- Email: [email protected]
Renewable energy
Renewable energy A technical overview Bent Serensen
The fluxes of renewable energy through the environment are outlined, and various conversion techniques are discussed, in terms of technological requirements and of basic efficiency limits. The current technical development stage is described and evaluated for the major renewable energy forms: solar, wind, waves, hydro and biomass. Keywords: Renewable energy; Conversion technology; Efficiency
The practical definition of renewable energy is a
flow of energy, that is not exhausted by being used. The primary renewable energy source on earth is thus solar radiation, because the earth-atmosphere system receives an amount of solar energy, which is essentially independent of the conversion processes that we may apply before allowing that energy to become re-radiated into space in the form of heat. On a more fundamental level, energy is of course a conserved quantity, but one that is degraded in quality by being converted into heat of lower temperature. Solar radiation is associated with the depletion of nuclear fuels in the sun, and it is basically the hugeness of this resource, that may allow us for practical purposes to consider solar radiation as a renewable energy source. In general, renewable energy may be taken to include the use of any energy storage reservoir, which is being refilled at a rate comparable to that of extraction. In this way, geothermal energy-use is renewable, as long as heat can flow into the sediment in question as fast as heat is being extracted. Fossil fuels would similarly be renewable energy sources, if they were only used at the average rate of Bent Sorensen is Technical Director, COWlconsult, Consuiting Engineers and Planners, 15 Parallelvej, DK-2800 Lyngby, Denmark, on leave from Institute of Mathematics and Physics, Roskilde University Center, DK-4000 Roskilde, Denmark. 386
fossilization of biomass. Since, however, this rate is very small, compared to current use, fossil fuels do not at present qualify as renewable. Figure 1 shows the main renewable energy flows available on earth. The various forms are based on solar energy in its different converted forms, on heat stored in the interior of the earth or being created by radioactive processes, and on gravitational energy in the planetary system. 1 An estimate of the amounts of renewable energy that may be recoverable in practice is presented in Table 1, with conventional fuel figures included for comparison. The flow of solar and solar-derived energy forms is not independent from the activities of man. The radiation fluxes are modified by changing the reflectivity of the earth surface, eg by urbanization and agricultural practices. Man's activities also change wind patterns and modify cloud coverage, thus again changing the radiation balance. Furthermore, the injection of pollution into the environment, as well as manmade structural changes, influence both radiation, heat and water flows.
Individual r e n e w a b l e energy sources and their utilization Although practically all renewable energy forms available on earth originate from solar energy, the 'pre-processing' by nature is often decisive for the cost and efficiency of human utilization. For example, hydropower is derived from water elevated by a range of processes, such as evaporation, transport and condensation of the water. These processes are driven by solar radiation and its derived forms ( a b s o r b e d radiation, wind, t e m p e r a t u r e differences). The following discussion will therefore depend on the type of renewable energy used for energy extraction, as well as on the method and technology used for each energy conversion step. Solar radiation may be used by thermal devices or by devices directly converting radiation into electric0301-4215/91/040386-06© 1991 Butterworth-HeinemannLtd
R e n e w a b l e energy - technical o v e r v i e w
Short-wavelength radiation
l
Extroterrestiol ,oorco,
Potential, latent, chemical and nuclear energy
Sensible heat energy
Kinetic energy
Loo,
THeot 122 500/reradiation
3 Lenergy OOOO
Long-wavelength radiation
~CU ob-
Atmosphere
~ -0
Hydrosphere
Upper lithosphere
Biosphere
Lower
lithosphere
F i g u r e 1. Natural energy flows in the e a r t h - a t m o s p h e r e system (unit TW).
tion process may therefore vary from just a little above ambient temperatures (solar space heating or hot water panels) to a couple of thousand degrees Celsius above ambient (parabolic concentrators). If
al or chemical energy. The thermal devices either absorb the radiation directly, or use reflections to first concentrate the radiation flux before absorption. The temperature associated with the absorp-
Table 1. Estimate of global energy resources at the surface of the earth. Resources
Estimated as recoverable
Resource base
Solar radiation Wind Wave Tides Geothermal flow Salinity gradients
1 000 TW 10 TW 0.5 TW 0.1 TW
90 000 TW 1 200 TW 3 TW 30 TW 30 TW 3 TW
Biomass standing crop Geothermal heat stored Kinetic energy stored in atmospheric and oceanic circulation Oil Natural gas Coal Fission resources Fusion resources
50 TWyears to ?
450 TWyears 10 ]~ TWyears 32 TWyears
300 180 930 90 0
to to to to to
2 500 TWyears 2 500 TWyears 7 000 TWyears 9 000 TWyears 10 ]j TWyears
N o t e : For renewable energy sources, the flows are given in TW, while for non-renew-
able resources, an estimated resource range (from proven and possible to ultimately minable) is stated in TWyears. Source: See, Jensen and Scrensen, op cit, Ref 4.
ENERGY POLICY May 1991
387
R e n e w a b l e energy - technical o v e r v i e w
the desired output is high quality energy, such as electric power, thermodynamics limits the maximum efficiency of conversion correspondingly: to just a few percent for a simple, fiat-plate collector and to about 85% for the best parabolic ones. However, for heating purposes requiring the elevation of temperature by just a few tens of degrees, the flat-plate collector may be a very suitable device. In the case of electricity production by absorbed solar heat, either a conventional thermodynamic cycle may be used, or more advanced concepts such as thermionic generators. 2 The advanced concepts may bring the efficiency closer to the thermodynamical limit, but usually at a higher cost. Direct conversion of solar radiation into electrical energy may be achived by a photovoltaic device. Depending on the degree of matching between the energy gap in the band structure of the electron energy levels, and the frequency spectrum of the incoming radiation, the overall efficiency of conversion may range from 10 and 30% (eg low for amorphous silicon, medium for crystalline silicon, high for gallium arsenide). Conversion of solar radiation into chemical energy may be accomplished by photosynthesis in green plants, yielding an efficiency of up to about 15%, considering both the mismatch between the solar frequency spectrum and the chlorophyll absorption spectrum, and also the losses in the chemical processes delivering the absorbed energy in a form suitable for growth of the plant. 3 Further losses are involved in using the chemical energy in the produced biomass for human purposes (food, burning for heat, transforming into biofuels such as biogas or ethanol, or waiting for fossilization into fuels such as coal, oil or natural gas). The associated conversion processes typically have practical efficiencies of the order of 50% or less, so that the combined efficiency in converting solar radiation into useful energy comes out below 10%. Turbines used in converting hydro energy usually have efficiencies over 90% at rated power level, but dropping off at lower loads. Wind turbines have a fundamental efficiency limit of 16/27 for a fixed turbine area. This limit is based upon the assumption, that only flow in one direction is important (ie neglecting radial and rotational flows). It takes into account the possibility of 'drawing in' air from an area larger than that swept by the rotor, and it takes into account the slipstream flow needed for continuity. In practice, fixed blade-pitch and fixed rotational velocity reduce the average efficiency, but values around 35% are realistic at locations with good wind conditions. Advanced tur-
388
bines may utilize rotational flows and thus achieve higher efficiencies, but presumably at higher cost. In any case, interference between turbines may become an important issue, if each turbine is made to extract power from an area much larger than the corresponding dimension of the device. Several devices for extracting wave energy have been proposed. Efficiencies can be of the order of 90%, although only for wave inputs close to the design specifications. Complex tuning mechanisms must be added, if such high efficiencies are to be maintained for a broader range of conditions. Figure 1 indicates that geothermal energy may be used in a renewable mode. However, current use is mainly reservoir-based and non-renewable. As the energy is in the form of heat, straight thermodynamical considerations determine the efficiency of conversion. Other, more exotic, renewable energy forms include energy in currents and tides, salinity or temperature gradients in oceans, and atmospheric electricity. Currents and tides may be exploited by conventional turbines, temperature gradients by thermodynamical engines, albeit of exceedingly low efficiency, due to small differences of temperature. Most of the renewable energy sources are characterized by strong variations in flow. Some are regular, eg the day-to-night and seasonal cycles of solar radiation, while others are difficult to predict in detail. For example, the power of wind energy is proportional to the third power of the wind speed, making traditional weather forecasts (typically working with intervals such as 5-8 miles/second wind speeds) fairly useless. Figure 2 shows some power duration curves for renewable energy sources, indicating the fraction of time, during which a certain level of power is available. Reservoir-based renewable energy, such as certain hydropower schemes or biofuels, are thus very important components of a system with a high penetration of renewable sources. 4 As a 25% subsidy in a fuel-based energy system, the intermittent nature of many renewable energy sources presents no problem. 5 The variability of renewable energy sources is not completely given by the power duration curves. They do not indicate, whether the lulls are regular or more or less unpredictable (eg diurnal solar cycle v passing cloud cover). For this reason, calculations of the performance of renewable energy conversion systems are best done with time series of actual data, using some simulation technique. The time series of data may consist of measured data, or of synthetic data (eg stochastically generated variations with specified averages and higher moments). 6
ENERGY POLICY May 1991
R e n e w a b l e energy - technical o v e r v i e w
Wave
Wind
200
4
\
Annual data (locationand heightaf measurement )
-----
!
E
Ris~,Denmark,56m
2 -\\ .\, i~
v kL
\
\
~
X
Winterseason Annualdata Summerseason
~. ~,
(uncertainty indata
~
is considerable]
I00 u_
\'\
. . . . . . Changi,Singapore,IOta ,
"--I- ~4~_z___~'__-'_r-Solar
1
\ \
Waterford,CN,USA,45m
.\ \ \ . 0
\
k ~
Ringhals, Sweden, 5am
NorthAtlantic 59°N' 19°W
, ...__ ,
"I-- ~ " " ~
x..
,
Currents
Annual data
1~\
0.8
7r7o7oi ;o oo
~-- 0.6 ~ \ \ E X
0.4
~danuory
southfacingsurface at 4 5 ° tilt
~
LL
Great Belt,Denmark Monthly data 0.8
July
Normal incidence radiation
0.6
(fullytrackingdevice) ~
E
hScOt:oernelir~o :~ir;~i:: an
v
\\
U_
0.4
\\X\
0.2
0 0
40
80
0.2
40
0
80
Fraction of time in which energy flux exceeds F
Figure 2.
P o w e r d u r a t i o n curves.
Source: See, S~rensen, o p cit, Ref 1.
Technical development stage In most areas of renewable energy conversion, the past 15 years have allowed for a first prototype development and subsequent commercial development. Several products have been tested in the marketplace, and many have not passed the test. As a result, we have today sorted out a limited range of technically viable products, in which we can place considerable confidence in terms of durability and performance. In some areas, the cost is currently too high for general application, and in others, there is
ENERGY POLICY May 1991
still technological progress to be made, before market entrance is possible. The main areas of renewable energy conversion equipment are briefly surveyed in the following, with emphasis on the status in regard to these questions. Flat-plate solar collector system for space heating and hot water productions have reached technical and economical viability in several parts of the world. Hot water systems are in widespread use in areas with low space heating requirements. Total heating systems with a solar component, for use at higher latitudes, have been technically perfected.
389
R e n e w a b l e e n e r g y - technical o v e r v i e w
They use selective surface panels for high performance, and collectors have been developed, which can withstand the harshness of humid climates. The cost of solar space heating systems is still higher than that of fuel-based alternatives (excluding the uncertain cost of adverse environmental effects), so the market penetration is low. Nevertheless, there is a production niche in several countries large enough to ensure continuing technical optimization. Photovoltaic collectors have gone through a continuous technological development, and have found an increasing number of marketplace niches. However, they are still too expensive for consideration as bulk power generation replacement. Along with the dominant crystalline silicon cells, parallel efforts in amorphous silicon technology and in highefficiency cells are ongoing. Solar concentrating devices are still in the experimental stage. Fairly large demonstration plants have been built, but since there is no clear evidence for an economic advantage relative to photovoltaic devices, the durability and maintenance-free operation of photovoltaic cells give them a wider perspective and in this way a competitive edge. Horizontal axis wind turbines have been technically perfected and have gained a fair position in the marketplace in selected areas (notably in Denmark and California). In areas of good wind conditions, they are competitive or near-competitive relative to fuel-based alternatives for bulk production of electricity, provided that the wind energy share is under about 25% of the demand (so that no dedicated energy storage is required), and provided that suitable maintenance facilities are locally available. The introduction of wind technology is currently spreading to other areas of the world, and the prospects for the next decade look bright. The utilization of waves and tides have been explored in a few prototype installations. Several problems have been encountered, and further development is needed in order to evaluate the potential, especially in the case of wave energy. Prototype geothermal energy installations aimed at using the resource in a renewable or nearrenewable fashion have been built. The aim has mostly been heating and hot water delivery, in contrast to the conventional, non-renewable steam turbine plants producing electric power. Hydro and biomass technologies continue to be in widespread use. Hydropower plants of extremely large size have been built, and considerable progress has been made on making smallscale plants costeffective. Biomass burning continues to be in widespread use, both in developing countries and in
390
forest-rich residential areas of industrialized nations (eg Sweden, Canada and New England), despite the adverse effects of over-exploitation of the forest resource in developing countries, and despite the severe environmental problems associated with smallscale intermittent-cycle burning of woodfuel. Biofuel production is ongoing on more than one level: in Asia, a large number of primitive labourintensive biogas plants have been installed. Not all of them are in working condition, and for those that are, their utility depends on the distribution of access to them among various population groups. Prototypes of high-technology, fully automated biogas plants have been built in countries such as Denmark. The experiences are mixed, but hope still exists that viable solutions will emerge. Liquid biofuels (alcohols) for energy-use are produced in a few countries, notably Brazil. In order to serve as gasoline replacement, the alternative fuel must be introduced to the distribution network, eg in the form of gasoline/ethanol blends, and automobile engines have to be prepared for running on the alternative fuel. This has impeded the plans for biofuel production in several countries, in conjunction with the uncertainty pertaining to investment recovery of such a large-scale commitment. The use of biomass to provide food continues to be a large area of application. The total use of renewable energy at present may be summarized in the following way: renewable energy is used in the form of food (the gross rate of biomass energy supply amounting to about 0.6 TW, woodfuel and waste burning (1.0 TW), timber (0.4 TW), paper and pulp industry inputs (0.06 TW), hydropower (0.3 TW), wind, solar, geothermal and tidal power (0.06 TW). All together, renewable energy accounts for about 25% of the total energyuse by human society, when counted in this way and excluding environmental heat gain. 7
The future The future share of renewable energy may increase from the present 25%, primarily by increasing the share of solar, wind and advanced biomass conversion. In the long term and with inclusion of energy storage facilities, the share of renewable energy could approach 100%. For the next decades, a doubling of the renewable energy contribution to 50% would be a very ambitious goal, but one that is technically feasible. One factor to consider would be land-use. Direct solar conversion systems integrated into rooftops could be introduced with no additional burden on
ENERGY POLICY May 1991
Renewable energy - technical overview
land-use, in contrast to centralized solar power plants (whether thermal or photovoltaic). Wind power plants make some disturbances on land-use, but generally do not exclude say agricultural use of the land between turbines. However, for practical reasons, densely populated regions would limit the number of turbine sites, and most large-scale wind developments in such regions are likely to be placed off-shore (test plants on shallow water sites are already in progress). Increased harvest of biomass for energy purposes does make a substantial impact on land-use, and would probably be restricted to marginal land, using special 'energy crops' on land unsuited for food production. The current round of energy discussions is critical for the future of renewable energy options. When the energy issue gained social and political importance during the mid-1970s, most of us were poorly prepared for such a discussion. Alternative energy sources had been neglected for two decades, due to the apparent abundance of cheap oil. New research programmes had to be established, but when the first substantial results began to appear around 1980, the public debate had already been turned away from the energy question. Presently, the energy issue has resurfaced as a part of the environmental debate: key problems are pollution from fossil energy-use and from nuclear accidents, and the possibility of global climate changes caused by the greenhouse gases emitted by fossil fuel combustion. This time, we can benefit from the research, development and commercialization efforts devoted recently to renewable energy techniques, and arrive at a much more qualified opinion regarding the realism of planning for a major contribution of renewable energy to future energy systems. ~ The political issue is clearly, how much value we are willing to attach to avoiding negative environmental and climatic impacts. Adding these 'indirect' economic costs to the evaluation, we find that some renewable energy options are already viable, while
ENERGY POLICY May 1991
other ones are close to viability and may automatically become viable, as soon as massive use of them becomes a reality. The real problem is the institutional arrangement, that will allow for this development. Who should make the investments in the small-size renewable energy devices aimed at decentralized energy production? One solution would be to let the present energy institutions (power utility companies, etc) finance installation of decentralized, renewable energy devices at the level of individual customers (rooftop solar panels, dispersed wind turbines, etc). However, the inertia in such institutions may be too big, implying the risk of seeing a half-hearted effort with too slow a rate of producing results. Alternatives include the establishment of new financing mechanisms, which will yield fair conditions for the establishment of decentralized energy systems, by truly reflecting their social benefits. lB. S¢~rensen, Renewable Energy, Academic Press, London, 1979. 21bid. 31bid. 4j. Jensen and B. Scrensen, Fundamentals of Energy Storage, Wiley, New York, 1984. 5B S~rensen, 'The regulation of an electricity supply system including wind energy generators', in Proceedings, 2nd International Symposium on Wind Energy Systems, Amsterdam 1978, Vol 1, pp Gl.I-1.8, BHRA Fluid Engineering, Cranfield, UK, 1979. 6B. S0rensen, 'A combined wind and hydropower systems', Energy Policy, Vol 9, No 1, March 1981, pp 51-55; A Study of Wind-Diesel/Gas Combination Systems, Energy Authority of New South Wales Report EA86/17, Sydney, Australia, 1986; 'Dachen Island's wind/diesel facilities', in , European Community Wind Energy Conference, Madrid, 1990, Stephens & Associates, Bedford, UK, 199(I, pp 688-691. 7Estimates are based on material in, op cit, Refs 1 and 4. Environmental heat is essential for human society but difficult to quantify (should one count the entire greenhouse heating of the earth-atmosphere system or only that of human habitats?). 8B. S0rensen, 'Renewable energy and development', in Proceedings International Conference on Renewable Energy and Local Production, Vol 1, pp 35-74, Danish Centre for Renewable Energy, Thy, Denmark, 1989; B. S0rensen, 'Energy and greenhouse strategies in Scandinavia', in Proceedings' Workshop on Energy and the Greenhouse Effect, Sydney 1990, Energy Research, Development and Information Centre, University of New South Wales, Australia, 199(I.
391
The fluxes of renewable energy through the environment are outlined, and various conversion techniques are discussed, in terms of technological requirements and of basic efficiency limits. The current technical development stage is described and evaluated for the major renewable energy forms: solar, wind, waves, hydro and biomass. Keywords: Renewable energy; Conversion technology; Efficiency
The practical definition of renewable energy is a
flow of energy, that is not exhausted by being used. The primary renewable energy source on earth is thus solar radiation, because the earth-atmosphere system receives an amount of solar energy, which is essentially independent of the conversion processes that we may apply before allowing that energy to become re-radiated into space in the form of heat. On a more fundamental level, energy is of course a conserved quantity, but one that is degraded in quality by being converted into heat of lower temperature. Solar radiation is associated with the depletion of nuclear fuels in the sun, and it is basically the hugeness of this resource, that may allow us for practical purposes to consider solar radiation as a renewable energy source. In general, renewable energy may be taken to include the use of any energy storage reservoir, which is being refilled at a rate comparable to that of extraction. In this way, geothermal energy-use is renewable, as long as heat can flow into the sediment in question as fast as heat is being extracted. Fossil fuels would similarly be renewable energy sources, if they were only used at the average rate of Bent Sorensen is Technical Director, COWlconsult, Consuiting Engineers and Planners, 15 Parallelvej, DK-2800 Lyngby, Denmark, on leave from Institute of Mathematics and Physics, Roskilde University Center, DK-4000 Roskilde, Denmark. 386
fossilization of biomass. Since, however, this rate is very small, compared to current use, fossil fuels do not at present qualify as renewable. Figure 1 shows the main renewable energy flows available on earth. The various forms are based on solar energy in its different converted forms, on heat stored in the interior of the earth or being created by radioactive processes, and on gravitational energy in the planetary system. 1 An estimate of the amounts of renewable energy that may be recoverable in practice is presented in Table 1, with conventional fuel figures included for comparison. The flow of solar and solar-derived energy forms is not independent from the activities of man. The radiation fluxes are modified by changing the reflectivity of the earth surface, eg by urbanization and agricultural practices. Man's activities also change wind patterns and modify cloud coverage, thus again changing the radiation balance. Furthermore, the injection of pollution into the environment, as well as manmade structural changes, influence both radiation, heat and water flows.
Individual r e n e w a b l e energy sources and their utilization Although practically all renewable energy forms available on earth originate from solar energy, the 'pre-processing' by nature is often decisive for the cost and efficiency of human utilization. For example, hydropower is derived from water elevated by a range of processes, such as evaporation, transport and condensation of the water. These processes are driven by solar radiation and its derived forms ( a b s o r b e d radiation, wind, t e m p e r a t u r e differences). The following discussion will therefore depend on the type of renewable energy used for energy extraction, as well as on the method and technology used for each energy conversion step. Solar radiation may be used by thermal devices or by devices directly converting radiation into electric0301-4215/91/040386-06© 1991 Butterworth-HeinemannLtd
R e n e w a b l e energy - technical o v e r v i e w
Short-wavelength radiation
l
Extroterrestiol ,oorco,
Potential, latent, chemical and nuclear energy
Sensible heat energy
Kinetic energy
Loo,
THeot 122 500/reradiation
3 Lenergy OOOO
Long-wavelength radiation
~CU ob-
Atmosphere
~ -0
Hydrosphere
Upper lithosphere
Biosphere
Lower
lithosphere
F i g u r e 1. Natural energy flows in the e a r t h - a t m o s p h e r e system (unit TW).
tion process may therefore vary from just a little above ambient temperatures (solar space heating or hot water panels) to a couple of thousand degrees Celsius above ambient (parabolic concentrators). If
al or chemical energy. The thermal devices either absorb the radiation directly, or use reflections to first concentrate the radiation flux before absorption. The temperature associated with the absorp-
Table 1. Estimate of global energy resources at the surface of the earth. Resources
Estimated as recoverable
Resource base
Solar radiation Wind Wave Tides Geothermal flow Salinity gradients
1 000 TW 10 TW 0.5 TW 0.1 TW
90 000 TW 1 200 TW 3 TW 30 TW 30 TW 3 TW
Biomass standing crop Geothermal heat stored Kinetic energy stored in atmospheric and oceanic circulation Oil Natural gas Coal Fission resources Fusion resources
50 TWyears to ?
450 TWyears 10 ]~ TWyears 32 TWyears
300 180 930 90 0
to to to to to
2 500 TWyears 2 500 TWyears 7 000 TWyears 9 000 TWyears 10 ]j TWyears
N o t e : For renewable energy sources, the flows are given in TW, while for non-renew-
able resources, an estimated resource range (from proven and possible to ultimately minable) is stated in TWyears. Source: See, Jensen and Scrensen, op cit, Ref 4.
ENERGY POLICY May 1991
387
R e n e w a b l e energy - technical o v e r v i e w
the desired output is high quality energy, such as electric power, thermodynamics limits the maximum efficiency of conversion correspondingly: to just a few percent for a simple, fiat-plate collector and to about 85% for the best parabolic ones. However, for heating purposes requiring the elevation of temperature by just a few tens of degrees, the flat-plate collector may be a very suitable device. In the case of electricity production by absorbed solar heat, either a conventional thermodynamic cycle may be used, or more advanced concepts such as thermionic generators. 2 The advanced concepts may bring the efficiency closer to the thermodynamical limit, but usually at a higher cost. Direct conversion of solar radiation into electrical energy may be achived by a photovoltaic device. Depending on the degree of matching between the energy gap in the band structure of the electron energy levels, and the frequency spectrum of the incoming radiation, the overall efficiency of conversion may range from 10 and 30% (eg low for amorphous silicon, medium for crystalline silicon, high for gallium arsenide). Conversion of solar radiation into chemical energy may be accomplished by photosynthesis in green plants, yielding an efficiency of up to about 15%, considering both the mismatch between the solar frequency spectrum and the chlorophyll absorption spectrum, and also the losses in the chemical processes delivering the absorbed energy in a form suitable for growth of the plant. 3 Further losses are involved in using the chemical energy in the produced biomass for human purposes (food, burning for heat, transforming into biofuels such as biogas or ethanol, or waiting for fossilization into fuels such as coal, oil or natural gas). The associated conversion processes typically have practical efficiencies of the order of 50% or less, so that the combined efficiency in converting solar radiation into useful energy comes out below 10%. Turbines used in converting hydro energy usually have efficiencies over 90% at rated power level, but dropping off at lower loads. Wind turbines have a fundamental efficiency limit of 16/27 for a fixed turbine area. This limit is based upon the assumption, that only flow in one direction is important (ie neglecting radial and rotational flows). It takes into account the possibility of 'drawing in' air from an area larger than that swept by the rotor, and it takes into account the slipstream flow needed for continuity. In practice, fixed blade-pitch and fixed rotational velocity reduce the average efficiency, but values around 35% are realistic at locations with good wind conditions. Advanced tur-
388
bines may utilize rotational flows and thus achieve higher efficiencies, but presumably at higher cost. In any case, interference between turbines may become an important issue, if each turbine is made to extract power from an area much larger than the corresponding dimension of the device. Several devices for extracting wave energy have been proposed. Efficiencies can be of the order of 90%, although only for wave inputs close to the design specifications. Complex tuning mechanisms must be added, if such high efficiencies are to be maintained for a broader range of conditions. Figure 1 indicates that geothermal energy may be used in a renewable mode. However, current use is mainly reservoir-based and non-renewable. As the energy is in the form of heat, straight thermodynamical considerations determine the efficiency of conversion. Other, more exotic, renewable energy forms include energy in currents and tides, salinity or temperature gradients in oceans, and atmospheric electricity. Currents and tides may be exploited by conventional turbines, temperature gradients by thermodynamical engines, albeit of exceedingly low efficiency, due to small differences of temperature. Most of the renewable energy sources are characterized by strong variations in flow. Some are regular, eg the day-to-night and seasonal cycles of solar radiation, while others are difficult to predict in detail. For example, the power of wind energy is proportional to the third power of the wind speed, making traditional weather forecasts (typically working with intervals such as 5-8 miles/second wind speeds) fairly useless. Figure 2 shows some power duration curves for renewable energy sources, indicating the fraction of time, during which a certain level of power is available. Reservoir-based renewable energy, such as certain hydropower schemes or biofuels, are thus very important components of a system with a high penetration of renewable sources. 4 As a 25% subsidy in a fuel-based energy system, the intermittent nature of many renewable energy sources presents no problem. 5 The variability of renewable energy sources is not completely given by the power duration curves. They do not indicate, whether the lulls are regular or more or less unpredictable (eg diurnal solar cycle v passing cloud cover). For this reason, calculations of the performance of renewable energy conversion systems are best done with time series of actual data, using some simulation technique. The time series of data may consist of measured data, or of synthetic data (eg stochastically generated variations with specified averages and higher moments). 6
ENERGY POLICY May 1991
R e n e w a b l e energy - technical o v e r v i e w
Wave
Wind
200
4
\
Annual data (locationand heightaf measurement )
-----
!
E
Ris~,Denmark,56m
2 -\\ .\, i~
v kL
\
\
~
X
Winterseason Annualdata Summerseason
~. ~,
(uncertainty indata
~
is considerable]
I00 u_
\'\
. . . . . . Changi,Singapore,IOta ,
"--I- ~4~_z___~'__-'_r-Solar
1
\ \
Waterford,CN,USA,45m
.\ \ \ . 0
\
k ~
Ringhals, Sweden, 5am
NorthAtlantic 59°N' 19°W
, ...__ ,
"I-- ~ " " ~
x..
,
Currents
Annual data
1~\
0.8
7r7o7oi ;o oo
~-- 0.6 ~ \ \ E X
0.4
~danuory
southfacingsurface at 4 5 ° tilt
~
LL
Great Belt,Denmark Monthly data 0.8
July
Normal incidence radiation
0.6
(fullytrackingdevice) ~
E
hScOt:oernelir~o :~ir;~i:: an
v
\\
U_
0.4
\\X\
0.2
0 0
40
80
0.2
40
0
80
Fraction of time in which energy flux exceeds F
Figure 2.
P o w e r d u r a t i o n curves.
Source: See, S~rensen, o p cit, Ref 1.
Technical development stage In most areas of renewable energy conversion, the past 15 years have allowed for a first prototype development and subsequent commercial development. Several products have been tested in the marketplace, and many have not passed the test. As a result, we have today sorted out a limited range of technically viable products, in which we can place considerable confidence in terms of durability and performance. In some areas, the cost is currently too high for general application, and in others, there is
ENERGY POLICY May 1991
still technological progress to be made, before market entrance is possible. The main areas of renewable energy conversion equipment are briefly surveyed in the following, with emphasis on the status in regard to these questions. Flat-plate solar collector system for space heating and hot water productions have reached technical and economical viability in several parts of the world. Hot water systems are in widespread use in areas with low space heating requirements. Total heating systems with a solar component, for use at higher latitudes, have been technically perfected.
389
R e n e w a b l e e n e r g y - technical o v e r v i e w
They use selective surface panels for high performance, and collectors have been developed, which can withstand the harshness of humid climates. The cost of solar space heating systems is still higher than that of fuel-based alternatives (excluding the uncertain cost of adverse environmental effects), so the market penetration is low. Nevertheless, there is a production niche in several countries large enough to ensure continuing technical optimization. Photovoltaic collectors have gone through a continuous technological development, and have found an increasing number of marketplace niches. However, they are still too expensive for consideration as bulk power generation replacement. Along with the dominant crystalline silicon cells, parallel efforts in amorphous silicon technology and in highefficiency cells are ongoing. Solar concentrating devices are still in the experimental stage. Fairly large demonstration plants have been built, but since there is no clear evidence for an economic advantage relative to photovoltaic devices, the durability and maintenance-free operation of photovoltaic cells give them a wider perspective and in this way a competitive edge. Horizontal axis wind turbines have been technically perfected and have gained a fair position in the marketplace in selected areas (notably in Denmark and California). In areas of good wind conditions, they are competitive or near-competitive relative to fuel-based alternatives for bulk production of electricity, provided that the wind energy share is under about 25% of the demand (so that no dedicated energy storage is required), and provided that suitable maintenance facilities are locally available. The introduction of wind technology is currently spreading to other areas of the world, and the prospects for the next decade look bright. The utilization of waves and tides have been explored in a few prototype installations. Several problems have been encountered, and further development is needed in order to evaluate the potential, especially in the case of wave energy. Prototype geothermal energy installations aimed at using the resource in a renewable or nearrenewable fashion have been built. The aim has mostly been heating and hot water delivery, in contrast to the conventional, non-renewable steam turbine plants producing electric power. Hydro and biomass technologies continue to be in widespread use. Hydropower plants of extremely large size have been built, and considerable progress has been made on making smallscale plants costeffective. Biomass burning continues to be in widespread use, both in developing countries and in
390
forest-rich residential areas of industrialized nations (eg Sweden, Canada and New England), despite the adverse effects of over-exploitation of the forest resource in developing countries, and despite the severe environmental problems associated with smallscale intermittent-cycle burning of woodfuel. Biofuel production is ongoing on more than one level: in Asia, a large number of primitive labourintensive biogas plants have been installed. Not all of them are in working condition, and for those that are, their utility depends on the distribution of access to them among various population groups. Prototypes of high-technology, fully automated biogas plants have been built in countries such as Denmark. The experiences are mixed, but hope still exists that viable solutions will emerge. Liquid biofuels (alcohols) for energy-use are produced in a few countries, notably Brazil. In order to serve as gasoline replacement, the alternative fuel must be introduced to the distribution network, eg in the form of gasoline/ethanol blends, and automobile engines have to be prepared for running on the alternative fuel. This has impeded the plans for biofuel production in several countries, in conjunction with the uncertainty pertaining to investment recovery of such a large-scale commitment. The use of biomass to provide food continues to be a large area of application. The total use of renewable energy at present may be summarized in the following way: renewable energy is used in the form of food (the gross rate of biomass energy supply amounting to about 0.6 TW, woodfuel and waste burning (1.0 TW), timber (0.4 TW), paper and pulp industry inputs (0.06 TW), hydropower (0.3 TW), wind, solar, geothermal and tidal power (0.06 TW). All together, renewable energy accounts for about 25% of the total energyuse by human society, when counted in this way and excluding environmental heat gain. 7
The future The future share of renewable energy may increase from the present 25%, primarily by increasing the share of solar, wind and advanced biomass conversion. In the long term and with inclusion of energy storage facilities, the share of renewable energy could approach 100%. For the next decades, a doubling of the renewable energy contribution to 50% would be a very ambitious goal, but one that is technically feasible. One factor to consider would be land-use. Direct solar conversion systems integrated into rooftops could be introduced with no additional burden on
ENERGY POLICY May 1991
Renewable energy - technical overview
land-use, in contrast to centralized solar power plants (whether thermal or photovoltaic). Wind power plants make some disturbances on land-use, but generally do not exclude say agricultural use of the land between turbines. However, for practical reasons, densely populated regions would limit the number of turbine sites, and most large-scale wind developments in such regions are likely to be placed off-shore (test plants on shallow water sites are already in progress). Increased harvest of biomass for energy purposes does make a substantial impact on land-use, and would probably be restricted to marginal land, using special 'energy crops' on land unsuited for food production. The current round of energy discussions is critical for the future of renewable energy options. When the energy issue gained social and political importance during the mid-1970s, most of us were poorly prepared for such a discussion. Alternative energy sources had been neglected for two decades, due to the apparent abundance of cheap oil. New research programmes had to be established, but when the first substantial results began to appear around 1980, the public debate had already been turned away from the energy question. Presently, the energy issue has resurfaced as a part of the environmental debate: key problems are pollution from fossil energy-use and from nuclear accidents, and the possibility of global climate changes caused by the greenhouse gases emitted by fossil fuel combustion. This time, we can benefit from the research, development and commercialization efforts devoted recently to renewable energy techniques, and arrive at a much more qualified opinion regarding the realism of planning for a major contribution of renewable energy to future energy systems. ~ The political issue is clearly, how much value we are willing to attach to avoiding negative environmental and climatic impacts. Adding these 'indirect' economic costs to the evaluation, we find that some renewable energy options are already viable, while
ENERGY POLICY May 1991
other ones are close to viability and may automatically become viable, as soon as massive use of them becomes a reality. The real problem is the institutional arrangement, that will allow for this development. Who should make the investments in the small-size renewable energy devices aimed at decentralized energy production? One solution would be to let the present energy institutions (power utility companies, etc) finance installation of decentralized, renewable energy devices at the level of individual customers (rooftop solar panels, dispersed wind turbines, etc). However, the inertia in such institutions may be too big, implying the risk of seeing a half-hearted effort with too slow a rate of producing results. Alternatives include the establishment of new financing mechanisms, which will yield fair conditions for the establishment of decentralized energy systems, by truly reflecting their social benefits. lB. S¢~rensen, Renewable Energy, Academic Press, London, 1979. 21bid. 31bid. 4j. Jensen and B. Scrensen, Fundamentals of Energy Storage, Wiley, New York, 1984. 5B S~rensen, 'The regulation of an electricity supply system including wind energy generators', in Proceedings, 2nd International Symposium on Wind Energy Systems, Amsterdam 1978, Vol 1, pp Gl.I-1.8, BHRA Fluid Engineering, Cranfield, UK, 1979. 6B. S0rensen, 'A combined wind and hydropower systems', Energy Policy, Vol 9, No 1, March 1981, pp 51-55; A Study of Wind-Diesel/Gas Combination Systems, Energy Authority of New South Wales Report EA86/17, Sydney, Australia, 1986; 'Dachen Island's wind/diesel facilities', in , European Community Wind Energy Conference, Madrid, 1990, Stephens & Associates, Bedford, UK, 199(I, pp 688-691. 7Estimates are based on material in, op cit, Refs 1 and 4. Environmental heat is essential for human society but difficult to quantify (should one count the entire greenhouse heating of the earth-atmosphere system or only that of human habitats?). 8B. S0rensen, 'Renewable energy and development', in Proceedings International Conference on Renewable Energy and Local Production, Vol 1, pp 35-74, Danish Centre for Renewable Energy, Thy, Denmark, 1989; B. S0rensen, 'Energy and greenhouse strategies in Scandinavia', in Proceedings' Workshop on Energy and the Greenhouse Effect, Sydney 1990, Energy Research, Development and Information Centre, University of New South Wales, Australia, 199(I.
391