  Near-field thermophotovoltaic systems functioning at 400$\sim$900~K based on
graphene-hexagonal-boron-nitride heterostructures and thin-film InSb $p$-$n$
junctions are investigated theoretically. The performances of two near-field
systems with different emitters are examined carefully. One near-field system
consists of a graphene-hexagonal-boron-nitride-graphene sandwich structure as
the emitter, while the other system has an emitter made of the double
graphene-hexagonal-boron-nitride heterostructure. It is shown that both systems
exhibit higher output power density and energy efficiency than the near-field
system based on mono graphene-hexagonal-boron-nitride heterostructure. The
optimal output power density of the former device can reach to
$1.3\times10^{5}~\rm{W\cdot m^{-2}}$, while the optimal energy efficiency can
be as large as $42\%$ of the Carnot efficiency. We analyze the underlying
physical mechanisms that lead to the excellent performances of the proposed
near-field thermophotovoltaic systems. Our results are valuable toward
high-performance moderate temperature thermophotovoltaic systems as appealing
thermal-to-electric energy conversion (waste heat harvesting) devices.
