Thermal wheels

[Helioechidna]

Hello all, firstly, thanks to Stefan for the invite to the site and the message on my you-tube posting.
The concept of fluid transfer heat engines is an alluring one, despite the scientific publications suggesting poor efficiency, there's an inate simplicity in the design that tends to suggest otherwise.
And that's what has kept my research going with the wheels.

As far as thermal efficiency goes my last test engine with 12 spheres has managed around 30-40% the Carnot limit.
This has been found by measuring the heat flux into and out of the bath under different conditions while it is under load.  I'm convinced that there is room for improvement on that figure. It is also a huge improvement on the published findings and I'll explain more about that soon.

The first thoughts most have is that a Stirling is probably more efficient, it probably is, but Stirlings need to be built and maintained by Swiss watch makers.  Stirling engines haven't been as widely adopted for exploiting low heat sources because of that reason. A wheel has no mechanical parts to wear out and can be left in field service theoretically for decades.

How I have calculated effieciency is from the standpoint of how much energy enters the wheel and how much work is performed as a consequence. If you are measuring how much energy enters the bath and compare the work done then the figure is significantly lower. I consider the bath as a separate system to the wheel, and view it as only one way of delivering energy to the wheel so despite the fact that a bath radiates and convects away 100's  of times more energy than the wheel absorbs for work, the closed Rankine cycle within the structure is the efficient part worthy of consideration at this point.

This is a perfect time to mention the solar option. Solar is inefficient because of the rate of heat tranfer into the vessel is almost matched by the rate of heat transfer out of the vessel by the heating of the thin layer of air on the vessel's surface. If the reverse side of the vessel is uninsulated then it becomes a greater radiator than the heated side and the gas inside your tank convects around from the warm side to the cold side blowing off energy almost as fast as you can absorb it.

That shouldn't discourage further research, in fact the rate of energy transfer is so finely tuned that it suggests there could be better ways of exploiting small heat differences with Rankine cycles.
But for simplicity of construction and the prospect of reliable service for decades, the wheel has some advantages over other techologies. I'd like to envisage a return to a Byzantinesque era of waterwheel powered civilization, except solar thermal powered wheels of all dimensions running silently and effieciently around us. Warm steam wafting from the wheel's surface into green houses and living areas, in warmer areas we'd live in the evaporativly cooled side of the wheel. Power stored as compressed air underground, the only by-products of power generation, cooled air and water from the compressors.

Just briefly, I'll mention a bit about achieving the measured efficiency, I want to publish the results so I can't give too much away. There are some basic design parameters to follow if you want to achieve a halfway reasonable efficiency with your wheel. Aim for higher rate of revolution rather than slower. Sounds obvious but it means that you can't use things like propane or R12. You end up wasting all your energy compressing the gas at the top of the cycle and the wheel becomes slow and slow enough that the mass of the tanks and the fluids cool off significantly, this wastes enourmous amounts of heat when having to heat it up again. Ideally you want the fluid to remain as thermally stable as possible and this equillibrium point be at the amount of heat required to just overcome the height of the fluid transfer tube. Because the speed needs to be as fast as possible the amount of mass being transfered must be small, so smaller tank volumes. The fluid too must undergo phase change sharply over this equillibrium point, and that's why we rule out low boiling solvents. So if we choose pentane/hexane (petroleum spirit) and aim for an equillibrium point of say 50 C (which is the knee of the PV curve for pentane/hexane), the height of the wheel is then determined by the bath temperature and ambient conditions.
The pressure difference due to height must be kept low in proportion to the pressure difference created by difference in temperature.  There are more improvements to be made but it you understand some of these simple design parameters you'll get better performance from the wheel than has previously been published.

[hansvonlieven]

G'day helioechidna,

Being somewhat familiar with the subject I must confess not having looked at developments in this area in recent years. Would it be too presumptious to ask for a few diagrams and some design parameters that will allow us to discuss the subject more fully.

I do not see efficiency as a real stumbling block as long as some useful energy can be extracted from these systems. The rest will follow once a few people start experimenting with these things.

Hans von Lieven

[Helioechidna]

Hi, here's what Stefan first posted a reference to:

http://www.youtube.com/watch?v=-fUlKBH1sY8

This is a model that I made in glass just to prove the point that you can get electricity from a thermal wheel.
The attachment shows the model used for thermodynamic measurements that I mentioned in the earlier post.

The basic principle is a wheel made up of opposing tanks that contain a fluid that evaporates then condeses over the temperature difference created in the bath below it and the air temperature at the uppermost position of the wheel.
Pressure is created from the evaporating fluid and pushes the remainder of the mas of liquid up the tranfer line to the top of the wheel which gives you an amount of potential energy = m*g*h. Mass of the liquid, g acceleration due to gravity and the the height the mass has gained. In this case the diameter of the wheel.

The Minto group is where we have been discussing and sharing files and will bring you or anyone up to speed about what has been developed so far.
http://tech.groups.yahoo.com/group/MintoWheel/

Just for the record I don't like using the term 'Minto' wheel as Wally didn't invent the device or make it work properly.

The design priciples that I've set out should get you a device that will at least perform decent work with temperatures  gained by solar means.
Let me know what you think?

P.S. Hans, where do you live? I'm in melbourne.

[hansvonlieven]

G'day Helioechidna,

Thanks for the picture, lovely job. I shall have a look at your group when I have a little more time than today.

I live in Sydney.

Greetings

Hans von Lieven

[Gearhead]

You can increase the efficiency by using an expanding and contracting gas to transfer liquid rather than heating all of the liquid.  Use an elastic barrier to prevent so much heat transfer. 

I have thought of using CO2 under several bar of pressure and bladders such as used in an accumulator.

It should be safe enough if not kept in an enclosed space.