Because of their practical energy storage, solar power towers have two features that are particularly desirable for utilities: flexible capacity factors and a high degree of dispatchability.
Power towers can be designed with annual capacity factors up to 60 percent, and as high as 80 percent in summer when the days are longer. Load dispatching capability of central receiver plants This means a power tower can operate at capacity for up to 60 percent of the year without using fossil fuel as a back-up, thus being able to deliver power during most peak demands. Without energy storage, the annual capacity factor of any solar technology is generally limited to about 25 percent. A solar power tower’s high capacity factors are achieved by building the solar portion of the plant with extra heliostats so that during daylight, sufficient energy is collected to power the turbine, while extra energy can be put into the thermal storage system. At night or during extended cloudy periods, the turbine is powered with stored thermal energy. The dispatchability of a solar power tower – its ability to deliver electricity on demand – is illustrated above, where three different parameters are plotted against time of day: the intensity of sunlight (insolation), the amount of energy stored in the hot-salt tank, and the output power from the turbine generator. In this example, sunrise on a winter’s day is around 7 a.m., and the intensity of sunlight rises quickly to reach its maximum at noon and drops off at sunset around 5 p.m.
The solar plant begins collecting energy shortly after sunrise and stores it in the hot-salt tank – the level of energy in storage increases during daylight hours. The turbine is brought on-line not at sunrise, but when the power is needed, in this example at 11 a.m. The output power of the plant is constant throughout the day, even though there are fluctuations in the intensity of sunlight. After sunset, the turbine continues to operate on energy from the storage tank; note the level of energy in storage declines after sunset. The turbine operates continuously until 9 p.m. using the thermal energy in storage. In the summer when the days are longer, the turbine would be able to operate a larger fraction of each day.
In designing a power tower, the size of the turbine, the fraction of the day it is in operation, and the period when it is operated are completely flexible. The plant’s efficient thermal storage system provides dispatchability, and by adjusting the size of the solar field and the size of the storage tanks, the capacity factor can be tailored to meet the specific needs of a utility.