3 ms·

Isn't 10 MW a unit of power and storing energy would be in MWhs or Joules? I understand that the grid needs to balance the power it is using, but to provide tha

Isn't 10 MW a unit of power and storing energy would be in MWhs or Joules? I understand that the grid needs to balance the power it is using, but to provide that power over time you need to store energy. Surprised an "electrical engineer in this industry" would use these terms, but maybe I just don't quite grok the lingo. An explanation would be great.

batmansmk 9y ago
You are correct, I used to work with batteries for 3 years, people count capacity in Ah or Wh, but never in W. More often in Ah. Why Ah instead of Wh? Because V is constant, making Ah proportional to Wh. Wh = Ah * V; V is the rated voltage the battery is designed for (12V for instance). By definition the capacity is how much energy you can get out of your battery at the rated voltage under standard temperature at a standard discharge rate. Voltage will go down as the battery depletes. When the voltage output is too low and cannot power the circuit anymore we consider the battery empty. Therefore, as we stop using the battery when V is too different, we can consider V constant over the usage period. So Ah = k x Wh, k = 1/ V being almost constant. Ah is a easier unit to manipulate in charge rates, lifecyle etc. One important note: due to technology limitations, energy one can get out of a batter greatly depends on the discharge rate (+-50% energy) and environment temperature (+-50% energy). Capacity cannot be taken literally as how much energy you can store and restitute. Design a grid that can still operate with lower voltage, charge at a lower rate and operate in hot weather and you suddenly have way more capacity (x3).
throwaway7645 9y ago
You're correct, but in the industry we really just talk in terms of Power at the transmission grid level and usually in millions of watts or MW. So if a coal plant is currently generating 2000 MW and you have a battery that can only consume 5 MW or 0.25%, then that isn't very much storage. Put another way, let's say the load in your region is 60 GW and you have 200 MW of energy storage. That is only a tiny fraction of storage relative to what the demand is.
- dmurray 9y ago
  So the main constraint on the battery is throughput, not storage capacity?
  - throwaway7645 9y ago
    Nope, the main issue is indeed the fact that it can't store a whole lot of energy (also important that they can't maintain a high output for too long), but we still talk in MW as that is what we're primarily concerned with (MW flow on a line going over the line's rating...not having enough generation (MW) to meet load (MW)...etc). Saying a battery can't store enough MW isn't scientifically accurate, but it is the slang of the industry if you will. If you worked in the water industry they're probably very concerned with water flow rates. In high voltage transmission we're concerned with the flow of power (MW (real power) & MVAR (reactive power))
  - srtjstjsj 9y ago
    You have the read in the implied math about the length of the demand cycle and how long it takes to fill capacity at the quoted throughput. for example, 12MWH battery can handle 12MW of peak throughput for 12 hours, and then discharge over 12 hours of trough throughput. "12MWH" and "12 hours" are ouputs to the battery formula, "12MW" is the output.

batmansmk 9y ago

You are correct, I used to work with batteries for 3 years, people count capacity in Ah or Wh, but never in W. More often in Ah. Why Ah instead of Wh? Because V is constant, making Ah proportional to Wh. Wh = Ah * V; V is the rated voltage the battery is designed for (12V for instance). By definition the capacity is how much energy you can get out of your battery at the rated voltage under standard temperature at a standard discharge rate. Voltage will go down as the battery depletes. When the voltage output is too low and cannot power the circuit anymore we consider the battery empty. Therefore, as we stop using the battery when V is too different, we can consider V constant over the usage period. So Ah = k x Wh, k = 1/ V being almost constant. Ah is a easier unit to manipulate in charge rates, lifecyle etc. One important note: due to technology limitations, energy one can get out of a batter greatly depends on the discharge rate (+-50% energy) and environment temperature (+-50% energy). Capacity cannot be taken literally as how much energy you can store and restitute. Design a grid that can still operate with lower voltage, charge at a lower rate and operate in hot weather and you suddenly have way more capacity (x3).

throwaway7645 9y ago

You're correct, but in the industry we really just talk in terms of Power at the transmission grid level and usually in millions of watts or MW. So if a coal plant is currently generating 2000 MW and you have a battery that can only consume 5 MW or 0.25%, then that isn't very much storage. Put another way, let's say the load in your region is 60 GW and you have 200 MW of energy storage. That is only a tiny fraction of storage relative to what the demand is.

dmurray 9y ago
So the main constraint on the battery is throughput, not storage capacity?
- throwaway7645 9y ago
  Nope, the main issue is indeed the fact that it can't store a whole lot of energy (also important that they can't maintain a high output for too long), but we still talk in MW as that is what we're primarily concerned with (MW flow on a line going over the line's rating...not having enough generation (MW) to meet load (MW)...etc). Saying a battery can't store enough MW isn't scientifically accurate, but it is the slang of the industry if you will. If you worked in the water industry they're probably very concerned with water flow rates. In high voltage transmission we're concerned with the flow of power (MW (real power) & MVAR (reactive power))
- srtjstjsj 9y ago
  You have the read in the implied math about the length of the demand cycle and how long it takes to fill capacity at the quoted throughput. for example, 12MWH battery can handle 12MW of peak throughput for 12 hours, and then discharge over 12 hours of trough throughput. "12MWH" and "12 hours" are ouputs to the battery formula, "12MW" is the output.