UPDATE: 26 August 2017 – A response to Jacobson et al.’s response has been published with further detail here.
Completely decarbonising energy systems is a difficult and complex problem. Wind and solar have the benefit of modularity, falling costs, and scalability. They are contributing to lowering the emission intensity of legacy electricity systems. But at high penetration, intermittency will be a major challenge.
Although there are potentially many long-term low-carbon options, the only other scalable low-carbon options are nuclear and coal with CCS. Nuclear faces many obstacles, including high capital cost and a lack of community consensus. CCS is proven technically, but unproven at scale. Bionenergy is frequently included in modelling but cannot be considered scalable given that human appropiation of net primary production is already too high. Marginal solutions abound, but scalable solutions are scarce.
A survey of the IPCC literature in AR5 Work Group 3 shows that some combination of nuclear, coal with CCS, and large scale bioenergy is going to be required. This is a difficult problem. Many models produce infeasible solutions. In contrast, Jacobson et al claim that intermittency is no barrier to 100% renewable energy for all energy including transport fuels. But how rigorous is Jacobson’s analysis? A recent rebuttal by Clack et al. in the Proceedings of the National Academy of Sciences (the same journal that ran the original Jacobson et al. paper) suggests not very.
Here is an abridged version of the abstract –
A number of analyses … have concluded that deployment of a diverse portfolio of clean energy technologies makes a transition to a low-carbon-emission energy system more feasible and less costly than other pathways. In contrast, Jacobson et al. argue that it is feasible to provide ‘low-cost solutions to the grid reliability problem with 100% penetration of WWS [wind, water and solar power] across all energy sectors in the continental United States between 2050 and 2055’, with only electricity and hydrogen as energy carriers. In this paper, we find significant shortcomings in the analysis. In particular, we point out that this work used invalid modeling tools, contained modeling errors, and made implausible and inadequately supported assumptions. Policy makers should treat with caution any visions of a rapid, reliable, and low-cost transition to entire energy systems that relies almost exclusively on wind, solar, and hydroelectric power.
I think the broader issue is that there is now a virtual industry producing these scenarios. For the non-energy specialist, there is really no way to establish the quality of the research. Many of these models are interesting and useful, but better decribed as thought experiments than serious evaluations. Most of the rigorous modelling is done outside of the public spotlight. The problem is that many of these are publicly promoted as ‘plans’ that simply require the right policy interventions.
There is also a dearth of critical evaluations of these scenarios. Researchers in this field don’t tend to publicly criticise the work of others, while energy professionals with hands-on expertise have no interest in publishing in the academic literaure. In this respect, the Clack et al. critique is unusual and perhaps a turning point in bringing more rigour to the field.
I’m not going to go through all the details of the original Jacobson paper, or the critique, other than use one example from the Jacobson paper to demonstrate it’s crippling failings. The example is the case of air travel, which is one of the most difficult challenges of future energy scenarios. There is no readily available substitute for jet fuel and avgas, other than biofuels. Biofuels could substitute for a small proportion of jet fuel, but not at scale. Here is what the current IPCC AR5 review says about air travel –
page 611 –
Air transport demand is projected to continue to increase in most OECD countries. Investments in high-speed rail systems could moderate growth rates over short to medium-haul distances in Europe, Japan, China, and elsewhere.
page 615 –
For large commercial aircraft, no serious alternative to jet engines for propulsion has been identified, though fuel-switching options are possible, including ‘drop-in’ biofuels (that are fungible with petroleum products, can be blended from 0 to 100 %, and are compatible with all existing engines) or hydrogen. Hydrogen aircraft are considered only a very long run option due to hydrogen’s low energy density and the difficulty of storing it on board, which requires completely new aircraft designs and likely significant compromises in performance. For small, light aircraft, advanced battery electric/motor systems could be deployed but would have limited range.
page 604 –
Although high potential improvements for aircraft efficiency are projected, improvement rates are expected to be slow due to long aircraft life, and fuel switching options being limited, apart from biofuels.
and page 605 –
Reducing transport emissions will be a daunting task given the inevitable increases in demand and the slow turnover and sunk costs of stock (particularly aircraft, trains, and large ships) and infrastructure.
Ok, so air travel is difficult challenge, so what does Jacobson et al say about resolving the challenges of air travel ?
we assume that 85% of transportation loads can be met with demand response or hydrogen … hydrogen will be used primarily for long-distance trucking, heavy ships, and aircraft … (and) can be produced by [wind,water,solar] electricity during any time of a year.
And when quizzed about the veracity of the claim, noted in a response to the rebuttal that –
… a 1,500-km range, four-seat hydrogen fuel cell plane already exists
So we learn that the claim that the entire fleet of aircraft will be economically  powered by renewable-fuelled hydrogen fuel cells within a few decades is predicted on the existence of hydrogen fuel cell 4 seater plane. And that’s it.