As passive house residential buildings and non-residential buildings are almost airtight, it´s very important to exchange the “used” indoor air by fresh outdoor air to keep the hygienic standards high. With the right technology a cheap and temperature neutral air conditioning is guaranteed.
Non-residential buildings like banks, schools, industry buildings etc. set even more requirements to the air conditioning as in those buildings the heat loads are produced by the equipment in the building like machines, computers and other working tools and don’t depend on the outdoor climate.
Technologies like free cooling are able to cool the room during the night and in the morning without additional cooling energy needs. The unit is equipped with EC-fans with an active air flow control which keeps the pressure conditions constant in all possible operational modes and conditions of the filters.
In public passive house buildings the VDI guidelines specify not only the optimal air amount balance and hereby the energy criteria but they also attach big importance to hygiene standards. These standards require special designs of the cooling systems which make it possible at all times to clean or even disinfect all components of the unit.
As a central AHU is installed in the technical room of the building, the architectural design of the room has to take the size of the unit into account. The system always has to be accessible for hygienic maintenance works.
Solutions of the defrosting system
The heating system is supposed to cover the heating needs of the building and simultaneously have enough reserve for the defrosting of the heat recovery units. There are two strategies for the defrosting process. One is preheating the outdoor air at temperatures lower than -5°C with the help of a heating coil on the outdoor air duct, which at the same time leads to overall pressure losses and therefore to higher energy loads of the supply air fan. This defrosting strategy usually uses a water-defrosting protection mixture which has a higher viscosity than water which leads to water pressure losses of approx. 40% which at the end requires more power consumption.
Another technology is smarter as the defrosting is depending on its demand. Sensors in extract air define the dew point of extract air. At outdoor air temperatures of 0°C and below the average temperature difference between outdoor air and exhaust air is defined and if the difference is below 0°C and the dew point temperature is above the average temperature, the system is defrosting automatically. In this case the extract air-exhaust air bypass is opened so that the warm extract air is heating up half of the heat recovery system until the exhaust air reaches temperatures beyond 0°C, afterwards the bypass dampers are closed again. This technology has got 2 energy advantages:
- during defrosting most of the heat recovery system is still running,
- demand-oriented defrost function.
Another thing that seriously affects the efficiency of the unit besides the season is the geographical location of it as the devices are working a lot with outdoor air. A risk of frosting occurs at temperatures below -5°C. Both defrosting technologies sum up to the following power demands:
Winter mode with preheating of the outdoor air:
FFM 3,603 kWh
Munich 11,363 kWh
Winter mode with demand-oriented defrosting:
FFM 2,142 kWh
Munich 5,720 kWh
Master inner loads
To solve the problem of not reaching needed cooling temperatures in offices or other big buildings, The Systemair company Menerga uses the adiabatic cooling strategies which is a big privilege of the recuperator. The cooling with water is especially helpful when it is about coping with high thermal loads produced by the internal equipment of buildings and to reach lower temperatures than would have been possible with normal cooling. The special design of the recuperator allows to cool down the air to a wished temperature by evaporating the water inside of the heat recovery system. The extract air is evaporated and cooled down to the wet bulb temperature. The warmth which is needed to evaporate the water is taken from the outdoor air and due to the special construction of the heat recovery system, the supply air temperature is cooled down. (h,x-diagrams are available).
Extract air = 26°C with 50% r.h. at 10,000 m³/h results in a cooling capacity of 45 kW. The room is cooled with the temperature difference between extract air and supply air which relates to a sensitive cooling capacity of 20,4 kW with which internal thermal loads can be led out of the building.
Menerga produces counterflow recuperators which are diverse in their use. Those are big units made of the microbiological material “polypropylene” which are certified for especially high air performances and work very energy efficient. The clever construction principle makes an access to the inside of the device for hygiene cleaning maintenance works possible.