S. Armstrong

A stand-alone photovoltaic supercapacitor battery hybrid energy storage system

Most of the stand-alone photovoltaic (PV) systems require an energy storage buffer to supply continuous energy to the load when there is inadequate solar irradiation. Typically, Valve Regulated Lead Acid (VRLA) batteries are utilized for this application. However, supplying a large burst of current, such as motor startup, from the battery degrades battery plates, resulting in destruction of the battery. An alterative way of supplying large bursts of current is to combine VRLA batteries and supercapacitors to form a hybrid storage system, where the battery can supply continuous energy and the supercapacitor can supply the instant power to the load. In this paper, the role of the supercapacitor in a PV energy control unit (ECU) is investigated by using Matlab/Simulink models. The ECU monitors and optimizes the power flow from the PV to the battery-supercapacitor hybrid and the load. Three different load conditions are studied, including a peak current load, pulsating current load and a constant current load. The simulation results show that the hybrid storage system can achieve higher specific power than the battery storage system.

Comparison of battery charging algorithms for stand alone photovoltaic systems

The battery is the most common method of energy storage in stand alone solar systems; the most popular being the valve regulated lead acid battery (VRLA) due to its low cost and ease of availability. Photovoltaics are not an ideal source for charging batteries as their output is heavily dependent on weather conditions. Therefore, when batteries are used in photovoltaic systems, the performance characteristics differ significantly from batteries used in more traditional applications and the battery life is usually shortened. In conditions of varying solar radiation and load profile the battery may experience a low state of charge (SOC). A low SOC for extended periods of time will cause increased sulphation, which severely reduces the life of the battery. Typically, steps are carried out to protect the battery and to charge the battery more effectively. Such methods include intermittent charging (IC), three stage charging (TSC) and interrupted charge control (ICC), among others. This paper quantifies the effectiveness of these three battery charging algorithms and evaluates their ability to maintain the battery at a high state of charge. The measurement setup is comprised of a solar simulator, which replicates the output of a large 50 W photovoltaic panel using a low power cell. Repeatable load and solar radiation profiles and temperature control are implemented using LabView so that identical operating conditions can be set up to compare the three battery charging systems.

Investigation of the harmonic response of a photovoltaic system with a solar emulator

This paper proposes a solar emulator to imitate the output of a solar panel and introduces a novel approach to maximum power point tracking (MPPT). It is difficult to analyse the performance of a photovoltaic system, therefore, an emulator circuit which models the solar array under controlled conditions is required. A DC halogen lamp, where the brightness can be adjusted as required, will illuminate a single solar cell. The output of the solar cell is amplified to produce the characteristics of a large solar panel. A new methodology, the harmonic detection method (HDM) is investigated which aims to locate the maximum power point by examining the harmonics produced at different locations on the current-voltage characteristic curve

Investigating the Effectiveness of Maximum Power Point Tracking for a Solar System

This paper investigates the effectiveness of maximum power point tracking (MPPT) and proposes a quantitative measure of MPPT efficiency. Using a vector methodology to track the direction and path of the sun throughout the day, the optimal solar tracking angle and angle of incidence of the suns rays are derived. The solar arrays output power is monitored, under sunny sky conditions, with and without the use of maximum power point tracking in order to study the difference in efficiencies and to quantify the benefits of maximum power point tracking. The paper presents results for the efficiency of MPPT under fixed horizontal solar panel conditions and optimal solar tracking

Effect of Wave Farm Aggregation on Power System Stability

There is a large and varied range of wave energy technologies presently under development, in varying stages of progress from model scale to pre-commercial prototypes. Similarly to offshore wind farms, it is anticipated that when these wave energy converters (WECs) ultimately reach commercial stage, they will be implemented in a farm layout, in order to improve economic feasibility and power quality. Transmission System Operator (TSOs) carry out power system dynamic simulations in order to investigate the effect of renewable energy farms on the stability and reliability of the grid under varying operating conditions. A detailed dynamic model of each individual WEC in the wave farm would be complex and computationally intensive; therefore aggregating the wave energy farm into a smaller number of models may be desirable for the TSO. This paper describes the methodology for simulating arrays of WECs in a power systems simulation tool, and examines the effects of aggregation on the load flow studies and stability analysis of the farm. The methodology is validated using DIgSILENT Power Factory simulations.

Assessing the Impact of the Grid-Connected Pacific Marine Energy Center Wave Farm

A new wave energy test site, Pacific Marine Energy Center-South Energy Test Site (PMEC-SETS) - is being proposed for construction off the coast of Newport, Oregon, U.S. The intermittent nature of wave energy presents a unique challenge for the transmission system operator, as the integration of a wave energy farm onto the electrical grid creates the potential for disturbance to the operation and stability of the grid. Consequently, this paper assesses the PMEC-SETS development and the impact of its connection to the Oregon (and the rest of the Western Interconnection) network, in terms of steady-state, dynamic, and transient characteristics. The electrical infrastructure analysis is implemented using two power system simulation tools: 1) PowerWorld and 2) DIgSILENT PowerFactory. Power-World is used to create an equivalent model of the transmission system, and DIgSILENT PowerFactory is used to examine the impact of the wave farm at the point of connection, under normal and faulted conditions. A case study of 20 wave energy converters is used to illustrate the results.

An initialisation methodology for ocean energy converter dynamic models in power system simulation tools

Environmental concerns, coupled with dwindling fossil fuel supplies, and growing government support, have resulted in increasing amounts of electricity generated from renewable energy. As the number of these installations rise, their influence on the behaviour of the electrical power system becomes more apparent. Transmission System Operators (TSO) carry out power system dynamic simulations in order to investigate the effect of renewable energy systems on the stability and reliability of the grid under varying operating conditions. Correct initialisation is an essential step in the dynamic model. This paper describes a three step methodology for correct initialisation of power systems dynamic model for an ocean energy system. The methodology is validated using DIgSILENT Power Factory simulations.