Stirling Engine Advantages and Disadvantages

Advantages

• High potential efficiency (Carnot limit). In practice the best Stirling engines can have efficiencies of 40% or more. The record for heat input to electrical power output is held by Stirling Energy Systems for one of their solar dish Stirling engines, at 31.25% of solar energy converted to grid-level electricity [40].

• Ability to run on any or multiple fuels. This is advantageous in several ways — it means that engines used in applications such as solar power generation can still produce electricity even when the sun is not shining if they are fed heat from an alternative source. This means more income generated by the plant in a given time

And reduced payback period for the operators. The ability to switch fuels without altering the engine also means that operators of Stirling engines are less vulnerable to rises in fuel prices or fuel shortages.

Reversible operation — if the shaft of a Stirling engine is powered with a motor then the Stirling engine can be used as a heat pump, shifting heat from the cold side to the hot side. It is possible to achieve extremely low temperatures by doing this, and there are a number of successful Stirling cryo-coolers.

Cleaner emissions even if burning fossil fuel for heat. This is because combustion is external to the engine and continuous, meaning it can take place under optimum burning conditions whereas an internal combustion engine burns the fuel discontinuously which is less efficient.

Quiet operation due to any combustion taking place externally to the engine, rather than gas expanding to the atmosphere.

Low vibrations due to the typically easy to balance designs and low operating speeds.

Low maintenance. They have less moving parts than an internal combustion engine and if well designed they can last for many hours between services or overhauls. In addition they are typically slower rotating and have fewer vibrations than an IC engine which also contributes to less wear. Typically, the only real points of wear are the piston rings, bearings and any other seals that are inherent with a particular design.

Smooth torque delivery — as illustrated in Figure 45 the instantaneous torque available from a 4 cylinder Stirling engine varies only slightly in comparison to that of a similarly performing 4 cylinder internal combustion engine.

Stirling Engine Advantages and Disadvantages

Figure 45: Comparison of torque vs. crank angle between a 4 cyl. petrol IC engine and a 4cyl. Stirling engine

• Cost — Stirling engines are not manufactured on large scales so the benefits in cost reduction of mass production are not realised. A large portion of the cost for any Stirling engine comes from the heat exchangers which must use typically expensive materials in order to transfer the required heat without corroding or deforming over time under heat and pressure.

• Lack of ‘throttle response’ — Stirling engines only operate effectively in a relatively narrow band of speeds and because increasing speed or power involves putting more heat into the engine there is ‘thermal lag’. These factors make Stirling engines unsuitable for applications such as car engines, though they are somewhat suitable to Stirling/electric hybrid systems.

• Long start-up time from cold. It takes time to get the heater up to operating temperature and the engine won’t produce peak power for some time after it actually starts.

• Typically not self-starting, i. e. require a motor to turn the shaft to start the cycle. This is not such a disadvantage when compared with an internal combustion engine as they require the same thing, but it still adds cost and complexity.

• Size and weight — Typically for a given power output in comparison with other engine types a Stirling engine is quite large and heavy. This is often due to the fact that the engine is internally pressurized, necessitating a robust housing. This also adds to the overall cost of the engine.

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