In this section, the results of the four introduced simulation scenarios are presented and evaluated. Furthermore, the optimisation problems for the final three study cases—which include the BESS installed on the marina site—are formulated and utilised in the simulations.
7.1. Base Scenario
The simulation of the base scenario was carried out for the analysed summer week (15–21 July 2019). The scenario assumes that all boats, docking in the Ballen marina, participate in the DR programme. Within this framework, sailors schedule their electricity usage in the most cost-efficient way, shifting their demand to low-price periods. Moreover, the flexibility factor is fixed as .
The simulation results of the Ballen marina’s DR—for all boats participating—are presented in
Figure 7.
The visual inspection of the plot indicates that the load factor has improved. Furthermore, the demand was properly shifted from peak to off-peak hours, leaving the total weekly demand unchanged. For the sake of clarity, the load shifting action is graphically presented in
Figure 8.
The results confirm that time zones of the TOU tariff have been correctly designed, making the peak shaving and valley filling action beneficial for sailors. During the noon valley, the load has been either decreased or slightly increased, depending on the profitability of each action for the particular day. The maximum price difference—between green and red zones—is equal to 0.18 EUR/kWh. Thus, the participating sailors would save this amount for every unit of shifted energy.
The total load during the analysed week is equal to 8167 kWh, whereas PV supplies 1510 kWh, accounting for 18.5% of the weekly demand. Hence, the remaining energy of 6657 kWh (81.5% of the weekly load) is imported from the public grid. Since the load is much higher than the PV generation during the simulation week, no energy export occurs.
The possible cost savings for the marina are due to importing the energy during periods of lower prices, thanks to load shifting. The energy supplied by the local PV generation is assumed to be free for the marina, whereas it does not affect the price offered to sailors. In this respect, the Elspot price, the energy import from the public utility—for the scenarios with and without DR—and the PV generation for the analysed week are presented in
Figure 9.
The opportunities for decreasing the marina’s energy cost—which depends on the hourly-varying Elspot prices—are visible during high-price periods when the energy import is minimised by the peak shaving action. Nevertheless, the maximum price difference for the investigated week is equal to only 0.02 EUR/kWh. For this reason, the direct cost savings for the marina are anticipated to be lower than for sailors, taking into consideration the different types of tariffs developed for both parties. Further, the key simulation results for all boats taking part in the load shifting—for the entire summer week—have been quantified and presented in
Table 2.
The relevant measure of flexibility is the shifted energy, calculated as a sum of all clipped peaks. Equivalently, this parameter is equal to the sum of all valley-filling actions since no energy conservation is taken into account. In this scenario, the shifted energy accounts for 4.2% of the total load. With the increase of this parameter, greater cost savings and demand curve flattening can be achieved.
The most significant DR implementation result is observable in the load factor, improving this parameter by 17.4%. This outcome is advantageous for the marina’s energy system on many levels. First of all, the future grid enhancements could be postponed, considering the possible peak demand increase during the next summer seasons. Secondly, the load shifting could mitigate possible voltage regulation problems caused by uneven load distribution between the piers. Moreover, the decrease in energy losses would be another positive effect of the improved load factor. Finally, in the future, more boats could be docked and charged in the marina without hindering the grid operation.
Despite the hourly-varying energy tariff for the marina, the marina’s electricity buying cost has been only slightly reduced, leading to 0.2% savings. In this regard, the direct cost reduction is insignificant compared to the potential additional cost of the DR implementation. However, the indirect benefits for the marina could be still relevant, such as the advantages of load factor increase.
Furthermore, the cost savings for sailors—under the TOU tariff—are equal to 2.1%, which is a relatively low result. Thus, the cost-efficiency of load shifting may be insufficient to convince sailors to participate in the DR programme. The price incentives would need to be accompanied by educational programmes, enhancing the knowledge about the ecological benefits of appropriate energy usage. Otherwise, further financial inducements should be offered to sailors, such as additional incentives or a further decrease in off-peak energy price; however, this may be not a valid business case for the marina.
The essential parameter of the developed DR model—having an impact on the outcomes—is the flexibility factor, constraining the maximum load-shifting action. The assumption of
has resulted in a reasonable load profile, which is likely to be achieved in the real-life price-based DR implementation. Nevertheless, it is important to study how changing this parameter affects the DR simulation results. For this purpose, the sensitivity analysis is carried out for five different values of
: 0%, 25%, 50%, 75%, and 100%. The flexibility factor of
indicates that no load shifting can be applied. Conversely,
signifies that a flat load curve is aimed towards, with the load factor approaching
. The results of the flexibility factor sensitivity analysis are presented in
Figure 10.
It is observed that the variations in the flexibility factor significantly affect the load profile shape. When this parameter is increased, the demand curve is flattened, obtaining almost a flat profile at average daily demand in the case of
. Nevertheless, it is anticipated to be practically infeasible to level out the profile to that extent. Thereafter, the simulation results are outlined in
Table 3.
With different flexibility factors, the shifted energy varies from 2.1% () to 8.5% () of the total load. Moreover, this parameter has a substantial impact on the load factor, improving it by 26.2% in the best scenario. Notwithstanding this, the effect of the flexibility factor on the marina’s and sailors’ energy cost is much lower, leading to at most 0.3% and 4.3% savings, respectively. The initially adopted factor of proves considerable improvement of the marina’s grid operation while maintaining realistic assumptions. Hence, this value is used for all subsequent simulations.
Afterwards, the implementation of BESS for increasing cost efficiency is examined.
7.2. Cost-Efficient Operation of BESS
As indicated in previous research [
28], the battery is underutilised in the summer period, being idle from 15 to 24 July. With the objective of minimising energy exchange with the public utility, the BESS needs not act during the peak tourist season since the load is significantly higher than the PV generation. Thus, no energy export is required to be prevented by its action. In the future, this issue could be mitigated by increasing the renewable generation capacity—in the form of PV units, wind turbines, or wave energy—which in turn would improve the operation of the marina’s ICES from both technical and economic perspectives. Nevertheless, the existing PV plant size and hourly-changing pricing scheme suggest the need for investigating the possibilities of different BESS scheduling—with the aim of minimising the energy cost.
The cost-efficient BESS operation—under the developed hourly-varying marina’s tariff—is determined, utilising the battery optimisation model presented in
Section 6. For this purpose, the battery’s objective function is reshaped: instead of minimising the energy exchange, the objective is defined as minimising energy cost for the marina. Moreover, the power balance constraint—with marina’s load, PV generation, BESS action, and power exchange—is included:
Thereby, the optimisation problem is formulated as:
In this context, the flowchart of the optimal cost-efficient battery operation—which comprises charging from the PV plant, pre-charging from the public grid, and supplying local demand—is presented in
Figure 11.
To avoid emptying the battery before midnight—when it cannot charge from the PV units—the beginning of the 24-h optimisation windows is shifted from 24.00 to 12.00. This way, new optimisation windows begin at the time of typically highest PV generation. With this approach, the optimisation needs to start 12 h before and stop 12 h after the analysed period—which is applied for all scenarios that include BESS. Since the battery is mostly empty during the summer period, the initial SOC for all forthcoming simulations is assumed as
, equal to 2.5%. The BESS operation is investigated, assuming a typical round-trip efficiency value of
. The simulation results of the cost-efficient BESS operation, along with the load and Elspot prices, are presented in
Figure 12.
Taking advantage of the hourly-varying energy prices, the battery utilisation increases from 0% to 12.5% during the summer week. Since the local PV generation is fully self-consumed by the load, the battery charges only from the public grid. The overall grid import increased by 0.3% from the base scenario as a consequence of the battery losses. It is clearly seen that the BESS pre-charges during periods of low load and low prices, whereas the discharging action is performed when load and prices are highest. The battery does not reach , as the periods of low prices—during which pre-charging from the public grid is profitable—do not last longer than 4 h. In this manner, the optimisation objective constrains the BESS to be fully pre-charged. During the day of low variations in the price (16 July 2019), the battery has remained inactive. This event indicates that the pre-charging action during that day would lead to battery losses, which would be more costly than the possible benefits from varying electricity prices. Nonetheless, the BESS was successfully activated on the other days of the week, proving minimisation of energy cost for the marina.
Subsequently, the boat flexibility and battery models are combined, with the aim to determine the optimal operation of the entire marina’s smart energy system.
7.3. Boat Flexibility and BESS
In the pivotal scenario of the Ballen marina’s grid operation, the boat flexibility should cooperate with the battery system, ensuring benefits for both marina and sailors. For this purpose, the models of DR and BESS are integrated, forming a single optimisation problem. First, the power balance constraint is modified, incorporating DR power, battery action, power exchange, and PV generation:
The objective function is designed to minimise the overall energy cost, defined as a sum of sailors’ and marina’s costs. This way, the benefits will be provided for both involved parties. Within this framework, the optimisation problem—combining DR and BESS operation—is formulated as:
The load-shifting profitability is dependent on the TOU tariff for sailors, whereas the cost savings for the marina are subject to the price variations of the Elspot-based tariff. In this manner, PV generation and battery action do not affect the energy prices for sailors. Nonetheless, the BESS is scheduled with respect to the shifted demand, coordinating its action with the DR programme. Hence, the boats and BESS are scheduled simultaneously to provide the lowest overall energy cost, combining the algorithms presented in
Figure 5 and
Figure 11.
The simulations of the integrated DR and BESS operation are carried out with the assumption of all boats participating (
) and the flexibility factor of
. The results for the summer week are presented in
Figure 13.
The obtained results are similar to the outcomes of the separate optimisation of the DR and BESS action. Nevertheless, minor changes in the battery operation can be observed as the marina’s demand profile is reshaped by the load shifting. Moreover, the optimal DR action is also only marginally different—since the optimisation windows’ beginning is shifted from 24.00 to 12.00. Further, the simulation results of four performed study cases are outlined in
Table 4.
It is observed that the boat flexibility has a greater impact on the marina’s energy cost compared to the BESS operation in the considered week. Moreover, the battery alone cannot improve the load factor—as well as the energy cost for sailors—since its action does not affect the shape of the demand curve. Nonetheless, the battery utilisation of 12.5% was achieved for the scenarios, which include BESS. Within this framework, the combination of DR and BESS deployment is the most beneficial scenario for both marina and sailors.
Finally, the integrated DR and BESS operation is analysed for two different seasons when grid operation characteristics are considerably divergent.
7.4. Late Summer and Late Autumn Seasons
The preceding analysis concerns the peak tourist season, with high load and consequently PV generation shortage. This study is extended with two seasons, during which the marina’s grid is facing different challenges. Essentially, the periods with low load and either high or low PV generation are the most interesting. In the first case, the energy export—due to PV production excess—is substantial, and the cooperation of DR and BESS could partly mitigate it. In the latter one, the PV production is low and should be reasonably distributed, with the aim to increase self-consumption and minimise energy cost.
In this manner, two representative weeks during different seasons are chosen for further analysis:
Late summer: 9–15 September 2019, low load (341 kWh) and high PV generation (1759 kWh).
Late autumn: 21–27 October 2019, low load (324 kWh) and low PV generation (355 kWh).
First, the simulations are carried out for the late summer week, utilising the developed optimisation algorithm (
35). The simulation results of the DR and BESS cooperation are presented in
Figure 14.
The late summer load profile demonstrates a considerably different shape compared to the previously examined peak tourist season week. The characteristic boat demand periods are not easily distinguishable, and the peak demand does not exceed 11 kW. Nevertheless, the DR action has successfully levelled out the main peak on 9 September, also shifting the load on the other days.
In this period, the battery acts as a buffer for excess PV generation. This way, the BESS charges only the amount of power that is necessary to supply the load until the end of the 24 h time horizon. The remaining surplus PV production is sold to the public grid, with profit for the marina. The self-sufficiency of 100% is obtained, with the battery utilisation of 63.1%.
Thereafter, the optimisation results of the DR and BESS action for the late autumn week—with low load and low PV generation—are shown in
Figure 15.
Similarly, the demand curve does not clearly indicate the peak and valley periods. However, the load factor is visually increased by the performed load shifting. The battery acts in a similar way as in the previous scenario. The self-sufficiency of 100% is achieved while locally consuming the entire PV generation. Furthermore, the BESS is utilised at 100%, being active for the entire simulation period. Within this framework, the developed optimisation strategy is concluded to be adequate also for periods with low load and high/low PV production.
Ultimately, the simulation results of the integrated DR and BESS optimisation for the analysed weeks are presented in
Table 5.
During the late summer week, the shifted energy accounts for 5.3% of the total demand. The initial load factor is relatively low, and as a consequence, the load shifting results in a 44.9% increase of this parameter. Moreover, the combined DR and BESS operation results in a 100% reduction in energy import and an 11.2% decrease in energy export. The high PV production leads to a negative weekly energy price for the marina, which is interpreted as a profit from selling the excess energy. The optimisation of boat flexibility and battery action results in a 412.5% increase in the marina’s profit for the considered week, corresponding to 33 EUR of additional earnings. Nevertheless, the cost savings for sailors amount to 2.7%—under the TOU tariff—since the shifted energy is relatively low.
On the other hand, the baseline load factor is improved by 16.0% for the late autumn week. The energy import and export are completely eliminated, ensuring free marina’s operation. Nevertheless, the sailors’ energy cost is decreased by only 1.9%, as a result of 4.6% shift in the weekly demand.
The observed enhancements of the marina’s cost efficiency—for both considered weeks—are mostly due to the minimisation of the energy exchange. Nonetheless, the benefits of boat flexibility are less unambiguous compared to the peak tourist season. Therefore, the DR programme implementation is concluded to be more vital for periods of higher boat demand.