As solar panels become more prevalent for renewable energy generation, figuring out effective ways to store the energy they produce is crucial. There are a few main methods for storing solar energy, each with their own advantages and disadvantages.
Batteries
One of the most common ways to store solar energy is in batteries. Typically, lead-acid or lithium-ion batteries are used. These batteries store the energy generated by solar panels during the day and discharge it as needed, including at night when the solar panels aren’t actively generating electricity.
Batteries provide a straightforward and efficient way to store solar energy for later use. They allow solar panel owners to reduce their reliance on the grid by storing excess energy produced during sunny parts of the day for use at other times. With enough battery storage capacity, it’s possible for some homes and businesses to detach from the grid entirely.
The main downsides of batteries are their still somewhat high costs, capacity limitations, and finite lifetime. While costs have come down, batteries represent a significant upfront investment. They also have physical size and weight constraints that limit total storage capacity. And batteries gradually degrade over repeated charge/discharge cycles, meaning they have to be replaced every 5-10 years in most solar installations.
Key Facts About Battery Storage
- Lead-acid batteries are cheaper but have shorter lifespans than lithium-ion
- Typical solar battery capacities range from 5-20 kWh for homes
- Larger scale battery installations can be in the megawatt-hours (MWh)
- Costs vary based on battery size but roughly $100-$300 per kWh is typical
- Can be paired with solar arrays of all sizes
- Require inverter to convert DC to AC for powering home appliances
Pumped Hydro Storage
Pumped hydro storage takes advantage of height differentials and gravity to store energy. Excess solar electricity is used to pump water from a lower reservoir up to an elevated reservoir. When energy is needed, the water is released from the upper reservoir back down to the lower one, spinning a turbine and generating electricity in the process.
Pumped hydro allows massive amounts of energy storage at a relatively low cost. It can respond very quickly to meet sudden spikes in energy demand. The long lifespan of pumped hydro installations also works well with the multi-decade lifespans of solar panels.
The catch is that pumped hydro requires very specific terrain with access to water reservoirs at different elevations. Suitable pumped hydro sites are geographically limited as a result. The installations also impact local ecosystems and water tables. And it isn’t as scalable down to small solar array sizes as batteries.
Key Facts About Pumped Hydro Storage
- Works by moving water between upper and lower reservoirs
- Ideal capacity is typically 500 MW to 1 GW
- 70-85% storage efficiency
- Long lifespan of 50 years or more
- Low maintenance but high initial construction costs
- Limited suitable sites based on terrain and water access
Hydrogen Storage
Hydrogen provides another medium for storing solar energy. Excess electricity can be used to split water molecules into hydrogen and oxygen through a process called electrolysis. The hydrogen can then be stored and later recombined with oxygen in a fuel cell to generate electricity when needed.
Hydrogen allows for long-term energy storage suitable for seasonal shifts. It also offers a carbon-free fuel that can be used to power vehicles or other equipment. The downsides are the relative inefficiency of converting electricity to hydrogen and back, and the specialized storage requirements for hydrogen.
Key Facts About Hydrogen Storage
- Works by splitting water into hydrogen and oxygen
- Fuel cells recombine them later to generate power
- Better suited for long-term storage compared to batteries
- Storage tanks require high pressure or very low temperatures
- Around 30% net efficiency over full cycle
- Still requires energy to produce and store hydrogen
Thermal Storage
Solar thermal energy can also be stored for later use. Solar heat is captured and used to heat up molten salts, oils, or other fluids. The thermal energy is held in storage tanks until electricity is needed. Heat exchangers transfer the heat to water to generate steam for driving turbines.
This allows solar thermal plants to dispatch power on demand. It also enables operation after sunset. Thermal storage only works for concentrated solar applications rather than PV panels, limiting its versatility.
Key Facts About Thermal Storage
- Fluids like molten salt retain heat very effectively
- Tanks are insulated to minimize losses
- Works with concentrated solar thermal plants
- Heat runs steam turbine so no inverter needed
- Lower costs but less flexibility than batteries
Flywheel Storage
Flywheels provide short-term storage by accelerating a rotor to a very high speed and maintaining its inertia. When energy needs to be drawn, the flywheel’s momentum is transferred to a generator. Flywheels can charge and discharge quickly making them suitable for rapid cycling.
However, flywheel storage capacity is limited. Friction losses also consume significant power over time. This makes flywheels impractical for anything beyond bridging short disruptions on the grid. Their high self-discharge rates also limit storage duration to minutes or hours.
Key Facts About Flywheel Storage
- Stores energy in a spinning rotor
- Very fast response times
- Used for short duration backup power
- Capacity from a few kWh to up to 25 kWh
- Discharges over time so unsuitable for long-term storage
- Works for grid stability but not daily solar storage
Compressed Air Storage
This approach uses electricity to compress air which is stored in underground caverns or pipes. To discharge the energy, compressed air is heated and expanded through a turbine. Sites with suitable geology are required, restricting locations.
While compressed air storage can hold large amounts of energy, the process has poor efficiency of around 40-60%. Costs are site-dependent but relatively low once constructed. Compressed air is better suited for large scale grid applications than small solar arrays.
Key Facts About Compressed Air Storage
- Compresses air in pipelines or caverns
- Stores energy efficiently but has major conversion losses
- Suitable geology required
- Works at scale of hundreds of MW
- Can help grid stability and peak load shifting
- Not practical for homes or small businesses
Conclusion
In summary, there are several ways to store the energy generated by solar panels for later use. Each method has its own pros and cons. Batteries provide an efficient and modular option suitable for home and small business use but have limits on capacity and lifespan. Pumped hydro can store huge amounts of energy but requires specific terrain. Hydrogen and thermal storage allow longer-term storage better suited for seasonal shifts. And compressed air and flywheels fill utility-scale grid stability needs.
The optimal solar energy storage solution depends on the specific application and context. But with energy storage, many sites can make effective use of solar power even when the sun isn’t actively shining. Ongoing innovation in storage technology will further increase the versatility and adoption of solar in the future. The combination of solar power generation and energy storage promises to play a major role in the renewable energy transition.
| Storage Method | Capacity | Efficiency | Discharge Time | Lifespan | Cost |
|---|---|---|---|---|---|
| Lithium-ion batteries | 5-20 kWh residential | 90%+ | Hours | 5-15 years | $100-$300 per kWh |
| Lead-acid batteries | 5-20 kWh residential | 80-90% | Hours | 3-5 years | $100-$200 per kWh |
| Pumped hydro | 500 MW to 1 GW | 70-85% | Hours to months | 50+ years | Site dependent |
| Hydrogen | Any size tank | 25-45% overall | Hours to seasons | Indefinite | Equipment + storage costs |
| Thermal storage | Hundreds of MWh | 70-90% | Hours to days | 20-30 years | Low costs per kWh |
| Flywheels | 5-25 kWh | 85-95% | Seconds to minutes | 20+ years | High per kWh |
| Compressed air | 50-350 MWh | 40-60% overall | Hours to days | 20-40 years | Site dependent |
