BUILDING INTEGRATED VENTILATION SYSTEMS MODELLING AND DESIGN CHALLENGES

Transcription

1 BUILDING INTEGRATED VENTILATION SYSTEMS MODELLING AND DESIGN CHALLENGES P. Heiselberg Ph.D. 1 ABSTRACT Today, attention has turned towards optimal use of sustainable technologies like natural ventilation. Buildings and ventilation systems are designed to interact with the outdoor environment and are utilizing the outdoor environment to create an acceptable indoor environment, whenever it is beneficial. In the majority of cases a combination of natural and mechanical ventilation systems would be beneficial depending on outdoor climate, building design, building use and the main purpose of the ventilation system as this both can reduce energy consumption and environmental load and at the same time ensure acceptable indoor climate. The last decade s extensive research carried out on the improvement of energy efficiency in buildings has focused on efficiency improvements of specific systems like ventilation, low energy cooling systems, daylighting, etc. Significant achievements are realised, and even if most ventilation technologies still offer opportunities for efficiency improvements, the greatest potential of the future and a prerequisite for achieving the potential energy savings is development of methods and technologies that optimizes the interaction of building components and energy systems like ventilation. The paper discusses both trends, gives examples of developed methods and technologies as well as discusses the research needed for continued development. INTRODUCTION Immediately after the energy crisis in 1973 attention was focused on thermal insulation, airtightness of buildings and heat recovery to decrease energy consumption of heating and cooling of buildings. Buildings were designed to be isolated from the outdoor environment with an indoor environment controlled by artificial lighting, mechanical ventilation and heating and cooling systems. Today, in the design of new buildings and retrofit of old buildings an integrated approach is used with focus not only on thermal insulation, airtightness and heat recovery but also on optimal use of sustainable technologies as passive solar gains, passive and natural cooling, daylight and natural ventilation. Buildings are designed to interact with the outdoor environment and they are utilizing the outdoor environment to create an acceptable indoor environment whenever it is beneficial. The extent to which sustainable technologies can be utilized depends on outdoor climate, building use, building location and design. Under optimum conditions sustainable technologies will be able to satisfy the demands for heat, cold, light and fresh air. In some cases supplementary mechanical systems will be needed and in other cases it will not be possible to use sustainable technologies at all. Research into building energy efficiency over the last decades has focused on efficiency improvements of specific building elements like the building envelope, including its walls, roofs and fenestration components (windows, daylighting, ventilation, etc.) and building equipment such as heating, ventilation, air handling, cooling equipment and lighting. Significant improvement have been made, and whilst most building elements still offer opportunities for efficiency improvements, the greatest future potential lie with technologies that promote the integration of active building elements and communication among building services. In this perspective Whole Building Concepts are defined as solutions where responsive building elements together with building services are integrated into one system to reach an optimal environmental performance in terms of energy performance, resource consumption, ecological loadings and indoor environmental quality. Responsive Building Elements are defined as building construction elements which are actively used for transfer of heat, light and air. This means that construction elements (like floors, walls, roofs, foundation etc.) are logically and rationally combined and integrated with building service functions such as heating, cooling, ventilation and energy storage. Natural ventilation Natural ventilation and passive cooling are sustainable, energy-efficient and clean technologies as far as they can be controlled. Buildings with natural ventilation are associated with less SBS-symptoms, 1 Per Heiselberg is professor at Hybrid Ventilation Centre, Aalborg University, Denmark - 1 -

2 than buildings with traditional ventilation systems, (Seppänen and Fisk, 2002) and natural ventilation systems are well accepted by occupants and should therefore be encouraged wherever possible. Natural ventilation can be used to provide fresh air for the occupants, necessary to maintain acceptable air quality levels, and to cool buildings in cases where the climatic conditions allow it. The successful application of natural ventilation techniques and the effectiveness of natural ventilation, i.e. its ability to ensure indoor air quality and passive cooling in a building, are determined by the prevailing outdoor conditions and microclimate (wind speed and direction, temperature, humidity and surrounding topography) as well as the building itself (orientation, number of windows or openings, size and location). For cooling purposes, the incoming air should be at a lower temperature that the indoor air temperature. However, even at higher temperatures, the resulting air flow inside the space can cause a positive effect on the thermal comfort conditions of the occupants, since it increases heat dissipation from the human body end enhances evaporative and convective heat losses. Natural ventilation techniques for cooling purposes are also very effective during the night hours, when outdoor air temperatures are usually lower than the indoor ones. Air humidity is the most important limiting factor for the application of natural ventilation techniques. High levels of humidity have a negative influence on thermal comfort. As a result, in regions with high relative humidity levels during summer, the use of conventional air-conditioning systems is necessary in order to remove water vapour from indoor air. Under such circumstances, natural ventilation during the day- or even night-time hours should be avoided. Hybrid ventilation Hybrid ventilation is a relatively new ventilation concept that utilises and combines the best features of natural and mechanical ventilation systems. Hybrid ventilation provides opportunities for innovative solutions to the problems of mechanically or naturally ventilated buildings: solutions that simultaneously improve the indoor environment and reduce energy demand. Natural and mechanical ventilation have developed separately over many years and the potential for further improvements is limited. But the combination of natural and mechanical ventilation opens a new world of opportunities. Hybrid ventilation systems can be described as systems that provide a comfortable internal environment using both natural ventilation and mechanical systems, but using different features of these systems at different times of the day or season of the year. In hybrid ventilation mechanical and natural forces are combined in a two-mode system where the operating mode varies according to the season, and within individual days. Thus the active mode reflects the external environment and takes maximum advantage of ambient conditions at any point in time. The main difference between a conventional ventilation system and a hybrid system is the fact that the latter has an intelligent control system that can switch automatically between natural and mechanical modes in order to minimize energy consumption. Naturally, expectations of hybrid ventilation performance will vary between different countries because of climate variations, energy prices and other factors. In countries with cold climates, hybrid ventilation can avoid the trend to use mechanical air conditioning in new buildings, which has occurred in response to higher occupant expectations, the requirements of codes and standards, and in some cases higher internal gains and changes in building design. In countries with warm climates, it can reduce the reliance on air conditioning and reduce the cost, energy penalty and consequential environmental effects of full year-round air conditioning. Both natural and mechanical ventilation have advantages and disadvantages. For natural ventilation systems one of the major disadvantages is the uncertainty in performance, which results in an increased risk of draught problems and/or low indoor air quality in cold climates and a risk of unacceptable thermal comfort conditions during summer periods. On the other hand, air conditioning systems often lead to complaints from the occupants, especially in cases where individual control is not possible. Hybrid ventilation systems have access to both ventilation modes and therefore allow the best ventilation mode to be chosen depending on the circumstances. Building integrated ventilation Building integrated ventilation is a technology that integrates responsive building elements with the ventilation system. The focus on the environmental impacts of energy production and consumption has provided an increased awareness of the energy used by fans, heating/cooling coils and other equipment in ventilation and air conditioning systems. An expectation of a reduction in annual energy costs has also been an important driving force for the development of natural and hybrid ventilation strategies. Available data from case studies provided in the international project IEA ECBCS-Annex 35 (Heiselberg 2002) show that a substantial energy saving has been achieved in a number of buildings, - 2 -

3 mainly because of a very substantial reduction in energy use for fans and a reduced energy use for cooling. This was achieved primarily by utilising the natural driving forces and the natural cooling potential of the outdoor air. Building elements like embedded ducts, multiple skin facades and exposed thermal mass was used to some extent to preheat and/or pre-cool ventilation air or to reduce the impact of high heat gains. But, due to limited knowledge on the integrated performance of these elements and the ventilation system aa well as appropriate simulation methods the system designs were far from optimised. Development, application and implementation of responsive building elements in building integrated ventilation systems are a possible next step towards further energy efficiency improvements in the built environment. Development of integrated solutions, Whole Building Concepts, where responsive building elements together with building services, like ventilation are integrated into one system, will provide opportunities for further improvement of environmental performance. New international research project In order to adress these issues the preparation phase of a new international research project was approved at the 54th IEA-ECBCS Executive Committee meeting in Prague in November The annex working title is Integrating Environmentally Responsive Elements in Buildings. The annex will address the following objectives: Define state-of-the-art of responsive building elements and their integration with building services Improve and optimise the integration of responsive building elements and building services Develop and optimise new building concepts with integration of responsive building elements, building services as well as natural and renewable energy strategies Develop guidelines for procedures and tools for detailed estimation of environmental performance of responsive building elements and integrated building concepts This paper describes the principles of building integrated ventilation technologies, gives examples of existing technologies and discusses both challenges and needs for future research in design and modelling of building integrated ventilation systems. RESPONSIVE BUILDING ELEMENTS In building integrated ventilation systems a natural or hybrid ventilation system is integrated with one or more responsive building elements (like floors, facades, roofs, foundation, embedded ducts, etc.). Responsive building elements are building construction elements that assist to maintain an appropriate balance between optimum interior conditions and environmental performance by reacting in a dynamic and integrated manner to changes in external or internal conditions or to occupant intervention, and by dynamically communicating with technical systems like ventilation. Examples of responsive building elements include: Facades systems (Double skin facades, adaptable facades, dynamic insulation) Roof systems Foundations (Earth coupling systems, embedded ducts) Storages (Active use of thermal mass, material (concrete, massive wood) core activation (cooling and heating), phase change materials (PCM)) Responsive building elements can be used to preheat and/or pre-cool ventilation air or to reduce the impact of high heat gains and thereby improves the performance of the integrated system by complementing the use of natural driving forces and natural cooling potential of outdoor climate with utilization and/or reduction of internal heat gains and solar radiation. AIR AND HEAT FLOW PROCESSES In building integrated ventilation the natural and mechanical air flow processes are integrated with the heat flow processes in the responsive building elements. As the two processes are dependent, knowledge of the combined air and heat flow process is necessary to estimate the need for mechanical heating and/or cooling support. The air flow process can be divided into different elements from air flow around buildings, air flow through openings, air flow in rooms to air flow between rooms in a building. As air flow passes different responsive building elements, different heat flow processes will affect the flow. In order to minimise the pressure loss in the ventilation system the velocity level in the system is low and the - 3 -

4 cross sectional area of the responsive building elements rather large. This means that the flow will be far from fully turbulent and therefore, it is very difficult to predict both the pressure loss and the convective heat transfer. Embedded ducts For horizontal embedded ducts the air velocity distribution will not be constant. Cold intake air will flow as a cold jet/gravity current in the bottom of the duct and warmer air will flow backwards in the top depending on the air flow rate through the duct and the amount of entrained air. The opposite will be the case in the summer period. The embedded duct on figure 1 is an example of an embedded duct from a school building. The fresh air is through a chimney let into one end of the duct and in the other end of the duct the air is from the top of the duct distributed via channels in the walls to the classrooms. FIGURE 1. Embedded duct under a school building. Left picture shows the intake af airt to the duct via a chimney. Middle picture shows the duct which is 2.1 m high and 1.5 m wide. Right picture shows air outlets to classrooms. Figure 2 shows the average air velocity distribution in the duct in a distance of 10 m from the inlet to the duct. The air velocity is highest at floor level and decreases rapidly with height. Fresh and cold air is flowing in the bottom half of the duct and warmer air returns in the top of the duct Height above floor [mm] ,15-0,1-0,05 0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 Average Air Velocity [m/s] FIGURE 2. Vertical average velocity distribution in embedded duct 10 m from air inlet. (NN 2003) Figure 3 shows the vertical average temperature distribution. There is a very strong vertical temperature gradient in the duct of 3.5 o C/m. As the outlets to the classrooms are positioned in the top - 4 -

5 of the duct, the supply air temperature to the classrooms will be much higher than the average temperature in the duct. 2,10 Height above floor [m] 1,80 1,50 1,20 0,90 0,60 0,30 0,00 6,00 8,00 10,00 12,00 14,00 16,00 Temperature[ C] FIGURE 3. Vertical average temperature distribution in embedded duct 10 m from air inlet. (NN 2003) Figure 4 shows the surface temperatures of the duct close to the intake of air from the chimney. It is seen that there is also a large temperature difference between the different surfaces. Therefore the radiant exchange of heat must also be considerable in the duct. FIGURE 4. Surface temperature distribution in embedded duct close to the air inlet from chimney. The flow conditions in the duct and thereby the convective and radiant heat transfer processes will depend on the outdoor conditions, the air flow rate and the season. Present heat transfer models do not take the air flow conditions in the duct into account and are therefore unable to predict the heat transfer proporly. In a study by Wachenfeldt (2003) the most commonly used convective heat transfer models for embedded ducts was compared with the measured heat transfer in am embedded duct in a Norweigian school building. The study showed that all the models underpredicted the convective heat transfer considerably

6 Ventilated double-skin facades Ventilated double skin facades are another often used responsive building element. Building environmental performance is very sensitive to the correct operation of the double skin façade, Gratia (2004). Therefore, it is very important that the complex heat and mass flow process in a ventilated double skin façade are modelled appropriately. This is especially important with regard to modelling of the air flow (velocity profiles, flow resistance and air flow rate) in the facade, Grabe (2002), as the predicted results are very sensitive to correct estimation of the air flow rate. Further experimental and numerical investigations are needed to improve models of ventilated double skin facades. VENTILATION MODELLING As the ventilation process and the thermal behaviour of the building are linked the development of analysis methods must take both aspects into consideration at the same time and include efficient iteration schemes. This is the case for all types of methods from simple decision methods, analytical methods, and multizone methods to detailed CFD analysis methods. A model that combines a thermal simulation model with a multizone air flow model will allow the thermal dynamics of the building to be taken into account and will improve the prediction of the performance of the ventilation system considerably. Such a model will be capable of predicting the yearly energy consumption for building integrated ventilation and will therefore be the most important design method for this type of ventilation system. Combined Thermal and Air Flow Modelling Methods Prediction of the yearly performance of building integrated ventilation systems requires combined thermal and air flow modelling methods. A number of different tools have been developed or are under development (ESP-r; CHEMIX; TRNSYS+COMIS; IDA). However, the experience of the ability of these tools to predict building integrated ventilation performance is still limited and several questions need to be answered like: How well are the ventilation system and especially the responsive building elements modelled? How well are loads, schedules and control strategies modelled? Can general guidelines on proper simulation be developed? How robust are the tools predictions, i.e. how dependent are the conclusions on the tool and boundary conditions used? A simulation exercise has been performed on a hybrid ventilated school building in Norway using ESP-r, see Wachenfeldt The simulation results were validated extensively against monitored data in the real building. All results indicate that the performance can be accurately predicted in the simulation model. It was possible to identify and quantify all major energy transport mechanisms in the building. However, similar accuracy cannot be expected in a design situation with very limited knowledge about the "future" building, as the model was calibrated with respect to both thermal bridges and the heat transfer in the embedded inlet duct. In addition, the existing fan frequency records enabled very good predictions of the airflow rate in the system at all times. In normal cases, such inputs would carry very high uncertainties. The combined thermal and air flow modelling method is a powerful tool for analysis of building integrated ventilation and its accuracy is sufficient for design. However, there is still a need for improvement, especially with regard to the simulation of control strategies, the simulation of impact of wind on the ventilation (opening) performance and of the heat transfer in responsive building elements like culverts, stacks, etc, where the airflow is far from fully turbulent. FUTURE CHALLENGES IN DESIGN AND MODELLING A ventilation system like natural ventilation has to be integrated with and designed together with the building as the performance depends on location, type and size of the openings in the building envelope and air flow pathways through the rooms in the building must ensure optimum use of natural driving forces and distribution of fresh air to all occupants. With the development of hybrid ventilation technology, where natural and mechanical ventilation systems are combined, the integration of building elements and the ventilation system began, primarily by using parts of the building for distribution of air to minimise the pressure loss and the energy use for fans. Following this development building integrated ventilation systems are a possible next step. By development of integrated solutions where active building elements together with building services, like natural and - 6 -

7 hybrid ventilation, are integrated into one system, will provide opportunities for further improvement of the environmental performance of buildings. In order to support and accelerate this development several challenges are still to be solved with the assistance of research and development. These challenges lie in all areas from design methods, ventilation components, control systems, sensors to analysis and evaluation methods. Building Elements and Ventilation Components There will be a need for development, application and implementation of responsive building elements that can be used to preheat and/or pre-cool ventilation air or to reduce the impact of high heat gains, and thereby improve the performance of the building integrated ventilation system by complementing the use of natural driving forces and natural cooling potential of outdoor climate with utilization and/or reduction of internal heat gains and solar radiation. These responsive building elements should be able to react in a dynamic and integrated manner to changes in external or internal conditions or to occupant intervention, and communicate dynamically with technical systems like ventilation. A number of elements are already implemented in building projects, like embedded ducts, ventilated double skin facades and hollow floor slab. However the knowledge on their actual performance and the modeling accuracy of the integrated heat and mass flow process is limited and research is needed to improve the understanding of the physical mechanisms before improved modeling and design methods can be developed and optimal operation strategies determined. Analysis and Design Methods With regard to modeling and analysis of natural, hybrid and/or building integrated ventilation a number of aspects of different models are identified for further development or improvement. The application of natural, hybrid and/or building integrated ventilation requires a careful design in the early design phases, and the scarcity of simple and fast design tools is one of the most important issues. Estimating the initial cost of natural, hybrid or building integrated ventilation systems in buildings can be quite difficult as the installation often consists of both mechanical installations and of building elements. Part of the investment in mechanical equipment is often shifted towards a larger investment in the building itself: increased room air volume per person, a shape favourable to air movement, a more intelligent facade/window system, etc. On the other hand the building might provide more useable (rentable) space, as space for plant rooms, stacks for ventilation channels, etc., is not needed. Recently a method for calculation of Life Cycle Costs (LCC) of natural ventilation systems have been developed, (Vik, 2003), which takes all these issues into consideration. This method can as well be applied on buildings with hybrid or building integrated ventilation systems. As integrated multi-zone air flow and thermal models are the most promising engineering methods for predicting yearly performance of building integrated ventilation systems, it is crucial to ensure that the fundamental governing equations are correct for air flows through buildings with large openings (Axley, 2002). It is necessary to further understand wind-driven flows through large openings in enclosures. It is expected that large eddy simulation and wind tunnel testing will continue to play an important role, in this respect, due to the very unsteady nature of the flows. It is not expected that large eddy simulation will become a daily engineering tool for ventilation engineers in the near future, but it can be a very useful tool for improving the fundamental understanding of air flows and for assisting in developing improved engineering methods. As an alternative method in such cases Kato (2004) has proposed a flow network model based on the power balance, i.e. the power loss along a stream tube in and around the building. The flow rate through each opening is generally expressed as a simple function of the pressure difference, such as the power law relationship. This pressure difference can be a result of wind pressure, stack pressure, fan-induced pressure, or a combination of all of these. The simulated performance of buildings and building integrated ventilation systems, that are designed to interact with the outdoor environment, to utilize the outdoor environment to create an acceptable indoor environment, whenever it is beneficial, and where occupants have a large degree of control, can be very sensitive to changes in assumptions made with regard to boundary conditions, input data, control strategies, user behaviour, etc. The simulated performance sometimes can be quite different from the actual one. It would be beneficial to develop guidelines and information on procedures and tools for simulation of environmental performance (energy use, resource use, ecological loading, indoor environmental quality) of this type of buildings and ventilation systems to evaluate solution robustness to changes in outdoor climate, user behaviour, load schedules, etc. Integration of generalised control strategies into integrated air flow/thermal models is important both to assist designers in choosing the most effective control strategies for a particular design, but also to get more reliable result. A possible development could also be combination of existing physical models, - 7 -

8 such as multi-zone methods, with stochastic models, to develop probabilistic methods that can handle the effects of human behaviour and uncertainties in boundary conditions and input parameters. CONCLUSION Development, application and implementation of responsive building elements in building integrated ventilation systems are a possible next step towards further energy efficiency improvements in the built environment. Development of integrated solutions, Whole Building Concepts, where responsive building elements together with building services, like ventilation are integrated into one system, will provide opportunities for further improvement of environmental performance of buildings. In order to support and accelerate this development several challenges are still to be solved with the assistance of research and development. These challenges lie in all areas from design methods, ventilation components, responsive building elements to analysis and evaluation methods. A number of responsive building elements are already implemented in building projects, like embedded ducts, ventilated double skin facades and hollow floor slab. However the knowledge on their actual performance and the integrated heat and mass flow process is limited and research is needed to improve the understanding of the physical mechanisms before improved design methods can be developed and optimal operation strategies determined. The combined thermal and air flow modelling method is a powerful tool for analysis of building integrated ventilation. However, there is a need for improvement, especially with regard to the simulation of the complicated air flow and heat transfer mechanisms in responsive building elements like culverts, stacks, multiple-skin facades etc, where the airflow is far from fully turbulent. It is also important to consider the sensitivity of simulated results with regard to boundary conditions, input data, control strategies, user behaviour, etc. REFERENCES Seppänen, O. and Fisk, W.J Association of Ventilation System Type with SBS Symptoms in Office Workers. Indoor Air, Vol. 12, No. 12, pp Heiselberg, P (ed.) Principles of Hybrid Ventilation. IEA ECBCS-Annex 35final report. Hybrid Ventilation Centre, Aalborg University, ISSN R0207. The booklet can be downloaded from Gratia, E., De Herde, A Optimal Operation of a South Double-Skin Façade. Energy and Buildings, Vol. 36, pp Von Grabe, J A Prediction Tool for the Temperature Field of Double Facades. Energy and Buildings, Vol. 34, pp Wachenfeldt, B.J Natural Ventilation in Buildings - Detailed Prediction of Energy Performance. Norweigian University of Science and Technology, NTNU, Trondheim. Ph.D. thesis 2003:72, Department of Energy and Process Engineering, ISBN NN Internal communication, Aalborg University. Vik, T.A Life cycle cost assessment of natural ventilation systems. Norweigian University of Science and Technology, NTNU, Trondheim. Ph.D. thesis 2003:14, Department of Architectural Design, History and Technology, ISBN Axley, J.W., Wurtz, E., Mora, L Macroscopic airflow analysis and the conservation of kinetic energy. Proc. of Roomvent 2002, The 7th International Conference on Air Distribution in Rooms, Copenhagen, 8-11 September. Kato, S Flow Network Model Based on Power Balance as Applied to Cross-Ventilation. International Journal of Ventilation, Volume 2, pp, February