Can Evacated Tubes Heat a Storage Tank?

[hobbyheater]

Does someone have experience that they would share about using evacuated tubes to heat a storage tank? The tank is 1000 gallons in volume, unpressurized, with internal heat exchangers for heat and demand hot water. The tank is extremely well insulated. The operating temperature range of the tank is 120 -170 F summer and 140-195 F winter. Winter heat loss is around 12000 btus per hour on average. The summer months largely supply domestic hot water and in the winter months, the temps have to be higher for efficient baseboard operation. I would like to be able to heat the tank over the shoulder months with solar and depend more on wood through the winter. On the wood side in the summer months, we go up to 10 days between fires and it requires a prolonged warm up period for the Jetstream boiler to prevent damage to its refractory base. Can the tubes provide the water at these higher temperatures ?
We know of a couple that live on a island not too far from here and they have a 20 tube open system and only 4 days last year they did not get a shower, though they said some were pretty cool!
We live on Northern Vancouver Island, BC with mild temperatures but lots of rain.

Allan

 

[GaryGary]

Hi,
Those are high temperatures for solar to maintain.
As the tank temperature goes up, the efficiency of solar collectors go down. This is just because the absorber has to run hotter and more heat is lost out the tubes or glazing.

You can use this calculator to see the effect of tank temperature on efficiency:
http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm

For example:
Using the Seido 16 evac tube collector, and assuming a 40F outdoor temperature, and full sun,

With a tank temperature of 100F, the collector efficiency is 47.2%

With a tank temperature of 190F, the collector efficiency is 38.8%

So, the collector would put out 22% more heat at 100F (which is a good space heating temperature) than it does at 190F.

The equivalent efficiencies for a Heliodyne selectively coated absorber flat plate would be 58.5% at 100F and 35.6% at 190F. That's not a typo, the flat plate is a good deal more efficient at lower temps.
While generally, I like flat plate collectors over evacs, this might be a case where the evac tube would be better because their efficiency drops off more slowly with temperature.

All this data is derived from the SRCC site, which tests collectors and is a good source of information. One thing to just mention in this regard is that the efficiency of evac tube collectors varies widely from brand to brand -- so, it pays to do some comparisons.

It would really help the solar efficiency if you could find an alternative that would work with lower temperatures.

The situation would, of course, be better in the summer -- a properly sized collector(s) could probably provide nearly all your demand through summer, and probably into fall.


Gary