Retro fit reversal - More efficient?

[Ripstop]

I have been talking over this with my friendly installer for a while now and am sure I can get much better efficiencies from my panel. At the moment I can just about push 2KWh at peak exchange under ideal conditions. The panel is 30 tube * 58mm but I reckon from doing some fag packet calculations this is only operating at about 66% efficiency (don't ask me to show the calculations    So in effect I am not extracting all I can into the tank.

I have a unique retro heat exchanger based on a shell and tube design but without the shell if you get what I mean. The tank is a SS 220L with the retro slotted in at the bottom, horizontally where the immersion element used to be. I worked out a mates East/West efficiency and he was getting 85% but then he does think that its output in KWh is a bit high, but thats much better than me.

So we were thinking.....what if we were to swap the Solar circuit and push it through the much more efficient boiler exchanger element in the tank which is coiled neatly round the bottom of the tank and then plumb the bolier through the retro element?

I think this would mean much better extraction rates into the cylinder and better thermal circulation with less stratification.

Is this madness or not?

Cheers
Rip.

[knighty]

the problem is (I guess) whatever efficency you gain for the solar coil.... you'll loose for your boiler ?  also might cause the boiler to shorty cycle - it turning off and on again all the time because it's outputting more heat then the retro fit coil can dump ?

can you not get a better retro fit col ?  one with more coils in it / more surface area etc.. ?

[dhaslam]

What are the temperature readings on a sunny day, ie is there a very big difference between the panel and the lower sensor on the tank?     Also what setting are you using on the pump?

[knighty]

also.... is it set up right so the controller knows the coil is in the bottom of the tank ?   sensors the right way around etc.. ?


at a guess... if the top of the tank is hotter than the bottom, and hotter than the pannels.... when really there might be lots of decent heat you can capture for the bottom of the tank ?

[ericw]

At the moment I can just about push 2KWh at peak exchange under ideal conditions. The panel is 30 tube * 58mm but I reckon from doing some fag packet calculations this is only operating at about 66% efficiency (don't ask me to show the calculations    So in effect I am not extracting all I can into the tank.

Where is the energy that is coming in to the system from the solar panel going if it is not going into your cylinder? 
As it cannot violate the laws of physics and simply disappear it can only be being lost through the insulation. 
The only ways you can reduce these losses is to increase the insulation or reduce the temperatures of the solar loop.
Unless you have a particularly poor coil, which needs a higher than normal temperature difference between the solar loop temperatures and the surrounding water in the cylinder to transfer this energy, the major component of mean temperature of the solar loop is set by the water surrounding the coil. So there is very little you can do to alter the situation by tweaking the coil.

[Ripstop]

Ok...today the readings were this...

Panel reached 83.6, bottom cylinder was 55 and transfer temp was 74 degrees. I always get about 10 degrees diff between the into cylinder value and TRF or outgoing cylinder value. Cylinder ended up at 60 top to bottom. I produced  7KWh's today which could have been more I guess if I had had a lower low cylinder value.

Even if the bottom of the cylinder is cold at say 20 degrees the panel may reach 40 or 50 and the TRF (cyl out value) will always be 10 degrees less than the panel.

Am I expecting too much?

The other issue is one of thermal currents in the tank. I am wondering if using the cylinder main coil is more effective than the retro coil? And does that actually make any difference.

Thanks for the comments....Its nice to get a balanced view
Cheers
Rip.

[KLD]

Rip,
Staying with Eric's physics perspective:

For your current coil's transfers efficiency, heat is put into the tank at a rate that depends on the difference of the average coil temp and the average surrounding tank temp. A quick sanity check: let's assume there is no loss between panel and tank, then your see a drop of 84 - 74 = 10°C across  the coil. At 3L/min flow rate this would translate to 2.1kW heating power. This is certainly in the right ball park. Trouble is, with your solar loop temp that hot, the losses through the pipe insulation between panel and tank will not be zero. Your coil, though, needs the 80 - 55 = 25°C delta T to transfer the 2.1kW. Short of changing the coil, there is nothing much you can do. However, the boiler coil -- if it's not a fast-recovery coil -- probably has the same issue.

So, you could: swap your special solar coil against another retro coil  with larger surface area; disconnect the boiler and use both coils for solar; put a dedicated solar tank in; just put up with it and enjoy the free heat you get! I know it's annoying from an engineering point of view, but then I suppose that's the compromise you make with a retrofit versus a dedicated system design.

Klaus

[dhaslam]

It takes roughly one kwh for every four degrees rise in temperature of your cylinder.   So if you heat  the cylinder to  60C from 10C you  have an input  of 12kwh and that is  without allowing for any hot water used.   You should be able to adjust the flow rate setting on the controller that  gives you an output figure close to the  actual.    It looks like your readings are only 50% of the actual output at present. 

[Ripstop]

Hmm thinking about what you have said....I have 12mm SS corrugated tubing on a 10 -12m run from the panel to the tank. I consider the insulation to be average or better than average. I have about 19mm on 50% of the runs and about 25mm on the other 50% due to double lagging.

So potentially my losses are worse than I thought. However according to Ericw its all going somewhere so maybe it is ALL going into the tank and my measuring is somewhat flawed. I need to reassess my system dynamics and check my measurements. I think that the output of my retro is dependent on both the ambient cylinder temp and circuit temperature. I am wondering if it has better dynamics at lower ambient cyliner temps.

Oh and btw I am running at 3l/min max 100% flow rate. My best ever day was 25/5/09 and I got 17.89 KWh which I thought was very good.

Thanks for the coments and I need to think a bit more.
Cheers
Rip.

[ericw]

Rip,

I've you are doing calculations on your system then this old thread might be of interest http://www.navitron.org.uk/forum/index.php/topic,5204.0.html
(I'm not sure anyone has checked it out so it could be a load of tosh)

It was set up for 20x47 tubes but as quick bodge you could change it to 40 tubes which would be near to your system. Or you could put in the proper numbers fom the SPF test results (www.spf.ch and search the Collectors section for Navitron)

It doesn't try to work out what is happening in the cylinder so you would need to do several runs at different starting temperatures to see what happens as the cylinder warms up.

It assumes that you have constant insulation thickness so you would need to kludge a way around that - the pipe loss figure output is deceptively small but when multiplied by the pipe length and temp difference the effect is significant.

[evan]

At the moment I can just about push 2KWh at peak exchange under ideal conditions. The panel is 30 tube * 58mm but I reckon from doing some fag packet calculations this is only operating at about 66% efficiency (don't ask me to show the calculations    So in effect I am not extracting all I can into the tank.

Where is the energy that is coming in to the system from the solar panel going if it is not going into your cylinder? 
As it cannot violate the laws of physics and simply disappear it can only be being lost through the insulation. 

I think the word "efficiency" might be misleading here.  It's more like "utilisation".  I.e., the panels and plumbing are working, but are heating a small volume of water to a high temperature.  The same system heating a larger volume of water (i.e. using a coil lower in the tank) will get more energy out.