SOLAR HEAT ENERGY STORAGE IN PHASE CHANGE MATERIALS full report
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SOLAR HEAT ENERGY STORAGE IN PHASE CHANGE MATERIALS
Developing efficient and inexpensive energy storage devices is as important as developing new sources of energy. Energy storage can reduce the time between energy supply and energy demand, thereby playing a vital role in energy conservation. It improves the energy systems by smoothening the output and thus increasing the reliability.
This paper deals with storage of solar thermal energy in materials undergoing phase changes. PCMs, which include salt hydrates, paraffins, non-paraffins, and eutectics of inorganic, are discussed. Heat storage in phase change materials (PCM) has an advantage of compactness and heat supply at constant temperature.
Energy storage is a key issue to be addressed to allow intermittent energy sources, typically renewable sources, to match energy supply with demand. There are numerous storage technologies that are capable of storing energy in various forms including kinetic energy, chemical solutions, magnetic fields, or other novel approaches.
PCMs absorb and emit heat while maintaining a nearly constant temperature. Within the human comfort range of 68Ã‚Â° to 86Ã‚Â°F (20Ã‚Â° to 30Ã‚Â°C), latent thermal storage materials are very effective. They store 5 to 14 times more heat per unit volume than sensible storage materials such as water, masonry, or rock.
Thermal energy can be stored in well-insulated fluids or solids. It can be generally stored as latent heat-by virtue of latent heat of change of phase of medium. In this the temperature of the medium remains more or less constant since it undergoes a phase transformation. Phase change storages with higher energy densities are more attractive for small storage.
Compared to different storage techniques for solar space heating and hot water production applications the operating temperature range for PCM is large, depending on the choice of material .The reason so as to why PCM is a suggested alternative to conventional storage
1. Thermal storage capacity per unit mass and unit volume for small temperature differences is high
2. Thermal gradients during charging and discharging is small
3. Simultaneous charging and discharging is possible with appropriate selection of heat exchanger
STORAGE IN PHASE CHANGE MATERIALS (PCM)
Phase change materials (PCMs) are "latent" thermal storage materials. They use chemical bonds to store and release heat. The thermal energy transfer occurs when a material changes from a solid to a liquid, or from a liquid to a solid. This is called a change in state, or "phase." Initially, these solid-liquid PCMs perform like conventional storage materials; their temperature rises as they absorb solar heat. Unlike conventional (sensible) storage materials, when PCMs reach the temperature at which they change phase (their melting point) they absorb large amounts of heat without getting hotter. When the ambient temperature in the space around the PCM material drops, the PCM solidifies, releasing its stored latent heat.
Heat storage in phase change has advantage of compactness, since the latent heat of most materials are large compared to their heat capacity over a temperature of order of 20 degrees .It has added advantage of heat supply at constant temperature .The various phase changes that can occur are melting, evaporation, lattice change etc.
The latent heat (enthalpy change) of transformation from one solid phase into another is generally small, but solid-gas and liquid â€œgas transitions have large heat of transformation, but large changes in volume make the system complex and impractical .The solid liquid transformations
Involve relatively small volume changes. These are available in a range of heats of fusion and transition temperatures. Some of the mixed fluoride salts exhibit large heats of fusion at melting points high enough for application in heat engines.
The hydrated salts that adsorb heat as they dissolve in their own water of crystallization come in the category of crystalline solid-liquid solution transformation. This process is similar to melting processes and heats of transition are of same order as the heats of fusion but there is no change in volume like in phase change materials. The heat of crystallization is released during the process of crystallization
There are a large number of organic and inorganic phase change materials (PCM) that meet the required thermodynamic and kinetic criteria for operation in desired temperature of 0-1400 C but many of them cannot be used due to the problems of chemical stability, toxicity, corrosion, volume change, availability at reasonable cost, etc
Solid-solid PCMs absorb and release heat in the same manner as solid-liquid PCMs. These materials do not change into a liquid state under normal conditions. They merely soften or harden. Relatively few of the solid-solid PCMs that have been identified are suitable for thermal storage applications. Liquid-gas PCMs are not yet practical for use as thermal storage. Although they have a high heat of transformation, the increase in volume during the phase change from liquid to gas makes their use impractical. The PCM applications described below are with liquid-solid materials.
TYPES OF PCMs
The PCMs fall in three categories:
Non paraffin organics
HEAT STORAGE CAPACITIES
Non paraffin organics 125-200 kJ/dm3
Salt hydrates 250 -400 kJ/dm3
Salt hydrates are characterized by X (Y) n .m H2O, where X (Y) n is an inorganic compound. These materials are preferred because of their high latent heat storage density. Salt hydrates such as sodium sulphate decahydrate and calcium chloride hexahydrate have suitable phase change temperatures for use as storage in space heating systems. These have the advantage of larger energy density.
The storage of heat in salt hydrates is in form of heat of fusion, which is latent heat of reaction. If latent heat of reaction is large latent heat storage has the advantage of making smaller systems. At certain temperature these materials release their water of crystallization and the solid remainder dissolves in it or in part.
Paraffinâ„¢s qualify as heat -of- fusion storage materials due to their availability in large temperature range and their reasonably high heat of fusion. Due to cost consideration, only technical grade paraffins may be used as PCMs in latent heat stores. Paraffins like other mineral oil products are complicated mixtures of several organic compounds and contain one major component called alkanes. The desirable characteristics that make them suitable to be used as PCMs are:
4. Congruent melting
5. Good nucleating properties
NON-PARAFFIN ORGANIC SOLIDS
This is the largest category of candidate materials for phase change storage some features of these organic materials are:
6. High heat of fusion
8. Low thermal conductivity
9. Varying levels of toxicity
10. Instability at high temperatures
11. Low flash points.
INORGANIC COMPOUNDS AND EUTECTICS
Apart from many inorganic salt hydrates there are many inorganic compounds, which undergo solid liquid phase transformation with high latent heat of fusion at higher temperature. Also apart from the pure compounds, eutectics of organic or inorganic compounds can be used to obtain the desired melting point .It is possible to get a fixed melting or freezing point eutectic mixture of inorganic salts.
Solar energy holds the key to futureâ„¢s non-exhaustive energy source. Effective utilization of
these resources requires effective storage. Heat storage using Ëœphase change materialsâ„¢ is a wise alternative. .
The main applications for PCMs are when space restrictions limit larger thermal storage units in direct gain or sunspace passive solar systems. Phase change materials may be used in solar domestic hot water heating or passive solar space heating systems.
Research is being conducted on methods of incorporating PCMs into other lightweight building materials such as plywood, as well as ceiling and floor tiles. Possible commercial applications include use in paving materials to minimize nighttime icing on bridges and overpasses, while also reducing surface damage from freeze-thaw cycling; outdoor wearing apparel for professionals (e.g., firefighters) or athletes exposed to extreme temperatures; and possible solar evaporator type heat pumps with thermal storage.
1. Processes and Materials of Manufacture by R.A. LINDBERG
2. SEMINAR TOPIC FROM :: edufiveseminar and presentationtopics.html
5. H.P Garg, S.C Mullick, Solar Thermal Energy Storage, D.Reidal publishing Co.
6. J.Prakash Solar Energy Fundamentals, TATA McGraw Hill
7. Britannica encyclopedia
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SEMINAR REPORT ON
‘SOLAR WATER HEATING SYSTEM WITH PHASE CHANGE MATERIAL’
Mr. GADHAVE PITAMBAR SUBHASH
The one of the most important part of a solar collector system is the solar tank. The solar collectors work daytime only, and its power depends on the weather. The hot water consumption in family houses is generally bigger in the evening and in the morning, so it is necessary to store the utilized energy. The collectors transform but not store the solar energy. The storage is necessary to accomplish in a heat-insulated tank, placed in tempered space. According the current architectural tendencies the boiler rooms are smaller, so the putting of the currently available solar tanks is very difficult. It is necessary to store the energy in a little space. The solution of the problem is the solar tank particularly filled with phase change material. This tank has smaller dimensions and bigger heat capacity than the conventional tanks. The other important advantage of a PCM solar tank is the possibility of the operating of the collectors at lower temperature. It could result a higher efficiency of the solar collector system.
The storage system consists of two simultaneously functioning heat-absorbing units. One of them is a solar water heater and the other a heat storage unit consists of PCM. The solar water heater functions normally and supplies hot water during the day. The storage unit stores the heat in PCM.s during the day and supplies hot water during the night and overcast periods. The storage unit utilizes small cylinders made of filled with paraffin (PCM) as the heat storage medium and integrated with a solar collector to absorb solar energy. The performance of this PCM based thermal energy storage system is compared with conventional sensible heat storage system and the conclusions drawn from them are presented.
1. Introduction 6
2. Phase change Materials 7
2.1 PCM in cooling & Heating 7
2.2 How it works 7
2.3 Applications 8
2.4 Advantages 8
3. Properties of PCM 9
4. Classifications of PCM 10
4.1 Organic phase change materials 10
4.1.1. Paraffin’s 11
4.1.2. Non-paraffin’s 11
4.2 Inorganic phase change materials 11
4.2.1. Salt hydrates. 11
4.2.2. Metallic’s 11
4.3 Eutectics. 12
5. Solar water heater with PCM 13
5.1 Solar Tanks 13
6. Experimental set up 14
7. Calculation of solidification 15
8. Experimental Result 16
9. Conclusion 19
10. References 20
Energy is essential for the existence of human life and plays vital role in the progress of the nation. However the past few years have witnessed a rapid growth in global population putting a tremendous burden on energy resources .In the present scenario the importance of available energy can’t be under Estimated. Also due to fast growth of the India’s economy. The country’s energy demand has grown to an average of 3.65% per annum over the past 30 years.
So it has become a need to harness alternate and renewable energy sources. Today India has one of the highest potential for the effective use of renewable energy sources .The country has also invested heavily in recent years in renewable energy utilization.
Solar energy being simple to use, clean, non polluting and inexhaustible has received wide spread attention in recent times. It provides well abundant energy source if utilized efficiently. But this energy is time dependent energy source with an intermittent character. Hence some form of thermal energy storage is necessary for more effective utilization of this energy source.
Phase Change Material (PCM) is one of the techniques to store this thermal energy in the form of latent heat. Inorganic phase change materials (PCM) are hydrated salts that have large amount of heat energy stored in the form latent heat which is absorbed or released when materials changes state from liquid to solid or solid to liquid .The PCM retains its latent heat without any change in physical or chemical properties over thousands of cycles. This PCM has wide range of applications; one of them is in the solar water heater.
One of the options is to develop energy storage devices, which are as important as
Developing new sources of energy. Energy storage not only reduces the mismatch between supply and demand but also improves the performance and reliability of energy systems and plays an important role in conserving the energy. It leads to saving of premium fuels and makes the system more cost effective by reducing the wastage of energy and capital cost.
2. PHASE CHANGE MATERIAL
These materials can store energy by the melting at a constant temperature. No material
has all the optimal characteristics for a PCM, and the selection of a PCM for a given
application requires careful consideration of the properties of various substances. Over
20,000 compounds and/or mixtures have been considered in PCM, including single component systems, congruent mixtures, eutectics and peritectics. The isothermal
operating characteristics (i.e. charging/discharging heat at a nearly constant
temperature) during the solidification and melting processes, which is desirable for efficient operation of thermal systems.
One of the most important aspects during the selecting of the material is the
conformable melting point and the high latent heat of fusion. The choice of the
substances used largely depends upon the temperature level of the application .
Residential, commercial and industrial buildings often have hot water requirements at
around 60 °C and bathing, laundry and cleaning operations in the domestic sector
generally need it at about 50 °C . The right melting point enables that the phase
changing comes off during every usage cycle. Thereby the latent heat could be fully utilized. According to the required temperature of the domestic hot water the melting point should be between 40 and 50 °C. Out of accordance with the conventional solar
tanks the temperature of the accumulation of heat is constant. Storage systems using
these heat accumulator materials can store the energy from the solar collector at lower
temperature level, too in winter. The stored energy can be used for pre-heating the cold
2.1 PCMs in Cooling and Heating
Based on the temperature requirement, a suitable PCM is selected. PCM
quantity is determined by the time of back up required and the total heat being generated / required in the system (external and internal) during the backup duration. PCM can be encapsulated in any convenient encapsulation and it can be reused multiple times.
2.2 How it works
For human comfort or to increase the efficiency of a system, temperature maintenance is very important. The ambient temperature of a place tends to increase due to the heat from the outside or due to the heat produced in-house. To capture the excess heat some equipments like
chillers, refrigerated air, etc. are being used. This heat can also be captured by energy storage device. PCM is a good energy storage material, which absorbs such excess heat. This excess heats melts the PCM.
This character of the PCM does not allow the temperature of the product to increase until the PCM melts completely. Thus for a particular period of time (until the PCM melts completely) we can maintain the temperature. Similarly when a heat source is being used for human comfort like solar water heater, a material like PCM that has high energy storage capacity (latent heat) than water (specific heat) can be used to increase the storage capacity in the same volume.
• Energy saving in Telecommunication shelters – while using natural energy
• Increasing life of Telecom equipment
• Reduce air-conditioning cost in building industry
• Increase hot water efficiency in solar water heaters
• Capture waste heat in boiler industry
• Cold storage/cold chain applications for horticulture
• Bio-Pharmaceuticals and vaccine transport
• Food / Poultry / Meat transport at specific temperatures
• Thermal wear for adverse climatic conditions
• Can be used as a room heater, same device / trolley can also be used as a cooler in
• Backup for room warmers / fireplace and so on
2.4 Advantages of PCMs
• Very High Latent Heat
• Can be tailor made for specific temperature requirements
• Easy handling and can be encapsulated in a variety of mediums
• Cheap and high stability over long periods of time
• Stable performance over repeated phase change cycles
• Limited / No supercooling
• Low / No flammability
• Non toxic
• Charge overnight & use during day as a cooler or charge in the daytime and use in the
night time as a warmer. It comes longer than oil radiator heaters
• The room heaters can be switched off in the night time – PCM can be used for 2 to 3
• PCM can be encapsulated in any form as per the customer’s requirement
3. PROPERTIES OF PCM
Phase change materials (PCM) are ‘‘Latent’’ heat storage materials. The thermal energy transfer occurs when a material changes from solid to liquid, or liquid to solid.
3.1. Thermal properties
(i) Suitable phase-transition temperature.
(ii) High latent heat of transition.
(iii) Good heat transfer.
Selecting a PCM for a particular application, the operating temperature of the heating or cooling should be matched to the transition temperature of the PCM. The latent heat should be as high as possible, especially on a volumetric basis, to minimize the physical size of the heat store. High thermal conductivity would assist the charging and discharging of the energy storage.
3.2. Physical properties
(i) Favorable phase equilibrium.
(ii) High density.
(iii) Small volume change.
(iv) Low vapor pressure.
Phase stability during freezing melting would help towards setting heat storage and high density is desirable to allow a smaller size of storage container. Small volume changes on
phase transformation and small vapor pressure at operating temperatures to reduce the containment problem.
3.3. Chemical properties
(i) Long-term chemical stability.
(ii) Compatibility with materials of construction.
(iii) No toxicity.
(iv) No fire hazard.
PCM can suffer from degradation by loss of water of hydration, chemical decomposition or incompatibility with materials of construction. PCMs should be non-toxic, nonflammable
and non-explosive for safety.
(iii) Cost effective.
4. CLASSIFICATION OF PCM
Fig. 1 Classification of PCM (1)
4.1 Organic phase change materials
Organic materials are further described as paraffin and nonparaffins. Organic materials include congruent melting means melt and freeze repeatedly without phase segregation and
consequent degradation of their latent heat of fusion, self nucleation means they crystallize with little or no super cooling and usually non-corrosiveness.
Paraffin is safe, reliable, predictable, less expensive and non-corrosive. The paraffins are waxes at room-temperature. These are hydrocarbons. Increasing the number of C-atoms increases the melting point too. The normal paraffins of type CnH2n+2 are a family of saturated hydrocarbons with very similar properties. Paraffins between C5 and C15 are liquids, and the rest are waxy solids. Paraffin wax is the most commonly used commercial organic heat storage PCM .
Paraffin waxes are cheap and have moderate thermal energy storage density but low
thermal conductivity and, hence, require large surface area
The non-paraffin organic are the most numerous of the phase change materials with highly varied properties. Each of these materials will have its own properties unlike the paraffin’s, which have very similar properties. This is the largest category of candidate’s materials for phase change storage.
4.2 Inorganic phase change materials
4.2.1 Salt hydrates
Salt hydrates may be regarded as alloys of inorganic salts and water forming a typical crystalline solid of general formula AB_nH2O. The solid–liquid transformation of salt hydrates is actually a dehydration of hydration of the salt, although this process resembles melting or freezing thermodynamically.
This category includes the low melting metals and metal eutectics. These metallics have not yet been seriously considered for PCM technology because of weight penalties.
A eutectic is a minimum-melting composition of two or more components, each of which melts and freeze congruently forming a mixture of the component crystals during crystallization
5. Solar water heater with PCM
Now a day’s importance of solar water heater system has been increased, but many people are unaware of the new techniques that, these solar water heaters can be used at evening or in winter season.PCM can be used in solar water heater, to increase its efficiency based on the following principle. During day time, the raised hot water which absorbs the energy from the Sun will be stored in a tank.
Fig. 2 PCM in solar water heating system (5)
When the tank is partially filled by PCM, the PCM will start to melt by absorbing the energy heat from hot water. The PCM can also be wrapped over the tank by using a PCM jacket. Thus the PCM can be charged during the day time. In the evening, in the absence of solar energy, the Temperature of the Hot water reduces by loss of heat to the environment. When the temperature goes below 580C, HS58 (type of PCM) will start to freeze. During its freezing, it will give up the heat to the water, and maintain the water’s temperature at not less than 580C. This increases the efficiency of the solar water heater by allowing availability of hot water at nights.
5.1 Solar tanks
The PCM tanks have two groups by the construction:
– PCM tanks with inner core, (fig. 3)
– PCM tanks with inner balls. (fig. 4)
The benefit of the tanks supplied with the inner core is the easy putting of the phase
change material. The disadvantage of the inner core is the small surface. The phase
change materials have small coefficient of thermal conductivity, so if the core has too
big diameter the process of the melting and the solidification will be slow by the thicker
and thicker solid layer on the inner surface of the core.
In the other type of the tanks the balls are filled with the phase change material. The
diameter of the balls is very small for the diameter of the tank. The balls are not fixed to the tank. It results a bigger heat exchange surface area between the water and the phase
change material. The biggest disadvantage of this construction is the requirement
special devices for making, filling and closing of the balls.
Fig. 3 Balls to encapsulate PCM (5) Fig. 4 Arrangement of the tubes filled with
paraffin and the solar heat exchanger (2)
6. EXPERIMENTAL SET –UP
The schematic of the experimental set-up is shown in Figure 5. It consists of the cylindrical TES tank, which holds the tubes with PCM, solar flat plate collector, flow meter, temperature indicator and a circulating pump. The photographic view of the experimental set-up is shown
in Figure 5. The stainless steel TES tank has a capacity of 48 liters, capable of supplying water for a family of four, with an internal diameter of 360mm and a height of 460mm. The tank has 25 tubes filled with PCM with 60mm outer diameter. Tank contains two plenum chambers on the top and the bottom of the tank and a flow distributor is provided on the top of the tank to maintain a uniform flow.
The tank is insulated with 50mm of glass wool and is provided with an aluminum cladding. It is considered that, on an average, the family would require 60 liters of heated water for their daily needs. This energy is stored as a mixture of sensible and latent heat of PCM, and sensible heat of water within the TES tank. We assume that the PCM store two-thirds of the energy while the remaining is stored as sensible heat of water. In the case of PCM less system, the same TES tank is used without having PCM shells.
Fig. 5. Experimental Set –Up (5)
1. Solar flat plate collector; 2. Pump;
3 & 4. Flow control valves; 5. Flow meter;
6. TES tank; 7. PCM capsules;
8. Temperature indicator; TP & Tf . Temperature sensors
Fig. 6 Photographic View of Experimental Set- up (5)
7. CALCULATION OF SOLIDIFICATION
The main problem of the operating of the PCM tanks is the low coefficient of thermal
Conductivity of the phase change material. During the discharging of the tank the PCM
solidifies to the inner surface of the tube. The thermal flux will be decreased by the thermal insulating effect of the thicker and thicker solid PCM layer. I have calculated the required time of the solidification of the paraffin. The equation of the thermal conductivity in tubes with two layers (layer 1 is the solid phase of the paraffin, layer 2 is the wall of the tube)
λPCM - coefficient of thermal conductivity of the paraffin
λw - coefficient of thermal conductivity of the tube
d1 - inner diameter of the solid paraffin layer on the inner surface of the tube
d2 - inner diameter of the tube
d3 - outer diameter of the tube
tw1 - temperature of the phase change
tw3 - temperature of the outer surface of the tube
During the solidification the value of d1 decreases from d2 to 0. The tw3-tw1 temperature
difference depends on the temperature of the water in the tank. With the equation we
can calculate the required time of the solidification.
The next diagram shows the decrease of the r1=d1/2 inner radius of the solid phase in function of time. The parameters are according to 60 mm tube diameter, 1 mm wall thickness, the PCM is paraffin, the material of the tube is stainless steel. The difference between the phase change temperature and the temperature of the outer wall of the tube is 2 °C
8. EXPERIMENTAL RESULTS
The lower difference between the temperature of the collector and the outer air results
higher collector efficiency.
The fig. 7 shows the irradiation and outer air temperature in a summer day.
Fig. 7 Irradiation, energy yield and air temperature in a typical summer day (2)
The paraffin has smaller specific heat capacity than the water, so it results higher
temperature in the PCM-tank, than the conventional solar tank in a summer day:
Fig. 8 Comparing the temperature in a conventional solar tank and a PCM-tank with
The fig.8 shows the result of a calculation of a PCM-tank with 70 kg water
around the tubes and 170 kg paraffin in the tubes. We used for the calculation the efficiency characteristic of our own-designed experimental flat collectors.
The temperature of the PCM-tank is higher than the conventional because of the lower
heat capacity of the PCM, so the collectors has to operate at higher temperature, and the
efficiency is lower because of the heat loss from the collector to the air. If our goal is
the higher efficiency we have to choose another PCM with lower melting point.
The melting point of the Glauber’s salt (Na2SO4 10H2O) is 32 °C , and the latent heat
is 250,9 kJ/(kgK). The heat capacity is 0,894 kJ/(kgK). The next figure shows the
calculation for the same day with Glauber’s salt:
Fig. 9 Comparing the temperature in a conventional solar tank and a PCM-tank with salt (2)
System efficiency is defined as the ratio of the amount of energy stored by the tank to the heat energy available from the solar radiation.
Fig. 10 Comparison of System Efficiency (5)
It is observed from the fig10 that the efficiency of the system without PCM is fluctuating over various periods of time, while the efficiency of the system with PCM is constant over the phase transition temperature and that it also shows a higher efficiency. Hence the system with PCM is more efficient.
The thermal behavior of the systems is investigated experimentally for various operating conditions. The effects of charging times, energy storage, time of solidification of PCM and efficiency of the systems are studied. These characteristics of both the systems are compared. At last it is concluded that :
The cost of the manufacturing of the tank is lower than the conventional tanks in trade with the same heat capacity and the space demand is much lower too. The other advantage of the PCM tank is the constant temperature during the heat accumulation. This constant temperature could be lower, it depends on the type of the PCM. The lower temperature of the heat accumulation permits the higher efficiency of the collectors at low external temperature.
Hence the systems with PCM are viable option for solar heat energy storage. Possessing considerable advantages over the systems without PCM, it can be used as an alternative to current domestic sensible solar water heating technologies.
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