Solar irradiation

The solar irradiation density at the earth's distance from the sun amounts to 1.3 kW/m2. Part of this is reflected, part of it is absorbed in the atmosphere. Thus, the irradiation density on the surface of the earth depends on the location. Under ideal conditions approximately 1 kW/m2 arrives in Central Europe. With seasonal changes, times of overcast sky, and night time this amounts to an average solar irradiation of about 1000 kWh per m2 and year (corresponding to 2 to 3 hours of ideal sunshine each day). The map below gives the irradiation distribution for the earth.

Figure 1: Solar irradiation map (Source: MySolar)
The solar irradiation resembles black body radiation of about 5800 K. The total radiated power of the sun may be estimated by the Stefan-Boltzmann law (radius of the sun: 6.9.108 m):

P = 4 p r2 s e T4 = 3.9.1026 W

The solar energy irradiated to the Earth is 5.1024 Joule per year. This number exceeds the present worldwide yearly energy consumption which is estimated to 5.1020 Joule. In the literature the energy consuption is frequently given in terms of Quads. The Quad is 1015 (one quadrillion) British Thermal Units (BTUs), where a BTU is1055 Joule (the BTU is defined as the amount of energy which increases the temperature of one pound of water by one degree Fahrenheit). Therefore, the annual global consumption is 500 Quads.

For the conditions of central Europe a PV system of 1 kWp (say an area of 10 m2 and an efficiency of 10%) will produce approximately 900 kWh of electrical energy. The expression Wp is a power rating for solar cells and modules and gives the output power under standard conditions (AM1.5 Spectrum with 1000 W/m2 and a cell temperature of 25odegree C).

The average electricity consumption of a 4-person family is about 4000 kWh, thus, solar energy has the potential to supply the average amount of energy, even in the moderate climate of Central Europe.

Two serious drawbacks arise: The storage and the required area. Daily and seasonal availability of solar energy is often contrary to the customers needs and not every family has access to 40 m2 where they can mount their solar modules. Nevertheless, storage solutions are developed and if existing buildings are equipped with solar systems a considerable amount of energy can be produced without consumption of additional area.


Previous Next