4. Results and discussion
4.1 Multi-storey residential building .1 Heating and Cooling loads
The results for Perth Weather are represented in Table 1 below:
Table 1: Comparison of Thermal Performance of Stella B17 in Perth weather
Unexpectedly, the modular construction is not performing as well as the traditional one despite the thermal insulation in the external walls and the double-glazed windows. These elements assist the thermal performance in winter but something is missing in summer. To understand what can ex- plain this difference in cooling load, the same two buildings were simulated in different climate than the Class 4 “warm temperate” of Perth (Building Code of Australia (BCA)). They were simu- lated in Port Hedland, Class 1 “Hot humid summer, warm winter” and in Melbourne, Class 6 “Mild temperate”. The results are represented in Table 2 and Table 3 below:
MODULAR TRADITIONAL
[MWh] [kWh/m2] [MWh] [kWh/m2]
Heating 26.59 4.94 39.78 7.39
Cooling 340.59 63.28 268.20 55.44
TOTAL 68.22 62.83
Table 2: Thermal performance of Stella B17 in Port Hedland compared to Perth
MODULAR PERTH PORT HEDLAND
[MWh] [kWh/m2] [MWh] [kWh/m2]
Heating 26.59 4.94 0 0
Cooling 340.59 63.28 1113.22 206.83
TOTAL 68.22 206.83
TRADITIONAL PERTH PORT HEDLAND [MWh] [kWh/m2] [MWh] [kWh/m2]
Heating 39.78 7.39 0 0
Cooling 298.39 55.44 1175.65 218.43
TOTAL 62.83 218.43
Table 3: Thermal performance of Stella B17 in Melbourne compared to Perth
MODULAR PERTH MELBOURNE
[MWh] [kWh/m2] [MWh] [kWh/m2] Heating 26.59 4.94 182.84 33.97 Cooling 340.59 63.28 119.16 22.14
TOTAL 68.22 56.11
TRADITIONAL PERTH MELBOURNE [MWh] [kWh/m2] [MWh] [kWh/m2] Heating 39.78 7.39 268.20 49.83 Cooling 298.39 55.44 83.53 15.52
TOTAL 62.83 65.35
In both cases, the modular version performs better than the traditional. Heating being not required in Port Hedland the result is not significant for the comparison, but this time the cooling load is lower for the modular building. On the contrary, the thermal performance in Melbourne follows the same trend as in Perth (modular performing better than traditional in winter but worse in summer) even if the overall performance is, this time, to the advantage of the modular building.
These results lead us to analyse the particularity of the summer conditions in these three locations, and the two major differences were the humidity and the variance between night and day tem- peratures. The latter made us think that the lack of thermal inertia in the modular building com- pared to the all-concrete (walls and floors) traditional equivalent didn’t allow to make the most of the lower nocturnal temperatures in Perth and Melbourne.
This hypothesis was verified by simulating internal walls made of bricks and finding 8% reduction in cooling load. The approach of using bricks in the modular building to increase thermal mass is technically feasible but not workable in terms of the construction process, cost and transport. The second factor that influences cooling energy demand is solar gain, which is the amount of heat that penetrates inside the building through the windows. Reducing solar gain can easily be achieved by tinting the windows as shown in Table 4 below. In summer, solar rays penetrate the building the most when the sun is the lowest in the sky, at sunrise and sunset, through the east facing and the west facing sides of the building.
Table 4: Thermal Performance of the modular building with East and West windows tinted
MODULAR MODULAR
TEST 4 TRADITIONAL
[MWh] [kWh/m2] [MWh] [kWh/m2] [MWh] [kWh/m2]
Heating 26.59 4.94 33.37 6.20 39.78 7.39
Cooling 340.59 63.28 280.36 52.09 298.39 55.44
TOTAL 68.22 58.29 62.83
This low cost and easy fix solution improved drastically the performance of the building in summer though it reduced slightly its performance in winter.
4.1.2 Carbon footprint
The Figure 1 below shows the carbon footprint, expressed as Global Warming Potential (GWP) of the modular building compared to the traditional equivalent:
The GHG from the building elements (foundations, envelop, fit-out, appliances, building services…) happen only once, at the construction stage, and their carbon footprint is broken down through-out the life of the building. However, the GHG from the production of the operating energy and water used during the life of the building happen every year. This graph reveals that over the life of a building, the impact of the operating energy, dominated by heating and cooling, is predominant in the whole carbon footprint of the building.
0 500 1000 1500 2000 2500 3000 3500 4000 4500
GWP [kgCO2-eq/Occ/yr]
MODULAR TRADITIONAL
Figure 1: GWP over the whole life of Stella B17 Modular vs Traditional equivalent
To better visualise the differences in construction between modular and traditional, we have re- moved from the graph the operating energy and water, as shown in Figure 2 below:
Looking at all the materials and services involved in fabricating, transporting and erecting these buildings, the modular building has a carbon footprint 6% more than it’s equivalent in traditional construction. To better understand this situation, the next graph Figure 3 shows the same compar- ison but this time focusing on the elements of the buildings that are different between the two construction methods:
In this graph, we have removed elements similar to the two buidings and we see that steel frame, plywood and bathroom structure; all three specific to the modular construction, account for 30% of these selected materials and services. The modules’ structure is made of steel frame and steel sheets for the floors (aggregated under “Steel frame” in the graphs). Moreover, this modular build- ing uses quite a lot of concrete for the ground floor and the two cores. Consequently, the carbon footprint of the structural elements of the modular building is higher that the concrete used in the traditional equivalent.
In a second order comes the additional carbon footprint of the double-gazing and the insulation in the walls, but these elements participate to reducing the operating energy which, as we saw, is more important than the carbon footprint of construction materials. Finally, the transport of the modules, from Melbourne to Perth for that particular project, also add to the overall deficit of the modular building.
-20204060800 100120
GWP [kgCO2-eq/Occ/yr]
Traditional Modular
Figure 2 Stella B17 Modular vs Traditional - Carbon Footprint of Selected Material and Services 200
4060 10080 120
Serv. Equip Fittings… Doors &… Carpet Steel frame Wall finish Concrete… Ceiling Plywood Concrete… Transport of… People & Equipm Insulation (wall) Bathroom finish Bathroom… Insulation (slabs) Floor Tiles Roof covering Bricks Mortar Formworks +…
GWP [kgCO2-eq/Occ/yr]
Traditional Modular
Figure 1 Stella B17 Modular vs Traditional - Carbon Footprint of Material and Services
For this particular project and without changing drastically the original design, one of the low hang- ing fruits would be to replace carbon intensive materials with a mainstream low carbon equivalent.
For example, replacing the carpets with a timber floor.
4.2 The detached houses