P2. Night Ventilation with Building Thermal Mass > Background

For certain building types, it is possible to take advantage of the storage capabilities of the building structure and shift a significant portion of a building's on-peak cooling requirements to off-peak periods, thereby reducing both energy and demand costs. This can be a significant demand management tool that does not require a major capital retrofit in a building. One of the more effective methods for pre-cooling a building involves the use of "free" cool night air to reduce cooling requirements for the next day. In reality, the use of outside air is not free, since energy is required to operate the air-handling fans. As a result, the best strategy involves a tradeoff between fan power and cooling provided. Currently, control products for employing nighttime ventilation do not exist.

Maximum potential savings associated with optimal control of building thermal mass as compared with conventional night setback control through the use of optimization routines applied to computer simulations of buildings and their associated cooling systems has been demonstrated. Results of this study showed there is a significant potential for reducing operating costs. The most dramatic relative savings over conventional control occur during times when nighttime ventilation cools the building zones and yet cooling is required during the day. Depending upon the building and HVAC system, the estimated daily savings in costs for cooling associated with nighttime ventilation were between about 10% and 50% when compared to conventional night setup control. The greatest savings occur in climates with large diurnal temperature swings, such as occur in dry regions of California.

Initial work in developing a control strategy for nighttime ventilation pre-cooling in conjunction with building thermal mass has been done. The strategy utilizes utility rate information along with simple estimates of building and HVAC system design characteristics. The method was tested in simulation and showed seasonal cost savings of up to 30% for dry climates, such as San Diego.

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