Experimental Investigation of A Latent Heat Storage System For Diesel Engine Waste Heat Recovery With and Without Cascaded Arrangement

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1 International Conference on Mechanical, Automobile and Robotics Engineering (ICMAR') Experimental Investigation of A Latent Heat Storage System For Diesel Engine Waste Heat Recovery With and Without Cascaded Arrangement M. Chinnapandiana, V. Pandiyarajan, and R. Velraj Abstract The exhaust gas from an internal combustion engine carries away about % of the heat of combustion. Waste heat recovery is one of the energy conservation options, which is as important as developing a new source of energy. Thermal storage plays a vital role in improving the overall efficiency of waste heat recovery from an internal combustion engine system. In the present work, a shell and finned tube heat exchanger integrated with an IC engine setup to extract heat from the exhaust gas and a thermal energy storage tank used to store the excess energy available is investigated in detail. The performance of the engine with and without heat exchanger is evaluated. In addition, the advantage of a cascaded latent heat storage system over the single storage tank system is also studied and a comparative performance has been made. Keywords Cascaded latent heat thermal storage, Heat recovery heat exchanger, Phase change material, Waste heat recovery T I. INTRODUCTION HE rapid industrial and economical growth in recent years in some of the thickly populated nations has stimulated the utilization of sustainable energy sources and energy conservation methodologies considering environmental protection. Nearly two-thirds of the input energy in a high capacity diesel engine is wasted through the exhaust gas and cooling system. It is imperative that a serious and concrete effort should be launched for conserving this energy through storage based Waste Heat Recovery (WHR) techniques, in order to improve the performance of the system and to reduce the impact of global warming. Thermal storage plays a vital role in improving the overall efficiency of waste heat recovery from an internal combustion engine system. storage not only reduces the mismatch between supply and demand, but also improves the performance and reliability of energy systems and plays an important role in conserving energy. M. Chinnapandian, Professor, Department of Aeronautical Engineering, St. Peter s University, Avadi, Chennai -, India, muthuchinnapandian@gmail.com V. Pandiyarajan, Assistant Professor, A.C. College of Technology, pandiyarajan_v@yahoo.com R. Velraj, Professor, Institute for Studies, Anna University, Chennai-, India, velrajr@gmail.com Among the various energy storage systems, latent heat storage systems are particularly attractive due to their highenergy storage system and isothermal behavior during the charging and discharging process. Although Latent Heat Thermal Storage (LHTS) systems possess desirable characteristics, the increase in thermal resistance during the solidification and melting of the phase change material (PCM) (during the charging and discharging process) varies the surface heat transfer rate, and hence, prevents its usage in any applications. A combined sensible and latent heat storage system (encapsulated PCM immersed in a liquid medium) avoids the above said problems. The PCM based storage system has a disadvantage, when the energy from the exhaust gas to be stored in a PCM that has a high melting temperature. This result in a high extraction and exit gas temperature from the heat exchanger, during the melting process and hence, lower charging efficiency. In order to improve the efficiency of the storage system, the cascaded method of storage is also attempted in the present work. Various studies carried out by researchers on WHR, PCM based Cascaded Latent Heat Storage (CLHS) system are reviewed and summarized. Morcos [] has studied the performance of shell and dimpled tube heat exchangers for waste heat recovery. The exchanger heat duty, overall heat transfer coefficient, effectiveness and tube side friction factor are investigated as functions of the tube surface geometry (plain or dimpled), the flow pattern (counter or parallel) in the tube, the Reynolds number and the shell side heat capacity. In the analysis, water is used as the tube side fluid and the exhaust gas from the diesel engine is used as the shell side fluid. Desai and Bannur [] experimentally studied the method of extracting waste heat from the exhaust gas of an IC engine. The heat can be recovered from the engine exhaust, which would otherwise go waste, especially in the case of continuous process plants. The authors designed and fabricated a shell and tube heat exchanger to extract the heat from the exhaust gas of a twin cylinder engine, as per TEMA and ASME standards. Yang et al [] studied the feasibility of WHR using a heat pipe exchanger for an automobile, using exhaust gas. Schatz [] introduced the concept of a heat battery, which stores engine waste heat using a PCM. The possibility of

2 International Conference on Mechanical, Automobile and Robotics Engineering (ICMAR') recovering waste heat from the engine coolant and storing it in a PCM heat battery is experimentally attempted. This stored heat is used during an engine cold start condition by transferring it from the PCM to the engine coolant, which ensures that the engine attains a high temperature at a faster rate. The concept of integrating thermal energy storage in motor vehicles has been proposed as an alternative short-term technological solution for controlling cold start emission. Korin et al [] have experimentally studied the reduction of cold-start emission from an IC engine by means of a catalytic converter embedded in a PCM. Vasiliev et al [] have investigated the heat storage system for pre-heating the internal combustion engine during start-up. Subramanian et al [] conducted an experiment on waste heat recovery from a diesel engine exhaust, and presented the advantages of a combined sensible and latent heat storage system. Soylemez [8] presented the thermodynamic feasibility analysis yielding a simple algebraic optimization formula, for estimating the optimum length of a finned pipe that is used for waste heat recovery. Fang and Chen [] presented a theoretical model for the performance of a shell and tube LHTS unit using multiple PCMs. The model is based on the enthalpy method. Numerical simulations are carried out to investigate the effects of different multiple PCMs on the melted fraction, stored thermal energy and fluid outlet temperature of the LHTS unit. As a result, the appropriate choosing of multiple PCMs is very significant for the performance improvement of the LHTS unit. Shaikh and Lafdi [] performed two dimensional simulation studies to investigate the impact of using different configurations of multiple PCM slab arrangements, with different melting temperatures, thermophysical properties and varied sets of boundary conditions on the total energy stored, as compared to using a single PCM slab. Michels and Pitzpaal [] studied the CLHS for parabolic trough solar power plants, with the introduction of expensive synthetic heat transfer oil, capable to increase the operating temperature from C up to C, and as the direct storage technology became uneconomical; they concluded that the CLHS system is a better solution. Seeniraj and Narasimhan [] observed a nearly uniform temperature at the HTF outlet for a longer period in the five PCM shell and tube arrangement. In the present work, heat recovery system consisting of a finned shell and tube heat exchanger and a Thermal Storage (TES) tank with paraffin as PCM storage material has been designed and fabricated for waste heat recovery from diesel engine exhaust. Castor oil is used as tube side fluid to extract heat from exhaust gas. Thermal performance of heat recovery heat exchanger and the storage system has been studied for charging process of PCM in the thermal storage tank for various engine load conditions. In addition, the advantage of a CLHS system over the single storage tank system is studied and a comparative performance has been made. II. EXPERIMENTAL SETUP The experimental setup consists of a twin cylinder, four stroke, water-cooled, Kirloskar make diesel engine. (bore 8. mm, stroke mm, rated power. kw at rpm) coupled to an electrical dynamometer, integrated with a Heat Recovery Heat Exchanger (HRHE) and a CLHS system. Fig. shows the photographic view of the experimental set-up. The heat recovery system is a shell-and-finned tube heat exchanger, made of mild steel and copper respectively, with shell side fluid being the exhaust gas, and tube side fluid being the castor oil. In general, the surface convective heat transfer coefficient for gases will be very low, and hence, the heat transfer surface on the gas side needs to have a much larger area for better heat transfer. Hence, a separate heat exchanger is designed with finned tubes in which the exhaust gas is allowed to pass through the shell side to achieve a higher surface area on the gas side. Four numbers of longitudinal copper fins are attached to each tube at equal intervals. The total surface area of the fins in the tube is.8 m, whereas the unfinned tube area is. m. All The HRHE is fitted into the exhaust pipe of the engine. The valve arrangement in the pipeline allows the exhaust gas from the engine to flow either to the heat exchanger or to the atmosphere. Castor oil is circulated using a gear pump through the tube side of the heat exchanger, and then allowed to pass through two TES tanks in a series in the case with cascaded arrangement as shown in Fig.. A provision has also been made to allow the HTF to directly flow through the second storage tank. The TES tanks are stainless steel cylindrical vessels of an inner diameter of mm and a height of mm. The storage tanks contain castor oil as the sensible heat storage medium, D-sorbitol filled cylindrical capsules as the latent heat storage medium in the first storage tank, and paraffin filled cylindrical capsules in the second storage tank. The first storage tank (High Temperature Storage Tank- HTST) contains kg of D-sorbitol filled in 8 capsules and kg of castor oil as the HTF, and the second storage tank (Low Temperature Storage Tank-LTST) contains kg of paraffin filled in 8 capsules and kg of castor oil. All the containers have a diameter of 8 mm and a height of mm. However, the container in the HTST has g of D-sorbitol, whereas the container in the LTST has g of paraffin. The cylindrical capsules are kept in a mild steel stand of a diameter of mm and a height of mm, having four wire-meshed decks at different heights. Twelve numbers of PCM capsules are placed in each deck. The temperatures at various locations are recorded using Cr/Al thermocouples (type K). Twelve thermocouples are placed in four different horizontal planes in the TES tank. Three thermocouples are placed uniformly in each plane. In addition, thermocouples are placed at the inlet and outlet of the HRHE and the TES tank. Castor oil from the HRHE enters the storage tank from the top and leaves at the bottom. A pump maintains the circulation of castor oil in this set-up. The TES tank is well insulated using glass wool, and is covered

3 International Conference on Mechanical, Automobile and Robotics Engineering (ICMAR') with aluminium cladding. A control valve fitted at the exit of the TES tank is used to vary the oil flow rate in the system. An orifice meter connected to the U-tube manometer is used to measure the volumetric flow rate of the air entering the engine. Fig. Photographic view of the experimental setup Fig. Schematic diagram of the experimental set-up I. RESULTS AND DISCUSSION The results obtained from the experimental investigation for the engine operated at various load conditions are studied in detail and presented. A comparative analysis has been made, for the results of the experiment conducted with a Single Storage Tank (SST) and the results of the Cascaded Storage System (CSS). Table shows the result of the performance and the heat balance analysis for the system considered at various loads. The important results are discussed with the following graphs. It is seen from the Fig. that when the HRHE is integrated with engine, a substantial amount of heat is recovered by the HRHE, which can be directly used for any application or it can be stored in the storage system as done in the present investigation. At % load the heat loss in the exhaust gas i decreases from. kw to. kw. As the load increase, a considerable decrease in heat loss is observed. The Efficiency of diesel engine with SST and CSS systems at various load conditions are shown in Fig.. A considerable improvement in energy efficiency is observed when the engine is integrated with heat recovery and storage system. Cascaded storage system further improves the efficiency compared to the single storage system. This is due to exhaust gas leaving at higher temperature when the high temperature PCM in the cascaded storage tank changes its phase. The charging rate (Ech-t) is defined as the average rate at which the heat is supplied to the TES tank at a particular load. It is the ratio of the total heat stored in the tank to the duration of charging process. The average charging rate at various load conditions is shown in Fig.. It is seen from the figure that as the load increases, the average charging rate increases in the cascaded storage system. At lower loads, the heat gain in the cascaded mode is partially lost due to the longer duration of the charging process associated with the surface heat loss from the tank. The percentage energy saved is indicative of the percentage of the fuel energy content saved, by introducing the storage system, when the system is employed, replacing the conventional one which requires either fuel or electric power. The percentage energy saved at different loads for the single storage tank and cascaded storage tank is shown in Fig.. It is the ratio of the energy stored in the storage system to the energy emitted by the fuel for the operation of the engine during the charging interval of time. It is a direct measure of the overall efficiency improvement of the system. The percentage of the energy saved varies from to % in the case of the single storage tank as the load increases from % to full load, and it varies from to % in the case of the cascaded storage system as the load increases from % to full load. At lower loads, the percentage of the energy saved is less, due to the high specific fuel consumption at part load condition, and also the higher heat loss during the charging process with the longer duration of charging. The above said results showing a higher average charging rate and the energy saved, reflect the efficiency of the cascaded storage system. In order to represent the energy level at every stage of the energy conversion and recovery process, the energy balance diagram for the diesel engine integrated with the cascaded latent heat storage system, are shown in Fig. at the full load condition. It is seen from the figure that the losses in energy due to the cooling water (.8 kw) and exhaust gases (.kw) are about % of the total input fuel energy, and nearly % of the energy is lost due to unaccounted factors. It is possible to utilize the energy present in the exhaust gas and cooling medium effectively, by using heat recovery systems to generate power, or to use it in other industrial processes. In the present work, when the heat recovery system is employed,. kw of energy is recovered out of. kw of the energy entering the heat recovery

4 International Conference on Mechanical, Automobile and Robotics Engineering (ICMAR') S. No 8 TABLE I ENERGY ANALYSIS A COMPARISON OF THE SINGLE AND CASCADED STORAGE SYSTEMS Descriptio n Load PCM Storage Type Fuel input (E f ) Brake power (W d ) energy loss in cooling water (E cw ) energy loss in exhaust without HRHE (E exgas ) energy recovery by HRHE (E exgas hr ) lost in exhaust with HRHE (E exgasl ) Charging rate of the storage medium (E ch ) Total energy usage from fuel input (E tf ) efficiency of diesel engine ( η d ) efficiency of integrated system (η i ) saved ( η s ) A B C D I* II* I II I II I II *I- Single storage tank * II-Cascaded storage System (Two numbers) Heat loss in the Exhaust Gas (kw) Efficiency (%)..... Without HRHE With HRHE Fig. Exhaust Gas with and without HRHE Diesel Engine IS w ith SST IS w ith CSS Fig. Efficiency of diesel engine with storage systems Average charging rate (kw) single storage tank cascaded storage tank Fig. Average charging rates at various engine load conditions with the single /cascaded storage system saved (%) single storage tank cascaded storage tank Fig. Percentage energy saved at various engine load conditions with the single /cascaded storage system

5 International Conference on Mechanical, Automobile and Robotics Engineering (ICMAR') Fig. flow diagram for a diesel engine integrated with the cascaded latent heat storage system system, which is about. % of the heat available in the engine exhaust at full load. Surprisingly, the rate of energy stored in the storage system (.8 kw) is higher than the energy recovered from the exhaust gas (. kw). This is due to the non-accounting of the latent heat energy in the water vapour present in the exhaust gas. However, when the temperature of the exhaust is brought down below oc, the water vapour present in the exhaust gas condenses and the released latent heat is stored in the storage medium. Hence, the rate of energy storage is higher than the rate of energy recovery from the HRHE. [] A. D. Desai and P.V. Bannur, Design, Fabrication and Testing of Heat Recovery System from diesel engine exhaust, Journal of Institution of Engineers 8() -8. [] F. Yang, X. Yuan and Lin G, Waste heat recovery using heat pipe heat exchanger for heating automobile using exhaust gas, Applied Thermal Engineering () -. [] O. Schatz, Cold start improvement by use of latent heat stores, Automotive Engineering Journal () 8-. [] E. Korin, R. Reshef, D. Tshernichovesky and E. Sher, Reducing coldstart emission from internal combustion engines by means of a catalytic converter embedded in a phase-change material, Proc. Institution of Mechanical Engineers () -8 [] L.L Vasiliev, V.S. Burak, A.G. Kulakov, D.A. Mishkinis and P.V. Bohan, Latent heat storage modules for preheating internal combustion engines: Application to a bus petrol engine, Applied Thermal Engineering () -. [] S.P Subramanian, V. Pandiyarajan and R. Velraj, Experimental analysis of a PCM based IC engine exhaust waste heat recovery system, International Journal ()() 8-.. [8] M.S. Soylemez, Optimum length of finned pipe for waste heat recovery, conversion and Management (8) -. [] Ming. F and C. Guangming, Effects of different multiple PCMs on the performance a latent thermal energy storage system, Applied Thermal Engineering () -. [] S. Shaikh and K. Lafdi, Effect of multiple phase change materials (PCMs) slab configurations on thermal energy storage, conversion and Management () -. [] H. Michels and R. Pitz-Paal, Cascaded latent heat storage for parabolic trough solar power plants, Solar 8() 8-8. [] R.V. Seeniraj and N.L. Narasimhan, Performance enhancement of a solar dynamic LHTS module having both fins and multiple PCMs, Solar 8(8)-. III. CONCLUSION A thermal energy storage system eliminates the intermittent and time mismatched demand and availability of any waste heat recovery system. In the present work, a finned shell and tube heat exchanger and a PCM based cascaded storage system were designed, fabricated and tested by integrating them with a diesel engine capacity of. kw. Nearly to % of the total heat that would otherwise be gone as waste, is recovered with this cascaded storage system. It is concluded from the results of the charging processes, that the cascaded storage system with the right combination of PCMs in the storage tank improves efficiency. Further, it is concluded from the present experimental investigation that it is possible to recover the heat, which is liberated from the fuel along with the exhaust gas during the burning of fuel (HCV-LCV) by decreasing the exhaust gas temperature below the saturation temperatures of the water vapour. Hence, in the selection of multiple PCM for cascaded arrangement, at least one PCM should be selected around to o C in order to extract the heat of condensation from the exhaust gas. REFERENCES [] V.H. Morcos, Performance of shell-and-dimpled-tube heat exchangers for waste heat recovery, Heat recovery systems and CHP 8() (88) -8.