Waste Heat

Question 1

What proportion of industrial energy consumption is currently wasted?

Answer 1

Between 20 and 50%

Question 2

How much industrial waste energy is though to be useable?

Answer 2

18 to 30%

Question 3

What is the efficiency of a conventional fossil fuel power plant?

Answer 3

~33%

Question 4

What is the maximum temperature generation of a combustion engine?

Answer 4

600^C

Question 5

What is the difference between active and passive heat recovery?

Answer 5

Passive simply transfers energy, incurring losses, while active adds energy to increase temperature or generate electricity

Question 6

Why is it difficult to decarbonise heating?

Answer 6

Electrical heating can be inefficient

Question 7

How much of the UK’s CO2 emissions, approximately, are generated by heating?

Answer 7

1/3

Question 8

How much more electricity would the UK need to generate if all heating demand was to be provided by electric heating?

Answer 8

50GW

Question 9

What are the 2 types of heat pump?

Answer 9

Compressor (active) and absorption (passive)

Question 10

What sources are typically used to maintain the temperature of a heat pump system?

Answer 10

Ground and water sources

Question 11

What is the difference between the Coefficient of Performance (COP) and Energy Efficiency Ratio (EER)?

Answer 11

COP used for heating, EER used for cooling, but both are effectively the same

Question 12

What is the Coefficient of Performance (COP)?

Answer 12

The ratio of useful heat transfer to mechanical work input

Question 13

How does the COP of an outdoor water source heat pump compare to an air source heat pump?

Answer 13

The COP is more consistent through seasonal variation but the peak COP is lesser

Question 14

What are the 3 types of heat storage?

Answer 14

Sensible
Latent
Thermochemical

Question 15

Where does a large amount of waste heat come from?

Answer 15

Inefficiencies in mechanical systems (friction)

Question 16

When is solar heating best to use?

Answer 16

For individual heating applications

Question 17

What is the difference between sensible and latent heat storage?

Answer 17

Sensible means there is no phase change in the heat carrier whereas latent makes use of the energetic variance as phase changes

Question 18

How does thermochemical storage compare to the other methods?

Answer 18

It has a higher storage capacity and efficiency but limited storage time

Question 19

How can sensible heat storage be made cheaper?

Answer 19

By scaling it up

Question 20

How do combined heat and power generation compare to typical generation methods?

Answer 20

Higher efficiency (~80%)
Lower fuel usage
20% lower cost
30% lower CO2 generation

Question 21

What are the main benefits and drawbacks of gas turbine CHP systems?

Answer 21

Low capital cost
High operating cost
Must be under constant operation for optimum efficiency

Question 22

Why has the efficiency of natural gas reciprocating engines reduced in recent years?

Answer 22

They are limited by strict emissions guidelines

Question 23

What are the main benefits and drawbacks of reciprocating engine CHP systems?

Answer 23

Can run on any fuel
High capital cost
Need regular maintenance
Resistant to up-scaling

Question 24

What is the equation for calculating the energy stored in a hot water tank?

Answer 24

E = PwCw * dT * V
E is energy
PwCw is the volumetric heat capacity

Question 25

What are the 6 underground thermal energy storage types?

Answer 25

Aquifer
Borehole
Tank
Pit
Cavern
Fracture

Question 26

What is the equation for drawdown at a thermal well?

Answer 26

S = (Q/2πT) ln (R/r)
Q is volumetric flow rate
T is transmissivity
R is distance between wells
r is radius of well

Question 27

How should heat storage wells be spaced?

Answer 27

At a minimum of 3 times the thermal radius apart

Question 28

In order, what are the 4 key components of a mechanical active heat pump?

Answer 28

Compressor
Condenser
Lamination Valve
Evaporator

Question 29

In a compression heat pump, changes in power requirements (m becomes m’) and the flow coefficient (K becomes K’) are related. What is the equation that describes this?

Answer 29

K’/K = (m/m’)^2

Question 30

What is the equation for ideal CoP?

Answer 30

1/(1-(Tsource/Tsupply))

Question 31

Give 2 examples of sector coupling

Answer 31

1. Using excess electricity from renewable sources to produce hydrogen, which can then
   be used as a fuel for transportation or to power heating and cooling systems.
2. Using waste heat from industrial processes to provide heating for homes and
   buildings.

Question 32

What is 1L in m3?

Answer 32

1x10^-3 m3

Question 33

I order, what are the 4 key components of an organic Rankine cycle?

Answer 33

Pump
Boiler
Turbine
Condenser

Question 34

What is the typical temperature range of industrial incineration?

Answer 34

650-1000^C

Question 35

How do you calculate the reliability of a system?

Answer 35

Total operating hours (T-S-U) / Total hours - scheduled outages (T-S)

Question 36

How do you calculate the availability of a system?

Answer 36

Total operating hours (T-S-U) / Total hours (T)

Question 37

What is the equation for thermal radius

Answer 37

rth = sqrt(cwQ/caHpi)
Q is injection rate
H is screen length
cw and ca are heat capacities of the water and rock respectively

Question 38

What is the equation for the thermal retardation factor?

Answer 38

R = ca/n*cw
n is porosity
cw and ca are heat capacities of the water and rock respectively

Question 39

Where in the ORC are irreversibilities introduced which mean that the ideal (Carnot) cycle is impossible?

Answer 39

Compression and Expansion are irreversible as they are not adiabatic (heat is transferred)