1、134 2010 ASHRAEABSTRACTThis paper summarizes the results of field testing to eval-uate the performance of simplified and optimal control strat-egies for shifting peak cooling loads and reducing buildingenergy costs through the utilization of both the passive andactive thermal energy storage systems
2、for an elementaryschool in Colorado. In particular, the performance of precool-ing of building thermal mass as well as real-time model-basedpredictive optimal controls is evaluated. The results indicate that the school is saving roughly 47%on its annual electricity costs by utilizing the TES operate
3、dusing their optimal control strategy. INTRODUCTIONShifting building cooling loads using thermal energy stor-age (TES) systems provides several advantages includingreduction of peak demands for the electrical utilities andreduction of operating costs for the building owners. Gener-ally, two types of
4、 TES systems are typically utilized in build-ings: passive and active. Passive TES systems utilize precooling strategies of thebuilding thermal mass during nighttime to shift and reducepeak cooling loads (Braun 2003). Simulation analyses of vari-ous precooling strategies have shown that energy cost
5、savingsof 10% to 50% and peak demand reductions of 10% to 35% arepossible by utilizing a preconditioning control strategy (Braun1990, Rabl and Norford 1991, Conniff 1991, Andreson andBrandemuehl 1992, Morris et al. 1994, Keeney and Braun1996, Chen 2001, Braun et al. 2001, Chaturvedi and Braun2002).
6、Experimental studies have also shown comparablelevels of cost savings and peak demand reduction (Braun et al.2001, Keeney and Braun 1997, Morris et al. 1994). Controloptimization geared toward specific outcomes can generallyincrease cost savings or peak demand reduction (Braun 2003).Active TES syste
7、ms refer to the use of chilled water or icetanks on the plant chilled water loop as a heat storage medium.Active TES systems provide load shifting by allowing thechiller plant to be run during unoccupied periods, storing theheat absorption capacity, and discharging it during occupiedand/or peak peri
8、ods to reduce the need for mechanical coolingof the chilled water loop. Chilled water tanks and ice storagetanks are the most common active TES equipment. Thedispatchable load shifting capacity with active TES systemsallows for a reduction in chiller size due to a reliable reductionin peak loads, an
9、d the lower chilled water supply temperatureallows for unique airside HVAC designs (Henze and Krarti2002). Several control strategies have been proposed for activeTES systems including chiller-priority, storage-priority,constant-proportion, and optimal controls (Henze 2003).While some active TES sys
10、tems in the field have been foundto be underperforming (Guven and Flynn 1992, Tran et al.1989), these systems have demonstrated overall cost savingsand increased energy consumption compared to systems with-out active TES (Sohn 1991, Henze and Krarti 1998, Ihm et al.2004). Simulation work on optimal
11、control of active TES hasshown that it is possible to reduce costs by as much as 20%without increasing overall energy consumption (Henze andKrarti 1998).The combined utilization of passive and active TESsystems has been investigated and found to be capable ofreducing costs by up to 45% when optimal
12、controls are consid-ered (Henze and Krarti 2002, Henze et al. 2004, Zhou et al.2005, Krarti et al. 2007). Field Testing of Optimal Controls of Passive and Active Thermal Storage Stephen Morgan Moncef Krarti, PhD, PEStudent Member ASHRAE Member ASHRAEStephen Morgan is a graduate student and Moncef Kr
13、arti is a professor and associate chair in the Civil, Environmental, and ArchitecturalEngineering Department at the University of Colorado, Boulder, CO.OR-10-015 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2
14、010, 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 135In this paper, the performance of combined passive andactive TES systems is investiga
15、ted through field testing ofvarious control strategies. The field testing is carried in anelementary school in Colorado equipped with an ice storagesystem. A simulation environment based Energyplus (Craw-ley, 2000), a detailed whole-building simulation program isused to determine the optimal control
16、 strategies (Zhou et al.2005, Krarti et al. 2007). First, the building and its coolingsystem is presented. Then, the testing procedures as well as thesimulation environment are briefly outlined. Finally, the test-ing results are summarized and discussed.BUILDING DESCRIPTIONThe field testing for the
17、TES control strategies has beencarryout in an elementary school located in Fort Collins, CO.This building, shown in Figures 1 and 2, part of the PoudreSchool District (PDS), was built in 2002 as a model of a high-performance school building. The school has a total floor areaof 65,000 ft2 including r
18、oughly 48,000 ft2is on the groundlevel, with the remaining 17,500 ft2on the second floor. Thereare 25 classroom spaces, a gymnasium, cafeteria, library,computer lab, and staff offices (see Figures 1 and 2). Figure 1 Basic floor plan for the elementary school (shaded areas represent second story spac
19、es).Figure 2 Elementary school (a) West faade and (b) East faade.(a) (b) 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or t
20、ransmission in either print or digital form is not permitted without ASHRAEs prior written permission. 136 ASHRAE TransactionsThe school is occupied Monday through Friday from 8:30am to 3:30 pm by 630 students and staff from August 24thtoMay 31st. The building is unoccupied from June 1stto August15t
21、hwhen staff returns to prepare for the academic year. Thebuilding is also unoccupied for two weeks over the holidays ofChristmas and New Years. Table 1 summarizes the basicfeatures of the elementary school. To cool the school, a chiller and a thermal energy storage(TES) system are utilized. The cool
22、ing setpoint is 24C (75F)during occupied periods from 8:30 am to 5 pm, Monday -Friday. During unoccupied periods, the temperature is set to32C (90F), effectively allowing the temperature to float.When the cooling system is not engaged (i.e. during off-season), the setpoint is left to float as well.T
23、able 1. Elementary School Building CharacteristicsCategory DescriptionOccupancy Schedule630 students and staff 8:30 am to 3:30 pm M-F Aug 24 to June 1 Unoccupied during holidaysExterior Wall Construction10 cm (4-in) brick faade, 2.5 cm (1-in) air gap, vapor barrier, 15 cm (6-in) metal stud with R-19
24、 batt insulation, brick or Roof Construction 1 cm(0.5-in) gravel, metal sheeting, 10 cm (4-in) rigid insulation, vapor barrierFloor Construction 10 cm (4-in) concrete slabGlazing TypeEye-level: 24 mm (0.95-in) Double Pane Glass - Conductivity = 0.9 W/m*K Shading Coeffi-cient = 0.66 Clerestory: 24 mm
25、 (0.95-in) double pane glassEquipment Power Density5 W/m2(0.47 W/ft2); Schedule 100% 9am-12pm, 1pm-3pm, 50% 8am-9am, 12pm-1pm, 3pm-4pm, 0% all other timesLighting Power Density 10 W/m2(0.94 W/ft2); Schedule 100% 8am-6pm, 10% all other timesConventional Cooling Setpoints 24C (75F) from 8am-5pm, 30C (
26、85F) all other timesHVAC System 7 AHUs combined 70 hp (52 kW) and 51,700 cfm with 22C (72oF) economizersChiller Characteristics Trane CGAFC 50 ton capacityIce Storage System Calmac 1500C 570 ton-hr unit 15% latent 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.
27、(www.ashrae.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 137The air-cooled chiller with a 50-
28、ton scroll compressor isrun only at night, charging the TES system when the buildingis unoccupied and the building electrical demand is at a mini-mum. The chiller is kept in assist mode during the day in orderto meet unexpected cooling loads. The TES system handles allcooling loads during the occupi
29、ed period. Typical operation isfrom 2 or 3 am until 7 or 8 am. Sometimes, the chiller is oper-ated earlier when additional charging of the storage tank isneeded. The chiller is never operated after 8 am when the inte-rior lighting and AHUs come on as the building starts to beheavily occupied by staf
30、f and students. The TES system, consisting of 3 ice-tanks with a totalcapacity of 570 ton-hr, is an internal melt ice-on-coil system.Its refrigerant is a water/glycol (brine) solution operated at -2.8C (27F). The TES system was sized to meet 100% ofcooling load for design-day conditions. Thus, it su
31、pplieschilled water to the cooling coils during occupied hours (on-peak demand-setting period). It is generally discharged duringoccupied period as needed to meet the building cooling load.The ice storage tanks are fully charged every night except onweekends. The tanks are located outside, buried un
32、derground.The TES system is charged fully every weekend, some-times operating from Saturday evening until Monday morn-ing, immediately prior to occupancy. This is possible becausethe building is completely unoccupied on the weekends, so theuse of the chiller during the day does not increase on-peake
33、lectrical demand. During periods of high cooling load, theTES is typically depleted just before the end of the occupiedperiod. This is in part due to the small chiller size and in partto the restrictions on demand due to outdoor lighting and othersmall evening loads. Figure 3 illustrates a typical w
34、eeklycharge cycle for the TES during a period of peak coolingdemand. TESTING PROCEDURETwo types of tests were carried out at the elementaryschool in addition to the tests that evaluate the performance ofthe existing control strategies. Due to demand constraints, thechiller was almost never used duri
35、ng occupied periods, andsince the ice storage system was so large the cooling energyconsumption was measured by monitoring the discharge of theice tanks. All system information was obtained from the PSDonline building automation system. Weather data wasobtained from a nearby weather station operated
36、 by the North-ern Colorado Water Conservation District (NCWCD). Theweather data included hourly averaged dry bulb temperature,wind speed, and solar radiation (Krarti et al., 2007).Precooling TestsThe first type of test was a simple precooling test to deter-mine the dynamic behavior of the building w
37、hen its passivethermal mass was utilized. Three of these tests were performedin May just before classes ended for summer recess. Two addi-tional precooling tests were completed at the end of August. Inthese tests, indoor temperature setpoints were determinedprior to the tests through a series of sim
38、ulation analyses andFigure 3 Typical weekly charging-discharging cycle for the elementary school. 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reprodu
39、ction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. 138 ASHRAE Transactionsdiscussions with the PSD energy manager. After determiningthe appropriate precooling temperatures and periods, zonetemperature setpoints were changed
40、 remotely for the specifiedperiods. Optimized TestsDuring the second half of August when the building isfully occupied, optimized and predictive tests were carried out.A simulation environment, briefly presented in the followingsection, is used to determine the optimal settings (Zhou et al.2005). Th
41、ese tests were performed for predictive updatewindows of 4, 6, and 24 hours. The simulation analysis wasexecuted based on the latest available weather data, and thetemperature setpoints were changed to reflect the optimizedcontrol determined by the simulation environment. Due tocontrol restrictions
42、for the cooling system, the ice storage wasdischarged to meet the cooling load for all occupied periods. SIMULATION ENVIRONMENTTo determine and implement various control strategiesincluding predictive optimal controls, a simulation environ-ment has been developed. The basic structure of the simulati
43、onenvironment is shown in Figure 4. A weather predictor is usedto generate future weather information that can be used by thesimulation environment (consisting of a whole building simu-lation program with internal optimization algorithms) todetermine the optimal temperature setpoints and/or charging
44、/discharging rates to minimize the building utility costs whilemaintaining comfort. A detailed description of the simulationenvironment as well as the weather predictor is presented byZhou et al. (2005) and Krarti et al. (2007). In this study, the school was modeled using EnergyPlusprogram (Crawley
45、et al 2000). The elementary school wasmodeled using the basic characteristics provided in Table 1.Both monthly and hourly calibrations of the simulation modelwere carried out. Figure 5 shows the results for the monthly calibration.The simulation model predictions and the utility data werewithin 5% f
46、or all the months. Hourly calibration of the simulation model predictionswere performed using hourly monitored data obtained duringregular building operation during a hot day. For this day, theTES, fully charged in the morning, was depleted throughoutthe day to meet the building cooling load. Zone s
47、etpoints wereFigure 4 Basic structure of the simulation environment. 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or trans
48、mission in either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 139set to float during the unoccupied periods and were set to 24C(75F) from 8 am to 5 pm. The building simulation modelpredicted reasonably well the actual building performance aspr
49、esented in Figure 6 for the indoor temperature variation andFigure 7 for the ice level in the TES tank. The discrepanciesbetween hourly model predictions and measured data arewithin 10%. The overall energy consumption predicted by thesimulation model was within 5% of measured values on anhourly and daily basis. FIELD TESTING RESULTSOnce the simulation model was calibrated, several testswere performed at the elementary school to evaluate variousTES control strategies. Conventional controls and basicpreconditioning tests were carried out first. Tests werecompleted for
