Abstract--Kitchen appliances consume large amount of energy

in residential houses. By using efficient appliances energy losses can be reduced. Most of the household appliances use DC internally except some appliances such as stove, refrigerator, dishwasher and microwave oven. The refrigerator and stove is one of the high power consuming kitchen appliances and it consumes large amount of energy. This work investigated on energy efficient home appliance design to run on low DC voltage and the purpose is to reduce energy consumption and losses. A combined refrigerator stove unit is designed to run on low voltage DC. The system is simulated with Matlab / Simulink and finally a prototype is built to analyze the performance.

Index Terms—combined refrigerator stove, Thermoelectric Module, refrigerator, Kitchen appliance, Low voltage DC appliance, peltier element.

I. INTRODUCTION uring the beginning of the nineteenth century the debate between AC and DC had started [1]. Tesla showed the

practical advantages of alternating current. Transformers made it possible to step up AC voltage easily, this allowed power to be transmitted over long distances with a low loss. This was not easy to achieve with Edison’s DC voltage and there were huge transmission losses. Tesla's practical results were the deciding factor, at least for the time being that an AC system was to prefer [1]. This debate again came into light due to recent development in power electronics [2] which gives a better utilization of existing transmission corridors with high voltage DC connections. High voltage DC transmission allows more power to be transmitted over a long distance with less losses compared to an AC transmission. Power electronics makes efficient and accurate control of electrical power possible. Efficient AC to DC, DC to AC and DC to DC conversion technology are now available on the market, where DC to DC conversion is more efficient than AC to DC conversion [3].

This work has been carried out at the Division of Electric Power

Engineering in Chalmers University of Technology and supported by CIT (Chalmers Indutriteknik). CIT is a foundation founded by Chalmers, providing knowledge on commercial terms. M. Amin and Y. Arafat are with Dept. of EEE, International Islamic University Chittagong, 154/A College road, Chittagong-4203, Bangladesh. (e-mail: amineeecuet@yahoo.com; yasir0202025@yahoo.com). S. Lundberg is with Chalmers University of Technology, 412 96 Göteborg, Sweden (e-mail: stefan.lundberg@chalmers.se). S. Mangold is with Stiftelsen Chalmers Industriteknik, Chalmers Science Park, SE-412 88 Göteborg, Sweden (e-mail: stephan.mangold@cit.chalmers.se).

Low voltage DC system is investigated for household application [4]-[6]. The devices run internally on DC, can be connected with Low voltage DC system without AC/DC rectification. Some high power appliances such as stove if connect with the low voltage DC system, energy losses in the feeder cable become high due to high current comparing with existing AC system. In this work a refrigerator and stove is designed which run on 48 volt DC. The combined refrigerator stove unit has three compartments. First one is refrigerator compartment, middle one is water tank compartment and third one is stove compartment. The heat extracted from the refrigerator is stored in the stove and is used for cooking or other purposes. Thermoelectric module (TEM) is used to pump heat energy from one side to another side. Two separate TEMs are used for refrigerator and stove. Water is used as a medium to transfer extracted heat of the refrigerator from refrigerator side TEM to stove side TEM. Stove side TEM extracts partial energy from that heat and remaining energy is stored in water tank which raises the temperature the of water. This hot water is used in other purposes to reduce energy consumption. To run the refrigerator on DC, a refrigerator model was investigated by using the peltier effect. The proposed model gives a robust, comparatively silent, harmful CFC (Freon) [7] free refrigerator. The Peltier effect uses electricity to pump heat. But the peltier effect is less energy-efficient than other methods. It is due to the fact that comparatively more energy is required for pumping energy from one side to another side of the peltier module. In this system the efficiency is increased by using the peltier element both cooling and heating purpose at a time.

II. MODEL DESIGN Thermoelectric module creates temperature difference on both sides of it by extracting heat from one side to another side when it is supplied by electric power. Two thermoelectric modules are used for the system. Refrigerator side TEM extracts heat from the refrigerator. To store that extracted heat as thermal energy storage in stove, paraffin is used which has the capability of latent heat storage by changing its phase from solid to liquid at 100⁰C. Stove side TEM is capable of extracting some energy from the extracted heat of refrigerator and the remaining energy is stored into the water.

1. Refrigerator side Thermoelectric module In this combined refrigerator-stove system, the refrigerator side thermoelectric module is used both as a refrigerator and as a heater. It pumps heat from the refrigerator, to cool it, to the hot side where it heats the water simultaneously for later use.

An efficient appliance for low voltage DC houseM. Amin, Member, IEEE, Y. Arafat, S. Lundberg, Member, IEEE and S. Mangold

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The module's Seebeck coefficient at the refrigerator (cold) side temperature [8], Tr is SMTr SMT 2 3 4 1

The module's Seebeck coefficient at the hot side (hot water) temperature, Tw is SMTw SMT 2 3 4 2

Where coefficients for a 71-cpl, 6-amp module, s1 = 1.33450 x 10-2, s2 = -5.37574 x 10-5, s3 = 7.42731 x 10-7, s4 = -1.27141 x 10-9 [8]. Temperature difference of this module is 3 The Seebeck coefficient in volts/°K can be evaluated SMT SMT 4

Subtracting SMTr from SMTw and divided by SMT SMT 2 3 4 5 SMT SMT 2 3 4 6 The module's resistance at the refrigerator (cold) side temperature, Tr is RMTr RMT 2 3 4 7

The module's resistance at the hot side (hot water) temperature, Tw is RMTw RMT 2 3 4 8

Where Coefficients for a 71-cpl, 6-amp module r1 = 2.08317, r2 = -1.98763 x 10-2, r3 = 8.53832 x 10-5 , r4 = -9.03143 x 10-8

[8]. The module resistance in ohms can be evaluated RMT RMT 9

2 3 4 10 The module thermal conductance at the refrigerator (cold) side temperature, Tr is KMTr KMT 2 3 4 11

The module thermal conductance at the hot side (hot water) temperature, Tw is KMTw KMT 2 3 4 12

Where coefficients for a 71-cpl, 6-amp module, k1 = 4.76218 x 10-1, k2 = -3.89821 x 10-6, k3 = -8.64864 x 10-6, k4 = 2.20869 x 10-8 [8]. The module thermal conductance in watts/°K can be evaluated KMT KMT 13

2 3 4 14 The input voltage to the module in volts is: 15 Hence the current of this module will be V SM 16

The input power in watt is:

_ V SM 17

The power pumped by this module in watt is:

12 18 Total rejected heat in watt is:

12 19

And _ 20

B. Stove side Thermoelectric module High temperature is necessary for the stove. It is not possible to get high temperature (>100 oC) using one single thermoelectric module when the refrigerator temperature is 0-4 oC. Two stage operation is required to get this high temperature difference. The purpose of the stove side module is to get high temperature and the energy from this module is stored in the paraffin. The module's Seebeck coefficient at the hot side (stove) temperature, Ts is SMTs SMT 2 3 4 21

Temperature difference of this module is 22 The Seebeck coefficient in volts/°K can be evaluated SMT SMT 23

SMT SMT 2 3 4 24

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The module's resistance at the hot side (stove) temperature, Ts is RMTs RMT 2 3 4 25

The module resistance in ohms can be evaluated RMT RMT 26 RMT RMT 2 3 4 27 The module thermal conductance at the hot side (hot water) temperature, Ts is KMTw KMT 2 3 4 28

The module thermal conductance in watts/°K can be evaluated as: KMT KMT 29

KMT KMT 2 3

4 30 The input voltage to the module in volts is: 31 The current of this module will be: V SM 32

The input power in watt is:

_ V SM 33

The power pumped by this module in watt is:

12 34 Total rejected heat in watt is:

12 35 Hence _ 36 Figure 1 presents the combined refregerator system. Left side is the refregeraot compartment, middle one is the hot water compartment and right side is the stove compartment. In this system, total rejected heat is the sum of heat pumped by stove side module and heat caused changing the temperature of tank water. Mathematically it can be written as

_ d∆T 37

∆T T T _ 38

Figure 1 System diagram

Where m is the weight of water, S is the specific heat of water; ∆Tw is the rise of water temperature and To_w is the initial water temperature.

d∆T _

d∆T 12

12 d∆T 1 – 12

12 39

Taking Laplace transformation of the equation (39), we get

s ∆T 1 – 12

12

∆T 1 1 – 12

12 40

Water temperature can be written as: T T _ ∆T 41

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III. SIMULATED RESULT The system is simulated for a 71-cpl, 6-amp module. The input voltage is 12 volt for each peltier element. The system is analyzed at steady state. The refrigerator temperature Tr is varied between 0 to 4 oC. The Stove temperature is kept near about 120 oC. Water temperature raises 20 oC to 60 oC. The hot water temperature is kept such that it cannot raise more than 60 oC. Figure 2 presents the variation of heat pumped by the module with module input current. In the both cases, heat pumped by the module increases with increasing of input current.

Figure 2 Heat pumped vs. current

Figure 3 presents the variation of heat pumped by the modules with temperature difference. Heat pumped decreases for both modules with increasing temperature difference. At the time supplying power to the module, the temperature difference is zero, it pumps maximum heat energy. In steady state the temperature difference is almost 60 oC, heat pumped by the module decreased.

Figure 3 Heat pumped vs. Temperature difference

Figure 4 presents the time variation of water temperature. Here the dash line is the actual temperature of the water and the solid line is the raise of the water temperature.

Figure 4 Time variation of Water Temperature

Figure 5 presents the time variation of input current. Initially refrigerator side temperature low, it consumes high current to keep the temperature 0 to 4 oC. When it reaches steady state, temperature difference is almost 60 oC, it consumes less current since it pumps less heat energy.

Figure 5 Time variation of Input current

Figure 6 presents the time variation of heat pumped, total rejected heat and heat pumped by these two module.

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IV. EXPERIMENTAL RESULT This experiment was performed to evaluate the

performance of the stove side TEM. In this experiment, there was no water inside the refrigerator. Initially the refrigerator side TEM was supplied by 48 V DC, stove tank was filled with 7 kg of paraffin, the temperature of the air inside the refrigerator was 17 ⁰C, the temperature of the heat sink of the refrigerator side TEM was 15 ⁰C and the temperature of the 9 l tank of water was 7 ⁰C. Figure 4.7 shows the temperatures of the refrigerator, water tank and paraffin for this experiment. The temperature of the air inside the refrigerator decreased from 17 ⁰C to 0 ⁰C within 6 minutes and it was further reduced to -18 ⁰C at the 32nd minute of the experiment when the temperature of the water in the water tank was at 31 ⁰C. This means that the hot side temperature of the TEM is almost 32 ⁰C. After this point, the refrigerator temperature started to increase with the increment of hot side temperature in order to maintain a constant temperature difference of 50 ⁰C between both side of the TEM as it is designed. The refrigerator air temperature reached to 0 ⁰C from -18 ⁰C after 60 minutes and in the mean time, the water tank temperature was increased to 41 ⁰C. The refrigerator temperature increased faster than the increment of the water tank temperature. This because of the high temperature difference between the refrigerator temperature and the surrounding room temperature, the leakage of thermal energy from the surroundings to the refrigerator was larger. The stove side TEM was supplied by 48 V DC at the 116th minute of the experiment when the temperature of the water tank reached to 50 ⁰C. As mentioned before, the aluminum stove tank was filled up with 7 kg paraffin for this experiment. The hot side temperature of the stove TEM took 254 minutes to increase the temperature from 30 ⁰C to 100 ⁰C and in the mean time the temperature of the water tank increases from 48 ⁰C to 72 ⁰C. The temperature gradient inside the paraffin is clear from the two curves of top paraffin temperature and bottom paraffin temperature as

shown in Figure 27. The curve is showing larger temperature gradient and it is due to the fact that the paraffin has low heat conductivity of 0.2Wm-1k-1.

Figure 7 Time variation of different temperature of the system

V. CONCLUSION Due to the fact that the thermal energy from the refrigerator

is stored and later used for heating the stove, the overall efficiency of the system is increased compared to a standard stove and refrigerator. To implement the proposed design practically, more research is recommended for improvement of insulation system to improve the efficiency of the whole system. More analysis is required to make the TEM more efficient for extracting energy and to make the unit cheaper. Design of automatic control system is also recommended for the proposed system.

VI. REFERENCES [1] Bruce Nordman1, Rich Brown, Chris Marnay “Low-voltage DC:

Prospects and Opportunities for Energy Efficiency” Lawrence Berkeley National Laboratory, November 16, 2007.

[2] Clark W. Gellings, " Edison Redux: the new ac-dc debate," Electric Power Research Institute, http://www.powerpulse.net/techPaper.php?paperID=130.

[3] Paajanen, Pertti; Kaipia, Tero; Partanen, Jarmo; , "DC supply of low-voltage electricity appliances in residential buildings," 20th International Conference and Exhibition on , vol., no., pp.1-4, 8-11 June 2009.

[4] Sannino, A.; Postiglione, G.; Bollen, M.H.J.; , "Feasibility of a DC network for commercial facilities," Industry Applications Conference, 2002. 37th IAS Annual Meeting. Conference Record of the , vol.3, no., pp. 1710- 1717 vol.3, 2002

[5] J. Pellis,” The DC Low Voltage house,” Eindhoven Univrsity of Technology, Netherland, Sept~ER 1997. http://www.ecn.nl/docs/library/report/1997/c97058.pdf

[6] Rodriguez-Otero, M.A.; O'Neill-Carrillo, E.;"Efficient Home Appliances for a Future DC Residence," Energy 2030 Conference, 2008. ENERGY 2008. IEEE , vol., no., pp.1-6, 17-18 Nov. 2008.

[7] http://www.brighthub.com/engineering/mechanical/articles/63281.aspx. Accessed date 25-01-2011

[8] http://thermal.ferrotec.com/technology/thermoelectric/thermalRef11/. Accessed date 25-01-2011

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VII. BIOGRAPHIES

Mohammad Amin (M’11) was born in Chittagong, of Bangladesh on August, 1985. He completed M.Sc. in Electric Power Engineering from Chalmers University of Technology. He is now with the dept. of Electrical & Electronic Engineering at International Islamic University Chittagong (IIUC). His field of interest is on Power Electronic, Power System, FACTS Devices, Renewable energy.

Yasir Arafat was born in Chittagong, of Bangladesh on March, 1985. He completed M.Sc. in Electric Power Engineering from Chalmers University of Technology. He is now with the dept. of Electrical & Electronic Engineering at International Islamic University Chittagong (IIUC). His field of interest is on Power Electronic, Power System.

Stefan Lundberg (S’04–M’06) was born in Göteborg, Sweden, in 1976. He received the Ph.D. degree in electrical engineering from Chalmers University of Technology, Göteborg, Sweden, in 2007. He is with the Department of Energy and Environment, Division of Electric Power Engineering, Chalmers University of Technology. His main area of interest is control and modeling of

wind parks.

Stephan Mangold was born in Bamberg, Germany on August 1, 1951 and a few weeks later the family moved to Sweden. He graduated and received his PhD degree in electronics from Chalmers University of Technology, Gothenburg, Sweden, in 1981. Dr Mangolds employment experience includes a position as development engineer at Diaphon Development AB, a small startup company, which was bought by the international 3M where he

continued as development manager for several years. In 1993 Dr Mangold founded a Science Centre “Experimentum” outside Gothenburg, which later got to be the base for Scandinavias largest Science Centre “Universeum” in Gothenburg. Stephan Mangold received the Super Innovator award from the Society Industrial Development in year 2004 and received the Gustaf Dalén Gold Medal from Chalmersska Ingenjörsföreningen in year 2005. He is currently working as project leader at Commercial Research and Development by Chalmers Industriteknik.

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