Power - Wind turbine - 390 kW - Isolated-grid / Canada

Case study assignment

You are a developer investigating the possibility of building a wind farm to provide electricity to an off-grid, island community. The community currently runs one or more of three installed 925 kW diesel generators, with an average fuel efficiency of around 4 kWh/litre, to meet its electricity requirements. You would like to approach the utility that operates this system, and offer to reduce their fuel consumption, when the wind is blowing, by feeding onto the community grid the output of six reconditioned 65 kW stall-regulated turbines.

You are aware that the diesel generators' operation at light load is not recommended because of reliability concerns, risk of failure, and premature generator ageing; and expect that the utility will be reluctant to operate their generators at a power level below 30% of their rated capacity. To accommodate this, your system will incorporate a dump load and automatic controller that will dissipate surplus wind farm output as heat whenever the output of the turbines is greater than the portion of the community load that is in excess of the minimum generator loading level.

Site information

The island, located about 10 km off the south coast of Newfoundland, Canada, has 700 inhabitants; it was formerly a remote fishing community. Winds are generally strong along this coast, and a weather station located around 23 km from the island indicates a long term average wind speed of 6.5 m/s at a 10 m measurement height. However, the Ramea Island seems to get similar average wind speed at 25 m (turbine's hub height). Winds tend to be significantly higher in winter than summer.

The annual electricity demand for the island is around 4,300 MWh; the peak load is 1,200 kW and the minimum load is around 200 kW. The estimated annual load duration curve for the grid is shown in the figure. Average monthly loads during the winter are nearly double those of the summer.

Financial information

You will ask the utility to compensate you on the basis of the fuel savings they achieve. The current delivered price of diesel fuel is around $0.775 per litre, and you expect this to rise at a rate slightly higher than the inflation rate. The utility will retain the greenhouse gas emissions reductions achieved by the project over its 20-year lifetime.

You anticipate financing around 32% of the project initial costs with debt. You expect to obtain a 15-year loan at a rate of 7.5%. Your company's income tax rate is 35%. You will depreciate the capital costs of the project according to a declining balance method at a rate of 30%. Due to the risks involved in this project, you will not embark on it unless it offers a high rate of return, i.e., exceeding 15%.

Prepare a RETScreen study, documenting any assumptions that you are required to make, and report on the significant conclusions from this analysis.

Solution

The worked-out solution is the data file selected from within the RETScreen Project Database. The user automatically downloads the Project Database file while downloading the RETScreen software.

Teacher's notes

• Average temperature and atmospheric pressure data from Burgeo, Newfoundland, have been used.
• The power curve for a Windmatic wind turbine has been used. This turbine is not in the RETScreen database, but results will be essentially unchanged if another 65 kW stall regulated turbine power curve (such as the Entegrity Wind Systems AOC 15/50) is used in its stead. Twenty-five metre lattice towers are typical for these relatively small machines.
• The wind energy absorption rate is included in Miscellaneous losses and is rather difficult to estimate based on the information provided. The job is simplified if it is initially assumed that the wind farm power output and the community load are completely uncorrelated. Since it is known that there is a positive correlation between load and wind speed, this assumption should cause underestimation of the absorption rate.
• A simple approach to calculating the wind energy absorption rate is outlined below. It should be noted that while this method happens to yield reasonable results in this case, it is not rigorous and could be seriously in error in other situations.
  • Set the wind energy absorption rate to 100% and calculate the average windfarm output based on the capacity factor provided by RETScreen (i.e., 31% capacity factor with 100% absorption x 390 kW rated capacity yields an average output of 121 kW). This means that the energy absorption portion of the miscellaneous losses are 0%, miscellaneous losses are 2-6%.
  • Find the "typical" power output of the combination of the windfarm and the generator operating at its minimum loading level (i.e., 121 kW plus 280 kW totals approximately 400 kW).
  • Assume that at community loads above 400 kW all the turbine output is used by the load, and below 400 kW it is all dumped. The load duration curve indicates that the load is below 400 kW around 35% of the time, implying an absorption rate of around 65 %.
• The wind energy absorption rate has been adjusted to 70% (i.e. miscellaneous losses of 30%) based on the observation that positive correlation in the windfarm output and the community load will cause the above two methods to underestimate the absorption rate. More accurate results could be obtained if seasonal average wind speeds and load duration curves were known.
• Operation and maintenance costs have been estimated by multiplying the renewable energy collected (803 MWh) by a lumped maintenance cost of $0.025/kWh.
• The electricity export rate is found by dividing the price of fuel, per litre, by the energy generated per litre of fuel, i.e. $0.19/kWh.
• The cost of $1,200/kW for refurbished wind turbines was used. This is considerably lower than new turbines of the same rating, which are around $2,200/kW.

Real project

Results

In 2004, Frontier Power Systems constructed a small windfarm near the community of Ramea, Newfoundland, Canada. The community of 700, a fishing village, is located on an island 10 km off the south coast of Newfoundland. Prior to the construction of the windfarm, all electricity was supplied by diesel generators, owned and operated by Newfoundland and Labrador Hydro. The windfarm produces 10 to 13% of the 4,300 MWh consumed annually by the community, thus reducing the amount of fuel purchased for the diesel generators. Newfoundland and Labrador Hydro pays Frontier Power Systems according to the diesel fuel savings realized by the wind farm. As an innovative renewable energy project, the Ramea system received financial assistance from Natural Resources Canada to test the new Wind-Diesel Integrated Control System (WDICS). For more information, view the RETScreen Video (2:02 minutes).

System description

The windfarm consists of six 65 kW wind turbines, a dump load, and an advanced automated control system. The turbines are reconditioned Windmatic WM15S stall regulated machines mounted on lattice towers; the hub height is 25 m. Used turbines were employed in order to keep initial costs down.

The WDICS controller is responsible for, among other functions, adjusting the power dissipated in a variable dump load. The load is adjustable in 1 kW increments between 0 kW and approximately 200 kW. The control system dumps wind farm output in order to keep the diesel generator operating at no lower than 30% of its rated capacity; below this loading, the generator operation is hazardous and unreliable, also the generator may wear prematurely. A sophisticated monitoring system monitors and logs the operation of the windfarm. The WDICS controller, supplied by Frontier Power Systems, is based on the technology developed by the Atlantic Wind Test Site, a Canadian government facility on Prince Edward Island.

Lessons learned

• The wind turbines do affect the operating regime of the diesel generators, but have little effect on the grid power quality (even during turbine connection).
• Significant delays may be encountered due to regulations, bureaucracy, and uncooperative weather. Land lease from the Crown, provincial and federal environmental assessments. Storms during winter impeded construction and low summer winds delayed commissioning.
• The utility prepares an Operating Agreement as part of any Power Purchase Agreement (PPA). The Operating Agreement defines the terms of the agreement between the utility and the Non-utility Generator for the operation, maintenance, and supply of power from the Generators facility. The operating agreement also defines the operating procedures and provides further clarification of the conditions of service outlined in the PPA. It states the minimum requirements for safe and effective parallel operation of the utilities system with the Generator's Facility and is intended for use by the Generator and the Utility when operating equipment which will have an effect on the other party's equipment or system.
• Wind-diesel systems are still a fairly novel technology, and problems are inevitable in early demonstration projects. The blade pitch setting of the turbines used in this project resulted in poor turbine output at high wind speeds and needed adjustment following commissioning. The use of reconditioned turbines, while decreasing costs, did impact reliability.
• Even in an area known for strong winds, there can be significant variation in the wind regime over a fairly short distance. When the scale of a wind-diesel project does not justify an on-site wind resource measurement, and data from a somewhat distant weather station is used instead, the project becomes more risky.
• Newfoundland and Labrador Hydro (NHL) had already automated the diesel power plants by using relatively new diesel-generator sets that are equipped with advanced digital control and protection systems. The diesel plant automation straightened the integration process of the wind and the diesel generation and saved additional "interconnection" costs.
• NHL has a "sharing-the-savings" policy which means, for example, if the wind project developer installs a project for $0.12/kWh and the avoided cost is $0.20/kWh then the power purchase agreement will be based on $0.16/kWh. Also, if the plant efficiency is 4 kWh/litre and the efficiency drops to 3.6 kWh/litre when the wind plant is connected, the power purchase agreement will shift the costs to the wind developer. However, for the Ramea wind-diesel project, the total avoided cost will be paid to the wind developer, since it is the first Canadian project to demonstrate the wind-diesel technology. The purchase price details and policy are normally given as part of Power Purchase Agreement (PPA) between the utility company and the non-utility developer.
• The use of refurbished wind turbines is critical in determining the economic feasibility of the project. The lower initial cost reduces the payback from 15 years to around 7. However, the availability of refurbished turbines and their cost may vary considerably.
• There is substantial financial risk involved when embarking on remote wind projects. A number of factors can easily put an end to a seemingly profitable venture, particularly at the construction and commissioning phase. Access and weather are major considerations in planning and construction of these projects and should not be underestimated when evaluating the overall risk involved.

The big picture

Wind systems have been integrated into only a small number of diesel generator-powered isolated grids. Canadian electric utilities have concerns on the reliability and power quality of the wind-diesel systems. The main concerns are dealing with an unproven technology which can adversely impact system stability and diesel generator operation. In some cases, the wind-diesel power plant may not be financially feasible due to the additional costs of control system upgrading and automation. Successful demonstration projects, designed with the utility's criteria in mind, are required to address reliability and performance concerns; also assist the utilities to gain expertise in control and operation of these systems. Utilities will take more interest in wind-diesel technology when it is clearly a reliable, cost-effective option for off-grid communities.

Photo

Wind turbines - 6 x 65 kW - Isolated island community, Newfoundland, Canada

References

• Brothers, Carl "Personal communication," Frontier Power Systems, PEI, 2006.
• Boone, Keith "Personal communication," Newfoundland and Labrador Hydro, 2006.
• Technology Early Action Measures (TEAM) "Third TEAM Progress Report 2003-2005", page 12, Website: http://www.team.gc.ca/english/.
• Marbek Resource Consultants Ltd., Survey of the Small (300 W to 300 kW) Wind Turbine Market in Canada, CANMET Energy Technology Centre - Ottawa, Natural Resources Canada, July 2005, 103 pp.