Energy

Question 1

What are some basic energy concepts?

Answer 1

Energy is the capacity or ability to do work and is a product of force and movement. Can be potential (e.g. compressed air, nuclear energy) or kinetic (e.g. wind, expanding gas, radiation, electricity).

Some basic energy concepts:

• Energy
• Work
• Thermal energy - heat content
• Temperature
• Power

Can also be found in different forms e.g. chemical energy, heat, etc.

Measured in joules or kWh.

Question 2

Thermal energy/heat versus temperature

Answer 2

Heat (quantity) is the sum of kinetic energy of the atoms and molecules that are part of the system giving its temperature. For e.g., heating is the transfer of heat from a hotter body to a colder through exchange of energy between particles bouncing into each other.

Temperature is a measure of heat (quality) which is related to the speed by which the atoms or molecules in a system move. The higher the speed, the higher the temperature.

Question 3

What is work?

Answer 3

Work is the transfer of energy performed when a force (energy with direction) is applied over a vertical distance. For e.g. when a 1kg object is moved to a shelf 1 metre higher, the min. force applied will be 10 Newton, and the corresponding work equalling 10 Joules will be performed. You work against the gravity of the Earth which is approx. 10 Newton per kilogram.

Question 4

What is power?

Answer 4

Power is the amount of energy (Joules) delivered per unit time
(second) and is measured in the unit Watt (W).

Power is time dependent.

Question 5

Energy and power if you have 1 lamp for 10 hours vs 10 lamps for 1 hour

Answer 5

Energy (in kilowatt-hours, for example) is the same in both scenarios, because 1 lamp used for 10 hours and 10 lamps used for 1 hour result in the same total energy consumption.

However, Power (in watts) is distributed differently:

In the first scenario, the power consumption is steady over a longer period (lower power but sustained).

In the second scenario, power consumption is higher in a shorter period (more intense but brief).

Question 6

Concentrated vs dispersed energy

Answer 6

Concentrated energy is energy that is packed into a small, dense form, making it easier to store, transport, and use. Fossil fuels (like coal, oil, and natural gas) are highly concentrated forms of energy.
Nuclear energy is another example, where a small amount of nuclear fuel can release a huge amount of energy.

Concentrated energy has higher energy density, is easier to store/transport, used efficiently in centralised systems like power plants, suitable for large technical systems. Concentrated high quality energy has the capacity to do useful work.

Dispersed energy is energy that is spread out over a large area or volume, making it less dense and often more challenging to capture and store efficiently. Solar energy is dispersed across large areas and requires many panels to collect significant amounts of energy.
Wind energy depends on large wind farms with turbines spread across wide areas. Tidal and wave energy also fall under this category.

Dispersed energy has low energy density, is more difficult to store or transport, requires larger infrastructure for collection.

Question 7

What happens to energy quality when entropy is increased?

Answer 7

Lower energy quality since entropy is a measure of order - disorder/disperse (as entropy increases the system becomes more disordered, and the energy becomes more spread out/dispersed).

Question 8

What is energy used for?

Answer 8

Energy is used for domestical and industrial uses. World energy consumption is increasing, with natural gas and coal making up a significant percentage. Hydro and other renewables for electricity reach an approximate of 15% (or 30% going up).

Question 9

What factors can influence changes in energy use?

Answer 9

• Population Growth
• Modernisation
• Economic development

All these factors lead to an increased need for energy.

Question 10

What is electrification?

Answer 10

Electrification of mobility, electricity for energy efficiency, electrical appliances, ICT, AI, etc.

Question 11

Steam turbine & generator: how does it work?

Answer 11

1. Boil water (using a fuel source such as coal or gas) (Chemical energy to thermal energy)
2. Create high-pressure steam
3. Steam is directed at turbine blades which spins turbine (the force of the steam causes the turbines to spin) (thermal energy to mechanical energy)
4. Turbine spins generator (which is connected by a shaft - as the turbine spins, it turns the generator). (Mechanical energy to electrical energy)
5. Inside the generator, coils of wire rotate in a magnetic field which generates electricity.
6. Steam passes through the turbine, is cooled and turned back into water (using a condenser). Feedwater is sent back to the boiler.

Losses include:
- cooling water losses
- flue gases
- heat radiation and losses
- mechanical friction

Question 12

How is electricity generated?

Answer 12

• From high temperature heat, e.g. fuels, geothermal, concentrated solar.
• From solar radiation, e.g. pv cells.
• From kinetic energy, e.g. hydro power, wind, wave, tidal.
• From chemical reaction, e.g. fuel cells.

Question 13

Nuclear energy

Answer 13

Nuclear reactors typically operate with lower steam pressure compared to other types of thermal power plants (like fossil fuel plants). Lower steam pressure means that the turbine doesn’t extract energy as efficiently from the steam, leading to a lower efficiency of about 30%. This means that only 30% of the thermal energy produced by the nuclear reaction is converted into usable electricity, with the rest being lost as waste heat.

• Optimism for future nuclear reactor designs (often called next-generation reactors), which aim to improve efficiency, safety, and sustainability. New reactor technologies may operate at higher pressures and temperatures or use advanced cooling systems, improving efficiency beyond the current 30%.

Question 14

The diagram shows the typical layout of a nuclear power plant:

Answer 14

Reactor Vessel: Contains uranium fuel rods, where nuclear fission generates heat.

Steam Generator: Heat from the reactor is transferred to water, turning it into steam.

Turbine and Generator: Steam drives the turbine, which is connected to the electric generator that produces electricity.

Condenser: Cools the steam back into water using a cooling system, often from a nearby water source like a lake or cooling tower.

Cooling Tower: Releases excess heat into the atmosphere through evaporated water.

Question 15

How does Geothermal power work?

Answer 15

Geothermal power uses the Earth’s internal heat to create steam that drives a turbine, generating electricity.

The process is sustainable since the water is recycled and reheated naturally by the Earth.

Geothermal systems typically work with heat sources at 170°C or higher, which is sufficient to produce steam and generate electricity.

Question 16

What is co-generation?

Answer 16

Combined production of electricity and useful heat. For example, in a cogeneration system, you can have electricity that is produced and used in-house or sold. The waste heat can then be recovered for useful purposes, such as industrial processes, heating buildings, heating hot water, and generating additional electricity.

Question 17

What is the difference between thermal power plants and CHP systems?

Answer 17

Thermal Power Plants focus mainly on producing electricity and have lower efficiency because of heat losses in cooling water.

CHP Systems focus on co-producing heat and electricity, with significantly higher efficiency (up to 80%) due to the use of both heat and electricity.

In summary, thermal power plants are efficient at large-scale electricity production but suffer from energy losses, while CHP systems offer higher overall efficiency by utilizing heat demand but are typically used in more localized, industrial settings.

Question 18

How does hydropower work?

Answer 18

Hydropower plants use the potential energy of stored water in a reservoir, converting it into kinetic energy as the water flows down through the penstock. This kinetic energy drives a turbine, which then spins a generator to produce electricity. It’s a renewable and clean form of energy since it doesn’t involve burning fuel and emits no greenhouse gases.

In this system, the energy comes from the natural water cycle, making hydropower a reliable and sustainable source of electricity.

Key steps:

Reservoir: Stores the water that provides the energy source.

Penstock: The pipe through which water flows to reach the turbine.

Turbine: Converts the kinetic energy of flowing water into mechanical energy.

Generator: Converts the mechanical energy of the spinning turbine into electrical energy.

Question 18

How do photovoltaic cells produce electricity?

Answer 18

Photovoltaic cells generate electricity by converting sunlight into a flow of electrons, producing DC electricity. This electricity is then converted into AC via an inverter, for use in homes, businesses, or for feeding into the power grid. Photovoltaic systems are a clean and renewable source of energy, contributing to sustainable energy solutions by harnessing the sun’s power.

This slide showcases the basic operation of PV cells, solar panels, and how these systems are integrated into the wider power network to produce sustainable electricity.

In grid-tied systems, the electricity generated can be shared with the wider power grid (shown in the diagram), reducing reliance on external power sources and even providing power back to the grid.
In off-grid systems, the electricity can be stored in batteries to be used later when sunlight is not available.

Question 19

PV reliability sunny vs cloudy day

Answer 19

Sunny Day: The PV system performs optimally, with a smooth and predictable output. The production increases steadily until it reaches its maximum potential in the middle of the day, following the sun’s intensity.

Partially Cloudy Day: On a cloudy or partially cloudy day, the power output is unstable. The production is interrupted multiple times due to cloud cover, causing sharp drops in output followed by quick recoveries when sunlight returns.

Question 20

How can hydrogen be used for energy production?

Answer 20

Hydrogen (H₂) can be burned in the presence of oxygen (O₂) to produce energy. This process releases energy and produces water (H₂O) as the only by-product, making it a clean energy source with no direct carbon emissions.

However, combustion relies on heat energy, similar to how fossil fuels are burned, though with less pollution.

Hydrogen can be used either in combustion or in fuel cells. Fuel cells offer a more efficient and cleaner alternative, as they convert hydrogen directly into electricity with water as the only by-product, without relying on the combustion process. Real-life applications, include powering vehicles like buses, contributing to zero-emission transportation.

Question 21

Hydrogen energy cycle (using electrolysis of water)

Answer 21

1. Making Hydrogen (Electrolysis):

Use electricity to split water (H₂O) into hydrogen (H₂) and oxygen (O₂).
This process creates clean hydrogen fuel, and the electricity can come from renewable sources like solar or wind.

2. Storing Hydrogen:

Hydrogen is compressed and stored in tanks, making it easy to transport for later use.

3. Using Hydrogen for Energy:

Burn the hydrogen (combustion) to make energy (for things like powering vehicles or heating buildings), with water as the only by-product. Or, use it in a fuel cell to create electricity directly, again with water as the only by-product.

Question 22

What is the challenge with balancing smart grids?

Answer 22

• Capacity factor is a measure of how much energy is produced compared to the maximum possible output (100%). Solar and wind energy have wide variations depending on the weather and season. Biomass, geothermal, and waste-to-energy are more consistent but may not always produce as much power. Hydropower also fluctuates based on rainfall and water availability.

The diagram shows a smart grid, which connects different energy sources (solar, wind, hydro, etc.) to supply electricity to various users like homes, businesses, and industries. The smart grid must therefore manage variable energy production (from solar and wind, for example) and balance it with energy demand in real time.

The grid includes energy storage systems (like batteries) and power management systems that help smooth out fluctuations in renewable energy production. Since renewable energy sources like solar and wind are not always available (the sun doesn’t always shine, and the wind doesn’t always blow), the grid must constantly adjust to balance energy production and consumption.

Question 23

Concept of electricity storage

Answer 23

• Electricity needs to be converted into another form of energy to be stored for later use, because it cannot remain in its electrical form without a continuous flow.

Since you can’t store electricity directly, it’s converted into either chemical energy (using batteries or hydrogen production) or potential energy (through pumped hydropower systems). These methods allow electricity to be stored and used when demand is high or renewable sources are unavailable.

Question 24

What are the key components of an electric transmission and distribution system?

Answer 24

Power Plant:

Electricity is generated at the power plant (on the left) through various means, such as burning fossil fuels, nuclear energy, or renewable sources like wind and solar.

Step-Up Transformer:

After generation, the electricity passes through a step-up transformer, which increases the voltage. High voltage is necessary for efficient long-distance transmission because it reduces energy losses along the way.

Transmission Lines:

The electricity is then transmitted over high-voltage transmission lines. These lines carry electricity over long distances from the power plant to the area where it will be used.

Step-Down Transformer:

Near the end of the journey, the electricity passes through a step-down transformer, which lowers the voltage to a safer and more usable level for homes or businesses.

Distribution Lines:

After the voltage is reduced, electricity is sent through distribution lines, which carry it to local transformers and residences.

Residence:

Finally, the electricity reaches homes or businesses at the appropriate voltage level for appliances, lighting, and other electrical needs.

As electricity travels through this system, some energy is lost, primarily due to resistance in the wires and inefficiencies in transformers. On average, 6-10% of the electricity generated is lost before it reaches end users.

Core losses refer to energy losses that occur within the transformers themselves. These losses typically account for 25-30% of all distribution losses in the system.

During times of low electricity demand (e.g., at night), system losses are mainly due to core losses, which can reduce total system losses to as low as 3%.

During peak demand (e.g., in the daytime when more electricity is used), line losses increase, and can reach between 10-15%, as more energy is transmitted through the system.

Question 25

Utilisation of waste and low-grade heat

Answer 25

• Waste heat from industry and other sources of a temperature are useful for something else
• Low-temperature heat can be upgraded using heat pumps

For example:

High-Temperature Geothermal: Used for electricity generation by harnessing steam from deep geothermal reservoirs.

Low/Medium Temperature Geothermal: Used for heating and cooling through geothermal heat pumps that utilize shallow ground heat. These systems are effective for residential and commercial buildings and offer a renewable alternative to conventional heating and cooling systems.

Question 26

What is low-grade heat?

Answer 26

Typically found at depths of 1 to 100 meters. The ground maintains a relatively constant temperature, which can be harnessed for heating and cooling.

Question 27

What is a heat pump, and how does it work?

Answer 27

Heat pumps work by transferring heat rather than generating it, making them highly efficient. They can provide both heating in the winter and cooling in the summer by reversing the cycle. This technology is environmentally friendly and uses less energy compared to traditional heating and cooling methods.

The system can pull heat from either the ground (through buried pipes in a geothermal heat pump) or from the air (through an outdoor air unit).

Key components:

• Evaporator: Takes in heat from outside (air or ground).
• Compressor: Increases the heat.
• Condenser: Releases heat inside the house.
• Expansion Valve: Cools the fluid so it can absorb more heat.

Question 28

How does the boiling point of water change as pressure increases?

Answer 28

The boiling point of water is the temperature at which the vapor pressure of the water equals the surrounding pressure. When the pressure is lower, the water requires less energy (and a lower temperature) to turn into steam. When the pressure is higher, more energy (and a higher temperature) is needed to reach the boiling point.

Higher Pressure = Higher Boiling Point: As the pressure increases, the boiling point of water also increases. At standard atmospheric pressure (around 100 kPa), water boils at 100°C (as indicated by the red dot on the graph).

Lower Pressure = Lower Boiling Point: At lower pressures (below 100 kPa), water boils at temperatures below 100°C. For example, at around 50 kPa, the boiling point is close to 80°C.

Very High Pressure: At pressures higher than atmospheric pressure, like 200 kPa, water boils at a temperature closer to 120°C.

Question 29

What is a heat exchanger, and how does it work?

Answer 29

A heat exchanger’s main purpose is to transfer heat from one fluid (liquid or gas) to another. These fluids are kept in separate compartments (like tubes or shells) so they do not mix, but they can still exchange heat through a heat transfer area, usually the wall of a tube.

The heat exchanger itself does not produce or lose energy. It only facilitates the transfer of heat from the hotter fluid to the cooler fluid. Energy is conserved, but heat is moved from one fluid to another.

Heat exchangers are commonly used in systems like:
- Air conditioners and refrigerators to transfer heat out of a space.
- Power plants to transfer heat between steam and water.
- Industrial processes to regulate temperature between fluids.

Question 30

Describe the concept of heat transfer.

Answer 30

Heat Transfer happens due to the difference in temperature. Heat flows from the hotter area to the cooler area until the temperatures are the same, and then the transfer stops. Heat always moves from areas of higher temperature to areas of lower temperature.

This process is fundamental to how heat moves in systems, including everyday situations like heating a room or cooling a drink, as well as in technical systems like heat exchangers or refrigeration units.

Question 31

Parallel flow vs. counter-current flow

Answer 31

In a parallel flow heat exchanger, both the hot and cold fluids enter the exchanger from the same side and move in the same direction.

The temperature difference between the two fluids is largest at the beginning, where heat transfer is the most effective. In the end, the two fluids will tend to reach similar temperatures.

Parallel flow is generally less efficient because the temperature difference between the fluids becomes smaller as they move forward, which limits the amount of heat transferred.

In a counter-current flow heat exchanger, the hot and cold fluids enter from opposite sides and flow in opposite directions.

The temperature difference between the two fluids is more consistent throughout the heat exchanger, allowing for more efficient heat transfer. Counter-current flow is much more efficient than parallel flow because the temperature difference between the fluids remains larger for a longer period of time, enhancing the overall heat transfer.

Question 32

What is a heat exchanger, and how does it work?

Answer 32

A heat exchanger transfers heat from one fluid (hot) to another fluid (cold), allowing the heat to be transferred without the fluids mixing.

Heat exchangers are used to transfer heat from one fluid to another, keeping the fluids separate. The shell and tube design is common for larger systems, while the plate heat exchanger is efficient and used for a variety of applications. Counter-current flow (as shown in the plate exchanger) increases heat transfer efficiency by maintaining a higher temperature difference throughout the process.

Question 33

Equation related to heat transfer:

Answer 33

Heat transfer (E - measured in joules) = coefficient (depends on the material properties and the system set-up e.g. how easily heat can pass through the barrier) X Surface Area (metre squared - through which heat is transferred) X driving force (e.g. temperature difference).

A larger surface increases the heat transfer because there is more contact between the two fluids. For example, a bigger heat exchanger will allow for more heat to be exchanged.

Question 34

[EXAM QUESTION] In the course, we talked about “heat pumps” and what they could be good for. What is it you can achieve with a heat pump and what good could it be used for? Give and explain an application. Notice: here we do not ask for the mechanisms on how the heat pump works. (3p)

Answer 34

A heat pump is a device used to transfer heat from one place to another. The key advantage of a heat pump is that it can efficiently move heat from a low-temperature area to a high-temperature area, making it useful for both heating and cooling applications. Heat pumps do not generate heat; instead, they use a small amount of energy (electricity) to transfer existing heat from one location to another.

One common application is residential heating and cooling. Heat pumps are widely used in homes to replace traditional heating systems like furnaces or boilers, which burn fuel to generate heat. Instead, heat pumps transfer heat from outside, making them more energy-efficient and environmentally friendly.

For example, a ground-source heat pump (also known as a geothermal heat pump) draws heat from the earth, which maintains a relatively constant temperature year-round. During the winter, the heat pump extracts this ground heat and distributes it throughout the home for heating. In the summer, the system can reverse, transferring heat from the home into the ground to cool the house.

This application is valuable because It reduces energy consumption compared to conventional heating systems.
It provides both heating and cooling in one system and it can significantly lower carbon emissions when used in place of fossil fuel-based heating.

Question 35

Globally, coal is the most important input source of energy for generation of electricity. In order of magnitude in per cent, how much is its contribution? (1p)

Answer 35

Globally, coal contributes to approximately 35-40% of electricity generation. This percentage has been declining in recent years due to increased usage of renewable energy sources and natural gas, but coal remains a significant input source for electricity generation worldwide.

Question 36

Natural gas is a fuel with a lower carbon footprint per energy output than for instance coal. Explain why. (1p)

Answer 36

Natural gas has a lower carbon footprint per energy output compared to coal because its combustion process produces less carbon dioxide (CO₂) per unit of energy generated.

This is due to the chemical composition of natural gas, which is primarily composed of methane (CH₄). When methane burns, it produces CO₂ and water (H₂O). Since methane has a higher ratio of hydrogen to carbon compared to coal, the combustion of natural gas generates more energy per unit of CO₂ produced.

Question 37

Electricity from solar and wind is growing rapidly all over the world but both have apparent
drawbacks compared to providing electricity from traditional thermal power plants fuelled with for
instance coal, oil, natural gas. What are these drawbacks and how could one reduce the problem with a dependency of electricity from solar and wind? (3p)

Answer 37

Solar and wind energy are intermittent and less predictable compared to traditional thermal power plants, as they depend on weather conditions and time of day. Additionally, energy storage challenges and the need for grid integration make their implementation more complex. Solutions include advancing energy storage technologies, diversifying energy sources, and building flexible grids with smart technology. In the short term, natural gas or other backup power sources can support the grid during periods of low renewable energy generation.