This group brings together the best thinkers on energy and climate. Join us for smart, insightful posts and conversations about where the energy industry is and where it is going.

10,376 Members

Post

The Catch-22 of Energy Storage

Pick up a research paper on battery technology, fuel cells, energy storage technologies or any of the advanced materials science used in these fields, and you will likely find somewhere in the introductory paragraphs a throwaway line about its application to the storage of renewable energy.  Energy storage makes sense for enabling a transition away from fossil fuels to more intermittent sources like wind and solar, and the storage problem presents a meaningful challenge for chemists and materials scientists… Or does it?


Guest Post by John Morgan. John is Chief Scientist at a Sydney startup developing smart grid and grid scale energy storage technologies.  He is Adjunct Professor in the School of Electrical and Computer Engineering at RMIT, holds a PhD in Physical Chemistry, and is an experienced industrial R&D leader.  You can follow John on twitter at @JohnDPMorganFirst published in Chemistry in Australia.


Several recent analyses of the inputs to our energy systems indicate that, against expectations, energy storage cannot solve the problem of intermittency of wind or solar power.  Not for reasons of technical performance, cost, or storage capacity, but for something more intractable: there is not enough surplus energy left over after construction of the generators and the storage system to power our present civilization.

The problem is analysed in an important paper by Weißbach et al.1 in terms of energy returned on energy invested, or EROEI – the ratio of the energy produced over the life of a power plant to the energy that was required to build it.  It takes energy to make a power plant – to manufacture its components, mine the fuel, and so on.  The power plant needs to make at least this much energy to break even.  A break-even powerplant has an EROEI of 1.  But such a plant would pointless, as there is no energy surplus to do the useful things we use energy for.

There is a minimum EROEI, greater than 1, that is required for an energy source to be able to run society.  An energy system must produce a surplus large enough to sustain things like food production, hospitals, and universities to train the engineers to build the plant, transport, construction, and all the elements of the civilization in which it is embedded.

For countries like the US and Germany, Weißbach et al. estimate this minimum viable EROEI to be about 7.  An energy source with lower EROEI cannot sustain a society at those levels of complexity, structured along similar lines.  If we are to transform our energy system, in particular to one without climate impacts, we need to pay close attention to the EROEI of the end result.

The EROEI values for various electrical power plants are summarized in the figure.  The fossil fuel power sources we’re most accustomed to have a high EROEI of about 30, well above the minimum requirement.  Wind power at 16, and concentrating solar power (CSP, or solar thermal power) at 19, are lower, but the energy surplus is still sufficient, in principle, to sustain a developed industrial society.  Biomass, and solar photovoltaic (at least in Germany), however, cannot.  With an EROEI of only 3.9 and 3.5 respectively, these power sources cannot support with their energy alone both their own fabrication and the societal services we use energy for in a first world country.

Energy Returned on Invested, from Weißbach et al.,1 with and without energy storage (buffering).  CCGT is closed-cycle gas turbine.  PWR is a Pressurized Water (conventional nuclear) Reactor.  Energy sources must exceed the “economic threshold”, of about 7, to yield the surplus energy required to support an OECD level society.

Energy Returned on Invested, from Weißbach et al.,1 with and without energy storage (buffering).  CCGT is closed-cycle gas turbine.  PWR is a Pressurized Water (conventional nuclear) Reactor.  Energy sources must exceed the “economic threshold”, of about 7, to yield the surplus energy required to support an OECD level society.

These EROEI values are for energy directly delivered (the “unbuffered” values in the figure).  But things change if we need to store energy.  If we were to store energy in, say, batteries, we must invest energy in mining the materials and manufacturing those batteries.  So a larger energy investment is required, and the EROEI consequently drops.

Weißbach et al. calculated the EROEIs assuming pumped hydroelectric energy storage.  This is the least energy intensive storage technology.  The energy input is mostly earthmoving and construction.  It’s a conservative basis for the calculation; chemical storage systems requiring large quantities of refined specialty materials would be much more energy intensive.  Carbajales-Dale et al.2 cite data asserting batteries are about ten times more energy intensive than pumped hydro storage.

Adding storage greatly reduces the EROEI (the “buffered” values in the figure).  Wind “firmed” with storage, with an EROEI of 3.9, joins solar PV and biomass as an unviable energy source.  CSP becomes marginal (EROEI ~9) with pumped storage, so is probably not viable with molten salt thermal storage.  The EROEI of solar PV with pumped hydro storage drops to 1.6, barely above breakeven, and with battery storage is likely in energy deficit.

This is a rather unsettling conclusion if we are looking to renewable energy for a transition to a low carbon energy system: we cannot use energy storage to overcome the variability of solar and wind power.

In particular, we can’t use batteries or chemical energy storage systems, as they would lead to much worse figures than those presented by Weißbach et al.  Hydroelectricity is the only renewable power source that is unambiguously viable.  However, hydroelectric capacity is not readily scaled up as it is restricted by suitable geography, a constraint that also applies to pumped hydro storage.

This particular study does not stand alone.  Closer to home, Springer have just published a monograph, Energy in Australia,3 which contains an extended discussion of energy systems with a particular focus on EROEI analysis, and draws similar conclusions to Weißbach.  Another study by a group at Stanford2 is more optimistic, ruling out storage for most forms of solar, but suggesting it is viable for wind.  However, this viability is judged only on achieving an energy surplus (EROEI>1), not sustaining society (EROEI~7), and excludes the round trip energy losses in storage, finite cycle life, and the energetic cost of replacement of storage.  Were these included, wind would certainly fall below the sustainability threshold.

It’s important to understand the nature of this EROEI limit.  This is not a question of inadequate storage capacity – we can’t just buy or make more storage to make it work.  It’s not a question of energy losses during charge and discharge, or the number of cycles a battery can deliver.  We can’t look to new materials or technological advances, because the limits at the leading edge are those of earthmoving and civil engineering.  The problem can’t be addressed through market support mechanisms, carbon pricing, or cost reductions.  This is a fundamental energetic limit that will likely only shift if we find less materially intensive methods for dam construction.

This is not to say wind and solar have no role to play.  They can expand within a fossil fuel system, reducing overall emissions.  But without storage the amount we can integrate in the grid is greatly limited by the stochastically variable output.  We could, perhaps, build out a generation of solar and wind and storage at high penetration.  But we would be doing so on an endowment of fossil fuel net energy, which is not sustainable.  Without storage, we could smooth out variability by building redundant generator capacity over large distances.  But the additional infrastructure also forces the EROEI down to unviable levels.  The best way to think about wind and solar is that they can reduce the emissions of fossil fuels, but they cannot eliminate them.  They offer mitigation, but not replacement.

Nor is this to say there is no value in energy storage.  Battery systems in electric vehicles clearly offer potential to reduce dependency on, and emissions from, oil (provided the energy is sourced from clean power).  Rooftop solar power combined with four hours of battery storage can usefully timeshift peak electricity demand,3 reducing the need for peaking power plants and grid expansion.  And battery technology advances make possible many of our recently indispensable consumer electronics.  But what storage can’t do is enable significant replacement of fossil fuels by renewable energy.

If we want to cut emissions and replace fossil fuels, it can be done, and the solution is to be found in the upper right of the figure.  France and Ontario, two modern, advanced societies, have all but eliminated fossil fuels from their electricity grids, which they have built from the high EROEI sources of hydroelectricity and nuclear power.  Ontario in particular recently burnt its last tonne of coal, and each jurisdiction uses just a few percent of gas fired power.  This is a proven path to a decarbonized electricity grid.

But the idea that advances in energy storage will enable renewable energy is a chimera – the Catch-22 is that in overcoming intermittency by adding storage, the net energy is reduced below the level required to sustain our present civilization.

BNC Postscript

When this article was published in CiA some readers had difficulty with the idea of a minimum societal EROI.  Why can’t we make do with any positive energy surplus, if we just build more plant?  Hall4 breaks it down with the example of oil:

Think of a society dependent upon one resource: its domestic oil. If the EROI for this oil was 1.1:1 then one could pump the oil out of the ground and look at it. If it were 1.2:1 you could also refine it and look at it, 1.3:1 also distribute it to where you want to use it but all you could do is look at it. Hall et al. 2008 examined the EROI required to actually run a truck and found that if the energy included was enough to build and maintain the truck and the roads and bridges required to use it, one would need at least a 3:1 EROI at the wellhead.

Now if you wanted to put something in the truck, say some grain, and deliver it, that would require an EROI of, say, 5:1 to grow the grain. If you wanted to include depreciation on the oil field worker, the refinery worker, the truck driver and the farmer you would need an EROI of say 7 or 8:1 to support their families. If the children were to be educated you would need perhaps 9 or 10:1, have health care 12:1, have arts in their life maybe 14:1, and so on. Obviously to have a modern civilization one needs not simply surplus energy but lots of it, and that requires either a high EROI or a massive source of moderate EROI fuels.

The point is illustrated in the EROI pyramid.4 (The blue values are published values: the yellow values are increasingly speculative.)

Finally, if you are interested in pumped hydro storage, a previous Brave New Climate article by Peter Lang covers the topic in detail, and the comment stream is an amazing resource on the operational characteristics and limits of this means of energy storage.

References

  1. Weißbach et al., Energy 52 (2013) 210. Preprint available here.
  2. Carbajales-Dale et al., Energy Environ. Sci. DOI: 10.1039/c3ee42125b
  3. Graham Palmer, Energy in Australia: Peak Oil, Solar Power, and Asia’s Economic Growth; Springer 2014.
  4. Pedro Prieto and Charles Hall, Spain’s Photovoltaic Revolution, Springer 2013.
Barry Brook's picture

Thank Barry for the Post!

Energy Central contributors share their experience and insights for the benefit of other Members (like you). Please show them your appreciation by leaving a comment, 'liking' this post, or following this Member.

Discussions

Ed Woode's picture
Ed Woode on Aug 25, 2014 4:08 pm GMT

Great article….first time I’ve been introduced to the EROEI concept. It made me think about energy optimization in water treatment systems in a wider perspective – here the energy saved over the lifetime of the system must at least be equal to the energy involved in constructing and operating the system.

What is your opinion on optimizing the grid itselt to eliminate the need for storage?

Cheers

Ed

 

 

Roger Brown's picture
Roger Brown on Aug 25, 2014 9:29 pm GMT

Without tyring to promote renewable energy as a solution to our problems I would like to point out that the claim about minimum EROEI is not correct. Consider the following fanciful example of energy production. A magician waves  his magic wand over barrel containing 10 gigajoules of energy and the contents of the container disappear and a moment later 10.1 gigajoues enter the container as the end results of the spell. It takes the magician exactly one second to work this spell. The net energy production rate is 3600*0.1=360GJ/Magican hour. If the magician tries to work the same spell on a 20GJ container it takes him two seconds to complete the spell so that his rate of production remains 360GJ/Magician hour. If you had an adequate supply of magicians you could support a very advance level of civilization at this rate of production in spite of the fact that EROEI=1.01. If you do not like the example of magical spells then you can suppose a machine which harvests energy from a parallel universe with the same energy input and output as above. In this the the rate of prodution is 360GJ/Engineer-hour. As long as the energy supply of your parallel universe holds out you are as good as gold.

Using EROEI as an economic figure of merit is an attempt to use the input energy as a proxy for the total resource cost of energy production. In a general qualitative way this assumption has some degree of validity. Not only does oil production from tar sands require more energy inputs per barrel produced, it also requires more labor, more capital equipment, more water, etc.  However, the level of economic production that can be supported by a given energy supply system depends in a complex way on the total resource cost of net energy production and on the efficiencty of energy use in the production of consumer goods and services, and not merely on EROEI or any other dimensionless energy ratio. 

Tim Havel's picture
Tim Havel on Aug 25, 2014 11:39 pm GMT

It’s important to understand the nature of this EROEI limit.  This is not a question of inadequate storage capacity – we can’t just buy or make more storage to make it work.  It’s not a question of energy losses during charge and discharge, or the number of cycles a battery can deliver.  We can’t look to new materials or technological advances, because the limits at the leading edge are those of earthmoving and civil engineering.  The problem can’t be addressed through market support mechanisms, carbon pricing, or cost reductions.  This is a fundamental energetic limit that will likely only shift if we find less materially intensive methods for dam construction.”

This paragraph shows rather clearly that the author does not know what he’s talking about. If I can make a battery that lasts forever and is 100% efficient (or even getting rather close thereto), it doesn’t matter how much energy was required to make it, it’s going to pay for itself (in EROEI terms) eventually. ‘nough said!

Roger Brown's picture
Roger Brown on Aug 26, 2014 12:04 am GMT

I assume that the expression ‘lasts forever‘ is hyperbole. Let’s be a little more specific and suppose that we develop a battery that lasts for 100 years. The low cost in energy and other resource terms implied by this life time only apply in long term equilbrium. That is if we have installed battery capacty uniformly over a 100 year time period, then we will have to retire and replace 1/100 of our fleet every year and the costs may be relatively low. If we try to ramp up the same technology rapidly over a two decade time period the costs will be much higher, a fact that is relevant if we are hoping to switch to new energy technology without a major disturbance to BAU functioning of the economy.

Clayton Handleman's picture
Clayton Handleman on Aug 26, 2014 3:02 am GMT

This reads like a hit piece on renewable energy.  Those who oppose solar have been trying to kill it with the EROI argument for decades.  It was valid early on but long ago was put to rest as processes got better.  But lets give them the benefit of the doubt and lets say the authors of the paper did their math right.  They use Germany which has the worst solar resouce of any major user of solar energy.  In the US we see massive deployments in the Southwest at roughly double the energy output of Germany on a per kw rated basis.  The paper uses an assumed PV lifetime of 25 years, which is commonly used but quite conservative for tier 1 modules.  They then wave their hand and suggest that the lifetime for modules significantly drops in the South of Germany and therefore negates any EROI gains from better resource.  They cite no source to validate this assumption.  Using the assumption of good solar resource one rises above their value of 7.  However the higher US CF in areas with little cloud cover offers a resource which requires less storage.  So that moves solar with backup, close to the value of 7 which they state.  However, as it becomes clear that the authors are looking for the weakest numbers, their value of 7 is suspect and the correct value is likely lower.

Wind is even better.  The author uses a (Capacity Factor) CF of less than 25%.  In the US CFs are much better.  Wind in the midwest is >30% CF and in the Great Plains currently is averaging 37%.  However, Texas has built transmission to areas where CFs are over 50%.  In KS there is a transmission line that looks promising which will give access to wind resource in excess of 50%, again over twice that of Germany.  The higher CF, of course, requires less storage so wind moves well above 7 even when buffered.

Germany is becoming the poster child for those who support renewables and those who oppose renewables.  Those who support renewables look at Germany and are astonished at the level of penetration achieved despite rather poor solar and wind resources.  Those who are opposed to renewables frequently look to Germany, as it begins to bump into penetration maxima, and suggests that other places will have the same difficulties.  The US is a vastly different case due to a variety of factors including::

1) Much higher CF resources for wind and solar.

2) Much larger area to draw from which decorrelates sources thus reducing storage requirements.

3) The US still has its nuclear fleet, we did not remove it in response to Fukushima

4) Accelerating adoption of EVs which charge at night matching well with the wind availability.  In other words, when looking at the growing market for night time charging and the fact that wind blows more at night, the CF is higher at that emerging peak load time.  This post has some useful graphics that clarify this point.

To suggest that Germany offers a case study for the rest of the world is cynically one sided at best. 

Nathan Wilson's picture
Nathan Wilson on Aug 26, 2014 4:58 am GMT

Germany is becoming the poster child for those who support renewables and those who oppose renewables.”

This is an important insight: renewables can probably be made to work in some locations, but not in Germany.  The Weißbach paper actually clearly states that implementing the solar resources as CSP instead of PV, and locating them in the desert fixes the EROI problem.  And as Clayton has shown, putting the wind farms in the resource-rich central US fixes wind’s EROI problem (a minor change to the paper). I would add that adding dispatchable fuel synthesis would further improve the situation by greatly reducing the need for electricity storage and eliminating the seasonal over-build factor (as well as supplying a portion of total energy demand with a more convenient energy form).

We can also conclude that two core assumptions of the clean energy movement are wrong:

  • renewable energy works everywhere – wrong, as demonstrated by the German examples in the paper.
  • Americans must sharply reduce our per capita energy consumption – wrong: the central and southwest US are awash in renewable energy, which is viable for EROI, and which can’t be transported beyond US borders and perhaps to the east coast (for economic and EROI reasons).

Furthermore, given that there are important regions of the world which cannot live on their renewables, the clean energy movement’s anti-nuclear stance should also be considered wrong.

Nathan Wilson's picture
Nathan Wilson on Aug 26, 2014 5:11 am GMT

optimizing the grid itselt to eliminate the need for storage

For that we would need a large amount of dispatchable loads.  This is a perfect fit for power-to-fuel syn-fuel production.  My favorite syn-fuel is ammonia, since due to its easy to obtain feedstocks (air and water) and ease of storage/transportation, it will likely always be the cheapest fuel possible when the primary energy source is solar, wind, or nuclear power (the Earth’s three largest inexhaustible energy sources).

see NH3 Fuel Association and NH3 – The Other Hydrogen

Roy Wagner's picture
Roy Wagner on Aug 26, 2014 2:34 pm GMT

Interesting to see EROEI as a topic without a comparrison of storage compared to the cost of spinning reserves. No mention of the comparrison to Peaker Gas Plants with capacity factors around 40%.

If energy storage can reduce the reliance on the present methods and technologies used to ensure the consistent supply of electricity the cost versus benefit argument is very different.

As storing electricity itself is impossible always requiring as some form of conversion chemical, thermal or potential such as pumped hydro.

What are the EROEI figures for idling power plants? or Gas Peaker plants which are paid for by the utility customers regardless of if they are used or not used.

The economic arguments should be which has the better EROEI energy storage or spinning reserves?

Pumped storage has an estimated service life in excess of 80 years surley this would triple the EROEI of a 25 year estimate?

All in all a very biased post.

 

Roger Brown's picture
Roger Brown on Aug 26, 2014 4:57 pm GMT

I was not appealing to magic or parallel universes. I was giving examples of ultra low labor cost energy sources and explaining why energy balance alone cannot not be used to analyze the economics of energy production. I realize that most of the people commenting here have no interest in this topic but are merely concerned in attacking or defending renewable energy. Nevertheless let me give one more example. Suppose we wish to convert to an economic system based on 100% nuclear energy and that a complete analysis of equilibrium requirements shows that one third of our gross energy output must be decidicated to manufacturing and maintaining nuclear power plants.  According to Charles Hall we can conclude, a priori, that this energy system cannot support industrial civilization based solely on energy balance considerations. I maintain that we need to know the  the resource opportunty cost (labor, fresh water, impacts on biodiversity, etc) of energy production plus information about the effiency with which energy can be converted in to useful products and services. This second area of information requires knowledge of the general resource base which supports economic production and knowledge of the state of human technological innovation. The hope of reducing economic analyis to a knowledge of dimensionless energy ratios is chimerical.

Bobbi O's picture
Bobbi O on Aug 26, 2014 6:37 pm GMT

 Thought provoking article but history has shown again and again that limitations based on current technology are always wrong. It’s always the unforseen discovery that makes present day forecast for what is possible look naive in hind sight.

Keith Pickering's picture
Keith Pickering on Aug 26, 2014 7:53 pm GMT

You seem to be admitting that solar is not economic in Germany, yet you castigate Weissbach for saying essentially the same thing. So if that’s bias, it’s a bias you seem to share.

The authors are German and they made assumptions (specifically regarding CF) that are typical for Germany. So?

But Weissbach et al. also provide us with a spreadsheet, so we can change those CF assumptions for other parts of the world. So let’s do that.

For solar, let’s change the annual full-output hours from 1750 (southern Germany) to 2190 (Mohave). Results? 

For field-type PV installation, EROI goes from 6.7 to 8.4; and buffered EROI goes from 2.3 to 2.6. Neither is a terribly significant change.

For wind, let’s change the annual full-output hours from 2000 (Germany) to 3500 (South Dakota). That changes EROI from 16 to 29, and the buffered EROI from 3.9 to 4.6.

So wind looks good in the US — but not wind-with-storage, even after addressing your objections. Which is sort of the whole point of the article.

Regarding the “larger area to draw on”, that’s essentially mythical. The only way that works is if states deliberately overbuild renewables (and I’m thinking particularly wind) on the expectation of export demand. But if there happens to be no export demand for those overbuilt renewables (because in the export markets it’s windy today too), you get curtailment — which drives CF right back down again, increasing cost and decreasing EROI.

 

Keith Pickering's picture
Keith Pickering on Aug 26, 2014 8:01 pm GMT

Oh it will? How will it do that, considering that a battery doesn’t actually create a single Watt of usable energy? 

Unless you can get a higher price for a Watt-hour of stored electricity than you can get for a Watt-hour of generated electricity, this is simply nonsense. And since the grid can’t tell whether a given Watt is stored or generated, such a situation will never occur.

Keith Pickering's picture
Keith Pickering on Aug 26, 2014 8:19 pm GMT

I was giving examples of ultra low labor cost energy sources and explaining why energy balance alone cannot not be used to analyze the economics of energy production.

Yes, but you were using an example that cannot ever occur in real life, so the criticism is valid. In the real world, nobody would be foolish enough to drill a well into a field with 100 million bbl of reserves, if it cost 99 million bbl of oil to drill that hole. Because in the real world, people know that the true size of the field is uncertain and the true cost of drilling the well is uncertain, and that kind of uncertainty would prevent any but the most reckless from such an investment. And even if the reserves and the cost proved to be accurate, the economic ROI would STILL be to low to attract investment.

One can argue about what the limit of EROI is for a functioning advanced civilization, but regardless of where one draws the line, it’s very hard to make a case that solar is above that line. And I’ve never seen anyone argue that in EROI terms solar is better than wind, and that being the case, why should we (as a society) be investing in solar?

 

Roger Brown's picture
Roger Brown on Aug 26, 2014 9:12 pm GMT

 “I realize that most of the people commenting here have no interest in this topic (i.e. EROEI as a tool of economic analysis) but are merely concerned in attacking or defending renewable energy.”


Thanks for verifying that the above statement was 100% correct.

Nathan Wilson's picture
Nathan Wilson on Aug 27, 2014 3:56 am GMT

In defense of central plains wind, I would point out that Weissbach’s adjustment for energy flow buffering can be improved significantly when large scale dispatchable fuel synthesis is used.  He assumes the need for 10 days of pumped hydro storage and uses 1.5 “over-capacity factors due to seasonal fluctuations” for wind and CSP.

With synthesis of a storable fuel of similar magnitude as the electrical energy demand, no margin for seasonal variation is needed (for central US wind or desert CSP).  For simplicity, we can assume all of the pumped hydro storage is replaced with dispatch of the fuel plant, and combustion of the stored syn-fuel.  I’ll assume the electricity-to-fuel conversion efficiency is 50%, and use an efficiency of 50% for fuel-to-electricity (Weissbach uses 33%, but this is more representative of German coal than liquid fuel).

Starting with Keith’s high quality wind EROI of 29, we get 29*1.5*.5*.5 = 10.9 for wind to fuel, 43.5 for wind to electricity, and 21.8 for electricity and fuel combined, assuming 2/3 of the wind power goes to fuel making.  So wind is fine and has margin for the things I’ve omitted.

For CSP, Weissbach gives a EROI value of 21 unbuffered.  With my adjustment of 1.5*.5*.5 for seasonal fuel making, the solar-to-fuel EROI would be 7.9.  I won’t correct the EROI for CSP with storage since Weissbach doesn’t provide data for molten-salt thermal energy storage, but I would note that it does not raise the cost of CSP significantly (storage cost is offset by a small cost for turbine/generator/cooling_tower), so I would doubt it would affect the EROI either.

Looking at the report’s EROI number of 4 for solar PV, moving the solar farms to the desert and removing the 2x seasonal overbuild are not enough to allow adequate EROI for fuel synthesis.  However, it is high enough to add supplemental electricity to a nuclear-dominated grid (this is a special case that only applies to warm climates with summer demand peaks). 

I don’t fault Weissbach for ignoring power-to-fuel.  The German vision for power-to-fuel ignores the huge potential for ammonia fuel, and instead revolves around capturing CO2 from coal-fired plants to make synthetic methane, which can be distributed and used with imported Russian natural gas (a strategy which is most appealing when the real goal is propping up the fossil fuel industry).

Nathan Wilson's picture
Nathan Wilson on Aug 28, 2014 2:53 am GMT

Not exactly, we still don’t have flying cars or a cure for the common cold.  Yes, there will be breakthroughs, but breakthroughs usually come in unexpected areas, not the areas we want or need.  For example, no one asked for the microwave oven or the internet, and the piston engine did not emerge from a desire to improve the coal-fired steam engine, but only occured in response to a new liquid fuel.

Throughout most of the 1900s, aviation went higher and higher; the thinner air allows aircraft to efficiently travel at faster speeds.  This progress ended with the economic failure of the Concord supersonic airliner.  All technology progresses only until a physical limit is hit; in the case of aviation, that limit is the speed of sound, above which drag abruptly jumps a couple of times higher, making supersonic travel inherently expensive.  In microelectronics, the limit is near atomic scale, so computers can continue to improve a bit longer.  For solar power, the limit relate to the diffuse nature of sunlight (around 1kW/sq.m at noon).  For batteries, the limit is the energy in chemical bonds (around 1-10 eV for all known chemicals, in contrast to 200,000,000 eV for typical nuclear bonds in fissile materials).

So no, solar PV and batteries storage won’t continue to improve forever; pumped hydro energy storage is already approximately mature.

Nathan Wilson's picture
Nathan Wilson on Aug 27, 2014 4:25 am GMT

What are the EROEI figures for idling power plants? …”

Good questions. The paper doesn’t spend much time on fossil fuel, but it says that for gas-fired electrical plants, the construction and decomissioning is only 2% of the energy cost (fuel is almost all of it), so low capacity factor would have negligible impact.  Idling and part-load operation reduce efficiency, so maybe the EROI drops from 28 to 25.

The paper does not do a great job of exploring different energy storage options (see my comment upthread on power-to-fuel); it gives the excuse that it is reporting data from other sources, and not much is available.

Roger Arnold's picture
Roger Arnold on Aug 27, 2014 10:02 am GMT

This touches on a fundamental problem with EROEI analysis. The whole concept of EROEI, when you really dig into it, turns out to be ill-defined. It treates different forms of energy as equivalent and fungible, when they’re not. The whole notion of energy return on energy investment runs afoul of the first law of thermodynamics — the conservation of energy.  All processes and activities, thermodynamically speaking, have an EROEI of exactly 1. Energy can neither be created nor destroyed.

“No, no” the defenders of EROEI will say, it’s useful energy return that we’re concerned with. Waste heat doesn’t count. But what qualifies as “useful” energy out? Thermodynamics has the rigorously defined concept of work output, but that’s not quite what the EROEI folks want to measure. They’d want to count the potential energy content of fuels as part of the “energy return” on oil well drilling or synthetic fuels production.  That fits intuitively for what you might want an EROEI figure to tell you, but it introduces subjectivity and a level of arbitrary decision making into the determination.

Even harder than determining what should count as energy return is quantifying the other side of the ratio: energy invested.  In your case of the 100 gigajoules of energy in the barrel doesn’t count; all that counts is the energy the magician uses in waving his magic wand to make the energy available. So you have an extremely high EROEI there. Similarly, for solar panels, the raw energy of the sun falling on the panels doesn’t count as energy invested. Only the energy used to manufacture, ship, install, and maintain the panels counts. But it’s easy to come up with borderline cases that challenge the rules one way or another.

In fairness to Dr. Hall and his students, they’ve put a lot of work into formalizing EROEI analysis and trying to make it a useful tool, at least for relative comparisons of competing energy resources. But my impression is that there’s still a lot of subjective judgement involved. By bending the rules and deciding where to draw the system boundaries, it’s easy to manipulate the analysis to make a particular approach look better or worse. It just depends on which way you want it to come out.

Roger Brown's picture
Roger Brown on Aug 27, 2014 5:14 pm GMT

2000 Kcal per day is 0.0003GJ/hour. Even if you assume a factor of 100 for the energy required to produce the food calories this input is a nit comapared to a production rate of 360GJ per hour. I deliberately made the production rate high so that it was not necessary to go into this level of detail in the analysis. But in fact it would be trivially easy to adjust the input output numbers to include other energy inputs.

The inital 10GJ from another source is needed only once and has no relevance to the long term equilibrium energy balance. I deliberate made this intial energy cost low compared to the net production rate (360GJ) per hour so that it would be obvious that this cost is a relatively unimportant consideration in this illustrative example of an extreme case.

Of course for real world alternative energy sources such as nuclear or solar the inital costs (in energy and other resource) of a rapid build out would have significant economic consequences.

 

 

 

 

Roy Wagner's picture
Roy Wagner on Aug 27, 2014 6:46 pm GMT

The questions where to point out what energy storage would be replacing.

Only a direct comparrison of the economic returns from a spinning reserve power plant or an energy storage system on a MW by MW basis would reveal their true EROEI.

Grid scale energy storage systems such as Pumped Hydro or CAES are capable of storing electricity regardless of source they are not tied to either renewables or fossil fuels.

So these arguments based on renewables plus storage are biased.

In remote locations without grid access energy storage or conversioninto locally used or transportable fuels may have more relevance.

 

 

 

Robert Bernal's picture
Robert Bernal on Aug 28, 2014 6:27 am GMT

We seek to replace fossil fuels, therefore EROI has all the relevance, since there are not many choices that can compete with FF’s at providing the resulting higher standards of living. The argument for FF’s is that they are needed to build the next better source before they become too expensive to do so.

Now would be a good time to invest in the future and build advanced nuclear – along with whatever best storage available for the applications such as clean liquid fuels for industry and motive, from air and water.

Robert Bernal's picture
Robert Bernal on Aug 28, 2014 7:08 am GMT

You need to fully understand the scope of his correct argument (concerning the first sentence, the earth moving is only one of many options). In a geologic blink of an eye, there will be only VERY expensive fossil fuels and an severely altered biosphere based on emprical physical and chemical evidence (and the known laws of both). Therefore (this will get winded), in an age where FF’s are for the most of us, depleted, we will have to provide power for the factory workers that build the farm equipment so that additional workers can build more factories so that we can have “everything” and even more factories (or energy intensive 3-d printing machines?) to build yet more wind and solar which, well you get the point. We won’t have the power to charge the everloving battery if we don’t secure the power to make it!

If it takes 2.5 years for solar to energy pay for itself and another 2,5 years for the batteries to provide the energy back that it took to build them also, then we’ll run into problems. The service might be well worth it (having to overbuild the solar in order to energy pay for more batteries so that we can transition from FF’s). However, if the batteries only last 10 years (which they don’t unless shallowly discharged each time) then we are again in trouble, having to build even more solar for their recycle. By then, we’ll be starting to recycle some of the solar as well… better build yet more panels.

It will take over a million sq miles of solar (alone, with storage) just to power 10 billion (at rather high standards), let alone these additional expenses. This is why we NEED the storage with the least embodied energy per unit capacity stored. Or, we can go with the much higher EROEI energy sources in the first place and use that to build all the RE and batteries we could ever need to help fill in…

Which is a cause of baseload disruption (as more solar, there must be a lower baseload threshold) and the need for yet more variable load following. Nuclear must power these storage options because it is the source with the highest eroei. It needs to back solar at night with its own created ammonia fuels.

Keith Pickering's picture
Keith Pickering on Aug 28, 2014 2:13 pm GMT

You’ve done an excellent job of proving it yourself. 

Robert Bernal's picture
Robert Bernal on Aug 28, 2014 7:27 am GMT

It takes many factories and highways and computers and so on to “power” a bunch of magicians. You’re saying that an EROEI of just over “1” is good enough to grow civilization? That would indeed require magic!

Nuclear gets the highest EROEI and we can say that corn ethanol gets the lowest. Ever compare the land size differences? Ok, lets say that the corn returns 1.1 per year after ALL energy inputs are considered (the fraction of the inputs for the cars that drive the employees that make the farm equipment, and so on). After a year, you will have people wanting (killing for?) to use this 0.1 just for survival (and the corn industry the other 1.0 just for continuance). No good.

However, if we did what you suggest and generated that 0.1 extra in a few seconds, well then, that must be the power of the atom!

Robert Bernal's picture
Robert Bernal on Aug 28, 2014 7:54 am GMT

Unfortunately, we need to remove heat from the depths of the oceans as well, lest a 20x more powerful greenhouse gas giant wakes up to thwart all our efforts. Sequester the excess CO2 into good soil by greening the deserts with biofuels, powered by a nuclear infrastructure, as the biofuels are obviously too low on the EROEI to do all that and power a civilization. And reflect light back to space in the meantime.

Robert Bernal's picture
Robert Bernal on Aug 28, 2014 7:59 am GMT

Great article, Thanks for posting here.

I have a question: about how many kWh does it take to make a kWh of storage for the various different battery types. I know this must be hard to find because of all the indirect inputs as well.

And, Nathan, I should already know, but how efficient is the process to convert heat and, or electricity into ammonia?

Thanks.

Roy Wagner's picture
Roy Wagner on Aug 28, 2014 8:03 am GMT

This is also a matter of perception consider the fact that renewables do not need storage.

The grid operators, utilities or energy consumers need storage to maximize the benefits of renewables

Currently the grid uses spinning reserves to meet it’s unpredictability issues which are created from the daily/hourly varying customer consumption, mechanical faliures or extreme weather events.  

As fossil fuels currently provide over 80% of electricity generation replacing fuel burning spinning reserves with energy storage systems is possibly the most logical next step.

As these facilities right now are the least efficient during normal operations 

The most efficient way to use energy storage is to store the most expensive electricity source in most cases that will be the fuel burning sources that cannot be ramped up or down quickly.

Allowing the cheapest source normally the fuel free sources such as wind or solar to be used first will keep average wholesale prices lower.

When there are peak usage conditions stored energy supplies generate a much higher price to match the increase in demand.

When conditions create a surplus which makes more sense to store or curtail the highest cost or lowest cost

 

Alistair Newbould's picture
Alistair Newbould on Aug 28, 2014 10:35 am GMT

Worth adding geothermal to those three Nathan? Dispatchable too. Currently geographically isolated but worth remembering about. 13% of NZ electricity generation according to this

http://www.nzgeothermal.org.nz/elec_geo.html

Drill a hole deep enough anywhere and you’ll hit magma. The limit would appear to be technological.

Steve K9's picture
Steve K9 on Aug 28, 2014 1:46 pm GMT

What advantage over alternatives does ammonia have that overcomes its corrosiveness and toxicity?  This is a question, not a critique.

Keith Pickering's picture
Keith Pickering on Aug 28, 2014 2:04 pm GMT

At the risk of soundin like a broken record ….

Batteries do not ever return the energy used to make them. Batteries cost energy to make, and they never return that energy back to society. The EROI of batties is negative (or actually, less than 1 … but the negative-number analogy is useful here), and always will be. The same is true of all energy storage technologies. Therefore the EROI of any generator-plus-storage technology will always be lower than the generator technology alone. The reason batteries are useful is not because they make energy (they do not) but because they allow time-shift of energy use.

In fact, the EROI of nearly every single thing society does is “negative”. We dont get energy back from hospitals. We (usually) don’t get energy back from manufacturing. Same with haircuts, retail sales, national defense, law firms, and roads. Agriculture is positive, but it declines with mechanization and today is dependent on outside energy for the productivity needed to feed society.

And the only way to run all those “negative”-EROI things is to have enough “positive”-EROI (actually, EROI greater than 1) in the system. So yes, there is a lower limit to EROI beyond which modern society will not function in the ways we have come to expect.

Keith Pickering's picture
Keith Pickering on Aug 28, 2014 2:11 pm GMT

You really should read Wiessbach’s paper, Roger. The authors make just that distinction, dividing both energy inputs and outputs into electrical vs. thermal, and accounting for them differently. In effect, they’re replacing “energy” with “exergy”. The graph presented here is the “classical” computation without the distinction, but the exergy-based way of doing it is, IMHO, a better way to go, for just the reasons you articulate.

Tim Havel's picture
Tim Havel on Aug 28, 2014 3:29 pm GMT

Sorry, I was using “EROEI” in the sense of Barnhart and Benson (“Can we afford storage? A dynamic net energy analysis of renewable electricity generation supported by energy storage”, Energy & Environmental Science, 2014, 7, 1538-1544; see “http://pubs.rsc.org/EN/content/articlehtml/2014/ee/c3ee42125b”). That article is far more carefully researched than the foregoing, and points out e.g. that wind is well in the clear if it is backed up by CAES. While the margins are admittedly much thinner for today’s silicon PV and e.g. lead-acid batteries, allow me to assure all readers here that the situation is rapidly evolving towards an EROEI that beats shale oil and other such unconventional fossil fuels. Which will soon be all we’ve got left!

Clayton Handleman's picture
Clayton Handleman on Aug 28, 2014 3:46 pm GMT

” While the margins are admittedly much thinner for today’s silicon PV and e.g. lead-acid batteries, allow me to assure all readers here that the situation is rapidly evolving towards an EROEI that beats shale oil and other such unconventional fossil fuels.”

 

Agreed, in fact, lead acid batteries have already been surpassed by Lithium Ion in terms economics as shown in this analysis.  However the cost of Lithium Ion used in the analysis is about double what it is today and Lithium Ion is expected to drop by about a factor of 2 or 3 from today’s by 2020.  And while that does not speak directly to EROEI, it is a reasonable assumption that some of the savings comes from reduced energy inputs into manufacture.

Roger Brown's picture
Roger Brown on Aug 28, 2014 5:50 pm GMT

Keith,

 

I apologize for my snide remark which was pointinless and unproductive.

 

Roger Brown's picture
Roger Brown on Aug 28, 2014 6:17 pm GMT

You’re saying that an EROEI of just over 1 is good enough to grow civilization?

 

I am saying that if the non-energy resource inputs were low enough then the economics would work. An energy source that required 1 hour of labor and a capital equipment of one wooden wand to produce 360GJ of net energy would support a lot of economic production. Of course no such energy source exists or is about to come into existence. My point is not that renewable energy is going to save us because the required resource inputs will drop indefinitely, but rather that the resource cost of net energy production is a better tool for economic analysis than EROEI.

 

Consider a biofuels example. Two farmer with 500 hectares of land are raising the same energy crop and producing the same gross output of fuel with an EROEI of 2. One of the two farmers improves his productivity by decreasing the required energy input by a factor of two while maintaining the same gross output of fuel. The other farmer switches to a new crop which produces twice the gross output of fuel with the same input of energy. Both farmers have increased EROEI from 2 to 4, but if the opportunity cost of agricultural land use is a dominating factor then the second farmer is outperforming the first because he produces twice the net energy on the same amount of land.

 

By the way, just for the record, my best engineering judgment is that an attempt to use biofuels to preserve and extend the automobile culture to 9 billion people would be ecologically disastrous. I also think that renewable energy by itself cannot support the same level of economic production to which the OECD countries have become accustomed. I mention these views in passing since an assumption seems to exist that if I criticize the analytical methods of an article attacking renewable energy it must be because I have unbounded faith in the economic potential of renewable energy.

 

Robert Bernal's picture
Robert Bernal on Aug 28, 2014 6:30 pm GMT

In the example of the farmers, I’m sure there is still room for efficiency improvements in order to lower the energy inputs of most the things and processes necessary, thus I must agree.

However, I also know that it would be nice to re-develop two kinds of molten salt reactor, one for electrical to grid (and for EV’s) and the other to make ammonia (for “everything” else). If the tractor used nuclear made synfuels, then even less land would be needed for fuel.

Robert Bernal's picture
Robert Bernal on Aug 28, 2014 6:40 pm GMT

I meant, how many units of energy are necessary to make a battery? Eventually (I would assume) the total of the stored energy WILL total to more than the amount of energy required to make it. Therefore, it can not be likened to a hospital (which can not even store energy). To me, it is more like a bank, costly, but necessary. I imagine that in nich applications, it wouldn’t matter if the battery never stored as much as was required to make it (as in a possible hospital application). But as a “green” application, they MUST deliver MORE, much more stored energy (the total of all discharges) than required to make them!

Robert Bernal's picture
Robert Bernal on Aug 28, 2014 7:07 pm GMT

Renewables wil need much backup. Thus we will need storage in order to supply energy at night when the wind isn’t blowing and in the day when it is cloudy. That storage should be ammonia made by the molten salt (or better) reactor (instead of natural gas made by pre-historic photosynthesis).

Roy Wagner's picture
Roy Wagner on Aug 28, 2014 10:55 pm GMT

The same place the to cheap to meter, safe nuclear energy resides if we are talking about over hyped and mythical promises.

Seriously they are both are proven technologies the fact is that CAES storage systems have been used successfully to increase the peformance of gas turbines.

 Bill gates has invested in Lightsail and SustainX, CAES can also utilize abandoned mines oil or gas wells or geological assets like salt caverns,

My personal favorite are the underwater airbag systems which have been proposed these have the lowest cost per kWh of any storage medium.

http://www.theengineer.co.uk/in-depth/the-big-story/compressed-air-energ...

Robert Bernal's picture
Robert Bernal on Aug 29, 2014 3:43 pm GMT

I believe that the “renewables only” crowd does not want to figure in the true costs of not utilizing the high EROEI of advanced nuclear, thus whether knowing or not, supporting the fossil fuels.

Michael Hogan's picture
Michael Hogan on Aug 29, 2014 5:56 pm GMT

As always it seems on this site, this overlooks a whole range of end-use energy storage options, primarily thermal energy storage, that are far less costly than any of the storage options assumed here. It also overlooks the potential for demand response, consolidation of balancing areas and investment in transmission – all of which are remarkably inexpensive options – to reduce the need for any form of energy storage in the first place. Finally, again as usual, this article overlooks the fact that any significant share of electricity production from nuclear – and if we went in the direction suggested by Mr. Brook (assuming that is practically realistic, which it is not) that share would be large indeed – requires equally significant investments in energy storage, as has already been the case in areas where significant investments in nuclear generation have taken place. This is all very interesting, but it’s irrelevant. The efficiency of solar has been increasing exponentially for many years and, as with computer processing power, that trend is likely to continue. Nuclear can meet a small percentage of our needs for near-zero carbon electricity but it is wildly unrealistic to assume we can do what needs doing without major investments in renewables. That will require continued improvement in technology to chip away at the EROEI, to be sure, but what it really needs is to be a lot smarter about how we approach this problem than is implied by this posting.

Clayton Handleman's picture
Clayton Handleman on Aug 29, 2014 9:01 pm GMT

Hmmm, I took something quite different away from Murials post.  But to each their own.

In any event, I favor a market driven approach that rewards people for load shifting and conserving over the current centrally planned Soviet Style monopolies that we call utilities.  I am a capitalist, I am an environmentalist, I am not a Marxist.

 

Roy Wagner's picture
Roy Wagner on Aug 29, 2014 9:29 pm GMT

In this case you are wrong my reply was just in the same fairytale like style as yours.

I do believe safe Nuclear energy is a requirement in any future energy supply scenario.

My hope is they can improve upon the methods and practices so this can happen.

Facts I would ask you to consider how many Nuclear Power stations have been cancelled or closed due to safety issues or operating costs in the recent history of the industry.

Please address the serious under estimating of the actual costs of construction ($ Hundreds of millions of dollars per plant) that seems to be the only way they can get approval.

How many exajoules of energy has Thorium generated since Nixon shut it down? 

Small modular reactors? Travelling wave ? Fusion? etc etc 

As for toxic waste where is that storage facillity?

I repeat CAES is a viable proven energy storage medium which can be used to store surplus energy by any renewable or any other surplus energy source.

Wind Geothermal and Wave energy can produce compressed air directly as this is a storeable form of energy (Potential energy) it can be produced continuosly without regard to Grid demand.

It is available almost instantaneously  to generate electricity if required multiple sources can use the same storage infrastructure. 

Existing technology is available with Capacities that can vary from a few miniutes to MWhr’s 

There is nothing fanciful or imaginary about it.

Just because the fossil fuel industry have been the first ones actually developing CAES into a reliable energy storage medium does not make it any less proven.

I suggest if you disapprove of Fossil Fuels for electricity generation you concentrate your efforts on vilfying them and support renewable energy as a better if not perfect alternative.

Blaming renewables on the lack of R&D or investment in Nuclear energy is wrong, it’s not a case of either/or for our future we must consider all the clean energy options equally.

 

 

 

 

 

James Thurer's picture
James Thurer on Aug 30, 2014 3:37 am GMT

References?

Robert Bernal's picture
Robert Bernal on Aug 30, 2014 8:05 am GMT

Theoritically, Any high EROEI source can be used… and MUST be used. Most of the time when I talk to ordinary people about “climate change”, they tend to change the subject at the mention of nuclear, even when I try to explain the molten salt reactor. They say we can do it all with solar, wind, hydro and biofuels. I ask, which has generated more electricity per unit of subsidy (just as I asked myself)? And which requires more land?

I find that an advanced nuclear is the answer because when I look up “eroei by energy source” (images) I find that nuclear (which is assumed to me to be the conventional LWR) is about half of wind. However, wind would need more storage, since it has less capacity factor. Nuclear would need some storage, but not nearly as much because it is more dependable (much higher CF). Storage can be in the form of thermal, electrical, potential and chemical. Also, advanced nuclear requires less materials and FAR less mining (than the LWR because it would consume much more of the fissile material AND be of a higher temp).

Some say an EROI of 75 and others only 10 (for nuclear). Anyways, here’s a graphic about the Molten salt reactor’s material inputs compared to the light water reactor’s. I would bet that it would have the highest EROEI of any energy source!

After considering that the advanced nuclear will use less materials (they do not require steam containment and can be mass produced (which is FAR more efficient), that nuclear is cheaper per unit of energy subsidized AND that it is more dependable (requiring less storage), it is clear that this is is one of the most efficient ways to transition from fossil fuels, especially, since it is meltdown proof and that the wastes (from the closed cycle) is MUCH less (and remains radioactive for just 1/1,000th as long as spent fuel, which, bty could also be used as fuel).

To me, this is pretty exciting science (and I’m not afraid).

In order to develop ALL the storage options that are convenient for the taskWe must develop the most dependable source with the highest EROEI.

Robert Bernal's picture
Robert Bernal on Aug 30, 2014 8:36 am GMT

Ok, I found the keyword: Energy Stored On Investment (and) battery. Proves you wrong (that “batteries do not ever return the energy used to make them”).

However, you’re almost correct. Batteries only fair from just 2, up to 10, whilst pumped hydro is 210. They say CAES is a whopping 240 (but I thought that needed NG).

http://news.stanford.edu/news/2013/march/store-electric-grid-030513.html

Keith Pickering's picture
Keith Pickering on Aug 30, 2014 2:31 pm GMT

CAES is a lot more difficult to implement that most people realize. I invite you to google “ISEP”, the Iowa Stored Energy Park, a planned CAES facility that was abandoned in the planning/assessment stage because no suitable site (for a very modestly-sized facility) could be found in the entire state of Iowa. This in spite of over-optimistic maps that indicated the entire state was underlain by CAES-friendly aquifer storage geology. Well, the aquifers are there, all right. But the sizes of the geological domes, the thickness of the porous layers, and the porousity of the layers combined to give all sites air volumes too small for investment.

This was a serious project by several Iowa utilities that were seriously looking for storage for Iowa’s booming wind generation. They sunk millions into ISEP before abandoning it.

Nathan Wilson's picture
Nathan Wilson on Aug 30, 2014 2:47 pm GMT

Good find Robert.  Energy Stored on Investment (ESOI) is just as relevant to the discussion as round trip efficiency (as Keith and Willem mention).

I had hoped that the claimed long life of the liquid metal sodium-sulfur battery and the vanadium flow battery would make them good ESOI performers.  But the Stanford article says no, lithium-ion leads with 10, and (the free abstract shows) NaS and VRB lag with only 6 and 3 respectively.

The Stanford article’s abstract ends with a statement that basically supports the conclusions of the Weissbach article: variable renewables only make sense when backed with flexible fossil fuel generation: 

As a result of the constraints on energy storage described here, increasing grid flexibility as the penetration of renewable power generation increases will require employing several additional techniques including demand-side management, flexible generation from base-load facilities and natural gas firming.

Nathan Wilson's picture
Nathan Wilson on Aug 30, 2014 4:28 pm GMT

Convention CAES made sense back in the 1970s when most natural gas was burned in low efficiency boilers or simple turbines.  But now that we have 60% efficient combined cycle natural gas plants (for about the same price), CAES just can’t compete.

The issue is that since conventional CAES takes electricity and natural gas as inputs, the calculated round trip energy efficiency depend on how you value the gas.  

For example, a 2005 proposed CAES system called Ridge Energy Storage for Texas would have had a natural gas usage rate  (“heat rate”) of 4500 btu/kWh (slightly better than the McIntosh Alabama plant at 4600 btu/kWh), and an electricity in/out ratio of 0.8.

If we value the natural gas like a GT gas turbine, i.e. using a heat rate of 7,500 bth/kWh, then the round trip energy efficiency of the electricity storage works out to 68%.  Pretty good.

On the other hand, if we use a natural gas value of 5,700 btu/kWh like a modern combined cycle gas plant, then the electrical round trip efficiency is just 36%, and a whopping 21% of the output energy comes from the natural gas.  So even if we build this plant just to get the fast throttling, the total natural gas usage is not much lower than with a combined cycle plant of the same output.

Using “Adiabatic CAES” replaces the natural gas-fired reheat with thermal energy storage.  There are currently no such plants anywhere in the world.  As described here, oil at 330C is one possibility. This system has a high cost for heat exchangers, and hot oil is an extreme fire hazard (leaks can spontaneously combust) – the DOE’s Solar One demo plant’s TES was destroyed in an oil fire.

There have been studies on using concrete for thermal energy storage, since it is cheap, but it has limited cycle life, and is neither a good heat conductor nor a good insulator.  Typically either the concrete is filled with expensive heat exchangers, or has open flow channels and is placed within a large and expensive pressure vessel.  Again no plants built to date.

The most cost effective thermal energy storage uses “solar salt” (a blend of sodium nitrate and potassium nitrate, see this presentation).  However, it’s usable temperature range (about 220-565 C) is not compatible with CAES, which really needs storage that can go down to room temperature.  

Intriguingly, sodium-cooled fast nuclear reactors like the IFR and the molten salt cooled reactor that China is developing are a good match for solar salt for thermal energy storage.  Such systems can be placed closer to demand centers than can CAES, which makes them much better for supporting distributed solar or simple demand variations.  (I still believe dispatchable fuel synthesis beats all storage).

Nathan Wilson's picture
Nathan Wilson on Aug 30, 2014 4:47 pm GMT

[from upthread] “What advantage over alternatives does ammonia have that overcomes its corrosiveness and toxicity?

From a safety standpoint, the reduced fire and explosion hazard relative to gasoline and hydrogen mean that ammonia achieves about the same overall safety rating as these fuels – see here.

The material compatibility issue is not particularly more difficult than with gasohol.  Some rubber gaskets won’t work, but others will.   No big deal.

But the big advantages of ammonia come when you compare it to other carbon-free fuels, or when you’re looking for an energy carrier for solar, wind, or nuclear power.  It’s got triple the energy density of 5,000 psi hydrogen, and can be stored in low cost 250 psi tanks.  The only feedstocks needed for its production are water and air, so there is no need for fossil fuels to provide a carbon source.

Pages

Get Published - Build a Following

The Energy Central Power Industry Network is based on one core idea - power industry professionals helping each other and advancing the industry by sharing and learning from each other.

If you have an experience or insight to share or have learned something from a conference or seminar, your peers and colleagues on Energy Central want to hear about it. It's also easy to share a link to an article you've liked or an industry resource that you think would be helpful.

                 Learn more about posting on Energy Central »