International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 15 (2016) pp 8547-8552 © Research India Publications. http://www.ripublication.com

Thermoelectric cooling system for internal combustion engine. Part 1: development of the technical aspects Nikolay Anatolyevich Khripach, Viktor Sergeyevich Korotkov and Igor Arkadyevich Papkin Moscow state university of Mechanical Engineering (MAMI) B. Semyenovskaya St, 38, Moscow, 107023 Russia.

direct conversion of thermal energy into electricity, is a highly relevant task.

Abstract In this article we present an analysis of heat balance of an internal combustion engine. This work shows that conversion of thermal energy, transferred from the combustion engine to the coolant, into electrical energy is a very promising research field. The device capable of converting thermal energy into electricity is a thermoelectric generator. Further, we examine the current trends in the development of thermoelectric generators, where the heat source is the cooling fluid of an internal combustion engine. On the basis of the analysis we propose to develop not just a separate thermoelectric generator, but a complete cooling system, which includes the generator as a component. Having done preliminary calculations we determined the composition and developed the concept of a thermoelectric cooling system for internal combustion engines.

EXTERNAL HEAT BALANCE OF ICE The external heat balance of an internal combustion engine [2] shows that a considerable share of the heat produced by burning fuel-air mixture gets discharged into the atmosphere with exhaust gases and absorbed in the cooling system. (1) Qall=Qe+Qeg+Qcool+Qnb+Qmisc , where Qall-total amount of heat produced by fuel combustion; Qe-heat equivalent to the efficient operation of the engine; Qeg-heat lost with the exhaust gases; Qcool-heat transferred to the cooling system; Qnb-heat lost due to chemical incompleteness of combustion; Qmisc-unaccounted heat losses.

Keywords: Thermoelectric generator, thermoelectric cooling system, direct conversion of thermal energy into electrical energy.

As shown in Figure 1, only 30% of the fuel energy is spent to produce useful work, approximately 30% is discharged into the atmosphere through the engine cooling system and up to 40% is discharged with the exhaust gases [3]. This creates a great potential for capturing and utilization of the heat, improving energy efficiency of stationary and mobile power plants and vehicle engines. A significant part of the heat energy dissipated into the atmosphere can be used for different purposes. Significance of individual components of the ICE heat balance depends upon operating conditions, type of the engine and amount of boosting.

INTRODUCTION The energy efficiency of the internal combustion engine (ICE), which is usually rated by fuel economy, primarily depends on the degree of conversion of fuel combustion energy into mechanical and/or electrical (in hybrid vehicles) energy. Modern technologies of managing the processes of ICE operation in motor vehicles approach the utmost limits of their development. Their further improvement will be accompanied by growing technical difficulties, while the result of their use will become less and less noticeable. It is necessary to introduce new technologies for conversion of fuel combustion energy, including the coolant heat expelled into the atmosphere, in order to further increase the energy efficiency of heat engine based power plants. One of such approaches towards sharp reduction of fossil oil fuels consumption, is to implement direct conversion of thermal energy into electrical energy by using thermoelectric generators (TEGs), based on thermoelectric elements that convert thermal energy into electricity at different power modes of engine operation. Paper [1] describes studies that have shown a reduction in fuel consumption of an internal combustion engine fitted with a thermoelectric generator on board the vehicle. TEGs also have a number of advantages over traditional electric machine power converters, namely no moving parts, high reliability and ease of maintenance. Hence, development of technical solutions to improve the efficiency of thermal power plants in transport vehicles by

Figure 1: External heat balance of internal combustion engine

An analysis of the heat balance enables to infer that utilization of the thermal energy of exhaust gases and the thermal energy passed into the cooling system has a good research potential.

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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 15 (2016) pp 8547-8552 © Research India Publications. http://www.ripublication.com high temperature and low temperature heat exchangers and heat pipes of different configurations. Paper [10] describes disadvantages of the design with heat exchangers and heat pipes. They include significantly higher price of the thermoelectric generator compared with a standard radiator because of the heat pipes, larger size and different mounting dimensions, which complicate installation of such thermoelectric generators inside the vehicle engine compartments. Therefore, paper [10] proposed a fundamentally different design of a thermoelectric radiator, which is shown in Figure 3.

GLOBAL TRENDS IN CONVERTING THERMAL ENERGY INTO ELECTRICAL ENERGY Currently, the global automakers pay more attention to the development of thermoelectric generators, where exhaust gases serve as the heat source. General Motors created a thermoelectric generator prototype [4] integrated into the exhaust system. Another prototype of a thermoelectric generator, shown in Figure 3, was developed by BMW [5]. The TEG prototypes are shown in Figures 2a and 2b, respectively.

a) prototype TEG by GM;

Figure 3: Thermoelectric radiator design. a)-front view; b)isometric view; c)-enlarged view; 1-flat pipe; 2-thermoelectric generator module; 3-cooling fins; 4-fan; 5-fan housing.

The main functional part of the thermoelectric radiator is the thermoelectric generator module. The thermoelectric module performs direct conversion of thermal energy from the cooling system into electrical energy. The cooling fluid passes through flat pipes (1) on two sides of which there are thermoelectric generator modules (2) with cooling fins (3) at the cold sides. Continuous supply of heat from the coolant to the hot sides of the thermoelectric generator modules and simultaneous dissipation of the heat into the air through the fins creates Seebeck effect, which allows producing electricity and feed it to the vehicle's power network or the battery. Besides the design of the thermoelectric radiator there are patented prototypes of thermoelectric radiators for ICE cooling systems, where the engine cooling system is considered as a whole. For example, paper [11] discusses two ways of heat recovery from the coolant, the scheme is shown in Figure 4. According to the first way the thermoelectric generator utilizes coolant temperature difference before and after the air heat sink. According to the second way the ICE cooling system is divided into a hot circuit, which includes an internal combustion engine, a water pump and a thermoelectric generator, and a cold circuit, which includes a radiator, another water pump and the thermoelectric generator.

b) prototype TEG by BMW. Figure 2: Prototype TEG

Currently the thermal energy dissipated by cooling systems of internal combustion engines in modern vehicles finds only partial use for climate control. However, this positive effect is only relevant during the cold season. Even in this case, most of the energy is dissipated to the atmosphere through the coolant and oil. There are several ways to use the waste heat removed by ICE cooling system. However, one of the most promising is to convert the heat into electricity and then use it in different systems of the vehicle (powering electrical systems, battery charging, etc.). To achieve this a thermoelectric generator is integrated into the cooling system. Its source of thermal energy is the coolant of the internal combustion engine. This kind of TEG is called thermoelectric radiator. In many studies, such as [6-9], the design of a thermoelectric radiator includes

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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 15 (2016) pp 8547-8552 © Research India Publications. http://www.ripublication.com air heat sink is used to create a second (cooling) circuit. The hot sides of the thermoelectric generating modules are connected to the cooling system of an internal combustion engine via a distributing valve, which allows regulating the rate of recovering thermal energy and, consequently, the output of the thermoelectric generator. The cold sides are in turn connected to a separate cooling circuit, which includes an own radiator and a pump. Paper [14] examines several options for recovery of thermal energy from the ICE coolant without introduction of additional pumps and radiators. In this case, the source of thermal energy for the thermoelectric generator is the coolant before the cooling radiator, or a portion of it, and the cooling media is the coolant after the radiator, or a portion of it. In the TEG system of thermal energy recovery, shown in Figure 6, the hot side of the thermoelectric generator modules is connected to the ICE cooling system through the distributing valve [15]. The cold sides are in turn connected to a separate cooling circuit, which includes an own radiator and a pump. The heat exchanger of the thermoelectric generator is a set of alternating thermally connected plates, in which hot and cold heat transfer media circulate.

Figure 4: An electricity generating device for a vehicle with internal combustion engine, which includes thermoelectric elements, thermally placed between the hot inlet and cold outlet of the heat exchanger.

Another option is a heat recirculation system, which includes a generator that produces heat while the vehicle is in motion [12]. The system also has a thermoelectric cooling module, which is connected to the heat generator. The module is used to convert the heat into electricity. The system also has a power control unit and a battery, which is charged by the thermoelectric cooling module. The scheme of this system is shown in Figure 5.

Figure 5: Heat recycling system for a road vehicle

A thermoelectric generator using the thermal energy of the combustion engine coolant is described in [13]. An additional

Figure 6: Thermoelectric generator

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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 15 (2016) pp 8547-8552 © Research India Publications. http://www.ripublication.com The output electrical power of the thermoelectric radiator at nominal ICE operating mode can be calculated according to the following expression: (2) NTEG = Q·ηTGM ,

DEVELOPMENT OF A THERMOELECTRIC COOLING SYSTEM FOR INTERNAL COMBUSTION ENGINE The conducted analysis shows that development of a thermoelectric cooling system, which includes a thermoelectric radiator, as a whole is a more promising research goal than development of a thermoelectric radiator alone. The following requirements have been identified during the analysis of the thermoelectric ICE cooling system: - Reliable operation in modern petrol and diesel engines of road vehicles; - Complete sealing when the device operates as a part of a mass-produced engine, equipped with a standard water pump and operated under regular loads; - Ease of regular maintenance work, involving replacement of the coolant (no air pockets, no need to dismantle system elements, etc.); - Compatibility of structural materials with coolants used in automobile engines; - Hydraulic and aerodynamic drag of the thermoelectric radiators in the developed model of the thermoelectric ICE cooling system must not deviate by more than 10% from the corresponding basic parameters of regular radiators; - The inside volumes/dimensions of the thermoelectric radiators must exceed similar parameters of regular radiators by more than 10%; - The developed model of a thermoelectric ICE cooling system must be electromagnetically compatible with other vehicle systems; - The design should use standard and common parts as much as possible; - Safety of maintenance personnel from electric shocks and burns caused by hot parts must be ensured.

where NTEG is the output electric power of thermoelectric radiator, kW; [eta] TGM is the efficiency of the thermoelectric generator modules; Q-thermal power transferred from the engine, kW. According to the equation of heat balance in the engine, the thermal power transferred from the engine at steady-state operating modes is determined by the following relationship: (3) Q  G  c  T , where G is fuel consumption of the thermal energy source through the engine, kg/s; c is specific thermal capacity of the thermal energy source, kJ/(kg•K); [delta]T is temperature difference of the heat source at the input and output of the engine, K Since one of the requirements for a thermoelectric radiator, formulated at the design stage, says that its interior and overall dimensions should not exceed similar parameters of regular radiators by more than 10%, it was decided to equip the thermoelectric ICE cooling system with a thermoelectric radiator which uses only the coolant as its source of thermal energy. Preliminary calculations showed that a thermoelectric cooler of this kind can convert thermal energy from an internal combustion engine into electrical energy providing not less than 0.7 kW of power. According to the preliminary calculations, radiators using oil or EGR as their heat sources would generate little electrical power if they remain within the preset overall size and internal volume limits. It should be noted also that each thermoelectric radiator requires installation of an extra fan into the thermoelectric ICE cooling system to cool the radiator. This, consequently, will consume additional energy to actuate the fans. Therefore, instead of the oil and EGR thermoelectric radiators we decided to add coolant-oil and coolant-EGR heat exchangers, designed to transfer the heat from the oil and EGR to the coolant. Later this additional heat is converted into electrical energy by the TEG. Thus, the engine will receive oil cooled to an optimal temperature, recirculating exhaust gases will be cooled down and flow into the intake manifold and then into the ICE, thereby reducing NOx emissions, and the coolant will be heated. In addition to the thermoelectric radiator and two heat exchangers, the thermoelectric ICE cooling system also includes the passenger compartment heater. To control the flow of coolant we decided to use a controlled solenoid valve instead of the classical thermostat, used in many refrigeration systems. There should be two solenoid valves because of inclusion of the passenger compartment heater: the first solenoid valve lets the coolant flow through the passenger compartment heater, the second valve sends the coolant

Expected requirements for technical parameters of the thermoelectric ICE cooling system have been identified as well: - Conversion of up to 5% of thermal energy dissipated by the thermoelectric radiator into electricity while maintaining the required temperature mode of engine operation; - Conversion of thermal energy produced by an internal combustion engine into electrical power of at least 0.7 kW. Thermoelectric ICE cooling system can have several integrated thermoelectric radiators, where the heat source can be not only the coolant but also lubricating oil of the ICE and recirculated exhaust gases (EGR). When EGR or oil are used as a source of thermal energy in a thermoelectric radiator, the cooling medium can be the coolant of the cooling system or the air. Preliminary calculations, done in order to determine the necessary components of the cooling system, showed the capacity of electric output of thermoelectric heaters. It is also necessary to consider the overall dimensions of the thermoelectric heaters when selecting thermoelectric modules.

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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 15 (2016) pp 8547-8552 © Research India Publications. http://www.ripublication.com through the long or short circulation path, depending on the coolant temperature. Both solenoid valves must be controlled by software. The system also includes an electric coolant circulation pump, an electric oil pump with a pressure relief valve, an expansion tank, pipes, connectors and a control system. In view of the decisions made we developed the concept of a thermoelectric ICE cooling system, shown in Figure 7.

cooling system and developed its concept. The developed thermoelectric cooling system will improve the energy efficiency of a heat engine, reducing fuel consumption and will generate electricity on board the vehicle.

ACKNOWLEDGMENTS This work was done under the grant agreement # 14.577.21.0184 dated October 27, 2015 with financial support of the Ministry of Education and Science of the Russian Federation. Unique identifier of applied research: RFMEFI57715X0184.

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[2] Figure 7: Schematic diagram of the internal combustion engine thermoelectric cooling system. 1-electric coolant circulation pump; 2-electric oil pump with a pressure relief valve; 3-coolant-oil heat exchanger; 4-coolant-EGR heat exchanger; 5-expansion tank; 6-passenger compartment heater; 7,8-solenoid valves; 9-thermoelectric radiator of the cooling system; 10-air fan for the thermoelectric radiator; 11TEG control unit; 12-system control unit; 13-electrical systems of the ICE and the vehicle.

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The thermoelectric ICE cooling system will operate as follows: the electric coolant circulation pump propels the coolant which passes through the small circulation path and, thanks to the presence of coolant-oil and coolant-EGR heat exchangers, heats up much faster. It also passes through the passenger compartment heater. When the required temperature is reached, the solenoid valve directs the coolant to the large circulation path with the thermoelectric radiator. The thermoelectric radiator has TEG modules inside, which generate electrical energy from the heat of the hot coolant due to temperature differences between their sides (the hot side is heated by the coolant while the cold side is cooled by the cooling fins and the thermoelectric radiator fan). Further work in this direction will include development of a mathematical model of a thermoelectric ICE cooling system, calculations and defining the design parameters of the system.

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[9] CONCLUSION This article presents a brief analysis of the systems implementing direct conversion of thermal energy from engine cooling systems into electrical power, which demonstrates relevance of this topic. Basing on existing designs of thermoelectric radiators for cooling systems, we defined the features and components of a thermoelectric ICE

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