3 ms·
Storage via gravitational potential energy is extraordinarily inefficient. Let's say you have a medium city, like Columbus, OH. This city purchases about 890 G
by credit_guy 4y ago
Storage via gravitational potential energy is extraordinarily inefficient.
Let's say you have a medium city, like Columbus, OH. This city purchases about 890 GWh of electricity per year [1] for costumers and street lights. Let's say they need to store 100 GWh. A kg of matter raised 1 meter stores about 10 joules, or watt-seconds. We need to store 100 times 3600 billion of watt-seconds, which is 360 trillion. You can trade mass for height. For example you can raise 36 trillion kg to a height of 1 meter. That's 36 billion tons or gigatons (GT). Or you can raise 3.6 GT to a height of 10 meters, just 36 MT to
a 1km hight.
Let's say you want to use blocks of stone. The total US production of blocks of stone in 2021 was 2.3 million tons [2]. You need 15 times as much to only cover Columbus, OH, and only assuming a 100% efficiency in the conversion of the energy from electricity to gravitational potential energy and back.
The only substance that can be useful for this type of storage is water. But even for water, you need the same huge quantities. 36 million tons of water is a lake with a surface area of 1 square kilometer and a depth of 36 meters. If you have some mountains around you, then you can do it. But Columbus, OH is out of luck. It does not have mountains, or even hills around it. Building an artificial hill that's 1000 meters tall, that has a 1 km2 lake at the top with a depth of 36 meters is a monumental undertaking, that was never attempted.
Of course, you can try to play around with the tradeoffs. Maybe you build a lower hill with a larger footprint. But how long is it going to take to offset the emissions from such a huge construction project?
Bottom line: pumped hydro makes sense, if geography allows you. If not, it is uneconomical by many orders of magnitude.
[1] https://www.columbus.gov/greenpower/ https://www.columbus.gov/greenpower/
[2] https://pubs.usgs.gov/periodicals/mcs2022/mcs2022-stone-dimension.pdf https://pubs.usgs.gov/periodicals/mcs2022/mcs2022-stone-dime...
- jrmg 4y agoThis city purchases about 890 GWh of electricity per year [1] for costumers and street lights. Let's say they need to store 100 GWh. Why do they need to store over 1/9th of their yearly energy usage?
- credit_guy 4y agoIf you switch to renewables, you need to store energy.
- jrmg 4y agoI’m questioning the amount (which is what the whole argument about this being impractical is based on). Do you really need to have a store of 11% of the city’s entire yearly energy usage? Like, 5 (almost 6!) entire weeks of energy? That seems ludicrously high to me.
- credit_guy 4y agoOk, let's say it's one week. Is that still ludicrously high? The scale of the challenge is the same. Storage via gravitational potential energy is not realistic, except if geography is on your side.
- jrmg 4y agoI think an entire week’s worth also seems high. But, yeah, on the whole you’ve convinced me. This does seem like a ludicrously inefficient form of energy storage.
- credit_guy 4y agoThat was my point, that gravitational storage is surprisingly inefficient. But since you challenged me on the 11%, I did some research, and now I think that storage may not even be needed (aside from the day-to-night storage via batteries, which is already economical nowadays). The EIA gives [1] the US electricity generation by month and source for 2020 and 2021. Solar and wind are small nowadays, but if you were to scale to cover the entire annual needs of the US only with solar and wind, you'd get a profile where on some months you get more than you need and on some months you get less. The longest stretch is in the Summer months (presumably because of AC). During those months, for 3 or 4 months in a row you produce less than you consume. You need about 6% or 7% of total annual consumption in storage to make up for the deficit. If we include conversion losses, and an additional buffer, then my original 11% sounds about right (although I just pulled it out of thin air). However, if your just use natural gas for those 6-7%, then you don't need storage at all. Cutting the fossil fuel generation from the current 60% to 6% means reducing emissions by more than 90% (lost of current generation is from coal, which emits about double the CO2 than the equivalent gas generation). You can also overbuild solar and wind a bit, let's say by 20%, and the deficit gets reduced from 6-7% to less than 3%. You can then balance the load using the hydropower that we already have. So, I think the whole storage problem may not be such a big problem after all. [1] https://www.eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_1_01 https://www.eia.gov/electricity/monthly/epm_table_grapher.ph...
- blablabla123 4y ago> Storage via gravitational potential energy is extraordinarily inefficient. I think the point is that what we do right now and what has been considered good practice is extremely inefficient, engineering-wise and economically.