Using off-peak precooling

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

All buildings have thermal mass, i.e., components with physical mass that acts as a thermal capacitor and changes temperature in proportion to the physical mass and its specific heat. However, most buildings are not operated to take advantage of this thermal mass. That is, they maintain a desired temperature setpoint, [T.sub.sp], throughout the building's operating hours and another [T.sub.sp] when the building is unoccupied (i.e., temperature setup and setback).

In contrast, a building precooling operational strategy cools the building prior to peak demand periods to reduce space cooling loads--and electric power demand--during peak demand periods. This strategy is analogous to chilled-water or ice-based thermal energy storage approaches (1) using the building's thermal mass to store "coolness" instead of chilled water or ice. As Braun (2) notes, the indoor temperature of a typical concrete construction building without air conditioning and external loads will rise approximately 1[degrees]F-2[degrees]F (0.6[degrees]C-1.1[degrees]C) per hour.

Thus, building precooling has the same goal as thermal energy storage: to reduce building electric costs by reducing peak electric demand and/or electricity consumption charges during peak electric demand periods.

Air conditioning and associated ventilation accounts for almost half of peak electric demand of commercial buildings, (3,4) so using off-peak electricity to provide a significant portion of space cooling can achieve considerable electricity cost savings.

Figure 1 illustrates the basic concept. In the early morning hours, the control system decreases the setpoint temperature, [T.sub.sp], to begin precooling the building in anticipation of the on-peak period. When the building is occupied in the morning, [T.sub.sp] is increased slightly but maintained at or near the lower bound of the acceptable indoor temperature range. This maintains the maximum precooling of building thermal mass while avoiding extensive use of cooling that would result in high electric demand during this period. Later in the day, the control system allows space temperatures to rise, allowing the thermal mass to discharge in an optimal way to meet a large portion of the space cooling load until the on-peak period ends. During this period, it is important to effectively manage the [T.sub.sp] profiles to avoid spikes in cooling power demand that compromise peak demand reductions. (6) At the end of the peak-demand period, [T.sub.sp] reverts to that used for a conventional strategy.

Precooling of building thermal mass can use either air conditioning or outdoor air to cool the building. AC-based precooling can provide large quantities of cooling under a range of outdoor conditions, i.e., even if the OA temperature, [T.sub.OA], or moisture levels exceed zone temperatures, [T.sub.Z], or acceptable indoor humidity levels.

An OA-based precooling approach operates the supply fan to provide 100% OA to precool the building when [T.sub.OA] is less than [T.sub.Z]. Since OA ventilation uses ventilation energy instead of mechanical cooling * energy, the actual temperature when OA ventilation operates must take into account this additional fan energy, i.e., it operates when [T.sub.Z] minus an offset factor based on fan energy exceeds [T.sub.OA]. Depending on the ratio of off- to on-peak electric rates and fan efficacy (i.e., W/cfm), this offset factor ranges between 2[degrees]F and 12[degrees]F (-17[degrees]C and -11[degrees]C), with a lower threshold for a higher ratio of electric rates and higher fan efficacies. (7)

OA ventilation typically works best in climates where nighttime OA temperatures fall appreciably below [T.sub.sp] and where the OA has relatively low moisture levels. Excess humidity would be stored in hygroscopic materials within the space and released as the building warmed up. These conditions are generally similar to those favorable for economizer operation.

Off-peak precooling impacts sensible cooling loads, but latent cooling capacity must still be provided to maintain comfort conditions. Depending on the reduction in sensible loads, conventional air-conditioning equipment may not have sufficient latent capacity to meet the latent load at all conditions. For buildings where moisture from OA ventilation is the primary latent load, a dedicated outdoor air system with energy recovery ventilation is an efficient way to address latent loads.

Effective control plays a decisive role in implementing nighttime precooling of commercial buildings. Specifically, an effective control algorithm must develop the ideal [T.sub.sp] profile for each zone in the building to minimize energy costs within a given utility rate structure without compromising occupant comfort. The need to avoid uncomfortable conditions in the pursuit of energy savings bears repeating, as employees' salaries in an office building are approximately two orders greater than energy costs. (8) Many commercial buildings have utility rate structures that include different on- and off-peak electricity consumption ($/kWh) charges in addition to peak electric demand charges. Furthermore, the key variables driving peak electric demand, such as temperatures, cannot be precisely known ahead of time. This optimization becomes particularly challenging in the case of ratcheted demand charges, i.e., where the peak demand over a period of several months is applied to all of those months. Consequently, such an optimization can be complex. (2,5)