How to Improve Thermal Comfort in an Office Environment
As temperatures rise across the globe and people are working for longer hours through the day, the need for a comfortable working environment and the concept of thermal comfort has increasingly grown. More electronics means additional heat in an interior space, and today’s work environment has led to a rapid increase in the necessity of air cooling and conditioning.
Yet, in spite of an advancement in air conditioning technology, thermal comfort in offices seems like a far-fetched dream. There are always employees who feel it’s too hot and open the windows while few others warm themselves with two pullovers in the neighboring cubicles. It is every employee’s dream to find a thermally optimal spot in an office. For example, recent studies show that women in workplaces feel on an average 3-deg C, colder than men. This has been primarily attributed to different metabolic rates. Thus, designing workspaces by recognizing the need for diversity can significantly improve employee satisfaction and enhance workspace dynamics.
A handwaving argument so far used by the HVAC industry for determination of the size of the cooling system needed (in BTUs) for a room with a roof height of 8 feet has been square foot x 25 + 1000 x Windows + 400 x person. This significantly neglects, however, modifications in office spaces due to cubicles, heating from computers and other electrical appliances in the microenvironment. In addition, this also disregards the effects of exposure and orientation of windows, shading, etc.
SimScale offers an effective cloud-based simulation tool for the design of office space architecture that can be used by HVAC designers, civil engineers, and architects alike to optimize workspaces. Simulations can provide a better estimate of the size of the HVAC systems required and also optimize their positions to minimize energy costs. Here it’s important to note that six major factors have been identified to define the concept of thermal comfort. Of these six, four constitute environmental factors while two are personal. Two personal factors are beyond the control of designers while the four environmental factors could definitely be considered during the preliminary design phases. The four environmental factors affecting thermal comfort are:
- Air temperature: temperature of the air in the room
- Air velocity: air movement can cause heat transfer from the body surface, smaller velocity implies less loss of heat
- Radiant temperature: heat loss is not only by convection but also by radiation
- Relative humidity: humidity in the micro-environment can also significantly affect living conditions
and the two personal factors influencing thermal comfort, which the designer cannot influence, are:
- Clothing: can be moderately controlled by the individual
- Metabolic rate: a factor that is beyond anyone’s control
Simulations using SimScale can be used for preliminary design and testing innovative systems like heating under the floor (or on the roof), passive instead of active cooling systems and so on. In this article, we will discuss some aspects that need to be considered and discuss small examples demonstrating the effects of these external conditions on thermal comfort.
Primary aspects to be considered
The optimization of air conditioning necessitates simulation of a thermodynamic process that includes airflow analysis in the office space considering at least the primary aspects like infusion of fresh and removal of stale air, heating infusion due to electronics / computers, seating position of employees, cubicles or separating partitions, exposure to outer environments (through windows and doors), insulation through walls etc. Some important aspects to consider for design of an HVAC system for an office space are:
- Placement of inlets and outlet point
The general idea is that hot air rises while cold air settles down displacing the hot air that originally sat there. However, the placement of the inlets and outlet ducts could prove quite consequential. Placing an outlet duct in the opposite direction can significantly increase the turbulence (or internal air velocity) in the space. Finding an optimal location for inlet and outlet could significantly reduce the energy costs necessary to remove/drive the hot and stale air.
- Exposure and orientation of windows
Most workspace walls can be considered to be well-insulated and to be at a constant temperature. However, depending on the orientation of the windows (either towards North or South) it could be an important aspect to consider not only the amount of sunlight but also heat addition from it. Office spaces with windows facing south are much more heated by sunlight compared to those with windows facing north.
The average sunlight, over a 24-hour period, globally is about 164 Watts/sq m. However, this would strongly vary based on the latitude and longitudinal of the location. The knowledge of the location can provide a seasonal estimate of the amount of heat from the sunlight. For example, around 40-deg N latitude (Munich is around 48-deg N), the average heat is 600 W/sq m in summer while 300 W/sq m in Winter. This can strongly affect both the performance and the internal conditions.
Construction of window structures and the depth of their placement can also slightly modify the exposure to sunlight due to the shadow of the structure.
- Insulation of the walls and roof
Generally, most of the internal walls are wooden and lightly insulated. In contrast, especially in colder countries, the outer walls are significantly insulated. Hence, the internal and external walls can be differentiated based on their insulation types. The internal walls can be thinner facilitating for heat transfer across rooms (or constant heat flux or zero temperature gradient). The external walls can be considered to be of constant temperature or also allow slight heat transfer. In addition, depending on the placement of the office in the building, either the roof can be considered to be constant temperature or could again be assumed to facilitate heat transfer.
- Presence of electrical appliances
The heat transfer from the electrical appliances can be modeled as a natural convective process. For the steady-state, they can be considered as a constant temperature source.
In addition, several other aspects like effects of a courtyard, external shading due to trees, a fountain in courtyard etc. can always be added and can also affect the overall thermal comfort. As a simple prototype, a convective heat transfer analysis of the SimScale office space is considered and discussed below.
Thermal Comfort and SimScale
This public project on SimScale provides a template for starters who intend to use SimScale for designing their HVAC system and understanding thermal comfort relative to an office space. The template is based on part of our own SimScale office layout. Thus, for designers, it is not necessary to start from scratch but one could directly modify the template for their usage. In this article, we demonstrate the applicability to modify this public project template to visualize the effects of different conditions generally encountered during the design process. The modified version of the project, for conditions described below, can be found in our public projects page here.
Fig 01: CAD file describing a simple office layout
The scenario considered here is a hot summer day when the outside temperature is about 30-deg C. The effect of cooling systems in such a condition is considered. The cooling system has a passive outlet and an active inlet. A more detailed tutorial on setting up such a problem can be found in this recording of the webinar on “Thermal Comfort of a Living Room”. The earlier two webinars on the same direction of air cooling and ventilation can also be found at here, and here.
The above CAD model is meshed using a hex-based mesh suitable for internal flow with a fineness at level 3 and computed over 16 cores. This results in a total of 1,506,294 nodes and 1,326,342 volume elements. A natural convection heat transfer process is chosen. For a preliminary design analysis, it would be sufficient to assume that the flow is laminar and has reached a steady state. In reality, the HVAC designers only need to consider the steady state rather than the initial transient state. Thus, the model described here is as near to reality as possible.
The material properties for air is imported from the SimScale library. The initial velocity is zero while the pressure is atmospheric pressure. The initial temperature in the room is set to be 30-deg C. The conditions of the room after about approximately 15 mins (1000 sec) is reviewed and discussed.
The customizations can start at the assignment of the boundary conditions. We compare two simple scenarios to start with. In the first case, the inlet for colder air is above while a passive outlet (outlet at atmosphere pressure) is provided at the bottom. In the second case, they are exchanged. In all these cases, the walls are considered to be adiabatic or otherwise do not conduct. Here you can find more information on this type of boundary condition.
Fig 02: Comparison of the temperature profile for variation in placements of inlets/outlets. Inlet is on top (and outlet on bottom) is shown on left while the opposite on right
Figure 2 shows the comparison of final temperature, with the first case on the left and the second on the right. Though they appear to be nearly similar, the second case might appear to be little better than the second. However, looking at the velocity profiles in Figure 3, it is quite evident that the first case facilitates better air circulation in the entire room, in addition to slightly higher air velocities. Overall, one can conclude that the second case shows more promise.
Fig 03: Comparison of velocity streamlines for variation in placements of inlets/outlets. Inlet is on top (and outlet on bottom) is shown on left while the opposite on right
Continuing with the customizations, consider the second case but now a different boundary condition is considered for the roof. For example, if the office was on the top floor and assuming the roof to be badly insulated, the roof can be almost as hot as outside on a hot summer day (say 30-deg C).
Figure 4 shows the variation of the temperature profiles. Case two with all walls (including roof and floor) adiabatic is still on the right, while the third case where the roof is heated up is on the left. As expected, there is a strong counter-current from the roof that is heating the room.
Fig 04: Comparison of temperature profile for an office space on an intermediate floor (right) and on the top floor (left)
The temperature distribution is different but the streamlines, shown in Figure 5, demonstrate a strong heat wave due to the heating of the roof that prevents faster heating of lower and far-end areas of the room. This is a common experience of most people living on the top floor.
Fig 05: Comparison of velocity streamlines for an office space on an intermediate floor (right) and on the top floor (left)
Instead of the roof, what if there was a not-so-well insulated outer wall or a wall made of glass? Here, again a constant temperature boundary condition is assigned to this wall. The comparison for such a scenario is as shown in Figure 6. Contrary, to the assumption that this could impede air cooling, such a scenario actually facilitates better cooling by increasing the flow of air across the room.
Fig 06: Comparison of velocity streamline for an office space with all walls adiabatic (right) and with one not-so-well insulated external wall (left)
Finally, the last case is regarding the orientation of the window. Generally, offices with windows facing south receive much more sunlight and heat compared to those facing north. For example: In Munich, the orientation of the sun, on an average, is at 64-deg in summer, 18-deg in winter and 41-deg in spring. Thus, the offices with windows to the South, receive an enormous amount of heat in summer compared to really cooler offices with windows to the North. This example is considered just to illustrate this difference. Fig. 07 shows the different velocity profiles in the two different orientations.
Fig 07: Comparison of velocity streamline for an office space with windows to south (right) and with windows to north (left)
The cases discussed above are by no means exhaustive but only a brief glimpse of the possibilities of the ability of applicability of such simulations using SimScale in HVAC. Such simulations can be used to obtain better customized cooling systems and optimization for an increase in overall thermal comfort and energy efficiency.
If you want to read more about how CFD is being applied in the HVAC industry, download this white paper for free: How to Ensure Thermal Comfort in Buildings with CFD