SOLAR THERMAL PANELS

Two basic designs of collector are available for domestic use:

• Flat Panel - 'FP'
• Evacuated Tube - 'ET'

Possible accreditation marks to look out for are 'Solar Keymark', 'Blue Angel RAL-UZ-73', SPF Quality Label and EN12975 tests. If you want to be in line for a grant then a listing on the Clear Skies or MicrogenerationScheme lists is a must.

Some makers require an annual drain-down of the system, I don't think this is a desirable attribute. All designs have a 'stagnation temperature' which should be as high as possible, commonly 270-290deg.C for ET types, commonly 200deg.C or less for FP designs: this is the maximum temperature which they should be allowed to reach before damage occurs, e.g. if the fluid is lost. Solar Thermal Panels are capable of delivering water at very high temperatures in high summer and systems may incorporate a cold water blending valve to reduce this to normal DHW temperature. Several suppliers offer packaged systems including Thermal Stores or other storage options.

Flat Panel designs vary in sophistication and complexity, the majority consist of a flat absorber plate attached to a snaking tube, within which flows a water-based fluid. Some tubes are of 'harp' configuration, as in the wires of a harp. They are normally covered by toughened or coated glass and have some or a lot of insulation within the frame of the panel. Cost around £1250-£1500 per panel plus installation. They seem to be:

• Easier to manufacture, it would be possible to produce a basic DIY flat panel without great difficulty but it would not be so efficient
• Less efficient than the vacuum tube designs, though it is difficult to be precise about this as comparable data is difficult to obtain
• Less costly to purchase
• Easier to integrate into a new or existing roof, but overall depth varies from 100mm. to 165mm.
• Larger in area than vacuum tube designs

The best designs incorporate a heat sink plate which is fixed in contact with the fluid tube: the plate heats up much faster and transfers the heat to the tube. The Agena Azur is slightly different with a collector composed of two closely-spaced embossed plates between which the fluid flows over the whole area. Many designs have special coatings on the glass which prevent heat escaping back out. Flat panels have been commonly used for many decades in sunny countries such as Greece and Spain, where they provide summertime hot showers, warm swimming pools and copious DHW.

Some Flat panels widely available in UK:

Stiebel-Eltron SOL27 / Viessmann Vitosol200F / Agena Azur6 / Consolar Plano26

Evacuated Tube, also known as Vacuum Tube designs are more complex and consist of a row of individual glass tubes connected to a common manifold at the top (at the bottom in one design). The tubes are mostly made of borosilicate glass (Pyrex) and contain a single or in some cases two microbore pipes inside; these are connected to the common manifold in different ways:

1. A 'dry' connection - the heat exchange between sealed 'heat pipe' and main fluid circuit takes place through a 'pocket' in the manifold and there is no contact with the main fluid circuit. These designs use a closed system in each tube, with evaporation of special fluid to transfer heat to the manifold followed by condensation back to the bottom

2. A 'wet' connection where heat exchange from the sealed 'heat pipe' is direct into the main fluid circuit;

3. A flow-and-return connection where the main circuit fluid is directed up and down inside the tube. In these designs the glass tube contains two microbore pipes or two concentric pipes: the fluid from the main circuit is pumped down the tube on one side and back up into the manifold on the other side.

Dry connections allow individual tubes to be changed in case of damage, without draining the collector or main circuit - though makers emphasise that the tubes are very robust and are tested to withstand hailstones. .  Evacuated tube designs seem to be:

• Complex to manufacture, it would not be possible to produce a DIY design
• More efficient than flat panel designs (one maker claims 30% more) but the same caveat applies regarding comparable data
• More costly to produce and consequently more expensive, around £1250 - £2000 per panel plus installation
• No thicker than most flat panels (some are much thinner), but they do not seem suitable for integrating into a pitched roof
• Smaller in area than flat panels
• Some seem to require more skill in fitting and commissioning than do most flat panels

This design is thought by some people to be more suitable for UK use since the ambient temperature has less influence on the performance of the collector and in some examples the tubes can be individually oriented to face the sun; this could be useful if the roof on which they are mounted is not at an ideal angle or orientation.

Some Evacuated Tube designs available in UK:

AMK Collectra OPC15H: twin flow/return pipes attached to an absorber plate within the glass tube

Consolar Tubo12: very thin pipes with manifold at the base; modular system in which units fit together to make desired collector area

Thermomax MS20:    single pipe sealed system with wet connection

Thermomax Solamax20:    sealed but divided pipe with flow and return side

Navitron SFB20: twin-walled tube  and SFC10: single-walled tube

Riomay Ecotube: single pipe with absorber plate

SolarUK LaZer2: twin pipes fitted into an absorber plate within the tube

Viessmann Vitosol 200T and 300:    single pipe

Rayotec/Schott ETC16:    single concentric pipe with flow-and-return and wet connection to manifold

SUMMARY

• Both ET and FP designs may be capable of providing up to 80% of DHW in a normal summer, much less in other seasons
• FP designs can be integrated into a pitched roof but take up more space
• ET designs may be preferable if the roof is not south facing or not with a pitch between 15deg. and 45deg.
• ET designs are more expensive but some can be repaired with relative ease
• ET designs are more varied than FP types, even though all look quite similar externally
• If you wish to try your hand at making a solar thermal collector it needs to be an FP design
• UK Govt. grants are currently set at a maximum of £400 per household, which is less than 20% of typical installation cost for a single panel

LINKS

SPF Swiss Test Institute:    www.solarenergy.ch

Construction Resources (Agena Azur):    www.constructionresources.com

Consolar:    www.consolar.co.uk

Chelmer Heating Services:    www.chelmerheating.co.uk

Hoval (quite detailed technical pages):    www.hoval.co.uk

Rayotec (Schott collectors):    www.rayotec.com

Solar Thermal Association:    www.greenenergy.org.uk

Thermomax (useful technical pages):    www.thermomax-group.com

SolarUK (Lazer2):    www.solaruk.net

Viessmann:    www.viessmann.co.uk/home_index.php

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