2. SOLAR ENERGY RESOURCE

2.1 Introduction

All our sources of energy derive ultimately from the sun. Some, such as Fossil and Nuclear, have stored that energy and concentrated it over a very long time, whereas Biomass, Hydro and Wind rely on conversion from solar energy over a short timescale. The underlying advantage of using solar energy directly is that it is free, abundant and inexhaustible. The total amount of energy radiated from the sun onto the earth's surface is more than 10 000 times the annual global energy consumption. With care, its use can also avoid degradation of the environment. Since both Solar Thermal and Solar Photovoltaic energies make use of the same source, this short section covers their common solar resource.

 

2.2 Solar irradiation

The sun is continuously radiating enormous amounts of energy. A fraction of that energy reaches the earth. The fraction of the energy from the sun that reaches the earth in just one day is still more than enough to cover the energy use of the world in a whole year. However, not all the energy of the sun that reaches the earth can be used effectively. Part of the sunlight is absorbed in the earth's atmosphere or reflected back into space.

The intensity of sunlight reaching the earth varies with time of day and year, location, and weather conditions. The total energy on a daily or annual basis is called irradiation and indicates the strength of the sunshine, typically expressed in kWh/m² per day.

To simplify calculations with irradiation data, solar energy is expressed in equivalents of one hour's bright sunlight. Bright sunlight corresponds with a radiative power of about 1,000 W/m² so one hour of bright sunlight corresponds with 1 kWh of energy per m². This is approximately the solar energy falling on a surface of one square meter perpendicular to the sun's rays on a cloudless day in summer. On a horizontal surface, this maximum value could only occur near the equator at midday.

 

Source: BP-Solarex

Figure 1: Annual Solar Radiation kWh/m²