Reviewed Paper Volume 2 Issue 8 April 2015 International Journal of Informative & Futuristic Research Review of Waste Heat Recovery From Refrigeration System Using Thermoelectric Generator Paper ID IJIFR/ V2/ E8/ 063 Page No. 2712-2721 Subject Area Key Words Mechanical Engineering Energy Crises, Wind Energy, Solar Energy, Hybrid Circuit, Charge Controller Kulkani Mahesh S 1 Jarag Sachin P 2 Ghutukade Santosh T 3 Kokare D.S 4 Kadam S J 5 Assistant Professor Department of Mechanical Engineering Bhartiya Vidyapeeth College of Engineering Kolhapur-Maharashtra Assistant Professor Department of Mechanical Engineering Bhartiya Vidyapeeth College of Engineering Kolhapur-Maharashtra Assistant Professor Department of Mechanical Engineering Bhartiya Vidyapeeth College of Engineering Kolhapur-Maharashtra Assistant Professor Department of Mechanical Engineering Bhartiya Vidyapeeth College of Engineering Kolhapur-Maharashtra Assistant Professor Department of Mechanical Engineering Bhartiya Vidyapeeth College of Engineering Kolhapur-Maharashtra Abstract In this paper we have taken the review of heat recovery from refrigeration system with the help of thermoelectric generator. At the condenser stage of refrigeration the refrigerant losses heat to surrounding this is waste heat. TEG is a device when thermal difference is applied on its two sides it produces the D.C. current as the potential difference on its two terminals. The waste heat from refrigerator system is used to produce electricity with the help of TEG. www.ijifr.com Copyright IJIFR 2015 2712
1. Introduction Today world is facing energy crises. The demand of energy is increasing day by day. In this situation there are only two options in front of us first is advancement in renewable energy sources and second is energy conservation through existing system. There are different ways by means we can conserve the energy likewise 1) waste utilization 2) energy management. Waste utilization: by this method we can recover waste heat through different tools. Areas of waste heat recovery are as follows, 1. Heat engines 2. Boilers 3. Gas turbines 4. Refrigeration systems In refrigeration system we can recover heat from: a) Ice plant b) Domestic refrigerator c) Window air conditioning system This paper summarizes the method for heat recovery through refrigeration system. With the help of following techniques we can recover the heat 1) By the aid of heat exchanger 2) By use of TEG 2. Overview of Thermoelectric generators 2.1 Thermoelectric Energy Conversion Technology Being one of the promising new devices for a waste heat recovery is thermoelectric generators (TEG) will become one of the most important and outstanding devices in the future. Within the recent years, the revival of interests into clean energy production has brought TEG technology into the attention of many scientists and engineers. Studied the potentials of thermoelectric technology in regards to fuel economy of vehicles by implementing thermoelectric (TE) materials available in the market. Hussainetal studied the effects of thermoelectric waste heat recovery for hybrid vehicles. Stoat and Milner explored the possibility of thermoelectric regeneration in vehicles in which they found out that the1.3kw output of the TE device could potentially replace the alternator of a small passenger vehicle. Stobart reviewed the potentials in fuel saving of thermoelectric devices for vehicles. They concluded that up to 4.7% of fuel economy efficiency could be achieved. From these articles, the understanding of TEG technology has been comprehensively discussed as a promising new technology to recover waste heat from refrigeration system. 2.2 Thermoelectric Principles Thermoelectric systems are solid state devices that can be used in two basic modes based on either the Peltier effect or the Seebeck effect, shown in Fig. The mode that uses the Peltier effect has 2713
current going through the TE module which causes absorption of heat on one side of the device and an expulsion of heat on the other. The mode that uses the Seebeck effect has a temperature gradient across a TE module that causes the TE module to generate an electric current. TE modules are made from alternating elements of n-type (negative) and p-type (positive) semiconductors connected electrically in series and thermally in parallel. This can be seen in Figs. Figure 1: Schematic showing cross-section of a typical multi couple thermo-electric module To measure the efficiencies of these TE modules to generate electricity, a term is defined called the dimensionless figure of merit (ZT). The equation to calculate the ZT value can be found in Eq. (1). Where S is the Seebeck coefficient, r is the electrical conductivity, T is the average operating temperature, and j is the thermal conductivity The current value of ZT for TE systems is around 1.0. To be competitive with current mechanical recovery systems, such as turbochargers, the ZT value for TE systems needs to be around 34. It should be noted that Srinivasan and Praslad have stated that the ZT needs to be equal to or greater than 8 to compete with conventional electricity generators or vapour compression refrigerators. Although these levels of ZT are not available at this time, lab studies of TE systems that use new materials have been recording ZT values that have exceeded 2 with evidence that it could be improved with additional research. It was estimated in 2008 by Heading et al. that higher efficiency TE systems will be commercially available within the next 5 10 yr. This rapid advancement of TE materials that has increased the ZT for operation around 500 _C has made TEG good candidates for waste heat recovery for automotive purposes. Figure 2:Electrical and thermal conduction paths of a multi couple thermo-electric module 2714
2.3 Seebeck Effect Figure 3: Seebeck Effect & Peltier Effect TE devices may potentially produce twice the efficiency as compared too there technologies in the current market. TEG is used to convert thermal energy from different temperature gradients existing between hot and cold end so far semiconductor into electric energy as shown in Fig. This phenomenon was discovered by Thomas Johann Seebeck in 1821 and called the Seebeck effect. The device offers the conversion of thermal energy into electric current in as impel and reliable way. Advantages of TEG include free maintenance, silent operation, high reliability and involving no moving and complex mechanical parts as compared to vapour compression cycle system which will be discussed in the next section of this study. In regard switch the applicability of TEG in modern engines, the ability of ICEs to convert fuel into use fuel power can be increased through the utilization of the mentioned device. By converting the waste heat into electricity, engine performance, efficiency, reliability, and design flexibility could be improved significantly. The fuel efficiency of gasoline powered, diesel and hybrid electric vehicles (HEVs) that utilize the power generation of IC engine is as low as 25% and conversely as much as 40% of fuel energy can be lost in the form of waste heat through an exhaust pipe [30]. An increase of 20% of fuel efficiency can be easily achieved by converting about 10% of the waste heat into electricity. Furthermore, secondary loads from the engine rivet rains can be eliminated with the help of TEG system, and as are salt torque and horse power losses from the engine can be reduced. This would help to reducing in e weight and direct the most of the increased power to the drive shaft, which would in turn help to improve the performance and fuel economy. Additionally, the possibility of minimizing the battery need sand exhaustion of vehicle battery life while permitting operation of specific accessories during engine off can be achieved by utilizing TEG. With no load (RL load not connected), the open circuit voltage as measured between points is: V = αδt Where V is the output voltage from the couple in volts [V], a [V C 1] the average Seebeck coefficient, and, DT [ C] the temperature difference across the couple, ΔT = Th Tc 2715
Where Th [ C] is the hot side of the couple and Tc [ C] the cold side of the couple.when a load is connected to the thermoelectric couple the output voltage (V) drops as a result of internal generator resistance. The current through the load is: Figure 4: Schematic of a typical thermoelectric device 2.4 Seebeck coefficients for some common materials In the table below are Seebeck coefficients at room temperature for some common, nonexotic materials, measured relative to platinum. The Seebeck coefficient of platinum itself is approximately 5 μv/k at room temperature and so the values listed below should be compensated accordingly. Seebeck coefficients at room temperature Table 1: Seebeck coefficients at room temperature Seebeck Coefficient Relative Material To Platinum (Μv/K) Selenium 900 Tellurium 500 Silicon 440 Germanium 330 Antimony 47 2716
Nichrome 25 Molybdenum 10 Cadmium, Tungsten 7.5 Gold, Silver, Copper 6.5 Rhodium 6.0 Tantalum 4.5 Lead 4.0 Aluminum 3.5 Carbon 3.0 Mercury 0.6 Platinum 0 (Definition) Sodium -2.0 Potassium -9.0 Nickel -15 Constantan -35 Bismuth -72 Carbon 3.0 Mercury 0.6 Platinum 0 (Definition) 3. Thermoelectric Material Conventional Thermoelectric Materials- Thermoelectric materials (those which are employed in commercial applications) can be conveniently divided into three groupings based on the temperature range of operation. Alloys based on Bismuth (Bi) in combinations with Antimony (An), Tellurium (Te) or Selenium (Se) are referred to as low temperature materials and can be used at temperatures up to around 450K. The intermediate temperature range - up to around 850K is the regime of materials based on alloys of Lead (Pb) while thermo elements employed at the highest temperatures are fabricated from SiGe alloys and operate up to 1300K. Although the above mentioned materials still remain the cornerstone for commercial and practical applications in thermoelectric power generation, significant advances have been made in synthesizing new materials and fabricating material structures with improved thermoelectric performance. Efforts have focused primarily on improving the material s figure-of-merit, and hence the conversion efficiency, by reducing the lattice thermal conductivity. Thermoelectric materials efficiency depends on the thermoelectric figure of merit, Z; a material constant proportional to the efficiency of a thermoelectric couple made with the material. Table 2: Temperature upper limit and the thermal efficiency of the semiconductor material pairs Material pair Temperature Limit (0 Celsius) Efficiency (%) BiTe 300 6 PbTe 600 9 SiGe 1100 11.5 2717
The maximum tested temperature differences between the cold and hot side of the TEG, and the produced maximum electrical power. 4. Applications Of Thermoelectric Generator i. Heat recovery from IC engine. ii. Heat recovery from boilers. iii. Heat recovery from gas turbine. iv. Heat recovery from all the refrigerating equipments like refrigerator, window air conditioning, Ice plant etc. 5. Heat Recovery From The Window Air Conditioning Vapour-compression refrigeration in which the refrigerant undergoes phase changes, is one of the many refrigeration cycles and is the most widely used method for air-conditioning of buildings and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for chilled or frozen storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. Oil refineries, petrochemical and chemical processing plants, and natural gas processing plants are among the many types of industrial plants that often utilize large vapour-compression refrigeration systems. Refrigeration may be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it elsewhere. A device that performs this function may also be called an air conditioner, refrigerator, air source heat pump, geothermal heat pump or chiller (heat pump). Figure 5: Single-Stage Vapour-Compression System 2718
Description of the vapour-compression refrigeration system The vapour-compression uses a circulating liquid refrigerant as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. Figure 5 depicts a typical, single-stage vapour-compression system. All such systems have four components: a compressor, a condenser, a thermal expansion valve (also called a throttle valve or metering device), and an evaporator. Circulating refrigerant enters the compressor in the thermodynamic state known as a saturated vapour [2] and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapour is then in the thermodynamic state known as a superheated vapour and it is at a temperature and pressure at which it can be condensed with either cooling water or cooling air. That hot vapour is routed through a condenser where it is cooled and condensed into a liquid by flowing through a coil or tubes with cool water or cool air flowing across the coil or tubes. This is where the circulating refrigerant rejects heat from the system and the rejected heat is carried away by either the water or the air (whichever may be the case). The condensed liquid refrigerant, in the thermodynamic state known as a saturated liquid, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic flash evaporation of a part of the liquid refrigerant. The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapour refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated. The cold mixture is then routed through the coil or tubes in the evaporator. A fan circulates the warm air in the enclosed space across the coil or tubes carrying the cold refrigerant liquid and vapour mixture. That warm air evaporates the liquid part of the cold refrigerant mixture. At the same time, the circulating air is cooled and thus lowers the temperature of the enclosed space to the desired temperature. The evaporator is where the circulating refrigerant absorbs and removes heat which is subsequently rejected in the condenser and transferred elsewhere by the water or air used in the condenser.to complete the refrigeration cycle, the refrigerant vapour from the evaporator is again a saturated vapour and is routed back into the compressor. Figure 6: P-H diagram for vapour-compression refrigeration system 2719
Figure 7: TEG Setup for refrigerating system Figure 8: TEG panel Figure 9: Thermo Electric Generator 2720
6. Conclusion Figure 10: Thermo Electric Generator i. Heat available at the condenser of refrigeration system we can recover. ii. E = m*(h2 h3) joule heat is exhausted from the condenser and (T2-T3) this much temperature gradient is there. iii. V = a*(t2-t3) volt potential difference can be produced with the help of TEG iv. This recovery of heat increases COP of refrigerating system. References [1] Abhilash Pathania Recovery of Engine waste heat for reutilization in air conditioning system in an automobile Global journal of research in engineering Mechanical and mechanics engineering, vol 12 Issue 1 version1.0 january 2012 [2] T. Endo, S. Kawajiri, Y. Kojima, K. Takahashi, T. Baba, S. Ibaraki, T. Takahashi, Study on Maximizing Exergy in Automotive Engines, SAE Int. Publication 2007-01-0257, 2007. [3] K. Nantha Gopal, Rayapati Subbarao, V. Pandiyarajan, R. Velraj, Thermodynamic analysis of a diesel engine integrated with a PCM based energy storage system, International Journal of Thermodynamics 13 (1) (2010) 15-21. [4] Hakan Özcan, M.S. Söylemez, Thermal balance of a LPG fuelled, four stroke SI engine with water addition, Energy Conversion and Management 47 (5) (2006) 570-581. [5] V. Johnson, Heat-generated cooling opportunities in vehicles, SAE Technical Papers, No. 2002, 2002-01-1969. [6] V Ganeshan, Internal Combustion Engine, Tata McGraw Hill Publishing Company Limited, Second Edition, pp 35, 606-670. 2721