By "now" they apparently mean "projects now being built." [...] (I wonder why the battery sizes are measured in watts rather than watt-hours?)
By "now" they apparently mean "projects now being built."
[F]ew standalone solar projects are being proposed anymore. We found only 23 solar projects in the queue that don’t include batteries, meaning that more than 90% of solar projects that have applied for interconnection have a battery component.
[...]
The average ratio of battery capacity to solar capacity was .89. Nor are the standalone battery projects small; the average capacity of a standalone battery project in the CAISO queue is 248 MW.
(I wonder why the battery sizes are measured in watts rather than watt-hours?)
Maybe they’re listing the “capacity” here as the peak (or sustained) load the facility can supply on battery power? It’s a bit of an odd choice because the next natural question is “248 MW for how...
I wonder why the battery sizes are measured in watts rather than watt-hours?
Maybe they’re listing the “capacity” here as the peak (or sustained) load the facility can supply on battery power? It’s a bit of an odd choice because the next natural question is “248 MW for how long?”
I was tangentially involved in engineering for batteries on grid scale solar projects. The use of batteries isn't for something like "saving" sunlight to distribute at night, but for the more...
I was tangentially involved in engineering for batteries on grid scale solar projects.
The use of batteries isn't for something like "saving" sunlight to distribute at night, but for the more mundane task of controlling output power fluctuation. If you had a storm roll in quickly, the power production might drop very quickly.
The load on the grid doesn't drop quickly, though, which means the non-solar power facilities need to pick up that load in a hurry, and it's not easy to (near-) instantaneously add 200+ MW of power.
It puts a tremendous stress on conventional plants, to the point that those power utilities or regulatory agencies will FINE your solar site for failing to comply with established power ramp rates. The faster your power changes - in either direction! - the steeper the fine.
If your power production goes from, say, 300 MW to 30 MW in 20 seconds, then you're dropping at a rate of about 13.5 MW per second. If the regulatory agency says you're only allowed to change 10 MW per second, then you're in violation and will be fined.
So the battery capacity isn't about long-term storage, to time shift solar to night production, but instead to pad out that power fluctuation so you're not in violation. In the example I made up, you'd need a battery system that can output 3.5 MW for 20 seconds to keep your company from getting fined.
Maybe I missed something but the math looks a little dodgy here? 300 MW dropping at 10 MW per second means that after 20 seconds, it seems like the output should be 100 MW? If the actual MW is 30...
Maybe I missed something but the math looks a little dodgy here? 300 MW dropping at 10 MW per second means that after 20 seconds, it seems like the output should be 100 MW? If the actual MW is 30 at that point then the difference to be made up is 70MW.
But I take your point. There's some peak rate that the battery needs to be able to supply, but not for very long.
Units get weird when talking about Watts over a duration. I think that Watts and Watt hours are unintuitive as units when talking about batteries and their capacities. I don’t understand why we...
Units get weird when talking about Watts over a duration. I think that Watts and Watt hours are unintuitive as units when talking about batteries and their capacities. I don’t understand why we don’t just talk about this stuff in terms of Joules and Joules per second or similar terms. That way we don’t have to talk about units normalized per unit time, and we don’t have to multiply by time units to cancel out the time units in the denominator. Unfortunately this seems like it’s a convention that is hard to get rid of (like Imperial units etc.).
You're basically "unit laundering" from power being a derivative (energy per time) to power being the base term and energy being the integral (accumulated power). I tend to think in Watt-hours for...
You're basically "unit laundering" from power being a derivative (energy per time) to power being the base term and energy being the integral (accumulated power).
I tend to think in Watt-hours for energy because I do like the "accumulated power" concept, but that also might be the fact that the pervasiveness of Watts as a unit leads me to think more in terms of power.
1 kilowatt hour could be 1 Watt for 1000 hours or 1 kilowatt for 1 hour, or 10 kW for (0.1 hr =) 6 minutes. And I understand that mathematically it converts okay to Joules, but again I think of spent energy as "how long have I been generating X power," or battery capacity as "how long could this provide X power?"
You're considering if they followed the allowable ramp rate for the same duration. In the example, for whatever reason (rain, solar eclipse, etc.) the solar field will only generate 30 MW. The...
You're considering if they followed the allowable ramp rate for the same duration. In the example, for whatever reason (rain, solar eclipse, etc.) the solar field will only generate 30 MW.
The solar field is allowed to output whatever it wants, down to zero. See also: night. It's not a matter of what the ultimate output power is, but how quickly you get to that output power, to give other utilities an opportunity to safely ramp up their production to make up for your deficit.
So if the solar field IS going from 300 to 30 MW in 20 seconds, on a hypothetical perfectly linear transition, then it's going to drop power at a rate of 270/20 = 27/2 = 13.5 MW per second.
The solar field, not wanting to get fined for exceeding the (hypothetical) 10 MW/s allowable ramp rate, needs to make up the difference there somehow. It could be batteries, it could be standby diesel generators, etc.
They don't need to maintain any particular ultimate output, they just need to get to whatever it is slowly, so as long as they're capable of SLOWING the rate of decline, they don't get fined.
So again, if they're allowed to drop at 10 MW/s, and they are dropping, without supplementation, at a rate of 13.5 MW/s, then they need to supplement that excess 3.5 MW, and they need to be able to sustain that supplement for however long the transient occurs.
Maybe think of it like stairs. If they're too steep, they could pose a trip/fall hazard, so the regulation is on stair pitch. As long as you have the appropriate steepness, you're allowed to build any number of steps. The step steepness regulation doesn't care or have anything to do with the elevation (power) difference between the two end points.
Okay, but if you have two flights of stairs using different pitches, and they start in the same place, they get further apart the further you go away from the starting point. Consider the...
Okay, but if you have two flights of stairs using different pitches, and they start in the same place, they get further apart the further you go away from the starting point. Consider the difference between a regular flight of stairs and a stepladder.
Or more simply, two lines on a graph going through the origin with different slopes.
Yeah correct. The horizontal distance in the stairs analogy is the time taken for the transient in the solar field. The too-steep stairs are the unmodified solar output, and the second set of...
Yeah correct. The horizontal distance in the stairs analogy is the time taken for the transient in the solar field.
The too-steep stairs are the unmodified solar output, and the second set of bolt-on stairs that pads the stair tread to a safe length is the surge generation capacity (battery).
If it's a line, y=mx + b, the battery supplements the slope to make it less steep. I'd like to put y=(m+battery)x + b, where the slope m is negative because power is falling and battery is positive. And again, the point isn't to get the combined slope to zero (steady power), it's to get the combined slope to an acceptable negative slope, where the magnitude is within some regulatory threshold.
Another example might be airbags. The point of the airbag isn't to prevent you from stopping, but instead to extend the time you have to slow down such that your body receives some acceptable (non-fatal) deceleration.
The battery slows the rate of power decline, because the solar field and the conventional utility are electrically coupled, and that electrical coupling becomes a mechanical coupling in the conventional generator. It takes time for the steam governor to open and admit more inlet power to the generator, and the solar field power drop is felt at the conventional utility as a sudden increase in demand. The sudden transient could stress the mechanical equipment, but also if the increase in demand outstrips the ability of the inlet governor to adjust then the mechanical power simply doesn't exist and the generated power fails to meet demand, resulting in a brownout.
The continuous power output of a lithium battery has a connection to capacity. A battery can typically charge and discharge in 20 minutes (excluding the slower charging ratr when the battery is...
The continuous power output of a lithium battery has a connection to capacity. A battery can typically charge and discharge in 20 minutes (excluding the slower charging ratr when the battery is almost full). For example a battery with capacity 100 Wh can discharge in 20 minutes at power output of 300W.
The discharge power/current is often expressed as a multiple of capacity and 3C ( 3 times capacity) is typical for li-ion and LiFePo4 batteries.
The charge/discharge rate gives you the limit at the battery cell level. At the pack level, the connections and power electronics may not be sized for this maximum power. So likely a battery pack with 248MW of power would be providing this much power for more than 20 minutes.
I agree that the article would be much more useful if they talked about both installed capacity and power.
This article has some examples of power and energy capacity: https://en.wikipedia.org/wiki/Battery_storage_power_station#Largest_grid_batteries From this and other examples in that table, the most...
In December 2020, Vistra Energy's Moss Landing Energy Storage Facility, on the site of the Moss Landing Power Plant, was connected to the grid. At the time, the 300MW/1.2GWh facility was by far the largest in the world
From this and other examples in that table, the most common power to capacity ratio is 1:4. Which means that the battery storage can fully discharge in 4 hours.
I'm surprised that Li-ion (which uses expensive metals in the cathode: Nickel, Manganese, Cobalt) is a more popular chemistry based on the amount of installed capacity compared to the cheaper lithium iron phosphate chemistry (LiFePo4). The only drawback of LiFePo4 is that it is less energy dense (ie, more volume and mass per 1KWh), which makes it less suitable for use in electric cars (though Tesla has started to put LiFePo4 into their standard range cars). The extra volume and mass does not matter for static storage. Compared to Li-ion it should be the chemistry of choice for grid energy storage: lasts longer, cheaper, safer: https://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery#Advantages_and_disadvantages
There are two Moss Landing facilities and the other one is being built by Tesla. It seems to be Li-Ion as well, but maybe they'll use LiFePo4 eventually.
There are two Moss Landing facilities and the other one is being built by Tesla. It seems to be Li-Ion as well, but maybe they'll use LiFePo4 eventually.
By "now" they apparently mean "projects now being built."
[...]
(I wonder why the battery sizes are measured in watts rather than watt-hours?)
Maybe they’re listing the “capacity” here as the peak (or sustained) load the facility can supply on battery power? It’s a bit of an odd choice because the next natural question is “248 MW for how long?”
I was tangentially involved in engineering for batteries on grid scale solar projects.
The use of batteries isn't for something like "saving" sunlight to distribute at night, but for the more mundane task of controlling output power fluctuation. If you had a storm roll in quickly, the power production might drop very quickly.
The load on the grid doesn't drop quickly, though, which means the non-solar power facilities need to pick up that load in a hurry, and it's not easy to (near-) instantaneously add 200+ MW of power.
It puts a tremendous stress on conventional plants, to the point that those power utilities or regulatory agencies will FINE your solar site for failing to comply with established power ramp rates. The faster your power changes - in either direction! - the steeper the fine.
If your power production goes from, say, 300 MW to 30 MW in 20 seconds, then you're dropping at a rate of about 13.5 MW per second. If the regulatory agency says you're only allowed to change 10 MW per second, then you're in violation and will be fined.
So the battery capacity isn't about long-term storage, to time shift solar to night production, but instead to pad out that power fluctuation so you're not in violation. In the example I made up, you'd need a battery system that can output 3.5 MW for 20 seconds to keep your company from getting fined.
Maybe I missed something but the math looks a little dodgy here? 300 MW dropping at 10 MW per second means that after 20 seconds, it seems like the output should be 100 MW? If the actual MW is 30 at that point then the difference to be made up is 70MW.
But I take your point. There's some peak rate that the battery needs to be able to supply, but not for very long.
Units get weird when talking about Watts over a duration. I think that Watts and Watt hours are unintuitive as units when talking about batteries and their capacities. I don’t understand why we don’t just talk about this stuff in terms of Joules and Joules per second or similar terms. That way we don’t have to talk about units normalized per unit time, and we don’t have to multiply by time units to cancel out the time units in the denominator. Unfortunately this seems like it’s a convention that is hard to get rid of (like Imperial units etc.).
You're basically "unit laundering" from power being a derivative (energy per time) to power being the base term and energy being the integral (accumulated power).
I tend to think in Watt-hours for energy because I do like the "accumulated power" concept, but that also might be the fact that the pervasiveness of Watts as a unit leads me to think more in terms of power.
1 kilowatt hour could be 1 Watt for 1000 hours or 1 kilowatt for 1 hour, or 10 kW for (0.1 hr =) 6 minutes. And I understand that mathematically it converts okay to Joules, but again I think of spent energy as "how long have I been generating X power," or battery capacity as "how long could this provide X power?"
You're considering if they followed the allowable ramp rate for the same duration. In the example, for whatever reason (rain, solar eclipse, etc.) the solar field will only generate 30 MW.
The solar field is allowed to output whatever it wants, down to zero. See also: night. It's not a matter of what the ultimate output power is, but how quickly you get to that output power, to give other utilities an opportunity to safely ramp up their production to make up for your deficit.
So if the solar field IS going from 300 to 30 MW in 20 seconds, on a hypothetical perfectly linear transition, then it's going to drop power at a rate of 270/20 = 27/2 = 13.5 MW per second.
The solar field, not wanting to get fined for exceeding the (hypothetical) 10 MW/s allowable ramp rate, needs to make up the difference there somehow. It could be batteries, it could be standby diesel generators, etc.
They don't need to maintain any particular ultimate output, they just need to get to whatever it is slowly, so as long as they're capable of SLOWING the rate of decline, they don't get fined.
So again, if they're allowed to drop at 10 MW/s, and they are dropping, without supplementation, at a rate of 13.5 MW/s, then they need to supplement that excess 3.5 MW, and they need to be able to sustain that supplement for however long the transient occurs.
Maybe think of it like stairs. If they're too steep, they could pose a trip/fall hazard, so the regulation is on stair pitch. As long as you have the appropriate steepness, you're allowed to build any number of steps. The step steepness regulation doesn't care or have anything to do with the elevation (power) difference between the two end points.
Okay, but if you have two flights of stairs using different pitches, and they start in the same place, they get further apart the further you go away from the starting point. Consider the difference between a regular flight of stairs and a stepladder.
Or more simply, two lines on a graph going through the origin with different slopes.
Yeah correct. The horizontal distance in the stairs analogy is the time taken for the transient in the solar field.
The too-steep stairs are the unmodified solar output, and the second set of bolt-on stairs that pads the stair tread to a safe length is the surge generation capacity (battery).
If it's a line, y=mx + b, the battery supplements the slope to make it less steep. I'd like to put y=(m+battery)x + b, where the slope m is negative because power is falling and battery is positive. And again, the point isn't to get the combined slope to zero (steady power), it's to get the combined slope to an acceptable negative slope, where the magnitude is within some regulatory threshold.
Another example might be airbags. The point of the airbag isn't to prevent you from stopping, but instead to extend the time you have to slow down such that your body receives some acceptable (non-fatal) deceleration.
The battery slows the rate of power decline, because the solar field and the conventional utility are electrically coupled, and that electrical coupling becomes a mechanical coupling in the conventional generator. It takes time for the steam governor to open and admit more inlet power to the generator, and the solar field power drop is felt at the conventional utility as a sudden increase in demand. The sudden transient could stress the mechanical equipment, but also if the increase in demand outstrips the ability of the inlet governor to adjust then the mechanical power simply doesn't exist and the generated power fails to meet demand, resulting in a brownout.
Thank you for providing some very specific and useful context.
The continuous power output of a lithium battery has a connection to capacity. A battery can typically charge and discharge in 20 minutes (excluding the slower charging ratr when the battery is almost full). For example a battery with capacity 100 Wh can discharge in 20 minutes at power output of 300W.
The discharge power/current is often expressed as a multiple of capacity and 3C ( 3 times capacity) is typical for li-ion and LiFePo4 batteries.
The charge/discharge rate gives you the limit at the battery cell level. At the pack level, the connections and power electronics may not be sized for this maximum power. So likely a battery pack with 248MW of power would be providing this much power for more than 20 minutes.
I agree that the article would be much more useful if they talked about both installed capacity and power.
This article has some examples of power and energy capacity: https://en.wikipedia.org/wiki/Battery_storage_power_station#Largest_grid_batteries
From this and other examples in that table, the most common power to capacity ratio is 1:4. Which means that the battery storage can fully discharge in 4 hours.
I'm surprised that Li-ion (which uses expensive metals in the cathode: Nickel, Manganese, Cobalt) is a more popular chemistry based on the amount of installed capacity compared to the cheaper lithium iron phosphate chemistry (LiFePo4). The only drawback of LiFePo4 is that it is less energy dense (ie, more volume and mass per 1KWh), which makes it less suitable for use in electric cars (though Tesla has started to put LiFePo4 into their standard range cars). The extra volume and mass does not matter for static storage. Compared to Li-ion it should be the chemistry of choice for grid energy storage: lasts longer, cheaper, safer: https://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery#Advantages_and_disadvantages
There are two Moss Landing facilities and the other one is being built by Tesla. It seems to be Li-Ion as well, but maybe they'll use LiFePo4 eventually.
Do you have more info on the Moss Landing facility and Tesla proposal?
They are both listed (in separate tables) on the first Wikipedia page that spacecowboy shared. Also, I posted an older article here.
Awesome! Thank you.