Solar volatility: interconnectors versus batteries?

Interconnectors cure renewables volatility

The solar energy reaching a given point on Earth’s surface varies by +/- 6% each year. These annual fluctuations are 96% correlated over tens of miles. And no battery can economically smooth them. Solar heavy grids may thus become prone to unbearable volatility. Our 17-page note outlines this important challenge, and finds that the best solutions are to construct high-voltage interconnectors and keep power grids diversified.

Solar variability: how much does solar energy vary by year?

How much does solar energy vary by year in typical locations? To answer this question, this data-file aggregates the average annual volatility of solar (and wind) resources across ten locations, mainly cities, in the United States.

Specifically, we find that the annual volatility of incoming solar radiation reaching ground level tends to vary by +/- 6% per year in a typical city, is 96% correlated across different locations within that city, and 50-70% correlated with other cities in the same region.

Workings are given in the data-file, while underlying data are from the excellent NREL NSRDB resource.

In order to smooth out annual solar volatility, we think the best options are non-correlated (i.e., diversified) energy sources, such as wind (also modelled), and other energy inputs (nuclear, hydro, gas, etc). For example, see our notes here and here.

Another excellent option is long-distance inter-connect power lines, as there is almost no correlation between the different annual insolation reaching, say, San Francisco and Houston, or New York and Seattle. For example, see our notes here and here.

Data in this file are useful for illustrating these arguments, and answering the question of ‘how much does solar energy vary by year?’. We can also run bespoke modelling for TSE clients using the NSRDB data, in which case, please contact us.

Power transmission: inter-connectors smooth solar volatility?

Can large-scale power transmission smooth renewables’ volatility? To answer this question, this horrible 18MB data-file aggregates 20-years of hour-by-hour solar insolation arriving at four cities in the US (Los Angeles, San Francisco, Phoenix and Houston). This is our starting point to assess volatility and intermittency.

For example, San Francisco receives an average annual insolation of 2,100 kWh/m2/year, however the hour-by-hour standard error is 141% of the hourly average, day-by-day standard error is 50% of the daily average, month-by-month volatility is 30% of the monthly average and year-by-year volatility is 9% of the annual average. Solar insolation is volatile.

It would be helpful for a stable grid to smooth each of these different time-periods of volatility. Hence the data-file models the impact of constructing large inter-connector transmission lines. The model uses a very simple rule: minimize the difference in solar output in the two inter-connected regions. For example if Region A has 10GW of output and region B has 5 GW (e.g., because it is cloudy that day), you could export 2.5 GW from Region A to Region B, and both would now have 7.5 GW.

Can power transmission smooth renewables volatility? Inter-connectors can have a phenomenal impact. Returning to our example of San Francisco, we find that a 2.5 GW inter-connector between 10 GW solar hubs in both San Francisco and Phoenix would smooth San Francisco’s volatility considerably: With the inter-connector, the hour-by-hour standard error is 124% of the hourly average level (down from 141%), day-by-day standard error is 36% of the daily average (down from 50%), month-by-month volatility is 24% of the monthly average (down from 30%) and year-by-year volatility is 4.4% of the annual average (down from 9%).

This is interesting and helpful because we think batteries may find it harder to smooth year-by-year volatility (it requires an enormous battery that only gets to discharge once every 2+ years). These arguments are laid out in our 17-page research report here. Workings are in the different tabs of the data-model linked below, including scripts we have used to manage the gargantuan data-sets, and apologies in advance for the large file size.

Energy security: volcanos versus solar panels?

Volcano impacts on solar power

This data-file aggregates data on c20 of the largest volcanic eruptions of the past 200-years, with a VEI of 5 or above, to quantify volcano impacts on solar power. Specifically, VEI denotes the ‘Volcanic Explosivity Index‘, a logarithmic scale that has become the standard for measuring volcanic eruptions.

For example, the 2010 eruption of Iceland’s Eyjafjallajökull volcano, which disrupted global air travel, was assessed at VEI=4, emitting 0.3km3 of tephra, 8km into the atmosphere. The VEI=5 eruptions in this data-file emitted an average of 3km3 of debris up to 20km into the atmosphere. Three VEI=6 eruptions (Pinatubo, Krakatoa, Novarupta) released 15-45km3 up to 40-80km upwards. Finally, the largest volcanic eruption since 1800 was Mount Tambora, resulting in the ‘year without a summer’ in 1816, ejecting 100km3 of material.

The data suggest that very large volcanic events occur every 30-years, on average, pushing aerosols into the upper atmosphere, dimming the sun, and thus cooling the climate by 0.5-1.2 C for 1-3 years.

Solar energy implications? So what happens as we build out solar generation in our energy mix? One technical paper has quantified how incoming solar energy measured by weather stations in the US (California, Colorado, New Mexico) fell by 20% in 1992, the year after the eruption of Mount Pinatubo, 12,000 km away, in the Philippines in 1991 (monthly data are also tabulated in this data-file).

This suggests a large systemic risk. Our roadmap to net zero sees 10% of all global energy coming from solar by 2050 (around 10,000 TWH per year). Once every thirty years, we might expect a large volcanic eruption to lower global solar insolation by c20%, thereby detracting 2,000 TWH pa from the world’s future energy system. This is the energy equivalent of Saudi Arabia’s oil simply disappearing from the market for 1-2 years. (data here)

Evaluating volcano impacts on solar power is not intended as an argument against solar. We need more solar in a decarbonizing energy system (research here). However, we think a resilient energy system likely needs to be diversified, with a large buffer of spare capacity, spanning across secure and non-correlated input sources.

Solar contacts: silver bullet?

Solar contacts silver and copper

The front contacts in today’s solar cells are made of screen-printed silver. Thus solar cells absorbed 11% of 2021’s silver market, and growing. Silver can be substituted with copper. But manufacturing is more complex and c5x more costly. So we expect a silver spike, then a switch. This 16-page note explains our outlook, and who benefits?

Manufacturing methods: an overview?

Overview of manufacturing methods

An of overview of manufacturing methods is given in this data-file, covering different means of upgrading, separating, heat-treating, drying, depositing, shaping and assembling different manufactured products.

In each case, we have aimed to quantify the relative costs, energy intensity, typical throughput volumes, an explanation of the process, and examples for how it is used.

Energy intensity varies vastly, and is 70% correlated with costs of the processes. But as a rule of thumb, a manufacturing process with <0.3 MWH/ton energy use is energy-light, while a process with >7MWH/ton energy use is energy-intensive.

Some of the lowest-cost methods are associated with the mining industry, where they are deployed at enormous scale (multi-MTpa), such as crushing, flotation and leaching; while screen-printing is one of the lowest cost assembly processes.

Conversely, some of the highest-cost methods are associated with the semi-conductor industry, involving the deposition of very thin and intricately positioned patterned layers on a substrate. These methods include photolithography, sputtering and vapor deposition.

The full data-file gives an overview of different manufacturing methods and is intended as a useful reference file or  ‘cheat sheet’, for decision-makers increasingly exploring new solar cells, battery recycling or materials used in the energy transition.

To read our latest commentary on manufacturing methods, please see our article here.

Solar trackers: efficiency improvements?

Solar Tracker Efficiency Improvements

This data-file quantifies solar trackers’ efficiency improvements. Depending on location, 40-90% of new utility-scale solar plants are being fitted with ‘trackers’ in the early-2020s; constantly re-positioning panels to face the sun, as it arcs across the sky, and as this arc varies season-by-season, due to the 23.5-degree tilt of the Earth on its axis of rotation.

Depending on the installation, trackers will typically add $0.1-1.0W to total costs, with a good ballpark estimate being a $0.2/W or c20% cost increase for utility-scale systems.

These trackers earn their keep by increasing productivity by 20-40% compared with fix-tile systems, which is quantified in this data-file, by aggregating the details from c30 technical papers and other sources.

A good rule of thumb is that a single-axis tracker adds 15% plus 0.35% per degree of latitude; while a dual-axis tracker adds 30% plus 0.25% per degree of latitude.

Latitude matters because the further a location is from the equator, the more variably the sun will arc across the sky in different seasons, especially during the summer solstice (worked examples are given in the data-file), comparing London, Singapore and Estonia, based on an excellent online resource here.

Solar trackers’ efficiency improvements are not evenly spaced throughout the year; performance additions may be around 40-50% higher in summer and c10-20% in the winter, versus typical fixed or horizontal baselines.

One of the leading providers of solar tracker systems is Array Technologies, assessed in our patent review here. Read our recent commentary on Solar trackers here.

TOPCon: maverick?

TOPCon solar cells efficiency gains

A new solar cell is vying to re-shape the PV industry, with 2-5% efficiency gains and c25-35% lower silicon use than today’s PERC cells. This 13-page note reviews TOPCon cells, which will take some sting out of solar re-inflation, tighten silver bottlenecks and may further entrench China’s solar giants.

Nexwafe: PV silicon breakthrough?

Nexwafe technology review

Nexwafe technology review. Nexwafe is developing a next-generation PV silicon technology called the EpiWafer process, growing standalone silicon wafers onto mono-crystalline seed wafers, with no need to slice ingots and surrender 30-50% of the PV silicon as ‘kerf’ sawdust.

This should improve the manufacturing efficiency, module efficiency and energy intensity of solar PV, and possibly also the costs. Silicon efficiency also matters more in the solar industry, to deflate future costs, after improved passivation has decreased the relative contribution of surface losses.

Data from the patent library shows how the resulting wafer can have 98% lower levels of oxygen impurities, and 4-5x narrower distributions of dopant distributions. 1.1% total efficiency gains are also targeted through a combination of optimizing wafer width and dopant distributions.

Our Nexwafe technology review found 60 filings and re-filings of 8 separate patents. We conclude that many underlying aspects of Nexwafe’s PV silicon ambitions are locked up with high-quality patents. However, our main surprise was the simplicity of the process, which is both a blessing and a curse (details in the data-file).

Company. Nexwafe spun out from Germany’s Fraunhofer Institute in 2015, has c40 employees, closed a €32M financing round in October-2021, lead by Reliance and including Saudi Aramco Energy Ventures.

First Solar: thin film solar breakthrough?

First Solar technology efficiency

First Solar Technology Review. This patent screenbreaks down First Solar technology efficiency. First Solar:  a solar panel manufacturer, listed in the US, founded in 1999, employing 6,400 people. It has capacity to manufacture 8GW of solar panels per year, using CdTe thin film technology.

Things to like about its technology are that they use about 60% less energy and emit about 60% less CO2 than photovoltaic silicon. First Solar also has manufacturing facilities in Ohio, satisfying the growing desire to re-shore the supply chain.

Efficiency is usually lower for CdTe solar, stated “up to 18.6%” for First Solar’s Series 6 cells, whereas we generally assume 20-25% for PV silicon. However, 70% of the patents in our review were focused on improving efficiency, and thus we might speculate whether improvements lie ahead.

There are also drawbacks to the technology and issues highlighted in the patent screen.

To read more more about our conclusions of our First Solar technology review, please see our article here. We think there is upside in the opportunity to re-shore materials that are increasingly important for the energ transition (here).

Copyright: Thunder Said Energy, 2022.