One of the most enduring criticisms of wind power is the inherent variability of the generation source and its accompanying requirements for ancillary services and backup power. Hydro has long been viewed as the perfect match for wind, providing the necessary balancing and reserves in an all-renewable package.
Wind and hydro both vary, but on opposite timescales. Wind varies minute to minute but is highly predictable over the long term while hydro is precisely controllable at any given moment but varies from season to season.
Hydro offers some benefits over thermal generation when considered as a wind enabler. Thermal plants lose efficiency and generate more emissions when they are run below full capacity. Hydro offers near real-time smoothing of wind’s variability along with cheap energy storage in the form of water in reservoirs.
The potential for hydro power to facilitate the integration of more wind resources appears obvious. Given that both technologies have been around for decades, why is there not more integrated wind-hydro generation in places like Europe and North America where both resources are abundant?
the wind+pumped storage proposition
A recent paper by two German academics, Leonhard and Grobe, presented at the 2004 General Meeting of IEEE’s Power Engineering Society, considered what might be termed a panacea scenario. The researchers looked at creating a fully sustainable power system in Europe using a “wind and water” approach combining a massive development of offshore wind and a corresponding expansion of pumped hydro resources. The authors quickly arrived at the conclusion that such a scheme was simply unattainable both in terms of cost and the environmental impact associated with building the enormous pumped storage capacity and transmission enhancements required. However, as the paper notes, such a system is technically feasible.
Another more recent paper by VA Tech Hydro examined the wind + pumped storage proposition in more detail, breaking the problem down by the size of the proposed system. Here the results became more encouraging. While the authors found that small (<20 MW), decentralized pumped storage facilities would not be sufficient to provide the balancing energy needed for a large influx of wind power, they were more optimistic about large pumped storage. In this case, the issue is not the capabilities of the facility itself but rather the surrounding transmission infrastructure. This should be a familiar refrain to those in the northeastern U.S. and other chronically congested areas-not being able to use available generation resources to their fullest extent because of transmission constraints.
The authors also looked at costs and noted that while pumped storage hydro systems have high initial capital requirements, they have relatively low ongoing costs and generally enjoy long operational life. Interestingly, while the economics of localized wind-hydro systems did not work out for the majority of circumstances, the authors pointed to one application where wind-hydro integration offers a compelling alternative to thermal generation: islands. Small island power systems usually rely on costly (and dirty) diesel generators. They are also often blessed with considerable wind resources, whether onshore or offshore. In such circumstances, a wind farm balanced by an appropriately sized pumped storage facility offers not only a vast environmental improvement over diesel generation, but cost savings as well.
Clearly, though, for the wind-hydro proposition to realize its potential, it will have to work on a larger scale. Despite the historical lack of wind-hydro projects, several North American utilities have begun to implement programs designed to leverage the complementary characteristics of wind and hydro power.
wind-hydro integration studies
Bonneville Power Administration currently has three customers using its network wind integration service to incorporate a total of more than 100 MW of capacity from the Nine Canyon and White Creek wind farms. Under the arrangement, BPA will use its hydro resources to provide balancing and backup power, provided the customer has a power purchase agreement in place along with a scheduling agent and firm transmission access. Demand charges and load variance charges remain in effect, but the energy derived from the wind turbines offsets hydro power that BPA would otherwise have provided.
The program works in principle, but BPA’s experience is indicative of what happens when competing interests vie for control of a hydro plant’s operations. Case in point: BPA is currently being forced to spill around 2,000 MW worth of water on the lower Snake and Columbia rivers to meet regulatory requirements for the declining salmon population in those areas. In fact, every year the agency gets operational instructions from the U.S. Ninth Circuit Court of Appeals in order to comply with provisions in the Endangered Species Act. Needless to say, this makes for a lot of legal wrangling and the judge in the case has now created a stakeholder task force that is charged with coming up with a long-term solution that will provide an alternative to the annual court ruling. The task force’s recommendations are due in September, and BPA hopes they will enable the organization to expand its wind integration program. For now, however, the service is officially on hold.
Hydro Quebec’s production and distribution companies signed an agreement in October to use their substantial hydro resources to balance nearly 1,000 MW of wind power that is currently being developed. Under the agreement, Hydro Quebec Production will provide 35 percent of the wind farms’ nameplate capacity, regardless of wind conditions. Then, on an annual basis, HQP will look at the actual amount of energy provided and either bill or credit Hydro Quebec Distribution for the difference, depending on whether the total is above or below the 35 percent threshold.
There is also a capacity payment provision in which HQP looks at the wind production during the 300 hours with the greatest load during the year. If the wind farms during those hours delivered less than 35 percent of their capacity, HQD is billed for the difference at a rate of CAN$80/kW. However, the agreement establishes a contract floor of 15 percent of capacity so that, given the nearly 1,000 MW total nameplate of the wind farms, the maximum charge HQD will incur would be for 200 MW (i.e., the difference between 15 percent and 35 percent of capacity) over those 300 hours.
Hydro Quebec estimates the cost for its integration program at around 0.5 CAN-/kWh. With the average cost of wind energy at 7.8 CAN-/kWh (including transmission) for the 1,000-MW development, this represents an additional cost of 6.4 percent. Whether this turns out to be an improvement over simply scheduling the wind power without a formal integration plan remains to be seen, but clearly this is a major initiative and could pave the way for similar efforts elsewhere.
Other utilities as well as the U.S. Department of Energy (DOE) are starting to look into the potential for wind-hydro integration. Manitoba Hydro is exploring it, for example, not because the utility needs additional generation-their domestic loads will be met until around 2020-but because the company sees lucrative export possibilities. Wind also would provide an alternative generation source during drought periods when more expensive thermal generation is required, but the economics for marketing the power to other regions have been the driver for further study. There are several states with renewable portfolio standards in place that could be served via Manitoba’s interconnection with the United States, making for a ready market.
Click here to enlarge image
In the United States, there are also three DOE-sponsored case studies going on now under the auspices of the Army Corps of Engineers, the Arizona Power Authority and Grant County (WA) Public Utility District. The National Renewable Energy Laboratory, Northern Arizona State University, EnerNex and Wind on the Wires are all involved in carrying out the studies, which should offer a more robust body of data on the subject of wind-hydro integration when they are completed.
The need for more information is key to the advancement of the wind-hydro proposition. Wind forecasting technology, for example, is improving but in BPA’s case it is used by each wind facility independently. Regional forecasting in which conditions over a wider area are shared between wind farms would be an improvement. Clearly, though, there are myriad details about combining wind and hydro power that must be worked out before the potential for these complementary renewables can be realized. Between government-sponsored investigations like those at DOE and commercial programs like Hydro Quebec’s, we may soon be able to apply a new level of understanding to the challenge.
Bob Fesmire is a communications manager in ABB’s power technologies division. He writes regularly on transmission and distribution, IT systems and other industry topics. The opinions expressed here are his own and do not necessarily represent those of ABB. Bob can be reached via email at email@example.com.