Page 83 - Zero Net Energy Case Study Buildings-Volume 1

WATSONVILLE WATER RESOURCES CENTER
Renewable On-Site Energy Supply: Solar Photovoltaic System
Based on initial energy modeling for the building’s predicted energy demand, the design team also determined the required size and specifications for the on-site renewable energy supply needed to achieve ZNE performance over the course of a year. A 70 kW (DC) system located on the south-facing, shallow-sloped roof of the office wing would meet the expected peak design load in summer and produce enough electric energy to ensure that the building achieved ZNE annual performance.
Because of the construction budget limitations, the client opted to seek an additional funding mechanism for the purchase and installation of this solar photovoltaic system. This separate process of funding and constructing the system required two years to complete, primarily for cost efficiency reasons. The City of Watsonville determined that the competitive bid process for the system would be optimized if the overall installation actually were larger than just the system for the Water Resources Center building. The City therefore bundled together a number of other facilities and locations as part of the separate system contract, including some of the plant load at the adjacent Water Operations Center. For this reason, it has become difficult to isolate the performance of the Watsonville WRC at the same resolution as the previous case studies in this monograph.
As a result, the solar photovoltaic system at the site today consists of a 96 kW (DC) array on the roof of the Water Resources Center office wing and a 254 kW (DC) array located on the ground along the northern border of the overall site. This combined system provides solar electric energy to both the Water Resources Center building and a 350 hp water recycling pump at the adjacent plant. Because there is only one utility meter for these two loads and the combined ground and roof solar arrays, it is not possible to distinguish between the energy supplied to the building and that supplied to the large pump. In addition, the energy use of the pump, essentially a type of process energy not normally “counted” as building load, varies quite a bit and depends on the demand for recycled water by local agriculture. In drought years, such as now occurring in California, the demand is higher than normal. Therefore, in any given year, more energy may be drawn by the pump than by the building from this combined system.
The net effect is that, even though the solar photovoltaic system now installed on the roof of the office wing would likely be sufficient (per the performance modeling) to produce a ZNE annual balance for the building alone, much of the power produced by the roof array is being used by the large water recycling pump. The measured data for 2013 shows this result (see discussion below).
In a change from the original design intent of providing a solar photovoltaic system on the roof sized to produce ZNE performance for the building in isolation, the City of Watsonville intention- ally combined the roof and ground arrays, and added the pump load, in order to make the com- bined energy load/supply systems perform at 80% of ZNE for the combined building and process loads. Under current utility rates and tariffs for such facilities, this is a common, economically prudent practice.
Controls
The building has a standard building management system (BMS) for control of heating, cooling and ventilation. CO2 sensors engage to control the mechanical ventilation. As noted above, the ventilation system in the laboratory is programmed to turn off during night hours and start up two hours before the building opens.
There is a standard switchover from heating operation to cooling operation at a selected point in the season. Interestingly, the office wing to this date has not used the cooling system. General lighting is dimmed in response to daylight availability.
CASE STUDY NO. 4
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