Although solar PV gets most of the attention, solar thermal (ST) for heating and cooling has been the more affordable technology for the domestic and small business sectors - that have been its majority adopters. ST's direct mode of heating, without requiring an electrical intermediary, suits these users, avoiding issues of electrical power management, grid integration etc.

Because much of humankind's power demand is associated with keeping people comfortable – warm or cool – in buildings, there is a large market for solar thermal in hot water, space heating, swimming pool and district heating applications, plus air conditioning and cooling. This accessible technology lends itself to both retrofitting and incorporation in to new buildings.

National and local Government incentives have supported substantial market penetration in such regions as Scandinavia, the Netherlands, Germany and Austria, much of the Mediterranean and parts of the USA. An outstanding example is that of Israel which, because it has actively embraced solar thermal technology since as far back as the 1950s and provided commensurate funding, now has ST heating in over 90% of its homes.

Legislation can help too. Spain has made solar water heating an obligation for new buildings nationwide, and cities like Barcelona and Murcia have added ordinances making such provision part of local planning procedures. A European movement, ProSTO, is encouraging the more widespread use of such ordinances to accelerate take-up of ST throughout the European Union.

Another plus for solar thermal is that, although often thought of in terms of individual premises, it has proved no less scalable than solar PV. Some of the world's largest ST installations are block and district heating schemes in Denmark, where a 12.9 thermal megawatt (MWth) scheme is one of three major systems on the island of Aeroe, and in Sweden and Germany.

Europe's SOLAGE programme recognises the value of economies of scale by encouraging the development of collective systems where arrays on numbers of premises share common heat exchange circuitry and other functions. China, a massive adopter and producer of solar thermal technology, also has large-scale schemes.

Not assured

With over 90,000 gigwatt hours of thermal energy now yielded annually by over 235m sq m of collector area deployed across the world, avoiding the emission into the atmosphere of over 40m tonnes of CO2 (estimate based on 2007 figures prepared by the Institute for Sustainable Technologies in Austria), solar thermal has become a major force in the renewables firmament. It is particularly well developed in China and Taiwan, which account for about half the installed base, plus Japan and India. Despite all this, continued success is not assured. In fact, in some regions the industry could be in danger of shooting itself in the foot.

Insiders regret that there are still too many commission-hungry salesmen selling sub-standard products and service, especially in the low-end of the market. A preponderance of small-scale applications leads to mixed quality in a minimally regulated market, a situation that is exacerbated by patchy engineering and installation practice.

Unfortunately, over the years there have been too many installations delivering something less than customer satisfaction, and critics have been able to question whether it is worth investing in small-scale ST solutions at all. Householders taking the plunge have learned that advertised payback times can be far removed from reality, having been based on misleading estimates of solar capture, coefficients of performance (COP), total installation costs – which should include integration with existing hot water systems – and through-life maintenance costs.

It is important to be realistic. Costs for installation to approved standards for roof integration can be substantial, and are increased still further by the need to provide safe roof access with scaffolding or other means. Then again, systems are rarely maintenance free, even though they may be advertised as being so. For instance, the water/antifreeze heat exchange medium that is specified for frost-prone climates may need checking and replenishing every couple of years; panel glazing will require cleaning, plumbing joints may become corroded and need re-making and reflective backing may need cleaning or renewing.

Evacuated tubes sometimes fail or succumb to hail or other damage and have to be replaced. In some cases, collectors may require a full overhaul with inter alia; re-blacking of flat plates; corrosion remediation; and cleaning to remove mould and bacteria from all elements including manifolds and piping.

A system may need power since, except in the case of thermo-syphon (convective) units, a pump will be needed to circulate the heat exchange fluid. Electrical power may be drawn from the main supply or provided by an accompanying solar PV element. Either way, there will be a cost.

And the industry could do more to educate customers in the nuances of intelligent demand management. Benefits can be minimal when customers fail to appreciate that the time hot water is used can make a big difference. This is because the greater the difference between the heat source temperature and that of the water being heated, the greater is the rate at which thermal energy is absorbed. (This follows from Newton's Law of Cooling, applied instead to heating.)

Therefore it might pay a householder to take a bath in the morning rather than late in the day, leaving a substantial differential for the sun to work on when the solar peak arrives. To improve awareness and results, there is much to be said for maintaining some after-sale contact with customers, avoiding the abrupt loss of communication that typically ensues once a sale or installation has been completed!

Education is also needed within the solar community itself. Not all designers and installers are sufficiently aware of the extent to which ST and conventional hot water systems differ, and the importance of effective system integration. Eco-architect Bill Dunster points out that ST system plumbing and heating controls must be well designed to take full advantage of solar hot water.

For instance, if a tank of water is heated every morning by a boiler before the sun has risen, there will be little input from the SHW system. A hot water tank large enough to absorb all collector output at peak solar times will avoid overheating while making best use of collector capability. On the other hand, while system designers should certainly include provision to avoid overheating, it is also advisable that water in storage tanks should periodically be heated to above 60 deg C, the temperature below which legionella bacteria can survive.

This might require a back-up form of heating. In any case, Dunster advises that the best use of ST, at least in temperate climates, is to pre-heat domestic HW supplies, the balance being supplied by another system.

As with solar PV it pays to site and orientate collectors carefully. According to collector manufacturer Sun Systems Inc, it is best in many mid and high-latitude countries to set the collector plate angle for best solar capture in the winter, since this is the time when optimisation is most crucial. In summer there is less need to maximise what is frequently an ample source of power.

In terms of their wider image, ST companies could do more to refute adverse propaganda, such as that from critics who claim, often on the basis of flawed assumptions, that embodied energy (that used in physically producing a system) is likely to outweigh the energy that the system will deliver during its service lifetime. Such assertions, typically aired in internet blogs, merit an informed response.

Controversy

Perceptions are not helped by an apparent lack within the industry of a united view on which technologies are the most effective. A case in point is the on-going controversy over the relative merits of flat plate and evacuated tube collectors.

Proponents of the latter argue that because each glass tube concentrates sunlight onto a central heat pipe collector, subsequent heat escape being prevented by a surrounding thermos-like double-glass layer with vacuum sealed between, efficiency is higher. With a circular tube, they point our, there is always part of the glass that is perpendicular to the sun's rays. Solar capture can occur, therefore, even when the sun is low in the winter and at the ends of each day.

Dr Alexander Eichwalder, sales and marketing director for Austria's GREENoneTEC Solarindustrie GmbH, a large-scale producer of both types of device, concedes that evacuated tube collectors can be superior in some circumstances. However, he recommends most customers to fit flat plate collectors because of their generally better price/performance ratio. Well engineered flat plate collectors, he says, are suitable for most standard applications, including public buildings as well as private homes.

He believes that coefficients of performance (COP) are an unhelpful measure of effectiveness except to scientists and engineers. End users, he says, are more concerned with the number of thermal kilowatt hours per year a given system will deliver, a measure that is readily translatable to money terms. In any case, collector efficiency alone has little meaning for users given that losses in the rest of the system can as much as halve overall efficiency.

Dr Eichwalder reports that some 92% of GREENoneTEC collector sales on the European mainland are of the flat plate type. He points out, though, that evacuated tube collectors have their place – hence the remaining 8% – and this is where output per area must be maximised. Thus, where roof space is limited, a south-facing orientation cannot be achieved or users require higher water output temperatures, these devices would be favoured. A disadvantage of evacuated tube technology, according to the Austrian expert, is that it does not scale up as readily as flat plate. The larger diameter manifolds required in collective-array applications are less conducive to fluid flow than the smaller manifolds that better match the serpentine tube heat elements of most flat plate collectors.

An anomaly in terms of the European picture is that evacuated tube collectors are more popular in the UK where they account for nearer 30% of all collector sales. This can be attributed partly to a climate characterised by low sun angles and diffuse sunlight, partly by a preponderance of small roofs and non-optimum orientations, and partly to the marketing policies of leading suppliers. The latter appear to have been influenced by a test series commissioned by the UK's Department of Trade and Industry (this government department has since been superseded), which appeared to favour the evacuated tube solution.

However, this research was allegedly marred by the very small sample of evacuated tube collectors tested, and unrepresentative selection of the best-engineered examples for testing. Findings of a more recent study carried out by the Solartechnik Prufung Forschung in Switzerland suggest that, in general, evacuated tube technology may have lower gross efficiency than flat plate and has no particular advantage except in the circumstances mentioned above. Because evacuated tube devices are typically more complex, with more separate parts and plumbed joints, they may be inherently less reliable. On the other hand, repair of a faulty unit is more likely to involve a single tube replacement than a whole collector.

Dr Eichwalder believes that the ratio of flat plate to evacuated tube sales in most of Europe will not change much in the medium term, unless the incidence of applications requiring higher heat output grows. He reports a rising trend in Europe for ST to move on from supplying just hot water to providing space heating support as well. Germany, which accounts for a large proportion of the European ST market, already sees half of all new installations being of this nature.

Economies of manufacturing scale have been bringing down the price of ST devices, just as they have with solar PV. Dr Eichwalder tells Renewable Energy Focus that GREENoneTEC alone produced 1.1m sq m of collector in 2008, a rise from less than 800,000 sq m the previous year. He expects output level to be little better than static in 2009, growth being restricted by shortage of credit, especially in southern European countries, and a delayed start to the present solar year due to heavy late-winter snow that fell across central Europe.

In general, economies of manufacturing scale are restricted by the need for different device specifications for different geographical regions. Systems for frosty climates require a glycol antifreeze addition to the primary water circuit. In countries where hail storms are experienced, flat plate collectors need to be protected by thick transparencies – some 3.2 mm of glass in the case of GREENoneTEC products for the home market. Low-iron borosilicate glass is used for high transmittance.

In some countries, notably the USA, South Africa and Australia where swimming pools are the dominant ST application, many unglazed collectors are used. In others where strong winds are a climatic feature, systems are engineered for winds up to 100 mph. Where blown sand is a hazard, evacuated tube devices need adequate space around the tubes to encourage air flow so that sand, dust and other detritus does not collect.

Evolved engineering

Although a basic technology, ST benefits just as much as solar PV from progressive engineering and design refinement. Products available today are more effective and durable than those of the industry's early days. The present state of evolution can be illustrated by reference to the latest products from GREENoneTEC (there are many reputable collector producers but the Austrian company is one of the biggest, with a wide product range.)

The HP160 Easy heatpipe system, which received its public debut at ISH 2009, Frankfurt, is a flat-panel roof-mounted product aimed at the domestic hot water market. The collector is integral with a water tank and, in contrast to the many thermo-siphon units seen in Mediterranean countries, the tank is well hidden by the collector so that the product is aesthetically acceptable.

High engineering standards are manifested in the 160-litre double-walled tank, the UV-resistant two-component adhesive used to bond all the joints, the design of the aluminium-coated absorber and the laser welding that ensures maximum transmission of heat between the absorber plate and the heater tubes. Spokesperson Dr Claudia Koenig states that the new product requires negligible maintenance and offers a yield 15% greater than that of a good thermo-siphon design. The aluminium frame is manufactured in-house on a robotic assembly line. Installing the device is a matter of folding out the frame, placing the system in the desired location and connecting up the hot and cold water lines.

The FK8231 Mediterrano Frame Collector is optimised for use in Mediterranean countries where both maritime and desert conditions pertain. It has a corrosion-resistant absorber, a selective coating that resists attack by sea air and an optimised ventilation concept to keep out blown sand. Polypropylene glycol included in the water heat transfer medium prevents freezing. A front glazing of tempered, low-iron solar safety glass protects the absorber system. With its aluminium frame, the product is engineered to resist 150 kph winds and 1.25 kN/sq m snow loadings.

To ensure reliability and consistent quality, the Mediterrano is based on a minimum number of individual components and is assembled on a robotic production line. A modular mounting system enables the collector to be used on both flat and pitched roofs. Collector area per module is 2.34 sq m with an absorber area of 2.14 sq m. The system weighs 82 kg and is 73 mm high.

A good product spread caters for multiple needs. An alternative to high end aluminium frames is to have wooden frames. The company produces these in two sizes.1.25 sq m and 2.5 sq m. Another frame model has 50mm thick mineral wool rear insulation. FK7000 is a tray collector series featuring an aluminium tray with positive-fit glass cover strips and silicone-free dry sealing to ensure long leak-tight service life. Façade collectors are designed for integration into building facades and do not require rear ventilation.

Where large ST surfaces are envisaged, customers can opt for GK3000 series collectors with standard dimensions of 5 sq m and 10 sq m. These larger sizes simplify roof connections and reduce installation workload, though a crane lift would typically be required for roof placement. The company also produces a traditional roof-mounted thermo-siphon model in which the collector is attached to a 140 litre or 280 litre accumulator tank.

ST underestimated

The extent to which solar thermal's contribution to the global energy supply dwarfs that of solar PV is often underestimated by energy policy makers. Moreover producing, installing and maintaining ST systems employs over 200,000 people worldwide. Will solar thermal emerge from the PV shadow and, Cinderella-like, begin to steal the attention that the hitherto more glamorous sister has attracted? Activity in terms of large utility scale concentrator projects that are outside the scope of this article (but see renewable energy focus May/June issue), suggests some progress in this direction. This may extend to lower-temperature collector technology as block and district heating schemes attain headline-grabbing size, helped by economies of scale.

However, it seems likely that mainstream low and medium-temperature collection technology applied to buildings will just keep moving steadily ahead, particularly as the industry improves its act and the recession completes its course. Given determination and vision, the ST sector should maintain its low-profile winning ways.