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In the Swiss context, the building energy refurbishment plays an important role. About one third of the residential building stock was built before 1946, and another third between 1946 and 1980 (data: BFS-OFS-UFS, 2016), with thus very different thermal standards than today. One of the goals of the 2050 Energy Strategy is hence to reduce the final energy consumption of buildings (all uses) from 100 TWh (2010-2015) to 55 TWh in 2050, while phasing out fossil fuels (75% of today’s heating demand) and direct electricity heating [25].

In the framework of the Swiss National Research Project NRP 54, Schalcher et al. [263] described the characteristics of the two economic categories of renovation, aiming either at conserving the value or at creating an added value. They estimated that the actual investments for the renovations aiming at conserving the building economic value are much lower than what would be necessary to implement standard overhauling measures to the existing Swiss building stock. Since this type of renovations mostly follows the supply-demand model, one of the possible interpretations made by the authors is that the refurbishment cycle does not correspond anymore to the economic reality or that the market does not provide enough value for the standard refurbishment cycles. Conversely, refurbishments aimed at creating an added value have a secondary role in terms of investments in the Swiss context as they are mostly limited to single-family detached houses. The study also dealt more specifically with energy refurbishments showing their significant economic impact because of their higher specific costs. Unfortunately, the analysis was based on the latest available data, which were from the period 1990- 2000. However, the authors assumed that the current and future potential for energy refurbishment could be actually higher because of the great augmentation of energy prices starting from 2004 and the effect of the Swiss Building Program (see Section 3.4.1.1).

In this context, it seems that energy refurbishment interventions have an immediate interest for building owners and a large potential for growing in Switzerland. Jakob [127] showed that energy refur- bishment of previously non-insulated building envelopes is profitable in most cases. Also considering a typical multi-family house of the construction period from 1975 to 1990, which had been already partially retrofitted, Jakob et al. [128] showed that improvements at the level of the building envelope can still be cost-effective. They also recommended the installation of PV systems due to quite favorable cost-effectiveness compared to other strategies. Similarly, Aguacil Moreno et al. [5] showed how the

Chapter 3. The case of Switzerland

integration of PV installations may determine a shorter payback time compared to a simple energy refurbishment. The reduction of the payback time due to PV installations could hence be an important trigger for large-scale, coordinated energy refurbishment interventions to the Swiss building stock. However, the methods currently available for assessing the potential for energy refurbishment interven- tions only partially address the question of the solar potential in the urban environment [128] and/or they are limited to building- [128, 251] or neighborhood-scale analyses [250, 251].

3.4.1.1 Federal incentive scheme

In 2010, the Swiss confederation introduced the “Building program” (“Gebäudeprogramm, Programme bâtiments, Programma edifici”, [3]) in order to reduce the energy consumption and CO2emissions of

the residential building stock. This program aimed at promoting energy refurbishment interventions for buildings built before 2000 through national-wide subsidies, and integration of renewable energy sources through canton-level incentives. In the first five years (2010-2014) of activity, 64180 energy refurbishment projects were subsidized.

This program been financed by the cantons and by the federal carbon tax, and the latter part is supposed to increase in the future [24]. The new Energy Act (EnG-LEne-LEne) as well as the revision of CO2 Act (CO2-Gesetz, Loi sur le CO2, Legge sul CO2, Art. 34) strengthened the Building program (ENFV-

OEneR-OPEn, Comments), by providing 1/3 of the tax income (capped to 450 MCHF/year) to the “Building program”. Investments in building energy saving interventions can be deducted from the taxable income. Starting from 2020, also demolitions costs associated to energy-efficient housing will be fiscally-deductible and the tax deduction could be deferred up to 2 years [24].

Depite these measures, the building retrofit rate is still low (0.9% per year) and it has been claimed that such incentive scheme does not encourage a faster refurbishment rate than the normal one [293]. In this context, it can be argued that the renewable energy transition cannot “wait” for the refurbishment of the building envelopes to install photovoltaic systems, while BIPV could hopefully encourage the refurbishment of the urban building stock.

3.5 Synthesis and discussion

This chapter has briefly reviewed the main Swiss policy and regulatory framework at the intersection of solar energy, building energy refurbishment and urban planning. We summarize and discuss here some aspects of the review that are particular relevant for the definition of the thesis motivation, which will be formulated in Section 4.4.

The importance of solar energy in urban environments is attested by the presence of these aspects not only in energy-related legislation, but also in the spatial planning one. The fact that both the Energy Act and the Spatial Planning Act have successfully gone through a direct democracy procedure also implies a strong support from the Swiss people to these matters. The policy for urban inner development and densification (as opposed to greenfield development) is also important, as it testifies the relevance of the existing built environment in the planned urban development.

To this regard, Swiss legislation also provides a strong commitment towards the balance between the promotion of solar energy and the conservation of the natural and built environments. In particular, building integration aspects - for example, the need for a “compact shape” (RPV-OAT-OPT, Art. 32) - are

3.5. Synthesis and discussion

detailed at one of the highest sources of law. These aspects will be further discussed in Section 4.2.4, but we can already stress out the importance of considering such criteria in solar potential assessments. In addition to the general considerations about electrification of heating and use photovoltaics (Chap- ter 1), we have seen the relevance of integrating building energy retrofitting with photovoltaic interven- tions also from a financial point of view in the Swiss context. Switzerland also encourages the use of solar energy and building energy refurbishment with a favorable regulatory and incentive framework, yet there are still some obstacles. We have seen how the facilitation framework for solar energy in buildings (notably, the building-permit waiver for “roof-adapted” solar systems) does not necessarily simply the procedure and is prone to conflicts. This causes a certain level of uncertainty in the approval process, which does not help the large-scale deployment of solar energy systems. We can also argue that the current incentive framework for solar energy and building energy refurbishment suffers from simplified allocation mechanisms, such as the waiting list. We have also seen that, despite the incen- tives, the refurbishment of the building stock has very low annual rates. This problem with allocation systems, which is common to many countries, will be further discussed in Section 4.3.4 and be part of the thesis target gaps listed in Section 4.4.

4

State of the art

This chapter provides an analysis of the state of the art in the core research areas investigated by this thesis. The literature review explores the latest research advancements in the assessment of the PV solar potential in urban areas and the subsequent use of this information in planning-support. Unlike the literature review conducted in the previous chapters, this review aims more specifically at identifying the research gaps in the fields, some of which will be addressed Part II and Part III.

4.1 Urban-scale simulation of solar irradiation

Interest towards solar energy assessments has been rapidly growing and many models have been developed recently. Freitas et al. [88] provides an extensive review of both computational models and tools used in solar radiation models1.

We conduct here a review on the solar radiation models that are available as a tool and their input/out- put data. In general, the larger the scale, the more simplified are the models applied and the less detailed are the input data. This is for example the case of regional-scale analysis conducted on the ba- sis of low-resolution raster data. Similarly, the scale often influences the resolution of the sensor points, on which the output solar radiation is calculated. In this sense, this review is aimed at highlighting the trade-off between:

• analysis accuracy vs computational cost/time;

• data resolution vs broad applicability (in areas where high-resolution data might not be available); Considering that analysis accuracy and data resolution both have a maximum reachable limit (due to computational power and/or sensor precision), we finally review the methods that have been used to account for the associated uncertainty (Section 4.1.5).

1_{As a reminder, in this section as elsewhere in the thesis, the word model is used to refer to very different concepts. In}

particular, it can apply either to mathematical and physical models (e.g., sky models and solar radiation models) or to geometrical models (e.g., 3D models, raster models). We will omit further specifications, as the context should make clear which usage applies.

Chapter 4. State of the art