Application and Control of Air-to-Air Energy Recovery Units Used properly energy recovery is an important tool for managing ventilation and energy efficiency.

Paul Ehrlich, Ira Goldschmidt & Angela Lewis Building Intelligence Group As published   September Issue - Column

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The application of energy recovery units allows for the ability to do total heat recovery and recover both sensible and latent heat from an exhaust air stream and use it to pre-condition outdoor air required for building ventilation.  There are many potential technologies available for energy recovery including:

• Heat recovery wheels with either sensible or total heat recovery (enthalpy) wheels.
• Plate heat exchangers that are either sensible only or total recovery – with the use of permeable membranes.
• Run around loops and heat pipes, which offer better isolation but only recover sensible energy.
  

The vast majority of applications today are using enthalpy wheels and that is what we are going to focus on, although the same concepts can be readily applied to other heat recovery technologies as well. 

In addition to saving energy the use of energy recovery unit (ERU) also allows for the ability to downsize equipment potentially reducing project costs.  While it makes sense to consider using an ERU for any project were there is supply and exhaust air, it is particularly desirable for applications that require a large percentage of outdoor air.  One example of this type of application is a large assembly area such as a school cafeteria or gym that may require ventilation for several hundred occupants.  For new construction the use of ERU’s is required under ASHRAE 90.1.  The 2007 version of the code, requires energy recovery for any air handler over 5,000 CFM that has more then 70% outdoor air with exceptions for dirty environments and for areas with minimal heating and cooling loads.  Under the 2010 version of 90.1 the requirements become more stringent and are driven by climate zone, air handler size, and percentage of outdoor air.  The codes also now require bypassing the use of the ERU when the air handler is in economizer, which leads us into an interesting discussion about proper control of energy recovery.

On the vast majority of existing building projects that we work on we are finding that the ERU has been designed to operate at all times that the air handler is in the occupied mode.  Operating in this manner is not always optimal since we end up always recovering energy from the exhaust air even when we don’t want to.  Ideally energy recovery should only be used when the outdoor air is either hot or cold and we can use the recovery to pre-heat or pre-cool the incoming air stream. So here are a series of rules to consider when writing sequences for new and existing ERU applications:

1. Economizer:  If conditions are right to do free cooling with outdoor air and the building has the ability to utilize an air side (or water side) economizer we do not want to be using energy recovery and instead want to bypass the unit if possible or at least stop the energy recovery.  Economizer changeover is usually determined based on outdoor air dry bulb or enthalpy often with a comparison against the return / exhaust air.  Continuing to operate energy recovery while in economizer results in adding heat load to the outdoor air stream increasing overall energy usage.
2. Heating Mode:  If the air handler is in the heating mode then we generally want to operate the energy recovery unit and to get as much heat from the exhaust air stream as possible.
3. Cooling Mode:  If the unit is in the cooling mode and we are utilizing the economizer (see above) then we do not want to do energy recovery.  On projects that are using dry bulb changeover for economizer we generally will keep the energy recovery off until the outdoor air temperature starts to approach the return air temperature.  That way we can avoid adding heat to the outdoor air when it is in the range of 55 – 70 degrees DB.
4. Frost Control:  In cold climates special consideration should be given to sensing for frost build up on the wheel during extremely cold weather and allowing the coil to defrost using exhaust air.
   

Ideally the ERU can be equipped with a bypass damper, which allows for air to be bypassed around the wheel (or the unit) when we don’t want to be doing energy recovery.  On many existing units there is not a bypass available.  In that case we generally will operate the supply and return fans at all times when the air handler is in operation but will shut off the rotation of the enthalpy wheel when we do not want to be doing energy recovery.  In order to keep the wheel from building up with dirt and moisture we “stir” the wheel – which involves turning it on for a few minutes each hour. 

Of course utilizing a variable speed motor for the wheel rotation and then modulating the wheel velocity based on discharge temperature is the ideal way to control an ERU. But of course this adds cost and complexity and other than for systems where you are doing demand controlled ventilation and dynamically varying the amount of outdoor air this is probably not a firm requirement.

Used properly energy recovery is an important tool for managing ventilation and energy efficiency.  But just like many other systems – proper control can make a large difference in the resulting efficiency and performance of these systems.

About the Authors

Paul and Ira first worked together on a series of ASHRAE projects including the BACnet committee and Guideline 13 – Specifying DDC Controls. The formation of Building Intelligence Group provided them the ability to work together professionally providing assistance to owners with the planning, design and development of Intelligent Building Systems. Building Intelligence Group provides services for clients worldwide including leading Universities, Corporations, and Developers. More information can be found at www.buildingintelligencegroup.com  We also invite you to contact us directly at Paul@buildingintelligencegroup.com or ira@buildingintelligencegroup.com

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