Synthesis of heat-integrated water networks: Integration of heat pumps

Abstract

A typical heat-integrated water network (HIWN) synthesis problem aims to minimise freshwater and utility consumption while establishing trade-offs between operating and investment costs by exploring different water and heat integration opportunities. Heat integration between hot and cold water streams is performed to save external heating and cooling utilities required to achieve the target temperatures of these streams. This paper proposes a mathematical programming approach to integrate water-to-water heat pumps into the HIWNs. A simple cascade heat pump system with isobutane as a natural working fluid enables a larger temperature lift between the heat pump source and the sink. The objective function of the proposed mixed-integer nonlinear programming (MINLP) model is to minimise the total annualised cost (TAC). The TAC includes operating costs for freshwater, heating/cooling utilities, electricity, and investment in heat exchangers and heat pumps. A sensitivity analysis is performed to evaluate the economic feasibility of the system by varying the heating/electricity cost ratio and depreciation period for the capital investment. The results show that additional utility savings can be obtained by integrating heat pumps into the HIWNs. For threshold problems, a heat pump is feasible for heating/electricity price ratios from 1.175, and heating utility is replaced by electricity for a 1:1 ratio. However, for pinched problems, the feasibility of the heat pump is shifted to lower heating/electricity price ratios, from 0.3 to 0.5, depending on the annualisation factor for the investment.