Stirling Engines and Micro-cogeneration Report

2.0 THE STIRLING ENGINE

2.1. History

The Stirling engine was invented by Scottish clergyman Robert Stirling in 1816 [1]. The original Stirling engine was designed as a competitor to the steam engine, but it never gained widespread popularity for large industrial applications [2]. Instead, in the late nineteenth century, a substantial number of smaller Stirling engines were built for applications requiring low to medium power [2]. In the early twentieth century, the small Stirling engines eventually gave way to electric motors and small internal combustion engines, and by the 1930s Stirling engines were largely relegated to trivial applications such as toys [2].

In the 1930s, the electronic company Phillips was seeking to expand sales of their radio products and needed a power source for locations where electricity was unavailable and battery supplies were unreliable [2]. The engineers at Phillips eventually picked the Stirling engine due its quiet operation and ability to run on a variety of fuels [2]. Phillips continued research into Stirling engines all the way through to the 1970s, but the company never produced a commercially viable radio powered by Stirling engines [2]. Instead, the Stirling engine found its way into niche applications such as cooling for cryogenics. Now in the twenty-first century, Stirling engines are beginning to find commercial viability once again, most notably for micro combined heat and power/cogeneration applications.

2.2. Principles of Operation

The Stirling engine is closed cycle heat engine. This means that the Stirling engine works by converting heat into mechanical output, and the fluid that does the mechanical work, called the working fluid, is normally enclosed within the engine and is not mixed with any other material [1]. A Stirling engine operates by cyclically moving the working fluid between two places with a temperature difference. The working fluid is compressed in the cold end and expanded in hot end, and the expansion of the fluid is what provides the mechanical work [1].

2.3. Key Components

The Stirling engine can be configured in several ways, but most Stirling engines have the following basic components:

  • A fixed mass of gas known as the working fluid which is normally sealed inside the engine. Ideally the gas should have low heat capacity so it expands a lot in volume when heated. Common working fluids are air, hydrogen, and helium [3].
  • A heat source. The source can be almost anything, since it does not come into direct contact with the working fluid or the internal parts of the engine. Some possible heat sources are solar, nuclear, and waste heat.
  • A heater to transfer the heat from the heat source to the working fluid. The heater needs to be effective at transferring heat to the working fluid but at the same time it should not introduce too much pumping loss (friction) to the working fluid. The heater also needs to withstand the high temperature of the heat source without deforming.
  • A regenerator. The regenerator is a device that sits between the cold and hot places of the Stirling engine so that the working fluid moves through it in both directions [1]. The regenerator acts as a temporary storage of heat, and its purpose is to help retain the heat within the engine instead of letting the heat dissipate in the colder parts, thereby improving the engine’s thermal efficiency. Ideally, the regenerator should take up as little volume as possible, introduce almost no pumping friction, has very high heat storage capacity, and has high thermal conductivity perpendicular (but not parallel) to the fluid flow [2].
  • A cooler or heat sink to cool the working fluid in the cold end of the engine.

2.4. Configurations

There are several configurations of Stirling engines, but the two basic types are the alpha and beta configurations.

2.4.1. Alpha Stirling

The alpha Stirling configuration, also known as the two-piston configuration, uses two pistons in separate cylinders [2]. One of the cylinders is heated by the heat source, while the other cylinder is cooled by the cooler, and the two cylinders are connected to each other through a pipe, allowing the working fluid to move back and forth between the cylinders [1]. The action of the alpha Stirling engine is as follows:

Alpha Stirling
Figure 1 - Animation of the Alpha Stirling Cycle
Source: http://en.wikipedia.org/wiki/Stirling_engine
  1. Most of the working fluid is in the hot cylinder. The fluid is heated and expands, and this in turn pushes the cold piston down to the bottom [1].
  2. The working fluid is at the maximum volume. The hot cylinder starts to push the fluid into the cold cylinder [1].
  3. Most of the working fluid is in the cold cylinder where it cools. The cold piston, powered by flywheel momentum or other piston pairs on the same drive shaft, compresses the fluid further [1].
  4. The working fluid is now at minimum volume. The fluid now expands in the hot cylinder where it will be heated and provide the power stroke of the engine [1].

2.4.2. Beta Stirling

The beta Stirling or single cylinder configuration arranges a single power piston above a displacer piston within the same cylinder [1]. The displacer piston does not extract power and is used to move the working fluid between the hot and cold parts of the cylinder [1]. The action of the beta Stirling engine is as follows:

Beta Stirling
Figure 2 - Animation of the Beta Stirling Cycle
Source: http://en.wikipedia.org/wiki/Stirling_engine
  1. The power piston compresses the gas while the displacer piston lifts up so that most of the gas is in the hot end [1].
  2. The gas is heated by the hot end and expands, pushing the power piston up for the power stroke [1].
  3. Powered by flywheel momentum, the displacer piston moves downwards and moves the working fluid to the cold end of the cylinder [1].
  4. The working fluid is cooled and then compressed by the power piston moving downwards [1].

2.5. Advantages and Disadvantages

Compared to the much more common internal combustion engine, the Stirling engine has several advantages:

  • The Stirling engine is more flexible in terms of energy source. The Stirling engine can run on almost any heat source and doesn’t have to be powered by combustion [3].
  • Stirling engines can be built to run more quietly than internal combustion engines [4].
  • Certain aspects of the Stirling engine are simpler (no valves are needed, less bearings and seals) and therefore the engine require lower maintenance [4].
  • The waste heat is easier to harvest compared to internal combustion engines [4].

Due to these advantages, Stirling engines are preferable over internal combustion engines for specialized application such as water pumping, submarine engines, and micro combined heat and power (micro-CHP). The application in micro-CHP is discussed in the next section of this report.

On the other hand, Stirling engines also have several significant disadvantages when compared to internal combustion engines:

  • Compared to internal combustion engines of the same rating, Stirling engines are heavier and have higher initial cost. One of the reasons is that Stirling engines require expensive heat exchangers which can withstand the high temperature in the hot end [4].
  • Stirling engines start slowly and needs time to warm up [1].
  • It is more difficult to adjust the power output of Stirling engines [1].

Because of these disadvantages, Stirling engines are not well-suited for automobiles where size, weight, throttle response, and quick starts are important [1]. However, Stirling engines may be feasible in hybrid-electric vehicles [1].

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