1、60 2009 ASHRAEABSTRACT A practical method for designing hybrid geothermal heat pump (GHP) systems that use closed-loop, earth heat exchang-ers installed in vertical boreholes is presented. The design diffi-culty with hybrid GHP systems is inherently an optimization problem that is best solved with a
2、 computer-based system simulation method. Many parameters can be optimized and there is no unique expression of the objective function. In this work the optimization problem is defined as balancing the annual thermal loads on the ground by minimizing the bore-hole heat exchanger length and supplemen
3、tal equipment size. The supplemental equipment examined in this work has been limited to flat plate solar collectors. The design method for GHP systems was developed from results of 62 detailed computer simulations. Three dimension-less groups containing key GHP design parameters were iden-tified us
4、ing the Buckingham Pi Theorem, and correlated with a fitted surface equation. With typical design parameters avail-able to a practitioner, the design method developed here can be used to estimate the total ground loop length for stand-alone GHP systems, along with the quantity of annual energy requi
5、red to balance the annual ground loads. With additional input parameters also readily available to designers, the area of a solar collector array can be calculated, along with the corresponding reduced borehole heat exchanger length. Solar collector array area is calculated using the utilizability m
6、ethod.INTRODUCTIONEnergy utilization in the built environment is of increas-ing concern, and geothermal heat pump (GHP) systems (also known as ground-source heat pump or Geoexchange systems) are now relatively well established as a means of significantly reducing energy consumption in space condi-ti
7、oning of buildings. This improvement in efficiency, however, generally comes at a higher first cost, which must be offset by lower operating and maintenance costs within an acceptable period of time to the building owner. As with most alternative energy systems, high capital cost is a significant ba
8、rrier to market penetration.One of the main goals in the design a GHP system is to properly size the total length of the ground-loop heat exchanger so that it provides fluid temperatures to the heat pump within design limits. Unlike with conventional heating and cooling systems, design of GHP system
9、s requires some type of life-cycle simulation due to the thermal storage effects of the earth. Annual heating loads in a building are rarely balanced with annual cooling loads, and thus thermal responses of the ground throughout the buildings life cycle must be considered. In heating dominated build
10、ings, annual imbalances in the ground load will lead to progressively lower heat pump entering fluid temperatures, and in cooling domi-nated buildings, progressively higher heat pump entering fluid temperatures will occur. These excursions will result in the heat pump efficiency to progressively det
11、eriorate and the equipment capacity to be ultimately compromised if the ground-loop heat exchanger (GLHE) is not large enough. Borehole fields designed for buildings with relatively large annual ground load imbalances can often be excessively large and costly, making vertical closed-loop GHP systems
12、 noncompetitive with conventional systems.The phenomenon of long-term temperature change in the subsurface due to GHP systems serving buildings with imbal-anced annual thermal loads has given rise to the concept of the A Design Tool for Hybrid Geothermal Heat Pump Systems in Heating-Dominated Buildi
13、ngsA.D. Chiasson, PhD, PE, PEng C. Yavuzturk, PhDAssociate Member ASHRAE Member ASHRAEA.D. Chiasson is an assistant professor in the Department of Mechanical and Aerospace Engineering, University of Dayton, Dayton, OH. C. Yavuzturk is an assistant professor in the Department of Mechanical Engineerin
14、g, University of Hartford, West Hartford, CT.LO-09-005 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in eit
15、her print or digital form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 61hybrid GHP system, where a supplemental component is utilized to effectively balance the annual ground loads. These systems permit the use of smaller, lower-cost borehole fields, but their desig
16、n adds to the complexity of the overall GHP design process because of the addition of another transient component of the system. For example, acceptable conditions for solar recharging of the ground in a heating-dominated building depends on solar availability and ground loop temperature. Consequent
17、ly, hybrid GHP systems should be analyzed on an hourly basis in order to fully understand their behavior. The fundamental task in designing hybrid GHP systems lies in properly balancing the size of the supplemental component and the size of the ground-loop heat exchanger, while optimizing the contro
18、l of the supplemental component. Current engineering design manuals such as Caneta Research, (1995), Kavanaugh and Rafferty (1997), Kavanaugh (1998), and ASHRAE (2003), developed from research conducted since the 1980s, mention the potential use of hybrid cooling tower/fluid cooler GHP systems, but
19、do not describe a design process for systems that utilize solar collectors for the thermal re-charge of the ground.Proper and reliable design of hybrid GHP systems is quite difficult and cumbersome without the use of a system simula-tion approach. Further, without an automated optimization scheme co
20、upled to the system simulation program, the design activity itself can become tediously impractical and time consuming. The use of system simulation for analyzing complex building systems is ever increasing, but the necessary computing resources are not at the disposal of every design practitioner,
21、nor is their use always economically justified. As new technologies and design concepts emerge, design tools and methodologies must accompany them and be made usable for practitioners. Without reliable design tools, reluctance of practitioners to implement more complex systems can become a significa
22、nt barrier. Therefore, the overall goal of the work presented in this paper is to develop a tool for the design of hybrid GHP systems in heating-dominated buildings that is useful to practitioners. The design tool is based on current “state-of-the-art” system simulation methods, cast in a format tha
23、t allows straight-forward use.BACKGROUND AND LITERATURE REVIEWA review of the literature reveals a number of works that could be regarded as hybrid systems. Research and develop-ment regarding coupled solar and geothermal systems has largely been administered by the International Energy Agency (IEA)
24、 through working agreement-Energy Conservation through Energy Storage (ECES), Annex 8 (Implementing Underground Thermal Energy Storage Systems) and Annex 12 (High Temperature Thermal Energy Aquifer and Duct Storage). In the United States, documented studies and reports in the literature dealing with
25、 hybrid GHP systems mainly pertain to cooling dominated buildings that utilize cooling towers. A summary of some of the heating dominated works is presented here.Recharging the ground with solar energy is not a new idea. Andrews et al. (1978) present qualitative evidence to suggest that coupling a g
26、round loop to a solar collector and a storage tank could improve system performance and reduce initial costs. The need for earth heat transfer models was iden-tified.Dalenback et al. (2000) present a preliminary study of a solar-heated low temperature space-heating system with seasonal storage in th
27、e earth for 90 single-family homes near Stockholm, Sweden. The total annual heating demand is 1080 MWh. The system performance was simulated using MINSUN and TRNSYS. The heating system is a sub-floor heating system designed for 30oC supply temperature. Sixty percent of the heating load is to be met
28、with the seasonal underground storage volume (with no heat pumps) and the remainder by electric heaters. The system consists of 3,000 m2of integrated roof solar collectors and a borehole storage volume of 60,000 m3, whose temperature varies between 30-45oC throughout the year.Seiwald and Hahne (2000
29、) describe a solar district heat-ing system with an underground storage volume located in southwest Germany. The solar contribution is about 50% of the total heat demand. At the time of their publication, the system consisted of 2,700 m2of flat plate solar collectors and a borehole storage volume of
30、 20,000 m3. The underground storage volume is planned for expansion to 63,000 m3and to be capable of long-term heat storage at temperatures of up to 80oC. Regular operation of the system began in January 1999.Reuss and Mueller (2000) describe another solar district heating system with an underground
31、 storage volume in Germany. The total annual space heating and hot water demand is 487 MWh. The system consists of 800 m2of roof-integrated solar collectors, a storage volume of 9,350 m3, and a buried 500 m3concrete water tank. Heat pumps are used to ensure the heating demand is met. The system perf
32、ormance was simulated with TRNSYS, and with government subsidies, the system operating cost was shown to be cheaper than conventional heating systems.Chiasson and Spitler (2001) present a system simulation approach to the design of a hydronic snow melting system for a bridge deck on an interstate hi
33、ghway in Oklahoma. A verti-cal, closed-loop GHP system was designed to meet the heating requirement. The massive concrete bridge deck slab was proposed to be used as a solar collector in the summer months to thermally “recharge” the ground by circulating fluid from the bridge to the ground. An appro
34、ximate 20% savings in the total GLHE length could be realized if the solar recharge design option is used.Chiasson and Yavuzturk (2003) examined the viability of using solar thermal collectors as a means to balance annual ground loads. A system simulation approach was used to model an actual school
35、building with typical meteorological year weather data for 6 U.S. cities with varying climates and insolation. Conventional and hybrid solar GHP systems were sized for each case for 20 years of operation with a minimum 62 ASHRAE Transactionsdesign entering fluid temperature of 0oC. Life-cycle cost a
36、nal-yses indicated that the drilling costs must exceed the range of $19.68/m to $32.81/m for hybrid solar GHP systems to be economically viable.METHODOLOGYThe design tool presented in this paper is based on corre-lation of dimensionless groups that are comprised of variables describing the design of
37、 geothermal heat pump systems. A total of 62 GHP system simulations were conducted in order to generate design data for subsequent correlation. This section describes the system simulations conducted and the development of the dimensionless groups.System SimulationsThe GHP system models were constru
38、cted in the TRNSYS modeling environment (SEL, 2000) using standard and non-standard TRNSYS library components. The system configuration for the base cases (i.e. with no supplemental hybrid component) is shown in Figure 1, and Figure 2 shows the system configuration for the hybrid cases. In all simul
39、a-tions, the building loads were pre-calculated as described below and read from a file into a water source heat pump component model.Component Model Description.Generic Buildings: Hourly heating and cooling loads on an annual basis for generic buildings were computed using eQuest (Hirsch, 2005), a
40、graphical user interface coupled to the DOE-2 simulation engine (York and Cappiello, 1981). Hourly loads were calculated for a school building and an office building in 10 cities in the United States and Canada: Albuquerque, NM; Anchorage, AK; Boise, ID; Dallas, TX; Halifax, NS; Houston, TX; Philade
41、lphia, PA; Raleigh, NC; Phoenix, AZ; and Vancouver, BC. In addition, hourly loads were calculated for a church and an apartment building for Halifax, NS and Houston, TX. Detailed descriptions of the buildings can be found in Chiasson (2007).Ground-loop Heat Exchanger Model: This component model is a
42、 modified version of that developed by Yavuzturk and Spitler (1999), which is a response factor (referred to as g-functions) model based on the work of Eskilson (1987). Modi-fications used in this present study are described by Chiasson (2007), and include replacement of a steady-state borehole resi
43、stance with hourly modeling of thermal storage of the u-tube fluid, and modeling of transient heat transfer within the borehole grout.Heat Pump Model: This component model describes the performance of a water-to-air heat pump that has been devel-oped for GHP system simulations. Inputs to the heat pu
44、mp model include the building loads, entering fluid temperature, and fluid mass flow rate. Quadratic curve-fit equations to Figure 1 System configuration as developed in the TRNSYS modeling environment for modeling stand-alone GHP systems.Figure 2 System configuration as developed in the TRNSYS mode
45、ling environment for modeling hybrid GHP systems with supplemental solar thermal collectors.ASHRAE Transactions 63manufacturers heat pump catalog data are employed to compute the heat of rejection in cooling mode, heat of absorp-tion in heating mode, and the heat pump energy consumption. Outputs pro
46、vided by the model include exiting fluid temper-ature, energy consumption, fluid mass flow rate, and coeffi-cient of performance (COP).Flow Controls, Pump, and Heat Exchanger Models: The hybrid GHP systems were modeled as a primary/secondary loop system as shown in Figure 2. The primary circuit is t
47、he building loop plus ground loop heat exchanger and the secondary circuit is the hybrid component loop. The flow circuits are separated by a plate type heat exchanger (TRNSYS component type 5) modeled with a constant effec-tiveness. The system is modeled in this way to mimic design practices and to
48、 allow different fluids to be placed in each loop. For example, the solar collector fluid is typically an aqueous solution of 50% propylene glycol to avoid extreme freezing conditions.Tee pieces, diverters, and pumps were modeled using TRNSYS standard library component models. Differential temperatu
49、re control schemes are specified with the differential controller model, which was used to activate pumps and valves. A simple control scheme was used to simulate a vari-able frequency drive on the primary building loop pump; the flow rate for the current hour of the simulation is scaled to the peak flow rate according to the ratio of the current hourly load to the peak building load. A maximum pump turn-down speed of 30% was assumed.Solar Collector Model: The solar collector component model is TRNSYS Type 73, a theoretical flat plate solar collector. A single-glazed solar collector wa
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