By Roy R. Stoecker, Ph.D., Principal Scientist, EEA, Inc.
James E. McAleer, M.B.A., Director, Strategic Planning, EEA, Inc.
Rising fuel cost, demand for carbon footprint reduction and closed-cycle cooling is simply incongruent. Thus the development and evolution of a once-through cooling hybrid, the substratum intake system. This new system (presently in pilot) allows steam electric stations to maintain their existing once-through systems while eliminating virtually all environmental impacts. In addition, it promises to increase overall plant efficiency while reducing condenser operations and maintenance costs.
It’s axiomatic that steam electric power stations with once-through cooling systems (OTC) use enormous amounts of water to cool their condensers. A 400 MW unit typically requires 175,000 gpm when operating at full load. The environmental problem with OTC is that planktonic organisms, such as fish eggs and larvae, suffer high mortalities in passing through the plant. The damage is caused by mechanical strains of pumping, rapid temperature rise and addition of biocides. The process is called entrainment.
The U.S. Environmental Protection Administration (EPA) attempted to regulate these impacts many years ago by creating Section 316b of the Clean Water Act, calling for a 90 percent reduction of entrainment. Up to the present, installation of best- available technology to reduce entrainment at plants equipped with OTC is closed-cycle cooling (CCC), or cooling towers. But cooling towers have their own set of problems, which can include high capital costs, high maintenance costs, reduction of plant efficiency and consequent increases in emissions among fossil-fueled units. For these reasons, utilities generally resisted converting from OTC to cooling towers.
As an alternative, the substratum intake system (SIS) uses substrate water from the saturated aquifer under a water body instead of surface water. The SIS is designed for plants that are underlain with a porous substrate such as sand. In practice, the SIS is a large well field of pipes and screens drilled into place by blind horizontal directional drilling. A booster pump and manifold deliver the substrate water to the plant’s forebay, reducing entrainment and impingement to zero.
Figure 1 on page 90 presents a schematic overview of an SIS installation. In effect, the SIS is a variant of OTC, the major difference being the water supply source. The system is composed of three pieces:
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The reservoir, under sea, lake or river bottom well field. Well screen pipes installed by horizontal directional drilling to a depth coincident with the most favorable aquifer stratum. The number, length and diameter of the well field pipes depends on the water quantity required by the facility.
Manifold and pumping station. The well collection system is connected to a manifold while a booster pump station pumps the water from the collection pipes to the underground shoreside facility.
Fail safe intake structure. Water collected by the SIS and pumped into the power plant is fed directly into the plant forebay where the circulating pumps operate normally. The forebay will be closed to surface water by a curtain wall and one-way flap valves that open should the booster pump fail. This gives the plant complete redundancy for cooling supply.
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The design and materials for SIS wells vary considerably depending on site-specific circumstances such as yield rate, grain-size distribution in the substrate and length and depth of the well. Well material options typically considered are:
- High density polyethylene (HDPE)
- Carbon steel
- Polyvinyl chloride (PVC)
- Fiberglas reinforced epoxy (FRE)
- Stainless steel
It’s expected that SIS wells will be principally made of HDPE, a highly durable pipe. HDPE also has low frictional losses because of the smooth walls. Stainless well screens will be utilized with the exact type selection based on substrate water chemistry and other factors.
Thermal Efficiency Gains (Fuel Savings). In temperate latitudes the summer substrate intake water is cooler than ambient surface water; therefore the SIS is expected to improve the thermal efficiency of the station during peak demand months. SIS imposes no parasitic load and engineers believe SIS can provide a boost in thermal efficiency during summer months. When compared to the cooling tower alternative, SIS will have even greater thermal efficiency benefits. This increase in thermal efficiency results in potential fuel savings for the plant. Preliminary calculations by thermal power plant engineers reveal that summer fuel savings (heat rate) would conservatively be 5 percent.
Reduced O&M. Because SIS-delivered water will be essentially sterile there will be no possibility of attaching or encrusting organisms fouling condenser tubes. SIS requires little or no maintenance but periodic inspections are recommended.
Carbon Footprint Reduction. Increased thermal efficiency means less fuel burned and reduced emissions. Assuming a 5 percent reduction in fuel consumed, SIS should enable a fossil fueled power generation station to make a considerable leap forward toward meeting greenhouse gas initiatives already underway as well as anticipated C02 reduction statutes.
Clean Water Act Advantages. The SIS completely eliminates entrainment of planktonic eggs and larvae and impingement of larger forms. During summer months in temperate latitudes thermal discharge plumes will be lessened or eliminated. Full flow SIS deployment would eliminate entrainment and impingement.
No Biocides. Because the substrate water is essentially sterile, there is no necessity for biocide to control slime buildup or attachment of encrusting organisms. This saves the costs of purchasing the biocides and is environmentally advantageous. It also eliminates residual chlorine monitoring.
Lower Capital Costs. Capital costs for installing an SIS are substantial and will vary widely depending on size, location, configuration, labor and material costs. However, when compared to installation costs of a cooling tower of equal cooling capacity, estimated SIS construction is potentially half that of CCC.
No Required Construction Outages. Since the SIS is an addition to the plant cooling system no reconfigurations are required. All of the existing water systems remain without alteration. The SIS can be installed with the plant remaining on-line.
Fail-Safe Design. Should a booster pump fail, tide gates will automatically open and the plant will revert to surface water cooling without interruption, a key consideration in nuclear plant applications.
In 2003 EEA Inc. engaged with two other companies that would contribute greatly to the development of SIS, well water engineering company JR Holzmacher Associates and hydro-geologists MC Environmental LLC. Once conceptually assembled, it was presented to KeySpan Energy, now National Grid, whose Environmental Management Department offered to fund a feasibility study. The study considered engineering, hydrogeology, ground water modeling, conceptual layout and potential system benefits.
The study concluded that the SIS design was feasible based on the substrate geological conditions, current technological achievements in horizontally directed drilling and the conceptual designed lateral subsurface intake structures. The SIS feasibility report also recommended undertaking a pilot/demonstration study that would have as its goals the design and installation of a prototype model with one or two lateral SIS pipes, pump, manifold and condenser simulator. Review by National Grid and the Long Island Power Authority confirmed the interest and support of a pilot/demonstration of the system. As development moved forward, SIS scientists and engineers presented the concept to other utilities, regulators and engineering firms. All expressed interest but stressed that a pilot was needed to prove the concept for eventual commercialization.
The pilot program is a fractional build- out of a full scale system. The overriding goal of the project is to build a pilot or demonstration system and then to conduct field testing. National Grid and LIPA are jointly funding the pilot project. The New York State Energy Research Development Authority also has joined the project with additional funding.
Completion of the pilot program is expected by late November 2008.
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Roy R. Stoecker, Ph.D., is co-founder and vice president of EEA, Inc. and has more than 30 years of experience in energy and environmental programs. He holds a B.S. in biology from Manhattan College and a Ph.D. in botany from the University of Hawaii. He is extensively published and the author of “Aquatic Studies at the Hudson River Center Site In: Estuarine Research in the 1980s: The Hudson River Environmental Society Seventh Symposium on Hudson River Ecology (1992).” James E. McAleer is director of strategic planning for EEA Inc. He holds a B.A. in social science from Fordham University and an M.B.A. in marketing from the Columbia University School of Business.