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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Anaël Mayeur: Plants were long considered to be a mere technical resource to serve humans, then as an aesthetic element that boosted the acceptability of planning projects. Now, they could be a way to integrate constructions into their environment more fully, provided that we consider their characteristics in greater detail. This is the subject of my doctoral research project, which focuses on the composition and study of the herbaceous seed combinations that promote biodiversity while continuing to meet the conventional technical challenges of revegetation.

The aim is to obtain seed mixes with a greater diversity of species than those commonly sold and used in large-scale developments, with a view to attracting small animals and pollinators in particular, but also to establishing competition against invasive exotic species. Some mixes are made up of seeds from the Végétal Local brand, the idea being to have sufficient genetic diversity for these species to enjoy enhanced resilience in the face of climate change, and a reduction in the use of inputs for their healthy growth.

Nicolas Durvaux: During the construction phases for new sections, we rolled out large-scale revegetation plans for the surrounding areas, with more than 17 million trees planted, as well as major sowing operations. We are currently working on maintaining this and incorporating new challenges such as abolishing the use of agrochemicals and applying reasoned mowing. We hope to promote and increase the potential for renaturation of the available green areas.

In addition, we have also observed that our need for mineralised operational surfaces has dropped by around 30% (around the operations centre, coating platforms, etc.) alongside technical and organisational developments. The opportunity arose to propose a renaturation of these spaces by demineralising unnecessary surfaces.

Renaturation raises questions regarding the creation of functional natural environments. Which levers do you use to achieve this?

Nicolas Durvaux: As the projects are smaller in size than those during the construction phases, each site needs to be engineered individually. Following the soil analysis phase, our partnership with the French Office National des Forêts enables us to conduct a study of several technical renaturation approaches for each plot. These approaches combine conservation measures of the existing sites such as meadows and hedgerows, the planting of local tree species that can adapt to climate change, and the creation of wetlands. For each project, these different approaches optimise the creation of natural areas and are approved by our in-house experts with a view to assessing the impact of feasibility, ecological interest and repercussions on maintenance.

Anaël Mayeur: To follow on from what Nicolas was saying, the composition and selection of seed mixes to be sown are part of the engineering work to be rolled out on every type of site. As part of my project, mixes with different functions are tested. Some are designed to be competitive against invasive exotic species, with others are intended to cover the plots quickly after being sown or to attract pollinators.

Let’s remember that the creation and upkeep of a site’s functions also depend on environmental factors that are not always under our control. It is therefore a good idea to preserve pre-existing natural elements where possible.

When a sowing operation is required, the use of wild and local seed mixes means that we can rely on species’ natural resilience to adapt to the changes in their environment. In this way, we give the better adapted plants a greater chance of survival in the event of disruptions. This leads to a sustainable population of plant species. We are in a long-term approach aimed at maintaining the newly created plant communities, and therefore their functions.

Both of your projects incorporate short supply chain development objectives in different ways. Which opportunities and obstacles have you come across?

Anaël Mayeur: Having a stock of wild and local seeds requires a sector with a wide range of skills and stakeholders. The technical pathway from collection to planting is complex and calls for local stakeholders with sound knowledge of the local area. We predict opportunities for the creation or specialisation of companies within this market, in addition to partnership opportunities for developers and stakeholders in this budding sector. Such partnerships would improve project acceptability and would also give rise to high-quality products and advice, in tune with the local conditions which planning projects must face.

Although the study of the social and economic factors is still in its infancy, two major obstacles to the widespread adoption of this approach are becoming apparent. The first concerns the cost of seed purchases, which may be prohibitive in comparison to mass-produced seeds. However, the removal of inputs and the reduction in seed quantities may offset these excess costs, for equivalent results (this remains to be demonstrated). The second obstacle concerns the supply of wild and local seeds. As the sector is still in its early development, not all regions are covered when you need to find a producer able to fulfil large orders, for example in the event of motorway construction and the creation of related compensatory areas.

Nicolas Durvaux: Short supply chains are primarily used in the deconstruction of mineralised sites. For the first project, we contacted local associations who were looking for materials. For the second project, we want to work with Granulat+® to optimise material recycling. Their presence across France means that chains can be shortened and recycling reduces the footprint of resources.

Our second ambition for short supply chains is to work on renaturation projects with local companies and building sites that bring the unemployed into the workforce. In addition to the pride of managing these projects in their local areas and seeing them grow, there is also the ambition of training new stakeholders in these approaches.

Lastly, the third challenge concerns the need to support local seedling creation sectors by conducting these projects with local plant species. This means planning future developments with a vision spanning several years.

Which stakeholders must be brought in to roll out your approach as widely as possible?

Nicolas Durvaux: The primary stakeholders are naturally project managers. We have to prove to them the benefits for biodiversity and convince them to agree to long-term projects.

These projects are rooted in local areas. Sometimes, local elected representatives wish to get involved by sharing the ambition or drive to support local employment, for the project’s completion or the arboriculture sector.

As some projects may need to extend their scope to include potentially workable land for certain types of agriculture, we must also foster dialogue upstream with chambers of agriculture and local farmers, to work with them to design projects that can meet the twofold environmental and sustainable production objective.

Anaël Mayeur: To second what Nicolas has said, it is necessary that project managers are involved as early as possible in the design phases so that plant requirements are planned ahead, to discuss the challenges they represent and to define the share of the budget that will be allocated to this. This can facilitate access to material by leaving stakeholders in the production sector the time to obtain a sufficient quantity to meet demand.

More generally, it is important to raise awareness among all stakeholders who use seed mixes, whether as part of the management of the sites operated, the construction of new developments or environmental restoration sites, so that the functions and importance given to plants on these sites can be reconsidered. Taking an interest in the origin of plants and the composition of commonly used seed mixes could play a part in improving the integration of projects in the current environmental challenges that local areas face.

For further information, you can watch the replay of the the Research & Solutions webinar on “The revegetation of road infrastructure” with Anaël Mayeur and Nicolas Durvaux (in French).

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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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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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Let the urban and the rural, the mineral and the vegetal, interact

Our wish is that the making of the city in the city makes it possible to preserve the existing agricultural and natural areas, that nature flourishes in the city, on the ground as on buildings, and that roofs become reservoirs of biodiversity, rainwater storages, urban heat island mitigation zones, social link catalysts, fruit and vegetable production areas.

Roof terraces are an under-exploited area in the city and yet of key importance: they can represent up to 32% of the horizontal surface of a city. Urban agriculture on rooftops is therefore an opportunity for local authorities to optimise built-up areas to integrate nature in the city, to participate in the climate resilience of cities, to recreate places where residents can meet and share while creating local jobs by offering functional and easily usable sites. Planners, developers and builders will see it as an asset for urban developments with high environmental and social added value, in line with the growing need of city dwellers to reconnect with nature.

Why a practical guide for those involved in the making of the city?

Developing urban agriculture on rooftops is not just greening the building, it is integrating a project, led by specific stakeholders, with its own operating model, its flows to manage, its needs to be anticipated. It is about understanding the challenges and diversity of urban agriculture, consolidating various skills to carry out the project, mastering different regulations, forging partnerships.

It is therefore to bring these different elements together within a single operational tool that we have written this guide, in the framework of the lab recherche environnement programme.We hope that our book will help local authorities, planners, developers, landlords, builders and managers of property assets to further develop urban agriculture on rooftops, in all its diversity, whether in the form of a shared garden, urban micro-farm, restaurant’s vegetable garden or productive urban farm.

 Order the book in paper or digital format

Structure of the practical guide

To this end, the book provides a review of current knowledge and practices, as well as a forward-looking vision of the building of tomorrow.

Then, all the criteria related to the layout of the rooftop for urban agriculture, whether technical, regulatory, related to flow management, project management, etc. are presented in a precise manner and accompanied by practical tips.

Finally, we offer methodological support to deploy these projects as well as summary sheets for each form of urban agriculture projects and for each production system. Twelve fact sheets presenting practices and feedbacks from rooftops in Paris, Lyon, Brussels and Besançon make it possible to adopt best practices.

Methodology – A guide designed with the players in the making of the city

To create this guide, we have:

• Analysed regulatory documents specific to rooftops or urban agriculture;
• Listed 170 agricultural rooftop projects around the world and analysed 70 French projects;
• Audited twelve sites in Paris, Lyon, Brussels and Besançon to better understand the constraints and present the details that make the difference in a project;
• Met many experts such as personnel from the technical departments of local authorities, construction and development stakeholders, project leaders and researchers;
• Led collaborative workshops on technical subjects;
• Benefited from the feedback of many contributors;
• Monitored urban agriculture projects during the construction phase.

Table of contents

Urban agriculture settles on the rooftops

1. Urban forms of agriculture
2. The advantages of making rooftops fertile
3. The peculiar ecosystem of the fifth facade
4. A shift towards “circular buildings”

Implementation guidelines

1. Getting to know the project area and defining it
2. Identifying the main elements of the roof structure
3. Identifying the necessary flows for a rooftop farm
4. What equipment for more functionality?
5. Anticipating the project management

Project methodology and fact sheets

1. The main project stages for a new or an existing building
2. Regulation and reference documents
3. Fact sheets presenting different forms of urban agriculture
4. Fact sheets presenting practices and feedbacks from rooftop agriculture projects
5. Bibliography

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The particular feature of agricultural production systems installed on terraced roofs is their location “above ground”, that is to say in an artificial environment. These arrangements can take various forms.

Growing trays are a system where the substrate is contained in various supports such as wooden boxes, geotextile or “smart pot” bags. The advantage of this technique is its rapid implementation, its modularity and its relative reversibility.

Crop beds concern a system similar to conventional green roofs, but allowing agricultural production. Their use can be compared to growing in the ground, but placed on a roof. It is a system that optimises the growing space while facilitating maintenance.

Hydroponics, aeroponics and bioponics are high-tech systems: all parameters (water oxygen, pH, temperature, etc.) are controlled to optimise plant growth through centralised computer management. Hydroponics represents a closed loop cultivation system where the roots of the plants are constantly in contact with a water circuit enriched in nutrients. In the case of aeroponics, the roots of plants are bathed in a permanent mist of nutrient solution. When we talk about bioponics, we mean that the nutrients inserted into the circuit are organic and biological.

Agricultural production systems installed on rooftops can be based on different techniques. Growing trays correspond to a system where the substrate is contained in various supports such as wooden boxes or geotextile bags. Crop beds can be compared to growing in full ground, but set out on a roof. Hydroponics, aeroponics and bioponics are high-tech systems: water oxygen, pH, temperature and other parameters are controlled to optimise plant growth through centralised computer management.

2. The four project families

Urban rooftop agriculture is extremely multifaceted, not only in terms of the growing techniques used, but also in terms of project forms.

Urban micro-farms are characterised by their multifunctionality: they rely on agricultural production to develop other initiatives (special activities, workshops, events, etc.) around themes linked to food, the environment and the rediscovery of nature. The diversification of activities allows them to multiply sources of income.

Shared gardens, a historical form of urban agriculture, are spaces maintained by a group of individuals who collectively produce food and other plants. Among these collective rooftop gardens, there are several forms: gardens shared by the inhabitants of a building or a neighbourhood, corporate vegetable gardens, educational vegetable gardens, therapeutic gardens.

Productive urban farms are companies or startups where the majority of income comes from the sale of the produce. To ensure the viability of their activity, they often offer services (training, visits, activities, etc.). The sale can be made in the form of baskets distributed through short supply chains, in organic outlets or grocery stores, on sites or in restaurants.

Restaurants with vegetable gardens follow the “farm-to-table model”, whereby fruits and vegetables grow on the rooftop of a restaurant, hotel, hospital or business where a cook, restaurateur or other manager uses the produce to supply the restaurant, canteen or bar downstairs. These arrangements make it possible to bring added value to cooked meals thanks to quality products, while enhancing the company’s image.

3. A particular microclimate

In the heart of a dense city, the temperature can be up to 3°C higher than on its outskirts. On a roof the temperature is even higher because of the increased exposure to the sun and the characteristics of the materials used in the roof. However, some roofs are in the shade of nearby taller buildings. It is therefore important to be familiar with the exposure of the building as well as the areas of shade before setting up a project involving crops.

Roofs are exposed to stronger winds than on the ground which can have an impact on crops and the substrate: dispersal, drying out, crops being swept away because their roots are only loosely anchored in the substrate… Likewise, for farmers and the visiting public, it is not always pleasant to be exposed to gusts of wind. It therefore seems useful to provide for improvements or equipment during the design of the project (on a new structure as on an existing structure) in order to reduce wind pressure.

4. Specific crops for our roofs

The urban heat island effect in the city makes it possible to consider growing exotic plants such as okra or acmella oleracea. On rooftops, the growing system is the main factor that determines which plants can be grown.

Hydroponic systems generally favour crops that are simple to grow and that sprout fairly quickly: leafy vegetables (salads, spinach, chard, etc.) and fruit vegetables (tomatoes, peppers, courgettes, etc.). However, the successive improvements of the systems and feedbacks show that it is possible to grow an increasing diversity of vegetable crops.  

For boxes and growing trays, the greater the height of the substrate, the greater the diversity of growable vegetable crops. On the other hand, the depth of the substrate makes it easier to withstand temperature variations and the high heat present on the roof.

Is pollution characteristic of these spaces?

The question that often comes to the fore is that of pollutants. Is produce grown on urban roofs bad for our health? To answer this question, we need to look at the three sources of pollution that exist in the city: water, air and growing substrates.

Most urban agriculture projects are supplied from the city’s drinking water network; which eliminates any possible risk of water pollution. In the case of water supply by rainwater recovery or filtration of a wastewater network, it must be subject to regular checks.

Air pollution levels depend on the distance from the emitting source, major roads or industries, and the height of the building. The first campaigns to analyse the presence of contaminants in fruits and vegetables from cultivated roofs and, in particular, that carried out on the rooftop of AgroParisTech on the 5th floor of the building in the 15th arrondissement of Paris, highlighted values below the regulations in force. For a site near sources of pollutants, it will still be necessary to carry out regular monitoring and perhaps adapt the type of plants grown.

6. Rooftop breeding

Animals in cities offer a number of functions that plants, grown in urban agriculture, cannot provide, such as the ecological maintenance of urban spaces or the production of natural organic fertilisers. However, although it may appear relevant, the introduction of animals into cities poses many more regulatory problems than do crops. Their production is subject to product sanitary regulations, as well as animal welfare and health standards.

L’Urban beekeeping activities continue to be set up on roofs, whether on that of the Paris Opera, the Galeries Lafayettes department store or the headquarters of major companies or in private homes. With the Labbé law and the “0 phyto” measure in municipalities, life seems quieter for bees in the city than in the countryside with food all year round thanks to the differentiated management of green spaces. But once again, the multiplication of hives must not compromise this positive state. We are beginning to see feeding problems as well as health problems for these urban bees, especially in Paris.

L’ Aquaponics represents the assembly of aquaculture and plant culture, all in a closed loop system. It is interesting today, because it makes it possible to reduce the expenses related to water by 80-90% compared to a traditional holding and requires little or no mineral inputs essential to the plants. Fish droppings become a source of food and a natural fertiliser for plants. Water saving, effluent management: aquaponics represents an ecological model of production. For this technology to participate significantly in the production of food, there remain many challenges, both technical and socio-economic.

We are increasingly seeing henhouses spring up in urban areas which are sometimes part of programmes to combat food waste in certain municipalities. Although it is still unusual to encounter chickens on a roof, valuing these unused spaces in small rearing areas in the heart of the city can be an opportunity to relocate animal products close to home.

Beehives on the roof of VINCI.Goats on the roof (Aubervilliers).A henhouse on the roof of a collective vegetable garden.

To find out more: Fanny Provent and Paola Mugnier, Agriculture urbaine : comment aménager une toiture-terrasse, the lab recherche environnement VINCI ParisTech, Eyrolles editions.

The co-authors will present this first practical guide from the lab recherche environnement (ed. Eyrolles) on 21 September 2020 on the occasion of the Lab Evening “Urban and vertical farmers”

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Retain heat and freshness

At the end of the 19th century, it was recommended to keep indoor temperature below 16°C for health reasons, probably because of the pollutants emitted by wood and coal heaters. Between 1986 and 2003, ADEME observed that expectations in terms of well-being were increasingly taking precedence over concerns about consumption costs with an average increase in the temperature of homes from 19°C to 21°C. More recent studies (CREDOC, 2012) show that 30% of households heat the living room to 21°C and 75% to more than 19°C. The bedrooms are heated to over 19°C in 50% of the housing units. When the temperature is restrictively limited to 19°C, which complies with the regulations, the occupants tend to equip themselves with auxiliary heaters. There is a gap between technical standards and social conventions which means that even the most efficient equipment does not produce the expected performance. Satisfactory winter thermal comfort in low-consumption housing requires new practices and, in particular, users should opt for heat conservation and recovery, for example by limiting the opening of windows and using sun exposure.

Other more proactive practices ensure thermal comfort in summer. Some habits, such as ventilating at night or closing windows and shutters during the day, can protect against the heat. Although these practices have been developed and are widely known in the south, they have yet to gain acceptance in the northern regions. In addition, certain factors such as outside noise and the risk of intrusion can prevent the occupant from ventilating at night.

Thermal comfort depends on the measured ambient temperature, but also on the perceived temperature which results, among other things, from humidity, air flows, colours of materials, furniture, light, clothing, food and activities. Like any subjective perception, the feeling of temperature is also related to socio-demographic and cultural factors such as gender, age, membership of a socio-economic or ethnic group. In a shared space, negotiations and compromises between occupants will determine whether everyone’s comfort expectations are met. Energy efficiency will also depend on what Dominique Desjeux calls the “war of buttons” to control lighting and the “war of fire” relating to heating.

High-performance technology is tailored to its user

Energy management systems are becoming increasingly sophisticated. But at the time of operation, a technical system escapes the control of its designers and it is the users who appropriate it. This appropriation depends on the complexity of the technical objects and their usability. The occupants give up using overly complex technical solutions that they cannot master. Also the past experiences of users and their technical skills have an impact. So, the person who is accustomed to collective heating is likely to be in difficulty in a new accommodation with individual heating. In a 2018 study on households suffering energy poverty, Christophe Beslay and Romain Gournet observed that certain fragile households had fallen into poverty and failure to pay energy bills following a move to a home where they could not use the consumption management equipment properly. The process of appropriating new technical systems only takes place when there is transmission of knowledge via user manuals and an on-site learning phase.

The challenge is to design housing for and around users. Occupants can actively contribute to energy efficiency, but they are above all users with their own priorities, daily routines, and household knowledge. Connected and intelligent housing are solutions to better take into account the inhabitants when the energy management systems are able to interact with the occupants, to adapt to their needs and to multiple action scenarios. Modelling, the “Internet of things” and machine learning will provide a better understanding of housing occupancy and uses. However, the support and participation of occupants in the design through surveys and workshops remain important to achieve a balance between environmental transition, well-being of inhabitants, financial costs and constraints of the territory.

FIND OUT more:

Christophe Beslay, “Pratiques sociales et modes d’habiter”, in Bruno Peuportier, Fabien Leurent, Jean-Roger Estrade (Coord.), Éco-conception des bâtiments et des infrastructures, Volume 2, Presses des MINES, Sustainable Development collection, 2018.

Christophe Beslay and Romain Gournet, Stratégies de traitement des impayés d’énergie. Survey of 14 households, Observatoire National de la Précarité Energétique, 2018.

CREDOC, 2012, Energy consumption survey.

Dominique Desjeux, Anthropologie de l’électricité. Les objets électriques dans la vie quotidienne en France, L’Harmattan, 1996.

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The energy efficiency of the building is based on solutions such as constructing compact housing, favouring solar gain and natural lighting, using insulating materials allowing air-tightness or heat storage, using double flow ventilation systems which recover and distribute heating, alternative energy sources (solar, geothermal, photovoltaic) and automatic consumption regulation devices. However, low-energy buildings often consume more than expected. According to a CEREMA study conducted between 2012 and 2016, 50% of efficient buildings consume a surplus of 10 KWhep per sq.m per year. In 25% of cases, this excess is 35 kWhep per sq.m per year. According to the study, the differences observed depend in part on user behaviour. As buildings become efficient, these deviations become increasingly significant compared to the overall performance of the project. They can easily reach 20 to 30% of the total consumption.

This discrepancy, due to an imprecise estimate of actual uses, was also observed by Eric Vorger, former doctoral student at MINES ParisTech and co-founder of Kocliko, a start-up that offers digital tools to optimise and guarantee the energy efficiency of buildings. “Human behaviour is simplistically modelled in building energy simulation software. However, its impact is considerable and it is the source of significant variations between simulation results and in situ measurements. Short-term predictions (of the order of 24 hours), used to regulate energy consumption in real-time, as well as medium-term predictions (one year), useful for sizing renovation operations, are marred by very significant uncertainties (often greater than 50%) because of this lack of knowledge of uses”.

The arrival of algorithms in eco-design makes it possible to analyse, understand, study and forecast user practices better and better and take them into account to optimise energy efficiency. As part of his PhD project, Eric Vorger, modelled the energy consumption of buildings resulting from the presence and activities of inhabitants. This occupancy model is coupled with the dynamic thermal simulation tool Pléiades+COMFIE, developed by MINES ParisTech with the support of the lab recherche environnement.
The presence of people in a home and their activities are valuable information that makes it possible to predict the opening or closing of windows and blinds, the use of lighting and domestic equipment, the occupation of certain rooms in the home, the number of people in a room and the metabolism of the occupants which influence the ambient temperature.

Eric has developed probabilistic models based on INSEE data (e.g. the “Time Use” surveys) and measurement campaigns. Thanks to these models, the same home can be simulated 10,000 times with, each time, occupants having different socio-demographic characteristics which determine different schedules. The second graph illustrates the average daily scenario of a virtual individual, produced from a series of simulations. Other simulations were carried out successively with the same dwelling and different types of household: a large family, a working couple, a retired person, etc. By taking into account the composition of households, more reliable forecasts of energy consumption are obtained, which allows contractors to commit to realistic energy efficiency targets and achieve them.

This design support tool enables design to be optimised thanks to realistic usage forecasts. There are therefore fewer design errors, and consumption and comfort forecasts are more reliable.  Kocliko has also developed solutions for energy management during the operational phase. Based on the observation that connected objects are now technology that is accessible at low cost, the start-up proposes to exploit the “Internet of things” in order to extend the collection of measurements in a housing unit: temperature, humidity percentage, electricity consumption, thermostat usage, etc. The machine learning algorithms developed by Kocliko combine this data to reconstruct actual uses and manage energy consumption. An energy saving of at least 20% can thus be achieved during the operating phase.

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