ASHRAE OR-16-C044-2016 In-Situ Testing of Shallow Depth Helical Heat Exchangers for Ground Source Heat Pump Systems.pdf

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1、 Francisco Javier Alvarez-Revenga is a Masters Student in the Department of Mechanical they are marked as 10, 11 and 12 in Figure 2. A trench was dug to collect all the pipes and lead them towards the mechanical room, which is located in the northeastern corner of the house print, in the basement. C

2、onsequently, there is approximately 128 ft (39 m) of horizontal runs, which were insulated with closed-cell foam insulation. Figure 2 Location of the geothermal boreholes at the residence. Boreholes marked as 1, 2, 3 and 4 correspond to the location of the deep vertical probes and 10, 11 and 12 repr

3、esent the location of the three helical heat exchangers Local soil formation conditions were determined through well logs and ground thermal response tests at the time of the building construction. According to the well log reports, the soil corresponding to the helical bores was classified as moder

4、ately plastic and sticky clay (CL). The conductivity test for the deep boreholes showed a thermal conductivity of 1.26 Btu/(hrftF) (2.18 W/(mK). Temperature sensors were located on the pipes just after they entered the mechanical room in order to measure the entering water temperature (EWT) and the

5、leaving water temperature (LWT). Data was acquired at 30 second intervals. A weather station was installed at the test site to collect outdoor conditions during the study. Some outdoor conditions were determined by using data from the National Oceanic and Atmospheric Administration databases (NOAA 2

6、015). Global horizontal solar radiation was retrieved from average hourly statistics provided by energy modeling software for the Gallatin Field weather station location, near Bozeman, MT (EnergyPlus 2015). Field Testing In-situ tests were performed during both heating and cooling seasons. The heati

7、ng and cooling mode tests spanned over 5 days (February 9th to 14th, 2015) and 3 days (August 2nd to 5th, 2014) respectively. During the heating mode test, the GSHP was initially switched on so it would take care of the heating load of the building using its own internal controls. The residence has

8、comprehensive building automation systems that allow the user to control the systems in a very flexible manner. These control sequences were configured in such way that the GSHP maintained a given setpoint in the 120 gal (450 L) buffer tank. From the buffer tank, a secondary loop connects the differ

9、ent building zones. In this case, a radiant floor heating (RFH) system was used to heat up the building spaces. This control strategy resulted in the system cycling and providing intermittent heating pulses. To explore the long term performance of the heat exchanger, a steady heat load was applied t

10、o the system, in a similar process to that of a formation thermal conductivity test (ASHRAE 2011). In order to achieve this, the digital control system was set to provide a steady supply of energy to the building. This control strategy leads to the system working constantly (compressors continually

11、running). The GSHP energy output was estimated based on the temperature of the ground being as steady as possible within reasonable ranges to ensure the loads could be sustained for long periods of time. The cooling mode test had a similar approach, but in this case only the steady state situation w

12、as explored. Conditions and parameters during the tests are summarized in Table 1. Table 1. Parameters During The Tests Description Value (SI Units) Value (I-P Units) Geothermal Fluid Total Mass Flow Rate in Loop (Heating Test Cooling Test) 0.63 0.57 kg/s 4990 4514 lbm/hr Specific Heat 4061.2 J/(kgK

13、) 0.97 Btu/(lbmF) Thermal Conductivity 0.48 W/(mK) 0.28 Btu/(hrftF) Kinematic Viscosity 2.710-6 m2/s 2.910-5 ft2/s Density 1023.9 kg/m3 63.92 lbm/ft3 Layout and Geometry of Heat Exchanger Type of Probe Helical Number of Probes 3 Spacing 3.4 m 11 ft Depth of installation 1.8 m 6 ft Length of Probe 3.

14、7 m 12 ft Diameter of Borehole 0.46 m 18 in Diameter of Helix 0.38 m 15 in Pitch between Turns 0.11 m 4.3 in Total Length of Pipe 40 m 131 ft Material of Pipe PEXa Inside Diameter 22.2 mm 0.875 in Outside Diameter 28.6 mm 1.125 in Density of Material 940 kg/m3 58.68 lbm/ft3 Thermal Conductivity of M

15、aterial 0.41 W/(mK) 0.24 Btu/(hrftF) Thermal Properties of the Ground Estimated Thermal Conductivity 2.18 W/(mK) 1.26 Btu/(hrftF) Estimated Specific Heat Capacity 1275.6 J/(kgK) 0.305 Btu/(lbmF) Estimated Density 1760 kg/m3 110 lbm/ft3 Backfilling Material Native Soil (No grout) Climate Data of Loca

16、tion Location Bozeman, MT Air Temperature From On-Site Data Acquisition System Solar Radiation From EnergyPlus Database Annual Mean Temperature 7C 44.6F Annual Variation (Semi-amplitude) 12.2C 21.9F The Model The independently developed CaRM numerical simulation tool was used. The CaRM simulation to

17、ol is based on the electrical analogy, where the domain (comprised of ground, grouting material and ground heat exchanger) is discretized with thermal capacitances and thermal resistances that link the thermal nodes (see Figure 3). Then, for each thermal node the heat balance equation is written, ob

18、taining a linear system with as many equations as unknown temperatures. The CaRM simulation tool has been highly improved from the first release reported in De Carli et al. (2010). In this study, the current version of this simulation tool (Zarrella and De Carli 2013) was used. The approaches define

19、d in this model account for the axial heat transfer (within both the ground and grouting material), the effect of surface conditions, convection, and short- and long-wave radiation heat transfer. These new features make it possible to investigate even short length borehole heat exchangers. The model

20、 can also be used for short-term analysis because the thermal capacitance of the borehole heat exchanger (both the grouting material and the heat carrier fluid) is considered. The actual geometry of the helical pipe (pipes length and pitch between turns) is modeled (see Figure 3-B). The details of t

21、he model are described in the reference (Zarrella and De Carli 2013). Figure 3 Approach of CaRM: A) overall approach, B) particular of helical pipe modeling (CaRM-He) Results Experimental Results Measured entering water temperature (EWT) and leaving water temperature (LWT) were plotted together with

22、 the outdoor air temperature and the horizontal solar radiation (see Figure 4 and Figure 5). EWT refers to the temperature entering the heat pump, thus it is the temperature of the fluid going out of the ground heat exchanger, and conversely, LWT is the temperature leaving the heat pump, so it is th

23、e temperature of the fluid entering the ground heat exchanger. Figure 4 5-day long heating test Figure 5 3-day long cooling test Model Validation For validating the model, LWT was used as the input to the numerical solution tool model (i.e. the temperature of the fluid entering the helical heat exch

24、anger). Conditions reported in Table 1 were used as parameters for the model as well. It was assumed the thermal conductivity of the ground was 1.26 Btu/hr-ft-F (2.18 W/(m K) as calculated in the previously mentioned formation thermal conductivity test report for the deep boreholes at the residence.

25、 Another assumption was that the heat losses along the insulated horizontal trench and inside the mechanical room were negligible. The calculated fluid temperature coming out of the ground heat exchanger (EWT) was plotted together with the measured values in Figure 6 and Figure 7. Figure 6 Compariso

26、n between measured and simulated EWT during the heating test: A) entire test, B) zoom in of hours 36 to 48. MBE = mean bias error. RMSE = root mean squared error Figure 7 Comparison between measured and simulated EWT during the cooling test: A) entire test, B) zoom in of hours 12 to 24. MBE = mean b

27、ias error. RMSE = root mean squared error Observations show that the simulations correspond well with the measured values, with a mean bias error (MBE) of -0.07F (-0.04C) and a root mean squared error (RMSE) of 0.49F (0.27C) in heating mode and a MBE of 0.12F (0.07C) and a RMSE of 0.13F (0.07C) in c

28、ooling mode. Discussion The CaRM tool can use either the inlet temperature at the borehole or the ground load as input values. For this validation, the temperature entering the helical heat exchanger was available and used as the input value. A comparison between inlet temperature and ground heat lo

29、ad as input in CaRM was reported in Zarrella A. et al. (2011). As it is well known, the ground heat load depends on the energy efficiency of the heat pump which in turn depends on the building A) B) A) B) heat load and return temperatures from both building and ground heat exchangers. For this purpo

30、se, the CaRM tool was developed to be coupled to a heat pump simulation tool where the entire model uses the building heat load and corresponding return fluid temperature as inputs. Future work would include further comparison of the model with more experimental results in different scenarios and fo

31、r longer periods of time. This makes the numerical simulation tool very interesting from the perspective of the designer and opens the door to explore important questions regarding the use of shallow depth helical heat exchangers coupled to GSHPs. For example, in an unbalanced climate like Bozeman,

32、MT where winter loads are significantly greater than the summer loads (i.e. 7701 Heating Degree Days vs. 254 Cooling Degree Days; base 65F (NOAA 2015), it is important to explore the long term behavior and the ground temperature trending over the years to ensure the system is appropriately sized and

33、 sustainable. One thing to take into account is the relatively high degree of uncertainty regarding the formation thermal conductivity, since it is strongly dependent on water content, temperature, and other factors. While there are formation thermal conductivity test procedures available (ASHRAE 20

34、11), they are not economical to implement and this can pose a challenge for keeping within a tight residential budget. As a typical alternative, the ground thermal conductivity can be estimated via tables from Chapter 34 of the ASHRAE HVAC Applications Handbook (ASHRAE 2011). Another possible source

35、 of error can be the experimental setup itself. It is very important to select appropriate sensors with the level of accuracy required for the test as well as the location and installation within the system. This paper explored the performance of a set of shallow depth vertical helical heat exchange

36、rs coupled with a ground source heat pump via in-situ testing. Experimental results were also compared to the simulations from a previously reported numerical model obtaining a good alignment. The performance of the helical ground heat exchanger depends, among other factors, on the temperature of th

37、e ground, which is influenced by the weather conditions at its working depths. The ability of the numerical model to simulate these conditions is remarkable, and poses an interesting opportunity for future implementation of design guidelines and/or software development to be used by the designers. A

38、cknowledgments The authors wish to thank the REHAU Company for sponsoring the Montana Ecosmart House Project and make this testing possible. Also, thanks to the Hoy family the homeowners for their generous support to the research team during the project. References ASHRAE. 2011. Chapter 34, ASHRAE H

39、andbook HVAC Applications. Atlanta: ASHRAE De Carli M., Tonon M., Zarrella A., Zecchin R. 2010. A computational capacity resistance model (CaRM) for vertical ground coupled heat exchangers. Renewable Energy 35(7) 1537-1550. EnergyPlus. 2015. EnergyPlus Energy Simulation Software. Weather Data. http:

40、/apps1.eere.energy.gov/buildings/energyplus/weatherdata_about.cfm Hino T., Ooka R. 2013. Development of a ground loop heat pump system augmented by solar collectors and nocturnal radiation. Proceedings of Clima 2013 Kemler, E.N. 1947. Methods of Earth Heat Recovery for the Heat Pump. Heating and Ven

41、tilating, Sept. 1947, pp. 69-72, New York National Oceanic and Atmospheric Administration (NOAA). 2015. Climate Data Online. http:/www.ncdc.noaa.gov/cdo-web/ Park H., Lee S.R., Yoon S., Shin H., Lee D.S. 2012. Case study of heat transfer behavior of helical ground heat exchangers. Energy and Buildin

42、g, vol 53, October 2012, pp. 137-144 Zarrella A., De Carli M. 2013. Heat transfer analysis of short helical borehole heat exchangers. Applied Energy 102 14771491. Zarrella A., Scarpa M., De Carli M. 2011. Short time step analysis of vertical ground-coupled heat exchangers: The approach of CaRM, Renewable Energy 36 2357-2367