Solar Furnace: Build an Engine with Water & Sunlight

[Ahassan54321]

TL;DR Summary

What factors determine the efficiency of a solar steam engine.

Hello All,
This is my first post on this forum and I am here because I need help with an idea that I have.

I am thinking of a pressurized vessel with two unidirectional valves. One for water intake, and another for steam release.
The vessel will have a small sealed borosilicate glass “window” that will let in the concentrated sunlight. The vessel will be placed so that the focal point of a significantly larger convex lens is inside the vessel.

One cycle of this engine will be as follows;

1) A specific volume of Water enters through the intake valve, and then that valve is sealed.

2) when all the water is converted to steam under pressure and superheated , the release valve opens and the kinetic energy from the steam turns a Tesla turbine which then turns a generator.

3) the release valve then shuts off again and intake valve opens up to bring the next batch of water.

I am thinking that the pressurized vessel should be made of a thermoinsulator and should be small enough in comparison to the lens that the time it takes to convert the water inside the vessel to steam is shortened enough that the engine can reliably work.

Please let me know if your thoughts and criticisms.

 

[berkeman]

Welcome to the PF.

Have you studied any basic thermodymics yet? If so, can you say what you think the most efficient cycle probably is for converting heat energy to mechanical energy? If not, can you think of some reasons why cooling down the reaction chamber for each cycle (by introducing a lot of cooler water) may be an issue for efficiency?

Also, have you done some reading about the typical ways that heat energy is converted to steam and mechanical energy currently in power plant operations?

 

[DaveE]

The thermal shock of injecting cold water into a super heated boiler is likely to cause you serious reliability problems.
My intuition (translation: I'm too lazy to actually figure it out) tells me that you really want a more continuous process. Perhaps you can weigh the boiler and inject water at a rate that keeps the weight constant, or use some other measure of the steam leaving to control the input flow.
Other than the energy source, how is your problem different than a steam engine circa 1900? Perhaps you should study what they did.

 

[Rive]

The vessel will have a small sealed borosilicate glass “window”...

Pointless: any window is just a weak point. Just heat up the surface, as everybody does.
I seriously recommend to look up existing DIY projects: it will spare you a lot of dead ends.

Also, please be very-very careful around the mirror, always. Concentrated sunlight is extremely dangerous.

Ps.: what you are building is not a 'furnace' but a solar powered steam engine. The difference is, that for the furnace you need concentrated light to reach high temperature (700+ C^o: for steam generation you need only efficient heat transfer, what means more surface at lower temperature 200+C^o. Of course, 200 degree will just cook your skin all the same within seconds, but in construction, it makes things a lot easier. Try to look up 'black body absorption' and that how it is related to cavities.

 

[sandy stone]

Don't overlook the usual cautions about pressure vessels.

 

[Ahassan54321]

Welcome to the PF.

Have you studied any basic thermodymics yet? If so, can you say what you think the most efficient cycle probably is for converting heat energy to mechanical energy? If not, can you think of some reasons why cooling down the reaction chamber for each cycle (by introducing a lot of cooler water) may be an issue for efficiency?

Also, have you done some reading about the typical ways that heat energy is converted to steam and mechanical energy currently in power plant operations?

Thank you for the response! The highest physics I have taken is non calculus general physics. I have taken a chemistry based thermodynamics course as well. But sadly I have forgotten everything.. it was 5 years ago. I will try to ponder on the points you mentioned and do research.

 

[Ahassan54321]

The thermal shock of injecting cold water into a super heated boiler is likely to cause you serious reliability problems.
My intuition (translation: I'm too lazy to actually figure it out) tells me that you really want a more continuous process. Perhaps you can weigh the boiler and inject water at a rate that keeps the weight constant, or use some other measure of the steam leaving to control the input flow.
Other than the energy source, how is your problem different than a steam engine circa 1900? Perhaps you should study what they did.

The reason I went the route that I did is because of the high pressures. I want there to be no back flow. Unless there is a way to prevent back flow at high pressures in a continuous system...You can imagine the input of new cold water as the water stops that a steam locomotive made back in the day. Thoughts?

 

[Ahassan54321]

Pointless: any window is just a weak point. Just heat up the surface, as everybody does.
I seriously recommend to look up existing DIY projects: it will spare you a lot of dead ends.

Also, please be very-very careful around the mirror, always. Concentrated sunlight is extremely dangerous.

Ps.: what you are building is not a 'furnace' but a solar powered steam engine. The difference is, that for the furnace you need concentrated light to reach high temperature (700+ C^o: for steam generation you need only efficient heat transfer, what means more surface at lower temperature 200+C^o. Of course, 200 degree will just cook your skin all the same within seconds, but in construction, it makes things a lot easier. Try to look up 'black body absorption' and that how it is related to cavities.

The reason that I deviated from what everybody else does is precisely the black body concept. I want as much radiation to be absorbed as possible so the sphere with a glass window serves as the cavity. I am confident that devising a window that is safe at high pressures is just limited by the complexity of engineering. Moreover, focusing the light inside the vessel enables me to insulate most of the external surface area. And therefore increasing the temp difference and also efficiency.

Other than it’s potential to not be an easy model to build, does the window concept have any other downsides?

I will be sure to check out the DIYs and thanks for the heads up, I’ll be careful!

 

[Ahassan54321]

Welcome to the PF.

Have you studied any basic thermodymics yet? If so, can you say what you think the most efficient cycle probably is for converting heat energy to mechanical energy? If not, can you think of some reasons why cooling down the reaction chamber for each cycle (by introducing a lot of cooler water) may be an issue for efficiency?

Also, have you done some reading about the typical ways that heat energy is converted to steam and mechanical energy currently in power plant operations?

Maybe it seems that efficiency might be an issue because I used the terms “engine” and “cycles” making it seem as if these cycles are happening rapidly. Try to think of them as replenishment of the boiler after maybe 4-5 minutes of steam release. I could also collect the steam via condensation and then feed it along with more water to the boiler. I could also coil the intake pipe around the vessel.

 

[Rive]

Other than it’s potential to not be an easy model to build, does the window concept have any other downsides?

It just don't worth the effort. The usual solution is to fix pipes onto the inner surface of the cavity. That gives a pressure vessel in one go (pipes are just excellent when it is about pressure) so no need for any complicated window at all.
It also gives you more surface.

 

[Ahassan54321]

It just don't worth the effort. The usual solution is to fix pipes onto the inner surface of the cavity. That gives a pressure vessel in one go (pipes are just excellent when it is about pressure) so no need for any complicated window at all.
It also gives you more surface.

Interesting!

 

[sophiecentaur]

pipes are just excellent when it is about pressure

After some initial use of 'fire tube' boilers, pretty well all steam locos used (and use) 'steam tube' boilers. Much safer and effective.
Using a distributed area of boiler means that the optics needn't be as precise. You do not need to focus the Sun's image accurately and a second reflector, behind the boiler, will reflect any spillage back onto the boiler tubes.
But all this stuff is known art and the OP will find loads of examples. Searching with Google Images is a good way of spotting designs; much better than looking at the wording of web page titles.