Ventilation: Thermal Efficiency and Health Aspects - Nearly Zero Energy Building Refurbishment - page 387

4.4 Ventilation Heat Recovery

Fan-supported systems, on the one hand, allow for the implementation a range of
heat recovery technologies, while on the other hand they require fan energy to
operate. The trade-off between both, taking into account some conversion between
heat loss and electrical power, frames the effectiveness of the heat recovery
technology in question. The amount of heat loss that can be recovered is, next to
the performance of the specific technology, determined by the climate and building
energy performance (Zhou et al. 2007 ; Juodis 2006 ; El Fouih et al. 2012 ).
Although some progress is made on the development of heat recovery systems for
systems relying on natural driving forces, their performance and practical feasi-
bility are still under debate (Hviid and Svendsen).
The two most widespread heat recovery technologies available are air-to-air
heat exchangers (Roulet et al. 2001 ; Fernandez-Seara et al. 2011 ; Lazzarin and
Gasparella 1998 ) and exhaust air heat pumps (EAHP) (Fehrm et al. 2002 ; Fra-
castoro and Serraino 2010 ; Sakellari and Lundqvist 2005 ; Wallman et al. 1987 ).
The thermal effectiveness of commercially available air-to-air heat exchangers
reaches up to 80 % (CEN 1997 ; WTCB 2012 ). Some of these systems include
enthalpy exchange (Nasif et al. 2010 ; Hemingson et al. 2011 ). Due to the small
diameters within these heat exchangers, filtering is usually necessary to prevent
excessive fouling and the associated loss in performance (Markowski et al. 2013 ).
The assessment of heat recovery ventilation can be made only taking into
account the intended ventilation (Laverge and Janssens 2012 ). The total heat
recovered annually by a heat recovery unit (HRU) can be calculated from the heat
content of the ventilation air, using the heating degree day data (ISO 2007 ; Day
2006 ).

Q HR ¼ Z
a

q c e g ðÞ DT ðÞ dt ð 1 Þ

With:
Q HR

Total annual heat recovered (J)

Density (kg/m 3 )

q

c Specific heat capacity (j/kgK)

e Effectiveness of HRU (-)

Flow rate (m 3 /s) g

t Time (s)

DT Indoor/outdoor temperature difference (K)

Assuming density, specific heat capacity and effectiveness to be constant over
the whole heating season and assuming 1,224 J/m 3 K to be the volumetric heat
capacity of air, normalized per m 3 /h, this can be transformed to:

q HR ¼ 24 1224 e HDD ¼ e 29376 HDD ¼ e q V ð 2 Þ