Renewable Resource Readings
I. Hydroelectric
Hydroelectric power has been used for hundreds of years to harness the power of
naturally flowing water, usually in the form of dams. Currently, hydropower provides 20% of the
world’s energy. Hydropower is a renewable resource, as the water cycle is driven by the sun.
Surface water is constantly replenished through evaporation, condensation and subsequent
precipitation back down to the Earth’s surface.
There are currently 3 different methods to generating hydroelectric power: dams, “run of
river,” and tidal/waves. Dams are barriers that impound or stop water flowing down a river or
stream and slows lets it out through special tunnels to generate electricity, provide water for
irrigation of crops and control frequent flooding. The creation of a dam creates a reservoir (lake)
behind a dam, which can be used for recreational activities such as boating, fishing, etc. Present
day dams range from large to small; the majority of energy is generate by large dams, although
China uses small-scale dams and still generates 50% of the world’s hydropower. Pennsylvania
currently has 23 dams which generate electricity.
A. Dams allow small amounts of water to flow from the reservoir through the intake
valve, through a small tunnel called a
penstock until it meets a turbine,
which is a machine with blades that
spins from the water flow. The
spinning blades create mechanical
(kinetic) energy, which is sent to the
generator, which transforms it into
electrical energy. This electrical
energy is stored in the powerhouse of
the dam and then eventually sent to
power lines for homes and
businesses. The water, after flowing
through the turbine, is released into
the river below the dam.
Dams provide relatively clean energy with minimal release of CO2 (from decaying
vegetation in reservoir), controls flooding, create recreational lakes, and is available 24 hours a
day. Disadvantages of dams include displacing people from their homes from the flooding during
the creation of the dam, ruining ecosystems/wildlife loss and disrupting the migrations of many
fish species (like salmon) who swim upstream to mate and spawn. There are also limited areas to
build dams.
B. Run of the river refers to a smaller version of hydropower generation. Water is diverted from
a main river into a pipe, which
runs downhill to a powerhouse to
use the force of the water to spin
turbines and generate electricity,
releasing water back into the
river. The downhill elevation uses
gravity to build up excess force
and pressure to spin the turbines
faster. Run of the rivers do not
use dams and therefore do not
face the same challenges that
dams do. The problem is that they
do not produce enough energy to
be commercially viable.
C. Tidal/wave hydroelectric power is generated from the force of ocean tides or waves flowing in
and out. The waves spin turbines in machines found either floating on the ocean or sitting on the
shoreline of estuaries. Originally, small dams were built in estuaries to harness the tide energy
but this destroys the valuable estuary ecosystems. Recently,
scientists have developed a floating barge filled with turbines
and generators to create electricity, which is transmitted to land
through underwater cables attached to the barge (see picture to
the right). Tidal power faces problems with resisting strong
ocean storms and inconsistent power generation on days where
the ocean tide is not strong.
B. Biomass/Biofuel/Biogas
Biomass is defined as any organic matter (living or once living) which contains
carbohydrates (sugars). Biomass fuels include wood, any sort of crop, garbage or waste. Humans
have been burning wood for energy since the days of the caveman. Third world countries still
use wood as their primary source of energy for heating their homes, cooking and other domestic
activities. Firewood can also be turned into charcoal for longer lasting energy. Problems with
wood burning include air pollution (soot), deforestation from people clearing forests for
firewood and a large amount of time invested in finding/transporting firewood for developing
countries. Wood is considered a renewable resource but it is not considered sustainable (meaning
we cannot replenish our wood supply at the same rate that we are consuming it).
Recently, we have been turning to alternative biomass sources for fuel, such as creating
biofuels (such as biodiesel and ethanol) and biogas.
Biofuels are created from vegetable or animal based oils such as canola/soybean oil, palm
oil, grease, sewage or leftover trash, which are refined down to their oils and used as fuel for
vehicles. Any type of plant or animal waste with a high oil content can be used. Biodiesel is a
type of fuel made from vegetable or animal oils that is often blended in with regular diesel fuel
(which is made from oil). Biodiesel is a cleaner alternative to diesel. Different vehicles are able
to utilize different levels of biodiesel as fuel (but not all vehicles can use diesel).
Problems with biodiesel include: losing oils/crops that we use for food, deforestation to
make room for biodiesel crops, often difficult to store/transport, difficult to grow. Biodiesel still
gives of CO2 when combusted in vehicles but it is considered carbon neutral because the plants
absorbed almost the same amount of CO2 when they are growing as they release when burned.
Ethanol is an alcohol created by fermenting the sugars found in plants. Ethanol is often
blended with gasoline (gas stations generally have 10% ethanol and 90% gasoline). The U.S.
currently uses corn to produce ethanol but crops such as sugarcane and sugar beets will work as
well. We are currently experimenting with cheap woody plants that can grow on soil where few
other crops can grow to allow for a cheaper fuel source. \
Scientists are also beginning to study the use of algae as a source for biodiesel and
ethanol. Algae grows 30x as fast as plants and stores up to the half of its total body mass as oil,
meaning there is a greater yield of oil to use as fuel per algae. We haven’t yet found a cheap way
to culture algae; present day facilities are extremely expensive to build and maintain but future
technologies will hopefully improve these facilities. Scientists hope to build algae plants next to
fossil fuel plants to utilize the CO2 waste from those plants to help the algae grow.
Biogas refers to when we capture methane gas
from fermenting animal waste, human sewage, or gas
released from landfills to use as fuel for heating or
cooking. Ordinarily, this gas is released as a greenhouse
gas, trapping heat on the Earth’s surface. Rural
communities in developing countries are investing in
digester tanks made from local materials, where they
allow animal waste to ferment and release gas. The
bacteria present in the manure will anaerobically (without
oxygen) break down the manure into gas and fertilizer.
The communities gain a source of gas for
cooking/heat, profit from selling the gas to nearby
communities, fertilizer from the fermented animal
waste and an extra profit from animals products (ex:
milk from cows). Gas does not produce smoke like
wood fuel does, which reduces the number of diseases
found in the community. People do not have to spend
as much time searching for firewood, which allows
them to use their time for other pursuits and frees up
children to go to school instead of gathering firewood.
Advantages for biofuels include using waste
that otherwise goes unused, it reduces our dependence
on oil, relatively low sulfur content in plants/animals
does not lead to acid rain (unlike fossil fuels which
contain a lot of sulfur), and they are carbon neutral
(equal CO2 taken in as plants as released when
burned). A major disadvantage is that the crops
which are usually used for food, are now being used
for fuel, leading to food shortages.
C. Solar Power
Solar energy from the sun has the potential to provide energy for 5 billion years, if we are
able to harness and store it. Essentially, most modern sources of energy come from the sun- coal,
oil, biomass- all of it originated as plants which were either eaten by animals who later decayed
or the plants themselves decayed and over time became a fossil fuel.
Solar energy can be used to heat buildings/water and provide electricity for lighting/appliances.
There are passive and active solar heating systems.
Passive heating systems is one in which a building or structure is designed to absorb heat
during the day and slowly release the heat at night. They also designed to absorb maximum heat
during the winter and reject heat in the summer. Design modifications include large windows on
the equator side of the house to absorb sunlight, having a lot insulation for the winter, overhang
on the roof to prevent excess heat during the summer months, and use building materials with
thermal mass (meaning they help to slow temperature changes). These modifications are called
passive because they do not use active mechanical systems; they simply transform solar heat into
usable heat (heat house, store heat in thermal mass for later).
Active solar systems concentrate sunlight to either produce
electricity or heat water. There are three main methods of active
solar system: photovoltaic cells, concentrated cells, and
concentrated photovoltaic.
Photovoltaic cells are the large blue solar panels you are
probably familiar with. Photovoltaic cells are essentially made of two silicon plates stacked on
top of each other and connected with a wire. Each plate
is laced with different elements to change the amount of
electrons. The top plate has an excess of electrons,
while the bottom plate has few electrons. This
difference in electrons through the wire that connects
the plates creates an electric current (electricity). The
problem with photovoltaic cells is that they are
incredibly expensive to make and currently inefficient.
We are researching making photovoltaic cells out of
more effective materials but they are not currently
ready for widespread use.
Concentrated solar cells contain mirrors that act like
magnifying glasses to amplify the sun’s power, without using
the expensive photovoltaic cells. There are two ways to use
concentrated solar cells: you can have a field full of mirrors
which rotate to reflect the sun to a single tower, which collects
the energy. It is still extremely expensive to create a tower that
is capable of handling intense heat from all the mirrors but it still less expensive than pure
photovoltaics. You can also have the mirrors funnel it to heat a small pipe of oil found between
the two mirrors. The oil is superheated and poured off into water, which is vaporized and turned
to steam. This steam powers the turbine (by causing the blades to spin) to generate electricity.
This method eliminates the major expense of photovoltaic cells but it is still too expensive to
consider on a large scale.
Finally, there are concentrated photovoltaics,
which basically combine the other two methods together
and currently the most cost effective method for solar
energy. We use huge mirrors to concentrate solar energy to
one small photovoltaic cell. Because the cell is small, this
cuts down on the majority of the costs while still reaping
most of the benefits.
The major disadvantage of solar energy is that the sun is (obviously) not present 24/7
and we need to find ways to store that energy for the nighttime. We do have some solutions (like
storing heat in molten salt, or storing steam pressure in large caverns in the ground releasing it
through turbines at nighttime) but they are currently too expensive to consider. While solar
energy is extremely clean (limited pollution) and we have an unlimited supply of solar energy as
long as the sun still burns, most countries consider it still too expensive to use on a nationwide
scale despite the long term benefits it would bring.
D. Geothermal Energy
Geothermal energy production uses the heat from the Earth to heat up fluid in cracks in
the ground to create stream to drive a turbine. Molten magma in the Earth’s core heats the
groundwater which seeps into cracks in the Earth’s crust to create steam, which we can
sometimes see on the surface as hot springs or geysers (these areas are often called hot spots).
We can also snake pipes throughout the crust and pump water or other fluid through the pipes to
be heated by the Earth’s core and turn to steam.
The steam is used to spin turbines to generate
electricity. The steam is then guided to a chamber
where it condenses back into the cold water. The
remaining colder water is pumped back into the
Earth’s crust for reheating. Geothermal energy
can be used either in large plants to generate
electricity to whole communities or for personal
heat pumps in your home. Because the ground
remains at a fairly constant temperature, it can be
used to cool down the house in summer and heat
it up in winter. The capital of Iceland receives all
of its heat from volcanic geothermal energy.
There are three major ways to utilize geothermal energy. First, you can tap into the areas
where water has seeped into the crust, is heated up and rises to the surface. You drill holes into
the crust to release and capture the steam to power our turbines. The problem with this method
lies in the fact that not all areas produce steam (some only heat up water below the boiling point)
and eventually, those areas that do have steam will run out. Scientists are concerned that when
we pump water back into the Earth to replenish the groundwater for steam that the aquifers will
become contaminated.
Secondly, you
can pump up the
extremely hot water
and depressurize
(called “flashing”) it to
create steam. These
flash steam plants are
useful for areas where
the Earth only heats
water up but cannot
create steam below the
Earth’s surface.
Lastly, you can create a binary cycle
system, which is when we use the hot water from
the Earth and use it to heat another liquid like
isobutane to steam. We choose another liquid
which boils at a lower temperature than water
(need less energy to create the steam). We then
return the water to the ground.
Advantages of geothermal energy include a lack of pollution, a constant source of energy
24/7 and less expensive to operate in the long run when compared to fossil fuel plants.
Governments are also offering tax breaks as incentives for business and home owners to utilize
this technology.
Disadvantages of geothermal energy are due to a difficult and extremely expensive initial
installation, pipes can corrode and pollute the water supply, hazardous gases are sometimes
released with the water and there is the possibility that the constant movement under the Earth’s
surface causes earthquakes. Another major detriment lies in the fact that not all areas are close
enough to the magma to produce steam, which makes them unsuitable for large scale geothermal
energy production. When heating your home using geothermal methods, you do not need to be in
a particularly hot spot of the Earth.
E. Wind
Wind turbines are major suppliers of clean energy for many developing
countries, such as India, and is found in developed countries such as the U.S.,
Europe and China. Wind turbines are essentially tall modern windmills used to
generate electricity, whereas windmills in the old days were used to mill grain
or pump water. The tallest can generate up to 6 million kWh of electricity each
year. To give you a sense of scale, the average American home uses about 7,200 kWh each year.
Wind turbines generally have
3 blades, which are connected to and
rotate around the nacelle, which is
the rectangular body on top of the
vertical tower. Inside the nacelle,
there is a gear box and a generator.
There are wind sensors on the outside
to move the wind turbine in response
to the direction and speed of the
wind. The nacelle and cables inside
the tower can twist to allow for the
twisting of the wind turbine. When
the wind spins the blades, the blades
spin a rotor, which is attached to a
shaft. The shaft spins the gear box, which is attached to the generator and increases the speed at
which the generator spins. This causes the wires in the generator to spin around a magnet,
generating electricity from mechanical energy. This electricity is sent down the cables to the
transformer box on the bottom of the tower. The transformer amplifies the electricity from about
690 volts to thousands of volts, which are then sent to the power grid.
Wind turbines are grouped together into a
wind farm to generate bulk electricity.
Wind turbines obviously can be found
areas where there are significant
wind speeds for the majority of the
year (see map below). Good wind
resources have an average annual wind
speed of at least 13mph. In PA, there
are currently 24 wind projects in
operation. The swirls on the map to
the left show the locations of those
wind farms.
Wind energy is the cheapest source of new power because they are easily and cheaply
built. There is no pollution associated with windmills and the farmers receive income for wind
turbines that they build or rent on their farmland. The manufacturing and maintenance of
turbines, as well as the transport of the generated electricity help to create jobs and income for
locals. With that being said, wind power still requires a higher initial cost than fossil-fueled
plants. If considered over the long term, the costs are significantly less than fossil fuel plants
because there is no fuel to purchase to continue the use of the turbines.
Other disadvantages include environmental concerns. Bats and migratory birds are killed
by flying into the wind turbines; scientists have been working to place wind farms out of
migratory patterns and the majority of these issues have been resolved. People also do not want
the turbines near their homes due to the white noise produced by the rotor blades and some
consider them unsightly. The major challenge to using wind power is that it is intermittent,
meaning that it is not always windy and it does not always provide electricity when it is needed.
They can only be placed in certain, often remote locations, which means that we need to find
ways to easily transport the energy generated to areas of demand (cities). We currently store the
electricity in batteries for storage.