Performance of a Coupled Cooling System with Earth-To-Air Heat Exchanger and Solar Chimney Haorong Li University of Nebraska-Lincoln, [email protected]

Performance of a Coupled Cooling System with Earth-To-Air Heat Exchanger and Solar Chimney Haorong Li University of Nebraska-Lincoln, Hli3@Unl.Edu

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln

Architectural Engineering -- Faculty Publications Architectural Engineering

2014 Performance of a Coupled Cooling System with Earth-to-Air Heat Exchanger and Solar Chimney Haorong Li University of Nebraska-Lincoln, [email protected]

Yuebin Yu University of Nebraska–Lincoln, [email protected]

Fuxin Niu University of Nebraska-Lincoln

Michel E. Shafik University of Nebraska at Lincoln, [email protected]

Bing Chen 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

Li, Haorong; Yu, Yuebin; Niu, Fuxin; Shafik, Michel E.; and Chen, Bing, "Performance of a Coupled Cooling System with Earth-to-Air Heat Exchanger and Solar Chimney" (2014). Architectural Engineering -- Faculty Publications. 89. http://digitalcommons.unl.edu/archengfacpub/89

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. Renewable Energy 62 (2014) 468e477

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Renewable Energy

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

Performance of a coupled cooling system with earth-to-air heat exchanger and solar chimney

Haorong Li a, Yuebin Yu a,*, Fuxin Niu a, Michel Shaﬁk a, Bing Chen b a Durham School of Architectural Engineering and Construction College of Engineering, University of Nebraska-Lincoln, Omaha, NE, USA b Department of Computer & Electronics Engineering, University of Nebraska-Lincoln, Omaha, NE, USA article info abstract

Article history: Buildings represent nearly 40 percent of total energy use in the U.S. and about 50 percent of this energy is Received 8 March 2013 used for heating, ventilating, and cooling the space. Conventional heating and cooling systems are having Accepted 12 August 2013 a great impact on security of energy supply and greenhouse gas emissions. Unlike conventional Available online approach, this paper investigates an innovative passive air conditioning system coupling earth-to-air heat exchangers (EAHEs) with solar collector enhanced solar chimneys. By simultaneously utilizing Keywords: geothermal and solar energy, the system can achieve great energy savings within the building sector and Earth-to-Air heat exchanger reduce the peak electrical demand in the summer. Experiments were conducted in a test facility in Solar chimney Coupled system summer to evaluate the performance of such a system. During the test period, the solar chimney drove 3 3 Cooling capacity up to 0.28 m /s (1000 m /h) outdoor air into the space. The EAHE provided a maximum 3308 W total cooling capacity during the day time. As a 100 percent outdoor air system, the coupled system maximum cooling capacity was 2582 W that almost covered the building design cooling load. The cooling capacities reached their peak during the day time when the solar radiation intensity was strong. The results show that the coupled system can maintain the indoor thermal environmental comfort conditions at a favorable range that complies with ASHRAE standard for thermal comfort. The ﬁndings in this research provide the foundation for design and application of the coupled system. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Research on the EAHE mainly focused on developing the mathematic model in the early days. One of the models presented In the United States, more than 114 million residence buildings treated the EAHE as a cylindrical two-dimensional heat transfer and 4.7 million commercial buildings consumed nearly 40 percent problem [11]. In doing so, a single cylindrical tube has been simu- of total energy use [1]. The building sector is one of the biggest non- lated in this model taking into account different parameters such as renewable energy consumers that are not utilizing much of the the ambient air temperature, the air velocity inside the tube, the renewable energy available in the nation. About 50 percent of this tube radius, and the air traveled distance along the length of the energy used in buildings is for heating, cooling, and ventilating tube. Although this model is considered as a very good beginning buildings [2]. At the same time, about 86 percent of energy used in for EAHE modeling, it only included the sensible heat transfer of air buildings is from the combustion of fossil fuel [3]. Accordingly, to the soil without taking into account the latent heat transfer [12]. research has being conducted to improve the operation efficiency Another model treated the inlet air temperature to the EAHE as a on the building sector [4e6] and find ways of utilizing renewable harmonic periodic sinusoidal input. In this model, a cylindrical energy sources and saving non-renewable energy [7,8]. In order to EAHE with adiabatic/isothermal boundary conditions submitted to utilize such a renewable energy source with the great heat capacity constant airflow condition with harmonic temperature signal has and high thermal inertia underground, many techniques have been been simulated. Based on that model, it has been shown that developed in the last decades such as ground source heat pumps depending on its thickness, the soil layer could induce either (GSHPs), earth-to-air heat exchangers (EAHEs) and so on [9,10]. an amplitude-dampening or phase-shifting regimes [10,13,14]. Accordingly, different experiments on the EAHE system, with various materials, sizes, types, functions, locations, experimental setups and scientific research objectives have been conducted e * Corresponding author. Tel.: þ1 402 554 2082. around the globe [15 18]. The early experiments results showed E-mail addresses: [email protected], [email protected] (Y. Yu). that the maximum effect for an EAHE is heavily dependent on the

Nomenclature t total total total vap vaporization Roman letter symbols w water cp constant-pressure specific heat of air, kJ/(kg$ C) EAHE earth-to-air heat exchanger h enthalpy, kJ/kg HR humidity ratio m mass flow rate, kg/s OA outdoor air Q rate of heat transfer, kW OAT outdoor air temperature T temperature, C RA room air V volume flow rate of airflow, m3/s RAT room air temperature SA supply air Subscripts SAT supply air temperature air air UUS undisturbed underground soil cooling cooling l latent Greek letters s sensible u humidity ratio, kg/kg system system r specific density, kg/m3 temperature difference between the ambient air and the soil sur- energy to provide a natural, feasible and alternative approach for rounding the EAHE. It has also been found that for a constant flow the cooling and ventilation of residential and commercial buildings. rate, the wider the tube the better the performance, which has been The paper is organized as follows: The principle of the experi- attributed to the increase of the surface area exposed to the soil. mental system, including the EAHE, facility, solar collector, and Furthermore, the thermal performance of the system has been solar chimney, is first introduced. Then, the experimental proce- enhanced by increasing the burial depth and length of the EAHE as dure for two years experiment is described. After that, we present well as reducing the airflow rate, which allows for a better heat the experimental results in detail, such as the underground soil transfer [11]. temperature, performance of the coupled system, and cooling ca- The other technique utilizing a non-renewable energy source pacity of the coupled system. The paper concludes with a brief that the current study focuses on is solar chimneys [19e23]. A solar discussion on the findings and future research direction. chimney is a natural draft device that utilizes energy produced by solar radiation to build up stack pressure, consequently, driving 2. The principle of the experimental system airflow through the chimney. As the solar chimney problem involves both fluid movement as well as heat transfer, the compu- The coupled system for providing cooling and ventilation in tation fluid dynamics (CFD) has been used as a tool to simulate the buildings is shown in Fig. 1. It includes a piece of EAHE pipe with airflow and heat transfer in the solar chimney in order to predict one end exposed to the outside and the other to the indoor. A solar the mass flow rate in the chimney [15,19]. The goal of these ex- chimney is connected to the building via a solar collector. Fig. 2 periments was to investigate the effect of varying the heat flux and further illustrates the plan view of the test facility. The cooling changing the chimney gap and inclination on the airflow and and ventilation process works in this way: the air inside the solar temperature distribution inside the solar chimney. It was shown collector is heated up as the solar radiation strikes the solar col- that by changing the chimney gap while maintaining other condi- lector; hence, the hot air migrates from the solar collector to tions, the airflow increased continuously with increasing chimney chimney bottom and rises to the top of the chimney. As the system gap. Results also showed that the airflow reached a maximum at a chimney inclination angle of around 45 for a 200 mm gap and 1.5 m high chimney, which was about 45% higher than that for a vertical chimney under otherwise identical conditions [21]. The effects of other chimney aspects (e.g. inlet size and glazing) on its thermal performance have also been investigated. For example, it was found that for a given chimney gap, the flow rate increase with inlet height due to decreasing entry flow resistance [22]. A different study pointed that increasing the solar collector surface area and the chimney height and diameter could generate more available chimney draft [23]. However, according to the current state of art, very few preliminary studies have connected an EAHE with a solar chimney to synergize the benefits for both cooling and ventilation. The uniqueness of this investigation is in the coupled system with solar energy and geothermal energy, and the analysis of synergetic performance for conditioning buildings. With the coupled system, we can achieve great energy savings within the building sector and reduce the peak electrical demand in the summer. It is necessary to conduct further investigations on the coupled system performance and to compare between the coupled system and the EAHE. For this reason, the research presents findings on utilizing two means of renewable energy sources namely geothermal and passive solar Fig. 1. Schematic diagram of a coupled system with EAHE and solar chimney. 470 H. Li et al. / Renewable Energy 62 (2014) 468e477

Fig. 2. The test facility building ﬂoor plan.

components are tightly connected, the hot air migration creates a dimensions are 15.2 m length, 4.52 m width, and 2.4 m height. The pressure difference and draws the ambient air through the EAHE; design cooling load is 2813 W based on the estimation using TRACE as the warm ambient air travels through the EAHE tube, it cools 700 software. down due to the heat exchange process that occurs with the underground soil, providing a fresh cool air draft to the test facility 2.3. Solar collector building. The solar energy research test facility is sealed with low air leakage. In this way a low-cost and environmentally friendly The solar collector was designed with an absorbing surface area passive cooling and ventilation is achieved. of 20 m2, considering that the solar irradiation is 400 W/m2 with a heat loss coefﬁcient of 8.5 W/(m2$C) and an absorptivity of 0.8. It is 2.1. Earth-to-air heat exchanger placed on the south side of the facility building, with one end linking to the testing room and the other to the solar chimney. Fig. 3 A culvert steel EAHE with 57 m length, 0.45 m diameter, and is the cross section view of the solar collector and the test facility. buried at a depth of about 3 m underground is installed at a testing The solar collector is placed 26 against the ground. The structure is facility of the University of Nebraska. The EAHE tube runs southeast also shown in Fig. 4. The solar collector is constructed by wood and to northwest. It starts with a slanted inlet of 8.53 m, then penetrates a special type of solar glass that traps the solar irradiation inside the the ground for 3 m and curves to a horizontal portion of about collector. All of its corners and angles are sealed with black tar, and 45.7 m, ﬁnally bends upwards for 3.65 m at a right angle to enter a layer of (R-5) insulation has been added to the bottom side of the the facility through its concrete slab near the south side of the collector. Above the wood and insulation, a layer of plywood is building. A damper is installed at the inlet to close and open the air applied and painted in black to prevent any solar irradiation losses path. from the sides of the collector.

2.2. Solar energy research test facility 2.4. Solar chimney

The test facility building, Solar Energy Research Test Facility, was The solar chimney was designed so that its available draft built on 1990 at the Allwine Prairie Preserve. It is a one-story overcomes the EAHE pressure losses as well as the negative thermal building sitting on a concrete slab with external dimensions of gravity effect of the air column inside the solar chimney, under the ﬂ 19.4 m long, 4.9 m wide, and 3 m high. It includes a testing room design ventilation air ow rate. Accordingly, the solar chimney where the experiments took place, a computer room for data logging and monitoring, and a bathroom. The main testing room

Fig. 3. A cross section view of the solar collector and the Solar Energy Research Test Facility-solar collector connection pipe. Fig. 4. Picture of solar collector. H. Li et al. / Renewable Energy 62 (2014) 468e477 471

during this testing period is presented based on the tested dates. Table 1 collects the main information about the experiment sensors, locations, type, and the corresponding accuracy.

4. Results and analysis

Chimney 4.1. Undisturbed underground soil testing

12.2 m During the coupled system experiments, a major check test for the undisturbed underground soil (UUS) has been carried out. We monitored the underground soil temperature profile at three different depths. For this test the three previously installed underground temperature reference thermocouples have been used Solar collector to record the UUS temperatures every 15 min. The UUS test lasted for a whole year (365 days); from January 1, 2008 till December 31, 2008. Fig. 6 shows the UUS temperature profiles at the three different depths with 0.61 m, 1.53 m and 2.9 m below the ground surface. Fig. 5. Schematic diagram of the solar chimney. As shown in Fig. 6, the deeper the depth of the thermocouple underground the less the temperature fluctuation. Moreover, by height and diameter were selected as 12.2 m and 0.457 m, knowing that July and August are the peak hot summer months in respectively, to provide an available draft of more than 5 Pa at a Omaha, Nebraska, it is clear that there is a phase shift between the temperature of 57 C. As illustrated in Fig. 5, the chimney was built ambient air temperatures and the underground temperature pro- of four steel spiral pipes; each pipe is 3.048 m in length, stacked file and this shift increases by the increase of the underground over each other. The chimney is placed over a 1.5 m steel stand depth. The maximum soil temperature at 2.9 m comes in the which in turn is bolted to a concrete foundation of 3 ft in depth. The middle of October, while that at 1.53 m comes in the middle of chimney is strengthened by nine steel wires; every three wires are August. At a depth of 2.9 m the UUS temperature fluctuates be- attached to the chimney at three different heights on the same tween a maximum temperature of 14.6 C and a minimum of 8.1 C, vertical line then stretched out and bolted into the ground away which verifies its good ability to be a heat sink for the EAHE heat from the chimney to increase its stability and resistance against the dissipation. Furthermore, it is found that the maximum tempera- wind. The three sets of wires form a Y shape in which the point of ture at this depth during the period from July 1, 2008 to August 31, intersection of all the Y shape is concentric with the chimney 2008 is 13.9 C while the minimum is 11.2 C. Since the coupled cylinder. system is designed for summer cooling to save energy and reduce peak demand, this temperature range suits very well for the 3. Experimental procedure and measurements purpose.

The main goal of this test is to evaluate the thermal performance 4.2. Performance analysis of the coupled system of the all passive, naturally driven, coupled system, through assessing the maximum cooling capacity, as well as the airflow rate, Fig. 7(a)e(c) show the profiles of indoor and outdoor thermal that it can provide for the testing facility building. In doing so the environmental conditions, including temperatures, relative hu- EAHE outlet damper was set to the 100% fully open position. The midity, and humidity ratios of the coupled system during summer data logging system was programmed to record, average, and log in 2008 (from August 14, 2008 to August 31, 2008). The facility was the weather data over a 15 min span during the whole testing cooled by the natural airflow driven by the solar collector enhanced period. The main measurements that have been recorded for the solar chimney. From Fig. 7(a)e(c), it is clear that the coupled system purpose of analyzing the cooling capacity of the naturally driven has damped all the indoor environmental conditions to an coupled system are: time and date, the indoor relative humidity (%), acceptable level during most of the days of summer 2008 testing outdoor relative humidity (%), supply air relative humidity (%), period. As shown in Fig. 7(a), while the outdoor air temperatures supply airflow rate (m3/s), solar collector airflow rate (m3/s), during the day time were above 25 C and high to 34 C, the indoor average indoor temperature (C), supply air temperature (C), and air temperature remained mainly from 21 Cto24C, which is outdoor air temperature (C). The system thermal performance considered to be comfortable for occupants in summer.

Table 1 List of experiment sensors.

Variable Measurement Location Type Accuracy

Temperature a. Soil temperature a. At depth of 0.61, 1.53, 2.9 m below the ground surface 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 airflow a. EAHE outlet inside the house Dwyer air velocity transmitters 3% b. Relief airflow b. Solar collector inlet 472 H. Li et al. / Renewable Energy 62 (2014) 468e477

25 a 40 depth=0.61m depth=1.53m depth=2.9m RAT OAT 35 20

30 ) 15 25

10 20 Temperature Temperature ( Temperature 15 5 10

0 5 2008/1/1 2008/3/1 2008/5/1 2008/7/1 2008/9/1 2008/11/1 2009/1/1 2008/8/13 2008/8/16 2008/8/19 2008/8/22 2008/8/25 2008/8/28 2008/8/31 Date Date 100 Fig. 6. Undisturbed underground soil (UUS) temperature proﬁles over the course. b 90

80 It deserves to point out that the stormy and windy outdoor 70 environmental conditions could have strong impact on the indoor relative humidity since the EAHE has limited latent cooling ca- 60 pacity. This is reﬂected from the indoor relative humidity and hu- 50 midity ratio in Fig. 7(b) and (c). The highest humidity ratio increased to 78% in extreme days when the outdoor air is stormy. 40

The humidity ratio exceeded ASHRAEs’ upper recommended hu- Relative humidity (%) 30 midity ratio limit 0.012 in few different days during the 2008 testing period. The statistics of the extreme indoor thermal envi- 20 RA-RH(%) OA-RH(%) ronmental conditions of summer 2008 testing period is further 10 collected in Table 2. The overall results show that the coupled 2008/8/13 2008/8/17 2008/8/21 2008/8/25 2008/8/29 2008/9/2 system maintained an acceptable indoor air temperature range of Date e (21.3 25.1 C) which complies with ASHRAE standard 55-2004. c 0.020 Accordingly, it is concluded that the coupled system has the ability to provide an acceptable indoor thermal environmental conditions ﬂ during the natural air ow test of summer 2008. However, the 0.016 windy and stormy outdoor air thermal environmental conditions could have a negative impact on the coupled system thermal performance. The ﬁndings imply that to ensure the performance of the 0.012 passive energy system and avoid violating thermal comfort re- quirements, a modest level of control might be needed. 0.008 4.3. Analysis of the cooling capacity

The couple energy system performance is further analyzed in Humidity Ration (kg/kg) 0.004 terms of the cooling capacity. It deserves to mention that the studied system is a 100 percent outdoor air system. All conditioning RA-H.Ratio OA-H.Ratio air from the EAHE is eventually exhausted from the solar chimney. 0.000 Therefore, from thermal condition point of view, two cooling ca- 2008/8/13 2008/8/17 2008/8/21 2008/8/25 2008/8/29 2008/9/2 pacities are included in this system: EAHE cooling capacity and Date coupled system cooling capacity. While the EAHE cooling capacity indicates the performance of the EAHE, it does not directly reflect Fig. 7. (a) Indooreoutdoor temperatures. (b) Indooreoutdoor relative humidity. (c) the available useful cooling capacity for the space. The differences Indooreoutdoor humidity ratios. are shown in the psychrometric chart (Fig. 8). With a high EAHE cooling capacity, it is possible to lower down the supply air tem- total cooling capacity is divided into two portions: the sensible perature and enthalpy with a given airflow rate. This will benefit cooling capacity and the latent cooling capacity, as given in Eq. (1). the coupled system cooling capacity. The total cooling capacity of the coupled system is determined by the supply air condition and ¼ þ airflow rate. This point is further discussed in Ref. [24]. Qtotal;cooling Qs;cooling Ql;cooling (1) Meanwhile, there are two heat transfer modes involved in the The total cooling capacity of any system or subsystem that uti- cooling process of the coupled system: the sensible heat transfer lizes air as the heat transfer medium can be formulated as shown in mode, which is involved exclusively in reducing or maintaining the Eq. (2): temperature of the air; and the latent heat transfer mode, which involves moisture removal from the air stream during the cooling ¼ r D process. The variables are illustrated in Fig. 8. Consequently, the Qtotal;cooling Vair air h (2) H. Li et al. / Renewable Energy 62 (2014) 468e477 473

Table 2 the coupled system and EAHE, total, sensible, and latent cooling Indoor environmental conditions for 2008 natural airﬂow test. capacities are formulated as follows: Environmental condition Max/Min Value Unit Date Time The coupled system total cooling capacity is formulated as:

RAT Max 25.1 C 8/14/08 00:00 ¼ r ð Þ Min 21.3 C 8/27/08 11:15 Qsystem;t Vair air hRA hSA (7) RA-RH% Max 78 % 8/28/08 12:30 Min 50 % 8/25/08 21:30 The coupled system sensible cooling capacity is formulated as: RA Hum. Ratio Max 0.01438 kg/kg 8/22/08 17:45 ¼ r ð Þ Min 0.008717 kg/kg 8/28/08 21:30 Qsystem;s Vair aircp TRAT TSAT (8) The coupled system latent cooling capacity is formulated as: À Á The sensible cooling capacity of any system or subsystem that ¼ r u u Qsystem;l hvapVair air RA;HR SA;HR (9) utilizes air as the heat transfer medium is formulated as follows: The EAHE total cooling capacity is formulated as:

Q ; ¼ V r cpDT (3) s cooling air air ¼ r ð Þ QEAHE;t Vair air hOA hSA (10) On the other hand, the latent cooling capacity of any system or subsystem (e.g. heat exchanger) that utilizes air as the heat transfer The EAHE sensible cooling capacity is formulated as: medium is formulated as shown in Eq. (4). ¼ r ð Þ QEAHE;s Vair aircp TOAT TSAT (11) ¼ Ql;cooling hvapmw (4) The EAHE latent cooling capacity is formulated as: The mass ﬂow rate of water condensation/vaporization (i.e. À Á Q ¼ h V r u u (12) moisture removal/addition) can be formulated as follows: EAHE;l vap air air OA;HR SA;HR By applying the above 6 formulas, from Eq. (7) to Eq. (12),onthe ¼ r Du mw Vair air (5) measured data of 2008 testing periods, the different cooling ca- Substitute Eq. (5) into Eq. (4), we obtain the expression for latent pacities of the coupled system and EAHE are calculated for the cooling capacity: natural airflow mode. Furthermore, the calculated cooling capacities are plotted as time series. Table 3 shows the values of the ¼ r Du constants that have been used in the calculations. The other vari- Ql;cooling hvapVair air (6) ables are obtained from the measurements. In order to determine the total, sensible, and latent cooling ca- Fig. 9(a)e(c) show the total, sensible, and latent cooling capac- pacities that the coupled system can provide for the testing facility ities of the EAHE during the natural airﬂow test of 2008, respec- building as well as the EAHE cooling capacity, Eqs. (2), (3) and (6) tively. During the night, since there is no solar strike and airflow are applied to the different system components. Consequently, through the EAHE, the cooling capacities are close to zero. From

CS: coupled system EAHE: earthe air heat exchanger TCC: total cooling capacity LCC: latent cooling capacity SCC: sensible cooling capacity

Outdoor air

C C -T E

H E

A A E

C E

C - -T L S C

C C

Room air

C Supply air

EAHE-SCC

CS-SCC

Fig. 8. Illustration of the cooling capacities in the system. 474 H. Li et al. / Renewable Energy 62 (2014) 468e477

Table 3 show a similar pattern. It is clear from both figures that the cooling Atmospheric air condition constants. capacities reached their maximum peaks during the day time due Contents Unit Value to the increase in the solar radiation intensity and the pulling power from the solar collector enhanced solar chimney. The findings Air enthalpy of vaporization, hvap kJ/kg 2468.4 Constant-pressure specific heat of air, cp kJ/(kg$ C) 1.005 are supported by the solar chimney analysis in Section 4.4. 3 Specific density of air, rair kg/m 1.293 However, this capacity changing pattern is not obvious in the latent cooling capacity from the EAHE. Fig. 9(c) shows the fluctuation on the latent cooling capacity from the EAHE system. It Fig. 9(a) and (b), it can be seen that during the day time, the EAHE sometimes even provides negative latent cooling capacity into the reached a maximum cooling capacity of about 3308 W, and a space and especially when the time gets close to the night. It means maximum sensible cooling capacity of about 2716 W. The total the outdoor air is possibly reversely humidified when it flows cooling capacity and the sensible cooling capacity from the EAHE through the EAHE. This is due to the following factors: First, since

a 4000

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Fig. 9. (a) EAHE total cooling capacity. (b) EAHE sensible cooling capacity. (c) EAHE latent cooling capacity. H. Li et al. / Renewable Energy 62 (2014) 468e477 475 the solar radiation drops close to zero when the sun goes down, the is because the outside air wind produces a draft through the EAHE. airﬂow rate through the EAHE drops very close to zero because the This draft was very humid because of the stormy outdoor air con- solar collector cannot collect any solar radiation when the sun sets. ditions, and consequently, condensation occurred inside the EAHE In addition, the air relative humidity sensors introduce errors when while this draft was migrating through it. This led to a reduction on the airflow rate is low. Third, the outdoor air temperature and the supply air relative humidity ratio. Because the outdoor hu- humidity ratio decrease during the night time. Thus, the difference midity ratio was relatively high with stormy outdoor air conditions, between the outdoor air humidity ratio and the supply air humidity the difference between the outdoor and supply air humidity ratios ratio become negative and produce inaccurate negative values for became positive during those nights. This also supports the afore- the EAHE latent cooling capacity. mentioned explanation of the negative EAHE latent cooling ca- It is also clear from Fig. 9(c), that the value of the EAHE latent pacities values during the night time. cooling capacity could be positive during the nights when the Fig. 10(a)e(c) show the total, sensible and latent cooling ca- outdoor air conditions were stormy and windy. This phenomenon pacities of the coupled system during the natural airﬂow test. From

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Fig. 10. (a) Coupled system total cooling capacity. (b) Coupled system sensible cooling capacity. (c) Coupled system latent cooling capacity. 476 H. Li et al. / Renewable Energy 62 (2014) 468e477

40 0.45 3 36 OAT Airflow rate to building m /s 0.40

32 0.35 28

) 0.30 24 /s)

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Temperature ( 0.15 12 Airflow rate(m 8 0.10

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Fig. 11. Outdoor air temperature-EAHE airﬂow rate.

Fig. 10(a) and (b) we can see that the coupled system reached a that the EAHE airflow rate followed the same pattern of the outdoor maximum total cooling capacity of about 2582 W, in the meantime air temperature. This is because the outdoor air temperature is in it reached a maximum sensible cooling capacity of about 1624 W. consistence with the solar radiation intensity in summer, which Compared these values with the design cooling load, it can be drives the solar chimney draft power and increases the EAHE concluded that the coupled system almost satisfies the test facility airflow rate. Fig. 12 shows the airflow rate to the solar collector with building design cooling load which is an extreme condition that the change of solar density. The stronger the solar density, the happens rarely. This implies the feasibility of the coupled system larger the airflow rate to the solar collector. In addition, a phe- performance during the natural airflow mode and its ability to nomenon of phase shift migration is found in the figure. When the provide required cooling capacity. Fig. 10(c) shows that the latent solar density was at the peak value, the airflow rate did not reach cooling capacity has some negative values during a few nights of the highest value. It delayed about one to one and a half hours. the 2008 natural airflow test. It is found that during these few More study is needed to investigate the matching among the solar nights the outdoor environmental conditions were stormy and radiation, airflow, and the internal load, and suggest any assisting windy. This leads to an increase in the outdoor air humidity ratio. device or safety factor to overcome the mismatching between the Consequently, even with condensation in the EAHE, which might load demand and the energy supply. As shown in Fig. 13 the total not be adequate for room air dehumidification, the supply air hu- cooling capacity of the coupled system is largely affected by the midity ratio is still higher than the room air humidity ratio during solar chimney air temperatures, where the coupled system total the night time. From Fig. 10(a)e(c), it is clear that the outdoor cooling capacity increases with the increase of the solar chimney air environmental conditions impact the performance of the coupled temperature and vice versa. The highest inlet air temperature after system. Furthermore, when the solar radiation increases the the solar collector can reach about 70 C while the outdoor air is in coupled system cooling capacity increases. This proves the natural the range from 15 Cto30C. The combined solar collector and controllability of the coupled system. solar chimney provides a great means to move the air. The overall performance proves the effectiveness of coupling the EAHE with 4.4. Solar chimney driving force the solar chimney technology and shows the effect of the solar radiation and the outdoor air temperature on the solar chimney An analysis of the solar chimney driving force during the 2008 natural draft and consequently the supply airflow free to the natural airflow test is conducted in this section. In Fig. 11 we can see building.

1200 0.35 Solar density (W/m2) Airflow rate to solar collector (m3/s) 1000 0.30 /s) 3

) 0.25 2 800 0.20 600 0.15 400 Solar density (W/m 0.10

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0 0.00 2008/8/18 00:00 2008/8/18 06:00 2008/8/18 12:00 2008/8/18 18:00 2008/8/19 00:00 Time

Fig. 12. Solar density-airﬂow rate to solar collector. H. Li et al. / Renewable Energy 62 (2014) 468e477 477

5000 80 Air temperature at the inlet of chimney ( ) Coupled system total cooling capacity(W)

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-40 0 2008/8/14 2008/8/17 2008/8/20 2008/8/23 2008/8/26 2008/8/29 Date

Fig. 13. Air temperature at the inlet of chimney vs coupled system total cooling capacity.

5. Discussions and conclusion [4] Yu Y, Liu M, Li H, Yu D, Loftness V. Synergization of air handling units for high energy efficiency in office buildings: implement methodology and performance evaluation. Energy and Buildings 2012;54:426e35. To utilize the solar energy and geothermal for free cooling, we [5] Liu M, Claridge DE, Turner ED. Continuous commissioningdguidebook devised a coupled passive energy system with a solar collector maximizing building energy efficiency and comfort. U.S. Department of ’ enhanced solar chimney and an earth-to-air heat exchanger. Energy s Federal Energy Management Program; 2002. DE-FG01-98EE73637. [6] Costa A, Keane MM, Torrens JI, Corry E. Building operation and energy Research has been conducted on the coupled system at the solar performance: monitoring, analysis and optimisation toolkit. Applied Energy energy research test facility to further investigate the performance 2013;101:310e6. of the coupled system. 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