Description

This document describes the creation of a 250 bar hydrogen filling station.
It is intented for small-scale hydrogen production and is thus designed to allow filling up (at least) 1 CNG tank with hydrogen at 250 bar of pressure. 
Any size of tank can be attached, but the standard size for use will be the 3 gallon, type 1, CNG tank.

A 3 gallon (or 11,35 l) CNG tank, filled with hydrogen at 250 bar, would be able to contain 8,51 kWh or hence 96 % of the energy in a 1 liter gasoline tank.
Calculation: 11,35 l x 0,003 kWh/l x 250 bar = 8,51 kWh / 8,83 kWh = 0,96 x 100 = 96% 

The station is also designed to use an electrolyser that generates hydrogen and oxygen separately and which also uses steel plates.
The reason why such an electrolyser is used is because this type of electrolyser is cheaper than regular electrolysers designed to make hydrogen (i.e. PEM cells). 
Since hydrogen is thus made (and not Brown's gas a.k.a. HHO), there is less risk of explosion when the gas is compressed by the compressor.

USE
A relatively small and cheap refill station like this could be useful for small enterprises (and perhaps individuals) to refill swappable hydrogen tanks. 
These tanks can then be used in either:
* single-fuel vehicles that are used for driving short distances with it after which they can be refilled again. This is useful for say vehicles operating in cities (i.e. delivery vehicles)
* dual-fuel vehicles. For instance cars that have a tank with liquid fuel for covering long distances with it, and a hydrogen tank for city use.

Given that the hydrogen is compressed at only 250 bar -rather than 350 bar or 700 bar as seen in other H2 filling station designs-, the compressor will need to work far less hard for the amount of hydrogen you can store in them.
This, as the energy needed for compression is does not have a linear relation with the amount of energy you store in the tanks; rather it's a polynomial function; 
see https://en.wikipedia.org/wiki/Polynomial#Polynomial_functions
and https://en.wikipedia.org/wiki/Proportionality_(mathematics)
). As such, by using this lower pressure (250 bar), the efficiency of the system as a whole drastically improves.
Other benefits are that the compressors you can use can be far cheaper (CNG) compressors, and that you could connect the 250 bar tank to internal combustion engines foreseen to run on CNG.

WORK OUT
The electrolyser to use will generally be a pulsed electrolyser that uses (316L, stainless) steel plates (rather than platinum and/or titanium plates).
(see https://hydrogengarage.com/pwm3g/ , http://www.free-energy-info.co.uk/Chapt10.html , http://www.hybridhydrotech.com/ ).
The separation of the hydrogen and the oxygen is possible as oxygen forms at the anode (+ side), whereas hydrogen forms at the cathode (- side).
Often, electromagnets are also used to assist the separation (oxygen has a negative charge, while hydrogen has a positive charge).

Since we'll need it able to fill up a 3 gallon tank, the electrolyser of the filling station would need to be able to generate 11,35 liter of hydrogen, and then compress this to 250 bar. 
The time to generate this 11,35 liter is best limited to 10 minutes or less, so the electrolyser needs to produce at least 0,189 liter per minute.
Inlet gas amounts to the compressor need to match with the outlet flow of the electrolyser.

A minimum gas flow for the compressor for an individual (= non-enterprise use) would be around 5m³/h. This is about 83 lph or about 1,4 lpm.
Many steel-plate electrolysers are able to generate 1,4 lpm so a set-up like this is easy to make. If the electrolyser generates too much gas, the production of the gas
can be reduced by lowering the amount of ampères given to the electrolyser.

Both the compressor and the electrolyser are to be powered via a conventional power plug that can plug in to the mains power grid.
This means that these devices would need to be able to run on the electricity that has voltages and frequencies dictated by your local power grid. 
The electrical power provided by the power companies should come from either renewable or nuclear sources. 
So, take a subscription that excludes that power can come from fossil fuel power plants.

We should be aware that the CNG tanks will weaken over time due to the use of hydrogen therein (hydrogen embrittlement). 
As such, we need to put a timestamp on the tank displaying the day hydrogen was first introduced to the tank. 
After a certain period following this day, the tank should be replaced.
Also, the type 1 tank should be an aluminium tank, not a steel tank.

The H2 filling station will have following components:
electrolyser - spark arrestor - compressor - spark arrestor - tank
This, out of safety reasons.
Also, again out of safety concerns, you should never have the filling station running when a human is nearby.
Only run it when there is no one around.