Using off-peak precooling

ASHRAE Journal, March, 2009 by Kurt Roth, John Dieckmann, James Brodrick

Specifically, in a "weak" time-of-use rate structure, ([double dagger]) the optimum operating strategy achieved negligible energy cost savings for the three thermal mass cases evaluated, while the savings in a "strong" rate structure ranged from 22% ("light" thermal mass) to 27% ("heavy" thermal mass). (11)

Finally, Henze, et al, (10) simulated four different building types in four different climates combined with an algorithm to optimize energy cost savings through control strategies that take advantage of the building's thermal mass.

As part of their study, they varied 10 parameters that impact the effectiveness of off-peak precooling control strategies. They found that the optimized control approach yielded whole-building energy cost savings ranging from 0% to 27%.

Simulations of a four-story building under different control strategies found that precooling achieved energy cost ([section]) savings of around 20%.13 When analyzed with a rate structure with a more moderate demand charge, (#) the energy cost savings was around 10%.

In Miami and Phoenix, the savings ranged from 10% to 18%, increasing as the degree of precooling increased. [parallel] Conversely, simulations for Seattle, which has a temperate climate with no difference between on- and off-peak electricity rates and low ($1.46/kW) demand charges, found that precooling strategies actually increased energy cost (and energy consumption).

The same study performed simulations where the on-peak indoor temperature was allowed to rise appreciably higher, i.e., to 77[degrees]F (25[degrees]C) instead of 73[degrees]F (23[degrees]C). Higher indoor air temperatures bring two energy consumption benefits: they increase the effective thermal capacitance of the buildings (larger temperature difference attained) and also decrease building cooling loads (by reducing the indoor-outdoor temperature difference). (7)

It would be expected to achieve appreciable energy savings without using a precooling control algorithm designed to exploit building thermal mass. This approach reduced energy costs by approximately 40%, or twice the energy cost savings of the other control strategies investigated. This validates the substantial value of allowing indoor air temperatures to rise significantly during peak demand periods.

Overall, there have been few field studies evaluating the impact of off-peak precooling. (2,14) One study applied conventional and nighttime precooling of two "nearly identical" four-story buildings. It found that the precooled building, with an interior cooled to 68[degrees]F (20[degrees]C) at night and indoor temperatures maintained at or below 71[degrees]F (22[degrees]C) prior to the peak demand period, had a 25% reduction in peak cooling load. Other field studies have not clearly demonstrated significant reductions. (2,14)

Market Factors

In buildings where off-peak precooling can reduce peak cooling demand, it can produce significant first-cost savings by enabling downsizing of the chilled-water plant as well as, potentially, the ventilation system. With chiller installed costs of approximately $400 to $600 per ton (15) and peak cooling load reductions on the order of 10% to 30% indicated by simulations, this control strategy could realize significant equipment cost savings.