1.7.4 Energy Recovery
Recover energy that would otherwise leave the building as waste, via energy transfer.
The most common heat recovery ventilation devices are flat plate air-to-air heat/energy exchangers, rotary heat and energy (enthalpy) wheels, and heat pipes. Heat pumps are an especially suitable method to remove heat from a heat source (air, water, process fluid, etc) and deliver it to another point or area of need. Hot gas heat exchangers installed in the hot gas line (between compressor and condenser) recovers heat from refrigeration systems. Hot drain heat exchangers are used to recover heat from drain water.
Thermal energy from air-to-air heat exchangers or general energy recovery systems is transferred from one fluid to another fluid through an impermeable wall. The fluids (air and/or water) do not mix. An informed evaluation can often reveal applications for harnessing waste heat using heat exchangers that are unique to a particular project. Energy recovery systems for commercial and institutional buildings are often complex, and require intense input from consulting engineers and coordination.
The additional capital costs include installation labor, additional ductwork or piping, filters, controls or energy management systems (EMS), and new pumps or fans. Capital savings come from lower load requirements, which allow smaller and less expensive HVAC systems. Operating savings come from lower utility bills.
For buildings that have high hot water requirements, such as hospitals, housing, restaurants or hotels, using recovered heat to heat or preheat water is very cost-effective. Hot drain heat exchangers are used to recover heat from drain water, preheating supply water before it goes into the hot water tank. Buildings with server rooms or other areas with high equipment loads can take advantage of excess heat from those rooms.
The size of heat exchanging elements is a function of the capacity and efficiency of the equipment. It is often large.