Energy intensity of rainwater harvesting systems: A review

Rainwater Harvesting Systems (RHS) are increasingly used in buildings to mitigate water shortage and rising prices of centralised water supply. Notwithstanding the benefits of RHS, they may also promote adverse impacts mainly related to the high consumption of energy. In this context, energy intensity (i.e. unit of energy per unit of water) is a crucial parameter for assessing the environmental feasibility of different RHS. However, only recently has attention been drawn to the connection between water and energy consumption, which has been prompted by the increasing importance of water security, energy efficiency and economic feasibility. This connection, known as the water-energy nexus, has been increasingly acknowledged as a key principal for water planning. The objective of this study is twofold: (i) to review the energy intensity data reported for RHS; and (ii) to outline strategies to enhance the energy performance of RHS in buildings. For the reviewed literature, the median energy intensity of theoretical studies (0.20 kWh/m³) was considerably lower than that described in empirical studies (1.40 kWh/m³). This implies that theoretical assessments of energy intensity may not sufficiently consider the energy used for pump start-ups and standby mode, as well as the true motor and pump energy efficiency. However, to some extent, this difference may also represent the amount of energy that can be reduced by optimising RHS design and operation. When comparing RHS to conventional town water supply systems, the reviewed empirical studies showed that RHS tend to be three times more energy intensive, although optimised RHS can have more comparable values. Ultimately, it is predominately the local characteristics, such as rainwater demand, building type (single-storey or multi-storey), RHS sub-systems design, potable water plumbing system design, and town water energy intensity, among other factors that will determine whether or not the environmental and economic performances of RHS are acceptable.