7. CONCLUSIONS AND FUTURE DEVELOPMENTS
7.1. Conclusions and summary of key findings
In view of the environmental sustainability issues that humanity is facing, mainly in cities, a thorough regeneration of the energy systems is required at all scales. The present Thesis aimed to justify the selection of the district scale as the optimal one to improve the energy performance of the built environment, increasing renewable energy integration and exploring new energy models such as demand side management on a district scale. The motivation was the fact that further research is necessary to understand the potential benefits of urban scale implementations, since most previous research has traditionally focused on individual buildings.
Supported by recently published studies, Chapter 1 of this Thesis established that the former approach, in which the Near Zero Energy concept mainly focused on the individual level performance, was not optimal. This was justified by the fact that looking beyond the individual level offers many advantages, such as the exploitation of the synergies between different buildings. In addition, most approaches on a large scale lack adequate representations of the buildings and make many simplifications in their calculations. Thus, further research was necessary to develop robust evidence on the behavior of innovative energy models at large scales.
7.1.1. Achieving accurate energy analyses on a district scale
In Chapter 2, the drawbacks of most current approaches for urban energy analyses (which make use of simplified methods) were highlighted. In addition, the chapter also made some progress to set up the means to achieve accurate evaluations of solar potentials, energy demands or demand side management opportunities on a district scale, which require dynamic interactions that are often disregarded in previous studies.
First, a study to determine what fraction of the electricity demand could be covered by using solar PV potentials at an urban and regional scale was presented. This was done by using CityGML 3D city models and real electricity consumption data of the municipalities involved. According to the results that were obtained, PV systems could generate 77% of the electricity consumption in the region by using all available roof space, or 56 % considering only roofs with a minimum surface area and insolation for an economically feasible PV installation. The main conclusion that was reached was that, when properly designed, PV systems could significantly decrease primary energy consumption and emissions at large scales, reaffirming their usefulness and their important role in future energy-efficient districts.
The next application that was included in Chapter 2 aimed to overcome the computational issues of evaluating mutual shading at urban level when using 3D models, since the number of surface interactions and radiation exchanges increase exponentially with the scale of a district. In addition, considering this matter is very important for any reliable energy analysis in an urban context. The previous approach on the evaluation of the PV potentials was very complete, but mutual shading was evaluated through reduction coefficients found in the literature due to the impossibility of evaluating all the interactions between such a huge amount of buildings. To overcome this issue, the new study introduced innovative tiling strategies that, when applied to 3D models, allowed to quantify the impacts of urban shading and multiple reflections
accurately. Two different case studies of different densities, Manhattan (New York) and Ludwigsburg (Germany) were chosen, quantifying the solar potentials, urban shading ratios and distinguishing between roofs and facades of different orientations. The results showed that for instance, high-density urban areas such as Manhattan might reduce their annual solar irradiance by up to 60% in facades and 25 % in roofs. In this case, using tiles of 500 meters and 200 meters overlap would be a minimum requirement to compute solar irradiance with an acceptable accuracy, whereas in medium density areas such as Ludwigsburg tiles of 300 meters width and 100 meters overlap would be sufficient.
Another of the possibilities offered by the use of 3D models is the evaluation of the thermal performance of all the buildings of a district or municipality, which is very important to improve energy efficiency at such scales. Urban 3D models enriched with semantic data on building’s use, year of construction or thermal properties allow quantifying measures to improve energy efficiency or the integration of renewable energies, offering an excellent support for establishing climate protection concepts and policies on a municipal and regional level. Chapter 2 takes a step in this direction, including a study on urban energy demand evaluation through 3D city models. This study allowed us to conclude that, with reliable input data such as updated 3D models as well as buildings’ construction and usage data, it is possible to quantify refurbishment scenarios or renewable integration for whole regions in an accurate way.
Even though tools such as SimStadt are able to calculate the energy demands of thousands of buildings in a region, as seen in the previous study, simplified calculations such as steady-state methods are frequently used. The interconnection between 3D models and tools that are able to assess the thermal performance of whole districts in an accurate and dynamic way is however necessary to assess time-dependent strategies such as demand side management on a large scale. To overcome these drawbacks of simplified energy performance simulations, a new open-source tool was developed in this Thesis, which allows performing dynamic simulations of whole districts in an accurate and user-friendly way. This was done by creating a plugin programmed in Python within the QGIS software, and an additional tool programmed in VBA that allows to simulate whole districts, considering any scenario that involves changing the characteristics of construction, ventilation or operating schedules. This tool is currently able to model any district of the world where open-source geometric data is available, but it could also be coupled with different 3D model formats, which would provide the necessary geometric information to be used as input for the HULC software. As a last step, Chapter 2 mentioned clustering techniques which would allow to reduce the study domain and choose the representative (or archetype) buildings of a district. The reason for this is that dealing with models that contain even hundreds of buildings is quite complex if their thermal performance wants to be evaluated in a dynamic way. In addition, when a district needs to be characterized accurately, a monitoring campaign should be planned to evaluate their real performance and use. However, the budgetary and technical constraints might only allow to monitor a few buildings, which is why clustering techniques should be used beforehand. Once the sample of buildings is reduced, detailed monitoring and modeling of the buildings could be carried out.
7.1.2. Exploring demand side management opportunities
Having already explored the advantages offered by urban scale analyses, the next issue addressed in this Thesis was the evaluation of the benefits that could be obtained from demand side management approaches, with an emphasis on the exploitation of the structural thermal energy storage capacity of buildings. To prove the benefits of DSM in buildings and districts, Chapter 3 presented two new applications.
The first application focused on the individual level performance of a dwelling located in a plus-energy district in Germany, evaluating the impacts of different dynamic pricing strategies by managing the activation of a heat pump in response to price changes. A validation of the model was carried out by using experimental data of the dwelling under study. In this study, evidence was obtained on the importance of
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better alternative than fixed thresholds. Savings up to 25 % were achieved by using optimal strategies, also increasing the self-consumption ratio of the household and without sacrificing the thermal comfort of the occupants noticeably. In summary, the outcomes of the study showed the great benefits that can be derived from the use of heat pumps and the thermal energy storage capacity of buildings within a DR framework. However, the results also emphasized the importance of looking at all the implications of using a certain strategy, since the decisions to choose optimal strategies should not rely solely on one aspect such as cost savings.
As a follow up to the previous work, the second application increased the scale and evaluated the benefits of allowing direct control of the heat pumps in a district through a cluster manager, to provide flexibility and obtain benefits from the participation in secondary reserve markets. Again, this was done by taking advantage of the structural thermal energy storage capacity of the buildings of the district. By performing simulations with a time resolution of 30 minutes, the study concluded that the DR potential is significant, but for profits to be made some changes are necessary before these approaches can be put into practice, even if the prices paid for providing flexibility were higher. Particular attention should be given to the activation times, and most importantly their duration, due to the minimum running time of the heat pumps. Nevertheless, their benefits with regards to providing flexibility for reserve markets were demonstrated.
7.1.3. Development of energy baselines
Once the benefits of widening the scale of energy analyses as well as the implementation of demand side management approaches were evidenced, Chapter 4 of this Thesis focused on defining a methodology for a proper characterization of districts through monitoring. From the point of view of the author, this is the best way in which a real district should be evaluated, reflecting the reality of the operation of the buildings with sufficient accuracy.
A novel proposal for obtaining energy baselines in residential buildings was developed and validated, both theoretically and experimentally. The proposed model, named QT, consists of a transfer function that is able to characterize any building based on measured data of its thermal performance. This model depends on the indoor air temperature, as a thermal response of the building, and the air-conditioning consumption, as response to the air-conditioning systems. Also, the model contemplates the dynamic effects due to the high thermal inertia of buildings, considering what is happening in the current time-step but also in the previous ones. The procedure is also sensitive to different operating conditions of the buildings, which may vary through time.
A dwelling monitored in a social housing district in southern Spain was used to validate the methodology, with very satisfactory results. Last of all, several possibilities for combining the QT model and DSM techniques were suggested for the application of the energy baselines.
7.1.4. Characterization of districts: application of the methodology
The author of this Thesis believes that one of the most distinguished advantages provided by the application of the energy baselines presented in Chapter 4 is the characterization of districts. In Chapter 5, the methodology that was developed was first applied to characterize a district. In this case, temperature and total electricity consumption measurements were available through a monitoring campaign, but with no disaggregation of the air-conditioning consumptions. A methodology is presented for such cases, obtaining satisfactory results from the application of the baseline models.
Frequently, improving the thermal comfort of buildings involves the implementation of thermal mitigation strategies by retrofitting the building stock. By contrast, an innovative study was proposed in Chapter 5 of this Thesis, which introduced the possibility of using surplus electricity to power the heat pumps of a social housing district to improve the thermal comfort conditions of the occupants. In this way, the benefits of
exploiting the rooftop PV potential of the district was assessed to mitigate extreme high and low temperatures, reducing also the energy bills of the occupants. Once the optimal strategies from an electrical and economical point of view were selected, the surplus electricity was used to power the heat pumps of the 235 dwellings of the district, heating or cooling the dwellings depending on the season. The results showed that the thermal comfort could be improved in average by up to 11% in winter and 26% in summer, or 24% and 44% respectively if the electricity was not used also to reduce the energy bills. In all cases, the thermal comfort differences among the dwellings of the district, which presented large disparities in the initial situation, were significantly reduced.
7.1.5. Exploiting the synergies between buildings through DSM approaches
Finally, Chapter 6 of the Thesis combined several of the elements, tools and strategies of the previous chapters. The purpose was to figure out whether further benefits could be obtained from the exploitation of the synergies between buildings, by implementing demand side management approaches on a district scale. Although in the literature it is frequent to find studies that focus on achieving cost savings by promoting energy sharing in districts, or others that use thermostatically controlled loads to perform demand side management at large scales, it is very unusual to find studies that combine both approaches, which motivated the study that was developed.
A case study was therefore proposed (a district in La Graciosa Island in Spain), in which a monitoring campaign allowed to know the real electricity consumptions of all the buildings in the district. The application included PV production, electrical storage, the use of the thermal energy storage capacity of the buildings as well as the possibility of sharing surplus electricity between different buildings. A novel rule- based algorithm, programmed in Fortran, was applied at 15 minutes intervals to encourage sharing surplus electricity between the prosumers of the district. The main purpose was to maximize the cost savings and the use of renewable energy. Different preconditioning strategies under 3 alternative time horizons were also evaluated.
Compared to the traditional situation in which each building self-manages its own importation and exportation of electricity, the proposed novel ESC concept demonstrated that it is possible to raise the cost savings, self-consumption and self-sufficiency ratios of a district in a similar way as batteries, but at less costs. Several scenarios showed that the new concept allowed to achieve significant savings, around 45%. However, a sensitivity analysis regarding the feed-in tariff showed that the results are very dependent on it. In general, the less the district is paid for selling the electricity, the more interesting it is to promote energy trading, since more locally produced electricity is retained within the district.
Apart from the economic benefits, the electricity exchanges with the power grid were also diminished, increasing the self-consumption ratio up to 42% and the self-sufficiency ratio up to 64%. Regarding the preconditioning strategies, the results showed that precooling was chosen instead of the normal schedule on around 66% of the summer days for all the considered time horizons, although the reductions of the air- conditioning costs were rather low. However, their use is justified by the fact that preconditioning measures are available at no additional costs.
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