There are several reasons explaining the mainstream use of renewable energy sources for power generation. First, there is no need to extract them from the Earth's interior, unlike fossil fuels. All that is needed is to buy and install special equipment, which is constantly getting cheaper due to high competition and the relative availability of materials and technologies. The second reason is environmental friendliness or rather the absence of environmental pollution due to the absence of CO2 emissions. And all these factors led to the third one, which is no less important - the state policy of stimulating the generation of electricity from renewable energy sources to moving to 100% "green" generation, which allows not only to take care of the environment, but also make money. In addition, the use of energy from "inexhaustible" sources provides users with a certain sense of independence from "dirty" fuels that are constantly becoming more expensive.
Like everything new, the RES electricity generation was not initially so popular. Politicians from different countries had to encourage people and enterprises to use "green" technology. To this end, the so-called "green" tariff was used, which allowed not only to consume "green" electricity, but also to sell it to the general network at a bargain price.
When the number of small and large stations - mostly solar and wind - reached a certain critical point, it became clear that the grids were not adapted to receive and transmit such volumes. It turned out that large stations are far from end consumers, while small private households generate electricity inconsistently because of natural features of the sun and wind. For example, at night, when the sun has already gone, everyone turns on electrical appliances. In cold weather everyone turns on the heaters, while sun is not enough and there is no wind at all. Thus, these peak needs should be covered by the same traditional gas- or coal-based plants.
However, there is no reason to worry. The majority of researchers almost have no doubt that the 100% use of RES is possible. This is proved by the experience of the countries that are on this way, despite the technical difficulties we will have to face and solve them sooner or later.
International experience in RES integration
System integration of renewable energy sources (RES) into the united power grid became a real challenge for many countries in due time, but they successfully got through it at a certain stage and already switched to more complex tasks that once again put the need for the integration of alternative energy sources. Now the Ukrainian energy infrastructure faces the similar problems.
In its study, the International Energy Agency (IEA) gives an example of Ireland. In 2003, the country introduced a moratorium on the development of wind energy due to fears about "security and stability of the energy system", while wind technologies generated 2% of the annual amount of electricity. Subsequently, the problems were analyzed, and a strategy for their solution was developed and implemented. Already in 2017, the share of wind power was 25% in Ireland. This figure is expected to reach 39% by 2023 without significant additional costs.
Another example - in China in 2016, the lack of throughput capacity for the electricity transmission to high demand centers, exessive energy supplies and regulations giving priority to coal-based electricity resulted in reducing approximately 56.2 terawatt-hours (TWh) of solar photovoltaic energy and wind energy. With every next year, the volume of such involuntary reductions was reduced due to the power grid preparation for the RES needs and the facilities of the "green" generation.
Similar circumstances forced other countries, such as Germany and the USA, to respond promptly to the accidents in the RES generation.
Why do these problems arise?
The point is that the electricity can be generated either by alternating or direct current. Many power generation technologies use rotary turbines to generate electricity producing alternating current. Some technologies, however, such as wind and solar photovoltaic, generate a direct current to be converted into alternating current using a special inverter device.
Another problem with the RES integration into the united power grid is that its infrastructure was designed long before the rapid boom of "green" technologies and was focused on large centralized facilities of the electricity generation, rather than on local, small or distributed as they are called now.
In traditional power grids, electricity is mainly supplied from large centralized power plants connected to the transmission network. Later, the voltage goes down to the distribution network, where connections on demand prevail, those that do not export energy back to the network.
Wind and solar plants technically differ from traditional forms of electricity production. Their highest capacity they can instantly feed into the grid depends on how much wind and sunshine are available at a specific moment, making the production indicators variable and only partially predictable.
Thus, in conjunction with the RES integration, there is a need for balancing the grid load indicators. This can be done through other types of generation. However, it is worth understanding that each type of generation has its own characteristics. Nuclear power plants, for example, relatively slowly increase or reduce the capacity to quickly meet the demand changes. At the same time, they are very powerful and have the ability to continuously generate electricity except for scheduled repairs of facilities. Coal power plants slowly increase and give out the capacity, it may take many hours to heat up machines and synchronize them so that the frequency of their production complies with the frequency of the power grid. Technologies are constantly under the development, and these indicators are becoming more flexible on many facilities.
How matters stand with the RES integration in Ukraine?
According to the Plan for the development of the transmission system for 2020-2029 published on Ukrenergo website, the admissible load fluctuations on the UPG (United Power Grid) should not exceed 500 MW, and there should be no possible fluctuations towards losses of over 1,000 MW. This will keep the frequency within the admissible limits - no more than ± 0,8 Hz of dynamic deviation.
In order to keep the balance of the grid, the above project proposes to involve more maneuverable gas generation to meet peak loads.
The authors of the project, thus, conclude: "... the introduction of WPP and SPP practically does not lower the need for traditional generation, because during a total maximum of loads in the UPG, SPP do not operate, while the WPP capacity should be taken considering the probability of windless weather in this time, which is peculiar to days with minimal temperatures, when the load in the UPG is the highest.
Ukrenergo is talking about limited capabilities for RES capacity integration into the UPG. So, at the end of this year, the UPG will be ready to intake 3,000 MW of RES capacity without further changes in the UPG structure. At the end of 2018, the total capacity of the installed RES facilities was 1,713 MW. One of the projected scenarios for the RES development provides for an increase in SPP capacity up to 3,050 MW and WPP capacity - up to 1,550 MW in 2020.
What are the RES system approaches?
To integrate large volumes of RES, there are two directions of action required. First, the equipping RES facilities with additional technologies to make them more suitable for integration. Secondly, these are the measures that will prepare the energy system to intake large volumes of RES.
The first category of measures provides for improved forecast of production and consumption of electricity, SPP and WPP can ensure various corresponding system services to stabilize the power system. Solar photovoltaic installations and wind turbines, for example, can provide voltage and frequency support by using the appropriate inverter technology.
The second package of activities involves preparing grids for the needs of RES integration. This process requires, in addition to technological and certain changes in administration approaches, day-ahead planning. Moreover, policies should be developed to accelerate the development and deployment of energy conservation technologies and to ensure the modernization of the common grid.