towards zero net energy homes

Spring 2009
Living Spaces
Homes and Landscapes of Western Massachusetts and Northern Connecticut


As discussed in my previous article on energy efficiency, insulating and air sealing the building shell are the initial steps in addressing a green home from an energy efficiency point of view.  The net energy used in the home can be further decreased and potentially reduced to zero if additional improvements are undertaken and energy is generated on-site. 

On-Site Renewable Energy

On-site renewable energy generation generally means harnessing the power of the sun or wind.  Both solar and wind resources are very site-specific.  If the target site has a good solar or wind exposure, then employing renewable generation options may be practical.  Using solar energy to generate electricity and heat water enables a home to create the power it needs without the burning of fossil fuels.  Using a renewable resource to supply all home energy needs is the goal of a zero net energy home.

The typical zero net energy home is still connected to the power grid.  The home uses the grid like a battery.  In the summer months, excess power is sold to the grid and in the low sunshine winter months, power is provided to the home by the grid.  A zero net energy home would generate enough power in the favorable times to offset the power used during unfavorable times.  Thus, the net energy used would equal zero over a twelve-month period.  Despite the fact that these homes typically still need external power generation in the winter months, the net zero energy concept is a monumental improvement over a typical home.

To achieve the net zero energy goal, all of a home's systems must be optimized to reduce electric energy demand to a level that is practical for current solar photovoltaic arrays to provide.  The building shell must be optimized (as described below), the mechanical systems must be efficient and properly sized, and the appliances must be very efficient.  Also, the energy-related habits of the owner/occupant must be reasonable.  For example, TV's, computers, and chargers should be unplugged when not in use.  Thermostats must be kept at a reasonable (68°F) level.

Building Shell Improvements

The building shell of net zero energy homes are typically insulated to a wall R-value of about R-42 and a ceiling value of R-72.  R-value measures the resistance to heat flow, with higher values indicating greater resistance to heat flow.  These super-insulated R-values are about twice that of a typical well insulated home.  In addition, the floor slab or foundation would be "double" insulated. 

The thermal efficiency of a window is measured by its U-value.  The U-value quantifies the flow of heat through the window.  The lower the better.  A typical "good" double pane window used today will have a U-value between 0.32 and 0.35.  The windows in a super-insulated building shell need to be much more efficient.  U-values of 0.12 to 0.18 are available for triple pane windows developed by smaller manufacturers.  These windows are two to three times more effective at reducing heat flow.  Larger window companies are developing more efficient windows but do not have any in this range yet.  The number, orientation, and size of windows are also critical.  Windows should be concentrated on the south to maximize passive solar gain and minimized on the north.

Mechanical Systems

Mechanical systems also play a large role in achieving net zero energy goals.  Solar hot water heating panels with an on-demand backup water heater are an integral part of achieving zero-energy.  Very efficient appliances are essential. 

The space heating and cooling loads of net zero energy homes are typically so small that conventional furnaces and boilers are unavailable because they are vastly oversized.  Mini-split heat pumps are a practical alternative, although they do not work well below 15°F.  At very cold temperatures, electric resistive heat is used as a backup for these systems. 

For an 1100 ft2 (three bedroom, one bathroom) home I toured recently in Turner's Falls, this electric heat consists of a small heating element in the ventilation system.  The projected heat load for this home is about 7500 Btu/hr (2200 watts) at -5°F.  In practical terms, this load could be satisfied by turning on (11) 100-watt light bulbs.  This example illustrates the dramatic effect of super-insulation on the energy required to heat a home.  It is easiest to measure net zero energy usage with an all electric home, but the backup fuel could also be propane, oil, or wood. 

Summary

If a homeowner is interested in driving energy usage lower, then a super-insulated home is the first step.  Second is minimizing energy use within the home.  The third step is the addition of on-site renewable energy generation.  These three steps taken together can result in a home that generates enough power to satisfy its own energy needs.  Additional information is available at PaulHuijing.com.


Note: This is a slightly longer version of the article published.