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The best ones I've found were 2-3 amps drain, which is still better than the Peltier effect coolers that draw 4 amps to cool down a small esky.

Nev

I looked into this for a fridge for our home converted campervan. The best I found was a decent sized fridge (for a campervan) and it drew 36Watts when running. It cost over 300 quid as well (we haven't got one). To size the solar system you'd need to guestimate how long the compressor might need to run and I've no idea how you'd go about that.

The problem is not only panels but batteries as well, because the sun only charges for a certain number of hours each day, but the fridge runs 24. I thing a solar/wind system would be better, (but I dont want to get Martin started).

Nev

Muddypause wrote:

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.

So if I spill 1 g of water on the table, does it take 2,260 joules of energy to make it just... evaporate?

Hi, I also converted my own campervan (VW LT35) and researched 12v fridges. I found the company 'Shoreline' boatfridge, (try google) They manufacture 12v fridgess using the regarded danfoss compressors. I brought an end of season fridge for about £350. From their website I see that the small 78L plus 5L icebox fridge (cost £385) its rated at, kwh 24 h 0.29! A fridge motor does not run constantly. I know from the sbmcc (self build motor caravan club, google also) that fridges of the danfoss type are well respected because of extremely low battery useage and quiet (almost silent) running. The American, 'waeco' is very expensive. With such low power requirements, perhaps a solar fridge is practical after all?

John

digiveg wrote:

Muddypause wrote:

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.

So if I spill 1 g of water on the table, does it take 2,260 joules of energy to make it just... evaporate?

Yep. And will cool the air and the table down in the process. That is the basis of an evaporative cooler that has been getting mentions on other threads, and Nev has detailed elsewhere. In order for a liquid to become a vapour, it has to absorbe energy.

Lets do some sums. Assuming none of the water is absorbed into the table, and it takes, say, 150 minutes for that water to evaporate. That's 2,260 joules needed in 9,000 seconds. So, it will have absorbed energy out of the air and from the surface of the table at a rate of about 0.25 joules per second - a quarter of a watt; 0.25% of the output of a 100W lightbulb.

This cooling effect of things as they evaporate has to be considered in things like carburettor design - carburettor icing was often a problem on some vehicles (talk to any Kawasaki motorcycle owner of twenty years ago); the petrol is forced to vapourise very quickly in a carb., and has to absorb energy from its surroundings in order for that to happen. This makes the body of the carburettor very cold, and promotes condensation to freeze on it, icing it up and stopping it from working. A similar problem occurs on a car that is converted to run on LPG (mine) - as the compressed liquid passes through the vaporiser to become a gas, the unit would freeze up, so it has to be plumbed into the cooling system of the car.

I am told that travellers crossing the desert know to keep their water in an animal skin, which is porous, and so allows evaporation, thus keeping the water inside cool. At first, you might think this is wasting water, but if you were to drink warm water in a hot environment, you are taking on more heat, which the body then has to loose. As the body's mechanism for doing this is to sweat (another example of evaporative cooling), then drinking warm water will make you sweat even more, which means you have to drink disproportionately more water. If you do the sums, keeping the water cool by letting it evaporate can, in fact, mean you need less of it.

My gran always told me that it is better to have a hot drink when you are hot, rather than a cold drink. Her theory was, as I understand that, that if you raise your internal temperature, your body responds by activating or advancing its own cooling mechanisms... pores open, breath becomes deeper, produce more sweat and saliva, and whatever else the body does in response to heat...

Mates always think I am nuts when I come in from the yards, hot and sweaty and turn the jug on for a coffee. Truth is though, I have one cup of coffee and am ready to head back out again in 10 minutes. Those same mates have chugged back two litres of cordial, water or whatever and are still flopping about whingeing!

So, I think maybe we can't compare terracotta or hide to humans, because we have internal systems that contribute to our response to environmental factors, and they don't.

I also have a different take on the conversion from liquid to gas, Muddy... Which could be completely wrong - so correct me if I am because this particular aspect is something I have been trying to get across to a mate of mine, and he keeps shaking his head like I'm talking a foreign language.

...LPG hits the vaporiser as liquid and comes through at near freezing point. In order for it to vaporise, it rotates through the vaporising chamber which must exist at a minimum temp (something like 20 degrees - not certain, can't remember). The reason it is suggested LPG converted cars be started on petrol, is to ensure that contact temp in the vaporiser. Should that temp be too low, the liquid temp over-rides operational temp and the vaporiser freezes, freezing the valve that permits passage to the carb and inhibiting proper flow. The car then runs like a bucket of shit. Once the vaporiser freezes up, the engine tries to draw on the petrol lines and starts behaving like a starving child. The solution is simple enough, in that the car should be started on petrol and the engine soon generates enough heat to ensure the vaporiser is ready to receive the LPG and do its job... BUT... if turkeys insist on starting the dang car on LPG and not allowing for shade when considering the starting temp the problem will persist!

Am I close?

Boots wrote:My gran always told me that it is better to have a hot drink when you are hot, rather than a cold drink. Her theory was, as I understand that, that if you raise your internal temperature, your body responds by activating or advancing its own cooling mechanisms... pores open, breath becomes deeper, produce more sweat and saliva, and whatever else the body does in response to heat...

I can sorta see some logic in that - it's analogous to the idea of eating a hot curry if you are in a hot place, because it makes you sweat more, and ends up cooling you down. But 'hot' in terms of a curry is not the same as 'hot' in terms of a hot drink. The real issue is if you are somewhere where the air temperature is higher than your ideal body temperature. In this situation, the body has to make itself cooler than its surroundings. Pretty well the only way it has of doing this involves sweating - becoming an evaporative cooler - which also involves losing water. If you drink something hotter than body temperature, you are taking on liquid, but also even more heat, which it is going to have to be dealt with by even more sweating. That's not to say that some people may be well equiped, or aclimatised, to do just that. I suppose it may also be the case that some people's bodies are adapted to produce less heat somehow.

Mates always think I am nuts when I come in from the yards, hot and sweaty and turn the jug on for a coffee. Truth is though, I have one cup of coffee and am ready to head back out again in 10 minutes. Those same mates have chugged back two litres of cordial, water or whatever and are still flopping about whingeing!

I have a feeling you may be one of those annoyingly irrepressable people who can't let a chap have a decent hour's tea break. But that quite apart, isn't it meant to be better to take on liquid little and often? Two litres of water all at once would prolly make most people want to sit down for a while.

So, I think maybe we can't compare terracotta or hide to humans, because we have internal systems that contribute to our response to environmental factors, and they don't.

I take your point - there are probably all sorts of considerations regarding metabolism, physiology, and even psychology to take into account. But in the end, there are some basic principles regarding how heat moves about, whether it involves terracotta or human being.

I also have a different take on the conversion from liquid to gas, Muddy

Uh-oh!

LPG hits the vaporiser as liquid and comes through at near freezing point.

Umm... freezing point? Freezing point of what? Propane freezes at around -190C. That's really really cold. Butane freezes at much higher temps, so is not added to LPG in cold climates. I suppose that may mean that you might have quite a lot of butane in your LPG, whereas we don't.

But the change in state from liquid to gas shouldn't get near to this freezing point, otherwise vaporisation just can't happen at a sufficient rate to make the engine run. In fact, if my figuring out is correct, it would reach a point of stasis at around -44C - it's boiling point. Below that temperature, it won't boil, so significant heat absoption will cease also. That implies that fuel freezing in this way is actually impossible; hey, I never thought of that before.

So, this bit is important -> When an LPG vaporiser freezes (or a carburettor), it is not the LPG (or petrol) that is freezing, but moisture in the atmosphere that is condensing and turning into water-ice, blocking the mechanism and even the valves of the vap. (carb). If the temperature gets too low, the rate of vaporisation will be too low to run the engine. But the fuel is not freezing.

In order for it to vaporise, it rotates through the vaporising chamber which must exist at a minimum temp (something like 20 degrees - not certain, can't remember).

AIUI, it turns into vapour simply as a result of the pressure change. Tank side of the vap. it is held at around 7 bar (~100 psi); engine side of the vap., it is at 1 bar. The valve in the middle regulates the rate of vaporisation. This vaporisation requires an energy input (the molecules must gain enough energy to occupy a much larger space), and this is what causes the vap. to lose heat and get cold.

My vaporiser is simply plumbed into the cooling system, in parallel with the car's heater matrix - there is nothing to regulate the temperature of the vap. at all.

The reason it is suggested LPG converted cars be started on petrol, is to ensure that contact temp in the vaporiser. Should that temp be too low, the liquid temp over-rides operational temp and the vaporiser freezes, freezing the valve that permits passage to the carb and inhibiting proper flow. The car then runs like a bucket of shit.

Yes and no. I start my car on LPG, and have done so with several inches of snow on it in the middle of a Scottish winter. Remember, that even cold water has some heat energy in it, and providing it circulates at a high enough rate, it can still impart heat to the vaporising liquid.

But I do hear that there are some cars that need to be run for a few minutes to prevent vap. freezing. Is this really a problem in Aus? Some conversions in the UK have stripped out the petrol altogether.

My concern is more to do with the fact that an unused petrol system may start to malfunction (injectors and pumps will gum up, etc.), and as LPG fuel guages are so notoriously inaccurate, switching to petrol is an occasional hasty necessity. So about once every couple of days I will start the car on petrol just long enough to hear the pumps whir into action, and then it's on to LPG.

Once the vaporiser freezes up, the engine tries to draw on the petrol lines and starts behaving like a starving child. The solution is simple enough, in that the car should be started on petrol and the engine soon generates enough heat to ensure the vaporiser is ready to receive the LPG and do its job... BUT... if turkeys insist on starting the dang car on LPG and not allowing for shade when considering the starting temp the problem will persist!

Am I close?

Look - I could be making all this up, y'know.

Well, no wonder he keeps giving me that look...

Best I just let him be!

Have just set up a small bar fridge with solar panels, took 2 panels, and inverter and 1 deep-cell battery. Overkill, I know, but this will be transfered to our dwelling soon, to use with our normal-sized fridge.

One battery is enough, as the fridge doesn't actully run 24 hours a day, it is on and off... interestingly, we have worked out our little inefficient bar-fridge will use the same or more power as our household-sized energy efficient super-insulated one...