CN110766193A - Electric heating combined system scheduling method considering heat transfer characteristics of heat exchanger - Google Patents
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Abstract
The invention relates to an electric-heat combined system scheduling method considering heat transfer characteristics of a heat exchanger, which comprises the following steps of: 1) constructing a heat transfer model inside a heat exchanger and an electric boiler operation model according to a structural diagram of a cogeneration device comprising an electric boiler and the heat exchanger; 2) acquiring an accurate model of heat transfer inside the heat exchanger according to a three-stage heat transfer process of the extracted steam inside the heat exchanger; 3) and constructing a scheduling model of the combined heat and power system, and finishing scheduling optimization of the combined heat and power system under the condition of considering the constraints of the electricity and heat supply network. Compared with the prior art, the method has the advantages of considering the steam extraction heat transfer process, being more accurate in model, being more practical in scheduling result and the like.
Description
Technical Field
The invention relates to the field of optimal scheduling of an electric heating combined system, in particular to an electric heating combined system scheduling method considering heat transfer characteristics of a heat exchanger.
Background
The thermoelectric generator set in the three north area has higher specific gravity and rich wind power resources. In the heating period in winter, the output of the thermoelectric unit is large, and the operation mode of 'fixing the power with the heat' of cogeneration greatly reduces the adjusting capacity of the heat supply unit and limits the flexibility of the electric power system, so that the phenomenon of wind abandoning of the electric power system is serious.
The traditional thermoelectric system is modeled by modeling the linear relation between the thermal output and the steam extraction amount under the condition of not considering the heat transfer process. One reason for such simplification may be that in power system analysis, electrical power balance is a common concern, and simplification makes it easier to solve system problems. However, only the extraction steam volume of the cogeneration unit is directly controlled, not the thermal output, and the heat exchanger extraction heat transfer process ultimately determines the actual thermal output of the cogeneration unit, so the traditional heat transfer model does not take into account the heat transfer process of the extraction steam and the complex relationship between the steam flow and the thermal output. In order to improve the energy utilization efficiency of the combined heat and power system, an accurate scheduling model considering the heat transfer characteristics of the heat exchanger needs to be constructed, the influence of the heat exchanger characteristics on scheduling is analyzed, and an idea is provided for the optimal scheduling of the combined heat and power system.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide an electric-heat combined system scheduling method considering the heat transfer characteristics of a heat exchanger.
The purpose of the invention can be realized by the following technical scheme:
a scheduling method of an electric-heat combined system considering heat transfer characteristics of a heat exchanger comprises the following steps:
1) constructing a heat transfer model inside a heat exchanger and an electric boiler operation model according to a structural diagram of a cogeneration device comprising an electric boiler and the heat exchanger;
2) acquiring an accurate model of heat transfer inside the heat exchanger according to a three-stage heat transfer process of the extracted steam inside the heat exchanger;
3) and constructing a scheduling model of the combined heat and power system, and finishing scheduling optimization of the combined heat and power system under the condition of considering the constraints of the electricity and heat supply network.
In the step 1), the heat transfer model in the heat exchanger is as follows:
wherein Q is the heat transferred by the heat exchanger, AhsAnd khsIs the heat exchange area and the total heat transfer coefficient, T, of the heat exchangerstm、TcdsInlet and outlet temperatures, T, of the steam, respectivelyhs,in、Ths,outRespectively the inlet temperature and the outlet temperature of the water.
In the step 2), the three-stage heat transfer process of the extracted steam in the heat exchanger comprises a gas sub-process, a gas-liquid sub-process and a liquid sub-process, and specifically comprises the following steps:
1) gas sub-process: steam temperature from TstmDown to the phase transition temperature TphsThe heat released in the process is Qg；
2) Gas-liquid sub-process: the entire latent heat of the steam is released into the water, during which the steam maintains the phase transition temperature TphsThe heat released in this process is Qg-l；
3) Liquid subprocess: the vapor is in the liquid phase and transfers heat from the phase transition temperature TphsRelease to TcdsThe quantity of heat released is Ql。
The accurate model of the heat transfer in the heat exchanger is specifically as follows:
Ahs＝Ag+Ag-l+Al
Q＝Qg+Qg-l+Ql
wherein A isg、Ag-l、AlEquivalent heat transfer areas, T, corresponding to the gas subprocess, the gas-liquid subprocess and the liquid subprocess respectivelyhs,1And Ths,2Respectively being intermediate temperature, kg、kg-l、klThe heat transfer coefficients corresponding to the gas sub-process, the gas-liquid sub-process and the liquid sub-process are respectively.
In the step 1), the electric boiler operation model specifically comprises:
Pb＝Qb/(3.6*δ)
wherein, PbFor consumption of electric power by electric boilers, QbThe delta is the electric heat conversion efficiency of the peak-shaving electric boiler.
In the step 3), the scheduling model of the combined heat and power system takes the minimum total coal consumption as a target function, and the constraint conditions comprise power supply balance constraint, heat supply balance constraint, output constraint of a straight condensing unit, output constraint of a wind turbine unit, climbing constraint of the unit and power flow constraint of a power grid.
The objective function of the scheduling model of the combined heat and power system is as follows:
wherein T is the total number of time periods Stp、SchpThe number of the thermal power generating units and the thermoelectric power generating units,
in order to respectively represent the coal consumption of the ith thermal power generating unit and the jth single-pumping type cogeneration unit,
respectively represent the ith thermal power generating unit and the jth combined heat and power generating unitthe force at the time t is exerted,
and the steam extraction quality of the jth combined heat and power generation unit at the time t.
The steam extraction quality of the cogeneration unit is calculated by a dichotomy.
Compared with the prior art, the invention has the following advantages:
firstly, on the basis of a general three-stage heat transfer model, a combined heat and power generation system considering the steam extraction heat transfer process is established, and an iterative method for calculating the accurate steam extraction mass flow required by meeting the given heat output requirement is provided, so that the heat output of a thermoelectric unit can be more accurately calculated by the method, a basis is provided for coordinating the output of the unit, compared with the traditional scheduling model considering the heat exchanger, the complete process of heat exchange inside heat exchange is not carefully considered, the three-stage heat transfer process provided by the invention can more truly reflect the heat transfer characteristics of the heat exchanger, the obtained scheduling model is more accurate, and the scheduling result is more in line with reality.
Decoupling the 'fixing power by heat' constraint, and simultaneously enabling an electric-heat combined system optimization scheduling model to be more accurate, reflecting the real situation of scheduling, and providing a basis for saving coal consumption and consuming renewable energy.
Drawings
Fig. 1 is a structural view of a heating cogeneration apparatus.
FIG. 2 is a model of a three stage heat transfer process with steam extraction.
Fig. 3 is an iterative method of extracting the steam mass flow.
FIG. 4 is a diagram of an electric heat integration system.
Fig. 5 shows the effect of heat exchanger area on the heat transfer process.
FIG. 6 is a graph of the effect of heat exchanger outlet water temperature on the heat transfer process.
Fig. 7 shows the output of the thermal power generating unit in three cases of the heat exchanger.
Fig. 8 shows the wind turbine output of three cases of heat exchanger.
FIG. 9 shows the effect of heat exchange area on heat transfer after the electric boiler is installed.
FIG. 10 is a graph showing the effect of the outlet water temperature of an electric boiler on the heat transfer process.
Detailed Description
The invention is described in detail below with reference to the figures and specific embodiments.
Examples
In order to decouple the 'fixing the power by heat' constraint and enable the optimal scheduling model of the electric-heat combined system to be more accurate, the invention focuses on a thermoelectric system model considering the steam extraction and heat transfer process, establishes a combined heat and power generation system considering the steam extraction and heat transfer process on the basis of a general three-stage heat transfer model, and provides an iterative method for calculating the accurate steam extraction mass flow required by meeting the given heat output requirement. Meanwhile, an electric boiler is additionally arranged in the system, a double-heat-source combined scheduling model comprising a thermoelectric boiler, a thermal power boiler, a wind power boiler and the electric boiler is established, the influence of different heat transfer characteristics on the consumption wind abandoning and running cost of the thermoelectric combined system is researched, and the optimized scheduling result of the system under the conditions of a single heat source and double heat sources is compared.
The invention provides an optimal scheduling method for an electric-heat combined system considering heat transfer characteristics of a heat exchanger, which comprises the following steps of:
1) building a structure diagram of a cogeneration device comprising an electric boiler and a heat exchanger, and describing a heat transfer process inside the heat exchanger and an electric boiler operation model;
2) constructing an accurate relation between a heat load and steam extraction quality in a three-stage heat transfer process in the heat exchanger based on steam extraction;
3) and constructing a scheduling model of the combined heat and power system, and considering the constraint of an electric and heat supply network to obtain the influence of the heat transfer characteristic of the heat exchanger on the scheduling of the heat and power system.
The method comprises the following specific steps:
1. firstly, constructing a cogeneration system of an electric boiler:
the cogeneration system comprises an extraction type cogeneration unit, a heat exchanger and an electric boiler, and the structure is shown in figure 1. The combined heat and power generation unit is connected with the heat exchanger, and after steam is pumped out from the steam turbine intermediate pressure cylinder section, the steam flows through the heat exchanger to exchange heat with the cold water pipeline, so that the heat energy requirement of a centralized heat supply user is met. In order to research an accurate model of three-level heat transfer of the heat exchanger, the invention explains the heat exchange principle of the heat exchanger according to the structure of the heat exchanger, and simultaneously, an electric boiler is additionally arranged to convert electric energy into heat energy to meet part of heat load of a heat sink.
2. Next, the structure of the general heat exchanger will be described
The general construction of the heat exchanger is shown in FIG. 1 by the dashed box, with the hot stream (extraction) flowing through the heat exchanger and delivering heat to the cold fluid, Tstm，TcdsDenotes the inlet temperature, outlet temperature, also T, of the hot fluid (steam)hs,in，Ths,outThe inlet temperature and the outlet temperature of the cold fluid (water) are shown. m isstmAnd mwaterIs the mass flow of extracted steam and water in the heat supply pipe network, AhsAnd khsIs the heat exchange area and the total heat transfer coefficient of the heat exchanger, Q represents the heat transferred through the heat exchanger, called the thermal output, and the heat transfer process of the heat exchanger can be described as:
the energy conservation equation of the cold and hot fluid shows that the heat flow releases energy, the middle transfer heat is equal to the cold flow absorbed heat in the heat transfer process, wherein cwater、cstmRespectively, the specific heat capacity of cold and hot fluids.
For a given cogeneration unit, extraction temperature and pressure are design parameters for the cogeneration turbine, so TstmIs a fixed value. Heat exchange area AhsIs also a design parameter of the heat exchanger, and the overall heat transfer coefficient khsRelated to the physical properties of the fluid. In general, a primary pipe network operates in a quality regulation mode, and the mass flow m of the primary pipe networkwaterThe temperature T of the water at the outlet of the heat exchanger is controlled by an operator mainly by adjusting the mass flow of the extracted steamhs,outTo meet the thermal demand, Ths,outChanges with the average temperature change of the environment, and therefore m is changed in a short timewaterAnd Ths,outA fixed value is maintained.
3. Electric boiler device
The electric boiler is a device for converting electricity into heat and plays an important role in a scheme of absorbing and abandoning wind. The constraint of 'fixing the power with the heat' of the cogeneration is decoupled by the electric boiler, so that the forced output of the thermal power plant can be reduced, and a space is made for the wind power to surf the internet. The electric boiler starts when having abandoned wind, absorbs partial wind-powered electricity generation heat supply, remedies the not enough part of heat supply that the thermoelectric generation group leads to owing to "decide the electricity with the heat" restraint, improves the flexibility of thermoelectric generation group, owing to abandon the wind heat supply of replacing thermoelectric generation group with the heat of using, sees short the journey, can also reduce entire system's coal consumption. The electric boiler is modeled as follows
Pb＝Qb/(3.6*δ)
Wherein, PbElectric power consumption for the electric boiler; qbThe heat power is output by the peak-shaving electric boiler; delta is the electric-to-heat conversion efficiency of the electric boiler, and is equal to 1.
4. The relationship between the thermal output and the steam extraction based on the three-stage heat transfer process is as follows:
because the heat exchangers have more complex relationships between the steam extraction amount and the thermal output of the thermal power plant, the relationships between the steam extraction amount and the thermal output of the thermal power plant change along with different operation conditions of the heat exchangers, and the previous research of simply linearizing the relationships cannot accurately describe the operation conditions of the cogeneration, so that more detailed research is needed.
To obtain the thermal output Q and the air extraction amount mstmIn this case, the heat exchanger is analyzed in detail for the internal processes of heat transfer, and generally in the heat exchanger, the extraction steam will undergo a complete phase change (from gas phase to liquid phase). The three sub-processes are respectively a gas process, a gas-liquid process and a liquid process, as shown in fig. 2.
First, the steam releases heat in the gas phase, the temperature of the steam being measured from the extraction temperature TstmDown to the phase transition temperature TphsThe quantity of heat released in the process is Qg. The steam is then in a gas-liquid state and releases all its latent heat into the water, during which the steam maintains a phase transition temperature TphsThe heat released in this process is Qg-l. In the last process, the vapor is in the liquid phase and transfers heat from TphsReleased to the temperature T of the condensatecdsThe quantity of heat released is Ql. Wherein, Ths,1And Ths,2Is the assumed intermediate temperature, k, of the water on the side of the pipeg，kg-l，klIs the heat transfer coefficient of the three sub-processes.
For the three stage heat transfer process with steam extraction, each heat transfer process is shown in FIG. 2 as Ag，Ag-l，AlAnd (3) representing the equivalent heat transfer area of each sub-process, and satisfying the following conditions:
Ahs＝Ag+Ag-l+Al
the total thermal power is equal to the sum of each heat transfer process:
Q＝Qg+Qg-l+Ql
first, according to the theory of heat transfer theory, each sub-heat transfer process satisfies:
Qg＝cstmmstm(Tstm-Tphs)
Qg-l＝rmstm
Ql＝Q-Qg-Qg-l
according to the principle of conservation of energy, the intermediate temperature T can be calculatedhs,1，Ths,2Temperature T at the inlet of the heat exchangerhs,inAnd temperature T of condensatecdsThen, there are:
Ths,1＝Ths,out-Qg/(cwatermwater)
Ths,2＝Ths,out-(Qg+Qg-l)/(cwatermwater)
Ths,in＝Ths,out-Q/(cwatermwater)
Tcds＝Tphs-Ql/(cwatermstm)
thus, the equivalent heat exchange area in each heat exchange process can be obtained:
known mwater，Ths,outIs given so that for a heat load power there is a given bleed air flow mstmAn iterative flow as in fig. 3 is obtained.
δ is the iteration threshold, mstepIs mstmThe superscript k is the number of iterations, mstm，maxAnd for the maximum steam extraction mass flow, according to known steam extraction parameters and heat exchanger parameters, taking the steam extraction mass of the first iteration as the minimum steam extraction mass, calculating the heat exchange area of each heat transfer sub-stage, judging whether the sum of the heat exchange areas of the three stages is greater than the total heat exchange area, continuing the iteration if the sum is greater than the total heat exchange area, taking the steam extraction mass of the current iteration as the maximum value if the sum is less than the total heat exchange area, taking the steam extraction mass of the last iteration as the minimum value, and calculating the accurate steam extraction mass in the range by using a bisection method until the accuracy requirement is met.
3. Finally, simulation verification is carried out
A simple electric-thermal combined system diagram is adopted to verify the influence of a heat transfer process on the scheduling of a thermoelectric combined system, a model shown in figure 4 is built on MATLAB, wherein 2 thermal power generating units G1 and G2, 1 thermal power generating unit and 1 wind power plant are additionally provided with an electric boiler to construct the thermoelectric combined system with double heat sources, the scheduling model established on the basis takes the minimum total running coal consumption as a target function, and the constraint conditions such as power supply balance constraint, heat supply balance constraint, output constraint of a straight condensing unit, output constraint of the wind power generating unit, climbing constraint of the unit, power grid flow constraint and the like are comprehensively considered.
Example 1: influence of no additional heat exchanger parameter on steam extraction quality
Heat exchange is a temperature difference driven process, so different heat exchanger outlet water temperatures are one factor considered in this example. Secondly, the properties of the heat exchanger itself, such as the heat exchange area of the heat exchanger, are another influencing factor. The effect of heat exchanger area and heat exchanger outlet water temperature on heat transfer process and extraction mass flow is therefore discussed in this example. And finally, a simple combined heat and power dispatching model of the wind power generation unit, the thermoelectric power generation unit and the conventional thermal power generation unit is utilized for research.
Fixed heat exchanger outlet temperature Ths,outFig. 5 shows the extraction mass flow required to meet a given thermal power requirement at different heat transfer areas, 90 ℃. As can be seen in the figure, under different heat transfer areas, in order to meet the same thermal power requirement, the required mass flow is completely different, when the mass flow is within the range of 0-11 h, the difference is more obvious, and the heat transfer area reflects the inherent heat transfer capacity of the heat exchanger, so that when the heat transfer area is larger, the heat transfer is more sufficient, the smaller the steam extraction amount required for meeting the thermal power requirement is, the smaller A ishs＝2971m2Can be taken as the limit of the heat transfer area of the heat exchanger to meet a given thermal power requirement. By contrast, it can be seen that if the heat exchanger is operated near its limits, more extraction steam will be required to meet the thermal energy demand, and it follows that it may be incorrect to disregard the heat transfer process when the actual operating conditions of the heat exchanger deviate significantly from the nominal conditions.
Fixed heat transfer area Ahs＝3500m2In fig. 6, the mass flow of the extracted steam required for satisfying the given thermal power requirement is shown at the outlet water temperature of different heat exchangers. It can be seen that the lower the outlet water temperature, the less extraction mass flow is required. This is because the heat transfer process is driven by the temperature difference between the cold and hot fluids, given the extraction temperature TstmAnd then, the lower the outlet water temperature is, the larger the temperature difference is, and the better the heat transfer effect is. The heat transfer process is influenced by adjusting the outlet temperature of the heat exchanger, so that the required steam extraction amount is influenced.
Example 2: influence of heat exchanger parameters on scheduling results
The influence of three different heat transfer processes on the system operation is compared
TABLE 1 three different cases of heat exchanger
| Case 1 | Ahs＝3500m2 Ths,out＝90℃ |
| Case 2 | Ahs＝3000m2 Ths,out＝90℃ |
| Case 3 | Ahs＝3500m2 Ths,out＝99℃ |
Fig. 7 shows the G1 group output of the thermal power unit for three cases, the heat transfer area of case 1 is larger than that of case 2, the extraction steam required under the same heat load is less, and the thermal output and the electric output of the thermal power unit are correspondingly reduced, so that the electric output of the conventional unit of the comparative case 1 is higher than that of case 2. The outlet temperature of the heat exchanger of case 1 is lower than that of case 3, and the required steam extraction is less and the thermal output and the electrical output of the thermoelectric unit are correspondingly reduced under the same thermal load, so that the electrical output of the conventional unit of case 1 is slightly higher than that of case 3.
Fig. 8 shows wind power output of three cases, which are respectively compared with wind power output of cases 1 and 2 (same heat exchanger outlet temperature and different heat transfer areas) and wind power output of cases 1 and 3 (same heat transfer area and different heat exchanger outlet temperatures), and especially in a period of heavy wind power generation at night, a heat exchanger with a larger area and a lower outlet water temperature can be obtained to increase wind power output, and more wind curtailment can be consumed.
TABLE 2 scheduling results for three cases
Under the same thermal load, the different heat transfer conditions affect the output of the thermoelectric power unit and the thermal power unit. It can be seen from Table 2 that the heat transfer characteristics ultimately lead toCoal consumption difference. From the table, the heat transfer area of the heat exchanger is increased by 500m2The coal consumption of the whole system is reduced by 3.69 tons of standard coal, and the steam extraction quality is reduced by 4.4%; the influence of the outlet temperature of the heat exchanger is larger, the outlet water temperature is reduced by 9 ℃, the coal consumption of the system is reduced by 4.79 tons of standard coal, and the steam extraction quality is reduced by 4.8 percent, so that the dispatching result of the system is more accurate only by considering the heat transfer parameters of the heat exchanger.
Example 3: the influence of the parameters of the heat exchanger after the electric boiler is additionally arranged on the scheduling result.
At the stage of heavy wind power generation and high heat load at night, the electric boiler bears a part of heat load, so that the output of the electric heating plant is low, and as can be seen from fig. 9, the whole influence trend of the heat exchange area on the steam extraction quality is unchanged, and the difference is more obvious at the load peak.
As can be seen from fig. 10, after the electric boiler is installed, the overall output of the thermoelectric power unit is reduced, the required steam extraction quality is reduced, and the overall trend of the influence of the outlet water temperature of the heat exchanger on the heat transfer process is unchanged.
Claims (8)
1. An electric-heat combined system scheduling method considering heat transfer characteristics of a heat exchanger is characterized by comprising the following steps:
1) constructing a heat transfer model inside a heat exchanger and an electric boiler operation model according to a structural diagram of a cogeneration device comprising an electric boiler and the heat exchanger;
2) acquiring an accurate model of heat transfer inside the heat exchanger according to a three-stage heat transfer process of the extracted steam inside the heat exchanger;
3) and constructing a scheduling model of the combined heat and power system, and finishing scheduling optimization of the combined heat and power system under the condition of considering the constraints of the electricity and heat supply network.
2. The dispatching method of the electric-heat combined system considering the heat transfer characteristics of the heat exchanger as claimed in claim 1, wherein in the step 1), the heat transfer model inside the heat exchanger is as follows:
wherein Q is the heat transferred by the heat exchanger, AhsAnd khsIs the heat exchange area and the total heat transfer coefficient, T, of the heat exchangerstm、TcdsInlet and outlet temperatures, T, of the steam, respectivelyhs,in、Ths,outRespectively the inlet temperature and the outlet temperature of the water.
3. The method for dispatching an electric-heat combined system with the heat transfer characteristics of the heat exchanger taken into consideration according to claim 2, wherein in the step 2), the three-stage heat transfer process of the extracted steam in the heat exchanger comprises a gas sub-process, a gas-liquid sub-process and a liquid sub-process, and specifically comprises the following steps:
1) gas sub-process: steam temperature from TstmDown to the phase transition temperature TphsThe heat released in the process is Qg；
2) Gas-liquid sub-process: the entire latent heat of the steam is released into the water, during which the steam maintains the phase transition temperature TphsThe heat released in this process is Qg-l；
3) Liquid subprocess: the vapor is in the liquid phase and transfers heat from the phase transition temperature TphsRelease to TcdsThe quantity of heat released is Ql。
4. The electric-heat combined system scheduling method considering heat transfer characteristics of a heat exchanger according to claim 3, wherein the accurate model of the heat transfer inside the heat exchanger is specifically as follows:
Ahs＝Ag+Ag-l+Al
Q＝Qg+Qg-l+Ql
wherein A isg、Ag-l、AlEquivalent heat transfer areas, T, corresponding to the gas subprocess, the gas-liquid subprocess and the liquid subprocess respectivelyhs,1And Ths,2Respectively being intermediate temperature, kg、kg-l、klThe heat transfer coefficients corresponding to the gas sub-process, the gas-liquid sub-process and the liquid sub-process are respectively.
5. The electric-heat combined system scheduling method considering heat transfer characteristics of a heat exchanger according to claim 1, wherein in the step 1), the electric boiler operation model is specifically:
Pb＝Qb/(3.6*δ)
wherein, PbFor consumption of electric power by electric boilers, QbThe delta is the electric heat conversion efficiency of the peak-shaving electric boiler.
6. The method for dispatching the electric-thermal combined system considering the heat transfer characteristics of the heat exchanger as claimed in claim 3, wherein in the step 3), the dispatching model of the electric-thermal combined system takes the minimum total coal consumption as an objective function, and the constraint conditions comprise a power supply balance constraint, a heat supply balance constraint, a pure condensing unit output constraint, a wind turbine output constraint, a unit climbing constraint and a power grid flow constraint.
7. The method for dispatching the combined heat and power system considering the heat transfer characteristics of the heat exchanger as claimed in claim 6, wherein the objective function of the dispatching model of the combined heat and power system is as follows:
wherein T is the total number of time periods Stp、SchpThe number of the thermal power generating units and the thermoelectric power generating units,
in order to respectively represent the coal consumption of the ith thermal power generating unit and the jth single-pumping type cogeneration unit,
to respectively represent the output of the ith thermal power generating unit and the jth cogeneration unit at the moment t,
and the steam extraction quality of the jth combined heat and power generation unit at the time t.
8. The method for dispatching the electric-heat combined system considering the heat transfer characteristics of the heat exchanger as recited in claim 7, wherein the steam extraction quality of the cogeneration unit is calculated by a dichotomy.
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