Debunking Evacuated Myths
I came across an interesting video the other day while surfing the internet. Here are a few thoughts that came to mind…
This particular type of evacuated tube, sometimes referred to as a heat pipe, appears to be very fragile and likely subject to thermal shock and breakage. The little white tip he inserts into the manifold is the heat exchanger from the heat pipe into the glycol. The glycol is then pumped to the water heater where it then goes through another exchanger into the tank water. That is, there are two heat exchangers between the collectors and the domestic hot water tank which reduces efficiency.
The absorber plate is aluminum wrapped around copper and held together with several clips. Aluminum-Copper absorber/tube combos are subject to galvanic corrosion. The mechanical wrapping with two or three clips is no where near as efficient as full contact brazing or welding.
The absorbers he is using are flat plate style. It actually looks like a single fin tube from a typical flat plate collector. Therefore, the spacing between tubes means the absorbing surface is not continuous, resulting in a lot more space required to get the same active area.
As mentioned in this earlier article of mine, a good flat plate collector is more efficient than an evacuated tube out to about 140F delta T.
Bare pipe at the exit of the manifold will waste a lot of energy right at the highest temp point of the array.
The claim that the evacuated tube collectors are twice as efficient as flat plate is nonsense. Perhaps he is obliquely referring to some very high temp point where this is true, such as a delta T of 220F. Maybe he is referring to one of the 20 yr old flat plate systems he showed in the video.
I found a decent explanation on YouTube of how a heat pipe evacuated tube collector is assembled. Notice in this video that the vacuum is inside a double tube like a thermos bottle. The bottle does not connect to the heat pipe, it only envelops it. This is much more durable than a copper pipe sealed to a single wall glass bottle. The metal to glass seal is always the problem. Very expensive and very fragile.
Note, the acetone boils at 130F, which means it will cut off below 130F. Not good in cloudy weather. A thermos bottle has two glass walls reflecting and scattering light, so its peak efficiency is lower than a single wall bottle sealed to the metal pipe.