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Heat Transfer

• Conduction: Conduction is how heat moves through materials on a molecular level. Heat is related to the vibration of a molecule, with hotter molecules vibrating more vigorously, which then hit into neighboring molecules and cause them to vibrate more vigorously and causes them to heat up.
• Convection: Convection is how heat circulates in liquids and gases. Warmer fluids become lighter because the vibrating molecules take up more space, and so they will rise over cooler fluids.
• Radiation: Radiation moves energy through electromagnetic waves, which are released from the sun and are where heat and visible light originally come from. For solar energy, two different spectrums are important, the visible light spectrum and the infrared spectrum, which is where heat comes from. When a wave strikes and object, its energy is either absorbed, reflected, or transmitted (passes through an object), depending on the properties of the object. Opaque objects absorb between 40 and 95% of incoming solar radiation, with darker colors absorbing more than lighter colors. Bright white objects can reflect 80-98% of incoming solar energy. Clear glass transmits 80-90% of solar radiation, but absorbs a higher percentage of infrared radiation, trapping heat while still transmitting visible light. Unlike conduction and convection, which have linear relationships, the relationship between temperature differential and radiation heat transfer rate is exponential
• Thermal capacitance: Thermal capacitance is the ability of a material to store heat. Thermal mass is a term that refers to materials that are good at storing heat, either through temperature or phase changes. Water, masonry materials, and salts are typically used.

Passive Heating

• Solar Chimneys
  • Have been used since Roman times and are still common in the Middle East.
  • This needs technical detail
• Trombe wall
  • Consists of a window and a thermal mass with an air pocket between them. When it is warm outside, air circulates in that air channel and the thermal mass takes some of that heat and eventually releases it into the house when it gets cold inside.
• Sun Spaces
  • Passive solar buildings typically do not have a large glass area that hits directly into the living space, they will use a sun space that has a large amount of glass and is properly shaded and that has another glass barrier between it and the living space, to create an intermediary temperature zone to lower the heat transfer by having two small temperature differentials as opposed to one large one.
  • These zones also create a \"thermal lock\" if used in an entryway, providing additional comfort by having an intermediary temperature zone and also preventing direct heat transfer between the interior and exterior every time a door is opened.
• Passive Solar Hot Water
  • Probably the oldest commercial solar systems, dating back to the start of the 20th century. On its most basic level, it consists of a water tank with glass on the side facing south on the roof or south side of a building. They usually use about a 40 gallon tank, which is glass lined and painted black with high temperature engine or barbecue paint. The box containing the tank is well insulated, using high R-value materials on all sides except the south side, which should contain 1 square foot of glass for every 2 to 2 1/2 gallons of water in the tank. The glass should also be of a high R-value. The plumbing runs should be as short as possible and well insulated to reduce loses and reduce the risk of freezing in the winter. The only downside to one of these systems is that if it is truly passive, there are no controls or backups, so you never know how hot your water will be and it may be cooler in the morning if the system isn't well insulated or you use more hot water than you produced in the day. Sometimes a water heater blanket with a thermostat is used as a backup. Plumbing diagram on (99)

Passive Cooling

• Reflective Structures
• Shading Structures
  • Overhangs and eaves can be designed to shade windows and doors in the summertime, but not in the winter Insert formula from 401 book
  • Awnings, shade screens, trellises, or climbing plants can all be affixed to buildings to lower the amount of sun entering the building, especially during the summer.
  • West-facing windows can also be shaded with vertical blinds or any of the above to prevent the low west sun from penetrating deep into west-facing rooms.
• Convective Cooling
  • The oldest, most straightforward convective cooling method is to use cool night air to drive out the warm air. If it is a breezy area, putting low vents on the side of the house the wind usually comes from and high vents on the opposite side will allow the cool night air to enter the house and force out warmer air. To get the best cooling rates, the exit openings should be 50 to 100% larger than the windward openings. If there are not strong winds at the site, low vents around the house, especially on the north side, and high vents near the peak of the house will allow for convective cooling as well. The higher the height difference, the faster the flow of air and the more cooling that will occur.
• Cooling Towers
• Evaporative Cooling
  • Evaporative cooling is the process used to cool most mammals through sweating. This can be applied in a home by passing incoming air through a moist filter or pad to raise the humidity of the air in the home while lowering the temperature.
  • In some extremely hot climates, where nighttime temperatures are still very hot, roof sprinklers can be used to send water trickling down the roof, which is then evaporated very efficiently because of the large surface area it occurs over, removing a large amount of heat from the top of the structure.
• Earth Tubes
  • Drawing outdoor air through a series of tubes buried underground and then dumped into the house. The tubes are made of a material that allows easy thermal transfer to take advantage of the room-temperature temperatures a few feet underground. The concern with this system is removing the stored thermal energy gain in the ground around the pipes by surface landscaping and watering. These tubes can supply cool air during peak load times even in the hottest climates.
• Four different strategies that may be used depending on the cite num=98>bioclimactic chart shown at http://oikos.com/esb/51/passivecooling.html (98) (even though some of these require fans and are not totally passive, they are still much better than conventional homes):
  • Natural ventilation: Using window openings and tall spaces called stacks with openings near the top and openings at the bottom of a building. This requires the building to be open during the day.
  • High thermal mass: These buildings would be kept tighter, but would have large amounts of thermal mass, up to 3 square feet of thermal mass exposed to the sun for each square foot of building area
  • High thermal mass with night ventilation: This is similar to above, but requires night ventilation to cool the mass at night, so the building is closed during the day, but open at night.
  • Evaporative cooling: This uses roof ponds or water reservoirs in the building in dryer climates to reduce the indoor air temperature by evaporating water.

A bit of both (Temperature stabilization)

• Green Roofs
  • Green roofs involve installing plants on the roof of a home to recycle waste water and to reduce temperature fluctuations by increasing the insulation value of the roof. A roof surface temperature can be reduced by 70 degrees in the summer by a green roof. A green roof with an air gap and radiant barrier below it can block 97% of the downward radiation from the sun.
• Roof Ponds
  • Developed by Harold Hay in the 1960s. They consist of a roof mounted water bladder with a movable insulating cover. The cover allows for control over heat exchange between the interior and exterior of the building. The bladder can absorb heat during the day from either the building or outside, depending on whether it is covered or not, and then the heat can be released at night.
• Thermal Mass
  • HVAC can be reduced through thermal mass, water, concrete, or brick often used, store heat during the day, release at night, levelizes temperature
• Insulation
  • The insulation of windows and doors is an important part of heat transfer rates. A solid door with no windows conducts heat about twelve times faster than a foam-filled Energy Star door. The older a door or window, the more likely it is to have leaks that allow outside air to filter into the building, causing HVAC systems to work much harder.
• Your home's landscaping can also be incorporated into your passive solar design by using foliage for shading during summer months, but still allowing sun in in the winter time.