Parametric real-time energy analysis in early design stages: a method for residential buildings in Germany
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Abstract
The greatest potential for optimizing the energy efficiency of buildings is in the early design stages. However, in most planning processes energy analysis is conducted shortly before construction when major changes to the design have a high cost impact. The integration of energy performance analysis in the early design stages is therefore highly desirable, but requires suitable tools able to quickly generate results that can help the planner optimize the building design. Parametric design approaches permit the effortless generation of many variants and therefore represent a suitable way of testing different alternatives in the early design stages. Most plug-ins for parametric design software currently rely on dynamic building performance simulation which provides detailed results, but requires computation times ranging from 20 s to 5 min. As optimization processes typically require several thousand simulations, the computation time can quickly amount to days. The approach presented in this paper proposes a real-time energy demand calculation based on a quasi-steady state method defined by the German standard DIN V 18599 which defines the national implementation of the European Directive on the Energy Performance of Buildings. The results are verified of tests on three residential reference buildings in Germany in comparison with an accredited commercial software product. An application example indicates the great potential for easy-to-use energy optimization in the early design stages.
Keywords
Real time Energy demand calculation Early design stage Parametric design Building design optimization ArchitectureList of symbols
- Qsink
Heat sinks
- Qsource
Heat sources
- QS
Solar heat gains
- Qsol,trans
Solar heat gains from transparent surfaces
- QI
Internal thermal gains (heat or cold)
- Qh,b
Balanced energy need for heating of the building zone
- QT
Transmission heat sinks
- QV
Ventilation heat sinks
- ΔQc,b
Heat stored in building components which is emitted in periods of reduced operation
- η
Utilization factor
- FX
Temperature correction factor
- Θe
External air temperature
- Θi
Reference internal temperature
- Θi,h,soll
Internal set-point temperature for heating during normal heating periods
- Θi,h
Internal temperature for reduced weekend operation
- Θu
Mean temperature in unheated space
- ΔΘEMS
Consideration of energy management system
- ΔΘi,NA
Permitted reduction in the internal temperature periods of reduced heating during the night
- Cwirk
Effective heat capacity
- fNA
Correction factor for reduced heating mode during the night
- tNA
Daily operation hours with reduced heating mode
- τ
Thermal time constant of a building zone
- atb
Ratio of indirectly heated areas to the total area
- FX
Temperature correction factor
- AG
Area of floor slab
- P
Perimeter of floor slab
1 Introduction
1.1 Background
Most European countries have implemented national regulations for building energy performance which comply with the demands of the European Directive on the Energy Performance of Buildings (EU 2010). In Germany, the Energy Saving Ordinance (EnEV) (Bundesregierung 2013) stipulates a monthly energy balance using a quasi-steady state method. The algorithms for calculating the energy demand are defined in DIN V 18599 (DIN 2011). For residential buildings, the older DIN V 4108 (DIN 2003) can at present still be used. Dynamic building performance simulation is only permitted for proof of protection against overheating in summer.
Stages in the architectural design process, based on Paulson Jr. (1976)
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Many boundary conditions need to be defined, but only limited information is available about the respective building in the early design stages.
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Inputting information on these boundary conditions requires extensive background knowledge, making it difficult for non-experts to apply building simulation during planning.
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The simulation is computationally intensive. Depending on the size of the building, the simulation can take between 20 s and 5 min on a standard PC. While this may be acceptable for a single simulation, the time required for simulating many variants, as needed for optimization, quickly multiplies to many hours or even several days—clearly too long for the early design stages.
Carlos and Nepomuceno (2012) describe the difficulties in applying dynamic simulation in the early design stages. Furthermore, they show that for moderate climates, quasi-steady state approaches can produce valid results. According to van Dijk et al. (2006), the monthly balancing of the simplified method is well suited for continuously heated buildings in warm, moderate and cold European climates. This would therefore be adequate for residential buildings which are mostly continuously heated. The time and effort required for dynamic simulations are such that the additional cost of these planning services often exceeds the budget for common residential buildings. Residential buildings, however, account for 75% of the floor area in Europe (Economidou et al. 2011). There is therefore a corresponding demand for simplified methods.
The algorithms used in quasi-steady state methods are simple enough for tools to be able to output the results in real time. Current commercial tools based on DIN V 18599 usually require the numerical input of areas in tabular form, but manual input quickly becomes very time-consuming when comparing more than one variant. Consequently, users often stop after calculating a few design variants. The full optimization potential of is therefore not exploited.
1.2 Parametric design and existing tools
Parametric definition of a cube with number sliders
A variety of parametric tools are available for 3D CAD programs: Grasshopper3D (GH) is a parametric tool with a graphical algorithm editor based on Rhinoceros (Rhino). Developed by David Rutten, it was first published in 2007. Since then, a large number of third party plug-ins and links to other software have been developed. For building performance simulation, a connection to EnergyPlus is provided by plug-ins such as Archsim (Dogan 2015), Honeybee (Roudsari et al. 2013) or Diva (Jakubiec and Reinhart 2011). Archsim and TRNSYS-Lizard (Frenzel and Hiller 2014) also provide an interface for TRNSYS. Currently, the only plug-in for a quasi-steady state method is based on ISO 13970 and connects GH to an Excel-based balancing software called Energy Performance Calculator (Ahuja et al. 2015). All these plug-ins export data from the parametric design software for external analysis and then re-import it for visualization. But exporting also introduces a delay and can be the cause of errors. Trials using several of these tools in student design projects showed that the process of exporting and importing acted as a barrier, even when the results are reported a few seconds later (Hollberg et al. 2016). Architects and engineers expect to have instant feedback on their designs, and they ask for a visualization of the results (Attia et al. 2013).
The main objective of this paper is the implementation of a quasi-steady state method based on DIN V 18599 in GH in order to enable parametrically controlled energy balancing in real time. The algorithms are programmed directly in GH, obviating the need to export information. The results can be displayed on a second screen parallel to the design screen, providing an instant visualization of the effects of changes to the design on the energy demand. In this way, the results can be directly used as design decision support. Furthermore, the parametric approach provides a basis for time efficiently optimizing the building design. The geometry can either be defined in GH or be drawn conventionally in Rhino for planners unfamiliar with parametric software.
2 Implementation of a quasi-steady state method in parametric software
Structure of the parametric tool
2.1 Input
Layers with colour codes for defining boundary conditions. *F x-value depends on geometric characteristics, see Sect. 2.2
Example of parametric material definition in GH
The climate of the site is defined by choosing one of the 15 different climatic regions in Germany as given in DIN V 18599-10. Data for a test reference year of 2010 are provided for each of these regions. Choosing the climatic region loads all the climate data, such as average temperatures or solar irradiation, into GH. The predefined climate is Potsdam, which is the reference climate according to EnEV.
The tool has currently only been developed for residential buildings. Depending on whether the building to be analysed is a single or a multi-family house, different user data are loaded from DIN V 18599-10 into GH. User data consist of heating set points, operating hours and internal loads, amongst others.
2.2 Calculation
In its current form, the tool implements the calculation of heat demand, which is most relevant for residential buildings in moderate climates. The calculation of cooling demand will follow in a later version. In the long term, a simplified approach for accounting HVAC systems could also be integrated.
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No mechanical air conditioning (ventilation, cooling, humidification)
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Single-zone model
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No cooling demand
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Global consideration of thermal heat bridges (ΔU WB = 0.05 W/m2 K)
2.3 Output
In order to provide the planner with insight into the calculation, partial results for sources and sinks are output for each module. This makes it possible to identify potential for improving the energy efficiency. Measures to increase energy efficiency can have a different impact depending on the type of building, the geometry and specific boundary conditions. Understanding the impact of individual measures, e.g. increasing the window area, on the total energy demand helps the planner to optimize the building design.
Screenshot of Rhino viewports for parallel modelling and result feedback
2.4 Verification
The application of the developed tool was verified using three reference buildings, all residential buildings of different types and sizes: a detached single family house (SFH), a multi-family house (MFH) and an apartment block (AB). A study by the Institut Wohnen und Umwelt (Loga et al. 2015) shows that those three categories correspond to the whole range of residential buildings in Germany. Furthermore, their geometric characteristics cover the range of typical building forms.
The tool’s calculation was additionally verified using two commercial software products based on DIN V 18599: “ZUB Helena® EnEV 2014” V 7.31 by ZUB Systems GmbH (S1) and “EnEV-Wärme&Dampf” V 15.38 by ROWA Soft GmbH (S2). Both software applications have been certified by a joint quality association “18599-Gütegemeinschaft e.V.” (Oschatz 2015).
Input parameters and boundary conditions
| Building component | U-value [W/(m2 K)] |
|---|---|
| Exterior wall, floor to exterior | 0.28 |
| Roof, ceiling to unheated roof | 0.20 |
| Cellar wall to unheated cellar, floor to unheated cellar | 0.35 |
| Window | 1.30 |
| Door | 1.80 |
| Boundary conditions | |
| Construction type | Medium heavy |
| C wirk | 90.00 Wh/(m2 K) |
| Thermal bridges (ΔU WB) | 0.05 W/(m2 K) |
| Minimum external air exchange (use) | 0.5 h−1 |
| Infiltration und windows (Cat. I/Blower door test) | 0.6 h−1 |
| Heating set point | 20 °C |
| Daily operational hours (t NA) | 17 h |
| Temperature setback (at night) (Δθi,NA) | 4 K |
| Internal loads q i | SFH: 45 Wh/(m2 d); MFH: 90 Wh/(m2 d) |
| Climate | Potsdam, Germany |
2.5 Modelling the reference buildings
Thermal model of reference buildings
Characteristic values of reference buildings
| SFH | MFH | AB | |
|---|---|---|---|
| Storey height | 2.70 m | 3.10 m | 2.77 m |
| Gross volume | 368.29 m3 | 3528.62 m3 | 15974.36 m3 |
| Net volume | 279.91 m3 | 2822.90 m3 | 12,779.49 m3 |
| Useable floor space AN | 117.86 m2 | 997.12 m2 | 5111.8 m2 |
| Envelope A | 310.97 m2 | 1446.14 m2 | 6331.43 m2 |
| A/V | 0.84 | 0.41 | 0.40 |
| Wall to exterior | 2649.13 m2 | 727.33 m2 | 102.28 m2 |
| Window (wall) | 1003.28 m2 | 206.68 m2 | 26.81 m2 |
| Roof | 1339.51 m2 | 226.50 m2 | 58.07 m2 |
| Ceiling to unheated roof | 0.00 m2 | 0.00 m2 | 41.99 m2 |
| Floor to exterior | 0.00 m2 | 20.67 m2 | 0.00 m2 |
| Cellar wall/floor to unheated cellar | 79.56 m2 | 182.03 m2 | 1339.51 m2 |
| F X (cellar wall/Floor to unheated cellar) | 0.7 | 0.65 | 0.65 |
| Floor to ground | 23.08 m2 | ||
| F X (floor to ground) | 0.45 |
3 Comparison of results
Comparison of annual heating demand
| SFH | MFH | AB | |
|---|---|---|---|
| Heating demand | |||
| S1 | 9110 kWh | 53,844 kWh | 235,186 kWh |
| S2 | 9481 kWh | 54,391 kWh | 235,000 kWh |
| T | 9545 kWh | 54,673 kWh | 238,849 kWh |
| Deviation | |||
| S1 | 100.0% | 100.0% | 100.0% |
| S2 | 104.1% | 101.0% | 99.9% |
| T | 104.8% | 101.5% | 101.6% |
Comparison of sinks and sources (%)
The high difference between S1 and S2 observed in the solar gains from windows (Qsol,trans) for the SFH can most likely be attributed to an input error, especially as this deviation does not appear for the other two building types. Nevertheless, it was impossible to find the error, despite carefully checking the input values. For the SFH and MFH, there are slight differences of solar gains through opaque components (Qsource,op). These gains only have a minimal effect on the total balance and are therefore negligible.
A significant deviation between the tool and S1 can be observed in QT and QV. The deviation is highest for the SFH. As a result, the deviation in Qh,b with 4.8% is also greatest for the SFH.
According to DIN V 18599-10, a factor for partly heated floor areas (atb) is defined, which equals 0.25 for single family houses and 0.15 for multi-family houses. atb is considered when calculating the indoor temperature for balancing single-zone models. This factor has not yet been integrated in the tool as in future multi-zone models seem to be the more effective model.
Comparison of annual heating demand excluding the factor for partly heated floor areas
| SFH (%) | MFH (%) | AB (%) | |
|---|---|---|---|
| Deviation | |||
| S1 | 100.0 | 100.0 | 100.0 |
| S1 B | 105.0 | 101.7 | 101.6 |
| T | 104.8 | 101.5 | 101.6 |
3.1 Sensitivity of F X-values
Deviation of results using simplified and exact F X-values
| Reference building | Annual heating demand with detailed F X-values (kWh/a) | Annual heating demand with simplified F X-values (kWh/a) | Deviation (%) |
|---|---|---|---|
| SFH | 9544 | 9544 | 0.00 |
| MFH | 54,673 | 55,402 | 1.33 |
| AB | 238,848 | 240,857 | 0.84 |
3.2 Example of application
To demonstrate the advantages of parametric input for the design of energy efficient buildings in the early design stages, variants of a MFH were generated and analysed using the tool. A five-storey building with a rectangular floor plan served as basis. A basement was not incorporated, and the U-values from Table 1 were used. The percentage of window area for the exterior walls was set to 20% for all façades.
Parametric variants of MFH
In the second step, the number of storeys was changed for the variant with a square floor plan. Three examples (B1–B3) with the same GFA are shown in Fig. 9. The minimum heating demand is achieved by variant B2 with three storeys. Although the ratio of A/V is bigger than in A2, the heating demand is slightly smaller.
4 Conclusion
The implementation of a quasi-steady state monthly balancing method in a parametric design software presented in this paper makes it possible to calculate a building’s energy demand in real time. A comparison of the developed tool’s results for three residential buildings with the results produced by an accredited commercial software product showed only small differences. As this new tool is conceived for the early design stages, this degree of differences is negligible.
The main advantage in comparison with commercial quasi-steady state tools is the parametric input of geometry, materials and boundary conditions. This allows the planner to easily change parameters and generate new variants. The primary advantage of the developed tool over other plug-ins for parametric design-based dynamic building performance simulation is the much improved computation time. While changing the design, the planner receives continuous feedback on the energy performance in real time. This provides the necessary basis for optimization processes, performed either manually by the planner or using computational optimizers, e.g. genetic algorithms.
The example application shows the great potential of being able to reduce energy demand by quickly and intuitively optimizing the base form. In this example, only one parameter was changed, but the approach can be applied analogously for the window layout or material selection. This demonstrates the great advantage of parametric control. In the future, additional case studies should be carried out to further validate the applicability.
The tool currently only calculates the heating demand of residential buildings. In future developments, further calculations, e.g. cooling demand, can easily be incorporated. The implementation of different thermal zones within a building is a further area of improvement that would enable the application of this method for non-residential buildings according to DIN V 18599. Currently, the tool is designed for application within Germany. In the future, international standards such as ISO 13790 (ISO 2008) or further national standards can be implemented to allow for a wider application.
Notes
Acknowledgements
This study was carried out as part of the research project ‘Integrated Life Cycle Optimization’ funded by the German Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety through the research initiative ZukunftBau.
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