Electricity Production from Small-Scale Photovoltaics in Urban Areas

Introduction

The photovoltaic systems consist of cells that convert sunlight into electricity. Each of these cells is manufactured by layers of a semi-conducting material, specially produced in order to develop electric field across the layers when the sun light is falling on the cell. This electric field causes the electricity flow and thus power is produced. The electric field is generated by the following mechanism (EPIA-Greenpeace, 2011; Luque & Hegedus, 2003; Sen, 2008): When the cell is exposed to light, photocarriers are generated and this carrier separation within the cell layers produces a photovoltage and charge motion produces a photocurrent, which runs in reverse through the diode junction as described in Goetzberger and Hoffmann (2005). The sun light intensity determines the amount of electrical power each cell generates. A photovoltaic system does not need bright sunlight in order to operate. It can also generate electricity on cloudy and rainy days from reflected sunlight (Rockett, 2010).

Photovoltaics present high interest in community due to fact that the sun is freely providing light in huge quantities and it is expected to be shining for the next hundred million years. There is more than enough solar irradiation available to satisfy the world’s energy demands. According to several studies (EPIA-Greenpeace, 2011; Luque & Hegedus, 2003; Sen, 2008; Šúri, Huld, Dunlop, & Ossenbrink, 2007), the Earth is exposed to enough sunlight to generate electricity with average value equal to 1,700 kWh/m² of land per year, using currently available technology. A large amount of statistical data on solar energy reaching earth’s surface is collected globally for many years and for many areas. For example, in the US National Solar Radiation, a database with 30 years of solar irradiation and meteorological data from 237 sites in the USA is available. The European Joint Research Centre (JRC) also collects and publishes European solar irradiation data from 566 sites (EPIA, 2011; Greenpeace, 2008). Figure 1 presents the solar irradiation and photovoltaic power generation potential in EU in kWh/kWp of installed power, as it is given by Šúri et al. (2007) and EPIA (2011). Higher solar irradiation corresponds to higher power generation potential. Europe receives around 1,200 kWh/m² of average annual energy while Middle East 1,800 to 2,300 kWh/m². Even though only a rather small part of solar irradiation can be used for electricity production, this “efficiency loss” does not actually waste a finite power resource, as the use of fossil fuels. However, the reduced efficiency has an impact on the cost of the PV systems (EPIA-Greenpeace, 2011;Sen, 2008; Šúri et al., 2007; EPIA, 2011).

Figure 1.

Solar irradiation and photovoltaic power generation potential in EU

(Source: Šúri et al., 2007).