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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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  1.  
    Hello,

    I'm reading David MacKay's Renewable Energy Without the Hot Air, and I'm after some advice about how to apply what I've learnt from it, thinking about my home and my parents'.

    My rural parents have about 4000m2 that could be devoted to fuel crops such as willow coppice. According to MacKay, willow coppice yeilds about 0.5W/m2, so about 48kWh/day, plenty for their heat and hot water. Could someone point me in the right direction for information about how we could make this happen - information on growing the willow, suitable back-boiler stoves etc.

    In the book, MacKay says that people who don't have access to biofuels should opt for a technology that is apparently banned from discussion on this forum, because the so much land would be needed to grow the biofuels for wood fired heating. To heat my urban maisonette would use about 2000m2 of land, whereas there's only 3000m2 per person of land in this country for agricultural production.

    It would be nicer to have a woodchip stove for central heating and backup DHW to the solar thermal system, than an ashp, but is this really sustainable? Or would it be ok if I became a vegan to offset my land-take from woodchip growing? What are your thoughts about this?
    • CommentAuthorsimeon
    • CommentTimeApr 4th 2009
     
    How much your parents use really depends on the size of their house and the state of their insulation. An average of 48kWh per day over the year sounds a little low as boiler efficiency may mean that you only get 35 - 40 kWh output. My consumption of gas energy (including cooking) is now down to 60kWh a day for our 4 bed Edwardian semi after a lot of insulation work. This is below the national average for a house of similar size and age.

    What is good though is that you have stated the amount of land required to fuel a house consumption from biofuel. I think 5 hectare would be preferable though. I doubt whether any more than a million hectare could be practically devoted to biofuel and that means that the country can only sustain about 200 000 homes from this renewable source.

    I think your parents would need another source of energy eg solar pv, solar thermal panels and wind turbine (if they live in a windy area).

    Personally, I am against the use of wood fuel in any built up area because of health impact and I doubt whether a conversion to veganism would free up sufficient land for biofuel production. Spend money on insulation which should reduce your energy consumption is the best advice.
  2.  
    Posted By: simeonHow much your parents use really depends on the size of their house and the state of their insulation. An average of 48kWh per day over the year sounds a little low as boiler efficiency may mean that you only get 35 - 40 kWh output. My consumption of gas energy (including cooking) is now down to 60kWh a day for our 4 bed Edwardian semi after a lot of insulation work. This is below the national average for a house of similar size and age.

    What is good though is that you have stated the amount of land required to fuel a house consumption from biofuel. I think 5 hectare would be preferable though. I doubt whether any more than a million hectare could be practically devoted to biofuel and that means that the country can only sustain about 200 000 homes from this renewable source.

    I think your parents would need another source of energy eg solar pv, solar thermal panels and wind turbine (if they live in a windy area).

    Personally, I am against the use of wood fuel in any built up area because of health impact and I doubt whether a conversion to veganism would free up sufficient land for biofuel production. Spend money on insulation which should reduce your energy consumption is the best advice.


    Simeon

    Your figures appear to be way of the mark. If you look at CHP schemes using woodchip you can expect to provide heat and electric for 15 homes on 1 hectare of sustainable ash coppice. Based on this your million hectares would provide 15million homes with power and heating not 200,000 and there is over 1 million hectares of wasteland/roadside verges in the country so food production would not be compromised.
    • CommentAuthormike7
    • CommentTimeMay 1st 2009
     
    Simeon dropped a '10' somewhere. A hectare is 10,000m2, so on MacKay's figure of 0.5W/m2 that'd be 5kW, or 120kWh/day per hectare.
    A million hectares would do for about 2 million homes like architectonic's parents'.
  3.  
    Mike 7

    I will stick with reality of 15 million homes for one million hectares rather than a calculated figure based on some questionable assumptions. Being a professor is no guarantee of the right answer
    • CommentAuthorsimeon
    • CommentTimeMay 1st 2009
     
    I stand corrected. It seems that on the basic figures that a million hectares could provide heating for 2 million homes.

    The present rate of primary energy consumption in the UK is about 1kW for home use. (This is not the energy rate delivered into your home but a figure that takes into account efficiency, fuel preparation and energy transport.) On that basis, a sustainable wood based CHP could provide total power for up to 2 homes per hectare.

    I do think that biomass energy is a poor use of land but you might as well develop it if you are doing nothing else with the land. The most efficient solar pv will provide up to 20W/m2 (take peak output and divide by ten) and this is 40 times that of biomass per m2.

    Why not cover a million ha with solar pv? (a rhetorical question)
  4.  
    Simeon

    Why is it a rhetorical question our CHP plant is co fired using both woodchip and solar thermal (more efficient than solar pv) similar to a co fired plant in Austria which has been generating since 2002.
    • CommentAuthormike7
    • CommentTimeMay 2nd 2009
     
    Posted By: renewablejohnMike 7

    I will stick with reality of 15 million homes for one million hectares rather than a calculated figure based on some questionable assumptions. Being a professor is no guarantee of the right answer


    I see I did not express myself clearly. I only meant to clarify the maths, not to support figures one way or the other.
    • CommentAuthormike7
    • CommentTimeMay 2nd 2009
     
    ...but now I come to look at it, maybe I do have an opinion. Questions, anyway:
    rewablejohn - where is this reality of 15 million homes being supplied by 1million hectares? Might that not also be a calculated figure based on some assumptions?

    Here's a few figures to pick at: Fuelwood yields of 5 to 10 oven dried tons/ha/yr are mentioned here and there (eg forestry commission), with 20 tons being mentioned as an extreme. Dry wood has a fuel value of about 4kWh/Kg. Taking the 10 ton figure, that would give us 40,000 kWh/yr, or 2,700kWh for each of your 15 homes, which sounds awfully little - given nothing has been deducted yet for efficiencies. Have I missed something?

    I see poor architectonics never did get much of an answer to his original question!
    • CommentAuthorralphpr
    • CommentTimeMay 2nd 2009
     
    I have exactly the same situation as architectonic (except they're clients rather than parents). I emailed Professor Mackay about his comments on Radio 4's more or less. He kindly rebutted my inquiry so I scuttled off to his book. When I further prodded him ,he argued thus:

    (quote)First, my book _is_ mainly about the low carbon vision.
    I advocate heat pumps as part of a low-carbon electricity future.
    I say "let's go for heat pumps and make zero carbon electricity", aiming to
    arrive by 2050.

    And second, my book contains the DATA that gives evidence
    that even with electricity being generated 100% from gas, heat pumps
    are still a good idea. If you have better DATA, please provide it.(unquote email with Prof Mackay yesterday)

    I am number crunching now for my site (using IES to crack a nut) but I am a little anxious about his arguments. Anyone got any better ones?
  5.  
    <blockquote><cite>Posted By: mike7</cite>...but now I come to look at it, maybe I do have an opinion. Questions, anyway:
    rewablejohn - where is this reality of 15 million homes being supplied by 1million hectares? Might that not also be a calculated figure based on some assumptions?

    Here's a few figures to pick at: Fuelwood yields of 5 to 10 oven dried tons/ha/yr are mentioned here and there (eg forestry commission), with 20 tons being mentioned as an extreme. Dry wood has a fuel value of about 4kWh/Kg. Taking the 10 ton figure, that would give us 40,000 kWh/yr, or 2,700kWh for each of your 15 homes, which sounds awfully little - given nothing has been deducted yet for efficiencies. Have I missed something?


    The only assumptions relate to consumer usage of 1kw electric and 5kw of heat per household. The point which you have missed is the co-generation being both woodchip and solar so the consumption of woodchip can be reduced by at least one third.Forestry commission yields are normally quoted for roundwood production whereas actual yields for CHP fuel can be far greater as all the brash can also be utilized.

    I see poor architectonics never did get much of an answer to his original question!</blockquote>
    • CommentAuthorJulian
    • CommentTimeMay 21st 2009
     
    Architectonics,
    woodchip fuelled heating may be sensible for your parents if they have the willow on or near their land. But if you live in an "urban maisonette" you might find that the fuel used to transport the very low density woodfuel from the countryside to your home far outweighed the benefits (if any) of using it in an urban setting.
    • CommentAuthorModerator
    • CommentTimeMay 26th 2009
     
    Posted By: architectonicsHello,

    I'm reading David MacKay's Renewable Energy Without the Hot Air, and I'm after some advice about how to apply what I've learnt from it, thinking about my home and my parents'.

    My rural parents have about 4000m2 that could be devoted to fuel crops such as willow coppice. According to MacKay, willow coppice yeilds about 0.5W/m2, so about 48kWh/day, plenty for their heat and hot water. Could someone point me in the right direction for information about how we could make this happen - information on growing the willow, suitable back-boiler stoves etc.

    In the book, MacKay says that people who don't have access to biofuels should opt for a technology that is apparently banned from discussion on this forum, because the so much land would be needed to grow the biofuels for wood fired heating. To heat my urban maisonette would use about 2000m2 of land, whereas there's only 3000m2 per person of land in this country for agricultural production.

    It would be nicer to have a woodchip stove for central heating and backup DHW to the solar thermal system, than an ashp, but is this really sustainable? Or would it be ok if I became a vegan to offset my land-take from woodchip growing? What are your thoughts about this?


    This book will throw you right off the track. get another book to read is my suggestion.
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