Solar Thermal Hybrid GSHP

Discussion in 'Hybrid Systems' started by Tom Bradford, Oct 16, 2016.

  1. Tom Bradford

    Tom Bradford Member

    I've read articles that conclude that solar thermal panels can economically "recharge" the ground loop in the heating season. For example, if the entering water is 35 Deg F. and the outside is sunny and 50F, solar thermal will contribute to the heating COP by routing water from the ground loop through the solar panels. Claims include the hybrid system has a lower life-cycle cost.

    To check on the economics of this hybrid arrangement, how does one size thermal solar system? Key parameters should by annual KWH/SF, Panel Efficiency (to heat water), flow rates, etc.

  2. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Yup. You want to know all of that, but why not bury enough pipe at the beginning?
  3. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Don't shade the ground from the sun's radiation by installing solar thermal panels. Let the sun strike the ground unimpeded.

    If you absolutely, positively gotta have solar panels, go for PV (photovoltaic) and let the extra electrons offset consumption.
  4. Tom Bradford

    Tom Bradford Member

    It's amazing how many studies that have been performed and papers published on different schemes to optimize, and thus reduce the cost of, the GSHP, especially the cost of the ground loop. The solar-assisted GSHP is applied to heating-dominated areas. The loop is sized for the cooling load. The ground temperature is balanced by adding heat to the loop or supplementing heat extracted from the loop by a heat source, e.g., solar thermal panels. I assume no one has tried this on this forum. Some of the articles, not including those referenced above, claim the the economics does not support the idea of solar-assisted. It looks like $40 to $60 per SF solar thermal panels and the added complexity and associated maintenance makes the integrated system unattractive.
  5. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    It never makes sense to put the heat in to the ground loop vs. the building to begin with. Unless you have an inordinate amount of waste heat from something.
  6. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    KISS (keep it simple st....)
    You loose so much energy to the ground, as much as you gain so much energy from the ground when needed, that it is the least economical way to increase your performance. As said before, get a wind turbine and/or a solar roof for the same price, and you not only run your system more efficient, but you are effectively at net zero.
  7. Tom Bradford

    Tom Bradford Member

    Appreciate the astute comments. My interest in a solar-assisted GSHP started when I learned that sometimes a "slightly" undersized GSHP has a better Life Cycle Cost than a oversized system. That led me to consider that flat plate collectors could provide thermal heat as a substitute for electric resistance. Expanding on that, flat plate solar thermal collectors could heat glycol water from the ground loop, circulated through the panel(s) when the sun shines, possibly by a PV-powered pump. But solar heat would only be useful in the heating months. The rest of the year it would just sit there with the pump off, maybe overheating, assuming there is no need for the heat somewhere else.

    You have helped me see that if I need to add heat to the house, solar PV is a better option over solar thermal. A quick survey shows PV systems are about twice the installed cost of flat plate collectors. But PV panels have more hours of useful energy per year - many more when you consider summer hours of sunshine. Even if there is a need for heat in non-winter seasons, solar PV with a heat pump should beat a solar thermal system in $ per unit heat provided. Thanks again for that insight.
  8. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    The "slightly undersizing" was a term for peak capacity to cover the coldest days of the year, a capacity you only need a limited amount of hours per year. A better performing ground loop does not significantly add to your peak capacity.
    However, that approach starts to fade away with variable speed technology, where increased units capacity is relatively cheap to install, and results in significantly higher efficiency over the season. In addition, given the much higher adaption rate of electrically driven heat pumps which is inevitable in the future, the "slightly undersizing" approach has to go away since we will not have enough electric power generating capacity to allow all the heat pump use electric resistance heat at the same time (coldest day of the year) .
    Deuce likes this.
  9. Douglas Nathe

    Douglas Nathe New Member

    I have thought of this too, but as some others mentioned there is too much loss to the ground that wont be recovered during a heating demand. Kills the efficiency of the collectors. I instead think that running a thermal storage tank in parallel to the ground loop is a better method. The storage tank is heated by the solar collectors. It would be important to ensure the EWT is not too high for the pump. You cant just take 120 degree water from a storage tank and use that as the EWT to the HP. Likely to cause damage. I would instead use a mixing valve. If the earth loop EWT is 50F and the max design EWT of the HP is 60F the mixing valve can utilize the storage tank and reduce the demand on the ground loop.

    I do still prefer using a larger ground loop to meet demands though.
  10. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    How large do you think the thermal storage tank should be? At what cost? If you do the math, you get to the point that 1000 gallons of 120F (if you get that high) are not really getting you that far.
    One can spend the money more wisely to increase operating efficiency.
  11. Douglas Nathe

    Douglas Nathe New Member

    With the federal tax rebates coming back I do see why anyone would attempt to use solar collectors. Geothermal would be better IMO, but if someone wants a DIY project or something I think it would work well.

    The only reasonable usefulness I see for adding a solar collector is if a currently installed geothermal system is struggling to keep up during peak heating demands.

    Here are some numbers I estimated for adding a solar collector.

    A typical 30 evacuated tube collector can collect 40,000 btu on a good day. For a storage tank I would use an old electric water heater because you can get one for next to nothing. Assuming that on a good day the solar collector can recharge the tank to 140 degrees then during the night the geothermal system draws heat out along with the earth loop until the tank is down to 50 degrees. A 90 degree drop at 1 btu per pound of water is a total heat output of 41,300 btu.

    By no means is this enough to heat a home in a heating dominant region. But if your system capacity is undersized this could work the supplement the earth loop when it cannot keep up. At least if expanding your EarthLink is not an option.
  12. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Indeed, all going well, you can store 41,300 BTUs in a 55 gallon tank if you heat it up from 50 to 140F. But that is usually enough to heat your house for about 1 hour during a cold night, maybe 70 minutes, not more.

    While warmer EWT increases your heat capacity, for example for a Waterfurnace dual stage 6 ton heat pump increasing your EWT from 30F to 40F will increase heat capacity by 14%, it is a very uneconomical way to increase peak system capacity.

    Lets say you are in Buffalo NY, some times pretty cold there, -8F was the peak in the last 5 years, and only 30 hours/year it is below 2F, most of the time between 2F and 0F Heatloss is linear, 65F outdoor is when you usually start heating, roughly 682 BTUs/degree when your house needs 45,000 BTUs at 2F. Now, if it drops down to -8F, that is 6,820 BTUs more, but it only happened for 1 hour duration. You pull about 65 million BTUs out of the ground over the annual heating season, a couple hundred thousand BTUS during peak time will not make much of a difference for the loop.
    The key to cover you during peak times is to invest a $1000 more for peak compressor capacity, your ground loop might drop to 30F instead of 31F during peak heating hours. Not very significant. An earth loop must be significantly undersized where a "earth loop cannot keep up"....

    Another way to look at it is that heat extraction from the loop is defined as GPM x delta T x 485 (constant). A larger compressor will increase your delta T between the loop and the ground, increasing heat extraction from the ground, but it will only do so for the very short peak time relative to the rest of the year. WA solar heater will inject BTUs into the ground more or less every day, keeping the ground warmer, but it usually does not very much in terms of added capacity during peak heating hours. A larger compressor is the preferred way to do that reliably.
  13. Allen Doerksen

    Allen Doerksen New Member

    Thank you for your comments. I find this very educational. My loops are over sided but the compressor must be undersized because I have to add heat with a electric boiler because my water temperature drops to low. Putting in PV solar panels is a good idea but the cost is very high. In Manitoba, Canada here we heat for 7 months out of the years. I'm a bit of a 'do it your self ' type of guy so I thought subsidizing my system with some solar heat would be the most cost effective. But maybe I'm barking up the wrong tree :)
  14. ChrisJ

    ChrisJ Active Member Forum Leader

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    Last edited: Feb 9, 2018

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