Lessons Learned – Sweetwater

• “Solar Ready” Path to ZNE. While zero net energy was a goal throughout the project, financial constraints eliminated the option to install PVs in the final construction documents. The massing and layout of the buildings were still optimized for maximum solar energy potential, and thanks to a power purchase agreement with Sonoma Clean Power, approximately half of the PV panels needed to reach ZNE were installed, with additional roof area available to meet ZNE goals in the future.

• Energy Model Occupancy Defaults not Suited for Project Type. Estimating occupancy patterns and plug load energy use in the design phase is notoriously difficult, and this was no exception in the case of Sweetwater. The default occupancy and plug load values in the energy modeling program, used for typical single family residences, did not anticipate the activity of the residents and staff in the buildings. Monitoring the plug loads and lighting activity energy use patterns in the residences can provide valuable feedback for other design teams developing living facilities for autism. However, no conclusions regarding resident energy use patterns are available at this time.

• Unanticipated Energy Loads. Programmatic elements tied to the operation of the farm, such as a greenhouse and a well pump, were added during design, which contributed to energy use that went beyond what the community building’s PV system was sized to offset. Fully anticipating the expansion of the energy needs for the farm would have helped the project get closer to reaching zero net energy.

• Heat Pump Performance. As part of the PG&E ZNE Pilot Program, Sweetwater benefited from a third party energy review over the course of 12 months. One finding from this study suggested that the primary-secondary piping design for the air-to-water heat pumps was potentially unnecessary given the variable speed capability of the heat pumps, and that one heat pump would be sufficient to carry the load of each of the buildings, which could improve the efficiency of the system.               

• Inconsistent Fan Operation. Another finding from the PG&E ZNE Pilot Program assessment was that there was inconsistent fan operation throughout the 12-month study. The design called for demand-controlled ventilation using a CO2 sensor, but the results of the study showed that the fan control strategy appeared to vary throughout the year with some continuous operation and some fan coil cycling coincident with heat pump operation. Operational improvements to better control fan operation were suggested and addressed at the close of the study.