Characterization of performance

Given the magnitude of heat needed to temper ventilation air, it is essential in high-performance housing to recover as much heat as possible. The effectiveness of this heat recovery (HR) can be characterized by its coefficient of performance (COP). This is the ratio between the useful energy extracted from the system and the energy used in the extraction process. To give a fair picture, this should be calculated in terms of primary energy. For heat pumps (HPs), the COP is often presented as delivered thermal energy (or power) QHEAT divided by the required electric energy (or power) PEL:

where QSOURCE is the heat energy (or power) which is withdrawn from the heat source.

In this case, COP has to be divided by the primary energy factor PE for electricity in order to obtain COP PE depends on the prevailing mix of power plants - that is, coal, oil, gas, nuclear, hydro, etc (see Appendix 1).

The COP (HP) of the installed device depends on its technical performance and operational conditions. From basic thermodynamics it can be learned that the COP HP can never exceed Carnot's value COPcarnot and is proportional to the inverse of the temperature difference between heat supply and heat source:

Accordingly, a heat pump works best when low heat temperature supply is required and high temperature heat sources are available.

Thermal efficiency. Thermal efficiency eHR relates to the proportion of waste or lost heat (sensible and latent) usefully recovered by the HR process. It is expressed as a percentage.

Latent heat recovery is the recovery of heat released by the condensation of (usually) water vapour.

Sensible heat recovery is the recovery of waste heat from dry air.

For the sake of simplicity, all subsequent explanations are restricted to the case of sensible HR only. The thermal efficiency is then:

where TSUPP, TROOM and TAMB are the temperatures of supply air, indoor air and fresh ambient air, respectively. If there is no heat loss of the HX unit to its surroundings, another expression for eHR is:

with Texh being the air temperature at the cold end of the exhaust air tract. Equations 10.14 and 10.15 lead to:

TSUPP = eHR • TROOM + (1 - eHR) • TAMB [1°.16]

This means that TSUPP and TEXH are both weighted means of indoor air temperature TROOM and fresh ambient air temperature TAMB. The better the efficiency eHR (approaching a value of unity of 1.0 from below), the closer TSUPP comes to TROOM and the more TEXH drops down to TAMB. An almost perfect HR thermally isolates a house from the ambient environment with regard to ventilation heat losses.

Electrical efficiency. Electrical efficiency eEE is proportional to the pressure drop in a ventilation system and to the ratio between the air flow rate VR and the electric power PE that is needed to effect this flow (for the propagation of mass). Electrical efficiency is given as a percentage:

Values of this ratio eEE depend on equipment (fan quality, ductwork and control). DC-powered fans are usually much better than fans with AC supply.

The inverse of this ratio (power divided by flow rate) describes the specific fan power (SFP). The unit is Wh/m3. A survey of data for German ventilation systems indicates that for a pressure drop of about 100 Pa, the range of specific energy demand values is between 0.2 and 0.8 W-h/m3. These values are important in order to decide on the energetic and economic viability of technical solutions. In high-performance housing, the specific fan power should be 0.45 Wh/m3 or less (< 0.15 Wh/m3 for exhaust fans only).

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