##### International Journal of Energy and Environmental Engineering
Original research

# Increasing wind energy penetration level using pumped hydro storage in island micro-grid system

Sheikh Mominul Islam

Author Affiliations

Department of Engineering, Memorial University of Newfoundland, St. John’s, Newfoundland and Labrador, A1B1V3, Canada

International Journal of Energy and Environmental Engineering 2012, 3:9  doi:10.1186/2251-6832-3-9

 Received: 15 October 2011 Accepted: 18 June 2012 Published: 18 June 2012

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

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.

##### Keywords:
HOMER; Hybrid; Electrolyzer; Pumped hydro

### Sizing of a pumped hydro storage system

Sizing of the hybrid power system was done with the help of the optimization software HOMER. The hydrogen energy storage system shown in Figure 7 was removed, and it was replaced with a battery model in HOMER. Overall efficiency of the battery was reduced to 70% to represent a pumped hydro energy storage system. (Typical efficiency of a pumped hydro system is 70%). HOMER sized the system by making energy balance calculations for each of the 8,760 h in a year. Simulated hybrid energy system electrical performance is shown in Figure 8.

Figure 8. Expected electrical performance of the proposed hybrid power system.

For each hour, HOMER compares the electric demand in the hour to the energy that the system can supply in that hour and calculates the flows of energy to and from each component of the system. The system cost calculations account for costs such as capital, replacement, operation and maintenance, fuel, and interest. In the HOMER simulation, it is assumed that the interest rate is 7% and the project lifetime is 25 years. With a condition of 40% renewable energy fraction, the optimized HOMER simulation results including sensitivity variables are shown in Figure 8. Renewable energy fraction for all cases is about 37%. From Figure 8, it is clear that yearly diesel consumption reduces with the battery size. The larger the battery sizes, the less diesel consumption. HOMER suggested that the best battery bank (consisting of Trojan T-105 batteries) should have 500 batteries. Based on the maximum power coming out of batteries, we selected a 250-kW converter between AC and DC buses. The nominal capacity of each T-105 battery is 1.35 kW h. Figure 9 describes the expected energy in and out of the battery storage system.

Figure 9. Expected energy in and out of the battery storage 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.

It appears that a storage system needs to be built on a hill. In the east side of Ramea, we noted a hill named the Man of War (see Figure 10).

Figure 10. Topographical map of Man of War hill.

It is a potential place where a hydro reservoir can be built at 63 m in height as shown in Figure 11.

Figure 11. Possible location of the pump hydro energy storage system in Ramea.

Access to this hill is not an issue as there is a telecommunication tower already installed for the community's communication on the next hill. The Man of War Hill is one of the highest hills in Ramea, and presently, nothing exists there. It has a significant area at the top and is very close to the water, i.e., required length of penstock would be minimal. In a hydro power plant, potential energy of the water is first converted to an equivalent amount of kinetic energy. The potential energy stored in the upper reservoir should be the same as the energy stored in the batteries suggested by HOMER. Figure 8 shows that we will need 500 Trojan T-105 batteries (each 1,350 W h). Hence, the potential energy stored in the upper reservoir should be as follows:

(1)

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.

The height of the reservoir will be 2 m which is a reasonable height for a man-made concrete-based covered pool-type tank. The size of the hydro turbine is determined by observing the output of the converter shown in Figure 12.

Figure 12. Turbine selection based on head and discharge.

From the HOMER time series data, it was found that during April, maximum power flow from the converter is 147 kW. Figure 13 shows expected output of the converter in June.

Figure 13. Output of converter in June.

It shows that we might be taking power from the reservoir for more than 3 h.

### Dynamic simulation of grid-connected PHS

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.

The reference sites [9-11] can be used for a selection of turbine for the pumped hydro storage project in Ramea. Dynamic simulations of the proposed Ramea wind-diesel-pumped hydro system were done to determine the expected power quality. Simulation was also done to find out if an addition of a 150-kW pump hydro unit will greatly impact the stability of the system. Figure 14 shows diesel-wind-hydro hybrid power system in Ramea simulated in SIMULINK.

Figure 14. SIMULINK representation of the diesel-wind-hydro hybrid power system in Ramea.

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.

In order to determine the system voltage and frequency variations, the following equations were used to generate a wind field [12].

(2)

Here, Vw is the wind speed, which is composed of the average wind speed component (Va) and turbulence component (Vt). The average wind speed value is chosen as 8.02 m/s; mw(t) is the random white noise which is used to generate the turbulence component within the wind field. Variable z is the turbine height, and Vo is the maiden wind speed. The wind speed characteristics are applied to the simulation, assuming a constant load of 500 kW at Ramea. During our simulation, it is considered that the pumping motor will operate for the first 5 s, and then, it will be switched off. At the 11th second, the hydro turbine will start, and it will operate for 5 s and then will switch off. It is understood that such fast switching will not happen in a real system. In a real system, such switching will happen in tens of minutes. It was not possible to do dynamic simulations for time in minutes. (a 1-min simulation took more than 3 days of computer time. The computer often hangs up, or simulation code crashes before simulation is done for 1 min.) Switching after 5 s well represents the transient in the system. Simulation for 24 s gave a good idea of expected transient in the system. Figure 15 shows power variations in the system.

Figure 15. Expected power changes in a transient condition in the Ramea hybrid power system.

From Figure 16, a voltage dip and a current surge can be seen for the first 5 s when induction motor is running the pump to store water in the upper reservoir.

Figure 16. Expected transients in the Ramea hybrid power system.

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% [13]. Some voltage variation can be counteracted by adjusting the power factor of the wind turbines [14]. Therefore, the expected voltage and frequency variations in the proposed Ramea wind diesel pump hydro generation system are within current power quality standard.

### Conclusions

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 [15] 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.

### Competing interests

The author declares that he has no competing interests.

### Authors’ information

SMI has completed his Master of Engineering degree from the Faculty of Engineering, Memorial University of Newfoundland. He has obtained his B.Sc. in Electrical and Electronic Engineering from Islamic University of Technology (IUT), Bangladesh. He has served as an instructor in Prime University and in American International University-Bangladesh. He worked as a Cell Planning Engineer for a mobile company in his country. His research interests are in HVac and HVdc power systems, renewable energy systems, hybrid power systems, control system, transmission line, distribution systems etc.

### Acknowledgements

The author thanks Newfoundland Labrador Hydro for providing the site load data and Harice Research Center MUN for providing financial support for this research.

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