Combined Thermoelectric Power Generation and ... - ScienceDirect

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ScienceDirect Energy Procedia 110 (2017) 262 – 267

1st International Conference on Energy and Power, ICEP2016, 14-16 December 2016, RMIT University, Melbourne, Australia

Combined thermoelectric power generation and passive vacuum desalination Sri Nag Mynenia, Abhijit Datea,*, Matthew Warda, Pritesh Gokhaleb, Matthew Gaya b

a School of Engineering, RMIT University, Melbourne 3000, Australia Faculty of Mechanical Engineering, Technische Universität Chemnitz, Germany

Abstract This paper proposes a passive water desalination system combined with thermoelectric power generation. Desalination is achieved by reducing the saturation temperature of saline water through vacuum. The vacuum is generated using 5 m of negative head (drop), which significantly reduces the boiling temperature of water. The thermoelectric generator unit (TEGU) uses four thermoelectric cells to generate electricity using Seebeck effect. The cold side temperature of TEGU is maintained by the reduced saturation temperature of saline water, and temperature for hot side can be provided by low grade heat source, e.g. solar, waste heat. The heat transfer through the system is a transient to steady state conduction, and theoretical calculations show that an absolute pressure of 50 kPa can be achieved in the system which reduces the saturation temperature to around 80 °C. Thermal resistance and heat flow networks are used to validate fresh water output, evaporation, and condensation rates based on input heat. The objective is to achieve proof of concept through experimental model, and thermal analysis. Further research is directed into elimination of noncondensable gases in the system and use of passive condensation techniques to reduce energy input. © Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license © 2017 2017The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. Keywords: Passive Vacuum Desalination; Thermoelectric Generator Unit; Seebeck effect; Saturation temperature; Low grade heat; Negative head; Sensible heat; Latent heat; Vaporization.

1. Introduction The increasing scarcity of fresh water is becoming a primary concern for most of the nations on the planet. At the current growth rate, the population on the planet is estimated to be 8.9 billion by 2050, and the critical level of water required for basic human needs is approximately at 1000 m3/capita, annually, according to Date, A. et. al [1]. The fresh water currently available on the planet accounts for only 3%, while the remaining 97% is sea-water. Nearly 700

* Corresponding author. Tel.: +61 3 9925 0612. E-mail address: [email protected]

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. doi:10.1016/j.egypro.2017.03.137

Sri Nag Myneni et al. / Energy Procedia 110 (2017) 262 – 267

million people, across 39 countries, lack access to fresh water, and this figure is forecasted to rise to 1.9 billion by 2050 according to Date, A. et. al [1]. The primary sources for 3% of fresh water are glaciers, rivers, lakes, groundwater, and rainfall. In dry countries such as Israel, Saudi Arabia, Australia, and parts of Africa, where the sources of fresh water are not available, the focus is on desalinating sea water. Therefore, desalination technology is vital for locations with fresh water shortages. This paper focuses on theoretical and experimental analysis of a desalination system that operates through passively created vacuum using 5 m negative head (drop). The system also incorporates a thermoelectric Generator Unit (TEGU) consisting of four thermoelectric cells and heat spreaders to generate electricity. Through this paper, a brief evaluation of existing technologies, design of proposed system, theoretical estimation, and experimental results are presented. The aim of the design is to generate a proof of concept with focus on passive vacuum generation, efficiency of TEG heat spreaders, evaporation of saline water at reduced saturation temperature, and fresh water output. Although the overall efficiency of the system is expected to be low compared to other desalination systems, significant reduction in energy input makes the system economical, and sustainable Nomenclature T R ܳሶ TEGU HE HS sw fw Ves cond amb sen lat TE

temperature (oC) thermal resistance (oC/W) rate of thermal energy (W) thermoelectric generator unit heating element heat spreader saline water fresh water vessel condenser ambient sensible heat latent heat thermoelectric cell

2. Overview of existing technology Until today, most of the operating desalination plants around the world utilize Reverse Osmosis (RO) technology which consumes large amount of electrical energy to force the sea-water across membranes. Chennan Li, Yogi Goswami, and Elias Stefanakos [2], reviews current desalination technologies which can utilize solar energy to reduce the dependence on electricity generated by fossil fuels. According to Chennan Li, et.al. [2], solar energy harvesting system is integrated to existing desalination processes such as Multi-Stage Flash(MSF), Electro-dialysis (ED), Heatpump desalination, Solar Stills, and so on, as a hybrid solution to reduce the energy consumption. But, the integrated hybrid systems still prove to be expensive, with the operation, and output of the plant depending on location and climate. According to Gude, V.G. and N. Nirmalakhandan [3], current desalination technologies either require high quality energy from burning fossil fuels or have limitations such as low efficiency, and low solar yield based on geographical location. The use of negative head to create a passive vacuum was tested in the experiments conducted by Al-Kharabsheh, and D. Yogi Goswami [4, 5]. The experiment uses a drop of 10 m to generate absolute pressure of 3.7 kPa. The prototype desalination system of Veera Gnaneswar Gude and Nagamany Nirmalakhandan [3], uses similar principle to create a vacuum, which enables desalination at low pressure and low temperature, so that the system is suitable to use low grade waste heat. The system uses a 10 m column of water to achieve a constant vacuum of 8.6 kPa which desalinates saline water at a low temperature of 50oC. The system was able to achieve the highest fresh water output of 6 liters per day. In the desalination system designed by G. Venkatesan, S.Iniyan, and Purnima Jalihal [6], vacuum in the evaporator chamber is generated using negative head, and the industrial waste sea water at high temperature is flashed in the evaporation chamber due to reduced saturation temperature. According to theoretical and experimental simulations of Mohammad Abutayeh, D. Yogi Goswami, and Elias K. Stefanakos [7], the desalination system was able to achieve 35 kPa of vacuum using a negative head of 10 m.

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3. Proposed system The proposed system in Fig. 1., developed using CATIA V5 software, consists of a steel evaporator section integrated with the PVC system of steam passage and condenser built by Ward, M. The Thermoelectric Generator Unit (TEGU) consists of lower heat spreader (hot side), upper heat spreader (cold side), and four thermoelectric cells connected in series and sandwiched between the heat spreaders.

Fig. 1. Proposed system developed using CATIA V5 software

The drain pipe used to create 5 m negative head is placed under the condenser where fresh water is collected. The heat input which is low grade waste heat is simulated using cartridge heating elements. Theoretically, the 5 m negative head creates a vacuum of 50 kPa in the system. Due to reduced pressure, the saturation temperature of the saline water in the evaporator decreases to 80 0C. Heat is transferred through TEGU to the saline water body. As the steady state heat transfer transitions to transient state heat transfer, nucleate boiling causes evaporation of saline water which generates fresh water vapor, as the brine gets deposited at the bottom of the evaporator. The fresh water vapor is condensed by copper condenser coil, and the fresh water generated is drained down to collection point to sustain the passive vacuum and, thereby making the system a continuous process. In TEGU, the upper heat spreader (cold side) is maintained at 80 0C due to contact with saline water body. The lower heat spreader (hot side) is maintained between 150 0C to 200 0C by varying heat input from the heating elements. The temperature difference across the thermoelectric cells generates voltage using the principle of Seebeck effect. 4. Theoretical model The theoretical model for the proposed system is developed using thermal resistance and heat transfer networks which are developed in light of research done by Date, A. et.al [1].


Tamb

Rcoolant Tcond,out Tboil Tamb

Tcond,in (RVessel)Loss

RVapour

Rcond

RSW (RUpper,

HS)Loss

Tamb

TUpper,HS RUpper, HS TTEGU,Cold RTEGU (RLower,

HS)Loss

Tamb

TTEGU , Hot RLower, HS

(THE)Total

Fig. 2. Thermal resistance network (left) and heat flow network (right)

Based on the networks in Fig. 2, the amount of heat transferred to saline water from the upper heat spreader, performance of TEGU, mass flow rate of vapour, heat rejected by the condenser, and mass flow rate of fresh water are calculated using the thermal resistance and heat flow formulas on Microsoft Excel. Heat losses from TEGU and the overall heat loss from the vessel are neglected in the theoretical calculations. 5. Experimental setup

Fig. 3. Vessel with insulation on evaporator (left), drain pipe to create 5 m negative head (center), data logger, power supply and condenser feed water (top right), TEGU with heating elements (bottom right)

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6. Results and discussion Three experiments were conducted with varying heat input. First experimental run with 75 W, second with 100 W, and third with 150 W. In each experiment, the amount of time taken to reach steady state boiling from transient boiling varied based on amount of heat input. Therefore, the experimental data was analysed for one hour of steady state boiling in each experiment. The data for experiemtal run with 100 W heat input is used below for discussion. However, the key output indicators are tabulated for all the three experiments.

Temperature (°C)

Vessel Temperature Profile (100W) 140 120 100 80 60 40 20 0 00:00

Cold side (TEGU) Hot side (TEGU) Saline vapour Condenser vapour cavity Saline water Condenser coil Fresh water Condenser inlet

00:14

00:28

00:43

00:57

01:12

01:26

Condenser outlet

Elapsed Time (HH:MM) Fig. 4. Temperature profile of the vessel with 100 W heat input

In Fig. 4, since the condenser temperatures, and fresh water temperature are overlapping, they are depicted in a separate graph in Fig. 5 below.

Condenser Temperature Profile (100W) Temperature (°C)

30.0 Condenser coil

25.0

Condenser inlet Condenser outlet

20.0 15.0 00:00

Fresh water

00:14

00:28

00:43

00:57

01:12

01:26

Elapsed Time (HH:MM) Fig. 5. Temperature profile of the condenser with 100 W heat input Table 1. Key output indicators of the three experiments for one hour of steady state heat transfer Heat input (W) Saline water input (ml) Fresh water output (ml) Temperature difference in TEGU (0C) Voltage in TEGU (V) Power generated by TEGU (W)

75 W 500 120 13 1.67 0.56

100 W 500 250 18.5 2.19 0.73

150 W 500 310 27 3.21 0.94

In Fig. 5, the condenser inlet, outlet and coil temperatures are similar throughout the experiment which indicates the poor heat transfer and heat rejection capacity of the condenser. Due to this, the rate of evaporation of saline water is much greater than the rate of condensation. During the experiments, it was observed that the gauge pressure of the


system gradually decreased to nearly 40 kPa from 50 kPa at the end of the steady state conditions. This is a result of higher rate of evaporation which increases the vapor pressure in the system, and thereby reducing the level of vacuum. The reduced vacuum in the system increases the saturation temperature of the saline water. The rise of saturation temperature of the water requires high amount of excess heat to initiate nucleate boiling. This increase in amount of excess heat can be seen in Fig. 4, where the cold side of TEGU is higher than the saline water temperature. Theoretically, both these temperatures should be nearly equal. The amount of excess heat required to cause nucleate boiling is proportional to the amount of heat input. Therefore, the amount of heat input into the system can be significantly reduced by minimizing the amount of excess heat. In order to reduce the amount of excess heat in the proposed system, it was clear from the experiments that the condenser needs to redesigned and increased in size for better heat rejection capacity. From Fig. 5, it can be observed that the temperature of condenser is rising during steady state. This is primarily due to circulation of condenser feed water from same container. During experiments, ice was periodically added to the feed water container to maintain steady temperature, but in order to improve the performance of the condenser, the input feed water should be separated from the outlet. Another reason for the rise in amount of excess heat can be attributed to heat losses from the TEGU and the adapter ring connecting the steel evaporator section to the PVC steam passage vessel. In Fig. 3, it can be seen that the TEGU lacks proper insulation. The overall insulation of the system, especially TEGU, needs significant improvement to minimize heat loss, and also to improve the power output from TEGU by maintaining a higher temperature difference between the cold and hot sides. The high rate of vapor generation caused film condensation on the condenser coil which further deteriorated the amount of heat transferred to the condenser feed water. The film condensation can be eliminated by making the surface of the condenser coil hydrophobic. There were also fluctuations observed in the measurement of fresh water levels caused mainly due to rise in gauge pressure of the system which could potentially cause errors in the measured fresh water output. 7. Conclusion Although the experiments performed on the proposed system successfully generated a proof of concept, major improvements to the system needs to be devised for improving the fresh water output, and power output from TEGU. The next iteration of the proposed system will be focused significantly on improving the condenser performance. References [1]Date, A., L. Gauci, and R. Chan, Performance review of a novel combined thermoelectric power generation and water desalination system. Solar Energy. 2015; 83, 256–269. [2] Li, C., Y. Goswami, and E. Stefanakos, Solar assisted sea water desalination: A review. Renewable and Sustainable Energy Reviews. 2013; 19: p. 136-163. [3] Gude, V.G. and N. Nirmalakhandan, Desalination at low temperatures and low pressures. Desalination. 2009; 244(1–3): p. 239-247. [4] Al-Kharabsheh, S. and D.Y. Goswami, Experimental study of an innovative solar water desalination system utilizing a passive vacuum technique. Solar Energy. 2003; 75(5): p. 395-401. [5] Al-Kharabsheh, S., Yogi, and D. Goswami, Analysis of an innovative water desalination system using low-grade solar heat. Desalination. 2003; 156(1–3): p. 323-332. [6] Venkatesan, G., S. Iniyan, and P. Jalihal, A desalination method utilising low-grade waste heat energy. Desalination and Water Treatment. 2015; 56(8): p. 2037-2045. [7] Abutayeh, M., D. Yogi Goswami, and E.K. Stefanakos, Theoretical and Experimental Simulation of Passive Vacuum Solar Flash Desalination. Journal of Solar Energy Engineering. 2013; 135(2): p. 021013-021013.

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