Perlite in a vacuum provides good heat insulation as is required for the the components in the SHE.
It minimizes conducted and radiated heat loss.
A further relevant point is that some Caribbean Islands, who do not have very much of other minerals, have a supply of perlite.
It is not documented that Africa has a large supply of perlite, therefore low income Caribbean Islands with perlite reserves would have a start up mineral resource that could benefit a budding African industrial economy.
At the time of this statement, perlite is cheap and easy to handle.
Perlite heat insulator - an health and environment neutral substance