Friction losses

The friction loss expected in an actual dwelling is determined by the plumbing characteristics and the flow rate of water. Details on how to estimate friction losses are provided in Cook et al. (2012), Swamee and Sharma (2008) and Tjandraatmadja et al. (2012). In domestic dwellings, rainwater tanks tend to be placed at a variety of locations and hence the distance between the rainwater tank and end-use appliances will vary with the design of the house, its land size and property layout. However, energy losses due to pipe friction as water travels through a rainwater system into a dwelling are considered to be minimal. Instead, the majority of the energy is used for running a pump for rainwater supply in the typical dwelling (Retamal et al. 2009; Tjandraatmadja et al. 2012). Retamal et al. (2009) estimated that losses in pumping energy in a typical domestic dwelling attributed to friction losses during delivery of water amounted to only 2%.

However, pump motor size can also impact on friction losses and care is required to prevent the oversizing of pumps for a required end use. For example, Cunio and Sproul (2009) reported an experiment where use of an oversized (high pressure) pump for the delivery of water to a toilet cistern resulted in 90%

pressure losses in friction alone. However, on the other hand, Tjandraatmadja et al. (2012) verified in a laboratory model house that the friction losses within the pipes for a given system set-up did increase with pump capacity, but rather ranged from 1% to less than 6% of the total pressure supplied for pumps of motor sizes 0.2, 0.45 and 0.75 kW.

Overall, whilst friction losses are expected to constitute only a minor energy loss component, they also reinforce the benefit of matching pump size to end use and system requirements.

6.3.7 other components

Pressure vessels can be used to mitigate the problem of inefficient pump operation, particularly where a pump is switching on and off frequently for short duration low flow rate water use events. A pressure vessel is a tank that contains an internal diaphragm which is partially filled with air as shown in Figure 6.14a. Pressure vessels are placed in-line after the pump and essentially store pressurised water. When the diaphragm in the pressure vessel is empty and a tap is turned on, the pump will start operating initially at maximum flow and will continue to pump until the pressure vessel is full as shown in Figure 6.14b. The next time a tap is turned on, water is drawn from the pressure vessel until the internal pressure reaches a threshold value, thereby delaying the start-up of the pump. The volume of water delivered by a pressure vessel is a function of the nominal volume of the vessel and the pump trigger pressure settings. However it is always less than the nominal volume as shown in Figure 6.15a.

Pressure vessels come in a range of sizes (nominal volumes of 5 L to over 1000 L). Small pressure vessels of 5 to 8 litres volume are sometimes installed with pumps to reduce the number of pump start-ups caused by small leaks in the water reticulation of a dwelling. The larger sizes are usually adopted for industrial applications.

A properly sized pressure vessel can reduce pumping energy by two mechanisms:

• It reduces the number of pump starts provided the volume required for a given end use is less than the water volume contained in the vessel.

• The flow rate at which the pressure vessel is filled is often greater than that at which an appliance is filled. This is particularly advantageous for low flow end uses such as filling a half-flush toilet cistern or brief operation of a tap for hand washing a glass of water (Tjandraatmadja et al. 2012).

Figure 6.14 Pressure vessel details: (a) Schematic diagram of pressure vessel components and (b) Pump operation with pressure vessel operation (Source: Tjandraatmadja et al. 2012).

Energy savings are generated if the volume of water that the pressure vessel provides is equal to or greater than the total volume of water required for an end use. For instance, a 5 L pressure vessel provides low volumes of water (1 to 2 L) before the pump starts, reducing stop-starts associated with small leaks in the system. However it offers negligible energy savings (Water Conservation Group, 2010). In comparison a larger pressure vessel, such as an 18 L pressure vessel with a 0.75 kW pump, can provide up to 6.3 L of water prior to the start-up of a pump, thereby reducing the energy used for rainwater pumping, particularly for low duration and high energy intensity water uses. This is shown in Figure 6.15b where the addition of an 18 L pressure vessel halves the specific energy required for low volume end uses such as the supply to a tap for a short duration or dishwasher operation (Tjandraatmadja et al. 2011).

Some caution is required regarding the set-up for pressure vessels. Installation of a pressure vessel in a system with a mains switching device can cause the pressure vessel to malfunction as the switching device controls the pump on-off cycling via a flow sensor (Retamal et al. 2008).

Figure 6.15 Impact of a pressure vessel on pump operation: (a) Volume of water released by pressure vessels of nominal volumes of 8 to 80 L coupled with pumps of 0.2 kW and 0.75 kW capacity; (b) Specific energy for rainwater supply to common end uses by a 0.75 kW pump with and without an 18 litre pressure vessel coupled. (Source: Tjandraatmadja et al. 2012).

6.3.7.2 Header tanks

A header tank is a localised storage vessel on the roof (or at similar height in a building). Water is pumped to the header tank and that supplies water by gravity to various end uses. This set-up is adopted in many parts of the world, for example, Asia, UK and South America. In Australia, header tanks can be found in commercial or industrial settings, but are not common in domestic residences.

Header tanks can provide large storage, (e.g., 100 to 300 L), reducing the number of pump start-ups compared to direct pump supply, and the pump operates at high flow rates to fill the header tank, increasing the energy efficiency. A properly sized header tank could provide the daily water needs of a dwelling with a single pump start-up per day, provided a proper switch is used. Thus in principle, header tanks could offer high energy savings for rainwater supply.

Laboratory studies in a simulated dwelling, shown in Figure 6.16, estimated that energy savings of 58%

to 79% could be achieved by a 300 L header tank when compared to direct supply to individual appliances by pumps ranging from 0.2 to 0.75 kWh motor capacity (Figure 6.17). This is equivalent to a reduction in the specific energy for rainwater supply ranging from 0.39 to 0.66 kWh/kL depending on pump size (Tjandraatmadja et al. 2012).

Figure 6.16 Simulated dwelling for evaluation of energy use for rainwater supply (a) Overview of major end uses (washing machine, toilet, taps and dishwasher), (b) Set-up for rainwater supply with a 300 litre header tank, (c) Rainwater tank supply and monitoring instrumentation.

Figure 6.17 Energy use for rainwater supply using a 300 litre header tank compared to direct supply of individual appliances (Source: Tjandraatmadja et al. 2012).

The caveat, however, is that a minimum height is needed for gravity supply from the header tank to provide the minimum pressure to open the solenoid valves that control water ingress into the appliances.

The installation adopted in Figure 6.14b placed the header tank at a height of 2.7 m above floor level, corresponding to the ceiling height, and provided a service pressure of <25 kPa, which was well below the minimum pressure requirements for operation of household appliances in Australia (31–100 kPa). However, by increasing the header tank to 5m, (higher than the ceiling height for a single storey dwelling) an adequate minimum service pressure of 50 kPa for toilet cistern filling would have been achieved. The appliances can also be manufactured to work on low pressures if there is a significant market for the industry to consider.

Therefore, in designing a header tank set-up, building design needs to allow adequate height for the header tank installation to generate sufficient hydrostatic pressure for solenoid operation (Tjandraatmadja et al. 2013). Alternatively, changes to the solenoid valve design in common appliances could be considered to allow for low pressure operation.

6.3.7.3 Different types of storages (under-floor bladders, gutter storage)

In urban areas, a variety of different rainwater storage methods have been adopted to overcome space constraints. For example, large plastic ‘bladders’ are sometimes used to store rainwater beneath the floor

of older style houses and have the advantage of a flexible shape in addition to making use of ‘dead’ space.

In one Sydney home with an under-floor bladder that was monitored, the rainwater system used a more powerful Venturi-style pump (a fixed-speed ejector pump that operates by creating a vacuum), to draw up the rainwater from beneath the house (Retamal et al. 2009). A combination of very efficient water use and a powerful pump caused the energy intensity of this particular system to be high at approximately 5 kWh/kL.

Some other innovative storage types also have the potential to reduce energy intensity, for example, gutter storages which provide some gravitational pressure, reducing pumping requirements (Retamal et al.

2009). However, other operational factors such as the potential mosquito breeding hazard may limit the uptake of such options. Overall, the type and location of storage also needs to be considered for optimising pumping energy.

6.4 reducIng energy use For rAInwAter systems – lessons From AustrAlIA

Since 2007, Australia experienced a high uptake of rainwater tanks in urban areas. Rainwater tanks are found in 23% of suitable dwellings across major capital cities increasing to 43% in selected capital cities. Rainwater tank installation is particularly strong in new dwellings, with approximately 57% of all dwellings less than 1 year old in south east Queensland connected to rainwater supply (Government of Queensland, 2009; ABS, 2010). Australia has also had the largest number of studies which examined the energy associated with rainwater pumping in urban settings.

Marsden Jacob Associates (2011), Stewart (2011), Gurung et al. (2012) and Gurung and Sharma (2014) have examined the life cycle costing of individual tanks systems. Electricity costs for operation of a rainwater tank are considered minor and estimated to represent only 2% of total operating costs over the life of the tank in Gurung et al. (2012). Ferguson (2011) recorded low energy consumption for rainwater supply in 52 dwellings in the Sydney area, with a median energy consumption of 62 kWh per dwelling per year. In monetary terms, this is equivalent to AUS$15 per year assuming an energy intensity of 1.48 kWh/

kL and current electricity prices of A$0.20 /kWh.

Notwithstanding the low costs associated with rainwater harvesting, the end use requirements for rainwater in urban dwellings in Australia cause pumps to operate well below their best energy efficiency point. Thus, there are significant benefits still to be gained by improving the energy efficiency in rainwater pumping through better matching end use service requirements and pump operation, and the use of ancillary devices such as pressure vessels and header tanks.

By adopting a smaller 0.2 kW pump instead of a 0.75 kW pump, electricity usage for rainwater supply in a typical dwelling can be reduced from 213 kWh per year to 66 kW per year, that is, a 73% reduction in energy consumption (Tjandraatmadja et al. 2012).

The use of devices such as pressure vessels and header tanks that could further reduce the energy consumption is uncommon, however their potential for energy savings has been demonstrated in laboratory studies (Tjandraatmadja et al. 2012) and in examples of dwellings where the energy intensity for pumping was 30–36% lower with the use of pressure vessels in-situ (see Section 6.3.7) (Retamal et al. 2009). The larger the pressure vessel with respect to the end use volume requirements, the least often the pump needs to start. However, almost no information is available to the public on the performance of pump and pressure vessels combinations.

Furthermore, header tanks coupled with an adequate level switch as previously discussed in section 6.3.7 have the potential to generate the largest energy savings of all ancillary devices and should be further examined.

In addition, significant improvements could be achieved by further investment in product design in areas such as the development of pumps customised for low flow urban end uses; improving the design of mains switching valve systems to reduce energy consumption, and facilitate their integration with pressure vessels; and the redesign of solenoid valves in household appliances for low pressure operation.

Investing in education, greater emphasis in current design and set-up of rainwater pumping systems followed by the development of benchmarks and guidelines that can inform and assist consumers to better select pumps could further reduce the energy requirements and lead to more sustainable practices.