Civil Engineering Reference
In-Depth Information
With:
q
HR
Total annual heat recovered per m
3
/h (Jh/m
3
)
Total ventilation heat loss (Jh/m
3
)
q
V
e
Effectiveness of HRU (-)
HDD
Number of heating degree days (HDD) (Kday)
HDD data for the whole EU are freely available from Eurostat (
2010
). In
accordance with the Eurostat definition of HDD (Eurostat), which assumes a
heating threshold of 15 C and an indoor temperature of 18 C, the number of
HDD for any given day is defined as 18 C minus the daily mean temperature,
whenever that daily mean temperature is below 15 C. The daily mean tempera-
ture is defined as the mathematical average of the minimum and maximum tem-
perature of that day. Based on this definition, the number of HDD for the EU
averaged over a 10-year period from 2000 to 2009 is 3,000.
The ventilation systems can be assumed to run at a constant rate, all year long.
This is a valid assumption since, although occupants tend to open windows during
summer (see above), thus increasing the total airflow rate, the system is rarely shut
down. The ability to shut the system down is even forbidden in some ventilation
standards (BIN
1991
). The (increase in) electric load for fan operation in the heat
recovery ventilation system is highly dependent on fan and ducting characteristics.
In addition, fan power will typically increase with higher HRU effectiveness.
Nevertheless, the use of specific fan power (SFP) allows for a straightforward
approach to it. The European standard EN 13779 (CEN
2007
) specifies SFP 3-4,
750-2,000 J/m
3
per fan, as default values for heat recovery systems.
The balance between the total heat recovered and the fan power difference is,
beside by system characteristics, strongly affected by climate and by the conver-
sion factor used to compare electricity consumption to fossil fuel consumption.
Primary energy is a widely accepted framework for this conversion. To calculate
the primary energy factors, the 2008 country energy balances reported by the
international energy agency (IEA
2008a
,
b
) were used. The primary energy factor
was calculated by dividing the sum of the primary energy input of electricity plants
and CHP plants by the net produced electricity. Since we assume the ventilation
systems considered to permanently run at a fixed rate all year long, average factors
were preferred to peak load factors. Due to the constraints of the first law of
thermodynamics, this factor cannot be inferior to 1. The availability of the data
limits the resolution of the calculated factors to country scale. Since, in spite of
efforts of the EU to integrate its electricity market, electricity production is still
mostly organized on a national scale, this is also the most logical scale within the
primary energy framework.
Based on the calculated total heat recovered annually by the heat recovery unit,
associated fan power difference and conversion factors, the equivalent HRU can be
calculated by:
=
f
g
q
V
e
eq
¼
e
f
g
q
V
DSFP
f
e
24
365
ð
3
Þ
