1、586 2009 ASHRAEABSTRACTThe potential for utilizing building thermal mass for load shifting and peak demand reduction has been demonstrated in a number of simulation, laboratory, and field studies. This project studied the potential of pre-cooling and demand limit-ing in a heavy mass and a light mass
2、 building in the Bay Area of California. The conclusion of the work to date is that pre-cooling has the potential to improve the demand responsive-ness of commercial buildings while maintaining acceptable comfort conditions. Results indicate that pre-cooling increases the depth (kW) and duration (kW
3、h) of the shed capac-ity of a given building, all other factors being equal. Due to the time necessary for pre-cooling, it is only applicable to day-ahead demand response programs. Pre-cooling can be very effective if the building mass is relatively heavy. The effective-ness of night pre-cooling und
4、er hot weather conditions has not been tested. Further work is required to quantify and demon-strate the effectiveness of pre-cooling in different climates. Research is also needed to develop screening tools that can be used to select suitable buildings and customers, identify the most appropriate p
5、re-cooling strategies, and estimate the benefits to the customer and the utility.INTRODUCTIONThe structural mass within existing commercial buildings can be effectively used to reduce operating costs through simple adjustments of zone temperature setpoints within a range that doesnt compromise therm
6、al comfort. Generally, the building is pre-cooled at night or in the early morning at moderately low cooling setpoint temperatures (e.g., 6870F) and then the cooling setpoints are raised within the comfort zone (below 78F) during peak periods. Heating setpoints must be left unchanged or lowered to a
7、void unwanted in-creases in heating system energy. The cooled mass and higher on-peak zone setpoint temperatures lead to reduced on-peak cooling loads for the HVAC equipment, which results in lower on-peak energy and demand charges. The potential for using building thermal mass for load shifting and
8、 peak demand re-duction has been demonstrated in a number of simulation, lab-oratory, and field studies (Braun 1990; Ruud et al. 1990; Coniff 1991; Andresen and Brandemuehl 1992; Mahajan et al. 1993; Morris et al. 1994; Keeney and Braun 1997; Becker and Paciuk 2002; Xu et al. 2004). This strategy ap
9、pears to have sig-nificant potential for demand reduction if applied within an overall demand response program because the added demand reduction from different buildings can be large.In the summer of 2003, Xu conducted a pre-cooling case study at an office building in Santa Rosa, California (Xu et
10、al. 2004). The research team found that a simple demand limiting strategy performed well in this building. This strategy in-volved maintaining zone temperatures at the lower end of the comfort range (70F) during the occupied hours before the peak period (8 a.m. to 2 p.m.) and floating the zone tempe
11、ra-tures up to the high end of the comfort range (78F) during the peak period (2 p.m. to 5 p.m.). With this strategy, the chiller power was reduced by 80 to 100% (1 to 2.3 W/ft2) during peak hours without having any thermal comfort complaints submit-ted to the operations staff (Xu et al. 2004). In t
12、he summer of 2004, Xu conducted pre-cooling tests along with online real-time comfort surveys to determine occupant reactions to the thermal conditions in the Santa Rose building and in a Sacra-mento office building. The results of the comfort surveys in two large test buildings indicate that occupa
13、nt comfort was Case Study of Demand Shifting with Thermal Mass in Two Large Commercial BuildingsPeng Xu, PE, PhDPeng Xu is a scientist and mechanical engineer at the Lawrence Berkeley National Laboratory, Berkeley, CA.LO-09-056 2009, American Society of Heating, Refrigerating and Air-Conditioning En
14、gineers, 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 form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 587maintained during th
15、e pre-cooling tests as long as the zone temperatures were between 70F and 76F (Xu 2006).Although these studies were quite successful and a large peak demand shed was achieved while maintaining occupant comfort, some key questions remained unanswered, including:What are the metrics of the building th
16、ermal mass and how are they determined? Thermal mass metrics refer to how fast the passive thermal storage can be charged and discharged. One example is the time constant for the whole building temperature change when the HVAC system is off.How can thermal mass be discharged more efficiently and mor
17、e smoothly with no rebound? Rebound hap-pens when the thermal storage is depleted before the end of the demand period and electricity demand reaches new high.How can a buildings pre-cooling potential be assessed and the potential economic savings be quickly deter-mined?What will be the comfort react
18、ion if the occupants are informed in advance of the test?What will be the occupants reaction if pre-cooling per-sists for a longer period and they have opportunities to adjust to the new thermal environment?The research team addressed several of these questions in this study and will address others
19、in subsequent studies in a multi-year effort to understand pre-cooling thermal mass as a Demand Response (DR) strategy for commercial buildings.The team systematically conducted more field tests for a longer period in two large commercial buildings before (Xu 2006). In that study, no comfort data we
20、re collected during the hot days and all tests were blind tests where the occupants were not informed in advance. If they were informed of the precooling tests and expected a temperature change, they might change their clothing level accordingly. Akin to commuting by mass transit or bicycle on a reg
21、ional air quality “Spare the Air day,” occupants may be willing to adjust to temporarily inconvenient or uncomfortable conditions that they know have long-term benefits. Since advance notice was thought to bias the tests, the tests in this study were conducted without notifying the occupants.FIELD T
22、ESTSTo address the questions listed above, the research team selected two buildings that had participated in the Auto-CPP (Critical Peak Pricing) pilot program, a study to demonstrate the capability of automated demand shed in buildings on CPP days (Piette et al. 2006). The selection was based on lo
23、cation, technical feasibility, and owner intentions to participate. The two buildings selected were one museum (CSSC) and one office (OSF), both in Oakland, California. A strategy similar to the demand-shifting strategy implemented before (Xu 2006), based on zone temperature reset, was used in both
24、buildings.There were several reasons for picking these two build-ings. First, they were both medium-sized buildings with full direct digital control (DDC) and so the zone temperatures set points could be changed directly. Second, CSSC is a heavy mass building and a large portion of the floor area is
25、 exposed concrete. OSF is a very light office building with full glazing on the west and east faade. Studying buildings at the two ends of the building mass spectrum provides the opportunity to test and verify the thermal mass metrics and methods that devel-oped in parallel. Third, the owners occupy
26、 both buildings, except one floor in OSF. The building owners and property management teams were innovative and interested in trying new ideas and methods to reduce their utility costs. More detailed building descriptions can be found in later sections of this paper.OCCUPANT SURVEYSDemand shifting a
27、nd load shedding strategies should be acceptable from the perspective of the building users so that employee productivity and customer satisfaction are not hampered. The team surveyed building occupants to learn about their comfort levels during the tests. Occupants were surveyed in the morning, ear
28、ly afternoon, and late afternoon to assess the effects of the pre-cooling period, the moderate shed period, and the high shed period.The web-based comfort survey had three pages, preceded by a welcome page. The welcome page informed the users of the purposes of the survey, its voluntary, confidentia
29、l, and anonymous nature, and the expected time to complete it. On the first survey page, the users were asked to fill in their office or cubicle number to identify their locations in the building for later analysis with temperature logs. The second survey page contained questions about the occupants
30、 current clothing and activity. This facilitated the calculation of their cloth value and metabolic rate, and to evaluate whether people take off/put on clothing as the temperature shifts in order to keep themselves comfortable. On the third page, two questions were asked. One is to assess sensation
31、 and comfort, and the other polled the respondents for their opinion of the effect of the temperature on their productivity. It should be noted that both questions are self- assessment questions rather than objective questions based on physical measurements. Both questions use seven-point scales for
32、 the users responses. The information collected in the survey, along with the detailed thermal measurements recorded in proximity to the occupants, also enabled us to calculate the Predicted Mean Vote (PMV)1for comparison with the actual comfort vote.Employees were asked by email to take the survey
33、at least twice per day (once in the morning and once in the afternoon) and more often if possible. The survey was brief and took 23 1.Predicted Mean Vote is the average expected comfort response (vote) based on indoor air climate conditions588 ASHRAE Transactionsminutes to complete on the first view
34、ing and about 1 minute thereafter. Although it would have been ideal to have all employees take the survey at frequent, specified times through-out the day, the reality of the typical office schedule made the success of this approach unlikely. Further, the research team were wary of demanding too mu
35、ch of the occupants. During the previous tests (Xu et al., 2004), the team had notified the occu-pants each time they wanted them to take the survey, and learned that some of the employees had found the multiple emails intrusive. During the 2005 tests, the team therefore attempted to minimize the co
36、mmunication impact. This was apparently a successful strategy at both Oakland sites, but there was low participation from the employees at the site.As a first step, an email was sent to all building occupants to explain the purpose of the survey and to ask the recipient to fill out the survey on the
37、 days before the pre-cooling tests to construct a baseline. Then a brief note was sent the day before a test or baseline day to remind people to participate.In some cases, the invitation was sent directly to the occu-pants. In others, a project contact in the building sent out the invitation. In gen
38、eral, it has been preferable to have the occu-pants receive the invitation from a known, respected person in the building, such as a supervisor or facilities manager. This can foster good response rates because it conveys a sense of importance and sanctions the taking of the survey during work-ing h
39、ours. However, such contacts were often busy or unavail-able, and preferred that the team send out the notifications.TESTS IN BUILDING 1: HEAVY MASS BUILDINGTest Site DescriptionThe museum is an 86,000-square-foot, state-of-the-art science and technology education facility on a 13-acre site in the h
40、ills of Oakland, California (see Figure 1). The building is a heavy mass building with lots of exposed concrete on the first floor. The walls are well insulated and the windows are small to have a better control of lighting levels inside.The cooling plant has a 230-ton centrifugal chiller with a var
41、iable pumping chilled water loop. There are eight air-handling units located on the roof using chilled water to condi-tion outside air and provide air circulation throughout the entire facility. Seven of them are single duct variable air volume air handling units and one is a constant volume unit. A
42、 newly installed DDC control system provides indoor comfort control.The building has independent HVAC systems serving each major exhibit area and the office area. CO2sensors are installed throughout the exhibit area and outside air ventilation rate is adjusted automatically to keep the CO2levels in
43、the zones within the desire ranges. The supply and return fans for the single duct system are equipped with variable frequency drives (VFD). There are about 40 zones in the building. Although the building is fully equipped with DDC, it had no global zone temperature adjustment capability before the
44、study. This function was added to the DDC systems program as part of this study.The buildings operation is typical of that of many muse-ums. The building is open to visitors from Tuesday through Sunday. Since all the CPP days are on weekdays, the CPP program is financially less attractive for this b
45、uilding than for other buildings since the load of this building on CPP days is lower than that on weekends. The daytime occupied hours are from 8 a.m. to 5 p.m. In normal operation, the HVAC system starts at 5 am and pre-heats or pre-cools the building until 8 am, depending on the outside weather c
46、onditions. Before the tests, no major faults in the mechanical system were apparent Figure 1 Museum (CSSC).ASHRAE Transactions 589in this building; however, some controllers had not been tuned properly and certain valves and dampers were oscillating during operation. There were no comfort complaints
47、 in either the office or the exhibit areas. The building operators had worked at the building for a long time and were quite confident and familiar with its mechanical system.Test StrategiesThe pre-cooling and zone temperature reset strategies that were tested are shown in Figure 2. The building in
48、average was normally operated at a constant setpoint of 72F (22.2C) throughout the startup and occupied hours. After 8 p.m., the system was shut off and zone temperatures started to float. Under normal operation, the setpoints in individual zones ranged from 70 (21.1) to 75F (22.2C), with an average
49、 value of about 72F (22.2C).The first strategy tested was termed pre-cooling + linear zonal reset. The HVAC system was turned on earlier in the morning than in normal operation to pre-cool the building to 68F (20.0C) from 3 a.m. to 7 a.m. Because the weather was relatively cool in the Oakland Hills location in the summer and the outside air temperature was in the low 60sF in the morn-ings, the HVAC system could cool the building with outside air using the economizers and no chiller operation. From 7 a.m. to 12 p.m., mostly occupied hours, all the zone temperature se
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