Homeowners are discussing with their architect the possibility of using either active or passive solar design to reduce their heating and/or cooling costs. Compare these two techniques.

Homeowners are discussing with their architect the possibility of using either active or passive solar design to reduce their heating and/or cooling costs. Compare these two techniques.

Restatement: Active versus passive solar design. 

Active Solar System:

• Collectors that collect and absorb solar radiation. Optimum collector orienta¬tion is true south (the highest apparent point in the sky that the sun reaches during the day-not necessarily magnetic south). Collector orientation may deviate up to 20° from true south without significantly reducing the perform¬ance of the system. Collectors should be tilted at an angle equal to latitude plus 15° for optimum performance. A collector receives the most solar radiation between 9:00 A.M. and 3:00 P.M. Trees, buildings, hills, or other obstructions that shade collectors reduce their ability to collect solar radiation. Even partial shading will reduce heat output. 

• Electric fans or pumps to transfer and distribute the solar heat in a fluid (liquid or air) from the collectors. 

• Storage system to provide heat when the sun is not shining. 

• May be more Bexible than a passive system in terms of location and installation. 

• Usually most economical to design active system to provide 40 - 80% of the home's heating needs. 

• Liquid systems heat water or an antifreeze solution in a hydronic collector, whereas air systems heat air in an air collector. 

• Liquid solar collectors are most appropriate for central heating. They are the same as those used in solar domestic water heating systems. Flat-plate collectors are the most common, but evacuated tube and concentrating collectors are also available. In the collector, a heat transfer or working Buid such as water, antifreeze (usually nontoxic propylene glycol), or other type ofliquid absorbs the solar heat. At the appropriate time, a controller operates a circulating pump to move the Buid through the collector. The liquid Bows rapidly through the collec¬tors, so its temperature increases only 10-20°F (5.6-11 °C) as it moves through the collector. The liquid Bows to either a storage tank or a heat exchanger for immediate use. Other system components include piping, pumps, valves, an expansion tank, a heat exchanger, a storage tank, and controls. 

• Air collectors produce heat earlier and later in the day than liquid systems. Therefore, air systems may produce more usable energy over a heating season than a liquid system of the same size. Also, unlike liquid systems, air systems do not freeze, and minor leaks in the collector or distribution ducts will not cause problems. Air collectors can be installed on a roof or an exterior (south-facing) wall for heating one or more rooms. These systems are easier and less expensive to install than a central heating system. They do not have a dedicated storage system or extensive ductwork. The Boors, walls, and furniture will absorb some of the solar heat, which will help keep the room warm for a few hours after sunset. Masonry walls and tile Boors will provide more thermal mass and thus provide heat for longer periods. A well-insulated house will make a solar room air heater more effective. Factory-built collectors and do-it-yourself for on-site installation are available. The collector has an airtight and insulated metal or wood frame and a black metal plate for absorbing heat with glazing in front of it. Solar radiation heats the plate that, in turn, heats the air in the collector. An electrically powered fan or blower pulls air from the room through the collector and blows it into the room(s). Roof-mounted collectors require ducts for supply¬ing air from the room(s) to the collector and for distribution of the warm air into the room(s). Wall-mounted collectors are placed directly on a south-facing wall. Holes are cut through the wall for the collector air inlet and outlets. Simple win¬dow box collectors fit in an existing window opening. They can be active (using a fan) or passive. A bafRe or damper keeps the room air from Bowing back into the panel (reverse thermosiphoning) when the sun is not shining. These systems pro¬vide only a small amount of heat, since the collector area is relatively small. 

•Local covenants may restrict options. For example, homeowner associations may not allow installation of solar collectors on certain parts of the house. 

Passive Solar Design

•Basic idea of passive solar design is to allow daylight, heat, and airflow into a building only when beneficial. The objectives are to control the entrance of sunlight and air Bows into the building at appropriate times and to store and distribute the heat and cool air so it is available when needed. 

•Four basic approaches to passive systems: First, direct gain-solar energy is transmitted through south-facing glazing. Works best when the south window area is double-glazed and the building has considerable thermal mass in the form of concrete Boors and masonry walls insulated on the outside. Second, indirect gain-a storage mass collects and stores heat directly from the sun and then transfers heat to the living space. The sun's rays do not travel through the occupied space to reach the storage mass. Third, isolated gain-passive solar concept, solar collection and storage are thermally isolated from the occupied areas of the building. This allows the collector and storage to function inde¬pendently of the building. Fourth, remote collection-includes a collector space, which intercedes between the direct sun and the living space and is dis¬tinct from the building structure. 

•Collectors usually receive the most sunlight when placed onto the roof. South-¬facing walls may also work. 

•Passive system does not use a mechanical device to distribute solar heat from a collector. 

•Example of a passive system for space heating is a sunspace or solar greenhouse on the south side of the house. 

•Passive system is simpler in design and less expensive to build. 

•May not be possible depending upon location of site. For example, building an effective sunspace may not be possible due to trees, other buildings in the way, etc. 

•Local climate, the type and efficiency of the collector(s), and the collector area determine how much heat a solar heating system can provide. 

•Most building codes and mortgage lenders require a backup heating system. Supplementary or backup systems supply heat when the solar system cannot meet heating requirements. They range from a wood stove to a conventional central heating system. 

•Passive solar buildings use 47% less energy than conventional new buildings and 60% less than comparable older buildings.