Welcome everybody. Last week we have looked into concept of energy

and the energy use of some major sectors. By now it will be clear for you all that our

energy need in the world is significant. So the big question is: Do we have enough

renewable energy sources available on planet earth to sustain our enormous energy need? This week, we will look at the most important

renewable energy sources. We will make rough estimations on what these

energy sources potentially can deliver in reference to our energy needs. We will do this using rough -back-of-the envelope-calculations. This week will give you some first insights

in what are the opportunities and limitations of the various energy source. Be aware that we only make rough estimations

this week. Later on during this course we will look in

more detail to the technological and system aspects of the various renewable energy sources. In this first video lecture we will take a

look at Hydro power. There are many large Hydro power plants around

the world. But most of them look like this. Water that contains potential energy is collected

in a reservoir behind a dam and flows to the intake .

The water is forced through the pipe or ‘penstock’ by gravity. The turbine in the dam is pushed in motion

and the potential energy is converted to mechanical energy. The turbine is connected to a huge electric

generator to convert the mechanical energy into electrical energy. The generator is connected via transformers

to the electricity grid to deliver electricity in the grid. Losses in the system are mainly caused by

friction at the edges of the pipe. The turbine and electrical losses also further

reduce the efficiency. When estimating the potential energy of Hydro

power, the most important factors are the available waterflow, either via melting snow

or rain, and the height difference of the water representing the potential energy to

be converted. As example we take a close look at the Itaipu

Dam on the border of Brazil and Paraguay to get a feeling of how to estimate the potential

of hydro energy. The ITAIPU dam is the second largest hydropower

plant on the planet. It has a total installed generation capacity

of 14GW, and has reached a record energy generation of 98,6 TWh per year or 0.27TWh per day. If we assume the generated energy is solely

used by the 200 million inhabitants of brazil, we arrive at total electricity generation

of 1.35 kWh for every person everyday. Let’s relate that back to the energy unit

of one person as we have discussed last week. One personal unit of energy was equal to 2.9

kWh per day. So, 1.35 kWh/person/per day would equal 0.45

man units of energy production per day. Another way of comparing the yield of any

renewable energy source is to look at the annual energy yield per rate power or in other

words how much kWh of energy is generated per year per Watt of installed capacity. In this case we have to divide 98.6 TWh by

the 14 GW of installed capacity. For the Itaipu dam we arrive at a little more

than 7 kWh/Watt/year. A value expressed in kWh/W/year will also

be used for other renewable technologies we introduce this week for comparison. Another parameter used in the industry to

compare the production of different powerplants is the capacity factor. It is an easy tool to estimate energy production

over a period of time. The Itaipu dam has a capacity factor of 0.80. Which is the annual production divided by

the installed capacity and the hours in a year (8760). So you could say on average the Dam produces

80 % of the time electricity on maximum capacity 80% . For fossil fuel plants like gas and coal plants

the capacity factor can be very close to one meaning it will run almost continuously on

installed capacity. This is something electricity grid operators

are very keen of to ensure stability on the grid. Other renewable energy sources like solar

and wind can have much lower capacity factors. We will look in to those later this week. For now we will make an estimate of the potential

of hydroelectricity W/m^2. Let’s work through an example. Let’s say a certain region has about 500mm

of rain per year and there is no meltwater from mountainsnow or glaciers. 500mm of rain per year, is equal to 0.5 m^3

of water per square meter. By multiplying that with the density of water,

which is 1000 kg/m^3, we get that per square meter of land there is 500 kg of water per

year. To calculate the potential energy of this

amount of water we will use this equation. Where E_pot is the potential energy in Joule,

M is the mass of the water, g is the gravitational constant of 9.8, and h is the height difference

between the source and the outflow of the dam. Which we will assume is 100m. For these numbers we get 490 Kilo Joules of

potential energy per square meter per year which is equal to about 0.14 kWh, so now we

know the energy density in terms of kWh/m^2/year. We can express this in power per area by dividing

by the total seconds in a year to get 0.015 W/m^2. However, this calculation assumes that all

potential energy is converted into electricity. In reality this is not the case and dependent

on the landscape surrounding the Dam. A hydroelectricity dam is usually build next

to a natural lake or water reservoir that collects the water fallen on a large area

through a network of rivers. The total area from which the reservoir collects

the water is referred to as the catchment area enclosed by the red line in this picture. If we know the size of this area we can estimate

the total energy potential of a certain region or dam. Now we are going back to the itaipu dam and

we will make a rough estimation, to verify it’s typical power and yield. The catchment area stretches an incredible

1.35 million square km. In this area the averaged annual rainfall

is 1650 mm/year. About 1000 mm per year of this water is evaporated

before it is passing the dam. The remainder equals a mass of 0.65 m^3 of

water. If we take the height of the dam, which is

196m, using the potential energy formula we arrive at 0.35 kWh/m^2/year. The overall conversion efficiency of potential

energy to electrical energy of this dam is estimated to be about 20%, including the collection

losses as well as the conversion efficiency of the turbines in the dam itself. With this conversion efficiency of 20% we

arrive at an achieved energy density of 0.07 kWh/m^2/year. If we now take a look at the actual data of

the dam, and divide the annual energy production which of the dam by the catchment area, we

find that the they are achieving approximately 0.073 kWh/m^2/year. This is almost the same energy density as

we found a moment ago, when calculating the potential and achievable energy density. However, we can also take the reservoir surface

area of 1350 square kilometers as a measure for the required land. This gives a good indication of the amount

of land that is sacrificed for the hydro power plant, and is often used when comparing the

impact of hydropower plants. When we do this we get a higher energy density

of 73 kWh/m^2/year. It is also interesting to take a look at the

surface power density or specific power for both defined areas. We can find this by dividing the energy density

by all the hours in a year. For the entire catchment area we get a power

density of 0.008W/m^2, whereas for the reservoir surface area we find a surface power density

of 8.3 W/m^2. As you can see, with some fairly simple rough

calculations, a general estimate can be made of the potential energy that can be gained

from this source. However, the energy potential is not the only

factor that should be considered. Also the immediate and long term effects on

the surroundings and the climate should be taken into account. Hydroelectricity mostly requires a dam to

be built, in order to create a reservoir. This reservoir has a huge impact on the local

flora, fauna, and landscape. In case of really large Hydroelectricity projects,

the local climate could even change, due to the change in the water cycle. One of the biggest benefits of Hydropower

is the consistency of the power source because of it’s high capacity factor. Although it is not always free from seasonal

intermittency. Also, because of the scale of the projects

and the lifetime of 30 to over 100 years, the carbon footprint per kWh of generated

energy, as well as the cost per kWh, are low. Typically the carbon footprint of Hydroelectricity

is 24 gCO2/kWh on average, according to research of the Intergovernmental panel on climate

change from 2014. This is much lower than coal fired power plants,

which have an average carbon footprint of 820 gCO2/kWh. However, the carbon footprint of hydropower

does greatly depend on each individual project. As you can see, there are a few major factors

that have to already be in place before hydropower can be considered. The most important factor is the availability

of water, and a mostly natural height difference. Also the environmental impact of a hydropower

plant should be taken into account. Because of the necessity of a natural feature

that allows for the easy implementation of Hydropower and the impact on the surroundings

of a dam, the global potential of new hydropower is relatively limited. In 2015 the estimated global installed capacity

was 1211 GW, producing approximately 3975 TWh of electricity. This represents approximately 16.6% of the

global electricity demand, and about 70% of all renewable electricity. We can also express the production of hydroelectricity

in our own units. If we divide it by the global population of

7.4 billion people and days in a year we get 1.47 kWh/person/day. This is a slightly more then half a human

unit. Estimations for the global potential of energy

for hydro go up to 25% of the global energy use. With these tools you can try to make your

own assessment of hydroelectricity. In the next video we will look into physical

potential wind energy.