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February 19, 2014 Archives

I've been reading through a student's report of his summer work placement. He  had a project on improving the performance of a heat exchanger used for getting rid of heat from a cryogenic cooler. The basic concept is that materials are being cooled to about 40 kelvin (that's 40 degrees above absolute zero, minus 233 degrees celsius, by removing heat from them. That heat needs to go somewhere, and the job of the heat exchanger is to dump it.

I was struck by the similarities of the problem of 'dumping waste heat' with that of 'dumping rubbish'.  What do we do with household rubbish - stuff we don't want that is generated by our day-to-day activities. Well, various things happen to it. 1. Some of it gets put in the pretty blue (in Waipa) recyling box and gets put out on a Thursday morning.  2. Some of it gets put into the equally pretty yellow pre-paid rubbish sacks and also goes out on a Thursday morning, and ends up in landfill :-( 3. Some of it accumulates in the garage, until such time that 1. or 2. applies (or it gets taken to the recycling centre / tip, which is the same as 1. or 2.) 

We do the same with waste heat. It is generated by just about everything that we do. Car engines generate waste heat, car brakes generate heat through friction, electrical appliances generate it, running up and down stairs creates it - basically it's an inescapable consequence of the second law of thermodynamics. Heat gets made, and usually we want to get rid of it. So, what do we do with it?

1. We can, if we're clever, recycle it. A sensibly-designed industrial plant will tap into the waste heat it makes to do useful things. Smart tumble dryers will use the waste heat in their exhaust to pre-heat the dry air being sucked into the machine, saving electricity. Heat engines can be put into effect where there is a consistent difference in temperature between two objects. 

2. We can dump it. That's what happens to most of it. We let it end up in cooling water, or the atmosphere, where we conveniently forget about it. However, unlike landfill, it's not usually a problem in itself (warm rivers near power stations might be, however). The amount we generate over the earth is pretty tiny compared to the amount that the sun gives us. The real issue is the amount of carbon dioxide and other greenhouse gases that have been generated in the process (plus, economically, the generation cost of the energy that is being wasted). 

3. We can store it and do something with it later. This isn't so easy, but can be done. We can exploit gels that have high latent heats, meaning that as they undergo a phase change they take in heat, and then as they undergo the reverse change they will give it out again. We can heat up objects with high heat capacity, keep them well insulated, and then release the heat later (e.g. night storage heaters). 

So does 'dumping' heat mean that we're treating energy in a similar manner to rubbish? Chuck it away and pretend it's not an issue. With the landfill problem, the first thing to address is not recyling, but simply not to consume so much stuff in the first place. If we did the same with heat, we'd have lower energy bills and lower greenhouse gas emissions. The two, and their problems, are perhaps not so wildly different. 

Here's a final thought then. My work emails (if I don't delete them) get stored somewhere in the world on a server belonging to a well-known and rather enormous company. How much power does it take to keep one of my emails on file? I don't know the answer to that one. If everyone in the world deleted their email and the data they really didn't need anymore, what difference does it make to the world's energy consumption? Anyone know?


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