Solar powered fan

[Dustin Smith]

I'm building a box to collect solar heat. It will have two small fans 12vdc using 1.25w each. Each will be powered by a 1.5w 12vdc solar panel. So I will build two identical circuits to run independently. I want to delay the start up of the fan, but I will have to be able to calculate what values to change based on my delay needed. Maybe 5min, may 10 min. Not sure. Also, I will need a filter for the current because it may have some pulses in it as the solar panel is a battery tender type. I can use capacitor for filter, can I also use one for a delay? How do I calculate the values for filter and delay? Or should I use a different way for the delay?

 

[Dustin Smith]

Oh yea, and I will probably insert a voltage regulator cause it will put out from 12 to 15v from the solar panel.

 

[kellys_eye]

A 1.5W solar panel will only output the stated power in full, direct, sunlight perpendicular to the panel surface and, even then, only if it is from a recommended manufacturer or supplier.

No smoothing necessary - the output is pure DC.

A motor (fan) will likely be enough load to prevent over voltage situations so no regulation required either.

Not sure of your timing needs - not very well explained.

 

[Bluejets]

I'm intrigued just what solar heat collection in a box is all about.

 

[Dustin Smith]

The output of the panel has been measured at over 2w in full sun. I don't want the fan on full if the sun is not full on my heat box, I don't want to blow cooler air. I found a m9nostable circuit using a 555 timer that I think I can use to operate a mossfet which controls the fan delay. I will try to delay it about 8 min and see how much heat I gain in that time. A heat box as I've called it, is simply a box with metal inside that collects the heat from the sun, and while it can operate passively, they work more efficiently with some added air flow.

 

[Audioguru]

The datasheet for the LM555 shows a graph with a monostable delay of 100 seconds maximum but you might get 3 or 4 minutes with a larger capacitor than the 100uF shown. A digital timer circuit would have much longer more reliable times.

 

[duke37]

I do not know what you are trying to achieve. If it is to control temperature, then do this directly using a thermistor and a comparator.

 

[Alec_t]

I can't see a timed delay being useful, given the variability of sunlight. Duke's suggestion to control the fans on a temperature basis seems a better bet.
What are you going to do with the heat you collect?

 

[Audioguru]

I think he want to control the temperature by having the solar powered fan to blow away the sun. Then when it gets too cool the sun comes back. Oh, instead blow away the clouds?

 

[Fish4Fun]

Hey Dustin!

You are **WAY** over-thinking this Firstly direct sunlight has ~1kW per square meter in energy readily transformed to low grade heat. Calculating heat gain, temperature rise etc, etc requires all kinds of assumptions and still leads to some mind-numbing math, but is fairly easily measured ;-) Here is the short version: The Maximum Efficiency will occur when the difference between the "Inlet" temperature and "Outlet" temperature approaches 0. I know, you think if the inside of the box is really hot and the air coming out of the "Outlet" is really hot that you will get "more heat" ... for a fixed energy input that simply isn't the case. Using solar power for your fans can help maximize efficiency, but you don't need to worry about "delays" and sensors .... HOWEVER you may well need to increase the size of your fans, LoL (and solar panels) to get the most out of your passive solar heater.

As the temperature inside your solar collector rises above the outside ambient temperature the thermal losses increase exponentially. With a 1m^2 collection area in Direct Sun the temperature inside the box will rise until the thermal losses are in equilibrium with the input energy. You want enough air flow to keep the ambient temperature inside your collector as close to the ambient indoor temperature as possible. I know ... sounds crazy, but that is the way it works.

Fish

 

[Bluejets]

I'm intrigued just what solar heat collection in a box is all about.

mmmmmmm....my suspicions were correct.