Multiple questions about loop design and AUX heating

Discussion in 'Geothermal Loops' started by Designer_Mike, Jan 29, 2011.

  1. Designer_Mike

    Designer_Mike Member

    I've been trying to educate myself on loop sizing and learn from mistakes others have made before I dive into the geothermal owners group.
    I try to "think out of the box" whenever possible and absolutely subscribe to KISS principles ...unless it's my hobby ;-)

    There are lots of questions from owners that have undersized loops or problematic systems.

    1) Is there any reason you could not install a standard water heater in the loop circuit? BTU are BTU regardless of the source and at low temperatures (say 35 degree water temp) a std water heater is going to be close to 100% efficient. Yes it would simply be an auxiliary heat source instead of the electric back-up coils but would be much cheaper to operate although more expensive initially.

    2) most designs I have seen have no reservoir tank (or a very small one) for the loop water. I would think it would make sense to have a "flywheel" to help absorb spikes and act as an expansion tank to keep air out of the loop (if plumbed in properly)

    3) I understand how turbulent flow greatly improves heat transfer in a heat exchanger but I find it hard to believe that a length of low conductivity plastic pipe running though even lower thermal conductive soil will benefit from high flow rates since it will take significantly more energy to produce the flow.

    I'll step back and prepare for the "silly newbie" answers I'm sure are out there.
  2. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Welcome, Mike

    1. Your idea is doable, but if your back up fuel is electricity why not skip the transportation costs involved with moving the water. Just put strip heaters in the system and you are done. If your back up fuel is anything else I would not take a chance on damaging a geo unit, or causing the machine to lock out trying to save itself. Put a water coil down stream of the geo coil and add the BTUH you need when you need it.

    2. I see no harm in the flywheel approach, but again to make it work you need to move the water or you are not accumulating anything. Moving water costs money. I have moved away from using buffering tanks on the delivery side of a system except for DHW.

    3. You got me there, I agree with you, but if someone can show me. I am able to learn.

    Attached is a photo of the hot water coil I mentioned tagged to a 4-ton WF.

    Attached Files:

    Last edited: Jan 29, 2011
  3. Looby

    Looby Member Forum Leader

    Besides violating your KISS principle, it makes no sense to heat the
    water circulating in the loop and then expect the heat pump to move
    100% of the added BTUs to the air coil. Some of the added heat will
    escape into the ground. Electric strip heaters (installed inside the
    heat pump cabinet) are inexpensive, simple, 100% efficient, require
    zero additional floor space, AND the required aux heat control logic
    is already present in the heat pump electronics.

    Still water is only slightly more conductive than HDPE -- and in
    pure laminar flow, the near-stagnant "boundary layer" of water at
    the pipe wall can be significantly thicker than the wall itself, thus,
    the water can be a greater impediment to heat transfer than the
    plastic pipe.

    That said, even at lower-than-recommended loop flow rates, the
    flow regime is "transitional" rather than purely laminar or strongly
    turbulent -- so, "loop turbulence" is always a matter of degree.

    WRT soil conductivity, it runs the gamut from "HDPE-like" to 10x
    better. Also, besides conductivity, there are considerations of
    soil heat capacity and water movement (diffusivity and advection)
    that can significantly affect underground heat transfer.

  4. Designer_Mike

    Designer_Mike Member

    Yes, it does violate my KISS principles....except this will be sort of a hobby and experiment for me :)
    I'm sure the heat pump I plan to install will have aux coils standard so I will most likely use those. I was just curious if there was any reason it might not work. I was specifically considering an alternate energy to heat the loop water which is another reason for the buffer tank.
    I originally considered using solar to add some BTU to the loop water when production was good. BUT after looking at the excess production I could get from my DHW heating panels even if I were to add a couple, the savings in winter would be insignificant. I also have access to a wood fired boiler for auxiliary heat source...I don't currently have it hooked up to anything and I really don't want to "need" to burn wood regularly.
    My thought was to have a somewhat large buffer tank for my loop water (say 500 gallons).

    I would definitely like to install a hot water coil as was shown above also.

    During the fall, I would turn a couple valves and allow the solar panels to heat the buffer tank POTENTIALLY to 160 deg or something. When the house called for heat, the fan would run and I would circulate the hot "loop" water through the coil. Of course the ground loops would be closed off from the tank and should be "warm" due to the summer of cooling the house anyway.
    As the tank cooled (below 90 deg) and the solar panels fell WAY behind (I expect that would happen fairly quickly depending on the weather) I would switch to running the heat pump. I'm sure it would be uber-efficient heating the house with 90 deg water. As those stored BTUs were used up, the loop circulation system would kick in and attempt to maintain the loop water above 50 degree.
    During the winter there will be zero excess production from the solar panels so all the heating would need to be done by the loop.
    Depending on my mood and the loop water temp, I may fire up the wood boiler, closing the loop valves and heating the buffer tank with wood. If temp was over 90 degrees, I'd skip the heat pump and simply use the HW coil again while the loop recovers.

    I's by no means keeping things simple!!!!! :eek:
    I'd be controlling everything with an industrial PLC with my own program so I can tweak as needed. (THINK HOBBY here)
    The larger buffer tank I use would give me more BTU storage. Having the tank at 160 degrees would give me the equivalent of ~20 hrs of heat pump operation which should get me through a couple of fall days and hopefully the solar panels could make up some of the required heat without turning on the compressor.

    During the early part of the cooling system my loop water would probably be around 35-40 deg so I could probably get enough cooling out of the direct water coil again and not run the compressor until my loop water got warm and the cooling load got higher.

    WOW...I think I have confused myself and it's my system:eek:
  5. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    You may have confussed yourself....

    .......but what you describe is possible and can be built to run off your 4G phone. I can do it off my gung ho samsung for combat use phone.

    You lack how to move BTUH, but your blue sky thinking has me buzzed.

    I can for see net zero housing throughout the earth, just takes time, thinking and money. At some point money as an issue goes away as we will have no other choice but to be good stewarts of our space ship earth.

    Allentown is not far, I used to race my dogsled team in Bradford.

    Take the wood firer boiler, when needed to the hot water coil. I am working on a way to get a Blackberry to stoke a boiler and an Android to cut and split wood.

    I am going to retire 09/09/2011 so hurry.


  6. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Just KISS by keeping any btu adding devices on the downstream (load side) side of the heat pump vs. upstream (source side). Lots of ways to do the former and is somewhat standard practice.
  7. Looby

    Looby Member Forum Leader

    Android has the CPU power to cut and split 300 mega cords per second,
    (and up to 500 MC/sec, if you write the code in assembly language)., all you need to do is cobble-up the actuator,

  8. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    There are upper limits to source side temps when unit is in heating...90 is about it. As a practical matter it is tough to keep compressor amps within limits and high side refrigerant pressure gets out of hand, which can't be good for durability.

    From WF Envision tables a 3 ton system sourced with 90 degree water delivers a full 4 tons of heating. Make sure the coil stays clean, CFM selections are on the high side, and the ductwork can accept the airflow. Possibly consider the upsized blower option if available if such operation is expected on a routine basis. Consider also the effect of high CFM settings on cooling - even with dehu option, (-15% CFM off selected CFM settings), dehumidification may be unsatisfactory.

    Another option mught be to slow source flow when source temp exceeds 80 (heating mode only), but that could get tricky from a controls and maintenance point of view.
  9. Designer_Mike

    Designer_Mike Member

    The Droid may have that capabilities, but if you try to use the voice recognition function, it will "burn down the house" instead of what you really asked it to do "split fire wood"....I can't wait until I can switch to the iPhone on verizon! (Ducking):p
  10. Verdae

    Verdae Member

    Hi Mike,
    I have a different take on integrating solar thermal in a heating dominated climate. There is a concept of managing the heat flux on a yearly basis. In a heating dominated climate, during the summer when your solar is stagnating and cooling the glycol, pump the heat into the ground. At the same time the hotter the ground the less efficient the geo is in cooling, so some if/than points are needed to avoid injecting heat into the ground from the solar at the same time you are at max cooling load. The final result is that the ground will be much warmer at the start of the heating season and the stored heat is recaptured. Naturally if the loop field has alot of water movement most or all of the heat will be lost and the system not worth it, but in this case the moving water will keep the loop temp up and injecting heat will not be needed.

    In a cooling dominated climate the solar thermal will not be needed and a dedicated water to water geo for domestic hot water will significantly improve operation efficiency.
  11. Designer_Mike

    Designer_Mike Member

    A friend told me to build a 60K gallon swimming pool and simply have it well insulated when they built it. Shut it down about mid august and cover the top with a bunch of insulation then start pumping BTU into it.

    My solar dealer told me he had heard of homes being built on a huge insulated sand bed with circulation piping built in. They did basically what you are saying and during the summer simply pumped the BTU into the huge sand storage and in the winter the would extract it.

    Both those options are economically off the table so I'll stick with something a bit more conventional.
    Once I get the system up and running and get some data, I think I could easily add more capacity to my buffer tank if I would like.
    I have a surplus 3000 gallon SS tank I could easily bury next to the basement, but what would be the most affordable insulation?.....While I'm installing the loop would be the time to do it!
  12. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Sorry the phone burned down the house.

    I will work on my programing skills in retirement.

    If I had a giant SS storage tank, rather than reinvent the wheel, (think OS 2), I would apply the tank to the delivery side of a system. The system would be hydronic. Water does not care how it gets warm and it is easy to move. I got banged for doing on line while posting math two days ago, but being fearless lets try again.

    If a gallon of water weighs______, and a BTU is the amount of energy required to raise 1 ___ of water 1*F then raising 3000 gallons of water 1*F would store how much heat?

    On to where to put the tank and how to insulate it. We also need to plan on how we are going to get the BTUs into the tank for storage. I would dig a hole in the basement to be deep enough to have the top of the tank or so, be level or a tad below the level of the floor. I would dig deep enough and add footers to keep the JHA happy and add 6" of styro foam foundation insulation to the bottom of the hole, (2" of styro is about R-10 so times three we have a base for the storage of around R-30). The tank would rest on the footers or piers. Do not forget drain tile to keep the warter out of the hole. Even SS will disolve in water. (We can use this water later.) Get the cast concrete wall guys to build a box around the tank. Leave enough space between the inside of the concrete wall and the out side of the tank to install say R-40 something. Vermiculite comes to mind. Put the tank in the hole and build the house.

    In the area over the storage tank we will not add radiant heat, we let the tank warm that area for us. Remember the numbers I guessed at for insulation, I chose them for a reason. Heat goes to cold every time. The bottom of the tank with its R-30 base insulation will be at a depth below grade of about 5'6" plus the height of the tank will be in the ground deep enough that it could pick up heat from the ground so only R-30. The sides are a differant story, so we insulate them well and pray for stratification in the tank.

    How is this going so far?

    I need a beer. BRB.
  13. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Finishing math

    3000gal x 8 lbs = 24,000 btu's for every 1F. Give or take some sig figs.

    Bloody imperial math!
  14. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Damn, I thought it would be more

    I used to read the Mother Earth News when John Shuttlecock published the rag here in Madison, OH, and got all excited about alt energy until the math raised its ugly head.

    The reason the engineers get the big bucks is that they do the math six or eight ways against center then double the numbers to CTA.

    Having had the wind taken out of my turbine somewhat, lets take the tank and say that it can hold 3,000,000 or so BTU. Lets fill the BTUs with solar thermal. Then we take Lloyds idea of a system designed for 100% in a heating dominated climate. Now we design a geo system using a water to water unit and a hydronic system to deliver the comfort.

    I think the stored BTUs will handle the aux heat needs.

    We now return this program to Mike's checkbook.
  15. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    All this energy

    Put all these efforts in to house construction and then heat it with a toaster oven.

    I had a snooty architect ask a table of us "if anybody has worked on a net zero home?". I have since, but at the time I said - "you mean a log home with a wood stove?".
  16. Designer_Mike

    Designer_Mike Member

    Yes, math is an ugly thing.
    BTU storage is not terribly economical. Or even remotely economical for that matter.
    A 3000 gallon tank....being generous will store about 3M Btu, but it would be pretty expensive even if I did everything myself, and the end result? I can store the equivalent BTU of ~30 gallons of heating oil!
    OR something like 565 Kw-hr of electricity to run a properly sized/designed geothermal heat pump for five or six days straight??
    Now if I could just get a solar panel that would generate fuel oil to cut down on storage requirements I'd be set.....make 3000 gallon of fuel oil over the summer and I'd be sitting nice and toasty all winter.:rolleyes:

    Checkbook is only one thing but if the math don't work, checkbook will stay closed!
  17. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I can think this stuff up

    all night long. The trick is getting it to work everyday.

    Chris: That is too funny.

    Mike: I always wanted to know what 3000 gallons of water was worth. I remember why they went to salts and waxes for storage. I do not think you would be very successfull using the tank.
  18. Looby

    Looby Member Forum Leader

    Uh, 26.41 x 10^6 joules ?

  19. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Starting to see more references to onsite storage for big buildings - freeze ice at night when power is cheap and thaw it for cooling during day. Apparently the loss of COP associated with lower temps (making ice instead of chilled water) is overcome by the cheaper power
  20. Designer_Mike

    Designer_Mike Member

    I have a number of customers that do this with different processing systems.
    Not only is electricity cheaper at night, air temp is much cooler (improving efficiency), and they only run the production lines one or two shifts max while they can make ice 24/7 if required.

    That 60K gallon swimming pool looks to be about the right size now!

    When we built our first home, we installed a heat pump (air source) and the contractor warned us of the difficulty making heat when it got below 20 degrees. He gave us the option of a storage tank to allow the unit to heat the water during the day (warmer outside) and utilize it at night but the efficiencies plus expense and space requirements made it prohibitive.
    Last edited: Jan 31, 2011

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