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Renewable Energy: Splitting thermal store from wood boiler

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    • CommentAuthorscrimper
    • CommentTimeJul 17th 2013
     
    Just exploring the alternatives to a GSHP for an upcoming renovation, and given that we've got quite a bit of wood etc, I thought I'd quickly explore the biomass options.

    The boiler itself would have to be sited 50-75m away in an outhouse (noise/mess/no room inside etc.) ... but would it be ok (or even preferable) to keep the actual thermal store inside the main house? I gather that solar thermal is quite a good companion to a wood boiler as it cuts down on the more sporadic summer use, and we have a nice big flat roof on the main house where we can site this. Basically want to keep all the pipework to a minimum ...

    p.s. another quick question - can anyone back up the claim I read elsewhere that wood pellets occupy just under twice the space of oil, for the equivalent amount of energy they contain? Can anyone out there help verify that!?
  2.  
    IMO 50m is too far to have the boiler from the house. the losses would be too much. Even with the best insulation (which by the way is v.expensive) you would still get what I would think of as unacceptable losses. If you must do this then (again IMO) you would be better off with the TS next to the boiler as you would then be pumping cooler water to the house (60 -70deg) as opposed to the boiler temp water to recharge the TS (up to 90deg.) The lower temp will give lower losses due to the lower differential between the pumped water and the ambient temp. But as I said at the beginning I wouldn't do it, its too far (IMO). Whilst there is mess with a biomass boiler there is no real noise from the boiler.
    • CommentAuthorscrimper
    • CommentTimeJul 17th 2013
     
    Thanks Peter - interesting.

    I keep reading about biomass being used in district heating solutions, so I'm surprised by your reaction to the distance involved in this one. But I take your point about the differentials.

    I guess if the TS was housed with the boiler, then I could always have a separate, smaller tank in the house for the solar thermal. Otherwise it would seem crazy to run the heat from the solar thermal on such a big round trip ...
    •  
      CommentAuthordjh
    • CommentTimeJul 17th 2013
     
    Posted By: Peter_in_HungaryIf you must do this then (again IMO) you would be better off with the TS next to the boiler as you would then be pumping cooler water to the house (60 -70deg) as opposed to the boiler temp water to recharge the TS (up to 90deg.) The lower temp will give lower losses due to the lower differential between the pumped water and the ambient temp.

    Except that the primary flow between boiler and thermal store only takes place whilst the boiler is firing. The secondary flow from the thermal store is either on-demand when a tap is opened (leading to extremely long delays before hot water flows and large associated losses) or else there is a circulation pump to get a quick response which means the losses are 24x7. Further, the standing losses from the thermal store itself can be useful if the store is in the house but are a complete loss if not.

    But I agree with you that the distance is too great in any case. I believe external furnaces are quite common in the US, so there might be useful information to be gleaned there.
    • CommentAuthorSeret
    • CommentTimeJul 17th 2013
     
    Just wondering, would the alternative GSHP have to be located in the same spot? If so you're looking at installing some very heavily insulated pipework anyway...
    • CommentAuthorscrimper
    • CommentTimeJul 17th 2013
     
    Good point about the secondary flow djh - that is a very strong argument for keeping it in the house just on its own, and compounded by avoiding the cost of a second tank for the solar thermal contribution. I shall try and find a suitable location that is perhaps 10-15m away!

    No Seret - got space for the GSHP & TS in the house, but not the Wood Boiler & TS ...
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 17th 2013
     
    Diesel is about 46 MJ/kg, wood is about 16 MJ/kg
    Density of Diesel is about 900 kg/m^3
    Wood Pellets are about 650 kg/m^3

    So that should be about 41.4 GJ/m^3 (11,500 kWh) for diesel
    and 10.4 GJ/m^3 (2,888 kWh) for wood pellets

    So about 4 times the storage volume needed, then you need to take the actual running efficiency of the two systems into account.
    • CommentAuthorscrimper
    • CommentTimeJul 17th 2013
     
    Thanks for those calculations ... yes, it was as I suspected ... sounded too good!
    • CommentAuthorsune
    • CommentTimeJul 17th 2013
     
    I get heat losses of around 20W/m for a preinsulated underground pipe with a 40mm flow and return in a 160mm jacket based on 70 flow, 50 return. Not all that much really. Well it depends on the scale of the boiler going in I guess. It would be a lot if it were a 5kW stove....
    A very common setup is a log boiler with store in a boiler room connected to the heating system in the house and a solar dhw tank in the house with priority to hot water.
    Simple.
    How come you are asking about pellet energy density do you plan to make pellets? It sounded like you had logs to me?
    If you have a log boiler then your store is usually pretty big to stop you having to load the thing all the time in which case its much better to have a smaller solar enabled dhw tank as well even if the boiler were in the house.
    ?
    • CommentAuthorscrimper
    • CommentTimeJul 17th 2013
     
    Thanks sune ... yes, could live with 20W/m!

    Asking about pellets because I would probably go for an SHT Thermodual system. Like many others, I don't want to be a slave to it and like the automation of pellets, but when I have the energy I will be very happy to make the savings & load it up with some of our own logs.

    Need to get the economics & geography of it all sorted before thinking about sizing the store(s) ... but do appreciate it's a pretty critical issue ...
    • CommentAuthorJeff B
    • CommentTimeJul 17th 2013
     
    Scrimper - I have an OPOP wood pellet boiler in my garage. This is an unsophisticated beast which requires manual cleaning, yet the noise and dust produced is minimal. What about building a small shed or lean-to at the back of the house? I have seen wood pellet boilers in small shipping containers too!

    As you are asking about the density of pellets I assume you are concerned about the potentail size of the storage container and not having a convenient place to site it?

    Jeff
    • CommentAuthorscrimper
    • CommentTimeJul 18th 2013
     
    Thanks Jeff ... good to know that these things aren't the medieval furnaces I sometimes I imagine them to be.

    I have seen the 'energy cabins' that Euroheat produce (at quite a price ...), so I will try and work something out along the same lines, with the same sort of footprint. Just a bit of a challenge to integrate it with the finished and approved plans for the existing house. In my dreams I'd have it all tucked away in a massive cellar, with two discrete trap doors for pellet & log deliveries ...

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