Net zero energy buildings produce more energy than they consume, and while they can function autonomously from the grid, they aren’t necessarily disconnected from it – they can use the connection to sell excess energy back to the utility companies. Sounds too good to be true? Can you really get paid to solarize your home? Well, David Knight from the Monterey Energy Group just might be able to help you get there. His take as an experienced engineer on which of these technologies really makes sense should carry a lot of weight with both designers and their clients.
David Knight outlined the latest in his Net Zero energy home projects for an audience of architects and builders at the AIA offices in San Francisco last Thursday. It’s unusual for a mechanical engineering to specialize in residential work, but he’s installed over 6,000 radiant heating systems. He did his first all-electric Net Zero Energy home 6 months ago and has completed 12 more in the past two months alone.
“The usual problems with alternative energy homes is that none of the subs talk to each other, and you really need to use a whole-house approach,” he said. “These projects need someone to fill the role as a coordinator, and that’s what we do.”
Can solar PV really produce enough power for an entire house? Yes it can, now. Solar technologies are improving, and buildings themselves are getting more efficient. The cumulative impacts of improvements in glazing, appliances, heating and cooling systems, etc. means that, according to Knight, that new buildings use half the energy that they did even 5 years ago. Even General Electric’s got a Net Zero home that uses hybrid energy sources.
Payback is another area that may be changing. Solar traditionally has been viewed as having a long payback period. However, California has highest energy rates in the country, and a tiered system where the more you use in a month, the more you pay per kWh, anywhere from 12 to 44 cents. Of course, this means that solar payback is that much faster.
And although the 2005 Title 24 code doesn’t give credits for solar-powered electric items like water heaters, the new 2008 Title 24 will. Currently, designs that call for electric heat or water heating take a huge hit in their compliance score, regardless of whether that electric power came from PG&E or from onsite energy production.
To size the power system, Knight goes through the home’s floor plan room by room to predict power usage for each appliance and for heating, cooling, and water heating as well. Exterior lights and devices are also factored in. With this information in hand, you can figure out the home’s projected power usage – and from that, you can figure out the square footage of the solar panels needed.
Knight’s approach is to balance the home’s projected energy consumption against its capabilities for onsite energy production. On the “consumption” side, factors such as better insulation or more efficient glazing act to reduce consumption. On the “production” side, there are not just one, but several options, some of which can be used in combination.
Solar Thermal is a good solution for swimming pools, but not necessarily for a home. The problem with Solar Thermal is there’s no way to store unused energy. And at the times you need it most, during the winter, is also when the sun’s power is the weakest and of the shortest duration. In order to take maximum advantage of the sun’s rays when the winter sun is low in the sky, the panels need a steep upward tilt, which can be an aesthetic problem. You need to have a sizeable onsite storage tank.
Grid-Tied Solar Electric
Grid-Tie Solar Electric systems can provide enough energy for the house’s needs, and can store unused energy for use at a future time or for resale back onto the grid. With a GTSE system, you get one bill a year (hopefully, a very small one). The panels can be low and flat against the roof, and no onsite storage is needed. They are easier to maintain than some other types of solar systems. There is no transmission loss, as with conventional power. Sized correctly and with enough roof space, they can indeed supply all of a home’s power needs. And because the panels can lie flat and don’t need to be tilted, they can even be built right into the roof to look like a skylight or other design feature.
Photovoltaic Thermal systems have PV panels on top of a flat thermal module that captures the superheated air and either turns it into hot water, or flows the warmed air through the house for heating. PV Thermal systems require an attic space with a south facing sloped roof.
Geothermal Heat Exchange
Geothermal Heat Exchange systems use buried coils of fluid to take heat from the earth, or put excess heat back into the ground.
- Pros: silent; good for larger homes.
- Cons: Expensive and complicated to install. They need a yard or other open area to bury the equipment in, making them less suitable for urban areas. And, California’s high energy prices means that their electric pumps can generate higher electricity costs.
- Mixed Blessings: They are better for colder climates than for California. There’s a tax credit for them, but David Knight believes it will be short-lived because it offers too much potential for cheating.
NZE systems cost about the same as LEED Platinum paperwork. And, while your mortgage interest is tax-deductible, your PG&E bill is not. So if you spend the money to upgrade your home, you can add the expenditures for new systems onto your mortgage for the portion that isn’t already covered by direct rebates.
About the author
Rebecca Firestone has been working in the Bay Area since 1998 as a technical writer, business content developer, architectural filing lady, marketing director, and sorcerer’s apprentice.