4.1 Energy

• Atmospheric CO2 is converted to organic matter (carbohydrates) by plants, plankton and algae in a process called photosynthesis.
• The endothermic process of photosynthesis is

  6CO2 + 6H2O -sunlight-> C6H1206 + 6O2
• The organic matter (carbohydrate) is converted back to CO2 by aerobic respiration. Aerobic respiration is C6H12O6 + 6O2 --> 6CO2 +6H2O
• In addition to aerobic respiration, carbon can be returned to the atmosphere include combustion or anaerobic/aerobic (oxygen) decomposition.
• Landfill uses anaerobic decomposition to return carbon but it takes up a lot of land and releases methane to do so. Methane is a greenhouse gas even more bad than carbon dioxide
• During the growth of plants to make biofuels, CO2 is removed from the atmosphere by photosynthesis. Later on, when the biofuel is formed and combusted. the same amount of CO2 is returned to the atmosphere. Hence, there is no net increase of CO2, and being carbon neutral
• Advantages of Biofuels as sources of heat energy

  • renewable (carbon neutral)
  • emit less harmful pollutants than fossil fuels (less soot, CO2, photochemical smog)
• Disadvantage of Biofuels

  • Non-renewable fossil fuels are being used to make them!
  • Lower energy density than fossil fuels
  • Need more frequent re-fueling of cars etc
  • Farming land can be used to grow crops, which could lead to food shortages
  • Water shortages
  • Excess fertilizers (to make plants for biofuels) causes environmental problems
• The advantages of carbon-based fuels as sources of heat energy are…

  • High energy density 
  • Large reserves, wide availability 
  • Cheap extraction 

  The disadvantages of carbon-based fuels as sources of heat energy are…

  • Non-renewable 
  • Combustion produces CO2 
  • Environmental habitat damage in extraction process
• The advantages of carbon-based fuels as sources of feedstock are…

  • Versatile for plastics, pharmaceuticals and chemical

  The disadvantages of carbon-based fuels as a source of feedstock are…

  • Depletion of resources for energy production 

  Feedstock is a raw material needed in industrial processes
• All renewable energy sources emit less harmful pollutants than fossil fuels. Biofuels such as biodiesel are made from oils from plants. During the growth of these plants CO2 is consumed via photosynthesis. When the biodiesel is burnt, the same quantity of CO2 is released into the atmosphere hence there’s no net increase in CO2, hence being carbon neutral. However, there is a net increase in CO2 from combustion of fossil fuels as there is no off-set from photosynthesis, contributing to more global warming.
• The longer the hydrocarbon chain, the higher the ratio of O2 required for complete combustion to occur. Since air is not pure oxygen, there’s insufficient O2 present for complete combustion to occur so soot is produced. 
• Incomplete combustion can produce soot and carbon monoxide. Carbon monoxide deprives the human body of oxygen leading to loss in consciousness and death. Soot contributes to visual pollution by blackening buildings whilst clogging air and fuel inlets reducing efficiency of engines. 
• Bomb calorimeters produce more accurate enthalpy values because…

  • Pure oxygen is used to combust the fuel rather than air, which consists of other elements. Thus, this ensures complete combustion occurs, allowing the fuel to release the maximum possible energy
  • Insulation minimises heat loss to the surroundings, ensuring all heat produced by the combustion is measured in the water.
• The advantages of photovoltaic cells compared to steam turbines are… 

  • No emission of greenhouse gases
  • Quiet operation 
  • Easily scalable for range of applications which require more electricity 
  • Renewable energy source

  The disadvantages of photovoltaic cells compared to steam turbines are…

  • Dependency on sunlight, which may not be present all the time 
  • Need energy storage such as batteries to manage supply and demand effectively
• The advantages of fuel cells compared to steam turbines (galvanic) are…

  • Constant production of electricity as long as fuel is available 
  • More efficient than burning fuels by transforming chemical to electrical energy directly with minimal energy losses 
  • Quiet operation 
  • Minimal maintenance 

  The disadvantages of fuel cells compared to steam turbines are…

  • Expensive due to costs of catalyst, membranes and the maintenance of high purity fuels and oxidants due to contamination can prevent form working
  • Need medium to high temperatures are needed for efficiency 
• The advantages of flow cells are…

  • Are rechargeable which allows them to have almost unlimited longevity  
  • The power required can be chosen via altering the number of flow cells 
  • The total energy output (how long energy will be delivered) can be chosen by altering the electrolyte storage volume. A larger volume of electrolyte means more reactant is available allowing the flow cell to produce energy for a longer period.
• Producing hydrogen from fossil fuels through steam + methane is currently cheaper. Producing hydrogen from biomass is renewable and reduces dependence on fossil fuels and can make up for agricultural waste production. Producing hydrogen from water using electrolysis can be very clean if powered by renewable energy sources as oxygen is its only byproduct.