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Wednesday, 24 February 2010 18:47
Page 5 of 6
4. Data analysis
4.1 Wiring ohmic loss
Wiring cables have been largely sized in order to minimise the ohmic losses. The resulting array resistance is calculated as about 30 mOhms Moreover, a HF filter including two coils in series had to be inserted in the coupling connections with the grid. (20 mOhm).The only way to estimate the real loss is to perform simulations over one year with different values. This shows that the overall ohmic loss is 1.5% for the cabling and the coils together.
4.2 Mismatch effects
The mismatch loss is strongly dependent on the operating voltage. The mismatch loss has been estimated at 4%.
4.3 Number of modules in series
The number of modules in series was optimised using PVSYST during the design, and was chosen as 20 modules. This optimum corresponds to the maximal yearly available production (or maximum system yearly efficiency).
With this optimum, the winter operating point will be below the MPP voltage, and during high irradiation in summer time it will pass over this point. The exact optimal balance of course requires the detailed simulations performed by PVSYST.
But by respect to design time, two voltage-sensitive parameters had to be revised according to measured data :
Fig.4 shows that with this new parameters, the optimal is now 21 modules in series, which should give a yield of 1002 kWh/kWp/year.
- The line voltage had been measured over a long time during an early PV experience on the same site (GAP, Univ. of Geneva), several years ago, and was found to be 630V on an average. No systematic measurements are available at the TPG, but they claim that they didn't modify the line. Nevertheless, the actual measured value is around 650V.
- For simulating the module temperature, we also used the usual thermal loss factor of 29 kW/m2K. But as discussed in section 4.4, on one hand the roof temperature increases with irradiation, and on the other hand the module ventilation is not so efficient as for tilted modules. Combining theses effects while using with true "meteorological" Tamb, the resulting equivalent k-value should be about 19 W/m2K.
Nevertheless the optimum curve is rather flat. The present setup of 20 modules in series should yield 989 kWh/kWp/yaer, i.e. a production loss of about 1.3% by respect to the optimum.
With Unom = 650V and k = 19 W/m2K. These simulations also quantifies the loss due to operating fixed voltage by respect to the MPP operation. This is limited to about 5000 kWh/year, i.e. about 3.3% of the production.
Last Updated on Thursday, 25 February 2010 13:25