Hacking the solar?

Thanks - I hadn't seen that graph and it makes sense that the panel can't produce max voltage at max amps.

A couple of additional thoughts;
1) I don't think we can float charge the 12V battery; even a light float charge could be a few hundred milliamps and this could drag the panel output too low and impact the normal vent fan operation.
2) We should be able to charge a secondary small gel cell battery
3) We could also run that beanie cap propeller that was mentioned earlier however, that's a little too geeky even for me.

So, yes, essentially, I believe that we can connect to the panel output with a conventional wire tap, route that voltage to a standard 12V solar controller (like the 7A one linked above) and use that controller to charge a secondary 12V battery. If we keep the average current draw below a few hundred milliamps, we shouldn't impact the standard panel functionality. It'll never now we're there.

As for what to do with that secondary battery...
1) Route to a new 12V power outlet for continuous charging of small electronics.
2) ??? thoughts ???

 

PS: any know what a typical float charger draws when topping off a 12V car battery?? I know that they try to maintain a fixed voltage but don't know the current that they output. I'm guessing that to it's in the 300 to 1000mA range for a 80AHr car battery.

 

There is an interesting description of the Prius solar panel system at the Kyocera corporate website Reliable Solar Power Products Born from Our Technology and Quality | KYOCERA Solar Report | KYOCERA

The Kyocera KC65T is the most similar standard flat panel that I could find. The specification sheet is at http://www.kyocerasolar.com/pdf/specsheets/KC65T.pdf

That panel is a little more powerful than the Prius unit, and obviously a different shape. But the electrical performance is quite close.

The power output of a solar panel is quite dynamic with regard to the amount of light available, the temperature, the orientation of the panel to the light, and the electrical load placed on it. It's certainly not like a lead-acid battery that puts out 12V to 13V over a wide range of loading.

The specification shows that the panel can operate from 0V (short circuit) to 21.7V (open circuit). There is a non-linear relationship between the output voltage and the available current. Maximum power is specified as occurring at 17.4V and 3.75A (65 watts). That's at STC, Standard Test Conditions. The more real world values are 15.3V and 3.01A (46 watts) at 800W/m2. Actually even that is optimistic since it assumes a temperature of 25 degrees C. The voltage drops to about 12V at 75 degrees C, bringing the output down to maybe 38 watts. That's for a flat panel perpendicular to the sun. So the real world production of a "65 watt" panel is usually below 40 watts. Note also that in the dark all solar panels output zero watts. It doesn't matter that it's still hot at 8:00 PM, no panel is going to produce significant power at that time of day.

It would be really interesting to know what the typical power output of the Prius solar panel is. My guess is that it is useful between maybe 10 watts and 40 watts. Below 10, the fan won't move, and above 40 is just not going to happen that often. Feel free to contribute real numbers to replace my WAGs...

 

RobH, you have it about dialed. In the solar business, my rule of thumb is that a Pv panel will put out it's name plate rating*the number of available hours of GOOD sun(that is perfect, no shadow, no shade, with the panel in the proper orientation south, and not too hot, typically around 4 hours a day) divided by 2(the 2 is the total system efficiency losses, charging ef, controller ef, wiring losses and inverter losses. So an 80 watt panel might put out ~ 80*4/2=160 wh/day, or enough to light a 15 watt cfl bulb for about 10 hours. I would guess that the Pv on the car doesn't average the full four hours due to shadows, shading, mis-orientation and most probably over temperature. Unknown by most folks is that even the shadow of a telephone wire can drop panel output to effectively zero! Pv output drops off dramatically with temperature (and goes up at the same rate as the temp drops). Most Pv specs are rated at 75f, and are installed with at least 6" of air space under them to help keep them cool, something the Prius roof option doesn't allow for. In the real world I would guess that the output would be considerably less than my calcs would indicate.

Just for the record,, I don't know what the solar option on the Gen III costs,, but a good quality name brand solar panel from a reputable dealer can be had for ~$2.58-3.00 a watt. So an 80 watt panel would cost ~$240. Somewhere I heard that the solar option was~$1500. so for the same $1500 I could by ~500 watts of Pv for my house,, or ~300 watts installed.

Personally as a way to "go green" it is a no brainer. Save the money on the car Pv and put it into something else. The net result would be much better use of money to reduce greenhouse gasses or reliance on fossil fuel.

Just my opinion,

Icarus

 

Almost certainly correct - a cogent and well reasoned analysis. On the other hand, it's a pretty neato way to keep the interior of your car a bit cooler And in all seriousness, there must be some societal advantage - admittedly vague and difficult to quantify, but real - in raising awareness and 'mainstreaming' alternative energy ideas.

 

The panel info did not come from Toyota. Toyota specifies it's a 58-60 watt panel.

Wayne

 

I don't disagree with the "neato" factor, as well as the mainstreaming idea. That said, it just seems a bit silly. Why not for example to spend the time/energy/money to engineer what is in essence a toy, spend the same time/energy/money for a solution with more benefit.

A simple Pv powered vent fan, hanging in an open window might move as much air, while parked and could probably be bought retail for under $50.

Icarus

 

Not in my experience. The sheer amount of power available from the built in panel, compared to the trivial amount in the tiny window units really puts this one in a whole different league. I'll grant you there are probably more efficient uses of materials than this panel, but it would be tough to beat for effectiveness in terms of cooling the interior. And when you figure the fuel saved by not having to run the AC to get that heat out later, it might not be such a frivolous use of resources as it first appears. One would have to actually run the numbers to know of course, but it is not an insignificant amount of energy to run the AC to have the same effect. I am a mechanical engineer with a specialty in heat transfer, so I may well run the analysis at some point. But for now, I'm just too busy enjoying my new car

 

From a technical and engineering point of view it would be interesting to see the numbers. If for example you parked the car in full sun, put up a silver windowshield cover, tinted the window, AND installed a small CFM solar window fan to change the air X times per hour, it would be curios to see the net heat gain, and the amount of btus of gasoline energy required to bring the car back to some base line temp.

Let's assume for the sake of this argument that the Pv roof maintains the inside temp at exactly the same as the OAT.(say 85f). (not likely however)How many btus would be required to bring the temp down to say 70f? Now run the scenario with the small window fan, no tint no shade cover. What would the interior temp be after X time, and how many btus would be required to cool it to the same 70F.

My hunch is that while the Pv is a cool factor, I think I would rather spend my "cool factor" money somewhere else.

T.

 

Like I said, from a strict cost/benefit analysis of how best to use the planet's resources you could very well be right. But in that scenario, one has to look hard at the manufacturing of any new item, including the Prius itself. How many miles do you have to drive at 50mpg to make back the energy expended to mine, process, form, transport, and assemble all the unobtanium for the batteries, steel, glass, etc? If your turned in "clunker" got 18mpg, it may not even pay back the expended energy to make the Prius over the entire life of the car, compared with keeping that old hunk of iron running forever on replacement parts. So while I acknowledge that you may be right about Pv on the roof of the car vs. other uses of money, the fact remains that the "cool factor" matters - human psychology cannot be removed from the equation. And it is damn nice to get into the car on a brutally hot day, when you've forgotten to put in your windshield screen and accidentally left the sunroof privacy shade open, and not have the skin blistered off the back of your legs. I'm just sayin... I like it and I don't feel the slightest bit guilty about it

 

No,, you are absolutely right! It's cool, and there is no reason to feel guilty about it. The fact remains that early adopters push the technology and in time drive down the cost. GenI Prius owners would have had a hard time justifying the "savings". The reality is by being an early adopter it allowed us to by "mainstream cars.

If adding a Pv helps people understand the value of Pv then the "loss leader" effect is good. I was merely pointing out that for my money, I would have spent it on something else.

Good luck, enjoy your GenIII. We get ~53 mph average over ~50k miles and couldn't be happier. If Toyota would make a light duty pick up on a Prius type platform I would buy one in a heart beat, and retire my '82 VW caddy pickup.

Icarus

 

The solar option is $1800 but you get the moon roof, most cars the moonroof is $900 to $1000. So that makes the solar option about $500, if the panel is $240 and that makes it $260 for installation and the better fan, controls and wiring. Not bad?

 

Thanks for the info RE: cost of the Pv. Since I have no use for a moon roof I would still opt for no moon roof, no Pv.

Icarus

 

I think I can beat it by craking a window for 30 seconds as I drive away. Not to mention, cracking a window is FREE. Another alternative is park under shade! Not always possible, but again, FREE. I still like the *IDEA* of the Pv panels, just not the implementation. There has to be something else that can be done to use that power in a truly unique and unbeatable way. So far, charging a small 12v to use as a separate source for mobile electronics etc., is the best idea I've heard.

 

and also with a fully charged 12v battery, there is no need for the prius to charge it.
so a small amount of fuel saved.

 

I think you might be surprised how much thermal mass the interior of a car really has and how much heat it holds. By dissipating heat continuously (relatively speaking) over a longish period, the panel on the IV really does discharge a lot of watts... and it does it without sap dripping on the car or lowering fuel economy, as both other solutions do.

 

To get back on topic about how to use the available solar panel (not whether it's a good idea to have one) but assuming that you do and want to put that energy to use, what are some of the ideas.

1) Charge an auxillary 12V sealed lead acid battery and use that battery to charge in-car electronics versus pulling from the standard 12V system (thus reducing gas-generated energy). Probability of success: very high - it's an independant system and the car doesn't even know that we've tapped into the panel.

2) Attempt to float-charge the 12V car battery to reduce gas-generated charging energy. Probability of success: high but requires some thinking to assure we don't screw up any of the standard car electronics or charging systems.

3) Attempt to charge the motive battery. Probability of success: low - too difficult, too expensive, limited gain, good chance of really messing things up.

4) ??? Other thoughts

 

How about this?

5) Route the panel output to a plug and use it to charge a larger bank of off-vehicle batteries that could be used to charge or UPS a house?

 

Hmm - definitely possible but the more current we pull from the panel, the lower it's output voltage and the more impact we have on it's standard function. I'd be worried if we pull more that a few hundred milliamps and that won't make much of a dent in a string of car batteries. With a 40W panel and in the spirit of "flying below the radar" so the factory solar vent controller doesn't get impacted, we need to think light duty. UPS for computer would seem okay. Emergency lighting systems, security systems, outdoor safety lighting, sump pump back-up battery, etc. all are in the 5 to 15 AHr range and my gut tells me are doable. I would think that a standard car battery needs something like 500mA to 1A average current to keep it topped off and that feels like too much impact on the panel to avoid some type of interaction with the vent function.

I need help understanding what the float current requirement look like for various sized batteries. There must have a graph of battery AHr versus average float charge current around somewhere.

Please keep the thoughts coming.

 

Well that changes everything... I didn't realize we were trying not to impact the original function. I thought the premise was that there were other functions more worthy than running a fan so we would be substituting the new function for the old. Not having measured anything, this is total speculation, but given the way the fan cuts out every time a cloud goes by, I'd guess there isn't much excess to tap at all... maybe enough to run a digital clock. If anything, the panel seems undersized for it's assigned task and I really don't see how you could pull any appreciable current away and not lose significant fan function out: