Achieving high renewable energy penetration in off-grid systems via low load diesel integration: a case study of King Island, Australia

Isolated power systems are increasingly pairing renewable generation with diesel technologies, to address the high cost and pollution of diesel-reliant systems. Within such a system renewable generation often provides the lowest cost of energy. Unfortunately, renewable resources are stochastic, with the inherent volatility a threat to power system flexibility. Power system flexibility is the ability of the power system to accept increasing levels of penetration from any single generation source, without compromising supply security. In principle renewable energy integration improves power system flexibility, via diversification of generation sources, however to address system stability, renewable energy integration must consider the range of services offered by conventional generation. A common solution involves hybridised diesel application. Under hybrid configurations, diesel generation remains the primary control of system frequency and voltage, with renewable generation able to reduce diesel fuel consumption. Despite commercial and environmental considerations promoting the use of renewable technologies, technical constraints typically limit the permissible renewable energy penetration to low levels (<30%) . Medium (>30%) and high (>60%) levels of renewable energy penetration introduce unique challenges, with loading and control of diesel infrastructure a key constraint to further renewable energy utilisation. In order to relieve such a constraint, the diesel generator/s must move outside optimal efficiency (kWh/litre) loading. As diesel generators throttle to lower and lower loading, and consequently lower efficiency, renewable generation is afforded increasing contribution. The lowest operating cost is typically found between 50-80% renewable energy penetration. In assessing such high renewable energy penetrations, operators must consider not only the relative costs of generation, but also the technical capability of the diesel equipment to run at such low loads.

Ideally diesel engines should be run above 40% of their maximum rated capacity. At low loads diesel generators can experience a number of operational issues, owing primarily to low cylinder temperature and pressure. At low temperatures poor combustion chemistry combines with poor piston ring sealing, historically resulting in glazing, soot formation and un-burnt fuel residue seepage. Operational performance can quickly deteriorate as issues compound to further degrade cylinder pressure and temperatures, with irreparable engine damage possible. Short periods of low load operation are permissible given the engine is brought up and held at high load for sufficient duration to purge accumulated carbon residue. Recent advances in diesel generator technology are however redefining low load thresholds with opportunity for hybrid power systems to increased system flexibility via modified diesel load limits. The University of Tasmania’s has explored both low load diesel, and variable speed diesel applications, having developed hybrid power system models to quantify the advantages of each practice. A low load diesel pilot is also currently underway within the King Island power system. This paper explores both simulated system performance, and the practicality of MW scale low load diesel application. A case study of the King Island power system is offered in defining the commercial and environmental benefits of low load diesel integration. In addition the interplay evident across low load diesel and other enabling technologies, such as energy storage, is discussed.