchanger effectiveness and turbomachine polytropic pack from a thermo-economic perspective for the two PTES system variants. Multi-objective thermo-economic optimisation of PTES systems at a fixed power capacity (10 MW) and discharging duration (6 h) was also conducted. It is found that helium is the best working fluid candidate for both PTES systems, and that the best options for the storage material are magnetite for PTES systems with STRs, and the combination of Hitec XL + Therminol 66 + Butane for PTES systems with LTSs. In the investigated design space for both systems, PTES systems with STRs are more attractive as the total purchase cost is lower for the same roundtrip efficiency as PTES systems with LTSs. Using the technique for order of preference by similarity to the ideal solution decision-making method, and a selected weighted matrix (1:1), the optimal solutions amongst the Pareto front solutions were determined. The optimal roundtrip efficiency and total purchase cost are 71.8 % and 37.7 M$ for PTES systems with STRs, and are 56.0 % and 36.0 M$ for PTES systems with LTSs, respectively. The conclusions and proposed approach can provide useful guidance for the further development, design and optimisation of PTES technology.
Collignon R, Caballina O, Lemoine F, et al., 2023, Heat transfer enhancement in wavy falling films studied by laser-induced fluorescence, International Journal of Heat and Mass Transfer, Vol: 202, Pages: 1-15, ISSN: 0017-9310
The characteristics of thin liquid films flowing down a uniformly heated and inclined plane are investigated, with heat transfer across the wavy films quantified using up-to-date optical measurement techniques based on laser-induced fluorescence (LIF). A planar two-colour LIF technique provides the temperature distribution inside the films, but requires a high degree of wave regularity for the spatial reconstruction. A pointwise adaptation of the aforementioned technique, with much finer temporal sampling, provides simultaneous measurements of the average temperature over the film height and of the film thickness. Despite the loss of spatial resolution, the latter technique can be applied to diverse situations, especially when the waves lose their regularity and have large amplitudes. With these two approaches, the enhancement of heat transfer due to surface waves is traced along the film flow. A growing thermal boundary layer is found close to the inlet of the flow (i.e., first few cm), but its thickness remains small relative to the film thickness. Therefore, the heat transfer coefficient (HTC) is observed to be insensitive to the shape and amplitude of the waves at the free surface. A critical distance is necessary for the thermal boundary layer to be thick enough to interact with the flow structures associated with the waves, and the critical length scales with the Peclet number of the flow based on the specific flow rate. Several experiments are conducted to quantify the influence of the main flow parameters that control the HTC, such as the Reynolds number, the inclination angle and the wave frequency. For moderate wave amplitudes, the internal structure of the film is insensitive to the wave dynamics, and the temperature distribution is essentially dominated by thermal diffusion in the direction normal to the heated wall. Classical Nusselt theory is found to be applicable to the unperturbed (flat) film flows with some limited adjustments to predict the heat t
Guo J, Song J, Lakshmi Narayanan SN, et al., 2023, Numerical investigation of the thermal-hydraulic performance of horizontal supercritical CO2 flows with half-wall heat-flux conditions, Energy, Vol: 264, Pages: 1-13, ISSN: 0360-5442
Thermo-hydraulic characteristics of supercritical CO2 (SCO2) flows in horizontal tubes with half-wall heat-flux conditions are investigated numerically, which is a common practice such as applications in solar parabolic trough collectors, while the heat transfer performance and the underlying mechanisms have not been fully understood. In heated flows, buoyancy acts to inhibit heat transfer when the top half of the tube wall is heated, however, when the bottom half of the tube wall is heated, this inhibition is alleviated, and the synergy between the temperature gradient and velocity fields improves thanks to the secondary flow in the near-wall region at the bottom wall. As a result, the heat transfer coefficient is ∼95% higher (on average) than in the case when the top half of the tube wall is heated. When the bottom half of the tube wall is cooled, buoyancy is expected to enhance heat transfer, while the synergy between the temperature gradient and velocity fields is supressed by the secondary flow in the near-wall region at the bottom of the tube. Conversely, when the top half of the tube wall is cooled, the buoyancy effect inhibits heat transfer, while the synergy between the temperature gradient and velocity fields is improved by the secondary flow in the near-wall region at the top of the tube, which eventually leads to an increase of ∼21% (on average) in the heat transfer coefficient relative to the case when the bottom half of the tube wall is cooled. Finally, the heat transfer discrepancy due to different heat flux conditions revealed in this study are employed in a heat exchanger model, indicating that the thermal performance of this device can be increased by ∼6% through an appropriate arrangement of the hot and cold flows without additional costs.
Wang K, Smith IK, Markides CN, 2023, Overview of low-temperature distributed heat and fundamentals, Power Generation Technologies for Low-Temperature and Distributed Heat, Pages: 1-48, ISBN: 9780128182376
This chapter presents an overview of low-temperature distributed heat from its definition, features, characteristics, and potential in various regions and sectors as well as the thermodynamic fundamentals of power cycles for heat recovery and conversion. Significant inconsistencies exist in the definition of the temperature boundaries that demark low-/medium-/high-temperature heat across the literature. A broader definition for low-temperature and distributed heat is proposed and adopted here, based on a comparison to the temperatures achieved by combustion and the scale of centralized power plants, which includes not only waste heat from various processes and sectors but also heat from renewable energy sources. Estimations of the waste heat potentials in the world and major regions showed that a significant share of energy consumptions is finally dissipated as waste heat, of which the majority is below 100°C. Thermodynamic fundamentals of power cycles for recovering waste heat are then presented, which highlights that matching the temperature changes of the heat source and the working fluid is important. Power cycles based on pressurizing, evaporating, expanding, and condensing volatile fluids, also referred to as two-phase cycles, can be superior in terms of conversion efficiency to those based on ideal or other single-phase gases that do not feature phase change thanks to their high work ratios and superior matching characteristics.
The special issue “VSI:AESMT′ 22“ aims to provide novel research papers in the field of the Renewable Energy as a part of the conference “Alternative energy sources, materials and technologies (AESMT′ 22)”. The conference was foreseen to be held in Veliko Tarnovo, Bulgaria. However due to the pandemic the conference was held on-line on 27–28 June 2022. The following distinguished scientists gave plenary reports during the conference: - Prof. Soteris Kalogirou (Editor-in-Chief of “Renewable Energy”, Cyprus); - Prof. André Thess (Director, German Aerospace Center, Institute of Engineering Thermodynamics, Germany). Some more important points about the AESMT'22 conference are as follows: - Representatives of 23 countries sent papers to the conference, namely from Belarus, Brazil, Bulgaria, China, Chile, France, Germany, India, Iran, Israel, Italy, Kazakhstan, Kuwait, Latvia, Poland, Portugal, Romania, Russia, Serbia, Spain, Tajikistan, Turkey, and United Kingdom; - The total number of the presented articles was 67 (25 of them orally, the others were posters); - After the preliminary review, 15 articles have been selected for submission in “Applied Thermal Engineering” journal.
Li Q, Jiang L, Huang G, et al., 2023, A ternary system exploiting the full solar spectrum to generate renewable hydrogen from a waste biomass feedstock, Energy and Environmental Science, ISSN: 1754-5692
A solar-driven system is proposed capable of hydrogen production from waste biomass with low carbon and water footprints. The ternary system consists of a membrane-based waste biomass concentrator (WBC), a biomass preconditioning reactor (BPR) integrated with an array of hybrid PV-thermal (PVT) collectors, and a flow electrolysis cell (FEC) equipped with a custom, high-performance electrode - NiMo alloy deposited onto Ni foam. An innovative full-solar-spectrum hybrid PVT reflector-concentrator was constructed to confirm performance; this enabled a thermal efficiency of up to ∼50% to be achieved when operating the BPR at 120-150 °C, while also converting ∼8% of the solar flux to electricity for the FEC. The solar-thermal BPR can reform recovered waste biomass (i.e., a sugar-containing liquid feedstock) into a bio-alcohol (5-hydroxymethylfurfural) with a yield of 25 mol%, with the transformed biomass then used to feed the anodic compartment of the FEC. Within the FEC, biomass electrolysis using the NiMo catalyst facilitated hydrogen production, offering a low energy consumption of 40-53 kW h kg−1, which is 16-28% more efficient than alkaline water splitting using Ni foam electrodes. The ternary system achieved a 7.5% overall solar-to-hydrogen efficiency, additional revenue from clean water production (with >80% water reclaimed), and a value-added chemical by-product (2,5-furandicarboxylic acid at a 3-10% yield from the waste sugar stream). This work presents a new route towards efficient and economically feasible renewable hydrogen production—a system which can underpin a circular economy.
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