Name the number of sement d for solar water heaters 
CREATE TABLE "season_18_2011_2012" (
  "series_ep" text,
  "episode" real,
  "segment_a" text,
  "segment_b" text,
  "segment_c" text,
  "segment_d" text
);
SELECT COUNT("segment_d") FROM "season_18_2011_2012" WHERE "segment_b"='Solar Water Heaters';