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Optimal Power Sharing between Photovoltaic Generators, Wind Turbines, Storage and Grid to Feed Tertiary Sector Users
No full textA paramount technical drawback of photovoltaic (PV) and wind power systems is the intermittency of energy production, which results in problems of stability of the electricity grid and power quality issues [1] [2] [3]. The effects of the separate installation of PV or wind power systems are well-known in literature [4] [5] [6]. In different countries, such as Denmark for wind, renewables have become the main sources of power: non-programmable RES share is currently around 50% of the national electric consumption. In Germany and Spain, this share is already higher than 20%, while in Italy this goal will be reached in few years [7]. In every case, a full transformation of the power system will be even more necessary: infrastructure, policies and markets have to be improved. To compensate for the intermittency of PV and wind generators, electrochemical storages are easy to install and manage in whatever site with respect to hydroelectric pumping systems which require large reservoirs. The widespread utilization could address the power balance of local loads and distributed generators mitigating their negative effects on the grid. For example, the power surplus from PV near midday could be used later to feed local loads. Nevertheless, storage is currently expensive and cannot solve the problem of the weak seasonal correlation between low demand and high non-programmable RES generation and vice versa. Therefore, it is fundamental to know what are the acceptable amounts of grid-connected RES and storages capacities. In particular, the maximum capacities of non-programmable RES must be defined so that there is no necessity of grid upgrade. Such an upgrade of distribution transformers and lines is a consequence of high active powers from distributed generation in reverse flow on radial networks originally designed to feed purely passive loads. Taking into account the previous general remarks, in the present PhD dissertation, the optimal power sharing between PV generators, wind turbines, storage and grid to feed tertiary sector (telecommunication equipment and offices) users is determined taking into account some technical end economic constraints. Therefore, the electrical consumers in this analysis are the owners of the PV generators, wind turbines and storage systems. These three technologies are used to meet a substantial amount of the demand, while the remaining power is provided by the distribution grid. In this way, the consumers became prosumers. First, each renewable source (sun and/or wind) is investigated in terms of hours of availability to estimate the total time in which PV and/or wind power productions occur along the whole year. Then, the primary goal of the prosumers is assumed to be the achievement of the best match between power profiles of loads and power profiles of generators. Such a best match is obtained thanks to an appropriate procedure to design the sizes of generators and storages. In this procedure, power ratings of PV and wind generators and energy capacities of batteries are chosen to reach the highest levels of self-consumption and the lowest levels of power exchange with the grid according to the load profile. In every case, the selected sizing solutions are cost-effective (Net Present Value NPV>0) and cannot create problems to the grid management (overloads of distribution lines and transformers are avoided). In Figure 1 1, it is present the scheme of the simulated systems: the main components are PV and wind generators, storage and electronic converters. They supply aggregations of tertiary-sector users with a peak of consumption corresponding to tens of megawatt, composed of many offices buildings and electronic equipment's. The core of the system is the DC bus, which connects all the renewables at the voltage imposed by the batteries. On the contrary, all the loads are in AC and can be fed by the grid, when there is no production and storage is empty. PV generators are connected to the DC bus by DC/DC converters operating as Maximum Power Point Trackers (MPPTs) to extract the maximum power. An inverter provides AC power to user loads: this converter is unidirectional, because the storage recharging is performed by PV and wind generators without the grid contribution. The reason is that storage is used to support renewables to feed loads and decrease grid-exchanges. Charge-discharge cycles performed to buy and sell energy at different prices are not allowed and batteries are recharged only when there is a surplus from wind and PV (with respect to the local loads). Simulations start from hourly load profiles and environmental parameters. Solar irradiance G, air temperature Ta and wind speed data u, with 1-minute time step, are measured with high accuracy instruments of meteorological stations. These stations are installed in five sites in Apulia (Southern Italy) and have a mutual maximum distance of 150 km. The meteorological data are inputs of energy production models (Figure 1 2): the sites are characterized by a similar annual solar radiation and high deviations in wind speed profiles. The first result is that the maximum levels of self-consumption, that can be reached according to the abovementioned constraints, are in the range 50÷61% of the energy consumption. A so high autonomy from the grid is achieved by installing high storage capacities: they permit to use more PV generators and wind turbines with negligible grid injections. If the goal is the maximization of the NPV, the costs of grid exchanges become the main driver: storage is expensive and to sell energy to the grid is less profitable than self-consumption. The best choice is to design the PV and wind generators to satisfy an amount of the total consumption; in this way, most of RES production is self-consumed and injections are low. In this case, in which storage is not used, the maximum levels of self-consumption result in the range 34÷41% of the load. At the end of the simulation part of my thesis, the data related to the five case studies are compared with the results of two additional simulations, in which loads and generators of all the five analysed sites are aggregated. In the first aggregation case, distributed storage is not present and capacity of PV and wind generators corresponds to the sum of the capacities of the five sites, previously calculated in case of maximization of NPV. The aggregation of generators and loads permits to increase the self-consumption from an average value of 37% up to 40% for the whole system (this result corresponds to that one of the best case study). In future works, this improvement could reach much higher values, if other different typology of loads (e.g. dwelling houses and factories) will be added in the aggregation. In the second aggregation case, a centralized storage is present and its capacity corresponds to the sum of the capacities of distributed storage calculated in the five independent sites in case of maximization of self-consumption. The capacity of PV and wind generators corresponds to the sum of the capacities of the five sites. The aggregation does not permit to achieve better results with respect to single cases, because the high installed capacity of storage already well manages surplus and deficit of energy. In addition to the above described simulation activity, during the whole PhD course, experimental work was performed on PV systems. Part of this experimental work was published in three journal papers [8] [9] [10] (I am co-author) and they are reported in Chapter 4. This experimental work was fundamental to study in details the operation and issues of PV systems. Thus, this work was significant for the achievement of simulation results described in the previous chapters. In the first part of the experimental work, it is defined a reasonable procedure, in terms of minimum type and number of tests and thus minimum duration (a few days), in order to identify the sources of poor performance and to solve or mitigate their negative effects. Such a procedure is based on experimental tests, partly on the PV-system site and partly in laboratory, and the suitable data processing, both before the experiments and after them [8]. The second part of experimental work is related to the electric characterization of the PV generator by tracking its I-V characteristic by connecting it to a capacitor. The curve tracer based on capacitive loads is used, because it is simpler, cheaper and scalable from module level to array level with respect to the use of a variable controlled load. Nevertheless, it is required a detailed sizing of the capacitive load to optimize the duration and the accuracy of the measurements. The practical setup of I-V curve tracers at module, string and array levels is addressed in this work for the main commercial technologies. Such tracers represent a tool to easily check the performance of PV systems at the beginning and during their operation, especially when poor performance occurs [9]. In Chapter 1, it is present a description of PV and wind power generation systems and electrochemical storage technologies. In Chapter 2, it is described the architecture of the simulated systems. The management of energy flows, the models of renewable generators, simulation constraints and sizing criteria are also described in detail in Chapter 2. In Chapter 3, inputs of the simulation and the energy and economic results of the different case studies are presented. Finally, the experimental work on PV system is reported in Chapter 4
A variable step size perturb and observe based MPPT for partially shaded photovoltaic arrays
No full textDEGRADATION TESTS AND CHARACTERIZATION PROCEDURES OF PHOTOVOLTAIC MODULES EXPOSED TO OUTDOOR CONDITIONS
No full textThis paper deals with the definition of test procedures specifically conceived to highlight the degradation of PhotoVoltaic (PV) modules and identify the mechanisms that are mainly responsible for this degradation. Several environmental and mechanical test cycles are applied to each set of PV modules under test and suitable characterization procedures are performed at the end of each test cycle, thus providing information related to the degradation rate. The applied stimuli are designed according to the measurements available for outdoor exposed PV plants and the laboratory results will be compared to the degradation estimated for these plants
Maintenance Activity, Reliability, Availability, and Related Energy Losses in Ten Operating Photovoltaic Systems up to 1.8 MW
No full textIn general, photovoltaic (PV) plants do not include components with moving parts and, as a consequence, they are wrongly considered maintenance-free. Thus, this article presents a maintenance, reliability, and availability analysis of ten PV systems, with different inverter configurations, in the context of the intermittent renewable energy source systems, including the wind farms. The first part of the analysis consists of the evaluation of reliability using failure rates from the literature. In the second part, these results are compared with data obtained from industrial maintenance reports in the years 2016–2018. Finally, the yearly energy losses and the availability of each PV plant are assessed
Degradation rate of eight photovoltaic plants: results during six years of continuous monitoring
Full text linkThe results of six years of continuous monitoring are presented in this paper that refer to eight outdoors PhotoVoltaic (PV) plants. The monitored plants are based on different technologies: mono-crystalline silicon (m-Si), poli-crystalline silicon (p-Si), string ribbon silicon, Copper Indium Gallium Selenide (CIGS) thin film and Cadmium Telluride (CdTe) thin film. Mono-crystalline silicon modules and thin-film modules are used both in fixed installation and on x-y tracking systems. The results are expressed in terms of degradation rate of the efficiency of each PV plant, which is estimated from the measurements provided by a multi-channel data-acquisition system that senses both electrical and environmental quantities. A comparison with the electrical characterization of each plant obtained by means of the transient charge of a capacitive load is also proposed. The capacitive-load technique has been implemented immediately after the installation of the PV plants and after 78 months of operation. The obtained results show that both the m-Si plants in fixed installation and on the tracking system had a negligible degradation, while p-Si and string-ribbon Si exhibited a moderate degradation. Higher was the degradation obtained for the thin-film based plants, with a worst behaviour of the plants installed on the tracking systems
A method for obtaining the I-V curve of photovoltaic arrays from module voltages and its applications for MPP tracking
Full text linkFor the purpose of control and monitoring of a Photovoltaic (PV) system its current-voltage (I-V) charac-
teristic curve is traced. Usually such a test involves the interruption of the normal operation of the PV
systems. In this paper a method for tracing the I-V curve from on-site measurements is proposed.
During the measurement of the characteristic curve the normal operation of the PV system is not inter-
rupted. The subjects of tracing the characteristic curve and Maximum Power Point Tracking (MPPT) of PV
arrays are generally dealt with separately but the proposed method performs the measurement of the
characteristic curve quickly and so it can also be utilized for MPPT purposes. Simulations and experi-
ments have been conducted to confirm the operation of the proposed metho
Hourly Simulation of Energy Community with Photovoltaic Generator and Electric Vehicle
Full text linkEurope has set the ambitious goal to become the first carbon-neutral continent by 2050. Therefore, it has undertaken several initiatives to promote the energy transition, including the active participation of citizens in the energy sector. In this context, recent European directives introduced the concept of energy community, whose members can consume, share, and store energy locally produced. This work proposes an energy and economic simulation of a renewable energy community powered by a 19.2 kWp photovoltaic system in the province of Cuneo, in Piedmont (Italy). The community consists of a prosumer, which owns the photovoltaic system and a charging station for electric vehicles, and other 17 energy users. Suitable indicators to assess the energy performance of the community (self-consumption and self-sufficiency) were evaluated starting from the estimated production and consumption power profiles. Then, an economic simulation was carried out to assess the economic return on the investment for the member who bore the initial costs and the annual economic savings for the others
Photovoltaic Power Prediction from Medium-Range Weather Forecasts: a Real Case Study
Full text linkThe aim of this work is to utilize weather forecasts with a lead time from 6 h to 30 h as input data of a photovoltaic (PV) model to predict the AC power production. In order to always use the last forecasts, the inputs are updated every time there are new data, e.g., every 6 h. The ability of the model is tested on a residential PV plant for which global irradiance and electrical power are measured. The typical indicators of forecast accuracy in the PV applications are used: mean bias error and mean absolute error for both irradiance and power. However, they are normalized with respect to the standard irradiance and the PV rated power. Their values are generally adequate in clear sky and overcast conditions, remaining around the 10% limit
Experimental assessment of degradation rate in photovoltaic modules
No full textA test procedure is described in this paper that is conceived to investigate the degradation mechanism of PV modules based on different technologies. Environmental and mechanical stress factors are applied to the modules under investigation and electrical and electroluminescence characterization procedures are implemented to assess the module performance. Preliminary results are reported that refer to the application of the proposed test procedures to two sets of p-Si modules
Irradiance Transposition Models from Horizontal to Tilted Plane for Photovoltaic Power Calculation
No full textThe accurate estimation of solar irradiance on inclined surfaces is essential for the assessment of Photovoltaic (PV) system performance and energy yield prediction. This paper presents a comparative analysis between three transposition models for converting solar irradiance from the horizontal plane to tilted surfaces. Additionally, two PV power models are analysed, starting from transposed irradiance while accounting for loss factors such as low-irradiance and thermal losses, degradation, and conversion losses. The performance of the models is investigated using measurements from a meteorological station, and from actual PV plants. Error metrics are evaluated to quantify deviations between modelled and measured profiles
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