Actual IAQ and energy-conservation benefits were monitored by multiple state agencies. This project demonstrated that a single properly designed 100-percent-outdoor-air system not only can meet the heating and cooling needs of any space efficiently, it can process, manage, and monitor the ventilation needs of any mix of occupancies efficiently using variable-volume ventilation down to the individual-room level.

SYSTEMIC INEFFICIENCIES

Outdoor-air ventilation is the largest heating and cooling load component in most facilities. In Wisconsin, envelope heating losses of Standard 90.1-compliant schools with classic VAV-reheat systems comprise only 6 to 8 percent of total heating-plant-capacity requirements. The remaining 92 to 94 percent is needed to offset the energy used to process and deliver air. This remaining capacity tends to be larger than that of outdoor-air loads because of the presence of other systemic inefficiencies, such as reheat and excessive reliance on refrigeration.

Outdoor-air economizers generally are regarded as inefficient because delivered air must be cooled to satisfy the space requiring the coldest air, then reheated for other areas. If outdoor air is introduced through a mixed-air path, the mixed-air temperature may need to approach outdoor ambient design conditions at minimum system flows to satisfy ventilation requirements. This occurs because VAV systems are thermally biased, designed to cool and deliver air at temperatures substantially lower than the space temperature. Therefore, because traditional VAV systems cannot utilize low-grade heat resources from people, lights, and equipment, they are treated as waste products, preventing energy from being recycled.

Lastly, HVAC systems rarely are designed to operate from enthalpy. They are controlled by temperature, a measure of sensible heat. Enthalpy is used only to initiate a switch between minimum and 100-percent outdoor air to minimize refrigeration energy use. HVAC systems typically do not have a mechanism to take advantage of latent energy reduction for cooling or humidification purposes. The only way to accomplish that is with the adiabatic phase change between liquid and vapor.

PRINCIPLES OF OPERATION

RGDD systems address the major causes of inefficiency in classic HVAC systems, such as difficulty processing and managing outdoor air. RGDD systems employ only 100-percent outside air, using no “return” air. Because all of the air is exhausted, the “return” path becomes a large central exhaust system, permitting exhaust from individual zones to be funneled through a common path in which contaminants are diluted to harmless levels before the air is stripped of its energy assets.

FIGURE 1. Two-stage air-to-air energy-recovery process employing three air paths.

RGDD systems are designed around a two-stage air-to-air energy-recovery process employing three air paths (Figure 1). First, in the supply-air stream, a primary heat exchanger employs an indirect evaporative-cooling process. A second heat exchanger can be a sensible, only, or total energy air-to-air unit. Located between the two heat exchangers is a direct evaporative process that chills the water passing directly over the evaporative media in lieu of using a cooling coil. During the cooling season, the air leaving the direct evaporative cooler is kept at approximately 55°F to maintain humidity using an apparatus dew-point control process.

Used by Willis H. Carrier,² this technique serves multiple purposes. First, the direct evaporative cooler acts as a wet-air scrubber, removing particulate and water-soluble gases from the air. Second, microbial contaminants are removed from the air as particulate, captured in the sump, treated with a non-chemical biological control system, and automatically flushed out of the system. Third, by eliminating the need for a cooling coil, parasitic fan energy losses are reduced substantially. Fourth, the “soft” nature of evaporative media acts as an effective noise-reduction system, typically eliminating the need for sound attenuators. Finally, the air is delivered at a constant dew-point temperature, automatically maximizing the thermodynamic impact of the adiabatic processes and minimizing the need to expend energy for the mechanical-refrigeration process, relegating refrigeration to the role of “cooling resource of last resort.”

Downstream of the fan, the supply-air stream is split into cold and ventilation decks, which deliver air at two temperatures. The ventilation deck is tempered through the secondary air-to-air heat exchanger using low-grade heat energy stripped from the building's exhaust. Supplemental heat is introduced into the exhaust path where a single source of heat provides the necessary energy for the primary heat exchanger's preheat, supplemental-heat, and frost-prevention features.

This energy exchange provides multiple benefits. First, it eliminates the need to expend virgin energy assets (in the form of energy-wasting reheat) to provide precision temperature and humidity control during the cooling season. Second, by depressing the dry- and wet-bulb temperature of the exhaust air, the exchange of energy amplifies the recoverability of latent cooling potential in the indirect evaporative cooler. This can reduce outdoor-air cooling loads up to 80 percent during hot and humid weather and eliminate the need for refrigeration in less extreme weather.

The RGDD System's design carefully considers how airflow is delivered to a space. It is suited for displacement strategies in which heat gain to a space is deferred to the exhaust-air path and can be displaced out of the building, rather than seen as a refrigeration load. Because both decks provide 100-percent outdoor air, actual ventilation rates provided through dual-duct VAV air-terminal units can be monitored.