Replies

1. GBA Editor
   Martin Holladay | Sep 14, 2017 02:34pm | #1

   Snarbs,
   Q. "Is it possible to have a large storage tank under the garage or possibly just heat the well water to store heat energy gained during the daytime with the Solar thermal panels and use the heat pump at night and overcast days to heat the house?"

   A. Heating the well water is problematic (because the water in your well is connected with the groundwater flowing through cracks in the rocks beneath the soil of Nova Scotia). However, it's certainly possible to store hot water in a very large insulated tank in your basement (or buried in your back yard).

   Many people have tried this. All have discovered a simple truth: the amount of energy than can be collected and stored by these methods has a very low economic value -- too low to justify the high cost of the hardware required to collect it.

2. Expert Member
   Dana Dorsett | Sep 14, 2017 03:34pm | #2

   When coupled with an water-to-water or water to air heat pump you can use UN-insulated tanks, and smaller tanks, taking advantage of the much higher "apparent volume" of the heat of fusion of the water in the tank once it's temperature drops to 0C. That makes it a pretty cheap tank, and the heat pump still operates at about a 4:1 efficiency or higher.

   The solar thermal can be cheap flat panels and will still run at very high efficiency even when it's -15C outside, since the output of the solar panel doesn't need to be heated to hydronic-heating temperature, it only has to be above freezing, which is a much lower lift.

   This has been done successfully in locations much colder than NS, and if designed properly it can be both cheaper and higher efficiency than the standard GSHP approach. But there is very real design risk (and design cost) that you don't have with pre-packaged cold climate ductless air-air systems (albeit at lower average efficiency).

   http://www.thermalbatterysystems.com/thermal-battery-systems/#.WbrXcsiGOUk

   http://www.thermalbatterysystems.com/featured-systems/ennis-montana/#.WbrYDMiGOUl

   See also:

   http://www.pmengineer.com/articles/92015-thermal-batteries-for-renewable-energy-systems

   Whether it's WORTH that much complexity is an open question. There may be more value putting the money into a higher-R tighter building envelope and heating/cooling with ductless mini-splits, with the power offset by rooftop photovoltaic panels (which unlike solar thermal, is getting cheaper and better every year.)

3. GBA Editor
   Martin Holladay | Sep 14, 2017 04:24pm | #3

   Snarbs,
   The most important sentence in Dana's comment is this one: "There may be more value putting the money into a higher-R tighter building envelope and heating/cooling with ductless mini-splits, with the power offset by rooftop photovoltaic panels."

   I'm willing to go farther than Dana. Here's my statement: Investments in a better thermal envelope (an envelope that has higher-than-code-minimum levels of insulation, minimal air leakage, and high-performance windows) along with an investment in a PV system will definitely yield more energy savings than a comparable investment in solar thermal equipment or insulated hot-water storage tanks.

4. 
   snarbs | Sep 14, 2017 05:26pm | #4

   Thanks for the responses! I was starting to get that feeling with the research I was doing. I figured storing the water in a large tank for (seasonal) would be a high cost with low payback. But thought a small tank just to offset nighttime heating and boost my hot water before either a hot water tank or on demand system.

   I'm not a huge fan of the mini split head look on the inside of houses and was wondering if its possible to use a ducted design with VRF or valves in the ducting to control temp in different zones (4 if possible). I also thought the ducted system could be an advantage for moving heat form the high passive solar area to the north side of the house/basement and also for distributing the wood stove heat.

   After my research into Photovoltaics I will definitely go that route.

   Thanks again for the responses. I've got some reading to do on those links.

   Cheers

5. Expert Member
   Dana Dorsett | Sep 14, 2017 05:35pm | #5

   "Investments in a better thermal envelope (an envelope that has higher-than-code-minimum levels of insulation, minimal air leakage, and high-performance windows) along with an investment in a PV system will definitely yield more energy savings than a comparable investment in solar thermal equipment or insulated hot-water storage tanks."

   But nobody is talking about "...insulated hot-water storage tanks...".

   The "thermal battery" concept use very cheap non-pressurized UN-insulated storage tanks, and LOW TEMPERATURE (even freezing temperature) solar thermal. Since I've never seen a cost breakdown or designed such a system myself it's a bit harder to make the comparative net energy or costs & savings assessment from my armchair perch. Storing large volumes of 0-10C water in a buried unpressurized uninsulated plastic tank is as lot cheaper and less lossy (it's usually "gainey", taking heat from the surrounding soil) than 50-60C water in a pressurized insulated tank.

   And running solar panels with 0-10C water in, 15-25C water out is dramatically more efficient at sub-zero outdoor temps than 40-45C-in, 60-70C-out. You get subtantially more heat per unit area out of a cheap flat-panel when operated at low temp than at domestic hot water temperatures, or slab-radiant heating temperatures.

   The cost of a decent 2-ton water-to-water heat pump is about the same as 2 tons of cold climate ductless, but that doesn't include the heat exchanger for the tank, or the pumps & design work, etc. So it's really hard for me to guess what the relative ROI of the whole system is, or what it is relative to sufficient PV + ductless to hit the same net annual power use. YMMV. To be sure there is a lot less design risk to go with PV + ductless.

   The average 2000kwh-5000kwh/month user in NS (which would be a house heated with heat pumps) pays about CDN $0.15/kwh, which is cheaper than the unsubsidized levelized cost of rooftop PV at the US average installed price. Rooftop PV may cost more in NS than the US average, but may also be subsidized (I've never looked into PV costs & subsidies for NS), so while it may be a net ENERGY savings, it may or may not be a net COST savings. Costs are continuing to fall year-on-year, and the subsidy picture is constantly evolving. Whether there's an economic rationale for installing PV as part of the initial build is something that would have to be investigated.

6. 
   user-2890856 | Sep 14, 2017 05:39pm | #6

   Drain back solar thermal , enough panels as opposed to cheap way out . Water furnace w/w hp . Coccoon insulated tank with setpoint hydronic mixing valve . feed the heat pump with 70* water , low resistance due to lack of ground loops , less pump energy required , Way less .

   You could easily have system COPs of greater than 10 with very little risk . It has been done very poorly in the past but there are several competent designers of simple system,s that work well . See Thorsten Chlupp , Fairbanks Alaska . You however do not need to go as far as he did , we're not talkin Fairbanks here . The latent capable thermal battery mentioned by Dana allows multiple sources that can be added at any time . Funny how the phase change to solid from liquid allows each pound of water to store an extra 140+- BTU per .

   Insulated or un insulated are both options . Oh yeah , and you can actually close doors and be comfortable everywhere instead of making concessions to be cheap . Maintains and repairs much more easily also

7. Expert Member
   Dana Dorsett | Sep 14, 2017 05:58pm | #7

   I was typing as response #4 was being posted...

   Regarding "...the mini split head look...", look again. Ducted & ductless air source heat pumps can definitely work in NS, and you don't have to be stuck with looking at wall-coil blobs to do it all with mini-splits.

   The Fujitsu xxRLFCD series dedicated mini-duct units have a specified output at -20C (which is roughly the 99% outside design temp for Shearwater or Amherst NS, most other locations have warmer 99th percentile bins than that). Flush mounted ceiling cassettes work just fine on a number of cold climate mini-splits, as do thin profile floor mounted wall coils.

   A 4 ton 8 zone multi-split using all ceiling cassettes or a combination of mini-duct cassettes & ceiling cassettes can handle a fairly substantial heat load at -20C. But cost & efficiency wise it probably won't take that much equipment for your house, especially if one cost-optimizes the thermal performance of the building envelope with better-than-code R-values & U-factors.

   Zoning ducted system duct valves tends to be less efficient and is more prone to increasing the actual load by air-handler driving room-to-room pressure differences that result in higher outdoor air infiltration rates. Any zones that can be served well with a right sized ceiling cassette or floor mounted coil is better off with a mini-split/multi-split solution, since it can't pressurize or depressurize the room.

   Heating other zones with 25-30C air from an over-solarized zone with a ducted air handler is less efficient than heating those remote zones with a heat pump. The temperature difference is too small to result in a high coefficient of performance with the power used by the air handler. Designing the house with some amount of heat transfer via natural convection can work, but only if you're happy with the sun-space being overheated. It's usually better/cheaper/more comfortable & cost-effective to use higher performance and somewhat smaller windows than you would in an all-out solar house design.

8. 
   SwitchgrassFarmer | Sep 14, 2017 06:18pm | #8

   Two, different, thoughts:

   1. I was in a friend's passive solar home a couple days ago. The home used a natural gas boiler and radiant heat, with mini-splits for cooling. Yea, I noticed the mini-splits, but they were innocuous and immaterial versus all of the other fine architectural features of the home.

   2. I live in a passive solar home with geothermal (ground source heat pump) HVAC. Lot's of hoops jumped through for the passive solar aspect including extra expense for glass, limitations on finishes to prevent sun damage, more complex overhangs. Works great, but if I had it to do over again, I think I would go with a combined solar thermal / ground source heat pump system instead (or mini-splits).

   Here is a paper describing such a combined system. http://ibpsa.org/proceedings/BS2009/BS09_2297_2305.pdf Google will find you other papers/discussions.

9. 
   snarbs | Sep 14, 2017 09:12pm | #9

   This has been a wealth of information here. The thermal battery concept seems to be a great way to store excess heat gain. I believe I will probably have more than sufficient heat in the daytime therefore storing the additional solar thermal in a tank that could be used at night makes sense to me.

   Richard mentioned an insulated tank with higher temps than the thermal battery ( This is what I was thinking for day to day heating when the sun is out) The higher temps must result in a more efficient heat pump.

   The thermal battery underground also seems like a solid idea for longer term heating conditions when the sun is not shining and I'll need to heat during the day and night.

   I'll look into the ductless heat pump options as they sound like you can conceal them quite well.

   I assume it would be possible to pump the heated refrigerant from the insulated tank or thermal battery to each head?

   Does a hybrid make sense in this situation?

   With regards to solar subsidies in Nova Scotia there currently are none or they're negligible (I believe there may be some for solar thermal but not PV). Even without the subsidies PV still makes sense which is a huge turning point and surprised me when I did the math.

   Thanks again for all the material. I have a lot of studying to do and hopefully can come up with the best solution!

10. 
    charlie_sullivan | Sep 14, 2017 09:55pm | #10

    If you want to combine heat pumps and solar-thermal-storage, one way to do it is to have two completely separate heating systems. One system is a mini-split heat-pump system, and the other is using radiators or under-floor water tubes with solar heated water in them. The two would be completely separate. The other way is to combine the systems and use either the heat pump or the solar to heat water, and to deliver the heat to the space with the same emitters regardless of where the heat originated. In that combined system, the heat pump would be something like the chilltrix, not a minisplit.

    The result could be very efficient, at least if you find one of the rare hydronic designers who knows how to minimize electric usage in the pumps, valves and controls. But it would also be very expensive. So it's only worth considering if you are interested in investing in it as an energy hobby or demonstration project, rather than just as a cost effective way to make a low or zero net energy house. If you low or zero net energy cost effectively, spend your money on the envelope, on mini-splits, and on PV instead.

    I don't mean to disparage the idea of an energy hobby project--and I don't mind providing advice on that. But I think it's worth consciously deciding and stating whether that's what you are going for.

11. 
    Jon_R | Sep 15, 2017 12:30am | #11

    Understanding that it's not cost effective, I'd like to see more discussion of the best way to achieve grid-independent net zero homes. Ie, off grid in a climate that requires heating and/or cooling with no burning of wood, propane, etc.

12. GBA Editor
    Martin Holladay | Sep 15, 2017 05:44am | #12

    Jon,
    If you change your requirements from a grid-connected home to an off-grid home, everything changes. Building an off-grid home that doesn't require propane or firewood is extremely challenging in a cold climate. (If this mental exercise allows players to purchase gasoline or diesel fuel, everything changes. With a big enough generator and unlimited gasoline or diesel, you can have all the heat and electricity you want.)

    Thorsten Chlupp has come closer than anyone else to achieving this goal -- but Chlupp has a grid-connected house, and Chlupp burns wood (as far as I know).

    If I were called to design the house you describe, I would advise the homeowner to look for land that is exposed to steady winds, so that a wind turbine could be installed. For a wind turbine to make sense, wind experts advise that the site has to be so windy that the wind is irritating to those who live there. That makes choosing such a site counter-intuitive -- because you have to buy land somewhere where human habitation is unpleasant.

    I've lived off-grid for 42 years. There is hardly any sun in the winter, so I make electricity in winter with a Honda generator that burns gasoline. I also use propane and firewood.

    If this mental exercise allows you to select the ideal climate, everything becomes easier. In the U.S., you would want to buy a building lot in San Diego, California, or almost anywhere in Hawaii.

13. 
    STEPHEN SHEEHY | Sep 15, 2017 07:45am | #13

    Snarbs: Don't forget that with a good building envelope, it doesn't take much to heat a home.
    My small zone 6 house in Maine probably costs around $4-500 per year to heat with minisplits, if I didn't have pv. That's at about 15¢ per kwh. While your ideas for storage are interesting, you probably will spend more money for a more complicated system than you would just using minisplits, ducted or not.
    For what it's worth, after they are installed, you don't notice the wall units any more than you notice the fridge or any other appliance.

14. Expert Member
    Dana Dorsett | Sep 15, 2017 11:19am | #14

    What Stephen Sheehy said. While the thermal battery stuff is fun to think about, the systems are complicated and carry considerable design risk. If it doesn't quite cut it, who will be on the hook for upgrading it to the point that it works, and maintaining it over time?

    In the end the surer-thing is to improve the building envelope to where it can be heated by reasonably efficient ductless heat pumps, and install rooftop solar. Both ductless air source heat pumps and small scale solar are now commodities manufactured in much higher volume than water-water heat pumps and solar thermal systems, with a growing amount of local contractors capable of properly installing, diagnosing, and maintaining the systems.

15. GBA Editor
    Martin Holladay | Sep 15, 2017 11:33am | #15

    Opinions are converging... We are achieving consensus. That's good.

16. Expert Member
    Dana Dorsett | Sep 15, 2017 12:09pm | #16

    In some ways I think the thermal-battery approach with low-temp solar thermal COULD become a commodity that's both cheaper then conventional ground source heat pump (GSHP) systems and potentially made competitive with mini-splits in colder climates, but it would likely take better capitalization along with some standardization of the design. At the moment every system is a full-custom design, with all the design risks that custom designs entail.

    I applaud those designers willing to dive-in, but can't actually recommend it as a solution here. I only mentioned it as a counterpoint to the more expensive and less efficient higher temp solar thermal solution SNARBS was contemplating. It's probably still cheaper than but just as efficient as better-class GSHP systems of similar output, but probably more expensive than ductless solutions.

    But for a ~2650' 2-story in NS it doesn't take a rocket-scientist architect to get to Net Zero Energy with ductless mini-splits and PV at at reasonable price point and a much lower risk, which is probably the "right" thing to do.

    It's worth simulating the house and all it's glazing in BeOpt to dial in the design for cost-optimal insulation, but importantly, to optimize the window type & sizing. A 2-story sun room could easily turn the house into an oven (even in winter) while increasing the peak heating load, if you're not careful.

    https://beopt.nrel.gov/

17. 
    Jon_R | Sep 15, 2017 02:56pm | #17

    If I did the math right, insulating to passive house levels (4755 Btu/ft2 per year) and creating a basement water tank occupying less than all of the basement would allow one to generate and store ALL of the heat in the Summer and use it throughout the Winter (space and water heating and tank losses). Using a reasonable ~6KW of PV (far less using a to-water heat pump).

    Sounds like Thorsten Chlupp would be completely successful if he were in a climate milder than Fairbanks. Without out-of-the-question costs or complexity.

18. 
    STEPHEN SHEEHY | Sep 15, 2017 03:36pm | #18

    Jon: I'm curious. Show us your work

19. 
    STEPHEN SHEEHY | Sep 15, 2017 03:37pm | #19

    How big a tank would you need?

20. GBA Editor
    Martin Holladay | Sep 15, 2017 03:37pm | #20

    Jon,
    Did you account for the rate of heat flow through the tank walls? Even with 4 inches of foam (call it R-24 insulation) surrounding the tank, the temperature of the water in the tank will drop quite a bit from July to November.

21. Expert Member
    MALCOLM TAYLOR | Sep 15, 2017 04:07pm | #21

    My only experience with tanks that big have been as cisterns for roof-top water collection. The cost for increased foundation structure, surface preparation and membrane were almost prohibitive.

    How would the storage work in the shoulder seasons? in our climate much of the very hot weather occurs right at the end of summer. Introducing a large mass emanating from below might be enough to tip us into needing a cooling source.

22. 
    Jon_R | Sep 15, 2017 10:57pm | #22

    A rough summary is 5M btu for space heating, 15M btu for tank losses and 5M btu for DHW. I simplified using 6 months of collection and 6 months of use and losses.

    A 32'x32'x8' tank (similar to a swimming pool) holds 500K pounds of water. So 50F delta-T to store 25M btus, with higher being practical (and necessary to use the heat without more energy).

    3000 sq ft of tank surface with 1 btu/hr/ft2 average heat loss = 13M btu/6 months.

    Using $75/cubic yard pricing for geofoam, I get $8,000 for 12" of insulation (say R45, or enough to average 1 btu/hr/ft2 at ~80F delta-T beginning of season).

    6KW for 7 hours/day = 26 Mbtu/6 months (OK, too optimistic and it needs to account for significant "Summer" tank losses). Fix with a heat pump and less PV. Might even get "free" cooling from it.

    Total cost - perhaps $50K? Larger homes have spent that on geothermal HP systems.

    Of course this is a worst case in terms of collection period. One will be able to collect a lot of PV solar over the 6 colder months.

    On the other hand, one can buy 10 Mbtu of propane for $300.