News

Solar-powered Technology for Water Quality Issues

Power Engineering
Thu, 10/15/2009
By H. Kenneth Hudnell, PhD

Electric generating utilities use water bodies for a variety of operational purposes. For example, reservoirs supply cooling water, the heated water is cooled in basins and contaminants are decomposed in wastewater ponds. Solar-powered circulation (SPC) technology provides an environmentally sustainable technology for solving water quality problems at power utilities without CO2 emissions, at negligible operational costs.

The SPC Unit

SolarBee Inc.’s SPC units comprise three pontoons supporting a framework of above-water, near-surface and underwater components.

Three 80-watt solar panels, a digital control box, an 18V, high-efficiency, direct-drive motor and accessories are mounted above water. A distribution dish, impeller and battery are suspended just below the surface. A flexible intake hose, 3 feet in diameter, is attached to the base of the impeller. A steel plate suspended 1 foot beneath the hose causes water in that density layer to be drawn in radially with near-laminar flow from long distances. Two moorings attached to the frame with chains maintain the spatial position of the unit.

The impeller operates continuously at 80 RPM unless prolonged periods of low light incidence cause the electronic controller to reduce the RPM or deactivate the system temporarily. The largest units pump approximately 10,000 gpm (14.4 M gpd) of water to the surface. A direct flow of approximately 3,000 gpm ascends through the intake hose and departs from the unit at low velocity immediately above and below the distribution dish. Another 7,000 gpm of induced flow ascends outside of the hose and departs from the unit without turbulence below the distribution dish. The units are constructed of 316 stainless steel and non-corrosive polymers, and are designed for a 25-year lifetime.

Water intake depth is adjusted using chains attached to the intake plate and secured to the frame. The level is determined by the water quality objective. For example, a shallow intake depth is used in heated-water cooling ponds and lakes to enhance evaporative heat loss. The intake is usually set near the bottom for odor control and oxygenation of the hypolimnion. Intakes in wastewater are generally set just above the anoxic zone where anaerobic bacteria in sludge digest some compounds.

Enhanced Evaporation and Heat Loss

Power utilities with open-water cooling systems typically send heated water to cooling basins before return to the cooling-source reservoir. Cooling is usually required for one of two reasons. First, National Pollutant Discharge Elimination System (NPDES) permits often specify a temperature that water cannot exceed when discharged into receiving waters. Second, the temperature of incoming cooling water directly affects the condenser backpressure and therefore, the utility’s fuel consumption and generation efficiency. The heated cooling water must be cooled before return to the reservoir to avoid increasing the temperature of the cooling-source water. Furthermore, warmer discharge water may flow on the surface for several miles to the cooling-water intake, thereby short-circuiting the normal mixing and cooling processes.

Cooling in water bodies occurs mainly through evaporation of surface water. Evaporation releases heat from the water body and cools a thin layer of water molecules at the surface. A convective current is formed such that the cooler molecules on the surface flow down as deeper and warmer molecules flow to the surface, thereby enabling evaporation to continue. However, the formation of convective currents is a slow process. SPC enhances the rate of evaporation by creating a circulation pattern that continually disperses warmer water across the surface.

A U.S. power utility deployed SPC units in a 6-foot-deep, 70-acre, four-arm serpentine cooling basin to model the increase in heat transfer rate during calm summer months. Measurements were made during four conditions:

No. 1: No SPC

No. 2: Four SPC units in one arm, but none in the corners of the arm

No. 3: Eight SPC units in two arms, but none in the corners of the arms 

No. 4: Eight SPC units in two arms with four of the units in the arms’ corners

A model was developed to estimate the increase in heat loss for conditions Nos. 2, 3 and 4, relative to condition No. 1. Measurements were made over four-hour periods during two to eight trials per condition. Model input parameters were water temperatures at the basin’s inlet and outlet, ambient temperature, flow rate, heat transfer rate, overall heat transfer coefficient and dimensionless temperature ratio. Mean wind speeds during conditions No. 1 through No. 4 were 8.5, 9.0, 6.6 and 8.9 mph, respectively.

The results indicated that heat loss increased by a mean of 1.34 F, 2.58 F and 4.73 F during the four-hour periods of conditions No. 2, No. 3 and No. 4, respectively. The enhanced cooling rate produced using SPC enables the utility to meet NPDES limits while reducing retention times.

Preventing Noxious Odor Events

Anaerobic bacteria at the bottom of many water bodies produce malodorous sulfur compounds such as hydrogen sulfide, a toxic and flammable gas. Large quantities of the compounds are produced in waters high in sulfate concentration. The compounds accumulate in the hypolimnion until quantities are sufficiently large to create a “release event” during which the gas bubbles to the surface. Such events may last for months, during which the “rotten egg” smell plagues surrounding areas.

Utilities commonly oxidize the compounds with chlorine, often in the form of sodium hypochlorite or hydrogen peroxide, to form compounds that do not cause foul odors or tastes. Aeration systems are also used to continually oxidize and transport small amounts of the compounds to the epilimnion and atmosphere. The operational costs of the chemical applications and electricity for aeration can be very significant.

An alternative approach is enriching the water with oxygen from the air using SPC. Oxygen is transported to the hypolimnion and small amounts of the sulfide compounds are continuously transported through the oxygen-rich epilimnion. This method of oxidizing the compounds prevents odor events and reduces operational expenses.

Sulfur compound odor events occurred for two to three months each year at a Midwestern power utility. The utility regularly applied large quantities of chlorine to oxidize the compounds in the 137-acre reservoir.

Two SPC units were deployed in 2007 with intakes extended into the deepest portion of the reservoir. Odor events have not occurred since SPC deployment. A manager at the utility reported saving $200,000 a year on chlorine applications since circulation was initiated.

Similar odor events occurred yearly at another U.S. power utility. The 50-acre pond is 30 feet deep in the north end, inclining to ground level at the south end. The pond water contains 22,000 mg/L sulfate, creating a chemical oxygen demand of 700 mg/L.

The utility considered adding an aeration system to the pond, but sought alternative solutions due to the estimated monthly expense of $2,000 to $3,000 for grid power. The utility chose SPC because one unit typically displaces 20 hp to 40 hp of grid energy required for aeration. At an average aeration displacement of 30 hp, one unit displaces about 25,000 watts of electricity. 

Two SPC units were deployed in the deeper end of the pond during 2005 to continually oxidize and de-gas the compounds. Odor events have not occurred since SPC deployment. 

Enhance Degradation of Anticorrosive Compounds

Nuclear power utilities periodically supplement water with pH control compounds such as hydrazine and ethanolamine (ETA) to prevent corrosion and passivate oxidized areas of iron pipes servicing steam generators. Both hydrazine and ETA increase pH, and hydrazine reacts with oxidized piping to form protective layers of magnetite, but both hydrazine and ETA are toxic to humans and aquatic organisms. Wastewater containing hydrazine and ETA must be treated prior to release because of NPDES and other environmental discharge standards for toxicity, oxygen demand and nitrogen concentrations.

Hydrazine and ETA are used at a northern U.S power utility. The wastewater is retained in a 1.2-acre, 6-foot-deep pond until discharge limits can be met. The treated water is injected into the ground where it travels to one of the Great Lakes through a direct vent. The long detention times required to meet discharge limits occasionally caused wastewater quantity to exceed pond capacity.

The utility installed a small SPC unit in the pond in 2002. Measurements of hydrazine and ETA concentrations indicated that SPC increased their degradation rates. The state dropped the requirements to measure hydrazine and ETA concentrations in 2006 because of the enhanced degradation rates. 

An environmental specialist at the utility said that the state was impressed because the utility initiated circulation without being required to do so; the circulator was solar powered; and the compound concentrations when released were well below those desired by the state. He also said that the utility commonly refers to SPC treatment when seeking changes in its permit.