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Question DetailsAsked on 4/30/2015

how cost efficient is solar powered radiant heating floors

I'm looking at building a new house. For providing overall electricity, solar power seems kind of expensive (long payback) but if I mix a solar powered water heater and radiant heat concrete floors, I'm wondering if it makes more sense then?

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Storing hot water for heating (on a diurnal or day to night cycle) is one of the more efficient solar heating energy storage systems, because you do not have to have massive and energy-wasting batteries to store the energy, and with well insulated tanks what you put in is about 98-99% recovered as heat, so in that sense it is an "efficient" system.

Can be "efficient" in terms of operation - but as you say, when you factor in capital investment and time value of that money, payoffs are typically in the 20 year plus range. Obviously, payback is faster if you can mount it so it gets optimal exposure (which for much of northern US and basically all of Canada means vertically on a wall, not on the roof), if you have intense sunlight (for photovoltaics) or intense sunlight and/or high air temperatures (thermal transfer system).

What makes or kills the deal for most people is being heavily personally committed to "green" living or living in a sunny area with veary high power bills or undependable public power grid; or on the other hand the high capitol cost, uncertainty of long-term operating and maintenance cost and efficiency, and the fact most people move every 3-5 years so they personally do not get the long-term payback, plus many homeowners do not want the hassle and uncertainly of an alternative power system.

Solar powered hot water heating systems can be effective at times - but generally only in locations with very high cost or undependable power, or places that combine seasonal solar availability with simultaneous heating demand like desert areas, where you can obtain the energy in the daytime sunlight and store it for use in the cold night. Takes a lot of highly insulated water storage for a diurnal system like that to work right - I ran the numbers for a government job once in Nevada, so lots of sunlight - but 2500SF building would have needed about a 1000 gallon tank to store enough heat (using thermal transfer - rooftop hot water tubes) to heat the building at night in their mild winter. Of course, using direct-heat photovoltaic heating or hot water thermal transfer coupled with a heat pump allows much higher temperature water which is more efficient for a baseboard or radiant floor heating system (less pumping cost), but then more capital cost, etc, etc.

Takes a lot of detailed cost estimating, a lot of contingency for the unknown and for contractors not being familiar with the work (plus they price up because this is "fad" work that the buyears "expect" to pay a premium for), and a lot of capital costs, so while it can work, usually it works out that the places it is pretty sure to work the best are the places where the energy is least needed for heating - Hawaii, Florida, equatorial regions, etc.

Saw a proposal once for solar hot water eneargy storage heating system in a very bright, sunny area with veay little cloud cover - unfortunately it was in Point Barrow, Alaska where you can't deny that the heating demand is high - but the prime heating demand period in winter was also during the time when there is zero or effectively zero direct sunlight - rendering the proposal a joke.

My recommendation to people, having worked on design and analysis (both for construction and to evaluate existing ones) of a number of alternative energy systems of all types, is they are basically not yet (and in many cases never will be) at the stage where they are either dependable enough or economic for residential use for the general public, and you risk a whole lot more than the relatively little $ you typically stand to POTENTIALLY gain.

I have seen cases where owners put $100,000 - 1,000,000+ of alternative energy systems into their houses for a few hundred to maybe $500-1000/year MAXIMUM possible savings. For instance - I pay a couple of thousand $ a year for TOTAL energy cost - heating, hot water, lighting, cooking, dishes and clothes washing/drying, etc. So, assuming an optimistic 20 year return period and figuring my time value of capital (or cost of a loan to build the system) at say 10%/year, without getting into fancy amortization schedules and such, to save say $1000/year (half annual total energy cost) I could afford to invest only about $8,500 - not many alternative power systems can even get started for that amount. And that is without any operating or maintenance expenses. Add in an unconservative say 5% per year operating and maintenance expenses, and 20 year payback period, to save $1000/year I could only afford to spend $6000 up front - and that is not putting any dollar value on the risk of buying the system or it not working right. Put more than about a 5% risk or estimate contingency on the project and it NEVER pays off.

And remember when looking at government or manufacturer or vendor websites, etc they are going to be giving you the rose-colored-glasses view, not the sort of numbers that an unbiased cost analysis would use with real time value of money, project and financing risks included, risk tolerance factored in, etc.

Answered 5 years ago by LCD

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