Seems like it could save money perhaps. Has this been considered in the past?
I believe there are quite a few misconceptions here:
When an aircraft is "pressurised", it means that at higher altitudes, the pressure inside the aircraft is higher than the pressure outside. At lower altitudes, the pressure is exactly the same inside and out.
Originally, aircraft weren't pressurised, and at higher altitudes the low pressure is a problem for the people on board (lack of oxygen, mostly), and for some hardware (due to the pressure differential).
So the pressure is artificially increased inside the aircraft beyond a given altitude (or more precisely, beyond a given outside pressure): when the aircraft takes off, the pressure goes down as the plane reaches higher altitude, until that pressure is reached, and from then on, the pressure outside continues to decrease while the pressure inside is maintained at that safe level.
So there's no reason to pressurise an aircraft on the ground. It has the same pressure inside as outside.
Even if it were useful, when on the ground, there are plenty of openings other than just the passenger door(s). Other doors are opened for catering and technical staff, cargo doors are open, and so on. So it would be impossible to maintain a different pressure in the aircraft and outside without completely breaking down all operations.
Also, having an "air lock" would require doors on each side. One door would be opened, people would enter the air lock, the door would be closed, the pressure would be changed, the other door would be opened, people would exit the airlock. Then the door would be closed again, pressure brought back to the other level, and the first door would be opened again before more people would be able to enter the lock. This would significantly hamper the flow of people embarking or disembarking.
Additionally, people are already annoyed enough by the changes in pressure on board planes which take quite a few minutes. Doing that in a few seconds to ensure relatively quick movement would just be horrible.
Finally, consider that many aircraft do not use jetways but stairs on the tarmac.
Apart from the practical implications, aircraft pressurise their cabins with the bleed air coming from the jet engines. This air is compressed by the engines and used for the combustions process. So there are no seperate air compressors. If you want to have the aircraft compressed at the ground, you will need to either keep the engines running or add equipment to the aircraft. The first thing is not beneficial and the latter will increase the fuel consumption of the aircraft.
From the picture it can be seen that the pressurization level of the cabin equals the pressure level at 7000ft. The means that if you keep this pressure then on the ground there is an underpressure in the aircraft, aircraft doors and seals are not designed with this in mind. The aircraft door for example is designed such that it can't be opened mid flight since the pressurisation of the cabin pushes the door outwards in the seals. This is the reason why aircraft doors and emergency hatches first move inwards before opening to swining outside of the aircraft.
To summarize, aircraft pressurization is there for the comfort of the passengers and is therefore kept at a comfortable level 7000ft. If the aircraft flies lower then, the inside pressure equals the outside pressure. And seconly, aircraft are not designed for the cabin to be of a lower pressure than the outside of the aircraft.
The only reason planes are pressurized is to allow the passengers the ability to breathe at flight altitude as if they were on the ground. If the plane is still or already on the ground, there would be no purpose in pressurizing it.
Most metal bodied aircraft pressurize their cabins to about 8000 feet cabin pressure altitude. If the outside air is at a pressure altitude below 8000 feet, the engines are just supplying air conditioning and circulation to the cabin, not necessarily added pressure.
The average human can breathe perfectly well below 15000 feet. Some may feel mild discomfort above 10000 feet. The aircraft engines breathe air and eat fuel best at altitudes of two to three times that. The airlines save money by flying at altitudes where they get the best fuel efficiency. Unfortunately, our bodies are not efficient like engines at that altitude. We would pass out or pass away. The average human has between 6 and 60 seconds of useful consciousness at flight altitudes.
The way to save money is to reduce the self-loading-cargo’s reliance on oxygen. Putting all the passengers asleep while wearing positive pressure oxygen masks would eliminate the need to pressurize the entire cabin 😜.
Below around 8000' MSL aircraft pressurization is roughly equal to the outside air pressure. Since most airports are below this elevation, there is no pressure differential once the airplane is on the ground, therefore no need or even possibility for an airlock.
It takes very little bleed air to pressurize when airborne so there isn't a significant cost in doing so. Furthermore, jetways aren't sealed so the additional cost in equipment and procedures to implement this plan even if it were possible or desirable would far outweigh any savings.
Besides the fact that this is unlikely to be kind on passengers to overpressurize, the passenger doors and the jetways aren't the only portal into the pressurized space. The luggage going in the belly and the servicing carts coming in the rear would also have to be accommodated. Trying to load and unload cargo through an airlock would slow things down a lot.
It's possible that such an effort would extend the service life of airframes, but I don't see how it would save any money. New pressurization equipment is needed while on the ground, portals would need extra material to interface with airlocks (increasing weight), and you'd need more time on the ground to move passengers and cargo. Together, those seem to all be extra costs, not money savers.