In this talk, after an introduction to thermoelectric (TE) properties, we analyze recently synthesized graphene nanoribbon (GNR) heterostructures that are obtained as extensions of pristine  armchair graphene nanoribbons (AGNRs). After simulating their band structure through a nearest-neighbor tight-binding model, we use the Landauer formalism to calculate the necessary TE coefficients, with which we obtain the electrical conductance G, thermopower S, thermal conductance Ke, and figure of merit ZT (using literature results for the phonon thermal conductance Kph), at room temperature. We then compare the results for the nanoribbon heterostructures with those for the pristine AGNR nanoribbons. The comparison shows that the metallic AGNRs become semiconducting (with much higher ZT values) after the inclusion of the extensions that transform them into heterostructures and that some heterostructures have higher values of ZT when compared to the semiconducting pristine AGNRs from which they  have originated.