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Re: Solar Air Cond

PostPosted: Thu Jan 09, 2014 10:12 am
by swampdweller
They're probably very good compressors but you need more than a good frigey, also necessary would be an experienced refrigeration engineer and a lot of time and money. Compressor volumetric efficiency data is probably available, but in an aircon system its the coils and fans that have the biggest effect on operating efficiency. You have to look at the overall system rather than individual components.

By the time you design, select and assemble indoor and outdoor coils, fans (with DC motors!) piping, refrigerant control device, reversing valve if you want heating as well as cooling (also DC - good luck finding one), filter assemblies, casings and controls, you could finish up with a beautiful and efficient machine, but my guess is you would be tweaking it until the day you die before you saved a single Watt. The good people at places like Daikin and Mitsubishi would probably assign about 5 R&D engineers to the job to make it work well and then spend upwards of $50,000 on compliance and efficiency testing begore they could market it.

At best a DC unit could save you the inverter efficiency losses, but while those loses are annoying, additional PV and inverter capacity to overcome them will be a lot cheaper than engineering a DC aircon from the compressor up.

Re: Solar Air Cond

PostPosted: Thu Jan 09, 2014 6:15 pm
by Warpspeed
As swampdweller says, there are quite a few rather esoteric dimensions to assessing overall refrigeration efficiency.

With respect to just the compressor itself, there are a few things you should know about piston refrigeration compressors generally.

They are designed to offer as constant a load as possible to the drive motor over a very wide range of flows and back pressures.
This is done deliberately so that the motor will never be overloaded under initial startup conditions, or when the high side pressure really climbs on a stinking hot day, or with clogged airflow through the condenser.

Basically the refrigeration compressor designer tries to get as flat a horsepower curve as possible under the widest possible operating conditions, and match that to the drive motor.

This is done by lowering the compression ratio, and deliberately fitting undersized valving to the compressor.
The undersized valves limit mass flow when the compessor starts up with the hot side and cold side at equal temperatures.
The lower compression ratio means that mass flow will gradually fall off as the high side to low side pressure differential climbs.

This all works wonderfully well at allowing a given motor to work under acceptible peak loading under extreme or transient operating conditions.
It also means that under ideal running conditions, pumping efficieny is not as high as it possibly could be.
This is a deliberate design feature, not a problem.
At least there is nothing you can really do about it.

Re: Solar Air Cond

PostPosted: Thu Jan 09, 2014 7:04 pm
by Smurf1976
There are commercially available "normal" air-conditioners being produced by major manufacturers which do have DC compressors. So that part of it certainly isn't a radical idea although the commercial units are simply running from AC mains power rather than using DC directly.

Re: Solar Air Cond

PostPosted: Thu Jan 09, 2014 7:09 pm
by Gordon-Loomberah
Warpspeed wrote:At least there is nothing you can really do about it.


You could get yourself a scroll compressor instead, better for efficiency and reliability

Re: Solar Air Cond

PostPosted: Thu Jan 09, 2014 7:38 pm
by Warpspeed
Centrifugals are definitely more efficient in very large sizes, but they don't work well in the very small sizes.

Big commercial airconditioning plants use centrifugals almost exclusively, along with very dense low pressure refrigerants.
All small airconditioners, refrigerators, and freezers run piston compressors at relatively high pressures.
Its realy the only practical solution.

Re: Solar Air Cond

PostPosted: Fri Jan 10, 2014 10:27 am
by swampdweller
Scroll compressors or rotary compressors of some type are now almost universal in small to medium sized air conditioners, some units have digital scrolls which vary the volume of the scroll chamber instead of using an inverter to vary compressor speed. All these technologies are basically sound and do a great job if you buy a quality unit.

Whichever compressor type, just use the EER and COP figures to select the most efficient unit for your application, using 4 or higher as a target EER OR COP. You may find that smaller units up to 5kW cooling capacity are more efficient, so don't oversize.

The most important thing of all, don't buy a unit that's too big or small! And reduce your aircon requirements with good insulation and window shading. Ask the dealer to calculate your cooling and heating requirements carefully, using an actual house plan rather than just Watts per Square metre or a rough guess (yes, some will do this). Just as a check, a reasonably well shaded and insulated house will require about 70 W/m2 cooling capacity in Hobart, ranging to 100 in Adelaide and 130 in Darwin, living areas only - don't calculate for the laundry, bathroom and so on.

The Your Home website has some excellent advice on air con and home efficiency generally: http://www.yourhome.gov.au/technical/index.html