I like entry #18 and how the inside of the E and D look like a power plug. Very clever. It took me a few times to see it... could you make the space a little bigger on the "D" side and see how it plays?
Also love the use of the triangle in #15 for "Delta"... could you try the white space you have to make the lines in the "E" looking a little like lightening bolts or live wire... not too pronounced or cheezy... just a little more like live power