Low Energy Pumping Systems for Rainwater Tanks

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1 School of Photovoltaic and Renewable Energy Engineering University of New South Wales Kensington, NSW, 2052 AUSTRALIA ABSTRACT The supply of low flow rate residential water appliances such as toilet cisterns and low flow shower heads by high pressure pumping systems such as found in rural dwellings is energy inefficient. This paper will present the experimental results of a study undertaken that compares the energy usage of a typical rainwater pumping system with a low pressure, low energy system. The paper compares a number of options for the delivery of water to a single toilet cistern in a rural dwelling including an existing conventional rainwater tank pumping system, a low pressure dedicated system and a gravity fed header tank system. The results indicate that a low pressure system is capable of delivering water with an energy intensity as low as 40 kwh/ml as compared to a typical conventional pump with an energy intensity of up to 1700 kwh/ml. Such low powered systems are ideally suited to be supplied from small photovoltaic systems driving a DC pump. Keywords - energy, efficiency, residential, domestic, rainwater INTRODUCTION Awareness of the need to conserve and manage water resources in Australia due to increasing shortages, population growth and climate change concerns has led to steps to improve water use efficiency, recycling and capture. One aspect of this change is the increasing use of water storage tanks in urban and mains connected dwellings to store both rainwater and recycled greywater. This water is typically used in an urban setting to supply toilets, gardens and in some cases laundry and hot water needs. Rainwater storage tanks are now a mandatory addition during residential construction or renovation in many areas. Sydney Water indicates that approximately 70% of the water supplied is for residential use (Sydney Water, 2005). It is believed that a significant proportion of this market could be addressed through the use of residential rainwater tanks. The use of on-site water storage requires the need for on-site pumping and hence the pumps used and energy intensity for delivering the water (usually expressed in kwh/ml) have become key parameters to be considered. Typical installations use high pressure pumps capable of generating close to mains pressure delivery (~ kpa, corresponding to about m head), with a wide range of flow rates depending on the application. These pumps are often rated at 500 W and sometimes upwards of 750 W (1 hp). When these pumps are used to deliver low flow rates at high pressure due to restrictive appliances such as toilet cisterns, low flow shower heads or washing machines, the energy intensity of water delivered starts to rival the energy requirements of desalination. Such applications require only low flow rates which can be supplied at significantly reduced pressure and hence yield energy savings. Pump motors are typically 240V AC, single phase and have efficiencies in the order of 25 30%. Previously published data regarding energy intensity for tank supply options vary considerably due to values being heavily dependent on specific designs and on

2 appliance and end use patterns. Marsden Jacobs & Associates reported energy intensity for rainwater tanks as 956 kwh/ml, whilst for desalination the figure was 4932 kwh/ml (Marsden Jacobs Associates, 2007). Lower figures for rainwater tanks (~300 kwh/ml) have been reported elsewhere (Coombes et al, 2002). Previous theoretical analysis of reduced pressure and flow tank pumping systems (Sproul 2007) have indicated possible energy savings in the order of 80%. This paper presents experimental data that demonstrates the energy savings achieved in a number of scenarios where a toilet cistern is supplied by a tank and pump arrangement. TYPICAL SYSTEMS A typical rainwater system for a domestic dwelling is shown in Fig. 1 (courtesy Sproul 2007). Source: Sproul 2007 Fig. 1: Typical rainwater tank system for domestic water supply. The delivery plumbing from a tank system is independent of any plumbing that is directly connected to mains supply. If mains supply is required to supplement the systems supplied by the water tank, it must be arranged in such a manner that ensures no backflow into the mains is possible. This is usually achieved by either filling the tank with a physically isolated feed (eg. gravity fall into the tank) or by a bypass valve designed to inhibit backflow. The option of filling the tank from the mains supply has the disadvantage of increasing the energy intensity of the water delivered since the mains supply is fed to the tank and then re-pumped again to supply the load. The advantage is that the delivery plumbing can be kept as a low pressure system utilising low pressure and cost fittings. The alternate bypass valve option saves the need for extra pumping but requires the tank delivery plumbing to meet normal mains pressure specifications. A rough theoretical analysis of the energy use of typical systems gives a taste of the possible savings that can be achieved in energy intensities through the use of more efficient designs. Even though the absolute amounts of energy used for a typical household per day are quite small, the relative energy intensities are considerable. As mentioned previously, the energy intensity for a typical tank supply system is reported to be in the order of kwh/ml. This is higher than the energy intensity of mains supply by Sydney Water (260 kwh/ml, Sydney Water 2006) even though mains water is transported tens or even hundreds of kilometres. Even though Sydney Water has the advantages of scale and often gravity, the energy intensity of the Solar09, the 47 th ANZSES Annual Conference Page 2

3 tank systems seem surprisingly high. The tank pump must develop relatively high pressure (30m head) to drive the water through the resistance of the half or three-quarter inch delivery plumbing network. The energy required by the pump is proportional to the pressure being delivered and hence large amounts of energy are consumed just to be wasted as frictional heat loss as the water flows through narrow pipes. If the plumbing for a single story dwelling has a net static head of say 1.5 metres, then over 95% of the energy generated developing a 30 metre pressure head is wasted! In comparison, energy efficient designs using low pressure, low power pumps together with large diameter plumbing can reduce the pressure head required for supply down to the vicinity of 5 metres yielding possible savings in excess of 90% for the energy intensity of the water delivered. EFFICIENT DESIGN It has been suggested in the preceding section that most of the energy input in a high pressure small diameter plumbing system is lost as frictional pressure drop across the system. Greater energy efficiency can be easily achieved for tank pumping systems by utilising larger diameter pipe (say 32 or 40 mm) and running at a lower pressure with a smaller pump. Pump fed appliances in urban environments such as toilets, drip irrigation and washing machines do not require high pressure since maximum flow rates are not critical. The use of efficient design such as doubling pipe diameter can save upward of 95% of pipe pressure losses (Cengel 2006) and save in the order of 90% of system energy intensity. As way of example, a typical residence with say 20 m of 12.5 mm diameter piping supplying an appliance at 20 litres/minute (eg. a bathroom tap), approximately 50 W of hydraulic power is required just to overcome pipe frictional losses (Sproul 2007). If the motor/pump has a wire to water efficiency of say 30% (common for medium sized tank pumps) then 160 W of electrical power is required to overcome the losses. Doubling the pipe diameter to 25 mm (one inch) reduces the electrical power required to just 6 W, representing a saving of over 95%. Combining larger diameter pipes with a low pressure delivery head of less than 5 m (by using a low power pump and/or gravity feed) can yield energy intensities in the order of 70 kwh/ml. If a residence used in the order of 400 litres/day for toilets, garden irrigation and washing, this corresponds to an annual energy use of 7 kwh/year which could be provided using a 10 W solar panel and 10 Ah battery. EXPERIMENTAL INVESTIGATION An experimental study was undertaken to compare the energy usage of a typical rainwater pumping system with that of low pressure, low energy options. As a possible typical use of urban rainwater supply, the filling of a toilet cistern by various means is considered. The systems compared consist of 1) a conventional tank pumping system using the existing small diameter pipe network to supply a rural dwelling toilet cistern, and 2) a number of low pressure large diameter pipe pumping scenarios using the same toilet cistern, a standard float valve test cistern and a number of pumps. A further theoretical investigation was considered for a medium pressure system pumping via large diameter pipes to a small roof header tank that then gravity feeds the same toilet cistern through the existing small pipe network. The systems will be characterised in terms of pressure, flow rates and electrical usage consumption. The energy intensity Solar09, the 47 th ANZSES Annual Conference Page 3

4 required to deliver the water (kwh / ML) can then be compared to each other and to other delivery systems such as mains pressure (Sydney Water) and desalination. Urban rainwater supply systems are via dedicated pipe delivery networks independent of mains supply to the remainder of the dwelling. This network is typically built around conventional high pressure (13 or 19 mm) plumbing driven by a high pressure pump simulating mains pressure. It is believed that such systems can still function well when adopting a low pressure design utilising a low power pump and/or gravity feed combined with a large diameter ( 25 mm) low pressure (and low cost) polypipe network such as used in garden irrigation. This cost saving can be offset against the additional cost of the small roof header tank option. Scenario 1 Conventional Pumping System The conventional case will be considered first for purposes of comparison. The exact energy intensity used and results obtained will be dependent on the particular dwelling under consideration. The existing rural dwelling has rainwater tanks that supply a single storey building via standard ¾ inch plumbing and a high pressure pump. The configuration has a measured static head of 0.8 metres to the cistern valve and an estimated 6.5 metres equivalent length high pressure 19 mm polypipe feeding the cistern. The system pressure/flow curve of the piping/cistern as seen by the pump at zero height was measured using pressure and flow meters while varying a throttling valve. The high pressure pump used was an Onga JSP W pressure switched household pump and the pressure/flow curve of the pump was obtained from manufacturer specifications. The pressure readings were adjusted to allow for a 1.5 m static head due to the tank water level. The resulting system and pump curves are shown in Fig 2. Fig. 2: Typical high pressure pump filling a toilet cistern The maximum flow rate measured for the system while filling the Kohler cistern was 4.5 l/min which agrees with the above determined curves. The pressure activated pump was activated for a total of 84 seconds to fill the cistern with capacity 6 litres (the flow rate varies as the float valve closes and extra runtime occurs until the pressure switch cuts out). The measured electrical input energy during this time was 10 Wh using a wattmeter. This yields an average energy intensity for filling the cistern of 1700 Solar09, the 47 th ANZSES Annual Conference Page 4

5 kwh/ml. This is clearly an extreme case of a high pressure low flow application but even so is clearly energy intensive compared to Sydney Water values of 260 kwh/ml. Scenario 2 Low Pressure Direct Connected System A low pressure system was then considered where the tank water is supplied directly to the toilet cistern via a one and half inch (40 mm) low density irrigation polypipe pumped from ground level. The static head from ground level to the cistern valve was 0.75 metres and the equivalent length of 40 mm plumbing used for this test scenario set at 30 metres to represent a building plumbing situation. This system was then driven by a low power low pressure pump to minimise energy use. The additional time to fill the cistern from reduced flow rate was not considered an issue. A temporary small tank was used as the source and kept at a fill level corresponding to a zero static head. The first pump tested was a Pondmate PM W centrifugal pump which has maximum flow of 25 l/min and a shut-off head of 1.8 m. The pump fed the same Kohler cistern as in scenario 1 but failed to fill the cistern at all. The cistern was a modern Kohler low flow quiet model designed for mains pressure and hence the valve would not open at all at pressures under approximately 2 metres head. Clearly such cisterns will need to be modified with suitable valves to allow low pressure flow or other model cisterns used. Tests on other types of float valves are discussed below. A second low power pump capable of developing higher pressure heads was tested. The pump used was a Shurflo model small 12 V 22 W diaphragm pump capable of being solar powered. The pump has a maximum flow of 3.7 l/min and maximum head of 10 m. The results of this test (see Tab. 1) yielded an energy intensity for delivery of 179 kwh/ml. This represents a significant performance improvement over the conventional case ( > 85% energy savings ) even though still using standard high pressure cisterns. The tests of the above two low power pumps were repeated using two other appliance options, one a simulated cistern using a low pressure rural trough float valve (similar to that found in older style toilets) and the other a standard flow cistern valve as found in any hardware store (the second cistern valve) replacement. For the low pressure trough float valve both small pumps performed satisfactorily. The float was placed at a height to yield a static head of 750 mm to match the previous tests. The measured results are shown in Tab. 1. Both pumps had roughly similar cistern fill times with flow rates of 3 l/min and 3.5 l/min. The Pondmate yielded an energy intensity of 71 kwh/ml and the Shurflo an intensity of 105 kwh/ml. The operating point of the trough float valve for both pumps is shown in Fig. 3. Solar09, the 47 th ANZSES Annual Conference Page 5

6 Fig. 3: Operating points of the low pressure pumps and the trough float valve The tests for the second cistern valve were then measured and the results are also tabulated in Tab. 1. The operating point of the second cistern valve for both pumps is shown in Fig. 4. The Pondmate yielded an energy intensity of 200 kwh/ml and the Shurflo an intensity of 115 kwh/ml. Fig. 4: Operating points of the low pressure pumps and the second cistern valve It should also be noted that the test scenarios were designed to not allow any artificial boosting of pressure from the static head height of the tank water. In practice the tank could supply a boost of up to 2 metres of head. Solar09, the 47 th ANZSES Annual Conference Page 6

7 Tab. 1: Experimental results of various configurations for scenario 2. Pump Appliance Supplied Electrical Input (Watts) Pressure Head (metres) Flow Rate (l/min) Energy Intensity (kwh/ml) Pondmate 1500 Kohler Cistern n/a Centrifugal 18 W Trough Valve Max Flow: 25 l/min Shutoff: 1.8 m Second Cistern Shurflo Kohler Cistern Diaphragm 22 W Max Flow: 3.7 l/min Trough Valve Shutoff: 10 m Second Cistern Scenario 3 Gravity Fed from Header Tank System Although the results of the direct connection scenario are impressive, the use of very low power pumps are only suitable where the system heads can be kept low. In practice this can be difficult as building plumbing can be routed over ceilings and up multiple stories resulting in significant static heads. Lengthy plumbing runs can also add significant friction head even with the use of 25 or 30 mm diameter piping. In reality system heads in excess of 5 m would not be uncommon. An energy efficient solution for such a scenario would be to install a small roof header tank that can gravity feed the toilet cisterns anywhere in the house and in multi-stories. The distribution plumbing from the header tank can be standard gauge and existing plumbing. The header tank is then filled as required (say a capacity of 10 toilet flushes) from an efficient float valve connected to a large diameter pipe (50 mm) and medium power tank pump capable of pumping to the required static head. Such a system is shown in Fig 5. The energy intensity and energy use of such a design purely depends on the dynamics of pumping to the header tank. Any inefficiencies or unnecessary loads & pressures on the distribution side is paid for free by gravity, the only drawback being a change to the cistern filling time. 4 m Gravity fed 2 m 1 m Fig. 5: Header Tank with Gravity Feed As a comparison, we can theoretically investigate such a system design. For the single storey dwelling being used for testing, consider a small header tank at a height of 4 metres, typical of the height within a single story roof space. The header tank would then feed the cistern system via say one inch polypipe. The tank would be filled via a two inch pipe direct from the tank using a medium power centrifugal pump. The tank Solar09, the 47 th ANZSES Annual Conference Page 7

8 would need an efficient high flow, low resistance float valve system capable of switching rapidly from full open to close. If the existing small diameter house pipe network was used to feed the toilet from the header, we can use the measured cistern system curve of Fig. 2 as a guide and expect such a header tank to fill the cistern at something like 1.5 l/min flow rate. The energy required to pump for such a system is simply the header filling energy. For large diameter pipes such as two inch diameter with a length of less than 10 metres, a simple calculation shows that the hydraulic resistance of the pipe even at flows of 100 l/min is only around 0.1 metres of head. The delivery pressure will be principally due to the static height plus the loss across the float valve. If an allowance of 1 m head loss for the valve is accepted, this gives a total system head of roughly 5.1 m. If a medium power low head high flow centrifugal pump such as the Pondmate PM18000P is considered, then examination of the pump performance curves would indicate a flow rate of roughly 110 l/min at a head of 5.1 m. Using the published power rating of the pump of 250 W input power, this yields an energy intensity for this solution of under 40 kwh/ml. The key to maximising the yield would be to select a properly matched and sized pump (pressure versus flow and efficiency) for the height of the header tank. This solution has the advantage of not requiring any modifications to the appliances in terms of valve flow rate adjustments. IMPLEMENTATION AND COSTS If we assume a traditional parallel rainwater delivery system for toilets, gardens and other appliances is already contemplated due to water restrictions, legislation, personal awareness etc, then only the difference in pricing of the systems and energy use need be considered in a costing analysis. The actual dollar savings in energy consumed by the low power option is minimal, so any costing benefits must come from initial installation. If we consider a domestic household would use say 200 litres a day of rainwater (this assumes a large urban tank and regular rainfall) this equates to 73 kl per year. Considering the energy intensity difference of the options and pricing domestic electricity at $0.15 per kwh, the annual energy cost savings will only be in the order of ten dollars. However, dollar savings can be made during the installation phase. Labour costs would be similar (apart from the header tank option) but material costs would be reduced. Low power pumps would typically cost less than half of a normal high pressure pump (compare for instance a HP Onga pump JSP110 for $589 and a Pondmate LP pump for $69 ( Low pressure 32 mm plumbing is also cheaper than 20 mm HP piping (roughly $1/m compared to $4/m based on private communication with retailers such as Tradelink). Typical savings can therefore be in the order of $500 or more for the low pressure option. The header tank option, including the cost and installation of the tank, would still give similar costs to the standard high pressure option. DISCUSSION AND CONCLUSION The energy intensity of the various water delivery options for domestic appliances is an important factor when considering the energy efficiency of the possible solutions. Even though the energy used in supplying domestic water to such appliances as toilet cisterns Solar09, the 47 th ANZSES Annual Conference Page 8

9 and garden irrigation is not high in absolute terms, the relative energy intensities and hence efficiencies of the options vary greatly. The energy intensities of the various options explored in this paper together with comparisons to mains supply and desalination are given in Tab. 2. Option Energy Intensity (kwh/ml) Tab. 2: Comparative energy intensities of various supply scenarios. High Pressure Conventiona l system Standard Cistern Pond/Shrflo Low Pressure Valve Pond/Shrflo Header Tank Sydney Water Desal /115 70/ It is clear that the traditional high pressure method of rainwater tank pumping is very energy intensive, over six times that of the mains supply by Sydney Water, with desalination being the only more energy intensive option for water supply. A desalination plant would be an energy intensive and expensive option in comparison to even traditional rainwater tank systems. For the average urban dwelling, the actual energy used in pumping rainwater with a conventional high pressure system is relatively small (approx. 70 kwh/year) equating to some $10 - $15 per year. However, the relative efficiencies of pumping through the use of low pressure systems as measured by energy intensity has been clearly shown. Savings of ~90% or greater have been easily demonstrated and have the added bonus of a reduced installation cost thus saving on building costs. The use of a header tank has the potential to push energy saving as high as 98% with the additional benefit of having essentially a fixed pumping head and flow rate. This simplifies the design of such systems. Although for the individual user the energy savings may appear modest, if large numbers of such systems are employed then there is the opportunity for a significant reduction in GHG emissions associated with water supply, as the low pressure systems are capable of supplying water at an energy intensity that is ~85% lower than that of mains supply. REFERENCES Çengel, Y.A. and Cimbala, J.M. (2006), Fluid Mechanics, Fundamental and Applications, McGraw Hill, New York. Coombes P.J., Kuczera G., and Kalma J.D., (2002). Economic, water quantity and quality results from a house with a rainwater tank in the inner city. Proceedings of the 27th Hydrology and Water Resources Conference. Melbourne, Australia. Marsden Jacobs Associates, The economics of rainwater tanks and alternative water supply options. Prepared for the Australian Conservation Foundation, Nature Conservation Council (NSW) and Environment Victoria. April (accessed May, 2009) Onga Pumps Data Sheets, html (accessed May, 2009) Solar09, the 47 th ANZSES Annual Conference Page 9

10 Pondmate Data Sheets, (accessed May, 2009). Shurflo Pump Data Sheets, (accessed May, 2009). Sproul, A.B. (2007), Energy Efficient Rainwater Systems, ANZSES Conference 2007, Alice Springs. Sydney Water, Annual Report (accessed May, 2009) BRIEF BIOGRAPHY OF PRESENTER Dr Alistair Sproul, BSc (Hons I) (Syd) PhD (UNSW) Alistair Sproul is Associate Professor and Postgraduate Coordinator within the School of Photovoltaic and Renewable Energy Engineering at UNSW. His current research interests are in the area of PV/energy systems for low energy buildings and highly efficient water pumping systems. He has worked in the area of photovoltaic research and R&D since 1985 in a range of positions with various companies (BP Solar, Pacific Solar) and research institutions (UNSW, Fraunhofer Institute for Solar Energy Systems). Since 2001 he has been strongly involved in developing and delivering the undergraduate program within the School of Photovoltaic and Renewable Energy Engineering at UNSW. Solar09, the 47 th ANZSES Annual Conference Page 10