DC Breeze - Q & A

Over the past several years I have seen several ads for various "12v DC air conditioners" but don't hear much about them - what's the problem?

Prior to the introduction of DC Breeze, the systems marketed as "12vdc or Battery Powered Air Conditioners" relied on a number of questionable designs which inevitably gave either inadequate performance, poor reliability or both.  In some cases these systems were nothing more than 12v pumps which circulated cool water (either from an ice chest or deeper water brought to the surface  with a hose going overboard) through a coil with a fan blowing over it.  To those unfamiliar with the thermodynamics of air conditioning such an approach may have sounded like a good way to get cheap A/C with little power draw.  Once they purchased the systems they inevitably found their performance to be unsatisfactory at best.

To illustrate the problem with such units let's consider the claimed performance of a typical one.  Innova Tech's "BayAir 12v Air Conditioner"  is  sold through West Marine for $1,250.00 and is promoted as providing 6,500 BTUs/hr of air conditioning with only 0.6 - 3.0 amps of current draw.  Further reading indicates that the unit cooled the air by circulating water through a ice chest filled with chipped block ice and a finned coil with a fan blowing over it.  While one can seriously question whether the design would ever allow it to absorb heat at claimed rate, let's assume that it does.  You will remember from your grade school science class that it takes 144 BTUs of heat to melt one pound of ice.  Given this fact, you can see that you would have to melt 45 lbs of ice per hour to actually accomplish 6,500 Btu/hr of air conditioning.  Assuming that 10% of the ice is going to melt by the time you get it to the boat, you need to buy and carry 50 lbs of ice for every hour you wish to air condition.  Not exactly a practical solution.

A few companies have attempted to build more sophisticated and functionally useful systems by packing conventional AC (alternating current) compressors with small, dedicated AC-to-DC inverters.  Unlike the water-circulators, these systems actually produced a reasonable amount of cooling power and didn't require you to break you back (and back) carting ice around.  The problem with this approach is poor reliability and energy efficiency. 

What is wrong with inverter-powered systems?

These systems have extremely high failure rates in the field and most customers end up feeling that they have been mislead about the energy consumption.  To understand the reliability problem you have to recognize that starting an induction (conventional AC) motor requires an electrical "surge" equal to 5x - 6x the full load running current.  If the normal running current of the compressor is 5 amps at 115vac then the surge will be 25 to 30 amps.  Converted to 12 volts, this means the inverter has to be able to absorb an input of up to 275 amps of current every time the compressor starts.  This power is only required for a brief instant but without it, the motor (compressor) won't start.  Since the input (12v) side of smaller inverters are not designed to accept this high level of current, the inverters rely on energy stored in electronic components known as "capacitors" to provide the needed surge power with each compressor cycle.  The more closely an inverter is sized to match the full-load power rating of the compressor, the harder the capacitors are stressed to provide the 5x+ inductive surge required to start it.  Over time this stress, particularly in high tropical temperatures, results in premature component failure.  The problem can be reduced by over-sizing the inverter.  However, doing so drives up the cost of the system and further reduces energy efficiency by increasing the electrical losses in the inverter electronics.  The high conversion efficiencies claimed by inverter manufacturers are only true at a very narrow power output (typically about 90% of full load).  When large inverters are used to power small running loads the efficiency of the electrical conversion falls off dramatically.

An additional problem can occur when the lower cost "modified" sine wave type inverters are used.  The shape of the wave form and fluctuating power levels increase the operating temperature of the compressors.  The higher temperature and square edges of the wave form cause early failure of the insulation on the motor windings.  This problem can be avoided by using a high quality "true sine wave" inverter but the cost is higher and the energy efficiency even lower for this type of inverter.

I already have an inverter on my boat. Why can't I just get a low-cost conventional 5,000 btu/hr air conditioner and run it off that?

You probably can, and it may, or may not, make sense to do that.  Remember, you are going to be faced with the same technical issues we have discussed above - reliability and efficiency.  If you have a small (1,000 - 1,500 watt) inverter of any type you are going to be putting a lot of stress on it to run the air conditioner and chances are it will lead to early inverter failure.  If you have a larger (2,000+ watt) "modified sine wave" type inverter it will probably hold up reasonably well to the stress of the inductive startup of the compressor.  However, the distorted wave form and voltages are likely to cause an early death of the air conditioning compressor.  It you have a large, high-quality "true sine wave" inverter you can probably run a small conventional air conditioner reliably.  The only penalty you will pay is high energy consumption.

Why is DC Breeze Different?

DC Breeze is an entirely new concept. The hermetically sealed compressor in the DC Breeze uses an advanced brushless DC motor rather than a conventional AC induction motor.  This allows the system to operate directly from DC power, eliminating the DC-to-AC inverter and the associated inductive start-up "surge". The result is an efficient, powerful, reliable cooling system that has all the advantages of convention A/C but can be powered from unconventional DC sources.

Glacier Bay, Inc. has an air conditioning option for their Micro HPS refrigeration system that also runs from DC power.  How is DC Breeze different?

Glacier Bay's Micro HPS refrigeration system has an Arctic Air option which also provides 5,000 BTUs of air conditioning from the same compressor used to provide refrigeration.  Unlike the DC Breeze which is a completely self-contained system, the Arctic Air option is what is known as a "split" system.  This means that the blower/evaporator unit is located remotely from the compressor and is interconnected with copper tubing.  The DC Breeze is a more traditional stand-alone configuration.  The Micro HPS option makes sense if you need both refrigeration and DC air conditioning.  If you already have refrigeration and only want to add air conditioning, the DC Breeze is the way to go.