  The conventionally independent power, water, and heating networks are
becoming more tightly connected, which motivates their joint optimal energy
scheduling to improve the overall efficiency of an integrated energy system.
However, such a joint optimization is known as a challenging problem with
complex network constraints and couplings of electric, hydraulic, and thermal
models that are nonlinear and nonconvex. We formulate an optimal
power-water-heat flow (OPWHF) problem and develop a computationally efficient
heuristic to solve it. The proposed heuristic decomposes OPWHF into
subproblems, which are iteratively solved via convex relaxation and
convex-concave procedure. Simulation results validate that the proposed
framework can improve operational flexibility and social welfare of the
integrated system, wherein the water and heating networks respond as virtual
energy storage to time-varying energy prices and solar photovoltaic generation.
Moreover, we perform sensitivity analysis to compare two modes of heating
network control: by flow rate and by temperature. Our results reveal that the
latter is more effective for heating networks with a wider space of pipeline
parameters.
