Page - 138 - in Clean Water Using Solar and Wind - Outside the Power Grid

138 Clean Water Using Solar and Wind: Outside the Power Grid At the cathode another catalyst (often nickel) causes hydrogen ions, electrons and oxygen to react: 4H+ + 4e− + O2 → 2H2O where the water is considered to be a by-product. In other words, the summary reaction is very simple: 2H2 + O2 → 2H2O A single cell will typically produce quite small voltages: 0.6– 0.7 V. Therefore, several cells must be connected in series to create a sufficient voltage to meet the requirement for the specific load. If cells are connected in parallel a higher current can be supplied. Connection of cells is called a fuel cell stack. The fuel cell will also produce water and heat as well as small amounts of emission, for example nitrogen dioxide (NO2). The energy efficiency is not high; between 40% and 60%. This means that storage of energy as hydrogen causes significant energy losses, even though in many cases the waste heat can be utilised. It should be emphasised that fuel cells have no moving parts. This makes them reliable sources of electrical energy. 10.6 PUMPED AND CLEANED WATER AS STORAGE Storing energy as water in elevated places has been known about and practised for centuries. In hydropower water can be stored in reservoirs as potential energy that can be converted to electricity. In many places pumped water storage is used to make use of excess energy. Then the potential energy of the water can be used to generate electricity when needed. This principle can also be used in stand-alone systems at the household level. Excess energy from solar PV or wind can be used to pump water to an elevated storage. Then the water can be used in two ways: • If the elevation is sufficiently high, then the water may drive a turbine and generator to generate electricity; • Otherwise the water can be used directly either for drinking (if  the quality is adequate) or for irrigation, using only gravity. Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user