I'll be making a "micro thermal power station" in the single-digit watt range at home (one-off, educational project. Bear with me). A heat source, likely ethanol, will cause a stirling engine to turn a DC motor, used as a generator. The topic of the educational part is to focus on efficiency. To this end I want to see where the energy is wasted on each stage. Measuring the electrical part (after the generator) is easy. The problem is the previous stages (heat→motion and motion→electrical). Also measuring the end-to-end efficiency is not hard (X ml of 95% ethanol is Y joules and so forth). The biggest unknown is the heat→motion part. I tried asking the stirling engine manufacturers and they haven't measured it. But if I figure out the generator part, then I can infer the heat→motion part.
Efficiency of DC motors when used as generators
I have several DC motors laying around at home, some DC (e.g. Mitsumi M36N-2), some BLDC (selected inrunners with Kv about 250). From what I've seen in datasheets like the Mitsumi's it is expected that the motors have the highest efficiency (when used as motors) when they are lightly loaded (around 10% of the stall torque). Is it reasonable to assume that the efficiency peak when used as generators will be in the same conditions?
Measuring efficiency with homebrew methods
Most of the motors I can use don't have datasheets, so their efficiency is unknown. I don't have a dynamometer, otherwise this part would've been easy. I came up with a method to measure the efficiency:
- attach a spool to the shaft of the motor;
- the spool has a known proof mass attached to the end of the thread;
- the motor is loaded with known resistance (if it's the BLDC, then three equal resistors);
- the whole setup is placed high near the ceiling;
- as the mass is released, it unspools the thread, rotating the motor. Record the output voltage with a DSO;
- integrate the power over time, and compare to the potential energy that the proof mass had before releasing;
- changing the mass or load can simulate different load conditions.
So I have something, but seems too cumbersome, and it also introduces other unknowns. I'm wondering about other approaches to this measurement problem. I may be missing something simpler. Any ideas?
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