Systems in a building autonomous
This section includes some minimal descriptions of methods, to give some feel for such a building’s practicality, provide indexes to further information, and give a sense of modern trends.
Water
There are many methods of collecting and conserving water. Use reduction is cost-effective.
Greywater systems reuse drained wash water to flush toilets or to water lawns and gardens. Greywater systems can halve the water use of most residential buildings; however, they require the purchase of a sump, greywater pressurization pump and secondary plumbing. Some builders are installing waterless urinals and even composting toilets that completely eliminate water usage in sewage disposal.
The classic solution with minimal life-style changes is using a well. Once drilled, a well-foot requires substantial power. However, advanced well-foots can reduce power usage by twofold or more from older models. Well water can be contaminated in some areas. The sono arsenic filter eliminates unhealthy arsenic in well water.
However drilling a well is an uncertain activity, with aquifers depleted in some areas. It can also be expensive.
In regions with sufficient rainfall, it is often more economical to design a building to use rain, with supplementary water deliveries in a drought. Rain water makes excellent soft washwater, but needs antibacterial treatment. If used for drinking, mineral supplements or mineralization is necessary.
Most desert and temperate climates get at least 250 millimetres (9.8 in) of rain per year. This means that a typical one-story house with a greywater system can supply its year-round water needs from its roof alone. In the driest areas, it might require a cistern of 30 cubic metres (7,900 US gal). Many areas average 13 millimetres (0.51 in) of rain per week, and these can use a cistern as small as 10 cubic metres (2,600 US gal).
In many areas, it is difficult to keep a roof clean enough for drinking. To reduce dirt and bad tastes, systems use a metal collecting-roof and a “roof cleaner” tank that diverts the first 40 liters. Cistern water is usually chlorinated, though reverse osmosis systems provide even better quality drinking water.
Modern cisterns are usually large plastic tanks. Gravity tanks on short towers are reliable, so pump repairs are less urgent. The least expensive bulk cistern is a fenced pond or pool at ground level.
Reducing autonomy reduces the size and expense of cisterns. Many autonomous homes can reduce water use below 10 US gallons (38 L) per person per day, so that in a drought a month of water can be delivered inexpensively via truck. Self-delivery is often possible by installing fabric water tanks that fit the bed of a pick-up truck.
It can be convenient to use the cistern as a heat sink or trap for a heat pump or air conditioning system; however this can make cold drinking water warm, and in drier years may decrease the efficiency of the HVAC system.
Solar stills can efficiently produce drinking water from ditch water or cistern water, especially high-efficiency multiple effect humidification designs, which separate the evaporator(s) and condenser(s).
New technologies, like reverse osmosis can create unlimited amounts of pure water from polluted water, ocean water, and even from humid air. Water makers are available for yachts that convert seawater and electricity into potable water and brine. Atmospheric water generators extract moisture from dry desert air and filter it to pure water.
Sewage
Resource
The approaches above treat human excrement as a waste rather than a resource. Composting toilets use bacteria to decompose human feces into useful, odourless, sanitary compost. The process is sanitary because soil bacteria eat the human pathogens as well as most of the mass of the waste. Nevertheless, most health authorities forbid direct use of “humanure” for growing food. The risk is microbial and viral contamination. In a dry composting toilet, the waste is evaporated or digested to gas (mostly carbon dioxide) and vented, so a toilet produces only a few pounds of compost every six months. To control the odor, modern toilets use a small fan to keep the toilet under negative pressure, and exhaust the gasses to a vent pipe.
Some home sewage treatment systems use biological treatment, usually beds of plants and aquaria, that absorb nutrients and bacteria and convert greywater and sewage to clear water. This odor and color free reclaimed water can be used to flush toilets and water outside plants. When tested, it approaches standards for potable water. In climates that freeze, the plants and aquaria need to be kept in a small greenhouse space. Good systems need about as much care as a large aquarium.
Electric incinerating toilets turn excrement into a small amount of ash. They are cool to the touch, have no water and no pipes, and require an air vent in a wall. They are used in remote areas where use of septic tanks is limited, usually to reduce nutrient loads in lakes.
NASA’s bioreactor is an extremely advanced biological sewage system. It can turn sewage into air and water through microbial action. NASA plans to use it in the manned Mars mission.
A big disadvantage of complex biological sewage treatment systems is that if the house is empty, the sewage system biota may starve to death.
Another method is NASA’s urine to water distillation system.
Waste
Sewage handling is not attractive, but it is essential for public health. Many diseases are transmitted by poorly functioning sewage systems.
The standard system is a tiled leach field combined with a septic tank. The basic idea is to provide a small system with primary sewage treatment. Sludge settles to the bottom of the septic tank, is partially reduced by anaerobic digestion, and fluid is dispersed in the leach field. The leach field is usually under a yard growing grass. Septic tanks can operate entirely by gravity, and if well managed, are reasonably safe.
Septic tanks have to be pumped periodically by a honey wagon to eliminate non reducing solids. Failure to pump a septic tank can cause overflow that damages the leach field, and contaminates ground water. Septic tanks may also require some lifestyle changes, such as not using garbage disposals, minimizing fluids flushed into the tank, and minimizing nondigestible solids flushed into the tank. For example, septic safe toilet paper is recommended.
However, septic tanks remain popular because they permit standard plumbing fixtures, and require few or no lifestyle sacrifices.
Composting or packaging toilets make it economical and sanitary to throw away sewage as part of the normal garbage collection service. They also reduce water use by half, and eliminate the difficulty and expense of septic tanks. However, they require the local landfill to use sanitary practices.
Incinerator systems are quite practical. The ashes are biologically safe, and less than 1/10 the volume of the original waste, but like all incinerator waste, are usually classified as hazardous waste.
Some of the oldest pre-system sewage types are pit toilets, latrines, and outhouses. These are still used in many developing countries.
Storm drains
Drainage systems are a crucial compromise between human habitability and a secure, sustainable watershed. Paved areas and lawns or turf do not allow much precipitation to filter through the ground to recharge aquifers. They can cause flooding and damage in neighbourhoods, as the water flows over the surface towards a low point.
Typically, elaborate, capital-intensive storm sewer networks are engineered to deal with stormwater. In some cities, such as the Victorian era London sewers or much of the old City of Toronto, the storm water system is combined with the sanitary sewer system. In the event of heavy precipitation, the load on the sewage treatment plant at the end of the pipe becomes too great to handle and raw sewage is dumped into holding tanks, and sometimes into surface water.
Autonomous buildings can address precipitation in a number of ways:
If a water absorbing swale for each yard is combined with permeable concrete streets, storm drains can be omitted from the neighbourhood. This can save more than $800 per house (1970s) by eliminating storm drains. One way to use the savings is to purchase larger lots, which permits more amenities at the same cost. Permeable concrete is an established product in warm climates, and in development for freezing climates. In freezing climates, the elimination of storm drains can often still pay for enough land to construct swales (shallow water collecting ditches) or water impeding berms instead. This plan provides more land for homeowners and can offer more interesting topography for landscaping.
A green roof captures precipitation and uses the water to grow plants. It can be built into a new building or used to replace an existing roof.
Electricity
Since electricity is an expensive utility, the first step towards conservation is to design a house and lifestyle to reduce demand. Fluorescent lights, laptop computers and gas-powered refrigerators save electricity, although gas-powered refrigerators are not very efficient. There are also superefficient electric refrigerators, such as those produced by the Sun Frost company, some of which use only about half as much electricity as a mass-market energy star-rated refrigerator.
Using a solar roof, solar cells can provide electric power. Solar roofs have the potential to 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. Solar cells have only small life-style impacts: The cells must be cleaned a few times per year.
A number of areas that lack sun have wind. To generate power, the average autonomous house needs only one small wind generator, 5 metres or less in diameter. On a 30 metre high 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 largest advantage of wind power is that larger wind turbines have a lower per-watt cost than solar cells, provided there is wind. However, location is critical. Just as some locations lack sun for solar cells, some locations lack sufficient wind for an economical turbine installation. In the Great Plains of the United States a 10 metre 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.
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).
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 by including a generator to recharge the batteries during extended fogs or other low-power conditions. 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.
Recent advances in passively stable magnetic bearings may someday permit inexpensive storage of power in a flywheel in a vacuum. Well-funded 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 finally allow the generation of electricity from biomass. Unlike direct incineration of biomass however, the method using a microbial fuel cell is completely emissionless. The plant can be chopped and converted as a whole, or it can be left alive so that waste saps from the plant can be converted by bacteria.