10.10 Photovoltaic (PV) Solar Energy + Storage

Jeff Simpson

Learning Objectives – By the end of this chapter, you should be able to do the following.

  • Explain how a photovoltaic panel or plant produces electricity.
  • Identify the parts of the US with the greatest solar potential
  • List advantages and disadvantages of photovoltaic energy.
  • Explain the importance of the duck curve and how solar + storage can mitigate issues.
  • Describe several possible long-duration energy storage options.
  • Explain how energy storage works to support the electric grid.
  • Distinguish between short-term and long-term energy storage.

Note: This section combines solar + storage. You will see why this is done as you read.


Solar energy is the major energy source driving life on earth and many human activities. Though only one billionth of the energy that leaves the sun reaches the earth’s surface, this is more than enough to meet the world’s entire energy needs. Solar energy represents an essentially unlimited supply of energy as the sun will long outlast human civilization on earth.

Unlike energy from fossil fuels, which almost always come from a central power plant, solar power can be harnessed locally. A solar water heater on the roof can provide all the water many houses need or heat a swimming pool. A set of photovoltaic solar panels on a home’s rooftop can provide electricity to the house, and with batteries, excess electricity produced by the panels can be stored and used when the sun is not out.

Society’s use of solar power on a larger scale is just starting to increase. Scientists and engineers have very active, ongoing research into new ways to harness energy from the Sun more efficiently. Because of the tremendous amount of incoming sunlight, solar power is being developed in the United States in southeastern California, Nevada, and Arizona.

Solar photovoltaic (PV) devices, or solar cells, change sunlight directly into electricity. PV uses semiconducting materials such as silicon to produce electricity from sunlight: when light hits the cells, the material produces free electrons that migrate across the cell, creating an electric current. Small PV cells can power calculators, watches, and other small electronic devices. Arrangements of many solar cells in PV panels and arrangements of multiple PV panels in PV arrays can produce electricity for an entire house. Some PV power plants have large arrays that cover many acres to produce electricity for thousands of homes. Hundreds of thousands of houses and buildings around the world have PV systems on their roofs. Many multi-megawatt PV power plants have also been built. Covering 4% of the world’s desert areas with photovoltaics could supply the equivalent of all of the world’s electricity. The Gobi Desert alone could supply almost all of the world’s total electricity demand.

Solar photovoltaic (PV) devices (or solar cells) change sunlight directly into electricity. Small PV cells can power calculators, watches, and other small electronic devices. Arrangements of many solar cells in PV panels and arrangements of multiple PV panels in PV arrays can produce electricity for an entire house. Some PV power plants have large arrays that cover many acres to produce electricity for thousands of homes. LINK

Solar panels on the International Space Station (ISS) while it was docked with the Space Shuttle Atlantis. PV provides all the power for the ISS.
Fig 10.10.1 – Solar panels provide all the power for the International Space Station (ISS), here docked with the Space Shuttle Atlantis. 


PV, developed in the US, was so expensive initially that it was used only where no other power sources would be feasible such as satellites or remote locations.

The four large panels shown in the image to the right are made of many smaller cells as shown below.


A PC cell.
Fig 10.10.2 – An individual PV cell.









Figure 10.10.3 - The price per watt of conventional (c-Si) solar cells has fallen since 1977. The price is starting to level off.
Fig 10.10.3 – The price per watt of conventional (c-Si) solar cells has fallen since 1977. The price is starting to level off.

Growth of solar around the world has been so rapid that a logarithmic scale must be used in the graph below.

Worldwide growth of photovoltaics on a semi-log plot since 1992
Fig 10.10.4 – Worldwide growth of photovoltaics since 1992 on a semi-log plot. Using a capacity factor of .89 for nuclear and .25 for PV, the PV deployed around the world is equivalent to 16 Palo Verde Nuclear Generating Stations (PVNGS). The world will add 190GW of PV in 2022, equivalent to adding almost 13 PVNGS in one year. LINK 


Fig 10.10.5 – In the last decade, US solar has grown at 33% per year thanks to strong federal policies like the solar Investment Tax Credit, rapidly declining costs, and increasing demand across the private and public sector for clean electricity. There are now more than 121 gigawatts (GW) of solar capacity installed nationwide, enough to power 23.3 million homes. 


Figure 10.10.6 – The US planned 39.7 gigawatts (GW) of new electricity generating capacity to start commercial operation in 2021. (This excluded distributed or rooftop PV. Those are not counted by EIA.) Solar accounted for the largest share at 39% followed by wind at 31%. About 3% of the new capacity will come from the new nuclear reactor at the Vogtle power plant in Georgia. For comparison, the output of the Palo Verde Nuclear Generating Station near Phoenix, AZ, USA is 3.9 GW. However, the capacity factor (the amount of time a plant produces energy) of a nuclear power plant is near 90% while Arizona PV will be closer to 25%. LINK


Levelized Cost of Energy
Fig 10.10.7 – As shown above, utility-scale solar is among the cheapest forms of electricity. Fossil fuels have externalized costs that are not included such as environmental damage and health effects. If the externalized costs of fossil fuels were included in this chart, the true cost of fossil fuels would be much higher. “C&I” means “commercial and industrial – locations with large areas of roof. 


Sources of US Electricity
Fig 10.10.8. The sources of electricity in the US have always changed. Only recently have non-hydro renewables begun to challenge the incumbent fossil fuels. Note that after growing for decades electricity use in the US has stayed fairly steady since the early 2000s. This may be due to a move toward more energy efficient appliances and LED light bulbs or a fluke as, again, the EIA does often has not included rooftop (distributed) solar production in many of their calculations, instead including only utility-scale solar.


Figure 10.10.10 - Non-hydro renewables are set to pass coal in electricity production in the US.
Fig10.10.9  – Non-hydro renewables are set to pass coal in US electricity production.

With all the above going for PV, it seems like solar power should be everywhere, but there are two items to consider.
1) There are so many existing fossil fuel power plants in the world, that even adding 190GW of world PV 2022 is just a bump. Making a real change will take many years of both growth in PV and in the decommissioning of older fossil fuel plants. This trend is continuing but is a transition. (LINK)
2) PV works only during daylight hours. As storage becomes bundled with PV, this limitation will be mitigated.


Video 10.10.1 – How Solar Energy Got so Cheap, and Why It’s Not Everywhere…Yet (7:53)

The video above makes the point that most “traditional plants suck at ramping up,”unable to go from zero to producing power in a short period of time. This is true with nuclear and coal. Nuclear power in the US most often operates at a constant output, rarely changing its output by much except when refueling. A coal plant may take from 3 hours to 2 days to ramp up. Neither nuclear or coal pair well with solar, because of this delay. But natural gas plants do. A gas peaking plant – intended to operate only during times of peak demand, usually in the afternoon and evening when solar contributions to the grid drop off and before people go to bed and require less electricity – does pair well with solar, and such plants can be operational in a few minutes. The West Phoenix gas plant can ramp up in under 3 minutes; it is essentially a jet engine attached to a turbine and generator. LINK

The video above discussed storing excess energy from solar for later use in flow batteries, pumped hydro, hydrogen, and gravity storage. Two less likely options include underwater air and compressed air storage. Other chemical storage methods include the generation of ammonia or methane, both which can be used as fuel for ocean shipping. 
Figure 10.10.10 – Storage increasingly is being paired with distributed PV on houses and commercial facilities. By storing excess rooftop solar production on-side, stress on the electrical grid is reduced creating a more stable grid and saving money for everybody. 


Some people point out that the making of PV cells and panels requires electricity, that dangerous chemicals are used in the manufacture of the panels, and that landfills will be over run with PV waste. The following video examines those claims made by critics of PV.

Video 10.10.2 – How Green Is Solar Energy Really? (9:02)

Many of the same people who are concerned about potential waste from PV are fine with mining coal, removing mountaintops, fracking, coal ash dumps, emissions from coal, oil and natural gas, and the carbon dioxide emitted from all fossil fuels.

Advantages of PV + Storage
1. At utility scale, PV is the least expensive form of electricity.
2. There is no fuel. It is renewable.
3. There are no emissions. This leads to lower hospitalizations and deaths and a cleaner environment.
4. It does not contribute to climate change.
5. Unlike large utility plants which are centralized, solar can be distributed, lessening the need for large transmission lines.
6. It is locally produced. More money stays local.
7. No large military is needed to protect our “national interests” that occur in other countries.

Disadvantages of PV + Storage
1. Solar is variable. The sun goes down. There are cloudy days. It is dispatchable only when coupled with storage.
2. PV can require large areas, though the area affected may not be more than the land affected by coal mining and ash waste pits. Also, rooftops can be used, reducing the need for cooling the structure underneath.



Roof of the car showing small solar panels
Fig 10.10.11 – Roof-mounted solar panels on a BYD F3DM plug-in hybrid car.



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