Search

Several emerging electrical energy storage technologies make use of packed-bed reservoirs to store thermal energy for subsequent conversion back to electricity. The present paper describes analysis and optimisation of such reservoirs under transient and steady-state cyclic operation. The focus is on thermodynamic issues, but a simple costing model is also included in order to determine the influence of cost factors on the main design parameters. A major part of the paper is devoted to segmentation (or layering) of the packed beds, which has previously been proposed as a means of simultaneously attaining high storage efficiency and full utilisation of the reservoirs. As illustrative examples, three different reservoirs are modelled, corresponding to the hot and cold thermal stores of a pumped thermal energy storage system, and a larger thermal store suitable for integration with adiabatic compressed air energy storage. [ABSTRACT FROM AUTHOR]

Highlights: [•] We test a thermally powered circulator based on two-phase thermofluidic oscillations. [•] Two heat exchanger designs and two power transmission lines are compared. [•] Increased fluid inertia in the load improves the maximum flow-rate and stalling head. [•] The pumping capacity depends on the heat uptake and the liquid oscillation amplitude. [•] The stalling head is correlated to the internal working fluid temperature amplitude. [ABSTRACT FROM AUTHOR]

This paper uses a whole system approach to examine system design and planning strategies that enhance the system value of electrifying heating and identify trade-offs between consumers' investment and infrastructure requirements for decarbonising heating in buildings. We present a novel integrated model of heat, electricity and gas systems, HEGIT, to investigate different heat electrification strategies using the UK as the case study from two perspectives: (i) a system planning perspective regarding the scope and timing of electrification; and (ii) a demand-side perspective regarding the operational and investment schemes on the consumer side. Our results indicate that complete electrification of heating increases peak electricity demand by 170%, resulting in a 160% increase in the required installed capacity in the electricity grid. However, this effect can be moderated by implementing smart demand-side schemes. Grid integration of heat pumps combined with thermal storage at the consumer-end was shown to unlock significant potential for diurnal load shifting, thereby reducing the electricity grid reinforcement requirements. For example, our results show that a 5 b£ investment in such demand-side flexibility schemes can reduce the total system transition cost by about 22 b£ compared to the case of relying solely on supply-side flexibility. In such a case, it is also possible to reduce consumer investment by lowering the output temperature of heat pumps from 55 °C to 45 °C and sharing the heating duty with electric resistance heaters. Furthermore, our results suggest that, when used at a domestic scale, ground-source heat pumps offer limited system value since their advantages (lower peak demand and reduced variations in electric heating loads) can instead be provided by grid-integration of air-source heat pumps and increased thermal storage capacity at a lower cost to consumers and with additional flexibility benefits for the electricity grid. Lastly, our results show that, regardless of consumers' investment and operation decisions, the UK electricity grid can reliably accommodate close to 50% of the heating demand, but this can be increased to about 75% by implementing smart operation schemes at the consumer end. • Value of smart electrification is examined from system and demand-side perspectives. • Coordinated system planning is key to accelerate electrification and lower system costs. • Consumer costs are reduced by sharing demand between electric heater and heat pump. • Air-source heat pump with storage has higher system value than ground-source heat pump. • Smart operation schemes can increase the optimal degree of electrification by 25%. [ABSTRACT FROM AUTHOR]

Several of the emerging technologies for electricity storage are based on some form of thermal energy storage (TES). Examples include liquid air energy storage, pumped heat energy storage and, at least in part, advanced adiabatic compressed air energy storage. Compared to other large-scale storage methods, TES benefits from relatively high energy densities, which should translate into a low cost per MW h of storage capacity and a small installation footprint. TES is also free from the geographic constraints that apply to hydro storage schemes. TES concepts for electricity storage rely on either a heat pump or refrigeration cycle during the charging phase to create a hot or a cold storage space (the thermal stores), or in some cases both. During discharge, the thermal stores are depleted by reversing the cycle such that it acts as a heat engine. The present paper is concerned with a form of TES that has both hot and cold packed-bed thermal stores, and for which the heat pump and heat engine are based on a reciprocating Joule cycle, with argon as the working fluid. A thermodynamic analysis is presented based on traditional cycle calculations coupled with a Schumann-style model of the packed beds. Particular attention is paid to the various loss-generating mechanisms and their effect on roundtrip efficiency and storage density. A parametric study is first presented that examines the sensitivity of results to assumed values of the various loss factors and demonstrates the rather complex influence of the numerous design variables. Results of an optimisation study are then given in the form of trade-off surfaces for roundtrip efficiency, energy density and power density. The optimised designs show a relatively flat efficiency vs. energy density trade-off, so high storage density can be attained with only a modest efficiency penalty. After optimisation, losses due to pressure drop and irreversible heat transfer in the thermal reservoirs are only a few percent, so round-trip efficiency is governed mainly by the efficiency of the compression and expansion processes: overall roundtrip efficiencies approaching those for pumped hydro schemes might be achievable whilst simultaneously attaining energy storage densities of around 200 MJ m –3 , but this is contingent upon attaining compression and expansion efficiencies for the reciprocating devices that have yet to be proven. [ABSTRACT FROM AUTHOR]

Three distinct pumped-thermal electricity storage (PTES) system variants based on currently available sensible heat storage materials are presented: (i) Joule-Brayton PTES systems with solid thermal reservoirs; (ii) Joule-Brayton PTES systems with liquid thermal stores; and (iii) transcritical Rankine PTES systems with liquid thermal stores. Parametric design optimisation is performed for each PTES system variant considering various system configurations, working fluids and storage media from a thermodynamic perspective. The results show that amongst the investigated systems, the recuperative transcritical Rankine PTES system with CO 2 as the working fluid and Therminol VP-1 as the storage material achieves the highest roundtrip efficiency of 68%. Further to the optimal thermodynamic performance of these system, their corresponding capital costs are also evaluated. The economic performance comparisons of selected optimal PTES designs reveal that the recuperative transcritical Rankine PTES system with CO 2 and Therminol VP-1 exhibits the lowest capital cost of 209 M$ for the given power capacity (50 MW) and discharge duration (6 h). The influences of the power capacity and discharge duration are also investigated, with results showing that the lowest power and energy capital costs are 3790 $/kW (discharge duration of 2 h) and 396 $/kWh (discharge duration of 12 h), respectively. [ABSTRACT FROM AUTHOR]

The organic Rankine cycle (ORC) is an effective technology for power generation from temperatures of up to 400 °C and for capacities of up to 10 MW el. The use of solar irradiation for driving an ORC is a promising renewable energy-based technology due to the high compatibility between the operating temperatures of solar thermal collector technologies and the temperature needs of the cycle. The objective of this review paper is to present and discuss the operation principles of solar-ORC technology and the wide range of solar-ORC systems that have been studied in the literature. Various solar thermal technologies that can drive the ORC are investigated, such as the flat plate collector, evacuated tube collector, compound parabolic collector, parabolic trough collector, linear Fresnel reflectors, dish concentrators and solar towers. Both simulation studies and experimental investigations are included in the study. Hybrid systems and different thermal storage techniques are also examined in detail. Moreover, systems with ORC which produce many useful outputs such as cooling, heating and fresh water are studied because they present high sustainability indexes. The limitations of the technology are also highlighted, along with critical suggestions aimed at steering future research in this field. The final conclusions indicate that the development of trigeneration and polygeneration systems with ORC sub-systems is a promising avenue, not only for the future development of solar-ORC technology but also for the development of renewable and sustainable energy systems in a broader context. • Solar driven organic Rankine cycles are summarized and discussed in detail. • Concentrating and non-concentrating solar thermal systems are included. • Parabolic trough collector is the best solar technology for power production. • The use of solar organic Rankine cycles in polygeneration is a promising idea. • There is a need for conducting future experimental studies in a great scale. [ABSTRACT FROM AUTHOR]

This paper presents a thermodynamic analysis and techno-economic assessment of a novel hybrid solar-biomass power-generation system configuration composed of an externally fired gas-turbine (EFGT) fuelled by biomass (wood chips) and a bottoming organic Rankine cycle (ORC) plant. The main novelty is related to the heat recovery from the exhaust gases of the EFGT via thermal energy storage (TES), and integration of heat from a parabolic-trough collectors (PTCs) field with molten salts as a heat-transfer fluid (HTF). The presence of a TES between the topping and bottoming cycles facilitates the flexible operation of the system, allows the system to compensate for solar energy input fluctuations, and increases capacity factor and dispatchability. A TES with two molten salt tanks (one cold at 200 °C and one hot at 370 °C) is chosen. The selected bottoming ORC is a superheated recuperative cycle suitable for heat conversion in the operating temperature range of the TES. The whole system is modelled by means of a Python-based software code, and three locations in the Mediterranean area are assumed in order to perform energy-yield analyses: Marseille in France, Priolo Gargallo in Italy and Rabat in Morocco. In each case, the thermal storage that minimizes the levelized cost of energy (LCE) is selected on the basis of the estimated solar radiation and CSP size. The results of the thermodynamic simulations, capital and operational costs assessments and subsidies (feed-in tariffs for biomass and solar electricity available in the Italian framework), allow estimating the global energy conversion efficiency and the investment profitability in the three locations. Sensitivity analyses of the biomass costs, size of PTCs, feed-in tariff and share of cogenerated heat delivered to the load are also performed. The results show that the high investment costs of the CSP section in the proposed size range and hybridization configuration allow investment profitability only in the presence of a dedicated subsidy framework such as the one available in the Italian energy market. In particular, the LCE of the proposed system is around 140 Eur/MWh (with the option to discharge the cogenerated heat) and the IRR is around 15%, based on the Italian electricity subsidy tariffs. The recovery of otherwise discharged heat to match thermal energy demand can significantly increase the investment profitability and compensate the high investment costs of the proposed technology. • A novel hybrid biomass/solar combined cycle with thermal energy storage is proposed. • Parabolic through collectors are combined gas turbine-ORC plants are integrated. • ORC system efficiency is optimized by using a genetic algorithm procedure. • Size of a two-tank thermal energy storage technology is optimized for system flexibility. • Solar hybridization increases efficiency and flexibility but also the levelized cost of energy. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • Analysis of portfolios of heat pumps, boilers and storage for zero-carbon heating. • Models of heating technologies are integrated into a whole-energy system model. • The model determines heating system configurations that minimise energy system cost. • Case studies suggest that cost-optimal heating portfolios are system-dependent. • Electric heat pumps supply 83–97% of heat; boilers and storage provide the rest [ABSTRACT FROM AUTHOR]

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 MW el), the lowest-cost system configuration included around 500 MW el of additional secondary generation capacity coupled to the nuclear power plants, with 4.5 GWh th 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. [ABSTRACT FROM AUTHOR]

Coffee roasting is a highly energy intensive process wherein a large quantity of heat is discharged from the stack at medium-to-high temperatures. Much of the heat is released from the afterburner, which is required to remove volatile organic compounds and other pollutants from the flue gases. In this work, intermittent waste-heat recovery via thermal energy storage (TES) and organic Rankine cycles (ORCs) is compared to combined heat and power (CHP) based on micro gas-turbines (MGTs) for a coffee roasting plant. With regard to the former, a promising solution is proposed that involves recovering waste heat from the flue gas stream by partial hot-gas recycling at the rotating drum coffee roaster, and coupling this to a thermal store and an ORC engine for power generation. The two solutions (CHP + MGT prime mover vs. waste-heat recovery + ORC engine) are investigated based on mass and energy balances, and a cost assessment methodology is adopted to compare the profitability of three system configurations integrated into the selected roasting process. The case study involves a major Italian roasting plant with a 3,000 kg per hour coffee production capacity. Three options are investigated: (i) intermittent waste-heat recovery from the hot flue-gases with an ORC engine coupled to a TES system; (ii) regenerative topping MGT coupled to the existing modulating gas burner to generate hot air for the roasting process; and (iii) non-regenerative topping MGT with direct recovery of the turbine outlet air for the roasting process. The results show that the profitability of these investments is highly influenced by the natural gas and electricity prices and by the coffee roasting production capacity. The CHP solution via an MGT appears as a more profitable option than waste-heat recovery via an ORC engine primarily due to the intermittency of the heat-source availability and the high electricity cost relative to the cost of natural gas. [ABSTRACT FROM AUTHOR]

• Unified thermo-economic optimisation framework for compressed-air energy storage. • Comparison of different configurations and materials at different scales. • Packed-bed stores perform better than liquid stores under the studied conditions. • Roundtrip efficiencies of up to 76% can be attained by these systems. • Costs of 140 $/kWh and 970 $/kW can be achieved for 50 MW and 300 MWh systems. 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. [ABSTRACT FROM AUTHOR]

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. • Scientific and engineering challenges of hybrid photovoltaic-thermal (PV-T) collectors. • Research gaps and various pathways for innovation of PV-T collectors and systems. • Design modifications, selective coatings, nanofluids and spectral splitting. • Carbon mitigation potential and pathways for global decarbonization with PV-T collectors. • Key challenges for the adoption of PV-T technology and recommendations. [ABSTRACT FROM AUTHOR]

• Thermo-economic evaluation is performed for sub-ambient thermally integrated PTES. • Efficiency up to 99% is achievable by sacrificing the storage capacity and cost. • Cold-storage medium flowrate affects the storage capacity, cost, and power output. • Continuous high flowrate external heat source can reduce the LCOS up to 0.17 $/kWh. • Proposed technology performs comparably to others without geographical limitations. 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 a high potential for medium-to-large scale energy storage with a unique hybrid nature for electricity storage and thermal integration. [ABSTRACT FROM AUTHOR]

• Thermo-economic assessments of concentrated solar power plants with thermal storage are presented. • Two steam accumulation options are compared taking Khi Solar One as a reference plant. • Comprehensive models of the power cycle, steam accumulation and concrete storage blocks are validated. • Adding concrete blocks increases the average efficiency by 13 %, from 23 % to 26 %, in discharging mode. • With an extra investment of $4.2 M, the projected net present value increases from $41 M to $71 M. In direct steam generation (DSG) concentrated solar power (CSP) plants, a common thermal energy storage (TES) option relies on steam accumulation. This conventional option is constrained by temperature and pressure limits, and delivers saturated or slightly superheated steam at reduced pressure during discharge, which is undesirable for part-load turbine operation. However, steam accumulation can be integrated with sensible-heat storage in concrete to provide higher-temperature superheated steam at higher pressure. In this paper, this conventional steam accumulation option (existing) and an integrated concrete-steam TES option (extended) are described and analysed, and their thermo-economic performance are compared taking the 50-MW Khi Solar One DSG CSP plant in South Africa as a case study. The results show that the extended option with five 10-m long, square cross-section concrete blocks, each with 3600 equally spaced tubes, provides an additional TES capacity of 177 MWh compared to the existing configuration as a result of utilising most of the available thermal power in the solar receivers. Moreover, the extended option delivers 58 % more electricity with a 13 % enhancement in thermal efficiency during TES discharging mode. With an estimated additional investment of $4.2M, the levelised costs of storage and electricity for Khi Solar One with the extended TES option are, respectively, 29 % and 6 % lower than those obtained with the existing TES option. With the extended TES option, the projected net present value of Khi Solar One increases by 73 %, from $41M to $71M, at an average electricity price of 280 $/MWh. [ABSTRACT FROM AUTHOR]

• The operation of a domestic air-source heat pump with thermal storage is optimised. • Operational cost, heat pump COP and PV self-sufficiency are used as objectives. • The heat pump is either coupled to a hot-water cylinder or two PCM thermal stores. • Operational costs can be decreased by up to 39% depending on the configuration. • Time-resolved results can be used to guide the development of smart controllers. Electricity-driven air-source heat pumps are a promising element of the transition to lower-carbon energy systems. In this work, operational optimisation is performed of an air-source heat pump system aimed at providing space heating and domestic hot water to a single-family dwelling. The novelty of this work lies in the development of comprehensive thermal network models of two different system configurations: (i) a standard configuration of a heat pump system coupled to a hot-water cylinder; and (ii) an advanced configuration of a heat pump system coupled to two phase-change material thermal stores. Three different objective functions (operational cost, coefficient of performance, and self-sufficiency from a locally installed solar-PV system) are investigated and the proposed mixed-integer, non-linear optimisation problems are solved by employing a genetic algorithm. Simulations are conducted at two carefully selected European locations with different climate characteristics (Oban in Scotland, UK, and Munich in Southern Germany) over four seasons represented by typical weather weeks. Comparison of key results against a conventional operating strategy reveals that the use of smart operational strategies for the operation of the heat pump and thermal stores can lead to considerable economic savings for consumers and significant performance improvements over the system lifetime. Optimising the operation of the standard configuration leads to average annual cost savings of up to 22% and 20% at the UK and German locations, respectively. The optimisation of the advanced configuration with the two PCM stores shows even higher potential for economic savings – up to 39% and 29% per year at the respective locations – as this configuration allows for greater operational flexibility, and high-electricity-price periods can be almost completely avoided. Depending on the objective function, configuration and location, the system seasonal coefficient of performance varies between 2.4 and 2.8. Lastly, a significant (up to four-times) increase in the fraction of heat pump energy demand covered by an appropriately-sized rooftop PV system is demonstrated, increasing from 8% to 34% at the UK location and from 6% to 24% at the German location. The analysis highlights trade-offs between the objective functions, while the time-resolved results can be used to guide the future development of smart controllers for these applications. [ABSTRACT FROM AUTHOR]

• Thermo-economic assessments of flexible nuclear power plants are presented. • Nuclear flexibility is enhanced with thermal energy storage and secondary power generators. • Thermodynamic modelling, design and sizing of components of the wider system are performed. • The flexible nuclear power is also integrated into a whole-electricity system model. • The energy system benefits of enhanced flexibility are quantified for a range of scenarios. The increasing penetration of intermittent renewable power will require additional flexibility from conventional plants, in order to follow the fluctuating renewable output while guaranteeing security of energy supply. In this context, coupling nuclear reactors with thermal energy storage could ensure a more continuous and efficient operation of nuclear power plants, while at other times allowing their operation to become more flexible and cost-effective. This study proposes options for upgrading a 1610-MW el nuclear power plant with the addition of a thermal energy storage system and secondary power generators. The total whole-system benefits of operating the proposed configuration are quantified for several scenarios in the context of the UK's national electricity system using a whole-system model that minimises the total system costs. The proposed configuration allows the plant to generate up to 2130 MW el during peak load, representing an increase of 32% in nominal rated power. This 520 MW el of additional power is generated by secondary steam Rankine cycle systems (i.e., with optimised cycle thermal efficiencies of 24% and 30%) and by utilising thermal energy storage tanks with a total heat storage capacity of 1950 MWh th. Replacing conventional with flexible nuclear power plants is found to generate whole-system cost savings between £24.3m/yr and £88.9m/yr, with the highest benefit achieved when stored heat is fully discharged in 0.5 h. At an estimated cost of added flexibility of £42.7m/yr, the proposed flexibility upgrades to such nuclear power plants appears to be economically justified with net system benefits ranging from £4.0m/yr to £31.6m/yr for the examined low-carbon scenarios, provided that the number of flexible nuclear plants in the system is small. This suggests that the value of this technology is system dependent, and that system characteristics should be adequately considered when evaluating the benefits of different flexible nuclear plant configurations and choosing the most cost-effective designs and operational characteristics. [ABSTRACT FROM AUTHOR]

[Display omitted] • Reaction kinetics of as-synthesised thermochemical sorbents are established. • Composite's dehydration activation energy drops by 13%. • Novel building envelope achieves space heating and air purification. • Clean air delivery rate of 41.8 m3/h is achieved for the proposed model. • Total efficiency including heating and air purification of 81% is obtained. In this paper, the dehydration (heat charge) and hydration (discharge) reaction kinetics of thermochemical sorbents synthesised in previous work by the author is established by using the isothermal method, with the aim of understanding their thermochemical conversion behaviour and developing reaction models for numerical simulations. The effects of temperature, reaction advancement, and vapour pressure are fully considered and employed in a thermochemical energy storage model. The derived dehydration reaction activation energies of the LiOH/LiCl@ expanded graphite (LiO2C1@EG and LiO3C1@EG) sorbents are 54.7 and 52.2 kJ/mol, respectively, which are lower than that of pure LiOH·H 2 O. To achieve the dual-function of space heating and air purification in an efficient manner, a novel solar building envelope combining thermochemical energy storage and photocatalysis is proposed and studied numerically based on the established reaction kinetics. Fresh air can be produced during solar harvesting. The porous wall, which is made of a composite sorbent, absorbs thermal energy to heat air near the wall and thus creates a chimney effect in the channel for continuous space heating. During discharge, the desorbed heat storage wall adsorbs the moist air and the hydration reaction enthalpy can be used again for air heating. The total efficiency including the equivalent formaldehyde degradation efficiency and the thermal efficiency is around 81% when the solar radiation is 600 W/m2. Results indicate that this passive building envelope can achieve a higher heat harvesting and utilisation efficiency in a more compact space compared to previous studies. Moreover, the influence of radiation intensity on air purification and thermal performance is investigated. The present work provides new insights and promotes the integration of passive solar building envelopes and thermochemical energy storage. [ABSTRACT FROM AUTHOR]

• Sophisticated techno-economic models of Pumped Thermal Energy Storage are developed. • Investigate various aspects of system design on efficiency and lifetime cost (LCOS). • Multi-objective optimization finds high-efficiency (>60%) and low-cost designs. • Pumped Thermal Energy Storage competes with Li-ion battery costs for >6 h storage. This article describes a techno-economic model for pumped thermal energy storage systems based on recuperated Joule-Brayton cycles and two-tank liquid storage. Models have been developed for each component, with particular emphasis on the heat exchangers. Economic metrics such as the power and energy capital costs (i.e., per-kW and per-kWh capacity) and levelized cost of storage are evaluated by gathering numerous cost correlations from the literature, thereby enabling estimates of uncertainty. It is found that the use of heat exchangers with effectivenesses up to 0.95 is economically worthwhile, but higher values lead to rapidly escalating component size and system cost. Several hot storage fluids are considered; those operating at the highest temperatures (chloride salts) improve the round-trip efficiency but the benefit is marginal and may not warrant the additional material costs and risk when compared to lower-temperature nitrate salts. Cost-efficiency trade-offs are explored using a multi-objective optimization algorithm, yielding optimal designs with round-trip efficiencies in the range 59–72% and corresponding levelized storage costs of 0.12 ± 0.03 and 0.38 ± 0.10 $/kWh e. Lifetime costs are competitive with lithium-ion batteries for discharging durations greater than 6 h under current scenarios. [ABSTRACT FROM AUTHOR]

• Techno-economic analysis of low-carbon heat supply options for UK supermarkets. • RIHC, an ASHP, and a thermal storage tank with advanced control is recommended. • Best investments provide a 16-to-18-year payback without a carbon price. • The UK renewable heat incentive subsidy (for ASHP) generates 89% of the 'savings' • When a £100/tCO2e tax is considered, paybacks are reduced to a 9–11 years. Eliminating the use of natural gas for non-domestic heat supply is an imperative component of net-zero targets. Techno-economic analyses of competing options for low-carbon heat supply are essential for decision makers developing decarbonisation strategies. This paper investigates the impact various heat supply configurations can have in UK supermarkets by using heat recovery principles from refrigeration systems under different climatic conditions. The methodology builds upon a steady-state model that has been validated in previous studies. All refrigeration integrated heating and cooling (RIHC) systems employ CO 2 booster refrigeration to recover heat and provide space heating alongside various technologies such as thermal storage, air-source heat pumps (ASHPs) and direct electric heaters. Seven cases evaluating various technology combinations are analysed and compared against a conventional scenario in which the building is heated with a natural gas boiler. The specific combinations of technologies analysed here contrasts trade-offs and is a first in the literature. The capital costs of these projects are considered, giving insights into their business case. Results indicate that electric heaters are not cost-competitive in supermarkets. Meanwhile, RIHC and ASHP configurations are the most attractive option, and if a thermal storage tank system with advanced controls is included, the benefits increase even further. Best solutions have a 6.3% ROI, a payback time of 16 years while reducing energy demand by 62% and CO 2 emissions by 54%. Such investments will be difficult to justify unless policy steers decision makers through incentives or the business case changes by implementing internal carbon pricing. [ABSTRACT FROM AUTHOR]

• Advanced exergy analysis of pumped thermal electricity storage systems is proposed. • The expander has the maximum exergy destruction rate in the recuperated system. • The largest avoidable exergy destruction arises in the recuperator and expander. • A quantitative analysis of system performance improvement potential is performed. • The modified overall exergetic efficiency increases substantially from 37% to 57%. Pumped thermal electricity storage is a thermo-mechanical energy storage technology that has emerged as a promising option for large-scale (grid) storage because of its lack of geographical restrictions and relatively low capital costs. This paper focuses on a 10 MW Joule-Brayton pumped thermal electricity storage system with liquid thermal stores and performs detailed conventional and advanced exergy analyses of this system. Results of the conventional exergy analysis on the recuperated system indicate that the expander during discharge is associated with the maximum exergy destruction rate (13%). The advanced exergy analysis further reveals that, amongst the system components studied, the cold heat exchanger during discharge is associated with the highest share (95%) of the avoidable exergy destruction rate, while during charge the same component is associated with the highest share (64%) of the endogenous exergy destruction rate. Thus, the cold heat exchanger offers the largest potential for improvement in the overall system exergetic efficiency. A quantitative analysis of the overall system performance improvement potential of the recuperated system demonstrates that increasing the isentropic efficiency of the compressor and turbine from 85% to 95% significantly increases the modified overall exergetic efficiency from 37% to 57%. Similarly, by increasing the effectiveness and decreasing the pressure loss factor of all heat exchangers, from 0.90 to 0.98 and from 2.5% to 0.5% respectively, the modified overall exergetic efficiency increases from 34% to 54%. The results of exergy analyses provide novel insight into the innovation, research and development of pumped thermal electricity storage technology. [ABSTRACT FROM AUTHOR]

• A model for heat recovery via refrigeration with thermal storage is presented. • Model informs decision-makers of the potential the solution has in supermarkets. • Fossil fuel use can be displaced with a small increase in electricity costs (2–3%). • Annual energy demand reduction of 17–18% is feasible with such systems. • Thermal storage increases the potential of heat recovery by 11–12%. The large amount of recoverable heat from CO 2 refrigeration systems has led UK food retailers to examine the prospect of using refrigeration integrated heating and cooling systems to provide both the space heating and cooling to food cabinets in supermarkets. This study assesses the performance of a refrigeration integrated heating and cooling system installation with thermal storage in a UK supermarket. This is achieved by developing a thermal storage model and integrating it into a pre-existing CO 2 booster refrigeration model. Five scenarios involving different configurations and operation strategies are assessed to understand the techo-economic implications. The results indicate that the integrated heating and cooling system with thermal storage has the potential to reduce energy consumption by 17–18% and GHG emissions by 12–13% compared to conventional systems using a gas boiler for space heating. These reductions are achieved despite a marginal increase of 2–3% in annual operating costs. The maximum amount of heat that can be stored and utilised is constrained by the refrigeration system compressor capacity. These findings suggest that refrigeration integrated heating and cooling systems with thermal storage are a viable heating and cooling strategy that can significantly reduce the environmental footprint of supermarket space heating provision and under the adequate circumstances can forsake the use of conventional fossil-fuel (natural gas) boiler systems in food-retail buildings. [ABSTRACT FROM AUTHOR]