Page 101 - Designing for Zero Carbon-Volume 2_Case Studies of All-Electric Multifamily Residential Buildings

THE PALO ALTO APARTMENTS CASE STUDY NO. 4
(Oppsoite Page, Top) Birds- eye view of the roofs of the two apartment buildings, showing the installation layout of the 178 solar PV panels. (Image from Google Earth)
Renewable On-Site Energy Supply
Solar Photovoltaic Systems
The solar photovoltaic system basically covers the rooftops of the two buildings (see the Google Earth image on opposite page) providing almost 70% of the electric power required by the thir- teen apartment units, including the EV charging loads. The panels are high-performance bifacial panels that take advantage of the light-colored roof membrane to collect solar energy from both sides of the panel. 178 Sunpreme GxB-370W panels comprise the 65.86 kW system. The DC power from these panels is efficiently converted to AC power by the system inverters, Fronius Symo, 15.0 kW and 12.0 kW.
Toward the end of 2021, a problem developed with the functioning of the inverters that led to a significant drop-off in the production of the solar PV system. Because of the ongoing pandemic and reporting errors, this situation has only recently been confirmed, and corrective measures have been initiated. The most recent year of recorded full production was 2020; 2023 is expected to be a return to that expected optimal production.
Design Analysis: Optimizing Zero-Carbon Design
Design Analysis: Embodied Carbon
An embodied carbon analysis was not done. Tight site constraints and city planning require- ments set the design as realized, requiring the concrete walls of the below-grade parking level, a high embodied carbon component.
Design Analysis: Energy Modeling and Operational Carbon
Energy-use modeling was undertaken as part of the Title-24 compliance documentation, using the California-mandated Energy Pro software (Version 5.1). The total annual energy use for each of the two buildings was calculated and Title-24 compliance was confirmed. The energy-use model predicted an annual energy consumption for the buildings, exclusive of any EV-charging energy use, of 55,000 kWh. For the 22,500 sq. ft. of conditioned building space, this translates to a predicted EUI of 8.3 kBtu/sq.ft. per year. (See the graph on the previous page.)
The annual energy production of the solar PV system was modeled using the PVsyst8 software. Analysis showed that 178 bifacial Sunpreme panels would deliver an estimated annual produc- tion total of 100,000 kWh, almost double the predicted annual energy consumption of the two buildings, and large enough to cover the loads created by the future fifteen EV chargers. In other words, the solar PV system as designed was expected to produce a project that was at least ZNE when fully operational.
The results of the PVsyst modeling analysis is shown in the graph on p. 89, along with the plot of the actual measured performance of the solar photovoltaic system in 2020, the last data re- corded when the solar PV system was operating properly in the latter half of that year. As can be seen, the actual performance closely tracked that predicted by the model for those six months.
8 PVsyst is advertised as a “preliminary design tool” for the estimating solar PV system energy production. https://www.pvsyst.com/
Designing for Zero Carbon: Volume 2
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