Variable renewable energy needs firm capacity to back it up for reliability reasons. What options exist to provide this firm capacity cleanly, affordably and reliably?
Actually, VRE does not necessarily need firm capacity (also called dispatchable power) to back it up at low- to moderate-penetration levels. Even at very high levels of deployment, VRE might not need dispatchable generation as a back-up, but it depends on the system configuration. A variety of options exist to integrate renewables into the grid and provide flexibility. Many of these options are described in the “Increase Power System Flexibility” Guide of the Clean Power Hub, and include things like adding battery storage, building new transmission, and using advanced weather forecasting in the dispatch planning process.
Traditional firm capacity may also be nice to have although it may not meet a country’s sustainability or development goals. Natural gas generators and impounded (dammed) hydropower are traditionally considered good sources of back up for VRE generation because they can respond quickly to changes in VRE output. However, natural gas generators emit greenhouse gases and nitrogen oxides, while large hydropower dams may upset ecosystem balance. Other types of dispatchable renewable generation include geothermal, biomass, and hydrokinetic plants; these options may be limited in scale or scope and have relatively high costs. Nuclear and coal are dispatchable sources that may also have high upfront costs and emissions, as well as being relatively inflexible.
Storage is becoming increasingly attractive as a short-term option to support reliable VRE generation. Battery storage based on lithium-ion chemistry is typically deployed to provide 2-8 hours of discharge. Alternative battery chemistries are under development than can provide longer duration output. And other storage fuels and technologies, including hydrogen, compressed air energy storage, and simple gravity systems, are increasingly available. Some researchers believe that VRE will soon become so inexpensive that electricity systems can be significantly “over-built” and a portion of the available energy curtailed as a least cost option.