Articles
 

From the May 1999 issue of . . .

The era of privatization and deregulation requires power producers to seek new and innovative strategies for cost-effective generation. Stephen J. Chippas, Paul R. Blaszczyk and Mark G. Jones describe how the proposed 70 MW Alta Mesa pumped storage hydroelectric project in southern California typifies this innovation and illustrates the vast opportunities currently available to power producers.

 
  The developer of the Alta Mesa Project, Mark Technologies Corporation of San Francisco, California, currently operates a 28 MW wind-turbine generation project on the site and plans to increase capacity to 54 MW in the near future. The current wind turbine project consists of 159 individual wind turbines, 117 turbines manufactured by Danwin, each rated at 160 kW, and 42 turbines manufactured by Vestas Wind Systems, each rated at 225 kW. The turbines have averaged 97% availability over the eleven years since the project was put into service. An additional 42 Vestas wind turbines, rated at 600 kW, will be installed on the site, bringing the total capacity of the site up to 54 MW.

The majority of the power generated by the wind turbines is produced during off-peak generation periods, limiting the effectiveness of the project to provide power during peak consumption periods. This has led Mark Technologies to look for a proven and cost-effective method to store the energy generated during off-peak hours and release it during periods of high demand. The use of hydroelectric pumped storage technology was a perfect solution.

The theory of a hydroelectric pumped storage project is the result of the age-old concept of supply and demand. Demand for electricity is higher during daylight hours and therefore those who can supply the energy can do so for a substantially higher price. Armed with this theory, engineers and economists alike developed the concept of using the cheaper electricity generated at night to pump water to upper reservoirs. There, it can be stored until demand and price are at their peak, at which time the water can be released and routed through hydroelectric turbines to generate electricity.

Increased utilization of renewable generation sources during peak demand periods reduces the need for
non-renewable sources.


The Alta Mesa hydroelectric pumped storage project utilizes the existing 1000-foot (about 300-metre) elevation change on the project site. A pair of reinforced concrete tanks will serve as the project reservoirs and a buried steel penstock will convey the water between the reservoirs and the powerhouse. A reversible pump/turbine will be used to pump water to the upper reservoir and to generate electricity when the water flows back to the lower reservoir.

From an economic standpoint, the theory of pumped storage is quite simple. With electricity costing roughly 57% less during non-peak hours, the additional energy required to pump the water is more than paid for through daylight generation. By using the abundant power produced by the wind turbines during non-peak hours to power the pumps and replenish the upper reservoirs, valuable and lucrative energy can essentially be stored until the high-demand hours.


The Alta Mesa pumped storage hydroelectric project

Mark Technologies approached Harza Engineering Company to discuss the feasibility of adding a hydroelectric pumped storage component to the project. The company has been designing hydroelectric projects since the inception of the company in 1920 and it has designed numerous hydroelectric pumped storage projects during that period, including the largest pumped storage project in the world, the 2100 MW Bath County pumped storage project. After meeting with Mark Technologies, Harza completed a preliminary feasibility study of the project and determined that the addition of a hydroelectric pumped storage component was feasible, and that the pumped storage component of the project could be designed and constructed within the budgetary constraints set out by Mark Technologies.

The Alta Mesa pumped storage project actually consists of two separate developments, the A-line project and the F-line project corresponding to the names of the wind turbine lines to which the respective upper reservoirs are nearest. The developments are essentially the same, consisting of an upper and lower reservoir with a steel pipeline, referred to as a penstock, connecting the reservoirs to each other via the powerhouse.

The reservoirs for the Alta Mesa pumped storage project will consist of round, prestressed concrete tanks. The tanks will be approximately 451 feet (135 metres) in diameter and will have a net inside depth of 50 feet (15.24 metres), making them two of the largest tanks ever constructed. The tanks will hold upwards of 56.2 million gallons of water, or roughly 7.5 million cubic feet (about 212,000 cubic metres). The reservoirs have been sized to accommodate a volume of water that will allow continuous operation of the turbine units for the anticipated six-hour period of peak demand. In order to reduce aesthetic concerns and to reduce the water temperature within the tanks, the reservoirs will be constructed in excavated pits so that the lower halves of the tanks are buried.

Based on preliminary economic analyses regarding the cost-effectiveness of providing a roof over the tanks, the reservoirs will be left open to the atmosphere. Although the loss of water due to evaporation may be substantial, the cost of purchasing any required make-up water over the life of the project has been estimated to be less than the cost of providing roofs on the reservoir tanks. Wildlife fences and other appropriate measures will be incorporated to protect the reservoirs from infiltration, as well as prevent unintentional entry of wildlife or visitors into the tanks.

The steel penstocks that will connect the upper and lower reservoirs to the powerhouse will be approximately 67 inches (1.7 metres) in diameter. The penstocks will be buried using the cut-and-cover method and will be provided with a cathodic protection system or an impressed current system to mitigate corrosion of the penstock. The decision to bury the penstocks was made based on comparisons of the relative construction and life-cycle costs associated with buried and surface mounted arrangements. In addition, any aesthetic concerns associated with the surface arrangement will be alleviated by burying the pipe.

The penstocks for the A-Line and F-Line developments will be located adjacent to each other for a majority of their respective lengths. The aim of this was to minimize the construction cost associated with the cut-and-cover techniques and to minimize the project's area of influence. However, in order to avoid a sector of private land not owned by Mark Technologies, approximately one thousand feet (about 300 metres) of the penstocks will cross Federal property. The penstocks will be completely buried and the affected Federal land will be returned to its original state upon completion of construction. However, the fact that public land is involved will require the issuance of a license by the Federal Energy Regulatory Commission (FERC).


Environmental considerations

When Mark Technologies were looking to expand the capacity of the project, sensitivity to environmental concerns was a significant factor. The additional generation and revenue benefits are not the only factors driving the decision to add a hydroelectric pumped storage component to the Alta Mesa project. Mark Technologies has always had a strong commitment to providing a clean source of energy and the addition of the hydroelectric component is a logical progression in its tradition of the production of clean energy. Wind turbines offer the benefits of inexpensive power generation with zero output of polluting gases, such as nitrous oxides, sulphur oxides and ozone, and particulate matter or heavy metals. It was the ability to generate needed electrical power without polluting the environment that led to the first stage of the Alta Mesa project.

The Alta Mesa hydroelectric pumped storage project may be unique among hydroelectric projects in that no rivers, streams or other bodies of water will be involved in the operation of the project. Typically, a pumped storage project utilized a river or other body of water as one of the project reservoirs. By avoiding the use of any bodies of water, the project will be able to avoid any of the typical environmental concerns related to habitat alterations, migration, and route disruption or stream-flow modifications. The reservoir tanks for the project will initially be filled by groundwater pumping from an on-site aquifer or by connection to a local municipal water supply. After the initial filling operation, the only water required for the project will be small amounts necessary to replace evaporated water.

Regulatory issues

The construction and operation of hydroelectric projects in the United States is regulated by the Federal Power Act (16 U.S.C. 817 - 'the Act'), Section 23(b)(1) of which states that FERC must grant a license for '. . . any person, State, or municipality, for the purpose of developing electric power, to construct, operate, or maintain any dam, water conduit, reservoir, power house, or other works incidental thereto across, along, or in any of the navigable waters of the United States, or upon any part of the public lands or reservations of the United States . . .' The Act further states that '. . . if no public lands or reservations are affected, permission is hereby granted to construct such dam or other project woks . . . upon compliance with State laws.'

As was discussed earlier in this article, the proposed Alta Mesa Project will not utilize any of the navigable waters of the USA and it will not use any surplus water or water power from any Government dam. The source of water for the initial reservoir filling and make-up water will be either from local groundwater sources or via a connection to a local municipal water supply. Based on these facts, the current configuration of the project would remove it from the majority of situations where the Federal Power Act would be applied and where a FERC license would be required for a project.

The only portion of the Act that could still apply to the project is ' . . . upon any part of the public lands or reservations of the United States . . .' As previously indicated, a section of the project's twin 67-inch (approx. 1.7 metres) diameter steel penstocks will need to pass over Federal property owned by the Bureau of Land Management ('BLM'). The amount of surface property disturbed during the installation would be less than 0.60 acres.

In order to determine whether a FERC license would be required, the project development team submitted a Declaration of Intention to FERC pursuant to the requirements published in the Code of Federal Regulations (18 C.F.R. 24.1). Based upon a subsequent meeting with FERC staff, it was determined that the project will need to apply for a FERC license. The process of obtaining a FERC license is expected to require a minimum of two years to complete.


Benefits and future applications

The benefits of the application of this unique 'closed-loop' pumped storage project include the increased peak energy generation capacity and increased utilization of existing energy generation facilities. By combining the renewable sources of generation, namely wind turbine energy, with a controllable form of energy storage, such as hydroelectric pumped storage, the renewable source generator gains the ability to control when power will be generated or released by the project.

An additional benefit realized by the increased utilization of renewable generation sources during peak demand periods is obvious in that it reduces the need for non-renewable sources. During periods of high demand for electricity, many of the utilities in the USA exceed their baseline capacity for generation. In order to supply the required power for peak demand periods, the utilities turn to rapid-response generating sources and the utilization of gas- or oil-fired combustion turbines and similar equipment is the method of choice for most major utilities. While this type of equipment is quite effective and efficient, it consumes non-renewable resources and produces combustion by-products that are released into the atmosphere during generation. The use of a clean source of peaking power, such as that produced by a hydroelectric pumped storage project, reduces the need for the combustion peaking equipment, and in turn the use of natural resources and the production of harmful emissions.

The marriage of a hydroelectric pumped storage project and renewable generation sources is by no means limited to wind turbines. Alternative sources of renewable energy, such as solar and geothermal, are also excellent candidates for the benefits associated with the addition of hydroelectric pumped storage.


Acknowledgements

The authors wish to thank M. Anthony Wolff, P.E., Senior Planning Engineer, Harza Engineering Company, for his assistance in the completion of this paper.

The article is based on a presentation made by Stephen Chippas at POWERGEN in Florida, December, 1998