1: Passive Solar Cooling | 2: Natural Ventilation | 3: Air Cooling
Passive Solar Cooling 1: Introduction
Is entirely passive solar cooling possible? Probably.
Is it feasible in a climate like Alice Springs? Probably not, for now at least.
Can passive solar design reduce energy consumption for cooling? Definitely!
The cooling and "thermal management" of buildings in hot arid climates involves some combination of five basic elements: thermal mass, insulation, shading, ventilation and evaporation.
While the two are sometimes confused, thermal mass and insulation are two quite different properties of building materials. Insulation is about blocking or preventing the transfer of heat altogether, while thermal mass is about absorbing and storing heat and delaying its transfer. Thermal mass is not insulation, and at some point during the day, uninsulated exterior walls start radiating the heat they have absorbed into the interior of the building. Ideally, the thermal mass would delay this point to around sunset, when doors and windows can be opened up for ventilation of interior heat to the exterior.
Effective natural ventilation at night is a key summer cooling strategy. Good crossflow ventilation through correctly sized and shaped rooms, doors and windows at night means that internal temperatures can be reduced throughout the night to a temperature close to the exterior temperature at dawn. For much of summer period (Oct-Mar), this temperature is in the low 20's or below. The basic strategy is to open all doors and windows at sunset, ventilate the house all night, and then close all doors and windows again at sunrise. The effectiveness of this strategy is enhanced by insulation (to keep exterior heat out longer), shading (to keep direct sunlight off walls and windows), and by design for good ventilation (rooms, doors and windows sized and located to maximise crossflow). In many hot arid buildings, occupants sleep outside at night, which is another form of natural ventilation.
This natural diurnal heating/cooling cycle breaks down when interior spaces heat up to uncomfortable levels during the day, when external temperatures remain higher than internal temperatures. Until the external temperature begins to drop in the late afternoon, cooling the building by ventilation alone is impossible. Where ventilation exists, however, it is possible to cool air easily and effectively by simply evaporating water into it. For every litre (ie, kilogram) of water evaporated into a mass of moving air, approximately 2.275 megajoules of heat is removed from the air in the process. This basic fact of nature underlies the modern evaporative cooler, and also a long tradition of effective "architectural" solutions to passive evaporative cooling system. Evaporative cooling is particularly effective in hot arid climates. The importance of evaporative cooling in the diurnal thermal cycle is that it allows interior spaces to be cooled effectively in the heat of the day.
In Alice Springs today, the primary means of ventilating (and cooling) buildings is the evaporative air-conditioner.
Natural ventilation refers to the movement of air into and out of buildings (via vents, ducts, towers, chimneys and infiltration), driven by some combination of the forces of buoyancy and air pressure. Buoyancy results from temperature and humidity differences between air masses, and may be either positive or negative. Hot dry air is lighter and tends naturally to rise, creating an up-draught, while cool moist air is heavier and tends to sink, creating a down-draught. Air pressure changes result from the effects of both buoyancy and the wind, the wind being the stronger force by far. The movement of the wind creates both positive and negative pressures, with positive pressures being found on the windward side of objects and negative pressures being found on the leeward side.
Considering the direction of airflow, there are two basic types of natural ventilation device: air entry and air exit devices. Air entry devices allow air into a building, air exit devices allow air out. In their most basic form, natural ventilation devices are simple vents allowing the free passage of air into and out of a building. For effective passive cooling, however, natural ventilation systems use of a variety of methods to improve the effectiveness of simple vents.
Natural ventilation is driven by differences in air pressure (wind) and/or buoyancy, and is improved by increasing the speed and volume of airflow achieved. , which is a function of good aerodynamic design and the strength of the natural forces involved. Good aerodynamic design can improve all forms of natural cooling, and involves factors including the correct sizing and placement of vents, ducts, doors, walls and windows, and the appropriate use of architectural features such as stairwells and ceilings. The strength of the buoyancy force is dependent on factors including the height of a structure and the degree of temperature and humidity difference between the air masses involved. The strength of air pressure forces are dependent primarily on wind speed (though buoyancy and active fans may also contribute in some cases).
Next: Natural Ventilation