Solar Resource

Sunlight is the fuel for all solar technologies, and the term ‘solar resource’ refers to identifying how much of it is available to a given collector area.  The most rudimentary analysis requires standing on your site with your arms up in a “Y” form and looking to see what obstructions there are in the field of vision between your arms. This basically represents the productive sun hours of 9am to 3pm.

There are professional devices, such as the Solar Pathfinder and Solmetric SunEye, that calculate the Total Solar Resource Fraction (TSRF) which accounts for shading, panel tilt and azimuth (direction relative to south).

Systems applying for Energy Trust Incentives must have 75% TSRF, meaning the solar array will have at least 75% of the sunlight available to a completely unshaded and perfectly oriented array on the same site.  iphone Apps such as Sun Tracker and Sun Seeker will also provide a solar assessment.

The tilt of the earth on its axis and our location north of the equator result in a specific sun path that changes throughout the year.  The sun is at its highest point at noon on the Summer Solstice, or June 21st.

Most photovoltaic and solar hot water systems are stationary, and having unobstructed solar access between the hours of 9AM and 3PM is ideal. Tracking systems usually follow the sun path from east to west during the day, and are seasonally adjusted for altitude angle.

Types of Collectors

Solar water heaters fall into 3 basic categories. Thermosyphon systems take advantage of the fact that warm water rises. These systems have so pumps and are mechanically the simplest of the three types. The water storage tank is above the collector coils, so if the system is on the roof it is essential to check whether your roof structure is adequate. Because the fluid being circulated is water, precautions must be made to prevent freezing or overheating. These systems work well in mild climates.

The second basic category of collector types is the drainback. Water is pumped up through the collector when there is enough sunlight to heat the coils. When conditions are not right for heating water, the pumps turn off and the water drains back to a reservoir, this takes care of freezing and overheating. A variation of this type uses heat transfer fluid that drains back to a reservoir on the back of the flat plate collector. The proximity of this reservoir to the coils reduces the amount of energy that the pump consumes.

The third type of collector places the dark coils inside evacuated tubes. These systems also rely on pumps to move the fluid. The advantage is reduced heat loss from the collector to the surrounding air. This would be important in a clear shy climate with cold daytime temperatures where solar heated water is being used for space heating. Evacuated tube systems are generally more expensive that drainback flat plate collectors.

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