Introduction

Concentrating solar—the other solar. Do we really need more than photovoltaics (PV) plus batteries? If we need hydrogen, do we need a pathway other than low-temperature electrolyzers? Is it time to abandon a sinking ship and jump on board the PV, batteries, and electrolyzer ban-wagon?

This special issue is about concentrating solar technologies (CST) that provide opportunities for highly efficient dispatchable and renewable power, for solar thermochemical water splitting to make hydrogen or carbon dioxide splitting to make carbon monoxide, and renewable syngas by combining the two. Other opportunities include solar reforming and solar gasification of either fossil inputs or biogenerated feedstocks, or combinations. These are all important opportunities. However, there are finite resources, finite time to get this energy transition right, and a decision to put resources in one pathway carries opportunity costs to others. Good leadership, good decision-making means knowing when (and when not) to cut losses and move on. Therefore, it is important that we examine this question and not rationalize continuing research that is a dead-end, but also not conclude that there are showstoppers and hence a dead-end just because there are difficulties, challenges, and even strong headwinds. Furthermore, it is essential that decision-makers have the right information to make the right decisions.

I believe that if we were to abandon ship now, it would be premature to do so. I believe that society would be missing a critical opportunity. An opportunity to increase the pace of the energy transition, to create new economic opportunity, to make the transition more energy just, and to have sustainable resources penetrate every aspect of the energy system to get to net zero carbon dioxide and then net negative in the latter half of the century. Net negative technologies gives society an opportunity to both accelerate the rate of decreasing net cumulative emissions and to mitigate the substantial risk that we will overshoot the Paris Climate Agreement target of <450 parts per million by volume (ppmv) CO2 in the atmosphere.1 We should never lose sight that the main driver for making this transition is the risk of climate change and not underestimate the scale of the problem, a scale that is so large it is difficult to appreciate fully.

The Scale of the Problem

Our atmosphere is ∼7.75 × 106 giga tons (GT = 109 metric tons) of gases. At 400 ppmv, the atmosphere contains ∼3100 GT of CO2. Global CO2 emissions (from fossil fuel, cement, and land use changes) is nearly 40 GT/yr, but cumulative (between 1750 and 2011) is 2040±310 GT and the increase has been 880±35 GT CO2. Land and vegetation has taken up ∼35% of these cumulative emissions and the oceans ∼25%. The remaining 45% accumulates in the atmosphere and will remain there for a very long time. Proven fossil reserves have the potential to release an additional cumulative ∼2800 GT emissions. Contrast that with what is necessary to meet the Paris agreement, which will require holding to a budget of ∼500 to 900 GT. Assuming a steady decreasing rate year over year (yoy), the global economy would need to reduce at a rate of ∼4.25% yoy, (30% and 50% in eight and 16 yr, respectively). Such decreases will certainly be quite challenging in the face of population growth and economic development, not to mention vested interests.

Given the scale of the CO2 (and other greenhouse gases) problem, how does that relate to the application scale? Few things match that scale and we need to be focusing on applications that can address the scale problem. Toward that end, we suggest that we might think about an application as niche if the scale is unlikely to be more than 100 × 106 metric tons (of CO2 mitigation) per year (100 MMT/yr) and scalable if it can approach 1 × 109 metric tons per year (1000 MMT/yr) by the end of the century. The range of more than 100 MMT/yr but less than 1000 MMT/yr is significant and any scale can be important, if production of the product (that replaces, reduces, recycles, or reuses CO2) leads to learning and cost reductions that in turn enable products of a larger scale and contributes to accelerating the transition. In looking at whether a pathway to a product is scalable, one should consider whether there are any demand limitations, but also whether there are any resource limitations to achieving scale. We are used to a world where critical resources are concentrated in certain regions of the globe and this has led to a great deal of geopolitical instability. We suggest that in considering scalability, we ask whether the resources are readily available, preferably domestically in many countries and able to provide economic opportunity to developing regions. CST excels over many over technologies by this metric in sun-rich regions of the world.

The Opportunities and Challenges

Large-scale industrial conversion of concentrated sunlight that stores chemical energy for later use and that transforms CO2 and water into infrastructure compatible hydrocarbon fuels and other carbon-based materials is an attractive option to facilitate a smooth and continuous energy transition, affecting the existing vehicle fleets and the existing built environment, and co-evolving as society advances further toward sustainability. However, such an option while certainly possible and with no apparent show-stoppers, it does have significant resource, economic, societal, and technical challenges before it will be practical, especially if it is going to achieve scale and meaningfully contribute to an economic and energy just transition.

We highlight a number of achievements we deem necessary: high solar energy-to-fuel system-level efficiency, low material intensity in solar collection, high material accessibility in many regions of the world, and good material durability; limited and no additional arable land use (no competition with food); and low water consumption. Opportunities to meet each of these challenges are already encouraging; this special issue addresses some of these opportunities.

Using the sunlight to re-energize CO2 both directly and in hybrids (with biomass or fossil feedstocks) can produce net lower and ultimately net neutral carbon-based fuels with most of the carbon in the initial feedstock making it into the fuel product. This measure of how much carbon makes it into the product refers to the atom efficiency (the ratio of carbon atoms in the product to the carbon atoms in the combined feedstock and energy inputs) a desirable goal is to achieve near unit atom efficiency, and thus little to no unintended byproduct CO2. Another advantage of CST is that it can unite renewable energy with both fossil and advanced biofuel, and could preserve an option for a low (and ultimately negative)-net-carbon future while affecting a smooth transition that maximizes utilization of installed infrastructure and existing expertise, and potentially any new investments in natural gas.

These CST (solar thermochemical) opportunities offer significant promise with affordable economics scalable to global demand that PV, batteries, and electrolyzers will not fulfil by themselves. There is an important complementary role for CST that will require innovations in the relative near-term. Despite known challenges, there are promising advances already happening and opportunities to leverage developments in related industry segments. Furthermore, such technologies could stimulate sustainable economic growth, create many high-quality jobs, and produce viable and scalable solar alternatives to crude oil as the feedstock and contribute to decarbonizing CO2 intensive steel, concrete, and aluminum or providing sustainable carbon-based alternatives. Nevertheless, there are no guarantees; momentum favors PV, batteries, and electrolyzers. Therefore, our field has no time to waste and besides the research and development, we must raise awareness on the promise.

Acknowledgment

The author would like to acknowledge helpful conversations with Drs. Gary Dirks, Ivan Ermanoski, Elisa Graffy, James E. Miller, Christian Sattler, and Nathan Siegel.

1

The Paris Climate Agreement is to achieve the goal of keeping the rise in global average temperatures below 2 °C, relative to pre-industrial levels. By deriving a correlation between temperature and CO2 concentration in the atmosphere, suggests that we must then keep the CO2 concentration below 450 ppmv. Alternatively, noting that the temperature rise is approximately linear in total CO2 emissions since pre-industrial provides a budget of less than ∼3000 GT. We have already emitted 2200 GT against that total budget.