194 LAND REQUIREMENTS FOR SOLAR AND COAL OPTIONS Before we begin our actual comparison, two additional introductory statements are needed: 1 Our comparison assumes that the amount of lan
Trang 1Land Requirements for the Solar and Coal Options
Author(s): Martin J Pasqualetti and Byron A Miller
Source: The Geographical Journal, Vol 150, No 2 (Jul., 1984), pp 192-212
Published by: Blackwell Publishing on behalf of The Royal Geographical Society (with the Institute of British Geographers)
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Trang 2The Geographical Journal, Vol 150, No 2, July 1984, pp 192-212
LAND REQUIREMENTS FOR THE SOLAR AND COAL OPTIONS
MARTIN J PASQUALETTI AND BYRON A MILLER
The absolute quantity of land committed as a function of various energy decisions is
an emerging issue of significance in energy planning Since the 1973 Arab oil embargo sensitized us to the need to develop renewable energy resources, the attention which consequent land commitments has received has often been directed
at various solar options This paper reports the approach and results of a land-use comparison over the entire energy-supply cycle between centralized solar-based and centralized coal-based electricity generation facilities in Arizona In this regard, the especially significant considerations for coal generation were found to be reclamation potential, transmission distances, and cooling water sources For solar generation, the most significant considerations were packing and tracking factors, and storage strategies The comparison includes photovoltaic and power tower systems, in terms of hectare/MWe*, as well as MWh/hectare over the life of the plant Contrary to common impression, the amount of land required by coal and solar electrical generation was found to be comparable Thus, as long as coal is considered a future energy choice, the issue of absolute land needs should not render the solar option at an automatic disadvantage
N RESPONSE TO clear national need and a generally heightened level of awareness and curiosity over the past ten years, we have witnessed a significant increase in the level of scientific interest and research into energy issues The response within geography is illustrated by an increasing number of publications and presentations on the topic (Blue, 1982) and the formation of an Energy Specialty Group within the Association of American Geographers
Geographers' involvement in energy research has been largely directed toward threats to environmental quality, partly because land use is at the foundation of the discipline as well as most energy-related environmental problems (Pasqualetti, 1981) Within this environmental sphere lies the issue of the actual land committed to various technology-specific energy decisions and options The low level of attention to this problem heretofore has resulted presumably from the cognitive view that the commodity of open space is plentiful in comparison with the land needed for energy development As the quantity of untampered space has decreased, however, the absolute quantity of land committed by our energy decisions has become a more significant matter
The issue of energy-related land commitments is a natural concomitant to an energy future which would rely to a large degree on naturally diffuse sources such as solar energy Although there are many relationships between solar energy development and land use (e.g., solar rights, street patterns, zoning), the most often mentioned (though misunderstood) relationship has been with regard to the land required for large centralized solar-electric systems; this land requirement is the topic of our paper Solar energy requirements are often mentioned as an environmental barrier to its widespread use in centralized power plants As examples: Solar energy 'suffers from an insoluble defect: it is a diffuse source of energy and requires a great deal of land surface for solar collectors' (Sutton, 1979 : 54); 'Because solar energy is so diffuse, "large" surface areas are required to collect it' (Landsberg et al., 1979 : 498); 'The environmental impacts associated with solar thermal conversion need not be severe The large amounts of land needed for large-scale, solar-electric generating plants would
be the most serious' (Kendall and Nadis, 1980: 132); 'The most significant environmental impacts of large solar thermal conversion plants would be their
* A key to the abbreviations used in this paper is included before the list of References Dr Pasqualetti is Associate Professor of Geography at Arizona State University in Tempe, Arizona 85287 Mr Miller is an Associate Planner for the City of Scottsdale, Arizona 85251 This paper was submitted for publication in October 1982
Trang 3LAND REQUIREMENTS FOR SOLAR AND COAL OPTIONS
land requirements' (National Academy of Sciences, 1979 : 366); 'The negative environmental effects of solar energy [include the] land use requirements which could compete with other, more attractive uses of land near populated areas ' (US Office
of Technology Assessment, 1977 : 1-19)
General comments such as those above stimulated us to seek more specific studies on the land-use needs of solar energy We found several Some of these studies are comparisons with other technologies (e.g., Caputo, 1977a, 1977b; DiNunno and Davis, 1976; Newsom and Wolsko, 1980; General Electric, 1976) Others give the amounts calculated for solar energy only (e.g., US DOE, 1977; Ouwens, 1976; Kash et al., 1976) Each of these studies suffers some shortcoming For example, DOE, Kash, and Ouwens do not compare the findings for solar energy with those of any other technology and thus their numbers tend to be isolated from the sense of reality which comparative analysis would offer The DiNunno and Davis studies do not indicate methodology One of the studies (Newsom and Wolsko (1980)) actually points out many of the deficiencies in some of the studies It said (p 7) that Caputo identifies 'only land for transmission and the total land for other purposes for each technology'; and that the General Electric study gives only a 'single figure for the total land use of each [technology]' However, the Newsom and Wolsko paper itself still cites from the others, is incomplete in methodological data, and does not itemize the land requirement for each energy phase for each technology Although some of the authors might have considered factors they do not mention, none of the papers explicitly addresses the wide variety of factors necessary for such an evaluation (e.g., load displacement, dispatch logic, capacity factors)
To muddle the issue further, the estimates offered in the papers cited above are given
in a variety of units, are based on vastly differing assumptions, and (not surprisingly) are significantly different from one another For example, the US Office of Technology Assessment (1977) indicated that photovoltaic arrays sufficient to satisfy 75 Quads of electricity would need 105 km2 of land The US DOE (1977) paper stated that a 1000 MWe peak output plant would require 11.7 x 106 m2 of land for cells of 8 per cent efficiency, 8.8 x 106 m2 for 10 per cent efficiency, and 5.8 x 106 m2 for 16 per cent efficiency Assuming a capacity factor of 0.3 and a 30-year plant lifetime, this translates
to 1300, 980 and 660 m2/MW-yr, respectively, it was stated Caputo (1977a) estimated
2000 m2/MW-yr for a centralized solar-thermal electric plant (compared, he indicated,
to 3000 m2/MW-yr for coal), and 3800 m2/MW-yr for photovoltaics (1977b) In an attempt to classify and improve the accuracy and comparability of quantitative results, our analysis explicitly discusses, coordinates, and justifies all pertinent factors and assumptions It also standardizes the results, both in terms of unit of area per megawatt and in terms of megawatt-hours per unit of area per 35 year period (i.e., the presumed lifetime of the plant) Each step in the energy development process is included (i.e., exploration, extraction, processing, generation, transportation, distribution, disposal),
as applicable
Inasmuch as the primary near-term competitor of the expanded use of solar energy will probably be coal, and also to keep the evaluation of solar energy in perspective, this paper compares the land-use requirements of solar-electric plants with coal-based facilities However, even though coal is the logical comparative resource, genetic and technical differences between the use of the two fuels do not allow the comparison to be completely analogous For example, coal is used both for base load and intermediate load electrical generation, while solar may offset electrical generation in base, intermediate, or peak loads, depending upon several factors such as those related to storage and dispatch Further, solar electricity generation in the short-term will be targeted to displace expensive peak and intermediate load generation, although a significant amount of base load capacity will be displaced as well (e.g., the Saguaro power tower's 1986 displacement is 39 per cent of oil and 61 per cent of coal (Weber, 1980)) As solar penetration becomes more significant, an increasing proportion of solar electricity generation will satisfy base load demand Thus, while the use of coal in our comparisons with solar energy is not absolutely ideal, no resource can be; nevertheleless, it can be used to help evaluate solar-electric strategies over the short and long terms
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Trang 4194 LAND REQUIREMENTS FOR SOLAR AND COAL OPTIONS
Before we begin our actual comparison, two additional introductory statements are needed: (1) Our comparison assumes that the amount of land necessary to provide the materials for solar electricity generation is comparable to the land necessary to provide the materials for coal mining, railroads, and coal-fired power plants*; (2) we address the land requirements of centralized power plants only Of the three types of solar power plants that would qualify under this heading (i.e., photovoltaic arrays, power towers, and solar satellite receiving stations), only photovoltaic arrays and power towers hold near-term promise for large-capacity utilization.**
Arizona is used as the site for our comparison for several reasons: it has large coal mines and several new coal-fired generating stations complete with substantial environmental documentation, ample open land for possible solar system siting, large and rapidly growing load centres, high rates of insolation, and keen interest and hopeful expectations regarding the future use of solar energy
Our comparison begins from the perspective of the quantity of land needed in terms
of generating capacity (hectares/megawatt)
Comparison of capacity-based land requirements
Land-use requirements of coal-fired power plants
Calculations in this paper have been based on 4965 MWe of coal-fired generating capacity located entirely within Arizona About 60 per cent of this capacity is on line and the remainder is under construction Environmental documentation pertinent to our evaluation is available for the entire 4965 MWe
Calculations of coal land commitments include six of the seven phases of electricity development: exploration, extraction, transportation, generation, transmission, and waste disposal (Coal processing/beneficiation, while important in eastern coal mining,
is not as significant in Arizona or most other western states The land required for breaking and sizing is part of the figure given for the mines.)
Coal supplied to Arizona power plants originates at several mines, and it varies widely in terms of ash, heat content, seam thickness, and sulphur content The mined land varies in reclamation potential The only commercial coal mining operations in Arizona are the two large mines on Black Mesa (Fig 1) Approximately 12 million tons
a year are mined on Black Mesa, two-thirds of which is taken from the Kayenta Mine to supply the Navajo Generating Station near Page, Arizona, and one-third of which supplies a power plant out of the study area The Cholla, Coronado, and Springerville Generating Stations are supplied from mines in west central New Mexico, just across the Arizona border
One of the key ingredients in calculating the amount of land which must be committed to coal-fired electrical generation is the reclamation potential of mined land This is a very site-specific matter, and it rests heavily upon one's definition of 'reclamation' In an oft-cited work on reclamation, the National Academy of Sciences
* Our assumption is based on a comparison of the material requirements of the Saguaro power tower proposed for a site near Tucson (111 MWe), the existing power tower near Barstow, California (10 MWe), and the Palo Verde Nuclear Generating Station near Phoenix A nuclear
plant was used in the comparison because nuclear data were more available and precise, and because a recent well publicized book on energy (Anderer, 1981) states that nuclear plants use even less concrete and steel than coal plants Thus, if solar and nuclear plants are relatively close
on their materials requirements, presumably coal plants and solar would be even closer The comparison indicates that solar energy material needs are greater than for coal-but not so much greater that the assumption of comparability becomes unreasonable In this regard, one should point out that the amount of material used in solar technology is declining rapidly (Weber, 1982; Brown, 1982)
** The factors discussed thus far are of significance to a determination of 'direct' land use only Several other, indirect, land-use factors are not considered: (1) concurrent land use: in some arrangements land beneath and between solar arrays may be usable; (2) socioeconomic impact: personnel needed for several phases of power development will require land for housing, schools, etc.; (3) indirect land use: vast areas of land are owned, leased, or reserved for rights of way for power plants and their related facilities
Trang 5LAND REQUIREMENTS FOR SOLAR AND COAL OPTIONS
Fig 1 Coal-fired power plants and coal mine location in Arizona
(NAS) differentiates between 'reclamation' and 'rehabilitation' The former implies that 'the site is habitable to organisms that were originally present or others that approximate the original inhabitants' (NAS, 1974 : 11) 'Rehabilitation' implies that the land will be 'returned to a form and productivity in conformity with a prior land-use plan including a stable ecological state that does not contribute substantially to environmental deterioration and is consistent with surrounding aesthetic values' (ibid.) In applying these definitions, the NAS concluded that rehabilitation of western coal lands which receive less than 250 mm (10 in.) of annual rainfall or which have high evapotranspiration rates pose a difficult problem because these lands require major sustained inputs of water, fertilizer, and management If rehabilitation of the drier sites
is allowed to occur naturally, it may be on a time scale that is 'unacceptable to society' (ibid., p 2)
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Thus, the matter of reclamation and rehabilitation is difficult to determine We assume that post-mining use of the land will be feasible, but it is impossible to state categorically how the land will compare in all ways to the pre-mined condition In order
to allow readers to insert their own impressions, we made our calculations based on both zero and 50 per cent reclamation of mined lands (We adhere to the term 'reclamation', along with its NAS definition, rather than the more restrictive term 'rehabilitation'.)
The area physically occupied by Arizona's power plants includes land for the powerhouse, cooling apparatus, coal storage and blending areas, laboratories, parking and other usual equipment and needs The amount of land needed for cooling exhibits the greatest variation At the Navajo site, cooling water is piped one mile (1.6 km) from Lake Powell Blowdown is discharged to lined ponds where it evaporates The Coronado and Springerville power plants will use well-water, cooling towers, and evaporation of blowdown The Cholla plant uses well-water to fill a 138 hectare cooling lake for Units 1-3 Arrangements for the disposal of waste ash vary from plant to plant All coal plants must dispose of ash, and each Arizona plant has land set aside for this purpose, although the plants might not actually use all such designated land Some of the ash is sold as pozzolana for the manufacture of concrete Sale is most economic when total transportation distances are low The Cholla plant currently sells virtually all
of its ash, although it still maintains a site for ash disposal A large parcel of land near the more isolated Navajo plant has been dedicated to the dumping of ash The coal-burning power plants in Arizona have all required the construction of several hundred km of transmission lines to link the plants with the principal load centres of the state A small percentage of the transmission rights of way are 'consumed' by tower pads and switching equipment
Reasonably precise figures are available for the amount of land directly affected by the coal-fired power plants in Arizona (Table I) In large part these figures have been taken from environmental impact statements, although some have been derived from base data presented in those documents The remainder have been estimated Footnotes to the table identify the source of each figure
The 4.03 ha/MW (9.96 ac/MW) of Table I is a total of the land disturbed directly For the generation site this includes the land actually occupied by the power plant operations The figure given for the transmission lines represents only the land disturbed by the towers and the access roads over the distance necessary to tie into the existing grid In the case of the Navajo power plant, transmission lines were not connected to the existing grid; rather, because the station was isolated and the nearest transmission lines did not have excess capacity, new transmission lines were built directly to the Phoenix Metropolitan Area The figures for the mines are for the land which will be disturbed directly during the lifetime of each power plant The figure for well fields and pipelines represents the land actually occupied by the pumps and pipelines The railroad figure is for land disturbed by railroad spurs built to connect the coal mines with the power plants
The figures in Table I represent the amount of land actually disturbed over the lifetime of the power plant The figure for coal mines is obviously large and includes all the land disturbed during the plant's lifetime Since our focus is on the long-term land-use impacts of electricity generation, and since the land disturbance which accompanies solar conversion is at a steady rate, we have elected to compare the final totals for each system before factoring in the reclamation aspect, rather than try to maintain a running total which would include factors for the amount of land to be mined, being mined, and reclaimed
Land-use requirements of solar power plants
Solar energy possesses characteristics which complicate the calculation of the land area required for a given electrical generation capacity Unlike coal, the land area required to intercept a given amount of solar energy is a direct function of basic earth-sun relationships: insolation variation with latitude, photoperiods which change diurnally and seasonally, and the continuous daily movement of the sun across the sky
In addition, daily and regional variations in incident solar radiation occur due to
Trang 7LAND REQUIREMENTS FOR SOLAR AND COAL OPTIONS
Notes to Table 1
a Including in generating station figures of 1012 ha (2500 ac)
b 51.5 km (32 mi) of pipelines at 5 ha/km (2 ac/mi) disturbed plus 4.0 ha (10 ac) disturbed for roads, reservoirs, etc
c 4481.7 m (14700 ft) x 14.0 m (46 ft) wide (based on average width of disturbance at Springerville generating station)
d 69.2 km (43 mi) spur; 95.5 ha (236 ac) disturbed by cuts; 117.4 ha (290 ac) disturbed by railroad bed
(27.4 km (17 mi) with Silverking to Goldfield 230 kv line)
(114.3 km (71 mi) with Cholla to Saguaro 500 kv line)
500 kv Coronado to Cholla
(41.9 km (26 mi) alone)
(80.5 km (50 mi) with Coronado to Kyrene 500 kv line)
based on 507.2 km (315 mi) of transmission corridor x 1.6 ha/km (6.4 acres/mi) disturbed by construction activities, construction roads and tower pads (shared corridors are counted 1/2)
f Salt River Project has negotiated contracts for the provision of 24.1 million metric tons (26.6 million tons) of coal from the Fish Lake Mine, Utah (1980-1984) Additional contracts must be negotiated to supply coal needs beyond 1984 At McKinley Mine, 17.8 ha are disturbed per 9
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million metric tons (1 million tons) of coal mined McKinley Mine coal characteristics: 5072 Kcal/Kg (9130 Btu/lb); 14.8% ash content, 0.6% sulphur content (Department of the Interior and Department of Agriculture, Final Environmental Statement-Coronado Project, August 1977) Coal characteristics and rate of land disturbance for Fish Lake Mine are not stated in the Coronado EIS Assuming the same rate of land disturbed per ton coal mined, 735.6 ha (1817 ac) will be disturbed at McKinley Mine and 69.2 ha (171 ac) will be disturbed at Fish Lake Mine
During 1984, Coronado power plant will use no more than 1.4 million metric tons (1.5 million tons) of coal each year Springerville power plant (1050 MW capacity) will use 3.3 million metric tons (3.6 million tons) of coal per year at full operation Adjusting for the higher heat content of McKinley Mine coal, this is equivalent to 3.188 million metric tons (3.507 million tons) of coal per year for the Coronado power plant Assuming Coronado will be in full operation after 1984 and will operate for 35 additional years, 2847.0 ha (7032 ac) will be disturbed at an undetermined coal mine Of the 111.4 metric tons (122.5 million tons) of coal needed after 1984, 18.2 million metric tons (20 million tons) will come from McKinley Mine, 93.2 million metric tons (102.5 million tons) from an undetermined mine Land disturbed at undetermined mine assumes rate of land disturbance at McKinley Mine
g 45.3 ha (112 ac) will be disturbed for limestone mining for the Coronado and Cholla power plants combined Land disturbed for limestone mining for Coronado based on the ratio of the
MW capacity of the Coronado power plant to the combined MW capacity of the Coronado and Cholla power plants
h Included in generating station figure of 850.2 ha (2100 ac)
i 41.9 km (26 mi) of pipeline disturbed at 5 ha/km (2 ac/mi) (disturbance rate based on Coronado EIS estimate)
j 18.3 km (11.4 mi) x 14.0 m (46 ft) average width of disturbance plus 5.6 km (3.5 mi) ash haul road with 23.8 m (78 ft) average width of disturbance
k 43.1 km (26.8 mi) x 30.5 m (100 ft) average width of disturbance of railroad and service road
1 Springerville Corridor (to New Mexico) (3 sets of towers 345 kv lines) 17.7 km (11 mi) long Coronado Corridor (to Coronado power plant) (1-345 kw line) 33.8 km (21 mi) long Coronado Corridor is serviced by railroad service road which runs parallel to transmission lines Land disturbance for the Coronado Corridor is included under 'railroad' Land disturbed for Springerville Corridor is based on 17.7 km (11 mi) x 2.5 ha (6.1 ac)/mi (rate of land disturbed for transmission corridors for Coronado power plant)
m Gallo Wash Mine, New Mexico Coal characteristics: 4941 Kcal/kg; (8894 Btu/lb); 14.63% ash content; 0.534% sulphur content: 4.5 cu m (5.9 cu yds) of overburden removed for each metric ton (1.1 ton) of coal mined (Westinghouse Environmental Systems Department, Springerville Generating Station: Applicant's Environmental Analysis, June 1, 1977) Springerville power plant will use 115.5 million metric tons (127 million tons) of coal over its 41 year lifetime; 25.5
ha (63 ac) are disturbed per 91 metric tons (1 million tons) coal mined
n Based on area to be mined for limestone for Coronado power plant
o Included in generating station figure of 413.4 ha (1021 acres)
p Water source is Lake Powell Water is pumped approximately 4.8 km (3 mi) at 5 ha/km (2 ac/mi) disturbed (disturbance rate based on estimate for pipeline disturbance at Coronado power plant) plus 4 ha (1 ac) disturbed at pumping station
q No station access road; road to ash disposal area approximately 1.9 km (1.2 mi) x 23.8 m (78 ft) average width of disturbance (width of disturbance based on road at Springerville power plant)
r 125.6 km (78 mi) x 30.5 m (100 ft) average width of disturbance (disturbance width based on railroad for Springerville power plant) 382.6 ha (945 ac) + 219.8 ha (543 ac) for areas, reservoirs, construction camps, and subballast area
s 1-500 kv line, to Boulder City, NV
442.8 km long (275 mi) long
1-500 kv line to Phoenix
402.5 km long (250 mi) long
disturbance based on 1288 km of transmission lines x 1.0 ha/km (4 ac/mi) disturbed
t Kayenta Mine on Black Mesa, AZ, coal characteristics: 5944 Kcal/kg (10700 Btu/lb); 7.93% ash content; 0.51% sulphur content (US Bureau of Reclamation Generating station; 97.2 ha/yr (240 ac disturbed/yr) x 35 years (Plant uses 5.6 million metric tons (6.2 million tons) of coal/yr)
u Included in generating station figure of 174.1 ha (430 ac)
v Based on estimate of area which will undergo accelerated change from meadow to xeric community due to groundwater pumping No estimate of area disturbed by pipelines is given in Cholla EIS
w Approximately 35.4 km (22 mi) railroad spur at McKinley Mine with 27.1 m (89 ft) average width of disturbance
Trang 9LAND REQUIREMENTS FOR SOLAR AND COAL OPTIONS
x 500 kv Cholla to Saguaro plant
225.4 km (140 mi) alone
114.3 km (71 mi) with Coronado to Kyrene (500 kv line)
230 kv Cholla to Coconino (near Flagstaff)
z See note g Area disturbed for limestone mining for Cholla based on the ratio of the MW capacity of the Cholla power plant to the combined MW capacity of the Cholla and Coronado power plants
atmospheric conditions Use of the sun to generate electricity must consider these irregularities
Solar energy use does not involve as many phases as coal 'Exploration' can be excluded because solar monitoring stations are small The 'extraction' and 'transportation' phases do not apply to solar energy The 'generation' phase requires consideration of the size of the array, insolation, conversion efficiency, energy storage, packing factors (i.e the ratio of collector area to land area), coverage factors (i.e percentage of collector area occupied by collection materials), tracking systems, topography, and other occupied land within the fence of a generating station The 'transmission' phase is required for solar electricity as it is for coal-fired electricity 'Waste disposal' does not apply to solar electricity
Land quantities required for solar conversion are not established as clearly as they are for coal, but the data needs are generally well known for each type of conversion In this paper, we have considered two conversion types: photovoltaic cells and power towers (Plate I) In order to make the necessary calculations, data are needed on solar insolation, conversion efficiencies, packing factors, coverage factors, tracking factors, storage, power conditioning, transmission lines, and additional land
Insolation.-The design point for solar facilities in the Phoenix area is noon on the summer solstice with a reference direct normal insolation of 1000 W/m2 (317 BTU/ft2/hr) (Weber, 1982) This is equivalent to 100 ha/MW (.246 ac/MW) It is from this insolation level that the related capacities of solar facilities in Arizona are currently derived While rated capacity can range between peak and average capacity depending
on the power conditioning and dispatch arrangement, it should be noted that rated capacity is usually significantly greater than the average capacity, as Figure 2 illustrates Efficiency.-The term 'efficiency' represents the percentage of captured insolation converted to electricity For photovoltaic arrays, we use 11 per cent (Kelly, 1978); for power towers, 20.8 per cent (Weber, 1980)
Packing factor.-The packing factor is the ratio of collector area to land area It is dependent on the type of tracking system, sensitivity of the collector to shading, the angle and direction of topographic slope, and latitude A packing factor of 1.00 would indicate that the collectors occupy all of the land with no spacing The packing factors
we use with regard to photovoltaic cells are taken from Masden (1978) He cites a range from 178 to 539, depending on the system In our calculations, we have incorporated three packing factors for photovoltaic systems: 300 was used for a vertical axis tracking system tilted 50? with respect to horizontal on flat land; 340 was used for a two-axes tracking system on flat land; and 539 was used for a two-axes tracking array positioned
on a south-facing 33.4? slope (equal to the latitude of Phoenix) The packing factor used for heliostats in a power tower arrangement is 230, based on design engineering conducted by Arizona Public Service Company for the repowering power tower under study at a site near Tucson, Arizona (Weber, 1980)
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PLATE I
Worker reflected in one of the 222 heliostat arrays at the power tower test facility at Sandia Laboratories, Albuquerque, NM The receiver tower itself, constructed here of concrete, is visible in the mirrors; receivers at other locations (e.g Barstow) will have concrete confined largely to the foundations of each heliostat with relatively little in the receiver
(Source: US Department of Energy)
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Power
Peak
Peak^ ' Array output
later use Rated '- ':"
an array is covered with the cells New techniques for the manufacturing of the cells should increase this to 95 per cent
Tracking factor.-The use of direct normal insolation assumes a two-axes tracking system which maximizes the insolation incident on the collector The cost of land, however, dictates that collectors be packed tightly, so tightly that some shading takes place The tracking factor accounts for this shading and for the decreased incident insolation when a single-axis tracking system is used For vertical axis, two axes, and two axes on a 33.4? slope system, the tracking factors we use are 900, 929, and 933 respectively (Masden, 1978)
Energy storage.-For a solar project, the need for storage and the amount necessary will depend on whether the unit is to provide base, intermediate, or peaking power, the penetration of solar energy in the generating capacity of a particular utility system, the incidence of inclement weather, economic considerations, the type of dispatch which is planned (e.g., immediate, delayed), and whether the facility is designed for stand-alone operation or is a repowering plant tied to an existing non-solar facility The storage type, capacity and function will affect the efficiency of electricity generation since all electricity which passes through storage will suffer loss This in turn affects land requirements; as efficiency is lowered, more land is required for a given output Power towers will have storage arrangements different from those of photovoltaic systems The Saguaro power tower, for example, will utilize a molten salt storage system all the time as a means to decouple the systems which interface with it: that is, 'storage acts as surge capacity which allows the receiver to operate independently from the salt/steam heat exchangers and vice versa' (Weber 1980 : 120) This prevents insolation variation from affecting the power output of the plant Used as storage per se the molten salt system will serve to shift the peak output to a time in the day when it will offset more expensive types of fuel The overall plant efficiency used in our calculations integrates this factor; we, therefore, have not included a separate storage phase in the calculations for power towers
201