Renewable Generators

Autonomous power systems can be based on renewable energy (RE) units, such as wind turbines, photovoltaic systems or small hydro power stations and fossil-fuel generators. Due to the intermittent nature of renewable energy, storage devices and/or appropriate demand management strategies are necessary when conventional generators are not included in the system. 

In the following, a description of the operational characteristics for renewable and conventional generation technologies, used in APS, will be provided. Based on this information, a technical overview of APS will be performed, with emphasis on stability and energy management issues. Finally, economic evaluation of APS design will be analysed in terms of the relevant optimisation problem configuration. 

Wind turbines driving induction generators (third case), are mostly used in APS applications where voltage is produced by other controllable power sources such as diesel generators. They are directly connected to the grid, and their rotational speed is defined by the grid frequency and their power production level. 

The power that can be produced by a wind turbine is defined by the following equation  

Cp is the aerodynamic efficiency factor. It is a function of the rotor blades design and angle as well as the relative speed of the rotor and wind (known as the tip speed ratio) 

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How long will it be before the United States has enough excess low-cost renewable generation that it can divert a substantial fraction to production of hydrogen ... 

A solution to avoid these situations is the implementation of limitations on the power produced from renewable generators, depending on the operating condition of the system. Such a measure can improve the power quality and stability of the APS. On the other hand, bearing in mind that the introduction of renewable sources in an APS fed by diesel generators aims at the improvement of the operational efficiency and the minimisation of the fuel utilisation, this solution is far from being optimal. 

An alternative or complementary solution to the introduction of storage is the utilisation of secondary controllable loads in the system that will be activated when the production of renewable generators exceeds the current demand. Such loads might be desalination units, water heating, house or district heating, etc. 

So far, issues related to the frequency stability of an APS have been presented. Maintaining voltage levels in the AC buses of an autonomous power system close to their nominal value is a similar problem. Such an objective can be obtained if equilibrium can be achieved between produced and consumed reactive power in the system. The influence of renewable generators on voltage control depends on the type of the interface used for the connection to the grid. Induction generators in wind turbines or small hydro plants, consume reactive power. A partial compensation can be achieved using local capacitors. Power electronic inverters used for the connection of photovoltaic systems and wind turbines are reactive power neutral. 

Fourth, electrolysis presents a path to hydrogen production from renewably generated electrical power. From an energy perspective, electrolysis is literally a way to transform electricity into fuel. Electrolysis is thus the means of linking renewably generated power to transport fuels markets. Currently, renewable solar, wind, and hydro power, by themselves, produce only electricity. 

In calling out the efficiency costs of alternating current/direct current (ac/dc) power conversion, one advantage of renewable power becomes worthy of note. Renewables generate dc power that can be applied to the dc-using electrolyzer cell stack without inversion. This incremental efficiency advantage associated with renewables may become material as the cost of power from renewables continues to drop. 

Electrolyzers typically operate from grid-quality power, so a new variety of power control and conditioning equipment needs to be developed in order for electrolyzers to operate efficiently from renewable sources. The prospect exists for good efficiency in converting renewable power to hydrogen, insofar as electrolyzers require direct current and renewables generate direct current, so there are no losses associated with ac/dc conversion. 

Hoekman, S., Broch, A., Robbins, C. and Purcell, R. (2010) CO2 recycling by reaction with renewably-generated hydrogen. International Journal of Greenhouse Gas Control, 4,44-50. 

VIA Metropolitan Transit in San Antonio, Texas, will acquire three Proterra allelectric battery buses with fast charge infrastructure powered by wind and solar renewable generation. 

Pletcher, D. (2010) Towards energy storage for renewable generation. ECS Transactions, 28 (16), 1-10. 

In South Korea, the Renewable Portfolio Standard that will come into effect in 2012 demands an increasing share of power generated from renewable sources and fuel cells. It is estimated that the impact of the RPS will be an annual installation of renewable generation capacity including fuel cells of more than 300 MW in 2012 and reaching more than 900 MW toward the end of the decade. 

Future electricity systems will take a very different form from the proven systems of the past, as decarbonisation targets will drive an ever greater reliance on renewable generation, coupled with interconnection of neighbouring power systems to increase resilience against the intermittent nature of many renewable energy sources. 

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