Prof. D. S. Strebkov, Dr. A. E. Irodionov.

All-Russian Research Institute for Electrification of Agriculture
VIESH, 1-st. Veshnyakovsky proezd, 2, Moscow, 109456, Russia
viesh@dol. ru

Basic principles of national, international and global solar power system designs and some results of computer simulation are considered. Rational siting of solar power generating units allows appreciably improving the diagram of the electric power generation, largely approaching to requirements of the consumer. The specified effect is reached by distribution of solar power stations in meridional direction so that the ending of insolation of a photoactive surface of one power station coincides with the beginning of insolation of panels of another, the nearest sunwise, station.

Principles of development and functioning of power engineering were designed in middle of 20-th century when a main problem was increase of energy generation with use of mineral fuel. The era of cheap energy on the basis of mineral fuel is coming to the end, and already today it is necessary to change approaches and directions of development of world power engineering to provide sustainable future development. It is obviously, that tendencies and prospects of development of energetics cannot be viewed in a separation from use of renewable clean energy sources what are the sun, wind, geothermal waters, biomass, energy of water-currents and series of others [1, 2, 3].

Despite its irregularity of operation as a result of known periodic and stochastic processes, solar photovoltaic generators on set of properties are most perspective for use in power industry. Thus in a number of countries solar power stations are used not only in an autonomous mode, but also directly connected to national electric power systems.

However, connection of high capacity solar stations to energy distribution networks may seriously complicate problems of power output and load matching and electromagnetic stability — which are the major tasks at designing and operation of reliable and effective electric power supply. Really, the power output of solar stations may vary dynamically and unpredictably for a control service of a distribution network. If the share of solar stations capacity in an electric power system is great, it will be difficult to compensate such fast power fluctuation and it may give rise to negative consequences both for generators of an electric power system, and for load.

The offered approach allows to not restrict the part of an installed power but to create a global electric power system in which solar stations will play the basic role.

Rational siting of solar power generating units allows appreciably improving the power output curve, largely approaching it to requirements of the consumer. The specified effect is reached by distribution of solar power stations in meridional direction so that the ending of insolation of a photoactive surface of one power station coincides with the beginning of insolation of panels of another, the nearest sunwise, station.

The distance between the adjacent solar stations in degrees of longitude should be no more than

15fhl + h) ,

where h1 and h2 — durations of daytime in latitudes in which stations are located, expressed in hours.

For a year-round and day-and-night operation it is durations of daytime in the winter solstice.

Changing an installed power and distance between stations in longitude, it is possible, in known limits, to correct a diurnal variations of an average system power output. If necessary, additional solar power substations may be included in the system for covering maximums of daily demand. The longitude, in which the substation should be located, is defined by time of passage of the corresponding maximum — at this time in desired longitude there should be midday. Siting of system solar power stations on either side of the equator will allow eliminating seasonal changes in an electricity generating — winter decrease in one hemisphere is compensated by summer growth of generation in the other.

Unique geographical features of Russia allow creating the solar power system with continuous and enough uniform electric power generating in a summer period. In the summer the sun does not sets on Russia, and the similar system generates energy day — and-night — the sun in succession illuminates photoactive surfaces.

Fig. 1. Siting of solar stations of the national electric power system.

Figure 1 shows the siting of two solar stations of the Russian national electric power system. One of stations is located on Chukot at 66°N and 173°W, second — in the Pskov region in the point with coordinates 57°30’N and 28°E. The probable need of the Russian Federation for the electric power by 2010 may will come up to 1100 Terawatt-hour (TWh). The system providing monthly electricity generation from March to August at a level 80 — 140 TWh may consist of two equal in power solar stations, 0.3 TW each. The diurnal variations of average power output of such electric power system are showed in Fig. 2. Computation is carried out for panels with polar axis tracking.

Solar panels of system generate electricity day-and-night during five months from April to August. During two more months — in March and September — interruptions are no more than 2 hours per day with a little bit greater irregularity of diurnal variations (Fig.2).

September

Moscow time

0,25 0,2 0,15 0,1 0,05 0

November 0 2 4 6

8 10 12 14 16 18 20 22 Moscow time

December

Moscow time

February

Moscow time

Fig. 2. Day variations of a mean power output (TW h) for national system consisting of two PV plants located in Chukot and Pskov region (0.3 TW each) with polar axis tracking panels.

Fig. 3. Siting of solar stations of the Afro-Eurasian electric power system.

It is possible to expand calendar duration of day-and-night work of the solar power system, as much as possible by increasing distance between its solar stations. For this purpose, for example, one of the solar stations may be located in the east of Russia, second in the West of Europe or in Africa. Figure 3 shows the solar power system with stations located in Russia (village Markovo, 64°40’N and 170°23’E) and in Mauritania in the point with coordinates 20°N and 10°W. The Afro-Eurasian solar power system, using panels with polar axis tracking, during seven months from March to September, day-and — night generates the electric power (Fig. 5). With station capacities in Mauritania is equal 1.0 TW and in Russia — 1.5 tW, the system annual power output is 6400 TWh.

For a year-round and day-and-night operation, longitudinal extent of one Russia or even whole continent is not enough — only the global solar power system is capable of such operation.

Figure 4 shows variant of stations siting of the global solar power system. Three solar stations evenly distributed in the meridional direction, the first power station is located in Bolson de Mapimi desert region (Mexico, 28°N and 104°W), second — in Libyan Desert (Libya, 25°N and 16°E), and the third — in Simpson Desert (Northern Territory of Australia, 25°N and 136°E). The electric power system is capable year-round and day-and-night to supply customers with "solar electricity".

The system consists of three each 2.5 TW-generating solar power stations with stationary panels, and its total annual power output is about 17800 TWh with high uniform of average diurnal variations (Fig. 6).

For placing of each such station with efficiency of photovoltaic cells of 20%, the territory with the area 44000 km2 (210 km x 210 km) is required. Naturally, there is no necessity to arrange compactly the panels of each station — the single requirement is proximity to the given meridian.

There is very high insolation level in Australia, but it is the remote continent, therefore instead of Australia the solar power station may be located in the same longitude in South Siberia, Russia. Because of short winter daytime, irregularity of a diurnal variations of

Fig. 4. Siting of solar stations of the global electric power system.

average power output of such system will be a little bit higher, and because of smaller insolation level, peak power of the Siberian solar station should be increased by 40%.

It is impossible to avoid influence of weather factors on a power output of solar stations. In stand-alone solar power supply for compensation of power fluctuation, standby generators and buffer energy storage units are successfully used. The modern buffer storages (electrochemical accumulators, capacitive stores, etc.) have excellent maneuverable performances — they automatically and very quickly pass from charge to discharge mode, but to create in a large electric power system the storage battery of sufficient capacity it is practically impossible.

The control system with space monitoring will allow using with global solar power system existing traditional power stations as standby generators. Depending on the type, traditional power stations have different maneuverable characteristics — for starting it is required from 2 — 3 minutes till several hours. More powerful power stations require, as a rule, the greater starting time.

Observation of a cloudy cover in vicinities of limited number extra-high-power solar plants by satellites will allow to predict a power output level and, if necessary, to specify the moment of starting for those or another standby generators. Such system will allow using traditional fuel or other renewable (biomass, hydro) power stations for compensation of solar array power output fluctuations.

Moreover, the global electric power system organized in a similar way is capable to operate generally without standby generators and buffer storages.

In the global solar power system some of a photoactive surfaces are always insolated, it generates the electricity continuously — depending on solar irradiance, the level of a power output varies only. Special interest represents time variations of a power output at worst-case conditions of insolation. Its minimal value in an annual cycle is the basis level of the diagram of the electricity generation, a warranted lower limit of a system power output.

).

With the specified probability all possible fluctuations of a system power output are higher than this level, and, as result, a part of reserve power stations, with equivalent installed capacity, may be completely put out from operation without damage to load.

Obviously, that the basis level of a power output depends not only on the lower level of insolation, but also on installed capacities of solar power stations. In principle, it is possible to create a global electric power system with stations of such peak power that even at worst-case conditions of solar irradiance the potential power output in each time interval will exceed needs of load. The most part of time, such system will generate excess energy, but exclusive maneuverable performances and ability of solar arrays to work uncertainly long in any modes — from idle up to maximum power — allow supporting a power output of PV system precisely at that level which is required and, that it is more important, without additional energy sources.

Practical realization of the project will demand solution on interdependent scientific, technical and economic problems. Some of them:

• Construction of solar global system with common control center;

• Interconnection of national electric power grids with the solar global power system;

• Providing Terrawatt capacity intercontinental flow of electric power [4].

Non-profit corporation Global Energy Network Institute (GENI), registered in USA is

actively engaged in similar projects, with an emphasis on linking local and remote renewable energy sources (wind, solar, hydro, geothermal, tidal and biomass). But build­up of the global solar power system will demand new approach to solution of these tasks. A serious problem may be intercontinental transfer of super-power energy fluxes on thousands kilometers, including underwater, with the minimal losses. Moreover, change of generating units siting and routes of power trunk may demand reorganization of national distribution grids.

As to photovoltaic generators, here efforts should be guided on solution of usual tasks — lowering of a specific cost, increase efficiency and resource of photovoltaic modules. Additionally, pace of production of unified photovoltaic modules should be enlarged on some orders.

It is obvious, that realization of the global photovoltaic project will demand long-term combination of efforts and mobilization of resources of all world community.

Conclusions

Solar power stations may be a basis of global electric power system — at rational siting of power stations, the system generate the electric power in year-round and day — and-night mode.

With use of a satellite monitoring a cloudy cover and a transparency of an atmosphere, the part of world system of traditional fuel power stations may be used for compensation of power output fluctuations as a result of the weather phenomena.

The global solar power system, with adequate peak power of solar stations, is capable to power supply all load demand under any conditions of insolation without use of buffer stores and standby generators.

1.