In stirling engines with high power to mass ratios, all the heater heads I've seen use many round(or oval) tubes to transmit the thermal energy from the heat source to the working gas. As the helium(or other gas) passes through these tubes, it acquires thermal energy. The purpose of these tubes is to increase the surface area to volume ratio. Why isn't the following done instead?
Make larger rectangular(or circular) tubes and use grid-like metal cross sections(or wire mesh) to increase the surface area. The outside rectangular or circular tube needs to be strong in order to withstand the high pressure of the gas flowing through it, but the inner grid-like metal cross sections(or wire mesh) do not need to be as strong since the pressure on all sides is uniform. Furthermore, the inner grid-like metal cross sections would transmit heat from the outer surface to the interior more quickly than the gas due to the metal's higher thermal conductivity. The goal of this scheme is to increase the speed at which heat is transmitted to the working gas while minimizing the material usage of the heater head but not sacrificing engine efficiency.
More generally, what methods can be used to increase the speed at which thermal energy is transmitted from the heat source to the helium working gas?
Increasing Thermal Conductivity
Re: Increasing Thermal Conductivity
The idea of the small tubes is that the gas gets heated to the core instantly, also its a simple system, the material is already there waiting to be used. With your tube, can you make it? And make it cheaply, the main problem quoted by the "experts" from other branches of engineering, ie., the ones that build internal combustion engines, is that the Stirling Engine is too costly to build, but thats only because they are not being built by the million as the petrol, and diesil engines are. Ian S C
Re: Increasing Thermal Conductivity
Thanks for the reply. One of my goals is to compare material usage per kilowatt in a sodium reflux receiver/stirling engine system based on Na/He heat transfer to the material usage in a steam boiler/steam turbine system based on H2O.
For the same efficiency, large steam turbines are about three to four times lighter than large stirling engines for the same generating capacity. However, a boiler/steam turbine system such as the one used by Bright Source Energy has a fatal flaw. The boiler, which is essentially a pressure vessel, has to hold the high pressure steam until it heats to an adequate temperature. Pressure vessels are expensive and their mass scales as a linear function of its volume according to Wikipedia. Furthermore, the thermal conductivity of steam is one tenth of the thermal conductivity of helium. This limits the flow rate of steam through Bright Source Energy's boiler. As a result, Bright Source Energy's boiler requires a tremendous amount of material usage. In comparison, the sodium reflux receiver requires far less material usage. My belief is that the material usage of a sodium reflux receiver/stirling engine system is probably half the material usage of a steam boiler/steam turbine system due to the tremendous mass of the steam boiler and the comparatively light weight of a sodium reflux receiver. The key element of this argument is that the thermal conductivity of helium is ten times greater than the thermal conductivity of steam. However, there was a claim that the steam boilers in power tower steam generation systems utilize some type of strange metal mesh/grid inside their boiler in order to speed up the transmission of heat to the steam inside the boiler. In some sense, this compensates for the low thermal conductivity of steam.
There are three possibilities.
1.)The effect of the metal mesh/grid is not that significant or is only significant because the boiler chamber is absolutely huge. In this case, sodium reflux receivers/stirling engines would still be much lighter than steam boiler/steam turbine systems per kilowatt of generating capacity.
2.)The effect of the metal mesh/grid is significant and the metal mesh/grid system could also be used in stirling heater heads. In this case, sodium reflux receivers/stirling engines would still be much lighter than steam boiler/steam turbine systems per kilowatt of generating capacity.
3.)The effect of the metal mesh/grid is significant and the metal mesh/grid system cannot be used in stirling heater heads. Only in this situation would my argument that sodium reflux receivers/stirling engines weigh less per kilowatt of generating capcity not hold.
Any thoughts on the significant issues?
For the same efficiency, large steam turbines are about three to four times lighter than large stirling engines for the same generating capacity. However, a boiler/steam turbine system such as the one used by Bright Source Energy has a fatal flaw. The boiler, which is essentially a pressure vessel, has to hold the high pressure steam until it heats to an adequate temperature. Pressure vessels are expensive and their mass scales as a linear function of its volume according to Wikipedia. Furthermore, the thermal conductivity of steam is one tenth of the thermal conductivity of helium. This limits the flow rate of steam through Bright Source Energy's boiler. As a result, Bright Source Energy's boiler requires a tremendous amount of material usage. In comparison, the sodium reflux receiver requires far less material usage. My belief is that the material usage of a sodium reflux receiver/stirling engine system is probably half the material usage of a steam boiler/steam turbine system due to the tremendous mass of the steam boiler and the comparatively light weight of a sodium reflux receiver. The key element of this argument is that the thermal conductivity of helium is ten times greater than the thermal conductivity of steam. However, there was a claim that the steam boilers in power tower steam generation systems utilize some type of strange metal mesh/grid inside their boiler in order to speed up the transmission of heat to the steam inside the boiler. In some sense, this compensates for the low thermal conductivity of steam.
There are three possibilities.
1.)The effect of the metal mesh/grid is not that significant or is only significant because the boiler chamber is absolutely huge. In this case, sodium reflux receivers/stirling engines would still be much lighter than steam boiler/steam turbine systems per kilowatt of generating capacity.
2.)The effect of the metal mesh/grid is significant and the metal mesh/grid system could also be used in stirling heater heads. In this case, sodium reflux receivers/stirling engines would still be much lighter than steam boiler/steam turbine systems per kilowatt of generating capacity.
3.)The effect of the metal mesh/grid is significant and the metal mesh/grid system cannot be used in stirling heater heads. Only in this situation would my argument that sodium reflux receivers/stirling engines weigh less per kilowatt of generating capcity not hold.
Any thoughts on the significant issues?
Re: Increasing Thermal Conductivity
I'm afraid that I tend to discount steam as a power source, unless you are generating Meg watts, either diesel engines, or gas turbine would be what I would think of as competition for Stirling Engines, as one would proberbly be limited to say 500 KW.
The tube hot end could consist of closed circuit tubes with Sodium inside circulating by thermosyphon, its liquid when heated. Its used in the exhaust valves of aircraft engines to conduct the heat away from the valve head. The engine I know is the P&W R-1830 14 cylinder radial engineas used in such aircraft as the DC-3. This technology was developed in the 1930s as the power of engines increased, and the problem of burnt exhaust valves became critical.
With the above system the gas would not circulate through the tubes but stay in the displacer cylinder. I would consider that the tryed system of fine tubes, with the gas passing through would be the most efficient. Ian S C
The tube hot end could consist of closed circuit tubes with Sodium inside circulating by thermosyphon, its liquid when heated. Its used in the exhaust valves of aircraft engines to conduct the heat away from the valve head. The engine I know is the P&W R-1830 14 cylinder radial engineas used in such aircraft as the DC-3. This technology was developed in the 1930s as the power of engines increased, and the problem of burnt exhaust valves became critical.
With the above system the gas would not circulate through the tubes but stay in the displacer cylinder. I would consider that the tryed system of fine tubes, with the gas passing through would be the most efficient. Ian S C
Re: Increasing Thermal Conductivity
Bright Source Energy does produce Megawatts. Their turbines are over 100 megawatts in capacity. These are gigantic plants. However, my belief is that they don't achieve anything with such a large scale. Their material usage per kilowatt is still greater than reflux receivers/stirling engines. I'm curious whether someone has a counterpoint to the argument I made.