Trái đất nhận được 174 petawatts (PW) của bức xạ mặt trời đến (sự phơi nắng) ở phía trên không khí.[1] Khoảng 30% được phản xạ trở lại không gian trong khi phần còn lại được hấp thụ bởi các đám mây, đại dương và vùng đất. phổ của ánh sáng năng lượng mặt trời ở bề mặt của trái đất là chủ yếu lây lan qua nhìn thấy được và cận hồng ngoại phạm vi với một vai nhỏ trong các cận tử ngoại | Hydrogen 105 pass through the PEM to the anode and so the process proceeds as long as current and water continue to be supplied 34 . In an ideal electrolysis cell a voltage of V if applied to the electrodes at 25 C will decompose the water into hydrogen and oxygen isothermally and the electrical efficiency will be 100 . A voltage as low as V will still decompose the water but now the reaction is endothermic and energy in the form of heat will be drawn from the cell s surroundings. On the other hand the application of a voltage higher than V will result in water decomposition with heat being lost to the surroundings 29 . The process becomes exothermic. Clearly maximum efficiency equates to the lowest voltage that results in hydrogen and oxygen being formed. But this operating regime draws a very low current from the source and hence a very slow rate of production of hydrogen per unit area of electrode surface which means that impractically large cells would be required to produce commercial quantities of hydrogen. As with all engineering processes a compromise is called for in this case between efficiency and production rate. Thus practical cells are operated at high temperature 900 C at voltages in the range V. For example a high temperature electrolysing cell operating at atmospheric pressure with a power input of 60 kW would generate 25 grams minute or 280 litres minute of gaseous hydrogen together with half this amount again of oxygen by volume 7 . This conversion rate from input power to volume of hydrogen is calculated on the basis of negligible thermal losses. The electric current required is 40 kA for a cell voltage of V. Individual cells can be combined in essentially two different ways to form a hydrogen production unit. These are tank type or filter press type 29 . In elec-trolysers of the tank type each cell with its anode cathode its own source of water and separate electrical connections is housed in a separate chamber .
