Performance comparison of two eutectic solder based latent heat storage systems during discharging

With a large capacity thermal storage system using phase change material (PCM), Concentrated Solar Power (CSP) is a promising technology for high efficiency of solar energy utilization. In a thermal storage system, a dual-media thermal storage tank is typically adopted in industry for the purpose of reducing the use of the heat transfer fluid (HTF). While the dual-media sensible heat storage system has been well studied, a dual-media latent heat storage system (LHSS) still needs more attention and study; particularly, the sizing of volumes of storage tanks considering actual operation conditions is of significance. In this paper, a strategy for LHSS volume sizing is proposed, which is based on computations using an enthalpy-based 1D model. One example of 60MW solar thermal power plant with 35% thermal efficiency is presented. In the study, potassium hydroxide (KOH) is adopted as PCM and Therminol VP-1 is used as HTF. The operational temperatures of the storage system are 390°C and 310°C, respectively for the high and low temperatures. The system is assumed to operate for 100 days with 6 hours charge and 6 hours discharge every day. From the study, the needed height of the thermal storage tank is calculated from using the strategy of tank sizing. The method for tank volume sizing is of significance to engineering application.

Thermal performance study of a solar-coupled phase changes thermal energy storage system for ORC power generation

Role of PCM addition on stratification behaviour in a thermal storage tank – An experimental study

Latent heat storage has high storage density with a smaller temperature swing. However many practical problems are encountered. A common problem in latent heat thermal storage is the poor conductivity of the phase change materials (PCMs). To increase the heat transfer rate and to reduce the charging time, extended surfaces such as fins are used. In the present work experiments are conducted to study the performance of latent heat thermal energy storage system (LHTS) to store heat energy in PCM (Paraffin) filled spherical capsules provided with following fin configurations: a) Internal hollow cylindrical fins, b) Internal solid cylindrical fins and c) No fins. Paraffin with a melting point of 610C was used in the investigation because it is appropriate for thermal storage applications. Water is used as heat transfer fluid (HTF) to transfer heat from source to TES tank. Time-based variations of the temperature distribution for HTF and PCM have been studied for different configurations of spherical capsules, different flow rates & inlet temperature of HTF. It is observed that when HTF inlet temperature is varied from 65°C to 70°C the charging time is reduced by 25% for without fins, 28% for internal hollow cylindrical fins and 23% for internal solid cylindrical fins. When flow rate is varied from 4 lpm to 6 lpm for same HTF inlet temperature of 70°C charging time is reduced by minimum percentage. The performance parameters such as instantaneous heat stored, cumulative heat stored and system efficiency are also studied.

In order to use solar radiation as thermal energy source, heat storage equipments result necessary in each application where continuous supply is required, because of the natural unsteady intensity of radiation during the day. Thermal solar collectors are especially suitable for low temperature applications, since their efficiency decreases when an high inlet temperature of fluid flowing through them is established. On the other hand, low temperatures and low temperature gaps, above all, make very difficult to use traditional sensible heat storing units (water tanks), because of the very large amounts of material required. In this work, a traditional sensible heat storage system is compared with a latent heat storing unit based on phase change materials (PCMs). As a case study, a 840 m3 greenhouse heating application was considered with an inside constant temperature of 18°C. It is thought to be heated by using single layer plate thermal solar collectors as energy source. Inlet temperature of the collectors fluid (HTF) was fixed at 35°C (little higher than melting temperature of PCMs) and a constant flux of 12 l/m2 hour was established as technical usual value. At these conditions, 215m2 solar panels exposed surface resulted necessary. The sensible heat storage system considered here is a traditional water tank storing unit equipped with two pipe coils, respectively for heat exchanges with HTF from collectors and water flux for greenhouse heating. Available DT for heat exchange is estimated as the difference of minimum HTF temperature (in outlet from the collectors) and the required water temperature for greenhouse heating. The latent heat storing unit is instead a series of copper rectangular plate shells which a phase change material is filled in (Na2SO4⋅10H2O). Heat transfer fluids flow through thin channels between adjacent plates, so that a large heat exchange available surface is achieved. The developed computational model (Labview software) permits to superimpose heat exchanges daily curves between heat storing materials and heat transport fluids (for both of the fluids and the heat storing equipments) on the energy supply/demand ones, respectively calculated on the basis of greenhouse energy demand and solar collectors dimensions, characteristics and efficiency. In this manner, units design is achieved by changing thermal energy storing units dimensions, in order that the corresponding heat exchange curves coincide with the previously calculated ones. Successively, among all the possible configurations, the ones showing lower units volumes and less amount of storing materials are chosen as the optimal design solutions. It has been proven that PCMs materials are much more suitable for low temperature applications than sensible heat storing materials (water). In the case of water tank, an about 15.8m3 total volume is required while for PCMs equipment the total volume of storing unit is reduced to about 2.2 m3, such as about seven times total volume less. Besides, according to the simplified and steady state model calculations, PCMs unit shows a better response to the hourly energy fluctuations of solar collectors and greenhouse demand than water tank unit. This is especially due to the high available exchange surface achieved in proposed arrangement.

Energy storage potential analysis of phase change material (PCM) energy storage units based on tunnel lining ground heat exchangers

Practical applications of Phase Change Materials (PCMs) often require their encapsulation in other materials, such as metals or plastics. This raises the issue of compatibility between PCMs and encapsulating materials, which has still not been sufficiently addressed. The study presented here follows existing research and provides experimental evaluation of the suitability of selected PCMs for proposed integration in building structures. Two organic PCMs, two inorganic PCMs and three representative plastics (polypropylene (PP-H), high density polyethylene (PE-HD) and polyvinylchloride (PVC-U)) were selected for compatibility tests. Evaluation of the results is based on the mass variations of the plastic samples during the test period. Plastic samples were immersed in PCMs and subjected to periodic heating and cooling (for 16 weeks) in a small environmental chamber simulating real operational conditions. The results show that the organic PCMs have a greater ability to penetrate the PE-HD and PP-H compared with the inorganic PCMs. The penetration of all PCMs was most notable during the first four weeks of the experiment. Later it slowed down significantly. Overall, the mass changes in PE-HD and PP-H samples did not exceed 6.9% when immersed in organic PCMs and 1.8% in inorganic PCMs. PVC-U samples exhibited almost negligible (less than 0.1%) mass variation in all cases.

Increasing the energy efficiency of the building is an alternative to reduce energy consumption while improving thermal comfort in order to confront the climate change and fossil fuel limitations. One of the options is the use of thermal energy storage as a practical way to save energy and improve its utilization. Thermal energy storage, classified into sensible heat storage and latent heat storage, is presented in this paper focusing on phase change materials (PCMs) as a promising solution to reduce energy consumption in buildings. The identification of different PCM classifications (organic, inorganic and eutectic PCMs), their particular characteristics and the candidate materials for building applications are reviewed. This review identifies key gaps in PCM deployment, such as the low thermal conductivity of organic PCMs, the supercooling of inorganic PCMs, and proposes research priorities. Active and passive latent heat storage technologies used in building applications are also summarized. For passive systems, which do not require conventional energy, the common methods of PCMs integration in building materials such as concrete, gypsum board, ceiling or floor are enumerated in this paper.

Multi-configuration evaluation of a megajoule-scale high-temperature latent thermal energy storage test-bed

With the energy challenges to come, development of renewable energy sources is becoming more and more topical. But, because these energies are irregularly supplied, their development is closely related to those of suitable energy storage systems. Among the existing technologies for storing thermal energy, latent heat storage appears to be of huge interest since it can lead to reduced size and cost. This paper reviews various recent patents on phase change materials and on latent heat storage systems over the past six years. Developments concerning the storage medium and also the entire storage system are thus addressed. An overview of existing materials and systems for latent heat thermal energy storage, namely for high temperature applications is also given, in order to identify the main issues in the development of such systems. The list of patents reviewed here is by no means supposed to be complete or extensive but shows nevertheless evolution trends concerning latent heat storage. An analysis of the repartition of the reviewed patents helps concluding about the expected future developments in this domain. Keywords: Hybrid PCM (physical and chemical bounding), PCM, PCM alloys, PCM emulsions, PCM macro encapsulation, PCM micro encapsulation, PCM slurries, thermal energy storage, solar water heater, cold-compression

Thermal performance comparison of three sensible heat thermal energy storage systems during charging cycles

Synthesis of Sn@SnO2 core-shell microcapsules by a self-oxidation strategy for medium temperature thermal storage

SummaryThermal energy storage (TES) is one of the significant technologies for reducing the discrepancy between ever‐increasing energy demand and utilization. Sensible and latent heat based TES concepts are the most widely implemented technologies for thermal applications. However, these two storage technologies have drawbacks of lower energy storage ability, higher thermocline degradation for sensible heat storage and lower thermal conductivity of the phase change material (PCM) causing higher charge and discharge times for latent heat storage. To counter these issues, a novel combined sensible‐latent heat storage system has been proposed. The concept of combined sensible‐latent thermal storage is to place the latent heat storage media above the sensible storage media in the storage container. In the present experimental study, an experimental investigation of the proposed storage system and conventional sensible storage system was performed during charge and discharge cycles. The proposed storage system used concrete spheres as sensible storage media and paraffin encapsulated capsules having the same diameter of 38 mm, as a latent heat storage media. A comparative assessment has been made for two different configurations based on temperature profile, storage capacity, charge and discharge times, energy transferred, and energy recovered. The experimental results reveal that the novel combined sensible‐latent heat storage has lower thermocline degradation during discharge and higher energy storage capacity relative to the sensible storage system. The discharge time and energy utilization is prolonged by adding the PCM capsules above the concrete spheres and also the temperature drop at outlet of the storage system has been reduced than the sensible storage. Based on the analysis, the novel combined sensible‐latent heat storage was found to have a better performance compared with the conventional sensible heat storage system.

A phase-change material (PCM) thermal energy storage (TES) has been studied as part of a project to develop a solar-based room-heating system for extreme cold areas using ANSYS Fluent. The purpose of using a PCM thermal storage is to overcome the intermittency of solar and provide room heating during nighttime as a sustainable and clean solution. A computational fluid dynamic (CFD) model is developed for a cubical TES unit having an array of cylindrical rods of stainless steel filled with inorganic PCM as thermal storage media. Heat-transfer fluid (HTF) selected for this study is water. In the present study, the diameter of cylindrical rods is optimized by comparing the melting characteristics of PCM with three different diameters, and thermal performance of the selected option is investigated and discussed. ANSYS Fluent software is used for numerical modeling of a commercially available inorganic PCM material Save-HS48 (Pluss Advanced Technologies, India) and the results were validated using experimental data provided by the PCM manufacturer. 3D transient option with solidification and melting in Ansys Fluent software is selected for the investigation. A four-hour charging time is assumed for the numerical modeling and discharging for a particular case was studied with suitable assumptions for full-cycle analysis. Thermo-physical properties of Save-HS48 PCM are found to be suitable for solar room-heating application and effectiveness of all the cases simulated in the present study provides suitable results for further experimental study of a PCM-TES unit.

A review on encapsulation techniques for inorganic phase change materials and the influence on their thermophysical properties