  Graphene---hexagonal-boron-nitride---InSb near-field structures are designed
and optimized to enhance the output power and energy efficiency of the
thermophotovoltaic systems working in the temperature range of common
industrial waste heat, $400~\rm K \sim 800~\rm K$, which is also the working
temperature range for conventional thermoelectric devices. We show that the
optimal output electric power can reach $3.5\times10^{4} \rm\ W/\rm m^2$ for
the system with a graphene---hexagonal-boron-nitride heterostructure emitter
and a graphene-covered InSb cell, whereas the best efficiency is achieved by
the system with the heterostructure emitter and an uncovered InSb cell
(reaching to $27\%$ of the Carnot efficiency). These results show that the
performances of near-field thermophotovoltaic systems can be comparable with or
even superior than the state-of-art thermoelectric devices. The underlying
physics for the significant enhancement of the thermophotovoltaic performance
is understood as due to the resonant coupling between the emitter and the cell,
where the surface plasmons in graphene and surface phonon-polaritons in
boron-nitride play important roles. Our study provides a stepping stone for
future high-performance thermophotovoltaic systems.
