Integrated Potential for Wind and Solar Energy in the Context of Sustainable Development of the Coastal Regions of Bulgaria ^†

Abstract

This study presents a comparative analysis of the potential for combined use of wind and solar energy in nine key coastal settlements on the Bulgarian Black Sea coast—Shabla, Balchik, Varna, Byala, Obzor, Nesebar, Burgas, Primorsko, and Tsarevo—selected for their diverse geographical and meteorological characteristics. The study evaluates the feasibility of implementing hybrid renewable energy systems by analyzing the average annual solar radiation and wind velocity for each location. A methodology based on physical and technical parameters is applied to determine the required installed capacity of photovoltaic systems to meet the average annual household electricity consumption of 6000 kWh. Concurrently, wind energy potential is assessed through theoretical and practical models using two turbine sizes (3 m and 6 m in diameter), which represent small-scale residential wind applications.

1. Introduction

With the accelerated depletion of fossil fuels and the need to limit carbon emissions, renewable energy sources (RES) are becoming a strategic priority for the sustainable development of the energy sector. Among them, wind and solar energy are the most affordable and technically mature technologies, suitable for both centralized energy and decentralized systems with local application [1].

Small wind turbines, especially those with a capacity of less than 10–20 kW, are used in the domestic sector, rural areas and off-grid systems. They offer the opportunity to reduce dependence on the electricity grid and ensure partial or complete energy independence. However, a major challenge in their use is the variability of the wind, which limits production in some periods [2,3,4].

To overcome these limitations, hybrid solutions combining wind and solar (photovoltaic) energy are increasingly being applied. Solar energy is particularly suitable for areas with seasonal activity—such as the Bulgarian Black Sea coast—where in the summer months, when the wind is weaker, solar panels compensate for the energy deficit [5,6,7]. This achieves a more even and predictable energy supply throughout the year.

The Bulgarian Black Sea Coast has favorable natural and climatic conditions for the development of renewable energy sources, especially wind and solar energy. The geographical location, proximity to the sea and open terrain create good conditions for constant air flow, while high levels of insolation guarantee stable solar radiation throughout most of the year [8,9,10].

Combining wind and solar energy in hybrid systems offers an optimal balance between the seasonal variability of the two sources. In winter, when sunlight decreases, wind activity along the Black Sea coast increases, and vice versa—in summer, solar production dominates. This allows the construction of energy-autonomous sites that can cover a large part of their own consumption without the need for expensive storage or backup generators.

This study performs a comparative analysis of the efficiency of wind turbines with a diameter of 3 m and 6 m in nine coastal settlements in Bulgaria. The annual electricity production, economic savings and the return on investment period are assessed. While many studies look at large RES parks or general regional indicators, this article focuses on an “average household” with consumption of ~6000 kWh/year and shows what sizing a ~4–5 kWp system in a coastal location would look like—a practical approach that is more accessible to households or small businesses.

2. Methodology of the Study

2.1. Methodology

The calculation of wind energy is based on the estimation of the kinetic energy of air masses passing through the area covered by the rotor blades of the wind turbine. The quantitative analysis process includes several stages related to physical dependencies, meteorological data, and technical characteristics of the facility.

The theoretical power P that can be extracted from the wind is calculated using the following formula:

$$P={\displaystyle \frac{1}{2}}\rho V^{3}A$$

(1)

where P—theoretical wind power (W), ρ—air density (kg/m³), A—the area through which the air passes (m²), i.e., the rotor area, V—wind speed (m/s).

According to Betz’s law, no more than 59.3% of the kinetic energy of the wind can be converted into mechanical energy by a wind turbine. This leads to the introduction of a coefficient of efficiency:

$$P_{\max }=0.593{\displaystyle \frac{1}{2}}\rho V^{3}A$$

(2)

However, in practice, the actual efficiency of modern turbines is around 35–45% of the total kinetic energy of the wind, due to technological and aerodynamic limitations.

The actual electrical power is obtained by multiplying the theoretical power by the overall efficiency coefficient $C_p$:

$$P_e=C_p{\displaystyle \frac{1}{2}}\rho V^{3}A$$

(3)

where $C_p$—overall efficiency coefficient (usually between 0.3 and 0.45).

To calculate the total amount of electricity produced for a certain period, the following formula is used:

$$E=P_e.t$$

(4)

where E—electricity produced (Wh or kWh), t—operating time (in hours).

The calculation of solar energy follows the following methodology.

For each location, the required installed capacity (in kWp) is determined to ensure that the system’s annual production covers a consumption of 6000 kWh. The calculation is performed using the following equation:

$${\mathrm{P}}_{\mathrm{r}eq}={\displaystyle \frac{E_{year}}{H\eta }}, kWp$$

(5)

where P_req—required installed power (kWp), H—average annual solar radiation (kWh/m²/year), η—system efficiency (≈0.75–0.90, depending on losses).

2.2. Experimental Data

For the practical application of the methodology, the following input data are required:

• Temperature and pressure (Figure 1 and Figure 2).
• Average wind speed at a certain height; in our case, 10 m (Figure 3).
• Geographical location and altitude (affect air density) (Figure 4).
• Rotor diameter (or radius)—most often, turbines with a power of 1–5 kW are used for a single-family house, which corresponds to a rotor diameter between 2.5 and 7 m, depending on the specific conditions.
• Average annual electricity consumption—it is assumed that an average household on the Black Sea coast consumes about 6000 kWh/year.
• Solar radiation—for each of the selected locations, approximate values of the average annual global solar radiation on a horizontal surface (kWh/m²) are used, obtained from public climate databases and literature (

  Table 1. Solar radiation at each point.

  Town	Solar Radiation
  	kWh/m2/year
  Shabla	1450–1550
  Balchik	1450–1550
  Varna	1250–1350
  Byala	1450–1550
  Obzor	1450–1550
  Nesebar	1450–1550
  Burgas	1500–1600
  Primorsko	1450–1600
  Tsarevo	1450

  ).
• System Efficiency Coefficient—the actual system output is assumed to be 90% of the theoretical maximum, taking into account losses from temperature, contamination, cable resistances, and non-ideal orientation.

3. Results and Discussion

3.1. Analysis of the Results

Based on the methodologies described in Section 2, an assessment of the possibilities for using wind and solar energy for domestic use for a single-family house has been made.

Figure 5, Figure 6, Figure 7 and Figure 8 present the results of the calculations of the theoretical and actual electrical power of wind turbines with diameters of 3 m and 6 m in nine coastal Bulgarian cities. The data are based on average annual wind speeds for each location and reflect both ideal conditions (without losses) and a realistic scenario with the efficiency factor included.

With a small rotor diameter (3 m), the theoretical power reaches maximum values in the northern part of the Black Sea coast—Shabla, Balchik, Varna and Byala—where it varies between 130 and 180 W. In the southern regions such as Burgas, Primorsko and Nessebar, the power drops below 80 W, which indicates a relatively weak wind energy potential in these areas.

After taking into account the real efficiency of the turbine (in this case 58%), the power values drop, as expected, by about 60–65%. However, the trend between cities remains the same—the northeastern regions retain their advantage. The actual power in Shabla and Balchik reaches approximately 100–120 W, while in Burgas and Primorsko it is below 50 W.

Increasing the rotor diameter to 6 m leads to a fourfold increase in the rotor area, which also drastically increases the theoretical power. In Shabla, Balchik and Byala, the values exceed 600–700 W. Even in less windy areas such as Obzor and Nessebar, a twofold increase is observed compared to the previous scenario.

Under realistic conditions and D = 6 m, the wind turbine could provide 400–500 W in cities with an average wind speed above 3.2 m/s. This makes the project for partial household electricity supply technically feasible with a suitable location. At the same time, in the southern Black Sea cities, the values remain low and unjustified from an economic point of view.

The required installed capacity of a photovoltaic system to cover the average annual electricity consumption of a household is calculated using the following data. Average consumption of a household: 6000 kWh/year. Average efficiency of a photovoltaic system: 90% (0.9).

Figure 9 and Figure 10 present the results for the annual energy of each of the considered systems, including a small photovoltaic power plant with a capacity of 6000 kWh/year.

All locations show energy yields in the range of 6000 to 6900 kWh/year, which suggests that cumulative production from several turbines is being considered or extrapolation has been made. Shabla, Balchik and Varna again stand out with the highest values (~6900 kWh), which is in line with their higher average annual wind speeds. Burgas, Primorsko and Nessebar remain at the bottom of the ranking (~6200 kWh/year).

Even with a smaller rotor diameter, location has a significant impact on the annual energy yield. The difference between the best and the weakest places is about 700–800 kWh per year, which is significant in long-term investment planning.

With a larger rotor (6 m), all annual production values increase significantly—in the range of 9000 to almost 12,000 kWh/year. The leaders are again Shabla, Balchik and Varna, where the yield exceeds 11,500 kWh per year. In the southern part of the Black Sea coast (Burgas, Primorsko, Nessebar), it reached below 10,000 kWh/year, which again indicates the lower local potential.

Increasing the diameter from 3 m to 6 m leads to an almost twofold increase in annual electricity production, which is logical due to the quadratic dependence of the area on the radius. This makes the investment in larger rotors economically more expedient—especially in areas with good wind conditions.

3.2. Economic Efficiency Analysis

Table 2. Annual savings and payback period at D = 3 m.

Location	Annual Saving	Payback Period
	BGN	Year
Shabla	2551.2	9.79931
Balchik	2408.85	10.3784
Varna	2380.38	10.50252
Byala	2408.85	10.3784
Obzor	2069.4	12.0808
Nesebar	1992.75	12.54548
Burgas	1795.65	13.92254
Primorsko	1806.6	13.83815
Tsarevo	2036.55	12.27566

and

Table 3. Annual savings and payback period at D = 6 m.

Location	Annual Saving	Payback Period
	BGN	Year
Shabla	2551.2	8.669225805
Balchik	2408,85	8.904898459
Varna	2380.38	9.326213486
Byala	2408.85	8.908445029
Obzor	2069.4	9.147866112
Nesebar	1992.75	9.255607049
Burgas	1795.65	9.489055188
Primorsko	1806.6	9.474235568
Tsarevo	2036.55	9.22580455

present the results of calculating the annual savings and the payback period, taking into account the following values: electricity price of 0.25 BGN/kWh; an investment of BGN 10,000 for 3 m; and BGN 25,000 for 6 m.

The results show that the annual savings from using a wind turbine are directly related to the local wind characteristics. In the most favorable areas—Shabla, Balchik, Varna and Byala—savings between BGN 2380 and 2551 per year are achieved, while in less windy areas such as Burgas and Primorsko the value drops to around BGN 1800.

The time required to recover the initial investment shows significant differences between the two cases:

• at D = 3 m: best return: Shabla—9.80 years; weakest: Burgas—13.92 years;
• at D = 6 m: best return: Shabla—8.67 years; weakest: Burgas—9.49 years.

4. Conclusions

The analysis shows that combining wind and solar energy offers an effective solution for sustainable power supply in the coastal regions of Bulgaria. With a rotor diameter of 6 m, an almost double increase in annual electricity production is observed compared to 3 m, which significantly improves the economic return. The best results are achieved in the northeastern regions, where wind speed and solar radiation are optimal. Calculations for annual savings and payback period show that such an investment is profitable in the long term, especially when implementing a hybrid system. The study supports the need for strategic development of decentralized energy solutions in coastal regions and demonstrates their applicability in the context of energy transformation and climate sustainability. The results show that the northeastern part of the coast (Shabla, Balchik, Byala) has the highest potential for electricity production, as well as the best economic parameters for return on investment. The combined use of wind and sun is an optimal solution for sustainable and independent energy supply to households in the region. The results can serve as a basis for energy planning in small settlements and municipalities along the Black Sea coast, which aim for sustainable development and energy independence. The study can be used by home users, designers and RES installers for realistic sizing of autonomous or semi-autonomous systems.