Application of water supply operation system to improve efficiency of hydraulic power generation

With a water supply area of 1,239 km² and population of 13.295 million, Tokyo is an extremely congested metropolis (Bureau of Waterworks 2017). The water supply transmission/distribution pipes throughout the metropolis have a total length of 27,038 km: more than two thirds the earth's circumference. To increase resilience to demand fluctuations and disasters, a number of water supply stations are maintained and the majority of transmission pipes constitute a dual supply network. Water transmission/distribution requires large amounts of electrical power, so, to improve the waterworks utilities' sustainability, efforts are made to improve equipment efficiency, operate services with consideration to energy saving, and use renewable energy sources (Bureau of Waterworks 2017; 2016a; 2016b; 2016c; 2014).

Tokyo is characterized by complex terrain with coastal lowlands, undulating plains, plateaus and mountainous areas. When transmitting water to distant points, the pressure can be excessive and pressure reducing valves are installed at supply stations in the middle of the network. In other words, the excess pressure of water flowing into supply stations is adjusted and thus unused. To overcome this, the installation of hydraulic power generation equipment is being promoted in place of pressure reducing valves, to convert the excess pressure to electrical energy. This is a kind of renewable use as it does not produce greenhouse gases and reduces the use of fossil fuels (Masuko et al. 2011; Tanaka 2018).

When installing hydraulic power generation equipment, the data on inflow rate and pressure at each water supply station are examined, in order to determine the turbine design and power output specifications. During examination, full use is made of data accumulated by the water supply operating system developed for Tokyo. Research was conducted so that the water supply operating system would be able to control the optimal inflow rate for power generation. This ensures that sufficient inflow of water for power generation is secure in the long-term without affecting the stability of the water supply. As a result, more effective use can be made of excess pressure.

Tokyo's water supply comes mostly from rivers – 78% from the Tone-Arakawa rivers, and 19% from the Tama River. Interchange between them is possible through the raw water connection pipelines. The aim is for effective use of water in relation to the condition of each river or dam, so, on the basis of this ‘raw water plan’, the daily volume of water distributed is predicted for the following month using results from previous years and seasonal changes.

Treatment plants are operated according to the predictions and the ‘main pipeline operating plan’ determined for transmitting water to water supply stations. If the transmission pipes are part of the dual network system, the routes used are determined on the basis of energy saving and maintenance conditions (Masuko et al. 2012).

Water station distribution pumps are operated on the basis of the ‘distribution pump operating plan’, which takes account of the pressure and volume patterns on weekdays and holidays. Distribution pipeline pressure is constantly maintained at over 20 m, allowing for pipe resistance and demand fluctuation.

The water supply operating system for the waterworks facilities integrates the ‘raw water plan’, the ‘main pipeline operating plan’ and the ‘distribution pump operating plan’ to improve demand prediction. The distribution reservoir operating function is derived from the demand prediction function as well as the water inflow plan. All of these plans are improved by taking into account factors such as the weather forecast, temperature, day of the week and national holidays. Revisions are also made constantly to adjust to real-time demand fluctuations. Figure 1 is an outline of the water supply operating system. Using influent conditions to maximize the hydraulic power generation equipment's potential, makes efficient power generation possible.

The hydraulic power generation equipment has been installed at four water supply stations where sufficient space was readily available – see Table 1.

Figure 2 shows the excess pressure status at Kasai water supply station, which is midway between Kanamachi treatment plant and Tokai water supply station. On average there is about 30 m excess pressure in the portion around Kasai water supply station. To operate the equipment, the influent rate possible for power generation has been determined by considering both turbine efficiency, fluctuations in influent pressure, and vibrations and noise in the turbine due to cavitation. The upper limit is 1.26 m³/s (4,550 m³/h), which is 90% of the maximum usage of 1.4 m³/s and the lower limit 0.58 m³/s (2,100 m³/h), or 42%.

Before installing the hydraulic power generation equipment, the distribution reservoir operation controlled the reservoir inflow rate in line with changing demand. As a result, the distribution reservoir level was kept constant – see Figure 3 (left). With this operating style and flow rates below the power generation threshold, the influent was bypassed and the energy in the excess pressure was unused. At flow rates above the upper power generation threshold, residual influent was also bypassed and energy in the excess pressure remained unused.

Distribution reservoir operation is now implemented, taking the hydraulic power generation equipment into consideration in the planning function of the water supply operating system. As a result, the period when the inflow rate is suitable for power generation is now longer (Figure 3, right). In addition, the influent that was ‘bypassed’ previously is now available for power generation. Effective use is thus made of the reservoir's capacity, like a hydroelectric dam, without affecting water supply stability, although the reservoir water level changes significantly. An example of the monitoring screen for this type of operation is shown in Figure 4.

If the actual level of demand begins to deviate from the predicted volume, the operating system automatically corrects the operating plan. This occurs even when the power generation equipment is running, which ensures both the stability of the water supply and use of the excess energy.

• P = power generated (kW)

• 9.8 = gravitational acceleration constant (m/s²)

• Q = inflow rate (m³/s)

• He = effective pressure drop (m)

• η = power generation efficiency

Here, the inflow rate Q in Equation (3) can be said to be controlled to the optimum for power generation. Depending on the turbine design, the power generation efficiency, η, is affected by the inflow rate, Q. This is particularly true for the Francis turbine, whose characteristic curve has a maximum – see Figure 5. In this case, it is possible to control the inflow rate, Q, so that η becomes optimal for power generation. The indicator that shows power generation efficiency is the original power generation unit, Wh/m³, i.e., the power generated per unit inflow (Wh/m³) increased in FY2015 compared to FY2014 and earlier. Power generation increased between FY2016 and FY2017, despite the decrease in flow volume – i.e., increasing power generation depends not only on increasing inflow volume but also on improving generation efficiency.

Where the transmission pipes are twinned, distribution reservoir operation is also possible. This is done by ensuring that the transmission pipe flow rate to the hydraulic power generation equipment is optimal for power generation while controlling the rate in the twin transmission pipe. These simple principles are enabling improvements in equipment utilization and power generation efficiency.

For the hydraulic power generation equipment at Kasai, the recovery period for the total cost – i.e., the sum of the equipment, construction and maintenance costs (CAPEX + OPEX) – through the gains from power generation was seven years. This period is based on the estimate prior to equipment installation and is expected to be reduced to six years.

The hydraulic power generation equipment at Minami-Senju and Kameido water supply stations was overhauled – Table 1 – 10 years after installation, in each case. No corrosion or abrasion defects were found and neither equipment set had ever broken down or experienced any other trouble. Even after total cost recovery, long-term power generation can be expected.

Application of the water supply operating system has enabled the amount of power generated to be increased and total costs to be recovered early. In other words, it has been demonstrated that the equipment utilization factor and power generation efficiency can be improved by combining the use of hydraulic power generation equipment and the capacity of distribution reservoirs. In addition, greenhouse gas emissions will be reduced for longer, and waterworks utility operations will be more sustainable.

As waterworks utilities become larger, purification plants and supply stations are also allocated to serve larger areas. This means that the use of pressure reducing valves, along with proper pump operation, becomes indispensable. Given Tokyo's undulating terrain, many pressure reducing valves are needed and the potential for hydraulic power generation is high. We will continue to install and use hydraulic power generation for the sustainable development.