Nowadays we are facing a considerable increase in demand for renewable sources of energy. This might be due to factors such as the rapid increase in terms of energy consumption (has become a tendency since Industrialization), the limited existence of fossil fuel resources or the soaring cost of fuels itself. The constant degradation of the environmental condition reinforces that necessity. In the recent times, renewable energy sources are considered alternative for the fossil fuel energy sources.

The use of renewable sources

 

“Renewable resources” are, by definition, all the natural components that can be obtained from Earth (its atmosphere or ground for example). Among these resources, renewable energy is included. It may comprehend a big variety of sources such as sunlight or solar energy, wind, biomass, green energy, tides or geothermal energy.

 

The most common application of renewable energy is the generation of electricity. Photovoltaic and Wind are considered the more promising renewable energy sources and are widely used in many countries for standalone applications or connected to a utility grid.

 

Considering the mentioned sources of renewable energy, some can be used directly to produce energy while others need to be converted in order to do so (indirect production). One of the advantages of renewable energy facilities is the fact that they generally require less maintenance when compared to traditional generators. Since they produce almost none or very few amounts of waste products (cases of carbon dioxide or other chemical pollutants originated by combustion), they have a much inferior impact on the environment.

Hybrid Systems

 

The unpredictable pattern of natural resources requires a combined utilization of sources in order to provide uninterrupted and reliable power supply to its users (technical difficulties may arise due to uncontrollable weather conditions like wind speed fluctuation, day & night behaviour, and summer and winter sun conditions. In other words, renewable energy is usually dependent on the weather conditions for its source of power input. For example, a hydro generator depends on rain to fill dams to supply flowing water, a wind turbine depends on the wind within a specific range of speed to turn the blades and run the turbine, and a solar panel depends on the position of the sun and, preferentially, no clouds to make electricity.

 

A combination of two or more renewable energy sources is more effective than the single source system in terms of cost, efficiency and reliability. We can easily reduce the need for fossil fuels by properly choosing a combination of renewable energy sources.

 

The combination of two or more energy sources, working together in order to compensate for each other is designated as a Hybrid energy system. The main advantage of a hybrid energy system is the enhancement of reliability of the hybrid energy system and cost-benefit of the system. Due to the fact that some renewable energy sources such as Solar Radiation and Wind are, most of the times, intermittent, and they are frequently combined with other power sources such as utility grid and diesel generators. The objective is to ensure the continuous supply of power.

 

Nowadays two types of hybrid systems are in operation: one combining the only production from renewable energies, which are ideal for applications in isolated systems and a second type that makes use of the production from diesel and gas generators.

 

Solar-Wind hybrid systems have as the main advantage the way they complement each other and the fact that they are exclusively renewable energy resources. The behaviour of solar radiation throughout the day follows an approximately constant pattern of production, reaching its peak at noon decreasing until sunset. The wind generator can serve as a complement to the system, in the periods where there is low or non-existent solar radiation. This characteristic gives this type of system a greater reliability in the matter of the continuity of electrical production over time.

 

Solar-wind-diesel/gas or simply solar-diesel/gas hybrid systems work similarly to the mentioned solar-wind system. This type of system has the advantage of reducing the consumption of fossil fuels. It is even more reliable, because diesel or gas generators work as a backup, thus ensuring the operation of the system even in periods when the remaining energy sources are not available or are not enough to guarantee the energy required.

 

Figure 1 – Hybrid Systems. On the left, a scheme of a solar-wind hybrid system and, on the right, a scheme of a solar-wind-diesel/gas hybrid system

 

Around the world, thousands of communities and industrial sites are however not powered by the utility grid. The traditional means of electrification in such locations is Diesel generation since it is a more conventional technology and more people trained in operation and maintenance.

 

Wind turbines and solar panels are the better known renewable energy devices used in hybrid power systems. Hybrid systems usually include energy storage, so they can deliver a certain amount of energy on demand. These systems provide a high degree of energy security through the mix of generation methods and can often incorporate a storage system (battery and/or fuel cell) or to ensure maximum supply reliability and security, a small fuelled generator (non-renewable).

 

Hybrid power systems exhibit higher reliability and lower cost of generation than those that use only one source of energy.

Case Studies

 

Some studies were conducted concerning the techno-economic assessment of an autonomous hybrid PV/diesel hybrid power system installed in a bungalow complex in Elounda, Crete. In remote areas which are far from the grids, electricity is supplied either by diesel generators or small hydroelectric plants. Under such circumstances, the supply of fuel becomes so expensive that hybrid diesel/photovoltaic generation becomes competitive with diesel-only generation.

 

The use of hybrid power systems in industries is becoming so usual and so advanced that in some cases its users started to require to design and model standalone systems with software such as MATLAB/SIMULINK, to evaluate their performance.

 

One example of such a study was considered to establish a comparison of a hybrid power system based on PV module, wind turbine and diesel generator with a PV/wind/battery hybrid power system. From the simulation results, it was identified that a hybrid connected system with the battery can perform better in some ways than the diesel connected system.

Implemented Hybrid System

 

As mentioned before, the industry is adapting and there are many cases already of implemented Hybrid Systems. In Jamaica, for example, it is installed as the world’s biggest hybrid system. It was installed by WindStream Technologies in an area very close to the coast and takes advantage of winds with an average speed around 96.5 km/h.

 

With this system, an output production of 106 MWh/year and a return of investment in less than 4 years are expected. The plant is designed to save the company approximately $2 million in energy costs over 25 years. The hybrid energy system consists of 50 SolarMills (patented product with solar panels and vertical axis turbines) making it the largest facility in the world.

 

EFACEC has also developed a Hybrid System Platform (called EFASOLAR3) which has the capability of integrating several energy sources such as diesel, photovoltaic, electrical grid, energy storage, wind and hydro. This system can be implemented in an existing installation or in a new project, ensuring a stable operation and optimizing the energy produced by the photovoltaic plant.

Hybrid Systems in Portugal

 

A hybrid system combining floating photovoltaics and hydroelectric power generation has been installed in the Alto Rabagão dam in Portugal (by EDP Group).

 

FPV systems are producing energy on areas that are unused otherwise. The pioneer project has an installed capacity of 220 kW and is expected to produce 332 MWh.

EDP Group Hybrid System

 

Figure 2 – EDP Group Hybrid System

Conclusion

 

Large-scale hybrid systems and solutions combined with heat pumps should be the main bets of the solar thermal industry for years to come.

 

The ISOL Index study conducted annually by the German Soll Agency in 18 of the world’s leading solar thermal markets analyses the responses of more than 340 manufacturers and distributors of the thermal solar present in these countries, in order to evaluate the business climate and expectations for this market. Portugal is one of the markets analysed. The expectation is to identify an increase in terms of this type of systems, due to the quick return of investments, cost savings and use of renewable sources than comparatively to fossil fuels don’t have a limited existence.