## Background

When thinking energy storage, most people will think of battery.
However, in some workshop, you can find 'air power tools' powered by compressed air.

It make ones wonder what are other means of storing energy.

It turns out, there are a whole range of options to store energy other than battery or fuel.
Some examples:
* lifting tons of concrete
e.g. [Energy Vault](https://www.energyvault.com/),
* melting salt (how does it store the energy?),
* spinning flywheel
* pumping water into dam
* and many more ways

## What are possible storage of energy?

| Storage   | Example  |
|:----------|:---------|
| Chemical  | battery, fossil fuel, etc. |
| Gravity   | lake/dam, concrete blocks  |
| Compressed air |    |
| Molten Salt   |    |
| Flywheel   | http://large.stanford.edu/courses/2020/ph240/barnett2/    |


A more extensive list is [wikipedia](https://en.wikipedia.org/wiki/Energy_storage).

## Stashed Questions

### How much typical energy cost?

To be looked around the internet. Some that are of interests:
* Electricity from the grid
* Electricity from battery
* Gas
* Fuel/petrol

### How much typical appliance running cost?

* Monitor, laptop, fridge, raspberrypi, smartphone, ...


### Is it feasible to replace lightbulb with gravity light

We pull up a weight and a mechanism will convert it to light throughout the night.

### If human regulate its body temperature and radiates heat, can we capture this heat?

If so, is it enough to power electronics that goes with people? like smartphone? or hearing aid.

### How about human's movement?

Good for charging while hiking, for example.

### Is there energy conversion that is more efficient?

Currently, energy distribution mainly means electrical grid, at least from my/consumer perspective. Is this conversion always efficient?

### Can we store energy in the form of photon?

You know, that particle that always escape or captured.
Is it possible for pure photon.
Sounds non-sensical.