Page - 114 - in Clean Water Using Solar and Wind - Outside the Power Grid

114 Clean Water Using Solar and Wind: Outside the Power Grid wide range of small-scale turbines, from ‘micro SWTs’ rated at less than 1 kW to ‘midi SWTs’ reaching 100 kW. They are commonly used as stand-alone electricity systems in off-grid locations. Obviously, if the location is not windy there is a lower load factor and a higher capital cost per kW. Another challenge is wind turbulence caused by obstacles in the surroundings. A high tower will reduce this turbulence but will increase the cost. Simply put, a turbine with a larger tower and a relatively larger rotor (compared to its maximum power output) will produce more energy per installed unit of capacity. The wind velocity on hills and ridges can be higher. On the other hand, the air density is lower at high altitudes. In cold climates the air is denser in winter, resulting in more wind energy. A problematic issue is that solar PV is an increasing competitor to wind power, as solar PV cost has dropped so dramatically during the last few years. For example, wind power water pumping can compete with solar power water pumping only at high wind speed and low solar irradiation value (Campana et  al., 2015). To find the most cost-effective solution for irrigation depends on the location. Another disadvantage of wind compared to solar PV is that the wind tower machinery needs frequent maintenance and repairs, which can be problematic considering the sub-Saharan conditions (Varadi et  al., 2018). 9.2 WIND POWER EFFICIENCY Wind speeds can change significantly in just minutes. The output of wind turbines is therefore variable. As already noted, the output grows with rising wind speed and is constant above the rated wind speed. Since the wind turbine cannot run at nominal speed continuously the real delivery of energy is expressed as the capacity factor. A wind turbine located offshore normally has a higher capacity factor than an onshore turbine. For example, onshore turbines in the UK have 26% while offshore wind has 35% (WEC, 2016, Chapter 10). In Denmark, a pioneering country in wind power, the power plants were running for 20–25% of the time in the 1980s and 1990s. In 2017 the offshore 160 MWe wind power park Horns Rev operated for some 3,500 hours or 40% of the time. The typical operating range for offshore is now 3,500–4,000 hours per year or 40–46%. Today large offshore wind Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user