Analysis of energy savings by free cooling in different european climates

In a wide range of conditions heat recovery in mechanical ventilation systems can give, when a convenient air flow control system is set up, an important contribution to cooling and humidity indoor comfort with low energy supply. Sometimes free cooling is useful to face up not only ventilation load but cooling load as well, taking advantage, directly or indirectly, of enthalpy difference between internal and external air during summer season. The paper reports on a summer analysis in different European climates of the energetic advantage of direct and indirect evaporative cooling techniques, in which external air flow is established by outdoor, indoor and inlet air enthalpies comparison. Psychrometric diagram has been divided into seven zones in function of external air conditions; for each zone external air flow control logic and obtainable free cooling effect have been developed and evaluated. As a function of indoor conditions, outdoor conditions (expressed hourly by Test Reference Year), inlet temperature, external ventilation air flow and building cooling load, energy savings with free cooling techniques have been calculated, considering water consumptions in the adiabatic saturators. Interaction of free cooling, sensible heat recovery and indirect evaporative cooling allows very interesting seasonal cooling load reductions, with the additional advantage to decrease the chiller cooling capacity. All the energetic advantages have been evaluated by an economic point of view, in terms of Net Present Worth and Discounted Payback Period of the investment.