Page 86 - Zero Net Energy Case Study Homes-Volume 2

CASE STUDY NO. 9
SILVER STAR APARTMENTS
ages on the grid. Closets and wiring are ready for the batteries to be installed, which is planned once additional funding is secured. A cost factor is that projects of this size that include elevators require 3-phase batteries and most are single-phase for the single-family residential market; typi- cally the 208V 3-phase batteries are significantly higher cost.
Energy Performance
Energy Modeling and Post-Occupancy Measurement
Energy Use—Modeling
Energy modeling was done during the design phase in 2013 for the purposes of code compli- ance. Since the project was seeking LEED Platinum certification, the model was run again in June, 2017, which was the “as-built” condition at the completion of construction. The California energy code compliance software was used in both cases for the modeling analysis, EnergyPro (version 6 for the final modeling).
The annual total amount of energy used was modeled to be 186,500 kWh, or an EUI of 18.8 (kBtu/sq.ft. per year). No breakdown into monthly energy use is available. The pie chart on the opposite page shows the annual energy use by category of use.
Energy Use—Post Occupancy Measurement
Energy use data was calculated from the single utility net meter for the project and the solar generation data as recorded by the PV system. Since the latter data was found to be faulty prior to March, 2019, (see discussion in the next section below), the reliable energy use data corresponds only to March through October of 2019. For the remainder of the full year of data, from November, 2019, through February, 2020, the energy use data has to be estimated. A simple method, given the uniformity of the energy use profile in this mild climate, is to estimate the monthly energy use for the four months as the same as the average for the previous eight months. This estimated energy use is showed dashed in the chart on the opposite page.
The total annual measured (and partially estimated) energy use is 185,850 kWh or 18.7 kBtu/ sq.ft. This number compares well to the modeled total of 18.8 kBtu/sq.ft. Achieving the project ZNE goal depends on the solar energy production at the site and whether it can reach this level for the year, namely approximately 186,000 kWh/year. This is discussed in the next section.
Energy Production versus Energy Use: Zero Net Energy Performance
There are six inverters on this project, which should be monitored regularly for indications of mal- function or failure. If one inverter fails, then the production from those panels is lost. The property owner and manager have not instituted a service agreement for monitoring or maintenance of the solar PV system (or solar thermal system).
The solar PV contractor has occasionally checked on the operation of the installed system as a professional responsibility. In a recent check, the contractor discovered that one inverter had failed and that a major portion of the system output had been lost prior to March, 2019. After advising the owner, the inverter was replaced and the system was restored to its full capacity.
As a result of this experience, the solar PV contractor has continued to monitor system perfor- mance, though still not under any service agreement, and has performed site inspections as well. The solar production data has been monitored and recorded since March, with some addi- tional power drops caused by local power outages and failure of some inverters to turn back on.
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Zero Net Energy Case Study Homes: Volume 2