Monday, September 18, 2017

A nation-sized battery, revisited

(Note: this is a draft. I will update it if any relevant objections are made).

Introduction

In an excellent blog post, Dr Tom Murphy examined whether it would be possible to power the entire USA using a combination of renewables and lead-acid batteries. He found that it would not be possible, because there is nowhere near enough lead in the earth's crust to make enough lead-acid batteries to compensate for the intermittency of renewables. Renewables occasionally don't produce power for 7 days in a row (during prolonged wind lulls, for example). As a result, it would be necessary to build enough lead acid batteries to power the country for 7 days to prevent the lights going off during those periods. However, the amount of lead in the earth's crust is not sufficient even to power just the USA for 1/3rd that long, to say nothing of the rest of the world. As a result, lead-acid batteries cannot compensate for the intermittency of renewables on a nationwide scale.

However, lead-acid batteries are not the only storage option available. We should investigate whether there are other storage options that have sufficient materials, not just whether lead-acid batteries have sufficient materials.

In this article I’ll investigate some other energy storage options. I'll try to determine if those options would be sufficient to power the entire USA during wind lulls. In all cases, I will assume that society needs 7 days of energy storage (or 336 billion kWh for the USA, as per Dr Murphy’s article) to prevent the lights from going out during occasional prolonged lulls in wind power.


Molten Silicon

First I will examine the possibility of using molten silicon as an energy storage medium. Molten silicon would be stored in insulated tanks, and heated up whenever the wind is blowing. When the wind isn’t blowing, hot air is drawn over the molten silicon and used to drive a turbine. The silicon doesn’t actually change temperature; instead it changes phase from solid to liquid when heat is added, and back from liquid to solid when heat is taken out, so the temperature remains constant at 1414 degrees C. This technology is already being pursued by a startup; see here.

Let’s find out if there is enough silicon in the earth’s crust to provide energy storage for the USA for 7 days. We’ll start by calculating how much silicon metal would be required. Murphy’s article says that we’d need 336 billion kWh to power the entire country for 7 days. Silicon has a latent heat of melting of 1.926 MJ/kg[2], which is equivalent to 0.535 kWh/kg, or 0.278 kWh/kg after subtracting waste heat losses (discussed further below). As a result, we would need 1.2 billion tonnes of silicon (336 billion / 0.278), which is a cube of silicon that’s 0.78 kilometers on a side (at 2.5 g/cm^3 [3]). Silicon is the primary ingredient of dirt, so we could gather a cube of silicon that’s 0.78 kilometers on a side from within a 10 kilometer radius around my house. That would be enough silicon to power the entire USA for 7 days. Furthermore, the silicon is not being “used up” at any rate, but could be re-melted, over and over again, for millions of cycles, with no degradation.

Of course, we’d also need gas turbines, in order to convert the heat back into electricity. However, gas turbines have already been scaled up and already provide much of the electricity generation for the world. Those are natural gas turbines, not hot air turbines, but their construction would be similar. I presume we can continue building gas turbines on a wide scale.

As a result, it is clear that we have vastly more silicon than we need to meet our energy storage requirements for the entire USA for all purposes, and can also build the requisite turbines on a wide scale.


Power-to-methane

Power-to-methane relies upon electrolyzing water to obtain hydrogen gas, then converting that hydrogen gas to methane using the Sabatier process. When the wind is blowing, methane gas is created. When the wind isn't blowing, that gas is converted back into electricity using existing natural gas turbines.

There is enough carbon in the Earth’s atmosphere to create the needed methane gas. We've been burning fossil fuels for more than a century now, so there is obviously enough carbon in the atmosphere to make a 7-day inventory of methane gas. Remember that carbon is not being “used up” during this process of synthesizing gas and burning it. This storage scheme is a closed cycle, in which carbon is taken from the atmosphere (actually, probably absorbed from the atmosphere into the oceans and then taken from there) and then re-released to the atmosphere. As a result, the maximum amount of carbon we would ever need is a 7 day inventory of methane, which obviously is a small fraction of the carbon we have emitted into the atmosphere over the last century.

There is also the question of how we could store 7 days worth of methane gas. However, that gas could be injected into the existing natural gas distribution network, which already is large enough to store months of gas. For example, California (where I live) can store 2 months of gas in the existing gas distribution network.[5]

It is clear that power-to-methane could be scaled up to provide 7 days of storage when the wind isn’t blowing.


Compressed Air Energy Storage

Compressed air energy storage relies upon compressing air when the wind is blowing. The compressed air can be stored in naturally-occurring underground caverns. When the wind stops blowing, the compressed air is released which powers a turbine and generates electricity.

There are enough caverns underground worldwide to hold the compressed air. The world is scattered with underground salt domes, salt caverns, porous rock formations, and aquifers. Here is a map of salt formations in Europe, for example[4].

There is one formation underneath Poland and Germany, for example, that appears to be 1,000 km long and 100 km wide. If it's 2 km deep, then it has a total volume of 200,000 km^3. Compressed air stores approximately 4 kWh/m^3 which is 800,000 billion kWh for the entire formation, whereas we need only 336 billion kWh for the USA according to Murphy's article. Obviously, it wouldn't be possible to convert an entire large underground salt region to a single compressed energy store. Still, if we could use even 0.1% of it, then we'd have more than enough energy storage for that region.

It is also possible to store compressed air in above-ground steel tanks in the regions which do not have suitable geography for underground storage.


Conclusions

There are already many solutions for storing 7 days worth of electricity. Those solutions are fairly low-tech and do not rely upon any technological breakthroughs. Furthermore, they could all be scaled up and do not face any material constraints.

The only objection to the low-tech storage mechanisms listed above is that they have fairly low round-trip energy efficiency. For example, molten silicon storage relies upon heating silicon to 1414 degrees centigrade to power a turbine, which implies a round-trip efficiency of approximately 50%. This means that approximately half the energy placed into storage would be lost as waste heat. Power-to-gas would probably be somewhat less efficient, at 40% or so (60% lost as waste heat). Compressed air could be somewhat higher, at 60-70%. In all cases, however, there would be considerable round-trip energy losses.

However, that drawback is not as important as it would seem. Those energy losses are incurred only part of the time, because most renewable electricity is delivered directly to the grid without ever being placed into storage. As a result, storage losses would constitute only a fairly small fraction of the total electricity generated. For example, if solar panels could provide enough electricity to meet 40% of electricity demand directly, without storage, then only 60% of electricity would need to be drawn from storage. In which case, we would need to overbuild solar panels by only 60% (not 100%) to compensate for storage losses with 50% efficient storage (0.40 electricity delivered directly, 0.60 to storage, and 0.60 to waste heat, which implies 1.60/1.00, which is 60% overbuilding). Waste heat losses would constitute only 37.5% (0.6/1.6) of all energy obtained from solar panels, not 50%.

As a result, the round-trip losses from energy storage would be less important, because those losses are incurred only part of the time. This is quite different from waste heat losses from coal power plants, for example. Coal power plants lose 65% of their energy as waste heat, 100% of the time. On the other hand, the round-trip losses from energy storage from renewables are only occasional, and so would represent a fairly small fraction of all electricity generated.

As a result, it is clearly possible to build a “nation-sized” energy storage mechanism with tolerable energy losses and at reasonable expense. We do not face material constraints on energy storage. No technological breakthroughs are required. We could build an energy storage system that would provide continuous, dispatchable power at all times from renewable sources.

One more thing. There are also newer electricity storage mechanisms being developed. For example there are new flow batteries being developed which use abundant materials (such as the iron flow battery described here, or the organic flow battery described here). Those flow batteries would have higher round-trip efficiency (like 70% or more) and could store large amounts of energy at the same time. If higher round-trip efficiencies could be achieved, then less overbuilding would be required. Overbuilding is unfortunate, and we should try to reduce it. However, even if those new flow battery technologies never reach commercialization, we still have other, lower-tech options which are perfectly workable and which impose modest energy losses relative to all electricity generated.


[1] https://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth%27s_crust.

[2] http://www.engineeringtoolbox.com/fusion-heat-metals-d_1266.html

[3] https://en.wikipedia.org/wiki/Silicon

[4] https://www.researchgate.net/figure/48693439_fig4_Figure-4-2-Salt-structures-and-cavern-storages-in-Europe

[5] https://californiahydrogen.org/sites/default/files/CHBC%20Hydrogen%20Energy%20Storage%20White%20Paper%20FINAL.pdf

*NOTE: I modified this article on Sept 21 and changed the overbuilding example from wind turbines to solar panels. I also re-worded the clumsy opening paragraph.

Thursday, September 14, 2017

Coal will never run out

The United States has 283 years of coal remaining, at present rates of usage, according to the EIA[1]. China, Russia, and Australia have similarly huge amounts.

However, that figure of 283 years remaining is for present rates of usage.  If coal usage declines, then the "hubbert curve" of remaining coal is flattened and pushed further out to the right. For example, if we were using only half the amount of coal per year as we do now, then we would have 566 years of coal remaining, not 283 years. Every reduction in coal usage will extend the amount of time remaining until depletion.

Coal usage has been declining fairly rapidly in the United States, for the last 5 years or so, because renewables and gas are so much cheaper now. Coal plants are being shuttered because coal is relatively more expensive now. In fact, coal usage is down 28% in the United States over the last 5 years, because so many coal plants have been shuttered[2]. As a result, the date of coal depletion is being pushed far out into the future. Since coal usage is down 28% already, the remaining time until coal is depleted has increased from 283 years to 388 years (= 283 / 0.72 - 5). In other words, we have “gained” 105 years of coal during the last 5 years.

This trend looks set to continue. New turbines are being developed ("supercritical CO2 turbines") to replace the old steam turbines in coal power plants. Those new turbines are 50% efficient instead of 33%, which pushes back the date of depletion another 262 years ( =393 * (0.5/0.33) ).  Any further encroachment of renewables would push back the date of depletion even further, and could easily result in millennia of coal before it runs out. For example, if coal plants are gradually replaced by wind turbines during the next few decades, until coal is used only for steelmaking and also during the time when the wind isn't blowing, then coal usage could decline by more than 50%, which would result in 1,190 years total until the coal runs out (= 595/0.5). That figure is without any electricity storage technology at all for when the wind isn't blowing. Coal could still be used for electricity generation when the wind isn't blowing, but we'd still have enough coal for 1,190 years.

Of course, there are several trends happening in the opposite direction also, which could push toward increased use of coal. First, the United States looks like it will phase out nuclear power over the next several decades, and some of that lost generation may be replaced by coal. Second, south Asia is growing quickly and will increase its usage of coal. However, both of those trends are temporary. Coal usage may bounce up and down, but in the long run, it is probably headed way downwards, which implies that remaining reserves will last far longer than reserve figures suggest.

Of course, it is possible that further technological developments will occur during the next few centuries, in which case the date of depletion for coal will be pushed even further outward. If flow batteries are commercialized, for example, then coal may not be needed for electricity generation at all. If new steel-making technologies continue to progress, then coal wouldn't be needed for that purpose either.

Our civilization has at least centuries to develop technologies such as flow batteries and alternative steel-making technologies. If those technologies are developed and commercialized in the next few centuries, then the remaining coal would be left in the ground as useless.

Technology is rapidly outpacing depletion. This time to depletion for coal keeps getting further and further away, and fairly rapidly. If this trend continues for long, then coal will never run out.


[1] https://www.eia.gov/energyexplained/index.cfm?page=coal_reserves

[2] https://www.eia.gov/coal/production/quarterly/pdf/t1p01p1.pdf

*NOTE: This post was revised several times during the 24 hours after its initial publication. I also updated this article on Sept 21 to use a more realistic example of how much electricity could be generated from renewables without storage.

Sunday, March 5, 2017

Dr Charles Hall is still totally wrong about EROI

Recently I saw an article on energyskeptic.com and found that Dr Charles Hall is still making the same incorrect claims. Once again, Dr Hall is claiming that civilization requires an EROI higher than a certain amount to feed its population and support advanced activities. Apparently, if EROI drops below a certain level, then civilization will revert to a primitive state. It could even result in mass starvation if EROI drops too low.

The following remarks are taken from the article:

[Summarizing his research] Pretty soon it looked like we needed an EROI of at least 10:1 to take care of the minimum requirements of society, and maybe 15:1 (numbers are very approximate) for a modern civilization.

Similar ideas are expressed in his earlier papers, such as Hall's "energy pyramid"[1] in Lambert & Hall[2].

Apparently, we require an EROI of at least 5:1 just to grow enough food to survive, whereas we require an EROI of at least 15:1 to have a modern, industrialized society, according to Dr. Hall and associates.

However, those numbers are clearly wrong. A decline in EROI from 15 to 5 does not imply a large reduction in net energy obtained. Civilization has several options for handling a decline in EROI, but the most obvious option is simply to divert some energy from consumption to investment, and thereby keep gross energy constant. If that option were pursued, then a decline in EROI from 15 to 5 would imply a decline in net energy of only ~14%, which can be shown using arithmetic ( (1-1/5)/(1-1/15) = ~86%) [2]. As a more extreme example, a decline in EROI from 1 billion down to 5 would imply a decline in net energy of less than 20% (19.999...). Such declines in net energy obviously don't imply the collapse of civilization, mass starvation, or a return to a medieval mode of life.

Presumably, the error here is to infer that modern civilization must have a proportionally higher EROI than primitive civilization, in order to gain more net energy to support more advanced activities. Dr Hall observes that the kung (a hunter-gatherer tribe) has an EROI of approximately 10. Presumably, Dr Hall infers that modern civilization must have a higher EROI to obtain more net energy.

From the article:
Lee's assessment of the traditional kung hunter gatherer life style implies an EROI of 10:1 and lots of leisure (except during droughts–which is the bottleneck).

However, Hall's inference is incorrect. Modern civilization doesn't just have a higher EROI than primitive societies; it also has a greater AMOUNT of gross energy which it can obtain. Primitive societies have too little energy they obtain, regardless of EROI. Even if the kung increased their EROI from 10 to 1 billion, it would result in less than 10% additional net energy, which presumably would make little difference. The problem is amount, not EROI. It is not possible to know how much net energy will be obtained from EROI alone.

The amount of net energy can be calculated using the following formula:

net = gross - gross/eroi

You will notice that it's not possible to solve the simple equation above using EROI alone. As a result, any remarks along the lines of "we require an EROI of at least x to have modern civilization" are incorrect. Without knowing how much GROSS energy is obtained, we cannot calculate how much NET energy will be obtained. Modern civilization runs on net energy, not a high EROI, so an EROI number by itself (without any indication of gross energy) provides no important information, unless the EROI ratio is lower than 1.

At present, the United States uses 6916 kg of oil equivalent capita, per year. Even if the average EROI ratio for the entire country dropped down to 3, the US would still have more net energy per capita than France[4]. The French are obviously capable of growing food, having education, and so on.

Dr. Hall then claims that we require an EROI of at least 3 to support modern transportation:

We found you needed to extract 3 liters at the well head to use 1 liter in the gas tank to drive the truck, i.e. an EROI of 3:1 was needed... But even this did not include the energy to put something in the truck (say grow some grain)

That claim is incorrect. Hall's paper in question[5] includes depreciation of all vehicles as an energy cost. It also includes all road construction. However, most vehicle depreciation occurs in personal vehicles which are used for discretionary trips. As a result, an EROI of 3 is not the minimum which civilization must have to deliver food in trucks, because civilization could curtail personal vehicles while retaining food delivery in trucks. Furthermore, the replacement rate of personal vehicles would decline proportionally as the rate of net energy to drive them declined. In other words, vehicle depreciation is not constant. As a result, the minimum EROI needed for modern transportation would be far lower than 3 because most of that energy investment could be curtailed without (thereby reducing the minimum EROI figure) without sacrificing anything essential.

It's not necessary for civilization to construct the entire first world edifice of cars and freeways before commencing any other activities. As a result, Hall's energy pyramid is incorrect, because the numbers contained in it would change as EROI declined, and also because those activities are not stacked on top of each other in the way implied by that diagram.

Dr. Hall then turns his attention to the idea of chaining energy sources. You could "chain" power plants (or "stack" them) and thereby achieve a higher aggregate EROI (This is discussed further on this blog, here). For example, if you had solar PV plant with an EROI of 2, you could use the output of that plant to build twice as many new ones, which would yield 4 units of energy for an initial investment of 1. Alternatively you could use the output from the initial plant to build 1.5 times as many new ones (which would yield 3 units of energy for an initial investment of 1) and then return the remaining net energy to society.

Dr. Hall addresses that issue in the same thread, as follows:

The problem with the "stacked" idea is that if you do that you do not deliver energy to society with the first (or second or third) investment — it all has to go to the "food chain" with only the final delivering energy to society.  So stack two EROI 2:1 technologies and you get 4:2, or the same ratio when you are done.

That is clearly incorrect. It is not necessary to devote the entire amount of gross energy obtained to building more solar panels. It would be possible to invest more than is required to replace existing solar panels, but less than the entire amount. This would still lead to exponential growth in net energy obtained (with any EROI higher than 1) while also providing energy for other purposes in the mean time. Exponential growth in energy supply would obviously allow us to obtain any amount until some limit (other than EROI) is reached. As a result, EROI is not proportional to net energy obtained by society.

Dr Hall also claims that solar PV with an EROI of 8 may not actually provide any net energy to society after including more factors as energy investments:

If the EROI [of solar] is 8:1 ... then it seems like you could make your society work. But let’s look closer. If you add in security systems, roads, and financial services and the EROI drops to 3:1 then it seems more problematic. But if you add in labor (i.e. the energy it takes to make the food, housing etc that labor buys with its salaries, calculated from national mean energy intensities times salaries for all necessary workers) it might drop to 1:1. Now what this means is that the energy from the PV system will support all the purchases of the workers that are building/maintaining the PV system, let’s say 10% will be taken care of, BUT THERE WILL BE NO PRODUCTION OF GOODS AND SERVICES for the rest of the population.

Of course, that implies that 7/8ths of the output of a modern power plant is devoted to the employees who work there and miscellaneous expenses like security cameras, roads to the plant, and so on. On its face, that number is highly implausible.  A modern solar PV power plant often has more than 250 MW nameplate capacity, which is equivalent to 47.5 MW continuously at a capacity factor of 0.19. Even if there were 2,000 employees who worked at the plant continuously for 30 years (which is false; solar plants have only a handful of employees), that would still imply more than 23.75 kw continuously per employee which is vastly higher than the total energy usage per capita of any industrialized country. That figure of energy usage includes the employees' discretionary consumption (such as taking plane trips to the Bahamas for vacation) which is not an energy investment.

There is another serious problem with Dr. Hall's ideas on this matter. Over and over again, Dr. Hall treats energy returns as energy investments. In doing so, he places terms in the denominator of the EROI fraction which belong in the numerator. For example, in his paper What is the minimum EROI that society must have?[5], he treats all personal vehicle depreciation as an energy investment. However, most personal vehicle travel is for discretionary trips and not for activities such as (say) gathering coal. As a result, such vehicle travel represents an energy return, not an investment. As another example, Hall repeatedly treats first world salaries for certain workers as energy investments; for example, in the remarks above, or in his book Spain's Photovoltaic Revolution[9], he treats salaries of power plant employees as energy investments.

If we treat things like first world salaries, discretionary car travel, vacations, etc as energy investments, then it would be possible to increase EROI greatly, by simply curtailing discretionary first-world activities somewhat for power plant employees. As a result, the EROI of those sources of energy would increase greatly as salaries declined, in which case, Hall's EROI figures would no longer hold. This implies that Hall's warnings about the decline of industrial civilization wouldn't hold either, because any decline in the first world incomes of power plant employees would cause a concomitant large increase in the EROI of power plants.

In conclusion. Dr. Hall's ideas and papers contain serious mathematical and logical errors which invalidate his analysis. He assumes that modern civilization must have a proportionally higher EROI than primitive civilization in order to obtain more net energy to support advanced activities. However, that assumption is clearly wrong, because modern civilization also has more gross energy than primitive civilization, and so would obtain vastly more net energy even with far lower EROI ratios. Furthermore, Dr Hall is throwing around numbers which are clearly implausible and which are refuted using straightforward arithmetic. What's more, Dr. Hall's criticism of the "stacked" energy source idea is incorrect, insofar as he wrongly assumes that society must devote either all of leftover energy, or none, to obtaining more energy. Finally, Dr. Hall repeatedly treats energy returns as investments, and in so doing, invalidates his other conclusions.

There is one more thing I should point out. These ideas are not new. Dr. Hall and his mentor (HT Odum) have issued warnings about declining net energy and imminent grim consequences to civilization, over and over again, since the early 1970s. Odum first warned in the early 1970s that all sources of energy then had perilously low and declining EROI (called "energy yield ratio" back then) [6]. Odum claimed repeatedly during the 1970s that nuclear reactors probably would not yield more energy over their lifetimes than was required to construct them and refine the Uranium. Odum also claimed at that time that the EROI of coal fired electricity was extremely low and declining. Dr Hall started warning in the early 1980s (during the oil crisis) that the EROI of oil was disastrously low and could decline to just above 1 fairly soon thereafter[7].  Dr Hall warned again, in 2009, that the EROI of oil and gas was perilously declining: "The fact that the EROI for global oil and gas extraction declined by nearly half from 1999 to 2006 is cause for concern."[8] Both Hall and Odum devoted much of their professional careers to issuing such warnings about almost all sources of energy, over many decades. These most recent warnings about the EROI of renewables are simply repetitions of earlier, failed predictions and warnings, applied to other sources of energy back then. Dr. Hall needs to explain why these ideas and methods have failed so badly in their predictions in the past, when applied to fossil fuels, but are still correct now when applied to renewables.

I have pointed out repeatedly, for several years, that Dr Hall's analysis contains mathematical errors. Dr Hall responds to this by being petulant and insulting:

First I would like to say that the bountiful energy blog post is embarrassingly poor science and totally unacceptable. As one point the author does not back his (often erroneous) statements with references. The importance of peer review is obvious from this non peer-reviewed post.

However, that simply does not address the mathematical errors I have pointed out.

If Dr Hall offers no relevant response to these objections, then his ideas are refuted.

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[1] https://i1.wp.com/energyskeptic.com/wp-content/uploads/2014/07/societys-hierarchy-of-energetic-needs-eroi-12-14.jpg

[2] Society previously needed to invest 1/15th of its energy on obtaining that energy, but must now invest 1/5th. Thus, its net energy has declined from 1-1/15, to 1-1/5, because of the increased energy investment. Thus, the ratio of net energy is (1-1/5)/(1-1/15), or ~86%.

[3] http://energyskeptic.com/2016/lambert-hall-energy-eroi-and-quality-of-life/

[4] http://data.worldbank.org/indicator/EG.USE.PCAP.KG.OE . The USA has an energy use per capita of 6916 kg of oil equivalent, which is 4613 net energy per capita with an EROI of 3. France has an energy use per capita of 3840 kg of oil per capita, which implies lower net energy per capita regardless of the EROI of France.

[5] What is the minimum EROI that society must have?, pp 42, table 2. Charles A. S. Hall, Stephen Balogh and David J. R. Murphy. Energies 2009, 2, 25-47.

[6] Energy Basis for Man and Nature. Howard T Odum and Elisabeth C Odum. MacGraw Hill, 1974.

[7] Petroleum Drilling and Production in the United States: Yield per Effort and Net Energy Analysis. Charles A.S. Hall, Cutler Cleveland. Science, 211, 4482, 576-579.

[8] A Preliminary Investigation of Energy Return on Energy Investment for Global Oil and Gas Production. Nathan Gagnon, Charles A.S. Hall, and Lysle Brinker

[9] Spain's Photovoltaic Revolution. Pedro A. Prieto and Charles A.S. Hall. Springer, 2013.