Solar water heating is an age old activity and to a first approximation one can say about it that “There is nothing new under the sun”.
In New Zealand almost all commercial solar water heaters are variants on the so called flat-plate tube-on-sheet collector. In these devices a blackened flat sheet of metal intercepts the sunlight. The heat is then transmitted to a set of tubes which are attached to the plate and in which the water being heated flows. Header pipes bring the water to the tubes and collect it from them. The absorber is enclosed in an insulated case with a transparent cover to create a complete solar panel.
Until about twenty-five years ago the absorber sheet was almost always made entirely of copper which is one of the best thermal conductors known. However price increases in copper began to make such collectors too expensive and the attention of designers turned to ways of achieving good collection efficiencies with cheaper materials.
Unfortunately there is nothing like copper for corrosion resistance in the actual waterways and even now most solar water heaters still have copper water pipes.
The next best readily available conductor for the sheet is aluminium whose conductivity, while not as good as that of copper, is still quite good. To offset the lower conductivity of aluminium one needs to use a thicker sheet or to put the tubes on the collector closer together. Both of these measures tend to offset the initial savings achieved by going away from a copper sheet. There are however a couple of other tricks which have been used to counteract the lower conductivities of metals other than copper.
One of these tricks is “selective coating”. In selective coating a thin layer of either finely divided nickel (black nickel) or chromium (black chrome) is formed on the surface usually by electrolysis. Such a layer has the property that it absorbs solar energy almost as well as a matt black surface but it is a poor emitter of energy at the temperature of the base metal. The net result is that, other things being equal, a selectively coated absorber exposed to sunlight will get hotter than a simple matt black one. This in turn means that the lower conductivity of an aluminium sheet can be compensated by having it run hotter so that the copper water tubes do not have to be put closer together and the sheet does not have to be made thicker. A second advantage of selective coatings is that they enable the collector to work better in poor conditions (weak sunshine), although this is not a major consideration in New Zealand. There are now on the market in New Zealand several collectors which use collector sheets with selective surface coatings.
For New Zealand conditions, even in the south of the South Island, nearly all solar energy gathering will take place in strong sunshine. This means that the theoretical gains to be had from selective coatings working in weak sunshine are only partially realised in practice. For this reason Thermocell has chosen to use BAC (Broadband Absorbtion Coating) technology for its collector surface. Selectively coated panels also tend to over heat water in strong summer sunshine whereas matte black panels tend to be self limiting.
Another way of compensating for the poor conductivity of the cheaper materials of construction is to use the so called heat-pipe effect. In a heat pipe, high thermal conductivity is achieved by using the evaporation and condensation of a volatile fluid to carry the heat along an evacuated tube.
Such a device has a thermal conductivity many hundreds of times that of the same cross section of pure copper. Thermocell has taken advantage of this phenomenon to construct solar collectors of mild steel in which the collector sheet itself is a flat plate version of a heat pipe. This “heat sheet” conducts the heat to the top of the panel where it is transferred in the heat exchanger section to a length of copper tube carrying the water. In addition to its very high thermal conductivity the heat sheet has the further advantage that its conductivity is only in one direction so energy can be transported from the collector sheet to the water tubes but not vice versa.
From the user point of view both of the methods outlined above make little or no change to the appearance of the collectors.
There is however another trend which does make a difference to the appearance of solar collectors. This is the design now offered by Thermocell in which the collector is incorporated into the structure of the building. Instead of building solar collectors as independent units which are then mounted on the roof of a house one can build a transparent section (typically glass glazing) into the roof and mount the absorbers in the roof space behind the glazing. In addition to the obvious cosmetic advantages this procedure offers several minor, but significant technical advantages:
- With only the glazed surface exposed the collectors are not so subject to wind cooling.
- The plumbing connections are not exposed to the weather and there is thus an extra degree of protection from that bane of all solar installations- frost.
- With an in-roof installation one can put in extra insulation which in an externally mounted system would add to the thickness and make the installation more obtrusive.
- The overall cost of a built-in system installed in a new house at the time of construction is about the same as, and sometimes a little less than, that of an externally mounted system. In some cases where transparent (eg polycarbonate) cladding is obtainable in the same profile as the roofing material it is possible to simply insert transparent sections of roofing over the collector area to achieve a significant overall cost reduction.
Solar water heating systems are available in simple thermosyphon or pump circulated versions. The thermosyphon arrangement requires no external energy input , but the geometry of the installation is critical in that the cylinder must be above the collector. Pump circulated systems, such as the Thermocell system, are not subject to such constraints, but of course require a small electrical input to the circulating pump.
The electrical energy used by the circulation pump ends up heating the hot water: the forced circulation of the water through the panels keeps the panels at a lower temperature than if they were in a thermosyphon system. The cooler the panels, the more efficiently they operate (true for any solar panel), and so the pumped system overall is more efficient than a thermosyhon system.
A final development in water heating, which is not strictly solar but is sometimes set up in a “solar assisted” manner, is the heat-pump. A heat pump is essentially a refrigeration system working in reverse. Instead of pumping heat from an evaporator inside the refrigerator to a condenser in the room the heat pump pumps heat from an evaporator in the atmosphere to a condenser attached to the hot water cylinder. For convenience the evaporator is sometimes mounted on the roof of the house where the sun helps to provide input to the evaporator. Despite the appearance of roof mounted heat pump evaporators, the heat pumps are really electrical heating systems which use electricity in a much more energy efficient way than simple conventional resistance heaters. There is not a lot to distinguish between a good solar system and a heatpump in terms of overall net consumer savings over a year.
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.
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.