NREL weird idea for long term energy storage

2022.04.11 – Ian Page

https://cleantechnica.com/2022/04/10/nrel-researchers-plot-energy-storage-under-our-feet/

I'm not sure what I think of this one!

The idea is to use fracked and uncapped wells. Pump natural gas into them under pressure and seal it up until it's needed. (Weird! Why not, not let the gas out of the well in the first place when fracking.)  Then use the pressure to drive a turbine. Finally pump the natural gas back in again (don’t burn it) 

Eventually replace the natural gas with something else such as nitrogen, CO2, or hydrogen. (Nitrogen has the advantage that when it leaks it doesn't matter)

There are some interesting features. When you compress gas, it gets hot. I guess that this heat is going to end up in the rock of the well and will cool off relatively slowly. This helps for when the gas is allowed to expand later as expansion takes heat and if there is some around in the rock to preheat it, the system will have a better round trip efficiency.

The problem with long term energy storage is three-fold.

Firstly capacity: 

Whatever is going to be stored has to be stored somewhere which usually involves a lot of capital expense. In this case that’s pretty cheap

Secondly energy carrier:

Whatever is physically storing the energy had better be really cheap. Water and air are preferred since there is no purchase cost and thus no interest to be paid on a debt to accumulate over time. You can't use air in a fracking well as it forms an explosive mixture with residual natural gas. Hydrogen (electrolytic) would be quite expensive. Probably the best is CO2. At least it's in a closed system with minimal leakage. It is also compressed to a liquid and thus a lot can be stored in a small volume. In addition, turning into a liquid actually stores phase change energy as well and helps to maintain pressure when generating electricity. (Depending on the temperature of storage as its triple point is quite low) 

Thirdly Power:

Generation equipment and grid connection is needed, which is a capital cost that is amortized over a relatively small and infrequent energy generation. The system can be used for the daily generation cycle to reduce this, but it's usually much less efficient and thus more costly than alternatives. In the seasonal market there are few alternatives and thus the price can be raised to cover costs, Power is often the key cost element. For example, in the UK we would need about 30Gw of stored energy for two weeks each year to cover all the shortage that may occur each year due to our climate. That would mean 30Gw of generating capacity idle for at least 48 weeks a year. The turbines are probably reasonably cheap (pressure not high temperature based, possibly Stirling engines?) but that's still a lot of equipment lying around unused.

There are currently no good answers for long duration storage. My current judgement is 

1. Extend the life of existing nuclear where safe (not new). Most of the damage has been done and it reduces the consumption of natural gas while renewables are growing.

2. Go all out to grow renewables.

3. As gas consumption drops, retain the gas power station assets, and store natural gas for use only as long duration storage. (A reduction of 90% in CO2 from power generation)

4. Eventually replace natural gas with biogas or hydrogen reusing the existing generating stations, grid connections, pipes, and storage.

5. When the turbines wear out (many years since they are only being used occasionally) replace them with some new magic long duration technology resulting from a couple of decades of research and development which may be fuel cells running on hydrogen. We can probably leave this one to the next generation to solve, provided ours solves the global warming problem.