Page 80 - Zero Net Energy Case Study Buildings-Volume 1

CASE STUDY NO. 4
WATSONVILLE WATER RESOURCES CENTER
Heating, Cooling and Ventilation
The preferred design approach for heating, cooling and ventilation for a building of this type is to minimize fan energy use normally associated with central plant systems. The design engineers also took the opportunity to capitalize on a steady source of cooled water available at the adja- cent water recycling plant.
The original concept therefore was to use the basic idea of a water-source heat pump for heating and cooling, with delivery of heating hot water or chilled water to a radiant slab system in both wings of the building. Ventilation in the office wing is provided separately by occupant-operated natural ventilation, backed up by mechanical ventilation via distributed air handling units that operate only if CO2 limits are exceeded. Known as dedicated outside air systems (DOAS), they utilize only small local fans for the backup ventilation, satisfying the objective of minimizing this type of electric energy use.
When the system was priced in mid-design, the cost was determined to be too high for heat pump equipment of the size necessary to meet both heating and cooling loads. For this reason, and because the client had not yet embraced ZNE as a confirmed project goal, a more conven- tional approach for radiant heating, namely a gas-fired condensing boiler, was incorporated into the design, while a much smaller heat pump is designated for the cooling operation. All of the advantages of the low-energy design approach were maintained, but the idea of a solution that did not utilize carbon-based fossil fuel was no longer viable.
This also would mean that a zero-net-energy measurement and verification would have to ac- count for the equivalent energy use by the gas-fired system. For the building to be designated as ZNE, measurement of total annual energy export from the site and the utility’s meter measure- ment of total annual energy import would have to show enough excess energy export to balance the equivalent energy used in the form of gas. In the absence of any balancing excess exported electric energy, the building would be only zero-net-electric-energy. This issue is discussed in detail later in this case study.
As noted, the fresh-air ventilation system is separate from the heating and cooling system. The ventilation system starts with occupant-operated openable windows when fresh air or cooling are desired. The interspersing of open office areas with closed offices optimizes the cross ven- tilation patterns for comfortable air movement. To increase the temperature range for perceived comfortable conditions, the office spaces have large, slow-moving room fans to increase gentle air movement.
In addition, for warm days when interior ventilation and air movement needs to be increased, air can be drawn up in the high space to ventilation “chimneys” visible on the roof, which function as outlets for the passive ventilation air draft.
The distributed air handling units located in attic space below these chimneys in the office wing provide mechanical ventilation when natural ventilation via the openable windows and other pas-
Diagram courtesy of WRNS Studio
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Zero Net Energy Case Study Buildings: Volume 1