A Round on Energy Recovery Wheels
As energy costs continue their upward spiral, Owners and Engineers are renewing their efforts to find ways to reduce building energy consumption. Consequently, the Energy Recovery Ventilator (ERV) is increasingly becoming a standard feature in heating, ventilating, and air conditioning (HVAC) design. Energy recovery ventilators are used to precondition outside air, using recovered building energy to do so. While useful on any building type, an ERV is most effective on those buildings with very high outside air requirements, such as schools, nursing facilities, hospitals, and laboratories.
“Energy recovery ventilators are used to precondition outside air, using recovered building energy to do so.”
The logic of an ERV is very straightforward: exhaust or relief air from the building is crossed through a heat exchanger with incoming outside air in order to pass heat between them. In the winter, the exiting building air gives up its heat and humidity (depending on the exchanger type) to the incoming cold winter air. In the summer, the exiting building air absorbs heat and humidity (again depending on the exchanger type) from the incoming air and acts to pre-cool it.
While there are multiple types of heat exchangers, the type most commonly used for commercial applications is the heat wheel. A heat wheel is a circular exchanger constructed of a tightly honeycombed metal matrix to which a desiccant-based heat absorbing chemical is adhered. The heat wheel is mounted between the exhaust and outside air streams so that the wheel is split into two pie-like halves, each air stream traversing over its own half. The wheel is motorized and is made to rotate between the exhaust and outside air streams, slowly carrying heat from one side to the other.
The heat wheel is efficient and cost effective; however, it can permit cross-over contamination from the exhaust-to-outside air stream. Because the wheel moves between the air streams, contaminants picked up from the exhaust stream can be delivered to the outside air stream. For most commercial applications, where the building exhaust is relatively innocuous, cross-over poses little hazard. On the other hand, for facilities such as nursing homes, hospitals, and laboratories, even minimal cross-over is a concern. To reduce this cross-over contamination, manufacturers have implemented improvements for these markets. They have improved the seals along the wheel and between the two air streams to prevent direct cross-over; they have designed purge sections to ‘clean’ the wheel as it moves from the exhaust side into the outside air side; and they have formulated additional chemical overlays that limit the size of contaminants the heat wheel can pick up.
The energy savings of the heat wheel is the underlying reason for its use. For a typical Pennsylvania summer, the use of a heat wheel based ERV can reduce the energy needed to temper ventilation air by up to 30%. During a Pennsylvania winter, the heat wheel reduces that energy consumption by over 50%. In this era of rising energy prices and global environmental consciousness, it is easy to understand why the energy recovery ventilator is becoming standard in many new designs.
– Julie K. Good, PE, LEED AP
