effectiveness, reliability and environmental resistivity silicon solar cells have prevailed upon various potential alternatives since about 20 years and their technical improvement is still progressing. A major goal in this area is to increase the solar cell efficiency. This can be achieved by two methods: • One method is to apply an optical reflector to the cell rear side to reflect the red portion of sun light which is not converted to electrical current. This method decreases the cell temperature in orbit by 10°C and has a corresponding efficiency increase of 5%. These cells are called BSR-cells (Back-Side- Fe Hector). • The second method is to apply a special doping procedure which generate an electric field in the silicon on the cells rear side. The effect of these BSF-cells (Back Side Field) is that the life time of the minority carriers is increased which again improves the cell efficiency. Finally both effects can be utilized simultaneously in BSFR-Cell. The efficiency of a BSFR-cell is approximately 14% (at 25°C). Due to the reduced radiation in LEO environment (the back side field is radiation sensitive), the BSFR-cell is a clear candidate for COLUMBUS. A very recent innovation in solar array technology at AEG is the other candidate for COLUMBUS. However, primarily for the option (foldable blanket). This innovation consists of a solar cell blanket which converts direct sunlight on the blanket front side, but is also sensitive for reflected sunlight from the earth (albedo) which significantly contributes to the total LEO radiation. Hence, the power generating elements are ‘bifacial' solar cells that are photoelectri- cally sensitive on front and rear side. These cells are bonded to an optically transparent solar array/blanket which allows the earth shine to penetrate into the solar cell from the blanket rear side. This increases the solar cell efficiency to approximately 16% (at 25°C). The final choice between these two cell types is subject to ongoing COLUMBUS study activities. 5. Power Conditioning and Energy Storage The subjects of this section are basic functions of the Primary Power Assembly as power conditioning, energy storage and the associated control of currents and voltages. All this is indispensable in order to convert solar array raw power such, that it is provided in a high quality version (regulated buses) to the users. A block diagram representing the basic topology of the Primary Power Assembly and its modularity is shown in Fig. 3. In sunlight the mainbus is regulated by a sequential switching shunt regulator (S3R) which controls all solar array sections by means of dump circuits which are connected across each section. In parallel the battery is charged via a Battery Charge Regulator (BCR) from the regulated mainbus. The BCR is a modulator buck regulator. The module dedicated current control loop is configured such that the battery is charged with constant power. For eclipse operation the battery is discharged via a modular Battery Discharge Regulator (BDR). The BDR works according to the SMART principle (series connection of buck- and push-pull stage). Again each module has its own current control loop in order to ensure well balanced current sharing.
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