School of Aerospace Mechanical Manufacturing Engineering, RMIT University, Melbourne, Australia

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1 Applied Mechanics and Materials Online: ISSN: , Vol. 393, pp doi: / Trans Tech Publications, Switzerland Experimental Investigation of Power Generation from Salinity Gradient Solar Pond Using Thermoelectric Generators for Renewable Energy Application Baljit Singh 1, 3, a, Jaisatia Varthani 2, Muhammed Fairuz Remeli 1, 3, Lippong Tan 1, Abhijit Date 1, Aliakbar Akbarzadeh 1 1 Energy Conservation and Renewable Energy (EnergyCARE) Group School of Aerospace Mechanical Manufacturing Engineering, RMIT University, Melbourne, Australia 2 Mechanical Engineering Dept., Faculty of Engineering, Universiti Selangor (UNISEL), Bestari Jaya Campus, Jalan Timur Tambahan, Bestari Jaya, Selangor Darul Ehsan, Malaysia 3 Faculty of Mechanical Engineering, Universiti Teknologi MARA (UiTM), Shah Alam, Selangor, Malaysia a baljit@salam.uitm.edu.my Keywords: Solar pond; Thermoelectric generator; Renewable energy; Power generation. Abstract. Low grade heat (<100 0 C) is currently converted into electricity by organic rankine cycle (ORC) engines. ORC engines require certain threshold to operate as the organic fluid generally boils at more than 50 0 C, and fails to operate at lower temperature. Thermoelectric generators (TEGs) can operate at very low temperature differences and can be good candidate to replace ORC for power generation at low temperatures. In this paper, the potential of power generation from TEG and salinity-gradient solar pond (SGSP) was investigated. SGSP is capable of storing heat at temperature up to 80 0 C. The temperature difference between the upper convective zone (UCZ) and lower convective zone (LCZ) of a SGSP can be in the range of 40 0 C 60 0 C. This temperature difference can be used to power thermoelectric generators (TEG) for electricity production. This paper present result of a TEG system designed to be powered by the hot and cold water from the SGSP. The system is capable of producing electricity even on cloudy days or at night as the SGSP acts as a thermal storage system. The results obtained have indicated significant prospects of such system to generate power from a low grade heat for remote area power supply. Introduction Solar pond is a simple and low cost solar energy system which collects solar radiation and stores it as thermal energy for a relative longer period of time. When solar radiation penetrates through the solar pond surface, the infrared radiation component is first absorbed in the surface mixed layer or upper convective zone. However, this heat is lost to the atmosphere through convection and radiation. The remaining radiation will subsequently be absorbed in the non-convective zone and lower convective zone before the last of the radiation reaches the bottom of the pond. In these ponds, the solution is heavier in the lower region because of higher salt concentration. As a result, the natural convection that takes place in normal ponds is suppressed. Solar radiation penetrating to the bottom region is absorbed there and temperature of this region rises substantially since there is no heat loss due to convection. The temperature difference created between the top and the bottom of the solar ponds can be as high as C. The collected and stored heat can be extracted and used for industrial process heat, space heating, and even power generation [1-4]. TEGs have no moving parts and can be customised to meet any power requirement [5]. It consists of two dissimilar materials, n-type and p-type semiconductors, connected electrically in series and thermally in parallel. Number of these thermoelectric elements is combined in series to All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-13/05/16,04:47:48)

2 810 Advances in Manufacturing and Mechanical Engineering form a thermoelectric generator module. Heat is supplied at hot side of the thermoelectric cell while the other end is maintained at a lower temperature by a heat sink. As a result of the temperature difference, Seebeck voltage is generated across the p-n junction that results in current flow through an external load resistance. The power output depends on the temperature difference, the properties of the semiconductor materials and the external load resistance. The relatively low conversion efficiency of thermoelectric modules ( 4%) has been a major factor in limiting their applications in electrical power generation and has restricted their use to specialised situations where reliability is a major consideration [6]. However, one exception is the thermoelectric recovery of waste heat when it is unnecessary to consider the cost of the thermal input [7]. Consequently, the low conversion efficiency is not a serious drawback. Two methods are available for converting low temperature heat into electricity-orc engines and TEG [8]. The ORC engine, although more efficient, has a number of moving components and a diaphragm which can be unreliable. Also, the threshold in temperature required for the ORC engine to operate makes it a poor candidate for power generation at low temperatures (<50 0 C). TEGs don t have any threshold to operate and it can operate even at very low temperature differences. As the SGSP at RMIT University experiences temperature fluctuations in the winter and summer, TEG promises a more viable solution to power generation as compared to the ORC engines. TEGs will not only operate at low temperature differences, with no moving parts, it can also be customised to meet any power requirement. Fig. 1: Temperature profile of the SGSP located at RMIT University. Fig. 1 shows the temperature profile SGSP at RMIT University for 19 months. The size of the solar pond is 50m 2. Due to the salinity gradient established in the pond, heat is trapped at the bottom of the pond due to the absence of convection current in the non convecting zone (NCZ). The temperature at the bottom of the pond is about 63 0 C and the top surface of the pond is only at 26 0 C. This enables the SGSP to provide both hot and cold water needed to produce electricity from the thermoelectric generator. Laboratory Setup and Testing Fig. 2 below shows the experimental setup used for this paper. 16 TEGs were attached to a 50mm square channel with a high thermal conductivity paste. Each side of the square channel were attached with 4 TEGs. Each TEG measured to be 40 X 40 mm and 3.8mm thick. All 16 TEGs are then connected in series instead of parallel to obtain the maximum electrical power. The square

3 Applied Mechanics and Materials Vol channel with TEGs is then suspended in a hot water urn. The hot water urn is then filled up with water until all 16 TEGs are submerged in the water. An electrical resistance heater was attached to a thermocouple sensor, which was dipped in the hot water urn. This thermocouple can be set to the required temperature and this allows the control of the hot water temperature in the hot water urn. Cold water is supplied by a tap located at the Renewable Energy Lab, RMIT University and the temperature was recorded at 25 0 C throughout the experiment. An electronic load was connected to the TEGs to obtain the maximum power by varying the resistance of the electronic load. Maximum power was obtained when the resistance of the electronic load matches the total resistance of the TEGs. All data were recorded by a data acquisition system and stored in the computer used with the experiment. As the temperature at the bottom of the solar pond can reach up to 90 0 C, the experiment was conducted with hot water at 40 0 C to 99 0 C, so that this temperature profile fits within the temperature profile of the LCZ zone of the solar pond at RMIT University. Cold inlet water Cold water outlet Hot water urn Electronic load Computer Data acquisition system Temperature sensor Fig.2 : Experimental setup for laboratory experiment. Results and Discussions Fig. 3 shows the graph for output power, P out in Watts [W] versus output voltage, V out [V]. As the temperature difference between the hot fluid and cold fluid increases, the amount of power generated also increases. The power curve in Fig. 3 was obtained by varying the resistance from the electronic load connected to the experimental setup. Maximum power was obtained when the resistance of the electrical load applied matched the resistance of the 16 TEGs connected in series used in the experimental setup. For hot water temperature of 40 0 C, 50 0 C, 60 0 C, 70 0 C, 80 0 C, 90 0 C and 99 0 C, the maximum power obtained was 0.15 W, 0.34W, 0.53W, 0.76W, 1.08W, 1.55W and 2.47W respectively. The cold water temperature at inlet for cooling was constant at 25 0 C. All maximum output was recorded at external load resistance of ohms, [Ω]. This indicated that the total resistance of the 16 TEGs connected in series was 27.34ohms, [Ω].

4 812 Advances in Manufacturing and Mechanical Engineering Fig. 3: Output power, P out in Watts [W] versus output voltage, V out [V]. Fig. 4 shows the plot of output current, I out [A] versus output voltage, V out [V]. The plot was obtained for hot water temperature of 40 0 C, 50 0 C, 60 0 C, 70 0 C, 80 0 C, 90 0 C and 99 0 C. The cold side temperature was maintained at 25 0 C. From figure 5, the relationship between output current, I out [A] and output voltage, V out [V] is linear. As the temperature difference between the hot and cold side of the fluid increases, the output current, I out [A] and output voltage, V out [V] increases proportionally. This is due to the higher amount of heat transfer for the higher temperature difference that results in increase of electrical power generated by the TEGs. Fig. 4: Output current, Iout [A] versus output voltage, Vout [V]. Fig. 5 below shows the relationship between the open circuit voltage, V oc [V] and hot water temperature, for the experimental setup. The relationship between open circuit voltage, V oc [V] and hot water temperature is linear. This shows that the open circuit voltage, V oc [V] will vary proportionally with the hot water temperature. The linear equation, obtained from the results of the experiment from fig. 5 is shown below. V oc = ΔT (1)

5 Applied Mechanics and Materials Vol Fig. 5: Open circuit voltage, V oc [V] versus hot water temperature, T [ 0 C]. The highest efficiency recorded was at 0.97% when the temperature was 99 0 C for the hot water in the hot water urn. The formula to calculate the efficiency is given below: η = P out / P in = [ x c p x (T out T in )+P TEG ] / (VI) heater (2) The input power of the heater is given by the voltage (V) and the current (I) applied for the different set of heating. The power output is calculated from the mass flow rate of cooling water ( ) with specific heat capacity (c p ) and the temperature difference between the cold water at inlet (T in ) and cold water at outlet (T out ). The output power also includes the power generated by the TEGs (P TEG ). The efficiency recorded here is low ( 1%) was due to the low grade heat (< C) used for the experiment. The maximum efficiency that can be obtained by the TEG is around 4 % when the hot side of the TEG is exposed to a temperature of C. For this experiment, the efficiency recorded was expected as the category of heat used was low grade heat. This experiment proves that TEG system is viable to be used to convert the low grade heat into electrical energy. Maximum power produced by the 16 TEGs used was at 2.47W. This output in power can be increased by increasing the number of TEGs used. Power generation can be maximised by increasing the number of TEGs. As the solar pond is a thermal storage for low grade heat, TEG system can be used to harness the solar energy continuously for electricity generation. The low efficiency of the system will not be a major hindrance for the system as the energy input in the form of solar energy is free. Conclusion From the results, it is shown that the TEGs are capable of producing power using the heat from SGSP. The amount of power produced is linear to the temperature difference across the TEG. The designed experimental rig was able to provide maximum power of 2.47 W which was obtained at 8.78 V and 0.28 A. In this case, the open-circuit voltage and the short circuit current values of 17.80V and 0.54 A respectively were obtained. The proposed system will be most suitable for small-scale applications of solar ponds for power generation.

6 814 Advances in Manufacturing and Mechanical Engineering References [1] A.A. El-Sebaii 1, M.R.I. Ramadan, S. Aboul-Enein, A.M. Khallaf, History of the solar ponds: A review study Renewable and Sustainable Energy Reviews, 15, (2011) [2] H.Tabor, Solar Ponds Solar Energy, 27 (1981) [3] Wafik A. Kamal, Solar pond literature analysis Energy Convers. Mgmt,32, (1991) [4] Randeep Singh, Sura Tundeeb, Aliakbar Akbarzadeh, Electric power generation from solar pond using combined thermosyphon and thermoelectric modules, Solar Energy, Volume 85, 2, (2011) [5] D.M.Rowe & Gao Min, Evaluation of Thermoelectric Modules for Power generation, Journal of Power Sources, 73(2), (1998) [6] Hongxia Xia, Lingai Luo and Gilles Fraisse, Development and Applications Of Solar-Based Thermoelectric Technologie, Renewable And Sustainable Energy Reviews, 11 (2007) [7] G.F. Rinalde, L.E. Juanico, E. Taglialavore,S. Gortari, M.G. Molina, Development Of Thermoelectric Generators For Electrification Of Isolated Rural Home, International Journal Of Hydrogen Energy, 35,( 2010) [8] Bertrand F. Tchanche, Gr. Lambrinos, A. Frangoudakis, G. Papadakis, Low-Grade Heat Conversion Into Power Using Organic Rankine Cycles A Review of Various Applications, Renewable and Sustainable Energy Reviews. 15 (2011)

7 Advances in Manufacturing and Mechanical Engineering / Experimental Investigation of Power Generation from Salinity Gradient Solar Pond Using Thermoelectric Generators for Renewable Energy Application / DOI References [1] A.A. El-Sebaii 1, M.R.I. Ramadan, S. Aboul-Enein, A.M. Khallaf, History of the solar ponds: A review study, Renewable and Sustainable Energy Reviews, 15, (2011) /j.rser [8] Bertrand F. Tchanche, Gr. Lambrinos, A. Frangoudakis, G. Papadakis, Low-Grade Heat Conversion Into Power Using Organic Rankine Cycles A Review of Various Applications, Renewable and Sustainable Energy Reviews. 15 (2011) /j.rser