ATHLONE INSTITUTE OF TECHNOLOGY

SCHOOL OF ENGINEERING

SEMESTER 2 EXAMINATIONS 2013

Summer Session

BACHELOR OF ENGINEERING in MECHANICAL ENGINEERING and

RENEWABLE ENERGY

AWARD YEAR

COMBINED HEAT AND POWER 3

External Examiner(s): Mr. Andrew O’Connell

Dr. Fergal Boyle

Internal Examiner(s): Dr. Niall Burke

Instructions to candidates:

Read all questions carefully.

All questions carry equal marks.

Answer Three out of Four questions.

Time Allowed: 2 hours

No. of pages including cover sheet: 12

Q.1 (a) In evaluating the economic viability of Combined Heat and Power Systems,

what is meant by the term “Spark Spread”?

(4 marks)

(b) What are the four strokes of an Otto cycle?

(4 marks)

(c) What are the conversion steps in transforming bio-energy into electrical

energy?

(4 marks)

(d) What are the typical CO2 emissions for a natural gas CHP system and how

does this compare with the CO2 emissions from national grid generated

electricity.

(4 marks)

(e) What is the difference between a heat engine and a heat pump?

(4 marks)

[20/60 marks]

Q.2 (a) Explain how a Combined Heat and Power (CHP) system operates and what

applications are CHP systems best suited.

(4 marks)

(b) What are the key contributors to the economic success of a CHP system?

(6 marks)

(c) Explain in detail, with the aid of a diagram, the components of a gas micro-

turbine, and evaluate the applications for this system.

(10 marks)

[20/60 marks]

Q.3 (a) Under what conditions may a gas turbine CHP system be an appropriate

choice for a site?

(6 marks)

(b) In the Brayton Cycle shown in the figure, calculate the maximum efficiency

that can theoretically be achieved using the temperatures provided.

(4 marks)

(c) Air enters the compressor of a gas turbine at 100kPa and 25°C. For a pressure

ratio of 5 and a maximum temperature of 850°C determine the back work ratio

and thermal efficiency using the Brayton cycle.

(10 marks)

[20/60 marks]

Fuel in

AC Power 33% Generator

1100°C

500°C

Exhaust Gases 67%

~20°C

Q.4 (a) Explain where heat is recovered on a reciprocating engine CHP system.

(4 marks)

(b) Detail some of the design characteristics/features that have made reciprocating

engines a leading prime mover for CHP systems.

(6 marks)

(c) A simple steam power plant operates on 30 kg/s of steam, as shown in Figure

1.

Neglecting losses in the various components, calculate:

(i) The boiler power input, ��� (2 marks)

(ii) The turbine power output, �� � (2 marks)

(iii) The condenser output, ��� (2 marks)

(iv) The pump power requirement, �� � (2 marks)

(v) The thermal efficiency and Carnot efficiency of the cycle. (2 marks)

[20/60 marks]

Figure 1

10 MPa

10 MPa 10 kPa

10 kPa

40 °C

40 °C

600 °C

TABLE 1.1

Supporting information extracted from “Steam – its generation and use”, 40th edition, edited by S.C. Stultz and J.B. Kitto

TABLE 1.2

Supporting information extracted from “Steam – its generation and use”, 40th edition, edited by S.C. Stultz and J.B. Kitto

TABLE 1.3

Supporting information extracted from “Steam – its generation and use”, 40th edition, edited by S.C. Stultz and J.B. Kitto

TABLE 2

Supporting information extracted from “Steam – its generation and use”, 40th edition, edited by S.C. Stultz and J.B. Kitto

TABLE 3

Supporting information extracted from “Thermodynamics for Engineers” by M.C. Potter and C.W. Somerton.

TABLE 4

Properties of Water (saturated Liquid)

Supporting information extracted from “Heat Transfer”, 9th edition by J.P. Holman

TABLE 5

Properties of Air at Atmospheric Pressure

Supporting information extracted from “Heat Transfer”, 9th edition by J.P. Holman

TABLE 6

Properties of Air

Supporting information extracted from “Fundamentals of Thermo-fluid Science”, 4th edition by Cengel et. al.