Complex sentence: Cogeneration is more thermally efficient use of fuel than producing process heat alone because in electricity production some energy must be rejected as waste heat, but in cogeneration this thermal energy is put to good use.
Split into simpler sentences: Cogeneration is more thermally efficient use of fuel than electricity generation alone. In separate production of electricity some energy must be rejected as waste heat, but in cogeneration this thermal energy is put to good use.