The County Council has invested over £16million in 34 such vehicles.
11 double decker buses have gone into service over the last year, joining 23 single decker hydrogen buses already in service.
The buses were built in the UK by Wrightbus and are run by operator Metrobus.
The group has a total of 54 hydrogen buses making it the largest zero emission hydrogen bus fleet in the UK.
The single deck buses can operate for 600 miles before they need refuelling.
Refuelling takes less than 10 minutes and they do not need to be refuelled or recharged throughout the day.
