CREATE TABLE table_76084 ("Name" TEXT, "Year commissioned" FLOAT, "Gross head  ( metres ) " FLOAT, "Installed capacity  ( megawatts ) " FLOAT, "Average annual output  ( million KWh ) " FLOAT)

What is the Year Commissioned of the power stationo with a Gross head of less than 18?
SELECT AVG("Year commissioned") FROM table_76084 WHERE "Gross head (metres)" < '18'
SELECT AVG("Year commissioned") FROM "table_76084" WHERE "Gross head (metres)" < '18'