Basic concepts 1 (TD) (3).pdf_presentation

Firma convenzione
Politecnico di Milano e Veneranda Fabbrica
del Duomo di Milano
Aula Magna – Rettorato
Mercoledì 27 maggio 2015
ARCHITECTURE - BUILT ENVIRONMENT – INTERIORS
BUILDING PHYSICS
A.Y. 2024/25 Prof. S. Ferrari
REVIEW OF THE BASIC CONCEPTS 1 (THERMODYNAMICS)

Prof. Simone Ferrari, D.ABC
The basis of the slides is taken from: “Thermodynamics: An Engineering Approach”, the 10th
Edition is available as e-book at the following platform link:
VitalSource Bookshelf
Remember that to access electronic resources from outside Polimi you need to set up the
Proxy service:
https://www.biblio.polimi.it/en/services/how-to/access-to-an-electronic-resource

THERMODYNAMICS AND ENERGY
Thermodynamics: The science of energy.
Energy: the ability to cause changes.
The name thermodynamics stems from the Greek words
therme (heat) anddynamis (power).
All activities in nature involve
some interaction between
energy and matter; thus, it is
hard to imagine an area that
does not relate to
thermodynamics in some
manner.

THE FIRST LAW OF THERMODYNAMICS
The first law of thermodynamics (the conservation of energy
principle) provides a sound basis for studying the relationships
among the various forms of energy and energy interactions.
The first law states that energy can be neither created nor
destroyed during a process; it can only change forms.
Conservation of
energy principle for
the human body.
A refrigerator operating with
its door open in a well-sealed
and well-insulated room
A fan running in a well-sealed
and well-insulated room
As a result of the conversion of electric
energy consumed by the device to heat, the
room temperature will rise.

FORMS OF ENERGY
Macroscopic forms of energy: Those a system
possesses as a whole with respect to some outside
reference frame, such as kinetic and potential
energies.
Kinetic energy, KE: The energy that a system
possesses as a result of its motion relative to some
reference frame.
Potential energy, PE: The energy that a system
possesses as a result of its elevation in a gravitational
field.
Microscopic forms of energy: Those related to the molecular structure of a system
and the degree of the molecular activity.
Internal energy, U: The sum of all the microscopic forms of energy.

SYSTEMS AND CONTROL VOLUMES
System: A quantity of matter or a region in space
chosen for study.
Surroundings: The mass or region outside the
system
Boundary: The real or imaginary surface that
separates the system from its surroundings.
The boundary of a system can be fixed or
movable.

Energy Change of a system
Potential energy
Kinetic energy
U = ….
Internal energy
(The sum of all the microscopic forms of energy)
Total energy of a system
The net change (increase or decrease) in the total energy of the system during a
process is
NOTE:
(buildings are stationary systems!!!)
The net change (increase or decrease) in
the total energy of a stationary system
during a process is equal to the change of
its internal energy ΔU
The macroscopic
energy of an object
changes with
velocity and
elevation.

Some Physical Insight to Internal Energy of a System
8
The internal energy of a
system is the sum of all forms
of the microscopic energies.
The various forms of
microscopic energies
that make up sensible
energy.
Sensible energy: The internal energy
associated with the kinetic energies of
the molecules.
Latent energy: The internal energy
associated with the phase change of a
system (solid-liquid-gas).
Chemical energy: The internal
energy associated with the atomic
bonds in a molecule (e.g., fuels).
Nuclear energy: The tremendous
amount of energy associated with the
strong bonds within the nucleus of the
atom itself.
Internal (total) = Sensible + Latent + Chemical + Nuclear
Internal (thermal) = Sensible + Latent

Closed systems
• The only two forms of energy interactions
associated with a closed system are
heat transfer and work.
• Closed system
(Control mass):
A fixed amount
of mass, and no
mass can cross
its boundary.

ENERGY TRANSFER BY HEAT
Temperature difference is the driving force for heat
transfer. The larger the temperature difference, the
higher is the rate of heat transfer.
The form of energy that is transferred between two systems (or a
system and its surroundings) by virtue of a temperature difference.
Energy is recognized as
heat transfer only as it
crosses the system
boundary.

THE THERMODYNAMIC TEMPERATURE SCALE
Comparison of (SI)
temperature scales.
The reference temperature in the original
Kelvin scale was the ice point, 273.15 K,
which is the temperature at which water
freezes (or ice melts).
Steam point: at 1 atm pressure 373.15 K
(100°C)
Kelvin scale (SI – International System of Units)
Comparison with (non-SI)
Rankine and Fahrenheit
temperature scales
Remark!

ENERGY TRANSFER BY WORK
– there must be a force acting on the boundary.
– the boundary must move.
Work = Force × Distance
Mechanical forms of work
The work associated
with a moving
boundary is called
boundary work.
Wb is positive → for expansion
Wb is negative → for compression
Work: The energy transfer associated with a force acting through a
distance.
– A rising piston, a rotating shaft, and an electric wire crossing
the system boundaries are all associated with work interactions

Nonmechanical Forms of Work
Electrical work: The generalized force is the voltage (the electrical potential) and the generalized
displacement is the electrical charge.
Magnetic work: The generalized force is the magnetic field strength and the generalized displacement
is the total magnetic dipole moment.
Electrical polarization work: The generalized force is the electric field strength and the generalized
displacement is the polarization of the medium.
ENERGY TRANSFER BY WORK

FUNDAMENTAL UNITS
The definition of the force units (Newton).
Work = Force × Distance
1 N x 1 m = 1 J
despite withdrawn from SI, for heat is still also used 1 Cal (or kcal) = 4186 J
The definition of the work (and heat) units (Joule).
W weight m mass g gravitational acceleration
The weight is a force!
the weight of a unit mass

FUNDAMENTAL UNITS
Watt, Joule (and Watthour)
W = J/s Ė, energy per unit of time (1 second)
also called “intensity of energy” or “power”
(n°of W) x (n°of seconds) = (n°of J) E, energy during time
(n°of W) x (n°of hours) = (n°of Wh) E, energy during time
Energy units conversion
(n°of Wh) x 3600 = (n°of J) (n°of J)/3600 = (n°of Wh)

In the absence of any
work interactions, the
energy change of a
system is equal to the
net heat transfer.
Energy Balance for Closed Systems
The energy change
of a closed system
during a process is
equal to the net
work and heat
transfer between
the system and its
surroundings.

The work (electrical, shaft) done on
an adiabatic system is equal to the
increase in the energy of the system.
During an adiabatic
process, a system
exchanges no heat with
its surroundings.
Energy Balance for Adiabatic (Closed) Systems

Energy Balance for Closed Systems
18
18
where:
Win 2 (-)
Wout 20 (+)
Qout 4 (-)
Qin 10 (+)
Example:
Qnet,in = +10 - 4 = + 6
Wnet,out = + 20 - 2= +18
∆U = +6 – (+18) = -12 (decreasing)
Formal sign convention: Heat transfer to a system and work done by a system are
positive; heat transfer from a system and work done on a system are negative.
∆U = (10+2) – (20+4) = -12 (decreasing)
Alternative to sign convention is to use the subscripts
in and out to indicate direction of heat and work.

Open systems
19
Open system (control volume): A properly
selected region in space
e.g., a DHW heater, the room in which it is
placed, the apt. in which the room is, the
building, the district, the town, etc.
Both mass and energy can cross the
boundary of a control volume.
An open system (a
control volume) with one
inlet and one exit.

Energy Balance for an open system

ENERGY TRANSPORT BY MASS
Flow work: The work (mech. energy) required to push the mass into or out of the
control volume. This work is necessary for maintaining a continuous flow through
a control volume.
Schematic for flow work.
The product pressure ×
volume has energy units.
The total flow energy (flow work + internal
energy of the fluid) is automatically taken
care of by enthalpy.
Enthalpy— A Combination Property

CONSERVATION OF MASS PRINCIPLE
Mass, like energy, is a conserved property, and it cannot be created
or destroyed during a process.
Closed systems: The mass of the system remain constant during a process.
Control volumes (open system): Mass can cross the boundaries, and so we
must keep track of the amount of mass entering and leaving the control
volume.
Mass is conserved even during chemical reactions.
Conservation of mass principle
for an ordinary bathtub.

Mass balance for a steady-flow process
During a steady-flow process, the total amount of mass contained within a
control volume does not change with time (mCV = constant).
Then the conservation of mass principle requires that the total amount of mass
entering a control volume equal the total amount of mass leaving it.
Conservation of mass principle for a two-inlet–one-outlet
steady-flow system.
For steady-flow processes, we are
interested in the amount of mass flowing per
unit time, that is, the mass flow rate.
Multiple inlets
and exits
Single
stream

Mass and Energy balances for a steady-flow process
Mass
balance
Energy
balance
= (kJ/s)

THE SECOND LAW OF THERMODYNAMICS
The second law of thermodynamics: it asserts that energy has quality as well
as quantity, and actual processes occur in the direction of decreasing quality of
energy.
Heat flows in the direction of
decreasing temperature.
Transferring heat to a paddle
wheel will not cause it to rotate.
Transferring heat to a wire will not generate
electricity.

THE SECOND LAW OF THERMODYNAMICS
Work can always be converted to heat directly and completely,
but the reverse is not true.

Bodies with relatively large thermal
masses can be modeled as thermal
energy reservoirs.
A source
supplies
energy in the
form of heat,
and a sink
absorbs it.
• A hypothetical body with a relatively large thermal energy capacity that can
supply or absorb finite amounts of heat without undergoing any change in
temperature is called a thermal energy reservoir, or just a reservoir.
• In practice, large bodies of water such as oceans, lakes, and rivers as well as the
atmospheric air can be modeled accurately as thermal energy reservoirs
because of their large thermal energy storage capabilities (or thermal masses).
THERMAL ENERGY RESERVOIRS

HEAT ENGINES
The devices that convert heat (partially) to work.
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.).
Part of the heat received by a heat engine is
converted to work, while the rest is rejected
to a sink.

HEAT ENGINES
They operate on a cycle.
For a cycle ∆E = 0,
thus Q = W
(Qin-Qout) = Wnet,out

HEAT ENGINES
Where:
< 1 always!!!

REFRIGERATORS AND (OR) HEAT PUMPS
Devices to transfer of heat from a low-temperature medium to a high-temperature one
In a household refrigerator, the freezer
compartment where heat is absorbed by
the refrigerant serves as the evaporator,
and the coils usually behind the
refrigerator where heat is dissipated to the
kitchen air serve as the condenser.
-20

The efficiency is expressed in terms of the
coefficient of performance (COP).
Air conditioners are basically refrigerators whose refrigerated space is a room or a
building instead of the food compartment.
Where:
Can the value of COPR be
greater than unity?
YES. That is, the amount of heat removed from the refrigerated
space can be greater than the amount of work input.

Split units have the cooling coil (evaporator, E) and fan
inside the room, whilst the more noisy compressor (C)
and condenser (CD) are included in the outdoor unit.
RAC (room air conditioner): a packaged unit which
can be installed in a window or an external wall
Refrigerator as Air
conditioner

Indoor air
Refrigerator as Air
conditioner
Outdoor air
34Outdoor air
Indoor air
When installed backward, an
air conditioner functions as a
Heat Pump

The efficiency is expressed in terms of the
coefficient of performance (COP).
COPHP > 1 always!!!
and, for fixed values of QL and QH:
Refrigerator Heat Pump

Reversible
air conditioners/HPs
Cooling mode
Heating mode
They employ a reversing valve to
reverse the flow of refrigerant from
the compressor through the
condenser and evaporation coils.
Some models have a reverse-cycle facility, to act as
(air-source) heat pumps for heating in winter.
NOTE: slide not taken
from the book!

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