Prairie Project Conserves Water and Energy

February, 2014

Lincoln -- Our building project on the north prairie was completed a year ago. It's a guest house that appears to be a barn, partly hidden in the tall grass prairie and wooded countryside. It is also a demonstration project that breaks with many building conventions to show how the application of simple principles of physics can conserve water and cut energy usage.

The building is connected to local electrical service but not to water and sewer lines. Water is provided by gravity flow of roof-collected rainwater into basement cisterns; the water is filtered for household use. Water is further conserved by reusing shower and bath water (graywater) for toilet flushing.

Heating the building in winter is aided by passive solar design; it complements radiant floor heat supplied by a geothermal heat pump. Cooling in summer is achieved with a cupola that serves as a solar chimney.

Many of these systems have been demonstrated elsewhere, although seldom together in one place. The solar cooling system may be unique in that it is assisted by the water-filled cisterns, a highly insulated building envelope, an air-tight basement, operable floor and ceiling vents in all rooms, and the substantial height of the cupola above the loft.

Here's how solar cooling works in the warm months: an air exchanger in the basement brings in fresh air from a vent low on the shady side of the structure; it cools the air somewhat in its chambers. A small fan in the exchanger blows the air out over the water-filled cisterns (which rest on the comparatively cool concrete basement floor) to provide an ambient basement air temperature around seventy degrees, somewhat under pressure due to the airtight basement. Meanwhile, the sun warms the upper part of the structure; heat rises quickly to exit the open cupola high above. The exiting air (and any wind blowing across the cupola) creates low pressure in the upper part of the structure. To cool any part of the building, the first floor vents are simply opened to allow the cool air from the basement to flow into the living spaces through the lower vents. As the air warms, it exits out the top vents into the loft and out through the cupola.

There are no ducts between levels or rooms to move air. The air simply moves according to principles of physics: heat rises and air moves from high pressure to low pressure. Needless to say, doors and windows are kept closed most of the time in the hottest months.

The first year's experience with our solar cooling system has been about as we predicted: the upper floors are ten to fifteen degrees cooler than the outside air during the heat of summer days. There are no air-conditioning bills to pay.

In cool seasons, the air exchanger brings in cold air that is warmed somewhat in its chambers, then circulated over the cisterns. The water now serves as a comparatively warm thermal mass to heat the basement air to about sixty five degrees. A large vent in the atrium floor above the basement is left open so any cold air in the structure can fall into it, and be warmed.

If you are still with me, you'll not be surprised to know this building has no lawn to water. Some might say it has no landscaping, but I'd say there is the best landcaping of all: natural plant succession (although we give the prairie some help with forbs like milkweed).

The project would be of interest to those who are concerned (as we all should be) about droughts, energy issues, and other environmental concerns.