Author:
Daniel Corbi Sánchez (NTT DATA Green Deal sustainability Unit).
Note: opinions in the articles are of the authors only and do not necessarily reflect the opinion of the EU.
In the building sector, and especially in offices and commercial areas, roofs have traditionally been considered to just be spaces to place equipment, such as Heating Ventilation and Air Conditioning (HVAC) and associated subsystems, cooling towers, generators, and small solar thermal installations for strict compliance with the regulations of Domestic Hot Water (DHW) generation with renewable energy.
Frequently, this high-density equipment on the roof has been selected due to oversizing from deriving high thermal loads because of an inefficient envelope with regards to the level of thermal insulation, air sealing and excessive solar radiation.
This is aggravated in old buildings, where successive implementations of new customers have occupied space on the terraces of their buildings to locate their own air conditioning, and for cooling systems for server rooms, etc.
There is an area of work for improvement, both in new buildings but especially in rehabilitation which includes redesigning roofs, recovering this ‘lost space’, not only from the point of view of enhancing PV installations, but also for the benefit of people to enjoy an open areas without the disturbing and annoying facilities that we can find in most of the actual roofs. This definition of a new space will allow for the reclaiming of former technical rooms (mainly located in basements) to create new areas, such as bicycle parking, electrical charging, parking lots, rainwater storage to take advantage of filtered water gathered from the upper green roof.
This redesign will not only require improving the envelope insulation potential and implementation of new facilities that will involve energy savings, with a greater increase of renewable energy generation, but also a necessary change due to new regulatory updates.
This article proposes a theoretical case study of building retrofit, such as the one shown in Figure 1.
Figure 1. Building before retrofit and Proposed building after retrofit. Credits: Daniel Corbí
Figure 2 shows that some of the usual facilities found in a typical building (left column), for example a 40-year-old office, may be switched with new features following retrofit (right column). This results in the freeing up a large amount of space for allocating new facilities, such as VRF (Variable Refrigerant Flow Heat Pump) HVAC (Heating, Ventilation and Air Conditioning) and complementary uses, such as a rainwater harvesting systems or a green roof with sufficient space to be developed.
Figure 2. Study case. Retrofit facilities progress
Legislation and strategies aiming to encourage new green roofs include the following:
This initiative aims to accelerate the vast and underutilised potential of rooftops to generate clean energy. This includes a proposal to gradually introduce an obligation to install solar energy in different types of buildings over the next few years, starting with new public and commercial buildings, but also includes residential buildings.
This initiative will be based on implementation of PV on buildings. Related initiatives are the Battery Alliance, ensuring that Europe can meet up to 90% of its demand with batteries produced in Europe by 2030.
REPowerEU (May 2022) is the European Commission’s plan to make Europe independent from fossil fuels well before 2030 and calls for increased ambition towards energy efficiency gains in buildings through the EPBD.
In this context the Commission has proposed to amend the Energy Performance of Buildings Directive (2018/844/EU) and include mandatory solar installations on rooftops in new buildings as shown in the following schedule:
Country scope (Spain’s example):
In its latest June-22 upgrade, the Spanish Building Technical Code (Código Técnico de la Edificación, known as CTE) set as compulsory to install a PV facility over roofs in every building type (including residential) and starting from roof surface of 1,000m2.
City scope (Madrid´s example):
In its latest June-21 upgrade, the Madrid´s buildings city hall code (Plan General de Ordenación Urbana de Madrid, known as PGOUM) encourage sustainable measures, like spaces to allocate sustainable mobility transport vehicles (such as bicycles), and setting green roofs and PV facilities by removing public regulation barriers because they won’t be reckoned in the gross surface – volume overall building calculations so thanks to this new public regulation more free space will be at disposal for the promotor.
Designing good practices for redesigning roofs will help sustainable certification schemes to permit more credits to be given and result in higher rankings. A brief description of these schemes and the credits available are set out below. There are numerous certification schemes that encourage sustainable roofs (including ILFI, DGNB, PassiveHaus, Minergie, EDGE, etc.), but for the purposes of this article only the three most widely used and well-known global schemes are covered in detail.
LEED (Leadership in Energy and Environmental Design) is an international symbol of excellence in sustainability and leadership in green construction, working on the triple bottom line (environmental, social, and economic). LEED was developed by the USGBC (United States Green Building Council) and is certified by a third party worldwide through the Green Business Certification Incorporation (GBCI).
Some credits in which a green roof with a PV facility helps to get better score (LEED BD + C V4.1 as sample):
WELL. The WELL Building Standard is a performance-based system for measuring, certifying, and monitoring features of the built environment that impact human health and wellbeing. The WELL Building Standard is third-party certified by the Green Business Certification Incorporation (GBCI), which administers the LEED certification program and the LEED professional credentials program.
Some credits in which a green roof with PV facility helps to get better score (WELL V2 as sample):
BREEAM (Building Research Establishment Environmental Assessment Methodology). This holistic approach to achieve ESG, health, and net zero goals is owned by BRE (Building Research Establishment, UK), which is a profit for purpose organisation with over 100 years of a building science and research background.
Some credits in which a green roof with PV facility helps to get better score (BREEAM NC as sample):
It has been considered the building case study shown in Figure 1, a five-storey building of 1,600m2 (40m x 40m) per level with a total of 8,000m2 of conditioned space and located in Spain. Where a refurbishment of the envelope and facilities will comply with the Spain energy regulation requirements.
Figure 3. Energy consumption case study (retrofit building) calculations.
To cover this electricity needs, a Photovoltaic system will be designed over the actual roof thanks to a steel pergola, to maximise the on-site renewable energy generation.
This steel pergola can be easily implemented on roof because it won´t add a noticeable weight, this way, structural building loads will be kept under limits.
Parameters considered in this case study:
Figure 4. Photovoltaic production case study (proposed retrofit building) calculations.
It is noted that in this theoretical case, it is possible to generate more renewable energy than the expected annual electricity consumption. Furthermore, if it was decided to use 80% of the roof surface dedicated to 500W PV panels with a Bi-facial technology (+15% with an 0.5 Albedo), the total energy generated could rise to an estimated +60% extra renewable generation, reaching an estimated 570,982 kWh/year.
This means that, in this type of medium storey building (from 4 to 5 levels) an Annual Net Zero Energy balance could be achieved in a warm European country or in a warm-cold country with quite a low envelope U-Value. All energy surpluses could be stored by batteries to use within the short term or fed back into the electricity network to be used by other energy consumers, or even stored in future hydrogen tank plant using an electrolyser to be converted back to electricity later through a hydrogen cell.
For the embodied CO2 calculation(3), it is considered that an average of 250KgCO2 is generated in the retrofit operation (demolition of former envelope and interiors including facilities, new façade, interiors, more efficient facilities, and newer redesigned roof).
Figure 5. Simple embodied carbon case study (retrofit building) calculations.
Further and more accurate analyses will have to be carried out to get a deeper insight into the total CO2 emissions during the Building Life Cycle (not only phases A1 to A5 but also B, C and D stages), but it is no longer impossible to realise that a life Cycle Net Zero Carbon balance is achievable.
Redesigning roofs is a compulsory exercise in every new and retrofit building project, to recover this forgotten space for people´s leisure and getting the maximum renewable energy generation.
Improved roof design will lead to compliance with future energy codes and enable higher scores in sustainable certifications as well as ease the path towards a Net Zero Energy and Carbon balance.
Figure 6. Example of a improved roof design, missing perfectly PV facilities, green areas and spaces for personal leisure(NTTDATA Offices, Barcelona, Spain). Credits: Daniel Corbí
