Subsurface Temperature Survey in Thailand for Geothermal Heat Pump Application

Transcription

1 日本地熱学会誌第 33 巻第 2 号 (2011) 93~98 頁 短 報 J.Geotherm. Res.Soc.Japan Vol.33.No.2(2011) P. 93~P. 98 Subsurface Temperature Survey in Thailand for Geothermal Heat Pump Application Youhei UCHIDA, Kasumi YASUKAWA, Norio TENMA, Yusaku TAGUCHI, Takemasa ISHII, Jittrakorn SUWANLERT and Oranuj LORPHENSRI (Presented 1 July 2010 at RE2010, Received 16 November 2010, Accepted 24 February 2011) Abstract The applicability of geothermal heat pumps in tropical region was investigated in this study based on subsurface temperature surveys. Although generally a geothermal heat-pump system may not have thermal merit for space cooling in tropics, there may be some places in tropical regions where the subsurface can be used as cold heat-source. In order to confirm this possibility, subsurface temperature surveys were widely conducted in the Chao-Phraya Plain, Thailand. In some cities, subsurface temperatures lower than monthly mean maximum atmospheric temperature by 5 or more over four months were identified. Thus underground may be used as cold heat-source even in parts of tropical regions. Keywords: geothermal heat pump system, space cooling, Thailand, subsurface temperature 1. Introduction In east Asian countries, where significant economical growth is expected so that energy saving and environmental protection will be major matters of importance for sustainable growth, the current number of geothermal heat pump (GHP) installation is quite limited and rapid growth of GHP installation is desirable. Although the subsurface heat can be directly used without a heat pump, the application of a heat-pump enables us to increase heat exchange rate with underground. It also allows us to control output temperature (ex, room temperature). Therefore in most cases, a heat exchange system with underground includes a heat pump and is called as a GHP system. GHP systems have been used for space heating only or for both heating and cooling. Since it is applicable in non-geothermal regions, the system is known as direct use of geothermal energy applicable anywhere. But it may not be true for tropical regions where the atmospheric temperature is stable throughout the year. Although cooling system is needed there, subsurface temperature may always be higher than atmospheric one because of geothermal gradient. Therefore underground is not suitable as cold heat-source and there is no advantage of subsurface heat exchange. High COP may not be expected in such cases. However, still there is a possibility of thermal merit for tropical regions if 1) seasonal change of atmospheric temperature exists, 2) daily change of atmospheric aaaaaaaaa Geological Survey of Japan, AIST, Higashi, Tsukuba, Ibaraki , Japan Department of Groundwater Resources, Thailand, 26/83 Ngamwongwan 54, Ladyao, Chatuchak, Bangkok 10900, Thailand CThe Geothermal Research Society of Japan,

2 temperature is rather high, and/or 3) the cooling effect of recharging groundwater flow on subsurface temperature is locally dominant than the heating effect of heat flux from a depth. Thus to understand local underground temperature distribution may be the first step for installation of geothermal heat-pump systems. For this purpose, vertical temperature profiling of observation wells were widely measured in Chao-Phraya Plain, Thailand. 2. Natural subsurface temperature and groundwater flow -In application of GHP systems- For designing a GHP system, information on groundwater is quite important. In case of open-loop GHP system, in which groundwater is pumped up for heat exchange at ground surface and re-injected after heat exchange, information on depth, temperature and flow rate of the aquifer is important. In case of closed-loop GHP system, in which heat exchange is conducted by circulating fluid in a heat exchange tube buried in underground, information on subsurface temperature and effective thermal conductivity is important. Since effective thermal conductivity is largely affected by existence of groundwater flow, as a consequence, information on subsurface temperature and groundwater flow is essential for both closed and open systems. Therefore in this section, effects of groundwater flow on GHP system will be explained. Subsurface temperature in natural state at a depth of 20 m or deeper is generally stable throughout the year and generally higher than year-average atmospheric temperature at the place. Fig. 1 schematically shows seasonal variation of atmospheric and subsurface aaaaaaaaaa Fig. 1 Seasonal variation of subsurface and atmospheric temperature for different climates. Solid lines: atmospheric, broken lines: subsurface. temperature at a depth of about 50 m. In cold climate regions, where subsurface temperature is mostly higher than atmospheric one, GHP system is quite suitable for space heating. In moderate (temperate) climate regions, where subsurface temperature is higher in the winter and lower in the summer, GHP system is useful for both space heating and cooling. In tropics, where space cooling is preferred, subsurface temperature is higher or approximately equal to atmospheric one and no advantage of GHP system can be seen (Yasukawa et al., 2009). However natural groundwater flow, which is controlled by topography of the ground surface and the subsurface boundaries with permeability changes, may perturb the subsurface thermal regime. At recharge zones, infiltration of precipitation disturb heat conduction from a depth, which lowers the shallow subsurface temperature while upward groundwater flow encourage the heat transfer from a depth at discharge zones. Therefore in an identical groundwater system, subsurface temperature at recharge zone is generally lower than that at discharge zone at the same depth. Thus temperature difference of a few degree may be achieved by groundwater flow. Groundwater flow has another important effect on subsurface heat exchange around a closed GHP system. Advection effect of groundwater flow reduces temperature rise/drop around the borehole during heat exchange, which otherwise degrades the system performance. Thus groundwater flow contributes to sustainable operation of GHP system. Therefore, if subsurface layers are effectively cooled by groundwater flow, in both natural state and operational period of a GHP system, GHP systems may be useful in tropical regions. 3. Theory and method Groundwater temperature measured in the observation wells is assumed to be the same as the subsurface temperature, because there is thermal equilibrium between water in borehole and the surrounding subsurface temperature. Temperature profiles are one-dimensional sequential data arrays, therefore, areally distributed temperature profiles provide three-dimensional subsurface information. Fig. 2 shows groundwater flow system and subsurface thermal regime (Domenico and Palciauskas, 1973). If there is no groundwater flow or static groundwater condition (Fig. 2a), subsurface thermal regime is governed by only thermal conduction and subsurface temperature gradient is constant (Fig. 2b). If it is assumed 94

3 that simple regional groundwater fl ow system due to topographic driving exists (Fig. 2c), thermal regime is disturbed by thermal advection owing to groundwater flow (Fig. 2d). In the groundwater recharge area, subsurface temperatures and gradients are lower than that of under static groundwater condition (Fig. 2b). Uchida et al. (2003) compiled shallow subsurface temperature-depth profiles at depths from 30 m to 300 m in selected Japanese basins and plains. They classified thermal data sets into four categories depending on the thermal regime and pointed out that main factor of subsurface temperature distribution is the effect of thermal transport due to regional groundwater flow system. The Chao Phraya Plain, Thailand, consists of the upper and the lower plains divided at Nakhon Sawan Province (in Fig. 3). The Bangkok Metropolis, situated in the lower plain, has land subsidence and sea water intrusion due to excessive extraction of groundwater by pumping. The lower plain extends about 200 km from the north to the south and about 175 km from the west to the east. It is bounded on the west by the mountain ranges about 100 km from the Chao Phraya River, on the east by mountainous terrains about 75 km from the river, on the south by the Gulf of Thailand and on the north by series of small hills which divide it from the Upper Plain. The mean annual precipitation is 1190 mm in the lower plain. The geological formations in and around Bangkok Metropolis are the basement complex and alluvial deposits constituting of aaaa Fig. 2 Subsurface thermal regime affected by a groundwater flow system (modified Domenico and Palciauskas, 1973), (a) static groundwater, (b) thermal regime under condition of (a), (c) simple regional groundwater flow system, (d) thermal regime under condition of (c). Fig. 3 Geological schematic around Bangkok city (upper) and hydrogeology from Ayutaya to Gulf of Thailand (lower), respectively (Sanford and Buapeng, 1996; Buapeng, 1990). aquifers in this area. Fig. 3 shows the hydrogeological conditions of the lower plain. The main aquifers in this area are unconsolidated deposits and subdivided into 8 aquifers. Total thickness of the aquifers is about 600 m. The upper plain extends about 250 km from the north to the south and about 140 km from the west to the east. The main rivers in the upper plain are Ping, Wang, Yom and Nan which flow from north to south and come into concurrence at Nakon Sawan Province. The mean annual precipitation in this area is 1202 mm. The alluvial sediments have been accumulated in the plain under the fluviatile environment since Tertiary, forming the total thickness of about 250 m. These sediments are classified into three aquifers (Sanford and Buapeng, 1996). 4. Subsurface thermal regime in the Chao-Phraya Plain The authors measured borehole temperatures using a digital thermistor thermometer with a precision of 0.01 in 2 m intervals to obtain temperature-depth profiles of 56 observation wells at depths from 20 m to 300 m from 95

4 2003 Fig. 4 Contour map of subsurface thermal gradients and typical temperature-depth profiles, (a) Bangkok, (b) Ayutthaya, (c) Nakhon Sawan, (d) Phitsanulok, (e) Sukhothai and (f) Kanchanaburi, respectively. 96

5 2003 to Fig. 4 shows distribution of subsurface thermal gradients in the Chao-Phraya Plain based on observed temperature-depth profiles. The distribution of subsurface thermal gradients indicates differences between the upper and the lower plains. Thermal gradients in the upper plain are generally low and most of them are lower than 1.0 /100 m. Some of them show negative gradients. Some observation points show thermal gradients of /100 m regionally, because there exists a hot spring in the vicinity, the temperature of which is 60 or higher. The thermal gradients in the lower plain, on the other hand, are generally high and show a clear tendency that thermal gradient is higher in the central area and lower in the peripheral areas. 5. Discussion Regional variation of subsurface temperature at depths from 20 m to 50 m of 3.4 was observed in the whole Chao-Phraya Plain. Fig. 5 compares atmospheric and subsurface temperature in depths between 20 to 50 m at (a) Bangkok, (b) Ayutthaya, (c) Nakhon Sawan, (d) Phitsanulok, (e) Sukhothai and (f) Kanchanaburi regions as shown in Fig. 4, respectively. Subsurface temperature data from the wells located near the sea, with extremely high temperature, are eliminated for Fig. 5 (a). Subsurface aaaaaa temperatures shown in Figs. 5 (d) and 5 (e) are identical because they are based on data from the same region, while the atmospheric temperatures are different. At Pitsanulok and Nakhon Sawan, subsurface temperature is lower than average of daily maximum atmospheric temperature in a month (monthly mean maximum atmospheric temperature; MMMAT) throughout the year. Its difference is bigger than 5 over four months. Also at Kanchanaburi, subsurface temperature is lower by 4 or more over four months with a largest difference of 10 in April. GHP system may be effective in these areas for space cooling especially in daytime. In Bangkok and Ayutthaya, subsurface temperature is lower than MMMAT for almost throughout the year, but the difference is smaller, so the performance of a GHP system may be lower than the other three areas. In Sukhothai, where subsurface temperature is higher than MMMAT for most of the year, underground may not be used as cold heat-source. 6. Conclusions A GHP system needs m deep heat exchange boreholes, therefore, temperature-depth profiles are useful for the installation of GHP system. The authors conducted borehole temperature measurement at 58 observation points to obtain groundwater temperature-depth profiles in the Cha Fig. 5 Comparison of atmospheric and subsurface temperature at each region. 97

6 Chao-Phraya Plain, Thailand. As a result, regional variation of subsurface temperature of 3.4 was observed in the whole Chao-Phraya Plain at depths from 20 to 50 m. In Nakhon Sawan, Pitsanulok, and Kanchanaburi in Thailand, subsurface temperature lower than monthly mean maximum temperature for 5 or more over four months was identified. Also in Bangkok and Ayutthaya, subsurface temperature is lower than monthly mean maximum temperature almost throughout the year. Thus underground may be used as cold heat-source even in parts of tropical regions. Acknowledgement This work is the revision and extension of the paper in RE2010 International Conference in Yokohama with the paper number P-Ge-2. The opportunity is gratefully acknowledged. References Buapeng, S. (1990) The use of environmental isotopes on groundwater hydrology in the selected areas in Thailand. Research Contract No. RB/4803/R1/R3, IAEA. Domenico, P. A. and Palciauskas, V. V. (1973) Theoretical analysis of forced convective heat transfer in regional groundwater flow. Geol. Soc. Amer. Bull., 84, Sanford, W. E. and Buapeng, S. (1996) Assessment of a groundwater flow model of the Bangkok Basin, Thailand, using carbon-14-based ages and paleohydrology. Hydrogeology Journal, 4, Uchida, Y., Sakura, Y. and Taniguchi, M. (2003) Shallow subsurface thermal regimes in major plains in Japan with references to recent surface warming. Physics and Chemistry of the Earth, 28, Yasukawa, K., Uchida, Y., Tenma, T, Taguchi, Y., Muraoka, H., Ishii, T., Suwanlert, J., Buapeng, S. and Nguyen, T. H. (2009) Groundwater temperature survey for geothermal heat pump application in tropical Asia. Bulletin of the Geological Survey of Japan, 60, 短報 タイ王国における地中熱ヒートポンプシステムのための地下温度調査 内田洋平 安川香澄 天満則夫 田口雄作 石井武政 Jittrakorn SUWANLERT Oranuj LORPHENSRI (2010 年 7 月 1 日 RE2010 にて発表,2010 年 11 月 16 日受付,2011 年 2 月 24 日受理 ) 概要地中熱利用システムは, 今後高いエネルギー需要増加が予想される東南アジアの国々でも省エネルギーシステムとして関心を寄せる関係者が多いが, 一般的に地下温度は年平均気温より高いため, 気温変化の小さい熱帯地方では地下温度が年間を通して気温より高く, 冷房の熱源として有効でないと考えられる しかし 熱帯地方であっても多少気温変化があり, 地下温度が比較的低い地域では, 地中熱を冷房の熱源として利用できる可能性がある このような仮説に基づいて, タイのチャオプラヤ平野の観測井において鉛直温度分布を測定し, 気温データと比較することで, 熱帯における地中熱冷房利用の可能性を調べた その結果, 熱帯地方でも地域によっては, 年間を通して地下温度は月平均の最高気温より 5 以上低く, 地中熱を冷房に利用できる可能性が確認された キーワード : 地中熱ヒートポンプシステム, 冷房, タイ王国, 地下温度 98