Drying Onions in areas of higher than usual humidity

All,
First time forum, long time email recipient.

We are conducting an Onion Growing trial in the Highlands of Papua New Guinea. At a little place called WAU.
I need to build an onion drying facility to handle about 5 tonnes of onions at a time.
no power, no gas and generators are expensive.
I can design the drying racks and cart rails inside a purpose built shed no worries.
Im thinking of using a Rocket style mass heater to run the drying process.

We need 2 things. Airflow and Heat.
i can regulate the Airflow with shutters on the walls no worries there.
My concerns are that the mass heater horizontal flue will need to be about 15 metres (or yards, no matter) long.
I am an experienced underfloor hydronic heating guy, so I know we could also install radiant tubes as well.
Our average temperature year round is about 26 degrees Celsius and i need to get the onions up to just under 40 degrees celsius and maintain that over a week period.

Has anyone got any ideas on how to regulate the heat?

My first thoughts are to get the mass up to temperature before placing in the onions and monitoring from there.

The rocket may also need a more convoluted flue.

I would appreciate any advice.

David

What about creating a wind tunnel on the side of a hill? Ive always wanted to do a clear plastic tunnel going up a hill into a drying shed and see how much flow it can generate. It’s low tech anyway but don’t know if it will work. Maybe if it’s steep and facing the sun.
Have you figured out anything that might work?
Good luck.