Thermal design can be a bit of an abstract art, often requiring the skills of an actuary to make any real sense out of mountains of data. However, the real key is being able to visualise in some way exactly what is going on in the building. This means being able to encapsulate in a graph or a diagram some pattern that defines what thermal state the building design is currently in, which then allows you to compare it with a pattern of the state you would like it to be in. That way, any changes that move the current pattern more towards the desired pattern are steps in the right direction - providing you with a clear validation mechanism by which you can objectively judge subsequent design decisions.
This is the main philosophy behind ECOTECT. Whilst this is commercial software, you can still download and use it unregistered in order to play with and investigate all the concepts discussed here. Also, whilst the screen-shots shown below feature graphs unique to ECOTECT, the fundamentals behind them are equally applicable to many other thermal analysis tools.
There are many ways to represent the thermal data from a building analysis. All have their application and are useful in some way. Different people will prefer to work with a different type of graph because it fits their particular conception of the process more and they are able to glean as much information as they need from it. Thus, a range of graphical displays are presented and discussed, starting with the most obvious and progressing to some less intuitive but far more revealing representations further down.
These graphs display hourly temperature patterns on a temperature/time graph, time running in the X-axis and temperature in the Y-axis. When multiple spaces are displayed, each is shown as a solid line in the specified zone colour. In order to show the influence of climatic factors on the temperature pattern of spaces, outside air temperature, solar radiation and wind speed are also displayed as dashed lines . The solar radiation scale is shown in the vertical axis down the right-hand side. It is therefore possible to see why there may be a sudden increase in temperature when the Sun comes out from behind a cloud or when a cooling breezes drops.
The primary information in such a graph is the comparison of temperature fluctuations. For example, in Figure 1 above this is a very hot day in which the outside temperature (blue) reached 44Ã‚Â°C at 2:00pm and hovered around 29-30Ã‚Â°C at night. The internal temperature (white) remained reasonably stable, rising slowly during the day to around 36Ã‚Â°C but falling quite rapidly around 5Ã‚Â°C when the Sun went down. The overall rate of change in the internal temperature and the 8Ã‚Â°C differential against the peak outdoor temperature suggests the moderating effects of thermal mass somewhere within the building.
Looking at the solar radiation line (yellow), the noticeable rise between 9 and 12am is not directly associated with a corresponding pattern in internal temperature, whereas the sudden fall between 6 and 7pm is. This suggests that a relatively exposed west-facing window is present in the design for which shading options should be revised. This also coincides with a drop in wind speed (green), however fluctuations earlier in the day did not noticeably affect temperatures so, though not conclusive, it is unlikely that this late afternoon fall was wind-induced.
Hourly Gains and Losses
Whilst temperatures are useful, it is often necessary to be able to track down exactly why a temperature change occurred. As this will be the result of the balance between heat gains and losses, a graphical display of each component is an invaluable tool. Such a graph displays time in the X-axis against energy flow (in Watts) in the Y-axis.
Conduction gains are shown in red and refer to flows through the external fabric due to the differential in air temperatures between inside and out. It is important to remember that this does not include the effects of increased surface temperatures due to sol-air effects as these are separated out and shown as the dotted red sol-air line. Solar gains in yellow refer to direct short-wave radiation into the space through transparent and translucent elements of the external fabric. Ventilation in green includes both infiltration and ventilation gains whilst internal gains, in blue, refer to the effects of lighting, equipment use and occupancy.
Finally, inter-zonal gains refer to heat flows from other adjacent spaces within the building and below. This is a special case in that heat exchanges with the ground beneath a floor slab are actually included in the inter-zonal gains, not the conduction gains, as the ground is actually considered an adjacent zone for the calculation of ground temperatures and edge effects.
The above graph shows the same day as Figure 1. The space is not air conditioned, hence the zero HVAC load. It is clear though that the major source of heat gains to the space are by conduction (red), gaining heat pretty well throughout the day. The second major source is solar gain (yellow) peaking, as predicted, late in the afternoon consistent with the presence of an exposed west-facing window. The sudden drop between 6 and 7pm conclusively shows that the corresponding temperature fall in Figure 1 occurred when the Sun set behind adjacent foliage.
Another interesting feature of this graph is the delay between the direct Solar gains and the indirect Sol-air gains (red-dotted). The initial rise indicates an average time lag of around 3-4 hours, however these gains appear to continue well after midnight - suggesting more of a 6-7 hour lag. This can be explained if a lightweight roof with a short lag was used - the initial rise in Sol-air gains corresponding to increased Sun on the roof at around 10-11am. The long-term storage late in the day being the effects of a double-brick cavity wall exposed to late afternoon Sun.
Finally, the only heat losses are due to inter-zonal gains (aqua). On such a hot day a building will always lose heat to the ground through the floor slab.
In a free running building it can be useful to see just how often an occupant is likely to be uncomfortable in any of its spaces. This is simply a matter of establishing a set of comfort conditions and then seeing how often and by how much each space departs from these conditions. The ECOTECT software allows the user to choose the algorithm by which the current comfort temperature is calculated, including both fixed band and adaptive comfort methods - see the comfort prediction topic for more details. This is a running monthly average temperature from with the hourly variation is measured in degree hours. The result is a graph as shown below with the vertical axis in thousands of degree hours (kDHr).
This graph shows that significant heat discomfort occurs for seven months of the year, from October right through the southern-hemisphere summer to April, whilst cooling discomfort is much less significant and lasts for only four months. Of itself this is not particularly useful information.
However, when you compare the same discomfort values of another design option, you simply check that both heating and cooling values have been reduced. If so, then the new option is a step in the right direction as it will lead to a more comfortable building.
Heating and Cooling Loads
In an air-conditioned building, the main criteria is usually overall heating and cooling loads. One assumes that the role of the installed HVAC system is to provide comfortable internal conditions throughout the entire occupied period - in which case you really want to minimise the cost of providing this service and reduce energy wastage as much as possible.
This basically means reducing the loads on the system as much as possible.
In its simplest form, HVAC loads are required to counteract the instantaneous losses or gains causing internal conditions to fall below or rise above a given temperature constraint, usually referred to as a thermostat range. Thus, they are basically the negative of the sum of all other heat flows.
Note that this gives a space load, not an actual energy load. This is because no details of the actual HVAC system are known to the model, only the amount of heating or cooling it has to supply to the space. Thus, for a given space load, an efficient system will use less energy meeting that demand than a relatively inefficient one. As the design of the actual HVAC system will most likely be the job of a consulting engineer, your role as the building designer is to provide that engineer with a building that requires the least amount of spaces loads possible. Thus, just as with the discomfort period graph above, any design option that reduces the overall heating an cooling requirement is a step in the right direction.
This is, of course, a bit of an over simplification - but it will hold true in almost all cases. The added complication has to do with the size of the system. A standard law of mechanics is that a very powerful machine operating well below its capacity will be less efficient at supplying the same output than a less powerful machine operating closer to its capacity.
Whilst it is always good to have some extra capacity in reserve, it is pointless if it is never used. It is actually the peak instantaneous heating and cooling loads that will determine the size of the installed HVAC system, not the annual total. Thus, in some cases it may be that a design option that leads to slightly higher overall annual loads, but significantly lower peak loads may allow you to down-size the required boiler/chiller such that the extra efficiency gained by it operating more often closer to its rated capacity compensates for the increased space load. This will likely be a very unusual situation, however it can occur and highlights why it is important that the building and HVAC system designers actually work in concert from the earliest stages of a project.
In addition to hourly temperature graphs, which provide a snapshot of building performance over only one day, it is possible to display statistical temperature data taken over the whole year. This data shows the number of hours each space spent at different temperatures throughout the year.
The vertical axis shows the hour count for each temperature while the horizontal axis shows the temperature.
The aim of this graph is to show the range and frequency of temperatures experienced both inside and outside the building. The dotted blue line shows the external air temperature whilst the solid white line shows internal temperatures. Multiple zones are displayed as solid lines arranged by zone colour. In the above graph, the outside air temperature spent only 600 hours at 24Ã‚Â°C whilst inside temperatures spent nearly 1000 hours at that temperature. Internal temperatures ranged between 8 and 34Ã‚Â°C whilst outside temperatures ran from 4-37Ã‚Â°C.
The shaded areas in the graph represent the selected comfort band for each zone. The aim is essentially to have the zone spend all its time at the centre of the dark area - at comfort conditions. Any time spent in either the blue or red shaded areas represents periods of occupant discomfort. Thus, any design decisions that compress the extents of the zone temperature range closer to the band centre or increase the amount of time spent within this area are a step in the right direction.
Monthly Average Hourly Fabric Gains
These graphs are currently unique to the ECOTECT software. They show hours of the day in the vertical Y-axis and months of the year in the horizontal X-axis. Each grid-square represents the gain for that hour averaged over the corresponding month. The result is an annual pattern of both diurnal and seasonal variation. The graph below shows the values of conduction gain through the building fabric (including sol-air driven gains) for Perth, Western Australia. As this is in the southern hemisphere, the hottest outdoor temperatures occur in summer from October to March.
You can see in this graph that the maximum conduction gains are occurring from around 2pm to 6pm on a February afternoon in the height of summer. Given that this is during the hottest part of the day, this is definitely not a good thing. Similarly, maximum heat losses occur between 4am and 10am on mid winter mornings - right when people are most likely to be getting up and feeling the cold most (if this were a house). Both of these issues are likely to drive the occupants to install both auxiliary heating and cooling.
The ideal situation would in fact be the complete reverse of this, maximum losses on a summer afternoon and maximum gains on a midwinter morning. Given the laws of physics it is unlikely that you will be able to achieve this, however the designer still has a fair degree of control over such problems. If we take this particular case, gains on a summer afternoon are most likely to be occurring as a result of solar radiation on the roof and west walls. Looking into it a little further and taking into account the time lag associated with thermally massive walls, the peak gains occurring at 2pm are more likely to have come from solar radiation falling on the east walls earlier in the morning gradually making its way through.
Thus some summer shading of the east walls (shading devices or carefully planted deciduous trees and vines), increased insulation of the roof to minimise gains when the Sun is directly overhead and shading of the west wall to minimise gains occurring later at night would be in order. To combat the early morning losses in winter, we could reduce the overall area of external windows, use thick full-length drapes to lessen both conductive and convective gains and seal the building against unwanted air infiltration.
With these simple changes maximum fabric gains in summer have been significantly reduced from around 1200W to around 450W and pushed back to between 4pm and 11pm at night - when outside air temperatures have fallen enough to allow the building to be opened up and the excess heat drawn away by night-time breezes. The early morning losses in mid winter have been similarly reduced.
We could even go further and experiment with double glazing and increasing the size of east facing windows to allow in some direct solar gains early on a midwinter morning. This would require significant care as such windows would need to be fully protected from early morning summer Sun. Each of these options could be easily evaluated using the above graph.
Monthly Average Hourly Solar Gains
Displaying fabric gains is useful as it is probably the only way to fully understand the effects of thermal mass on the building response. However the same process can be undertaken for each component of the overall heat flow in a building. For example, the following graphs isolate the two components of solar radiation, indirect gains (Figure 9) through the fabric due to sol-air temperature effects and direct gains (Figure 8) through transparent elements in the external fabric.
A comparison of the direct and indirect gain patterns above clearly show the time lag effects of the external walls. It is quite clear that the exposed east wall is the source of delayed fabric gains from around 2pm summer afternoons. Ventilation, internal and inter-zonal gains can be similarly isolated for study.