Ellen Kathrine Hansen loses credibility right off the bat, with this opening sentence:
Judging by looks alone, you’d never guess that the simple one-and-a-half-story house on a residential street outside Århus, Denmark, is anything more than an ordinary single-family home.
No, judging by looks alone, I’d guess that that was some kind of Scandinavian eco-friendly house — and it is:
Specialized windows, tight insulation, and a climate-control system minimize the need for electricity and heating. The sun handles the rest: Solar panels, solar thermal collectors, and the Home for Life’s south-facing orientation allow the house to generate enough electricity and heat to make it carbon neutral.
So, how much does a carbon-neutral home cost?
Our first prototype cost about US $700 000 to build, not including the design and planning.
I’m sure a big chunk of that expense comes from economically inefficient photovoltaics and automated window blinds:
The house generated 800 kilowatt-hours of electricity last August, used just a bit more than half of it, and fed the rest back to the grid.
There are two basic ways to reduce the cost of heating and cooling a home:
One approach is to design houses with small windows and thick walls filled with insulation; this strategy prevents the sun from overheating the interior, cuts down on air-conditioning in the summer, and reduces heat loss in the winter. But it doesn’t make for a delightful living experience. The people living in one such house complained to me that it was so heavily insulated you couldn’t even hear birds singing outside.
So we decided to build a house that didn’t wall itself off like a fortress from the sun but instead invited sunlight and fresh air in. In a word, that means windows. Our test house has about double the window area of an ordinary Danish house. We chose specialized panes with two or three layers of glazing, which in the cooler months reduces the heat escaping from the inside while allowing lots of heat and daylight to enter. In fact, the windows alone deliver half of the heating needed in the winter.
The windows’ frames also add insulation. They’re made of a brand-new type of polyurethane (the stuff that foam is made of) strengthened with thin glass threads.
Engineers at Velfac, a VKR subsidiary, tested more than 200 materials before finding one that was at once highly insulating and durable and had a pleasing surface finish. Because of the material’s strength, a weather-resistant frame can be made with just a slim sheet of this polyurethane.
The large windows cut down on the amount of indoor lighting and mechanical ventilation needed — good news for our net-zero-energy goal. But sometimes we need to keep the interior heating in check. To do so, a roof overhang on the south side provides shade when the sun is high in the summer, and shutters and blinds on both sides of each window regulate the transmittance of heat and provide privacy.
To further reduce the risk of overheating, we programmed the windows to open on their own to let in fresh air. Sensors in every room track the temperature, carbon dioxide levels, and humidity, and a weather station on the roof monitors outside conditions. Our control system, from another VKR company, WindowMaster, uses that information to decide when to lower the solar screens or slide open selected panes. These automated adjustments of the windows, rather than traditional air-conditioning and heating, provide the bulk of the house’s temperature control.
The big win comes from passive solar heating:
In total, the Home for Life ought to use about 60 percent of the energy of a traditional single-family house in Denmark: 15 kWh per square meter per year for lighting, household appliances, and running the active components of the house and 32 kWh/m² per year for hot water and heating. It’s the latter where the Home for Life really stands out: Its heating consumption is just half that of an ordinary Danish home. Once all the systems are fine-tuned, we estimate that the house will generate a surplus of about 9 kWh/m² per year.
The shape of the house made a big difference. Its overall surface area was kept to a minimum because that is a major factor in heat loss. In addition, the tip of the roof is tilted to the north, which increases its surface facing south. That side of the roof is covered with solar panels, solar thermal collectors, and skylights, each of which plays an important part in determining the house’s overall energy budget.
First, let’s look at the electricity. The 50 m² of polycrystalline solar panels generate about 5500 kWh a year. That’s 20 percent more electricity than the house needs, although in winter it does draw some power from the electricity grid. These solar cells, with 13 percent efficiency, aren’t the best on the market, but they’re a good compromise for the price.
Then there’s the heating, which comes in through the windows or the solar thermal collectors. The 6.7 m² of collectors catch the sun’s rays on copper plates installed on the lowest part of the roof. Underneath the plates, copper pipes circulate a fluid that absorbs the heat of the plates, converting 95 percent of the sun’s energy into heat. The collectors can catch indirect sunlight, too, so the house still has heat on cloudy days.
Should more interior heating be needed, we use an air-source heat pump. In one common configuration of this type of pump, air passes through a heat exchanger placed outside the house to transfer the air’s warmth to a liquid. The liquid travels to an electrically powered compressor inside the house, which applies pressure to raise the fluid’s temperature further. In general, a heat pump is far more energy efficient than conventional oil or electric heating, and it has lower CO2 emissions, too. But the pump’s performance depends heavily on the amount of heat contained in the air; when it’s cold outside, these heat pumps aren’t efficient.
To avoid that problem, we used a heat pump designed by another VKR subsidiary, Sonnenkraft, which uses the solar collectors to preheat the cold winter air before it reaches the heat pump. The pump can now easily produce 20°C water even when the outside air is below freezing. After the liquid is compressed, the heat travels through pipes in the floors and to radiators. In all, our solar collectors and pump can produce about 8000 kWh’s worth of heat a year.