Energy Transformation: Renewable Energy - Physics (Grade 10)

Read the Device by Its Path

Energy transformation is the change of energy from one form into another. The word "transformation" does not mean energy is created from nothing. Energy still follows the law of energy conservation, but its form and transfer path change.

The U.S. Energy Information Administration (EIA) explains that energy is not created or destroyed when used, but changes form. EIA also explains that every conversion produces useful energy and energy that is not useful for the device's purpose. EIA's Laws of Energy page can be opened through eia.gov.

The cleanest way to read an energy technology is to write its path.

\text{energy source} \rightarrow \text{initial form} \rightarrow \text{conversion device} \rightarrow \text{useful energy}+\text{dissipated energy}

Dissipated energy is not lost energy. It spreads to the surroundings, for example as heat, sound, vibration, or reflected light.

Conversion Paths in Power Plants

The diagram below helps us ask one simple question: what does energy from nature become before we use it?

mermaid

In hydropower, the U.S. Department of Energy (DOE) explains that water flows through a penstock, spins turbine blades, and the turbine spins a generator that produces electricity. DOE's explanation of how hydropower works can be opened through energy.gov.

In wind power, DOE explains that wind turns turbine blades, the rotor spins a generator, and the generator creates electricity. DOE's page on how wind turbines work can be opened through energy.gov.

In a photovoltaic solar panel, EIA explains that a photovoltaic cell converts sunlight directly into electricity. However, only photons absorbed by the semiconductor material provide energy to generate electricity. EIA's Photovoltaics and Electricity page can be opened through eia.gov.

Technology	Main conversion path	Energy often dissipated as
Hydropower plant	$\text{gravitational potential of water}\rightarrow\text{water motion}\rightarrow\text{turbine motion}\rightarrow\text{electricity}$	shaft heat, sound, vibration
Wind turbine	$\text{wind kinetic energy}\rightarrow\text{turbine motion}\rightarrow\text{electricity}$	sound, vibration, component heat
Photovoltaic solar panel	$\text{solar radiation}\rightarrow\text{electricity}$	reflected light and heat
Biomass or biogas	$\text{chemical energy of fuel}\rightarrow\text{heat}\rightarrow\text{turbine motion}\rightarrow\text{electricity}$	waste heat, exhaust gases, sound
Geothermal power	$\text{thermal energy of Earth fluid}\rightarrow\text{moving steam or fluid}\rightarrow\text{turbine motion}\rightarrow\text{electricity}$	waste heat and system friction

DOE explains that biopower technologies convert biomass fuels into heat and electricity through burning, bacterial decay, or conversion into gas and liquid fuels. DOE's biopower explanation can be opened through energy.gov.

DOE also explains that geothermal power plants use hot fluid or steam from below Earth's surface to drive turbines and produce electricity. DOE's Geothermal Electricity Generation page can be opened through energy.gov.

Efficiency Tells Us the Useful Part

Because not all input energy becomes the output we need, we use efficiency. Efficiency compares useful energy with input energy.

\eta=\frac{E_{\text{useful}}}{E_{\text{input}}}\times 100\%

Symbol guide:

Symbol	Meaning
$\eta$	efficiency
$E_{\text{useful}}$	energy that matches the device's purpose
$E_{\text{input}}$	energy received by the device

Suppose a panel receives $800 \text{ J}$ of light energy and produces $160 \text{ J}$ of electrical energy. Its efficiency is:

\begin{aligned} \eta &= \frac{E_{\text{useful}}}{E_{\text{input}}}\times 100\% \\ &= \frac{160 \text{ J}}{800 \text{ J}}\times 100\% \\ &= 20\% \end{aligned}

The energy that does not become useful electricity is:

\begin{aligned} E_{\text{dissipated}} &= E_{\text{input}}-E_{\text{useful}} \\ &= 800 \text{ J}-160 \text{ J} \\ &= 640 \text{ J} \end{aligned}

So, an efficiency of $20\%$ does not mean the remaining $80\%$ disappears. The remaining energy moves into the surroundings, for example as heat and light that the panel does not absorb.

EIA uses energy efficiency to describe the technical performance of energy conversion and energy-consuming devices. EIA's Energy Efficiency and Conservation page can be opened through eia.gov.

Do Not Stop at the Device Name

Device names often make energy transformation sound too simple. Solar panels, wind turbines, generators, and biogas reactors are not "energy makers". They redirect energy paths.

When you see this	Ask this
solar panel	how much radiant energy becomes electricity?
wind turbine	is the wind fast enough to rotate the rotor?
hydropower plant	how large are the water flow rate and height difference?
biomass	is the feedstock sufficient and managed sustainably?
geothermal heat	does the site have enough heat, fluid, and flow pathways?

By tracing the conversion path, you can evaluate renewable-energy technology more carefully: what the source is, what the initial energy form is, what the device transforms it into, and which part is actually useful.