Storing Renewable Energy

[ALine]

Question: Why is it hard to store renewable energy? Can't you just hook it up to a rechargeable battery grid?

[mistermack]

50 minutes ago, ALine said:

Question: Why is it hard to store renewable energy? Can't you just hook it up to a rechargeable battery grid?

You can, and they do, but it's not very economic. Rechargeable batteries are either very expensive or not very efficient. Or a bit of both. 

Lead Acid batteries, like in a car, can last a long time. But that's in a car, where they get very little use, and are kept fully charged all the time. If you use them for power, you have to keep charging them up, and running them down. They don't last very long, once you start using them like that. Also, you don't get the same energy out that you put in. They waste a lot, and you lose more energy transforming the current backwards and forwards to AC/DC high voltage/low etc.

So it can be a useful option, but only where the alternative is expensive.

[pzkpfw]

Can be done: https://electrek.co/2018/01/23/tesla-giant-battery-australia-1-million/ (here used mainly as a smoothing mechanism.)

But as above, current battery technology just isn't what it needs to be, to be cost effective in the long run to be the main storage mechanism.

 

(One alternative that always amused me, hydroelectric dams where water is pumped back up into the reservoir when power is cheap, to be used later when the power wasn't so cheap. Could also be used to "store power" from renewables.)

Edited August 13, 2018 by pzkpfw

[pavelcherepan]

3 hours ago, mistermack said:

But that's in a car, where they get very little use

I agree with what you wrote, except this ^. 

Every time a spark plug discharges somewhere between 12-45 kV, it takes the energy directly from the battery which is then recharged by alternator. Therefore, battery gets a lot of use, it has to supply energy to every spark plug many times per second and keep recharging as well. It gets pretty busy.

Edited August 13, 2018 by pavelcherepan

[John Cuthber]

19 minutes ago, pavelcherepan said:

I agree with what you wrote, except this ^. 

Every time a spark plug discharges somewhere between 12-45 kV, it takes the energy directly from the battery which is then recharged by alternator. Therefore, battery gets a lot of use, it has to supply energy to every spark plug many times per second and keep recharging as well. It gets pretty busy.

When a car engine is running, the power for the spark plug comes directly from the alternator.
It's also not much power, so Mistermack's statement is correct.

[mistermack]

3 hours ago, John Cuthber said:

When a car engine is running, the power for the spark plug comes directly from the alternator.
It's also not much power, so Mistermack's statement is correct.

That's right, and it's easy to check. Check the voltage across your battery when the car is running and it will read about 13.25. Check it with the engine off and it's about 12.5, so when the car's running, the battery is receiving charge, not discharging.

Anyway, my car's a diesel. 

4 hours ago, pzkpfw said:

(One alternative that always amused me, hydroelectric dams where water is pumped back up into the reservoir when power is cheap, to be used later when the power wasn't so cheap. Could also be used to "store power" from renewables.)

They built an installation like that in Wales, about forty or fifty years ago. Of course, it wouldn't make sense to use hydro power to pump water back up.  But at the time, it wasn't cost effective to power down some of the generation plant at night or other periods of low demand, so they had to store power some way, or dump it. I think modern generating plant is more responsive now, but it might still make sense to store wind energy that way.

[timo]

As stated before, battery storage is already used in the energy system. Just not to systematically balance renewable power generation. The most common application is balancing the discrepancy between electric power forecasts used for the operation planning and actual power generation and load. I believe that the Tesla battery mentioned by pzkpwf falls in this category. On a smaller scale, batteries are often used within private/business premises to maximize the use of self-generated photovoltaic power or as backup power source in case of failures (link).

The topic of storing renewable energies usually comes up in the context of electricity generation from wind power and solar photovoltaic power (PV). Often in contexts like "the weather dependency of wind and PV make it impossible to create a reliable electric power supply" or "we need to solve the storage problem before we can build renewable power systems". The short answer why batteries are not considered as the storage solution is that they are too expensive for really large amounts of renewable wind and PV generation:

• Battery capacity costs money
• Fully-renewable power systems need large storage capacities - in the order of a month of electric power supply
• It is more economical to create synthetic fuels to satisfy this backup storage demand

For a more detailed explanation, keep on reading ...

 

Detail Explanation

I'll use Germany as an example, because I have data for it at hand. At least the European weather conditions (and almost certainly also the northern American weather conditions) are sufficiently similar, anyways. Also, I will focus on wind and PV generation as well as batteries and synthetic fuel generation as possible storage options. There are many other options for generation and storage, and some can be very relevant for specific areas (pump storage in Norway, hydro power generation in Iceland, concentrated solar power in northern Africa, ...), but wind, PV, battery and hydrolysis are somewhat universally-relevant options.

Wind and PV power generation depend on the weather and vary significantly over time, whereas the power demand (=load) is relatively stable. This creates an imbalance between generation and load that must be match by additional demand (or exports), curtailing of the power generation or additional generation (or imports). The following image shows an example of an optimized renewable energy system with onshore wind, offshore wind and PV as power generation, gas power as additional generation option, hydrolysis and methanation (the creation of hydrogen and subsequently methane from water and carbon) as additional flexible demand, no import/export, and curtailing where needed (called "Lost Generation", here).

 

As you can see, batteries do play a role in the balancing: They are systematically charged by the daily PV peaks and discharged in the evening. The reason they do not cover all of the balancing demands is ultimately a question of economic viability and the cost of battery capacity. The high-frequency daily charging/discharging makes very good use of the battery capacity since the annual energy throughput is roughly 365 times the installed capacity - up until you have so much capacity that you don't fill it up every day and get diminishing returns. There are, however, lower-frequency storage demands, i.e. longer times of effective surplus or deficit power. An example are the 3-5 days of low wind power generation around hour 5800 in the plot above. Even the annual generation of wind power changes by ~10% between years and may have to be stored for bad years. The events occur relatively rarely, but are associated with lots of energy.

 

For a fully-renewable power generation, the total capacity required of the storage system is significant. For the example system used in the plot above, 45 TWh of electric energy (-equivalent) storage are needed. Assuming 300 €/kWh battery costs and 10 years battery lifetime that means 1350 * 10^9 €/a costs for the storage alone. But the total cost of the German electricity system today is only 100 * 10^9 €/a. For low-frequency, high-capacity storage needs, the creation of synthetic fuels via hydrolysis (for Hydrogen) and possibly further steps (for Methane or theoretically even fluid fuels) is more suitable. It is associated with high losses, but fuels are very efficient to store in large quantities of energy over long time.

So instead of using batteries alone, a mix of storage options is used. Highly efficient but capacity-limited battery storage for high-frequency storage and low-efficiency high-capacity synthetic fuel storage for the low-frequency storage. In the example of the plot above, the battery capacity is 120 GWh (i.e. 0.3% of the total capacity demand), but has an annual throughput of 30 TWh, whereas the fuel storage is 45 TWh with an annual electric energy output of 55 TWh (for reference, the total annual demand is 450 TWh).

 

[Carrock]

Storing energy with trains going up and down a hill sounds ridiculous but it's already becoming commercial.

https://www.aresnorthamerica.com/article/4875-advanced-rail-energy-storage-using-trains-to-store-power

Quote

The technology has already attracted the interest of the Valley Electric Association Inc., which will host a $40 million, 50-MW rail energy storage plant in Nevada with interconnection to the California ISO.
"The basic concept is: How do I move mass with the force of gravity?" ARES CEO James Kelly said in an interview. "It finally dawned on us to use 100-year-old technology, and that's electric railroads, and to add modern digital control systems to automate electric railroads for storage."

 

[Enthalpy]

Two variations on the computation by Timo (nice to see you), which change the 45TWh figure hence the cost.

Generating electricity in one country isn't necessary and does not correspond to present-day practice. If the wind doesn't blow in whole Germany, it does in Scotland, Brittany, Aquitaine or Galicia. Electricity is presently transported, typically on such distances, as the market is continent-wide.

There is no need to store an amount of (Germany's mean  consumption) 64GW over 29 days as the 45TWh imply. Even if the electricity came from Germany alone, wind wouldn't stop for that long. Europe-wide, you won't have more than 1 day without wind. That would be 1.5TWh storage for the country. It wastes some electricity but is globally cheaper.

The Powerwall costs slightly over 340€/kWh but is a small unit for houses. It is guaranteed for 10 years so it will last rather 20 years.
https://www.tesla.com/powerwall
The utility-sized Powerpack is hopefully cheaper per kWh
https://www.tesla.com/powerpack
https://en.wikipedia.org/wiki/Tesla_Powerwall#Powerpack_specifications
"should" cost 220€/kWh.

1.5TWh and 220€/kWh cost 330G€ every 20years or 16G€/year, not 1350G€/year. This is affordable and much less than what the inhabitants pay for electricity.

====================

The other point is that batteries are only one solution. It's a mature one, already in use at substantial scale, but not necessarily the cheapest one.

I have good hope that flywheels are cheaper per stored kWh than batteries. The store-restore cycle is more efficient from night to day, less efficient over a week
https://www.scienceforums.net/topic/59338-flywheels-store-electricity-cheap-enough/
and Prof. Seamus Garvey's underwater bags look cheap too and has been experimented
https://www.offgridenergyindependence.com/articles/3358/compressed-air-energy-storage
there are few more ideas.

Edited September 18, 2018 by Enthalpy

[timo]

On 9/18/2018 at 9:32 PM, Enthalpy said:

Two variations on the computation by Timo (nice to see you), which change the 45TWh figure hence the cost.

Generating electricity in one country isn't necessary [...]. Electricity is presently transported, typically on such distances, as the market is continent-wide.

There is no need to store an amount of (Germany's mean  consumption) 64GW over 29 days as the 45TWh imply. Even if the electricity came from Germany alone, wind wouldn't stop for that long. Europe-wide, you won't have more than 1 day without wind. That would be 1.5TWh storage for the country.

All of what you said is at least arguably true (including the part about technological alternatives, that I did not cite). I took the battery example from the introductory slides of a lecture on renewable energies, where it was meant as an approximation and a starting point for the students to possibly try objections on. One of the key factors for the calculation is, of course, the capacity required. For simplicity (and readily availability of a suitable plot from the same lecture ), let's restrict this to fully-renewable systems:

• The required capacity relative to the load is indeed influenced by the size of the area - it goes down with area, just as you stated. Perhaps not down to 1 day.
• While the "night with no wind" is a picture that everyone understand easily, it is too simple to understand why we need storage. "Several days with not enough wind in the region" is much better but less intuitive. And I have seen cases in which evidence suggested that in an economically-optimized calculation the storage demand is influenced by the annual fluctuations in renewable generations.
• It also strongly depends on the amount of renewable generation: If you accept that storage demand is driven by prolonged times of insufficient renewable generation, rather than complete absence, then it is clear that this demand gets smaller if you install more generation than the total electric energy demand suggests.Simply said (and ignoring power limits and efficiency losses for now): You have a trade-off between extra generation costs and extra storage costs.

 

The image above is based on 8-year historic weather and load data with some fixed assumptions about the share of wind and PV generation (usually 1:2 or 1:3 in terms of energy)  and a fixed spatial distribution of the PV panels and wind parks. Black curves correspond to isolated German systems, the blue curves are the corresponding extremes of loss-less capacity-unbound electricity transmission in Europe (defined as roughly the EU). The horizontal axis is the potential for electricity generation relative to the demand, the vertical one the required capacity for useable energy in units of days of mean power demand.

The solid curves correspond to a storage that is perfectly efficient and not limited by input power. Detailed calculations of optimal scenarios, which also consider topics like adaptive demand that I did not cover in these posts for the purpose of keeping complexity down, end up with a generation ratio of around 1.1 to 1.3. So 7 days for the Ger scenario and 4 days for the Eur scenario may look realistic. The dashed curves correspond to 65% efficiency loss on power intake (35% return efficiency), which is realistic for chemical storage. The most prominent effect is that the location of the diverging capacity requirements shifts from 1.0 to some larger value.  Lastly, the dotted lines show the capacity requirements with the additional constraint that the maximum charging power is 50% of the mean load power.

 

As I hopefully made clear by now, the amount of required capacity depends on a lot of factors (some discussed and some more). More complex calculations tend to find a mix of long-term storage (cheap capacity, bad efficiency) and short-term storage (expensive capacity, good efficiency), but the mix depends on a lot of details, e.g. the assumed technology costs, which directly play into the the trade-off between extra installation and extra storage. The 30 days I used were the result of one of these calculations; I might indeed want to do the Europe-equivalent alongside to see the results there.

 

1.5TWh and 220€/kWh cost 330G€ every 20years or

16G€/year, not 1350G€/year. This is affordable and much less than what the inhabitants pay for electricity.

I strongly doubt the 1-day capacity requirement for reasons hopefully made a bit more clear in my statements above. But even if the calculations came up with 16 G€/a, which is about 16% of the total cost of electricity including grid fees and taxes, there is one thing that I want to comment on that may or may not be clear to everyone: That number is on top of the other costs, not the new costs. With the ridiculously high number I estimated the difference is irrelevant (13 times as expensive vs. 14 times as expensive) but for smaller numbers this should be considered.

 

The other point is that batteries are only one solution. It's a mature one, already in use at substantial scale, but not necessarily the cheapest one.

I agree that there are a multitude of storage solutions. My current favorite (just for the coolness) is underwater pump storage, which may in fact be the technical realization of the compressed air bags you linked to. For me, who usually considers the energy system from an abstracted point of view, all of the alternatives to synthetic chemical fuels fall into the "limited by capacity costs" class of short-term storage and become just another technical realization of a battery. Redox-Flow batteries are the best candidate for not being capacity limited that I currently see (but have not investigated). For a company in the market, that wants to optimize revenue on the percentage-margin, the choice of technology of course may be very relevant. But they usually have a very different approach for decision making in the first place.

[Enthalpy]

Fraunhofer's storage spheres pump water out of pressure-resisting constructions. They need some material (typically concrete) to keep the vacuum.

A seducing aspect of the underwater bags is that, to store compressed air at depth, the bladders resist no pressure at all. So to say, the Ocean's pressure keeps the air under pressure in the bag.

Though, underwater bags need some weight to stay in the depth. Unless the installer finds heavy rocks or sand at the seabed, he'll have to bring concrete, and in significant amounts too.

One other aspect is that water is easier to pump and turbine with decent efficiency than air to a high pressure ratio, and better, storage in the spheres can transport just electricity to and from the depth.

==========

Build extra production capacity: sure. When storage is obviously too expensive, this is a part of the solution. Wind electricity is already cheaper than nuclear one and is continues to get cheaper, so more wind turbines are affordable.

The other part of the solution is to transport electricity. Between Scotland, Greece and Romania, the distance covers one depression width, so we won't lack wind at all locations simultaneously. Even from Scotland to Galicia, we won't have under 5 Beaufort at all locations for more than one day.

[ScienceNostalgia101]

Here's what I've been thinking about lately. If solar collectors can store excess energy as molten salt...

 

...and electricity can be used to melt metal...

 

... why can't they use "molten metal storage" and/or "molten salt storage" from excess electricity as a whole to store energy?

 

Or alternatively, what about desalinization of water? California has massive problems with shortages of clean water, and desalinizing water would strain the power grid, so why not use excess energy to store up huge reservoirs of desalinized water so you can shut down desalinization plants when electricity is scarce?

Edited October 20, 2018 by ScienceNostalgia101

[mistermack]

I think the salt is heated directly by solar energy. You lose a lot when you convert energy to and from electricity, but not so much from direct heating.

The desalination idea sounds good, but I think it's probably already being done. A lot of big users of electricity get a better price at off peak times, and adjust their use to suit.