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Manuela
Almeida | Marco Ferreira | Ana Rodrigues |
Dep. of Civil Engineering | Dep. of Civil Engineering | Dep. of Civil Engineering |
University of Minho,
Portugal | University of Minho,
Portugal | University of Minho, Portugal |
malmeida@civil.uminho.pt | marcoferreira@civil.uminho.pt | anarocha32846@yahoo.co.uk |
In Europe, the urban areas are responsible
for 70% of the final energy consumption [1]. In an attempt to reduce these numbers,
many initiatives have been developed, mainly within the scope of the Climate
Change Package, where the main targets are the reduction of the carbon
emissions, the increase of the production of energy based on renewable energy
sources and increase the energy efficiency, with the well-known goal of 20% for
each [2].
In Portugal, the energy efficiency goals
include the renovation of public buildings, such as schools, in order to
promote a more efficient management and better serve the local community, [3]
while improving the comfort conditions and the energy performance and reducing
the operation costs. The basic school buildings are owned and managed by the
municipalities, which assume the responsibility for all the costs related to
their use, maintenance and renovation.
Having joined the Covenant of Mayors initiative,
a European movement involving local and regional authorities, voluntarily
committing to increase energy efficiency and use of renewable energy sources in
their territories, the municipality of Vila Nova de Gaia developed an Action
Plan for Energy Sustainability aiming to exceed the European Union 20% CO2 reduction objective by 2020.
Within this Plan, the municipality included
an action, called School Buildings CO2 Zero, where all
school buildings under the municipality management, must present zero carbon
emissions until 2020 [4].
Vila de Nova the Gaia municipality has 110
schools under their supervision, which, according to 2013 data, resulted in an
energy consumption of 2.8 GWh and 1239 Ton of carbon
emissions [5].
In Portugal, many school buildings were
built between 1940 and 1970 and are still being used, with a significant number
of these schools based on a model, known as P3, which was inspired in a
Scandinavian design. These buildings present pathologies related to thermal discomfort,
indoor air quality and signs of degradation.
Figure 1. P3 School general plan of
ground floor.
The average indoor temperature varies
between 15°C and 18°C during winter and 26°C and 29°C during summer and the CO2 concentration is frequently above the regulation limit,
overcoming 4000ppm several times during the day. These problems have been
noticed in similar buildings placed in different locations, which mean that
this is a common pathology in school buildings [5, 6, 7].
Figure 1 shows the general plan of a P3 school, with U shape. In the figure,
number 6 refers to classrooms and number 5 is a multipurpose area.
The renovation is currently in the project
phase, where it has been important to analyse different solutions and choose
the most cost effective way of reaching the goal of zero carbon emissions.
The main purpose, from the technical
perspective of energy and emissions reduction, is to pave the way for the
elimination of the CO2 emissions and improve the overall
energy performance of the building. This reduction will allow savings on the
energy bills supported by the municipality and will cooperate in the
implementation of the Action Plan for Energy Sustainability. Besides, it is
intended to improve the comfort conditions and the indoor air quality to assure
the users’ health conditions and the optimization of the environmental
conditions for the students.
For the present intervention, there is a
constraint regarding the building integrated technical systems (BITS) for
heating and cooling. As these systems have been recently replaced, they will be
kept as they are in this first phase of the renovation process and their
replacement by systems based on renewable energy sources will only occur in a
later intervention, closer to the end of the systems lifetime.
The most cost effective way to reduce significantly
the emissions, resulting from the use of buildings is very often improving the
buildings envelope to a certain level and using efficient technical systems
based on renewable energy sources [8].
The selection of the most cost effective
package of measures has been done with a life cycle costs approach based on the
cost optimal method, introduced by EC Delegated Regulation (EU) No 244/2012 of
16 January 2012, supplementing Directive 2010/31/EU of the European Parliament
and of Council on the Energy Performance of Buildings.
The method requires the calculation of the energy
performance of the building with each one of the considered renovation packages.
The energy calculations were based on the Portuguese thermal regulation that is
based on ISO-13790. Then, the global costs were calculated using the net
present value (including investment costs, energy costs, maintenance costs and
replacement costs).
The comparison of the results can be made
comparing the primary energy and the global cost of each renovation package. Figure 2 shows a
generic representation of a cost-energy curve where the cost optimal solution
is identified.
Figure 2. Generic results of the cost
optimal calculations.
The school was built in 1970 and it can be
divided in two different zones, considering its occupation and energy use
patterns: the classrooms zone and the multipurpose area, which is used as refectory
and indoor playground.
Figure 3. View of the East façade of
the school, relative to the classroom area.
The classrooms are distributed in two
floors, with 6 classrooms in each floor, with a total acclimatized area of 733 m²
and a floor to ceiling height of 2.7 m. The multipurpose area has only one
floor with a floor to ceiling height of 5.2 m and an acclimatized floor
area of 317 m². The total acclimatized area is 1 050 m². Figure 3 shows
the school East facade.
Concerning construction solutions, the building
has cavity walls without insulation, fibre cement tiles on the roof and simple
glazing with aluminium frame windows. The glazing area corresponds to 41% of
the vertical opaque area, which is very significant and with a great impact on
the building thermal performance.
For heating the classrooms, electric
storage heaters (with 2.4 kW/each and an efficiency of 1.0) that absorb
energy during the night and release it during the day were recently installed,
taking advantage of the reduced price of the electricity with the night tariff
(when Portugal has a surplus of production). For DHW, in the toilets, there are
electric heaters with 50 l storage tank (with nominal capacity of 1.5 kW).
In the multipurpose area there is an HVAC
system for heating and cooling (COP=3.00/EER=3.50, nominal capacity of 6.50 kW)
and in the kitchen, for DHW, there is a gas heater with 23.6 kW and an
efficiency of 80%.
The classrooms do not have a cooling system
or mechanical ventilation.
The internal lighting is mainly based on fluorescent
lamps with an average capacity of 58 W each.
Data collected from energy monitoring before
renovation allowed identifying the distribution of the energy use and the main
physical pathologies.
Regarding the energy use, main consumers
are lighting, appliances and heating. Regarding the indoor air quality, the monitoring
has shown that the concentration of CO2 is above the adequate
values most of the time.
In order to solve these problems in a cost
effective way, several measures have been analysed individually, as well as several
packages of those renovation measures, due to the trade-offs and synergies that
can result from their combination.
In accordance with this methodology, the
solution chosen for the renovation of this school resulted from the combination
of each cost optimal measure that is associated to each building element.
Following, a description of the
intervention on each of the main building elements is presented.
For the exterior walls, the solution includes
placement of external insulation over the existing facades, consisting of EPS
(expanded polystyrene) with 6cm of thickness, covered by Viroc® boards (compressed and dry mixture of pine wood particle and
cement). This solution, besides significantly reducing the thermal losses and
consequently the heating energy needs, improves the comfort conditions, solves
the thermal bridges problems eliminating building pathologies and creates a
façade with low maintenance costs.
For the roof, the chosen solution includes
removing the existing tiles and the introduction of rock wool with 10cm,
covered by new steel sheets. The inclusion of insulation reduces the heat losses
optimizing the heating systems behaviour and preventing summer overheating.
As the glazing area is a very significant part
of the building envelope in this building, the replacement of the existing
windows is not only necessary, because of their state of degradation, but also
an important measure due to its impact on the energy performance of the
building. Therefore, new PVC windows with double-glazing (6 mm+16 mm+6 mm)
will be installed. The cavity between the layers of glass is filled with argon.
This solution leads to a U-value of 0.7 W/m²°C. Besides the thermal
characteristics of the glass and frames, in the classrooms, shading devices will
be placed outside above the windows, consisting on horizontal plats to control
shading and to drive natural lighting into the interior ceiling.
To assure the interior air quality, a hybrid
ventilation system has been designed and will be installed. The air intake is
promoted by the adoption of ventilation grids under the windows and the air
exhaust is done on the opposite side of the rooms, through the roof. Mechanical
extraction in the exhaust area will allow the increase of the air renovation
rate whenever the CO2 sensors detect a concentration
above the desired values.
In the multipurpose area, the air quality
is already controlled due to the installed HVAC system.
The existing systems have been recently installed
so, their replacement is not an option at the moment. Closer to the end of
their lifetime, in a 2nd phase, it is planned that the heating
systems can be replaced by systems fully based on renewable energy, namely a
biomass boiler.
LED lamps with 20 W each will replace
the lighting. This solution allows reducing the energy consumption and
producing less heat, reducing internal gains that are a problem during summer.
There are also appliances in the classrooms
and in the kitchen that are not intended to be replaced within the current
renovation process, but only when their replacement moment arrives. Appliances
with the highest efficiency level should replace these.
These actions allow reducing very
significantly the energy use, but the final energy values are yet far from the zero
energy level. To fill this gap and get closer to the zero emissions goal,
photovoltaic panels will be installed to produce electricity from renewable
sources. These panels will produce energy for the school, mainly for lighting
and appliances.
In brief, the adopted energy renovation
features are the following:
·
External insulation on the
walls and roof;
·
PVC framed double glass
windows;
·
Hybrid ventilation with natural
crossed ventilation through ventilation grids and mechanical exhaustion
controlled by CO2 sensors;
·
LED based lighting;
·
Photovoltaic panels.
Table 1 presents the U-values for the elements of the building before and
after the renovation. The ground floor solution is kept as it is, with a
U-value of 1.89W/m²y.
Table 1. Buildings thermal
characterization before and after renovation.
Element | U-Value [W/m².y] | |
Before | After | |
Walls | 1.19 | 0.40 |
Roof | 1.40 | 0.38 |
Windows | 5.20 | 0.70 |
Comparing the results achieved with the
several renovation measures, the energy efficiency measure with the highest
impact is the replacement of the windows (due to their large area and
substantial reduction of the U-value), followed by the replacement of lighting.
Besides these measures, only photovoltaic panels allow higher reductions of the
non-renewable primary energy use as well as the future replacement of the
heating system by a wood pellets based system.
The chosen renovation solution allows improving
the comfort conditions, assuring the indoor air quality, saving energy and significantly
reducing the carbon emissions. With the contribution of the photovoltaic panels,
the electricity use is not far from zero but the high conversion factor for
electricity in Portugal (2.5 kWhEP/ kWh)
still leads to a primary energy use of 39.73 kWhEP/m2y. Table 2 presents the energy use,
primary energy and carbon emissions before the renovation and the estimated use
after the renovation (phase 1) and also after replacing the heating system
(phase 2).
Table 2. Summary of the renovation
impact.
Energy use [ kWh/m²y] | Before | After (Phase 1) | After (Phase 2) |
Appliances (Electricity) | 16.6 | 2.6 | 2.6 |
Lighting (Electricity) | 20.2 | 3.2 | 3.2 |
Heating (Electricity) | 47.6 | 4.8 | 0.0 |
Cooling (Electricity) | 0.5 | 1.0 | 1.0 |
Ventilation (Electricity) | 0.0 | 1.4 | 1.4 |
Cooking & DHW (Nat. Gas) | 7.3 | 7.3 | 7.3 |
Primaryenergy [ kWh/m²y] | 219.4 | 39.7 | 27.7 |
Emissions [kgCO2eq/y] | 1 4127 | 3 260 | 2 535 |
After the global renovation, 72% of the
energy needs will be fulfilled based on renewable energy and the energy use for
appliances and lighting will be reduced in 84%, when compared to the values
before renovation. Concerning the primary energy and emissions, the reductions
to be achieved are presented in Table 3.
Table 3. Summary of total reduction
of non-renewable primary energy and emissions.
Reduction | |
Non-renewable primary energy | 87% |
CO2eq emissions | 88% |
Regarding costs, the chosen renovation
solution has high investment costs but in a life cycle perspective, it has
lower global costs than the base scenario (restoring the building functionality
without improving the energy performance).
Figure 4 shows the costs for the thirty years’ life cycle. Analysing the figure,
it is noticed that the investment costs are clearly higher in the chosen
renovation but the operating costs are lower.
Figure 4. Distribution of the global
costs during thirty years.
It is expected that the users’ comfort will
improve significantly and the problems related to the quality of the air will disappear
without the need of complex HVAC systems that have been widely used in this
type of buildings during recent years.
The Portuguese thermal regulation has energy
reference requirements for the buildings’ envelope, thus the chosen renovation
presents solutions for the walls, roof and windows that are in accordance with
the reference U-values. It is noteworthy that the U-value of the windows is quite
low when compared to the reference value.
According to the calculations, this
intervention will also allow significantly reducing the energy consumptions and
the carbon emissions in a cost effective way during the building’s life cycle. The
energy bills of public buildings under the municipality´s supervision are quite
heavy, and the reduction of these numbers, through investment on energy
efficiency improvement, is an added value for the municipality, that can use
those savings for other purposes.
Further energy efficiency measures are
possible, but reduced improvements are achieved with high investments costs and
more photovoltaic panels will decrease the efficiency of the system due to the
lack of synchronism between the electricity generation and its use.
Although the zero emissions level has not yet
been achieved, the full planned renovation is considered a good compromise
between this goal and the cost-effectiveness of the intervention. The
renovation procedures analysed and presented in this paper do not constitute a
single pilot project as the municipality intends to replicate these measures in
the many similar schools in need of renovation in the country.
[1]European
Investment Bank (IEB), 2012. ELENA– European Local Energy Assistance;
[2]Fundo Social Europeu, 2014 Metas de Portugal
para 2020 e situação em 2013.
[3]Parque Escolar, 2014. Enquadramento estratégico.
[4]Energaia, 2011. Plano de Ação para a
sustentabilidade energética de Vila Nova de Gaia.
[5]Gaiurb, 2014. Guião de apoio a ações de
reabilitação energética do parque escolar do município de Vila Nova de Gaia –
Relatório Preliminar.
[6]LNEC 2010. Net Zero Energy School – Reaching
The Community, Escola Secundária de Vergílio Ferreira, condições ambientais no
período de Inverno de 2010. Relatório 180/2010 – ES/LNEC.
[7]LNEC 2010. Net Zero Energy School – Reaching
The Community, Escola Secundária de Vergílio Ferreira, condições ambientais no
período de meia – estação de 2010. Relatório 269/2010 – ES/LNEC.
[8]IEA
EBC, 2015. Methodology for Cost-Effective Energy and Carbon Emissions
Optimization in Building Renovation (Annex 56).
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