Modelling the impact of energy intensity on the economic and environmental costs of internally plumbed rainwater tanks systems

Abstract

Australian capital cities are forecast to face decreased average annual rainfall by 2030 under mid-emissions climate scenarios. With two-thirds of Australia's population already residing in these growing major cities, urban water resources pressure will undoubtedly increase in the years ahead. Methods to augment traditional reservoir based water supply will be required to maintain water security. Internally plumbed rain water tank systems (IPRWTs), supplying water at a decentralised level, are one potential alternate supply source. Incentive schemes and legislation have seen IPRWTs become increasingly prevalent in metropolitan areas since widespread drought from 2000-2009. For example in Queensland water savings targets were mandated for all newly constructed homes. The most common means of meeting these targets was through the use of IPRWTs, which when installed were required to supply garden irrigation, clothes-washers and cistern flush events. Studies to date have been focused on maximising potable savings, such that little research has investigated the energy intensity and greenhouse gas emissions associated with the operation of IPRWTs. This is a worthy consideration given that they are typically thought of as green, sustainable solutions. This paper examines the energy intensity and greenhouse gas cost of IPRWTs under a number of modelled usage scenarios, utilising high-resolution end-use level data from a recently conducted IPRWT monitoring study in South East Queensland. This modelling, facilitated through the use of specialised software, MATLAB and spread-sheet software, has determined the average annual energy consumption of IPRWTs, taking into account the effect of climatic conditions on water consumption. This energy consumption leads to electricity and carbon based costs to homeowners and the general community, which have been quantified at an end-use level. In the interests of system optimisation, the net economic and environmental impacts of configuration changes to IPRWTs have been considered, such as not plumbing in toilet cisterns. For standard 2.8 person households preliminary evidence indicates that based only on operational costs, more end-uses are preferable, due to higher yields. However, when plumbing and environmental costs are taken into account it has been found that it may be worthwhile to plumb in only the most energy efficient rainwater end-uses (irrigation and clothes washing). For larger households, or those consuming large volumes of water in regular irrigation, energy intensity of water supply can be significantly lowered by only plumbing in efficient end-uses, while yearly water savings (from avoided mains costs) fall only by a small margin. For retrofitters and new home builders the extra cost incurred when plumbing in toilet cisterns to rainwater supply should be considered. Existing system owners should note the large increase in system energy intensity when small leaks develop from poor maintenance, usually from toilet cisterns, as this can lead to significantly greater economic and environmental costs over the lifetime of a rainwater system and reduce the viability of cistern end-uses.