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  Ventilation Efficiency
 

Introduction l

Ventilation Efficiency  l

Thermal performance of buildings  l

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Ventilation systems and heat recovery  
    Ventilation thermal energy loss in buildings is becoming increasingly important due to the more progressively installation of thermal isolation. Since the mechanical ventilation systems require an additional consumption of energy (electricity), equip such units with a heat recovery device aimed at raising the global energy efficiency is almost mandatory. Indeed, the purpose is to recover the energy from the extract air for pre-heating or pre-cooling the inlet air. Among the main causes of the heat recovery inefficiencies are leakages and parasitic shortcuts within the ventilation system and leakage on the buildings’ envelope.    
   
Thus, the global heat recovery efficiency of the ventilation system mainly depends on the internal and external recirculation in the unit, on the air infiltration and exfiltration (through the buildings’ envelope), on the specific fan power and on the efficiency of the heat recovery unit itself (often, only this nominal efficiency is taken into account). As example, the Figure shows the measured global heat efficiency in function of the efficiency of the heat recovery unit itself
Figure: Global heat recovery efficiency in function of the nominal efficiency of the heat recovery unit
 
   
    Only a positive value of the specific net energy saving (SNES) per cubic meter of supplied air indicates a net gain in thermal energy, this figure is calculated by:


  where:
  • is the ventilation heat loss, based on average internal and external temperature during the heating season;
  • fr is the part of the fan power recovered as heat in the supply air. This factor f ris close to one for supply fans and zero for exhaust fans;
  • fp is a production factor, accounting for the fact that the production of 1 kWh of electrical energy requires much more primary energy;
  • electric power used by the fan to blow air
  • h G is the global efficiency of the heat recovery system
Measurements carried out in thirteen ventilation systems at fourteen buildings in Switzerland (by using the tracer gas dilution method), revealed that thermal energetic savings in several cases were small or even negative. Major leakages and internal recirculation have been observed as well as losses of heat-bearing air leaving the building through the envelope instead of passing the respective heat recovery unit. In the case of small ventilation units, thermal energy savings up to 830 kWh (during an entire season) were calculated even assuming the best technical performances (airtight building, 90% of efficiency in the heat recovery unit itself and a specific fan power of 0.25 Wh/m 3). From an economic and energetic point of view, such ventilation units are disadvantageous and hard to recommend.

Certainly, not only the technical design specifications of the heat recovery units should be improved, but also the isolation and airtightness techniques in buildings. As well, the architectural design of buildings should allow the best performance of the ventilation and heat recovery systems whenever possible.

   
   
Diagnosis of Air Handling Units (DAHU)
 
   
    DAHU is a powerful methodology that guides the diagnosis of Air Handling Units (AHU) in buildings in order to detect deficiencies causing energy consumption excesses and excessive financial expenses and user comfort reduction. Such methodology was developed within the framework of the European project AIRLESS and is based on fifteen years of experience in the field in Switzerland and at international level. Basically, DAHU follow three major tasks: measures planning, execution of measures and interpretation of the results. Measures are performed by using the tracer gas dilution method which allows determining airflows, ventilation efficiencies, AHU efficiencies and heat recovery systems efficiencies. Such method consists in injecting a tracer gas at carefully chosen locations in the AHU and subsequently measuring its concentration to determine all the wanted airflow rates.

For further information contact Flavio Foradini (email: firstname.lastname@e4tech.com) from E4tech

References:

C.-A.Roulet, F. Foradini, and P.Cretton. Use of tracer gas for diagnostic of ventilation systems. in Healthy Buildings'. . Budapest.

C.-A. Roulet, F. Foradini, and L. Deschamps. Measurement of Air Flow Rates and Ventilation Efficiency in Air Handling Units. in Indoor Air. . Edinburgh.

C.-A. Roulet, F. D. Heidt, F. Foradini, M.-C. Pibiri. Real Heat Recovery with Air Handling Units. CISBAT . Lausanne ( Switzerland).

   
         
         
 
 
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