Energy self-sufficient households with photovoltaics and electric vehicles are feasible in temperate climate

Fig 7. Required energy in alternative cases and produced energy in self-sufficient cases in 2050. Produced energy is provided with a rooftop PV efficiency of 27.2%. https://doi.org/10.1371/journal.pone.0227368.g007 Fig 8. Total costs per person (4 people per building in SFBs, 20 people per building in MFBs). Self-sufficient cases show the range for PV efficiency varying between 22.1% and 27.2%. For sensitivity of self-sufficient cases to costs (including cost of the PV system), see Fig 11 below. Assuming battery cost of 265 CHF per kWh with a fixed additional 2,740 CHF for battery system setup. Alternative cases show costs assuming fuel, electricity, and gas costs ranging between + and– 20% of the assumptions in Table 4. The grid-based cases show range of costs assuming a selling price of electricity to the market of 0.16 CHF/kWh (scarce scenario) and 0 CHF/ kWh (abundant scenario), and a purchasing price from the market of 0.31 CHF/kWh (scarce scenario) and 0.21 CHF/kWh (abundant scenario). https://doi.org/10.1371/journal.pone.0227368.g008 Energy self-sufficient households with photovoltaics and electric vehicles are feasible in temperate climate PLOS ONE | https://doi.org/10.1371/journal.pone.0227368 March 4, 2020 16 / 25

differently than assumed here, the most economically attractive option is full electrification while remaining connected to the grid. Grid-based cases with batteries are marginally, but not substantially more expensive; this is because we assume electricity can always be sold back to the local grid operator, so the battery brings little economic advantage. In reality, batteries would likely be desirable for distribution network balancing purposes. Moving from fully elec- trified but grid-connected houses to fully self-sufficient houses carries a major cost burden. Again, the cost of self-sufficiency per person is significantly lower in multi-family houses due to more efficient use of the installed infrastructure. In Fig 8, self-sufficient cases vary only mar- ginally, based on PV efficiency varying from of 22.1% to 27.2%. Alternative cases have a larger range as the prices for energy were changed more drastically. The grid-based scenarios vary based on the scarce and abundant electricity supply scenarios. While generally, a conventional electricity demand and a driving behavior in a rural context result in the highest costs, the cost differences between different self-sufficient cases are comparatively narrow. As we will see fur- ther below in the sensitivity analyses, other factors such as varying discount rate have an even higher impact on overall cost, but this impact is the same across the separate scenarios. Note that we assume no explicit subsidies whatsoever, in neither the self-sufficient nor

alternative cases, with the exception of implicit subsidies embedded in fossil fuel costs [79]. Given the share of costs we see, the introduction of subsidies on storage technologies and/or heat pumps would appear most effective if a government wants to actively encourage self-suffi- cient households. On the other hand, terminating implicit fossil fuel subsidies or introducing levies on fossils fuels would decrease the relative attractiveness of the non-renewable cases (see Fig 6C). Sensitivity analyses To examine the sensitivity of results to a range of realistic solar irradiance profiles, we test all cases for each year in the period from 2000 to 2015 with daily data for the location of Worb, Bern, which we consider representative for a site in the Alpine foothills of Switzerland. Results show that the highest electricity production was in 2003 while the worst year was 2013, but in all cases, the overall energy balance is still positive (including full hydrogen production for self-sufficiency). Thus, we consider the results robust to year-on-year variability in PV system productivity. Exemplary for the SNPU case, and for all years 2000 to 2015, the variability of daily PV generation and the variability of weekly (seven-daily) net electricity balance consider- ing electricity demand are shown in Fig 9 (see the optimized cases in S4 Data). Results regarding the sensitivity of net energy balance to changing a range of technical assumptions are shown in Fig 10, assuming a rooftop PV efficiency of 27.2%, and no hydrogen

production (i.e., fulfilling design parameters for net zero-energy buildings). This efficiency was chosen so that all net energy balances are positive for comparison. It is clear that irradiance and available area have the largest impact on the net energy balance (adjustments 1, 2, 12, and 13). Changes to the demand profile, including demand changes driven by climate change, have a lesser impact (adjustments 3, 4, 5, 11), or comparatively insignificant impact (adjust- ments 6, 7, 8, 9, 10). Retrofitted buildings deviate more than new buildings from the baseline assumptions because they are more affected by changes as they have a smaller available PV area and a slightly higher energy demand. Results of the cost sensitivity analysis are depicted in Fig 11. The main sensitivity is to the cost of the storage components: batteries, fuel cell, electrolyzer and storage tank (adjustments 1, 2, 3, 5, 7 in the figure). The strong effect of battery costs is partly due to our assumption that they must be replaced after 15 years in operation, i.e. before end of the total system lifetime of 20 years, which increases expenditures. The higher storage tank price (adjustment 7) of 880 Energy self-sufficient households with photovoltaics and electric vehicles are feasible in temperate climate PLOS ONE | https://doi.org/10.1371/journal.pone.0227368 March 4, 2020 17 / 25

CHF/m3 instead of 600 CHF/m3 is based on a business-as-usual scenario by Marchenko and Solomin (71). A 50% higher PV cost only lets costs increase by 3.4–4.4% in SFB and 4.9–6.7% in MFB cases. In contrast, the discount rate has an effect ranging from –5.3% to +16.6% across all cases when varied from 0% to 6% (from its 4% base case value, adjustments 4 and 8 in the figure). We do not investigate lower than base case costs for the storage and PV components as the base case already assumes cost reductions from the current cost of these components. However, we investigate heat pumps at two thirds of the base case cost, finding that they would reduce overall costs by about 10%. Taken together, the technical and cost sensitivity analyses paint a clear picture. Technical feasibility, i.e. the net energy balance, is most affected by the ability to collect incoming sunlight as electricity, which in turn is influenced by available area and PV system performance. In contrast, economic feasibility, i.e. total system cost, is most affected by the cost of the various storage components of the system. Fig 9. Variability of generation and net electricity balance the years 2000 to 2015 based on data for Worb, Bern, for the SNPU case a. Daily photovoltaic electricity generation. b. Seven-daily average net electricity balance. https://doi.org/10.1371/journal.pone.0227368.g009 Energy self-sufficient households with photovoltaics and electric vehicles are feasible in temperate climate PLOS ONE | https://doi.org/10.1371/journal.pone.0227368 March 4, 2020 18 / 25

Discussion and conclusion We show that under a wide range of conditions, assuming full electrification of heating and transport, both single-family and multi-family buildings can be net zero energy buildings. In addition, a surprising number of cases are also able to function as true zero energy buildings and achieve full self-sufficiency. We also show that grid-connected and fully electrified cases remain substantially less expensive and are almost always less expensive than corresponding cases that retain the use of fossil fuels for heating and transportation. Thus, there is a clear case for full electrification, but there is a substantial premium to be paid for full self-sufficiency. The sensitivity analyses point out that the technical feasibility which our results show is robust to varying assumptions, including to the consideration of realistic PV generation profiles. Cost sensitivity analyses show that in absence of the expected cost declines in batteries, electrolyzers, tanks and fuel cells (i.e., all storage-related components), costs would be up to 50% higher. Sys- tem costs are less sensitive to PV costs. This result underscores that the cost premium for self- sufficiency will depend primarily on the cost of the storage components that make self-suffi- ciency possible. The higher cost may not be prohibitive where self-sufficiency is desired for technical (e.g., remoteness) or social (e.g., desire for buildings perceived as ecologically friendly) reasons. The differences between SFB and MFB cases are interesting in two ways in

Fig 10. Technical sensitivity analysis for cases with SFBs (top) and MFBs (bottom). The bars show the range of changes across all rural-urban and conservative- profound scenario combinations, for either new (blue) or retrofitted (red) buildings. The base assumption in all cases is rooftop PV efficiency of 27.2% and no consideration of hydrogen production; the numbered adjustments are specific changes made to these assumptions. Additional energy demand due to climate change (adjustment number 5) is 25% of the median electricity demand, as defined by Nipkow (34). For larger rooftops (adjustment 13), we consider SFBs with only two stories but the same floor area, and MFBs with only three stories but the same floor area, resulting in an increase of the roof size of 50.0% on SFBs and of 66.6% on MFBs. https://doi.org/10.1371/journal.pone.0227368.g010 Energy self-sufficient households with photovoltaics and electric vehicles are feasible in temperate climate PLOS ONE | https://doi.org/10.1371/journal.pone.0227368 March 4, 2020 19 / 25

particular. First, MFBs fail to achieve full self-sufficiency across a wider range of cases, simply because they have less area available for PV installations, relative to their electricity demand. However, MFBs are also substantially more cost-efficient on a per-person basis as investment into components is spread across more inhabitants. Thus, where MFB cases do achieve the potential for self-sufficiency, they ought to be economically more attractive. The wide range of possible parameters to vary meant that assumptions for the creation of the self-sufficient cases needed to be restricted, for example, the assumption that only one third of one facade side is available, or the assumptions we made about the oversizing of hydrogen storage systems to address longer-term variability of PV generation. Based on our sensitivity analyses the results appear robust nevertheless, and we consider our assumptions to generally be conservative. A major uncertainty that we cannot fully address is the economic attractiveness of energy self-sufficiency by 2050. Under our base assumptions, it seems that grid-connected and fossil-powered cases have a cost advantage over self-sufficient cases. How- ever, we cannot forecast 2050 prices, especially not for fuels. Furthermore, we know that con- tinued reliance on fossil fuels is incompatible with preserving a stable global climate. Thus, it is highly likely that government intervention either through bans of or taxes on fossil fuels will change the price balance. Our analyses suggests that with even a relatively modest increase of

fossil fuel costs, some of the self-sufficient scenarios become cost-competitive with grid-con- nected fossil-fired cases. Future work could also investigate the vulnerability of self-sufficient buildings to extreme weather conditions in more detail. By analyzing each weather year indi- vidually, we do not transfer stored hydrogen at end-of-year to the following year; this would result in additional supply security in reality. However, the effect of long dark winter periods could also be more substantial in particularly bad years, given our coverage of weather only from 2000–2015 occur that one year in the future has a worse solar yield than the worst year in the analyzed period. In such cases, hydrogen would have to be purchased on the market or hydrogen backup system size would have to be further increased. Additional work is also needed on performing full environmental impact assessments of self-sufficiency including all upstream energy and material use for different cases, as well as modeling cases located in other destinations worldwide with adjusting the data. Finally, we do not model optimal control of EV charging in combination with building-integrated storage components–implementing Fig 11. Cost sensitivity analysis of self-sufficient cases, showing the range of percent change in costs across all self-sufficient cases for each of the specific numbered adjustments made to the default cost assumptions, and differentiated between new/retrofitted buildings by color and between SFBs/MFBs by color intensity. https://doi.org/10.1371/journal.pone.0227368.g011

Energy self-sufficient households with photovoltaics and electric vehicles are feasible in temperate climate PLOS ONE | https://doi.org/10.1371/journal.pone.0227368 March 4, 2020 20 / 25

smart technologies for this may well reduce the required size of batteries and hydrogen storage in buildings, and thus bring costs down substantially. Overall, we find that self-sufficient households are technically feasible across a wide range of scenario combinations. Their financial attractiveness depends on a variety of costs, particu- larly those of storage components, as well as on discount rates and primary energy prices by 2050. Given the perceived attractiveness of completely energy self-sufficient households, it is likely that more such buildings will be constructed even if fully electrified but grid-connected households are substantially less expensive. Falling costs of storage and political will to push for self-sufficiency may well in combination result in such buildings become more common than currently expected. Supporting information S1 Dataset. Definitions of cases and technological assumptions. File name S1_Method- s_TechPotential_And_AltCases. (XLSX) S2 Dataset. Definition of cases with hydrogen production to fulfill requirements of a true zero energy building. File name S2_Methods_Scenarios_Baseline_Optimized. (XLSX) S3 Dataset. Definition of cases with a hydrogen production 1.75 times the amount deter- mined to supply net-negative months. File name S3_Methods_Scenarios_BackupStorage175. (XLSX) S4 Dataset. Definition of cases which are connected to the grid and still use batteries for short-term storage. File name S4_Methods_GridBased_withBatteries. (XLSX) S5 Dataset. Definition of cases which are connected to the grid but do not use batteries for

short-term storage. File name S5_Methods_GridBased_withoutBatteries. (XLSX) Acknowledgments We thank Anthony Patt for helpful comments on developing the grid-connected cases. Stefan Pfenninger acknowledges funding from the Swiss Competence Center for Energy Research − Supply of Electricity (SCCER SoE), contract number 1155002546. Author Contributions Conceptualization: Ursin Gsto¨hl, Stefan Pfenninger. Data curation: Ursin Gsto¨hl. Funding acquisition: Stefan Pfenninger. Investigation: Ursin Gsto¨hl, Stefan Pfenninger. Methodology: Ursin Gsto¨hl, Stefan Pfenninger. Supervision: Stefan Pfenninger. Visualization: Ursin Gsto¨hl, Stefan Pfenninger. Writing – original draft: Ursin Gsto¨hl. Energy self-sufficient households with photovoltaics and electric vehicles are feasible in temperate climate PLOS ONE | https://doi.org/10.1371/journal.pone.0227368 March 4, 2020 21 / 25

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