Page 126 - Zero Net Energy Case Study Homes-Volume 2

CASE STUDY NO. 11 SOL LUX ALPHA
(Right) View of bifacial solar PV canopy above the sixth floor roof deck.
(Photo: Matt Abrams,
SkyHi Aerial Photography, www.skyhiaerial.com)
(Opposite page, top) View of wall-mounted inverters at roof deck;
(Opposite page, bottom) Wall- mounted battery units located in the ground-level garage.
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Construction
The most unusual phase of the construction process was the arrival and placement of the manu- factured wall panels on the six levels of the structural frame. Since there was no site access from any of the sides, the heavy panels could only be lifted by crane from the street side of the narrow site. The builder had to obtain a city permit to block the street for specified periods of time when the crane was in use.
Renewable On-Site Energy Supply
On-Site Solar Photovoltaic System
The solar PV system was limited to the roof level because of the potential for new building de- velopment at all sides of the lot, thus blocking access to the sun for any vertical solar facades. To allow for a usable roof deck area, the solar panels were made into an overhead canopy which covers the entire roof deck and common service areas.
Usually, solar PV arrays at the roof level are required to have a 3’-0” setback for roof access, but this requirement was waived because the array was placed on the raised canopy. This allowed for a larger system and the corresponding higher solar energy production. Even with this maxi- mum area of solar PV array, equal to the lot footprint area of 2,500 square feet, there would have to be enough energy generated to meet the project goals for ZNE performance of building and electric vehicles over the course of a year for all four condominium units and the common space. With high energy-efficiency designed into the building using the Passive House standards, it re- mained to choose the solar PV panels with a high power production to meet the calculated loads.
A “bifacial” solar PV panel5 was selected to maximize output per unit area of the PV array. (A bifacial panel harvests solar energy from the backside of the panel, making it highly productive when used as a canopy—about a 20% boost in power output.)
The total PV array then consists of 96 panels forming the roof canopy, divided into 21 panels for each of the four condominium units and 12 panels for the common spaces and elevator. The output of each bifacial panel is approximately 450-500 watts, so the total output of the system assigned to each condominium unit is about 10 kW and the remaining 6 kW is assigned to the common area. The maximum power production of the total system is approximately 46 kW.
On-Site Energy Storage: Battery Component
For each condominium unit, there are three Tesla Powerwall batteries that total 41.5 kWh of available energy storage when fully charged. For the condominium unit alone, this is enough energy for two to three days without recharging the batteries—ample resiliency for any utility power outage. The sun continues to shine, automatically providing periods of re-charge, so the condominium unit could potentially operate normally off-the-grid indefinitely.
The EV charging presents a different set of conditions. Its battery has a relatively high storage capacity (75kWh for a Tesla) and the car alone can drain the house battery array completely in a short time by fully recharging. So, if the car charging occurs at night, then the house battery could be drained by morning, which may not be the best schedule for battery use in terms of utility rates and time-of-day use.
5 Sunpreme Maxima GxB 380, http://sunpreme.com/products-main/
Zero Net Energy Case Study Homes: Volume 2