Holidaying off-grid?

So here at Low Energy Supermarket Ltd we are always looking at ways to rely less on electric hook ups and see if you can really holiday “off grid”.

Below is a blog from one of our staff Mark……

Having already tried most caravan sites for kids holidays and finding no one has any availability, we thought we’d see how we can survive “off grid” for a weekend, and here is how we went on……..

I’ve invested quite a bit on the caravan fitting a solar panel, a Victron MPPT charge controller, a Victron Smart Battery Monitor, and a 100ah lithium battery, our first off-grid trip quickly realised that the gas-powered fridge was not working. The wife was not happy at having no ice cubes and I was in trouble…… The fridge was working fine when connected to the grid electricity supply a week earlier, so there is nothing apparently wrong with the cooling circuits.

We have an Elddis caravan and the factory-fitted fridge is a Thetford N175 (now discontinued), but the principles of this absorption refrigeration are the same as other makes and models.

Once home I could investigate…..

Initially, we opened the vents behind the fridge and removed the covers to inspect the gas burner.  Whilst the fridge was running on gas, we could see that there was a clean blue flame and the bottom of the flue pipe was getting hot. The heat was also rising from the refrigerant pipework attached to the flue.  All appeared to be working okay, so we gave the burner a quick brush and tried the fridge again. This time we fitted a temperature data logger inside the fridge, but all this recorded was a steady drop in temperature outside the caravan as we were monitoring overnight.  No fridge cooling was apparent – absolutely nothing at all.

Blip was us opening the door to see if it was getting colder!
Above, Gas Burner, flue pipe insulation pushed up slightly.

We were then told that maybe the gases had ‘separated’ or maybe crystallised inside the pipework and a common cure is to take the fridge out and turn it upside down.  Now, these fridges are not light – circa 40kg however, 6 screws later it slid out effortlessly and we left it upside down for maybe 30 minutes, then, reinstalled and tested again on gas.  Still no improvement.  We really hoped this was going to work! The back of the fridge was very rusty – surprising what the elements do to elements!

Inversion – did not work this time.  Maybe it does in some situations?

It was evident that this initial cleaning of the burner and inversion had not made any difference, so time to make some further investigations.  I inspected the top of the flue to ensure the gases were escaping okay, it looked fine.  I pulled the flame diffuser out from inside the gas flue pipe and it was apparent that this has been very hot in the past judging by the colouration.  I thought maybe this gas flame diffuser was mounted a little high in the flue pipe, so I dropped it down slightly by perhaps 1 cm to help the heat dissipation into the refrigerate pipework.  I fully removed the gas burner for inspection and found a small amount of rust inside although this wasn’t really obstructing the burner.  Everything was put back together, the fridge tested again overnight, same result, no cooling whatsoever.  All this time, I kept checking the small blue flame produced by the burner which appeared to be working okay.  However, it was on this occasion that I noticed the thermocouple wasn’t always directly in the small blue flame and this was occasionally causing firing the automatic igniter as the fridge controls obviously thought the flame had gone out.  I thought this was a little strange but the cover was off the burner, so I thought maybe it was a slight breeze causing it.

Above, clear flue pipe (looking down)

Rust and debris clear (was that fluff the blockage?)

Next was to get a bit more technical are using a thermal imaging camera that I borrowed from work (thanks Low Energy Supermarket!). If the fridge is working fine when connected to the mains electricity supply, what does the heat distribution look like at the back of the fridge?  Obviously, I need to get the same heat distribution from the gas burner.  I was very surprised (shocked actually) at just how much heat the 240v electric supply put into the back of the fridge.  It is right that the ammonia and hydrogen within the pipework boil to start the cooling cycle. After perhaps 45 minutes, it was possible to hear the boiling within the pipe.  The heat travelled upwards within the pipework, towards the top heat diffuser, where any remaining heat in the pipe was very quickly dissipated. The temperature measured at the top of the flue pipe was much higher than I expected, somewhere in the region of 115 to 140°C, so do not test this with your fingers!  As it appears to be a vapor traveling up inside this pipe, as opposed to a liquid, the temperature drops away very quickly towards the top of the fridge.  Presumably, the cooling reaction then takes place as these gases drop down through the refrigerator pipework and this is the way this type of absorption refrigerator works.

Above – visible rear photo of the fridge.

Fridge running on 240V Mains after 1 hour

The cooling pipe above the flue gets really hot, 114deg C in this photo, but close up was almost 140deg C, and this is the back of a fridge which is cold inside.  Crazy how this works…

It was clear from this that the gas heater on, this fridge needs to get a lot more heat into the system.

So, again, the burner was fully removed for closer inspection.  This time, the burner was blasted with compressed air from every conceivable angle.  The air was blown into the burner/flame outlet, it was also blown through the venturi holes just after the gas jet, the compressed air was also blown in the direction of the gas flow, through the gas jet within the burner.  The jet obviously controls the amount of gas delivered.  At no point was anything physically pushed through the small jet inside the burner which could potentially cause damage and unintentionally increase gas flame size which could be dangerous. The burner was fully cleaned out using compressed air alone.

The fridge was reassembled, put back into position, electrics and gas supply reconnected. The fridge was then turned onto the gas power supply and success!!  Within 50 minutes, the temperature inside the freezer compartment was down to 0°C, within 90 minutes, it was at -8° centigrade, the wife could have her ice cubes!

Strong blue flame and a much bigger flame after jet cleaning, with a healthy red-hot thermocouple.

Out of curiosity, this is what the 12V DC and 240 AC heating elements look like with the flue cover removed.  I wanted to check that the heat is conducted into the refrigerant pipework in the same way for the gas as it is from the electric heating.  Basically, the heating elements and the gas flue pipes are both welded onto refrigerant pipework to get them to conduct the heat into it.

Conclusion

It was all down to the effectiveness of the gas burner flame.  Although I did initially have a nice clean blue flame, it was nowhere near big enough.  There must have been either a piece of debris or a build-up of tiny particles of dirt partially blocking the gas jet. On re-assemble and turning the fridge gas burner back in, it was immediately apparent that the flame was at least double the size it was before, and still a good clean blue flame. The flame was also making a roaring sound which was not there before.

This cooling effect was significantly faster and much more powerful cooling than when I was on the mains electric supply. It is therefore apparent that if this fridge was situated in an area of very hot weather, then it is likely to work more effectively on the gas power supply rather than on the mains power supply.

Thanks for reading!

Mark

PS The electric worked brilliantly – i’ll do a blog on that next week!

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