Seite - 124 - in Clean Water Using Solar and Wind - Outside the Power Grid

124 Clean Water Using Solar and Wind: Outside the Power Grid • Diurnal variations: The solar power has a diurnal variation, so the cycle time is one day. To fully compensate for variations in this timescale the buffer storage capacity needs to be several kWh per kW of renewable energy capacity. • Bridging periods of bad weather: This is a period where the skies are overcast and there is hardly any wind. To store capacity during such a period is naturally much more challenging than with diurnal variations. To afford this storage capacity may be unrealistic in a low-income region. • Seasonal variations: In a cold climate there is a large temperature difference between cold and warm seasons. Thus, the need for energy storage for the winter is high. On the other hand, the difference between the coldest and warmest month in a tropical or subtropical country is much less. Therefore, the need to store energy for the coldest month is not as great. The most important storage technologies are listed here. They are suitable for different time and energy scales and not all of them are appropriate for a low budget. • Batteries: For short variations it is common to use batteries. Car lead-acid batteries have been used for decades and are a viable alternative in low-income countries. However, the requirements for the battery for a renewable energy system are different than for a car. Battery storage is described in 10.3–10.4. The discharge time is several hours. • Flywheels: The electric power can be stored in a flywheel storage. In other words, the electric energy is converted into kinetic energy, coming from a motion of a spinning mass, or a rotor. Most modern high-speed flywheel energy storage systems consist of a rotating cylinder, supported by magnetic bearings. The flywheel operates in a vacuum to reduce drag. A motor-generator unit will convert the electric energy to rotation; and to utilise the stored kinetic energy the motor-generator unit is used in the opposite direction to generate electric power. Flywheel storages are common in aerospace and telecommunications. Typical timescales for discharge are in the range of seconds to minutes. • Compressed air: A compressed air system uses excess electric energy to compress ambient air and store it under pressure in a Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user