VENTILATION HEAT RECOVERY WITH COOLING
Wind cowl Silencer
Heat exchanger Heat is recovered from exhaust air
Circulating fluid between heat exchanger and heat pump
Preheating or precooling of intake air
Room heating Hot tap water
Figure 8: A representation of the NVHRC demonstration project at a sports hall in Denmark
H HST CST
HP
extraction with a heat pump on the air discharge. The net energy consumption has been
calculated in line with the Danish building regulations, where electricity is weighted as being 2.5 times as expensive as heat. In all four building types, NVHRC had the lowest net energy consumption (see Figures 4 to 7). These examples show that projected energy savings of up to 50% appear to be possible with NVHRC. The key reasons for
this are: l NVHRC uses mainly natural driving forces for ventilation. As previously mentioned, an axial fan is used for backup ventilation on days with low natural driving pressure, but it uses very little energy: an estimated 0.03 to 0.07 kJ/
cubic metre when in use. With balanced mechanical ventilation, the usage is estimated at 1.8 to 2.1 kJ/cubic metre.
l The operational coefficient of performance (COP) for the heat pump has been calculated at an impressive 4 to 5.
l The heat from the outlet air is also recovered in summer time (in Denmark), mainly for heating domestic water. An electricity price of 0.2 euro (£0.16)
per kWh and a price of heat at 0.07 euro (£0.06) per kWh have been used in the calculations. The full results of the research project are available at
www.nvvk.dk, (available only in Danish). As the above shows, the concept of heat
Demonstration project shows benefits of NVHRC
A demonstration plant has been set up in a sports hall in Fynshav, Denmark. Figure 7 shows the plant running in heat-recovery mode, with air going via intake units in the facade. The units are capable of both preheating and precooling the inlet air. Wind cowls placed on the
rooftop draw air out of the building. A heat exchanger placed just before the wind cowl cools down the exhaust air, and a circulating fluid
48 CIBSE Journal July 2011
transfers the extracted heat to the heat pump in the basement. From here the heat is distributed to the heating system. The circulating cold liquid
can also be re-routed to a cold water storage tank, which can be cooled down to 10C. The cold water from the tank can be used for cooling the intake air. The ventilation capacity of
the plant is up to 8,000 cu m/h. The heat pump has a maximum of 25 kW thermal.
The air handling unit on the demonstration plant
recovery from discharge air in mechanical ventilation systems is not a new one; however, this solution provides the benefit of recovering the heat that would otherwise be lost through natural ventilation. Using low pressure loss heat exchangers
in the natural ventilation discharge, in combination with heat pumps, the system allows the building to maintain a more effective energy balance and shows potential to reduce its energy costs and lifetime operational carbon footprint.
l Morten Stender ChriStenSen is formerly of Esbensen Consulting Engineers,
www.esbensen.dk For more information, please contact Tommi Haferbier Nielsen at
thn@esbensen.dk This article first appeared in REHVA Journal,
www.rehvajournal.com
www.cibsejournal.com
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