Figure 1 shows the schematic diagram of offshore wind power for hydrogen production and onshore hydrogen storage for ammonia production.

Wind power generation is mainly utilized for subsequent hydrogen production, providing power sources for the hydrogen transmission system. Based on the current research on wind power generation technology, the relationship between wind power generation capacity and wind turbine units can be obtained as follows:

$P_{\text{WF}}=\{\begin{array}{c}\frac{P_W^N{v}^3}{v_W^N{}^3-v_{in}^3}-\frac{P_W^N{v}_{in}^3}{v_W^N{}^3-v_{in}^3},v_{in}\le v\le v_W^N\\ \frac{1}{2}{\eta }_F{\rho }_K{B}_W{F}_p{v}^3,v_W^N<v\le v_{out}\end{array}$ (1)

$P_{\text{WF}}$—the output power of the wind turbine

${\eta }_F$—the efficiency of the wind turbine

$B_{\text{W}}$—the radius of the wind turbine blades

$S_t^{HT}=\left(1-{\varphi }_{q,bat}\right)S_{t-1,HT}+\left[P{}_{t,ch}^{\text{q}}{\eta }_{q,ch}-P_{t,dis}^{\text{q}}/{\eta }_{q,dis}\right]\times \text{Δ}t$ (2)

$S_t^{HT}$—The ammonia storage quantity of the ammonia storage tank at time t

${\varphi }_{q,bat}$—Self-release ammonia loss rate

${\eta }_{q,ch}$—Hydrogen filling efficiency

${\eta }_{q,dis}$—Hydrogen release efficiency

$P_{t,ch}^{\text{q}}$—Hydrogen charging power of hydrogen storage tank

$P_{t,dis}^{\text{q}}$—Hydrogen discharging power of hydrogen storage tank

The ammonia unit is mainly used in ammonia fuel cells and in the operation of thermal power units. For AFC, considering the energy of the battery, it can generate electrical energy and output power. The operation model is as follows:

$\{\begin{array}{c}{P}_{F,DC}=\frac{U_{FN}{\varsigma }_F{V}_{\text{NH3}}{N}_A}{1800V_m{C}_0}\\ Q_{DC}=Q_{FH,in}{\eta }_{C,\text{NH3}}\end{array}$ (3)

$V_{\text{NH3}}$—The volume of ammonia gas at normal temperature

${\eta }_{C,\text{NH3}}$—Ammonia storage tank ammonia storage efficiency

$U_{FN}$—Rated output voltage

$Q_{DC}$—Fuel cell energy

$Q_{FH,in}$—Input hydrogen fuel chemical energy

${\varsigma }_{\text{F}}$—The energy conversion efficiency of the fuel cell is set at 45%

Other units include models of methane reactors and carbon capture systems, etc. The hydrogen produced by offshore wind power is used not only in the ammonia production process and storage but also in the methane reactors and gas-hydrogen blended CHP units.

The methane reactor can mix hydrogen transported from offshore platforms with carbon dioxide to provide the required natural gas for CHP units and gas boilers. The specific model is as follows:

$\begin{array}{c}\{\begin{array}{c}{M}_{t,MR}^{\text{g}}={\eta }_{MR}{M}_{t,MR}^{{\text{H}}_2}\\ M_{\text{min,}MR}^{{\text{H}}_2}\le M_{t,MR}^{{\text{H}}_2}\le M_{\text{max,}MR}^{{\text{H}}_2}\\ \text{Δ}{M}_{\text{min,}MR}^{{\text{H}}_2}\le M_{t+1,MR}^{{\text{H}}_2}-M_{t,MR}^{{\text{H}}_2}\le \text{Δ}{M}_{\text{max,}MR}^{{\text{H}}_2}\end{array}\end{array}$ (4)

$M_{t,MR}^{{\text{H}}_2}$—The amount of hydrogen energy transported by sea and input into MR

$M_{t,MR}^{\text{g}}$—The output power of natural gas from the MR unit

${\eta }^{MR}$—The energy conversion efficiency of MR

$M_{\text{max,}MR}^{{\text{H}}_2}$, $M_{\text{min,}MR}^{{\text{H}}_2}$—To determine the upper and lower limits of hydrogen energy input for MR

$\text{Δ}{M}_{\text{max,}MR}^{{\text{H}}_2}$, $\text{Δ}{M}_{\text{min,}MR}^{{\text{H}}_2}$—For the upper and lower limits of the climbing slope of MR

Hydrogen production at sea, in addition to large-scale ammonia production, is also used for efficient hydrogen energy utilization. Some hydrogen is introduced into gas-hydrogen blended CHP units to ensure cleaner fuel and reduce environmental impact. Current experiments show that the maximum hydrogen volume ratio blended into natural gas for safe and stable gas turbine operation is 20%. The specific model is as follows:

$\{\begin{array}{c}{\varpi }_t=\frac{P_{t,CHP}^{{\text{H}}_2}}{F_{{\text{H}}_2}}/\left(\frac{P_{t,CHP}^{{\text{H}}_2}}{F_{{\text{H}}_2}}+\frac{P_{t,CHP}^{\text{g}}}{F_{{\text{CH}}_2}}\right)\\ 0\le {\varpi }_t\le 20\text{\%}\\ P_t^{CHP}=\left({\varpi }_t{F}_{{\text{H}}_2}+\left[1-{\varpi }_t\right]F_{{\text{CH}}_2}\right)\left(\frac{P_{t,CHP}^{{\text{H}}_2}}{F_{{\text{H}}_2}}+\frac{P_{t,CHP}^{\text{g}}}{F_{{\text{CH}}_2}}\right)\end{array}$ (5)

${\varpi }_t$—Hydrogen blending ratio of gas

$P_{t,CHP}^{{\text{H}}_2}$—The hydrogen gas input to the CHP unit through the hydrogen blending device for gas

$P_{t,CHP}^{\text{g}}$—The power of natural gas input to the CHP unit through the hydrogen-blended natural gas injection device

$P_t^{CHP}$—The total power of the mixed gas of the CHP unit

$F_{{\text{H}}_2}$—The lower calorific value of hydrogen gas

$F_{{\text{CH}}_2}$—The lower calorific value of natural gas

$F_{\text{mix}}$—The lower calorific value of the mixed gas

In the CHP unit equipped with carbon capture devices [9] [10] , the electricity generated is used in two places. One part is supplied to the carbon capture device for its operation, and the remaining part is used for other electrical equipment. The specific model is as follows:

$\{\begin{array}{c}{P}_{cct}=P_{CHP}^f-P_{qt}^E\\ P_{cct}=L_B^{CCT}+{\eta }_{CCT}{\lambda }_t^{CCT}{Q}_p^C\\ Q_p^C={\vartheta }_C{P}_{chp}^f\end{array}$ (6)

$P_{cct}$—Power consumption for carbon capture

$P_{CHP}^f$—The generating power of the CHP unit

$P_{qt}^E$—The net output power of the carbon capture unit

$L_B^{CCT}$—Base energy consumption power

${\eta }_{CCT}$—The energy consumed by the carbon capture unit when capturing CO₂ in the carbon capture unit system

$Q_p^C$—Carbon emissions

${\lambda }_t^{CCT}$—The carbon capture efficiency of the carbon capture unit

${\vartheta }_C$—The unit carbon emission intensity of the carbon capture unit

The low-carbon scheduling model for offshore wind power and onshore ammonia synthesis aims to minimize total operating costs while considering the operation and maintenance of energy storage devices. It optimizes day-ahead scheduling and backup plans for offshore wind power hydrogen production, onshore hydrogen consumption, ammonia synthesis, thermal power units, and carbon capture devices.

The formula for the total operating cost of the system is as follows：

$C_T=C_{PE}+C_{LM}+C_{TP}+C_{CC}+C_{CE}$ (7)

This paper presents the IES low-carbon optimization scheduling test system, consisting of the IEEE-33-node power system and the 23-node thermal system. The problem is solved using MATLAB 2021a with the Gurobi solver via the Yalmip toolbox, with a 24-hour scheduling period. Four scenarios will be analyzed for the low-carbon optimization scheduling model based on offshore wind power hydrogen production and onshore ammonia synthesis:

An analysis was conducted on the four aforementioned scenarios. The optimization results for each scenario are presented in the table below.

As shown in Table 1 , Scenario 4 has the lowest total operating cost, 42.93%, 25.23%, and 9.48% lower than the other scenarios, respectively. This indicates that the introduction of hydrogen and ammonia units has improved new energy consumption capacity and economic efficiency.

In terms of carbon emissions, Scenario 4 shows a significant reduction compared to the other scenarios, with reduction rates of 49.96%, 48.36%, and 40.01%, respectively. This highlights the advantages of the proposed comprehensive energy system optimization in achieving low-carbon benefits.

Scenario 1 directly integrates offshore wind power into the energy system, causing high wind power discard and electricity purchase costs during shortages. In contrast, Scenario 2 incorporates offshore wind power hydrogen production, including production, transmission, and storage, which reduces wind power discard by 18% and lowers electricity purchase costs. This demonstrates that offshore wind power hydrogen production technology lowers operating costs and enhances renewable energy utilization.

As shown in Figure 3 , Scenario 2 reduces wind power discard by about 18% compared to Scenario 1. Scenario 1 does not produce hydrogen, and the hydrogen production values in Figure 3 are from Scenario 2. During the periods from 1:00 to 7:00 and 19:00 to 24:00, the difference in wind power discard is positive, indicating that the output devices in Scenario 1 are increasing. The difference between predicted and actual wind power utilization shows an overall downward trend, but during most periods, the difference in wind power discard is negative, as the electricity load demand is higher during the daytime. Compared to Scenario 1, during the 8:00 to 19:00 period, the electricity load demand is high, and the wind power discard is much lower than in the morning and evening. Therefore, Scenario 2, with offshore wind power hydrogen production, compresses the excess electricity into hydrogen and transports it to the land, greatly reducing wind power discard during this period. Scenario 2 can also store excess hydrogen in onshore storage tanks and release it when electricity demand is higher, such as from 10:00 to 11:00 and 19:00 to 20:00. However, compared to Scenario 1, more energy (electricity and thermal energy) needs to be purchased, increasing the purchase cost. Thus, compared to Scenario 1, Scenario 2 reduces wind power discard but still experiences it, while the total cost is relatively lower.

Analysis of the Impact of Ammonia Production on Thermal Power Plant Systems:

Scene 3 has a total operating cost reduction of approximately 17.4% compared to Scene 2. Scene 2 does not have a coal-fired power plant with ammonia blending combustion. The coal consumption costs of the coal-fired power plants in Scene 2 are all generated by Scene 3. Scene 3 has configured an ammonia utilization link on the basis of Scene 2, which requires more hydrogen consumption. However, it reduces carbon emissions in coal-fired power generation and reduces carbon emissions by approximately 13.92% during the entire ammonia blending combustion process, demonstrating the effectiveness of the ammonia blending combustion link of the coal-fired power plants in the integrated energy system in reducing carbon emissions.

Compared with Scenarios 1 and 2, Scenarios 3 and 4 have introduced ammonia blending combustion in thermal power units. Due to the increase in ammonia production, the use of thermal power units has been promoted, and the carbon emissions of thermal power generation have been reduced. The output curves are shown in Figure 4 . In most time periods, the output situations of Scenarios 3 and 4 are similar and show an upward trend overall. During the 9:00 - 12:00 period, considering the hydrogen production situation in Scenario 2, the demand for electricity load is greater, and a large amount of hydrogen is released for use in CHP units. The output situation of thermal power units shows a slight decrease in process. The thermal energy scheduling results are shown in Figure 4 . The thermal power units in the integrated energy system are mostly cogeneration units. During their operation, they release heat to provide heat load for the entire system and also reduce the heating energy consumption of the system. While enhancing the flexibility of system heating, it also promotes the system to generate further economic benefits.