1. Introduction:

Solar energy is the most promising inexhaustible heat energy source for the present and future needs of mankind. Energy is the backbone of all human activities on the earth. In recent years, energy demand has increased due to the high-energy consumption in different fields (e.g. Electricity Generation, Industry, Transportation, Household, etc.…). Fossil fuels have served and fulfilled all human needs of energy for long era, these fossil fuels caused huge damages to the environment that led to the most of the recent environmental problems, which are the global warming and the danger of ice melting in the north and south poles.

In addition, the prices of these fossil fuels increased from last year and it is expected to continue increasing in coming years because energy demand is increasing while fossil fuels in reserves are decreasing. Therefore, other sources of energy must be developed in order to take the role of fossil fuels one of the major problems with the solar energy is its intermittent nature. So to balance the energy supply and demands, a heat energy storage system is necessary. There are three main methods of thermal energy storing systems, that is, sensible, latent, and combined sensible and latent heat storage system. The thermal energy storage systems using both sensible and latent heat storage methods are gaining a lot of importance nowadays, due to their high thermal energy storage capacity per unit volume and isothermal behaviour during charging and discharging processes.

The solar energy incident over the atmosphere is 1017watts in which the incidence on the earth's surface is 1016 watts. The total power demand all over the world is 1013watts.The solar radiation in a normal sunny day is around 1Kw/m2 [1]. From this figure, it is clear that the energy available is 1000 times more than that we required. If we can utilize 1% of this energy, it will be 10 times the total world will require. The diameter of the sun is 1.39 x106 and the earth is about 1.27 x 104.The mean distance between the two is 1.50 x 108 km [1]. The sun is a hydrodynamic spherical body of extremely hot ionized gases called plasma. The energy is produced by the process of thermonuclear fusion. The temperature inside the sun is estimated at 8 x10⁶ °K to 40 x 10⁶ °K. The energy is emitted by the fusion of hydrogen to helium [2]. Solar energy being simple to use, clean, non polluting and inexhaustible have received widespread attention in recent times and provides a well abundant energy source if utilized efficiently. But this energy is a time dependent energy source with an intermittent character [3, 4, 5, 6 and 7], hence some form of thermal energy storage (TES) is necessary for more effective utilization of this energy source. Phase Change Materials (PCM) are 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 of latent heat which is absorbed or released when materials changes state from liquid to solid or solid to liquid [8 and 9].

Phase Change Materials (PCMs) are one of the techniques to store this thermal energy in the form of latent heat. Organic phase change materials (PCMs) are the hydrated salt that has a large amount of heat energy stored in the form of latent heat which is absorbed or released when materials changes state from liquid to solid or solid to liquid. The PCMs retains its latent heat without any change in physical or chemical properties over thousands of cycles. PCMs have wide range of applications; one of them is in the solar water heater.

One of their 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.

2. Solar Water Heater:

Hot water is required for many purposes and the sun can be used effectively, efficiently and economically to provide this heat. The warming effect of solar radiation is obvious and it is well known that a container of cold water, left exposed to the sun will be raised in temperature. Solar water heating systems are designed to make convenient use of this phenomenon. Solar water heating is the conversion of sunlight into renewable energy for water heating using a solar thermal collector. Solar water heating systems comprise various technologies that are used worldwide increasingly. In a “close-couple” SWH system the storage tank is horizontally mounted immediately above the solar collectors on the roof. No pumping is required as the hot water naturally rises into the tank through thermo siphon flow. In a “pump-circulated” system the storage tank is ground –or floor-mounted and is below the level of the collectors; a circulating pump moves water or heat transfer fluid between the tank and the collectors. Solar water heating systems are designed to deliver hot water for most of the year. However, in winter there sometimes may not be a sufficient solar heat gain to deliver sufficient hot water. In this case gas or electric booster is used to heat the water.

Some systems use an electric pump to circulate the fluid through the collectors. Solar water heaters can operate in any climate. Performance varies depending, in part, on how much solar energy is available at the site, but also on how cold the water coming into the system is. The colder the water, the more efficiently the system operates [10, 11, 12 and 13].

2.1. Types of solar water heater:

· Passive and Active

· Direct and Indirect

Passive systems rely on the natural buoyancy or thermosyphon effect created when the temperature of the water in the collector rises, causing the water itself to rise as it becomes less dense with increased temperature, thus inducing circulation in the circuit.

Solar thermal systems in their simplest form consist of a solar collector and a storage tank. These systems are termed passive, whilst those that contain circulating pumps are known as active systems. The active closed loop and open loop solar water heater was shown in figures1 and 2.

Fig. 1. Active, closed loop solar water heater

Fig. 2. Active open loop system solar water heater

3. Solar Energy and Thermal Energy Storage:

Solar energy is the origin of all known forms of energy. Fossil fuels such as oil, coal, and natural gas were produced by photosynthetic processes, after that, complex chemical reactions took place and the decaying vegetation was subjected to very high temperatures and pressures for a long time. In addition, the sun produces the wind energy because wind is produced as a result of the difference in temperature between the various regions of the earth.Solar energy can be converted into different forms of energy, either to thermal energy or to electrical energy. Solar energy is converted directly into electrical power only by photovoltaic; photovoltaic is a device that converts the direct solar radiation into direct electrical current. On the other hand, solar collector converts solar energy into thermal energy. The solar collector is a device that absorbs the direct solar radiation and converts it into thermal energy. Solar collectors have three types: (1) Flat plate collectors, which produce temperature lower than 100℃, (2) Evacuated tube collectors, which produce temperature between 100 ℃and 200℃, and (3) Parabolic trough collectors, which collect a huge amount of solar radiation and concentrate it on a small area in order to generate thermal energy at very high temperatures up to 1000℃.

3.1. Different methods for thermal energy storage:

Energy storage is storing energy during the time when excess energy is available in order to be used later. In other words, energy storage is used to correct the mismatch between the time of energy supply and energy demand.

3.2. Storage of Thermal Energy:

Thermal energy can be stored in form of sensible heat or latent heat or combination of sensible and latent heat.

a) Sensible Heat Storage

Heating a liquid or a solid, without changing phase: This method is called sensible heat storage. The amount of energy stored depends on the temperature change of the material. Water is known as one of the best materials that can be used to store thermal energy in form of sensible heat because water is abundant, cheap, has a high specific heat, and has a high density. In addition, heat exchanger is avoided if water is used as the heat transfer fluid in the solar thermal system.

b) Latent Heat Storage

Latent heat storage uses the latent heat of the material to store thermal energy. Latent heat is the amount of heat absorbed or released during the change of the material from one phase to another phase. Two types of latent heat are known, latent heat of fusion and latent heat of vaporization

Latent heat of fusion is the amount of heat absorbed or released when the material changes from the solid phase to the liquid phase or vice versa, while latent heat of vaporization is the amount of thermal energy absorbed or released when the material changes from the liquid phase to the vapour phase or vice versa. Indeed, latent heat of vaporization is not paid attention for latent thermal energy storage applications because of the large change in the volume accompanied by this type of phase change. The amount of thermal energy stored in form of latent heat in a material is calculated by

Q=m*LH

Q is the amount of thermal energy stored or released in form of latent heat (kJ), m is the mass of the material used to store thermal energy (kg), and LH is the Latent heat of fusion or vaporization (kJ/kg).

The amount of thermal energy stored as latent heat depends on the mass and the value of the latent heat of the used material. Materials used to store thermal energy in form of latent heat are called phase change materials.

The amount of energy stored (E) in this case depends upon the mass (m) and latent heat of fusion (λ) of the material. Thus,

E=mλ

4. Phase Change Materials (PCM):

All materials are phase change materials. The most important difference between these materials is the phase change temperature. Each material makes its phase change at different temperature. In addition, each material has a different value of latent heat and thermal conductivity. The main drawback of most of phase change materials is their low thermal conductivity that decreases the heat transfer rate. The most important feature for the selected phase change material is to have its phase change temperature fitted to the application temperature range. Indeed, there is no specific material that is called as an ideal material to be used as a phase change material. Energy storage is the most fundamental requirement of all solar energy systems.

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 20,000 compounds and/or mixtures have been considered in PCM, including single component systems, congruent mixtures, and tactics. 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 [14,15,16 and 17]. Phase change materials offer the best solution to this fundamental requirement, thereby resolving the problem met during the time of peak demand. The phase change material and the PCM process is shown in figures 3 and 4.

PCMs latent heat storage can be achieved through 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 due to the large volumes or high pressures required to store the materials when in their gas phase.

Fig.3.Pcm Material

Fig. 4. Classification of Phase Change Materials

Organic phase change materials are classified as paraffin and non-paraffin. Organic PCMs are characterized by their ability to melt and freeze many times without phase segregation and degradation of their latent heat of fusion as shown in figure 4.

1) Paraffin:

Paraffin of type CnH2n+2 is a group of saturated hydrocarbons with very similar properties, Paraffin between C5 and C15 are liquids and the rest are waxy solids, Paraffin wax is one of the most popular organic heat storage PCM for commercial applications, it consists of a straight chain hydrocarbon having melting temperatures ranging between 23℃and 67℃.

Following are some of the advantages and disadvantages of paraffin

Advantages:

a) Paraffin is available in a large temperature range.

b) Paraffin have no tendencies to super cool.

c) Paraffin is chemically stable.

d) Paraffin waxes show high heats of fusion.

e) Paraffin waxes don’t segregate

f) Paraffin waxes are safe

g) Paraffin is non-corrosive.

Disadvantages:

a) Low thermal conductivity

b) High volume change between the solid and liquid phases

c) Commercial paraffin does not have sharp exact melting points.

d) Paraffin is flammable

e) Pure paraffin is expensive

2) Non-Paraffin:

The non-paraffin organic PCMs are characterized by their varied properties; each of these materials has its own properties. Abhatetal. Buddhi and Sawhney have conducted an extensive survey and recognized a number of esters, fatty acids, alcohols, and glycols suitable for thermal energy storage. These organic materials are subdivided into fatty acids and other non-paraffin organic.

B) Inorganic Phase Change Materials:

Inorganic phase change materials are classified as salt hydrates and metallic

1) Salt hydrates

Salt hydrates consist of a salt and water that combine in a crystalline matrix when the material solidifies. There are many different salt hydrates having melting temperature ranges between 15°C - 117°C, Salt hydrates are considered as the most important group of PCMs that have been studied for application in latent thermal energy storage systems.

2) Metallic:

Metallic include the low melting metals and metal eutectics. Metallic have not been strongly studied as PCM for latent heat storage because of their heavy weights. For the applications that weight is not an important issue while volume is an important parameter, metallic are attractive because of their high heat of fusion per unit volume.

C) Eutectics:

The eutectics consist of two or more components where each of them melts and freezes congruently forming a mixture of a component that crystal during crystallization process. Usually, eutectics melt and freeze without segregation. During melting process, both components liquefy at the same time without possibility of separation

5. Experimental Investigation:

Figures 5 show the schematic diagram and photographic view of the experimental setup developed for the investigation. The insulated thermal energy storage tank has a capacity of 100 liters (400mm diameter and 1000mm height) and is filled with 75mm diameter spherical capsules as shown in Figure 5. The PCM used in the experiment is OM53 (melting temperature of 60 ± 2⁰cand latent heat of fusion of 213 KJ/Kg) and material of the spherical capsules used in the experiments is high density polyethylene (HDPE).The TES tank is divided into three segments; that is, at 𝑥/𝐿 = 0.33, 0.66, and 1.0 (𝐿 is length of the TES tank, mm; 𝑥 is the axial distance from the top of the TES tank, mm; 𝑥/𝐿is the dimensionless axial distance from the top of the TES tank)along its axial direction; the resistant temperature detectors(RTDs) with an accuracy of ±0.3⁰C are placed at the inlet, outlet, and three segments of the TES tank to measure the temperatures of HTF. Another three numbers of RTDs are inserted into the PCM capsules and they are placed at three segments of the TES tank to measure the temperatures of PCM. The RTDs are connected to a temperature indicator, which provides instantaneous digital outputs (Figure 5).A flow meter with an accuracy of ±2% is used to measure the flow rate of HTF and a centrifugal pump (500 lit/hour) is employed to circulate the HTF through the storage tank.

Fig.5. Experimental setup

Figure 5 shows the Schematic of the experimental setup (1) solar flat plate collector, (2) pump, (3) and (4) flow control valves, (5) flow meter, (6) TES tank, (7) PCM capsules, and (8) temperature indicator, 𝑇𝑝, and 𝑇𝑓: temperature sensors (RTDs).

The performance of the charging of TES is studied using 2 lit/min, 4 lit/min, and 6 lit/min flow rates with varying inlet HTF temperatures. Initially, the energy is stored inside the capsules as sensible heat until the PCM reaches its melting temperature. As the charging process proceeds, energy storage is achieved by melting the PCM at a constant temperature. Finally, the PCM becomes superheated. The energy is then stored as sensible heat in liquid PCM. Temperatures of the PCM and HTF are recorded at an interval of 12minutes.Thecharging process is continued until the PCM temperature reaches the value of 61⁰C.Batchwise discharging of TES is studied with different discharge flow rates, that is, 2 lit/min, 4 lit/min, and 6 lit/min, keeping the constant cold water inlet, that is, 2 lit/min and 30⁰C. A certain quantity of hot water (6.67 lit) is withdrawn from TES tank and the tank is again filled with cold water of quantity equal to the amount of water withdrawn. Again, after a time interval of 15 minutes, allowing transfer of energy from PCM to HTF, another 6.67 lit of water is withdrawn from the TES tank. This process is continued until the water (HTF) outlet temperature reaches 34^°C.The table 1 shows the design specification of solar water heater and Table 2 shows the thermo physical properties of pcm.

Table 1 Design specification

Material for collector	Stainless steel
Length of the collector	2m
Width of the collector	1m
Area of the collector	2m²
Air gap between glass plate and collector	5cm
Total quantity of PCM	11kg
Material of PCM balls	High density polyethylene

Table 2 Thermo physical properties of pcm (om53)

PROPERTY	VALUE
Freezing Temp (^oC)	53
Melting Temp (^oC)	54
Latent Heat (kJ/kg)	250
Liquid Density (kg/m³)	0.87
Solid Density (kg/m³)	0.91
Liquid Specific Heat (J/g.k)	2.3 to 2.4
Solid Specific Heat (J/g.k)	1.6 to 1.7

6. RESULTS AND DISCUSSIONS:

The temperature histories of HTF and PCM at three segments of the TES tank i.e.at X/L=0.33,0.66 and 1 are shown in figure 5. Figures10 and 11 represent the temperature variation of the HTF inside the storage tank for a mass flow rate of 2kg/min and 4kg/min. It is observed from the figure that the temperature of the HTF at all segments increases gradually until it reaches the temperature of 60or 62^oC and then it remains linearly constant around 64^oC for a period of 45 min during which the PCM undergoes phase change at 60±1^oC.After that the HTF temperature increases up to 70 or 71^oC.

A figure 8 and 9 represents the temperature variation of PCM during the charging process for the mass flow rate of 2kg/min and 4kg/min respectively. It is seen from the figure that the PCM temperature increases gradually at the beginning of the charging period and remains nearly constant around 60^oC during melting process and increases sharply during heating of liquid PCM. Also, it is noted from the figure that the PCM in the first segment is completely charged nearly 85% of the total charging time. The charging process is terminated when the PCM temperature in all the segments reaches 63^oC. It is also observed from both the figure that there is no significant temperature difference between each segment from top to bottom of the storage tank during the sensible heating of the solid PCM and also during phase change period. The region that the water temperature in the storage tank to increase gradually in according with inlet temperature of HTF supplied from the solar collector and the PCM temperature also increases gradually along with HTF temperature. From the temperature histories it is inferred that in the present system, the heat transfer rate possible from the HTF to the PCM in the storage tank is higher than the heat receiving rate of HTF from the solar collector. Hence it is possible to reduce the charging time further by solar collector surface area. The variation of absorber plate and glass plate temperature is shown in figures 6 and 7.

Time (min)

Fig. 6. Time Vs Absorber Plate Temperature (06.04.2015 or 07.04.2015

Time (min.)

Fig. 7. Time Vs Glass plate Temperature (06.04.2015 or 07.04.2015)

Fig. 10 and 11 illustrate the effect of varying the mass flow rate of HTF (2and 4 kg/min) during the charging of the storage tank. Increase in mass flow rate has a large influence on the phase transition process of PCM. As the flow rate increases the time required for the complete charging becomes smaller. It is seen from the fig that the charging time has decreased by 16 % when the flow rate is increased from 2 to 4kg/min. This is because an increase in the fluid flow rate (2 to 4 kg/min) translates into an increase in surface heat transfer coefficient between the HTF and PCM capsules .Hence the mass flow rate has significant effect on the time for the storage tank.

Time (min.)

Fig. 8. Time Vs PCM Temperature at 2 lit/min (07.04.2015)

Time (min.)

Fig.9. Time Vs PCM Temperature at 4 lit/min (06.04.2015)

Time (min.)

Fig. 10. Time Vs Heat transfer fluid Temperature at 2 lit/min (07.04.2015)

Time (min.)

Fig. 11. Time Vs Heat transfer fluid Temperature at 4 lit/min (06.04.2015)

Time (min.)

Fig. 12. Time Vs Discharging Water Temperature (06.04.2015 and 07.04.2015)

Time (min.)

Fig. 13. Time Vs Discharging PCM Temp (06.04.2014 and 07.04.2015)

Figure 10 shows the instantaneous heat storage in the storage tank during the charging process for various mass flow rates of the HTF. This is estimated based on the instantaneous inlet and outlet temperature of the HTF.It is observed that during the inlet period of charging the instantaneous heat is high and it is decreasing 50 to 60 minutes. This drop in the heat stored is due to the decrease in temperature difference between the HTF and the temperature of the storage tank .As the charging process proceeds , the PCM starts melting and the heat stored remains almost uniformly due to the constant temperature difference between the HTF and the storage tank. This is the major advantage of a combined system where a uniform rate of charging and discharging is possible for a longer period, which will be useful in many applications.

Figure 11 shows the system efficiency of the storage system for various mass flow rates of HTF, system efficiency is defined as the amount of energy stored in the storage tank and the heat energy available from the solar radiation. As the time increases the system efficiency, decrease during sensible heating of the solid PCM and it remains nearly constant during the phase change period and then it further decrease during sensible heating of liquid PCM .This is due to the fact that as charging proceeds the temperature difference between the HTF and the PCM in the storage tank decreases. This decreases the amount of heat transferred to the storage tank and thus the energy stored decreases with increase in time. Also the increase in HTF temperature, the inlet of solar collector decreases the heat absorption rate for the collector. In addition, as the time increases the temperature of the water in storage tank which results in increasing heat loss from the storage tank. This also contributes to the decrease in system efficiency. The temperature histories of PCM and HTF during the discharging process (heat recovery) for batch wise discharging method are reported.

Fig. 13 represents the temperature histories of PCM during batch wise discharging process. It is seen from the figure that the temperature drop is large until the PCM reaches its phase transition temperature. As the hot water in the storage tank is losses its sensible heat due to the mixing of inlet water at a temperature of 32^oC .After that, the temperature drop in the PCM is negligible for a long duration as the PCM releases its latent heat. The PCM temperature is nearly constant for the duration of 40minutes as the inlet water is supplied intermittently to extract heat from the storage tank .After complete solidification of the PCM, its temperature start decreasing, however the rate of temperature drop is not as high as in the binning of discharging process. This is due to low temperature differences between the PCM and HTF inlet temperature, though the solid PCM releases its sensible heat.

Fig. 12 shows the temperature histories of HTF during batch wise discharging process. The rate of heat recovery is large at the beginning of discharging process and decreases with time because of the change in the thermal resistance of the solidified layer of the PCM and decrease in temperature difference between solidified PCM and HTF. The discharging experiments are carried out by batch wise method. This method of discharge permits the complete utilization of heat in the storage tank. In the case of batch wise discharging process, a certain quantity (6.67 lit) of hot water is withdrawn from the storage tank and the same amount of cold water is filled in the storage tank. Withdrawn of hot water is stored in the bucket having capacity of 6.67 litres and the average temperature of the hot water in the bucket is measured. The optimum retention period is 20minutes between batches. The batches of withdrawing hot water are continuous till the outlet temperature reaches 34^oC. The average temperature of the total withdrawn hot water is approximately 45 ± 2^oC.It is found that 12 batches of 6.67 litres of hot water at an average temperature of the 45^oC can be obtained in a period of 240 minutes from the TES tank is a capacity of 100 litres that contains 87 litters of water and 13 litters of OM53.

7. CONCLUSION:

· The performance of the solar collector can be enhanced by increasing the area of the absorber plate and increasing the size of the water tubes.

· The performance of solar water heater with PCM can be improved by selecting the appropriate PCM with high thermal storage capacity.

· The performance of absorber plate can be enhanced by fixing at better orientations.

· During the late evening hours like 8 pm to 8.30 pm, the backup would be better i.e...)., the heat transfer fluid temperature would be reasonably above the atmospheric temperature which in turn fulfil the domestic applications on that time.

· The solar water heater unit with thermal energy storage system is best suited during off sunshine hours, even though that cost will be higher.

· The performance of system with thermal energy storage is better than other systems.

· It is also improved by increasing the quantity and quality of PCM and also heat transfer area.

· The mass flow rate has a significant effect on the heat extraction rate of the solar collector, which in turn affect the rate of charging of the TES tank.

· Combined with sensible and latent storage concept reduces the size of storage tank appreciably compared to conventional storage system.

· The combined heat storage system employing batch wise discharging of hot water from the TES tank is best suited for applications where the requirement is intermittent.

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Received on 06.08.2016 Modified on 15.04.2016

Research J. Science and Tech. 2016; 8(4):190-198.

DOI: 10.5958/2349-2988.2016.00028.0