By Michael Ambrose

The California power crisis of 2001 seemed to foreshadow a grim future made up of expensive and unreliable electricity. In response, companies all over the state looked at ways to cut back on power consumption and find alternative sources for electricity. Some users took more far-reaching steps: In November 2001, voters in San Francisco approved a $100 million plan for solar panels, energy efficiency, and wind turbines for public facilities.

During the crisis, the East Bay Municipal Utility District, or EBMUD, the Oakland-based water utility where I work, discovered that one of its largest electrical demands was its own corporate headquarters. As a result, EBMUD is now taking a radical step to cut back on its electrical purchases: It's installing a cogeneration system on the roof of the administration building. The project, when completed later this year, will use microturbines for combined cooling, heating, and power.

EBMUD might not be the first company you'd think of as an electricity producer. Formed in 1923, EBMUD is a publicly owned utility dedicated to providing water service in portions of two counties in the San Francisco Bay Area. The water supply system covers 325 square miles and serves some 1.3 million customers. A wastewater treatment system, added in 1950, today serves about 640,000 customers.

The MicoGen heat exchangers (shown during installation) use exhaust gas from the microturbines to recover heat in the form of hot water.

But EBMUD is no stranger to power generation. There are two hydroelectric facilities that together generate nearly 40 megawatts. In addition, a byproduct of the wastewater treatment system, methane, is used to produce a modest amount of power. Waste gas from the plant's digesters is supplemented by natural gas and is used to generate about 4 MW for use at the wastewater plant. This on-site generation helps defray electric power purchases.

Following deregulation of the electrical power industry in California and the spike in prices that followed, EBMUD reviewed its overall energy demands. The water system as a whole uses a significant amount of energy, and the feeling was that there must surely be room for some efficiencies. The pumping plants alone use large amounts of electricity to move the water from the treatment plants to reservoirs in the hills. (Water flows via gravity back to the customers.)

It turned out, however, that most of the energy demand is distributed throughout the service area. The water treatment and distribution system consists of six treatment plants, about 155 distribution pumping plants, and many more storage tanks and reservoirs. The aggregate electrical load is significant, but very dispersed throughout the service area.

FINDING A SOLUTION AT HOME

We discovered that the site of the largest single energy demand—and the highest energy costs of all the facilities at the district—was, surprisingly, the administration building. The headquarters houses about 600 people in a steel-framed structure with large window exposures and moderate insulation. The building spaces are conditioned through a combination of gas-fired boilers, electric chillers, and direct expansion units. The energy load is unusually high because of 24-hour operations and a high density of computers.

Faced with a large draw—and with soaring electricity prices—EBMUD decided to explore the addition of more generating capacity at the site that needed it most, headquarters. We eventually settled on a cogenerating facility: a set of microturbines fueled by natural gas, with the waste heat from the microturbines used for hot water and building cooling loads.

The primary equipment in the cogeneration system consists of 10 60-kW Capstone C60 microturbines. Considering the amount of power provided, the microturbines have a small physical presence. Each turbine is housed in a cabinet 30 inches wide and 77 inches deep. The microturbines produce very low levels of carbon monoxide, hydrocarbon, and nitrogen oxide, even without emission controls. And they are relatively quiet machines, averaging about 70 decibels at 30 feet.

The turbines are rated for 28 percent efficiency, but the energy savings really appear when cogeneration is added. In addition to electricity, the administration building has a large hot water and HVAC demand, which is met through gas-fired boilers. Through heat exchangers, the exhaust gas from the turbines is used to make hot water, which is routed to a single-stage absorption chiller and then sent to the rest of the building. This system will allow EBMUD the flexibility to choose between gas and electricity and heating versus cooling.

ELECTRICITY ISN'T IN CHARGE

In spite of the 600-kW combined rating for the turbines, EBMUD isn't turning into a power company. Because an agreement with the local utility allows the turbines to be used only for cogeneration, the system will be controlled by waste heat demand, not by electrical demand. In essence, it's a heating system that produces electricity as a by-product.

Installation is nearly complete, with testing having begun back in April, although the California power crisis has now abated. The combined system is about 70 percent efficient.

Since the natural gas for the turbines can be purchased from the local utility at reduced rates because it is a cogeneration system, the project will pay for itself in six to eight years. And some day, if needed, electrical power generated may be sent to the utility grid.

Michael Ambrose is a senior mechanical engineer for the East Bay Municipal Utility District in Oakland, Calif.



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