The Stirling Engine.

A ‘Stirling Engine’ is in the family of heat engines. It is a closed cycle regenerative hot air (or other permanent gas) engine.. Closed cycle means that there is a fixed volume of the ‘working fluid’ in the system. There is no intake, there is no exhaust.

Since the Stirling engine is a closed cycle, it contains a fixed mass of gas called the “working fluid”, most commonly air, hydrogen or helium. In normal operation, the engine is sealed and no gas enters or leaves the engine. No valves are required, unlike other types of piston engines. The Stirling engine, like most heat-engines, cycles through four main processes: cooling, compression, heating and expansion. This is accomplished by moving the gas back and forth between hot and cold heat exchangers. The hot heat exchanger is in thermal contact with an external heat source, e.g. a fuel burner, and the cold heat exchanger being in thermal contact with an external heat sink, e.g. air fins. A change in gas temperature will cause a corresponding change in gas pressure, while the motion of the piston causes the gas to be alternately expanded and compressed.

The gas follows the behavior described by the gas laws which describe how a gas’s pressure, temperature and volume are related. When the gas is heated, because it is in a sealed chamber, the pressure rises and this then acts on the power piston to produce a power stroke. When the gas is cooled the pressure drops and this means that less work needs to be done by the piston to compress the gas on the return stroke, thus yielding a net power output.

When one side of the piston is open to the atmosphere, the operation is slightly different. As the sealed volume of working gas comes in contact with the hot side, it expands, doing work on both the piston and on the atmosphere. When the working gas contacts the cold side, the atmosphere does work on the gas and “compresses” it. Atmospheric pressure, which is greater than the cooled working gas, pushes on the piston.

To summarize, the Stirling engine uses the temperature difference between its hot end and cold end to establish a cycle of a fixed mass of gas expanding and contracting within the engine, thus converting thermal energy into mechanical power. The greater the temperature difference between the hot and cold sources, the greater the potential Carnot cycle efficiency.

Pros and Cons of Stirling Engines

Pros

• They can run directly on any available heat source, not just one produced by combustion, so they can be employed to run on heat from solar, geothermal, biological, nuclear sources or waste heat from any industrial process.

• A continuous combustion process can be used to supply heat, so most types of emissions can be greatly reduced.

• Most types of Stirling engines have the bearing and seals on the cool side of the engine; consequently, they require less lubricant and last significantly longer between overhauls than other reciprocating engine types.

• The engine mechanisms are in some ways simpler than other types of reciprocating engine types, i.e. no valves are needed, and the fuel burner system can be relatively simple.

• A Stirling engine uses a single-phase working fluid which maintains an internal pressure close to the design pressure, and thus for a properly designed system the risk of explosion is relatively low. In comparison, a steam engine uses a two-phase gas/liquid working fluid, so a faulty relief valve can cause an over-pressure condition and a potentially dangerous explosion.

• In some cases, low operating pressure allows the use of lightweight cylinders.

• They can be built to run very quietly and without an air supply, for air-independent propulsion use in submarines or in space.

• They start easily (albeit slowly, after a warm-up period) and run more efficiently in cold weather, in contrast to the internal combustion which starts quickly in warm weather, but not in cold weather.

• A Stirling engine used for pumping water can be configured so that the pumped water cools the compression space. This is, of course, most effective when pumping cold water.

• They are extremely flexible. They can be used as CHP (Combined Heat and Power) in the winter and as coolers in summers.

• Waste heat is relatively easily harvested (compared to waste heat from an internal combustion engine) making Stirling engines useful for dual-output heat and power systems

Cons

Power and torque issues

• Stirling engines, especially those that run on small temperature differentials, are quite large for the amount of power that they produce (i.e. they have low specific power). This is primarily due to the low heat transfer coefficient of gaseous convection which limits the heat flux that can be attained in an internal heat exchanger to about 4 - 20 W/(m*K). This makes it very challenging for the engine designer to transfer heat into and out of the working gas. Increasing the temperature differential and/or pressure allows Stirling engines to produce more power, assuming the heat exchangers are designed for the increased heat load, and can deliver the convected heat flux necessary.

• A Stirling engine cannot start instantly; it literally needs to “warm up”. This is true of all external combustion engines, but the warm up time may be shorter for Stirlings than for others of this type such as steam engines. Stirling engines are best used as constant speed engines.
• Power output of a Stirling tends to be constant and to adjust it can sometimes require careful design and additional mechanisms. Typically, changes in output are achieved by varying the displacement of the engine (often through use of a swashplate crankshaft arrangement), or by changing the quantity of working fluid, or by altering the piston/displacer phase angle, or in some cases simply by altering the engine load. This property is less of a drawback in hybrid electric propulsion or “base load” utility generation where constant power output is actually desirable.

Gas Choice Issues

• Hydrogen’s low viscosity, high thermal conductivity and specific heat make it the most efficient working gas, in terms of thermodynamics and fluid dynamics, to use in a Stirling engine. However, given the high diffusion rate associated with this low molecular weight gas, hydrogen will leak through solid metal, thus it is very difficult to maintain pressure inside the engine for any length of time without replacement of the gas. Typically, auxiliary systems need to be added to maintain the proper quantity of working fluid. These systems can be a gas storage bottle or a gas generator. Hydrogen can be generated either by electrolysis of water, or by the reaction of acid on metal. Hydrogen can also cause the embrittlement of metals. Hydrogen is also a very flammable gas, while helium is inert.

• Most technically advanced Stirling engines, like those developed for United States government labs, use helium as the working gas, because it functions close to the efficiency and power density of hydrogen with fewer of the material containment issues. Helium is relatively expensive, and must be supplied by bottled gas. One test showed hydrogen to be 5% absolutely (24% relatively) more efficient than helium in the GPU-3 Stirling engine.
• Some engines use air or nitrogen as the working fluid. These gases are less thermodynamically efficient but they minimize the problems of gas containment and supply. The use of Compressed air in contact with flammable materials or substances such as lubricating oil, introduces an explosion hazard, because compressed air contains a high partial pressure of oxygen. However, oxygen can be removed from air through an oxidation reaction, or bottled nitrogen can be used.

Size and Cost Issues

• Stirling engine designs require heat exchangers for heat input and for heat output, and these must contain the pressure of the working fluid, where the pressure is proportional to the engine power output. In addition, the expansion-side heat exchanger is often at very high temperature, so the materials must resist the corrosive effects of the heat source, and have low creep (deformation). Typically these material requirements substantially increase the cost of the engine. The materials and assembly costs for a high temperature heat exchanger typically accounts for 40% of the total engine cost.

• All thermodynamic cycles require large temperature differentials for efficient operation; however, in an external combustion engine, the heater temperature always equals or exceeds the expansion temperature. This means that the metallurgical requirements for the heater material are very demanding. This is similar to a Gas turbine, but is in contrast to a Otto engine or Diesel engine, where the expansion temperature can far exceed the metallurgical limit of the engine materials, because the input heat-source is not conducted through the engine; so the engine materials operate closer to the average temperature of the working gas.

• Dissipation of waste heat is especially complicated because the coolant temperature is kept as low as possible to maximize thermal efficiency. This increases the size of the radiators, which can make packaging difficult. Along with materials cost, this has been one of the factors limiting the adoption of Stirling engines as automotive prime movers. However, for other applications high power density is not required, such as Ship propulsion, and stationary microgeneration systems using combined heat and power

The Stirling engine has promise for many uses. As more research is applied to the idea, more uses will be found.