3.6 Sensitivity analysis
3.6.3 Sensitivity to heat demand
Finally, we investigate the sensitivity of the system to SH and DHW demands (absolute value and respective shares, as well as SH distribution temperature), in particular concerning the applicability of the analysed system for building retrofit. Besides the previously analysed base case (Base’), with a 68.5 kWh/m2 total heat demand (70% DHW / 30%
SH, with a low 30°C distribution at 0°C outdoor), we will consider the following heat demand scenarios, which all re-late to multi-family buildings situated in Geneva (Table 3:2): (i) the same low-energy building as Base’ (i.e. the same low SH demand) but with a reduced DHW demand (Base’ / low DHW), resulting in a 49.1 kWh/m2 total heat demand (72% of Base’ ); (ii) a retrofitted building from the 60’s with an excellent envelope (Retrofit / best case), with a total heat demand similar to Base’ (106%) but approximately even shares of SH and DHW, as well as an intermediate SH distribution temperature (40° at 0°C), as monitored by the authors in a currently running case study (Tornare et al., 2016); (iii) a less performant retrofitted building from the 60’s (Retrofit / intermediate), with a 97.6 kWh/m2 total heat demand (142% of Base’ ), as observed in a previously monitored case study (Mermoud et al., 2012); the SH distribu-tion temperature is set at the same intermediate level (40° at 0°C); (iv) the same less performant retrofitted building (Retrofit / reference), with a high SH distribution temperature (50°C at 0°) as actually monitored in the case study (Mermoud et al., 2012); (v) the same building before retrofit (No Retrofit), with a 110.0 kWh/m2 total heat demand (202% of Base’) and a high distribution temperature (50°C at 0°), as observed in the same case study.
For all of these scenarios, sizing of the system components occurs as follows (Table 3:2). The nominal HP capacity (at 0°C evaporator input / 35°C condenser output) is adjusted so as to match the maximal hourly load of combined SH and DHW demand. Correspondingly to Base’, the solar collector area is set to 3.32 m2 per kW of HP capacity. As a consequence of the variation in heat demand and maximal load, the specific solar collector area hence varies between 0.09 and 0.23 m2 per m2 heated area. Finally, the SH and DHW storage capacities are proportional to the respective maximal hourly loads, accordingly to the values of the Base’ scenario (SH: 150 L/kW; DHW: 50 L/kW for the lower part of the storage + 40 L/kW for the upper part of the storage). For all these scenarios, Table 3:2 summarizes the heat demand and distribution values, as well as the component sizing values. As a complement, the hourly SH and DHW demand load curves are given in Annex G. Simulation results are presented in Figure 3:11, detailed values in Annex B.
Base' Base' low DHW
Retrofit best case
Retrofit intermed.
Retrofit
reference No-retrofit
Heat Qdhw kWh/m2 47.7 28.3 34.6 28.3 28.3 28.3
demand Qsh kWh/m2 20.8 20.8 37.8 69.3 69.3 110
Qdem kWh/m2 68.5 49.1 72.4 97.6 97.6 138.3
Pdhw.max W/m2 31.4 18.6 22.8 18.6 18.6 18.6
Psh.max W/m2 11.1 11.1 18.1 34.3 34.3 54.4
Pdem.max W/m2 38.7 25.8 35.0 49.8 49.8 69.8
Psh.0°C W/m2 6.8 6.8 11.5 21.1 21.1 33.5
Tsh.off °C 15 15 17 17 17 17
Heat Tsh.0°C °C 30 30 40 40 50 50
distribution Tsh.15°C °C 28 28 30 30 30 30
Components Php W/m2 37.6 25.8 35.0 49.8 49.8 69.8
Asol m2/m2 0.125 0.086 0.116 0.166 0.166 0.232
Vst L/m2 4.53 3.34 4.75 6.73 6.73 9.66
Table 3:2 Sensitivity to building heat demand: scenario parameters.
Towards a SPF of 5? Numerical sensitivity analysis for various building demands
Figure 3:11 System performance, sensitivity to building heat demand (in parentheses: SH distribution temperature – DHW/SH ratio).
Generally speaking, the following observations can be made. With the adopted sizing factor of 3.32 m2 per kW of HP capacity (i.e. per kW of maximal heat demand load), the simulated scenarios present similar shares of input energy flows. One observes in particular a very limited use of direct electric heating (Edir in the range of 0.7 – 1.1 kWh/m2, i.e.
in all cases 1% of the total annual heat demand). The total electricity demand Etot remains in all cases in the range of 24 to 33% of the total annual heat demand Qdem. However, given the variation of Qdem , Etot presents important varia-tions in terms of its absolute value: from 16.0 kWh/m2 for the base case, to 30.7 kWh/m2 for the considered reference building retrofit, respectively 45.2 kWh/m2 for the non-retrofitted building. Correspondingly, the specific solar collec-tor area (expressed in terms of m2 per m2 heated area) varies from 0.13 for the base case, to 0.17 for the reference building retrofit, respectively 0.23 for the non-retrofitted building. Besides the issues of specific electricity demand and system cost, the available roof area may hence become a strongly limiting factor for this kind of system, especially in the case of existing buildings. Ultimately, we observe the following variation of the SPFhp in function of the SH dis-tribution temperature: 3.5 for both scenarios at 30°C (0°C outdoor), 3.3 for both scenarios at 40°C, 2.9 for both sce-narios at 50°C. For these 3 pairs, the correspondent SPFsys is around 4.2, 3.7 and 3.1 respectively. Within these pairs,
0
Towards a SPF of 5? Numerical sensitivity analysis for various building demands
Finally, for all six heat demand scenarios, and similar to the analysis in section 3.6.1, we also investigate the sensitivity of the system performance to the sizing of the solar collectors (m2 per kW of HP capacity, with an HP designed for 100% coverage of the demand). Results are presented in Figure 3:12.
Figure 3:12 System performance (top and middle) and specific solar collector area (m2 per m2 heated area) (bottom) as a function of specific solar collector area (m2 per kW of HP capacity, with HPsized for 100% demand coverage).
As already pointed out, the SPFsys depends on the SH distribution set point (Figure 3:12, top). In all cases, and for Ge-neva’s weather conditions, a sizing factor of 3 m2 solar collector per kW of HP capacity (HP sized for 100% coverage of heat demand) seems to be a good compromise between system size and system performance. Below 3 m2/kW the drop in collector temperature results in deterioration of the HP performance and eventually to direct electric heating due to HP failure, resulting in severe deterioration of the SPFsys. Above this threshold, doubling of the collector area leads to a 10-20% increase of SPFsys.
2.0 2.5 3.0 3.5 4.0 4.5 5.0
0 1 2 3 4 5 6 7
SPFsys
0 10 20 30 40 50 60
0 1 2 3 4 5 6 7
Total electricity (kWh/m2)
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45
0 1 2 3 4 5 6 7
Solar collector area (m2sol/m2)
Solar collector area (m2sol/kWhp) Base' (30°C - 70/30)
Base' / low DHW (30°C - 60/40) Retrofit / best case (40°C - 50/50) Retrofit / intermediate (40°C - 30/70) Retrofit / reference (50°C - 30/70) No-retrofit (50°C - 20/80)
Towards a SPF of 5? Numerical sensitivity analysis for various building demands
The associated electricity consumption Etot depends strongly on the building heat demand (Figure 3:12, middle). Dou-bling of the collector area leads to relatively modest electricity savings of 2 - 4 kWh/m2, which might not be worth-while in regard of the additional investment costs.
Similarly, the collector area depends strongly on the building heat demand (Figure 3:12, bottom). For multifamily buildings which do not benefit from a low SH and DHW demand (up to around 70 kWh/m2), implementation of such a system may be restricted not only by the associated electricity consumption, but also by the available roof area or the related investment costs.
In conclusion, the following best case scenarios can be highlighted. With around 0.10 m2 solar collectors per m2 heat-ed area, a newly constructheat-ed building with low SH and DHW demand (around 20 + 30 = 50 kWh/m2) and a low SH distribution temperature (30°C), only requires 12 kWh/m2 of electricity for covering of the entire heat demand (SPFsys
= 4.0). Similar results are obtained for buildings with a SH + DHW demand of around 70 kWh/m2, as is the case of a new building with a higher DHW demand (for which Etot = 17 kWh/m2 and SPFsys = 4.1) and of best case retrofit with a SH distribution at 40°C (for which Etot = 20 kWh/m2 and SPFsys = 3.7). For buildings with a higher SH demand and/or a higher SH distribution temperature, a lower SPFsys and a higher electricity consumption have in all cases to be ex-pected.
Finally, a SPFsys of 5 and an associated Etot of 10 – 15 kWh/m2 could potentially be achieved, but only in newly con-structed buildings with a high efficient envelope, a low SH distribution temperature, and with a collector area of at least 0.20 - 0.25 m2 per m2 heated area (depending on the DHW demand). However, the additional collector area may not be worthwhile given the rather small associated electricity saving, not to mention that such a collector area would not fit on buildings with more than 4 storeys.
Towards a SPF of 5? Numerical sensitivity analysis for various building demands