Generation Reliability for Isolated Power Systems with Solar, Wind and Hydro Generation

Licentiate thesis, 2003

This licentiate thesis deals with reliability issues for small isolated power systems. The focus is on power systems in remote areas based entirely on renewable energy sources. The sources investigated in this thesis are mainly solar and wind power; stochastic models for these energy sources are presented. The solar power model is based on the solar declination and cloud coverage. In addition a Markov model for simulating cloud coverage data is presented. The wind power model is based on two Markov chains, one for low wind and one for high wind speeds. Both the solar and the wind models use measurements as input but by using normalised wind speed measurements a more general model is obtained. There are still, but minor, needs for site-specific meteorological measurements. The presented models of solar and wind power are together with some simple models of hydro power, storage and load used for case studies. Since the solar model is dependent on the location, a site in Africa is used. Timbuktu in Mali was chosen for its subsahara climate and the fact that Mali is considered a developing country. Twelve cases were studied with combinations of solar, wind and hydro power both with and without storage possibilities. A system with only wind as power source has a higher overall availability than one with only solar power. But since all the solar power is available during daytime the availability is higher for solar power during daytime. By adding storage capability the overall availability will be higher for solar power because more efficient use of the storage. Combining the power sources with small hydro power (only 10 % of total maximum load) the availability will be significantly increased. The hydro power will then supply the load during low load hours. A combination of all three power sources gives a high reliability because during the high daytime load all three sources are available and during nighttime both wind and hydro are available. The largest availability problems are during mornings and evenings when the load is high but the solar power has a low availability. This effect is season dependent. To be able to use exclusively renewable energy sources a combination of sources is needed to secure the reliability of the supply.