18-01-2013, 04:34 PM
EXPERIMENTAL INVESTIGATION ON HEAT RECOVERY FROM DIESEL ENGINE EXHAUST USING THERMAL STORAGE SYSTEM
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INTRODUCTION
The exhaust gas from an internal combustion engine carries away about 30% of the heat of combustion. The energy available in the exit stream of many energy conversion devices goes as waste, if not utilized properly. The major technical constraint that prevents successful implementation of waste heat recovery is due to its intermittent and time mismatched demand and availability of energy. In the present work, thermal energy storage tank used to store the excess energy available is investigated in detail. A combined sensible and latent heat storage system is designed, fabricated and tested for thermal energy storage using cylindrical phase change material (PCM) capsules. The performance of the engine without heat exchanger is evaluated. It is found that nearly 10–15% of fuel power is stored as heat in the combined storage system, which is available at reasonably higher temperature for suitable application. The performance parameters pertaining to the heat exchanger and the storage tank such as amount of heat recovered, heat lost, charging rate, charging efficiency and percentage energy saved are evaluated and reported in this paper.
There exists today a worldwide concern about the best ways of using the depletable sources of energy and for developing techniques to reduce pollution. This interest has encouraged research and development efforts in the field of alternative energy sources, cost-effective use of the exhaustible sources of energy, and the use of the usually wasted forms of energy.
As the fuel prices continues to escalate, the relevance of efficient energy management is apparent to companies everywhere, from the smallest concern to the largest multinationals. The methods and techniques adopted to improve energy utilization will vary depending on circumstances. But the basic principles of reducing energy cost relative to productivity will be same. A large number of industrial processes covering most industrial sectors, use significant amounts of energy in the form of heat, which are rarely efficient .Thus there is considerable scope for the use of heat exchangers and other form of heat equipment to enable waste heat to be recovered .The potential savings possible are greatest for the temperatures ranges from 200C to 500C. The developed countries are the pacesetters in energy consumption, discharging at the same time vast amounts of waste energy .The industry in these countries consumes the largest share of energy.
BACK GROUND
In all energy conversion methods due to thermodynamic constraints and other reasons, large quantity of heat available in the exit stream goes into the atmosphere without proper utilization and these result in a major drop in efficiency. Air preheater using Waste Heat Recovery (WHR) system and cogeneration are successful techniques to improve the overall thermal efficiency of a system to a certain extent. However, there is still a large potential to store and utilize the exit stream energy by the efficient implementation of suitable WHR systems and improve the overall thermal efficiency.
DESCRIPTION OF PRESENT WORK
Thermal storage units have received greater attention in solar and waste heat recovery thermal applications, because of the large heat storage capacity and their isothermal behavior during charging and discharging process. The major technical constraint, which prevents successful implementation of heat recovery system, is intermittent time mismatched demand and availability. In order to overcome the above constraint WHR device integrated with thermal storage unit can be adopted. Thermal energy storage provides one practical means of storing energy during availability and use this energy when needed.
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.
METHODOLOGY:
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.
RESULTS AND DISCUSSIONS
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.
CONCLUSION
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.