If power is approx linear with rpm then the energy excess is modest  . 10 mW x 20,000/1000 = 200 mW. || If you MUST limit energy, and it's not obvious why you need to, a voltage regulator will let the alternator produce voltage BUT only dissipate Valt x Iload. In most alternators Voc_RPM_max is several times V_rpm_useful_min BUT not eg 20 x as high. eg an alternator may make 5V at desired load but 20V at full RPM. So using a voltage regulator and only drawing what you need till only increase power by 4 x(in this example) Pl= Il x 5V. Palt = 20V x Il.

This is an interesting question and probably has a good solution available once you supply all the information that is relevant that you know but that, so far, we don't.

SO - this is a brief interim answer based on what you have told us. If you add more detail and explanation the answer can be improved. Otherwise, this is about as good an answer as you can reasonably expect with the level of detail provided. (Others MAY give you an even better answer but you have no right at this stage to expect it :-) ).

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If power available is approx linear with rpm then the energy excess is modest.
10 mW x 20,000/1000 = 200 mW.
In most situations 200 mW wiould not be hard to dissipate with modest temperature rise.

Also, the alternator need not dissipate all the power that it CAN generate. Just because it CAN produce 200 mW does not mean that it MUST.

If you MUST limit energy, and it's not obvious why you need to, a voltage regulator will let the alternator produce voltage BUT only dissipate Valt x Iload. In most alternators Voc_RPM_max is several times V_rpm_useful_min
BUT not eg 20 x as high.
eg an alternator may make 5V at desired load but 20V at full RPM.
So using a voltage regulator and only drawing what you need till only increase power by 4 x(in this example)
P_load = I_load x 5V.
P_alt = 20 V x I_load

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