INTRODUCTIONCodes require the introduction of outdoor air for mechanically-ventilated commercial buildings, to help assure that pollutants and bio-effluents introduced into the indoor air will be diluted and flushed out. When the outdoor air is very humid, hot, or cold, substantial energy is required to temper it. Energy recovery ventilation (ERV) systems exchange heat (often both sensible heat and water vapor) between the outgoing exhaust air and the ventilation air being brought in. Under appropriate conditions, this allows reducing the capacity of the HVAC system and saves energy. Heat and energy recovery wheels are the most commonly applied ERV systems, and they are rapidly increasing their market.
HVAC systems control temperature, humidity, and other conditions of the air in a building. In 2006, commercial buildings used about 2.8 quads of energy for space heating, ventilation, and space cooling. This is about 34 percent of the total energy consumption by the U.S. commercial sector.
Pre-conditioning the (outdoor) ventilation air can achieve significant reductions in the space heating and cooling loads. Ventilation systems can be fitted with heat/energy recovery devices to transfer energy between the supply air and the exhaust air to pre-condition the intake air.
Exhaust energy recovery technologies include energy recovery loops, heat pipes, plate exchangers, and rotating wheel air-to-air heat exchangers. Heat recovery devices transfer sensible heat between the supply and exhaust airstreams by making use of the temperature difference between the two airstreams. In contrast, energy recovery devices transfer both sensible and latent heat (also known as heat of vaporization) by exploiting both the temperature difference and the difference in humidity levels between the two airstreams. Where there are large humidity loads, energy recovery devices can recover more energy than heat recovery devices.
CURRENT STATUSThe North American energy recovery ventilation market is estimated to have earned revenues of $324.6 million in 2006, which is expected to grow to $778.7 million in 2012. Early manufacturers faced a lack of understanding by HVAC practitioners, credibility and maintenance issues. Manufacturers have since improved the technology, addressing many reliability and maintenance issues. The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) has also implemented Standard 1060 for rating device effectiveness, increasing confidence in them.
DRIVING FACTORSBesides the energy efficiency benefits of commercial energy recovery ventilation systems, there are also significant non-energy benefits that help to drive the development of the market.
Growing Concern about Indoor Environmental Quality.Increasing ventilation is a recognized way to improve indoor environmental quality. However, there are many cases where the outside air needs to be conditioned prior to circulation, leading to increased energy consumption. The use of energy recovery ventilation systems can help to increase ventilation rates while mitigating the increase in energy consumption.
Load Reduction.Energy recovery ventilation systems introduce pre-tempered air to the main air conditioning units. This reduces the capacity required. Smaller units generally cost less, and thus heat/energy recovery devices may result in “first cost” savings. For example, reducing the capacity of a single-zone rooftop air conditioner from 20 tons to 18 tons is estimated to reduce the cost by about $4,700, which is about 23 percent of the installed price. This can offset the additional costs of energy recovery ventilation components.
Standards and Recommendations.Government and non-government organizations promote adoption. The American Society of Heating, Refrigerating and Air-Conditioning Engineers’ (ASHRAE’s) Standard 90.1 prescribes the use of energy recovery ventilation systems with at least 50 percent recovery effectiveness for individual fan systems designed with fan capacities of 5,000 cfm or greater and a minimum outdoor air supply of at least 70 percent. The federal government mandates energy recovery ventilation systems in federal buildings and recommends that schools and small businesses consider the use of energy recovery ventilation. Several utilities have implemented incentive programs to promote these devices. They include Florida’s Progress Energy, Texas’ Austin Energy, Vermont Gas, and Wisconsin’s Focus on Energy.
Newer buildings generally have more efficient envelopes. This reduces the sensible heat load relative to the latent heat load that results from internal sources and ventilation air. But single-stage unitary equipment deals particularly poorly with high humidity loads when outdoor temperatures are mild. This can result in a loss of humidity control, which may in turn lead to issues for occupants, and even damage the structure. Energy recovery ventilation systems can help stabilize humidity levels.
LIMITING FACTORSOf course, energy recovery devices use energy themselves, at least to overcome the air pressure drop across the devices. The value of the recovered energy must offset the parasitic losses of the fans that overcome the pressure drops. Energy recovery ventilation systems are least effective when the temperature and humidity of the outside air are similar to that of the circulating air, and where the ventilation load is relatively small (in which case economizers and natural ventilation may be more appropriate). Conversely, they are most effective at peak demand times, when energy may be most highly valued and tightly constrained. The EPA has prepared an application map showing areas where schools could potentially benefit from the use of energy recovery ventilation systems (www.epa.gov/iaq/schooldesign/saves.html#ERV_System_Application_Map).
BARRIERS TO IMPLEMENTATIONEnergy recovery devices can save money by reducing equipment loads. However, at the design phase, they may suffer from being viewed as “additional” equipment that would add to the cost of the HVAC system. Since the HVAC designer may not understand the potential first cost savings that arise from being able to reduce the capacity of the air handling equipment, the decision to use energy recovery ventilation systems should be made early in the design process, as their use could have significant impacts on sizing and design of other HVAC equipment. For example, when an economizer is used together with an energy recovery ventilator, the ventilator must be controlled together with the economizer and the operation adjusted to provide for the economizer. If no economizer is used, the operation of the energy recovery device needs to be controlled to prevent over-warming the air, and air bypasses may be useful.
Even where apparently cost-effective, energy recovery devices may also not be appropriate for some applications. For example, they are hard to apply if the supply and exhaust airflow ducts are widely separated for some reason (such as central supply but distributed exhaust). The availability of design guides would be helpful in evaluating the possibility of using energy recovery ventilation systems.
In addition, there are few detailed case studies of commercial applications of energy recovery ventilation systems available to assist HVAC designers.
RECOMMENDED NEXT STEPSTools such as design guidance need to be prepared to help HVAC designers evaluate energy recovery ventilation systems, and to give designers more confidence when recommending their use. As far as possible, the tools should take a system approach to take into account cross effects due to interactions among various HVAC system components (e.g., the installation of energy recovery ventilators could allow smaller capacity chillers).
Detailed case studies of commercial applications of energy recovery ventilation systems are also somewhat lacking in the public domain. A compilation of these case studies would help encourage HVAC designers to consider recommending energy recovery ventilation systems in commercial applications.