The 2014 Solar Decathlon in Europe asked university teams to design and construct a house built for sustainability. This year’s winner, Delft University’s Prêt-à-Loger, realized that reusing is more sustainable than rebuilding, so the team designed a retrofit skin that turns an existing house into a net-zero energy home.

The “skin” consists of passive and active solar elements, including a steel and glass three season porch whose windows can be closed in the winter to create a greenhouse effect and opened in the summer to allow natural ventilation. The skin makes year-round gardening a possibility in any climate. The steel skeleton is pretty much self-supporting, so the house doesn’t need to bear unreasonable structural loads. On the active side, building-integrated photovoltaics (BIPV) produce enough electricity to meet or exceed the home’s requirements.

Photovoltaics

The BIPV system is a trade-off between letting in natural light and capturing enough sunlight to generate an adequate amount of electricity. A combination of roof and window mounted monocrystalline PV panels provides up to 4.875 kW while letting 35% of the sunlight into the interior. A PVSyst simulation estimates that the system will produce about 3.8 MWh each year. Of course, those estimates depend on the orientation of the original house as well as local shading effects. My ballpark calculations suggest that the PVSyst estimate is based on an unshaded south-facing location, so the reported numbers represent a best-case scenario.

Lighting and Electricity

All electrical lights are energy-efficient LED bulbs, and solar tubes provide additional natural daylighting. An innovative plug-and-play cabling design allows the lighting to be reconfigured and expanded easily, and a radio frequency control system lets the user adjust the lighting wirelessly, further simplifying the wiring installation. All electrical appliances are chosen primarily based on their energy-efficiency ratings. Small motors control the glasshouse windows and shading, and ventilation fans and a heat recovery unit (a heat exchanger) provide fresh air and climate control.

Water

Almost one-third of all domestic water use goes right down the toilet - literally. To eliminate this waste, the house incorporates a rainwater collection system that’s used for toilet flushing. Low-flow faucets and showerheads also contribute to water conservation.

Water is heated by a solar-thermal system that’s installed under the PV panels, serving a dual purpose of cooling the panels (increasing their efficiency) and heating water. The solar water heating meets up to 90% of the hot water requirements; an air-sourced heat pump supplements the system in the winter.

Heating and Cooling

The “skin” consists of a warm (south-east) side and a cool (north-west) side. Passive heating and cooling techniques are used as much as possible. The warm side includes controllable shades to reduce incoming sunlight in the summer but allow it in the winter. The cool side adds a thick layer of insulation.

One of the basics of passive heating and cooling is the use of a large thermal mass. This is difficult to do in a retrofit since the structure already exists, so the designers included an under-the-floor box filled with a phase change material (PCM) to store excess heat during the day and radiate it at night. In the summer, the excess heat can be ventilated through the chimney, bypassing the heat exchanger.

What’s It Like to Live Inside?

Here’s a brief overview of the home’s features and comments from the people who live in one of the first homes with a skin:

“Prêt-à-Loger” is French for “Ready to Stay,” and after seeing the description and amenities, I think I’d be ready to stay in one!

^{Images and video courtesy of Prêt-à-Loger}