The participants also recognised the importance of developing a simplified tool to visualise why low exergy systems would be advantageous in some energy chains compared to high exergy systems. This tool should be easy to use and show the exergy flow through a system or energy chain. Finally, two Pre-design tools were developed (named as "the Pre-design tool" and "the Educational tool"). They are briefly described in this chapter, and more detailed descriptions can be found in the User-Guides of these tools (François et al. 2004a and b). Both the User-Guides and the tools are included in the CD-version of this Guidebook.
Both tools give outputs of similar format. The main difference is that the Pre-design tool is even more easy-to-use than the Educational tool, but it is therefore also less flexible. The ease-of-use is based on the drop-down lists, which are used more frequently in the Pre-design tool. In the Educational tool, the user has more options for the input parameters, but here the user has to know more facts about the building and its systems. The Educational tool does an analysis of a single moment, as the Pre-design tool gives an estimate of the annual energy and exergy demand of the building.
The exergy chain is described in chapter 2.1.1. An important step in the entire analysis is the estimation of the energy demand of the actual building. The heat demand is a key figure in the analysis, it corresponds to the building’s exergy load. A low exergy load means a thermally good constructed building envelope. The energy requirement for the service equipment is then estimated. (Schmidt 2004)
The main focus in this guidebook is on the system "building", whose system border of the to be analysed here encompass the building envelope.All energy (or exergy) flows over the border are indicated in Figure 22. For the balance of energy flows through the building, all possible effects has to be taken into account, even the extraction and production of the energy carrier. The calculation of energy flows caused by a building starts much earlier. (Schmidt 2004)
Figure 22. Energy (or exergy) flows over the systems boundaries of a building (Schmidt 2004)
For a deeper analysis of the energy flows in a building, a closer focus on the building services system is needed. The entire flow from the source to the sink, as indicated in Figure 23, must be taken into consideration. All energy flows from the left hand side, i.e. from the source, via a number of HVAC-components and the building structure itself, to the ultimate sink, the outdoor environment. Imperfections and losses in the different steps through the building are regarded, as well as the need for auxiliary energy. Energy, mainly in form of electricity, is needed to drive additional pumps and fans for the operation of the system. (Schmidt 2004)
Figure 23. Energy utilisation in building services equipment (according DIN 4701-10 2001, modified)
The detailed presentation of the tool can be found in the document called "User-Guide for the Educational Tool for Energy and Exergy analyses of Heating and Cooling Applications in Buildings" (François et al. 2004a).
All steps of the energy chain - from the primary energy source, via the building, to the sink (i.e. the ambient environment) - are included in the analysis (Figure 24). The entire tool is built up in different blocks of sub-systems for all important steps in the energy chain. All components, building construction parts, and building services equipment, have sophisticated input possibilities, as described further down. Heat losses in the different components are regarded, as well as the auxiliary electricity required for pumps and fans. The electricity demand for artificial lighting and for driving fans in the ventilation system is also included. On the primary energy side, the inputs are differentiated between fossil and renewable sources. The steady state calculation for this heating case is done in the direction of the development of demand.
Figure 24. Energy utilisation in building services equipment, the modelling method for the Pre-design tool. The energy flows are shown form source to sink, in accordance to DIN 4701-10, modified (Schmidt 2004).
Although the analysis follows the same main principles as European Standards do, the tool is aimed at calculations under steady state design condition, not at annual energy use calculations (the user can enter mean values of climatic data to represent seasonal or annual average conditions).
The tool mainly presents seven parts of an excel worksheet. The worksheet is divided into seven different sections for the input and calculation of values. Project data and boundary conditions for the analysis are questioned in the first section. In tool section 2 the heat losses due to transmission through the building envelope and ventilation are estimated. Tool sections 3 and 4 follow, with the estimation of the possible heat gain, both solar and internal, which are to be subtracted from the heat loss. Tool section 5 sums up the losses and gains, a heat balance according to the first law of thermodynamics is set up. The choices for the building service equipment for heating are made in tool section 6. Energy sources, i.e. the boiler / the heat generation part and the actual emission system must be chosen. In tool section 7 the exergy analysis is finally drawn out. All sections are described in more detail in the User-Guide (François et al. 2004a).
The user can run this tool for example to study:
The form of results is presented here using the results from a detailed analysis performed on the ZUB office building (see case example DEU 1). Flows of energy and exergy have been calculated and are displayed in a diagram. The analysis has been done under steady state conditions. All detailed input values and boundary conditions are given in the User-Guide (François et al. 2004a).
The results are displayed as demands and losses by components (Figures 25 and 26). In this diagram, it is easier to understand where inefficiencies occur and possible steps for a further increase in the system efficiency may be indicated.
It is clearly shown that the greatest imperfections occur in two energy transfor transformation components, the primary energy transformation and the heat generation component.
The following diagrams, Figures 27 and 28, depict other possibilities of where losses happen and system imperfections occur. On the left-hand side of the diagrams, the supplies are shown and on the right hand side, the covered demand and losses are shown. These occur in the indicated places and are covered by shown components or sources.
Figure 26. Exergy losses / demands and energy utilisation by components of the ZUB building. The losses of useable energy and the consumption of exergy in every component are shown as bars.
Figure 27. Energy supplies / gains and utilisation for the ZUB building
Figure 28. Exergy supply and demands for the ZUB building
3.2 Pre-Design tool for heating and cooling
The objective of the "Pre-design Tool for Energy and Exergy Analyses of Heating and Cooling Applications in Buildings" is to provide designers or users with a simple tool that shows a holistic image of the energetic-flow and exergetic-flow patterns of various low temperature heating and high temperature cooling systems in buildings. For this purpose, the tool includes a program that estimates the heating and cooling requirements of a building using the well-known modified-bin method (ASHRAE HANDBOOK fundamentals).
Description of the toolThe heating and cooling requirements are estimated using descriptive data where the user should enter the physical characteristics of the building and the characteristics of the site. Simplified energy and exergy analyses are carried out on the system components of selected heating and cooling technologies. The exergy analysis is carried out using estimated quality factors of the energy flow in the system.
In the building context, the system includes in general the ambient air, the exterior building envelope, the interior air and the heating or cooling technology characterised by their different components such as the emission subsystem, the distribution subsystem, the storage subsystem and so on.
It is important to emphasise that the tool is not intended to carry out sophisticated energy and exergy analyses that require more detailed information on the technologies, the subsystem configurations, the operating parameters and the energy vectors used, so that all the exergy components could be taken into account and calculated precisely. This will require more sophisticated software, which is beyond the objective set to the tool.
The tool mainly presents six excel worksheets:
More information on the contents of these worksheets and on the use of the tool can be found in the User-Guide of the tool (François et al. 2004b). It is included in the CD-ROM version of this Guidebook. Here, only some characteristics are given.
In order to provide a simple and easy-to-use tool, the inputs are mainly entered from drop-down lists on the Input Parameters worksheet. All the required input parameters for the computations are introduced and explained. Typical values are provided for each parameter and since the methodology uses qualitative estimates on some parameters, the assumed values are also indicated in different tables.
For example, the heating and cooling applications are selected from drop-down lists. There are twenty-three options available for the heating generation and twelve options for the cooling technology.
The results are presented on three worksheets, 4. to 6.
Heating and cooling load worksheet
The "Heating and Cooling Load" worksheet is a report that summarises all the characteristics of the site conditions and the parameters entered by the user. The output for the building design heating and cooling loads as well as the building heating and cooling energy demands are also shown. In addition, the model calculates the corresponding exergy quantity for each variable by using a quality factor depending on outside and inside air conditions. The quality factor for the design exergy load is calculated using a constant design temperature. The quality factor for the exergy demand is calculated using variable temperature bins.
Heating worksheet
The "Heating" worksheet is a report that summarises the heating application assessment from both the energy and exergy situations. All the components of the heating process are indicated, as selected by the user, with their characteristics and estimated parameters. The worksheet produces the energy and exergy analyses as flow patterns along the different subsys tems. These analyses are presented in two formats, as tables and curves in the provided figure. The extent of the exergy losses in each subsystem can be deduced from the figure.
Cooling worksheet
Similarly to the "Heating" worksheet, the "Cooling" worksheet is a report that summarises the cooling application assessment from both the energy and exergy situations. All the components of the cooling process are indicated, as selected by the user, with their characteristics and estimated parameters. The worksheet produces the energy and exergy analyses as flow patterns along the different subsystems. These analyses are presented in two formats, as tables and curves in the provided figure. The extent of the exergy losses in each subsystem can be deduced from the figure (see Figure 29).
