All sources of energy have some level of intermittency, they are not always available when needed. The reliability range is wide, utility supplied electric or gas energy seldom fails while we know that the sun is not available to operate photovoltaic (PV) or thermal systems at night. Our needs for energy also vary from critical full time (example needed), to critical when needed (example needed), to use it while it is available with no problem with loss of operation (example needed).
Key to reliable power is redundancy, having multiple sources of power. For utilities this usually means multiple generating stations and multiple transmission paths to ensure delivery of power. For applications not connected to a reliable power grid, the alternate power source can be small emergency generators or battery backup.
Most renewable energy sources, solar and wind being the main sources, are intermittent by their very nature. If continuous power is required, they need to be backed up by energy storage or generators. The most common combination is PV power systems with batteries. In these systems the PV system is designed to provide enough energy to meet the energy needs and recharge the battery. As such the size is based the worst case during the year with safety factors added for year to year differences, longest expected periods of low sunshine, and the expected degradation of equipment as it ages. The result is rather high energy costs because at many times of the year the energy produced by the PV array exceeds needs and is not harvested.
More popular these days are PV systems that are utility connected and any excess energy is either banked with the utility (net metering) or sold to the utility (net billing). Since most of these utility connected systems generate energy only when the utility is functioning, there is also the ability to design the system to include energy storage. There are many options, see the AZSC separate article on (title and article needed covering AC vs DC coupled storage, demand reduction, backfeed limiting, etc. based on 2019 products).
There are many aspects to energy storage, size can range from very small (battery or capacitor) to very large (pumped hydro), many technologies, safety aspects, to name a few. There is no easy way of describing this wide range of technologies, but we will start with some definitions.
There are two basic terms for defining storage, Power and Energy:
Watt: A unit of Power, one Watt is the rate at which work is done when one ampere (A) of current flows through an electrical potential difference of one volt (V). Items like light bulbs are rated in watts. Power is the rate at which energy is generated or consumed and hence is measured in units (e.g. watts) that represent energy per unit time. Electrical demand is a power term, usually measured in kilo-Watts (kW).
Watt-hours: A unit of Energy, defined as power over a time interval. One watt of power for one hour is one Watt-Hour. Often used with the prefixes Mega for 1-million (mWh) and Kilo for 1-thousand (kWh).
The suitability of a storage technology is determined primarily by its power and energy capacity and the rate at which these can be stored and delivered. Other characteristics to consider are round-trip efficiency (how much energy is lost from charging and discharging), cycle life (how many times the technology can charge and discharge at a particular depth of discharge [e.g., 80% or 100%]), safety, and ramp rate (how fast the technology can respond to a command).
Other metrics to consider include specific energy and specific power. Specific energy is a measure of energy per unit mass of an energy storage system. Specific power is defined as the power per unit mass of an energy storage system. When specific power and energy are high, the weight of the energy storage system tends to be lower per kW/kWh. See the Figure below for a comparison of specific power and specific energy for each of the many of the energy storage technologies presently available. Optimal characteristics of energy storage technology include high specific energy and specific power, but these features are often costly and not necessary for every application.
The above graphic covers battery technologies from the point of view of large scale energy storage. It was extracted from a good report that is worth reading: ENERGY STORAGE TECHNOLOGIES WHITE PAPER by the Port of Long Beach
The basic physics of energy storage
There are three basic ways to store energy; electrochemical, mechanical and electrical.
Electrochemical methods basically change one element or compound into another wherein the process is reversible (the energy can later be recovered). Examples include breaking water into its components of hydrogen and oxygen (H₂O into H₂ and O₂) and lead-acid batteries (PbO₂ + Pb + 2H₂SO₄ into 2PbSO₄ + 2H₂0), more on these later.
Mechanical methods utilize physical forces such as gravity (pumped hydro), angular momentum (flywheels), pressure (compressed air), and heat.
Electrical storage includes capacitors and inductors. Some energy storage combine elements of these ways.