Calorific value (CV) is a measure of heating power and is dependent upon the composition of the gas. The CV refers to the amount of energy released when a known volume of gas is completely combusted under specified conditions.
The CV of gas, which is dry, gross and measured at standard conditions of temperature and pressure, is usually quoted in megajoules per cubic metre (MJ/m3).
Gas passing through the National Grid pipeline system has a CV of 37.5 MJ/m3 to 43.0 MJ/m3, with the exception of Stornoway which receives liquid petroleum gas.
Why is Calorific Value Important?
Knowledge of the CV of natural gas is an essential part of the day to day activities of National Grid as this information is used to determine the amount of energy transported by National Grid. CV information is provided daily to gas shippers and suppliers, which is then used to bill gas consumers. This data is also used by National Grid to determine transportation charges for gas shippers and suppliers.
How is Calorific Value Measured?
The CV of natural gas is measured continually using process gas chromatographs.
Process gas chromatographs separate natural gas into its constituent compounds (i.e. methane, ethane, carbon dioxide, etc.) and measure the amount of each in the gas. The physical characteristics of each component, as defined by ISO 6976, are programmed into the chromatograph and an overall CV is derived from the measured composition.
The determination of the CV of gas is carried out in accordance with international standards and the Gas (Calculation of Thermal Energy) Regulations, as amended in 1997. These regulations stipulate when and where the CV of gas is measured and the type of instrument to be used. The regulations are enforced by Ofgem, who also perform audit checks on the primary data.
Where is Calorific Value Measured?
The calorific value of natural gas is measured at reception terminals and other locations on the National Grid pipeline system
At present the CV of gas is measured at over 110 different locations.
Calculation of Thermal Energy
The amount of energy consumed by a customer is calculated using the following formula:
The United Kingdom mainland has been subdivided into thirteen charging areas.
A daily CV average for each charging area is provided by National Grid to the gas shippers and suppliers, and is calculated as follows:-
The volumes of all inputs into the charging area are measured on a daily basis and a daily CV average determined for each input.
The daily CV average for the charging area is then calculated by summing the product of the CV and volume for all the inputs and dividing by the total volume of gas entering the charging area.
CV = (38.2 x 6) + (40.2 x 1) + (39.6 x 3) (total energy)
(6 + 1 + 3) (total volume)
The maximum daily CV average for the charging area permitted by the regulations is equal to 1.0 MJ/m³ above the lowest measured daily CV average of the inputs into the charging area.
All domestic customers and most industrial customers are billed on the basis of the daily CV averages for the charging area in which their premises are situated.
Some very large consumers of gas (e.g. gas fired power stations) have CV measuring apparatus installed on the pipeline leading to their premises, allowing full accounting for the energy delivered.
Scottish Independents, Harris and Lewis
In addition to the thirteen charging areas there are a small number of communities in Scotland, with isolated pipeline systems where the billing CV is based on a declared CV, which is set in advance. National Grid ensure that the average CV of gas supplied to these consumers is never less than the declared value.
Operations and Trading National Grid
The Energy Tracking section of Network Operations, National Grid based at Warwick, is responsible for the determination and provision of statutory CV information.
If you require further information, please contact a member of the Energy Tracking section at the following location:
National Grid House
Telephone CV Helpline: 01926 654642
(for calorific value enquiries only)
For CV and Gas Quality related queries only, please mail firstname.lastname@example.org.