Converting thermal energy to mechanical (gravitational potential or kinetic) energy

So far we have discussed energytransfers

Mechanical Energy -> Thermal Energy Thermal Energy -> Thermal Energy (conduction, convection, radiation)

A very important process in the energyEconomy is the conversion

Thermal Energy -> Mechanical energy

This is what happens in a car, and ina coal fired electrical generating plant.

(In cars and generating plants the completeSequence is

Chemical energy -> Thermal Energy -> Mechanical Energy

We only discuss the second step here.)

A very important fact about conversionsof Thermal to Mechanical energy is that it

CAN NEVER TAKE PLACE COMPLETELY.SOME OF THE OUTPUT FROM AN ENGINEWHICH CONVERTS THERMAL ENERGY INTOMECHANICAL ENERGY BY DOING WORKALWAYS OUTPUTS SOME THERMAL ENERGYAS WELL

How the conversion is done.

Usually with a working fluid, usually a high pressure gas. The thermal energyis used to heat the gas. When the gasis heated, its temperature, and it pressurego up. (Pressure is the force that a gasexerts on each square meter of the wallsof its container.)

Then the high pressure gas is used to pusha piston (as in the lab and in your car). Therod on the rising piston does the mechanical work.

Or the high pressure gas is directed at theblades of a turbine, which then turns, doingthe mechanical work (as in an electrical generatingplant.

The laboratory this week illustratesthe first way. This is sometimescalled a reciprocating engine.

Thermal energy in, lifting weight

Thermal energy in, piston rising

Thermal energy out, piston compressing air

Thermalenergy out, piston andweightcompressingair

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

If, in the lab experiment, the piston plusthe weight had mass 0.113kg and thepiston rose 1.5cm how much work didthe heated gas do?A. (0.113x1.5) joulesB. 0.113x1.5x9.8 joulesC. 0.113x.015/9.8 joulesD. 0.113x.015x9.8 joules

Answer D. .016 joules

If .19 joules/degree K are required to heat the gas from the low temperatureof 23 C to the high temperature of 60 C, thenhow much thermal energy is minimally required as input during the lifting of the weight?

A. 0.19 x (60-23) joulesB. 0.19x (60-273)joulesC. 0.19 x 60x(1-296/333)D. (60-23)/.19 joule

Answer A. =.7.03 joules

The other common way to convert thethermal energy to mechanical energyis a turbine.

It is basically just a propeller: Thegas (for example steam) is expelled athigh pressure from a nozzle after being heatedthe stream of gas is directed at theturbine, where the gas molecules of bounceoff the blades of the turbine, causing themto turn.

This kind of engine is used in electrical generatingplants (whether fired by coal, natural gas ornuclear 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.

Schematicview ofan engine

QC is never zero

The ratio of the output mechanicalenergy to the input thermal energyis called the EFFICIENCY of theengine

Efficiency =

(Output Mechanical Energy)/(Input Thermal Energy)

The Maximum Possible Efficiency of such an engineIs

(1-Tc/TH)

Where the temperatures are in K.

Use of fossil fuels to generate thermal energyfrom chemical energy, often followed bypartial conversion to mechanical energy.

We will discuss

Coal: mainly used in the US to drive turbinesfor the generation of electricity

Petroleum: for which the biggest applicationsare in transportation and

Natural gas: for which the direct use of thethermal energy for heating buildings is thebiggest application.

In a coal fired electrical generating plantsteam enters the turbines at about 540C(1000 F) and condenses at the cold endafter driving the turbines which run the electricalgenerator at about 35C (95F). What is themaximum 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 notachieve efficiencies this high. 30% ismore 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 millionbtu from a ton of coal, how much coal does it use per day?(1btu=1055joules)A.3600x24x1580x106/(.35x1055x25x106)tonsB. 25x106x3600x24/(1055x1580x106x.35) tonsC. 3600x24x1580x106/(.65x1055x25x106)tonsD. 25x106x3600x24x1055/(1580x106x.65) tons

A. 14,800 tons/day (!)

= about 150 railroad cars full

If the US economy uses 23 quads of thermalenergy from coal per year and the efficiencyof coal generating plants is 35%, thenapproximately how many such 1500 Megawatt plants would be required?

1btu=1055 joules, 1 year =3x107seconds,1 quad = 1015 btu

A. (23x1015 )(1055)/(0.35(1.5x109 )(3x107))=1540 B.(23x1015 )(1055)/((1.5x109 )(3x107))=530 C.1055(1.5x109 )(3x107)/(23x1015 )(0.35)=5900 D.( 23x1015 )(0.35)(1055)/((1.5x109 )(3x107))=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 2005http://www.eia.doe.gov/cneaf/coal/page/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 CO2 emissions/yr

(about 20 tons/person)

US Department of Energy, Energy Information Agency (EIA) EIA-906/EIA-920

In automobiles are rated for FUEL efficiencywhich is a related, but different, concept.

The FUEL EFFICIENCY of a car is the averagenumber of miles you can drive it per gallon.

Petroleum, mostly transportation

If the fuel efficiency of your car is 35 miles pergallon how much energy is being provided toyour engine for each mile you drive. (1.25 x 105 btu are available per gallon of gasoline.) A. 1.25 x 105 btu B 1.25 x 105/35 btuC. 1/ 1.25 x 105 btu D. 35/ 1.25 x 105 btu

Answer B: =3571 btu/mile

If the thermal efficiency of your car is 30% how much energy is being usedto push the car forward per mile(if the gasoline is providing 1.25 x 105/35 btuper mile)A..30x 1.25 x 105/35 btuB. 1.25 x 105/35 btu/.30C. 30/ 1.25 x 105/35 btuD. 1/ 1.25 x 105/(35 x.30)btu

.30x 1.25 x 105/35 btu=1071 btu

Answer A.

What force is the engine providing todrive the car forward in the last example(in Newtons). The energy provided ina mile was .30x 1.25 x 105/35 btu=1071 btu1 btu=1055 joules. 1 mile=1600 metersA. 1071/1055x1600) NewtonsB. 1071x1600/1055 NewtonsC. 1071x1055/1600 NewtonsD. 1071/1600 Newtons

Answer C: 706 newtons

What power is the engine producing at 35mph in this car? (The force was 708Newtons). 1 pound(lb)=4.4 newtons,1 mile = 5280ft, 1 horsepower=550ft-lb/s

A. (708/35)x(3600/5280)x4.4/550 hpB. (708x35)x(5280/3600)x(1/(4.4x550))hpC. (708x35)(3600/5280)x4.4x550hpD. (708/35)x(5280/3600)x4.4/550hp

Answer:

B. = 15hp

In the energy economy, thermal energy is obtained for conversion tomechanical 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'sprediction

___ actual

US oilproduction

Gas consumption US 2003

Natural Gas US production

In summary,

Most of the natural gas use (about a quarterfossil fuel use, is for direct heating)

Oil and coal are used almost entirely forthermal to mechanical energy conversionwith the attendant efficiency limitations.

(The second law limitation on thermal efficiency forengines does not apply to direct heatinguses of thermal energy. )

Table 7-1, p. 209

Fig. 7-5a, p. 211

Fig. 7-5b, p. 211