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Analysis of Third Hybrid Test Bench Sensitivity Run

Michael M.D. Ross
RER Renewable Energy Research

Full Text Report
Link to CETC-Varennes

Acknowledgements:

Research conducted on behalf of the Photovoltaics and Hybrid Systems Program at the CETC-Varennes (Natural Resources Canada) with partial funding from the Panel on Energy Research and Development (PERD).

Citation:

Ross, Michael M. D. Analysis of Third Hybrid Test Bench Sensitivity Run. Report to CETC-Varennes (Natural Resources Canada). Montreal, Qc: RER Renewable Energy Research, 2006.

Summary:

The photovoltaic array and genset of the CETC hybrid system test bench supplied power to a variable load, via a battery when necessary, over a two month period. This test examined the operation of a hybrid system with a moderate solar fraction and relatively infrequent starting of the genset, achieved by a low genset start state-of-charge threshold and longer genset run times.

This test was the third in a series of “sensitivity” tests intended to investigate the influence of a range of common operational parameters—like solar fraction and the state-of-charge at which the genset starts and stops (see [Ross, 2005b]). In many ways, the test was similar to the second sensitivity test. In certain respects, however, it was different: the loose cell connection present in that test had been fixed; the load was lower; the genset start was based on a method for estimating the battery state-of-charge; the genset was preferentially started at 4 AM; and the load followed a diurnal pattern of variation throughout this test.

The genset ran 12 times in this test. From one run to the next there was significant variation in the energy and charge returned to the battery. For example, the maximum charge returned to the battery was 219 Ah, and the minimum was 112 Ah. Part of this variation can be explained by variation in the estimate of the state-of-charge at the time the genset was started. Further variation can be explained by error in the estimation of the state-of-charge. The remaining variation can largely be explained in terms of the effects of partial state-of-charge cycling. It is shown in this test that if the PV array raises the state-of-charge above the level achieved between the two prior genset runs, then when the genset runs it will tend to raise the battery state-of-charge to a higher level than during the previous genset run. Otherwise, the reverse will occur.

The high resistance of the connection between the array and the test bench results in the array having a more rounded IV curve, at least as seen from the battery. The nominal maximum power point voltage of the array is quite low, so as the battery state-of-charge and voltage rise, the array tends to operate at voltages above its maximum power point, especially when array temperatures are relatively high (e.g., 50 ºC in this test). Normally this would result in a significant decline in the array power reaching the load and the battery. In our system, wiring losses compensate in part for this decline: as array current declines, so do the wiring losses. This demonstrates how wiring losses may reduce the potential benefits of maximum power point tracking.

The decline in the charge and energy returned to the battery in each genset run of the second sensitivity test can be explained in terms of partial state-of-charge cycling, and it is unlikely that a loose cell connection was related to this decline. Similar behaviour is evidenced in the existing test.