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The 2005 World Sustainable Building Conference, 01-068 Tokyo, 27-29 September 2005 (SB05Tokyo)

EXERGY: THE STEP BEYOND THE “”- CONSCIOUS DESIGN - A NEW LOOK AT SUSTAINABLE BUILDING

Dietrich Schmidt Tekn. Dr.1 Masanori Shukuya Prof.2

1 Fraunhofer-Institute for Building Physics, Project Group Kassel, Gottschalkstrasse 28a, 34127 Kassel, , [email protected]. 2 Laboratory of Building Environment, Musashi Institute of Technology, 3-1, Ushikubo-Nishi 3-chrome, Tsuzuki-ku, Yokohama, 224-0015, Japan.

Keywords: energy efficient building, sustainable building, analysis

Summary It is often claimed that energy is consumed. This is not only the case in everyday conversation but also in scientific discussion associated with so-called energy and environmental issues. This claim conflicts with the fact that the total amount of energy is conserved even though forms of energy may change from one to another. But, it is confusing to use one of the most well established scientific terms, energy, to mean “to be conserved” and “to be consumed” simultaneously. This is why we need to use the concept exergy to really understand what is consumed. Over the last two decades, various so-called “energy saving” measures have been conceived, developed and implemented in building envelope systems and also, in their associated environmental control systems, such as lighting, heating, cooling, and ventilating systems. There is still a large “saving potential” left, due to the fact that the demand of buildings accounts for more than one third of the world’s energy demand. Most of the energy supplied is utilized for room conditioning, to heat or cool the room space at a temperature of about 20°C. An optimization of the exergy flows in buildings can help in identifying the potential of an increased efficiency in the energy utilization. This kind of optimization leads us to rationalize heating and cooling systems that can make smart use of a small difference, let’s say a few degrees, between the thermal and the room temperature. We call such systems “low exergy systems” and they will allow for new ways towards the better utilization of energy in building stock. 1 Introduction The growing concern of environmental problems, such as global warming, which have been linked to the extended use of energy, has increased both the importance of all kinds of so-called “energy saving measures”, and the necessity for an increased efficiency in all forms of energy utilization. The and the need to fulfill it are steps in this direction. There have been a lot of efforts made to make buildings and the processes related to them, such as domestic hot water production, more efficient and to reduce the use of fossil energy sources in the built environment. An example of this is in Germany, where the approach has been taken with low energy and passive houses, in which almost no surplus heating from the is needed to keep the houses at comfortable levels, even in harsh winter conditions. Also, research related attempts, such as high technology zero energy houses, have been made. One aim in all efforts has been to save natural resources and fossil energy sources with the key of creating energy conscious and comfortably built environments. Today, all estimations of the energy use in buildings, i.e. calculations of the heating or cooling loads of rooms and buildings, as well as temperature calculations, are based on so-called energy balances. This is in reference to the known principle (the first law of thermodynamics), which states that energy is conserved in every device or process and it can not be destroyed or consumed. At the same time, the terms “” and “energy savings” are widely used. When such expressions are used, we implicitly refer to “energy” as the energy available from fossil or condensed uranium. These sources of energy are dissipated in everyday life. Various, so-called “energy saving” measures, and their associated environmental control systems such as heating, cooling and lighting systems, have been conceived, developed and also implemented in building envelope systems. National policies and codes have been influenced by these ideas, too. The question remains, what is really consumed under the law of energy conservation? And furthermore, is it sufficient to “save” energy, to be “energy conscious”, or to make sustainable houses with sustainable “energy” systems?

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The authors of this paper believe there is a lack in understanding of how energy systems in buildings . To enhance the understanding of the nature of energy flows in systems, we can use the so-called principle of entropy generation or the second law of thermodynamics, in addition to the energy conservation principle. In every process where energy or matter is dispersed, entropy is inevitably generated. In combining these two important principles, the concept of exergy should be used. The exergy concept can explicitly show what is consumed in energy utilization processes. In other words, exergy is the concept which quantifies the potential of energy to cause changes or to do work. In particular, the method of exergy analyses is found to provide the most correct and insightful assessment of the thermodynamic features of any process and to offer a clear, quantitative indication of both the irreversibilities and the degree of matching between the resources used and the end-use energy flows (Sciubba and Ulgiati 2005). The strongest point of the concept lies in the possibility of finding both the right energy source of a needed quality for a certain use is and the source for an increase in so-called energy efficiency. Exergy can be regarded as the valuable part of energy. Energy is everywhere around us, but in forms we can not use, or at least not directly. So, in this regard, we must be concerned about the quality of the energy flows. The use of high quality sources, like or fossil fuels, is for high quality purposes only. These are, for example, lighting or driving machines. But, for low quality purposes, like heating or cooling of indoor spaces, we should not use the named high quality energy sources. In the theory of thermodynamics, the concept of exergy is stated to be the maximum work that can be obtained from an energy flow or a change in a system. The exergy content expresses the quality of an energy source or flow. This concept can be used to combine and compare all flows of energy according to their quantity and quality. Exergy analysis is commonly used in, for example, the optimization processes of power stations. The use of the concept of entropy, as previously suggested (Gertis 1995), also complies with the exergy concept. Apart from the efforts made to improve energy efficiency in buildings, the issue of gaining an overall assessment and comparing different energy sources still exists. Today’s analysis and optimization methods do not distinguish between the different qualities of energy flows during the analysis. An assessment of energy flows from different sources is first done at the end of the analysis by weighting them with the primary energy factors. In the building codes of a number of countries, these problems have been solved via the transformation of all energy flows to the primary energy demand. The primary energy factors necessary for the calculation are not based on analytical ground or thermodynamic process analyses, yet they have been derived from statistical material and political discussion (DIN 4701-10 2001). Buildings still account for more than one third of the world’s primary energy demand (ECBCS 2002) and most of the energy is used to maintain room temperatures at around 20°C. In this sense, because of the low temperature level, the exergy demand for applications in room conditioning is naturally low. In most cases, however, this demand is satisfied with high quality sources, such as fossil fuels or electricity (Baehr 1980a). To open up the field of thermodynamic theory for building designers, user-friendly tools and methods are required to show the different dependencies. Architects and engineers, who are familiar with the energy codes, should easily be able to use this tool to obtain better and more exergy efficient buildings, thereby improving energy utilization in the overall building system (Schmidt 2004a; Sakulpipatsin, Boelman and Schmidt 2005) . The research work described in this paper is related to the international co-operation work of the “LowExNet”, the network of the international society of low exergy systems in buildings, which is a continuation of the activities of the Annex 37 working group of the International Energy Agency (Annex 37 2004). 2 Insight gained from the exergetic viewpoint The concept of exergy analysis in buildings proposed here can be used effectively to develop and evaluate energy-use strategies. The use of the proposed concept will enhance the development and optimization of energy systems in buildings. In many cases, some improvements in energy utilization can be achieved without any detailed or elaborate analysis, simply through common sense, good housekeeping, and leak plugging practices. But, as system configurations become more complex and good engineering methods are needed, it becomes apparent that a more in-depth analysis is required, which goes far beyond simple energy bookkeeping (Moran 1989). As illustrated here, the energy conservation concept alone is inadequate for an understanding of some important aspects of energy resource utilization. For this, exergy, that is derived from the two basic concepts and the associated environmental temperature must be used, in addition to energy calculations. The concept of 'primary energy use' may be reasonable in estimating the amounts of input to the systems in question. However, one cannot reveal where, within the systems, the consumption occurs and how the potentials of energy are used effectively or ineffectively. A clear picture of where the potential for a further increase in an efficient energy use can be found will be obtained by using a combined energy and exergy analysis. This is done in engineering thermodynamics; for example, in analyzing power stations (see Ahern 1980 and Moran & Shapiro 1998). This method has been used for the analysis of buildings (Schmidt and Shukuya 2003). The only differences are in the aim of the optimization procedure: power stations should maximize the electricity output as much as possible from a The 2005 World Sustainable Building Conference, Tokyo, 27-29 September 2005 (SB05Tokyo) given flow of primary energy/exergy. In buildings where people live, the most important thing is to have rational energy utilization patterns which enhance occupants’ well-being within the built environment. For the following study of building environmental control systems, such as heating or cooling, steady state conditions are assumed. Energy and matter are supplied into the system to make it work. In- and outputs are the same, according to the laws of energy and mass conservation. The energy flow through the building envelope is constant in time under steady state conditions. In the case of heating, heat transmission occurs from the warm interior to the cold ambient environment, across the building envelope (Figure 1).

Energy

Exergy

Entropy

Figure 1 Energy, entropy, and exergy flow through a building wall (Shukuya and Hammache 2002) This is accompanied by an increasing flow of entropy. The entropy of a substance is a function of the temperature and pressure. A certain amount of entropy is generated by this process, due to irreversible processes inside the building envelope. This generated entropy has to be discarded to the surroundings, i.e. the outdoor environment. It is important to recognize that the energy flowing out of the building envelope is not only accompanied by a destruction of exergy, but also by an increased flow of entropy. Disposition of generated entropy from a system allows room for feeding on exergy and consuming it again. This process, which underlies every working process, can be described in the four fundamental steps summarized in Table 1. Heating and cooling systems are no exception in this case (Shukuya 1998): All processes work according to these laws. The human body is no exception here and nor are the systems used in the built environment as shown in Figure 2 (Shukuya 1998; Saito and Shukuya 2001). Table 1. The four steps of the exergy-entropy process. 1. Feed on exergy 2. Consume exergy 3. Generate entropy 4. Dispose entropy

Exergy Exergy

Entropy

Entropy

Figure 2 The exergy-entropy-process on two systems: the human body and a building It is important to note that our whole natural and man-made environment functions in the fundamental four steps of the exergy-entropy-process. As described above, a certain amount of entropy is generated due to exergy consumption within the systems. This generated entropy must be discarded into the surrounding, The 2005 World Sustainable Building Conference, Tokyo, 27-29 September 2005 (SB05Tokyo) namely the outdoor or in the end the space. All energy flows in real systems are not only characterized by a decreased flow of exergy, but also by an increased flow of entropy. Disposing of the generated entropy from the system makes room for feeding on exergy and consuming it again. In other words, the entropy sink is as important as the exergy source in all systems. The flow of energy through our environment could be displayed in the way schematically drawn in Figure 3. By designing buildings and the related environment control systems according to these basic findings, an increase in energy efficiency will follow.

Exergy

The Universe

Global Environment

Regional Environment

Urban Space

Built Environment Room Space

Entropy

Figure 3 The environmental systems as nesting structures. 3 Low Exergy Buildings It can be shown by exergy analysis applied to buildings that the greatest fraction of the total supplied exergy for heating in buildings is consumed when heat is generated from other sources, e.g. fossil fuels like . Parts of these losses occur during , extraction, and transformation in power stations or in heat generation, e.g. in the boiler. Only a small fraction of the exergy consumption happens within the buildings (Schmidt and Shukuya 2003).

Figure 4 Exergy use in a building for different system alternatives. In all cases, the biggest losses happen within the boiler, and heat distribution and emission are of minor importance. The smallest fraction is used for the heating of the room space, but the entire exergy use in a building is influenced by that value (Johannesson 2004 and Schmidt 2004). This means that our known energy systems consume more exergy than needed for a certain purpose. There is a clearly shown larger potential for exergy saving measures than for energy “savings” (Johannesson 2004). The 2005 World Sustainable Building Conference, Tokyo, 27-29 September 2005 (SB05Tokyo)

Figure 5 Energy use for the same cases and legend shown in Figure 4 (Schmidt 2004). Since a great fraction of the supplied exergy is consumed within the heat generator, namely the boiler in this case, one would suggest the use of a better or perfect boiler. This is done for the case boiler. As shown, even with the best possible boiler, with an energy efficiency of 100 % (see Figure 4, dark thin solid line), the exergy consumption is still large. In contrast, if more insulation is used, the energy analysis in Figure 5 shows an improvement (half dark dotted line), but the exergy analysis shows that this measure really helps to reduce the exergy consumption of the entire system, even if the improvement is hard to visualize (different between dark solid and half dark dotted line) in Figure 4 (Schmidt 2004; Schmidt and Shukuya 2003). To use the supplied exergy most efficiently, we have to design our heating systems with as low supply temperatures as possible. In most cases, low exergy consumption within a component coincide with a low inlet temperature, that means that the energy is supplied at a low temperature level. There are examples of such systems already in the market, such as thermally activated building constructions. There are some types of floor heating systems or waterborne systems where heating or cooling pipes are put into the concrete slab construction. Another one is the airborne hollow core deck system, where tempered air first circulates inside the construction, thereby heating or cooling the rooms and then is released as fresh supply air to the rooms (Johannesson 2004). There are many more system alternatives, which are showcased in the LowEx Guidebook (Ala-Juusela et al 2004 and Annex 37 2005).

3.1 Examples: New Buildings Today, there already exist a number of so-called Low-Exergy-Buildings. Careful planning with a coordinated building envelope and services, the use of passive effects to heat, cool or ventilate the buildings, in combination with the reduction of internal (e.g. by low energy office equipment) and external loads (e.g. by efficient external sun shadings), are characteristics of successful building concepts. There are twenty-seven new built buildings with different uses showcased and described in detail in the final report of the IEA Annex 37 (Ala-Juusela et al 2004).

Figure 6 Examples of a new office building in Germany (Hauser et al 2005), equipped with thermally activated floor and ceiling systems and a ground heat exchanger for cooling, and residential buildings in the , equipped with wall and floor heating systems (Annex 37 2005). The 2005 World Sustainable Building Conference, Tokyo, 27-29 September 2005 (SB05Tokyo)

The case examples presented here or in the report of the IEA Annex 37 show the wide variety of applications of LowEx systems. They also demonstrate the flexibility of the systems with regard to the exergy source. Their heating or cooling systems use exergy from the sun, the ground, the network as well as the electricity or gas grid. Together with the findings from literature studies and occupant surveys, the case examples give strong evidence that in addition to the desired heating or cooling effect, LowEx systems can provide occupants with a comfortable, clean and healthy indoor environment.

3.2 Examples: Retrofitted Buildings The existing building stock is very important to focus on, the renewal of the building stock is very slow, and if we neglect the possibilities for low exergy systems in the existing buildings, the total effect will not be as large as we hope for. There are some examples of LowEx systems in existing buildings and there are also few examples of historical buildings with a cultural heritage, which means an even greater challenge. There are special issues to take into consideration to realize LowEx systems in existing buildings. The age of the building is not such an important issue when considering the possibilities for applying LowEx systems. The important aspects are the degree of protection of the building, the building type, the scale of renovation, replacement of installation and the type of LowEx system to be applied. Even though the low temperature heating systems are functional systems with lots of advantages, we need to keep in mind that when, we are talking about retrofits, there are also some technical limitations. For example, in old houses, the walls are not always that good, and one can encounter really poor U-values, i.e. bad insulation standards. If this is the case, floor heating is not efficient enough to meet the heating demand (Schmidt and Ala-Juusela 2004).

Figure 7 Examples of a retrofitted church building in Slovenia, equipped with a wall tempering system and a wooden residential building in Norway, equipped with advanced waterborne floor, wall and ceiling heating systems (Annex 37 2005). 4 International collaboration - the network LowExNet Since the IEA Annex 37 working group experts considered it very important to continue working with the topics described above, the International Society for Low Exergy Systems in Buildings was formed to further promote the use of the exergy concept to pursue more sustainable buildings. The general objective of the LowExNet is to promote the rational use of energy by means of facilitating and accelerating the use of low valued and environmentally sources for the heating and cooling of buildings. Specific objectives are: • To investigate potentials for replacing high valued energy (e.g. fossil fuels and electricity) with low valued energy sources, and to assess the impact on global resources and the environment • To assess existing technologies and components for low exergy heating and cooling in buildings, to enhance the development of new technologies, and to provide the necessary tools for the analysis and evaluation of low exergy systems • To develop strategic means for the introduction of low exergy solutions in buildings through the implementation of case studies, design tools and guidelines. In order to reach these objectives, the LowExNet organizes activities, such as workshops, seminars and presentations, in combination with other international events, such as conferences in the field of energy use in buildings and sustainable buildings. At present there are members form 13 countries from different places in the world who are bringing the low exergy approach in building further. A lot of material, the LowEx Guidebook and all other deliverables of the Annex 37 are also available via the internet homepage of the LowExNet (www.lowex.net). The 2005 World Sustainable Building Conference, Tokyo, 27-29 September 2005 (SB05Tokyo)

5 Conclusions The classical exergy analysis enables us to identify the location, to understand the origin, and to establish the true magnitude of waste or loss. Exergy analysis is therefore an important tool for the design of thermal systems since it provides the designer with answers to two important questions of where and why the losses occur. The designer can then proceed forward and work on how to improve the thermal system. Even if the method is a powerful tool, it has not yet been widely used for building applications. There are already many low exergy technologies available in the market. Low temperature systems successfully combine both traditional and innovative new approaches for heating. Usually the heat is transferred into the room through air or liquid circulation systems and the same system can often be used for both heating and cooling. These systems are commercially competitive, even if they are not chosen because of their high energy/exergy efficiency. But for reasons such as reduced story height, better comfort and reduced investment costs. Research shows that people living in buildings with low temperature heating systems are very satisfied with the indoor air quality. In particular, levels are considered to be higher than in buildings with a traditional conventional heating system. Low temperature systems can utilize a variety of sources of heat including district heat, , electricity or bio-, gas and oil (preferably in combined heat and power (CHP) systems). So the user is not constrained by choices made in the planning phase. Low temperature heat distribution/emission systems have often an operating life of at least 30-40 years, e.g. thermally activated building components are part of the construction itself, during which time the user benefits from the economic advantages offered by flexibility of fuel choice. The life cycle costs of a low temperature heating system are about the same as of a traditional system. Although the initial investment might be slightly higher, the system offers increased flexibility in terms of fuel choice and increased energy efficiency. There are a large number of demonstration projects which show the wide variety of possibilities to apply low exergy heating and cooling systems in buildings. There are examples of low exergy systems in dwellings and offices, but also in a museum and a concert hall. A collection of example buildings can be found in the final report, LowEx Guidebook, of the IEA Annex 37 (Ala-Juusela et al 2004). The application of low exergy systems provides many additional benefits besides a more flexible energy supply, such as: improved thermal comfort, improved indoor air quality and reduced energy use. These aspects should be further promoted to increase the application of low exergy systems for heating and cooling of buildings. There is still a great need for further research and development activities in the LowEx field. For the research part the question of an optimization method to come to a suitable temperature levels and combinations in a total system perspective in buildings are not sufficiently solved. There are a lot research needed to explore new or not commonly used exergy resources for the use in the built environment, such as the ground (e.g. using ground coolness for cooling), water (e.g. using ground-, sea- or river water as a cooling source), sky (e.g. using the radiation to a clear sky at night for cooling), snow or others. Moreover there are also tasks to conduct for the practice, for the development. The design of heat- in the market is not optimal for the use in LowEx systems, a further development and optimization is required, as well as the design of combined radiant cooling systems with ventilation open way for further development activities. The building regulations and energy strategies should take the quality of energy into account more than today. 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