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Global Renewable Energy ResourcesAyhan Demirbaş aa Department of Chemical Engineering , Selcuk University , Konya, TurkeyPublished online: 19 Aug 2006.

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Energy Sources, Part A, 28:779–792, 2006Copyright © Taylor & Francis Group, LLCISSN: 1556-7036 print/1556-7230 onlineDOI: 10.1080/00908310600718742

Global Renewable Energy Resources

AYHAN DEMIRBAS

Department of Chemical EngineeringSelcuk UniversityKonya, Turkey

Renewable energy sources (RES) supply 14% of the total world energy demand.RES are biomass, hydropower, geothermal, solar, wind, and marine energies. Therenewables are the primary, domestic and clean or inexhaustible energy resources.The percentage share of biomass was 62.1% of total renewable energy sources in1995. Large-scale hydropower supplies 20 percent of global electricity. Wind powerin coastal and other windy regions is promising as well.

Keywords renewable energy, global, inexhaustible resource, future of renewableenergy

Energy resources will play an important role in the world’s future. The energy resourceshave been split into three categories: fossil fuels, renewable resources, and nuclear re-sources (Demirbas, 2000a). Renewable energy sources (RES) are also often called alter-native sources of energy. RES that use domestic resources have the potential to provideenergy services with zero or almost zero emissions of both air pollutants and greenhousegases.

Currently, RES supply 14% of the total world energy demand. RESs contributed2% of the world’s energy consumption in 1998, including 7 exajoules from modernbiomass and 2 exajoules for all other renewables (UNDP, 2000). The RES are readilyavailable in nature. RES are biomass, hydropower, geothermal, solar, wind, and marineenergies. The renewables are the primary energy resources. Renewable energy is a cleanor inexhaustible energy like hydrogen energy and nuclear energy. RES occur naturallyin the environment and, therefore, should never run out. They also produce lower ornegligible levels of greenhouse gases and other pollutants when compared with the fossilenergy sources they replace. RES are derived from those natural, mechanical, thermal,and growth processes that repeat themselves within our lifetime and may be relied uponto produce predictable quantities of energy when required. RES reduced environmentaleffects compared to fossil fuels. Renewable technologies like water and wind powerprobably would not have provided the same fast increase in industrial productivity asfossil fuels did (Edinger and Kaul, 2000).

Oil and gas are expected to continue to be important sources of energy (Nakicenovicet al., 1998). The share of renewable energy sources is expected to increase very signif-icantly (to 30–80% in 2100). Hydropower and traditional biomass are already important

This study has been supported by Scientific Research Project (BAP in Turkish initials) ofSelcuk University.

Address correspondence to Ayhan Demirbas, Selcuk University, Muh. Mim. Faculty, Depart-ment of Chemical Engineering, 42031 Konya, Turkey. E-mail: ayhandemirbas@hotmail.com

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Table 1Global renewable energy resources

Capacity Approx. annual outputResource (MW) (TW h/year)

Modern biomass 35,000 185Wind 20,000 50Geothermal 8200 44Small hydro 3000 15Solar photovoltaic 1200 1Solar thermal 350 0.2

Source: UNDP, 2000.

factors in the world’s energy mix, contributing about 18% of the total world energyrequirements, whereas the renewables contribute only about 2% of the present world pri-mary energy use. Biomass, wind, and geothermal energy are commercially competitiveand are making relatively fast progress (Fridleifsson, 2001). Known energy reserves ofthe world are presented in Table 1. Table 2 shows the energy production and consumptionin the world. Table 3 summarizes the status of various renewable technologies, and alsoprovides information on trends in cost and production.

Any country wishing to exploit its RES needs to gather statistics in order to determinehow much resource is available, the current rate at which it is being exploited, and theeffectiveness of any policy measures to encourage uptake of the various technologies.While there is a limited number of RES, there are a large number of technologies allowingthe exploitation of these sources, most of which are still at the R&D stage or have notyet reached commercial maturity.

About 98% of carbon emissions result from fossil fuel combustion. Reducing use offossil fuels would considerably reduce the amount of carbon dioxide produced, as wellas reducing the levels of the pollutants. Indeed, much of the variation in cost estimatesto control carbon emissions revolves around the availability and cost of carbon-freetechnologies and carbon-reducing technologies, such as energy efficiency and energyconservation equipment. This can be achieved by either using less energy altogether, or

Table 2Energy production and consumption in the world (1998)

ResourcesProduction(Quad Btu)

Percent oftotal production

(%) Total consumption

Oil 152.0 40.0 73.60 million barrels/dayNatural gas 85.5 22.5 82.20 tcf/yearCoal 88.6 23.3 5.01 billion tons/yearNuclear 24.5 6.5 2.30 trillion kWh/yearHydroelectric 26.6 7.0 2.60 trillion kWh/yearBiomass 2.5 0.7 196.00 billion kWh/year

Source: Tewfik, 2004.

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Table 3Global status of various renewable energy technologies

Technology

Energyproduction

(1998)

Turnkeyinvestment cost

(US $/kWh)

Potential futureenergy cost

(¢/kWh)

Biomass energyElectricity 160 TWh (e) 900–3000 4–10District heating >700 TWh (th) 250–750 1–5Ethanol 420 PJ — —

Geothermal energyElectricity 46 TWh (e) 800–3000 1 or 2–8Heat 40 TWh (th) 200–2000 0.5–5

HydroelectricityLarge 2510 TWh (e) 1000–3500 2–8Small 90 TWh (e) 1200–3000 3–10

Marine energyCurrent 0.6 TWh (e) 1700–2500 8–15OTEC Unclear 1500–3000 UnclearTidal Unclear 2000–3000 5–7Wave Unclear Unclear Unclear

Solar heatLow-temperature 14 TWh (th) 500–1700 2 or 3–10

Solar photovoltaicElectricity 0.5 TWh (e) 5000–10000 5 or 6–25

Solar thermalElectricity 1 TWh (e) 3000–4000 4–10

WindElectricity 18 TWh (e) 1100–1700 3–10

e = electricity; th = thermal.Source: UNDP, 2000.

using alternative energy resources. Much of the current effort to control such emissionsfocuses on advancing technologies that emit less carbon (e.g., high efficiency combustion)or no carbon such as nuclear, hydrogen, solar, wind, geothermal, or other RES, or onusing energy more efficiently and on developing innovative technologies and strategies tocapture and dispose of carbon dioxide emitted during fossil fuel combustion. Substantialprice reductions in the past few decades have made some RES competitive with fossilfuels in certain applications in growing markets. Main renewable energy sources and theirusage forms are given in Table 4. Table 5 shows the percentage share of each renewableenergy source in 1995. As seen in Table 5, the percentage share of biomass was 62.1%of total renewable energy sources in 1995.

Biomass Resources

Biomass is the name given all the earth’s living matter. It is the general term for materialderived from growing plants or from animal manure. It is a rather simple term for

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Table 4Main renewable energy sources and their usage forms

Energy source Energy conversion and usage options

Hydropower Power generationModern biomass Heat and power generation, pyrolysis, gasification, digestionGeothermal Urban heating, power generation, hydrothermal, hot dry rockSolar Solar home system, solar dryers, solar cookersDirect solar Photovoltaics, thermal power generation, water heatersWind Power generation, wind generators, windmills, water pumpsWave Numerous designsTidal Barrage, tidal stream

all organic materials that stem from plants, trees, crops and algae. Plants absorb solarenergy, using it to drive the process of photosynthesis, which enables them to live. Thesolar energy, which is stored in plants and animals, or in the wastes that they produce,is called biomass energy. This energy can be recovered by burning biomass as a fuel(Tewfik, 2004). The average majority of biomass energy is produced from wood andwood wastes (64%), followed by municipal solid waste (24%), agricultural waste (5%)and landfill gases (5%) (Demirbas, 2000b). Biomass can be economically produced withminimal or even positive environmental impacts through perennial crops.

Biomass, as an energy source, has two striking characteristics. Firstly, biomass isthe only renewable organic resource also one of the most abundant resources. Secondly,biomass fixes carbon dioxide in the atmosphere by photosynthesis. Direct combustionand cofiring with coal for electricity production from biomass has been found to be apromising method in the nearest future. Biomass thermochemical conversion technologies

Table 5Percentage share of each renewable

energy source in 1995

Resource Percentage

Biomass 62.1Wood in household 30.3Wood in industry 12.7Power station 11.8Municipal solid waste 3.7District heating 1.8Biogas 1.0Liquid fuels 0.8

Hydro 33.6Geothermal 3.2Wind 0.7Solar 0.4

Source: Demirbas, 2004b.

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such as pyrolysis and gasification are certainly not the most important options at present;combustion is responsible for over 97% of the world’s bioenergy production (Demirbas,2004a). The supply is dominated by traditional biomass used for cooking and heating,especially in rural areas of developing countries.

About 200 years ago biomass in the form of wood was the major source of energy.Biomass is a renewable resource. It can be used in similar ways to fossil fuels. It isreadily available worldwide. In the developing world biomass is still the major source ofenergy. In Brazil large numbers of cars run on alcohol rather than petrol. In the Westernworld, people are developing ways of using biomass as an alternative to fossil fuels.There is a large biomass plant in Sweden, and in the United Kingdom attempts are beingmade to develop a power station that will run solely on wood from a nearby farm. Theworld production of biomass is estimated at 146 billion metric tons a year, mostly wildplant growth. Some farm crops and trees can produce up to 20 metric tons per acre ofbiomass a year. Types of algae and grasses may produce 50 metric tons per year (Cuff andYoung, 1980). Biomass accounts for 35% of primary energy consumption in developingcountries, raising the world total to 14% of primary energy consumption. In the future,biomass has the potential to provide a cost-effective and sustainable supply of energy,while at the same time aiding countries in meeting their greenhouse gas reduction targets.

Biomass, mainly now represents only 3% of primary energy consumption in industri-alized countries. However, much of the rural population in developing countries, whichrepresents about 50% of the world’s population, is reliant on biomass, mainly in theform of wood, for fuel. Total annual production of biomass is 2,740 Quads in the world(Ramage and Scurlock, 1996).

Biomass energy (firewood, agricultural residues, animal wastes, charcoal, and otherderived fuels) currently represents approximately 14% of world final energy consump-tion, a higher share than that of coal (12%) and comparable to those of gas (15%) andelectricity (14%). Biomass is the main source of energy for many developing countriesand most of it is noncommercial. Hence, there is enormous difficulty in collecting re-liable biomass energy data. Yet good data are essential for analyzing tendencies andconsumption patterns, for modeling future trends and for designing coherent strategies.Data on biomass energy use and production are also needed to evaluate whether its useenhances or damages the environment, and to measure its effect on global emissions ofcarbon dioxide. Biomass and wastes are the most important renewable energy sources inthe world. They are of significant importance not only because they contribute to sus-tainable energy supplies, but because energy recovery limits methane emissions to theenvironment.

The energy dimension of biomass use is importantly related to the possible increaseduse of this source as a critical option to tackle the global warming issue. Biomass is gener-ally considered as an energy source completely CO2-neutral. The underlying assumptionis that the CO2 released in the atmosphere is matched by the amount used in its produc-tion. This is true only if biomass energy is sustainably consumed, that is, if the stockof biomass does not diminish in time. This may not be the case in many developingcountries.

The importance of biomass in different world regions is given in Table 6. As shownin Table 6, the importance of biomass varies significantly across regions. In Europe,North America, and the Middle East, the share of biomass averages 2–3% of total finalenergy consumption, whereas in Africa, Asia, and Latin America, which together accountfor three-quarters of the world’s population, biomass provides a substantial share of theenergy needs: a third on average, but as much as 80–90% in some of the poorest countries

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Table 6The importance of biomass in

different world regions

RegionShare of biomass in

final energy consumption

Africa 60.0South Asia 56.3East Asia 25.1China 23.5Latin America 18.2Europe 3.5North America 2.7Middle East 0.3

of Africa and Asia (e.g., Angola, Ethiopia, Mozambique, Tanzania, Democratic Republicof Congo, Nepal, and Myanmar). Indeed, for large portions of the rural populations ofdeveloping countries, and for the poorest sections of urban populations, biomass is oftenthe only available and affordable source of energy for basic needs such as cooking andheating.

Wood-related industries and homeowners consume the most biomass energy. Thelumber, pulp, and paper industries burn their own wood wastes in large furnaces andboilers to supply 60% of the energy needed to run the factories. In our homes, weburn wood in stoves and fireplaces to cook meals and warm our residences. The energyconversion efficiency of an efficient fuelwood stove (or efficient stove) is estimated tobe 20–30% compared to about 10% efficiency of traditional fuelwood stoves. Therefore,efficient stoves can reduce fuelwood consumption significantly.

Biomass is burned by direct combustion to produce steam, the steam turns a turbineand the turbine drives a generator, producing electricity. Gasifiers are used to convertbiomass into a combustible gas (biogas). The biogas is then used to drive a high efficiency,combined cycle gas turbine (Dogru et al., 2002). Biomass consumption for electricitygeneration has been growing sharply in Europe since 1996, with 1.7% of power generationin 1996. The use of wood and wood waste as fuel in 1995 is given in Figure 1.

Biomass resources include various natural and derived materials, such as woodyand herbaceous species, wood wastes, bagasse, agricultural and industrial residues, wastepaper, municipal solid waste, sawdust, biosolids, grass, waste from food processing,animal wastes, aquatic plants and algae, and so on. The major organic components ofbiomass can be classified as cellulose, hemicelluloses, and lignin. The average majority ofbiomass energy is produced from wood and wood wastes (64%), followed by municipalsolid waste (24%), agricultural waste (5%), and landfill gases (5%) (Demirbas, 2000b).

There are three ways to use biomass. It can be burned to produce heat and electricity,changed to gas-like fuels such as methane, hydrogen, and carbon monoxide, or changedto a liquid fuel. Liquid fuels, also called biofuels, include mainly two forms of alcohol:ethanol and methanol. The most commonly used biofuel is ethanol, which is producedfrom sugarcane, corn, and other grains. A blend of gasoline and ethanol is alreadyused in cities with high air pollution. However, ethanol made from biomass is currentlymore expensive than gasoline on a gallon-for-gallon basis. So, it is very important for

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Figure 1. Use of wood and wood waste as fuel in 1995 (IEA).

scientists to find less expensive ways to produce ethanol from other biomass crops. Today,researchers have found new ways to produce ethanol from grasses, trees, bark, sawdust,paper, and farming wastes.

Hydro Energy Sources

The water in rivers and streams can be captured and turned into hydropower, also calledhydroelectric power. Hydropower is also inexpensive, and like many other renewableenergy sources, it does not produce air pollution.

Large scale hydropower provides about one-quarter of the world’s total electricitysupply, virtually all of Norway’s electricity and more than 40% of the electricity usedin developing countries. The technically usable world potential of large-scale hydro isestimated to be over 2200 GW, of which only about 25% is currently exploited.

There are two small-scale hydropower systems: micro hydropower systems (MHP)with capacities below 100 kW and small hydropower systems (SHP) with capacity be-tween 101 kW and 1 MW. Large-scale hydropower supplies 20 percent of global electric-ity. In the developing countries, considerable potential still exists, but large hydropowerprojects may face financial, environmental, and social constraints (UNDP, 2000).

The two small-scale hydropower systems, which are being discussed in this sectionare the sites with capacities below 100 kW (referred to as micro hydropower systems)and sites with capacity between 101 kW and 1 MW (referred to as small hydropowersystems). Micro hydropower (MHP) systems which uses cross flow turbines and peltonwheels, can provide both direct mechanical energy (for crop processing) and electricalenergy. However, due to design constraints, turbines up to a capacity of 30 kW aresuitable for extracting mechanical energy. Of the total installed capacity of about 12 MWof MHP systems, half is used solely for crop processing. The most popular of the MHPsystems is the peltric set, which is an integrated pelton turbine and electricity generationunit with an average capacity of 1 kW.

MHP systems are sometimes described as those having capacities below 100 kW,mini hydropower plants are those ranging from 100 to 1,000 kW and small hydropower(SHP) plants are those that produce from 1 to 30 MW.

Dams are individually unique structures and dam construction represents the biggeststructures in basic infrastructure within all nations (Novak et al., 1996). Today, nearly

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Table 7Current and estimated development of electricity generation from

hydropower in the world

LocationHydro scale

(market area)Current deployment

1995 (TWh/year)Estimated deploymentin 2010 (TWh/year)

EU + EFTA Large hydro 401.5 433Small hydro 40 50% of SHP 9.1 10.1

CEE Large hydro 57.5 83Small hydro 4.5 16% of SHP 7.3 16.2

CIS Large hydro 160 388Small hydro 4 12% of SHP 2.4 3.0

NAFTA Large hydro 635 685Small hydro 18 25% of SHP 2.8 3.5

OECD Pacific Large hydro 131 138Small hydro 0.7 3% of SHP 0.5 2.1

Mediterranean Large hydro 35.5 72Small hydro 0.5 0.7% of SHP 1.4 1.0

Africa Large hydro 65.4 147Small hydro 1.6 3% of SHP 2.4 2.0

Middle East Large hydro 24.8 49Small hydro 0.2 1Total hydro 25 50% of SHP 0.8 2.0

Asia Large hydro 291 1000Small hydro 42 100% of SHP 12.6 9.1

Latin America Large hydro 461.5 990Small hydro 3.5 10% of SHP 0.8 1.0

Worldwide Large hydro 2265 3990Small hydro 115 220Total hydro 2380 4210% of SHP 4.8 5.2

NAFTA: The North American Free Trade Agreement; EU/EFTA: The Europe Union/The Euro-pean Fair Trade Association; CEE: The Central and Eastern Europe; CIS: The Commonwealth ofIndependent States; OECD: The Organisation for Economic Co-Operation and Development.

Source: Adıgüzel and Tutus, 2002.

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500,000 square kilometers of land are inundated by reservoirs in the world, capableof storing 6,000 km3 of water. As a result of this distribution of fresh water in thereservoirs, a small but mesurable changes have occurred in the world. The total insalledcapacity of hydropower is 640,000 MW (26% of the theoretical potantial) generatingan estimated 2,380 TWh/year in the world and producing nearly 20% of the world’stotal supply of electricity. The current and estimated electricity generation of the worldfrom the hydropower is given in Table 7. Some 27,900 MW of the total hydropower isat small scale sites, generating 115 TWh/year (Gleick, 1999; Demirbas, 2002; Penche,1998). The NAFTA countries are, now, the biggest producers, with latin Amarica andEU/EFTA regions, but it is estimated that Asia will be generating more hydroelectrictythan NAFTA countries at the end of the next decade (Table 7).

There is no universal consensus on the definition of small hydropower. Some coun-tries of the European Union such as Portugal, Spain, Ireland, Greece, and Belgium accept10 MW as the upper limit for installed capacity. In Italy the limit is 3 MW, in France8 MW, in UK 5 MW, in Canada 20–25 MW, and in the United States 30 MW, however,a value of up to 10 MW total capacities is becoming generally accepted as small hy-dropower in the rest of the world. If total installed capacity of any hydropower system isgreater than 10 MW, it is generally accepted as a large hydropower system (Adigüzel andTutus, 2002; IASH, 2004; ISHA, 2004; UNIDO, 2003; Cunningham and Atkinson, 1998;Kueny, 2003). Small hydro can be further subdivided into minihydro usually defined as<500 kW and microhydro is <100 kW. Small hydro installation limits are given inTable 8. The definition of microhydro or small-scale hydro varies in different countries.Small hydropower is one of the most valuable energy to be offered to the rural commin-utes’ electrification. Small hydroelectricity growth can decrease the gap of decentralizedproduction for private sector and municipal activity production. Small-scale hydropowersystems supply the energy from flowing or running water and convert it to electricalenergy. The potential for small hydropower systems depends on the availability of waterflow where the resource exists. If a well-designed small hydropower system is establishedanywhere, it can fit in with its surroundings and will have minimal negative impacts on

Table 8Classification of hydropower accepted in various countries according

to the installed capacity

Country

Micro scalehydropower

(kW)

Mini scalehydropower

(kW)

Small scalehydropower

(MW)

Large scalehydropower

(MW)

USA <100 100–1000 1–30 >30Canada — — 1–25 >25China — <500 0.5–25 >25Russia <100 — — —France 5–5000 — — >8India <100 101–1000 1–15 >15UK — — <5 >5Brazil <100 100–1000 1–30 >30Turkey 1–100 101–1000 1–10 >10Various <100 <1000 <10 >10

Sources: Bennett, 1990; Cunningham and Atkinson, 1998; Adıgüzel and Tutus, 2002.

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the environment. Small hydropower systems allow achieving self-sufficiency by usingthe scarce natural water resources. These systems supply low cost energy production thatis being applied in many developing countries in the world (Romas and De Almedia,1999, 2000).

Geothermal Energy Sources

As an energy source, geothermal energy has come of age. Geothermal energy for elec-tricity generation has been produced commercially since 1913, and for four decades onthe scale of hundreds of MW both for electricity generation and direct use. The utiliza-tion has increased rapidly during the last three decades. In 2000, geothermal resourceshave been identified in over 80 countries and there are quantified records of geothermalutilization in 58 countries in the world. Table 9 shows the status of geothermal energy.

In Tuscany, Italy, a geothermal plant has been operating since the early 1900s.There are also geothermal power stations in the USA, New Zealand, and Iceland. InSouthampton (UK) there is a district heating scheme based on geothermal energy. Hotwater is pumped up from about 1,800 meters below ground.

Direct application of geothermal energy can involve a wide variety of end uses, suchas space heating and cooling, industry, greenhouses, fish farming, and health spas. It usesmostly existing technology and straight forward engineering. The technology, reliability,economics, and environmental acceptability of direct use of geothermal energy have beendemonstrated throughout the world. Electricity is produced with geothermal steam in21 countries spread over all continents. Low temperature geothermal energy is exploitedin many countries to generate heat, with an estimated capacity of about 10,000 MWthermal.

As an energy source, geothermal energy has come of age. Geothermal energy forelectricity generation has been produced commercially since 1913, and for four decades

Table 9World’s top countries using geothermal energy in direct uses

Country Installed (MWt) Production (GWh/a)

China 2282 10531Japan 1167 7482USA 3766 5640Iceland 1469 5603Turkey 820 4377New Zealand 308 1967Georgia 250 1752Russia 308 1707France 326 1360Sweden 377 1147Hungary 473 1135Mexico 164 1089Italy 326 1048Romania 152 797Switzerland 547 663

Source: Fridleifsson, 2001.

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on the scale of hundreds of MW both for electricity generation and direct use. The uti-lization has increased rapidly during the last three decades. In 2000, geothermal resourceshave been identified in over 80 countries, and there are quantified records of geothermalutilization in 58 countries in the world.

Geothermal energy is clean, cheap, and renewable, and can be utilized in variousforms such as space heating and domestic hot water supply, CO2 and dry-ice productionprocess, heat pumps, greenhouse heating, swimming and balneology (therapeutic baths),industrial processes, and electricity generation. The main types of direct use are bathing,swimming, and balneology (42%), space heating (35%), greenhouses (9%), fish farming(6%), and industry (6%) (Fridleifsson, 2001).

Solar Energy Sources

Energy from the sun is called as solar energy. The heat that builds up in a car whenit is parked in the sun is an example of solar energy. Solar power is our most directrenewable energy source. This energy comes from processes called solar heating (SH),solar home heating (SHH), solar dryer (SD), and solar cooker (SC), solar water heating(SWH), solar photovoltaic (SPV: converting sunlight directly into electricity), and solarthermal electric power (STEP: when the sun’s energy is concentrated to heat water andproduce steam, which is used to produce electricity).

Solar dryers are used for drying fruits and spices. The three most popular types ofSD are box type, cabinet type, and tunnel type. Box type uses direct heat for dehydration.In cabinet type dryers, air heated by the collector dehydrates the food product, whereasin tunnel type forced air circulation is used to distribute heat for dehydration. Cabinetand tunnel type dryers yield a high quality of dried products but they are very bulky andcostly compared to the box type dryers. Of about 800 dryers disseminated so far, 760 areof the box type (Pokharel, 2002).

Cookers of both the box and concentric disk design have been disseminated byvarious agencies. It is estimated that out of about 100 solar cookers disseminated, onlya few are of the concentric type.

Solar energy systems are solar home system, solar photovoltaic (SPV) systems,solar water heating (SWH) systems, solar dryers, and solar cookers. These systems areinstalled and managed by a household or a small community. A solar home system is aPV system with a maximum capacity of 40 W. These systems are installed and managedby a household or a small community.

Photovoltaic (PV) systems, other than SHH systems, are used for communication,water pumping for drinking and irrigation, and electricity generation. Like wind powermarkets, PV markets have seen rapid growth and costs have fallen dramatically. The totalinstalled capacity of such systems is estimated at about 1000 kW. Solar photovoltaicsand grid-connected wind installed capacities are growing at a rate of 30 percent a year(UNDP, 2000). Figure 2 shows the world PV module shipments from 1990 to 2000.

Wind Energy Sources

Using the wind to produce electricity by turning blades on a wind turbine is called windenergy or wind power. Renewable energy from the wind has been used for centuries topower windmills to mill wheat or pump water. More recently large wind turbines havebeen designed that are used to generate electricity. This source of energy is nonpollutingand freely available in many areas. Wind turbines are becoming more efficient. The costof the electricity they generate is falling.

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Figure 2. World PV module shipments from 1990 to 2000 (UNDP, 2000).

There are wind farms around the world. Because the UK is on the edge of theAtlantic Ocean it has one of the best wind resources in Europe. Offshore wind farmsin coastal waters are being developed because winds are often stronger blowing acrossthe sea. Turbines can produce between 500 kW and 1 MW of electricity. Productionof wind-generated electricity has risen from practically zero in the early 1980s to morethan 7.5 TWh per year in 1995. Cumulative generating capacity worldwide has topped6500 MW in late 1997 (Garg and Datta, 1998). Figure 3 shows the growth in world windturbine installed capacity.

An advantage of wind turbines over some forms of renewable energy is that they canproduce electricity whenever the wind blows (at night and also during the day). In theory,wind systems can produce electricity 24 hours every day, unlike PV systems that cannotmake power at night. However, even in the windiest places, the wind does not blow allthe time. So while wind farms do not need batteries for backup storage of electricity,small wind systems do need backup batteries.

Wind power in coastal and other windy regions is promising as well. By any measurethe power in the wind is no longer an alternative source of energy.

Figure 3. Growth in world wind turbine installed capacity.

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Global Renewable Energy Resources 791

Other Renewable Energy Sources

Marine energy, municipal solid waste, gas from animal wastes (biogas), landfill gas,hydrogen, and peat energy are the other RES. Marine energy sources are current, tidal,ocean thermal energy conversion (OTEC), and wave technologies.

The tides are caused by the gravitational pull of the Moon, and to a lesser extent theSun, on the oceans around the world. Barrages are built in river estuaries that have largetidal ranges, such as the River Severn in the UK. The world tidal resource is difficultto estimate. The world wave resource is between 200 and 5000 GW mostly found inoffshore locations (Garg and Datta, 1998). Wave energy converters fixed to the shorelineare likely to be the first to be fully developed and deployed, but waves are typically 2–3times more powerful in deep offshore waters than at the shoreline.

The OTEC is an energy technology that converts solar radiation to electric power.OTEC systems use the ocean’s natural thermal gradient to drive a power-producing cycle.As long as the temperature between the warm surface water and the cold deep waterdiffers by about 20 K, an OTEC system can produce a significant amount of power. Theoceans are thus a vast renewable resource, with the potential to help us produce billionsof watts of electric power.

Wave energy can be harnessed in coastal areas, close to the shore. The first patentfor a wave energy device was filed in Paris in 1799, and by 1973 there were 340 Britishpatents for wave energy devices. There has been one such device working on the islandof Islay in Scotland since the early 1990s, producing 75 kW of electricity. By comparisonto wind and PV, wave energy and tidal stream are very much in their infancy. Currently,around 1 MW of wave energy devices is installed worldwide, mainly from demonstrationprojects.

Hydrogen, produced by passing an electrical current through water, can be usedto store solar energy and regenerate it when needed for nighttime energy requirements.Hydrogen can be produced by pyrolysis from biomass (Arni, 2004). It can be burned toproduce heat or passed through a fuel cell to produce electricity.

Landfill gas contains about 50% by volume methane. Producing energy from land-fill gas improves local air quality, eliminates a potential explosion hazard and reducesgreenhouse gas emissions to the atmosphere.

Conclusion

Renewable energy sources (RES) supply 14 percent of the total world energy de-mand. RES are biomass, hydropower, geothermal, solar, wind and marine energies.The renewables are the primary, domestic and clean or inexhaustible energy resources.The percentage share of biomass was 62.1% of total renewable energy sources in1995.

Renewable energies have been the primary energy source in the history of the hu-man race. The first renewable energy technologies were primarily simple mechanicalapplications and did not reach high energetic efficiencies. Industrialization changed theprimary energy use from renewable resources to sources with a much higher energeticvalue such as coal and oil. The share of renewable energy sources is expected to increasevery significantly. Hydropower and traditional biomass are already important factors inthe world’s energy mix, contributing about 18% of the total world energy requirements,whereas the renewable energies contribute only about 2% of the present world primaryenergy use.

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792 A. Demirbas

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