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The Resilience Hub & Portland Maine Permaculture Message Board › SOLAR THERMAL SYSTEM TECHNOLOGY & DESIGN CONSIDERATIONS


A former member
Post #: 101
I am currently taking EFFICIENCY MAINE's certificate course for Solar Thermal designers/installers.

They've compressed a full week's course into two days.

If some of you would like your questions answered, it would help me keep the large amount of material fresh in my mind while I take the certificate test.

This includes 'what kind of system is best for my home/business, etc.' 'component evaluation and selection' , system do's and don'ts, system safety, vac. tubes vs. flat panel decisions, generating enough hot water for radiant heating systems, etc.

Post your questions on here, so others can learn.

user 5846522
Portland, ME
Post #: 97
Frank, what specifics regarding a site do you need about my (or any) location to start a discussion in this regard?
user 5846522
Portland, ME
Post #: 98
Do you need a geologic survey?
A former member
Post #: 102

You would need a basic solar orientation for your home...which includes those aspects--roofs, adjacent lawns which get the greatest amount of sunlight, and available surface area.

Ideally, you should have a true south- not magnetic south, facing roof with a 49 degree pitch + 11-15 degrees for full winter sun orientation; and enough surface area and support strength for two or three, 4x8 flat panels.

Your attic should be able to support and enable a two pipe run to and from the panels down to the present hot water heating system.

You should have enough room in the basement to hold an air expansion tank, circulation pump, control panel and a solar storage and exchange tank--the panels and pipes are filled with a 50-50 or less mix of distilled water and glycol which is pumped from the outflow of the panels to a coil in the bottom of the storage tank. This coil can either heat the potable water that is piped at the top of the storage tank to the water heater---your conventional heater or an on-demand gas heater, or in a dual coil system heat a transfer medium, which heats a coil at the top of the storage tank.

There are a number of variations in design necessitated by the layout of your current system, i.e. attaching an external heat exchanger to your present hot water heater; using a thermosyphon or water drainage system; using double walled vac. tubes to save space on the roof; using a flush or tilt wall mount against the side of the house, building a pergola and using the vac. tube array for the top, and using different sized panels for architectual harmony.

That's just hot water.

When you get into radiant heating systems, the overall size and complexity increases.

You may need to support and install 8-15 panels; plus the piping runs; plus the pump(s) & controllers, Plus larger storage & transfer tanks, plus heat load sheds or some other way of getting rid of the excess heat in the late spring...summer...early fall i.e. use a flush wall mount that receives full sun in winter and in summer when the sun is higher in the sky, is shaded by a roof overhang or tree branches.

The other half of the puzzle is matching BTU output over an annual cycle at your best location to your BTU use over an annual cycle. Size the system for two adults and your kids come home for the Holidays and bring their kids, you will need to supplement the solar system .

So you will need to determine how many BTU's you use over a year to heat hot water, electric bills, etc. We tend to estimate use, and provide excess capacity; but remember too much hot water is a problem in the summer when overheated panels can be tubs and pools are great ways to dump that excess hot water.

Start with your BTU use over an annual cycle and then locate the sunniest place to safely put panels.. Start from there.

Geologic survey? usually GOOGLE earth gives me some idea of tree shading; but for external mounts, piping is run in trenches and that may necessitate some kind of soil analysis...tend to use 4" pvc with two insulated pipes inside, but freezing and frost heaves are a concern.....however, there are few lawn mounts; nearly all are on roofs or garages.
user 5846522
Portland, ME
Post #: 99
frank thank you for your response.

i learned about some details and/or concerns i had not previously thought of. i did misread your initial post i saw geo-thermal when it was solar thermal hence my question. again thanks for the info.
Lisa F.
Group Organizer
Portland, ME
Post #: 640
Penny - We're almost done installing our solar hot water system and ended up going with a rack of evacuated tubes on the roof (vs flat plate) plumbed to a locally-made heat exchanger and a super-insulated tank in the basement. Our roof pitch was no where near the "ideal" but with the tubes we could orient the rack exactly how we wanted it - the way the tubes are installed allows wind to sail through and the rack of tubes is not very heavy, either. Will post photos in the next few days. We optimized the angle for the winter sun and oriented it to solar south (about 18 degrees off magnetic).

By the time we get our state and fed incentive$ for the system we should be under $5000 investment for a fairly generously sized system.
A former member
Post #: 103
The decision over whether to go with flat plates or Vac tubes was discussed thoroughly at the training session.

Double walled, vac tubes have some distinct advantages:

  • smaller foot print on the roof; less glycol in the manifold;
  • Easy to replace a broken or leaky tube, homeowner can do it :) the heat rod is tricky;
  • They do capture more light and from different angles reducing the requirement on precisely siting the collectors on a lat.of 44 degrees + or - 11 degrees for summer/winter sun orientation.
  • there is more attention to insulating materials for rods and to safety valves for temp & pressure.

The only down side were snow and ice build ups on the glass tubes since they don't get hot....the solution is to have at least an inch between tubes and to make sure they are mounting near the top of the roof, where there is less likelyhood of snow buildup.

We got lectured on only purchasing components which have been independently tested and E100 certified, and warned about the inefficiency of using an external heat exchanger. Only three or four tanks were recommended.

Basically, an in-tank, bottom coil for the coolant was what was recommended. Coolant leaks into potable water were to be avoided at all costs; and double coil systems were stressed as an alternative, i.e. the bottom coil circulates the hot glycol through a transfer medium like distilled water; which heats a coil of potable water in the top of the tank that goes to the hot water heater.

Designs change radically if you are pre-heating water for a radiant heating system.
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