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Renewable energy capability to save carbon emissions
Pergamon
RENEWABLE
PII: S0038-092X ( 96 ) 00136-3
ENERGY
CAPABILITY
Solar Energy Vol. 57, No. 6, pp. 485-491, 1996 Copyright© 1997 ElsevierScienceLtd Printed in Great Britain. All rights reserved 0038-092X/96 $15.00+0.00
TO SAVE CARBON
EMISSIONS
D. C O I A N T E t~ and L. B A R R A ~ ENEA - - Italian National Agency for New Technology, Energy and the Environment, Casaccia Research Centre, via Anguillarese 301, 00060 Rome, Italy (Received 13 March 1996; revised version accepted 9 September 1996) (Communicated by Ari Rabl) Abstract--The present R&D approach to new renewable energy sources includes a drawback which could negate their environmental significance. New renewable energies are affected by a technical limitation because of the random intermittent nature of their power generation which hinders them from fully expanding into the electricity market. As a consequence, the contribution which renewable electric energy sources make is just significant in terms of world electricity generation and only marginal in terms of total energy consumption. Thus, in spite of expectations, the practical achievable amount of environmental benefits arising from new renewable energy would not be enough to counteract the environmental crisis. It is known that the intermittence of energy supply can be removed by implementing grid-tied power systems, adding a further stage aimed to chemically store the intermittent solar energy by producing clean synthetic fuels. Until now this chance was considered of little importance, on the contrary, it should become a compulsory solution so that renewable energy can acquire an actual and environmentally consistent significance. Copyright © 1997 Elsevier Science Ltd.
sources because of the conflicting aspects with agricultural food use. This limit, instead, appears less severe for wind energy and substantially negligible for photovoltaic systems. As a matter of fact, in the first case, land occupation is very marginal and the wind power production is largely compatible with other agricultural use. For the second case, there are two items which play a more favourable role: (1) much higher global efficiency (already partially reached) in converting solar radiation directly; (2) the possibility of using barren desert lands. The first item tends to use less land for each generated power unit. The second provides a different solution, largely sufficient and not in conflict with agricultural production. Therefore, with ever larger demands for clean energy in the long term, wind power and, above all, photovoltaic conversion of solar radiation will likely play a most important role among new renewable energy sources. According to this picture, the present development strategy of renewable electricity sources assumes the adoption of large and land-diffused PV and wind energy systems in grid-tied applications as the most suitable perspective model for giving a contribution to mitigate the environmental crisis. The expansion process of renewable electricity is deemed to proceed through a gradual penetration into power markets, starting from
1. I N T R O D U C T I O N
As the world population grows, energy production and consumption will increase. In the same way, world pollution will rise unless suitable measures are taken to counteract this phenomenon. At any rate, future environmental constraints will require a massive exploitation of solar energy, in all its conventional and new forms, namely: biomass, thermal energy, wind energy and photovoltaic energy. As a consequence, we can foresee in the long term increasing importance of these new renewable energy sources. In addition, since the demand for electric energy is growing all over the world, we expect in the future a large prevalence of electrical applications within the market share assignable to renewable energy. As a consequence, because of the more mature state of technology, the most significant power contribution for the mid term will likely arise from the new use of biomass and hydropower plants. However, the complex processes of solar energy conversion, which these power sources are based on, imply that the global efficiency, from solar radiation to generated power, is very low. So large occupation of territory is required for generating significant power amounts. Thus, the availability of suitable land surfaces has to be considered as the most serious limiting factor for these tAuthor to whom correspondence should be addressed. tlSES member. 485
486
D. Coiante and L. Barra
the present situation of remote village electrification, roof-top applications and grid-tied supporting and peaking systems until reaching the bulk power market (Taschini and Iannucci, 1992). Of course, all that may occur is a corresponding progressive lowering of system prices, which makes achieving the energy cost effectiveness in each subsequent market niche possible. The competitiveness of renewable electricity is considered fully achieved when the penetration reaches the bulk power market, since only this factor has a magnitude sufficient for supporting the production necessary for a large-scale economy. Finally, at this stage, renewable electricity is judged to be a real energy option, capable of supplying a significant contribution to the energy balance and, as a consequence, of mitigating the environmental crisis. This latter point is a complex subject and, of course, would merit a proper evaluation in terms of complete cycle analysis of systems. In such a case, the environmental impact of renewable energy itself, although small, should be accounted for. Nevertheless even a more limited approach, which considers only the capability of renewable energy to replace fossil fuel consumptions, can lead to interesting results about the coherence of present development strategies in reference to the amount of expected environmental benefits. The present article discusses this limited picture, showing that environmental constraints require from renewable energy sources much more than a simple electric energy contribution, which, even if significant, is unfortunately insufficient to afford world climate changes. 2. RENEWABLE ENERGY A N D THE POWER MARKET
2.1. Technical limit In accordance with the present approach, direct grid connection (without any intermediate stage of energy storage) has to be assumed as an obligatory solution for large-scale applications of renewable energy systems for technical and economic reasons. Unfortunately, the straight connection of random intermittent power generators to a conventional grid can lead to a relevant worsening of system stability. In practice, when the amount of intermittent tied power becomes significant, in comparison with the operative power level, the system stability can be strongly jeopardized because of abrupt variations of power generation. This
leads to a technical limit on the amount of renewable power to be grid connected. Therefore, in order to preserve the power reliability level, when tied intermittent power overcomes the limit, the grid must add further conventional reliable generators or conventional energy storage subsystems, which will be devoted principally to backup operations. As a consequence, the intermittent power capacity exceeding the upper technical limit increases the cost of the added renewable energy significantly, because the extra cost of energy backup systems must be charged upon the exceeding renewable power capacity (WEC, 1994).
2.2. Barrier to penetration into the bulk power market In order to make the subject more understandable, let us introduce for a given grid "i" a relative parameter, ki < 1, so that the maximum power capacity of intermittent renewable electric generators, which can be grid tied directly, will be (Pmax)i = ki(Pa)i
( 1)
where (Pa)i is the conventional power capacity presently active in the "ith" grid. Consequently, in order to transfer the discussion from the single grid to the bulk power market, we should consider the sum of all grid power capacity installed in the world, [ P = ~ i (Pa)i], and account for all the limits of eqn (1). That is Vmax= E (Pmax)i= E k,(Po),=KP /
(2)
i
where K (K< 1) is the weighted mean of ki values, that is, the mean share of world power market, accessible to renewable electric power systems. Thus the technical limit of eqn (1) for penetration into a single power grid can be conceptually generalized through eqn (2), admitting that the intermittent renewable energy systems will be prevented from penetrating beyond a fraction K of the total installed power in the world. According to present strategy, which assumes a future scenario where the most important and promising application of renewable energy in the electricity market is that of grid-tied systems, the above technical limitation will assume the meaning of a market barrier. In general, this result has not been considered worth worrying about as the market margin appears so large in comparison with the present market size that
Renewable energy capability to save carbon emissions the limit presence does not affect the current sale volume. On the contrary, since K has in general a small size, the consequence could be worrying in perspective with regard to renewable energy amount, which can actually contribute to the global energy balance. As a consequence, the penetration limit can lead to heavy limitations of achievable benefit from new renewable energy sources on environmental balance (Coiante and Barra, 1992). In conclusion, the diffusion into use of renewable power systems could be limited by this barrier, which does not allow it to fully profit by the whole available extension of the bulk power market. 3. R E N E W A B L E E N E R G Y A N D ENVIRONMENTAL BALANCE
3.1. Renewable energy contribution
In order to establish the actual significance of renewable power, as it arises from the realization of the present strategy, it is convenient to perform some simple calculations about the practical achievable amount of clean energy. Remembering e q n ( 2 ) , let us express the nominal power, P (in kW), and recall the practical definition of the load factor for power generators as L F = (AEP)/(8760 P)
(3)
where (AEP) is the annual energy production, expressed in kWh, and 8760 is the number of hours in a year. Let us indicate with R L F and C L F the respective average load factors of renewable power plants and conventional ones. Referring to the subject of primary energy consumption of fossil fuels in relative terms, we are now able to obtain, with some simple calculations, the maximum expected share of energy contribution of the new renewable energy sources as (Emax)/(E) = H K [(RLF)/(CLF)]
(4)
where, in accordance with eqn (2): Emx is the maximum electric energy contribution achievable from intermittent renewable energy sources expressed in terms of equivalent primary energy, for example, in equivalent oil tons (toe); E is the world consumption of primary fuel energy in toe; H represents the global fraction of conventional electric energy production (in toe) with respect to E. Thus, anticipating a result, which will be better specified later, since H < 1,
487
K < 1 and, in general, ( R L F ) < ( C L F ) , as a first consequence of eqn (4), we have (Emax)/(~) << 1
(5)
That is, in relative terms of primary energy, the contribution achievable from intermittent renewable energy sources for electricity will not be other than very small. 3.2. Assumptions and hypotheses
In order to fix a number in the previous conclusion, it is necessary to define the value of all parameters of eqn (4). Their exact definition is quite complicated, as it would require the detailed knowledge of all the world energy systems. Nevertheless, in order to have an approximate evaluation, sufficiently good for our aim, we may consider the following facts and hypotheses. (1) The load factor of fuel-fired conventional power plants is nearly consolidated. At present it can reach an average value of about 0.6, corresponding to 5250 operative hours at nominal power. The nuclear and hydropower plants can have larger load factors. Nevertheless, considering the relative weights in the global electricity production, we are not far wrong in assuming the same approximate value also for this type of power plant. (2) The load factor of renewable energy system ranges from 0.15 to 0.35 dependent on both the type of energy source and the renewable energy availability at the site of the plant installation. (3) The penetration limit of grid-tied intermittent power systems in a given grid is a matter of serious discussion among experts. In accordance with the published literature (Van Wijk, 1992; Kurihara, 1993), we can reasonably assume for our conceptual aim that values for ki and K in eqn (2) range from 0.1 to 0.5. (4) The determination of electricity penetration coefficient, H, requires the knowledge of electricity production from conventional power plants in each country with reference to respective primary energy consumption. As an example, this investigation was performed by authors examining the consumptions of the year 1991 for OECD countries (BP, 1992). The result is reported in Fig. 1. The picture shows the electricity consumption share in terms of equivalent fuel energy, arising from the present power
488
D. Coiante and L. Barra 0,8
uJ r,.< 2:: U)
_o
0,6
0,4
uJ _.l LU
0,2
0,0 A U S A U T B & L C A N F R A G E R GRE I T A J A P N E T S P A S W E S W I T U R
UK USA
OECD COUNTRIES SOURCES: BP & OECD/IEA
Fig. 1, Share of electricity production from non-renewable energy sources in the year 1991 for OECD countries. Only the 16 highest values are represented in the figure.
systems (that is, including the geo-hydroelectric production and nuclear electricity) for each OECD country. Since the OECD countries are presently consuming about 52% of world energy needs, the average value among the electricity penetration shares in these countries is deemed quite representative of the present world situation. In addition, considering the development trend of the other countries, the OECD data can be assumed as a future target also for developing countries. On the basis o f these considerations, looking at Fig. 1, a mean value of 0.45 can be seen, which may be assumed as representative for H.
3.3. Renewable energy amount and environmental benefit Concerning the capability of new renewable energy systems to curb carbon emissions, we have to focus our attention mainly on the possibility of replacing fossil hydrocarbons. Therefore we must take energy contributions from hydro and nuclear power, which are carbon free, into particular account. To this aim, let us introduce a further parameter, C, which expresses the ratio between the sum of hydro and nuclear contributions and the total energy consumption, that is C = (hydro + nuclear)/E
(6)
In this way the fossil hydrocarbon component of energy balance, EC, will be expressed as EC=(1-C)E
(7)
and a more suitable expression of eqn (4) is obtained:
(Emax)/(E C ) [H K/( I - C)][(RLF )/(CLF)] =
(8) This equation allows us to easily calculate in relative terms the order of magnitude of the possible contribution, of intermittent renewable energy sources, to hydrocarbon replacement. According to the amount of hydro and nuclear power installed in each country, C can vary from 0 to 1. For the above examined OECD countries, in the year 1991, C values range from 0 for Denmark to 0.74 for Norway, with an average global value of about 0.23. Thus, summing up in eqn (8) all these considerations and the above assumptions, the maximum contribution achievable from intermittent renewable energy sources can be expressed in a simplified parametric form by
(Ema,,)/(EC)~O.97(RLF)K
(9)
where K c a n vary from 0.1 to 0.5, while ( R L F ) in practice can assume values from 0.1 to 0.3. Figure 2 shows a plot of eqn (9) for three values of the load factor, mainly regarding the wind energy and photovoltaic power plants.
Renewable energy capability to save carbon emissions
489
./
14
,Z,
12-
i RLF~
10 ~
i.......
RLF = 0.25
J ~
..... .ik ~
0
20
~ ' ~
J
f
....4
RLF = 0.1
30
40
50
INTERMITTENT POWER GRID PENETRATION (%)
Fig. 2. Maximum renewableenergycontribution in percentageof total fossil fuel energy needs vs. the share of intermittent electric power acceptablein grid-tied applications, for differentvalues of renewable load factor.
The lowest curve is representative of the worst case among practicable PV applications, while the highest is related to the best case of wind turbines installed in good wind sites. Looking at Fig. 2 we can agree about the fact that all the possible contribution from intermittent renewable energy sources will be located within the layer enclosed between the lowest and highest curves. The intermediate line can be considered representative of the best situation for PV systems, which can ideally occur in the case of concentrating the electric generation o f large-scale grid-tied PV systems in the sunniest countries, where R L F can reach values as high as 0.25. In this most favourable case, we can see the maximum contribution of hydrocarbon replacement, arising from intermittent PV power systems, will amount to about 12%, provided the grid-tied power can penetrate up to 50% of the total conventional installed power. Apart from considering this latter aspect as technically unlikely, because K in practice is confined within 20-25% (Milborrow, 1995) (which corresponds in Fig. 2 to an energy contribution of about 5%), the general conclusion is that, because o f the intermittent nature of the sources, the future global contribution of renewable power in terms of fossil hydrocarbon replacement could be less significant than expected and, as a consequence, the environmental improvements could also be marginal.
4. ENVIRONMENTAL SIGNIFICANCE
4.1. Necessity of energy storage The disappointing result, so clearly summarized in Fig. 2, arises substantially from the numerical results of eqns(4) and (8) as the product of three factors, H, K and [(RLF)/(CLF)], each of them smaller than 1. Therefore, any technical device which can increase the value of each parameter leads to a situation improvement. This can be achieved by introducing, into the system layout, a suitable stage of energy storage, which can stabilize the intermittence of the renewable energy flow. The opportunity of storing intermittent renewable energy, i.e., photovoltaic, in the form of chemical energy (e.g., hydrogen) is a result already acquired (Schnurnberger et al., 1988). Moreover, the present discussion intends to quantitatively indicate the necessity of this solution in relation to the logical self-consistency of renewable energy development strategy. In order to understand this point, let us stress the subject, assuming the most favourable case of the possibility of a seasonal energy storage. In this case, we can consider that the intermittent power of the renewable energy source is transformed into a fully stable power source, capable of supplying the load with the required reliability. This means that the renewable load factor is now equivalent to the conventional one, that is, [ ( R L F ) / ( C L F ) ] ~ I . At the same time, the acquired power stability allows us to
490
D. Coiante and L. Barra
overcome the penetration limit in grid-tied applications. In turn, a larger penetration into grid power capacity makes it possible for renewable energy to occupy more and more significant shares of the bulk power market, so K can largely increase up to values, in principle, near to unity (fully acquired substitutionality). At this point, the renewable power kWh has the same technical quality as conventional sources (same reliability). In this case, eqn (4) becomes
(Emax)/(E) ~ H
( 1O)
Therefore, the renewable contribution can, in principle, grow to the level of electricity penetration sharing. This can be considered to be a very ambitious and likely unachievable goal, nevertheless, as we are going to see, it is not yet sufficiently ahead of future global climate constraints.
4.2. Environmental necessity for synthetic ecofuel production As a matter of fact, according to the average value of Fig. 1, we can roughly assume that the H value is about 45%. So, even if electrical consumption, for the sake of absurdity, were fully satisfied with clean renewable electricity, the largest part of the remaining world energy needs would continue to be served with pollutant fossil fuel energy. Thus, the confinement of new renewable energy sources within the sector of electricity production has to be considered as an additional drawback, which can strongly limit the environmental contribution of renewable energy. This is more commonly known as the "electricity ghetto effect" (Scott, 1995), which accounts for the fact that other important energy services (e.g., heating, cooking, transportation, etc.) are better satisfied by energy carriers, other than electricity, mainly by fossil fuels. Therefore, if the renewable energy is to become a really significant solution for the environmental crisis, it must acquire the ability to escape from the "electricity ghetto" and push its expansion, in addition to the electricity market, into the fossil fuel market. The present line of biomass development, which considers extensive plant cultivation for energy production, especially those aimed at biofuel production (Laurent and Delmon, 1992), provides the chance for identifying a viable path for approaching the solution, since the biofuel cycle can already be considered in perfect agreement with this new strategic view. Profiting by the analogy with a biofuel pro-
duction line, let us consider the following possible process for producing ecofuels from the other renewable energy sources. The primary intermittent renewable energy (wind energy or sun radiation), available on the site, is captured and converted into a secondary form of energy (electrical power), which can be used in the respective market sector or addressed to a second stage, where it can be transformed to chemical energy in the form of a synthetic fuel (for example hydrogen by water electrolysis or methanol Hz-derived, or other synthetic hydrocarbons). All these products, named ecofuels or solar fuels, would be able to suitably substitute for the fossil fuels in all their energy applications without increasing the atmospheric carbon content. As a matter of fact H2 burning does not produce carbon at all and other synthetic hydrocarbons return to the atmosphere during the combustion of the CO2 sequestered during the previous synthesis. The adoption of this process allows the realization of an additional path, different from the electricity one, capable of enhancing the market expansion. According to this solution, the renewable energy can escape from the "electricity ghetto" and directly penetrate the fossil fuel market, so the substitution of ever larger amounts of pollutant energy with clean ecofuels can finally lead to the expected significant environmental benefits.
4.3. Recovery of added costs The addition of a second stage to renewable power systems for transforming and storing the intermittent renewable energy increases the final cost of energy unit. But, in principle, the added costs can be recovered by accounting for the environmental credit assignable to ecofuels. This subject is presently a matter of animated discussion among the experts, and its definition would merit a consideration certainly larger than this article allows. At any rate, summing up the first results, it can be considered that, when the external societal costs of the whole fossil fuel energy cycle are suitably internalized, the real cost of a conventional energy unit should be increased by a consistent social factor, likely ranging from 2 to 4 according to the results of different analyses (Hohmeyer, 1988; Hohmeyer and Gartner, 1992; Coiante and Barra, 1995). Since all cost projections consider the competitiveness of grid-tied renewable power kWh in the electricity market to be achievable, the social factor also represents a
Renewable energy capability to save carbon emissions
491
p r o s p e c t i v e a v a i l a b l e m a r g i n for recovering the e x t r a cost o f ecofuel p r o d u c t i o n ,
tion, as significant as we expect, in o r d e r to m i t i g a t e the climate changes.
5. C O N C L U S I O N
REFERENCES
In a d d i t i o n to the d r a w b a c k linked to the low value o f surface energy density, the present d e v e l o p m e n t strategy for new r e n e w a b l e energy sources is affected by a technical l i m i t a t i o n caused b y the r a n d o m i n t e r m i t t e n t n a t u r e o f p o w e r generation. This prevents the r e n e w a b l e energy f r o m fully e x p a n d i n g into the electricity m a r k e t . A s a consequence, the r e n e w a b l e energy c o n t r i b u t i o n to w o r l d electricity g e n e r a t i o n will be small. In a d d i t i o n , since the r e n e w a b l e energy p r o d u c t i o n is m a i n l y confined within the electricity sector a n d this l a t t e r a c c o u n t s only for a b o u t 45% o f w o r l d energy c o n s u m p t i o n , the c o n t r i b u t i o n shall be c o n s i d e r e d ever m o r e m a r g i n a l in terms o f t o t a l energy balance. Thus, in c o n t r a s t to the e x p e c t a t i o n o f a consistent e n v i r o n m e n t a l recovery f r o m r e n e w a b l e energy a p p l i c a t i o n s , the p r a c t i c a l achievable a m o u n t o f e n v i r o n m e n t a l benefit will also be m a r g i n a l . T h e b a r r i e r limiting the e x p a n s i o n o f renewa b l e energy s h o u l d be r e m o v e d b y i m p l e m e n t i n g the grid-tied p o w e r systems, which the present m a r k e t strategy is b a s e d on, a d d i n g a further stage a i m e d at chemically storing the intermittent solar energy a n d p r o d u c i n g clean synthetic fuels. T h e e x t r a costs, i n c u r r e d b y this solution, can be recovered by a c c o u n t i n g for the environm e n t a l benefit. In conclusion, the possibility o f m a k i n g large a m o u n t s o f s u b s t i t u t i o n a l ecofuels, arising f r o m i n t e r m i t t e n t r e n e w a b l e energy, available in the fuel m a r k e t has to be c o n s i d e r e d as a necessary c o n d i t i o n for the r e n e w a b l e energies to really m a k e a c o n t r i b u -
BP (1992) Statistical Review o f Worm Energy 1992. British Petroleum Company, London. Coiante D. and Barra L. (1992) Can photovoltaics become an effective energy option? Solar Energy Mater. Solar Cells, 27, 79-89. Coiante D. and Barra L. (1995) A practical method to evaluate the real cost of electrical energy. Int. J. Energy Res., 19, 159-168. Hohmeyer O. (1988) Social Costs o f Energy Consumption. Springer, Heidelberg, pp. 103-104. Hohmeyer O. and Gartner E. (1992) The Costs o f Climate Change. Commission of the European Community Report, Fraunhofer Institute for Systems and Innovation Research, Freiburg. Kurihara I. (1993) Capacity value evaluation of photovoltaic power generation. In Proc. Advisory Group Meeting IAEA, 26-28 April 1993, Vienna, pp. 43-59. Laurent E. and Delmon B. (1992) Feasibility of the partial and full hydrorefining of bio-oils produced by rapid thermal pyrolysis studies with model compounds. In
SE ~1:6-0
Proc. 7th E.C. Int. Conf. on Biomass for Energy and Industry, Florence, 1992, Hall D. O., Grassi G. and
Scheer H. (Eds). Ponte Press, Bochum (GE), pp. 177-185. Milborrow D. (1995) What happens when the wind stops blowing? Wind Directions, 14, 7-9. Schnurnberger W., Seeger W. and Steeb H. (1988) Selected technical hydrogen production systems. In Hydrogen as an Energy Carrier, Winter C. J. and Nitsch J. (Eds). Springer, Berlin, pp. 209-248. Scott D. S. (1995) Interpreting the architecture of the energy system. In Proc. WEC 16th Congress, WEC Committees (Eds). Tokyo, Japan, pp. 89-108. Taschini A. and Iannucci J. J. (1992) Potential of photovoltaic systems for present and future electric utility applications. In Proc. IEA/ENEL/Pacific Gas & Electric Executive Conf. on Photovoltaic Systems for Electric Utility Applications, 2-5 December, 1990, Taormina, Italy,
OECD (Eds), pp. 51-60. Van Wijk A. J. M. (1992). Capacity credit of wind power in The Netherlands. Electric Power System Res., 23, 189-193. World Energy Council (1994) New Renewable Energy Resources. Kogan Page, London, pp. 120 121.
RENEWABLE
PII: S0038-092X ( 96 ) 00136-3
ENERGY
CAPABILITY
Solar Energy Vol. 57, No. 6, pp. 485-491, 1996 Copyright© 1997 ElsevierScienceLtd Printed in Great Britain. All rights reserved 0038-092X/96 $15.00+0.00
TO SAVE CARBON
EMISSIONS
D. C O I A N T E t~ and L. B A R R A ~ ENEA - - Italian National Agency for New Technology, Energy and the Environment, Casaccia Research Centre, via Anguillarese 301, 00060 Rome, Italy (Received 13 March 1996; revised version accepted 9 September 1996) (Communicated by Ari Rabl) Abstract--The present R&D approach to new renewable energy sources includes a drawback which could negate their environmental significance. New renewable energies are affected by a technical limitation because of the random intermittent nature of their power generation which hinders them from fully expanding into the electricity market. As a consequence, the contribution which renewable electric energy sources make is just significant in terms of world electricity generation and only marginal in terms of total energy consumption. Thus, in spite of expectations, the practical achievable amount of environmental benefits arising from new renewable energy would not be enough to counteract the environmental crisis. It is known that the intermittence of energy supply can be removed by implementing grid-tied power systems, adding a further stage aimed to chemically store the intermittent solar energy by producing clean synthetic fuels. Until now this chance was considered of little importance, on the contrary, it should become a compulsory solution so that renewable energy can acquire an actual and environmentally consistent significance. Copyright © 1997 Elsevier Science Ltd.
sources because of the conflicting aspects with agricultural food use. This limit, instead, appears less severe for wind energy and substantially negligible for photovoltaic systems. As a matter of fact, in the first case, land occupation is very marginal and the wind power production is largely compatible with other agricultural use. For the second case, there are two items which play a more favourable role: (1) much higher global efficiency (already partially reached) in converting solar radiation directly; (2) the possibility of using barren desert lands. The first item tends to use less land for each generated power unit. The second provides a different solution, largely sufficient and not in conflict with agricultural production. Therefore, with ever larger demands for clean energy in the long term, wind power and, above all, photovoltaic conversion of solar radiation will likely play a most important role among new renewable energy sources. According to this picture, the present development strategy of renewable electricity sources assumes the adoption of large and land-diffused PV and wind energy systems in grid-tied applications as the most suitable perspective model for giving a contribution to mitigate the environmental crisis. The expansion process of renewable electricity is deemed to proceed through a gradual penetration into power markets, starting from
1. I N T R O D U C T I O N
As the world population grows, energy production and consumption will increase. In the same way, world pollution will rise unless suitable measures are taken to counteract this phenomenon. At any rate, future environmental constraints will require a massive exploitation of solar energy, in all its conventional and new forms, namely: biomass, thermal energy, wind energy and photovoltaic energy. As a consequence, we can foresee in the long term increasing importance of these new renewable energy sources. In addition, since the demand for electric energy is growing all over the world, we expect in the future a large prevalence of electrical applications within the market share assignable to renewable energy. As a consequence, because of the more mature state of technology, the most significant power contribution for the mid term will likely arise from the new use of biomass and hydropower plants. However, the complex processes of solar energy conversion, which these power sources are based on, imply that the global efficiency, from solar radiation to generated power, is very low. So large occupation of territory is required for generating significant power amounts. Thus, the availability of suitable land surfaces has to be considered as the most serious limiting factor for these tAuthor to whom correspondence should be addressed. tlSES member. 485
486
D. Coiante and L. Barra
the present situation of remote village electrification, roof-top applications and grid-tied supporting and peaking systems until reaching the bulk power market (Taschini and Iannucci, 1992). Of course, all that may occur is a corresponding progressive lowering of system prices, which makes achieving the energy cost effectiveness in each subsequent market niche possible. The competitiveness of renewable electricity is considered fully achieved when the penetration reaches the bulk power market, since only this factor has a magnitude sufficient for supporting the production necessary for a large-scale economy. Finally, at this stage, renewable electricity is judged to be a real energy option, capable of supplying a significant contribution to the energy balance and, as a consequence, of mitigating the environmental crisis. This latter point is a complex subject and, of course, would merit a proper evaluation in terms of complete cycle analysis of systems. In such a case, the environmental impact of renewable energy itself, although small, should be accounted for. Nevertheless even a more limited approach, which considers only the capability of renewable energy to replace fossil fuel consumptions, can lead to interesting results about the coherence of present development strategies in reference to the amount of expected environmental benefits. The present article discusses this limited picture, showing that environmental constraints require from renewable energy sources much more than a simple electric energy contribution, which, even if significant, is unfortunately insufficient to afford world climate changes. 2. RENEWABLE ENERGY A N D THE POWER MARKET
2.1. Technical limit In accordance with the present approach, direct grid connection (without any intermediate stage of energy storage) has to be assumed as an obligatory solution for large-scale applications of renewable energy systems for technical and economic reasons. Unfortunately, the straight connection of random intermittent power generators to a conventional grid can lead to a relevant worsening of system stability. In practice, when the amount of intermittent tied power becomes significant, in comparison with the operative power level, the system stability can be strongly jeopardized because of abrupt variations of power generation. This
leads to a technical limit on the amount of renewable power to be grid connected. Therefore, in order to preserve the power reliability level, when tied intermittent power overcomes the limit, the grid must add further conventional reliable generators or conventional energy storage subsystems, which will be devoted principally to backup operations. As a consequence, the intermittent power capacity exceeding the upper technical limit increases the cost of the added renewable energy significantly, because the extra cost of energy backup systems must be charged upon the exceeding renewable power capacity (WEC, 1994).
2.2. Barrier to penetration into the bulk power market In order to make the subject more understandable, let us introduce for a given grid "i" a relative parameter, ki < 1, so that the maximum power capacity of intermittent renewable electric generators, which can be grid tied directly, will be (Pmax)i = ki(Pa)i
( 1)
where (Pa)i is the conventional power capacity presently active in the "ith" grid. Consequently, in order to transfer the discussion from the single grid to the bulk power market, we should consider the sum of all grid power capacity installed in the world, [ P = ~ i (Pa)i], and account for all the limits of eqn (1). That is Vmax= E (Pmax)i= E k,(Po),=KP /
(2)
i
where K (K< 1) is the weighted mean of ki values, that is, the mean share of world power market, accessible to renewable electric power systems. Thus the technical limit of eqn (1) for penetration into a single power grid can be conceptually generalized through eqn (2), admitting that the intermittent renewable energy systems will be prevented from penetrating beyond a fraction K of the total installed power in the world. According to present strategy, which assumes a future scenario where the most important and promising application of renewable energy in the electricity market is that of grid-tied systems, the above technical limitation will assume the meaning of a market barrier. In general, this result has not been considered worth worrying about as the market margin appears so large in comparison with the present market size that
Renewable energy capability to save carbon emissions the limit presence does not affect the current sale volume. On the contrary, since K has in general a small size, the consequence could be worrying in perspective with regard to renewable energy amount, which can actually contribute to the global energy balance. As a consequence, the penetration limit can lead to heavy limitations of achievable benefit from new renewable energy sources on environmental balance (Coiante and Barra, 1992). In conclusion, the diffusion into use of renewable power systems could be limited by this barrier, which does not allow it to fully profit by the whole available extension of the bulk power market. 3. R E N E W A B L E E N E R G Y A N D ENVIRONMENTAL BALANCE
3.1. Renewable energy contribution
In order to establish the actual significance of renewable power, as it arises from the realization of the present strategy, it is convenient to perform some simple calculations about the practical achievable amount of clean energy. Remembering e q n ( 2 ) , let us express the nominal power, P (in kW), and recall the practical definition of the load factor for power generators as L F = (AEP)/(8760 P)
(3)
where (AEP) is the annual energy production, expressed in kWh, and 8760 is the number of hours in a year. Let us indicate with R L F and C L F the respective average load factors of renewable power plants and conventional ones. Referring to the subject of primary energy consumption of fossil fuels in relative terms, we are now able to obtain, with some simple calculations, the maximum expected share of energy contribution of the new renewable energy sources as (Emax)/(E) = H K [(RLF)/(CLF)]
(4)
where, in accordance with eqn (2): Emx is the maximum electric energy contribution achievable from intermittent renewable energy sources expressed in terms of equivalent primary energy, for example, in equivalent oil tons (toe); E is the world consumption of primary fuel energy in toe; H represents the global fraction of conventional electric energy production (in toe) with respect to E. Thus, anticipating a result, which will be better specified later, since H < 1,
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K < 1 and, in general, ( R L F ) < ( C L F ) , as a first consequence of eqn (4), we have (Emax)/(~) << 1
(5)
That is, in relative terms of primary energy, the contribution achievable from intermittent renewable energy sources for electricity will not be other than very small. 3.2. Assumptions and hypotheses
In order to fix a number in the previous conclusion, it is necessary to define the value of all parameters of eqn (4). Their exact definition is quite complicated, as it would require the detailed knowledge of all the world energy systems. Nevertheless, in order to have an approximate evaluation, sufficiently good for our aim, we may consider the following facts and hypotheses. (1) The load factor of fuel-fired conventional power plants is nearly consolidated. At present it can reach an average value of about 0.6, corresponding to 5250 operative hours at nominal power. The nuclear and hydropower plants can have larger load factors. Nevertheless, considering the relative weights in the global electricity production, we are not far wrong in assuming the same approximate value also for this type of power plant. (2) The load factor of renewable energy system ranges from 0.15 to 0.35 dependent on both the type of energy source and the renewable energy availability at the site of the plant installation. (3) The penetration limit of grid-tied intermittent power systems in a given grid is a matter of serious discussion among experts. In accordance with the published literature (Van Wijk, 1992; Kurihara, 1993), we can reasonably assume for our conceptual aim that values for ki and K in eqn (2) range from 0.1 to 0.5. (4) The determination of electricity penetration coefficient, H, requires the knowledge of electricity production from conventional power plants in each country with reference to respective primary energy consumption. As an example, this investigation was performed by authors examining the consumptions of the year 1991 for OECD countries (BP, 1992). The result is reported in Fig. 1. The picture shows the electricity consumption share in terms of equivalent fuel energy, arising from the present power
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uJ r,.< 2:: U)
_o
0,6
0,4
uJ _.l LU
0,2
0,0 A U S A U T B & L C A N F R A G E R GRE I T A J A P N E T S P A S W E S W I T U R
UK USA
OECD COUNTRIES SOURCES: BP & OECD/IEA
Fig. 1, Share of electricity production from non-renewable energy sources in the year 1991 for OECD countries. Only the 16 highest values are represented in the figure.
systems (that is, including the geo-hydroelectric production and nuclear electricity) for each OECD country. Since the OECD countries are presently consuming about 52% of world energy needs, the average value among the electricity penetration shares in these countries is deemed quite representative of the present world situation. In addition, considering the development trend of the other countries, the OECD data can be assumed as a future target also for developing countries. On the basis o f these considerations, looking at Fig. 1, a mean value of 0.45 can be seen, which may be assumed as representative for H.
3.3. Renewable energy amount and environmental benefit Concerning the capability of new renewable energy systems to curb carbon emissions, we have to focus our attention mainly on the possibility of replacing fossil hydrocarbons. Therefore we must take energy contributions from hydro and nuclear power, which are carbon free, into particular account. To this aim, let us introduce a further parameter, C, which expresses the ratio between the sum of hydro and nuclear contributions and the total energy consumption, that is C = (hydro + nuclear)/E
(6)
In this way the fossil hydrocarbon component of energy balance, EC, will be expressed as EC=(1-C)E
(7)
and a more suitable expression of eqn (4) is obtained:
(Emax)/(E C ) [H K/( I - C)][(RLF )/(CLF)] =
(8) This equation allows us to easily calculate in relative terms the order of magnitude of the possible contribution, of intermittent renewable energy sources, to hydrocarbon replacement. According to the amount of hydro and nuclear power installed in each country, C can vary from 0 to 1. For the above examined OECD countries, in the year 1991, C values range from 0 for Denmark to 0.74 for Norway, with an average global value of about 0.23. Thus, summing up in eqn (8) all these considerations and the above assumptions, the maximum contribution achievable from intermittent renewable energy sources can be expressed in a simplified parametric form by
(Ema,,)/(EC)~O.97(RLF)K
(9)
where K c a n vary from 0.1 to 0.5, while ( R L F ) in practice can assume values from 0.1 to 0.3. Figure 2 shows a plot of eqn (9) for three values of the load factor, mainly regarding the wind energy and photovoltaic power plants.
Renewable energy capability to save carbon emissions
489
./
14
,Z,
12-
i RLF~
10 ~
i.......
RLF = 0.25
J ~
..... .ik ~
0
20
~ ' ~
J
f
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RLF = 0.1
30
40
50
INTERMITTENT POWER GRID PENETRATION (%)
Fig. 2. Maximum renewableenergycontribution in percentageof total fossil fuel energy needs vs. the share of intermittent electric power acceptablein grid-tied applications, for differentvalues of renewable load factor.
The lowest curve is representative of the worst case among practicable PV applications, while the highest is related to the best case of wind turbines installed in good wind sites. Looking at Fig. 2 we can agree about the fact that all the possible contribution from intermittent renewable energy sources will be located within the layer enclosed between the lowest and highest curves. The intermediate line can be considered representative of the best situation for PV systems, which can ideally occur in the case of concentrating the electric generation o f large-scale grid-tied PV systems in the sunniest countries, where R L F can reach values as high as 0.25. In this most favourable case, we can see the maximum contribution of hydrocarbon replacement, arising from intermittent PV power systems, will amount to about 12%, provided the grid-tied power can penetrate up to 50% of the total conventional installed power. Apart from considering this latter aspect as technically unlikely, because K in practice is confined within 20-25% (Milborrow, 1995) (which corresponds in Fig. 2 to an energy contribution of about 5%), the general conclusion is that, because o f the intermittent nature of the sources, the future global contribution of renewable power in terms of fossil hydrocarbon replacement could be less significant than expected and, as a consequence, the environmental improvements could also be marginal.
4. ENVIRONMENTAL SIGNIFICANCE
4.1. Necessity of energy storage The disappointing result, so clearly summarized in Fig. 2, arises substantially from the numerical results of eqns(4) and (8) as the product of three factors, H, K and [(RLF)/(CLF)], each of them smaller than 1. Therefore, any technical device which can increase the value of each parameter leads to a situation improvement. This can be achieved by introducing, into the system layout, a suitable stage of energy storage, which can stabilize the intermittence of the renewable energy flow. The opportunity of storing intermittent renewable energy, i.e., photovoltaic, in the form of chemical energy (e.g., hydrogen) is a result already acquired (Schnurnberger et al., 1988). Moreover, the present discussion intends to quantitatively indicate the necessity of this solution in relation to the logical self-consistency of renewable energy development strategy. In order to understand this point, let us stress the subject, assuming the most favourable case of the possibility of a seasonal energy storage. In this case, we can consider that the intermittent power of the renewable energy source is transformed into a fully stable power source, capable of supplying the load with the required reliability. This means that the renewable load factor is now equivalent to the conventional one, that is, [ ( R L F ) / ( C L F ) ] ~ I . At the same time, the acquired power stability allows us to
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overcome the penetration limit in grid-tied applications. In turn, a larger penetration into grid power capacity makes it possible for renewable energy to occupy more and more significant shares of the bulk power market, so K can largely increase up to values, in principle, near to unity (fully acquired substitutionality). At this point, the renewable power kWh has the same technical quality as conventional sources (same reliability). In this case, eqn (4) becomes
(Emax)/(E) ~ H
( 1O)
Therefore, the renewable contribution can, in principle, grow to the level of electricity penetration sharing. This can be considered to be a very ambitious and likely unachievable goal, nevertheless, as we are going to see, it is not yet sufficiently ahead of future global climate constraints.
4.2. Environmental necessity for synthetic ecofuel production As a matter of fact, according to the average value of Fig. 1, we can roughly assume that the H value is about 45%. So, even if electrical consumption, for the sake of absurdity, were fully satisfied with clean renewable electricity, the largest part of the remaining world energy needs would continue to be served with pollutant fossil fuel energy. Thus, the confinement of new renewable energy sources within the sector of electricity production has to be considered as an additional drawback, which can strongly limit the environmental contribution of renewable energy. This is more commonly known as the "electricity ghetto effect" (Scott, 1995), which accounts for the fact that other important energy services (e.g., heating, cooking, transportation, etc.) are better satisfied by energy carriers, other than electricity, mainly by fossil fuels. Therefore, if the renewable energy is to become a really significant solution for the environmental crisis, it must acquire the ability to escape from the "electricity ghetto" and push its expansion, in addition to the electricity market, into the fossil fuel market. The present line of biomass development, which considers extensive plant cultivation for energy production, especially those aimed at biofuel production (Laurent and Delmon, 1992), provides the chance for identifying a viable path for approaching the solution, since the biofuel cycle can already be considered in perfect agreement with this new strategic view. Profiting by the analogy with a biofuel pro-
duction line, let us consider the following possible process for producing ecofuels from the other renewable energy sources. The primary intermittent renewable energy (wind energy or sun radiation), available on the site, is captured and converted into a secondary form of energy (electrical power), which can be used in the respective market sector or addressed to a second stage, where it can be transformed to chemical energy in the form of a synthetic fuel (for example hydrogen by water electrolysis or methanol Hz-derived, or other synthetic hydrocarbons). All these products, named ecofuels or solar fuels, would be able to suitably substitute for the fossil fuels in all their energy applications without increasing the atmospheric carbon content. As a matter of fact H2 burning does not produce carbon at all and other synthetic hydrocarbons return to the atmosphere during the combustion of the CO2 sequestered during the previous synthesis. The adoption of this process allows the realization of an additional path, different from the electricity one, capable of enhancing the market expansion. According to this solution, the renewable energy can escape from the "electricity ghetto" and directly penetrate the fossil fuel market, so the substitution of ever larger amounts of pollutant energy with clean ecofuels can finally lead to the expected significant environmental benefits.
4.3. Recovery of added costs The addition of a second stage to renewable power systems for transforming and storing the intermittent renewable energy increases the final cost of energy unit. But, in principle, the added costs can be recovered by accounting for the environmental credit assignable to ecofuels. This subject is presently a matter of animated discussion among the experts, and its definition would merit a consideration certainly larger than this article allows. At any rate, summing up the first results, it can be considered that, when the external societal costs of the whole fossil fuel energy cycle are suitably internalized, the real cost of a conventional energy unit should be increased by a consistent social factor, likely ranging from 2 to 4 according to the results of different analyses (Hohmeyer, 1988; Hohmeyer and Gartner, 1992; Coiante and Barra, 1995). Since all cost projections consider the competitiveness of grid-tied renewable power kWh in the electricity market to be achievable, the social factor also represents a
Renewable energy capability to save carbon emissions
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p r o s p e c t i v e a v a i l a b l e m a r g i n for recovering the e x t r a cost o f ecofuel p r o d u c t i o n ,
tion, as significant as we expect, in o r d e r to m i t i g a t e the climate changes.
5. C O N C L U S I O N
REFERENCES
In a d d i t i o n to the d r a w b a c k linked to the low value o f surface energy density, the present d e v e l o p m e n t strategy for new r e n e w a b l e energy sources is affected by a technical l i m i t a t i o n caused b y the r a n d o m i n t e r m i t t e n t n a t u r e o f p o w e r generation. This prevents the r e n e w a b l e energy f r o m fully e x p a n d i n g into the electricity m a r k e t . A s a consequence, the r e n e w a b l e energy c o n t r i b u t i o n to w o r l d electricity g e n e r a t i o n will be small. In a d d i t i o n , since the r e n e w a b l e energy p r o d u c t i o n is m a i n l y confined within the electricity sector a n d this l a t t e r a c c o u n t s only for a b o u t 45% o f w o r l d energy c o n s u m p t i o n , the c o n t r i b u t i o n shall be c o n s i d e r e d ever m o r e m a r g i n a l in terms o f t o t a l energy balance. Thus, in c o n t r a s t to the e x p e c t a t i o n o f a consistent e n v i r o n m e n t a l recovery f r o m r e n e w a b l e energy a p p l i c a t i o n s , the p r a c t i c a l achievable a m o u n t o f e n v i r o n m e n t a l benefit will also be m a r g i n a l . T h e b a r r i e r limiting the e x p a n s i o n o f renewa b l e energy s h o u l d be r e m o v e d b y i m p l e m e n t i n g the grid-tied p o w e r systems, which the present m a r k e t strategy is b a s e d on, a d d i n g a further stage a i m e d at chemically storing the intermittent solar energy a n d p r o d u c i n g clean synthetic fuels. T h e e x t r a costs, i n c u r r e d b y this solution, can be recovered by a c c o u n t i n g for the environm e n t a l benefit. In conclusion, the possibility o f m a k i n g large a m o u n t s o f s u b s t i t u t i o n a l ecofuels, arising f r o m i n t e r m i t t e n t r e n e w a b l e energy, available in the fuel m a r k e t has to be c o n s i d e r e d as a necessary c o n d i t i o n for the r e n e w a b l e energies to really m a k e a c o n t r i b u -
BP (1992) Statistical Review o f Worm Energy 1992. British Petroleum Company, London. Coiante D. and Barra L. (1992) Can photovoltaics become an effective energy option? Solar Energy Mater. Solar Cells, 27, 79-89. Coiante D. and Barra L. (1995) A practical method to evaluate the real cost of electrical energy. Int. J. Energy Res., 19, 159-168. Hohmeyer O. (1988) Social Costs o f Energy Consumption. Springer, Heidelberg, pp. 103-104. Hohmeyer O. and Gartner E. (1992) The Costs o f Climate Change. Commission of the European Community Report, Fraunhofer Institute for Systems and Innovation Research, Freiburg. Kurihara I. (1993) Capacity value evaluation of photovoltaic power generation. In Proc. Advisory Group Meeting IAEA, 26-28 April 1993, Vienna, pp. 43-59. Laurent E. and Delmon B. (1992) Feasibility of the partial and full hydrorefining of bio-oils produced by rapid thermal pyrolysis studies with model compounds. In
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Proc. 7th E.C. Int. Conf. on Biomass for Energy and Industry, Florence, 1992, Hall D. O., Grassi G. and
Scheer H. (Eds). Ponte Press, Bochum (GE), pp. 177-185. Milborrow D. (1995) What happens when the wind stops blowing? Wind Directions, 14, 7-9. Schnurnberger W., Seeger W. and Steeb H. (1988) Selected technical hydrogen production systems. In Hydrogen as an Energy Carrier, Winter C. J. and Nitsch J. (Eds). Springer, Berlin, pp. 209-248. Scott D. S. (1995) Interpreting the architecture of the energy system. In Proc. WEC 16th Congress, WEC Committees (Eds). Tokyo, Japan, pp. 89-108. Taschini A. and Iannucci J. J. (1992) Potential of photovoltaic systems for present and future electric utility applications. In Proc. IEA/ENEL/Pacific Gas & Electric Executive Conf. on Photovoltaic Systems for Electric Utility Applications, 2-5 December, 1990, Taormina, Italy,
OECD (Eds), pp. 51-60. Van Wijk A. J. M. (1992). Capacity credit of wind power in The Netherlands. Electric Power System Res., 23, 189-193. World Energy Council (1994) New Renewable Energy Resources. Kogan Page, London, pp. 120 121.