Renewable Power Generation and Energy Storage
While new topologies and designs of electrical machines and power converters are being developed continuously as part of this research it is equally important to consider the use of existing products where possible. To successfully integrate and control a variety of generating and storage equipment in a system it is important to understand intimately the characteristics of each system element as well as the interaction between them. For that reason commercial products are often operated under operating conditions that may not been envisaged by the manufacturer. Using laboratory tests the suitability of such equipment has been established and, where necessary, adapted in collaboration with manufacturers.
- Integration of wind power in diesel networks
- Network interface for flywheel energy buffer
- Modular PV/grid interface with common DC busbar
- Control of stall-regulated variable speed wind turbines
- Advanced maximum power tracking of PV systems
- Integration of high-penetration PV generation in electrical distribution systems
- High-efficiency DC-DC converters for fuel cells and ultra-capacitors
- Operation and control of micro-grid systems
For information about potential areas of post-graduate and collaborative research please contact Dr Bleijs.
As part of the PV research two systems have been installed on University buildings. The first PV system, designed and part-funded under the EU UNIVERSOL project, is a 11.2 kWp research and demonstration system installed on an adjacent laboratory roof , consisting of 64 mono-crystalline modules, flexibly arranged into 7 sub-arrays of different sizes and topologies. This system is extensively instrumented to monitor meteorological and electrical variables, using a dedicated data acquisition and data storage system. Click here to download further information. The second PV system of 38.2 kWp is integrated in the new David Wilson Library building and uses 3 different silicon cell technologies (mono- and poly-crystalline and amorphous thin film) for different locations. This system is also being monitored to analyse the performance of each technology.
In addition to the operational PV systems the research group has available a wide range of facilities and equipment for academic and industrial research, such as:
- Wind Turbine Simulator (software-driven hardware-in-the-loop system)
- Module-size Halogen Solar Simulator
- Programmable PV Array Simulator (including partial shading)
- Motor/generator sets of various power ratings
- DC and AC variable speed drives and inverters
- Ballard Fuel Cell
- Battery Test Cell (including multiple charge/discharge unit)
- Instrumented Diesel Generator Set
- Ultra-capacitor Bank
- Programmable Power Supplies and Load Banks
- DC and AC machines (cage and slip-ring induction, PM and wound-field synchronous)
- Power Analysers
In remote locations such as rural areas and island communities the cost of electricity supplied from diesel generators (DG) is often very high due to the costs of fuel transportation. In many cases such locations have an excellent wind regime, making the use of wind turbine generators (WTG) an attractive option. Due to the variability of the wind, parallel operation of a DG and a WTG may affect the voltage and frequency stability of the local grid. Studies have been carried to identify equipment and control methods that can assure stable and economic operation of wind-diesel systems. It has also been shown that small amounts of energy storage can eliminate or reduce the need for frequent start-stop operation of the diesel generator with the associated fuel and wear penalties.
A flywheel is eminently suited as an energy buffer to filter the variable output power of wind turbines, connected to a weak grid or isolated diesel generators. To exploit the energy storage capacity of a flywheel a bi-directional variable-speed interface between the grid and the flywheel is required. Continuously variable transmissions (CVT) and power-electronic variable speed drives (VSD) have been investigated, including the design of suitable controllers for power smoothing.
Both current-source and voltage source inverters have been tested for this application; using back-to-back voltage source inverters with vector control and dedicated power controllers, fast and accurate control of active and reactive power flow has been achieved.
When medium size PV systems are integrated in buildings the sub-arrays often experience different levels of insolation due to their orientation and shading from nearby objects, resulting in sub-optimal performance and loss of energy yield. A novel PV/grid interface has been developed, consisting of modular current-fed DC-DC converters, a common high-voltage DC link and an industrial 3-phase inverter with adjustable power factor and grid supervision. Each converter contains a fast and accurate maximum power point (MPP) tracker and the system control includes power feedforward to minimise DC bus voltage fluctuations. Excellent MPP tracking over a wide input voltage range has been demonstrated.
To make best use of the available energy wind turbines must operate at variable speed related to the prevailing wind speed. In wind speeds below the rated value the control must be aimed at extracting maximum energy, while above the rated wind speed the rated power or torque must be maintained. Because direct measurement of the wind, as seen by the turbine rotor, is not possible the control system must use other information to achieve the control objectives. Various algorithms have been tested through simulation and are being implemented on a hardware-in-the-loop wind turbine simulator consisting of software-controlled DC motor drive and an industrial bi-directional AC cage generator drive.
Maximum power tracking algorithms for PV systems may fail to operate correctly under rapidly changing insolation conditions due to the array capacitance. Under partial shading conditions where multiple maxima are present these algorithms are often also unable to find the global maximum. Advanced algorithms have been developed that are able to recognise such situations and find the true maximum power point. These algorithms have been implemented in a fast DSP controller and tests using our solar simulator have confirmed correct operation under adverse conditions.
As the cost of PV cells and modules continues to fall the penetration of PV generators from domestic and industrial installations will increase, in particular in regions of the world with high solar radiation. The integration of a large numbers of PV generators in the local distribution system may lead to operational problems related to voltage level, stability and power quality. Investigations are undertaken to establish the relation between network configuration, penetration level and load pattern on these issues, based on models of existing local networks, and to find appropriate measures to eliminate or reduce these effects.
Fuel cells produce a DC voltage that depends on the load current and cannot be used directly to supply a DC or AC load. For application in automotive propulsion systems or in micro-grids in combination with other generators and loads it is necessary to convert the output voltage to a constant (and preferably higher) DC voltage level. In view of the high costs of fuel cells and the hydrogen fuel it is essential that this conversion process is highly efficient. Similar requirements apply to the use of ultra-capacitors, with the additional need for very fast response. New power-electronic circuit topologies and control methods are under investigation using a combination of detailed modelling, simulation, hardware construction and micro-processor control implementation.
Micro-grid systems consist of a number of generators (both dispatchable such as diesel generators and fuel cells, and non-dispatchable such as wind generators and PV generators) and loads (including deferrable), together with storage devices (batteries, flywheels and ultra-capacitors), that operate in low-voltage AC or DC networks. Micro-grids can be operated interconnected to the power grid or in islanded mode. The key function of micro-grids is to ensure stable operation under fault conditions and network disturbances. While the micro-grid offers technical and economic benefits the increased penetration of distributed generators may lead to technical challenges such as over or under voltages at the point of connection, protection malfunction, increase in short circuit levels and power quality problems. The power flows inside the micro-grid and through the interconnection to the power grid must be accurately controlled to avoid frequency and voltage control problems. The aim of this research is to investigate the role that the use of power-electronic converters and micro-controllers can play in ensuring stable operation under all conditions.
- A selection of relevant publications is shown below.
- J A M Bleijs: 'Wind turbine dynamic response – difference between connection to large utility network and isolated diesel micro-grid'. IET Renewable Power Generation, Vol.1, Issue 2, June 2007, pp.95-106
- J A M Bleijs, F Hardan & A J Ruddell: ‘Flywheel Energy Storage System for Wind Power Smoothing in Weak and Autonomous Networks’. Proc ‘Wind Power for the 21st Century’ Conference, Kassel, Germany, Sept 2000, pp.270-273
- A G Dutton, J A M Bleijs, H Dienhart, M Falchetta, W Hug, D Prischich & A J Ruddell: ‘Experience in the Design, Sizing, Economics, and Implementation of Autonomous Wind-Powered Hydrogen Production Systems’. International Journal of Hydrogen Energy, 25, no.8, May 2000, pp.705-722
- J A M Bleijs & J A Gow: ‘Fast Maximum Power Point Control of Current-Fed DC-DC Converter for Photovoltaic Arrays’. IEE Electronics Letters, 37, no.1, pp. 5-6, Jan. 2001
- J A M Bleijs & A L U Mayere: ‘PV Systems at the University of Leicester: Performance and Developments’. Proceedings 22nd European Photovoltaic Conference, Milan, Sept. 2007, pp. 3089-3093
- Y Liu & J A M Bleijs: ‘Intelligent MPPT Control of PV Panels using a DSP Based DC/DC Converter, tested with a Flexible Solar Emulator’. Proceedings 22nd European Photovoltaic Conference, Milan, Sept. 2007, pp. 3059-3063
- J A M Bleijs: ‘Continuous Conduction Mode Operation of the Three-Phase Diode Bridge Rectifier with Constant Load Voltage’. IEE Proc on Electric Power Applications, Vol.152, No. 2, March 2005, pp.359-68
- O A Ahmed & J A M Bleijs: ‘High-efficiency DC-DC Converter for Fuel Cell Applications:Performance and Dynamic Modelling', Proceedings IEEE ECCE 2009, San Jose, September 2009, 8p.
- For further information on solar energy:
- For wind power: