Thursday, September 11, 2008
How it works
So the installers buttoned up all the final issues and I'm left with my solar water heater.
First note the black cables coming down from the roof into the black box. Those are the glycol lines transferring heat from the panels into an heat exchanger inside the monster 120 gal water tank. The black box they run through is control computer and glycol pumps. The bright cyan tank is the glycol overflow for when the glycol expands when it becomes very hot. There's a clear container on the floor; this is the last ditch glycol expansion container if for some reason the cyan tank fills up and the pressure in the line goes above 6 bar. Since we installed the system, it has got to 110 degree at my house and system pressure got to 5.5 bar (my panels were 375F). still, no overflow.
The pipes onto of the tank are the hot and cold water. note the cross connect between the pipes; there's a valve on the hot line which only allows 120 degree F water out of the system. the valve just mixes in cold to always output 120 F water.
the tank is factory set to 140F, which means that the tank will heat the water inside to 140F . once the water is 140 (which in the summer happens around 11am) the pumps stop pumping the glycol through (raising the bar pressure as the panels continue to heat).
From what I can tell, if you want hot water when the tank is colder then 120, then the electric backup kicks in and heats the water from whatever temp it is at to 120. You don't need a whole new electrical service like you normally would for an electric tankless system because you're heating water which is already hot and not from the cold tap. That said, I'm sure that the whole thing would fall apart if I were to use both showers and wash some close in hot water; i.e. the amount of flow out you could achieve while keeping the water hot is probably limited since you only have 20 amps to play with.
I should also note, that the coldest I've seen the water at is 89F the day after we got the system and we hit a cold snap. the water in the tank didn't have a chance to heat up until the next day, which was also overcast but good enough to get the tank to 120. even when the tank was at 89F, we still have hot water at 120F.
When I got my first PGE utility bill I was ready to see a significant savings; but that's not what happened. Instead, I ended up paying more for additional electricity used by the system then the natural gas the system was not using; what happened...... I'll explain this in my next post.