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I recently received a relatively polite but very firm email taking an air conditioning company (and myself) to task. The complaint was that the (advertised) power drawn by his motorhome’s reverse cycle air conditioner was less than its specified ability to cool. This, he stated, was contrary to the basic laws of physics. Further, he complained, I make general such incorrect statements in my various published books and writings.
As vaguely similar claims are made by all air conditioning vendors there is no point in identifying the particular brand involved except to mention the manufacturer is well known and respected in this field.
The reverse cycle air conditioner concerned is claimed by its manufacturer to have a cooling capacity of some 3200 watts, yet is claimed to draw only 1465 watts in doing so.
That, stated the respondent, implied that the device generates twice the power it consumes. This, he continued, was impossible - stating that the basic laws of physics prevent a theoretical efficiency of more than 100% and a practical efficiency of a lot less. (Wattage is used both as a measure of energy used, the ‘work’ that energy usage performs).
Were the claims to be valid, the respondent continued, the world’s energy problem could be solved by using multiple air conditioners each drawing 1465 watts and somehow harnessing their 3200 watt outputs to drive electric turbine generators.
Whilst our respondent clearly has some knowledge of physics, he had wrongly assumed that refrigerant air conditioners work in the reverse way to that of electric ovens, ie that instead of electrical energy being used directly to generate heat, it is somehow used to generate ‘cold’. And were that to be how air conditioners actually work, his complaint would have been absolutely valid: and both I and the world’s air conditioning industry would have needed to do some urgent apologising. But they don’t.
Like fridges, refrigerative air conditioners do not generate heat or ‘cold’ as such. They are heat pumps that move heat from where it is not wanted to somewhere where it (hopefully) does not matter. They work more or less like this:
Heat and temperature are related but they are not the same thing. If an inflated ball made of some heat resistant material is heated to (say) 150 degrees C, it is at the same temperature as (say) an iron cannon ball of the same size and heated to the same temperature. But the amount of heat energy contained in the football is negligible compared to that in the cannon ball.
The heat in the cannon ball is held within its very dense solid iron interior and will retain heat for many hours. The heat in the football is held mainly within its compressed (but still non-dense) air and will cool within minutes. In either case, the amount of heat energy is proportional to the objects’ respective density.
If we increase that density (by compressing the object) the heat energy too becomes denser and that object’s temperature rises. This was elegantly described by French physicist J C Charles in the 1700s. Two centuries on, Rudolph Diesel realised this principle could be exploited to ignite fuel in an engine. His resultant diesel engine compresses indrawn air by twenty or so times - resulting in peak temperatures of well over 500 degrees C.
A reverse cycle air conditioner utilise's this principle to move heat. It basically consists of two inter connected coils of tubing filled with a refrigerant substance. This substance is pumped through them by a compressor. One coil is inside the area to be cooled, and the other is outside that area. Using a cycle of compression and then expansion, heat (via warm air blown over the inside coil) is transferred to the refrigerant substance where it is dissipated to atmosphere via the outside coil. In the process the refrigerant substance constantly moves between gaseous and liquid form.
The system thus works not by using energy directly to cool or heat, but by using it to move heat from one place to another. In essence, an air conditioned room is like a room-sized fridge. Indeed, if it’s vented to the exterior as it should be, a fridge becomes an (inefficient) air conditioner if you leave its door open.
As the total absence of heat energy implies a temperature of -273 degrees, even a Hobart winter’s air contains a lot of heat. A reverse cycle air conditioner can thus work also as a heater by, as its title implies, running it ‘back to front’. Then, it will shift more heat energy (in watts) than it consumes (in watts). In both cases the energy needed depends on the ‘before and after’ temperature difference and the efficiency of the pumping process.
By way of illustration and contrast a 1000-watt electric bar fire produces just under 1000-watts of heat energy.
The measure of an air conditioner’s efficiency is called the ‘Coefficient of Performance’ (usually abbreviated to COP). It is the ratio of energy used to the amount of heat moved – in watts.
In the case of our respondent, the air conditioner concerned was claimed to require 1465 watts to move the equivalent of 3200 watts of heat. The COP was thus 2.1. As buyers cannot reasonably be expected to work all that out, the COP is more commonly expressed as a Star Rating. These ratings begin at a COP of 2.3. Each further increase of 0.3 results in a further star. Energy Smart (WA) states: 'Typically, COPs vary from a low of 1.7 to a high of about 3.3 for domestic air conditioners’ (ie four star). A few now exceed 4.5.
For reasons that, to me at least, are unclear most RV air conditioners tend to be at the lower end of the efficiency scale. A survey of ten typical air conditioners currently marketed (in Australia for RVs), show an averaged COP of 1.62 (ie zero stars). The recently released Air Command Heron Q however scores one star. It is claimed to produce 3100 cooling watts for an energy consumption of 1265 watts.
The DC Airco range (12 and 24 volt) units, however, have a COP of well over 5. This places them beyond the top of Australia’s star rating scale. The 12-volt 9000RM version is claimed to produce 2650 (cooling) watts for a consumption of 470 watts and has a COP of 5.6: the 24-volt version is even better. (Some promotion for the 12-volt unit quotes consumption as 430 watts but the vendor says that this is a printing error.)
For clarity I need to add that the above refers to refrigerative (ie compressor type) fridges and air conditioners - not to those that work on the absorption cycle.
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