Constructing a net zero energy home in Edmonton, Alberta is extremely challenging because this is a cold place. Thankfully, it's also a sunny place. Still, in order to achieve the net zero standard, factors such as energy efficiency, insulating values and solar energy collection need to be maximized.
Peter the builder and I came up with the idea of an adjustable solar awning system that will increase by about 10% the amount of energy that the Mill Creek NetZero Home (MCNZH) produces annually.
I just wrote a proposal to help fund the prototype solar awning that we will put on the MCNZH. Here are some excerpts. Sorry for the formal tone, and warning: the following may be for eco-nerds only.
Passive Solar Energy
The MCNZH is designed to passively collect a large amount of solar heat through its south-facing windows. In fact, the 3642 kWh of solar heat that will be captured through the windows represent 46% of the annual space heating required by the MCNZH. With such a large area of south-facing windows, though, there is considerable risk of the home overheating.
In a home as well-insulated as the MCNZH will be, there are “…severe restrictions in the amount of south windows that can be used without excessive overheating” (CMHC, 2005, p.53). “Increasing the window overhang[, however,] allows for additional south glazing” (CMHC, 2005, p.54). In order to remain comfortable for its occupants, then, the south windows must be shaded by overhangs.
The installation of overhangs presents a compromise between maximum solar energy capture and occupant comfort. HOT2000 simulations show that the MCNZH will capture 314 kWh less annually if equipped with overhangs (HOT2000 is a residential building performance evaluation software distributed by Natural Resources Canada (Natural Resources Canada, n.d.)).
The solar awning prototype will eliminate this compromise, maximizing solar energy capture and providing occupant comfort at the same time.
Photovoltaic Electricity Generation
The MCNZH needs to provide itself with over 8800 kWh of heat and electricity annually. Much of this energy will be provided by an active solar hot water system and a ground source heat pump, but the remaining kWh will need to be provided by photovoltaic (PV) panels.
PV panels produce the most electricity when the sun is striking them at a 90° angle. Because it is so far north, Edmonton experiences a large degree of variation throughout the year in the angle of incidence at which the sun strikes an installed PV panel. On the day of the summer solstice (June 21), a PV panel must be tilted at a XX degree angle (not sure what angle yet) to be producing at its maximum capacity (all other variables considered). Alternatively, on the day of the winter solstice (December 21), a PV panel must be tilted at XX degrees.
When a PV panel is installed in a fixed location on a building in Edmonton, it is usually tilted at an angle of approximately 53°. This represents a compromise between the winter and summer months that maximizes electricity production.
The solar awning prototype will eliminate this compromise by being adjustable. The solar awning prototype will increase the electricity produced by the MCNZH by 10%-15%, or 400 – 600 kWh annually.
Any design element that has only one function is probably a mistake or a missed opportunity. We ought to strive for multiple and diverse functions of each element so we pay once and get many benefits.
Amory B. Lovins, Rocky Mountain Institute
The solar awning prototype is a system that will provide two functions at once for the MCNZH. Covered in PV panels, the awnings will efficiently produce electricity and provide shade for the windows during the hotter months. They will be installed above the first and second floor windows of the home, and be adjusted four times a year by the homeowner. The solar awning prototype will provide a benefit of 714-914 kWh of energy to the MCNZH. This represents an increase of 8%-10% of energy generated when compared with the same home with regular awnings and the PV panels fixed in place on the roof.
During the summer months, the awnings will be set at an angle of between 25° and 30° (see picture). The summer adjustment will also slide the awnings forward so that they act as shade-providing overhangs for the windows. In addition to providing shade, the PV panels will generate electricity more efficiently because they are properly angled towards the sun.
Solar Awnings in July. The widows are fully shaded and the PV panels are perpendicular to the sun.
During the winter months, the awnings will be set at an angle of between 70° and 90° (see picture). The winter adjustment will pull the awnings up and away from the windows so that they don’t provide any shade. This will maximize passive solar heat capture through the windows, and increase solar electricity production by optimally tilting the PV panels towards the sun.
Solar Awnings in December. The windows are unshaded and the PV panels are perpendicular to the sun.
The solar awning idea is an elegant way to squeeze out an extra 10% of solar energy for this house. Plus I think they look pretty cool. I can't wait to see them in action.
(cross posted at www.raisingSpaces.com)
CMHC (2005). Tap The Sun: Passive Solar Techniques and Home Designs.
Natural Resources Canada. (n.d.). Retrieved October 16, 2007 from http://22.214.171.124/residential/personal/new-homes/r-2000/standard/hot2...