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In hot weather my loft can be almost 20 掳C warmer than my cellar. Is there anything useful or interesting I can do with this? The cellar in question is three floors below the loft.
鈥� The best thing you can do with the 20 掳C temperature difference between loft and cellar is to make sure you store your food and drink in the cellar and hang your washing up to dry in the attic.
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鈥淭he best thing you can do is store your food and drink in the cellar and hang up your washing in the attic鈥�
In the grand scheme of things, a 20 掳C temperature difference is not very big. If you want to extract mechanical energy from it, then the maximum possible efficiency is that given by a . Running between a hot reservoir of temperature of say 30 掳C (303 kelvin) and the cellar at say 10 掳C (283 K), then the fraction of the heat flow that can be extracted as useful work can be calculated by dividing 283 by 303 and subtracting the answer from 1. This gives 0.066 or, in this case, 6.6 per cent. This means that for every 100 joules of heat energy that flowed from attic to cellar, you would only be able to extract 6.6 joules as work. In reality it would be even less than this since this is the upper thermodynamic limit for a perfect machine.
This efficiency limit is why practical engines always use high-temperature sources of energy, such as coal combustion. Running between combustion flames at 800 掳C (1173 K) and cooling water at 30 掳C (303 K) gives a much greater theoretical efficiency limit of 74 per cent. This is also the reason why using underground hot rocks as an alternative source of energy is a challenge for engineers. The temperatures of the extracted fluids are often only in the range of a few hundred degrees C, so thermal efficiencies are low.
Simon Iveson, Faculty of Engineering and Built Environment, University of Newcastle, New South Wales, Australia
鈥� Thermocouples are a proven way of generating energy from a temperature difference between a heat sink and source; they have been used in spacecraft for this purpose and are both practicable and reasonably efficient.
In both cellar and loft, old central heating radiators 鈥� the bigger, the better 鈥� might be used to feed heat between thermocouples attached to them and their surroundings. Copper and aluminium are potential materials for the connections between loft and cellar. Ideally, several pairs of cables connecting the thermocouples in series would generate a useful voltage for charging batteries in cellphones, cameras or emergency lighting.
Chris Collins, Llandrindod Wells, Powys, UK
鈥� Brew beer in the loft, and store the product in the cellar. Job done.
David Everton Nottingham University, UK