Hybrid systems are the combination of two or more energy sources which balance their own characteristics for the purpose of higher system reliability. Since they are currently in an experimental phase, there are only a few systems of this kind found worldwide (mostly a part of NASA projects).

Hybrid systems can be the following:

• fuel cells in combination with gas turbines or microturbines,
• Stirling engine in combination with a solar power plant,
• wind power plants in combination with energy storage and e.g. reciprocating engines, turbines or fuel cells,
• reciprocating engines or microturbines in combination with energy storage (e.g. flywheels).

Fuel cells in combination with gas turbines or microturbines can achieve 60-70% electric efficiency. Fuel cell efficiency can be increased if they operate with a pressure higher than atmospheric. If the pressure is of sufficiently high value, it is possible to integrate the cells with a gas turbine. In such a hybrid integration, the turbine gas compressor is used for increasing the pressure in a fuel cell. Rejected fuel cell heat (which contains 50% fuel energy as unreacted fuel and waste heat) is returned into the turbine where it expands in order to provide more energy.

Stirling engine in combination with a solar power plant is a small hybrid system of 5 to 25 kW capacity. As such it is ideal as a freestanding unit and as a replacement for diesel generators. Units with rated power from 1 to 20 MW can be used as network facilities.

Wind power plants with energy storage and e.g. reciprocating engine, turbine or fuel cells are used in remote locations without electric power network. The first hybrid technology project is a wind power plant in combination with hydrogen on the island of Utsira in Norway, put into operation in September 2003. There are 10 households (the smallest settlement in Norway) which had big problems with reliable electricity supply due to their remote location. One wind power plant is a hybrid power plant used for settlement supply while the other power plant delivers all generated energy into the network. Under good weather conditions, wind power plants generate sufficient energy for settlement supply and obtaining hydrogen through electrolysis while the surplus is delivered into the network. If weather conditions are not favourable, stored hydrogen is used for generating electricity in fuel cells.

Reciprocating engines or microturbines in combination with energy storage (e.g. flywheels) represent a reliable electricity supply reserve.