Levels of passive solar energy use for homes:
Many detached suburban houses can achieve reductions in heating expense without obvious changes to their appearance, comfort or usability. This is done using good siting and window positioning, small amounts of thermal mass, with good but conventional insulation and occasional supplementary heat from a central radiator connected to a water heater. Sunrays may fall onto a wall during the daytime, which will radiate heat in the evening.
Historically, most "passive solar" approaches have depended on near-daily solar capture and storage, only expected to maintain temperatures through a few days and nights. These are now termed "short-cycle passive solar". More recent research has developed techniques to capture warm-season solar heat, convey it to a seasonal thermal store for use months later during the cool or cold season. This is referred to as "annualized passive solar." This requires large amounts of thermal mass. One technique buries water-proof insulation in 7-metre skirts around the foundation, and buries loops of plastic pipe or ducts under the foundations and slab. The "skirts" of insulation prevent heat leaks from weather or water.
A "purely passive" solar-heated house would have no mechanical furnace unit, relying instead on energy captured from sunshine, only supplemented by "incidental" heat energy given off by lights, candles , other task-specific appliances (such as those for cooking, entertainment, etc.), showering, people and pets. The use of natural air currents (rather than mechanical devices such as fans) to circulate air is related, though not strictly solar design.
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While many solar and wind projects continue to follow the centralized model, current fuel cell technology keeps many of these types of projects on a scale that makes them fit more appropriately into a decentralized model. Fuel cells work pretty much “by turning pretty much any feedstock (natural gas, biogas, propane, even coal) into electricity and heat. Because there’s no combustion inside the fuel cell, there are no emissions. Fuel cells rely on an electro-chemical reaction. Because the cells are built to take advantage of the heat they generate, they are much more efficient than traditional fossil-fuel plants”, and are considerably smaller than coal-fired power plants.