However, if you are going to maximum efficiency, then you'd ideally also need a means of detecting the amount of insolation so that you could turn this feature off if there's not enough sun to get anything meaningful out of the panels.
The panels already detect the insolation for you, in terms of what flow rate is achieved at the required temperature (e.g. what percentage duty cycle the pump achieves) and the tank detects it in terms of the rate of increase of temperature at various heights.
No, you have not understood what I mean. Reading the temperatures for given flow rates etc doesn't tell you what you need to know - they only tell you what inputs you're getting for the given hydraulic parameters.
What you need to know is "given the amount of insolation, am I better running the collector at high temp and heating the top of the store, or running it at a lower temperature and heating the whole store ?"
Assuming you start off with the system "cold" and wait for the sun to come up, initially it would make sense to heat the whole store - because I'd imagine for a while you just wouldn't get anything meaningful out of the collector if you wait for it to get hot. But, without a (fairly complex ?) model of the system, you don't know what would happen if you switched modes - you can't go on header temperature since you're artificially holding it down by cooling it with the lower cylinder coil.
About the only proxy I can think of would be to look at the pump duty cycle, and rather crudely assume that :
When bottom heating, if pump duty/flow rate goes above some threshold (and the store top is not already hot enough) then you can probably switch to top heating.
When top heating, if pump duty/flow rate drops below some lower threshold (or the store top is hot enough) then you can probably switch to bottom heating.
It would probably do, but would not be optimal.
The focus on efficiency is misplaced, in my opinion. It is useful in a low solar-fraction system, but once the designed solar fraction increases it is best to optimise usable heat (i.e. exergy) rather than any old heat (energy) in my view. And that's what top-loading does.
Either you gain more heat that you are using, in which case I can see case for top-down heating - but you'll still end up heating the whole store.
Or you'll gain less heat than you are using, in which case you'll be topping it up and all heat gained will subtract from your alternative source of heat. In such cases, you'll be topping up, so if you go for the higher efficiency option then you do less topping up.
Again it comes down to knowing how the output of the panels varies with collector temperature for differing amounts of insolation - do Navitron have such information for their systems ?
The information is available for *every* panel. That's what is printed on the certificates. That's why the certificates exist. There's a reasonable explanation of how they are used at
http://www.apricus.com/html/solar_collector_efficiency.htmI think it's well worth spending some time playing with the numbers from different collector types whilst varying the light levels, to get a feel for what the parameters mean.
Great, but I'll need to look at that when I've got time to get my head round it - and get the puter to plot a few graphs so I can visualise the variables I'm interested in.
