Inexpensive Home Building

Cut through the jargon and nonsense of home building and house construction by starting from zero dollars and trying to figure best-value bang-for-your-buck when choosing construction methods or construction materials. My research might answer some of your questions and at other times perhaps you have the knowledge or experience to post the answers to my questions and thereby help others too. The goal is an affordable and sustainable home for all.

Wednesday, May 09, 2007

Passive Cooling Solar Cooling: Use Heat To Cool

Previous: "Best Ways To Cool Your Home."

The previous article covered how the best "cooling" is not to heat in the first place (avoiding heat, so actual cooling is unecessary). This article now covers how to remove any heat that does occur, using as many passive cooling techniques as possible.

Solar heat for home heating is fine but you tend to want more of it when there is less of it (in winter). The advantage of solar cooling or heat-powered cooling is, in a way, the hotter it gets, the cooler it gets.

Yes, you can use heat to cool by taking advantages of natural forces to passively pump air and transfer heat.

A solar chimney is a thermal chimney that would work if heated at night--but the Sun is the usual heat source. Even wind power is solar power insofar as the Sun drives the wind by creating temperature and pressure differences. Even evaporative cooling in the shade is solar power insofar as the Sun creates the ambient heat.

Passive Cooling Techniques (many overlap):

  • Solar chimney / thermal chimney / stack effect / convection vertical ventilation / heat-powered pump: A heated chimney or vent stack collects and ejects heat from the home, which creates a vacuum suction that draws cooler replacement air in from a lower vent. Any top vent (especially on a domed or peaked ceiling) or at least a high open window (transom, clerestory) combined with a shaded low vent (e.g. in door bottom out of sunlight) will use convection.
  • Wind chimney / windcatcher / wind tower / Persian bagdir / wind-powered pump: Whereas the solar chimney can create its own wind by forcing a convection cycle, the wind chimney/catcher does not create its own wind but does work at night. The windcatcher uses the Coanda effect (boundary-layer attachment—e.g. your finger diverts faucet water) or Venturi effect to siphon air from the home. You can open/close apertures according to wind direction to cause a downdraft if desired.
  • Venturi tube / Bernoulli principle / cross ventilation: This is another example of using constrictions to create pressure differentials or vacuum effects to suck air. I list this separately from the last entry but think of applications akin to a horizontal wind chimney. Any cross ventilation by opening opposing windows can be good.
  • Evaporative cooler / swamp cooler / desert cooler / passive down-draft cooltower / spring house / Persian yakhchal: Water absorbs heat to evaporate, which is why you sweat to cool yourself. Air traveling over water cools. Use in low-humidity conditions and guard against health hazards from standing water (mosquitoes, mildew, Legionnaire’s Disease, etc.).
  • Geothermal cooler / Earth cooler: Below the frost line can be 20-50 degrees cooler than surface summer air (the principle behind the root cellar, fruit cellar, and Persian yakhchal). Guard against standing water dangers when humid air in pipes drops below the dew-point temperature and condenses water.
  • Solar ice-maker / solar refrigerator / solar air-conditioner: This variation on evaporative cooling often uses fancy plumbing for the Sun to power a closed condensation-expansion cycle similar to that of a conventional electric refrigerator.
  • Lunar refrigerator / moon cooler / night radiant refrigerator: Use solar-oven principles in reverse to shed heat toward the blackest night sky (do not actually point at the Moon).
  • Passive Annual Heat Storage (PAHS) / Annualized Geo Solar (AGS) / Trombe wall / Morse wall / thermal mass / thermal battery / thermal well / thermal sump: Whereas a PV solar panel with battery diverts and stores light for later use, and a rainwater catchment with cistern diverts and stores water for later use, a thermal battery/well diverts and stores heat for later use. Trombe/Morse walls usually are thick adobe/masonry walls or water containers that absorb daytime heat and then release heat at night to moderate (average out) the daily internal air temperature (use low thermal mass in areas with little day-night temperature change). Larger masses can moderate variations over days or longer. PAHS/AGS are much larger versions of the Trombe wall to moderate temperature over the entire year. PAHS/AGS usually specify a large, engineered, subterranean area and might take a few years of supplemental heat to "charge" the system. Thermal "storage" is not actually static but controls the direction and speed of heat by selection of materials and fluid/air flow.
You might augment these passive cooling methods with minimal active interventions such as circulation fans, or water pumps, or opening/closing vents/windows by hand or with automatic sensors.

Use these methods in any combination with radiators (high surface area e.g. fins), heat exchangers (high conductance), and thermal masses (high heat capacity) to pump, transfer, and store heat as needed.

Tuesday, May 01, 2007

"Cool Roof" High Albedo Reflects Sunlight

Part of: "Best Ways To Cool Your Home."

A "cool roof" combines high solar-light reflectance (albedo) with high thermal emmittance. The US Environmental Protection Agency (EPA) estimates the following savings in money ($ dollars) and electricity (kWh): The EPA labeled these figures as "annual" savings although that seems low compared to its other statement that a cool roof can save 20-70% of annual cooling consumption based on "over 10 buildings in California and Florida":