## 2 Operation Mode of Renewable Energy (RE) and Battery Switch Station (BSS)
### Joint Operation Mode  
The RE company would aggregate distributed wind power, photovoltaic and other RE resources in its jurisdiction. The RE is divided into two parts according to the purpose. One part of the RE is sold to the distribution company at a purchase price determined by the distribution company. In order to guide the RE's output in each period, the distribution company adopts the TOU pricing strategy. The RE company will try to increase revenue by selling electricity to the distribution company during the high-price periods. The other part of the RE is sold to BSS to charge the batteries at the charging price determined by the RE company. In order to take advantage of the BSS's energy storage capabilities, the BSS is allowed to sell the electricity stored in the batteries back to the RE company for a profit, and the discharging price is also determined by the RE company. Therefore, the energy flows between the RE company and the BSS in both directions. In order to guide the charging and discharging behavior of the BSS reasonably, the TOU pricing strategy is adopted by the RE company to determine the charging price and the discharging price. To facilitate the subsequent analysis, we assume that the charging price and discharging price are the same during a same time period, which are collectively referred to as charge/discharge prices. The operating mode of the system is shown in Figure 1.  
Since the RE company and BSS belong to different investors, there is no subordinate relationship between them. Although the RE company has no right to directly control the charging and discharging behavior of the BSS, it can indirectly affect the charging and discharging decision of the BSS by adjusting the charge/discharge prices. After obtaining the next day's RE purchase prices for each period of time decided by the distribution company, the RE company will set the charge/discharge price for each period of the next day accordingly. On the one hand, it could guide the BSS to operate as a controllable load to promote the consumption of RE. On the other hand, the BSS would be regarded as a special ESS, which is allowed to discharge in reverse in order to increase the electricity sales revenue of the RE company by load shifting.  
The BSS will set the charge/discharge schedule according to the battery swapping demand after the charge/discharge price is decided by the RE company. The BSS could maximizes its revenue in the following ways: On the one hand, BSS will charge the batteries at low-price periods to satisfy the battery swapping demand with the charging cost as low as possible; on the other hand, the BSS will store some RE at the low-price period and sell it back to the RE company at the high-price period, using the difference between the charge/discharge prices at different times to make a profit. In order to realize the zero carbon emission of EVs, BSS signs a cooperation agreement with the RE company that BSS could only charge the batteries by RE.  
Under the framework of this cooperation, the distribution company does not obtain economic benefits through direct participation in cooperation. In fact, its benefits lie in the increased controllability of RE generation through the collaboration of the RE company and BSS. Since the distribution company could indirectly adjust the output power of RE by setting the RE purchase price, the impact of randomness and uncontrollable characteristics of RE on the grid could be reduced.  
Figure 1: Block diagram of cooperation between renewable energy (RE) company and battery switch station (BSS).