Increasing wind energy penetration level using pumped hydro storage in island micro-grid system
© Islam; licensee Springer. 2012
Received: 15 October 2011
Accepted: 18 June 2012
Published: 18 June 2012
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© Islam; licensee Springer. 2012
Received: 15 October 2011
Accepted: 18 June 2012
Published: 18 June 2012
Ramea is a small island in southern Newfoundland. Since 2004, it has a wind-diesel hybrid power system to provide power for approximately 600 inhabitants. In this paper, wind speed data, load data, and sizing of pumped hydro system at Ramea, Newfoundland are presented. The dynamic model of wind turbine, pumped hydro system, and diesel generator are included in this paper. The dynamic model is simulated in SIMULINK/MATLAB to determine the system voltage and frequency variation and also to visualize different power outputs. Sizing of pumped hydro system indicates that a 150-kW pumped hydro storage system can be installed in Ramea to increase the renewable energy fraction to 37% which will reduce non-renewable fuel consumption on this island. Also, it is found that a pumped hydro energy storage system for Ramea is a much better choice than a hydrogen energy storage system. Such a system will have a higher overall efficiency and could be maintained using local technical expertise, therefore, a more appropriate technology for Ramea.
Three Caterpillar D3512 diesel engines (Peoria, IL, USA) are the main power source of the isolated wind-diesel power plant . Diesel generators are with the following specifications: 4.16 kV, 1,200 rpm, and 925 kW with a power factor of about 0.85. Each of the diesel units (CAT 3512, 1,400-hp engine with a 925-kW generator) is equipped with an automatic voltage regulator and a governor system. Two frequency control modes are used including (1) a speed-droop characteristic for fast load following capability and (2) an isochronous mode for load sharing and frequency regulation. One or two of the three diesel generators are normally required to supply the local community load. Parallel operation and cycling periods of the diesel generators are coordinated by the diesel generator's master controller. The wind energy system consists of six Windmatic wind turbines (OR, USA), a 200-kW controllable dump load, and six capacitor banks. The Windmatic WM15S is a horizontal axis, two-speed, upwind turbine which uses two induction generators, a 65-kW and a 13-kW unit for the energy conversion and direct connection to the distribution system. A 30-kvar fixed capacitor bank is connected in parallel with the output of each wind turbine to partially compensate for the reactive power needs of the induction generators. The start-up of the 65-kW wind turbines is currently assisted by the diesel plants; each wind turbine operates as a motor until it is accelerated beyond synchronous speed, at which point it begins to generate power. The diesel plant also provides the balance of the reactive power, while the capacitor banks are switched on/off to correct the wind plant's power factor to above 0.9.
Annual average wind speed at 10 m in height in Ramea is about 6.08 m/s. This report presents an overview of present pump hydro storage facilities in the world. Analysis of the present wind diesel hydrogen hybrid power system is included. A new pumped hydro storage system is proposed for the community. Based on the recent load and wind data, sizing of a pumped hydro storage (PHS) system is presented. For sizing and analysis, the National Renewable Energy Lab software called HOMER  is used. After determining the size of the PHS, the dynamic modeling and simulation of the PHS were carried out to determine the expected system voltage and frequency variations. System design and analysis is presented along with some future policy suggestions. Several studies and simulation have been carried out on pumped hydro storage system [3–7] in the past years.
The new Ramea wind diesel hydrogen system is still under construction. In 2009, we visited the site and collected the site and system data. An analysis of the current configuration of Ramea hybrid system was done using HOMER.
Excess electricity from the wind turbines is converted to hydrogen using an electrolyzer. Hydrogen is compressed and stored in three large tanks. Stored hydrogen is used to generate electricity when needed. All system parameters including costs were obtained from NL Hydro. System expected performance was analysed, and it is presented in Figure 5. It shows that the expected renewable fraction is 37% and the cost of electricity would be $0.248/unit. One 925-kW diesel is always running. A second diesel would be needed only for 12 h in a year. A hydrogen generator will run only for 702 h in a year. Expected electrical performance is shown in Figure 5. It shows that the hydrogen generator will contribute only 1% of electricity to the system, while the electrolyzer will consume 10% of the system electricity. In other words, most of the electricity taken by the electrolyzer will be wasted in the system.
It shows that wind turbine will produce 37% of the total energy while the remaining 63% will come from the diesel plant. Expected diesel consumption is also shown in Figure 8, and it is less than what is shown in Figure 5. Figure 8 shows that most of the electricity will be produced by wind energy during the winter season, and less energy should be expected during the summer season. Therefore, during summer, most of the electricity demand will be met from the diesel. Figure 8 also shows that the total electrical energy required for that area is 4,281,096 kW h/year, and 37% of that will be met by the wind energy including the system peak demand. Figure 8 shows the monthly statistics and expected frequency histogram of battery state of charge. On average, the battery will be 70% charged (i.e., upper reservoir will be 70% full). The battery will be most used in July and August, while it will be least used in May. Effectively, this battery represents a pump hydro energy storage system with an overall efficiency of 70%. During our visit to Ramea in September 2009, we looked for small ponds or lakes which can be used for a pumped hydro installation. We found two small ponds whose elevation is a few meters above sea level. A head of few meters is not good for a pump hydro installation. The hills in Ramea can be used for such a purpose.
For a head, h = 63 m, required reservoir size will be V = 3,932 m3 (equivalent to 500 batteries). Figure 10 shows the topographical location of Man of War Hill in Ramea. From Figure 10, it can be seen that about 2,000-m2 area is available to build a hydro storage reservoir.
It shows that we might be taking power from the reservoir for more than 3 h.
The amount of power available from a micro hydro power system is directly related to the flow rate, head, and the acceleration due to gravity. From the above maximum daily average power of 147 kW to approximately 150 kW, the usable flow rate can be calculated using the equation . Here, P = power output in kilowatts (150 kW), Q = usable flow rate in cubic meter per second, H = gross head in meters (63 m), g = 9.81 m/s2, and η = hydro turbine efficiency which is equal to 70%. From the above equation, we determined the flow rate Q = 0.347 m3/s. The minimum operating time for the hydro turbine will be equal to 3,932 m3/0.347 m3/s = 11331 s = 3.14 h. Figure 11 shows a possible site for a pump hydro facility. The proposed pumping and generating station could be about 100 m from the top reservoir and about 60 m from the existing 4.16-kV transmission line.
Hydro electricity generation is considered as an established renewable technology. A water flow from an upper to a lower level represents a hydraulic power potential. Pumped storage plants utilize a reversible pumping turbine to store hydro energy during off-peak electricity hours by pumping water from a lower reservoir to an upper reservoir. This stored energy is then used to generate electricity during peak hours, when electricity is costly to produce, by flowing water from the upper to the lower reservoir. The pumped hydro storage system will store excess energy during the off-peak time which will be produced by six 65-kW wind turbines, three 100-kW wind turbines, and a diesel plant. The stored energy will be used to produce electricity during peak times throughout the day. Hydro turbines can be broadly categorized into either impulse or reaction turbines. Figure 12 shows a guide for selection of hydro turbine. For the Ramea site, the expected flow rate is 0.347 m3/s, and head is 63 m; therefore, the best selection from Figure 12 is a pelton- or turgo-type turbine.
From Figure 8, it can be seen that only one diesel generator is running at a time; that is why dynamic simulations were done, considering only one diesel in the system. Figure 14 shows 390- and 300-kW blocks representing all 65-kW and all 100-kW wind turbines in Ramea, respectively. Here, the pump is considered as a 150-kW centrifugal pump with induction machine, and hydro turbine is considered as a 150-kW unit with a synchronous machine coupled to the system bus. Community load is considered as a constant load, and the system was simulated for 24 s. It took a quad core processor-based computer more than 3 h to complete one 24-s simulation. The system was studied for a varying wind speed.
The diesel engine tried to maintain the power balance by running faster, and the system frequency increased to 61 Hz. At t = 5 s, the pump was switched off, and the system came back to its original state within a few seconds. At the 11th second, the hydro turbine is switched on for 5 s. This leads to another transient in the system. Figure 16 also shows the expected variation in the system voltage, frequency, and current during this transient. It can also be noted in Figure 16 that a small variation in the wind speed is not leading to any significant transient in the system. Figure 15 shows the expected power variation in the system. It can be noted in Figures 15 and 16 that simulation was not perfectly converging between t = 11 s and t = 16 s. Significant time was spent to resolve this issue, but this remained unresolved. Various methods of integration were tried but all led to similar results.
Figure 15 shows the power output from all wind turbines. Wind plant operation results in fluctuating real and reactive power levels and will result in voltage and current transients. Changes in the mean power production and reactive power needs of a wind turbine can cause a steady state voltage and frequency change in the connected grid system. The voltage and frequency variation at load end is given in Figure 16. From Figure 16, it can be seen that the expected voltage variation and the frequency variation due to wind speed variation is negligible although switching of large load or a hydro generation unit will cause large voltage and frequency variations in the system. According to power quality standard EN 50160, the voltage variation at the customer's end in remote locations is required to be within +10% . Some voltage variation can be counteracted by adjusting the power factor of the wind turbines . Therefore, the expected voltage and frequency variations in the proposed Ramea wind diesel pump hydro generation system are within current power quality standard.
The current hybrid power system in Ramea consists of a diesel plant, wind turbines, and a hydrogen-based energy storage system. Our analysis indicates that yearly contribution from the hydrogen storage to the system will be less than 1%. We propose a pumped hydro based energy storage system for Ramea. Our study shows that a 4000 m3 water reservoir could be built on the Man of War hill in Ramea for excess wind energy storage. This reservoir will have a head of 63 m and can use sea water. A 150 kW pumping and turbine station could be built near the base of Man of War hill next to the available water and already existing transmission line. Our analysis indicates that such a pumped hydro energy storage system will lead to a 37% renewable energy fraction and will result in acceptable electrical transients in the system. Using a synchronous machine-based turbine unit and an induction motor-based pumping unit, a pumped hydro energy system can be directly connected to the system. A variable speed system  is also a possibility. We believe such an energy storage system can be built and maintained using locally available technology and manpower. Therefore, we recommend a full feasibility study of such an energy storage system for Ramea, Newfoundland. We also believe that pumped hydro energy storage is the best energy storage option for many diesel communities in Newfoundland and Labrador.
The author thanks Newfoundland Labrador Hydro for providing the site load data and Harice Research Center MUN for providing financial support for this research.