Space Emperor God Sigma

The year is 2050 and an energy crisis is Earth's main concern. A brilliant scientist in Trinity City develops the Trinity energy, a new energy powerful enough to supply the entire world. The alien empire of the Elda learn of this energy and plot to seize it by force. They attack Earth's space colony on Jupiter's moon of Io and occupy it. Meanwhile, the Trinity energy has been applied to three giant robots that combine to form God Σ, a mighty robot led by its main pilot, martial artist Toushiya Dan. With its mighty sword, it now stands between the peace of Earth and the power of the Elda forces. However, later in the series it's revealed that the Elda forces came from the future, where earthlings attacks Elda's home planet. Using advanced time-machine the Eldans tries to seize the secret of Trinity energy to save their home planet.