The most populous state is a testbed for how far renewables can go
When the sun is shining and the wind is howling, renewables can provide sustainable power to metropolitan and rural communities alike. It’s a tempting lure for countries of all sizes looking to embrace green grids, but concerns remain regarding the reliability, consistency and economic viability of renewable energy sources.
A “green grid” is an electric grid fully or partially supplied by renewable energy. Portugal, for example, has a fully green grid that powers their entire country, as well as part of neighboring Morocco, with electricity generated from renewable energy sources like hydropower. Scotland gets 70% of its electricity from renewables. The United States, however, is a different animal.
With over 325 million people and an energy policy caught between ever-conflicting federal and state agendas, the world’s third largest country gets pulled in a lot of directions. Close to 15% of Americans live in California. California is a prime example of a state with plans for a green grid. By 2025, California hopes to have at least 1.5 million zero-emission vehicles on the road. By 2030, 50% of retail electricity is expected to come from renewable sources.
Several years in, California serves as an example of what can work and what can’t in a green-grid state. This information is useful for other state and countries looking to adopt renewable energy as a primary source of energy.
The peaking power problem
Thanks to some ambitious planning and crucial state subsidies, many cities in California have already achieved renewable baseload power supply. That being said, peaking power presents another issue altogether.
Currently, gas-fired power plants supply most of the energy during peak demand while also acting as a backup power supply in case of emergencies, such as battery storage failure for solar power or adverse weather conditions that hinder renewable generation.
The burden on these gas-fired power plants is huge. California Independent System Operator (CAISO), which administers the vast majority of the state’s electric power system and transmission lines, found that cities can experience a more than 50% increase in electricity consumption between 4 and 7 p.m. This rapid ramp is almost entirely powered by the massive industrial gas turbines that drive the gas-fired plants. The frequent starting and stopping of these gas turbines in response to small or large peaking power demand is detrimental to the machinery and results in more frequent maintenance that, in turn, drives delays and costs.
By providing solely peaking power functions, these gas turbines can experience 50% load factors, significantly lower than the industry average of 85%. Load factor, sometimes called capacity factor, is the ratio of actual power output to theoretical output if it were possible to operate the plant at full nameplate capacity. An 85% industry-wide average is simply a result of the inability for a plant to run constantly over the course of a year.
Issues with efficiency and cost
The underutilization and stress of these gas turbines is an unforeseen consequence of solar-supplied baseload power generation. Battery technology needs to store solar during the day for use at night. That technology has come far but remains expensive. Because solar plants achieve only 20% to 25% load factors when compared to the 85% average for gas-fired plants, they must have a substantially higher capacity to generate comparable power.
For example, if the electricity demand in a region is 24,000 MWh per day, meaning the hourly net capacity is 1000 MW, then a gas-fired power plant would need a gross capacity of close to 1200 MWh to compensate for its 85% load factor. On the other hand, solar-generated electricity would need 4000 to 5000 MW of gross capacity per hour due to its lower load factor of 20% to 25%. This massive increase in gross capacity, along with the inherent higher costs of solar and its accompanying battery technology, can result in 10 times the capital cost of solar power generation when compared to gas.
Solar-powered baseload electricity coupled with gas-fired peaking electricity can be an inefficient utilization of machinery and capital. Additionally, simple cycle gas plants are predominately used for solely peaking power generation instead of the more powerful and lower-emission combined-cycle plants. This is because combined cycle plants operate the gas turbine-driven Brayton cycle and the heat recovery and steam turbine system of the Rankine cycle. The full process can take over half an hour, making it inefficient as a flexible power source for low-energy demand or emergency power.
Although combined-cycle power plants can achieve up to 50% greater efficiency than simple cycle plants, there are limitations in using them for peaking power demand. For example, the heat recovery steam generator (HRSG), which produces superheated steam by deriving power from the exhaust gas of the gas turbine, cannot simply start and stop to accommodate peak demand. The HRSG must run constantly due to its complicated arsenal of heat exchangers, economizers that heat water close to its point of saturation, evaporators that produce saturated steam and superheaters.
The costs of running the HRSG are taxing on combined-cycle plants with applications for peaking power demand. The economics of combined-cycle plants, the attainable energy efficiency and the significant reduction in emissions all depend on the plant operating as frequently as possible. Therefore, combined-cycle plants only gain their edge against simple cycle plants if they are used for baseload power demand.
Realistically, simple cycle plants must be used in conjunction with renewable-driven baseload power for a system that is reliable and economically feasible. Although renewable-sourced baseload power will result in zero emissions, the utilization of simple cycle gas-fired plants instead of combined-cycle plants reduces the overall environmental impact of the system.
Economics, equipment wear, zero-emission and efficiency aside, using solar and gas for power generation will undeniably have a positive impact on the environment. For sunny states like California looking to reduce their carbon footprint, this combination can provide an ideal balance between reliability and renewability.
Balancing power and emissions
The renewable wave is swelling alongside a push toward expanding gas-powered turbine capabilities by maximizing utilization and managing maintenance with fixed intervals and inputs. Together, these two waves are crashing on California’s coast by offering environmentally sustainable power generation solutions with the potential to decarbonize the global economy.
“A one-size-fits-all approach doesn’t work anymore,” said Stefan Jansson, Siemens Gas and Power product commercialization manager. “Plant operators deserve competitive value propositions that provide efficiency and cost security.”
Siemens seeks to provide fuel flexibility solutions for their industrial-grade gas turbines. Hydrogen and natural gas co-firing in gas turbines is emerging as a dual-fuel option for combined-cycle power plants.
Siemens long-term goal is to reach the ability to fire 100% hydrogen with its dry low emission system, which would mean zero CO2 power production. A combination of hydrogen storage production – when solar and/or wind power go overcapacity – and hydrogen firing in a high-efficiency combined-cycle plant during under capacity periods from solar/wind, is a future vision Siemens hopes becomes reality.
Reducing emissions without sacrificing power is a constant challenge for the energy industry. To tackle that challenge, operators that power California metropolitan residences and industries will most likely have access to a variety of gases, such as pipeline-quality gas, wellhead gas or process gas. They’ll also need the ability to switch between these different gaseous fuels. Gas turbines with close to 60% net efficiency and the reliability to run on different fuel sources will be able to compete with renewables in terms of power and by providing a versatile and low emission solution to the power generation industry.
Siemens is one company with equipment designed to combat emissions while maintaining power. The SGT-800, for example, provides up to about 165 MW of power at 58.5% combined cycle net efficiency in a configuration with two SGT-800 gas turbines and one steam turbine, Siemens said. The gas turbines offer low NOx and CO emissions without the need for water. Its efficiency also helps minimize the CO2 footprint. SGT-800 combustion hardware has built-in flexibility and durability so that it can withstand wide changes in gas compositions, the company said.
“It’s all about balance,” Jansson said. “Operators are pushing to reduce emissions while simultaneously over-firing when spot prices jump during peak demand. Increasing power output to take advantage of market prices without increasing maintenance levels is one of the biggest challenges in the industry today.”
In response to support operators maximizing the use of their assets in this challenge, Siemens has developed component lifetime algorithms for its aftermarket support. Lifetime algorithms adapt maintenance and operations according to the operator’s needs, based on site unique factors. Siemens utilizes fleet experience to develop algorithms that monitor and forecast the lifecycle of critical components by considering the operating profile, ambient data and component properties through big data management. This enables an operator to fit the maintenance cycle into low-season periods while maximizing output during high-season demand.
Peaking power solutions
For peaking power, simple cycle gas-fired plants with the ability to start and stop almost instantly will give the power generation industry the stability it needs to embrace renewables.
Although aeroderivative gas turbines are typically designed for aviation power, their lightweight profile and reliability make them well equipped for simple cycle plants. Siemens SGT-A65 aeroderivative gas turbine provides over 60 MW and 41.3% to 43.8% simple cycle fuel efficiency. It also can cold start and reach full power in under 5 minutes, according to the company.
California will likely install several aeroderivative gas turbines in its simple cycle power plants to accommodate the rush of solar power generation. Starting in 2020, new homes in California must have net-zero energy needs. This means that these new homes must produce enough electricity, usually via solar, to be self-sufficient. The country’s largest state by population averages 80,000 new homes per year. About 15,000 of the 80,000 new homes install solar.
If 2020 yields the same housing starts as today, the number of new solar-powered homes per year will quintuple from today’s levels for a total of 80 000 new solar-powered homes. This annual addition will drastically increase the solar power production of the state as well as save these new homes an average of US$80 per month on their electricity bills. For example, the average US household consumes 11 000 kWh per year, which is about 30 kWh per day. Usage rates are lower in California, with the average California household only consuming 7000 kWh per year – 19kWh per day – due to temperate year-round weather. The average solar panel generates about 1 kWh per day meaning a California household would need about 19 solar panels to be energy self-sufficient.
With the addition of 80,000 new homes providing self-sufficient power and likely adding extra power to the grid, California could produce too much renewable power. The state already offloads extra power to neighboring Nevada and Arizona. On particularly sunny days, California has been known to pay its neighbors to take electricity off their hands. Neighboring states receiving free electricity is simply uneconomic for California since the state is already paying a premium for more expensive solar power generation.
California’s citizens are feeling the brunt of what it really costs to live in a renewable-reliant state. The average kWh cost of electricity in the United States is 13.2 cents. In California, it’s close to 20 cents, meaning that citizens without rooftop solar must pay 50% more for their electricity than the national average. Most Californians who choose to drive internal combustion engine cars must pay a US$0.473 state excise tax and a 2.25% sales tax on gasoline. California state income taxes are around 8% for the middle class and peak at a marginal tax rate of 13.3%.
The rush of solar installations, both commercially and residentially, will flood the Golden State with renewable baseload power generation, reducing emissions and pushing California closer towards its aspirational green grid. It remains to be seen whether the cost of renewable power generation, which falls on California residents in terms of taxes and laws, is worth the environmental impact of solar.
Takeaways from California
Not every state or country has the sunshine or aspiration for green grids. For those that do, cities across California all provide miniature case studies of the successes and failures of green baseload power grids. As battery technology, solar panel and solar inverter costs come down, the industry will likely look closer at renewable power generation. For now, the efficiencies and low emissions of combined-cycle gas-fired power plants make it difficult to argue any other way.
Monitoring California may be a good idea for those who like to keep a finger or two on the pulse of the power generation industry. Despite if you agree or disagree with California’s approach, the state provides the perfect sandbox for the latest applications and advancements of gas turbine technology and solar-supplied baseload power.
The author: Danny Foelber is the owner of Eau Claire Writing, a freelance writing company that specializes in oil and gas content. Foelber is also an oil and gas and marine industry writer.