A COMPARISON OF PASSIVE SOLAR AND MECHANICALLY DRIVEN EARTH-TO-AIR HEAT EXCHANGERS FOR COOLING BUILDINGS

A COMPARISON OF PASSIVE SOLAR AND MECHANICALLY DRIVEN EARTH-TO-AIR HEAT EXCHANGERS FOR COOLING BUILDINGS Michel Shafik Building System Solutions Inc. 6825 Pine St C-11 Omaha, NE 68106 mshafik@bssi.bz James Devaney Department Computer & Electronics Engineering University of Nebraska Lincoln - Omaha Campus 1110 South 67th Street Omaha, NE 68182 jdevaney2@unl.edu Dr. Bing Chen Department Computer & Electronics Engineering University of Nebraska Lincoln - Omaha Campus 1110 South 67th Street Omaha, NE 68182 bchen1@unl.edu ABSTRACT The research presented compares a fully passive solar driven geothermal air-conditioning strategy to an active forced air driven geothermal air-conditioning system; in either case ambient air propagates tough the Earth-to-Air Heat Exchanger (EAHE) for the purpose of chilling our Solar Energy Research Testing Facility (SERTF) building, located in Bennington Nebraska. The all passive system employs two techniques namely EAHE and solar chimney. This full scale demonstration is then used to evaluate the performance of the coupled system (i.e. EAHE coupled with solar chimney) for the two cases, first: passive solar driven air flow, second: mechanically driven forced air flow, 24 hours per day. The system performance is evaluated tough assessing the maximum air flow rate (CFM) and cooling (in /, kwatts, and tons) that the system can provide to the building during hot summer time for each case study. Total Cooling Capacity of the system under these two scenarios have been calculated using standardized equations which account for latent and sensible cooling achieved. The experimental results are then compared to building cooling load coefficients generated with a simulation using the TRACE 700 software package. Thermal environmental comfort conditions for human occupancy are also evaluated using American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) standard 55-2004. 1. INTRODUCTION The main objective of the current research is to study and compare the passive solar driven system to a mechanically driven forced air system for a feasibility and efficiency analysis. Thermal environmental conditions for human occupancy are evaluated using ASHRAE standard 55-2004. All in pursuit of developing an optimal perforg full scale demonstration of an all passive cooling and ventilation system coupled at the SERTF at the University of Nebraska [1]. 1.1 Cooling a Building with an EAHE The EAHE is simply a culvert pipe buried at a certain depth underground [2] in which hot ambient summer air is drawn tough one end of the pipe (inlet) and migrates to the other end (outlet) which is connected to the building to be cooled. While the hot air migrates from the inlet side to the outlet side of the EAHE, heat is dissipated from the air to the earth due to the low earth temperature [3] and thermal conductivity of the loess clay soil. Previous research conducted at the SERTF only considered the sensible heat transfer of air to the soil without taking into account the latent heat transfer [4]. Many researchers have studied the EAHE from different points of view, some focused on developing 1

mathematical models to predict the performance on an EAHE, and others conducted different types of experiments to evaluate the heating/cooling potential of an EAHE, as well as understanding the main parameters that affects its design [5, 6]. Hollmuller has modeled the inlet air temperature (i.e. ambient temperature) to the EAHE as a harmonic periodic sinusoidal input [7]. He has also developed a tee diesonal heat transfer model which account for sensible and latent heat exchanges [8, 9]. 1.2 Moving Air with a Solar Chimney The solar chimney is a vertical or inclined air channel; the bottom end is connected to the building for cooling and ventilation. This air channel generates air movement by buoyancy forces and stack ventilation, drawing cooler air tough the building in a continuous cycle in which hot air rises and exits from the top of the chimney [10]. In other words, a solar chimney is a natural draft device which utilizes energy produced by solar radiation to build up stack pressure, consequently, driving airflow tough the chimney [11]. Nugroho realized the the solar chimney problem involves both fluid movement as well as heat transfer, it was found that the computational fluid dynamics (CFD) is a good tool to describe the system numerically [12]. Thermal performance of the solar chimney is greatly impacted by the chimney gap (width). In an experimental study on a solar chimney of 6.6 ft (2 m) height and variable gap width ranging 3.9 inches - 3 ft (0.1-1 m), it was found that when the chimney gap is small 7.9 inches (0.2 m) the flow is upward. However, when the gap is large 1.6 ft (0.5 m), air flows upwards near the heated walls due to the buoyancy effect but near the center of the chimney air flows downwards. In other words, the mass flow rate inside the chimney increases as the chimney gap increase to a certain level (about one-tenth of the chimney height) then decreases as the gap further increases [13]. This phenomenon was also observed in a numerical study showing that there is an optimum chimney width at which a maximum ventilation flow rate can be achieved. This phenomenon was attributed to the occurrence of back flow at the outlet of the chimney [14]. 1.3 Coupled system (EAHE & Solar Chimney) These two techniques are further investigated in this research to study the capability of developing an environmentally friendly and energy saving system that could provide cooling for residential buildings during hot summer days to reduce the current summer electrical peak loads experienced the Midwestern portion of the United States. The most current research presented involves estimating the cooling load of the SERTF with a software simulation. The results of this simulation are used in a parametric comfort zone analysis based on monitoring system installed at the site. Introducing mechanically driven forced air increases the performance of the EAHE by forcing air tough; this pushes the limits of the previously researched passive air system and reveals the limitations of the demonstration. The system is monitored under various scenarios over the course of four cooling seasons. A sensor array containing 81 Thermocouples, 20 Moisture Sensors, four Humidity Sensors, tee air flow meters, and two Pyranometers logs data every 15 utes. An analysis of two scenarios: Passive Solar Driven and 24 hour Forced Air have been performed during this time and will be discussed. 2. BACKGROUND A full scale experimental setup was installed at the SERTF; utilizing an EAHE by burying a 188 ft (57 m) culvert steel pipe, which creates a cool draft tough the test facility when coupled to a solar chimney [11]. Heat from the ambient air is dissipated into the soil surrounding the EAHE during the migration from the outside into the testing facility. The EAHE enters the floor of the SERTF, for the purpose of chilling the testing facility. Fig. 1: The undisturbed underground soil temperature profiles over the course of the year 2008 [1]. Tee thermocouples have been buried on the property away from the EAHE for the purpose of analyzing the ground temperature behaviors of undisturbed soil. These thermocouples are buried at the testing facility two, five, and 9.5 feet underground and are observed for an entire year as the team prepares the rest of the site for testing. Results of this test are very promising. 2

Figure 1 depicts a dampening effect as the temperature measurements are observed further into the ground, centering around 51 F - 54 F. This information infers if the team dug deep enough; a depth exists where temperature is 51 F - 54 F year round (noal change in amplitude with 0 phase change) [8]. The two foot location reflects trends comparable to ambient temperature (maximum in summer and imum during winter). A phase shift of approximately 90 is observed for the sensor monitoring 9.5 underground (maximum in fall and imum in spring). 3. SYSTEM OVERVIEW Figure 2 shows the diagram of the coupled system. An EAHE is coupled with a solar chimney and solar collector to provide cooling and ventilation for the SERTF. The cooling and ventilation process can be illustrated as follows: the air inside the solar collector is heated up as the solar radiation strikes the solar collector; hence, the hot air migrates from the solar collector to the bottom of the chimney and then rises to the top, otherwise known as the stack effect. [11] As the system components are connected as tight as possible, this hot air migration draws the ambient air tough the EAHE; as the warm ambient air travels tough the length of the EAHE tube it cools down due to the heat exchange process that occurs with the underground soil, providing a fresh nice cool air draft tough the test facility building. The SERTF is sealed as much as possible to reduce air leaks. In this way a low cost and environmentally friendly passive cooling and ventilation is achieved [1]. It should also be mentioned weep holes are drilled along the bottom of the EAHE which sits on a layer of crushed rock to prevent accumulation of condensation. Table 1: Building Design Load Calculated Values Design Component Total Cooling Load Sensible Cooling Load Latent Cooling Load Fan Size Calculated Value 9,389 5,583 3,805 (0.8 Tons) (0.47 Tons) (0.32 Tons) 3/4 HP motor 2,000 ft3 flow 0.0414 pressure in H 2 O Table 2: Building Design Load Parameters Design Parameter Indoor Design Temperature Outdoor Design Temperature Value 75 F 93.4 F Relative Humidity (RH) 50 % Floor Area 740 ft 2 Wall R-Value Roof R-Value Slab R-Value 15.625 ft2 F 19.445 ft2 F 11.37 ft2 F Windows U-Value 0.181 ft 2 F 4. BUILDING COOLING LOAD SIMULATION Table 2 shows the main design calculated cooling load values calculated by the TRACE 700 software package to be compared with the actual measured coupled system cooling capacities. It should be mentioned that the test facility building is a one story building and the test room dimensions are 49.875ft (15.2m) length, 14.83ft (4.52m) width, and 7.75ft (2.36m) height with a floor area of 740ft 2 (68.75m 2 ). It should also be pointed out that it is an unoccupied space with no internal loads. Moreover, all the windows were covered with R-5 Styrofoam insulation during the 2008 and 2009 testing periods to imize the solar gain. This explains why the calculated design cooling load and airflow rate are relatively small compared to the floor area [1]. 5. MATHEMATICAL REPRESENTATION There are two heat transfer modes involved in the cooling process: the sensible heat transfer mode, which is involved exclusively in reducing or maintaining the temperature of the air; and the latent heat transfer mode, which involves moisture removal from the air stream during the cooling process. Consequently, the total cooling capacity is divided into two portions: the sensible cooling capacity and the latent cooling capacity, this is represented mathematically as follows: The following equations are used to quantify the total amount of air conditioning achieved while perforg the two test scenarios [15]. 3

Fig. 2: System overview of the cooling process by the coupled system [1]. Where: EAHE = Earth-to-Air-Heat-Exchanger The coupled system total cooling capacity is formulated as: Q system,t = 60ρ air CF M EAHE (h RA h SA ) (1) ρ air = specific density of air, lbm ft 3 CF M EAHE = EAHE supply air mass flow rate, ft 3 h RA =room air enthalpy, lbm The coupled system sensible cooling capacity is formulated as: Q system,s = 60ρ air CF M EAHE C Pair (T RAT T SAT ) (2) h SA =supply air enthalpy, lbm C pair = constant pressure specific heat of air, lbm F T RAT = average room air temperature, F And the latent cooling capacity is formulated as: Q system,l = 60ρ air h fg CF M EAHE (ω RA ω SA ) (3) T SAT = supply air temperature, F h fg = air enthalpy of vaporization, lbm ω RA = average room air humidity ratio, grains ω SA = supply air humidity ratio, grains 4

Variable ρ air Table 3: Constants Value 0.0764 lbm ft C pair 0.24 lbm F h fg 1061.2 lbm 1 ton of cooling 12,000 5.1 Passive Mode Analysis The purely passive solar cooling method mates of the cooling potential of the EAHE and with the natural air movement of a solar chimney. The coupled system provides cooling to the SERTF building without any external inputs. From Figure 3 and 4 it is clear that the coupled system reached a maximum total cooling capacity of about 7,450 / (0.63 tons of cooling), A maximum sensible cooling capacity of 4,860 / (0.41 tons of cooling ). Comparing these values with the design cooling load, measured data verifies the coupled system almost satisfied the test facility building design cooling load 0.8 tons of cooling, which is an extreme condition which rarely happens. This implies the feasibility of the coupled system performance during the natural airflow mode and its ability to provide a sufficient amount of cooling during standard operating conditions for the Midwestern United States. It is clear from both figures that the cooling capacities reach their maximum peak during the day time due to the increase in the solar gain inside of the solar collector, see figure 2. Fig. 3: Coupled System Total Cooling Capacity (2008 Passive Air Test) [1]. Fig. 4: Coupled System Sensible Cooling Capacity (2008 Passive Air Test) [1]. Figure 5 shows that the latent cooling capacity has some negative values during a few nights of the 2008 natural airflow test. It was found that during these few nights the outdoor environmental conditions were stormy and windy, leading to an increase in the outdoor air humidity ratio. Otherwise under standard peak cooling conditions experienced in the daytime hours; the coupled system does provide consistent contributions to latent cooling load. From Figure 3, 4 and Figure 5 it is clear that the outdoor environmental conditions impact the performance of the coupled system. Furthermore, when the solar radiation increases, the coupled system cooling capacity increases, which could potentially prove a natural controllability of the coupled system [1]. Fig. 5: Coupled System Latent Cooling Capacity (2008 Passive Air Test) [1]. 5

5.2 Forced Air Analysis, 24 hours/day The forced air test occurred during the summer of 2009. A 2,000 CFM, 3/4 HP, 0.0414 H 2 O fan was turned on at 3:15 PM 8/6/2009 and was left on continuously until 11:45 AM 8/15/2009, a 9 day test. Length of time of this test was detered by local weather patterns of Bennington, NE. Drawing in cool night air gave the soil surrounding this EAHE a recharging effect. Our system was left in a passive cooling mode before and after the 9 day test. Passive Cooling mode means the air was allowed to propagate tough the system by naturally creating a suction with the solar collector and solar chimney on the buried tube. Passive mode extracts about 1/4 1/2 ton of cooling during daylight hours of hot sunny days. Fig. 6: Coupled System Total Cooling Capacity (2009 Forced Air Test) [1]. Fig. 8: Coupled System Latent Cooling Capacity (2009 Forced Air Test) [1]. 6. COMFORT ZONE ANALYSIS The coupled system thermal comfort analysis starts with presenting the environmental thermal comfort conditions recommended by ASHRAE standard 55 of the year 2004 for thermal comfort (the most current). Thereafter, a comparison of the coupled system, testing facility building, indoor and outdoor environmental conditions is carried out. In this comparison the indoor and the outdoor temperatures, relative humidity, and humidity ratios of the natural and forced airflow modes of the coupled system are plotted as time series on spreadsheets. Empirical data (ie. enthalpy and humidity ratio) are generated with the Engineering Equation Solver (EES) software package. Consequently, the compliance of the indoor environmental conditions, of the testing facility building, with ASHRAE standard 55-2004 [16] during the natural and forced airflow modes. Fig. 7: Coupled System Sensible Cooling Capacity (2009 Forced Air Test) [1]. Fig. 9: Acceptable Range Of Indoor Thermal Environmental Conditions [16]. 6

Table 4: Results Fig. 10: Indoor Outdoor Temp Passive Air 2008 [1]. Result Passive 2008 Forced 2009 Max Cooling Capacity Airflow Ambient Temp Avg Indoor Temp Ambient Humidity Ratio Indoor Humidity Ratio 7,437 15,275 0.62 tons 1.27 tons 02.18 kwatts 4.47 kwatts : 0 ft3 max: 440 ft3 : 0 m3 sec max: 0.21 m3 sec : 51.8 F max: 92.1 F : 11 C max: 33.4 C : 70.4 F max: 77.2 F : 21.3 C max: 25.1 C : 1652 ft3 max: 1803 ft3 : 0.78 m3 sec max: 0.85 m3 sec : 59.9 F max: 96.6 F : 15.5 C max: 35.9 C : 67.6 F max: 82.4 F : 19.8 C max: 28 C : 0.00616 : 0.00989 max: 0.01583 max: 0.03407 : 0.00872 : 0.00944 max: 0.01438 max: 0.01787 8. CONCLUSION Fig. 11: Indoor Outdoor Temp Forced Air 2009 [1]. Figure 9 shows the acceptable range of indoor environmental conditions recommended by ASHRAE standard 55-2004 [16]. It is clear that the thermal comfort zone limits are between 71 F (21.7 C) and 82.5 F (28.1 C). Moreover, it shows that the upper recommended humidity ratio limit is 0.012, with no recommendation for the lower humidity limit. Based on ASHRAE recommended comfort zone limits, the indoor environmental conditions of the coupled system have been evaluated as shown in Figure 10 [17, 18]. 7. RESULTS Table 4 shows a comparison of the results achieved during the passive solar and mechanically driven EAHE tests of 2008 and 2009 respectively. The results show forcing air tough the EAHE increases the overall cooling capacity and increased airflow to the SERTF for ventilation. On the other hand the passive solar driven test of 2008 provided overall lower average indoor temperatures and humidity. Test results generated within the University of Nebraska in 2004 show that the coupled system can provide an effective form of air-conditioning [19]. The system was then evaluated to detere if the system is capable of providing acceptable indoor thermal environmental comfort conditions. During the natural airflow test of 2008, the indoor environmental conditions recorded and monitored within the SERTF complied with the ASHRAE standard 55-2004 for thermal comfort [16]. Moreover, it provided 0.63 tons of cooling, which almost covered the simulated building design cooling load (0.8 tons, extreme design condition estimation from TRANE 700 software package). With additional reduction of building air infiltration, the airflow of the coupled system (i.e. EAHE and solar chimney) would increase; which would also increase the overall cooling capacity for the passive solar mode. During the forced air test of 2009 the fan extracted much more cooling than required based on the design parameters established in section 4. Therefore, the system could not comply with humidity and sensible temperature requirements of ASHRAE standard 55-2004 under the forced airflow, although it provided 1.27 tons of cooling which is even more than the simulated building load requirements (0.8 tons, extreme 7

condition estimation from TRANE 700 software package). The indigenous loess clay soil proved to be a good heat sink at a depth of 9.5ft where its temperature fluctuates in the range of 46.5 F - 58.2 F (8.1 C - 14.6 C), see figure 1. The systems ability to dissipate heat into the underground soil for heat absorption is a future research topic of interest. In conclusion, the coupled system proved to be a feasible cooling system which complied with ASHRAE standard 55-2004 for Thermal Environmental Conditions for Human Occupancy under the passive solar chilling strategy, with room for improvement. 9. ACKNOWLEDGMENTS Nebraska Center for Energy Sciences Research (NCESR). Gang Wang, Ph.D., P.E. - Assistant Professor of the Department of Civil, Architectural and Environmental Engineering at University of Miami. Joerg Henkel - Project Manager of SERTF. Haorong Li, Ph.D. - Associate Professor, The Durham School, Department of Architectural Engineering. Suat Irmak, Ph.D. - Associate Professor of Biological Systems Engineering and Interim Director of UNL Water Center, University of Nebraska - Lincoln, Department of Biological Systems Engineering. 10. REFERENCES (1) Shafik, M., Feasibility Study of Solar Driven Underground Cooling System, Master s thesis, University of Nebraska - Lincoln., 2010 (2) Bojic, M., N. Trifunovic, G. Papadakis, and S. Kyritsis, Numerical simulation, technical and economic evaluation of air-to-earth heat exchanger coupled to a building, Energy, 22(12), pp. 1151-1158, 1997 (3) Bansal, V., R. Misa, G. D. Agarwal, and J. Mathur, Performance Analysis of Earth-pipe-air-heat-exchanger for summer cooling, Enegry and buildings, 2009 (4) Newman, B. C. T. W., M.A. and J. Maloney., Analysis of the performance of earth contact cooling tubes, Master s thesis, University of Nebraska - Lincoln, 1983 (5) Al-Ajmi, F. F. and D. L. Loveday, The cooling potential of earth-air heat exchangers for domestic buildings in a desert climate, Building and Environment, 41(3), pp. 235-244, 2006 (6) Pfafferott J, H. S. and J. M., Design of passive cooling by night ventilation: evaluation of a parametric model and building simulation with measurements, pp. 971-983., 2003 (7) Hollmuller, P. and B. Lachal, Cooling and preheating with buried pipe systems: monitoring, simulation and economic aspects, Energy and Buildings, 33(5), pp. 509-518, 2001 (8) Hollmuller, P., Analytical characterization of amplitude-dampening and phase-shifting in air/soil heat-exchangers, International Journal of Heat and Mass Transfer, 46(22), pp. 4303-4317., 2003 (9) Hollmuller, P. and B. Lachal., Buried pipe systems with sensible and latent heat exchange: validation of numerical simulation against analytical solution and longterm monitoring, pp. 411-418., 2005 (10) Bacharoudis, E., M. G. Vrachopoulos, M. K. Koukou, D. Margaris, A. E. Filios, and S. A. Mavrommatis, Study of the natural convection phenomena inside a wall solar chimney with one wall adiabatic and one wall under a heat flux, 27., 2007 (11) Wang, G., B. Chen, M. Liu, J. Henkel, and R. S., Analysis, design, and preliary testing of solar chimney for residential air-conditioning applications, in A Solar Harvest: Growing Opportunities., 2004 (12) Nugroho, A. M., M. H. Ahmad, and H. T. Jit, Evaluation of parametrics for the development of vertical solar chimney ventilation in hot and humid climate, in 2nd International Network for Tropical Architecture conference, at Cistian Wacana, 2006 (13) Bouchair A., Solar chimney for promoting cooling ventilation in southern Algeria., Building Service Engineering, Research and Technology, 15(2), pp.81-93., 1994 (14) Gan, G. and S. B. Riffat, A numerical study of solar chimney for natural ventilation of buildings with heat recovery, Applied Thermal Engineering, 18(12), pp. 1171-1187., 1998 (15) McQuiston, F., P. J.D., and S. J.D., The Heating, ventilating, And Air Conditioning: Analysis and Design. 6th Edition, John Wiley & Sons, Inc., 2005 (16) American Society of Heating, Refrigerating and Air-Conditioning Engineers, Thermal environmental conditions for human occupancy, ASHRAE, Atlanta, supresedes ANSI-ASHRAE-55, 1992., 2004 (17) American Society of Heating, Refrigerating and Air-Conditioning Engineers, 1999 ASHRAE Handbook: HVAC Applications., 1999 (18) American Society of Heating, Refrigerating and Air-Conditioning Engineers, 2001 ASHRAE Handbook Fundementals., 2001 (19) Wang, G., B. Chen, M. Liu, and J. Henkel, Analysis, design, and preliary testing of an earth contact cooling tube for fresh air conditioning, in A Senior project., 2004 8