An overview of ThermalEnergy Storage and it’s Applications Abstract This report reviewsthe concept of thermal energy storage, it’s types, it’s benefits and studiessome case-studies of energy storage globally. It reviews thermal energy storagein buildings using sensible, latent heat and thermochemical energy. Sustainableheating and cooling with Thermal Energy Storage in buildings can be achievedwith the use of phase change materials in active systems and seasonal storage. Introduction In the present day, the developmentof sustainable technologies is a huge issue. One of the key issues is thedemand for environmentally friendly energy generation and consumption.
ThermalEnergy Storage is quite attractive now because of the energy economy and itssolution to store and re-use the same energy. TES can be very beneficialbecause they have good efficiency, reliability and it leads to less pollution,running costs and better economics. (HG.
, 1942) Thermal energy storage is atechnology that stores thermal energy so that it can be used later forheating and cooling applications and power generation. They used particularlyin buildings and industrial processes. There are two properties of heatwhich are crucial to understand when discussing about Thermal Energy Storage. Sensibleheat is related to changes in temperature of a gas or object with nochange in phase. Latent heat is related to changes in phase betweenliquids, gases, and solids. (Alvaro de Gracia, 2015) Fig. 1(left). Image showingthe difference between sensible and latent heatFig.
2(right). Graphexplaining the heating and cooling cycle for a latent heat curve Water has one of the highest thermalcapacities. When liquid water isput into ice cube trays and placed in the freezer, the water gives off energy asthe water becomessolid ice. (A. Safaria, 2017) Type 1. Sensible Heat Storage Sensibleheat it the Heat stored bychanging the temperature of a storage medium(such as water, air, oil, rock beds, bricks, concrete or sand). This heat isproportional to the mass, the specific heat capacity of the storage medium andthe temperature rise. (Mazlan Abdul Wahid, 2016) Below are some ofthe applications for sensible heat storage.
Fig. 3 shows how a typical solar water heater system works. Solar collector heats the hot water supply which is then transferred to the water pipes. An indirect mode involves having a heat exchanger to help the process run smoother. Fig 4. Shows the system of an aquifer thermal energy storage. The heat exchanger transfers heat in the summer to the groundwater which is pulled up in the winter for heating and cold groundwater is injected back into the ground. Fig 5.
Shows the system of using water and stone to heat cool air that is injected into the collector. After passing through insulated, water and stone warm air is released. Fig. 3. Schematic of domestic hot water system withelectrical heater(a) direct mode (b)indirect mode. Fig. 4. Schematic of aquifers.
Fig. 5.Usingwater & stone as storage media Borehole thermal storage systemThe drake landing solar communitylocated in Alberta, Canada started operating since 2007. The distinguishingfeature about the community is that more than 97% of the heating comes fromsolar thermal panels on the garage roofs.
The heat is stored in long-term andshort-term borehole thermal storage system. The 144 boreholes are drilled to37m into the earth. The system works oninter-seasonal heat storage system which means during the summer 1.
5 mega-wattsof thermal power is generated and stored for heating in winter. (Drake Landing Solar Community, n.d.) Fig 6(left). Drakecommunity – aerial view showing solar panels on garages. Fig 7(right). The scheme ofthe borehole thermal storage used in Drake communityType 2: LatentHeat Storage The storage of the heat released orabsorbed during phase change of materials is called latent heat storage.
Thematerial stores heat while changing phase however the temperature remainsconstant. These materials used in latent heat storage are known as phase changematerials or PCM. The most commonly used materials areorganic solutions especially for low temperature applications (between 0 ºC and130 ºC). Paraffin compounds are used in insulation for walls and other areas.inorganic compounds such as salts are used for temperatures about 150 ºC.
Thereare also many eutectic compounds used for applications above 250 ºC. Thediagrams below highlight the various PCMs available and their meltingtemperatures. (Mazlan Abdul Wahid, 2016) Fig.
8(left). Types of phase change materials Fig. 9(right).
The enthalpy and melting temperatures of various PCM. The cycle of a Phase Change MaterialThe PCM absorbs heat when thetemperature rises above a certain level and stores it. At this point of time,there is change in the phase of the material. When the surrounding temperaturedecreases, the PCM solidifies and the heat is released back into theenvironment returning to its initial state. A material like paraffin is verylong-lasting as shown in the picture below, the properties of the material doesnot change even after many cycles. (Mazlan Abdul Wahid, 2016) Fig.10(left).
Cycle of aPCM.Fig.11(right). SEM imagesof microencapsulated paraffin at different thermal cycles.
Some applications for Phase Change Materials PCMs are used in various applicationsincluding but not limited to gypsum boards, dry walls, under-floor electricalheating system, clay bricks, concrete, waste heat recovery, air conditioning, thermalcomfort in vehicles, clothing, solar cooking, solar power plants, cooling offood and thermal energy storage. (Mazlan Abdul Wahid, 2016) Fig. 12. applications of phase change materials.Phase change materials usedfor cooling (Mazlan Abdul Wahid, 2016) Fig. 13. Cooling of PCMs using water in a fanFig. 14.
Cooling (a) and regeneration mode (b) of theventilated cooling ceiling with PCM Andasol Solar Power StationThis power station located in Spainwas one of the first plant to use parabolic throughs. It generates aconcentrated solar power of 150 MW. TheAndasol plant uses the technology of phase change materials to store thermalheat. It has large tanks of molten salt for storing the thermal heat. This heatis released back into the plant during the winter to generate electricity.
Hence, this plant is able to generate high amounts of electricity irrespectiveof the season. (Power-Technology, n.d.) Fig. 15(left).
Ariel viewof the Andadol solar power stationFig. 16(right). View of themolten salt storage tankType 3: ThermochemicalStorage Thermochemical storage is simple the storingof heat energy in chemical bonds.
A reversible chemical reaction has two parts, endothermic and exothermic. Asshown in Fig17, the salt hydrate uses heat to absorb the heat energy that isstored to form anhydrous salt solution and water. This process is known asendothermic or dehydration. The second part of the reaction is when thereaction is reversed where water and anhydrous salts are forced to react toform salt hydrate and heat. This reversed reaction is called exothermic orre-hydration process. (Yate Ding, 2013)Fig. 17.
The dehydrationand re-hydration process of a chemical reaction SolarThermal Group at the Australian National UniversityThe Australian National Universityhas been working on a thermochemical storage of solar energy. They have deviseda system which uses 20 m2 paraboloidal shaped solar collectors to collectradiation. The heat is then sent to an ammonia synthesis reactor whichdissociates ammonia into Hydrogen and Nitrogen. During night time, the hydrogenand nitrogen is made to react forming the original Ammonia and giving off heat.This way, concentrated solar energy is used to achieve 24h heatproduction. (Solar Thermal, 2015) Fig 20 (right): An image of the parabolic solar collector Fig.
18. The process of thethermochemical storage of solar energy Fig. 19.
The reaction of ammonia Analysis of the three types of thermal storage The threetypes of thermal energy storage vary in terms of their cost, density,temperature range and the application of the design. (Alvaro de Gracia, 2015) Fig. 26.
Methods of thermalenergy storage. (a) Sensible heat, (b) latent heat, (c) thermochemicalreaction. Sensible Heat Storage Phase Change Storage Thermochemical Storage Sensible heat energy storage is advantageous due to its low cost Phase-change heat storage can be used widely at a reasonable cost Cost efficient storage materials The losses are often increased significantly at high energy density (high temperature) High energy density Higher energy density than sensible or latent heat storage systems. Large temperature range (RT to > 1000 °C) Very little temperature variation during the charging and discharging processes Losses only occur during charging and discharging, but not over time, making these systems preferable for long term storage Small scale designs – used mainly for residential. Used for large scale designs such as thermal storage of power plants. Flexible systems – They can be movable and can vary depending on the of storage capacity and thermal power required. Other types of ThermalEnergy Storage The process of ice-based thermal storage is the following: At night using low-cost electricity, a chiller unit cools and ethylene glycol solution to below freezing temperatures. (Energy Storage Model, n.
d.) The solution is circulated through tubes in a tank freezing the water held within During the day, the ice melts cooling the solution in the tubes This chilled solution is moved through a heat exchange coil where it cools the air Ice-based Technology The process is as follows: 1. Charge: Off-peak or excess electricity is used to power an air liquefier, which produces liquid air. 2. Store: The liquid air is stored in tank(s) at low pressure 3. Discharge: To recover power the liquid air is pumped to high pressure, evaporated and heated. The high-pressure gas drives a turbine to generate electricity.
(Liquid Air Energy Storage Technology, n.d.) Fig.21. Process of the ice-based technology Liquidair energy storage Fig. 22.Process of the liquid air energy storage LAEG is the idea of using highpressure gas (mainly liquid nitrogen also called liquid air) to drive a turbineto generate electricity.
It has more than 30 years of lifetime, low technologyrisk, used for large scale energy storage. Compressed air energy storage is the system where off peak low-cost electricity is stored and used during peak hours. (CAES technology, n.d.) 1. In the charging process, the system uses electricity to convert it to mechanical motion and where it moves the pump and energy is sent to the underground gas reservoir.
In the discharging process, the reverse happens, the pump is enabled pushing the gas towards the turbines and electricity is generated Compressedair energy storage Fig. 23. Process of the compressed air energy storageConclusions Thermalenergy storage is quite efficient in storing energy, being used in small scalebuildings to large-scale power plants all over the world. There has beensignificant development and research in the use of Phase Change Materials. Thiscould also be due to the rise of awareness of energy consumption and itsenvironmental impacts. Thermal energy storage is necessary when strategizingfor energy sustainability and the use of materials intelligently. (Alvaro de Gracia, 2015) It is especiallyrequired for the storage of solar energy, which is the most renewable source ofenergy.
Thermal energy storage projects in the world Accordingto the DOE Global Energy Storage Database, the number of global projectinstallations have increased substantially over the last couple of years. Thereare 192 projects with the total storage of about 3.21 GW, with Spain having thehighest energy storage projects. (DOE Global Energy Storage Database , n.d.) Fig. 24.
Screenshot of theDOE global energy storage database for thermal energy storage. Whilethermal storage is growing substantially, other kinds of storage have alsoincreased over the years with pumped hydro storage taking most of the percentof energy storage. Today, there are 1,067 global energy storage projects with85.20GW of total energy stored, with China having the highest energy storageprojects. (DOE Global Energy Storage Database , n.d.
) Fig. 25. Screenshot of theDOE global energy storage database for energy storage.The future of thermalenergy storageThere is alot of research happening with regards to Thermal Energy Storage, especiallywith Phase change materials and the usage of small amount of salts to storemore energy. The recent development of PCM encapsulated technologies hasbrought a lot of interest in the field. (Mazlan Abdul Wahid, 2016)Another very interesting aspect oflatent heat storage, is the mathematical modelling and numerical simulations tothe phenomenon. There is good amount of experimentation to incorporate thephase change materials concept by the control of software simulations.
(Alvaro de Gracia, 2015)