The Drainage Services Department of the Hong Kong Special Administrative Region Government is committed to helping mitigate global climate change. Over the years, energy-saving and renewable energy facilities have been installed in some of its plants. With the official launch of Hong Kong’s “Harbour Purification Plan Phase II A”, the Drainage Services Department has installed a hydraulic turbine power generation system at Stonecutters Island Sewage Treatment Plant (the sewage treatment plant with the largest sewage treatment capacity in Hong Kong), which uses the hydraulic energy of flowing sewage to drive the turbine generator, and then generates electricity for the use of facilities in the plant. This paper introduces the system, including the challenges encountered in the implementation of relevant projects, the considerations and characteristics of system design and construction, and the operation performance of the system. The system not only helps to save electricity costs, but also uses water to reduce carbon emissions.
1 Project introduction
The second phase A of the “Harbour Purification Plan” is a large-scale plan implemented by the Hong Kong Special Administrative Region Government to improve the water quality of Victoria Harbour. It was officially put into full use in December 2015. Its scope of work includes the construction of a deep sewage tunnel with a total length of about 21km and 163m below the ground, to transport sewage generated in the north and southwest of the island to the Stonecutters Island Sewage Treatment Plant, and to increase the treatment capacity of the sewage plant to 245 × 105m3/d, providing sewage treatment services for about 5.7 million citizens. Due to land limitations, Stonecutters Island Sewage Treatment Plant uses 46 sets of double deck sedimentation tanks for chemically enhanced primary treatment of sewage, and every two sets of sedimentation tanks will share a vertical shaft (that is, a total of 23 shafts) to send the purified sewage to the underground drainage pipe for final disinfection, and then to the deep sea.
2 Relevant early research and development
In view of the large amount of sewage treated by Stonecutters Island Sewage Treatment Plant every day and the unique double-layer design of its sedimentation tank, it can provide a certain amount of hydraulic energy while discharging the purified sewage to drive the turbine generator to generate electricity. The Drainage Services Department team then carried out a relevant feasibility study in 2008 and conducted a series of field tests. The results of these preliminary studies confirm the feasibility of installing turbine generators.
Installation location: in the shaft of sedimentation tank; Effective water pressure: 4.5~6m (the specific design depends on the actual operating conditions in the future and the exact position of the turbine); Flow range: 1.1 ～ 1.25 m3/s; Maximum output power: 45~50 kW; Equipment and materials: Since the purified sewage still has certain corrosivity, the selected materials and related equipment must have adequate protection and corrosion resistance.
In this regard, the Drainage Services Department has reserved space for two sets of sedimentation tanks in the sewage treatment plant to install a turbine power generation system in the expansion project of the “Harbour Purification Project Phase II A”.
3 System Design Considerations and Features
3.1 Generated power and effective water pressure
The relationship between electric power generated by hydrodynamic energy and effective water pressure is as follows: electric power generated (kW)=[density of purified sewage ρ (kg/m3) × Water flow rate Q (m3/s) × Effective water pressure H (m) × Gravity constant g (9.807 m/s2)] ÷ 1000
× Overall system efficiency (%). The effective water pressure is the difference between the maximum allowable water level of the shaft and the water level of the adjacent shaft in the flowing water.
In other words, the higher the flow velocity and the effective water pressure, the greater the power generated. Therefore, in order to generate more power, one of the design goals is to enable the turbine system to receive the highest water flow speed and effective water pressure.
3.2 Key points of system design
First of all, in terms of design, the newly installed turbine system must not affect the normal operation of the sewage treatment plant as much as possible. For example, the system must have appropriate protective devices to prevent the upstream sedimentation tank from overflowing the purified sewage due to incorrect system control. Operating parameters determined during design: flow rate 1.06 ～ 1.50m3/s, effective water pressure range 24 ～ 52kPa.
In addition, since the sewage purified by the sedimentation tank still contains some corrosive substances, such as hydrogen sulfide and salt, all turbine system component materials in contact with the purified sewage must be corrosion resistant (such as duplex stainless steel materials often used for sewage treatment equipment), so as to improve the durability of the system and reduce the number of maintenance.
In terms of power system design, since the power generation of sewage turbine is not completely stable for various reasons, the entire power generation system is connected in parallel with the grid to maintain reliable power supply. The grid connection shall be arranged in accordance with the technical guidelines for grid connection issued by the power company and the Electrical and Mechanical Services Department of the Hong Kong Special Administrative Region Government.
In terms of pipe layout, in addition to the existing site restrictions, the need for system maintenance and repair is also considered. In this regard, the original plan of installing the hydraulic turbine in the settling tank shaft proposed in the R&D project has been changed. Instead, the purified sewage is led out of the shaft by a throat and sent to the hydraulic turbine, which greatly reduces the difficulty and time of maintenance and reduces the impact on the normal operation of the sewage treatment plant.
In view of the fact that the sedimentation tank occasionally needs to be suspended for maintenance, the throat of the turbine system is connected to two shafts of four sets of double deck sedimentation tanks. Even if two sets of sedimentation tanks stop operation, the other two sets of sedimentation tanks can also provide purified sewage, drive the turbine system, and continue to generate electricity. In addition, a place has been reserved near the shaft of 47/49 # sedimentation tank for the installation of the second hydraulic turbine power generation system in the future, so that when the four sets of sedimentation tanks operate normally, the two turbine power generation systems can generate power at the same time, reaching the maximum power capacity.
3.3 Selection of hydraulic turbine and generator
Hydraulic turbine is the key equipment of the whole power generation system. Turbines can generally be divided into two categories according to the operating principle: pulse type and reaction type. Impulse type is that the fluid shoots to the turbine blade at high speed through multiple nozzles, and then drives the generator to generate energy. The reaction type passes through the turbine blade through the fluid, and uses the water level pressure to drive the generator to generate energy. In this design, based on the fact that the purified sewage can provide low water pressure when flowing, Kaplan turbine, one of the more appropriate reaction types, is selected, because this turbine has high efficiency at low water pressure and is relatively thin, which is more suitable for the limited space on site.
In terms of generator, permanent magnet synchronous generator driven by constant speed hydraulic turbine is selected. This generator can output more stable voltage and frequency than asynchronous generator, so it can improve power supply quality, make parallel grid simpler, and require less maintenance.
4 Construction and Operation Features
4.1 Grid parallel arrangement
The grid connection shall be carried out in accordance with the technical guidelines for grid connection issued by the power company and the Electrical and Mechanical Services Department of the Hong Kong Special Administrative Region Government. According to the guidelines, the renewable energy power generation system must be equipped with anti islanding protection function, which can automatically separate the relevant renewable energy power generation system from the distribution system when the power grid stops supplying power for any reason, so that the renewable energy power generation system cannot continue to supply power to the distribution system, so as to ensure the safety of electrical engineering personnel working on the grid or distribution system.
In terms of synchronous operation of power supply, the renewable energy power generation system and distribution system can be synchronized only when the voltage intensity, phase angle or frequency difference are controlled within acceptable limits.
4.2 Control and protection
The hydraulic turbine power generation system can be controlled in automatic or manual mode. In automatic mode, the shafts of sedimentation tank 47/49 # or 51/53 # can be used as the source of hydraulic energy, and the control system will start different control valves according to the default data to select the most appropriate sedimentation tank, so as to optimize the hydraulic turbine power generation. In addition, the control valve will automatically adjust the upstream sewage level so that the sedimentation tank will not overflow the purified sewage, thus increasing the power generation to the highest level. The turbine generator system can be regulated in the main control room or on site.
In terms of protection and control, if the power supply box or control valve of the turbine system fails or the water level exceeds the maximum allowable water level, the hydraulic turbine power generation system will also automatically stop operation and discharge the purified sewage through the bypass pipe, so as to prevent the upstream sedimentation tank from overflowing the purified sewage due to system failure.
5 Performance of system operation
This hydraulic turbine power generation system was put into operation at the end of 2018, with an average monthly output of more than 10000 kW · h. The effective water pressure that can drive the hydraulic turbine power generation system also changes with time due to the high and low flow of sewage collected and treated by the sewage treatment plant every day. In order to maximize the power generated by the turbine system, the Drainage Services Department has designed a control system to automatically adjust the turbine operation torque according to the daily sewage flow, thereby improving the power production efficiency. Figure 7 shows the relationship between the power generation system and the water flow. When the water flow exceeds the set level, the system will automatically operate to generate electricity.
6 Challenges and Solutions
The Drainage Services Department has encountered many challenges in carrying out relevant projects, and has formulated corresponding plans in response to these challenges,
7 Conclusion
Despite various challenges, this set of hydraulic turbine power generation system was successfully put into operation at the end of 2018. The average monthly power output of the system is more than 10000 kW · h, which is equivalent to the average monthly power consumption of about 25 Hong Kong households (the average monthly power consumption of each Hong Kong household in 2018 is about 390kW · h). The Drainage Services Department is committed to “providing world-class sewage and rainwater treatment and drainage services to promote the sustainable development of Hong Kong”, while promoting environmental protection and climate change projects. In the application of renewable energy, the Drainage Services Department uses biogas, solar energy and the energy from the flow of purified sewage to generate renewable energy. In the past few years, the average annual renewable energy produced by the Drainage Services Department is about 27 million kW · h, which can meet the energy needs of about 9% of the Drainage Services Department. The Drainage Services Department will continue its efforts to strengthen and promote the application of renewable energy.
Post time: Nov-22-2022
