energy consumption in rainwater tank systems

The energy associated with household scale rainwater pumping is a function of the pump characteristics, the system set-up, and the household water use patterns (Retamal et al. 2009). The energy can be expressed as the ‘total energy’ consumed over a period of time for pumping (e.g., kWh per year) or as ‘specific energy’, that is, the energy required to pump a set amount of water (e.g., kWh per kL of rainwater).

The tank location relative to where water is used and the land topography can influence the head required and the associated energy usage. Gardner et  al. (2006) and Beal et  al. (2008) examined the energy for rainwater pumping for 4 to 6 properties located on a steep slope in Brisbane, Australia, where household rainwater tanks received back-up supply from a communal tank at the base of the slope. This study found that the energy required for water supply from the rainwater system was between 2.1 and 3.8 kWh/kL. Retamal et al. (2009) found the energy footprint of household-scale rainwater harvesting systems varied from 0.9 to 2.3 kWh/kL for 8 households equipped with various rainwater pump and mains backup systems, different numbers of occupants, and diverse rainwater end uses.

The large variability observed between dwellings is a reflection of the wide range of rainwater systems configurations (mainly pump type/size) and the demand characteristics of the different households (low flow or high flow appliances/end uses). It is also an indication that the energy requirements for rainwater pumping can be optimised for dwellings through appropriate system design and configuration set-up.

Analysis by Umapathi et al. (2013), based on a monitoring study of 20 homes in South East Queensland, found that rainwater systems with automatic switching devices for topping up tanks with mains water had significantly less energy demand than systems using trickle top-up.

Small pumps used for rainwater supply are typically much less efficient than large pumps used for bulk water supply (Retamal et al. 2009). In Australian capital cities, pumping for centralised water supply typically has an average energy demand of 0.9 kWh/kL, with a range between 0.09 and 1.85 kWh/kL

depending upon pumping distance and lift required for each city (Kenway et al. 2008). This places the median energy for household rainwater supply of 1.5 to 1.8 kWh/kL within the upper range of centralized gravity fed water supply. However, when compared to other alternative sources such as recycled water or desalinated water, which use on average 2.8 kWh/kL (Knights et al. 2007) and 3.5 kWh/kL (Apostolidis et al. 2011) respectively, rainwater supply is the least energy intensive option for alternative water supplies.

1.5 conclusIons

Rainwater harvesting for local water supply is not a new concept as there is well documented evidence of its practice since the first urban societies evolved. This chapter has reviewed rainwater harvesting practices around the world, with a particular emphasis on the drivers behind the adoption of rainwater harvesting.

There is a particular focus on the Australian experience with rainwater systems, which is due to the rapid uptake of rainwater systems over the last decade. This uptake was in response to growing pressures on mains water supply systems due to an extended drought and growing population.

Rainwater tanks can provide reliable and affordable water supply in rural and remote areas where piped water supply systems are not available. In developing countries, rainwater can supply an ‘improved’ drinking water source where surface water is contaminated by faecal pathogens, or good quality groundwater is not readily available. In modern cities, rainwater tanks are now being implemented under integrated urban water management concepts, to reduce the volume of mains water consumed for non-potable household uses. This substitution concept is based on fit for purpose water quality to reduce unnecessary treatment costs.

This chapter has outlined some of the issues that are faced when considering the potential for rainwater harvesting systems as a secondary source of non-potable water in modern cities. These issues include understanding the likely yield from rainwater systems, both for optimising the design of the rainwater system as well as estimating the likely impact on reducing mains peak flows. There is also the need to manage the risks associated with the use of rainwater, which requires an understanding of its likely quality as well as improved guidelines on how to manage risks. The chapter has highlighted the challenges of evaluating the cost-effectiveness of rainwater harvestings relative to other alternative water sources, as well as centralised water supply systems. Whilst rainwater harvesting often has a higher cost per unit of water supplied than other sources (e.g., recycled water), consideration of externalities, such as mitigating the environmental impact of stormwater discharge to receiving waters, can increase the cost to benefit ratio. However, economic analysis has found that under most assumptions the operating and capital costs of installing a rainwater tank as a secondary water source will be greater than the benefits. The challenge is to identify the urban contexts and rainwater tanks configurations that are best suited to maximise the benefits relative to the costs. This challenge is explored in greater detail in the subsequent chapters of this book.

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Alison M. Vieritz, Luis E. Neumann and Stephen Cook

AbstrAct

Mathematical models that simulate the performance of rainwater systems are important in supporting decisions on the suitability of rainwater systems to not only meet potable and non-potable water demands, but also reduce discharge of stormwater and associated pollutants to the environment. A variety of modelling tools have been developed over the last few decades to support the design of rainwater systems and optimise the combination of connected roof area and storage size to best meet demands. However, the selection of the rainwater system modelling approach can influence the predicted outcomes, with different choices of time-step, algorithms and simulation length significantly influencing the results.

This chapter presents a generalized model of a rainwater tank, and discusses its components in relation to the different rainwater tank models that have been developed around the world. We then explore the impact of key model parameters, including connected roof area, tank storage size, rainfall loss factor, water demand and climate inputs, as well as the choice of time-step and simulation length on the output, using the rainwater tank model described by Mitchell (2007). We demonstrate how each of these parameters can affect the simulated results of the tank behaviour. Finally, we show that using an ‘average tank’ behaviour to represent the collective behaviour of a large number of identical tanks in an urban area (also known as spatial lumping) can significantly overestimate the reliability and yield of the rainwater system. The errors introduced by such approaches are discussed using examples of yield and overflow estimation.

Keywords: Rainwater tank model; yield estimation; overflow estimation; model time-step; spatial averaging.

2.1 IntroductIon

It may be a little surprising to observe that rainwater tank systems have been adopted in many modern cities even though reticulated (mains) water supplies exist in these areas. There are a number of reasons for this. Firstly, they provide a secondary, non-potable water source to complement reticulated drinking water supply. Water scarcity and the need to reduce demand on the drinking water system has been the

Chapter 2

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