What really affects the energy efficiency of a ventilation system? Does energy consumption depend solely on the efficiency of heat recovery?
When selecting a ventilation system, calculations are made for design conditions, i.e. the most unfavorable ones that may occur. The ventilation system is designed only for winter conditions, while the air conditioning system is also designed for summer conditions. For example, these may be parameters for winter: te=-20°C and RH=100%, while for summer: te=+30°C and RH=45%. To calculate the energy consumption of the ventilation system, such selections must be made precisely for all 8,760 hours in a year! Doing this task manually is practically impossible, which is why alternative models have been created based on degree-hours of heating, cooling, humidification, and dehumidification. These models certainly contain many simplifications, do not allow for creating schedules for the operation of the system, and sound rather intimidating :)
Fortunately, we have computers that love to perform calculations in loops. In the IX CHART program, there is a tool that allows such calculations to be performed for 60 cities in Poland based on climatic data from several years. This tool is called the Energy Consumption Calculator (K.Z.E.).
But we haven't yet answered the fundamental question - what does the energy efficiency of a ventilation system depend on? The obvious answer is that it depends on the efficiency of the heat recovery exchanger - which is true with a small "but". This "but" concerns the hydraulic resistance of the heat recovery exchanger. If an increase in the efficiency of the exchanger causes an increase in resistance, we need to carefully examine the final effect of our work, because it may turn out that although we use less heating or cooling energy, we will pay more for the energy to drive the fan. An increase in resistance of the heat recovery exchanger can occur, for example, by increasing the density of the fins or its length. To prove this, let's do calculations for two scenarios. The general assumptions will be as follows:
Climatic data for the city of Katowice.
Amount of fresh air: 2,000 m3/h.
End of heating season temperature: +12°C.
Start of cooling temperature: +20°C.
Efficiency of supply and exhaust fan: 65%.
Water heater - heat produced from gas - price of 1 kWh of energy: PLN 0.21.
Freon cooler - cold produced from electric energy with an ESER efficiency of 2.6 - price of 1 kWh of refrigeration energy: PLN 0.23.
The price of electric energy to drive the fans is PLN 0.6 per 1 kWh.
The system operates in a 24/7, 365-day schedule.
Result of the calculations: Heating energy consumption Eg=12925.9 kWh Cooling energy consumption Ech=6538.9 kWh Fan energy consumption Ew=12728.2 kWh
Total annual energy cost: 11855.3 PLN
Variant 2: We increase the efficiency of the heat recovery exchanger by increasing the plate density, which causes the efficiency to increase by 5% and the hydraulic resistance of the supply and exhaust fan to increase by 100Pa. Heat recovery exchanger efficiency of 85% in winter and 75% in summer. Total resistances of the air handling unit - supply fan 1100Pa, exhaust fan 800Pa.
Calculation result: Heating energy consumption Eg=9694.5 kWh (a 5% increase in efficiency resulted in a decrease of 3231 kWh, which represents a 25% reduction in annual heating energy demand!) Cooling energy consumption Ech=6520.4 kWh (a 5% increase in efficiency resulted in a decrease of only 18.5 kWh, which represents only 0.3% of the annual cooling energy demand) Fan energy consumption Ew=14225.6 kWh (a 100 Pa increase in hydraulic resistance caused an 11.8% increase in fan energy consumption, which is 1497 kWh)
It may seem that it is worth doing, but the price of electricity is three times higher!
Total annual energy cost is 12070.9 zł
Conclusion? Despite a significant reduction in heating energy consumption (-25%), a relatively small increase in efficiency (+5%), it is not economically viable to do so. Therefore, it is important to be able to quickly calculate air handling units taking into account all parameters.
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