Storage of Fluctuating Renewable Energy

Introduction

The fluctuating energy supply at different places of the Earth is a natural phenomenon since the most important energy source on planet Earth is the Sim. that is, solar energy. Due to the rotation of the Earth, the Sim shines on different points of our planet at varying times and intensities. From our point of view, the Sim is shining always in a different way. that is, the solar energy supply is fluctuating at different places on the Earth. Nature has already provides storage of the fluctuating solar energy via photosynthesis, which has a modest efficiency of only 1% (Olah et al.. 2005), and the result is vegetation. Therefore, the storage of energy is also a typical natural phenomenon.

Humans have been the beneficiaiy of this natural phenomenon. Making a fire with wood uses stored solar energy. Utilization of biomass, however, is limited by the finite area of arable lands (~ 10% share of the world (World Energy Council. 2016)). Nowadays, other renewable energy sources, like solar panels and wind turbines, are developing very rapidly. Production of renewables and their consumption is increasing in the world, as illustrated in Figure 1. These devices, however, produce electricity in a fluctuating manner due to weather conditions. Electricity consumption is also a fluctuating phenomenon due to tunes of day or night and seasons. The efficient and large-scale usage of renewables can be managed if there are efficient storage alternatives at every different level, that is, large, medium and/or small scale.

In this chapter, an overview about different energy storage alternatives of different scales and different principles is given, including:

• • thermal way of storage,
• • mechanical ways of storage, like water reservoirs, compressed air, and fly wheels,
• • chemical ways of storage: (i) batteries (ii) reversible reactions (iii) hydrogen economy and (iv) CO,-based circular economy producing energy carrier molecule, e.g., methanol, methane,
• • biological way of storage, typically algae-based technologies.

Storage capacity and volume required for different energy systems

Figure 2 shows storage capacities and discharge tunes of the different kinds of energy storage alternatives. As it is presented, batteries and fly wheels har e lower storage capacity. The highest value belongs to the

Storage of Fluctuating Renewable Energ)’ 325

Figure 1. Increasing trend of renewable energy production in the World (BP, 2017; Deak et al., 2018).

Figure 2. Storage capacity of different storage systems (based on Specht et al., 2009).

Table 1. The volume of storing 10 MWh electrical energy (Teichman et al., 2012).

Synthetic Natural Gas (methane). Table 1 shows the volumetric energy density of energy storage for 10 MWh energy (Teichman et al., 2012). As can be seen, the pumped hydro needs the highest volume and the hydrocarbon needs the smallest volume due to its high carbon content.

Thermal Storage

The oldest form of energy storage. The heat can be stored as sensible heat or latent heat.

Storage using sensible heat

The housing structures do that job and protect us against external hot or cold temperatures. In the industry, e.g., steel industry, so-called recuperators are applied. These devices contain heat resistant solid material, e.g., sliamot brick or fire brick, which is heated up with flue gases and then the accumulated heat is used for the preheating of feed gases.

Storage using latent heat of melting

The latent heat method is more powerful than the use of sensible heat. Molten material can store heat more efficiently: A large amount of heat can be stored in a small volume. Smaller volume but more heat.

A rapidly developing area of this heat storage option is a microcapsule-based solution where the melting material is closed in a microcapsule. There are many alternatives for the melting material to match its melting point to the stored heat features. To find or produce the appropriate material in the microcapsule is the subject of many research projects. Such materials can be paraffin, modified hydrocarbons, polymers, etc. The microcapsule can find many application areas, including tempering of buildings, especially in the desert, where significant money can be saved on the air conditioning or other utility requirements.

Mechanical Storage: Water Reservoirs

Pumped hydro storage is coupled with hydropower turbines for generating electricity. As can be seen in Table 1, the hydro needs the highest volume to store the same amount of energy. In pumped hydro storage, in the case of electric energy surplus, water is pumped to the upper reservoir and, in the case of energy requirement, the water is let down into the lower reservoir. Energy is released due to the change in position: water potential energy becomes kinetic energy. Kinetic energy turns a turbine which turns a generator which generates electricity. The efficient operation of such a pumped hydro power station requires pump-turbine/motor-geuerator assemblies, as shown in Figure 3.

Pumped hydro energy storage can be built in places where the geographical situation is suitable. This is the reason why the first pumped hydro power stations were built in Italy and Switzerland. It is important to note that the hydro power stations require a relatively high capital investment.

The current hydro power storage capacities represent about 2.5-10% of the total electricity production in countries where hydropower is used (e.g., USA, Europe, Japan). Considering, however, the advantages of the hydro power storage, more power stations are expected to be built.

Figure 3. Schematic view of pumped hydro power station.

Chemical Storage

Batteries

Batteries are the best-known and most widespread tools for electrical energy storage. As shown in Figure 2, the batteries belong rather to lower or middle capacity storage devices compared to the other options. Many different batteries aiming to get a better and more suitable device have been developed. Typical features are the voltage, energy density, efficiency, cycles and price. The development is continuous, motivated by the promise of this method.

There are practically uncountable types of batteries. Some important ones are: Lead-acid battery, Nickel-Cadmium (Ni-Cd) batteiy, Nickel-Metal hydride (Ni-MeH) batteiy, Lithium-ion (Li-Ion) battery. Advantages and disadvantages are listed in Table 2 (ADB, 2018).

Table 2. Comparison of batteries, advantages-disadvantages.

Table 3. Data of different batteries (Rechargeable battery, 2019).

The different features of the batteries can be quantified if we define certain numbers that represent their performance. These numbers help to compare and rank the batteries. Such a comparison is shown in Table 3.

The comparison of the numbers explains why the Li-ion battery is selected for the electric cars. The partner of the Li in the cathode can be Cobalt Oxide (or Lithium Cobaltate), Manganese Oxide (or Lithium Manganate). Iron Phosphate, Nickel Manganese Cobalt (or NMC) and Nickel Cobalt Aluminum Oxide (or NCA).

It is an important issue for batteiy producers and researchers to create a battery that can store large amounts of electrical energy and can be charged in a very short time. Currently the charge time ranges from half an hour (quick charge to 80% of battery capacity) to 24 hours; in comparison, the “charge” time of gasoline cars is a few minutes.

A special issue is the problem posed by trucks. The issue can be summarized by the statement of an average truck driver: "We do not want to transport batteries but goods’'. That is, the energy requirement of trucks is much higher than that of the passenger cars and the energy content of the hydrocarbons is still much higher (30 times better, see Table 1) than that of the best batteiy. So, if we want to use electric cars/trucks on a large scale these are the challenges to be solved, that is, stored energy, energy density and charging time.

Reversible chemical reactions

There are reversible chemical reactions where the hydrogen is reversibly created and, if needed, released. Equation (1) shows an example of such a reaction (Mizsey et al., 1999). Methyl cyclohexane is reduced to toluene where 3 moles of hydrogen are released.

The reversible reaction has a positive balance, which means the reaction is capable of energy storage.

Teichman et al. (2012) proposed an efficient reaction system for hydrogen storage in a reversible reaction. N-ethylcarbazol is hydrogenated and dehydrogenated. The molecule is capable of storing 9 moles of hydrogen. Octene is also proposed (Teichman et al., 2012). The basic idea is the same: Solar cells are producing electricity which is used for water electrolysis. The produced hydrogen is chemically bound and released if needed. The hydrogen is then consumed in fuel cells to produce electricity and fulfill energy requirements. This method is patented but only for household scale.

The energy storage with reversible chemical reactions is capable of storing the fluctuating energy that is typical for certain renewable ones. It assumes that the renewable energy is available in the form of electricity. The electricity is transferred into hydrogen via water electrolysis. The system has advantages and disadvantages (Table 4).

As a conclusion, the energy storage of industrial scale is the SNG (Synthetic Natural Gas), see Figure 2, and this technology is the Power-to-gas method.

Table 4. Evaluation of energy storage with reversible chemical reactions.

Figure 4. Retrofit installation of solar cells on a private house (Mizsey’s photo of lus own home, 2019).