Not all solar coils created equal?

[Jaded]

I've been mooching about for a new vented cylinder but can't fathom why each supplier has similar recomendations about cylinder volume for panel size but apparently completely different standards regarding solar coil area.
What am I missing, sage advice welcome. 

[martin]

All coils are not equal - some are just coiled tube, some are finned (giving a larger surface area), some are less efficient because they're stainless steel instead of copper, copper thermal conductivity varies according to the "type", and then opinions vary as to what is the optimum area....
Personally, I'm lazy and have always followed the sage advice from the fellas at Newark who make the "house brand" tanks - they've never let me down yet - http://www.newarkcoppercylinder.co.uk/

[desperate]

Some coils use smooth pipe, some corrugated or finned, and some bundles of 10 or 8mm micro in parallel, I'm guessing but probably different designs call for different areas.

Desp

PS Martin types quicker than me

PPS welcome to the Forum.

[Brandon]

not equal indeed.

The requirement for a clear skies cylinder solar coil was that it was 0.58m²

some cylinders still use that size to this date, and they are often in stainless.

I use newark stores and cylinders a lot, as they fit a 1.2m² copper coil as a minimum (if you are installing what would be regarded as a larger than normal array, then speak to them, and they will put in a longer (and hence with greater area) coil.

whilst the stainless coil will work, it will not transfer as quickly, and therefore the panel return will be warmer, the panel will therefore get hotter for a give insolation and flow rate, this will help the heat transfer at the cylinder end (it being related to conductivity, surface area and delta T) but the system as a whole will be less efficient, as the heat loss through the pipes will also increase in line with the temperature for the same reason.

So the poor client who has the aforementioned stainless cylinder will see higher panel temps initial than his neighbour with the better unit, but an overall lower energy capture.

[Jaded]

Thanks for the quick replies and the welcome note.
Am I correct in thinking that there should be a correlation between solar coil area and the panel header area, I'm sure I saw that mentioned somewhere but have been unable to find it again?

 

Edit:

I guess you've allready anwered this question. On balance, solar sizing is all about following accepted practice and general guidelines.

[titan]

All coils are not equal - some are just coiled tube, some are finned (giving a larger surface area),

But how much better is a finned coil. Manufacturers figures are misleading, Newark includes the fins as the surface area of the coil which is incorrect. The surface area should be the area the heating fluid is in contact with. ie the internal surface area of the pipe. I was looking for a suitable cylinder for a heat pump, a few manufacturers quote 3m2 coils but they are including the fins the actual pipe contact area is 1.6m2. Has anyone seen performance figures of the various coil options.

[martin]

It's not wrong, dishonest or misleading in any way, they are quoting "surface area", not the internal capacity of the pipe - simple physics will show that if other factors are the same, more heat will pass from the coil into the surrounding liquid with a finned coil (with a larger surface area) than just a pipe....

[titan]

they are quoting "surface area", not the internal capacity of the pipe - simple physics will show that if other factors are the same, more heat will pass from the coil into the surrounding liquid with a finned coil (with a larger surface area) than just a pipe....

You clearly have problems reading I said " The surface area should be the area the heating fluid is in contact with. ie the internal surface area of the pipe not the internal capacity. I don't have a problem with fins  but I do when they include them as part of the coil area. Incidental other manufactures do produce 3m2" coils that is a coil with an internal surface area of 3m2 not Newark's 1.6m2 coil with fins described as 3m2. I would like to see some data to show how each coil compares, Newark can't provide it but they say it is the same how do they know.

[Brandon]

I have only ever seen newark state 1.6m²

[wookey]

Indeed, although finned pipe of 1.6m2 total surface area will have a slightly lower heat transfer rate than 1.6m2 of actual pipe. Not sure how much lower.

[bigj]

Does all the above meen that the 1m2 finned stainless steel coil in a DPS TS is pretty useless?

[Brandon]

But how much better is a finned coil. Manufacturers figures are misleading, Newark includes the fins as the surface area of the coil which is incorrect. The surface area should be the area the heating fluid is in contact with. ie the internal surface area of the pipe. I was looking for a suitable cylinder for a heat pump, a few manufacturers quote 3m2 coils but they are including the fins the actual pipe contact area is 1.6m2. Has anyone seen performance figures of the various coil options.

If it were only the surface in contact with the fluid that allowed heat transfer, then why do radiator manufactures bother with fins?  Be the CH rads or automobile type rads?

[titan]

If it were only the surface in contact with the fluid that allowed heat transfer, then why do radiator manufactures bother with fins?  Be the CH rads or automobile type rads?

Because fins help with the heat transfer increasing the surface area,  I am not disputing that, although both your examples are used in air which is a poor heat conductor. My comments are about  how the coils are described by various manufactures. If heat transfer rates were used for the description the surface area would be meaningless as it would account for different materials conductivity and finning. I spoke with most of the  manufactures about a cylinder for my heat pump, they all said they did special 3m2 coils but only one did have a 3m2 coil all the others had much smaller finned coils but none could provide any data to support how effective their designs were compared with a true 3m2 coil.

[Justme]

When I was looking for a TS / multi use cylinder I contacted lots of makers. I was looking for a bespoke unit so wanted to have usage performance stats for their units.

NONE of them would quote what the output dhw temperature would be at a fixed flow rate through a unit of X deg C evenly through the tank with out adding the complexity of stratification.

Now that cant be that hard to computer model can it?

The closest I got was from a Navitron employee saying that it would work as their system worked at 60c with a smaller dhw coil.

I reckon I could get close just using the real world usage of our 20 tube SFD unit I have, as we know that a tank temp of 50c will let you have 2 hot showers with reasonable flow rates but is to cold for filling the sink fast to wash up (but I do like washing up water to be so hot most people would need gloves and cooling it down some).

[Brandon]

most coils are rated at W/K, allowing you to do the extrapolation required for your desired delta T and therefore your optimum flowrate.

Some store manufactures will say it is an x kW coil as a head line, but then you need to look mat the primary and secondary temperatures, and the flow rates.  This is where a lot of the misinformation comes from in the anti thermal store camp, they believe that you absolutely MUST maintain your store at 85^oC as that is what the manufactures say, (indeed they do say that , but merely to be able to claim say a 30l/min flow at x ^oC).  Obviously a lower primary temp will suffice if you are content with either a lower flow rate, lower temp or both.