Stirling work could prove a world beater

A potentially world-beating source of ‘green’ energy has been unveiled. Tom Shelley reports



Small Stirling engine generators, each mounted at the focus of a steerable reflector dish, could become economically viable as a major source of sustainable energy by using sodium vapour as a working fluid.
To this end, Dr Shaiq Haq, professor of mechatronic engineering at the Air University in Islamabad, Pakistan, has been conducting studies, using a former satellite dish antenna with a 3.5 square metre surface area, covered in coated, aluminium foil, and a small Stirling engine placed at its focus. With a solar insolation of 1,600W per square metre, Haq explains this high amount of energy input means “we can afford a less efficient engine”. He also reveals that, in an experiment placing a soft drinks can at the focus, this easily reached 700ºC.
The system uses a microcontroller and small motors to make the concentrator follow the calculated sun’s position during the day, after which it slowly returns to its initial position during the night. “We don’t use an active tracking system,” he states, “because it wastes a lot of energy.”
Small Stirling engines are readily available. For example, ‘Coffee Cup’ engines are available for $119 from the American Stirling Company (http://www.stirling.com) and for just £29.99 from http://www.101gear.com. However, while these may demonstrate the principles, they are essentially toys. On the other hand, high-technology Stirling engine coolers are used by the military in infrared imaging and heat-seeking systems, while Stirling engines are used in some Swedish torpedoes. But developing low-cost heat engines that could provide a useful amount of power for a household remains a challenge.
Since Stirling engines involve a piston moving back and forth, it makes sense to combine them with a spring and design them to work at mechanical resonance. It also makes sense to have them generate power from a linear alternator – a magnet moving back and forth through a coil – rather than add the cost, complexity and losses of energy incurred by converting the reciprocal to rotating motion.
But the big problem with Stirling engines always has been, and still is, the need to transfer heat in and out of a gas phase. The demonstration models available on the Internet all run on air and there are a number of development designs that run on gas mixtures based on hydrogen and helium. While abundant in outer space, this is a distinctly limited resource on earth, most of it being produced as a by-product of natural gas production from wells in Texas, Oklahoma and Kansas.
Following his soft drinks can experiment, Haq moved on to making small Stirling engines to take its place, typically producing 10 to 50W, running on air. He then came up with the idea of running them on sodium vapour.
Sodium as a heat transfer medium is a concept that comes from the nuclear industry, with liquid sodium used as a coolant in the first generation of fast breeder reactors, none of which, it seems, are now operating. Interest in using it as a heat transfer medium in the nuclear industry, however, continues, not only as liquid, but also as a combination of liquid and vapour. Sodium boils at 883ºC at atmospheric pressure, but produces significant amounts of vapour at somewhat lower temperatures. According to one report, using a combination of liquid and vapour allows an enthalpy gain for heat transfer purposes of 4933kJ/kg – about three and a half times what is possible with liquid sodium. While explosively reactive with water, sodium vapour is routinely used in low- and high-pressure sodium vapour motorway and street lights all over the world. It can be contained by nothing more highly technological than borosilicate glass, the same material used in much of the world’s domestic ovenware.
The other heat transfer medium that might have potential for low-cost power generating engines placed in solar concentrators is ammonia and water. This is the brainchild of Russian engineer Alexandr Kalina, now living in the US. Large-scale experiments and demonstrations show that it is a much better fluid combination to work with than water and steam, still the most favoured fluid for converting external heat energy into mechanical motion and electrical energy.
Eventually, Haq foresees engines at the focus of his concentrator, producing just over 1kW - which requires them to be only 20% efficient. This could be more than enough to power lighting in a house based on LEDs and high-efficiency lamps - plus a television, plus a cooler – if not an energy-hungry refrigeration cycle air conditioner. While high levels of sunlight do not occur in the UK, there is an awful lot of waste heat, which many engineers are looking to Stirling engines as a useful way of utilising.

Stirling engine cycle
Stage 1. Isothermal expansion using external heating
Stage 2. Isovolumetric heat removal. The working gas passes through a regenerator, heating the regenerator and cooling the gas
Stage 3. Isothermal compression. The compression space is usually intercooled
Stage 4. Isovolumetric heat addition. The compressed gas flows back through the regenerator and picks up heat

Pointers
* Stirling engines are already demonstrable as cheap devices that can run on very low- grade heat sources
* Much development work has been undertaken on such systems in the US, particularly by Sandia Labs and Stirling Energy Systems, as in the system shown in our image, but until now, their high capital cost has prevented their wider use.
* In the sunny climate, of Pakistan, using much cruder and less expensive construction, a low-cost solar concentrator has been devised that produces 700ºC at its focus
* While Stirling engines have been shown to be a good way of extracting energy from such a source, the effort now is to improve their efficiencies, particularly by trying novel working fluids. Sodium vapour looks promising, but there are also other fluid mixtures to investigate, such as ammonia and water

Email: shaiq@mail.au.edu.pk

Author
Tom Shelley

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Do you have any comments about this article?
I can see that stirling engine can be very very attractive considering high cost of oil not to mention the green house effect it produces that we really must re-consider. I live in the Philippines where we get a lot of sunshine almost all year round. My question is that, is it really not yet possible to mass produce stirling engine 1 square meter in size or about 1,600 watt power input can be practical to use? or really how much can it cost.

Comment John Mendoza, 05/06/2010
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