• Energy Research

AbstractThis paper presents the preliminary operational results of two 3.5 kWp CPV systems using triple junction III-V solar cells and a two-axis tracking mechanism. The plant is installed in the campus of the Università Politecnica delle Marche (Ancona, Central Italy). The concentration optics consists of a primary Fresnel lens and a secondary reflective optics with an overall geometrical concentration ratio of 476 X. An experimental measurement setup acquires the main plant operating and ambient quantities; the paper reports the first months of plant operation with particular focus on the influence of the available radiation and the ambient temperature on the performance of the system. The electric output has a linear trend with the available direct normal radiation while ambient temperature has a minor effect on the performance of the CPV systems; also the influence of the Air Mass coefficient is reported.

Abstract The paper presents a cradle-to-grave life cycle assessment for two domestic solar hot water systems. The first consists of polypropylene unglazed solar panels coupled with a 300-l storage tank; the second one consists of a traditional system with glazed solar panels coupled with a thermal storage of the same volume. Life cycle assessment was conducted according to the Eco-Indicator 99 methodology, Egalitarian Approach, yielding 49.7 and 18.3 eco-indicator points for the glazed and unglazed panels systems, respectively. In addition, for each domestic solar hot water system, the energy, CO 2 and economic payback times were calculated. In order to take into account the influence of local climate on the solar panels yield evaluate, the systems performance was simulated for three different locations: Rome, Madrid and Munich. The payback times were evaluated with respect to both natural gas and electrical boilers. The Energy Payback Time of the unglazed panel system ranges between 2 and 5 months, that of the glazed panel between 5 and 12 months. The CO 2 Payback Time of the unglazed panel system ranges between 1 and 2 months, that of the glazed panel between 12 and 30 months. The economic payback time, if compared with natural gas boiler, is in the range 9–11 years/8–13 years for the system with unglazed/glazed panels, respectively; if compared with the electrical boiler, it is in the range of 3–4 years for the system with unglazed panels and 4 years for that with glazed panels. The different national costs of natural gas and/or electricity play an important role in the economic payback times. Indeed, in Munich, the smaller energy savings achieved with the renewable systems are offset by the higher costs of these commodities.

Abstract This work analyzes and compares the effects of the secondary optics on the performance of a triple junction solar cell used in a compact HCPV prototype unit. The HCPV system is composed of triple junction III-V (Ga0.5In0.5P, Ga0.99In0.01As and Ge) solar cells that have a circular shape with an active area of 4.15 mm2. The optics consists of a primary PMMA square Fresnel lens (75 mm – side) with constant pitch and a refractive secondary optic (RTP) made of dielectric material. The overall geometrical concentration ratio is 1300×. The tracking system is a tip-tilt type two-axis mechanism driven by stepper motors. The HCPV secondary optics were firstly designed at Polo Tecnologico Andrea Galvani and then tested on the prototype unit at the Engineering Faculty of Universita Politecnica delle Marche in Ancona, Central Italy. The aim of the paper is to present the numerical and experimental performance results of two different secondary optics and to assess the effects of the main construction parameters (i.e.: the geometry of the secondary optics and the distance between the two optics) on the concentration efficiency. Moreover, two different 3 J cell receiver types were tested, the Insulated Metal Substrate technology (IMS) and the Direct Bonded Copper (DBC) technologies. The experimental tests were performed under real outdoor operating conditions, therefore also the Direct Normal Irradiance (DNI) was measured. In general, the free-form optics showed significant improvements in terms of overall irradiance and homogeneity. In the best configuration the electric efficiency achieved 39.55%, neglecting the primary optics losses, thus confirming that the presented system setup is able to reach the highest standards of CPV technology performance.

Abstract Solar radiation is a variable energy source and the mismatch between the availability of such source and the domestic energy demand is a paramount challenge to deal with. For this reason, in this work a 4.08 concentration ratio portable solar box cooker coupled with a thermal energy storage (TES) based on a phase change material (PCM) was characterized through outdoor experimental tests. The TES is a double-wall stainless steel vessel, with the annular volume filled with 2.5 kg of erythritol. The portable solar box cooker was tested under 4 different experimental conditions: without load, with water, with silicone oil, and with silicone oil inserted in the erythritol-based TES. The load tests were divided into a heating and a cooling phase, in order to evaluate the cooker performance in absence of solar radiation. Results showed that equipping the portable solar box cooker with the erythritol-based TES allowed to extend the average load cooling time, in the range 125–100 °C, of around 351.16%.

Abstract The paper presents the operational results of a real life residential microgrid which includes six apartments, a 20 kWp photovoltaic plant, a solar based thermal energy plant, a geothermal heat pump, a thermal energy storage, in the form of a 1300 l water tank and two 5.8 kW h batteries supplying, each, a couple of apartments. Thanks to the thermal energy storage, the solar based thermal energy plant is able to satisfy the 100% of the hot water summer demand. Therefore the thermal energy storage represents a fundamental element in the management of the residential demand of thermal energy. It collects renewable thermal energy during day-time to release it during night-time, effectively shaving the peak of the thermal energy demand. The two electric storages, on the other hand, provide the hosted electrical subsystems with the ability to effectively increase the self-consumption of the local energy production, thus lowering the amount of energy surplus to be sold back to the grid, and increasing the self-sufficiency of the microgrid. For instance, the storage has supported self-consumption up to the 58.1% of local energy production with regard to the first battery, and up to the 63.5% with regard to the second one. Also, 3165 and 3365 yearly hours of fully autonomous activity have been recorded thanks to the first, and the second battery respectively. On the other hand, the yearly average efficiency amounts to 63.7%, and 65.3% respectively, for the first and second battery. In the second part of the paper we propose a computational framework to evaluate the overall performance of the microgrid system, while accounting different operating conditions and energy management policies. From this perspective, the framework acts as a useful modeling and design tool, to assess the opportunity of employing alternative energy management system topologies and strategies. Eight different configurations, with growing complexity, have been derived from the original system on purpose. The simulations, carried out based on real data related to one-year time period, have provided results showing that, the higher the integration level of electrical and thermal storage is, the higher degree of self-sufficiency can be achieved by the microgrid, and, in turn, the more consistent the yearly energy saving become. Nevertheless, despite the energy cost reduction achievable with the availability of storage systems in the Leaf House, their high investment cost made them not really profitable at the current price conditions for devices and energy purchase.

Abstract Design and planning of low carbon cities and districts must consider the synergies between all the energy networks available. Energy systems optimal design thus assumes a critical importance in determining both costs and environmental impact of operating such districts. This is particularly true following the concept of Local Energy Community, with a single entity representing both the demand and the manager of the energy generation assets. This paper proposes an innovative model for the optimal design of an energy community aiming at lowering its carbon footprint. The community is modeled as a network of spatially dislocated energy hubs, each with its own demand of electricity, heating and cooling energy. The model aims at defining the optimal mix of energy systems, thermal and electric energy storages and energy network infrastructures needed to satisfy the district’s users energy demands. The model is validated using energy demand data from the Nanyang Technological University campus in Singapore by analyzing three scenarios. In the first one, the optimization goal is purely economic and it aims at minimizing the overall cost of operating the district. The second and third scenarios focus on reducing the carbon footprint of the district by imposing an additional constraint, which limits the overall primary energy consumption. In all the scenarios the algorithm chooses to partially or totally connect the five sites with a district cooling network and take advantage of cold thermal storage, proving their potential in hot climates. In the first scenario, the advantages of the district cooling solution are mainly related to the savings in the capital cost of electric chillers that partially offset the cost of the district cooling network; indeed, district cooling network allows the sites to share cooling power thus achieving a reduction in chillers total installed size of 33%. In the second scenario, in order to meet the target of 10% reduction of the overall primary energy, the optimal solution also requires the installation of a photovoltaic system. In the third scenario, imposing a 20% reduction of the overall primary energy, also a natural gas fed trigeneration plant comes into play.

Abstract Microturbines (MGTs) are a relatively new technology that is currently attracting a lot of interest in the distributed generation market. Particularly interesting is their use as backup source for integrating photovoltaic panels or/and wind turbines in hybrid systems. In this case the sensitivity to ambient conditions of the MGT adds to that of the renewables and the knowledge of the effects of ambient conditions on its performance becomes a key subject both for the sizing of the energy system and for its optimal dynamic control. Although the dependence of medium/large gas turbines performance on atmospheric conditions is well known and documented in literature, there are very limited reports available on MGTs and they regard only global parameters. The paper aims at filling this lack of information by analyzing the ambient temperature effect on the global performance of an MGT in cogeneration arrangement and by entering in detail into its machines’ behavior. A simulation code, tuned on experimental data, is used for this purpose. Starting from the nominal ISO conditions, electrical power output is shown to decrease with ambient temperature at a rate of about 1.22%/°C, due to a reduction of both air density and volumetric flow. Meanwhile, thermal to electrical power ratio increases at a rate of about 1.30%/°C. As temperature increases compressor delivers less air at a lower pressure, and the turbine expansion ratio and mass flow reduce accordingly. With the in-use control system the turbine inlet temperature reduces at a rate of 0.07%/°C with respect to its ISO condition value.

The constant operation of water electrolyzers prevents degradation caused by operational fluctuations, preserving performance. This study introduces a MILP-based design framework for hybrid energy storage systems, integrating photovoltaic systems, Lithium-ion batteries, and alkaline electrolyzers operating at constant rated conditions. The framework targets energy-independent residential users, fully meeting electrical loads while incorporating spatial analysis via a GIS-based management module. Applied to the Italian context, the framework uses historical data for residential end-users with 1.5−3.0 kW electrical loads. The results indicate photovoltaic systems sized between 3.0−4.5 kW, Lithium-ion batteries with 6.0−7.0 kWh capacity, and alkaline electrolyzers sized at 100−260 W for daily loads of 2.8−6.0 kWh. Lithium-ion batteries account for approximately 60 % of the total system cost. A levelized hydrogen cost of 12−19 €/kg is required to cover the overall investment costs. Additionally, the system offers environmental benefits, with CO 2 emission reductions of approximately 0.35 to 0.83 tons per user annually.

AbstractDue to their non-deterministic behaviour, renewable energies are defined non-dispatchable and they are largely coupled with thermal energy storage (TES) systems to overcome the problem of matching energy production and demand. Hence, interest on TES is growing in energy conservation field, especially while combined with demand side management (DSM) concept, being DSM the need of shaping the electricity consumption of the final user on the basis of grid requests. In this work an existing installation of a TES system coupled with heat pumps is presented. A dynamic simulation model was built up and validated by means of experimental data for the summer season cooling requirements. The simulations performed were used to show the load shifting potential of such storage and energy and cost savings were assessed.