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The German solar thermal industry

The generation of heat using solar energy is a technology with a long-standing track-record. In many private households, hotels, hospitals etc. solar collectors are successfully deployed to heat water and support central heating systems. The first systems for process heat generation and solar cooling have also been installed.

The technology at a glance

Solar thermal systems for domestic water heating in single-family homes:

1) Collector
2) Solar storage tank
3) Boiler
4) Solar station
5) Hot water consumer (e.g. shower)

 

 



Solar thermal collectors absorb sunlight, convert it into thermal energy and pass it on to a heat transfer fluid. In systems with forced circulation, the fluid is pumped into the hot water tank via a pipe system. For this purpose, a circulation pump is activated by the controller as soon as the temperature exceeds that of the storage tank. The solar heat is transferred to the water tank via a heat exchanger. The cooled liquid flows back to the collector, where it is reheated. In the winter, conventional energy sources provide additional heat. In Germany these tend to be gas or oil boilers. However, wood pellet boilers have also become increasingly popular.

Thermosiphon systems use gravity to transport heat to the storage tank. For this reason, the storage tank must always be installed in a higher position than the collector. These systems are mainly used in southern countries and do not require a pump or any controls. While being technically simple and reliable, they are seldom deployed in conjunction with larger collector surfaces and central hot water generation.

The simplest form of collector is the unglazed plastic absorber. In this case, water is pumped through hoses or black plastic mats, and is used to heat open-air swimming pools. Temperatures of 30 °C to 50 °C are typically achieved with this method.

Flat plate collectors are the type most widely used in Europe. These consist of a metal or wooden casing, with thermal insulation on the base and sides, and a glass cover on the top. The collector contains the coated absorber, to the base of which small pipes are welded. Heat transfer fluid flows through these pipes, between the collector and the hot water storage tank. Collectors deployed in Germany normally operate in a temperature range of between 60 °C and 90 °C, whereby they are capable of achieving well in excess of 200 °C when circulation comes to a standstill.

Evacuated tube collectors are capable of operating with greater efficiency, because thermal losses are sharply reduced by the strong negative pressure in the glass tubes. A variety of designs are available: the absorbers either lie in a tube in the vacuum, or a double glass tube with a vacuum is mounted onto a heat conduction plate. A collector comprises a number of these tubes.

Another type of flat plate collector is the so-called air collector. Here, air is used as a heat transfer medium and used to heat halls and buildings, or water, alternatively, in conjunction with air-water heat exchangers. The heated air can also be used directly to dry agricultural produce.


Market development in Germany

The number of solar thermal systems and the related production of heat will continue to increase in the coming years.
To date, about 5 % of German households use solar thermal energy, and over one million solar thermal systems are already installed on German roofs. The total installed solar energy output in Germany at the end of 2006 amounted to some 5,750 megawatts. Over the course of the next five years the German Solar Industry Association (BSW) expects over EUR 10 billion to be invested in new solar energy systems in Germany for the generation of hot water. Medium-sized enterprises and craftsmen in particular will benefit from this market development. There are approximately 5,000 German solar thermal energy companies and craftsmen, employing around 20,000 people. In 2006 sales of EUR 1.2 billion were recorded, while the market grew by almost 60 %.

Overview of the solar thermal markets in Europe. This compares installed thermal outputs in megawatts and the market shares relative to the European market.
Germany is the market leader by a large margin.

There is some 800 million square metres of roof surface in Germany that is suitable for solar utilisation. Further utilisation potential exists in the form of façades and open spaces. To date, the market has been dominated by small-scale systems used to heat domestic water and to boost the central heating systems of private residential buildings. These tend to have a collector surface area of between 6 and 15 square metres. Solar technology for larger systems intended for use on apartment buildings, hotels, hospitals, retirement homes and halls of residence etc. has greatly improved in recent years. Its increased use is anticipated in the future. As part of the demonstration programme “Solarthermie 2000” (solar thermal energy 2000), 10 solar district heating plants with a collector surface area of up to 3,000 square metres and seasonal heat storage tanks containing up to 12,000 cubic metres of water were installed. The solar thermal energy generated over the summer is stored until the winter months, and can then be used to heat the connected buildings. Plants with over 30 square metres are rarely installed, despite the fact that they are more thermally efficient and therefore more cost-effective. Depreciation periods are one reason for the low take-up of this potential. The maximum depreciation period applied within the business sector is four years. This is only about one half of the depreciation period of high-efficiency solar PV systems. It is often forgotten in this regard that after a longer depreciation period, good solar power systems continue to supply energy for many more years, which is practically free of charge.


Framework conditions

Solar thermal energy plants are technically advanced, environmentally friendly and protect the climate. In addition, they improve supply security because they reduce dependency upon oil and gas imports. In comparison to current oil and gas prices, however, they are often more expensive. For this reason the German government encourages their use through a grant under the so-called Market Incentive Programme.

In order to accelerate distribution in future, further systematic expansion programmes are to be set up in the near term.

In addition to smaller, decentralised plants, new opportunities are presented by district heating networks, into which large-scale solar collector arrays can feed solar thermal energy. In Germany a great deal of work is already being done on projects of this type. Some of these are also capable of using large seasonal heat tanks, storing the solar thermal energy generated in the summer for use during the winter months.

Outlook

Along with ecological aspects such as climate protection and pollution reduction, there are also economic arguments for the increased use of solar thermal energy plants. Due to the steady rise in prices for oil and gas, alongside the growing dependence on oil and gas imports from unstable regions, solar thermal energy is set to become economically viable not just for producing hot water, but also for supporting central heating systems. In Europe, solar thermal energy plants are likely to become a ubiquitous part of the heating technology deployed in all buildings.

Another highly promising application is solar thermal cooling, where the heat from solar collectors is used to drive thermal refrigerators. Over 100 such plants are already being successfully operated across Europe. A number of companies and research institutes are working to develop this technology further, to make it more compact, more cost-effective and suitable for smaller-scale applications.

 

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