California peak and off peak demand in California. Solar produces power when it’s least needed: from 7 am to 4 pm during Off Peak and Super Off Peak time frames. Adding more solar power makes the problem worse, requiring even more solar PV power and other plants to shut down more often.
[ California solar generation has reached the point where it’s producing so much power at the time of day when it’s least needed that it has to be shut down during the sunniest time of the year. This is because year round, solar generates power when there is the least demand, and the least power when demand is highest.
Notice in the figure above that peak demand occurs after 4 p.m., which according to the California ISO, is “when the sun is setting and solar output is declining. During July and August supplies are even more limited during peak hours”. Except for July and August on the weekends, supply surplus occurs during “super off-peak” hours from 10 to 4 PM – which is when solar generation is at its highest. In addition, surplus conditions occur this same time period in March and April weekdays while weather is still mild and there’s no need for air-conditioning.
Because solar PV is so seasonal, it provides from 2% in winter, to 10% in summer of California’s daily needs — but not when most needed, and at times, far more than what is needed, so solar PV and/or other power generation has to be shut down. Additional solar PV power only makes the problem worse. Solar thermal with energy storage would help, but it’s mostly “smoke and mirrors”, less than a quarter have storage, and most of the time produce less than half a percent of daily power needs for California.
Alice Friedemann www.energyskeptic.com author of “When Trucks Stop Running: Energy and the Future of Transportation”, 2015, Springer]
July 12, 2016. On the path to deep decarbonization: Avoiding the solar wall. Op-Ed by Sally Benson and Arun Majumdar Co-Directors, Stanford Precourt Institute for Energy.
If California continues to rely mostly on solar resource for meeting the 2030 50% Renewable Portfolio Standard, the total statewide solar-generating capacity would reach 30 to 40 GW under peak production, according to a report by Energy and Environmental Economics Inc. (E3).
Under these conditions, on a sunny day, for most of the year, California would be generating more electric power than it needs during the middle of the day from solar energy alone.
E3 calculates that this large amount of over-generation could be a problem 23% of the time, resulting in curtailment of 8.9% of available renewable energy, with marginal overgeneration by solar PV of 42-65 percent.
In other words, California could hit the solar wall.
And this does not even consider that midday demand is likely to decrease due to the installation of additional residential and commercial solar PV systems “behind the electricity meter.”
Consequences of hitting the solar wall
Just a decade ago it would have been nearly unthinkable that during the middle of the day solar energy could provide more electricity than an economy as large as California’s needs. But supportive policies, rapid scale-up and decreasing costs make this possibility a reality today. While from some perspectives this is very encouraging, in reality, there are consequences for hitting the solar wall. For example:
- Reliance on so much solar energy would require rapid ramping capacity for more than 10s of GW of natural gas power plants from 4:00-6:00 p.m., when the sun is going down and electricity demand goes up as people return home.
- Large back-up capacity from natural gas plants or access to other sources of dispatchable electricity would be required for days when the sun isn’t shining.
- Zero marginal-cost solar generation could squeeze out other valuable low-carbon electricity sources that can provide baseload power. For example, natural gas combined cycle plants, geothermal energy and nuclear power that cannot operate during these times at zero marginal cost.
- Large-scale curtailment of solar PV during times of overgeneration, which will reduce the value of solar capacity additions to investors.
- Real-time pricing during times of overgeneration could limit or eliminate the net-metering advantage of PV on residential and commercial-scale installations.
There is no doubt that California’s solar energy potential is invaluable, but we must take steps to avoid the solar wall.
A few of the suggested things to do:
- Ensure adequate capacity of rapid ramping natural gas plants to provide reliable supply during the morning and evening hours as the sun rises and sets.
- Increase energy storage to avoid curtailment of solar overgeneration during peak production periods. For now, few financial incentives exist for large-scale pumped-hydropower or compressed air storage projects. Levelized costs of small-scale storage in batteries range from about $300 to more than $1,000/megawatt-hour (MWh) depending on the use-case and the technology. These are expensive compared to pumped-hydro storage at $190 to $270/MWh. For comparison, gas peaker plants have a levelized cost of $165 to $218/MWh. The business case for battery storage will be limited until prices come down significantly. Both R&D and scale-up will be needed to reduce costs.
- Use electrolysis to produce hydrogen fuel to augment the natural gas grid, generate heat and power with fuel cells, or power hydrogen vehicles. However, compared to storing electricity in batteries, hydrogen-based storage systems that combine electrolysis and fuel cells are about three times less efficient. In addition, today, these technologies are expensive, and significant cost reductions will be required to make them competitive alternatives.
Issues with renewable power:
- Short steep rams when the ISO must bring on or shut down generation resources to meet an increasing or decreasing electricity demand quickly, over a short period of time
- oversupply risk: when more electricity is supplied than needed
- decreased frequency response when less resources are operating and available to automatically adjust electricity production to maintain grid reliability.
To balance unpredictable, intermittent renewables ISO needs flexible resources that can sustain an upward or downward ramp, change ramp directions quickly, react suddenly to meet expected operating levels, start with short notice from a zero or low electricity operating level, stop and start many times a day, respond for a defined period of time.
For example, figure 1 shows a net load curve for the January 11 study day for years 2012 through 2020. This curve shows the megawatt MW amounts the ISO must follow on the y axis over the different hours of the day shown on the x axis. Four distinct ramp periods emergy.
#1 Ramp of 8,000 MW upward (duck’s tail) starting 4 a.m.
#2 Ramp down at 7 a.m. when sun comes up and solar generation starts (belly of the duck).
#3 At 4 p.m. solar generation ends, ISO must dispatch resources to meet the 11,000 MW ramp up (arch of the duck’s neck)
#4 Finally a ramp down after around 6 p.m. until the next morning
Figure 1. Net load – January 11, winter ramp duck
The summer has even more extreme requirements – the system needs to supply an additional 13,000 MW within just 3 hours to replace the electricity lost by solar power as the sun sets.
The more renewable energy added to the grid, the more likely it is that more electricity will be generated than needed. Since the system frequency must be within a very tight band around 60 hertz, solar and wind sudden over or underproduction can cause a blackout if not compensated for. When there’s too much production of electricity, this drops wholesale prices to zero or even negative, which causes generators to have to paly utilities to take the energy. This is a situation to be avoided if possible. The middle of the day is when it is most likely solar will produce too much electricity. ISO would like to see this electricity exported, more electric cars, encourage users to consume when this happens, and more energy storage.
More California ISO links
Daily Renewables Watch (daily renewable stats back to 4-20-2010)