Presentation on theme: "1 8.Generation of Electricity 9. Basic Thermodynamics Maxine Narburgh CSERGE N.K. Tovey ( ) M.A, PhD, CEng, MICE, CEnv Н.К.Тови М.А., д-р технических наук."— Presentation transcript:
1 8.Generation of Electricity 9. Basic Thermodynamics Maxine Narburgh CSERGE N.K. Tovey ( ) M.A, PhD, CEng, MICE, CEnv Н.К.Тови М.А., д-р технических наук Energy Science Director CRed Project HSBC Director of Low Carbon Innovation NBSLM01E Climate Change and Energy: Past, Present and Future 2010
2 8. Generation of Electricity - Conventional Diagram illustrates situation with coal, oil, or nuclear Gas Generation is more efficient - overall ~ 45% Overall efficiency ~ 35% Largest loss in Power Station
3 8. Generation of Electricity - Conventional. Pump Multi-stage Turbine Generator Boiler Condenser Simplified Diagram of a generating set includes boiler, turbine, generator, and condenser Superheated Steam 563 o C 160 bar Steam at ~ 0.03 bar Why do we condense the steam to water only to heat it up again?. Does this not waste energy? NO!! But we must wait until the Thermodynamics section to understand why?
4 8. Generation of Electricity - Conventional Chemical Energy Coal / Oil / Gas Electrical Energy Heat Energy Boiler Turbine Generator Mechanical Energy Electricity used in Station Power Station 100 units 38 units 90 units 3 units 90% 95% 48% 41 units
5 Why not use the heat from power station? - it is typically at 30 o C? Too cold for space heating as radiators must be operated ~ 60+ o C What about fish farming - tomato growing? - Yes, but this only represent about 0.005% of heat output. Problem is that if we increase the output temperature of the heat from the power station we get less electricity. Does this matter if overall energy supply is increased? 8. Generation of Electricity - Conventional.
6 8. Generation of Electricity - CHP Overall Efficiency - 73% Heat is rejected at ~ 90 o C for supply to heat buildings. City Wide schemes are common in Eastern Europe
Electricity Act blinked our approach for 35 years into attempting to get as much electricity from fuel rather than as much energy. Since Privatisation, opportunities for CHP have increased on an individual complex basis (e.g. UEA), unlike Russia A problem: need to always reject heat. What happens in summer when heating is not required? Need to understand basic thermodynamics 8. Generation of Electricity - Conventional.
8 9. Introduction to Thermodynamics N.K. Tovey ( ) M.A, PhD, CEng, MICE, CEnv Н.К.Тови М.А., д-р технических наук Energy Science Director CRed Project HSBC Director of Low Carbon Innovation 8 NBSLM01E Climate Change and Energy: Past, Present and Future 2010
9 9. Elementary Thermodynamics - History. Newcomen Engine pushes piston up 3) At end of stroke, close steam value open injection valve (and pumping rod down) 4) Water sprays in condenses steam in cylinder creating a vacuum and sucks piston down - and pumping rod up 2) Open steam valve 1) Boil Water > Steam Problem: Cylinder continually is cooled and heated. 9
10 9. Elementary Thermodynamics - Watt Engine. Watt Engine 1) Cylinder is always warm 2) cold water is injected into condenser 3) vacuum is maintained in condenser so suck out exhaust steam. 4) steam pushes piston down pulling up pumping rod. Higher pressure steam used in pumping part of cycle. 10
11 9. Elementary Thermodynamics. Thermodynamics is a subject involving logical reasoning. Much of it was developed by intuitive reasoning nd Law of Thermodynamics - Carnot st Law of Thermodynamics - Joule Zeroth Law - more fundamental - a statement about measurement of temperature Third Law - of limited relevance for this Course 11
12 9. Elementary Thermodynamics. Carnots reasoning Water at top has potential energy Water at bottom has lost potential energy but gained kinetic energy 12
13 9. Elementary Thermodynamics. Carnots reasoning Water looses potential energy Part converted into rotational energy of wheel Potential Energy = mgh Theoretical Energy Available = m g (H 1 - H 2 ) Practically we can achieve % of this H1H1 H2H2 13
14 9. Elementary Thermodynamics. Carnots reasoning Temperature was analogous to Head of Water Energy Temperature Difference Energy (T 1 - T 2 ) T 1 is inlet temperature T 2 is outlet temperature Just as amount of water flowing in = water flowing out. Heat flowing in = heat flowing out. In this respect Carnot was wrong However, in his day the difference was < 1% 14
15 9. Elementary Thermodynamics. Joule 1849 Identified that Lost Heat = energy out as Work Use a paddle wheel to stir water - the water will heat up Mechanical Equivalent of Heat Berlin Demonstration Symbols W - work Q - heat Over a complete cycle Q = W Heat in +ve Heat out -ve Work in -ve Work out +ve FIRST LAW: You cant get something for nothing 15
16 9. Elementary Thermodynamics. Schematic Representation of a Power Unit Heat Engine Heat In Q 1 Heat Out Q 2 Work Out W First Law: W = Q 1 - Q 2 so efficiency But Carnot saw that Heat Temperature What do we mean by temperature? Which should we use? Kelvin? Rankine, Reamur, Fahrenheit, Celcius, 16
17 9. Elementary Thermodynamics. Is this a sensible definition of efficiency? If T 1 = 527 o C ( = = 800K) and T 2 = 27 o C ( = 300K) Note: This is a theoretical MAXIMUM efficiency 17
18 9. Elementary Thermodynamics. Second Law is more restrictive than First It is impossible to construct a device operating in a cycle which exchanges heat with a SINGLE reservoir and does an equal amount of work on the surrounds This means Heat must always be rejected Second Law cannot be proved - fail to disprove the Law If heat is rejected at 87 o C (360K) By keeping T 2 at a potentially useful temperature, efficiency has fallen from 62.5% 18
19 9. Elementary Thermodynamics. The Practical efficiency will always be less than the Theoretical Carnot Efficiency. To obtain the "real" efficiency we define the term Isentropic Efficiency as follows:- Thus "real" efficiency = carnot x isen Typical values of isen are in range % Hence in a normal turbine, actual efficiency = 48% A power station involves several energy conversions. The overall efficiency is obtained from the product of the efficiencies of the respective stages. 19
20 9. Elementary Thermodynamics. EXAMPLE: In a large coal fired power station like DRAX (4000MW), the steam inlet temperature is 566 o C and the exhaust temperature to the condenser is around 30 o C. The combustion efficiency is around 90%, while the generator efficiency is 95% and the isentropic efficiency is 75%. If 6% of the electricity generated is used on the station itself, and transmission losses amount to 5% and the primary energy ratio is 1.02, how much primary energy must be extracted to deliver 1 unit of electricity to the consumer? 20
21 9. Elementary Thermodynamics. ( ) - ( ) Carnot efficiency = = 63.9% so overall efficiency in power station:- = 0.9 x | combustion loss x | Carnot efficiency 0.75 x | Isentropic efficiency 0.95 x | Generator efficiency 0.94 | Station use =
Elementary Thermodynamics. Transmission Loss ~ 91.5% efficient Primary Energy Ratio for Coal ~ 1.02 Overall efficiency 1 x x = = units of delivered energy 1.02 i.e. 1 / = 2.90 units of primary energy are needed to deliver 1 unit of electricity. 22
23 9. Elementary Thermodynamics. How can we improve Carnot Efficiency? Increase T 1 or decrease T 2 If T 2 ~ 0 the efficiency approaches 100% T 2 cannot be lower than around o C i.e K T 1 can be increased, but properties of steam limit maximum temperature to around 600 o C, (873K) 23
24 9. Elementary Thermodynamics. In this part of the lecture we shall explore ways to improve efficiency We need to work with thermodynamics rather than against it The most important equation: What if we could use Q 2 effectively?
25 9. Applications of Thermodynamics - CHP Overall Efficiency - 73% Heat is rejected at ~ 90 o C for supply to heat buildings. City Wide schemes are common in Eastern Europe
26 Pipes being laid in streets in Copenhagen Most towns in Denmark have city wide schemes such as these Pipes like these were recently laid in UEA to new Thomas Paine Building Ways to Respond to the Challenge: Technical Issues Combined Heat and Power
27 9. Applications of Thermodynamics. Combined Heat and Power Engine Generator
28 Working with Thermodynamics. Heat Pumps Schematic Representation of a Heat Pump. IT IS NOT A REVERSED REFRIGERATOR. Schematic Representation of a Heat Pump Heat Pump Heat Out Q 1 Heat In Q 2 Work IN W A Heat Pump is a reversed Heat Engine: NOT a reversed Refrigerator If T 1 = 323K (50 o C) and T 2 = 273K (0 o C)
29 Working with Thermodynamics. A heat pump refrigerator consists of four parts:- Heat Pumps and Refrigerators 1) an evaporator (operating under low pressure and temperature) 3) a condenser (operating under high pressure and temperature) 4) a throttle value to reduce the pressure from high to low. 2) a compressor to raise the pressure of the working fluid Throttle Valve Compressor Condenser Evaporator
30 Throttle Valve Condenser Heat supplied to house Evaporator Heat extracted from outside Low Temperature Low Pressure High Temperature High Pressure Responding to the Challenge: Technical Solutions The Heat Pump Any low grade source of heat may be used Typically coils buried in garden Bore holes Example of roof solar panel Compressor A heat pump delivers 3, 4, or even 5 times as much heat as electricity put in. We are working with thermodynamics not against it.
31 Types of Heat Pump Heat Source airwaterground Heat Sink airair to airwater to air ground to air waterair to water water to water ground to water solidair to solidwater to solid ground to solid For Space Heating Purposes: The heat source with water and the ground will involve laying coils of pipes in the relevant medium passing water, with anti-freeze to the heat exchanger. In air-source heat pumps, air can be passed directly through the heat exchanger. For Process Heat Schemes: the source may be a heat exchanger in the effluent of one process
33 Keith Tovey ( ) Н.К.Тови M.A., PhD, CEng, MICE, CEnv Energy Science Director: Low Carbon Innovation Centre School of Environmental Sciences, UEA. Rotary Club of Norwich Recipient of James Watt Gold Medal