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Aug 272017

From the moment we gained energy autonomy and with the Algarve summer heat approaching fast, I was due to make a fridge. Why buy one if you can make one, right?

The reasons why I wanted a DIY fridge:

  • After electric heating, a fridge and/or freezer is the biggest energy drain in a house (save for an aircon if you need the luxury?)
  • and seeking energy efficiency (to maintain energy autonomy), you really need a top loader ("chest" freezer/fridge style), not one with a door
  • But to be quick when you open the fridge or freezer, obviously you need to see where is what!
  • Yet without a front window you can't
  • Now get that: No company offers a top loader fridge or freezer with a front window to see what's inside! rolleyes

Hence why I wanted to build a fridge myself, and way cheaper than even the inferior fridges are. SunFrost by the way are not "the world's most efficient refrigerators" as they claim on their site, even a "cheap" German Steca PF166 uses just 22kWh/year or 60Wh a day - what an incandescent bulb uses per hour! - to maintain under 5C / 41F for food safety (at 20C / 68F ambient). No SunFrost achieves that (and there may now be fridge/freezers that use yet less energy). But even the Steca is out of my budget, and like all other fridge/freezers it doesn't have a window to see what is where inside. rolleyes

DIY acrylic fridge - copper heatsink and copper extender bar
Copper is best to dissipate heat:
copper heatsink and copper extender bar

I figured that if I use the best materials, I can build a both energy-efficient and convenient fridge: a top loader with a front window. The best material to insulate the fridge?

I am not a physicist but Google allows us to study anything we want, and so I found out that:

  • a vacuum between multiple panes of acrylic is the best insulator (save for the insane cost of building an aerogel fridge)
  • a vacuum means: no air
  • it is impossible to create a vacuum at home
  • BUT you can try to reduce the amount of air in a closed space
  • the applicable formula is: Air Density = Atmospheric pressure / Temperature

So I figured, to reduce air density between multiple acrylic panes (in order to get towards a vacuum) I cannot reduce atmospheric pressure (I have no such powers), but I can temporarily increase temperature. If I then quickly rolleyes close off the acrylic panes and the air inside cools down, it creates a partial vacuum. And so that's what I did:

DIY fridge welding acrylic panes
Removing fingerprints between the window panes
DIY fridge heating air between acrylic panes
Heating the air between the window panes


DIY acrylic fridge box
After making the acrylic fridge, next task:
preparing for outer insulation

However, working alone and without suitable equipment I wasn't able to close off the acrylic box anywhere near the word "quick". mrgreen

Whatever!, I thought. My FIVE acrylic panes should do the trick (keeping the foods behind the fridge window cold, that is). So then my next task was to get some thick insulation all around the acrylic box that I had managed to make. Foolishly, just a few months earlier I had thrown away the material that the vendor used to protect the solar panels during transport - which seems would have been excellent now. But it was gone: in a tiny house, space is sparse.

This led to the worst of all my DIY fridge building decisions: I ended up buying bottle after bottle of PU spray foam - and that me who doesn't want toxic material in the house! - so much was needed to get my desired 15cm or 6.9in space for insulation filled up! Needless to say, I grew ever more frustrated with this project. rolleyes

DIY fridge with acrylic front window
Final prototype

This next image shows the final DIY acrylic fridge prototype. The plan was to ultimately box it all very nicely (maybe wood, maybe metal) - except for the fridge window of course. grin

But it didn't reach that stage anymore: I did use the fridge for a while, the way you see it here, with a peltier device run directly from the battery bank without the losses of an inverter, but when the Algarve summer hit with a vengeance (up to 40C / 104F) it finally dawned on me that I really need a compressor fridge.

Unfortunately, the cheapest DIY compressor fridge unit that I could find cost £400 (pounds, yes), else I would have built the fridge with a compressor from the start: Even with a superbly efficient design it seems a peltier fridge cannot achieve food-safe temperatures in the summer.

My DIY acrylic fridge indeed was superbly efficient: The fridge was so well insulated that at 24C / 75.2F ambient (nighttime) the inside fridge temperature increased by only 0.2C per hour without electricity consumption (no peltier at that point). Another time at 26C / 78.8F ambient I recorded just 0.55C temperature increase per hour despite three opens to prepare dinner. grin

So yeah, dear SunFrost: "The world's most efficient refrigerator"? And way more convenient than any on the market? You see one here. Build it with a compressor and it will work well during hot summers too.

What happened to my prototype?

Peltier element - thermoelectric element
Peltier buried in insulation material

After a good night's sleep (lol) - realizing that I had ended up with the most toxic insulation material of my entire tiny house bundled up around my food storage! - I ultimately decided to get rid of the outer box with the PU foam. Another reason was: The peltier element that was buried deep inside the insulation BROKE! Thus I would have had to open up the insulation anyway, in order to replace the peltier.

When I dismantled the outer box I managed to break parts of the 2mm thin acrylic sheets making up the fridge (not the 5mm thick window panes). So now, save for all the copper parts etc, I would have to reorder all the material which I won't do.

Finally I decided to just buy a good value stock fridge/freezer. Yeah right! rolleyes

I bought the Mobicool B40 for €250/$300, and overall I am very pleased with the value for money:

    Keep fridge in the shade at all times
    Keep fridge in the shade at all times
  • The B40 can run on either the peltier at 12V or the compressor at 230V (here)
  • with the compressor too, it is amazingly quiet (even I can sleep easily with it!)
  • the insulation could and should be thicker though: in the summer it turns on every 10 min, for just under 2 min
  • thus you must ensure that this fridge/freezer sits in the shade at all times!
  • it is just about big enough to stock food for a week for the dog and myself (so basically for him, so much he eats)
  • with the compressor it draws around 93Wh, but starts up with 400Wh to over 500Wh (always varies)
  • with the peltier it only used 46Wh but ran all the time in summer, and even so, it couldn't reach more than 19C below ambient
  • The B40 can be used as either a fridge or a freezer (to minus 14C / 6.8F in my test)
  • or it can be both fridge and freezer if you know where to put an insulation barrier inside wink
  • If you open it during hot ambient temperature, it obviously rapidly condenses the humidity of the air as water and/or ice on the walls and bottom (there is no drain plug)
  • Worst of all, it does not maintain the desired temperature range, even after you've figured out the right dial position: the built-in thermostat clearly is not a digital thermostat like I am using myself
  • However, this budget fridge does work even on bumpy unpaved and cobblestone roads, surprise!

This was a failed project: Setting out for a quality DIY fridge, but (for now) ending up with a quality stock fridge.

Next mobile tiny house living update is here

adult GSDWill you give back a bit today?

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