## 1 Introduction  
Over the past ten years, power generation from coal has been responsible for more than 40% of the global energy production [1]. The International Energy Agency (IEA) estimates a further increase in coal-based electricity production due to the industrialization of developing countries [2]. At the same time, concerns related to climate change [3; 4; 5], resource depletion, as well as supply insecurity and fossil fuels' price volatility are growing [6]. These aspects have led to the development of certain protocols and climate agreements between countries, as for example the Kyoto Protocol and the Paris Agreement [7; 8]. In Spain, renewable electricity production decreased during the recent economic crisis, while coal use increased. This situation has been changing in the last few years; in 2016, coal-fired power generation constituted 14.5% of the electricity generation in Spain, while the respective figure for renewable energy was 41.1% [9]. Despite the growing importance of renewable energy, it still has drawbacks that need to be addressed. The main disadvantages of renewable technologies are the lack of continuity in the supply, relatively high cost, and system complexity [10; 11] when compared to fossil fuel plants. Hybrid power plants appear as a promising alternative that could address current energy challenges, both in the short and medium term. Hybrid plants are based on the combination of two or more resources with the goal of benefitting from the advantages of each of the incorporated technologies. Hybrid plants operate with different energy sources that complement each other to obtain a desired power output [12; 13]. Hybrid plants may combine conventional fossil fuel power plants with renewable energy plants or combine two or more renewable energy sources [14]. Some of the possible technologies that could form a hybrid power plant are wind and solar photovoltaic (PV); hydropower and solar PV; concentrating solar thermal and coal/natural gas; wind and coal/natural gas; coal/natural gas and biomass; wind and diesel; or wind, solar PV, and diesel. By using complementary energy sources in the same power plant, the hybrid plants can reduce emissions related to the combustion of conventional fuels, alleviate random behavior [15; 16], and achieve greater supply stability [2; 17; 18; 19]. In the last few years, several research articles on the hybridization of coal-fired power plants with renewable energy sources have been published in the literature. [PERSON] et al. [2] and [PERSON] et al. [20] proposed the hybridization of coal plants with fuel cells. [PERSON] et al. [21] studied the effect of coupling wind energy with coal. [PERSON] et al. [22] studied the design of a hybrid photovoltaic-wind-diesel energy system at coastal areas in Bangladesh in order to achieve the maximum power output from renewable sources while keeping the cost of electricity to a minimum. [PERSON] et al. [23] made a techno-economic analysis of a smart grid solar-wind-diesel system to support a small community in Brisbane (Australia), concluding on its economic and environmental feasibility. Concerning the hybridization of coal plants with biomass, [PERSON] et al. [24] analyzed the range of bioliquids that can be burned with coal, and [PERSON] et al. [25] considered the torrefraction of biomass and coal. Nevertheless, the most developed, examined alternative for coal hybridization is concentrated solar power that focuses radiation on small areas (receivers) through mirrors [26]. With regard to concentrated solar power, there are four available options: the parabolic through, the Fresnel reflectors, the dish Stirling, and the solar power tower [27]. [PERSON] et al. [28] designed a dynamic model of a hybrid plant combining concentrated solar and combined cycle technologies, and made a comparison with a conventional combined-cycle power plant using the case study of a plant in Almeria, Spain. [PERSON] et al.