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Absorption cooling

In light of the electrical resistance heating and wasted engine heat talks, it got me to ponder about this topic. 60% to 70% of the heat energy from burning fuel is not used to turn the crank. Why don’t we use exhaust heat for refrigeration? It doesn’t make sense to further tax the crank with an AC compressor. Absorption refrigeration works as long as there’s heat provided by the exhaust.

Better yet, it can be the ultimate cold air intake when we don’t need or want air conditioning. This allows the engine designers to bump up compression ratio to improve engine efficiency. With today’s variable valve timing and knock sensors, the system can revert to low compression ratio operation when we need to cool the cabin instead of the intake

Combined cycle engines are actually quite common in electric utility power plants. Most common is the Brayton-Rankine combined cycle where a gas turbine (Brayton cycle) is the top end of the cycle and its exhaust is used as the heat source for the boiler of a steam turbine (Rankine cycle).

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I wonder what the cost difference would be between the current AC system and an equivalent absorption cooling system. My concern isn’t so much about non recurring costs, since that would be quickly paid if millions of vehicles used it. I’m more interested in recurring costs. If the absorption cooling system is even a few bucks more, that might prevent it from finding its way into production.

Perhaps if anyone here is familiar with absorption chillers, they would be kind enough to run some numbers to see how big and heavy a system would need to be to replace an automotive A/C. I have not thought about absorption chillers since a single homework assignment in college 40 years ago.

Absorption refrigerators were common at one time when gas (propane) was very cheap and electric power expensive or unavailable. In rural South Africa such fridges were kerosene powered. My brother in law grew up on a poultry farm which bought propane in bulk and their kitchen had as Servel gas fridge based on the Platen Munters refrigeration cycle. The fridge was very quiet and no more bulky than regular unit.

However, the unit was taken off the market because of excessive CO generation and the fact it consumed oxygen in an enclosed space and the CO2 had to be vented as well. However this need not be an issue when using waste heat from the exhaust system of a car.

All well and good to use the exhaust heat to run AC but keep a few things in mind. 1) Most people want immediate AC (here in Florida we sure do!) and the warm-up would be an issue. 2) Turbochargers are becoming common and you will lose some of that heat energy to the turbo. 3) The heat is used to warm up the catalytic convertor so it scrubs the exhaust further affecting 1). and 4) Engine makers know this, too, and as engine efficiency rises, and they are, the exhaust heat declines.

I know the first rule of brainstorming is not to criticize the idea but you still need to take the constraints into account, IMHO

This is a bit analogous to the use of vacuum to run lots of accessories like HVAC doors and cruise control. As efficiency rose vacuum declined and those features became electrical. Which is what AC is already becoming. Electric and hybrid cars like the Prius, have electrically driven AC compressors.


Mustangman beat me to it. The lag to reasonable output would be unacceptable to the vast majority of consumers. I recall reading somewhere that an auto AC system is the equivalent of a 2 ton home system so it can quickly cool down a roasting cabin…I use a heat pump on my pool to maintain temperature. It is super efficient but it needs a long time to achieve temp as compared to a gas heater.

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I grew up with a non vented Servel gas refrigerator. No wonder I’m not quite right.
I was always fascinated by a flame making the box cold. There was a market for used gas refrigerators for use in cabins with no electricity. A co-worker in the 1970’s had a gas powered central AC unit cooling his house. The compressor was located at the rear of the lot with long underground lines to the air handler in the basement. That may have been to reduce noise.

Gas refrigerators are still used in RV’s and I think reefer trucks hauling food.

Heck, there are DUAL units that run on electricity or gas for RV’s. If you have hookups, the 'fridge runs on electricity and gas if you are in the boonies. I did have a Peltier effect solid-state 'fridge that ran off 12 volts. It is intended for tailgating or RVing and takes a long time to get cold.

I think railroad cars uses to be aie conditioned using the absorption principle. With the steam locomotives, there was plenty of steam to cool the cars in the summer and heat them in the winter. When the diesel-electric locomotives came along, the passenger locomotives had a steam generator. Amtrak now cools and heats the cars electrically with a generator in the locomotive…

@Mustangman You are right about the reefer trucks. They run on diesel & are used to keep food frozen if necessary or keep non frozen food like fresh produce from freezing in cold country.

In discussing topics like this it might help to remember the basics: heat propagates from a warmer body to a cooler body. Hot exhaust cannot absorb heat from a cooler gas. It can only warm it up.

AC systems work by compressing a refrigerant, removing the heat generated by the compression (compress matter and it releases heat energy), and then reexpanding it wherein it absorbs heat energy (cools), and feeding it through a heat exchanger wherein it absorbs heat. There’s no way to use hot gas to absorb heat unless it’s temperature is reduced to below that desiring to be cooled.

More fundamentally, “temperature” is simply a method of measuring atomic activity. A highly active atom can transfer some of its energy to a less active atom, in effect “warming” it, but it cannot cause a less active atom to become even less active (cooler). The more active atom will always stimulate the less active atom.

I have been wondering why no one takes advantage of exhaust heat to operate a steam turbine powered alternator.

Heat is a source of energy. You can change energy from one form or another. Therefore, you can change heat energy into a form useful to cool an automobile.

The main problem is entropy. Spontaneous reactions only occur if they promote entropy (randomness), and heat is the most random form of energy. This is why you can always produce heat at impressive efficiency percentages, but heat engines have such low efficiency, turning heat into motion.

You could use the heat directly to boil a low-boiling-point liquid, or you could use a Sterling engine to make motion from heat, then spin an A/C compressor.

I guess my question would be “why would you?” You’re still dealing with circulating a refrigerant - you’re just using the exhaust heat to make the refrigerant do its thing. How would this be more efficient than circulating a refrigerant via an engine-driven device?

Also, we’ve got many, many years of research into making car air conditioners work, and work very well.

A principle from software that more software companies should follow is that you should be very mindful of throwing out the old code to start fresh, because starting fresh means years and years of smashing bugs. I imagine it would be similar in this case - starting with a wholly different refrigeration technology would require lots of R&D just to make it do what we can already do with our current systems.

There would need to be a compelling reason to make the switch.

The start up lag, as mentioned will prevent this from happening. It might make sense for a long distance truck operator, but not for daily short distance commuters.

Heat is not a source of energy. It’s a FORM of energy. It comes from energy released from atomic structures banging into one another (such as the heat energy that gets released when compressing matter) and from bonds being broken between atomic particles (fission and combustion). The bonds are under tension, they contain energy. Atoms bond by sharing valance electrons, and when these bonds are broken the energy entrapped in the bonds can be released as heat energy, electrical energy, and even photons.

Fusion is generally thought to be energy released by forcing atoms together, but that isn’t the whole story. Energy from fusion is released from the forcing of atoms together causing atomic structures to break. The bonds within the atomic structure holding the protons and neutrons together contain a great deal of energy, and when broken they break free with enough energy to start a chain reaction.

If you can explain how an atomic structure with a lower activity level than another can transfer some of its energy to an atom with a higher energy level, I’m listening. I’m sure there are more than a few physicists that would also be interested.

@mountainbike You are just adding energy to the refrigerant with heat instead of a compressor. Thermodynamics had to be the weirdest class that i took in college. Common sense doesn’t apply.

It may not be more efficient then those compressor type. However, I was just wondering why are we simply letting wasted exhaust heat go out the tail and put another load on the crank instead of using exhaust heat to do something useful

Thank you, Senor Pedant :wink:

If you can explain how an atomic structure with a lower activity level than another can transfer some of its energy to an atom with a higher energy level, I’m listening.

Been there, done that. OLD tech! (Predates compressors for refrigeration.) You heat a compressed, liquid refrigerant until it boils. Then you do all of the typical stuff and exchange heat. The technology has been around 100 years; I’m surprised you never heard of it!

That’s correct, you’re transferring energy from the cabin air to the refrigerant, which after having been compressed and having had its heat energy (residual and from compression) removed has been expanded, wherein it then can absorb the heat energy from the cabin air.

I would argue that common sense absolutely does apply. The trick is to remember the fundamentals. If one remembers that energy can be changed in form and transferred from more active atoms to less active atoms, atomic activity being the very essence of what heat is, the rest all simply makes sense.

I should add that how the laws apply at the subatomic level at near absolute zero, one facet of “quantum mechanics”, is not yet fully understood. Absolute zero is that theoretical point where all atomic activity ceases. Scientists have come very close to absolute zero in laboratory conditions and found that atoms begin to behave in ways not yet understood. The energies that hold subatomic particles together are also a subject of great fascination to me, and contain within them the basis of theories predicting things like “quarks” and all that other weird stuff. The scientist at CERN are, I suspect, still a long way from understanding these forces enough to begin to use them actively. There’s a whole lot of atoms left to break apart, a whole lot of theories yet to prove. There’s a whole lot we don’t yet know.