Fossil fuels-going the way of the dinosaur?

• Total resource vs proved reserve– Total resource is the amount of a resource that is known to exist– Proven resource is the amount that is recoverable under current

economic and technical conditions– They are not equal!– Remember “technically recoverable includes both!

• Barriers to untapped resources– Restrictions on offshore oil drilling– Strip mining of coal-environmentally a bad idea– Tar sands mining has been referred to as the “most destructive

project on Earth”– Risks associated with fracking

• How much does any one (or combination of )“solution” contribute to global climate change?

• No mater how you look at it, fossil fuels follow a Hubbert type curve, they will run out! It is a question of when, not if.

• Understanding and managing the potential risks

Heat Engines

• How do we get the heat energy of the fuel and turn it into mechanical energy?

• Simply put we combine the carbon and hydrogen in the fuel with oxygen.

• 2 reactions that occur are– C + O2 CO2 + heat energy– H2 + O H2O + heat energy

• This process is just the reverse of photosynthesis.

Just a little chemistry

• For example, the the equation for burning heptane looks like:– C7H16 + 11O2 7CO2 + 8 H2O +1.15 X 106 calories per

100g of Heptane• 1.15 x106 is called the heat of combustion for

heptane. Every hydrocarbon has such a number• It is the maximum amount of energy for a certain

amount of mass of a substance you can extract.• It represents the energy from the sun stored in

the fuel since ancient times

So what is a heat engine?

• A heat engine is any device that can take energy from a warm source and convert it to mechanical energy

• Efficiency: not all of the energy from the burning of the fuel goes into the production of energy. Heat is lost as waste heat and needs to be disposed of.

• For example, most energy generating plants are located near bodies of water or have cooling towers which are used to carry off waste heat.

Diagram of a heat engine

How well does one work?

• Your car often carries off waste heat via its cooling system. But your car recycles some of that heat—how?

• No heat engine will perfectly convert all the heat energy to mechanical energy.

• We need to quantify the efficiency and designers of heat engines work to maximize this efficiency.

Carnot and his cycle

• Sadi Carnot created an efficiencey measure for a heat engine, now named after him (Carnot Efficiency).

• Always less than 100%• Simply put it is the percentage of the energy

taken from the heat source which is actually converted to mechanical work.

Carnot Efficciency

• Efficiency = work done/energy put into the system

• In terms of the flow of heat (Q) energy this becomes : [(Qhot - Qcold)/Qhot ]X 100%

• Now energy is not easy to quantify, but temperature is, and since we know the Kelvin T scale is true measure of energy, we can express the efficiency in terms of temperature.

Carnot Efficciency

• So our efficiency, in terms of T becomes:– Carnot Efficiency = [(Thot - Tcold)/Thot ]X 100%

– Or with some algebraic wizardry we get Carnot efficiency = [1- (Tcold/Thot) ]X 100%

Example: for a coal fired electric power plant, the boiler temperature = 825K and the cooling tower temperature is 300k. So [1-(300/825)] X 100% = 64%

Carnot Cycle• From an initial state A, the gas is placed in

contact with the hot temperature reservoir (Th) and expands isothermally (keeping T = Th = constant) to some state B. During this isothermal expansion heat Qh flows into the gas from the hot temperature Th.

• From state B, the gas undergoes an adiabatic expansion to state C. No heat is exchanged during this expansion. Expanding an insulated gas means work is done at the "expense" of the internal energy. That means the gas will have a lower temperature. This is the cold temperature Tc.

• At state C, we place the gas in contact with the cold temperature heat reservoir (like a large tank of water) and do an isothermal compression to state D. In compressing the gas, work is done on the gas by the outside. But the temperature remains constant -- meaning the internal energy U of the gas remains constant. For this to happen, heat Qc is given out to the cold temperature heat reservoir.

• From state D we do an adiabatic compression back to state A. Remember, "adiabatic" means insulated so there is no heat exchange.

Figure 2

Figure 1

Adiabatic –constant pressureIsothermal-constant temperature

So how can we make this work for us:The Steam Engine

• Concept of a heat engine was revolutionary-if the heat energy could be turned into mechanical energy, human and labor could be replaced cheaply and more efficiently.

Simple steam engine• Water is heated in the

boiler and steam forces piston up

• At the valve, steam escapes into the cooling tower, where it cools and condenses.

• Cool water is pumped back into boiler, T drops and piston drops, until sufficient steam is created to cause the process to repeat.

A little history

• First writings on the power of steam are from Hero of Alexandria (10-70 CE).

• The aeolipile (known as Hero's engine) was a rocket-like reaction engine and the first recorded steam engine.

• He also created an engine that used air from a closed chamber heated by an altar fire to displace water from a sealed vessel; the water was collected and its weight, pulling on a rope, opened temple doors.

• Taqi al-Din in 1551 and Giovanni Branca in 1629 both created experimental steam engines.

More History• Thomas Savery (1650-1715), in 1698, patented the

first crude steam engine.• Based on Denis Papin's Digester or pressure cooker

of 1679. Papin’s device was for extracting fats from bones in a high-pressure steam environment, which also renders them brittle enough to be easily ground into bone meal.

• Savery had been working on solving the problem of pumping water out of coal mines

• Thomas Newcomen created the atmospheric engine, which was relatively inefficient, and in most cases was only used for pumping water out of deep mines

Newcomen’s atmospheric engine

Watt’s Steam Engine• Improvement upon

Newcomen’s• Used 75% less coal than

Newcomen's, and was hence much cheaper to run.

• Watt developed his engine further, modifying it to provide a rotary motion suitable for driving factory machinery.

• This enabled factories to be sited away from rivers, and further accelerated the pace of the Industrial Revolution.