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The experimental thermal characterization during charging and discharging of a prototype compact latent heat thermal energy storage system (LHTESS) with an embedded horizontally oriented finned mul...

This briefing reviews progress that was made at the 26th United Nations Climate Change Conference of the Parties (COP26) held in Glasgow, Scotland, UK over 1–12 November 2021. The context of the global climate change challenge is outlined, along with the aspirations of the major participating international groups. An overall balance sheet is provided that gives an assessment of the achievements and disappointments in the outcomes of COP26. This assessment sets a backdrop to what needs to be achieved when the Parties next meet at COP27 in Sharm El-Sheikh, Egypt in 2022 to address both immediate and longer-term climate change mitigation, adaptation and climate finance.

Mobilized-Thermal Energy Storage (M-TES) systems, are an attractive alternative solution to supply heat to distributed heat users by recovering and transporting the low-temperature industrial waste heat (IWH) by vehicular means, have the potential to reduce both the CO2 emissions and costs of energy consumption and lead to more efficient industrial activities as well as improve the quantity of low-carbon energy consumed for heat generation in the residential sector. This paper provides a state-of-the-art review of Phase Change Materials (PCM) applied in M-TES systems. The concept of the M-TES system is briefly described and summarized, including available IWH sources, heating and cooling facilities (for distributed end users), and the main features of two different types of M-TES containers. Recent research achievements in the field have been reviewed, focusing on developed prototypes, experimental and numerical studies, and economic and environmental evaluations. Finally, the barriers to the application of M-TES and possible solutions are discussed. The review highlighted that direct-contact M-TES storage systems can have up to 60% shorter charging and discharging periods when compared to indirect-contact M-TES storage systems with similar storage capacities. Using heat transfer enhancement techniques such as graphite additives, the charging and discharging period for indirect-contact M-TES container can be shortened by up to 74% and 67%, respectively, and that the charging time of direct-contact M-TES can be reduced by up to 29%. The use of M-TES to provide heat can significantly decrease the primary energy requirement, exergy losses and the CO2 emissions by up to 95%, 60% and 93%, respectively, compared to conventional heating facilities using fossil fuels. Recommendations for future research are presented, providing insights of where the current research in the M-TES field is heading and highlights the key challenges that remain to be resolved.

Paraffin has been one of the most potential phase change materials (PCMs) in low-temperature latent heat storage. However, the low thermal conductivity of paraffin restricts its thermal performance and limits its large-scale applications. Adding a suitable proportion of expanded graphite (EG) into the paraffin can enhance the heat transfer rate, thus improve the thermal efficiency of the whole heat storage system. In this paper, five paraffin/EG composites with 2 wt%, 5 wt%, 10 wt%, 15 wt% and 20 wt% of EG were prepared to study their long-term characteristics. Effects of thermal cycling (up to 100 thermal cycles) on the thermophysical properties were studied, such as, phase transition temperature, latent heat, chemical compatibility, thermal stability and thermal conductivity. The paraffin/EG composite has the potential for converting intermittent electricity directly into heat, therefore, the effect of thermal cycling on electrical conductivity was also studied. The results showed that both phase transition temperature and latent heat decreased slightly after 100 thermal cycles, within 1% and 3% respectively. Thermal cycling changed the distribution and interaction between the paraffin and EG resulting in a lower temperature thermal decomposition process. Long-term thermal cycling also had a more negative effect on the increase rate in thermal and electrical conductivity achieved for higher EG content.

A primary drawback of solar thermal technologies, especially in a domestic setting, is that collection of thermal energy occurs when solar irradiance is abundant and there is generally little requirement for heating. Thermochemical Energy Storage (TCES) offers a means of storing thermal energy interseasonally with very little heat loss. A combination of Solar Thermal Collectors (STC) and TCES systems will allow a variety of different heating applications, such as domestic space and hot water heating as well as low temperature industrial process heat applications to be met in a low carbon way. This paper describes and assesses the feasibility of two novel technologies currently under development at Loughborough University; i) an evacuated flat plate STC and ii) composite TCES materials, coupled together into a system designed to store and supply thermal energy on demand throughout the year. The predicted performance of an evacuated flat plate STC is described. The objective of this paper is to evaluate the economic, energy and carbon saving potential of conceptual STC + TCES systems suitable for domestic use. This research uses experimental results from Differential Scanning Calorimeter tests to evaluate the total enthalpy, dehydration enthalpy and sensible component enthalpy of composite TCES materials. The experimental results along with predicted performance of STC are used within a developed model to assess key metrics of conceptual STC + TCES systems feasibility, including; charging time, payback time, cost/ kWh, energy savings and CO2 savings. Preliminary results suggest the combination of these two technologies has significant potential for domestic applications.

Results of an extensive experimental investigation performed to study the effect of different values of wall heat flux in a rectangular PCM (phase change material) test cell on the melting process are presented. A new experimental system consisting of a rectangular cross-section test cell formed from polycarbonate sheet, copper plates and mica heaters was constructed. During experiments uniform wall heat flux (q″wall = 675, 960 and 1295 W/m2) were applied to both the left and right sides of the test cell. Thermocouples were used to measure the temperature at different locations inside the PCM and on the surface of the copper plates and an infrared camera was used to measure the polycarbonate sheet external surface temperature distribution. The results show the expected strong correlation between the magnitude of wall heat flux and the melt fraction in the PCM as it drives the convective heat transfer. The transparent polycarbonate wall makes it possible to observe the location of the solid/liquid interface and determine melt fractions. The experiments have produced a significant experimental data set for the validation of numerical models simulating the solid/liquid phase change process and PCM melting in geometrical configurations relevant to, for example, latent heat thermal energy storage systems.

This study experimentally investigates the effect of different values of wall heat flux intensity on the melting of RT44HC Phase Change Material (PCM) in a rectangular test cell. A new novel experimental test rig to provide accurate data for the validation of numerical models of phase change was developed. The designed and constructed test rig consists of a horizontal rectangular cross-section test cell formed from polycarbonate sheet with copper plates and mica heaters to provide controlled uniform wall heat flux. Experiments were performed for three constant uniform wall heat flux values (q″wall = 675, 960 and 1295 W/m2) applied to both left and right sides of the test cell. An imaging technique was used to visualize and record the movement of the solid-liquid interface using a Canon EOS DSLR Camera. The results obtained show a strong correlation between the magnitude of wall heat flux which drives the convective heat transfer and melt fraction development in the PCM. The results also show that increasing the input power from 675 W/m2 to 960 W/m2 to 1295 W/m2 reduces the total time for the melting process by 26.3% and 42.10% respectively. The raw data set comprised of measured temperatures and observation of melt fraction development provide a useful data set for validation of numerical models aiming to simulate the melting process in a rectangular cross-section test cell.

Incorporating PCMs into traditional building envelops is an effective way to minimise indoor temperature fluctuations and maximise thermal comfort in summertime. PCMW is of particular interest due to its ease of installation. This paper presents a theoretical analysis in which both outdoor air temperature and solar radiation are considered to predict the optimal melting temperature of an internal PCMW for passive indoor temperature control. Key factors influencing the selection of optimal melting temperature are studied including outdoor temperature, solar radiation, window size, air change rate (ACH), and external wall and window U-values. A case study was carried out on a lightweight building using real weather data for the summer months in the UK. The simulation was developed in EnergyPlus with the potential to reduce energy use while maintain the desired internal temperatures determined. The optimal melting temperature was found to be 23.4 °C during summer in the UK from a theoretical analysis, which showed good agreement with the numerical simulations performed. In summer, the percentage energy savings can be as high as 40% compared with a similar building without PCMWs and the period which temperatures are maintained within the thermal comfort range can be improved by up to 7.2%.

Hermeticity of vacuum edge-sealing materials are one of the paramount requirements, specifically, to the evolution of energy-efficient smart windows and solar thermal evacuated flat plate collectors. This study reports the design, construction and performance of high-vacuum glazing fabrication system and vacuum insulated glazing (VIG). Experimental and theoretical investigations for the development of vacuum edgeseal made of Sn-Pb-Zn-Sb-AlTiSiCu composite in the proportion ratio of 56:39:3:1:1 by % (CS-186) are presented. Experimental investigations of the seven constructed VIG samples, each of size 300mm·300mm·4 mm, showed that increasing the hot-plate surface temperatures improved the cavity vacuum pressure whilst expediting the pump-out hole sealing process but also increases temperature induced stresses. Successful pump-out hole sealing process of VIG attained at the hot-plate set point temperature of 50˚C and the approximate cavity pressure of 0.042 Pa was achieved. An experimentally and theoretically validated finite volume model (FVM) was utilised. The centre-of-pane and total thermal transmittance values are calculated to be 0.91 Wm-2K-1 and 1.05 Wm-2K-1, respectively for the VIG. FVM results predicted that by reducing the width of vacuum edge seal and emissivity of coatings the thermal performance of the VIG is improved.

Thermochemical Energy Storage (TCES) materials can be used to store and release thermal energy during times of high demand for applications such as Domestic Space Heating (DSH) or Domestic Hot Water (DHW). Novel composite materials have been created by 2 methods, i) impregnating zeolites with MgSO4 using a modified wetness impregnation method and ii) forming zeolites impregnated with MgSO4 into 4mm pellets using a pellet preparation method and a binder, these composite pellets have been named ZMK. Differential Scanning Calorimetry (DSC) measurements were used to assess the composite materials energy density and performance ratio. The ZMK pellets were also tested on a larger (200g) scale using an in-situ open fixed bed reactor. It was found that ZMK dehydrates at a temperature of 150˚C with a dehydration enthalpy of 715 J/g, with an overall performance ratio of 85% at a 10mg scale. ZMK was found to have a volumetric energy density of 415MJ/m3 and a specific energy density of 702J/g at a 200g scale when hydrated (discharged) with 68% RH air at 20˚C, making it a suitable candidate for interseasonal TCES.

A numerical study of melting of Lauric acid in a vertical rectangular cross-section enclosure was performed with FLUENT 18.2. The enclosure was subject to a constant heat flux on one side of 500, 750 and 1000 W/m2. For model validation purposes simulations were initially performed of experimental systems in the literature with predicted values compared to experimental measurements. Predictions indicate that during the initial stage of melting, conduction is the dominant mode of heat transfer, subsequently replaced by convection when there is sufficient liquid PCM. The simulations show that as the magnitude of heat flux is increased, average wall temperature increases and melting time reduces. The predicted results indicated that melting time decreases by 28.5 % as the wall flux increases by 50 % from 500 to 750 W/m2. The time required for melting reduces by about 50% when the wall heat flux is increased from 500 to 1000 W/m2.

With the increasing awareness of building energy efficiency, indoor environment quality for human wellbeing and working efficiency, efforts have intensified in to inventing intelligent building components. This paper provides a first step in developing a novel multi-effect smart window system, which achieves enhanced energy efficiency and an improved indoor luminous environment by integrating a Transparent Insulation Material (TIM) structure incorporating a Thermotropic material. This system automatically regulates the admittance of solar heat and natural light into the building by responding to a changing environment while taking advantage of the increased thermal resistance and scattered daylight of window integrated TIM. A comprehensive workflow via EnergyPlus and RADIANCE was used to accurately predict the luminous and energy performance of applying the smart window system on a typical south-facing office under selected climates (London, Stockholm, Rome and Singapore). The effect of the optical properties and transition temperature of thermotropic material on building performance was explored in detail. Annual simulation results predict that, with a careful selection of the Thermotropic material properties, installing the TT PS-TIM window system is able to yield up to a 27.1% energy saving when compared with a conventional double glazed window, under the modelled Rome climate. TT PS-TIM windows also provide dynamic daylight control, resulting in increased daylight availability with the percentage of working hours that fall into the UDI500–2000 lx range increasing to 62.3%. The results of this research provide guidance for the next step of the material design and development that seek to balance energy efficiency and solar and daylight control through the use of thermotropic materials.

To improve the energy efficiency of an industrial process thermochemical energy storage (TCES) can be used to store excess or typically wasted thermal energy for utilisation later. Magnesium carbonate (MgCO3) has a turning temperature of 396 °C, a theoretical potential to store 1387 J/g and is low cost (~GBP 400/1000 kg). Research studies that assess MgCO3 for use as a medium temperature TCES material are lacking, and, given its theoretical potential, research to address this is required. Decomposition (charging) tests and carbonation (discharging) tests at a range of different temperatures and pressures, with selected different gases used during the decomposition tests, were conducted to gain a better understanding of the real potential of MgCO3 for medium temperature TCES. The thermal decomposition (charging) of MgCO3 has been investigated using thermal analysis techniques including simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA/DSC), TGA with attached residual gas analyser (RGA) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) (up to 650 °C). TGA, DSC and RGA data have been used to quantify the thermal decomposition enthalpy from each MgCO3.xH2O thermal decomposition step and separate the enthalpy from CO2 decomposition and H2O decomposition. Thermal analysis experiments were conducted at different temperatures and pressures (up to 40 bar) in a CO2 atmosphere to investigate the carbonation (discharging) and reversibility of the decarbonation–carbonation reactions for MgCO3. Experimental results have shown that MgCO3.xH2O has a three-step thermal decomposition, with a total decomposition enthalpy of ~1050 J/g under a nitrogen atmosphere. After normalisation the decomposition enthalpy due to CO2 loss equates to 1030–1054 J/g. A CO2 atmosphere is shown to change the thermal decomposition (charging) of MgCO3.xH2O, requiring a higher final temperature of ~630 °C to complete the decarbonation. The charging input power of MgCO3.xH2O was shown to vary from 4 to 8136 W/kg with different isothermal temperatures. The carbonation (discharging) of MgO was found to be problematic at pressures up to 40 bar in a pure CO2 atmosphere. The experimental results presented show MgCO3 has some characteristics that make it a candidate for thermochemical energy storage (high energy storage potential) and other characteristics that are problematic for its use (slow discharge) under the experimental test conditions. This study provides a comprehensive foundation for future research assessing the feasibility of using MgCO3 as a medium temperature TCES material. Future research to determine conditions that improve the carbonation (discharging) process of MgO is required.

Thermal energy storage (TES), specifically heat storage, may have a key role to play in supporting the achievement of the UK’s future decarbonization targets for heat and electricity. TES systems can be designed to collect and store low-quality heat (such as low-temperature industrial process heat), which can either be used directly or at a later time to produce hot water and space heating for buildings.

Vacuum flat plate (VFP) solar thermal collectors exhibit excellent optical and thermal characteristics due to a combination of wide surface area and high vacuum thermal insulation offering a high performance and architecturally versatile collector with a variety of applications for industrial process heat and building integration. A vacuum flat plate solar collector consists of a solar absorber in a flat vacuum enclosure comprising glass or glass and metal covers sealed around the periphery with an array of support pillars to maintain the separation of the enclosure under atmospheric pressure. The edge seal must be both mechanically strong and hermetic to ensure the durability of the internal vacuum over collector lifetime. This presents several challenges for the fabrication of flat vacuum enclosures. In this study a novel sealing technique is presented using a tin-based alloy, Cerasolzer 217, to create the vacuum seal between two glass panes and an edge separating spacer. The sealing process is undertaken at temperatures ≤250 °C allowing the use of thermally tempered glass panes. The mechanical strength of the edge seal was investigated using a tensometer. It was demonstrated that the bond between glass and edge spacer was sufficiently strong to withstand induced stresses in the edge seal region. The edge seal was leak tested using a conventional Helium mass spectrometer leak detector and was shown to possess leak rates low enough to maintain an adequate vacuum pressure to supress conductive and convective heat transfer in the collector. A finite element method (FEM) is developed and validated against the experimental results and employed to predict the stresses in different regions of the enclosure. It was found that the mechanical strength limits of the seal and glass are higher than the stresses in the edge seal region and on the glass surface, respectively.

Heat losses from a flat panel solar collector can be significantly reduced by lowering the internal pressure to

Creating a vacuum (

Non-concentrating solar thermal collectors are generally available in two forms, flat plate or evacuated tube. Recently a third configuration, the evacuated flat plate, has attracted interest due to enhanced performance and aesthetic characteristics. By isolating a solar absorber in a vacuum space (

The concept of an evacuated flat plate (EFP) collector was proposed over 40 years ago but, despite its professed advantages, very few manufacturers have developed commercial versions. This situation suggests both technical difficulties in manufacturing a competitively-priced sealed for life panel and a lack of awareness of the benefits of such panels. This paper demonstrates an evacuated flat plate simulation that closely models experimental efficiency measurements. Having established the validity of the model, it compares published data for a commercial EFP collector with predictions for an optimal design to investigate whether any further efficiency improvement might be possible. The optimised design is then evaluated against alternative solar energy devices by modelling a number of possible applications. These comparisons should inform choices about solar options for delivering heat: EFP collectors are well-suited to some of these applications. Evacuated flat plate collectors are a possible alternative to concentrating collectors for Organic Rankine Cycle power generation. The annual output for all the modelled collectors was found to be a quadratic function of delivery temperature: this enabled a novel optimisation of ORC source temperature. Predictions for concentrating and non-concentrating ORC plant are compared with a PV/thermal alternative. The ORC output is significantly less than a PV panel would achieve; applications needing both heat and power are better served by PVT panels. This is an original and novel result.

The need to develop renewable heat sources for domestic space heating is a well known problem, for solar thermal systems mismatch between generation and load is a major issue, and thermochemical interseasonal heat storage offers a solution to this problem. Recent research has shown that using an absorbent material as a host for salt hydrates can be advantageous in achieving a high energy density material while alleviating the problematic practical characteristics, such as agglomeration, which salt hydrates typically possess. In this paper results are presented for a 13X molecular sieve which was tested to determine its potential for interseasonal domestic thermochemical energy storage alone and as a host material for Magnesium Sulfate (MgSO 4 ). Two different impregnation preparation methods have been utilised in our experiments, (i) a wetness impregnation method and (ii) a new method in which 13X molecular sieve powders and MgSO 4 are formed into pellets with use of a binder. The materials produced by each method were tested against each other and compared to a zeolite-Y material to assess which is the best candidate material for thermal energy storage. The impact of ion exchange on the energy storage potential of the 13X materials was also investigated. Analysis of the materials characteristics and thermal performance was conducted using a Differential Scanning Calorimeter (DSC), Thermogravimetric Analyser (TGA) coupled with a Residual Gas Analyser (RGA), Scanning Electron Microscope (SEM) with Energy Dispersive X-ray (EDX) spectroscopy and a custom built fixed bed 200 g in-situ hydration and dehydration chamber to assess the materials performance on a larger scale. The results demonstrate that the thermochemical storage potential of the 13X molecular sieve was enhanced following a Mg 2+ ion exchange process, resulting in a maximum increased energy storage of approximately 14% (65 J/g) compared to standard non treated 13X pellets.

The UK domestic solid-wall housing stock is a particular focus by reason of its considerable space-heating energy consumption than in any other sector because these are so far anticipated to be hard-to-heat. Existing UK solid-wall houses, which have both heritage values and historic fabric, are being improved but yet they tend to have preventable heat loss through windows. This paper takes a comparative analysis approach to envisage space-heating supply required for achieving thermal comfort temperatures and attainable solar energy gains to households with the retrofit of the thermal transmittance (U-value) of a new composite edge-sealed triple vacuum glazing i.e. 0.33 Wm-2K-1. 3D dynamic thermal models (timely regimes of heating, occupancy, ventilation and internal heat gains) of an externally-insulated solid-wall detached house with a range of existing glazing types along with triple vacuum glazing with frame areas are modelled. The predictions of varying window-to-wall ratios on space-heating energy and solar energy gains for winter months are analysed. The results have shown by increasing WWR from 5% to 59%, the winter months space-heating energy supply to the solid-wall detached house slightly decreases with triple vacuum glazed windows whilst a gradual rise was noticed with single glazed windows. A small increase in solar gains, i.e. 5.6 kWh, was predicted for a room with a triple vacuum glazed window compared to a triple glazed air filled window. However, with similar increment to WWR, the house achieved higher solar gains with single glazed windows. The notable winter and annual space-heating energy savings of 14.58% (EUR 49.2) and 15.31% (EUR 105.4), respectively, were obtained with a solid-wall detached house retrofitted with triple vacuum glazed windows compared to single glazed windows. In the overall fabric heat loss, the heat loss calculations show a prominent reduction from 12.92% to 2.69% when replacing single glazed windows to triple vacuum glazed windows. However, the space-heating energy cost savings can be more significant when the solid-wall insulation is improved to 2010 UK building regulations. Simulated results show a negligible space-heating and cost savings with a house of double argon glazed windows compared to double glazed air filled windows. Solar gains do contribute in reducing the space-heating load but during summer months it could cause overheating and may be inconvenient to households and cooling is essential in such circumstances. Thus, the triple vacuum glazing, if manufactured at the mass production level with cost-effective airtight sealing materials and improved fabrication methods, is a great opportunity in reducing building energy consumption and has a potential to increase window-to-wall area ratios without a risk of overheating during summer months.

A considerable effort is devoted to devising retrofit solutions for reducing space-heating energy in the domestic sector. Existing UK solid-wall dwellings, which have both heritage values and historic fabric, are being improved but yet they tend to have meagre thermal performance, partly, due to the heat-loss through glazings. This paper takes comparative analyses approach to envisage space-heating supply required in order to maintain thermal comfort temperatures and attainable solar energy gains to households with the retrofit of an experimentally achievable thermal performance of the fabricated sample of triple vacuum glazing to a UK solid-wall dwelling. 3D dynamic thermal models (timely regimes of heating, occupancy, ventilation and internal heat gains) of an externally-insulated solid-wall detached dwelling with a range of existing glazing types along with triple vacuum glazings are modelled. A dramatic decrease of space-heating load and moderate increase of solar gains are resulted with the dwelling of newly achievable triple vacuum glazings (having centre-of-pane U-value of 0.33 Wm-2K-1) compared to conventional glazing types. The space-heating annual cost of single glazed dwellings was minimised to 15.31% (≈USD 90.7) with the retrofit of triple-vacuum glazings. An influence of total heat-loss through the fabric of solid-wall dwelling was analysed with steady-state calculations which indicates a fall of 10.23% with triple vacuum glazings compared to single glazings.

In this study, an experimental setup is developed to assess the thermal performance of a compact Latent Heat Thermal Energy Storage System (LHTESS) prototype during the charging/discharging stages. The LHTESS consists of a shell and horizontally oriented multi-tube heat exchanger and a commercially available paraffin wax RT44HC, which has a phase change temperature between 41°C and 43 °C as the energy storage medium. The testing campaign evaluated the influence of several operating conditions including the heat transfer fluid (HTF) volume flow rate and inlet temperature on the LHTESS power input and output, melting and solidification time and the energy stored and released. From the experimental results, it was observed that increasing the HTF inlet temperature has a significant effect on charging time compared to changing the HTF volume flow rate. When the LHTESS was charged using a fixed HTF inlet temperature of 60 °C, the charging process period took 296.3 min, 233.5 min, 204.8 min and 197.8 min when the HTF volume flow rate is 3.0, 4.5, 6.0 and 7.5 L/min. However, when the LHTESS was charged at HTF volume flow rate of 4.5 L/min, the results show that the charging completion time for HTF inlet temperatures of 55°C, 60 °C and 65°C are 316.6, 233.5 and 209.67 min, respectively. The results from the experimental analysis showed that the discharge time was significantly longer than the charging time due to an ever-growing layer of solid PCM around the external surface of heat exchanger throughout the discharging process which reduces the heat transfer coefficient between the PCM and HTF. This did not change substantially with the changing HTF volume flow rate.

Summary Recent theoretical studies have predicted that adiabatic compressed air energy storage (ACAES) can be an effective energy storage option in the future. However, major experimental projects and commercial ventures have so far failed to yield any viable prototypes. Here we explore the underlying reasons behind this failure. By developing an analytical idealized model of a typical ACAES design, we derive a design-dependent efficiency limit for a system with hypothetical, perfect components. This previously overlooked limit, equal to 93.6% under continuous cycling for a typical design, arises from irreversibility associated with the transient pressure in the system. Although the exact value is design dependent, the methodology we present for finding the limit is applicable for a wide range of designs. Turning to real systems, the limit alone does not fully explain the failure of practical ACAES research. However, reviewing the available evidence alongside our analytical model, we reason that underestimation of the system complexity, difficulty with the integration of off-the-shelf components, and a number of misleading performance claims are the primary reasons hindering ACAES development., Graphical abstract, Energy Systems; Energy Management; Energy Storage

Latent heat storage in molten salt mixtures has been considered as a promising method for medium temperature solar thermal storage and industrial waste heat storage. The binary sodium/lithium nitrate salt mixture is one of the potential candidate materials. However there is limited information on their thermal performance except for their phase diagrams. In this paper, two binary salts, NaNO3–LiNO3 (46–54%) and NaNO3–LiNO3 (40–60%), were investigated to assess their suitability for medium temperature heat storage. The thermal properties and long term stability under multiple cycles of each binary salt pair were investigated using Differential Scanning Calorimetry (DSC). The chemical stability at elevated temperatures was tested using a Thermogravimetric Analyser (TGA). Both binary salts analysed have suitable melting temperatures (just under 200°C) with relatively high latent heat values (>220kJ/kg), both exhibiting good thermal and chemical stability.

A comprehensive review of phase change materials (PCMs) with phase transition temperatures between 0 and 250 °C is presented. From that review, organic compounds and salt hydrates seem more promising below 100 °C and eutectic mixtures from 100 to 250 °C. Practical indirect heat exchanger designs for latent heat storage systems were also assessed and feasible heat enhancement mechanisms reviewed. The focus on this temperature range is due to potential CO 2 emissions reduction able to be achieved replacing conventional heating and cooling applications in the domestic, commercial and public administration sectors, which represented around a quarter of the UK’s final energy consumption in 2015.

In this study, the thermal performance of latent heat thermal energy storage system (LHTESS) prototype to be used in a range of thermal systems (e.g., solar water heating systems, space heating/domestic hot water applications) is designed, fabricated, and experimentally investigated. The thermal store comprised a novel horizontally oriented multitube heat exchanger in a rectangular tank (forming the shell) filled with 37.8 kg of phase change material (PCM) RT62HC with water as the working fluid. The assessment of thermal performance during charging (melting) and discharging (solidification) was conducted under controlled several operational conditions comprising the heat transfer fluid (HTF) volume flow rates and inlet temperatures. The experimental investigations reported are focused on evaluating the transient PCM average temperature distribution at different heights within the storage unit, charging/discharging time, instantaneous transient charging/discharging power, and the total cumulative thermal energy stored/released. From the experimental results, it is noticed that both melting/solidification time significantly decreased with increase HTF volume flow rate and that changing the HTF inlet temperature shows large impacts on charging time compared to changing the HTF volume flow rate. During the discharging process, the maximum power output was initially 4.48 kW for HTF volume flow rate of 1.7 L/min, decreasing to 1.0 kW after 52.3 min with 2.67 kWh of heat delivered. Based on application heat demand characteristics, required power levels and heat demand can be fulfilled by employing several stores in parallel or series.

Advancement in hermetic (vacuum-tight) edge-sealing materials has been one of the challenges since decades because of the existing cost, use of hazardous substance and complexity-to-construct issues in vacuum glazing. This paper presents novel experimental findings with designs and methods developed to construct and analyse thermal performance of the fusion edge-sealed vacuum glazing. The novel concept of fusion edge-seal consists of forming a thin glass-metallic rigid textured layer, in which the formation processes and experimental glass-metallic textured surface bonding property tests of 15 samples are microstructurally analysed using FIB-SEM and optical microscopy and succeeded the correct mixture of B2O338-Sn62 wt%. Experimental analyses of at least 60 samples conducted using different techniques and Pb-free materials, among which five vacuum glazing samples of various designs and techniques discussed in this paper. The fusion edge-sealed vacuum glazing, constructed with bonded Sn62-B2O338 wt% surface textured fused with Sn90-In10 wt% alloy at 450 °C, achieved at the hot-plate surface heat induction of 50 ± 5 °C and the cavity vacuum pressure of 8.2 · 10−4 Pa. Validated 3D FEM employed and the centre-of-sheet and total thermal transmittance values of fusion edge-sealed vacuum glazing (sample ‘A5’), area of 300 · 8300 mm with 10 mm wide fusion edge-seal, predicted to be 1.039 and 1.4038 Wm−2K−1, respectively.

The concept of an evacuated flat plate collector was proposed over 40 years ago but, despite its professed advantages, very few manufacturers have developed commercial versions. The absorber is a key component of a flat plate collector: in the context of an evacuated panel, absorber design poses a number of technical challenges.\ud \ud A flooded panel absorber has been designed for use in evacuated flat plate solar collectors. The aim was to obtain higher efficiency, in a low out-gassing material, than would be possible using a conventional serpentine tube design.\ud \ud Initial plans for a micro-channel plate were modified when optimisation analysis showed that a flooded panel could achieve as good performance with easier fabrication. The absorber plate is made from hydroformed stainless steel sheets welded together and features an array of through-holes for the glass supporting pillars with the square panel sub-divided into two rectangles connected in series for ease of fabrication and better flow distribution. The coolant flow was modelled in Star-CCM+. FEM simulations based on tensile test data informed the choice of sheet thickness and weld radius around the holes to withstand the 1 bar pressure differential.\ud \ud Hydroforming is an effective method for producing sheet metal components, e.g. plates for heat exchangers or solar absorbers. As a thermal engineering experimental technique, the tooling is significantly cheaper than press tools since the mould does not need a matching die. In a research context, the ability to form plates in-house and explore profile and tooling options at low cost is very useful and might find application in other fields such as experimental heat exchangers.\ud \ud A hydroforming facility was built using 85 mm thick steel sheet and a 25 MPa hydraulic pump. This proved highly effective at forming 0.7 mm stainless steel sheet. A total of eight absorbers were fabricated and successfully leak tested using helium. Two variants were made: one kind for use in enclosures with a metallic rear tray, the other for enclosures with glass on both sides. The collector efficiency factor is estimated to be 3% higher than for commercial tube-on-plate designs.

Building Integrated PV and Concentrating PV can generate electricity onsite and provide savings in materials and electricity costs, as well as protecting buildings from weather. In this paper, a novel truncated stationary asymmetric compound parabolic photovoltaic concentrator with a geometric concentration ratio of 2.0 has been designed and experimental characterised. The designed system is suitable for building façade application, especially for vertical façade. It has wide acceptance half angles of 0° and 55°, this acceptance angle range enables the concentrator to operate year-round at its geometric gain in most of the UK and EU climatic condition. A comprehensive indoor test was carried out to evaluate the electrical and thermal characterisation of the developed Building Façade Integrated Asymmetric Compound Parabolic Photovoltaic concentrator (BFI-ACP-PV) system, and also the factors that affect the power output of the developed system. The experimental results showed that the developed BFI-ACP-PV system has the potential to increase the power output per unit solar cell area by a factor of 2, when compared with a non-concentrating PV system. Subsequently, a Phase Change Material (PCM) system was integrated to the rear of the BFI-ACP-PV system to moderate the PV temperature rise and maintain good solar to electrical conversion efficiency. It was found out that the electrical conversion efficiency for the BFI-ACP-PV coupled PCM system was increased by over 5% compared with a similar system with no PCM integrated at the rear, when the incident solar radiation intensity was 280 W/m2, this value increased by over 10% for an incident solar radiation intensity of 670 W/m2.

The concept of an evacuated flat plate collector was proposed over 40 years ago but, despite its professed advantages, very few manufacturers have developed commercial versions. This paper demonstrates the reduction in heat loss coefficient and increase in efficiency resulting from evacuating a flat plate collector: it is hoped that these results will stimulate interest in the concept. Evacuated tubes are now mass-produced in large numbers; evacuated flat plate collectors could in principle replace these tubes if the technical difficulties in creating extended metal-glass seals can be overcome. The experimental experiences described here should indicate targets for future research. Two different designs of evacuated flat plate solar thermal collector, each with a 0.5 × 0.5 m flooded panel black chrome plated absorber, were tested under a solar simulator. The cover glasses were supported by an array of 6 mm diameter pillars. Inlet and outlet temperatures were monitored via PT100 RTDs and glass temperatures were measured using thermocouples. Inlet temperature was controlled by a fluid circulator connected to a header tank with a Coriolis mass flow meter to measure fluid flow rate. Testing was conducted indoors with and without the use of a fan to cool the top cover glass. The test conditions spanned the range 200 < G < 1000 W/m 2 , 0⩽T M ⩽52°C. Evacuating the enclosure reduced the measured heat loss coefficient by 3.7 W/m 2 K: this was a close match to predictions and corresponds to an increase in aperture efficiency from 0.3 to 0.6 at T M /G=0.06m 2 K/W. The poor efficiency under non-evacuated conditions was due to the black chrome absorber coating being less selective than commercial panel coatings. The solder seals were developed from experience with vacuum glazing but the increased gap led to reliability issues. A vacuum pump maintained the enclosures under a high vacuum ( < 0.1 Pa) during testing. The enclosure based on a thin rear metal tray proved to be more effectively sealed than the more rigid enclosure with glass on both sides: the latter developed leaks as the front to rear temperature difference increased. The biggest challenge in the manufacture of evacuated flat plate collectors is to ensure a long-term hermetic seal such that no pumping is required.

© 2017 The Author(s) A solar simulator has been designed and built for testing prototype (0.5 × 0.5 m) flat plate thermal collectors. An internally reflecting light tube generates multiple virtual images of the four halogen floodlights to ensure uniform illumination. Ray-tracing simulations were used to choose the tube dimensions and maximum allowable clearance. Illumination measurements agree well with these predictions. The visible & near IR spectrum appears to follow a black body curve. In the absence of a “cold sky” IR filter there is a secondary, long wavelength IR spectral component that causes heating of the cover glass on a solar flat plate collector. The cover glass temperature can be maintained at typical outdoor levels using a cooling fan. The design would be well suited to LED illumination. Simulation of solar collector response to this spectrum shows that an efficiency based on pyranometer readings is approximately 1% higher than would be obtained with an AM1.5 spectrum.

Transparent insulation materials (TIMs) can combine the advantages of opaque insulation with solar collection when applied to building facades. Mathematical models developed to calculate the optical performance of a transparent insulated (TI) system are presented. The TI-system modelled consisted of a 6 mm outer glass pane, a 22 mm wide polymethylmethacrylate (PMMA) capillary cell section and an 8 mm inner glass pane. When solar beam radiation passed through the TI-system, the solar transmittance, absorptance and reflectance that occurred in the system were calculated. Optical interactions at each layer in the TI-system were numerically modelled for five incidence angles. A direct to diffuse transmittance was calculated for solar radiation passing through the PMMA capillary cells with an incidence angle, θ i , in excess of 0°. The calculated transmittance for the PMMA capillary cells at 0° incidence angle was 0.8264 with overall solar transmittance of up to 0.66 calculated for the whole TI-system, a value which is consistent with that found in previous research. This paper presents a method for calculating the optical properties for multiple PMMA capillary cells, encapsulated between two glass panes for different solar incidence angles, with input of relevant data from other sources. This method can also be adapted for other types of glazing systems.

Solar thermal collectors for buildings use a heat transfer fluid passing through heat exchange channels in the absorber. Flat plate absorbers may pass the fluid through a tube bonded to a thermally conducting plate or achieve lower thermal resistance and pressure drop by using a flooded panel or microchannel design. The pressure drop should be low to minimise power input to the circulating pump. A method is presented for choosing the optimum channel hydraulic diameter subject to geometric similarity and pumping power constraints; this is an important preliminary design choice for any solar collector designer. The choice of pumping power is also illustrated in terms of relative energy source costs. Both microchannel and serpentine tube systems have an optimum passage diameter, albeit for different reasons. Double-pass and flooded panel designs are considered as special microchannel cases. To maintain efficiency, the pumping power per unit area must rise as the passage length increases. Beyond the optimum pumping power the rise in operating cost outweighs the increase in collector efficiency.

This study presents a new dispatch model (SCSP) for a CSP plant based on a dynamic programing algorithm. The purpose is to investigate the cost of balancing a CSP plant in the Spanish electricity market. Results are presented for a parabolic plant in the Spanish market for years 2009, 2010 and 2011 using solar availability data at the Plataforma Solar, Andalucia, Spain. The variation of balancing cost with solar multiple (SM) and number of storage hours (Nh) is analysed and results for two different optimisation cases presented. The first uses day-ahead forecasts for both solar availability and market prices. The second uses day-ahead solar availability and within-day market price forecasts. Both cases are settled in the balancing market. Key results include that the balancing cost decreases with increased SM and Nh and that balancing costs can be 2.2% to 9.5% of the plants gross income. For all SM and Nh, balancing costs are a function of season, being lower in summer than winter driven by increased load-factor in summer. During the year Quarter 3 has a lower balancing cost than Quarter 2 due to a closer match between forecast and actual solar availability. Optimising against within-day prices costs more than with day-ahead prices resulting from more balancing energy traded at a less favourable price than day-ahead. It is envisaged that the numbers presented in this study will provide an aid to policy makers when constructing tariffs to support future CSP development.

This paper presents an overview of the Malawi energy situation and the potential of renewable energy resources including solar, wind, biomass, hydro and geothermal. Despite a range of efforts by local and international stakeholders to increase access to modern energy sources in the country, 89 per cent of Malawi׳s energy is still sourced from traditional biomass mainly fuel wood. Only 8 per cent of the population in Malawi have access to electricity but installed capacity of electricity generation is lower than demand. This leads to load shedding by the electricity supplier; consequently electricity supply in Malawi is unreliable and micro and macroeconomic activities are significantly affected. Solar, non-traditional biomass (crop residues and forest residues not burnt on three stone fireplaces, and biogas), hydro, wind and geothermal are potential energy resources that could enhance Malawi׳s energy security. However, unreliable financing mechanisms for large scale energy projects; shortage of trained human resource; lack of coordination among local institutions; unclear regulation enforcement; and sometimes political governance impede sustainable delivery of energy projects. The Malawi energy policy targets and drivers are also discussed in the paper. Based on the prevailing energy situation, a PESTLE analysis is provided in this paper outlining a novel thinking for addressing the political (P), economic (E), social (S), technological (T), legal (L), and environmental (E) challenges that constrain the development of renewable energy technologies in Malawi.

Vacuum glazing Finite element model Finite volume model Low-emittance coating Triple vacuum glazing Hybrid vacuum glazing Electrochromic vacuum glazing abstract Vacuum glazing consists of two parallel glass sheets with a narrow vacuum gap in between. The sheets are separated by an array of support pillars under the influence of atmospheric pressure imposed on the external surfaces of the two glass sheets. The vacuum gap sealed by a sealant (either solder glass or indium alloy) minimizes the air heat conduction and convection across the glazing. One or two high performance low emittance (low-e) coatings deposited on the internal surface(s) within the vacuum gap reduces the radiative heat transfer to a very low level. The heat transmittance of 0.80 W m � 2 K � 1 and 0.86 W m � 2 K � 1 at the central area of vacuum glazing with two low-e coatings and support pillars of 0.25 mm and 0.4 mm in diameter were achieved and experimentally characterized using the guarded hot box calorimeters by the University of Sydney and the University of Ulster, respectively. If combined with solar control glazing such as electrochromic (EC) glazing, it has a great potential to significantly reduce the load of heating in winter and cooling in summer and to provide thermal comfort for the occupants. If combined with the third glass sheet forming a gas filled or second vacuum gap, the heat transmittance will be further reduced. In this work, the basic characteristics and fabrication process of vacuum glazing are reviewed. The potential performance of vacuum glazing, EC vacuum glazing and other hybrid vacuum glazing with second either gas filled gap or vacuum gap are presented.

The performance of Pimlico District Heating Undertaking (PDHU), in London, with an annual heating load of 50,000 MWh was analysed throughout 2012. Half-hourly data for the system were investigated to determine the natural gas consumed, operation of the 2500 m3 accumulator, the two 1.6 MWe combined heat and power (CHP) engines, the three 8 MW boilers, the electricity import and export and the consumers' heat consumption. These data were used to characterise the current performance in detail, and an energy flow diagram for the system energy flows was generated. The modulating efficiencies of the boilers varied from 84% to 91%, whereas the CHP engines performed with a modulating near constant electrical efficiency of 40%, but with a thermal efficiency that decreases with higher load. The current operation of the plant is compared across 10 scenarios. These scenarios were compared while (i) using the accumulator more effectively to let the boilers operate at full load only and (ii) using the provided maintena...

This paper presents the experimental performance analysis of a latent heat thermal energy storage system (LHTESS) designed for domestic hot water (DHW) applications. The designed, fabricated and characterised thermal store comprised of a vertically oriented multi-pass tube heat exchanger in a rectangular cross-section container filled with phase change material (PCM) paraffin wax RT44HC. The experimental investigation evaluated the heat transfer within the system, measured the transient temperature distribution, determined the cumulative thermal energy stored, charging and discharging time and the instantaneous charging and discharging power. The experimental work was conducted under controlled experimental conditions using different heat transfer fluid (HTF) inlet temperatures and different volume flow rates for store charging and discharging. It was found that during charging natural convection in the melt played a significant role. During discharging thermal conduction dominates and natural convection has an insignificant impact on the LHTESS performance. This is due to the development of a solid layer of PCM around the heat transfer tubes which increases the thermal resistance and reduces heat transfer to the liquid PCM. Higher HTF inlet temperature during charging significantly decreased store charging time. Increasing HTF inlet temperature from 60 to 70 °C shortened the charging time by 3.5 h, a further increase to 80 °C decreased melting time by a further 2 h.This study illustrates the extent to which LHTESS, and heat exchanger designs need to be improved to meet the desired charge/discharge time requirements for most short-term storage applications and that a broad range of domestic and commercial heat demands can be fulfilled by assembling several LHTESS units to operate in parallel.

A primary drawback of solar thermal technologies, especially in a domestic setting, is that collection of thermal energy occurs when solar irradiance is abundant and there is generally little requirement for heating. Thermochemical Energy Storage (TCES) offers a means of storing thermal energy interseasonally with very little heat loss. A combination of Solar Thermal Collectors (STC) and TCES systems will allow a variety of different heating applications, such as domestic space and hot water heating as well as low temperature industrial process heat applications to be met in a low carbon way. This paper describes and assesses the feasibility of two novel technologies currently under development at Loughborough University; i) an evacuated flat plate STC and ii) composite TCES materials, coupled together into a system designed to store and supply thermal energy on demand throughout the year. The predicted performance of an evacuated flat plate STC is described. The objective of this paper is to evaluate the economic, energy and carbon saving potential of conceptual STC + TCES systems suitable for domestic use. This research uses experimental results from Differential Scanning Calorimeter tests to evaluate the total enthalpy, dehydration enthalpy and sensible component enthalpy of composite TCES materials. The experimental results along with predicted performance of STC are used within a developed model to assess key metrics of conceptual STC + TCES systems feasibility, including; charging time, payback time, cost/kWh, energy savings and CO2 savings. Preliminary results suggest the combination of these two technologies has significant potential for domestic applications.

An analysis of the performance of four cooling approaches to meet daytime office cooling requirements for an open plan office was undertaken using a finite-volume model implemented in Matlab. The four cooling approaches simulated were: a conventional HVAC system (A), using extra ventilation on demand (B), using cool night time air to charge a store with RT15 (C) and charging a store with RT8HC using night time off peak electricity (D). Charging the thermal store with cool night time air reduced the yearly cooling demand by 39% compared to the conventional HVAC cooling system. Mechanical ventilation on demand achieved a reduction of 34% compared to the same conventional system. Using off peak electricity to charge the PCM store achieved a reduction of 15%, and successfully shifted daily cooling demands into night time operation during spring and autumns months. Due to the high capital cost of the thermal stores, the lowest cost of energy was obtained using extra ventilation on demand, with a cost of 84.45£/MWh for an assumed 20-years of operation.

Heat loss through windows of buildings is one of the significant causes of high-energy consumption that require intervention to advance their thermal performance in a move towards net-zero energy buildings. High-performance triple vacuum glazing features ultra-low heat loss that maintains the transparency, regardless of tiny pillar spots, and slimness due to its narrow evacuated cavities. This study reports the design and development of high-vacuum system with the modified vacuum cup which provides an effective pump-out hole sealing and has an achievable vacuum pressure of 4.02x10-5 Pa. A new dual-edge seal (low temperature melt 186˚C) consists of Cerasolzer CS186 alloy as a main and J-B Weld epoxy steel adhesive as a support seal is developed and the triple vacuum glazing (area of 300 x 300mm) samples are fabricated, in which the vacuum pressure of 4.8x10-2 Pa is achieved. A 3D FEM of the fabricated triple vacuum glazed is developed and the centre-of-pane and overall U-value of 0.33Wm-2K-1 and 1.05 Wm-2K-1, respectively, are predicted.

This paper presents a performance evaluation of two domestic space heating systems that use air source heat pumps (ASHP) to replace conventional boilers with air source heat pumps (ASHP). The storage system consisted of encapsulated PCM spheres in a packed bed with twice the storage capacity over the temperature range of 40 and 65°C achievable with hot water. The simulations predicted a potential CO2 reduction of 23% for the detached dwelling and 20% for the semi-detached dwelling, operating the heat pumps in economy 10 electricity i.e a low tariff at times between 00.00-05.00 and 13.00-16.00; with current grid emission values by successfully offsetting the electrical load to meet the heat demand.

PurposeTransparent insulation materials (TIMs) have been developed for application to building facades to reduce heating energy demands of a building. The purpose of this research is to investigate the feasibility of TI‐applications for high‐rise and low‐rise office buildings in London, UK, to reduce heating energy demands in winter and reduce overheating problems in summer.Design/methodology/approachThe energy performance of these office building models was simulated using an energy simulation package, Environmental Systems Performance‐research (ESP‐r), for a full calendar year. The simulations were initially performed for the buildings with conventional wall elements, prior to those with TI‐systems (TI‐walls and TI‐glazing) used to replace the conventional wall elements. Surface temperatures of the conventional wall elements and TI‐systems, air temperature inside the 20 mm wide air gaps in the TI‐wall, dry‐bulb zone temperature and energy demands required for the office zones were predicted.FindingsPeak temperatures of between 50 and 70°C were predicted for the internal surface of the TI‐systems, which clearly demonstrated the large effect of absorption of solar energy flux by the brick wall mass with an absorptivity of 90 percent behind the TIM layer. In the office zones, the magnitude of temperature swings during daytime was reduced, as demonstrated by a 10 to 12 h delay in heat transmission from the external façade to the office zones. Such reduction indicates the overheating problems could be reduced potentially by TI‐applications.Originality/valueThis research presents the scale and scope of design optimisation of TI‐systems with ESP‐r simulations, which is a critical process prior to applications to real buildings.

A detailed experimental study was undertaken to analyse the natural convective heat transfer in CPC cavities, a complex function of collector orientation, geometrical aspect ratios and thermal boundary conditions at the enclosure walls. Results are reported for CPC solar collectors with full-, three quarter- and half-height reflectors, C R = 2 and a 100 mm wide flat plate absorber. Experiments were conducted using a purpose built solar simulator under controlled lab environment employing realistic boundary and thermal conditions. The effects of simultaneous tilting of the solar collectors about both transverse and longitudinal axes, truncation of the reflector walls and inlet water (collector heat removal fluid) temperature on the natural convective heat flow characteristics inside the CPC cavity have been determined. It is concluded that the correlations developed for prediction of natural convection characteristics in rectangular, annuli and V-trough enclosures are not appropriate for application to CPC solar collectors with divergence ranging from 150% to 300%. Based on the experimental data a correlation is presented to predict the natural convection heat loss from the absorber plate of solar collectors for a range of water inlet temperatures.

A set of technological interventions has been identified which are suitable for applications in the majority of existing UK dwellings and which would significantly reduce their carbon footprint. The dwelling types are semi-detached, detached and terraced. The paper considers two scenarios, viz. i) using present-best technologies and ii) looking at the likely-future technologies. The research has assessed the effectiveness of the selected technological interventions. These reduce heating demands and as a result there is a decrease in carbon dioxide emissions. It is concluded that the total CO2 emitted because of space and water heating can be reduced by up to 86, 87 and 86% respectively for the semi-detached, detached and terraced dwellings using likely-future technologies and by up to 78, 77 and 82% using present-best available technologies. This assumes that the proposed mechanical ventilation system used in conjunction with a heat-wheel heat recovery system (MVHW) runs on green electricity generated by ...

An extensive indoor experimental characterisation program to investigate the heat loss from a point focus Fresnel lens PV Concentrator (FPVC) with a concentration ratio of 100× was performed for a range of simulated solar radiation intensities between 200 and 1000 W/m 2 , different ambient air temperatures, and natural and forced convection. From the experimental program it was found that the solar cell temperature increased proportionally with the increase in simulated solar radiation for all experimental tests, indicating that conductive and convective heat transfer were significantly larger than the long wave radiative heat transfer within and from the FPVC system. For the simulated worst case scenario, in which the FPVC system was tested under a simulated solar radiation intensity of 1000 W/m 2 and ambient air temperature of 50 °C with no forced convection, the predicted silicon solar cell efficiency in the FPVC system was reduced to approximately half that at standard test conditions.