Technical Concepts and Overview

Question 1

What does the first law of thermodynamics state?

Answer 1

“Energy is neither created nor destroyed but converted from one form to another”

Question 2

What does the second law of thermodynamics state?

Answer 2

“A device operating continuously cannot export only work from an isolated source of heat”

Question 3

What implications for the design of energy systems does the second law of thermodynamics have?

Answer 3

The design must account for the inevitable loss of heat energy as they operate

Question 4

Even though energy is conserved . . . the flow of energy is []

Answer 4

irreversible

Question 5

To happens to the “grade” of energy as it flows through a process?

Answer 5

It deteriorates

Question 6

What is “exergy” as a measure of energy grade?

Answer 6

The theoretical maximum extractable work from a flow of energy

The most energy that can be EXERTED from a flow at any one time

Question 7

What happens to the exergy of a heat source as it’s temperature approaches infinity?

Answer 7

Exergy nears pure energy, it is said to be very high grade

Question 8

Explain first law efficiency

Answer 8

accounts only energy arriving at the process, irrespective of grade

n = required energy / total derived energy input

Question 9

Explain primary energy efficiency

Answer 9

replaces energy delivered from a primary source such as gas, sunlight etc. by the primary energy required to produce it.

Essentially processed energy over the source energy

n = required energy / “primary equivalent of delivered energy” input

Question 10

Explain exergy efficiency

Answer 10

compares actual consumption against that required by a thermodynamically ideal process

n = required exergy / total exergy input

Question 11

Energy systems accept [] energy flows and transforms them to produce one or more flows of a different []

Answer 11

accept one or more energy flows

flows of a different grade

Question 12

Why is exergy efficiency useful?

Answer 12

Helps to understand where exergy is being destroyed in an energy process

Question 13

In a conventional power station, where is the exergy loss typically located?

Answer 13

between combustion flame and the steam entering a turbine

Question 14

What does CCGT stand for?

Answer 14

combined cycle gas turbine

Question 15

Why is a CCGT better than a conventional power station?

Answer 15

a CCGT uses gas turbines to take exergy from the gas flame and pass cooled down gases to drive a secondary steam turbine. (Uses much more of the available energy)

Question 16

What does a heat pump do?

Answer 16

uses power to take energy from a cold source and pass heat to a hot source. The inverse of a heat engine

Question 17

What is a Carnot engine?

Answer 17

An ideal heat engine

Question 18

What is the equation of a Carnot engine?

Answer 18

Q^. = W^. +q^. all without loss of exergy in the process

Question 19

A Carnot engine takes energy at a rate of Q^. from [] and discharge waste at rate q^. to []

Answer 19

heat source Th (T hot)

heat sink Tc (T cold)

Question 20

The higher the [] and the lower the [] the more [] is converted to work

Answer 20

higher the source temp

lower the sink temp

think of the waterfall with a waterwheel in it analogy

Question 21

In a heat pump, energy is taken from [] and power is supplied to raise its [] and discharge heat at a higher []

Answer 21

taken from a low grade heat source

raise its grade

discharge heat at a higher temp

Question 22

An example of a heat pump is

Answer 22

A refrigerator

air conditioning unit

air to air heat pump

Question 23

How does an air conditioning unit work?

Answer 23

removes heat from a room, discharges heat at a higher temp to exterior

Question 24

How does an air to air heat pump work?

Answer 24

take zero-grade heat from ambient

raises its grade by adding the exergy of its shaft power to the flow of energy into the building

Question 25

Carnot efficiency is

(explanation and formula)

Answer 25

Carnot efficiency is the theoretical maximum efficiency of a heat engine

n =W. / Q. = 1 - Tc / Th

Question 26

What is meant by combined heat and power or cogeneration?

Answer 26

the use of a heat engine or power station to generate electricity and useful heat.

Opting for a more useful temp of “waste” heat as opposed to high efficiency power output

Question 27

What is primary energy?

Answer 27

raw fuel entering the economy

Question 28

What is secondary energy?

Answer 28

The fuel and power reaching final consumers

Comes after some processing

Question 29

What do we mean when we refer to delivered energy?

Answer 29

the sum of all energy that is metered as arriving at the final use

Question 30

Even delivered energy may need to be converted to [] energy when it arrives on site

Answer 30

useful energy

Question 31

What is the emergency reserve for the national heating demand?

Answer 31

10-14 days

Question 32

What is the national reserve for the national transport energy demand?

Answer 32

2 months

Question 33

What is the national reserve for the national electricity demand?

Answer 33

almost none due to the difficulty of storing electricity (size constraints of large batteries)

Question 34

What is a capacity factor?

Answer 34

ratio of average generation rate to installed capacity

Question 35

Why doe we have a levelized cost of energy?

Answer 35

LCOE is a standardised way for industries and governments to compare the cost of different energy systems

Question 36

What is the equation for LCOE?

Answer 36

LCOE = (OPEX + CAPEX) / E

OPEX - operating expense
CAPEX - capital expenditure
E - energy produced

Question 37

What is the unit for LCOE?

Answer 37

£/kWh

Question 38

What is the rough LCOE for onshore wind?

Answer 38

0.04 ~ 0.05

Question 39

What is the rough LCOE for fixed offshore wind?

Answer 39

0.1 (double that of onshore)

Question 40

What is the rough LCOE for concentrating solar power?

Answer 40

0.1

Question 41

What is the rough LCOE for floating offshore wind?

Answer 41

0.18 almost double of fixed offshore

Question 42

What is the rough LCOE for tidal

Answer 42

0.29 ~ 0.33 expensive

Question 43

What is the rough LCOE for wave energy

Answer 43

0.38 ~ 0.48 most expensive

Question 44

for non-household consumers, what is the average price of electricity?

Answer 44

LCOE 0.1

Question 45

although energy is conserved, not all forms of energy are inherently useful. What is important about an energy flow?

Answer 45

It’s grade

Question 46

What are the national energy system’s “vectors”

Answer 46

capacity must always exist
supply and demand must be met on a suitable timescale