The gas turbine
The main aspects to consider are:
Similarities:
Reactor construction, size and shape, cooling, internal power control, both gas turbines use a recuperator, industrial heat exchanger can be applied, both systems can use a free power turbine, the same nuclear reactor control by the negative temperature coefficient, both fit into a space of 10 x 10 x 10 meters, both can have a one spool or two spool turbine with a free power turbine (the PBMR Co (SA) project uses a two spool helium turbine with a free power turbine) and for logistic support both are suitable for a pool-management system.
Helium gas turbines directly coupled to an inherently safe and well-proven nuclear heat source.
Differences
| Differences | Closed cycle | Open cycle |
|---|---|---|
| 1. Gas turbine | helium | air |
| 2. Gas turbine | new design | adapted existing gas turbine |
| 3. Generator | helium cooled | air cooled |
| 4. Heat dump in cycle | yes | open air |
| 5. Intermediate Heat Exchanger (IHX) | no | yes |
| 6. Control of the installation | mass flow | mass flow |
| 7. Efficiency calculated by | 30 °C | 15 °C |
| 8. Efficiency | higher | lower due to pressure losses |
| 9. Efficiency | at 15 °C comparable | |
| 10. Pressure | losses comparable | |
| 11. Magnetic bearings | compulsory | advisable |
| 12. Amount of helium | more | less |
| 13. Number of rotating seals | none | at least one (helium ventilator) |
| 14. Inlet air | only for reactor | cooling for GT and reactor cooling |
| 15. Funnel arrangements | reactor cooling | reactor cooling and GT outlet,but no gaseous emissions |
| 16. Starting | using generator | using existing starting system |
| 17. Response to power changes | comparable | |
| 18. Weight total installation | comparable | |
| 19. Volume total installation | comparable | |
| 20. Vibrations | comparable | |
- Ad 1 and 2) As said the open cycle could be more easily realised, using existing but adapted gas turbines, such as turbo expanders, using the results of studies for gas turbines with an external heat source (see miscellaneous E) GT-2004-53336).
- Ad 3) Air-cooled generators are available.
- Ad 4) no comment
- Ad 5) Intermediate heat exchanger. The existing recuperator designs must be studied for a suitable design. The possibility of radio-active dust entering the helium circuit and through a leak of one or more pebbles in the gas flow of the gas turbine is always mentioned, however the helium ventilator which was used in Jülich, never showed radio-active contamination.
- Ad. 6) The demanded electricity on the generator must be transmitted to the control of the electromotor driving the compressor. This will result in a reduction of the air mass flow, but as has been explained, not in a change in temperature. This, due to the negative temperature coefficient, will stay at 800 °C. So the gas turbine will slow down.
- Ad 7, 8 and 9) There is always a lot of attention paid to the efficiency of an energy conversion unit. In this case the efficiency of the open air cycle gas turbine will be reduced due to the increase of the resistance in the system caused by the IHX. Any efficiency calculations considering gas turbines depend heavily on the inlet temperature of the compressor. In the case of the closed-cycle system this is dictated by the temperature of the air or water used in the heat dump. Most calculations use 30 °C, being the temperature of the sea and rivers in the tropics. For an open-cycle, the ISO norm applies for the inlet conditions, being 15 °C and 1013 mbar pressure.
- Ad 10) It is obvious we have to study and to work out the effects of adding an IHX and taking away the heat dump and the cooling of the generator.
- Ad 11) Magnetic bearings are a well-proven technology nowadays. The usage is compulsory in the case of a helium closed cycle system, because of the effects of lubrication oil entering the helium system and so the nuclear fuel.
- For an open air cycle this seems unnecessary, although to reduce maintenance it is still advisable.
- Ad 12) Helium is an expensive gas. It is also not common knowledge that, due to the importance of this gas for Zeppelins for military applications, there are still restrictions on the procurement.
- Ad 13) Helium is a one atomic gas, so it is difficult to seal. This especially applies to rotating seals. As has been explained in the case of a closed-cycle helium system, rotating seals to the open air can be prevented. In the case of the indirect cycle there is at least one rotating seal, the seal between the electromotor of the helium ventilator and the actual ventilator.
- Ad 14) In the case of the closed-cycle helium HTR-GT the reactor cooling is driven by natural draft. In case of the open air cycle the gas turbine needs filtrated air as transport medium to bring the reactor energy from the IHX to the turbines.
- Ad 15) In addition to the reactor outlet cooling construction, the gas turbine will have a funnel construction as well.
- Ad 17) This seems logical in view of the extra weight of the Intermediate Heat Exchanger.
Conclusions
In view of the predicted doubling of the world population, the doubling of the energy consumption per capita and the further increase in the shortage of fresh water in the next 50 years, the search for more environmentally friendly and controllable energy conversion systems is a necessity.
“Burning” uranium is the next logical step in combustion.
- “Burning” uranium in an inherently safe way and, indirectly in an open-cycle adapted gas turbine, is possible.
- Timely innovation is a necessity for a company or industrial branches to survive.
- All technologies required are available. However study and engineering by all parties involved are needed to find the optimal solutions.
The next logical step for small-scale inherently safe nuclear energy conversion can be taken!