Page 99 - Zero Net Energy Case Study Buildings-Volume 2

WEST BERKELEY BRANCH LIBRARY
CASE STUDY NO. 10
Natural Ventilation
The objective of the passive design approach for ventilation and cooling is to use wind pressure and the bouyancy of air to drive air movement through the building instead of using electricity for fans. Normally, cross ventilation via openable windows is an easy mechanism to design into buildings; when combined with good building control systems and a programmed sequence of operations, this can be done automatically and optimally for efficient heating and cooling. In this case study building, however, the high noise levels on the street side of the project site, com- bined with the code restrictions on openings in the zero-setback sides of the building, required a different approach.
The commercial district zoning ordinance required a two-story façade at the street, but this pro- vided an opportunity to use a chimney effect to draw air through the building. The prevailing wind off of the nearby San Francisco Bay creates negative pressure at the back of the false-front structure of the façade, where several air ducts are embedded. These vertical ducts act essen- tially as wind chimneys that naturally draw air through the spaces from the north side due to the pressure difference, where automatically controlled operable windows act as the air inlets. (See diagram above.)
The amount of air moving through the spaces is determined by exterior temperature, interior CO2 levels and the heating or cooling mode of operation in effect. Wind characteristics determine the need for the operation of low-power backup fans. (See the description of the Mixed Mode of Op- eration on the following page.) The building uses this natural ventilation system at all times, mini- mizing outdoor air flow to that required by code for fresh air during heating and cooling modes and maximizing the air flow when outdoor air temperatures can be used directly for cooling.
Particularly in mid-winter when the building is in heating mode, the injection of fresh air into the occupied space could create uncomfortable drafts. For this reason, the automatically controlled operable windows are equipped with hydronic convection heating grilles7 on the interior. This proven system heats the incoming air to room temperature within inches of the window, thereby avoiding any cold draft effects. It is this feature that allows passive ventilation to be used year- round for fresh air.
7 Runtal hydronic column radiators, http://www.runtalnorthamerica.com/residential_radiators/ column-r-radiators.html.
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