Energy when the sun doesn't shine
and the wind doesn't blow
Solar and Wind power are often seen as a complete carbon-free energy source that can provide unlimited power. Unfortunately there are times when the sun doesn't shine and when the wind does not blow, so something must be able to supply power when the arrays and wind farms cannot. There are also places where the sun is not shining for months on end or the wind is very inconsistent.
Pumped water storage, pressurized gas tanks, and batteries are the prime choices to fill in when a carbon-free source is required to act as back up. Another option is to produce a carbon-neutral energy source such as methane derived from captured carbon and water.
Pumped Water Storage
Pumped water storage is probably one of the oldest techniques to store energy. For electrical power plants, the idea is to use it when peak power demands or no energy production periods demand energy. Most systems have some type of energy source (a flowing river, a solar array, a wind farm) and use power collected during the same period electricity is being delivered to pump the water up a hill to a reservoir. When the main source is not producing or producing to o little, the water in the reservoir is released to flow through a turbine which creates electricity from the flowing water.
Pressurized gas cylinders
Compressed Air Energy Storage (CAES) plants use the same principle as pumped water. But instead of pumping water up a hill to a reservoir, in a CAES plant, ordinary air is compressed and stored under pressure in an underground cavern. When electricity is required, the pressurized air is heated which causes it to expand. The expanded air drives a turbine to generate electricity. The compression causes heat and the decompression causes cooling, both of which reduce the efficiency of the system.
In one sense, batteries would seem to be logical choice except that batteries big enough to run a city seems very difficult to imagine. In the early stages of solar power, lead acid batteries were used but they are not very effective and power down slowly losing voltage. The invention of the lithiumion battery has changed that situation. Lithium batteries can hold a charge extremely well, although they do tend to heat up and can burst into flames, so they must be designed to account for that problem. Perhaps the most successful company so far to develop large battery arrays is Tesla. Their Powerpack 2 is advertised with a low cost, high efficiency and high power density utility-scale inverter.
Between one and 20 Powerpacks can be connected inverter and systems canaccommodate multiple inverters, enabling flexible system sizing and design scalable from 200 kWh to 100+ MWh.
These are not inexpensive storage devices. The advertised price is about $2.5 million per MW capacity. Is it possible to power an entire city during the dark hours from a solar array or windless periods from a wind farm? This is a brand new entrepreneurial experiment so only time will tell, but the concept of a large battery farm is already a reality.