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Department for Business, Energy and Industrial Strategy

Thermal Energy Storage

1.1    Scope

The scope of this technology is for commercial and industrial thermal energy storage (TES) products including applications for power generation, heating and cooling.

1.2    Definitions

TES is a technology that stocks thermal energy by heating or cooling a storage medium so that the stored energy can be used at a later time for heating and cooling applications and/or power generation.

Thermal energy storage is available in a range of different designs and efficiencies. The Energy Technology List (ETL) Scheme aims to encourage the purchase of higher efficiency products.

The ETL Scheme covers two categories of TES products:

  • Small Industrial/ Large commercial 
  • Large Industrial 

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:   

  • Have a capacity of no less than 5 MWh

1.3.2    Performance requirements

Products shall have:   

  • An energy density of no less than 50 kWh/m3 
  • An expected lifespan no less than 15 years
  • A round trip energy efficiency of no less than 85%

1.4    Measurement and Calculations

1.4.1    Measurement standards

There are no appropriate BS EN and/or IEC standards for testing TES products to determine conformity with the above requirements. Therefore, the test requirements specified in section 1.4.2 are to be used to determine product performance, and verified in line with section 1.5.

1.4.2    Test Requirements

In line with the eligibility and performance requirements above:

  • The TES performance test for capacity and energy density shall be determined by the manufacturer depending on the application of the system and fully evidenced and verified at the point of application.
  • The lifespan of the TES product shall be provided by the supplier and fully evidenced and verified at the point of application.
  • The roundtrip efficiency shall be calculated and verified to determine the energy output that the TES product can deliver, relative to the amount of energy input into the TES product during charging and discharging. The following procedure shall be followed. The TES product shall be:
    • Discharged to its minimum available energy level
    • Charged to its full available energy at rated input power
    • Discharged at the rated power of the system in accordance with system specifications and operating instructions

The roundtrip efficiency will be the ratio of total output energy divided by total input energy over one charging/discharging cycle using rated input and output power.

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 first three routes can be found in the ETL Testing Framework.

1.6    Conformity testing

Products listed on the ETL may be subject to the scheme’s conformity assessment 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 first technical review of these criteria will consider:

  • The availability of specific test and measurement standards for TES product performance.
  • The suitability and appropriateness of the product categorisation
  • Uptake of the TES technology listed on the ETL.
  • Self-discharge rate.
  • Market developments for the technology in the UK.
  • Use of distributed TES with solar PV and wind

Review of performance requirements, in particular whether round trip efficiency can be tightened.