Water Distribution Strategy

An energy assessment was previously done for a fruit farm in the Western Cape. It was found that 78% of all the electricity consumed was for pumping irrigation water from the water sources to the orchards. This case study is about the follow-on project to optimize the cost of water distribution on the farm.

All the water used for irrigation is from rainfall. The water is collected in 8 buffer dams/reservoirs which is also topped up from 10 boreholes. A network of pumping systems distributes the water from these sources to the orchards, as shown in the diagram below.

 

The cost of water distribution was obtained from the cost of electricity, the energy intensity (kWh/m3) and the flow rate per pumping system. This cost per cubic meter from primary sources to reservoir is given in the graph below. It shows a variation between 50c/m3 and R2.00/m3, with the higher cost water sources clearly indicated.

The same analysis was done for the secondary distribution of water (from reservoir to orchard). In this case the results were normalised to take into effect the differences in pumping heights and pumping distances. The results are shown in the graph below. The variation in cost is between 20c/m3 and 80c/m3.

The final step was to link the primary and secondary chains to obtain the total cost for water distribution to the orchards. This analysis also included the volume of water pumped from each primary and secondary source. The results in the graph below show the total cost per cubic meter for the orchards serviced from the secondary sources. It is clear that there is a huge variation in water distribution cost between the different applications.

These results were then used to develop a water distribution strategy to minimize the total energy cost for water distribution on the farm. The strategy included aspects such as:

  • Using the “correct” amount of water per orchard. Too much water is a waste of energy cost and water, and too little water may reduce the production output of the orchard.
  • Give preference to the lower cost pumping systems for water distribution.
  • Improve the efficiency of higher cost pumping systems and distribution lines.
  • Utilize the lower cost storage dams/reservoirs more than others.
  • Combine electricity supply points to lower the unit cost of electricity.
  • Use solar power solutions to replace the more expensive electricity supply points.

Pumping System Optimization

Two Oceans Aquarium is located at the V&A Waterfront in Cape Town. The purpose of the aquarium is to display marine life to the public. It is important that the water in the tanks are clear for display purposes to enhance the experience of the visitor. Animal health is also a major concern and the condition of the water influences the animal health. Filtration systems are used to clean the water, and the circulation through the filters are enabled by pumping systems for each display tank.

The focus of the assessment was to closely examine the pump system at Two Oceans Aquarium, make a series of measurements and refer to the on-site personnel for information regarding the specifications of the system as well as the methods of control. This information gathered would then be analysed to determine the potential for optimisation as well as quantifying any financial benefits that could be achieved in terms of cost of implementation and ROI (return on investment)

The pumping system at the predator tank has three 15kW pumps running in parallel. Each pump is circulating water through two banks of filters, 7 filters on each bank. Each pump is equipped with VSD control and the contents of the 2 million litre tank is circulated through the filters every 5 hours

The purpose of the filtration system is to provide clean water which is important for animal health and visitor experience. The clarity of the water is determined by the bio load in the tank, feeding of animals, top-up seawater, the effectiveness of the filter and the flow rate of water through the filter.

A schematic diagram of the system is given below. The suction side has two lines. A strainer is installed before each pump to avoid large rocks or stones to enter the pump. A pressure gauge is available before and after the pump. The pumps are heavily throttled to avoid cavitation. Cavitation should be avoided because it causes bubble sickness at the animals. Pressure gauges are also installed before and after the filter banks. The return flow is combined into a header and filtered water return to the tank via one line.

Figure 1 Schematic diagram of predator tank system

Discussion of Pump Efficiency Results

It should be noted again that the filtration media has been changed a few months ago, from sand to the OC-1 media. The OC-1 media has less resistance to water flow through the filters. The reduction of back pressure has caused cavitation, and this is avoided by heavy throttling of the pumps. There are currently two levels of inefficiency in the pumping system:

  • At the current operating point, the pump is running at 43,7% efficiency. The 15kW pump motor is consuming 8,6kW of power while the optimum is 4,6kW for the same flow and head conditions.
  • The system requires a head of 4,2m from each pump to operate under current conditions. In order to avoid cavitation, the pump is throttled to provide a head of 10,5m. This is another inefficient condition and a waste of energy.

The energy waste is graphically shown in the graph below. Energy is represented by a Constant x Head x Flow, which can be represented by various areas on the graph. The energy use at the current condition A is shown by the outer rectangle. The middle rectangle B indicates the optimum energy use at the current condition, which is 47% less than the actual consumption. The bottom rectangle C indicates the energy that the system requires to work under current conditions. This is only 22% of the actual consumption.

Figure 2 Energy Waste

 

The alternative solution shows that a higher flow rate can be obtained with a lower pump speed combined with less throttling. The maximum flow rate at 45Hz was 116m3/hr before cavitation sets in. By reducing the pump speed to 39 Hz a flow rate of 136m3/hr could be reached before cavitation sets in.

The results show that the change in filtration media from sand filter to OC-1 filtration media had a severe impact on pumping system efficiency. The pumps and motors are oversized for the new filtration media and must be throttled to avoid cavitation. The effect is that 25,8kW is consumed by the pumps for a total flow rate of 316 m3/hr through the filters. The same flow rate can be achieved with only 5,7kW consumption if more suitable pumps are installed.
A summary of the estimated saving of 78% and recommendations are contained below: