Energy

5.2.1 Energy concept

The south façade has a big window ratio of 50 per cent. Spacious rooms to the south let solar gains enter deep into the building. The other façades have only small windows. The building has the following technical features:

• highly insulated air-tight building envelope: U-value(average): 0.26 W/m2K; pressuration test (50 Pa): 0.6 h-1;

• minimized thermal bridges;

• energy efficient windows: U-value (including frame): 0.9-1.07 W/m2K;

• efficient ventilation with heat recovery (80 per cent efficiency) and ground preheating;

• solar collectors for DHW and heating: 40.5 m2;

• wood pellet heating system: 9-25kW, 70 per cent coverage;

• photovoltaics (PV) on the roof, grid-connected single crystal silicon solar cells: 1.44 kWp; and

• efficient appliances.

The specific space heating demand is 11.6 kWh/m2a. Demand for DHW amounts to 15.9 kWh/m2a. The primary energy demand for the system is 19.1 kWh/m2a. These values refer to the heated net living area.

Figure 5.2.1 Ventilation

Figure 5.2.2 Control system

Figure 5.2.3 Energy supply

Figure 5.2.1 Ventilation

Figure 5.2.2 Control system

Figure 5.2.3 Energy supply

5.2.2 Building envelope

Due to the compact building volume and the strict structural and thermal separation of heated and unheated zones, the insulation layer could be applied continuously, thus minimizing thermal bridges.

The prefabricated back-vented exterior walls achieve an excellent U-value of 0.13 W/m2K, with a construction thickness of less than 40 cm.

For fire safety, the northern façade is covered with fibre-cement panels. The other façades are faced with vertical white-stained wooden siding.

The windows with a Heat Mirror TC 88 inner film are filled with krypton. The U-value of the glass is 0.7 W/m2K, the g-value is 0.5. Depending on the window size, the U-value of the entire window with a wooden frame varies between 0.9-1.07 W/m2K.

Source: Beda Bossard, BARBOS Büro für Baubiologie Bauökologie and Energie

Figure 5.2.5 Roof

Figure 5.2.8 Floor to cellar

Source: Beda Bossard, BARBOS Büro für Baubiologie Bauökologie and Energie

Figure 5.2.6 Wall

Source: Beda Bossard, BARBOS Büro für Baubiologie Bauökologie and Energie

Figure 5.2.7 Terrace

Source: Beda Bossard, BARBOS Büro für Baubiologie Bauökologie and Energie

Figure 5.2.8 Floor to cellar

Notes: U-values (W/m2K): 1: exterior walls; 2: roof; 3: terrace; 4: floor to cellar; 5: window (glass);

6: window (including frame), mean value; 7: average U-value building envelope. Source: Beda Bossard, BARBOS Büro für Baubiologie Bauökologie and Energie

Figure 5.2.9 U-values

Source: Beda Bossard, BARBOS Büro für Baubiologie Bauökologie and Energie

Figure 5.2.10 Supply air with heater coils (top right) and absorbing ducts (middle right)

Source: Beda Bossard, BARBOS Büro für Baubiologie Bauökologie and Energie

Figure 5.2.11 Solar collectors and PV Installation on the roof

Source: Beda Bossard, BARBOS Büro für Baubiologie Bauökologie and Energie

Figure 5.2.11 Solar collectors and PV Installation on the roof

5.2.3 Roof (from outside to inside)

• Humus substrate (extensive planting);

• protective and drainage felt;

• roofing membrane;

• moisture and wind barrier;

5.2.4 Exterior walls (from inside to outside)

• moisture and wind barrier;

5.2.5 Terrace

• moisture and wind barrier;

• timber/concrete structure = 12.0 cm;

5.2.6 Floor to cellar

• moisture and wind barrier;

5.2.7 Ventilation

The supply air is preheated in polyethylene pipes buried in the ground. The pipes, with a diameter of 200 mm and a length of 4 m x 25 m, are buried at a depth of 1.8 m. The ground preheating prevents the heat exchanger from freezing during the winter.

The air is further tempered by heat recovered from the return air via a cross counter-flow heat exchanger with an efficiency of 80 per cent. The air is then transported to the apartments via shafts in the party walls. Each apartment has a separate supply air and return air trunk.

The supply air is further tempered to a maximum of 46°C by heater coils and supplied through the floor and the wall outlets. The return air is exhausted in the kitchen and bathrooms, and is led outdoors via the heat recovery system.

During the summer, the heat recovery system can be left out by using a bypass.

5.2.8 Heat production and distribution

The heating energy for the Wechsel apartment building is produced by a wood pellet heating system and by solar collectors.

The wood pellets heating power can be varied between 9 kW and 25 kW. The annual heat production from the wood pellets amounts to 25.5 kWh/m2a. This comes up to 70 per cent of the entire heating energy demand of 36.5 kWh/m2a. This is the sum of 14.5 kWh/m2a for space heating and 22 kWh/m2a for DHW heating.

The 18 solar collectors on the roof are arranged in two arrays and have a tilt angle of 45°. The total absorber area amounts to 40.5 m2. The solar fraction is 30 per cent.

The solar collectors and wood pellet heating system both supply a combi-boiler, whereas the collectors have priority. The wood pellet heating system only turns on when the collectors cannot supply the demand.

The hot water in the boiler provides the apartments' DHW and is used to heat up the supply air to a maximum of 46°C through a water/air heat exchanger.

The heat is distributed by the fresh air supply. Radiators in the bathrooms provide additional comfort.

5.2.9 Electricity

The Wechsel apartment building uses 15.7 kWh/m2a for household electricity and 3.1 kWh/m2a for electricity for technical systems (pumps, lift and control system).

The 12 m2 roof-mounted PV system is grid connected. It uses single crystal silicon solar cells. The total nominal output is 1.44 kWp. The eight series-connected modules are oriented directly to the south and have a tilt angle of 30°. During the first year, the installations yielded 1694 kWh. The PV electricity is used to dehumidify the unheated cellar.

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