1、74 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 appropria
3、tely sizing supple-mental equipment. The supplemental equipment examined in this work has been limited to direct-contact evaporative cool-ing towers. The design method for GHP systems was developed from results of 91 detailed computer simulations. Three dimension-less groups containing key GHP desig
4、n parameters were iden-tified using 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
5、 quantity of annual energy required to balance the annual ground loads. With additional input parameters also readily available to designers, the capacity of a cooling tower can be calculated, along with the corresponding reduced borehole heat exchanger length. Cool-ing tower capacity is calculated
6、using an annual equivalent full load hour concept.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 r
7、educing energy consumption in space condi-tioning 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 sy
8、stems, high capital cost is a significant barrier to market penetration.One of the main goals in the design of a GHP system is the proper sizing of the total length of the ground-loop heat exchanger so that it provides fluid temperatures to the heat pump within design limits. Unlike with conventiona
9、l heating and cooling systems, design of GHP systems 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 c
10、ycle must be considered. In heating dominated buildings, 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 may resul
11、t in the heat pump equipment capacity being 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 noncompe
12、titive 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 Cooling-Dominated BuildingsA.D. C
13、hiasson, PhD, PE, PEng C. Yavuzturk, PhDAssociate Member ASHRAE Member ASHRAEA.D. Chiasson is an assistant professor, Department of Mechanical and Aerospace Engineering, University of Dayton, Dayton, OH. C. Yavuzturk is an assistant professor, Department of Mechanical Engineering, University of Hart
14、ford, West Hartford, CT.LO-09-006 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 either print or digital
15、form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 75hybrid 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 design adds to the complex
16、ity of the overall GHP design process because of the addition of another transient component to the system. For example, acceptable conditions for supplemental heat rejection to the atmosphere in a cooling-dominated building depends on weather conditions and ground loop temperature. Consequently, hy
17、brid GHP systems should be analyzed on an hourly basis in order to be able to fully assess their behavior. The fundamental task in designing hybrid GHP systems lies in properly sizing the supplemental component and the ground-loop heat exchanger, using an appropriate control algorithm for system ope
18、ration so that annual heat rejection and extraction loads in the ground can be balanced. Current engineering design manuals such as Caneta Research, (1995), Kavanaugh and Rafferty (1997), and ASHRAE (2003), developed from research conducted since the 1980s, mention the potential use of hybrid coolin
19、g tower GHP systems, but do not describe a detailed design process for these systems.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 coupled to the system simulation
20、 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, nor is their use always econom
21、ically 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 significant barrier. Therefore, the ove
22、rall goal of the work presented in this paper is to develop a tool for the design of hybrid GHP systems in cooling-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 that allows straight-forward use.
23、 It should be noted however that the design practice of balancing ground loads in no way means that the system is economically optimal as the method is based on minimizing ground loop length under the assumption that the ground loop is the most costly portion of the system. One could choose to optim
24、ize a system based on life-cycle cost using a system simulation approach. However, attempting to find an optimum life-cycle cost is fraught with uncertainties in economic indi-cators and future energy prices. For this reason, a design approach of minimizing total ground loop length is preferred, and
25、 the practicing engineer could make economic calculations using the sizing results of the design tool developed here. Furthermore, as an added benefit, system hybridization while balancing ground loads annually by shifting the unbalanced portion to a supplemental heat rejection unit removes an impli
26、citly built-in constraint in life span of energy-efficient operation in such systems. The design method for hybrid GHP systems proposed in this paper was developed from results of detailed computer simulations, using three dimensionless groups containing key GHP design parameters based on the Buckin
27、gham Pi Theo-rem. With typical design parameters available to a practitioner, the design method can be used to estimate the total ground loop length for stand-alone GHP systems, along with the quantity of annual energy required to balance the annual ground loads. With additional input parameters als
28、o readily available to designers, a method for the calculation of the cool-ing tower capacity is presented, along with the corresponding reduced borehole heat exchanger length. BACKGROUND AND LITERATURE REVIEWCaneta Research, Inc. (1995) discusses the advantages of hybrid GHP applications with respe
29、ct to reducing capital costs and optimizing available surface area relative to a stand-alone GHP systems. A design procedure is suggested that sizes the capacity of the supplemental component based on the differ-ence between the monthly average cooling and heating loads of a commercial building. The
30、 ground loop is sized to meet the building heating loads while the cooling load in excess of the heating load is met through supplemental heat rejection. Caneta Research, Inc. (1995) also suggests that it may be advantageous to operate the supplemental heat rejecter during night-time hours for stora
31、ge of low temperature energy (cold storage) in the ground. Other control strategies discussed include set point control of heat rejection based on an upper limit of heat pump entering fluid temperatures, and the possi-ble year around operation of the rejecters in southern climates.Kavanaugh and Raff
32、erty (1997) discuss hybrid GHP systems from a perspective of cost containment of large loops designed to meet 100% of the cooling loads. The advantage of hybrid GHPs in areas of limited land availability is also stressed. Kavanaugh and Rafferty (1997) suggest a design procedure, which is to size the
33、 ground loop for the heating loads, and size the supplemental heat rejecter for the differ-ence in the cooling load.Kavanaugh (1998) revises and extends the design proce-dures recommended by ASHRAE (1995) and Kavanaugh and Rafferty (1997) for cooling tower design in hybrid systems. The revisions inv
34、olve balancing the heat flow to the ground on an annual basis in order to limit heat buildup in the borehole field, based on a set point control of the ground loop temper-ature. The revised procedure is applied to a multi-story office building in three different climates, and the results indicated t
35、hat hotter climates are more appropriate for the cooling tower hybrid application since the savings in required bore length are much more significant than moderate and cold climates.Phetteplace and Sullivan (1998) describe a case study that has been undertaken to collect performance data from an ope
36、r-ating hybrid cooling tower GHP system at a 2,230 m2military base administration building in Fort Polk, LA. The authors 76 ASHRAE Transactionsreport that for the monitoring period, approximately 43 times as much heat was rejected to the ground as was extracted, indicative of a highly cooling-domina
37、ted building. The system consists of 70 vertical closed-loop boreholes, each 61.0 m deep with a 3.30 m spacing. The supplemental heat rejecter is a 275 kW cooling tower, and is controlled with a set-point to activate the cooling tower when the heat pump exiting fluid temperatures reach 36.0oC. The r
38、elative energy consumption of the major system components over the study period is provided where the heat pumps account for 77% of the total energy consumption, the circulating pumps for 19%, the cool-ing tower fan for 3.0% and the cooling tower pump for 1.0%.Singh and Foster (1998) report on first
39、 cost savings that resulted from using a hybrid cooling tower GHP system on the Paragon Center building located in Allentown, PA and an elementary school building in West Atlantic City, NJ. Hybrid GHP systems were installed in these buildings because required loop lengths could not be installed due
40、to drilling difficulties at the Paragon center building and insufficient land area at the elementary school.Yavuzturk and Spitler (2000) use a system simulation approach to compare the advantages and disadvantages of various control strategies for the operation of a hybrid cooling tower GHP in a sma
41、ll cooling-dominated office building. The control strategies examined were to operate the cooling tower under the following conditions: (1) when the exiting fluid temperature from the heat pumps exceeded a set point of 35.8oC, (2) when the entering fluid temperature to the heat pumps exceeded a set
42、point of 35.8oC, (3) when the differen-tial temperature between the entering fluid to the heat pumps and the ambient air wet-bulb exceeded 2.00oC, (4) when the differential temperature between the entering fluid to the heat pumps and the ambient air wet-bulb exceeded 8.00oC, (5) when the differentia
43、l temperature between the exiting fluid from the heat pumps and the ambient air wet-bulb exceeded 2.00oC, and (6) on various night-time schedules at various times of the year in order to chill the ground. The most beneficial control strategy found by Yavuzturk and Spitler (2000), based on 20-year li
44、fe-cycle cost analyses, was to operate the cooling tower when the differential temper-ature between the exiting heat pump fluid temperature and the ambient air wet bulb temperature exceeded a set point. This resulted in operation of the cooling tower under the most advantageous weather conditions.Ra
45、mamoorthy et al. (2001) use a system simulation approach to find the optimal size of a supplemental cooling pond operating with a GHP system serving a cooling-domi-nated office building in two climates. Heat rejection to the pond was accomplished using a similar control strategy to the “best” one fo
46、und by Yavuzturk and Spitler (2000). That is, to reject heat to the pond based on a differential temperature between the source (the heat pump exiting fluid in this case) and the sink (the pond in this case). A sensitivity analysis was conducted on the actual differential temperature value, and it w
47、as found that varying this differential temperature was negli-gible in impacting heat pump energy consumption. Although significant ground-loop size reductions were found, the study identified the need for an automated optimization scheme to find the true optimum pond area and ground loop length, as
48、 well as to facilitate the design procedure.TESS (2005) developed a sub-hourly simulation model of a small commercial building at a military base in the South-eastern United States. The model identifies and implements a series of hybrid configurations and control schemes, and simu-lates each configu
49、ration for a 20-year life span for each of the control strategies and system parameters. An optimization algorithm is used to select the system and control strategy that resulted in the lowest life-cycle cost, considering first costs, energy and demand costs, water and water treatment costs, and maintenance costs. A comparative discussion of the simula-tion results, including a comparison of the systems and control strategies, and a description of the optimized system are provided. Epstein and Sowers (2006) document the heating up of the vertical borehole field at the Richard Stockton Col
