]The Silent Waste
Every day, enormous amounts of heat are generated in industrial plants, power stations, and ship propulsion systems. But a large portion of this remains unused – it escapes unnoticed into the environment. This silent waste is one of the biggest efficiency problems of our time: Valuable energy is lost while, at the same time, the demand for electricity and climate-friendly solutions is increasing.
- Over 160,000 GWh of industrial waste heat are wasted annually in Germany – often at temperatures above 500°C, even though it could be used directly for electricity generation.
- More than 9,000 GWh of electricity from renewable energy sources are curtailed or sold at negative prices every year – a potential that can be recovered through storage in thermal storage and reconversion to electricity in the Heat2Power engine.
Why hot gas engines today?
The conversion of heat into mechanical work is a universal principle that has been used since the 19th century. Today it is gaining new significance: Any heat source – whether fossil, renewable, or industrial waste heat – can serve as a starting point for electricity generation. This flexibility is crucial, especially in times of energy transition and decarbonization.
Objectives and application areas of modern Heat-to-Power concepts
Classic Stirling engine technology forms the basis for a new generation of heat-to-power systems. While traditional Stirling engines were long used only in niche applications, the further development towards the modern heat-to-power concept shows that their thermodynamic principles can now be used in entirely new performance ranges.
The goal is to create robust and efficient systems in the 500 kW to 15,000 kW range that can convert all types of heat sources into electricity. These concepts are particularly suitable for:
- Reconversion of excess heat from renewable energy generation
- Utilization of industrial waste heat (e.g., cement, glass, and steel plants)
- Stationary power generation with biomass or combined heat and power plants
- Ship propulsion systems with reduced fuel consumption and alternative fuels
This makes it clear: The Stirling engine is not a thing of the past, but provides the basis for the Heat2Power-Engine, which overcomes classic limitations and opens up new fields of application.
Advantages and disadvantages of Stirling engines
Classical Stirling engines of modern design have undeniable advantages, but have not been able to prevail for the important applications because they have also had system-related, typical disadvantages.General advantages of classic Stirling engines
- Many possible energy sources for heat generation.
- Comparatively simple, robust design with a long service life.
- External combustion is continuous. Modern burner systems result in very low emissions.
- Since they produce neither explosion nor exhaust noise, Stirling engines are quiet.
- Low lubricating oil consumption.
- Stirling engines generate high torque at low speeds.
General disadvantages of classic Stirling engines
- Stirling engines have an unfavorable power-to-weight ratio; therefore, they are practically only used in stationary applications with constant speed, constant torque, or constant power.
- Loss of efficiency because the real process deviates significantly from the ideal process.
- In classic Stirling engines, the maximum operating 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 emerged at the beginning of the 19th century. Numerous designs existed. The most famous is that of the brothers Robert and James Stirling from 1816. Today, when people talk about a hot gas engine, they usually mean the Stirling engine. In his patent, Stirling describes the use of the regenerator (economizer) for an air engine, which was also intended for other applications, such as furnaces, to save fuel.
Alexander Kirk Rider is the only person who achieved successful mass production of a Stirling hot gas engine, type Alfa. From 1870 onward, he sold around 80,000 of these machines.
The practical use of hot air engines was largely limited to low-power applications. At the beginning of the 20th century, approximately 250,000 Stirling engines were in use worldwide, for example, to power water pumps and small appliances. From the 1920s onward, Otto, diesel, and electric engines became increasingly widespread, gradually displacing these historic Stirling engines from the market.
In the mid-20th century, various efforts were made to further develop the Stirling engine. Today, it is primarily used in combined heat and power plants, as a generator in private households, in space travel, and as an air-independent propulsion system for submarines.
The Franchot hot air machine has two
double-acting cylinders on one crankshaft.
1860
1860
1860
The open machine used the expanded air to fuel the fire.
By 1878, 1,300 units had been built under license.
1869
The cold and hot areas are strictly separated.
1875
This hot air engine from England uses
the displacer as a regenerator.
Performance Comparison of Heat Engines
| Efficiency levels of existing systems: | ||||
|---|---|---|---|---|
| Tmax [K] |
Tmin [K] |
ηCarnot | ηeff | |
| internal combustion engines | 2775 | 1275 | 0.54 | 0.36 |
| Classic Stirling engine | 800 | 400 | 0.50 | 0.25 |
| Gas Turbine ("Micro Turbine") |
1775 | 975 | 0.45 | 0.30 |
| HP-Steam-Turbine | 600 | 400 | 0.60 | 0.45 |
| Calculated efficiencies of the Heat2Power Engine: | ||||
| Tmax [K] |
Tmin [K] |
ηCarnot | ηeff | |
| Example 1 | 800 | 400 | 0.50 | 0.30 … 0.40 |
| 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 … 0.50 |
| Example 5 | 850 | 280 | 0.67 | 0.53 … 0.57 |
| Example 6 | 900 | 400 | 0.56 | 0.33 … 0.47 |
| Example 7 | 900 | 350 | 0.61 | 0.37 … 0.52 |
| Example 8 | 950 | 400 | 0.58 | 0.35 … 0.49 |
| Example 9 | 950 | 350 | 0.63 | 0.38 … 0.54 |
| Example 10 | 1000 | 350 | 0.65 | 0.50 … 0.55 |
| Example 11 | 1100 | 350 | 0.68 | 0.55 … 0.58 |
| Example 12 | 1100 | 280 | 0.75 | 0.60 … 0.64 |
|
1=Heat2Power engine, 2=Internal combustion engine, 3=Gas engine, 4=Micro gas turbine, 5=Gas turbine |
More technical details: Explanations regarding efficiency
The Carnot efficiency defines the theoretical upper limit for the conversion of heat into work:
ηCarnot = 1 − Tmin / Tmax.
The effective efficiency (ηeff) is lower because real machines have unavoidable losses: Friction, heat radiation, exhaust losses, flow losses and limited regenerator efficiency.
Classic Stirling engines typically achieve only about 50% of their Carnot efficiency. The Heat2Power engine is designed for a performance factor of 75–85% and can therefore achieve significantly higher efficiencies.
The principles of modern heat-to-power concepts
„Things need to be made as simple as possible. But not simpler.“ (A. Einstein)
The thermodynamic principles of the Stirling engine form the basis, but modern heat-to-power concepts build upon them. Through simple mechanics, optimized equipment, and adiabatic process control, the Heat2Power-Engine is created, which unlocks higher efficiencies, lower losses, and flexible application possibilities.
The Stirling engine remains a relevant concept; the Heat2Power engine adopts its thermodynamic principles, develops them further, and transcends their known limitations. This topic forms the core of this website and is explained in detail on the following pages – from the fundamentals of classical technology to specific technical innovations.
The classic Stirling engine technology forms the starting point, on which the Heat2Power-Engine is based and its further developments become understandable.