Experimentation with a water tank including a PCM module

Storage of heat is seen as a major issue for the development of solar energy for house heating and cooling under all climates. Most of the storage systems available on the market use water as the storage medium. The idea studied here was to add a phase change material (PCM) module at the top of a hot-water storage tank with stratification. An experimental solar pilot plant was constructed to test the PCM behaviour in real conditions. The PCM module geometry adopted was to use several cylinders. A granular PCM–graphite compound was chosen as the PCM for the experiments presented here.     

Thermal energy storage in building integrated thermal systems: A review. Part 1. active storage systems

Energy consumed by heating, ventilation and air conditioning systems (HVAC) in buildings represents an important part of the global energy consumed in Europe. Thermal energy storage is considered as a promising technology to improve the energy efficiency of these systems, and if incorporated in the building envelope the energy demand can be reduced. Many studies are on applications of thermal energy storage in buildings, but few consider their integration in the building. The inclusion of thermal storage in a functional and constructive way could promote these systems in the commercial and residential building sector, as well as providing user-friendly tools to architects and engineers to help implementation at the design stage. The aim of this paper is to review and identify thermal storage building integrated systems and to classify them depending on the location of the thermal storage system.     

Corrosion of metals and salt hydrates used for thermochemical energy storage

Solar energy can be efficiently used if thermal energy storage systems are accordingly designed to match availability and demand. Thermal energy storage by thermochemical materials (TCM) is very attractive since these materials present a high storage density. Therefore, compact systems can be designed to provide both heating and cooling in dwellings. One of the main drawbacks of the TCM is corrosion with metals in contact. Hence, the objective of this study is to present the obtained results of an immersion corrosion test following ASTM G1 simulating an open TCM reactor, under humidity and temperature defined conditions. Four common metals: copper, aluminum, stainless steel 316, and carbon steel, and five TCM: CaCl₂, Na₂S, CaO, MgSO₄, and MgCl₂, were studied. Aluminum and copper show severe corrosion when combined with Na₂S, aluminum corrosion is more significant since the specimen was totally destroyed after 3 weeks. Stainless steel 316 is recommended to be used as a metal container material when storing all tested TCM.     

Performance analysis of a latent heat storage system with phase change material for new designed solar collectors in greenhouse heating

The continuous increase in the level of greenhouse gas emissions and the rise in fuel prices are the main driving forces behind the efforts for more effectively utilize various sources of renewable energy. In many parts of the world, direct solar radiation is considered to be one of the most prospective sources of energy. In this study, the thermal performance of a phase change thermal storage unit is analyzed and discussed. The storage unit is a component of ten pieced solar air collectors heating system being developed for space heating of a greenhouse and charging of PCM. CaCl₂6H₂O was used as PCM in thermal energy storage with a melting temperature of 29 °C. Hot air delivered by ten pieced solar air collector is passed through the PCM to charge the storage unit. The stored heat is utilized to heat ambient air before being admitted to a greenhouse. This study is based on experimental results of the PCM employed to analyze the transient thermal behavior of the storage unit during the charge and discharge periods. The proposed size of collectors integrated PCM provided about 18–23% of total daily thermal energy requirements of the greenhouse for 3–4 h, in comparison with the conventional heating device.     

In situ thermal and acoustic performance and environmental impact of the introduction of a shape-stabilized PCM layer for building applications

Energy consumption in buildings accounts for up to 34% of total energy demand in developed countries. Thermal energy storage (TES) through phase change materials (PCM) is considered as a promising solution for this energetic problem in buildings. The material used in this paper is an own-developed shape stabilized PCM with a polymeric matrix and 12% paraffin PCM, and it includes a waste from the recycling steel process known as electrical arc furnace dust (EAFD), which provides acoustic insulation performance capability. This dense sheet material was installed and experimentally tested. Ambient temperature, humidity, and wall temperatures were measured and the thermal behaviour and acoustic properties were registered. Finally, because of the nature of the waste used, a leaching test was also carried out. The thermal profiles show that the inclusion of PCM decreases the indoor ambient temperature up to 3 °C; the acoustic measurements performed in situ demonstrate that the new dense sheet material is able to acoustically insulate up to 4 dB more than the reference cubicle; and the leaching test results show that the material developed incorporating PCM and EAFD must be considered a non-hazardous material.     

Development of a ventilation system utilizing thermal energy storage for granules containing phase change material

We have developed an experimental ventilation system that features direct heat exchange between ventilation air and granules containing a phase change material (PCM). Measurement of outlet air temperature when the inlet air temperature was periodically varied to simulate changes of outdoor ambient air temperature showed that the outlet air temperature was stabilized and remained within the phase change temperature range. This effect is expected to be useful in practical ventilation systems. The potential of such systems for reducing ventilation load was examined through computer simulation for eight representative cities of Japan. This revealed how different temperature conditions would affect required heat storage capacity.     

Performance comparison of a group of thermal conductivity enhancement methodology in phase change material for thermal storage application

Phase change materials (PCM) are able to store thermal energy when becoming liquids and to release it when freezing. Recently the use of PCM materials for thermal energy storage (TES) at high temperature for Concentrated Solar Power (CSP) technology has been widely studied. One of the main investigated problems is the improvement of their low thermal conductivity. This paper looks at the current state of research in the particular field of thermal conductivity enhancement (TCE) mechanisms of PCM to be used as TES. This work considers a numerical approach to evaluate the performance of a group of TCE solutions composed by particular configurations of two of the principal TCE systems found on the literature: finned pipes and conductive foams. The cases are compared against a single PCM case, used as reference. Three different grades of graphite foams have been studied, presenting a charge time 100 times lower than the reference case for the same capacity. For fins two materials are analyzed: carbon steel and aluminum. The charge times of fin cases are from 3 to 15 times faster, depending on the amount and type of material employed. The internal mechanisms are analyzed to understand the results and locate possible improvement.     

Liquid metal gallium laden organic phase change material for energy storage: An experimental study

This paper presents the experimental study on the thermophysical behavior, thermal cyclic characteristics and energy storage performance of liquid metal (LM) laden in organic solid-liquid phase change material (PCM) for energy storage. In this view, Gallium (Ga) is added into D-Mannitol (DM) with a weight fraction of 0.1% and 0.5% by dispersion technique using a ball mill. Repeated melting/freezing cycle was carried out for 350 cycles and the samples were characterized using Differential Scanning Calorimetry (DSC), Thermogravimetry Analysis (TGA) and Fourier Transform Infrared (FTIR). The DM/Ga composite PCM showed enhanced thermal conductivity of ～8.4%, ～27.8% for 0.1 and 0.5 wt % Ga as compared to pure DM. XRD studies reveal that the pure DM exhibited β polymorphic phase while TGA and FTIR analysis confirm the thermal reliability and chemical stability of composites in the temperature range of 50–200 °C. Non isothermal crystal kinetic study proved that the addition of Ga increased the crystallization rate due to heterogeneous nucleation effect and leads to the reduction in subcooling temperature of the PCM. The experimental setup results to test the charging and discharging performance of the composite PCM revealed that the total time for one complete cycle reduced from 97.48 min for pure DM to 84.73 min and 63.92 min for DM-Ga composite with 0.1 wt % and 0.5 wt % respectively. Based on the results obtained, D-Mannitol based composites could be recommended as potential PCM candidates for solar heat and industrial waste heat recovery application due to its high energy density capacity, thermal/chemical stability and good heat transfer performance.     

Experimental research on thermal characteristics of PCM thermal energy storage units

To explore thermal management integration in electric vehicles (EVs), a phase change materials (PCMs) thermal energy storage unit using flat tubes and corrugated fins is designed. The investigation focuses on the thermal characteristics of the PCM unit, such as the temperature variation, heat capacity, and heat transfer time, etc. Meanwhile, the heat storage and release process will be influenced by different inlet temperature, liquid flow rate, melting point of the PCM, and the combination order of the units. Under the same inlet temperature and flow rate condition, the PCM unit with higher melting point enters the latent heat storage stage slowly and enters the phase change melting release stage quickly. Furthermore, the heat storage and release rates increase with increasing liquid flow rates, but the effects are diminishing in the middle and later periods. The multiple PCM units with different melting temperatures are cascaded to help recycle low-grade heat energy with different temperature classes and exhibit well heat storage and release rates.     

Key performance indicators in thermal energy storage: Survey and assessment

Thermal energy storage (TES) is recognised as a key technology for further deployment of renewable energy and to increase energy efficiency in our systems. Several technology roadmaps include this technology in their portfolio to achieve such objectives. In this paper, a first attempt to collect, organise and classify key performance indicators (KPI) used for TES is presented. Up to now, only KPI for TES in solar power plants (CSP) and in buildings can be found. The listed KPI are quantified in the literature and compared in this paper. This paper shows that TES can only be implemented by policy makers if more KPI are identified for more applications. Moreover, close monitoring of the achievements of the already identified KPI needs to be carried out to demonstrate the potential of TES.     

The novel use of phase change materials in refrigeration plant. Part 3: PCM for control and energy savings

Phase change materials (PCMs) have been recognized as energy storage tanks since the 1980s. The ‘tank’ has been introduced into the refrigeration system to enable its capacitance to take account of fluctuations in the daily cooling load. However, this part of the paper will present a novel control purpose of using PCM in refrigeration systems. The novel application of PCMs in refrigeration systems at different positions in the refrigeration cycle circuit with a shell and tube structure has been investigated extensively by the novel mathematical model presented in part 2 of the paper. The results show that for energy savings, PCMs at different positions give coefficient of performances (COPs) up to 8% through lowering the sub-cooling. PCMs also improve the system COPs up to 4% and 7% for the thermostatic expansion valve (TEV) and orifice systems, respectively, by minimizing the superheat. Further benefits such as system stabilization were also observed in this investigation.     

High-chain fatty acid esters of myristyl alcohol with even carbon number: Novel organic phase change materials for thermal energy storage—1

High-chain fatty acid esters have not been investigated for their thermal properties as phase change materials (PCMs) in thermal energy storage. A series of high-chain fatty acid esters of myristyl alcohol (1-tetradecanol) were synthesized via esterification of lauric, myristic, palmitic, stearic and arachidic acids under vacuum and in the absence of any catalyst. The esterification reactions were studied by FT-IR spectroscopy. A differential scanning calorimeter (DSC) and a thermo-gravimetric analyzer (TGA) were intensively used to determine the thermal properties of the introduced thermal storage materials. The thermal properties were given in terms of phase change temperature, enthalpy, specific heat (C_p) and thermal decomposition temperature with related statistical data. The thermal reliability of the novel organic PCMs was investigated by thermal cycling with 1000 thermal cycles with respect to the thermal properties of the original synthesized PCMs. In addition to the synthesized esters, one commercial product was also investigated. The DSC analyses indicated that the melting points of the novel organic PCMs were between 38 and 53 °C with phase change enthalpy above 200 kJ/kg. The effect of chemical structure of the materials on thermal properties was also discussed. The results showed that these materials were favorable for low temperature heat transfer applications with superior thermal properties and reliability.     

High-chain fatty acid esters of myristyl alcohol with odd carbon number: Novel organic phase change materials for thermal energy storage—2

A series of high-chain fatty acid esters of 1-tetradecanol (myristyl alcohol) were synthesized via esterification of 1-tridecanoic, 1-pentadecanoic, 1-heptadecanoic and 1-nonadecanoic acids under vacuum and in the absence of catalyst. The esterification reactions were controlled by FT-IR spectroscopy. Differential scanning calorimeter (DSC) and thermo-gravimetric analyzer (TGA) were intensively used to determine the thermal properties of the presented novel organic phase change materials (PCM). The thermal properties were given in terms of phase change temperature, enthalpy, specific heat (C_p) and thermal decomposition temperature with related statistical calculations. The thermal reliability of the synthesized PCMs, which is an important property for utilization, was determined via measuring the change in thermal properties after 1000 thermal cycles. The DSC analyses indicated that the melting points of the novel organic PCMs were between 40 and 50 °C with phase change enthalpy above 200 kJ/kg. The results showed that these thermal storage materials were favorable for low temperature heat transfer applications with superior thermal properties and reliability among the known PCMs.     

Thermal cycle testing of calcium chloride hexahydrate as a possible PCM for latent heat storage

In order to study the changes in latent heat of fusion and melting temperature of calcium chloride hexahydrate (CaCl₂·6H₂O) inorganic salt as a latent heat storage material, a thousand accelerated thermal cycle tests have been conducted. The effect of thermal cycling and the reliability in terms of the changing of the melting temperature using a differential scanning calorimeter (DSC) is determined. It has been noticed that the CaCl₂·6H₂O melts between a stable range of temperature and has shown small variations in the latent heat of fusion during the thermal cycling process. Thus, it can be a promising phase change material (PCM) for heating and cooling applications for various building/storage systems.     

Thermal performance of PCM thermal storage unit for a roof integrated solar heating system

The thermal performance of a phase change thermal storage unit is analysed and discussed. The storage unit is a component of a roof integrated solar heating system being developed for space heating of a home. The unit consists of several layers of phase change material (PCM) slabs with a melting temperature of 29 °C. Warm air delivered by a roof integrated collector is passed through the spaces between the PCM layers to charge the storage unit. The stored heat is utilised to heat ambient air before being admitted to a living space. The study is based on both experimental results and a theoretical two dimensional mathematical model of the PCM employed to analyse the transient thermal behaviour of the storage unit during the charge and discharge periods. The analysis takes into account the effects of sensible heat which exists when the initial temperature of the PCM is well below or above the melting point during melting or freezing. The significance of natural convection occurring inside the PCM on the heat transfer rate during melting which was previously suspected as the cause of faster melting process in one of the experiments is discussed. The results are compared with a previous analysis based on a one dimensional model which neglected the effect of sensible heat. A comparison with experimental results for a specific geometry is also made.     

Polyurethane/graphite nano-platelet composites for thermal energy storage

In this work new polyurethane-based phase change materials containing segments of poly(ethylene glycol) with average molar mass of 8000 g/mol with and without chain extender and modified with graphite nano-platelets have been fabricated and characterized. Structure, morphology and phase behaviour of these solid-solid phase change materials were investigated, as well as the thermal stability and conductivity. The heat of phase transition was in the range of 118.0–164.5 J/g for polyurethane without chain extender and 128.0–148.5 J/g for polyurethane with chain extender. The highest heat of phase transition and crystallinity were found for system modified with 0.3% of graphite nano-platelets in polyurethane without chain extender. Modulated differential scanning calorimetry results showed some changes in the phase transition behaviour and the crystallinity of the polyurethane matrix due to graphite nano-platelets confinement effect. Enhancements in the thermal stability in polyurethane modified with graphite nano-platelets, attributed to the barrier effect, were found based on thermogravimetric analysis data. The thermal conductivity increased with an increase of graphite nano-platelets content for both polyurethane systems, with and without chain extender, which is important for modern thermal energy storage applications.     

Modification of urea–sodium acetate trihydrate mixture for solar energy storage

The system urea–sodium acetate trihydrate has been mentioned in the literature as an energy storage system. Due to its low melting point (30 °C), the system is not suitable for use in a hot climate. Modifying the system composition is shown in the present work through adding lead acetate trihydrate to the binary mixture in the ratio of 16.6% with the object of raising its melting point to a practical value. A melting point of 44.5 °C could be reached for the new system.The mixtures of urea–sodium acetate trihydrate and urea–sodium acetate trihydrate–lead acetate trihydrate are tested in the present work as phase change storage mixtures and comparison between both mixtures was carried out.The results showed that the system composed of urea–sodium acetate–lead acetate stored 286 kJ/kg of the storage mixture.     

MSWI bottom ash for thermal energy storage: An innovative and sustainable approach for its reutilization

The management of Municipal Solid Waste (MSW) is a very important issue that must be dealt by the perspective of the 3 Rs (Reuse, reduce, recycle. MSW incineration bottom ash (BA) accounts for 85–95% of the total solids that remained after incineration. Finding suitable alternatives for its revalorization is very attractive, especially in terms of environmental sustainability. Thermal energy storage (TES) is a complementary technology of renewable energy. The aim of this study is to evaluate the thermophysical properties of weathered BA (WBA) in order to find suitable alternatives for its valorisation. Several samples of WBA were collected from a waste-to-energy facility located in Tarragona (Spain). This facility sieved to different size fractions. Each size fraction was characterized with respect its physicochemical properties and the energy density (ρ_en) was calculated. The results obtained showed that this residue is a suitable alternative candidate material for TES as it presents an energy density of 1461.2 kJ m⁻³ K⁻¹ for the 1–2 mm fraction. In addition, the low cost of this type of residue (0.6€ per t) and its high production rate per year makes it an attractive and sustainable alternative for TES.     

State of the art on high-temperature thermal energy storage for power generation. Part 2—Case studies

Power generation systems are attracting a lot of interest from researchers and companies. Storage is becoming a component with high importance to ensure system reliability and economic profitability. A few experiences of storage components have taken place until the moment in solar power plants, most of them as research initiatives. In this paper, real experiences with active storage systems and passive storage systems are compiled, giving detailed information of advantages and disadvantages of each one. Also, a summary of different technologies and materials used in solar power plants with thermal storage systems existing in the world is presented.     

Study of a metal hydride based thermal energy storage system using multi-phase heat exchange for the application of concentrated solar power system

Thermal energy storage system is of great significance for the concentrated solar power system to keep the balance between power generation and demand. Metal hydride based thermal energy storage system is regarded as a promising method due to its good reversibility, low cost, and no by-product. Multi-phase heat exchange has much higher heat transfer coefficient than single-phase fluid heat exchange, thus facilitating the steam generation. In this study, a two-dimensional model of the metal hydride reactor using multi-phase heat exchange is proposed to estimate the performance and its feasibility of application in the concentrated solar power system. The results show that the velocity of the heat transfer fluid should match well with the thermal conductivity of the metal hydride bed to maintain the heat flux at a relatively constant value. The match of thermal conductivity of 3 or 5 W/(m·K) and fluid velocity of 0.0050 m/s results in the heat flux up to about 19 kW/m², which is increased by 3 orders of magnitude than single-phase heat exchange. In the thermal energy storage system, the reheating cycle is recommended to improve the utilization of the thermal energy. The efficiency of the system could be improved from 18.4% to 30.8% using the reheating cycle. The increased efficiency is comparable to the previously reported efficiency of 39.2%. Besides, the operation strategy of raising the steam temperature by increasing the hydrogen pressure or the superheater temperature is suggested for the system to obtain higher efficiency.