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The feasibility of using circulating groundwater as renewable energy sources for air-conditioning in Taipei basin
Renewable Energy 39 (2012) 175e182
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
The feasibility of using circulating groundwater as renewable energy sources for air-conditioning in Taipei basin Chihping Kuo*, Hungjiun Liao Department of Construction Engineering, National Taiwan University of Science & Technology, No. 43 Sec. 4 Keelung Road, Taipei 10699, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2011 Accepted 28 July 2011 Available online 30 August 2011
To reduce the uplift pressure of groundwater, many wells, deep into the Chingmei gravel stratum underneath Taipei city, were installed during construction of Taipei MRT underground stations. These wells can also be used to establish a circulating groundwater system for air-conditioning. To study the feasibility of this circulating cooling system and the thermal response of the Chingmei gravel stratum, some groundwater parameters of the Chingmai gravel stratum were studied such as pumping and recharging capacities, flow direction, and flow velocity etc. Two small scale in-situ discharge tests and numerical simulations were used for this purpose. However, the tests could not be performed for a long period to prove the purpose, the computer modeling provided the predicted long time response of the stratum. The discharge tests on the wells confirmed the discharge capacity of the Chingmei stratum and the numerical results also indicate that circulating groundwater with a steady temperature can be provided to cool down air-conditioners, as long as groundwater is flowing at a slow velocity. This study confirms the feasibility of using the groundwater of the Chingmei gravel stratum as a steady and clean cool source for air-conditioners. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Groundwater Thermal response Chingmei gravel stratum Air-conditioner Urban heat-island effects
1. Introduction Due to global warming and urban heat-island effects, the mean daily summer surface temperature of Taipei city has been rising, from about 28.0 C in 1960 to 29.5 C in 2005 [1]. From the energy point of view, air temperature ascent of 1 C causes more electrical power consumption about 600,000 kW in Taiwan [2]. Airconditioners have become a necessity in summer to cool down the indoor temperature. But the heat exhausted from the airconditioners will further increase the air temperature and make the situation worse [3]. To mitigate this problem, the exhaust heat might be alternatively discharged into the underground reservoir in the Chingmei gravel stratum underneath Taipei city. During construction of the Taipei MRT subway system, many wells which are more than 50 m deep were installed to reduce the uplift pressure of groundwater. These wells provide a good passage for groundwater to circulate to and from the Chingmei gravel stratum after completion of underground station construction. To make use of the groundwater stored in the Chingmei gravel stratum, a circulating groundwater system utilizing these wells can be established to provide a cooling device for air-conditioning (Fig. 1). It is understood * Corresponding author. Tel.: þ886 2737 6560; fax: þ886 2737 6606. E-mail addresses: [email protected], [email protected] (C. Kuo). 0960-1481/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2011.07.046
that over pumping groundwater can result in large scale ground subsidence. So, this system is designed only to circulate groundwater to cool down air-conditioners. No groundwater is pumped out of the underground reservoir. Although using an underground reservoir as a geothermal resource has been commonly adopted in countries located in high latitudes, its use in countries located at lower latitudes needs some modifications. Unlike the study on Aquifer Thermal Energy Storage (ATES) system [4,5], the proposed system focuses on two subjects: one is the discharge capacity of the Chingmei gravel stratum; the other is the steady supply of circulating groundwater with a low temperature. To evaluate the feasibility of this cooling system for air conditioners, some basic properties of the Chingmei gravel stratum are to be studied such as the pumping and recharging capacity, the velocity and direction of groundwater flow etc. To simulate the groundwater flow of the Chingmei gravel stratum and the thermal response to the circulating groundwater, a conceptual model for the entire Taipei basin is adopted here. Two numerical programs were used: one is SHEMAT (Simulator for Heat and Mass Transport [6]), the other is MODFLOW (Modular ThreeDimensional Groundwater Flow Model [7,8]). The former is a 3-D finite-difference numerical program used to simulate the heat transport behavior (i.e., diffusion and dissipation) of groundwater and the thermal response of the stratum. SHEMAT is an easy-to-use
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valve Higher temperature circulating water
Air-conditioner
Lower temperature circulating water Heat exchanger G.L.
Warm groundwater
Cool groundwater valve Piezometric level
pump
Warm groundwater
Cool groundwater
Fig. 2. Geographical structures of Taipei basin, observation wells locations, Geological investigating boreholes, and study area.
Chingmei Gravel Stratum
Fig. 1. Schematic diagram of circulating groundwater system; major devices to use groundwater as a cooling source for air-conditioning including circulating wells, pump and heat exchanger.
tool, handles coupled processes of groundwater flow combined with heat transfer, to simulate the thermal field of an aquifer but it is unable to simulate hydrogeological behavior in detail. For example, the groundwater flow (i.e., velocity and direction) generated by various thickness or inclined ground layers is unabled to be evaluated by SHEMAT. To compensate for this problem, MODFLOW, a widely used software to simulate various hydraulic systems and groundwater flow model, was used to determine the groundwater flow parameters first, before they are passed to SHEMAT to do the follow-ups. A Taipei MRT underground station construction site located in the center of the basin was chosen as the field test site to verify the numerical results obtained from SHEMAT and MODFLOW analyses.
from Xindian river; and sandy layers are deposited by the Dahan river. Basically, the sandy and clayey deposits are underlain by thick gravel deposits in the Taipei basin. Based on boring log data, a schematic diagram of the basin profile can be drawn, as in Fig. 3. Sungshan stratum consists of alternative sandy and clayey layers; the Chingmei stratum is an independent gravel stratum; the Wuku stratum and the Panchiou stratum are also gravel strata. The geological information indicates that the Chingmei stratum is an alluvial fan gravel stratum which covers the entire Basin. Besides, a varve layer, which consists of alternative layers of mud and clay, is found beneath the Chingmei stratum. In this study, the Sungshan stratum and the varve layer are considered to be aquitard layers (i.e., a layer with a much lower permeability than other layers) bounding the Chingmei stratum as indicated by Teng et al. [9]. The major intake zones of groundwater in Taipei basin are located around the midstreams of the Xindian river and the Dahan river [10]. In the eastern part of the Taipei basin, the river water and rain water are able to infiltrate through the sediments to the underlying aquifers. But the clayey sediment from the Keelung river has low permeability. So the vertical recharge of groundwater is limited in the Keelung river area. The secondary groundwater intake zone in Taipei basin is from the edges of the basin, such as the surrounding mountains, hills, and tablelands [11]. Finally, Hong
2. Conceptual groundwater model of Taipei basin 2.1. Description of Taipei basin Taipei basin (Fig. 2) covers approximately 243 km2 in area with an average elevation of 20 m above sea level. The basin is surrounded by Datun volcanoes in the north; Linkou tableland in the west; and hills and mountains in the east and the south. Several major rivers meander through the Taipei basin: the Tanshui river, the Keelung river, the Xindian river and the Dahan river. Sediments deposited from different rivers vary from one another. Generally, soft clays are deposited by the Keelung river; gravels are deposited
Fig. 3. Schematic diagram of profiles in the section along deposited direction (from Xindain in the southeast to Tanshui in the northwest of the basin) and the possible distribution of subsurface stratums.
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et al. [10] pointed out that the groundwater table of Taipei basin is highly related to the base flow from the Xindan river and concluded that the groundwater is mainly recharged by the Xindan river. As estimated by Chen [12], there is about 68.4 billion m3 of groundwater stored in the Chingmei gravel stratum. The temperature of groundwater ranges from 24 C in winter to 28 C in summer [13]. It can be a conditions related to the resource for the cooling medium needs of air-conditioners. Therefore, groundwater such as pumping and recharging capacity, flow direction and velocities are important features needed to evaluate the feasibility of the proposed groundwater circulation system. 2.2. Groundwater flow model of Taipei basin The groundwater flow model of the Taipei basin was established using a MODFLOW model. MODFLOW is a modular 3-D finitedifference groundwater flow model which can simulate groundwater flow under complex hydraulic conditions with various hydrological processes. In particular, the code model UCODE [14] of MODFLOW version 5.3 can auto-calibrate the environment to increase the accuracy of the conceptual groundwater flow model. The conceptual groundwater flow model of the Taipei basin was divided vertically into three layers: the upper one is an aquitard (Sungshan stratum), the middle one is the confined aquifer (Chingmei stratum), and the lower one is an aquitard also (i.e., varve). The three layers in the model were divided into 2500 cells with 50 columns and 50 rows. The mesh is square in shape covering an area of 500 500 m2. The change in terrain and thickness for each layer in space can be determined by interpolation from the boring log data provided by the Central Geological Survey (CGS) of Taiwan. Three types of boundary condition were assigned: (1) the estuary of the Tanshui river is regarded as the constant-head boundary (i.e., groundwater level ¼ sea level ¼ 0); (2) no infiltration occurs in the hills on the eastern and southern sides, the volcano on the northern side, and the tableland on the western side are regarded as the zero-flow boundaries (i.e., groundwater flow ¼ 0); and (3) the midstreams of the Xindian stream and the Dahan stream are regarded as the recharging boundaries. Hong et al. [10] indicated that such a recharging boundary could best correspond to the observed groundwater data of the Taipei basin. In this study, eight observation wells No.115, 116, 117, 114, 113, 122, 111,
177
and 123 (Fig. 4) are regarded as recharge boundaries; the hills, volcano, and the tableland are regarded as zero-flow boundaries. To calibrate the model, monthly groundwater level data from the 23 observation wells, provided by the Water Resources Agency, covering January 1997 to December 2001, were used. To verify the model, monthly groundwater level data from the 23 monitoring wells from January 2002 to December 2002 were used. The hydraulic parameters of the aquifer were determined from the pumping tests and checked with the numerical results published by Tsao et al. [15]. To examine the suitability of these parameters, the values of 0.12e3.20 m2/min and 0.00192 as suggested by Tsao et al. [15] were taken as the initial values of Transmissivity and Storage coefficient respectively for the inverse modeling. Furthermore, the spatial distribution of the conductivities was estimated by the Kriging method for regional scale. After the above parameters had been inversely calibrated in the numerical model, a zonation distribution could be applied to scale up the calibration parameters [16e18]. The zonation distribution was adopted to establish the spatial patterns of the flow and the recharge rates. Following this process, the amount of groundwater recharge to the basin can be calculated. Since the exact amount of groundwater recharge to the basin cannot be accurately measured, the conceptual model UCODE [14] was used to calibrate the zonation distribution and the transmissivity. Model calibration was carried out through auto-calibration and artificial trial-and-error by matching observed groundwater levels and simulated groundwater levels. Spatial distributions of groundwater flow were estimated at different times. After the above exercise, the model can be used to simulate the groundwater flow vector and head contours (Fig. 4) for the initial approximate determination of local groundwater flow. 3. Simulating the thermal response of aquifer 3.1. Studied subject During the construction of a Taipei MRT underground station (O8 station), 22 deep wells were installed to reduce the uplift groundwater pressure from the Chingmei gravel stratum. The plane view of the site and the layout of test wells are shown in Fig. 5. These wells were to be sealed off after the construction. To make the best use of these wells, the study will evaluate the feasibility of converting these wells to become flow passages as part of the
Fig. 4. Plane view of hydrogeological model meshes and flow boundaries; groundwater flow vector and head contours in the end of simulation.
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Fig. 5. Plane view of the MRT O8 station site and the layout of wells involved in discharging test and small scale thermal test.
cooling system for air conditioners. These wells have a diameter of 0.8 m and are about 80 m deep. The design pumping rate of each well was 6 m3/min. But not all 22 wells were operated at the same time. The exact number of wells actually operated varied with the construction sequence and the need for dewatering. Normally, 3 to 6 wells were operated as a group. The Chingmei stratum is located from G.L.-54.6 to 86.75 m beneath the O8 station. In between ground surface and Chingmei stratum lies the Sungshan stratum which has alternating layers of silty sand and silty clay. Underlying the Chingmei stratum is a layer of varve clay with a thickness of about 1.2 m. The cooling capacity needed for the cooling towers of the O8 station is 1184 kW.
area. Meanwhile, a line of virtually discharging wells were placed at the inflow boundary. The parameters used in this model were as follows: effective porosity ¼ 0.25 and conductivity ¼ 102 cm/s [19], thermal capacity ¼ 1.875 MJ/m3 K and conductivity ¼ 1.308 W/ m K [20].
3.2. Local groundwater flow model The local groundwater flow model adopted for the O8 station area was extracted from the conceptual model of the entire Taipei basin. Using the local model, the flow direction and quantity of groundwater were calculated. The direction of groundwater flow under the study area was from east to west (Fig. 4). The water budget in and outside the study area was computed by MODFLOW. The average water budget during the simulated period was about 1.95 102 cmd (m3/day) through a cross section of 1500 m2. It is equivalent to 3.5 104 cmd for the whole basin. The value of the water budget in the study area can be converted into groundwater flux (i.e., groundwater velocity) of 13 cm/day. 3.3. Local heat transport model Based on the local groundwater flow model, a heat transport model established from the SHEMAT program was used to simulate the heat propagation behavior which resulted from discharging water. 3-D grids consisting of 122 120 3 cells were used. In the center area (2000 m 2000 m), each cell represented an area of 1 m 1 m; outside the center area, each cell represented an area of 10 m 10 m or 20 m 20 m depending on the location. The upper and lower layers were treated as an aquitard, and the middle layer was treated as a confined aquifer. However, SHEMAT can only be used in a layer of constant thickness for the current version. So, a mean value of 30 m was adopted to approximate the thickness of the aquifer. To simulate the flowing groundwater condition in the Chingmei stratum with the SHEMAT program, a line of virtual pumping wells was placed at the effluent boundary of the study
Fig. 6. The well dimensions and schematic arrangement of in-situ heat transport test.
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179
Fig. 7. Observed data and simulated results of discharging test in O8 station.
4. In-situ test for verification and discussion on results Two in-situ discharging tests were performed at the O8 station to verify the suitability of hydrological and heat transport models established by the above mentioned process. One was done in summer; the other was in winter. To reduce the interference from other pumping wells, both tests were carried out during the period when no other wells were pumping at the site. 4.1. Discharging test to verify local groundwater flow model In order to test the discharging capacity of the Chingmei stratum and to estimate the transmissivity of the local Chingmei stratum around the test wells, an in-situ discharging test was performed. Four wells namely PW7, PW8, PW9, and PW14 were used in this test, one for discharging and the others are for observation (Fig. 5). The well dimensions are the same as PW21 and PW22 shown in Fig. 6. During the discharging process, the rise of water level in the observation wells was recorded. As shown in Fig. 7, up to 7 m3/min of water flow rate can be successfully discharged by gravity force only through a well 80 cm in diameter and a 30 m long screen
opening in the Chingmei gravel stratum. The transmissivity (T) was computed from the above observed data by using Kozeny’s formula [21]. An average value of T ¼ 1.53 m2/mim was obtained from this test. By comparing the observed data and MODFLOW results (Fig. 7), it can be seen that the water level of wells PW8 and PW9 simulated by MODFLOW is in good agreement with the observed data; but MODFLOW overestimates the observed water level of wells PW14 and PW7. This indicates that the values of hydrological parameters can vary significantly in a short distance. Gravel soil with low-permeability may exist near wells PW14 and PW7. 4.2. Small scale thermal test for local heat transport model To simulate the thermal response of Chingmei gravel stratum to the circulating cooling water from air-conditioners, a package airconditioner was used in this small scale heat transport test. The layout and arrangement of the test facilities are shown in Figs. 5 and 6, respectively. The heat exchange capacity of this air conditioner was able to increase the temperature of circulating groundwater by 3 C at a flow rate of 0.146 m3/min. Four temperature sensors (RTD1 to RTD4) were installed to measure the
Fig. 8. Measured and simulated results for in-situ heat transport test.
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Fig. 9. Simulated temperature of pumped water (intake at PW22) at different groundwater flow velocities.
temperature of circulating groundwater, aquifer, and ground respectively. The distance between pumping well (PW22) and recharging well (PW21) was 15.4 m. An observation well (PS1023) was located next to PW21. The elapsed time of the test was 3 days and the test results are shown in Fig. 8. During the test, it took about 1.5 h for the circulating water from air conditioner to increase the water temperature in the discharging well to a steady value of 27 C (RTD2). The groundwater temperature inside the pumping well (RTD1) and in the Chingmei gravel stratum (RTD3) was about the same and was equal to 24 0.2 C. Basically, the groundwater temperature was not affected by the cooling water from the package air-conditioner. The air temperature during the test period increased from 15.3 C to 16.4 C. The in-situ test was simulated with a numerical model and the simulated results are plotted in Fig. 8. No noticeable temperature change was observed at well PW22 (pumping well) from the numerical analysis over a three-day period either. Even after 900 days, the recharging groundwater with higher temperature still has no influence on the pumping well located 10 m away (under a groundwater flow velocity of 13 cm/day). 4.3. Predicting ground thermal response to air conditioner of O8 station To eliminate the 1184 kW of heat generated by the airconditioners of Taipei MRT O8 station, the required cooling capacity of the medium calculated by thermal equilibrium equation is as follows: temperature increase of cooling water ¼ 5 C, flow
rate ¼ 3.4 cmm (m3/min). To carry the heat away from the O8 station, the flow characteristics of the groundwater in Chingmei gravel stratum will be accounted for and evaluated. Two groundwater flow velocities, 103 cm/day (no flow) and 13 cm/day, were adopted in the analysis to simulate the no flow and slow flow groundwater conditions respectively. The distance between pumping and recharge well was 15.4 m. Over a period of 900 days of water circulation, the simulated temperature change of the aquifer at the intake of pumping well PW21 is shown in Fig. 9. It can be seen that the groundwater temperature at PW21 under the no flow groundwater condition keeps rising to 31.5 C after 900 days. However, the temperature under slow flow (13 cm/day) groundwater condition quickly rises to 27.7 C after 15 days, drops slightly down to 27.6 C and then remains at this temperature. The worst location of circulating wells based on the groundwater flow directions might be installing the pumping well at the downstream. The dissipated heat to the discharging well at the upstream may have a quicker impact on the pumping well and downgrade the cooling capacity essentially. The simulation for the above location of circulating wells and under the same groundwater flow velocity (13 cm/day) was made. It shows the temperature at PW21 quickly rises to 31.5 C after 10 days. Therefore, to avoid choosing the pumping well at the downstream location may reduce the impact of the dissipated heat from the pumping well. The distributing pattern of groundwater temperature after 900 days circulation is shown in Fig. 10. During water circulation, the temperature around the recharge well increases gradually. Obviously, the shape of the temperature distribution diagram changes
Fig. 10. 2-D view of long-term simulated temperature distribution at different groundwater flow velocities.
C. Kuo, H. Liao / Renewable Energy 39 (2012) 175e182
181
Fig. 11. 2-D view of simulated short-term temperature change and distribution at different groundwater flow velocities, temperature level scale same as Fig. 10.
with groundwater flow velocity. For the no flow groundwater condition (¼103 cm/day), the heat accumulates around the recharge well (Fig. 11). Shortly, the heated water from the recharge well will be pumped up again. So the temperature of circulating groundwater will increase gradually. As a result, the effectiveness of heat exchange between circulating groundwater and air conditioner will decrease. In comparison, when the groundwater flow velocity is equal to 13.1 cm/day, the shape of temperature distribution changes (Fig. 11). The heated water from the recharge well is unable to reach the pumping well. So the water temperature from the pumping well can remain a steady 27.6 C over a period of 900 days. In other words, the groundwater in the Chingmei gravel stratum can be a good cooling source for the air conditioners of the O8 station.
designed thermal capacity of the cooling tower for MRT O8 station is 1184 kW (is equivalent to 336.8 RT refrigerating capacity). The original design is to use four cooling towers: two to carry away 315 kW of heat each and the other two to carry 277.12 kW each. The comparisons of electrical power consumption, water consumption, and facilities space requirement between two systems are listed in Table 1. The result shows that the power consumption of the two systems is about the same. However, water consumption and space requirements are significantly reduced with the groundwater circulation system. The last but not the least is that in the circulating water model no exhaust heat is discharged to the air. It can help a great deal to mitigate the heat-island effect in Taipei basin. 5. Conclusions
4.4. Comparison of power consumption between two systems To evaluate the energy efficiency of this groundwater circulation cooling system, the electricity consumption and the required space for facilities are compared with the traditional cooling towers. The Table 1 Comparison of electricity power consumption, water consumption, and installation space requirement between two systems (MRT O8 station). Item Cooling capacity Circulating water request Power request
Required flow rate Temperature rising Pump lift Pump for circulating water Motor inside cooling tower Electricity consumption
Water consumption Space required for equipment
Cooling tower
Circulating well
1184 kW 3.4 cmm þ5 C 9m 72 HP
30 m 82 HP
14 HP
e
64.15 kW hr
61.30 kW hr
4.4 m3/day 52 m3
0 0.62 m3
The feasibility of using a circulating groundwater system to cool down the air-conditioners is evaluated here. The efficiency of this system is related to the groundwater direction, flow velocity, and thermal response of the aquifer. Based on the numerical modeling, in-situ discharging test results and a small scale groundwater circulation test, the following conclusions can be made: 1. Due to the variation of hydrogeological condition of the Chingmei gravel stratum, a groundwater flow model using MODFLOW was established from boring log data provided by Central Geological Survey (CGS) and calibrated by groundwater data provided by Water Resource Agency (WRA). The in-situ discharging test was used to verify the local hydraulic parameters (i.e., transmit and storage) determined from numerical analysis. An east-to-west groundwater flow direction and a flow velocity of 13 cm/day were obtained from this study. 2. The heat transport model using SHEMAT together with the hydraulic parameters determined from the MODFLOW model were employed to simulate the thermal response of the aquifer. The thermal response of the Chingmei gravel stratum to the air conditioners of the O8 station, was simulated by the proposed
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model. It shows that the heated water from the recharge well is unable to reach the pumping well (pumping rate ¼ 3.4 cmm) located 15.4 m away when the groundwater flow velocity is equal to 13 cm/day. So the water temperature from the pumping well can remain a steady 27.6 C over a period of 900 days. Compared with the small scale heat transport test results, they are in good agreement with the simulated results. No water temperature increase in the pumping well or in the aquifer was found. In other words, the groundwater in Chingmei gravel stratum can be a good cooling source for the air conditioners of the O8 station. 3. The power consumption of using cooling towers and circulating groundwater to cool air-conditioner is about the same. However, water consumption and space requirements are significantly reduced with the groundwater circulation system. 4. No exhaust heat is discharged to the air by using the proposed system, that can help a great deal to mitigate the heat-island effect in Taipei basin. References [1] Chen TC, Wang SY, Yen MC. Enhancement of afternoon thunderstorm activity by urbanization in a valley: Taipei. Journal of Applied Meteorology and Climatology 2007;46:1324e40. American Meteorological Society. [2] Lin JM. Urban heat island effect and its environmental implications. Journal of Ecology and Environmental Sciences 2010;3(1):1e15. [3] Hsieh CM, Aramakia T, Hanakia K. Estimation of heat rejection based on the air conditioner use time and its mitigation from buildings in Taipei City. Building and Environment 2007;42:3125e37. [4] Bridger DW, Allen DM. Designing aquifer thermal energy storage systems. ASHRAE Journal 2005;47:S32e8.
[5] Dickinson JS, Buik N, Matthews MC, Snijders A. Aquifer thermal energy storage: theoretical and operational analysis. Geotechnique 2009;59:249e60. [6] Clauser C, editor. Numerical simulation of reactive flow in hot aquifers, SHEMAT and processing SHEMAT. Germany: Springer; 2003. [7] Harbaugh AW, McDonald MG. Programmer documentation for MODFLOW-96, an update to the U.S. geological survey modular finite-difference groundwater flow model. Open-File Report: 96e486. USGS; 1996. [8] Chiang WH, Kinzelbach W. 3D-groundwater modeling with PMWIN. Germany: Springer; 2005. [9] Teng LS, Lee CT, Peng CH, Chu JJ, Chen WF. Origin and geological evolution of the Taipei basin, northern Taiwan. Western Pacific Earth Sciences 2001;1:115e42. [10] Hong NM, Tan CC, Lin ST, Tsay TS. Uncertainty analysis of groundwater exchange at boundaries in Taipei basin. Geophysical Research Abstracts; 2007:9. European Geosciences Union. [11] Chia YP, Chang MS, Liu WY, Lai DC. Hydrogeological characteristics of Taipei basin. Taipei basin subsurface geology and engineering environment, central geological survey monograph. In: Special publication, vol. 11; 1999. p. 393e406. [12] Chen WF. The groundwater in Taiwan. Taipei: Sinobooks; 2005. [13] Liao HJ, Kuo CP, Wang CS. Using circulating groundwater as cooling medium for air conditioner at NTUST. In: The 17th Southeast Asian Geotechnical Conference, 2010; p. 409e412. [14] Poeter EP, Hill MC. Documentation of UCODE, a computer code for universal inverse modelling. US Geological Survey, Water-Resources Investigations Report 98e4080; 1998. [15] Tsao YS, Lin CN, Tan YC, Mao AS. Simulation and application of mathematical groundwater model in Taipei Basin. Taipei: MOEA; 1985. [16] Gau HS, Liu CW, Chen JS, Tsao YS. Estimation of optimal yield in Yunlin area of Taiwan using decision analysis. Journal of the American Water Resources Association 1998;34:1295e309. [17] Jang CS, Liu CW. Geostatistical analysis and conditional simulation for estimating the spatial variability of hydraulic conductivity in the Choushui River alluvial fan, Taiwan. Hydrological Processes 2004;18(7):1333e50. [18] Yeh WWG. Review of parameter identification procedures in groundwater hydrology. Water Resources Research 1986;22:95e108. [19] Freeze RA, Cherry JA. Groundwater. Englewood Cliffs NJ: Prentice-Hall; 1979. [20] Tindall JA, Kunkel JR. Unsaturated zone hydrology for scientists and engineers. London: Prentice-Hall; 1999. [21] Kozeny J. Hydraulik. Germany: Springer-Verlag; 1953.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
The feasibility of using circulating groundwater as renewable energy sources for air-conditioning in Taipei basin Chihping Kuo*, Hungjiun Liao Department of Construction Engineering, National Taiwan University of Science & Technology, No. 43 Sec. 4 Keelung Road, Taipei 10699, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2011 Accepted 28 July 2011 Available online 30 August 2011
To reduce the uplift pressure of groundwater, many wells, deep into the Chingmei gravel stratum underneath Taipei city, were installed during construction of Taipei MRT underground stations. These wells can also be used to establish a circulating groundwater system for air-conditioning. To study the feasibility of this circulating cooling system and the thermal response of the Chingmei gravel stratum, some groundwater parameters of the Chingmai gravel stratum were studied such as pumping and recharging capacities, flow direction, and flow velocity etc. Two small scale in-situ discharge tests and numerical simulations were used for this purpose. However, the tests could not be performed for a long period to prove the purpose, the computer modeling provided the predicted long time response of the stratum. The discharge tests on the wells confirmed the discharge capacity of the Chingmei stratum and the numerical results also indicate that circulating groundwater with a steady temperature can be provided to cool down air-conditioners, as long as groundwater is flowing at a slow velocity. This study confirms the feasibility of using the groundwater of the Chingmei gravel stratum as a steady and clean cool source for air-conditioners. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Groundwater Thermal response Chingmei gravel stratum Air-conditioner Urban heat-island effects
1. Introduction Due to global warming and urban heat-island effects, the mean daily summer surface temperature of Taipei city has been rising, from about 28.0 C in 1960 to 29.5 C in 2005 [1]. From the energy point of view, air temperature ascent of 1 C causes more electrical power consumption about 600,000 kW in Taiwan [2]. Airconditioners have become a necessity in summer to cool down the indoor temperature. But the heat exhausted from the airconditioners will further increase the air temperature and make the situation worse [3]. To mitigate this problem, the exhaust heat might be alternatively discharged into the underground reservoir in the Chingmei gravel stratum underneath Taipei city. During construction of the Taipei MRT subway system, many wells which are more than 50 m deep were installed to reduce the uplift pressure of groundwater. These wells provide a good passage for groundwater to circulate to and from the Chingmei gravel stratum after completion of underground station construction. To make use of the groundwater stored in the Chingmei gravel stratum, a circulating groundwater system utilizing these wells can be established to provide a cooling device for air-conditioning (Fig. 1). It is understood * Corresponding author. Tel.: þ886 2737 6560; fax: þ886 2737 6606. E-mail addresses: [email protected], [email protected] (C. Kuo). 0960-1481/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2011.07.046
that over pumping groundwater can result in large scale ground subsidence. So, this system is designed only to circulate groundwater to cool down air-conditioners. No groundwater is pumped out of the underground reservoir. Although using an underground reservoir as a geothermal resource has been commonly adopted in countries located in high latitudes, its use in countries located at lower latitudes needs some modifications. Unlike the study on Aquifer Thermal Energy Storage (ATES) system [4,5], the proposed system focuses on two subjects: one is the discharge capacity of the Chingmei gravel stratum; the other is the steady supply of circulating groundwater with a low temperature. To evaluate the feasibility of this cooling system for air conditioners, some basic properties of the Chingmei gravel stratum are to be studied such as the pumping and recharging capacity, the velocity and direction of groundwater flow etc. To simulate the groundwater flow of the Chingmei gravel stratum and the thermal response to the circulating groundwater, a conceptual model for the entire Taipei basin is adopted here. Two numerical programs were used: one is SHEMAT (Simulator for Heat and Mass Transport [6]), the other is MODFLOW (Modular ThreeDimensional Groundwater Flow Model [7,8]). The former is a 3-D finite-difference numerical program used to simulate the heat transport behavior (i.e., diffusion and dissipation) of groundwater and the thermal response of the stratum. SHEMAT is an easy-to-use
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valve Higher temperature circulating water
Air-conditioner
Lower temperature circulating water Heat exchanger G.L.
Warm groundwater
Cool groundwater valve Piezometric level
pump
Warm groundwater
Cool groundwater
Fig. 2. Geographical structures of Taipei basin, observation wells locations, Geological investigating boreholes, and study area.
Chingmei Gravel Stratum
Fig. 1. Schematic diagram of circulating groundwater system; major devices to use groundwater as a cooling source for air-conditioning including circulating wells, pump and heat exchanger.
tool, handles coupled processes of groundwater flow combined with heat transfer, to simulate the thermal field of an aquifer but it is unable to simulate hydrogeological behavior in detail. For example, the groundwater flow (i.e., velocity and direction) generated by various thickness or inclined ground layers is unabled to be evaluated by SHEMAT. To compensate for this problem, MODFLOW, a widely used software to simulate various hydraulic systems and groundwater flow model, was used to determine the groundwater flow parameters first, before they are passed to SHEMAT to do the follow-ups. A Taipei MRT underground station construction site located in the center of the basin was chosen as the field test site to verify the numerical results obtained from SHEMAT and MODFLOW analyses.
from Xindian river; and sandy layers are deposited by the Dahan river. Basically, the sandy and clayey deposits are underlain by thick gravel deposits in the Taipei basin. Based on boring log data, a schematic diagram of the basin profile can be drawn, as in Fig. 3. Sungshan stratum consists of alternative sandy and clayey layers; the Chingmei stratum is an independent gravel stratum; the Wuku stratum and the Panchiou stratum are also gravel strata. The geological information indicates that the Chingmei stratum is an alluvial fan gravel stratum which covers the entire Basin. Besides, a varve layer, which consists of alternative layers of mud and clay, is found beneath the Chingmei stratum. In this study, the Sungshan stratum and the varve layer are considered to be aquitard layers (i.e., a layer with a much lower permeability than other layers) bounding the Chingmei stratum as indicated by Teng et al. [9]. The major intake zones of groundwater in Taipei basin are located around the midstreams of the Xindian river and the Dahan river [10]. In the eastern part of the Taipei basin, the river water and rain water are able to infiltrate through the sediments to the underlying aquifers. But the clayey sediment from the Keelung river has low permeability. So the vertical recharge of groundwater is limited in the Keelung river area. The secondary groundwater intake zone in Taipei basin is from the edges of the basin, such as the surrounding mountains, hills, and tablelands [11]. Finally, Hong
2. Conceptual groundwater model of Taipei basin 2.1. Description of Taipei basin Taipei basin (Fig. 2) covers approximately 243 km2 in area with an average elevation of 20 m above sea level. The basin is surrounded by Datun volcanoes in the north; Linkou tableland in the west; and hills and mountains in the east and the south. Several major rivers meander through the Taipei basin: the Tanshui river, the Keelung river, the Xindian river and the Dahan river. Sediments deposited from different rivers vary from one another. Generally, soft clays are deposited by the Keelung river; gravels are deposited
Fig. 3. Schematic diagram of profiles in the section along deposited direction (from Xindain in the southeast to Tanshui in the northwest of the basin) and the possible distribution of subsurface stratums.
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et al. [10] pointed out that the groundwater table of Taipei basin is highly related to the base flow from the Xindan river and concluded that the groundwater is mainly recharged by the Xindan river. As estimated by Chen [12], there is about 68.4 billion m3 of groundwater stored in the Chingmei gravel stratum. The temperature of groundwater ranges from 24 C in winter to 28 C in summer [13]. It can be a conditions related to the resource for the cooling medium needs of air-conditioners. Therefore, groundwater such as pumping and recharging capacity, flow direction and velocities are important features needed to evaluate the feasibility of the proposed groundwater circulation system. 2.2. Groundwater flow model of Taipei basin The groundwater flow model of the Taipei basin was established using a MODFLOW model. MODFLOW is a modular 3-D finitedifference groundwater flow model which can simulate groundwater flow under complex hydraulic conditions with various hydrological processes. In particular, the code model UCODE [14] of MODFLOW version 5.3 can auto-calibrate the environment to increase the accuracy of the conceptual groundwater flow model. The conceptual groundwater flow model of the Taipei basin was divided vertically into three layers: the upper one is an aquitard (Sungshan stratum), the middle one is the confined aquifer (Chingmei stratum), and the lower one is an aquitard also (i.e., varve). The three layers in the model were divided into 2500 cells with 50 columns and 50 rows. The mesh is square in shape covering an area of 500 500 m2. The change in terrain and thickness for each layer in space can be determined by interpolation from the boring log data provided by the Central Geological Survey (CGS) of Taiwan. Three types of boundary condition were assigned: (1) the estuary of the Tanshui river is regarded as the constant-head boundary (i.e., groundwater level ¼ sea level ¼ 0); (2) no infiltration occurs in the hills on the eastern and southern sides, the volcano on the northern side, and the tableland on the western side are regarded as the zero-flow boundaries (i.e., groundwater flow ¼ 0); and (3) the midstreams of the Xindian stream and the Dahan stream are regarded as the recharging boundaries. Hong et al. [10] indicated that such a recharging boundary could best correspond to the observed groundwater data of the Taipei basin. In this study, eight observation wells No.115, 116, 117, 114, 113, 122, 111,
177
and 123 (Fig. 4) are regarded as recharge boundaries; the hills, volcano, and the tableland are regarded as zero-flow boundaries. To calibrate the model, monthly groundwater level data from the 23 observation wells, provided by the Water Resources Agency, covering January 1997 to December 2001, were used. To verify the model, monthly groundwater level data from the 23 monitoring wells from January 2002 to December 2002 were used. The hydraulic parameters of the aquifer were determined from the pumping tests and checked with the numerical results published by Tsao et al. [15]. To examine the suitability of these parameters, the values of 0.12e3.20 m2/min and 0.00192 as suggested by Tsao et al. [15] were taken as the initial values of Transmissivity and Storage coefficient respectively for the inverse modeling. Furthermore, the spatial distribution of the conductivities was estimated by the Kriging method for regional scale. After the above parameters had been inversely calibrated in the numerical model, a zonation distribution could be applied to scale up the calibration parameters [16e18]. The zonation distribution was adopted to establish the spatial patterns of the flow and the recharge rates. Following this process, the amount of groundwater recharge to the basin can be calculated. Since the exact amount of groundwater recharge to the basin cannot be accurately measured, the conceptual model UCODE [14] was used to calibrate the zonation distribution and the transmissivity. Model calibration was carried out through auto-calibration and artificial trial-and-error by matching observed groundwater levels and simulated groundwater levels. Spatial distributions of groundwater flow were estimated at different times. After the above exercise, the model can be used to simulate the groundwater flow vector and head contours (Fig. 4) for the initial approximate determination of local groundwater flow. 3. Simulating the thermal response of aquifer 3.1. Studied subject During the construction of a Taipei MRT underground station (O8 station), 22 deep wells were installed to reduce the uplift groundwater pressure from the Chingmei gravel stratum. The plane view of the site and the layout of test wells are shown in Fig. 5. These wells were to be sealed off after the construction. To make the best use of these wells, the study will evaluate the feasibility of converting these wells to become flow passages as part of the
Fig. 4. Plane view of hydrogeological model meshes and flow boundaries; groundwater flow vector and head contours in the end of simulation.
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Fig. 5. Plane view of the MRT O8 station site and the layout of wells involved in discharging test and small scale thermal test.
cooling system for air conditioners. These wells have a diameter of 0.8 m and are about 80 m deep. The design pumping rate of each well was 6 m3/min. But not all 22 wells were operated at the same time. The exact number of wells actually operated varied with the construction sequence and the need for dewatering. Normally, 3 to 6 wells were operated as a group. The Chingmei stratum is located from G.L.-54.6 to 86.75 m beneath the O8 station. In between ground surface and Chingmei stratum lies the Sungshan stratum which has alternating layers of silty sand and silty clay. Underlying the Chingmei stratum is a layer of varve clay with a thickness of about 1.2 m. The cooling capacity needed for the cooling towers of the O8 station is 1184 kW.
area. Meanwhile, a line of virtually discharging wells were placed at the inflow boundary. The parameters used in this model were as follows: effective porosity ¼ 0.25 and conductivity ¼ 102 cm/s [19], thermal capacity ¼ 1.875 MJ/m3 K and conductivity ¼ 1.308 W/ m K [20].
3.2. Local groundwater flow model The local groundwater flow model adopted for the O8 station area was extracted from the conceptual model of the entire Taipei basin. Using the local model, the flow direction and quantity of groundwater were calculated. The direction of groundwater flow under the study area was from east to west (Fig. 4). The water budget in and outside the study area was computed by MODFLOW. The average water budget during the simulated period was about 1.95 102 cmd (m3/day) through a cross section of 1500 m2. It is equivalent to 3.5 104 cmd for the whole basin. The value of the water budget in the study area can be converted into groundwater flux (i.e., groundwater velocity) of 13 cm/day. 3.3. Local heat transport model Based on the local groundwater flow model, a heat transport model established from the SHEMAT program was used to simulate the heat propagation behavior which resulted from discharging water. 3-D grids consisting of 122 120 3 cells were used. In the center area (2000 m 2000 m), each cell represented an area of 1 m 1 m; outside the center area, each cell represented an area of 10 m 10 m or 20 m 20 m depending on the location. The upper and lower layers were treated as an aquitard, and the middle layer was treated as a confined aquifer. However, SHEMAT can only be used in a layer of constant thickness for the current version. So, a mean value of 30 m was adopted to approximate the thickness of the aquifer. To simulate the flowing groundwater condition in the Chingmei stratum with the SHEMAT program, a line of virtual pumping wells was placed at the effluent boundary of the study
Fig. 6. The well dimensions and schematic arrangement of in-situ heat transport test.
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179
Fig. 7. Observed data and simulated results of discharging test in O8 station.
4. In-situ test for verification and discussion on results Two in-situ discharging tests were performed at the O8 station to verify the suitability of hydrological and heat transport models established by the above mentioned process. One was done in summer; the other was in winter. To reduce the interference from other pumping wells, both tests were carried out during the period when no other wells were pumping at the site. 4.1. Discharging test to verify local groundwater flow model In order to test the discharging capacity of the Chingmei stratum and to estimate the transmissivity of the local Chingmei stratum around the test wells, an in-situ discharging test was performed. Four wells namely PW7, PW8, PW9, and PW14 were used in this test, one for discharging and the others are for observation (Fig. 5). The well dimensions are the same as PW21 and PW22 shown in Fig. 6. During the discharging process, the rise of water level in the observation wells was recorded. As shown in Fig. 7, up to 7 m3/min of water flow rate can be successfully discharged by gravity force only through a well 80 cm in diameter and a 30 m long screen
opening in the Chingmei gravel stratum. The transmissivity (T) was computed from the above observed data by using Kozeny’s formula [21]. An average value of T ¼ 1.53 m2/mim was obtained from this test. By comparing the observed data and MODFLOW results (Fig. 7), it can be seen that the water level of wells PW8 and PW9 simulated by MODFLOW is in good agreement with the observed data; but MODFLOW overestimates the observed water level of wells PW14 and PW7. This indicates that the values of hydrological parameters can vary significantly in a short distance. Gravel soil with low-permeability may exist near wells PW14 and PW7. 4.2. Small scale thermal test for local heat transport model To simulate the thermal response of Chingmei gravel stratum to the circulating cooling water from air-conditioners, a package airconditioner was used in this small scale heat transport test. The layout and arrangement of the test facilities are shown in Figs. 5 and 6, respectively. The heat exchange capacity of this air conditioner was able to increase the temperature of circulating groundwater by 3 C at a flow rate of 0.146 m3/min. Four temperature sensors (RTD1 to RTD4) were installed to measure the
Fig. 8. Measured and simulated results for in-situ heat transport test.
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Fig. 9. Simulated temperature of pumped water (intake at PW22) at different groundwater flow velocities.
temperature of circulating groundwater, aquifer, and ground respectively. The distance between pumping well (PW22) and recharging well (PW21) was 15.4 m. An observation well (PS1023) was located next to PW21. The elapsed time of the test was 3 days and the test results are shown in Fig. 8. During the test, it took about 1.5 h for the circulating water from air conditioner to increase the water temperature in the discharging well to a steady value of 27 C (RTD2). The groundwater temperature inside the pumping well (RTD1) and in the Chingmei gravel stratum (RTD3) was about the same and was equal to 24 0.2 C. Basically, the groundwater temperature was not affected by the cooling water from the package air-conditioner. The air temperature during the test period increased from 15.3 C to 16.4 C. The in-situ test was simulated with a numerical model and the simulated results are plotted in Fig. 8. No noticeable temperature change was observed at well PW22 (pumping well) from the numerical analysis over a three-day period either. Even after 900 days, the recharging groundwater with higher temperature still has no influence on the pumping well located 10 m away (under a groundwater flow velocity of 13 cm/day). 4.3. Predicting ground thermal response to air conditioner of O8 station To eliminate the 1184 kW of heat generated by the airconditioners of Taipei MRT O8 station, the required cooling capacity of the medium calculated by thermal equilibrium equation is as follows: temperature increase of cooling water ¼ 5 C, flow
rate ¼ 3.4 cmm (m3/min). To carry the heat away from the O8 station, the flow characteristics of the groundwater in Chingmei gravel stratum will be accounted for and evaluated. Two groundwater flow velocities, 103 cm/day (no flow) and 13 cm/day, were adopted in the analysis to simulate the no flow and slow flow groundwater conditions respectively. The distance between pumping and recharge well was 15.4 m. Over a period of 900 days of water circulation, the simulated temperature change of the aquifer at the intake of pumping well PW21 is shown in Fig. 9. It can be seen that the groundwater temperature at PW21 under the no flow groundwater condition keeps rising to 31.5 C after 900 days. However, the temperature under slow flow (13 cm/day) groundwater condition quickly rises to 27.7 C after 15 days, drops slightly down to 27.6 C and then remains at this temperature. The worst location of circulating wells based on the groundwater flow directions might be installing the pumping well at the downstream. The dissipated heat to the discharging well at the upstream may have a quicker impact on the pumping well and downgrade the cooling capacity essentially. The simulation for the above location of circulating wells and under the same groundwater flow velocity (13 cm/day) was made. It shows the temperature at PW21 quickly rises to 31.5 C after 10 days. Therefore, to avoid choosing the pumping well at the downstream location may reduce the impact of the dissipated heat from the pumping well. The distributing pattern of groundwater temperature after 900 days circulation is shown in Fig. 10. During water circulation, the temperature around the recharge well increases gradually. Obviously, the shape of the temperature distribution diagram changes
Fig. 10. 2-D view of long-term simulated temperature distribution at different groundwater flow velocities.
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181
Fig. 11. 2-D view of simulated short-term temperature change and distribution at different groundwater flow velocities, temperature level scale same as Fig. 10.
with groundwater flow velocity. For the no flow groundwater condition (¼103 cm/day), the heat accumulates around the recharge well (Fig. 11). Shortly, the heated water from the recharge well will be pumped up again. So the temperature of circulating groundwater will increase gradually. As a result, the effectiveness of heat exchange between circulating groundwater and air conditioner will decrease. In comparison, when the groundwater flow velocity is equal to 13.1 cm/day, the shape of temperature distribution changes (Fig. 11). The heated water from the recharge well is unable to reach the pumping well. So the water temperature from the pumping well can remain a steady 27.6 C over a period of 900 days. In other words, the groundwater in the Chingmei gravel stratum can be a good cooling source for the air conditioners of the O8 station.
designed thermal capacity of the cooling tower for MRT O8 station is 1184 kW (is equivalent to 336.8 RT refrigerating capacity). The original design is to use four cooling towers: two to carry away 315 kW of heat each and the other two to carry 277.12 kW each. The comparisons of electrical power consumption, water consumption, and facilities space requirement between two systems are listed in Table 1. The result shows that the power consumption of the two systems is about the same. However, water consumption and space requirements are significantly reduced with the groundwater circulation system. The last but not the least is that in the circulating water model no exhaust heat is discharged to the air. It can help a great deal to mitigate the heat-island effect in Taipei basin. 5. Conclusions
4.4. Comparison of power consumption between two systems To evaluate the energy efficiency of this groundwater circulation cooling system, the electricity consumption and the required space for facilities are compared with the traditional cooling towers. The Table 1 Comparison of electricity power consumption, water consumption, and installation space requirement between two systems (MRT O8 station). Item Cooling capacity Circulating water request Power request
Required flow rate Temperature rising Pump lift Pump for circulating water Motor inside cooling tower Electricity consumption
Water consumption Space required for equipment
Cooling tower
Circulating well
1184 kW 3.4 cmm þ5 C 9m 72 HP
30 m 82 HP
14 HP
e
64.15 kW hr
61.30 kW hr
4.4 m3/day 52 m3
0 0.62 m3
The feasibility of using a circulating groundwater system to cool down the air-conditioners is evaluated here. The efficiency of this system is related to the groundwater direction, flow velocity, and thermal response of the aquifer. Based on the numerical modeling, in-situ discharging test results and a small scale groundwater circulation test, the following conclusions can be made: 1. Due to the variation of hydrogeological condition of the Chingmei gravel stratum, a groundwater flow model using MODFLOW was established from boring log data provided by Central Geological Survey (CGS) and calibrated by groundwater data provided by Water Resource Agency (WRA). The in-situ discharging test was used to verify the local hydraulic parameters (i.e., transmit and storage) determined from numerical analysis. An east-to-west groundwater flow direction and a flow velocity of 13 cm/day were obtained from this study. 2. The heat transport model using SHEMAT together with the hydraulic parameters determined from the MODFLOW model were employed to simulate the thermal response of the aquifer. The thermal response of the Chingmei gravel stratum to the air conditioners of the O8 station, was simulated by the proposed
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model. It shows that the heated water from the recharge well is unable to reach the pumping well (pumping rate ¼ 3.4 cmm) located 15.4 m away when the groundwater flow velocity is equal to 13 cm/day. So the water temperature from the pumping well can remain a steady 27.6 C over a period of 900 days. Compared with the small scale heat transport test results, they are in good agreement with the simulated results. No water temperature increase in the pumping well or in the aquifer was found. In other words, the groundwater in Chingmei gravel stratum can be a good cooling source for the air conditioners of the O8 station. 3. The power consumption of using cooling towers and circulating groundwater to cool air-conditioner is about the same. However, water consumption and space requirements are significantly reduced with the groundwater circulation system. 4. No exhaust heat is discharged to the air by using the proposed system, that can help a great deal to mitigate the heat-island effect in Taipei basin. References [1] Chen TC, Wang SY, Yen MC. Enhancement of afternoon thunderstorm activity by urbanization in a valley: Taipei. Journal of Applied Meteorology and Climatology 2007;46:1324e40. American Meteorological Society. [2] Lin JM. Urban heat island effect and its environmental implications. Journal of Ecology and Environmental Sciences 2010;3(1):1e15. [3] Hsieh CM, Aramakia T, Hanakia K. Estimation of heat rejection based on the air conditioner use time and its mitigation from buildings in Taipei City. Building and Environment 2007;42:3125e37. [4] Bridger DW, Allen DM. Designing aquifer thermal energy storage systems. ASHRAE Journal 2005;47:S32e8.
[5] Dickinson JS, Buik N, Matthews MC, Snijders A. Aquifer thermal energy storage: theoretical and operational analysis. Geotechnique 2009;59:249e60. [6] Clauser C, editor. Numerical simulation of reactive flow in hot aquifers, SHEMAT and processing SHEMAT. Germany: Springer; 2003. [7] Harbaugh AW, McDonald MG. Programmer documentation for MODFLOW-96, an update to the U.S. geological survey modular finite-difference groundwater flow model. Open-File Report: 96e486. USGS; 1996. [8] Chiang WH, Kinzelbach W. 3D-groundwater modeling with PMWIN. Germany: Springer; 2005. [9] Teng LS, Lee CT, Peng CH, Chu JJ, Chen WF. Origin and geological evolution of the Taipei basin, northern Taiwan. Western Pacific Earth Sciences 2001;1:115e42. [10] Hong NM, Tan CC, Lin ST, Tsay TS. Uncertainty analysis of groundwater exchange at boundaries in Taipei basin. Geophysical Research Abstracts; 2007:9. European Geosciences Union. [11] Chia YP, Chang MS, Liu WY, Lai DC. Hydrogeological characteristics of Taipei basin. Taipei basin subsurface geology and engineering environment, central geological survey monograph. In: Special publication, vol. 11; 1999. p. 393e406. [12] Chen WF. The groundwater in Taiwan. Taipei: Sinobooks; 2005. [13] Liao HJ, Kuo CP, Wang CS. Using circulating groundwater as cooling medium for air conditioner at NTUST. In: The 17th Southeast Asian Geotechnical Conference, 2010; p. 409e412. [14] Poeter EP, Hill MC. Documentation of UCODE, a computer code for universal inverse modelling. US Geological Survey, Water-Resources Investigations Report 98e4080; 1998. [15] Tsao YS, Lin CN, Tan YC, Mao AS. Simulation and application of mathematical groundwater model in Taipei Basin. Taipei: MOEA; 1985. [16] Gau HS, Liu CW, Chen JS, Tsao YS. Estimation of optimal yield in Yunlin area of Taiwan using decision analysis. Journal of the American Water Resources Association 1998;34:1295e309. [17] Jang CS, Liu CW. Geostatistical analysis and conditional simulation for estimating the spatial variability of hydraulic conductivity in the Choushui River alluvial fan, Taiwan. Hydrological Processes 2004;18(7):1333e50. [18] Yeh WWG. Review of parameter identification procedures in groundwater hydrology. Water Resources Research 1986;22:95e108. [19] Freeze RA, Cherry JA. Groundwater. Englewood Cliffs NJ: Prentice-Hall; 1979. [20] Tindall JA, Kunkel JR. Unsaturated zone hydrology for scientists and engineers. London: Prentice-Hall; 1999. [21] Kozeny J. Hydraulik. Germany: Springer-Verlag; 1953.