A wave energy conversion system (100a,b), said system comprising: a float (101, 101a,b, 501a,b) and a power generation drum with a drum housing (102, 102a,b, 502a,b,c) rotationally attached to the float (101, 101a,b, 501a,b) by a shaft attachment (105a,b, 505a,b); wherein the shaft attachment (105a,b, 505a,b) allows an at least partially rotational movement of the drum housing (102, 102a,b, 502a,b,c) and the float (101, 101a,b, 501a,b) relative to each other; wherein the drum housing (102, 102a,b, 502a,b,c) comprises a cable reel (103a,b) with a cable (104, 104a,b) having a first end part connected to the cable reel (103a,b) and a second end part, said cable reel (103a,b) being an integral part of or connected to the drum housing (102, 102a,b, 502a,b,c) to rotate with the drum housing (102, 102a,b, 502a,b,c); wherein the drum housing (102, 102a,b, 502a,b,c) comprises a spring system, said spring system comprising a torsion spring with a first end part non-rotationally connected to the shaft attachment (105a,b, 505a,b) and a second end part operatively connected to the drum housing (102, 102a,b, 502a,b,c), wherein for at least one connection mode of operation a rotation of the drum housing (102, 102a,b, 502a,b,c) builds up a tension in the spring, thereby applying a torque to the shaft attachment (105a,b, 505a,b); and wherein the second end part of the torsion spring is configured for being operatively connected to the drum housing (102, 102a,b, 502a,b,c) in a first mode of operation to build up a tension in the spring by rotation of the drum housing (102, 102a,b, 502a,b,c) in a direction for unwinding the cable (104, 104a,b) from the cable reel (103a,b) and to rotate the drum housing (102, 102a,b, 502a,b,c) in the opposite direction for winding up the cable (104, 104a,b) on the cable reel (103a,b) by a release in the tension of the spring; characterised in that the torsion spring is operatively connected to the drum housing (102, 102a,b, 502a,b,c) for shifting between the first mode of operation and a second mode of operation, wherein for the second mode of operation the second end part of the spring is operatively connected to the drum housing (102, 102a,b, 502a,b,c) for building up a tension in the spring by rotation of the drum housing (102, 102a,b, 502a,b,c) in a direction for winding up the cable (104, 104a,b) on the cable reel (103a,b). a float (101, 101a,b, 501a,b) and a power generation drum with a drum housing (102, 102a,b, 502a,b,c) rotationally attached to the float (101, 101a,b, 501a,b) by a shaft attachment (105a,b, 505a,b); wherein the shaft attachment (105a,b, 505a,b) allows an at least partially rotational movement of the drum housing (102, 102a,b, 502a,b,c) and the float (101, 101a,b, 501a,b) relative to each other; wherein the drum housing (102, 102a,b, 502a,b,c) comprises a cable reel (103a,b) with a cable (104, 104a,b) having a first end part connected to the cable reel (103a,b) and a second end part, said cable reel (103a,b) being an integral part of or connected to the drum housing (102, 102a,b, 502a,b,c) to rotate with the drum housing (102, 102a,b, 502a,b,c); wherein the drum housing (102, 102a,b, 502a,b,c) comprises a spring system, said spring system comprising a torsion spring with a first end part non-rotationally connected to the shaft attachment (105a,b, 505a,b) and a second end part operatively connected to the drum housing (102, 102a,b, 502a,b,c), wherein for at least one connection mode of operation a rotation of the drum housing (102, 102a,b, 502a,b,c) builds up a tension in the spring, thereby applying a torque to the shaft attachment (105a,b, 505a,b); and wherein the second end part of the torsion spring is configured for being operatively connected to the drum housing (102, 102a,b, 502a,b,c) in a first mode of operation to build up a tension in the spring by rotation of the drum housing (102, 102a,b, 502a,b,c) in a direction for unwinding the cable (104, 104a,b) from the cable reel (103a,b) and to rotate the drum housing (102, 102a,b, 502a,b,c) in the opposite direction for winding up the cable (104, 104a,b) on the cable reel (103a,b) by a release in the tension of the spring; characterised in that the torsion spring is operatively connected to the drum housing (102, 102a,b, 502a,b,c) for shifting between the first mode of operation and a second mode of operation, wherein for the second mode of operation the second end part of the spring is operatively connected to the drum housing (102, 102a,b, 502a,b,c) for building up a tension in the spring by rotation of the drum housing (102, 102a,b, 502a,b,c) in a direction for winding up the cable (104, 104a,b) on the cable reel (103a,b).