--- title: DIY Fridge date: 2015-07-23T01:44:40Z source: http://longlucas.bravesites.com/diy-fridge tags: solar, offgrid --- **Refrigeration is the major energy consideration for self containment. Most people choose a three way fridge running mostly on propane or a compressor fridge with substantial solar and battery equipment to power it.** ** ** **My approach is different.** **It is much simpler, more efficient, and cheaper to store "cold" than to store electricity.** **No need for excess solar with the expense and windage of the panels.** **No need for an excess of large, heavy and expensive battery banks.** **No need to park in the sunlight.** ** ** **What's the catch? You ask!** **Well, you have to build your own fridge. You cannot buy one off the shelf at the moment. They are all modifications of very inefficient household fridges and freezers with varying levels of quality and energy demands. ** **I first tried this approach twenty five years ago and have been using it ever since. Would never consider anything else now for mobile self containment.** ** ** **If you have to buy off the shelf, I would be inclined to choose a gas fridge, although I have never owned one.** **Unless, you want to make some substantial modification to that compressor fridge and be able to toss some of that excess solar and battery power. I am a fan of solar but not so much, of heavy and expensive batteries. **       **September 2007** **I came across this link today.** **It seems that [OzeFridge][1] might have just what you need if you would rather not build from scratch as I have done. Complete charged units with holding plate ready to fit to your insulated cabinet.**     **The above OzeFridge link led to an extensive discussion on the CMCA website about the advantages of holding plates.**   **Julian said: Quote: "Just an idle thought / glimpse of the future - with all these numbers it looks technically feasible to run a well insulated 100 litre eutectic holding plate fridge with BD35  directly from a less than 60W solar panel with no battery and very basic regulator. If there's not much sun it probably means the fridge doesn't need as much cooling. The panel size is dictated more by the amps needed for the motor rather than the amp hours for the fridge - the small BD35 is probably ultimately too big."**   **(BD35 is the smallest Danfoss DC compressor.)**   **Julian I do like your vision.  Domestic refrigerators have changed little over the last eighty years or so in basic operation design. Why should they when you can run a double door monster in the house for less than a hundred bucks a year. Changes to this basic design for the needs of boats and RVs has only scratched the surface of what must be possible.**       CMCA members are able to access the  technical article, I wrote in 2006 for the Wanderer: [FRIDGES FOR SELFCONTAINMENT][2] Pictures here will show some stages of the fridge-freezer construction. The only thing I had to buy was the urethane insulation ($100) and epoxy (see my post JOYS OF EPOXY) The rest came from disposal bins.( spotted gum here as well as the table and other bench tops) I chose to use a 240 volt system but all the following will apply just as well to a 12 volt setup. I have tried for four inches of urethane foam all around and two inches on the top. The lid gets pretty thick with four inches and heat gain through the top is less of a problem. The bottom of the box is the freezer and contains the ethylene glycol holding plate which I made from a stainless cafeteria serving tray. It starts here, bench testing the works from the icemaker. You can see the fingers freezing up. Owner said it was not working when he tossed it, but it made a tray of ice cubes in 15 minutes, held under  the fingers you see. The condenser has a powerful fan which seems to swith on about 10 seconds per minute when the compressor runs. I cut a hole in the bus side to face this condesser and fan. Air comes out the grill you see under the sink. ![][3] ![][4] The olive oil can was for my test of holding plate size. The evaporator beside it shows the finger protusions which made ice cubes in the original ice maker. I filled the olive oil can with half antifreeze and half water. Then the evaporator was pushed in. Three hours on the compressor and the antifreeze mixture was a frozen gel and at minus 13 degrees. ![][5] One problem I am working on, is the stratification of cold air at the bottom of box. I am trying a small muffin fan to circulate this air when the compressor is running. A better solution would be a thick aluminum plate, immersed in the freezer holding plate and extending up the side of the fridge. You can see the ledge above the stainless bowl. A plywood lid fits here to separate the freezer from the fridge. ![][6] You can see the mesh type drywall tape on the bottom of the lid. This is great stuff for taping the corners for epoxying. Much easier to use than fibreglass. I am still playing around with the seal on the drop in lid. I want to make sure humidity does not diffuse in to the box. With the lid in place you have a normal bench top. ![][7] My choice of a powerful 240 volt compressor results in two hours a day being a sufficient run under normal conditions once the fridge and freezer and contents have been cooled down. This initial cooling can be done at home on mains supply before you leave. You also have an easy system to run when ever mains power is available, but I am a free camper and have designed for that purpose. If you spend the money for a Danfoss compressor system at 12 volts, you will have greater efficiency but longer running times with the much smaller compressor. Then you are in to the issue of how many storage batteries and how many solar cell panels and the chargers etc. I have to run a, hopefully, quiet generator, for two hours. I might also get by with a single house battery. UPDATE JUNE 2007 Before you read further, I finally tested my fridge above on a generator. LaWrie came around and we checked out his little Honda 10i. What a great little machine. Started my 240 volt compressor just fine despite the required initial draw of nearly eight amps for a fraction of a second Even did so on the superquiet eco throttle setting. Am I ever impressed! There goes the budget as this is one item I will buy new. Will get the 5 year warranty too, because Hondas are not really a repairable item when you check out the unbelievable high prices they want for even a small part. Will donate Mike's gift of the Danfoss fridge to Julian for him to have a play with. Could never imagine forking out a thousand bucks to replace a failed Danfoss compressor. I think Danfoss and Honda must chat to each other. Geez. I was quoted double the price of one of Val's little fridges using a Danfoss compressor for just a replacement compressor, so look for a long warranty on these as well.   CHANGES MADE FEB 2008 I did some experimenting to improve the holding plate. The goal was to increase the cold holding capacity and to improve the rate of heat transfer from the evaporator for efficiency. The first trial with beer cans was less successful than hoped for. Two problems were that the shape of the beer cans reduced the volume of eutectic available for phase change and the Sikaflex was not quite up to the pressure of the expansion. The soft gel packs solved both problems.  I cut up a thick aluminium pot to place between the evaporator fingers and the gel packs. Aluminium is a good conductor of heat, water and ethylene glycol is not. The stainless tray to the upper regions is an attempt to improve cold air stratification. When the bowl you see below has another stainless tray sealed on top, there will be room for a dozen cans of beer filled with beer, together with room for meat and fish. The plywood partition then sits on top of the supports you see below to separate the freezer compartment from the fridge compartment. Depth looks a bit foreshortened in the  picture. The space to the right is tall enough for  a three litre milk  container. Plenty of room in this fridge for a week or more of fresh food stuffs. First let me state that if I were building a motorhome fridge again, I would make a couple of changes. My system has cost very little, well under a couple of hundred dollars as I salvaged a discarded 240 volt closed system from an ice cube maker. This compressor is powerful at 550 watts and has the ability to take the fridge holding plate down in a short time. Tests indicate that a run of an hour at a time is probably most efficient to allow time for heat to be absorbed through out the glycol bath. A small Danfoss 35 compressor would not present this problem as it would likely operate at a rate slow enough for suitable heat dissipation, hour after hour. A single solar panel, running this Danfoss compressor during available sunlight could store cold in a similar holding plate. No need to store energy in heavy expensive batteries. If the sun hides for a few days the holding plate will keep all cosy cold until it shines again. No need for the generator. To construct such a fridge, I would look for a used or case damaged fridge with Danfoss compressor, to salvage the working sealed system. Heck, even buying a new one to tear apart would save you a bundle over trying to buy the parts separately. With a closed sealed system, you will not need to pay a refrigeration mechanic to assemble and charge the components for you. Gone are the days when the handy man was permitted to do all of this for themselves. The picture below show the soft freezer gel packs in the holding plate. The beer cans were abandoned in favour of the gel packs. The gel packs get a greater volume of eutectic fluid in the same available space. ![][8] In my original boat fridge I constructed the holding plate evaporator from the core of a car radiator. Heaps of surface for the heat exchange in the glycol bath, but a 50% mixture may have had limited change of state for that latent heat capacity. I think that change of state might take place at the microscopic level, with the water in the glycol, but have no idea of the ratios involved. ![][9] March 18 2008 It has been a little over five full days since the fridge compressor last ran. Tonight the lower box temperature had risen to 2 degrees suggesting that the last of the phase change in the holding plate had occurred. As this is my third night on the computer and TV and LED lights, without driving, it is about time to charge up my 70 Ah AGM house battery as well. Some of the energy for this also came from my starting batteries during this time of staying put. Had I driven for a bit each day, the charging would have been handled by the alternator and inverter charger set up for the house battery. So I am running the generator for an hour as I write this, charging batteries and cooling the fridge. I will run the generator an hour each day from now on, until mains becomes available again, and the cycle repeats. You can use the fridge and freezer to suit requirements. To keep your ice cream hard, it would be necessary to run the compressor every day, maybe for an hour. I am satisfied with the freezer content just remaining solid, as they are there only for relatively short term, so the 0 degree plateau suits my needs at the moment. To do without need of a generator you could build a holding plate fridge from a sealed system using a Danfoss BD35 compressor. That is the smallest. A solar panel of sufficient capacity to run the compressor during sunlight hours could cool down the holding plate. The compressor could be turned off when the sun does not shine and the stored cold in the holding plate takes over. House battery capacity could be sized for other needs as it would not be needed to store energy for compressor operation. Some battery capacity may assist the compressor in the very short term during it's sunlight hours operation. We can do some solar panel size calculations but I suspect that a single panel for the fridge needs could be sufficient. This could be part of an overall plan for energy efficiency and sufficiency at modest cost. Modest, because you do not need a lot of solar or a lot of battery capacity. Key points would include: Effective design of shape, size and insulation of the fridge box with at least a 10 litre holding plate At least 80% of the holding plate contents designed to undertake a complete phase change Use of LEDs for lighting Modest size of LCD TV Efficient laptop For most people the other uses of electrical energy are very modest provided it is not used to cook anything or heat anything.   **Holding plate holdover performance** The graph below was plotted in October 2013 to show how long the fridge/freezer would store cold after the compressor was shut off. It represents the temperature of the holding plate measured between it and a wine skin with 4 litres of water to add to the hold over time. You can see that freezer content would remain frozen solid until the seventh day when it would then start to thaw. Because of the fridge design, the temperature in the fridge part of the box remains suitable for such content even when the holding plate dropped to -300C. The lettuce will remain unfrozen and the milk will not sour.****  ![][10] HERE IS ANOTHER STORY Part of the story relates to the problem of determining battery state of charge from voltage readings alone. It can be quite useless information. Firstly, state of charge can vary by ten percent or so for just one tenth of a volt difference in the measured voltage. Now we have to assume that you have had your meter calibrated to ensure such accuracy. Try a few DMMs and see if you get a consistent reading to one tenth of a volt and you will see part of the problem. There are other problems every bit as serious.   Val has a Waeco on sale ,cheap, half the price of the Danfoss BD35 compressor alone. I am about to go out to the bus and check out a little fridge with a Danfoss compressor. A friend donated it. Bit of work to adapt it to good efficiency. Will save four hundred bucks at Mr Val's though for a Waeco, ready to go.   It does seem very difficult to get the battery state of charge message out to the non believers. It is use your brain and pay attention, or spend the four hundred bucks for the thinking electronics. I had the same problem many years ago with my DIY decent boat fridge which worked. Many of the other boaties were hopping from port to port trying to get their underperformers fixed. LEDs are a similar story. Have to drag em along and say, look for yourself. Is this whole area lighting or not? All this great light and single digit watts. Theory is fine and we have lots of it discussed. Have to adjust that theory to fit the empirical data though. It is a slow process getting the word out. Have to understand the reader too. He has to figure out who to trust. THREE HOURS LATER Now for a bit of the empirical stuff. Checking on that free fridge which has been running a few hours now and cooling well. All data at 12 volts. Fridge drawing 4.6 amps Charger putting 4.8 amps in to 70 amp-hour Fullriver AGM. Charger is the hundred dollar solid state job from Jaycar. Now, the Jaycar charger has no trouble putting out its rated 12 amps so the above data gives me a bit of an idea of the state of charge of my Fullriver. Charger was putting in the 12 amps three hours ago with the fridge also running. Next, I am gonna check out running the Jaycar charger from my 350 watt modified sine wave inverter ( $30 on eBay I think) connected to my 24 volt starting batteries, also with the fridge running. I have heard the theory Will post the results when I do. OK Fridge has been running FOUR HOURS now Fridge and charger both showing 4.6 amps. Logical I guess as the current is probably going straight to the fridge. Now if I thought a voltmeter reading would tell me anything useful, I could measure the battery voltage. I figure I already know it's state of charge so why mess with useless data. I switched the Jaycar charger off for half an hour to use some of my battery capacity. Connected up my non sine wave inverter to the charger and switched on. No Sparks or loud noises or claps of thunder. Charger running as before. Fridge still 4.6 amps Charger doing it's 12 amp thing Feed to inverter 6 amps and dropping. Hang on you say, not possible. Remember that the inverter is running from 24 volts. Back to the charger, couple of minutes now and it is down to 9 amps. Explains that dropping inverter current. Another FIFTEEN MINUTES! Fridge drawing 4.5 amps Charger putting out 4.7 amps Inverter drawing 3.7 amps (Think 7.4 amps in 12 volt terms) QED Now what do we have: Fridge with Danfoss compressor FREE Charger from Jaycar $100 Inverter from eBay $ 30 Fullriver AGM from Mr Val $170 My bus already has a serious fridge. See Picture above If I use the guts of this one to make a decent little box it will cool down nicely while driving. Might be just the thing for the lobsters and icecream. Fresh road kill maybe.![][11] Now, I am gonna back off just a smidgen on how I have been saying that voltage is useless data for state of charge. Not quite, especially for AGMs, but you have to be very sure how to interpret it and understand the conditions when you take the measurement. It is not like your fuel guage. Just again on my choice of a  MSW inverter. I did some testing with the charger running from the pure sine wave of the mains supply and also from the MSW inverter. The inverter did just a bit better. If that surprises you, it is in fact logical when you consider how the input of the charger accepts the different ac waveforms. GAS BOTTLES The gas bottle box below is fireproof and vapour proof to the inside of the bus. Access is only possible standing on the ground outside. This is an early stage in the construction and shows the way the bottles are secured by a circular cutout to snugly fit the bases. It is impossible for the bottles to move inside the box and there is no need for additional securing hardware. It was done this way on my boat and there was never a problem crossing oceans. Vent holes and pipe routing is through the floor of the box as per the standards. ![][12] [1]: http://ozefridge.com.au/ [2]: http://www.cmca.net.au/pages/members/technical_articles/index.php?article_id=100 [3]: http://myimages.bravenet.com/319/446/510/2/Fridgexsealedxunitxsnapshot.png [4]: http://myimages.bravenet.com/319/446/510/2/Fridgexconstructionxdetailxscreenshot.png [5]: http://myimages.bravenet.com/319/446/510/2/evaporatorxtestingxfourxinchxscreenshotx.png [6]: http://myimages.bravenet.com/319/446/510/2/fridgexlidxscreenshotx.png [7]: http://myimages.bravenet.com/319/446/510/2/winexskinsxscreenshotx.png [8]: http://myimages.bravenet.com/319/446/510/2/Freezerxbagsxscreenshot.png [9]: http://myimages.bravenet.com/319/446/510/2/Finishedxfridgexbottomxscreenshotx.png [10]: http://longlucas.bravesites.com/files/resized/122528/560%3B478%3Bf8852030dff026cfbadb0372ea80e34f7bf55658.jpg [11]: http://images.bravenet.com/common/images/smilies/1_grin.gif [12]: http://myimages.bravenet.com/319/446/510/2/propanexlockerxscreenshotx.png