Humidification on large-scale systems has traditionally been done with fossil-fuel based boilers or steam generators. There is a need for an electrified solution, and a leading technology for this is adiabatic humidification.
Adiabatic is the process where heat does not leave or enter a system. Heat is often seen as temperature (sensible heat), but moisture within air is also a form of heat (latent heat). Adiabatic humidifiers introduce water to air in a way that allows the air to absorb the water to become a gas. During this process, the water changes phase from small liquid droplets to gas. This phase change requires energy input, which happens when the air gives up sensible heat (measured temperature) to absorb the water. While the temperature of the air drops (reducing sensible heat), the moisture content in the air increases (raising latent heat). The resultant energy change is zero. The air has not gained or lost energy but has instead shifted the form of heat energy from sensible to latent. Since this results in no change in the total energy of the air, the process is called adiabatic. This process of humidifying air is called Adiabatic Humidification.
We learned that when the air absorbs moisture from an adiabatic humidifier, it loses sensible heat (measured temperature). This means that when adiabatic humidifiers are operating, they reduce the temperature of the air they are injecting into. The amount of temperature drop that occurs depends on how much moisture is added to the air. The higher the humidity level needs to be, the more moisture is added, and therefore, the higher the temperature drop will be. The moisture is added to the airstream through nozzles that are pressurized with RO-quality water, which comes from a high-pressure pumping system. Output is modulated through staging valves.
Physics still dictates that the energy to humidify needs to come from somewhere. Because the sensible heat in the air reduces, it means the air temperature will drop – and any heat that needs to be added to the air to compensate for this becomes the energy source of the humidification process. This means the energy source of the heating coils within the air handling system is the energy source for humidification. A fossil-fuel-driven steam-based humidification system might convert 85% of the potential heat from the fuel source to usable energy for generating steam. This would be a COP of 0.85. An electric resistance steam boiler or steam generator would be able to transfer 1kW of heat into the water for every 1kW of electrical input, thus having a COP of 1. With the adiabatic humidification process fueled by the heating coils within the air handlers, the COP of the heating source ultimately becomes the effective COP of the humidification system. If the hot water system utilized air-to-water heat pumps as the heating source, this might result in an annual average COP of ~2.5 for the New England climate zone. If HW/CHW heat recovery is used, or geothermal, COP can exceed 3. With a COP of 3, the humidification process can be completed using 1/3 of the energy of alternative electric humidification systems.
On very small-scale systems (small, individual rooms), adiabatic humidification might not be the best fit, and for this, electric steam generators should be considered. For medium-sized systems, adiabatic humidification should be considered, and on large-scale central systems, adiabatic humidification is the leading solution. Common applications are laboratory and healthcare buildings, where indoor air quality is critical, and humidity levels are maintained throughout the heating season to be within ASHRAE’s recommended ranges. ASHRAE Approves the Use of Atomizing Humidifiers in Healthcare Facilities.
Adiabatic humidification offers a way to humidify buildings without using fossil fuels as the source of energy and can even take advantage of the high efficiency associated with hot water systems using heat pumps as the source of heat.
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