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Architectural Engineering -- Faculty Publications Architectural Engineering

2014 Investigation of a Coupled Geothermal Cooling System with Earth Tube and Yuebin Yu University of Nebraska–Lincoln, [email protected]

Haorong Li University of Nebraska-Lincoln, [email protected]

Fuxin Niu University of Nebraska-Lincoln

Daihong Yu University of Nebraska-Lincoln, [email protected]

Follow this and additional works at: http://digitalcommons.unl.edu/archengfacpub Part of the Architectural Engineering Commons, Construction Engineering Commons, Environmental Design Commons, and the Other Engineering Commons

Yu, Yuebin; Li, Haorong; Niu, Fuxin; and Yu, Daihong, "Investigation of a Coupled Geothermal Cooling System with Earth Tube and Solar Chimney" (2014). Architectural Engineering -- Faculty Publications. 88. http://digitalcommons.unl.edu/archengfacpub/88

This Article is brought to you for free and open access by the Architectural Engineering at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Architectural Engineering -- Faculty Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Applied Energy 114 (2014) 209–217

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Applied Energy

journal homepage: www.elsevier.com/locate/apenergy

Investigation of a coupled geothermal cooling system with earth tube and solar chimney ⇑ Yuebin Yu a, Haorong Li a, Fuxin Niu a, Daihong Yu b, a Durham School of Architectural Engineering and Construction, College of Engineering, University of Nebraska-Lincoln, Omaha, NE, USA b Department of Architectural Engineering, Lawrence Technological University, Southfield, MI, USA highlights

We investigated a coupled geothermal system with earth tube and solar chimney. Three experiment tests were conducted in a sequence as passive, active, and passive. We analyzed indoor air condition, cooling capacity, and soil temperature. The system is feasible to provide cooling for free and supply a high airflow rate. Preliminary control and soil saturation and recovering should be considered. article info abstract

Article history: We present a systematic study of a coupled geothermal cooling system with an earth-to-air heat exchan- Received 12 April 2013 ger and a solar collector enhanced solar chimney. Experiments were conducted with an existing test facil- Received in revised form 5 September 2013 ity in summer to evaluate the performance of the system, in terms of passive cooling capability, active Accepted 17 September 2013 cooling capability, and soil thermal capability. Correspondingly, three different tests were carried out in 43 days in a sequence, from a passive cooling mode to an active cooling mode, and then back to a pas- sive cooling mode. The results show that the coupled geothermal system is feasible to provide cooling to Keywords: the facility in natural operation mode free without using any electricity. The solar collector enhanced Earth-to-air solar chimney can provide more airflow to the system during the daytime with a stronger solar intensity. Earth tube Solar chimney The thermal sensation analysis based on predicted mean vote and predicted percent of dissatisfied people Coupled system indicates that the indoor air condition under the natural airflow stage was more acceptable in terms of Cooling than that of the forced airflow stage. The cooling capacity of the coupled system drops Active and passive quickly after the one week forced airflow test due to the underground soil temperature increase. It takes the soil over two weeks to fully recover from the thermal saturation after the forced air test. In addition, the underground soil temperature test results indicate that the underground heat dissipation in the hor- izontal level was greater than that in the vertical level. The findings suggest that a minimum level of con- trol on the system and consideration on soil saturation is needed to further improve the overall performance. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction changer (EAHE) is such a ground-coupled device that conditions the ambient air for various cooling, heating and ventilation pur- The utilization of geothermal energy can be an open system poses. EAHEs have been applied in agricultural facilities and green- with direct water exchange [1–3] or a closed system with a med- houses in the United States over the past several decades [6]. ium circulating in the heat exchangers [4,5]. A ground-coupled In the ambit of bioclimatic architecture, where a building has a heat exchanger (GCHX) is an underground heat exchanger that direct connection with nature, EAHEs have been assigned an can capture heat from and/or dissipate heat to the ground. They increasingly important role in cooling primary air free for buildings use the earth’ near constant subterranean temperature to warm during hot weather [7]. Air passing through the buried pipes in an or cool air or other fluids for residential, agricultural or industrial EAHE system can be either from outdoor air or circulation air. To uses. Among the different types of GCHXs, an earth-to-air heat ex- assess the performance of EAHEs, Krarti and Kreider [8] proposed a simplified analytical model. The transfer problem was ⇑ treated as a transient problem without condensation, Corresponding author. Tel.: +1 248 204 2586. assuming a periodic variation of both air source and ground E-mail addresses: [email protected] (Y. Yu), [email protected] (D. Yu).

0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.09.038 210 Y. Yu et al. / Applied Energy 114 (2014) 209–217

Nomenclature

t total Roman letter symbols ACP above cooling pipe fcl ratio of clothed body surface area to nude body surface EAHE earth to air heat exchanger area GCHX ground-coupled heat exchanger h enthalpy, kJ/kg HR ratio 2 NCP next to cooling pipe hc convectional heat transfer coefficient, W/m K M metabolism, W/m2 OA outdoor air Pa partial water vapor pressure, Pa OAT outdoor air temperature Q rate of heat transfer, kW PMV predicted mean vote T temperature, °C PPD predicted percent of dissatisfied people V volume flow rate of air flow, m3/s RA room air W external work, W/m2 RAT room air temperature SA supply air Subscripts/acronyms SAT supply air temperature air, a air cooling cooling Greek letters 3 cl cloth q specific density, kg/m mrt mean radiant temperature system system

temperatures. They validated the model against measured data is a natural draft device which utilizes energy produced by solar and applied it for parametric analysis on cooling capacity and air radiation to build up stack pressure, consequently, driving airflow temperature with the pipe diameter and air velocity. through the chimney [20]. As the solar chimney problem involves Without considering the moisture transfer in the air, EAHE is both fluid movement as well as heat transfer, it was found that the like the other GCHXs, where the tubes and eventually soil are uti- computation fluid dynamics (CFD) is a good tool to describe the lized as a heat sink/source. The heat transfer rate of a GCHX is crit- system numerically [21,22]. Other techniques have also been used ical since it determines the overall performance of a system that beside the CFD to simulate the solar chimney systems. Two-dimen- couples a GCHX [9–11]. Akpinar and Hepbasli [9,10] and Hepbasli sional conservation equations for mass, momentum and energy et al. [10] investigated the performance of a GCHX coupled heat were used to describe the system. The conservation equations pump system. They concluded the heat extraction rate of the sys- were then solved by finite difference-control volume numerical tem was impacted by a GCHX in terms of the depth below grad, method [23]. When a solar chimney is used in conjunction with pipe size, spacing, and soil type beside other system parameters. EAHE, and heating with natural airflow can be Nam et al. [11] developed a numerical model to predict heat ex- obtained. change rates of a GCHX in a coupled system. It was later Despite the research findings, very few experimental studies applied to find the optimum design for an office building in Tokyo, have been carried out to systematically evaluate the performance Japan. Zeng et al. [12] utilized a quasi-three-dimensional model for of an EAHE-solar chimney coupled geothermal system in natural the vertical GCHX heat transfer analysis. By comparing the single and forced airflow conditions. In addition, research about the soil U-tube with double ones, they concluded that the borehole with temperature saturation and recovering of such a system, which double U-tubes is superior to the single U-tube regarding the heat could strongly impact the operation and performance, was not re- transfer rate. ported. This research conducted three field tests and systematic Understanding the soil temperature field underground can analysis of an EAHE-solar chimney coupled geothermal system. guide the reasonable operation of a GCHX coupled system and The paper starts with a brief introduction of the test facility fol- avoid the low performance due to the heat transfer attenuation. lowed by a description on the experimental procedure. After that, During the last decade, investigations about the soil temperature the room air condition, thermal comfort, cooling capacity, and soil field and heat transfer have been conducted by researchers thermal capability are discussed. The paper concludes with a dis- [13,14]. Some researchers obtained the soil temperature distribu- cussion on the findings and future research directions. tion under the nature condition and intermittent operation, ana- lyzed the variation of the soil temperature and heat balance of the system, and put forward the method of calculation of the tem- 2. The test facility perature for heat extraction or injection utilizing the numerical and experimental method [15–17]. 2.1. Introduction of the test facility Just like other GCHXs, an EAHE is a passive device that provides a thermal coupling between the medium fluid and the soil. A test facility for an EAHE-solar chimney coupled geothermal Additional components are needed to provide the driving force system was constructed in Omaha, Nebraska, USA. The design of for the medium fluid. In a pure passive design of an EAHE condi- tioning system, a solar chimney may be used to pull the air through Table 1 the tubes. A solar chimney is a vertical or inclined air channel Main design calculated values. where its bottom end is connected to the building to be cooled and ventilated [18]. This air channel generates air movement by Design component Calculated value Unit buoyancy forces and stack ventilation, drawing cooler air through Total cooling load 2.8 kW the building in a continuous cycle in which hot air rises and exits Sensible cooling load 1.6 kW from the top of the chimney [19]. In other words, a solar chimney Latent cooling load 1.2 kW Y. Yu et al. / Applied Energy 114 (2014) 209–217 211 the test facility was based on a steady-state model calculation [24]. temperatures above and next to the EAHE. Table 2 shows the A culvert steel EAHE 57 m long with 0.45 m diameter was buried at instruction of all sensors. The multiple sensors measuring under- a depth of about 3 m below grade. The test facility is a one-story ground soil temperatures were located above and next to the cool- building with external dimensions of 19.4 m length, 4.9 m width, ing pipe (EAHE) with different distances. Fig. 2 provides the and 3 m height. It includes a main testing room with dimensions sectional view of the buried soil temperature sensors. 15.2 m length, 4.52 m width, and 2.4 m height. The design cooling load shown in Table 1 based on the TRACE 700 software. A solar 2.3. Data acquisition and logging collector was designed and constructed with an absorbing surface area of 20 m2 considering that the solar irradiation is 400 W/m2 All the measurement instruments were connected to the data with a heat loss coefficient of 8.5 W/(m2 °C) and an absorptivity collection modules, which in turn were connected to a computer of 0.8. The solar collector was constructed by wood and a special system. The LabVIEW software package (version 6) was used to type of solar glass that traps the solar irradiation inside the collec- create a customized program that helped in calibrating, controlling tor. The solar chimney height and diameter were selected as and monitoring the sensors. All the data collection modules and 12.2 m and 0.457 m respectively to provide an available draft of the data loggers were synchronized to log and store an average more than 5 Pa at a temperature of 57 °C. measurement every 15 min. The measurements were then stored Fig. 1 illustrates the schematics of the coupled geothermal sys- and organized daily as notepad files on the computer system for tem. As shown in Fig. 1 an EAHE is coupled with a solar chimney system performance analysis. and solar collector to provide cooling and ventilation for the solar energy research test facility. The cooling and ventilation process 3. Experimental procedure works when the air inside the solar collector is continuously warmed up by the solar radiation. The air temperature difference The experiments were conducted in summer, from July to Sep- between the inlet of solar collector and the outlet of the solar tember, 2007. In the 43-day testing period, both the natural and chimney generates a buoyancy effect to draft the air from the in- forced cooling modes were carried out to analyze the performance door to the solar chimney. The indoor therefore becomes negative of the coupled geothermal system. The effect of the air on the pressurized due to the air pull strength and results in continuous underground soil temperature and the recovery ability of soil tem- air flowing from the outdoor into the EAHE. As the air enters the perature were investigated. This testing period has been divided EAHE, it is conditioned through the heat and mass transfer process into three time portions. with the tube and soil. The conditioned air provides the cooling capacity to the space and removes the load [19]. In a forced cooling The first test was conducted in a passive/natural cooling mode. mode, a installed in the path of the tube is turned on to drive It lasted for 14 days, started on July 24th, 2009 and ended on the air. August 6th, 2009. The main goal of the first test was to run the all passive, naturally driven, coupled system and analyze 2.2. Specs of sensors its performance for space cooling. The second test was conducted in an active cooling mode. It The main measurements that had been recorded for analyzing lasted for 9 days, started on August 7th, 2009 and ended on the performance of the coupled system are: the indoor relative August 15th, 2009. The second test had two main goals: the first humidity (%), outdoor relative humidity (%), supply air relative was to evaluate the thermal performance of the fan assisted humidity (%), supply air flow rate (m3/h), solar collector airflow coupled system, through assessing the maximum cooling rate (m3/h), average indoor temperature (°C), supply air capacity that it can provide for the testing facility building. temperature (°C), outdoor air temperature (°C) and underground The second goal was to create soil saturation and study how

Sun

Sensor Abbreviations: OAT: outdoor air temperature SAT: supply air temperature RT: SAF: supply air flow RAF:relief air flow OAT Solar chimney Test facility building

RT

RAF Outdoor air Solar collector

SAT SAF

Earth to air heat exchanger

Fig. 1. Schematic diagram of the cooling process by the coupled geothermal system. 212 Y. Yu et al. / Applied Energy 114 (2014) 209–217

Table 2 Instruction of sensors.

Measurement Location Type Accuracy Temperature a. Soil temperature a. As in Fig. 2 Type ‘‘T’’ thermocouples ±0.1 °C b. Indoor air temperature b. Between the EAHE outlet and the solar collector inlet c. Outdoor temperature c. Near the EAHE tube inlet d. Supply air temperature d. At the EAHE outlet e. Exhaust air temperature e. At the chimney inlet Relative Humidity a. Indoor air temperature a. Between the EAHE outlet and the solar collector inlet Humidity measurement ±2.5% b. Outdoor temperature b. Near the EAHE tube inlet c. Supply air temperature c. At the EAHE outlet Airflow rate a. Supply air flow a. The EAHE outlet inside the house Dwyer air velocity transmitters ±3% b. Relief air flow b. The solar collector inlet

3500

0.3m-ACP ACP: Above cooling pipe 3000 Distance from Sensor location NCP:Next to cooling pipe 2500 /h)

cooling pipe(CP) relative to CP 3 0.3m-ACP 2000 Natural airflow Forced airflow Natural airflow

0.15m-ACP 1500

0.15m-NCP 1000 Earth to air heat exchanger Airflow rate (m 0.3m-NCP 500

0 2009/7/24 2009/7/31 2009/8/7 2009/8/14 2009/8/21 2009/8/28 2009/9/4 0.15 0.15 Date

Fig. 2. Location of underground soil temperature sensors. Fig. 3. Airflow rate during the test.

long it will take the fan assisted coupled system to thermally and save fan power for the coupled geothermal energy system. saturate the soil surrounding the EAHE. For this test a constant The second test was in a forced airflow mode with the constant speed fan had been mounted on the top of the EAHE outlet, and speed fan turned on continuously. The airflow rate kept a constant it had been set to run continuously during the whole 9 days of value at 2750 m3/h. testing. Fig. 4 shows the comparison of indoor and outdoor air temper- The third test was conducted in a passive cooling mode. It lasted ature and Fig. 5 shows the comparison of indoor and outdoor air for 20 days, started on August 16th, 2009 and ended on Septem- humidity for the first two test periods, respectively. It can be seen ber 4th, 2009. The main goal of the third test was to determine from Figs. 4 and 5 that, under the natural airflow condition, the in- how long it will take the soil surrounding the EAHE to recover door air temperature fluctuated in a narrow range of 21–24 °C, and from the thermal saturation and return back to its original tem- the indoor air relative humidity was also maintained in between perature range. For this test the assisting fan had been turned 50% and 70%. For most of the time in the natural passive cooling off and the system was allows to run naturally. mode, the indoor air environment satisfied the ASHARE standard 55-2004 for acceptable thermal comfort [25]. This pattern of room air temperature and relative humidity changed after the active 4. Experimental results and analysis cooling mode was initiated. When the assisted fan was turned on from August 7th and the EAHE was utilized with a forced airflow, 4.1. Room air condition analysis

The EAHE and solar chimney coupled system can be a purely 40 passive system or equipped with a simple control system for space OAT RAT . In order to analyze the performance of the cou- 35 pled geothermal system under different types of airflow, which were natural and forced airflow mode, an assisted constant airflow 30 fan was installed. Fig. 3 shows the trended airflow rates in three 25 different testing stages. The first and third tests were natural air- flow mode from July 24th to August 6th in 2009 and August 16th 20 September 4th, respectively. During the natural cooling mode,

3 3 Temperature (ºC) the airflow rate varies from 0 m /h to maximum 500 m /h. The 15 peak value of the ventilation air occurred during the daytime when Natural airflow Forced airflow high solar intensity was strong. The peak airflow is close to a 10 desired value in design condition for a conventional forced air 2009/7/23 2009/7/27 2009/7/31 2009/8/4 2009/8/8 2009/8/12 2009/8/16 conditioning system, assuming 12.78 °C supply air temperature Date in summer. This demonstrates that the solar collector enhanced solar chimney is capable to provide a significant pulling force Fig. 4. Indoor and outdoor air temperature comparison. Y. Yu et al. / Applied Energy 114 (2014) 209–217 213

100 Table 3 Thermal sensation. 90 80 PMV 3 2 1 0 +1 +2 +3 70 Thermal Cold Cool Slightly Neutral Slightly Warm Hot sensation cool warm 60 50 40 30 either the positive or negative direction, PPD increases. The range Forced airflow

Relative humidity (%) Natural airflow of PMV with the satisfied thermal sensation scale referred from 20 ASHRAE is from 0.5 to +0.5. The corresponding PPD value was 10 OA-RH(%) RA-RH(%) 10.2% [25]. Eqs. (1)–(4)give the calculation formulas for PMV and 0 PPD [26]. 2009/7/23 2009/7/27 2009/7/31 2009/8/4 2009/8/8 2009/8/12 2009/8/16 Date PMV ¼ 0:028 þ 0:3033e0:036M n 3 Fig. 5. Indoor and outdoor air humidity comparison. ðM WÞ3:05 10 ½5733 6:99ðM WÞPa 0:42½ðM WÞ58:151:7 105Mð5867 PaÞ h 30 8 4 0:0014Mð34 TaÞ3:96 10 fcl ðTcl þ 273Þ Natural airflow Forced airflow Natural airflow 28 i o 4 RAT SAT ðTmrt þ 273Þ fcl hcðTcl TaÞ ð1Þ 26

24 4 2 PPD ¼ 100 95 e0:03353PMV 0:2179PMV ð2Þ 22 where, 20 Tcl ¼ 35:7 0:028ðM WÞ Temperature (ºC) 18 ()"# ðT þ 273Þ4 0:155 I 3:96 108 fcl cl fcl h T T 3 16 cl 4 cð cl aÞ ð Þ ðTmrt þ 273Þ 14 2009/7/24 2009/7/31 2009/8/7 2009/8/14 2009/8/21 2009/8/28 ( pffiffiffiffiffiffiffiffi 2:38 ðT T Þ0:25 for 2:38ðT T Þ0:25 P 12:1 V Date h ffiffiffiffiffiffiffifficl a cl a ffiffiffiffiffiffiffiffiair c ¼ p 0:25 p 12:1 V air for 2:38ðTcl TaÞ 6 12:1 V air Fig. 6. Comparison between indoor and supply air temperature. ð4Þ In this study, the thermal resistance of clothing was set as 0.5 the indoor air temperature had a significant fluctuation in the clo and the human activity as 45 W. The other variables, including range of 20–27.5 °C and the indoor air humidity fluctuated sharply the air velocity, were based on real measurements or calculated with a higher value and in a bigger range. The reason is reflected in values. The final results are plot in as is Figs. 7 and 8. It can be seen Fig. 6 where the comparison of room air temperature and supply from Fig. 7 the average PMV of indoor condition in the natural air- air temperature is plot. It can be seen from this figure the supply flow stage was 0.18, and almost all fell in the range of 0.5 to +0.5. air temperature was almost in the range of 16–21 °C during the The average PMV in the forced airflow stage was 0.85, which ex- natural passive airflow stage. However, during the forced airflow ceeded the ASHARE comfort standard. The main thermal sensation stage the peak of supply air temperature was boosted up. For most was slightly warm. When the outside air temperature was high the of the day time, the supply air temperature was higher than 22 °C. thermal sensation appeared to be warm. The PPD in the forced air- Sometime the temperature even exceeded the designed value flow stage was in the range of 10–40%. The maximum PPD reached which resulted in the thermal comfortable problem. As a 100% up to 70%. The results show that the indoor air condition under the outdoor air system, the high EAHE capacity does not necessary lead natural airflow condition was more comfortable than that of the to high cooling capacity. The results suggest that there exists an forced airflow condition. optimal value that the airflow rate not only enhances the cooling capacity of the EAHE but also provides a high cooling capacitance 4.3. Thermal capacity analysis to the coupled system. When a forced fan is applied with the EAHE coupled cooling system, a minimum control on the fan speed or With a fixed cooling load, the indoor air condition of the cou- position is needed to increase the flexibility, balance the pled system is determined by the total cooling capacity and the cooling capacity demand and supply, and create more stable room supply air condition, which is mainly driven by the outdoor air air thermal comfort. condition and the airflow rate, rather than the extracted energy from the EAHE. This is not difficult to understand since this system 4.2. Thermal comfort analysis is a 100% outdoor air system. When the supply air temperature is higher than the desired room air temperature, any high cooling Predicted mean vote (PMV) and predicted percent of dissatis- capacity will be useless and actually serve against the cooling pur- fied people (PPD) can be used to evaluate the indoor thermal com- pose. More discussion of this and the self-regulating feature in pas- fort. PMV represents the predicted mean vote of a large population sive operation mode can be found in [19]. of people exposed to a certain environment. PMV is derived from The total cooling capacity of any system or subsystem that uti- the physics of heat transfer combined with an empirical fit to sen- lizes air as the heat transfer medium can be formulated as shown sation. The thermal sensation scale is referred to as PMV scale, as in the following equation: shown in Table 3. PPD is the predicted percent of dissatisfied people for different PMVs. As PMV changes away from zero in Q cooling;t ¼ V airqair Dh ð5Þ 214 Y. Yu et al. / Applied Energy 114 (2014) 209–217

2.5 6000 Natural airflow Forced airflow Natural airflow 2.0 4000 1.5 2000 1.0 0

PMV 0.5 -2000 0.0 Cooling capacity (W) -0.5 -4000 Natural airflow Forced airflow Natural airflow -1.0 -6000 2009/7/24 2009/7/31 2009/8/7 2009/8/14 2009/8/21 2009/8/28 2009/7/24 2009/7/31 2009/8/7 2009/8/14 2009/8/21 2009/8/28 2009/9/4 Date Date

Fig. 7. PMV of the indoor condition during the tests. Fig. 10. Coupled system total cooling capacity.

remove a higher cooling load from the space. However, the coupled 80 system sometimes provided negative cooling to the space in the 70 Natural airflow Forced airflow Natural airflow forced airflow mode. This is because the EAHE could not accommo- date the high flow rate with the given outdoor air condition. For 60 Standard PPD PPD example, when the outdoor is humid or extremely hot, the air 50 could not be conditioned well by the EAHE to a point that can still 40 cool the space. Another reason that contributes to the situation is

PPD (%) 30 the soil saturation problem. Combing Figs. 6 and 10, it can be seen that during the forced cooling test, the average supply air 20 temperature was moved up and therefore the cooling capacity as 10 shown in Fig. 10 becomes invalid to bring the room air to the 0 desired conditions. The higher room air temperature was also par- 2009/7/24 2009/7/31 2009/8/7 2009/8/14 2009/8/21 2009/8/28 tially resulted by the high outdoor air temperature during the Date forced air test period. In addition, the values of cooling capacity in the third stage are smaller than those in the first stage due to Fig. 8. PPD of the indoor condition during the tests. the decrease of EAHE cooling capacity after forced airflow condi- tion. Fig. 11 shows the relationship of the airflow rate and EAHE to- tal cooling capacity on August 3rd, 2009. It can be seen from the 60000 figure that the airflow rate and EAHE total cooling capacity have 50000 Natural airflow Forced airflow Natural airflow the similar trend and they are relative with the solar radiation intensity. The airflow rate and EAHE total cooling capacity reached 40000 the peak at about 3 pm on that day. The peak cooling capacity from 30000 the system was more than the design load of 2800 W. Therefore, 20000 the EAHE can satisfy the cooling requirement to realize the free cooling. 10000

Cooling capacity (W) 0 4.4. Underground soil temperature analysis -10000

2009/7/24 2009/7/31 2009/8/7 2009/8/14 2009/8/21 2009/8/28 2009/9/4 The cooling energy for the space air conditioning of this coupled Date system comes from the ground soil. In order to study the impact of the EAHE on the soil temperature, multiple soil sensors were bur- Fig. 9. EAHE total cooling capacity. ied around the EAHE tube. Fig. 12 shows the underground soil tem- perature profiles at 9 m away from the test facility building at Consequently, total cooling capacities of the coupled system three different heights 0.3 m, 1.2 m, and 2.1 m above the EAHE and EAHE can be obtained based on the airflow rate, density, and during the entire testing period. It is clear from the figure that, enthalpy difference. regardless the depths, the soil temperature increased drastically The coupled system total cooling capacity is formulated as: during the forced airflow testing period. From Fig. 12, it can be seen that the change of soil temperature of location closer to the EAHE

Q system;t ¼ V airqairðhRA hSAÞð6Þ tube was more severe than that farther from the tube. For example, the underground soil temperature at 0.3 m above the EAHE pre- The EAHE total cooling capacity is formulated as: served at a temperature of about 17.8 °C during the first passive cooling period then it rose up to 21.1 °C during the forced airflow Q ¼ V airq ðhOA hSAÞð7Þ EAHE;t air test and even crossed the temperature curve of 1.2 m above the Figs. 9 and 10 show the calculated results for the EAHE and cou- tube. Afterwards it gradually settled down at a temperature about pled system total cooling capacity. It can be seen from Figs. 9 and 18 °C in two weeks. The soil temperature at 1.2 m above the EAHE 10 that the EAHE and coupled system total cooling capacity in both tube was also elevated by about 0.8 °C after the forced air test. This the first and third testing stages had a smaller value than that of phenomenon is called the ground thermal saturation, where the the second testing stage. The forced air from the fan to some ex- underground temperatures rise above their normal temperatures tents enhanced the overall cooling capacity and might be able to during the time of the year. It should be mentioned that it took Y. Yu et al. / Applied Energy 114 (2014) 209–217 215

1200 3500

Design load=2800 3000 1000 Air flowrate (m3/h) 2500 EAHE total cooling capacity (W)

/h) 800 2000 (W) 3 y 1500 acit

600 p

1000 ca g

400 500 Air flowrate (m Air flowrate Coolin 0 200 -500

0 -1000 2009/8/3 00:00 2009/8/3 06:00 2009/8/3 12:00 2009/8/3 18:00 2009/8/4 00:00 Date

Fig. 11. Airflow rate vs EAHE total cooling capacity.

7000 24 2.1m-ACP 1.2m-ACP 0.3m-ACP airflow rate 6000 22 5000

20 4000

18 3000

2000 Temperature (ºC) 16 Natural airflow Forced airflow Natural airflow 1000 14 0 12 2009/7/24 2009/7/31 2009/8/7 2009/8/14 2009/8/21 2009/8/28 2009/9/4 Date

Fig. 12. The underground soil temperature at three different heights above EAHE. the forced airflow test few hours to thermally saturate the sur- vertical and horizontal direction respectively. Fig. 14 shows the rounding ground with this design, while it took the ground over underground soil temperature at the location of 0.3 m above and two weeks to recover from thermal saturation. next to EAHE in vertical and horizontal direction respectively. As Figs. 13 and 14 show the underground soil temperature at two seen from these two figures, when the airflow entered the forced different directions to EAHE. Fig. 13 plots the underground soil airflow stage from natural airflow, the temperatures increase in temperature at the location of 0.15 m above and next to EAHE in the horizontal level was higher than that in the vertical level.

40 25 30 24 20 23

10 22 Temperature (ºC) 0 21 -10 20 -20 19

Temperature (ºC) -30 18 -40 17 -50 16 OAT 0.15 m-ACP 0.15 m-NCP -60 15 2009/7/24 2009/7/31 2009/8/7 2009/8/14 2009/8/21 2009/8/28 Date

Fig. 13. The underground soil temperature at two different directions with 0.15 m away from EAHE. 216 Y. Yu et al. / Applied Energy 114 (2014) 209–217

24 0.3 m-ACP 0.3 m-NCP 23 Natural airflow Forced airflow Natural airflow 22

21

20

19 Temperature (ºC) 18

17

16 2009/7/24 2009/7/31 2009/8/7 2009/8/14 2009/8/21 2009/8/28 2009/9/4 Date

Fig. 14. The underground soil temperature at two different directions with 0.3 m away from EAHE.

The results indicate that the underground heat dissipation in the 2. The indoor air condition is more stable in passive cooling mode horizontal level was greater than that in the vertical level. This than in active cooling mode. As a 100% outdoor air system, might be because that the soil temperature at horizontal level of when a forced air mode is used to improve the overall cooling the EAHE tube is generally lower compared to that at vertical level capacity, a preliminary control is needed on the fan or damper above the tube. Therefore, a higher gradient leads to higher heat to ensure that the operation can maintain an acceptable supply dissipation in that direction. Moreover, Figs. 13 and 14 show that air condition with an increased cooling capacity. the underground temperatures followed the outdoor air tempera- 3. The underground heat dissipates faster in the horizontal level ture pattern especially during the forced airflow test and with a than the vertical level. The soil around the tube in the horizon- less damping degree than the period before and after the forced tal level may saturate sooner than the vertical direction above airflow test. the tube due to the higher temperature gradient. Combining the information from the previous analysis on soil 4. The surrounding soil of the earth tube may saturate and takes temperature, in order to improve the cooling capacity from the long to recover if the heat is over-extracted from the soil in a geothermal system and avoid thermal saturation around the EAHE forced air mode. The design of the earth tube can be further tube, enhancement on heat transfer around the tube and along the improved by, for example, welding rods around the tube and horizontal direction might be necessary. Since the main resistance extend the effective volume of soil to the system. comes from the soil side, a possible measure is to weld metal rods around the tube and extend the supporting volume of soil to the As a self-regulating system on solar and geo-thermal energy system. utilization for room air-conditioning, the overall performance such as the indoor thermal comfort could be vulnerable to many factors, 5. Discussions and conclusions including the internal load condition, the solar radiation, and the outdoor air temperature. Following studies on the preliminary con- In this paper, an experimental study of a coupled geothermal trol of the coupled EAHE system for improved thermal comfort cooling system is presented. In this system, an earth-to-air tube conformance during the off-design conditions will be presented. is coupled with a solar collector enhanced solar chimney to achieve free space cooling in summer. 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