|Absorption cooling, adsorption cooling and steamed potato. |
This page is aimed at the novice, it may be pretty boring to others.. I have no formal qualifications in refrigeration, physics or tea making. The following information could be inaccurate or just plain wrong.
*To save space I'll mainly use the term absorption and absorber even though sometimes this could also be adsorption and adsorber*
There are two main types of absorption cooling cycle. One is a continuous cycle where heat can be applied continuously and the cold part of the system always stays cold. This is the cycle used in gas/kero fridges. The other system is an intermittent cycle where there are two distinct phases. There is a generation phase where the generator (which is the absorber later) is heated and the receiver( which is the evaporator later) is at close to ambient temperature and a cooling phase where the absorber is near ambient and the evaporator is cold. Each system has its positives and negative.
So how does an absorption fridge work? First some basic kitchen physics.
Before we talk about absorption let's talk about a simple compressor fridge.
In the above text I've explained how we can cool water by boiling in at low pressure, we've also seen how we can heat things by condensing vapor into liquid. So let's make a simple cooler to keep the milk for the tea fresh. Let's make a small thermally insulated box and inside it place a flask of water (the evaporator) with a pipe running from the top of the flask to a pump outside the box.. We run the pump, lower pressure, boil the water and lower the temperature. We might call this an intermittent compressor fridge because we need to stop it once in a while to refill the water.
Since this is a hassle we fit another container (condenser) to the outlet from the pump and condense the steam back into water, this container will become hotter due to the latent heat of the condensation. The condenser is at a higher pressure than the evaporator. We now have a heat pump of sorts. Finally we run a pipe back into the evaporator via a valve which maintains a pressure difference between the evaporator and condenser, this might just be a narrow tube. Now we have a continuous cycle, this is how most fridges work. Usually the refrigerant won't be water and the pressures in the system will be much higher. The containers will usually be tubes to improve heat flow but the principle is the same.
And now what you came here for, the absorption stuff.
To build an intermittent cycle fridge we don't have to use a mechanical pump to lower the pressure in the evaporator. Just say we had something that absorbed the water vapor, some chemical with a hunger for water that would grab enough water molecules the keep the vapor pressure low enough to allow the water to boil at low temperatures. Such chemicals exist, one of them is a salt called Lithium Bromide(LiBr). If we replace the pump and condenser with a container of LiBr and remove the air - presto we have absorption cooling (in theory anyway). This cooling phase is the evaporation/absorption part of the cycle. The system would continue to cool till either we run out of water or the LiBr has absorbed all the water it can. What now? We remove the contraption from the box and heat the LiBr, it changes from being an absorber to being a generator. We cool (as best we can) what was the evaporator and it becomes the condenser/receiver. This cooling may just be exposure to room temperature air or a dip into cool water. During this part of the cycle water is released by the LiBr and condenses back into the receiver ready for another cooling cycle. If we can heat the LiBr enough we can remove all the water, but at lower generator temperatures we must be content with only removing some of the water. Note that the pressure is governed by the receiver temperature. At the generator temperatures we can expect from a passive solar collector, we might only change the concentration of the LiBr solution by 5% to 10 %. Unfortunately LiBr is too expensive to build a practical intermittent system and as it uses water as the refrigerant it can't be used to make a freezer, also the working pressures are very low and hard to maintain. intermittent cycle absorption fridges need far more absorber and refrigerant than continuous cycle ones. For a more practical cycle see the icyball patent. This uses an ammonia/water cycle, ammonia is the refrigerant and water the absorber, the system works at much higher pressures. Also see - Solar Powered Refrigeration by means of an Ammonia/water Intermittent Absorption Cycle. There are more complicated intermittent cycle systems, for example we might spilt the receiver into two allowing the condensation to occurs outside of the fridge and the evaporation to occurs inside. One might also build a pseudo continuous cycle by using multiple generator/absorbers. This could be useful in an adsorption system where the adsorber is a solid and unsuitable for a continuous cycle.
Now let look at doing a continuous cycle absorption fridge.
In the continuous cycle fridge we discussed above we used a mechanical pump to maintain a pressure difference across a pressure reducing valve with the aim of boiling a liquid as it passed from the high pressure to low pressure parts of the system, we want to do something similar now using absorption techniques. We arrange two containers of LiBr/water solution so that the solution can circulate between to two, probably using a pair of tubes. We heat one container (the generator) and it generates gas (water vapour) - raising the pressure in the container with respect to the cold one. In the process the solution becomes more concentrated. The pressure increase will also force some solution across to the other container causing a difference in the liquid levels. We can run a pipe from the top of the generator taking the hot gas into a condenser to liquify it. The liquid then passes through a pressure reducing valve and boils in the low pressure side of the system. This gives us the desired cooling effect. The low pressure gas then travels into the cooler container (absorber) where it is absorbed by the LiBr solution. The LiBr is circulated from one container to the other, absorbing gas on the cool side and releasing it on the hot side. The transfer of heat from one container to the other by the LiBr solution is undesirable and can be reduced with the addition of a counter flow heat exchanger. A heat exchanger between the warm water entering the evaporator and the cold gas leaving it may also improve efficiency.
The diagrams above are simplified and may look nothing like a real fridge. A real fridge would probably use tubes and fins in many places. The size of the various parts could vary considerably along with their shape and placement. I hope this has been useful just the same.
There a zillions of possible absorber/refrigerant pairs, many are still untested (or undiscovered) - none that I know of are perfect. The perfect pair would be cheap, nontoxic, non corrosive, environmentally safe, have a convenient working pressure, low absorber specific heat, high refrigerant latent heat and so forth. A continuous cycle requires a liquid absorber, an intermittent cycle can use either a liquid absorber or a solid adsorber.
The working pressure is governed by the refrigerant, the pressure will be what ever is required to make the refrigerant boil at the desired evaporator temperature and will be higher in the generation phase for intermittent cycle or in the generator in a continuous cycle. The ideal working pressure is probably lightly above atmospheric pressure. This is because a slight leak of refrigerant out of the system will degrade performance less than a leak of air into the system.
Some working pairs include, Ammonia/water, Ammonia/Calcium chloride(solid), Lithium bromide/water, Zeolite/water and Activated charcoal/methanol.
Is it worth while?
Comparing absorption and electric (compressor based) fridges is not a simple matter. In my opinion the photo-voltaic (PV) electric fridge is better in most remote power situations. While the absorption fridge is simple in concept one must consider things like backup systems for cloudy days (eg. gas boost), reliability, portability and so on. The niches for solar absorption may include :-
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