An innovative multi-source energy system for small-scale combined heat and power (CHP) applications was analyzed. The integrated apparatus consists of a biodiesel-fired organic Rankine cycle (ORC), a photovoltaic unit (PV) and a wind turbine (WT). The three subsystems work in parallel to fulfill the electric and thermal demand of a block of 40 dwellings that is part of a smart community. A conventional packaged firetube boiler fuelled by natural gas [74
] satisfies the heat request when the ORC supply is insufficient while the thermal surplus is adopted for the production of biodiesel through a pre-existing transesterification process of waste cooking oils (WCOs) [35
], which is collected from the smart community (including dwellings, restaurants, refectories, etc.) and it is considered with no cost. The ORC system is able to work at part loads to fulfill the energy demand of the users. To this purpose, a parametric investigation was done and Figure 7
shows the electric and thermal performance as a function of the ORC electric load. The partial load characteristics were found in accordance with previous works and the minimum load was 40%, corresponding to the minimum evaporation temperature [1
]. At full load the electric and thermal efficiencies were equal to 14.6% and 70.5%, respectively, while the corresponding values were 8.2% and 77.0% at minimum. The EUF
dimensionless parameter was always higher than 83.5% and a negligible influence of the electric load was noticed.
Afterwards, the ORC nominal power was defined as adopting a parametric investigation and a thermal-driven strategy was adopted to guarantee the users heat demand on an hourly basis. Particularly, Figure 8
highlights the thermal nominal power and the annual balance in terms of self-consumption, integration and surplus as a function of the ORC electric power. As predictable, the higher the nominal power, the higher the self-consumed energy. Nevertheless, it is evident that 90% of the heat demand was provided when the ORC thermal power (PORC,th
) was equal to 106 kWth
and a slight influence on the self-consumed thermal energy was present for larger units. On the other hand, the surplus moved from 68% to 178% when PORC,th
passed from 106 to 200 kWth
. Consequently, the ORC nominal power has been defined in order to assure that the thermal excess and the boiler integration are equal [28
]. The selected unit presents a nominal electric power equal to 12.7 kWel
while the thermal power was 63.5 kWth
. The corresponding thermal self-consumption reached 68% whereas the surplus and integration were 32%. The ORC unit satisfies 41.1% of the electric load while the electric excess that is injected to the grid was 8.8%.
The selected ORC apparatus was combined with production photovoltaic systems and wind turbines in order to increase the performance of the CHP system and improve the system global efficiency (Figure 1
). To this purpose, nine wind turbines were taken into account (PWT
= 1–60 kWel
) while the peak power of photovoltaic subsystem (PPV
) ranges from 0.31 to 63 kWel
(1–200 modules). According to Equations (11) and (12), the minimum distance between dwellings and the wind turbine ranged from 90.5 (PWT
= 1 kW) to 241.3 m (PWT
= 60 kW) to satisfy national noise level requirements whereas the total PV modules surface was between 1.63 m2
= 0.31 kW) to 326.1 m2
= 63 kW). As expected, the wind turbines and photovoltaic unit present an intermittent operation during the year depending on the weather conditions. As an example, Figure 9
compares the yearly performance for WT and PV systems of similar maximum power on hourly basis.
3.1. Multi-Variable Optimization
Several ORC-PV-WT arrangements were compared in order to define the most suitable configuration of the multi-source integrated system. To this purpose, a multi-variable optimization was adopted. The optimization criterion aimed at maximizing the electric self-consumption and minimizing both the payback period and the electric surplus (t-S-s approach).
shows the three parameters for all the investigated configurations while Figure 11
a,b illustrate the corresponding projections on the coordinate planes (t-s and S-s planes, respectively). The ideal point refers to the minimum payback period (tmin
= 6.9 years), the maximum self-consumption (Smax
= 64.5%), and the minimum surplus (smin
= 8.8%). The investigation shows that the sole biodiesel ORC system presents the minimum electric self-consumption (Smin
= 41.1%), surplus and payback period. Conversely, the integration assures a noticeable upsurge in the electric demand fulfillment, with percentages larger than 50% when at least 10 kW wind turbine was installed, independently on the PV peak power.
The maximum self-consumption was 69.9% when PWT
= 60 kW and PPV
= 63 kW. It is noteworthy that the higher the self-consumption, the higher the electricity injected to the grid (Figure 4
b). Furthermore, the payback period increased progressively with the self-consumed electricity when at least 10 PV modules (3.15 kWp
) were installed. Consequently, a proper trade-off should be identified and the minimum distance criterion to the ideal point was used to select the optimal multi-source integrated system [21
]. Specifically, the analysis recommends integrating the selected ORC subsystem (PORC,el
= 12.7 kWel
= 63.5 kWth
) with 6.3 kWel
photovoltaic unit and 10 kWel
wind turbine. The optimized multi-source energy equipment provides 86.9 MWhel
for the ORC alone) per year and it guaranteed 56.1% of the dwellings electric demand whereas the electric excess was 30.8% and the payback period was equal to 7.7 years (Figure 8
). ORC, WT and PV subsystems provided 57.3%, 30.2% and 12.5% of the overall electric energy production, respectively. In this case, the acoustic influence area of the wind turbine to fulfill the national noise regulations was about 65,600 m2
, with a minimum distance between apartments and WT lower than 145 m.
The performances of the selected multi-source energy system are compared with the systems selected adopting two parameters optimization (Table 5
), referring to the ideal points in Figure 10
and Figure 11
. The surplus minimization constraint was imposed for all the optimization criteria. The optimized configurations present the same ORC unit (PORC,el
= 12.7 kW), as already observed, while the nominal power of the wind turbine and photovoltaic subsystems were different (Figure 12
). Specifically, when the payback period was not considered, the S-s optimization suggests a total electric power equal to 46.6 kW, with a 20 kW wind turbine and 13.9 kWp
PV unit. In this case the maximum self-consumption (61.1%) was guaranteed but the electric surplus was high (58.5%) and the minimum distance between dwellings and wind turbine to maintain sound pressure levels lower than 40 dB was equal to 162.4 m. Conversely, the adoption of the t-s approach assured a significant decrease in the electric surplus (10.1%) but the integration from the grid reached 56.9% owing to the absence of wind turbine and the negligible share of the photovoltaic unit (1.9 kWp
). The t-S-s approach permitted to obtain a similar self-consumption of the S-s criterion (56.1%). However, the electric excess and the payback period were reduced. Furthermore, the comparison with the single ORC unit shows that the integration of the different technologies assured an increase in terms of operating hours (from 5899 to 8760 h/year).
The comparison between the performances of the optimized systems in terms of marginal differences is proposed in Figure 13
. To this purpose, the t-S-s criterion was assumed as the reference. A significant increase (+60.7%) in the total electric power was observed when the economic aspects were beyond the optimization goal (S-s approach) whereas lower rises in the self-consumption (+5.0%) and total investment cost (+28.8%) were registered. This configuration guarantees the maximum global efficiency (24.0%) and primary energy saving (40.0%), and the shares of ORC, WT and PV subsystems to the electric production were 30.2%, 46.2% and 23.6%. When the t-s optimization was adopted, the payback period was lower than 7 years and a decrease in the initial investment equal to 34.7% was registered. On the other hand, a noticeable electric integration (+30%) from the grid was necessary and the global efficiency dropped to 10%. Similar results were found for the system configuration that guarantees the minimum payback period. In particular, the results demonstrated that for the investigated innovative multi-source energy system the sole economic optimization was not able to promote an efficient development of renewable DES and the proper balance of the electric grid. In this case, the global efficiency and the electric production reduced significantly with respect to the proposed optimized system (−7.5% and −38.9%, respectively), and a negative impact in terms of both system self-sufficiency and greenhouse gas emissions was produced owing to the higher electric integration from the grid (+11.1%), the lower injection to the grid (−19.5%) and the 2019 renewables share in the national electric mix (about 35%) [77
To evaluate in more details the possible advantages of the multi-source integration, the t-S-s optimized configuration was compared to the single-source units characterized by similar nominal power (Pt-S-s
= 30.2 kW, PWT
= 30 kW and PPV
= 29.9 kW). Figure 14
highlights the percentage energy balances and the fuel consumptions for the selected systems. A noticeable increase in the electric and thermal production was noticed when the integrated system was selected. Furthermore, the integration of ORC permits to overcome the non-programmable nature of WT and PV technologies and to increase the flexibility of the CHP apparatus with the possibility to reduce the fuel consumption. Specifically, the natural gas consumption moves from 36,285 m3
/year for the WT and PV alone to 11,160 m3
/year when the ORC system was adopted.
In this case, the yearly biodiesel request was equal to 47.8 t/year and permitted to satisfy both the thermal and the electric demand of domestic users. In this way the decrease in fuel consumption was equal to 22.0 tons of oil equivalent with respect to the single-source systems.
3.2. Monthly Energy Balance
Finally, the monthly energy balances of the selected integrated system (t-S-s criterion) to satisfy the electric and thermal loads of a block of 40 dwellings are shown in Figure 15
. The thermal load was significantly influenced by the heating period (from December to March in Palermo city, in line with the Italian legislation [78
]. Conversely, the dwellings electric demand presents the highest values (larger than 376.2 kWhel
/month/apartment) during the summer period, due to the cooling request, whereas analogous demands were observed in the other months (about 150 kWhel
a illustrates that the multi-source energy system never fulfilled the global electric demand and 43.9% integration is necessary on an annual basis. The withdrawal from the grid was registered also from October to March although the electric production was higher than the energy request. In fact, the thermal and electric loads were not concurrent during the day. The optimized system satisfied 78.9% of the electric demand from September to May whereas the electric surplus reached 53.9%. On the other hand, a significant integration (70.4%) is noticed from June to August owing to the air conditioners load with a small fraction of the electric production (3.8%) injected to the grid.
The investigation (Figure 15
b) shows a noticeable thermal excess from May to September (about 78% of the total production). This thermal surplus was used to partially satisfy the thermal demand of the transesterification process for the production of biodiesel from waste cooking oils. During the winter, the auxiliary boiler is fundamental to meet the peak thermal demand. In particular, the yearly natural gas consumption was close to 11,000 m3
and a very low thermal surplus was found. In this way, the proposed multi-source hybrid CHP system guaranteed a primary energy saving index larger than 30% and an energy utilization factor equal to 83.8%.
The work demonstrated that the optimized integration of different technologies (i.e., ORC, WT and PV) and different final users (i.e., domestic and industrial consumers) permitted to overcome the limits of the particular technologies (e.g., intermittency and stochasticity of wind and solar energy) and allowed a significant primary energy saving as compared to the corresponding single-source energy systems. The proposed integrated solution offers interesting opportunities to achieve the nearly zero energy building (NZEB) target in the residential sector with positive effects in terms of operating costs and environmental impact. To this purpose, the advantages in terms of saving emissions will be highlighted in future work.