(2)
These values are to be broken down by 9% ultraviolet, 49% visible, 42% Infrared.
Energy received from the Sun balances the energy that Earth loses back into space, maintaining a stable average temperature. The incoming solar radiation is 342 watts/m2, of which only 168 reach the ground and 107 W/m2 will be reflected Solar radiation, 67 W/m2 absorbed by atmosphere. For thermal equilibrium, the reradiated power must also be 342 watts/m2. In fact, 235 watts/m2 represents outgoing longwave radiation and 107 watts/m2 is reflected solar radiation are both dispersed in the space. Infrared radiation from Earth’s surface is 390 W / m², of which only 235 are dispersed in the space. The remaining about 155 W / m² are infrared radiation from the atmosphere (greenhouse gases effect) and returned to Earth together with the energy previously stored by the atmosphere (67 W / m²), vapor condensation heat (78 W / m²) and the transfer rate from the convention (24 W / m²). All these quantities provide 324 W / m² of return radiation.
Table 2 shows Global solar radiation that reaches the surface of the ground as sum of three components:
Atmospheric conditions |
Solar Radiation | clear sky | Fog | cloudy | Yellow solar disc | White solar disk | Just visible sun | thick fog | Covered sky |
Global | 1000W/m2 | 600W/m2 | 500W/m2 | 400W/m2 | 300W/m2 | 200W/m2 | 100W/m2 | 50W/m2 |
Direct | 90% | 50% | 70% | 50% | 40% | 0% | 0% | 0% |
Diffused | 10% | 50% | 30% | 50% | 60% | 100% | 100% | 100% |
Table 2: Energy on Earth’s surface (in W / m2 per year).
2.0 Energy storage
Energy storage represent as a link between renewable power, and 24/7 reliability. For unmanned application power supply and demand must be balanced at any given moment. Energy storage plays an important role in this balancing. When there is more supply than demand, the excess electricity generation can be used to power storage devices. When demand is greater than supply, storage facilities can discharge their stored energy to the load. Energy harvesting model (EHM) is presented in []. Due to the random nature of the environmental energy, the harvested energy needs to be stored in an energy buffer (rechargeable battery and supercapacitors). The equation explain, as at any moment in time, the available energy should be greater or equal to the required energy for supporting the load (utilizes harvested and stored energy):