Introduction to Cycle Analysis.pptx

Introduction to
Cycle Analysis

Heat Engines
Work can easily be converted to other forms of
energy, but converting other forms of energy to work
is not that easy. The mechanical work done by the
shaft shown in figure, for example, is first converted
to the internal energy of the water. This energy may
then leave the water as heat. We know from
experience that any attempt to reverse this process
will fail. That is, transferring heat to the water does
not cause the shaft to rotate. From this and other
observations, we conclude that work can be
converted to heat directly and completely, but
converting heat to work requires the use of some
special devices. These devices are called heat
engines.

Heat Engines
Heat engines differ considerably from one another, but
all can be characterized by the following
1. They receive heat from a high-temperature source
(solar energy, oil furnace, nuclear reactor, etc.).
2. They convert part of this heat to work (usually in the
form of a rotating shaft).
3. They reject the remaining waste heat to a low-
temperature sink (the atmosphere, rivers, etc.).
4. They operate on a cycle.

Heat Engines
Thermal efficiency – the fraction of the heat input that
is converted to network output is a measure of the
performance of the heat engine.

The Second Law of Thermodynamics
1st statement : (Kelvin-Planck Statement of the Second
Law)
- It is impossible to construct a heat engine that operates in
a cycle, receives a given amount of heat from a high-
temperature body, and does an equal amount of work.
This implies that it is impossible to build a heat engine
that has a thermal efficiency of 100%. The thermal
efficiency of practical heat engines typically ranges from
10 to 40%. Thus in practice, some portion of the heat
supplied from a high- temperature source is always
rejected to a low-temperature sink.
- “ it is impossible for any heat engine to convert heat
completely into work”
- There is no 100% efficient engine

The Second Law of Thermodynamics
2nd statement : (Refrigerator Statement)(Clausius
Statement)
- It is impossible for any refrigerator to transfer heat from a
cooler region to a hotter region without doing work
- There is no workless refrigerator
- Heat don’t flow from cold to hot body on its own

Carnot Cycle
- First proposed in 1824 by French engineer Sadi Carnot
- The theoretical heat engine that operates on the carnot cycle is called the Carnot
heat engine.
- The carnot cycle is composed of four reversible process – two isothermal and two
adiabatic

Carnot Cycle
Four Reversible Process of a Carnot Cycle:
1-2: A reversible isothermal expansion during which heat is
transferred from the high- temperature reservoir to the working
fluid
2-3: A reversible adiabatic expansion during which the
temperature of the working fluid decreases from the high
temperature to the low temperature
3-4: A reversible isothermal compression during which heat is
transferred from the working fluid to the low-temperature
reservoir
4-1: A reversible adiabatic compression during which the
temperature of the working fluid increases from the low
temperature to the high temperature

Reversed Carnot Cycle
The Carnot heat-engine cycle just described is a totally
reversible cycle. Therefore, all the processes that comprise
it can be reversed, in which case it becomes the Carnot
refrigeration cycle. This time, the cycle remains exactly
the same, except that the directions of any heat and work
interactions are reversed: Heat in the amount of QL is
absorbed from the low-temperature reservoir, heat in the
amount of QH is rejected to a high-temperature reservoir,
and a work input of Wnet.in is required to accomplish all this.

Carnot Heat Engine
- Idealized heat engine with the maximum possible efficiency
- is a device that operates in a cycle and produces a net positive work while exchanging heat
across its boundaries. The measure of performance for a Carnot engine is called the thermal
efficiency and is defined as the ratio of the desired effect (the net work output) to the energy
input required to produce the desired effect.
- Efficiency only depends only on the temperatures of the hot and cold reservoirs
𝑒 =
𝑊
𝑄𝐻
= 1 −
𝑇𝐿
𝑇𝐻
=
𝑇𝐻 − 𝑇𝐿
𝑇𝐻
; 𝑛𝑜𝑡𝑒 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑖𝑠 𝑎𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒

Carnot Refrigerator
- A Carnot refrigerator is used to maintain the temperature of a refrigerated space at a
temperature lower than the temperature of the environment. The measure of its
performance is called the Coefficient of Performance (COP), which is defined as the
ratio of the desired heat transfer to the net energy input required to produce the
desired effect.
𝐶𝑂𝑃 =
𝑄𝐿
𝑊
==
𝑇𝐿
𝑇𝐻 − 𝑇𝐿
; 𝑛𝑜𝑡𝑒 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑖𝑠 𝑎𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒

Example:
1. Heat is transferred to a heat engine from a furnace at a rate of 80 MW. If the rate of waste heat
rejection to a nearby river is 50 MW, determine the net power output and the thermal efficiency for this
heat engine.
2. A carnot heat engine receives 500 KJ of heat per cycle from a high temperature source at 652 deg C
and rejects heat to a low temperature sink at 30 deg C. Determine the thermal efficiency of this carnot
engine and the amount of heat rejected to the sink per cycle.
3. A refrigerating system operates on the reversed Carnot cycle. The higher temperature of the
refrigerant in the system is 120 deg F and the lower is 10 deg F. The capacity is 70.4 KW. Determine the
COP, heat rejected from the system, net work.
4. A heat engine that pumps water out of an underground mine accepts 700 KJ of heat and produces 250
KJ of work. How much heat does it reject , in KJ?
5. A refrigerator used to cool a computer requires 1.2 KW of electrical power and has a COP of 1.8.
Calculate the cooling effect of this refrigerator, in KW.

Introduction to Cycle Analysis.pptx

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Introduction to Cycle Analysis.pptx