## 3 Results and Discussion
### Electricity Prices  
Generally, larger shares of variable renewable energy in the demand (\(f_{\rm VRE}\)) lead to lower electricity prices, especially when coupled with low fossil fuel generation [68]. This is a direct consequence of the European electricity marked design, which is based on marginal costs and the merit order principle. Figures 9 and 10 show the relationship between day-ahead electricity prices in Croatia to the shares of fossil fuel (\(f_{\rm fossil}\)) and variable renewable energy (\(f_{\rm VRE}\)) generation in the EU-27, during 2024.  
Electricity prices in Croatia increase with the share of fossil fuel generation but decrease with the share of variable renewable energy generation. In 2024, wind and solar PV generation was very low on several instances, when dark and windless weather conditions engulfed the European continent, causing tight electricity supply. These events are called _Dunkelflaute_ in the German language and are characterized by large-scale high-pressure weather systems and very dense cloud cover [69; 70]. Most recently, the _Dunkelflaute_ event occurred on 5-7 November and 11-12 December 2024.  
Generally, the _Dunkelflaute_ event lasts only a few hours; however it is accompanied by tightening electricity supply and distinct spikes in electricity prices. In Croatia, electricity prices exceeded 600 \(\upbar\)/MWh for a total of 14 h during the _Dunkelflaute_. During the heatwave of July 2024, wind and solar PV generation were average in Croatia; however high cooling demand drove electricity prices between 300 and 600 \(\upbar\)/MWh for a total of 31 h. High electricity prices are a signal that flexible generators and consumers should  
Figure 8: Annual renewable oversupply as a function of the wind-to-solar PV capacity ratio (\(P_{\rm wind}/P_{\rm solar}\)). Data projected for Croatia in 2030.  
be promoted, along with improved interconnections between market areas. Low-carbon dispatchable capacity and long-term storage systems should be developed to effectively manage heatwaves and _Dunkelflaute_ events.  
When wind and solar PV generation is high (\(f_{\rm VRE}\geq 50\%\)) and fossil fuel generation is low (\(f_{\rm fossil}<15\%\)), electricity prices are low and can even drop below zero. Negative electricity prices are driven by the rapid expansion of renewable energy, especially solar PV and wind, coupled with limited grid flexibility and insufficient storage capacity. It happens because inflexible generators choose to continue producing at negative prices rather than ramp down and restart at a later hour. Negative electricity prices typically occur in the afternoon hours of summer months when energy demand is low, but solar PV generation is very high, and occasionally at night during winter months due to high wind output. In recent years, the frequency of negative electricity prices has increased, and these periods have also prolonged [71].  
Figure 10: Relationship between electricity prices in Croatia and the share of fossil fuel and VRE generation in the EU-27, data for nighttime hours (22:01-7:00) during 2024.  
Figure 9: Relationship between electricity prices in Croatia and the share of fossil fuel and VRE generation in the EU-27; data for daytime hours (7:01-22:00) during 2024.  
As shown in Figure 10, electricity prices during nighttime hours (22:01-7:00) are more stable than daytime prices. Furthermore, nighttime prices are better correlated to the share of fossil fuel generation. Generally, the supply side matches better the demand side, overnight, and baseload power plants operate without excessive oscillations from VRE supply. Electricity prices range between 0 and 200 \(\upupup\)/MWh for most of the time, but electricity prices below 50 \(\upupup\)/MWh occur exclusively when the share of VRE is \(f_{\mathrm{VRE}}\geq 20\%\).