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?

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.

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

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.

...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!

<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>

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.