4.6 Simulation results
4.6.2 Effect of limited roof and ground area
From this point onward, limitations intrinsic to the resource type will be taken into account. At a building scale, air, lake, river and groundwater can be considered as unlimited heat sources (see section 4.2.1 ). However, geothermal and solar are space extensive, i.e. the amount of heat supplied by the heat source to the HP is subject to available roof or ground area in the premises of the building.
In order to take into account this limitation, we consider 2 cases:
An available roof area of 0.2 m2 per m2 heated floor area, corresponding to a “low-rise” multifamily building in Geneva (4 storeys, under hypothesis of an 80% ratio between the available roof area, in m2, and the storey specific heated area, in m2 per storey).
An available roof area of 0.1 m2 per m2 heated floor area, which corresponds to a “high-rise” building in Ge-neva (8 storeys);
As a reference, Geneva multifamily buildings have in average 5.8 storeys (+/-2.6 standard deviation), against 4.1 sto-reys (+/- 1.6) in Switzerland (according to a recently developed geo-dependent heat demand model of the Swiss build-ing stock, Schneider et al., 2016).
For simplification purposes, the available ground area for the boreholes is considered to be equal to the available roof area (0.2 or 0.1 m2/m2). This simplification means that we consider the same 80% ratio between available ground area and the storey specific heated area.
With this limitation, and for each building type, solar and geothermal systems are downsized as follows:
For the solar HP system, the collector area Asol is defined as the minimum value between: a) the previously defined Asol,0 , in absence of roof limitation (Table 4:6) and b) the new limited roof area (0.2 m2/m2 or 0.1 m2/m2)
For the geothermal HP system, the ground area Ageo is defined as the minimum value between: a) the previ-ously defined Ageo,0 , in absence of ground limitation (Table 4:6) and b) the new limited ground area (0.2 m2/m2 or 0.1 m2/m2). Note that, in this study, the geothermal HP system is also limited to the 250 m borehole length defined in section 4.4.2.
Potential and constraints of available heat sources in relation to various building demands.
Consequently, the solar HP is downsized according to eq. (22), and the geothermal HP according to eq. (23):
𝑃ℎ𝑝 = 𝐴𝑠𝑜𝑙
𝐴𝑠𝑜𝑙,0𝑃ℎ𝑝,0 (22)
𝑃ℎ𝑝 = 𝐴𝑔𝑒𝑜
𝐴𝑔𝑒𝑜,0𝑃ℎ𝑝,0 (23)
As for the storage, the SH and DHW storage capacities are the same as in section 4.4.2 for they depend of the heat demand, which remains unchanged.
Direct electric heating is assumed to cover all auxiliary heat needs, in particular when the downsized HP is not able to cover 100% of the demand.
Table 4:7 summarizes the component sizing values, in relation with the building heat demand values.
Limit*
(m2/m2) New New low DHW
Retrofit best case
Retrofit intermediate
Retrofit
reference No-retrofit
Solar Pnom,hp N/A 37.6 25.8 35.0 49.8 49.8 69.8
(W/ m2) 0.2 37.6 25.8 35.0 49.8 49.8 66.8
0.1 33.3 25.8 33.3 33.3 33.3 33.3
Asol N/A 0.113 0.077 0.105 0.149 0.149 0.209
(m2/ m2) 0.2 0.113 0.077 0.105 0.149 0.149 0.200
0.1 0.100 0.077 0.100 0.100 0.100 0.100
Geothermal Pnom,hp N/A 37.6 25.8 35.0 49.8 49.8 69.8
(W/ m2) 0.2 37.6 25.8 35.0 28.5 28.5 24.0
0.1 24.2 22.2 18.0 14.3 14.3 12.0
Ageo N/A 0.078 0.078 0.117 0.155 0.155 0.194
(m2/m2) 0.2 0.078 0.078 0.117 0.155 0.155 0.194
0.1 0.078 0.078 0.100 0.100 0.100 0.100
Table 4:7 Sizing of system components for the different building types, taking into account the roof/ground area limitation Pnom,hp HP nominal capacity (at 0°C evaporator input / 35 °C condenser output for water HP)
Asol solar collector area, considering roof surface availability
Ageo geothermal footprint, considering ground surface availability and a borehole length of 250m Limit* available roof or ground surface in m2 per m2 of heated floor area of the building
Potential and constraints of available heat sources in relation to various building demands.
SPFsys and yearly Esys consumption are presented in Figure 4:12 for a low-rise building (0.2 m2/m2). Obviously, when comparing this figure with Figure 4:11, only solar and geothermal values are subject to change.
For geothermal, no downsizing is required, so SPFsys and Esys are the same as without ground limitation. For solar both the SPFsys and Esys are almost unchanged. In fact, only the No-retrofit building type suffers a very slight decrease of solar area (from 0.21 to 0.20 m2/m2), with insignificant impact.
As said previously, all other resources are not affected by the limited roof and ground area.
For the high-rise building (0.1 m2/m2), the results are presented in Figure 4:13. Both for geothermal and solar, down-sizing is noticeable for the Retrofit intermediate building (Esys increase of 6% and 7%) and for the Retrofit reference building (Esys increase of 11% and 6%). The downsizing effect is more important for the No-retrofit building (Esys in-crease of 43% and 25%). Note that the inin-crease ratios of Esys are below the downsizing ratios of Ageo (35%, 35% and 48%) and Asol (33%, 33% and 52%). This results from the components being sized for the standard weather heat de-mand, which is higher than the 2010 heat demand (see section 4.3), meaning that the components are slightly over-sized for 2010 building demands, which in turn explains the offset between the downsizing ratios of Asol and Ageo and the increase ratios of Esys.
As far as geothermal HP systems are concerned, note that preceding results are linked to the previously defined as-sumptions: i) an available ground area identical to the available roof area. ii) boreholes with a 250 m length and a 6 m inter-axial distance, leading to Ageo,0 of 2.1 – 3.3 m2 per kW HP. In this respect, note that boreholes with water instead of antifreeze lead to twice higher sizing values (4.1 – 6.1 m2 per kW HP), so that downsizing has disastrous effects in the low-rise No-retrofit building, as well as in the Retrofit intermediate, Retrofit reference and No-retrofit high-rise buildings (see Annex J).
Finally, in the case of the No-retrofit building, with HP downsizing in the order of 50%, Edir reaches values above 15 kWh/m2, against 1 – 4 kWh/m2 for the Retrofit intermediate and Retrofit reference buildings (see Annex K). In this regard, systems which are not sized to cover 100% of the heat demand usually have a backup heater (ex. gas boiler) instead of direct electric heating. For such bivalent heating systems (not analysed in this study), the SPFsys makes no more sense and alternative performance indicators would have to be used (renewable heat fraction, CO2 emissions,
…).
Potential and constraints of available heat sources in relation to various building demands.
Figure 4:12 System performance with a limited roof/ground area of 0.2 m2 per m2 of heated area
(low-rise building).
Air Geothermal Lake River Groundwater Solar
0.0
New New Retrofit Retrofit Retrofit No-retrofit
Esys[kWh/m2]
Potential and constraints of available heat sources in relation to various building demands.