Flat collectors are by far the most common type of collector. In essence, they are dark-coloured absorbers through which water flows in pipes, highly insulated on every side except for the front, which is covered with low-E glass (see above). Cool water flows in at the bottom and heated water flows out at the top. Several flat-plate collectors can be linked together – either in series or in parallel. They come in grades of sophistication, which can be simply classified as glazed or unglazed.
Basic, unglazed
The basic unglazed solar collector snakes the narrow pipework – or ‘solar absorber’ – in a layout that maximizes exposure to the sun for a given area. Unglazed panels are mainly only found in the US and Australia.
Glazed
This takes the same layout principle and adds a box, low-E coated glass and insulation.
Figure 3.9 An unglazed solar collector typically
used for swimming pool heating.
Source: US Department of Energy
Figure 3.10 An unglazed collector on a hotel in Mexico.
Figure 3.11 A normal flat-plate collector has the same layout
but is glazed and insulated. Fins can be attached to the pipes and painted matte black to absorb the heat.
Source: US Department of Energy Flat-Plate Collector Enclosure Flow tubes Absorber plate Insulation Inlet connection Glazing frame Glazing Outlet connections
Figure 3.12 Glazed flat-plate collectors in a passive
system on the roof of a school in Bangalore, India.
Source: http://servamaticsolarparts.com
Figure 3.13 A passive, batch solar water heater. Source: US Department of Energy
Passive, Batch Solar Water Heater
Batch collector
Spigot drain valve (for cold climates) Bypass valves Cold water supply Solar storage/ backup water heater Hot water to house
Figure 3.14 Batch solar heaters in California. Source: © http://servamaticsolarparts.com
Also known as the breadbox, or integral collector storage system, batch heaters include one or more tanks or tubes in an insulated glazed box. The tanks are black or painted with TISS paint. The simplest possible system involves painting a tank black, putting it in a crate, insulating it on all sides except the one pointing at the sun and covering the sun side with glass or plastic. Cold water enters the tank at the bottom. The heated water inside the tank rises to the top of the collector and is drawn into the building as needed or into a storage tank. A curved mirror can be arranged around the tank to reflect more heat onto it, and in this case the tank is only insulated at the back.
Batch heaters should be used in places where freezing is infrequent. They heat the whole tank in the collector at once so take longer to heat a little water for morning use. Since the tank is outside, it will cool down more quickly. Therefore, batch heaters are most appropriate where water is going to be used later in the day. Modern batch solar collectors come in a dome covered not with glass but a material especially designed for transmitting
solar energy while holding in heat, for example twin-walled GE Lexan®. The
Figure 3.18 Evacuated tubes contain vacuums to retain more
heat. Fins, or a cylindrical surface, that maximize the area of collector directly facing the sun are attached to, or enclose, the fluid-containing tubes. This increases the period over which heat can be collected, both during a day and throughout the year. The surface is given a coating, which absorbs more heat radiation frequencies. Evacuated tubes are more efficient than flat-plate collectors mainly because they do not re-emit as much heat thanks to the vacuum within the tubes.
Source: US Department of Energy
Evacuated-tube collector Evacuated tube Glazing Cross section Inflow Reflector Outflow
Outer glass tube Absorbing coating Inner glass tube Fluid tubes Copper sheet Evacuated space
Figure 3.17 Cross-section through an evacuated-tube
collector.
Source: US Department of Energy Hot vapour rises to
Heat Pipe Tip
Cooled vapour, liquifies and returns to bottom of pipe
to repeat cycle
Evacuated tube
Copper Heat Pipe Non-toxic liquid
Figure 3.16 Tube collectors on a 211 apartment block
in Wezembeek, Belgium, yielding 6.6 MWh per year.
Source: IEA-SHC Figure 3.15 Evacuated tubes.
where it is circulated to a heat exchanger coil in the storage tank. There are two main types of evacuated-tube collector:
direct flow – the fluid in the closed loop is circulated through the piping of
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the absorber, transferring its heat through a normal heat exchanger; heat pipes – the absorbed heat is transferred in a closed loop by the
•
evaporation of the circulating fluid at the sun-heated end and its condensation at the manifold. The use of these phase changes means much more heat can be transferred.
The most common evacuated tube on the market at the moment is the Sydney tube collector, which is a direct flow collector consisting of two glass tubes fused together. The vacuum is located between the two tubes, and the outside of the inner tube is coated with the selective absorber. Inside the inner tube, the heat is removed by a copper pipe embedded in a cylindrical heat transfer fin, which conducts the heat to the manifold. There is no wet connection between the outer tube and the heat conductor, so if the external part of the tube is damaged it can be easily replaced.
Evacuated tubes or flat-plates?
In Europe, evacuated tubes occupy about 30 per cent of the market. Elsewhere in the world, the figure is more like 8 per cent, except for China. China is the market leader in vacuum tube collector production, and most tubes in the world are installed there. Because of Chinese companies’ strong marketing strategies, tubes are on the increase elsewhere, too. Although the COPs for evacuated tubes are higher than for flat-plate collectors, in practice the difference is not huge in most circumstances. This is because the efficiency of the collector by itself is often only a small part of the efficiency of the entire system. What is important to the end user is the number of kilowatt-hours per year that a given system will deliver. Evacuated tubes perform marginally better, all other things being equal, in conditions where:
roof space is limited;
•
it isn’t possible to use an exact south-facing
•
orientation;
the location is at a higher latitude;
•
there is less direct sunlight – the prevailing
•
conditions are mostly diffuse or cloudy; a higher output is required for space heating,
•
too;
there is a demand for higher temperatures.
•
A disadvantage of evacuated tubes is that they do not scale up as readily as flat-plates. This is because they have larger diameter manifolds, which are less conducive to fluid flow than the smaller ones in the serpentine heat elements of most modern flat-plate collectors. Flat-plate Figure 3.19 The manufacture of evacuated tubes.
Kalt Architects. Seventeen separate installations supply the 17 apartments with 40% of their domestic hot water needs. The double-skinned facade also offers privacy and restricts noise.
Source: © IEA-SHC
collectors are generally preferred where they are cheaper and where there are clear skies for much of the year. Evacuated tubes perform over more of the year – including colder months. With both types of plates, bear in mind that they are potentially vulnerable to damage from hailstones, although flat-plate collectors are tested to be hailstone proof under the standard EN12975.
Evacuated tubes can work in a system with combi boilers but will require the installation of a storage tank. There are then two options: either the tank output feeds into the combi boiler, having preheated it, or bypasses it when a sensor detects it is hot enough. The option chosen depends on the exact boiler model.