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Author Topic: How do I connect a hot water cylinder thermostat?  (Read 6636 times)
aa44
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« on: July 31, 2010, 06:20:05 PM »

I have planning permission for a small (1 - 2kw) wind turbine.  I am not an electrical engineer so I want to keep things simple.  I would like to connect the wind turbine to a small battery bank and use a charge controller to switch between the batteries and a dump load.  I am fairly happy about that side of the system as I can see all the relevant bits and pieces on several web sites.

The primary purpose of the batteries would be to power an immersion heater (say 24v) in my thermal store.  Looking at ebay I can see plenty of 24v immersion heaters for sale but none with a thermostat.

Could anybody help with the following questions:

1.  Can I buy a suitable thermostat off the shelf?  (The tank might be something like an Akvaterm with plenty of suitable bosses).
2.  Does the thermostat require a power supply?
3.  Do I position the thermostat (circuit wise) between the battery bank and the immersion heater?

Sorry if this has been covered elsewhere but I couldn't find it.


Thanks
Andrew
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DaveB275
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« Reply #1 on: July 31, 2010, 06:49:54 PM »

Hi Andrew.

I am hoping to do something similar in the future. My understanding of this suggests you do not want a thermostat on a dump load immersion, since if it tripped, it may release the load off the turbine, thus letting it freewheel. Defeats the purpose of the dump load. I'm sure I read somewhere that if a thermostat IS included, it must be bypassed / not used.

I'm sure someone more knowledgable will be along to advise.

Dave

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Alan
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« Reply #2 on: July 31, 2010, 08:09:44 PM »

Hello Andrew

Quote “I would like to connect the wind turbine to a small battery bank and use a charge controller to switch between the batteries and a dump load. “

If as quoted the turbine feeds the charge controller to charge the batteries. When the batteries are charged the power is diverted to a dump load there is no problem. ( not the immersion heater ) But the power is wasted. ?

Quote “The primary purpose of the batteries would be to power an immersion heater (say 24v) in my thermal store. “

If the thermal store is a unvented system ( water tank with no header tank or overflow pipe ) a fail safe thermostat will be required. ( Fail safe – normal on / off thermostat and a high temperature cut off device which if operated requires to be manually reset. )

If it is open vented ( It has a header tank and an overflow pipe ) there is not a legal requirement for a fail safe thermostat. It would be wise to have one.

A conventional thermostat would not be suitable for switching the current from a low voltage battery system. ( It would arc and probably catch fire )

A conventional thermostat could be used to switch a solid state relay or contactor designed for switching direct current.

Bit more info required.

Regards

Alan
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Baz
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« Reply #3 on: July 31, 2010, 08:39:47 PM »

You prtty much never want to run an immersion off a battery unless it was the size of a submarine power pack since your aim should be to discharge it only 10% of capacity.
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johnrae
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« Reply #4 on: July 31, 2010, 08:49:20 PM »

"Small battery bank" and "immersion heater" should be considered mutually exclusive terms and not used in the same sentence.  
Say your immersion heater is rated at 1kW and your battery bank is 24volts (your statement) then amperage will be in the order of 42 amps.  ie 42AH/Hr   Assuming that you require around 1kWHr per day, you will in effect consume some 42AH per day.   On the basis that you should base your battery bank in the basis of say 10% discharge then your bank would require a capacity in the order 420AH.   This is not a "small battery bank"

You'd be better to consider feeding the immersion heater directly from your wind-turbine and when the water is hot enough, use the thermostat to divert the power to an alternative dump load.

When I was designing my system, (1kW) the dump load was the central heating circuit.  Satisfied tank opens the central heating coil to the tank and starts the circulating pump so that heat was effectively extracted from the tank.   As it is, my turbine performance is less than ideal (too much shading by buildings) so over-heating is a figmant of the imagination.

jack
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aa44
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« Reply #5 on: July 31, 2010, 09:19:37 PM »

Thanks for all the replies guys.  Really helpful but slightly depressing.

The more I read the more I get the impression that, for somebody who is not an electronics whizz, trying to use a small wind turbine just seems to be too hard.  Connecting the turbine directly to the immersion seems to be an engineering challenge that few have got right.  If I need a big battery bank then connecting it that way seems to be prohibitively expensive.

Am I missing something here or am I trying to do something that these machines just aren't designed to do?

Sorry for the moan but I do find it frustrating that I have a nice windy site in Shetland and I have planning permission for a small turbine but there does not seem to be any simple way of making use of this situation.
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johnrae
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« Reply #6 on: July 31, 2010, 09:26:19 PM »

Don't be despondant.  Simply connect the turbine to the immersion and use the thermostat to divert to a dump load as I suggested. 
If you haven't yet bought your turbine, buy a 48 volt unit - less amps, lighter wire etc.  48volt/1kW immersion heaters with universal 2.25"BSP bosses are available as standard.
jack
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aa44
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« Reply #7 on: July 31, 2010, 10:03:25 PM »

Thanks Jack.  Sounds like a great idea but would I be right in thinking that there is a certain amount of electronics wizardry in there that requires a bit of home brew skill rather than buying it off the shelf?
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Baz
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« Reply #8 on: July 31, 2010, 10:08:44 PM »

Thermostats. The ones available to attach externally to tanks and pipes are for signalling to gas boilers and not capable of any current handling.
The ones on mains immersions are for higher current say 15A but AC only. The DC from your typical rectified 3 phase turbine is very switch unfriendly. You need to use a relay specially designed for high current DC.
The next problem is that the load resistance for 1kW is so low that in light wind it acts as a huge brake on the generator and the turbine blades are stalled. You really need a mutitstage load.

If you are able to learn just enough about electronics to make some of the circuits that are often published that switch a relay as the voltage from your turbine rises then you have a way forward.
Then you will need a number of resistors. A 3kW mains immersion run at 48v is just 150W. Too small you think? However mostly you will only be getting 100-300W anyway, though for 10 hours at a stretch (so much better than PV). Fit 2 in your tank will give you a shower or two a day. Another in a small oversink 7 litre water heater will preheat kettle water and do the washing up.  These are the ones that get regular use but you still need a few more for the gales.
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Alan
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« Reply #9 on: July 31, 2010, 10:24:05 PM »



Its not difficult If all you want to do is heat the water.

Go for the highest voltage three phase A.C. turbine you can get. Then your cable costs, controls will be cheaper.

Use solid state relays for switching the heater on / off

Feed the out put of the three phase solid state relay into a three phase bridge rectifier. Use the direct current to power the heater with direct cables ( Do not wire through a temperature stat. )

You don't want the heater connected to the turbine until the blades are rotating at a speed where power can be used. If the heater is connected to the heater directly the wind speed will have to be higher than required to enable the turbine to rotate. ( Your power is being lost )

Three phase solid state relays need a direct current voltage to enable them to turn on. The minimum turn on voltage is about three volts.

With a full wave bridge rectifier connected across any two phases of the wind turbine a D.C. Voltage is produced. Feed this voltage through a variable resistor ( This resistor will allow you to manually set when the heater is switched on / off. This allows the turbine to rotate with no load, when a voltage is produced the solid state relay switches the heater on ) depending on what voltage turbine yo get you may need a zenor diode fitted across the solid state relay so that over voltage does not cause damage to the relay.

You could have as many relays as you want switching loads on / off as required and at what ever voltage you decide from the turbine. A second relay could be used as a dump load if the water tank gets to hot.

This control voltage can be wired in series with tank thermostats to control temperature.

You could use pulse width modulation circuits to keep the load on the turbine in proportion to available power if required.

When you know what turbine you are getting more info could be given

Regards

Alan
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aa44
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« Reply #10 on: July 31, 2010, 10:41:32 PM »

Thank you very much for the replies guys.  It's not the simple answer I want but it's not impossible either.  Time for a bit of a think.

Thanks again for your help.
Andrew
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johnrae
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« Reply #11 on: July 31, 2010, 10:57:09 PM »

Andrew
Here's a simple switching circuit that'll apply the immersion load once the turbines up to speed (to avoid the dead brake effect).
The relay is a miniature 24 volt unit
The 48DC comes from a 3-phase bridge rectifier stack.

You will use the immersion thermostat to control the diversion system, whatever that may be.  Since it's only a signal control there should be no issues about it having to carry substantial current.

Not very efficient, but if you have a ready water supply you could simply fit a solenoid valve such that, when the tank gets hot, some of the hot water is dumped and replaced by cold water.  Is that wails of derision i hear.  Well that's what we had to do in the "good old days" when we had coal back boilers and blazing fires.  When the damper was open and the water tank rumbled it was a case of running off some of the water into the kitchen sink.  In those days H&S was common sense; how did we survive before H&S was invented.

jack




* Turbine SCR Control.jpg (41.74 KB, 708x387 - viewed 1438 times.)
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Alan
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« Reply #12 on: July 31, 2010, 11:37:10 PM »

Thingy drawing.





Regards

Alan
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aa44
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« Reply #13 on: July 31, 2010, 11:43:08 PM »

Thanks very much.   Cheesy
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