Refrigeration and Air conditioningsystems are directly or indirectly responsible for present energy crisisproblem as their use in household, commercial and transportation sector areincreasing rapidly.
Now-a-days power cuts are very often due to accidents, orcould be due to implementation of demand side management schemes to shift powerusage to avoid high loads by the electricity supplier, or by the user to shifttheir electricity usage to off-peak pricing periods (electrical load shifting)and it is important to maintain regular temperatures inside cold storagefacilities and cold transport vehicles. A major contribution to the heatloadings for a cold store comes from heat penetrating the walls. Therefrigeration system removes this heat load, but if there is a power failure,cooling is not provided to the stored product. Thermal Energy storage systems(TES) will use phase change materials for storage of heat and cold at shiftedtime.
Phase change material (PCM) melts within a narrow temperature range, andabsorbs a large amount of energy while in the transition state, thus minimizingthe rise in the environment temperature. PCM with a suitable meltingtemperature may be used to provide thermal capacity to maintain suitableinternal temperature during power failure. PCM may also be used in loadshedding applications to shift electricity usage to an optimum time.
Many ColdThermal Energy Storage (CTES) systems have gained attention in recent years.Cold Storage is a special kind of room, the temperature of which is kept verylow with the help of machines and precision instruments. This paper aims tocombination of cold store using with the inclusion of PCM. The effect on thechange in air temperature during loss of electrical power (and thus loss ofcooling) is investigated and validated for a cold store and then extended topredict the transient performance of a typical cold store. The energy used canhave different sources, which are renewable and non-renewable. Especially solarenergy is not continuous and thus heat storage is necessary to supply heatreliably.
When solar collectors are used to heat domestic hot water, thestorage also matches the different powers of the solar collector field, whichcollects the energy over many hours of the day, to meet the demand of a hotbath that is filled in only several minutes. Sensible heat storage is used forexample in hot water heat storages or in the floor structure in under floorheating. An alternative method is changing the phase of a material. Thebest-known examples are ice and snow storage.
The storage of thermal energy inthe form of latent heat in phase change materials (PCMs) represents anattractive option for low and medium temperature range energy applications.Wide ranges of PCMs have been investigated, such as paraffin wax, salt hydratesand non-paraffin organic compounds. The economic feasibility of employing alatent heat storage material in a system depends on the life span and cost ofthe storage materials. In other words, there should not be major changes in themelting point and the latent heat of fusion with time, due to thermal cycles ofthe storage materials. For latent heat storage, commercial grade PCMs ispreferred due to various reasons, such as low cost and easy availability.
Thisis one of the important aspects for a PCM-based energy storage system.Eliminating the problems of super cooling, phase separation and stability overa long period of application is an important criterion for the successfulapplication of suitable PCMs for thermal energy storage systems. The latentheat over the sensible heat is clear from the comparison of the volume and massof the storage unit required for storing a certain amount of heat.
It showsthat inorganic compounds, such as hydrated salts, have a higher volumetricthermal storage density than the most of the organic compounds due to theirhigher latent heat and density. The various PCMs are generally divided intothree main groups: organic, inorganic and eutectics of organic and/or inorganiccompounds. Organic compounds present several advantages such as nocorrosiveness, low or no under cooling, possess chemical and thermal stability,ability of congruent melting, self-nucleating properties and compatibility withconventional materials of construction. Subgroups of organic compounds includeparaffin and non-paraffin organic. Technical grade paraffin has beenextensively used as heat storage materials due to wide melting/solidificationtemperature ranges and has a relatively high latent heat capacity. They alsohave no sub cooling effects during the solidification as well as small volumechange during the phase-change process.
They are chemically stable, nontoxicand non-corrosive over an extended storage period. Widely used non-paraffinorganics, as latent heat storage materials are fatty acids like lauric,myristic, palmitic and stearic acid. Their advantages are a possibility forreproducible melting and solidification behavior and little or no sub coolingeffects. Disadvantages of organic compounds include lower phase-changeenthalpy, low thermal conductivity and inflammability. Inorganic phase changesof materials are a perspective way of thermal energy storage. Big latent heat,good thermal conductivity and inflammability are the main advantages of inorganicmaterials. But they cause corrosion and suffer from loss of H2O. Incongruentmelting and super cooling are the biggest problem with their exploitation.
During melting and freezing there are precipitations of other phases which donot take part in next process of charging and discharging. The use of phasechange materials (PCMs) in energy storage has the advantage of high energydensity and isothermal operation. I. Energystorage systemThermalenergy storage (TES), also commonly called heat and cold storage, allows thestorage of heat or cold to be used later. To be able to retrieve the heat orcold after some time, the method of storage needs to be reversible. Sensibleheat By far the most common way of thermal energy storage is as sensible heat.
A sensor can detect this temperature increase and the heat stored is thuscalled sensible heat. Latent heat if heat is stored as latent heat, a phasechange of the storage material is used. There are several options with distinctadvantages and disadvantages.
The phase change solid-liquid by melting andsolidification can store large amounts of heat or cold, if a suitable materialis selected. Melting is characterized by a small volume change, usually lessthan 10 %. If a container can fit the phase with the larger volume, usually theliquid, the pressure is not changed significantly and consequently melting andsolidification of the storage material proceed at a constant temperature. Uponmelting, while heat is transferred to the storage material, the material stillkeeps its temperature constant at the melting temperature, also called phasechange temperature.Heat transfer in PCM storage is characterized by a movingsolid–liquid interface, generally referred to as the “moving boundary” problem.It is a transient, non-linear phenomenon. Analytical solutions for phase changeproblems are only known for a couple of physical situations which possess asimple geometry and simple boundary conditions, as nonlinearity poses majordifficulties in moving boundary problems. Neumann originated the mostwell-known precise analytical solution for a one-dimensional moving boundaryproblem, called the Stefan problem.
Some analytical approximations forone-dimensional moving boundary problems with different boundary conditions arethe quasi-stationary approximation, perturbation methods, the Megerlin methodand the Heat-balance-integral method. It has been assumed here that the meltingor solidification temperature is constant. However, for example technical gradeparaffin has a wide temperature range at the points where melting andsolidification occur. Phase change problems are usually solved with finitedifference or finite element methods in accordance with the numerical approach.The phase change phenomenon has to be modeled separately due the non-linearnature of the problem. A wide range of different kinds of numerical methods forsolving PCM problems exist. The most common methods used are the enthalpymethod and the effective heat capacity method.
III. Characteristics and Classification ofPCMPCMs latent heat storage can be achievedthrough solid–solid, solid–liquid, solid–gas and liquid–gas phase change.However, the only phase change used for PCMs is the solid– liquid change.
Liquid-gas phase changes are not practical for use as thermal storage.Liquid–gas transitions do have a higher heat of transformation thansolid–liquid transitions. Solid–solid phase changes are typically very slow andhave a rather low heat of transformation. PCM stores 5 to 14 times more heatper unit volume than conventional storage materials such as water, masonry orrock. The classification of PCM is shown in figureFigure-1Classification of PCMIV. PropertiesAffecting TESS A. Melting Point Meltingpoint is the temperature at which the first crystal of the material collapses.
It is imperative to have the melting point of TESD within the temperature rangeof application. The melting point as such does not affect the energy storagecapacity of a material. However, as a phase change is involved in melting, theinclusion of melting point in temperature range of application can permit theuse of phase change as an on-off switch. Melting point lower as well as higherthan the temperature range of application prohibits the use of the material inTESS.
B. Heat of Fusion Heatof fusion also known as enthalpy of fusion or latent heat of fusion is a veryimportant property useful in selecting a TESD. It refers to the amount ofthermal energy that a material must absorb or evolve in order to change itsphase from solid to liquid or vice versa.
Large values of heat of fusion aid inincreasing the efficiency of TESS. C. Heat Capacity Heatcapacity refers to the amount of energy per molecule that a compound can storebefore the increase in its temperature. This energy is generally stored intranslational, vibration and rotational modes.
Thus materials with greaternumber of atoms in its composition are expected to have higher heat capacity. D. Thermal Conductivity ThermalConductivity measures the ability of amaterial to conduct heat. Greater values of k imply an efficient heat transfer.
Thermal conductivity is a property which needs to be optimized. Since thermalconductivity is phase dependent property, it is important to know values in both the solid as well as molten phases.It has been observed that most of molten materials exhibit much higher valuesof thermal conductivity as compared to that in their solid state. E. Density Densityof a material refers to its mass per unit volume. Density values can readily bemeasured using densitometers. Materials with higher density thus occupy lessspace which in turn increases the energy storage capacity. Materials with highdensity obviously possess higher energy storage capacity but many of them showa significant decrease in density in their molten state.
RequiredProperties of a PCM S. No Properties Selection Criteria 1. Thermal Properties a. Suitable Melting Temperature b. High Latent Heat of Fusion c. Good Heat transfer Rate 2. Physical Properties a.
Favorable Phase Equilibrium b. High Density c. Small Volume Change 3.
Chemical Properties a. Long Term Chemical Stability b. Compatibility with Construction Materials c. Non-Toxicity 4.
Economical Properties a. Cost Effective b. Abundant in Nature c. High Scale Availability V. Futuristicapplication OF PCM Manyresearch articles concluded that PCM is used for increasing the performance andefficiency of solar panels. Available solar panels efficiency is very less andone of key reason is high temperature loss.
Maximum efficiency of solar panelsonly exists at Standard Test Condition (STC). According to STC during theworking of solar panels temperature cell should be 25oC in another statement italso states that efficiency is decreased by 0.5% for an increase in each 1oCfrom the nominal temperature of 25oC. A high class thermal management leads toincrease the efficiency of solar panels through significant amount. PCM can usefor the thermal management in solar panels. Some other use of PCM likeapplication in ceiling fan for cooling of room, cooling of motorcycle helmet,applications in textiles for providing high class human comfort, combatvehicles for controlling the high internal temperature, Cold Storage plant etc.34-37&43.
VI. Phase Change MaterialApplications ü Building andconstructionü Energy storageü Shipping andtransportationü Others(Textiles, Protective clothing) VII. Selection of PCM Solid liquid PCMs are useful becausethey store a relatively large quantity of energy over a narrow temperaturerange, without a corresponding large volume change and currently appear to beof greatest practical value. A good design of latent thermal energy storagerequires the knowledge of PCM and the latent exchange process especially themelting and solidification process. VIII. Propertiesof Selected PCM Selected PCM Calcium chloride Hexahydrate (CaCl2.6H2O) Paraffin wax Melting temperature 27 o C 36 o C Heat of fusion 190.
8 KJ/Kg 146.9 KJ/Kg Thermal conductivity 0.540 W/m K 0.464W/m K Density 1562 kg/ 1828 kg/ IX. Hot Air OVER VIEW X.
ExperimentDescriptionAt the beginning, phase change materialsare in idle mode when they are purchased from the chemical industry. Soon afterit was incinerated to the temperature of 50 degree Celsius as the passivecrystals of PCM melts. At this moment, the PCM is made active. Then the aluminumpackets are used to seal the PCM and its better conductivity is the ultimatereason for its preference.
The control volume of air is affected by very smallextent and it is equalized by increasing the fans speed. On comparing theentire control volume, the reduction in control volume is insignificant.Thespeed of the fan and heat absorbed by the PCM are the two governing factorsbased on which the adjustment of mass flow rate is done. According to theprevalence of temperature in the PCM, the heat is efficaciously transferred bythe water within certain time.
The PCM material used is PARAFFIN WAX whose latentheat is 180 KJ/Kg K and the melting point is 29 Degree Celsius.When the fan isset in motion, the discharge of air takes place from top of the housing to theperson remaining at rest beneath the fan. The air flow is enhanced and the airis squeezed downwards due to the arrangement of blades in angles. The postulateof ceiling fan is that the hot air flows upward and air at ambient temperatureis squeezed downwards.
Because of the installation of PCM a lot of the fanshousing, the hot air which is directed up passes through the perforated holesin the circular disc of the PCM. Owing to the contact of PCM with hot air, theforced convection process takes place. The PCM which is placed a lot of fanshousing absorbs the heat of air because the melting point of PCM is lesser thanambient temperature. The high latent heat of fusion of PCM per unit massresults in small amount absorbing huge amount of heat energy. Due to the highspecific heat, it provides additional sensible heat storage and also avoid sub cooling.Water flows through the aluminum piping which absorbs the latent heat ofmelting of PCM, thereby carries away the heat to outside of the home. Thedischarge of water to the environment is comparatively low which flows like apattern of droplets of water and hence there is no difficulty in letting anddischarging of water.
PCM which is at low temperature absorbs the heatcontinuously from the hot air and the cooled air is squeezed downwards by thefan blades. The cooling effect is similar to AC whereas it eradicatesenvironmental hazards like ozone depletion and global warming. Theself-regulating valve present in the inlet of the insulated piping is keptslightly open so that the flow will be laminar and it is automatically adjustedby sensing temperature of PCM and the fan’s speed.
When the fan’s speed andtemperature is high then Due to the high conductivity of aluminum, heattransfer is effective and room temperature keep on decreasing. Figure-2- Experimental Setup Duct with CaCl2 Duct with paraffin waxXI. Resultand Discussion The temperature distribution of PCMs andrecording during discharging process. Performancecharts are shown in below. From that result we can select the PCM which givescomfort cooling than normal condition. Also it is economically low cost.Figure-3 Performance on Day 1Figure-4Performance on Day 2 Figure-5 Performanceon Day 3Figure-6 Performance on Day 4 Figure-7Performance on Day 5 XII.
ConclusionThe aim of this work was to verify PCM usageon free air cooling testing using experimental method. The result show, thatPCM could be useful for high temperature climatic condition for passivecooling. PCM usage significantly helps to increase thermal inertia ofbuildings. During the summer days in a controlled temperature of 25 degreeCelsius condition using PCM together with night ventilation. Different PCM with different peak temperaturescan be incorporated in a building to envelope to ensure thermal comfort fordifferent indoor temperature. The next step would be comparison of the resultsobtained in experiments on similar duct with theoretical calculation.
Suchexperiments are planned to be carried out in the further test methods.