A strong interest in building thermal physics was evident with many universities from the early 1900s (Haberl 2004). From the 1940s the development of building thermal theory moved to newly established building and national research organisations. The Carslaw and Jaeger book, ―Conduction of Heat in Solids‖ (1947), documented the parallel path method, which is still in use today. At the Building Research Congress in 1951, various articles discussed the processes, practicalities and problems associated with the use of a single heat path method (Bruckmeyer 1951; Mackey 1951; Mackey & Wright 1944, 1946). As early as 1942, researchers were using the analogy of electrical theory (Billington 1951; Paschkis 1942; Paschkis & Baker 1942; Van Gorcum 1950) or hydraulic theory (Leopold 1948a, 1948b) to describe the heat flow through solid materials. For various reasons, the electrical analogy had become the predominant approach by the early 1950s.
As early as 1953, Australia researchers from the CSIRO, were publishing methods and principles for calculating the internal temperatures of buildings, in an ever-changing external
Holden and others from the CSIRO commenced the development of what has become the AccuRate software in use today. At the same time, they were developing the electrical analogy (Figure 3.2) and the use of matrix algebra to account for the multi-variate inputs required to model the heat flows in a building (Clarke 2001; Davies 1974; Holden 1963; Muncey, R & Holden 1967; Muncey, R & Spencer 1966, 1969). As the capacity of computers increased throughout the late 1960s and early 1970s, the matrix method, as in Figure 3.3, was further developed to include many more inputs (Milbank & Harrington-Lynn 1974; Rao & Chandra 1966).
Figure 3.2 – Muncey & Spencer Matrix calculation method
(Muncey, R & Spencer 1969, p. 228)
Figure 3.3 – Matrix Heat Flow and an electrical analogy
(Muncey, R 1979, p. 93)
As soon as computers became useful for building theory applications, these same national research organisations commenced developing building thermal simulation programs (Haberl 2004). The first of these building thermal simulation programs had limited input and output capabilities, as they were dependent on their state of computer technology. However there was ongoing debate and growth of knowledge on calculating the room temperature within buildings. With the developing capacity of computers to perform a greater number of calculations in the early 1970s, the interest in and capacity to broaden the HER software accelerated (Clarke 2001; Isaacs, T 2005). Government and industry funded projects were established to develop detailed building simulation programs (DSP). These early DSPs were the predecessors to the current range of House Energy Rating computer programs.
In Australia, the first formal detailed simulation program developed by the CSIRO was completed in the mid 1970s and was named STEP. The STEP program was able to model a
single room for each hour for a period of three days. Over the following decades, as computer capabilities increased and major improvements to programming were made, the next generations of the software became ZSTEP 1 to 3, CHEETAH, CHENATH and NatHERS (Ahmad, Q & Szokolay 1993; Delsante 1988, 1996, 1997; Delsante, Stokes & Walsh 1983; Landman & Delsante 1987; Walsh, PJ & Delsante 1983). Throughout this evolution the capabilities of the software improved, as follows:
- Number of subfloor, internal and roof zones able to be modelled increased to 99
- The simulation calculates the zone temperature for each hour of a full year
- A climate file with hourly input variables was introduced
- The ground model for concrete slab-on-ground floored buildings was developed
- The ground model for platform-floored buildings was developed
- A simplistic model for the calculation of heating and cooling loads was developed
In the early 1990s, Federal and State agencies within Australia agreed to develop a National House Energy Rating Scheme and subsequently the CSIRO developed the CHEETAH software further, to meet the requirements of this scheme (Delsante 1996, 2005e). The program was reviewed and improved to meet the imminent energy rating requirements for new residential buildings (CSIRO 1997; Thwaites 1995). Throughout these improvements the program maintained its single dimension thermal modelling methodology (Boland 2002), which has been found to have between a 22% and 41% discrepancy from two and three dimensional models (Adjali et al. 2000; Belusko, Bruno & Saman 2010; Stazi et al. 2007).
The NatHERS program, which principally used the CHEETAH thermal simulation engine, had modules tested with the IEA BESTEST validation method in the early 1990s (Ahmad, Q & Szokolay 1993; Delsante 1995b, 1996). This validation was in response to concerns raised by industry about the imminent use of the software to produce star ratings for regulatory approval of house designs (Henriksen 2003). As a result of the BESTEST validation a range of improvements was made. This established the first generation of NatHERS with the CHENATH simulation program operating behind the AccuRate front end user interface. As
version number of the AccuRate program (ABCB 2006c; AGO 2004a; Chen, White & Wonhas 2010; Delsante 1989, 1993, 1996, 2005a, 2005e; Delsante & Mason 1990; Energy Partners 2007; Isaacs, T 2005, 2008; Lee et al. 2005; Lee, Snow & Stokes 2005; Li & Delsante 2001; Li et al. 2001; Li, Delsante & Symons 2000; Marker 2005; NatHERS 2009a). Elements of the software that were improved included:
- Improved materials library
- Improved windows and roof windows library and modelling
- Improved ventilation model to suit modes of natural ventilation
- Improved modelling of platform-floored subfloor zones
- Improved roof space modelling
- Improved ground model
- Increased number of climate zones
- Improved internal solar radiation modelling
After these improvements were completed the program was validated once again via the BESTEST method and the results classed the software as satisfactory (Delsante 2005a, 2005e). Williamson (2009) noted however, that the BESTEST method did not include any assessment of the natural ventilation models. The results of the calculated energy requirement for heating and cooling of a low-mass building are shown in Figure 3.4 and Figure 3.5, respectively. These two figures illustrated that the BESTEST validation method was allowing a substantial variation between programs (Kummert, Bradley & McDowell 2004) of approximately 1.2MWh for heating and 1.8MWh for cooling. The BESTEST 600 building is a single-roomed building of 48m2 (Judkoff, R & Neymark 1995; NeymarkJudkoffAlexander et al. 2008). If the heating and cooling values were considered to be of a similar nature to those in the Australian NatHERS Star Ratings, as in Table 3.2 (ABCB 2006b), the allowable variance between programs could have a dramatic impact on a house‘s energy star rating. The broad range allowable in the energy calculations may be a result of software developmental legacy from the time of much less capable computers. It would be expected with more
modern computer capability and a greater understanding of building physics, that the range variance would be tightened. In Australia this concern can be attributed to the simplifications in algorithms in the software, as acknowledged by the CSIRO (Delsante 1996).
Figure 3.4 – BESTEST results for low-mass annual heating requirement
(Delsante 2005d)
Figure 3.5 – BESTEST results for low-mass annual cooling requirement
Table 3.2: BESTEST and NatHERS heating & cooling values for type 600 building
Allowable BESTEST Variation
NatHERS Equivalent 48m2
Effective Star Value
Launceston Sydney
Heating 1.2MWh 25KWh/m2.annum 0.4 Stars 1.2 Stars
Cooling 1.8MWh 38KWh/m2.annum 0.5 Stars 1.8 Stars
Combined Total 63KWh/m2.annum 0.9 Stars 3.0 Stars
Note: based on the 4.0 to 5.0 Star rating step (ABCB 2006b)
Internationally, many detailed simulation programs have come under tighter scrutiny than AccuRate as their capability to predict room temperature and to calculate energy requirements to meet ever increasing building thermal performance guidelines, has been questioned (BREDEM 2006; Crawley et al. 2005). One of the BESTEST validations of the ENERGYPLUS software in 2004 (Henninger & Witte 2004) revealed some dramatic differences between the twelve software programs used for the comparative validation (Figure 3.6 & Figure 3.7). For the low-mass building the variations were up to 2.4MWh in annual cooling energy and 1.5MWh in annual heating energy. In many of the BESTEST reports there was discussion with regard to calculated average values for minimum, maximum and mean temperatures (Judkoff, R & Neymark 1995; Neymark & Judkoff 1997). All energy calculations by the programs were based on the actual varying minimum and maximum values, when the heating or cooling requirement was invoked and not the mean or average temperatures. It was these daily extremes which were of greatest importance for validation purposes.
Figure 3.6 – EnergyPlus BESTEST results for low-mass annual heating requirement
(Henninger & Witte 2004)
Figure 3.7 – EnergyPlus BESTEST results for low-mass annual cooling requirement
(Henninger & Witte 2004)
One problem with the software comparison approach for housing in many parts of Australia was that there was a reduced need for controlled heating, ventilation and air-conditioning on a daily basis (Kordjamshidi, King & Prasad 2005). The Australian climate can, (depending on building fabric), allow a house to operate without the use of heating or cooling for some portions of the day, but may require heating or cooling, at times of minimum and maximum
predominant heating requirement, a difference in the calculated minimum temperature would have dramatic impact on the energy calculation and the resultant star rating. What was evident to industry and of concern to the government and the CSIRO, was the need to validate the AccuRate program empirically, for the purpose of modification, improvement or calibration of the program (TPC 2005). The methods to validate the software empirically were investigated and these are discussed below.