The present description relates generally to the field of heating, ventilation, and air conditioning (HVAC) systems. The present description relates more particularly to systems and methods for air conditioning a building using a rotatable energy recovery wheel. The present description relates more particularly still to systems and methods for cooling a data center using a rotatable energy recovery wheel.
An energy recovery wheel is a type of heat exchanger positioned within two separate air streams (e.g., supply and exhaust air streams of an air handling system) in order to transfer heat energy from one of the air streams to the other. Energy recovery wheels are often called thermal wheels, rotary heat exchangers, enthalpy wheels, or heat wheels. Typically, an energy recovery wheel is rotated such that a portion of the wheel moves successively between the two air streams. As the energy recovery wheel rotates, heat is absorbed into the wheel from the warmer of the two air streams (e.g., during a first half of the rotation) and rejected from the wheel to the cooler of the two air streams (e.g., during a second half of the rotation).
The heat transfer provided by an energy recovery wheel can be “sensible” heat transfer (i.e., an exchange of energy or enthalpy which results in a change in air temperature with no transfer of moisture between the two air streams), “latent” heat transfer (i.e., an exchange of energy or enthalpy which does not necessarily result in a change in air temperature and which transfers at least some moisture between the two air streams), or a combination of sensible heat transfer and latent heat transfer (i.e., an exchange of energy or enthalpy which results in both a change in air temperature and a transfer of moisture between the two air streams).
Typical buildings suitable for human occupancy (e.g., office buildings, residential buildings, etc.) generally require ventilation. Energy recovery wheels have traditionally been used with such buildings to extract the “energy” that was expended to condition the indoor air before it is exhausted to make room for the new “fresh” outside air. Data centers do not require ventilation and may actually be harmed by the humidity or pollution levels in the outside air. Energy recovery wheels can be used to provide indirect free cooling for a data center without introducing humidity or contaminants into the data center.
The change in air stream temperature resulting from the use of an energy recovery wheel may be proportional to the temperature difference between the two air streams (i.e., before the air streams pass through the energy recovery wheel) and may depend on the thermal efficiency of the energy recovery wheel. However, it is often difficult and challenging to predict the actual change in temperature resulting from the use of an energy recovery wheel because the actual heat transfer may be susceptible to various uncontrolled and/or unmeasured variables. Previous implementations of energy recovery wheels typically rely on a measurement of the air temperature outside the building to determine whether the change in temperature caused by the energy transfer wheel will be sufficient to cool the building to the desired temperature. However, such estimates can be inaccurate and lead to suboptimal system control and wasted energy.