Nothing but hot air?

Research in focus August 2012

The sun is shining, the thermometer has jumped to 30°C, and a gentle wind is blowing – it's a beautiful summer's day, the perfect day for a visit to the lake or outdoor pool. But when the weather is so lovely outdoors, conditions often deteriorate indoors. Offices, for example, often become overheated and stuffy – yet this is precisely where most employees spend up to 80 percent of their working day. In the summer, offices generally get too hot, while in winter the level of humidity tends to be too low, something that can lead to dry eyes, irritated mucous membranes and symptoms of fatigue. In many workplaces, colleagues at adjacent desks may have very different ideas about what temperature is comfortable, leading to constant arguments about whether windows should be open or closed. Poor conditions in the work environment can have a negative impact on performance, concentration and general well-being. The discomfort many people feel is clearly linked to the conditions they experience indoors (only German). At the Fraunhofer Institute for Building Physics IBP, a team of researchers working in the Energy Efficiency and Indoor Climate department led by Thomas Kirmayr is attempting to tackle this problem by analyzing specific indoor climate situations. The scientists investigate and develop optimized ventilation concepts to improve thermal comfort in indoor environments.

“Pinpoint the problem, analyze the causes, and then conduct simulations, concept development and implementation. That's the procedure we follow for existing buildings to analyze the indoor environment and optimize it accordingly,” says Kirmayr. The Fraunhofer scientists start by establishing an overview of the indoor climate situation. To do this, they perform an on-site analysis of current conditions in the respective location while also taking seasonal factors into account. “Take the example of an office building: If you have similarly high temperatures both inside and outside the building, and no wind to get the system moving, then even opening a window might not help. You're still going to see a drop in concentration and performance,” explains Kirmayr, a qualified industrial engineer. “When you are planning new buildings or renovation work, it is also important to take into account seasonal difference in temperature – from cold winters to hot summer days – because these factors also have a clear impact on people's sensation of comfort,” he continues.
Furthermore, some complaints such as dry eyes, irritated mucous membranes, poor concentration and dizziness can also be caused by emissions, which is why the scientists also have to take these factors into consideration in their investigations. To do this, the Fraunhofer researchers take air samples and send them to their colleagues in the department of Environment, Hygiene and Sensor Technology who analyze their composition. This enables the scientists to identify and quantify volatile organic compounds and selected aldehydes and ketones.
Once the researchers have ruled out the presence of elevated and/or harmful emissions, the next step is to assess the ventilation of the indoor environment and the factors that affect thermal comfort. The on-site investigations focus on factors such as air flow, air change rate, temperature and air humidity, all of which have a significant impact on comfort. With the aid of a tree-like mobile measuring station specially designed for this purpose by experts from the Fraunhofer IBP, the researchers can obtain a detailed insight into the on-site conditions. The measuring system consists of multiple sensors which record the humidity, the air temperature and the heat radiated by the surrounding walls as well as the movement of the air, performing all of these measurements at multiple levels. The comprehensive readings provided by this equipment play an important part in helping the scientists to build up a full picture of the indoor climate situation.
The next step is to determine the rate of air change, in other words the rate at which stale air is replaced by fresh air. On hot, still summer days, many offices struggle to take in enough fresh air even when the windows are open. Natural ventilation depends heavily on natural outdoor phenomena that move air around, such as the wind. This makes it difficult to model, which is why Kirmayr’s team relies on a range of short-term and long-term indoor climate measurements. These enable them to measure the air change rate while taking into account changing outdoor climate conditions. The most common technique is a tracer gas measurement, a specialist method of recording air change rates and air age in indoor environments. This involves injecting a non-reactive gas into the room which has a relatively low concentration in ambient air and measuring how quickly the mixture of air and tracer gas ‘thins’, in other words how quickly the concentration diminishes as a result of the indoor air being replaced by fresh air from outdoors. The researchers can use the results of these measurements to reliably determine air change rates and air age. By using two tracer gases, the researchers can even measure the rate of air change between two adjoining spaces. By performing long-term measurements over a number of weeks, the scientists obtain more detailed data which take into account different occupancy situations, user behavior and day/night variations as well as fluctuations in outdoor climate. The next step is to create indoor ventilation simulation models on the basis of these results. By using these simulation models to compute various possible solutions, scientists can determine the optimum rate of air change in the indoor environment they are investigating. These assessments based on verified measurement techniques help the decision-makers in a company to decide how to proceed. “Whether you are constructing a new building or renovating an old one, it makes sense to evaluate the impact of a measure before you spend money on implementing it. That's why reliable simulations are so essential to our work,” says Kirmayr. As well as helping scientists to visualize empirical data, simulations also make it possible to demonstrate and evaluate both current conditions and potential solutions. “That’s particularly important in cases where companies need to invest a considerable amount to improve their indoor climate situation,” Kirmayr adds.
The most detailed way of investigating ventilation scenarios is to use Computational Fluid Dynamics (CFD) simulation models. These enable indoor climate specialists to analyze the mixing of the air in various different ventilation scenarios and evaluate how novel approaches might improve the situation. CFD also provides the data required for detailed calculations of air flows and the air turbulence that can cause annoying drafts. This rich array of data enables scientists to work through different indoor climate concepts in order to come up with a solution that offers the greatest potential for the subject of their investigation, for example for a typical office environment. This is where companies often face a conflict of objectives as they try to focus on two things at the same time: Cost efficiency – i.e. the cost of equipping the offices and the building – on the one hand, and optimum performance and effectiveness of the employees who work in these offices on the other. “That's precisely why these factors should be considered at the earliest possible stage, ideally when the building is still being planned,” Kirmayr emphasizes. “That's the best way of achieving maximum comfort, high user acceptance and, ultimately, optimum performance, and it avoids companies incurring additional costs later on to resolve conflicts and carry out renovations.”
Once the problems have been pinpointed, it is often possible to come up with solutions that benefit everyone. For example, a mechanical ventilation concept with optional heat recovery could provide the perfect alternative to constantly opening and closing windows. Unlike the option of manually opening windows, a mechanical ventilation system ensures optimal ventilation at all times regardless of the outdoor weather conditions and can also be used to filter undesirable substances out of the air. “One essential thing you do need is a precise, carefully engineered plan for the required level of comfort. Offices can also benefit from a separation between the ventilation system and the heating and cooling processes, because that allows you to find better solutions to the problem of dry winter air without requiring additional active humidification. But optimum comfort also depends on other factors such as the form and arrangement of the air diffusers, intuitive operation, and allowing users to have some control over the settings,” says Kirmayr. These kinds of systems also have a positive effect on energy bills as well as benefiting users: “In large buildings with mechanical ventilation systems you can achieve heat recovery rates of between 70 and 80 percent; in small buildings you can push that to 90 percent.” Hoping to combine the advantages of window ventilation and mechanical ventilation systems, researchers at the Fraunhofer IBP are also experimenting with innovative hybrid ventilation systems. By using intelligent control algorithms and automated windows, they have already shown that this method can also meet the goals of cutting energy consumption while achieving high air quality and good thermal comfort.
Ultimately, a comfortable indoor environment depends on many different factors, as Kirmayr explains: “We know from experience that careful planning can prevent many problems and extra costs further down the line. The best approach is to take a holistic view which takes into account outdoor climate conditions, architecture, and the technical components and materials you are using. If you do end up experiencing indoor climate problems in an existing building, the methods we use can often provide effective relief at a very reasonable cost.”
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