This is following on from my recent post on the greenhouse solar installation.
To use any excess panel energy available during the cooler months, I've built electrical heating elements to connect directly to the 'dump load' circuit from the charge controller. There are no proprietary heating elements available for the power and voltage I require, and even if there were then they'd likely be prohibitively expensive.
The first job was to source some suitable resistance wire. Lots of types available on eBay but not too many which are insulated, so I opted for enamelled 0.7 mm diameter Isotan wire (aka Konstantan) available from a seller in Norway.
The reason for buying insulated wire was that it could be formed either in or around metal pipe or ducting. In the end, I opted for inserting it in small-bore copper tubing, a coil of which I had lying around the workshop.
However, this wasn't the only choice for the design - I'd originally envisaged simply wrapping the insulated wire around the outside of a 22 mm diameter copper pipe, but I didn't have any to hand and B&Q wanted £17 for a 2 metre length....
I unrolled the small-bore copper tubing coil and measured its length at 7.2 m. A few basic calculations were carried out to establish the heating power available from the lengths of tube and wire I had.
It seemed that making two identical 145 W heaters and then connecting them in parallel would make the best use of the materials in achieving the dump load power I was after.
One limiting factor in the design is the allowable temperature of the wire, which should not exceed around 500 C otherwise the enamel insulation is in danger of breaking down.
Allowing for connections, the wire length will be around 50 mm longer than the tube, i.e. 3.65 m. Since I only finalised the design and calculations after having received the first 5 metre length of resistance wire, this means buying another length for the second heater, which has been ordered but not yet delivered.
However, I could proceed with building the first heater. The copper tube length was cut in half and the inside edges carefully smoothed off to avoid scraping the enamel insulation away when inserting the resistance wire.
The wire was then inserted through the tube, and a couple of short lengths of insulation sleeve fitted into the pipe ends around the wire, for extra protection. These sleeves were taken from the wiring of GU10 spotlights, the lamp casings of which I'd used to make the low-voltage MR16 spotlights fitted around the outside of our house.
|tube, wire and insulation sleeve....|
The copper tubing was marked out, a 180 degree bend made at around 900 mm in from one end, and this length inserted inside a steel pipe. The outside of the steel pipe was then used as a former to wind a helical heating coil.
|winding the tube coil....|
The coil was slid off the former and the central straight section tied to the coil with copper wire at a few positions, for extra rigidity during handling.
The wire ends were trimmed, stripped of enamel and connected up to a ceramic terminal block. This block was mounted on the aluminium strip of the 'cold wire' connection taken from one of the old GU10 lamp fittings. The whole termination assembly from the lamp was re-used in its entirety. A simple clamp arrangement also helped to hold the tube ends at the connection.
|connectors and clamp....|
So, here's the finished heating element.
It was initially tested for an hour or so on a 12V battery - I don't want to connect it as a dump load to the charge controller until I've also built the second heater.
Under 12V, it pulled around 2.5 A (as it should have done !) i.e. it was only putting out around 30 W, getting hand hot in the process. When connected to the panels at 26 V load voltage, it will draw up to 5.5 A (depending on the prevailing light conditions) to generate up to its design output of 145 W.
The two heaters will be suspended on wire loops hung underneath the lower slatted shelf in the greenhouse, and the warmed air allowed to naturally convect. It's not really a lot of heat, but it should help to take the chill off any plants above and it's a lot better than simply not using the available energy.
Total expenditure for this one heater was £6.49, for the 5 m length of resistance wire and a 15A ceramic terminal block. Everything else required we already had lying around.
Simple, cheap and hopefully also effective at the panel voltage.....