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And before you get too exercised, please read the post, date 9 Feb 2006, titled "All Tools Suck".

Tuesday, May 06, 2008

Help Me Learn: How to Design a Solar Charging CIrcuit

I have an general interest in sustainable power systems (my home has a 5-star rating and a 3Kw PV system) but I am not an electrical engineer and have no useful understanding of electrical circuit design beyond very basic stuff (I know what a resister is but I couldn't reliably tell you how resistance relates to current and voltage).

I have two projects in mind that I'd like to pursue, both of which require a bit more knowledge than I have and I have no idea where to go to get the knowledge--all of my resources are focused on small-scale electronics (digital circuits, basic occilators, etc.).

My first project is to create a water feature that is variously powered by wind, solar, humans, etc. where all the different power sources contribute to charging a storage system which then drives an electrically-powered pump of some sort. I'm thinking of something like a 6-volt marine battery, something that can hold a good charge and produce enough current to drive a beefy motor.

What I don't know is how to design a charging circuit that will feed the battery from multiple input sources.

The other project I'm thinking about is modifying an RV to be electrically driven so that it could be, as much as possible, solar powered (e.g., for traveling about the American West during summer). That is, building an electric RV that would run off batteries for cruising and be recharged by a combination of solar, auxiliary generator (presumably a diesel engine that could run on waste vegetable oil or the most ecologically sound fuel available at the moment), or grid connection when parked.

It would need to enable a 200- to 300-mile range on a single charge to account for the (almost) worst case where you have no solar input and must recharge overnight from a campground. The worst case is no solar input and no grid access, so you'd have to run the generator in order to get to the nearest power source (or wait out the clouds without your beer getting too warm).

Some obvious questions are:

- Assuming an Airstream RV (chosen to minimize drag, even though they're frightfully expensive), how much energy would be required to provide a 200-mile range at 55 MPH?

- Assuming a more affordable typical RV, what would the cost from drag be?

- Given current solar panel technology, what output could be expected from the maximum area one could reasonably attach to an Airstream? Would it make sense to include some sort of fold-out panel system for use when parked (e.g., you're stopped for the afternoon at some tourist spot)?

- Assuming worst case of no solar input and no access to the grid, what size of generator would be needed to enable direct operation of the vehicle at say 40 MPH?

All of this would go to answering the first question, which is "is this even practical with today's generally-available and affordable technology?" If the answer to that is "no", then what advances would be required to make it affordable?

We could start with the presumption of a 50,000 USD budget, which is about what it costs to buy a full-sized conventional RV. So if I bought a used one and refit it, could that even be done for that budget?

Another consideration is the value of not buying fuel. With gasoline pushing 4.00USD a gallon and diesel already over that as of May 2008, a 3000-mile trip at say 8 MPG starts to add up pretty fast. That's roughly 1500 USD in fuel costs for that trip. At 4 dollars a gallon, I can recoup 15,000 USD in investment in 10 years of driving vacations. If fuel was at European rates that payback would of course be much higher (and it seems reasonable to expect that U.S. fuel will climb to approach European rates over the next 10 years simply because of both market pressures and increasing social acceptance of the true cost of our life styles in the face of global warming).

So I'm wondering if anyone can provide pointers to resources, online or otherwise, where I could start developing the necessary knowledge to start answering these questions?

I don't think any of this is particularly challenging from either a design or implementation aspect, I just have no idea how to go about learning about it efficiently....