Exergy analysis applied to building design Poppong Sakulpipatsin, MSc, Elisa Boelman, Dr. Eng. MBA, Building Technology Section, Faculty of Architecture, Delft University of Technology, Berlageweg 1, 2628 CR Delft, The Netherlands; email: p.sakulpipatsin@bk.tudelft.nl e.c.boelman@bk.tudelft.nl

Dietrich Schmidt, Tekn. Dr., Fraunhofer Institute for Building Physics, Project Group Kassel, Gottschalkstr. 28a, 34127 Kassel, Germany; email: dietrich.schmidt@ibp.fraunhofer.de

KEYWORDS: energy, exergy, building design

SUMMARY: Exergy analysis offers the potential of minimizing energy resource depletion, and is useful for assisting building designers in choosing building and building service concepts. Exergy recognizes that the energy that is carried by substances can only be used down to the level that is given by the environment. Unlike energy, exergy is not subject to a conservation law. This paper presents an outline of an exergy analysis tool for buildings and building services, including a new graphic input interface being developed with the aim of enhancing its user-friendliness. The idea underlying this interface is that energy engineers and building designers may approach the same contents from different perspectives. The tool illustrates energy and exergy flows in a building, including energy conversion steps in the building and building services, and exergy dissipation via the building envelope as a result of heat transmission and ventilation. The main advantage of this tool is that it enables a designer to compare energy and exergy flows for different building and building service configurations, from primary energy conversion through to the heating system and building envelope.

1. Introduction Exergy analysis has been utilized in the optimization of thermal processes in power plants and in industry. However, energy systems in buildings are designed based solely on the energy conservation principle. This principle alone does not provide a full understanding of important aspects of energy use in buildings, e.g. matching the quality levels of energy supply and end-use; fully expressing the advantages of using passive (e.g. thermal insulation, window design) and ambient energy (e.g. heat pumps) in buildings. From this viewpoint, exergy analysis is an important link in understanding and designing energy flows in buildings.

Recently, the exergy concept has been applied to the built environment as well (Shukuya 1994, Gertis 1995, Asada and Shukuya 1999, Nishikawa and Shukuya 1999, Jenni and Hawkins 2002, and Schmidt and Shukuya 2003). Some researchers (Rosen 2001 and Wall 2001) have also used the exergy concept in a context of sustainable development. In the last few years, a working group of the International Energy Agency has been formed within the Energy Conservation in Buildings and Community Systems programme: Low Exergy Systems for Heating and Cooling of Buildings (Annex 37, 2002 and Ala-Juusela, 2004). The overall objective of the Annex was to promote the rational use of energy by means of low valued and environmentally sustainable energy sources. This annex is being followed up by the international LowExNet group, which works towards providing knowledge on and tools for exergy analyses to be applied in the built environment (LowExNet 2004).

This paper presents an outline of a spreadsheet-based exergy analysis tool (Schmidt, 2004) and a new graphic input interface being developed to enhance its user-friendliness. The tool is meant to facilitate the practical application of exergy into building design. It does so by helping building and building-services designers develop insight into combinations of design options that can lower the total exergy consumption of a building and its associated building services. The interface is structured so as to allow users to input more detailed information on the parameters they are most familiar with. For example, a building designer

could focus more on varying building size and orientation, and/or building envelope configuration. A building services designer, on the other hand, could concentrate more on building occupancy schedules, indoor and outdoor air temperatures, and building service configurations. The new graphic interface is being developed mainly for building technology graduate students, and allows building materials to be inputted in more detail. The excel-based tool allows building-service and energy data to be inputted and viewed in more detail.

2. Outline of the exergy analysis tool

2.1 Energy and exergy flows From a building services perspective, energy supplied to the active heating systems in a building flows from the primary energy source (e.g. a fuel) via the building services to the building envelope, and is ultimately dissipated in the outdoor environment, as shown in Figure 1. The spreadsheet-based tool is built up in different blocks of sub-systems, each of which represents an important step in the energy flow (see Figure 1). The tool estimates the energy demand of a building operating at steady state design conditions, based on the German National Standard (DIN 4701-10), the German Energy Conservation Regulation (EnEV) and the European Standard EN ISO 13790 (EN ISO 13790).

FIG 1: Energy flow model underlying the spreadsheet-based tool (Schmidt, 2004)

Input data include general information on building shell construction and building services specifications. Calculations take into consideration heat losses in the different components as well as the auxiliary electricity required for pumps and fans. The electricity demands for artificial lighting and for driving fans in the ventilation system are also taken into consideration. On the primary energy side, the inputs are differentiated between fossil and renewable sources. On the building envelope side, average values for passive solar energy gains are estimated from standard solar radiation data, window size and glass type. Average heat losses through the building envelope are estimated based on standard design temperatures and overall heat transfer coefficients.

2.2 Outline of the energy and exergy calculations The main calculation module was previously developed as a spreadsheet-based model, according to the flow shown in Figure 1 (Schmidt, 2004).

Some values generated by the input interface (e.g. areas; heat transfer coefficient) are sent to the calculation module in order to calculate the heat demand of the building. The heat demand is balanced by the heat supply from the building services, which is delivered by from the selected emission system. The heat demand is summed up with the heat losses and auxiliary energy involved in the energy supply, so as to show the energy magnitudes through the entire system.

The exergy consumed during each step of the process is calculated by subtracting the exergy levels before and after the process, and by adding an amount of the auxiliary exergy. The exergy losses mainly depend on temperature changes and on heat losses occurred during the process.

Calculation results show the energy and exergy needed in each step of the process, according to the energy type (e.g., heat, electricity).

3. Building-centred approach to exergy analysis From a building designers viewpoint, a building may be regarded as a shell allowing its occupants to interact with (or shelter from) the outdoor environment. Natural resources (e.g. sunshine, wind) may supply passive heat and cold (energy) to a building, as far as the building envelope can be designed for providing the desired indoor climate conditions. Additional energy needs may be met by building services, in the form of heat and electricity (see Figure 2).

FIG 2: Building-centred energy flow model underlying the graphic interface

Quite often, building shape and function are more direct concerns of building designers than energy efficiency although the resulting design decisions are likely to have a significant impact on building energy performance.

3.1 Graphic user interface The graphic user interface (GUI) developed for the exergy analysis tool builds on the assumption that many building designers are visually oriented and likely to design the building shell in more detail than the associated building services. Hence, the GUI foresees the possibility to input more details on building dimensions and building materials than the spreadsheet tool does. The spreadsheet tool, on the other hand, provides a comprehensive overview of the numerical input data and equations used. The GUI is structured around outdoor environment, building and building services (see Figure 2), while the spreadsheet tool is organized in terms of the energy flows from source (fuels or renewable sources) to sink (outdoor environment), passing through the building services and building envelope (see Figure 1).

Figure 3 indicates how the information flow is structured between the users, the calculation module in the spreadsheet, and the GUI.

user inputinterface

construction material properties, buildingservices data, indoor and outdoor climate data

outputinterface

calculation

FIG 3: Information flow in the exergy analysis tool

The GUI is designed so as to enable the users to interact with the spreadsheet-based calculation via input and output parameters which are generally familiar to building design professionals.

The input interface is divided into three parts, namely: outdoor environment, building, and building services.

With regard to the outdoor environment, the interface requires general information on the site (e.g. a city where the building will be built) and season, to generate mean exterior temperature and solar radiation values by querying climate data form the database (Hoare, 2005). As for the building services, the building designer can select components from drop-down menus, following the structure of the energy flow model shown in Figure 1. Specific details on the building services can be added but require toggling to the calculation part in the spreadsheet. The building part, which is the main part of the interface, is explained in the next section.

3.2 Input of building specifications The building part plays a big role in the input interface because it is the part that building designers are most familiar with. A building designer can input building dimensions and orientation (see Figure 4), and may define details of construction materials, e.g. thickness and thermal conductivity (see Figure 5). The building details input in the GUI are used for generating aggregate values trough calculations and libraries, before being sent to the spreadsheet calculation module. The details of building material properties (Schalkoort and Luscuere, 2003) and the interface basic structure are derived from an interface developed at the Delft University of Technology (Dijk and Luscuere, 2002), which is currently used as a tool for teaching building services to architecture students.

FIG 4: building geometry input interface

The building geometry input module allows a designer to descriptively enter building shape and orientation, without needing to enter numerical values. At the conceptual building design stage, often a rough building form is first conceived by the architect. Dimensions and orientation are often approximate, and likely to be adjusted several times in the following stages. At the conceptual design stage, calculations of numerical values such as building volume and floor area are likely to be experienced as cumbersome, particularly if they have to be repeatedly input in order to check the energy performance of different design alternatives. In order to facilitate the input of building geometry information, the interface allows a user to easily change the aspect ratio of a simple box-like model. A user can input a small building with a maximum volume of 1125 m3 (Dijk and Luscuere, 2002) and eight possible orientations. Figure 4 shows an example of building geometry input. A building designer can get an impression of what the building looks like, and how different building shapes could result in the same building volume or area.

The building component input interface is used for entering details of building shell components: outer walls, ceiling and ground floor. Energy and exergy calculations are performed at the building level, so partitions and other components inside the building do not get taken into account.

When entering component specifications, building designers can specify by hand which materials are used, including thickness and sequence (up to six layers), or they can select from standard constructions. It is also possible to specify details of a window and a door in a wall (see Figure 5). These data are used for calculating heat transfer coefficients, and then are inputted into the calculation module. A temperature correction factor is calculated depending on the orientation defined in the building geometry input interface. The temperature correction factor functions as a modifier of mean indoor and exterior temperatures.

FIG 5: building component input interface (wall)

The interface also calculates the heat transfer coefficient and area of a window and/or a door in the wall, for which a user needs to input only the material names and object dimensions.

The building component input interfaces for ceiling and floor work similarly. The ceiling and ground floor are modelled as flat horizontal plates that are perpendicular to the wall. A ceiling shape that is different from the model (e.g. gable, shed, and hip roofs) can be incorporated by selecting a different temperature correction factor among a number of default values.

An opportunity to input descriptive details of building components is significant for building designers, because dimensions and materials often have a more concrete meaning for them than just numerical values of heat transfer coefficients. Also, when the building component becomes more complex, the calculations can become more difficult and time-consuming for the building designer. Figure 5 shows some values generated by the interface and sent to the spreadsheet-based calculation module.

The building services input interface (see Figure 6) allows building designers to select building services components along the energy supply chain (see Figure 1). The input data are restricted, as it is assumed that most building designers are somewhat less familiar with specifics. Detailed specification of the selected equipment might be too specific for typical building designers. The interface offers a list of available building services components, using default values for their specifications. If the designers want to change some details of these specifications, they can change these values directly on the interface, as they wish. The interface also provides advice on good combinations of building service components, based on the inlet and outlet temperatures of each sub-system. The module for giving these recommendations is currently under development.

FIG 6: building services input interface

. Example of energy and exergy calculation results nstruction and building onstruction data were

n

m3

4An example is used to illustrate a potential use of the tool. Technical data for coservices are for a room in a typical residential building (see Table 1). Detailed centered to the tools input interface. On the other hand, the details for the selected building services components were provided by the interface to the calculation module as default values. The case has beetaken from (Schmidt and Shukuya 2003) and calculated for a winter period.

Table 1: building system

Room dimension 6 X 6 X 3

Boiler Standard boiler (efficiency = 0.98 and supply temperature at 90 C)

ture radiators, with supply at 70C and return at 60C

n

all

win

Emission High tempera

Ventilatio Natural ventilation, with an air exchange rate of n = 1.5 h-1

Exterior w U = 0.4 W/m.K, A = 18 m W WU = 2.2 W/m. K, A = 9 m Window win

The results of energy utilizati for the whole process are visualized in Figure 7 8. The diagram rized by types of energy and exergy, such as

n

gher during heat generation than during the other processes. On the

re.

on and exergy consumptionand Figure in Figure 7 shows results categoheat, electricity and solar energy. The different line slopes indicate the different intensities of exergy consumption, per subsystem. For the illustrated case, major losses occur in the primary energy transformation, where primary energy from a fuel is transformed into electricity, and in the heat generatioinvolving combustion in a boiler.

Figure 8 shows the relative differences between exergy consumption and energy utilization. It can be seen that this difference is significantly hiother hand, for the processes during which heat enters the building at the surface of the emission system and leaves the building via its envelope, the difference between exergy consumption and energy utilization is relatively small. This is because these processes take place relatively close to environmental temperatuAs a whole, this system involves relatively large exergy consumption in the boiler, for high-temperature combustion. This high-temperature heat is subsequently degraded and supplied to a heat emission system, delivering heat at a relatively low temperature, in order to keep an indoor space at a temperature rather close to the exterior temperature.

0500

1000

1500

2000

2500

3000

1 2 3 4 5 6 7 8Components

Ene

rgy

/ Exe

rgy

(W)

system energy totalheat energyelectrical energyincl. free /rene. ener.incl. internal & sloar gainssystem exergy totalexergy incl. gains

Generation Emission Room Air EnvelopeDistributionStoragePrim. Energy transform

FIG 7: Exergy and energy flows through the different components

0

200

400

600

800

1000

1200

1400

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Primary energytransform

Generation Storage Distribution Emission Room air EnvelopeComponents

Ene

rgy/

Exe

rgy

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exergy lossenergy loss

FIG 8: Energy loss and exergy consumption of the components

The example presented above discussed energy and exergy consumed in different sub systems of the building energy chain, considering heat supply, distribution, emission and final dissipation through room air and the building envelope. The results showed where and how much energy was depleted. The tool can be used for similar analysis in different building energy systems. For example it was employed to study system flexibility and the possible integration of renewable sources into building systems (Schmidt, 2004), by varying heat generation methods and types of fuel. The results showed how the same heat demand could correspond to different exergy needs. This difference, which was not clearly shown in the corresponding energy analysis, showed how many renewable sources (e.g. solar heat) could be utilised more efficiently at low temperature levels.

5. Conclusions The exergy analysis tool was developed to analyze energy and exergy chains in buildings. It can be used to compare the impact of improvements in the building envelope to improvements in the building services.

The graphic user interface for the tool was designed with consideration to a building designers perspective. Two concepts are applied for the interface development. Firstly, basic design parameters (e.g. building geometry and materials) are descriptively inputted into a graphic interface, which generates values compatible with the input formats required by the spreadsheet-based calculation model. This can substantially reduce the burdensome work of preparing technical data, for use in the program, which building designers may be less familiar or concerned with. Secondly, some detailed values of building service components are incorporated as default values and referred to by the respective component name. If they wish to do so, building designers can change these values by hand for their calculations. This is likely

to ease designers use of the tool, allowing them to gain insight into more efficient ways of optimizing energy systems in their building. Future work includes putting the tool into broader use in order to perceive responses from the users. Input data (e.g. physical properties, building service specifications, climate data) for the interface also need to be extended.

6. Acknowledgements This publication was made possible by a research fellowship from the Delft University of Technology.

7. References Ala-Juusela M. (ed.) 2004, Guidebook to IEA ECBCS Annex 37 - Low Exergy Systems for Heating and Cooling of Buildings, VTT Technical Research Centre of Finland, Finland.

Annex 37. 2002. International Energy Agency Low Exergy Systems for Heating and Cooling of Buildings Annex 37, Web Homepage, http://www.vtt.fi/rte/projects/annex37/Index.htm.

Asada H. and Shukuya M. 1999, Numerical Analysis of Annual Exergy Consumption for Daylighting, Electrical-Lighting and Space Heating/Cooling Systems, 6th International IBPSA Conference, September 1999, Kyoto, Japan.

Dijk E.J. van and Luscuere P. 2002, An architect friendly interface for a dynamic building simulation program, Sustainable Building 2002 Conference, Oslo, Norway, September 23-25, 2002, paper No.513.

DIN 4701-10. 2001. Energy Efficiency of Heating and Ventilation Systems in Buildings Part 10: Heating, Domestic hot Water, Ventilation. German national Standard. Berlin:Deutsches Institut fr Normung e.V.

EN ISO 13790: 2004, Thermal Performance of Buildings Calculation of energy use of Space Heating. International Organization for Standardization.

Gertis K. 1995, Realistische Betrachtung statt ideologoischer Wnsche. Niedrigenergie- oder Niedrigentropiehuser? Clima Commerce International CCI, Promoter Verlag Karlsruhe, Germany, Vol 29, No 4, pp. 134-136.

Hoare R. 2005, WorldClimate, Buttle and Tuttle Ltd, http://www.worldclimate.com/

Jenni J. and Hawkins A. 2002, Der Unterschied zwischen Temperatur und Energie Oder was kann mit exergiegerechter Speicherladung und entladung erreicht werden? HeizungKlima, Switzerland, Vol 29, No 3, 2002, pp. 8-11.

LowExNet 2004. Network of the International Society for Low Exergy Systems in Buildings, Web Homepage, http://www.lowex.net.

Nishikawa R. and Shukuya M. 1999, Numerical Analysis on the Production of Cool Exergy by Making Use of Heat Capacity of Building Envelopes, Sixth International IBPSA Conference (BS'99), Kyoto Japan, pp. 129-135.

Rosen M. A. and Dincer I. 2001, Exergy as the confluence of energy, environment and sustainable development, Exergy, An International Journal, Volume 1, Issue 1, 2001, pp. 3-13.

Schalkoort T.A.J. and Luscuere P. 2003, Klimatinstallaties,Publicatieburo, Faculteit Bouwkunde. TU Delft.

Schmidt D. 2004, Design of Low Exergy Buildings- Method and a Pre-Design Tool. In: The International Journal of Low Energy and Sustainable Buildings, Vol. 3 (2004), pp. 1-47.

Schmidt D. and Shukuya M. 2003, New Ways Towards Increased Efficiency in the Utilisation of Energy Flows in Buildings. Proceedings to the International Building Physics Conference 2003 September 14-18 2003, Leuven, Belgium.

Shukuya M. 1994, Energy, Entropy, Exergy and Space Heating Systems. Proceedings of the 3rd International Conference Healthy Building 94, 1994, Vol. 1, pp 369-374.

Wall G. and Gong M. 2001, On Exergy and Sustainable Development, Part I: Conditions and Concepts, Exergy An International Journal, Vol. 1, No. 3, pp 128-125.

IntroductionOutline of the exergy analysis toolEnergy and exergy flowsOutline of the energy and exergy calculations

Building-centred approach to exergy analysisGraphic user interfaceInput of building specifications

Example of energy and exergy calculation resultsConclusionsAcknowledgementsReferences