What should be the optimum DC to AC ratio in a solar power PV plant?


Many times solar project developers asks this question that,how much should be the DC installed capacity  to get  a 5MW AC output from the plant .As a rule of thumb a typical plant lose about 10 % of its energy while converting this energy from DC to AC .In case of NVVN (NTPC Vidyut Vyapar Nigam Ltd) PPA. The capping is not on the installed capacity, it is on the generated energy and the PPA restricts the developer to inject energy beyond 21 % PLF. For example  a typical 5 MW plant  can generate maximum energy  9198Mwh  per year and anything generated beyond this capping will not be paid by NVVN as part of the PPA . This pose a question that ,why developer should spend extra money on modules if they are not going to be paid beyond certain limit of energy generated .At the same time many developers feel that they have invested a lot of money in Land ,Power evacuation and BOS systems (Inverter ,Transformer ,switch-gear etc..).If they want to maximize there profits ,they need to be consider some more investments by adding few extra modules to maximize there output from the investment. They have anyway done in the ac side of the system .The below graph shows yield of a typical 1MW plant at different DC-AC ratio.

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DC to AC ratio Is governed by various factors apart from PPA for example in higher operating temperature of a PV plant at 50°C  will generate about 10 % less power as compared to plant operating at 25°C  .A typical 5 MW plant (rated output at 25°C  ) will utilize 4.45MW,if the same plant is operating at 50°C  (Radiance 1000W/m2) with temperature coefficient of -0.4%/°C  .{(5MW(1+(50°C  -25°C )(-0.4%/°C)  )}.

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Hence it is important to take in to consideration the operating temperature  of the side while deciding DC to AC power ratio of the plant. In case of above example it is assumed the plant used poly crystalline module with temperature coefficient of -0.4%/°C  .As shown in above figure even a -0.1%/°C  of the module temperature coefficient can reduce 2.5%  energy yield of a typical 5MW  plant. Hence while selecting an appropriate module the temperature coefficient should be taken in to consideration to optimize the power output of the plant.

By,

Dr Sanjay Vashishtha  & Rishikesh Muthyal

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