Autonomous building

Using a solar roof, solar cells can provide electric power. Solar roofs can be more cost-effective than retrofitted solar power, because buildings need roofs anyway. Modern solar cells last about 40 years, which makes them a reasonable investment in some areas. At a sufficient angle, solar cells are cleaned by run-off rain water and therefore have almost no life-style impact.

Many areas have long winter nights or dark cloudy days. In these climates, a solar installation might not pay for itself or large battery storage systems are necessary to achieve electric self-sufficiency.^[11] In stormy or windy climates, wind turbines can replace or significantly supplement solar power.^[12] The average autonomous house needs only one small wind turbine, 5 metres or less in diameter. On a 30-metre (100-foot) tower, this turbine can provide enough power to supplement solar power on cloudy days. Commercially available wind turbines use sealed, one-moving-part AC generators and passive, self-feathering blades for years of operation without service.

The main advantage of wind power is that larger wind turbines have a lower per-watt cost than solar cells, provided there is wind. Turbine location is critical: just as some locations lack sun for solar cells, many areas lack enough wind to make a turbine pay for itself. In the Great Plains of the United States, a 10-metre (33-foot) turbine can supply enough energy to heat and cool a well-built all-electric house. Economic use in other areas requires research, and possibly a site survey.^[13]

Some sites have access to a stream with a change in elevation. These sites can use

During times of low demand, excess power can be stored in batteries for future use. However, batteries need to be replaced every few years. In many areas, battery expenses can be eliminated by attaching the building to the electric power grid and operating the power system with net metering. Utility permission is required, but such cooperative generation is legally mandated in some areas (for example, California).^[13]

A grid-based building is less autonomous, but more economical and sustainable with fewer lifestyle sacrifices. In rural areas the grid's cost and impacts can be reduced by using single-wire earth return systems (for example, the MALT-system).

In areas that lack access to the grid, battery size can be reduced with a generator to recharge the batteries during energy droughts such as extended fogs. Auxiliary generators are usually run from propane, natural gas, or sometimes diesel. An hour of charging usually provides a day of operation. Modern residential chargers permit the user to set the charging times, so the generator is quiet at night. Some generators automatically test themselves once per week.^[14][15]

Recent advances in passively stable magnetic bearings may someday permit inexpensive storage of power in a flywheel in a vacuum. Research groups like Canada's Ballard Power Systems are also working to develop a "regenerative fuel cell", a device that can generate hydrogen and oxygen when power is available, and combine these efficiently when power is needed.

Earth batteries tap electric currents in the earth called telluric current. They can be installed anywhere in the ground. They provide only low voltages and current. They were used to power telegraphs in the 19th century. As appliance efficiencies increase, they may become practical.

Microbial fuel cells and thermoelectric generators^[16][17] allow electricity to be generated from biomass. The plant can be dried, chopped and converted or burned as a whole, or it can be left alive so that waste saps from the plant can be converted by bacteria.

Heating

Most autonomous buildings are designed to use insulation, thermal mass and passive solar heating and cooling. Examples of these are trombe walls and other technologies as skylights.

Passive solar heating can heat most buildings in even the mild and chilly climates. In colder climates, extra construction costs can be as little as 15% more than new, conventional buildings. In warm climates, those having less than two weeks of frosty nights per year, there is no cost impact.

The basic requirement for passive solar heating is that the solar collectors must face the prevailing sunlight (south in the Northern Hemisphere, north in the Southern Hemisphere), and the building must incorporate thermal mass to keep it warm in the night.

A recent, somewhat experimental

In annualized geo solar systems, the solar collector is often separate from (and hotter or colder than) the living space. The building may actually be constructed from insulation, for example, straw-bale construction. Some buildings have been aerodynamically designed so that convection via ducts and interior spaces eliminates any need for electric fans.

A more modest "daily solar" design is practical. For example, for about a 15% premium in building costs, the Passivhaus building codes in Europe use high performance insulating windows, R-30 insulation, HRV ventilation, and a small thermal mass. With modest changes in the building's position, modern krypton- or argon-insulated windows permit normal-looking windows to provide passive solar heat without compromising insulation or structural strength. If a small heater is available for the coldest nights, a slab or basement cistern can inexpensively provide the required thermal mass. Passivhaus building codes, in particular, bring unusually good interior air quality, because the buildings change the air several times per hour, passing it through a heat exchanger to keep heat inside.

In all systems, a small supplementary heater increases personal security and reduces lifestyle impacts for a small reduction of autonomy. The two most popular heaters for ultra-high-efficiency houses are a small heat pump, which also provides air conditioning, or a central hydronic (radiator) air heater with water recirculating from the water heater. Passivhaus designs usually integrate the heater with the ventilation system.

Cogeneration systems produce hot water from waste heat. They usually get the heat from the exhaust of a generator or fuel cell.

Heat recycling, cogeneration and solar pre-heating can save 50–75% of the gas otherwise used. Also, some combinations provide redundant reliability by having several sources of heat. Some authorities advocate replacing bottled gas or natural gas with biogas. However, this is usually impractical unless live-stock are on-site. The wastes of a single family are usually insufficient to produce enough methane for anything more than small amounts of cooking.

Cooling

Annualized geo solar buildings often have buried, sloped water-tight skirts of insulation that extend 6 metres (20 ft) from the foundations, to prevent heat leakage between the earth used as thermal mass, and the surface.

Less dramatic improvements are possible. Windows can be shaded in summer. Eaves can be overhung to provide the necessary shade. These also shade the walls of the house, reducing cooling costs.

Another trick is to cool the building's thermal mass at night, perhaps with a whole-house fan and then cool the building from the thermal mass during the day. It helps to be able to route cold air from a sky-facing radiator (perhaps an air heating solar collector with an alternate purpose) or evaporative cooler directly through the thermal mass. On clear nights, even in tropical areas, sky-facing radiators can cool below freezing.

If a circular building is aerodynamically smooth, and cooler than the ground, it can be passively cooled by the "dome effect." Many installations have reported that a reflective or light-colored dome induces a local vertical heat-driven vortex that sucks cooler overhead air downward into a dome if the dome is vented properly (a single overhead vent, and peripheral vents). Some people have reported a temperature differential as high as 8 °C (15 °F) between the inside of the dome and the outside.

A truck trailer refrigerator operating from the waste heat of a tractor's diesel exhaust was demonstrated by NRG Solutions, Inc. NRG developed a hydronic ammonia gas heat exchanger and vaporizer, the two essential new, not commercially available components of a waste heat driven refrigerator.

A similar scheme (multiphase cooling) can be by a multistage evaporative cooler. The air is passed through a spray of salt solution to dehumidify it, then through a spray of water solution to cool it, then another salt solution to dehumidify it again. The brine has to be regenerated, and that can be done economically with a low-temperature solar still. Multiphase evaporative coolers can lower the air's temperature by 50 °F (28 °C), and still control humidity. If the brine regenerator uses high heat, it also partially sterilises to the air.

If enough electric power is available, cooling can be provided by conventional air conditioning using a heat pump.

Food production

Food production has often been included in historic autonomous projects to provide security.^[20] Skilled, intensive gardening can support an adult from as little as 100 square meters of land per person,^[21][22] possibly requiring the use of organic farming and

Greenhouses are also sometimes included.^[20][23] Sometimes they are also outfitted with irrigation systems or heat sink systems which can respectively irrigate the plants or help to store energy from the sun and redistribute it at night (when the greenhouses starts to cool down).^[20][24]

See also

Notes