The receiver pipe is one of the most important parts of the system because it can have a large impact on system efficiency. A receiver pipe has a very difficult job. It must absorb large amounts of sun radiation to heat up but must be well insulated to keep this heat from escaping. The most efficient way to do this is to use a glass vacuum tube.
A glass vacuum tube allow virtually all the sun radiation in but once it heats up the pipe it is trapped. Heat is lost only 3 ways conduction, convection and radiation. For conduction and convection there is a vacuum gap so no median to transfer heat through by contact or flow. For radiation the vacuum tube is also very useful. Glass in only transparent to certain wavelengths of light, mostly visible. As things heat up they release shorter and shorter frequencies of radiation. This is why when metal is very hot it gets brighter and red. The sunlight that is coming into the receiver pipe is mostly around the visible range and passes right through the glass. It then heats the receiver pipe to 150-300C. At this lower temperature the wavelength of the radiation is much longer then visible light and is reflected by the inside of the glass and back to the metal receiver pipe. So in this way the right system could let more the 90% of the light energy into the receiver pipe and would let less than 10% of the heat escape at high temperatures. I am also considering just trapping air between the metal pipe and a glass tube (see design choice number 3). This system would be simpler but would not insulate as well. A stale air test was done up to 199C.
This is a thick pyrex tube that was used for stale air testing.
The graph below shows the transparency of Pyrex glass at different wavelengths. It also shows the wavelengths of radiation emitted from an object at 250C. Only the small area where both curves overlap is the radiation that can escape through the glass tube. The rest of the heat radiation is trapped.
The metal receiver pipe is also very important. It must absorb as much of the sun energy as possible and turn it into heat with very little light reflected. The metal pipe must be a good conductor so it easily transfers it's heat on the outside surface to the inside to heat the oil. Heat fins can be added on the inside of the pipe to help transfer the heat to the oil.
In early tests a galvanized steal pipe with a diameter of 2.36 inches was used as the receiver pipe. This pipe was chosen only because of its low cost. It is actually meant to be used as a fence post. It was used in most of the early tests (see glass insulation test). This pipe was sanded and spray painted flat black with high heat car exhaust pipe paint. The flat black colour is the colour that absorbs the most sunlight.
The picture below shows the difference of the light reflected from a piece of cloth insulation (on the right side) and the receiver pipe. Both are receiving the same amount of focused sunlight but the pipe is absorbing much more of the light.
For the solar thermal car several improvements on this metal receiver pipe can be made. A metal can be chosen that is a better thermal conductor. Copper is the best conductor but is very expensive. Aluminium would be a good choice as it is a good conductor, light weight and not too expensive. Heat transfer fins could also be installed on the inside of the pipe. A better coating could be used that absorbs more light.