2. Introduction
In the last decade, the use of PV systems connected to the grid has undergone a considerable increase, due to both the incentive mechanisms and the progressive decrease in installation costs of over 10% per year [
1]. Currently, however, the reduction in incentive tariffs in most EU countries and the increasing cost of electricity withdrawn from the grid, have led to identify the self-consumption of electricity production from PV systems as a real solution of economic gain for consumers in their role as prosumer (producer–consumer). The interest in self-consumption, defined as the ratio between the electric energy produced and directly consumed on site and the total energy produced by the PV system, instantaneous and/or deferred, is mainly due to the increase in profits, as it decreases the energy withdrawn from the grid and the relative costs as well as the PV energy injected. The exchange with the grid is, in fact, an economic loss for the end user, in addition to the reduction in problems created to the electricity grid [
1,
2].
The growing introduction of Nearly-Zero Energy Buildings (nZEBs) in the built environment, in view of the objectives set by EU directives and national regulations in previous years, also implies the need to intervene in the optimization of plants powered by renewable sources, such as photovoltaics, improving their design, integration, performance and maximizing the energy and cost-effectiveness as well as the producibility on-site. More and more buildings are designed as power generators with a PV cladding which, as an active building material, produces renewable energy. From the first age of solar roofs, we are moving towards a complete active building skin where all facades and building surfaces are photoactive, and PV becomes part of the aesthetics and building skin technology. This change of architectural paradigm also shifts the attention in terms of energy behavior, since energy is produced all day, from surfaces with various orientations and increasingly with variable operating conditions affected by the surrounding built environment.
The present work concerns an analysis on a pilot Swiss BIPV building, Palazzo Positivo located in Chiasso-Canton Ticino, which has been renovated according to Passivhaus and Minergie standards by transforming all the building skin into a new active PV cladding producing energy. By using the real production data derived from an annual monitoring of the building, we intend to define a method that permits the assessment of the energy and economic effectiveness of different BIPV skin possible for configurations (separating and recombining only some facades or roofs as active PV parts, in simulated scenarios) set as reference scenarios (we will call them merged clusters,
MC) and highlight the limits/potentials of the optimization. Today, many pilot BIPV buildings are realized [
3] but a deep critical technical-economic convenience on the role/effectiveness of each part of the building skin to become active is not yet known. Some methodologies in literature have recently defined the cost effectiveness by means of similar assumptions and based on a life cycling cost approach, but these methodologies are not verified in a real and monitored case study [
4].
The paper is organized as follows. In
Section 3, the state of the art is analyzed, highlighting both objectives and points of innovation of the project.
Section 4 describes the solar system features.
Section 5 illustrates the methodological framework, aimed to clarify the assumptions and methods of analysis for assessing the economic and energy benefits in relation to current and future possible scenarios. The development of the analysis and the results are summarized within
Section 6.
Section 7 concludes the work by describing the implications of the results.
3. State of the Art and Motivation
On 30 November, 2016, with the aim to promote the use of smart technology in buildings, the European Commission proposed an update to the Energy Performance of Buildings Directive to streamline existing rules and accelerate building renovation [
5]. The Directive (2018/844/EU) amending the Energy Performance of Buildings Directive, revised, entered into force on 9 July, 2018. This revision introduces targeted amendments to the current Directive aimed at accelerating the cost-effective renovation of existing buildings, with the vision of a decarbonized building stock by 2050 and the mobilization of investments. The revision also supports smart technologies and technical building systems [
6,
7]. Solar installations, on-site and integrated, are an opportunity for increasing energy performance in buildings. In the new Energy Performance of building Directive, important provisions for solar in buildings have been considered [
8]. In this framework, thus, BIPV is confirmed as a strategic approach for prosumers and small/medium-scale installations which should drive the energy transition in future decades. In the Commission Recommendation (EU) 2019/1019 of 7 June, 2019 on building modernization (Official Journal of the European Union, 21.06.2019), it is stated that the performance of technical building systems has a significant impact on overall building energy performance and should therefore be optimized.
The growing interest in the self-consumption of electricity production has generated in the last few decades the development of numerous research on the theme of optimization and maximization of Self-Sufficiency Rate (
SSR) and Self-Consumption Rate (
SCR) of photovoltaic systems, both in the residential and tertiary sectors. Even in absence of incentives, the self-consumption has numerous advantages: it contributes to the reduction in production peaks, helps to avoid overloads on the network and allows a better control of electricity purchase costs [
9]. The
SSR, however, is defined as the percentage of coverage of electricity production needs, which is similar to the
SCR only in the case of an ideal nZEB building, with a perfect balance between photovoltaic generation and electrical requirements.
The methods investigated for the optimization of the value of self-consumption are different, although they can mainly be traced to two types: the use of storage systems (batteries) or the movement of active loads, defined as Demand Side Management (DSM). These strategies can be used separately or combined, leading to different results [
8,
10]. The aim is minimizing the dependence from the grid of currently connected PV systems, in order to maximize the consumption of locally-produced PV energy, reducing costs and avoiding peaks and grid instability.
In the first case (accumulation systems), optimization strategies are based on the storage of the surplus of electricity generated by the PV system and on its deferred use, avoiding the input into the grid. However, today, despite the increase in the percentage of self-consumption due to the use of batteries, the continued high costs of this system do not allow us to define it as a convenient tool [
11,
12,
13,
14,
15,
16,
17,
18,
19]. Numerous studies focus on the optimization of self-consumption by inserting batteries. Some deal with the correlation between the increase in
SCR and the size of the battery to be installed, others with its long-term management and with the technical–economic benefits that can be obtained. The study [
20] illustrates the influence of battery storage size on PV self-consumption in a grid-connected residential building. It shows that self-consumption reaches a “plateau” at a certain value of storage size equal to the average amount of daily PV-consumption/production. Moreover, for all analyzed storage sizes, the achievable amount of energy self-consumption is considerable: by increasing the battery size in terms of energy from 0 to 32 kWh, the SSR increases by 36 percentage points in winter and 51 in summer [
20]. In [
15], a similar result is achieved for a case study in Germany, where the increase is 27 percentage points (from 38% to 65%) with a 7 kWh battery [
21]. In the latter case, Truong et al. [
15] analyze the technical and economic benefits of the Powerwall designed by Tesla on a residential building in Germany, estimating the conditions that lead storage to become economically viable. Additionally, [
22] shows a strategy for maximizing the self-consumption, using a simulated Vanadium Redox Flow (VRF) battery (demonstrator), showing the possibility of reaching a value of self-consumption equal to 100%, from a value of 65%, through the local use of the energy produced, avoiding its injection into the grid, covering about 75% of total consumption. The same VFR battery technology is used in [
23] with the aim of achieving system self-sufficiency thanks to its sizing capacity independent of nominal power. Always referring to residential buildings, the study conducted by Weniger et al. [
13] leads to important results for the sizing of battery capacity. In fact, with a PV plant sized with 1 kWp/MWh, a single residential building can reach a self-consumption value equal to 30%. The addition of 1 kWh/MWh of the battery capacity increases the SCR and the SSR to almost 60%, while the further increase in accumulation does not lead to appreciable results, showing that the correct capacity value must be based on the daily requirement [
13]. As in other studies, the case study is a nZEB multi-family residential building in which the goal is to reduce as much as possible the electricity injected or withdrawn from the grid, giving priority to self-consumption and storage of the energy surplus produced.
Numerous studies investigate the topic of Demand Side Management (DSM) as an alternative to storage to optimize the self-consumption value of a system. This strategy, however, is mostly convenient in the case of residential buildings. In fact, when office and commercial buildings are considered, where the load profiles follow fixed characteristics during the year, the mismatch of needs makes it more difficult to achieve feasibility [
9]. In this case, as proposed by Martín-Chivelet et al. [
9], optimization is carried out by simulating the differently oriented façade combinations that provide better output in terms of
SCR and
SSR. Sánchez et al. [
11] also studied the performance of poorly oriented BIPV systems, but the study is not designed to evaluate the benefits in terms of self-consumption. As for batteries, even DSM cases in the literature are numerous for residential buildings. The concept of DSM concerns the temporary shift of energy demand by the user, due to the use of more energy-intensive equipment, such as dishwashers, washing machines and tumble driers. A large part of energy waste can in fact be charged to a non-virtuous behavior of users, which necessarily implies a lack of overlap between the needs profile and the production profile. Optimizing this profile, translating the load curves in an appropriate way, can therefore be a good way to increase self-consumption. Numerous references in the literature aim to improve self-consumption in residential buildings by combining the strategies previously exposed: DSM and storage [
1,
2,
10,
14,
24,
25].
Castillo-Cagigal et al. [
26], define the concept of ADSM (Active Demand Side Management) as the automation control of user load management, potentially useful in the residential sector for the possibility of being combined with additional functions relating to the comfort and safety of the building. The interesting results of his study are the following: the discovery that the influence of storage systems in the energy balance has no linear growth based on its capacity but reaches an asymptotic level for high storage capacities.
The final assumption is, therefore, that the use of the ADSM strategy is the same as the use of a small electrical energy storage system and the combination of the two strategies significantly improves the direct use of photovoltaic production.
The investigations previously analyzed are mainly based on simulated PV production models created on the basis of real meteorological conditions. The only reference of real monitoring of both consumption and electrical production of a BIPV system is not aimed at optimizing the values of self-consumption and self-sufficiency. In [
14], the power generation of the BIPV system and the building loads were investigated by monitoring the building to verify the worst array causing the reduction in generation performance and to verify the achievement of the purpose of zero-energy buildings.
From an economic perspective, the investigations previously analyzed show that the BIPV system and storage installation are becoming attractive investments due to decreasing trends in costs and feed-in tariffs, also in the case of energy efficient retrofitting.
Due to the assumption that, actually, electricity tariffs, PV system cost and system efficiency do not allow us to define BIPV solar energy technology, a cost-effective option is reported in some studies, as in the research of Radhi H. [
27]. According to this, the only way such a system will be convenient is if the electricity tariff increases or if the total system cost drops drastically. However, as Scognamiglio A. specifies in her studies, in the case of the BIPV system, the economic evaluation is more complex because not only the direct costs of BIPV installation should be considered, but also the benefits deriving from PV energy production and the avoided costs of the traditional building materials [
28].
Other studies focus on the evaluation of the cost-effectiveness of BIPV for the building skin, in addition to the energy performance [
29,
30]. In [
29], Hammond, G. P. et al. study the performance of a domestic BIPV system utilizing energy analysis, environmental life-cycle assessment (LCA) and economic analysis. The results illustrate that the systems are unlikely to pay back their investment over the 25 year lifetime, even if the energy analysis determines that the system paid back its embodied energy in just 4.5 years. They also demonstrate the importance of government support to the future uptake of BIPV.
Bonomo et al. illustrate a new methodology for evaluating the cost-effectiveness of BIPV, based on a life-cycle costing approach, assessing the whole building envelope solution, taking into account the multi-functionality of the system and, therefore, all the advantages of BIPV [
29].
The economic sustainability of BIPV technology is a crucial aspect of its feasibility and market success. So far, the main efforts towards cost-effectiveness have been focused, similarly to conventional PV, on minimizing the final installation price per kWp. When using PV, the economic analysis and the afford ability assessment are generally focused on the energy payback time and on the return of investment [
31]. The present study will base the analysis of a BIPV solution on the following innovative aspects beyond the state of art:
Multi-functionality. BIPV is considered as a building material, which replaces a conventional layer of the building skin, thus providing a cost saving due to the cost of the replaced building cladding;
Unitary concept of BIPV and building envelope. The energy and economic assessments are conducted considering the BIPV system together with the whole building, as a unique system.
Life-cycle costing. The balance of costs and benefits is extended to the entire life cycle of both PV and the building envelope.
A project-based approach. The analysis is performed for a specific case study in which characteristics and peculiarities are known and provided by real data by avoiding uncertainties on simulated cost and energy parameters. Accordingly, the paper will focus the analysis on the energy performance and the cost effectiveness of the BIPV and BAPV systems installed in the case study building.
6. Analysis and Results
In this section the results concerning the technical-economic assessment, according to the assumptions and reference scenarios described in the previous chapters, are presented and discussed. The goal was to bring to light how the different MCs affected the energy and cost-effectiveness of the whole building in different ways. To make the results more clear, the discussion and results were grouped in the thematic analysis as follows:
energy and cost effectiveness,
comparison between BAPV and BIPV scenarios,
business model alternative to self-consumption: FiT,
electrical storage implementation: Scenario 2.
As abovementioned, the outcomes were the results of the method actualized for a specific case study. In
Table 4, an overview of the analysis after 20 years, considering Scenario 1, is shown.
It is worthy to note the position of MC9 (building without PV) and the current status of MC1.
6.1. Energy and Cost Effectiveness
The quantification of the energy effectiveness of each MC is represented within this paragraph by the SSR, namely the amount of energy produced with a renewable source and self-consumed compared with the global amount of energy needed during the year for the building. This value is directly related to the PV energy production value.
However, as emerging from
Figure 8, a high value of
SSR itself did not entail a low value of
TCE during the solar system lifetime (in this study 20 years; the lifetime value was an approximated value based on the average performance guarantee on 80% of nominal power under standard test conditions of some examples of PV modules of different brands and different technologies. The lifetime of a PV system could be longer). The
MC1, representing the current state of the solar system installed on the building was the solution with the higher
SSR. However, in terms of
TCE, it did not represent the most affordable solution.
Figure 8 represents a comparison between the
SSR and the total cost in 20 years. Solar solutions without accessories (mainly tracker and balconies), with a high final yield and self-consumption rate were the most effective from an economic point of view, such as
MC7,
MC8 and
MC5.
For the MC9, MC10 and MC11, the SSR was zero because of the total injection of the energy produced into the grid.
6.2. Comparison between BAPV and BIPV Scenarios
Three different
MCs were compared in order to evaluate the effectiveness of building integrated/added PV systems. The analysis on BAPV (
MC5) and BIPV (
MC3) systems showed typical aspects. The BAPV systems had a high value of PV energy production, corresponding with high yearly average revenues and
SSR, which, despite the extra cost of the system (modules, cables, structures, etc.), was high. Namely, the extra cost used in the calculation corresponded to the entire PV plant cost, since in a BAPV system no building system was replaced. On the other hand, the BIPV system,
MC3, had a restrained energy production due to the non-optimal exposition, but a lower initial extra-cost as indicated in
Table 5.
The most affordable solution in terms of TCE was represented by the MC8, the solution combining BAPV on the roof and BIPV on facades where the TCE was significantly lower. Within this analysis, the MC5 (only BAPV) was more affordable than other BIPV or mixed BIPV/BAPV solutions.
6.3. Business Model Alternative to Self-Consumption: FiT
The MC9, the solution without any PV on the building, was not convenient, according with the TCE, in comparison with more than half of the MC due to the high price of the energy purchased from the grid. However, in comparison with the current configuration, both in self-consumption (MC1) and FiT (MC10), the TCE of the MC9 was about 20% and 15% lower, respectively.
The MC with
Feed in Tariff (
MC10 and
MC11), showed different results. Only
MC11 showed a low value of
TCE, which represents a good convenience of this business model (
Table 6). The energetic benefit in terms of
SSR was not calculated because the energy produced with PV was all injected into the grid. In spite of the low value of
TCE, it is necessary to underline that the
FiT for the energy injected into the grid was considered 0.28 CHF/kW, as paid by the electric society in 2013 for the building, according to Swiss
FiT skills. Nowadays, it is important to remark that the subsidies are no longer available in most EU countries. Self-consumption also shows feasible scenarios in a non-subsidized situation.
6.4. Electrical Storage Implementation: Scenario 2
In this paragraph, the scenario concerning the storage implementation is discussed. Within the first part, we will provide an insight on the correlation between the batteries’ capacity and the effect on building self-consumption and self-sufficiency, along with the main assumptions adopted for this scenario, with the goal to also define an optimal size of storage.
6.4.1. Battery Implementation. Preliminary Assumptions and Results for MC1
The values of both
SCR and
SSR and of electric injected and withdrawn energy from the grid were calculated and the output values of the various capacity scenarios were compared, with the goal to assess the effectiveness of battery implementation, as shown in
Figure 9 and
Figure 10 (for the
MC1).
In
Figure 9 the correlation between the storage capacity and the resulting energy injected into the grid and withdrawn is reported. The graph of the
SSR and
SCR (
Figure 10) shows a non-linearity between the storage capacity and the
SCR and
SSR, resulting, instead, in an asymptotic trend towards a limit value as the storage capacity increases. In fact, when a capacity of about 50% of the load profile was reached, further capacity of storage led to negligible increases in the values of
SSR and
SCR. Oversized batteries, in fact, could not be fully charged or discharged. As storage capacity increased, the
SCR varied from a minimum of 54% to a maximum of 90%, while the
SSR varied from a minimum of 35% to a maximum of 58% (
Table 7).
This means that, for this size of PV system which included all building skin surfaces, the energy produced can be consumed almost totally, with the use of optimal accumulation systems, while self-sufficiency can reach a maximum percentage of about 58%.
Even the values of injection and withdrawal from the grid had an asymptotic profile towards a minimum limit, remaining almost unchanged from the achievement of 50% of storage capacity.
From this point of view, the capacity of the battery which optimized the values of
SSR,
SCR, energy input and energy taken from the grid is considered equal to 50% of the total daily electrical load, as already found in other studies in the literature [
1,
9,
10,
12,
13,
18,
19,
21]. In addition, in this case, the optimization method of the battery capacity took into account the
TCE value, also becoming a cost-optimal sizing. For each scenario, in fact, Step 2 was developed to compare the different MCs according to the main parameters: PV energy production, final yield,
PbP and
TCE after specific years. The final yield stood for the ratio between the energy produced and the nominal power of the PV system (kWh/kWp).
The total costs over 20 years included
initial investment costs for the PV plant,
battery installation costs,
PV revenues for 20 years,
storage revenues for 14 years.
The results of the analysis are reported in
Table 8 that shows the comparative values of
SSR, SCR and
TCE, considering the period of 20 years calculated for the
MC1.
In conclusion, the optimal sizing of the battery, from an economic point of view (
TCE) and for the
MC1, would be assuming
Cbat = 0.3
Eload, as
Figure 11 shows.
6.4.2. Results of Battery Implementation in the Various Construction Typologies
Within
Figure 12 is shown the value of the
TCE of the
MC at the change of the capacity of the battery (
Cbat). The optimal sizing of the battery changed according to the
MC.
The analysis showed that for half of the cases analyzed, a storage system, with a capacity included between 10% and 30%, will be affordable by 2020 (
MC1,
MC4,
MC6,
MC7). These
MCs represented the configurations with higher PV production. Within these cases, an improvement of the
SCR permitted the self-consumption of a higher amount of energy in comparison with the
MC where the energy production was low. As possible to notice from
Figure 13, the
MC1,
MC4,
MC6 and
MC7 showed a low
SCR at low capacity of the storage system, while, considering the high PV production, the
SSR was the highest compared with the other analyzed
MC. However, the installation of a storage system, also for the
MC where batteries reduce the
TCE, entailed low benefits considering the reduction in
TCE in 20 years. A further reduction in the batteries’ cost would permit the considerable reduction in the
TCE. Although in these conditions and for this case study a storage system is not recommended, in a few years the batteries could be a solution that permits the reduction in the costs of a PV system.
The “energy” comparison in terms of
SSR and
SCR (
Figure 13 and
Figure 14) showed that, except for the
MC2 and
MC3, where the
SCR was already 100%, in all cases, the battery increased the values of self-sufficiency by about 30% at least, and the values of
SSR by about 15%, averagely. The maximum percentage increase was in
MC6, by about 38%, in terms of
SCR, and by about 21% in terms of
SSR. In
MC2,
MC3,
MC4,
MC5,
MC7 and
MC8, the
SCR reached the value of 100%.
7. Conclusions
The paper presented and discussed the results of a comparative analysis on a multifamily building, focusing on alternative scenarios for energy and economic effectiveness of building skin retrofit with PV parts. By assuming real monitored data for the case study, we analyzed and compared, in a life-cycle standpoint, the main construction and energy related design strategies affecting cost and energy effectiveness of the whole building. The main parameters relevant for BIPV skin design, including both construction aspects (orientation, surface typology) and energy strategies (such as self-consumption/sufficiency, storage and combination with BAPV) were highlighted and discussed in dedicated scenarios. This analysis permitted us to define both approaches and conclusions that are scalable for other case studies to correlate in detail the construction/architectural choices, typical of the architectural stage, with the energy and cost results which are crucial in order to establish the intervention feasibility for BIPV systems.
In Scenario 1 (current building load profile without storage) the following conclusions emerged:
Comparison between BAPV and BIPV: the most affordable solution in term of TCE is represented by the MC8, the solution combining BAPV on the roof and BIPV on facades, where the high value of final yield in roof is combined with the low extra cost in façade.
Affordable values of TCE can also be reached with only BIPV or BAPV (MC2 and MC5, respectively). It shows, once again, that cost effectiveness is reached through a balanced combination of final yield, investment cost, SSR, SCR and revenues.
Energy and cost-effectiveness: The most effective solar solutions from energetic and economic perspective are represented by building skin configurations in which the accessories (mainly trackers and balconies) are not used. This can be explained due to the high investment cost and the low irradiation.
The cost-effectiveness is higher by about 25% in BAPV solutions MC5, MC7 and MC8 (on the rooftop and garage) due to the lower investment cost in comparison with the building state of the art (MC10). However, it has to be noticed that self-sufficiency is very low due to the limited rooftop area so that the scenario is not relevant from a nearly-zero energy building perspective.
The solution without any PV surface in the building skin (MC9), even though it does not correspond to the target of nearly-zero energy building with on-site energy production, is about 15% more convenient than MC10 (state of the art) but not convenient in comparison with most of the optimized solutions. This marks the need for optimizing building skin design with costs and building energy needs.
Solutions with only BIPV surfaces on the façade (MC2 and MC3) are average solutions in terms of cost effectiveness. Although the final yield of these solutions is low, the investment cost (ECBIPV) of the PV installation is also low, therefore supporting their cost convenience.
Business models with FiT: if compared with the self-consumption business model, subsidies with FiT, that are no longer available in most EU countries, would support a more convenient scheme (MC1 vs MC10).
In Scenario 2 (including storage) the main conclusions are
Optimal storage size: a correct sizing of the battery capacity can make significant increases to the values of self-consumption and self-sufficiency, as well as minimizing the values of power flows between the household and the grid.
However, an excessive increase in the battery capacity corresponds to insignificant increases in both self-consumption and self-sufficiency, and reductions in the values of energy injected and withdrawn from the public grid reaching for both horizontally asymptotic values. In addition, an oversized battery, where a large part of its capacity would remain unused, leads exclusively to an inconvenient increase in installation costs, without energy benefits, as we have seen in the figure that shows the comparison between the capacity of the storage system and the TCE for the MC.
Oversized solar systems benefit from the installation of a storage system with a capacity included between 10% and 30%. The installation of a battery permits a considerable increase in the amount of electricity self-consumed reducing the TCE (MC1, MC4, MC6 and MC7). However, the benefits induced by a storage system are not worthwhile within a retrofitting effort. A further reduction in the batteries’ cost would permit a considerable reduction in the TCE. Although in these conditions and for this case study a storage system is not recommended, in a few years the batteries could be a solution that permits the reduction in the costs of a PV system. No further links with the building construction in comparison with those found within Scenario 1 have been found.
This study provides a practical reference for researchers and professionals in finding the technical approaches and strategies supporting appropriate business models or at least a clearer value proposition project-related to BIPV systems. Outcomes permitted us to show, in a real case study, how BIPV design choices related to the building skin (including different building design options, surface orientations, technologies, price levels, etc.) are closely related to the specific building typology and energy system (HVAC, load profile, etc.) by directly affecting the total life cost, self-consumption/sufficiency and cost-effectiveness. It emerged as the two major components in view of the full decarbonization of the built environment are closely related. Retrofitting existing buildings to increase the energy efficiency is not independent from a transition to energy positive buildings producing electricity, covering their heating and cooling needs and contributing to the grid stability with sustainable, renewable energy technologies as part of the building skin design.
Further development will permit the extension of this methodology to other BIPV case studies, in order to perform a sensitivity analysis of the main parameters that influence the integration of solar systems in building skin and finding other patterns of the main technical approaches supporting appropriate value proposition BIPV systems.