CHPQA guidance note 28: The determination of Z ratio
Summary
DESNZ publishes guidance on calculating Z ratios for Combined Heat and Power schemes with steam turbines, defining how to measure the trade-off between power generation and heat supply when steam is extracted at different pressures. The guidance provides reference tables showing Z ratios from 3.9 to 8.1 depending on turbine size (2-50+ MWe) and steam pressure (2.4-21.7 bar). Operators must determine their specific Z ratio through measurement or calculation, with fallback to tabulated values if testing is not feasible.
Why it matters
This is technical guidance for existing regulations rather than new policy — it clarifies how CHP operators should calculate efficiency parameters for CHPQA certification. As such, it affects compliance methodology for existing schemes but does not change the underlying incentives or market structure for CHP deployment.
Key facts
- •Z ratios range from 3.9 to 8.1 depending on turbine size and steam pressure
- •Applies to CHP schemes with fully or partially condensing steam turbines
- •Turbine efficiency assumptions: 65% (2-5 MWe) to 84% (50+ MWe)
- •Steam export pressures from 2.4 to 21.7 bar(a) covered in reference tables
Areas affected
Memo
## What this is about
DESNZ has updated guidance on calculating Z ratios for CHP schemes using steam turbines, addressing a technical compliance requirement rather than introducing new policy. The Z ratio measures the trade-off between electricity generation and heat supply when steam is extracted from turbines at different pressures—a critical parameter for determining CHP efficiency under the Combined Heat and Power Quality Assurance (CHPQA) programme.
This guidance matters because CHP operators must demonstrate specific efficiency thresholds to qualify for business rate relief, enhanced capital allowances, and exemption from the Climate Change Levy. Getting Z ratio calculations wrong affects both regulatory compliance and the financial viability of CHP investments, particularly for schemes with pass-out steam turbines where operators can vary the balance between power and heat output.
## Key points
Reference Z ratio tables: The guidance provides standardised Z ratios ranging from 3.9 to 8.1, varying by turbine size and steam export pressure. Larger, more efficient turbines have lower Z ratios—a 50+ MWe turbine operating at 21.7 bar has a Z ratio of 3.9, while a 2-5 MWe turbine at 2.4 bar reaches 8.1. These reflect typical thermodynamic efficiencies from 65% for smaller units to 84% for large installations.
Measurement methodology: Operators must determine site-specific Z ratios through testing or calculation rather than relying solely on manufacturer specifications. The preferred measurement approach involves fixing boiler output while incrementally increasing steam extraction, observing changes in power generation. This requires careful coordination to avoid disrupting industrial processes or creating safety risks.
Calculation requirements: Where measurement testing isn't feasible, operators can calculate Z ratios using steam flow measurements, pressure and temperature data, and thermodynamic analysis via steam tables or ASME97 software. The calculation method needs at least two of three mass flow measurements (steam in, steam out, condensate out) plus power output data.
Multiple pressure handling: For schemes with multiple steam export pressures, operators must calculate weighted average Z ratios based on mass flow proportions at each pressure level. Fully condensing turbines use the steam pressure before the turbine inlet as the reference point.
Fallback provisions: When neither testing nor calculation is practical, operators can use tabulated values from the guidance, interpolating between listed steam pressures as needed. However, they must provide written justification for why site-specific determination wasn't possible.
Evidence requirements: All CHP schemes must supply evidence of their applicable Z ratio regardless of the determination method. The guidance explicitly states that manufacturers' specifications don't represent normal operating conditions and require correction for performance degradation over time.
## What happens next
This is updated technical guidance for existing CHPQA regulations rather than new policy requiring consultation or parliamentary approval. CHP operators should review their current Z ratio determinations against the updated methodology and reference tables, particularly if they're using outdated manufacturer data or haven't conducted recent performance testing.
Schemes approaching CHPQA renewal or facing compliance reviews should prioritise site-specific Z ratio determination through measurement or calculation. The guidance includes a worked calculation example in Appendix 4 that operators can use as a template for their own assessments.
DESNZ hasn't indicated any transition period, suggesting the guidance takes immediate effect. However, existing CHPQA certificates remain valid until their scheduled renewal dates, giving operators time to implement the updated methodology before their next compliance assessment.
The timing aligns with broader government efforts to improve CHP scheme monitoring and ensure that efficiency claims reflect actual operational performance rather than theoretical design parameters. This technical clarification supports the policy objective of directing CHP incentives toward genuinely efficient installations while maintaining clear compliance pathways for existing schemes.
Source text
For CHP Schemes which include fully or partially condensing (pass-out) steam turbines, Power Efficiency will decline as steam extraction increases for a given fuel consumption, so there is a balance between increasing heat recovery and reducing power output. The trade-off between heat to site and power for these CHP Schemes is known as the Z ratio. CHPQA Guidance Note 28 v2 Page 1 © Crown Copyright 2026 GUIDANCE NOTE 28 THE DETERMINATION OF Z RATIO Definition and Use of Z Ratio GN28.1 For CHP Schemes which include fully or partially condensing (pass-out) steam turbines, Power Efficiency will decline as steam extraction increases for a given fuel consumption, so there is a balance between increasing heat recovery and reducing power output. The trade-off between heat to site and power for these CHP Schemes is known as the Z ratio. The Z ratio can be derived from actual measurements of the CHP Scheme in question or may be determined by calculation. The Z ratio depends on the pressure and temperature of the steam supplied to site, the steam turbine generating set’s thermodynamic (isentropic) and mechanical efficiencies and the vacuum (or pressure) maintained in the condenser. Rigorous calculation requires knowledge of all of the above parameters. GN28.2 Table GN28-1 shows Z ratios for different steam export pressures and for turbine thermodynamic efficiencies that are typical in operation for steam turbines in the size ranges shown. This Table is illustrative as the Z ratio is influenced by the steam turbine loading and the condition of the turbine blades. Each CHP Scheme must determine, and supply evidence of the Z ratio that is applicable to its operation. Manufacturers’ specifications are not generally representative of normal operation (but can be corrected for fall-off in performance since new). GN28.3 Where there is more than one pass-out or steam export pressure, the Z ratio should be the mean of the values at each pressure level weighted in proportion to mass flow. For a fully condensing steam turbine (no pass-out) the steam export pressure should be taken as the steam pressure before the steam turbine. CHPQA Guidance Note 28 v2 Page 2 © Crown Copyright 2026 Table GN28-1 Typical Z ratios for given steam turbines and steam pressures Steam turbine size range 2 to <5 MWe 5 to <10 MWe 10 to <25 MWe 25 to <50 MWe Above 50 MWe Typical thermodynamic (isentropic) efficiency 65% 70% 75% 80% 84% Steam export pressure 21.7 bar(a) 5.0 4.7 4.4 4.1 3.9 14.8 bar(a) 5.4 5.0 4.7 4.4 4.2 11.4 bar(a) 5.7 5.3 4.9 4.6 4.4 7.9 bar(a) 6.1 5.7 5.3 5.0 4.7 3.8 bar(a) 7.2 6.7 6.3 5.9 5.6 2.4 bar(a) 8.1 7.5 7.0 6.6 6.3 DETERMINATION OF Z RATIO GN28.4 To determine the Z ratio by measurement requires some careful manipulation of the CHP operation and requires that adequate metering of power and steam outputs is in place. Each site must decide how to carry out the test without disrupting process operations or putting plant or personnel at risk. In general, it is suggested that the start point should be with the condensers at the highest load possible. Ideally, the boilers supplying the turbine steam should be on fixed output (firing set point) to give a constant flow, pressure and temperature at the turbine inlet. The steam extraction or export can then be increased in small steps thereby reducing the flow to the condensers, and the changes in steam export and power generation observed. If some or the entire site heat load can initially be provided by standby boiler plant, which can then automatically be backed off as the extraction is increased, this will enable the test to cover the greatest operating range. It may otherwise be necessary to vent surplus steam during the test to permit a reasonable range of condenser loading to be covered. GN28.5 Where there is more than one pass-out pressure it may be possible to determine the Z ratio for each pressure by keeping the other pass-out(s) constant. Otherwise, a single experiment will enable the mean isentropic efficiency to be determined and then the individual Z ratios may be determined by calculation. GN28.6 Determination of the Z ratio by calculation also involves some on-site measurements but avoids the need to alter the turbine operation. Measurements of at least two of the three mass flows (steam in, steam out and condensate out) and the power output are necessary. In addition, the pass-out steam pressure and temperature and condenser vacuum (pressure) are required. CHPQA Guidance Note 28 v2 Page 3 © Crown Copyright 2026 The calculation requires the use of steam tables, a steam enthalpy-entropy diagram, or software such as the ASME97 steam tables. ➢ Refer to Appendix 4 of the Guidance Notes for an example calculation of Z Ratio. GN28.7 Where it is not possible to determine the Z ratio based on steam turbine performance testing a statement giving the reasons is required. In such cases, the Z ratio may be taken from Table GN28-1, interpolating, if necessary, between the steam pressures tabulated.