Thermal energy storage can be achieved in the form of sensible heat of a solid or liquid medium, latent heat of a phase change substance or by a chemical reaction. The choice of storage media depends on the amount of energy to be stored in unit volume or weight of the medium and the temperature range which is required for a given application.
The experimental setup consists of a six cylinders Ashok Leyland engine, heat recovery heat exchanger and thermal storage system. Fig shows a schematic diagram of the experimental setup.
The engine used for this work is a four stroke, water cooled, six cylinder Diesel engines. The rating of the engine is 82 hp at 1500rpm. The engine is mounted on the bed with suitable connections for fuel and cooling water supply. The engine is coupled with a generator to vary the load on the engine.
It consists of a vertical cylindrical shape heater core made of mild steel, with a circumference of 0.3m and an active length of 0.45m. A copper tube of size 0.01m is wound over this heater core at gradual intervals across its length. The copper tube is connected into the thermal storage tank that is filled with water and phase change material, and is made in the shape of a coil, inside the tank. The above said setup is fitted in the exhaust pipe of the engine to extract the waste heat from engine exhaust gas, using water as heat transfer fluid. The water inside the copper tube flows with natural Circulation. Fig shows the schematic diagram of the heat recovery heat exchanger.
The storage tank is a stainless steel vessel of diameter 0.25m and height 0.3m. It contains water as the sensible heat material and paraffin as the latent heat material. Hence it is called combined sensible and latent heat storage system. The water also acts as the heat transfer fluid to extract the heat from the flue gas. The tank is filled with 40 spherical containers made of low density polyethylene(LDPE) having diameter 0.05m and each spherical container contains approximately 100 grams of paraffin. The thermal storage tank is well insulated by using fibre coir to prevent heat radiation to the surroundings. In this paper, the experimental results are enumerated in the form of various graphs of exhaust gas temperature variation. Variations of temperature of the storage and other performance parameters under various loads on the engine are studied. Based on these graph interferences are given for various observations
It is already seen that as the load increases the exhaust temperature also increases. Hence, when the load on the engine is increases, the exhaust temperature increases. However, initially for some period of time, the engine and auxiliaries will absorb part of the incremental heat till the system attains steady state. Thereafter the temperature of exhaust gas coming from the engine will be approximately at a constant temperature.
The heat in the exhaust gas is extracted in the HRHE by circulating water from the storage tank. The time required to attain 65°C at the outlet of the storage tank is 240 minutes at no load condition and 180 minutes at 40 amps and 140 minutes at 60 amps load condition respectively. This is due to the increased heat extraction rate at higher loads. It is also evident from the graphs that at all load conditions the rate of increase in temperature is appreciable up to a temperature of 60degreeC to 65degreeC. It is due to the fact that when the temperature reaches 60degreeC, the paraffin in the storage tank start changes its phase and for the phase large amount of heat is taken from the water and this reduces the rate of increase in temperature of water.
Fig shows the temperature variation of the water in the storage tank at the selected thermocouple locations. The temperature measurements are taken at 6 different locations (i.e., at three different heights and two radial locations at each height) in the storage tank. It is seen from the graphs that at any time, there is small difference in temperature between the top and bottom thermocouples. This is due to stratification caused by the density difference of the hot and cold water.
The fuel consumed by the engine is noted in order to calculate the heat carried away by the exhaust gas. It is seen that the fuel consumption is increases as the load on the engine increases. The fuel consumed is 8 lit/hr at no load and 11lit/hr at 40 amps load and 13lit/hrat 60 amps load.
VARIATION OF HEAT CARRIED AWAY BY EXHAUST GAS FOR DIFFERENT LOADS ON THE ENGINE
VARAITION OF TEMPERATURE IN ‘C WITH TIME IN MINUTES FOR DIFFERENT LOADS ON THE ENGINE
VARIATION OF CHARGING RATE FOR DIFFERENT LOADS ON THE ENGINE
VARIATION OF CHARGING EFFICIENCY FOR DIFFERENT LOADS ON THE ENGINE
Based on the results obtained, the following conclusions are drawn. Approximately 0.2% of the energy in the fuel or 6 to 7% of the energy in the exhaust waste heat can be recovered using such a HRHE system can be stored in the storage tank depending on the load on the engine. The percentage of heat recovered can be increased further by increasing the surface area of the HRHE. The charging efficiency of the storage tank and the percentage energy saved can be improved further with proper insulation. A combined storage system overcomes the main drawback of sensible storage system by exhibiting isothermal behavior. The higher heat capacity of the combined system reduces the size and space requirements compared to conventional storage.