CURRENT TECHNOLOGY STATUS
Cells : There are currently 7 alternative solar cell technologies that are under development internationally. These can be categorized into non thin film and thin film. The non thin film will include concentrator, wafer silicon, grown silicon sheet and deposit silicon sheet. The thin film category includes single junction amorphous silicon, multi junction amorphous silicon and poly crystalline thin film based on cadmium and copper indium diselinide.
Spectacular developments have taken place in solar cells over the year resulting in increased efficiency and lowering of cost internationally. Out of the various technologies crystalline silicon (single crystalline and multi crystalline) and amorphous silicon have been commercialized.
The developments in single crystalline have reached a stage where efficiency of 24% have been achieved on small area cells. The most advanced developments is in the form of PERL (Passivated emmiter rear locally diffuse) cell. The other promising developments relate to the concept of laser grooved buried contact cells. On commercial scale, the cell efficiencies of 14% and above are routinely attained internationally. Against this the developments in India have achieved commercial efficiencies upto 11 to 12%.
In the multi crystalline cells, the highest efficiency achieved till date is of about 17.8 % (2x2 cm area) using high quality cast sub strates. In India multi crystalline cells of acceptable quality are not yet manufactured.
Amorphous silicon technology developments have revolved around quality manufacturing technology, low cost production and improvements in cell reliability. High film production type of machines have resulted in almost continuous feed throughs of the substrate materials. Single junction cells have reached efficiencies of 12 to 12.5% in small areas and 10.5% (on 30 x 40 cm). multijunction devices have reached efficiencies of 13.7% and large sub modules have surpassed 9% efficiency levels. Currently starting efficiencies of 10% after degradation can be realized on stacked cell structures. Panel efficiencies on commercial scale are available with stable efficiencies of 6% . In India the module efficiency on pilot scale have indicated efficiencies of 5%.
In poly crystalline thin film cells, the most advanced candidates at present are cadmium telluride and copper indium diselinide. The highest total area efficiency of 13.6% (1.08 sq. cm) has been achieved on cadmium telluride internationally. In the case of copper indium diselinide the highest active area cell efficiency of 14.1% has been reported (3.5 sq. cm). in India the developments are in very nascent stages.
For concentrator cells research results have indicated efficiencies of 34% for mechanically stacked GaAs-On-GasSb cells operated at 100 X. In silicon concentrator cell development, stable efficiencies of the order of 26%have been achieved at 100x (1x1.2 sq.cm) important progresses have been made in the development of concentrator components. The latest developments is in the design of point focused concentrator modules. In India the researches in concentrator cells have been minimal upto now.
Tremendous advances have been reported in tandem cell structures with noticeable efficiency improvement compared to their single junction counterparts with efficiency increase of above 15 to 17%. One of the new developments reported is in the area of spheroidal solar cells which may result in low cost cell. Current efficiency achievements are reported at11.3% efficiency.
BALANCE OF SYSTEMS : the most important BOS (Balance of systems) in SPV is electronic BOS. Internationally very reliable electronic BOS are available based on solid state circuitry and programmable microprocessor based systems with several features. The Indian electronic BOS are most prone to failures due to unreliability of component quality, lack of proper design features and poor tolerances.
STORANGE SYSTEM : In batteries, internationally various new kinds are being developed with minimum of 10 years life time, total maintenance free operations, high energy efficiency and deep discharge characteristics. These technologies are in the advanced stages of development. In India the use is mostly made of lead acid storage batteries. Some manufactures have recently come out with deep discharge tubular plate industrial battery version for PV applications. However, these do need frequent maintenance. Back