I love travelling in my Hi-Ace van and for the last few years me and my wife and been travelling north for the winter to sunny, windy Far North Queensland as the Trade Winds kick in and Sydney gets cold and shitty.
The humble inverter was the first big lurch toward freedom. No longer dependent on landing in a caravan park every night we were free to drive and compute and kite where ever the winds of fortunes blew us. However, we are not fully disconnected from the Matrix. We still need to service out ice dependency. 2-3 times a week, 2 x 5kg bags of ice go into the Esky to replace the buckets of cold water relentlessly invading every available orifice of our food stuff. The water, usually wafting of gently rancid milk, claims the labels of every can, bottle or jar not designed total immersion. Damn you to hell esky!
As with all problems to be solved I undertook a short research program and the university of Google and as a foundation member of Over Engineering Anonymous I soon found myself fascinated by this age old art of refrigeration.
RV fridge options do exist but the energy intensive nature of them puts me way off them:
Compressor fridges are a very good option. But they are expensive and energy intensive and I would need to upgrade the 100Ah battery or add solar to the van to make sure that we didn't use up the battery and be unable to use it for running out laptops and phones and printer when working on the road.
3 way fridges I believe also have some merits and are widely used. They do however, need a constant heat source which means putting it outside to use gas and running of our precious limited battery where it is not particularly efficient. Moreover, unlike the compressor fridge it cools relative to ambient temperate so probably still needs to be used in conjunction with some ice.
In the course of finding out about the fridges and finding out how they worked I came across a lot of research on a different type of absorption fridge that introduced me to a lot of very exciting ideas and has the potential of a very DIY fridge or ice maker to be knocked up in the garage.
The absorption process in a 3 way fridge uses either ammonia or a lithium salt dissolved in water. The use of these material makes it tricky or risky for the DIY'er. Ammonia, well it can kill or blind you and lithium bromide is difficult to get hold of as I believe the the presence of lithium means the material is restrict because of its use as a 'psychiatric drug'.
In these fridges the refrigerant is initially dissolved in a liquid that is in a low pressure environment. By heating the liquid the refrigerant is driven out of the solution as a vapour which in the process of changing state and by the powers invested in it by the laws of thermodynamics takes with it heat from its surroundings and the cooling effect on your beer ensues. The vapour then passes through a condenser where it expels much of the heat and then drips back down into the original solution from whence is came and the whole process continues. This is fittingly called a 'vapour adsorption' process.
The total amount of energy it can extract from the system is the amount of energy in addition to the heat source required to vapourise the refrigerant less the amount of heat that can't be expelled through the condenser.
The research that I came across is for an absorption fridge that uses water as the refrigerant and instead of the refrigerant being absorbed back into the solution straight after clearing the condenser it uses a variety of common moisture absorbing materials such as activated carbon, silica gel (like in pharmaceuticals or vitamin table bottles) or Zeolite. The role of these 'desiccant' materials is to take the water vapour (which is the refrigerant) out of the system and prevent the pressure from building up as this would increase the temperature at which evaporation would occur until eventually the evaporation would stop and so would the cooling of the fridge contents.
Unlike the ammonia or salt/water vapour absorption fridges the process is not continuous. The cooling happens as long as there is evapouration of the water happening. The ultimate limit on this is the amount of desiccant material there is because once it is saturated with water it can no longer prevent the pressure rising and the cooling slowing down until it stops. In practical systems this can be 12 hours. This compared to relatively short duration of the boiling /condensing/ absorbing process in the 3 way fridges.
This is both a pro and a con. On the pro's side this means there is 12 hours of cooling occuring with no energy input to the system. Practically this means no burning, no electrically heating, no noise just chillin'. Awesome. The con is that the process then needs to be reversed by heating up the desiccant material to drive the water back into the system. During this time there is no evaporation going on so no further chilling but the cold water or ice created during the cooling cycle acts as a cold 'bank' to keeps things cold while the system is reset.
One of the very exciting things about this is the multitude of 'low grade heat' sources that can be used to reverse or 'regenerate' the system. Any source of heat capable of 150-250 degrees will do. Solar collectors, heat off the car engine, wood gas burner, bio gas off a digester or LPG. Most exciting is a solar collector which some of the research suggests has real promise and especially if you use methanol instead of water at the refrigerant then the temperatures that can be easily reached with a collector are more than sufficient to 'desorb' the methanol out of the desiccant. further, the 12 hours cycle time of these fridges matched perfectly with the day time/ night time cycle. Spooky!
All of this has got me very excited about the prospect of making a DIY solar powered fridge or even more appealing is a solar ice maker to replace our 2x3x 5kg habit and allow us to finally be free of the matrix!!!
As the science is so interesting I am planning on running a few experiments to see all of this in action and come up with some real parameters for a prototype fridge. As luck would have it I over engineered a pressure and temperature controller using the truely outstanding single chip computer/ microcontrol developed by Geoff Graham called the Maximite. The add on box I built has 4 temperature sensors and vacuum pressure sensor and all this can be display real time and the data logged. This is just what I need to run experiments to fine tune the requirements for a solar ice maker.
Bring it on!!!
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