solar fridge?

[paradox]

I have recently since leaving hm forces bought a 1978 vw camper van.

It has a 3 way fridge fitted that will work on 240 volt 12 volt or butane gas it puzzled me how a fridge (cold) can work of burning gas(hot) then i thought if heat can make things cold how can i build a fridge to make things cold from solar energy?

[Muddypause]

This has always puzzled me too. I can still remember getting our first fridge in the early 60s, and that used a gas flame to make things cold, too.

I'm a bit hazey on all the details, but basically, there is amonia dissolved in water, and hydrogen gas, in a network of pipes, condensers and boilers. The gas flame raises the temperature of the amonia solution until the amonia boils off. This then recondenses elsewhere, only to re-evaporate again when it mixes with the hydrogen. It's this re-evaporation that makes things cold (ask any school physics student about the Latent Heat of Evaporation - they'll be delighted to tell you about it). Then the amonia vapor is re-absorbed by the water, separating it from the hydrogen, and the whole cycle can start again.

Easy enough, innit? Aparently, this cycle was invented by Einstein, so I reckon we can be fogiven for not understanding it. Presumably he was just warming up for the Theory of Relativity at this stage.

Your 3-way fridge must be a very complicated bit of kit, because the above cycle is completely different to the regular compressor heat pump cycle that electric fridges use.

[Earthmother]

We have a combi cool 3 way fridge for camping and wondered the same!

[paradox]

thanks for your replys people can anyone shed any more light on the subject?

[Martin]

the last part of your question - about how to do it using solar - one of "Nev's" evaporative coolers would be the simplest! - it too uses the dreaded "latent heat" thingy -
my clear as mud explanation! - when water changes state from water to ice or from water to vapour, it either uses or releases lots of "extra" energy - it's almost as if it needs a bloody great shove to get it to change!
so, at it's simplest, you shove the things you want to keep cold in a sealed box, pop it into a trough of water, and cover it with a cloth, and pop it in the sun! - as the water evaporates, it needs lots of "extra" heat to help it to change state, which it takes from it's surroundings (including your box!)
It also explains why steam scalds are so horrible - quick change of state - dumps all the extra heat into you!
If I remember right, it takes something like 16 times the energy to get something to boil than it does to raise the same volume of water by 1 degree!

[Martin]

that's raised an interesting thought - would vegetables cook just as well at one degree below boiling? - if so, it may be a way of saving lots of energy!
If you go up a mountain, it boils at a lower temperature - so..........
errmmm?

[Stonehead]

that's raised an interesting thought - would vegetables cook just as well at one degree below boiling? - if so, it may be a way of saving lots of energy!
If you go up a mountain, it boils at a lower temperature - so..........
errmmm?

This is going to get very complicated....

If the temperature is below boiling, then the cooking time will be longer. But even then, you can still find yourself eating or drinking something that's tepid and this is what happens as you go up in altitude.

At the same time, baking gets thrown out of kilter as doughs rise faster (whether with year or baking powder). As a result, you go higher up, you need to allow less time for yeast doughs to rise or use less baking powder. You also need to up the temperature in both cases.

I can't remember the exact height at which altitude starts making a different but it's something like 3000-4000ft. I'm sure Google will give the answer, not least because many Americans live above those heights.

As for the rest, my brain is frazzled after a long day so I'll leave it to someone else!

[Martin]

I'm probably missing something, but it SEEMS sensible - don't waste all that extra energy to get it to change state!
I started to try doing the sums, and the "tilt" sign came up - I'd got the pan boiling, allowed for the extra heat to get it to change state, but then you have the fact that only a certain proportion will be changing state at any one time........

[Stonehead]

I'm probably missing something, but it SEEMS sensible - don't waste all that extra energy to get it to change state!

That's why pressure cookers save energy - no change of state, but higher temperatures and faster cooking times.

But back to solar powered fridges, why not a solar-powered thermoacoustic fridge? IIRC the sound levels generated by a typical rock concert are sufficient to cool slightly, while higher levels can actually freeze water. Basically, you bounce sound waves through a stack of metal plates, some get hotter, some colder, and you attach heat exchangers to the ends of the stack.

No, I keep correcting this so I'm really going to have to go!

[Shirley]

I want a pressure cooker...

Are they all created equal.. or are some better than others??

[Stonehead]

I want a pressure cooker...

Are they all created equal.. or are some better than others??

Have a look at http://fastcooking.ca/energy_savings_pr ... cooker.htm. US and marketing for a particular brand, but it explains it quite well and, yes, higher pressures are better.

Goodnight. Finally, yes!!!

[Martin]

So, I've got my nice cauliflower, at sea level - if I cook it in a thermostatically controlled saucepan at 99 degrees that doesn't boil, will it taste any different to one cooked at 3,000 feet at 99 degrees that does boil?

[Muddypause]

If I remember right, it takes something like 16 times the energy to get something to boil than it does to raise the same volume of water by 1 degree!

You may have misunderstood that a little, Martin. There isn't a sort of 'boil threshold' that has to be crossed before the water boils. What happens is that the water will not get any hotter than 100 degrees, no matter how much energy you put into it (assuming 1 bar pressure). Any energy that you put into the water after it has reached 100 degrees will be used in turning the water into water vapour.

However, the maths is this:

It takes about 4.2 joules of energy to raise 1 g of water by 1 degree, and about 2,260 joules to turn that g of water into water vapour.

But unless you alter the gas tap, when water gets to 100 degrees, energy used at the same rate will be used in boiling the water (turning it into vapour) as you used to get it up to that temperature. You are not suddenly having to apply extra energy to boil the water. The extra boiling energy is simply a reflection of the fact that it takes a long time to turn water into water vapour.

For example, if it takes two minutes to raise an electric kettle of water through 70 degrees, it will have taken 4.2 x 70 joules of energy per g of water. A typical electric kettle will have brought about 1 litre of water to the boil in that time, and used around 300kJ (ignoring insulation losses). But it will then take a further 15 minutes to boil the kettle dry. That's probably why you're thinking it takes the extra energy - but the kettle hasn't been 'turned up' to make the water boil in the first place, and boiling the kettle dry is not something we try to do much of, normally, anyway.

When you bring a pan of water to the boil, you do not suddenly need to turn the gas up hugely to keep it boiling - you are using energy at the same rate (unless you alter the setting). In practice, the difference between keeping water at 99 degrees and keeping water at 100 degrees is more to do with maintaining heat losses than making it evaporate, so there may not be a huge difference.

It might be far better to design an insulated pan, or a cooker that doesn't use a lot of energy just heating the air.

[Boots]

Hey Paradox,
Try this - as to the hows and whys...

http://www.monash.edu.au/pubs/eureka/Eu ... ridge.html

You can contact Eric through Deakin, his details are here

http://www.deakin.edu.au/scitech/et/sta ... ame=erichu

[jondy]

Solar powered fridge? :-
Buy a 12volt compressor fridge, power it with a 12volt battery and charge the battery with a solar panel.

12v compressor fridges as used in caravans and boats are not the same as the 3 way variety, 240v, 12v, gas. The 3 way variety is a very heavy user of 12v battery power so not much use for solar charging. 12v compressor fridges are expensive but work very well and consume very little battery power. From memory and a bit of a guess, about 1 amp hour or less. Powering a domestic 240v fridge from a battery with inverter would require a lot of solar panels, I guess.

John