Wind-light-fire ammonia system power margin optimization scheduling strategy for recovering ammonia production waste heat

Abstract

The invention relates to a wind-light-fire ammonia system power margin optimization scheduling strategy for recovering ammonia production waste heat, and provides a thermal power generating unit-ammonia energy system and a waste heat recovery overall power regulation margin model thereof in order to solve the problems that the total energy abandoning amount of renewable energy is increased and the system power regulation margin is insufficient. And then considering the influence of the environment temperature on the waste heat recovery efficiency of the ammonia energy system, constructing a double-layer optimization scheduling model taking the maximum power regulation margin of the wind-light-fire-ammonia combined system as an upper layer target and the minimum total cost as a lower layer target, improving the power regulation margin of the combined system, and promoting the consumption of renewable energy sources.

Description

Technical Field

[0001] The present invention relates to the field of power margin adjustment operation of a combined system, and in particular to a power margin optimization scheduling strategy for a wind-solar-thermal-ammonia system for recovering waste heat from ammonia production. Background Art

[0002] With the implementation of carbon peak and carbon neutrality measures, the power system based on renewable clean energy is developing rapidly. With the continuous increase in installed capacity and grid-connected capacity of wind and solar power, the volatility and intermittency of wind and solar power output have brought severe challenges to the regulation capacity and wind and solar power consumption level of the combined system. The traditional power system relies solely on thermal power units for output regulation, and its characteristics such as small regulation margin, slow response rate and high carbon emissions do not meet the regulation requirements of the new power system. Therefore, improving the system power regulation margin and wind and solar power consumption is of great significance to achieving low-carbon economic operation of the system. Summary of the invention

[0003] In response to the problems of increasing total amount of abandoned renewable energy and insufficient system power regulation margin in my country's existing power system, the present invention innovatively proposes a power margin optimization scheduling strategy for wind, solar, thermal and ammonia systems that recovers waste heat from ammonia production. The strategy improves the upper and lower power regulation margins of the combined system by coordinating the thermal power units with the ammonia energy system and recovering the waste heat generated in the ammonia production process, while reducing the carbon emissions of the system to achieve low-carbon economic operation of the combined system.

[0004] The technical solution to realize the present invention is: a power margin optimization scheduling strategy for a wind-solar-thermal-ammonia system for recovering waste heat from ammonia production, and the specific steps are as follows:

[0005] 1) Modeling of wind-solar-fire-ammonia combined system

[0006] This paper adopts the new energy consumption mode of wind-solar-thermal-ammonia combined power supply to build a wind-solar-thermal-ammonia combined system. The ammonia energy system consists of electricity-to-ammonia and ammonia fuel cells, among which electricity-to-ammonia includes electrolytic cells, pressure swing adsorption nitrogen production and ammonia production plants.

[0007] When wind and solar power generation is high and energy is wasted, the electricity-to-ammonia equipment uses excess electricity to electrolyze water to produce hydrogen and synthesize ammonia, which is stored by the ammonia energy system to achieve the "electricity-ammonia" transformation. At the same time, the steam turbine of the thermal power unit recovers the waste heat generated in the ammonia production process through the heat storage tank and the heat exchanger, which helps to improve the energy efficiency of the system. When the electricity load is peak, the ammonia fuel cell converts ammonia energy into electricity, fills the power gap and meets the electricity load demand, and achieves the "ammonia-electricity" transformation. At the same time, the combustion of ammonia mixed with thermal power units is conducive to reducing the carbon emissions of the system. The ammonia energy system absorbs power when wind and solar power are high, and releases power when wind and solar power output is low.

[0008] 2) Two-layer optimal dispatch of wind-solar-thermal-ammonia combined system considering regulation margin balance analysis

[0009] This paper uses abandoned wind and solar power to convert into ammonia for energy storage, solves the problem of insufficient system regulation margin through the coordination of thermal power units and ammonia energy system, and analyzes the balance of power regulation margin of the combined system. The maximum power regulation margin of the wind-solar-fire-ammonia combined system is used as the objective function of the upper optimization model, and the lower optimization model considers the impact of ambient temperature on the waste heat recovery efficiency of the ammonia energy system, with the goal of minimizing the total cost of the combined system, to achieve low-carbon economic operation of the wind-solar-fire-ammonia combined system.

[0010] 2.1) Joint system power regulation margin balance analysis

[0011] After considering the power-to-ammonia and waste heat recovery of the ammonia energy system, a balance analysis is conducted on the power regulation margin of the combined system. Changes in the system power regulation demand caused by net load fluctuations will affect the system power supply. Method 1 does not consider the ammonia energy system and its waste heat recovery, and only relies on thermal power units to provide power regulation margin to deal with system uncertainty, which may face penalties for wind and solar power abandonment or load loss. Method 2 considers the ammonia energy system and its waste heat recovery, and converts wind and solar power into ammonia for storage through power-to-ammonia conversion, which can provide additional downward regulation margin for the combined system; recovering the waste heat released during the synthesis of ammonia and supplying it to the steam turbine of the thermal power unit through a heat exchanger can provide additional upward regulation margin for the combined system.

[0012] 2.2) Upper-level optimal scheduling model considering the adjustment margin of the joint system

[0013] This paper uses the coordination between thermal power units and ammonia energy systems to smooth out the random fluctuations of wind and solar power and improve the wind and solar power absorption capacity. From the perspective of the balance of supply and demand of system power regulation margin, a regulation margin supply and demand model is established to optimize the power regulation margin to cope with the adverse effects brought about by large-scale wind and solar power access to the grid.

[0014] 2.2.1) Supply Modeling

[0015] The regulation margin supply of the wind-solar-thermal-ammonia combined system comes from the output of the ammonia energy system and the thermal power unit. Considering the physical characteristics of the thermal power unit, the ammonia energy system and the remaining heat recovery, a power regulation margin supply side model is established to characterize the ability of the combined output of the ammonia energy system and the thermal power unit to provide the system with up and down regulation margins, thereby improving the wind and solar power consumption level. The system's up regulation margin supply and down regulation margin supply models are shown in equations (1) and (2), respectively.

[0016]

[0017] Where: ΔP t su and ΔP t sd are the upper and lower regulation margins that the system can provide during period t; n A and nT They are ammonia energy system and thermal power unit respectively; and are respectively the upward adjustment margin and downward adjustment margin that the ammonia energy system can provide during period t; and They are respectively the upward adjustment margin and downward adjustment margin that the thermal power unit can provide during period t; and are the output power of the ammonia energy system and the thermal power unit during period t; P re,t is the output power equivalent to the waste heat recovery at the thermal power unit side during period t; and are the maximum output power and minimum output power of the ammonia energy system respectively; and are the maximum output power and minimum output power of the thermal power unit respectively; and They are the upward climbing capacity and downward climbing capacity of the ammonia energy system respectively; and They are the upward climbing capacity and downward climbing capacity of thermal power units respectively.

[0018] 2.2.2) Demand Modeling

[0019] There is similarity or complementarity between renewable energy generation power and load power, and the power regulation margin on the demand side is determined by the net load power. Therefore, a power regulation margin demand model is established for the net load power, and the net load power upper regulation margin demand and lower regulation margin demand model are shown in formula (3).

[0020]

[0021] Where: ΔP t lu and ΔP t ld are the required value of the upward adjustment margin of the net load power and the required value of the downward adjustment margin of the net load power in period t respectively; P l,t is the power demand of the system load during period t; is the maximum wind power output of the system during period t; is the maximum photovoltaic output of the system during period t.

[0022] The change in net load power reflects the system's power response demand for dispatchable resources. The output of dispatchable units must track the net load power to maintain the balance of system power.

[0023] 2.2.3) Objective Function

[0024] According to the above description of the supply and demand adjustment margins, the system adjustment margin index is defined as the difference between the adjustment margin supply and the adjustment margin demand. Correspondingly, the adjustment margin also has two directions, upward and downward, as shown in formula (4).

[0025]

[0026] Where: and They are the upward power regulation margin and downward power regulation margin of the joint system in period t respectively.

[0027] The objective function of the upper optimization model is to maximize the power regulation margin of the joint system, as shown in formula (5).

[0028]

[0029] Where: Decision variables P w,l,t and P pv,l,t are the power directly supplied to the electric load by wind power and photovoltaic power in period t; t is a 0-1 variable, λ t =1 means the system adjusts upwards during period t, λ t =0 means the system is adjusted downward during period t.

[0030] 2.2.4) Constraints

[0031] The solution of the upper optimization model must satisfy the following constraints:

[0032] ① Wind power and photovoltaic output constraints:

[0033]

[0034] ② Output constraints of thermal power units:

[0035]

[0036] ③ Ammonia system output constraints:

[0037]

[0038] ④ Constraints on abandoned wind and solar power

[0039] In order to avoid a large amount of wind and solar power abandonment when optimizing the overall power regulation margin of the thermal power unit-ammonia energy system and the remaining heat recovery, this paper introduces the maximum allowable wind and solar power abandonment rate constraints, namely:

[0040]

[0041] Where: k w,t ,k pv,tare the wind curtailment rate and solar curtailment rate in period t respectively; P w,t ,P pv,t are the output power of wind power and photovoltaic power in period t respectively; They are the maximum allowable wind power and photovoltaic power abandonment rates, respectively.

[0042] ⑤ Power constraints of electricity-to-ammonia conversion:

[0043] This paper assumes that the power-to-ammonia conversion rate is always above 50% of its equipment capacity.

[0044]

[0045] Where: P P2A,t is the power used for electricity-to-ammonia conversion in wind and solar power generation during period t, It is the maximum capacity of the electric-to-ammonia conversion equipment.

[0046] ⑥ Waste heat recovery output constraints:

[0047]

[0048] Where: is the predicted power of waste heat recovery of the ammonia energy system during period t.

[0049] ⑦Electric load power balance constraints:

[0050] The power balance constraints of the electric load that must be met in any period of time are:

[0051]

[0052] 2.3) Lower-level optimization scheduling model considering the waste heat recovery efficiency of ammonia production

[0053] Waste heat recovery in the ammonia energy system can affect the power regulation margin of the combined system, but the waste heat recovery efficiency is affected by many factors, among which the ambient temperature is the most important. In order to more accurately quantify the heat provided to the steam turbine of the thermal power unit by the ammonia production process, this paper considers the impact of ambient temperature on the waste heat recovery efficiency, takes the minimum total cost of the combined system as the lower optimization goal, and realizes the low-carbon economic operation of the combined system under the condition of maximum regulation margin.

[0054] 2.3.1) Waste heat recovery model

[0055] At present, the Haber-Bosch process is usually used to synthesize ammonia in industry. This process uses high temperature (300-500°C) and high pressure (150-300atm) conditions to react nitrogen and hydrogen to synthesize ammonia under the action of an iron-based catalyst. The ammonia production reaction is shown in formula (13).

[0056] N2+3H2→2NH3 (13)

[0057] In the ammonia production process, the reaction required for ammonia synthesis will release a large amount of heat energy. The ammonia production plant stores the heat released in the Haber ammonia synthesis reaction through a heat storage tank. The heat stored in the heat storage tank can be expressed as:

[0058]

[0059] Where: Q in,t is the heat recovered from the waste heat to the heat storage tank during period t; η P2A is the heat release ratio used for heating in the ammonia plant; γ is the thermal power released by generating unit NH3; is the mass of ammonia produced by the synthetic ammonia process during period t; and are the power delivered to the ammonia energy system by wind power and photovoltaic power station during period t; ω P2A is the unit energy consumption of synthetic ammonia.

[0060] The heat stored and released by the heat storage tank during period t are expressed as:

[0061] Q ch,t =max{Q in,t -Q out,t ,0} (16)

[0062] Q dis,t =max{Q out,t -Q in,t ,0} (17)

[0063] Where: Q ch,t Q is the heat stored in the heat storage tank during period t; dis,t Q is the heat released by the heat storage tank during period t; out,t It is the heat delivered from the heat storage tank to the heat exchanger during period t.

[0064] 2.3.2) Relationship between ambient temperature and waste heat recovery efficiency

[0065] During the waste heat recovery process, the heat in the heat storage tank can be transferred to the thermal power unit through the heat exchanger to supply the steam turbine. Considering the influence of ambient temperature on the waste heat recovery efficiency, the heat injected into the steam turbine of the thermal power unit is shown in formula (18).

[0066]

[0067] Where: is the heat injected from the heat storage tank into the steam turbine of the thermal power unit during period t; are the mass flow rates of cold water and hot water respectively; c p is the specific heat capacity of water; T c,o,t is the cold water temperature at the heat exchanger outlet during period t; T c,i,t is the ambient temperature during period t; T h,i,tis the hot water temperature at the heat exchanger inlet during period t; T h,o,t is the hot water temperature at the heat exchanger outlet during period t.

[0068] The output power equivalent to waste heat recovery on the thermal power unit side can be expressed as:

[0069] P re,t =m re,t η re q coal (19)

[0070] Where: m re,t is the equivalent coal consumption Q of waste heat recovery of thermal power units during period t re,t Quantity; η re is the thermal efficiency of the thermal power unit; q coal It is the lower calorific value of coal.

[0071] 2.3.3) Relationship between ambient temperature and waste heat recovery efficiency

[0072] The optimization objective of the lower model is the total cost of the wind-solar-fire-ammonia combined system. The mathematical expression of the objective function is shown in formula (20).

[0073] F1=min(F2-F re ) (20)

[0074]

[0075] Where: F1 is the total cost of the joint system; F2 is the operating cost of the joint system; F re is the waste heat recovery income; C w is the wind power operation cost; C pv is the photovoltaic operation cost; The operating cost of thermal power units; The operating cost of the ammonia energy system; is the carbon emission cost; C eq Environmental protection cost; C cf For penalty costs.

[0076] ① Waste heat recovery benefits

[0077] Thermal power units can recover the waste heat generated by the ammonia synthesis reaction in the ammonia production plant, which can reduce the coal consumption and carbon emissions of the thermal power units. The waste heat recovery income is equivalent to the sum of the reduced coal consumption costs and carbon emission costs of the thermal power units.

[0078]

[0079] Where: c coal is the unit price of coal; is the CO2 emission per unit of coal combustion; is the penalty cost per unit of carbon emissions.

[0080] ②Operation costs of wind power and photovoltaic power

[0081]

[0082] Where: c w ,c pv are the unit power generation cost coefficients of wind power and photovoltaic power respectively.

[0083] ③Operation cost of thermal power units

[0084] The thermal power unit in this article can be mixed with ammonia for combustion to adjust the output, where ammonia and coal powder are replaced according to the principle of isothermal replacement. The actual coal consumption of the thermal power unit is:

[0085]

[0086] Where: a, b and c are the consumption characteristic coefficients of thermal power units respectively; is the lower heating value of ammonia; m coal,t and are the coal consumption and ammonia content of the thermal power unit during period t respectively.

[0087] The operating cost of the thermal power unit is:

[0088]

[0089] ④ Ammonia system operating costs

[0090] The operating cost of the ammonia energy system in this article includes the operating cost of electricity-to-ammonia and the operating cost of ammonia fuel cells. The electricity consumed during the operation of the ammonia energy system comes from wind power and photovoltaics, and its cost has been included in the power generation cost of wind power and photovoltaics. Therefore, the operating cost of electricity-to-ammonia is the equipment maintenance cost, including the maintenance cost of the electrolytic cell, the equipment in the ammonia production plant, and the nitrogen production equipment. The operating cost of the ammonia energy system can be expressed as:

[0091]

[0092] Where: c P2A is the unit power maintenance cost of the power-to-ammonia equipment; c sofc P is the operating cost of ammonia fuel cell per unit of electricity generation; sofc,t is the output power of the ammonia fuel cell during period t; C P2A is the operating cost of electricity-to-ammonia conversion; C sofc The operating cost of an ammonia fuel cell.

[0093] ⑤Carbon emission costs

[0094] After the thermal power unit is modified by ammonia blending and waste heat recovery, the carbon emissions of the thermal power unit can be reduced. The reduction is the sum of the CO2 emissions of replacing coal combustion with ammonia combustion of equal calorific value and the CO2 emissions of replacing coal combustion with waste heat recovery. The carbon emission cost of the system is:

[0095]

[0096] Where: E is the carbon emission quota; e G Provide carbon emission quotas for power supply units; is the CO2 emission in period t.

[0097] ⑥ Environmental protection costs

[0098] Thermal power units emit SO2 and NO into the environment during operation. x For polluting gases, the corresponding units need to pay a certain amount of environmental protection tax to the state. The environmental protection cost is:

[0099]

[0100] Where: K is the tax amount to be paid per pollution equivalent; m S ,m N SO2 and NO per unit of coal combustion x Emissions; η S ,η N Environmental protection devices remove SO2 and NO x Efficiency; J S ,J N SO2 and NO x The pollution equivalent number.

[0101] ⑦Penalty Cost

[0102] The penalty costs for abandoning wind and solar power and losing load are:

[0103]

[0104] Where: c pa The penalty cost for abandoning wind and solar power; c lol Penalty costs for units abandoning wind and solar power; is the abandoned air volume in period t; is the amount of abandoned light during period t; is the load loss during period t.

[0105] 2.3.4) Constraints

[0106] The solution of the lower optimization model must satisfy the following constraints:

[0107] ① Power balance constraints of electricity-to-ammonia conversion:

[0108]

[0109] Where: P P2A,t is the electric power consumed by the electric-to-ammonia conversion during period t.

[0110] ② Constraints on ammonia blending ratio:

[0111] At present, the proportion of ammonia mixed with coal in thermal power units is defined according to the proportion of calorific value generated by combustion, which can be expressed as:

[0112]

[0113] Where: β t is the ammonia blending ratio of the thermal power unit during period t.

[0114] Under the current energy structure, using thermal power units to burn coal with a large proportion of ammonia is not the best choice. Therefore, it is necessary to restrict the proportion of coal-ammonia:

[0115] 0≤β t ≤β max (37)

[0116] Where: β max The maximum allowable coal-to-ammonia ratio. Due to the high ignition temperature of ammonia and the slow flame propagation speed, after comprehensive consideration of many factors, this paper sets the upper limit of the coal-to-ammonia ratio for thermal power units at 20%.

[0117] ③ Thermal power unit climbing constraints:

[0118]

[0119] Where: It is the output power of thermal power unit in the next period of time t.

[0120] ④ Ammonia energy system climbing constraints:

[0121]

[0122] Where: It is the output power of the ammonia energy system in the next period of time t.

[0123] ⑤Heat storage constraints:

[0124] In order to keep the initial and final states of the heat storage tank conserved within a scheduling cycle, the following heat storage constraints should be met:

[0125]

[0126] 2.4) Model solution

[0127] This paper uses the YALMIP toolbox to call the CPLEX solver to solve the two-level optimization scheduling model of the joint system in this paper, and outputs the optimal solution when max||x(k+1)-x(k)||2≤ε.

[0128] The beneficial effects of the power margin optimization scheduling strategy of the wind-solar-fired ammonia system for recovering waste heat from ammonia production according to the present invention are as follows:

[0129] 1. In order to address the problem of insufficient regulation capacity of the existing wind-solar-thermal combined system, the present invention models the supply and demand regulation margin, proposes an overall power regulation margin model for the combined system, and improves the upper and lower power regulation margins of the combined system through the coordination between the thermal power units and the ammonia energy system.

[0130] 2. In response to the high carbon emission problem caused by the wind-solar-fire combined system, the present invention considers the waste heat recovery of the ammonia energy system as the output gain of the thermal power unit, indirectly reducing the coal consumption of the thermal power unit, thereby reducing the system's carbon emissions. At the same time, considering the impact of ambient temperature on the waste heat recovery efficiency, the low carbon and operational adaptability of the combined system are improved.

[0131] 3. After considering the power-to-ammonia and ammonia combustion technologies of thermal power units, the wind and solar power consumption levels are significantly improved, reducing the system's wind and solar power abandonment rate, thereby reducing the total system cost and achieving the economic operation of the wind-solar-fire-ammonia combined system. BRIEF DESCRIPTION OF THE DRAWINGS

[0132] Figure 1 This is a schematic diagram of the wind-solar-fire-ammonia combined system structure;

[0133] Figure 2 It is a schematic diagram of power regulation margin balance analysis of the joint system;

[0134] Figure 3 It is a schematic diagram of the optimization solution process;

[0135] Figure 4 This is a schematic diagram of waste heat recovery and increased power generation under Example 3;

[0136] Figure 5 It is a schematic diagram of the power regulation margin of the joint system under Example 2 and Example 3;

[0137] Figure 6 It is a schematic diagram of the optimization scheduling results under different examples;

[0138] Figure 7 This is a schematic diagram of wind and solar power consumption under different calculation examples;

[0139] Figure 8 This is a schematic diagram of ammonia production and carbon emissions under Example 3;

[0140] Fig. 9is a schematic diagram of ambient temperature and waste heat recovery efficiency; DETAILED DESCRIPTION

[0141] The following is combined with Figure 1-9 The present invention is further described in detail with specific implementation modes, and the specific implementation modes described herein are only used to explain the present invention and are not used to limit the present invention.

[0142] 1. Modeling the wind-solar-thermal-ammonia combined system

[0143] This paper adopts the new energy consumption mode of wind-solar-thermal power generation and constructs a wind-solar-thermal-ammonia combined system. Figure 1 The ammonia energy system consists of electricity-to-ammonia and ammonia fuel cells, where electricity-to-ammonia includes electrolytic cells, pressure swing adsorption nitrogen production and ammonia production plants.

[0144] When wind and solar power generation is high and energy is wasted, the electricity-to-ammonia equipment uses excess electricity to electrolyze water to produce hydrogen and synthesize ammonia, which is stored by the ammonia energy system to achieve the "electricity-ammonia" transformation. At the same time, the steam turbine of the thermal power unit recovers the waste heat generated in the ammonia production process through the heat storage tank and the heat exchanger, which helps to improve the energy efficiency of the system. When the electricity load is peak, the ammonia fuel cell converts ammonia energy into electricity, fills the power gap and meets the electricity load demand, and achieves the "ammonia-electricity" transformation. At the same time, the combustion of ammonia mixed with thermal power units is conducive to reducing the carbon emissions of the system. The ammonia energy system absorbs power when wind and solar power are high, and releases power when wind and solar power output is low.

[0145] 2. Two-layer Optimal Dispatch of Wind-Solar-Thermal-Ammonia Combined System Considering Regulation Margin Balance Analysis

[0146] This paper uses abandoned wind and solar power to convert into ammonia for energy storage, solves the problem of insufficient system regulation margin through the coordination of thermal power units and ammonia energy system, and analyzes the balance of power regulation margin of the combined system. The maximum power regulation margin of the wind-solar-fire-ammonia combined system is used as the objective function of the upper optimization model, and the lower optimization model considers the impact of ambient temperature on the waste heat recovery efficiency of the ammonia energy system, with the goal of minimizing the total cost of the combined system, to achieve low-carbon economic operation of the wind-solar-fire-ammonia combined system.

[0147] 2.1 Analysis of power regulation margin balance of joint system

[0148] After considering the waste heat recovery of the power-to-ammonia and ammonia energy systems, a balance analysis is conducted on the power regulation margin of the combined system, such as Figure 2As shown. The change in system power regulation demand caused by net load fluctuations will affect the system power supply. Method 1 does not consider the ammonia energy system and its waste heat recovery, and only relies on thermal power units to provide power regulation margin to deal with system uncertainty, which may face penalties for wind and solar power abandonment or load loss. Method 2 considers the ammonia energy system and its waste heat recovery, and converts wind and solar power into ammonia for storage through electricity-to-ammonia conversion, which can provide additional downward regulation margin for the combined system; recovering the waste heat released in the process of synthetic ammonia and supplying it to the steam turbine of the thermal power unit through a heat exchanger can provide additional upward regulation margin for the combined system.

[0149] 2.2 Upper-level optimal scheduling model considering the adjustment margin of the joint system

[0150] This paper uses the coordination between thermal power units and ammonia energy systems to smooth out the random fluctuations of wind and solar power and improve the wind and solar power absorption capacity. From the perspective of the balance of supply and demand of system power regulation margin, a regulation margin supply and demand model is established to optimize the power regulation margin to cope with the adverse effects brought about by large-scale wind and solar power access to the grid.

[0151] 2.2.1 Supply Modeling

[0152] The regulation margin supply of the wind-solar-thermal-ammonia combined system comes from the output of the ammonia energy system and the thermal power unit. Considering the physical characteristics of the thermal power unit, the ammonia energy system and the remaining heat recovery, a power regulation margin supply side model is established to characterize the ability of the combined output of the ammonia energy system and the thermal power unit to provide the system with up and down regulation margins, thereby improving the wind and solar power consumption level. The system's up regulation margin supply and down regulation margin supply models are shown in equations (1) and (2), respectively.

[0153]

[0154] Where: ΔP t su and ΔP t sd are the upper and lower regulation margins that the system can provide during period t; n A and n T They are ammonia energy system and thermal power unit respectively; and are respectively the upward adjustment margin and downward adjustment margin that the ammonia energy system can provide during period t; and They are respectively the upward adjustment margin and downward adjustment margin that the thermal power unit can provide during period t; and are the output power of the ammonia energy system and the thermal power unit during period t; P re,t is the output power equivalent to the waste heat recovery at the thermal power unit side during period t; and are the maximum output power and minimum output power of the ammonia energy system respectively; and are the maximum output power and minimum output power of the thermal power unit respectively; and They are the upward climbing capacity and downward climbing capacity of the ammonia energy system respectively; and They are the upward climbing capacity and downward climbing capacity of thermal power units respectively.

[0155] 2.2.2 Demand Modeling

[0156] There is similarity or complementarity between renewable energy generation power and load power, and the power regulation margin on the demand side is determined by the net load power. Therefore, a power regulation margin demand model is established for the net load power, and the net load power upper regulation margin demand and lower regulation margin demand model are shown in formula (3).

[0157]

[0158] Where: ΔP t lu and ΔP t ld are the required value of the upward adjustment margin of the net load power and the required value of the downward adjustment margin of the net load power in period t respectively; P l,t is the power demand of the system load during period t; is the maximum wind power output of the system during period t; is the maximum photovoltaic output of the system during period t.

[0159] The change in net load power reflects the system's power response demand for dispatchable resources. The output of dispatchable units must track the net load power to maintain the balance of system power.

[0160] 2.2.3 Objective Function

[0161] According to the above description of the supply and demand adjustment margins, the system adjustment margin index is defined as the difference between the adjustment margin supply and the adjustment margin demand. Correspondingly, the adjustment margin also has two directions, upward and downward, as shown in formula (4).

[0162]

[0163] Where: and They are the upward power regulation margin and downward power regulation margin of the joint system in period t respectively.

[0164] The objective function of the upper optimization model is to maximize the power regulation margin of the joint system, as shown in formula (5).

[0165]

[0166] Where: Decision variables P w,l,t and P pv,l,t are the power directly supplied to the electric load by wind power and photovoltaic power in period t; t is a 0-1 variable, λ t =1 means the system adjusts upwards during period t, λ t =0 means the system is adjusted downward during period t.

[0167] 2.2.4 Constraints

[0168] The solution of the upper optimization model must satisfy the following constraints:

[0169] ① Wind power and photovoltaic output constraints:

[0170]

[0171] ② Output constraints of thermal power units:

[0172]

[0173] ③ Ammonia system output constraints:

[0174]

[0175] ④ Constraints on abandoned wind and solar power

[0176] In order to avoid a large amount of wind and solar power abandonment when optimizing the overall power regulation margin of the thermal power unit-ammonia energy system and the remaining heat recovery, this paper introduces the maximum allowable wind and solar power abandonment rate constraints, namely:

[0177]

[0178] Where: k w,t ,k pv,t are the wind curtailment rate and solar curtailment rate in period t respectively; P w,t ,P pv,t are the output power of wind power and photovoltaic power in period t respectively; They are the maximum allowable wind power and photovoltaic power abandonment rates, respectively.

[0179] ⑤ Power constraints of electricity-to-ammonia conversion:

[0180] This paper assumes that the power-to-ammonia conversion rate is always above 50% of its equipment capacity.

[0181]

[0182] Where: P P2A,t is the power used for electricity-to-ammonia conversion in wind and solar power generation during period t, It is the maximum capacity of the electric-to-ammonia conversion equipment.

[0183] ⑥ Waste heat recovery output constraints:

[0184]

[0185] Where: is the predicted power of waste heat recovery of the ammonia energy system during period t.

[0186] ⑦Electric load power balance constraints:

[0187] The power balance constraints of the electric load that must be met in any period of time are:

[0188]

[0189] 2.3 Lower-level optimization scheduling model considering waste heat recovery efficiency of ammonia production

[0190] Waste heat recovery in the ammonia energy system can affect the power regulation margin of the combined system, but the waste heat recovery efficiency is affected by many factors, among which the ambient temperature is the most important. In order to more accurately quantify the heat provided to the steam turbine of the thermal power unit by the ammonia production process, this paper considers the impact of ambient temperature on the waste heat recovery efficiency, takes the minimum total cost of the combined system as the lower optimization goal, and realizes the low-carbon economic operation of the combined system under the condition of maximum regulation margin.

[0191] 2.3.1 Waste heat recovery model

[0192] At present, the Haber-Bosch process is usually used to synthesize ammonia in industry. This process uses high temperature (300-500°C) and high pressure (150-300atm) conditions to react nitrogen and hydrogen to synthesize ammonia under the action of an iron-based catalyst. The ammonia production reaction is shown in formula (13).

[0193] N2+3H2→2NH3 (13)

[0194] In the ammonia production process, the reaction required for ammonia synthesis will release a large amount of heat energy. The ammonia production plant stores the heat released in the Haber ammonia synthesis reaction through a heat storage tank. The heat stored in the heat storage tank can be expressed as:

[0195]

[0196] Where: Q int is the heat recovered from the waste heat to the heat storage tank during period t; η P2A is the heat release ratio used for heating in the ammonia plant; γ is the thermal power released by generating unit NH3; is the mass of ammonia produced by the synthetic ammonia process during period t; and are the power delivered to the ammonia energy system by wind power and photovoltaic power station during period t; ω P2A is the unit energy consumption of synthetic ammonia.

[0197] The heat stored and released by the heat storage tank during period t are expressed as:

[0198] Q ch,t =max{Q in,t -Q out,t ,0} (16)

[0199] Q dis,t =max{Q out,t -Q in,t ,0} (17)

[0200] Where: Q ch,t Q is the heat stored in the heat storage tank during period t; dis,t Q is the heat released by the heat storage tank during period t; out,t It is the heat delivered from the heat storage tank to the heat exchanger during period t.

[0201] 2.3.2 Relationship between ambient temperature and waste heat recovery efficiency

[0202] During the waste heat recovery process, the heat in the heat storage tank can be transferred to the thermal power unit through the heat exchanger to supply the steam turbine. Considering the influence of ambient temperature on the waste heat recovery efficiency, the heat injected into the steam turbine of the thermal power unit is shown in formula (18).

[0203]

[0204] Where: is the heat injected from the heat storage tank into the steam turbine of the thermal power unit during period t; are the mass flow rates of cold water and hot water respectively; c p is the specific heat capacity of water; T c,o,t is the cold water temperature at the heat exchanger outlet during period t; T c,i,t is the ambient temperature during period t; T h,i,t is the hot water temperature at the heat exchanger inlet during period t; T h,o,t is the hot water temperature at the heat exchanger outlet during period t.

[0205] The output power equivalent to waste heat recovery on the thermal power unit side can be expressed as:

[0206] P re,t =m re,t η re q coal (19)

[0207] Where: m re,t is the equivalent coal consumption Q of waste heat recovery of thermal power units during period t re,t Quantity; η re is the thermal efficiency of the thermal power unit; q coal It is the lower calorific value of coal.

[0208] 2.3.3 Relationship between ambient temperature and waste heat recovery efficiency

[0209] The optimization objective of the lower model is the total cost of the wind-solar-fire-ammonia combined system. The mathematical expression of the objective function is shown in formula (20).

[0210] F1=min(F2-F re ) (20)

[0211]

[0212] Where: F1 is the total cost of the joint system; F2 is the operating cost of the joint system; F re is the waste heat recovery income; C w is the wind power operation cost; C pv is the photovoltaic operation cost; The operating cost of thermal power units; The operating cost of the ammonia energy system; is the carbon emission cost; C eq Environmental protection cost; C cf For penalty costs.

[0213] ① Waste heat recovery benefits

[0214] Thermal power units can recover the waste heat generated by the ammonia synthesis reaction in the ammonia production plant, which can reduce the coal consumption and carbon emissions of the thermal power units. The waste heat recovery income is equivalent to the sum of the reduced coal consumption costs and carbon emission costs of the thermal power units.

[0215]

[0216] Where: c coal is the unit price of coal; is the CO2 emission per unit of coal combustion; is the penalty cost per unit of carbon emissions.

[0217] ②Operation costs of wind power and photovoltaic power

[0218]

[0219] Where: c w ,c pv are the unit power generation cost coefficients of wind power and photovoltaic power respectively.

[0220] ③Operation cost of thermal power units

[0221] The thermal power unit in this article can be mixed with ammonia for combustion to adjust the output, where ammonia and coal powder are replaced according to the principle of isothermal replacement. The actual coal consumption of the thermal power unit is:

[0222]

[0223] Where: a, b and c are the consumption characteristic coefficients of thermal power units respectively; is the lower heating value of ammonia; m coal,t and are the coal consumption and ammonia content of the thermal power unit during period t respectively.

[0224] The operating cost of the thermal power unit is:

[0225]

[0226] ④ Ammonia system operating costs

[0227] The operating cost of the ammonia energy system in this article includes the operating cost of electricity-to-ammonia and the operating cost of ammonia fuel cells. The electricity consumed during the operation of the ammonia energy system comes from wind power and photovoltaics, and its cost has been included in the power generation cost of wind power and photovoltaics. Therefore, the operating cost of electricity-to-ammonia is the equipment maintenance cost, including the maintenance cost of the electrolytic cell, the equipment in the ammonia production plant, and the nitrogen production equipment. The operating cost of the ammonia energy system can be expressed as:

[0228]

[0229] Where: c P2A is the unit power maintenance cost of the power-to-ammonia equipment; c sofc P is the operating cost of ammonia fuel cell per unit of electricity generation; sofc,t is the output power of the ammonia fuel cell during period t; C P2A is the operating cost of electricity-to-ammonia conversion; C sofc The operating cost of an ammonia fuel cell.

[0230] ⑤Carbon emission costs

[0231] After the thermal power unit is modified by ammonia blending and waste heat recovery, the carbon emissions of the thermal power unit can be reduced. The reduction is the sum of the CO2 emissions of replacing coal combustion with ammonia combustion of equal calorific value and the CO2 emissions of replacing coal combustion with waste heat recovery. The carbon emission cost of the system is:

[0232]

[0233] Where: E is the carbon emission quota; e G Provide carbon emission quotas for power supply units; is the CO2 emission in period t.

[0234] ⑥ Environmental protection costs

[0235] Thermal power units emit SO2 and NO into the environment during operation. x For polluting gases, the corresponding units need to pay a certain amount of environmental protection tax to the state. The environmental protection cost is:

[0236]

[0237] Where: K is the tax amount to be paid per pollution equivalent; m S ,m N SO2 and NO per unit of coal combustion x Emissions; η S ,η N Environmental protection devices remove SO2 and NO x Efficiency; J S ,J N SO2 and NO x The pollution equivalent number.

[0238] ⑦Penalty Cost

[0239] The penalty costs for abandoning wind and solar power and losing load are:

[0240]

[0241] Where: c pa The penalty cost for abandoning wind and solar power; c lol Penalty costs for units abandoning wind and solar power; is the abandoned air volume in period t; is the amount of abandoned light during period t; is the load loss during period t.

[0242] 2.3.4 Constraints

[0243] The solution of the lower optimization model must satisfy the following constraints:

[0244] ① Power balance constraints of electricity-to-ammonia conversion:

[0245]

[0246] Where: P P2A,t is the electric power consumed by the electric-to-ammonia conversion during period t.

[0247] ② Constraints on ammonia blending ratio:

[0248] At present, the proportion of ammonia mixed with coal in thermal power units is defined according to the proportion of calorific value generated by combustion, which can be expressed as:

[0249]

[0250] Where: β t is the ammonia blending ratio of the thermal power unit during period t.

[0251] Under the current energy structure, using thermal power units to burn coal with a large proportion of ammonia is not the best choice. Therefore, it is necessary to restrict the proportion of coal-ammonia:

[0252] 0≤β t ≤β max (37)

[0253] Where: β max The maximum allowable coal-to-ammonia ratio. Due to the high ignition temperature of ammonia and the slow flame propagation speed, after comprehensive consideration of many factors, this paper sets the upper limit of the coal-to-ammonia ratio for thermal power units at 20%.

[0254] ③ Thermal power unit climbing constraints:

[0255]

[0256] Where: It is the output power of thermal power unit in the next period of time t.

[0257] ④ Ammonia energy system climbing constraints:

[0258]

[0259] Where: It is the output power of the ammonia energy system in the next period of time t.

[0260] ⑤Heat storage constraints:

[0261] In order to keep the initial and final states of the heat storage tank conserved within a scheduling cycle, the following heat storage constraints should be met:

[0262]

[0263] 2.4 Model Solution

[0264] This paper uses the YALMIP toolbox to call the CPLEX solver to solve the two-level optimization scheduling model of the joint system in this paper, and outputs the optimal solution when max||x(k+1)-x(k)||2≤ε. The solution process within the complete two-level optimization scheduling cycle is as follows: Figure 3 shown.

[0265] In this example, the installed capacities of wind power, photovoltaic power and thermal power units are 500MW, 200MW and 300MW respectively, and the maximum power of electricity-to-ammonia is 100MW. The load data and wind and solar power generation data used in the example are taken from a typical daily scene in Xinjiang. The system is simulated with a 24-hour cycle and a 1-hour step size.

[0266] In order to verify the feasibility and effectiveness of the model proposed in this paper, the following three examples are set for simulation verification:

[0267] Example 1: Wind-solar-thermal-ammonia combined system without considering P2A.

[0268] Example 2: Consider the P2A wind-solar-thermal-ammonia combined system.

[0269] Example 3: Considering the ammonia energy system and the wind-solar-fire-ammonia combined system with residual heat recovery.

[0270] The situation of waste heat recovery of ammonia energy system and increased power generation of thermal power units is as follows Figure 4 As shown in the figure, it can be seen that during the dispatching period, the ammonia energy system recovered a total of 133.86MWh of heat and supplied it to the steam turbine of the thermal power unit, which increased the power generation of the thermal power unit by 53.54MW.

[0271] Figure 5 The comparison results of the upper and lower regulation margins of the system in Example 2 and Example 3 are given. As can be seen from the figure, in the periods of 14:00-16:00 and 17:00-20:00, the upper regulation margin of the system in Example 2 is insufficient, and 7.54MW and 12.91MW loads need to be removed. Because during this period, the system's electrical load demand is high, but the output of renewable energy is low, and the thermal power units have been fully output, when the demand exceeds the supply, the system's upper regulation margin is insufficient. In this paper, the thermal power units output at 50% of the installed capacity, and the electricity-to-ammonia operation process is considered. At this time, the total power supply power of the system is lower than the load demand power, so Example 2 has sufficient lower regulation margin. Example 3 considers the waste heat recovery of ammonia fuel cells and ammonia energy systems based on Example 2. The excess wind power output during the low electricity consumption period is converted into ammonia for energy storage, and the waste heat generated during the ammonia production process is recovered and supplied to the steam turbine of the thermal power unit to provide electrical power to the system, which increases the upward regulation margin by 1.58% and 1.70% during the 14:00-16:00 and 17:00-20:00 periods, respectively. At the same time, the ammonia fuel cell provides additional downward regulation space for the system, so the downward regulation margin of Example 3 is better than that of Example 2.

[0272] Figure 6 The power dispatch results of Examples 1 to 3 are given. This paper assumes that the heat energy recovered from the waste heat of the ammonia energy system is all supplied to the steam turbine of the thermal power unit. Since the unit power generation cost of the ammonia energy system is higher than that of the thermal power unit, the output of the thermal power unit is adjusted first at the upper level. As can be seen from the figure, the thermal power unit in Example 1 is relatively stable, and the output of the thermal power unit is the lowest only in the period of 11:00-12:00. The reason is that the photovoltaic output reaches the maximum during this period. In order to absorb more photovoltaic power generation, the output of the thermal power unit is reduced. Example 2 considers electricity to ammonia. In the period of 1:00-7:00, the wind power output is large. The excess wind power is converted into ammonia through electricity to ammonia and supplied to the thermal power unit for combustion, which reduces the output of the thermal power unit during this period. Example 3 considers ammonia fuel cells based on Example 2. During the entire dispatch cycle, the power generation of the ammonia energy system replaces part of the output of the thermal power unit, reducing the output of the thermal power unit by 270.56MW.

[0273] Table 1 gives a comparison of the cost structure and wind and solar abandonment rates under different examples to analyze the superiority of the scheduling strategy in this paper. As can be seen from the table, after the introduction of electricity-to-ammonia in Example 2, the excess wind and solar power can be converted into ammonia for storage, reducing the wind abandonment rate from 14.86% to 0.77%, and the solar abandonment rate from 10.00% to 0.20%; and by replacing part of the coal with coal-to-ammonia technology, carbon emissions are reduced by 0.76 kilotons. Therefore, the introduction of electricity-to-ammonia technology helps to improve the low-carbon and economic efficiency of the wind-solar-fire-ammonia combined system. After considering the waste heat recovery of the ammonia energy system and ammonia fuel cells, Example 3 further improves the wind and solar consumption level, and the wind and solar abandonment rates are reduced to 0.04% and 0.02% respectively. By recovering the waste heat generated in the ammonia production process and supplying it to the steam turbine of the thermal power unit, the amount of coal burned is further reduced, and the carbon emissions are reduced by 0.21 kilotons on the basis of Example 2. Therefore, the introduction of waste heat recovery of the ammonia energy system and ammonia fuel cells further improves the economy and low-carbon efficiency of the system.

[0274] Table 1 Comparison of cost and abandonment rate of different examples

[0275]

[0276] The actual consumption of wind and solar power is as follows Figure 7 As shown in the figure, since Example 1 does not consider electricity-to-ammonia conversion, in the entire dispatch cycle, wind power and photovoltaic power respectively absorbed 6767.71MW and 845.16MW. Example 1 chose to abandon wind power during the periods of 1:00-13:00 and 23:00-24:00 when the overall power demand of the system was low and the wind power was large, and chose to abandon photovoltaic power during the period of 12:00-14:00 when the photovoltaic power reached its peak, resulting in a large amount of clean energy waste. Therefore, Example 2 considered electricity-to-ammonia conversion, converting excess wind and photovoltaic energy into ammonia energy for storage, which greatly increased the wind and photovoltaic absorption capacity to 7887.71MW and 937.16MW, respectively, an increase of 16.55% and 10.89%, respectively, indicating that electricity-to-ammonia conversion can effectively improve the wind and photovoltaic absorption rate of the combined system. Case 3 considers the waste heat recovery of the ammonia energy system and the ammonia fuel cell, increasing the wind and solar power consumption of the system during the period of 1:00-7:00 to 7945.41MW and 938.86MW, respectively, an increase of 0.73% and 0.18%, respectively. This is because the main function of the waste heat recovery of the ammonia energy system is to reduce the carbon emissions of the system, which is part of the simulation. Figure 8 Although the improvement in the wind and solar power consumption level is not significant, it still has a certain positive effect on the wind and solar power consumption.

[0277] The ammonia production and carbon emissions of the ammonia energy system are as follows: Figure 8 As shown in the figure, during the dispatching period, the ammonia energy system produced a total of 114.14 tons of ammonia to provide to the thermal power units and ammonia fuel cells, and recovered the waste heat generated by the ammonia energy system and supplied it to the steam turbine of the thermal power unit, reducing the carbon emissions of the thermal power unit by 53.80 tons. This shows that the ammonia energy system provides ammonia for the combined system to directly reduce coal consumption, and also provides heat energy for the steam turbine of the thermal power unit to indirectly reduce the coal consumption of power supply. Therefore, the introduction of waste heat recovery from the ammonia energy system can significantly improve the low-carbon nature of the combined system.

[0278] In order to further analyze the influence of ambient temperature changes on the waste heat recovery efficiency of the ammonia energy system, the relationship between the ambient temperature of the ammonia energy system waste heat recovery and the waste heat recovery efficiency is given, such as Fig. 9 As shown in the figure, it can be seen that in the periods of 1:00-6:00 and 15:00-24:00, the ambient temperature gradually decreases, the temperature difference increases, and the waste heat recovery efficiency gradually increases. In the period of 7:00-14:00, the ambient temperature gradually increases, the temperature difference decreases, and the waste heat recovery efficiency decreases. By considering the ambient temperature, the combined system can be more adaptable, can operate efficiently in a wider range of ambient temperatures, and reduce the impact of environmental changes.

[0279] The above is only a preferred embodiment of the present invention. It should be pointed out that, for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A power margin optimization scheduling strategy for a wind-solar-thermal-ammonia system that recovers waste heat from ammonia production, the specific steps of which are as follows: 1) Modeling of wind-solar-fire-ammonia combined system This paper adopts the new energy consumption mode of wind-solar-thermal-ammonia combined power supply to build a wind-solar-thermal-ammonia combined system. The ammonia energy system consists of electricity-to-ammonia and ammonia fuel cells, among which electricity-to-ammonia includes electrolytic cells, pressure swing adsorption nitrogen production and ammonia production plants. When wind and solar power generation is high and energy is wasted, the electricity-to-ammonia equipment uses excess electricity to electrolyze water to produce hydrogen and synthesize ammonia, which is stored by the ammonia energy system to achieve the "electricity-ammonia" transformation. At the same time, the steam turbine of the thermal power unit recovers the waste heat generated in the ammonia production process through the heat storage tank and the heat exchanger, which helps to improve the energy utilization efficiency of the system. When the electricity load is peak, the ammonia fuel cell converts ammonia energy into electricity, fills the power gap and meets the electricity load demand, and achieves the "ammonia-electricity" transformation. At the same time, the combustion of ammonia mixed with thermal power units is conducive to reducing the carbon emissions of the system. The ammonia energy system absorbs power when wind and solar power are high, and releases power when wind and solar power output is low. 2) Two-layer optimal dispatch of wind-solar-thermal-ammonia combined system considering regulation margin balance analysis This paper uses abandoned wind and solar power to convert into ammonia for energy storage, solves the problem of insufficient system regulation margin through the coordination of thermal power units and ammonia energy system, and analyzes the balance of power regulation margin of the combined system. The maximum power regulation margin of the wind-solar-fire-ammonia combined system is used as the objective function of the upper optimization model, and the lower optimization model considers the impact of ambient temperature on the waste heat recovery efficiency of the ammonia energy system, with the goal of minimizing the total cost of the combined system, to achieve low-carbon economic operation of the wind-solar-fire-ammonia combined system. 2.1) Joint system power regulation margin balance analysis After considering the power-to-ammonia and waste heat recovery of the ammonia energy system, a balance analysis is conducted on the power regulation margin of the combined system. The changes in system power regulation demand caused by net load fluctuations will affect the system power supply. Method 1 does not consider the ammonia energy system and its waste heat recovery, and only relies on thermal power units to provide power regulation margin to deal with system uncertainty, which may face penalties for abandoning wind and solar power or losing load. Method 2 considers the ammonia energy system and its waste heat recovery, and converts wind and solar power into ammonia for storage through electricity-to-ammonia conversion, which can provide additional downward regulation margin for the combined system; recovering the waste heat released during the synthesis of ammonia and supplying it to the steam turbine of the thermal power unit through a heat exchanger can provide additional upward regulation margin for the combined system. 2.2) Upper-level optimal scheduling model considering the adjustment margin of the joint system This paper uses the coordination between thermal power units and ammonia energy systems to smooth out the random fluctuations of wind and solar power and improve the wind and solar power absorption capacity. From the perspective of the balance of supply and demand of system power regulation margin, a regulation margin supply and demand model is established to optimize the power regulation margin to cope with the adverse effects brought about by large-scale wind and solar power access to the grid. 2.2.1) Supply Modeling The regulation margin supply of the wind-solar-thermal-ammonia combined system comes from the output of the ammonia energy system and the thermal power unit. Considering the physical characteristics of the thermal power unit, the ammonia energy system and the remaining heat recovery, a power regulation margin supply side model is established to characterize the ability of the combined output of the ammonia energy system and the thermal power unit to provide the system with up and down regulation margins, thereby improving the wind and solar power consumption level. The system's up regulation margin supply and down regulation margin supply models are shown in equations (1) and (2), respectively. Where: and are the upper and lower regulation margins that the system can provide during period t; n A and n T They are ammonia energy system and thermal power unit respectively; and are respectively the upward adjustment margin and downward adjustment margin that the ammonia energy system can provide during period t; and They are respectively the upward adjustment margin and downward adjustment margin that the thermal power unit can provide during period t; and are the output power of the ammonia energy system and the thermal power unit during period t; P re,t is the output power equivalent to the waste heat recovery at the thermal power unit side during period t; and are the maximum output power and minimum output power of the ammonia energy system respectively; and are the maximum output power and minimum output power of the thermal power unit respectively; and They are the upward climbing capacity and downward climbing capacity of the ammonia energy system respectively; and They are the upward climbing capacity and downward climbing capacity of thermal power units respectively. 2.2.2) Demand Modeling There is similarity or complementarity between renewable energy generation power and load power, and the power regulation margin on the demand side is determined by the net load power. Therefore, a power regulation margin demand model is established for the net load power, and the net load power upper regulation margin demand and lower regulation margin demand model are shown in formula (3). Where: and are the required value of the upward adjustment margin of the net load power and the required value of the downward adjustment margin of the net load power in period t respectively; P l,t is the power demand of the system load during period t; is the maximum wind power output of the system during period t; is the maximum photovoltaic output of the system during period t. The change in net load power reflects the system's power response demand for dispatchable resources. The output of dispatchable units must track the net load power to maintain the balance of system power. 2.2.3) Objective Function According to the above description of the supply and demand adjustment margins, the system adjustment margin index is defined as the difference between the adjustment margin supply and the adjustment margin demand. Correspondingly, the adjustment margin also has two directions, upward and downward, as shown in formula (4). Where: and They are the upward power regulation margin and downward power regulation margin of the joint system in period t respectively. The objective function of the upper optimization model is to maximize the power regulation margin of the joint system, as shown in formula (5). Where: Decision variables P w,l,t and P pv,l,t are the power directly supplied to the electric load by wind power and photovoltaic power in period t; t is a 0-1 variable, λ t =1 means the system adjusts upwards during period t, λ t =0 means the system is adjusted downward during period t. 2.2.4) Constraints The solution of the upper optimization model must satisfy the following constraints: ① Wind power and photovoltaic output constraints: ② Output constraints of thermal power units: ③ Ammonia system output constraints: ④ Constraints on abandoned wind and solar power In order to avoid a large amount of wind and solar power abandonment when optimizing the overall power regulation margin of the thermal power unit-ammonia energy system and the remaining heat recovery, this paper introduces the maximum allowable wind and solar power abandonment rate constraints, namely: Where: k w,t ,k pv,t are the wind curtailment rate and solar curtailment rate in period t respectively; P w,t ,P pv,t are the output power of wind power and photovoltaic power in period t respectively; They are the maximum allowable wind power and photovoltaic power abandonment rates, respectively. ⑤ Power constraints of electricity-to-ammonia conversion: This paper assumes that the power-to-ammonia conversion rate is always above 50% of its equipment capacity. Where: P P2A,t is the power used for electricity-to-ammonia conversion in wind and solar power generation during period t, It is the maximum capacity of the electric-to-ammonia conversion equipment. ⑥ Waste heat recovery output constraints: Where: is the predicted power of waste heat recovery of the ammonia energy system during period t. ⑦Electric load power balance constraints: The power balance constraints of the electric load that must be met in any period of time are: 2.3) Lower-level optimization scheduling model considering the waste heat recovery efficiency of ammonia production Waste heat recovery in the ammonia energy system can affect the power regulation margin of the combined system, but the waste heat recovery efficiency is affected by many factors, among which the ambient temperature is the most important. In order to more accurately quantify the heat provided to the steam turbine of the thermal power unit by the ammonia production process, this paper considers the impact of ambient temperature on the waste heat recovery efficiency, takes the minimum total cost of the combined system as the lower optimization goal, and realizes the low-carbon economic operation of the combined system under the condition of maximum regulation margin. 2.3.1) Waste heat recovery model At present, the Haber-Bosch process is usually used to synthesize ammonia in industry. This process uses high temperature (300-500°C) and high pressure (150-300atm) conditions to react nitrogen and hydrogen to synthesize ammonia under the action of an iron-based catalyst. The ammonia production reaction is shown in formula (13). N2+3H2→2NH3 (13) In the ammonia production process, the reaction required for ammonia synthesis will release a large amount of heat energy. The ammonia production plant stores the heat released in the Haber ammonia synthesis reaction through a heat storage tank. The heat stored in the heat storage tank can be expressed as: Where: Q in,t is the heat recovered from the waste heat to the heat storage tank during period t; η P2A is the heat release ratio used for heating in the ammonia plant; γ is the thermal power released by generating unit NH3; is the mass of ammonia produced by the synthetic ammonia process during period t; and are the power delivered to the ammonia energy system by wind power and photovoltaic power station during period t; ω P2A is the unit energy consumption of synthetic ammonia. The heat stored and released by the heat storage tank during period t are expressed as: Q ch,t =max{Q in,t -Q out,t ,0} (16) Q dis,t =max{Q out,t -Q in,t ,0} (17) Where: Q ch,t Q is the heat stored in the heat storage tank during period t; dis,t Q is the heat released by the heat storage tank during period t; out,t It is the heat delivered from the heat storage tank to the heat exchanger during period t. 2.3.2) Relationship between ambient temperature and waste heat recovery efficiency During the waste heat recovery process, the heat in the heat storage tank can be transferred to the thermal power unit through the heat exchanger to supply the steam turbine. Considering the influence of ambient temperature on the waste heat recovery efficiency, the heat injected into the steam turbine of the thermal power unit is shown in formula (18). Where: is the heat injected from the heat storage tank into the steam turbine of the thermal power unit during period t; are the mass flow rates of cold water and hot water respectively; c p is the specific heat capacity of water; T c,o,t is the cold water temperature at the heat exchanger outlet during period t; T c,i,t is the ambient temperature during period t; T h,i,t T is the hot water temperature at the heat exchanger inlet during period t; h,o,t is the hot water temperature at the heat exchanger outlet during period t. The output power equivalent to waste heat recovery on the thermal power unit side can be expressed as: P re,t =m re,t the re what coal (19) Where: m re,t is the equivalent coal consumption Q of waste heat recovery of thermal power units during period t re,t Quantity; η re is the thermal efficiency of the thermal power unit; q coal It is the lower calorific value of coal. 2.3.3) Relationship between ambient temperature and waste heat recovery efficiency The optimization objective of the lower model is the total cost of the wind-solar-fire-ammonia combined system. The mathematical expression of the objective function is shown in formula (20). <h2 style=";text-align:left;direction:ltr">F1 = min(F2-F<h2 style=";text-align:left;direction:ltr"> re <h2 style=";text-align:left;direction:ltr"> ) (20) Where: F1 is the total cost of the joint system; F2 is the operating cost of the joint system; F re is the waste heat recovery income; C w is the wind power operation cost; C pv is the photovoltaic operation cost; The operating cost of thermal power units; The operating cost of the ammonia energy system; is the carbon emission cost; C eq Environmental protection cost; C cf For penalty costs. ① Waste heat recovery benefits Thermal power units can recover the waste heat generated by the ammonia synthesis reaction in the ammonia production plant, which can reduce the coal consumption and carbon emissions of the thermal power units. The waste heat recovery income is equivalent to the sum of the reduced coal consumption costs and carbon emission costs of the thermal power units. Where: c coal is the unit price of coal; is the CO2 emission per unit of coal combustion; is the penalty cost per unit of carbon emissions. ②Operation costs of wind power and photovoltaic power Where: c w ,c pv are the unit power generation cost coefficients of wind power and photovoltaic power respectively. ③Operation cost of thermal power units The thermal power unit in this article can be mixed with ammonia for combustion to adjust the output, where ammonia and coal powder are replaced according to the principle of isothermal replacement. The actual coal consumption of the thermal power unit is: Where: a, b and c are the consumption characteristic coefficients of thermal power units respectively; is the lower heating value of ammonia; m coal,t and are the coal consumption and ammonia content of the thermal power unit during period t respectively. The operating cost of the thermal power unit is: ④ Ammonia system operating costs The operating cost of the ammonia energy system in this article includes the operating cost of electricity-to-ammonia and the operating cost of ammonia fuel cells. The electricity consumed during the operation of the ammonia energy system comes from wind power and photovoltaics, and its cost has been included in the power generation cost of wind power and photovoltaics. Therefore, the operating cost of electricity-to-ammonia is the equipment maintenance cost, including the maintenance cost of the electrolytic cell, the equipment in the ammonia production plant, and the nitrogen production equipment. The operating cost of the ammonia energy system can be expressed as: Where: c P2A is the unit power maintenance cost of the power-to-ammonia equipment; c sofc P is the operating cost of ammonia fuel cell per unit of electricity generation; sofc,t is the output power of the ammonia fuel cell during period t; C P2A is the operating cost of electricity-to-ammonia conversion; C sofc The operating cost of an ammonia fuel cell. ⑤Carbon emission costs After the thermal power unit is modified by ammonia blending and waste heat recovery, the carbon emissions of the thermal power unit can be reduced. The reduction is the sum of the CO2 emissions of replacing coal combustion with ammonia combustion of equal calorific value and the CO2 emissions of replacing coal combustion with waste heat recovery. The carbon emission cost of the system is: Where: E is the carbon emission quota; e G Provide carbon emission quotas for power supply units; is the CO2 emission in period t. ⑥ Environmental protection costs Thermal power units emit SO2 and NO into the environment during operation. x For polluting gases, the corresponding units need to pay a certain amount of environmental protection tax to the state. The environmental protection cost is: Where: K is the tax amount to be paid per pollution equivalent; m S ,m N SO2 and NO per unit of coal combustion x Emissions; η S ,η N Environmental protection devices remove SO2 and NO x Efficiency; J S ,J N SO2 and NO x The pollution equivalent number. ⑦Penalty Cost The penalty costs for abandoning wind and solar power and losing load are: Where: c pa The penalty cost for abandoning wind and solar power; c lol Penalty costs for units abandoning wind and solar power; is the abandoned air volume in period t; is the amount of abandoned light during period t; is the load loss during period t. 2.3.4) Constraints The solution of the lower optimization model must satisfy the following constraints: ① Power balance constraints of electricity-to-ammonia conversion: Where: P P2A,t is the electric power consumed by the electric-to-ammonia conversion during period t. ② Constraints on ammonia blending ratio: At present, the proportion of ammonia mixed with coal in thermal power units is defined according to the proportion of calorific value generated by combustion, which can be expressed as: Where: β t is the ammonia blending ratio of the thermal power unit during period t. Under the current energy structure, using thermal power units to burn coal with a large proportion of ammonia is not the best choice. Therefore, it is necessary to restrict the proportion of coal-ammonia: 0≤β t ≤β max (37) Where: β max The maximum allowable coal-to-ammonia ratio. Due to the high ignition temperature of ammonia and the slow flame propagation speed, after comprehensive consideration of many factors, this paper sets the upper limit of the coal-to-ammonia ratio for thermal power units at 20%. ③ Thermal power unit climbing constraints: Where: It is the output power of thermal power unit in the next period of time t. ④ Ammonia energy system climbing constraints: Where: It is the output power of the ammonia energy system in the next period of time t. ⑤Heat storage constraints: In order to keep the initial and final states of the heat storage tank conserved within a scheduling cycle, the following heat storage constraints should be met: 2.4) Model solution This paper uses the YALMIP toolbox to call the CPLEX solver to solve the two-level optimization scheduling model of the joint system in this paper, and outputs the optimal solution when max||x(k+1)-x(k)||2≤ε.

Images