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close this bookDesign Handbook on Passive Solar Heating and Natural Cooling (HABITAT, 1990, 162 p.)
close this folderVIII. Final design evaluation
View the document(introduction...)
View the documentA. Steady-state evaluation
View the documentB. 5-star design rating system a correlation technique
View the documentC. Monthly mean indoor temperature
View the documentD. CHEETAH - Thermal behaviour and energy load simulation model

D. CHEETAH - Thermal behaviour and energy load simulation model

This computer program is available to designers to provide a quick and easy method to evaluate building designs. It will provide either temperature profiles for showing the building conditions under actual weather conditions as shown in figure 106 or heating and cooling loads for specified Indoor temperature conditions as shown in figure 107. CHEETAH uses the response factor method as the basis of the calculations and was developed over many years as a program called ZSTEP by the CAIRO Division of Building Research. It is a little different from the conventional response factor method and has been fully described in the journal, Energy and Buildings. The program can be used on an IBM PC or compatible (either XT, AT or 386 machine: one year of weather data can be processed on a 386 type machine in four minutes) with at least 512K of memory, two floppy disk drives or a hard disk and an appropriate maths co-processor. A colour graphics card and colour monitor are most desirable but not essential.

The program is fully menu driven with a comprehensive easy-to- use help system. Its features are listed in the user manual as follow:

(a) Hourly temperatures and heating and cooling energy requirements are calculated for periods ranging from one day to a full year:

(b) The building may be subdivided into as many as 10 zones. Calculations need only be done for the zones of interest, while still taking into account the interactions of all zones:

(c) Libraries of common building materials and building sections are always available for describing a building:

(d) Occupant behaviour (e.g., the operation of curtains and shading devices) may be taken into account to a limited extent;

(e) Data input is fully interactive. with immediate error checking;

(f) Context-sensitive help, often quite detailed, is available at any stage.

This section illustrates some of the output possible, based on the house plan In figure 105. This house plan was used as the basis of the 5-star design rating system described earlier in this book. The designer enters the physical data about the building and its occupants' lifestyle using the interactive menus that consider the building In detail. At the early design stage, the designer needs only to enter rough approximations of the areas etc. to get a preliminary Indication of the bulidings performance. As the design is refined, so too can the input data file be modified and refined through the menu system. The program allows the designer to investigated the behaviour of the building over a hot or cold period of a day. a week, a month or the whole year. The performance on cloudy or clear days can also be studied. On completion of the calculations the results can be viewed In a graphical form as temperature curves or a bar chart of heating or cooling energy loads, as shown in figures 106 and 107.


Figure 104. Winter-time dally temperature swings in various constructions


Figure 104 continued

The two design options in figure 106 are described as follows. The plan and building design are the same except that "Lech House 1" is uninsulated and has a carpeted timber floor over a vented air space, whilst "Lech House 34" has a carpeted concretes-lab floor on the ground and an Insulated roof (R2.0) and walls (R1.0). The three days shown are for a cold winter period In the country area of western New South Wales (ambient temperatures of approximately -4C to 15C). The main effect of the change Is warmer conditions at night. The sharp drop during the day Is the result of specifying that the occupants should open the windows when the indoor temperature rose to 25C and should leave them open until it dropped to 20C. The designer has available a number of choices about user-lifestyle patterns. The heating and cooling loads for "Lech House 1" in figure 107 are self explanatory.

As the design develops, it is possible to study various design options to help make better decisions. Figure 108 illustrates a case where external coatings with different surface absorptivities (0.2, 0.6 and 0.9) are compared on a simple test building. From this type of data. one can decide whether a particular surface colour is satisfactory from a thermal design viewpoint.

Many students of architecture in Australia are being taught how to use this package and there are a number of designers who find it helpful in the designing process as they attempt to balance the thermal design requirements with other design factors. Design weather data are available for many locations in Australia and also for London (Kew). Additional locations are being added to the collection as the need arises. Any weather data on floppy disk that have been prepared for SERI-RES can be converted quickly and easily by either the distributor of CHEETAH or a user with computer experience.


Figure 105. Floor plan of the house used as the example building


Figure 106. Graphs of temperature comparisons between ambient and the two house configurations based on the plan in figure 105


Figure 107. Sensible heating and cooling loads (MJ) for Lech House 1 at Wagga. New South Wales


Figure 108. A comparison of the indoor temperature effect of different surface colours on a test room over three warm days
Note: Abs = surface absorptivities