How do I convert hydro inflow data from cumecs to GWh/day?

  • Last post 09 January 2019
Matthew Keir posted this 03 August 2016

This is not as simple as you might think – conversion factors can change due to:

  • changes to the infrastructure (rerunnering, draft tube modification etc),
  • consent changes allowing different lake levels or flows (higher or lower head, different minimum flow conditions etc), or
  • even changes in operational regime (operating more or less time at the peak runner efficiency, higher or lower head etc).

The hydrological modelling dataset (HMD) records these changes through time for the main hydro sites. This information is recorded in the infrastructure and constraint attributes file. We work together with generators, the system operator and Opus to maintain this record.

As a user of this information, you need to consider where your interest lies. Are you interested in:

  • the energy from inflows (or storage) to an individual stage in a hydro scheme, or
  • the energy from inflows to the entire hydro scheme assuming that all the inflows can be captured and used for generation. 

It is worth noting that individual hydro schemes can have multiple owners or participants managing the generation stations in the scheme.

To make this easy we have derived the specific energy of water for each reservoir or catchment considering both the energy produced by the single stage and for the entire scheme. The specific energy, as we have used it, is the electrical energy that one (1) cubic meter of water can generate and provide to the electricity system.

Let’s use Taupo as an example to convert average flow from cumecs to GWh/day

A cumec is a volumetric flow rate; it is one cubic metre per second (m3/s).

From the file ‘Specific_energy_(derived_for_scheme).csv’ you can filter by 'site' to yield Taupo (TPO) data. A range of values exist for specific energy that have been derived from the infrastructure and hydro constraints data. Each value represents a distinct set of conditions, and these values are recorded with start dates and end dates.

The current value is recorded with no end date – for our TPO example, it is 0.6865 kWh/m3.

So, if the average flow rate for a day into the Taupo reservoir is 100 m3/s, then a volume of 8,640,000 m3/day would have flowed into Taupo on that day. Multiplying this daily flow volume by the specific energy gives us a flow of 5.9 GWh/day.

We are interested in receiving feedback on the HMD. Please let us know if anything is unclear or you notice any errors or omissions.

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Matthew Keir posted this 09 January 2019

Sorry, I've just updated the links in the post above.

Also, note that data for 2017 is now available. I'll put a post up on this next week once I've clarified some tributary flow data.



SpdJoe posted this 09 January 2019

Hi Matthew,

Where would I be able to locate the 'Specfic_energy' csv flie?

Thank you.