Mechanical accumulator of electrical energy. Alternative energy sources. Compressed gas energy storage
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All the NEs considered above had an electromechanical control device, which determined their low maneuverability.
Figure: 2.7. NEE connection diagrams:
a - shunt; b - linear
Electric energy storage units (EES) are connected to the EES by means of a controlled valve converter *, the power reverse time of which is 0.01 s, which determines their high maneuverability, and, consequently, the possibility of complex use in EES.
* Since the accumulation of electrical energy is possible only with direct current.
Electric energy storage devices include:
fuel cells (FC);
electrochemical storage batteries (EAB);
superconducting inductive storage (SPIN);
capacitive storage (EH).
There are two ways of connecting the NEE to the power system - shunt and linear, the corresponding circuits are shown in Fig. 2.7, a, b.
Let us consider in more detail the blocks of electric energy storage units.
NEE control device.
It can be made according to a three-phase bridge circuit, which has high technical indicators and has proven itself in the operation of existing high-power converters. The number of bridges in the IEE control device is determined both by the actually feasible power of the thyristor bridge and by the operating considerations discussed below. 
Figure: 2.8. Diagram of sequential connection of 12-pulse converter modules that make up the control unit:
1 - accumulating element; 2 - switch; 3- phase-to-phase reactor; 4 - converter bridge; 5- transformer; 6 - three-phase network
Each bridge is connected to the AC mains through a separate transformer. In order to provide a 12-pulse conversion mode, which has a number of advantages over the six-pulse one (less ripple of DC voltage, better harmonic composition of AC voltage, etc.), the secondary windings of one half of the transformers are connected in a "triangle", and the other in a "star" (fig. 2.8).
To increase the power factor of the NEE, which is determined by the angles of regulation and switching of the converter device, as well as the degree of distortion of the shape of the AC voltage curve, various compensating devices are connected to the station's AC buses - synchronous compensators, static thyristor compensators, filter compensating devices. Reactive power consumption can be reduced by dividing the converter into a series of modules connected in series.
During operation, the control angles of all modules, except one, are maintained at 0 °. One of them has an angle determined by the required voltage. All modules with a zero angle require only a minimum reactive power for switching.
In fig. 2.8 shows a possible diagram of a converter designed to reduce reactive power consumption. The converter is a series connection of 12-pulse modules containing power transformers. Each module is rated at 4.5 kV and consists of two 6-pulse bridges connected in parallel with a phase-to-phase current balancing reactor. Two modules have current values \u200b\u200bof 50 kA, two others - 30 and 20 kA. For example, at the maximum AE current of the storage, each 6-pulse bridge provides a constant current of 25 kA. If the 12-pulse module is short-circuited with a mechanical switch at zero voltage and then disconnected from the three-phase network, the overall efficiency of the converter will improve, since the forward voltage drop across the four series-connected thyristors is eliminated.
The value of the output active power of the NEE should in all modes of its operation be determined by the system requirements and not depend on the changing voltage at the AE itself. One way to ensure that this condition is met is by adjusting the valve control angles. The use of controlled converters as a connecting link between the AE and the AC network makes it possible, due to the corresponding change in the angles of switching on the valves during the charge-discharge cycle of the NEE, to implement practically any power regulation law. In this case, the power on the AC voltage buses will depend on the ratio between the voltage on the AE and the back-EMF of the converter, determined by the value of the control angles. However, this control method has several limitations. Since the power of the IEE conversion device can reach several hundred megawatts, the bridge arms must be assembled from series-parallel connected valves. In order to limit overvoltages, active-capacitive damping circuits must be connected in parallel to them. With deep regulation of converters, reverse voltage surges appear on the shoulders of the bridge and its individual valves. The parameters of the damping chains necessary for their limitation become unacceptable due to the power losses in them. This problem remains with other protective devices (eg avalanche diodes). The use of thyristors in high-power converter installations further increases the number of valves in the bridge arms and imposes more stringent requirements on their protection devices. 
Figure: 2.9. Switching circuit of UU converters 
Figure: 2.10. External characteristic of the converter
On the other hand, with deep symmetric regulation due to the phase shift of the current relative to the voltage on the station buses, the reactive component of the power prevails.
To compensate for it, an unacceptably high power of compensating devices is required (in the limit equal to the power of the station). These circumstances make it difficult to regulate within a wide range of control angles. Their values \u200b\u200bcan be increased due to the use of alternate control of converters, in which one part of the bridges operates in rectifier mode, and the other in inverter mode. With such an asymmetric control law, it is possible to expand the regulation limit of the output voltage of the converter with an acceptable power factor of the station. However, it is apparently impossible to completely assign the function of control of the IEE to the regulation of the angles of switching on the valves. It is advisable to combine it with other methods of ensuring the independence of the power on the IEE buses from the voltage on the AE.
In fig. 2.9 shows a diagram of the UU NEE (for the case when the station's converter device consists of two bridges), which makes it possible to change the back-EMF of the converter (depending on the voltage on the AE) by switching the bridges from parallel to serial connection when the NEE is charged and, vice versa, when his discharge. It is applicable for any number of converter bridges in the station. The anode of each bridge must be connected through switching devices with the anode and cathode of the previous bridge along the current flow and the anode of the next one, and the cathode with the anode and cathode of the next bridge along the current flow and the cathode of the previous one.
Let us consider the operation of the IEE in the inversion mode, since it is in this mode that it is important to ensure the independence of the power on the storage buses from the voltage on the AE.
Let us consider the external characteristic of the converter for the case when the value of the active power on the ac voltage buses is kept close to constant. At the initial moment (at the maximum AE voltage), the converter operates with series-connected bridges. Maintaining a given discharge current is ensured by adjusting the inverter control angles (points 1-2 in Fig. 2.10). At the moment of decreasing the voltage on the AE to a value at which it is possible to maintain this current value due to the operation of one bridge (point 2), the bridges are switched from a serial connection to a parallel connection, which corresponds to the transition from point 2 of the external characteristics of the converters to point 3. In this case the currents flowing through the converter bridges, and, therefore, the current and power of the station on the alternating voltage buses do not change, since the primary windings of the transformers are connected in parallel. The position of point 4 is determined by the percentage of AE underutilization.
The total number of bridges of the station should be determined by the permissible limit of control of the valve control angles and the specified AE utilization factor. The circuit (see Fig. 2.9) is built in such a way that in the inversion mode, when switching, the stations are not disconnected from the EPS and the switching devices do not cut off the operating direct current. Therefore, their manufacture will not cause additional difficulties. Short-term overloads of bridges during switching do not exceed the permissible ones for DC / DC converters.
The described scheme, in combination with the regulation of the valve control angles, allows maintaining the required active power supplied by the station, up to the complete discharge of the AE without interrupting the power supply. With its help, it is possible to ensure the independence of the consumed active power from the voltage on the AE and in the mode of its charging (when the bridges operate in the rectifier mode), but with the station disconnected from the EES for the time of re-switching.
Another way to regulate the power of the NEE is to connect the AE to the station converter in parts. For this, the AE must be divided into sections, each of which is connected independently of each other to the DC voltage buses of the converter device. At the same time, the power of the station fluctuates around a given average value; fully charged or discharged sections must be disconnected from the converter before the next connection. Sufficiently fine crushing of the AE into sections in combination with the regulation of the control angles of the transducer will reduce the unevenness of the change in the active power of the AE during the operation cycle to an acceptable level.
Other well-known methods of regulating the charge-discharge circuits of capacitor banks (using transformers with voltage regulation under load, switching battery capacitors from series to parallel and vice versa, connecting converters to the AC network through inductive-capacitive static converters, using compensated converters as converting devices with artificial switching of the valve current, etc.) require special consideration.
Thus, an IEE with a control device based on a 12-pulse converter when using the methods discussed above will meet all the requirements for sources of peak power in an EPS.
Let us now turn to consideration of possible types of storage devices for NEE.
Electrochemical energy storage. Electrochemical energy storage or electrochemical storage battery is one of the most common types of storage.
An electrochemical storage battery (EAB) consists of many cells connected in series and in parallel. Its charge occurs during off-peak hours, and its discharge occurs during peak load hours. During the charging process, electricity is electrochemically converted into chemical energy. In a discharge, the stored energy is released in a reverse reaction. A lot of work has been done to improve the EAB. It turned out that lead-acid batteries can also be used in EES. However, the cost of such items is high. New types of batteries are based on the use of chemical reactions of materials such as zinc, sulfur, sodium, etc., which are available in sufficient quantities and are relatively cheap. Tests of zinc chloride batteries operating at low temperatures are showing encouraging results. Among the batteries that require higher temperatures for operation, we can mention sodium-sulfur and lithium-sulfur. Laboratory tests of sodium-sulfur EAB are especially successful.
Characteristics of promising types of batteries for balancing load peaks are given in table. 2.3.
Electrochemical storage batteries have an efficiency reaching 65-70%. It is expected that promising batteries will have a service life of about 20 years with a specific investment in the installation of about $ 150 / kW and a specific energy consumption of 250 kWh / m3.
The disadvantages of EAB are a limited number of charge-discharge cycles (no more than 500), short energy storage time and negative environmental impact.
Table 2.3
Material used as cathode, anode |
Electrolyte |
Temperature, ° С |
Possible |
Possible |
Lead oxide |
||||
Zinc - chlorine |
Water solution |
|||
Sodium - sulfur |
||||
|
Lithium - sulfur |
Energy efficiency concepts are becoming more and more relevant against the background of technological development. This is due to the fact that energy efficiency as such has moved from the category of additional and often exclusive properties to the rank of one of the key consumer characteristics of the product. Suffice it to recall the simplest batteries that are used in digital technology, electrical equipment, power tool equipment, etc. There are also larger areas of application of energy storage systems, for which energy efficiency is especially important. And this request resonates with specialized manufacturers who produce energy storage devices with improved performance.
General information about energy storage
There are many permanent and inexhaustible sources of energy in nature, which is used to serve the various needs of mankind. But for its final use, it must go through many stages of processing and accumulation. This function is performed by power plants and substations. Their immediate tasks include the generation of energy with acceptable characteristics for use, as well as its transformation and distribution. The main energy supply infrastructure for residential buildings, industrial facilities, engineering equipment and other responsible consumers is implemented through stationary power grids. They are constantly supplied, but today the demand for autonomous equipment, devices and electrical appliances is growing steadily. Especially for such consumers, a capacitive energy storage is used, which is independent but conditionally - at certain intervals it must also be charged from the same stationary networks. The simplest example of such a storage device is a telephone battery. For example, a Li-Ion cell can have a capacity of about 2000-3000 mAh. It will be enough for several hours or days of autonomous operation of the serviced device, depending on its model. But after exhausting this volume, the battery must be connected to a 220 V outlet for recovery.
Mechanical drives
This category of drives has the longest history of existence. To illustrate such devices, one can cite gravitational systems as an example. Today they are almost never used, but overhead doors with counterweights were widespread in the past. They use the energy of the load, which is accumulated and returned at the right time in one form or another - it depends on the design of the drive. In addition to ordinary goods, liquid also acts as an active storage element. The advantages of such systems include design flexibility. Engineers could use an extensive network of pipelines, passing through which the water gave energy to the associated tanks. In our time, such energy storage devices are presented in the form of pumped storage stations. True, liquid storage devices are characterized by a short storage time, since the water evaporates and requires regular renewal.
Kinetic storage
This group is mainly represented by oscillatory mechanisms in which the accumulation process is realized through reciprocating, rotary or linear movements of the same weight. A feature of such structures is that, if necessary, the return of energy will also be carried out not continuously, but in portions - in steps. A classic example of a kinetic storage device is a mechanical watch. In this case, the "charge" is produced by winding the mechanism, followed by a gradual release of energy from the spring pendulum. A more modern interpretation of the kinetic mechanisms is the gyroscopic accumulator. The energy store in this case is based on a rotating flywheel with an impact function. Such systems are used in hydraulic and pneumatic engineering.
Heat storage
From a technological point of view, this is the simplest example of energy accumulation, with the processes of which a person is found everywhere. A metal fence heated in direct sunlight already becomes a heat accumulator, as it retains it in its structure. Also, other materials can act as heat accumulators. The efficiency of their work in this capacity will depend on the specific and volumetric heat capacity. For example, the heat capacity of water is 4.2 kJ, while for steel it is small - only 0.46 kJ. And yet, when it comes to purposeful accumulation, metal or oil storage devices are more often used. This decision is justified by the desire to optimize the design. Modern convectors and radiators are predominantly made of steel and aluminum. Again, some models are filled with materials that are more beneficial in terms of retention of thermal energy.
Electrical energy storage
The most massive form of energy is electricity. Therefore, this category is developing most actively, offering more and more perfect solutions. At the moment, the most common accumulator of electricity is a radio-technical capacitor. It is characterized by a high rate of return and energy storage, without limiting work processes by the environment. For example, most models can be used in hot or cold environments. Again, for optimization purposes, the electrical energy storage units are filled with special electrolytic cells with a high specific capacity.
Chemical storage
During the operation of such storage devices, a chemical reaction occurs. The source of energy in this case will be the very organization of conditions for this reaction and ensuring the activity of the components involved. Moreover, the output can generate energy of different types. For example, hydrogen can be released from water during direct electrolysis. Most often, with such storage methods, it is the fuel that is released. It can be transformed within the complex for providing a chemical reaction or transferred to the consumer in its original form. Therefore, energy storage devices can also act as converters, although such an expansion of functions technically complicates the system.
Electrochemical storage
This type of drives, as the name implies, is combined or hybrid. Since chemical reactions are highly efficient and cheap, they were logically combined with the task of generating the most popular type of energy - electricity. The active element in such devices is the electrolyte. In particular, the energy storage for the phone is usually made on the basis of lithium-ion or lithium-polymer cells. The same applies to battery packs for power tools. According to their characteristics, these are quite profitable batteries, distinguished by decent performance, high capacity and small size. But electrochemical batteries have a limited number of charge-discharge cycles, which is their main disadvantage.
Modern solutions
Leading high-tech companies are also promoting the capacitive battery business. For example, Tesla engineers have created a Powerwall 2 unit weighing 122 kg, based on the same lithium-ion batteries. This unit is modular and can store about 13.5 kWh. LG offers similar developments. For example, the Chem RESU system has a capacity of about 10 kWh, but in other performance characteristics it is not inferior to the Tesla unit. This battery is a universal energy storage device that can be used both in everyday life and in industry in production. The main thing is that the power meets the requirements for the consuming systems.
Conclusion
In the segment of energy storage units, different directions of technological development also stand out. They are united by only one thing - meeting the requirements of end users. For example, electric energy storage devices for small-sized apparatus and equipment must meet the requirements of reliability and reliability. The wide market for digital technology is more likely focused on compact sizes of drives and increasing their capacity. It is obvious that it is not easy to combine all the listed qualities in one device, therefore the developers still strive to initially orient their products to specific areas of application.
For the period of a network outage or its breakdown, electric energy storage devices are widely used for the home. They are mainly installed in private houses and are constantly connected. This allows for quite a long time to receive electricity sufficient for lighting and other urgent household needs.
As a rule, these devices are used to generate electricity in unconventional ways. In such cases, interruptions in its supply occur, and the storage devices successfully compensate for the temporary lack of energy. At their core, these are batteries that can be charged and discharged.
Storage device
However, energy storage devices perform broader functions than a conventional battery. They are complex, integrated structures capable of not only storing energy, but also making it fit for further use.
These devices occupy one of the leading positions in the market for alternative energy devices. They are based on lithium batteries. They consist of a charger or charge controller, a voltage converter () and a control system. The storage design allows replacing a large number of equipment for emergency systems and in alternative power supply. Most of the models are designed to work not only from a stationary network, but also from solar panels. Their average power output is 5 kilowatts. For normal operation, the device simply needs to be connected to the network.
Application of energy storage devices
Most often, electric energy storage devices for the home are used in individual households. First of all, they serve as the main power sources in emergency situations and centralized power outages. With the help of these devices, it is possible to add power for an individual energy economy during the period of load sagging during peak hours in general distribution networks. Very often, an electric energy storage device installed at home or in the country can significantly improve the quality of energy supply.
Nowadays, many consumers use expensive household appliances and appliances. Voltage surges often cause breakdown and failure. A lot of problems can be avoided by using drives. A stable voltage is created to ensure the stable operation of electrical appliances. There is no need for short-term switching on of the generator. It becomes possible to use tariffs with different rates.
The chain of the technological cycle of electricity production necessarily includes such a link as a storage (battery). In traditional methods of generating electricity, energy reserves are accumulated in a preliminary, "non-electrical" form, and this link - an energy storage unit, is located directly in front of the electric generator.
The reservoir of the hydroelectric power plant is designed to accumulate the potential energy of river water in the gravitational field of the Earth, raising it to a certain height with the help of a dam. The thermal power plant accumulates in its storage facilities the reserves of solid or liquid fuel necessary for uninterrupted operation, or supplies natural gas through the pipeline, the calorific value of which guarantees the required energy supply. The cores of reactors of nuclear power plants represent a stock of nuclear fuel with a certain resource of nuclear energy available for use.
Constant power mode is available for all listed types of power generators. The amount of energy produced is regulated within wide limits, depending on the level of daily energy consumption. Alternative sources (wind, tidal, geothermal, solar energy) cannot provide the guaranteed constant power of the generator at the level required at the moment. The storage, therefore, is here not so much a storage of resources as a damping device that makes energy consumption less dependent on fluctuations in the power of the source. The energy of the source is accumulated in the storage device, and later consumed, as needed, in the form of electrical energy. Moreover, its price largely depends on the cost of the drive.
A characteristic feature of the storage device in alternative energy sources is also the fact that the energy accumulated in it can be spent for other purposes. Thus, for example, they can be used to generate strong and super-strong magnetic fields.
Units of energy and the ratio between them adopted in physics and power engineering: 1 kWh, or 1000 W 3600 s - the same as 3.6 MJ. Accordingly, 1 MJ is equivalent to 1 / 3.6 kWh, or 0.278 kWh
Some common energy storage devices:
Let's make a reservation right away: the given review is not a complete classification of energy storage devices used in the power industry; in addition to those considered here, there are thermal, spring, induction, various other types of energy storage devices.
1. Storage capacitor type
The energy stored by a 1 F capacitor at a voltage of 220 V is: E \u003d CU2 / 2 \u003d 1 2202/2 kJ \u003d 24 200 J \u003d 0.0242 MJ ~ 6.73 W h. The mass of one such electrolytic capacitor can reach 120 kg. The specific energy per unit mass turns out to be just over 0.2 kJ / kg. Hourly drive operation is possible at a load within 7 W. Electrolytic capacitors can last up to 20 years. Ionistors (supercapacitors) have a high energy and power density (about 13 Wh / l \u003d 46.8 kJ / l and up to 6 kW / l, respectively), with a resource of about 1 million recharge cycles. The indisputable advantage of the capacitor storage is the ability to use the stored energy in a short period of time.
2. Accumulators of the gravitational type
Impact-type energy accumulators store energy when lifting a pile driver with a mass of 2 tons or more to a height of about 4 m. The movement of the moving part of the pile driver releases the potential energy of the body, imparting it to an electric generator. The amount of energy produced E \u003d mgh in the ideal case (without taking into account friction losses) will be ~ 2000 10 4 kJ \u003d 80 kJ ~ 22.24 Wh. The specific energy per unit mass of a copra woman turns out to be 0.04 kJ / kg. Within an hour, the drive is capable of providing loads of up to 22 watts. The mechanical structure has an expected life of over 20 years. The energy accumulated by the body in the gravitational field can also be spent in a short period of time, which is the advantage of this option.
The hydraulic accumulator uses the energy of water (weighing about 8-10 tons) pumped from the well into the tank of the water tower. In reverse motion, under the action of gravity, the water rotates the turbine of the electric generator. A conventional vacuum pump without any problems allows you to pump water to a height of 10 m. The stored energy E \u003d mgh ~ 10000 8 10 J \u003d 0.8 MJ \u003d 0.223 kWh. The specific energy per unit mass turns out to be 0.08 kJ / kg. The drive's load for an hour is in the 225W range. The drive can last 20 years or more. The wind engine can directly drive the pump (without converting energy into electrical energy, which is associated with certain losses), the water in the tower's tank, if necessary, can be used for other needs.
3. Accumulator based on a flywheel
The kinetic energy of a rotating flywheel is determined as follows: E \u003d J w2 / 2, J means the intrinsic moment of inertia of a metal cylinder (since it rotates around the axis of symmetry), w is the angular velocity of rotation.
With radius R and height H, the cylinder has a moment of inertia:
J \u003d M R ^ 2/2 \u003d pi * p R ^ 4 H / 2
where p is the density of the metal - the material of the cylinder, the product pi * R ^ 2 H is its volume.
The maximum possible linear velocity of points on the surface of the cylinder Vmax (is about 200 m / s for a steel flywheel).
Vmax \u003d wmax * R, whence wmax \u003d Vmax / R
Maximum possible rotational energy Emax \u003d J wmax ^ 2/2 \u003d 0.25 pi * p R2 ^ 2 H V2max \u003d 0.25 M Vmax ^ 2
The energy per unit mass is: Emax / M \u003d 0.25 Vmax ^ 2
The specific energy in the case of a cylindrical flywheel made of steel will be about 10 kJ / kg. A flywheel with a mass of 200 kg (with linear dimensions H \u003d 0.2 m, R \u003d 0.2 m) stores energy Emax \u003d 0.25 pi 8000 0.22 0.2 2002 ~ 2 MJ ~ 0.556 kWh. The maximum load provided by the flywheel storage during an hour does not exceed 560 W ... A flywheel may well last 20 years or more. Advantages: rapid release of stored energy, the ability to significantly improve performance by selecting the material and changing the geometric characteristics of the flywheel.
4. Storage in the form of a chemical storage battery (lead-acid)
A classic rechargeable battery, having a capacity of 190 Ah at an output voltage of 12 V and 50% discharge, is capable of delivering a current of about 10 A for 9 hours. The energy released is 10 A 12 V 9 h \u003d 1.08 kWh, or approximately 3.9 MJ per cycle. Taking the mass of the battery equal to 65 kg, we have a specific energy of 60 kJ / kg. The maximum load that the battery can provide for an hour does not exceed 1080 watts. The warranty period for a high-quality storage battery is within 3 - 5 years, depending on the intensity of use. It is possible to directly obtain electric power from the battery with an output current of up to thousands of amperes at an output voltage of 12 V, which corresponds to the automotive standard. The battery is compatible with many devices designed for a constant voltage of 12 V, converters 12/220 V of various output powers are available.
5. Accumulator of pneumatic type
Air pumped into a steel tank with a volume of 1 cubic meter up to a pressure of 40 atmospheres performs work under conditions of isothermal expansion. Work A performed by an ideal gas under conditions T \u003d const is determined according to the formula:
A \u003d (M / mu) R T ln (V2 / V1)
Here M is the mass of the gas, mu is the mass of 1 mole of the same gas, R \u003d 8.31 J / (mol K), T is the temperature calculated on the absolute Kelvin scale, V1 and V2 are the initial and final volume occupied by the gas (at this V2 / V1 \u003d 40 when expanding to atmospheric pressure inside the tank). For isothermal expansion, the Boyle-Mariotte law is valid: P1V1 \u003d P2 V2. Let us take T \u003d 298 0K (250С) For air M / mu ~ 40: 0.0224 \u003d 1785.6 moles of the substance, the gas does work A \u003d 1785.6 8.31 298 ln 50 ~ 16 MJ ~ 4.45 kWh per cycle. The walls of the tank, designed for a pressure of 40-50 atmospheres, must have a thickness of at least 5 mm, in connection with which the mass of the drive will be about 250 kg. The specific energy stored by this pneumatic accumulator will be equal to 64 kJ / kg. The maximum power provided by the pneumatic accumulator during an hour of operation will be 4.5 kW. The guaranteed service life, like most drives, based on the performance of mechanical work by their structural parts, is from 20 years. The advantages of this type of storage: the ability to locate the tank underground; the reservoir can be a standard gas cylinder using the appropriate equipment, the wind turbine is able to directly transmit the movement to the compressor pump. In addition, many devices directly use the stored energy of the compressed air in the reservoir.
Here are the parameters of the considered types of energy storage devices in the summary table:
|
A type energy storage |
Estimated performance |
The value of the stored |
Specific energy (per unit mass of the device), kJ / kg |
Maximum load when the drive is operating for an hour, W |
Life expectancy, |
|
Condenser type |
battery capacity 1 F, |
24,2 |
within 20 |
||
|
Pile type |
weight of a copra woman 2000 kg, maximum |
0.04 |
not less than 20 |
||
|
Gravity hydraulic type |
liquid mass 8000 kg, height difference 10 m |
0.08 |
not less than 20 |
||
|
Flywheel |
cylindrical steel flywheel mass |
2000 |
not less than 20 |
||
|
Lead-acid battery |
battery capacity 190 Ah, |
3900 |
1080 |
minimum 3 maximum 5 |
|
|
Pneumatic type |
steel tank capacity tank weight 2,5ts compressed air pressure 40 |
16000 |
4500 |
not less than 20 |
Mechanical storage (MH), or a mechanical energy accumulator, is a device for storing and storing kinetic or potential energy with its subsequent release to perform useful work.
As for any type of energy storage device (EE), the characteristic modes of operation of the MN are charge (accumulation) and discharge (energy return). Storage energy serves as an intermediate MN mode. In the charging mode, mechanical energy is supplied to the MN from an external source, and the specific technical implementation of the energy source is determined by the type of MN. When the MN is discharged, the main part of the energy stored by it is transferred to the consumer. Some part of the accumulated energy is spent on compensating for losses that occur in the discharge mode, and in most types of MN - in storage modes.
Since in a number of storage installations the charge time D3 can be much longer than the discharge time (r3 "g), a significant excess of the average discharge rate is possible. RP over average power P3 charge MN. Thus, it is permissible to accumulate energy in the MP using relatively low-power sources.
The main types of MN are subdivided into static, dynamic and combined devices.
Static MN store potential energy through elastic changes in the shape or volume of the working fluid or when it moves against the direction of gravity in a gravitational field. Solid, liquid or gaseous working fluid of these MNs has a static state in the energy storage mode, and the charge and discharge of NEs are accompanied by the movement of the working fluid.
Dynamic MN accumulate kinetic energy mainly in rotating masses of solids. Conditionally, storage devices of accelerators of charged elementary particles, in which the kinetic energy of electrons or protons, cyclically moving along closed trajectories, is stored can also be attributed to dynamic MPs.
Combined MN store both kinetic and potential energy. An example of a combined MN is a super flywheel made of high-strength fibrous material with a relatively low modulus of elasticity. When a given MI rotates, the potential energy of elastic deformation is stored in it along with kinetic energy. When extracting the accumulated energy from such a MN, the use of both of its types is achieved.
In terms of the level of specific accumulated energy per unit mass or volume of the storage element, dynamic inertial MNs are significantly superior to some other types of NEs (for example, inductive and capacitive storage). Therefore, MN are of great practical interest for a variety of applications in various branches of technology and scientific research.
Certain types of MP have found by now large-scale application in the electric power industry, for example, guide - Roof storage installations of power plants. Charging - The discharge cycle of their work reaches tens of hours.
For inertial MPs, short-term discharge modes are characteristic. The extraction of energy from the MP is accompanied by a decrease in the angular velocity of the flywheel to the permissible level. In some cases, braking can occur up to a complete stop of the flywheel. Possible "shock" discharges, characterized by one-time or cyclical withdrawal of stored energy, and due to the large angular momentum and short discharge time of the MN, the decrease in the angular velocity of its rotor is relatively small, although the power supplied can reach sufficiently high values. In this MN mode, special requirements are imposed on ensuring the strength of the shaft. Under the influence of the torque, dangerous shear stresses arise in the shaft, h. the kinetic energy of the rotor is converted into the potential energy of elastic deformation of the torsion of the shaft. To overcome the above difficulties, elastic or frictional couplings are provided in individual MH designs.
Static MNs preserve the stored energy while in a stationary state. The carriers of potential energy in them are elastically deformed solids or compressed gases under excess pressure, as well as masses raised to a height relative to the earth's surface. Typical examples of static MN are: stretched or compressed springs, rubbers; gas accumulators and pneumatic accumulators; impact devices of various pile drivers, for example, for driving piles, using the energy of the masses in a raised state; reservoirs of pumped storage power plants, tanks of water-pressure installations. Here are the main energy ratios and characteristic parameters of some typical devices.
Consider an MN with elastic elements.
We believe solid state the system is linear, then the elastic storage element has constant stiffness (or elasticity) N= Const. Force acting on him F\u003d Nx proportional to linear deformation x. Elementary work perfect when charged with MH dW\u003d Fdx. Total stored energy
W = J Fdx \u003d J Nxdx \u003d NAh2 / 2-FaAh / 2, Oo
WhereAh is the resulting deformation, limited, for example, Admissible tension ar material; Fn = NAh - the applied force.
Let's estimate the specific energy Wya \u003d Wj M, per unit mass M \u003d yV\u003d ySh spring or rod volume V and section S, whose material has a density y and works to break within the limits of Hooke's law a \u003d xfE, moreover X* \u003d xfh- relative deformation, E-module of elasticity (Young), G ^ Gp. Introducing da \u003d Edx„ we can write DW\u003d Fhdx* \u003d Fhdo/ E and dWya \u003d dW/ ySh \u003d Fda/ ySE, whence at C \u003d F/ S find
Wya \u003d] (aljE) da \u003d a2J (2jE).ABOUT
For steel we will accept springs with „\u003d 8 108 N / m "E \u003d 2 , 1-1011 N / m2, y \u003d 7800 kg / m3, then Wya ^200 J/ kg. AnaA logical calculation for technical rubber gives ^ beats ^ 350 J / kg, however, due to the hysteresis nature of the dependence F= F(X) In the charge-discharge cycle, the resulting losses and heating lead to TO gradual aging (destruction) of rubber, instability and deterioration of its elastic properties.
Gas storage the system is in a mechanically non-equilibrium state with respect to the environment: when the temperatures of the system and the environment are equal (T \u003d T0C) system pressure p\u003e p0, c, therefore the system can do work. The reserve of elastic energy compressed in a cylinder volume V gas is
W \u003d P (vdp \u003d v (p2-pi) .. (4.1)
According to (4.1), per unit mass M of any compressed gas, there is a specific energy
Wya \u003d W / M \u003d V (p2-Pl) IM \u003d Aply. (4.2)
Based on (4.2) at K \u003d 1m3, the value W- WysM numerically equal to pressure drop Ap \u003d p1-p1. For example, if A /? \u003d 250 105 Pa (initial pressure p! \u003d 105 Pa), then IL \u003d 25-106 J, regardless of the chemical composition of the gas. The maximum value of Wya during expansion of compressed gas to zero pressure at a given temperature according to the Mendeleev - Clapeyron equation PV- MvRyT is
Wya\u003d WlM \u003d RyTI ", (4.3)
Where c \u003d M / Mts - molar mass (kg / kmol); Ry & ~ 8.314 kJ / (kmol K) - universal gas constant at Тх273 К; /? "105Pa; Mm is the number of kilomoles in a gas of mass M.
It can be seen from (4.3) that the use of light gases in ML is most effective. For the lightest gas, hydrogen (μ \u003d 2 kg / kmol) at T \u003d 300 K, the specific energy is ~ 1250 kJ / kg (or 1250 J / g). In (4.3), the pressure is not explicitly included, since Wya is determined by (4.2) by the ratio of the excess gas pressure to its density. The latter with increasing pressure and Г \u003d const increases linearly (in the isothermal process PV= Const). It should be noted that the high pressures that are reasonable for the effective application of the MN under consideration cause, for strength reasons, a significant mass of gas cylinders, taking into account which the value of Wya of the installation as a whole can be reduced by almost an order of magnitude compared to fVya from (4.2), (4.3). The strength assessment of the cylinders can be carried out using the Design Relationships § 4.5.7.
Consider gravitational energy storage devices.
The gravistatic gravitational energy of the Earth's gravity (at the level of Orya) is estimated by a rather high indicator "beats \u003d 61.6 MJ / kg, which characterizes the work necessary for the uniform movement of a body with a mass of Mx \u003d Kg from the earth's surface into outer space (for comparison, we will indicate that this value PVya is approximately twice the chemical energy of 1 kg of kerosene). When lifting a load with a mass M to the height h \u003d x2 - xl stored potential energy
W \u003d jgMdx \u003d gMh , (4.4)
Where M \u003d const, g \u003d 9.8l m / s2. According to (4.4), the specific energy Wya\u003d Wj M\u003d gh depends only on the height h. The stored energy is released when the load falls and the corresponding useful work is performed as a result of the transformation of potential energy into kinetic one. The highest specific kinetic energy in nature when falling can be developed by meteorites, for which Wya ^ 60 MJ / kg (excluding energy consumption for friction in the atmosphere).
The direct use of gravistatic forces generated by natural masses is practically impossible. However, by pumping water into raised artificial reservoirs or from underground reservoirs to the surface, a sufficiently large amount of potential energy can be accumulated for large-scale applications in electric power systems. If the level difference h \u003d 200 m, then, based on the mass of water M \u003d 103 kg, the stored energy according to (4.4) is equal to I\u003e "\u003d 1962 kJ, specific energy Wya\u003d WjM= 1.962 kJ / kg.
Consider inertial kinetic MN.
In principle, kinetic energy can be stored for any movement of the mass. For uniform translational motion of a body with a mass M with speed v kinetic energy W\u003d Mv2 / 2. Specific energy Wya\u003d W/ M \u003d v2 j2 depends (quadratically) only on the linear velocity of the body. A body moving with the first cosmic speed km / s has a specific
Energy Wyax32 MJ / kg.
For a variety of energy and transport applications, rotational MNs are rational - inertial MNs (flywheels). The stored kinetic energy W \u003d J & / ~ is determined by the square of the angular velocity Q \u003d 2nn (P - speed) and moment of inertia J flywheel relative to the axis of rotation. If the flywheel has a radius r and mass M = yV (V-volume, at - material density), t °
J ^ Mr2 / 2 \u003d yVr2j2 and W \u003d n2Mr2n2 \u003d n2yVr2n2. The corresponding specific energy (per unit M or V) is FV/ M\u003d n* r2n2 , J / kg and lV0ya\u003d W/ V\u003d n2yr2n2 , J / m3. The values \u200b\u200bof Q and n for a given size r are limited by the linear peripheral speed v \u003d Q.r \u003d 2mr, associated with the permissible breaking stress of the material ar. It is known that the voltage a in a disk or cylindrical rotor MH depends on v2. Depending on the geometric shape of the metal flywheels, they are characterized by permissible limit speeds at the periphery of approximately 200 to 500 m / s.
Stored energy, in particular for a thin rim flywheel, W\u003d Mv /2 (Mis the mass of the rotating ring). Specific energy Wya\u003d W/ M \u003d v2 /2 does not depend on the size of the ring and is determined by the ratio of the parameters Op / y of its material (see Sec. 4.5.1, where it is shown that v2 \u003d opjY). It should be noted that a similar pattern for Wya ~ avjу also takes place in inductive energy storage (see Ch. 2), although they differ significantly from MN in physical nature. In the general case, in the manufacture of MN storage elements, it is necessary to use materials with increased Gp / y\u003e 105 J / kg. The most suitable materials are high-strength alloy steels, titanium alloys, as well as light aluminum alloys (duralumin type) and magnesium alloys (electron type). Using metallic materials, it is possible to obtain the specific energy MN up to Wm \u003d 200-300 kJ / kg.
Designed to create flywheels with especially high specific energies (super flywheels), fine-fiber materials can theoretically provide the following levels of the Wya index: glass filaments - 650 kJ / kg, quartz filaments - 5000 kJ / kg, carbon fibers (with a diamond structure) -15000 kJ / kg ... The filaments (or tapes made from them) and adhesive resins form a composite structure, the strength of which is lower than that of the original fibers. Taking into account the fastening elements in real super - flywheels, the values \u200b\u200bof Zhud are practically achieved less than the indicated ones, but still relatively higher than in other Variety of MN. Super flywheels allow peripheral speeds up to v "1000 m / s. The technical implementation of such devices requires special conditions. For example, It is necessary to install a flywheel in an evacuated housing, since the indicated values v correspond to supersonic velocities in air (Mach number Ma\u003e 1), which in the general case can cause a number of unacceptable effects: the appearance of compaction shocks in air and shock waves, a sharp increase in aerodynamic drag and temperature.
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AND-mass on a rigid string; b- elastic rim |
Multilayer fiber super flywheels have a fairly high reliability and are safer in operation than solid flywheels. Under unacceptable loads caused by inertial forces, only the most stressed outer layers of the fiber composite structure of the super flywheel are destroyed, while the destruction of the massive flywheel is accompanied by the scattering of its torn parts.
The combination of the properties of static and dynamic MN takes place in various devices. The simplest of these is the oscillating pendulum. The cyclic process of mutual transformation of potential energy into kinetic energy can be maintained for a sufficiently long time if the losses in the pendulum mechanism are compensated.
Let us consider illustrative examples of MNs that store kinetic and potential energy at the same time when charged. They demonstrate the fundamental possibilities of joint practical use of both types of accumulated mechanical energy. In fig. 4.1, and weight shown M, revolving around the center ABOUT on an absolutely rigid string of length /, deflected from the vertical position by an angle cf. Linear Velocity v corresponds to the rotational motion M along a circle of radius g. Potential energy of the load Wn\u003d gMh due to its rise to a height h as a result of rejection. The kinetic energy of the load is 1FK \u003d 0.5 Mv2 . The load is acted upon by the force F \u003d F „+ Fr. Its inertial component is FK \u003d Mv lr\u003e the value of the gravitational component F T \u003d gM. Since F „/ Fr \u003d r2 / rg \u003d tan (D, insofar as Wn/ Wk \u003d 2h/ rtg^>. If Uchest ^! that A \u003d / (l - coscp) and r \u003d / sincp, then / y / r \u003d (1 - coscp) / sinср. In this way, W„L lFK \u003d 2coscp / (l + cos (p), and in the case cp-\u003e 0 we obtain Wn / WK-\u003e 1. Consequently, at small angles cp, the stored energy fV \u003d JVK + Wn can be distributed at equal frequencies (W The value of Wn can be increased , if you fix the load on an elastic suspension (bar or string).
Another example of joint accumulation W„ and Wk a rotating fine-rim flywheel serves as (Figure 4.1, b), which had elasticity (stiffness) N. The tension in the rim ^ p \u003d NAI is proportional to the elastic elongation A / \u003d 2n (r - r0) caused by inertial forces AFr \u003d AMv2 / r, distributed Nymi along the rim circumference with radius r. Equilibrium of a rim element with a mass of 2DM \u003d 2 (A // 2l;) A (p is determined by the relation 2A / v \u003d 2A / 7 (() sinAcp ^ Ai ^ Acp, whence 0.5 Mv2 \u003d 2K2 (r - r0 ) N. Therefore, the kinetic energy of the rim lVK \u003d 2n2 (r - r0 ) N. Since the stored potential energy)