A Guide to Energy-Efficient Multi-Story Buildings

A lot goes into ensuring that a multi-story residential building is energy efficient, as this case study demonstrates.

When people are dissatisfied with the thermal environment, their productivity, capacity of concentration, well-being, and health can be negatively affected. Thus, ensuring thermal comfort for any new building design project through its HVAC system, as well as positioning of windows, doors, stairs, and other components, is paramount.

When it comes to green buildings, the challenge is even bigger, as other factors—such as energy consumption or noise and air pollution—need to be kept to a minimum simultaneously. Several factors determine if a building is “green,” including:

• Having HVAC systems with low energy consumption.
• Employing renewable energy.
• Efficiently using resources.
• Proper indoor air quality.
• Measures against pollution.
• Recycling.

Both in the case of green and standard buildings, energy efficiency is essential, and finding a middle ground between this and thermal comfort is one of the most commonplace tasks for engineers and architects.

The main tool for accurately testing these two elements for a building’s design is numerical simulation with computational fluid dynamics (CFD). This method allows users to investigate elements such as airflow, temperature distribution, pressure field, wind speed, and air exchange rate more quickly and efficiently.

First Steps

In this project, a residential building design was virtually tested with the goal of finding the right capacity settings for its HVAC system in order to ensure thermal comfort in winter. To that end, a computational fluid dynamics (CFD) simulation was performed online to determine a suitable heating capacity of the three-story building in order to guarantee the occupants’ thermal comfort while maintaining recommended indoor air quality.

To quantify the thermal comfort of the occupants, two quantities can be calculated through the results of the CFD simulation. These values are predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD), and they determine the probability that an occupant feels cold or warm. 

The ASHRAE 55 standard defines the PMV as “an index that determines the mean value of votes of a group of occupants on a seven-point thermal sensation scale.” 

The PMV takes into consideration different factors—the predicted occupant metabolic rate, clothing insulation, temperature, airspeed, mean radiant temperature and relative humidity.

Once the PMV is determined, the PPD—“an index that establishes a quantitative prediction of the percentage of thermally dissatisfied occupants determined from PMV” (i.e., people that may feel too warm or too cold)—can be defined. 

The PPD indicates the percentage of people that could experience a condition called local discomfort. There are a few factors causing local discomfort, including draft or lack of airflow, but the resulting consequence is the undesired cooling or heating of an occupant’s body. In the presented case, these factors will be taken into account to assess the level of thermal comfort but only the PMV value will be used as a measure.

What Does the CAD Model Show? 

The model presented includes three apartments of about 190 square feet on top of one another, separated by 4-inch slabs. At the ground floor level, there is also a 136-square-foot office space that has its own independent access. In each apartment, there are two people, and in the office there is one. 

The furniture—beds, wardrobes, kitchen counters, table chairs—are represented in their simplest form in order to reduce the simulation’s complexity while keeping a level that does not impact the results’ accuracy.

The airflow will be simulated in the three apartments and the office, through four distinct air volumes. The heat could be transferred from one air volume to the other by thermal conduction through the floors and ceiling. The slabs between the apartments are assumed to be plain blocks of concrete.

The case scenario shows the residential building in winter conditions, with an outdoor temperature of negative 20° C and a humidity rate of 50 percent.

The building is relatively new and has good insulation of its main components. The insulation quantity used for this project is the thermal transmittance (or U-value) and is described as per EN ISO 6946 as the rate of transfer of heat through a material. This can be a single material or a composite. The table below summarizes the U-values used in this project.

Heating Strategy

The main objective of this project is to guarantee the thermal comfort of the dwelling’s occupants; this heating power selection is essential in the design process. There are many heating strategies available to the architect and the HVAC engineer in order to reach an acceptable and uniform temperature in the apartments. 

The strategy adopted in this project is to implement radiators at different locations throughout the rooms, typically under the windows. The hot air that the radiators generate raises and acts as an air shield against the cold air at the surfaces of the windows, and enters through small gaps to reach the center part of the rooms where the occupants are most likely to be present. 

Using the U-values, surface areas, and heat transfer coefficients (external and internal) of the building’s components, one can approximate the heat power necessary to reach a temperature of 69.8°F taken as a reference for thermal comfort temperature. The summary of the calculations is shown in the table below for each level.

It can be observed that in this approximation, the heat being transferred from one apartment to the other through the thermal conduction of the slabs has been neglected. The power generated by each individual radiator can then be determined by the pro-rata of each individual room surface area to the total level surface area.

The second approach consists of installing underfloor heating that will generate an even temperature distribution in the rooms. Both of these heating methods will be implemented and compared in this project.

Improving Indoor Environments

To maintain the indoor air quality in the dwellings, and prevent the stagnation of harmful compounds such as carbon monoxide, air must be constantly renewed. In recent residential buildings, such as the one presented in this case study, this air renewal is performed by mechanical ventilation means in the form of extracting units placed at different locations around the apartment, typically in bathrooms and kitchens. 

The air introduced in the room would come from different air intakes, located as far as possible from extracting units, to maximize the volume under the stream and taking into account “zone air distribution effectiveness” as per ASHRAE 62.1. It recommends, for example, a supply of air from the ceiling for better effectiveness.

One of the most-used ventilation rate measures is the outdoor air rate calculation presented in the ASHRAE 62.1 standard for indoor air quality. Therefore, the indoor air quality can be ensured by maintaining sufficient air renewal. 

The minimum outdoor air rate, which is the amount of air that needs to be introduced in the apartments, is defined by ASHRAE 62.1 as:

[From ASHRAE 62.1 and for a residential dwelling unit, Rp is 2.5 L/s and Ra is 0.3 L/s.m2, for a 58m2 space occupied by two people. This gives a Vbz of 21.5 l/s.

As a baseline, the outdoor airflow rate will be distributed equally among the three extracting units for each flat (7.2l/s or 8.8g/s of air)—one in the kitchen, one in the bathroom, and one in the bathroom. The air at the intake, from the outdoor, is filtered. It has come through a double-flow controlled mechanical ventilation (CMV) in order to heat up its temperature by heat exchange with the exhausted air. It is set to a temperature of 15°C.

Thermal Comfort Analyzed

As presented above, the PMV results use values taken directly from the CFD results (surface temperature, velocity, and air temperature) and inputs from the environment and people (clothing coefficient, metabolic rate, and humidity). In this project, and extracted from the 

ASHRAE 55 standard, a winter clothing coefficient of 1, a “cooking/cleaning” metabolic rate of 1.2 and a humidity of 50 percent are chosen as inputs for the calculation of the results.

Here is an explanation of the results: 

The average temperature for each apartment and office show acceptable results, with small error to the target temperature of 69.8°F, demonstrating a great correlation between the analytical and the numerical approach.

In the images below, the temperature distribution in the apartments and office helps to identify hot spots such as in the bathroom on the second floor or the TV room on the first floor. The layout of the rooms in each apartment, as well as the location of the inlets/outlets, and radiator greatly impact the heat distribution. One can observe hot spots around the radiator and colder zones at the windows with no radiator underneath—i.e., in the bedrooms. 

For the ground floor apartment and the office, the temperature remains mostly evenly distributed, with locally low temperatures, expected in the windows’ vicinity

On the first-floor apartment heat map, one can observe that the TV room is warmer by 1-2 degrees than the rest of the apartment, at about 68.9°F, indicating that the radiator delivers too much power. The TV room is the warmest space in the apartment. A more evenly distributed temperature could be achieved by moving some of the heat power from the TV room to the bedroom.

The second-floor apartment shows a better-distributed temperature than the first floor. However, there is a hot spot in the kitchen (left part of the apartment). This can be correlated with the warmer first floor TV room where heat is transferred through the slabs to the upper level. 

The simulation of transfer of heat through the concrete slabs helps understand the importance of building materials and their properties. Slabs with high heat resistance would limit this effect and therefore contribute to keeping the heat within one apartment.

The PMV slices, at about four feet above each apartment and office floor, shows what a satisfactory thermal comfort map looks like, with very little variance of the PMV value throughout. It can be observed that the occupants would rather feel neutral in terms of thermal comfort and are within the recommended range of PMV as per ASHRAE 55 (negative 0.5 to 0.5).

With minimal values for outdoor rate change at the extracting units, the resulting flow results show low-velocity values (below .65 feet/s) and are therefore considered to have an insignificant adverse impact on the PMV values. 

The flow pattern, however, coupled with temperature plots, highlights the heat curtain phenomenon formed by the radiator under the windows. This can be seen in the foreground slice of the picture below, where hot air rises to the ceiling of the second-floor bathroom, preventing cold air from penetrating deeper inside the room. The rear slice shows a situation with no radiator under a window, in the bedroom of the same apartment. The cold air can flow directly towards the center of the rooms participating in the overall low temperature.

This phenomenon impacts the average temperature in the room and therefore the thermal comfort of the occupant. In the 20th century, when the insulation of windows was poor (high U-values), this effect was particularly desired, which is why radiators have been traditionally installed under the windows.

Tool for Predicting Energy Consumption Works

As shown in this project, CFD simulation is a valuable tool in accurately predicting energy consumption, leading to a more environmentally friendly building while guaranteeing a suitable level of thermal comfort. 

The hand calculation values for evaluating the radiator heat power for each level were confirmed by the CFD results, leading to an average value of 69.4°F from the three apartments and office. This value is close to the one predicted in the calculation (negative 1.01 percent error margin). 

With temperature plots and flow pattern visualization, some hot spots and areas of low temperature were identified and linked to specific phenomena such as the hot air curtains created by the radiators.  The thermal comfort PMV value indicates that the results for the occupants of the spaces is within a range of negative 0.5 to 0.5 (slightly cold to slightly warm).

This analysis could be extended further and applied to different aspects. One example is the study of different U-values for the components and their impact on the energy expenditures of the heaters. In other words, assessing the impact on energy and potential saving if, for example, new, better-insulated windows were installed in a building. 

A second example could be to propose designs with different inlet and outlet positions and evaluate their impact on the heat and flow distribution. A third would be to investigate the effect of an underfloor heating. 

All these ways of improving the design—whether it is existing or at a concept stage—to achieve acceptable levels of thermal comfort and minimizing energy expenditure, are all possible through an iterative design process with CFD simulation.

Arnaud Girin is technical marketing specialist for SimScale. He has a mechanical design background and has worked for six years on design performance optimization with CFD and FEA tools. He is currently involved in simulation projects for multiple industries, with a focus on architecture, engineering and construction (AEC).

FAQs

Why Is thermal comfort in a building so important? 

When people are dissatisfied with the thermal environment, their productivity, capacity of concentration, well-being, and health can be negatively affected. Thus, ensuring thermal comfort for any new building design project through its HVAC system, as well as positioning of windows, doors, stairs, and other components, is paramount.

What is a PMV Index?

PMV is an index that determines the mean value of votes of a group of occupants on a seven-point thermal sensation scale. 

Why should you use computational fluid dynamics (CFD) simulation in building design?

Building professionals should use CFD in an interactive design process to achieve acceptable levels of thermal comfort and minimize energy expenditure for building occupants. The simulations help determine a suitable heating capacity while maintaining recommended indoor air quality.