1、512 2010 ASHRAEABSTRACTA study for the optimization of the ground thermalresponse in hybrid geothermal heat pump systems is presented.The design difficulty with hybrid geothermal heat pump sys-tems is inherently an optimization problem that is best solvedwith a short time-step system simulation meth
2、od. Many param-eters can be optimized, and there is no unique expression of theoptimization objective function. In this study, the optimizationproblem is defined as balancing the annual thermal loads onthe ground by minimizing the borehole heat exchanger lengthand supplemental equipment size. The su
3、pplemental equip-ment examined in this research work has been limited to flatplate solar thermal collectors for heating-dominated applica-tions and direct-contact evaporative cooling towers in cool-ing-dominated applications. Optimal control and operatingstrategies for the annual thermal load balanc
4、e in the groundare discussed. Sensitivity analyses are conducted for theassessment of differential temperature control strategy impact-ing the magnitude of the ground heat transfer, and the supple-mental equipment design.INTRODUCTIONThe hybridization of geothermal heat pump systems (alsoknown as gro
5、und-coupled or ground source heat pumpsystems) is accomplished by incorporating supplemental heatrejection or addition equipment, such as cooling towers, fluidcoolers, boilers, and solar collectors, with the ground heatexchanger (GHE) loop. More generally, hybridization ofgeothermal heat pump system
6、s could conceivably include thecoupling of any heat source or sink to a GHE loop. Hybrid-ization thus allows for part of the building thermal load to beexchanged via the supplemental equipment before heat trans-fer with the ground takes place.In non-hybridized geothermal heat pump (GHP) systemsthat
7、serve heavily heating- or cooling-dominated buildingthermal loads, an annual thermal imbalance of the groundthermal loads will occur. For instance, in heating-dominatedbuildings a non-hybridized geothermal heat pump system willon an annual basis extract more energy from the ground thanreject to it,
8、causing the average temperature of the groundvolume to decrease over time. As the average ground temper-ature decreases, the thermal quality of the heat source for theheat pump cycle is degraded (a heat source at a progressivelylower temperature), causing the coefficient of performance(COP) of the h
9、eat pump to deteriorate. Similarly, in cooling-dominated buildings more energy is rejected to the groundthan extracted from it, and on an annual basis the averageground temperature will increase, resulting then in a thermallydegraded heat sink (a heat sink at a progressively highertemperature) for t
10、he heat pump cycle. Thermal imbalanceconditions in the ground will cause the GHP system to operateat increasingly reduced capacities, and may ultimately resultin system failures due to continuously deteriorating heat pumpCOP. In order to avoid failure without hybridization in heat-ing- or cooling-do
11、minated buildings, ground heat exchangerloops must be sized to satisfy annual peak heating and coolingloads for the entire life span of the system, which requiresexcessively large and costly ground heat exchanger loops andborehole fields.The significance of hybridization lies in the fact that itmay
12、be used to completely balance ground thermal loads on anannual basis, thus not allowing sink/source thermal degrada-tion of the heat pump cycle to occur. Furthermore, balancingOptimization of the Ground Thermal Response in Hybrid Geothermal Heat Pump SystemsA.D. Chiasson, PhD, PE, PEng C.C. Yavuztur
13、k, PhD, CEMMember ASHRAE Member ASHRAED.W. Johnson, PhD, PE T.P. Filburn, PhD, PEA.D. Chiasson is an assistant professor in the Department of Mechanical and Aerospace Engineering, University of Dayton, Datyon, OH. .C.and T.P.Filburn is an associate professor in the Department of Mechanical Engineeri
14、ng, University of Hartford, West Hartford, CT. D.W. John-son is an assistant professor in the Department of Civil and Environmental Engineering, University of Texas San Antonio, San Antonio, TX.OR-10-054 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashra
15、e.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 513ground loads annually by shifting the unbal
16、anced portion to asupplemental heat transfer unit removes an implicitly built-inlimitation in life span of energy-efficient operation in suchsystems. A thermal balance in the loading of the groundvolume via ground loop heat exchangers is achieved so that onan annual basis the magnitude of energy ext
17、racted equals themagnitude of energy rejected, ensuring maximum and mini-mum heat pump entering fluid temperatures (temperature ofthe heat transfer fluid returning to the heat pump from theground) to remain constant within an acceptable range for theoperation of the heat pump cycle at designed effic
18、iencies.Thermal balancing of the ground loads implicitly sizes theground loop heat exchanger loop for the less dominant build-ing load at the allowable heat pump entering fluid tempera-tures, and as a consequence hybrid systems permit the use ofsmaller, lower-cost borehole fields. However, the desig
19、n ofhybrid systems adds to the complexity of the overall GHPdesign process because of the addition of another transientcomponent to the system. For example, as building loadsdisplay a time-dependent behavior, acceptable conditions forsupplemental heat rejection to the atmosphere in a cooling-dominat
20、ed building and for solar recharging of the ground ina heating-dominated building are also time-dependent func-tions of weather conditions, solar availability and ground looptemperature. Consequently, hybrid GHP systems are bestanalyzed on an hourly basis (as typical weather data are alsoavailable i
21、n hourly time-steps) for the accurate and reliableassessment of the overall system thermal behavior.The accurate design of hybrid GHP systems is essentiallyan optimization problem as sizing of the supplemental compo-nents (required solar collector area, cooling tower or fluidcooler capacities) and t
22、he GHE loop length stipulate themanagement of multiple degrees of freedom on multiplesystem design parameters under constraint conditions ofannual thermal load balance in the ground at a desired enteringheat pump fluid temperature range. In addition, the design ofhybrid GHP systems must use an appro
23、priate control algo-rithm for system operation for load balancing in the ground.Clearly, proper, accurate and reliable design of hybrid GHPsystems is quite difficult and cumbersome without the use ofa detailed system simulation approach. Furthermore, withoutan automated optimization scheme coupled t
24、o the systemsimulation program, the design activity itself can becometediously impractical and time-consuming.The overall goal of the work presented in this paper is todevelop an optimization approach for the design of hybridGHP systems in heating- and cooling-dominated buildingsthat effectively bal
25、ances ground thermal loads, based oncurrent “state-of-the-art” system simulation methods. Itshould be noted however that balancing ground loads may ormay not yield an economically optimized system as theapproach is based on minimizing ground loop length under theassumption that the ground loop is th
26、e most costly portion ofthe system. One could choose to optimize a system based onlife-cycle cost using a similar system simulation approach.However, attempting to find an optimum life-cycle cost isfraught with uncertainties in economic indicators and futureenergy prices. For this reason, a design a
27、pproach of minimiz-ing total ground loop length by balancing annual ground loadsis preferred, and economic calculations may still be madeusing the sizing results of the optimization approach proposedin this paper.BACKGROUND, AND SURVEY OF LITERATUREA review of the literature reveals a number of prim
28、arystudies in hybrid systems. A significant majority of the reportsdeal with methods and case studies for cooling-dominatedapplications while works for heating-dominated cases appearto be somewhat understudied.For heating-dominated applications, Andrews et al.(1978) presented one of the relatively e
29、arlier studies thatprovides qualitative evidence to suggest that coupling aground loop to a solar thermal collector and a storage tankcould improve system performance and reduce initial costs ofground heat exchangers, and identifies the need for detailedground heat transfer models. More recently, Da
30、lenback et al.(2000) conducted a preliminary study of a solar-heated lowtemperature space-heating system with seasonal storage in theground for single-family homes near Stockholm, Sweden,using a system simulation approach. Other solar district heat-ing systems with an underground storage volume ared
31、escribed by Seiwald and Hahne (2000) and Reuss and Muel-ler (2000). In the Seiwald and Hahne study, the solar contri-bution is about 50% of the total heat demand and the systemconsists of 2700 m2of flat plate solar collectors and a boreholestorage volume of 20,000 m3. The Reuss and Muller (2000)stud
32、y discusses a system that consists of 800 m2of roof-inte-grated solar collectors, a storage volume of 9350 m3, and aburied 500 m3concrete water tank. In both of these works, thesystems performance was assessed using a system simulationapproach; and with government subsidies, the system operat-ing co
33、st was shown to be less than conventional heatingsystems. The design of a hydronic snow melting system for abridge deck on an interstate highway was presented by Chias-son and Spitler (2001). The design was developed using anhourly system simulation approach. A vertical, closed-loopGHP system was de
34、signed to meet the heating requirement.The massive concrete bridge deck slab was proposed to beused as a solar collector in the summer months to thermally“recharge” the ground by circulating fluid from the bridge tothe ground. Chiasson and Yavuzturk (2003) examined theviability of using solar therma
35、l collectors as a means tobalance annual ground loads. Again, a system simulationapproach was used to model an actual school building at sixdifferent U.S. locations. Conventional and hybrid solar GHPsystems were sized for each case for 20 years of operation witha minimum design entering fluid temper
36、ature of 0C.For cooling-dominated applications, Caneta Research,Inc. (1995) provides a discussion of the advantages of hybridGHP applications with respect to reducing capital costs and 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published i
37、n ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. 514 ASHRAE Transactionsoptimizing available surface area relative to a stand-aloneGHP
38、 systems. Caneta (1995) proposes a design procedure thatsizes the capacity of the supplemental component based on thedifference between the monthly average cooling and heatingloads of a commercial building, and makes suggestions withrespect to system control and operating strategies. In anotherstudy
39、, Kavanaugh and Rafferty (1997) discuss hybrid GHPsystems from a perspective of cost containment of large loopsdesigned to meet 100% of the cooling loads. The study alsostresses the advantage of hybrid GHPs in areas of limitedland availability, and suggest a design procedure, which sizesthe ground l
40、oop for the heating loads, and size the supplemen-tal heat rejecter for the difference in the cooling load. A laterstudy by Kavanaugh (1998) revises and extends the designprocedures recommended by ASHRAE (1995) and Kavan-augh and Rafferty (1997) for cooling tower design in hybridsystems. The revisio
41、ns involve balancing the heat flow to theground on an annual basis in order to limit heat buildup in theborehole field, based on a set point control of the ground looptemperature. The results of the study indicated that hotter cli-mates are more appropriate for the cooling tower hybrid appli-cation
42、since the savings in required bore length are much moresignificant than moderate and cold climates. Furthermore, anumber of case studies of actual hybrid GHP systems are dis-cussed by Phetteplace and Sullivan (1998), Singh and Foster(1998), Chiasson (2005), and Epstein and Sowers (2006). TheEpstein
43、and Sowers (2006) study is especially interesting as itdocuments the heating up of the vertical borehole field at theRichard Stockton College, NJ. Temperature measurements ofthe subsurface were recorded at various depths throughout thegeologic profile over time, and a gradual increase in temper-atur
44、e of 11.0C was observed. Ultimately, a cooling tower wasadded to the system in order to remove the thermal energybuildup.Various simulation-based approaches have also beentaken in the design and analysis of optimal control and oper-ation of hybrid GHP systems for cooling-dominated applica-tions.Yavu
45、zturk and Spitler (2000) use a system simulationapproach to compare the advantages and disadvantages ofvarious control strategies for the operation of a hybrid coolingtower GHP in a small cooling-dominated office building. Thisstudy examined a series of operating strategies for the coolingtower and
46、various night-time schedules at various times of theyear in order to re-cool the ground, and recommends a controlstrategy, based on 20-year life-cycle cost analyses, to operatethe cooling tower when the differential temperature betweenthe exiting heat pump fluid temperature and the ambient airwet bu
47、lb temperature exceeds a set point.A later study by Ramamoorthy et al. (2001) also uses asystem simulation approach to find the optimal size of asupplemental cooling pond operating with a GHP system serv-ing a cooling-dominated office building in two climateregions. Heat rejection to the pond was ac
48、complished using asimilar control strategy to the “best” one found by Yavuzturkand Spitler (2000); that is, to reject heat to the pond based ona differential temperature between the source (the heat pumpexiting fluid) and the sink (the pond). A sensitivity analysiswas conducted on the actual differe
49、ntial temperature value,and it was found that varying this differential temperature wasnegligible in impacting heat pump energy consumption.Although significant ground-loop size reductions were found,the study identified the need for an automated optimizationscheme to find the true optimum pond area and ground looplength, as well as to facilitate the design procedure.In a later work, Wrobel (2004) presented details of aninstalled hybrid ground source heat pump system for spaceconditioning, water heating and deicing in an independentliving center for seniors, and described a parametric
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