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Distributed Renewable Energy Systems for Rural Applications
Cop),,';);ht © IF.-\C 11th Tr;enn;,, 1 Wo rld Congress, T " llinn, Eston;", l 'SSR , I'I<)()
DISTRIBUTED RENEWABLE ENERGY SYSTEMS FOR RURAL APPLICATIONS P. K. Patwardhan 328 Linking R oad, Khar, BOil/ba.\', -IOU U52 , India
Abstract. One of the major prob Iems faced in remote areas is the supp Iy of conventional electricity and this is particularly true of some 5,60,000 villages which exist in a country like India, The conventional distribution techniques are too expensive and inadequate. Th i s Paper presents State-of-Art in var i ous strateg i es adopted for providing e nergy sources in such areas. One specific area has been developed in detail. Solar Thermal area, which is particularly suited for a country like India . This provides stand-alone and distributed energy sourc e, not only for application in hinter land and rural areas, but could be used also for large complexes of human habitat. The paper would give details of current work in this area and some of the new proposals which are suggested, which could contribute to a high degree in the overall energy generation in the country. Currently employed are conventional means, such as, Hydro / Thermal and Nuclear. Contribution from alternative sources could be as high as 1% of the total energy generation by the year 2000 . The advantages of such systems in the context of environment protection and preservation of ecological balance would also be highlighted. Keywords. Solar energy, developing countries, heliostats, micro-processor steam turbines, molten salts, modeling, composites, pollution, eco-system.
these projects. indigenously.
INTRODUCTION Per capita consumption of total energy of any country has a direct correlation with its Gross Domestic Product and is thus a measure of its economic development. In India , nearly 40% of tota I energy requ i rements are met from non-commercial source like fire wood, agricultural waste, cow-dung, animal power and manual labour. The remaining 60% is classified as commercial energy. About 30% of this is contributed by electricity wh ich is a I most 18 % of the tota I energy requ i rement. India has kept up a reasonably good growth record for the power sector.
Thermal
Expertise
is
control,
fully
available
(Coal)
Amongst non-renewables are coal and nuclear. Of the 112 bi II ion tonnes of coal reserves estimated, 26 billion are proven and only 24 billion good coking grade coal . 50,000 MW(e) of thermal power is planned, for situating these at pit heads, consum i ng 160 mill i on tonn e s annua 11 y . Interestingly, a 1000 MW(e) thermal station operat ing at 60% capac i ty factor needs over 10,000 tonnes of coal daily which corresponds to a del ivery of 5 to 7 train loads every day! The environmental problems of coal are also ser ious. Because of SO, hazard, acid rain and also the increased atmospheri c content of CO , lead i ng to an i rrevers i b Ie Gre e nhouse effect. An increase of even 2°C in global temperature can lead to disastrous consequences.
Even though the e Iectr i ca I energy generated every year has multiplied nearly thirty-fold during the last three decades , the country's per capita commercial energy consumption is less than a tenth of the world average . Not surprisingly, our economic growth has resulted in the demand for electricity outstripping its avai labi I ity . Today the shortfall is estimated to be nearly 10 to 15%.
Nuclear: Under nuclear option an ambitious program of 10,000 MW(e) is planned by year 2000 A.D. It is c Iaimed that all poss i b Ie safety aspects against radiation have bee n implemented with minimal risk to the population . This will include ultimate large scale power generation through Fast Breeder Technology.
India's Ambitious Programme An ambitious programme is on for trebling this capacity raising it to 120,000 to 150,000 MW(e) by 2000 A.D. This will include utilisation of all forms of energy options, including renewable sources, in which hydro power tops the list.
RENEWABLE SOURCES OF ENERGY Hydro Power Now let sources.
Of the 75,000 MW(e) potential only 13,000 MW(e) is tapped . It is planned to increase to 21,000 to 40,000 MW(e) (30 - 50%) by the 1990's . The main problems are long gestation periods 6 - 10 years, in addition to mUltipurpose nature of
us
look
at
the
scenario
of
renewable
Renewab I e energy sources have attracted i nterest and attention over the past decade and promise to play a , growing role in the world's
32 1
energy futu re. Factors lead i ng to increased awareness and need of the renewables include :
-
Heliostat operations / safety
-
Composite beam power and flux density
*
-
Power trans i ents
Non-vulnerabi I ity Lower Prices
to
supply
Distributions
- Targeted sun tracking
*
Econom i es of Sca Ie
*
Advances in New Energy Technologies
*
Non-Finite & Diversified
*
Modularity & Small Scale
*
Oil
*
Displacement
and
and
Resource
Conservation .
-
Mi rror reflectance
-
Ma i ntenance
-
Heliostat control electronics
-
Encoders
-
Or i ve motors
-
Mi rror
-
Computer control of elec . power.
faces
and
Likely Public" Investor support. Similar Centres are a I so ope rat i ng in Japan, France as well as in Spain and have provided very valuable experience on their uti I ity and performance.
The above considerations are applicable to both Photo-thermal as well as Photo-voltaice devices. In this Paper we will only consider possible applications of Solar Thermal. In this category, there are three compet i ng techno Iog i es.
*
Central Receiver
*
Parabolic Dish Systems
*
Parabol ic Trough
There have been adverse views on the quest i on whether r e newable sources of energy are readi Iy avai lable at competitive costs. Some of these it appears have been dispeled by the recent work on the So lar Therma I Systems and has bu i I t up a certain degree of confidence in this energy alternativ e. POTENTIAL CONTRIBUTION OF RENEWABLE ENERGY RESOURCES
Parabolic Dish Type with Point Focus : These are modular and can provide high efficiency electrical power in the range of 25 to 50 KW per module.
The earth receives som e 178,000 TeraWatt-year (5 . 4 mill ion quads) per year of therma I energy from the sun. Another 35 TwYrlyr (1000 quads/yr) of energy enters the atmosphere and the oceans from the bedrock of the Earth. The tidal movements in the oceans r e present some 3 TWyr / yr (90 quads / yr) of energy. By comparison, the world consumed some 9.6 TWyr/yr (290 quads) of energy in 1980.
Parabol ic Trough Collectors : These are su i ted for process heat in industrial applications, whereas dish type could be used as I imited power generating stations. Central Receiver
Limitations : The third type are Earth receiving Solar Collectors, which employ a Single Receiver on the top of a tower, on which solar I ight is focussed by a large number of movable two-axis tracking mirrors or, hel iostats spaced over a large area concentrat i ng sol ar energy on the tower. Th i s is particularly suited for a tropical country like India, where many areas have 10 to 12 hours of sunshine round the year.
Our abi I ity to harness these energy sources, however, is limited by the low energy supply densities of renewables, their geographical distribution and the competing uses of land and other resources. Much of this energy is so diffuse as to make energy convers ion techn i ca 11 y i mpractical or, economically unattractive for years to come. 12 wattOne TeraWatt-year (lTWyr) = 10 Note --years of energy; 1 TWyr / yr 30 quadrillion BTUs (quads) = 30 quads; 1 quad = 10 15 BTU).
These have very high operating temperatures in the range of 1000· to 1500 0 C or even higher and can provide anywhere between 1 MW electrical to 100 to 200 MW electrical power. Coupled with suitable storage systems, this third type could be very suitable for applications in remote areas and even in large clusters of human dwell ings to prov i de e Iectri ca I energy. These are i nexpensive, relatively easy to operate and are pollution free and hence do not cause any env i ronmenta I hazards. Considerable work has nal Laboratory USA in US and elsewhere tantial amount of Plant
-
Hel iostats Mechanical systems
-
Electrical systems
-
Controls
-
Focus and al ignment
In this regard very us e ful material has become available under the National Solar Thermal Technology Programme under the auspices at U.S. Department of Energy, office of Conservation and Renewable Energy. Washington D. C . (A five year R&D profile 1986-1990. Some of their findings are covered in the following pag e s.
been done at SANDIA Natio and also in other locations which have provided subsOperating Experience.
The following components and sub-systems received special attention for performance luation :
-
OPERATIONAL EXPERIENCE AND PEFORMANCE EVALUATION OF CENTRAL RECEIVER AND OTHER SOLAR THERMAL SYSTEMS
have eva-
Highlights:
322
*
Those research activities which cut across all of the solar thermal options, such as mater i a I s and concentrator deve Iopment, are especially important so also Technology Transfer activities.
*
Hel iostats upto 150 square meters have been built and systems containing more than 1800 hel iostats have been constructed and operated.
*
*
The abi I ity of solar thermal technology to in increments provide production capacities of anywhere from 10 to 100 Megawatts wi II be a key advantage.
po 11 utants. to $15 / m'.
Within the central receiver option, system concepts utilizing water / steam, molten nitrate salt, and liquid sodium as heat-transfer fluids have reached a good level of understanding. The earl iest and most detai led work has been on water / steam. The 1 O-MW (e) Central Receiver Pilot Plant at Barstow, California (Solar One) has demonstrated the concept of electric power product ion us i ng the steam-Rank i ne convers i on cycle with water serv ing as the receiver heat transfer fluid. 750-kWe Albuquerque, New Mexico, the Centra I Receiver Molten Sa I t Electric Experi ment (MSEE) has demonstrated the technical feasibility of mol ten salt as a heat transfer fluid.
The Luz trough / electric installation in Barstow, California, the first phase (13.8 MWe net) of what is already the world's largest solar thermal electricity generating station, has recently initiated operation. The system uses strings of evacuated-tube, insulated receivers which raise the heat transfer fluid temperature to 310°C before it enters the steam generator. A fossi I hybrid boost is used to superheat the steam and the efficiency of the system.
*
Some activities span almost the entire Solar Thermal Technology area in appl icabi I ity (e.g. stressed membrane concentrator research) and hence deserve extra emphas is.
are
like Iy
to
come
down
Most of these receivers operate at temperatures Iess than 600°C, with annua I effi cienc i es between 75 and 90 %. $80 / m' for central receivers. Central receiver research is focused on finding the best cost / performance systems (and hence receivers) in two d i st i nct ranges upto 600 ° C, and greater than 800°C. Above 800 ° C other receivers with heat transfer media such as sol id particles and carbonate salts are being investigated, and basic receiver element testing has been initiated.
While salt and sodium are not as widely used as a heat transfer fluid as water, they have severa I advantages. The i r better heat transfer characteristics allow higher flux (and hence smaller and more efficient receivers) and the fluids need not vaporize. Perhaps their biggest advantage is that they can be used directly as a very efficient thermal energy storage medium.
*
costs
2. Concentrators. For central receiver concentrators, the advent of a lightweight flexible polymer or sol-gel surface reflective material could reduce large (150m') stressed membrane hel iostat costs to $40 / m', just 1 / 10th of the cost of the heliostats at Solar One, if manufactured in large quantities. Reductions in weight and wind loading may allow the major concentrator structural elements to be fabricated from composite materials. Research on concentrator drive mechanisms is also being conducted; a 50% cost reduction in drives for the next generation of concentrators is anticipated from this effort.
* In
*
The
3. Energy Convers i on Techno Iogy. The pr i nc i pa I tasks (1) the investigation of direct conversion dev ices, and (2) the deve Iopment of lower-cost, higher-efficiency energy transport and storage. Recent heat engine comparison studies suggest that Ki nemat ic-St i r ling and organ i c-Rank i ne cyclebased systems exhibit the potential of meeting near-term cost goals. Also the direct conversion I iquid metal thermo-electric and thermochemical processes have shown particular promise of meeting long term solar thermal cost goals and are the object of an ongoing development and testing program. 4. Transport and storage. The primary objective of the transport and storage task is the development of h igh-effi c i ency • The goa I s of the tests are to resolve technical uncertainties related to molten salt sub-syst e ms and components and to provide a technical base for molten salt solar plants. Storage systems utilizing oil/rock, liquid sod i um and mo Iten nitrate sa I t have a 11 been successfully demonstrated.
Central Receiver / Electric: Current status for central receiver / electric is based upon a 100-MWe rated capacity, a 50% capac i ty factor, and an annua I effi c i ency of 17%. Two collector fields contain a total of 9300 95m' glass / metal heliostats costing $150 / m'. Two molten salt cavity receivers are used, each with a maximum rating of 320 MWt. Receiver outlet temperature is 560°C, and receiver costs are $80 / m'. A molten salt/steam heat exchanger provides super-heated steam to dr i ve the steam-Rank i ne turbine-generator. A dual-tank hot and cold molten salt thermal storage system has a capacity of 2600 MWht. Total overnight installed system cost is $2900 / kWe peak.
Proveness and Innovations It is believed that solar thermal systems in the i r present state have demons trated the techn i ca I capabi I ities and potential to become viable economic options in the 1990's.
*
Some of the i nnovat i ve concepts cu rrent Iy being investigated are a holographic concentrator, a number of collector wind avoidance approaches, and photo-enhanced catal ysi s. Ho Iograph i c concentrators with no mov i ng parts that are able to track the sun and concentrate the sun's rays appear feasible.
New Developments The investigation of photo-enhanced catalysi s and solar-assisted bond breaking at the University of Houston, the solar production of fuels and chemicals at the University of New Hampsh i re, and the sol ar detox i fi cat i on of hazardous wastes at the Un i vers i ty of Day ton . These efforts will provide the technology base to assess the use of concentrated sunl ight in chemical conversion. The end result may be important new app I i cat ions of sol ar energy for the manufacture of fuels, chemicals and electricity.
1. Optical materials. The current focus is on devel.op ing silvered pol ymer and si Ivered steel reflective surfaces which wi II match the performance and durabi I ity of state-of-the-art laminated silvered glass technology. Reducing the wei ght will cut costs by ha I f assoc i ated with silvered glass, additional benefits will accrue due to ease of fabrication, installation and hand ling. For polymer reflectors, research and env i ronmental testing are underway to characterize degradation mechanisms induced by ultraviolet radiation as we II as effects caused by water, air and
* 323
Automatic controls plant performance
will also allow maximum during startup, shutdown,
and periods cloudiness.
*
of
transient
conditions
such
*
as
Ope rat i ng schemes and clean i ng mechan isms are under deve Iopment to m i n i m i ze losses due to reflectivity and absorptivity degradation and to reduce the costs of cleaning and mainta i n i ng mirrors and rece i ver su rfaces .
Cost Estimates and Future Profi le Current capabi I ities for central receiver electric systems are an annua I effi c i ency and cap i ta I cost of 17% and $2900 1 kWe peak. The long term goal is 22 % annual efficiency and $1000/kWe peak and the near-term (five-year) goal is to achieve a capital cost reduction to $1800/kWe peak while improving system annual efficiency to 20 %.
TECHNOLOGY PRIORITY On the basis of Performance, it is essential to consider seriously this energy alternative on pr i or i ty bas is. Further work of system eng ineering employing better techniques and modelling, efficient control systems, material study, particularly for appl ications in storage of energy, rapid heat transport systems, development of better materials to replace glass such, as stretched membrane materials, which will provide light weight, inexpensive tool ing and ease for manufacture are some of the areas which would require further attention . The possibi I ity of connecting of such electrical Generating Plants, with National Grid would lead to advantages in terms of providing peaking power in load flow management. In the mid-21st century by which time most of the fossil resources would be highly depleted, Solar energy via Solar Thermal route, appears to be a very attractive electrical energy development alternative. A Developing country I ike India should accord it the status of a Technology Thrust Area.
For intermediate-load coal plants (capacity factory range of 0.4 to 0.5), the Ieve I i zed energy cost can be seen to range from si ightly under $0.501 kWh to nearly $0.08 / kWh . The solar thermal system goal selected for electric appl ications was SO.05 / kWh. Some of the Solar Thermal Operating Plants and their Performance: The pace-setting French program culminated in the unprecedented 1 MW solar thermal furnace at Odeillo in the eastern Pyrenees. This innovative faci I ity was designed and is till used for experiments requiring extremely high temperatures (upto 4000 0 C) in exceptionally clean environments. The Odeillo facility was the first solar thermal faci I ity to produce electricity whi le connected to a utility grid. It was also the first facility to use a field of free-standing heliostate operating und e r automatic control. Solar One Performance: Location Barstow, California, U. S.A. Solar has exceeded many of its original design performance specifications.
During its Experimental Test and Evaluation Phase, Solar One was successfully operated in all its steady-state operating modes. Transitions to and from each steady-state mode were accomplished, and, during the course of test i ng, sign i fi cant improvements were made in the plant I s start-up and shutdown times. Considering the first-of-a-kind nature of the plant and the high level of technology involved, Solar One has operated very well.
One and
* The requirement for production of 10 megawattselectric net was exceeded by a peak production of 12.1 Megawatts. Similarly, the required 7 megawatts net generation from storage was exceeded by an output of 7.3 megawatts. The plant has also successfully operated down to 0.5 megawatts which is considerably lower than the des i gned m i n i mum operat i ng producti on level of 2 megawatts. The minimum sunlight threshold for operation was designed as 450 watts per square metre, yet the plant has operated in direct solar radiation levels as low as 300 watts per square meter. In an e ndurance test, the receiver and storage systems kept the turbine continuously on-line for 33.6 hours and generated 127 megawatthours net. Solar One was designed to have 95 % of the heliostats available at anyone time. Between April 1982 and April 1983, 98 % of the heliostats were ava i Iab Ie for ope rat i on. Th i s percentage increased to 99 % du ring Apr i I 1983. The establ i shment of the sharp thermal gradient (thermocl ine) needed for the storage system has been verified. Gradients of 49°C I meter have been measured. Equally important is the very low rate of heat loss from the storage tank. The tank heat loss has been measured at 1.3%/ day .
324
DISTRIBUTED RENEWABLE ENERGY SYSTEMS FOR RURAL APPLICATIONS P. K. Patwardhan 328 Linking R oad, Khar, BOil/ba.\', -IOU U52 , India
Abstract. One of the major prob Iems faced in remote areas is the supp Iy of conventional electricity and this is particularly true of some 5,60,000 villages which exist in a country like India, The conventional distribution techniques are too expensive and inadequate. Th i s Paper presents State-of-Art in var i ous strateg i es adopted for providing e nergy sources in such areas. One specific area has been developed in detail. Solar Thermal area, which is particularly suited for a country like India . This provides stand-alone and distributed energy sourc e, not only for application in hinter land and rural areas, but could be used also for large complexes of human habitat. The paper would give details of current work in this area and some of the new proposals which are suggested, which could contribute to a high degree in the overall energy generation in the country. Currently employed are conventional means, such as, Hydro / Thermal and Nuclear. Contribution from alternative sources could be as high as 1% of the total energy generation by the year 2000 . The advantages of such systems in the context of environment protection and preservation of ecological balance would also be highlighted. Keywords. Solar energy, developing countries, heliostats, micro-processor steam turbines, molten salts, modeling, composites, pollution, eco-system.
these projects. indigenously.
INTRODUCTION Per capita consumption of total energy of any country has a direct correlation with its Gross Domestic Product and is thus a measure of its economic development. In India , nearly 40% of tota I energy requ i rements are met from non-commercial source like fire wood, agricultural waste, cow-dung, animal power and manual labour. The remaining 60% is classified as commercial energy. About 30% of this is contributed by electricity wh ich is a I most 18 % of the tota I energy requ i rement. India has kept up a reasonably good growth record for the power sector.
Thermal
Expertise
is
control,
fully
available
(Coal)
Amongst non-renewables are coal and nuclear. Of the 112 bi II ion tonnes of coal reserves estimated, 26 billion are proven and only 24 billion good coking grade coal . 50,000 MW(e) of thermal power is planned, for situating these at pit heads, consum i ng 160 mill i on tonn e s annua 11 y . Interestingly, a 1000 MW(e) thermal station operat ing at 60% capac i ty factor needs over 10,000 tonnes of coal daily which corresponds to a del ivery of 5 to 7 train loads every day! The environmental problems of coal are also ser ious. Because of SO, hazard, acid rain and also the increased atmospheri c content of CO , lead i ng to an i rrevers i b Ie Gre e nhouse effect. An increase of even 2°C in global temperature can lead to disastrous consequences.
Even though the e Iectr i ca I energy generated every year has multiplied nearly thirty-fold during the last three decades , the country's per capita commercial energy consumption is less than a tenth of the world average . Not surprisingly, our economic growth has resulted in the demand for electricity outstripping its avai labi I ity . Today the shortfall is estimated to be nearly 10 to 15%.
Nuclear: Under nuclear option an ambitious program of 10,000 MW(e) is planned by year 2000 A.D. It is c Iaimed that all poss i b Ie safety aspects against radiation have bee n implemented with minimal risk to the population . This will include ultimate large scale power generation through Fast Breeder Technology.
India's Ambitious Programme An ambitious programme is on for trebling this capacity raising it to 120,000 to 150,000 MW(e) by 2000 A.D. This will include utilisation of all forms of energy options, including renewable sources, in which hydro power tops the list.
RENEWABLE SOURCES OF ENERGY Hydro Power Now let sources.
Of the 75,000 MW(e) potential only 13,000 MW(e) is tapped . It is planned to increase to 21,000 to 40,000 MW(e) (30 - 50%) by the 1990's . The main problems are long gestation periods 6 - 10 years, in addition to mUltipurpose nature of
us
look
at
the
scenario
of
renewable
Renewab I e energy sources have attracted i nterest and attention over the past decade and promise to play a , growing role in the world's
32 1
energy futu re. Factors lead i ng to increased awareness and need of the renewables include :
-
Heliostat operations / safety
-
Composite beam power and flux density
*
-
Power trans i ents
Non-vulnerabi I ity Lower Prices
to
supply
Distributions
- Targeted sun tracking
*
Econom i es of Sca Ie
*
Advances in New Energy Technologies
*
Non-Finite & Diversified
*
Modularity & Small Scale
*
Oil
*
Displacement
and
and
Resource
Conservation .
-
Mi rror reflectance
-
Ma i ntenance
-
Heliostat control electronics
-
Encoders
-
Or i ve motors
-
Mi rror
-
Computer control of elec . power.
faces
and
Likely Public" Investor support. Similar Centres are a I so ope rat i ng in Japan, France as well as in Spain and have provided very valuable experience on their uti I ity and performance.
The above considerations are applicable to both Photo-thermal as well as Photo-voltaice devices. In this Paper we will only consider possible applications of Solar Thermal. In this category, there are three compet i ng techno Iog i es.
*
Central Receiver
*
Parabolic Dish Systems
*
Parabol ic Trough
There have been adverse views on the quest i on whether r e newable sources of energy are readi Iy avai lable at competitive costs. Some of these it appears have been dispeled by the recent work on the So lar Therma I Systems and has bu i I t up a certain degree of confidence in this energy alternativ e. POTENTIAL CONTRIBUTION OF RENEWABLE ENERGY RESOURCES
Parabolic Dish Type with Point Focus : These are modular and can provide high efficiency electrical power in the range of 25 to 50 KW per module.
The earth receives som e 178,000 TeraWatt-year (5 . 4 mill ion quads) per year of therma I energy from the sun. Another 35 TwYrlyr (1000 quads/yr) of energy enters the atmosphere and the oceans from the bedrock of the Earth. The tidal movements in the oceans r e present some 3 TWyr / yr (90 quads / yr) of energy. By comparison, the world consumed some 9.6 TWyr/yr (290 quads) of energy in 1980.
Parabol ic Trough Collectors : These are su i ted for process heat in industrial applications, whereas dish type could be used as I imited power generating stations. Central Receiver
Limitations : The third type are Earth receiving Solar Collectors, which employ a Single Receiver on the top of a tower, on which solar I ight is focussed by a large number of movable two-axis tracking mirrors or, hel iostats spaced over a large area concentrat i ng sol ar energy on the tower. Th i s is particularly suited for a tropical country like India, where many areas have 10 to 12 hours of sunshine round the year.
Our abi I ity to harness these energy sources, however, is limited by the low energy supply densities of renewables, their geographical distribution and the competing uses of land and other resources. Much of this energy is so diffuse as to make energy convers ion techn i ca 11 y i mpractical or, economically unattractive for years to come. 12 wattOne TeraWatt-year (lTWyr) = 10 Note --years of energy; 1 TWyr / yr 30 quadrillion BTUs (quads) = 30 quads; 1 quad = 10 15 BTU).
These have very high operating temperatures in the range of 1000· to 1500 0 C or even higher and can provide anywhere between 1 MW electrical to 100 to 200 MW electrical power. Coupled with suitable storage systems, this third type could be very suitable for applications in remote areas and even in large clusters of human dwell ings to prov i de e Iectri ca I energy. These are i nexpensive, relatively easy to operate and are pollution free and hence do not cause any env i ronmenta I hazards. Considerable work has nal Laboratory USA in US and elsewhere tantial amount of Plant
-
Hel iostats Mechanical systems
-
Electrical systems
-
Controls
-
Focus and al ignment
In this regard very us e ful material has become available under the National Solar Thermal Technology Programme under the auspices at U.S. Department of Energy, office of Conservation and Renewable Energy. Washington D. C . (A five year R&D profile 1986-1990. Some of their findings are covered in the following pag e s.
been done at SANDIA Natio and also in other locations which have provided subsOperating Experience.
The following components and sub-systems received special attention for performance luation :
-
OPERATIONAL EXPERIENCE AND PEFORMANCE EVALUATION OF CENTRAL RECEIVER AND OTHER SOLAR THERMAL SYSTEMS
have eva-
Highlights:
322
*
Those research activities which cut across all of the solar thermal options, such as mater i a I s and concentrator deve Iopment, are especially important so also Technology Transfer activities.
*
Hel iostats upto 150 square meters have been built and systems containing more than 1800 hel iostats have been constructed and operated.
*
*
The abi I ity of solar thermal technology to in increments provide production capacities of anywhere from 10 to 100 Megawatts wi II be a key advantage.
po 11 utants. to $15 / m'.
Within the central receiver option, system concepts utilizing water / steam, molten nitrate salt, and liquid sodium as heat-transfer fluids have reached a good level of understanding. The earl iest and most detai led work has been on water / steam. The 1 O-MW (e) Central Receiver Pilot Plant at Barstow, California (Solar One) has demonstrated the concept of electric power product ion us i ng the steam-Rank i ne convers i on cycle with water serv ing as the receiver heat transfer fluid. 750-kWe Albuquerque, New Mexico, the Centra I Receiver Molten Sa I t Electric Experi ment (MSEE) has demonstrated the technical feasibility of mol ten salt as a heat transfer fluid.
The Luz trough / electric installation in Barstow, California, the first phase (13.8 MWe net) of what is already the world's largest solar thermal electricity generating station, has recently initiated operation. The system uses strings of evacuated-tube, insulated receivers which raise the heat transfer fluid temperature to 310°C before it enters the steam generator. A fossi I hybrid boost is used to superheat the steam and the efficiency of the system.
*
Some activities span almost the entire Solar Thermal Technology area in appl icabi I ity (e.g. stressed membrane concentrator research) and hence deserve extra emphas is.
are
like Iy
to
come
down
Most of these receivers operate at temperatures Iess than 600°C, with annua I effi cienc i es between 75 and 90 %. $80 / m' for central receivers. Central receiver research is focused on finding the best cost / performance systems (and hence receivers) in two d i st i nct ranges upto 600 ° C, and greater than 800°C. Above 800 ° C other receivers with heat transfer media such as sol id particles and carbonate salts are being investigated, and basic receiver element testing has been initiated.
While salt and sodium are not as widely used as a heat transfer fluid as water, they have severa I advantages. The i r better heat transfer characteristics allow higher flux (and hence smaller and more efficient receivers) and the fluids need not vaporize. Perhaps their biggest advantage is that they can be used directly as a very efficient thermal energy storage medium.
*
costs
2. Concentrators. For central receiver concentrators, the advent of a lightweight flexible polymer or sol-gel surface reflective material could reduce large (150m') stressed membrane hel iostat costs to $40 / m', just 1 / 10th of the cost of the heliostats at Solar One, if manufactured in large quantities. Reductions in weight and wind loading may allow the major concentrator structural elements to be fabricated from composite materials. Research on concentrator drive mechanisms is also being conducted; a 50% cost reduction in drives for the next generation of concentrators is anticipated from this effort.
* In
*
The
3. Energy Convers i on Techno Iogy. The pr i nc i pa I tasks (1) the investigation of direct conversion dev ices, and (2) the deve Iopment of lower-cost, higher-efficiency energy transport and storage. Recent heat engine comparison studies suggest that Ki nemat ic-St i r ling and organ i c-Rank i ne cyclebased systems exhibit the potential of meeting near-term cost goals. Also the direct conversion I iquid metal thermo-electric and thermochemical processes have shown particular promise of meeting long term solar thermal cost goals and are the object of an ongoing development and testing program. 4. Transport and storage. The primary objective of the transport and storage task is the development of h igh-effi c i ency • The goa I s of the tests are to resolve technical uncertainties related to molten salt sub-syst e ms and components and to provide a technical base for molten salt solar plants. Storage systems utilizing oil/rock, liquid sod i um and mo Iten nitrate sa I t have a 11 been successfully demonstrated.
Central Receiver / Electric: Current status for central receiver / electric is based upon a 100-MWe rated capacity, a 50% capac i ty factor, and an annua I effi c i ency of 17%. Two collector fields contain a total of 9300 95m' glass / metal heliostats costing $150 / m'. Two molten salt cavity receivers are used, each with a maximum rating of 320 MWt. Receiver outlet temperature is 560°C, and receiver costs are $80 / m'. A molten salt/steam heat exchanger provides super-heated steam to dr i ve the steam-Rank i ne turbine-generator. A dual-tank hot and cold molten salt thermal storage system has a capacity of 2600 MWht. Total overnight installed system cost is $2900 / kWe peak.
Proveness and Innovations It is believed that solar thermal systems in the i r present state have demons trated the techn i ca I capabi I ities and potential to become viable economic options in the 1990's.
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Some of the i nnovat i ve concepts cu rrent Iy being investigated are a holographic concentrator, a number of collector wind avoidance approaches, and photo-enhanced catal ysi s. Ho Iograph i c concentrators with no mov i ng parts that are able to track the sun and concentrate the sun's rays appear feasible.
New Developments The investigation of photo-enhanced catalysi s and solar-assisted bond breaking at the University of Houston, the solar production of fuels and chemicals at the University of New Hampsh i re, and the sol ar detox i fi cat i on of hazardous wastes at the Un i vers i ty of Day ton . These efforts will provide the technology base to assess the use of concentrated sunl ight in chemical conversion. The end result may be important new app I i cat ions of sol ar energy for the manufacture of fuels, chemicals and electricity.
1. Optical materials. The current focus is on devel.op ing silvered pol ymer and si Ivered steel reflective surfaces which wi II match the performance and durabi I ity of state-of-the-art laminated silvered glass technology. Reducing the wei ght will cut costs by ha I f assoc i ated with silvered glass, additional benefits will accrue due to ease of fabrication, installation and hand ling. For polymer reflectors, research and env i ronmental testing are underway to characterize degradation mechanisms induced by ultraviolet radiation as we II as effects caused by water, air and
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Automatic controls plant performance
will also allow maximum during startup, shutdown,
and periods cloudiness.
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Ope rat i ng schemes and clean i ng mechan isms are under deve Iopment to m i n i m i ze losses due to reflectivity and absorptivity degradation and to reduce the costs of cleaning and mainta i n i ng mirrors and rece i ver su rfaces .
Cost Estimates and Future Profi le Current capabi I ities for central receiver electric systems are an annua I effi c i ency and cap i ta I cost of 17% and $2900 1 kWe peak. The long term goal is 22 % annual efficiency and $1000/kWe peak and the near-term (five-year) goal is to achieve a capital cost reduction to $1800/kWe peak while improving system annual efficiency to 20 %.
TECHNOLOGY PRIORITY On the basis of Performance, it is essential to consider seriously this energy alternative on pr i or i ty bas is. Further work of system eng ineering employing better techniques and modelling, efficient control systems, material study, particularly for appl ications in storage of energy, rapid heat transport systems, development of better materials to replace glass such, as stretched membrane materials, which will provide light weight, inexpensive tool ing and ease for manufacture are some of the areas which would require further attention . The possibi I ity of connecting of such electrical Generating Plants, with National Grid would lead to advantages in terms of providing peaking power in load flow management. In the mid-21st century by which time most of the fossil resources would be highly depleted, Solar energy via Solar Thermal route, appears to be a very attractive electrical energy development alternative. A Developing country I ike India should accord it the status of a Technology Thrust Area.
For intermediate-load coal plants (capacity factory range of 0.4 to 0.5), the Ieve I i zed energy cost can be seen to range from si ightly under $0.501 kWh to nearly $0.08 / kWh . The solar thermal system goal selected for electric appl ications was SO.05 / kWh. Some of the Solar Thermal Operating Plants and their Performance: The pace-setting French program culminated in the unprecedented 1 MW solar thermal furnace at Odeillo in the eastern Pyrenees. This innovative faci I ity was designed and is till used for experiments requiring extremely high temperatures (upto 4000 0 C) in exceptionally clean environments. The Odeillo facility was the first solar thermal faci I ity to produce electricity whi le connected to a utility grid. It was also the first facility to use a field of free-standing heliostate operating und e r automatic control. Solar One Performance: Location Barstow, California, U. S.A. Solar has exceeded many of its original design performance specifications.
During its Experimental Test and Evaluation Phase, Solar One was successfully operated in all its steady-state operating modes. Transitions to and from each steady-state mode were accomplished, and, during the course of test i ng, sign i fi cant improvements were made in the plant I s start-up and shutdown times. Considering the first-of-a-kind nature of the plant and the high level of technology involved, Solar One has operated very well.
One and
* The requirement for production of 10 megawattselectric net was exceeded by a peak production of 12.1 Megawatts. Similarly, the required 7 megawatts net generation from storage was exceeded by an output of 7.3 megawatts. The plant has also successfully operated down to 0.5 megawatts which is considerably lower than the des i gned m i n i mum operat i ng producti on level of 2 megawatts. The minimum sunlight threshold for operation was designed as 450 watts per square metre, yet the plant has operated in direct solar radiation levels as low as 300 watts per square meter. In an e ndurance test, the receiver and storage systems kept the turbine continuously on-line for 33.6 hours and generated 127 megawatthours net. Solar One was designed to have 95 % of the heliostats available at anyone time. Between April 1982 and April 1983, 98 % of the heliostats were ava i Iab Ie for ope rat i on. Th i s percentage increased to 99 % du ring Apr i I 1983. The establ i shment of the sharp thermal gradient (thermocl ine) needed for the storage system has been verified. Gradients of 49°C I meter have been measured. Equally important is the very low rate of heat loss from the storage tank. The tank heat loss has been measured at 1.3%/ day .
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