I know the ejector has to be very precise in order to work, and you gain about 10% efficiency average and sometimes better by eliminating most of the throttling losses you get from a TXV or cap tube. I thought of getting one out of a junked Prius for my GSHP. Keep us posted on your success!
If you don't need potable water rated pumps, check out the Grundfos Alpha and Alpha2-self adjusting to maintain a given amount of head regardless of changing circumstances, and about $145 for iron and about $250 for stainless. They take from 5 to 45 watts.
Laing/Bell & Gossett make potable and non-potable ECM pumps-they come in several types that aren't PWM controllable but very efficient from 11-60 watts.
All of these are much more efficient than the usual Taco/B&G induction motor pumps. The Grundfos might already do what you want out of the box (unless the fun is in the designing, of course).
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"I‘d put my money on the sun and solar energy. What a source of power! I hope we don‘t have to
wait until oil and coal run out before we tackle that." Thomas Edison, 1847 — 1931
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