Solar energy storage using latent heat storage techniques and application

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1 Available online at IOSRD International Journal of Engineering Journal home page: Solar energy storage using latent heat storage techniques and application K Siva Prasad 1*, S M Bharani Deepak 1, K Vinyl Jai 1, G Surendhar 1 1,2,3,4 Department of Mechanical Engineering,Vel Tech High Tech Dr.Rangarajan Dr.Sakunthala Engineering College,India *Corresponding author, address: [email protected] Abstract: The objective of the present work is to investigate experimentally the thermal behavior of latent heat short term Thermal Energy Storage (TES) unit. A TES unit is designed and constructed and is integrated with a solar collector to study the performance of the storage unit. Water is used as Heat Transfer Fluid (HTF) to transfer heat from the solar collector to the TES tank and Inorganic Phase Change Material (PCM) Sodium Thiosulphate Pentahydrate is used as latent heat storage material. A mathematical model was developed for TES unit with Sodium Thiosulphate as LHS storage media. The time variations of storage unit temperatures in closed-loop active solar waterheating system are obtained with Sodium Thiosulphate as LHS storage while charging and discharging at mass flow rates of kg/s, 0.4 kg/s, and 0.66 kg/s. The significance of time wise variations of HTF and PCM temperatures during charging and discharging processes are discussed. It was observed that as mass flow rate increases from kg/s to 0.66 kg/s, heat extraction rate from solar collector is also increasing, and thus the charging time of storage unit is decreasing respectively. Thus this method could well be used as an thermal water heater for short term applications. Keywords: Latent Heat Capacity, Thermo chemical energy storage, Phase change materials 1. Introduction Energy storage is critically important in the success of any intermittent energy source in meeting demand. For example, the need for storage for solar energy applications is severe, especially when the solar availability is lowest. Energy storage systems have an enormous potential to increase the effectiveness of energy conversion equipment use and for facilitating large-scale fuel substitutions in the world s economy. They can contribute significantly to meeting society s needs for more efficient, environmentally benign use in building heating and cooling. Energy storage technologies are also a strategic and necessary component of the efficient utilization of renewable energy sources and energy conservation. TES deals with the storing of energy by heating, cooling, melting, solidifying or vaporizing a material, the thermal energy becoming available when the process is reversed. Thermal energy storage can be done in three different methods: a. Sensible heat storage b. Latent heat storage and c. Thermo chemical storage 1.1. Sensible heat storage In sensible heat storage (SHS), thermal energy is stored by raising the temperature of a solid or liquid. SHS system utilizes the heat capacity and the change in temperature of the material during the process of charging and discharging. The amount of heat stored depends on the specific heat of the medium, the difference in temperature and the amount of storage material. Water appears to be the best SHS liquid available because it is freely available and has a high specific heat. However above C, oils, molten salts and liquid metals, etc. are used. For air heating applications rock bed type storage materials are used. The amount of sensible heat stored is given by Q p = mc dt Where m= mass of storage material,kg C p = Specific heat of storage material,kj/kg.k dt =Temperature difference, K 91 Page

2 1.2. Latent heat storage Latent heat storage (LHS) is based on the heat absorption or release when a storage material undergoes a phase change from solid to liquid or liquid to gas or vice versa. The storage capacity of the LHS system with a PCM medium is given by Q = Tm Ti mc p dt + ml + T f Tm mc p dt Where L = Latent heat of storage material, kj/kg 1.3. Thermo chemical energy storage Thermo chemical systems rely on the energy absorbed and released in breaking and reforming molecular bonds in a completely reversible chemical reaction. In this case, the heat stored depends on the amount of storage material, the endothermic heat of reaction, and the extent of conversion. Q = a mδ r h r 2.Phase change material A phase change material (PCM) is a substance with a high heat of fusion which, while melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy. Heat is absorbed or released when the material changes its phase from solid to liquid and vice versa 2.1 Types of PCM a. Organic b. Inorganic c. Eutectic 3. Objectives of the project a. Design of components of solar thermal energy storage system like pump, solar flat plate collector and thermal storage unit. b. Modeling of solar thermal energy storage system with Sodium Thiosulphate as latent heat storage (LHS) material. c. Experimentation Inorganic PCM (Sodium Thiosulphate) as latent heat storage (LHS) material. 4. Design of experimental setup The set up consists of a solar flat plate collector, thermal energy storage unit, a water tank and a pump as shown. The collector is well insulated and collects the solar energy and transfers heat energy to the HTF, water. This hot water passes to the thermal energy storage unit though a copper coil and transfers heat from copper coil to the storage media. HTF passing through the coil loses heat energy and is stored in the storage unit during charging (day time). During discharging (night time or early morning) cold water is passed through storage unit to recover the stored energy. Fig. 1. Block Diagram of Solar Thermal Energy Storage System 92 Page

3 4.1 Conditions considered for PCM selection are a. Melting temperature in desired temperature range b. High latent heat of fusion c. High thermal conductivity, specific heat and density d. Small volume change on phase transformation e. Non-toxic, non-flammable and non-explosive material f. Low cost g. Large-scale availability h. Less Literature Available on selected material i. Important physical properties that determine the TES capacity of PCM j. Specific Heat Capacity (C p ) k. Melting Point (T m ) l. Latent Heat of Fusion (L) 5. Energy equation for pcm as latent heat storage material Consider storage tank is filled with salt hydrate (Na 2 S 2 O 3 5H 2 O). The energy balance equation for PCM storage tank can be obtained by adding latent heat term to. Hence Energy balance equation for the storage tank with PCM while charging and discharging continuously Charging During charging, PCM gets heat from HTF with a mass flow rate of and temperature of T co coming from collector as shown in Fig 3.6. By absorbing the energy supplied by HTF, PCM will undergo phase change and energy is stored as latent heat. By supplying heat energy further PCM temperature will increase and energy is stored as sensible uring charging there will be no heat recovery the system.energy 5.2. Discharging Fig. 2 Charging of storage unit with PCM as LHS material D During the night time, HTF with mass flow rate L and with temperature T i will be entering into storage unit and collects heat from storage unit as shown. While discharging there will be no heat addition. 93 Page

4 6. Experimental setup Fig. 3. Discharging of storage unit with PCM as LHS material Experimental set up consists of a solar flat plate collector and a thermal energy storage unit, a water tank and a pump as shown in Fig 4.5. The collector is well insulated and collects the solar energy and transfer heat energy to HTF. This hot water passes to the thermal energy storage unit. Storage unit consists of a PVC pipe of diameter 160 mm with thickness 5 mm and length 45 mm, insulated with thermocol containing copper coil of 10 mm diameter. The pitch diameter of copper coil is 80 mm. At different sections of the copper coil, thermocouples (T 1, T 2, and T 3 ) are attached for measuring the temperature of the heat transfer fluid (HTF) and thermocouples (T 4, T 5 and T 6 ) are placed from centre of the cylinder in radial direction for measuring storage media temperature at three locations. Placements of thermocouples in the storage unit are shown in fig 4.6. In the storage unit, HTF passes though the copper coil and heat is transferred to the storage media surrounding the copper coil. Thus the heat energy is stored in the storage unit. Below figures shows different parts of set up. Fig. 4. Solar Flat Plate collector 94 Page

5 Fig. 5. Pump Fig. 6. Thermal Storage Unit Fig. 7. Experimental Setup of Solar thermal energy Storage system 95 Page

6 7. Results of latent heat storage system Experiments are conducted at mass flow rates of kg/s, 0.4 kg/s and 0.66 kg/s during charging and discharging process for LHS system to study the performance of solar thermal energy storage unit. 7.1 Charging The temperature histories of LHS media (Na 2 S 2 O 3.5H 2 O) at different locations of the TES tank are recorded for every 15 min. during charging process (heat addition) from 8.00 a.m. to 3.00 p.m. for different mass flow rates of kg/s, 0.4kg/s and 0.66 kg/s are shown Graph: 1 Temperature distributions of LHS media (Na 2 S 2 O 3.5H 2 0) during discharging process with a mass flow rate of 0.4 kg/s Graph : 2 Temperature distribution of LHS media (Na 2 S 2 O 3.5H 2 0) during charging process with a mass flow rate of 0.6 kg/s Above figure shows the temperature distribution of LHS storage media with all the three mass flow rates. It is seen that as mass flow rate increases charging time is decreasing. For the charging of LHS storage media to 65 o C lower mass flow rate kg/s takes nearly 6.5 hours while mass flow rate 0.66 kg/s is getting charged in 4.5 hours. Hence it is observed that mass flow rate has significant effect on the charging time of storage unit. 96 Page

7 Graph.3 Temperature distributions of LHS media (Na 2 S 2 O 3.5H 2 0) during charging process with three mass flow rates 7.2. Discharging The temperature histories of LHS media at different locations of the TES tank are recorded for every 5 min. during discharging process (heat recovery) from 5.00 p.m. to 7.00 p.m. for different mass flow rates of kg/s, 0.4 kg/s and 0.66 kg/s are shown. Graph : 4 Temperature distribution of LHS media (Na 2 S 2 O 3.5H 2 0) during discharging process with a mass flow rate of kg/s Graph : 5Temperature distribution of LHS media (Na 2 S 2 O 3.5H 2 0) during discharging process with a mass flow rate of 0.4 kg/s Graph : 6 Temperature distribution of LHS media (Na 2 S 2 O 3.5H 2 0) during discharging process with a mass flow rate of 0.6 kg/s 97 Page

8 Graph below shows the temperature distribution of LHS storage media with all the three mass flow rates. It is seen that as mass flow rate increases discharging time is decreasing. For the discharging of LHS storage media lower mass flow rate kg/s takes nearly 100 min. while mass flow rate 0.66 kg/s is getting discharged in 80 min. Hence it was observed that mass flow rate has significant effect on discharging time of storage unit also. Graph : 7 Temperature distribution of LHS media (Na 2 S 2 O 3.5H 2 0) during discharging process with three mass flow rates Where as in LHS system it takes 90 min. for discharging the heat stored in one day. In LHS system charging and discharging occurred slowly and storage capacity of LHS system is more than that of the SHS system. 8. Conclusion Charging and discharging experiments are conducted with continuous flow on the solar thermal storage unit to study its performance. The temperature histories of the heat transfer fluid and storage media during charging and discharging process for different flow rates kg/s, 0.4 kg/s, 0.66 kg/s are discussed. Mass flow rate has significant effect on the heat extraction rate from the solar collector, which in turn affects the rate of charging of the TES tank. It was observed that as mass flow rate increases from kg/s to 0.66 kg/s heat extraction rate from solar collector is also increasing, and thus the charging time of storage unit is decreasing respectively. A thermal energy storage system has been developed for the use of hot water at an average temperature of 38 C for domestic applications using latent heat storage concepts. References [1] Atul Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi: Review on thermal energy storage with phase change materials and applications, Renewable and Sustainable Energy Reviews, vol.13, No 2, 2009, [2] N. Nallusamy, S.Sampath, R.Velraj, Enhancement of solar thermal energy storage performance using sodium phosphate of a conventional solar water-heating system, Renewable Energy, vol.32, [3] S. D. Sharma, D. Buddhi and R. L. Sawhney, Accelerated Thermal Cycle Test of Latent heat storage materials, Solar Energy, vol. 66, [4] Murat Kenisarin, and KhamidMahkamov, Solar energy storage using phase change materials, Renewable and Sustainable Energy Reviews, vol.11, [5] T. Kousksou, F. Strub, J. CastaingLasvignottes, A. Jamil, J.P., and Be de carrats: Second law analysis of latent thermal storage for solar system, Solar Energy Materials & Solar Cells, vol.91, [6] Felix Regin, S.C. Solanki, J.S. Sain: Heat transfer characteristics of thermal energy storage system using PCM capsules: A review,renewable and Sustainable Energy Reviews,Vol. 12, [7] AhmetKoca, Hakan,F. Oztop, TanselKoyun, and Yasin Varo, Energy and exergy analysis of a latent heat storage system with phase change material for a solar collector, Renewable Energy, vol.4, Page