In many industrial plants, energy leaves the site unused via the stack: as hot exhaust gas from furnaces, dryers or firing processes. Forman and colleagues (2016) estimate that around 72 percent of the primary energy used worldwide is lost after conversion, much of it as heat.

Part of this heat can be converted into power. Which technology suits depends mainly on the temperature. In this article I compare the common processes and show where the externally fired micro gas turbine has its strengths.

Why does temperature decide?

The Carnot efficiency describes the maximum amount of power that can be obtained from heat. It depends only on the temperature of the heat source and of the surroundings. With waste heat at 150 °C and 25 °C ambient, the theoretical limit is around 30 percent. At 900 °C it is around 75 percent. Real plants achieve considerably less, but the ratio remains: hot waste heat is more valuable.

That is why it pays to look at the temperature level before talking about technology. Exhaust gases from glass, ceramics or metal processes are often at several hundred to over a thousand degrees. Waste heat from cooling water or compressed air, on the other hand, is usually below 100 °C.

Which processes are there?

ORC plant

The Organic Rankine Cycle works like a steam power plant, only with an organic working medium that evaporates even at low temperatures. ORC plants are mature and common for sources of about 100 to 350 °C. Quoilin and colleagues (2013) give a good overview of technology, costs and typical efficiencies.

Steam turbine

Steam is the classic working medium of power plant engineering. At high temperatures and large outputs, the steam turbine is efficient. In the small output range, however, boiler, water treatment and operation quickly become complex.

Stirling engine and thermoelectrics

Stirling engines use heat from outside and are interesting for small outputs, but are not widespread on the market. Thermoelectric generators have no moving parts but only achieve low efficiencies.

Externally fired micro gas turbine

Here the waste heat heats clean, compressed air in a high-temperature heat exchanger. This air drives the turbine, which generates power via a generator. The turbine never comes into contact with the exhaust gas of the heat source. In their review article, Al-attab and Zainal (2015) describe the strengths of this design: it can use heat from sources that are not suitable for direct combustion in the turbine.

Guide values for orientation. The limits depend on manufacturer, design and heat source.

When does the externally fired micro gas turbine pay off?

The technology plays to its strengths with very hot waste heat that occurs continuously. Our ClinX Heat is designed for exhaust gases of about 900 to 1,000 °C. Per module it generates 150 kW of power (gross) and provides up to 450 kW of heat.

Advantages in operation

• Air as the working medium: no water cycle, no water treatment, no refrigerant.
• Air bearings without lubricating oil and without oil changes.
• The warm exhaust air of the turbine is available as useful heat, for example for drying or heating.
• No intervention in the production process: the waste heat is branched off and used.

Combination with ORC

The processes are not mutually exclusive. Downstream of ClinX Heat, an ORC plant can still convert the remaining heat at a lower temperature level into power.

Checklist: is your waste heat suitable?

Before talking about technology, it is worth looking at the heat source. We clarify these questions in an initial conversation:

• Temperature: how hot is the exhaust gas at the point where it can be tapped?
• Quantity: how large is the volume flow, i.e. how much hot exhaust gas is produced?
• Availability: does the process run around the clock or only in shifts? More operating hours mean more power.
• Composition: does the exhaust gas contain dust or aggressive components? This determines the material and design of the heat exchanger.
• Space: is there room for a 40-foot container near the heat source?
• Consumers: what can power and residual heat be used for on site?

What determines economic viability?

The greatest influence comes from operating hours and the electricity price you save with your own power. The more hours the heat source runs per year and the more power you consume yourself, the faster the plant pays off. Then there is the value of the residual heat if it can be used in the plant, for example for drying or heating. There are also funding programmes for energy efficiency in industry for waste heat recovery projects.

Frequently asked questions

From what temperature can waste heat be converted into power?

ORC plants work from about 90 °C, though with low efficiency. The higher the temperature, the more power can be obtained. ClinX Heat is designed for exhaust gases at 900 to 1,000 °C.

What is the difference between ORC and ClinX Heat?

An ORC plant uses an organic working fluid in a closed cycle and is suitable for lower temperatures. ClinX Heat uses air as the working medium and is intended for high-temperature waste heat. Both can be combined.

Does the production process have to be modified?

Not with ClinX Heat. The plant is connected to the existing hot gas flow, and the main process continues unchanged.

What happens with dust-laden exhaust gases?

ClinX Heat tolerates a dust content below 10 mg/m³. If it is higher, a filter can be installed upstream. We clarify this during project planning.

Is power from waste heat below 100 °C worthwhile?

It is technically possible but usually difficult economically because efficiency is very low. At low temperatures it often makes more sense to use the heat directly, for example for heating or via a heat pump.

Does ClinX Heat need water?

Not for power generation: the turbine’s working medium is air. Water only comes into play where you use the residual heat via a heating water circuit.

Sources

• Forman, C. et al. (2016): Estimating the global waste heat potential. Renewable and Sustainable Energy Reviews 57, 1568 to 1579.
• Quoilin, S. et al. (2013): Techno-economic survey of Organic Rankine Cycle (ORC) systems. Renewable and Sustainable Energy Reviews 22, 168 to 186.
• Al-attab, K. A., Zainal, Z. A. (2015): Externally fired gas turbine technology: A review. Applied Energy 138, 474 to 487.