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Book ChapterDOI

Energy Loss by Thermal Conduction and Natural Convection in Annular Solar Receivers

01 Jan 1978-pp 423-432

Abstract: An effective device for the collection of solar energy is the so-called parabolic-cylindrical solar collector. In this device, a circular receiver tube is enclosed by a concentric glass envelope and situated along the focal line of a parabolic trough reflector. The heat transfer processes which occur in the annular space between the receiver tube and the glass envelope are important in determining the overall heat loss from the receiver tube. In typical high temperature receiver tube designs the rate of energy loss by combined thermal conduction and natural convection is of the same order of magnitude as that due to thermal radiation, and can amount to approximately 6% of the total rate at which energy is absorbed by the solar collector. The elimination of conduction and natural convection losses can significantly improve the performance of a large collector field.
Topics: Parabolic trough (61%), Heat transfer (60%), Thermal conduction (59%), Natural convection (58%), Solar energy (58%)

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nt
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ENERGY
LOSS
BY
THERMAL
CONDUCTION
AND
NATURAL
CONVECTION
IN
ANNULAR
SOLAR
RECEIVERsl
by
1
. k d
1'
2
A.
C.
Ratze
,
C.
E.
H1c
ox,
an
D.
K.
Gart
1ng
Fluid
and
Thermal
Sciences
Department
Sandia
Laboratories
Albuquerque,
NM
87115
USA
presented
at
Fifteenth
International
Thermal
Conductivity
Conference
Ottawa,
Ontario,
Canada
The
work
discussed
in
this
paper
was
supported
by
the
United
States
Energy
Research
and
Development
Administion.
Member~
u[
Llu.:!
Technicu.l
St.:1ff
~
ASTE
R
OIS
T
RIBhJT.IO
.N
OF
THIS
DOCU
ME C b
.
NT
IS
UNLJMIT£0

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INTRODUCTION
An
effective
device
for
the
collection
of
solar
energy
is
the
so
called
parabolic-cylindrical
solar
collector.
In
this
device,
a
circular
r.eceiver
tube,
with
a
suitable
selective
coating,
is
en-
closed
by
a
concentric
glass
envelope
and
situated
along
the
focal
line
of
a
parabolic
trough
reflector.
The
heat
transfer
processes
which
occur
in
the
annular
space
between
the
receiver
tube
and
the
glass
envelope
are
important
in
determining
the
overall
heat
loss
from
the
receiver
tube.
In
typical
high
temperature
receiver
tube
designs
the
rate
of
energy
loss
by
combined
thermal
conduction
and
natural
convection
is
of
the
same
order
of
magnitude
as
that
due
to
thermal
radiation,
and
can
amount
to
approximately
6%
of
the
total
rate
at
which
energy
is
absorbed
by
the
solar
collector.
The
elimination
of
conduction
and
natural
convection
losses
can
signifi-
c·antly
improve
the
performance
of
a
large
collector
field.
In
this
paper,
several
techniques
useful
for
the
reduction
of
energy
loss
by
thermal
conduction
and
natural
convection
are
considered.
'.lhe
receiver
configuration
chosen
for
study
is
typical
of
those
used
in
the
Solar
Total
Energy
System
at
Sandia
Laboratories.
The
receiver
tube
has
a
"black
chrome"
selective
coating
and
is
2.54
em
in
outside
diameter.
The
inside
diameter
of
the
glass
envelope
is
approximately
4.4
em.
Typical
operating
temperatures
of
the
receiver
tube
and
glass
envelope
are
approximateli
573
K
and
373
K,
respectively.
CONDUCTION
HEAT
LOSS
Of
the
three
modes
of
heat
transfer,
the
most
significant
heat
loss
savings
for
an
annular
receiver
can
be
accomplished
by
limiting

..
-2-
conduction
losse~.
Convection
losses
are
negligible
so
long
as
the
annular
space
is
properly
sized.
Radiation
losses,
being_
pri-
marily
fixed
by
the
receiver
tube
selective
surface
properties,
are
more
difficult
to
reduce.
Variations
in
electroplating
parameters
to
reduce
the
receiver
thermal
emittance
properties
may
result
in
lower
solar
absorptivity
and
perhaps
poor
durability
_properties.
Attempts
to
limit
heat
transfer
through
the
annular
space
will
:be
discussed
in
the
f6llowing
sections.
Techniques
studied
include
{1)
evacuation
of
the
annulus
gas,
(2)
oversizing
the
arinular
space,
.and
(3)
using
gases
other
than
air
for
the
heat
transfer
medium.
·Effect
of
Vacuum
A
revie\v
of
the
literature
on
vacuum
technology
indicates
that
the
thermal
conductivity
of
a
gas
i~
a
function
of
the;
mean
free
p·ath
of
the
gas
molecule~
[1,
2,
3].
An
expression
relating
the
mean
free
path
of
a
gas
to
the
enclosure
pressure
and
gas
temperature
is
where
T,
P,
and
o
are
given
in
deg
K,
mm
Hg,
and
em,
respectively.
For
a
given
gas,
the
relative
magnitudes
of
the
molecular
mean
free
path
and
the
annulus
gap
determines
whether
the
effective
heat·
transfer
coefficient
for
thermal
conduction
is
(1)
.independent
of
annulus
pressure,
(2)
a
function
of
the
annulus
pressure,
or
(3)
negligible.
The
governing
equation
for
the
effective
heat
transfer
coefficient
for
the
annular
space
is
[1]
k
kef~
r.Ln(r
/r.)
+.bA(r./r
+
lf
~
0
~ ~
0
(2)

Citations
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Journal ArticleDOI
Devander Kumar, Sudhir Kumar1Institutions (1)
Abstract: Solar parabolic trough collector (SPTC) is a well-known solar thermal system applied for solar electric generation. Nowadays, major attention is directed toward improving the performance of solar thermal systems with optimization of solar field production. In this research work, a comprehensive thermo-optical modeling has been proposed to evaluate the performance of a mini-level SPTC considering various heat equilibriums with the environment. Here, receiver wall temperature is considered as the base for modeling. Collector consists of a non-evacuated receiver tube with black paint coating and enveloped with glass cover. Available meteorological data in terms of global and diffuse solar insolations, air temperatures and wind speeds have been used as inputs for performance evaluation of SPTC with horizontal and inclined aperture planes. The validation of the proposed analytical model is justified with existing experimental results and yielded a close agreement. The developed model is successfully applied to a SPTC in order to estimate the through-out year performance characteristics in terms of water temperature rise, heat energy generation, optical and thermal efficiency for the climactic conditions of Bhiwani. The results enlighten that using 0.010 kg/s mass flow rate of water and aperture area of around 1.34 m2, collector achieved maximum rise in water temperature 11.1 °C and 12.2 °C on horizontal and inclined planes, respectively in the month of April. The uppermost heat energy generation is found to be 2.38 kW h/day in May on horizontal plane, 2.46 kW h/day in April on inclined plane. The maximum optical efficiency is attained as 72.26% and 72.4% on horizontal and an inclined plane respectively. The peak instantaneous thermal efficiency is accomplished as 66.78% in July on horizontal plane, 65.77% in September on inclined plane of collector.

22 citations


Dissertation
07 Dec 2011
Abstract: Access to safe and adequate water supply and improved sanitation is a fundamental need and a basic human right vital for health and dignity of all people. The situation has therefore called for concerted effort which has been expressed in the global commitments reiterated in the Millenium Development Goal (MD G), to reduce to half the population of people who lack access to safe and adequate water supply by 2015 and provide safe and adequate water for all by the year 2025. There are various techniques that have been used • to disinfect water and make it safe to drink. These include: chlorination, ultra-violet disinfection, use of ozone gas, pasteurization and mixed-oxidant gaseous systems which is the most recent technology. Conventional water treatment methods rely heavily on chemicals, high energy consumption, use of expensive equipments and huge capital investments. Although they are suitable where large amount of water is required, they are not suitable for most rural settings existing in Kenya today that are normally scattered making centralized system a very expensive venture. Direct application of heat is one of the oldest and most reliable method of water disinfection. This study has pursued the concept of heating water using solar thermal energy to kill disease causing organisms, mainly E. coli which is one of the major indicator offeacal pollution in water. The direct heating of water was investigated using a 2m2 flat plate collector with a 2element plane mirror reflector used to concentrate solar radiation from an aperture area of 4m2 Two experimental collectors were investigated; one having 20mm diameter galvanized pipe with a 26 gauge stainless steel absorber sheet and the other with 18mm diameter copper pipe with a 32 gauge aluminium absorber sheet. In both investigations, collector without reflectors was used as control experiment. Thermal performance tests have been conducted using continuous flow operation at controlled flow-rate (30±3l/hr and 40±3I1hr) and using pasteurization tests with thermostatic valve installed with a valve opening temperature of 82°C. In both the experimental systems under investigation it was observed that the use of mirror increased the day average instantaneous efficiency by up to 10% during continuous flow operation and up to 100% in output of pasteurized water discharged via the thermostatic valve. Water quality tests were done for both raw and pasteurized water samples using presumptive test for total coliform and Eijkman test for confirmation of E. coli in water. There was no E. coli found in pasteurized water. Data analysis was done using both descriptive and inferential statistic. An economic analysis of the solar pasteurization system has indicated an annual saving of up to 5tonnes of wood for collector with reflectors and an aperture area of 4m2 having a daily pasteurization output of up to 100liters for a day under clear sky condition with an average solar radiation of 720W/m2, ambient temperature of 28°C and average wind speed less than 2m/s. Thus, the adoption of this study would not only reduce the number of r:'~sesof water borne diseases such as cholera and typhoid which have been identified as major cause of morbidity in Kenya by the Health Sub-sector Strategic Plan (19992004) but also lead to a reduction in environmental degradation, green-house gas emissions and health effects associated with inhaling smoke.

1 citations


References
More filters

Book
01 Jan 1949
Abstract: Scientific foundations of vacuum technique , Scientific foundations of vacuum technique , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

1,727 citations


Journal ArticleDOI
Thomas H. Kuehn1, Richard J Goldstein1Institutions (1)
Abstract: Correlating equations for heat transfer by natural convection from horizontal cylinders to a cylindrical enclosure are obtained using a conduction boundary-layer model. The correlation is valid for heat transfer by conduction, laminar flow and turbulent flow. The results approach the correlation for heat transfer from a free horizontal cylinder as the outer cylinder diameter becomes infinite and for quasi-steady heat transfer to fluid within a horizontal cylinder as the inner cylinder diameter approaches zero. Horizontal concentric, eccentric and arrays of cylinders within the outer cylinder are geometries included in the correlation.

239 citations


"Energy Loss by Thermal Conduction a..." refers methods in this paper

  • ...In Figure 5, the cross-hatched area indicates the region occupied by the experimental data as compiled by Kuehn and Goldstein [6]....

    [...]


Journal ArticleDOI
David K. Gartling1Institutions (1)
Abstract: A numerical procedure based on the finite element method is developed for the analysis of general two-dimensional problems in free/forced convection heat transfer. The dicretization of the field equations through use of the Galerkin method is described. Solution methods for both types of steady state convection are presented. The utility and accuracy of the described method is demonstrated through the solution of diverse examples illustrating both forced and free convection analysis. The use of temperature-dependent material properties and the inclusion of solid body conduction effects are also demonstrated.

59 citations


Journal ArticleDOI
01 Jul 1934

49 citations


ReportDOI
01 Jul 1976
Abstract: This report presents in some detail the various significant factors which influence the design of parabolic-cylindrical solar collectors.

35 citations