Design Data

The American Society for Heating, Refrigeration and Air Conditioning Engineers (ASHRAE 2005) gives a standard set of guidelines for setting dry bulb temperature with coincident wet bulb temperature (or the other way around) to provide thermal comfort in particular locations. The summer design conditions are set based on historical maximum temperatures and coincident humidity measurements after discounting the extreme values (0.4%, 1% and 2% levels of exceedance of both dry and wet bulb temperatures and the coincident values of the other variable). On the other hand, the summer design conditions is set by the Australian Institute for Refrigeration, Air Conditioning and Heating (AIRAH 2007) by investigating the dry and wet bulb temperature values recorded at 3 p.m.

We have temperature data for the major capital cities in Australia, but not exactly in the correct format for our needs. As an example, the data for Adelaide was gathered at one location until 1977 with this location then changing. The most difficult aspect to deal with though is the fact that the wet bulb temperature is not recorded, only the dry bulb temperature and relative humidity. The standard method of estimating wet bulb temperature has been to use psychometric tables or charts. However, Stull (2011) derived an empirical formula for the estimation based on the dry bulb temperature and relative humidity, using a technique called gene expression programming. It is given as:

(2.7) 2.2.1 The

ASHRAE Approach

Table 2.7 presents the data for all capital cities. Data for these cities ranged from 1993–2012 to 1997–2012. For this approach, the 0.4%, 1% and 2% levels of seasonal temperature are calculated from the cumulative distribution function for the whole season. The second and third columns list the values of the dry bulb temperature and corresponding wet bulb temperature. These values are normally used to represent the outside summer conditions in the selection and sizing of comfort air conditioning equipment for buildings. The last two columns are the design conditions used when the humidity level is more significant than the temperature in equipment selection such as cooling towers. The different exceedance level choice depends on how critical is the particular situation in maintaining the indoor temperature when the outside conditions exceed the design conditions.

24 A Framework for Adaptation of Australian Households to Heat Waves

Table 2.7: ASHRAE summer design temperatures at different levels of exceedance for all cities

Note: DB = dry bulb; CWB = corresponding wet bulb; WB = wet bulb; CDB = corresponding dry bulb

2.2.2 The

AIRAH Approach

The traditional method used by AIRAH relied on the temperatures at 3 p.m. every day of the summer for the years in which records were gathered. The design temperatures are those which are exceeded 10 times per annum. Table 2.8 shows the results for all capital cities. However, these data contradict currently presented data within the AIRAH standard. The dry bulb (DB) design condition for the AIRAH standard delivers similar results to the ASHRAE method at the 2% level, and the design wet bulb (WB) condition is consistent with the 0.4% level from the ASHRAE method.

Table 2.8: AIRAH summer design temperatures for all capital cities

City DB CWB WB CDB

A Framework for Adaptation of Australian Households to Heat Waves 25 Note: DB = dry bulb; CWB = corresponding wet bulb; WB = wet bulb; CDB = corresponding dry bulb In the AIRAH handbook, there is another criterion that should be calculated. This is the critical process design temperature, defined as the DB and WB temperatures that are individually exceeded 0.25% of the hours of plant operation. For continuous operation, values are given for all capital cities in Table 2.9.

Table 2.9: AIRAH summer design temperatures for critical processes

City DB CWB

Adelaide 40.4 23.4

Perth 39.6 24.6

Darwin 34.1 27.7

Brisbane 34.5 26.3 Melbourne 38.9 23.1

Sydney 34.9 24.2

Hobart 34.1 20.6

Note: DB = dry bulb; CWB = corresponding wet bulb

2.2.3 Adjusting Design Temperatures to Suit

Climate

2030/2070

Due to the lack of availability of baseline data, accurate projections for design data could not be determined. However, indicative values are provided.

The suggested changes in the annual maximum and minimum temperature are presented in Table 2.10. These changes are based on the methods used to determine future temperature projections for the TMY.

26 A Framework for Adaptation of Australian Households to Heat Waves

Table 2.10: Increases in annual maximum and minimum temperatures due to climate change

Accurately determining future design temperatures involves using the same historical data on which the TMY is based. With a suitable data set, the appropriate method to develop future design temperatures involves firstly taking the summer dry bulb temperatures and fitting a beta distribution. This distribution is defined by Equation 2.8.

(2.8)

This distribution is appropriate since it is bounded above and below and allows for skew. Boundary conditions could be determined by applying a heuristic and simply adding and subtracting a certain percentage of the range of known temperatures to the historical minimum and maximum. There are also more systematic methods using extreme value theory. Once the parameters α and β, are estimated, the corresponding quantiles of 0.4%, 1% and 2% probabilities of exceedance can then be determined in line with the ASHRAE method. A relatively simple method to estimate these quantiles for 2030 and 2070 would be to assume that the parameters, α and β, do not alter but only the minima and maxima, as per Table 2.10. Therefore, the new quantiles could be estimated from the probability density function.

A more systematic method would be to perform the alterations to all the historical data that had been performed on the TMYs, and to then construct a frequency distribution of temperatures as was completed for the present design data estimation. This type of alteration would be performed on both dry bulb and wet bulb temperatures, and a similar procedure to what was done to construct present design data would be performed.

To demonstrate this approach, an analysis was conducted on the current and future TMY data. Table 2.11 shows the current and future design data based on the TMY with the increases in maximum DB temperature applied to 2030 and 2070 data.

Comparing the current DB temperatures to the ASHRAE values confirms how the TMY represents a typical year rather than encompassing the extreme periods.

Due to the delays and concerns related to the baseline data, the correct approach to determining future design data was not possible. In the absence of this approach, indicative future design temperatures can be determined by simply adding the predicted maximum temperature increases to all DB conditions determined by using the ASHRAE method only. Table 2.12 presents these approximate values.

A Framework for Adaptation of Australian Households to Heat Waves 27 Table 2.11: Examples of current and future design data determined from the TMY

City Percentage Current 2030 2070

Adelaide 0.4 39.3 40.1 40.9

1 37.8 38.7 39.4

2 36.4 37.3 38

Perth 0.4 37.5 38.7 39.6

1 36.4 37.6 38.5

2 35.3 36.5 37.4

Darwin 0.4 34.1 35.6 36.4

1 33.6 35.1 36

2 33.2 34.7 35.5

Brisbane 0.4 31.2 32.8 33.5

1 30.7 32.2 33

2 30.1 31.7 32.4

Melbourne 0.4 33.9 34.6 35.5

1 32.6 33.4 34.3

2 31.4 32.2 33

Sydney 0.4 32.7 34 34.4

1 31.5 32.8 33.2

2 30.4 31.8 32.2

Hobart 0.4 27.6 28.6 29

1 26.1 27.1 27.6

2 24.8 25.8 26.3

The CSIRO report provides a single value for the change in relative humidity for each location. This change is very small on average equating to +0.2% for 2030 and +0.6% for 2070, across all cities presented. This change has an almost negligible impact on the WB temperature. Given the uncertainty relating to the predicted DB design temperature, the corresponding WB temperature was found, assuming a constant relative humidity. Furthermore, the CSIRO report does not provide sufficient data to predict future design WB temperatures. Table 2.12 presents the corresponding WB temperatures for future years on this basis.

It should be emphasised that the data presented in this section are insufficiently developed and further work is needed to determine correct design data for the future.

However, in the absence of other data, this information can be used as a guide in air conditioning design.

28 A Framework for Adaptation of Australian Households to Heat Waves

Table 2.12: Indicative ASHRAE 2030 and 2070 summer design temperatures at different levels of exceedance for all cities

City Percentage 2030 2070

Note: DB = dry bulb; CWB = corresponding wet bulb