## 6 Simulation and Analysis
### Cross-RIES Optimization Results and Analysis
#### 6.4.3 Four Carbon Emission Reduction Policy Scenarios
This indicates that with continued growth in the load demand, the cross-RIES system in the GC-CET-CRIES scenario has more significant emission reduction advantages. This is because as the load demand increases, CP-RIES is forced to use grid electricity, with poorer carbon abatement benefits, because of the fluctuating and limited nature of the renewable energy output, whereas carbon- and green-certificate-trading mechanisms can effectively adjust the allocation of the electricity, heating, and cooling loads among the RIES systems within the cross-RIES system. This allows for meeting both the carbon emission targets and load demand. To further explore the impacts of the underlying mechanisms of carbon emission reduction policies on carbon reduction in the cross-RIES system, Figure 11 presents the load allocation rates and carbon emissions of each RIES system in the four carbon-emission reduction policy scenarios. It can be observed that with the implementation of carbon emission reduction policies, the load allocations of the individual RIES systems in the cross-RIES system shift from the predominantly fossil-fuel-based FP-RIES to the renewable-energy-based CP-RIES. Taking the year 2040 as an example, the load allocation rate of CP-RIES gradually increases in the carbon emission reduction policy scenarios, starting from 31.22% in the Reference-CRIES scenario to 34.01% in the GC-CRIES scenario, 37.2% in the CET-CRIES scenario, and 39.18% in the GC+CET-CRIES scenario. Correspondingly, the carbon emissions of FP-RIES decrease gradually to 31.34%, 27.42%, 25.07%, and 23.40% in the respective scenarios. This shift in the load allocation results in a significant reduction in carbon emissions for the entire system in the order of Reference-CRIES, GC-CRIES, CET-CRIES, and GC+CET-CRIES. It is evident that carbon- and green-certificate-trading mechanisms optimize the allocation of the load between renewable energy and traditional fossil energy sources through market mechanisms, thereby achieving the low-carbon nature of the cross-RIES system. This further confirms the superiority of carbon-trading  
Figure 10: The carbon emissions in four carbon emission reduction policy scenarios. mechanisms over green-certificate-trading mechanisms in terms of emission reduction. It is important to note that in the four carbon-emission reduction policy scenarios, the load allocation rate of the HP-RIES remains relatively stable, without any significant changes. This is because HP-RIES is a hybrid system, with both renewable and fossil energy sources, and the market mechanism plays a neutralizing role in load regulation within the HP-RIES system. To analyze the impacts of carbon emission reduction policies on carbon emissions and economic costs, Figure 12 presents the variations in the carbon intensity and marginal abatement costs in the different market-based carbon emission reduction policy scenarios. It can be observed that the carbon intensity ranks from the highest to the lowest as the Reference-CRIES, GC-CRIES, CET-CRIES, and GC+CET-CRIES scenarios. Compared to the Reference-CRIES scenario, the GC+CET-CRIES scenario reduces the carbon intensity by 35.35%, 33.96%, 34.43%, 32.89%, and 31.36% in the years 2020, 2025, 2030, 2035, and 2040, respectively. It is worth noting that GC-CRIES exhibits a significantly higher carbon intensity compared to those of CET-CRIES and GC+CET-CRIES, while the difference between CET-CRIES and GC+CET-CRIES is minimal. As for the marginal abatement costs, the scenario without any policy intervention shows significantly higher costs, while the differences among the other three scenarios are relatively small, ranked from the highest to the lowest as GC+CET-CRIES, CET-CRIES, and GC-CRIES. Considering the above observations, it further confirms that carbon trading has a more significant emission reduction effect compared to green certificate trading, albeit with a slight increase in abatement costs.