1. Introduction
Energy efficiency in the building sector is a central topic both for policymakers and international institutions in the field of research [
1]. In the European Union (EU), all new buildings starting from 2021 must comply with Nearly Zero Energy Building (NZEB) standards; such an obligation has already been in force since 2019 for public buildings as per the Energy Performance of Buildings Directive [
2]. In addition, other directives on energy efficiency [
3] and renewable energies [
4] endorse all countries to improve their building stocks through initiatives and specific funding. Such a tendency towards more sustainable and efficient buildings, both residential and tertiary ones, were recently supported and promoted by the European Green Deal [
5].
Increasing the energy efficiency of buildings and their self-consumption brings other benefits to the overall energy system such as the reduction of the electricity demand in peak hours and a better management of the grid as reported by Jradi et al. [
6]. To this aim, several solutions have been proposed that include photovoltaic (PV) systems, thermo-photovoltaic (PV-T) for electricity and thermal generation and co-generation and tri-generation using gas, solar or biomass as the main driver, fuel cells and Stirling engines, investigated for instance by Calise et al. and Seddiki and Bennadji [
7,
8]. However, the implementation and optimization of systems that include two or more energy sources to provide heating, cooling, domestic hot water (DHW) and electricity at the same time are still in their early stages and require further efforts in the definition of the best strategies and solutions for each type of building [
9,
10]. The vast majority of works in the literature relate to residential buildings for which extensive reviews on different existing alternatives can be found [
1,
11].
On the contrary, non-residential tertiary buildings especially public buildings and offices have only recently gained more attention despite their high energy consumption and the need for specific measures, as stated for instance in the EU Energy Efficiency Directive [
3]. Among the solutions proposed for increasing the energy efficiency of such buildings, the intervention and retrofitting of facades and windows are the most common as it has been demonstrated that they can reduce by 20–30% the energy consumption of the building [
12,
13]. However, it has been proven that recommended management strategies based on passive measures including shading and the reduction of daylight bring about a reduction of visual and thermal comfort as highlighted by Ballarin et al. [
14], thus needing caution in the application. Indeed, adding shading devices reduces the amount of light indoors, which can negatively affect productivity in the workspace. In addition, in order to move towards the NZEB standard such interventions are not sufficient and must be complemented with a renewable-driven energy system [
14,
15]. A methodology for such an assessment was proposed for sub-tropical climates by Melgar et al. [
16] and it is based on the evaluation of the energy demand of the building and the calculation of the energy produced by each generator.
Among the available analyses specifically devoted to office buildings, the use of water produced in the surroundings is exploited by Park et al. [
17] for heating, cooling and DHW production, with a related reduction in energy consumption of the examined buildings in the range of 8–27% and a reduction in CO
2 emissions of 40%. Another solution based on a multi-generation system including PV-T collectors and a ground-source heat pump is presented by Bae et al. [
18] for reaching the Zero Energy Building (ZEB) standard in Canadian and South Korean climates. The energy savings estimated were about 60% with a reduction in the bill up to 130% annually thus also showing the economic feasibility of such solutions. In Braun et al. [
19], a solution with PV-T collectors as a generation source for heating and electricity coupled with a reversible heat pump was reported. The results showed that high solar fractions and primary energy savings could be achieved if the utilization of generation sources for covering all of the energy demand of the office (heating, cooling, ventilation and electricity) was exploited. In addition, an economic analysis was carried out and proved that the costs for such a system were comparable with conventional solar thermal and solar electrical cooling systems. In Mohamed et al. [
20], different multi-generation systems for co-generation and tri-generation applied to an office building in Helsinki were evaluated. The results of [
20] showed that co-generation systems based on biomass were the most cost-effective alternative whereas, from an energy point of view, the tri-generation biomass system was the desirable option.
From this context, it is clear that there is yet to be a systematic and wide assessment on multi-generation systems for office buildings especially for what concerns the increase of the share of renewables. Accordingly, within the SolBio-Rev project, an integrated system for supplying the different thermal and electricity demands of an office building was developed, which relied mainly on solar and biomass energy as sources. The system’s architecture and its application to residential buildings were recently published [
21], showing the possibility of supporting a strong increase of the share of renewables in residential buildings across Europe. In this paper, the evaluation of the potentialities of this system for providing heating, cooling, DHW and electricity in office buildings in three different climates (Madrid, Berlin, Helsinki) is presented. The energy flows for the different cases are presented and the possibility of reaching 100% renewables is discussed. Indeed, the presented analysis differs from the common literature because it is not focused on giving rigorous dynamic information on the investigated buildings but rather presents a critical analysis on the perspective of polygeneration systems towards the contribution to the decarbonization of the building stock.
3. Operating Modes
The different sections of the analyzed multi-generation system can be managed in multiple ways according to building requirements, climate and season. Indeed, the development of optimized control strategies will be the focus of future activities. In the following the main operating modes are presented, which consider only basic control strategies to serve as the starting point for the present evaluation of the system potentiality in different climates. The main feature that differentiates the installation in warmer climates (where the cooling demand during summer season is relevant and the ambient temperature is high) from the colder climates (with limited cooling demand in the summer) is the presence of the adsorption module. Such a difference is considered in the following operating modes.
DHW can be directly produced from solar heat if the temperature level of the solar collectors is higher than 45 °C or by the biomass boiler.
Space heating can be achieved from solar heat (if there is no need for DHW or if the temperature in the storage tank is lower than the 65 °C needed for DHW but higher than 35 °C) otherwise by the heat pump or the biomass boiler. The heat pump can work in dual source mode using either ambient heat as an evaporation source or low temperature solar heat (15–20 °C).
Electricity is mainly sourced from the grid. However, if the biomass boiler and the heat pump are not needed to supply thermal power for space heating, a biomass-driven ORC operation is possible and, therefore, the electricity can be produced by the ORC. Alternatively, if the temperature level from the solar collectors is enough for driving the TEGs (50–60 °C), this component can be used for this purpose.
DHW is mainly produced from solar heat.
Space cooling is produced by the reversible heat pump either as a stand-alone component (colder climates) or as a cascade system with the sorption chiller (warmer climates). In this latter case, the evaporator of the adsorption module is connected to the compressor of the heat pump to reduce the temperature lift and therefore increase the efficiency of the heat pump.
Electricity is mainly sourced from the grid. In colder climates, if there is no cooling demand the electricity can be supplied also by the ORC using solar heat as the driver. If the temperature level is not enough to drive the ORC, solar thermal fluid can be passed through the TEGs thus exploiting them for the electricity supply.
DHW is directly supplied by solar heat with the biomass boiler working as a back-up unit.
Space heating demand is covered either by solar thermal collectors or the dual source heat pump mode.
Electricity can be provided by TEGs or the ORC. The main heat source for the ORC is the biomass boiler (in colder climates) or solar heat (in warmer climates).
4. Methodology and Main Assumptions
The methodology followed for the evaluation of the potentiality of the multi-generation system proposed is schematically presented in
Figure 2. The main focus of the study was the energy analysis and the results are presented in the next sections. The primary goal of the analysis was to assess whether the potentiality exists to reach a high share of renewables (i.e., up to 100%) with the solar-biomass system proposed.
The first step of the analysis was to evaluate the age, distribution and energy demand profiles of offices in different European climates. Characterizing the structure of offices and in general non-residential buildings is complex due to the large varieties of different structures and the poor availability of material in the literature. In the next sections the potential of the multi-generation system is evaluated in non-residential buildings using the office building typology as a reference where construction characteristics and the energy demands were collected from the project ENTRANZE [
25] for the cities of Madrid, Berlin and Helsinki. These data were calculated through simulations considering as a reference a five-floor office building with a surface-to-volume S/V ratio of 0.33 and a net heated area of 2400 m
2. The DHW demand was obtained from the Standard EN 15316-3-1; the room set-point for winter operation was 21 °C (30% humidity) whereas the room set-point considered in the summer was 26 °C and with 70% of humidity. In both seasons, a ventilation rate of 0.8 air changes/hour was considered.
At the same time, a simplified model was realized in TRNSYS18 [
26] for the calculation of the heat available from solar collectors and the correspondent temperature levels for the different locations and times of the year. Further information about the model is reported in
Appendix A. The model was structured so that four finite temperature ranges could be obtained as the outlet of solar collectors: 80–90 °C, 60 °C, 35 °C and 15 °C. These corresponded to the nominal temperature for the adsorption chiller/ORC operation, DHW provision, space heating and solar-assisted operation of the heat pump, respectively.
Finally, a prioritization scheme was defined to identify which generation source should be preferred for each load. The leading idea behind such a scheme was to prioritize renewable energy over other sources and, among the renewable sources used, to prioritize the solar one over the biomass one. This concept is presented in
Figure 3 and can be schematically summarized as follows:
- -
Heating demand. (1) Solar heat is the preferred source because it is the least dispatchable resource and the one with the lowest environmental impact. In order to activate the direct space heating supply from solar heat, a minimum temperature level of 35 °C on the solar collectors is needed and up to 80 °C if there is not a contemporary demand for DHW. (2) The heat pump in the solar source operation is the second choice where solar operation indicates the use of solar heat at 15 °C as a heat source for evaporation. (3) The biomass boiler, which is basically sized and operated as renewable back-up heating source.
- -
DHW. (1) Solar heat represents the preferred heat source with (2) the biomass boiler as back-up when solar heat is not available.
- -
Cooling demand. In colder climates all of the cooling demand is covered with the reversible heat pump. In warmer climates, if there is enough solar heat to drive the adsorption module, the operation of the heat pump in cascade operation is preferred in order to minimize the electricity consumption. If there is not enough heat, the reversible heat pump as a stand-alone component is used.
- -
Electricity generation. In warm climates, (1) TEGs are the preferred system whereas (2) the ORC works only in intermediate seasons because high temperature heat is needed to drive the sorption module during the summer. In cold climates, (1) the ORC is the preferred generation method especially during summer followed by (2) TEGs. There is a third option that is not considered in the present analysis, which is the biomass-driven operation of the ORC. The case studied represents, then, the worst-case scenario (the most conservative one), in terms of the share of renewables because the increased operation of the ORC would reduce the electricity consumption from the grid and therefore from fossil fuels.
Main Assumptions
Starting from the inputs described above, the systematic assessment of the hybrid system for different scenarios was carried out. For the calculation, several assumptions were made:
The flow rate of the thermal fluid of the solar generation section was continuously varied to keep the set outlet temperature.
The efficiency of solar collectors was calculated with the TRNSYS model described in
Appendix A and was based on the data supplied for commercial components.
The EER (energy efficiency ratio) of the heat pump was 3:3 for the solar-assisted operation, 2:5 for the stand-alone operation in the summer and 4:0 for the cascade operation in the summer, corresponding to the performance of a commercial unit taken as reference. The Coefficient of Performance (COP) of the sorption module (thermal efficiency = cooling provided/heat input) was 0.5 [
22,
27], the efficiency of the ORC was 4% [
28] and the efficiency of the TEGs was 2.5% [
29]. The efficiency of the cascade adsorption module, ORC and TEGs was considered to be constant.
All of the efficiencies of the components were considered constant, regardless of their size and operating conditions (i.e., ambient temperatures and part load). Despite being simplistic assumptions, as cautionary values (a low to medium range of expected ones) were used and given the main aim of the analysis was a reference for the evaluation of renewable penetration in the tertiary sector, they did not represent a limiting factor for the obtained results.
The ORC was operated only if solar energy was available whereas the biomass-based operation was not considered. This underestimated the potentiality of the system in terms of overall renewable energy that could be self-consumed but allowed a simplification of the analysis that could be refined once more sophisticated controls were applied.
The storages in the system (short-term storage and buffer) were sized in order to compensate for daily fluctuations of the temperatures thus guaranteeing a constant inlet to the user for the hours of the day in which heating, cooling or DHW demand was needed.
The electricity produced was used only for the parasitic consumptions of the system (i.e., the pumps and the auxiliaries of the dry cooler) whereas the other internal loads were not part of the analysis.
A summary of the selected cases and main assumptions is given in
Table 1.
6. Discussion
6.1. Layout Adaptation According to Climatic Conditions
A first evaluation was done on the design and research guidelines towards the application of such a complex and flexible system in office buildings. In order to do so, a techno-economic analysis might be the most appropriate tool but the combination of technologies currently proposed is still far from the market and cost-based information on most components (i.e., the solar collectors with integrated TEGs, the reversible heat pump) cannot be estimated with sufficient confidence. Accordingly, economic considerations have not been used as criteria for the evaluation of the system. Nonetheless, it is worth remarking that the results presented here indicated the promising potential of the system, which could boost research and increase the maturity of the system towards a practical application.
Figure 9 and
Figure 10 show the contributions of the different heating sources for space heating and DHW purposes. All of the examined cases (i.e., locations and extension of solar collectors) were reported to better compare the different cases. It was noticed that, passing from the southern location (Madrid) to the northern one (Helsinki), the contribution of the heat pump to the overall space heating supply changed. Indeed, for the selected control strategy, the heat pump was operated only at high efficiency when heat at low temperatures (15–20 °C) from solar collectors could be exploited. As in northern countries the available heat in the winter is usually low, using the heat pump for space heating is not convenient. Therefore, while in Madrid the share of heating produced by the heat pump was 20–35% (according to the number of solar collectors installed), it was only around 10% in Berlin and almost zero in Helsinki. However, it is worth remarking that the specific energy mix for electricity in Helsinki and Berlin included a larger share of renewables [
37,
38,
39] than in Madrid. This meant that an optimization of the system with the scope of maximizing the use of renewables (both direct for self-production and indirect from a country-specific energy mix) might be to increase the operating hours of the heat pump during winter. As discussed in the previous section, the contribution of biomass was needed only for 50% of roof surface coverage with solar collectors in Madrid. In Berlin and Helsinki, biomass was needed to cover from 40% to 50% of the share for space heating. In this case, the system for very cold climates could be simplified, avoiding the installation of the heat pump and relying only on solar collectors and biomass for space heating purposes. However, this conclusion should be supported with a cost-benefit analysis that is not included in this study.
The DHW demand could be efficiently and to a large extent covered by using solar energy in all climates including Helsinki. It is worth stating, however, that such a result is extremely dependent on the building type considered; office buildings are characterized by a low DHW request. Considering instead the DHW demand for residential purposes or for other tertiary buildings such as hotels, it might be significantly higher therefore reducing the flexibility of the operation of the proposed hybrid system.
To deeper understand the possibility of the system in terms of renewable energy utilization, the electricity produced and supplied by the grid is shown in
Figure 11 for all of the six scenarios investigated. It is necessary to remark that both the operating hours of the TEGs and the ORC might be underestimated; as explained in
Section 4, the ORC in the present analysis was run only when there was solar heat available whereas in the real system implementation it would be possible to also use the biomass boiler when there was not any contemporary need for space heating or DHW. Similarly, the TEGs were operated only with solar heat at 50–60 °C but they could also be operated in “night mode”, i.e., exploiting heat rejected at low temperatures and heat from the short-term storage (at T < 60 °C) during the night for electricity production [
40]. Both in Berlin and Helsinki for the smaller collectors’ surface, the TEGs were not operated. This was due to the fact that the heat at the temperature level needed for TEGs operation was mainly used for DHW according to the prioritization scheme in
Figure 3. The contribution of the ORC instead was 9% in Berlin and 11% in Helsinki for 50% solar collectors, showing a good potential for the application of such a system. For the larger collectors’ surface, in Helsinki only the TEGs were operated. This was due to the lack of a significant amount of heat at 90 °C even in the summer needed to drive the ORC whereas more operating hours were guaranteed by TEGs. In this climate, the use of a biomass-driven operation of the ORC could represent the best solution to reduce the electricity drained from the grid. Berlin represented the most balanced case (with the larger extension of solar collectors) because up to 30% of renewable electricity out of the total needed for the auxiliaries could be produced equally distributed between TEGs (14%) and the ORC (15%) even under the oversimplified management applied. In Madrid, a large share of heat at high temperature was needed to drive the cascade chiller. Therefore, for the 50% coverage scenario, only the TEGs were operated whereas a balance between the ORC and TEGs existed for the 100% case and up to 35% of overall electricity used came from self-production.
Such results prove the great flexibility of the proposed hybrid system, which can be easily adapted to all of the different scenarios and, if properly managed, can efficiently exploit different generation sources for a non-residential user allowing long operating hours on renewables only. The results will also be used as a starting basis for the definition of an advanced control system. In order to properly manage the multi-generation system proposed, artificial intelligence-based techniques can be used that define the scheduling and operating hours of components based on weather forecasts, predicted energy demand and energy prices using self-consumption as the objective function.
6.2. Comparison with Residential Buildings
The methodology and energy analysis that were presented in the previous sections were also applied by the authors for evaluating the alternatives and possible share of renewables in residential buildings [
21]. The main peculiarity of office buildings over residential ones is the lower DHW demand compared with the heating/cooling one. In the case of residential buildings, two different scenarios were considered; i.e., standard buildings (representative of the current building stock distribution) and new and renovated buildings considering the nZEB directives in force for the residential stock at a European level.
One significant difference in the operation of the system for the two cases is that for office buildings the SFheating is higher than in residential ones but the SFDHW is lower because a higher share of solar heat is needed to cover the space heating. Moreover, at a building level, the integration from the heat pump is mainly used for cooling whereas in offices it is used to some extent (20% to 35% of overall annual demand) for heating.
This indicates that in office buildings different layouts and operational strategies should be considered. For instance, if the focus of the installation is to cover heating and DHW demand with the highest solar fraction possible, components such as TEGs are not needed in the installation, which can then be simplified. On the contrary, due to the need for biomass integration, it is also possible to design the system in order to increase the self-production of electricity through the ORC (mainly in Madrid and Berlin) or TEGs (mainly in Madrid and Helsinki).
If a comparison is made on the total share of renewables achieved in both cases, the results for offices and buildings are comparable as the lower demand covered by renewables in offices for electricity and DHW is compensated by the higher solar share for heating, further demonstrating that the flexibility of the proposed system goes in the direction of an increased self-consumption and decentralized energy generation.
6.3. Towards a 100% Renewable Scenario
The ultimate goal of the study here presented is the answer to the following question: can we use a multi-generation system such as the one presented to consume 100% renewable energy? In order to answer such a question, the amount of renewable energy over the total energy demand (share of renewables) to supply heating, cooling, DHW and electricity (for the heat pump and auxiliaries only) is shown in
Figure 12 for the different solar collectors’ rooftop covering. For a fair comparison, the contribution of the electricity taken from the grid for the heat pump and auxiliaries was weighted considering the following share of renewable generation for the national electrical grid: 47% in Germany [
41], 46% in Finland [
42] and 33% in Spain [
43]. What is clear is that even with the simplifications applied, a share of renewables of 100% for DHW is possible in all locations and more than 80% of the heating request is covered only using renewable energy with a share up to 92% in Berlin and Helsinki. In Madrid, especially for the larger collectors’ installation, even cooling is almost totally covered by using solar heat. The relatively low share of renewables for cooling in Helsinki and Berlin should not be misinterpreted; as shown in
Section 5.1, the cooling demand in Helsinki is practically zero and therefore having more than 90% of renewables for heating and DHW already indicates that the path towards a fully sustainable building is reached. Similarly, the cooling demand in Berlin is only 7% of the overall heating demand and therefore the share of the non-renewables on the absolute amount is low as well. As shown also is that the share of electricity, which is another 5% of the overall energy demand in Berlin and Helsinki, can be more efficiently produced.
It is then possible to state that the road for reaching a full operation with renewables with the proposed solution is close, thus entirely proving that the use of multiple generation sources and a creative and flexible configuration with adaptable components and management is the key for the sustainable cities of the future.
In this regard, it is worth noticing that the solar-biomass-sourced system introduces a great complexity compared with standard systems and there is a need for several additional components. To cover for this extra complexity, a strong action at a regulatory/policy framework level is needed through financial aids (i.e., incentives for equipment purchasing) and energy efficiency and distributed renewable generation at a building level. This is indeed the path indicated by the EU in the Clean Energy for all Europeans package [
44] where a set of energy efficiency/renewable generation targets is indicated and the commitment towards financial instruments to achieve the green transition are reported. This would greatly facilitate the market penetration and the economic sustainability of the solution proposed here. At the same time, strong research is needed at a control level; the great level of complexity calls for a control strategy that combines multiple objectives and the need to include model predictive features [
45,
46,
47] in order to achieve an optimized operation and, at the same time, relieve the burden on the electric grid and increase revenues for the building owner.
7. Conclusions
The present paper discusses the application of a solar-biomass system for heating, cooling, DHW and electricity production applied to office buildings. The system mainly consists of solar collectors with TEGs, a biomass boiler and a reversible heat pump/ORC that, in summer mode, can be operated as a cascade system by coupling it to an adsorption module.
A literature survey on the energy demand of the building stocks and a simplified TRNSYS model to define available solar heat at various temperature levels were used as the input for an energy analysis. The results were reported showing the energy flows between the various components of the system. Six scenarios were considered; an installation in Madrid, Helsinki and Berlin with solar collector surfaces of 50% and 100% of the total rooftop area. The focus of the analysis was the identification of the potentiality of the system in terms of share of renewables achievable and showed that, in all locations, 100% of DHW was obtained with renewable energy, mainly exploiting solar heat. In Madrid, where the cascade chiller operated in the summer, up to 90% of the cooling demand could be covered using the solar-driven adsorption-compression system. The share of renewables for heating varied between 70% in the case of Madrid (where solar heat represented the predominant contribution also in the winter) and up to 92% for Berlin and Helsinki (where the majority of the heating request is supplied by the biomass boiler). Future activities will be devoted to the further development of components and to the definition of advanced control logics with the aim of increasing even more the amount of renewable energy exploited towards a 100% scenario.