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  1. #1

    Using a heat pump with a high temperature storage tank?

    I’ve read about how a new type of heat pumps systems that is attached to high temperature storage tank. These are being used in Europe and Iceland /Greenland with great success but don’t know if anyone in Connecticut or the Northeast has done anything like this.

    How I understand these systems works is the heat pump draws the available heat from either air or ground/geothermal pretty much year round at any opportunity and pumps the heat through a heat exchange and concentrator (?) into a high temperature storage tank.

    Later that stored high temperature heat is drawn through the heat exchanger as needed to warm a tank of ‘low’ temperature hot water to warm the house and the domestic hot water. The method of distributing the heat inside the house can be any of the conventions methods such as baseboard, in floor heat, forced hot air, etc.

    The high temperature storage tanks are supposed to be “off-the-shelf” equipment used for solar house and various industrial uses. These tanks are super insulated and are filled with different types of heat storage material like ‘molten salts’ or different mixes of sand and oils, etc. and hold the heat at many hundreds (thousands?) of degrees so that a very large amount of BTUs can be stored in a small footprint.

    When the house needs cooling the heat pump draws the heat out of the house through the normal means and instead of ‘dumping’ the heat outdoors instead sends it back into the high temperature storage tank for later use in the cold winter months. Should the high temperature storage tank ever reach its maximum storage capacity, i.e. it becomes ‘full’ any excess heat that is pulled from the house during the cooling season is ‘dumped’ outdoors in the conventional means.

    It sounds like the heat pump can be a smaller unit since it only needing to ‘fill’ the storage tank to its max capacity at a slow ‘trickle charge’ rate over a whole summer season and therefore only needs to be sized just to take care of the ‘instant’ cooling needs of the house on a hot summer day rather than sized to try and heat the house on a cold 40F winter day.

    With the stored heat it also sounds like it does not appear that the outdoor winter temperature matters much since the house is getting it's heat from the storage tank and not the outdoor air so no ‘back up’ system would be needed except if there was a very long cold period where more heat was used than was stored. This also assumes the storage tank(s) were sized large enough and were ‘filled up’ before the cold period arrived. I’m not sure how this system works if it is first installed during the winter heating season where the system would not have had time to harvest and store any summer heat yet.

    There sounds like a lot of calculations have go into designing this kind of system to figure out how much heat can be collected and what size tanks (‘low’ temp and ‘high’ temp), the different heat exchangers, pumps, controllers, etc.

    I’m not sure I’ve got all the details right, but I'm wondering if any one builds and install these types of systems.

    Thanks
    Tom

  2. #2
    Join Date
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    You got a link, I've never heard of what you described.

  3. #3
    Sorry, I don't have a URL. I read about it in a British science magazine "NewScientist".

  4. #4
    Join Date
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    There are many variations of this theme, but the storage is based around phase change materials more commonly known as "PCM", you pick a temp at which you want the substance "PCM" to change state, at this point, because of the latent heat of fusion, maximum energy is required with out a change in temp. It is not uncommon, to have different PCM in the same tank.
    I do not know of a heat pump that can produce 1000 degree temperatures. how ever a small co-generation (power and heat) based around the sterling cycle may have the possibility.
    There are possible problems with PCM, basically many do not regenerate indefinitely as the desire rate.
    The term of "thermal reloading" can also apply

  5. #5
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    Most compressors would have a problem getting the storage media to 200°F. Their oil would cook.
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    How many times must one fix something before it is fixed?

  6. #6
    To jtrammel and barbar, Thank you both for the reply.

    I believe the high temp is obtained by going through several cycles or stages of 'stepping up' the temp.

    I found the URL, but the article is behind a paywall.
    http://www.newscientist.com/article/...evolution.html

    Is there anyone who has done this kind of system?
    Last edited by TomInCT; 04-28-2013 at 09:01 AM. Reason: typo

  7. #7
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    They do make water to water heat pumps that can be toed to a ground loop (goethermal) These make hot water in winter and cool water in summer. The maximum water temps even special high temp models can manage is about 160F. For higher effciency, you want to keep temps under 110F, and use radiant floor heat when possible. There are soem advantages ot usiang a storage tank, but unless you have special demand rate pricing for electricity, it's cost prohibitive. A large poly storage tank will run you 1/2 the cost of the system.

    For cooling, you can also store water for peak loads and even make ice to put all run time during lower off peak rates at night (this is doen in some cities in large commercial office buildings). Ice storage however requires a special controls and a storage tank.


    What you are describing is an attempt to balance cooling and heating loads and using a smaller ground loop for the balance. You however need ot have balance heating and cooling degree days. Few climates have this. So you'd need to add a boiler for lower temperatures. Keep in mind that with ground source heat pumps, your essentially doing this. You using the ground as a large heat sink. Except that some heat can still "bleed" off to surround soil well beyond the loop field. I assume you'd have a hot tank and a cold tank and hte heat ump transfers heat between the two depending on the season and demand. So it tried to keep the cold tank 10-60F (uses glycol, or) and the hot tank over 90-140F.

    Now for the math. To store 1 ton of heating or cooling, you multiple 500 * gallons storage * delta T of temp storaged vs. minimum useable temp (80F heating, 55F cooling). Lets take my home. Lets say I used about 1100 therms for heating. That's about 110MM BTU's or 32,000 kwhrs (scary to think if I had stright electric resistance heating). Lets say my cooling load is 1/2 that and I make up the remainder using a HE boiler and also gain some heat from waste heat the compressor gives off (comrpressors are cooled by their refrigerant). So 16,000 kwhrs or 55MM BTU's. I would need a 2200 gallon storage tank with a 50F delta T in heating and cooling... and again a small boiler when it gets below maybe 15F outside (in my climate at least). Since you cold water is so cold, you might as well use ice storage, so the storage tank is about I think 1/3 the size but you run at 32F. The boiler could also make domestic hot water.

    I don't see enough advantage of this over utilizing ground loops to accomplish the same thing. That's a lot of addd complexity. I wonder if you're comfusing geothermal system in those locations that have hot thermal wells due to underground vulcanic activity. IN some locations I think if you drill deep enough the ground temp can boil water ,so you get free steam and can use an absorbtion chiller for any cooling you need.

  8. #8
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    With all that money spent, you'd be better off designing a home to have minimal heat loss to begin with and desing it with a lot of thermal mass by using walls with high thermal mass and insulating it heavily form the outside. Done properly all you need is cooling for dehumidification, ventilation, and internal loads and you only desing to an average 1 or 2 day temperature, since it would take 24-48 hours to heat up or cool of the home.
    A very small water to water heat pump could manage this and supply chilled and warm water. Heating and cooling could be supplies by the fresh air ventilation unit. You just heat or cool the air a little above ambient as needed.

    The design I forsee is concrete walls up to the 2nd floor joists, then either concrete block above that or another slightly thinner concrete wall, again insulated on the outside. Then stucco over that. Basement floor also gets insulated under the slab.

    While we're doign all concrete, you could also go ahead and do poured concrete floors and install radiant loops in them and circulate warm or cool water as needed, then jsut use a ventilating dehumidifier. Ahh and house built like a fortress without squeaky floors.

  9. #9
    To motoguy128,

    Thanks for the detailed reply. A well insulated house sounds like a must do. I'm not sure about the cost of all that concrete though. The size of the storage tank(s) and complicated system sounds like it might be more costly than one of the more traditional methods.

  10. #10
    Join Date
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    Higher mass materials are defnitely more expensive no good way around that. We quit using plaster in the 60's for interior walls for that reason once gypsum wall board became common. Drywall isn't a suprerior material, it's Just a lot less expensive and requires a lot lower skill level to install. There's no real advantage to stick frame homes over masonry, except cost and speed of construction. Wood has all sorts of issues like dimensional tolerances and stability, warping, sagging over time, flammability, tolerance to moisture, stiffness. It takes a real craftsmen to build a frame home that is not only quare and plumb during construction, but remains that way several years later as the wood ages.

    Sometimes it's best to keep it simple until turnkey systems are more proven.

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