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Electricity storage

 

The problem with wind and solar to an even greater extent is seasonal, daily, and hourly variability.  Electricity demand is also variable but in a far more predictable way; again seasonally, daily and hourly. These demand and supply peaks and troughs hardly ever match.  In particular winter demand peaks in southern latitudes occur when solar incidence is low or non-existent.  Wind does blow at night but often when demand is very low.

I have explained (in Renewable Electricity, elsewhere on this website) that even in the best wind provinces it is available for generation for only about a third of the time and that this limits the practical contribution that wind can make to total generation to around 30%.  This capacity factor limits solar to much less as it is dark for half the time and overcast or raining for another proportion. 

Let's take a real example: unlike much of Australia wind resources are of a very high quality in parts of South Australia and Tasmania. This has encouraged a great deal of wind investment there.  In 2016 in South Australia (just 5% of the national electricity market) wind generation reached 4,292 GWh, from over 700 installed turbines with a combined capacity of 1.6 GW of wind generation. 

As expected, a simple calculation based on hours in a year gives us an average capacity factor of 31%. The remainder of the time the State was dependent on other sources of electricity. In this case, fossil fuels and two interlinks to the neighbouring State of Victoria allowing the wind farms to dispose of unwanted energy when local demand is low and for consumers to get power from interstate when there is no wind.  Nevertheless this swap was in the wind generator's commercial interest as the farms produced 33% of the State's generation, the surplus, that enjoyed a substantial financial subsidy, being exported.

But fluctuations in the wind; high air-conditioning demand; and major storms, together with the closure of base load fossil generation; and the limitations of the interlink, resulted in serious blackouts.

As I previously discussed, an effective, efficient and low capital means of energy storage could dramatically change this. But it needs to be on a very large scale. You can easily see that to move to 90% wind power in South Australia first they would need another 1400 turbines and then a means of storing two thirds of their energy for when the wind is not blowing: 8.6 terawatt hours (TWh).

The intrinsic cost of wind energy, when not subsidised by means of large-scale generation certificates (LGCs), is already very much higher than the present alternative fossil-based sources, as demonstrated by the price difference between South Australia (33% wind, 60% fossil) and Queensland (0.05% wind, 94% fossil) in the following table.

 National Electricity Market

SA

 Vic

 NSW

 Qld

 Price maximum (A$/MWh)

2,650

 290

299

270

 Price Minimum

57

46

68

49

 Approximate median

540

100

140

100

 LGC price (current wind subsidy - $ per MWh)

84

84

84

84

 Annual generation 2014-15 (TWh) 13 56.5 64.3 68.2
Electricity market situation in March 2017 (prior to closure of Victoria's major fossil based generation plant)
Note that Qld is considerably less populous than NSW but exports its cheaper electricity to the larger State
Sources: Office of the Chief Economist; NEM data dashboard; LGC closing rates 20/3/17

 

Given this high cost of wind generated electricity, the additional cost of any electricity storage would need to be trivial to avoid totally uneconomic electricity price increases.  South Australian electricity distributors already pay four to five times the national average for their wholesale electricity.

At the moment the only commercial large scale (terawatt) method for storing energy is pump-storage, associated with hydroelectric schemes.  The best of these lose over 30% the energy in pumping water up hill and then letting it run back through the turbines, an added cost.  Pump storage is primarily used for providing extra generation when consumer demand is high; not for smoothing wildly fluctuating generation from wind or solar farms. 

In Australia hydro-generation is far too small and unresponsive to compliment a large-scale contribution of renewable energy.  The entire capacity of the Snowy Mountains Hydroelectric Scheme is only 7.8GW. Pump storage is a tiny fraction of this but an increase is within reach of complementing the presently small contribution of wind generation, albeit at a cost to consumers or taxpayers.

Due to falling panel costs and improved efficiency, photovoltaic (PV) solar generation may now be cheaper per kWh than wind. But the lower capacity factor and rapid fluctuations in output when it is cloudy require even higher levels of storage to make solar competitive as a total energy solution (see Why buy electricity when you can make your own? above). Any storage solution like batteries needs to be highly distributed to be sufficiently responsive; ideally connected and matched to particular photovoltaic solar panels.

Indeed many off-grid PV solar and wind installations use batteries.  So why not use them to allow more wind and solar for the grid in general?

 

Convert to batteries

 

 

Batteries

Batteries store electricity by converting it to chemical energy and then back.  An ideal battery would store electricity with no losses, for example heat being generated during charging or discharge. It would be constructed from plentiful; non-toxic chemicals; and would be light weight to enable its use in vehicles and hand held devices. It would work in any climate and safely store chemical energy at a similar density to high explosives like TNT.   As in any battery the chemical reaction must be reversible. This should be possible many thousands of times over, before the storage qualities are significantly degraded, because a thousand cycles is only three years of daily use.  And recharging should be as fast as possible - certainly no more than the discharge time.

Since I first wrote this significant advances have been made in battery technology and these have been quickly commercialised. Sometimes too quickly resulting in fiery mobile phones and damaged aircraft. 

For more than a century our best rechargeable battery was the lead-acid accumulator, like that in your conventional car; truck; or buss. Then, at last, in the last decade of the 20th century, new materials based on rare-earths offered new avenues of research.  Commercial nickel metal-hydride (NiMH) batteries, that met many of the desired criteria, became a reality.  Nickel is less dense than lead and the new batteries packed twice as much energy per kilogram as lead-acid cells.  As an added bonus they could be small or large and had no messy liquid acid or fumes. This improvement opened the way to many new types of hand tools and electric vehicles.  

For many years lithium shimmered as the 'Holy Grail' of battery technology. Lithium is the lightest metal and the least dense solid element. It's also highly reactive, with the greatest electrochemical potential and energy-to-weight ratio of any element in the periodic table. 

But lithium metal is so reactive that it spontaneously bursts into flame and early attempts to build a safe and practical battery ended in tears.  Nevertheless work went on, involving many improvements until, in 1985, Akira Yoshino assembled the first practical rechargeable lithium-ion battery at the Kawasaki Laboratory of the Asahi Kasei Corp in Japan.  His new battery avoided using lithium metal directly, making it less unstable, yet provided another doubling in energy density over NiMH.

With volume, the battery prices quickly fell, so that new opportunities and markets, like little drones and more complex telephones, proliferated.  The trouble is that Li-ion batteries are still rather dangerous. Higher current energy density comes with a risk, as Boeing discovered on their new 787; as did UPS when a battery fire brought one of their planes down.  Samsung too had to withdraw their Samsung Galaxy Note 7 recently due to higher density batteries catching fire. Further increase in energy density is likely to make them even more dangerous. 

Among these new markets for batteries are electric cars. 

 

Hybrid car

 

The electric car maker Tesla is well aware of the dangers inherent in the lithium-ion (Li-ion) batteries it uses.  At the present time a Tesla P100D can get a third of the way from Sydney to Brisbane carrying an 85 kWh lithium-ion battery weighing 544 kg (around half a ton).  To obviate the risk of fire or explosion the Tesla 'battery' is not a single unit but consists of 7104 small individual Li-ion cells organised in 16 modules, separated across a wide pan below the car. 

It would be nice to drive all the way to Brisbane without a lengthy recharging stop. But the risk would be significantly magnified if the battery had four times the present energy density, let alone the density required to go head-to-head with conventional car with a full tank of petroleum.

Although petroleum has one hundred times more potential energy than a super-battery, that energy is difficult to release all at once. This is because the fuel requires three times its weight in oxygen to combust and there generally isn't that much oxygen around.  That's the reason conventional cars seldom explode, although there are tens of thousands of crashes every year.  It's actually quite difficult for movie makers to make cars explode. To get a nice explosion in the movies, the fuel is usually atomised by a real explosive because, in real life, unless atomised by the crash, there's generally not enough oxygen close-by to sustain more than a relatively unimpressive fire. 

On the other hand, batteries contain all the chemicals required to release their stored energy.  They're chemically self-contained and if shorted or become unstable in a fire, like TNT, they can release their stored energy almost instantaneously.  At the moment the Tesla battery has about the same energy as 75 kg of TNT split into over seven thousand little cells.  Quadrupling this stored energy, say to get to Brisbane on one charge, would add an extra punch to every cell and put the battery up there with explosive potential of a 500 lb bomb. This is not just a Tesla problem.  Any battery capable of this level of energy density, approaching that of petroleum, will have this problem.

Despite recent failures, an attractive feature of Li-ion technology, compared to other very high energy batteries, is its relative safety.

Tesla now manufactures stationary energy storage systems for off-grid solar.  These significantly reduce the risks, compared to an automotive accident or a mobile phone, but they are often much larger.  It's not entirely clear what might happen in the case of a building fire or collapse or flooding - presumably Tesla have engineered the units with a worst case scenario in mind, as the larger units have the energy storage of a very large bomb indeed.

It's tempting to think that with Li-ion technology we have found the Holy Grail of batteries - all we have to do is get the price down - and mass production is the answer.   But the batteries still fall well short of our ideal, as outlined above particularly in terms of cycle life, efficiency (energy lost to heat) and cost.

Here are some features of present generation of, advanced, high energy density, batteries: 

Nickel metal-hydride (NiMH)
Energy density
Specific power
Charge/discharge efficiency
Self-discharge rate
Low self-discharge types:
Cycle durability

140–300 Wh/L
250–1,000 W/kg
66%-92%
13.9–70.6% (per month)
1.3–2.9% (per month) at 20 °C
180–2000 cycles
Lithium-ion
Energy density
Specific power
Charge/discharge efficiency
Self-discharge rate

Cycle durability

250–676 Wh/L
~250-~340 W/kg
80–90%
8% (per month) at 21 °C

400–1200 cycles

Source: Wikipedia: NiMH - Lithium-ion 
Note in particular the cycle durability.  Longer battery life is one reason that Toyota uses NiMH cells in it's hybrid vehicles. 
Others are higher intrinsic safety and greater specific power.

 

Cost remains the greatest issue of all. 

According to Wikipedia, in 2017 the price of energy storage using lithium-ion cells has dropped to less than $200 million per GWh.  This is the price Tesla pays.  They will cost you a lot more.

So some proposed batteries as a solution to South Australia's woes. 

Let's say that just 5% of battery storage would avoid blackouts during periods of peak load when the wind drops.  Yet in this example just 100 GWh of battery storage, to smooth out wind fluctuations would cost an eye-watering twenty billion US dollars, way more than a new power station.  And if these batteries were discharged several times a day to cover drops in the wind they would soon need replacing.  With a maximum life of 2,000 cycles the batteries would only last half a dozen years, probably less, like your mobile phone or car battery.  All of these costs end up on the consumer's electricity bill.

As it turned out the political populists in South Australia were just talking tokenism: 100MWh of battery storage.  This is equivalent to just 1% of the average daily generation of the State's wind farms - about a thousandth of that required to make a difference, should the wind drop or equipment fail during peak demand.

 

Battery investment

 

In other contexts, like PV solar, batteries are a partial answer (see above).  But although great strides have been made in battery technology costs are still, at least, one order of magnitude too high.  Energy losses also remain a significant factor, as high energy batteries lose up to 20% of the energy stored as heat and, more gradually, most of their stored energy by leakage over a year.  

 

Super-capacitors

As we have seen, with repeated heavy use few batteries retain their storage capacity for more than five years. Super-capacitors store electricity as charge without a chemical conversion.  Thus they have lives of many decades and can be repeatedly charged and discharged.  But their energy density is still less than a battery so they presently cost more.  Further, unlike a battery, a capacitor's voltage falls more or less linearly as they discharge so they need additional electronics to compensate for this; adding to cost and degrading their intrinsic efficiency.  

A possibility I once considered (and now publicly declare as the likelihood of patenting the idea is now nil) is that a capacitive layer could be incorporated in future vapour-deposited thin film PV solar panels.  Such an innovation could extent their output during periods of darkness and to meet demand peaks thus reducing dependence on batteries for energy smoothing. 

There are many applications in which their advantages, like long life, outweigh the higher cost over a battery but it is unlikely capacitors will entirely replace batteries for large scale energy storage any time soon. 

 

Other solutions

Other methods like heat storage in molten salt or hydrogen storage are costly and lose too much energy (30% to 70%) in storage and/or in the recovery to electricity.

There is a discussion of the Spanish experience using hot salt for storage, in the article on Spain.

My brother is an advocate of mechanically lifting weights.  Others suggest pressurising gas to drive water through a turbine, for example in underwater bags.  But in each case, capital cost and conversion efficiency remain significant hurdles.

 

 

 

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