Wind-diesel power systems can vary from simple designs, where wind turbines are connected directly to the diesel grid with a minimum of additional features, to more complex systems. Two overlapping concepts define the system design and required components: the amount of energy that is expected from the wind system (system penetration) and the decision to use thermal loads and/or a storage device to remedy system energy fluctuations. Given today’s technology, these issues are usually determined by the system designers as a starting point for overall system design. These concepts are described in the following section.
When incorporating renewable-based technologies such as wind onto a diesel grid, the amount of energy that will be obtained from the wind resource relative to the diesel generators must be determined, because this will dictate which components will be used. A three level classification system has been developed that defines different levels of penetration on the grid. These classifications, defined as low, medium, and high penetration, separate systems along power and system control needs (see table 1 below).
|Penetration Class||Operating Characteristics||Instantaneous Penetration||Average Penetration|
|LOW||• Diesel runs full-time • Wind power reduces net load on diesel • All wind energy goes to primary load • No supervisory control system||< 50%||< 20%|
|MEDIUM||• Diesel runs full-time • At high wind power levels, secondary loads are dispatched to insure sufficient diesel loading or wind generation is curtailed • Requires relatively simple control system||50%–100%||20%–50%|
|HIGH||• Diesels may be shut down during high wind availability • Auxiliary components are required to regulate voltage and frequency • Requires sophisticated control system||100%–400%||50%–150%|
Table 1: Penetration Class of Wind-Diesel Systems (Proposed by Steve Drouilhet)
Wind-Diesel Power System Configurations
The wind farm in Kotzebue is one of many low-penetration systems that have been installed worldwide. Low-penetration systems vary from small to relatively large isolated grids. Some large grids, such as those found in certain areas of the United States and Europe, reach a wind power penetration that would classify them in the same category as low-penetration systems. In low-penetration systems, the wind turbines act as just another generation source, requiring no special arrangements.
The control technology required at this level of generation is trivial, especially given the control, flexibility, and speed of modern diesel and wind systems. In many systems, no form of automated control is required; the wind turbines act under their existing controllers, and an operator monitors all system functions. Because the diesel engines are designed to allow for rapid fluctuations in power requirements from the load, the addition of wind has very limited impact, if any, on the ability of the diesel control to supply the remaining difference. Issues of spinning reserve, a term used to represent the availability of instantaneous system capacity to cover rapid changes in system load or energy production, are addressed by the allowable capacity of the diesel engines, which in many cases can run at 125% rated power for short periods of time with no adverse impact.
Systems with larger ratios of wind power fall into this category. The concept is that by allowing power penetrations above 50%, any under-loaded diesel generators in power plants consisting of multiple generators will be shut off and, if necessary, a smaller unit will be turned on. This in turn will reduce plant diesel consumption and diesel engine operation. It might also make the system vulnerable to potential shortfalls, assuming the loss of one or more of the wind generators or diesel engines. In addition, with a large penetration of energy being produced by the wind turbines, it will become harder for the operating diesel units to tightly regulate system voltage and maintain an adequate power balance. There are options to insure that the high-power-quality requirements of the power system are maintained, even with half of the energy provided by wind. Some of these options include power reduction capabilities within the wind turbine controller, the inclusion of a secondary load to insure that no more than a specified amount of energy will be generated by the wind, installation of capacitor banks to correct power the factor, or even the use of advanced power electronics to allow real-time power specification.
Spinning reserve on medium-penetration power systems requires experience with regard to proper power levels and system commitments, but is not considered technically complex. Such spinning reserve issues should be handled on a case-by-case basis. They can be partially resolved through the use of advanced diesel controls, the installation of a modern, fuel-injected diesel engine with fast start and low-loading capabilities, controlled load shedding or reduction, power forecasting, and proper system oversight. Combined with the use of variable-speed or advanced-power conditioning available on many modern wind turbines, the control requirements of medium-penetration systems are relatively simple. The ability to provide high power quality in medium-penetration power systems has been demonstrated for years in a number of critical locations. The most notable examples are the military diesel plants on San Clemente Island and Ascension Island, and the power systems in Kotzebue, Toksook Bay and Kasigluk, Alaska. All of these systems have experienced power penetration at or above the guidelines set for medium penetration systems.
Although demonstrated on a commercial basis, high-penetration wind-diesel power systems require a much higher level of system integration, technology complexity, and advanced control. The principle of high-penetration systems is that ancillary equipment is installed in addition to a large amount of wind capacity (up to 300% of the average power requirements), so that the diesel can be shut off completely when there is an abundance of wind power production. Any instantaneous wind power production over the required electrical load, represented by an instantaneous penetration over 100%, is supplied to a variety of controllable secondary loads. In these systems, synchronous condensers, load banks, dispatchable loads (including storage in the form of batteries or flywheel systems), power converters, and advanced system controls are used to insure power quality and system integrity. Spinning reserve is created through the use of short-term storage or the maintenance of a consistent oversupply of renewable energy. Although these systems have been demonstrated commercially, they are not yet considered a mature technology and have not been demonstrated on systems exceeding 200 kW average load. Wind-diesel systems that employ the high-penetration system are operating in St. Paul and Wales. Because of the large overproduction of energy, high penetration wind-diesel systems are economically feasible only if there is a use for the additional energy generated by the wind turbines. In the case of Alaska, this extra energy can be used to heat community buildings and homes (thermal energy), displacing fuel oil. Another use could be to power electric or hybrid cars, ATV’s, and snow machines.
Storage use in high penetration wind applications
Until recently, it was assumed that high penetration wind-diesel systems without storage were only theoretically possible. This is no longer the case. Commercially operating short-term storage and no storage systems have been installed in recent years, demonstrating that both technology choices are viable.
In systems incorporating storage, the storage is used to cover short-term fluctuations in power. During lulls in wind generation, the battery bank or other storage device supplies any needed power. If the lulls are prolonged or the storage becomes discharged, a diesel generator is started and takes over supplying the load. Studies have indicated that most lulls in power from the wind are of limited duration, and using storage to cover these short time periods can lead to significant reductions in the consumption of fuel, generator operational hours, and generator starts. The storage system does not necessarily need to be able to carry the full community load, since in larger systems the storage is only used to smooth out lulls in wind energy or to buy enough time to start a standby generator. In these cases, the storage capacity should be approximately the same size as the smallest diesel and have an accessible capacity up to 15 minutes.
In wind-diesel applications, the requirements for storage systems will depend on local wind resources, the costs of different components, capacity and power response times, and the power system performance required. Different storage options are discussed in a separate section of this report, but care should be taken to insure that the storage technology selected meets the specific needs of the particular wind-diesel application.
All high-penetration systems, with and without storage, have been installed in northern climates where the extra energy can be used for heating buildings or water, displacing other fuels. In these systems, it may be wise to install uninterruptible power supplies (UPS) on critical loads. Although only a limited number of systems have been installed, the concept is economically attractive and has the potential to drastically reduce fuel consumption in remote communities in Alaska.
Links and Resources
- Alaska Wind-Diesel Applications Center. The Alaska Center for Energy and Power (ACEP) at the University of Alaska has developed a Wind-Diesel Applications Center. This Center represents a partnership between ACEP, AEA, several other state organizations involved in wind-diesel technologies, and NREL. The Center’s mission is to advance technology in wind energy and wind-diesel integration for the benefit of Alaskans.
- Fay, Ginny, Katherine Keith, and Tobias Schwörer. "Alaska Isolated Wind-Diesel Systems: Performance and Economic Analysis." June 2010. http://www.iser.uaa.alaska.edu/Publications/wind_diesel10022010.pdf. Ginny Fay and Tobias Schwörer of ISER and Katherine Keith of ACEP prepared this document for Alaska Energy Authority and Denali Commission, assessing the performance and economic feasibility of wind-diesel systems in rural Alaska.
- Other papers, presentations, and reports can be found in the ACEP Database. Use search term "wind diesel."