Fool’s Gold, Fool’s Green [Gas in Transition]
What you (think you) see is not always what you get
Anyone who grew up, as I did, in a part of the Western US where prospectors once roamed in search of gold should know what “fool’s gold” is. A relatively common mineral, iron pyrite, its shiny crystals may resemble gold nuggets. Their appearance sometimes misled neophyte prospectors into believing they had struck it rich. On presenting their finds at the local assayer’s office, they looked – and no doubt felt – like fools.
These days, in the realm of clean energy, there’s something analogous that we should watch out for. I call it “fool’s green”: products and technologies that appear to be green, but on closer examination, aren’t. And in this case, there’s more at stake than just prospector’s egos. When energy policy makers are mistaken about a technology’s “greenness”, it can have serious long term consequences for the planet’s ecosystems.
Analogies are never perfect. Despite their superficial similarities in appearance, the difference between gold and fool’s gold is unambiguous. “Greenness”, however, isn’t a clearcut either-or attribute of a product or technology. It comes in degrees. Solar PV panels, for example, deliver usable energy from sunlight, with no carbon emissions. That’s green, isn’t it? Yet solar panels come with environmental costs. The production of polysilicon that underpins China’s dominance of solar cell manufacturing owes its low cost, in large part, to dirty coal-fired power plants. The plants are built nearby to provide China’s polysilicon factories with cheap, reliable, 24/7 power. Renewables can’t do that. Moreover, polysilicon production is only one link in a complex chain of resource extraction and carbon-emitting steps leading to installed solar panels. So are solar panels truly “green”? Or are they “fool’s green”?
Life cycle analysis has shown that on balance, modern solar PV panels do warrant a “green” label. Depending on location, it now only takes a few years of operation for panels to repay the resource and carbon debt of their genesis. But there is a substantial debt to repay. PV panels are not as green as advertised. They’re best for applications like well pumping that have low capital costs and the flexibility to consume intermittent energy on an as-available basis. For other applications, PV systems accrue further demerits on their “green-ness” scorecard. More resources must be spent for the backup power or energy storage capacity required to make them usable in an on-demand energy economy.
The green hydrogen conundrum
Electrolytic hydrogen presents a different set of issues. Whether the hydrogen is true green or fool’s green depends on the state of the power grid at the time the hydrogen was produced. Green hydrogen is usually thought of simply as hydrogen produced by electrolysis of water using renewable energy. But being produced using renewable energy isn’t enough. Until such time as the grid is fully decarbonised, energy used to produce truly green hydrogen must be surplus. There must be no competing load being served by a carbon-emitting energy resource that the energy used for hydrogen could have served instead. If there is, then the clean energy directed to hydrogen production will necessarily be offset by production from fossil resources that would not otherwise have been needed. The hydrogen isn’t green at all; it’s fool’s green.
The carbon footprint for fool's green hydrogen is substantial. It can be figured in various ways. There are complications in the modelling when real considerations like transmission limits and available energy storage capacity are factored in. But by any reasonable calculation, the figure is high.
The thermodynamic minimum energy required to produce a kilogram of hydrogen by electrolysis at standard conditions is about 39 kWh. State of the art PEM units are 80% efficient; that means 49 kWh of DC power/kg hydrogen. It would be a bit over 50 kWh AC power from the source of generation. That’s a lot of energy. Producing it from a gas combustion backing turbine at 40% thermal efficiency takes 450 MJ of thermal energy from natural gas. The combustion energy from natural gas is 50.6 MJ / kg, so we need to burn 8.9 kg of natural gas to make one kg of hydrogen by electrolysis.
8.9 kg of natural gas translates to 24.5 kg of emitted CO2/kg H2. It’s fair to argue that only 60% of that should be charged to the production of hydrogen; the H2 produced can return 40% of the electricity used to produce it when the hydrogen is used later to generate power. Hence the carbon footprint of electrolytic H2 made using non-surplus renewable energy comes to 14.7 kg CO2/kg H2.
It’s instructive to compare that 14.7 kg with the carbon footprint for hydrogen produced by other means. The overall steam methane reforming reaction (SMR) is:
CH4 + 2H2O + heat ⇒ CO2 + 4H2
It takes 16 grams of CH4 (16 being the molecular weight of methane) and 36 grams of steam to produce 8 grams of H2 and 44 grams of CO2. The mass ratio of CO2 to H2 is 5.5 to 1. That’s only 37% of the carbon footprint of fool's green hydrogen -- before any effort to capture the CO2 from the process. With CCS at a 90% capture level, the carbon footprint would be only 0.55 kg of CO2 or 3.7% that of fool’s green hydrogen.
Knowledgeable advocates of green hydrogen may at this point be crying “foul”. My figure above of 16 grams of CH4 for 8 grams of H2 is based on chemical stoichiometry. It’s substantially lower than what’s commonly reported for steam methane reforming. In addition, I only consider CO2 emissions, and ignore the CO2-equivalent of fugitive methane emissions. Very high estimates for the latter are the basis for Robert Howarth and Mark Z. Jacobson’s notorious assertion that using natural gas for heating or power generation is as bad as burning coal.
It’s quite true that figures commonly cited for the amount of methane input to hydrogen output with SMR are substantially higher than the 2 to 1 that I cited above. The SMR reaction is strongly endothermic -- which is to say it requires heat input to drive it forward. In the context of an oil refinery, where hydrogen is needed for desulfurization and hydro-cracking of crude oil, the most expedient way to supply that heat is by combustion of some of the natural gas feedstock. The SMR reaction is carried out inside catalyst-packed tubes residing within a high temperature gas-fired furnace. It’s not the most efficient way to deliver thermal energy to drive the SMR reaction, but it’s cheap and easy -- especially when the cost of dumping a bit more CO2 into the atmosphere is ignored.
The figures that Howarth and Jacobson cite in their hatchet job on blue hydrogen translate to a 3.65 to 1 ratio of methane to hydrogen. That’s well above the minimum that would be needed if combustion of methane were used efficiently to drive the SMR reaction, but it’s not out of line with what’s used in cheap SMR reactors. It’s equivalent to 10 kg of CO2/kg of hydrogen. Even that is less than the 14.7 kg CO2 for fool’s green hydrogen. And it’s possible to do better.
In practice, nothing says that the enthalpy to drive the SMR reaction has to be supplied by combustion of methane in an inefficient oven. In fact, when cheap renewable energy is available, there are strong advantages to supplying it by ohmic heating integrated directly into the catalyst-packed reaction tubes. An article in the 24 May 2019 issue of the AAAS journal Science details the advantages. The process is termed electrified methane reforming (EMR).
With EMR, no extra feed gas is burned to supply heat. Hence there is no flue gas, and no CO2 needing to be captured from it. The only CO2 is what’s produced in the reforming reaction per se. Virtually all of that is separated from the hydrogen output in the PSA step (Pressure Swing Absorption) just upstream of the hydrogen output. Hence, in addition to generating less CO2 to begin with, EMR facilitates capture of the CO2 that is generated.
The danger of hype
The danger of all the hype about green hydrogen as the clean energy carrier of the future is that it may skew policy priorities in a counterproductive direction. Bad hype-inspired policy makes it likely that much of what’s being invested for green hydrogen will actually go into production of fool’s green hydrogen. The money and materials invested won’t constitute merely an inefficient use of resources; manufacture and operation of these systems will actually increase net power consumption and net carbon emissions. It will not reduce them at all.
There’s a good reason that much of the electrolytic hydrogen produced under a green hydrogen banner will be fool’s green. It’s a matter of economics. Part of the cost of electrolytic hydrogen is the cost of capital for the associated electrolysis equipment. That cost is significant even if the equipment can be utilised at near 100% CF (capacity factor). It’s crippling if the equipment can only be utilised on occasion when renewable energy production would otherwise be curtailed. Yet just such restricted operation is required for true green hydrogen.
Given the strong economic incentive that exists to utilise electrolysis equipment at the highest practical CF, we should not be surprised to see electrolytic hydrogen producers doing just that. It’s a safe bet that the electrolysis capacity now ramping up for a green hydrogen boom will mostly be operated at close to 24/7. The systems will draw power from the grid, irrespective of whether there is a momentary surplus of renewable energy. Minimising the cost of the hydrogen produced is necessary in order to support the claim that “green” hydrogen will soon be cheaper than hydrogen from fossil fuels. Belief in that claim is crucial for businesses to secure the subsidies they seek.
How it could play out
Undoubtedly electrolysis operations will be throttled back when actual energy shortfalls loom. Electrolysis is, after all, a discretionary load, and operation as a “virtual battery” lends credibility to the contention that these facilities are valuable green energy assets. However, the shortfalls that will trigger throttling will be shortfalls in the overall energy supply, not shortfalls of clean energy alone. As a result, electrolysis will frequently be adding load to the grid at times when that load can only be balanced by added supply from fossil fuels.
One might ask how electrolytic hydrogen producers could expect to get away with that type of operation and still label their product “green”. Well, they could do it the same way that numerous companies now get away with claims to be “powered 100% by clean energy”. They’d purchase sufficient clean energy credits to cover the energy they consume in making hydrogen. Clean energy credits are largely an accounting gimmick employed for greenwashing. However, It’s a gimmick that’s become established. There’s precedent.
Clean energy credits allow the purchaser to “call dibs” on a certain amount of the output from a participating clean energy provider. It doesn’t matter when or where the clean energy is actually produced -- beyond a requirement that the provider can’t sell credits beyond the amount of energy they’ll be supplying to the grid. It’s a neat way to sidestep the difficulties of transmitting and storing intermittent power. Unfortunately, climate change doesn’t care about accounting gimmicks. It only “cares” about the radiative forcing from elevated atmospheric levels of CO2 and other greenhouse gases. If we allow fool’s green hydrogen to pass as green, the result will be to increase net power consumption and global carbon emissions.
Keeping ourselves honest
It’s not my intent here to attack green hydrogen. Not when it’s truly green. There are ways to produce truly green hydrogen. But when we adopt policies to promote production of green hydrogen, they will have to be carefully drafted. If they don’t recognize and address the difference between green hydrogen and fool’s green hydrogen, it will be green hydrogen’s “evil twin” that will profit. And it will be at the expense of the global climate.
We might attempt to address the problem by requiring that, in order to qualify for subsidies, electrolytic hydrogen production facilities must be powered by dedicated clean energy systems. If systems are configured so that electrolyzers are physically unable to use power from the grid, then their operation can never require increased generation from grid-connected fossil fuel resources. Many of the green hydrogen projects going forward do in fact involve dedicated RE resources. But policies mandating that approach are not what we want.
One reason such policies are undesirable is that there will be periods when the production potential of grid-connected renewables actually will exceed available load. Even if that condition arises for only a few hours a week, we’d still like to be able to tap the excess capacity for green hydrogen. Otherwise it’s wasted.
Beyond that reason, there’s a deeper consideration. Powering an electrolysis facility from dedicated resources isn’t enough to ensure production of truly green hydrogen. Yes, isolation prevents the electrolysis load from directly increasing fossil fuel use; it also allows the system to be operated at the duty cycle supported by the dedicated resources. That will be less than it would be for full-time operation from the grid, but more than it would be for operation limited to periods when clean energy resources would otherwise be curtailed. It would allow a better return on capital. However, it incurs an “opportunity cost” that hinders reduction in carbon emissions. Clean energy resources that are dedicated to electrolysis are, ipso facto, not available for replacing generation from fossil fuels. Yet replacement of generation from fossil fuels will always exceed production of hydrogen in reducing carbon emissions.
The way to green is blue
Short of operating electrolysis plants only when there is surplus capacity from net zero energy resources, the only way to produce hydrogen without raising carbon emissions is via capture and sequestration of carbon emissions from non-electrolytic production. E.g., from producing blue hydrogen from natural gas. That can be done, however.
The blue hydrogen produced can be used in building most of the distribution, storage, and utilisation infrastructure needed for transition to a sustainable green hydrogen economy. Green hydrogen advocates aren’t actually wrong; green electrolytic hydrogen from water will eventually be cheaper than blue hydrogen from natural gas or coal. But we have to fully decarbonize our electrical energy economy first. Otherwise, we’ll be producing fool’s green hydrogen at the expense of climate change mitigation. And the cheapest, fastest, and surest way to do that is to embrace blue hydrogen.
The sooner we start, the sooner we'll get to where we can begin transitioning to the real green hydrogen economy.