Since the way in which absorption heat pumps work was initially difficult for me to wrap my head around, and is probably hard for other people to understand as well, I'll first describe how the easiest to understand absorption heat pump works.

This is a two pressure, single stage, water / liquid desiccant absorption heat pump. The water is the refrigerant, the desiccant is the absorber.

First, we start out with high pressure high temperature gaseous refrigerant. It passes through the condensor, and turns into high pressure high temperature liquid refrigerant. Then, the water goes through the expansion valve, becoming low temperature low pressure mixed phase (some water, some steam) fluid. The water/steam mix goes through the evaporator, and changes the rest of the way to steam. So far, not very different from a vapor compression heat pump, using water as the refrigerant.

The steam enters the absorber, where it encounters highly concentrated liquid desiccant. Because of how very little water the desiccant has, it absorbs the steam. The now dilute liquid desiccant then goes through a pump, which increases it's pressure and moves it into the generator.

In the generator, heat is added to the dilute liquid desiccant, and the water boils out. The now concentrated liquid desiccant is returned to the absorber. The steam that boiled out flows to the condensor.

In summary, it's a lot like a vapor compression heat pump, but with the compressor replaced by the absorber/generator pair.

One of the limitations of such a heat pump, is that the colder one desires the evaporator to operate at, the higher the temperature that the generator needs to be heated to. This is because for a lower temperature in the evaporator, the pressure there needs to be lower. For the desiccant to absorb lower pressure steam, it needs to be more concentrated. For the desiccant to be more concentrated, it needs to be boiled at a higher temperature to reach that concentration.

(Higher temperature operation of the condensor will also require a higher temperature heat source for the generator, but for a simpler reason: for steam to condense in a hotter condensor, it needs to be at a higher pressure. For water to boil out of the dilute liquid desiccant at a higher pressure, it needs to be hotter.)

The core of this idea is: If the steam were mechanically compressed either between the evaporator and the absorber, or between the generator and the condensor, the required temperature of the heat source applied to the generator would be reduced.

Taking capital cost into consideration, it would be cheaper to insert mechanical compression between the generator and the compressor (where the refrigerant is already at a higher pressure), than between the evaporator and absorber (since compressing low pressure gas requires a costlier compressor than compressing high pressure gas).

In effect, change from one stage of compression (the absorber/generator pair) to two stages of compression (the absorber/generator pair, plus a mechanical compressor).

Why is this useful? Because the resulting refrigeration cycle consumes less mechanical energy than a pure reverse rankine cycle, and can work with a lower temperature heat source than a pure absorbtion cycle.

This means... if the heat source is a solar thermal collector, it can be a cheaper design (flat plate, instead of evacuated tube), that produces less hot temperatures.

Or, if the heat source is a car engine's waste heat, a smaller, cheaper, lighter, heat exchanger could be used for extracting primary heat from the exhaust or coolant.
-- goldbb, Sep 03 2009

Absorb vs adsorb http://www.madsci.o...077064138.Ch.r.html
[normzone, Sep 08 2009]

Oh, and I have searched the web for absorbtion compression hybrid cycles... they all seem to be "conventional" two stage heat pumps, except that one full cycle of one type or the other is done, followed by a full cycle of the other type.

E.g., a vapor compression cycle moves heat from low temperature to medium temperature, then an absorbtion cycle moves heat from medium temperature to high temperature.

Those types of hybrids require a greater capital cost than a single combined cycle, since theye require two evaporators, two condensors, and two expansion valves.
-- goldbb, Sep 03 2009


This liquid desiccant you speak of... what is it exactly?
-- swimr, Sep 06 2009


i'm too tired to absorb your excellent tutorial, sadly, but on the last point: car waste heat reuse is being thoroughly researched by Real Scientists. Why it's taking so long to get to industry, makes me briefly consider changing careers and commercializing something. Then I remember how tired I am.
-- white, Sep 07 2009


// This liquid desiccant you speak of... what is it exactly? //

Concentrated sulphuric acid would work.

While the math behind this may be attractive, real-world inefficiencies, capital cost and the law of diminishing returns mean that there's a point where clawing back that last electron-volt of energy isn't worth the hassle. Adding mass and complexity to a vehicle is not necessarily the yellow brick road to higher efficiency .....
-- 8th of 7, Sep 07 2009


Sulfuric acid would probably work fairly well as an absorbent... provided you don't mind the added expense of making all the parts which contact it acid proof.

Two alternative liquid desiccants, either of which would likely result in lower capital cost, are lithium bromide, and calcium chloride.

And swimr, in answer to your question, "what is it exactly," a desiccant is any substance which either absorbs or adsorbs water vapor.
-- goldbb, Sep 08 2009


Absorb vs adsorb (link).
-- normzone, Sep 08 2009


I didn't say "or" because I was unsure, but rather because a (solid) chemical which adsorbs water, and a (liquid) chemical which absorbs water, can both be thought of as desiccants. :)
-- goldbb, Sep 10 2009



random, halfbakery