by David Pimentel, G. Rodrigues, T. Wane, R. Abrams, K. Goldberg,
H. Staecker, E. Ma, L. Brueckner, L. Trovato, C. Chow, U. Govindarajulu, and
(Originally published in BioScience -- Vol. 44, No. 8, September 1994)
Solar energy technologies, paired with energy conservation, have the
potential to meet a large portion of future US energy needs
The United States faces serious energy shortages in the near future. High energy
consumption and the ever-increasing US population will force residents to confront
the critical problem of dwindling domestic fossil energy supplies. With only
4.7% of the world's population, the United States consumes approximately 25%
of the total fossil fuel used each year throughout the world. The United States
now imports about one half of its oil (25% of total fossil fuel) at an annual
cost of approximately $65 billion (USBC 1992a). Current US dependence on foreign
oil has important economic costs (Gibbons and Blair 1991) and portends future
negative effects on national security and the economy.
Domestic fossil fuel reserves are being rapidly depleted, and it would be a
major drain on the economy to import 100% of US oil. Within a decade or two
US residents will be forced to turn to renewable energy for some of their energy
needs. Proven US oil reserves are projected to be exhausted in 10 to 15 years
depending on consumption patterns (DOE 1991a, Matare 1989, Pimentel et al. 1994,
Worldwatch Institute 1992), and natural gas reserves are expected to last slightly
longer. In contrast, coal reserves have been projected to last approximately
100 years, based on current use and available extraction processes (Matare 1989).
The US coal supply, however, could be used up in a much shorter period than
the projected 100 years, if one takes into account predicted oil and gas depletion
and concurrent population growth (DOE 1991a, Matare 1989). The US population
is projected to double to more than one-half billion within the next 60 years
(USBC 19921). How rapidly the coal supply is depleted will depend on energy
consumption rates. The rapid depletion of US oil and gas reserves is expected
to necessitate increased use of coal. By the year 2010, coal may constitute
as much as 40% of total energy use (DOE 1991a). Undoubtedly new technologies
will be developed that will make it possible to extract more oil and coal. However,
this extra extraction can only be achieved at greater energy and economic costs.
When the energy input needed to power these methods approaches the amount of
energy mined, extraction will no longer be energy cost-effective (Hall et al.
Fossil fuel combustion, especially that based on oil and coal, is the major
contributor to increasing carbon dioxide concentration in the atmosphere, thereby
contributing to probable global warming. This climate change is considered one
of the most serious environmental threats throughout the world because of its
potential impact on food production and processes vital to a productive environment.
Therefore, concerns about carbon dioxide emissions may discourage widespread
dependence on coal use and encourage the development and use of renewable energy
Even if the rate of increase of per capita fossil energy consumption is slowed
by conservation measures, rapid population growth is expected to speed fossil
energy depletion and intensify global warming. Therefore, the projected availability
of all fossil energy reserves probably has been overstated. Substantially reducing
US use of fossil fuels through the efficient use of energy and the adoption
of solar energy technologies extends the life of fossil fuel resources and could
provide the time needed to develop and improve renewable energy technologies.
Renewable energy technologies will introduce new conflicts. For example, a
basic parameter controlling renewable energy supplies is the availability of
land. At present more than 99% of the US and world food supply comes from the
land (FAO 1991). In addition, the harvest of forest resources is presently insufficient
to meet US needs and thus the United States imports some of its forest products
(USBC 1992a). With approximately 75% of the total US land area exploited for
agriculture and forestry, there is relatively little land available for other
uses, such as biomass production and solar technologies. Population growth is
expected to further exacerbate the demands for land. Therefore, future land
conflicts could be intense.
In this article, we analyze the potential of various renewable or solar energy
technologies to supply the United States with its future energy needs. Diverse
renewable technologies are assessed in terms of their land requirements, environmental
benefits and risks, economic costs, and a comparison of their advantages. In
addition, we make a projection of the amount of energy that could be supplied
by solar energy subject to the constraints of maintaining the food and forest
production required by society. Although renewable energy technologies often
cause fewer environmental problems than fossil energy systems, they require
large amounts of land and therefore compete with agriculture, forestry, and
other essential land-use systems in the United States.
Assessment of renewable energy technologies
Coal, oil, gas, nuclear, and other mined fuels currently provide most of US
energy needs. Renewable energy technologies provide only 8% (Table 1).
The use of solar energy is, however, expected to grow. Renewable energy technologies
that have the potential to provide future energy supplies include: biomass systems,
hydroelectric systems, hydrogen fuel, wind power, photovoltaics, solar thermal
systems, and passive and active heating and cooling systems.
Biomass energy systems
At present, forest biomass energy, harvested from natural forests, provides
an estimated 3.6 quads (1.1 x 10 18 Joules) or 4.2% of the US energy supply
(Table 1). Worldwide, and especially in developing countries, biomass energy
is more widely used than in the United States. Only forest biomass will be included
in this US assessment, because forest is the most abundant biomass resource
and the most concentrated form of biomass. However, some biomass proponents
are suggesting the use of grasses, which on productive soils can yield an average
of 5 t · ha-1 yr-1 (Hall et al. 1993, USDA 1992).
Although in the future most biomass probably will be used for space and water
heating, we have analyzed its conversion into electricity in order to clarify
the comparison with other renewable technologies. An average of 3 tons of (dry)
woody biomass can be sustainably harvested per hectare per year with small amounts
of nutrient fertilizer inputs (Birdsey 1992). This amount of woody biomass has
a gross energy yield of 13.5 million kcal (thermal). The net yield is, however,
lower because approximately 33 liters of diesel fuel oil per hectare is expended
for cutting and collecting wood and for transportation, assuming an 80 kilometer
roundtrip between the forest and the plant. The economic benefits of biomass
are maximized when biomass can be used close to where it is harvested.
A city of 100,000 people using the biomass from a sustainable forest (3 tons/ha)
for fuel would require approximately 220,000 ha of forest area, based on an
electrical demand of 1 billion kWh (860 x 109 kcal = 1 kWh) per year (Table
2). Nearly 70% of the heat energy produced from burning biomass is lost in the
conversion into electricity, similar to losses experienced in coal fired plants.
The area required is about the same as that currently used by 100,000 people
for food production, housing, industry, and roadways (USDA 1992).
The energy input/output ratio of this system is calculated to be 1:3 (Table
2). The cost of producing a kilowatt of electricity from woody biomass ranges
from 7¢ to 10¢ (Table 2), which is competitive for electricity production
that presently has a cost ranging from 3¢ to 13¢ (Table 2; USBC 1992a
). Approximately 3 kcal of thermal energy is required to produce 1 kcal of electricity.
Biomass could supply the nation with 5 quads of its total gross energy supply
by the year 2050 with the use of at least 75 million ha (an area larger than
Texas, or approximately 8% of the 917 million ha in the United States) (Table
However, several factors limit reliance on woody biomass. Certainly, culturing
fast-growing trees in a plantation system located on prime land might increase
yields of woody biomass. However, this practice is unrealistic because prime
land is essential for food production. Furthermore, such intensely managed systems
require additional fossil fuel inputs for heavy machinery, fertilizers, and
pesticides, thereby diminishing the net energy available. In addition, Hall
et al. (1986) point out that energy is not the highest priority use of trees.
If natural forests are managed for maximal biomass energy production, loss
of biodiversity can be expected. Also, the conversion of natural forests into
plantations increases soil erosion and water runoff. Continuous soil erosion
and degradation would ultimately reduce the overall productivity of the land.
Despite serious limitations of plantations, biomass production could be increased
using agroforestry technologies designed to protect soil quality and conserve
biodiversity. In these systems, the energy and economic costs would be significant
and therefore might limit the use of this strategy.
The burning of biomass is environmentally more polluting than gas but less
polluting than coal. Biomass combustion releases more than 100 different chemical
pollutants into the atmosphere (Alfheim and Ramdahl 1986). Wood smoke is reported
to contain pollutants known to cause bronchitis, emphysema, and other illnesses.
These pollutants include up to 14 carcinogens, 4 cocarcinogens, 6 toxins that
damage cilia, and additional mucus-coagulating agents (Alfheim and Ramdahl 1986,
DOE 1980). Of special concern are the relatively high concentrations of potentially
carcinogenic polycyclic aromatic hydrocarbons (PAHs, organic compounds such
as benzo(a)pyrene) and particulates found in biomass smoke (DOE 1980). Sulfur
and nitrogen oxides, carbon monoxide, and aldehydes also are released in small
though significant quantities and contribute to reduced air quality (DOE 1980).
In electric generating plants, however, as much as 70% of these air pollutants
can be removed by installing the appropriate air-pollution control devices in
the combustion system.
Because of pollutants, several communities (including Aspen, Colorado) have
banned the burning of wood for heating homes. When biomass is burned continuously
in the home for heating, its pollutants can be a threat to human health (Lipfert
et al. 1988, Smith 1987b).
When biomass in the form of harvested crop residues is used for fuel, the soil
is exposed to intense erosion by wind and water (Pimentel et al. 1984). In addition
to the serious degradation of valuable agricultural land, the practice of burning
crop residues as a fuel removes essential nutrients from the land and requires
the application of costly fossil-based fertilizers if yields are to be maintained.
However, the soil organic matter, soil biota, and water-holding capacity of
the soil cannot be replaced by applying fertilizers. Therefore, we conclude
that crop residues should not be removed from the land for a fuel source (Pimentel
Biomass will continue to be a valuable renewable energy resource in the future,
but its expansion will be greatly limited. Its use conflicts with the needs
of agricultural and forestry production and contributes to major environmental
Liquid fuels are indispensable to the US economy (DOE 1991a). Petroleum, essential
for the transportation sector as well as the chemical industry, makes up approximately
42% of total US energy consumption. At present, the United States imports about
one half of its petroleum and is projected to import nearly 100% within 10 to
15 years (DOE 1991a). Barring radically improved electric battery technologies,
a shift from petroleum to alternative liquid and gaseous fuels will have to
be made. The analysis in this section is focused on the potential of three liquid
fuels: ethanol, methanol, and hydrogen.
Ethanol. A wide variety of starch and sugar crops, food processing wastes,
and woody materials (Lynd et al. 1991) have been evaluated as raw materials
for ethanol production. In the United States, corn appears to be the most feasible
biomass feedstock in terms of availability and technology (Pimentel 1991).
The total fossil energy expended to produce 1 liter of ethanol from corn is
10,200 kcal, but note that 1 liter of ethanol has an energy value of only 5130
kcal. Thus, there is an energy imbalance causing a net energy loss. Approximately
53% of the total cost (55¢ per liter) of producing ethanol in a large,
modern plant is for the corn raw material (Pimentel 1991). The total energy
inputs for producing ethanol using corn can be partially offset when the dried
distillers grain produced is fed to livestock. Although the feed value of the
dried distillers grain reduces the total energy inputs by 8 % to 24%, the energy
budget remains negative.
The major energy input in ethanol production, approximately 40% overall, is
fuel needed to run the distillation process (Pimentel 1991). This fossil energy
input contributes to a negative energy balance and atmospheric pollution. In
the production process, special membranes can separate the ethanol from the
so-called beer produced by fermentation. The most promising systems rely on
distillation to bring the ethanol concentration up to 90%, and selective-membrane
processes are used to further raise the ethanol concentration to 99.5% (Maeda
and Kai 1991). The energy input for this upgrading is approximately 1280 kcal/liter.
In laboratory tests, the total input for producing a liter of ethanol can potentially
be reduced from 10,200 to 6200 kcal by using membranes, but even then the energy
balance remains negative.
Any benefits from ethanol production, including the corn by-products, are negated
by the environmental pollution costs incurred from ethanol production (Pimentel
1991). Intensive corn production in the United States causes serious soil erosion
and also requires the further draw-down of groundwater resources. Another environmental
problem is caused by the large quantity of stillage or effluent produced. During
the fermentation process approximately 13 liters of sewage effluent is produced
and placed in the sewage system for each liter of ethanol produced.
Although ethanol has been advertised as reducing air pollution when mixed with
gasoline or burned as the only fuel, there is no reduction when the entire production
system is considered. Ethanol does release less carbon monoxide and sulfur oxides
than gasoline and diesel fuels. However, nitrogen oxides, formaldehydes, other
aldehydes, and alcohol--all serious air pollutants-- are associated with the
burning of ethanol as fuel mixture with or without gasoline (Sillman and Samson
1990). Also, the production and use of ethanol fuel contribute to the increase
in atmospheric carbon dioxide and to global warming, because twice as much fossil
energy is burned in ethanol production than is produced as ethanol.
Ethanol produced from corn clearly is not a renewable energy source. Its production
adds to the depletion of agricultural resources and raises ethical questions
at a time when food supplies must increase to meet the basic needs of the rapidly
growing world population.
Methanol. Methanol is another potential fuel for internal combustion engines
(Kohl 1990). Various raw materials can be used for methanol production, including
natural gas, coal, wood, and municipal solid wastes. At present, the primary
source of methanol is natural gas. The major limitation in using biomass for
methanol production is the enormous quantities needed for a plant with suitable
economies of scale. A suitably large methanol plant would require at least 1250
tons of dry biomass per day for processing (ACTI 1983). More than 150,000 ha
of forest would be needed to supply one plant. Biomass generally is not available
in such enormous quantities from extensive forests and at acceptable prices
If methanol from biomass (33 quads) were used as a substitute for oil in the
United States, from 250 to 430 million ha of land would be needed to supply
the raw material. This land area is greater than the 162 million ha of US cropland
now in production (USDA 1992). Although methanol production from biomass may
be impractical because of the enormous size of the conversion plants (Kohl 1990),
it is significantly more efficient than the ethanol production system based
on both energy output and economics (Kohl 1990).
Compared to gasoline and diesel fuel, both methanol and ethanol reduce the
amount of carbon monoxide and sulfur oxide pollutants produced, however both
contribute other major air pollutants such as aldehydes and alcohol. Air pollutants
from these fuels worsen the tropospheric ozone problem because of the emissions
of nitrogen oxides from the richer mixtures used in the combustion engines (Sillman
and Samson 1990).
Hydrogen. Gaseous hydrogen, produced by the electrolysis of water, is another
alternative to petroleum fuels. Using solar electric technologies for its production,
hydrogen has the potential to serve as a renewable gaseous and liquid fuel for
transportation vehicles. In addition hydrogen can be used as an energy storage
system for electrical solar energy technologies, like photovoltaics (Winter
and Nitsch 1988).
The material inputs for a hydrogen production facility are primarily those
needed to build a solar electric production facility. The energy required to
produce 1 billion kWh of hydrogen is 1.3 billion kWh of electricity (Voigt 1984).
If current photovoltaics (Table 2) require 2700 ha/1 billion kWh, then a total
area of 3510 ha would be needed to supply the equivalent of 1 billion kWh of
hydrogen fuel. Based on US per capita liquid fuel needs, a facility covering
approximately 0.15 ha (16,300 ft2) would be needed to produce a year's requirement
of liquid hydrogen. In such a facility, the water requirement for electrolytic
production of 1 billion kWh/yr equivalent of hydrogen is approximately 300 million
liters/yr (Voigt 1984).
To consider hydrogen as a substitute for gasoline: 9.5 kg of hydrogen produces
energy equivalent to that produced by 25 kg of gasoline. Storing 25 kg of gasoline
requires a tank with a mass of 17 kg, whereas the storage of 9.5 kg of hydrogen
requires 55 kg (Peschka 1987). Part of the reason for this difference is that
the volume of hydrogen fuel is about four times greater than that for the same
energy content of gasoline. Although the hydrogen storage vessel is large, hydrogen
burns 1.33 times more efficiently than gasoline in automobiles (Bockris and
Wass 1988). In tests, a BMW 745i liquid hydrogen test vehicle with a tank weight
of 75 kg, and the energy equivalent of 40 liters (320,000 kcal) of gasoline,
had a cruising range in traffic of 400 km or a fuel efficiency of 10 km per
liter (24 mpg) (Winter 1986).
At present, commercial hydrogen is more expensive than gasoline. For example,
assuming 5¢ per kWh of electricity from a conventional power plant, hydrogen
would cost 9¢ per kWh (Bockris and Wass 1988). This cost is the equivalent
of 67¢/liter of gasoline. Gasoline sells at the pump in the United States
for approximately 30¢/liter. However, estimates are that the real cost
of burning a liter of gasoline ranges from $1.06 to $1.32, when production,
pollution, and other external costs are included (Worldwatch Institute 1989).
Therefore, based on these calculations hydrogen fuel may eventually be competitive.
Some of the oxygen gas produced during the electrolysis of water can be used
to offset the cost of hydrogen. Also the oxygen can be combined with hydrogen
in a fuel cell, like those used in the manned space flights. Hydrogen fuel cells
used in rural and suburban areas as electricity sources could help decentralize
the power grid, allowing central power facilities to decrease output, save transmission
costs, and make mass-produced, economical energy available to industry.
Compared with ethanol, less land (0.15 ha versus 7 ha for ethanol) is required
for hydrogen production that uses photovoltaics to produce the needed electricity.
The environmental impacts of hydrogen are minimal. The negative impacts that
occur during production are all associated with the solar electric technology
used in production.
Water for the production of hydrogen may be a problem in the arid regions of
the United States, but the amount required is relatively small compared with
the demand for irrigation water in agriculture. Although hydrogen fuel produces
emissions of nitrogen oxides and hydrogen peroxide pollutants, the amounts are
about one-third lower than those produced from gasoline engines (Veziroglu and
Barbir 1992). Based on this comparative analysis, hydrogen fuel may be a cost-effective
alternative to gasoline, especially if the environmental and subsidy costs of
gasoline are taken into account.
For centuries, water has been used to provide power for various systems. Today
hydropower is widely used to produce electrical energy. In 1988 approximately
870 billion kWh (3 quads or 9.5 % ) of the United States' electrical energy
was produced by hydroelectric plants (FERC 1988, USBC 1992a). Further development
and/or rehabilitation of existing dams could produce an additional 48 billion
kWh per year. However, most of the best candidate sites already have been fully
developed, although some specialists project increasing US hydropower by as
much as 100 billion kWh if additional sites are developed (USBC 1992a).
Hydroelectric plants require land for their water-storage reservoirs. An analysis
of 50 hydroelectric sites in the United States indicated that an average of
75,000 ha of reservoir area are required per 1 billion kWh/ yr produced (Table
2). However, the size of reservoir per unit of electricity produced varies widely,
ranging from 482 ha to 763,000 ha per 1 billion kWh/yr depending upon the hydro
head, terrain, and additional uses made of the reservoir (Table 2). The latter
include flood control, storage of water for public and irrigation supplies,
and/or recreation (FERC 1984). For the United States the energy input/output
ratio was calculated to be 1:48 (Table 2); for Europe an estimate of 1:15 has
been reported (Winter et al. 1992).
Based on regional estimates of land use and average annual energy generation,
approximately 63 million hectares of the total of 917 million ha of land area
in the United States are currently covered with reservoirs. To develop the remaining
best candidate sites, assuming land requirements similar to those in past developments,
an additional 24 million hectares of land would be needed for water storage
Reservoirs constructed for hydroelectric plants have the potential to cause
major environmental problems. First, the impounded water frequently covers agriculturally
productive, alluvial bottomland. This water cover represents a major loss of
productive agricultural land. Dams may fail, resulting in loss of life and destruction
of property. Further, dams alter the existing plant and animal species in the
ecosystem (Flavin 1985). For example, cold water fishes may be replaced by warm
water fishes, frequently blocking fish migration (Hall et al. 1986). However,
flow schedules can be altered to ameliorate many of these impacts. Within the
reservoirs, fluctuations of water levels alter shorelines and cause downstream
erosion and changes in physiochemical factors, as well as the changes in aquatic
communities. Beyond the reservoirs, discharge patterns may adversely reduce
downstream water quality and biota, displace people, and increase water evaporation
losses (Barber 1993). Because of widespread public environmental concerns, there
appears to be little potential for greatly expanding either large or small hydroelectric
power plants in the future (Table 3).
For many centuries, wind power like water power has provided energy to pump
water and run mills and other machines. In rural America windmills have been
used to generate electricity since the early 1900s.
Modern wind turbine technology has made significant advances over the last
10 years. Today, small wind machines with 5 to 40 kW capacity can supply the
normal electrical needs of homes and small industries (Twidell 1987). Medium-size
turbines rated 100 kW to 500 kW produce most of the commercially generated electricity.
At present, the larger, heavier blades required by large turbines upset the
desirable ratio between size and weight and create efficiency problems. However,
the effectiveness and efficiency of the large wind machines are expected to
be improved through additional research and development of lighter weight but
stronger components (Clarke 1991). Assuming a 35% operation capacity at a favorable
site, the energy input/output ratio of the system is 1:5 for the material used
in the construction of medium size wind machines (Table 2).
The availability of sites with sufficient wind (at least 20 km/in) limits the
widespread development of wind farms. Currently, 70% of the total wind energy
(0.01 quad) produced in the United States is generated in California (Table
3; AWEA 1992). However, an estimated 13% of the contiguous US land area has
wind speeds of 22 km/in or higher; this area then would be sufficient to generate
approximately 20 % of US electricity using current technology (DOE 1992). Promising
areas for wind development include the Great Plains and coastal regions.
Another limitation of this energy resource is the number of wind machines that
a site can accommodate. For example, at Altamont Pass, California, an average
of one turbine per 1.8 ha allows sufficient spacing to produce maximum power
(Smith and Ilyin 1991). Based on this figure approximately 11,700 ha of land
are needed to supply 1 billion kWh/ yr (Table 2). However, because the turbines
themselves only occupy approximately 2% of the area or 230 ha, dual land use
is possible. For example, current agricultural land developed for wind power
continues to be used in cattle, vegetable, and nursery stock production.
An investigation of the environmental impacts of wind energy production reveals
a few hazards. For example, locating the wind turbines in or near the flyways
of migrating birds and wildlife refuges may result in birds flying into the
supporting structures and rotating blades (Clarke 1991, Kellett 1990). Clarke
suggests that wind farms be located at least 300 meters from nature reserves
to reduce this risk to birds.
Insects striking turbine blades will probably have only a minor impact on insect
populations, except for some endangered species. However, significant insect
accumulation on the blades may reduce turbine efficiency (Smith 1987a).
Wind turbines create interference with electromagnetic transmission, and blade
noise may be heard up to 1 km away (Kellet 1990). Fortunately, noise and interference
with radio and television signals can be eliminated by appropriate blade materials
and careful placement of turbines. In addition, blade noise is offset by locating
a buffer zone between the turbines and human settlements. New technologies and
designs may minimize turbine generator noise.
Under certain circumstances shadow flicker has caused irritation, disorientation,
and seizures in humans (Steele 1991). However, as with other environmental impacts,
mitigation is usually possible through careful site selection away from homes
and offices. This problem slightly limits the land area suitable for wind farms.
Although only a few wind farms supply power to utilities in the United States,
future widespread development may be constrained because local people feel that
wind farms diminish the aesthetics of the area (Smith 1987a). Some communities
have even passed legislation to prevent wind turbines from being installed in
residential areas (Village of Cayuga Heights, New York, Ordinance 1989). Likewise
areas used for recreational purposes, such as parks, limit the land available
for wind power development.
Photovoltaic cells are likely to provide the nation with a significant portion
of its future electrical energy (DeMeo et al 1991). Photovoltaic cells produce
electricity when sunlight excites electrons in the cells. Because the size of
the units is flexible and adaptable, photovoltaic cells are ideal for use in
homes, industries, and utilities.
Before widespread use, however, improvements are needed in the photovoltaic
cells to make them economically competitive. Test photovoltaic cells that consist
of silicon solar cells are currently up to 21% efficient in converting sunlight
into electricity (Moore 1992). The durability of photovoltaic cells, which is
now approximately 20 years, needs to be lengthened and current production costs
reduced about fivefold to make them economically feasible. With a major research
investment, all of these goals appear possible to achieve (DeMeo et al. 1991).
Currently, production of electricity from photovoltaic cells costs approximately
30¢/kWh, but the price is projected to fall to approximately 10¢/kWh
by the end of the decade and perhaps reach as low as 4¢ by the year 2030,
provided the needed improvements are made (Flavin and Lenssen 1991). In order
to make photovoltaic cells truly competitive, the target cost for modules would
have to be approximately 8¢/ kWh (DeMeo et al. 1991).
Using photovoltaic modules with an assumed 7.3% efficiency (the current level
of commercial units), 1 billion kWh/yr of electricity could be produced on approximately
2700 ha of land (Table 2), or approximately 0.027 ha per person, based on the
present average per capita use of electricity. Thus, total US electrical needs
theoretically could be met with photovoltaic cells on 5.4 million ha (0.6% of
US land). If 21% efficient cells were used, the total area needed would be greatly
reduced. Photovoltaic plants with this level of efficiency are being developed
(DeMeo et al. 1991).
The energy input for the structural materials of a photovoltaic system delivering
1 billion kWh is calculated to be approximately 300 kWh/m2. The energy input/output
ratio for production is about 1:9 assuming a life of 20 years (Table 2).
Locating the photovoltaic cells on the roofs of homes, industries, and other
buildings would reduce the need for additional land by approximately 5% (USBC
1992a), as well as reduce the costs of energy transmission. However, photovoltaic
systems require backup with conventional electrical systems, because they
function only during daylight hours.
Photovoltaic technology offers several environmental advantages in producing
electricity compared with fossil fuel technologies. For example, using present
photovoltaic technology, carbon dioxide emissions and other pollutants are negligible.
The major environmental problem associated with photovoltaic systems is the
use of toxic chemicals such as cadmium sulfide and gallium arsenide, in their
manufacture (Holdren et al. 1980). Because these chemicals are highly toxic
and persist in the environment for centuries, disposal of inoperative cells
could become a major environmental problem. However, the most promising cells
in terms of low cost, mass production, and relatively high efficiency are those
being manufactured using silicon. This material makes the cells less expensive
and environmentally safer than the heavy metal cells.
Solar thermal conversion systems
Solar thermal energy systems collect the sun's radiant energy and convert it
into heat. This heat can be used for household and industrial purposes and also
to drive a turbine and produce electricity. System complexity ranges from solar
ponds to the electric-generating central receivers. We have chosen to analyze
electricity in order to facilitate comparison to the other solar energy technologies.
Solar ponds. Solar ponds are used to capture solar radiation and store it at
temperatures of nearly 100°C. Natural or man-made ponds can be made into
solar ponds by creating a salt-concentration gradient made up of layers of increasing
concentrations of salt. These layers prevent natural convection from occurring
in the pond and enable heat collected from solar radiation to be trapped in
the bottom brine.
The hot brine from the bottom of the pond is piped out for generating electricity.
The steam from the hot brine turns freon into a pressurized vapor, which drives
a Rankine engine. This engine was designed specifically for converting low-grade
heat into electricity. At present, solar ponds are being used in Israel to generate
electricity (Tabor and Doran 1990).
For successful operation, the salt concentration gradient and the water levels
must be maintained. For example, 4000 ha of solar ponds lose approximately 3
billion liters of water per year under the arid conditions of the southwestern
United States (Tabor and Doran 1990). In addition, to counteract the water loss
and the upward diffusion process of salt in the ponds, the dilute salt water
at the surface of the ponds has to be replaced with fresh water. Likewise salt
has to be added periodically to the heat-storage zone. Evaporation ponds concentrate
the brine, which can then be used for salt replacement in the solar ponds.
Approximately 4000 ha of solar ponds (40 ponds of 100 ha) and a set of evaporation
ponds that cover a combined 1200 ha are needed for the production of 1 billion
kWh of electricity needed by 100,000 people in one year (Table 2). Therefore,
a family of three would require approximately 0.2 ha (22,000 sq ft) of solar
ponds for its electricity needs. Although the required land area is relatively
large, solar ponds have the capacity to store heat energy for days, thus eliminating
the need for back-up energy sources from conventional fossil plants. The efficiency
of solar ponds in converting solar radiation into heat is estimated to be approximately
1:5. Assuming a 30-year life for a solar pond, the energy input/output ratio
is calculated to be 1:4 (Table 2). A 100 hectare (1 km2) solar pond is calculated
to produce electricity at a rate of approximately 14¢ per kWh. According
to Folchitto (1991), this cost could be reduced in the future.
In several locations in the United States solar ponds are now being used successfully
to generate heat directly. The heat energy from the pond can be used to produce
processed steam for heating at a cost of only 2¢ to 3.5¢ per kWh (Gommend
and Grossman 1988). Solar ponds are most effectively employed in the Southwest
Some hazards are associated with solar ponds, but most can be prevented with
careful management. For instance, it is essential to use plastic liners to make
the ponds leakproof and thereby prevent contamination of the adjacent soil and
groundwater with salt. Burrowing animals must be kept away from the ponds by
buried screening (Dickson and Yates 1983). In addition, the ponds should be
fenced to prevent people and other animals from coming in contact with them.
Because some toxic chemicals are used to prevent algae growth on water surface
and freon is used in the Rankine engine, methods will have to be devised for
safely handling these chemicals (Dickson and Yates 1983).
Solar receiver systems. Other solar thermal technologies that concentrate solar
radiation for large scale energy production include distributed and central
receivers. Distributed receiver technologies use rows of parabolic troughs to
focus sunlight on a central-pipe receiver that runs above the troughs. Pressurized
water and other fluids are heated in the pipe and are used to generate steam
to drive a turbogenerator for electricity production or provide industry with
Central receiver plants use computer-controlled, sun-tracking mirrors, or heliostats,
to collect and concentrate the sunlight and redirect it toward a receiver located
atop a centrally placed tower. In the receiver, the solar energy is captured
as heat energy by circulating fluids, such as water or molten salts, that are
heated under pressure. These fluids either directly or indirectly generate steam,
which is then driven through a conventional turbogenerator to yield electricity.
The receiver system may also be designed to generate heat for industry.
Distributed receivers have entered the commercial market before central receivers,
because central receivers are more expensive to operate. But, compared to distributed
receivers, central receivers have the potential for greater efficiency in electricity
production because they are able to achieve higher energy concentrations and
higher turbine inlet temperatures (Winter 1991). Central receivers are used
in this analysis.
The land requirements for the central receiver technology are approximately
1100 ha to produce 1 billion kWh/yr (Table 2), assuming peak efficiency, and
favorable sunlight conditions like those in the western United States. Proposed
systems offer four to six hours of heat storage and may be constructed to include
a back-up alternate energy source. The energy input/output ratio is calculated
to be 1:10 (Table 2). Solar thermal receivers are estimated to produce electricity
at approximately 10¢ per kWh, but this cost is expected to be reduced somewhat
in the future, making the technology more competitive (Vant-Hull 1992). New
technical advances aimed at reducing costs and improving efficiency include
designing stretched membrane heliostats, volumetric-air ceramic receivers, and
improved overall system designs (Beninga et al. 1991).
Central receiver systems are being tested in Italy, France, Spain, Japan, and
the United States (at the 10-megawatt Solar One pilot plant near Barstow, California;
Skinrood and Skvarna 1986). Also, Luz's Solar Electric Generating System plants
at Barstow use distributed receivers to generate almost 300 MW of commercial
electricity (Jensen et al. 1989).
The potential environmental impacts of solar thermal receivers include: the
accidental or emergency release of toxic chemicals used in the heat transfer
system (Baechler and Lee 1991); bird collisions with a heliostat and incineration
of both birds and insects if they fly into the high temperature portion of the
beams; and--if one of the heliostats did not track properly but focused its
high temperature beam on humans, other animals, or flammable materials--burns,
retinal damage, and fires (Mihlmester et al. 1980). Flashes of light coming
from the heliostats may pose hazards to air and ground traffic (Mihlmester et
Other potential environmental impacts include microclimate alteration, for
example reduced temperature and changes in wind speed and evapotranspiration
beneath the heliostats or collecting troughs. This alteration may cause shifts
in various plant and animal populations. The albedo in solarcollecting
fields may be increased from 30% to 56% in desert regions (Mihlmester et al.
1980). An area of 1100 ha is affected by a plant producing 1 billion kWh.
The environmental benefits of receiver systems are significant when compared
to fossil fuel electrical generation. Receiver systems cause no problems of
acid rain, air pollution, or global warming (Kennedy et al. 1991).
Passive heating and cooling of buildings
Approximately 23% (18.4 quads) of the fossil energy consumed yearly in the
United States is used for space heating and cooling of buildings and for heating
hot water (DOE 1991a). At present only 0.3 quads of energy are being saved by
technologies that employ passive and active solar heating and cooling of buildings
(Table 2). Tremendous potential exists for substantial energy savings by increased
energy efficiency and by using solar technologies for buildings.
Both new and established homes can be fitted with solar heating and cooling
systems. Installing passive solar systems into the design of a new home is generally
cheaper than retrofitting an existing home. Including passive solar systems
during new home construction usually adds less than 10% to construction costs
(Howard and Szoke 1992); a 3-5% added first cost is typical.1 Based on the cost
of construction and the amount of energy saved measured in terms of reduced
heating costs, we estimate the cost of passive solar systems to be approximately
3¢ per kWh saved.
Improvements in passive solar technology are making it more effective and less
expensive than in the past. In the area of window designs, for example, current
research is focused on the development of superwindows with high insulating
values and smart or electrochromic windows that can respond to electrical current,
temperature, or incident sunlight to control the admission of light energy (Warner
1991). Use of transparent insulation materials makes window designs that transmit
from 50% to 70% of incident solar energy while at the same time providing insulating
values typical of 25 cm of fiber glass insulation (Chahroudi 1992). Such materials
have a wide range of solar technology applications beyond windows, including
house heating with transparent, insulated collectorstorage walls and integrated
storage collectors for domestic hot water (Wittwer et al. 1991).
Active solar heating technologies are not likely to play a major role in the
heating of buildings. The cost of energy saved is relatively high compared with
passive systems and conservation measures.2
Solar water heating is also cost effective. Approximately 3% of all the energy
used in the United States is for heating water in homes (DOE 1991a). In addition,
many different types of passive and active water heating solar systems are available
and are in use throughout the United States. These systems are becoming increasingly
affordable and reliable (Wittwer et al. 1991). The cost of purchasing and installing
an active solar water heater ranges from $2500 to $6000 in the northern regions
and $2000 to $4000 in the southern regions of the nation (DOE/ CE 1988).
Although none of the passive heating and cooling technologies require land,
they can cause environmental problems. For example, some indirect land-use problems
may occur, such as the removal of trees, shading, and rights to the sun (Schurr
et al. 1979). Glare from collectors and glazing could create hazards to automobile
drivers, pedestrians, bicyclists, and airline pilots. Also, when houses are
designed to be extremely energy efficient and airtight, indoor air quality becomes
a concern because air pollutants may accumulate inside. However, installation
of welldesigned ventilation systems promotes a healthful exchange of air
while reducing heat loss during the winter and heat gain during the summer.
If radon is a pollutant present at unsafe levels in the home, various technologies
can mitigate the problem (ASTM 1992).
Comparing solar power to coal and nuclear power
Coal and nuclear power production are included in this analysis to compare
conventional sources of electricity generation to various future solar energy
technologies. Coal, oil, gas, nuclear, and other mined fuels are used to meet
92% of US energy needs (Table 1). Coal and nuclear plants combined produce three
quarters of US electricity (USBC 1992a).
Energy efficiencies for both coal and nuclear fuels are low due to the thermal
law constraint of electric generator designs: coal is approximately 35% efficient
and nuclear fuels approximately 33% (West and Kreith 1988). Both coal and nuclear
power plants in the future may require additional structural materials to meet
clean air and safety standards. However, the energetic requirements of such
modifications are estimated to be small compared with the energy lost due to
The costs of producing electricity using coal and nuclear energy are 3 ¢
and 5¢ per kWh, respectively (EIA 1990). However, the costs of this kind
of energy generation are artificially low because they do not include such external
costs as damages from acid rain produced from coal and decommissioning costs
for the closing of nuclear plants. The Clean Air Act and its amendments may
raise coal generation costs, while the new reactor designs, standardization,
and streamlined regulations may reduce nuclear generation costs. Government
subsidies for nuclear and coal plants also skew the comparison with solar energy
technologies (Wolfson 1991).
Clouding the economic costs of fossil energy use are the direct and indirect
US subsidies that hide the true cost of energy and keep the costs low, thereby
encouraging energy consumption. The energy industry receives a direct subsidy
of $424 per household per year (based on an estimated maximum of $36 billion
for total federal energy subsidies [ASK 1993]). In addition, the mined-energy
industry, like the gasoline industry, does not pay for the environmental and
public health costs of fossil energy production and consumption.
The land requirements for fossil fuel and nuclear-based plants are lower than
those for solar energy technologies (Table 2). The land area required for electrical
production of 1 billion kWh/year is estimated at 363 ha for coal and 48 ha for
nuclear fuels. These figures include the area for the plants and both surface
and underground mining operations and waste disposal. The land requirements
for coal technology are low because it uses concentrated fuel sources rather
than diffuse solar energy. However, as the quality of fuel ore declines, land
requirements for mining will increase. In contrast, efficient reprocessing and
the use of nuclear breeder reactors may decrease the land area necessary for
Many environmental problems are associated with both coal and nuclear power
generation (Pimentel et al. 1994). For coal, the problems include the substantial
damage to land by mining, air pollution, acid rain, global warming, as well
as the safe disposal of large quantities of ash (Wolfson 1991). For nuclear
power, the environmental hazards consist mainly of radioactive waste that may
last for thousands of years, accidents, and the decommissioning of old nuclear
plants (Wolfson 1991).
Fossil-fuel electric utilities account for two-thirds of the sulfur dioxide,
one-third of the nitrogen dioxide, and one-third of the carbon dioxide emissions
in the United States (Kennedy et al. 1991). Removal of carbon dioxide from coal
plant emissions could raise costs to 12¢/kWh; a disposal tax on carbon
could raise coal electricity costs to 18¢/kWh (Williams et al. 1990).
The occupational and public health risks of both coal and nuclear plants are
fairly high, due mainly to the hazards of mining, ore transportation, and subsequent
air pollution during the production of electricity. However, there are 22 times
as many deaths per unit of energy related to coal than of nuclear energy production
because 90,000 times greater volume of coal than nuclear ore is needed to generate
an equivalent amount of electricity.3
Also, and as important, coal produces more diffuse pollutants than nuclear
fuels during normal operation of the generating plant. Coal fired plants produce
air pollutants-- including sulfur oxides, nitrogen oxides, carbon dioxide, and
particulates--that adversely affect air quality and contribute to acid rain.
Technologies do exist for removing most of the air pollutants, but their use
increases the cost of a new plant by 20-25% (IEA 1987). By comparison, nuclear
power produces many fewer pollutants than do coal plants (Tester et al. 1991).
Transition to solar energy and other alternatives
The first priority of a sustainable US energy program should be for individuals,
communities, and industries to conserve fossil energy resources. Other developed
countries have proven that high productivity and a high standard of living can
be achieved with considerably less energy expenditure compared to that of the
United States. Improved energy efficiency in the United States, other developed
nations, and even in developing nations would help both extend the world's fossil
energy resources and improve the environment (Pimentel et al. 1994).
The supply and demand for fossil and solar energy; the requirements of land
for food, fiber, and lumber; and the rapidly growing human population will influence
future US options. The growth rate of the US population has been increasing
and is now at 1.1 % per year (USBC 1992b); at this rate, the present population
of 260 million will increase to more than a half billion in just 60 years. The
presence of more people will require more land for homes, businesses, and roads.
Population density directly influences food production, forest product needs,
and energy requirements. Considerably more agricultural and forest land will
be needed to provide vital food and forest products, and the drain on all energy
resources will increase. Although there is no cropland shortage at present (USDA
1992), problems undoubtedly will develop in the near future in response to the
diverse needs of the growing US population.
Solar energy technologies, most of which require land for collection and production,
will compete with agriculture and forestry in the United States and worldwide
(Table 2). Therefore, the availability of land is projected to be a limiting
factor in the development of solar energy. In the light of this constraint,
an optimistic projection is that the current level of nearly 7 quads of solar
energy collected and used annually in the United States could be increased to
approximately 37 quads (Ogden and Williams 1989, Pimentel et al. 1984). This
higher level represents only 43% of the 86 quads of total energy currently consumed
in the United States (Tables 1 and 3). Producing 37 quads with solar technologies
would require approximately 173 million ha, or nearly 20% of US land area (Table
3). At present this amount of land is available, but it may become unavailable
due to future population growth and increased resource consumption. If land
continues to be available, the amounts of solar energy (including hydropower
and wind) that could be produced by the year 2050 are projected to be: 5 quads
from biomass, 4 quads from hydropower, 8 quads from wind power, 6 quads from
solar thermal systems, 6 quads from passive and active solar heating, and 8
quads from photovoltaics (Table 3).
Another possible future energy source is fusion energy (Bartlett 1994, Matare
1989). Fusion uses nuclear particles called neutrons to generate heat in a fusion
reactor vessel. Nuclear fusion differs from fission in that the production of
energy does not depend on continued mining. However, high costs and serious
environmental problems are anticipated (Bartlett 1994). The environmental problems
include the production of enormous amounts of heat and radioactive material.
The United States could achieve a secure energy future and a satisfactory standard
of living for everyone if the human population were to stabilize at an estimated
optimum of 200 million (down from today's 260 million) and conservation measures
were to lower per capita energy consumption to about half the present level
(Pimentel et al. 1994). However, if the US population doubles in 60 years as
is more likely, supplies of energy, food, land, and water will become inadequate,
and land, forest, and general environmental degradation will escalate (Pimentel
et al. 1994, USBC 1992a).
Fossil energy subsidies should be greatly diminished or withdrawn and the savings
should be invested to encourage the development and use of solar energy technologies.
This policy would increase the rate of adoption of solar energy technologies
and lead to a smooth transition from a fossil fuel economy to one based on solar
energy. In addition, the nation that becomes a leader in the development of
solar energy technologies is likely to capture the world market for this industry.
This assessment of alternate technologies confirms that solar energy alternatives
to fossil fuels have the potential to meet a large portion of future US energy
needs, provided that the United States is committed to the development and implementation
of solar energy technologies and that energy conservation is practiced. The
implementation of solar technologies will also reduce many of the current environmental
problems associated with fossil fuel production and use.
An immediate priority IS to speed the transition from reliance on nonrenewable
energy sources to reliance on renewable, especially solar based, energy technologies.
Various combinations of solar technologies should be developed consistent with
the characteristics of different geographic regions, taking into account the
land and water available and regional energy needs. Combined, biomass energy
and hydroelectric energy in the United States currently provide nearly 7 quads
of solar energy, and their output could be increased to provide up to 9 quads
by the year 2050. The remaining 28 quads of solar renewable energy needed by
2050 is projected to be produced by wind power, photovoltaics, solar thermal
energy, and passive solar heating. These technologies should be able to provide
energy without interfering with required food and forest production.
If the United States does not commit itself to the transition from fossil to
renewable energy during the next decade or two, the economy and national security
will be adversely affected. Starting immediately, it is paramount that US residents
must work together to conserve energy, land, water, and biological resources.
To ensure a reasonable standard of living in the future, there must be a fair
balance between human population density and energy, land, water, and biological
We thank the following people for reading an earlier draft of this article,
for their many helpful suggestions, and in some cases, for providing additional
information: A. Baldwin, Office of Technology Assessment, US Congress; A. A.
Bartlett, University of Colorado, Boulder; E. DeMeo, Electric Power Research
Institute; H. English, Passive Solar Industries Council; S. L. Frye, Bechtel;
M. Giampietro, National Institute of Nutrition, Rome, Italy; J. Goldemberg,
Universidade de Sao Paulo, Brazil; C. A. S. Hall, College of Environmental Science
and Forestry, SUNY, Syracuse; D. O. Hall, King's College, London, United Kingdom;
S. Harris, Oak Harbor, WA; J. Harvey, New York State Energy Research and Development
Authority; B. D. Howard, The Alliance to Save Energy; C. V. Kidd, Washington,
DC; N. Lenssen, Worldwatch Institute; L. R. Lynd, Dartmouth College; J. M. Nogueira,
Universidade de Brasilia, Brazil; M. G. Paoletti, University of Padova, Italy;
R. Ristenen, University of Colorado, Boulder; S. Sklar, Solar Energy Industries
Association; R. Swisher, American Wind Energy Association; R. W. Tresher, National
Renewable Energy Laboratory; L. L. Vant-Hull, University of Houston; Wang Zhaoqian,
Zheijan Agricultural University, China; P. B. Weisz, University of Pennsylvania,
Philadelphia; Wen Dazhong, Academia Sinica, China; C. J. Winter, Deutsche Forschungsanstalt
fur Luft und Raumfahrt, Germany; D. L. Wise, Northeastern University, Boston;
and at Cornell University: R. Barker, S. Bukkens, D. Hammer, L. Levitan, S.
Linke, and M. Pimentel.
Advisory Committe on Technology Innovation (ACTI). 1983. Alcohol Fuels: Options
for Developing Countries. National Academy Press, Washington, DC.
Alfheim, 1., and T. Ramdahl. 1986. Mutagenic and carcinogenic Compounds from
Energy Generation Final Report No. NP-6752963 (NTIS No. DE 86752963). Center
for Industriforkning. Oslo, Norway.
Alliance to Save Energy (ASE). 1993. Federal Energy Subsidies: Energy, Environmental,
and Fiscal Impacts The Alliance to Save Energy, Washington, DC.
American Society for Testing Materials (ASTM). 1992. Standard guide for radon
control options for the design and construction of new low rise residential
buildings. Pages 1117-1123 in Annual/ Book of American Society for Testing Materials.
E1465-92. ASTM, Philadelphia, PA.
American Wind Energy Association (AWEA). 1991. Wind energy comes of age. Solar
Today 5: 14-16.
1992. Wind Technology Status Report. American Wind Energy Association, Washington,
Baechler, M. C., and A. D. Lee. 1991. Implications of environmental externalities
assessments for solar thermal power plants. Pages 151-158 in T. R. Mancini K.
Watanabe and D. E. Klett, eds. Solar Engineering 1991. American Society of Mechanical
Engineering, New York.
Barber, M. 1993. Why more energy? The hidden cost of Canada's cheap power.
Forests, Trees and People Newsletter No. 19: 26-29.
Bartlett, A. A. 1994. Fusion: An illusion or a practical source of energy?
Clearinghouse Bulletin 4(1): 1-3, 7.
Beninga, K., R. Davenport, J. Sandubrue, and K. Walcott. 1991. Design and fabrication
of a market ready stretched membrane heliostat. Pages 229-234 in T. R. Mancini,
K. Watanabe and D. E. Klett eds. Solar Engineering 1991. American Society of
Mechanical Engineering, New York.
Birdsey, R. A. 1992. Carbon storage and accumulation in United States forest
ecosystems. General Technical Report WO 59. USDA Forest Service, Washington,
Bockris, J. O. M., and J. C. Wass. 1988. About the real economics of massive
hydrogen production at 2010 AD. Pages 101-151 in T. N. Veziroglu and A. N. Protsenko,
eds. Hydrogen Energy Vll. Pergamon Press, New York.
Chahroudi, D. 1992. Weather panel architecture: a passive solar solution for
cloudy climates. Solar Today 6: 17-20.
Clarke, A. 1991. Wind energy progress and potential. Energy Policy 19: 742-755.
DeMeo, E. A., F. R. Goodman, T. M. Peterson, and J. C. Schaefer. 1991. Solar
Photovoltaic Power: A US Electric Utility R & D Perspective. Edited by 2.1.
P. S. conference. IEEE Photovoltaic Specialist Conference Proceedings, New York.
Dickson, Y. L., and B. C. Yates. 1983. Examination of the Environmental Effects
of Salt-Gradient Solar Ponds in the Red River Basin of Texas. North Texas State
University, Denton Institute of Applied Sciences, Denton, TX.
Department of Energy (DOE). 1980. Health Effects of Residential Wood Combustion:
survey of Know ledge and Research. DOE, Technology Assessments Division, Washington,
1990. The Potential of Renew able Energy. SERI, DOE, Golden, CO.
1991a. A Annual Energy Outlook with Projections to 2010. DOE, Energy Information
Administration, Washington, DC.
199lb. 1989 International Energy Annual/. DOE, Washington, DC.
1992. Wind Energy Program Overview. National Renewable Energy Laboratory, DOE,
DOE/Conservation Energy (CE). 1988. Passive and Active Solar Domestic Hot Water
Systems. FS-119. DOE, Conservation and Renewable Energy Inquiry and Referral
Service, Washington, DC.
Energy Information Administration (EIA). 1990. Electric Plant Cost and Power
Production Expenses. EIA, Washington, DC.
Electric Power Research Institute (EPRI). 1991. Photovoltaic system performance
assessment for 1989. EPRI Interim Report 65-7286. EPRI, Los Angeles, CA.
Food and Agriculture Organization (FAO). 1991. Food Balance Sheets. FAO, Rome.
Federal Energy Regulatory Commission (FERC). 1984. Hydroelectric Power Resources
of the United States. FERC, Washington, DC.
1988. Hydroelectric Power Resources of the United States: Developed and Undeveloped.
FERC, Washington, DC.
Flavin, C. 1985. Renewable Energy at the Crossroads. Center for Renewable Resources,
Flavin, C., and N. Lenssen.1991. Here comes the sun. Pages 10-18 in L. Brown
et al., ed s. State of the World 1991.Worldwatch Institute, Washington, DC.
Folchitto, S. 1991. Seawater as salt and water source for solar ponds. Sol.
Energy 46: 343-51.
Gibbons, J. H., and P. D. Blair. 1991. US energy transition: on getting from
here to there. Physics Today, 44: 22-30.
Gommend, K., and G. Grossman. 1988. Process steam generation by temperature
boosting of heat from solar ponds. Solar Ponds 41: 81-89.
Hall, C. A. S., C. J. Cleveland, and R. L. Kaufmann. 1986. Energy and Resource
Quality: The Ecology of the Economic Process. Wiley, New York.
Hall, D. O., F. Rosillo-Calle, R. H. Williams, and J. Woods. 1993. Biomass
for energy: supply prospects. In T. B. Johansson, H. Kelley, A. K. N. Reddy,
and R. H. Williams, eds. Renewable Energy. Island Press, Washington, DC.
Holdren, J. P., G. Morris, and 1. Mintzer. 1980. Environmental aspects of renewable
energy. Annual Review of Energy 5: 241-291.
Howard, B., and S. S. Szoke. 1992. Advances in Solar Design Tools. 5th Thermal
Envelope Conference, December 1992. US Dept. of Energy, ASHRAE, Clearwater,
International Commission on Large Dams (ICLD). 1988. World Register of Dams.
International Energy Agency (IEA). 1987. Clean Coal Technology. OECD Publications,
1991. E??ergy Statistics of OECD Countries. IEA, Paris.
Jensen, C., H. Price, and D. Kearnev. 1989. The SEGS power plants: 1989 performance.
Pages 97-102 in A. H I anney and K. O. 1 und, eds. Solar Engineering 1989. American
Society of Mechanical Engineering, New York.
Kellet, J. 1990. The environmental impacts of wind energy developments. Town
Planning Review 61: 139-154.
Kennedy, T., J. Finnell, and D. Kumor. l 991. Considering environmental costs
in energy planning: alternative approaches and implementation. Pages 145-150
in T. R. Mancini, K. Watanabe and D. E. Klett, eds. Solar Engineering 1991.
American Society of Mechanical Engineering, New York.
Kohl, W. L. 1990. Methanol as an Alternative Fuel Choice: An Assessment The
Johns Hopkins Foreign Policy Institute, Washington, DC.
Lipfert, F. W., R. G. Malone, M. L. Dawn, N. R. Mendell, and C. C. Young. 1988.
Statisical Study of the Macroepidemiology of Air Pollution and Total Mortality.
Report No. BNL 52122. US Dept. of Energy, Washington, DC.
Lynd, L. R., J. H. Cushman, R. J. Nichols, and C. E. Wyman. 1991. Fuel ethanol
from cellulose biomass. Science 251: 1318-1323.
Maeda, Y., and M. Kai. 1991. Recent progress in pervaporation membranes for
water/ethanol separation. Pages 391-435 in R. V. M. Huang, ed. Pervaporation
11 Membrane Separation Processes. Elsevier, Amsterdam, the Netherlands.
Matare, H. F. 1989. Energy: Fact and Future. CRC Press, Boca Raton, FL.
Mihlmester P. E., J. B. Thomasian, and M. R. Riches. 1980. Environmental and
health safety issues. Pages 731-762 in W. C. Dickinson and P. N. Cheremisinoff.
eds. Solar Energy Technology Handbook. Marcel Dekker,New York.
Moore, T. 1992. High hopes for high-power solar. EPRI Journal/ 17 (December):
Ogden, J. M., and R. H. Williams. 1989. Solar Hydrogen: Moving Beyond Fossil
Fuels. World Resources Institute, Washington, DC.
Peschka, W. 1987. The status of handling and storage techniques for liquid
hydrogen in motor vehicles. International Journal of Hydrogen Energy 12: 753-764.
Pimentel, D. 1991. Ethanol fuels: Energy security, economics, and the environment.
Journal of Agricultural and Environmental Ethics 4: 1-13.
Pimentel, D. 1992. Competition for land: development, food, and fuel. Pages
325-348 in M. A. Kuliasha, A. Zucker and K. J. Ballew, eds. Technologies for
a Greenhouse-constrained Society. Lewis, Boca Raton, FL.
Pimentel, D., L. Levitan, J. Heinze, M. Loehr, W. Naegeli, J. Bakker, J. Eder,
B. Modelski, and M. Morrow. 1984. Solar energy land and biota. SunWorld 8: 70-73
Pimentel, D., M. Herdendorf, S. Eisenfeld, L. Olanden M. Carroquino, C. Corson,
J. McDade Y. Chung, W. Cannon, J. Roberts, L. Bluman, and J. Gregg. 1994. Achieving
a secure energy future: environmental and economic issues. Ecological Economics,
Schurr, S.M., J. Darmsradter, H. Perry, W. Ra msay, and M. Russel 1. 1979.
Energy in America's Future: The Choices Before Us. A Study Prepared for RFF
National Energy Strategies. The Johns Hopkins University Press, Baltimore, MD.
Sillman, S., and P. J. Samson. 1990. Impact of methanol-fueled vehicles on
rural and urban ozone concentrations during a region-wide ozone episode in the
mid-west. Pages 121-137 in W. L. Kohl, ed. Methanol as an Alternative Fuel Choice:
An Assessment. The Johns Hopkins Foreign Policy Institute, Washington, DC.
Skinrood, A. C., and P. E. Skvarna. 1986. Three years of test and operation
at Solar One. Pages 105-122 in M. Becker, ed. Solar Thermal Receiver Systems.
Springer Verlag, Berlin.
Smil, V. 1984. On energy and land. Am. Sci. 72: 15-21.
Smith, D. R. 1987a. The wind farms of the Altamont Pass area. Annual/ Review'
of Energy 12: 145-183.
Smith, D. R., and M. A. Ilyin. 1991. Wind and solar energy, costs and value.
The American Society of Mechanical Engineers, IOth Annual Wind Energy Symposium:
Smith, K. R. 1987b. Biofuels, Air Pollution, and Health: A Global Review. Plenum
Press, New York.
Steele, A. 1991. An environmental impact assessment of the proposal to build
a wind farm at Langdon common in the North Pennines, U.K. The Environmentalist
Tabor, H. Z., and B. Doran. 1990. The Beith Ha'arva SMWe solar pond power plant
(SPPP)-Progress Report. Solar Energy 45: 247-253.
Tester, J. W., D. O. Wood, and N. A. Ferrari. 1991. Energy and the Environment
in the 2 Ist Century. The MIT Press, Cambridge, MA.
Twidell, J. 1987. A Guide to Small Wind Energy Conversion Systems. Cambridge
University Press, Cambridge, UK.
US Bureau of the Census (USBC). 1992a. Statistical Abstract of the United States
1992. US Government Printing Office, Washington, DC.
1992b. Current Population Reports. January ed. USBC, Washington, DC.
US Department of Agriculture (USDA).1992. Agricultural Statistics. US Government
Printing Office, Washington, DC.
Vant-Hull, L. L. 1992. Solar thermal receivers: current status and future promise.
American Solar Energy Society 7: 13-16.
Veziroglu, T. N., and F. Barbir. 1992. Hydrogen: the wonder fuel. International
Journal of Hydrogen Energy 17: 391-404.
Voigt, C. 1984. Material and energy requirements of solar hydrogen plants.
International Journal of Hydrogen Energy 9: 491-500.
Warner, I L. 1991. Consumer guide to energy saving windows. Solar Today 5:
West, R. E., and F. Kreith. 1988. Economic Analysis of Solar Therma/ Energy
Systems. The MIT Press, Cambridge, MA.
Williams, T. A., D. R. Brown, J. A. Dirks, K. K. Humphreys, and J. L. La Marche.
1990. Potential impacts of CO2 emission standards on the economics of central
receiver power systems. Pages 1-6 in J. T. Beard and M. A. Ebadian, eds. Solar
Engineering 1990. American Society of Mechanical Engineering, New York.
Winter, C.-l. 1986. Hydrogen energy-Expected engineering break-throughs. Pages
9-29 in T. N. Veziroglu, N. Getoff and P. Weinzierl, eds. Hydrogen Energy Progress
Vl. Pergamon Press, New York.
Winter, C.-J. 1991. High temperature solar energy utilization after I 5 years
R&D: kick-off for the third generation technologies. Solar Energy Materials
Winter, C.-l., and 1. Nitsch, eds. 1988. Hydrogen as an Energy Carrier: Technologies,
Systems, and Economy. Springer Verlag, Berlin.
Winter, C.-l., W. Meineke, and A. Neumann. 1992. Solar thermal power plants:
No need for energy raw material-only conversion technologies pose environmental
questions. Pages 1981-1986 in M. E. Arden, S. M. A. Burley and M. Coleman, eds.
1991 Solar World Congress: Proceedings of the Biennial Congress of the International
Solar Energy Society. Vol. 2, part 11. Pergamon Press, Oxford, UK.
Wittwer, V., W. Platzer, and M. Rommell. 1991. Transparent insulation: an innovative
European technology holds promise as one route to more efficient solar buildings
and collectors. Solar Today 5: 20-22.
Wolfson, R. 1991. Nuclear Choices. MIT Press, Cambridge, MA.
Worldwatch Institute. 1989. State of the World. Worldwatch Institute, Washington,
1992. State of the World. Worldwatch Institute, Washington, DC.