The life cycle assessment answers this question by providing a comprehensive analysis of all phases in the life cycle.

Electricity generation

UOne approach is to assess average impacts per kWh of electricity generated by different production lines. The results can be used to compare renewable lines (hydro-power, wind, photovoltaic and biogas) to fossil and fissile generation (coal, gas, nuclear). The figure below, taken from the ecoinvent 3.4 database, shows different indicators in relative value compared to the maximum value (this value includes other production lines such as fuel oil which are not represented on the chart for greater clarity). For example, coal-fired power plants have the most greenhouse gas emissions. Emissions from gas-fired plants are two times lower, and are reduced by a factor of more than 20 with nuclear and renewable generation. The primary energy indicator represents the total for renewable and non-renewable lines.

Coal is the production line with the greatest impact on climate, and also on human health, eutrophication and waste. It also has a high level of impact on primary energy consumption and water, and on biodiversity. Biogas requires the most land and primary energy, while its impacts on health and biodiversity are high. Nuclear power generates the most radioactive waste and uses the most water, while the indicators for primary energy and to a certain extent resources are high. Photovoltaic energy consumes the most resources, though that being said silicon is an abundant material (Saint-Aubin, 2019) and the ecoinvent data does not yet incorporate recycling to any great extent. Some wind turbines use rare earth magnets, but according to the French Energy Management Agency (ADEME) (2020) potential high pressure on rare-earth elements does not appear to compromise the development of wind power, particularly due to alternative technologies for electric generators (asynchronous generators or synchronous generators without a permanent magnet). The photovoltaic and wind lines also consume more metals per kWh but are currently making great efforts to achieve a large share of end-of-life recycling: 90% of the mass of wind turbines is re-usable or recyclable, and this share is set to rise to 95% from 2024. The recycling rate for photovoltaic modules is 94.7% according to PV Cycle, which collects panels at no cost to the owner. The use of energy and materials needed for the energy transition was studied by Hertwich (2014), who demonstrated that the large-scale roll-out of photovoltaic, wind and concentrated solar lines reduces the environmental impacts of electricity generation.

The major growth in the renewable energy sector has also led to a significant cost reduction, due to scale effects and industrial maturity, and correlatively environmental impacts, cf. the figure below from Besseau (2019) for photovoltaic energy and Sacchi et al. (2019) for wind power.

The values given above are averages, but when studying an urban project, it is more precise to calculate production on the basis of local climate data and to consider the life cycle of the fittings (production, renewal and end-of-life) according to the way they are integrated into the building and the local situation. An hour-based dynamic life cycle assessment can also be used to analyse the reduction of environmental impacts over time, according to the season, the day of the week and the time of day (Roux et al., 2016 et 2017). Some renewable energy sources are intermittent, but the great reliability of weather forecasts allows for proper management of the electronic system. 

The example below uses a positive-energy house in the climate of the Île de France (Greater Paris) region (COMEPOS, 2019). A 30 m² surface area of photovoltaic modules on the roof is necessary for production to balance consumption over a year. In order to assess the interest of this system, a variant without photovoltaic modules was considered in comparison, using modules produced in France or in China. The integration of photovoltaic modules reduces primary energy consumption and the quantity of radioactive waste generated, without bringing about a significant increase of the other environmental indicators. The impacts are slightly greater for modules produced in China, which is related to the electricity generation required for the production of the modules and shipping.

Renewable energy generation preserves energy resources and avoids the production of radioactive waste which must be stored for tens of thousands of years. The integration of photovoltaic modules in a building is therefore in line with the principle of sustainable development.

Part of the electricity generated by the photovoltaic system is exported to the grid. Should it be stored in power packs to be consumed in the building itself? Power packs give rise to additional environmental impacts and are not necessary, with the exception of a few specific cases (a remote site for example): excess production can generally be consumed in neighbouring buildings (in this case it is known as collective self-consumption) or purchased by an aggregator as national consumption is currently always greater than renewable generation. Pooled and optimised storage on the scale of the grid (e.g. gas production using electricity) is preferable to many power packs on an individual level.

According to the calculation planned for the RE2020 environmental regulation, the exported energy is recorded with a reduced primary energy coefficient for photovoltaic generation above 10 kWhef/m², which does not correspond to physical exchanges. An electricity system model designed by Roux et al. (2016 et 2017)and approved on the basis of data from RTE (France’s transmission system operator) shows that over the lifespan of a PV system installed today in mainland France (exclusive of island areas that are far from the national grid), exported electricity avoids conventional production. Therefore, there is no need to modify the physical assessment by artificially reducing the equivalence in primary energy of the exported electricity. Penalising the integration of photovoltaic systems into buildings results in a paradox: to meet the objectives of the energy transition, hectares of forests are being cut down to install photovoltaic power plants on the ground, rather than leveraging available surface areas on roofs. Given this penalisation of photovoltaic systems in the regulations, some recommend the construction of buildings without photovoltaic modules, which would be subsequently installed. This results in compounded impacts of a conventional roof and of the modules; the opposite of what a correctly conducted LCA should encourage.

Some methods only include the share of modules corresponding to the percentage of the building’s self-consumption, to offset the fact that exported electricity is not accounted for. The result of these LCAs then depends on the percentage of self-consumption, which itself depends on the scale on which the assessment is conducted. This percentage is higher on the scale of a local area for which a collective self-consumption system is rolled out than on the scale of a single building. The environmental performance (e.g. greenhouse gas emissions) of a building therefore varies depending on the scale, which does not correspond to a physical reality as it is the same building and the same photovoltaic system. Considering that exported electricity avoids generation on the grid and the corresponding environmental impacts provides greater coherence as the building’s performance no longer depends on the scale under study. This methodological choice is also consistent with the calculation of a module’s energy and environmental payback time. Also, all photovoltaic modules make up the sides and the roof of a building: there is no reason to only consider some of them.

Heat production

In much the same way as photovoltaic electricity generation, a solar thermal system can be analysed through a life cycle assessment, considering its production, renewal, end-of-life and its generation of heat calculated by digital simulation. The payback time in terms of energy or CO2 is generally lower than two years (Ardente, 2005).

The use of wood for energy generation reduces greenhouse gas emissions provided that the forest is sustainably managed, i.e. without deforestation. However, wood burning emits dust and volatile organic compounds that are harmful to health. The use of fuel wood is prohibited for this reason in cities such as Paris. A set of environmental indicators should be analysed rather than basing the decision on a carbon footprint alone. “Air pollution” and “water pollution” indicators in the RE2020 regulation under-estimate the toxicity of certain chemical substances (by a factor of 50,000 for dioxins, grouped together with other, less toxic volatile organic compounds). The regulation requires manufacturers to record 168 flows (substances or groups of substances) in the INIES database, whereas the ecoinvent international database includes around 4,000, which brings about a more precise assessment of impacts on health and biodiversity.

Integration in an urban project

The integration of renewable generation options in an urban project must complement an approach focused on energy sobriety (raising user awareness of good behaviours), bioclimatic design (thermal insulation, passive solar gains) and energy efficiency (efficient fittings for heating, air conditioning, domestic hot water, lighting and ventilation).

In a dense neighbourhood, the number of floors to a building is too high for solar power generation to provide sufficient energy and to balance consumption. It is therefore possible to work with a renewable electricity supplier, and possibly a heat network that includes renewable generation or geothermal energy.

The challenge of local electricity and heat generation is similar to that of food. Urban agriculture can only provide part of food requirements, which are mostly met by a broader area. The same goes for energy requirements. For example, the city of Hamburg has entered into a partnership with the neighbouring region of Schleswig-Holstein (Hambourg, 2021) to enjoy a supply that is 100% renewable electricity by 2035.

Given the long lifespan of buildings and infrastructure, forward-looking scenarios concerning all energy sources should be integrated: development of the share of biogas and hydrogen in the gas network, development of the electricity production mix (Roux et al., 2016b), and of the generation mix in district heating networks.

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What are the market trends for photovoltaics and building-integrated photovoltaics (BIPV) and are there prospects for technological developments? 

Romain Besseau: Globally, the photovoltaic (PV) sector has enjoyed exponential growth up to now. Installed capacity is clearly on the rise year on year, but we are also seeing it occur at a faster pace each year.

In mainland France, the scenarios recently presented by RTE, France’s transmission system provider, all take into account a significant increase in photovoltaic energy generation. This ranges from 70 GWc of installed capacity by 2050, i.e. a seven-fold increase compared to 2018 in a scenario with a high renewal of nuclear power plants, to more than 200 GWc, which means installed power multiplied by 22 in a scenario without any new nuclear plants and a small-scale extension of the existing nuclear fleet. Whichever decision is made, the photovoltaic industry will have a role to play in the energy future of mainland France. This role will be even more significant in the overseas territories.

Lastly, PV energy self-consumption is enjoying development, particularly thanks to a drop in PV energy production costs. It is becoming increasingly frequent that it is more profitable to consume PV energy that can be produced locally rather than electricity from the grid. Widespread individual or collective self-consumption meets part of energy requirements. This share will grow when consumption matches PV production on the same time level, and therefore sun exposure.

Pauline Grougnet: The photovoltaic ecosystem is very dynamic, supported by increasingly stringent regulations, committed stakeholders and citizens and a wide range of applications and services.

In France, large ground-based solar power plants pull up the photovoltaic market, but practices and searches for land are changing. Indeed, the conversion of former sites that are demilitarized, industrial or polluted is also favoured. A striking example is the commissioning of a 15MWc plant in October 2020 on the former site of the “AZF” factory.

Solar power will also be rolled out on a massive scale on building roofs, encouraged by the French Climate and Resilience Law which requires that, from 1st January 2023, new buildings used for commercial, craft and industrial purposes in addition to warehouses and hangars, with a surface area greater than 500 m², and office buildings with surface areas greater than 1000 m², will have to vegetalise or collect solar power over 30% of their surface.

This law will affect the photovoltaic shade structures installed on any new car parks built from 1st January 2024 and will concern 100% of their surface.

Other means of installing major production capacity are also being targeted by the leading energy producers and suppliers (Akuo Energy, EDF, TotalEnergies, etc.), as are agrivoltaics and floating PV systems.

To a lesser degree, building-integrated photovoltaics (BIPV) [1]is emerging at various paces in European Union Member States. Switzerland., Germany and Austria have many references, but self-consumption, electric mobility of occupants and citizens and the new tariff arrangement of 8 October 2021 are set to contribute to large-scale development in France.

As regards technology, new formats of silicon cells are emerging on the market. For the time being, the final size has not been the subject of consensus within the profession, but the 158mm cell will be withdrawn in favour of 161mm, 182mm and even 210mm cells! However, given the current situation of supply difficulties, particularly for semi-conductor materials, industrial players are considering alternatives to create photovoltaic materials and cells such as organic films for instance.

Lastly, a wide range of services rounds off these developments: PPAs (Power Purchase Agreements), third-party financing, green and citizen energy cooperatives, the rental of photovoltaic modules for individuals, etc.

Photovoltaic technology raises questions regarding its carbon footprint. Which solutions have been developed by researchers and companies to ensure a positive carbon balance?

Romain Besseau: The issue of PV’s carbon footprint is important and must be studied seriously. While it’s true that once installed a PV system does not give rise to any greenhouse gas emissions (GHG), PV panels do not grow on trees. It takes energy and materials to produce the components of a PV system, and that indirectly causes GHG emissions. The estimation of the industry’s carbon footprint must take into account emissions that occur from the extraction of raw materials to the end of the system’s life. To achieve this, we use a method called the life cycle assessment (LCA). Many LCAs in the photovoltaic industry result in carbon footprint estimates ranging from 40 to 100 gCO2eq/kWh. For reference, the carbon footprint of the European electricity mix is around 400 gCO2eq/kWh.

Actually, PV carbon footprint estimates are based on old data sets, which results in an over-estimation of the industry’s carbon footprint. As part of my research work, I was able to demonstrate that the data sets considered for the LCA correspond to the PV industry’s performance in 2005. This is problematic given the progress made by the PV industry since this period when it was just starting out. I developed a model configured to take into account the improved efficiency of PV models, the reduction of the mass of components such as inverters, the performance of the silicon refining process and PV cell production process. The model shows how this brings about a two- or three-fold reduction in the PV industry’s carbon footprint. The levers to further reduce it are to continue to improve panel yield, the efficiency of crystalline silicon production processes and the use of electricity that is as close to zero-carbon as possible for the production of these cells, a process which still consumes a lot of energy, though much less than in the past.

Pauline Grougnet: We can go further in our efforts to reduce this footprint by incorporating PV systems as a functional element of a building’s envelope and not just as an addition, a complement with the sole purpose of producing energy. The photovoltaic envelope approach pools resources (i.e. integration systems) and uses surfaces and applications that were planned in the construction design, whether or not they are active!

Which focal points should be considered regarding environmental impacts other than carbon?

Romain Besseau: GHG emissions are far from being the only environmental issue to be considered. The LCA is used to study both the impact on climate change and the impacts on ecosystems, human health and resources in the broadest sense, covering land, water and minerals.

As discussed before, PV’s carbon footprint is low over its life cycle. PV will cause impacts on ecosystems and human health, particularly during the extraction of raw materials and in refineries and factories that produce PV modules.

There are many preconceived ideas about the use of rare earth magnets in PV panels. The overwhelming majority of the market is based on silicon cells and this technology does not require any rare earth elements. Some specific technologies use rare earth elements, but for extremely specific applications such as the space industry which uses multilayer panels. The PV industry does, however, use mineral resources such as copper for the production of wires and inverters, and steel and aluminium for PV module mounts. This is why these systems need to be recycled efficiently.

The PV industry may have a significant spatial footprint when installed on the ground. Yet the potential in building roofs is far from being exhausted, and this is even more the case for building façades. I believe it is important to promote the installation of PV systems on surfaces that are already urbanised. Lastly, there are considerations and experiments underway to develop joint uses between PV electricity generation and agriculture.

One aspect that is worth discussing and assessing is the impacts that are potentially caused by the weather dependency of PV generation. Storage facilities may become necessary to meet consumption demand, which adds a layer of environmental impacts.

Pauline Grougnet: Thanks to this photovoltaic envelope approach, PV also boasts positive spin-offs that are sometimes largely unknown! Naturally, the primary advantage is that by using surfaces in addition to the roof, the PV installation’s installed capacity is increased, thereby contributing to the creation of positive-energy houses, without having to resort to extra land coverage. Next, the existing solutions proposed by BIPV industry players offer a wide range of aesthetic finishes in line with the surrounding area, particularly when the building must fit aesthetic and landscape constraints of protected sites. Lastly, in semi-transparent applications, PV solutions can improve comfort for occupants and reduce energy requirements. Sensibly positioned on a curtain wall, a double skin façade or an atrium, PV cells will not only generate electricity but will act like thousands of tiny sun-breakers. Firstly, these solutions let natural light in, while controlling glare and heat transmitted, resulting in enhanced comfort for occupants. Secondly, by optimising the layout of cells, air-conditioning loads can be reduced by 20% to 25% under an atrium or glass roof.

What are the benefits of a dialogue between researchers and companies and what could come of these discussions?

Romain Besseau: I had the opportunity of working in the photovoltaic sector before moving onto research, which gave me an in-depth knowledge of the industry. I became aware that the data used to assess the industry’s carbon footprint was obsolete. Without this knowledge, I would probably not have questioned how representative these data sets are and would have used them as they were. Fostering dialogue between researchers and companies prevents such situations. Lastly, research also needs funding. It can provide expertise to companies and create value through informed decisions.
Pauline Grougnet: Researchers and companies must continue to work together. Research must be connected to the market, to practices, to the “real life” of projects and the users and consumers of studies and results. Supported by research, companies remain aware of the techniques and technologies that are being developed and in doing so can anticipate changes in their practices, methods and organisation to remain in control and competitive. If companies do not have the time or resources to do so, in particular for consulting firms like ActivSkeen, it is really in their interest to consult researchers or more broadly institutes such as the lab recherche environnement to update their databases, explore case studies and develop simulation tools. This saves time, and also puts them in a position to offer more robust technical assistance to their customers, who are often not well-versed in this field. Thanks to Romain’s work, and above all thanks to its dissemination and accessibility for non-experts, ActivSkeen has been able to update its data and methods to assess the carbon footprint of the photovoltaic systems that we scale and recommend every day. There are many developments following on from this dialogue, but broadening the scope of the environmental footprint analysis by considering integration systems and energy uses would be an interesting avenue to pursue.

[1] Definition of BIPV (Building-Integrated Photovoltaics): Replacing the conventional materials used in the building’s envelope by photovoltaic materials that serve exactly the same function as the materials replaced.

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The global food system causes massive environmental impacts, and faces the challenge of feeding an even larger, more urbanized population in the coming decades. Urban agriculture (UA) is a type of alternative agriculture, which may have environmental and social benefits, and comes in a large diversity of forms. These environmental benefits and impacts can be modeled with life cycle assessment (LCA). Application of LCA to UA is relatively recent, and has not undergone the same methodological reflections and adaptations that LCA of other sectors has.

In her thesis project, Erica Dorr pursued the following goals :

1. investigating what LCA tells us about the environmental performance of UA, and
2. investigating how best to apply LCA to UA.

She performed a review and meta-analysis of UA LCAs, and reviewed literature on the development of LCA for agriculture in general. She did LCAs of nine urban farms and gardens in Paris, France and the Bay Area, California, USA, and (with the FEW-meter project) analyzed resource use and food production at 72 UA case studies. She finally summarized and generated knowledge on the environmental performance of UA, and created a methodological framework to improve consistency and completeness in UA LCAs.

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David Damichey: Cubik-Home produces three-dimensional concrete modules off-site. Each one measures up to 10 metres in length and 4 metres in width. The modules are partially or fully prefabricated in our plant and are assembled horizontally and vertically for two-floor buildings right now. This will be expanded to three-floor buildings in the near future and will ultimately be rolled out for constructions with up to eight floors.

The advantages of this process derive from a remote technical platform, which offers a range of personalisation options and reflects the image of a solid property. We are aware that concrete is criticised for its environmental impact. We strive to turn this into a strength by working with next-generation concretes. Consumers are increasingly receptive to a reduction in products’ long-term carbon footprints, via the circular economy.

In addition, time constraints have risen in line with customers’ level of expectation. The desire to own property now comes by browsing on a tablet. Speaking of time constraints, off-site construction is an obvious way of cutting construction times as off-site processes allow for forward planning. In terms of quality, the Cubik-Home method brings about improved overall cost control and performance management thanks to more in-depth preliminary studies.

Bruno Peuportier: Mass production means that more time can be spent on studies in the design phase, compared to small-scale building projects. This is the phase in which the most impactful decisions are made regarding the building’s environmental performance. The design can therefore be optimised, and even personalised depending on what the project owner wants.

Off-site construction provides enhanced quality control thanks to the work environment and production checks. This is particularly true for features such as thermal insulation and airtightness. There is also more scope for recycling.

Pooling resources over a large number of buildings also helps in the selection of suppliers, for example for low-carbon cements, recycled steel, construction components and fittings. This selection process includes environmental criteria.

2. Which scientific and business-specific challenges are met by the collaborative project between Cubik-Home and MINES Paris Psl, launched as part of the lab’s Research & Solutions programme?

Bruno Peuportier: For our research team, it was interesting to model an innovative industrial process including inputs and outputs, and in particular water and energy consumption, as well as waste. Defining an appropriate benchmark to which the product can be compared as part of a life cycle assessment raised a few questions. Should we consider parameters that are completely identical? How should conventional materials be selected? Can a standard construction site be defined? Are all usage scenarios the same? Which assumptions should be considered for the end of the life cycle?

The most motivating aspect of this type of research is definitely having to identify avenues for improvement. We had to review the choice of materials, components and equipment, and study material-saving options, the way in which the various elements can interact to reduce consumption and impact while ensuring a high level of comfort.

David Damichey: When you are creating an innovative industrial solution, it is difficult to rely on existing life cycle assessment benchmarks. As we have always been interested in these aspects, we probably could have moved forwards ourselves if we had more time. However, to adopt a method that quickly leads to the definition of relevant levers and indicators, working with experts in this field was an obvious choice for us.

It is always possible to adapt a production line to make it more efficient, but this is inevitably more expensive than adopting good practices upstream. During a discussion with VINCI employees, I asked them to put me in touch with the MINES Paris PSL Research Centre for Energy Efficiency of Systems. Our first contacts were very surprising, as we did not speak the same language, but we all made an effort to understand each other and the fact that we took part in the webinar on 11 May is a testament to its success.

3. There are many stages in the life cycle assessment. It therefore considers impacts related to the building’s construction, use, renovation and demolition phases. Which of these phases has the most impact in terms of CO2 emissions and which actions will be rolled out to mitigate this impact?

Bruno Peuportier: As with most buildings and estimating a life span of 100 years, the main contributions to impacts are found in the building’s use phase (heating, domestic hot water, electricity, drinking water production and waste water treatment). Improving energy performance, with, for example, the use of a geothermal heat pump, reduces this contribution and gives greater importance to building materials, although the share related to these products can be cut by using lower-impact materials.

David Damichey: As Bruno said, the main contribution to the building’s footprint is in the use phase. However, our role as a committed industrial player is not to remain a spectator, but to take action on our level to mitigate the impacts of the process and the product. Let’s not forget that the most inexpensive energy and resources are those we don’t consume. A building with a more efficient and intelligent envelope has fewer energy requirements, thereby preserving resources. From the outset, we strove to reduce the thickness of the walls and slabs which are 5 and 7 cm thick respectively.

We were thinking of working with lower-carbon, bio-sourced materials, but this move came more quickly thanks to our collaborative work with the Mines research centre. To give but a few examples, we are now using Cem III cement to produce the modules. This next-generation cement has even become a replacement and has been extended to all Francioli’s production. In the future, we are also going to use bio-sourced materials for insulation and fittings.

4. Which research and development prospects are open to Cubik-Home to go even further with its environmental goals?

Bruno Peuportier: To leverage the proposed concept most effectively, the option of using foundations (stakes) to power a geothermal heat pump needs to be studied in greater depth. The aim is to develop expertise to roll out this solution on its most appropriate scale, and to assess performance more precisely. Other improvements may be explored such as the option of using bio-sourced fibres instead of steel. Ultimately, the goal is to achieve a carbon-neutral solution, without shifting the impact to health, biodiversity or resources.

David Damichey: Off-site construction addresses the question of building differently with beneficial effects for environmental objectives, such as a better use of resources and their reduced consumption in construction. This also applies to waste production, as there is less waste than on a conventional building site. The market is clearly developing and offers strong potential for meeting carbon reduction targets.

As we are currently at the stage of pre-series production, we are focusing on the product’s economic and environmental optimisation. Without wanting to disclose our ambitions, a building that remains movable over time offers many advantages in terms of the circular economy.

Off-site construction will not replace conventional construction, but it will round off the range of available solutions by developing industrial prefabrication tools.

For further information, you can watch the replay of the Research & Solutions webinar on “The environmental opportunities of off-site construction” with David Damichey and Bruno Peuportier (in French).

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Peri-urban farms with short food supply chains are currently enjoying strong growth and represent the leading type of urban agriculture for the supply of fresh produce in towns and cities. This type of farm includes horticultural areas and many new small-scale projects based on alternative systems such as bio-intensive market gardening and permaculture (CC by 2.0 - Sustainable sanitation).Urban community gardens and urban mini-farms are two other forms of urban agriculture that are currently being developed. Community gardens combine social, educational and food-related goals and are projects that attract a participation from the greatest number of urban dwellers (Les Vignoles gymnasium, Fanny Provent).Urban micro-farms can be multi-purpose or focus primarily on production (Hotel Pullman Eiffel Tower, Paris).Less common in French towns, greenhouses, indoor agriculture and low-tech and high-tech systems entail greater investment costs (Agripolis farm, Paola Mugnier).These production systems are not very attractive for urban dwellers and, as regards greenhouses, raise issues related to the landscape (an integrated greenhouse on one of the buildings of the former Anderlecht abattoirs, Paola Mugnier).

Paola Mugnier and Fanny Provent also describe multifaceted and multi-purpose production systems. Their practical guide « Urban agriculture: how to implement it on rooftops and terraces? » (from which photos #2 to #5 are taken), as well as the Exp’au urban agriculture consulting company and agriculturalization indicators, feature among the approaches rolled out, based on researcher knowledge, to assist and equip developers when selecting project content, in addition to project implementation and management. Another important aspect for developers is the issue of urban contaminations. The REFUGE methodology has been designed to conduct an assessment of health risks based on a historical survey of the site and ground investigations. This type of assessment reveals a lack of risk or identifies when a site is in a grey area, as is often the case, which may be used by rolling out a health control plan or through farming methods that do not use the ground soil.

The prospects for the development of urban agriculture are set to be extensive and varied. Many flat roofs represent untapped high-potential land resources. A number of new requirements are emerging in relation to the analysis of soil bearing capacity, an improved integration of greenhouses in buildings, optimised management of flows and uses and lastly the incorporation of urban agriculture in circular economy scenarios.

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We spend more than 80% of our time indoors. Life cycle assessments (LCAs) help us to target the sources of environmental impacts over a building’s entire life cycle. However, the so-called “use” phase only currently considers energy consumption, while occupants are directly exposed to pollutants in the indoor air. Building materials, surface coatings, furniture and even the detergents used for cleaning emit chemical compounds. Specifically, volatile organic compounds (VOCs) may be toxic and could even cause cancer.

Objectives

The objective of this thesis is to develop a methodology to calculate the impact of indoor air quality (IAQ) on human health and to incorporate this into the building’s LCA. The impacts of emissions from materials and activities will be calculated using an indicator already present in the building’s LCA, with a view to proposing a design assistance tool. Through a comparison of materials and a scaling of ventilation systems, this tool will limit the impacts on occupants. However, increasing ventilation to reduce IAQ impacts results in increased energy consumption, which means that solutions must be considered to identify the optimum level between high air quality and low energy consumption.

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1. Processes taking place in the system under study

The processes related to energy and water consumption vary greatly, depending on building use (housing, offices, etc.). Simplifying the calculation of the environmental balance sheet by considering annual water consumption does not result in any major errors as the technologies used to produce drinking water and treat waste water change little over time (and water consumption is much the same whether or not it rains). The same cannot be said for electricity, the production mix of which varies considerably (cf. data from RTE, France’s transmission system operator). A dynamic model, for example with hourly time steps, significantly improves the precision of the calculation: the difference between static and dynamic LCA may reach 40% (Roux et al., 2016a).

Beyond these variations over a day, week or year, there are also long-term variations (over several decades) related to the energy transition (Roux et al., 2016b). There are therefore plans to adjust the electricity generation mix by increasing the share of renewable energy sources. This is also the case for gas. Climate change also brings about a change in air-conditioning and indeed heating requirements.
Simplifying assumptions may be considered to prevent the calculation process from becoming overburdened. For example, knowing whether a product was produced six months or one year before the building work does not appear to have a significant impact on the LCA results.
Waste processing after a building’s long life span will most likely be different to what occurs today, which may give rise to a comparison of scenarios, as part of a sensitivity analysis, or an uncertainty estimate. Generally speaking, the processes taking place in the distant future are more uncertain. As regards these future processes, as a precaution, potential improvements resulting in impact reductions may not be factored in as they are only assumed and remain uncertain. This in turn leads to considering an overestimate of impacts (e.g. the impacts of current processes), supposing that the environmental performance of the processes will be improved in the future.

2. The life cycle inventory

The variation in processes leads to a variation in the life cycle inventory (cf. RTE data on CO₂ emissions per kwh generated in France ).

Assessing an inventory hour by hour would make the calculation too unwieldy, and is only justified if characterization factors (used to convert emissions to impacts) vary greatly over time. This may be the case for volatile organic compound emissions, which break down according to the level of sunlight to form ozone which is harmful to health. In general, emissions do not vary according to sun exposure, therefore it is deemed sufficient to use an annual average in most cases. Wood heating causes more emissions in winter, when there is a lower level of sunlight, which may give rise to a more precise analysis if there is data on the variation of characterization factors according to sunlight.

3. Impact indicators

Certain physical or chemical phenomena such as the breakdown of some substances in the air, water or earth may vary over time according to weather conditions. As mentioned above, this is the case for the formation of photochemical ozone. Impacts may be calculated on an hourly basis according to sunlight (over a typical climatic year, for instance). This requires a modelling of characterization factors, and therefore supplementary research.

As regards global warming, the use of indicators as recommended by the IPCC does not take into account a potential variation of the impact according to the date of the emission. Certain exploratory methods consider a discount rate as in economic studies (one kilogram of CO2 emitted today is worth more than one kilogram emitted in fifty years) or no longer count impacts beyond a set cut-off point (e.g. in 100 years). This type of approach is highly questionable and there is no consensus of opinion within the scientific community. These methods give less importance to impacts affecting future generations, which runs counter to the principle of sustainable development. If impacts are not considered beyond a timeframe of 100 years[1] , does this equate to leaving future generations an “environmental debt”? This is a political choice, that cannot be justified by science.

[1] The indicators developed by the IPCC operate with a rolling time horizon, for example 100 years for Global Warming Potential, GWP100. The effects of an emission are integrated over the 100 following years, regardless of the date of this emission, and not solely over a set time horizon of 100 years from a building’s construction date. By considering this set time horizon, the effect of an emission taking place 50 years after construction is only integrated over the following 50 years, instead of 100 years, which underestimates the impact on future generations.

Examples comparing regulatory dynamic LCA and physical dynamic LCA

• 5-cm increase in glass wool for block walls (internal insulation and plasterboard)

Over one square metre, a 5-cm increase in thickness represents roughly 600g of glass wool, the production, transportation and implementation of which generates around 0.82 kg of CO₂ in year 0 of the life cycle.

Increasing thickness from 15 cm to 20 cm brings about a reduction in annual heating requirements of around 2 kWh per m² of wall for a typical individual house in the Paris region. If heating is generated by a heat pump (the most common system under the 2012 thermal regulations for buildings and most probably under the 2020 environmental regulations), the corresponding reduction in electricity consumption is around 0.8 kWh (per year). The reduction in greenhouse gas emissions is 200g of CO₂per kWh of electricity saved with the physical dynamic model (for heating, with an increased use of more carbonated resources in peak periods) and 79g CO₂/kWh with the regulatory calculation in the first year. This amount decreases over the years according to the regulatory “dynamic” indicator, but remains constant in the physical LCA (the impact on global warming does not depend on the emission date).

The result is presented in the graph below:

The initial CO₂ emission is gradually offset by the energy saved through thermal insulation. However, according to the regulatory calculation, it takes 15 years to achieve this offsetting, whereas the carbon payback time is only 5 years with the more physical calculation. Ultimately, over the 50 years, the insulator will cut emissions 7 times more than the quantity emitted for its production according to the physical calculation, compared to only 1.5 times more with the regulatory calculation.

• Comparison between double and triple glazing

The production of one square metre of glazing generates around 11kg of additional CO₂ for triple glazing compared to double glazing, but triple glazing (in this example installed on the north-facing façade of a typical individual house in the Paris region) cuts heating requirements by roughly 15 kWh and consumption by approximately 6 kWh (if a heat pump is used). The assessment of triple glazing compared to double glazing is therefore as follows for the two methods (the impacts related to a replacement of the window after thirty years are taken into account):

If a life span of 30 years is considered for the glazing[2], there is no added benefit of triple glazing according to the regulatory calculation as it results in higher overall emissions over the building’s entire life cycle. Conversely, the physical calculation demonstrates that triple glazing “pays for itself” in less than ten years, and that the reduction of emissions over the life cycle is roughly four times the amount initially emitted.

In both cases (insulation thickness and selection of glazing), the regulatory calculation shifts the environmental optimum towards a lower energy performance compared to the physical calculation. Physical dynamic LCA considers the increase in electricity consumption impacts in the colder months (more carbonated generation in peak periods), while the regulatory “dynamic” LCA decreases them over the years (global warming is no longer considered beyond 100 years). Due to this decrease in impacts, the regulatory calculation gives less value to energy-saving technologies or local renewable energy generation.

[2] Value considered in the regulatory database and in the physical LCA.

In short

As a dynamic thermal simulation may be used in addition to the regulatory approach, LCAs may be conducted on a more physical basis with a view to assisting project design. The physical dynamic LCA rolled out in the Pleiades ACV EQUER tool considers time variations for energy consumption, electricity generation processes and related environmental impacts (seasonal, weekly and hourly variations, long-term prospective scenarios).

Bibliography

Roux C., Schalbart P. and Peuportier B., 2016a. Accounting for temporal variation of electricity production and consumption in the LCA of an energy-efficient house, Journal of Cleaner Production 113 (2016) 532-540

Roux C., Schalbart P., Assoumou E. and Peuportier B., 2016b. Integrating climate change and energy mix scenarios in LCA of buildings and districts, Applied Energy 184 (2016), pp. 619-629

Peuportier B., Les matériaux et l’exploitation, deux leviers indissociables pour réduire l’empreinte carbone du bâtiment (www.lab-recherche-environnement.org)

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