Converting thermal energy to mechanical (gravitational potential or kinetic) energy So far we have discussed energy transfers Mechanical Energy -> Thermal Energy Thermal Energy -> Thermal Energy (conduction, convection, radiation) A very important process in the energy Economy is the conversion Thermal Energy -> Mechanical energy This is what happens in a car, and in a coal fired electrical generating plant.

(In cars and generating plants the complete Sequence is Chemical energy -> Thermal Energy -> Mechanical Energy We only discuss the second step here.) A very important fact about conversions of Thermal to Mechanical energy is that it CAN NEVER TAKE PLACE COMPLETELY. SOME OF THE OUTPUT FROM AN ENGINE WHICH CONVERTS THERMAL ENERGY INTO MECHANICAL ENERGY BY DOING WORK ALWAYS OUTPUTS SOME THERMAL ENERGY AS WELL

How the conversion is done. Usually with a working fluid, usually a high pressure gas. The thermal energy is used to heat the gas. When the gas is heated, its temperature, and it pressure go up. (Pressure is the force that a gas exerts on each square meter of the walls of its container.) Then the high pressure gas is used to push a piston (as in the lab and in your car). The rod on the rising piston does the mechanical work. Or the high pressure gas is directed at the blades of a turbine, which then turns, doing the mechanical work (as in an electrical generating plant.

The laboratory this week illustrates the first way. This is sometimes called a reciprocating engine.

Thermal energy in, lifting weight Thermal energy in, piston rising Thermal energy out, piston compressing air Thermal energy out, piston and weight compressing air

The working fluid can be gasoline vapor (in a car), air (as in the lab) or water vapor (steam) as in a coal fired generating plant.

If, in the lab experiment, the piston plus the weight had mass 0.113kg and the piston rose 1.5cm how much work did the heated gas do? A. (0.113x1.5) joules B x1.5x9.8 joules C x.015/9.8 joules D x.015x9.8 joules

Answer D..016 joules

If.19 joules/degree K are required to heat the gas from the low temperature of 23 C to the high temperature of 60 C, then how much thermal energy is minimally required as input during the lifting of the weight? A x (60-23) joules B. 0.19x (60-273)joules C x 60x(1-296/333) D. (60-23)/.19 joule

Answer A. = joules

The other common way to convert the thermal energy to mechanical energy is a turbine. It is basically just a propeller: The gas (for example steam) is expelled at high pressure from a nozzle after being heated the stream of gas is directed at the turbine, where the gas molecules of bounce off the blades of the turbine, causing them to turn. This kind of engine is used in electrical generating plants (whether fired by coal, natural gas or nuclear energy).

The turbine generator consists of a series of steam turbines interconnected to each other and a generator on a common shaft. There is a high pressure turbine at one end, followed by an intermediate pressure turbine, two low pressure turbines, and the generator. As steam moves through the system and loses pressure and temperature it expands in volume, requiring increasing diameter and longer blades at each succeeding stage to extract the remaining energy. The entire rotating mass may be over 200 tons and 100 ft (30 m) long.

Schematic view of an engine Q C is never zero

The ratio of the output mechanical energy to the input thermal energy is called the EFFICIENCY of the engine Efficiency = (Output Mechanical Energy)/(Input Thermal Energy) The Maximum Possible Efficiency of such an engine Is (1-T c /T H ) Where the temperatures are in K.

Use of fossil fuels to generate thermal energy from chemical energy, often followed by partial conversion to mechanical energy. We will discuss Coal: mainly used in the US to drive turbines for the generation of electricity Petroleum: for which the biggest applications are in transportation and Natural gas: for which the direct use of the thermal energy for heating buildings is the biggest application.

In a coal fired electrical generating plant steam enters the turbines at about 540C (1000 F) and condenses at the cold end after driving the turbines which run the electrical generator at about 35C (95F). What is the maximum possible efficiency of such a system? A.1-35/540 B.1-308/813 C. 1-95/1000 D. 308/813

Answer: B. 1-(308/813)=.62 or 62% In fact coal fired generating plants do not achieve efficiencies this high. 30% is more typical.

Mojave Generating Station, a 1,580 MW coal power plant near Laughlin, Nevada

Consider the power plant illustrated earlier. It outputs 1580 MegaWatts (millions of watts). If it operates at an efficiency of 35%, and one gets (from the table at the back of your book) 25 million btu from a ton of coal, how much coal does it use per day?(1btu=1055joules) A.3600x24x1580x10 6 /(.35x1055x25x10 6 )tons B. 25x10 6 x3600x24/(1055x1580x10 6 x.35) tons C. 3600x24x1580x10 6 /(.65x1055x25x10 6 )tons D. 25x10 6 x3600x24x1055/(1580x10 6 x.65) tons

A. 14,800 tons/day (!) = about 150 railroad cars full

If the US economy uses 23 quads of thermal energy from coal per year and the efficiency of coal generating plants is 35%, then approximately how many such 1500 Megawatt plants would be required? 1btu=1055 joules, 1 year =3x10 7 seconds, 1 quad = btu A. (23x10 15 )(1055)/(0.35(1.5x10 9 )(3x10 7 ))=1540 B.(23x10 15 )(1055)/((1.5x10 9 )(3x10 7 ))=530 C.1055(1.5x10 9 )(3x10 7 )/(23x10 15 )(0.35)=5900 D.( 23x10 15 )(0.35)(1055)/((1.5x10 9 )(3x10 7 ))=190

Answer: D.

* Generation of hundreds of millions of tons of waste products, including fly ash, bottom ash, flue gas desulfurization sludge, that contain mercury, uranium, thorium, arsenic, and other heavy metals * Acid rain from high sulfur coal * Interference with groundwater and water table levels * Contamination of land and waterways and destruction of homes from fly ash spills such as Kingston Fossil Plant coal fly ash slurry spill * Impact of water use on flows of rivers and consequential impact on other land-uses * Dust nuisance * Subsidence above tunnels, sometimes damaging infrastructure * Coal-fired power plants without effective fly ash capture are one of the largest sources of human-caused background radiation exposure * Coal-fired power plants shorten nearly 24,000 lives a year in the United States, including 2,800 from lung cancer[44] * Coal-fired power plants emit mercury, selenium, and arsenic which are harmful to human health and the environment[45] * Release of carbon dioxide, a greenhouse gas, which causes climate change and global warming according to the IPCC and the EPA. Coal is the largest contributor to the human-made increase of CO2 in the air[46] Environmental problems associated with coal use:

Mountain top removal coal mining in Boone County, West Virginia

Fig. 7-14, p. 224 Reserves of Coal

Fig. 7-15, p. 225

US coal production /acr/acr_sum.html#fes1

Minnesota coal generating plants (2005) 46 plants 5676 MW of generating capacity 43.8% of total electrical generating capacity 34.9 million tons of CO 2 emissions/yr (about 20 tons/person) US Department of Energy, Energy Information Agency (EIA) EIA-906/EIA-920

In automobiles are rated for FUEL efficiency which is a related, but different, concept. The FUEL EFFICIENCY of a car is the average number of miles you can drive it per gallon. Petroleum, mostly transportation

If the fuel efficiency of your car is 35 miles per gallon how much energy is being provided to your engine for each mile you drive. (1.25 x 10 5 btu are available per gallon of gasoline.) A x 10 5 btu B 1.25 x 10 5 /35 btu C. 1/ 1.25 x 10 5 btu D. 35/ 1.25 x 10 5 btu

Answer B: =3571 btu/mile

If the thermal efficiency of your car is 30% how much energy is being used to push the car forward per mile (if the gasoline is providing 1.25 x 10 5 /35 btu per mile) A..30x 1.25 x 10 5 /35 btu B x 10 5 /35 btu/.30 C. 30/ 1.25 x 10 5 /35 btu D. 1/ 1.25 x 10 5 /(35 x.30)btu

.30x 1.25 x 10 5 /35 btu= 1071 btu Answer A.

What force is the engine providing to drive the car forward in the last example (in Newtons). The energy provided in a mile was.30x 1.25 x 10 5 /35 btu= 1071 btu 1 btu=1055 joules. 1 mile=1600 meters A. 1071/1055x1600) Newtons B. 1071x1600/1055 Newtons C. 1071x1055/1600 Newtons D. 1071/1600 Newtons

Answer C: 706 newtons

What power is the engine producing at 35mph in this car? (The force was 708 Newtons). 1 pound(lb)=4.4 newtons, 1 mile = 5280ft, 1 horsepower=550ft-lb/s A. (708/35)x(3600/5280)x4.4/550 hp B. (708x35)x(5280/3600)x(1/(4.4x550))hp C. (708x35)(3600/5280)x4.4x550hp D. (708/35)x(5280/3600)x4.4/550hp

Answer: B. = 15hp

In the energy economy, thermal energy is obtained for conversion to mechanical energy Is supplied mainly by Fossil fuels (about 80% of total energy use) and Nuclear energy (about 8%)

Fig. 7-7, p. 213 US oil consumption 2003

----- Hubert's prediction ___ actual US oil production

Gas consumption US 2003

Natural Gas US production

In summary, Most of the natural gas use (about a quarter fossil fuel use, is for direct heating) Oil and coal are used almost entirely for thermal to mechanical energy conversion with the attendant efficiency limitations. (The second law limitation on thermal efficiency for engines does not apply to direct heating uses of thermal energy. )

Table 7-1, p. 209

Fig. 7-5a, p. 211

Fig. 7-5b, p. 211