Direct Energy, 2018a

96 5.3 Magnetohydrodynamics
This amount of power is tiny. While this device can make a useful sensor,
it will not make a useful energy harvesting device. It generates tens of
picowatts of power, and a 1mA current must be supplied to generate the
power.
5.3 Magnetohydrodynamics
A magnetohydrodynamic device converts magnetic energy to or from electrical
energy through the use of a conductive liquid or plasma. Similar to
the Hall eect, the fundamental physics of the magnetohydrodynamic eect
is described by the Lorentz force equation, Eq. 5.1. The dierence is that
the magnetohydrodynamic eect occurs in conductive liquids or plasmas
while the Hall eect occurs in solid conductors or solid semiconductors. Another
related eect, which is also described by the Lorentz force equation,
is the electrohydrodynamic eect, discussed in Sec. 10.6. The dierence
is that the magnetohydrodynamic eect involves magnetic elds while the
electrohydrodynamic eect involves electric elds.
Matter can be found in solid, liquid, or gas state. A plasma is another
possible state of matter. A plasma is composed of charged particles, but a
plasma has no net charge. When a solid is heated, it melts into a liquid.
When a liquid is heated, it evaporates into a gas. When a gas is heated, the
particles will collide with each other so often that the gas becomes ionized.
This ionized gas is a plasma [3]. When ions in either a conductive liquid
or a plasma ow in the presence of a magnetic eld perpendicular to the
ow of ions, a voltage is produced.
This magnetohydrodynamic eect was rst observed by Faraday in 1831
[3]. In the 1960s, there was interest in building magnetohydrodynamic devices
where the conducting medium was a plasma. These devices typically
operated at high temperatures, in the range of 3000-4000 K [60]. Progress
was limited, however, because few materials can withstand such high temperatures.
More recently, engineers have used this principle to build pumps,
valves, and other devices for microuidic systems [61] [62]. These room
temperature devices can control the ow of conducting liquids through the
use of an external magnetic eld.