The Cerebral Forum
Solar Projects => Energy Storage Projects => Topic started by: Gabriel on June 13, 2013, 09:50:43 AM
In the winter, it is easy to use the energy from a heliostat since light can be converted directly to heat simply by shining it on, for example, a wall. At this time of the year though, it's hot enough that I'm far more interested in cooling things down than heating them up.
Here's a neat project that is actually able to do exactly that, use heat for cooling. It's called A Solar Absorption Ice Maker. There is a fair amount of information on them scattered around the net, but this is the first PDF I found that actually explains the process in plain English.
It's something that I definitely want to try at some point. You figure if I already have a heliostat array set up, then I wouldn't need to build the parabola part of this ice maker. The rest of it actually looks relatively simple.
I once did a "quick" test with a chest freezer to see how long it would take to freeze about 5 gallons of water. I wish I had written it down, but I think it took about 4 days to completely freeze all of the ice. It also took roughly an equal amount of time to thaw again. This is why I posted this in the "Energy Storage Projects" section of this forum. If you can use the extra energy available on a sunny day to make enough ice for extended food storage, then you are (sort of) also storing energy. ("sort of" because ice is more of an absence of energy, yeah I'm confused too.)
It would be interesting to see if anyone else has tried something like this.
I've run across that pdf before and found it intriguing too. I was thinking about making a small version of it but couldn't find pure ammonia. Apparently it's not available for purchase to the general public, since it is so dangerous. I also wondered about the calcium chloride in the collector pipe - I know that cacl is extremely corrosive, so I'm surprised they've gotten away with using it. You say that you tried something similar - where'd you get the ammonia?
Aside from general air conditioning, this sort of thing would be great for pumping up the output of a solar powered sterling engine.
It was just a normal, unmodified chest freezer. What I was trying to do was to see how feasible it would be to remove the battery from a solar powered fridge. So you would still have a regular solar panel to power the freezer during the day, but instead of using a battery to power the freezer at night you would just freeze a bunch of ice to help it make it through the night (or ideally several days).
In a round about way, that's sort of what led me to the Solar Absorption Ice Maker.
I don't know where you would get ammonia, and I haven't even bothered to look yet. It would be nice to find something that is less dangerous though. Perhaps water and lithium bromide? http://www.brighthubengineering.com/hvac/66301-water-lithium-bromide-vapor-absorption-refrigeration-system/ (http://www.brighthubengineering.com/hvac/66301-water-lithium-bromide-vapor-absorption-refrigeration-system/)
I think one potential advantage of using heliostats instead of the fixed parabola with this ice maker is the fact that you can remove the heat just by telling the heliostat to reflect the light somewhere else. That way the device will have the chance to go into its "night" cycle multiple times throughout the day. If this were the case, you could probably get by with using less ammonia (or whatever else that might be used) and still make the same amount of ice.
I was giving some thought to this cooler again today, (because it has been another very hot day). I decided to get a rough idea of how much cooling the generated 10lbs of ice mentioned in the above PDF could do.
First I figured out how many joules 1 gallon of frozen water can absorb. Here is a quick and dirty paste of the MatLab code.
JoulesAbsorbedBy1GallonOfFrozenWater = 3780 * 4.186 * 80 %grams of water in one gallon * number of joules in 1 calorie * number of calories removed to freeze water
This comes out to...
JoulesAbsorbedBy1GallonOfFrozenWater = 1265846.4 Joules
Since 10lbs of ice = about 1.2 gallons of ice, the above solar ice maker will generate enough ice to absorb 1519015.68 Joules.
To keep things modest, I looked up the smallest air conditioner that I could quickly find. It had a rated output of 5000BTU*, which is supposedly enough to cool a small 150 square foot room.
5000BTU is equal to 5275279.26 Joules
To figure out how long the air conditioner would have to run to provide an equivalent amount of cooling when compared to the solar ice maker, we do 1519015.68/5275279.26 = 0.2879 hours
So the small air conditioner provides the same amount of cooling in about a half an hour as the 140 ft^2 (or 13 m^2) Solar Ice Maker took all day to do.
Obviously, some improvements would have to be made for this to compete with the already well established air conditioning systems.
Who is up for a challenge??!!!
*Note: It is assumed knowledge that you know that BTU actually stands for BTU / hour in this context. (It is of course always safer never to assume knowledge.)
http://en.wikipedia.org/wiki/Absorption_refrigerator (http://en.wikipedia.org/wiki/Absorption_refrigerator) states that the vapor compression cycle has a coefficient of performance that is 5 times better than the vapor absorption cycle. This means that it takes 5 times more energy to produce the same cooling effect. The 5000 BTU/ hour AC unit (1500 watt equivalent) has a typical coefficient of performance of 2 to about 5. To power this small AC unit you would need 1500/2 to 1500/5 or 750 to 300 watts. Full sun I think is 1000 W per square meter. So on an energy basis with a perfect collector and generator, you would need .3 to .75 square meters to power the small AC unit. For the absorption unit you would need 1.5 to 3 square meters. It looks like Steven has about 8 Square meters in his Icemaker. In theory at least, we could do much better and with a smaller collector. Thanks for sharing this interesting technology!