Performance Improvement of High-temperature ... - Science Direct

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ScienceDirect Energy Procedia 105 (2017) 1211 – 1218

The 8th International Conference on Applied Energy – ICAE2016

Performance improvement of high-temperature silicone oil based thermoelectric generator Tongcai Wanga, Tongjun Liua, Weiling Luana,*, Shan-Tung Tua, Jinyue Yanb,c a

Key Laboratory of Pressure Systems and Safety (MOE), School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China b School of Business, Society & Engineering, Mälardalen University, SE-72123 Västerås, Sweden c School of Chemical Science, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden

Abstract The recent advances in waste heat recovery technologies have provided great opportunities for energy conversion efficiency improvement. This paper proposed a metal foam filled thermoelectric generator (TEG) for the utilization of liquid waste heat resource. A prototype was designed and constructed to study the performance enhancement due to metal foam inserts. High-temperature oil based experiment was conducted to investigate the TEG performance in higher liquid temperature. The influences of hot oil inlet temperature and cold water flow rate were proved to be key operating parameters for the TEG performance. Specially, net power output and net power enhancement ratio were presented to assess the overall net power output performance. The metal foam filled TEG was demonstrated to outperform the unfilled TEG both in power generation efficiency and net power performance. In the experiments, the maximum power generation efficiency and net power enhancement ratio of metal foam inserted TEG were 2.49% and 1.33, respectively. © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. Keywords: waste heat recovery, thermoelectric generator, heat exchanger, metal foam, high-temperature oil

1. Introduction Recently, the rising energy costs and strict greenhouse gas emission policies are promoting energy utilization in more efficient ways. Energy conversion efficiency improvement and renewable energy implementation are regarded as two of the most promising solutions for current energy problems. Although renewable energy capacity continues to rise, fossil fuels are still the dominant resource of

* Corresponding author. Tel.: +86-21-64253513; fax: +86-21-64253513. E-mail address: [email protected]

1876-6102 © 2017 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 scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.416

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Tongcai Wang et al. / Energy Procedia 105 (2017) 1211 – 1218

today`s energy consumption. In the traditional fossil fuel heat utilization process, about 20-50% of industrial energy consumption is wasted in the form of hot exhaust gases, cooling water, dissipated heat and heated products [1]. Due to recent advances in waste heat recovery (WHR) technologies, energy conversion efficiency improvement by recovering waste heat from the industrial process has attracted a lot of attentions. However, one of the greatest barriers to the mass WHR is the technical difficulty in recovering the low temperature waste heat (below 230 oC), which actually accounts for the majority of the industrial waste heat [2]. Thermoelectric generator (TEG) is a promising low temperature WHR technology, due to the ability of directly utilizing the temperature difference between waste heat and the environment for thermoelectric power generation [3]. Moreover, its merits, such as simplicity, compactness and applicable temperature universality, make TEG especially suitable for low temperature WHR. Currently, most of the TEG systems for low temperature WHR are operated below thermal-toelectricity efficiency of 3% [4-7]. Aranguren et al. [4] built a TEG prototype with 48 TEMs and two different kinds of heat exchangers (finned heat sinks and heat pipes). Power output of 21.56 W and average efficiency of 2.2% were achieved. Merotto et al. [5] integrated the conventional TEMs with a novel catalytic combustor fuelled with propane/air mixture. This system was able to produce 9.86 W of electrical power with a thermal to electrical conversion efficiency of 2.85%. Remeli et al [6] set up a heat pipes assisted TEG system for low temperature WHR. A conversion efficiency of 2.02% at a temperature difference of 86.7 oC and the approximate heat exchange effectiveness of 0.48 were acquired. Recent advances in nanotechnology have enabled great improvement of thermoelectric material performance that might achieve much higher conversion efficiencies [8-10]. In addition, lots of efforts have been put on the TEG system optimization researches. Among the TEG system optimization cases, heat transfer enhancement through flow channel inserts is an effective method. Amaral et al. [11] investigated the thermoelectric power gain of a kind of TEG with different panel inserts at fixed thermal input conditions. The net power, which was proposed to evaluate power enhancement of the panel inserts, came out to be higher than the unfilled TEG in the liquid-to-liquid situation. Similarly, Lesage et al. [12] studied the performance enhancement effect of spiral and panel flow channel inserts on a liquid-to-liquid TEG. It is founded the panel inserts performed better than the spiral inserts. Metal foams, which were proved to be excellent heat transfer enhancement inserts in many other studies [13-15], are supposed to result in obvious performance enhancement for TEG. Lu et al. [16] conducted a simulation to compare the heat transfer enhancements of metal foams and offset-strip fins for an exhaust-based thermoelectric generator. The use of metal foams was demonstrated to outperform the offset-strip in both the total power output and the conversion efficiency at the expense of higher pressure drop. Unlike TEG using waste heat in gas form, liquid waste heat based TEG with flow channel inserts was demonstrated to produce smaller pressure drop [17]. Currently, most of the liquid-to-liquid TEG prototypes used hot water as the waste heat source [17-19]. However, the low temperature of hot water has limited the thermoelectric power generation ability, which is closed related with the temperature difference of the heat exchange fluid. Considering the adverse pressure drop problem of metal foams filled gas-to-liquid TEG, this paper proposes a liquid-to-liquid TEG prototype for low temperature WHR. To achieve larger temperature difference, high-temperature silicon oil was adopted in the experiment. Metal foams (MF) of small pore density were filled in the flow channels to enable thermoelectric generation enhancement at a relative low pressure drop. Specially, net power output and net power enhancement ratio was presented to assess the influence of metal foams on the TEG net power generation performance. This kind of high-temperature silicon oil based TEG is supposed to outperform gas-to-liquid TEG both in the total power output and net power enhancement. 2. Methodology

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2.1. Power generation efficiency Thermoelectric power generation efficiency ( KTEG ) is used to evaluate the electricity generation ability of TEG for WHR. It is defined as the ratio of thermoelectric power output ( PTEG ) to the heat transfer rate through TEM hot side ( Qh ):

KTEG

PTEG / Qh # PTEG ˄ / Qc PTEG˅

(1)

where Qc is the heat transfer rate of the cold water and can be calculated as follows:

Qc

U cVc cpc (Tco Tci )

(2) in which, U c , Vc , cpc , Tci and Tco are cold water density, cold water volume flow rate, water specific heat at constant pressure, cold water inlet and outlet temperature, respectively. 2.2. Net power output and net power enhancement ratio Considering the pressure drop problems caused by metal foam inserts, net power output (Pnet) and net power enhancement ratio (Eh) ware put forward to assess the net energy benefit of the high-temperature liquid-to-liquid TEG. The net power output equals to thermoelectric power output minus the work loss due pressure drop. It is formulized as,

Pnet

PTEG W

(3)

where W is the work loss due to pressure drop. It is calculated as,

W

'phVh 'pcVc

(4) in which ' ph and ' pc is the pressure drop of hot oil and cold water, respectively. Net power enhancement ratio (Enet) is presented to evaluate the net power performance enhancement of MF-filled TEG. It is defined as, Enet

PMF / PUF

(5)

where PMF is the net power output of MF-filled TEG, PUF is the net power output of unfilled TEG. 3. Experimental setup (b)

Fig. 1 (a) schematic diagram of the experimental TEG system; (b) schematic of the TEG prototype

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An experimental system was built to investigate the performance of high-temperature silicone oil based TEG system and evaluate the influence of the metal foams inserts. The schematic diagram of the experimental TEG system is illustrated in Fig. 1a. This system was comprised of three layers of plate heat exchanger (HE), two layers of thermoelectric modules (TEMs), a hot oil flow loop, a cold water channel and a series of testing equipment. It used high-temperature silicone oil (Huber SilOil M20.196/235.20) and cold water as heat exchange fluid. The hot oil circulation was performed by Huber unistat cc400-3 oil pump. This set of hot oil supply system could heat the hot oil as high as 235 oC. As is illustrated in the diagram, hot oil flowed through the middle HE layer and then went back into the oil pump. The flow rate of hot oil was controlled by flow valve and measured by volumetric method. In terms of cold water flow channel, the volume flow rate was controlled by flow meters with accuracy of ±1.5% according to the manufacturer. Cold water flowed through the connected bottom and top HE layers. Pressure drop of both the hot oil and cold water was measured by differential pressure gauges with accuracy of ±2%, respectively. The temperature of hot air and cold water were measured by embedded K-type thermocouples within the accuracy of ±0.75%. PT100 within accuracy of ±0.15 oC were mounted to test the side surface temperature of the every HE layers. The voltage and current were collected by data acquisition system (Agilent 34970A) with the accuracy of ±0.005% and ±0.055%, respectively. The detailed structure of TEG prototype is sketched in Fig. 1b. This TEG prototype was constructed by plate HE and TEMs. The plate HE flow channels were made of thin steel plates with the thickness of 1.5 mm. The dimensions of each flow channel were 200 mm × 83 mm × 13 mm. Open-cell copper metal foams with the porosity of 0.96 and pore density of 5 PPI was adopted to enhance the TEG performance. In order to fill metal foams into the flow channels, one side of the channels was sealed with heat-resistant gasket and flanges. The middle layer of HE was used as hot oil flow channel. Correspondingly, the top and bottom layers were connected and used as cold water flow channels. Two layers of Bi2Te3 TEMs (TEP-1-142T300) were sandwiched between hot and cold flow channels. All the TEMs were connected in series. Silicone grease was spread between TEMs and HE layers to reduce the thermal resistance. Four mechanical clamps were employed to fasten the TEG. The dimensions of the assembled TEG were about 200 mm × 83 mm × 77 mm. To maximize the heat loss, insulated aerogel felt was wrapped around the assembled TEG. 4. Results and Discussions 4.1. System performance of hot oil based TEG prototype

Fig. 2. Variation of hot oil and cold water inlet and outlet temperature of MF-filled TEG


The system performance was first tested at a series of hot oil inlet temperature, when the volume flow rates of hot oil and cold water were fixed at 16.36 L/h and 12 L/h, respectively. Fig. 2 shows the temperature variation of hot oil inlet, hot oil outlet, cold water inlet and cold water outlet of the MF-filled TEG. The hot oil temperature was controlled in a ladder rising trend from room temperature to 188 oC. The maximum temperature difference of hot oil inlet and cold water inlet was as high as 165 oC at the highest hot oil inlet temperature. At each given temperature, the system was tested for about 10 minutes in a stable working condition. It can be observed that the hot oil temperature quickly rose to the given temperature. Moreover, the hot oil outlet temperature and cold water outlet temperature varied correspondingly to the variation of hot oil temperature. The performance stability of the experimental prototype is demonstrated. 4.2. Influence of hot oil inlet temperature (a)

(b)

Fig. 3. Open circuit voltages (a) and power output (b) as a function of hot oil inlet temperature

In order to investigate the influence of thermal input condition, the inlet temperature of hot oil was changed at a stable flow rate of 16.36 L/h. In this experiment, the cold water flow rate was fixed at 12 L/h. Fig. 3a indicates the open circuit voltage relation with hot oil inlet temperature. It was shown that the open circuit voltages increased with the raise of hot oil inlet temperature. Unlike the hot water based TEG system, the silicone oil can be heated to much higher than 100 oC. The MF-filled TEG outperformed unfilled TEG in open circuit voltage performance for the high-temperature oil-to-water heat exchange. When the hot oil inlet temperature was set at 188 oC, the maximum open circuit voltages of MF-filled and unfiled TEGs went up to 35.9 V and 31.2 V, respectively. Thus, increase the hot fluid temperature was an effective way to improve the performance of TEG. Based on the circuit theory, the maximum output power can be achieved when the load resistance is equal to TEM resistance. Fig. 3b shows the power output relation with hot oil inlet temperature. It is indicated that the increase of hot oil inlet temperature enhanced the power output in an exponential way. At the maximum hot oil inlet temperature, the maximum power output of MF-filled and unfilled TEG were 8.2W and 6.2 W, respectively. The application of metal foams brought about obvious power output promotion for the high-temperature oil based TEG. 4.3. Influence of cold water flow rate The influence of cold water flow rate on the TEG performance was investigated at fixed thermal input condition. The hot oil inlet temperature and flow rate were set at 188 oC and 16.36 L/h for the fixed thermal input condition. Fig. 4a describes the influence of cold water flow rate on the TEG open circuit

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voltages. The metal foam inserts was demonstrated to substantially improve the TEG open circuit voltages in all the cold water flow rate range. Unlike the obvious influence of hot oil inlet temperature, increasing cold water flow rate only slightly changed the open circuit voltages. By rising cold water flow rate from 8 L/h to 25 L/h, the open circuit voltages of MF-filled and unfilled TEGs increased slowly from 34.6 V and 30.0 V to 39.3 V and 34.7 V, respectively. (b)

(a)

Fig. 4. Open circuit voltages (a) and power output (b) versus cold water flow rate

The relation between power output and cold water flow rate is shown in Fig. 4b. Similar with the open circuit variation trends, the power output went up slightly with the growth of cold water flow rate. The maximum power output of MF-filled and unfilled TEGs were 9.8 W and 7.7 W, respectively. 27.3% power output enhancement was exhibited by filling metal foams in the high-temperature oil-to-water TEG. 4.4. Power generation efficiency and net power output performance Fig. 5a shows the power generation efficiency variation with the changing of cold water flow rate. It is obvious that metal foam inserts brought about distinct power generation efficiency improvement for the high-temperature oil-to-water TEG. The improved efficiency indicated that MF-filled TEG can generate more thermoelectric power than the unfilled TEG at the same thermal input condition. In addition, increasing the cold water flow rate could further result in the power generation efficiency improvement. Taking the MF-filled TEG for example, raising the water flow rate from 8 L/h to 25 L/h caused the power generation efficiency increasing from 2.19% to 2.49%. (a)

(b)

Fig. 5. Power generation efficiency (a) and net power output performance (b) as a function of cold water flow rate


The net power output was used to evaluate net power generation benefit after subtracting work loss due to pressure drop. Moreover, net power enhancement ratio was proposed to compare the net power output performance of MF-filled and unfilled TEGs. Fig. 5b presents the net power output and net power enhancement ratio of the high-temperature oil-to-liquid TEG. It was demonstrated that MF-filled TEG outperformed the unfilled TEG not only in the power output performance but also in the net power output performance. For the liquid waste heat, work loss due to the implementation of metal foams in TEG was not a noticeable barrier. The net power output increased with the augment of cold water flow rates for both MF-filled and unfilled TEGs. Conversely, net power enhancement ratio dropped with the increase of cold water flow rate. With the increasing of cold water flow rate, the net power enhancement ratio decreased to 1.28. The maximum net power enhancement ratio of 1.33 was achieved when the cold water flow rate was set at 8 L/h, indicating a 33% enhancement of net power output due to the use of metal foam inserts. Thus, MF-filled TEG was demonstrated to be an effective method for recovery of hightemperature liquid waste heat. 5. Conclusions A metal foam filled TEG prototype was designed and constructed for WHR. The experiment is based on hot silicon oil to simulate the high-temperature liquid waste heat. The performance of MF-filled and unfilled TEGs were compared. The influences of hot oil inlet temperature and cold water flow rate were investigated to improve the operating parameters. In the experiments, the maximum power output of MFfilled and unfilled TEGs were 9.8 W and 7.7 W, respectively. Specially, net power output and net power enhancement ratio were proposed to evaluate the net power benefit of the MF-filled TEG. Metal foams were demonstrated to play an important role both in the power output enhancement and net power output enhancement. The maximum power generation efficiency of the MF-filled TEG was calculated to be 2.49%. Moreover, the MF-filled TEG could produce 33% net power improvement over the unfilled TEG. It is demonstrated that MF-filled TEG was a feasible way for high-temperature liquid waste heat recovery. Acknowledgements The authors gratefully acknowledge the financial support from the National Nature Science Foundation of China (51172072, 51475166) and National Basic Research Program of China (2013CB03550). Financial support from the program of China Scholarship Council is acknowledgment. References [1] Johnson I, Choate W T, Davidson A. Waste Heat Recovery: Technology and Opportunities in US Industry. BCS, Inc., Laurel, MD (United States), 2008. [2] Ziviani D, Beyene A, Venturini M. Advances and challenges in ORC systems modeling for low grade thermal energy recovery. Appl Energ 2014, 121: 79-95. [3] Rowe DM. CRC handbook of thermoelectrics. Boca Raton, FL: CRC Press; 1995. [4] Aranguren P, Astrain D, Rodríguez A, et al. Experimental investigation of the applicability of a thermoelectric generator to recover waste heat from a combustion chamber. Appl Energ 2015, 152: 121-130. [5] Merotto L, Fanciulli C, Dondè R, et al. Study of a thermoelectric generator based on a catalytic premixed meso-scale combustor. Appl Energ 2016, 162: 346-353. [6] Remeli MF, Tan L, Date A, et al. Simultaneous power generation and heat recovery using a heat pipe assisted thermoelectric generator system. Energ Convers Manage 2015; 95: 201-9.

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Biography Tongcai Wang is a Ph.D candidate in the School of Mechanical and Power Engineering, East China University of Science and Technology (ECUST). He got his bachelor degree from ECUST in 2011. His research interest is waste heat recovery through thermoelectric generator.