MCZNH Solar Awnings (summer and winter positions)

The Mill Creek NetZero Home (MCNZH) will have a ground-breaking solar awning installed on its south face (introduced in Solar Awning Part 1). Essentially, it's a movable awning made out of photovoltaic (PV) modules. It will serve two functions: to shade the south windows in the summer and fall, and to tilt the modules so that they are always at an optimum angle to the sun. An analysis of the solar awning's net energy benefit follows.

Overheating

There is a real risk that homes with large amounts of south glazing will overheat during the summer, fall and spring. One way to mitigate this risk is to build overhangs over the south windows. CMHC provides software (called the Design Checker), with its book Tap The Sun: Passive Solar Techniques and Home Designs (CMHC, 2005) that enables home owners to calculate how large a home’s overhangs should be to avoid overheating.

According to the Design Checker, “overheating is considered to occur if the temperature of the home [exceeds] the set-point heating temperature by more than 4º C. Overheating is considered excessive if it exceeds 4% of the hours in a given month... In virtually all cases, October was found to be the critical overheating month” (CMHC Design Checker Software).

Solar Awnings Increase Usable Solar Gain

The MCNZH was simulated using the Design Checker. Because of the MCNZH’s very high percentage of south glazing versus floor area (11.7%), the Design Checker recommended that the overhang width be 6.5 feet, with a header height of 0 feet in order to avoid significant overheating throughout the critical overheating month of October (it still reports 7% hours of overheating, but the owners of the MCNZH are willing to tolerate slightly more overheating hours than recommended). The south windows would be 74% shaded in October between 10 am and 2 pm.

The shade that these overhangs provide does prevent overheating, but it also reduces the amount of usable solar energy entering the MCNZH during the heating season. Two HOT2000 simulations were conducted: one with the recommended overhangs, and one with no overhangs at all. HOT2000 reported that the MCNZH built with the recommended fixed overhangs will require an additional 1154 kWh per year of supplemental energy. So, the solar awnings, if designed to shade the south windows by about 74% in October, will provide a benefit of 1154 kWh of space heating since they can be made to not cast any shade during the heating months of November - March.

The MCNZH solar awnings will probably have three settings, summer, winter, and swing season. The ideal swing season angle hasn't yet been determined. Here are some pictures of the different shadows with the summer and winter angles:

MCNZH Solar Awnings in October. Set at the summer angle, they shade almost 40% of the south windows. In the swing season angle (spring and fall), they will need to shade the windows upwards of 70% in order to fully minimize overheating.

MCNZH Solar Awnings in late November. They have been moved out of the windows' way for the winter because the risk of overheating is low, and the sun is needed in the house to provide heat.

Solar Awnings Increase PV Electricity Production

To maximize electrical power output from a PV array on June 21, the optimum tilt angle would be roughly 30° from horizontal, because at that angle the modules are perpendicular to the sun. On a winter day, say December 21 when the sun is lower in the sky, the optimum angle would be closer to 75° from horizontal. Being able to adjust the tilt on a PV array on a seasonal basis should result in the increase of power production of roughly 20% with the same module area.

The solar awning prototype will increase the electricity produced by the MCNZH by 20%, or about 800 kWh annually.

MCNZH Solar Awnings in July. They are providing full shade to the windows, and they are tilting the PV modules to a near-optimum angle for maximum electricity production.

Net Energy Benefit

The MCNZH solar awnings will provide a net energy benefit of approximately 1954 kWh annually, greatly increasing the home's chances of reaching its goal of using NetZero energy.

(cross posted at raisingspaces.com)