Totally Tubular

You see them everywhere in China, ungainly water tanks jutting above the roof lines of homes and apartment buildings, the mark of a developing country where indoor plumbing has been slow to arrive. Or at least this is what I initially thought.

But solar hot water collectors, or evacuated-tube collectors in particular, are a fabulous solution to rising electricity costs, where hot water is responsible for as much as 30% of a domestic utility bill.

You see this thermosiphon version (above) of ET solar all over China, but particularly around Shanghai and south to Shenzhen along coastal China. There must be hundreds of ET solar hot water factories making these units, serving the large demand within China and now for the export market.

I took a factory tour once of a joint venture company, Sino-British, that made a basic unit for approximately the equivalent of $900. It couldn’t possibly supply enough hot water for an American family of four in a temperate climate like that in Shanghai. But in a clime as hot and with as much insolation as south Florida it probably could.

((For scientific verification of average solar resource by location go to: http://www.nrel.gov/rrdc/pvwatts/))

Flat-plate collectors are more familiar in south Florida for their simplicity and lower cost as a pool heating source. But evacuated-tube collectors, thought to be best for mild climates, are going to eat into market share as they get out into the marketplace, even though they cost more.

How ET technology works is simple: those long tubes are double-glazed glass (tube within a tube), at the very center of which is a black absorber plate and tube which carries a heat-transfer fluid into a manifold at the top. The insulation provided by a vacuum between the two tubes gives ET a greater efficiency curve, meaning that the difference between inlet temperature and ambient temperature causes less heat loss and thus greater efficiency in the ET over the flat-plate.

The ET may be easier to install than a flat-plate, all things being equal, because the tubes can be inserted last. And especially good for south Florida, they can be removed before an advancing hurricane.

The biggest problem I foresee with ET is the residential aesthetics: ET systems are ugly, no less in the U.S. than in China. All the better if the water tank can be burrowed within the envelope of the house, but shiny tubes are still not as pleasing to the eye as the dark sheen of a plate.