STOP CASSINI

Draft Minutes of Meeting with Dr. Strobl, ASE on August 12, 1997
taken and translated by Regina Hagen, Darmstädter Friedensforum;
German text released by Dr. Strobl, ASE, on August 22, 1997
with the provisio that ASE management give their consent

Meeting time: 90 minutes

Participants:

Regina Hagen, Darmstädter Friedensforum
Wolfgang Schlupp-Hauck, Friedens- und Begegnungsstätte Mutlangen
Dr. Gerhard Strobl, Head of Development High Efficiency Solar Cells at Angewandte Solarenergie - ASE GmbH, Heilbronn

At the beginning of the meeting, Dr. Strobl gave information about ASE. ASE was a joint venture of NUKEM GmbH and Damiler-Benz Aerospace AG (DASA) from 1994. Since 1996, ASE has been a 100 % subsidiary of NUKEM and therefore of the Rheinisch-Westfälische Energieversorgung (RWE) [one of the major German electricity companies].

At the Heilbronn location, solar cells for space missions have been produced since 1964; shortly afterwards they started development and production of solar cells for terrestrial applications. Aerospace applications are a separate ASE profit center. Germany is a rather small market for terrestrial applications. Wordwide, about 50 % of the silicon solar cells for space missions are supplied by ASE. To gain a larger share of the US market for terrestrial applications, ASE took over a US company and founded ASE Americas Inc.

Next, we discussed current possibilities for using solar energy for deep space missions. The sequence of the various topics was random. We discussed the following issues:

Gallium arsenide (GaAs) is gaining market share, especially in the US; the development is mainly a spin-off from military applications.
Contracted by ESA [European Space Agency], ASE began research on solar cells for deep space missions up to the distance of Jupiter in 1991; at that time, no specific mission to apply these cells had yet been considered. The research was started because RTG technology is not available for European space missions and alternative electricity sources for deep space missions were not yet in sight. In addition, solar cells are much cheaper than plutonium generators. The research and development contract with ESA covered the years 1991 to 1996.

The ESA press release of 1994 which has been mentioned so often referred to the technical breakthrough that 25 % efficiency had been reached under deep space conditions.

Due to the distance from the sun (earth: 1 AU [astronomical unit], Jupiter 5.2 AU, Saturn 9.54 AU), only 3.7 % of the light that reaches earth is available at Jupiter; at the distance of Saturn appr. 1 % of earth’s light intensity is left. In 1994, ASE managed to solve the problem of the feared „broken knee" effect, which usually occurs under LILT (low [light] intensity, low temperature) conditions. Since the 70s, this effect had caused the failure of all attempts to develop solar cells for deep space missions. Therefore, electricity for deep space missions could only be provided with RTGs prior to this breakthrough.

To develop the Si cells, ASE’s R&D team had an ESA (appr. DM 2 million) as well as an additional ASE research budget. The R&D effort resulted in single specimens of the new Si cell. By „stripping" this cell of specific features, a whole product family for various applications may be established. The most complex „king version" of this product family, the HI-ETA® NR LILT cell [NR for non-reflective], will be offered to ESA for the Rosetta mission to the comet Wirtanen.

In parallel to ASE’s research effort, GaAs cells were developed by CISE, an Italian research institute. This research was also contracted by ESA with ASE as the prime contractor. (CISE is located in Milano, Italy, and have their focus on research; they have but a small production line.) The GaAs cells developed by CISE have the same efficiency as ASE’s SI cells, but they weigh considerably more.

In the meantime, ASE produced 1,000 of the HI-ETA solar cells which will be used by ESA for the formal qualification program under realistic conditions. No decision has been taken yet as to which cells will be used for Rosetta. For the production and test phase, part of the Rosetta budget has been provided by ESTEC (the Dutch development section of ESA).

For technical reasons, 25 % efficiency is the maximum possible efficiency for solar cells; Dr. Strobl believes it will be difficult to provide solar cells with the same efficiency for the distance of Saturn as there is less light available.
Rosetta is to be equipped with 50 m² of solar panels. That’s the largest panels ever used for European satellites. In order to preserve energy for later use batteries are to be used.
Dr. Strobl pointed out that to his knowledge NASA declared that solar energy should be preferred for new deep space missions if this is technically feasable.
Most ASE solar cells for space applications are bought by governmental or private customers. They are mainly used for satellite missions near to the earth. Generally, deep space missions are very rare and are not financially rewarding (so far, Rosetta has been the only deep space project for ASE.)
Apart from space applications, ASE solar cells are also applied for terrestrial usage, for exapmle in solar car roofs.
Solar cells for Saturn missions have yet to be developed. Even when this has been achieved, NASA could not use them for the current Cassini mission under any circumstances. Cassini is so heavy that the additional weight of the solar panels would prevent any launch regardless of the type of launch rocket used. In concrete terms this means that a solar Saturn mission should be designed according to the amount of energy that could be provided by alternative electricity sources. Whether an appropriate mission would make sense from an economical point of view, Dr. Strobl cannot say.
There is too little money available to develop the next solar cells generation for deep space missions. The R&D budgets of ASE come from the profit of solar cells production as well as from governmental customers.
There is a considerable spin-off from solar cells for space applications to terrestrial applications, hardly ever visa-versa. The reason for this is that high efficiency and low weight are extremely important for space missions. Although this means that the solar cells for space missions are rather expensive, on the whole they reduce the price for space missions - and make them possible.

ASE has only a few competitors in the solar cell market for space missions. There is one competitor in Japan and two or three in the US. To date, ASE is the only company which is rigorously engaged in the field of solar cell production for space missions. As a result of the satellite technology, this market has been mostly commercialized. Yearly ASE output of solar cells for space missions amounts to appr. 500,000.

[Some comments in square brackets were included during translation to help non-German readers understand the context.]

Published online with permission of the author.

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Last modified September 22nd, 1997.
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