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ELEC3915C
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
TURN OVER
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]
