An evacuated tube solar collector is composed of hollow glass tubes. The air between the tubes is pumped out, while the outside of the tubes are heated, creating a vacuum. This mechanism creates excellent insulation, trapping the heat inside the tube, making solar hot water evacuated tubes highly efficient.
An absorber coating inside the tube absorbs the solar radiation. Evacuated tube solar collectors heat the transfer fluid (often simply water) moving through the collector and then to the hot water storage tank. In cooler climates a heat exchanger is used to separate the potable water from the non-toxic anti-freeze in the collector.
Generally, evacuated tube collectors are used to heat greater volumes of water, or to heat water to higher temperatures, as is often the case in commercial applications. These types of collectors can also be used for steam production and solar space heating.
Efficiency of Evacuated Tube Collectors
The vacuum is a very effective insulator that keeps the water inside the collector hot while the actual tubes are only a few degrees warmer than ambient air temperature. This low heat loss makes these the most efficient of all solar thermal collectors.
An evacuated tube collector can work well in cloudy conditions and in temperatures as low as -40°F. This characteristic is most advantageous in colder climates, or during the winter, where other solar collectors may not produce as much energy. The cylindrical shape also allows the collector to catch ground reflected radiation, known as the “passive tracking effect,” when facing true south.
Nevertheless, a less efficient collector (such as a flat plate collector) is often sufficient for a residential hot water heating system in warmer climates, as the added solar radiation makes up for any efficiency loss. But the higher efficiency makes an evacuated tube system preferable when collector space is limited.
Cost and Durability of Evacuated Tube Collectors
The cost of evacuated tube collectors can be significantly more than flat plate collectors – though the additional up-front cost may be made up for in utility bill savings. Because flat plate collectors are usually more cost-effective for residential solar water heating systems, solar hot water evacuated tube collectors are more commonly used for commercial applications.
Evacuated tube solar collectors have been around since the 1980’s and most have a similar warranty to flat plate collectors at around 20 years. But these types of collectors tend to need more repairs for the following reasons:
- the thin glass (less than 2 millimeters thick) is more susceptible to breaking, and
- the reliance on a vacuum seal means that over time, the seal can be lost
Despite these issues, the tubes can be replaced individually without having to replace the entire collector.
Maintenance of evacuated tube collectors is slightly more complicated than for other types of collectors. It is true that an evacuated tube solar water heater can be easily cleaned by falling rain water. But snow tends to melt more slowly off these collectors, and due to the fragility of the glass, it cannot be removed with a snow rake. Thus the practicality of cleaning the solar collector can vary depending on climate.