Back To The Future !

Forget everything you thought you knew about Stirling engines!

Stirling heat engines have been known as "Heat-to-Power" units since the 19th century. For many applications, these historic machines and their successors, the "classic" Stirling engines, remained in the shadow of internal combustion engines and turbines. However, the end of these aggregates is now in sight.

In the future, they could be replaced for many applications by a New Stirling Installation with lower emissions and energy consumption, more flexibility and better efficiency at higher output levels

A warning in advance: The technology described here has absolutely nothing in common with the currently available Stirling engines for low power, nothing in common with the handicraft engines from the online shop or with the mini CHPs for home use. Except the name.

Why Stirling engines today?

Because there is now a further development of the old technology that eliminates the previously known disadvantages and outperforms current systems.

As a reminder: Stirling engines work with external combustion or any heat source. That is exactly what we need in today's times of energy transition and decarbonization. The New Stirling Engine Technology can be used to replace large combustion engines and turbines and to tap unused heat sources for generating electrical energy.

Should Stirling engines be used in cars?

No. Cars will be electrified or will run on H2 in the future. The New Stirling Engine Technology is intended for high to very high capacities and is therefore predestined for stationary or ship-based use.

What is special about the New Stirling Engine Technology?

The patented New Stirling Technology is designed as the best Heat-to-Power solution in terms of efficiency, power output and pollutant emissions. The known thermodynamic process has been optimized by fundamentally re-thinking and re-designing the machine.

What is the objective and what are the best fields of application for the New Stirling Technology?

  • In the power range from approx. 500 kW to 15000 kW, better efficiencies are to be achieved with simpler equipment and problem-free operation.
  • If energy generation should or must continue to be based on combustion, then this must be done with the highest mechanical / electrical efficiency and also enable the use of non-fossil fuels. The so-called "methane slip" should be avoided.
  • Peak or excess output from regenerative energy generation can best be stored as heat ("Power-To-Heat") and best converted back into electricity ("Heat-to-Power") with a New Stirling Engine.
  • The use of previously unused waste heat must also be promoted. For example, these occur in large quantities in cement, glass, aluminium and steel works. This must be used in the interests of climate protection and in view of the future enormous increase in demand for electricity in the form of electricity. It will now be profitable and feasible to convert previously unused waste heat directly into electricity.
  • Conventional ship drives with piston engines or turbines generate immense amounts of CO2 and pollutant emissions. Here too there is now an alternative to reduce fuel consumption and use alternative fuels.

[ Overview of the preferred applications of the New Stirling Engine Technology]

Advantages and Disadvantages of Stirling Engines

Classic Stirling engines have undeniable advantages, but they could not prevail for the important applications because they also had so far system-related, typical disadvantages.

General Advantages of classic Stirling Motors

  • Many possible energy sources to generate power.
  • The combustion is continuous and thus results in favorable exhaust gas values.
  • Stirling engines are quiet, they produce neither explosion nor exhaust noises.
  • Low consumption of lubricating oil.
  • Stirling engines generate high torques at low speeds.

General Disadvantages of classic Stirling Motors

  • Stirling engines have an unfavorable power-to-weight ratio, which is why they are practically only used stationary with constant speed, constant torque or constant power.
  • Efficiency loss because the real process deviates significantly from the ideal process.
  • With classic Stirling engines, the maximum working temperature is limited by the materials used. In practice, the working gas can hardly be heated above 800 K.
  • Compression loss and limited regenerator efficiency.

Some like it hot

Hot gas engines appeared in the early 19th century. There were numerous designs. The best known is that of the brothers Robert and James Stirling from 1816. When people talk about a hot gas engine today, they actually mean the Stirling engine. In his patent, Stirling describes the use of the regenerator (economiser) for an air engine, but it was also intended for other applications such as stoves etc. to save fuel.

Alexander Kirk Rider is the only one who was able to realize a successful mass production with an Alfa type Stirling hot gas engine. From 1870 he sold around 80,000 machines.

Robert Stirling
Robert Stirling
Patent drawing of the Stirling engine from 1816
Patent drawing of the Stirling engine from 1816
1833-Ericsson-Motor from 1833
Ericsson’s "Caloric Engine" from 1833
Franchot-Engine from 1938
Franchot-Engine from 1938
The Franchot hot air machine has two double-acting cylinders on a crankshaft
Charles-Louis-Félix Franchot
Charles Louis Félix Franchot
Stillman-Engine from 1860
Stillman engine from 1860
Wilcox-Engine from1860
Wilcox-Engine from 1860
Wilcox-engine from1860
Wilcox-Engine from 1860
Shaw’s Hot air engine from1860
Shaw’s Hot air engine from1860
The open machine used the expanded air to feed the fire
Luftexpansionsmaschine von Lehmann von 1867
Lehmann air expansion machine from 1867
By 1878, 1,300 examples were built under license
Roper-Motor from1869
Roper-Motor from 1869
Rider-Motor von 1875
Rider Motor from 1875
The cold and hot areas are strictly separated from each other
Rider-Motor from 1875
Rider Motor from 1875
Alexander Kirk Rider
Robinson-Motor from 1881
Robinson hot air engine from 1881
This hot air engine from England uses the displacer as a regenerator.

The practical use of hot-air engines has been largely limited to low-power applications. At the beginning of the 20th century, around 250,000 Stirling engines were in use around the world, for example to drive water pumps and small appliances. From the 1920s onwards, petrol, diesel and electric motors became more widespread and increasingly pushed these historic Stirling engines out of the market.

In the middle of the 20th century, new attempts were made by various parties to further develop the Stirling engine. Nowadays it is mainly used in CHP units, as a power generator in private households, in space travel and as an air-independent propulsion system (AIP) for submarines.

Does the New Stirling Engine have the potential to outperform the competition?

Thermal efficiencies of existing technologies:
ηCarnot ηeff
Combustion engines 2775 1275 0.54 0.36
Classic Stirling machine 800 400 0.5 0.25
Gas turbine
("Micro Turbine")
1775 975 0.45 0.3
HP-Steam turbine 550 45 0.6 0.45
Expected efficiency of the New Stirling Technology:
ηCarnot ηeff
Example 1 800 400 0.5 0.3 ....0.4
Example 2 800 350 0.56 0.34 ...0.45
Example 3 850 400 0.53 0.32 ...0.45
Example 4 850 350 0.59 0.35 ...0.5
Example 5 900 400 0.56 0.33 ...0.47
Example 6 900 350 0.61 0.37 ...0.52
Example 7 950 400 0.58 0.35 ...0.49
Example 8 950 350 0.63 0.38 ...0.54
Example 9 1000 350 0.65 0.5 ... 0.57
Example 10 1100 350 0.68 0.55 ... 0.60
comparison of efficencies
Efficencies of various types of engines

1=New Stirling-Installation, 2=Combustion engine, 3=Gas engine, 4=Micro gas turbine, 5=Gas turbine
The specification of an efficiency only makes sense if the associated power and speed are mentioned. It is best to show the efficiency in a map. In this respect, the above values are only for orientation.

Classic Stirling engines only achieve 50% of their Carnot efficiency, the effective efficiency is correspondingly lower.
Internal combustion engines can have higher peak efficiencies, but in most operating states the efficiency is significantly lower. Cars avoid almost completely this peak area.

The larger the motor, the greater the maximum achievable efficiency. Ship engines reach up to 50%

[Question] What is the significance of the Carnot-Efficiency (1-Tmin/Tmax) ?

[Answer] The Carnot Efficiency defines the maximum efficiency within the range of the possible achievable temperatures, hence, the limit of the feasable. The New Stirling Technology does not only extend this range but also extends the imaginable efficiency within this physical limitation.

[Question] What makes the difference between Carnot efficiency and effective efficiency?

[Answer] The effective efficiency is smaller than the Carnot efficiency because the machine does not work without losses. These losses are (partly with Stirling engines, partly with internal combustion engines, partly with both):

  • Friction work
  • Heat radiation, wall heat losses
  • Flow losses
  • imperfect isothermal process or imperfect combustion
  • Overflow, leakage
  • Flushing losses
  • Gas exchange losses

The relationship between an ideal and a real machine is called the quality level. From the table above it can be seen that the quality of combustion engines and micro-turbines is around 65% and that of modern classic Stirling engines around 50%. This means that the losses are 35% and 50%, respectively. The New Stirling Engine Technology aims to achieve a quality level of 75 ... 85%.

The principles of the New Stirling Technology

"You have to make things as simple as possible. But not even simpler" (A. Einstein)

The New Stirling Technology follows the same principle of thermodynamics as classic machines, but the design of the equipment has been drastically changed. It is based on three main principles:
  • 1:1 implementation of the theoretical thermodynamic process
  • Apparatus and thermodynamic optimization
  • Simple mechanical construction
Before the New Stirling Technology is explained in more detail, please refer to the overview of the existing "classic" technology in order to understand in depth the changings.

[General Technology and Thermodynamics of Stirling motors]

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  • Dipl. Ing. Thomas Seidenschnur

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