This page http://www.viking-house.co.uk/ says:

“For the cost of a sewage connection they generate 10-12kWh per kg of waste.”

10 kWh/kg = 36 MJ/kg. (Checked with a calculator but I still wonder if I've slipped a decimal point somewhere).

Compare this with the energy content of petrol:

https://en.wikipedia.org/wiki/Gasoline#Energy_content

of 42.4 MJ/kg.

It doesn't sound physically impossible but does seem rather optimistic. Even assuming that's the dry mass of the waste there's still lots of other chemicals included, most notably oxygen and nitrogen which don't contribute (at least not in the same way).

http://www.newton.dep.anl.gov/askasci/chem03/chem03702.htm

The VH page then goes on to say:

“An average family producing 50kg of waste per week continuously generates 4kWh of electricity and 6kWh of heat.”

Right, is that 4 kW and 6 kW or 4 and 6 kWh/week? When will this ongoing muddle stop?

50 kg/week x 12 kWh/kg = 600 kWh/week = 3.57 kW. Neither interpretation seems to make sense.

Maybe the original 10-12 kWh/kg was just for the electricity and the heat is extra. For 6/4 = 1.5 times as much heat that would mean that the extraction would be 36 MJ/kg × (1 + 1.5) = 90 MJ/kg which is over twice the amount of energy in petrol. I'd be more than a little sceptical.

No reason why you cannot use a CHP system running on gas if you have deep enough pockets

http://cleanergy.com/wp-content/uploads/2014/06/cleanergy_brochure_chp_lowres.pdf

It's not from this lot: https://en.wikipedia.org/wiki/Steorn is it?

Seriously, good luck with it and I hope there's something useful there though I'm deeply sceptical that it's quite as powerful as your page seems to say but even a tenth of the output would be interesting. Please sort out the units so we can at least be sure of what's being claimed. However, while I think it's great to talk about such things I think it's a bit unfair to answer somebody asking for specific advice for a particular project with something as speculative as this.

I'm really intrigued by this.

Ed, i think the company is on the link below, he's making some amazing claims on his linked in page, great if it all works

http://uk.linkedin.com/pub/graham-taylor/1b/335/8a6

http://www.future-energies.co.uk/future_energies/1,1,387.html

Posted By: RobL0.13 W/m2

Seems about right, I think that sugar cane is the best you can currently get.
Why I never understand people wanting to grow trees to burn, stick some PV and get 8 times the yield. Even storing it in batteries is going to be better, though probably a higher up front cost. No idea what a reasonable chainsaw, log spitter, WBS and timber store costs, probably not much different than a PV system with a couple of kWh of lithium storage.

Posted By: SteamyTea
Why I never understand people wanting to grow trees to burn, stick some PV and get 8 times the yield. Even storing it in batteries is going to be better, though probably a higher up front cost. No idea what a reasonable chainsaw, log spitter, WBS and timber store costs, probably not much different than a PV system with a couple of kWh of lithium storage.

Maybe because people want to be warm. A couple of kWh of storage isn't going to heat anything much and the sun doesn't shine for very long in winter.

And trees are pleasant to look at in the 10-20 years that they're growing!

Posted By: billtAnd trees are pleasant to look at in the 10-20 years that they're growing!

True, but you can't burn them while they are growing. And you can't replant where you have just cut them down.

Posted By: renewablejohnEnergy from oil seed rape is not just in the oil thats produced.

You don't get that much more even of you dig the whole lot up and burn it, even after gasification.

Posted By: SteamyTeastick some PV and get 8 times the yield

May have got my decimals in the wrong place.
Trees at best turn about 0.25% of the suns energy into usable biomass, so that will be about 2.38 kWh/m^2
PV can turn about 10%, or about 95 kWh/m^2
So trees are about 1/40th as efficient.


As for storing the energy as logs for winter use, the charging time is rather long, about 40 years.
True that there are some coppice crops that can be harvested every year, one of the better ones being miscanthus. You get about 14 tonnes per hectare, or 1.4 kg/m^2 and it has an energy content of about 17 MJ/kg (4.72 kWh/m^2). That is about 0.5% efficiency. Or about a 20th of what PV can do. The charging time is still 11 months though.

<blockquote><cite>Posted By: SteamyTea</cite>Bit of cross over there John.
It is still an appalling efficiency compared to PV.</blockquote>

I do really question your efficiency figures see below quoting 5-6%

http://rsta.royalsocietypublishing.org/content/295/1414/477

But then I think where trying to compare Apples and Pears

ie Annual efficiency for biomass compared to summer efficiency for PV. Peak summer output for PV may well be 15-18% but in the depth of winter it is more likely to be zero.

Posted By: renewablejohnI do really question your efficiency figures see below quoting 5-6%

"The maximum efficiency of solar energy conversion in plant production is 5-6%, but plants grown under usual field conditions do not achieve this degree of conversion. The highest yielding crops convert solar energy into plant material with an efficiency of 1-2%, but the average yields of the major crops and forests indicate considerably lower efficiencies. The average efficiency of solar energy conversion on a global scale is estimated as about 0.15%."

Now you read maximum of 5-6%, while I see the average efficiency of 0.15%.

Strange that.

Posted By: SteamyTeaI base my efficiency on how many kWh of solar energy hits a square metre of ground, then work out what I can get out of it with different technologies. I use 10% for PV, or just over half what is currently available.

How do you work yours out.

As for peak summer PV, do many plants grow when there is little sunshine? That was my quip about 'long charging time'.

PV at 15-18% based on solar of 1000 per m so halve and round up to give 10% yet our friend Mackay reckons only 100 per m. All sounds very iffy to me.

As I have said before plants will grow with very little sunshine it is heat which determines growth rates. Many plants will actually shut down in bright sunlight.

<blockquote><cite>Posted By: SteamyTea</cite>Ed
How about making a large, flat roofed shed, putting PV on the top, LED lighting of the right frequency on the inside, plant the willow and burn the extra 16 MWh/year (about 3 times what I use that is).

Actually, I have a better idea, let RenewableJohn have a go at it<img src="/forum114/extensions/Vanillacons/smilies/standard/bigsmile.gif" alt="" title=""></img></blockquote>

No need to you can forget the LED and just purchase one of these greenhouses

http://www.polysolar.co.uk/Case-Studies/greenhouses

Posted By: Ed Davieshttp://mammothwillow.co.uk/shop/index.php?main_page=product_info&products_id=93" rel="nofollow" >http://mammothwillow.co.uk/shop/index.php?main_page=product_info&products_id=93says

“An acre field will give you around 5-6 tonnes of burnable wood chips or logs and kindling (at 30% moisture). In energy terms this equates to 1400 litres of heating oil or 16,000 kWh of gas.”

An acre is 4047 m². Just taking the 16,000 kWh (assuming it means per year) at face value that's 1826 W/acre or 0.45 W/m².



According to PVGIS 150 W (roughly a m²) of PV laid flat on my site in Caithness would produce 116 kWh/year or 13.2 W/m².

Sloping the PV would be cheaper but thinking about spacing, shading, electricity prices at different times of the year, etc, is left as an exercise for the reader.

Hope your happy with your solar panel in the middle of winter producing zero output at least I would be warm and generating my own electric on demand using my wood.

Posted By: renewablejohnThere actually a bit better performance then that due to better absorption of reflective light so better performance on dull days

Horizontal modules give pretty good performance on cloudy days. I noticed a solar farm on the way upcountry that had the modules at about 15°. Can pack a lot more in and probably give a better yield than south facing 'optimal angle' modules. May also have been a condition of connection.

How much timber/biomass do you need to burn to keep your place warm, then we can make a direct comparison with PV.

Posted By: renewablejohnPointless to do a biomass comparison

Not pointless at all. It is a comparison between two technologies, not between two different houses. It makes no difference where the fuel comes from, it still has an energy content.
It would be interesting to know how many tonnes of timber you have to shift and what land area that took up. Simple enough to work out from there.

Is your new PV system going to have separate monitoring on the two aspects? Would be interesting to see how they differ as not many people put them on a North facing roof, though not many people put them on at such a shallow angle.

Posted By: SteamyTeaNot pointless at all. It is a comparison between two technologies, not between two different houses. It makes no difference where the fuel comes from, it still has an energy content.

What bit of "the sun doesn't shine much in winter" don't you understand?

You won't get enough energy to be usable from PV in the winter unless you have an enormous array and an enormously expensive and resource intensive store to go with it.

It's a quite pointless discussion anyway, as both energy sources use much too much land area to be useful to anyone who doesn't own several hectares - so completely irrelevant to 99.9% of the population.