Hi everyone, I have a 30 tube split system with 300lt tank in central victoria. My question being if i set the solar return pump on the minimum setting my evap tube manifold reachs temps over 100 deg cel an keeps climbing now by doing this my hot water heats up nocibly faster an hot over a day as opposed to haveing the solar return pump run faster but heats the tank slower but only have the manifold temp 10/20 hoter than the tank temp. So by running my return pump slower so i dont cool down my manifold with the colder water comeing from the hot water tank am i doing damage to the manifold running it that hot?
solar evacuated tube optimal manifold operating temperature
(17 posts) (8 voices)-
vicsolaruser
MemberPosted Monday 18 Sep 2017 @ 4:16:21 am from IP # -
The science of heat collection is quite interesting. I have been playing with pump speeds as part of my solar hydronic system and am also finding best results with slower pumps speeds. That said, my temperatures are much lower than yours as my collectors are only ever fed water at about 22 degrees and at the output is somewhere in the low 30s so nowhere near the temps you describe.
As far as damage is concerned, the collectors are usually able to handle stagnation temperatures which can go far above 100 degrees C. I have seen my collectors at close to 180 degrees after some time sitting without water. I haven't noticed any difference in performance so I think they are ok.
Posted Monday 18 Sep 2017 @ 7:30:18 am from IP # -
arttt
MemberI would have though from a first principles basis that the lower the manifold temp the more efficent the heat transfer and the less losses from the pipes.... maybe.
There would be greater pump energy use at high flow rates.
Higher flow rates also mean the water in the tank travels through the collector many times and gets mixed in the tank so no stratification and therefore it takes longer to get hot water. Slower flow can deposit hot water at the top of the tank and it stays there ready for use straight away.Posted Monday 18 Sep 2017 @ 9:08:41 am from IP # -
100 will not damage the manifold slower pump speed allows the water to absorb more heat from the manifold for return to the tank where it will stratify.evac tubes work differently from flat plates as the heat transfer is from steam to liquid so is absorbed by the cooler circulating water very efficiently even at high manifold temps, the steam in the heat pipe will be many times hotter than the water it is heating.
Posted Monday 18 Sep 2017 @ 8:38:36 pm from IP # -
vicsolaruser
MemberOk people so by running my return pump on the slowest seeting my tank has heated up faster but like i said my manifold hit 100deg an kept riseing once the manifold temp hit 150deg on 15deg overcast day the pump has stoped due to over temp an now minifold at 190 an temp due no flow an the only way is to fix is wait till end of day or manualy get on roof an pull temp sensor out to cool down.
So after all that if run return pump on next setting i get constant twmp in manifold about 10/20 deg above my tank temp but loose an estimated 40% reduction in heating time. E.g 20+deg lost potential temp now the elec boster will have to do extra every day. So i conclude the pump speeds on lowara solar return pumps dont offer enough of a speed seeting range to allow people to gain all the potential efficiency out of there evac tube solar system a approx 40% loss of efficiency also in addition i think there is a need to adjust pump speeds between season's to maximize efficiency of evac tube systemsPosted Tuesday 19 Sep 2017 @ 2:36:50 am from IP # -
arttt
MemberI think you are mistaken if you think there is a 40% reduction in heating time.
This is what i think is happening.
With the fast pump setting you will be increasing the whole tank temp lets say from 20C to 40C.With the slow pump setting you will be increasing half the tank from 20C to 60C and the other half will stay and 20C.
the slow pump gives the impression of a tank of hot water sooner but they are both roughly just as efficient to get to a full tank of 60C water.
So the numbers are a guess but it gives you the idea of what is happening.
If the above isnt what you are seeing please try and explain how you are coming up with "40% loss of efficiency" and what data that is based on.
Posted Tuesday 19 Sep 2017 @ 3:31:18 am from IP # -
Benny
MemberIt does sound odd that at the slower pump speed the manifold gets a lot hotter and yet reportedly the main tank heats up quicker. If the manifold is getting hotter doesn't it suggest that more of the absorbed heat from the tubes is staying up there on the roof and not being transferred to the tank - so how would it heat up more quickly ? There will be greater heat losses from the manifold area if its a lot hotter.
arttt's explanation sounds reasonable.Posted Tuesday 19 Sep 2017 @ 3:47:48 am from IP # -
vicsolaruser
MemberYes i agree there is some temp stratification diff in the tank but eventually the heat spots dispersed to the rest of the tank but i am confidentin my calculations. the hole tank heats up faster an i have checked this useing multiple temp sensor ports over the tank to confirm this. the slower flow allows the manifold to heat up an keep heating up an not be cooled down by water from tank. At moment my pump goin at aprox 5ltr min on second lowest seeting with a return temp about 10/20 deg above tank temp slowly comeing down to within 5deg of tank temp causing the pump to stop wait for manifold to heat up again an start pumping for few minutes till it cools down again. now with the pump set at about 2. 5 lt a min the slowest seeting the manifold can heat up an continue to heat up with out the flow criticaly controling manifold temp.
Resulting in return temps 60deg+ diff between manifold an tank temps. yes there would be some heat loss from excces temps in manifold but wen the water retuning 3 or 4 times hoter at half the pace your already winning my problem being the temp continues to rise in manifold an not level out resulsting in the pump stoping due to over temp. Now i know this seeting is to slow because the temp keeps riseing out of control an doesnt level out over the course of a day, and the next seeting to fast because i can see a drop manifold temps. so this leads me to believe in my calculations that their could be better efficiency gained by by more variable pump speed options on lowara solar return pumps to beter suit peoples location,climate an season of the yearPosted Tuesday 19 Sep 2017 @ 5:01:01 am from IP # -
vicsolaruser
MemberMy calculations took into account many variables and were based testing the tank at 10min intervals over the couses of a day useing data from multible temp sesnsor ports, pump speeds , ambient temps cloud cover the diff being the slowest seting resulted in overall tank temp average increse of about 1 to 1.2 deg every 10min opposed to increase of aprox .6 to .8 deg every 10 min on the next fastest seeting
Posted Tuesday 19 Sep 2017 @ 5:21:14 am from IP # -
vicsolaruser
MemberYes the pump stop starting doesnt help things but I think is still proves there a big efficiency gains to be made in some evac tube systems if solar return pumps were more variable
Posted Tuesday 19 Sep 2017 @ 6:18:50 am from IP # -
Benny
MemberSounds like you're being very thorough vicsolar and I can't argue with your findings - although a bit more punctuation in your posts would help (-;
Perhaps you can work out your own motor speed controller ? Have you worked with Arduino etc ? Maybe you could separately monitor the manifold temperature - run at low speed initially but when temp exceeds say 90C turn to high speed until it returns to say 70C ? The arduino could drive a relay or SSR to simulate the speed control switching. I suppose the ultimate might be to analog control the pump speed to keep the manifold at a particular optimum temperature like 90C.
Another option might be to introduce some restriction in the pumping line - so even when you set it to high speed (5L/min) it actually flows at say 3.5L/min.Posted Tuesday 19 Sep 2017 @ 11:57:28 pm from IP # -
Greg
MemberWhen my solar controller died I replaced it with a new one with greater functionality. I hope to find a reasonably priced speed controller solar pump that will take advantage of this functionality
From the manual
RPM control output: (speed control is activated), the control system attempts to maintain a constant temperature difference between collector and tank. The pump performance is continuously adjusted, based on the temperature difference flow rate of pump is increased or reduced.Posted Wednesday 20 Sep 2017 @ 2:24:01 am from IP # -
vicsolaruser
MemberHi again thank you to all people for your advice an comments it bein very helpful. also thanks for being patient with my punctuation an spelling. I struggle at the best of times let alone useing one of these touch screen phones.
So after taking into consideration every ones advice an just spending thousands on a new system to find out that its not as efficient as it could be i think in the short term my cheapest way out is to install a manual restriction valve i.e simple ball valve. Also in mean time on cold days around up to about 13 deg il set the pump on seting 1 most other times il just leave it on seting 2 or 3. Also greg's comment about a solar controller that controls pump speeds im intrigued greg if possible could you post the brand of controler you replaced your old 1 with, much appreciated thanks.Posted Wednesday 20 Sep 2017 @ 6:04:07 am from IP # -
Greg
MemberVic, I'm surprised your solar system doesn't have an adjustable flow control valve on it already. I'd add one of those in preference to a ball value as it will allow more accurate and quantifiable figures. The unit I have is from Misol (hit up e bay) but there are probably many others. I've been unable to find a variable speed motor for what I consider a reasonable cost.
Posted Wednesday 20 Sep 2017 @ 10:16:02 pm from IP # -
AndyG
Membervicsolaruser said:
. Also greg's comment about a solar controller that controls pump speeds im intrigued greg if possible could you post the brand of controler you replaced your old 1 with, much appreciated thanks.I have an Apricus evacuated tube system and the controller is a Resol Deltasol BS4 controller.
It allows you to set the fine tune the pump speed (at least with a grundfos 15-20 CIL2 pump) so that the pump speed increases in increments of 10% as the difference between the manifold and tank temperature increases.Posted Saturday 23 Sep 2017 @ 7:00:16 am from IP #
