Wind

Phone 4 Energy

Ultimate Guide to Power Efficiency

Get Instant Access

Hydro-electric power The sun evaporates every year on average 980 litres of water from every square metre of the Earth's surface, in total 500,000 km3 (see Figure 1.14). About 22 per cent of the solar radiation energy reaching Earth is needed to drive this water cycle. Nearly 20 per cent of the evaporated water rains down on landmasses, where the majority evaporates again. About 40,000 km3 flows back to the oceans in rivers or groundwater. This is equal to more than one billion litres per second. Technically, the energy of this flow can be used.

About 160 EJ is stored in rivers and seas, which is equivalent to roughly 40 per cent of the global energy demand. About one-quarter of that energy could be technically exploited, so that nearly 10 per cent of the global energy demand could be provided free of carbon dioxide emissions by hydro-electric power. The potential for hydro-electric power in Europe is already relatively well exploited, whereas large unexploited potentials still exist in other regions of the world.

The history of using hydro power reaches back many centuries. Initially, watermills were used to convert hydro power into mechanical energy.

Figure 1.15 Principle of a Hydro-electric Power Plant

Electricity generation from hydro power started at the end of the 19th century and has achieved technical sophistication. A weir creates a height (or 'potential') difference (also called a 'head') between the water before and after the weir (see Figure 1.15). This potential difference can be utilized by a power plant. The water flows through a turbine, which transforms the potential energy into mechanical energy. An electric generator converts this into electricity. Depending on the head height and flow rate, different turbines are used. Common turbines are the Pelton, Francis or Kaplan turbines. Finally, a transformer converts the generator voltage to the grid voltage. The power output:

of the power plant can be calculated from the efficiency of the generator vq and the turbine Vt, the density Pw of the water (pw ~ 1000 kg/m3), the head H (in m), the gravitation constant g (g = 9.81 m/s2) and the flow rate Q (in m3/s).

Table 1.8 Contribution of Hydro-electricity to the Net Electricity Generation in Different Countries

Country Paraguay

Norway

Brazil

Iceland

Venezuela

Austria

Canada

Share (%) 100

99

89

83

75

71

60

Country Russia

China

Australia

US

Germany

UK

Netherlands

Share (%) 19

17

8

7

4

1.4

0.1

Source: DOE, 2003

Note: Left: Upper Reservoir; Right: Lower Reservoir, Penstock and Surge Shaft

Figure 1.16 Pumped-storage Hydro-electric Power Plant in Southern Spain near Malaga

In addition to electricity generation in river or mountain power plants there are so-called pumped-storage hydro-electric power plants (see Figure 1.16). These power plants can be used for electricity storage. In times of excess power generation, a pump transports water in a storage basin to a higher level. When the water flows back, a turbine and a generator can convert the potential energy of the water back into electricity.

Hydro-electric power is, apart from the use of biomass, the only renewable energy resource that covers a noticeable proportion of the global energy demand. The resources and the contribution of hydro-electricity to the electricity supply vary from country to country as shown in Table 1.8.

The Itaipu hydro-electric power plant shown in Figure 1.17 is situated in the border area between Brazil and Paraguay. It is at present the largest hydroelectric power plant in the world. It has a rated capacity of 12.6 GW and generated 24.3 per cent of the electricity demand of Brazil and 93.6 per cent of that of Paraguay in the year 2000. The total electricity generation in the same year was 93.4 billion kWh. Table 1.9 shows the enormous dimensions of the Itaipu power plant. However, an even larger plant is under construction: the

Table 1.9 Technical Data of the Itaipu Hydro-electric Power Plant

Reservoir

Generator units

Surface 1350 km2 Maximum height

Extent 170 km Overall length

Volume 29,000 km3 Concrete volume

196 m 7760 m

Number

Rated power

8.1 million m3 Weight

3343/3242 t each

Source: Itaipu Binacional (2003)

Source: Itaipu Binacional (2003)

Figure 1.17 Itaipu Hydro-electric Power Plant

Source: Itaipu Binacional (2003)

Figure 1.17 Itaipu Hydro-electric Power Plant

Three Gorges hydro-electric power plant in China will generate 18.2 GW when completed.

Such large hydro-electric power plants are not uncontroversial because they also have a negative impact on nature and local conditions. For the Three Gorges power plant in China, several hundred thousand people had to be relocated. An example of adverse environmental effects is the Aswan dam in Egypt. It stopped the Nile from flooding, and hence from replenishing the nutrients in the intensively farmed flood planes. The artificial irrigation required to make up for the missing fertilization caused salination of the ground and harvests deteriorated. The area around the estuary also is affected by increasing soil erosion.

Before planning large hydro-electric power plants, the advantages and disadvantages should be considered carefully. On the one hand, hydro-electric power is a technology that can generate electricity without carbon dioxide emission at very low cost. On the other hand, there are negative impacts as detailed above. Small hydro-electric power plants can be an alternative. Their negative impacts are usually much smaller, but their relative costs are much higher.

The tidal power plants described on p21 also utilize hydro power. Other types of power plants that use hydro power are wave or ocean current power plants. However, these power plants are still at the prototype stage at present.

Biomass Life on Earth is possible only because of solar energy, a substantial amount of which is utilized by plants. The following equation describes, in general, the production of biomass:

Table 1.10 Efficiencies for Biomass Production

Oceans 0.07% Woods

Fresh water 0.50% Maize

Man-made landscape 0.30% Sugarcane

Grassland 0.30% Sugar beet

Biomass

Dyes such as chlorophyll split water molecules H2O using the energy AE of the visible sunlight. The hydrogen H and the carbon dioxide CO2 taken from the air form biomass CkHmOn. Oxygen O2 is emitted during that process. Biomass can be used for energy in various ways. Such use converts biomass back again to CO2 and H2O. However, this conversion emits as much CO2 as the plant had absorbed from the atmosphere while it was growing. Biomass is a carbon dioxide-neutral renewable energy source as long as the resource is managed sustainably.

A comparison of biomass production with other energy conversion processes is based on the estimated efficiencies of various plants. This efficiency describes what percentage of solar energy is converted to biomass. The average efficiency of global biomass production is about 0.14 per cent.

Table 1.10 shows some specific efficiencies of different methods of biomass production. The efficiency is calculated based on the calorific values given in Table 1.11 of the biomass grown in a certain area over a given time, which is then divided by the solar energy incident in this area during the same period of time.

Biomass usage can be classified as use of organic waste or agricultural residues and the cultivation of purpose-grown energy plants. Biomass can be used, for instance, in combustion engines, typically combined heat and power (CHP) plants. These CHP plants are usually smaller than large conventional coal or gas power plants, because it is important to minimize biomass transportation distances. Therefore, these power plants usually have a capacity of a few MW. Figure 1.18 shows a power station that is fired with residues from olive oil production.

Table 1.11 Calorific Values c

f Various Biomass Fuels

Fuel (anhydrous)

Lower calorific value (LCV)

Fuel (anhydrous)

LCV

Straw (wheat)

17.3 MJ/kg

China reed

17.4 MJ/kg

Non-flowering

Colza oil

37.1 MJ/kg

plants (wheat)

17.5 MJ/kg

Ethanol

26.9 MJ/kg

Wood without bark

18.5 MJ/kg

Methanol

19.5 MJ/kg

Bark

19.5 MJ/kg

Petrol (for comparison) 43.5 MJ/kg

Wood with bark

18.7 MJ/kg

Source: Markus Maier/Steffen Ulmer

Figure 1.18 Biomass Power Plant Using Residues of Olive Oil Production in

Southern Spain

Source: Markus Maier/Steffen Ulmer

Figure 1.18 Biomass Power Plant Using Residues of Olive Oil Production in

Southern Spain

The potential of fast-growing energy plants such as Chinese reeds or colza is very high. Even in densely populated industrial countries such as Germany, these could provide about 5 per cent of the primary energy demand without competing with food production. In the energy supply of many developing countries, biomass has a share of up to more than 90 per cent. In industrial countries, the revival of biomass use is in sight. However, some countries use more biomass than can be sustainably grown, causing enormous problems for nature. Furthermore, the cost of the technical use of biomass is usually higher than that for fossil fuels.

For the technical use of biomass there are several possibilities. Besides firing a power plant, it can be liquefied or converted to alcohol. In some regions, biomass is already used intensively as motor fuel. For instance, in Brazil, alcohol has been produced from sugar cane for some decades now. The use of bio-fuels is also increasing in other countries. The major advantage of biomass in comparison to solar or wind energy is that the stored bio-energy can be used on demand. Therefore, biomass will be an important resource to smooth fluctuations in solar and wind energy in a future climatically sustainable energy supply.

Low-temperature heat Solar radiation heats up the surface of the Earth as well as the atmosphere. Temperature differences between the atmosphere and the Earth's surface cause compensatory air flows that are the source of wind power as described below. The Earth stores solar heat over hours, days or even months.

Drive power from heat source to heat source

Compressor

Steam

Liquid

Low temperature Low pressure

Vaporizer

Compressor

High pressure High temperature)

Condenser

Steti in

Liquid

to heating system to heating system from heating system from heating system

Expansion valve

Figure 1.19 Principle of a Compression Heat Pump

Heat pumps can technically utilize the low-temperature heat in the ground and air as already mentioned in the section on geothermics. An exact division of such low-temperature heat into solar energy and geothermic energy is very difficult.

Figure 1.19 shows the operating principle of compression heat pumps that utilize low-temperature heat. A compressor, which is driven by an electric motor, or a gas or petrol motor powered with external energy, compresses a vaporous working medium. Pressurizing the working medium also results in significant heating. A condenser removes the heat from the working medium until it becomes a liquid. The heat at the higher temperature level is used for room or domestic water heating. The pressurized working medium expands back across an expansion valve and reaches the vaporizer. Heat from the low-temperature heat source vaporizes the working medium again and the compressor completes the cycle.

Depending on the working medium and pressure, a heat pump can even provide useful heat at high temperatures from sources with temperatures below 0°C. Ambient air, groundwater or soil can be heat sources. The mechanical energy W needed by the compressor is usually smaller than the useful heat Qout provided. The ratio of both values is called coefficient of performance (COP) as defined by:

where P represents the electrical or mechanical power supplied to the compressor. The ideal or Carnot coefficient of performance gc of the heat pump is dependent on the temperature difference of the target temperature T2 (the high temperature to the heating system) and the lower final temperature T1:

From this equation is can be seen that low temperature differences and high temperatures of the heat source are essential for high COP values. However,

COP values are usually much lower than the ideal value. They are in the range 2.5-4 for electric heat pumps and between 1.2 and 2 for gas motor heat pumps. Low COP values destroy the ecological benefits of heat pumps. If a heat pump used for room heating (with a COP of 3) is driven by electrical energy from conventional power stations with an average efficiency of 33 per cent, the primary energy demand is the same as that of a condensing boiler with an efficiency of nearly 100 per cent. Earlier heat pumps used CFCs as the working medium, which had harmful effects on the ozone layer and the climate. Today alternative working fluids exist.

If renewable energy resources provide the mechanical power, a heat pump can produce useful heat that is climatically neutral. Therefore, the heat pump could play a more important role in a future climatically sustainable energy supply.

Wind energy More than 100 years ago, wind power had a dominant role in the energy supply of many countries. Technically advanced windmills ground corn or pumped water. In the US thousands of Western Mills were used in agriculture, but all these windmills were mechanical systems. Wind generators providing electricity started to enter the market in the early 1980s in Denmark and the US. In Germany wind power had an unexpected boom in the 1990s, making Germany the largest wind power market in the world. In 2002, the German wind power industry achieved a turnover of nearly €4 billion and created more than 45,000 new jobs. Altogether, 15,797 wind generators with a total capacity of 15,327 MW and an electricity generation potential of 30 TWh/year were installed in Germany by mid-2004 (Ender, 2004). This is equivalent to nearly 6 per cent of Germany's electricity demand. If Germany continues with the same growth rates as in the late 1990s, it will cover more than 10 per cent of its electricity demand in a few years. More recently, Spain started a similar exploration of its wind potential.

Although Germany only has limited areas suited for setting up wind farms, the potential is considerable. Excluding conservation areas and allowing for safe distances to settlements due to noise considerations, 53.5 GW could be achieved from installations onshore. This capacity could produce 85 TWh/year, 15 per cent of the electricity demand. The German offshore potential is 23.6 GW, which could produce 79 TWh/year. The potential in other countries is even higher. In the UK, wind power could produce well above 1000 TWh/year, which is much more than the total British electricity demand. Also the US could cover its entire electricity demand using wind power. Chapter 5 describes in detail the use of wind power for electricity supply.

Was this article helpful?

0 0
Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

Get My Free Ebook


Post a comment