Energy storage is one of the fundamental elements in renewable energy production systems. This is especially important when energy is produced unsystematically, as is the case in photovoltaic systems, where the availability of solar energy varies throughout the year - from season to season, day to night, and day to day (different weather conditions). The energy consumption according to time of day and time of year is illustrated in Fig. 1. To balance the different time patterns of solar loads and production, energy storage must be included in almost all stand-alone power systems.

Requirements for electrical energy storage systems [1]:

• high energy efficiency,
• long service life,
• good charging performance even at very low currents,
• low self-discharge rate,
• high power availability,
• easy to estimate the state of charge and the state of wear,
• safe overcharge or deep discharge behavior,
• voltage and capacity easy to increase through series and parallel connection,
• small voltage difference between charging and discharging,
• fast charging capability,
• no capacity loss effect,
• low explosive potential,
• high operational reliability,
• easy to recycle,
• low toxicity of materials,
• low cost.

Energy storage is definated here as any method that allows you to store electricity produced.
Currently, professional energy storage in the world is based on special power plants [2]. According to the RES Act, an energy storage is a separate device or a set of devices for storing energy in any form, not causing emissions that are a burden to the environment, in a way that allows at least partial recovery of energy.

Storage of solar electricity

Storage of electricity in the electricity grid
It is possible to consider the electricity grid as a way to store solar energy. During periods of excessive power production, the excess is fed into the grid from where it can be withdrawn at any time, but only at \( 80\% \), the remaining \( 20\% \) is counted as a "charge" due to the power company for this form of energy storage. The free flow of energy between the grid and the consumer is made possible by a two-way meter. This form of photovoltaic installation is on-grid. Of course, it does not allow you to achieve energy autonomy, but it allows you to use electricity generated in your own home without having a battery bank.

Electricity storage - other ways
Other ways to store electricity are available in electric and magnetic fields, and by converting it into electrochemical energy, mechanical energy, chemical energy, and thermal energy. Some of this stored energy can be used by households and others in industrial applications.

Conversion to Electrochemical Energy
The most intuitive and simple method of energy storage is battery charging. The batteries currently used are lead-acid, nickel-cadmium (NiCd), nickel-metal-hydride (NiMH), and lithium-ion [3]. Batteries store direct current at a voltage, such as 12 volts. In order to use the energy stored in batteries, it is necessary to connect them to electrial devices that convert direct current to alternating current, that is, the use of inverters.
Issues regarding battery energy storage require discussion of several more topics, such as the maximum and minimum voltages for receiving and delivering current to the battery.

Conversion into mechanical energy
Large power plants with gas turbines use compressed air as a way to store energy. During the day, when electricity production is the highest, the air is compressed. When there is demand on the grid, this compressed air is used to produce electricity. Due to low efficiency, such energy storage is not cost effective for small plants.
Electricity can be stored using pumped storage plants, equipped with pump-turbines, which pump water into a higher reservoir during lower electricity demand. And the energy thus stored is recovered by a reversible pump-turbine during periods of increased energy demand. Pumped storage plants are ideal for storing huge amounts of energy over long periods of time, taking power out of the system during times of oversupply and supplying it during times of increased demand. The efficiency of pumped storage ranges from \( 65 \) to \( 85\% \), which is very good compared to other energy storage technologies.

Conversion to kinetic energy
In the short term, energy can be stored in the form of the kinetic energy of a spinning flywheel ( Fig. 2 ) [2]. These are often used in internal combustion engines (with less than 6 cylinders) to equalize their operation, so the technology is fairly well mastered. But apart from a few applications (e.g. in hybrid cars) on a larger scale they do not appear.

Figure 2: G2 front2 (G2 flywheel). Photo NASA, CC0 license, source: Wikimedia Commons.

Chemical storage of electricity - hydrogen production
One way to produce hydrogen is through electrolysis. The advantage of energy storage in the form of hydrogen is that it can be stored for long periods of time and has a high energy density (three times that of gasoline and more than 100 times that of batteries). This gives it the potential to be used as a vehicle fuel. The use of hydrogen in fuel cells offers the possibility, like flywheels, of responding to rapid changes in energy demand on the grid.
An alternative to storing energy as hydrogen is to store it as methane. It is easier to store for long periods of time, and can be used directly in existing gas-fired power plants. The disadvantage is further energy losses in the methane generation process, which make the total amount of energy recovered less than \( 38\% \) [4]. Methane generation is attractive for storing excess energy produced by photovoltaic power plants as well as wind [5]. Installations converting excess energy into methane have been built in Denmark and Germany.

Storage of electricity in supercapacitors
A supercapacitor is a type of high-capacity electrolytic capacitor. It works by accumulating charge on a double electrical layer that forms at the boundary between the electrolyte and the electrode. The carbon nanotubes used allow for a large surface area, and this increases the capacitance of the capacitors. The advantage of supercapacitors [6] over batteries is higher energy density. This means that they can give off energy with high power. Moreover, their huge advantage is the small decrease in performance characteristics and long service life (up to 20 years of operation).

Magnetic Field Utilization
The use of a magnetic field as an energy store for direct current requires a superconductor. The magnetic field created by the current in a superconductor persists indefinitely, and thus energy can be stored in it for any length of time. Unfortunately, superconductors require low temperatures, or cooling, which is very expensive. The efficiency of such a system is on the order of \( 95\% \), unfortunately storing large amounts of energy is not possible [7].

Storage of electrical energy - thermal processes
Excess energy can be stored by cooling air to \( -195_{}^{o}\textrm{C} \) which causes it to condense. The volume of air decreases a thousand times, and storing it becomes much simpler than storing compressed hydrogen. When needed, the liquefied air can be heated, turning it into a high-pressure gas that can be used to drive a turbine that generates electricity. Using waste heat from a power plant can increase its efficiency to \( 70\% \) [8], [9].
There are a number of methods to store energy in the form of temperature differences. In most applications, however, this energy is not later converted to electricity, but rather it is used in its stored form (e.g., for heating).
Methods that allow the storage of electrical energy include the heat pump. A heat pump can be used to store energy in the following way: two tanks with a large thermal capacity are connected by a pump. When there is little demand for electricity, heat is pumped into the hot tank. At times of higher demand, the pump is turned off and instead turbines are run to generate electricity through the temperature difference. British company Isentropic is developing this method, predicting that it could become a competitor to pumped storage power plants, with the costs of $35/MWh and energy recovery efficiencies of \( 72-80\% \) [10].