As the only 2-axis commercial scale solar thermal concentrator on the market, Solarbeam is the only solar thermal collector designed to produce solar hot water at high temperatures (above 55°C) in a wide range of climates, all day, all year around.
Why flat plate systems can’t compete in high temperature production
Flat plate panels are simply not designed for this kind of performance. This is especially true during cold days, and the fact that they typically keep working during cloudy periods is no help for high temperature applications.
The SRCC rates flat plate and tube collector efficiency based on two factors. One is the temperature difference between outdoors and the water entering the collector. As the difference grows, the efficiency of flat plate’s drops quickly. The other factor is power of the sun or irradiance. These two factors are considered together. When the temperature difference is large, a drop below full sun reduces efficiency even more dramatically.
For example, a typical panel that is 70% efficient for ambient temperature 20°C and collector inlet temperature 20°C, and full sun (800 W/m2), is:
· Only 35% efficient for ambient temperature 0°C, collector inlet temperature 60°C, and full sun (800 W/m2). (for full sun, this holds for any scenario where outdoor to inlet temperature difference is 60°C)
· Only 20% efficient for the same temperatures, if irradiance is 600 W/m2, a light level typically hitting the none-tracking panels outside of a few hours a day, even on sunny days.
· 0% efficient (not capable of producing) in cloudy conditions, irradiance of 400 W/m2, for the same temperature difference, or at full sun, if the ambient temperature is -20C.
Solarbeam produces solar hot water during full sun, which as the above demonstrates is necessary for producing hotter water with any system. Because the collector area is so small (10”x10”), and the sun concentration level is so high (2000 times), Solarbeam is 82% efficient regardless of the ambient or inlet water temperature, even if it is -20°C outside and the collector inlet temperature is 80°C.
Solarbeam efficiency is also not noticeably affected by increased wind speeds (above 3.5 m/s), which even further reduce flat plate performance.
Why evacuated tube systems can’t compete for high temperature production
While SRCC test results show evacuated tube collectors drop in efficiency more slowly than flat plates as temperature differences increase and irradiance drops, there are a number of studies that show this performance often fails to translate to the real world. In winter conditions evacuated tubes have been shown to hold snow and frost late into the day because they are so well insulated (Lane, 2004, Solar Hot Water Systems). This drastically reduces performance. Snow has been shown to stay on tubes for days after a storm, and most tube manufacturers prohibit simple cleaning methods like using brooms to clean tube off. Freeze thaw cycles also pose a danger to the tubes integrity, as do overheating problems.
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