CHPQA appendix 4: Determination of Z-ratio by plant performance test
Summary
DESNZ provides worked example for calculating Z-ratios through plant performance testing for combined heat and power qualification assessment. The example demonstrates solving simultaneous equations from three test conditions to determine specific electricity generation rates for different steam extraction points. This determines the Z-factor that measures efficiency loss when diverting steam for heat rather than electricity generation.
Why it matters
Technical guidance that clarifies existing methodology rather than changing it. Z-ratio calculations directly affect CHP qualification for business rates relief and climate change levy exemption — administrative precision matters for scheme eligibility.
Key facts
- •Example shows Z-factors of 4.54 for medium pressure and 6.34 for low pressure steam extraction
- •Weighted mean Z-factor calculated as 5.52 based on annual average steam flows
- •Method requires three independent test conditions to solve simultaneous equations
- •Steam conditions: MP at 15 bar/295°C, LP at 4 bar/165°C
Areas affected
Memo
What this is about
DESNZ has published a worked example showing how CHP operators can calculate Z-ratios through plant performance testing rather than using standard tabulated values. The Z-ratio measures the electricity generation lost when steam is diverted from turbines to provide heat - a critical parameter for determining CHP scheme efficiency and qualification for tax reliefs.
The guidance walks through testing a dual pass-out steam turbine system, solving simultaneous equations from three operating conditions to derive specific electricity generation rates for medium-pressure steam (47.76 kWh/tonne), low-pressure steam (110.76 kWh/tonne), and condenser steam (230.86 kWh/tonne). This yields Z-factors of 4.54 for MP steam and 6.34 for LP steam, with a weighted average of 5.52.
Key points
Testing methodology requires three independent datasets to solve for three unknowns: steam flows to MP pass-out, LP pass-out, and condenser. The example uses controlled variations in pass-out flows while maintaining constant total steam flow to the turbine.
Data normalisation is essential for accurate comparison. The guidance normalises all three test cases to 84.0 te/h total steam flow before solving the simultaneous equations, accounting for minor variations in operating conditions during testing.
Z-factor calculation follows established thermodynamics: - MP steam (15 bar/295°C): 829 kWh/tonne heat delivery, 183.1 kWh electricity forgone = Z-factor 4.54 - LP steam (4 bar/165°C): 762 kWh/tonne heat delivery, 120.1 kWh electricity forgone = Z-factor 6.34 - Weighted average based on annual steam flows: 5.52
The weighted average reflects operational reality rather than peak design conditions. With annual averages of 28.7 te/h MP steam and 34.3 te/h LP pass-out (including 4.0 te/h for deaerator/condensate heating), the calculation weights LP steam more heavily in the final Z-factor.
Site datum temperature matters for heat calculations. The example uses 10°C as the site datum, subtracting 42 kJ/kg from steam enthalpy values to determine usable heat content. This detail affects Z-factor precision and scheme qualification margins.
Performance testing offers customisation over standard values. Rather than using generic Z-factors from CHPQA lookup tables, operators can demonstrate their specific plant efficiency characteristics through testing. This matters for marginal schemes where standard assumptions might understate actual performance.
Quality standards remain implicit but critical. The guidance assumes operators follow proper testing protocols - stable operating conditions, accurate instrumentation, representative operating points. Poor test data invalidates the entire calculation chain.
What happens next
Implementation is immediate - operators can use this methodology for current CHPQA applications and annual returns. The guidance clarifies existing rules rather than introducing new requirements, so no transition period applies.
Testing costs versus benefits require evaluation. Performance testing demands significant resources - instrumentation, stable operating periods, engineering analysis. Operators should compare potential Z-factor improvements against testing costs and scheme value uplifts.
Documentation standards will matter for CHPQA assessments. While the guidance shows the calculation method, operators submitting performance test results will need comprehensive supporting evidence - test protocols, instrumentation specifications, raw data, quality assurance procedures.
No further consultations are flagged on Z-ratio methodology. This appears to close a technical gap in existing guidance rather than signal broader CHPQA reforms. The March 2026 publication date suggests routine guidance updates rather than urgent policy changes.
Integration with broader CHP policy remains unchanged. Z-ratio calculations feed into overall efficiency assessments for business rates relief and climate change levy exemption, but this guidance doesn't alter those frameworks or thresholds.
Source text
Example 4 outlines determination of Z-ratio by plant performance test. CHPQA Guidance Note Appendix 4 v3 Page 1 © Crown Copyright 2026 APPENDIX 4 EXAMPLE 4 - DETERMINATION OF Z-RATIO BY PLANT PERFORMANCE TEST A test was carried out in which the two pass-out streams from the steam turbine in a Scheme were altered in turn whilst endeavouring to maintaining a constant steam flow to the ST. The flow of LP steam to the deaerator and condensate heater was estimated at 4.0 te/h. This was assumed to remain unchanged. Since there are two pass-out streams and a condenser (vacuum exhaust) steam, we need to derive the specific electricity generation (kW per tonne/h of steam, i.e. kWh/tonne) for each stream. To solve for three unknowns three independent sets of data are required. The three sets of test data are: Total steam te/h LP steam to site, te/h MP steam to site, te/h Generation kWe 1. Initial operation 82.9 28.0 25.5 9,471 2. Increased MP pass-out 84.0 28.0 30.0 10,056 3. Increased LP pass-out 83.5 33.0 25.0 10,255 Step 1: Derive LP pass-out (= LP steam to site + 4.0 te/h) and flow of steam to condenser Total steam te/h LP pass-out te/h MP pass-out te/h Condenser steam te/h Generation kWe Case 1 82.9 32.0 25.5 25.4 10,626 Case 2 84.0 32.0 30.0 22.0 10,056 Case 3 83.5 37.0 25.0 21.5 10,255 Step 2: Normalise data to a total steam flow of 84.0 te/h Total steam te/h LP pass-out te/h MP pass-out te/h Condenser steam te/h Generation kWe Case 1 84.0 32.425 25.838 25.737 10,767 Case 2 84.0 32.000 30.000 22.000 10,056 Case 3 84.0 37.221 25.150 21.629 10,317 Step 3: Calculate the kWh/tonne of steam to MP pass-out, LP pass-out and condenser by solving 3 simultaneous equations Solution of simultaneous linear equations Form: W = aX + bY + cZ Equation 1 10,767 = 32.425 a + 25.838 b + 25.737 c Equation 2 10,317 = 37.221 a + 25.150 b + 21.629 c CHPQA Guidance Note Appendix 4 v3 Page 2 © Crown Copyright 2026 Equation 3 10,056 = 32.000 a + 30.000 b + 22.000 c Eliminate a 8,987.6 = 32.425 a + 21.909 b + 18.842 c 10,189.6 = 32.425 a + 30.398 b + 22.292 c Equation A 1779.4 = 3.929 b + 6.895 c Equation B 577.4 = -4.560 b + 3.445 c Eliminate b -497.4 = 3.929 b + -2.968 c -2276.8 = 0.000 b + -9.863 c c = 230.86 Substitute in Equation A b = 47.76 Solve Equation 1 for a a = 110.76 Check in Equation 2 10,317 = 4,122.7 + 1,201.2 + 4,993.1 OK Check in Equation 3 10,056 = 3,544.4 + 1,432.8 + 5,078.8 OK Step 4: Heat to site per tonne/h of MP and LP pass-out MP pass-out condition: 15 bar / 295°C, specific enthalpy 3,027 kJ/kg (datum 0°C) Site datum 10°C, water specific enthalpy 42 kJ/kg Heat to site in MP pass-out = (3,027 – 42) kJ/kg = 2,985 kJ/kg = 2,985 MJ/tonne = 2,985 𝑀𝐽 𝑡𝑜𝑛𝑛𝑒× 1 3.6 𝑘𝑊ℎ 𝑀𝐽 = 829 kWh/tonne LP pass-out condition: 4 bar / 165°C, specific enthalpy 2,786 kJ/kg (datum 0°C) Site datum 10°C, water specific enthalpy 42 kJ/kg Heat to site in MP pass-out = (2,786 – 42) kJ/kg = 2,744 kJ/kg = 2,744 MJ/tonne = 2,744 𝑀𝐽 𝑡𝑜𝑛𝑛𝑒× 1 3.6 𝑘𝑊ℎ 𝑀𝐽 = 762 kWh/tonne Step 5: Determine Z factor for MP and LP pass-out for 1 tonne/h increased MP pass-out 𝐼𝑛𝑐𝑟𝑒𝑎𝑠𝑒𝑑 ℎ𝑒𝑎𝑡 𝑡𝑜 𝑠𝑖𝑡𝑒= 829 𝑘𝑊ℎ𝑡ℎ 𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑖𝑛 𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛= 230.86 𝑘𝑊ℎ𝑒−47.76 𝑘𝑊ℎ𝑒= 183.10 𝑘𝑊ℎ𝑒 𝑍−𝑓𝑎𝑐𝑡𝑜𝑟= 829 𝑘𝑊ℎ𝑡ℎ 183.1 𝑘𝑊ℎ𝑒= 4.54 For 1 tonne/h increased LP pass-out 𝐼𝑛𝑐𝑟𝑒𝑎𝑠𝑒𝑑 ℎ𝑒𝑎𝑡 𝑡𝑜 𝑠𝑖𝑡𝑒= 762 𝑘𝑊ℎ𝑡ℎ 𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑖𝑛 𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛= 230.86 𝑘𝑊ℎ𝑒−110.76 𝑘𝑊ℎ𝑒= 120.10 𝑘𝑊ℎ𝑒 CHPQA Guidance Note Appendix 4 v3 Page 3 © Crown Copyright 2026 𝑍−𝑓𝑎𝑐𝑡𝑜𝑟= 762 𝑘𝑊ℎ𝑡ℎ 120.10.1 𝑘𝑊ℎ𝑒= 6.34 Step 6: Determine weighted mean Z-factor Annual average MP steam to site = 28.7 tonnes/h Annual average LP steam pass-out = (30.3 + 4) = 34.3 tonnes/h Annual average total pass-out = (28.7+34.3) = 63.0 tonnes/h 𝑊𝑒𝑖𝑔ℎ𝑡𝑒𝑑 𝑚𝑒𝑎𝑛 𝑍−𝑓𝑎𝑐𝑡𝑜𝑟= [(28.7 𝑡𝑜𝑛𝑛𝑒𝑠 ℎ ) × 4.53] + [(34.3 𝑡𝑜𝑛𝑛𝑒𝑠 ℎ ) × 6.34] 63 𝑡𝑜𝑛𝑛𝑒𝑠 ℎ = 𝟓. 𝟓𝟐