Evaporative Cooling of Buildings: Improving Energy Efficiency

Evaporative cooling of buildings can significantly improve energy efficiency

Roof ponds can be an an effective way to improve energy efficiency on buildings in Rio de Janeiro. Photo by i-sustain, licensed under creative commons.

Water on the surface of a building has a tendency to evaporate. For every gram of water that evaporates, roughly 2500 J of heat energy is consumed. Wetting a building therefore helps to remove heat – in a process that is analogous to human sweating [1].

Historically, water has been used in the form of fountains and cascades to improve the thermal comfort of buildings.

Evaporation of water helps to passively cool buildings, reducing the energy needed for air conditioning. When combined with other passive design techniques, adequate thermal comfort might be achieved without air conditioning.

Indirect Evaporative Cooling

When evaporative processes are used to cool an element of the building, which then acts as a heat sink, this is known as indirect evaporative cooling.

Cooling is provided whilst keeping the evaporative process outside, which avoids elevating the indoor humidity level. Direct evaporative cooling cools outside air through evaporation, and brings this air into the building. The drawback of this method is that indoor humidity levels are increased. This is not appropriate for Rio de Janeiro, so direct evaporative cooling techniques should be discounted.

Methods of evaporative cooling include roof pond systems (which can also lose heat by radiation and convection) and water spraying.

Roof Ponds

In the simplest form, this involves placing a pond on the building roof. As water evaporates from the pond, heat is consumed. This cools the roof, which acts as a heat sink and absorbs heat from the interior of the building.

A more refined method involves insulating the pond during the daytime, to prevent solar gain. During the day water in the pond absorbs heat, cooling the ceiling below. At night, water is circulated over the insulation. Heat is then removed by means of evaporation, convection and radiation (to the black body night sky). Such systems have been successfully trialled in the hot, humid climate of Mexico, where cooling of 10-13 deg C below outside air has been reported [1].

Obviously, roof ponds require additional structural support and there may be public health concerns associated with standing water (mosquitoes etc.).

Water Spraying

Water spraying is a simple and inexpensive way of reducing the solar heat gain of buildings in the tropics [2]. Water is typically sprayed onto roofs for 40 seconds every five minutes. A small amount of external power is required to pump the water up to the roof, but the energy required for this is minimal compared with the additional cooling that is attained [3]. While roof spraying has some potential for cooling in hot humid climates, the effect is relatively small – a reduction of indoor air temperature of 1-4 deg C has been reported. Studies show that roof spraying is a less effective cooling method under cloudy conditions [1], [4].

Roof ponds, and to a lesser extent roof spraying, should be considered for buildings in Rio de Janeiro.

References

  1. T. Chenvidyakarn, “Review Article : Passive Design for Thermal Comfort in Hot Humid Climates,” Journal of Architectural/Planning Research and Studies Volume 5. Issue 1., 2007. [Online]. Available: http://www.ap.tu.ac.th/jars/download/jars/v5-1/01 Review Article.pdf.
  2. S. Chungloo and B. Limmeechokchai, “Application of passive cooling systems in the hot and humid climate: The case study of solar chimney and wetted roof in Thailand,” Building and Environment, Sep-2007. [Online]. Available: http://gse.cat.org.uk/downloads/passive_cooling.pdf. [Accessed: 23-Oct-2012].
  3. Arizona Solar Center, “Passive Solar Heating & Cooling Manual, Part 3 of 4.” [Online]. Available: http://www.azsolarcenter.org/tech-science/solar-architecture/passive-solar-design-manual/passive-solar-design-manual-cooling.html. [Accessed: 23-Oct-2012].
  4. W. Wongsuwan, T. Fongsamootre, and M. O. T. Cole, “Experimental Studies on the Roof Pond House under Tropical Climatic,” 2006. [Online]. Available: http://www.en.kku.ac.th/enjournal/th/images/stories/files/published/33No-5.pdf.