Flexible heat and power generation in retrofitted oxy-fuel CCGTs with electrolysis-derived O <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"> <mml:msub> <mml:mspace width="0.1em"/> <mml:mtext>2</mml:mtext> </mml:msub> </mml:math> and H <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"> <mml:msub> <mml:mspace width="0.1em"/> <mml:mtext>2</mml:mtext> </mml:msub> </mml:math> co-firing