Hybrid Systems

Hybrid systems are being developed to improve on the performance of an individual DG device. By combining two or more DG systems, the overall fuel utilization efficiency can be increased or the system can be more reliable. Because these systems are under development, there are few actual performance values available.

The SOFC/gas turbine hybrid system can provide electrical conversion efficiencies of 60 to 70%. Solid oxide fuel cell/gas turbine concepts rely on the principle that fuel cell efficiency and reaction speed will improve when the fuel-cell stack operates above atmospheric pressure. By operating the fuel cell stack at 4 atmospheres or higher, it is possible to integrate the fuel cell with a gas turbine. In this hybrid arrangement, the gas turbine compressor is used to pressurize the fuel cell, then the hot exhaust from the fuel cell stack, which still contains 50% of the fuel's energy (as unreacted fuel and waste heat), is fed back into the turbine, combusted and expanded to extract more energy. Energy recovered from a recuperator is used to help heat inlet air for the fuel cell stack and the compressor.

Several companies are working to develop Stirling engine/solar dish hybrid systems. These kinds of hybrid systems are small, with typical outputs in the range of about 5 to 25 kW. This size makes dish/Stirling hybrids ideal for stand-alone or other decentralized applications, such as replacement of diesel generators. Larger dish/Stirling plants with outputs of 1 to 20 MW could be developed to meet moderate-scale grid-connected applications. Dish/Stirling hybrids can also be designed to run on fossil fuels for operation when there is no sunshine.

Wind turbines can be used in combination with energy storage and some type of backup generation (i.e., reciprocating engine, turbine, or fuel cell) to provide steady power supply to remote locations not connected to the grid.

Energy storage devices such as flywheels are being combined with IC engines and microturbines to provide a reliable backup power supply. The energy storage device provides ride-through capability to enable the backup power supply to get started. In this way, electricity users can have an interruption-free backup power supply.