Advantages and Disadvantage of Stirling engines

Advantages of Stirling engines

* The heat is external and the burning of a fuel-air mixture can be more accurately controlled.

* 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 or nuclear sources.

* A continuous combustion process can be used to supply heat, so emission of unburned fuel can be greatly reduced.

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

* The engine as a whole is much less complex than other reciprocating engine types. No valves are needed. Fuel and intake systems are very simple.

* They operate at relatively low pressure and thus are much safer than typical steam engines.

* Low operating pressure allows the usage of less robust cylinders and of less weight.

* They can be built to run very quietly and without air, for use in submarines or in space.

* They start easily and run more efficiently in cold weather, features lacking in their internal combustion cousins.

* A Stirling engine which is pumping water can be configured so that the pumped water cools the cool side. 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 winters and as coolers in summers (cryocooling).



Disadvantages of Stirling engines

* Some Stirling engine designs require both input and output heat exchangers, which must contain the pressure of the working fluid, and which must resist any corrosive effects due to the heat source. These increase the cost of the engine, especially when they are designed to the high level of "effectiveness" (heat exchanger efficiency) needed for optimizing fuel economy. Fuel economy may not be an issue with the advantages of using unlimited but unusual fuel sources that a Stirling engine can make use of.

* Stirling engines that run on small temperature differentials are quite large for the amount of power that they produce, due to the heat exchangers. Increasing the temperature differential (and pressure) allows smaller Stirling engines to produce more power.

* Dissipation of waste heat is especially complicated because the coolant temperature is kept as low as possible to maximize thermal efficiency. This drives up the size of the radiators markedly, which can make packaging difficult. This has been one of the factors limiting the adoption of Stirling engines as automotive prime movers. (Conversely, it is convenient for domestic or business heating systems where combined heat and power (CHP) systems show promise. ref)

* A "pure" 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 run, constant speed engines.

* Power output of a Stirling is constant and hard to change rapidly from one level to another. 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 mass of entrained working fluid (generally helium or hydrogen). This property is less of a drawback in hybrid electric propulsion or base load utility generation where a constant power output is actually desirable.

* Hydrogen's low viscosity, high thermal conductivity and specific heat makes 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. 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. Helium must be supplied by bottled gas. Some engines use air as the working fluid which is less thermodynamically efficient but minimizes the problems of gas containment and supply. 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. Hydrogen is also a very flammable gas, while helium is inert. Compressed air can also be explosive because it contains a high partial pressure of oxygen. Oxygen can be removed from air through an oxidation reaction, or equivalently, bottled nitrogen can be used.