Some of the concepts and technologies used in the Solar Decathlon solar houses are tried and true, while others have seldom-or never-been tried. Learn more about these old and new ideas below.
Why is energy efficiency important? Because it's much cheaper to save energy than to make it. An excellent place to start making a home more energy efficient is with good insulation, keeping out unwanted heat and cold.
R-value is a standard rating for heat transfer resistance. The higher the value, the better the insulation. U-value is the opposite, a measure of heat transfer, so the lower values are better. U-values are generally used for rating windows. For more information, visit the U.S. Department of Energy's (DOE) Office of Energy Efficiency and Renewable Energy's (EERE) Insulation and Air sealing page.
These energy-efficient lamps use less electricity to provide lighting levels comparable to conventional (incandescent) lamps. These bulbs also use less electricity and generate less heat, which means your cooling system doesn't need to work as hard in the hot summer months, reducing the electricity consumed by air conditioners. CFLs work in standard incandescent fixtures. For more information, visit the EERE Types of Lighting Compact Fluorescent Lamp fact sheet.
These are prefabricated panels typically made of foam insulation sandwiched between sheets of oriented strand board or other building material. SIPs, which are used in many of the solar houses in the Solar Decathlon, offer superior insulation (typically R-4 per inch) and ease of construction. For more information visit the EERE Structural Insulated Panels fact sheet.
Low-emissivity (low-e) coatings for windows, invented and commercialized in the 1980s, have revolutionized window technology. Thin, transparent coatings of silver or tin oxide permit visible light to pass through, but also reflect infrared heat radiation back into the room. This reduces heat loss through the windows in winter. Low-e windows are available for different climate zones and a variety of applications. For more information, visit the EERE Energy Savers Windows page.
Light-emitting diodes are semiconductors that emit light when voltage is applied to them. Widespread in use for display panels of electronic equipment, they are just beginning to be used for general lighting. LED lighting is currently quite expensive, but uses very little electricity, has very long life spans, and provides great flexibility in color and intensity. Several Solar Decathlon houses feature LED lighting.
Radiant in-floor heating turns a home's floor into a radiator by piping hot water either through a cement floor or under other types of floors. Slower, but steadier than forced-air or even standard radiators, several Decathlon homes use it to take advantage of solar water heating systems. For more information, visit the EERE Radiant Floor Heating fact sheet.
Effectively sealing or tightening a home against air leakage is one of the most cost-effective ways to increase energy efficiency. But to insure good indoor air quality, "exchanging" about one-third of the air in the house with outside air per hour is generally recommended. One way to maintain energy efficiency while providing fresh air is to use ERVs, which use heat exchangers (like little radiators) to transfer heat from the outgoing air to the incoming air, or vice versa, as needed. For more information, visit the EERE Energy Recovery Ventilation Systems.
Absorption chilling is a refrigeration technology that uses heat instead of mechanical compression. The desired cooling comes from one fluid being absorbed by another. Heat must then be applied to separate the two fluids to continue the cycle. Use thus far has largely been in commercial buildings with natural gas or industrial waste heat providing the necessary heat. For more information, visit the EERE Absorption Cooling page.
Considerable amounts of solar energy can be captured for desired heating in the cool months, while avoiding unwanted heating in the hot months. This can be achieved without active mechanical systems simply by properly siting and designing a home. South-facing windows, for example, can let in a lot of heat from winter sun, while large overhangs keep out that solar heat in the summer when the sun is higher in the sky and the heat is not desired. Similarly, proper window and skylight placement can provide appropriate light for home activities, reducing the amount of electric lighting that is needed. All the soar houses in the Solar Decathlon incorporate these design principles. For more information visit the National Renewable Energy Laboratory's (NREL) Passive Solar Heating and Daylighting for Homeowners page.
PV systems are semiconductor devices that generate electricity by absorbing light energy, triggering electrical current flow from an electron-rich material to an electron-deficient one. Most of the solar homes in the Solar Decathlon use crystalline-silicon PV modules, the most common type, but other types of PV cells include thin films of various compositions. For more information, visit EERE's Photovoltaic Basics page.
Solar thermal collectors use solar energy to heat water or a transfer fluid such as antifreeze flowing through a "collector" designed to capture sunlight. The heated water is then usually stored for domestic hot water use, but several of the Decathlon homes also use it for space heating. Most Solar Decathlon houses use better-insulated (but higher cost) evacuated-tube collectors for higher efficiency. For more information, visit the EERE Solar Collector section.
It takes energy to change a solid to a liquid or a liquid to a gas (for example, melting ice or boiling water). Conversely, there is energy embodied in the liquid or gas, as it gives up heat energy when it liquefies or solidifies. While melting and freezing ice is not a very effective heat storage system, certain other materials with higher melting points can store heat (or cold) effectively. In the case of water and space heating in some Solar Decathlon homes, excess hot water or exhaust air is routed through the phase-change material, cooling the water or air and melting the material. When heat is needed, cool water or intake air is run through the phase-change material, absorbing heat from the solidifying material.
Fuel cells electrochemically combine hydrogen and oxygen to generate electricity at an anode and a cathode along the lines of a battery (plus heat). The opposite reaction, hydrolysis, uses electricity to split water into hydrogen and oxygen. With an appropriate power source, the two processes can be combined as an energy storage system. For solar homes in the Solar Decathlon home, this means using excess PV power when the sun is shining to make hydrogen, and then when electricity (and maybe heat) are needed and the sun isn't shining, using the hydrogen in a fuel cell to generate that electricity.