How Does A Solar Grid Work At Night?
Solar power harnesses energy from the sun to generate electricity. Solar panels convert sunlight into direct current (DC) electricity. Inverters then convert the DC electricity into alternating current (AC) electricity that can be used in homes and businesses and fed back into the grid. While solar power provides clean, renewable energy, one of the key challenges is how to continue supplying power when the sun goes down or is obscured by clouds. This article provides an overview of how solar grids work to provide electricity even at night.
Solar Panels and Inverters
Solar panels contain photovoltaic cells that convert sunlight directly into direct current (DC) electricity. The photovoltaic effect causes electrons in the cells to be knocked loose when photons from sunlight hit the panels, generating an electric current. More intense sunlight exposure results in more electricity generation.
The DC power generated by solar panels then flows to an inverter, which converts the DC electricity into alternating current (AC) electricity that is compatible with the electric grid and can be used to power homes and businesses. Inverters play a crucial role in linking solar panel systems to the grid. Pure sine wave inverters provide the highest quality AC power while modified sine wave inverters are more affordable but can cause compatibility issues with some devices.
Energy Storage
Energy storage is crucial for solar power systems to provide electricity at night when the sun is not shining. There are several methods used to store excess solar energy during the day for use at nighttime [1]. The most common is battery storage, usually using lithium-ion batteries. These store electricity chemically and release it when needed. Battery storage allows solar homes to disconnect from the grid and provides backup power during outages [1]. However, batteries can be expensive upfront. Other lower-cost storage options include pumped hydro storage which uses excess solar power to pump water uphill into a reservoir, then releases it through hydroelectric turbines at night. Thermal energy storage heats or cools a material during the day that can provide heating/cooling at night. Hydrogen production uses electricity to split water into hydrogen and oxygen, then the hydrogen is stored and later converted back into electricity in fuel cells when needed.
Grid-tied Solar Systems
Grid-tied solar systems are connected to the main electrical grid. They generate electricity from sunlight using solar photovoltaic (PV) panels. The electricity produced is converted by an inverter and sent directly into the home’s electrical system to power appliances and lighting. Any excess electricity that is not immediately used is exported to the grid [1].
During the day when the sun is shining, the solar PV system generates more electricity than a home uses. This excess electricity is pushed out to the larger utility grid, effectively running the electric meter backwards and providing credit to the homeowner. At night or on cloudy days when solar production drops, the home will pull electricity from the grid utility [2].
This two-way flow of electricity allows grid-tied systems to feed excess solar power to the grid during daylight hours when production is high. Then as production drops in the evening, the home can seamlessly draw power from the grid as needed. This integration with the grid provides continuous electricity around the clock.
Net Metering
Net metering policies allow solar customers with grid-tied systems to receive credit for excess electricity they generate and send back to the grid. Net metering enables solar customers to, in effect, sell their excess power generation to their utility at the retail electricity rate. With net metering, a bidirectional electricity meter tracks both electricity consumed from the grid and excess solar electricity sent to the grid. During times when the solar panels produce more electricity than the home is using, the excess is fed onto the grid, and the customer’s meter spins backwards to provide a credit in kilowatt-hours. At night or during periods of low solar production, the customer buys electricity from the grid and the meter spins forward as normal. At the end of each billing cycle, the customer is only charged for their net electricity usage – the difference between what they consumed from the grid and what they sent back. This greatly improves the value proposition of installing solar panels by providing bill savings from excess solar generation.
Base Load Power Plants
Traditional base load power plants like nuclear, coal, hydroelectric, and geothermal play an important role in complementing solar energy to meet nighttime demand. As the Penn State College of Earth and Mineral Sciences explains, base load plants provide steady, low-cost power around the clock. Though solar can meet a large portion of daytime energy needs, base load plants help fill the gap at night when solar panels aren’t generating electricity.
According to the National Electrical Manufacturers Association, nuclear and hydroelectric plants are common base load sources in the U.S. grid. Geothermal can also provide consistent baseload power. These plants have high capacity factors, meaning they can reliably generate electricity 24/7. While solar and wind offer inexpensive, clean energy during sunny and windy periods, turning to base load plants at night maintains grid reliability.
As renewables expand their role in energy grids, base load plants help integrate intermittent resources. Base load capacity balances the variability of solar and wind generation. Energy storage systems, like batteries and pumped hydro storage, are another solution for supplying power when the sun isn’t shining. But base load plants continue to provide an important foundation, especially for overnight and prolonged low-solar periods.
Solar Forecasting
Solar forecasting refers to predicting how much solar energy will be generated by PV systems at a certain place and time. Forecasting models allow grid operators to anticipate the variable output of solar panels and effectively schedule other power sources like natural gas and battery storage to meet demand. There are three main techniques for solar forecasting:
Statistical models use historical performance data and weather forecasts to predict solar output. Time series analysis and machine learning algorithms are commonly used. These models excel at near-term forecasts up to 6 hours ahead.
Physical models simulate the physical processes involved in converting sunlight to electricity. They incorporate weather data like cloud cover and solar irradiance to model energy yield. Physical models provide greater accuracy for intra-day forecasts up to 72 hours ahead.[1]
Ensemble forecasts combine multiple statistical and physical models to leverage the strengths of each approach. The varied projections are aggregated into a consensus forecast that is more robust and accurate than any single model. Ensemble techniques offer the best performance for day-ahead predictions.
By integrating solar forecasts into unit commitment and economic dispatch processes, grid operators can schedule the optimal mix of power sources ahead of time to match net load. This enables the seamless integration of solar energy while maintaining reliability and minimizing costs.
[1] https://www.gridx.ai/knowledge/what-is-solar-power-forecasting
Demand Response
Some utilities implement demand response programs to reduce electricity usage during times of peak demand or when solar generation is low. These programs provide incentives for customers to voluntarily reduce energy consumption during specific times. This helps lower the stress on the grid and reduce the need for supplemental fossil fuel generation when solar production is insufficient to meet demand.
For example, a utility may request that customers reduce usage between 5 pm and 8 pm on hot summer evenings when air conditioning load is high but solar generation is dropping off. Customers who lower their consumption during the requested timeframe may receive bill credits, cash payments, or other rewards. The utility benefits by avoiding the need to ramp up expensive natural gas peaker plants when solar power declines in the evening.
According to an analysis by the Brattle Group, demand response programs paired with renewable energy sources like solar can lead to substantial cost savings. They estimate $8 billion per year in avoided system costs by using demand response to shift just 10% of peak demand to other times (Source). Intelligent utilization of demand response allows utilities to reliably and cost-effectively integrate more solar power onto the grid.
Microgrids
Microgrids are localized grids that can disconnect from the traditional grid and operate autonomously. Many microgrids integrate solar PV and battery storage to continue operating during grid outages or at night. For example, according to a report by the U.S. Department of Energy, Fort Carson in Colorado has a microgrid with a 13MW solar array and a 30MW battery that can provide backup power for over 12 hours (https://www.energy.gov/). The Sterling Municipal Light Department in Massachusetts has a 4MW/3.9MWh battery integrated with their existing solar installation that provides power to municipal facilities at night (https://www.nrel.gov/). Microgrids with solar+storage allow communities and facilities to maintain resilient operations 24/7.
Conclusion
In summary, there are several techniques that allow solar grids to continue providing power even when the sun goes down. Solar panels produce energy during daylight hours that gets stored in batteries, allowing it to be used at night. Grid-tied solar systems with net metering send excess solar power to the grid during the day, which homeowners can then draw from at night. Grid operators can accurately forecast solar production and shift other power plants to fill in when solar dips. Demand response reduces energy usage during peak demand periods. And microgrids can disconnect from the main grid and operate autonomously using local solar and storage. With the right combination of these solutions, along with steady base load power plants, a solar grid can effectively and reliably operate 24 hours a day.
