Thursday, 25 September 2014

Building a Simple Electrical Heater - Part 2

Recent additions - electrical heater box and growlight panel, both on test

I've put this on a separate post to my original, because the whole design has now become a little more complex !

After building the two identical electrical resistance heating elements, I spotted a 6" (150 mm) 24 VDC fan at our local car boot sale a couple of weeks ago.  This was snapped up for £2.  

the fan....

With this addition, I thought I could maybe improve on my original free-convection idea and wire up this fan in parallel to our two heating elements, to push the air over the heaters.

One reason for doing this is that the heater resistance wires may tend to overheat in very bright sunlight conditions, in the absence of a cooling airflow, especially now that we've added a fourth solar panel and more power to the array. 

The first step was to construct a heater box.  An 8' x 4' sheet of 11 mm thick OSB and cut into the basic panel sizes we wanted by B&Q.  It cost £16 for the board, and the cutting guy was very obliging and made many more cuts for us than the six or so that are usually allowed for free.

After doing a bit more design work in AutoCAD, I finished off the profiles required with a handsaw and made the basic shell.  The overall length corresponds to the distance between the supporting framework of our greenhouse slatted shelves.

basic box structure...

I added a few horizontal baffles, made a detachable blower box with additional counter-directional baffles - these were cut from short pieces of white plastic guttering I had in the shed - and then we painted these sub-assemblies with the remnants of old tins of white gloss we had lying around in the house.  

The gloss paint is necessary to protect the box woodwork when we water the plants sitting on the greenhouse shelves above - any run-off will simply pass through the heater box onto the soil below.

painted heater box with longitudinal baffles
blower box with lateral baffles

It took several days for the gloss paint to dry and fully harden-off, but I had plenty of other paid work / things to do and so I wasn't in any particular hurry.

If it's not too clear as yet what I'm trying to achieve here (maybe you're baffled...sorry... ) then here's a cross section through the centre of the fully assembled heater box...

airflow through the heater...

The idea is that the airflow from the fan does not have any direct path through the heater box, but rather that the flow is either diverted and / or arrested by the baffles, and therefore the air is forced to spill out over the length of the longitudinal gaps and has no other exit route than to pass directly up through the two coiled heating elements.

We mounted the assembled heater box into the greenhouse shelf frame.  The lower shelf itself had been removed for access, and because it was made from very wide planking I've now cut some much narrower slats to allow increased air circulation (see the photo at the very top).

I also made a couple of aluminium brackets to support the heating elements, assembled everything in the box, wired it all up and it was ready for some first testing.

Box in position and heating elements installed....

blower box with fan, fitted below.....

Both heaters are individually wired so I can experiment with parallel and series connections by swapping wires over near the dump load terminal block, without needing to remove the shelf slats for access to the terminals inside the heater box.

So the heaters were first hooked up in series to the solar controller dump load terminals, and I forced the controller into dump mode - if the controller is in the 'float' condition, i.e. the battery voltage is somewhere between the lower and upper set points, then you can use momentary switches on the panel to toggle between battery charging or not.

On the day of installation it was well into the afternoon before I had everything set up, so they weren't peak load conditions.

The heaters were initially tested without the fan, and seemed to be working OK.  

Solar panel voltage and current on first heater test...

Under these particular afternoon light conditions, the heating elements were pulling around 3.5 A at 33 V = 115 W.

So then I connected the fan, and immediately encountered a problem.  Although the fan spun up to speed OK, after just a few seconds the charge controller switched itself back into charge mode.  I toggled it back to the dump load, but it immediately tripped itself out again and back into charge.  This happened every time the fan was connected.  The battery voltage at the time was in excess of 13 V, and so why the controller wouldn't remain in dump mode is a mystery.

It must be some characteristic of the fan motor that's causing this trip, because the heating elements work fine without the fan connected.  I tried the fan without the heating elements in the circuit, and encountered the same problem, but the fan works fine when it's coupled directly to the batteries.

I need to study the circuit diagram and try to figure out what the charge controller is seeing that causes it to trip back into charge.   I thought the only sensing criterion was the battery voltage, but there's obviously something about the fan device and / or the way I have it all connected up that's fooling the controller.

I also spoke to my brother on the subject, who has an telecoms / electronics background, and he suggested, in the absence of any technical data on the fan, simply putting a capacitor across the fan motor connections to take out any electrical noise the controller might be seeing, and so we'll first source a suitable device on eBay and try that - hopefully a cheap and easy fix.

However, it can wait until next week - we're away for a long weekend...

In the meantime, the heaters will just have to operate without fan assistance in free-convection mode....

Update 04-Oct-14

That simple capacitor fixed the tripping problem !

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