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Abstract

The energy performance directive mandates that all new buildings in Europe shall be "nearly zero energy" buildings. New, innovative solutions have arisen both on the demand and on the supply side of the market that aim at much more ambitious energy and environmental performance levels than nearly zero at an affordable price. The basic principle of a carbon neutral building is that the CO 2 emission shall not be positive over the whole life cycle. In our definition, during its life time the CO 2 emission avoided due to the energy produced by photovoltaics should exceed the operational emissions and cover the CO 2 emissions caused by the production and maintenance of the building. The paper presents the latest scientific results of an innovative, prefabricated compact house system. We define carbon neutrality and the system boundaries of our research after an overview of the range of approaches presented in the international literature. We analysed one geometrical design with several construction options using dynamic energy simulation, life cycle assessment (LCA) and life cycle costing (LCC). First, a Building Information Model (BIM) of the building was created in Archicad to quantify the bill of materials. Energy simulation was performed with DesignBuilder for the climate of Hungary. LCA was carried out with a self-developed tool based on the Ökobaudat and ecoinvent databases. LCC was also performed in a calculator based on the methodology defined in 244/2012/EU. The results show that it is possible to realise a compact home that is carbon neutral for the whole life cycle and the use of renewable materials is favourable due to biogenic carbon sequestration.
1
A CARBON NEUTRAL COMPACT HOUSE CONCEPT SUPPORTED WITH LCA, LCC
AND DYNAMIC ENERGY SIMULATION
Péter Medgyasszay
1, 2
Dóra Szagri
2
Ádám Bihari
3
Benedek Kiss
2
Balázs Nagy
2
Miklós Horváth
4
Jose Dinis Silvestre
5
Zsuzsa Szalay
2
1
Belső Udvar Architect Research and Expert Office | Hungary
2
Climate Change and Building Energy Research Group, Department of Construction Materials and Technologies,
Budapest University of Technology and Economics | Hungary
3
Naturarch Contruct Ltd | Hungary
4
Department of Building Services and Process Engineering, Budapest University of Technology and Economics |
Hungary
5
CERIS, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1,1049-001, Lisboa, Portugal
Corresponding author: belsoudvar@belsoudvar.hu
Keywords
Net Zero Carbon Emission, dynamic energy simulation, LCA, LCC, compact house
Abstract
The energy performance directive mandates that all new buildings in Europe shall be „nearly zero energy” buildings. New,
innovative solutions have arisen both on the demand and on the supply side of the market that aim at much more ambitious
energy and environmental performance levels than nearly zero at an affordable price. The basic principle of a carbon neutral
building is that the CO
2
emission shall not be positive over the whole life cycle. In our definition, during its life time the CO
2
emission avoided due to the energy produced by photovoltaics should exceed the operational emissions and cover the CO
2
emissions caused by the production and maintenance of the building.
The paper presents the latest scientific results of an innovative, prefabricated compact house system. We define carbon neutrality
and the system boundaries of our research after an overview of the range of approaches presented in the international literature.
We analysed one geometrical design with several construction options using dynamic energy simulation, life cycle assessment
(LCA) and life cycle costing (LCC). First, a Building Information Model (BIM) of the building was created in Archicad to quantify the
bill of materials. Energy simulation was performed with DesignBuilder for the climate of Hungary. LCA was carried out with a self-
developed tool based on the Ökobaudat and ecoinvent databases. LCC was also performed in a calculator based on the
methodology defined in 244/2012/EU.
The results show that it is possible to realise a compact home that is carbon neutral for the whole life cycle and the use of
renewable materials is favourable due to biogenic carbon sequestration.
P. Medgyasszay et al
2
1.1 INTRODUCTION
The rapid change in the Earth's climate on a global scale is one of the greatest challenges facing humanity today. International
conventions have been established to slow down this process, which threatens the very existence of humanity [1]. In these
conventions, the reduction of greenhouse gas emissions, including CO
2
, has been defined as an objective. The construction and
operation of buildings is a major emitter and has significant potential for emission reductions. According to the calculation of the
International Energy Agency (IEA), the buildings and construction are still responsible for 36% of global final energy use and 39%
of energy-related carbon dioxide emissions [2].
Countries around the world have made significant efforts and achievements in the last 20 years to reduce the energy demand of
operating buildings. There have also been significant developments in customer demand and the availability of affordable
technologies, driven by legislative requirements. In addition to opportunities to improve energy efficiency, investment
opportunities to enable local energy production have also emerged. Operational energy demand has been decreasing, and the
embodied energy associated with the construction of buildings has increased in proportion. These developments have led to the
emergence of new targets and concepts focusing directly on reducing CO
2
emissions. There is no standard for low CO
2
buildings,
but several concepts exist.
The prefix "climate/CO
2
/GHG/carbon" instead of "energy" is used in the designations to express that the concept is linked to
greenhouse gas (GHG) emissions instead of energy consumption. The concepts use the terms "nearly zero/zero/neutral" to
express the degree of climate neutrality that is to be achieved. The terminology "net/absolute", often defined only in brackets,
refers to whether a periodic-based - typically annual - accounting (net) or real zero emission (absolute) is the requirement [3, 4].
In the case of a "net" balance, some emissions are associated with the process (e.g. construction or operation), but there are
offsets over the lifetime of the process that result in zero emissions by the end of the period. Offsets are classified in the literature
into three categories: 1) Net balance approach (e.g. electricity generated from solar energy) 2) Economic compensation, (e.g.
climate trade) 3) Technical reduction (e.g. CO
2
removal from the atmosphere) [3, 4].
An essential feature is the terminology "balance", which refers to the whole life cycle approach. During operation, the boundary
of the operational energy and the environmental load involved in the study can be drawn in several places, for example, energy
use regulated or unregulated by the energy performance regulations, only building-related or also user-related energy use or
even mobility can be considered. It also can be different where the boundary of the building system is drawn. Building structures
are almost always part of the calculation, but mechanical installations, lighting, user-related, small domestic appliances and
furniture are not necessarily part of the analysis. When calculating the environmental balance, it is important to consider what
emissions are associated with the process (energy use, material production) and the assumed calculation period.
In this paper, the process and the first steps of a market-driven product development and the experiences of the preliminary
study are presented, answering the question within which system boundaries a (net) zero GHG emission balance building can be
constructed with prefabricated building elements.
1.2 METHOD
The aim of the research project was the complex optimisation of a new product of a manufacturing company, from the product
idea to the finished product. The new product has approx. 90 m
2
built-up area, designed with three rooms. The building has to
meet two main criteria: (1) it shall be realised as a compact house, with factory prefabrication and fast on-site installation; (2) it
shall achieve carbon neutrality, which means that the building has to offset enough CO
2
during its operation period to cover the
CO
2
emissions during its installation.
In the applied research initiated by the manufacturing company based on market needs, we followed the process shown in Figure
1. The definition of the main objectives was followed by a brainstorming with the participation of the manufacturer, the architects
and the building experts to determine the first variants for the analysis. A Building Information Model (BIM) of the building was
created in Archicad to quantify the bill of materials. Energy demand was calculated with a dynamic energy simulation tool, which
was the input for the environmental and economic assessment.
The building variants examined in the preliminary study are shown in Table 1. All versions had the same external dimensions for
transportability (Figure 2).
P. Medgyasszay et al
3
Figure 1. Flow chart of first steps of the research project
Table 1. Description of the examined versions
Code of investigated variant Description of the variants
BU-OPT-01 Based on the research team's previous project experience, assumed to be the best
energy- and GHG emission version
CC-V01 The architectural form and use of materials preferred by the client
BU-COST-OPT-01 Based on the research team's previous project experience, assumed to be the most cost
optimal version
Figure 2. Analysed geometry a) SketchUp model b) Dynamic simulation model
The building structures of the variants are presented in Table 2. All versions have a cross laminated timber (CLT) structure. The
main difference is in the type and thickness of insulation materials and claddings. Several technical systems for the operation of
the buildings have been investigated. Due to the nature of the project, air-to-water heat pumps for heating-cooling-hot water
production, building-mounted photovoltaic (PV) systems for local energy production, and natural ventilation with slot ventilation
were designed.
In the study, we take into account all external and internal building structures and the building services system. We do not
consider built-in furniture, sanitary ware or other mobile objects, the choice of which is the decision of the homeowner. The
duration of the Reference Study Period (RSP) was 30 years, during which no major renovation was assumed, but the necessary
replacements were taken into account. The lifespan of a building and certain structures may be longer than 30 years, but only the
first 30 years are considered in this study. During the use phase, heating, cooling, hot water production, lighting and the
consumption of appliances was calculated.
P. Medgyasszay et al
4
Table 2. Materials in the examined buildings
BU-OPT-01 CC-V01 BU-COST-OPT-01
Foundation ground screws ground screws ground screws
External floor CC_Floor_CLTF1_16cmSteico
- 12 cm CLT
- foil: Rothoblaas BARRIER ALU
200
-16 cm Steico Protect
- foil: Rothoblaas TRASPIR 115
- 0,9 cm laminated board
- modified bituminous sheet
CC_Floor_CLTF1_18cmSteico
- 12 cm CLT
- foil: Rothoblaas BARRIER ALU
200
-18 cm Steico Protect
- foil: Rothoblaas TRASPIR 115
- 2,1 cm laminated board
- modified bituminous sheet
CC_Floor_CLTF3_16cmEPS
- 12 cm CLT
- foil: Rothoblaas BARRIER ALU
200
- 16 cm Graphite EPS Bachl
- foil: Rothoblaas TRASPIR 115
- 2,1 cm laminated board
- modified bituminous sheet
External wall CLTW1_14Steico
- 10 cm CLT
- 14 cm Steico Protect L
- 0,5 cm thin-layer finish render
CC_Wall_CLTVW1-
T_16cmSteico
- 10 cm CLT
- 16 cm Steico Protect
- foil: Rothoblaas TRASPIR 115
- 3 cm airgap
- 2,5 cm Thermowood
CC_Wall_CLTVW3_16cmEPS
- 10 cm CLT
- 16 cm Graphite EPS Bachl
- 0,5 cm thin-layer finish render
Partition wall CC_Wall_CLT10
- 10 cm CLT
CC_Wall_CLT10
- 10 cm CLT
CC_Wall_CLT10
- 10 cm CLT
Flat roof CC_Roof_CLTR2_16cmSteico
- 12 cm CLT
- foil: Rothoblaas BARRIER ALU
200
- 16 cm Steico Protect L
- foil: Rothoblaas TRASPIR 115
- 0,15 cm BAUDER Thermoplan
membrane
CC_Roof_CLTR2_21cmSteico
- 12 cm CLT
- foil: Rothoblaas BARRIER ALU
200
- 21 cm Steico Protect L
- 0,9 cm laminated board
- foil: Rothoblaas TRASPIR 115
- 2 layers BAUDER Thermoplan
membrane
- Sedum green roof
CC_Roof_CLTR6_20cmEPS
- 12 cm CLT
- foil: Rothoblaas BARRIER ALU
200
- 20 cm Grafit EPS Bachl
- foil: Rothoblaas TRASPIR 115
- 0,15 cm BAUDER Thermoplan
membrane
Pitched roof CC_Roof_CLTR1_16cmSteico
(pitched)
- 12 cm CLT
- foil: Rothoblaas BARRIER ALU
200
- 16 cm Steico Protect L
- foil: Rothoblaas TRASPIR 115
- 3,9 cm airgap / LVL
- 2,5 cm planking
- TRASPIR 3D coat
- 0,05 cm Lindab SRP Click
CC_Roof_CLTR1_20cmSteico
(pitched)
- 12 cm CLT
- foil: Rothoblaas BARRIER ALU
200
- 20 cm Steico Protect L
- foil: Rothoblaas TRASPIR 115
- 3+3 cm airgap / LVL
- 2,5 cm planking
- TRASPIR 3D coat
- 0,05 cm Lindab SRP Click
CC_Roof_CLTR1_20cmEPS
(pitched)
- 12 cm CLT
- foil: Rothoblaas BARRIER ALU
200
- 20 cm Graphite EPS Bachl
- foil: Rothoblaas TRASPIR 115
- 3+3 cm airgap / LVL
- 2,5 cm planking
- TRASPIR 3D coat
- 0,05 cm Lindab SRP Click
Windows Wood, Triple Glazed Wood, Triple Glazed Wood, Triple Glazed
External doors Wood Wood Wood
Internal doors Wood Wood Wood
Shading outside, Blind with high
reflective slats
outside, Blind with high
reflective slats
outside, Blind with high
reflective slats
Dynamic simulation
Dynamic simulations were carried out to define the building’s performance. We selected to use DesignBuilder 6 software with
the CTF method, which is a sensible heat only solution and does not take into account the moisture storage or diffusion in the
elements. During the modelling, the thermal zones were set according to the plan of the building, and each room represents one
thermal zone (Figure 2b). We used a generated weather file for the weather conditions (Hungary, near Lake Balaton). The
structural layers were assigned according to Table 2; the windows have a 3-layer glass structure with Argon filling and a Low-E
layer with painted wooden frames; external shading was also part of the setup. The technical building system was modelled in a
simplified way within the software, equipped with heating, cooling and natural ventilation. The simulations provided sufficient
information on the performance of the building and helped us compare and optimise the models and structural variations.
P. Medgyasszay et al
5
Life Cycle Assessment (LCA)
The environmental impact of the building was assessed with the standardised methodology of Life Cycle Assessment (LCA),
according to ISO 14040 [5] and EN 15804 [6]. The whole life cycle was considered from material manufacturing (A1-A3),
installation (A4-A5), through replacements (B4) and operational energy use (B6) during the use phase to the end-of-life (C2-C4).
Other stages were neglected due to the lack of data or because they were not relevant. For timber products, a -1/+1 approach
was adopted, which means that carbon sequestration is included in module A, but the carbon is released at the end-of-life in
module C. Recycling/reuse potential was not considered, but the exported energy from the photovoltaic systems was included in
module D. The exported energy avoids the impacts of grid electricity and this was assumed to offset the emissions from the
production and end-of-life stage, hence achieving a net zero carbon building. The necessary size of the PV panel was determined
with an iterative process, also considering the manufacturing impact of the panels. Life cycle impact assessment data were taken
from the German Ökobaudat and the ecoinvent database due to the limited availability of Environmental Product Declarations in
Hungary. As most of the applied materials are imported from abroad, using international data is assumed not to compromise the
accuracy of the evaluation. Several impact categories were considered, but here only the results of the Climate change - Global
Warming Potential (GWP) are presented, as this is the relevant indicator for assessing carbon neutrality.
Life Cycle Costing (LCC)
In the course of the research, a full life cycle cost analysis (LCC) was also carried out to compare the different variants. The most
frequently used method of LCC calculation is defined by the European Commission Regulation (EU) No. 244/2012 on the definition
of cost-effective buildings and the
Guidelines and methodological annex of this regulation [7]. The essence of the method is to
apply the approach to the calculation period in order to find the best building structural - building engineering solution from the
economic point of view. We are looking for the solution that is neither the cheapest investment nor the cheapest operation but
which requires the least cost during the examined life cycle. The total cost over the entire life cycle is called the
global cost”.
The material cost of the materials, construction products and structures used and the related labour costs calculated on the basis
of standard time came from the following sources:
- TERC Etalon online cost estimation database based on the Consolidated Construction Standards System (YOUR data
warehouse) 2020 data;
- manufacturer prices, supplier prices.
The values are values per m
2
, net amounts, in Euro, and based on the Hungarian market prices.
1.3 RESULTS
Dynamic simulation
Figure 3 shows the total energy consumption of the three models. Overall, the differences between lighting, domestic hot water
and “other” energy consumption from appliances are due to the minor differences in the size of the three-dimensional model.
During the modelling, we created the buildings based on the external dimension, keeping in mind the transportability of the
elements. It means that if thinner structural layers are chosen, the heated floor area of the building increases accordingly to a
minimal extent and decreases with thicker structures. The energy consumption for space cooling and heating is more variable:
the BU-OPT-01 model resulted in the highest heating energy consumption, where the thermal insulation on the wall (14 cm of
wood fibre) and in the roof structure (16 cm of wood fibre) had the smallest thickness. The BU-COST-OPT-01 structure had the
lowest heating energy consumption, where increased thermal insulation was used with 16 cm of Graphite EPS on the wall and 20
cm for the roof. Almost identical results were obtained for the cooling; the mixed operation mode (natural ventilation and
machine cooling) did not result in significant differences in the cooling energy consumptions. These results were a good starting
point for the further LCA and LCC analysis of the building to determine the optimal structure.
P. Medgyasszay et al
6
Figure 3. Annual energy consumption of the models according to the dynamic energy simulation
Life Cycle Assessment (LCA)
The Global Warming Potential (GWP) of the three variants for the whole life cycle is shown in Figure 4. In every design, the GWP
is negative in the product stage due to the large quantity of cross laminated timber with high carbon storage. CC-V01 has the
highest carbon storage because in this version even the insulations are thick bio-based materials, while BU-COST-OPT-01 has the
lowest due to the use of conventional insulations. The carbon is assumed to be released at the end-of-life stage (C). The effect of
the construction process (A4-A5) and replacement (B5) is negligible because only the exchange of the heat pump and
waterproofing is necessary during the 30-year calculation period. The operational energy use (B6) is zero, as in a yearly balance
the energy generated by the photovoltaic panels exceeds the energy need for heating, cooling, etc. The rest of the PV energy is
exported to the grid, which is represented by the negative values in module D. Every variant has 6.2 kWp PV panels installed,
which is nearly sufficient to achieve a net zero balance. Summing up all the life cycle phases including the exported energy, the
highest GWP belongs to the BU-OPT-01 and the lowest to CC-V01. A zero net balance could be achieved by increasing the size of
the PV panel to 6.8, 6.4 and 6.7 kWp in the three variants, respectively.
Figure 4. Global Warming Potential of the three variants for the whole life cycle a) in the life cycle stages b) total
1702
687
2661
2064 2133
1233
741
2553
1994 2047
1098
714
2621
2040 2101
0
500
1000
1500
2000
2500
3000
3500
heating cooling dhw lighting other
kWh
BU-OPT-01 CC-V01 BU-COST-OPT-01
-60000
-40000
-20000
0
20000
40000
60000
80000
BU-OPT-01 CC-V01 BU-COST-OPT-01
GWP (kg CO2-eq)
A1-A3: Product A4-5: Constr. process
B1-B5: Use B6: Operational energy
C1-4: End of life D: Exported energy
0
5000
10000
15000
20000
25000
30000
35000
BU-OPT-01 CC-V01 BU-COST-OPT-01
GWP (kg CO2-eq)
Total A + B + C + D
P. Medgyasszay et al
7
Life Cycle Costing (LCC)
The calculation of the cost of the different variants for each structure and the entire buildings was carried out. During the study,
a standardized cost line was obtained for the main structural and technical elements: walls, flat roof, pitched roof, raised floor,
facade glazed windows, external and internal doors, heating system, lighting and solar panels.
The significant difference is in the investment cost (Figure 5). As expected, the BU-COST-OPT-01 model became the most cost-
effective due to the significantly lower price of polystyrene insulation. The difference in operating costs is mainly due to heating
costs, but it is not significant compared to global costs. Revenue from the sale of surplus energy from the solar system was taken
into account in operating cost as a negative cost. The residual value and the renovation cost can be considered almost identical
for the three variants.
Figure 5. Life Cycle Cost results of the three variants
1.4 CONCLUSIONS
This paper investigated the possibilities for achieving a net zero greenhouse gas emission compact house. A net balance approach
has been adopted where the emissions from the manufacturing, construction and end-of-life processes are offset by the avoided
grid impact from the exported photovoltaic energy. The net zero balance was achieved by a very high energy efficiency, the
application of bio-based materials and 6-7 kWp of photovoltaic panels for a house with a floor area of around 90 m
2
. From the
analysed three building variants, the version with bio-based insulation achieved the lowest environmental impact but had the
highest investment and life cycle cost. In this study, a relatively short calculation period of 30 years was considered without any
P. Medgyasszay et al
8
major renovations. The building life time would normally far exceed this period, which means that in reality the building would
remain carbon negative until the end-of-life when CO
2
would be released.
Acknowledgements
MooB - Multi-objective life cycle optimisation of sustainable and innovative construction materials and buildings (Project 2018-
2.1.15-TÉT-PT-2018-00005) has been implemented with the support provided from the National Research, Development and
Innovation Fund of Hungary and from FCT, Fundação para a Ciência e Tecnologia, Portugal. The research was also supported by
Project FK 128663 implemented with the support provided from the National Research, Development and Innovation Fund of
Hungary, financed under the FK_18 funding scheme.
References
[1] United Nation: Paris Agreement. UN Climate Change Conference (COP21)
[2] UN Environment and International Energy Agency (2017): Towards a zero-emission, efficient, and resilient buildings and
construction sector. Global Status Report 2017.
[3] Satola, D., Balouktsi, M., Lützkendorf, T., Wiberg, A. H., & Gustavsen, A. (2021). How to define (net) zero greenhouse
gas emissions buildings: The results of an international survey as part of IEA EBC annex 72. Building and Environment, 192 (October
2020), 107619. 1-17.
[4] Lützkendorf, T., & Frischknecht, R. (2020). (Net-) zero-emission buildings: a typology of terms and definitions. Buildings
and Cities, 1(1), 662–675.
[5] ISO 14040, Environmental management — Life cycle assessment — Principles and framework, (2006).
[6] EN 15978, Sustainability of construction works - Assessment of environmental performance of buildings - Calculation
method, (2011).
[7] EU, Commission Delegated Regulation (EU) No 244/2012, supplementing Directive 2010/31/EU of the European
Parliament and of the Council on the energy performance of buildings by establishing a comparative methodology framework
for calculating cost-optimal levels of minimum energy performance requirements for buildings and building elements. Official
Journal of European Union. (2012). 21.3.2012
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Full-text available
Article
The concept of (net) zero greenhouse gas (GHG) emission(s) buildings is gaining wide international attention and is considered to be the main pathway for achieving climate neutrality targets in the built environment. However, there is an increasing plethora of differing terms, definitions, and approaches emerging worldwide. To understand the current progress of the ongoing discussion, this study provides an overview of terms, definitions, and key features from a review of 35 building assessment approaches. The investigation identified that 13 voluntary frameworks from 11 countries are particularly characterised by net zero-carbon/GHG emissions performance targets, which are then subject to a more detailed analysis. The review was organised in the context of the project IEA EBC Annex 72 on “Assessing Life Cycle Related Environmental Impacts Caused by Buildings”, which involves researchers from over 25 countries worldwide. In the current dynamic political surroundings and ongoing scientific debate, only an initial overview of this topic can be presented. However, providing typologies and fostering transparency would be instrumental in delivering clarity, limiting misunderstanding, and avoiding potential greenwashing. To this end, this article categorises the most critical methodological options—i.e., system boundaries for both operational and embodied GHG emissions, the type of GHG emission factor for electricity use, the approach to the “time” aspect, and the possibilities of GHG emission compensation—into a comprehensive framework for clarifying or setting (net) zero GHG emission building definitions in a more systematic way. The article concludes that although variations in the existing approaches will continue to exist, certain minimum directions should be considered for the future development of harmonised (net) zero GHG emissions building frameworks. As a minimum, it is recommended to extend the usual scope of the operational energy use balance. At the same time, minimum requirements must also be set for embodied GHG emissions even if they are not considered in the carbon/GHG emissions balance.