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Forum Index : Other Stuff : Poor mans Eutectic Fridge
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| mattma Newbie  Joined: 01/10/2012 Location: AustraliaPosts: 25 |
Hi I wanted to float an idea and see whether people thought it may have enough legs to give it a try. The idea is for a solar powered fridge for use in our Hiace van that is decked out as a camper. I really like the idea of thee eutectic fridges I have read about where the compressore run at their most efficient ( high) rpms and freeze down containers full of fluid with freezing point well below zero (eutectic plates). The fridges as I understand only need to run for a few hours a day to store up the cold in the plates. Quieter, more efficient so less battery. Problem is they are expensive, need additional batteries and I can't really vent the heat from the compressor effectively inside the van. So the idea I am tossing around at the moment is to move away from the integrated eutectic plates and create a roof mounted freezer to freeze down container of eutectic fluid that could then be placed in the esky and swapped out as they thaw and refrozen. My idea is to take the refrigeration circuit out of a conventional bar fridge (compressor, evaporator...), build a very well insulated box to be the new freezer cabinet and drive it off a solar panel and inverter ( running only when the sun shines, no batter storage). Effectively this would be a bar fridge running as a enlarged freezer only cabinet - although 10 L would probablu suffice as this would be equivalent to about 2 5kg bags of ice which does us fine for a couple of days. The swappable plates I would just make out of stainless steel oven dishes and use either salty water or those blue freezer packs immersed in coolant. Would this work? Is there any easy way to estimate how long it would take to freeze a certain volume of plate? What are some of the other aspects that I should think about in the design? Looking forward to hearing peoples input. Cheers Matt |
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| Dubyagee Newbie Joined: 15/06/2007 Location: United StatesPosts: 21 |
I wonder if this could be integrated into the design proposed? http://en.wikipedia.org/wiki/Icyball Regards, Dubyagee |
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Tinker Guru  Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Matt, I have an eutectic fridge on my sail boat. The eutectic container has the evaporator coils inside so I think your idea of freezing something like it externally from the insulated fridge does not sound practical to me. You can get eutectic fridges which look just like the conventional ones - Autofridge is one brand - that would do you. Now, keep in mind, if you want to power such a fridge from solar only you have to do your utmost to keep losses at a minimum and efficiency at a premium. Swapping cold containers around does not come into that equation. You need _very_ thick insulation, I use 75mm on my boat fridge which is purely solar powered. 75mm is not enough, 100mm would have been better. My compressor (Danforth) is rated for a 125 liter fridge compartment, I only have around 40 liters. And that requires about 140W of solar panel power to keep the contends at fridge temperatures ( < 5 deg. C ) on the average summer day in Perth. To freeze anything I would have to fire up the small 12V generator or plug in shore power. The eutectic fridges, including mine, are really designed to run from engine power for perhaps a couple of hours a day. This would completely freeze the eutectic fluid and, with excellent insulation, one can get into the eutectic holdover temp range. As it is with mine, solar power gets it only down to fridge temperatures, especially since the solar panels are almost never optimally oriented. Back to the drawing board for you perhaps?   Klaus |
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| mattma Newbie Joined: 01/10/2012 Location: AustraliaPosts: 25 |
Thanks for the ideas and input. Much appreciated. Tinker, if the box is very well insulated why is it that the external evaporator coil be would be less effective in cooling the storage plates? Just thinking out loud it seems that the same amount of heat would be removed per unit volume of refrigerant flowing around as the operating temperatures throughout the circuit would all be the same. Is it to do with practical limitations on insulation in the cabinet? If I understand what you said about needing to fire up an external power source to get freezer temperatures it it because you need extra power to run the compressor at higher speeds so that in a fixed amount of time you have a greater volume of refrigerant having passed through the evaporator. So, presumably being able to leave the compressor running for a longer period of time at the lower rates would do the same thing? In this roof top ice maker idea the frozen blocks would last around 3 days in the esky and so there would be a window of 3 days to freeze around 10kg of water ( and then swap the thawed plates in the exky with the newly frozen ones. To run the numbers I guess I need to know the amount of energy that the cooling circuit can remove in a given time at a given rpm. This sounds like the most practical way to get this is to measure the rate that a certain volume of water cools in it? Cheers Matt |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Matt, what do you think is more efficient: cooling one insulated box or cooling two with the same energy input. You are aiming for the second example Klaus |
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norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
Todays 12/24 volt compressor(Danfoss) fridge/freezers are capable of running the compressors at varied RPM`s. Basically using a solar - battery combination with one of these compressors, allows you to run at full revs and continuous whilst the sun is powering your panels than switch over to lower revs non-continuos (thermostat or timer controlled) of a night. ie at full revs comp is drawing approx 5amps at lowest revs approx 1.5amps whilst running. Size your solar-battery combination with this in mind. In a very loose sense you will be using the battery as a Eutectic , and in a mobile situation without using a motor driven compressor as Tinker explains, that is the way to go. Just google the amount of heat required to be removed from water to turn it into 10kgs of ice and you will see that using solar panels in a mobile application, is just not feasible. Go with solar-battery-12/24v compressor fridge/freezer combination(some are made with a eutectic plate). Load whilst the sun shines and enjoy a cold drink all the time. Employ the KISS principle. We come from the land downunder. Vic |
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| mattma Newbie Joined: 01/10/2012 Location: AustraliaPosts: 25 |
Hi Many thanks for the input everyone. Norcold, a quick follow up question. If the Danfoss compressors only draw 5amps at full speed and from what I have read on RV forums once the eutectic plates are frozen, the compressor in a circa 50L cabinet only needs to run at full speed for 4-5 hours a day wouldn't solar only with say a 150W panels be enough to keep it cool without the need for a battery? Cheers |
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| norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
Unfortunately the danfoss comp will not run directly from a solar panel, the features built into its electronics, to protect the comp,(they hate anything but pure DC within a narrow voltage range) will not allow it to run direct. I know of no way around this without a battery, perhaps super caps may do the job, but have not tried or heard of anyone trying. Perhaps one of the electronic guru`s on the 4M know the way around this without a battery. We come from the land downunder. Vic |
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yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
I aint no electronics guru but I will have a go at it. The power module that drives the "brushless DC" motor is the key. You should be looking for a fridge that has a danfoss module with Adaptive Energy Optimization to get the lowest wattage. Danfoss 12/24v Power Module for BD50F/35F w/AEO (3-pin) part No 101N0320 There is two resistors fitted to the thermostat wires that control how the compressor runs and battery management. fitting a 220 Kohm resistor between two terminals lets the module run without a battery with a voltage range of 9.6 volts to 31.5 volts, specifically for running from solar panels. leaving the second resistor out activates the AEO function. I think a cap would help with morning startup for sure. Typical performance for the BD35F compressor is around 480 watts per day in an average small fridge (120 litres) with a freezer compartment.Average run time of about 30 % on cool days. The official rating is at 21 degrees C. Adding an extra 50 mm of insulation will halve the run time to 15% and watts to 240 a day. My take on this is you would have no problem running a freezer, get it down to -23C and it would probably be at -17C when the sun got up the next day. With a fridge that has a good sized freezer compartment, it would go close as well. I don't think that something that is all fridge would make it through the night, the margin is too small. You really need to stay between 1 and 5 degrees. too cold damages the vegies and around 7 degrees is to hot for safely holding meat. I personally think this whole eutectic fluid thing is a bit of a con, if the vent hole between the freezer compartment and the fridge is closed off a bit more with a bit of insulation and the freezer compartment is run closer to its minimum temperature and full of food or ice the effect will be the same. Also the lower the temperature the air flow around the heat exchanger area is, the better, there is a physical limit to the difference between ambient temp and possible minimum freezer temp. I'm confused, no wait... maybe I'm not... |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
There is another alternative. Danfoss used to make a solar power module for the older DB35K fixed speed compressor. They probably still sell it. check this PDF out Solar BD compressor voltage range is 10 to 45 volts only catch is this compressor is not suitable for mobile use and it does not have the soft start feature like the DB35F. edit OK sorry, stuffed up there, if I had read the PDF instead of going from memory, I would have seen that the module works with the latest compressor SWEET! I have not had much luck finding an Australian supplier for replacement power modules. The last price I got was around $670 locally vs $280 inc freight from OS. Danfoss compressor datasheets PDF 1.1MB I'm confused, no wait... maybe I'm not... |
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| norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
Well I`ll be. Correction Mattma- the older BD danfoss compressors will not run directly off solar(non adaptive module) to my knowledge.(better add that disclaimer). Many thanks for that yahoo2, I`ve been out of the game awhile. Like you, I don`t have much confidence in Eutectic in mobile 12v systems.(but even in my era the "gimmick" sold thousands) Different proposition in motor driven. You need quantity of eutectic thus weight and much more than 60 watt comps. Better to have a few more stubbies as eutectic. We come from the land downunder. Vic |
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| norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
outbackmarine.com.au carry the Veco range (Italian) of refrig. gear Amongst these I notice a small board that is connected to the BD35F with the non-adaptive electronics, claimed to alter the RPM of comp automatically in line with load. They also have a electronic unit for the old 4pin comp. We come from the land downunder. Vic |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
This has been a good little chat. I have been chasing a fault in a fridge for a few months and it has just occurred to me that I haven't checked the battery management resistor. Good to know about the veco stuff Norcold, anything that prompts danfoss to sharpen their pencil on parts is a good thing. There used to be a minimum order on power modules of 25 units, so only the service agents stocked them and they would not sell them, install only. $$$$ I'm confused, no wait... maybe I'm not... |
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| norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
Don`t like the chances of Danfoss Australia ,sharpening their pencils. My association with them was from when they were Russel Armstrong, I bought comps (BD3`s) by the pellet, than Danfoss Aust took over and service went downhill. A few good fellows gave good service but they moved on. Always had the impression their products being top notch, they didn`t need service. Although out of it now am keenly following the Chinese copies, they`ll get it right. Have an old customer(25yr old unit) approached me a month or so back, his electronics had smoked. That is when I found the Veco replacement electronics, will be following it up once he has tested it with the coming winter season. We come from the land downunder. Vic |
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| mattma Newbie Joined: 01/10/2012 Location: AustraliaPosts: 25 |
Wow. Thanks for all the great information. It seems like there technically it will be possible to create the solar only roof top ice maker. Keys seem to be very well insulated cabinet and hacking the power unit. In order to replace the compressor in an existing fridge with a 12V one, am I right in assuming the removal and re-connection need to be done by someone qualified (I'm in AU). Is the local fridge repair shop likely to be able to do this or is this a more specialized offering requiring someone with specific qualifications? Thanks again for the great thread. Matt |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Technical and practical can be worlds apart. For example, I remember from my school days that 1 Kg of water needs about 100Kj of energy removed to get from classroom temp to 0 C, then 335Kj to phase change it from a liquid to a solid and another 100Kj removed to get to -25 C. So you can see that there is a massive difference between freezing water and keeping something that is already frozen, cold. Roughly 5 to 10 times the heat needs to be extracted. Every compromise you make in efficiency for the cooling unit, insulation, ambient temp, compressor run time and airflow at the condenser will make it less likely to work. I guess Norcold and I are both saying consider the alternatives before you spend your hard earned. perhaps pack a good esky with frozen stuff and chuck in a slab of dry ice before you leave. Buy frozen gear along the way. Run a fridge with a freezer compartment and add insulation to that. Budget to go to the pub and have a counter meal towards the end of the trip. My parents had a van custom built with all the gear, they done one big trip through the top end of WA for 6 months, since then, it has sat in the shed for 30 years. My mum tells me that it is cheaper and easier for them to use cabins or B&B's than get the van out for a short trip.  I don't know, I don't do holidays, its too exhausting for me.I'm confused, no wait... maybe I'm not... |
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| mattma Newbie Joined: 01/10/2012 Location: AustraliaPosts: 25 |
Thanks again Yes agree that the alternatives are very attractive and will most likely be the solution. However, I thoroughly enjoy learning about and tinkering with with projects made of found and re-purposed parts and refrigeration is something that has got my interest at the moment. Its has been great to get some much rich input to ground my thinking about the latest project. I'd like to be able to be off grid for about a week so maybe something that can slow the melt of ice might have more pay off. The esky I have holds ice well for 3-4days so focusing on stretching that seems like a more fruitful first project. Thanks again for all the help (and data which is just gold!) |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Sorry about this Matt, I wrote a reply to one of your questions,  and never posted it  ha.. ha...
Here it is, Better late than never I guess 
I'm confused, no wait... maybe I'm not... |
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