Energy saving inverter/refigerator hack!

[alpine44]

The annoying thing with DC to 110AC inverters is their idle draw. In some cases, like a small refrigerator, the idle draw of a bigger inverter can exceed the power consumption of the appliance over the course of a day.

I just modified a free (roadside find) 110V office refrigerator so that its thermostatic switch does not turn on the compressor but instead a cheap 12VDC to 110VAC 300W inverter. Now, the inverter is only on when the refrigerator demands power and the compressor is wired directly to the output of the inverter.

Voila, no more need to spend close to a grand on a 12VDC refrigerator that only consumes electricity when running. The same could be done with an inexpensive 110V chest freezer.

[john61ct]

Yes lots of HowTo threads on doing this, including 12V thermostat.

But only 240V Euro/antipodes fridges have energy efficiency like 12V compressor fridges, so

IMO not worth it if living off-grid mostly-solar not driving frequently

[alpine44]

Originally Posted by john61ct

Yes lots of HowTo threads on doing this, including 12V thermostat.

Sound ideas tend to gain popularity.

Originally Posted by john61ct

But only 240V Euro/antipodes fridges have energy efficiency like 12V compressor fridges, so

IMO not worth it if living off-grid mostly-solar not driving frequently

We should ask ourselves whether the operating voltage or the country of purchase are direct reasons for the efficiency difference or whether there is some other causality that follows the laws of physics.

The Kenmore office fridge has a condenser hidden beneath the side walls. You can feel them getting pretty warm when the fridge runs. What separates the space you are trying to cool from this hot area is less than one inch of injected foam. Cheap to make but certainly not efficient to operate.

My marine fridge with 12VDC swing compressor has 2" of insulation and the condenser coil is separated from the fridge by another 2" of air. After studying several compessor spec sheets, l guesstimate that the refrigeration components account for maybe 10% of the lower electricity consumption and that the major efficiency gain comes from better insulation of the cold space. Even that could be improved in the door area.

The next experiment with the free office fridge will be to extract the refrigeration components and putting the evaporator (the plate getting cold) in a very well insulated chest, custom built for my food storage needs and the available space in my vehicle.

You can buy refrigeration kits for building marine 'ice boxes' but these kits are as expensive as the good 12VDC fridges/freezers due to the low production volume. Let's see how less costly, high-volume components perform in a carefully designed context.

[john61ct]

Originally Posted by alpine44

We should ask ourselves whether the operating voltage or the country of purchase are direct reasons for the efficiency difference or whether there is some other causality that follows the laws of physics.

Certainly nothing to do with either.

All down to regulatory requirements, mandates for ever-increasing energy efficiency letting free enterprise finding the HowTo,

in countries that haven't allowed the regulated industry to cripple the process by capturing the bureaucracy.

[Drew Bru]

Originally Posted by alpine44

The next experiment with the free office fridge will be to extract the refrigeration components and putting the evaporator (the plate getting cold) in a very well insulated chest, custom built for my food storage needs and the available space in my vehicle.

You can buy refrigeration kits for building marine 'ice boxes' but these kits are as expensive as the good 12VDC fridges/freezers due to the low production volume. Let's see how less costly, high-volume components perform in a carefully designed context.

That sounds like an interesting project, I'd be curious to see what the results are when you complete it!

[brokedown]

I measured my inverter's idle draw as 0.6a, about 7 watts. I'm much more offended by the losses in converting DC to AC than the idle draw, but with that said I keep it turned off and my refrigerator is a 12 volt unit.

[alpine44]

Originally Posted by Drew Bru

That sounds like an interesting project, I'd be curious to see what the results are when you complete it!

If you look at the formulas governing the energy demand of a refrigerator/freezer, you'll see that the R-value of the box and the coefficient of performance (COP) of the refrigeration components have the same leverage on the result.

It is easy and cheap to double and even triple the R-value of the box but impossible to double the COP of a typical compressor refrigeration system. For several hundred bucks worth of components and a lot of design and testing effort you may be able to raise the COP of a standard, mass-produced system by 20-30% on a good day.

So the low hanging fruit is clearly to improve the insulation of the cold space.

The same applies to heating or cooling of the entire bus; insulation offers the best bang for the buck.

[alpine44]

Originally Posted by brokedown

I measured my inverter's idle draw as 0.6a, about 7 watts. I'm much more offended by the losses in converting DC to AC than the idle draw, but with that said I keep it turned off and my refrigerator is a 12 volt unit.

What is the rated output and efficiency of your inverter?

My box van came with a 1000W pure sine inverter, connected to the dual staring batteries. I accidentally left the inverter on and it drained the batteries in less than two days to the cutoff voltage of the inverter, which -according to Murphy's law- is lower than what the 7.3L diesel needs for starting.

If my math does not fail me, a well insulated 8 cuft fridge would consume about 5W (~5.5W input @ 90% inverter efficiency) continuously to maintain 40F at 80F ambient. If we assume that the fridge has a 100W compressor and factor in startup/shutdown losses, the duty cycle would be somewhere between 10 and 20%. That means, the idling inverter would consume roughly as much as the running fridge (80% of 7W= 5.6W).

I will report the actual numbers once I buy a fancier DC power meter.

[alpine44]

Originally Posted by john61ct

Certainly nothing to do with either.

All down to regulatory requirements, mandates for ever-increasing energy efficiency letting free enterprise finding the HowTo,

in countries that haven't allowed the regulated industry to cripple the process by capturing the bureaucracy.

The reason why we have minimum efficiency standards for air conditioners but not for refrigerators is not some corporate conspiracy but the fact that fridges hardly make a dent in the overall residential energy consumption. Why would anyone care about the efficiency of a fridge that sits in an energy-hogging McMansion?

Also, the story of Tesla leads me to believe that ultimately not government mandates (that were there before) but compelling and game-changing products (instead of lame 'álternatives') change consumer choices.

[joeblack5]

I agree with the additional insulation. Yes new 12/24 fridges are in $600 to $1000 range but good used one are more around $100.


The motors of these are mostly danfoss Bd35 that is designed to be more efficient then the ordinary fridge motor. Either way... insulation is the main part of the package...and that means a separate condensor that is not part of the outer skin of the fridge /freezer.


We added additional insulation and a condensor cooling fan to our Norcold. The consumption data is some where in the Elf bus and Dory build.


You must have had a good inverter or very good fridge to get away with a 300 watt inverter.What inverter did you use. My 500 watt pure sinewave did not reliably start a normal older under counter fridge.



Johan

[gs1949]

Interesting thread. Thanks to everyone who has posted.

[Doktari]

Good thread. Im going to refine my energy usage and system. Im cycling my batteries much more than necessary leaving the inverter on most the time. Picked up a DC fridge to install. Maybe I’ll super-insulate it.

[Native]

Remember, you have to insulate the "cold area" and not the whole thing. On most high production units, this will take a bit of design work and effort. I am not saying it can not be done. I am not saying it should not be done. Just wanting y'all to be aware it is not as simple as putting the refrigerator in an insulated cabinet.

[bus-bro]

This thread just tells me to get a 12 v refrigerator and run it direct from the batteries. I'd go with a portable one. Anyone got recommendations?

[alpine44]

Originally Posted by bus-bro

This thread just tells me to get a 12 v refrigerator and run it direct from the batteries. I'd go with a portable one. Anyone got recommendations?

Your conclusion is exactly the opposite of what I tried to suggest with this thread.

If you want to throw money at the problem rather than creativity, then Engel is the gold standard for portable 12VDC fridges/freezers. Be prepared to spend well over $500 even for a gently used one. (I have been looking for a while on CL and fleabay).

You can occasionally find a good deal on a non-portable Norcold marine/RV fridge, like Joeblack5 mentioned. Verify that you get a compressor-driven unit since Norcold also makes absorption fridges that are very inefficient on electricity but may be run on propane.

[alpine44]

Originally Posted by Native

Remember, you have to insulate the "cold area" and not the whole thing. On most high production units, this will take a bit of design work and effort. I am not saying it can not be done. I am not saying it should not be done. Just wanting y'all to be aware it is not as simple as putting the refrigerator in an insulated cabinet.

Yes, the condenser and compressor need to be exposed to ambient air. The metal grid on the back of a fridge is the condenser and the compressor is the black, roundish thing on the bottom rear.

[alpine44]

Originally Posted by joeblack5

I agree with the additional insulation. Yes new 12/24 fridges are in $600 to $1000 range but good used one are more around $100.


The motors of these are mostly danfoss Bd35 that is designed to be more efficient then the ordinary fridge motor. Either way... insulation is the main part of the package...and that means a separate condensor that is not part of the outer skin of the fridge /freezer.


We added additional insulation and a condensor cooling fan to our Norcold. The consumption data is some where in the Elf bus and Dory build.


You must have had a good inverter or very good fridge to get away with a 300 watt inverter.What inverter did you use. My 500 watt pure sinewave did not reliably start a normal older under counter fridge.



Johan

Johan, I hooked the free office fridge up to a 1000W Xantrex Prosine inverter and it never indicated more than 200W draw but the display may nor update quickly enough to show a higher startup spike. OTOH, the label on the Kenmore freebe indicates 1.9A @ 110V.

I have a Chinese 300/600W pure sine inverter on order and will report the results once it arrives.

My immediate access fridge will be a very efficient marine Norcold with 12V swing compressor. It is on the small side but fits under my stove/baking oven and can be easily reached from the driver seat.

I wanted another fridge/freezer in the back of the vehicle that can run on the 60V battery bank so I needed to go through an inverter anyway. Buying a R134a inverter compressor for 60VDC and building the system from scratch did not look cost effective.

The intent of this thread was to explore what can be done with inexpensive, mass-produced components like a 110VAC refrigeration system and a common DC to 110VAC inverter.

The first idea was to get rid of the inverter idle draw and then I realized that spending some thought and little money on insulation improvement may be preferable over buying a fancy niche-market product when it comes to cooling/heating in general. Just google "Sunfrost worth it?"

[john61ct]

Originally Posted by bus-bro

This thread just tells me to get a 12 v refrigerator and run it direct from the batteries. I'd go with a portable one. Anyone got recommendations?

I agree. Engel for the win, very efficient and robust, can last for decades even with harsh treatment.

Get two identical, either or both can act as freezer or fridge.

[alpine44]

Originally Posted by john61ct

I agree. Engel for the win, very efficient and robust, can last for decades even with harsh treatment.

Get two identical, either or both can act as freezer or fridge.

If I needed something that I can throw in the back of my new $20,000+ Side-by-Side for the next weekend with my equally well heeled buddies, I would be delighted to spend the coin on an Engel and have it overnighted to my doorstep.

However, I am dealing with some interesting challenges trying to fit a kitchenette, dinette, queen bed, shower, and flushing toilet into a 14' long, 7.5' wide and 7' high box. And that box with wheels, engine, and everything else that makes it a functional vehicle cost me about 1/3 of the retail price of the fancy fridge/freezer.

So, before we hit the "Add to cart" button let's ask what a fridge/freezer is anyway. First and foremost, a well insulated box. Plus a compressor, a condenser, an evaporator plate, and a few copper pipes. This is not rocket science and were are not trying to beat Elon Musk to Mars if we would dare to build our own fridge.

Companies like Engel do not make any of the refrigeration components in house. They get better pricing from the OEMs because of their purchase volume but they do not use anything that you could not buy directly.

What they produce in house is the insulated box plus handles, wheels, and a lot of marketing. From the sales pictures and videos it looks like they use 2" of insulation at most.

I hope it became clear here that increasing the insulation by 50% will surpass any efficiency gain that could possibly be had with a fancy compressor, which Engel claims as their magic ingredient.

If the geek talk about COP, R-value, and heat transfer is over your head, then ask your grand parents how long a bag of ice lasted in what they called an 'ice box' back then. You will be amazed what good insulation can achieve. Our regular household fridges with their thin walls are fancier but atrociously inefficient and nobody cares due to the relatively low cost of electricity.

I am not trying to knock Engel; they make solid products that have their merit for some needs. But why do people buy a skoolie, add insulation, and then install a mini-split instead of just buying an RV with AC? Why are commercial freezers built from insulated wall panels and then outfitted with off the shelf refrigeration units? In both cases you get exactly what you want in terms of size, shape and features - often with better efficiency than the one-size-fits-none products.

[john61ct]

In which case, follow the usual practice for boats, build a custom box whatever shape you like, 4-6" insulation, then add an off the shelf 12V compressor kit.

Unless you're plugged into shore power most of the time, it really is worth spending a bit more to maximize energy efficiency.

IMO, obviously each owner should do what they like, just sharing information and opinions here.