Almost every part of the world uses drying to preserve food. While
quite effective, it's also quite energy intensive. According to
Wikipedia, in some industrialized countries, as much as 12% of
industrial energy is consumed in drying processes.
My idea uses a partial vacuum (like freeze drying),
but with pressures
and temperatures which aren't as low as those used in freeze drying.
The first part of the idea is to put the food being dehydrated on racks
which allow gas circulation; there will probably be blowers or fans to
improve convection even more. Actually, any equipment normally
used for hot air drying would probably work.
This racks are then put into a pressure vessel, which is then sealed.
Gas (air or steam) from this vessel is circulated via a blower past a
heat exchanger, where the gas acquires heat, then back into the
vessel. This acquired heat is passed on to the food being dehydrated,
which of course causes further drying.
Gas from the vessel is, simultaneously, moved via compressor into the
inside of the heat exchanger; compression increases the temperature
of the gas, and the heat exchanger cools the gas. After the initial air
removal stage, the gas will be almost pure steam; the cooling of
steam will of course condense it to water.
The fluid then flows from the heat exchanger into a gas/liquid
separator. A float valve determines whether to remove liquid (via a
condensate pump) or gas (via a vacuum pump).
The temperature is controlled as follows:
When the temperature in the vacuum chamber is higher than desired,
the main compressor is cycled off. Gas will continue to flow into the
heat exchanger, but via a check valve which bypasses the
compressor. As the condensate pump and vacuum pump remove fluids
from the separator, the overall system pressure and temperature
drop, eventually causing the compressor to cycle back on.
When the temperature in the vacuum chamber is lower than desired,
the vacuum pump is deactivated (or valves are used to allow it to
assist the compressor at moving gas into the heat exchanger), and, if
necessary, an auxiliary heater is activated to heat the gas that has
been blown past the outside of heat exchanger and is about to enter
the vacuum chamber. By not removing gas from the system, and by
adding heat, the overall temperature and pressure rise.
For maximum energy efficiency, the thermostat should be set to allow
a wide temperature range.
This idea has one big advantage over both regular hot air drying and
freeze drying -- namely, it's much more energy efficient.
There's also one big advantage over regular hot air drying -- the
temperature is much lower. Many foods are damaged by heat, and
this idea avoids that.
The idea has one big disadvantage compared to hot air drying -- the
equipment will cost more to construct, and it's hard to say how long
the energy payback would be. The cost of equipment will be quite
similar to that of a freeze-dryer.
The idea has one disadvantage compared to freeze drying -- in freeze
drying, there's a phase change of the food's liquid to a solid, followed
by desublimation, which a different effect from evaporation, due to
the lack of surface tension during desublimation. This idea (like
conventional hot air drying) uses evaporation, not desublimation, so
some things will shrink as they dry which wouldn't occur with freeze
drying.