Week 1-5 - Tutorial Questions

Question 1

Explain what renewable energy sources are and where they come from.

Answer 1

-Energy derived by nature
-Constantly replenished

Come from Sun, heat from earth, energy from solar, wind, ocean, hydropower, biomass, geothermal resources etc

Question 2

Name the 6 main forms of renewable energy.

Answer 2

• Wind power
• Hydropower
• Solar energy
• Biomass
• Ocean
• Geothermal Energy

Question 3

What are the forms of renewable energy that do not draw their energy from solar radiation?

Answer 3

*Geothermal
*Tidal

Question 4

Which are the most important atmospheric greenhouse gases contributing to global warming?

Answer 4

*Water Vapour
*Carbon Dioxide (CO2)
*Methane

Question 5

Describe the Greenhouse Effect and how it leads to a global temperature rise.

Answer 5

• Earths surface warms up due to the
  absorption of solar energy, cools down at night by radiating part of its
  energy into deep space as infrared radiation.
• Carbon dioxide (CO2), water vapour, and trace amounts of some other gases such as methane act like a blanket and keep the earth warm.
• However, human activities make a difference in CO2 concentration,
  thereby causing global temperatures to rise.

Question 6

Name 3 things that are needed for combustion

Answer 6

• Oxygen
• Fuel
• Heat

Question 7

Name 3 types of heat transfer and how they transfer heat

Answer 7

Convection - Between solid surface and gas or liquids at different temperatures

Radiation - Through electromagnetic waves

Conduction - Directly on surfaces

Question 8

Explain Perfect, Good and Incomplete Combustion and provide the chemical reactions.

Answer 8

• Perfect Combustion - Occurs when the perfect amount of oxygen is present, reacts with fuel (Hydro Carbons or H2C). the biproducts are Water, Nitrogen and carbon dioxide. No carbon monoxide produced
  Chemical Reaction:
  Fuel + Air = Water + Carbon Dioxide + Nitrogen + heat
  H2C + O2,N2 = H2O + CO2 + N2 + Heat
• Good Combustion - Same as Perfect Combustion only it has excess O2 so therefore has an excess output of O2
• Incomplete Combustion - Occurs when there is not enough oxygen, biproducts are carbon monoxide and smoke.
  Chemical Reaction:
  Fuel + Air = Carbon monoxide + Smoke
  H2C + O2,N2 = CO + Smoke

Question 9

Explain how a basic heat pump works

Answer 9

• Refrigerant starts as a cool liquid
• As it passes through evaporator it absorbs heat
• Refrigerant vaporizes due to its low boiling point.
• Vapour enters an electrically driven
  compressor - Rises temp and pressure
• Vapour enters a heat exchanger called the Condenser - Turns into warm liquid
• then passes through an expansion valve where it losses pressure and temperature as it expands. The whole process then
  restarts.

Question 10

(A) In Hydro-power, what does the term head mean and (B) what are the typical heights of Low, medium and high head pressures?

Answer 10

(A) head is the difference in height between the inlet and outlet surfaces.
Low - <30m
Medium - 30m-300m
High - 300m-1000m

Question 11

How can electricity from other renewable energy sources be used to make hydro-power dams more carbon neutral and allow the storage of off peak renewable
energy.

Answer 11

• When water levels is low, the energy stored is low. So therefore we need to pump more water from the tailrace of the dam system back up to the reservoir to refill the dam.
• requires the use of electricity to drive the pumps
• pumps are driven by electricity from renewable energies such as wind or solar, then water which is pumped back to the reservoir essential stores some of the electrical energy used to drive the pumps as potential energy in the height of the water.
• Since the energy form the wind or solar energies is generally wasted at off peak
  times, storing this energy in the head height of the water prevents some of this
  renewable energy from being lost as it will create more electricity when the water
  is released from the dam.

Question 12

Explain the difference between reaction turbine and impulse turbine.

Answer 12

Reaction turbines - uses change in pressure to generate power

Impulse turbine - Uses change in velocity of water jet to produce power

Question 12

What is the difference between a reservoir (dam) hydro-electric system and a Run of River hydro-electric system?

Answer 12

In a reservoir hydro-electric system the main energy source comes from a large
man made reservoir of still body water in a high location such as a mountain.
* head = Medium to High.
* The water contained in the reservoir can come from rivers or from pumping from
another location such as a lake.
* all year round.

In a Run of River hydro-electric system the main energy source comes from the flow
of the water in the river and the drop of the river created by a small dam, weir or
rapids.
* head = low.
* Since the supply of the water in the river is generally seasonal, a consistent all year
round supply of energy cannot be guaranteed.

Question 13

Explain the working principle of a Run of River micro-hydropower system

Answer 13

Run-of-the-river micro hydropower system converts the kinetic energy of moving water to electricity in 5 steps:
1. Water conveyance or pressurized pipeline (penstock) delivers the flowing
water from the river to the power station.
2. Turbine or waterwheel transforms the energy of flowing water into
mechanical energy.
3. The Generator transforms the mechanical energy into electricity. A voltage
regulator is used to provide a steady voltage as water supply fluctuates.
4. Wiring delivers the electricity to appliances.
5. Water is returned to the River downstream.

Question 14

What are the biomass sources of Ethanol?

Answer 14

Ethanol can be produced from a variety of biomass materials, containing
sugar, starch and cellulose. The best-known feedstock under three categories
are:
(1) Sugars: sugarcane, sugar beet, sweet sorghum, grapes, molasses
(2) Starches: maize, wheat, barley, potatoes, cassava, rice
(3) Cellulose: wood, straw, stems of grasses, bamboo, sugarcane bagasse.

Question 15

Describe the process of anaerobic digestion which converts biomass into
biogas.

Answer 15

Biogas is produced from wet biomass through a biological conversion
process that involves bacterial break down of organic matter by microorganisms to methane, c02 and water
* The process is known as ‘anaerobic digestion’ which proceeds in three
steps: i) Hydrolysis ii) Acid formation iii) Methane formation
* i) Hydrolysis:
* Organic waste of animal and plants contains carbohydrates in the form of cellulose, hemicellulose and lignin
* A group of anaerobic micro-organisms (cellulolytic bacteria / hydrolytic bacteria) breaks down complex organic material into simple and soluble organic components, primarily acetates.

Acid formation:
converted into simple acetic acid.
Mathane formatrion
Methane formation:
* The bacteria decompose compounds with a low molecular weight.
* Acetic acid from the Acid Formation stage becomes the substrate strictly
for anaerobic methanogenic bacteria, which ferment acetic acid to
methane and CO2
.
* Gas production is stable for pH between 6.6 and 7.6.
* Biogas consists of CH4
, CO2 and traces of other gases such as H2
, CO,
N2
, O2 and H2S.
* Gas mixture is saturated with water vapour.
* The methane content of biogas is about 60% which provides a high
calorific value to find use in cooking, lighting and power generation

Question 16

Describe the thermo-chemical conversion of biomass using the pyrolysis process.

Answer 16

Thermo-chemical conversion process of decomposing biomass at various combinations of pressure and temperature to obtain biofuel.
* Biomass is heated at high pressure at temperatures 500-900°C in absence
of oxygen, or partially combusted in a limited oxygen supply, to produce a
hydrocarbon rich gas mixture, an oil-like liquid and a carbon rich solid
residue (charcoal).
* Pyrolytic or ‘bio-oil’ produced can easily be transported and refined into a
series of products similar to refining crude oil.
* There is very little waste product, the conversion efficiency is high (82%)
depending upon the feedstock used, the process temperature in reactor
and the fuel air ratio during combustion.

Question 17

Name two forms of Direct Solar Energy.

Answer 17

• Solar Thermal
• Solar Photovoltaic

Question 18

Name two forms of indirect Solar Energy.

Answer 18

• Biomass
• Wind
• Waves

Question 19

With the aid of a diagram, describe a flat-plate solar thermal collector.

Answer 19

Consists of 4 parts:
*Metallic flat absorber plate - Black surface usually made of copper/steel/aluminium

• Tubes or channels - soldered to absorber plate. takes away heat from absorber plate
• Transparent toughened glass sheet cover - used to reduced convection losses
• Fibre glass insulation - at bottom and sides to reduce heat loss

Question 20

With the aid of a diagram, describe the heliostat field collector.

Answer 20

Device that includes a plane mirror which turns so as to keep reflecting sunlight toward the target.

use hundreds or thousands of small reflectors to concentrate
the sun’s rays on a central receiver placed atop a fixed tower.

Question 21

With the aid of a diagram, explain the mechanism of parabolic trough solar
collector.

Answer 21

(Reflectors) are curved in one dimension to focus the sun’s rays onto a pipe running down through the trough (Absorber Tube).

• The focused heat energy from the trough is then transferred into a high temperature
  mineral oil which is used to generate steam for electricity generation

Question 22

What are the three main components of a pumped solar water heater and
provide a description of these components

Answer 22

1) The collector panel is mounted on a pitched roof facing the sun. It normally consists of a steel plate bonded to copper pipe through which water circulates. The plate is painted with a special black paint and covered in a single pain of glass to maximise heat transfer.

2) The storage tank generally serves as the normal domestic hot water cylinder and is
insulated with fibre glass or foam, the hot water from the solar collector circulates through a heat exchanger at the bottom of the tank. An immersion heater is also included as an auxiliary heat source.

3) The pumped circulation system keeps the water flowing through the solar collector
pipes and transfers the heat from the panel to the storage tank. Temperature sensors
detect when the collector is becoming warm and turn on the pump