Solar array design for solar car

[solar_car]

hello,
our team has a solar car project, and i am in charge of the solar array system. Now we have an average silicon solar cell of Voc 0.6 V and Isc 6 A, thus 3.5 W of power. We are given 6 sq. meter of area to cover with solar cells. This corresponds to around 2300 standard sized solar cells. We want to have a module of 36 cells and 2 bypass diode; this corresponds to Voc of 21.6 V, Isc of 6 A, and power of around 130 W. Our battery system has a minimum of 72 V, maximum of 126 V, and nominal of 100 V. So, we want each panels to have a minimum Voc of 150 V. This corresponds to around 7 module per panel. If we chain all modules in series we will have per module; Voc of 150V, Isc of 6 A, and power of 900 W. Our MPPT has variable input and output voltage ranges but is restricted to a minimum of 9 A input current.

Problem 1 > Given a module Voc of 21.6 V and Isc of 6 A, is there any kind of parallel and series module configuration to get Voc of 150 V and Isc of 9 A per panel.

My assumption and problem > Parallel configuration of 2 modules will output 12 A, we use a current divided circuit (w\ resistors) to reduce the 12 A to 9 A (max MPPT input current). But, say, if the parallel modules had total current output of 9 A together, we don't want to lose this current to the resistor. I have read in many articles of solar car teams using series and parallel module configuration to get desired Voc and Isc. It seems to me to be impossible arrange the module i have stated in parallel configuration while limiting the current to 9 A.

 

[gnurf]

Problem 1: You can either put your 6A/21.6V modules in series, which would increase the voltage in steps of 21.6V, or parallel, where the current increments in steps of 6A. Not much more to say about that I think.

If the output of your MPPT converter is 72V, and the input is 6A@120V (assuming the panel MPP is 80% of Voc), you should ideally (i.e., Pout = Pin) have around 6A*120V/72V = 10A charging current available. As the battery voltage increases you'll gradually lose this extra charging current. But, at least you'll maximize the system performance during the period you really need it (when the battery is flat). I don't know your battery charging requirements, but maybe there is some kind of trickle charging period towards the end of the charge when the battery approaches its maximum of 126V, and thus a lower charging current is not a big problem?

I know I've neglected your minimum 9A requirement, but maybe this will stimulate your thought process in some way.

Other random thoughts:
- Why is you're MPPT restricted to a minimum 9A of input?
- Dissipating power in resistors makes little sense (to me at least) in a MPPT system
- What about three strings of 6A solar power (for a total of 18A) and two batteries in parallel on the output (which would charge each battery with 9A)?

 

[solar_car]

1. The MPPT operation manual states maximum input current of 9 A.
2. The resistor current dividing circuit in between the MPPT and solar panel is not feasible. Although it can limit current to 9 A when current from panel is greater than 9 A, it will also eat up the extra current when the current from panel is lesser than 9 A ( loss of power).
3. we cannot put our battery bank in parallel because we need a nominal of 100 V for the motor controller.
4. There is a tricke charging period when battery approaches max voltage.
5. Is there any MPPT you know of for solar cars that have high range of input voltage, input current, and output voltage.
6. We know a solar cell will never reach it maximum Isc and Voc due to irradiance. Should we still perform our analysis, design, protection circuitry (fuses, contactors, relay) on the basis of rated Voc, Isc (rule of thumb: 20% greater than rated for protection/safety) or down it by couple percantage (like when you took 80% of Voc).

thanks for your reply by the way...