What innovations does Power and Energy Conversion involve?
The Power Systems Division carries out a number of R&D activities, with different strategies according to the various components of a spacecraft power supply.
Historically and still now, the space industry has been the driver of solar cells development and the current triple junction cells offering a 30% efficiency under out-of-atmosphere conditions are the best performing products. This area is also characterised by a permanent search for increased performance, which does not come as a surprise: the most prominent feature of a spacecraft is its solar generator, so better efficiency means reduced size and mass, both beneficial to competitiveness. The Division is therefore driving and coordinating the European efforts in this direction, by supporting the development of cells including 4 to 6 junctions. This increase in junction numbers is justified by the fact that each one can be tuned to optimally convert into electricity a "slice" of the solar spectrum (i.e. a color, if we include infrared and ultraviolet in colors). Efficiencies close to 40 % are not out of reach in a 5 to 10 years horizon.
A solar generator is not made only of solar cells but also of structural parts, mechanisms, harness, ... and in these domains, too, the Division is supporting the industry in optimising their products toward more and more watts per kilogram.
Opposite to photovoltaics, the terrestrial applications of high performance batteries are so numerous and so demanding that they justify worldwide a R&D budget orders of magnitude the one ESA can afford. As a consequence the Division focusses on what is necessary to turn a terrestrial product into a space qualified one and, punctually, investigates the use of advanced materials or processes which cost cannot be afforded by terrestrial batteries but could be by space systems, provided that it is overcome by the saving on mass, i.e. on launch cost. This was typically the case, in the mid 80s, for the Nickel-Hydrogen technology, extremely expensive at battery level but allowing to divide the mass by 2 with respect to the previous Nickel-Cadmium. Today Li-ion has ruled out NiH2 and is still making significant progress but technologies like Lithium-sulfur (LiS) are serious candidates for its replacement.
Power conditioning, distribution and conversion
This field is extremely large, ranging from components to circuits, units and architectures. As a consequence a number of different strategies are used, which can just be exemplified here:
- On parts level, the development of radiation resistant integrated circuits, implementing the most repetitive functions found in power units, like distribution by electronic current limiters or Pulse Width Modulation regulator for converters. This reduces the effort for designing a unit as well as this of ESA for review.
- At circuit level, the promotion of recurring schematics avoiding to re-invent, project after project, the same functionality with a different implementation each time, with the same benefit as above
- At unit level the accent is made on the standardisation of the power interfaces, so that units manufacturer and user can rely on a stable and well defined specification
- The Division works at exploring and, when found interesting, promoting the implementation of new architectures and/or new topologies. The solar array Maximum Power Point Tracker developed for Rosetta or the B2R topology patented in 2011 are a good example of this approach, enabling or improving a full class of missions.
For missions beyond the Jupiter orbit or at the surface of bodies where illumination conditions are poor (e.g. Mars polar regions or our Moon, with its two weeks long darkness periods) the combination of solar generator and batteries is no longer effective. Nuclear sources remain the only available solution and ESA has undertaken in 2009 the preliminary development of thermal and power sources based on a radioisotopic heat source of Americium 241.
Last update: 17 March 2015