Seite - 58 - in Clean Water Using Solar and Wind - Outside the Power Grid

58 Clean Water Using Solar and Wind: Outside the Power Grid be converted from DC to AC. This is performed in an inverter, a power electronics device. A DC pump, however, does not need an inverter. DC pumps are more common for small-scale applications, typically less than 3 kW. This is adequate for a single household or for a small irrigation systems. DC motors tend to have higher efficiency than corresponding AC motors. Traditionally it has been simpler to control the speed of a DC motor. Today, however, it is more economical to install an AC motor with an electronic variable speed controller. Furthermore, DC motors have many moving parts that are expensive to replace. They are supplied with brushes and commutators that have a limited operational life and must be replaced regularly. An AC motor with power electronics controller is both more reliable and more economical. It has no brushes and has a rugged design. Inverters represent a proven technology over a very wide range of power levels and usually have very high efficiency: 98% or higher. Inverters for solar PV systems are often subject to harsh conditions, exemplified by operation during many sun hours combined with temperatures higher than 40°C. Dust is another challenge. When a small inverter is coupled directly to a PV module and mounted on the rear side the conditions may be severe. The temperature may reach up to 80°C. Under these conditions the electronics must be ready to operate for a period comparable with the lifetime of the PV modules, at least 20 years. Therefore, it is important to have a design margin that will ensure long-term reliability. The individual components must meet the harsh conditions. Given an adequate design the useful life of an inverter will exceed 20 years. There is an internationally accepted standardisation of test procedures for PV system components. The standard IEC1215 defines the electrical and thermal properties of the components and can give an indication of the expected lifetime. The pump also needs a controller that can adapt the power directed to the pump with the power delivered from the energy source. Generally, a controller should be provided with a voltage protection capability that will shut down the pump if the supplied voltage is too low or too high. In Chapter 11 we analyse more details concerning the control equipment. Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user