May 21, 2019
Hybrid systems for maximum autonomy! A hybrid system is an energy production system that uses more than one production method to meet the required energy. These systems combine electricity that comes mainly from local and renewable energy sources, such as photovoltaics, wind turbines, hydroelectric systems and biomass, exploiting the geographic advantages of the area. Applications are also frequent where a renewable energy source is used next to a conventional one, as with oil generators, batteries, and conversion generators. The purpose of these systems is to combine different energy sources in such a way that there is a constant and constant supply of energy. Hybrid systems find application in areas where network connection or fuel transportation are uneconomic options. They also provide future network connectivity and can be useful as an effective power solution in case of holiday interruptions even for specialized consumers. They are dynamic systems as they are designed to alternate between available sources of energy or even to combine them at the same time, thus depending on the minimum of exogenous factors. Under Law 3468/2006, as a hybrid system or otherwise a hybrid station is defined as each power station that:
It uses at least one form of renewable energy.
The total energy absorbed by the network on an annual basis shall not exceed 30% of the total energy consumed to fill the storage system of that system. The maximum power output of the station’s renewable energy sources may not exceed the installed capacity of the station’s storage units plus up to 20%. One of the main benefits of using hybrid systems is to ensure the reliability of the system, as renewable sources of energy depend heavily on weather conditions, which are highly varied. In addition, hybrid systems contribute to the diversification of energy sources and to de-dependence on specific conventional energy production methods. Finally, hybrid systems achieve the most economical operation of conventional units, as they either do not work at all or operate at a fixed load. Such systems provide energy of the same quality as the grid in the range of 1kW to many hundreds of kW, they can feed from a very small system, such as a transponder antenna up to whole areas. Classification of hybrid systems can be done on the basis of two factors. The former refers to whether the system will be interconnected with the grid or whether the system will be autonomous, while the second criterion refers to the size of the project and whether it concerns individual houses or entire areas. In most cases, a hybrid system is built with the aim of autonomy and especially in areas that are not economically viable or possible to connect to the grid.
Domestic hybrid systems
Standard residential hybrid systems, whether for residential autonomy or when intended to be connected to the grid, consist of a combination of a photovoltaic panel, wind energy, biomass or conventional energy sources.
Photovoltaic – Wind hybrid systems
Photovoltaic panels are a clean and reliable technology. They are superior to other technologies as they require minimal maintenance if they do not have moving parts, and their source of energy is natural light. Wind energy is the energy of wind coming from the movement of atmospheric mass gases. Greece’s total operating wind potential can cover a large part of its electrical needs. The performance of a wind turbine depends on its size and wind speed. Size is a function of the needs it needs to serve and varies from a few hundred to several million Watt. The performance of such a system that combines photovoltaic panels with a wind turbine depends directly on the wind and solar potential of each region, which vary locally and in time. The dynamics of one source can overcome the weakness of the other in a certain period of time. This is clear if we take into account the fact that, in most areas, more solar wind energy is available in the summer months, while the opposite happens during the winter. In such systems, their dimensioning is of great importance because, due to the lack of meteorological data, the assessment of wind potential is difficult, as the wind is affected by multiple factors, such as location, soil morphology, potential obstacles and other local actors. During the operation of the system, the demand for energy may be less than that produced. In this case, the energy left over from the generated wind turbine together with the energy generated by the photovoltaic panels is stored by means of power converters and battery charge regulators. By contrast, if demand exceeds the power generation from the wind turbine, then the surplus is covered by the photovoltaic, while if the desired load is still not reached, then the energy stored in the batteries is used.
Photovoltaics – Biomass
Biomass is organic matter such as various plant and forest residues, animal waste and plants from energy crops. Thus, biomass is a renewable energy source produced by plants under the photosynthesis process. The most common uses of biomass are for the production of thermal and electrical energy and can be used to meet energy needs, but also for the production of liquid biofuels, such as bioethanol and biodiesel. In Greece, the main applications concern the production of thermal energy in agricultural and forestry industries, domestic heating, and the production of biodiesel. Thus, biomass can be combined with photovoltaic panels to meet the needs of a house in electricity, but also for space heating, a process that is usually energy-intensive.
Photovoltaics – Conventional
To qualify as a hybrid system it is not necessary that both forms of energy used are renewable. A system that is extensively used is the combination of photovoltaic panels with conventional generators using either gasoline or diesel. In this case, the design of the system should be done with great care, so that the project is economically viable. A gasoline generator is more economical but has a shorter lifetime and a higher operating cost. Instead, a diesel generator has a lower purchase cost, but its use is damaging to the environment because of the gases produced by burning oil. Characteristically, at a low, virtually zero load, the oil generator uses 30% of the fuel it would use at the maximum load. This problem is eliminated by combining it with photovoltaics and batteries, so as to normalize electricity production and reduce costs. For these reasons it is preferable to use a Diesel generator. Thus, in the normal operation of the hybrid system, photovoltaics provide the required power to the load through the power converter. Additional energy from photovoltaics charges the batteries through a charging regulator until the batteries reach the maximum permissible charge level. The main purpose of using batteries is to store or supply energy according to demand over a given period of time. The generator only goes into operation if the demand for the load can not be covered by photovoltaics and batteries.
Photovoltaics – Thermics
The combination of photovoltaic and thermal panels is a relatively recent technology in which the production of electrical & thermal energy is integrated into one body to cover all types of building needs. In typical photovoltaic panels, the efficiency factor falls with the rise in temperature. Instead, the PV-T hybrid panels benefit by absorbing the unnecessary heat of the photovoltaic section to produce thermal energy. This function works beneficial for the photovoltaic section, which operates at ideal temperatures and therefore produces up to 50% more energy than a corresponding simple photovoltaic panel. In this way, electricity production is combined with the production and storage of hot water or air. This water can then be used for daily needs (hot water) to heat the building either with conventional radiators or with underfloor heating or in the form of air conditioning for both heating and cooling. In addition, any application using hot water can benefit. If air is used as a refrigerant, this heated air can be used to heat rooms.
Photovoltaics – Geothermy
In this case, solar systems may be used to support the geothermal network. The combined heat and power systems for hot water production and space heating can cover from 20% to 40% the needs of a home in heating and in hot water depending on the size of the collector surface, the volume of the tank, the meteorological data of the area and the characteristics of the residence. Such a system uses hot water from the solar thermal systems or the hot water of a system, such as the one in the above category, to supply the geothermal system with hot water and hence to heat the space. A new technology that has been developed over the past year and is still not commercially available is the combination of photovoltaic panels with geothermal energy. In this case, excess heat from solar energy produced in the summer can be stored using a geothermal pump on the ground and reused in the cold months through a heat pump. In this way, not only does the efficiency coefficient of the solar panels increase, but also the optimal use of heat is ensured. Even for autonomous buildings that are not connected to the PPC network, the use of hybrid systems at reasonable cost covers our energy needs without the use of costly and polluting means, such as oil generators. The hybrid systems most used in residences consist of photovoltaic panels and a wind turbine.
Area Electrification
Hybrid systems are not only used to house individual homes but also to power entire areas. In this case, electricity is produced by a combination of means of production and then fed to consumers by using a local network. A hybrid power generation system is defined as any stand-alone power system in which more than one energy source is integrated with the necessary supporting equipment, including energy storage, to provide electricity to the grid or its location. The key components that constitute a hybrid power system are:
(a) power plants,
(b) the energy storage unit,
(c) the power generation unit and the coordination of the various available options for optimal coverage of needs.
The most commonly used production technologies can be conventional fueling power generators, which can quickly respond to the required load, although they may be noisy and economically inexpensive. In areas with good wind potential, significant amounts of energy can produce wind turbines. However, due to the stochastic nature of the wind, it is necessary to connect the wind turbines with other technologies or to store the energy to achieve a steady supply. For this reason, they are used very often in hybrid systems. Photovoltaic panels are easy to install and maintenance-free, and are ideal for installation in areas where fuel supply can be a problem, but require a lot of space to install a system with sufficient power, and, moreover, power generation from them is not stable. Hydroelectric projects provide a more stable and reliable supply of electricity than other renewable energy sources. Hydroelectric power plants are not characterized by major energy discontinuities and even small projects (<10MW) can be viable, especially if combined with some other energy source. In addition to the use of other systems viable is the exploitation of very small hydroelectric formations of the order of 200kW or even smaller. Significant electricity generation can arise from the use of biomass as a fuel as the available sources of fuel are numerous and vary from agricultural and forest residues, energy crops, as well as biomass conversion technologies such as combustion and gasification. Finally, for the large production of electricity from hybrid systems, other less widely used technologies such as hydrogen cells, centralized solar systems and sea power systems (waves and tides) can be used. The volatility of certain energy sources, particularly renewables, may result in incomplete supply demand being met by production itself. Moreover, any system generators are not able to cope directly with a sharp increase in demand. To overcome such problems, there must be some energy storage in the system. The most common storage means are batteries, while other methods include electrochemical systems with external storage such as fuel cells, electrical systems such as capacitors and mechanical systems with the example of pumping water. The most used option in large hybrid systems and of great interest is pumping hydroelectric systems, especially in seaside areas or in areas close to large aquatic bodies. In this case, when the renewable source offers the system more energy than required, water is pumped from the sea into tanks at a higher altitude. When the demand for electricity is greater than the supply, then the hydroelectric system is activated and the water from the tanks returns to the sea, through turbines and generators, thereby generating direct electrical energy. This is the basis for many hybrid projects in Greece today. In many cases, the combination of wind turbines with hydroelectric systems has been used, and the combination of wind turbines with photovoltaic panels is not rare. Two of the largest hybrid power generation projects are located in Ikaria and Rethymno. More specifically, the Ikaria hybrid power plant consists of two Small Hydroelectric Stations (MOSIS Preesperas, 1.05 MW and MOSIS Kato Preesperas, 3.1 MW), a pump station (with 12 pumps of 250 KW each) and the Wind Turbine Wind Park with three wind turbines with a total power of 2.7 MW. The successful operation of the project will allow a large increase in the penetration of energy from Renewable Sources into the island’s electrical system. The second project is located in the Amari area of Rethymno and is expected to have a total power of 50MW and will meet the energy needs of Rethymnon by 65% while further energy upgrading of the power station of the hybrid station up to 100MW is possible.
The project consists of 2 wind parks of total power 81MW, a pumping station with 10 pumps of 36MW each and a controlled production station with a capacity of 50MW and pumping 108MW. Finally, the project consists of two tanks, one being in the municipality of Rethymnon and the other one being the reservoir of the river dam in the Municipality of Amari.
