Energy-water management and minimal cost solution in residences

Abstract

Energy and water insecurities are global challenges with wide impact because of their necessities, wide utilisation and interconnection with other challenges such as food insecurity, climate change and ecosystem collapse. Therefore, there is an urgent need for sustainable and economic solutions to these problems. The need for these solutions has sparked a renewed interest in energy-water nexus, a concept that defines the mutual relationship between energy and water. Energy-water nexus has been studied extensively at provincial, national, international and global levels but only a few studies have explored energy-water nexus at the residential level. Studies on rainwater harvesting (RWH), greywater recycling (GWR), water desalination and other alternatives conclude that the RWH and GWR systems are the most economical, sustainable and suitable solutions to water insecurity in residences. Additional water security, water savings and reliability will be achieved with integrated rainwater harvesting and greywater recycling (RWH-GWR) system. Despite this knowledge, no previous work has investigated optimal tank sizing, optimal operations and the interplay between tank sizing and operation of the integrated RWH-GWR system. Therefore, this study investigates the design and performance of an integrated RWH-GWR system in residence to improve its economic attractiveness. An optimisation model is formulated to minimise the storage volume of the water tanks and operational cost of the proposed RWH-GWR system subject to technical and operational constraints including the time-of-use (TOU) electricity tariff. This model is applied to a practical case study of a single-family building in Durban, South Africa. The optimisation problem is solved, and simulation results are compared to the baseline energy and water consumption.

A mixed binary linear programming problem is developed and solved by the solving constraints integer programming (SCIP) solver interfaced in MATLAB. The simulation results validated the effectiveness of the proposed model. It produces the optimal size of the water tanks and system operation. It also validates that accurate tank sizing, system operation and increased non-potable water utilisation will improve the economic attractiveness of the integrated RWH-GWR system. Sensitivity analyses are carried out to evaluate the robustness of the proposed model to fluctuations in water demand, rainfall intensity, electricity pricing and discount rate. Performance and economic analyses of the integrated RWH-GWR system sized by optimisation and Rippl methods of tank sizing are carried out to determine the most economic tank sizing methods.