Figure 3.15 One possible implementation of a Stirling engine.

Real efficiency departs from from the theoretical because:

1. The efficiency of mechanical heat engines increases with increasing compression ratio. The Stirling engine is no exception (see Problem 3.1). The internal volume of the heat exchangers and the regenerator causes the compression ratio to be smaller than if heat could be applied directly to "Space #1."

2. The combustion gases leaving the input heat exchanger are still quite hot and carry away considerable energy. To recover part of the latter a preheater is used as explained previously.

3. The regenerator cannot operate ideally and return all the heat deposited in it during part of the cycle.

4. There are frictional losses when the working fluid is transferred from one space to the other through the heat regenerator. This is the main reason for using hydrogen as a working fluid, as it leads to minimum frictional losses.

5. Ideally, the motion of the pistons should be intermittent, but this is difficult to implement in machines operating at reasonably high rpm. One has to make a compromise in the piston programming.

6. The limited time for the heat exchanges in the working fluid leads to temperatures that never reach the desired steady state levels.

The power output of a Stirling engine can be controlled by either adding or removing working fluid. To reduce power, a compressor removes some working fluid from the engine and stores it at high pressure in a holding tank. To increase power, gas from this tank can be quickly delivered back to the engine. The input heat exchanger temperature is continuously monitored, and this information is used to control the fuel flow.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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