Page 64 - Zero Net Energy Case Study Buildings-Volume 3

CASE STUDY NO. 13
LA ESCUELITA EDUCATION CENTER
(Facing page) Diagrams of three modes of operation of the classroom system. (Courtesy SVA Architects)
done to estimate the effectiveness of the design, both in the parameters of the air flow and in the comfort conditions for the occupants. Computer models for this system were not available that would be reliably predictive as the basis of the design.
The second innovative system was a displacement ventilation system that was used in the class- rooms to provide fresh air, heating and cooling with a minimum use of fan energy, utilizing air properties in a similar way to the Great Room in the individual classroom spaces. In this case, fresh air is delivered to the classroom by a local fan unit (essentially a “dedicated outside air sys- tem” or DOAS unit that provides 100% outside air). Like the air supply in the Great Room, natural air properties are used in conjunction with a large diameter room fan to create thermal comfort conditions and appropriate air movement through the space. Heating and cooling are accom- plished through three modes of operation. (Refer to the operation diagrams on the next page.)
• Heating Mode (Cold Season)
The outside air from the DOAS unit is heated via hot water coils in the parallel fan-pow- ered VAV boxes with blended return air. The hot water is piped from the gas-fired boilers. When the air enters the classroom near the floor, the air naturally rises toward the ceiling. The large, slowly-moving room fan (approximately 20% of full speed) mixes the warm air in the room. The DOAS air is exhausted locally through the relief air louvers as newly warmed outside air continues to enter the room.
• Cooling Mode (Swing Season)
When space cooling is needed in the classroom and outside air is cool enough, such as during the swing seasons of fall and spring, the DOAS unit simply supplies 100% cool outside air. The air rises due to natural buoyancy and the heat of the occupants via the process known as “displacement ventilation”. The large room fan remains off during this mode of operation to allow displacement ventilation to occur naturally. (For the same rea- son, if an occupant opens a window for natural ventilation, the large room fan automati- cally shuts off.)
• Cooling Mode (Warm Season)
The DOAS unit supplies 100% outside air, which is at warm temperatures that normally require mechanical space cooling. However, in the marine climate of Oakland, adequate thermal comfort conditions can be provided by the large room fan operating at a fairly high speed.
Some user “training” is required since the operation of the large room fan can be counter-in- tuitive: fan on (though low-speed) during the winter and fan off when bringing in outside air for cooling during the swing seasons.
The Administration space has a system similar to the classroom spaces, except that the large room fans are manually controlled and there is no displacement ventilation (the outside air is supplied at the ceiling level).
In addition to these modes of operation during occupied hours, the system also operates in “Swing Season” mode (DOAS unit on, large room fan off) at night during the entire cooling sea- son to pre-cool the building in advance of the cooling demand on the following day. This night ventilation operating strategy is an integral part of the cooling system, with the added thermal mass in the walls as discussed above in the section, Building Envelope. Sensors located in the concrete mass of the floors permit good control of the amount of night ventilation so that the building is not over-cooled by morning, requiring heating in the early hours for occupant comfort.
Heating is provided to the Great Room by a separate air handling unit with economizer for mini- mum outdoor air control that is interlocked with the Cool Tower so that the latter is closed when the heating air handling unit is operating.
The heating, cooling and ventilation systems for all spaces are designed with the large room- sized fans as an integral component of each. The design team worked with the manufacturer of the large fans to develop fan designs that were larger and delivered more uniform air movement. They built a full-scale mockup in a classroom-sized space to optimize the design, changing the dimension of the fan blade, its curvature and height above the floor, until the air motion in any direction was essentially uniform. The optimum location of the fan was found to be ten feet above the floor and two feet below the ceiling. These dimensions also proved to be good vertical dis- tances for the desired daylight penetration.
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Zero Net Energy Case Study Buildings: Volume 3