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It is often said that there is nothing new under the sun; Perhaps this article will demonstrate that there have been many inventions but few have seen any development but may still have a commercial future.

The Stirling Engine

The Stirling engine was invented in 1816 by the Rev. Robert Stirling who sought to create a safer alternative to steam engines, whose boilers often exploded due to the high steam pressures and the limitations of the materials available at the time.

Like other heat engines the Stirling engine converts heat energy into mechanical energy. The essential features of the Stirling engine however are that it is a closed cycle, external combustion engine. This means that it uses a fixed amount of working fluid, usually air, but other gases may be used, enclosed in a sealed container and the heat consumed by the engine is applied externally. This allows the engine to run on just about any heat source including fossil fuels, hot air, solar, chemical and nuclear energy. It can also work with very low temperature differentials, as low as 7°C, between the heat source and the heat sink so that it can be powered by body heat and even the steam from a cup of coffee!

Since it can use heat from a constant flame and does not depend on explosions as in an internal combustion engine, the engine runs silently.
The Stirling engine is therefore a highly-efficient heat engine.

Since its invention, there have been many variations, all stemming from Stirling’s original patent in the 1800s. The term hot air engine is a catch-all for any heat engine that uses the expansion and contraction of air under the influence of a temperature change to convert thermal energy into mechanical work. In the example shown, the engine is fired using the included denatured alcohol burner.

Heat from the alcohol burner is applied to the end of an air-tight glass cylinder. The air in the cylinder is permanently sealed inside the engine. This fixed amount of air is heated and cooled, causing it to expand and contract, thus driving the piston. As the displacer moves up, the air in the cylinder moves down to the hot side, so the air pressure inside the cylinder increases, causing it to push up on the piston. Alternatively, as the displacer moves down, the air in the cylinder moves up to the cool side, causing the pressure inside the cylinder to decrease, and pulling the piston down.
This process repeats over and over again, setting the wheel in motion.

One initial plus point for the Stirling engine is that it is largely silent running. Are there any Stirling engines in commercial use or being developed for commercial use?

Nuclear PowerThere is a potential for nuclear-powered Stirling engines in electric power generation plants. Replacing the steam turbines of nuclear power plants with Stirling engines might simplify the plant, yield greater efficiency, and reduce the radioactive by-products. A number of breeder reactor designs use liquid sodium as coolant. If the heat is to be employed in a steam plant, a water/sodium heat exchanger is required, which raises some concern as sodium reacts violently with water. A Stirling engine obviates the need for water anywhere in the cycle.

US government labs have developed a modern Stirling engine design known as the Stirling Radioisotope Generator for use in space exploration. It is designed to generate electricity for deep space probes on missions lasting decades. The engine uses a single displacer to reduce moving parts and uses high energy acoustics to transfer energy. The heat source is a dry solid nuclear fuel slug and the heat sink is space itself.

Automotive engines

There have been at least two automobiles exclusively powered by Stirling engines that were developed by NASA, as well as earlier projects by Ford and American Motor Companies.

The main difficulties involved in using the Stirling engine in an automotive application are start-up time, acceleration response, shutdown time, and weight, not all of which have ready-made solutions. Many people believe that hybrid electric drive systems can bypass all of these setbacks. In November 2007, a prototype hybrid car using solid bio-fuel and a Stirling engine was announced by the Precer project in Sweden.

The NASA vehicles were designed by contractors and designated MOD I and MOD II. The MOD II replaced the normal spark-ignition engine in a 1985 four-door Chevrolet Celebrity hatchback. In the 1986 MOD II Design Report the results show that the highway petrol mileage was increased from 40 to 58 mpg and the urban mileage from 26 to 33 mpg with no change in gross weight of the vehicle.

Start-up time in the NASA vehicle took up to 30 seconds, while Ford’s research vehicle used an electric heater placed directly into the hot air mix to get the vehicle started in only a few seconds.

Geothermal energy

Some believe that the ability of the Stirling engine to convert geothermal energy to electricity and then to hydrogen may well hold the key to replacement of fossil fuels in a future hydrogen economy. This belief was also founded on research conducted at Los Alamos Labs that began as a hot dry rocks research, but later calculated the near limitless energy potential from molten rock on one side of a Stirling engine and ocean water on the other.

Although currently the most feasible source of commercial electrical generation is solar, long-range predictions show advances in deep drilling and development of methods to work with molten rock could yield exponential levels of clean energy generation for thousands of years.

Marine engines

However, in terms of current practical uses of the Stirling engine, the best examples can be found in submarines. An illustration of such an engine simply shows a large enclosed unit without any external moving parts. For example, Kockums (formerly SAAB), the Swedish shipbuilder, has built at least eight commercially successful Stirling powered submarines during the 1980s.

As of 2005 it has started to carry compressed oxygen with them. With their air-independent propulsion systems such as fuel cell or Stirling, they stay submerged much longer than conventional submarines.

Since Stirling engines can be built to be essentially silent, they’re a natural match for powering submarines.

Kockums added a Stirling engine to its Gotland and Södermanland classes of submarines. Small submarines pose an elusive threat to naval forces operating in relatively shallow waters, particularly amidst islands. Knowing this the Swedish Navy has a long tradition of building relatively small submarines optimized for the confined waters of the Baltic and particularly among the thousands of islands in the Swedish archipelago.

It was the first navy to introduce the modern form of Air Independent Power (AIP) by converting a Nacken Class submarine in 1989, followed by the Gotland Class in 1995, which had AIP by design. Its closed circuit Stirling engines generate enough power to propel the submarine at slow speeds. This allows the submarine to operate without the main diesel generators, which recharge the batteries. Thus AIP-equipped submarines add persistent to their threat profile, multiplying the dangers to a conventional navy operating in their arena. It was enough of a game-changer that the US Navy leased HSwMS Gotland for two years to exercise against the emerging threat.

In summary, Stirling Engines are more suited to the delivery of steady and silent power, but less suited to rapidly changing demands such as motor cars/cycles. Given the need for steady, silent power delivery, perhaps there is an aviation application just waiting to be developed alongside the development of battery-powered aircraft.