Tidal, Wave & Biomass

Question 1

How does tidal power come about?

Answer 1

Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun causing the oceans to bulge.

As the Earth rotates, the local water levels rises and falls twice in a day. A tidal cycle is approximately 24 hours 50 minutes.

The Sun also creates a smaller pull on the oceans (bigger but much further away). Throughout the year when the gravity of the Sun and Moon act together in the same direction there is a large tide and when they pull in different directions the tides are smaller.

Question 2

How do tidal barrages work?

Answer 2

Similar to a conventional hydro-electric scheme with a small hydrostatic head but with huge volumes of water flowing through the tidal estuary to drive turbines built into the barrage. The water is fed through the barrage slowly to generate electricity.

Question 3

What is limiting the implementation of tidal barrages at more sites in the UK?

Answer 3

The potential environmental damage to the local river ecosystems.

Question 4

What type of turbines are most popular for tidal power generation?

Answer 4

Kaplan turbines. They are well suited to low head height and high volume flows typical of those found in tidal flows which are typically <12 m.

Question 5

Describe submerged turbines used for tidal power. What are the advantages & disadvantgages?

Answer 5

Similar to wind turbine technologies but mounted underwater on the sea bed or estuary floor. Designs include horizontal axis and vertical axis.

There is a constant flow therefore, no yawing mechanisms is required and there is minimal environmental impact. However, being underwater makes installation and maintenance more difficult.

Question 6

Where do waves come from?

Answer 6

As the wind blows, pressure and friction forces agitate the water surface and gravity acts as a restoring force.

These forces transfer energy from the air to the water, forming waves

Question 7

Where in the world have the best wave power potential?

Answer 7

The temperate zones (30-60 degrees latitude) where strong storms occur.

Question 8

What technologies development are in place for wave power?

Answer 8

1. Use of a shore based oscillating water column
2. Using the motion of joints in an articulated structure to drive hydraulic rams that power motors
3. Floating buoys that use the kinetic energy of the buoy’s rise and fall to drive a turbine
4. Trapping and compressing air in successive waves to build enough compression to drive a turbine
5. Using pressure differences under wave crests to drive water flows through turbine chambers

Question 9

How does an oscillating water column work to generate wave power?

Answer 9

Waves surge into a chamber open to the sea and force air through a Wells turbine, which can generate power from air flowing in either direction.

As the waves recede, air is drawn back into the chamber via the turbines, generating more power.

Question 10

How do articulated structures work to generate wave power?

Answer 10

As waves pass under the device the cylinder joints pivot and move hydraulic rams, pressuring oil and storing potential energy.

This high-pressure oil is released through hydraulic turbines, driving generators inside the device.

Question 11

How does a floating buoy work to generate wave power?

Answer 11

A buoy is moored to the seabed so it sits on or below the water surface.

The buoys rotate back and forth about a long linkage as the waves pass over them. The front edge of the duck matches the wave motion. The oscillating motion is used to generate power.

Question 12

What is meant by biomass?

Answer 12

Usually refers to organic matter such as timber and crops but also organic waste products bring used to generate heat and power.

Question 13

Why is biomass considered as sustainable and carbon neutral?

Answer 13

The carbon released in the combustion process is offset by the carbon trapped in the organic matter by photosynthesis during its growth.

Question 14

Why is biomass, as it is currently used, often not fully carbon neutral?

Answer 14

Because carbon that was previously captured is still being released into the atmosphere. To be truly carbon-neutral we need to make sustainable use of plants or trees as fuel, and replant them as we harvest them (using carbon neutral methods) - so carbon is reabsorbed in a virtuous cycle.

Question 15

Name some popular biomass sources in the UK.

Answer 15

• Short rotation coppice of willow and/or poplar, Miscanthus (Elephant Grass), Reed Canary Grass and Oil Seed Rape.
• Straw, Grain husks, Waste wood , Forestry products, Animal wastes e.g. slurry and chicken litter
• Landfill gases, domestic waste, industrial waste products, alcohol fuels

Question 16

How is algae used to create biomass?

Answer 16

The algae use photosynthesis to convert CO2 and sunlight into oxygen and biomass.

This is done in either high rate algal ponds or photobioreactors. Because the cells grow in aqueous suspension they have more efficient access to water, CO2 and dissolved nutrients.

Most of this new biomass is in the form of oil.

Question 17

What are the two categories of conversion processes that are used to utilise biomass as a fuel?

Answer 17

1. Release the chemical energy directly, in the form of heat which can be converted to electricity (e.g., biomass boilers)
2. Convert it to another form, such as liquid biofuel or combustible biogas. These processes can be thermal, chemical or biological.

Question 18

What is the benefit of burning a range of biomass/biofuel sources?

Answer 18

The potential to reduce fuel costs and increasing security of supply. They produce less pollutants when burned.

Question 19

What are the main 4 thermal processes used to convert biomass into another form?

Answer 19

1. Torrefaction - thermo chemical treatment of biomass in absence of O2 at 200 to 320 °C to remove water and volatiles e.g. torrefied pellets or briquettes
2. Pyrolysis - chemical decomposition of biomass by heating to produce gas and liquid products and a solid residue higher in C content e.g. charcoal
3. Gasification - converts biomass, into CO and H2 (syngas) by reacting with a controlled amount of O2 and/or H2O(g)
4. Combustion – burning in oxygen to produce CO2, H2O, ash and heat

Question 20

How can algae oil be used as a standard fuel for applications such as aircrafts and vehicles?

Answer 20

It can act as feedstock for an oil refinery where methods like hydrocracking or hydrogenation can be used to transform the oil into standard fuels like gasoline and diesel.

The chemical process is called trans-esterification where it reacts with an alcohol to generate glycerol and biodiesel which can be used in un-modified diesel engines.

Question 21

What biochemical processes can be used to convert biomass into another form?

Answer 21

Enzymes or bacteria and other micro-organisms can be used to break down biomass in processes such as anaerobic digestion, fermentation and composting.