Water energy

The basic process of utilizing water energy to produce electricity is described as water flowing through the  turbines that spin in order to transfer kinetic energy in water into electricity that supplied to the grid. In fact, the water power represented as Hydro power, Wave and Tidal. In this case, we mainly focus on the hydro power which is widely used around the world.

Hydro Energy

Approximately 1/3 solar incident upon earth  produces evaporation from sea and land. When condensation occurs in the atmosphere, most of this energy is given up in the form of latent heat and re-radiated to earth. Approximately 0.1% is available due to precipitation onto high land which could be harnessed as hydro energy source when it falling through hydro turbines (As shown in video below).

According to the Hong Kong Observatory in 2012, the global annual rainfall is about 1.4767 m. The surface area of the earth is about 5.11 x 10¹⁴ m². So the annual precipitation is about 7.5459 x 10¹⁴ m³.
The energy that is available to the power plant is equal to the potential energy, PE (Joules) in the stored water, give by the equation:

PE = mgh 

Where

m = mass of the water (kg)

g = acceleration due to gravity (10 m²/s )

h = average height of the land above sea level (800 m)

Therefore the total potential energy  = 6.0367 x10²¹  J

Divided by number of second in 1 year:

Power = 1.914 x 10¹⁴ W

Multiply by number of hours in one year:

Power = 1.6767 x 10¹⁸ Wh = 1676.7 TWh

It is equal to nearly twice mankind's primary energy consumption. However, only a small part of it could be used as energy source.

Reference

Nickl Elsa. Etc (2010)Changes in Annual Land-Surface Precipitation Over the Twentieth and Early Twenty-First Century

http://www.weather.com/outlook/weather-news/news/articles/yearly-rainfall-2011_2012-01-06
http://www.kidzworld.com/article/1375-hydro-energy#

http://www.hko.gov.hk/wxinfo/pastwx/ywx2011.htm

iha_activityreport_2012_web.pdf

Wind energy

Wind energy is another kind of renewable energy as it is clean and operates without emitting greenhouse gases. Basically, wind power is the conversion of kinetic energy of wind into useful form of energy through turbines (Figure 1). Normally, it is used for producing electricity. According to the calculation made by Hurley (2009), the world 's wind resources are estimated to be 50000 TWh/year. In fact, the total world primary energy consumption in 2008 is about 21283 TWh (IEA, 2010). The total available global wind resource on land is therefore more than adequate to supply a very significant proportion of the overall world's energy demand.

Figure 1. Structure of wind turbine

As most renewable energy sources without emission of pollutants such as SOx and NOx they will form the basis of any long-term sustainable energy supply system.  By 2020, taking EWEA projections that 180GW of wind energy would be generating 425 TWh per annum, wind power will save about 215 million tonnes CO2, 261,000 tonnes SO2 and 333,000 tonnes NOx comparing with the supply system depends on fossil fuels.

However, the disadvantages of using wind energy is obvious. Firstly, it cause visual impact on the surrounding. For example, the wind turbine would lead to light flash when the Sun is low in the sky east or west of the turbines. Secondly, the large wind turbines that used by power plants would threat the life of bird. A study in 2001 in the United States have estimated that each wind turbine has an average fatalities of 2.2 for birds. Finally, wind turbine would also create aerodynamic noise during the operation. Basically, the noise level increase with the speed of rotation in the turbines. So modern turbines operates with low speed to alleviate it.

Reference

Hurley Brain (2009). How Much Wind Energy is there? Wind Site Evaluation Ltd.

IEA Key energy statistics 2010 (2010).

WWEA (2011). World Wind Energy Report 2010.WWEA Head Office Charles-de-Gaulle-Str. 5 53113 Bonn Germany

EWEA (2010). WIND ENERGY AND THE ENVIRONMENT. Renewable Energy House 26, rue du Trône  B-1000 Brussels  Belgium 

Photovoltaics

Photovoltaic (PV or solar cells) is a method of converting direct solar radiation into electricity supply. Normally, it is made from semiconductors (such as Si and Silicon). The photovoltaic panel could be divided into three layers: Junction region, n-type (extra electron) and p-type (extra hole). The solar radiation would cause the electrons separated from the atoms in the junction region. Then the electrons would move to one side (n-type) when holes to the other side. Current would be generated when wires are connected with the PV panels (Figure 1).

Figure 1. Working principle of PV panels

At present, Si is widely used as the material of solar panels. And there are three main types of Si PV module: Monocrystalline Si, Polycrystalline Si and Amorphous Si thin-film. Among these types, Monocrystalline Si has the largest efficiency when it is expensive to produce electricity, while Amorphous Si thin-film is the efficient one with a lowest production price.
The main reasons of using PV system are:
1. It is reliable when there are no moving parts in the whole system.
2. It has a very low operation and maintenance costs as the origin fuel of PV system is solar radiation.
3. It has no noise and no pollution to the environment during operation.
4. The direct converting from solar energy into electricity makes the PV system more efficient without transmission losses.

Nowadays, solar PV generates electricity in well over 100 countries and continues to be the fastest growing power-generation technology in the world. Between 2004 and 2009, grid-connected PV capacity increased at an annual average rate of 60%. According to the globe status report from Renewable 2010, solar PV accounts for about 16 percent of all new electric power capacity additions in Europe in 2009 (Figure 2).

Figure2. Solar PV-Existing world capacity (2004-2009)

Reference

DTI: Photovoltaics in Buildings - A Design Guide, Report No. ETSUS/P2/00282/REP, 1999
CIBSE TM25: Understanding Building Integrated Photovoltaics, 2000

G. Boyle: Renewable Energy: Power for a Sustainable Future, Oxford University Press, Oxford, 2003
T. Markvart: Solar Electricity, John Wiley & Sons, Chichester, 2000