Solar Heating and Cooling Needs New Materials

Solar thermal systems have undeniably benefitted from continual incremental improvements, but not much about the most commonly used flat-plate collector seems to fundamentally alter. A module containing a black, glass-covered metal absorber plate harvests solar radiation. This is backed by tubing (generally made of copper or aluminium), through which water or other heat exchange fluid is pumped into a building's space heating or hot water system.

According to those developing a new breed of collectors, however, the use of materials such as copper bring significant downsides that could be eliminated by replacing them with a new generation of advanced, high-performance polymers.

An International Energy Agency (IEA) Solar Heating & Cooling (SHC) Programme task force group has spent the last four years plotting a plastic revolution in solar thermal.

The international research group involved in SHC Task 39 highlighted a wide range of potential benefits if the copper and other metals used in components such as the absorber plate and tubing could be removed. These benefits include the need for fewer separate materials and therefore manufacturing processes, lower weight and freedom from the sharp price fluctuations associated with copper in particular.

Polymer components could be manufactured more cheaply and flexibly thanks to the well-established process of extrusion, which could easily produce the exact dimensions needed to integrate collectors with buildings of all shapes and sizes.

Components would be lighter to transport and easier to install, and could possibly even come in a variety of attractive hues to do away with the 'any colour as long as it is black' approach of conventional collectors.

For all the virtues of their proposed plastic systems, however, the Task 39 group was faced with the fact that metals are used in solar thermal collectors for a very good reason. The extremes and fluctuations of UV radiation and temperature that a solar thermal system must withstand would prove too much for most polymers, especially over a required service life that spans at least 20 or more years.

 An Aventa solar thermal development in Oslo (Source: Dahle & Breitenstein)

The key temperature for a polymer operating in solar thermal is around the 160°C mark. This is the maximum that a system would have to cope with under stagnation conditions (the point at which the thermal process produces the highest operating temperatures), even though the base temperature requirements for domestic space heating and hot water are significantly lower.

Alongside detailed technical investigation, the IEA group's work was broadly split into two main areas designed to overcome these limitations.

One looked at using cheaper, readily available commodity plastics and adding some sort of overheating protection 'failsafe' mechanism to prevent them being exposed to temperatures they cannot deal with.

A second strand of investigation concentrated on the development of more sophisticated high-performance polymers with the properties needed to withstand the demands of solar thermal systems. Norwegian company Aventa, a participant in Task 39, is a notable pioneer in this area and says it is close to bringing a commercial polymer-based system to market.

Founded as a corporate entity in 2005 – but building on work underway since the early 1990s at the University of Oslo's physics department – Aventa has earmarked 2011 as the year in which polymeric solar thermal breaks into the mainstream market.

Aventa has collaborated with US group Chevron Phillips Chemical to develop a high-performance polyphenylene sulphide (PPS) polymer able to cope with the 160oC stagnation level temeperature and remain stable and reliable for the lifetime of a commercial system.

The Norwegian company's all-polymer collector, based on Chevron Phillips' Xtel PPS alloy, is currently in the final stages of testing and performance certification. Accordingly, Aventa says it plans to begin volume production this year.

John Rekstad, the company's chairman and a professor of physics at Oslo, says the crucial virtue of the material is its ability to operate effectively in a thermal environment, unaided by overheating protection, while also offering all the manufacturing flexibility that comes with the extrusion process.

'Of course Teflon, for example, can sustain temperatures of 340°C. The problem with Teflon is you can't extrude it or make products out of it in the way we need to,' says Rekstad.

According to Rekstad, the need to establish the Aventa polymer's ability to operate effectively over the lengthy service periods needed for solar thermal systems is one reason that the path to commercialisation has been a relatively slow one. The company has worked its way through the process of acquiring the necessary certification needed to enter the market and carried out the tests required to 'satisfy ourselves that we can stick to our promises,' says Rekstad. 'That takes time, but it is absolutely necessary to gain acceptance in the market.'

With those hurdles almost overcome, Aventa hopes to expand production capacity to 40,000 m² of collector surface this year and has developed a new extrusion die in conjunction with its manufacturing partners, which will make the polymer components on its behalf.

The end result is a system that can more than hold its own in terms of performance and cost, claims Rekstad. 'The base cost for production per unit can be reduced to a level of almost half per unit of conventional collectors,' he says.

In terms of overall system efficiency – the measure that Aventa prefers to use – Rekstad says the company's device is equivalent to conventional collectors. The absorber plate, for example, is made of extruded twin wall sheets. Even though its thermal conductivity is inferior to copper's, Aventa says the improved efficiency of its heat transfer process results in a performance that is equally robust.

In the Aventa system even the glass collector cover is replaced by a 10 mm sheet of UV-protected polycarbonate, removing the problems associated with glazing large surface areas.

Of course, the ultimate test of polymers in solar thermal applications will be acceptance by their end-consumers. These are ultimately homeowners and commercial building operators but, more urgently, the construction and building design industries. 'This is very important to create the shortest possible path from production to installation,' says Rekstad.

A sign of the construction industry's interest came in late 2010 when OBOS, Norway's largest building co-operative, took a 23% stake in Aventa, becoming its largest shareholder. 'They want to use our system in a number of projects, because they see it as a way of combining solar with the processes they are used to in building and construction,' says Rekstad. 'They can integrate these elements into buildings more easily than is the case with conventional collectors,'.

OBOS will run its own trials by building two identical houses, one with an Aventa solar thermal system and the second with an air-water heat pump. The two buildings will then be monitored to discover which delivers the best energy efficiency and cost performance.

At the time of its share acquisition, OBOS said it saw no reason why Aventa should not go on to become a leading player in the European solar thermal industry. For that to happen it will have to help kick-start a market that, by the admission of many in the industry, is struggling to make sufficient progress at the moment and like a number of other renewable energy sectors actually contracted in Europe last year.

Rekstad claims that for too long solar thermal technology has remained in the shadow of the PV sector, which has enjoyed the lion's share of government promotion, taxpayer subsidies and publicity. 'Market figures tell a story of impressive worldwide expansion, but this just has not got the same attention as PV, for example,' says Rekstad. He continues: 'Yet total energy production for solar thermal is eight or nine times bigger than the total output of energy from all electricity-producing solar technologies installed worldwide.'

Rekstad believes part of the reason is the lack of interest in solar thermal among major power utilities, a situation that he claims is easily explained: 'The energy companies' reason for existing is to sell energy. Solar thermal is all about needing to use less energy.'

Rekstad has been evangelical about the benefits of solar thermal heating for far longer than polymers have been part of the picture. He built Norway's first solar thermal house in 1977 in what he admits some saw as 'a crazy experiment' and has monitored the results ever since.

'I've lived in the house for 34 years and in that time we've had to pay €120 to replace the pump. That has been the only cost and in that time we have saved an average of 7.5 MWh annually. I'm not considered that crazy anymore.'