A second application of Pressure Controlled Heat Pipes (PCHPs) is their ability to switch power between different reactors at high temperatures for In-Situ Resource Utilization (ISRU). The lunar soil consists of approximately 43% oxygen that is contained within the oxides of the lunar soil. Oxygen is produced by heating the regolith to 1050°C, then flowing hydrogen through the regolith. The hydrogen reacts with the oxygen in the lunar regolith to produce water. The water is then electrolyzed to produce oxygen, and the hydrogen recycled back into the process. The mass of the overall system can be minimized if one solar concentrator supplies a constant rate of power to two reactors, with the power switched from one reactor to the other as fresh batches of regolith are added. (Pressure Controlled Heat Pipe Solar Receiver for Regolith Oxygen Production with Multiple Reactors)
For this application, constant power at 1050°C needs to be directed into the two reactors. At certain times during the cycle, excess energy needs to be dissipated. For long term reliability, the power switching mechanism should operate at much lower temperatures. Two pressure controlled heat pipes can achieve this, as shown in Figure 1. Solar energy enters into a central cavity, surrounded by two PHCPs. A constant conductance heat pipe transfers heat from each pressure controlled heat pipe to the associated reactor. Each PCHP has a linear actuator and bellows, which are used to control the Non-Condensable Gas (NCG) front location, and therefore control the power delivered to each reactor. Moving the bellows in reduces the reservoir volume, and moves the NCG gas front lower in the SRHP, reducing the power on supplied to the associated reactor. Moving the bellows out increases the condenser area, supplying more heat to the associated reactor. When the bellows are moved all of the way out, the front moves into the secondary condenser, so that excess heat can be radiated away. The NCG protects the bellows and driver mechanism, since their operating temperature is around 300°C, versus the 1050°C operating temperature for the remainder of the PCHP.
The projected operational scenario for the start up and operation of a multiple reactor system is as follows; see Figure 2:
Step 1: Assume that the SRHP is at 1050 C, and the gas front extends below the CCHP evaporators on both the Left Hand Side (LHS) and the Right Hand Side (RHS) reactors, minimizing the heat transfer to the reactors. At this point, fresh, cold (600°C) regolith is added to the RHS reactor; see Figure 2(a). Without the NCG front, the temperature of the entire system would drop, as heat was rapidly transferred into the cold regolith. By moving the NCG front below the CCHP, the power can be regulated, keeping the SRHP at 1050°C. In the figure, we have assumed that 3 kW is available to heat up the cold regolith.
Step 2: The piston is withdrawn, and the NCG front moves up the CCHP evaporator; Figure 2(b). As the regolith warms up, the driving ΔT between the SRHP and the regolith gradually decreases. By moving the NCG front, the entire 3 kW of heat can still be transferred into the reactor.
Step 3: At some point during the warm-up, the driving ΔT between the SRHP and the regolith becomes so low that the regolith can no longer receive the 3 kW of power available. As shown in Figure 2(c), the NCG gas front starts to expose the secondary condenser, allowing the excess heat to radiate away. Using the secondary condenser to reject waste heat eliminates the requirement to defocus the solar concentrator. Once the RHS reactor is at the operating temperature, only 1 kW of power is required to maintain the temperature.
Step 4: At this point, hydrogen flows through the RHS reactor to produce oxygen. Only 1 kW is required to maintain the RHS, and 3 kW can be used to heat up the LHS reactor. As shown in Figure 2(d), 3 kW is supplied to the LHS. Initially, the entire 3 kW enters the reactor. As the regolith temperature increases, excess heat is radiated from the secondary condenser.
When the LHS reactor is at temperature, the RHS regolith is spent. The LHS reactor is used to produce oxygen. At the same time, fresh regolith is added to the RHS reactor, and the process repeats.
The system setup is shown in Figure 3, before the entire system was insulated. The bellows are each driven by a linear stepper motor. During testing, the total power was held constant at approximately 3.5kW. This power represents the constant power the system would receive from a solar concentrator during actual operation.
The test results are shown in Figure 4 and Figure 5. In Figure 4, four temperatures are shown on the primary y-axis for each the left hand side (LHS) and right hand side (RHS) reactors.
- PCHP vapor temperature (TC1 & TC26)
- Vapor temperature at the entrance of the secondary condenser, TC13 & TC38
- Vapor temperature in the CCHP condenser, TC19 & TC50 (indicates when NCG front enters the secondary condenser)
- Average “regolith” temperature
Figure 5 shows an animation of the experimental data. The non-condensable gas position is controlled with the bellows/stepper motors so that the system can accept a constant power, and direct it to the LHS and RHS, as required.
Figure 5: Animation showing how heat is Initial start-up of the two pressure controlled heat pipes. The NCG is shown in yellow.