sizing resistor for solar fan

Question

background; i'm an art student tinkering with solar. mostly learned from youtube and internet. i think i'm ok with hooking up solar panel with dc fan and dc light. no battery, no charge controller. now trying to refine my existing solar system, and trying to understand about resistors.

i connected two solar panel in parallel (at max; 400w, 48v, 10amp per solar panel) to 3 dc fan (rated 24v, 160w, 6.7amp each) in series. 100% DC system. no battery, no solar charge controller. direct connect. So the max voltage from solar panel is 48v on a bright sunny day. the fans reach up to 16v each (48v / 3). it works well. strong BUT noisy.

i would like to add a resistor to reduce the voltage of each fan from 16v to 10v. it will be a lot quieter. effectively reduce the voltage from 48v to 30v.

[note; i could add another fan (from 3 to 4 fan) to reduce the voltage but the size does not fit. aesthetically, a resistor is also more pleasing]

Question;

a) what size of resistor (i.e. watt and ohm) would achieve the voltage drop in this circuit.

b) does it matter where i put it on the circuit ?

c) is there any other way to drop the voltage other than adding a resistor ? [adding a 4th fan discussed above already]

all help / insight gratefully appreciated.

Answer 1

a) I will make the assumption that your fans behave linearly (ie current is proportional to voltage, ie they behave like a resistor (in steady state)).

I will also suppose the solar panels are a constant voltage source (not completely true, but it's the best approximation I can make without the full datasheet of your panels: the voltage usually drops somewhat when current increases)

So, in your current setup, you have 48V/3=16V per fan.

If your fans draw 6.3A at 24V, then they will draw I_16=6.316/24 = 4.2A at 16V, and I_10=6.310/24=2.6A at 10V

So if you have a total voltage U=48V, and 3 fans each at 10V, you have Ur=48-3*10=18V to drop in the resistor.

So the resistor has Ir=2.6A (same current through all components in series), and Ur=18V.

So according to Ohm's law, R=Ur/Ir=18/2.6= 6.9 Ω

The power dissipated in the resistor will be Pr = R * Ir²=6.9 * 2.6²=46.6W

So you need a resistor about 6.9 Ω, with a power rating above 46.6W

Please note that you should take some decent margin on the power rating, and make sure for what conditions it is rated (sometimes the rating is given for forced air circulation or with a big heat sink, sometimes at the rated power, the temperature of the resistor is several hundreds °C, ...). If in doubt, once you found a likely resistor (with a good datasheet), ask for confirmation that it will do.

b) No, it doesn't matters where you put it in the circuit, as long as it is in series with the fans (it can be between the + of the panel and the fans, between 2 fans, or between the fans and the -)

c) yes, there is another way : using a DC/DC converter, taking 48V as input and outputting 10V : the big pros are that the voltage will not drop if the panels are not producing 100% (as long as they provide enough power), and that your are not dissipating 50W in heat (I suppose you want the fans to feel cooler, so adding a heater in your home is contra-productive). As 10V is not a very usual output level, you will probably end up with a controller for which you can control the output voltage (and therefore the speed of your fans). The main cons is that it will be more expensive.

Other recommendations:

• be careful about solar panels, they produce a lot of power. Always put a blanket over them when doing connections (or do them at night)
• if you put the resistor outside, you will avoid heating your home
• if you add a power switch in parallel to the resistor, you can switch between quiet mode and more powerful but noisy. Just make sure the switch is rated for enough DC current (I insist on DC: many switches are not rated for DC at all, or only for lower currents).
• add a fuse near your solar panels
• the resistor is likely to get quite hot at best, very very hot at worse: make sure nobody can touch it, and nothing flammable is nearby
• you might want to add a little 24V fan in parallel to the resistor to cool it down (nb: if the fan draws 200mA max, it shouldn't influence much the voltage of the fans, otherwise, it's best to re-do the computation once you choose your little fan)