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Department for Business, Energy and Industrial Strategy

Screw Expanders

1.1    Scope

Screw expanders are a particular category of saturated steam to electricity conversion equipment. This equipment is designed to convert waste or excess wet or saturated steam, from a specific process, into electrical power. Energy is removed from the steam as the steam expands and pressure reduces across the expander. Such energy is converted into shaft power energy and then to electricity. The technology features a screw rotor which turns as steam passes into the expander reducing the pressure. The rotors generate a positive torque on the drive shaft which is used to rotate the electrical generator.

1.2    Definitions

Screw expanders are specifically designed to convert wet or saturated steam, from a specific process, into electrical power by means of a closed thermodynamic power cycle that does not involve the internal combustion of fuel.

Screw Expanders are available in a range of efficiencies. The Energy Technology List (ETL) Scheme aims to encourage purchase of higher efficiency products, which can realise substantial reductions in carbon emissions when used to reduce the use of electricity from the mains supply.

To be eligible for inclusion on the ETL, products shall meet the requirements as set out below.

1.3    Requirements 

1.3.1    Eligibility requirements  

To be eligible, products shall:

  • Utilise waste or excess steam source from a process (i.e. steam is not produced for the primary purpose of power generation).
  • Use wet or saturated steam at the inlet (i.e. not superheated steam).
  • Be designed to use water or steam as the thermal working fluid (i.e. product shall not use any thermal working fluid applicable to Organic Rankine Cycle).
  • Not exceed 700kWe power output at standard conditions.
  • Be designed to provide three-phase electricity output.
  • Not incorporate any form of combustion equipment, including boost burners.
  • Be designed and include fittings for permanent installation.
  • Have an appropriate Conformity Assessment mark. 

1.3.2    Performance requirements 

Eligible products shall meet or exceed minimum net electrical efficiencies as set out in Table 1.1. Minimum net electrical efficiencies shall be achieved across the specified range of inlet and outlet pressures.

Table 1.1    Net Electrical efficiency thresholds Screw Expanders

Standard conditions for the measurement of net electrical efficiencies   Inlet Pressure Test Point (barA)
8 11 15
Product Outlet Pressure Test Point (barA) Minimum Net Electrical Efficiency%
Screw Expanders 2 >= 3.5 >= 4.4 >= 5.3
5 N/A >= 2.5 >= 3.5

1.4    Measurement and calculations

1.4.1    Performance metrics

The Net Electrical Efficiency is defined as: 

$$Net\ Electrical\ Efficiency\ (\%)=\ \frac{Net\ Electrical\ Output\ (kWe)}{Inlet\ Enthalpy\ \left(kJ/kg\right)\ast Inlet\ Mass\ Flow\ Rate\ (kg/s)}$$

Net electrical output is defined as the electrical output minus any electrical input into the product. Inlet enthalpy is the enthalpy of the steam entering the product. The inlet mass flow rate is the flowrate of the steam as it enters the product. 

1.4.2    Measurement Standards and Test Requirements

The required minimum performance shall be determined as mentioned below. 

Product performance shall be demonstrated by calculating the net electrical efficiency from measurements of net electrical output, inlet enthalpy drop and inlet mass flow rate. Net electrical efficiencies shall meet or exceed the threshold minimum efficiencies as set out in Table 1.1. 

Products can either be tested in an accredited laboratory or on-site as part of a user acceptance testing process. For the purposes of compliance and verification of compliance with the ETL requirements, measurements and calculations shall be made using harmonised standards or other reliable, accurate and reproducible method, which takes into account the generally recognised state-of-the-art methods.  User acceptance testing reports shall be considered along with the manufacturers and supplier’s internal user-acceptance testing procedure. This may or may not include references to elements of BS EN 60953-2: 1996: Rules for steam turbine thermal acceptance tests. Wide range of accuracy for various types and sizes of turbines.

The product shall not exceed the threshold electrical power output of 700kWe at standard conditions. 

The test report shall include (or be accompanied by): 

     a) Manufacturer’s design data for the product
     b) Details of the testing methodology (including any standards used) used to determine product performance
     c) A copy of the published performance data for the product

1.5    Verification for ETL Listing 

Any of the following testing routes may be used to demonstrate the conformity of products against the requirements:

  • In-house testing – Self-tested and verified or cross-checked by an independent body
  • Witnessed testing
  • Independent testing
  • Acceptance Tests or Field Trials

Further information regarding the routes can be found in Guidance Note 5 on the ETL product testing framework[1].

1.6    Conformity testing

Products listed on the ETL may be subject to the scheme’s conformity testing programme in order to ensure listed models continue to meet the ETL requirements.

1.7    Review  

1.7.1    Indicative review date

This specification is scheduled to be reviewed during the 2024/25 review cycle. 

1.7.2    Illustrative future direction of the requirements

The next technical review will consider:  

  • Increasing performance thresholds.