CSIRO timing offset error in several weather elements

A critical review of the CSIRO Weather and Climate Data (Part 4) 

The CSIRO developed a set of Representative Meteorological Year (RMY) weather and climate data sets as the baseline for the organisation’s work in creating so-called “predictive” weather files that can be used to investigate the impact of climate change on building energy consumption (further information on this important work is available at https://acds.csiro.au/future-climate-predictive-weather). The RMY data are presented in the EnergyPlus Weather (.epw) format, transcribed from the Australian Climate Data Bank (ACDB) which is the basis for climate information in the Nationwide House Energy Rating Scheme (NatHERS) software tools and provides data for 70 geographic climate zones across Australia.

These climate data sets have been made freely available by the CSIRO since August 2021, and have become the de facto standard for building energy modellers seeking to demonstrate compliance with the energy efficiency requirements of the National Construction Code (NCC) along with a variety of other applications. Thus the accuracy of the RMY data sets has significant implications for the energy efficiency of Australia’s future building stock, and Exemplary Energy have undertaken a timely review the CSIRO weather and climate data sets ahead of the 2022 publication of the NCC.

Our critique has already highlighted several major shortcomings with the data sets, and discussions of the first three issues can be accessed by clicking on the following links:

1. Reliance on weather data ending in 2015 for the characterisation of a warming climate ;

2. A 30-minute error in solar data in the .epw format; and

3. A lack of coincident precipitation data despite the .epw format expressly inviting it.

The differences between the .epw and ACDB formats mean that the transcription is non-trivial and is grossly flawed regardless of the method. For example, solar radiation data in the ACDB format is timestamped at the centre of the time period (each hourly data point representing 30-minutes either side of the timestamp), whereas in the .epw format it is timestamped at the end of the period. These data should always be integrated from the original high frequency observations.

On the other hand, the transcription of instantaneous1 elements such as dry bulb temperature, dew point and wind speed should be straightforward. However, the CSIRO method appears to introduce a 60-minute offset error in several weather elements including dry bulb, dew point, atmospheric pressure and wind.

The issues arising from these errors need to be considered by policymakers and modellers alike. In mid-November 2021, we advised our colleagues at CSIRO and the Australian Department of Industry Science, Energy and Resources (DISER, responsible for the NCC) of these findings but they have yet to even add a warning to the distribution website. We will continue to work with them to avoid further propagation of the errors and offer our support to improve the data going forward. We urge users and policymakers to be mindful of these issues as modelling inaccuracies arising now are embedded in building operations for many years to come.

In the interests of full disclosure, we note that Exemplary Energy offers high quality climate and weather data, including ersatz future climate data, that avoid the issues of the CSIRO datasets. These are available for modellers demonstrating NCC compliance through the JV3 pathway (simulating a compliant reference building as well as the actual building being proposed), along with non-regulatory applications in design and optimisation and resilience testing of buildings and energy systems.

1 Most observations (i.e. those other than solar radiation and precipitation) are actually averaged from a series of high frequency measurements taken over a period on the order of a few seconds, or in the case of wind observations a period of ten minutes. For our purposes, these are taken is representing the instantaneous conditions at the time of the timestamp.

Updating the PV system model for EWEI Analysis

When we first published the Exemplary Weather and Energy Index (EWEI) in Exemplary Advances in 2014, it included “an SAM model of a typical 3 kWpeak solar PV system designed by GSES.” In a context where grid connection was novel and home batteries were only used in remote area (grid isolated) systems, the most cost effective size was 1.5 kWpeak – small enough for the household to consume all that it produced when it produced it. But Exemplary and GSES were both aware that grid connection was coming fast and elected for a system double that size; looking to “future proof” the PhotoVoltaic index in a wide development context. But growth has continued apace.

In the July 2021 edition of “Exemplary Advances” we referred to a “Renew Economy article” reporting an increase in rooftop solar systems to an average of 8.7 kW. At that time Exemplary, continuing its partnership with GSES, updated the system size for the EWEI simulations from a 3 kW system, to account for the past continuous increase in capacity of Australian PV deployment and energy generation trends that better reflect the market.

However, GSES recommended a 5 kW PV system for our model, citing that although the average Australian PV system size has reached the 8 kW mark, this figure is skewed by a relatively small number of high powered 3 phase systems. As most residential owners use a single phase power supply, a more accurate representation would be 6.6 kWDC array connected to a 5 kWAC inverter. Also this power capacity is the default limit for a single phase inverter to constrain the effects of power imbalance. This limit is imposed by AS/NZS 4777.1 and also by network operators, who have the authority to provide exceptions where they see fit.

Exemplary’s PV system model now has the parameters of SMA 5000TL inverter with Trina Honeyblack 330WDC panels. Upon comparison with the earlier model, the monthly energy outputs showed an increase by around 120% for all the four EWE analysis locations. We also performed comparative studies to understand how the new and the original PV system models stand when compared to the long term average climate data (Reference Meteorological Year – RMY) and projected future climate data (Ersatz Future Meteorological Year – EFMY).

The example shown above is based on a concatenated year of recent months and charts a sensitivity analysis exercise for Canberra; similar exercises were done for three other capital cities of Brisbane, Perth, and Sydney. The modest shift in the percentage differences validates the PV system model update in our EWE analysis late last year to provide meaningful continuity to our percentage comparison based on the latest weather for our readers and solar PV owners while switching to a more currently recognizable system size. This enables a fuller understanding of the performance of their installed PV systems when compared to the long term average weather and its estimated performance in future weather conditions.

Ultimately, it must be noted that while the Exemplary Weather and Energy Index analysis is focused on non-residential buildings, the residential PV installations are still dominating the market and it is thus our focus for the PV analysis of the EWEI. Additionally, efficiencies of scale for PV systems are modest, as corroborated by this sensitivity analysis. Therefore the results obtained in the analysis, expressed as percentage differences, continue to be indicative for all fixed-array residential and commercial rooftop systems of near-north orientation.

Met Bureau Disseminates New Solar Irradiation Data

On 13 April the Bureau advised us that their satellite-derived hourly solar irradiation data has now been updated to the end of 2021 with some enhancements on the service offered until July 2019 when it was suspended due to the illness and untimely death of its key scientist Dr Ian Grant (as reported in “Exemplary Advances” 2019 December under Vale Dr Ian Grant).

For almost three years, that tragic loss has thwarted our aspiration to update our weather and climate files at the end of each calendar year. We have already begun our work to make the new data available to the building and renewable energy industries by the end of this financial year. We will regularly report on our progress through “Exemplary Advances”.

CSIRO Confirms its Ostrich-like Do-nothing Response on Climate Data

On 7 April, CSIRO wrote (apparently to all those who had accessed their wonky climate data) advising improvements to the terminology with the misleading subject line, “CSIRO projected weather files for building energy modelling – update”. Rather than warn the data users of the fatally flawed transcription errors in the current climate files and the projected future climate files (neither of them are weather files) the email ended with the massively disappointing and irresponsible line:

“Note that the data within the text files is unchanged.”

Exemplary will now be making formal complaints to the Department of Industry, Science, Energy and Resources (DISER) and the Australian Building Codes Board (ABCB) asking them to require the CSIRO to label the files with an unfitness warning until the series of errors have been corrected, as a matter of urgency.