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Solar Ammonia in a Climate Crisis: Almost Certainly the Most Affordable of All Low-Carbon Shipping Fuels

By Miguel R.L. (aka Lambda), ETH Zürich engineer

Time is fast running out to keep global surface temperatures below the 1.5°C limit we have set for ourselves. In fact, we are now headed for a catastrophic 2.7°C surface temperature rise by century’s end. Much has been written about decarbonizing ground transportation such as cars and trucks or increasing the shares of renewables in the electric grid. We also know how to improve building efficiency and substantially clean up many industries. With so many wonderful technologies in existence and ready to go, the pace of change in the midst of this climate emergency is both infuriating and downright shameful.

In this article, we’ll primarily be looking at one sector in particular: the shipping industry. It emitted 2.5% of global emissions, or 880 million metric tons (Mt) of CO2 in 2019, the same year in which, for comparison purposes, Germany produced 681 Mt of CO2. It currently is a relatively small portion of total GHG emissions, but one that is made harder to eliminate by the long lifetimes of large ships (25–35 years) and the need for a high specific energy storage medium.

Not choosing the most economical, environmentally sound and technically feasible option runs the risk of locking us into long-term wasteful practices that we collectively cannot afford. It’s for this reason that we’ll compare the environmental, economic and engineering feasibility aspects of two of the most commonly considered contenders in this race: biofuels and green ammonia. A distinction will be made about the short and medium time spans that will be critical to try to save ourselves from rapidly degrading ecosystems and destructive climatic changes.

Hopefully, it will become clear to the reader why I think ammonia is likely to be the winner for the shipping industry. Fuels that will not be examined in detail include: (a) hydrogen, (b) liquefied natural gas (i.e. methane CH4), whether of renewable or fossil origin, and (c) methanol. Hydrogen has transportation and storage issues that are unlikely to be resolved anytime soon, although it might possibly displace ammonia in the long term. LNG is burdened by hard-to-eliminate fugitive methane emissions and emits CO2 during use. Like all hydrocarbon synfuels produced in a carbon-neutral manner, methanol requires either costly CO2 direct air capture (DAC) or biogas. Conventional Li-ion batteries lack the required energy density for all but the shortest of shipping routes, and the commercialization timeline and economics of re-smeltable metal-air batteries are uncertain at this time.

In the course of this article series, the implications of scaling both biofuels and fully renewable ammonia to large majority shares of shipping fuels will be examined. Sources will be included for the readers to peruse at their leisure. Respectful, constructive criticism is welcome since this is intended to be an effort in elucidating the best solutions and a small contribution in attempting to get them implemented. There will be some numbers, without which this would not be an analysis but an opinion piece with unverifiable claims. This first part will mainly look at the scaling issues of biofuels as they are likely to exist within the next decade at least. Part 2 will examine lifecycle emissions of biofuels and solar ammonia, while the feasibility of cheaply synthesizing and distributing ammonia using solar energy and electrolysis will be explained in part 3. Part 4 will wrap up with a cost comparison of all relevant fuels.

Where biofuels come from

Biofuels as they are produced today are divided into two main categories, bioethanol and biodiesel, and are further distinguished by their origin. First-generation (1G) biofuel feedstocks stem from feed crops and edible oils. Second-generation (2G) feedstocks are sourced from non-edible oil, waste oils and grease (biodiesel), and lignocellulosic sources (ethanol). Third-generation (3G) feedstocks are unfeasible as of 2020, as their GHG emissions surpass those of fossil fuels.

Let’s look at the current origin of biofuel feedstocks. The EOCD-FAO Agricultural Outlook 2020–2029 report states:

“At present, about 64% of ethanol is produced from maize, 26% from sugarcane, 3% from molasses, 3% from wheat, and the remainder from other grains, cassava or sugar beets. About 77% of biodiesel is based on vegetable oils (37% rapeseed oil, 27% soybean oil, and 9% palm oil) or used cooking oils (23%). More advanced technologies based on cellulosic feedstocks (e.g. crop residues, dedicated energy crops, or woods) do not account for large shares of total biofuel production.”

Furthermore, the report goes on to explain that advanced non-1G feedstocks will not substantially grow their share in the coming years:

“Global biofuel production will continue to be dominated by traditional feedstocks, despite the fact that increasing sensitivity to the sustainability dimension of biofuel production is observed in many countries (Figure 9.3).”

At this point, we should note that the report’s definition of traditional feedstocks includes waste vegetable oils, which are usually classified as 2G feedstocks. Waste oils seem like a good idea until you realize that they only make up 23% of biodiesel feedstock (see above). Meanwhile, high demand for soybean oils for the biofuel industry is already pushing up soybean prices in the food industry.

The more important takeaway from the report, however, is the share of traditional feedstocks (91.4%), as well as the absolute energy generation from biological sources: 4340 PJ or 1’205’552 GWh. Keep this number in mind for later.

Area: a key measure

Let’s also investigate the space requirements of biofuel cultivation as it stands today. The reader is encouraged to come up with more rigorous sources for this topic, as I had some difficulty in coming up with my own. Nevertheless, the ones listed below should be sufficient for a reasonable estimate. I’ve compared with other lower-quality sources, and my numbers are within the ballpark; some are higher, some are lower. We’ll examine Brazilian bioethanol and biodiesel production, which is some of the most productive on Earth on a land area basis. Conversion of sugars to ethanol is also simpler and less energetically intensive than for biodiesel. The numbers are as follows (also see here and here):

Now let’s compare the above power density figures with a couple of fairly recent utility-scale PV projects. Also included is a hypothetical PV project in Chile’s Atacama Desert, extrapolated from California’s numbers, using the same system-level solar-to-electricity efficiency:

Solar Star’s power density numbers may raise eyebrows if compared to the newer project in Egypt, but given the use of SunPower panels, which are some of the most efficient on the market, the above numbers should not be too surprising. The ground coverage ratio (GCR), the ratio of module to land area, could also be higher for Solar Star.

We can observe that even higher power density numbers are feasible. For example, the Echuca solar farm in Australia, completed in December of 2019, does not have a tracking system and instead plasters the ground in a configuration reminiscent of an extended accordion. I suspect that solar modules have gotten so cheap that, even if each solar module produces less energy than in a single-axis tracker configuration, the savings in installation labor and time, tracker costs, and land costs start to accrue. If this configuration, deployed by Australia’s 5B, is expected to be good enough for the 10 GW, $20 billion Australia–Singapore Power Link project, it ought to be sufficient for any potential solar farm in Atacama as well.

For the upcoming map, I nonetheless assumed the more conservative numbers and extrapolated Solar Star’s numbers to Atacama (Echuca would halve the required area). It should also be noted that Solar Star uses 20.1% efficient modules. But LONGi, which in 2021 became the world’s largest PV company by sales revenue and market value, achieved a c-Si cell-level efficiency of 26.3% in October of 2021. It stands to reason that we will soon see 23% efficient c-Si modules, especially if we consider that companies such as SunPower and Kaneka have already surpassed 24% efficiency for small prototype modules. Such a switch would decrease the required area by 14%.

The summary of the above tables is that solar in the best locations is easily 30× as power dense as the most efficient of biofuel crops. Using Echuca numbers and newer solar modules would raise this figure to about 60× the power density. Accounting for realistic solar-to-ammonia synthesis inefficiencies (which will be analyzed in Part 3), we still achieve a power density that is 20–40× as great. This figure does not factor in the 15–20% yield increases estimated possible from tight PV-electrolyzer integration, the kind that has the potential to avoid multiple lossy power conversions. While such integration would not decrease the required synthesis energy, it would certainly lessen the cost and required area.

Let me rephrase for maximum clarity: on an equal chemical energy basis, the best solar ammonia synthesis installations today occupy about 2.5–5% the area that sugarcane ethanol does. The gap widens further if we substitute bioethanol with biodiesel.

On their own, these numbers are damning enough for biofuels, but especially disastrous when we consider what type of land area is being converted for energy production: biodiverse cerrado that could aid in the fight against climate change, or sun scorched deserts of great beauty but generally poor in biodiversity. And, unfortunately, it seems that human-induced desertification is only going to stress the survivability of the former and increase the prevalence of the latter. 

Scale matters

Let’s now look at scaling up biofuels to worldwide shipping and aviation consumption. In 2019, the year before the pandemic, shipping consumed 221 million metric tons of oil equivalent (Mtoe), or about 2.57 million GWh. Therefore, 2019 worldwide biofuel production only accounts for about 47% of global shipping energy demand.

That is, all the world’s biofuel production provides less than half of the global shipping fleet’s energy consumption. Scaling up to worldwide demand would require about 536’000 km2, or productive, frost and drought-free farmland equivalent to Romania and Oman put together (or slightly smaller than Botswana).

With aviation’s energy density requirements being even stricter than those of shipping, we will again require energy dense fuels, at least for the foreseeable future. Aviation energy consumption of both non-OECD and OECD countries claimed 3.74 million GWh in 2019. As a result, current liquid biofuels energy production can only account for 19.1% of current combined shipping and aviation demand. However, aviation energy demand is projected to rise from about 13 quadrillion BTUs (quads) in 2019 to about 29 quads by 2050.

Battery electrification will certainly displace a portion of the total fuel demand, but the exact quantity is obviously uncertain. Battery electric aircraft the likes of Heart Aerospace’s 400 km (250 mi) E19, or Wright Electric’s 1290 km (800 mi) Wright 1 are very welcome developments and will be critical to electrify short-haul flights in the coming years. However, with planned commercialization in the years 2026 and 2030, respectively, and short-haul commercial passenger flights under 1500 km only accounting for about a third of aviation’s emissions, batteries will have a minor impact in denting aviation’s emissions for at least the next decade. For similar reasons, the battery electrification share of shipping is estimated at about 5% by 2040 by some sources. And while IRENA projects an approximate halving of shipping’s energy intensity by 2050 [figure 30], the overall energy demand remains approximately flat. Let’s look at these areas graphically, by representing the areas with countries and superimposing the outlines over Brazil:

For comparison purposes, let’s also graphically juxtapose the predicted energy use in 2050 for both industries put together:

An image is worth a thousand words, isn’t it? Just as a reminder, these figures assume production of sugarcane ethanol only. Producing the required biodiesel for aviation is more inefficient still. We have also neglected any embodied energy from the fertilizer used to grow crops, as well as any energy for farming machinery or the processing of fuel, such as the approx. 0.1 kg of methanol required per liter of biodiesel. Some among you might be quick to point out that the figure unwisely compares PV electricity with chemical energy. That’s actually not true. The areas represented by North Macedonia, Togo, and Iceland result after scaling the actual PV areas with the electricity conversion losses to ammonia. In other words, we are comparing chemical energy with chemical energy.

So there you have it. In this first part, we’ve established that biofuels, if scaled to actual consumption figures, would create huge monoculture wastelands that we could be returning to nature. In the second part, we will briefly discuss alternate forms of biofuel production and put any doubts about biofuel emissions superiority to rest. Part 3 will look at the feasibility of cheaply synthesizing and transporting solar ammonia, showing that neither electrolyzer capital costs nor capacity factors pose an issue. Part 4 compares solar ammonia costs with both biofuels and bunker fuel and summarizes the discussion.

Featured image by Thai Subsea Services Ltd, retrieved from Valentin Schönpos from Pixabay

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