The photo below this message shows a Helios Heater. I took this photo before putting the heater in my pocket the other night. I took it along and wanted to activate it to warm my hands a little by the end of a viewing session. This is a portable hot pack that can be activated and reused. I was wondering how efficient this was. By efficiency I mean how much heat and energy is required to charge it vs. how much you'll get back and be able to use to warm you're hands or body with.
I looked it up on Google, just to see if anyone had anything to say about the efficiency of this but found a PDF file that was an advertisement for a different Helios heater which was an IR electric heater that heated objects using IR heat. The IR heater actually used a parabolic reflector to aim the IR energy at objects and was supposed to be very efficient, up to 92% efficient. Converting electricity to usable heat. We would not want to aim an IR heater at the telescope when we are using it, but might be able to aim it at people in the observatory, but that's the subject of a different post.
So back to my question, "How efficient is this little automatic heat pack?" It uses a fluid which is a salt compound and is in a liquid state when it's charged. To charge it you have to boil it. The crystals liquify and a trigger event which I believe may be the release of oxygen from an aluminum tab will cause the crystals to reform and give off heat in an exothermic reaction. The energy from the phase change from liquid to solid gives off heat. 130F to be precise.
To charge them you need to boil them in water. You need to use boiling water as a way to transfer heat into them without melting the cover and they suggest you may want to have a small towel or something in the pan of water as well to keep the packet from sticking to the bottom of the pan or overheating and melting. You need to boil the smaller ones for 5 minutes or so in boiling water which is 212 F. I can't talk about energy efficiency and creating a calculation about efficiency unless I make some assumptions. If we make the assumption that you are using 6 times the weight in water as your small water bath then you need to generate at least 6 times the difference in the water temperature for units of energy to get the water to 212. If we ignore the energy needed to keep the water boiling we have about 840 units of energy to get the water bath to 212F. I'm making a lot of assumptions use about units of energy to heat up an 8 once small heater to 212 and cause the crystals to liquify. This of course is a really brief and quick guess and it will actually take more energy. But I'm giving a conservative guess at how many units of energy. I'm assuming one unit of energy per degree per ounce and this really isn't a BTU or a fixed unit, just some energy unit. But if we use 212 F and the F difference as the unit for energy we are getting back and stick with degrees in the same unit, the ratios of all our calculations will give us a basic ratio of efficiency and it doesn't have to be BTU's it can just be basically units of energy like a BTU.
I'm also not calculating the amount of energy that is required or figuring out how much additional energy is needed for the phase change, to actually melt the crystals. I'm treating this as if it's just being heated up like water, but it's more probable that there is a huge input of energy to cause the phase change to happen, which is not included in my charge calculations. This makes the calculations more conservative, so it's really less efficient if we are using 6 times the water. There are a lot of things that could be happening over time. Water could evaporate as it boils, the water might not boil at 212 F and a host of other variables. A real experiment would provide different figures and perhaps give a better account.
So we have all these disclaimers above and likely more for my calculation. How hot is hot? I'm using 70F as the temperature of the pack when we start and figure we need 140 degrees added to get the pack to change, if you're wondering.
For using the pack. I have one that has been spent sitting right in front of my computer. If I put it on my forehead it feels cool to the touch. It's room temperature. Our body won't feel heat unless it's warmer than we are. Which is normally 98.6. So usable heat from the pad is about 30 degrees from 130 Degrees down to about 100. This means we have a basic formula for my little 8 ounce hot pack (and I've guessed the weight of this.)
212 - 70 = 142 units to heat up the water from 70 to 212.
6 times the water mass to give us a bath. 6 * 142 = 852 units of energy. Without worrying about phase change added requirements. Basically the water boils and we figure that is enough, but there is more than this happening so this is very conservative.
Our pack is 1/6th the mass of the water. It doesn't matter what it's size is. We'll just say it's giving off 30 degrees or units per pack. Or returning 30 units of energy but requiring 852 units to heat it.
So my conservative energy estimate for this hot pack is 852/30 for efficiency or about 3% return on the energy put into the boiling water. Clearly from an energy perspective this is a rich man's heater. It's wasting more than 90% of the original energy. You could use it as a hot pack the minute you've heated it, but it would still provide very little return on the original energy. Clearly a hot water bottle or electric heater would be far more efficient.
But the big advantage to this is heat on demand and you don't need electricity or a battery with you. It's a crystal heat battery using phase change characteristics and a chemical reaction to release stored energy.
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