## 5 Discussion
### The Effects of CSP and EH in Energy Services
#### 5.1.1 The Influences of CSP and EH on Unit Commitment  
Figure 7 shows the unit commitment decisions based on stochastic scenario 3.  
In Strategy 1, thermal units are the only controllable resources to cope with the renewables uncertainty. The power output of thermal unit is constrained by the ramping limit, the operation space and the reserve requirement. It is noted in Figure 7a that thermal units need to ramp up quickly to counterbalance the decreased power output of the PV during the late peak hour. Therefore, as can be seen from Figure 7, in order to meet the power output constraints, large number of generators need to be turned on and operate at low power during abundant solar energy periods, creating the minimum generation constraint. The minimum generation constraint could be figured out as a flat line from 12:00 to 16:00 in Figure 7a. This operation not only increases the generation cost but also influences the penetration of renewable energy.  
In Strategy 2, the dispatchable resource CSP is incorporated into the power system. Comparing Strategy 1 with Strategy 2 in Figure 7a-c, the conclusions could be summarized as follows. Firstly, as shown in Figure 5a and the related analysis, the solar power-driven PV and CSP station are complementary in their capability to serve different parts of load demand. It is noted in Figure 7a that the incorporation of the CSP could satisfy the load need in the late peak hour and thus the ramp rate and the range requirement of thermal units are reduced considerably from around 1200 MW to 800 MW in a few hours (16:00-20:00).  
Figure 6: CSP reserve planning results at stochastic scenario 3, including: (**a**) the total scheduled up reserves and reserve deployment in CSP stations; (**b**) the total scheduled down reserves and reserve deployment in CSP stations.  
Secondly, as can be seen from Figure 7a, flexible CSP could reduce the minimum generator constraint from about 1100 MW in Strategy 1 to 800 MW in Strategy 2. In the short term, the reduction means that fewer thermal units need to operate at low power at noon, which is illustrated in Figure 7b,c. In the long term, the capability of CSP to provide firm capacity could potentially alternate the inflexible base load generating sets. The reduction of requirements for thermal units could accommodate greater utilization of renewables, and decrease the renewables spillage. The detailed comparison of renewables spillage is shown in Figure 8.  
Finally, because of the power fluctuation of renewables, fast start-up and shut-down of the thermal units from 4:00 to 7:00 are worth noting in Strategy 1 as shown in Figure 7a,b. However, the incorporation of the CSP could smooth the power output of thermal units through controlling the large-capacity TES system. Therefore, the constant number of thermal units are needed in Strategy 2 which significantly reduces the start-up costs and has economic benefits.  
In Strategy 3, the EH is equipped with a CSP station. From Figure 7a, the peak-valley difference in Strategy 3 is the minimum among the three strategies (Strategy 1: 1160 MW, Strategy 2: 986 MW, Strategy 3: 748 MW). During the daytime when the renewables resources are abundant and the electricity price is low, the EH equipment is employed to convert redundant power output to thermal energy stored in the TES for further use. Although the added EH would slightly increase the fuel costs  
Figure 7: Unit commitment results in three strategies: (**a**) the power supply of traditional thermal units; (**b**) the generation schedule of thermal units; (**c**) the number of thermal units turned on.  
of thermal units in the daytime, it could further utilize the room of the TES and decrease the generation cost and electricity cost during the late peak hour.