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Saturday, February 11, 2017

Australian Air Conditioning

This blog was triggered by an item on our National Radio when they crossed to Kerry-Anne Walsh in Australia.  She reported that there is a heat wave in Aus with temperatures reaching and even exceeding 45C.  At the same time, she reported that the Ausi electrical generation network can't cope with the load so they are instituting rolling load shedding* just when the people most need their air conditioning.

*Power grids are set up to shut down sectors when demand exceeds the power they can generate. Not pleasant if you are in the sacrificed sector when the temperatures are lethally hot and your air conditioner no longer works.

This is not just a matter of convenience or comfort.  When the wet-bulb* temperature rises above about 33C, a human can't cool off any more and such temperatures become fatal.

* A wet bulb temperature is a combination of temperature and humidity.  At 100% humidity and a temperature of 33C, you can no longer cool the body by sweating or radiation.  As the air gets drier and drier, you can tolerate higher temperatures because your sweating mechanism becomes more effective.  It has been suggested that with Climate Change, areas of the earth will become uninhabitable unless the people have air conditioning.  

Kerry-Anne also mentioned that part of the problem is the emphasis in Aus on Wind power as part of the grid generation and when you depend on wind, it doesn't always blow when you need it most.  ie - during one of these heat waves.  This got me to thinking.  Let me make a side step for a moment.

When I lived in Gazankulu in South Africa, I set up a fish farm  for one of the local sub tribes.  It was fed water from a near by lake and the water was pumped by a MonoPump powered by an array of solar panels.  A mono pump is a positive displacement pump* so if it turns a little it pumps a little water and if it turns a lot it pumps a lot.  (unlike centrifugal pumps that need full speed to pump their water).  It had a DC motor. DC motors can be set up to turn in proportion to the amount of power they receive.  Note that AC motors must have their full power or they tend to burn out.

*A piston pump is one example of a positive displacement pump.  It's output is proportional to the number of rotations it makes.  Centrifugal pumps which are the ones most used and which are powered by Alternating Current (AC) must operate at their rated speed to be effective.

When the sun came up in the morning and touched the panels the pump started to operate slowly and as the sun rose, it pumped more and more.  Why do I mention this.

The two things you have to take from this story is that with suitable electronics, the rpm of a DC motor is in proportion to the amount of power you feed it and a positive displacement pump will pump in proportion to it's rate of rotation.

An air conditioner, in essence,  is nothing more than a gas pump and two fans.  Let me divert again and explain a touch of physics.  I hope no physicist are reading this.  They would have a conniption fit at my explanation.  I apologize right at the beginning but if you are not into physics, the explanation makes more sense this way.

When you compress a gas it heats up and if you have somewhere cooler that the gas, the heat will flow to this cooler location.  Think of it as if you are squeezing the heat out of the gas.  If you compress it more and more, at some point it will condense into a liquid and a lot of heat will be squeezed out as the molecules come much closer together in the liquid.  When you let off the pressure and let the liquid evaporate and in addition, let the resulting gas expand, it cools and can absorb heat from its environment.  This is an air conditioner.

You have the compression outside the house and have a fan blowing outside air across the radiator which contains the condensed, hot liquid (or compressed gas).   You then pipe the liquid into the house in a pipe and let it expand in a second radiator with a fan blowing across it. The expanded gas cools and cools the air in the house.  The pump takes the gas and once more compresses it.  Now let's pull this together.

All you need is a few solar panels on your roof pointing North*.  They are connected directly to an air conditioner with  3 DC motors (to power the gas compressor and the two fans).

I live in the southern hemisphere

For the most part, it is hot when the sun shines.  Yes I know there are some hot cloudy days but at a 95%+ level, Sunny = Hot.  So now you have an air conditioner that works harder and harder the higher the sun is both on a daily and a seasonal basis.  You can even slant the panels a little toward the West to take care of the continuing heating of the air even after the sun has reached it's zenith. 

Better still, you are not putting extra strain on the grid when it is under its greatest strain.  I would actually state this a little differently.  You become independent of "The Man" and take care of your own needs without dependence on a central power source.

Have a well insulated house and you can forget about your air conditioning.  It works automatically and works hardest when you most need it.

Incidentally, most air conditioners can work in reverse.  That is to say, heating the house.  If you are in the central plains of America where there are lots of sunny days in the winter but it is very very cold, you can switch your air conditioner to heat the house.  It won't work very well on cloudy days but you will have to cut less wood to get you through the winter.

Apropos, no one would ever suggest that only one source of renewable power is the answer to weaning ourselves off fossil fuel.  Each has it's advantages and together they are much more effective than any single source.  And single technologies are more effective  if they are geographically distributed.  That is the advantage of our existing national grids.  They can bring power from areas where the wind blows or the sun shines to where the wind  is not blowing or the sun is not shining at the moment.

If there is one country in the world with abundant renewable energy it is Australia.  I don't know why they burn coal at all.  Must be political.  It certainly isn't technical. 

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