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Solar thermal rabbit hole

dar_laa | Posted in Energy Efficiency and Durability on

A friend’s attempt to get natural gas to his contemporary house project ended without service.  Climate zone 6.  Meanwhile the radiant floor tubing is already installed.  I thought perhaps now this is a great chance for a solar thermal packaged system.   But no installers are doing it around here anymore and Solar PV is down to below $3 per watt (less the $13k battery system). 

So I’m going to suggest a small, fire tube combi boiler with LP gas.   Along with a PV system and battery.
(a 2,500 sq.ft. “pretty good house” in terms of the envelope)
Unless someone can promote a Stiebel Eltron solar thermal system with evacuated tubes?

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  1. Expert Member
    NICK KEENAN | | #1

    There are two main problems with solar thermal. First is that when you need the most heat is precisely when the least solar energy is available. Which makes sense because the sun is what warms the surface of the planet. So you have to decide whether to size your system for the coldest days of the year, which means the rest of the year it is dramatically oversized.

    The second problem is that sunshine is variable and there isn't any good technology for storing heat. Solar enthusiasts used to talk a lot about "thermal mass" but that was all fantasy.

    A couple of technological and regulatory innovations have driven the stake through the heart of solar thermal. The first is net metering, which solves the problem of energy storage, and coupled with dropping prices for photovoltaic cells makes rooftop solar electric very attractive.

    The second is that heat pump technology has really improved. Since heat pumps have coefficients of performance significantly higher than one, you get more heat out than the energy you put in. A rooftop PV solar system coupled with a heat pump produces more heat than a solar thermal array of the same size. And if you can store the electricity to the grid using net metering you can get that heat when and where you want it.

    I would look into air-to-water heat pumps if you already have the piping in the floor. I imagine if it was installed with solar in mind it's designed for low-temperature water, which is where heat pumps excel.

  2. Expert Member
    Akos | | #2

    Depending on how PGH the house is, you might be in the range of being able to heat the entire house with a pair of Sanden CO2 heat pumps:

    These can do dual duty of space heat as well as hot water. There is a bit of engineering needed to make these work efficiently. They need relatively cold return water and a large buffer tank with good stratification, this should be no problem with floor heat. A pair can do 16000BTU, which should be within the range of a 2500 sqft R20/40/60 PGH.

    Of course, if you have good net metering and sufficient PV panel, you can skip all that and just use a resistance boiler. With cheap electricity, the operating cost of this is probably comparable to propane in most areas.

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