• Energy Research
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Abstract Small Mediterranean islands are remote, off-grid communities characterized by carbon intensive electricity systems coupled with high energy consuming desalination technologies to produce potable water. The aim of this study is to propose a novel dynamic, multi-objective optimization approach for improving the sustainability of small islands through the introduction of renewable energy sources. The main contributions of our approach include: (i) dynamic modelling of desalination plant operations, (ii) joint optimization of system design and operations, (iii) multi-objective optimization to explore trade-offs between potentially conflicting objectives. We test our approach on the real case study of the Italian Ustica island by means of a comparative analysis with a traditional non-dynamic, least cost optimization approach. Numerical results show the effectiveness of our approach in identifying optimal system configurations, which outperform the traditional design with respect to different sustainability indicators, limiting the structural interventions, the investment costs and the environmental impacts. In particular, the optimal dynamic solutions able to satisfy the whole water demand allow high levels of penetration of renewable energy sources (up to more than 40%) to be reached, reducing the net present cost by about 2–3 M€ and the CO2 emissions by more than 200 tons/y.

Abstract In recent years, distributed energy systems (DESs) have been recognized as a promising option for sustainable development of future energy systems, and their application has increased rapidly with supportive policies and financial incentives. With growing concerns on global warming and depletion of fossil fuels, design optimization of DESs through economic assessments for short-run benefits only is not sufficient, while application of exergy principles can improve the efficiency in energy resource use for long-run sustainability of energy supply. The innovation of this paper is to investigate exergy in DES design to attain rational use of energy resources including renewables by considering energy qualities of supply and demand. By using low-temperature sources for low-quality thermal demand, the waste of high-quality energy can be reduced, and the overall exergy efficiency can be increased. The goal of the design optimization problem is to determine types, numbers and sizes of energy devices in DESs to reduce the total annual cost and increase the overall exergy efficiency. Based on a pre-established DES superstructure with multiple energy devices such as combined heat and power and PV, a multi-objective linear problem is formulated. In modeling of energy devices, the novelty is that the entire available size ranges and the variation of their efficiencies, capital and operation and maintenance costs with sizes are considered. The operation of energy devices is modeled based on previous work on DES operation optimization. By minimizing a weighted sum of the total annual cost and primary exergy input, the problem is solved by branch-and-cut. Numerical results show that the Pareto frontier provides good balancing solutions for planners based on economic and sustainability priorities. The total annual cost and primary exergy input of DESs with optimized configurations are reduced by 21–36% as compared with conventional energy supply systems, where grid power is used for the electricity demand, and gas-fired boilers and electric chillers fed by grid power for thermal demand. A sensitivity analysis is also carried out to analyze the influence of energy prices and energy demand variation on the optimized DES configurations.

Abstract The methodology, the site and the dataset as well as the emissions scenario considered in the weather file definition influence the numerical evaluation of efficiency measures resilience. With a complete statistical and critical approach, the paper analyzes the importance of these aspects by means of a residential case study simulated in Benevento, a city of south Italy. Using data monitored from 2015 to 2020, a current weather file is built with different methodologies. The comparison indicates that there is not repeatability of the year chosen as a reference for the various months and thus the resolution of the building energy balance could bring different results. Some future climate projections are also generated on medium (2050 s) and long (2080 s) term considering different emission scenarios. With long term projection, the heating degree days are reduced also of − 21% meanwhile the cooling degree days are more than double compared with the current condition. This suggests a remarked transition towards a dominant cooling climate for Benevento. Moreover, when the climate change is considered, the insulation intervention and the installation of double glazed low emissive window is not resilient because the heating energy need decreases also of −56%, but the cooling energy need increases of + 62% (2080 s). If the efficiency measures include also the cool roof and the external shadings, the cooling demand could be reduced until –33% in some scenarios (e.g. RCP 4.5-50th percentile) and increased (+31%) in some others (e.g. 2080 s).

Abstract This paper describes the results from a performance comparison of different types of HVAC systems in a low energy residential building. There were five different HVAC systems which were investigated and evaluated: Ground Couple Heat Pump (GCHP); Ground Water Heat Pump (GWHP); Air-to-Water Heat Pump (AWHP); Air-to-Air Heat Pump (AAHP); and Boiler & Split (B&S) systems applied respectively for heating and cooling seasons. These HVAC systems were implemented in one residential-complex which consisted of a three-story block subdivided into 15 apartments with total floor area of 1050 m2. Year-round dynamic simulations were carried out using Energy Plus based on three different climatic conditions from northern, central and southern Italy. The climates of cities Milan, Rome and Palermo were used respectively in order to estimate potential energy savings among HVAC systems for each site location. It was found that in Milan, AAHP & AWHP heat pump systems save 14.8% and 23.3% primary energy respectively compared to the B&S system, while GCHP and GWHP systems save 59.6% and 62.6% respectively. A techno-economic analysis for a twenty year period was also carried out for each specific case. It was observed that the GWHP case becomes economically feasible after 9 years with respect to other cases for the Milan case study.

Polygeneration systems with thermal energy storage represent promising solutions to achieve energy saving and emissions reduction in the civil sector. The definition of customer-oriented design and operation strategies represents a most challenging task, in order to maximize the profitability and make the investment attractive. A large potential is often recognized for the installation of centralized plants serving a cluster of buildings located over a small area; in such cases the design problem becomes extremely complex and the analyst needs reliable instruments to identify the optimal solution. This paper in two parts presents a scientific tool for the optimization of design and operation for complex polygeneration plants serving a number of buildings with heat, cooling and electricity. The method is flexible with respect to boundary conditions as concerns the power exchange with the public grid, the tariff structure and the normative constraints. In Part I of the paper the method is described, focusing the attention on the most conceptual aspects. The Mixed Integer Linear Programming algorithm is also presented and error analyses are performed for each of the figures adopted, in order to assess the robustness of the method. In Part II of this paper the tool will be extensively applied to some explicative case studies, in order to better clarify its potential in supporting energy analysts and decision makers.

Abstract The fast economic and population growth in the African continent will lead to an important increase in demand for energy and water resources. Unfortunately, very few studies have addressed water use for energy production in Africa. This study focuses on water consumption and withdrawals throughout the different stages of energy production (fuel production, power plant construction and operation) in African countries. An in-depth analysis of water loss through evaporation in hydropower reservoirs is also performed due to the important role it plays in many countries and its severe impacts on electricity generation during the increasingly frequent droughts in Africa. The results indicate that in the year 2016, a total of 42 billion cubic meters of water was lost through evaporation in hydropower reservoirs compared to 1.2 billion cubic meters from all the other fuel types combined. Oil extraction and refining dominate water use for fuel production and non-hydro renewable energies have an almost negligible impact on the overall water use (10 million cubic meters). Fuelwood is shown to be a high consumer of water accounting for 4.5 billion cubic meters. The use of non-hydro renewable energies instead of fossil fuels can contribute significantly to reduce water use while covering the growing energy needs in Africa. Modern technologies that substitute fuelwood use in households would also reduce the impacts on water resources. The hydropower potential remains largely untapped in several regions of the continent. Nevertheless, new hydropower developments need to be carefully considered especially in regions characterized by severe water scarcity.

Abstract The legislation of various European countries imposes limits on the demand for building heating and cooling in order to reduce the primary energy consumptions. Moreover, the legislation prescribes that a fraction of the demand for building cooling, heating and power must be met through renewable energy sources. Among renewable energy systems, wind power, solar photovoltaic, solar thermal energy, solar cooling and heat pumps (though only “partially” renewable) have to be mentioned. In this framework combined heat and power (CHP) systems can provide a further solution to reduce the primary energy consumption. Due to the availability of different technologies, a key factor is the choice of the allocation strategy which allows the division of the energy demands among the various technologies in order to minimize the primary energy consumption. Since the cost of the technologies and the actual tariff and incentive scenarios depend on the specific country and may lead to not optimal allocation strategies in terms of primary energy consumption, these economic parameters are not taken into consideration in the analysis. Therefore, the obtained solutions represent a target which the policies should aim to achieve. This paper aims to develop and apply a methodology for the optimal allocation of the demand among CHP and renewable energy systems, with the aim of minimizing the primary energy consumption, by accounting for legislative constraints. The methodology is then applied to different climatic scenarios to evaluate the effects of a variation of the demand and technology characteristics on the allocation of the loads. Moreover, an analysis on the combined effects is presented. Finally, some guidelines are obtained.

Abstract As energy availability and demand often do not match, thermal energy storage plays a crucial role to take advantage of solar radiation in buildings: in particular, latent heat storage via phase-change material is particularly attractive due to its ability to provide high energy storage density. This paper analyzes the performance of a building-integrated thermal storage system to increase the energy performances of solaria in a cold climate. A wall opposing a highly glazed facade (south oriented) is used as thermal storage with phase change materials embedded in the wall. The study is based on both experimental and simulation studies. The concept considered is particularly suited to retrofits in a solarium since the PCM can be added as layers facing the large window on the vertical wall directly opposite. Results indicate that this PCM thermal storage system is effective during the whole year in a cold climate. The thermal storage allows solar radiation to be stored and released up to 6–8 h after solar irradiation: this has effects on both the reduction of daily temperature swings (up to 10 °C) and heating requirements (more than 17% on a yearly base). Coupling of the thermal storage system with natural ventilation is important during mid-seasons and summer to improve the PCM charge-discharge cycles and to reduce overheating. Results also show that cooling is less important than heating, reaching up to 20% of the overall annual energy requirements for the city of Montreal, Canada. Moreover, the phase change temperature range of the material used (18–24 °C) is below typical summer temperature levels in solaria, but the increase in thermal capacity of the room alone can reduce annual cooling requirements by up to 50%.