Croatia has a total installed electricity generation capacity of 3,763 MW, including 2,097 MW of hydropower and 1,666 MW from thermal power plants (including a nuclear power plant, jointly owned with Slovenia) [1]. The domestic demand exceeds domestic production and about 30% of additional power is covered from imports. The Croatian government intends to restructure, liberalize and privatize the energy sector to emphasize compatibility with the European Union, whilst also planning to increase the domestic production capacity, especially with natural gas and coal as the fuel [2]. For Southern Croatia, the installation of an 800 MW coal-fired power plant at Ploče is proposed, albeit severely opposed by eco-activists [3], [4]. The main concerns relate to the potential of the Neretva river valley for ecological food production, to the possible pollution of the river, to the emission of hazardous pollutants (mainly fine dust) and to the negative impact of such a power plant upon the tourism in the region. The thermal power plant will moreover necessitate the transport of hundreds of tons of coal per day. Although modern coal-fired thermal power plants implement highly efficient techniques for dust abatement and for the reduction of common pollutants (e.g. SO₂), the CO₂ emissions of a coal-fired power plant remain a major issue. Unless an expensive CO₂ capture is foreseen and depending on the quality of the coal, CO₂ emissions will vary between 0.5 and 1 ton CO₂/MWh.

As elsewhere, public concern, mostly about adding coal-fired power plants, has generated uncertainty in the electricity market, whilst creating a very favorable context towards new Renewable Energy projects, with a focus on using solar energy [5]-[7].

In March 2007, the European Union targeted 20% renewable energy for 2020, with special emphasis on small scale units.

Due to the attractive solar irradiation in Southern Croatia, new power plant projects offer the possibility to build them as hybrid plants by adding solar energy within the production. Such a hybrid solution, where solar energy will partly replace the coal resource, will not only reduce the pollutant emissions (including CO₂), but will moreover create an additional tourist attraction pole, as is the case with the Gemasolar solar power plant in Southern Spain [8]. An additional advantage results from the fact that the power block (boiler, turbines) will be available within the thermal power plant, thus only the investment in heliostats, solar receiver and heat carrier circuits need to be considered.

The present paper investigates the potential of solar energy in the Southern Croatian Dubrovnik-Neretva county and its possible contribution in a hybrid set-up. Such a hybrid solution of a traditional fossil fuel fired thermal power plant with a supply of solar energy during the daytime and during the night time if thermal energy storage is foreseen, will certainly create a more positive assessment by the public in comparison with a sole fossil fuel power plant.

To enhance the rate of this solar energy development, it is necessary to update the insights, the tools and the technical/economic analysis. Within the solar energy technologies, photovoltaics (PV) to a large extent and Concentrated Solar Power technology to a lesser extent, have been widely investigated and applied. PV draws a significant focus with a guaranteed future in view of the ongoing technical improvements and cost reduction [7], [9], [10]. Concentrated Solar Power Plants are gaining increasing interest, mostly by using the Parabolic Trough Collector (PTC) system, but with Solar Tower Collectors (STC) progressively occupy a significant market position. The large-scale STC technology was successfully demonstrated by Torresol in the Spanish Gemasolar project on a 19.9 MW_el scale [8]. The varying solar radiation flux throughout the day and throughout the year remains a main problem for all solar energy technologies. A more efficient operation requires the incorporation of two technologies, i.e. Thermal Energy Storage (TES) to cover non-sunny periods and Backup Systems (BS) or Hybrid Operation when solar energy or TES supply are not available. The combination of both systems facilitates a successful continuous and year round operation. To determine the optimum design and operation of the CSP throughout the year, whilst additionally defining the capacity of TES and required BS, an accurate estimation of the daily solar irradiation is needed, accounting for hourly irradiation fluxes and direct irradiation, since a CSP plant will only accept Direct Normal Irradiance (DNI) in order to operate.

Most of the countries, except those above latitude 45 °N or below latitude 45 °S, are subject to an annual average irradiation flux in excess of 1.6 MWh/m² [6]. The procedure, outlined in the present paper, combines previous theoretical and experimental findings into a general method of calculating the hourly beam (direct) irradiation flux and applying it to the design of CSP applications.

The present paper will hence:

• Briefly review the CSP technologies and their advantages;

• Estimate the hourly beam irradiation flux from available monthly mean global irradiation data for 2 selected European locations, being Sevilla as reference for the Gemasolar project [8] and Dubrovnik, as reference for the Dubrovnik-Neretva County;

• Select an appropriate plant configuration and perform a preliminary design;

• Estimate the Levelized Electricity Cost for the hybrid potential in Southern Croatia.

Concentrated Solar Power (CSP) generates electricity by using heat provided by concentrated solar irradiation. Zhang et al. [6] reviewed available CSP technologies, including Parabolic Trough Collector (PTC), Solar Power Tower (SPT), Linear Fresnel Reflector (LFR), Parabolic Dish Systems (PDS) and Concentrated Solar Thermo-electrics (CSTe). Sunlight is reflected to a receiver (either a tower in SPT or focus heat capture pipes in PTC), where heat is collected by a thermal energy carrier (primary circuit) and mostly used via a secondary circuit to power a turbine. PTC and SPT have been built and developed around the world since 1981, with capacities ranging from 0.5 MW to more than 300 MW [6]. Additional large-scale CSPs (2014-2017) will have capacities of 100's MW_el.

Short-term, direct Thermal Energy Storage stores excess heat collected in the solar field by the Heat Transfer Fluid (HTF), thus avoiding losing the daytime surplus energy while extending the production after sunset. Long term thermal energy storage is less obvious, since it involves storage covering months of low solar irradiation. Currently, only sensible heat is stored. The significant improvement by using latent heat storage (phase change materials) or even chemical heat storage (reversible endothermic/exothermic synthesis) is under development [11]-[13]. In current TES, liquids (oils, molten salts) are used for sensible heat capture and storage. Indirect storage uses solid heat absorbers and a secondary HTF circuit. SPT plants commonly use a BS to regulate production and guarantee a nearly constant capacity. The reference Gemasolar CSP uses molten salts (<565 °C), a steam boiler and advanced Rankine cycle turbine. After discounting the parasitic energy use (mostly molten salt circulation, storage and boiler), the resulting conversion efficiency is about 16%. With novel HTF materials, such as powders, the temperature range will be extended to well above 750 °C, with reduced parasitic energy use and an expected overall conversion around 20% [14]-[17]. The measures to enhance the efficiency of CSP plants are in full development. The Ploče initiative could certainly benefit from the expected conversion increase of 20 to 25%.

The paper develops the underlying equations to calculate the daily total irradiation, the direct (beam) and diffuse irradiation. Having established the annual, monthly and daily levels of direct (beam) solar irradiation, its impact on the power yield of the CSP can be assessed. The projected overall efficiency of CSP plants was included in a CSP performance simulation. Initial simulation results are illustrated for a 12 MW_el Solar Power Tower project, with molten salts as HTF and operating in a hybrid way with the projected coal-fired thermal power plant at Ploce, in the Dubrovnik-Neretva County. In the assessed example, solar generation will account for ~45 GWh/year, with possible higher yields due to improvements in HFC, solar tower design and reduced parasitic electricity consumption. The results demonstrate the potential of adding a CSP in a hybrid operation of the coal-fired power plant of Ploče.