What Happens To Unused Solar?
Solar energy is the radiant light and heat from the Sun that is harnessed using various technologies such as solar panels to generate electricity and thermal energy. Solar power is a renewable energy source that produces no greenhouse gas emissions during operation. With the growth of solar panel installations in recent years, particularly on residential rooftops, an emerging issue is how to handle excess or unused solar energy that is sometimes generated. This occurs when solar power production exceeds a home’s or grid’s real-time usage and storage capacity. Excess solar presents both challenges and opportunities if not properly managed. This article will examine what happens to unused solar energy, causes, impacts, and potential solutions.
Solar Energy Production
Solar power production varies throughout the day and year due to the position of the sun. Solar panels produce the most electricity when they are directly facing the sun at a 90 degree angle. The hours of peak solar production are called “peak sun hours.” Peak sun hours are defined as hours when solar irradiance (sunlight) reaches an average of 1,000 watts per square meter on a solar panel surface. The number of peak sun hours per day depends on location, time of year, and weather conditions.
In the continental United States, there are 4-6 peak sun hours per day on average. Production is highest from spring through early summer when the days are longer. According to one source, solar production peaks in June. Areas like the Southwest can see up to 7 peak hours in the summer months, while winter months have far fewer peak hours. Production is also impacted by cloud cover, pollution, and other weather conditions that can limit irradiance.
Supply and Demand
Solar energy often experiences periods of supply and demand mismatch because you cannot accurately predict the amount of solar energy that will be produced at any given time (Nel Hydrogen, 2022). The amount of solar energy produced depends on variable factors like weather and time of day. But demand for electricity also fluctuates throughout the day. This mismatch between the supply of solar power and demand for electricity on the grid can lead to an excess supply of solar energy at certain times.
For example, solar energy peaks at midday but demand may not peak until late afternoon or evening when people return home from work. According to PV Magazine, India is projected to see a 20% drop in annual utility-scale solar installations in 2023 partly due to supply-demand mismatches from factors like module shortages and project delays (PV Magazine, 2023).
Grid Interconnection
Solar power connects to the electric grid through a process called interconnection. This allows solar panels to send excess electricity they generate back to the utility grid. The connection point where a solar system ties into the grid is called the point of interconnection.
To connect to the grid, solar systems must meet certain technical standards for safety and reliability. They typically include an inverter to convert the DC power from solar panels into usable AC power that synchronizes with the grid frequency. The inverter enables two-way power flows between the solar system and grid.
While integrating more solar power has benefits like clean energy, it also poses grid integration challenges. Solar output varies with weather and time of day, which makes balancing electricity supply and demand more complex (Ye-Min). The intermittent nature of solar can lead to stability issues on the grid. Solar variability also makes forecasting and scheduling much harder for grid operators (RatedPower).
Large amounts of solar power can lead to oversupply during peak generation times like midday. This requires curtailing or otherwise using the excess solar electricity. Grid upgrades and modernization may also be needed to handle greater solar penetration without compromising reliability (RatedPower). Overall, significant coordination and planning is necessary to integrate substantial solar capacity onto the existing electric grid infrastructure.
Excess Solar Scenarios
Excess solar energy is most often generated during peak production hours when solar panels are operating at maximum capacity. This typically occurs on sunny weekend middays during the spring and summer months when electricity demand is lower. Homes with solar panels may produce more renewable energy than they can use during these high production times.
For example, on a sunny Saturday afternoon in July, many homeowners are away from the house and electricity needs are minimal. However, the sun is shining brightly on the rooftop solar array, enabling it to generate surplus clean energy. This excess solar production feeds into the grid for other homeowners and businesses to utilize.
According to the U.S. Energy Information Administration, weekends have the lowest average electricity demand. Weekday demand peaks in the late afternoon and early evening as people return home from work. So weekends often lead to the greatest excess solar generation. Seasonally, solar panels generate the most surplus electricity during the sunny spring and summer months when the days are longer.
Curtailment
Curtailment refers to limiting or reducing the output of a solar energy system. This happens when there is an oversupply of solar power being fed into the grid, and the grid cannot absorb all of the excess generation. According to experts, curtailment can occur for a few key reasons:
Economic curtailment happens when the price of electricity in wholesale markets is lower than the compensation rate received by the solar generator. In these cases, it makes economic sense for the solar operator to curtail some of their output rather than sell it at a loss on the wholesale market (1).
Transmission congestion can also cause curtailment when there is not enough capacity on transmission lines to transport excess solar power to areas of demand. Congestion forces grid operators to restrict solar output in order to maintain reliability of the grid (2).
Finally, minimum generation constraints, which require traditional baseload power plants like coal and nuclear to generate at a certain minimum level in order to operate safely and efficiently, can force curtailment of solar. When solar output meets minimum demand requirements, baseload plants cannot turn down further, resulting in oversupply and curtailment (3).
(1) https://www.gridx.ai/knowledge/solar-curtailment
(2) https://blog.ucsusa.org/mark-specht/renewable-energy-curtailment-101/
(3) https://www.caiso.com/documents/curtailmentfastfacts.pdf
Storage Solutions
Energy storage is a key enabling technology for the effective utilization of solar power. While solar energy is only produced during daylight hours, energy demands continue after sunset. Solutions are needed to be able to store excess solar generation during the daytime and deploy it when needed.
Batteries are the most common form of storage used with solar installations (SEIA). Lithium-ion batteries in particular are well-suited for solar applications due to their high efficiency, long cycle life, and rapidly declining costs. By storing excess solar generation during low demand periods and dispatching it during high demand periods, batteries can reduce peak demand charges and provide backup power.
Beyond batteries, there are some other promising storage technologies for solar energy. Thermal energy storage using molten salt can efficiently store heat for later electricity generation. Pumped hydro storage can store energy by pumping water uphill into reservoirs during low demand periods. Hydrogen production via electrolysis is another potential solution for long-term solar energy storage (Department of Energy).
A combination of storage solutions tailored to specific grid needs and solar plant configurations will enable increased solar penetration and utilization. Continued innovation and cost declines will make storage increasingly viable.
Alternative Usage
Excess solar energy can be utilized in various ways besides just feeding it back into the grid. Three popular alternative uses for surplus solar power are charging electric vehicles, powering electrolysis to produce hydrogen, and desalination of water.1
Installing solar-powered EV charging stations is a great way to make use of extra solar generation. The EV batteries act as storage devices, capturing excess energy that can be used to power the vehicles when needed. This helps balance demand with fluctuating solar supply while promoting sustainable transportation.
Surplus solar electricity can also be used for electrolysis to split water into hydrogen and oxygen. The hydrogen can then be stored and used as a clean fuel for transportation, heating, or generating electricity in fuel cells. This provides grid stabilization services by shifting the excess solar supply.
Solar desalination is the process of using solar energy to remove salt from seawater to produce fresh, potable water. This is an extremely useful application in arid regions with limited natural freshwater resources. When solar supply exceeds demand desalination operations can be ramped up to utilize the excess electricity.
Future Outlook
Solar power is predicted to continue growing at a rapid rate globally. According to the International Energy Agency (IEA), solar PV and wind additions are forecast to more than double by 2028 compared to 2022 levels, breaking records year after year. The IEA predicts solar PV capacity growth will average 123 GW per year from 2022-2028, compared to just 61 GW per year during 2010-2021 (IEA).
In the US specifically, the Solar Energy Industries Association (SEIA) estimates that solar capacity will triple between 2020 and 2030, reaching over 30% of total electricity generation by 2035. They predict solar will account for 20% of all electricity generation in the US by 2025 (Energy5).
To accommodate this massive growth and prevent curtailment of excess solar generation, various strategies are being implemented. These include developing more flexible grids, coupling solar with storage, using excess solar for hydrogen production or desalination, and creating regional power markets to share renewable resources across wider geographies (Arka360).
Conclusions
In summary, the growth of solar energy can sometimes outpace demand on electric grids. Utilities use solutions like curtailment and storage to manage excess solar production. However, curtailment wastes clean energy while storage adds costs. Alternative usage scenarios like electric vehicles, hydrogen production, and desalination can provide productive outlets for surplus renewable generation. Overall, the solar industry must continue innovating and planning ahead to integrate large amounts of solar smoothly. Addressing excess solar now will enable the continued growth of this sustainable energy source.
Managing excess solar production is an important issue to tackle. With thoughtful policies and technology, utilities can transition existing infrastructure to leverage excess solar instead of curtailing it. Investing in solutions today will allow solar to reach its full potential as a mainstay of our energy portfolio while accelerating the renewable energy transition.
