Living Box – Prefabricated modular wood-house system - Part VI - KEP energy Magazine - KEP energy

Living Box – Prefabricated modular wood-house system

English translation from a paper published in 'Bollettino degli Ingegneri n. 6/2016 pp. 3-18'

Energy efficiency design

A building is a system that induces impacts at energy, economic and environmental level, both at local scale and at global scale. The control of these impacts is a part of the wider scenario of sustainable development, and can be implemented through the Zero Energy Building concept. Recently, this concept has been transposed from the research field to the regulation field, following the introduction of NZEB requirement as European Union (EU) binding target for the new buildings, starting from 2018/2020.

The NZEB requirement introduction is framed into the EU strategy, aimed to the energy dependence reduction and the greenhouse gases cut. Consequently, it must be understood as a measure useful to energy efficiency increase in the building sector.

The report “Energy 2020 – A strategy for competitive, sustainable and secure energy” indicates that the energy efficiency is the fulcrum of the EU strategy in 2020, finalized to decoupling the energy demand from the economic growth. The report “Action plan for Energy efficiency: realising the potential” highlights the relevant energy efficiency potential economically suitable. The energy efficiency targets achievement, under the condition of cost-efficiency, is one of the most relevant topics that cross the overall EU action in the energy and climate fields.

Starting from these considerations, the Living Box design has been oriented towards a high energy efficiency level, in order to make the design in agreement with the nZEB requirement, which in a short time will be introduced in the building design field. The system energy efficiency is due to the convergence of three design strategies: passive strategies, aimed to control the energy needs for heating and cooling, particularly referred to the building envelope; active strategies, aimed to control the energy needs for heating and cooling, particularly referred to the technical plants; installation of on-site renewable energy sources devices, aimed to minimize the overall Living Box impact.

Lacking specific targets towards which address the design, or rather not knowing the Living Box geographic location, the building energy efficiency has been assessed in three different climate areas: the city of Bolzano, taken as representative of the northern Italy climate; the city of Florence, taken as representative of the central Italy climate; the city of Reggio Calabria, taken as representative of the southern Italy climate. The different latitude of these cities, that is the different climate area, imposes an adaptation of the building envelope and technical plants components, anyway keeping the fundamental design layout. In the northern Living Box version, it has been pay attention to the building envelope thermal insulation, and have been boosted the solar gains through the glazed components, in order to cut the building heating need. Contrarywise, in the southern Living Box version, it has been pay attention to the ventilation rate, and have been hampered the solar gains through the glazed components, in order to minimize the building cooling need.

Building envelope design

In the following are described the main features of the building envelope components.

1 – Beared wall (“A-type” cell): the wall has 22 cm total thickness, and it is formed by a timber frame that supports a double layer of Oriented Strand Board (OSB) panels, having each one 2 cm thickness. In the interspace between the OSB panels is placed a thermal insulation layer, having 16 cm thickness. The wood fibre has been selected as thermal insulation material, in order to meet the thermal transmittance requirements and the thermal capacity requirements. The wood fibre is one of the least environmental impacting materials (as confirmed from its environmental footprint indices PEI, GWP, AP). The wall thermal property can be adapted to the specific climate area in which Living Box is built through the material density variation. The internal wall surface finish is formed by painted plasterboard panels;

2 – Bearing wall (“Type-B” cell): the wall has 32 cm total thickness, and it is formed by a cross laminated solid timber plate, having 10 cm thickness. It is coupled with a thermal insulation layer formed by wood fibre panels, having 10 cm thickness, placed outside. Both the external and internal wall surface finishes are formed by painted plasterboard panels.

3 – Roof covering: the slab has 32 cm total thickness, and it is formed by a timber frame that supports a double layer of Oriented Strand Board (OSB) panels, having each one 1,5 cm thickness. In the interspace between the OSB panels is placed a thermal insulation layer, having 25 cm thickness. The internal slab surface finish is formed by plasterboard panels, whilst the external one is formed by mineralized wood wool, having 4 cm thickness, (beyond the Building Integrated PhotoVoltaics (BIPV) devices, as explained in the paragraph 4.3).

4 – Slab on ground: the slab is similar to the roof covering, but the position of the surface finish is internally/externally inverted;

5 – Windows: the windows are formed by timber frame, having 68 x 80 mm section, and double stratified glazing, having 6/7-15-6/7 mm thickness and low-emissive coating. The thermal transmittance is equal to 1,5 W/m2K.

Some optional components have been developed in order to manage the solar radiation. They are installable on the building as a function of climate needs or architectural needs. Among them there are internal packable curtains and orientable laths brise-soleils. The Living Box is featured from a perimetric projection placed at the roof covering height. Its variable depth, ranging from 0,8 m to 1,2 m, has been sized for each sample city, checking that at the summer solstice the beam solar radiation part is totally shield, whilst at the winter solstice it is totally free, as shown in Figure 15 in reference to the city of Florence.

Technical plant design

In agreement with the current EU strategy, finalized to the energy end-uses electrification, and in order to simplify the technical plant layout, all the building energy demand for heating, cooling and DHW production is supplied from the electricity. The energy end-uses electrification is a EU strategy aimed to connect at the same time the demand side with the supply side both from power generation and from distributed generation, possibly implementing the smart-grid technologies. So, the Living Box sustainability has been understood as alignment with this strategy.

The air-conditioning plant is able to serve as heating, cooling, ventilation, humidification and dehumidification device. In this way it is possible the total control of the indoor thermo-hygrometric parameters and of the Indoor Air Quality (IAQ), giving to the users a high comfort level. The plant is fuelled from an air-air invertible heat pump, through a ducts grid placed at the intrados of the roof covering and some air diffusers. The kitchen and the bathroom are each one equipped with forced extraction fans. The ACS production is fuelled from an air-water heat pump, connected with a boiler having 150 l storage volume.

In the single living unit configuration (single-family detached dwelling), the various devices are placed in one of the two “B-type” cells. In the multiple living units configuration (multi-flats block building), the various devices are placed in one dedicated “A-type” cell, close to the staircase in order to facilitate the link with the flats.

Renewable energy sources devices

The nZEB target implies that a building is able to produce an energy amount almost equal to the one consumed. From a building-grid interaction point of view, it means that the Living Box should put in the grid an electricity amount almost equal to the taken one. Consequently, a photovoltaic plant has been calculated, placed over the building roof (“A-type” cell). As a consequence of the roof planarity the thin-film Cd-Te (Cadmium Tellorum) modules have been detected as suitable technology, because they are able to produce electricity also in non-optimal position. The modules are coupled with aluminium lath, spliced in opera, in order to obtain a continuous layer, which serve both as roof and as energy producer.

The photovoltaic devices placed over the Living Box roof are architectonically integrated in the roof covering (BIPV configuration). The annual electricity production estimation has been carried out through the “PVGIS”, which is a calculator developed from the Joint Research Centre of the European Commission, published at the following URL: http://re.jrc.ec.europa.eu/pvgis/.

The photovoltaic technology is the one way technically suitable for the on-site electricity production, at building scale. The sustainability has been understood as “distributed generation value”, and not as photovoltaic devices Life-Cycle.

The photovoltaic plant is composed from 36 modules, having each 87,5 Wp. The total installed power capacity is equal to 3.150 Wp. From the calculation, it results that the annual electricity production is about 3.710 kWh in the city of Bolzano, 3.860 kWh in the city of Florence and 4.610 kWh in the city of Reggio Calabria. Over the Living Box roof (“B-type” cell) are also installed some solar thermal devices, used as DHW production, sized in order to fuel an approximately 60% average share of the annual DHW need.

Building energy assessment

The building energy assessment has been carried out in compliance with the system standard UNI EN ISO 13790:2008, and in compliance with the related components standards. The calculation methodology is semi-steady. The calculation time-step is one month. The assessment has been carried out both through spreadsheets and on-line calculators. The weather data of the three sample cities are derived from UNI 10349:1994.

The energy assessment outcomes confirmed that the Living Box is able to achieve a high energy efficiency level, that is the energy performance established as target in order to meet the nZEB requirement.

The building energy balance is positive in each of the three sample cities, or rather the Living Box is beyond the nZEB concept, becoming an Energy Positive Building. However, it should be considered that the analysed configuration (single living unit) is particularly favourable, because the ratio between solar surface and climatized area is 1:1. From this point of view, the combination among a series of living units implies an inevitable building energy performance worsening, and consequently a reduction of the overall building electric need supplied from renewable energy sources.