Running an AC fridge efficiently

[CaneyJ]

I’ve got an unmodified domestic mains freezer running as a fridge using an external temperature controller [which is easy to setup] to consume just 12-13AH per day including the Inverter and factoring inefficiencies.

 

Compared to my current Waeco CF-50, it uses around half its power in a day. Here’s how.

 

I took a LEC 32L counter-top freezer with thicker wall insulation than its fridge counterpart and hooked it up to a digital temperature control relay with a 10-Amp switch rating that runs off a separate 12v supply.

 

The temperature controller can cut the mains feed to the freezer via an inline AC socket and plug which goes between the Inverter’s output and the freezers AC cord.

 

It was setup to cut the delivered AC to the freezer every time the thermal probe [slipped through the door seal on a thin wire] dropped back to 4°C.

 

Observations showed it switched on the AC power feed when the temperature reached a steady 6°C.

 

The freezer would run for about 5 mins only around once per hour and cut the compressor at 4°C before the temperature bottomed-out at around 3°C.

 

I plan to combine this freezer with a TBS Powersine PS600-12 Inverter which has a standby mode that only draws 0.4W when there is a zero-watt power demand [therefor using only 1AH in 24 hours!].

 

 

 

Initial power consumption results using the converted freezer on a mains watt-meter

 

When running; the freezer’s compressor draws around 52-watts average [with a 659W max peak].

 

Over 24-hours it consumed only 0.123KwH!

 

This worked out that the converted freezer was on only 10% of the time or around 5 - 6 mins for every hour.

 

This was with the freezer [now acting fridge] half-full of food, pre-run down to temperature before readings began, the door never being opened and within a 21° environment all day

 

The quoted yearly consumption on the standard LEC freezer is 146KwH per year. As a converted fridge with the external thermostat set to 4°C it now only uses 45KwH in a year!

 

As a comparison on efficiency; I hooked up the watt-meter to our old and large Hotpoint fridge / freezer combi at home and reported a daily draw of around 1.4KwH over 24 hours or 511KwH in a year.

 

The mains watt-meter must also be factored-in as that itself had a constant power draw of 2 Watts.

 

I also measured the 12V power consumption of the digital control relay [which was running off a separate 12V power supply] separately using a multi-meter and found it drew around 0.1 Amps or 1-watt constantly.

 

When you also factor-in the 0.4-Watt hourly draw of the inverter in its standby mode I could count the 0.123KwH reading as including both the temperature relay controller and the Inverter by subtracting the watt-meter’s 2-watt constant consumption.

 

This means the total setup on a warm day uses around 11AH in 24 Horus!

 

Even when factoring in that the Inverter might only be 80% efficient when running, that is still a total battery consumption of no more than 12-13AH in a day. Suck-it 12V fridges.

 

 

Making the setup even more efficient using a simpler [& cheaper] inverter using a 2^nd heavy-duty [high-amp] relay and an optional bypass switch or a third [remote-controlled] relay.

 

Instead of using an expensive Inverter with a low-standby current like the TBS [or a Victron Phoenix 800]; another method I’ve thought of is using the digital temperature controller to instead switch on & off another high-amp relay connected in-line to the Inverter’s DC power input cable instead of the fridges AC power.

 

This means that instead of the inverter being on all the time with its parasitic standby current, the temperature controller can send a command to the heavy-duty relay to switch on the inverter on-demand.

 

All this would leave is the focus on getting something that is reasonably efficient at its DC-AC conversion but without the worry that it has a high standby current .

 

Something like a cheaper Sterling 800W Quasi Inverter or a Chinese Pure-Sine Inverter that is rated to at least 700W+ for around £120 should do the job here for much less than a 12V fridge.

 

As a final bonus to this setup; a 12V remote control activated bypass relay could be fitted between the temperature controllers 12V switched input and the heavy-duty relays switched input to allow the inverter to be switched on independently of the temperature controllers demand so it can be briefly used for other short-term uses such as running a small twin-tub washing machine, a travel Iron or hair straighteners.

 

It would have to be brief use items as eventually the inverter will have to go back on fridge duty.

 

[pig]

Nice. Do you have more details of the 12v digital control relay please?

[CaneyJ]

Google MH1210A. They come in 2 versions. One requires a 220V AC supply and there is the one I use that uses a 12V DC supply. Both switch a 10-Amp rated source which can be either AC or DC voltage.

[Guest]

Very interesting post. Cheers.

[nicknorman]

Interesting idea. Obviously most of the benefits come from the better insulation of the freezer. However your tests were with the door permanently closed. Clearly if you open the door regularly (every time you have a cuppa) then the benefits will be less because the efficiency of the actual heat extraction is probably similar or slightly worse than a 12v fridge. But despite that, I can see that it would give you a saving.

 

For best energy performance then, one presumes that a 12v freezer adapted as you have done, would be best (no inverter losses) but of course the outlay for the 12v freezer is in the "ouch" range.

Edited September 22, 2015 by nicknorman

[CaneyJ]

I bought my LEC fridge almost brand new second hand for £35. The thermostat was £8 on eBay.

 

New they are £100 as are most basic domestic freezers.

 

Alternatively, buy a Waeco B40 hybrid chest freezer and run it on its mains compresser rather than its crap 12v mode peltier with the same mods I've done so you don't lose all your cold air when opening.

 

2nd hand B40 hybrid fridges also go on ebay for cheap.

[nicknorman]

I bought my LEC fridge almost brand new second hand for £35. The thermostat was £8 on eBay.

New they are £100 as are most basic domestic freezers.

Alternatively, buy a Waeco B40 hybrid chest freezer and run it on its mains compresser rather than its crap 12v mode peltier with the same mods I've done so you don't lose all your cold air when opening.

2nd hand B40 hybrid fridges also go on ebay for cheap.

Yes, I should have said "12v compressor freezer"

[Phil Ambrose]

Yes interesting but can't help wondering if fitting a 12v fridge with more insulation (to match freezer spec) could possibly produce similar results.

Phil

[CaneyJ]

Well here's shorelines option

 

http://www.shoreline-marine.co.uk/freezers/chest-freezers/mini-chest-freezer-cf35

 

Basically taken my LEC freezer , flipped it on its back and fitted a danfoss. That'll be £565!

 

I'll stick with my £200 freezer and inverter combo.

[b0atman]

Yes interesting but can't help wondering if fitting a 12v fridge with more insulation (to match freezer spec) could possibly produce similar results.

Phil

but who makes one now if the boating chandlers would pull their finger out and tell manufacturers to supply freezer level insulated fridges then there would be a terrific surge of buying My freezer hardly runs keeps at -22c whereas fridge is nearly always on .I have been freezing 4l bottles of water and putting into larder fridge top shelf as a short term solution.The biggest problem with posters idea is door no shelf for milk bottle would a fridge door fit ?

Why not fit a Danfoss external refrigerator thermostat and series wire into original stat wires.

 

With regards to Nicks point of door opening all contents would be in drawers holding cold in.

[KenK]

The other option is to buy a efficient fridge in the first place.

There is a huge difference in terms of electrical consumption between and A+ fridge and an A+++ approximately twice as much per year.

There are only a few manufacturers making A+++ fridges, I have been doing the research as mine is on its last legs and I'm looking for a replacement.

No connection with the manufacturer except they support my wife's favourite football team but these are the most efficient I have found so far.

 

Liebherr TP1720 147 ltr. 63 kWH per annum. £340-£350. AC 240 V

 

Liebherr TP1760 154 ltr. 64 kWH per annum. £400-£405. AC 240 V

 

These are both Larder fridges no freezer compartment, something which on the boat we have rarely if ever used.

 

Ken

[CaneyJ]

I guess people miss the point that you don't need to spend £400+ on a top class AC fridge or a rubbish insulated 12v fridge when a £8 mod to a £50-£100 bargain basement freezer yields better results.

 

Remember that an A+++ fridge still isn't worth it if your 3000W+ bells and whistles inverter sucks 20-30Amps a day on its own.

 

If you don't have a fancy low-draw inverter that's why I can up with the other option of using a heavy duty relay in conmbo to switch on and off the inverter on demand instead.

[pig]

Google MH1210A. They come in 2 versions. One requires a 220V AC supply and there is the one I use that uses a 12V DC supply. Both switch a 10-Amp rated source which can be either AC or DC voltage.

Thanks.

[Phil Ambrose]

I guess people miss the point that you don't need to spend £400+ on a top class AC fridge or a rubbish insulated 12v fridge when a £8 mod to a £50-£100 bargain basement freezer yields better results.

 

Remember that an A+++ fridge still isn't worth it if your 3000W+ bells and whistles inverter sucks 20-30Amps a day on its own.

 

If you don't have a fancy low-draw inverter that's why I can up with the other option of using a heavy duty relay in conmbo to switch on and off the inverter on demand instead.

Understand you are being careful with money but for me I'm happy with my 110ltr larder fridge which is 12v and likewise my 80ltr freezer also 12v, I don't need an inverter and don't have to fiddle around making and modifying bits because my cold kit just works. Never have to think about it other than de-frosting the freezer on occasion.

Phil

[Bro]

Simply now 'cause I'm no leccy! What advantage would the proposed set-up have over my taking the serviceable 12v Danfoss BD35F unit out my existing (rubbish) fridge and fitting it to a well insulated freezer cabinet?

[CaneyJ]

Based on cutting the consumption to a third [45KwH]of the LEC freezers original [146KwH] and that the fridge equivlant of the LEC uses around 106kwh; putting a danfoss in a freezer box should cut its current consumption by at least half of whatever it was before.

 

Rehousing it whilst more costely would at least be more attractive than encasing it in ugly celotex and duck-tape which was my original idea for my Waeco CF-50.

[ditchcrawler]

encasing it in ugly celotex and duck-tape which was my original idea for my Waeco CF-50.

Which is what I did, filled the gap either side of the fridge with Celotex, put bubble wrap/ally foil insulation as used for radiators between the Condenser and the back of the cabinet and also above the motor/compressor, that still leaves the inefficiently lagged door but not a lot I could do with that short of putting an insulated skin on the front which would stick out into the galley. As I had now removed the recommended air gap around the fridge to allow cooling I put 2 fans in the floor blowing cool air up behind it 24/7

[Bro]

Simply now 'cause I'm no leccy! What advantage would the proposed set-up have over my taking the serviceable 12v Danfoss BD35F unit out my existing (rubbish) fridge and fitting it to a well insulated freezer cabinet?

 

Based on cutting the consumption to a third [45KwH]of the LEC freezers original [146KwH] and that the fridge equivlant of the LEC uses around 106kwh; putting a danfoss in a freezer box should cut its current consumption by at least half of whatever it was before.

 

Rehousing it whilst more costely would at least be more attractive than encasing it in ugly celotex and duck-tape which was my original idea for my Waeco CF-50.

 

You're losing me. Question was "what advantage?" Put another way,

Can you say:-

(1) For battery usage, which of the two systems (yours v's the rehousing option) would use least electricity and in rough percentage terms by how much?

(2) For cost, which of the two options would be best? Bear in mind here you seem to be considering fitting an expensive inverter (TBS Powersine PS600-12) and in my case the cost of any inverter needs be factored in as I don't have one.

 

Not trying to be awkward, just trying to understand and find what would suit me best.

[OldGoat]

 

I’ve got an unmodified domestic mains freezer running as a fridge using an external temperature controller [which is easy to setup] to consume just 12-13AH per day including the Inverter and factoring inefficiencies.

 

When running; the freezer’s compressor draws around 52-watts average [with a 659W max peak].

 

 

 

 

Your post is most interesting as I'm struggling with the same problem.

 

Particularly measuring the peak power.

Did you do that with the Efergy unit above? That model is not listed (well at first glance), most of those listed are similar to the Current Cost unit that I use at home.

Like you I need the peak power to decide on which inverter to buy. Your reading indicates that a Victron 800W unit would do the job - with a bit to spare.

 

TIA.

[b0atman]

Absolutely brilliant post time fridges moved on and became more efficient.

One point to watch out for some modern fridges are putting the heat exchanger into the casing as an auto defrost so fridge reaches low temp cut off battling the heat then heat carries on defrosting and raising internal temp. Efficiency out of the window.

[OldGoat]

Absolutely brilliant post time fridges moved on and became more efficient.

One point to watch out for some modern fridges are putting the heat exchanger into the casing as an auto defrost so fridge reaches low temp cut off battling the heat then heat carries on defrosting and raising internal temp. Efficiency out of the window.

I challenged one manufacturer (Beko, I think) whose techie said the condenser was 'now' next to the compressor and fan assisted. As most / many fridges are drop shipped to the customer, there aren't many chances to see an actual unit.

 

On another post Liebherr are very well respected in the 'trade' but are not usually seen in the domestic market 'cos they're v expensive!

Edit: Oh, yes they are Currys etc list them - yes they cost more than others but (some models) don't appear to be that much more efficient. Too big for my boat...

Edited September 23, 2015 by OldGoat

[CaneyJ]

 

Your post is most interesting as I'm struggling with the same problem.

 

Particularly measuring the peak power.

Did you do that with the Efergy unit above? That model is not listed (well at first glance), most of those listed are similar to the Current Cost unit that I use at home.

Like you I need the peak power to decide on which inverter to buy. Your reading indicates that a Victron 800W unit would do the job - with a bit to spare.

 

TIA.

 

Whatever watts your compressor is at when running; times that at least by ten for the peak compressor start load to size-up an Inverter. In my case however, the peak load was over 13 times the actual running watts [52W] though the label on the back rated the freezer at 60 watts which is closer to that x10 Ratio considering the peak start-up measured in at 659W.

 

I’m pretty confident of the peak-watt figure. Though my efergy watt-meter doesn’t have a very quick reading resolution, it was on for a whole day logging multiple freezer starts so I’m pretty sure it captured the max watt load.

 

As a comparison; I tried it on my large fridge-freezer at home. That draws around 120-130 Watts when running but took 1.3KW to start it so the fridge size and wattage needed is generally proportional.

 

For a modern under-counter fridge I would say that an 700-800W Inverter is the minimum requirement.

In response to Bro; what 12V fridge do you have?

 

The easiest, quickest and cheapest improvement would be to use the external insulation and fan method mentioned by ditchcrawler.

 

I would add that if your 12V fridge is a few years old, your Danfoss compressor might also need a re-gas. You could also make sure your evaporation coils are clean & free of dust.

 

I would still run it off an external digital thermostat controller and use it to switch the fridges 12V feed on and off [as well as any external DC fans so they don’t waste power running 100% of the time] as old manual fridge thermostats are fickle to say the least.

 

Here is a list of items required to set-up an efficient AC system using any common inverter

 

· <£100 - Currys Essentials 60L Chest Freezer [as an example!] You could buy any freezer but if you can fit a chest-type in your boat, it is better for efficiency.

This model for example uses 153Kwh and based on my results above, should drop consumption running as a fridge by about a factor of 3.2 so should only require 48KwH in a year running at 4°C.

This breaks down into 132-watts [or 11AH] daily. Bump this figure up to around 13-14AH per day based off of an 80% efficient Inverter.

 

· £113 – Sterling Pro Power Q [quasi-sinewave] 12V Inverter [uK based company support with a reasonable brand image compared to a unknown Chinese suppliers] – This should run most things including small power tools except induction items like cordless rechargeable electric toothbrushes.

 

· £21 – Durite 120-Amp heavy-duty relay [P/N: 0-727-18] - This isn’t needed if you can rewire the on-off toggle switch of the inverter itself directly to the digital temperature controller.

Otherwise; this gets installed in-line to the Inverters positive DC power input cable. The Inverter’s toggle switch must be left in it’s on position.

The secondary terminals on this heavy-duty relay are controlled by the digital temperature controller [or via a bypass switch when operating other items].

 

· £8 – MH1210A 12V Powered Digital Temperature Controller [relay]

 

· £4 - REUK 0.8Amp 12V voltage regulator [this is needed so that the digital temperature controller isn’t fried when your leisure batteries are being charged at 15V+ on a cold day]

 

· £2 - Torpedo [temperature controller bypass] switch = £2 or……. a remote control 12V relay = £18]

 

With this setup, the unmodified domestic freezer is simply plugged into the inverter.

 

The total cost for this system is around £250 which is half that of your average new 12V fridge but this includes both an easily replaceable domestic freezer [fridge] which itself doesn’t need to be physically modified and you gain the benefit of an inverter on-board for some extra luxuries.

 

Re-housing your old 12V compressor would cost about the same as this setup but keeping it as a fridge in a fridge-style cabinet would use more power. Just look at the Amp-hour figures quoted by Shoreline and Waeco on their fridges.

[Bro]

 

 

Whatever watts your compressor is at when running; times that at least by ten for the peak compressor start load to size-up an Inverter. In my case however, the peak load was over 13 times the actual running watts [52W] though the label on the back rated the freezer at 60 watts which is closer to that x10 Ratio considering the peak start-up measured in at 659W.

 

I’m pretty confident of the peak-watt figure. Though my efergy watt-meter doesn’t have a very quick reading resolution, it was on for a whole day logging multiple freezer starts so I’m pretty sure it captured the max watt load.

 

As a comparison; I tried it on my large fridge-freezer at home. That draws around 120-130 Watts when running but took 1.3KW to start it so the fridge size and wattage needed is generally proportional.

 

For a modern under-counter fridge I would say that an 700-800W Inverter is the minimum requirement.

In response to Bro; what 12V fridge do you have?

 

The easiest, quickest and cheapest improvement would be to use the external insulation and fan method mentioned by ditchcrawler.

 

I would add that if your 12V fridge is a few years old, your Danfoss compressor might also need a re-gas. You could also make sure your evaporation coils are clean & free of dust.

 

I would still run it off an external digital thermostat controller and use it to switch the fridges 12V feed on and off [as well as any external DC fans so they don’t waste power running 100% of the time] as old manual fridge thermostats are fickle to say the least.

 

Here is a list of items required to set-up an efficient AC system using any common inverter

 

· <£100 - Currys Essentials 60L Chest Freezer [as an example!] You could buy any freezer but if you can fit a chest-type in your boat, it is better for efficiency.

This model for example uses 153Kwh and based on my results above, should drop consumption running as a fridge by about a factor of 3.2 so should only require 48KwH in a year running at 4°C.

This breaks down into 132-watts [or 11AH] daily. Bump this figure up to around 13-14AH per day based off of an 80% efficient Inverter.

 

· £113 – Sterling Pro Power Q [quasi-sinewave] 12V Inverter [uK based company support with a reasonable brand image compared to a unknown Chinese suppliers] – This should run most things including small power tools except induction items like cordless rechargeable electric toothbrushes.

 

· £21 – Durite 120-Amp heavy-duty relay [P/N: 0-727-18] - This isn’t needed if you can rewire the on-off toggle switch of the inverter itself directly to the digital temperature controller.

Otherwise; this gets installed in-line to the Inverters positive DC power input cable. The Inverter’s toggle switch must be left in it’s on position.

The secondary terminals on this heavy-duty relay are controlled by the digital temperature controller [or via a bypass switch when operating other items].

 

· £8 – MH1210A 12V Powered Digital Temperature Controller [relay]

 

· £4 - REUK 0.8Amp 12V voltage regulator [this is needed so that the digital temperature controller isn’t fried when your leisure batteries are being charged at 15V+ on a cold day]

 

· £2 - Torpedo [temperature controller bypass] switch = £2 or……. a remote control 12V relay = £18]

 

With this setup, the unmodified domestic freezer is simply plugged into the inverter.

 

The total cost for this system is around £250 which is half that of your average new 12V fridge but this includes both an easily replaceable domestic freezer [fridge] which itself doesn’t need to be physically modified and you gain the benefit of an inverter on-board for some extra luxuries.

 

Re-housing your old 12V compressor would cost about the same as this setup but keeping it as a fridge in a fridge-style cabinet would use more power. Just look at the Amp-hour figures quoted by Shoreline and Waeco on their fridges.

 

 

Thanks CaneyJ for the further explanation.

 

You ask what 12V fridge I have. It is LEC with Danfoss BD35F unit which I thought to rehouse in a new freezer cabinet. Your chest freezer suggestion would be best in conserving temp. when opened to access contents, though upright freezer cabinets typically have drawers not shelves to store contents and as such would not suffer quite so much as open shelf design used for fridges.

 

One further question (last?). Are cooling units used in AC freezers so much different than the 12V Danfoss unit and is there much difference in efficiency between the two?

[OldGoat]

Oh dear (in reply to post #22) - I thought your figure was an actual measurement taken from the efergy socket meter, rather than a reasoned guess.Not a criticism, but I've seen all sorts of figures bandied abut here and elsewhere. Danfoss give a peak draw for their DC units but zilch for the A/C types.

 

The Efergy is no longer available and there have been no s/h units on fleabay since June. There are other units around but a bit vague - 'overload power' is mentioned by one - but what is meant by 'overload'. Given that the peak power draw occurs for a 'few' milliseconds, I doubt whether the unit is sufficiently sensitive - quick acting - to capture that amount.

 

Any comments?

 

(I accept your other points - but first I should decide on which inverter might power the fridge (in my case a fridge freezer as commanded by The Management)

[OldGoat]

 

Thanks CaneyJ for the further explanation.

 

You ask what 12V fridge I have. It is LEC with Danfoss BD35F unit which I thought to rehouse in a new freezer cabinet. Your chest freezer suggestion would be best in conserving temp. when opened to access contents, though upright freezer cabinets typically have drawers not shelves to store contents and as such would not suffer quite so much as open shelf design used for fridges.

 

One further question (last?). Are cooling units used in AC freezers so much different than the 12V Danfoss unit and is there much difference in efficiency between the two?

 

Looking at the Danfoss specs: for the BD 35 (and 50), it's a quite different machine. Being a DC unit it runs at different speeds depending on the demand.

An AC basic fridge can't do that.

 

At home we have a Samsung Inverter larder fridge - seems strange to have an inverter in a 240v unit. No explanation - but it does 'sip' the power - so there must be something clever in it somewhere. A quick google infers there may be some variable frequency trickery going on.