The uneven distribution of river flow across the territory, its intra-annual and long-term variability make it difficult to meet the needs of the population and the national economy for the required amount of water. This is especially acute in low-water years and seasons. The problem is solved by regulating river flow with reservoirs and ponds.
Reservoir is an artificial reservoir designed to regulate river flow, i.e. redistribution in time, with the aim of using it more efficiently for the needs of the national economy.
Large reservoirs, as a rule, have a complex (multi-purpose) purpose: hydropower, water supply, water transport, recreation, flood protection. The most efficient use of water resources is ensured by a cascade of reservoirs operating in a single system.
Small reservoirs and ponds are used to supply water to the population and certain industries or agriculture.
More than 2,500 large reservoirs with a volume of more than 100 million km 3 each have been created around the globe. Most of them are located in North America (36% or about 900). There are approximately 100 such reservoirs in Russia, the largest of which are Bratskoe, Krasnoyarsk, and Zeyaskoe.
The system of reservoirs on the river is called cascade.
Reservoirs can be divided into types according to the nature of the bed, the method of filling it with water, geographical location, location in the river basin, and the nature of flow regulation.
By the structure of the basin reservoirs are divided into:
· river type or valley, the bed is part of a river valley. They are distinguished by their elongated shape and increasing depths from the top to the dam.
· Lake type or basin type, these are spring-loaded, i.e. regulated, lakes and reservoirs located in isolated lowlands and depressions, in bays, estuaries fenced off from the sea, as well as in artificial excavations.
According to the method of filling with water reservoirs are divided into:
· Zaprudnye, when they are filled with water from the stream on which they are located
· Liquids, when water is supplied to them from a nearby watercourse or reservoir.
By geographical location:
· Mountain, built on mountain rivers, they are usually narrow and deep and have pressure, i.e. the magnitude of the increase in water level in the river as a result of the construction of a dam to 300 m or more
· Foothills, have a head height of 50-100 m
· Plains usually wide and shallow, head height no more than 30 m.
By the nature of flow regulation:
· Multi-year regulation (redistribution of runoff between low- and high-water years)
· Seasonal (redistribution of runoff within a year between low- and high-water seasons)
· Weekly (redistribution of flow during the week)
· Daily regulation (redistribution of flow during the day)
The nature of flow regulation is determined by the purpose of the reservoir and the ratio of the useful volume of the reservoir and the amount of river water flow.
The shapes and sizes of reservoirs are characterized by the same morphometric characteristics as lakes. They also depend on the degree of filling of the reservoir and are “tied” to a certain value of the water level, but, unlike lakes, the water level in the reservoir is regulated and the course of the level is determined by the nature of the regulation.
When designing reservoirs, for each of them levels are established (set) corresponding to certain phases of the hydrological regime, the so-called design levels.
· Normal retaining level NPU, the level that is reached by the end of the filling period in an average year in terms of water content and can be maintained by the dam for a long time
· Forced support level FPU, which occurs in rare cases, for example, during high water or floods, is held for a short time, exceeds the FSL by 0.5-1 m
· Trigger level. Trigger levels include: the daily (dispatcher) trigger level, which is achieved during normal operation of the reservoir; the level of maximum production, which is achieved only in dry years
· ULV dead volume level, the maximum possible decrease in the water level in the reservoir, below which release is impossible. The volume of the reservoir located below the ULV is called dead volume.
The volume located between the ULV and the NPU is called useful volume of reservoir PO.
The sum of useful and dead volumes gives the total volume or capacity of a reservoir.
The volume enclosed between the NPU and FPU is called reserve volume .
According to the morphometric features of the basin characteristic areas are identified:
ü Lower – near the dam (always deep-water);
ü Medium – intermediate (deep water only at high levels);
ü Upper – shallow (located within the flooded channel and floodplain);
ü Area of support wedging out.
The boundaries are arbitrary and depend on the amplitude of level fluctuations
In dry, dry years, water flow in rivers decreases, and the need for water for irrigation and public water supply increases. A decrease in water consumption entails a decrease in electricity production at hydroelectric power stations, worsening water supply conditions, a decrease in water quality and other adverse consequences. Seasonal fluctuations in river flow are characterized by a sharp decrease in water flow in rivers in winter, when the need for electricity is usually greatest; The demand for water for industrial water supply in winter usually does not decrease. A summer decrease in water flow is unfavorable for irrigation, shipping and other water consumers and water users.
For the most complete and economical use of water resources and adaptation of the water yield regime to the needs of various sectors of the national economy, the flow of water reservoirs is regulated.
TYPES OF RESERVOIR
Lakes are natural reservoirs. Under natural conditions, the lake regulates its flow without human intervention. The maximum flow of the river flowing out of the lake is several times less, and the minimum is much greater than the total inflow from the rivers flowing into the lake. If, when backing up the lake with a dam, its level is increased or the river bed at its source is cleared, or both of these measures are carried out, then the regulating capacity of the lake will increase and it will be possible to increase the minimum flow of the river flowing from the lake above the natural one.
Most often it is necessary to create artificial reservoirs. To create a reservoir, a dam is built in the river bed, which backs up the river. At the same time, the floodplain and the surrounding area are flooded. When designing and constructing reservoirs, it is imperative to comprehensively study all the positive and negative consequences of the construction of reservoirs. When placing them, it is necessary to reduce in every possible way the area of flooding of valuable agricultural land. On lowland rivers, the flood area can be quite large. For example, the area of the Kuibyshev reservoir on the Volga is 6450 km 2. On rivers of the plain type, due to the small slope of the river, reservoirs are of great length - up to 200-300 km. With flat banks, the width of the reservoir sometimes reaches 40-50 km. On mountain rivers, due to the large slope of the river and steep banks, a large volume of reservoir can only be obtained with a high dam height, which, however, does not cause large flooding of the territory.
In hydropower engineering, reservoirs are distinguished according to their location relative to a given hydraulic installation:
1) upland, located on the river or its tributaries above a given hydroelectric power station;
2) own, i.e. formed by structures that are part of this hydroelectric power station;
3) grassroots, located below this hydroelectric power station.
RESERVOIR VOLUME
The normal retaining level (NRL) is the highest level at which, according to stability conditions, the normal operation of retaining structures is calculated (Fig. 3-1). NPU can be maintained indefinitely.
Forced level (FLU) is a level that can be allowed for a short time when exceptionally large floods or high waters occur, having a probability lower than the calculated one, which was accepted for normal operating conditions.
The lowest drawdown level (LL) of a reservoir is called the dead volume level (LDL).
The volume of water in the reservoir between the NPU and the ULV is called the useful or working volume. The volume of water below the SLV is usually not used to regulate flow and is called dead volume.
The total volume of the reservoir at NPL is equal to the sum of the useful and dead volumes. Between the NPU and FPU marks there is a reserve volume of the reservoir, which is used to receive and transform floods and floods of rare recurrence. The sum of the working, reserve and dead volumes gives the total volume of the reservoir at FPU. To determine the volume of the reservoir using topographic plans of the area, planimeter the areas between the corresponding horizontal lines and the dam alignment. Based on these data, an area curve F = f(Z) is constructed, showing the dependence of the surface area of the reservoir F on the elevation Z. Then, for each increment in elevations, the increment in volume A Y is calculated and a dependence curve V = f (Z) is constructed, which is called a static volume curve reservoirs (Fig. 3-2). On lowland rivers with high flow rates, curves of the free surface of the water in the reservoir are constructed. These backwater curves, at the same water level at the dam, will have a greater curvature and a higher water level at the end of the backwater curve, the greater the inflow flow (Fig. 3-3). In these cases, a curve of dynamic volumes of water in the reservoir V = f (Z, Q) is obtained.
In some cases, it is taken into account that the porous soils of the shore and bed of the reservoir absorb water when the water level rises and release it back when the water level decreases, which is equivalent to an increase in the actual capacity of the reservoir.
The Owen Falls reservoir, located in Uganda, Kenya and Tanzania, has the largest total volume of 205 km3 and the largest surface area of 76 thousand km2. The largest reservoir is on the river. Volta (Ghana), the total volume of which is 148 and the useful volume is 90 km3. In terms of useful volume, the Nasser reservoir on the river is in third place in the world. Nile (Egypt), created with technical assistance from the USSR. In table Table 3-1 shows data on the largest reservoirs of the USSR with a useful volume of more than 10 km3.
WATER LOSSES FROM RESERVOIRS
Loss of water from the reservoir occurs due to evaporation, filtration and subsidence of ice on the banks during the winter drawdown of the reservoir. For the hydroelectric power station, the water taken from its upper pool for irrigation, water supply, ship locking, etc. is also “lost.”
Evaporation. With the creation of reservoirs, evaporation increases. Total evaporation losses are determined by the product of the surface area of the reservoir Fв and the thickness of the evaporated water layer hB.
As can be seen from table. 3-2, specific water losses due to additional evaporation in Central Asia are 15 times greater than in the north of the European part of the USSR.
Of all the reservoirs in the south of the European part of the USSR, losses due to additional evaporation from reservoirs average more than 10 km3 per year.
Filtration. There are losses of water due to filtration through the body of the dam, under it and bypassing it through the thickness of the soil and through leaks in dam gates and turbine guide vanes.
When the reservoir is drained in winter, ice settles on the banks. In the spring, the ice melts and replenishes the water runoff. But for annual regulated reservoirs, spring replenishment usually only increases the volume of idle discharges through the dam. Thus, the subsidence of ice on the shores represents losses for the energy sector.
There are basic and special types of flow regulation.
1. Main types of regulation
The main types of flow regulation include perennial, annual, weekly and daily.
Long-term regulation allows, in low-water years, to increase water consumption and electricity generation by hydroelectric power stations due to the runoff of high-water years. With long-term regulation, the reservoir is filled with excess runoff from high-water years and is emptied during a number of low-water years. All water consumers and water users are interested in long-term regulation, but its implementation requires a large volume of reservoir.
For deep long-term regulation, a useful reservoir volume equal to one to two average annual river flows is required. Partial long-term regulation is possible even with a reservoir capacity of about 50% of the average annual flow.
Annual regulation redistributes runoff throughout the year in accordance with the needs of water users and water consumers. In high-water seasons, the reservoir is filled, and in low-water seasons, it is emptied. The regulatory cycle is one year. The required volume as a percentage of the average annual flow ranges from 3-10% with partial flow regulation to 40-60% with full flow regulation.
For the energy industry, weekly and especially daily regulation, carried out in accordance with weekly and daily fluctuations in the load of power systems, are of great importance.
Daily regulation with a relatively constant water influx ensures uneven water consumption by the hydroelectric station, following daily fluctuations in the load of the power system. The required volume of the pool or daily regulation basin is determined by calculation (see § 5-2). The approximate volume is from 5 to 10% of the daily throughput of all hydroelectric power plant turbines. If only daily regulation is carried out at a hydroelectric station, then the regulation cycle lasts one day and by the end of the day the water level in the pool or pool returns to its original position. With daily regulation, the hydroelectric power station covers the peaks of the daily load schedule.
Weekly regulation allows you to increase the capacity and energy production of hydroelectric power plants on weekdays by reducing the used flow on weekends, when the load in the power system decreases. For weekly regulation, a reservoir volume of 50-100% of the daily throughput capacity of all hydroelectric power plant turbines is required.
2. Special types of regulation
Special types of regulation include:
a) Compensatory regulation, which can be carried out by an upstream reservoir in order to compensate for the unevenness of inflow from the intermediate catchment area between the reservoir sections and the hydroelectric power station. When there is a small inflow from the intermediate catchment, increased releases are given from the compensating reservoir and vice versa. If a large reservoir has its own hydroelectric power station, then it is possible to carry out compensatory annual and even long-term regulation of the electricity generation of several; Hydroelectric power stations located on different watercourses, but connected to a common electrical network. Thus, the reservoir of the Bratsk hydroelectric power station carries out compensatory regulation of energy production by the Yenisei and Angarsk hydroelectric stations.
b) Transformation of floods and floods. If the peak portion of the flood is retained in the reservoir, the maximum flow through the dam will be reduced. This will make it possible to reduce the spillway structures of the waterworks, reduce flooding on the river below the reservoir, etc.
c) Emergency use of the reservoir. In the event of an accident in the power system, a hydraulic station can quickly take on additional load and use up a specially provided reserve or part of the working volume of the reservoir from its reservoir. After the accident is eliminated, the additional consumed volume is restored by reducing the load of the hydroelectric power station or due to the nearest flood.
Lecture 9. Useful volume of the reservoir. Justification of the optimal reservoir drawdown depth.
9.1 Useful volume of annual regulation reservoir
The main task of the annual regulation reservoir is to increase the amount of energy and power of the hydroelectric power station during the low-water period of the year due to excess water retained in the reservoir during floods. The first issue that must be resolved by us is the issue of dividing the entire volume of the annual regulation reservoir into two parts - useful and dead volumes. Having the full volume of the reservoir, it is necessary to divide it into these two volumes, i.e., decide the issue of determining the depth of drawdown of the reservoir or set the ULV mark. When solving this problem, we will assume that the FSL mark of the reservoir is known and that the reservoir can always be filled during a flood, with the exception of very rare cases when particularly unfavorable hydrological conditions occur. We will not take these cases into account for now.
The task is to find the maximum drawdown depth of the reservoir at which the greatest energy effect at the hydroelectric power station can be obtained. If we accepted that the reservoir can be filled every year, then here we can consider the period of emptying the reservoir separately. The general solution to the problem can be obtained as follows.
As the emptying of the reservoir increases, the amount of energy that is obtained from the use of the reservoir also increases. This energy depends only on the depth to which the reservoir will be drained, and practically does not depend on the length of time during which the reservoir is emptied, the magnitude of the transit flow or other factors.
The generation of a hydroelectric power station in the presence of a reservoir can be represented as consisting of two parts: electricity generation due to the transit flow of the river flowing during the drawdown of the reservoir, and generation due to the drawdown of the reservoir
E hydroelectric power station =E IN +E TR (11-2)
The amount of transit energy from a hydroelectric power station depends, of course, on the volume of transit flow, i.e., on the magnitude of transit water flows and the duration of the period of emptying the reservoir. But it also depends on the pressure, i.e., on the drawdown depth of the reservoir. Finally, it depends on the operating mode of the reservoir. With rapid drawdown of the reservoir at the beginning of the low-water period, as, for example, shown in Fig. 1.1, most of the transit flow passes at a relatively low pressure and therefore produces a small energy effect. The drainage is mainly at the end of the low-water season, shown in Fig. 1.2, allows you to pass almost the entire transit flow at high pressure and, therefore, receive more energy from the hydroelectric power station.
Rice. 1.1 Fig. 1.2
Let us know the hydrological characteristics of the low-water season, during which the reservoir is emptied. By setting some simple conditional mode of operation of a hydroelectric power station (for example, when regulating the flow rate of water passed through the turbines of a hydroelectric power station to a constant value), we can determine the amount of energy that the hydroelectric power station produces at various operating levels by the end of the low-water season. The results of such calculations can be presented graphically in the form of a curve of the dependence of the transit energy of a hydroelectric power station on the depth of drawdown of the reservoir, shown in Fig. 2
The energy characteristics of the reservoir are plotted on the same graph. The deeper the reservoir is drained, i.e., the larger its useful volume is assumed to be, the greater the amount of energy obtained from the reservoir and the less transit energy becomes. The decrease in transit energy is explained by a decrease in pressure as the drawdown of the reservoir deepens. By summing up the energy of the reservoir and transit energy for different drawdown depths, we obtain the total energy value of the hydroelectric power station for the entire period of emptying the reservoir. It is obvious that for the given hydrological conditions and for the adopted regulation regime, the reservoir drawdown depth at which the hydroelectric power station produces the greatest amount of energy turns out to be the most profitable. Further deepening of the drawdown of the reservoir, although it would increase its useful volume and the regulated flow used by the hydroelectric power station, the pressure would decrease so much that the total amount of energy generated by the hydroelectric power station would not increase, but decrease.
If the characteristics of the transit flow change during the emptying of the reservoir, then the curve of the dependence of transit energy on the depth of drainage of the reservoir will take a different form and take a different position on the graph. In Fig. In Fig. 2, the dotted line shows such a curve obtained with a reduced transit flow, and the curve of the total energy of the hydroelectric power station for this case. From the above graph it can be seen that in this case the most advantageous value of the useful volume of the reservoir increases. This is very easily explained by the fact that with a decrease in transit flow, the energy of the reservoir accounts for the majority of the total energy of the hydroelectric power station. Let us note here that a change in the magnitude of the transit flow can occur even if the hydrological characteristics remain unchanged, but with a change in the duration of the emptying of the reservoir.
If we, without changing the volume and distribution of transit flow, adopted a different regulatory regime, then the shape of the curve of the dependence of transit energy on the depth of the maximum drawdown of the reservoir at the end of the low-water season would change. At the same time, the shape of the total curve of the total energy of the hydroelectric power station would also change. Consequently, we would have received a different mark for the most profitable reservoir drawdown. If the control regime changes significantly during reservoir emptying, the change in the most advantageous reservoir drawdown depth can also be significant. From Fig. 1.1 and 1.2 it follows that when the reservoir is emptied early, its deep drawdown is less profitable than when it is emptied late.
The influence of various conditions on the position of the most favorable drawdown depth of a reservoir, which has a certain characteristic of its own, was discussed above. But comparing different reservoirs with each other, it is not difficult to see that the maximum depth of their drawdown, under the same other conditions, depends on the type of their characteristics - volumetric and energy. For example in Fig. Figure 3 plots the volumetric characteristics of two reservoirs that have the same total volume at the same FSL level. From this graph it is clear that at the drawdown depth indicated in the drawing, the useful volume of the reservoir is A makes up the majority of its total volume. For reservoir B at the same drawdown depth, the useful volume is only a small part of the entire total volume of the reservoir. Further deepening of the drawdown noticeably increases its useful volume and, therefore, gives a large energy effect, while for the reservoir A deepening the drawdown only very little increases the regulated water flow.
For a mixed dam-diversion scheme, water-energy calculations to determine the most advantageous reservoir drawdown depth are carried out in the same way as for a dam scheme. These calculations must, of course, take into account the entire head, both that created by the dam and that created by the diversion. It is clear that in a dam-diversion scheme the useful volume of the reservoir constitutes a significantly larger part of its total volume, and the dead volume is much smaller than in a dam scheme. The dead volume may be negligible.
From all that has been said above, let us dwell on one very important point. For a given reservoir, the most advantageous drawdown depth depends very much on the volume of transit flow. But in different hydrological years, the volume of transit flow during the low-water season, when the reservoir is emptied, is not the same. Consequently, the drawdown depth of the reservoir should also be different in different hydrological years.
If we had the opportunity to obtain a fairly reliable forecast of the natural flow regime of the river for the upcoming low-water season, then a preliminary determination of the most favorable depth of reservoir drawdown in each individual year would not present any fundamental difficulties. However, in the absence of a preliminary forecast of river flow, it becomes impossible. But if it is impossible for practically every single year to establish its own special most advantageous reservoir drawdown depth, then this means that one depth of maximum reservoir drawdown, identical for all years, should be determined, regardless of the difference in hydrological characteristics in all these years.
Of particular importance is the use of an annual regulation reservoir to increase the energy and power of hydroelectric power stations during emptying of the reservoir in low-water years. Therefore, determining the maximum drawdown depth of a reservoir should be carried out with a small volume of transit flow. In this case, we get, as can be seen from Fig. 2, a greater depth of the maximum drawdown of the reservoir, which we will consider the same for all hydrological years. With this solution to the problem, in high-water years, the amount of energy generated by a hydroelectric power station may turn out to be slightly less than the maximum possible. But, as we will see later, the energy lost in this way can be, if not completely, then partially compensated if we apply different regulatory regimes in different hydrological years. Indeed, in high-water years, there is no need to quickly empty the reservoir at the beginning of the low-water season, as in low-water years, since the large amount of transit flow allows the hydroelectric station to operate with the power required for the energy system, no less than the guaranteed one, while taking only relatively a small amount of water. At the end of the low-water season, when only a small part of the transit flow remains unused, the drawdown of the reservoir can be quickly brought to a constant maximum level, as a result of which additional energy will be obtained.
Although we have come to the conclusion that the maximum working depth should be determined based on a low-water year, this conclusion cannot yet be considered complete, since we still need to decide which year should be chosen from among the low-water ones as the design year. The choice of the calculation year cannot, of course, be made arbitrarily, since the calculation year must meet certain conditions, i.e., the conditions for the best use of hydroelectric power plants in the energy system. Of the two main requirements imposed by the energy system on hydroelectric power plants, in this case the first one is of greatest importance - achieving the greatest security in the operation of the energy system. Methods for satisfying the second main requirement of the energy system - the largest amount of energy generated by hydroelectric power plants - will be discussed further.
Taking as the initial condition for determining the most favorable maximum depth of reservoir drawdown the achievement of the greatest security in the operation of the energy system, we at the same time resolve the issue of the reservoir emptying regime, which we previously accepted as conditional. Since the greatest security, as we established earlier, is achieved when the hydroelectric power station operates according to a guaranteed power schedule, it follows that the regulation regime during emptying of the reservoir must correspond to the operation of the hydroelectric power station according to this schedule.
If the composition of the existing energy system is known, then a guaranteed schedule of the average daily power of a hydroelectric power station can always be constructed. Having a fairly complete hydrological characteristic of the river, we can calculate the regulation for a long series of years when the hydroelectric power station operates according to a guaranteed schedule of average daily power. As a result of this calculation, a graph of changes in the water level in the reservoir over all these years will be obtained. In Fig. Figure 4 shows combined curves of changes in water level in the reservoir over several years, and here only those sections of the curves that are of interest to us in this case are highlighted, i.e., those related to the time of emptying of the reservoir.
The shallower the year, the greater the amount of water that needs to be taken from the reservoir to obtain guaranteed power at the hydroelectric power station. Therefore, the shallower the year, the deeper the reservoir is drained. However, in especially low-water years, no deep drawdown of the reservoir will make it possible for the hydroelectric power station to operate according to a guaranteed schedule during the entire period of emptying the reservoir due to a significant decrease in pressure during deep drawdown. The curve of changes in water level in the reservoir for such a case is shown in Fig. 4 dotted line. Obviously, in such particularly low-water years, disruptions to the normal operation of the energy system cannot be avoided. Therefore, we exclude all such years from further consideration.
From among the remaining years, we will take the year with the least water, in which the depth of the reservoir drawdown is greatest. If we had used the reservoir to a lesser extent this year, the hydroelectric power station would not have been able to operate according to the guaranteed schedule due to a lack of water. A deeper drawdown of the reservoir this year is not required to obtain guaranteed capacity and it cannot provide additional energy, since the operation of a hydroelectric power station with an average daily capacity greater than the guaranteed one would lead to premature emptying of the reservoir and an excessive reduction in the pressure of the hydroelectric power station. Thus, we come to the conclusion that the drawdown depth we obtained is the limit to which the annual regulation reservoir can be emptied annually. That part of the total volume of the reservoir, which is contained between the maximum drawdown mark and the NPL mark, represents the useful volume of the reservoir.
When determining the maximum drawdown depth of the reservoir using the method described above, we took the guaranteed schedule of the average daily power of the hydroelectric power station as the initial condition. But since a hydroelectric power station, which has a reservoir of annual regulation, simultaneously carries out daily regulation, then during the hours of the daily peak load of the energy system it must develop a power greater than the daily average. With deep drawdown of the reservoir and a significant decrease in pressure, the available power of the hydroelectric power station can be equal to or even greater than the guaranteed average daily power. In such cases, despite the fact that the hydroelectric power station can operate according to the guaranteed average daily power schedule, a disruption of the normal operation of the energy system still occurs. Consequently, in this case, the maximum drawdown mark for the annual regulation reservoir, up to which it must be emptied annually, must lie higher than that which we determined earlier. Based on the annual schedule of the guaranteed peak power of a hydroelectric power station and the characteristics of the turbines installed on it, it is not difficult to determine what minimum pressure is necessary to have and, therefore, what water level in the reservoir must be maintained on any calendar date for the entire time the reservoir is emptying. The dependence of the required minimum water level in the reservoir on time is plotted in Fig. 5. The same graph shows a curve of the dependence of the water level in the reservoir on time when the hydroelectric power station operates according to the guaranteed schedule of average daily power. Of all such curves in Fig. 5 shows only two. One of them, shown as a solid line, was obtained in a hydrological year when the drawdown of the reservoir by the end of the low-water season exactly coincides, as can be seen from the graph, p. permissible drawdown under the condition of obtaining the required peak "power at the hydroelectric power station. This depth of drawdown of the reservoir should be considered as the limiting one, i.e., as the one to which the reservoir is emptied annually. In the same drawing, the dotted line shows the curve of the dependence of the water level in the reservoir on time , which was previously adopted to determine the operating depth based on the average daily power.
The greatest load on the energy system, when a hydroelectric power station is required to participate in the power balance with its entire total displacement capacity, in most cases does not coincide in time with the greatest emptying of the reservoir. The annual peak load of the energy system usually occurs at the end of December and beginning of January, and the complete emptying of the reservoir occurs at the end of the low-water season, i.e. in the spring, before the onset of the flood. In this regard, during the deepest drawdown of the reservoir, the guaranteed peak power is slightly less than the maximum. This makes it possible to increase reservoir utilization by the end of the low-water season. Such a case is shown in Fig. 5.
For low-pressure hydroelectric power plants, in which the pressure and available power of the hydroelectric power station depend on fluctuations in the water level in the downstream, when determining the maximum drawdown depth of the reservoir, the unsteady nature of water movement in the downstream of the hydroelectric power station during daily regulation should be taken into account. A large, but short-term increase in the load of a hydroelectric power station does not have a significant effect on the magnitude of the pressure and, consequently, the available power of the hydroelectric power station. Therefore, for low-pressure hydroelectric power plants, peak operating power mode and frequency regulation in the energy system seem to be more advantageous, since they make it possible to slightly increase the useful volume of the annual regulation reservoir used by such hydroelectric power plants, and at the same time the amount of energy generated by the hydroelectric power station.
If the displacement capacity of a hydroelectric power station includes reserve power, in particular, if a load reserve of the energy system is installed at the hydroelectric power station, then its value should, of course, be taken into account when determining the operating depth of the annual regulation reservoir, which is permissible under the condition of obtaining the required peak power at the hydroelectric power station.
Limitations on the drawdown depth of an annual regulation reservoir can be caused by other reasons than those noted above and which depend on the characteristics of the turbines installed at the hydroelectric power station. One of these additional reasons may be siltation of the reservoir with sediment, filling not only the dead volume determined by energy conditions, but also part of the useful volume of the reservoir. Another example of limiting the drawdown depth of a reservoir can be found in dam-diversion schemes. Such a case is shown in Fig. 6.
If the dam of such a hydroelectric power station is high enough, then the reservoir could have a very large usable volume if the maximum drawdown mark is determined on the basis of the energy calculations outlined above. In this case, the pressure diversion tunnel would have to occupy the height position shown in the dotted line in the drawing. But then, with a long tunnel length and a large slope, the internal pressure in its lower part at the connection with the turbine pipeline would be extremely large at a time when the reservoir is filled to the low level. This would require strengthening and therefore increasing the cost of the tunnel lining, which may not be economically viable. For this reason, to reduce the internal pressure in the tunnel, it has to be located higher, as shown in the drawing with solid lines. Since the ULV in the reservoir must be higher than the water intake openings of the diversion, this leads to a decrease in the useful volume of the reservoir.
Also, restrictions on the drawdown depth of an annual regulation reservoir may be caused by the operating conditions of other water consumers.
Finally, when choosing the reservoir drawdown depth, the equipment that will be installed at the hydroelectric power station should be taken into account. There is no turbine that works equally well at a head of 100 m and 50 m. In general, the ratio of the minimum pressure to the maximum turbine pressure for RO turbines is 0.6; for vertical submarines and PLDs – 0.5; for horizontal PL-0.35. This means that if you divide the minimum pressure at the selected operating depth by the maximum pressure of the proposed equipment, you should get a number no less than the indicated ones. For example, if the pressure at a hydroelectric power station at a NPL is 110 m, then when installing a RO115 turbine, the reservoir should be activated at no lower than 46 m, (115 * 0.6 = 69 m), less can be activated (and for rigid-blade turbines, the smaller the pressure fluctuations, the better), more - no.
9.2 Justification of the optimal reservoir drawdown depth
Above, we have already considered the energy characteristic or electricity generation curve for the period of emptying the reservoir, depending on the depth of the reservoir drainage.
For ease of perception, the total energy was obtained as the sum of two components: the energy of the transit runoff and the energy due to the drawdown of the reservoir.
It was noted that the value Esrab grows to a certain limit h O , after which the decrease in pressure is not compensated by an increase in the used flow and the total production decreases.
If the transit flow changes during the period of reservoir drawdown, then the position of the curve will also change. The dotted curve corresponds to a smaller value during the operation of the transit flow. Such a decrease may be a consequence of both lower water content and a shorter period of reservoir drawdown. The curve of the total energy of the hydroelectric power station also took a new position. The maximum output in this case corresponds to a different operating depth h o1 .
The curve of the total annual electricity generation of the hydroelectric power station in Fig. 1 has a similar character. 7
However, from a comparison of the two mentioned curves it is clear that the maximum annual production occurs at a lower final drawdown depth than the production during the period of emptying the reservoir. This is due to the fact that during the period of filling the reservoir, production decreases due to a decrease in both pressure and flow.
In Fig. The 7-dotted curves show the generation of hydroelectric power plants, taking into account additional generation at other hydroelectric power stations of the cascade. Taking into account the effect at the underlying stations of the cascade, the operating depth ensuring maximum output is greater.
So, each combination of initial conditions (transit flow, operation mode and duration, cascade scheme, etc.) corresponds to its own reservoir operation depth, at which the maximum values of the guaranteed annual electricity generation of the hydroelectric power station will occur.
However, this operating depth cannot be definitively accepted as optimal. Analysis of the graphs presented above provides only the zone within which the optimal reservoir drawdown depth should be sought. To substantiate it, in addition to changes in energy indicators, it is necessary to take into account other consequences of the drawdown of the reservoir.
Along with an increase in output, provided and installed capacity, an increase in useful volume leads to an increase in costs. Thus, a deeper final drawdown of the reservoir is associated with a greater depth of water intake and an increase in the cost of gates and hydraulic structures. An increase in the installed capacity of the designed hydroelectric power station is also associated with additional capital investments and costs. . These are the costs of expanding the hydroelectric power station building, increasing the total power of generators, electrical parts, turbine equipment, etc.
Additional costs for turbine equipment are caused by an increase in wheel diameter or the number of turbines. Both measures are used to increase the installed power and compensate for the decrease in the available power of the turbines when the design pressure decreases due to deeper drawdown of the reservoir. With a cascade scheme for using a watercourse, an increase in the useful volume of the reservoir of the designed hydroelectric power station may lead to the feasibility of increasing the installed capacity at the lower hydroelectric power stations of the cascade. This also involves additional capital investment and costs.
Finally, with the integrated use of a watercourse, additional capital investments and costs for related activities may be required. Thus, additional capital investments caused by an increase in the depth of reservoir drawdown when moving from one option to another represent the sum
Additional costs and reduced costs are determined similarly. All economic indicators are used in calculations taking into account the time factor. Accordingly, for the option of the designed hydroelectric power station, replacement options are calculated, for which changes in capital investments, costs and expenses are also determined during the sequential transition from the previous option to the next one.
In general, the costs of the replaced options or their changes A3 replacement represent the sum of the costs (or increments) of the replaced power plants, fuel and related activities
The minus sign at the second term can occur when transitioning between options in the zone from /g 0G od To ha (Fig. 7), i.e. when, with increasing operating depth, the power continues to increase, and the annual output is already beginning to decrease.
However, it must be borne in mind that a decrease in output is not always identical to a decrease in fuel economy. The fact is that specific fuel economy b w in different seasons of the year it is different, in particular, in winter, higher than in the spring-summer period. Therefore, with an increase in production during the period of reservoir drawdown (in winter) and a decrease in it during the period of filling reservoirs, despite the general decrease in annual production, the total fuel economy may not decrease, but increase. To correctly assess this component of costs, it is obviously necessary to carry out calculations to determine fuel savings separately by season.
All indicators for the replaced options should be determined according to the full effect on the designed hydroelectric power station and other hydroelectric power stations of the cascade, taking into account the different times of its receipt and use.
The rationale for the optimal drawdown depth of the reservoir is made according to one of the following conditions:
according to the equality of cost increment when the operating depth changes by the value Δh
according to the equality of the payback period for additional capital investments of a standard value with an increase in operating time by Δh
When designing hydraulic structures with reservoirs of long-term regulation, it is necessary to additionally carry out calculations to determine the timing of its initial filling and the regime for the hydroelectric station to reach its design energy output.
- artificial reservoirs, created, as a rule, in river valleys for the accumulation and storage of water for use in the national economy.
Reservoirs have similarities with and: with the first - in appearance and slow water exchange, with the second - in the progressive nature of water movement. At the same time, they also have their own distinctive features:
- Reservoirs experience significantly greater fluctuations in water levels throughout the year than rivers and lakes, which are associated with artificial regulation of flow - accumulation and discharge of water;
- water flow leads to less heating of water than in lakes;
- small reservoirs freeze earlier, and large ones - later than rivers, but both open later than rivers;
- the mineralization of reservoir waters is greater than that of rivers, etc.
People began to build the first reservoirs that served to irrigate fields even before our era in the valleys of the Nile, Tigris and Euphrates, Indus, Yangtze, etc. In the Middle Ages, reservoirs were no longer only in Asia and Africa, but also in Europe and America. In modern times, reservoirs began to be used not only for irrigation, but also for industrial water supply and for the development of river transport. In modern times, another function of reservoirs has been to generate electricity.
A huge number of reservoirs were built after. From that time until today, their number around the world has increased fivefold. It was during this period that the largest reservoirs in the world were created. Reservoir creation peaked in most regions of the world in the 1960s, followed by a gradual decline.
Currently, more than 60 thousand reservoirs are in operation around the globe.
The main parameters of reservoirs are the surface area, water volume, depth and amplitude of fluctuations in water levels under operating conditions.
The area of the water surface of all reservoirs in the world is 400 thousand km 2. The Victoria Reservoir (Owen-Fole) in East Africa (Uganda) is considered the largest in terms of surface area. It also includes Lake Victoria (68,000 km 2), the level of which rose by 3 m as a result of the construction of the Owen-Fole dam on the Victoria Nile River in 1954. The second place is occupied by the Volta Reservoir, located in the Republic of Ghana (West Africa). Its mirror area is 8482 km2.
The length of some of the largest reservoirs reaches 500 km, width - 60 km, maximum depth - 300 m. The deepest reservoir in the world is Boulder Dam on the river. Colorado (average depth 61 m).
The total volume of the world's reservoirs is 6,600 km 3 , and the useful volume, that is, suitable for use, is 3,000 km . 95% of the water in reservoirs comes from reservoirs with a volume of more than 0.1 km 3 . The largest reservoir in terms of water volume is also the Victoria Reservoir (204.8 km 3). The Bratsk Reservoir, located on the Angara River, follows it (169.3 km 3).
Based on the volume of water and the area of the water surface, reservoirs are divided into large, very large, large, medium, small and small.
The largest reservoirs have a total water volume of more than 500 km 3 . There are 15 of them in total. They are found in all regions of the world except Australia.
According to their genesis, reservoirs are divided into valley-river, lake, located at groundwater outlets, in river estuaries.
For reservoirs lake type(for example, Rybinsk) is characterized by the formation of water masses that are significantly different in their physical properties from the properties of tributary waters. Currents in these reservoirs are most associated with winds. Valley-river reservoirs (for example, Dubossary) have an elongated shape, the currents in them, as a rule, are runoff; The water mass is close in its characteristics to river waters.
Purpose of reservoirs
For a specific purpose, reservoir waters can be used for irrigation, water supply, hydropower generation, navigation, recreation, etc. Moreover, they can be created for a single purpose or for a set of purposes.
More than 40% of reservoirs are concentrated in the temperate zone of the Northern Hemisphere, where most economically developed countries are located. A significant number of reservoirs are also located in the subtropical zone, where their creation is associated primarily with the need for land irrigation. Within the tropical, subequatorial and equatorial zones, the number of reservoirs is relatively small, but since large and largest ones predominate among them, their share in the total volume of all reservoirs is more than 1/3.
The economic importance of reservoirs is great. They regulate flow, reducing flooding and maintaining proper river levels throughout the rest of the year. Thanks to a cascade of reservoirs on rivers, unified deep-water transport routes are created. Reservoirs are areas for recreation, fishing, fish farming, and waterfowl breeding.
But along with the positive significance of the reservoir, they cause undesirable but inevitable consequences: flooding of lands above the dam, especially rich floodplain meadows; flooding and even waterlogging of lands above the dam in the zone of influence of reservoirs due to rising groundwater levels; drainage of lands below the dam; deterioration of water quality in reservoirs due to a decrease in self-purifying ability and excessive development of blue-green algae; Reservoir dams prevent fish from spawning, causing damage to fisheries, etc.
At the same time, the construction of reservoirs causes irreparable harm to nature: flooding and underwatering of fertile lands, swamping of adjacent territories, processing of banks, dehydration of floodplain lands, changes in microclimate, genetic migration routes of fish in rivers are interrupted, etc. In addition, their construction in flat areas is associated with deforestation and the need to resettle many thousands of people. Of course, we are talking more about large reservoirs here.
Useful volume Wplz. We clarify the net of the reservoir, taking into account the loss of water from the reservoir due to evaporation, filtration and ice formation. To do this, we first determine the total volume of the reservoir Wsr in each month and the area ssr.
So, the total volume of the reservoir
W = Wplz. net + Wmo,
where Wmo is the dead volume of the reservoir.
Due to the fact that there is no data on water turbidity in the task, we calculate the dead volume approximately. Let's assume that
Wmo? 0.1· Wpl. = 0.1·7.484 = 0.7484 million m3.
The values of the total volume are recorded in column 2 of Table 3.
Then we determine the monthly average volumes of the reservoir Wav, with which, using topographic characteristics, we find the surface area w.
Evaporation losses are calculated for each month using the formula
where hi is the evaporation layer.
The calculation results are entered in column 6 of Table 3.
Filtration losses Wf in each month are found using the formula
Wф = сi·kф·ni,
where kf = 0.003 m/day,
ni - number of days in a month.
We enter the results in column 7 of Table 3.
Ice formation losses
Wl = 0.9 kl hl (schn - schk),
where 0.9 is the relative weight of ice;
kl is the coefficient of gradual increase in the thickness of the ice cover, equal to approximately 0.65;
hl - average long-term ice thickness at the end of freeze-up;
schn and schk are the surface area of the reservoir at the beginning and end of freeze-up.
We distribute the volume of losses Wl over the winter months (column 8 of table 3), and then find the amount of water losses (column 9 of table 3).
Taking into account these losses, the surpluses will decrease and the deficiencies will increase (columns 11 and 12 of Table 3), so the useful gross volume will be
Wbr = 9.578 million m3.
The discharge will decrease accordingly: 16.348 million m3
Then the total volume of the reservoir will be
Wtotal = Wmo + Wfr + Wfr = 0.7484 + 9.578 + 0 = 10.326 million m3.
Characteristic levels and capacities of the reservoir
The main characteristics of reservoirs are:
normal retaining level FPU, m;
ULV dead volume level, m;
catastrophic retaining level KPU, m;
total reservoir volume W, million m3 or km3;
useful volume of the reservoir Wplz, million m3 or km3;
dead volume of the reservoir Wmo, million m3 or km3;
volume of reservoir forcing Wfs, million m3 or km3;
reservoir capacity coefficient = Wplz/Wо,
where Wо is the average long-term flow.
NPL - the water level to which the reservoir is filled under normal conditions.
The total volume of the reservoir W is the volume enclosed between the bottom of the reservoir bowl and the water surface at the NPL mark. The full volume W is not entirely used to regulate flow. The lower part of the reservoir, designed to maintain minimum water levels and sediment sedimentation in it, is called the dead volume Wmo and cannot be drained.
The volume of the reservoir enclosed between the water surfaces at the NPU and ULV levels is called the useful volume - Wplz. During periods of high water it is filled, and during periods of low water it is emptied. The volume enclosed between the water surfaces at the NPU and KPU marks is called the forcing volume. KPU is a catastrophically backed-up level during the period when exceptionally high water flows or floods pass through the hydraulic system. The volume, forcing Wfs serves to reduce the amount of discharge flow through the hydraulic unit.
Figure 2. Main elements of the reservoir
The formation of a reservoir causes changes in the water flow regime. In the upper pool these changes mainly come down to the following:
water levels rise and depths increase, which is associated with flooding of the territory within the reservoir bowl;
Current speeds decrease, resulting in the loss of a significant portion of precipitation;
The water surface increases, resulting in an increase in evaporation, which leads to an increase in the salinity of the water in the reservoir.
The following changes occur in the downstream: high water and flood flows decrease and low water increases; and erosion of the riverbed below the hydroelectric complex occurs. In addition to the indicated changes in the watercourse in the upper pool, the following occur: flooding of the territory within the reservoir bowl; flooding of lands adjacent to the reservoir and collapse of the banks of the reservoir under the influence of waves.
In addition to the constant flooding of the lands occupied by the reservoir within the FSL, the economic use of which is impossible, temporary flooding of the territory above the FSL is observed during catastrophic floods and floods, from the surge of water by the wind onto the banks and from rising water levels during congestion and jams. Economic use of temporarily flooded lands is possible. When flooding occurs, groundwater rises, which sharply worsens the conditions for economic use of land and requires drainage measures.
We find characteristic water levels and their marks using the topographic characteristics of the reservoir:
The NSL corresponding to the filling Wfull = 10.326 million m3, at the level of the NSL = 131.8 m of the dam is equal to
NPU = NPU - PP = 131.8 - 120.0 = 11.8 m;
The dead volume level at the level of ULV = 121.2 m is equal to
ULV = ULV - PP = 121.2 - 120.0 = 1.2 m;
The forced support level of the FPU is equal to
FPU = NPU + 2.0 = 13.8 m,
where PP is the mark of the dam base.
