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Author Topic: optimum switch on temp solar tubes  (Read 511 times)
BARRAQUILLO
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« on: June 10, 2018, 03:52:27 PM »

looked through posts but cant find answer. what is the more efficient. To set solar tubes switch on temp a lot higher than the store , thus switch on less often or lower/more often. can only a certain amount of heat be exchanged. the rest simply flows back to the collector and is in effect wasted. i hope that makes sense.
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pdf27
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« Reply #1 on: June 10, 2018, 07:12:26 PM »

Doesn't make much difference - heat energy going back to the tubes stays in the fluid (the tubes aren't going to cool it) and will return again to the cylinder on the next pass. The cooler the heat transfer fluid the more energy you can extract from the sun, but it isn't a huge difference and to keep it running cooler you need the circulation pump to work a little harder.
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brackwell
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« Reply #2 on: June 11, 2018, 08:02:20 AM »

The colder the water entering the tubes the more efficient is the system. As the circulating water temp increases so do the heat losses and in the ultimate the losses equal the gain at somewhere above 100C.   In summer does not make much difference as the solar gain is usually more than required. In  winter the losses have increased due to colder ambient temps and the potential gain is also less. In these conditions the warmer water in the tubes needs to get to the tank where the heat is captured.  So a compromise particularly in winter between pump running losses and thermal losses. A difference of 5C between tank exit temp and panels exit temp is a good ball park setting.
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BARRAQUILLO
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« Reply #3 on: June 11, 2018, 09:23:17 AM »

thank you for your opinions. The reason i ask is because i am located in southern spain. i only have a 320ltr store. i have the collector and store set at about a 35deg difference. however despite this meaning the collector  reaches 100+ for a lot of the day the temperature of the water in the store does not get above 55c by sundown[base temp @ sunrise 45c]. so in an entire day i only gain 10deg of heat with out any large extractions of hw. i have tried smaller differentials but as mentioned by pdf27 the pump seemed to run too hard.
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brackwell
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« Reply #4 on: June 11, 2018, 10:12:55 AM »

So the base temp at sunrise is 45C and you have a delta T of 35C  ie the pump does not come on until 80C ??  Why would one do that?   In effect you are operating the tubes at 80C + when the heat losses are getting significant

So you have a 320l tank of hot water before you start.  The pump is operating to keep the tubes cooler so whats wrong with that. Not sure i understand what your concern is. If you dont want the pump to run you need to cover/remove some of the tubes in summer.  If you want to capture more heat you need to reduce the delta T to a lower no. 

Where are you measuring the temps remembering if you are only measuring at the bottom of the tank then after a day of heating the top of the tank will be a lot higher and will be even hotter at the top than your sunrise temp.   
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todthedog
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« Reply #5 on: June 11, 2018, 10:50:44 AM »

I had mine set at a 5C difference between the exit of the tubes and the tank temperature,with a motorised valve set at 85C to dump into a heat dump radiator, not a regular occurance in Finistere!
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dimengineer
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« Reply #6 on: June 11, 2018, 12:21:38 PM »

thank you for your opinions. The reason i ask is because i am located in southern spain. i only have a 320ltr store. i have the collector and store set at about a 35deg difference. however despite this meaning the collector  reaches 100+ for a lot of the day the temperature of the water in the store does not get above 55c by sundown[base temp @ sunrise 45c]. so in an entire day i only gain 10deg of heat with out any large extractions of hw. i have tried smaller differentials but as mentioned by pdf27 the pump seemed to run too hard.

Agree with all the others. That's way, way too high a temp difference. Don't worry about the pump, it is perfectly capable of running all day, every day.
Try 10/5
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21 tube Kloben Panel/250L Megaflow, 1.68kWp Solar PV - 7 x 240W Sanyo Panels. Morso Squirrel WBS
BARRAQUILLO
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« Reply #7 on: June 11, 2018, 03:00:00 PM »

the store has 3 temp gauges, top,middle,bottom. because the the sun is so strong the tubes picks up heat very quickly. if set to just 5deg difference the pump would be on continuously. i don't necessarily have a problem with that but my question was which is a more efficient heat transfer, higher or lower differential . it is not a case of needing to cover up some of the tubes. the problem is only achieving an approximate 10 deg rise in the store temp even with abundant sunshine. however i was a little anxious about the possible ware on the pump if it was running continually during the daylight hours[it does get hot but i guess it would with hot water running through it].   
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dimengineer
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« Reply #8 on: June 11, 2018, 04:02:11 PM »

Given its a low power pump - basically a central heating pump, it is designed to run all day. It is almost certainly better for it to keep running than the cycle on/off.
The temperature difference self balances anyway - If you've got max sun, the delta T will rise above 10C (assuming a 10/5 set up) until the heat transfer into the tank balances with the heat gain from the tubes. Just keep it simple.
Lower delta T and therefore temp is better. The system is more efficient, less heat loss to the outside world.
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21 tube Kloben Panel/250L Megaflow, 1.68kWp Solar PV - 7 x 240W Sanyo Panels. Morso Squirrel WBS
sbchapman
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« Reply #9 on: June 11, 2018, 04:11:31 PM »

Hi, not immediately comparible but useful for background hopefully: I have 3.2m2 ET heating ~200l water in the UK using a DeltaSol controller. I adjust the differential seasonally - winter switch off (DF) =6C and switch on (DO) = 8C; summer off (DF) = 10C, or even sometimes up to 12C or 14C, and summer on (DO) between 10C and 14C.

In summer, setting DF too low can result in very high store temps (requiring active active cooling to prevent stagantation) so it's a balancing act as you don't want to miss energy when it's cooler. You can never cover all eventualites (eg several days of sunshine an dlimited heat demand) and therefore need overheating protection so you can drive the system harder, but it's a question of trial and error as all systems have different flow rates, insulation, stratifiaction characteristics and heat demand.

What controller do you have and if its a Resol, what are the other paramters set to (eg collector max, pump speed etc)
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daveluck_uk
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« Reply #10 on: June 11, 2018, 04:29:09 PM »

Hello I'm in Galicia,

With a 475 ltr tank and 2 banks of tubes I start the pump when the manifold temperature is  12 hotter than the bottom of the tank it then runs until the delta is 7. In the summer the pump is running all day everyday and the tank hits 80+. To ensure it doesn't get anywhere need that I dump the excess via the central heating radiators.

The pump runs at 28w so in a 12 hour day burns up less than 400w to produce 400+ litres of scalding hot water! I reckon youll do something similiar down South so I think that is fairly efficient. In terms of cost 1kwh up here costs about 35c including tax and distribution costs. It is almost free hot water!


Sorry forgot to mention, on a good day I get in excess of 30 temp increase in the tank.


« Last Edit: June 11, 2018, 04:31:49 PM by daveluck_uk » Logged
brackwell
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« Reply #11 on: June 11, 2018, 04:33:32 PM »

The temps you quoted 45C and 55C where are you measuring these.  You need to quote the temps of all 3 positions to get a full picture.  The low position temp will be very dependent on hot water draw off ie the cold water added at the bottom.  I have difficultly seeing how the panel temp can get to 100C and the hot water only to 55C  and wonder if it indicates air in the system.
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BARRAQUILLO
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« Reply #12 on: June 11, 2018, 05:00:11 PM »

its puzzling me too. but i do periodically check for air locks. a slightly different question. my understanding is that the solar travels through the bottom of the tank via a coil. if it is a closed system[the solar not the store] how is the solar charged with water initially?........also, thanks for all your responses.
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daveluck_uk
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« Reply #13 on: June 11, 2018, 05:39:01 PM »

its puzzling me too. but i do periodically check for air locks. a slightly different question. my understanding is that the solar travels through the bottom of the tank via a coil. if it is a closed system[the solar not the store] how is the solar charged with water initially?........also, thanks for all your responses.

Via the fill loop?

You might want to post some pictures to show what system you have.

As an aside...I thought my system was OK and had plenty of fluid in it. Requarly checked for air locks etc. Then one day no heat transfer and and a seized pump. Still no air coming from the air bleeder and still showing 2 bar pressure. When I drained it out there was very, very little fluid and not very pleasant.

Are you 100% certain you have fluid and no air traps?

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