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Old 15-08-2013, 01:45   #1
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Isotherm ASU or Not?

I was looking at the Isotherm Cruise 85 and Cruise 85 ASU Refrigerators and the price between the two here is around $800usd.

Now for that much I can get another windgen of course I would have to figure out where to put a 3rd one. Or I could add more solar to the 700w I already have. I could even add more batteries to the 2k ah on board.

So what is the advantage of going with the ASU? is it really worth that much extra? From what I read it is like a 30% savings or approx 18ah in my calculations.

I mean power is always an issue but is it going to save me that much over adding a couple hundred watts more in solar panels considering you can find them for around $2usd per watt?

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Old 15-08-2013, 02:09   #2
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Re: Isotherm ASU or Not?

Asu simply is a way of speeding up the compressor and storing the cold. It does not reduce the total power consumption, but lets you store the energy when power is abundant such as running the motor.
Its a good idea, but I think you could spend the money more wisely. If you can add more solar panels you can definitely spend the money better.

Incidentally the compressors in the standard (non ASU) fridge can be modified to have two speeds quite simply with a bit of DIY electronics. This does not give all the advantages of ASU but should be a useful modification (although I have never done this myself) to cool the fridge down quicker when power is abundant especially when adding warm goods to the fridge.

As a simple step on a non ASU fridge you can turn the thermostat to a colder setting, and/or add goods, when there is plenty of power such as the middle of the day when the batteries are regulating due to the solar panels.

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Old 15-08-2013, 06:38   #3
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Re: Isotherm ASU or Not?

A small peltier cooler (NOT fridge) consumes 40W but it cycles only 25-35% of the time. So it asks some 50 A every day. This in turn asks for equivalent juice from the solar, which implies 2 x 100W 22% efficient solar panels plus a regulator and a battery. So to say a cooler that costs you 100 units asks for further 700 units in solar, regulation and storage...

Mind you not everyday is sunny, even in sunny countries.

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Old 15-08-2013, 07:33   #4
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Re: Isotherm ASU or Not?

The purpose ASU running compressor when it might not be called for by thermostat would be to store surplus energy that can not be stored in battery bank. The problem is unless this system has a large eutectic holding plate very little actual energy can be stored only one Btu per pound per degree. True energy storage only results when liquids inside refrigerator are phase changed into ice. With this process 144 Btu of energy per pound of ice. Remember when this small compressor is running it produces only 200 to 500 btu of energy per hour. If someone believes the speed up function has some value it can be accomplished with a small $3 relay attached to engine warning system or simply by lowering thermostat when engine is running.

The other function of ASU to reduce compressor speed this one is different than other BD compressor speed controllers as speed is determined by temperature instead of compressor cycle times. The trouble I found when testing ASU for my book is it lacked a wide enough control range for tropical weather in a six cu ft box. An investment of $5 in plug in resisters used to preset compressor speed will eliminate the need for automatic speed reduction gadget. The main reason I dislike these units is they are unfriendly for troubleshooting when compressor will not run, it takes a very good technician to determine if ASU or Danfoss control is the problem.

No one has a crystal ball that will tell you in your ice box their conversion refrigeration system is going to reduce your daily energy bill by 30% if you buy their gadget. Secret to best daily energy consumption is to maintain desired box temperature at slowest compressor speed with fewest running hours per day. If you must spend those extra dollars buy alternative energy Wind or Solar for your boat.
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