Comparison of Biomass and Solar Energy Yields

Basic Biomass and Solar Energy Calculations

These are very important calculations. More below.

On Mon, Aug 4, 2008, Greg Walker <greg@rtd.gb.com> wrote:

We were talking about energy yield from biomass. These are some of the figures that I have:-

If we start with 1 cubic metre of fresh cut timber, its cellulose content will yield 1476 kilowatt hours of fire power. Moisture content will lower the temperature at which that can be delivered. Cellulose content would be about 450 Kg.

Convert that material to charcoal, & you can get 135 Kg, which will give you 1150 kilowatt hours of fire power.

Convert it to Pyrolysis Oil and you get 315 Kg (25% moisture content), giving 1480 kilowatt hours of fire power.

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That is an amazing yield. Is this proven technology, or are you talking of a theoretical value? Please reference this if you can.

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If we take woodland growing at yield class 15, it will deliver 15 cubic metres of timber per hectare, per annum – on average. <

Tell me more about growing classes. I heard from sustainable forestry people back in Wisconsin, where I lived before, that the sustainable yield is about 2000 lb/year/acre - or about 4 cubic meters/hectare/year. Is your value for heavy-input plantations, or natural, integrated forests?

> So 1 hectare should give us say 22,000 KWH of fire power per annum, about 2.5 KWH per hour, or an average output from burning the biomass created, of 2.5KW. That’s 2.5 KW per Hectare, averaged for the whole year. Burn that to make electricity, & you're down to 250 Watts/Ha.

Compare that with peak sun power of 1 KW per square metre. That's 10,000 KW/Ha. Allow for night & low sun, & you're down to 2,500 KW/Ha.

I don't know how much sunshine we can get into water with solar collectors, but, I guess the bottom line is “Solar power conversion doesn’t have a lot of competition from biomass”. Of course, after that there's still the same ~ 90% loss of converting the steam to electricity. <

Right.

> I understand that public utilities charge for $0.10 per KWHr for electricity in the US, but in the UK it’s about twice that – or it was until a few months ago, I think. It might have moved quite a lot since then. That puts biomass sales value at $200 per Ha per annum when converted to electricity.

If you can get say 5 bhp from 30 square metres of solar collector, peak – that’s 0.125 KW per square metre peak. It works out at 300 KW/Ha, averaged over the year. Lets go back to our electricity valued at $0.10 per KWHr. 1 KW year = 8760 KWHr = $876 per KW year. So our 30 square metre solar collector should deliver 5 x 876 x .75 = $3,285 per annum. 300 KW/Ha works out at $250,000 per hectare per annum. Wow. <

The cost for that amount of concentrator will be $1.2M based on linear extrapolation of our present system cost - which excludes labor cost because of DIY construction. This means 5 year payback.

Now to reality. For the first prototype with straight steam generation and no heat recovery, we're expecting 500W (2% overall efficiency) of usable power - based on Mike Brown's 1 hp steam engine and required steam flow rate. That's a start, and I think, at that scale, we can reach 15% overall efficiency if we are good - or 3.5 kW of usable peak power.

At that rate, the payback time will be 4 years.

Overall, on economic terms - the harnessing of solar energy is viable. On ecological terms, it is absolutely essential.