Design and Fabrication of Alternate Energy Storage Device using PCM – Mechanical Project

ABSTRACT
The use of a latent heat storage system using phase change materials is an effective way of storing thermal energy. In this project a PCM based tube in shell heat exchanger is designed and fabricated. The project focuses on the temperature distribution pattern of the phase change material during the process of charging and discharging the results are obtained by experimentally .Paraffin wax has been used as the phase change material. This type of thermal energy storage system can be used as a medium to store energy and can be used further.

INTRODUCTION
The continuous increase in the level of greenhouse gas emissions and the increase in fuel prices are the main driving forces behind efforts to more effectively utilize various sources of renewable energy. In many parts of the world, direct solar radiation is considered to be one of the most prospective sources of energy. One of the options is to develop energy storage devices, which are as important as developing new sources of energy. The storage of energy in suitable forms, which can conventionally be converted into the required form, is a present day challenge to the technologists. The use of a latent heat storage system using phase change materials (PCMs) is an effective way of storing thermal energy and has the advantages of high-energy storage density and the isothermal nature of the storage process. There are large numbers of PCMs that melt and solidify at a wide range of temperatures, making them attractive in a number of applications. The project includes testing of a small heat exchanger, in order to establish the effectiveness of using paraffin wax as a suitable phase change material. The paraffin wax is incorporated in the heat exchanger, which acts as thermal energy storage device. This device will then be tested on a test rig for water heating and other applications.

Objective: The objective of this project is to design and fabricate a tube in tube, phase change material (PCM) based heat exchanger, which can act as a thermal energy storage device, and hence can be incorporated in solar water heater. The thermal energy storage device will act as a short term energy storage device.

EXPERIMENTAL SETUP

PCM in solar water heater:

Working: During sunshine period, valve 1 is kept open and valve 2 is kept closed. The cold water from the storage tank goes through the flat plate solar collector, absorbing heat energy from the solar radiations. It then passes through the PCM heat exchanger, where it loses its heat to the phase change material. It then goes back to the storage tank. In this way, the PCM gains heat energy which will then be used to heat water during non-sunshine period.
During non-sunshine period, valve 1 is kept closed and valve 2 is kept open. The cold water from the storage tank goes through the PCM heat exchanger, absorbing heat energy from the heat stored in the phase change material. It then goes back to the storage tank. By this way cold water is heated with the help of heat stored in the PCM.

Designing PCM Based Heat Exchanger: The calculation for the PCM based heat exchanger is done in the following steps:

Amount of hot water required: Usually, in a typical household, during non-sunshine period, requirement of water is around 3 buckets. Taking volume of each bucket as 20 litres, total requirement will amount to 60 litres. However for our experimental purpose we consider designing of a heat exchanger to heat 10 kg of water. The same approach may be adopted to design a system of higher requirement.

Amount of heat energy to be stored: During winter season, the average temperature at which the water is available in the tanks is around 15-20oC. Temperature of water required for comfortable bathing during winter season is around 40oC. Hence the amount of temperature difference we need to attain for comfortable bathing is 20-25oC.

Initial temperature of water Ti = 15oC
Final (desired) temperature of water Tf = 40oC
Tf –Ti = 25oC
Heat capacity of water, Cv = 4.187 kJ/kg
Hence, amount of heat required to carry out the above transition = Q
Q = mwater x (Tf –Ti) x Cv
i.e. Q = 10 x 25 x 4.187 = 1046.75 kJ
Hence, amount of energy needs to be stored = 1046.7 kJ

Selection of suitable heat exchanger: A tube in shell type heat exchanger is the simplest choice of heat exchanger. Incorporation of phase change material is also easy in this type of exchanger. The phase change material is incorporated in the outer shell of the heat exchanger and the heat transfer fluid (HTF), which is water in this case, flows from the inner tube.

Selection of suitable phase change material: Selection of a suitable phase change material is very important and is the fundamental requirement of our project. The paraffin wax has high latent heat capacity of 206 kJ/kg. The phase transition temperature of the wax is 40oC to 50oC, which is suitable for our requirement. The change in volume of wax from solid to liquid is negligible. The wax is chemically stable and does not affect any component of the heat exchanger. Above all, wax is available at low cost.

CONCLUSION
Paraffin wax is a good PCM for energy storage in latent heat storage system. It has a suitable transition temperature range of 45-55°C and a relatively high latent heat of 206 kJ/kg. In addition, it does not exhibit any subcooling.
A simple tube-in-tube heat exchanger system can be used for energy storage with reasonable charging and discharging times. The melting was more at the top and nearer to the inner tube. The solidification was rapid at the point which was nearer to the inner tube carrying heat transfer fluid.
The given arrangement can be used to store thermal energy in solar water heating application. The cost to incorporate the system is also very economical. The primary objective of using paraffin wax as a PCM material has been successfully experimented. This project is highly feasible and more than the feasibility this project can overcome the energy crisis to greater extent.