solar losing heat

tipperary

Hi all

I am looking for some advice on heat losses in my solar system. I have a resol controller, and recently got a unit for it to record temperatures. I have graphed the recorded temperatures, and attached the graph for yesterday.

The graph shows the temperature at the base of the cylinder dropping sharply as the termperature of the solar panel drops, steadying off at about 22 degrees (probably around room temperature).

Is this temperature drop normal? Also, if anyone notices anything else strange from the graph that would be impacting on the performance of the panels I would appreciate your input.

shane0007

It looks like to do with the recording parameters of the controller. The top of your cylinder would not have dropped to zero degrees. That is an impossibility. I reckon the recordings switch off at that time and zero out. Check the actual temperature of the water temperature on the controller and check the next day recordings and see if they differ. If they do, then the above applies.

fclauson

I think the comment is about the 6pm/7pm temp drop

did some one use some hot water at that time - cold water is fed int othe bottom of the tank and so the lower sensor would pick this up while the top sensor would stay close to "hot" until the cold water reached it

quentingargan

Normally I would suspect some leakage on the non-return valve. However, there is a long period of falling temperature when the panel is still hotter than the cylinder. So that ain't it.

More likely is that there is heat being lost to the central heating coil, or being list at some point in the cylinder between the upper and lower sensors. The most common fault I have seen is that the central heating coil has exposed pipework losing heat, or that the pipework from the central heating siphons heat uphill. The pipes should, ideally, fall slightly as they come out of the cylinder. Feel these pipes and see if there is heat in them at a time when the heating has been off for sometime. (like 5.00pm).

I would suspect that this is also losing heat all day long. In recent weather the bottom of your cylinder should be a lot hotter, unless you have a very small solar array.

tipperary

Hi

thank you very much for all for your replies.

Yes, it is the temperature drop at around 6pm that I was referring to. There was some hot water being used around this time alright, so that could have been the reason. That does makes sense alright though. I will try to keep an eye on the temps and what's being using the hot water for a bit and see how things go.

Quentin, I didn't get a chance to check the boiler flow and return today before the heating came on, but will check these pipes during the week. In relation to the temperature of the cylinder, I have about 7m2 of flat panels on the roof, feeding into a 500 litre cyliner (thanks to the genius of a builder we had!). That said, the previous day the temperature had been 60 degrees at the bottom of the cylinder, before a similar drop in temp at around 6pm or 7pm.

Condenser

Your cut out temp for the solar pump may be set too low. The panels should be above a certain temp before the pump engages and should disengage once they drop below this temp at night. If its too low, the heat you generated during the day may be dumping back out your panels in the evening.

Buffman

fclauson wrote: »

did some one use some hot water at that time - cold water is fed int othe bottom of the tank and so the lower sensor would pick this up while the top sensor would stay close to "hot" until the cold water reached it

This would be my thinking on it too, I've noticed a good drop in the bottom temperature if somebody uses the hot water. As said, this is due to the cold feed feeding into the bottom, as even a small amount of CW entering can cause the temperature to drop.

Saying that, if this is a 'typical' graph and nobody is using the hot water at that time, it may be cause for concern. You could possibly test it by turning off the electric element for a day and seeing how the top cylinder temperature performs.

quentingargan

Condenser wrote: »

Your cut out temp for the solar pump may be set too low. The panels should be above a certain temp before the pump engages and should disengage once they drop below this temp at night. If its too low, the heat you generated during the day may be dumping back out your panels in the evening.

The graph shows the pump coming on and off, and the panel temperature is usually 4 to 8 degrees above the cylinder temperature when the pump is running, so I think all that end is OK.

I also don't think it is water use. The drop in temperature is gradual and almost linear. More interestingly, it seems to settle at about 22C which is close to room temperature, but a lot warmer than the panel.

I would look to heat loss within pipework still... Though diagnosis over the internet is a very approximate science:rolleyes:

shane0007

fclauson wrote: »

I think the comment is about the 6pm/7pm temp drop

did some one use some hot water at that time - cold water is fed int othe bottom of the tank and so the lower sensor would pick this up while the top sensor would stay close to "hot" until the cold water reached it

The temperature drop between 6 - 7 is only the panel temperature. The top of the cylinder does not drop until near midnight. As I said previously, check the temperature at midnight and see if they are the same. If not, it is as I suspect, the reading is zeroing out as it is only reading until midnight as a 24 hour cycle.
If you look at the graph, the cylinder temperature began at 60C and ends at zero. Check the next day and see if it is back at 60 or thereabouts or is it zero.
The pump is behaving as it should be. I would just question is if it is variable.
Your panel temperatures are a bit low. What was the weather like on the day of recording. If it was during the hot blaze we just had, then something is amiss. If there was leakage back through to the panel, the panel temperature would be higher at off times.
What make of panel do you have and perhaps you could post a photo of the installation within the hot press.

liveandnetural

hi tipp
what system are you using to record the temps and give you that graph

tipperary

Thanks all for your replies. Thanks to work commitments, I haven't been able to do much checking of the boiler pipes yet.

It is the drop in the bottom of the cylinder at around 6pm that I am concerned about. The drop at 12pm is just due to the way the excel sheet is set up - some days have one less reading than other days. The drop at this time of the day seems to happen most days, but then again we would be washing babies bottles etc around this time too. I will try to narrow down the possibilities over next weekend, and hopefully the weather will heat the cylinder up enough for me to be able to see any temp drops!

I have a Resol Deltasol solar controller, so I got a vbus/lan unit to allow the temps to be recorded. This saves the data as text files, so I used excel to generate the graphs.

shane0007

If the temperature drops at the bottom of the cylinder, this is due to a draw out of the top of the cylinder, i.e. opening a tap. This is perfectly normal.

championc

Hi Tipp

I have to say I am somewhat in agreement with Condenser.

Something doesn't quite add up for me. You say you have 7m2 of Flat Plates and you have a 500l store. With the top of your store recording 60 deg C, then this proves that you are not trying to heat the whole store with the Flat Plates - only a portion of it. My guess is that your store is connected to your CH and you have an upper coil on the store connected to the CH so this just leaves the lower part being heated by the FP's.

From what I can tell from zooming into your graph, the controller turns ON the pump when the temp sensor FP's are 5 deg above the bottom of tank temp and turns it OFF when just 2 deg above. I would recommend you changing this to 8 ON and 4 OFF so there is no way that any cool glycol can be circulated around you system.

In addition, with 7m2 on the roof, and with only one temp sensor, you suddenly have alot of heated liquid being circulated and so I would expect the temp to rise much higher at the roof sensor before it suddenly starts to fall. Remember, you have multiple collectors and it "should" take a good minute or two before the cooler liquid gets circulated around the system and finally hits your roof sensor.

Are your panels linked in Series / daisy chained (not parallel) i.e. One links into another into another ?

tipperary

Hi all

I managed to check the temp of the central heating pipes around 6pm, and none of them felt hotter than they should be. I have also been monitoring the exact time of the 6pm temp drop, and have concluded that as advised, it is just normal temp drop when hot water is being used. I had been thrown by the extent of the temp drop for the relatively little hot water used, but as has been said, even when just a small amount is used, it will be replaced by cold water at the bottom of the cylinder. I guess there is no way of telling how the temperature varies over the height of the tank.

championc, my cylinder is a triple coil cylinder, with solar connected at the bottom. Central heating and back boilder are connected about half way up. I have the heating set up to heat the top of the cylinder to 60 degrees every morning to ensure there is enough hot water for showers in the morning/during the day. I must try to fine tune the timing of this to make sure I get the greatest benefit from the solar heat.

I will try changing the pump on/off temps as suggested and see if it makes much difference.

The panels are linked in series.

Thanks everyone for your help on this.

quentingargan

Hmmm. Doesn't make sense to me. Were all of the pipes coming off the cylinder stone cold? Or just not piping hot?

The system seems to be hugely under-performing. Even though you have a 500L cylinder, it should be performing a lot better than it seems to be.

shane0007

Now that you have said you have a triple coil cylinder, I presume you have a solid fuel input. If so, where is the pipe stat located for the solid fuel circulating pump? Is it close to the cylinder?

tipperary

Hi. The pipes weren't stone cold, they felt to be around the ambient temp in the hot press.

Is there any websites that would give an idea of what sort of performance I should be getting at this time of year? I know it's probably hard to be definitive what with varying cloud cover and so on, but it would be nice to get some idea of what I should be expecting. I have attached a graph of today's performance (up to 10.30pm) which may or may not give more information.

quentingargan wrote: »

Hmmm. Doesn't make sense to me. Were all of the pipes coming off the cylinder stone cold? Or just not piping hot?

The system seems to be hugely under-performing. Even though you have a 500L cylinder, it should be performing a lot better than it seems to be.

tipperary

shane0007 wrote: »

Now that you have said you have a triple coil cylinder, I presume you have a solid fuel input. If so, where is the pipe stat located for the solid fuel circulating pump? Is it close to the cylinder?

Shane

the stat for the solid fuel pump is located beside the stove. Our house is an 'upside down' house, with the cylinder located downstairs, and the stove located upstairs. There are non-return valves fitted on the flow and return to the stove so I had kinda discounted this as being a source of heat loss - although I could be wrong.

shane0007

That is a whole other issue..... a solid fuel stove cannot be installed in that way!

How and ever, your graph is showing perfectly normal temperatures for both collector temperatures and top of cylinder temperatures for this time of year. I would not worry about performance from that graph.

If you wish to calculate the solar gain for your house you can do so here http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php#
You can enter the co-ordinates of your house with the angle of inclination of your roof, etc. It will only be an average but it will give you a fair idea. It is accurate to 10% over a 10 year period which is pretty spot on.

Once you have your energy (kwh) you can then calculate the temperature delta T.

KW = Volume x Specific Heat Capacity of Water x ΔT / Time (Secs)
KW = Volume x 4.186 x 60 (70 – 10) / 3,600 (1 hour)

or

Time (Secs) = Volume x Specific Heat Capacity of Water x ΔT / KW
Time (Secs) = Volume x 4.186 x 60 (70 – 10) / KW

shane0007

Forgot to mention, don't worry that the site is for solar pv, it gives accurate solar radiation based 1sqm over that particular designated zone, i.e. your house. From that, you can calculate the rest.

championc

Looking at your graph, how much of your 500l do you reckon you are heating ? You have raised the temp from 15 deg C to 55 deg C in less that 6 hours. It seems strange that it takes 2 hours for the incoming New water to register it's rising main temperature. I would have expected more of a sudden drop down.

One thing you could try is using the 6pm amount of water at noon to see the effect.

Another suggestion - can you monitor the mid tank temperature. If you always feed the top of the tank from 05:00 to 08:00 and expect it therefore always to be 60 deg C, then is there any point in monitoring it (other than for the control of a valve or control switch). The other puzzling thing here is that it has taken 3 hours to increase a small amount of water in the top of your tank by about 6 or 7 deg C.

Mothman

My 14yo 6m2 flat panels have met my demand for past 3 weeks or so. Have used no back up.
Demand includes daily bath, showers and washing up. Family of 5.

Looking at your graph I'd be concerned at the loss from top of tank during day. I put extra insulation around my tank to arrest the loses and paid extra attention pipes near tank.

Also when the bottom of my tank gets to near to tank top temp, the top of tank heats up. This didn't seem to happen in your later graph though perhaps there was too much cloud about to raise panel temp sufficiently on the day

championc

Mothman wrote: »

Looking at your graph I'd be concerned at the loss from top of tank during day

Looking at it Mothman, it loos like a loss of 1 deg C every 2 hours which doesn't seem too bad to me at all. What sort of loss rate do you have on your system ?

Mothman wrote: »

Also when the bottom of my tank gets to near to tank top temp, the top of tank heats up. This didn't seem to happen in your later graph though perhaps there was too much cloud about to raise panel temp sufficiently on the day

I was initially thinking that too Mothman, but it looks like the sun just stopped shining as it was about to reach parity


The only other comment for you Tipp is
You are using your CH to heat the top of your tank to 60 deg C - Daily. If you lose about 5 deg C between morning and midnight, and since scalding point is about 45 deg C, then there's no point in heating your store, with CH Cash Costing energy, any higher than 50 deg C. The only consequence of this action is less cold water being mixed with the Hot, at the Anti-Scald valve.

Mothman

championc wrote: »

Looking at it Mothman, it loos like a loss of 1 deg C every 2 hours which doesn't seem too bad to me at all. What sort of loss rate do you have on your system ?

Currently if top of tank is at 60C in evening, it is about 58C in morning. I haven't been around much since we stopped lighting stove 3 weeks ago so this is based on just a couple observations. I don't have monitoring equipment to out put graphs
I expect water to stay hot for 3 to 4 days which carries us through most cloudy spells.
The tank is a recent installation after the original failed. The insulation was very poor on the replacement, so much so that I couldn't keep my hand on the outside...it was too hot! I lagged it with 100mm of wool, 200mm on top.