  In this paper, we develop an approximate wide-bandwidth upper bound to the
absorption enhancement in arrays of metaparticles, applicable to general
wave-scattering problems and motivated here by ocean-buoy energy extraction. We
show that general limits, including the well-known Yablonovitch result in solar
cells, arise from reciprocity conditions. The use of reciprocity in the
stochastic regime leads us to a corrected diffusion model from which we derive
our main result: an analytical prediction of optimal array absorption that
closely matches exact simulations for both random and optimized arrays under
angle/frequency averaging. This result also enables us to propose and quantify
approaches to increase performance through careful particle design and/or using
external reflectors. We show in particular that the use of membranes on the
water's surface allows substantial enhancement.
