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What is an Passive Solar Cooling ?

Reduce Internal Heat Gain:
Use lights sparingly, Turn lights off when not in use, Remove light bulbs in areas where they’re not needed to avoid overlighting, Turn water heater temperature town to 120oF, Install water heater insulation blanket, Insulate hot water pipes, Eat more cold meals in the summer, Cook outside, Use the microwave in the summer, Bake at night, Run exhaust fan when cooking, Use the cold or warm water settings on washing machine, Wash clothes at night, Hang clothes on outside line, Dry larger loads, Close off utility room, Open window to utility room when the clothes dryer is in use during summer, Turn computers and other electronic devices off when not in use, Watch TV more sparingly, Unplug TV and stereo when not in use, Plug TV and stereo into power strip and turn off when not in use, Turn off furnace pilot light during the cooling season, Let pets spend more time outside in the summer, Spend more time outdoors on porches and patios, Take shorter showers, Open window when showering, Run exhaust fan when showering, Install an efficient showerhead, Hand wash dishes, Switch off drying option on dishwasher

Reduce External Heat Gain:
Plant shade trees, Build artificial shade structures such as arbors and trellises, Install awnings, Install and use window shades, Seal cracks in building envelope, Upgrade insulation, Replace energy-inefficient windows, Repaint with a lighter color, Replace roof shingles with lighter ones or metal roofing or Spanish tiles, Install radiant barriers


Purge Heat:
Use natural ventilation early and late in cooling season and as much as possible during the height of the cooling season, if your climate permits this, Purge heat at night in dry climates, Install and use window fans, Install attic fan, Install whole house fan, Improve efficiency of air conditioning system (seal ducts, replace dirty filters, shade air conditioner, etc.), Replace inefficient air conditioners with more efficient models or evaporative coolers if you are in a dry climate, Install an air-source heat pump, Use fans, Install and use ceiling fans

If there are only light breezes at the site, natural convection can still be used to ventilate and cool a house as long as the outdoor air is cooler than the indoor air at the peak of the house. Since warm air rises, vents located at high points in the interior will allow warm air to escape while cooler outdoor air flows in through low vents to replace it (Fig. 23). The coolest air around a house is usually found on the north side, especially if this area is well shaded by trees or shrubs and has water features. Cool air intake vents are best located as low as possible on the north side. The greater the height difference between the low and high vents, the faster the flow of natural convection and the more heat mitigation can occur.

Another convective cooling strategy is the drawing of outdoor air is drawn through tubes buried in the ground and dumped into the house. Made of material that allows easy thermal transfer, these tubes are buried several feet deep to avoid the warmer daytime surface temperatures. Warm outdoor air entering the tube gives up its heat to the cooler earth, and cools substantially before entering the house . Thermal saturation of the surrounding earth must be addressed, by means of surface landscaping and watering, thereby removing the gained thermal energy from the tube/earth transfers. Though condensation is rarely a problem in dry climates, such tubes should be sloped slightly and have adequate drainage to insure that water build-up doesn't block the passage of air. The intake end should be screened and placed in a shady spot away from foot traffic. When properly built and sized, these underground tubes can supply cool air during the peak load daytime even in the hottest climates.

Heat Gain Control

Many of the principles and techniques of passive solar heating are adaptable to natural cooling. Insulation and weather-stripping that prevent heat loss in the winter will also retard heat gain during summer. Movable insulating shutters for winter nighttime containment of heat gain can also be used to reduce summertime daytime heat gains. Inside the house, thermal mass such as masonry walls and floors, act as "heat sponges", absorbing heat and slowing internal temperature rise on hot days, and can be cooled down by nighttime ventilating (at the beginning and end of the summer season) and by use mechanical cooling during off-peak cost hours (nighttime). Suitably placed near a window, skylight, or vent, the same thermal mass can be exposed to cool night air to release the heat absorbed from the space earlier in the day. Finally, earth integrated buildings, embedded into the ground, benefit from the lower difference between interior and exterior surface temperature.

For optimum summer cooling, a building's surroundings should be designed to minimize summer sunlight striking external surfaces, and to prevent surrounding area heat re-radiation and reflection. Great temperature differentials between desert exterior conditions of 110+ degrees and 78 degrees required for interior comfort can be tempered using "thermal decompression" zones that become increasingly more effective as one nears the building. Mitigation of undesirable summer direct sun and thermal impacts is achieved through use of vegetation i.e. deciduous trees which interrupt the summer sun's direct path, and ground covers which prevent ground reflection as well as keep the earth's surface cooler thereby preventing re-radiation. One moves out of intense direct sun and heat through vegetation that filters sunlight and shades the ground; then through a more densely filtered zone with ground covers; then through a patio area with vegetation, trellises and water features; into a tempered building entry ("thermal lock"); and finally into the building proper. This movement, 110 degrees stepping down in stages to 78 degrees, allows the body to adjust properly, and provides the best means of arriving at a lesser differentiation between the building's perimeter wall interior and exterior surface temperatures. It is this difference, between interior and exterior surface temperature, that exacerbates the amount and rate of heat flow through the material. Glazing should be minimized on the roof and the east and west walls where summer sunlight is most intense.

Intense direct solar impacts from the sun rising in the east are equal to those of the setting west sun. The reason we feel the setting sun impact more is due to the added thermal impact of the earth reradiating the heat it has gained during the day. The summer sun is much higher in the sky and has a negative impact on skylights and roof windows and lead to enormous solar heat gains. They should not be used in hot climates unless they are insulated and/or shaded. Vertical south facing glass (windows, clerestories, etc.) with overhangs or shades, present fewer problems but are still adversely affected by exterior air temperature. A horizontal overhang or an awning above a south window is an inexpensive, effective solution. If it protrudes to half the window height (Fig. 21), such an overhang will shade the window completely from early May to mid-August, yet allow for winter sun access. A trellis with deciduous vines can be used. Another good strategy is the use of deciduous trees that shade the south face and roof during the summer. All these shading methods work equally well with Trombe walls, water walls, greenhouses, and other south-wall passive solar collector strategies.

Building System For Passive Solar Systems

A. Orientation

Good orientation is essential for an energy efficient structure. The effects of bad orientation can be remedied to some extent - but doing so can be expensive. The best orientation is for a building to be longer east-west that it is north-south, with

a major yard to the south, and most of the windows facing south and a few facing north, east, or west. This orientation will maximize solar heating in winter and minimize summer overheating by making best use of the seasonal difference in sun path. This was well understood in Ancient Greece and Rome where houses were solar oriented and even communities were laid out for good solar access. In the later days of Rome legal action could be brought to maintain solar access to keep a home warm in winter.

A structure with less than ideal shape can work well if some glazing is on the south side and the other windows are properly designed.

B. Insulation

The second step in building or remodeling for energy efficiency is reducing unwanted conductive heat loss (or gain). Insulation is the key - not only for the walls and ceiling, but also for the foundation or slab perimeter, windows, doors, and the people inside. Most homes are woefully under-insulated. Typical wall insulation levels are still R-13 to R-19 in many areas of the country, but they should be at minimum R-30 and R-50+ is much better. Straw bale buildings offer R-50 walls at the same cost as conventional buildings with R-19. Ceiling or roof insulation should also be R-50+ in most areas. Double pane windows are a minimum and high performance windows or double pane plus storm windows are usually cost effective. Insulated drapes and shutters are very effective on windows and sky lights.

C. Weatherization

Infiltration losses are as important as conductive losses and careful weatherizing is necessary. This includes both the obvious problems of weather-stripping doors and windows and also the more general problems of caulking and sealing building joints, access holes, and other areas where unwanted infiltration occurs. Infiltration may easily account for half of the heat loss in a well insulated but poorly weatherized house. The infiltration rate on a typical house is around 1.5 air changes/hour, but can go significantly higher if the wind is blowing. With careful attention to detail the air exchange can be reduced to 0.2 air changes/hour. The goal is to have controlled ventilation so the air comes in when and where you want it and is fresh and healthy. A super-insulated solar house will perform so well it is often possible to have several windows slightly open almost all winter. In very cold areas an air-to-air heat exchanger is desirable for ventilation during the coldest periods. The heat exchanger warms the incoming fresh air with warm stale outgoing air.

D. Shading

Shading and solar control is critical to prevent overheating in the summer. The importance of shading is best illustrated with an example. In the Central Valley near Sacramento, California just 55 square feet of west facing glass will add as much as 55,000 BTU to a house on a summer afternoon. A substantial air conditioner will be required to offset this heat gain. Running an air conditioner at this peak period of electrical demand may require almost 2 kw of added generation capacity to be built, which will cost the utility $2,000+. The homeowner also has to pay for the air conditioner and the energy for its operation over the life of the house, which may also exceed $2,000+. This entire cost could be avoided completely by proper orientation of the house, or largely avoided by use of an exterior shade costing less than $100.

Shading or solar control is relatively easy if the house is oriented properly. Overhangs and wing walls can shade the south windows. The other windows can be shaded with awnings, wing walls, wide overhangs, exterior shades, shade screens, arbors or trees.

E. Landscaping

Very substantial savings in energy use for heating or cooling may be realized by altering the environment the house sits in. Landscaping can provide shading in the summer and wind protection in the winter. Landscaping can also channel cooling breezes in summer. The most important areas for control are the east and west windows and walls.

Glossary About Passive Solar Systems

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