A weekend in Northants
We spent the weekend with friends in Northamptonshire. The trip had a land, air and water theme. The land element was a walk in the woods through the Forestry Commission’s Salcey Forest. The walk was, frankly, rather boring, being on flat land with paths hemmed in by trees and consequently no views or variety. We revived ourselves with tea and cake in the café and then embarked on the air part of the visit, a stroll up a gently inclined walkway that eventually reached treetop height. This was more interesting and we were rewarded with views from the 20m high platform at the end. It was fascinating to be able to examine the tops of the trees close up. The conifer bark looked amazing.
Finally, after a pub lunch we visited Stoke Bruerne on the Grand Union Canal and had a stroll about, peering at the many canal boats and into the mouth of the Blisworth Tunnel before returning to the canal museum.
Stoke Bruerne was surprisingly touristy for a village in the middle of the country. Almost every building along the canal bank in the heart of the village is a pub, restaurant, café or shop. The canal clearly attracts a lot of business.
A free heat-powered fan
While I am still working out how to build a motor controller for my electric spinning machine, dear husband has decided that he’s going to make a Peltier-driven fan to sit on the woodburning stove I made out of a gas cylinder a few years ago. The stove (being without the internal baffles that commercial stoves have in order to harvest as much heat as possible from the combustion products) loses a lot of heat up the flue. The flue gets very hot as a consequence, and we’ve thought for a while that we ought to put a fan behind it to blow the warm air that collects at the back of the fireplace into the room. We even acquired a plastic cooling fan from an old PC someone was throwing away, with the intention of building a small Stirling engine to power it. There are quite a few designs for Stirling engine-driven stove fans on sites such as Instructables, and even some commercially available products that look quite good but are rather expensive. But since the stove cost next to nothing to build, buying a proprietary fan offends my thrifty principles, even if the running costs will be zero.
Why zero, do you ask? Well, whether the fan is powered using a Peltier device or by making use of the Stirling cycle, it will operate using “free” energy. Some of the waste heat that is there in abundance will be converted to kinetic energy, via electrical energy in the case of the Peltier solution. This is, therefore, the thermal equivalent of a hydraulic ram: a little energy is employed to get some more of the energy to where it is needed – warming us on cold winter evenings.
The two principles are described below.
The Peltier–Seebeck effect
Nearly 200 years ago a German called Seebeck discovered that, if you connect two different metals at two places and keep each of those junctions at a different temperature, there will be a potential difference (voltage) between them. This is how thermocouples work as temperature-measuring devices. A few years later, a Frenchman called Peltier demonstrated the opposite effect: if you apply a voltage across the two junctions, then their temperatures will differ and heat will be transferred from one to the other. Nowadays, whichever function is to be performed (exploiting a thermal difference to generate electricity, or using electricity to run a heat pump), the devices generally bear M Peltier’s name instead of Herr Seebeck’s and small ones can be bought very cheaply.
The Stirling cycle
This is a particular type of thermodynamic cycle, in other words (in simple terms), it turns work into heat or vice versa in a repeated, cyclical manner. It was invented by a Scot, Robert Stirling, at around the same time as Peltier and Seebeck were experiencing their own flashes of inspiration. Unlike most of the thermodynamic cycles we come across in everyday life (eg the diesel cycle in diesel-engined vehicles), Stirling engines are external combustion – the heat comes from an external source. In my day, mechanical engineering students were taught that Stirling engines were not of much practical use except for limited applications such as cryogenics. Perhaps as a result of increasing interest in waste heat recovery, they seem to be enjoying something of a resurgence. They are also popular with hobbyists and model engineers because, unlike an internal combustion engine, there is no need for valves and they are therefore relatively easy to build. One variant, known as the beta type Stirling engine, can even be made without the upper, tight-fitting piston by making use of a rubber diaphragm.
Dear husband has bought a very smart-looking piece of apparatus for the princely sum of £14.99 that is aimed at the overclocking community, i.e. gamers who want to run their PCs faster than the makers intended. It consists of a heat sink, a fan and copper heat pipes to transfer the heat, much like the one used in this Instructables project. Now I will have to make a Stirling engine fan so we can see which works best. We have a gas-fired stove in another room, as well as my home-made wood burner, which means that both fans will get used, if they work.