In this era of increasing energy prices and heightened awareness of the environment, we simply cannot afford to ignore the useful energy contained in building exhaust and excess exfiltration. In some cases, energy recovery is required by code or by a state program. In other cases, it is supported by utility incentives or a green building initiative. In any case, there are real benefits to the building owner, the operator, and the occupants that can be used to enhance your business and your bottom line.
Outdoor AirERV is a different approach to familiar mechanical ventilation, which is a requirement in virtually all commercial buildings; system design begins with the required (or the desired) quantity of outdoor air for dilution. Refer to ASHRAE Standard 62-2001 and your local code for the minimum rates of outdoor air. Keep in mind that increased rates of outdoor air in design are a boon to indoor air quality (if excess humidity is mitigated, as it is with enthalpy recovery) and can provide for possible future changes in use or design occupancy for the space.
Free Energy?The theory of ERV is straightforward: capture the energy already used to condition inside air before it leaves the building in the exhaust. An air-to-air heat ex-changer (AAHX) uses this waste energy in the exhaust air to precondition outside ventilation air before it is introduced into the system or directly to the space. The energy doesn't need to be pushed or pumped; the basic physics tells us that heat flows from warm to cold and water moves from regions of high humidity to regions of low humidity.
By arranging the airstreams to flow in opposite directions, 60 percent to 80 percent of the delta is commonly exchanged (recovered). ERV is not free. However, the only significant energy input is to the fans in order to overcome the pressure loss of the AAHX. (Wheel motors are generally so small that they do not matter in energy savings or efficiency calculations.)
Common types of AAHX include heat pipes, plates, permeable plates, heat wheels, and enthalpy wheels. They are not equal, but all have their place in the set of solutions you can offer your customers. Heat pipes, plates, and heat wheels (without desiccant coatings) recover temperature only. They are most useful in heating only applications where winter humidification is not necessary or provided.
Coil run-around loops (strictly speaking, these are air-to-water-to-air heat exchangers) also recover temperature only and are less efficient due to the required additional energy input for the circulating pumps. While popular and useful in many laboratory and industrial applications, they are seldom employed for general space conditioning applications.
In an enthalpy wheel, as the heat exchanger rotates from one airstream to the other, it transfers water molecules from the wetter to the drier side. In summer, the cool, dry surfaces of the heat exchanger strip heat and moisture from the entering outside air. In winter, those same, now relatively warm and moist surfaces heat and humidify the ventilation air.
In an enthalpic plate, water molecules are allowed to move through a membrane to transfer to the opposing airstream. Sharing some of the characteristics
of enthalpy wheels, they generally are less efficient at transferring humidity and are more widely used in residential applications where outside air is a smaller percentage of the load.
Owner/operator benefits of ERVs include reduced demand for electricity and fuels, lower utility bills, and improved humidity control. Potential benefits to the contractor include increasing customer satisfaction and loyalty, greater margins available when selling complete systems, and improved market share.
Efficiency Equals Reduced DemandRecently published guidance (ARI Guideline V) allows designers to calculate the efficiency of HVAC systems incorporating ERV. While we are accustomed to seeing EER numbers of 9 to 11 for commercial direct exchange (DX) cooling equipment, the RER (recovery efficiency ratio, like EER in units of Btuh per watt) for enthalpy recovery typically ranges from 70 to 120, depending on design conditions.
More importantly, a system comprised of a 10-EER DX packaged unit and an enthalpy wheel with 30-percent outside air will have a combined efficiency (CEF as defined by Guideline V) of 12 to 14. The reduced load of a more efficient system at peak not only reduces demand charges for the building operator, but it may be an important resource for the power company.
Check with your local utility to see what prescriptive or custom programs may provide incentives for ERV installation; the client may be excited to learn that you can offer a superior system without extra cost. ERV efficiency also reduces the total load on the HVAC system, meaning that equipment often can be significantly downsized, increasing value and speeding payback.
Immediate payback is often achieved in densely occupied buildings (schools, theatres) in hot and humid climates. Guideline V also provides guidance on the proper sizing of unitary equipment used with ERV.
Lower Utility BillsWinter or summer, energy recovery means that the balance of the HVAC system has less work to do in conditioning the outside air. Shifting the majority of the outside air's load from the cooling and heating plant to the ERV means less electricity, less gas, and less oil is used, reducing energy bills.
In colder climates, the majority of the savings will come on the heating side; in hot, humid climates, cooling savings will dominate. As important as these energy savings are today, they will be even more important tomorrow; ERV can be part of a strategy hedging against future utility rate hikes. ERVs should not be considered in those rare and pleasant climates where annual energy savings would be exceeded by the fan energy to operate the AAHX.
Improved Humidity ControlA unique property of enthalpy recovery is that it exhibits a variable sensible heat ratio. In other words, when all the work is sensible (as when cooling on a relatively dry day), the enthalpy wheel will only do cooling (not dehumidification). On the other hand, on a day when it is 75 degrees and raining, the wheel will do no sensible work (temperatures are the same inside and out), but will dehumidify based on the difference between outdoor air at 100 percent relative humidity (RH) and 50 percent RH indoor air.
As recent research has demonstrated, combining an enthalpy wheel with a standard DX packaged unit results in the ability to control humidity better at both design and part loads. ERV allows standard systems to meet the potentially challenging new humidity control requirements in ASHRAE Standard 62.1.
Equipment AvailabilityEnthalpy wheels are available today in a wide variety of products: stand-alone energy recovery ventilators, dedicated outdoor air systems, standard air handler options, custom air-handling units, integrated unitary packages, accessories for rooftop and indoor unitary packaged equipment, wall-hung air conditioners, and components for site-built systems.
Once the almost exclusive realm of custom and specialty manufacturers, ERV is provided by nearly all major manufacturers of HVAC and ventilation equipment and is supported by knowledgeable sales engineers, manufacturer representatives, and distributors.
Wellford is vice president of marketing for Airxchange Inc., a developer and manufacturer of energy recovery ventilation systems. He can be reached at email@example.com.
Publication date: 05/10/2004