CN110766193A - Electric heating combined system scheduling method considering heat transfer characteristics of heat exchanger - Google Patents

Abstract

The invention relates to a scheduling method for an electric-heating combined system considering the heat transfer characteristics of a heat exchanger. Electric boiler operation model; 2) According to the three-stage heat transfer process of extraction steam inside the heat exchanger, obtain an accurate model of heat transfer inside the heat exchanger; 3) Build a combined heat and power system scheduling model, considering the constraints of electricity and heat network Complete the scheduling optimization of the combined electric heating system. Compared with the prior art, the present invention has the advantages of considering the extraction steam heat transfer process, the model is more accurate, and the scheduling result is more practical.

Description

A scheduling method for combined electric and heat systems considering heat transfer characteristics of heat exchangers

technical field

The invention relates to the field of optimal scheduling of an electric-heating combined system, in particular to a scheduling method for an electric-heating combined system that considers the heat transfer characteristics of a heat exchanger.

Background technique

The proportion of thermal power units in the Three North Regions is relatively high, and wind power resources are abundant. During the heating period in winter, the thermal power unit has a large output, and the cogeneration operation mode of "setting electricity by heat" greatly reduces the adjustment capacity of the heating unit, limits the flexibility of the power system, and leads to the curtailment of the power system. The phenomenon is more serious.

In the modeling of traditional thermoelectric systems, the relationship between heat output and steam extraction is modeled as a linear relationship without considering the heat transfer process. One reason for this simplification may be that in the analysis of power systems, power balance is generally concerned, and simplification makes it easier to solve system problems. However, only the steam extraction volume of the cogeneration unit is directly controlled, not the heat output. The heat transfer process of the heat exchanger extraction steam ultimately determines the actual heat output of the cogeneration unit, so the traditional heat transfer model does not Consider the heat transfer process of extraction steam and the complex relationship between steam flow and heat output. In order to improve the energy utilization efficiency of the combined heat and power system, it is necessary to build an accurate scheduling model considering the heat transfer characteristics of the heat exchanger, analyze the influence of the heat exchanger characteristics on the scheduling, and provide ideas for the optimal scheduling of the combined heat and power system.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a scheduling method for a combined electric and heat system considering the heat transfer characteristics of the heat exchanger in order to overcome the above-mentioned defects of the prior art.

The object of the present invention can be realized through the following technical solutions:

A method for scheduling a combined electric-heating system considering the heat transfer characteristics of a heat exchanger, comprising the following steps:

1) According to the structure diagram of the cogeneration device including the electric boiler and the heat exchanger, build the heat transfer model inside the heat exchanger and the operation model of the electric boiler;

2) Obtain an accurate model of heat transfer inside the heat exchanger according to the three-stage heat transfer process of the extraction steam inside the heat exchanger;

3) Construct the dispatching model of the combined heat and power system, and complete the scheduling optimization of the combined heat and power system considering the constraints of the electricity and heat network.

In the described step 1), the heat transfer model inside the heat exchanger is:

Among them, Q is the heat transferred by the heat exchanger, A ^hs and k ^hs are the heat exchange area and total heat transfer coefficient of the heat exchanger, T ^stm and T ^cds are the inlet temperature and outlet temperature of the steam, respectively, T ^hs,in , T ^hs,out are the inlet and outlet temperatures of the water, respectively.

In the described step 2), the tertiary heat transfer process of steam extraction in the heat exchanger includes a gas sub-process, a gas-liquid sub-process and a liquid sub-process, specifically:

1) Gas sub-process: the steam temperature drops from T ^stm to the phase transition temperature T ^phs , and the heat released in this process is Q ^g ;

2) Gas-liquid sub-process: all the latent heat of the steam is released into the water, during this process, the steam maintains the phase transition temperature T ^phs , and the heat released in this process is Q ^gl ;

3) Liquid subprocess: the vapor is in the liquid phase and releases heat from the phase transition temperature T ^phs to T ^cds , and the released heat is Q ^l .

The exact model of the heat transfer inside the heat exchanger is specifically:

A ^hs =A ^g +A ^gl +A ^l

Q=Q ^g +Q ^gl +Q ^l

Among them, A ^g , A ^gl , and Al are the equivalent heat transfer areas corresponding to the gas sub-process, the gas-liquid sub-process and the liquid sub-process, respectively, T ^{hs,1 and} T ^hs,2 are the intermediate temperatures, k ^g , k ^gl and k ^l are the heat transfer coefficients corresponding to the gas sub-process, the gas-liquid sub-process and the liquid sub-process, respectively.

In the described step 1), the electric boiler operation model is specifically:

Pb=Q ^b /(3.6*δ)

Among them, P ^b is the electric power consumption of the electric boiler, Q ^b is the thermal power output by the peak-shaving electric boiler, and δ is the electric-heat conversion efficiency of the electric boiler.

In the step 3), the combined heat and power system scheduling model takes the minimum total coal consumption as the objective function, and the constraints include power supply balance constraints, heat supply balance constraints, pure condensing unit output constraints, wind turbine output constraints, unit climbing constraints and Grid flow constraints.

The objective function of the dispatching model of the combined heat and power system is:

为分别代表第i座火电机组和第j座单抽式热电联产机组的煤耗量，

为分别代表第i座火电机组和第j座热电联产机组t时刻的出力，

为第j座热电联产机组的t时刻的抽汽质量。Among them, T is the total number of time periods, S ^tp and S ^chp are the number of thermal power units and thermal power units,

is the coal consumption of the i-th thermal power unit and the j-th single-extraction cogeneration unit, respectively,

are the outputs of the i-th thermal power unit and the j-th cogeneration unit at time t, respectively,

is the extraction steam quality of the jth cogeneration unit at time t.

The extraction steam quality of the cogeneration unit is calculated by the dichotomy method.

Compared with the prior art, the present invention has the following advantages:

1. On the basis of the general three-stage heat transfer model, a cogeneration system considering the heat transfer process of extraction steam is established, and an iterative method for calculating the precise extraction steam mass flow required to meet the given heat output demand is proposed. The method can more accurately calculate the heat output of the thermal power unit, and provide a basis for coordinating the output of the unit. Compared with the traditional scheduling model that considers the heat exchanger, it does not carefully consider the complete process of the heat exchange and the internal heat transfer. The three-stage heat transfer proposed in the present invention The process can more truly reflect the heat transfer characteristics of the heat exchanger, the obtained scheduling model is more accurate, and the scheduling results are more realistic.

2. Decoupling the constraint of "determining electricity by heat", and at the same time making the optimal dispatching model of the combined electric and heating system more accurate, reflecting the real situation of dispatching, and providing a basis for saving coal consumption and consuming renewable energy.

Description of drawings

Figure 1 is a structural diagram of a heating cogeneration device.

Figure 2 shows the model of the three-stage heat transfer process of extraction steam.

Figure 3. Iterative method for extracting steam mass flow.

Figure 4 is a diagram of the combined electric heating system.

Figure 5 shows the effect of heat exchanger area on the heat transfer process.

Figure 6 shows the effect of water temperature at the outlet of the heat exchanger on the heat transfer process.

Figure 7 shows the output of thermal power units in three cases of heat exchangers.

Figure 8 shows the output of wind turbines in three cases of heat exchangers.

Figure 9 shows the effect of heat exchange area on the heat transfer process after installing an electric boiler.

Figure 10 shows the effect of the water temperature at the outlet of the installed electric boiler on the heat transfer process.

Detailed ways

The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

Example

In order to decouple the constraint of "determining electricity by heat" and at the same time make the optimal scheduling model of the combined electric and heat system more accurate, the present invention focuses on the thermoelectric system model considering the extraction steam heat transfer process, and on the basis of the general three-stage heat transfer model , establishes a cogeneration system considering the extraction steam heat transfer process, and proposes an iterative method for calculating the exact extraction steam mass flow required to meet a given heat output requirement. At the same time, an electric boiler is installed in the system, and a combined dispatch model of dual heat sources including thermal power, thermal power, wind power and electric boiler is established to study the influence of different heat transfer characteristics on the absorption of abandoned wind and operating costs of the combined heat and power system. The optimal scheduling results of the system in the case of heat sources.

The invention provides an optimal scheduling method for a combined electric-heating system taking into account the heat transfer characteristics of a heat exchanger, comprising the following steps:

1) Build a structure diagram of a cogeneration device including an electric boiler and a heat exchanger, and describe the heat transfer process inside the heat exchanger and the operation model of the electric boiler;

2) Based on the three-stage heat transfer process of the extraction steam in the heat exchanger, the precise relationship between the heat load and the extraction steam quality is constructed;

3) Constructing the dispatching model of the combined heat and power system, and considering the constraints of the electricity and heat network, the influence of the heat transfer characteristics of the heat exchanger on the dispatching of the heat and power system is obtained.

Specific steps are as follows:

1. First build a cogeneration system with electric boilers:

The cogeneration system includes extraction steam cogeneration units, heat exchangers, and electric boilers. The structure is shown in Figure 1. The cogeneration unit is connected to the heat exchanger. After the steam is extracted from the middle pressure cylinder section of the steam turbine, the steam flows through the heat exchanger to exchange heat with the cold water pipeline to meet the heat energy demand of the central heating users. In order to study the accurate model of the three-stage heat transfer of the heat exchanger, the present invention explains the heat exchange principle of the heat exchanger according to the structure of the heat exchanger, and at the same time installs an electric boiler to convert electric energy into heat energy to meet the heat load of some retail households.

2. Next, describe the structure of the general heat exchanger

The general structure of the heat exchanger is shown in the dotted box in Figure 1. The heat flow (extraction steam) flows through the heat exchanger and transfers heat to the cold fluid. T ^stm , T ^cds represent the inlet temperature of the hot fluid (steam), the outlet Temperature, also ^Ths,in and ^Ths,out represent the inlet temperature and outlet temperature of the cold fluid (water). m ^stm and m ^water are the mass flow of extraction steam and water in the heating pipe network, A ^hs and k ^hs are the heat exchange area and total heat transfer coefficient of the heat exchanger, Q represents the heat transferred through the heat exchanger, called The heat output, the heat transfer process of the heat exchanger can be described as:

The energy conservation equation of the hot and cold fluids in the above formula shows that in the process of heat transfer, the heat released by the heat flow and the heat transferred in the middle are equal to the heat absorbed by the cold flow, where c ^water and c ^stm are the specific heat capacities of the cold and hot fluids, respectively.

For a given cogeneration unit, the extraction steam temperature and pressure are the design parameters of the cogeneration steam turbine, so T ^stm is a fixed value. The heat exchange area A ^hs is also a design parameter of the heat exchanger, while the overall heat transfer coefficient k ^hs is related to the physical properties of the fluid. In general, the primary pipe network works in the quality regulation mode, and the mass flow m ^water of the primary pipe network remains unchanged for several months or even the entire heating season. The operator mainly controls the heat exchanger by adjusting the mass flow of the extraction steam. The outlet water temperature ^Ths,out meets the thermal demand, and ^Ths,out varies with the average temperature of the environment, so m ^water and ^Ths,out keep a fixed value for a short time.

3. Electric boiler device

The electric boiler is a device that converts electricity to heat, and plays an important role in the scheme of absorbing abandoned air. Decoupling the “heat-based electricity” constraint of cogeneration through electric boilers enables thermal power plants to reduce their forced output and make room for wind power grids. The electric boiler starts when there is abandoned wind, absorbs part of the wind power for heat supply, makes up for the insufficient heat supply of the thermal power unit due to the constraint of "fixing electricity with heat", and improves the flexibility of the thermal power unit. , in the short term, it can also reduce the coal consumption of the entire system. The model of the electric boiler is as follows

P ^b =Q ^b /(3.6*δ)

Among them, P ^b is the electric power consumption of the electric boiler; Q ^b is the thermal power output by the peak-shaving electric boiler; δ is the electric-heat conversion efficiency of the electric boiler, which is approximately equal to 1.

4. The relationship between heat output and steam extraction based on the three-stage heat transfer process:

Due to the existence of heat exchangers, there is a more complex relationship between the steam extraction volume and the thermal output of the thermal power plant, and the relationship between them changes with the different operating conditions of the heat exchanger. The previous studies on the simple linearization of their relationship are no longer A more detailed study is required to accurately describe the operation of cogeneration.

In order to obtain the relationship between the heat output Q and the extraction volume m ^stm , this example analyzes the internal process of heat transfer in the heat exchanger in detail. Generally, in the heat exchanger, the extraction steam will undergo a complete phase change process (from gas phase to liquid phase) . The three sub-processes are gas process, gas-liquid process, and liquid process, as shown in Figure 2.

First, the steam releases heat in the gas phase, the steam temperature drops from the extraction temperature T ^stm to the phase transition temperature T ^phs , and the heat released in the process is Q ^g . The steam is then in a gas-liquid state and releases all its latent heat into the water, during this process, the steam maintains the phase transition temperature T ^phs , and the heat released in this process is Q ^gl . In the last process, the steam is in the liquid phase and releases heat from T ^phs to the condensate temperature T ^cds with a heat release of Q ^l . where T ^hs,1 and T ^hs,2 are the assumed intermediate temperatures of the pipe-side water, and k ^g , k ^gl , and k ^l are the heat transfer coefficients of the three subprocesses.

For the three-stage heat transfer process of steam extraction, each heat transfer process is shown in Figure 2, and A ^g , A ^gl , and A ^l are used to represent the equivalent heat transfer area of each sub-process, which satisfies:

A ^hs =A ^g +A ^gl +A ^l

The total thermal power is equal to the sum of each heat transfer process:

Q=Q ^g +Q ^gl +Q ^l

First, according to the heat transfer theory, each sub-heat transfer process satisfies:

Q ^g =c ^stm m ^stm (T ^stm -T ^phs )

Q ^gl =rm ^stm

Q ^l =QQ ^g -Q ^gl

According to the principle of energy conservation, the intermediate temperatures T ^hs,1 , T ^hs,2 , the heat exchanger inlet temperature T ^hs,in and the condensed water temperature T ^cds can be calculated, then there are:

T ^hs,1 =T ^hs,out -Q ^g /(c ^water m ^water )

T ^hs,2 =T ^hs,out -(Q ^g +Q ^gl )/(c ^water m ^water )

T ^hs,in =T ^hs,out -Q/(cwa ^ter mwa ^ter )

T ^cds =T ^phs -Q ^l /(c ^water m ^stm )

In this way, the equivalent heat transfer area of each heat transfer process can be obtained:

It is known that m ^water , ^{Ths, out} are given, so for a heat load power, there is a given suction flow m ^stm , and the iterative process as shown in Figure 3 is obtained.

δ is the iteration threshold, m ^step is the increment of m ^stm , the superscript k is the number of iterations, m ^{stm, max} is the maximum extraction steam mass flow, according to the known extraction steam parameters and heat exchanger parameters, take the first iteration The extraction steam quality is the minimum extraction steam quality, calculate the heat exchange area of each heat transfer sub-stage, and judge whether the sum of the heat exchange areas of the three stages is greater than the total heat exchange area. The extraction steam quality of the iteration is the maximum value, and the extraction steam quality of the previous iteration is the minimum value, and then the accurate extraction steam quality within this range is calculated by the dichotomy method until the accuracy requirements are met.

3. Finally, carry out simulation verification

A simple combined electric-heating system diagram is used to verify the influence of the heat transfer process on the dispatching of the combined heating and power system. The model shown in Figure 4 is built in MATLAB. There are two thermal power units G1 and G2, one thermal power unit and one wind farm, plus A combined heat and power system with dual heat sources is constructed by installing electric boilers. The dispatch model established on this basis takes the minimum total operating coal consumption as the objective function, and comprehensively considers power supply balance constraints, heat supply balance constraints, pure condensing unit output constraints, wind turbine output constraints, Constraints such as unit ramp constraints, grid power flow constraints, etc.

Example 1: Influence of parameters of no heat exchanger on extraction steam quality

Heat exchange is a temperature differential driven process, so different outlet water temperatures of the heat exchangers are a factor considered in this example. Secondly, the nature of the heat exchanger itself, such as the heat exchange area of the heat exchanger, is another influencing factor. Therefore, in this example, the effects of the heat exchanger area and the water temperature at the outlet of the heat exchanger on the heat transfer process and the extraction steam mass flow are discussed. Finally, a simple combined heat and power dispatching model of wind power, thermal power unit and conventional thermal power unit is used for research.

With a fixed heat exchanger outlet temperature ^Ths,out = 90°C, Figure 5 shows the extraction steam mass flow required to meet a given thermal power requirement at different heat transfer areas. It can be seen from the figure that under different heat transfer areas, in order to meet the same thermal power demand, the required mass flow is completely different. In the range of 0 to 11h, the difference is more obvious, and the heat transfer area reflects the The inherent heat transfer capacity of the heat exchanger, therefore, when the heat exchange area is larger, the heat exchange is more sufficient, and the steam extraction required to meet the thermal power demand is smaller, A ^hs = 2971m ² can be used as the heat exchanger to meet the given heat Limits of heat transfer area for power requirements. It can be seen from the comparison that if the heat exchanger works near its limit, more steam extraction will be required to meet the heat energy demand. It can be seen that when the actual working state of the heat exchanger deviates significantly from the rated working condition, the heat transfer is not considered. The process may be incorrect.

With a fixed heat transfer area A ^hs = 3500m ² , the extraction steam mass flow required to meet the given thermal power requirements is shown in Figure 6 under different water temperatures at the outlet of the heat exchanger. It can be seen that the lower the outlet water temperature, the less extraction steam mass flow is required. This is because the heat transfer process is driven by the temperature difference between the hot and cold fluids. After a given extraction steam temperature T ^stm , the lower the outlet water temperature and the greater the temperature difference, the better the heat transfer effect. By adjusting the outlet temperature of the heat exchanger, the heat transfer process is affected, thereby affecting the required extraction steam volume.

Example 2: Influence of heat exchanger parameters on scheduling results

The effects of three different heat transfer processes on system operation were compared

Table 1 Three different cases of heat exchangers

Case 1 A^hs=3500m² T^hs,out=90℃ Case 2 A^hs=3000m² T^hs,out=90℃ Case 3 A^hs=3500m² T^hs,out=99℃

Figure 7 shows the output of thermal power unit G1 in three cases. The heat transfer area of case 1 is larger than that of case 2. Under the same heat load, less steam is required for extraction, and the thermal output and electrical output of the thermal power unit are also reduced accordingly. Therefore, compared with case 1 The electric output of the conventional unit is higher than that of Case 2. The outlet temperature of the heat exchanger in case 1 is lower than that in case 3. When the heat load is the same, less extraction steam is required, and the thermal output and electrical output of the thermal power unit are also reduced accordingly. Therefore, the electrical output of the conventional unit in case 1 is slightly higher than that in case 3. .

Figure 8 shows the wind power output of the three cases, comparing case 1 and case 2 (same heat exchanger outlet temperature, different heat transfer area) and case 1 and case 3 (same heat transfer area, different heat exchanger outlet temperature) Wind power output, especially during the nighttime wind power generation period, can be compared to a larger area of heat exchanger and a lower outlet water temperature to increase the output of wind power, and to absorb more wind.

Table 2 Scheduling results of three cases

Under the same heat load, different heat transfer conditions affect the output of thermal power units and thermal power units. It can be seen from Table 2 that in the case of the same heat load, due to the heat transfer characteristics, the coal consumption will be different eventually. It can be seen from the table that the heat transfer area of the heat exchanger is increased by 500m ² , the coal consumption of the whole system is reduced by 3.69 tons of standard coal, and the extraction steam quality is reduced by 4.4%; The coal consumption of the system is reduced by 4.79 tons of standard coal, and the extraction steam quality is reduced by 4.8%. It can be seen that only by considering the heat transfer parameters of the heat exchanger, the scheduling results of the system are more accurate.

Example 3: Influence of heat exchanger parameters on scheduling results after installing electric boilers.

In the stage of large wind power generation and high heat load at night, the electric boiler bears a part of the heat load, so the output of the electric heating plant is low. It can be seen from Figure 9 that the overall influence trend of the heat exchange area on the extraction steam quality remains unchanged. The difference is more pronounced during peak load.

It can be seen from Figure 10 that after the installation of the electric boiler, the overall output of the thermal power unit decreases, and the required extraction steam quality decreases, while the overall trend of the effect of the water temperature at the outlet of the heat exchanger on the heat transfer process remains unchanged.

Claims (8)

1. a method for dispatching a combined electric and heating system considering heat transfer characteristics of a heat exchanger, is characterized in that, comprises the following steps: 1) According to the structure diagram of the cogeneration device including the electric boiler and the heat exchanger, build the heat transfer model inside the heat exchanger and the operation model of the electric boiler; 2) Obtain an accurate model of heat transfer inside the heat exchanger according to the three-stage heat transfer process of the extraction steam inside the heat exchanger; 3) Construct the dispatching model of the combined heat and power system, and complete the scheduling optimization of the combined heat and power system considering the constraints of the electricity and heat network. 2. A kind of electric-heat combined system scheduling method considering heat transfer characteristics of heat exchanger according to claim 1, is characterized in that, in described step 1), the heat transfer model inside heat exchanger is:

Among them, Q is the heat transferred by the heat exchanger, A ^hs and k ^hs are the heat exchange area and total heat transfer coefficient of the heat exchanger, T ^stm and T ^cds are the inlet temperature and outlet temperature of the steam, respectively, T ^hs,in , T ^hs,out are the inlet and outlet temperatures of the water, respectively. 3. a kind of electric-heat combined system scheduling method considering heat transfer characteristics of heat exchanger according to claim 2, is characterized in that, in described step 2), the three-stage heat transfer process of extraction steam inside heat exchanger Including gas sub-process, gas-liquid sub-process and liquid sub-process, specifically: 1) Gas sub-process: the steam temperature drops from T ^stm to the phase transition temperature T ^phs , and the heat released in this process is Q ^g ; 2) Gas-liquid sub-process: all the latent heat of the steam is released into the water, during this process, the steam maintains the phase transition temperature T ^phs , and the heat released in this process is Q ^gl ; 3) Liquid subprocess: the vapor is in the liquid phase and releases heat from the phase transition temperature T ^phs to T ^cds , and the released heat is Q ^l . 4. The method for scheduling a combined electric-heating system considering heat transfer characteristics of a heat exchanger according to claim 3, wherein the precise model of the heat transfer inside the heat exchanger is specifically: A ^hs =A ^g +A ^gl +A ^l Q=Q ^g +Q ^gl +Q ^l

Among them, A ^g , A ^gl , and Al are the equivalent heat transfer areas corresponding to the gas sub-process, the gas-liquid sub-process and the liquid sub-process, respectively, T ^{hs,1 and} T ^hs,2 are the intermediate temperatures, k ^g , k ^gl and k ^l are the heat transfer coefficients corresponding to the gas sub-process, the gas-liquid sub-process and the liquid sub-process, respectively. 5. a kind of electric-heat combined system scheduling method considering heat transfer characteristics of heat exchanger according to claim 1, is characterized in that, in described step 1), electric boiler operation model is specifically: P ^b =Q ^b /(3.6*δ) Among them, P ^b is the electric power consumption of the electric boiler, Q ^b is the thermal power output by the peak-shaving electric boiler, and δ is the electric-heat conversion efficiency of the electric boiler. 6. The combined heat and power system scheduling method considering the heat transfer characteristics of the heat exchanger according to claim 3, wherein in the step 3), the combined heat and power system scheduling model takes the minimum total operating coal consumption as the objective function , the constraints include power supply balance constraints, heat supply balance constraints, pure condensing unit output constraints, wind turbine output constraints, unit ramping constraints and grid power flow constraints. 7. A kind of combined heat and power system scheduling method considering heat transfer characteristics of heat exchanger according to claim 6, it is characterized in that, the objective function of the combined heat and power system scheduling model is:

Among them, T is the total number of time periods, S ^tp and S ^chp are the number of thermal power units and thermal power units,

is the coal consumption of the i-th thermal power unit and the j-th single-extraction cogeneration unit, respectively,

are the outputs of the i-th thermal power unit and the j-th cogeneration unit at time t, respectively,

is the extraction steam quality of the jth cogeneration unit at time t. 8 . The method for dispatching a combined electric-heating system considering the heat transfer characteristics of a heat exchanger according to claim 7 , wherein the extraction steam quality of the combined heat and power unit is calculated by a dichotomy method. 9 .

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