ABOUT THE EXPEDIENCY OF USING ELECTROCHEMICAL STORAGES IN POWER SUPPLY SYSTEMS FOR NON-TRACTION CONSUMERS OF RAILWAYS FROM RENEWABLE ENERGY SOURCES

Annotation. This paper focuses on the problem of analyzing expediency of using electrochemical storages in power supply systems for non-traction consumers of railways from renewable energy sources (RES). It is quite obvious that RES implementation in any national industry as a proper distributed energy source is too problematic if there is no possibility to accumulate it in good supply. The aim of this paper is to select the most appropriate electrochemical storage device for power systems for non-traction consumers of railways from RES. The methodology of research is based on modern methods of computational mathematics, statistics and information analysis using modern computer technology. Consequently, the analysis helps conclude that 4 LiFePO

124 mulation being too expensive currently. Thus, it is necessary to identify features of the tendencies as well as their potential to be used by railway transport.

Analysis of previous research and publications.
It is known [1] that energy resources largely determine the economic situation both in Ukraine and in the modern world as a whole. For Ukraine, solving the problems of energy saving and energy efficiency has become one of the top priorities. Since the railway is today the main consumer of electrical energy, the reduction of energy intensity and the energy component of the prime cost of freight and passenger transportation is one of the determining factors for effective development. The power supply system of railway transport is an important link in the technological cycle of production, transmission and consumption of electricity [1,2]. The problem of reducing the consumption of electric energy in the power supply systems of non-traction consumers can be partially solved through the introduction of RES, however, the problem of accumulation of generated electricity by the above sources remains unresolved [2].
Currently, there is a wide range of energy storage systems, however, the most widespread are electrochemical storage systems. The latter have found application in traction and non-traction power supply systems for urban and suburban rail transport in the USA, EU countries, Japan and Russia, while the energy consumption of storage systems used in transport does not exceed 1000 kW·h and is determined by the characteristics of the load and the tasks solved in this case [3].
Moreover [3][4][5], electric energy storage systems in the power supply systems of railway transport solve the following tasks: ensuring the required voltage level in case of emergency or technological shutdown; stabilization of the voltage level at the point of connection; increasing the efficiency of the use of regenerative braking; increasing the energy efficiency of transportation.
The aim of this paper is to select the most appropriate electrochemical storage device for power systems for non-traction consumers of railways from RES.

Possibilities of lead-acid storage cells.
Start from the first abovementioned means to accumulate energy, i.e. electrochemical storage cell. In 1859, G. Plante invented an effect of energy concentration within a lead-acid medium owing to which the storage cell showed up. Using it, K. Faure developed the first efficient electrochemical device in 1881. Despite the fact that later other accumulator types have been produced, a lead-acid one is still popular.
The storage cell is based upon a galvanic pair which EMF is almost 2 Volts. Now, a lead-acid storage cell (LASC) is composed in such a way to provide 6 or 12 Volts. Then, the storage cells are integrated into blocks to apply a value of the latter   where А and n are empiric coefficients.
However the dependence is widely used now, it is typical for the specified discharge current since in terms of heavy current or small current it is in significant errors as for the SC capacity determination.

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The capacity depends heavily upon the environmental temperature as well. In terms of temperature variation, decrease or increase in its amount can also be described with the help of the known dependence where 1 2 t and t are the temperature values in the mentioned sequence as for time; and α is temperature coefficient of the capacity.
However, self-discharge rate is more important characteristic of LASC to apply it. In the context of dioxide-lead electrode it is described with the help of the equa- The rate is identified by means of oxygen release on it. In this context, if the electrode is of tetragonal modification than SC discharge lasts twice slower to compare with the rhombic one. Consideration of the fact that its prolonged operation life (in terms of charge/discharge cycles) results in gradual α modification to β helps conclude that self-discharge rate of the electrode increases.
As for the lead electrode (i.e. anode), its electrochemical process, determining mainly SC self-discharge rate, is described using the equation 4 4 2 .

Pb НSO PbSO H е
Along with the cell charging, increase in sulfate ion concentration takes place.
In turn, the process results in the decreased potential of the electrode.
According to [8], So called floating charge is the most expedient measure for the use when the storage capacity is connected in parallel with load and charge/discharge is of chaotic nature. It is almost impossible to implement the procedure for LASC. Moreover, a procedure of regenerative charging applied for urgent cases when SC is close to complete discharge is absolutely inacceptable. Nevertheless, SC cannot remain discharged since it will result in sulfation of plates and considerable decrease of its capacitance or even failure. 127 Thus, availability of significant self-discharge of LASCs, stipulating their low efficiency and productivity as well as operation complexity in the case of chargedischarge processes, makes lead-acid accumulating devices undesirable within the electric power supply grids with wind and solar energy sources.
Alkaline energy batteries and their differences. Accumulating devices, differing from lead-acid electrochemical systems, came to existence early in the 20 th century owing to the invention by Thomas Edison in 1901. He proposed nickel-iron secondary energy source consisting of oxide-nickel cathode, iron anode, and alkaline electrolyte. Current-forming process of such an accumulator is described by means of the expression To compare with LASC, only water is lost in the process of the nickel-iron (NI) accumulator discharge. Electrolyte ( NaOH or КOH alkalis) is responsible for a charge transfer through first-class conductor and second-class conductor phase separation. As for the processes on a cathode of the electrochemical systems, they are more complex. First, the fact is stipulated by their solid phase nature as well as the availability of β -and γ -modifications of NiOOH .
As a result of the mentioned features, a process of NiFe battery charge form oxides with more than 3+ oxidation nickel rate. Generally, the processes, taking place on a cathode, are described as follows:  ) Ni OH , a process of NiOOH iterating should be introduced gradually not more than by 80%. The matter is that if concentration of alkaline or charging current is high then NiOOH γ − modification with less density may rise. That results in electrode swelling [9].
After the accumulator is discharged by half, secondary process starts. Then, high nickel oxides, formed while charging, start resolving with oxygen release. That becomes one of the reasons of rapid self-discharge of just charged oxide-nickel cathodes.
Anode process is not less complex [10]. Hematite ( 2 3 or thermally reduced iron are applied to manufacture the electrode. Dibasic or tribasic iron hydroxide form (i.e. FeO − anions may form which results in the electrode failure.
Like with the previous accumulator type, anode also has a tendency to decomposition. The process takes place in alkaline solution with hydrogen release. In addition to the situation, anode decomposition process with oxygen uptake is also possible. The abovementioned stipulates considerable self-discharge of the accumulator. On the whole, the self-discharge process of nickel-iron accumulator are described as follows: owing to high internal resistance, NiFe batteries can remain serviceable even if undurable short circuits take place or severe degree of discharge.
As for the NiFe battery charging, then, according to [11], it is done with the help of small direct current during 6-7 hours until a potential within each element of the battery achieves 1.75.....1.9 V. It is the moment when active electrolyte "boiling" starts within each element. Charging of the accumulators should involve constant control of the electrode temperature as well to prevent it from exceeding +40 0 С. If that happens, it is necessary to reduce charging current. Accelerated charging of alkaline accumulators is followed by intensive gas release. Hence, their use in electric power supply systems with RES is problematic involving further studies.
Generally, railway transport applies NiFe batteries as the batteries for diesel locomotives. Such studies are even known [12] proposing their charging with the help of two-stage current with the accumulated energy control. Transition to another value of charge current is recommended depending upon the voltage value and CS temperature.
When the SC achieves its nominal capacitance while charging, oxygen release starts on a cathode. Then the oxygen diffuses through a porous retainer to anode recovering on it. The closed oxygen cycle helps stabilize pressure inside the battery.
Hydrogen starts releasing on a cathode, which left its all active components, during a SC charge. The reaction is as follows The hydrogen diffuses through porous electrode up to anode surface oxidizing on it.
Once again, the reaction of hydrogen formation on a cathode and its oxidization on anode while the battery recharging implements the closed type as for the hydrogen as well. Moreover, it helps stabilize pressure in the case too. Hence, the NiMH batteries may be manufactured in an airtight form.
As for the material, applying to produce anode and determining NiMH battery characteristics, it is an alloy able to absorb oxygen amount being quite larger to compare with proper one (thousandfold). Hydrogen absorption and desorption in the process of SC charging/discharging result in almost 15% decrease/increase in crystalline lattice of the selected alloy. Usually, the latter consists of one metal, absorbing hydrogen exothermally, and another one, forming hydrides with heat absorption.
To compare with prior accumulating facilities, slow charge of NiMH battery Rise in lithium SCs is a very rapid process. According to [13], only in 2000-2015 the total annual capacitance of lithium batteries, entered a market, increased from 10000 up to 60000 МW·h to compare with those ones, considered before, which remained at their previous level being 10000 МW·h. Mainly, the growth falls at various needs of traction electric transport [14][15].
Currently, the three types of lithium SCs are known: with metal lithium electrode and liquid electrolyte; rechargeable lithium-ion cells; and lithium-pol ones.
In terms of the former, current-formation electrochemical reaction on anode is performed according to the equation: As for the rechargeable lithium-ion cells, the reaction follows the expression: 6 6 .

CHARGE
Concerning the methods to charge lithium storage cells, the majority of their manufacturers recommend performing the process during 2-3 hours according to a two-stage procedure. First, it is required to support minor direct current until the specified voltage value is achieved; then, the value should be supported as a constant one. In this case, stage one is 70-80% of the storage cell state of charge. However, the idea of rapid charge has already been confirmed especially in terms of lithium-ion batteries [48]. The paper mentions the fact that a SC, which walls are covered with metal foil, was charged up to 80% of its capacitance for 14 minutes. It withstood more than 500 cycles of such charging conditions.
Thus, in this regard lithium batteries, which basic specifications are listed in Table 1, have significant potential as for their use by electric power supply systems with the renewable energy sources as well.  Consequently, the analysis helps conclude that Conclusions. Lead-acid SCs, having significant self-discharge, low efficiency, and short useful life prevent from applying so-called floating or regenerating charge-discharge modes, are not applicable for the mains. Especially, it concerns railway power supply systems.
Despite the fact that alkaline SCs can be almost discharged for a long time, withstand high charge/discharge current, and remain serviceable even if undurable short circuits or deep discharge degrees take place, they have considerable selfdischarge (up to 60%). In addition, some of them (i.e. NiCad and NiMH storage cells) have memory effect as well. Moreover, DC energizes them for 6-7 hours making their use problematic in supply systems with RES.
High specific power characteristics of lithium storage cells, longer useful life to compare with other electrochemical devices, insignificant self-discharge, lack of memory effect, and available positive practices concerning rapid charge/discharge make them the most appropriate facilities to be used as electric power accumulation in the mains with RES.
LiFePO4 storage cell is the most advantageous among lithium batteries. However, its operation should involve constant control of a charge level of each element ISSN 1562-9945 (Print) ISSN 2707-7977 (Online) 134 and balance of voltage value on them. Thus, complicated system to control them is required.
Anyway, use of the mentioned LiFePO4 storage cell within the supply system of railways (with RESs or without them) is complicated heavily through voltage falls of them when traction loads are connected since they shorten useful life of the accumulator.