https://en.wikipedia.org/wiki/List_of_pumped-storage_hydroel...
This sounds good to me and it claims have 86% efficiency.
Since kinetic energy = 1/2 m * v ^ 2 , the a slight change in velocity on the trains that are already moving fast at any point in time, could store a lot of energy (i.e. for 2 identical weight trains, from 0km/s to 1km/s is a smaller change in stored kinetic energy than from 100km/s to 101km/s ! [in fact the latter increase of 1 km/s stores 201 times as much kinetic energy than the former: ((101 * 101) - (100 * 100)) / ((1 * 1) - (0 * 0))]
Now that I think of it, this could probably explain why our local trains are suffering more and more irregular arrival times :) but why would it be kept secret or hidden in plain sight? perhaps all the negative news about negative prices for renewable energy during energy flood is just manufacturing consent to keep price hikes for the plebs palatable, or a kind of white lie to offset their airplane travels...
The trains move on a track 5.5 miles long at an inclination of 8 degrees. thats a height difference of sin(8deg) * 5.5miles * 1.609344km/mile = 1.232 km, now it may be hard to find a steep cliff 1.2km high, but you could use a smaller cliff and heavier weights, think of the steep section at the start of an amusement ride (they will probably be better equipped with safety for such systems anyway since they are used to designing crazy rides for human consumption). no need for train and electrical tracks 5.5 miles long, since the motor can reside on the top part of the cliff/hill...
https://arstechnica.com/science/2017/03/german-institute-suc...
I'd assume the only thing they look at is not to go above the pressure differential that the dome/sphere can sustain.
Norway is having great success with this strategy on their hydro plants.
The Balkans region, along the Adriatic coast, offers an interesting prospect: using the sea as a lower basin with mountaintop reservoirs. This is a rare topology, particularly near large populations.
What the results of localised salinisation might be is a concern though.
Bath County Pumped Storage Station in the US. Huge place.
https://www.dominionenergy.com/about-us/making-energy/renewa...
Besides, district heating produced on power from the grid is taxed as if produced on coal, as the power has no traceability. This makes it really expensive, unless you are directly hooked up to a solar or windfarm.
That's the law in Denmark at least.
PS: I worked in the administrative building just next door to Skærbækværket! :)
Sounds like there is room for improvement. E.g. an agreement with an energy provider and tracking when it was used on demand as a sink for excess energy instead of base-rate heating.
You can make methane gas from pure CO2 and hydrogen period. The reaction is exothermic, it's just the Sabatier method. You could however use electricity to split water into hydrogen and oxygen.
...and you thought you were heating with renewable energy. You were not. Much of the "surplus" power on the grid comes from coal plants (especially in Denmark!) that can't throttle down, so someone burns coal, converts the heat to electricity at an efficiency of 40% or so, and you turn it back into heat.
No we didn't. As a mathematical modeller and software developer for their inhouse production optimization software, I was perfectly aware of what was happening.
> Much of the "surplus" power on the grid comes from coal plants (especially in Denmark!)
Most of the power plants owned by Ørsted are bio-converted and runs primarily on wood chips and pellets. Unless you are talking about surplus energy imported from Germany, what you say simply isn't true.
Besides, the huge amounts of surplus energy that often came from germany were caused by their massive open sea wind farms, making the energy pretty green.
Ørsed operated almost exclusively combined heat and power plants, meaning that they can produce heat and power concurrently. The utilization of energy was well above 90%, when we ran the plants this way, because we cooled the plant with the district heating water, instead of sea water. The theoretical maximum is ~98%.
We also never planned for pure power production, only to turn it into heat again. That would be monumentally stupid.
We had many, highly skilled engineers and power traders, and they absolutely knew what they were doing.
When I worked at Ørsted, we did this all the time. We had a huge electric kettle, which we used to produce district heating water.
But there are many more things you can do with surplus energy. You can make methane gas from pure CO2 and hydrogen when you apply electric power as a catalyst.
The methane can then be eaten by bacteria to provide protein powder to be used as supplements to feeding livestock.
Or, we can yank CO2 out of the atmosphere and store it as ethanol. I think this was discovered last year or so? Ethanol is a really good "battery", and it only discharges as much CO2 as was captured by the surplus energy used to make it, making it a clean fuel that only exhausts CO2 and sterile water.