Hybrid Power Systems incorporate several electricity generating components with usually one major control system which enables the system to supply electricity in the required quality.
Components for electricity generation can utilize renewable energy sources like wind turbines, photovoltaic, solar thermal, hydro power, wave power or biomass power stations, etc. Furthermore, fossil power plant like diesel generators, gas turbines or fuel cells etc. can be added.
The term Hybrid Power System does not give any information about the size of the energy system. Generally, Hybrid Power Systems are considered to supply loads in the size of several watts up to several megawatts. They usually supply island networks that are not connected to an integrated grid covering countries or even continents – but represent small grids with a limited number of consumers. Due to the resulting fluctuating consumption pattern several specific features are required concerning the electricity supplying Hybrid Power System.
In integrated electricity grids the load equalizes due to the large number of consumers and its statistical application. This is how base, medium and peak load are defined which are covered by dedicated base, medium and peak load power plant to minimize the electricity cost price. Base load is needed continuously 24 hours per day, medium load is required in consecutive 3 to 6 hours and peak load is required in shorter sequences. Unfortunately, this cost effective procedure cannot be transferred to Hybrid Power Systems in most of the cases. On the contrary, Hybrid Power Systems have to cope with much more severe short term variations in power demand. Thus, different energy management structures have to be applied. These energy management structures vary with the size of the Hybrid Power System depending on the financially optimal system design.
In island networks it is essential to integrate one component that is responsible for frequency and voltage stabilization. In small systems up to 50 kW inverters and battery systems are applied for frequency and voltage stabilization; in lager systems continuously running synchronous generators with controllable engines are more cost effective. The further the technical progress of power electronics the more systems in the higher power class are controlled by inverter technology, too. Depending on the magnitude of the Hybrid Power System, different storages for equalization of load variations are applied. For Megawatt class systems pumped storage plants are most appropriate, for medium sizes of several hundred kilowatts the application of compressed air storage plant and for small scale systems the application of battery storages is advisable considering the economic point of view.
In larger systems, often geographical constraints prevent the application of appropriate storages. Thus, in these cases the storage component is replaced by a dynamically controlled generator driven by a fuel engine. The necessity for this operation results from the requirement to match demand and supply in electrical grids for each moment in time for stable grid operation. In order to avoid short term voltage drops and flickers power storages are applied. These storages provide high power for short periods. In contrast, the stored energy in such storages is rather small. Fly wheel storages, capacitors and special kinds of batteries belong to this group of storages.
Photo 1: Hybrid Power System on Sagar Island, India including a small scale wind turbine and PV,
Copyright: Nico Peterschmidt, inensus GmbH
In the range of up to 30 kW mainly classic DC-coupled systems are established; in larger systems AC-coupling is more common. However, even in small scale systems the little bit more complex but easily extendable AC-coupled systems are gaining market share. Even mixed AC and DC systems are being offered. The discussion about advantages and disadvantages of specific system types is being continued.
In general, Hybrid Power Systems have to meet different requirements depending on the appliances served, the consumer behaviour, the consumer’s demands on the power quality and the energy sources available locally. While mobile phone antennas need to be supplied with almost constant power of high quality, small villages have a fluctuating and usually growing energy demand while short term power outages are not critical. The integration of wind power at a gusty site requires different features of a Hybrid Power System than a continuously operating hydro power plant. To realize cost efficient power supply with the required power quality an individual system design considering all site specific aspects is essential.
For Hybrid Power Systems all areas without electricity supply from integrated networks but demand for electrification can be identified as potential markets. Large potential for rural electrification especially with renewable energy sources can be found in developing countries. Unfortunately, the market for such systems has not materialized to a substantial scale yet due to a lack of structures in financial and political aspects. Furthermore, in many countries the required infrastructure for assembly, operation and maintenance of complex technical systems is not available locally.
In 2005 the World Wind Energy Association Hybrid Systems Working Group was established. Its aim is to provide a platform for discussions about all problems concerning Hybrid Power Systems paving the way for increased Hybrid Power System utilization.