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.
SCHEMATIC OF 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.