Radiant Energy Flux

For a solar array to work, it must face in the right direction.  The maximum power is transferred when the array is facing perpendicular to the rays.  The maximum theoretical power produced by a solar array is given by:

Power (W) = Intensity of radiation (W m^-2) × area (m²)

In Physics code:

P = FA

If the array is at an angle to the radiation, the equation becomes:

P = FA sin q

In practice the efficiency is only about 10 %, so the area for a given power has to be 10 times what it should theoretically be.

Resistance and Temperature

Ohm's Law tells us that the resistance remains constant as long as the temperature remains constant.  However as the temperature increases, we find that the resistance changes.

If the material has a positive temperature coefficient, the resistance goes up.  This is because the molecules vibrate more and there is a greater chance of a collision.  The graph shows what happens with a metallic conductor.

The resistance is related to the temperature by the equation:

R = R₀(1 + aq)

For which the physics codes are:

• R - the resistance at any temperature (W);

• R₀ - the reference resistance (W);

• a - the temperature coefficient (^oC^-1);

• q - temperature change (^oC).

If the material has a negative temperature coefficient, the resistance goes down.  This is because in a semi-conductor material, more electrons are released.

For a semi-conductor, the relationship is not so straight-forward.

Heating and Cooling

Heat is transferred by three processes:

• Conduction - heat energy is passed on by collisions with molecules in a solid lattice.

• Convection - hot fluids expand get less dense.  They rise to the top of the fluid and pass the heat energy on.  They cool, become more dense and fall again.

• Radiation - heat is passed out as infra-red electromagnetic radiation.

In space, only radiation can get rid of excess heat.  The molecule density is far too low for convection to work.

The energy flow to and from materials is given by the equation:

Energy transferred (J) = mass (kg) × specific heat capacity (J kg^{-1 o}C^-1) × temperature change (^oC)

In physics code:

DE = mcDq

In satellites, heat is transferred to heat sinks through heat pipes.  The heat sinks themselves:

• are painted black as this is a good emitter of radiation;

• have a large surface area.