All the associated chilled water distribution pumping equipment is also variable speed. There are 11 variable speed distribution pumps with 2,200 total motor horsepower. The campus chilled water related electricity demand is controlled and leveled by a 4.4 million gallon, stratified, chilled water thermal energy storage tank having 35,000 ton-hours of usable chilled water storage and a peak discharge rate of up to 14,500 gpm or approximately 8,000 tons at a 13°F differential temperature.
Peak load for the campus system reached 14,500 tons in 1995 with the annual consumption expected to exceed 30 million ton-hours. For four full months per year, all campus cooling is provided by free cooling through plate and frame heat exchangers using cooling tower or lake cooling water from a small, shallow, on-campus lake. The lake water based free cooling is a small, winter only version of DWSC which operates at about 0.15 kWh/ton-hour. This compares to the average annual efficiency of the overall, campus wide system, not including the in-building pumping, of about 0.7 kWh/ton-hour.
The six, existing hermetic chillers cannot be economically converted to non-CFC refrigerants. Three are over 30 years old, two over 20 years old and one is 11 years old. Chiller no. 7 can be converted relatively easily to a HFC refrigerant. Current annual refrigerant consumption is under 1% of the total machine charge. Should the University decide to pursue conventional cooling methods in the future, chiller no. 7 will be converted to a HFC refrigerant and all of the other chillers will be replaced over the next 10 years.
2000 tons of new cooling load were added in 1995, representing a nearly 14% increase in one year. August 1995 chilled water consumption for the entire system was up 24% as compared to 1994. New cooling loads are planned through the year 2000 and continued construction and renovation are expected to add 12,000 to 22,000 tons over the next 35 years. The load factor on the system is expected to continually decrease as new loads are added and existing loads are renovated (due to variable volume, air economizer cycles and digital controls). Today the load factor is about 26%, by year 2029 it is expected to decrease to 20%.
Operating costs continue to rise each year due mostly to electricity tariff increases, capitalization of new capacity and increased consumption. Current average costs per kWh are $0.08/kWh, well above the national average for industrial electricity.
Because of the uncertainties facing the campus chilled water system due to CFCs, load growth and aging equipment, a major study effort has been undertaken over the last 18 months to determine Cornell's path for the future. Two major paths have emerged, continuing with conventional chillers (referred to as the Base Case) or a DWSC based system using nearby Cayuga Lake as the heat sink.
The Base Case required a dedicated effort by an engineering team separate from the evaluation of DWSC. All large chiller options were evaluated and screened based upon expected duties (on-peak, off-peak and base load) and CorneH's costs for energy (steam, gas and electricity). The mixture of equipment selected included variable speed, electric motor driven, base loaded. centrifugal chillers and constant speed, electric motor driven, peaking, centrifugal chillers. Although the conventional chiller path is known technology, it does include risks and complications. The risks include plant siting. underestimation of future growth, future regulation, inflation and electric rates.
For these and other reasons, the DWSC path could provide long term cost and operational stability for Cornell. As a result, Cornell has studied this alternative in significant detail from engineering, environmental and community acceptance perspectives.
There are many design challenges associated with the lake water intake and outfall structures and piping situated in as much as 220 feet of water and resting on very soft, un-consolidated sediments. The intake structure must be rugged, cleanable and must minimize the entrainment of a small freshwater shrimp, called Mysids. To keep the entrainment of Mysids to a minimum, the lake water intake structure will extend up off the lake bottom about 15 - 20 feet, with the intake positioned horizontally. The intake water velocity will be very low across the screen and the intake will be lighted with low levels of light, above 0.0001 lux, to repel the Mysids which are light averse. The materials of construction for the intake will be copper and it will be removable for cleaning.
In order to take advantage of HDPE as a pipe material, the intake has been sized for the largest internal diameter commercially available today, 4 feet. Should a larger HDPE pipe diameter become available in the next few years, it would be used as the 48" intake piping is currently limiting the capacity of the system. The total length of the intake piping may approach 9000 feet.
The outfall piping and outfall diffuser are expected to be constructed of HDPE, concrete or ductile iron, also 4 feet in diameter. The design of the diffuser would result from detailed, near-field, thermal modeling of the outfall water discharged into lhe lake. Unlike a more conventional thermal discharge, there will be significant periods of time during the year when the 50-55°F lake return water will be colder than the shallow waters into which it is returned. It is hoped that the outfall diffuser would be located within 500 feet from shore, in about 8 - 10 feet of water.
Site conditions limit the size of the heat exchange facility located at the lake shore. As a result, the facility will be split into two pieces. At the edge of the lake, a small, low profile pump house would contain the lake water pumping equipment and the mechanical pig cleaning access points. Because of the cleanliness of the lake water, no significant filtration is expected to be required.
Across a shore road about 500 feet away from the lake water pumping facility, a second much larger facility would house the heat exchangers and the chilled water pumping with variable speed drives. This main facility would include the overall plant control room, chilled water side-stream filters, water treatment equipment and all necessary office and shop space. The chilled water transmission lines would also terminate at this facility.
The chilled water transmission piping to and from campus represents approximately 49% of the total construction cost for the project. Twin, 36 inch diameter, direct buried, coated and welded carbon steel pipes will be used having operating pressures of nearly 300 psig. This high system pressure is due to the elevation differences between the campus and the lake. The piping would be backfilled with sand to protect the coating. It would also be cathodically protected to mitigate external corrosion with chemical treatment used to protect the interior. This pipeline work would be very similar to gas transmission main work and would be complicated by the proposed routings in and through the city streets.
Early in the study of DWSC, Cornell felt that the dominant public concern about the project would be its potential effect on the ecology of Cayuga Lake. To answer this significant question, Cornell commissioned Stearns & Wheler, a Cazenovia, New York consultant, to perform an environmental investigation and assessment. Stearns & Wheler concluded from the results, as documented in their December 1994 interim report, there are no significant adverse impacts on the Cayuga Lake ecosystem associated with the DWSC concept that could not be adequately mitigated. The environmental study of lake ecology has focused on 4 major areas: thermal impacts, water chemistry impacts, biotic impacts and construction impacts.
Thermal Impacts
The DWSC system heat input into the lake will be on average 5% of the current heat input from an existing, 350 megawatt, coal fired power plant also located on the lake. At peak periods, the DWSC will introduce less than 20% of the input of the fossil plant. The DWSC system will initially be less than 1 millionth of the sun's input annually to the lake. Less than 1% of the hypolimnion volume would be circulated through the DWSC plant on an annual basis. The results of preliminary hydrodynamic thermal modeling suggests that hypolimnetic withdrawal and epilimnetic discharge will have only small, transient impacts on the temperature structure of the lake and that no changes in Cayuga Lake's period of thermal stratification are expected. A detailed analysis of the thermal impacts will be undertaken in the next project phase to provide additional guidance regarding the near-field impacts associated with the outfall plume.
Cayua Lake Water Chemistry Impact
Recirculation of the hypolimnetic water to the epilimnion would not have an adverse impact on the lake's ecosystem. There are only minor differences in concentrations of chemicals such as phosphorus, dissolved oxygen, ammonia, total organic carbon and silica between the upper and lower waters. Phosphorus is the most important limiting nutrient for phytoplankton growth in the lake and the DWSCs relative contribution to total phosphorus concentration from internal recycling is small as compared to external point and non-point sources.
Biotic Impacts
Generally, the subtle changes attributable to the DWSC are expected to be lost in the background variability of more significant biological factors due to competition and natural predators. Zooplankton in the immediate vicinity of the plant intake would be entrained and experience mortality; but, investigations at existing power plants have concluded that there is no impact on lake-wide populations.Two species require additional study to understand potential impact and plan for mitigation. The small light adverse freshwater shrimp (Mysids Relicta) is an important food of juvenile lake trout and if pulled into DWSCs intake in sufficient numbers their lake-wide population could be impacted. It is possible that a low wattage light source located at the intake, will force the Mysid to stay outside the field of DWSC intake influence. Colonization of DWSC lake water components by Zebra and Quagga mussels is expected. A plan for mussel mitigation that will include periodic mechanical cleaning through the use of pipe "pigs" and identification of control technologies. Also chemical injection and bio-monitoring strategies permissible by regulatory agencies, will be developed in the next project phase.
A periodic DWSC surface discharge could be used to scour the lake shore, cleaning the sedimentation blockage that currently impedes more active recreational potential. The DWSC discharge could be modified for use in an artificial river that has attractive application for various water sports such as canoeing and white water kayaking; University and community interests could be well served by developing this alternative. The local city school district, which also must phase out their use of CFCs, will derive a cooling benefit from DWSC in exchange for granting the university critical pipe routing easements. The City of Ithaca, in exchange for authorizing extensive rights-of-way through City streets, could accomplish a variety of municipal infrastructure upgrade projects at reduced cost if built concurrent with DWSC such as: the repair of roads, replacing sidewalks, upgrading deficient water and sewer lines and improving water flow for fire service. Encouraging partnerships that benefit the community and nurturing project stake-holders will result in a broad base of public support that is critical to realizing a successful project.
Other DWSC projects have investigated opportunities including: irrigation potential, municipal water supplies, aquifer recharging and the flushing of storm sewer lines.
The DWSC can also be used to demonstrate a creative way for society to reduce its impact on the environment. If sufficient interest in DWSC develops on a regional or national level, it would be appropriate to formulate regulatory guidelines for its use. A DWSC Guidance Plan, as a result of intensive collective study by scientists and engineers, could provide the necessary framework for the sound development of this promising technology.
Should Cornell choose a DWSC based chilled water system, it will be poised to enter the 21st century with a large campus utility system that utilizes a renewable resource without any compromise in the quality of the service it delivers at potentially significantly less operating cost. Cornell would be the first to tap the deep water source cooling resource of a fresh water lake, placing it squarely among the leaders in environmental stewardship in the world. Should Cornell choose the Base Case conventional chiller path, it will do so knowing that it has done so after careful consideration of the alternatives and with the most cost effective combination of chiller technology available today.