Electrically-powered, environmentally friendly, with a speed of 20,000 rpm. These are some of the features of an unusual compressor unit designed for the sensitive field of large-scale natural gas storage - the product of a European industrial partnership which is a model of its kind.
The big natural-gas distribution companies
buy much of their stock in summer, when prices are lowest. Before
they can sell it off in winter, they have to store it in huge natural
caverns. In order to accommodate such large volumes, however, the
latter must be reduced. This is done by means of motorcompressors,
i.e. compressors driven by electric motors. Traditionally, between
the motor (running at a maximum of 3,000 rpm, determinated by the
50Hz frequency of electric current in Europe) and the compressor
(running at between 6,000 and 20,000 rpm in order to improve efficiency)
a gearbox needs to be installed. All of which creates efficiency,
reliability and maintenance problems.
The contribution made by MOPICO
After looking at ways of addressing this issue in the late eighties, engineers developed the new concept of the electric compressor unit, which was hailed as revolutionary. Thanks to a special device - a frequency controller - the requisite rotation speeds are attained and the gearbox can be dispensed with.
In this early application, christened MOPICO (Motor Pipeline Compressor) and designed for transporting gas, the motor and the compressor form a single unit; the rotating part is supported by magnetic bearings, allowing the system to operate without oil, and hence without causing pollution.
MOPICO does nevertheless have certain limitations, due to the integration of the motor and the compressor. It isn't suitable for corrosive gases and it can't develop the large pressure ratios needed for applications such as gas storage.
New advances with HOFIM
In 1990, a new project was launched under the BRITE-EURAM programme. Known as HOFIM (High Speed Oil Free Intelligent Motorcompressor), it brought together Belgian, French, British and Swiss partners.
This time, the engineers decided to separate the compressor and motor again, in order to attain the necessary pressure ratios, while still retaining the concept of an oil-free motor and compressor (mounted on magnetic bearings).
For Dominique Gilon, coordinator of HOFIM, "the success of such a project hinges as much on the quality of the human relations between the various participants as on the technical skill of the firms involved. It's vital that the partners have confidence in each other, from the preliminary discussion phase of the project right through to the final tests - where things don't always go exactly as planned..."
World first: conclusive tests
The HOFIM compressor unit was built in 1992. Fully automated and remotely controllable, it runs at 20,000 rpm and has an output of 2MW. Extremely safe, non-polluting, economical and far more compact than traditional installations, HOFIM consumes 15% less energy than conventional solutions and requires very little maintenance.
In collaboration with the German firm EWE, a new
project christened HIT (Hofim Industrial Test) was launched the
following year under the THERMIE programme. Its objective was to
install the prototype at the Nüttermoor industrial plant. A
400,000 m³ cavern was filled with 40 million cubic metres of
gas. In order to compress the latter, a pressure of 150 bar had
to be obtained, i.e. 150 times atmospheric pressure.
Once again HOFIM passed the test with flying colours. "The motorcompressor has accumulated 7,500 operating hours to date," comments Dominique Gilon. "That may not seem much - considering that it was commissioned three years ago, but you have to remember that, being designed to fill storage caverns with gas, the motor only works during the summer months. This type of compressor unit is already attracting a great deal of interest and we've received numerous offers which are currently under discussion."
The partners have registered patents in Europe and the United States. This type of direct-drive motorcompressor can be used for many other applications besides gas storage, and represents a market worth an estimated ECU 25 million per annum.