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Second law analysis of a diesel engine waste heat recovery with a combined sensible and latent heat storage system
Energy Policy 39 (2011) 6011–6020
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Second law analysis of a diesel engine waste heat recovery with a combined sensible and latent heat storage system V. Pandiyarajan a, M. Chinnappandian b, V. Raghavan a, R. Velraj a,n a b
Institute for Energy Studies, Anna University, Chennai-600 025, India Department of Mechanical Engineering, St. Peter’s Engineering College, Avadi, Chennai-600054, India
a r t i c l e i n f o
abstract
Article history: Received 19 September 2010 Accepted 30 June 2011
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. The major technical constraint that prevents successful implementation of waste heat recovery is due to intermittent and time mismatched demand for and availability of energy. The present work deals with the use of exergy as an efficient tool to measure the quantity and quality of energy extracted from a diesel engine and stored in a combined sensible and latent heat storage system. This analysis is utilized to identify the sources of losses in useful energy within the components of the system considered, and provides a more realistic and meaningful assessment than the conventional energy analysis. The energy and exergy balance for the overall system is quantified and illustrated using energy and exergy flow diagrams. In order to study the discharge process in a thermal storage system, an illustrative example with two different cases is considered and analyzed, to quantify the destruction of exergy associated with the discharging process. The need for promoting exergy analysis through policy decision in the context of energy and environment crisis is also emphasized. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Waste heat recovery Thermal energy storage Phase change material
1. Introduction Energy is an important entity for the economic development of any country. Transportation, manufacturing, commercial, and social activities, and agriculture, all now consume energy at a large and growing rate. The demand for energy is increasing exponentially all over the world. Today’s industrial developments are based upon abundant and reliable supplies of energy. The rapid industrial and economical growth in India and China, where more than one-third of the population of the world is present, has increased the need for energy in the recent years. Considering environmental protection, and in the context of the great uncertainty over future energy supplies, attention is focused on the utilization of sustainable energy sources and energy conservation methodologies. A large quantity of hot flue gases is generated from boilers, furnaces, I.C. engines, etc. If some of this waste heat could be recovered and put into use, a considerable amount of primary fuel
n Corresponding author. Tel.: þ91 44 22358051, 22359187; fax: þ 91 44 22351991, mobile: þ91 99625 37765. E-mail addresses: [email protected], [email protected] (V. Pandiyarajan), [email protected] (M. Chinnappandian), [email protected] (V. Raghavan), [email protected], [email protected] (R. Velraj).
0301-4215/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.enpol.2011.06.065
could be saved. Waste heat recovery (WHR) devices and cogeneration are successful energy recovery 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 thermal energy by the efficient implementation of suitable WHR systems, and improvement of the overall thermal efficiency. Large capacity diesel engines are one of the most widely used power generation units. Nearly two-thirds of the input energy is wasted through the exhaust gas and cooling water of these engines. It is imperative that a serious and concrete effort should be launched for conserving this energy through waste heat recovery techniques. Such a system would ultimately reduce the overall energy requirement. Apart from the fast depleting nature of fossil fuels, the combustion of these fuels leads to considerable thermal and environmental pollution, which is threatening our eco system. The energy in the cooling water is usually considered as waste due to its low temperature level. However, much attention is focused upon the exhaust gas waste heat, and several methods are suggested to recover it. In a four stroke diesel engine, the temperature of the exhaust gas is approximately 400–500 1C at full load conditions. Hence, it is possible to recover a large quantity of useful heat from these exhaust gases. Researchers have carried out several experimental and theoretical investigations on the application of the diesel engine exhaust for industrial process heating and driving absorption refrigeration systems and heat pumps. Rabghi et al. (1993)
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Nomenclature
Subscripts
PCM HRHE TES WHR LCV cp m T Dtc L s h rpm
ex hrhe moil mpcm Tpc cw o i f Tg1 Tg2 Tw1 Tw2 ch d lt s
Z c E A
phase change material heat recovery heat exchanger thermal energy storage waste heat recovery lower calorific value [kJ kg 1] specific heat at constant pressure [kJ kg 1 K 1] mass [kg] temperature [K or 1C] duration of charging [s] latent heat of fusion of paraffin [kJ kg 1] specific entropy [kJ kg 1 K 1] specific enthalpy [kJ kg 1] revolution per minute first law efficiency (energy efficiency) [%] second law efficiency (exergy efficiency) [%] energy [kW] exergy [kW]
presented the waste heat in different branches of industry, and new ways to recover the discharged heat from industrial equipment. It was concluded that there exist numerous opportunities for recuperating and using waste heat. Mostafavi and Agnew (1997) have carried out a thermodynamic analysis to evaluate the performance of a diesel engine integrated with an absorption refrigeration unit. Desai and Bannur (2001) experimentally studied the method of extracting waste heat from the exhaust gas of an IC Engine. They designed a shell and tube heat exchanger, fabricated to extract the heat from the exhaust gas of a diesel engine. It was observed that such a heat exchanger had an effectiveness of 80–82%. Subramanian et al. (2004) have done an experiment on waste heat recovery from diesel engine exhaust, and they mentioned the advantages of a combined sensible and latent heat storage system. Lee et al. (2010) have done an experiment on the effects of secondary combustion on efficiencies and emission reduction in the diesel engine exhaust heat recovery system. They pointed out that the use of the secondary combustor and heat exchanger results in 80%, 35% and 90% reduction of carbon monoxide (CO), nitrogen oxide (NOX) and particulate matter (PM), respectively. Pandiyarajan et al. (2011) have done an experiment on waste heat recovery from diesel engine exhaust, using a finned shell and tube heat exchanger, and a PCM based thermal energy storage system. They found that nearly 10–15% of fuel power is stored as heat in the combined storage system, which is available at a reasonably high temperature for suitable application. The major technical constraint that prevents the successful implementation of a heat recovery system is the intermittent and time mismatched demand for and availability of energy. In order to overcome the above constraint, WHR systems should be integrated with energy storage units. Among the number of energy storage methods, thermal energy storage (TES) is one of the key technologies for energy conservation, and therefore, is of great practical importance. Dincer and Rosen (2002) have pointed out that TES systems can contribute significantly to meet the society’s needs for more efficient, environmentally benign energy use in building heating and cooling, aerospace power, and utility applications. TES is perhaps as old as civilization itself. Since recorded time, people have harvested ice and stored it for later use. Large TES systems have been employed in recent years for numerous applications, ranging from solar hot water storage to building air conditioning systems. TES technology has only
exhaust gas heat recovery heat exchanger mass of castor oil mass of paraffin phase change temperature of PCM cooling water reference state initial final temperature of exhaust gas entering HRHE temperature of exhaust gas leaving HRHE temperature of the engine cooling water at inlet temperature of the engine cooling water at outlet charging diesel engine energy lost saved
recently been developed to a point where it can have a significant impact on modern technology. Thermal energy storage systems have an enormous potential to increase the effectiveness of energy conversion equipment use, and for facilitating large scale fuel substitutions in the world’s economy. In many situations the operation of the diesel engine may not be continuous, and also there are some applications which require energy intermittently. In such situations, thermal storage systems are highly suitable to bridge the time-mismatches between the availability of and the demand for energy. The energy can be stored in the form of the sensible heat of a solid or liquid medium, or the latent heat of a phase change substance. Sensible heat storage (SHS) units have very low heat capacity per unit volume. On the other hand, latent heat thermal storage (LHTS) units are particularly attractive due to their high energy storage capacity and isothermal behavior during the charging and discharging processes. The disadvantage of latent heat storage is its low charging and discharging rates, owing to the low thermal conductivity of the phase change material (PCM). The low charging and discharging rates can be improved by the enhancement of an overall heat transfer coefficient between the PCMs and the heat transfer fluid; the enhancement can be attained by the use of finned tubes or metal filings as filler materials. However, a combined sensible and latent heat storage system is a better alternative, which offers the advantages of both sensible and latent heat storage systems with a small loss in volumetric heat capacity. The major problem of variation in the surface heat transfer rate due to the poor thermal conductivity of the phase change material in a latent heat thermal storage system is eliminated in a combined sensible and latent heat storage system. There are several studies carried out by various researchers on the development of the PCM, heat transfer analysis, heat transfer enhancement methods, and their applications. A detailed review of the use of the PCM and other topics mentioned above, are presented by Zalba et al. (2003), Sharma et al. (2009) and Jagadheeswaran and Pohekar (2009). The idea of the second law analysis of thermal storage systems is recent, but there is little doubt that the analysis leads to a proper evaluation of the performance of a given system relative to the best that could possibly be achieved. Najem and Diab (1992) have done a thermodynamic energy and exergy analysis of a diesel engine to identify the sources of the losses in useful energy within the components of the diesel engine. They concluded that
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in the exergy analysis there is a loss of 50% and 15% in the chemical availability of the fuel and the losses incurred in the cooling water/exhaust gases, respectively. Whereas this percentage loss is in opposite manner in the energy analysis, i.e., 15% loss while extracting energy from the fuel and 50% loss in the cooling water/exhaust gases loss. This shows that the major exergy loss occurs during the conversion of fuel in to heat. Smith and Few (2001) conducted a second law analysis for a domestic purpose cogeneration plant with a heat pump. They discussed the development of the combined heat and power concept, and described the construction of an experimental unit while identifying the areas of improvement needed in the performance of the plant, using the second law that could not be examined by the first law of thermodynamics. Butcher and Reddy (2007) presented the performance of a waste heat recovery power generation system based on the second law analysis for various operating conditions. Soylemez (2008) presented the thermodynamic feasibility analysis yielding a simple algebraic optimization formula, for estimating the optimum length of a finned pipe that is used for WHR. Kamate and Gangavati (2009) have concluded that there is a remarkable difference between the energy and exergy efficiency of a cogeneration power plant with an increase in the steam inlet pressure and temperature. The exergy analysis on sensible heat storage was first studied and documented by Bejan (1978), and he indicated that the amount of exergy loss for the best second law efficiency is about 50% of the supplied exergy, and he defined a new parameter called the entropy generation number to make the comparison between two systems. The pioneer work of using the exergy analysis for the evaluation of phase change thermal storage was introduced by Bjurstrom and Carlson (1985). Krane (1987) adopted the second law analysis for the optimum design and operation of the thermal energy storage system, while Adebiyi (1991) conducted the second law study on a packed bed energy storage system using the PCM, and both the authors confirmed the importance of the second law analysis for design optimization. Domanski and Fellah (1996) performed an analysis for a complete charging and discharging cycle of a phase change thermal storage system consisting of two storage units in a series. They have designed the capacity of each unit, and sized it in such a way that the both units must change phase, over the same period of time. Ramayya and Ramesh (1998) have performed an exergy analysis of latent heat storage systems, with the sensible heating and sub cooling of the PCM. They concluded that the optimum phase change temperature for the overall cycle is higher for the latent heat storage system with sensible heating and sub cooling, compared to latent heat storage alone. Abraham et al. (2006) proposed a methodology for the evaluation of exergy analysis based entropy added tax and further emphasized the introduction of this methodology will force the manufacturers, to improve the efficiency of the components. Koca et al. (2008) performed the energy and exergy analysis for a latent heat storage system with CaCl2 6H2O as the PCM for a flat plate solar collector, and they observed the net energy and exergy efficiencies to be 45% and 2.2%, respectively. Dincer (2002a) presented a detailed investigation of the energy and exergy analysis of a sensible heat storage system with illustrative examples, and demonstrated how the exergy analysis provides a more realistic and meaningful assessment than the conventional energy analysis in the performance of a sensible energy storage system. Further, Dincer (2002b) explained the utilization of exergy as an efficient tool for an energy-policy making application. He used the energy and exergy concept for various actual processes, and discussed the role of exergy for energy and environment policy-making activities from several key perspectives. Kanoglu et al. (2007) presented an extensive
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overview to explain the energy and exergy efficiencies for improved energy management in power plants. In the present work, a thermodynamic energy and exergy analysis has been carried out on the experimental setup consisting of a finned shell and tube heat exchanger and a thermal energy storage (TES) tank with paraffin as the PCM for waste heat recovery from a diesel engine exhaust. Castor oil is used as the heat transfer fluid (HTF) to extract heat from the exhaust gas, and it also serves as a sensible heat storage (SHS) medium. The HTF flows through the tube side and the exhaust gas flows through the shell side of the heat exchanger. Experimental investigation has been carried out for the charging process of the PCM in the TES tank for various engine load conditions. The major objective of the present work is to perform the energy and exergy analysis to measure the quantity and quality of energy extracted from a diesel engine and to analyze the charging process in a storage system using experimental measurements. The importance of the exergy analysis and the parameters used for the energy and exergy evaluation are presented. The energy and exergy balance for the overall system is quantified and illustrated, using energy and exergy flow diagrams. In addition, two case studies are explained in detail, to illustrate the performance of the storage system with respect to different discharge requirements. The quantitative results provided in the present work will be useful for the engineers and manufacturers to identify the area required for attention for further improvement and the results of many such researches on energy conversion and recovery devices in the long run will be useful for the policy makers, to device a methodology to introduce environment based taxation on energy conversion and recovery based devices.
2. Thermodynamic analysis of system It is important to consider the quantity and quality of energy for better energy utilization. Designing efficient and cost effective systems, which also meet environmental conditions, is one of the foremost challenges that the engineers face. With finite natural resources and large energy demands, it becomes increasingly important in today’s world, to understand the mechanisms which degrade energy resources, and to develop systematic approaches for improving systems, and thus, also to reduce the impact on the environment. This can be accomplished by applying both the first and second laws of thermodynamics, which are the standard measure to understand a system’s behavior, and to acquire practical and theoretical knowledge in designing commercial systems. The use of this analysis allows the identification of losses that degrade the quality and quantity of energy transfer. Exergetics combined with economics may provide a useful solution for the systematic study and optimization of systems. It is the systematic approach that can be used to identify areas of the real losses of valuable energy in a thermal device. The exergy analysis uses the conservation of mass and energy principles, together with the second law of thermodynamics for the design analysis of energy systems. In a diesel engine, the first law analysis identifies significant energy losses because of cooling, and the heat lost in the exhaust gas of the engine. But when these losses are analyzed using the exergy method, the actual exergy loss is insignificant compared to the thermodynamic losses within the engine (Najem and Diab, 1992). The present work is aimed at illustrating the capability of the exergy analysis to provide a systematic approach to pinpoint the waste and unrecoverable energy associated with a diesel engine integrated with the TES system. This study gives a significant insight that will provide the direction for improvement in energy utilization. The parameters required for the energy and exergy
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analysis are obtained from the experimental investigation, the experimental setup and the methodology used in this connection are explained in the following section. 3. Experimental setup and methodology The experimental setup consists of a twin cylinder water cooled diesel engine (make: Kirloskar, bore 87.5 mm, stroke 110 mm, four stroke, rated power 7.4 kW at 1500 rpm) coupled to an electrical dynamometer, and integrated with a heat recovery heat exchanger (HRHE) and a thermal energy storage system. Fig. 1 shows the schematic diagram of the experimental setup. The heat recovery system is a shell and finned tube heat exchanger, made of mild steel and copper, respectively. The HTF flows through the tube side and the exhaust gas flows through the shell side of the heat exchanger. 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 1.008 m2 whereas the unfinned tube area is 0.5625 m2. The HRHE is fitted into the exhaust pipe of the engine. The exhaust gas from the engine is allowed to flow either to the heat exchanger or to the atmosphere by using valves. Castor oil is circulated using the gear pump through the tube side of the heat exchanger and is passed through the thermal energy storage tank. The TES tank is a stainless steel cylindrical vessel of an inner diameter of 450 mm and a height of 720 mm. The layout of the thermal storage tank is shown in Fig. 2. It contains castor oil as the sensible heat storage medium, and paraffin filled cylindrical capsules of a diameter of 80 mm and a height of 100 mm, as the latent heat storage medium. The storage tank contains 15 kg of paraffin filled in 48 capsules and 55 kg of castor oil. Each container has 320 g of paraffin. The thermo-physical properties of paraffin are given in Table 1. The cylindrical capsules are kept in a mild steel stand of a diameter of 430 mm and a height of 640 mm, having four wiremeshed layers at different heights. Twelve numbers of the PCM capsules are placed in each layer. 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. The engine cooling water temperatures at the inlet and outlet are also recorded. 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 setup. The TES tank is well insulated using glass wool and covered with aluminum cladding. A control valve fitted
Fig. 1. Schematic diagram of the experimental setup.
Fig. 2. Layout of thermal storage tank.
Table 1 Thermo-physical properties of the PCM (paraffin). Property
Value
Latent heat of fusion Specific heat capacity (solid) Specific heat capacity (liquid) Thermal conductivity (solid) Thermal conductivity (liquid) Density (solid) Density (liquid) Phase transformation temperature range
214 kJ/kg 1.85 kJ/kg K 2.384 kJ/kg K 0.4 W/m K 0.15 W/m K 856 kg/m3 775 kg/m3 58–60 1C
at the exit of the TES tank is used to vary the oil flow rate in the system. An orifice meter which is connected to the U tube manometer is used to measure the volumetric flow rate of air entering the engine. The experiments are conducted by operating the engine at various load conditions. An electrical dynamometer is used to vary the load on the engine. First, the experiment is conducted at 25% load condition. Initially the exhaust gas is not allowed to flow through the heat exchanger to avoid carbon deposition on the tube surface. After a short duration from the start of the engine, the exhaust gas is allowed to pass through the shell side of the heat exchanger, while ensuring the oil circulation through the tube side. A tachometer is used to measure the speed of the engine to ensure the rated speed of 1500 rpm. The ammeter and voltmeter readings of the electrical dynamometer are taken to find the brake power. The pressure difference in the orifice meter is observed from the U tube manometer, which is used to measure the mass flow rate of the air entering the engine. The fuel consumption of the engine is measured by counting the time required to consume a constant mass of diesel fuel of 0.042 kg (50 cm3).
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The temperature readings are continuously monitored in the TES tank, and the inlet and outlet of the HRHE and TES tank. The above said measurements are used to evaluate the heat recovered, the charging rate and charging efficiency. Several experiments are conducted to check the repeatability of the results. The experiments are conducted for 50%, 75% and full load conditions. An experiment is also conducted to determine the heat loss coefficient (UL) of the insulated storage tank and to evaluate the heat retention time of the heat storage tank. The results along with the evaluated parameters are analyzed and discussed in the following section.
4. Thermodynamic energy and exergy analysis This section explains the energy and exergy analysis of the diesel engine integrated with the HRHE and PCM based thermal energy storage system. The exergy analysis is carried out with the assumption that the changes in the potential, kinetic, electromagnetic and electrostatic exergy are negligible. The reference state considered in the exergy analysis for the atmospheric conditions is 1 atm, and the temperature is 307 K (34 1C).
The input and output energies of each system are considered, and the performance parameters of the diesel engine integrated with the HRHE are evaluated using the following equations (Eqs. (1)–(10)). Input thermal energy to the diesel engine: (thermal energy content of the fuel) _ f LCV Ef ¼ m
ð1Þ
Thermal energy carried by the exhaust gases: _ ex cp,ex ðTg1 Tgo Þ Eex ¼ m
ð2Þ
Thermal energy recovered by the HRHE: _ ex cp,ex ðTg1 Tg2 Þ EHRHE ¼ m
ð3Þ
Thermal energy lost to cooling water: _ cw cp,w ðTw2 Tw1 Þ Ecw ¼ m
ð4Þ
The charging rate 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 and the duration of charging, and is evaluated using fmoil cp,oil ðToil,f Toil,i Þ þ mpcm Lþ mpcm cp,pcm ðTpcm,f Tpcm,i Þg Ech,t ¼ kW Dtc
ð5Þ
The charging efficiency is defined as the ratio of the change in the energy content (DEC) in the storage tank to the actual energy supplied during the charging process (Qa). It is evaluated using
Zc ¼
DEc Qa
ð6Þ
where DEc ¼ Ef Ei and Qa ¼ DEc þQr , Ef the Energy content of the storage tank at the end of the charging process with respect to the environment, kJ; Ei the energy content of the storage tank at the beginning of the charging process with respect to the environment, kJ; Qa the actual energy supplied for charging, kJ; Qr the (UL AS LMTD) Dt, kJ; UL the time averaged overall heat loss coefficient when the temperature changes from 34 to 120 1C, kW/m2 K; and Dtc the time of charging, s. Total useful energy: Et ¼ Wd þEch,t kW
ð7Þ
Zi ¼
Et Ef
ð9Þ
Percentage energy saved: Es ¼
fmoil cp,oil ðToil,f Toil,i Þ þmpcm L þmpcm cp,pcm ðTpcm,f Tpcm,i Þg _ f LCVDt m
ð10Þ
4.2. Performance parameters used in the exergy analysis The following are defined and used to study the performance of the waste heat recovery system, and the TES based on exergy principles. The input and output exergies of each system and the parameters connected to the second law analysis are evaluated using the following equations (Eqs. (11)–(20)). The input exergy to the diesel engine (chemical availability of fuel); Kotas (1995) _ f LCV kW Af ¼ 1:04m
ð11Þ
Exergy lost in the exhaust gas without the HRHE:
Exergy recovered in the HRHE: T _ ex cp,ex ðTg1 Tg2 ÞTo ln g1 AHRHE ¼ m kW Tg2 Exergy lost to cooling water: T _ w cp,w ðTw2 Tw1 ÞTo ln w2 Acw ¼ m kW Tw1 Exergy present in the oil: Toil,f kJ Aoil ¼ moil cp,oil ðToil,f Toil,i ÞTo ln Toil,i Exergy present in the PCM Tpcm,f Apcm ¼ mpcm cp,pcm ðTpcm,f Tpcm,i ÞTo ln Tpcm,i To þ mpcm L 1 Tpc
ð12Þ
ð13Þ
ð14Þ
ð15aÞ
ð15bÞ
where Tpc is the phase change temperature of paraffin (333 K). Total exergy charged in the storage system: Ach ¼ Aoil þ Apcm kJ
ð15cÞ
Exergy charging rate of the storage system: Ach,t ¼
Ach
Dtc
kW
ð15dÞ
Charging efficiency of the system:
cch ¼
Ach Ach þAloss
ð16Þ
Total useful exergy: At ¼ Wd þ Ach,t kW
ð17Þ
Exergy efficiency of the diesel engine:
cd ¼
Wd Af
ð18Þ
Exergy efficiency of the integrated system:
ci ¼
At Af
ð19Þ
Exergy saved:
Thermal energy efficiency of the diesel engine: W Zd ¼ d Ef
Overall thermal energy efficiency of the integrated system:
_ ex fðhi ho ÞTo ðsi so Þg kW Aex ¼ m
4.1. Performance parameters used in the energy analysis
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ð8Þ
As ¼
Ach Af
ð20Þ
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The parameters evaluated using the above expressions for the present system, are analyzed and presented in the following section. 5. Results and discussion The parameters defined in the previous section for the energy and exergy analyses are evaluated at various engine load conditions (Tables 2 and 3). Based on the above values various performance curves are drawn and their significance is explained in this section. Fig. 3 shows the energy and exergy efficiencies of the diesel engine, with and without considering the integrated heat recovery system. The exergy efficiency of the diesel engine is slightly lower than the energy efficiency for the same power output without considering the integrated system. This is due to the fact that the chemical availability of fuel which is considered as the input in the exergy analysis, is slightly higher than the calorific value of the fuel which is considered as the input in the energy analysis. Further, it is seen from the graph that the exergy efficiency of the integrated system is considerably lower than its energy efficiency. This is due to the low exergy stored in the storage tank at a low temperature level. However, it is possible to reduce the difference in the energy and exergy efficiency, by storing the energy at a higher grade by selecting a suitable phase change material. On comparing the performance of the diesel engine with and without the integrated system, the energy efficiency has improved considerably and there is only a marginal increase in the exergy efficiency with the integrated system. This is due to the energy storage at a lower temperature level in the storage tank. The energy and exergy recovered by the HRHE through the exhaust gas are shown in Fig. 4 and the values are given in Tables 2 and 3. It is observed that the total energy recovered from
the exhaust gas varies from 1.24 to 3.35 kW at various loads. However, only 0.31–1.23 kW of the exergy of fuel is available to produce useful work. This shows that the actual exergy losses are insignificant compared to the irreversibility losses in the engine. Fig. 5 shows the energy and exergy efficiency during the charging process, accounting for the energy and exergy losses associated during the process. The energy efficiency of charging varies from 98.13% at 25% load to 99.34% at the full load condition. The exergy efficiency of charging varies from 97.94% at 25% load to 99.28% at the full load condition. The exergy efficiency is slightly lower than the energy efficiency. At higher loads, the charging efficiency is high. This is due to the shorter
Efficiency, %
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50
En Eff (DE)
45
Ex Eff (DE)
40
En Eff (IS)
35 30
Ex Eff (IS)
25 20 15 10 5 0 25
50
75
100
Load, % Fig. 3. Energy and exergy efficiencies of the diesel engine and integrated system.
Table 2 Energy analysis. Description
A
B
C
D
Load [%] Thermal energy content in the fuel input (Ef) [kW] Brake power (Wd) [kW] Thermal energy loss in cooling water (Ecw) [kW] Thermal energy loss in exhaust gas without HRHE (Eex) [kW] Thermal energy recovery by HRHE (EHRHE) [kW] Thermal energy lost in exhaust gas with HRHE (Elt) [kW] Charging rate of the storage medium (Ech,t) [kW] Total energy usage from the fuel input (Et) [kW] Energy efficiency of charging (Zch) [%] Thermal energy efficiency of diesel engine (Zd) [%] Overall thermal energy efficiency of integrated system (Zi) [%] Percentage energy saved (Es) [%]
25 12.69 1.89 4.24 2.14 1.24 0.90 1.32 3.22 98.13 14.92 25.34 10.42
50 16.22 3.79 5.30 2.67 1.67 1.01 1.85 5.64 98.62 23.35 34.76 11.41
75 19.83 5.68 6.53 3.72 2.53 1.19 2.82 8.50 99.11 28.66 42.87 14.21
100 24.96 7.31 7.82 4.55 3.35 1.20 3.81 11.12 99.34 29.27 44.55 15.28
Table 3 Exergy analysis. Description
A
B
C
D
Load [%] Chemical availability of fuel (Af) [kW] Brake power (Wd) [kW] Exergy lost in cooling water (Acw) [kW] Exergy lost in exhaust gas without HRHE (Aex) [kW] Exergy recovered from exhaust gas by HRHE (AHRHE) [kW] Exergy lost in exhaust gas with HRHE (Alt) [kW] Exergy charging rate of storage medium (Ach,t) [kW] Total exergy used from fuel input (At) [kW] Exergy efficiency of charging (Cch) [%] Exergy efficiency of diesel engine (Cd) [%] Exergy efficiency of integrated system (Ci) [%] Percentage exergy saved (As) [%]
25 13.20 1.89 0.03 0.39 0.31 0.08 0.15 2.04 97.94 14.35 15.47 1.12
50 16.87 3.79 0.05 0.56 0.47 0.10 0.21 3.99 98.47 22.45 23.68 1.22
75 20.62 5.68 0.08 0.98 0.85 0.13 0.31 6.00 99.02 27.56 29.08 1.52
100 25.96 7.31 0.12 1.36 1.23 0.13 0.43 7.73 99.28 28.14 29.78 1.64
V. Pandiyarajan et al. / Energy Policy 39 (2011) 6011–6020
Fig. 4. Energy and exergy recovered by the HRHE.
99.5 Energy Eff
Efficiency, %
99
Exergy Eff
98.5 98 97.5 97 25
50
75
100
Load, % Fig. 5. Energy and exergy efficiencies of the charging process.
Fig. 6. Energy and exergy saved with load.
duration required for the charging process at higher loads, and hence, the loss encountered during the charging process is low. Fig. 6 shows the energy and exergy saved due to the integrated heat recovery system at various load conditions. It is seen from the figure that the energy saved is much higher at all loads, compared to the exergy saved. This is due to the quantity of energy recovered from the exhaust gas, which is at a higher temperature, and stored at a lower temperature level, that reduces the quality of energy stored in the storage system. Further, it is seen that the energy and exergy saved increases as
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the load increases. This is due to the shorter duration of the charging process at higher loads. In order to represent the energy and exergy level at every stage of the energy conversion and recovery process, the energy and exergy balance diagrams for the diesel engine integrated with the PCM based storage system are shown in Figs. 7 and 8 at full load condition. It is seen from Fig. 7 that the losses in energy due to the cooling water (7.82 kW) and exhaust gases (4.55 kW) are about 50% of the total input fuel energy, and nearly 21% 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, 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, 3.35 kW of energy is recovered out of 4.55 kW of energy entering the heat recovery system, which is about 74% of the heat available in the engine exhaust at full load. Surprisingly, the rate of energy stored in the storage system (3.81 kW) is higher than the rate of energy recovered from the exhaust gas (3.35 kW). This is due to the non-accounting of the latent heat energy in the water vapor present in the exhaust gas. However, when the temperature of the exhaust is brought down below 100 1C, the water vapor 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 similar trend is observed at all load conditions (Table 2). However, it is seen from Fig. 8 that the exergy stored in the storage tank is much lesser than the exergy recovered from the HRHE, as the quality of the energy stored in the storage tank is much lesser than the quality of the energy recovered from the HRHE. It is further observed from Fig. 8 that 17.21 kW of exergy which is 66% of the exergy available in the fuel, is destroyed due to irreversibilities in the engine. The integration of the HRHE and the PCM based storage system saves 0.43 kW of exergy from 1.36 kW of exergy in the exhaust gas of the diesel engine. On comparing the energy (3.81 kW) and exergy (0.43 kW) saved in the storage system from Figs. 7 and 8, it is seen that the percentage of exergy saved is much lesser because of irreversible losses due to the quality of the energy degradation, while the heat transferred over a finite temperature difference, and the energy stored in the storage tank is at a low temperature (120 1C). However, it is possible to increase the percentage of exergy saved by increasing the temperature at which energy is stored, by selecting a PCM with a higher phase change temperature. This may reduce the energy efficiency of the heat recovery heat exchanger, as the exit temperature of the HRHE will be higher. Hence, in order to increase the energy and exergy efficiency, energy should be stored at a higher to lower temperature cascaded arrangement and multiple PCMs. In order to study the discharge process in a thermal storage system, an illustrative example with two different cases is considered and analyzed, to quantify the destruction of exergy associated with the discharging process. In addition, during the storing period there will be some heat loss from the surface of the storage tank. This loss is evaluated using the overall heat loss coefficient along with other measured parameters for the storage system, and is used to determine the energy/exergy loss analysis carried out in the following case study. 5.1. Illustrative example A heat exchanger coil immersed in the thermal energy storage tank is used to extract the energy from a storage system that contains 19,450 kJ of thermal energy after a storage period of 5 h. The average surface overall heat loss coefficient of the storage tank is 0.92 W/m2 K. The energy is extracted from the storage tank for two different applications. In the first case, 200 l of water is heated from 35 to 50 1C and in the second case 100 l of water is
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Fig. 7. Energy flow diagram for a diesel engine integrated the with TES unit.
Fig. 8. Exergy flow diagram for a diesel engine integrated with the TES unit.
heated from 35 to 65 1C. The energy and exergy efficiencies of the TES tank for the storing and discharge processes for the above two cases, are evaluated and given below. [The heat content of the storage tank and the overall heat loss coefficient values correspond to the present experimental investigation. The remaining data given in the above example are for a hypothetical case.] 5.2. Energy calculation (i) Thermal energy available in the TES tank immediately after the charging process (given): ET ¼ 19,450 kJ
(vi) Amount of energy recovered during the discharge process for case (ii): Ed,C2 ¼ mw Cp,w ðTw,d3 Tw,d1 Þ ¼ 100 4:1868 ð6535 1CÞ ¼ 12,560kJ
(vii) Efficiency during discharge for both case (i) and case (ii):
Zd,c ¼
Ed,C1 ðorÞ Ed,C2 12,560 ¼ 71:6% ¼ 17,540 Ea
5.3. Exergy calculations (i) Exergy available in the TES tank immediately after the charging process: Ach ¼ Aoil þ Apcm ¼ 2170 kJ
(ii) Thermal energy loss during the storage period in the TES tank: Qloss,s ¼ U A LMTD ¼ 0:92 1:427 80:79 ¼ 1910 kJ (iii) Storage efficiency (energy retention capacity)
Zs,d ¼
ET Qloss,s 194501910 ¼ 90:18% ¼ 19450 ET
(iv) Thermal energy available after 5 h of storage duration: Ea ¼ ET Qloss,s ¼ 19,4501910 ¼ 17,540 kJ (v) Amount of energy recovered during the discharge process for case (i): Ed,C1 ¼ mw Cp,w ðTw,d2 Tw,d1 Þ ¼ 200 4:1868 ð50351CÞ ¼ 12,560 kJ
(ii) Exergy loss during the storage period: T Al,s ¼ moil Cp,oil ðTd,i Td,f ÞTo ln d,i Td,f T þ mpcm Cp,pcm ðTd,i Td,f ÞTo ln d,i Td,f 393 ¼ 55:2 2:75 ð120109:8Þ307 ln 382:8 393 þ 15:36 2:384 ð120109:8Þ307 ln ¼ 400 kJ 382:8 (iii) Storage efficiency (exergy based):
cs,d ¼
As Al,s 2170400 ¼ 81:5% ¼ 2170 As
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(iv) Exergy available after 5 h of storage: Aa ¼ As Al,s ¼ 21702400 ¼ 1770kJ (v) Amount of exergy recovered during the discharge process for case (i): T Ad,C1 ¼ mw Cp,w ðTd,w2 Td,w1 ÞTo ln d,w2 Td,w1 323 ¼ 336 kJ ¼ 200 4:1868 ð5035Þ307 ln 308 (vi) Amount of exergy recovered during the discharge process for case (ii): T Ad,C2 ¼ mw Cp,w ðTd,w3 Td,w1 ÞTo ln d,w3 Td,w1 338 ¼ 614 kJ ¼ 100 4:1868 ð6535Þ307ln 308 (vii) Exergy efficiency during the discharge process for case (i):
cd,C1 ¼
Ad,C1 336 ¼ 18:98% ¼ 2170400 As Al,s
(viii) Exergy efficiency during the discharge process for case (ii):
cd,C2 ¼
Ad,C2 614 ¼ 34:68% ¼ 2170400 As Al,s
It is seen from the above illustrative example that the energy efficiency for both the cases is the same, i.e., 71.6%. However, the exergy efficiency for the second case is high (34.68%) when compared to the exergy efficiency of the first case (18.98%). This is due to the fact that in the second case, water is raised to a higher temperature, due to the lower mass associated with the heating process. A lower mass with a lower heat capacity increases the temperature of water at a faster rate, and hence the temperature potential difference between the source and the water decreases at a later stage of heating (when compared to the first case) that increases the exergy efficiency. In the present investigation, castor oil is used as the heat transfer fluid for charging the PCM in the TES tank. Since the phase change temperature of the PCM is 60 1C, water also can be used as the HTF. However, as the oil is easily heated to a higher temperature by transferring the heat from the exhaust gas, the exergy efficiency achievable with castor oil is high. Hence, it is construed that during any heat exchange process both the streams should attain the closure temperature (i.e. the minimum DT between two streams) at a faster rate. This could be achieved in two ways.
A heat exchanger with a maximum possible heat transfer surface area should be used. _ p Þ may be A heat transfer fluid with a lower heat capacity ðmc chosen for the given application, when there is a choice in selecting the HTF.
6. Conclusion The first and second laws of thermodynamics are employed to analyze the quality and quantity of energy in the diesel engine and the thermal energy storage system. The energy and exergy analyses enable us to develop a systematic approach that can be used to identify the sites of the real losses of valuable energy in
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thermal devices. The first law analysis shows that significant losses occur in the exhaust gas and cooling water. However, when analyzed using the exergy method, it is found that the actual exergy losses are insignificant compared to the irreversibility losses in the engine. It is observed from the energy analysis that 15.2% of the total energy from the fuel input is saved using the TES system. However, from the exergy analysis, it is identified that only 1.6% of the chemical availability of the fuel is saved. This indicates that improving the performance of the engine is more important than the recovery of low-grade energy. It is concluded from the results that the selected phase change material should have a high melting temperature, and fluids like oils can be used as heat transfer fluids to store heat at a higher temperature. Also, from the illustrative example it is concluded, that during any heat exchange process, both the streams should attain the closure temperature at a faster rate. It is possible to meet this requirement either with a high heat transfer surface area of the heat exchanger or by selecting a low heat capacity _ p Þ heat transfer fluid. ðmc In order to increase the energy and exergy efficiencies, energy should be stored with a cascaded arrangement using multiple PCMs, from a higher temperature to a lower temperature. In addition, with the view of curtailing fossil fuel usage and encouraging energy efficient and energy recovery devices, the Government can levy tax on the usage of fossil fuels, based on the level of exergy destruction associated with the fuel during the energy conversion and utilization. If the policy decision, based on entropy based environmental taxation is introduced, there will be a good promotion in the heat recovery devices in the industries.
References Abraham, J., Suganthi, L., Samuel, A.A., 2006. Entropy added taxing methodology: a case study with domestic refrigerators in India. International Journal of Exergy 3 (2), 150–163. Adebiyi, G.A., 1991. A second law study on packed bed energy storage system utilizing phase change materials. ASME Journal of Solar Energy Engineering 113, 146–156. Bejan, A., 1978. Two thermodynamic optima in the design of sensible heat units for energy storage. ASME, Journal of Heat Transfer 100, 708–712. Bjurstrom, H., Carlson, B., 1985. An exergy analysis of sensible and latent heat storage. Heat Recovery Systems 5 (3), 233–250. Butcher, C.J., Reddy, B.V., 2007. Second law analysis of a waste heat recovery based power generation system. International Journal of Heat and Mass Transfer 50, 2355–2363. Desai, A.D., Bannur, P.V., 2001. Design, fabrication and testing of heat recovery system from diesel engine exhaust. Journal of Institution of Engineers 82, 111–118. Dincer, I., 2002a. Thermal energy storage as a key technology in energy conservation. International Journal of Energy Research 26, 567–588. Dincer, I., 2002b. The role of exergy in energy policy making. Energy Policy 30, 137–149. Dincer, I., Rosen, M.A., 2002. Thermal Energy storage systems Applications. John Wiley & Sons, Chichester (England). Domanski, R., Fellah, G., 1996. Exergy analysis for the evaluation of a thermal storage system employing PCMs with different melting temperatures. Applied Thermal Engineering 16 (11), 907–919. Jagadheeswaran, S., Pohekar, S.D., 2009. Performance enhancement in latent heat thermal storage system: a review. Renewable and Sustainable Energy Reviews 13, 2225–2244. Kamate, S.C., Gangavati, P.B., 2009. Exergy analysis of cogeneration power plants in sugar industries. Applied Thermal Engineering 29, 1187–1194. Kanoglu, M., Dincer, I., Rosen, M.A., 2007. Understanding energy and exergy efficiencies for improved energy management in power plants. Energy Policy 35, 3967–3978. Koca, A., Oztop, H.F., Koyun, T., Varol, Y., 2008. Energy and exergy analysis of a latent heat storage system with PCM for a solar collector. Renewable Energy 33, 567–574. Kotas T.J., 1995. The exergy method of thermal plant analysis. Reprint edition, Krieger, Malabar, FL. Krane, R.J., 1987. A second law analysis of the optimum design and operation thermal energy storage system. Heat Mass Transfer 30 (1), 43–57. Lee, D.H., Lee, J.S., Park, J.S., 2010. Effects of secondary combustion on efficiencies and emission reduction in the diesel engine exhaust heat recovery system. Applied Energy 87, 1716–1721.
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Mostafavi, M., Agnew, B., 1997. Thermodynamic analysis of combined diesel engine and absorption refrigeration unit—naturally aspirated diesel engine. Applied Thermal Engineering 17 (5), 471–478. Najem, A.L.N.M., Diab, J.M., 1992. Energy–exergy analysis of a diesel engine. Heat Recovery Systems & CHP 12 (6), 525–529. Pandiyarajan, V., Chinnappandian, M., Malan, E., Velraj, R., Seeniraj, R.V., 2011. Experimental investigation on heat recovery from diesel engine exhaust using finned shell and tube heat exchanger and thermal storage system. Applied Energy 88 (1), 77–87. Rabghi, O.M.A.L., Beirutty, M., Akyurt, M., Najjar, Y., Alp, T., 1993. Recovery and utilization of waste heat. Heat Recovery Systems & CHP 13 (5), 463–470. Ramayya, A.V., Ramesh, K.N., 1998. Exergy analysis of latent heat storage system with sensible heating and sub cooling of PCM. International Journal of Energy Research 22, 411–426.
Sharma, A., Tyagi, V.V., Chen, C.R., Buddhi, D., 2009. Review on thermal energy storage with phase change materials and applications. International Journal of Renewable and Sustainable Energy Reviews 13, 318–345. Smith, M.A., Few, P.C., 2001. Second law analysis of an experimental domestic scale cogeneration plant incorporating a heat pump. Applied Thermal Engineering 21, 93–110. Soylemez, M.S., 2008. Optimum length of finned pipe for waste heat recovery. Energy conversion and Management 49, 96–100. Subramanian, S.P., Pandiyarajan, V., Velraj, R., 2004. Experimental analysis of a PCM based IC engine exhaust waste heat recovery system. International Energy Journal 5 (2), 81–92. Zalba, B., Marin, J.M., Cabeza, L.F., Mehling, H., 2003. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Applied Thermal Engineering 23, 251–283.
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Energy Policy journal homepage: www.elsevier.com/locate/enpol
Second law analysis of a diesel engine waste heat recovery with a combined sensible and latent heat storage system V. Pandiyarajan a, M. Chinnappandian b, V. Raghavan a, R. Velraj a,n a b
Institute for Energy Studies, Anna University, Chennai-600 025, India Department of Mechanical Engineering, St. Peter’s Engineering College, Avadi, Chennai-600054, India
a r t i c l e i n f o
abstract
Article history: Received 19 September 2010 Accepted 30 June 2011
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. The major technical constraint that prevents successful implementation of waste heat recovery is due to intermittent and time mismatched demand for and availability of energy. The present work deals with the use of exergy as an efficient tool to measure the quantity and quality of energy extracted from a diesel engine and stored in a combined sensible and latent heat storage system. This analysis is utilized to identify the sources of losses in useful energy within the components of the system considered, and provides a more realistic and meaningful assessment than the conventional energy analysis. The energy and exergy balance for the overall system is quantified and illustrated using energy and exergy flow diagrams. In order to study the discharge process in a thermal storage system, an illustrative example with two different cases is considered and analyzed, to quantify the destruction of exergy associated with the discharging process. The need for promoting exergy analysis through policy decision in the context of energy and environment crisis is also emphasized. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Waste heat recovery Thermal energy storage Phase change material
1. Introduction Energy is an important entity for the economic development of any country. Transportation, manufacturing, commercial, and social activities, and agriculture, all now consume energy at a large and growing rate. The demand for energy is increasing exponentially all over the world. Today’s industrial developments are based upon abundant and reliable supplies of energy. The rapid industrial and economical growth in India and China, where more than one-third of the population of the world is present, has increased the need for energy in the recent years. Considering environmental protection, and in the context of the great uncertainty over future energy supplies, attention is focused on the utilization of sustainable energy sources and energy conservation methodologies. A large quantity of hot flue gases is generated from boilers, furnaces, I.C. engines, etc. If some of this waste heat could be recovered and put into use, a considerable amount of primary fuel
n Corresponding author. Tel.: þ91 44 22358051, 22359187; fax: þ 91 44 22351991, mobile: þ91 99625 37765. E-mail addresses: [email protected], [email protected] (V. Pandiyarajan), [email protected] (M. Chinnappandian), [email protected] (V. Raghavan), [email protected], [email protected] (R. Velraj).
0301-4215/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.enpol.2011.06.065
could be saved. Waste heat recovery (WHR) devices and cogeneration are successful energy recovery 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 thermal energy by the efficient implementation of suitable WHR systems, and improvement of the overall thermal efficiency. Large capacity diesel engines are one of the most widely used power generation units. Nearly two-thirds of the input energy is wasted through the exhaust gas and cooling water of these engines. It is imperative that a serious and concrete effort should be launched for conserving this energy through waste heat recovery techniques. Such a system would ultimately reduce the overall energy requirement. Apart from the fast depleting nature of fossil fuels, the combustion of these fuels leads to considerable thermal and environmental pollution, which is threatening our eco system. The energy in the cooling water is usually considered as waste due to its low temperature level. However, much attention is focused upon the exhaust gas waste heat, and several methods are suggested to recover it. In a four stroke diesel engine, the temperature of the exhaust gas is approximately 400–500 1C at full load conditions. Hence, it is possible to recover a large quantity of useful heat from these exhaust gases. Researchers have carried out several experimental and theoretical investigations on the application of the diesel engine exhaust for industrial process heating and driving absorption refrigeration systems and heat pumps. Rabghi et al. (1993)
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Nomenclature
Subscripts
PCM HRHE TES WHR LCV cp m T Dtc L s h rpm
ex hrhe moil mpcm Tpc cw o i f Tg1 Tg2 Tw1 Tw2 ch d lt s
Z c E A
phase change material heat recovery heat exchanger thermal energy storage waste heat recovery lower calorific value [kJ kg 1] specific heat at constant pressure [kJ kg 1 K 1] mass [kg] temperature [K or 1C] duration of charging [s] latent heat of fusion of paraffin [kJ kg 1] specific entropy [kJ kg 1 K 1] specific enthalpy [kJ kg 1] revolution per minute first law efficiency (energy efficiency) [%] second law efficiency (exergy efficiency) [%] energy [kW] exergy [kW]
presented the waste heat in different branches of industry, and new ways to recover the discharged heat from industrial equipment. It was concluded that there exist numerous opportunities for recuperating and using waste heat. Mostafavi and Agnew (1997) have carried out a thermodynamic analysis to evaluate the performance of a diesel engine integrated with an absorption refrigeration unit. Desai and Bannur (2001) experimentally studied the method of extracting waste heat from the exhaust gas of an IC Engine. They designed a shell and tube heat exchanger, fabricated to extract the heat from the exhaust gas of a diesel engine. It was observed that such a heat exchanger had an effectiveness of 80–82%. Subramanian et al. (2004) have done an experiment on waste heat recovery from diesel engine exhaust, and they mentioned the advantages of a combined sensible and latent heat storage system. Lee et al. (2010) have done an experiment on the effects of secondary combustion on efficiencies and emission reduction in the diesel engine exhaust heat recovery system. They pointed out that the use of the secondary combustor and heat exchanger results in 80%, 35% and 90% reduction of carbon monoxide (CO), nitrogen oxide (NOX) and particulate matter (PM), respectively. Pandiyarajan et al. (2011) have done an experiment on waste heat recovery from diesel engine exhaust, using a finned shell and tube heat exchanger, and a PCM based thermal energy storage system. They found that nearly 10–15% of fuel power is stored as heat in the combined storage system, which is available at a reasonably high temperature for suitable application. The major technical constraint that prevents the successful implementation of a heat recovery system is the intermittent and time mismatched demand for and availability of energy. In order to overcome the above constraint, WHR systems should be integrated with energy storage units. Among the number of energy storage methods, thermal energy storage (TES) is one of the key technologies for energy conservation, and therefore, is of great practical importance. Dincer and Rosen (2002) have pointed out that TES systems can contribute significantly to meet the society’s needs for more efficient, environmentally benign energy use in building heating and cooling, aerospace power, and utility applications. TES is perhaps as old as civilization itself. Since recorded time, people have harvested ice and stored it for later use. Large TES systems have been employed in recent years for numerous applications, ranging from solar hot water storage to building air conditioning systems. TES technology has only
exhaust gas heat recovery heat exchanger mass of castor oil mass of paraffin phase change temperature of PCM cooling water reference state initial final temperature of exhaust gas entering HRHE temperature of exhaust gas leaving HRHE temperature of the engine cooling water at inlet temperature of the engine cooling water at outlet charging diesel engine energy lost saved
recently been developed to a point where it can have a significant impact on modern technology. Thermal energy storage systems have an enormous potential to increase the effectiveness of energy conversion equipment use, and for facilitating large scale fuel substitutions in the world’s economy. In many situations the operation of the diesel engine may not be continuous, and also there are some applications which require energy intermittently. In such situations, thermal storage systems are highly suitable to bridge the time-mismatches between the availability of and the demand for energy. The energy can be stored in the form of the sensible heat of a solid or liquid medium, or the latent heat of a phase change substance. Sensible heat storage (SHS) units have very low heat capacity per unit volume. On the other hand, latent heat thermal storage (LHTS) units are particularly attractive due to their high energy storage capacity and isothermal behavior during the charging and discharging processes. The disadvantage of latent heat storage is its low charging and discharging rates, owing to the low thermal conductivity of the phase change material (PCM). The low charging and discharging rates can be improved by the enhancement of an overall heat transfer coefficient between the PCMs and the heat transfer fluid; the enhancement can be attained by the use of finned tubes or metal filings as filler materials. However, a combined sensible and latent heat storage system is a better alternative, which offers the advantages of both sensible and latent heat storage systems with a small loss in volumetric heat capacity. The major problem of variation in the surface heat transfer rate due to the poor thermal conductivity of the phase change material in a latent heat thermal storage system is eliminated in a combined sensible and latent heat storage system. There are several studies carried out by various researchers on the development of the PCM, heat transfer analysis, heat transfer enhancement methods, and their applications. A detailed review of the use of the PCM and other topics mentioned above, are presented by Zalba et al. (2003), Sharma et al. (2009) and Jagadheeswaran and Pohekar (2009). The idea of the second law analysis of thermal storage systems is recent, but there is little doubt that the analysis leads to a proper evaluation of the performance of a given system relative to the best that could possibly be achieved. Najem and Diab (1992) have done a thermodynamic energy and exergy analysis of a diesel engine to identify the sources of the losses in useful energy within the components of the diesel engine. They concluded that
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in the exergy analysis there is a loss of 50% and 15% in the chemical availability of the fuel and the losses incurred in the cooling water/exhaust gases, respectively. Whereas this percentage loss is in opposite manner in the energy analysis, i.e., 15% loss while extracting energy from the fuel and 50% loss in the cooling water/exhaust gases loss. This shows that the major exergy loss occurs during the conversion of fuel in to heat. Smith and Few (2001) conducted a second law analysis for a domestic purpose cogeneration plant with a heat pump. They discussed the development of the combined heat and power concept, and described the construction of an experimental unit while identifying the areas of improvement needed in the performance of the plant, using the second law that could not be examined by the first law of thermodynamics. Butcher and Reddy (2007) presented the performance of a waste heat recovery power generation system based on the second law analysis for various operating conditions. Soylemez (2008) presented the thermodynamic feasibility analysis yielding a simple algebraic optimization formula, for estimating the optimum length of a finned pipe that is used for WHR. Kamate and Gangavati (2009) have concluded that there is a remarkable difference between the energy and exergy efficiency of a cogeneration power plant with an increase in the steam inlet pressure and temperature. The exergy analysis on sensible heat storage was first studied and documented by Bejan (1978), and he indicated that the amount of exergy loss for the best second law efficiency is about 50% of the supplied exergy, and he defined a new parameter called the entropy generation number to make the comparison between two systems. The pioneer work of using the exergy analysis for the evaluation of phase change thermal storage was introduced by Bjurstrom and Carlson (1985). Krane (1987) adopted the second law analysis for the optimum design and operation of the thermal energy storage system, while Adebiyi (1991) conducted the second law study on a packed bed energy storage system using the PCM, and both the authors confirmed the importance of the second law analysis for design optimization. Domanski and Fellah (1996) performed an analysis for a complete charging and discharging cycle of a phase change thermal storage system consisting of two storage units in a series. They have designed the capacity of each unit, and sized it in such a way that the both units must change phase, over the same period of time. Ramayya and Ramesh (1998) have performed an exergy analysis of latent heat storage systems, with the sensible heating and sub cooling of the PCM. They concluded that the optimum phase change temperature for the overall cycle is higher for the latent heat storage system with sensible heating and sub cooling, compared to latent heat storage alone. Abraham et al. (2006) proposed a methodology for the evaluation of exergy analysis based entropy added tax and further emphasized the introduction of this methodology will force the manufacturers, to improve the efficiency of the components. Koca et al. (2008) performed the energy and exergy analysis for a latent heat storage system with CaCl2 6H2O as the PCM for a flat plate solar collector, and they observed the net energy and exergy efficiencies to be 45% and 2.2%, respectively. Dincer (2002a) presented a detailed investigation of the energy and exergy analysis of a sensible heat storage system with illustrative examples, and demonstrated how the exergy analysis provides a more realistic and meaningful assessment than the conventional energy analysis in the performance of a sensible energy storage system. Further, Dincer (2002b) explained the utilization of exergy as an efficient tool for an energy-policy making application. He used the energy and exergy concept for various actual processes, and discussed the role of exergy for energy and environment policy-making activities from several key perspectives. Kanoglu et al. (2007) presented an extensive
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overview to explain the energy and exergy efficiencies for improved energy management in power plants. In the present work, a thermodynamic energy and exergy analysis has been carried out on the experimental setup consisting of a finned shell and tube heat exchanger and a thermal energy storage (TES) tank with paraffin as the PCM for waste heat recovery from a diesel engine exhaust. Castor oil is used as the heat transfer fluid (HTF) to extract heat from the exhaust gas, and it also serves as a sensible heat storage (SHS) medium. The HTF flows through the tube side and the exhaust gas flows through the shell side of the heat exchanger. Experimental investigation has been carried out for the charging process of the PCM in the TES tank for various engine load conditions. The major objective of the present work is to perform the energy and exergy analysis to measure the quantity and quality of energy extracted from a diesel engine and to analyze the charging process in a storage system using experimental measurements. The importance of the exergy analysis and the parameters used for the energy and exergy evaluation are presented. The energy and exergy balance for the overall system is quantified and illustrated, using energy and exergy flow diagrams. In addition, two case studies are explained in detail, to illustrate the performance of the storage system with respect to different discharge requirements. The quantitative results provided in the present work will be useful for the engineers and manufacturers to identify the area required for attention for further improvement and the results of many such researches on energy conversion and recovery devices in the long run will be useful for the policy makers, to device a methodology to introduce environment based taxation on energy conversion and recovery based devices.
2. Thermodynamic analysis of system It is important to consider the quantity and quality of energy for better energy utilization. Designing efficient and cost effective systems, which also meet environmental conditions, is one of the foremost challenges that the engineers face. With finite natural resources and large energy demands, it becomes increasingly important in today’s world, to understand the mechanisms which degrade energy resources, and to develop systematic approaches for improving systems, and thus, also to reduce the impact on the environment. This can be accomplished by applying both the first and second laws of thermodynamics, which are the standard measure to understand a system’s behavior, and to acquire practical and theoretical knowledge in designing commercial systems. The use of this analysis allows the identification of losses that degrade the quality and quantity of energy transfer. Exergetics combined with economics may provide a useful solution for the systematic study and optimization of systems. It is the systematic approach that can be used to identify areas of the real losses of valuable energy in a thermal device. The exergy analysis uses the conservation of mass and energy principles, together with the second law of thermodynamics for the design analysis of energy systems. In a diesel engine, the first law analysis identifies significant energy losses because of cooling, and the heat lost in the exhaust gas of the engine. But when these losses are analyzed using the exergy method, the actual exergy loss is insignificant compared to the thermodynamic losses within the engine (Najem and Diab, 1992). The present work is aimed at illustrating the capability of the exergy analysis to provide a systematic approach to pinpoint the waste and unrecoverable energy associated with a diesel engine integrated with the TES system. This study gives a significant insight that will provide the direction for improvement in energy utilization. The parameters required for the energy and exergy
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analysis are obtained from the experimental investigation, the experimental setup and the methodology used in this connection are explained in the following section. 3. Experimental setup and methodology The experimental setup consists of a twin cylinder water cooled diesel engine (make: Kirloskar, bore 87.5 mm, stroke 110 mm, four stroke, rated power 7.4 kW at 1500 rpm) coupled to an electrical dynamometer, and integrated with a heat recovery heat exchanger (HRHE) and a thermal energy storage system. Fig. 1 shows the schematic diagram of the experimental setup. The heat recovery system is a shell and finned tube heat exchanger, made of mild steel and copper, respectively. The HTF flows through the tube side and the exhaust gas flows through the shell side of the heat exchanger. 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 1.008 m2 whereas the unfinned tube area is 0.5625 m2. The HRHE is fitted into the exhaust pipe of the engine. The exhaust gas from the engine is allowed to flow either to the heat exchanger or to the atmosphere by using valves. Castor oil is circulated using the gear pump through the tube side of the heat exchanger and is passed through the thermal energy storage tank. The TES tank is a stainless steel cylindrical vessel of an inner diameter of 450 mm and a height of 720 mm. The layout of the thermal storage tank is shown in Fig. 2. It contains castor oil as the sensible heat storage medium, and paraffin filled cylindrical capsules of a diameter of 80 mm and a height of 100 mm, as the latent heat storage medium. The storage tank contains 15 kg of paraffin filled in 48 capsules and 55 kg of castor oil. Each container has 320 g of paraffin. The thermo-physical properties of paraffin are given in Table 1. The cylindrical capsules are kept in a mild steel stand of a diameter of 430 mm and a height of 640 mm, having four wiremeshed layers at different heights. Twelve numbers of the PCM capsules are placed in each layer. 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. The engine cooling water temperatures at the inlet and outlet are also recorded. 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 setup. The TES tank is well insulated using glass wool and covered with aluminum cladding. A control valve fitted
Fig. 1. Schematic diagram of the experimental setup.
Fig. 2. Layout of thermal storage tank.
Table 1 Thermo-physical properties of the PCM (paraffin). Property
Value
Latent heat of fusion Specific heat capacity (solid) Specific heat capacity (liquid) Thermal conductivity (solid) Thermal conductivity (liquid) Density (solid) Density (liquid) Phase transformation temperature range
214 kJ/kg 1.85 kJ/kg K 2.384 kJ/kg K 0.4 W/m K 0.15 W/m K 856 kg/m3 775 kg/m3 58–60 1C
at the exit of the TES tank is used to vary the oil flow rate in the system. An orifice meter which is connected to the U tube manometer is used to measure the volumetric flow rate of air entering the engine. The experiments are conducted by operating the engine at various load conditions. An electrical dynamometer is used to vary the load on the engine. First, the experiment is conducted at 25% load condition. Initially the exhaust gas is not allowed to flow through the heat exchanger to avoid carbon deposition on the tube surface. After a short duration from the start of the engine, the exhaust gas is allowed to pass through the shell side of the heat exchanger, while ensuring the oil circulation through the tube side. A tachometer is used to measure the speed of the engine to ensure the rated speed of 1500 rpm. The ammeter and voltmeter readings of the electrical dynamometer are taken to find the brake power. The pressure difference in the orifice meter is observed from the U tube manometer, which is used to measure the mass flow rate of the air entering the engine. The fuel consumption of the engine is measured by counting the time required to consume a constant mass of diesel fuel of 0.042 kg (50 cm3).
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The temperature readings are continuously monitored in the TES tank, and the inlet and outlet of the HRHE and TES tank. The above said measurements are used to evaluate the heat recovered, the charging rate and charging efficiency. Several experiments are conducted to check the repeatability of the results. The experiments are conducted for 50%, 75% and full load conditions. An experiment is also conducted to determine the heat loss coefficient (UL) of the insulated storage tank and to evaluate the heat retention time of the heat storage tank. The results along with the evaluated parameters are analyzed and discussed in the following section.
4. Thermodynamic energy and exergy analysis This section explains the energy and exergy analysis of the diesel engine integrated with the HRHE and PCM based thermal energy storage system. The exergy analysis is carried out with the assumption that the changes in the potential, kinetic, electromagnetic and electrostatic exergy are negligible. The reference state considered in the exergy analysis for the atmospheric conditions is 1 atm, and the temperature is 307 K (34 1C).
The input and output energies of each system are considered, and the performance parameters of the diesel engine integrated with the HRHE are evaluated using the following equations (Eqs. (1)–(10)). Input thermal energy to the diesel engine: (thermal energy content of the fuel) _ f LCV Ef ¼ m
ð1Þ
Thermal energy carried by the exhaust gases: _ ex cp,ex ðTg1 Tgo Þ Eex ¼ m
ð2Þ
Thermal energy recovered by the HRHE: _ ex cp,ex ðTg1 Tg2 Þ EHRHE ¼ m
ð3Þ
Thermal energy lost to cooling water: _ cw cp,w ðTw2 Tw1 Þ Ecw ¼ m
ð4Þ
The charging rate 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 and the duration of charging, and is evaluated using fmoil cp,oil ðToil,f Toil,i Þ þ mpcm Lþ mpcm cp,pcm ðTpcm,f Tpcm,i Þg Ech,t ¼ kW Dtc
ð5Þ
The charging efficiency is defined as the ratio of the change in the energy content (DEC) in the storage tank to the actual energy supplied during the charging process (Qa). It is evaluated using
Zc ¼
DEc Qa
ð6Þ
where DEc ¼ Ef Ei and Qa ¼ DEc þQr , Ef the Energy content of the storage tank at the end of the charging process with respect to the environment, kJ; Ei the energy content of the storage tank at the beginning of the charging process with respect to the environment, kJ; Qa the actual energy supplied for charging, kJ; Qr the (UL AS LMTD) Dt, kJ; UL the time averaged overall heat loss coefficient when the temperature changes from 34 to 120 1C, kW/m2 K; and Dtc the time of charging, s. Total useful energy: Et ¼ Wd þEch,t kW
ð7Þ
Zi ¼
Et Ef
ð9Þ
Percentage energy saved: Es ¼
fmoil cp,oil ðToil,f Toil,i Þ þmpcm L þmpcm cp,pcm ðTpcm,f Tpcm,i Þg _ f LCVDt m
ð10Þ
4.2. Performance parameters used in the exergy analysis The following are defined and used to study the performance of the waste heat recovery system, and the TES based on exergy principles. The input and output exergies of each system and the parameters connected to the second law analysis are evaluated using the following equations (Eqs. (11)–(20)). The input exergy to the diesel engine (chemical availability of fuel); Kotas (1995) _ f LCV kW Af ¼ 1:04m
ð11Þ
Exergy lost in the exhaust gas without the HRHE:
Exergy recovered in the HRHE: T _ ex cp,ex ðTg1 Tg2 ÞTo ln g1 AHRHE ¼ m kW Tg2 Exergy lost to cooling water: T _ w cp,w ðTw2 Tw1 ÞTo ln w2 Acw ¼ m kW Tw1 Exergy present in the oil: Toil,f kJ Aoil ¼ moil cp,oil ðToil,f Toil,i ÞTo ln Toil,i Exergy present in the PCM Tpcm,f Apcm ¼ mpcm cp,pcm ðTpcm,f Tpcm,i ÞTo ln Tpcm,i To þ mpcm L 1 Tpc
ð12Þ
ð13Þ
ð14Þ
ð15aÞ
ð15bÞ
where Tpc is the phase change temperature of paraffin (333 K). Total exergy charged in the storage system: Ach ¼ Aoil þ Apcm kJ
ð15cÞ
Exergy charging rate of the storage system: Ach,t ¼
Ach
Dtc
kW
ð15dÞ
Charging efficiency of the system:
cch ¼
Ach Ach þAloss
ð16Þ
Total useful exergy: At ¼ Wd þ Ach,t kW
ð17Þ
Exergy efficiency of the diesel engine:
cd ¼
Wd Af
ð18Þ
Exergy efficiency of the integrated system:
ci ¼
At Af
ð19Þ
Exergy saved:
Thermal energy efficiency of the diesel engine: W Zd ¼ d Ef
Overall thermal energy efficiency of the integrated system:
_ ex fðhi ho ÞTo ðsi so Þg kW Aex ¼ m
4.1. Performance parameters used in the energy analysis
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ð8Þ
As ¼
Ach Af
ð20Þ
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The parameters evaluated using the above expressions for the present system, are analyzed and presented in the following section. 5. Results and discussion The parameters defined in the previous section for the energy and exergy analyses are evaluated at various engine load conditions (Tables 2 and 3). Based on the above values various performance curves are drawn and their significance is explained in this section. Fig. 3 shows the energy and exergy efficiencies of the diesel engine, with and without considering the integrated heat recovery system. The exergy efficiency of the diesel engine is slightly lower than the energy efficiency for the same power output without considering the integrated system. This is due to the fact that the chemical availability of fuel which is considered as the input in the exergy analysis, is slightly higher than the calorific value of the fuel which is considered as the input in the energy analysis. Further, it is seen from the graph that the exergy efficiency of the integrated system is considerably lower than its energy efficiency. This is due to the low exergy stored in the storage tank at a low temperature level. However, it is possible to reduce the difference in the energy and exergy efficiency, by storing the energy at a higher grade by selecting a suitable phase change material. On comparing the performance of the diesel engine with and without the integrated system, the energy efficiency has improved considerably and there is only a marginal increase in the exergy efficiency with the integrated system. This is due to the energy storage at a lower temperature level in the storage tank. The energy and exergy recovered by the HRHE through the exhaust gas are shown in Fig. 4 and the values are given in Tables 2 and 3. It is observed that the total energy recovered from
the exhaust gas varies from 1.24 to 3.35 kW at various loads. However, only 0.31–1.23 kW of the exergy of fuel is available to produce useful work. This shows that the actual exergy losses are insignificant compared to the irreversibility losses in the engine. Fig. 5 shows the energy and exergy efficiency during the charging process, accounting for the energy and exergy losses associated during the process. The energy efficiency of charging varies from 98.13% at 25% load to 99.34% at the full load condition. The exergy efficiency of charging varies from 97.94% at 25% load to 99.28% at the full load condition. The exergy efficiency is slightly lower than the energy efficiency. At higher loads, the charging efficiency is high. This is due to the shorter
Efficiency, %
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50
En Eff (DE)
45
Ex Eff (DE)
40
En Eff (IS)
35 30
Ex Eff (IS)
25 20 15 10 5 0 25
50
75
100
Load, % Fig. 3. Energy and exergy efficiencies of the diesel engine and integrated system.
Table 2 Energy analysis. Description
A
B
C
D
Load [%] Thermal energy content in the fuel input (Ef) [kW] Brake power (Wd) [kW] Thermal energy loss in cooling water (Ecw) [kW] Thermal energy loss in exhaust gas without HRHE (Eex) [kW] Thermal energy recovery by HRHE (EHRHE) [kW] Thermal energy lost in exhaust gas with HRHE (Elt) [kW] Charging rate of the storage medium (Ech,t) [kW] Total energy usage from the fuel input (Et) [kW] Energy efficiency of charging (Zch) [%] Thermal energy efficiency of diesel engine (Zd) [%] Overall thermal energy efficiency of integrated system (Zi) [%] Percentage energy saved (Es) [%]
25 12.69 1.89 4.24 2.14 1.24 0.90 1.32 3.22 98.13 14.92 25.34 10.42
50 16.22 3.79 5.30 2.67 1.67 1.01 1.85 5.64 98.62 23.35 34.76 11.41
75 19.83 5.68 6.53 3.72 2.53 1.19 2.82 8.50 99.11 28.66 42.87 14.21
100 24.96 7.31 7.82 4.55 3.35 1.20 3.81 11.12 99.34 29.27 44.55 15.28
Table 3 Exergy analysis. Description
A
B
C
D
Load [%] Chemical availability of fuel (Af) [kW] Brake power (Wd) [kW] Exergy lost in cooling water (Acw) [kW] Exergy lost in exhaust gas without HRHE (Aex) [kW] Exergy recovered from exhaust gas by HRHE (AHRHE) [kW] Exergy lost in exhaust gas with HRHE (Alt) [kW] Exergy charging rate of storage medium (Ach,t) [kW] Total exergy used from fuel input (At) [kW] Exergy efficiency of charging (Cch) [%] Exergy efficiency of diesel engine (Cd) [%] Exergy efficiency of integrated system (Ci) [%] Percentage exergy saved (As) [%]
25 13.20 1.89 0.03 0.39 0.31 0.08 0.15 2.04 97.94 14.35 15.47 1.12
50 16.87 3.79 0.05 0.56 0.47 0.10 0.21 3.99 98.47 22.45 23.68 1.22
75 20.62 5.68 0.08 0.98 0.85 0.13 0.31 6.00 99.02 27.56 29.08 1.52
100 25.96 7.31 0.12 1.36 1.23 0.13 0.43 7.73 99.28 28.14 29.78 1.64
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Fig. 4. Energy and exergy recovered by the HRHE.
99.5 Energy Eff
Efficiency, %
99
Exergy Eff
98.5 98 97.5 97 25
50
75
100
Load, % Fig. 5. Energy and exergy efficiencies of the charging process.
Fig. 6. Energy and exergy saved with load.
duration required for the charging process at higher loads, and hence, the loss encountered during the charging process is low. Fig. 6 shows the energy and exergy saved due to the integrated heat recovery system at various load conditions. It is seen from the figure that the energy saved is much higher at all loads, compared to the exergy saved. This is due to the quantity of energy recovered from the exhaust gas, which is at a higher temperature, and stored at a lower temperature level, that reduces the quality of energy stored in the storage system. Further, it is seen that the energy and exergy saved increases as
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the load increases. This is due to the shorter duration of the charging process at higher loads. In order to represent the energy and exergy level at every stage of the energy conversion and recovery process, the energy and exergy balance diagrams for the diesel engine integrated with the PCM based storage system are shown in Figs. 7 and 8 at full load condition. It is seen from Fig. 7 that the losses in energy due to the cooling water (7.82 kW) and exhaust gases (4.55 kW) are about 50% of the total input fuel energy, and nearly 21% 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, 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, 3.35 kW of energy is recovered out of 4.55 kW of energy entering the heat recovery system, which is about 74% of the heat available in the engine exhaust at full load. Surprisingly, the rate of energy stored in the storage system (3.81 kW) is higher than the rate of energy recovered from the exhaust gas (3.35 kW). This is due to the non-accounting of the latent heat energy in the water vapor present in the exhaust gas. However, when the temperature of the exhaust is brought down below 100 1C, the water vapor 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 similar trend is observed at all load conditions (Table 2). However, it is seen from Fig. 8 that the exergy stored in the storage tank is much lesser than the exergy recovered from the HRHE, as the quality of the energy stored in the storage tank is much lesser than the quality of the energy recovered from the HRHE. It is further observed from Fig. 8 that 17.21 kW of exergy which is 66% of the exergy available in the fuel, is destroyed due to irreversibilities in the engine. The integration of the HRHE and the PCM based storage system saves 0.43 kW of exergy from 1.36 kW of exergy in the exhaust gas of the diesel engine. On comparing the energy (3.81 kW) and exergy (0.43 kW) saved in the storage system from Figs. 7 and 8, it is seen that the percentage of exergy saved is much lesser because of irreversible losses due to the quality of the energy degradation, while the heat transferred over a finite temperature difference, and the energy stored in the storage tank is at a low temperature (120 1C). However, it is possible to increase the percentage of exergy saved by increasing the temperature at which energy is stored, by selecting a PCM with a higher phase change temperature. This may reduce the energy efficiency of the heat recovery heat exchanger, as the exit temperature of the HRHE will be higher. Hence, in order to increase the energy and exergy efficiency, energy should be stored at a higher to lower temperature cascaded arrangement and multiple PCMs. In order to study the discharge process in a thermal storage system, an illustrative example with two different cases is considered and analyzed, to quantify the destruction of exergy associated with the discharging process. In addition, during the storing period there will be some heat loss from the surface of the storage tank. This loss is evaluated using the overall heat loss coefficient along with other measured parameters for the storage system, and is used to determine the energy/exergy loss analysis carried out in the following case study. 5.1. Illustrative example A heat exchanger coil immersed in the thermal energy storage tank is used to extract the energy from a storage system that contains 19,450 kJ of thermal energy after a storage period of 5 h. The average surface overall heat loss coefficient of the storage tank is 0.92 W/m2 K. The energy is extracted from the storage tank for two different applications. In the first case, 200 l of water is heated from 35 to 50 1C and in the second case 100 l of water is
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Fig. 7. Energy flow diagram for a diesel engine integrated the with TES unit.
Fig. 8. Exergy flow diagram for a diesel engine integrated with the TES unit.
heated from 35 to 65 1C. The energy and exergy efficiencies of the TES tank for the storing and discharge processes for the above two cases, are evaluated and given below. [The heat content of the storage tank and the overall heat loss coefficient values correspond to the present experimental investigation. The remaining data given in the above example are for a hypothetical case.] 5.2. Energy calculation (i) Thermal energy available in the TES tank immediately after the charging process (given): ET ¼ 19,450 kJ
(vi) Amount of energy recovered during the discharge process for case (ii): Ed,C2 ¼ mw Cp,w ðTw,d3 Tw,d1 Þ ¼ 100 4:1868 ð6535 1CÞ ¼ 12,560kJ
(vii) Efficiency during discharge for both case (i) and case (ii):
Zd,c ¼
Ed,C1 ðorÞ Ed,C2 12,560 ¼ 71:6% ¼ 17,540 Ea
5.3. Exergy calculations (i) Exergy available in the TES tank immediately after the charging process: Ach ¼ Aoil þ Apcm ¼ 2170 kJ
(ii) Thermal energy loss during the storage period in the TES tank: Qloss,s ¼ U A LMTD ¼ 0:92 1:427 80:79 ¼ 1910 kJ (iii) Storage efficiency (energy retention capacity)
Zs,d ¼
ET Qloss,s 194501910 ¼ 90:18% ¼ 19450 ET
(iv) Thermal energy available after 5 h of storage duration: Ea ¼ ET Qloss,s ¼ 19,4501910 ¼ 17,540 kJ (v) Amount of energy recovered during the discharge process for case (i): Ed,C1 ¼ mw Cp,w ðTw,d2 Tw,d1 Þ ¼ 200 4:1868 ð50351CÞ ¼ 12,560 kJ
(ii) Exergy loss during the storage period: T Al,s ¼ moil Cp,oil ðTd,i Td,f ÞTo ln d,i Td,f T þ mpcm Cp,pcm ðTd,i Td,f ÞTo ln d,i Td,f 393 ¼ 55:2 2:75 ð120109:8Þ307 ln 382:8 393 þ 15:36 2:384 ð120109:8Þ307 ln ¼ 400 kJ 382:8 (iii) Storage efficiency (exergy based):
cs,d ¼
As Al,s 2170400 ¼ 81:5% ¼ 2170 As
V. Pandiyarajan et al. / Energy Policy 39 (2011) 6011–6020
(iv) Exergy available after 5 h of storage: Aa ¼ As Al,s ¼ 21702400 ¼ 1770kJ (v) Amount of exergy recovered during the discharge process for case (i): T Ad,C1 ¼ mw Cp,w ðTd,w2 Td,w1 ÞTo ln d,w2 Td,w1 323 ¼ 336 kJ ¼ 200 4:1868 ð5035Þ307 ln 308 (vi) Amount of exergy recovered during the discharge process for case (ii): T Ad,C2 ¼ mw Cp,w ðTd,w3 Td,w1 ÞTo ln d,w3 Td,w1 338 ¼ 614 kJ ¼ 100 4:1868 ð6535Þ307ln 308 (vii) Exergy efficiency during the discharge process for case (i):
cd,C1 ¼
Ad,C1 336 ¼ 18:98% ¼ 2170400 As Al,s
(viii) Exergy efficiency during the discharge process for case (ii):
cd,C2 ¼
Ad,C2 614 ¼ 34:68% ¼ 2170400 As Al,s
It is seen from the above illustrative example that the energy efficiency for both the cases is the same, i.e., 71.6%. However, the exergy efficiency for the second case is high (34.68%) when compared to the exergy efficiency of the first case (18.98%). This is due to the fact that in the second case, water is raised to a higher temperature, due to the lower mass associated with the heating process. A lower mass with a lower heat capacity increases the temperature of water at a faster rate, and hence the temperature potential difference between the source and the water decreases at a later stage of heating (when compared to the first case) that increases the exergy efficiency. In the present investigation, castor oil is used as the heat transfer fluid for charging the PCM in the TES tank. Since the phase change temperature of the PCM is 60 1C, water also can be used as the HTF. However, as the oil is easily heated to a higher temperature by transferring the heat from the exhaust gas, the exergy efficiency achievable with castor oil is high. Hence, it is construed that during any heat exchange process both the streams should attain the closure temperature (i.e. the minimum DT between two streams) at a faster rate. This could be achieved in two ways.
A heat exchanger with a maximum possible heat transfer surface area should be used. _ p Þ may be A heat transfer fluid with a lower heat capacity ðmc chosen for the given application, when there is a choice in selecting the HTF.
6. Conclusion The first and second laws of thermodynamics are employed to analyze the quality and quantity of energy in the diesel engine and the thermal energy storage system. The energy and exergy analyses enable us to develop a systematic approach that can be used to identify the sites of the real losses of valuable energy in
6019
thermal devices. The first law analysis shows that significant losses occur in the exhaust gas and cooling water. However, when analyzed using the exergy method, it is found that the actual exergy losses are insignificant compared to the irreversibility losses in the engine. It is observed from the energy analysis that 15.2% of the total energy from the fuel input is saved using the TES system. However, from the exergy analysis, it is identified that only 1.6% of the chemical availability of the fuel is saved. This indicates that improving the performance of the engine is more important than the recovery of low-grade energy. It is concluded from the results that the selected phase change material should have a high melting temperature, and fluids like oils can be used as heat transfer fluids to store heat at a higher temperature. Also, from the illustrative example it is concluded, that during any heat exchange process, both the streams should attain the closure temperature at a faster rate. It is possible to meet this requirement either with a high heat transfer surface area of the heat exchanger or by selecting a low heat capacity _ p Þ heat transfer fluid. ðmc In order to increase the energy and exergy efficiencies, energy should be stored with a cascaded arrangement using multiple PCMs, from a higher temperature to a lower temperature. In addition, with the view of curtailing fossil fuel usage and encouraging energy efficient and energy recovery devices, the Government can levy tax on the usage of fossil fuels, based on the level of exergy destruction associated with the fuel during the energy conversion and utilization. If the policy decision, based on entropy based environmental taxation is introduced, there will be a good promotion in the heat recovery devices in the industries.
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