Increasing the manoeuvrability of power units of the thermal power plants due to applying the hydrogen-oxygen systems

The article describes a method for providing additional peak power of a power-unit with help of an additional energy circuit involving a peak turbine and a hydrogen–oxygen system for energy storage and generation. This method is described on the example of K-300-240 turbine power unit. The proposed technical solutions result in the nominal, most economical mode of a power-unit operation during a day. Consequently, the specific fuel consumption is decreased, and reliability of equipment operation is increased. We have developed the technology for high-pressure gases generating that is realized in the membrane-less electrolysers. Thus, there is no need in the use of expensive and energy-intensive compressor equipment for the energy storage systems. That means that the cost of power unit modernization is reduced. Taking into account the environmental and economic factors, the use of hydrogen–oxygen systems for energy storage at the power generating enterprises provides a value of the energy return coefficient that is practically the same as for hydro-storage power plant. But capital costs of power unit modernization are significantly lowered and negative impact on the environment is minimal.


Introduction
The power industry of Ukraine operates under conditions of a shortage of manoeuvrable capacities. Which count only 9% of the total installed capacity whereas their required minimum must be of 20%-23% to ensure the industry stable operation. The daily difference between the maximum and minimum load of the country's energy system (night 'dip') in wintertime is 7400 MW.
In summertime, this difference decreases by 35%-40%. Covering the 'peak' part of the load daily schedule by 40% is provided by the units of hydroelectric power plants and hydro-storage power plants (HSPPs), and the rest part is covered due to the thermal power plant (TPP) operating in a manoeuvrable mode. It leads to the need for a daily shutdown of up to 15 power units for 4-6 h in the autumn-winter period that causes dramatic increase of fuel consumption and reducing the reliability of their operation. At the same time, the TPP power units will be used for a long time to cover peak and semi-peak loads due to objective reasons, mainly caused by a need to support the basic operating mode of the nuclear power plants [1][2][3][4][5][6][7].
The total capacity of solar and wind power plants that were put into operation during 2018-2020 amounts only 7% of the installed energy system power. It further exacerbates the problems of imbalance in the schedules of energy generation and consumption, along with operation reliability of the heat-power equipment operating in the manoeuvrable mode taking into account the high degree of wear and tear operation. Accordingly, when TPP operates in variable modes, the turboelectric generators failure rate increases in 2.5-2.7 times compared to TPP operation in the basic mode [8,9]. Significant decrease of reliability of other main elements of the power generating facilities has been recorded as well. In this regard, it is relevant to develop such technical solutions of the power units which provide additional power generation during the peak consumption period in the most economical basic mode of their operation.
The average annual utilization rate of the installed capacity of Ukrainian TPPs in 2013 was only 31% while the world indicator is 40%. Currently, 26 units with a total capacity of 6.7 GW have already been operating for 200-250 thousand hours (that is nearly the limit of physical wear), 43 units with a total capacity of 10.4 GW are in operation for 250-300 thousand hours, and 11 units with a capacity of 150 and 200 MW (2 GW) have exceeded the limit of their service life (300 thousand hours). The installed capacity of the TPPs in Ukraine is about 30 GW, but in 2015 its actual capacity was only 21.8 GW due to downtime and repair works, as well as conservation of the outdated equipment. The power industry of Ukraine operates under conditions of uneven load schedule. 'Night failure' of power consumption was about 7 GW, which practically remained in 2018-2020. The lack of sufficient manoeuvrable capacities forces one to use the thermal power under variable load conditions that results in decreasing the operational reliability of this power equipment, and it is the reason of its faster wear and tear [10].
One of the ways to solve this problem is the modernization of equipment, which provides for an individual approach when choosing the technical measures to increase manoeuvrability and efficiency of the existing power units. The peculiarity of hydrogen as a fuel is that the product of its combustion in oxygen is superheated steam which is an actuating medium for the modern steam turbine plants. The use of the hydrogen and oxygen reaction for additional generation of the superheated steam makes it possible to provide a deeper and more efficient control of the electric power generating equipment at a constant capacity of the basic steam generator operating in the most economical nominal mode.
The aim of this study is to develop and analyse a combined cycle of a steam turbine plant using hydrogen-assisted generation of the superheated steam for increasing manoeuvrable characteristics of the power units to cover the peak loads in the Ukrainian power system. This aim is achieved by improving the turbine plant technological schemes and by increasing the thermodynamic parameters of the cycles by the use of hydrogen technologies.

Formation of a technological scheme for an increased manoeuvrability steam turbine power unit
The proposed method provides this effect due to the use of two turbines in the power generating system. One turbine is the base, and another is the additional 'peak' one. The steam flowing from the steam generator is redistributed between both turbines. The increase in power plant efficiency is provided not only due to increasing the temperature during the cycle and approaching the actuating medium expansion process to a thermodynamically more efficient one, but also as a result of steam humidity decrease [11][12][13][14]. The gas-dynamic and erosion operating characteristics of the last stages of low-pressure cylinder (LPC) turbine are also improved.
Burning the hydrogen and oxygen, when the steam is introduced into the combustion chamber to maintain a technically acceptable temperature level for the structural elements, covers a wide range of combustion kinetics problems. This is an independent problem that goes beyond the scope of the presented material and is considered in the specialized publications including [15,16].
The papers [11,17] describe the general principles of hydrogen and oxygen implementation to increase the power plants productivity without reference to the specific technological schemes of the steam turbine units.
One of the objectives of this work is selection of a steam turbine power unit for developing the recommendations as to its modernizing and improving the operational characteristics under maximum preservation of the current equipment. It is necessary to analyse the ways of adapting the operation of a steam turbine power unit when using hydrogen steam superheating. As a result of this analysis, we have selected the most common in Ukraine power unit with K-300-240 steam turbine.
In the case of a rational choice of capacities of the base and peak turbines, the technological scheme of the power unit is preserved by almost 80%-85%. The changes relate mainly to the hydrogen infrastructure which includes a system of hydrogen and oxygen generation and storage. An additional turbine is needed as well. It may be any of the existing gas turbine units having the similar operating parameters.
Modernization of a steam turbine power unit according to the proposed scheme will result in minimization of the costs per unit of additionally generated power not more than USD 200-250 kW −1 .
The technical realization of the proposed method for the power equipment increasing manoeuvrability in relation to 300 MW power unit is imaged in figure 1. Figure 2 shows the T-S-diagram of K-300-240 turbine operation cycle when this turbine has got a high-temperature hydrogen-oxygen infrastructure.
High-temperature steam is formed as a result of the hydrogen and oxygen reaction in a steam boiler and then it is mixed with the main stream. The temperature of the actuating medium increases ensuring an increase in the thermodynamic cycle efficiency when the power plant is operating in the peak load mode. Technological scheme of the combined highly manoeuvrable steam turbine unit: 1-high pressure electrolyser; 2-hydrogen storage system; 3-oxygen storage system; 4-high-temperature mixing-type reheater; 5-additional economizer; 6-base steam boiler; 7-peak turbine with electric generator; 8-turbine high pressure cylinder; 9-turbine medium pressure cylinder; 10-double-flow low pressure cylinder of the turbine; 11, 12, 13-high pressure heaters; 14-capacitor; 15-ejector system; 16-heaters of water heating system; 17-driver of the feed pump; 18-deaerator; 19-low pressure heaters. According to the scheme for steam turbine plant operation (figure 1), a part of the steam is sent to a high-temperature mixing-type reheater 4 from the standard extraction devices after expansion in the high pressure cylinder (HPC) 8 (process 2-3 in figure 2). And being mixed with a high-temperature product of hydrogen and oxygen combustion, it heats up from the temperature corresponding to point 3 (figure 2) to a higher temperature (point 7 in figure 2). As a result of the heat drop appearance in the peak turbine 7 (figure 1; process 7-8 in figure 2), the steam flow acquires a temperature of 710 K (point 8 in figure 2).
And as a result of flow mixing with the steam leaving the medium pressure cylinder (MPC) 9 (figure 1), where the temperature is of 500 K (point 5 in figure 2), its temperature increases up to 536 K (point 9 in figure 2) and such flow enters the low-pressure turbine part. Due to additional volume of higher temperature steam (process 8-9 in figure 2) coming from the peak turbine 7 (figure 1), the power of the base turbine low-pressure part is increased.
As it follows from the diagram, the specific work created in the peak turbine 7 (figure 1) is equal to the area of 4-7-8-5 in figure 2. The additional amount of energy produced in the turbine low-pressure part corresponds to the area bounded by the lines connecting points 9-10-6-5. In the practical implementation of the power unit combined scheme operation, the parameters, and the flow rate of the actuating medium for the steam turbine cycle should be selected taking into account the limitation on the maximum allowable power transmitted to the base generator. According to the regulatory documents for a unit with K-300-240 turbine and TGV-300 electric generator, the increase in power should not exceed 25 MW [18].
We consider the power unit technological scheme supplemented not only by an electrolyser and the connecting steam pipelines with the appropriate shut-off and control valves but by a remote hydrogen-oxygen steam generator with a reheater.
Thus, the steam flow from the HPC extraction will be heated up to ∼1073 K at pressure of 3.5 MPa and then to feed the peak turbine having an autonomous electric generator. The hydrogen-oxygen steam generator is also equipped with an economizer compensating the heating of feed water when the high-pressure heater (HPH) is switched off during the peak power generation mode.
The additionally installed equipment is shaded in figure 1. To generate the peak power by such operation scheme, we use only steam of the second extraction from the HPC of the base turbine. In this case, the steam flow rate through the intermediate reheater and through the MPC of the main power plant does not change, and all connections of the thermal circuit elements are retained with exception of the disconnected HPHs 11 and 12 (figure 1).
Taking into account the parameters of steam in the additional circuit, a gas turbine which provides the corresponding flow rate of the working fluid can be used as a prototype of the peak turbine. For a quick increase in power, the elements of the peak turbine flow path should be placed in a thin-walled well-insulated casing with a light rotor to ensure high dynamic characteristics [19]. In this case, the time to reach the additional power generation mode will not exceed 300-400 s. In the future, it is possible to implement the design providing the supply of a steam-hydrogen-oxygen mixture directly to the flow path of the satellite turbine to ensure releasing the chemical energy of the hydrogen oxidation reaction in the inter-disk space of the flow turbine. Such measure makes it possible to realize the steam expansion process as close as possible to an isothermal one. So, the thermodynamic efficiency will be increased by 3.5%-4.0%.

Results and discussion
3.1. Technological support for generating hydrogen and oxygen to provide the steam high-temperature superheat Stable and safe functioning of the Unified Energy System of Ukraine requires involving the flexible capacities [20] to regulate daily and seasonal loads, as well as to increase the energy system capacity share that is received from the renewable energy sources. Hydrogen is considered as the primary source of energy for manoeuvring capacities [21].
An important component of the problem of hydrogen efficient implementation in the steam turbine plants is the development of a technology for hydrogen and oxygen generating at high pressure to eliminate the use of the expensive and energy-intensive compressor equipment.
The presence of metal moving parts in the compressor equipment operating in medium of high-pressure hydrogen or oxygen causes the safety, reliability, and labour-intensive maintenance problems. Vibrations and noise are also common undesirable features of mechanical compressors [22][23][24].
The A.M. Pidhorny Institute of Mechanical Engineering Problems of National Academy of Ukraine has already developed the highly efficient high-pressure electrolyser to accumulate energy generated by the power system during the periods of the reduced electricity consumption. The energy is accumulated in the form of hydrogen and oxygen and further used during power shortages. The main advantages of the developed electrolyser and electrolysis technology are in their high energy efficiency. Namely, to obtain 1 m 3 of hydrogen and 0.5 m 3 of oxygen, the electrolyser consumes 3.85-4.0 kWh that is 20%-25% lower than such characteristic of currently used electrolysers. The use of chemically active electrodes and separation in time of the electrolytic gassing processes made it possible to implement a membrane-less design of an electrolysis cell and to obtain gas pressures limited only by the strength of the electrolyser's body parts and fittings used [25][26][27][28].
The scheme of electrolyser arrangement is presented in figure 3. The level of pressure in the electrolyser achieved in practice is of 20.0 MPa [26]. The results of the comparative analysis show that the proposed electrolyser surpasses the currently used ones by its technical perfection, the ease of installation and maintenance, reliability, and safety [29][30][31][32][33][34][35][36]. The electrolyser is of modular design. Its additional advantage is in the absence of the parts containing platinum group metals.  The paper considers an experimental electrolyser generating 0.5 m 3 h −1 of hydrogen. Its design involves the standard 40 l cylinders with electrode stacks placed into them [37].
When a module involves a container with an internal volume of 1 m 3 as the electrolysis cell body with electrode stacks in it (figure 4), the productivity of this module will be of 6-7 m 3 h −1 of hydrogen.
An appropriate set of such modules will provide the necessary performance of the electrolysis plant and the filling of the gas storage system in the inter peak period.
The selected ratios of 1:5 between the durations of the base (10 h) and peak (2 h) operating modes of the electrolysis plant give the possibility for accumulating the required amount of hydrogen generated during 10 h of the base operation.
The calculated operation characteristics of the upgraded K-300-240 turbine in the peak power generation mode are presented in table 1.
The results of thermodynamic analysis indicate that the total increase in productivity of the K-300-240 power unit is of 15% and amounts of 46.9 MW, when an additional steam turbine circuit with a hydrogen steam generator is introduced. For that, 21.8 MW is generated by an additional power circuit with a highly  Since the pressure level in the condenser depends on organization of the steam condensation process, then, beside of the phase transformation process on the tubes, it is advisable to use the condensation mechanism on the drops of cold water, which is introduced through the nozzles into the condenser neck. This mechanism is realized by injection of the cooled condensate, which is prepared using the proposed thermal-transforming cooling system. Figure 5 illustrates the principle of this system operation as the flow diagram.
The reduced pressure in the condenser is created by the ejector. It operates due to steam from the LPC extraction. The mixed in the ejector steam is supplied to the low-pressure heaters (LPH-1, LPH-2).
The performed calculations showed that it is possible to lower the pressure by 1 kPa implementing the thermo-transforming technology of deepening the vacuum in the condenser (spraying of the cooled condensate water). That means to ensure the absolute pressure at the level of 3 kPa. For the K-300-240 turbine, decreasing the pressure from 4 to 3 kPa will provide power increasing by 1.12 MW.
Equipping the steam turbines with the devices for vacuum deepening is expedient, first of all, in cases when water is cooled in the cooling towers in summer.

Analysis of the practical aspects of these technical proposals implementation
To assess the weight and size characteristics of the hydrogen and oxygen storage system, we considered the way to increase the power of the K-300-240 turbine power unit by 15% during its peak operation. To cover the morning (2 h) and evening (2 h) periods of peak energy consumption, the volume of accumulated energy should provide generation of 93.8 × 10 3 kWh. To generate such amount of energy in the modernized power unit, it is necessary to use 3.5 × 10 4 m 3 of hydrogen and 1.75 × 10 4 m 3 of oxygen.
When storing these gases in the containers at a pressure of 20.0 MPa, the storage volumes will be of 180 m 3 and 90 m 3 , respectively. It is advisable to use the standard rail tank cars for the gas storage system. To ensure safety and reliability of operation, it is necessary to provide for their underground placement in a specially reinforced concrete shell. Such shell must be tested on strength with one and a half-fold safety factor when pressure inside the container is nominal.
To obtain such amounts of hydrogen for 10 h (the interval between the peak consumptions), the electrolyser productivity should be of 3.6 × 10 3 m 3 h −1 . When using the electrolyser modules providing a capacity of 7.8 m 3 of hydrogen per hour, their number in the electrolysis plant will be of 460 units. Consequently, the total area occupied by the power generation system will be of ∼260 m 2 . In this case, the energy return coefficient will reach 67% that practically corresponds to the performance of the HSPP. And the mass and size characteristics of the hydrogen-oxygen systems for energy generation and storage are quite acceptable for practice. In the energy system of Ukraine, the reduction of the load of power units in the inter-peak period is 15%-20%.
For the considered here option with a base turbine of 300 MW capacity, 45-60 MW of electrical energy is released during unloading. Thus, the electrolysis plants consuming 14.4 MW to generate 3.6 × 10 3 m 3 h −1 of H 2 and 1.8 × 10 3 m 3 h −1 of O 2 can operate during this period.
The volume of gases generated during 10 h is sufficient to obtain a peak power of 46.9 MW. Thus, implementing a steam turbine cycle with hydrogen superheating of the steam we have the thermodynamic efficiency of using the energy of hydrogen fuel in 1.5-1.7 times higher than when using the natural gas in the peak gas turbine plants. The dynamic characteristics are practically equal.
The results of this analysis indicate that levelling the power consumption schedule is more economical when using the high-pressure electrolysers for the modernized TPP than creating the gas turbine plants or pumped storage power plants, especially taking into account the environmental factors of their operation.
A further increase of technical and economic characteristics of the combined power plants having the hydrogen energy storage can be achieved by improving design of the flow paths of currently used turbines. The results of fundamental studies of the thermal and gas-dynamic processes in the elements of the turbine flow path, carried out on the basis of the developed methods for calculating the three-dimensional steam flow, make it possible to create the blade rows of the more advanced design. These blades may be used both for modernizing the flow paths of the operating turbines and for designing the new generation equipment.
The efficiency may be increased for the HPC up to 88.5%, for the MPC up to 94% and for LPC up to 89%-90% [38,39] due to the following complex of measures:-creating the blades with an inverse spin;-banding of the rotor blades of all stages with the non-contact seals;-implementing the structures with rational geometry of contours in the LPC;-as well as the improving the moisture removal system.
Thus, the systematic approach to use the research results in the field of thermal and gas dynamics, thermal physics, strength, and reliability of the turbomachine structures gives the possibility for providing a large-scale introduction of the innovative technologies for modernization of the existing power equipment. In this case, the cost of 1 kW of the installed peak power will not exceed USD 200-250.
To compare, the same indicator for peak gas turbines is in the range of USD 350-600. The cost of 1 kW of the installed capacity at HSPP reaches USD 1200-1300. The advantage of the proposed approach lies not only in the cost of the installed capacity unit but also in the fact that there is no need to alienate significant land resources with the associated negative environmental impact.

Conclusions
The scientific and technical proposals are developed to modernize the power equipment concerning the operation of the K-300-240 power unit during the day in the nominal mode, both when generating peak power and when carrying a base load. The proposed measures provide a decrease in specific fuel consumption, an increase in the reliability of equipment operation and decrease in the wear rate of the main equipment. An additional advantage of the proposed approach is the maximum use of the existing infrastructure of the TPP, including the use of hydrogen in the cooling system of turboelectric generators.
The developed technology for generating the high-pressure gases and the developed prototype membrane-less electrolysers exclude the use of the expensive metals and compressor equipment for the energy storage systems. Thus, the cost of modernization is decreased. The equipment for electrochemical energy storage can be produced by the domestic machine-building plants with the subsequent expansion of the scope of its implementation at the power units of 500-800 MW capacity.
Taking into account the environmental and economic factors, the use of hydrogen-oxygen systems for energy storage at the power generating enterprises provides the energy return coefficient that is practically the same as such performance for HSPP. But capital costs of power unit modernization are significantly lower and negative impact on the environment is minimal.
The proposed technical solutions provide: reduction of capital costs for smoothing the peak loads in comparison with the pumped storage and gas turbine power plants; higher fuel utilization coefficient; fast load growth, which is commensurate in time with the dynamics of highly manoeuvrable gas turbine engines. Implementing the program of modernization of the power units at TPPs and Heat Electro-power Stations taking into account the proposed technical solution should significantly reduce the deficit of peak capacities in the energy system of Ukraine.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).