Dedicated Outdoor Air Systems (DOAS) condition the outdoor ventilation make-up air separately from the return air from the conditioned space. This approach to handling ventilation make-up air results in superior humidity control by dealing with the primary source of humidity in most buildings – ambient humidity carried in by the ventilation air – directly at its source. When the DOAS removes enough extra moisture from the make-up air to handle the building interior load, energy savings can be obtained by running the separate, sensible cooling only, interior cooling system at higher evaporating temperature, improving the energy efficiency. Further energy savings are realised by providing only the amount of ventilation air necessary and by using enthalpy recovery for the building exhaust air to pre-cool the make-up air.
Background
It is common practice in commercial building air conditioning to combine ventilation make up air with return air from the building, condition (cool or heat) this air as needed, and distribute the conditioned air to the interior space, with or without zoned temperature control.
This approach to handling ventilation make-up air has received considerable attention in past several years. The impetus for this attention has been the growing realisation of the penalties and difficulties involved in meeting ASHRAE Standard 62 (Ventilation for Acceptable Indoor Air Quality) requirements throughout the conditioned space of a commercial building, with effective humidity control, particularly in the context of energy efficient approaches such as variable air volume.
In practice, vapour compression cycle air conditioning systems are often unable to maintain comfortable humidity levels in the conditioned space, when humid ventilation make-up air is mixed with building return air prior to the cooling coil.
Handling the treatment and distribution of ventilation make-up air and of return air from the occupied space with separate, parallel systems offers a number of potential advantages over conventional VAV systems that help overcome the problems discussed above. Many of these advantages directly result in significant energy savings.
The ventilation make-up air system can be sized and operated to provide the ventilation air flow rate required by code (e.g., ASHRAE -STD 62) to provide acceptable indoor air quality and provide this flow rate regardless of the interior temperature, without any need to oversize the ventilation rate. The ventilation rate can be constant, or it can be varied based on the building operating/occupancy schedule or in response to the actual occupancy (on a real time basis). Moreover, a DOAS allows easy verification that the system supplied the minimum OA quantities to different portions of a building. Energy recovery heat exchange between the make-up air and exhaust is readily implemented in this configuration, reducing peak cooling and heating loads to condition make-up air.
The predominant humidity load in most commercial building in most climate areas is the humidity brought in with the ventilation make-up air (in hot weather). Consequently, the entire humidity load for the building can be handled efficiently by separately conditioning the make-up air so that excess ambient humidity is removed (along with additional capacity to cover internal moisture sources).
With the ventilation make-up air separately conditioned, with the entire building humidity load handled in the process, the recirculated indoor air conditioning system can be operated to maintain temperature control. Because this is intended for sensible cooling only, the cooling can be operated at a higher than normal temperature (approximately 55oF evaporating temperature vs. 40oF to 45oF, typically) preventing moisture condensation and increasing the COP of the compressor. In addition to providing independent temperature and humidity control, this is an ideal situation for VAV. The conditioned air flow rate is varied in proportion to the net cooling or heating load, saving significant amounts of blower power during the large proportion of the year when full heating or cooling capacity is not required. Meanwhile, the parallel ventilation make-up air system continues to deliver the appropriate amount of air for IAQ purposes. Note that this applies to both chilled water based systems and to DX systems.
Performance
DOAS achieves energy savings via three primary factors –
- Optimal use of the ventilation air provided (allowing compliance with ASHRAE 62 with the minimum quantity of outdoor air),
- Ready use of enthalpy recovery to precool the outdoor air, and
- Allowing the interior load to be handled at higher refrigerant temperature and COP.
The combination of a DOAS with a sensible cooling only VAV system saves energy by reducing total ventilation air flow and by handling sensible cooling loads more efficiently. In a DOAS, ASHRAE 62 ventilation requirements can be met with less ventilation air flow due to the inherent precision of the DOAS in delivering required ventilation flows in the aggregate and in the individual zones in the building. In space cooling mode, energy saving include the benefit of higher chilled water temperature for the sensible portion of the load.
In addition to energy savings, DOAS systems provide superior indoor humidity control over a wide range of outdoor temperature and humidity levels. This can prevent mold growth and promote healthier indoor conditions. On the other hand, in contrast to a conventional VAV system, DOAS generally precludes economizer operation at levels above and beyond those needed to satisfy OA requirements, as the DOAS would most likely not include additional ventilation capacity.
Cost
A general perception exists that replacing one single purpose system with two parallel systems – the DOAS and the interior thermal load systems will result in increased installed equipment costs due to installation of additional (more) equipment. In new construction or major renovations this is not necessarily accurate.
Perceived barriers to market adoption of technology
To the extent that use of DOAS is viewed as a means to enhancing the performance of zoned, VAV, and/or DCV system designs, first cost is also an issue, as well as the contractor’s willingness to sell and estimate costs for zoning jobs. However, energy savings will certainly complement the payback period of the capital cost.
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