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Energy storage projects are typically described in terms of maximum power (megawatts) and hours of supply at peak power. Multiplying hours and megawatts gives m
by philipkglass 5y ago
Energy storage projects are typically described in terms of maximum power (megawatts) and hours of supply at peak power. Multiplying hours and megawatts gives megawatt hours, a unit of energy. Gigawatts and gigawatt hours are just different magnitude units for the same power and energy values, like meters and millimeters.
Most electrical grids experience peak demand in the early evening when people come home from work, turn on lights, cook dinner, etc. This peak period is usually on the order of 4 hours. This early evening period is also when solar output drops toward zero.
Energy storage projects in CA are currently generally provisioned for 2-4 hours of storage to ensure a good match to this daily evening demand peak and dropping solar output. Storage is still expensive so it doesn't make financial sense to add capacity that isn't going to be used most days.
Some of these projects publish detailed information about power and energy. Others just publicize power, but given what is known about the California market their energy capacity can still be estimated to a reasonable degree.
- danans 5y ago> Gigawatts and gigawatt hours are just different magnitude units for the same quantities, like meters and millimeters. I liken it more to distance vs speed. Power, like speed, is a rate, in particular the instantaneous rate at which energy is transferred. Expressed using the same scheme we use for speed (distance per time : power per time), power is watt-hours per hour (or 0.0036 megajoules per hour). I'm guessing we express power differently because James Watt codified the standard unit for power and the standard for electrical energy came from the time-integral of his definition of power.
- philipkglass 5y agoI meant the "giga" units relative to their "mega" counterparts. Same physical quantities but multiplied by 1000.
- danans 5y agoHmm, in that case your comparison should be from MW to GW, not GW to GWh, right?
- philipkglass 5y agoI referred to megawatts and megawatt hours in my first two sentences. "Gigawatts and gigawatt hours" was indicating counterparts to MW and MWh. Though apparently that grammatical construction was not as clear as I initially believed.
- danans 5y agoAh, OK. Thanks for clarifying!
- vondur 5y agoThis is what confuses many people, especially with solar and battery storage. "I've got 400 watts of solar feeding into my 400AH of batteries with a 3000w inverter" Most people will have no idea how much usable power they get and how much energy storage they need to meet their needs. (this example is mostly from an RV perspective)
- danans 5y agoIn general, people are not used to formally separating power and energy as concepts, much less as units of measure. This makes perfect sense because most of the measurements of things we own or consume are counted in units like area, volume, and energy, which are inherently more tangible than an instantaneous rate like "power". Speed is a bit of an exception to this, but I think that has to do with the ubiquity of speedometers in cars. To the extent that people broadly understand power as distinct from energy they tend to do so in a mechanical, not an electrical context, and in less precise terms ("That's a powerful truck"). However, some exceptions to this are light bulbs and electric space heaters, where people generally understand power as being proportional to brightness and heat output. In a way, it's an incredible testament to the engineering of electrical systems that people have been able to largely ignore the difference between power and energy, or just need to flip a circuit breaker if something exceeds a limit.
- Ajedi32 5y agoAmp hours is particularly annoying IMO because you can't convert it to energy without knowing the voltage.
- imtringued 5y agoThe reason that battery capacity is measured in amp hours is that you can't know the exact voltage of the battery as it drops with lower levels of charge.
- danans 5y agoNewer large-capacity battery systems seem more likely to specify battery capacity in (k)Wh. I think this is because they come with sophisticated battery management systems that keep the battery within a narrow charge (20%-80%) range, so therefore they can be assured of the bounds of the voltage. Also, Li-ion and LiFePO4 batteries have a much different discharge curve than earlier chemistries [1], with much more constant voltage up to 80% discharge. 1. https://www.powertechsystems.eu/wp-content/uploads/sites/6/2018/11/LiFePO4-vs-Lead-Acid-Discharge-Curve-EN.png https://www.powertechsystems.eu/wp-content/uploads/sites/6/2...
- jeffbee 5y agoEverything about these storage projects is available as public records from the CPUC, down to the site plan drawings.
- philipkglass 5y agoThanks! I've been noticing these projects mostly from press releases that surface through Google News, and those don't always provide both numbers. It's good to know that there is a detailed source of open data.