Demand Shifting With Thermal Mass in Large Commercial Buildings: Field Tests, Simulations and Audits

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The principle of pre-cooling and demand limiting is to pre-cool buildings at night or in the morning during off-peak hours, storing cooling in the building thermal mass and thereby reducing cooling loads during the peak periods. Savings are achieved by reducing on-peak energy and demand charges. The 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.

In Summer 2003, a pre-cooling case study was conducted at the Santa Rosa Federal Building. It was found that a simple demand limiting strategy performed well in this building. This strategy involved maintaining zone temperatures at the lower end of the comfort range (70°F) during the occupied hours before the peak period and floating the zone temperatures up to the high end of the comfort range (78°F) during the peak period. With this strategy, the chiller power was reduced by 80 to100% (1 to 2.3 W/ft2) during peak hours from 2 pm to 5 pm without having any thermal comfort complaints submitted to the operations staff.

Although the initial study was quite successful, some key questions remained unanswered, including: What was the actual comfort reaction? What is the effect of extended (nighttime) pre-cooling on the following day peak shed? What will happen in really hot weather?

In order to address these questions, field tests were performed in two buildings in 2004. In addition to further testing at the Santa Rosa Federal Building, tests were performed in a medium size office building in Rancho Cordova (McCuen Center One Building). A key feature of the 2004 study was the comfort survey. A web-based comfort survey instrument was developed and used in the field tests to assess thermal sensation, comfort and productivity ratings in these two buildings. To supplement the field tests, EnergyPlus computer simulation models were built for the two buildings and used to estimate the impact of various pre-cooling strategies on peak demand. In addition, a set of buildings were audited to assess their suitability for pre-cooling in terms of their building materials and control system and the willingness and ability of the building staff to implement pre-cooling strategies. These audits provide a preliminary assessment of customer acceptability and market readiness of pre-cooling.

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