Biodiesel may be an alternative to fossil fuel, however, it still is a one-way street.
I found a report on the EECA website, CRL energy report on life cycle asssessment of tallow biodiesel, and it's quite interesting to read. As usual, rather confusing data display but the bottom line is clear: By using tallow for biodiesel generation, we can save something around 50% of green house gas emissions.
Sounds fantastic, however, what the report does not hilite and explore is the fact that even a reduction by 50% of diesel emissions won't cut the cheese long term. Plus it does not provide an answer to the question of how much tollow would be required to produce all that biodiesel to actually replace all diesel by biodiesel. Who knows whether we have enough land and animals to do so?
The report states that we can get 36.0 kg tallow out of a cattle. It also states that one kg of tallow would give 1kg of biodiesel.
"A higher heating value energy density for biodiesel of 39.95 MJ/kg" is the key: With 1kg of tallow, we can can therefore achieve a yield of 39.95 MJ.
Quote: "There is also sufficient feedstock of domestic tallow to meet the government’s proposed biofuels sales obligation to 2012".
According to http://www.nationmaster.com/graph/ene_gas_oil_imp-energy-gas-diesel-oils-imports, New Zealand did import 709,000 tons of diesel in 2005 - quite some time ago and I would think that this has increased.
So that is 709,000,000 kg. To replace that - let's assume for now the energy density would be the same to offset a wee bit the increased consumption - we would need 709,000,000 kg tallow and that is 709,000,000/36kg cattle = 19,694,444
According to http://www.beef.org.nz/statistics/sld0011.asp, we had total of 9.61 million beef and dairy cattle in NZ. That still leaves us short of about 10 million. Which I guess could possibly made up of sheep, however a sheep only gives 2.8 kg of tallow compared to 36 kg for cattle. So we would need an additional of approx 10*36/2.8 million sheep = 128 million sheep.
According to http://www.beeflambnz.com/main.cfm?id=31&nid=291, we have about 33 million sheep. However, number of sheep are rather disappearing (http://tvnz.co.nz/national-news/sheep-jokes-aside-kiwis-lose-their-friends-2517446).
Would leave us still short of 95 million sheep. And we have not yet accounted for any New Zealand manufactured diesel which does not show up on the import list....
And yet, we are still burning fuel, be it fossil or bio diesel. If every Indian and Chinese would do that, we would have desaster.
Food for thought.