California solar power and the “duck curve”

Last week as I wrote in Solar about to go mainstream,  I told you about one state where solar has already gone mainstream with over 10% of homes already having solar panels on their roofs. That state, Hawaii, not only has abundant sunshine, it also has the highest regular electric rates in the country. This week the discussion moves to the continental United States and the one state with more solar power facilities than all the others put together, California.

Map of states with a renewable portfolio standardCalifornia, Hawaii, 35 other states and the District of Columbia have created Renewable Portfolio Standards specifying a minimum level of renewable fuel sources for their electric utilities. In seven of the states the Renewable Portfolio Standards are voluntary, rendering them merely symbolic.

In 30 states, however, the standards are mandatory. The Illinois standard, adopted in 2007 is for 25% of electricity from renewable sources by 2025, sometimes known as a 25 x 25 standard. In Michigan, the standard is 10% from renewable sources by 2015. California has probably the nation’s most aggressive Renewable Portfolio Standard requiring 20% of all electric power from renewable sources in 2010 and 33% in 2020.

Cummulative California Photovoltaic Installed Capacity
Solar power installations in California more than doubled last year.

Of course, solar cells can only generate power when the sun shines. There’s little power in the early morning hours or late afternoon, and none at all after the sun goes down. Coal burning power plants typically have capacity factors greater than 80%, with only short shutdowns for maintenance. Wind turbines have capacity factors of about 40% depending on location. Solar cells are rated for direct sunlight on a clear day at noon and generate less all other times of the day, when it’s cloudy or even when the panels are dusty. A capacity factor of only 12% to 13% is typical.

California ISO Solar Power Duck GraphModels of the effects of future solar power generation in California have resulted in the California “duck graph.”

Solar power reduces the amount that needs to be generated from other sources during the day, but drops off rapidly after 4 pm and does nothing to help meet peak power requirements between 6 and 10 pm. As more solar power is installed, the belly of the duck gets deeper and the utility must bring up other sources of power quicker after 4 pm. Coal power plants can’t start and stop quickly and nuclear power is steady all day long. Winds tend to pick up in the hours around dusk and dawn as the temperatures change, but wind is unpredictable. That leaves natural gas (and hydro power to some extent) to make the quick transition.

Aerial photo of Ivanpah thermal solar power facilityOne potential solution is thermal solar power. California built the world’s largest thermal solar power generating facility in the Mojave Desert last year. Thousands of mirrors reflect sunlight to water tanks at the top of tall towers, turning the water to steam which drives turbines to generate electricity. One of the advantages is that the steam stays hot for hours after the sun goes down and the facility continues to generate power into the evening hours.

However, more facilities like the one pictured above at Ivanpah, California are unlikely to be built for several reasons:

  1. The facility is killing approximately 2,000 birds per month, heating them to 800 degrees as the fly through the air.
  2. Pilots flying overhead are complaining about the blinding glare.
  3. The cost of regular silicon solar cells continues to come down. While thermal solar was expected to be cheaper than regular solar panels when it was built, that is no longer the case.
  4. The project is only generating ¼ of the amount of power that was expected. The project investors, including Google, who got a $1.6 billion federal loan to build the facility are now asking for $539 million federal grant to help pay off the loan.

Solar vs grid power costRemember this graph from last week?

The price of large utility-scale solar power facilities has come down much faster than smaller residential rooftop setups. The major difference is the cost of inverters which turn the direct-current from the solar panels into alternating current and synchronize it to the grid. For a residential system, these electronics can be more expensive than the silicon panels themselves. The inverters are a much smaller portion of the cost for large systems. As a result, most solar power installations in the US last year were utility-scale systems.

Solar power installed in 2013 by state

Many environmentalists talk about the “distributed generation” of rooftop solar systems and the end of the grid as we know it. With a distributed generation model, the grid becomes less and less important as power is generated in the same neighborhood (or even the same building) as it is needed. But even as solar power escalates around the country, the reality is very different. Three out of four homes are not well suited for solar, either due to roof geometry, shade, or other issues. And solar power installations are increasingly large-scale facilities far from populated areas where land is cheap, feeding directly into the grid. Wind power too, is mostly located in rural areas, thousands of feet from the nearest home. We will need the current electrical grid for a long time to come. In fact, due to the variability of renewable power sources like wind and solar, the grid will need to be even more robust than it is today.

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