Practices

Performant ontwerp van een passief school via thermische dynamische simulaties

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At the request of the Royal Atheneum in Etterbeek (Belgium), evr-architecten is designing a new kindergarten (+/- 1000m²) as extension of the existing school complex. The starting point is an integrated sustainability approach with special attention for social, spatial and ecological criteria. Therefore and in co-operation with 3E, an economically sound energy concept is sought, where a healthy and comfortable indoor climate is achieved. The passive house standard1 is considered as ambition level, with specific requirements for the building physics and the technical installations. The total net heat demand must be lower than 15kWh/m².year and the total primary energy consumption can not be higher than 120kWhp/m².year. In passive schools, with a high occupancy ratio and therefore high internal heat gains, controlling the summer comfort is a crucial element in the optimisation of the energy concept. Additionally, the building physics (insulation and air-tightness) and ventilation are optimised, so that a conventional heating system becomes superfluous. The high ventilation flow, which is needed in a school environment, makes heating with ventilation air possible. The net energy need for heating of a regulatory compliant school (“E100” level, measuring the maximum primary energy consumption compared to a reference level) of that size is calculated by means of a simulation program at approximately 100 kWh/m²year (Achten, 20072). The designed passive school has a total heat demand of 10 kWh/m².year and is therefore 90% more energy efficient than a theoretical E100 school.

1.     Introduction

While searching for an economically sound energy concept for the new kindergarten of the Royal Atheneum in Etterbeek (Belgium), the pre-design made by architect office evr-architecten is evaluated and optimised. Using dynamic thermal simulations, both the building physics (insulation thicknesses, glazing types,…) and technical installations aspects (ventilation with heat recovery, solar thermal energy systems,…) are analysed, and for each modification the users comfort is controlled. The economical, energetic and ecological impact of several relevant energy concepts is considered and the optimal energy concept is then developed as a full pre-design.

Evr-architecten already has several years of experience in building low energy and passive buildings. Its pre-design is therefore already strongly optimised. A large overhang roof is foreseen on the Southern and Western glazed facades, and the building is foreseen as a heavy construction in order to create sufficient thermal mass. This study focuses specifically on controlling the summer comfort. In passive schools, with a interne heat gains, with a high occupancy ratio and therefore high internal heat gains, this is a crucial element in optimising the energy concept.

2.     Thermal Dynamic Building Simulations

In order to optimise the energy concept from energy and economic viewpoints, 3E uses dynamic simulation models: a thermal and geometrical building model is composed, which makes it possible to conduct a comprehensive and detailed analysis of the thermal behaviour of the building. For each time increment (1h), the heat transfer and ventilation are simulated, taking into account:

-          Actual and local meteorological data (temperature, solar irradiation, relative humidity,…),

-          Building characteristics (composition of the building physics, internal heat gains, technical installation,…)

-          Shadows if relevant (surrounding buildings, woods, specific obstacles,…)

-          And pre-defined users profiles (presence profiles, temperature set points,…).

The results of the dynamic simulations include, for each time increment, heat demand, cooling demand, comfort parameters for the complete building or for a specific zone.

 In this energy concept, the focus lies specifically on potential comfort problems and on an optimised indoor air quality, because of their crucial importance in a school. Therefore, the dynamic simulations are conducted for a standard year in the area (Brussels, Belgium) and for the hot summer year of 2003. The data are provided by Belgium’s Royal Meteorological Institute (“Koninklijk Meteorologisch Instituut van België” KMI).

3.     Thermal modelling of the Reference Design

Thermal and geometrical building model

The model is made based on the received drawings and consists in several thermal zones, as illustrated in Figure 1. These zones combine areas with the same function, and therefore comparable users’ profiles, internal heat gains, heating and cooling set points, ventilation profiles etc. The following zones are identified:

-          Multi-purpose area

-          Class room (1 zone per class)

-          Storage

-          Entrance

-          Sanitary facilities.

The total effective surface of the building is 1034 m².

Figure 1: 3D visualisation of the building model. different colours respresent various thermal zones

Characterisation of the building physics

The total heat loss surface and the U-value of each building element is shown in Tabel I.

Building element

Surface

U-value

Floor in contact with the ground

611 m²

0.13 W/m²K

External wall

421 m²

0.13 W/m²K

Roof

727 m²

0.12 W/m²K

Window

206 m²

0.79 W/m²K

Sun pipe

4.5 m²

1.99 W/m²K

Doors

3.9 m²

0.76 W/m²K

Tabel I heat loss surfaces and U-values per building element.

Shadowing

The school is situated in the middle of a large number of high trees. These broad-leaved trees provide natural shadowing for the various façades during the summer. During the winter, the foliage is much less dense and the shadowing is also significantly diminished. Therefore, both summer and winter simulations are conducted in order to approach the actual situation most closely.

Figure 2 Partial view of the existing school grounds surrounded by large trees

Internal heat gains

The internal heat gains have a large influence on the total energy demand. These are generated by the people present in the building, but also by lighting and electrical equipment such as computers, printers,… For small children, we suppose a sensible heat gain +/- 50W/kleuter and a latent heat gain of 30W/kleuter. For lighting, a gain of 10W/m² in the class rooms and 8W/m² in the multi-purpose area are considered. The heat gains from electrical equipment can be considered negligible in kindergartens.

Infiltration and ventilation

The model takes into account infiltration from outdoor air and therefore the air-tightness of the building. In the simulations, an air-tightness n50=0.6h-1 is considered, which is the requirement for passive buildings. Experience shows that this value is achievable for the proposed building physics composition.

Besides infiltration, mechanical ventilation is also considered. For the simulations we consider high-efficiency energy recovery (85%). Ventilation occurs through a central system with 1 VAV-loops, where the air flow is either distributed to the class rooms (depending on a timer) or to the multi-purpose area (depending on occupancy). For the simulations, we consider a loss flow of 15% to the closed circuit. The flows are calculated based on the design occupancy, following the related European standard NBN EN 137793.

In the multi-purpose area, the opening and closing of doors is taken into account through natural ventilation.

4.     Results for the reference design

Energy needs

As no cooling is foreseen, we focus on the analysis of the heat demand. The heat demand is defined by the heat losses vs. heat gains. Heat gains consist in internal gains and solar gains. As mentioned in §3, the school is located in the middle of a large number of high trees, and shadowing plays an important role.

Figure 3 shows the difference in solar gains with and without shadowing  of the surrounding trees. Trees in the summer (with shadowing) block approximately half of the incident solar radiation. The absence of foliage in the winter makes it possible to take advantage of unchanged solar gains during the heating season.

Figure 3: solar gains with and without shadowing (blue = with shadowing, red = withiout, green = yearly hypothesis

In a passive building, the influence of those solar gains is very important. The difference between the simulated heat demand, with and without shadowing, amounts approx. 3kWh/m²year. In order to evaluate the total heat demand, we consider the non-shadowed situation in wintertime (October to March) and the shadowed situation in summertime (April to September). This is illustrated in green in Error! Reference source not found.. The composed total heat demand amounts less than 10kWh/m²year, which is largely below the maximum value of van 15 kWh/m²year.

Summer comfort

In passive buildings, besides the energy need, the evaluation of thermal comfort for users is also critical. Requirements related to thermal comfort are defined by European NBN EN ISO 77304 en CR 17525 standards. One of the requirements is the total absolute number of overshoot hours. A maximum of 100h above 26°C and 20h above 28°C (during human presence) is considered as threshold for an acceptable comfort. Figure 4 shows the absolute number of overshoot hours in the kindergarten for several rooms during a standard climatic year. The absolute number of hours above 26°C amounts more than 300h, those above 28°C amount more than 100h: the thermal comfort is insufficient.

Figure 4: absolute number of overshoot hours for class room 2 (purple), class room 1 (brown) and the multi-purpose area (yellow)

5.     Optimising the design

Improvement measures

The possibilities for optimisation of the design in order to avoid overheating in summer are related to building physics and technical installations.

Building physics measures focus on reducing solar gains, through less glazing, external solar shading or solar control glass.

Additional solar shading is not very functional because of the large shadowing by surrounding trees, and the reduction of the glazing surface has a direct impact on the solar gains in the winter. Therefore we have first considered the impact of technical installation measures such as the implementation of a by-pass and night cooling. The conclusions will confirm whether additional building physics measures are needed.

Technical installation optimisation

A first option consists in the addition of a by-pass on the heat recovery, a second option consists in a by-pass complemented by night cooling. Night cooling foresees half of the total ventilation flow (775 m³/h) for the multi-purpose area. The other half is distributed among the ten class rooms (77.5 m³/h per room).

The multi-purpose area is the most subject to overheating because of the high occupancy. Figure 5 shows the number of overshoot hours in this area for the various options.


Figure 5: number of overshoot hours for the various technical options in the multi-purpose area

Summer comfort is clearly improved, with less than 100h hours above 26°C and exactly 20h overshoot hours above 28°C. The impact of the night cooling is limited compared to the by-pass because of the small air flow that can be achieved with the ventilation group. Over-dimensioning of the ventilation group would have an adverse effect on the electircity consumption of this group.


Figure 6: number of overshoot hours for the various technical options in the most Southern located class room

Comfort in the class rooms becomes good thanks to the implementation of a by-pass combined with night cooling, as there are only 17h of overshoot hours above 26°C (see Figure 6). Additionally, simulations show that this can be further reduced by a factor two, by taking into account the opening of windows when the outdoor temperature allows it. Because this attitude can not be controlled, it is not further taken into account.

Building physics optimisation

The by-pass and night cooling are just sufficient to achieve acceptable summer comfort during a standard climatic year in the region. During a hot summer however, this will not be sufficient. Therefore, building physics measures which can have a positive impact are also examined.

In order to create the desired transparency, both the Southern and the Western façades of the multi-purpose area are fully glazed. Addition of opaque parts (U=0.21W/m²K) to reduce this glazing surface by 10% (first option) and 20% (second option) are considered. Both options include by-pass and night cooling.

Figure 7 shows the number of overshoot hours in the multi-purpose area for the standard climatic year. The number of hours above 28°C can be reduced to approx. 15h. This is 15% better than the option with only a by-pass and night cooling. However, the standard climatic year provides only a general view, and does not say much about extreme situations, such as the summer of 2003.


Figure 7: number of overshoot hours for the various facade options in the multi-purpose area for a standard climatic year

Figure 8 shows the number of overshoot hours in the multi-purpose area for the summer of 2003.


Figure 8: number of overshot hours for the various façade options in the multi-purpose area for the summer of 2003

The number of overshoot hours above 28°C increases in this extreme situation to more than 50h, which again is not acceptable. However, the simulations suppose that the multi-purpose area if fully occupied during the pauses. This is a strong overestimation in the summer period, because in sunny and dry weather the children will play outside, and not in the multi-purpose area.

When simulations are run under the assumption that three quarter of the children play outside in the summer months, the number of overshoot hours above 28°C is reduced to 10h.

The various improvement options, both related to building physics and to technical installations, do have a negative impact on the heat demand. However, appropriate regulation can limit the impact of the by-pass and night cooling. Additional, simulations show that the lower U-value of the opaque parts in the multi-purpose area largely compensates the reduced solar gains. The optimised building has therefore an identical heat demand (+0.5% difference) compared to the original design.

6.     Conclusions

An economically sound energy concept is sought for the new kindergarten of the Royal Atheneum in Etterbeek (Belgium), with the passive house standard as ambition level. The total net heat demand must therefore be lower than 15kWh/m².year and the total primary energy consumption can not be higher than 120kWhp/m².year.

Because of the high occupancy ratio and therefore high internal heat gains, controlling the summer comfort is a crucial element in the optimisation of the energy concept.

A first optimisation consists in the implementation of a by-pass on the heat recovery and the optimal use of the thermal mass through the application of night cooling via the hygienic ventilation group. This reduces the number of overshoot hours above 28°C from 250h to 20h per year in a standard climatic year.

A second optimisation is implemented the building physics level. Opaque parts are integrated in the fully glazes façades of the multi-purpose area, reducing further the number of overshoot hours above 28°C to 15h per year.

In extreme situations such as the hot weather of the summer of 2003, both improvement measures are insufficient if the multi-purpose area is considered fully occupied. However, when supposing that three quarter of the children will play outside in the summer months, the comfort can be guaranteed by the implementation of both measures, even in extreme weather conditions.

The designed passive school has a total heat demand of 10 kWh/m².year and is therefore 90% more energy efficient than a theoretical regulatory compliant school (“E100” level measuring the maximum primary energy consumption compared to a reference level).

The energy study shows that the use of dynamic simulations to develop an energy concept provides the opportunity to optimise a building in such a way that a sufficient summer comfort is achieved without installation of active cooling. This represents important savings in economic and energetic terms.

ReferenCES

1.      http://www.passivhaus.org.uk/index.jsp?id=668 consulted 01/06/2010

2.      Achten K, Coppye W. (2007), Handleiding energiezuinige nieuwbouw voor lokale overheden, deel 3: achtergrond maatregelen. In opdracht van Vlaams Energie Agentschap (VEA)

3.      NBN EN 13779 (2004), Ventilation for non-residential buildings- Performance requirements for ventilation and room-conditioning systems.

4.      NBN EN ISO 7730 (2005), Ergonomics of the thermal environment - Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria

CR 1752 (1999), Ventilation for buildings. Design criteria for the indoor environment

Acronym of the case

Passive School Etterbeek

Lessons learnt

An economically sound energy concept is sought for the new kindergarten of the Royal Atheneum in Etterbeek (Belgium), with the passive house standard as ambition level. The total net heat demand must therefore be lower than 15kWh/m².year and the total primary energy consumption can not be higher than 120kWhp/m².year. Because of the high occupancy ratio and therefore high internal heat gains, controlling the summer comfort is a crucial element in the optimisation of the energy concept. A first optimisation consists in the implementation of a by-pass on the heat recovery and the optimal use of the thermal mass through the application of night cooling via the hygienic ventilation group. This reduces the number of overshoot hours above 28°C from 250h to 20h per year in a standard climatic year. A second optimisation is implemented the building physics level. Opaque parts are integrated in the fully glazes façades of the multi-purpose area, reducing further the number of overshoot hours above 28°C to 15h per year. In extreme situations such as the hot weather of the summer of 2003, both improvement measures are insufficient if the multi-purpose area is considered fully occupied. However, when supposing that three quarter of the children will play outside in the summer months, the comfort can be guaranteed by the implementation of both measures, even in extreme weather conditions. The designed passive school has a total heat demand of 10 kWh/m².year and is therefore 90% more energy efficient than a theoretical regulatory compliant school (“E100” level measuring the maximum primary energy consumption compared to a reference level). The energy study shows that the use of dynamic simulations to develop an energy concept provides the opportunity to optimise a building in such a way that a sufficient summer comfort is achieved without installation of active cooling. This represents important savings in economic and energetic terms.

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