# Solar Energy Questions and Answers – Solar Thermo-Mechanical Systems -2

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This set of Solar Energy MCQs focuses on “Solar Thermo-Mechanical Systems -2”.

1. How does an ideal Rankine cycle work?
a) Heat-addition process does not occur at constant temperature
b) Heat-addition process occurs at temperature
c) Heat-addition process does not occur at constant pressure
d) Heat-addition process occurs at constant pressure

Explanation: Like Carnot cycle, Rankine cycle is a heat engine phase cycle. It determines how working fluid is used to convert energy from one form to another. The heat-addition process in Rankine cycle does not occur at constant temperature.

2. Which of the following best describes the process of an ideal Rankine cycle?
a) Working fluid is heated in boiler → exhaust vapour is condensed → vapour runs the turbine → liquid residue is pumped back into boiler
b) Working fluid is heated in boiler → vapour runs the turbine → exhaust vapour is condensed → liquid residue is pumped back into boiler
c) Vapour runs the turbine → working fluid is heated in boiler → exhaust vapour is condensed → liquid residue is pumped back into boiler
d) Exhaust vapour is condensed → Working fluid is heated in boiler → vapour runs the turbine → liquid residue is pumped back into boiler

Explanation: In an ideal Rankine cycle, the working fluid is heated in boiler and converted to vapours which is used to run the turbine. The exhaust vapours from the turbine is condensed to form a liquid residue which is pumped back into the boiler to complete the cycle.

3. Why is the efficiency of Rankine cycle less than that of Carnot cycle?
a) Because of the working fluid
b) Because of the turbines
c) Because all the heat is not supplied at upper temperatures
d) Because all the heat is supplied at upper temperatures

Explanation: The efficiency of Rankine cycle is less than that of Carnot cycle because all the heat is not supplied at upper (higher) temperatures. However, is has a high work ratio and its steam consumption is less than Carnot cycle.

4. How can the efficiency of Rankine cycle be improved?
a) By supplying heat at higher temperatures
b) By supplying heat at lower temperatures
c) By rejecting more heat
d) By using a part of rejected heat

Explanation: Rankine cycle has lower efficiency that Carnot cycle because all the heat is not supplied at upper temperatures. The efficiency of Rankine cycle can be improved by using a part of rejected heat in heating the liquid.

5. How does Stirling cycle differ from Carnot cycle?
a) The two adiabatic steps in Carnot cycle are replaced by two constant volume steps
b) The two adiabatic steps in Carnot cycle are replaced by two constant temperature steps
c) The two isothermal steps in Carnot cycle are replaced by two constant volume steps
d) The two isobaric steps in Carnot cycle are replaced by two constant volume steps

Explanation: The working of Stirling cycle is similar to that of Carnot cycle except for two steps. The two adiabatic steps in Carnot cycle are replaced by two constant volume steps.
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6. In Stirling cycle, the heat addition and rejection take place at _________
a) constant pressure
b) constant temperature
c) two different temperatures
d) multiple pressures

Explanation: In Stirling cycle, the heat addition and rejection take place at constant temperatures. A suitable gas or air is used as working fluid and the turbine, compressor and heat exchanger are closely coupled.

7. Which of the following best describes Brayton Cycle?
a) Exhaust gas → hot compressed gas → expands through turbine by doing work → heat exchanger → rejection of heat → gas goes to compressor
b) Exhaust gas → hot compressed gas → expands through turbine by doing work → heat exchanger → gas goes to compressor → rejection of heat
c) Hot compressed gas →expands through turbine by doing work → exhaust gas → heat exchanger → rejection of heat → gas goes to compressor
d) Heat exchanger → exhaust gas → hot compressed gas → expands through turbine by doing work → gas goes to compressor → rejection of heat

Explanation: In Brayton Cycle, the hot compressed gas expands through the turbine by doing work. The exhaust gas from the turbine is passed into heat exchanger to reject heat. Then this gas is fed into the compressor to be compressed and used again.

8. Which of the following governs the efficiency of any practical solar thermo-mechanical system?
a) Photovoltaics
b) Rankine Cycle
c) Brayton Cycle
d) Properties of working fluid

Explanation: Properties of working fluid govern the efficiency of any practical solar thermo-mechanical system. Rankine and Brayton Cycles are techniques to implement heat engines to convert the input energy into a usable output energy.

9. The choice of working fluid depends on the operating temperatures in the boiler.
a) True
b) False

Explanation: The choice of working fluid depends on the operating temperatures in the boiler. It also depends on the condenser being used and the type of heat engine.

10. What is the main problem in a solar thermo-mechanical system as the collection (operating) temperature increases?
a) Efficiency of collection system and engine decreases
b) Efficiency of collection system decreases but engine efficiency increases
c) Efficiency of collection system and engine increases
d) Efficiency of collection system increases and engine efficiency decreases

Explanation: The main problem in a solar thermo-mechanical system is that as the collection (operation) temperature increases efficiency of the collection system decreases. However, the engine efficiency increases as the temperature of working fluid increases.

11. The practical heat engine _______ theoretical efficiency.
a) has greater efficiency than
b) has the same efficiency as that of
c) has lesser efficiency as compared to the
d) always has minimum efficiency equal to that of

Explanation: The practical heat engine used in solar thermo-mechanical systems always has lesser efficiency as compared to the theoretical efficiency. It can never exceed the theoretical limit due to practical limitations.

12. What is the bare minimum requirement for the construction materials used in solar thermo-mechanical systems?
a) Contaminate the working fluid
b) Withstand heat from the working fluid
c) Withstand pressure from the working fluid
d) Withstand both, heat and pressure from working fluid

Explanation: The bare minimum requirement from the construction materials used in a solar thermo-mechanical system is that they withstand both, heat and pressure from the working fluid. The materials should not contaminate the fluid.

13. Which of the following affects the installation of a solar thermo-mechanical system?
a) Availability of land
b) Availability of photovoltaics
c) Availability of sunlight and clouds
d) Availability of rain

Explanation: Availability of land affects the installation of solar thermo-mechanical system. Sunlight is important but most places on earth receive sunlight. Clouds and rain are a hinderance to the system.

14. What is the minimum vapour pressure (in kPa) of the working fluid to drive a turbine?
a) 300
b) 700
c) 100
d) 50

Explanation: The minimum vapour pressure of the working fluid to drive a turbine is 700 kPa. The preferred vapour pressure 2000kPa for useful work.

15. Organic vapours are used small plants and low powered solar thermo-mechanical systems.
a) True
b) False

Explanation: Organic vapours are used small plants and low powered solar thermo-mechanical systems. This is simply driven by the demands and finances.

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