Accuracy of CV determination systems for calculation of FWACV Dave Lander Update 12 th October 2011.

Accuracy of CV determination systems for calculation of FWACV

Dave Lander

Update 12th October 2011

Overview

Based on work previously carried out October 2006

Examines how consumers gas bills are estimated

Examines how the accuracy of all of the inputs into the calculation affects the overall accuracy of the gas bill

Poses questions about:

• fairness

• the appropriate level of acuracy

Accuracy of CV determination systems -

Introductory concepts: error, uncertainty, bias...

Uncertainty

• "Parameter that characterises the spread of values that could reasonably be attributed to the measurand."

• Range and an associated probability

Error

• Measured result minus a “true” value

Bias

• Mean value of a distribution of errors.

• Associated with an agreed set of conditions (each showing an error)


The Charging Area CV

Charging area CV is calculated as the Flow weighted average CV

Subject to a 1 MJ/m3 cap

Uncertainty in FWACV arises from:

• Uncertainty in measurement of CVs and flows

• Variation in the CV of the sources of gas


The Charging Area CV

Consumer A receives high CV gas “all the time”

• For him the FWACV is biased

Consumer B receives low CV gas “all the time”

• For him the FWACV is biased

FWACV delivers zero bias in charging area energy

CV cap limits the exposure of consumer B


A

B

The Consumers’ Energy Bill

Energy = quantity of gas x representative calorific value

Quantity is expressed as volume at reference conditions

• Consumer:

• actual metered volume x conversion factor

• conversion factor is provided in the Regulations

Representative calorific value represents the CV of the gas seen by the consumer

• Consumer:

• average of charging area CVs over the billing period

• determined through use of approved CVDDs


Sources of Error, bias and Uncertainty

FWACV

• Daily volumes at Network Offtakes

• Error, bias in daily volumes

• CVs at Network Offtakes

• Error, bias in CVs

Actual gas quality received

• Variation in gas quality

• “Location” uncertainty

Quantity of gas

• Error, bias in domestic meter

• Error, bias in conversion factor


A

B

Estimating error, bias and uncertainty

Principles suggested by Marcogaz Energy Measurement Working Group

• Provides guidance on implementation of OIML Recommendation “Gas Metering”

• Estimates errors and bias in each component of measurement, which are then combined arithmetically to provide and overall bias in energy measurement

• Estimates uncertainties in bias for each source, which are then combined in quadrature to provide an overall uncertainty in bias.

• Sources: measurement instrumentation; fixed factors; representative CV calculation


Estimating error, bias and uncertainty

Domestic meter bias and uncertainty

Fixed factor bias and uncertainty

• Compare with average and variance in pressure, temperature, altitude

Matrix of FWACV scenarios:

• Uncertainty in CV determination at NTS Offtakes

• 0.125%, 0.25%, 0.5% (i.e. 0.05, 0.10, 0.20 MJ/m3)

• Uncertainty in NTS offtake metering

• 1%, 4%


Results: Consumers’ energy bills

Current situation

• MPE in CV determination is 0.25%

• MPE in Offtake volume metering is 1%

Overall bias is close to zero (-0.081%), because:

• Daily CVs and volumes, and hence FWACV, assumed to be unbiased

• Small bias arises from assumptions in fixed factor in the Regulations

Expanded uncertainty in bias is 5.8%

• 61% of variance arises from temperature variation

• 25% of variance arises from CV variation (i.e. 1 MJ/m3 cap)

• 9% of variance arises from domestic meter

• 0.06% of variance arises from FWACV uncertainty


Results: Consumers’ energy bills

Current situation

• MPE in CV determination is 0.25% [0.5%]

• MPE in Offtake volume metering is 1%




Expanded uncertainty in bias is 5.817% [5.822%]




• 0.06% of variance arises from FWACV uncertainty [0.22%]


Results: Consumers’ energy bills (impact of biomethane)

Current situation

• MPE in CV determination is 0.25% [biomethane 10 MJ/m3, or 25%]

• MPE in Offtake volume metering is 1% [biomethane 3%]




Expanded uncertainty in bias is 5.817% [5.818%]




• 0.06% of variance arises from FWACV uncertainty [0.08%]



Points for discussion

Overall, consumer billing is largely unbiased, provided assumptions about CV measurement and domestic and offtake metering are appropriate. (This can be part of a specification.)

Some consumers experience bias and are under- or over-billed, largely because of temperature CV variation.

This is as fair as the current system can get; suppliers and gas transporters don’t gain. The cap limits the exposure of the worst affected (although arguably at the expense of bias in LDZ energy).

Doubling the uncertainty in CV determination at NTS Offtakes has little impact.

Uncertainty in CV determination at small entry points is unlikely to have significant impact (although yet to be modelled).

Cheap and cheerful CV measurement in Smart meters?

Typical Inferential-type CVDDs uncertainty in GCV

GasPT2 – 0.2-0.75%, depending on CO2 content

EMC 500 – 0.2-0.5%

Gas-lab Q1 – 0.4%

Title of Presentation -

Accuracy of CV determination systems for calculation of FWACV Dave Lander Update 12 th October 2011.

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