Evaporative Cooling: Requirements

Introduction | Status Quo | Alternatives | Requirements | Load Minimisation | Conclusions | Bibliography

Evaporative cooling has two basic requirements, water (for evaporation) and airflow (for ventilation). Water consumption is directly proportional to the amount of cooling generated, and so remains constant regardless of whether the ventilation component is driven by active or passive means, as the latent heat of vapourisation of water is constant. As Perlmutter et al (1996) determined, some efficiency could be gained by improvements in the manner in which water is introduced into the air stream, but ultimately the only manner in which to significantly reduce water consumption is to reduce the cooling load as discussed in the following section (Load Minimisation).

Scope for improving the efficiency of evaporative coolers themselves therefore appears to lie mainly in the minimisation or elimination of the electrical energy consumed by the generation of airflow and pumping/spraying water. Energy for the pumping/spraying component could easily be provided by a 12 volt DC photovoltaic system, a relatively straightforward modification that does not require further elaboration here.

Assuming a cross-sectional area of 4m², a cooling tower would require an average airflow velocity of 0.52 m/s to achieve 20ACH over 375m³ (150m² x 2.5m), producing a ventilation rate of 2.08m3/s. Pearlmutter et al (1996) determined that, for practical purposes, the strength of natural drafts created in cooling towers is "negligible", but that with the assistance of wind-catchers (in their specific location) airspeeds of up to 3.75m/s are achievable. Elliot (2007) claimed a ventilation rate 1.17 to 3.77 m³/s (4200 to 13,570 m³/hr) using both a wind-catcher and wind-escape.

Air exit devices such as wind-escapes and solar chimneys can clearly enhance natural ventilation by creating negative pressure with the building envelope. In cases where wind velocity and/or solar irradiance were temporarily insufficient to induce adequate ventilation rates, an evaporative cooling system could be temporarily assisted by an electric fan, creating a "hybrid" system which may be primarily passively ventilated but can optionally draw on a PV-powered active ventilation as required.

Thus, the primary technical challenge with respect to passive evaporative cooling systems does not appear to be in finding the means to generate ventilation, but rather in developing the means to manage changing wind and solar conditions and to smooth out their volatility.

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