Panagiotis Bakas

Impact of PV module mismatch on the PV array energy yield and comparison of module, string and central MPPT

With PV technology advancing in high pace the need for better understanding solar cell behavior from component up to system level is evident. Electrical mismatch between solar cells and thus PV modules is one effect that has not been studied extensively. In addition, several techniques for reducing the mismatch losses have been developed, such as distributed MPPT and module sorting. Therefore, a study of mismatch losses will be presented along with a comparative study of the different mismatch mitigation techniques.

Energy storage sizing for large scale PV power plants base-load operation - comparative study & results

With greenhouse gas emissions increasing and global environmental policies changing, renewable energy sources have gained significant attention over the last 15 years. However, the intermittent nature of renewable generation makes its integration to the grid a challenging task. Thus, energy storage systems are usually proposed as a way to overcome this problem. This paper deals with energy storage for very large PV power plants (larger than 100MW). Different operating strategies have been defined for the PV power plant and the required energy storage capacity was estimated for each of them via a sizing methodology that will be briefly described. In addition, six areas of favorable solar radiation characteristics have been chosen as case studies to evaluate the presented methodology. Finally, a time series analysis was conducted in order to estimate the confidence interval for which both PV generated energy and energy storage are sufficient to fulfill the specific operating strategy. This analysis was performed for each operating strategy and location and thus, a comparative study will be presented.

Solar PV array-inverter matching considering impact of environmental conditions

Sizing of photovoltaic (PV) generators is an important issue which might affect the PV system overall yield and performance ratio. The procedure is realized according to inverter specifications with the most important one being the operating voltage range. The parameter that is affected by this specification is the number of PV modules per string and a software tool was developed in order to study the effects of this parameter on the system performance. Moreover, the PV generator voltage is highly affected by the ambient temperature of a specific location. Thus, the optimum number of PV modules per string will differ in locations with different environmental conditions. The importance of PV string sizing is high, since improper string length can lead to significant deficiencies on the overall inverter performance. A study of the effects of PV string size on annual system energy production and yield has been conducted for several locations in Europe with significantly different weather as well as for locations with extreme climatic conditions such as high altitude and desert areas.

Techno-economic evaluation of alternative configurations for very large scale PV power systems including energy storage

PV systems are already mature, especially for small and medium size applications. However, in the last couple of years larger PV power plants in the range of 100MW have been installed and set in operation. An important question that arises for very large scale PV power systems is the optimization of the overall system configuration, especially due to the very high initial costs and the financing of such projects. Several concepts in the literature have been investigated regarding efficiency and technical feasibility; however it is also important to evaluate their economic feasibility, using commonly accepted metrics. In this paper four different system configurations are presented, with the typical system configuration of central inverters and medium voltage transformers used as a reference. The other three alternatives include high power DC-DC converters connected in series or in parallel, as well as cascaded DC-AC inverters. In addition, taking into account current developments in energy storage, new possibilities for combining PV with energy storage for large scale plants are rising, in order to deal with the grid integration of ever increasing PV installations. Therefore, a comparison is performed among a PV power plant with and without energy storage for different operation scenarios. The results show clearly the increase of cost and indicate the need for new evaluation metrics to include the expected benefits from the integration of energy storage to PV power systems.

Photovoltaic string configuration for optimal inverter performance

In the recent years photovoltaic installations have been growing rapidly both in numbers and in size. The main reason for this growth has been the generous subsidies offered by policy makers. As financial incentives are now being gradually removed, PV plant manufacturers are in quest for more efficient plant configurations that eventually reduce the investment cost and increase the annual energy yield. This paper presents a methodology for optimally sizing the PV panel strings (series-parallel connection of PV generators) for any specific inverter equipment and for a given location weather dataset. The methodology is based on some new PV plant performance metrics presented here. Finally, the concept of maximum power point of the inverter (MPPTi) is introduced.

Energy storage integration into grid connected utility-scale photovoltaic (PV) systems

This paper takes into account energy storage sizing results from previous research activities regarding base-load implementation of an energy storage system integrated into a PV power plant, for six locations of favorable meteorological characteristics [1]. The main target is to evaluate scenarios of integrating energy storage in different positions of a grid connected PV power plant in terms of transmission cost and transmission losses. The analysis deals with different transmission voltage levels and proposes where and how energy storage should be included within a very large scale utility PV power plant for base-load operation and after taking into account the drivers for using Energy Storage. Furthermore, this paper analyses different power electronics (PE) interface techniques for integration of energy storage into grid connected utility-scale applications. Various alternatives are identified and presented in this paper, while advantages and disadvantages for all scenarios are highlighted.

Comparison of PV systems with maximum DC voltage 1000V and 1500V

The purpose of this work is the comparison of two different photovoltaic (PV) arrays with different maximum permissible voltage from a technical point of view. In addition, inverter input specifications are taken into account since they affect the PV array design. The comparative cases include PV arrays with inverters of 1000 and 1500 V maximum input voltage. The most commonly used are those with 1000V maximum voltage, but it is claimed that by using those with 1500 V maximum voltage there is potential for more cost effective PV systems. One of the PV array design parameters that are affected by the inverters maximum voltage rating is the number of PV modules that can be connected in series. In this study it is shown how the ratio of number of PV modules per string (Nms ratio) between the two comparative cases is related to the reduction of the PV array cabling and number of combiner boxes (CB). In addition the comparison was iterated for various Nms ratios and for three inverter power ratings of 800kW, 1.5MW and 3MW. Finally, the savings in conductor mass and number of CB of the PV array with respect to the Nms ratio of the two comparative studies are presented.

A Review of Hybrid Topologies Combining Line-Commutated and Cascaded Full-Bridge Converters

This paper presents a review of concepts for enabling the operation of a line-commutated converter (LCC) at leading power angles. These concepts rely on voltage or current injection at the ac or dc sides of the LCC, which can be achieved in different ways. We focus on the voltage and current injection by full-bridge (FB) arms, which can be connected either at the ac or dc sides of the LCC and can generate voltages that approximate ideal sinusoids. Hybrid configurations of an LCC connected at the ac side in series or in parallel with FB arms are presented. Moreover, a hybrid configuration of an LCC connected in parallel at the ac side and in series at the dc side with an FB modular multilevel converter is outlined. The main contribution of this paper is an analysis and comparison of the mentioned hybrid configurations in terms of the capability to independently control the active (P) and reactive power (Q).

Hybrid topologies for series and shunt compensation of the line-commutated converter

This paper presents two concepts for enabling the operation of a line-commutated converter (LCC) at leading power angles. The concepts are based on voltage or current injection at the ac side of an LCC, which can be achieved in different ways. However, this paper focuses on the voltage and current injection by series-connected full-bridge cells that can generate voltages that approximate ideal sinusoids. Thus, hybrid configurations of an LCC connected at the ac side in series or in parallel with full-bridge cells are presented. Finally, these hybrid configurations are compared in terms of voltage and current rating.

Evaluation of a very large scale PV power system with energy storage for capacity firming

Current technology developments in energy storage and PV systems raise new possibilities for combining them, in order to deal with grid integration of PV systems. This paper studies the operation of a very large scale PV power plant, which includes an energy storage system for capacity firming. The energy storage is distributed in the PV plant, i.e. many energy storage units are connected in parallel with each PV array before the central solar inverter. The benefits of this configuration are explained in the paper, whose scope is to evaluate the complete system. A tool has been created in order to perform the sizing of the energy storage, according to a desired operation and evaluate the system using the LCOE.