Heat pipes Heat-sheets
In its simplest form, a heat-pipe (Dunn & Reay 1994) is a sealed tube containing a small quantity of a volatile liquid (such as water) with no air or other "permanent" gas present. If such a pipe is placed vertically and the lower end is heated, liquid will evaporate and the vapour so formed will travel to the cooler parts of the pipe where it will condense and give up its latent heat of vaporisation. The condensate will then run back to the heated end where it can re-evaporate. This is illustrated below:
Because the heat transfer within the pipe comes from boiling liquid and condensing vapour, both of which processes have inherently very high heat transfer coefficients, and because the amount of material which has to move from one end of the pipe to the other is small the effective thermal conductivity of the heat-pipe is very large. To illustrate the magnitude of these quantities imagine that the heat-pipe is transmitting one kilowatt using water as the working fluid. The mass flow would be just under 0.5 g/s. At a temperature of 100 °C in a 20 mm diameter pipe this would correspond to a vapour velocity of about 2.5 m/s.
In more sophisticated versions, the pipe contains a capillary wick to assist the return of the liquid from the condenser end to the evaporator end. Such pipes will work without the aid of gravity, for example in spacecraft. However, for terrestrial applications the far cheaper and simpler two-phase thermosyphon, as the gravity return heat-pipe is usually known, is often adequate.
The main useful characteristics of the two-phase thermosyphon are:
(1) the thermal conductivity is extremely high: about a thousand or more times that of copper, (2) the thermal conductivity is almost independent of the metal that the heat-pipe is made from,
(3) the device acts as a thermal diode. That is, the conduction is very high in one direction (upwards) and very low in the other (downwards),
These characteristics make heat-pipes useful wherever a large amount of heat needs to be conducted through a small cross-section. They have been used in cooling space-craft components, in cooling plastics-forming dies, for the construction of air-to-air heat exchangers for industrial and domestic energy recovery, and in cooling electronic components mounted in confined spaces. One of the most spectacular applications has been the cooling of the support columns for the trans-Alaska oil pipeline to prevent the melting of the permafrost at their bases.
Thermocell has developed a flat-plate version of the heat-pipe (Foot, Wallace & Williamson 1981a,b) which extends the range of application. The lightweight flat-plate heat-pipe, which we call a "heat-sheet", consists of two sheets of metal seam-welded together at the edges and carrying a pattern of indentations.
The indentations create a vapour space within the heat-sheet which is evacuated and into which the working fluid is introduced. The form in which we have used the heat-sheet to date has been as a two-phase thermosyphon. The first commercial application of the heat-sheet is our solar water-heating collector.
The heat-sheet, made of sheet steel, takes the place of the copper or aluminium absorber sheet of a conventional flat-plate collector. The thermal conductivity is sufficiently high that one only needs a small heat exchanger of copper tube along the upper region of the collector to transfer the collected heat to the water. From a user point of view, the collector is the same as a conventional flat-plate solar collector but is significantly cheaper for a given area of collector. The advantages of this construction are:
(1) lower cost per unit area of collector, (2) much less copper used,
(3) light weight,
(4) significant savings during frost protection.
This last feature is a result of the fact that the water-way is at the top of the panel. When water is circulated through the system to protect the waterway from freezing in frost conditions the thermal diode effect means that there is very little conduction from the waterways to the rest of the panel. The remainder of the panel does not require protection since the working fluid has a very low freezing point.
Shown below is our heat-sheet solar panel.
