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Green Building Bible, Fourth Edition
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    • CommentAuthorseprice
    • CommentTimeDec 15th 2011 edited
     
    Hi, can anyone help me - I am trying to model the potential heat gain from a solar thermal store of soil under a house using PHPP! Heat will be delivered to the soil using a bog-standard solar thermal system. I am struggling to find the basic facts - firstly how hot can a solar thermal system run? i.e. I am currently assuming a cut off of 70 degrees C in the pipes, but I know I could deliver enough energy to take the soil up to 140 degrees in the summer (!!), but then my solar thermal system wouldn't work at these temperatures would it?? Can I assume an average temp of 70 degrees C for the whole solar store (it's only 36m3 and it will be insulated)? How quickly will the heat move away from the soil around the pipes in the solar store?

    Secondly I am assuming that the whole solar store will need to be insulated around the bottom and sides, to minimise heat loss to the surrounding earth. Will I then need insulation on the top as well as this is directly below the house and could potentially be 70 degreesC!? I don't want to be too hot in the summer.

    Just in case there are any techy Passivhaus people out there- here's a passivhaus question: I am currently overwriting the 'monthly average ground temperature' values in the 'Ground' worksheet (which I have calculated from the specific heat capacity of the soil and energy delivered by the solar thermal system etc. etc.) for the area under the floor which is solar store- I have another 'Ground' worksheet for the rest of the house. This means the monthly method doesn't work (as I've got two lots of 'monthly average ground temp' data),so I have to use the annual method. Any comments on this- would you do it another way, or would you rely on the annual method?

    Thanks for your help everyone

    Cheers
    Sarah
    :bigsmile:
    • CommentAuthorseascape
    • CommentTimeDec 15th 2011
     
    Can't help on using PHPP but searching these threads maybe helpful - in one of them it gives a rate at which heat moves through the earth:

    Seasonal heat storage paper
    Passive annual heat storage by John Halt
    Passive House with no tapes/membranes,no ducts,no back up heating system
    Seasonal Heat Store
  1.  
    Hi Sarah, How big is the house? What will be in the store? How many m2 of Solar will you have? This is what we're doing!
    •  
      CommentAuthordjh
    • CommentTimeDec 16th 2011
     
    Posted By: sepriceHi, can anyone help me - I am trying to model the potential heat gain from a solar thermal store of soil under a house using PHPP! Heat will be delivered to the soil using a bog-standard solar thermal system. I am struggling to find the basic facts

    It sound like you either haven't tried searching or have a poor technique. Please supply more details of what you've tried so we can help diagnose your problem.

    firstly how hot can a solar thermal system run? i.e. I am currently assuming a cut off of 70 degrees C in the pipes, but I know I could deliver enough energy to take the soil up to 140 degrees in the summer (!!), but then my solar thermal system wouldn't work at these temperatures would it??

    You can run a solar system very hot. google for 'thermal oil' or 'process heat' together with solar

    Can I assume an average temp of 70 degrees C for the whole solar store (it's only 36m3 and it will be insulated)? How quickly will the heat move away from the soil around the pipes in the solar store?

    Heat moves slowly in soil so you can't use an average for the store. You need to take account of the differences.

    Secondly I am assuming that the whole solar store will need to be insulated around the bottom and sides, to minimise heat loss to the surrounding earth. Will I then need insulation on the top as well as this is directly below the house and could potentially be 70 degreesC!? I don't want to be too hot in the summer.

    Those are all design choices - people have built all variations. You need to read about their experiences. The links seascape gave provide a starting point to find them.

    Just in case there are any techy Passivhaus people out there- here's a passivhaus question: I am currently overwriting the 'monthly average ground temperature' values in the 'Ground' worksheet (which I have calculated from the specific heat capacity of the soil and energy delivered by the solar thermal system etc. etc.) for the area under the floor which is solar store- I have another 'Ground' worksheet for the rest of the house. This means the monthly method doesn't work (as I've got two lots of 'monthly average ground temp' data),so I have to use the annual method. Any comments on this- would you do it another way, or would you rely on the annual method?

    PHPP can already cope with multiple different surfaces under the house - basements versus ground etc so I expect it should be fairly easy to plug in monthly figures for a 'hot' area. I wouldn't trust the annual method alone - it can give significantly different results from the monthly method (and is not allowed in some circumstances).

    I would do all the modelling of the heat store in some other way and then just plug the results into PHPP.
    • CommentAuthorseprice
    • CommentTimeDec 16th 2011
     
    Thanks for all the comments. Seascape, I have tried searching threads before I wrote the post and I did find some useful information - I will look out for that 'rate of heat flow' though. There seems to be a wealth of information on interseasonsal heat storage but very few numbers/facts, for example, how much of the annual heat demand will it provide?

    VIking house - brilliant, I had been meaning to ring you about this as I had noticed you were installing such systems and I've read all the papers on your website. Who's the best person to speak to?

    djh - thanks for the comments. I know that heat moves slowly in the soil, but won't the soil heat up more each year and plateau out? I started by accounting for the difference by having 70 degrees at the bottom of the solar store where my pipework is and 30 degrees at the top of the store where the house is, but then I realised I was just guessing at the temp gradient, so it prob isn't v accurate! Anyway it's beside the point as PHPP requires an average temperature for the ground.

    Thanks all
    Sarah
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 16th 2011 edited
     
    You can calculate it in exactly the same way that you would for a normal solar hot water system. All you have to do is change the figures. It is easy to get lost in the calculations though.
    First thing you need to know is the solar resource, the air temperatures and ground temperature for your area. I recommend weekly values as monthly is a bit crude during the shoulder months.
    Then you need to know the heat load of your house (actually you should know this already as you cannot estimate anything without it).
    Then the size and shape of store, the insulation levels and the specific heat capacity of the earth (try 800 J.kg^-1.K^-1 as dry earth is about that).
    Then you need to find a suitable ET system that can collect enough energy to deliver to the store and at the temperature you need, this is dependant on the mass of the working fluid as well as the losses.

    Then start by modelling without any heat losses. This then gives you the ideal. Then you start 'adding in' in the losses from both the heat store and solar system. Treat the ground and air as infinite heat sinks. Remember that the higher the temperature the higher the losses and they are not linear.

    Had a quick google and soil thermal conductance seems to be 0.3 W.m^-1.K^-1
    • CommentAuthorGavin_A
    • CommentTimeDec 18th 2011
     
    I'd be wary of the thermal expansion problems from a soil based store running at those temperatures below a house.

    While not really designed to operate at those temperatures as such, most conventional solar water heating systems will run at those temperatures, the main problem I can see would be the pump, which would usually be expecting to only ever see temps of around 100degC or lower as it's on the return side of the system from the water tank. Your return flow would be significantly higher than this, which would likely cause problems with some pumps.
    • CommentAuthorfclauson
    • CommentTimeDec 18th 2011
     
    All I can say is that the ground will NEVER get to 70 degrees let alone any higher - the losses are just so great

    SteamyTeam and I built a spread sheet which showed that the store would hardly move in temp - and is some instances if the insulation between your screed and slab are not thick enough then the store will suck heat from the house (i.e. work in reverse to what you expect)

    You also have the issue of getting the heat out of the pipes into the soil/soil material - the temp next to the pipe might rise but once its risen you cannot pass as much heat to it and the thermal conductivity of most store materials is very poor

    Viking has an approach which gives some benefit - but the thermal dynamics of it all are very complicated and very situation dependant.

    At the last PHPP conference there was a worked model of this where they used the store as a heatpump source - this was expensive and only sort of worked

    You mention 36M3 of the store - what is its surface area - this is much more important. A store of 36M3 in a perfect sphere has a surface area much smaller surface area than a 1cm thick slab of 36M3 - surface area is key

    Hope this has not put you off

    If a house is built to a passive standard then its heat demand will be very low - the occupant’s usage will vary the heat demand the most. I would spend all your efforts getting the build right and then pay a little to heat the place to the temp you want.

    Heating of a passive house is a whole other topic - the PHPP model makes certain assumptions - again it’s about surface are and heat distribution. Some would have you believe you can heat a house with the odd towel rail or two - the answer is may be - but maybe not. Its about the shape again. Would you cook a Sheppard pie by getting one corner very hot and hope the rest would get warm by convection & conduction!!
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 18th 2011 edited
     
    Posted By: fclausonWould you cook a Sheppard pie by getting one corner very hot and hope the rest would get warm by convection & conduction!!

    Isn't that how it is done, the Sheppard never complained, said he had to run of to 'tend his sheep' :bigsmile:
    • CommentAuthorDavipon
    • CommentTimeDec 18th 2011
     
    Shepherds, surely?:devil:
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 18th 2011 edited
     
    Yes, his name was Alan (though that may have 1 silent P):shamed:
    • CommentAuthorDavipon
    • CommentTimeDec 18th 2011
     
    It would for the astronaut:bigsmile:
    • CommentAuthorfclauson
    • CommentTimeDec 19th 2011
     
    So I should have said Cottage pie instead :cry:
    •  
      CommentAuthorJSHarris
    • CommentTimeDec 19th 2011
     
    <blockquote><cite>Posted By: fclauson</cite>So I should have said Cottage pie instead</blockquote>

    Isn't cottage pie a different thing? My understanding is that shepherds pie is made with lamb or mutton mince and cottage pie is made with beef mince.
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 19th 2011 edited
     
    Cottage pie uses sliced potatoes rather than mashed as well I think.
    Regional variations cause havoc. Down here, the filling is wrapped in pastry and has to survive the drop to the mine floor.
    • CommentAuthorfclauson
    • CommentTimeDec 19th 2011
     
    To bring it back to the original discussion which of the two is a better interseasonal store then
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 19th 2011
     
    Huge insulated tank of water inside the house. If you can control the the temp loss so that the surface temperature is 35C, then it could be a novel method of heating.
    •  
      CommentAuthorJSHarris
    • CommentTimeDec 19th 2011
     
    <blockquote><cite>Posted By: SteamyTea</cite>Huge insulated tank of water inside the house. If you can control the the temp loss so that the surface temperature is 35C, then it could be a novel method of heating.</blockquote>

    Bit warm for a swimming pool.............

    Which reminds me, didn't the GD house with the heated glass have an indoor swimming pool? Maybe that was being used as an inter-seasonal thermal store to overcome the losses from the heating system.
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 19th 2011
     
    Posted By: JSHarrisBit warm for a swimming pool


    Not for me :surfing:
    •  
      CommentAuthordjh
    • CommentTimeDec 19th 2011
     
    Posted By: SteamyTeaHuge insulated tank of water inside the house. If you can control the the temp loss so that the surface temperature is 35C, then it could be a novel method of heating.

    Indeed. I don't know what the surface temperature would be but the heat loss rate from a 'seriously well-insulated' 'bl**dy big' tank of 'very hot' water is comparable to the heat needed by a passivhaus. :bigsmile:

    Sadly, such a tank and the associated collectors probably cost a 'whole h*ll of a lot' more than a gas boiler :cry:
  2.  
    Here is some practical research into a water based seasonal store.

    http://scanhome.ie/research/solarseasonal.php
    • CommentAuthorseprice
    • CommentTimeDec 21st 2011
     
    Dear All,

    Thanks for all your time in answering my query. It has all been very interesting an helpful.

    Cheers
    • CommentAuthorDavipon
    • CommentTimeDec 21st 2011
     
    Seprice. Before you bale out, is it worth using the obviously developable space under the house as a heat store and heat dump through a ground source type of set up, backing it up with solar thermal for DHW?
  3.  
    Hi Serpice, if you can heat the ground beneath a Passive House to 20 degrees then your house isn't losing any heat to the ground, this reduces the heating demand by 25-30%.
    • CommentAuthorwookey
    • CommentTimeDec 24th 2011
     
    seprice - a bog standard (ET) solar system can run up to about 200C, but it's not providing any power at that point as it's losing it all to the surroundings. Loses as a proportion of collected heat rise quickly as the system fluid temp rises, typically reaching equilibrium somewhere between 140 and 200C (where it will be collecting, say, 2kW and dissipating 2kW at the same time).

    Given a sufficient heat exchanger the system fluid temp (when running) is controlled almost entirely by the store temp. Having a heat exchanger that gets all the available heat into soil requires a very large one as it's such a poor conductor. It's easy with water.

    I too find this subject (interseasonal stores) fascinating and would like to hear how you get on. It does seem to be something which is possible, but quite tricky to get right, and is usually deemed not cost-effective on individual houses.

    Tony - how are your measurements going? How are other existing projects doing? Viking - do you have annual numbers for some of your builds now? As djh said - quite a few of these have been built now of one sort or another. I hope we were getting some idea of what did and didn't work in practice.
    • CommentAuthortony
    • CommentTimeDec 26th 2011
     
    Mine is working, not as well as I had hoped. But then my first two winters were the two coldest in recent memory and outside temperatures fell well below my design assumptions. Cold has depleted the store temperature more than I had expected especially last year when it was so cold before Christmas. This year has been way better. I am holiday so can't give numbers but will when I get back. Thete is some info on my website <tonyshouse.info>.
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