Bring one liter of water to the boil in a large saucepan, add the hibiscus flowers and the sugar, peeled pieces of ginger and the juice of the lemon to the boiling water, stir briefly and turn off the stove. Stir occasionally, leave to stand for 10 minutes and then sieve off the flowers and pieces of ginger. Finally add the remaining two liters of water and half a liter of orange juice and refrigerate.