Deregulation in the power sector, liberalization of the electricity market, tendency to connect markets across Europe, introduction of incentives for construction and use of renewable energy sources (RES) and - consequently - their increasing presence in electricity systems, have taken a decisive impact on wholesale electricity prices. In addition, the introduction of CO2 taxes has further increased overall electricity prices. On the other hand, it reduced the profits of fossil fuel thermal power plants (TPPs). Also, in order to meet the prescribed requirements in the field of environmental protection, TPPs must apply pollution prevention systems that are very expensive, both in terms of investment and exploitation. As an alternative, investments in the hydro sector should be considered. However, this option is characterized by other issues; In contrast to the long-term construction and exploitation of hydro capacity, there are fluctuations in electricity prices on the day-ahead market which affect the work schedule and revenues of these facilities. To investigate this problem, hourly and daily prices in the regional electricity market were analysed. Trends in wholesale electricity prices on the day-ahead market have been determined. Their relevant values were estimated and used as one of the sets of input data for cost-benefit analysis and sensitivity analysis of the case study - future reversible hydropower plant (RHPP) "Bistrica". The impact of fluctuations in electricity prices on the day-ahead market was emphasized. The aim of this research was to draw a conclusion as to whether the movement of these prices is in favour of the construction of possible RHPPs in Serbia. Based on the conducted analyses, it can be concluded that the upward trend in the level of wholesale electricity prices (which began in 2019 and is expected to continue in the future considering the EU regulatory policy regarding CO2 emissions taxes) may positively affect the profitability of investment in RHE Bistrica (i.e. to shorten the payback period), but not so dramatically, since with the increase in electricity prices, the costs for pumping also increase.

Increased climate changes and environmental pollution cause a decrease of available quality water quantities and increase differences in precipitation amounts by seasons. The question is how to provide sufficient quantities of water to supply the population, agriculture, industry, and electricity generation. Sources of quality water are most often located in areas far from the centers of consumption, so it is necessary to construct large water management systems. In order to mitigate the variations in the available quantities of water during the year, reservoirs of significant, seasonal capacity must be built. The power system stability is additionally endangered by the growing impact of renewable sources with extremely variable electricity generation, which is manifested both on an intraday and seasonal level. Therefore, seasonal accumulations, with multi-purpose and even multiple use of the same amount of water, represent a winning option and a stronghold on which power supply, economic and social security and stability should be built. In the case of Serbia, apart from seasonal ones, there are also a spatial differences in the quantities and quality of water in the river basins. In the areas of Zapadna and Velika Morava, Šumadija, Kolubara and Belgrade, the required quantities of water for the supply exceed their own resources. On the other hand, the Drina basin is rich in water of significantly higher quality, and the possibilities of leveling the waters of river Lim are extremely limited. This paper reaffirms the old idea of building a multi-purpose hydro system „Lim-Zapadna Morava“, which would solve these problems. Beside those in power industry, other positive effects that would be achieved by the realization of this strategic project are also presented in this paper.