What is the lowest numbered hydroelectric power when the renewable electricity demand is 21.5%?
CREATE TABLE "renewable_electricity_in_spain_g_wh_2006" (
  "autonomous_community" text,
  "hydroelectric_power" real,
  "wind_power" real,
  "solar_power" real,
  "biomass_power" real,
  "solid_waste_power" real,
  "total_renewable_generation" real,
  "total_electricity_demand" real,
  "pct_renewable_of_total_electricity_demand" text
);
SELECT MIN("hydroelectric_power") FROM "renewable_electricity_in_spain_g_wh_2006" WHERE "pct_renewable_of_total_electricity_demand"='21.5%';