Could it be possible to create a pentane steam engine? I'm questioning if there's a way to make use of the principle of steam engines again.
The idea of a  steam engine is a little primitive and outdated maybe, but the principle isn't that at all
if you ask me.
In the 19th century, they used water for steam engines and I assume this fluid was chosen because of the wide availability and low costs.
The cons of using water is that you have to burn fuel like coal for heat because of the relatively high boiling point of water.
Back in those days, those fuels were still widely available, but obviously those times have changed.
The thing is, if you use a fluid with a much lower boiling point you'd have to add only a little bit of energy onto it to make it boil.
So to obtain the best efficiency, you'd take a fluid which has a boiling point near the environment temperature.
For example, pentane boils at 309K / 36C. (I don't know which substance is more suitable)
I don't know how much energy you gain when this substance boils, but if you have a closed circuit for the fluid and a radiator, you could in theory build an engine which runs indefinitely.
Because the energy you gain from the boiling substance is big enough to generate the little heat which is needed to make it boil.
Maybe this is a stupid question, and i suppose this has been considerated a long time ago.
But i'm just curious about it. And then again, the simple or weird ideas are the ones who actually work. :p
Thanks for any response.
 A: Pentane is used as a medium in low-enthalpy heat engines. 
See, for example, David Banks. Introduction to thermogeology,
The efficiency of such engines is quite low, as it has an upper limit of the Carnot efficiency at the operating temperatures. Nevertheless, if you have a temperature difference between two regions that you can exploit without too much expense, it can be worthwhile.
A: I have given the exact same question some thought recently. To make progress I worked through what would happen with methanol instead of pentane. What I did was work through what would happen after condensing, and the work that could be done by the resulting vacuum, ignoring vapour pressure. I worked out you should get about 5% efficiency.
Remember this is thermal efficiency from what can be a near limitless supply of low grade heat. Condensing colder and using higher pressure may mean butane can be used. Turbines also need a makeover, would like to see them on "air" bearings. Any oil would end up contaminating things. With computer simulations now should be able to 3D print something good 1st time.
As a retired person I can add that my belief is that Carnot efficiency is an oversimplification. Basically if you want to get anything working you will have to fund it yourself, because no business would sensibly fund work against an established theory.
Beyond the boiling point there is the critical point or something, where the pressure is such that gasification is instant from liquid with no added energy when the pressure is reduced. Using butane may be better for this. Physicists in refrigeration -heat pump people- should know exactly what to do to get things working and keep them safe.
The development can mirror the development of steam engines in the industrial revolution. To begin with they used pistons and air pressure and the efficiency was about 4%. Then they used high pressure steam and turbines and they got closer to 50%.
A: this topic has been in my mind for a long time. The mindset is always to use a specific part of the energy to convert for e.g electricity.
Think about your homes in colder regions. In Germany we have good grade fireplaces with 80% efficiency. But 20% goes to the chimney. The heat (tested many times with Thermo cams is around 150-200 °Celsius. so working backwards the airvolumes which the fireplaces usually need for combustion and hence convection drafts in the chimney, this 20-30 % losses matches to the energy given off by the firewoord. Lets call a 10kw fire (e.g 2.5KG wood) burns for an hour. 80% is given to your room or eventually partially to the water jacket), the rest 2 KW escapes to the chimney (as this is your actor to draw in new air for combustion/oxygen replenish.
Now you could take a little fan and divert the offgas into a thermal heat exchanger with heat resistant fluid (thermal oil, engine oil or whatever). Next step you could use PENTANE (boiling point 50°C and heat up to 150-200°C). Now use the pressure and expansion to drive a little turbine (screw/scroll/piston) or whatever suits for the convenient size and take it out as electricity. Upon partial condensation, use an economizer to cool down to create the heat sink required to liquify the PENTANE back to under 50°. The rest energy use it to heat water for your central heating or shower/bath water. In case this is already higher in temperature in a buffer central tank, then use it for a radiator to heat your rooms. then possibly coming out with 25-30°C rest heat. Again use this rest energy in another tank + heat exchanger to preheat the incoming council water (in winter this is in between 6-10°C). Like this the efficiency is a tremendous upgrade. And not even high tech. Not sure why nobody is using it. Possibly have to create this myself in the garage. Only scary part is the Pentane maybe escaping some seals in the turbines and hence the risk of fire eventually. But all this could be engineered out. Greets from Germany, Bernd
