Testing Sorbent Heaters IN Epiphyte: Results
This is a followup to the previous testing with the heater prototype. In this case, I used the actual heating system that has been built into the Epiphyte Sorbent Panel. This system consists of a pair of Nichrome heating wires, each with a total length of about 65 inches, zig-zaged across each outer surface of the sorbent.
One of the wires can be seen in this picture, taken before the sorbent was poured into the well on top of the wire.
Thermocouples were attached to the heater wires at several locations, and two more were embedded in the middle of the sorbent. Although ideally the embedded thermocouples were intended to be located right in the center of the sorbent, these sensors in practice had to be left to float in the sorbent, so there’s no guarantee they were still dead center after the panel was installed in the system and the testing had begun.
The entire sorbent panel (2 ft. square, with an 8-inch square sorbent container in the middle) was installed in the Epiphyte plenum.
Since we don’t have a proper hole drilled in the faceplate yet, I had to route the wires through the gap between the faceplate and the plenum opening, so it didn’t close completely. Since we are not testing CO2 capture yet, that doesn’t really matter.
Test Procedure
The heating system is under the control of the processor, with a PC as the user interface. There is also a small optical panel for moment-by-moment monitoring of the temperatures, but all the data was captured by the PC and stored in a file for plotting and analysis.
Three tests were done: (I) Only Heater 1 activated; (II) Only Heater 2 activated; (III) Both heaters activated. In each case, the heaters were driven with a PWM control waveform (see post above for more information) at 1kHz and 80% duty cycle. During each test, thermocouples on each wire and in the middle of the sorbent were read at 5-second intervals.
After the wire temperature reached 150C, power to the heater(s) was shut off and the system was allowed to cool, with or without the fan.
Results
For Case I, Heater 1 was active. After the heater was turned off, the system was allowed to cool for a few minutes, then the fan was turned on at speed level 3 to finish cooling faster. Results are plotted here:
Some interesting observations are immediately apparent. Compared to the earlier testing with the prototype heater operating in open air, the heaters in the system took much longer to heat up. This is likely due to absorption of the heat by the sorbent, combined with a (parasitic) absorption by the metal mesh, grid, and frame of the panel. This latter effect is important to keep in mind when designing the next system, as it affects the efficiency.
The other observation is that the sorbent heats up much more slowly than the wire. This is evidently due to a combination of the material’s heat capacity and relatively low heat conduction. In order for the sorbent to reach and maintain a high-enough temperature for desorption, the heater would have to be on for longer, but a more sophisticated control system is required to allow the wire temperature to reach a maximum safe temperature and then stay there while the sorbent heats.
Case II is shown here, with only Heater 2 on; in this and the following case, the fan was turned on as soon as the heater was turned off, which is why the time scale of this graph is shorter than the first:
This result is qualitatively different from Case I. Although the active wire behaves similarly, the sorbent heats much more slowly, and in fact its temperature stays very close to that of the inactive heater wire. My guess is that the sorbent thermocouple has shifted so that it is much closer to heater wire 1 than to wire 2; but the only way to verify this would be to painfully disassemble the whole panel and rebuild it.
In Case III, both heaters are on:
It can be observed that the sorbent heats somewhat faster than in Case I but not as fast as might be expected for double the heating power; this appears to be more evidence that the sensor is not ideally located.
Next Steps
The next step is to modify the control software to heat the sorbent to its desorption range (100-120C) in a controlled and efficient manner without exceeding a safe temperature anywhere. This will likely entail a software PID controller; and the different time scales for the heater wire compared to the sorbent would indicate the need for a cascade control design.