Case Study: Bombard in ReVISION House Vegas
In designing and building a net-zero-energy-home, the energy that the home uses annually must be exactly balanced with the energy the home can generate over that same year. This relatively simple equation is not as easy or obvious to resolve at it may seem. The first step is to understand what the energy uses will likely be for a house in a given climate zone, then devise strategies to assure that the home and its systems are as energy efficient as practical with the goal of ultra-low energy consumption. After strategies have been selected and adopted targeting the low-energy goal, and the annual energy use is predicted through energy-modeling, a properly sized renewable power system can be specified.
For the ReVISION House Vegas, the net-ZEH equation took on an additional variable: that of accomplishing zero-energy in the context of retrofitting a house that has already been built. The original equation stays the same but the efficiency options become more limited because we are working with an actual house and not a blank sheet of paper. And that existing house, according to EnergyGauge USA energy modeling, used 15,692 kilowatt-hours (kWh) a year. Add to that natural gas consumption of 478 therms a year, and total energy consumption is the equivalent of 29,451 kWh annually. If left as-is, the ReVISION House would need a cost prohibitive 18kW PV array covering virtually the entire roof surface to generate that much power in a year!
After a careful review of potential energy efficiency strategies, a “whole-house” package of improvements was selected which lowers the total energy consumption to 8,756 kWh a year; a 70% reduction. Working with Bombard Renewable Energy and SunPower, the project team selected a SunPower SPR-210 system rated at 4.871 kW AC/5.67 kW DC output. Consisting of 27 SunPower-210BLK modules and a SunPower SPR-6000M inverter, the estimated annual AC power output is 10,716 kWh/yr when mounted on the ReVision house with its specific latitude, solar conditions, roof slope and orientation. With this on-site generation and the home’s estimated energy consumption, the annual net-energy consumption is predicted to be zero, resulting in a true net-zero-energy home!
The SunPower 210 solar panels are the latest generation of silica based panels with efficiencies up to 16.9% compared to other contemporary panels with efficiencies of 12%. This means the SunPower panels convert up to 50% more of the incident solar energy into electricity, allowing for a smaller array in area for a given output, and more flexibility in placement. A unique mounting system is also used: SolarMount®/S-5 ™ by UNIRAC. The SolarMount clips attach directly to the vertical seam of the metal roof meaning that no additional holes need to be placed in the roof surface for panel attachment. Considering that the ReVision house employs 27 SPR-210 panels, that’s a lot of holes…and potential leaks… avoided.
For the net-zero-energy equation to work, one other important component is required: net-metering. Net metering is provided through the participating utility, in this case NV Energy, in which energy produced by the PV system in excess of what the house is using at any one given moment flows back into the electricity grid with the homeowner getting credit for those kilowatts. When the sun goes down, the homeowner buys the kilowatts they need as normal. In essence this allows the grid to act as a battery back-up for the solar system, without the cost or maintenance of a physical battery storage rack.
Steven Winter Associates and Building America will be monitoring the energy consumption and PV generation of the ReVISION House Vegas for several months to calibrate and confirm engineering assumptions and energy modeling results.