As I sat on my comfy new seats near the now-refurbished stove, I couldn’t help thinking that the kitchen was missing an essential appliance. Unlike the vast majority of homes in the UK, my boat didn’t have a refrigerator. Most boaters and caravaners will probably tell you that a refrigerator is an expensive luxury, because they need a large amount of gas or electrical power. A refrigerator will draw somewhere between 2 and 10 amps per hour at 12 volt (depending on how new and efficient it is) and when you’re responsible for producing your own power, you learn that keeping your milk cool might not be as easy as you think. The initial cost of a new fridge is quite high, and it’s not something that I considered a high priority when I first started living on the boat. I still didn’t think the fridge was an essential appliance, but I decided to fit one just in case it became necessary.
I managed to get my hands on a 25 year old Electrolux 3-way (12v/240v/gas) fridge, which was incredibly inefficient. It drew about 8 amps per hour and the gas connection was pretty unsafe, but it used an Einstein engine rather than a compressor. This appealed to me because Einstein engines are completely silent, and I really didn’t want the noise of a fridge motor keeping me awake at night. It was also free, and I felt confident that I could improve its performance with a few simple modifications.
The first modification was to add a thermostat, so that the cooling engine would shut off when it reached a particular temperature. I already had a Velleman kit that triggered a small relay when the temperature fell to a certain level, but that was the opposite of what I needed it to do. If I had used the kit without modification, it would be powering the relay when the fridge was at the correct temperature and turning it off when it got warm. It would be more efficient (and better for the relay) if it only needed to trigger the relay if the fridge was too warm.
I hacked the circuit so that it would work in the opposite way, and replaced the standard relay with something that could handle the power that the fridge would need (If you’re trying this yourself, reversing the operation of the thermostat is just a matter of swapping the connections to pins 12 & 13 on the LM324).
I drilled a small hole in the back of the fridge and fed a suitable thermistor inside, which I connected to the kit. I removed the gas fittings from the fridge because they would never be used, and I replaced the gas control knob with a potentiometer to control the Velleman kit. I left the 240v controls in place, because it’s possible that I might want to remove the fridge or use it on mains electricity at some point. I also added a small LED, so that I could see when the fridge was drawing power from the batteries.
I connected the fridge to an ammeter and recorded the power consumption over the space of an hour. Without the control circuit, the fridge drew just under 8 amps per hour. With the thermostat fitted, the fridge cut in and out several times during the test, and the end result was that less than 4 amps per hour. I did notice that the relay tended to ‘flutter’ when the temperature of the fridge was close to the cut off point, so I added a capacitor across the relay, which stopped the annoying buzz. So far, I’d spent no money on the project (because I’d already got enough spare parts stashed away in the workshop) and I’d cut the power consumption of the fridge in half.
I’d learned from the internet that adding an additional layer of foam insulation around a fridge would improve it’s efficiency, so I purchased a sheet of 1 inch thick polystyrene insulation and cut it to fit around the sides and bottom of the fridge. Thicker insulation might have been more effective, but I wanted to fit the fridge into a gap behind the kitchen cupboard and I knew I didn’t have much extra space to work with.
When I fitted the fridge into the space, I raised it from the floor on a wooden shelf so that there would be enough air-flow to the serve the cooling matrix, and I found that the insulation foam was just thick enough to wedge the fridge in place and stop it from moving around. With an 8 amp peak load, I needed to run more 6mm2 power cable to to the fridge – but this was a short run and I had just enough left over to do the job.
All that was left was to cut a wooden panel and some trim to make the whole project look neat. With a full fridge, all of the modifications I had made had dropped the power consumption down from 8 amps to 2-3 amps. In the heat of summer, the solar panel can replenish the power needed by the fridge several times over. In the winter, I didn’t need to use the fridge because nature is cold enough. A sealed plastic box stashed somewhere on the front or back deck is perfectly adequate to keep most goods chilled -if not frozen- when the weather is cold. I also found that cutting a small hole in the insulation behind one of the kitchen cabinets makes a perfectly serviceable larder under the sink without chilling the rest of the boat.