Fuel - replacing oil with bio-fuels

The calorific value of petrol is approximately 10 kWh per litre, diesel is about 11 kWh per litre and jet fuel is about 10.5 kWh.
(As a family we currently use about 20 litres per week in our diesel powered people carrier, which translates to approximately 30 kWh per day - that's 50% higher than our electricity usage but much lower than our currently rather excessive gas usage.)

Much is being made of the possibilities of bio-fuels to replace oil based products.  These are fuels produced from plants that are processed and converted into oils - which can then be refined in a similar manner to traditional oil to produce petrol, diesel and jet fuel equivalents.  I still have a lot to learn about this whole industry - but there are a number of challenges to making this a viable approach for replacing fossil fuels for vehicles.

So called first generation bio-fuels have received a lot of bad press because of the land area that they take up.  Farmers have converted land previously used to grow food because of the grants available - resulting in shortages of certain food crops and pushing up food prices.  The simple fact of the matter is it takes a LOT of land area to fill a petrol tank.  Taking some facts from Mackay's Book, 1 hectare of rape will produce about 1200 litres of bio-diesel.  That would run our car for a year at our current rate of consumption.  There are about 33 million vehicles registered in the UK - as a first order of approximation we'd need 33 million hectares of rape seed or similar.  The only slight problem is that the total area of the UK is only 24 million hectares - and a good portion of that couldn't be farmed!  And we haven't even started thinking about planes yet!

There are a couple of easy to read sites promoting the use of bio-fuels for aviation.
http://www.flyonbiofuels.org/?gclid=CJPgjLTHr50CFYIA4wodlnSohQ
http://enviro.aero/Default.aspx
In particular they promote the use of algae as a more efficient and less environmentally damaging way of converting sunlight into fuel.  The benefit is that algae don't (necessarily) take up land area that can be better used for food production.  They can grow in all sorts of fresh and saltwater environments.  The catch however is that to grow them efficiently you need to feed them with concentrated CO2 - of which ironically there is a limited supply.

The aviation industry currently uses about 250 billion litres of aviation fuel (kerosene or Jet-A) per year.  The above sites suggest that 1% of this could come from bio-fuels by 2016 and as much as 15% by 2020 and 50% by 2040.  These figures seem wildly optimistic to me - although perhaps genetic engineering or some other technique can improve on the efficiency of the growth process.  Currently a farm in the US feeding algae with 10% concentrated CO2 produces 0.01 litres per square metre of bio-diesel per day.  That's about 36500 litres per hectare per year, or about 30 times more than rape seed.  Better, but a long way short of good enough.  To get to 125 billion litres just for aviation will require 3 million hectares of algae farms.