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Energy recovery is an option for organizations using a direct exchange system that's increasingly being written into building codes and ASHRAE standards. There are two basic types: airside and waterside.
According to Ron Holdaway, an engineer at Smith Seckman Reid, airside energy recovery is the process of using the building's exhaust air to pre-condition the ventilation air for the building. The exhaust air passes through an energy recovery device such as a rotating heat wheel, a fixed plate heat exchanger, or a run-around coil. The energy from the exhaust air stream is transferred to or from the outside airside. In the summer, the temperature of the warmer, humid outside air is reduced before entering the air conditioning system, which reduces air conditioning system tonnage. In the winter, the cold outside air is warmed before heaters heat the air to maintain the space temperature.
A water-cooled system relies on chillers to cool the water, which is then distributed to air-handling units to condition the space. The heat from the air-handler coils is returned to the chiller where the head is returned to the air via fans or a cooling tower. Holdaway notes that waterside energy recovery systems capture some of the heat from the building before it is rejected. This recovered energy is often used to temper the building's heating hot water.
"Some form of water-side heat recovery is being used in most large buildings today," Holdaway says. "Best design practice today is to use dedicated heat recovery chillers to recover heat from building's return chilled water and produce heating and domestic hot water."
The results of such a set-up yield much higher efficiencies. If a typical high-efficiency gas boiler is 90 percent, efficiency, a dedicated heat recovery chiller is 300 percent efficient or greater to produce the same hot water, Holdaway notes.