Desiccants are materials that attract and hold moisture, and desiccant air-conditioning systems provide a method of drying air before it enters a conditioned space. With the high levels of fresh air now required for building ventilation, removing moisture has become increasingly important. Desiccant dehumidification systems are growing in popularity because of their ability to remove moisture from outdoor ventilation air while allowing conventional air-conditioning systems to deal primarily with control temperature.
Desiccant dehumidification is a new approach to space conditioning that offers solutions for many of the current economic, environmental, and regulatory issues being faced by facility managers. Indoor air quality is improved through higher ventilation rates, and achieving those fresh air make-up rates becomes more feasible with desiccant systems. At "low load conditions" outdoor air used for ventilation and recirculated air from the building have to be dehumidified more than they have to be cooled.
Properly integrated desiccant dehumidification systems have become cost-effective additions to many building HVAC systems because of:
Their ability to recover energy from conditioned air that is normally exhausted from buildings.
The lower cost of dehumidification when low-sensible load, high-latent load conditions are met.
The greater comfort achieved with dehumidified air.
The promotion of gas cooling for summer air-conditioning by utilities in the form of preferential gas cooling rates.
High electric utility demand charges, which encourage a shift away from conventional, electrically driven air-conditioning (which requires a heavy daytime loading).
Desiccant systems offer significant potential for energy savings (0.1 to 0.4 quads nationwide). They also inhibit microbiological growth by maintaining lower humidity levels. Better control of humidity prevents moisture, mildew, and rot damage to building materials.
Desiccant dehumidification is particularly attractive in applications where building exhaust air is readily available for an energy-recovery ventilator (ERV, or "passive" desiccant system) or where a source of waste heat from other building operations is available to regenerate an "active" desiccant system.
To dehumidify air streams, desiccant materials are impregnated into a lightweight honeycomb or corrugated matrix that is formed into a wheel. This wheel is rotated through a supply or process air stream on one side that is dried by the desiccant before being routed into the building. The wheel continues to rotate through a reactivation or regeneration air stream on the other side that dries out the desiccant and carries the moisture out of the building. The desiccant can be reactivated with air that is either hotter or drier than the process air.
"Passive" desiccant wheels, which are used in total ERVs and enthalpy exchangers, use dry air that is usually building exhaust air for regeneration. Passive desiccant wheels require additional fan power only to move the air and the energy contained in the exhaust air stream. However, passive desiccants cannot remove as much moisture from incoming ventilation air as active desiccant systems and are ultimately limited in sensible and latent capacity by the temperature and dryness of exhaust air leaving the building.
"Active" desiccant wheels use heated air and require a thermal energy source for regeneration. The illustration above shows the operational characteristics of active and passive desiccant wheels. The advantage of active desiccant wheels is that they dry the supply air continuously-to any desired humidity level-in all weather, regardless of the moisture content of the building's exhaust air. They can be regenerated with humidity control to any desired level.