Solar thermal power plants in the world: The experience of development and operation

The main areas of large-scale development of solar energy are: —conversion of solar energy into low-grade heat, and using the latest in heating systems of residential, municipal facilities, public and industrial buildings that consume energy such as temperature capacity; —conversion of solar energy into electricity through photovoltaic and thermodynamic converters. This report provides short information of the dynamics of the creation and operation of solar power plants (SPP) with the thermodynamic conversion, and the criteria for reducing cost of electricity produced from them.


INTRODUCTION
In the world practice a view to obtaining electrical energy with the help of thermodynamic conversion of solar energy (SE), the following technologies are used: parabolic through collector (PTC), linear Fresnel, dish Stirling and solar tower [1].
On the basis of the abovementioned technologies today in the developed countries of the world are created big capacities solar power plants. In [1][2][3] are considered indicators the operational SPP solar tower and parabolic trough which capacity 100 MW and above.
SPP with the thermodynamic conversion of SE operated mostly in the range of 35° S-35° N, but on research of executed in [4] PTC with the orientation of the optical axis of the north-south for the winter declination of the Sun can also be used in areas between 40° S-40° N.
Together with it, on a row with the purely thermodynamic transformation of SPP, are created integrated solar combined cycle (ISCC) power plants. In Table 1, the lists of the ISCC with PTC installed in the period from 2009 to 2015. Accordingly, in Table 2 is a list of the planned ISCC to 2020 worldwide.
According to the data [5], in the world leaders in terms of development and operation of SPP are USA, Spain, South Africa, Morocco, Chile, China, India, and Israel. The total number of operation and under construction until 2019 SPP are shown in Table 3.
As can be seen from Tables 1 and 2, the capacity of solar part of ISCC is from 5 to 20% of the total capacity of the whole plant [3]. Table 4 shows the technical and economic indicators of the solar power plant with thermodynamic conversion based on different technologies.
As it is seen in Table 4, solar thermal power plants based on PTC dominate around the world.
Created in the world ISCC based on PTC are generally designed as in Fig. 1. ISCC consists of the gas turbine with natural gas as combustible fuel, heat recovery steam generator (HRSG), steam turbine, cooling system and the solar field from PTC. The plants presented in Tables 1 and 2 are under construction according to the schematic diagram provided on Fig. 1 [6].
Generalization of operating experience shows that in ISCC, the electric solar capacity does not account for more than 15% of the electric steam turbine capacity of the ISCC. This limitation is necessary in order to avoid considerable negative effects on the Rankine cycle efficiency during times of no or low solar irradiation.
The steam turbine of the ISCC has to be designed for maximum solar heat, i.e. it will be larger than in a CC with the same gas turbine. Hence, at operating points with no solar irradiation the steam turbine will operate in part load conditions whereas in the CC it would operate at full load.
Usually, from 100 to 85% of the nominal load the efficiency of the steam turbine is approximately con-stant. Hence, by limiting the Electric Solar Capacity to 15% the negative effects of increased part load become negligible [7].
As it is specified in [8,9], now the electricity produced by large photovoltaic systems in Germany costs cheaper, than 9 Euro cents for kW h that is comparable with the electric power developed by coal and gas thermal power plants at prime cost from 5 to 10 Euro cents for kW h. Prime cost of the nuclear energy developed by the modern nuclear power plants (NPP) according to them makes 11 cents for kW h. Fig. 3 is shown Global CSP Capacity (MW e ) by technology and status [10].

On the
Cost price of the electric energy, generated by solar thermal power plants is in range of 12-18 Euro cents for kW h that does it to one of the least expensive options for increase in the generating capacities (Fig. 2). According to forecasts cost price of the electric energy generated on solar thermal power plants in Central and Southern Europe, as expected, will decrease to 8-12 Euro cents for kW h by 2030 (Fig. 4) [9]. Table 5 lists the current and expected costs of the main systems of a typical solar thermal power plant [11].
The following criteria is expected to reduce costs by Table 5: A) Solar Field 1. Collector with larger Aperture (trough) 2. Improved optics through higher accuracy heliostats, improved field layout (tower) 3. Advanced assembly procedure, industrialized assembly, industrial automatization in manufacturing; (sub) supplier standards; standardized design 4. Higher reflectivity, higher cleanliness 5. Improved durability 6. Improved absorber coating 7. Wireless power supply and control (heliostat) 8. Improved optics through higher accuracy heliostats, improved field layout (tower) 9. Improved O&M procedures B) Thermal Storage 1. Direct storage concept (HTF = Storage Medium) 2. Higher temperature difference 3. Adapted thermal storage materials   3. Adapted turbine design (for daily start-up) 4. Improved control and O&M strategies/procedures.
Based on the foregoing can be stated that the use of ISCC power plant with PTC in countries with hot climate, today the world is one of the perspective ways for the production of electricity as from the power energy point of view and from an economic point of view.
Today in the republic is developed the Roadmap "Republic of Uzbekistan: Solar Energy Development" in assistance with the Asian Development Bank (ADB). According to the optimistic scenario in Uzbekistan until 2031, it will be installed 4 GW capacity of solar power plants, where is planned construction of solar PV plants and ISCC based on PTC. Based on the abovementioned Roadmap is scheduled construction of the first ISCC in Navoi region with a total capacity of 130 MW [12,13].
Taking into account the our country climatic conditions by us proposed the following scheme-technological solution for the creation of ISCC based on PTC in the Republic.
In Fig. 5 it is presented ISCC based on PTC which consists of two gas turbine units, two HRSG, solar heat steam generator, steam turbine unit and the solar field from PTC [14].
Designed power plants of this type can work in three modes: ISCC mode at solar hours (even with one gas turbine), conventional combined cycle mode at non solar hours and gas turbine mode when the steam turbine is not functioning.
According to results of preliminary calculations, in the presence of the territory for the construction of the solar field (at the rate of 2 hectares per 1 MW) modernization of thermal power plants with CCGT (Navoi, Tashkent and Talimardjan), the total capacity of about 2 GW today, combining them with solar field  of ISCC power plant with PTC (~14% efficiency), will provide the SE share from 13 to 15% of the total plant capacity and natural gas savings will be from 418 to 482 mln m 3 per year, which in turn will lead to a reduction in CO 2 emissions per year from 106.1 to 139.2 mln kg. CONCLUSIONS Our future, substantially, depends on application of technological innovations. According to forecasts, within the next decades value and a share of SE in the general process of energy production will grow rapidly. These technologies reduce not only CO 2 global emission, but also give necessary stability to power energy, doing it less dependent on limited reserves of fossil fuel. Heat recovery steam generator 1 Heat recovery steam generator 2