DIRECT POWER GENERATION FROM HEAT WITHOUT MECHANICAL WORK

Transcription

1 PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 11-13, 213 SGP-TR-198 DIRECT POWER GENERATION FROM HEAT WITHOUT MECHANICAL WORK Kewen Li, Changwei Liu, and Pingyun Chen China University of Geosciences, Beijing 29 Xueyuan Road, Beijing 183, China ABSTRACT Most of the current geothermal power generation technologies transfer thermal energy to mechanical work and then to electricity. In this study, a direct heat to electricity (DHE) technology, without going through mechanical energy, was used to harvest low enthalpy geothermal work. A power generation system has been built using thermoelectric generator (TEG) modules. Experiments have been conducted to measure the output power at different conditions such as different temperature differences. TEG modules manufactured with different materials have also been tested. The power generator had an installed power of 5 W at a temperature of around 2 o C. An output power of over W has been generated with a temperature difference of 8 o C. The power generated by the thermoelectric system is almost directly proportional to the temperature difference between the hot and the cold sides. The cost of the DHE power generator is close to that of photovoltaics (PV) in terms of equivalent energy generated. INTRODUCTION According to World Energy Assessment (WEA, 2), geothermal has the largest resource among all types of renewable energies. Among the vast number of geothermal resources, a great portion may be characteristic of low temperature (<15 o C). Most of the co-produced geothermal energy associated with oil and gas fields may be in the range of low temperature (Erdlac, 27; Li, et al., 27; Bennett, et al., 211; Xin, et al., 212). The frequently-used technology to generate electricity by using this type of low enthalpy geothermal energy is Organic Rankine Cycle (ORC) binary power generator. A noteworthy example is the 25 kw ORC plant in Chena Hot Springs, Alaska, which produces electricity from a very low temperature (74 C) geothermal resource (Erkan, et al., 27). Compared with solar and wind systems, geothermal energy has many advantages, including having weather resistance, good base load, great stability, and high thermal efficiency (for high temperature geothermal resource). However the fact is that the total capacity installed of geothermal power lags behind solar and wind. The installed power of PV and wind until 211 were 7 and 24 GW, respectively. According to GEA (Geothermal Energy Association), the total geothermal power installed in the world was about 11.2 GW until May 212. The average annual growth rate of geothermal power was about 2% while that of PV was about 58% during the same period of and up to 74% in 211 only (REN 21, 212). Li (213) discussed the reasons causing the low growth rate of geothermal energy. The main reasons may be high initial investment, long payback and construction time, difficulty to assess resource, and difficulty to modularize. Li (213) also pointed out the possible directions to speed up the growth of geothermal power. One of the solutions may be the large-scale utilization of TEG technology. Since 1821, many researches have investigated the application of thermoelectric materials. Thacher (27) developed a thermoelectric power generator using car exhaust heat. The maximum power output reached 255W. Hsu (211) developed a lowtemperature waste heat system to utilize the car exhaust heat. When the engine rate boosted to 35 RPM, 12.4 W of maximum power output was obtained at an average temperature difference of about 3 K. On the aspect of numerical modeling, Freunek, et al. (29) described a new analytical model for thermoelectric generators and found that the influence of the Peltier heat on the output power was about 4%. Eisenhut and Bitschi (26) derived an analytic model based on convective heat sources. Liu (212) presented the designs of electricity generators based on thermoelectric effects using heat resources

2 of small temperature difference. Karabetoglu, et al. (212) reported the method to characterize a thermoelectric generator at low temperatures. Suter, et al. (212) established a numerical model for a 1kWe thermoelectric stack for power generation, which may help define the configuration and operating parameter range that are optimal from a commercial standpoint. In this study, we built a power generation system by using TEG technology and conducted experiments to measure the output power at different temperature differences and various other conditions. We also tested TEG modules manufactured with different materials. The cost of the power generator using TEG technology was estimated and the results showed that TEG technology was competitive to PV technology. ADVANTAGES OF TEG TECHNOLOGY Thermoelectric generation technology, as one entirely solid-state energy conversion method, can directly transform thermal energy into electricity by using thermoelectric transformation materials. A thermoelectric power converter has no moving parts, and is compact, quiet, highly reliable and environmentally friendly. Therefore, the whole system can be simplified and operated over an extended period of time without maintenance. In addition, it has a wider choice of thermal sources. It can utilize both the high- and low-quality heat to generate power. The low-quality heat may not be utilized effectively by conventional methods such as ORC technology. Table 1 lists the size and the approximate cost of each module. Module 5 was the most expensive TEG and Module 4 was the cheapest one. Table 1: Property of different TEGs. Type Size(cm2) Cost(US$) Module 1 Module 2 Module 3 Module 4 Module We kept the temperature on the hot side at about 2 by using a digital thermostat oil bath and used the tap water as the liquid on the cold side with a temperature of about 2. The temperatures of both hot and cold sides were measured and the results are shown in Figure 2. The temperature was measured by using two micro-thermocouples with very thin tips. Obviously, the temperature on the hot side of the modules was stabilized at about 18 and that on the cold side at about 4. The increase in the temperature on the cold side from 2 to 4 was because of the heat conduction. The temperature difference was stabilized at around 14. The results illustrate that the test system for thermoelectric power generation was stable and ready for experiments cold side temperature POWER TESTS OF DIFFERENT TEG MODULES We tested five different modules with different semiconduct materials in order to find the TEG with the maximum output at a specific temperature difference. Figure 1 shows the schematic of the module tests. The TEG module was clamped tightly in between two containers, one was the hot side with a high temperature and another was the cold side with a low temperature. Temperayure/ 14 hot side temperature Time,S Figure 2: Temperatures on the hot and cold sides of the module With the stable temperature difference of 14, we measured the output power of the five different TEG modules. The results are shown Figure 3. Three out of the five thermoelectric modules generated power more than 4.5W: Modules 2, 3, and 4. Figure 1: Schematic Figure 1: Schematic of the module test. The power ratio (power generated by each TEG module divided by the cost) was calculated and the results are shown in Figure 4. Obviously, Module 4, with the cost of 3.4 dollar each, has the highest power-cost ratio.

3 Note that Module 5 with the highest cost generated less power and yielded the lowest power-cost ratio at a temperature difference of about Power,W 4 Module-1 3 Module-2 Module-3 2 Module-4 Module Time,S Figure 3: The power generated from different modules Table 2: Property of the heat-conducting media Thermal conductivity Medium Name Cost(US$) (W/m K) Silicone Film Graphite Sheets Silicone Grease We conducted the measurements of the output power for Module 4 when different heat-conducting media were used. The high temperature on the hot side was provided by a thermostatic heating station and the hot side temperature was kept at about 8. Tap water still served as the cold side. At the same time, we chose two different size modules for the test: 4mm 4mm and 5mm 5mm. The power test results are shown in Figure 5. The power generated is proportional to the module s area. In this study, the 4mm 4mm size modules were used for the experiments in next section Module-2.8 Module-3.6 Module-4 5cm 5cm Module cm 4cm Module-1 1 Power,W Power-Cost Ratio,W/$ Time,S Figure 4: The power-cost ratio of different TEG modules However, the above test results do not imply that Module 5 was without value because Module 5 was manufactured for operating temperatures as high as 3. We will measure the output power of Module 5 at higher temperature differences in the near future. POWER TESTS USING DIFFERENT HEATCONDUCTING MEDIA Heat-conducting medium between the ceramic plate and liquid block plays a very important role for the TEG systems. Both thermal conductivity and cost should be taken into account. In this study, we chose some commercially available media for the comparison test. The property data of the heatconducting media are shown in Table 2. NONE Grahite Sheet Silicone Film Different Thermal mediums Silicone Grease Figure 5: The effect of area and heat-conducting medium type on output power According to the results shown in Figure 5, the modules with silicon grease generated 1.424W and 2.146W for different size, respectively. The silicon grease with a lower thermal conductivity performs better than other media with a higher thermal conductivity. The reason might be due to the difficulty for the other two types of heat-conducting media to remove air between two plates. Note that the thermal conductivity of air is.23w/m K. On the other hand, silicone grease can adhere at the interface tightly, so it help the thermoelectric modules dissipate heat and generate more power. However, the silicone grease has an obvious disadvantage: it does not last a long enough time for a period of practical power plant life. Silicone film is not recommended because of its high cost. The graphite sheet may be the most suitable

4 medium due to the high thermal conductivity and low cost but the air gap weakens the heat-conducting performance. We used trace amount silica gel which can tolerate 2 to adhere graphite sheet at the module s surface. This method not only takes advantages of the graphite merit, but also avoids the air gap s negative effect. EXPERIMENTAL RESULTS AND DISCUSSION The thermoelectric power generation system was operated with 1 hot water and 2 tap water. The 1 hot water was supplied by a thermostatic water bath. The power generated by the system at a temperature difference of about 8 is shown in Figure 8. One can see that the power is stabilized at around W. 2 We designed a 5W TEG power generator after the above experimental study. Its schematic is shown in Figure Poewer,W EXPERIMENT SET-UP Time,s Figure 8: Power generated from the TEG power system vs. time As shown in Figure 9, ten 15W bulbs (direct current) were lit up. Figure 6: The Schematic of the thermoelectric power generation system We then built the 5W TEG power generator which was composed of 1 thermoelectric modules (Module 4). 13 liquid blocks (containers) and some connection boxes were also used in this system. Figure 7 shows the picture of the TEG power generator before wrapped by insulation material. Hot Liquid Inlet Thermoelectric Power Generation Cold Liquid Outlet Cold Liquid Inlet Hot Liquid Outlet Figure 9: The thermoelectric power generation system lighting up ten 15W bulbs We also measured the power output at different temperature differences using the TEG power generator. The purpose was to establish the relationship between power output and temperature difference d. The experimental results are shown in Figure 1. Figure 7: The thermoelectric power generation system before operating

5 the Los Angeles Basin". Geothermal Resources Council, (211) Eisenhut, C. and Bitschi, A.: Thermoelectric conversion system based on geothermal and solar heat, (IEEE),p ,26. Power,W y = x R² = Temperature Difference, 8 1 Figure 1: Relationship between power and temperature difference. One can see that the power output increases with the increase in temperature differences almost linearly. We can approximate the power output at a specific temperature difference. For example, a power output of 5 W will be reached at a temperature difference of about 2. Note that the slope of the power curve shown in Figure 1 increases with the increase in temperature difference. The relationship between power output and temperature difference looks like exponential, which is of great significance. We estimated the total cost of the DHE power generator and found that the cost is close to the PV s cost in terms of equivalent energy generated. Note that the values of the capacity factor for PV and DHE were considered. The capacity factor for PV is about 14% and that of DHE (or TEG technology for geothermal energy) is around 9%. CONCLUSIONS According to the current study, the following preliminary conclusions may be drawn: (1) A power generator has been built using TEG modules and tested. The power generator had a power of 5 W (predicted using experimental data). An output power of over W has been generated under a temperature difference of 8 (hot side temperature was about 1 and the cold side was 2 ). (2) The electricity generated by the thermoelectric system is almost directly proportional to the temperature difference between the hot and the cold sides. (3) The cost of the DHE power generator is close to the PV s cost in terms of equivalent energy generated (the capacity factor was considered). REFERENCES Bennett, K., Horne, R.N. and Li, K.: "Power Generation Potential from Coproduced Fluids in Erkan, K., Holdman, G., Blackwell, D., and Benoit, W.: Thermal Characteristics of the Chena Hot Springs Alaska Geothermal System, PROCEEDINGS, Thirty-Second Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 22-24, 27. Freunek, M., Mueller, M., Ungan, T., Walker, W. and Reindl, L.M.: New Physical Model for Thermoelectric Generators, Journal of electronic materials, Vol. 38 (29),p Hsu, C.T., Huang, G.Y., Chu, H.S., Yu, B. and Yao, D.J.: "Experiments and simulations on lowtemperature waste heat harvesting system by thermoelectric power generators". Applied Energy, Vol. 88 (211), p Karabetoglu, S., Sisman, A., Fatih Ozturk, Z. and Sahin, T.: "Characterization of a thermoelectric generator at low temperatures". Energy Conversion and Management, Vol. 62 (212), p Li, K.: Comparison of Geothermal with Solar and Wind Power Generation Systems, Proceedings, 38th Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February11-13, 213. Li, K., Zhang, L., Ma, Q., Liu, M., Ma, J., and Dong, F.: Low Temperature Geothermal Resources at Huabei Oilfield, China, GRC Trans. V. 31 (27). Liu, L.: "Large-scale Ocean-based or Geothermal Power Plants by Thermoelectric Effects" Renewables 212: Global Status Report, REN21 (Renewable Energy Policy Network for the 21st Century). Available at: Suter, C., Jovanovic, Z. and Steinfeld, A.: "A 1kW thermoelectric stack for geothermal power generation Modeling and geometrical optimization". Applied Energy, (212). Thacher, E., Helenbrook, B., Karri, M. and Richter, C.J.: "Testing of an automobile exhaust thermoelectric generator in a light truck". Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, Vol. 221 (27), p

6 WEA (2): World Energy Assessment Report: Energy and the Challenge of Sustainability. 5 p. Xin, S., Liang, H., Hu, B. and Li, K. :" Electric power gerneration from low temperature coproduced geothermal resources at HuaBei Oilfield" in Thrity-Seventh Workshop on Geothermal Reservoir Engineering Stanford University. Erdlac, Jr., R.J., Armour, L., Lee, R., Snyder, S. Sorensen, M., Matteucci, M., and Horton, J.: "Ongoing Resource Assessment of Geothermal Energy from Sedimentary Basins in Texas," Proceedings of 32 nd Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California, January 22-24, 27.