ENERGY EFFICIENCY OF THE RECEIVER OF SOLAR ENERGY [615838]

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ENERGY EFFICIENCY OF THE RECEIVER OF SOLAR ENERGY
IN THE TERRITORY OF THE REPUBLIC OF SERBIA

dr Miodrag Kova ćevi ć1, dr Ivan Tasi ć, dr Jasmina Pekez
Technical College of Applied Sciences in Zrenjanin, Republic of Serbia
University of Novi Sad Technical Faculty "Mihajlo P upin" Zrenjanin, Republic of Serbia
University of Novi Sad Technical Faculty "Mihajlo P upin" Zrenjanin, Republic of Serbia

ABSTRACT

The energy efficiency (EE) of the system for heatin g water from collector to accumulator solar
boiler is in the range of 35 to 55 (%) depending on the performance of the collector, the materials us ed
for the production of solar collectors and the mann er of installation and maintenance. Low energy
efficiency (EE) values refer to solar collectors th at have poor design and thermal insulation
characteristics. Such collectors, as a rule, have l ow values of absorption, and the heat emissivity va lues
from the absorbent surface are significantly lower. Collectors that have such characteristics are
collectors whose absorbers do not have good selecti ve characteristics, therefore the value of the
radiation emission coefficient is close to the valu e of the coefficient of absorption of radiation. Th is
feature is directly influenced by the number and ty pe of transparent coverings.

1. INTRODUCTION

Solar collectors are also tested for efficiency in conditions of equality of external air temperature
and temperature of absorber – fluid in absorber (ze ro efficiency). Zero efficiency is not relevant for
a serious assessment of the efficiency of the solar collector. For this type of estimation, the
characteristic of the efficiency curve, curve (or e quation) of the dependence of the energy
efficiency of the collector on the relationship bet ween the difference in the characteristic
flux/absorber and ambient temperature temperatures and solar radiation is very important for
this type of assessment.

The crucial feature for selecting a solar collector from the point of view of its efficiency
is that efficiency that is valid for the operation of the solar collector in dynamic (real) working
conditions. The amount of heat that can be used wit h 1 (m 2) collector is about 900 (kWh).
Vacuum heat collectors have considerably greater ef ficiency, which manifests significantly
during colder periods. The energy efficiency (EE) o f vacuum solar collectors is based on the
thermal insulation of the absorber. The energy effi ciency of the system for heating water with
vacuum collectors is about 40 (%) higher than the s ystem with flat plate solar energy transfer
if one year is observed. The price of installation of vacuum collector systems is almost 50 (%)
higher than the price of installation of systems wi th classical receivers of solar energy.
Bearing in mind these properties, vacuum collector systems are recommended for enclosures
in buildings where there is a continuing need for h ot water, or where large quantities of hot
water are required. [4]

The number of sunny hours in Serbia goes in averag e from a bit less of 2.000 hours (in
the North) to more than 2.300 (in the South). It is a larger value than in the most European
countries, but the solar potential is not used. The potential of solar energy presents 16,7% of
overall usable potential of Renewable Energy Source s (RES) in Serbia. The energy potential

1Miodrag Kovacevic, Djordja Stratimirovica 23, 23 00 0 Zrenjanin, Republic of Serbia,
+ 381 63 564 125, miodrag.kovacevic@vts-zr.edu.rs

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of solar radiation is for about 30% bigger in Serbi a than in Central Europe. The average daily
energy of global radiation for a flat surface durin g winter period goes from 1,0 kWh/m 2 in the
North and 1,7 kWh/m 2 in the South, and during summer season between 5,4 kWh/m 2 in the
North and 6,9 kWh/m 2 in the South. The most favorable areas in Serbia r ecord a great number
of sunny hours, and the yearly ratio of real irradi ation and overall possible irradiation is close
to 50%. Serbia has one of the best solar resources in Europe. Solar radiation in average is
bigger for about 40% of the European average. The l owest measured values of solar radiation
in Serbia could be compared to the highest values i n the countries leading in the use of solar
radiation, such as Germany and Austria. For compari son, the average value of solar radiation
for the German territory is around 1.000 kWh/m 2, while for Serbia it is 1.400 kWh/m 2. The
number of sunny hours in Vojvodina goes from a bit less than 2.000 hours (Western part) up
to 2.100 hours (Eastern part). According to "Valent in Energie Software – TSol Pro 4.5" the
average annual value of global radiation for horizo ntal surface is between 1.294 kWh/m 2 on
the North of AP Vojvodina and 1.350 kWh/m 2 on the South of Vojvodina, and 1.281 kWh/m 2
on the West and up to 1.294 kWh/m 2 on the East of Vojvodina. This shows that on the s ame
source, the average yearly value of sun radiation o ver a horizontal area for the territory of AP
Vojvodina is around 1.300 kWh/m 2. The average daily energy of global solar radiatio n on
horizontal surface at the territory of AP Vojvodina goes from 1,0 – 1,4 kWh/m 2 during
January, and from 6,0 – 6,3 kWh/m 2 during July. At the territory of AP Vojvodina, the annual
average of daily solar radiation energy on the surf ace leaned towards south under the angle of
30° results with 4,0 – 4,6 kWh/m 2.[1]

2. ANALYSIS OF ENERGY EFFICIENCY OF SOLAR COLLECTOR S

When examining the energy efficiency of solar colle ctors in dynamic operating
conditions it has been established that the heat tr ansfer factor on the efficiency of the receiver
is significantly influenced by the effectiveness of the receivers and the mass flow of the
working fluid. The heat transfer factor from the re ceiver, under the same test conditions, is
higher in the absorber whose tubes are in the form of a serpentine than in the receiver whose
pipes are of the absorber in the form of a pipe reg ister. In the case of an absorber of the type
of pipe register, the factor of the discharged heat from the receiver is increasing if the mass
flow of the fluid increases. With this type of abso rber, by increasing the distance between the
absorber tube, the heat removal factor from the rec eiver decreases until the diameter of the
absorber tube has a significant effect on the incre ase in the heat removal factor from the
receiver.

In the case of an absorber whose tube is in the sh ape of a serpentine, the increase in
flow affects the increase in the heat removal facto r from the receiver. The heat removal factor
has a higher value in the turbulent flow of the wor king fluid than in the laminar flow regime.
The fluid flow regime can be influenced by an incre ase in the diameter of the tube of the
absorber. By increasing the distance between the ab sorber tube, the heat removal factor
decreases. The effectiveness of the receiver is in the function of constructive characteristics,
and its mathematical form varies depending on the d esign of the absorber – that is, the concept
of the performance of the absorber tube. By analyzi ng terms that define the effectiveness of
the receiver, it can be concluded that all the para meters that depend on the construction of the
receiver receivers for a particular receiver of sol ar energy, except for the convective loss
coefficient that affects the thermal losses from th e front – receiving side of the receiver and
which depends on the inclination of the receiver, w ind, temperature of the absorber and
ambient temperature. Wind velocity and ambient temp erature are climatic parameters, so the
temperature of the absorber and, consequently, the thermal losses can be influenced by

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regulating the flow of fluid through the absorption chambers – pipes. By increasing the mass
flow of the working fluid, the temperature of the a bsorber decreases, thereby reducing the
heat losses. In order to gain insight into the exis ting state of energy efficiency of flat heat
receivers of solar energy on the market, a comparat ive analysis of 10 representative types of
constructions has been carried out, which have been widely used in practice. The energy
efficiency analysis was performed on the basis of t he efficiency curve which is part of the
project and certificate of the receiver of solar en ergy.

The general energy efficiency equation is given in the manufacturer's attestation and has
a form suitable for graphic presentation of the typ e:

GTaΔ−=1 0ηη (1)

Based on the consideration of the analyzed represe ntative receiver constructions, it has
been found that the receiver's efficiency varies fr om 45 to 75% at ΔT / G of 0.05 K/m 2W,
depending on the design parameters. The results of the study show that thermal losses in
different types of receivers are more dynamic than optical efficiency, and that these changes
are caused by changing the value of the receiver's effectiveness.

Total receiver losses are the most influential on the receiver's effectiveness. They
make heat losses from the front, rear and side side s of the receiver, with the influence of heat
losses on the front of the receiver dominant. Thus, the specific loss of heat from the upper side
of the receiver is directly dependent on the coeffi cient of convective heat transfer between the
absorber and the transparency, the heat loss coeffi cient of radiation from the absorber to the
transparency, and the temperature of the transparen cy and the total heat loss coefficient from
the receiving side, that is, the coefficient of con vective heat losses (from the banners) due to
the wind and the heat loss coefficient by radiating the banner to the sky. The character of its
change depends on the working conditions and the co nstructive parameters of the receiver and
is consistent with the physical character of the ch ange in the influence parameters.
The coefficient of convective heat transfer between the absorber and the transparency
increases by 23% with an increase in the temperatur e of the absorber from 40 to 100 ˚C (at the
absorption coefficient of 0,95) and 29% (in the abs orption coefficient of 0,10). The coefficient
of convective heat transfer between the absorber an d the transparency depends on the type of
gas in the interface between the absorber and the t ransparency and the inclination of the
receiver.

The coefficient of heat loss by radiation between the absorber and the transparency, in
the case of a single transparency receiver, with an absorption coefficient of 0.95, increases
with an increase in the absorber temperature from 4 0 to 100 ˚C by 53% – if there is no air
flow and 43 % – at air velocity of 10 m/s. The tota l coefficient of heat losses from the
receiving side of the receiver, in a calm air, and an increase in the temperature of the absorber
from 40 to 100 ˚C , increase from 26,6% (in the abs orption absorption coefficient of 0,95) to
24,5% apsorber absorption coefficient of 0,10). The coefficient of heat loss by radiating the
banner to the sky increases with an increase in the temperature of the absorber and,
consequently, the transparency by 18% at wind speed s of 0 m/s and 7,5% for wind speeds of
10 m/s, with an apsorber temperature rise of 40 to 100 ˚C, at a coefficient of absorption of the
absorber of 0,95. The coefficient of convective hea t losses from the windscreen transparency
depends on the wind speed and the tilt of the recei ver of the solar energy.

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The practice has shown that at an absorber temperat ure of 40 ˚C the absorber turns about
60% of the received solar radiation into heat. By b ringing the temperature of the absorber to
the ambient temperature, it positively affects the reduction of both convective and heat loss
due to radiation, and this can be achieved by regul ating the mass flow of the working fluid.
For the transfer of heat from the absorber to the w orking fluid, the constructive concept of the
absorber is important, ie, the size of its contact surface with the working fluid and the way of
achieving the connection of the absorber tube and a bsorbent plate, the coverage of the total
surface of the absorber with the absorbent tubes an d the conductive characteristic of the
material of the absorber. On the other hand, we sho uld aim to increase the coefficient of heat
removal from the absorber, which depends on the eff ectiveness of the receiver and the mass
flow of the working fluid.

By testing the efficiency of the solar energy rece iver with steel, aluminum and copper
absorber, thickness 0,15 mm, 0,30 mm and 0,60 mm, i t was found that the receiver with a
copper absorber was 0,6 mm had the highest energy e fficiency value under the test conditions.
By replacing the steel absorber with a thickness of 0,15 mm aluminum of the same thickness,
the energy efficiency of the receiver increased by 17%. By replacing the aluminum absorber
with a thickness of 0,15 mm – copper, the energy ef ficiency was increased by 6,5%. When the
thickness of the absorber was increased to 0,3 mm, the replacement of the steel absorber with
aluminum resulted in an increase in efficiency by 9 ,5%, and the replacement of the aluminum
absorber with copper, increasing the efficiency of the receiver by 5%. By increasing the
thickness of the absorber to 0,6 mm, the efficiency increased by 3% when the steel absorber
was replaced by aluminum, and 1,5% when the aluminu m absorber was replaced by copper.
The receiver with an aluminum absorber with a thick ness of 0,6 mm had the same efficiency
as a receiver with a copper absorber with a thickne ss of 0,3 mm.

The increase in the value of the absorption coeffi cient of the materials of the absorber,
under the conditions of the tests, with values of 8 0 W/Mk (steel) to a value of 380 W/mK
(copper), results in an increase in the energy effi ciency of the receiver by 13%. The
dependence of optical efficiency on the coefficient of conduction (in W/mK) is defined by the
expression:

2 6 4
0 10 13 , 1 10 824 , 7 553 , 0 λ λ η ⋅⋅−⋅⋅+=− − (2)

and the dependence of the thermal loss coefficient of the coefficient of conduction by the
expression:

2 6
1 10 325 , 7 00502 , 0 07413 , 4 λ λ ⋅⋅−⋅ + =−a (3)

Determining the number of absorbent tubes on the ab sorption board, which causes an
increase in the energy efficiency of the receiver, is reduced to the analysis of the distance
between the absorption tubes. In this context, a be tter solution with a smaller distance
between the pipes is better, as this increases the exchange surface and shortens the heat
transfer path. Reducing the distance between the ex changer pipe is achieved by increasing the
number of absorber tubes. The research found that t he receiver with an absorber having 21
hoses has the highest value of efficiency, is the l east distance between the tube, and hence the
largest exchange surface, under the conditions of t esting. The efficiency of the receiver of
solar energy increases with the increase in the num ber of pipes of the absorber, so by
changing the number of pipes from 4 to 21, the effi ciency of the absorber by 21% is increased

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for the investigated case, but the efficiency with the 16 pipe absorber is only 1% higher when
the number of pipes increased to 21. The dependence of the change in optical efficiency on
the number of absorber tubes is determined by the e xpression:

2
0 00123 , 0 04146 , 0 39427 , 0 n n ⋅ −⋅ + =η (4)

and the dependence of the heat loss coefficient on the number of tubes of the absorber by the
expression: 2
1 01124 , 0 36754 , 0 32296 , 2 n n a ⋅ −⋅ + = (5)
2
1 00129 , 0 07151 , 0 66729 , 5ta ta l l a ⋅ +⋅ − = (6)

In the case of a receiver whose tube is a serpentin e absorber tube, the energy efficiency is
increased by 6%, by changing the mass flow rate fro m 0.9 [kg / min] to 1.66 [kg / min] under
certain test conditions. The dependence of energy e fficiency on the mass flow of the working
fluid, for the receiver whose tube is the absorber in the form of a serpentine, is defined by the
expression:

2
410 * 34848 , 5 1303 , 0 18485 , 44 ⋅−⋅
⋅ −⋅+ = m m η (7)

Constant mass flow of working fluid causes an incre ase in the temperature of the
outgoing fluid, which results in a decrease in ener gy efficiency due to increased heat losses.
This is especially true when the medium temperature of the working fluid is higher than the
ambient temperature. The carried out testing found that in order to increase the temperature of
the outlet fluid from 25 to 70 ˚C, the efficiency d ecreased by 35% under precisely determined
conditions of testing.

0 5 10 15 20 25 30 35 40 0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 η
To[0C]

Figure 1: The dependence of solar collector efficie ncy on ambient temperature

3. CONCLUSIONS

The influence of the ambient air temperature chang e on the efficiency of the receiver
was carried out under conditions with precisely det ermined test conditions where the ambient
air temperature from 15 ˚C increased to 36 ˚C. The test found that the receiver's efficiency

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was increased by 30%, which is the result of a decr ease in the thermal losses of the receiver
due to a decrease in the difference between the mea n temperature of the working fluid and the
ambient temperature. Based on the results of the re search carried out on the type of fluid that
shows the highest efficiency of the receiver, under precisely determined test conditions, it has
been found that the working fluid – a mixture of wa ter and ethylene glycol, with an ethylene
glycol content of 16%, has a 0,8% higher efficiency than when a mixture of water with a
propylene glycol content of 16% was used as the wor king fluid under the same test
conditions. It has been found that the type of work ing fluid has a negligible influence on the
energy efficiency of the receiver of solar energy.

The results of the performed tests of more favorab le values of the basic structural
characteristics from the aspect of energy efficienc y can be applied to all types of structures
with a tube-type absorber type.
The study found that:

1. Increasing the number of pipes of absorbers from 4 to 21 increases also efficiency up
to 21%;
2. By replacing the steel absorber with a thickness of 0,15 mm with an aluminum
absorber of the same thickness, the efficiency of t he receiver by 17% was increased,
and the replacement of the aluminum absorber with c opper the same thickness
increased the efficiency by 6,5% and
3. Energy efficiency was increased by 8.4%, while incr easing the thickness of thermal
conductivity 0.04 W/mK, from 10 to 60 mm) for the e xamined case of a flat receiver
of solar energy.

Other constructive characteristics have negligible influence on the energy efficiency of
the receiver of solar energy. For concrete cases of flat heat receivers of solar energy in
exploitation, when it is not possible to change the constructive parameters, increasing the
energy efficiency of the receiver can be achieved b y regulating the mass flow of working fluid
and accumulation – consumption.

Conducted studies of the effect of the change in t he flow on the efficiency of the receiver
of solar energy have been determined that:

1. By increasing the mass flow rate from 4 kg/min to 3 0 kg/min, the efficiency increased
by 15,4% for the receiver with the absorber in the form of a tube register and
2. Increasing the mass flow from 0,9 kg/min to 1,66 kg /min, energy efficiency increased
by 6% for the receiver whose tube is absorbent in t he form of a serpentine.

References

[1] STUDY ON THE ESTIMATION OF OVERALL SOLAR POTEN TIAL – SOLAR ATLAS AND THE
POSSIBILITY OF "PRODUCTION" AND USE OF SOLAR ENERGY ON THE TERRITORY OF AP
VOJVODINA, AP of Vojvodina, Provincial Secretariat for Energy and Mineral Resources, Novi Sad, 2011.
[2] Pekez, J., Lambi ć, M., Grbi ć N., Comparative indicators of the influence parame ters on the energy efficiency
of solar collectors, Energy Technology, Vol. 3, No 1-2, 2006, 6-8, ISSN 1451-9070
[3] Pekez, J., Lambi ć, M., Tasi ć I., Increasing the Energy Efficiency of Solar Coll ectors by Controlling the Mass
Flow of a Working Fluid, Energy Technology, Vol. 2, No 3, 2005, 49-51, ISSN 1451-9070.
[4] Lambi ć, M. et al., Energy Efficiency, Serbia solar, Zrenj anin, 2004 .