Electronic – Large Active Area phototransistors? And can solar cells be a replacement

I do not know what "blue cell which looks like cracked glass" you are talking about, but I have seen cells on sale rated up to 173 W/m² nominal power - these where best quality, selected cells. The ratings are however usually given as open circuit voltage times short circuit current, and even with the best cells, the most power you are ever going to get even with the cell facing directly the sun, is 70% of that, which is about 120 W/m². But that nominal power you are only going to get at full sunlight with the cell facing exactly in the direction of the sun. Yesterday I was making tests with some small cell around 14:00, and when the cell was in horizontal position, I was able to get about 67% of the maximum power it had facing the sun. However when the cell was in shade, the output was only about 6% of the maximum power.

If you are interested in getting 12 V & 40 A only at clear sky, if you buy the best quality cell of rated nominal power 173 W/m² (real maximum power 120 W/m²), the rough calculation would be:

$$12 \mbox{ V} \times 40 \mbox{ A} = 480 \mbox{ W}$$

$$\frac{480 \mbox{ W}}{120 \mbox{ W}/\mbox{ m}^2 \times 67\%} \approx 6 \mbox{ m}^2$$

That might fit on a van if it is a moderately large one, but if you want it to provide that power also when there are clouds, or any shade, you will need a panel at least 10 times as large. If you don't buy the highest quality selected cells, but go for most common panels, they have only 114 W/m² nominal rated power, the required area will be 50% more.

If you would want to supply that 480 W round the clock, you will need batteries, account for the loss of power while charging (10% - 50% depending on what batteries and charge controllers you use), and take into account that sun only shines a small part of the day.

I read that from a panel in horizontal position, in London, UK, in December, you get on average 0.434 Wh of energy per each nominal W of panel power per day. In July the value is about 10 times as much Wh per day.

That means to provide 480 W continuously even in December, you would need a panel of nominal power 26544 W, which would be at least 154 m² in size. And that's not taking into account loss on charging the battery.

The solar cells will provide some current when illuminated. This can be applied to charge the battery. It doesn't matter if something else is connected to the battery at the same time that is drawing current, you still get to use the power from the solar cells effectively. In fact, it's more efficient to power the end load directly than to first store the energy in a battery and later have the battery power the load. The point of the battery is to be able to use the power when it's needed, not just when the solar cells can provide it.

For example, let's say the solar cells can produce 1A in full sunlight, and this 1A can be applied to charging the battery. Now let's say a load is turned on that draws 400mA. The 1A from the solar cell will be split so that 400mA drives the load, leaving 600mA left to charge the battery. This is perfectly fine. If the load were more than the solar cell current, this would still work. Let's say the load takes 1.5A. 1A of that would come straight from the solar cell, and 500mA from the battery.

In other words, there is nothing wrong with the solar cell current relieving some of the load on the battery as apposed to charging the battery and then having the battery supply the load current. As I said before, that is actually more efficient since there is some loss in storing and the retreiving energy from the battery.

Knowing when the battery is full and what to do about it is a separate topic. Some battery types are quite fussy about how they are charged. For those you will need a active circuit to monitor the battery voltage and possibly temperature to decide what current to charge it with or what voltage to hold it at.

Lead-acid batteries are pretty forgiving in this area and don't have a problem with continuous charging even when full. The simplest solution is to arrange and size the solar cells such that they can only produce the maximum float voltage at the maximum current the battery can take when full. Then you can just connect the solar cells accross the battery and be done with it. This will charge the battery much more slowly than it is capable of when it is low, but is a simple setup guaranteed not to damage it.

If the solar panel size or cost is important, you need to use the energy from it more efficiently. In that case I'd use a switching power supply driven by a microcontroller that takes into account what current the solar cells can put out, the battery voltage, and possibly the battery temperature to decide what current, if any, to charge it with. This gets complicated. You have to read the battery datasheet carefully and implement the charging regime accordingly.