all about hydro

How it Works

Juneau is fortunate to have hydroelectric generation for its primary source of electric power. Our hydro is non-polluting, zero-carbon, and renewable. In 2007, over 98% of the electric power generated by AEL&P was produced by hydro. In 2008, even though we will have more diesel generation than normal, we will still get 95% of our power from hydro.

The “fuel” for a hydroelectric generating plant is water. Electric power is generated by high pressure water in a pipe (penstock) shooting against a water wheel (turbine) thus spinning the turbine and turning the electric generator.

Mother Nature provides the water that fuels the hydro plant in the form of rain and snow. The water that is used in the plant is stored in natural reservoirs (like Snettisham) or man-made reservoirs (like Salmon Creek). This is a good thing, because we can use water stored in the reservoirs to generate power even when stream flow is low, such as during dry periods, or during the winter.

Our reservoir basins act as large funnels. Precipitation that falls in the basin eventually flows into the reservoir (except for the small amount that soaks into the ground or evaporates). Water flows into the reservoir from rainfall or snowmelt, and water is taken out of the reservoir for power generation.

The Annual Hydro Cycle

The above graph helps illustrate our annual reservoir cycle. Our water year starts in October. By that time of the year our reservoirs are at their maximum level, preferably completely full. As the weather gets colder, the precipitation that falls in the basin is mostly snow, and so water flow into the reservoir decreases. During the winter and spring, typically from November to mid-May, more water is taken out of the reservoir for power generation than flows in, so the water level in the reservoir drops.

By mid-May our reservoirs are at their lowest level. At that time the snowpack that has been accumulating in the basin begins to melt, stream flow increases, and more water flows into the reservoir than is taken out for power generation, so the reservoir level begins to rise. The reservoir rises due to the melting snow until roughly August. From August through October we rely on our fall rains to finish filling the reservoirs.

Recent History

On the above graph the green line represents our target reservoir elevation throughout the year. We call it the “rule curve”. It is good for the actual water level to be above the rule curve, because that means we have more water in storage than we expected. If the actual water level falls below the rule curve we are in dangerous territory and at a risk for running out of water in the reservoirs, thus needing to supplement our hydro with diesel generation.

Water year 2006 (black line) was a good one. We were way above the rule curve all year long. With the warm, wet weather of December 2005 and January 2006, reservoir levels remained very high. Even the minimum elevation in May 2006 was much higher than the rule curve. The reservoir filled completely, and even overflowed in September and October 2006.

At the beginning of water year 2007 (October 2006 -- the blue line) the reservoir was still full, but as it got cold in November the reservoir level began to fall. Winter 06-07 was colder than normal. That cold weather resulted in a record breaking snowfall, but it also resulted in higher electric loads for heating homes and businesses. With so much water being taken out of the reservoir to meet power demands, and so little flowing in due to low inflow, the reservoir level was actually below the rule curve by May 2007.

As the record snowpack melted the reservoir filled rapidly in June and July 2007. August 2007 was quite dry, and the reservoir didn’t fill as rapidly, actually falling below the rule curve in August and staying below through September 2007.

We started water year 2008 (October 2007) below the rule curve, and the reservoir never filled that fall. With water levels so low, it was clear that we would need to supplement our hydro generation with some diesel generation this winter.

The twist

Electric demand in Juneau is highest in the winter and lower in the summer. Our loads have grown to the point that even with average precipitation, we will not have enough water in the reservoir to make it through the winter, and will need to supplement with diesel generation. But our loads are light enough in the summer that with average precipitation we will completely fill and actually spill water from the reservoir. It sounds odd, but we can run diesel generation in the winter and still spill from the reservoir in the fall. This means it will still be possible to sell surplus energy during the summer, since spilled water equates to surplus energy.