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Solar water heating update

tomandsophie

By tomandsophie
in Boat Equipment

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smileypete Veteran II

smileypete

Posted
Posted

Tom,

 

I would give the supplier of your panel a call and explain about pump flow and insulation (or lack thereof) and see what they say.

 

Otherwise you could be whistling in the wind worrying about pump speed when the problem lies elsewhere.

 

IF that is IF the supplier does think that slower pump speed will help then post here for some suggestions.

 

cheers,

Pete.

 

PS Chris W, how do you work out that a 120W panel is needed to run a pump that probably uses ~12W?

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smileypete Veteran II

smileypete

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Posted (edited)
Have spoken to the suppliers and they say that the flow rate is far too fast. They recommend a maximum of 3 litres/min.

I will be buying a new pump soon and will post the results on here...

 

Hi,

 

OK, I would try running the 12v pump on something like this first:

 

http://cgi.ebay.co.uk/Universal-In-Car-Ada...1QQcmdZViewItem

 

If the pump runs on 5 or 6v it will take very approximately 1/4 of the power and give 1/4 of the flow.

 

So if the pump moves 12lpm at 12V it will move approximately 3lpm at 6V.

 

cheers,

Pete.

Edited by smileypete
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chris w Veteran II

chris w

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Posted

Simply dropping the volts by using a regulator as suggested probably won't work as the pump may not start on only 6v or so. That's why I suggested much earlier a PWM voltage regulator which pulses the 12v at 500Hz (in my design) but has a variable on/off duty cycle. So if the 12v pulse is only ON for say 30% of the time, the "average" voltage will be about 4v but it will be supplied as 12v pulses ON for 600uS and OFF for 1.4mS and so on.

 

Chris

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Gibbo Veteran II

Gibbo

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I looked it up :P

 

As usual it's more complicated.

 

The amount of heat transferred is the temperature rise multiplied by the volume of water. You can't just consider the temperature of the flowing water. To say "it will get hotter" is therefore completely pointless and irrelevant.

 

The amount of water is important. So faster water flow means less temperature rise but more volume of water heated, so the total amount of heat transferred remains the same.

 

However the rate of heat transfer is highest when there is the greatest temperature differential.

 

If the flow of water is very fast then there will be a big temperature difference between the solar panel and the circulating water. This will transfer the maximum amount of heat from the solar panel into the circulating water (heat rise multiplied by volume - stop prattling on about "it will get hotter if it's slower" - you also have to consider the volume of water). However the temperature difference in the calorifier will be minimal, so the tranfer of heat there will small.

 

If the flow is very slow then the temperature difference in the solar panel will be minimal so there will be very little heat transfer there. However there will be a big temperature difference in the calorifier so that will get the best heat transfer in there.

 

The ideal flow rate is therefore when the temperature difference between the flowing water and solar panel EQUALS the temperature difference between the flowing water and the calorifier.

 

Too fast and it's not ideal. Too slow and it's not ideal. MUCH too fast or MUCH too slow and it won't really do anything.

 

So obviously a compromise has to be found as these temperature differences will vary depending upon how hot the solar panel is and how hot the calrofier is.

 

The ideal would be to measure the calorifier temperature, the solar panel temperature and the flowing water temperature, then adjust the flow rate to keep the two differences the same.

 

Makes perfect sense now I think about it.

 

Gibbo

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smileypete Veteran II

smileypete

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Yep, the pump does run at a maximum of 1.3a. So the cigarette lighter adapter might be a good thing to try seeing as I have one at home. I could at least give it a go and see what happens...

 

Hi,

 

Would be worth checking the output of the adapter, the pump will draw less current at 4.5V/6V/9V but it still might be more that the adapter can provide.

 

Chris:

 

I've found that 12V PC fans will run quite happily on 4.5V or 6V so there is a reasonable chance a pump based on the same type of motor will. Even if not there is a very good chance it will run on 9V

 

cheers,

Pete.

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Gibbo Veteran II

Gibbo

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I've found that 12V PC fans will run quite happily on 4.5V or 6V so there is a reasonable chance a pump based on the same type of motor will. Even if not there is a very good chance it will run on 9V

 

cheers,

Pete.

 

Or get someone to throw this together......

 

http://www.smartgauge.co.uk/bits/pump.bmp

 

It will provide a full 12 volt power pulse to the pump for about 1 second to get it fired up properly then pulse width modulate it at *about* 50/50 mark/space ratio running the pump at *about* half its current speed.

 

I just did that whilst I was drinking my soup :P

 

Gibbo

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barge sara Community Regular

barge sara

Posted
Posted

Haven't seen this suggested yet so here goes.

 

We wanted solar water heating, but all the manufactured panels I found had to be installed 'standing up' at at least 30 degrees or so, whereas on a boat I would want it flat on the roof. This is particularly the case for the vacuum tube type.

 

Are you trying to install it flat - could that be preventing heat moving to the manifold?

 

Regards,

 

John. MV Sara.

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chris w Veteran II

chris w

Posted
Posted (edited)
..........The ideal would be to measure the calorifier temperature, the solar panel temperature and the flowing water temperature, then adjust the flow rate to keep the two differences the same.Gibbo
That's why in my first post on this topic I mentioned that for domestic radiators the temperature difference should be 11degC to ensure the radiator emits its quoted output power.

 

Chris

 

Pete

 

PC fans are very low torque and so will start up very easily. You wouldn't try to stop a water pump impeller with your fingers (if it were exposed somehow) as it's a high-torque motor. A PC fan will stop easily if the fan is touched.

 

Chris

Edited by chris w
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ChrisPy Veteran

ChrisPy

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I looked it up B)

 

As usual it's more complicated.

 

The amount of heat transferred is the temperature rise multiplied by the volume of water. You can't just consider the temperature of the flowing water. To say "it will get hotter" is therefore completely pointless and irrelevant.

 

The amount of water is important. So faster water flow means less temperature rise but more volume of water heated, so the total amount of heat transferred remains the same.

 

However the rate of heat transfer is highest when there is the greatest temperature differential.

 

If the flow of water is very fast then there will be a big temperature difference between the solar panel and the circulating water. This will transfer the maximum amount of heat from the solar panel into the circulating water (heat rise multiplied by volume - stop prattling on about "it will get hotter if it's slower" - you also have to consider the volume of water). However the temperature difference in the calorifier will be minimal, so the tranfer of heat there will small.

 

If the flow is very slow then the temperature difference in the solar panel will be minimal so there will be very little heat transfer there. However there will be a big temperature difference in the calorifier so that will get the best heat transfer in there.

 

The ideal flow rate is therefore when the temperature difference between the flowing water and solar panel EQUALS the temperature difference between the flowing water and the calorifier.

 

Too fast and it's not ideal. Too slow and it's not ideal. MUCH too fast or MUCH too slow and it won't really do anything.

 

So obviously a compromise has to be found as these temperature differences will vary depending upon how hot the solar panel is and how hot the calrofier is.

 

The ideal would be to measure the calorifier temperature, the solar panel temperature and the flowing water temperature, then adjust the flow rate to keep the two differences the same.

 

Makes perfect sense now I think about it.

 

Gibbo

 

.......................... blimey !! :P:D

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smileypete Veteran II

smileypete

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Posted
Haven't seen this suggested yet so here goes.

 

We wanted solar water heating, but all the manufactured panels I found had to be installed 'standing up' at at least 30 degrees or so, whereas on a boat I would want it flat on the roof. This is particularly the case for the vacuum tube type.

 

Are you trying to install it flat - could that be preventing heat moving to the manifold?

 

Regards,

 

John. MV Sara.

 

I think this is very relevant!

 

cheers,

Pete.

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smileypete Veteran II

smileypete

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Posted
Or get someone to throw this together......

 

http://www.smartgauge.co.uk/bits/pump.bmp

 

It will provide a full 12 volt power pulse to the pump for about 1 second to get it fired up properly then pulse width modulate it at *about* 50/50 mark/space ratio running the pump at *about* half its current speed.

 

I just did that whilst I was drinking my soup :P

 

Gibbo

 

Any news on the homebrew alternator controller?

 

This morning I came up with a simple 'remote' battery charger - high power laptop adapter and a carefully calculated length of bell wire :D

 

cheers,

Pete.

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Gibbo Veteran II

Gibbo

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Posted
Any news on the homebrew alternator controller?

 

I've done some of it but I keep geting sidetracked.

 

This morning I came up with a simple 'remote' battery charger - high power laptop adapter and a carefully calculated length of bell wire :P

 

I know a boat electircal engineer who does a similar trick with carefully chosen bow thruster charge cable sizes so the fuses can't blow if the bow thruster battery is flat.

 

Gibbo

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