Air to air heat recovery: assessment of temperature efficiency

For the assessment of air to air heat recovery systems in ventilation systems, efficiency seems to be the major evaluation parameter, although other parameters such as pressure drop, frost behavior and power consumption of fans can play an important role. Efficiency of a heat exchanger can be expressed in many forms; temperature efficiency, moisture efficiency, enthalpy efficiency, primary energy rate, etc. When speaking of temperature efficiency, an important difference should be made between the efficiencies at the supply and exhaust sides respectively. Supply side efficiency will increase with increasing supply temperature due to inefficient fans and/or heat gains through a badly insulated casing. In case of these parasitic energy flows one cant speak of real energy recovery; the supply efficiency can be used to evaluate comfort at the air supply openings. Exhaust side efficiency accounts for these parasitic energy flows more correctly and is a better expression to evaluate energy performance of the AHU with heat recovery. Worldwide, many different efficiency measuring methods exist, according to various standards. The conditions at which to measure efficiency vary; flow moisture temperature conditions, acceptable leaks, etc; all leading to hardly comparable results. There is a strong need for more uniform testing conditions, as well as a clear understanding of what efficiency means and how to use it in system evaluation. Apart from instantaneous performance, some standards also include a seasonal system performance, including effects of varying flow rates en temperatures, moisture conditions and defrosting. This approach will help to assess the global energy performance of the system, as a part of a real building.