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Q & A: boiling water with an air conditioner

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Most recent answer: 01/16/2017
Q:
Is it possible to use an air conditioning system to boil water and therefore power a steam turbine to generate electricity, I notice my Ac heats at nearly 4kw for 1kw of electricity. by drawing heat from the atmostphere. if this would work wouldnt it cool the atmosphere and also generate free electricity
- William (age 36)
UK
A:
One of your ideas, to use the heat from air conditioners, makes sense and has many applications. The other, to generate free electricity, turns out to violate basic laws.

You're absolutely right that the heat output of the air conditioner coils is several times larger than the energy used. With an efficient commercial system, that factor can be more than five. You can buy heat pump systems that work as air conditioners in the summer and in reverse, pumping heat into a house, in the winter. Those are much more efficient than simple electrical heaters. Even when the electrical power comes from fossil fuels, they can be more fuel-efficient than gas heaters, especially in mild climates. Since electrical power can also come from cleaner sources than fossil fuels, heat pumps can become really big improvements over other heating systems.

So the general idea of pumping heat rather than just generating it is indeed correct. It should be much more widely used in order to reduce the need for fossil fuel use.

The idea of using an electrically powered heat pump to boil water to drive a turbine to generate electricity will never work, however. The energy output is always less than the input, for fundamental reasons.

It's not particularly practical even to use heat pumps instead of electrical coils to boil water. The reason is that the efficiency of the heat pump drops as the temperature difference between the hot and cold sides gets bigger. That's the same reason that houses with heat-pump heating systems need some backup heaters if they are located in places with serious winters.

Mike W.

(published on 07/10/2012)

Follow-Up #1: solar-powered heat pumps

Q:
Hello Mike,I've had the same idea as William, about using heat pump to boil water, then use vapor and run a turbine and also use thermoelectric material that turns heat into electricity so we could create a loop and generate electricity.First, I disaprove your point saying that "free electricity" from this violates thermodynamic laws, since its notting else but solar energy Since it uses solar heat to generate electricity, just like solar panel exept we would use ambient heat instead of solar radiance.I've read that thermal electric central can have up to 60% efficiency turning hot water into electricity.And that Heat Pump can have up to 500% efficiency in a high temperature environement.ε th�orique = T2 / (T2 - T1) [T �tant exprim� en KelvinIf you use this device in the desert which in daylight heat can be up to 45C and if you wanna boil water up to 105C your efficiency would be according to this formula up to 530% into a perfect design with 0% lost.I've made lots of research and the best heat pump are rated at 85C for output water heating. Why are you saying that its "impossible" to heat it further more ? I don't quite like the word impossible ... and i'd like more information about this.Thanks !
- Francis Veilleux (age 23)
Quebec, Canada
A:

Ordinary ambient heat in an environment at approximately uniform temperature cannot work. Ultimately all these engines are driven by something being out of equilibrium. That sort of ambient heat is already in equilibrium. 

Now you mention solar heat input. That can make one spot hotter than another, exactly the condition needed to get that temperature difference you mention needed to get some thermodynamic efficiency. With a mirror or lens, you can make those temperature differences large and get good efficiency while the sun is shining. This is the very principle used in solar thermoelectric plants. These actually have an advantage over photovoltaic cells since the heat stored in the hot liquid can be used overnight, without needing a separate energy storage system like a battery. Spain has been a particular leader in developing such plants (https://en.wikipedia.org/wiki/Solar_power_in_Spain), making good use of their frequent clear skies. Under somewhat cloudier conditions, you can't focus the light so ordinary photovoltaics work better.

Mike W.


(published on 01/16/2017)

Follow-up on this answer.