Page 18 - A Comprehensive Guide to Solar Energy Systems

10 A CoMPREHENSIVE GUIdE To SoLAR ENERGy SySTEMS

1.5 The Difficulties With Harnessing Solar Energy

The two main issues with harnessing solar PV energy are the following:

• Solar energy is perceived to be a dilute form of energy. As an example, in order to
produce an average 1 GW of electricity (the size of a large fossil fuel power-station)
from PV cells, in a hot tropical part of the world, peak mid-day sun energy level
−2
2
of (1200 W m ), the solar farm would be about 20–25 km . This includes the solar
panels, spacing between them, access roads and corridors often left for wildlife
mitigation. In a temperate country, such as the United Kingdom, each solar farm
would probably have to more than twice this size to achieve a power of 1 GW.
Although this seems a lot of land, if we account for the area occupied from coal
surface mining, the coal life cycle can use more land than photovoltaics [32]. of
course when PV are installed in roof-tops they do not use any additional land.
• It also happens that in many countries, especially in the temperate parts of the world,
when the sun is shining, the public demand for electricity is low. Peak demand on
some national grids is usually in the early morning and evening when the sun is either
not shining or its energy is weak. It is estimated that in temperate climates on average
a solar farm produces useful electricity about 20% of the time, while in regions of very
strong solar irradiation (e.g., the Atacama desert in Chile, the PV capacity is in excess
of 35%).
These two issues make it difficult to feed solar PV electricity into a national grid (Chap-
ter 15). This is largely because electricity generation and electricity consumption must be
in balance at all times, even over such short times as seconds. However, large PV power
plants that are utility-friendly, providing reactive power and ancillary services to the grid,
have been developed [33]. Nevertheless, for supporting large penetrations of solar, storage
facilities will be needed. This can take many forms [31]; at the moment the less expensive
energy storage systems are pumped hydro and compressed energy storage, while the price
of batteries (especially Li-ion ones) are being reduced catalyzed by increasing markets for
electric vehicles.

1.6 Is Harnessing Solar Energy Cost Effective?

The efficiency of solar energy systems is rated according to their performance under a
−2
standard test irradiance of 1000 W m , which corresponds to the maximum irradiance
expected on a clear day in summer at moderate latitudes. The actual level of solar irradi-
ance will depend on the latitude and local climatic conditions, but the annual average
−2
solar energy density lies in a range from 100 to 350 W m for most locations. The capacity
factor for solar collectors (actual output power/rated output dC power) therefore lies at
10%–35% depending on location. The cost of producing electricity from solar PV energy
compared to the cost of producing electricity from coal has been estimated by Lazard [34]
and the results are in Table 1.2.