There has been already been a lot of excitement (and heated debate) about the contribution biomass can make to our energy needs. The fact is that although biomass is renewable, it’s not unlimited. There simply will never be enough to meet all our energy needs. We have to choose what to use it for.

While estimates vary wildly, a recent paper published by the German Biomass Research Centre suggests that global biomass output should be kept between about 16EJ and 40EJ per year in 2050 (an exajoule, EJ is 1018 joules). This recommendation considers factors such as sustainable land use, biodiversity and future food demand. In other words, it’s what we can safely produce on this planet. But 40EJ is only about 8% of our current – and growing – global energy demand.

So we need to use biomass wisely, keeping it for situations where other forms of renewable energy won’t work. Transport requires dense energy because the weight of the fuel needs to be dragged around inside the vehicle. And as planes are never going to be run on batteries or photovoltaics, aviation should be a high priority for biomass energy in a clean energy future.

Air traffic global energy consumption is currently about 8EJ and is expected to continue growing at around 3% per year – suggesting consumption could be as high as 20EJ by 2050. With biomass output at 40EJ, that leaves little biomass for other uses.

The amount left over must also take into account energy required to make the aviation fuel, in other words the energy returned on energy invested (EROEI). Creating aviation fuel from biomass is an energy intensive process likely to have an EROEI as poor as 50%. (That compares with about 20% EROEI for difficult to extract shale oil).

So if we’re lucky enough to have any biomass left over from our aviation needs, what should be our next priority? Maritime transport (currently using 7EJ and growing) is also a tough sector to move to electric battery power. Brazil is prioritising land transport but even with a concerted effort, over a number of decades, has managed to supply just 17% of its transport network with biomass.  And global demand is 100EJ – far more than biomass can cope with.

A quick look at the numbers makes the need for choices clear. Yet the current approach doesn’t take global priorities into account. Outside Brazil, biomass resources are typically used for heating, generating electricity, fuelling cars, and, in developing countries, for cooking. Surely we need to prioritise uses that are difficult to meet with other clean options?

Looking at the numbers, I’d suggest we should be prioritising aviation, efficient rural heating and cooking, and shipping in that order. That doesn’t leave much for cars, cooling or combined heat and power in cities – uses I’d suggest biomass isn’t best suited to.

What does this mean for designers and engineers? The next time we set ourselves the challenge of creating a low carbon building or district we should let global biomass levels influence our design thinking.