plan for DIY solar thermal domsestic HW pre-heater

With domestic hot water energy averaging about 5kWh/day (measured), this is about 25% of our winter energy total, and about 60% summer energy total. Time to build a solar-thermal collector for domestic-water pre-heat. Total budget ~ $1k

Existing system plans out there are poor. Mostly too complicated, too toxic & too expensive.

Here is the my plan I'll post here for criticism. Maybe it's bad too. Please see comments at the bottom of this posting.



This plan uses no antifreeze of any type, and relies on drain-back when the pump is off. The parabolic mirror collector is oriented vertically because we live at a latitude of 50 degN, and get lots of snow and frost in the winter. Vertical works best & stays clean in our climate.

- collector area : 1.5 m2 (1400W maximum instantaneous power)
- the collector will mount high on the SW corner of the house, and thus be capable of sun-tracking from 9:00 AM till sunset (10:30 pm in summer)
- the heat-exchange tank will sit on a wood stand in the basement. The tank will be fully insulated so it does not radiate heat to the cool basement air in summer (valuable)
- the mirror will be made from sona-tube, coated with epoxy resin. mirrored with adhesive reflective film.
- Parabolic mirror type collector is easy to rotate for tracking, and is supposed to harvest most effectively in cold/sunny temperatures. That's us!
- the collector pipe at the parabolic focus will be air-insulated via a 1-1/2" x  8' sealed clear polycarbonate tube. These are inexpensive and available from electrical suppliers.
- all pipe will be 1/2" copper, insulated. The exchanger secondary coil will be 50' of 1/2" soft-copper with separators.
- the drain-back solenoid will be "open on de-energize" type. This will energized closed whenever the pump is running, and open (drain-back) whenever the pump stops.

The controller / energy measure instrument I build (I'm a geek)

acknowledging inspirations for this idea
www.builditsolar.com/Projects/SpaceHeating/SolarShed/solarshed.htm
www.mountainelectric.ca/hotbox
www.greenpowerscience.com/SHOPREFLECTIVEBUY.html
M
Edited by mountain - 1/31/10 at 8:00pm

Thanks for raising some important points Russ.

re water conditioning for the circulating loop
  We live in a limestone area where the soil is predominantly glacial silt. So precipitating out the calcium may be important to minimize buildup in pipes and maximize pump life. I'll research this further. I don't want to buy a water softener, and the primary loop will be closed-loop. No glycol because I want to be able to drain the primary loop water into the town sewer without feeling guilty. I'm also very cheap... a born dirtbag.

Thermal rating of polycarbonate tubes : these things are sold as glass-containment for flourescent light-tubes. They are rated for max +480 DegF, and are UV stable. If they don't work I'll be surprised. www.thefind.com/pets/info-clear-polycarbonate-fluorescent-tube-lamp for web info. These are available cheap at any electrical supplier. If it melts I move to glass. I'll seal the ends, and add a fitting to pull a bit of vacuum in the tube.

My estimates for collector sizing and energy expectations are based on my existing 2 panels, both of which I measure instantaneous power (in kW) and accumulated energy (in kWh). For these 2 panels, maximum power is measured at 750W/m2 (glass) & 800W/m2 (polycarbonate). Theoretical max power is 950W/m2. For the 2.2m2 glass collector, maximum daily energy harvested is 3kWh/m2. The vertical polycarbonate collector yields slightly higher. it's been a cloudy grey winter, and Nov thru Jan, we have averaged daily energy harvest of 0.87 kWh/day/m2
So, with a 16 sq-ft mirror (1.48m2), I expect maximum power of 5kW, and a minimum average energy harvest of 1.3 kWh/day with a fixed mirror. Much more energy (but not power) once I get the tracking working, and also more in the summer. Total 8' height is close to the maximum space I've got.

My combined controller w/energy measure unit is good & getting better. Maybe I'll refine & add it to my product line some day. But I'm busy enough these days with work that actually pays.

I looked at the George Plhaka's site a while back. His costs look about right. He is located in southern Ontario ( Toronto is 42 DegN), whereas I am at 50.1 DegN. Imagine his system if it snows a foot overnight. Imagine a big wind-storm (s***!). We get occasional big-winds and frequent snow. His mirrors must get awfully frosty at night unless they are covered. I like his parabolic support ribs. Maybe I'll buy a set from him.

Propylene glycol price is $105 for a 3.8 litre jug, and I'm awfully cheap
Bad whiskey is cheaper, and you can invite friends over & drink it afterwards.

I wonder what RV winterizing antifreeze is? Probably methyl alcohol.

Fawlty Towers re Manuel
Sybil : "Why do you keep him around?"
Basil : "Because he's cheap... and keen to learn!"