Saxony-based lighting manufacturer Narva has developed a new, cost-effective technology for highly efficient vacuum tube collectors. They are well suited to solar-powered hot water generation in commercial enterprises and can still obtain heat from solar radiation even under less than favourable conditions. 11/2007
Vacuum tube collectors are considered the best available technology for producing heat from sunlight. Even on cold winter days and with low to moderate radiation, they still deliver hot water from solar energy. The fact that so far they only represent about 10% of the market for solar collectors in Germany is attributable to their high price, according to Roland Digel, solar power expert at the Deutsche Bundesstiftung Umwelt (DBU): ‘Vacuum tube collectors are effective but expensive.’
All that is about to change with the introduction of a new, highly automated production process which also uses more cost-effective materials than before. It was developed with DBU support by Narva Lichtquellen GmbH + Co. KG of Brand-Erbisdorf, Saxony. Underlining the significance of this investment, Digel explains: “We hope this innovative technology will result in a reduction in the price of vacuum tube collectors and that they will then be able to gain wider acceptance in the market. At the same time, we hope to be able to enhance the efficiency level even further.”
One factor the lighting manufacturer had to consider for the new process was the need to develop a new, machine-compatible solution for the vacuum-sealed connection between the outer glass tubes of the collector and the water-conducting copper tube. The solution consists of a sheet made of iron alloy, which is soldered onto the copper tube and bonded to the glass tube by means of an ingenious glass treatment process. The resultant patented glass-metal bond is impervious enough to maintain the vacuum in the interior of the tube collector for its expected service life of at least 15 years. According to Narva’s managing director Gerhard Mientkewitz, the quality of the vacuum is such that the particles of air remaining in it are rendered virtually incapable of transferring any heat from the metal parts in the interior of the tube collector to the glass wall. Narva already had experience with a similar bonding technology from its fully automated production of fluorescent tubes.
Thanks to a new concept for the exterior glass casing, the cost of materials for tube collectors should fall by 20 per cent. In the past, special high-grade glass was used to ensure that the solar radiation would pass through the glass layer with as little impediment as possible before heating a layered copper sheet inside. Now, lighting manufacturers use simple, cost-effective plain glass. To give it comparable optical transmissivity to that of the special glass, it is coated in a dipping tank with an ultra-thin film of silicon dioxide. As solar technology manager Wilfried Schaffrath explains, the individual collector thus achieves “very good” optical efficiency: 85 per cent of the solar energy is converted into usable heat.
The production line initially began partially automated operation in June 2007. ‘Narva currently produces approximately 10,000 vacuum tubes a month, and the trend is distinctly upward’, managing director Gerhard Mientkewitz reports. Over the next five months, additional production stages that currently require manual action will also be automated.
In Germany, partner companies Philippine GmbH & Co. KG of Lahnstein and MP-TEC GmbH & Co. KG of Eberswalde assemble the vacuum tubes into solar modules, which they also market themselves. Narva Trade Solartechnik GmbH in Meppen was established to market the tubes internationally. It already has branches in several European countries and the USA. Narva is looking for additional companies there to assemble the vacuum tubes into solar modules. This is already being done successfully in the Czech Republic with Regulux spol. s.r.o. and in Poland with Hewalex.
By way of development goals, Mientkewitz cites reducing the specific energy costs of solar collectors and opening up new application fields in the high-temperature arena. ‘We want to offer energy prices that compare favourably with oil and gas’, Narva’s managing director explains. He sees collectors with Narva vacuum tubes – capable of producing hot water up to 90°C – as particularly well-suited to the provision of process heat in laundry, paintshop, drying and air conditioning applications. ‘Of course, the tubes can also perfectly well be used for providing domestic hot water and heating’, Mientkewitz says. Together with its Swedish partner ClimateWell AB, Narva is currently constructing two air-conditioning plants in Germany and Sweden. These will one day be capable of delivering the energy collected in the solar collector for both heating and cooling purposes.