UNIVERSiTY COLLEGE LONDON·
·.EXAMINATION FOR INTERNAL STUDENTS
MODULE CODE ELEC3915
ASSESSMENT . ELEC3915C

PATTERN

MODULE NAME Renewable Energy

,
DATE Friday 18 May 2018

TIME 10:00

TIME ALLOWED 3 hrs

This paper is suitable for' candidates who attended classes for this
module in the following academic year(s):
Year
,
Suitable for all candidates
. EXAMINATION PAPER .CANNOT BE REMOVED FROM THE EXAM HALL. PLACE EXAM
PAPER AND ALL COMPLETED SCRIPTS INSIDE THE EXAMINATION ENVELOPE
2016/17-ELEC3915C-001-EXAM-Electronic and Electrical Engineering 43

© 2016 University College London

TURN OVER
ELEC3915

Conversions needed:
3412 BTU per kWh 2205. pound per tonne
Equations needed:
n 1O8 = 23.45 COS (21r__+__ _) 1 (2 2)E=ZPA VI -V2 vwt=Fvwt . COHOs =-tanatano365.25
1 T/2 1 T/2 21!
, - f cos mtdt =- f cos-tdt
. T· T 24Average of a 24 hour cosme curve: -T/2 -T/2
Answer question #1, and any 3 ofthe remaining 4 questions,
for a total of 4 questions answered. Each question is worth 25 marks.
1. REQUIRED A wealthy residential development of some 1000 homes is to be constructed in a
. beautiful city in Spain at latitude 37° north latitude. Eachhome is expected to carry a daily load
as shown in Fig. 1.1.
2
kW
1.5 r t~1
1 i (
.-r"
! I'\. ./ \
0.5
,/
12 idnight 6AM 12 noon .6PM 12 midnight
, ,Figure 1.1: Load curve
(a) Estimate the total energy required by all homes in the housing development in kWhr-yr-1•
[2 marks]
(b) The Sun Chart for this region appears in Fig. 1.2.
0
90 ~(c)C:un:':JI"=.oc::Io=re':IgOn=::;s';IRu:C:::LIIIII1I11111111111111"1
80°
700
600
a
~ 50° "
~
.~
r;;
~ -lao
'0
if}
300
20~
100
r
Sponsor: rl'O li-t----+-+-+-+,-+-+-+-+--+--+--+---+-+-+-+-+-+------'1 f;u\;}~~ I~~~~g~~n{': 0 HI~--+--+--+!----r.l=2+..IP ..Mj--f----1---t---+--+I--+--+--+--+---+----1
~al(i.~a. SpaIn
'­
'j\ I r.,; !
'I IX i ~E.. I.X \' .
fit / \ /f I'\. II D, X\' i
i \ \ IV i I .~. V I .L
1 Y 1\ 1/\ I ~~ h! V I
! II 'I.. I II ry i I \/\ \, \ 1X \\ I
I I 'I 1/\ I/'I/A' 'I 1~Y\ A II\, i
I
I
I
i
j2.\
'11 IVI ~j:i- --. """" 1 PM
:300 GOo 901:1 L20 0 1500 1800 2LOo 2400 270 0 300 0 3300 360 0
East <-- Solar AZImuth --> West
Figure 1.2: The Sun Chart for the City
Page 1 of8 TURNOVER
ELEC3915

(i) Estimate the hour of sunrise on the day of the Spring Equinox. State the date ,used.
[2 marks]
(ii) For how many hours is the sun above the horizon on that day? [1 mark]
(iii) At what azimuth and tilt angle should a solar panel be placed to capture maximum sun
throughout that day? Ifyou don't know when the Spring Equinox is, pick any day
• I ~m~~
(iv) Estimate the amount of solar energy that falls on a squ~e meter of panel that directly
faces the sun at noon on this day, but does not track the sun during the rest of the day.
Recall that the sun makes a cosine arc through the sky. Be sure to recall the sun's
average intensity on the surface ofthe Earth. Don't worry about declination or latitude;
we want just a simple estimate. [3 marks]
(c) From the daily load curve suggest an average power flow for the morning, the day, the !light
(three values). [2 m~ks]
(d) The idea is to have solar panels supply home load during the day and, at the same time,
charge batteries. The batteries will not be in the homes; they will be in a special location
somewhere in the development. The batteries must have enough storage capacity to handle
the total load of the community for 2. nights and a day, in case ofa rain storm. Answer these
questions to design the system for the community:
(i) Calculate the total energy needed to be supplied by the solar panels while the sun is out.
[3 marks]
(ii) How many Tesla PowerPack batteries are required? Each one supplies 210 kWh.
. ~m~~
(iii) How many solar panels for each house are needed, given that each panel has a peak
output of250W? [4 marks]
(e) Draw the power flow diagram for one house. Assume the batteries ~e in a public location
outside the house, so there is no need to include the battery in the diagram. Include all other
major components of the system. [3 marks]
CONTINUE Page 2 of8
ELEC3915

2. [25 marks] This question intends to discover your understanding of the effects of renewabies on
the electrical transmission and distribution power lines. .
(a) What is the essential, principal difference between a centralized electrical system and a
decentralized system? [2 marks]
(b) What is meant by a 3-phase electri~al power system? Why are electrical grids designed
using them? [3 marks]
(c) A transmission line can be several lOs to 100s of kilometres long. A generator at the origin
, feeds power into a transfOriner with (j00 volts on the primary side and 132,000 volts on the
secondary side, single phase.
. (i) If the generator is rated 120 MYA 3-phase, calculate the current on both sides of the
transformer. Assume all three phases are balanced. [4 marks]
(ii) Calculate the active, reactive and apparent VAN =VBN =VCN == 132,000vpower transferred to the load, ZA, in the
ZA =2B =Zc =1000+ j400 Ohmsfollowing transmission 3-phase system
Zo =0shown in Fig. 2.1, with the given values.
[5 marks]
.. 111 UneA "~
-_.+llllLB I UneB
Linse
Nsutmlline
Figure 2.1: A 3~phase circuit.
. (iii) Calculate the inductance ofthe load, ZA, at 50 Hz. [2 marks]
(iv) Reactive power on the transmission causes the curren~ to increase and therefore losses
to increase. Can a solar PV power plant placed onthe transmission line help this
situation, and if so how? [3 marks]
(d) The distribution power lines that go from house to house down a city block is called a
"feeder"; they can be several kilometers long and many homes and businesses act as
impedance loads all along the way.
(i) What happens to the voltage along the' length ofthe feeder? Why does this occur?
. D~~~
(ii) Ifhomeowners add solar systems to their homes at different places along the feeder,
what happens to the voltage? [2 marks]
(iii) Suppose a business on the end of the line uses a large number of electrical motors.
What happens to the current and the power factor on the line? [2 marks]
Page 3 of8 TURNOVER
ELEC3915

3. [25 markslThis question asks about conventional fossil fuel power plants and asks for a
comparison with co-generation power plants.
(a) A conventional fossil fuel electrical power plant uses a steam turbine.
(i) Draw a block diagram showing its essential components. [5 marks]
(ii) How fast is the generator spinning in RPM to produce 50 Hz output? How many poles
are there in the generator? [2 marks]
(b) The waste heat of the conventional power plant is still at a high enough temperature to be
useful. An alternative syst~m is ~hown in part (b) of Fig. 3.1:
(a)
(b)
A"L~:,yt~%f;~]?r~
Figure 3.1: A Conventional Power Plant with (b) and without (a) Cogeneration
(i) What is the efficiency of the power plant by itself shown (a) of Figure 3.1 ? [1 mark]
(ii) What is the efficiency of the boiler by itself in part (a)? [1 mark]
(iii) What is the efficiency of the combined system in part (b)? [2 mark]
(c) Pennsylvania State University (Penn State) in the United States uses a combined heat and
power plant (CHP) system for heating and for electrical power, as showt:I in figure 3,1 (b).
Some 6 MWe is generated by the steam turbine and 200 buildings are heated with the excess
waste heat through 17 miles of underground piping. The steam is at 550°C and the cool
water coming out of the turbine is at 2200 C.
.(i) Draw a block diagram of the probable layout of the essential components of the system.
[5 marks]
(ii) Since Pennsylvania has vast quantities of coal, the system was originally built to use
coal as its source of heat for the boilers. What would you recommend to the campus
supervisors in order to reduce carbon emissions? Remember to include ideas for the entire
campus. [4 marks]
CONTINUE Page 4 of 8
ELEC3915

(iii) Estimate the amount of coal in tonnes required to run the plant at Penn State for a day, if
. .
coal has a heat content of 14,000 ~TU per pound. The plant never goes off. Assume the
synchronous generator .atlached to the steam turbine is 95% efficient. Assume the steam
turbine is 35% efficient. [5 marks]
Page 5 of8 TURNOVER
ELEC3915

4. [25 marks] This problem seeks your understanding of wind and wind turbines.
The theoretical power coefficient for wind turbines is given in Fig. 4.1:
" IU.U.l3J
!': fl592~93
O.S
. .
.
•..•D....................................................•.. ~_~
.
.
0.4
0.2
0.1 0.2 0.3 004 0.5 0.6 0.7 . 0.8 o.g; V.,
,=~
• V,
1
Figure 4.1: Power Coefficient as a function of the wind speed ratio.
The specifications for a standard 3 MWe wind turbine are shown in Figure 4.2:
3500

3000

2500

~ 2000
..
.. ~ 1500
Do
1000

500

o
o 3 4 5 6 7
Power Curve V90-3.0 MW

Air Density 1.225

./
/
/
/
./
~
B 9 10 11 12 13 14 15 16
Wind Speed [m/s)
1i 1B 19 20 21 22 23 24 25
Figure 4.2: Specifications for the turbine.
(a) Indicate the "cut-in" speed, the "rated" speed and the "cut-out"· speed of the wind.
[3 marks]
(b) Calculate the length ofthe blades, if the wind is blowing at the rated speed and the power
coefficient is 0.5. Assume the blades extract 60% of impinging wind energy. [3 marks]
(c) The blades of this turbine are shaped in the standard way.
(i) Describe how the blades are shaped. [3 marks]
(ii) Describe how the blades begin to move from rest and what causes them.to begin
moving. [3 marks]
/
(iii) Draw a diagram which shows the position of the blades with respect to the wind as the
blades are moving at rated 'speed: [3 marks]
CONTINUE Page 6 of8
ELEC3915

(d) Calculate the force on a single blade when the turbine is spinning at rated speed, and the
power coefficient is 50%. [3 marks]
,(e) This wind turbine uses an asynchronous generator,operating at 50 Hz and generates 1000
vAC. It does so with a power factor of 0.96.
(i) What are the main differences between synchronous and asynchronous generators?
'[4 marks]
(ii) What current does this turbine deliver to the grid at its output? [3 marks]
Page 7 6f8 TURNOVER
"
ELEC3915
5. [25 marks] Write a short response to the following statements: .
(a) "There is plenty of oil, coal and natural gas to last several hundred years. There is no need to
worry about running out of fuel resources". [5 marks]
(b) "The climate is always changing. It is what it is and there is nothing we can do about it.
[5 marks]
(c) "Uranium is the densest energy resource known. We really should make more use of it."
[5 marks]
(d) "The Duck Problem is a serious one. It is preventing quick replacement of the centralized
grid system with a decentralized one." . [5 marks]
(e) "Prosperity requires energy. By simple arithmetic: GDP has to grow at the same rate as the
population, and it can't grow unless everyone uses at least the same amount of energy they
have been using." [5 marks]
END OF PAPER .Page 8 of8