By Stephen J. Pargeter
Originally published in the August 2012 issue of Today’s Facility Manager
Studies have proven that outdoor air ventilation creates a healthful work environment. However, as these intake rates increase, so does the size, cost, and operating expense of the heating, ventilation, and air conditioning (HVAC) system. Facility managers (fms) are therefore caught between two powerful forces—the need to lower energy costs and the need to meet or exceed outdoor air ventilation regulations for occupant health and comfort.
During the ventilation process, large amounts of energy are wasted as HVAC systems replace indoor air with fresh outdoor air multiple times per day. The heating or cooling energy is continually lost in the exhaust air stream while new energy must be expended to condition entering outdoor air. Those who fail to capture wasted energy will continue to incur high energy costs.
To address this challenge, one option for fms is a site recovered energy technology such as energy recovery ventilation (ERV). Site recovered energy is any energy recovered on-site and reused to reduce the demand for more energy. Designed to operate with new or existing HVAC units, the ERV technology provides a means to cut energy costs without compromising outdoor air ventilation requirements.
While there are a number of air-to-air heat exchangers used in ERV, the most commonly used type in commercial and institutional buildings is a rotary heat exchanger (also known as an energy recovery wheel or enthalpy wheel). This wheel resolves the conflict between indoor air quality (IAQ) and energy conservation by recovering site energy normally wasted in exhaust air. It transfers energy by rotating between incoming (outdoor) and exhaust air streams to transfer heat and moisture from one air stream to the other, recycling up to 80% of this energy by preconditioning outdoor air for significantly reduced HVAC load and operating cost.
Total energy saved depends on the wheel’s effectiveness and the difference in temperature and humidity between the two air streams. A bigger differential means larger energy savings.
For new and replacement projects, energy recovery costs are typically offset by lower first costs for an HVAC system. And significant reductions in fuel consumption provide healthy returns for the life of the HVAC system—instant to two-year paybacks in most North American climate zones. An energy recovery wheel may also be added to an existing HVAC system to improve efficiency.
HVAC systems with ERV improve control over humidity to minimize mold and mildew and reduce the risk of IAQ complaints and sick building syndrome, especially in humid climates. Meanwhile, many gas and electric utilities offer rebates for energy recovery to reduce peak demand and encourage energy conservation.
Organizations Using ERV
To meet new building codes, Dillard’s, a large apparel and home furnishings retailer, sought to increase outdoor air ventilation rates by a factor of three at its store in Palm Beach, FL. The two story building required approximately 50,000 cfm (cubic feet per minute) of outdoor air. Rather than add capacity to meet the increased load, management at Dillard’s installed rooftop units with integrated energy recovery wheels. By reducing the load by 160 tons, the retailer was able to choose units with that much less capacity. And by reducing peak demand by 215 kilowatts (kW), the company qualified for utility rebates that effectively paid for the wheels. The use of ERV is now standard for all Dillard’s stores.
Another example of this technology involves a major renovation at the Providence Career and Technical Academy (PCTA) in Providence, RI. The engineering team there chose to install a dual temperature, two pipe HVAC system with energy recovery wheels to maintain control of humidity during the shoulder season. The improved efficiency has yielded an estimated savings of $28,000 per year.
Another ERV adopter is the Eddy Village Green in Cohoes, NY, a nursing home complex of 16 housing units, each with 12 bedrooms. Each of those units has a 13 ton HVAC system with an integrated energy recovery wheel that supplies 3,500 cfm of outside air. In addition to an estimated upfront combined savings of $20,208 from installing smaller packaged units, the facility hopes to recover an estimated ongoing savings of $23,168 based on local utility rates.
Evaluating ERV Systems
Fms who are interested in finding out about the potential of ERV can begin by looking for the following characteristics in a system. These include products that feature:
- rating and certification by the Air-Conditioning, Heating and Refrigeration Institute (AHRI);
- high humidity (not just heat) transfer capability;
- easy accessibility and cleaning; and
- performance modeling software.
Fms should also look for established providers of these systems. Most mainstream HVAC equipment manufacturers offer energy recovery wheel options for new or existing applications.
The energy demand posed by outdoor air ventilation requirements is a pressing issue among fms eager to save money while still providing clean, healthy buildings. Energy recovery wheels offer a way to cut energy costs and supply ample outdoor air ventilation while also garnering a high return on investment.
Pargeter is vice president, Product Engineering, at Airxchange, a Rockland, MA manufacturer of energy recovery wheels. He holds a BSc (Honors) degree in Production Engineering and Production Management and has served on the board of directors of the Air-Conditioning, Heating and Refrigeration Institute (AHRI).
Thanks for the information! I’m trying to find ways to make my HVAC more energy efficient. I thought that the information about how the wheel effectiveness in my HVAC affects the amount of energy it uses. What are some things that I can do to make the wheel more effective and improve the difference in temperature and humidity between the two air stream? My HVAC has been using more energy, so it would help to know how to make it work more efficiently in ways other than by just cleaning out the filter.
RenewAire ERVs also provides sensible and latent energy recovery without inheriting the risk of cross contamination. As it uses static plate exchange cores it does not have the same maintenance implications as energy recovery wheels. However, we have used and been satisfied with both technologies. The important consideration is why throw away usable energy to immediately use more. Certainly Greenland would appreciate us all taking a harder look at these technologies.
Great Article. We agree with the author that Energy Recovery should be seriously considered. In addition to energy savings, the reduced cooling and heating equipment requirements will help justify the added expense of the energy recovery. The cost premium for energy recovery is typically minimal when considering the cooling tonnage and heating plant reduction that can be realized.
As stated in the Article, total energy recovery wheels also provide continual dehumidification by helping to dehumdify the outside air on a continual basis. Large volumes of outside air required by ASHRAE 62.1 and Code, make it more difficult to control humidity when total energy recovery is not implemented.
Some of the applications we have been using energy recovery include Casinos, Skilled Nursing Facilities, Offices, Adult Day Care, K-12 Schools and Universities.
We typically specify total energy recovery wheels. In applications where contaminated air could be encountered, wheels should be avoided. With that said, we have successfully utilized toilet exhaust recovery with wheel type systems on multiple applications. Fan and ducting arrangements should be considered carefully when toilet exhaust is used. We typically specify the fans to be in a “Push-Pull” arrangement to minimize cross contamination between the exhaust and outside air streams. The “Push-Pull” arrangement has the outside air supply fan push through the wheel while the relief/exhaust fan pulls air through the wheel assembly. The “Push-Pull” arrangement is only viable for 100% outside air systems when recirculation of return air is not required.
When comparing energy recovery versus demand ventilation, the added advantage of energy recovery is continual ventilation at a higher level helping to purge the building of any contaminants that may have accumulated in the building. Demand ventilation also requires an extensive quantity of carbon dioxide sensors and more complicated controls when multiple spaces are served by the system. For example, a large school should have carbon dioxide sensors in every classroom if demand ventilation controls are provided.
Hopefully, this comment isn’t too long winded and proves helpful.
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