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11CONTENTSCLASSIFICATION AND PRINCIPLES OF STORAGE OF SENSIBLE HEATHEAT AND TEMPERATURESOLAR PONDSENSIBLE HEAT STORAGELIQUID MEDIA STORAGE SOLID MEDIA STORAGE (PACKED BED STORAGE) DUAL MEDIA STORAGE

2CLASSIFICATION AND PRINCIPLES OF STORAGE OF SENSIBLE HEAT

3DISADVANTAGES OF SHS SYSTEMSLow heat storage capacity per unit volume of the storage medium Non-isothermal behavior during heat storage (charging) and heat release (discharging) processes4HEAT AND TEMPERATUREHeat - in the physical sense - is a form of energy and can be stored in a variety of ways and for many different applications. A characteristic property of heat is its temperature: according to this it can be distinguished in low-temperature heat and high-temperature heatLow-temperature heat is usually applied for domestic hot water supply; it is usually stored in small hot water tanksHigh-temperature heat is applied for all sorts of power processes and in chemical engineering5SOLAR PONDThe solar pond is a special arrangement for an easy store of solar heat. This heat is absorbed at the bottom of the pond and heats the brine there. The salt concentration in the pond increases from top to bottom so that density increases and natural convection is suppressedGround Stores, Multiple Well Stores, Aquifers and Storage Reservoirs are seasonal stores which are supposed to transfer summer heat into the winter demand6REQUIREMENTS OF STORELong service life, non-corrosive, non-toxic, non-flammable - large heat storage capacity High thermal diffusivity and heat diffusivityCapability to withstand charging / discharging cycles without loss in performance, store capacity or change in structure Wide availability, simple handling, storage in simple containersLow cost

7SENSIBLE HEATSensible heat is heat exchanged by a body or thermodynamic system that changes the temperature, and some macroscopic variables of the body, but leaves unchanged certain other macroscopic variables, such as volume or pressure.The sensible heat of a thermodynamic process may be calculated as the product of the body's mass (m) with its specific heat capacity (c) and the change in temperature (T)Qsensible = mcT8SENSIBLE HEAT STORAGE

In this type of storage, thermal energy is stored by virtue of heat capacity and the change in temperature of the material developed during the process of charging and discharging. The temperature of the storage material rises when energy is absorbed and drops when energy is withdrawn. The charging and discharging operations, in a sensible heat-storage system, can be expected to be completely reversible for an unlimited number of cycles over the lifespan.

9CLASSIFICATION BASED ON HEAT STORAGE MEDIA10LIQUID MEDIA STORAGE

Of available liquids, water can be considered to be the most suitable liquid media for storage below 100C. With its highest specific heat water is the most commonly used medium in a sensible heat storage system. Approximate size is to use about 75 to 100 litres of storage per square meter of collector areaWater storage tanks are made from a variety of materials like steel, concrete and fiberglass. The tanks are suitably insulated with glass wool, mineral wool or polyurethane11If the water is at atmospheric pressure, the temperature is limited to 100C. It is possible to store water at temperature a little above 100C by using pressurized tanksIn order to reduce the costs, an alternative way, which is being examined for large-scale storage, is to use naturally occurring confined underground aquifers which already contain water Since the investment required is a series of openings for injecting and withdrawing water, it is expected that storage costs for such systems would be low12UNDERGROUND SENSIBLE STORAGE SYSTEM

13SHORT TERM SENSIBLE HEAT STORAGE BY WATER

14SCHEMATIC DIAGRAM OF A LONG TERM HEAT STORAGE SYSTEM

15LONG TERM THERMAL STORAGE IN UNDERGROUND LAYERS

16ADVANTAGESIt is abundant and inexpensive.It is easy to handle, non-toxic and non-combustible.Its flow can take place by thermo-siphon action. It has high density, high specific heat, good thermal conductivity and low viscosity. Can be used both as storage medium as well as working medium (thus eliminating the need for a heat exchanger, e.g., in a space-heating system) Charging and discharging of heat can occur simultaneously. Control of water system is flexible.

17DISADVANTAGESLimited temperature range (0C to 100C). Corrosive medium Low surface tension (i.e., leaks easily)

18Though water is the best choice as a heat-storage medium in a space-heat system; other liquids such as oils, liquid metals and molten salts have also been used in solar thermal power plants. Heat transfer oils are used in sensible heat storage systems for intermediate temperatures ranging from 100 to 300 C. Eg: Dowtherm and Therminol. The problem associated with the use of heat transfer oils is that they tend to degrade with time. The use of oils also presents safety problems since there is a possibility of ignition above their flash point19THERMOPHYSICAL PROPERTIES OF SELECTED LIQUIDS FOR SENSIBLE HEAT STORAGESl no:Medium Temperature Range (C)Density (kg/m3)Heat capacity (J/kgK)1Water 0 to 100100041902Thermional 66 9 to 343 750 2100 3Hitec (molten salt) 142 to 540 1680 1560 4Engine oil Up to 160 888 18805Lithium 180 to 1300 5104190 6Sodium 100 to 760 960 1300 7Octane (organic liquid)Up to 126 7042400 20SOLID MEDIA STORAGE (PACKED BED STORAGE)

Energy can be stored in rocks or pebbles packed in insulated vessels. This type of storage is used very often for temperatures up to100C in conjunction with solar air heaters. It is simple in design and relatively inexpensive.Approximate sizing is to use 300 to500 kg of rock per square meter of collector area for space heating applications. Rock or pebble-bed storages can also be used for much higher temperatures up to 1000C.

21Magnesium oxide (magnesia), aluminum oxide (alumina) and silicone oxide are refractory materials, and they are also suitable for high-temperature sensible heat storage. Bricks made of magnesia have been used in many countries for many years for storing heat. They are available in the form of devices with electric heater elements embedded in the bricksIt utilizes the heat capacity of a bed of loosely packed solid materials such as rocks, metals, concrete, sand, bricks, etc., to store energy22SOLID MEDIA PROPERTIES FOR SENSIBLE HEAT STORAGESl noMaterialDensity(kg/m3)Specific Heat(J/kgK)Heat Capacity (J/m3 K)1Aluminium27078962.42552Aluminium oxide39008403.27603Aluminium sulphate27107502.03254Brick16988401.42635Brick magnesia300011303.39006Concrete224011302.53107Cast iron79008376.612323Sl noMaterialDensity(kg/m3)Specific Heat(J/kgK)Heat Capacity (J/m3 K)8Pure iron78974523.56949Calcium chloride25106701.681710Copper89543833.429411Earth (wet)170020933.558112Earth (dry)12607951.001713Potassium chloride19806701.326614Potassium sulphate26609202.447215Sodium carbonate251010902.735916Stone, granite26408202.164817Stone, limestone25009002.250018Stone, marble26008002.080019Stone, sandstone22007101.562024ADVANTAGESRocks are abundant, low cost, easy to handle, non-toxic and non-combustible High storage temperatures are possible Rocks act both as heat transfer surface and storage mediumHeat exchanger can be avoided No freezing problem No corrosion problem25DISADVANTAGES Storage volumes are large High pressure drop needs high capacity air blowerSimultaneous charging and discharging are not possible26DUAL MEDIA STORAGE

In dual media Storage, both solid and liquid sensible heat storage materials are used. These solid and liquid materials may be combined in a number of ways Dual media storage unit or (thermocline is based on the partial replacement of the liquid storage medium by a cheaper material, which is usually a solid storage medium. This dual-media concept requires only a single tankThe thermocline zone reduces the volumetric storage capacity of a dual media storage unit27ConclusionA) Comparison between different heat storage mediaSENSIBLE HEAT STORAGEWaterRocka) Operating Temp. Range choiceLimited (0-100C)Largeb) Specific HeatHighLowc) Thermal ConductivityLow, convection effects improve the heat transfer rateLowd) Thermal Storage Capacity per unit mass and volume for small temp differencesLowLowe)Stability to thermal cyclingGoodGoodf)AvailabilityOverallAlmost Overallg)CostInexpensiveInexpensive28B) Comparison of heat transfer properties and life of different types of thermal storesSensible heat StorageWaterRocka) Required heat exchanger geometrySimpleSimpleb) Temperature gradients during charging and dischargingLargeLargec) Thermal stratification with effectExistent, works positivelyExistent, works positivelyd) Simultaneous charging appropriate discharging exchangerPossibleNot Possiblee) Integration with solar heating/coo ling systemsDirect integration with water systemsDirect integration with air systemsf) Cost of pumps, fans, etc.LowHighg) Corrosion with conventional materials of constructionCorrosion eliminated through corrosion inhibitorsNon Corrosiveh) LifeLongLong29

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