CC BY
2УДК: 621.43.016
ANALYZING THE EFFICIENCY OF DIFFERENT LIQUID-BASED COLLECTORS AND THE KEY FEATURES OF COMMON SELECTIVE COLLECTOR SURFACE
KARIMOVA SALIMA ELAMONOVNA
Karshi State University, Uzbekistan
Abstract. This article examines the efficiency of various liquid-based solar collectors, focusing on their thermal performance and the impact of selective surfaces on energy absorption. Liquid-based collectors, including flat-plate, evacuated tube, and concentrating collectors, are evaluatedfor their ability to convert solar radiation into usable thermal energy. Key parameters such as heat transfer efficiency, temperature differentials, and fluid dynamics are analyzed to determine optimal operating conditions. Additionally, the role of selective coatings—designed to enhance solar absorption while minimizing thermal radiation losses—is explored. This research contributes to the ongoing efforts to improve renewable energy systems and reduce reliance on fossil fuels.
Key words: Solar energy, liquid-based collectors, efficiency, wind energy, thermal energy, fuel
and energy resources, thermal power plants,thermal radiation, renewable energy.
INTRODUCTION
As the global demand for renewable energy sources continues to rise, solar thermal technology has emerged as a pivotal solution for harnessing solar energy. Among the various solar energy systems, liquid-based collectors play a crucial role in converting sunlight into thermal energy, which can be utilized for heating applications in residential, commercial, and industrial settings. These collectors operate by circulating a heat transfer fluid—typically water or a glycol mixture—through a series of pipes or channels that absorb solar radiation.
The efficiency of liquid-based solar collectors is influenced by several factors, including design configuration, materials used, and environmental conditions. Flat-plate collectors, evacuated tube collectors, and concentrating collectors each possess unique characteristics that affect their thermal performance. Understanding these differences is essential for optimizing their application in diverse climates and settings.
Any energy source that is classified as an "alternative energy source" is that because, at one time it was not selected as the best choice. If the original choice of an energy source was a proper one the use of an alternative energy source would make sense only if some condition has changed. This might be:
1. Present or impending nonavailability of the present energy source
2. Change in the relative cost of the present and the alternative energy
3. Improved reliability of the alternative energy source
4. Environmental or legal considerations
To some, an alternative energy source is a nondepleting or renewable energy source, and, for many it is this characteristic that creates much of the appeal. Although the terms "alternative energy source" and "renewable energy sources" are not intended by this writer to be synonymous, it will be noted that some of the alternative energy sources discussed in this section are renewable. [1].
It is also interesting that what we now think of as alternative energy sources, for example solar and wind, were at one time important conventional sources of energy.
Conversely, natural gas, coal, and oil were, at some time in history, alternative energy sources. Changes in the four conditions listed above, primarily conditions 2 and 3, have led us full circle from the use of solar and wind, to the use of natural gas, coal, and oil, and back again in some situations to a serious consideration of solar and wind. [2].
In a strict sense, technical feasibility is not a limitation in the use of the alternative energy sources that will be discussed. Solar energy can be collected at any reasonable temperature level,
stored, and utilized in a variety of ways. Wind energy conversion systems are now functioning and have been for many years. Refuse-derived fuel has also been used for many years. What is
important to one who must manage energy systems are the factors of economics, reliability, and in some cases, the nonmonetary benefits, such as public relations.
Government funds tax incentives in the alternative energy area dropped sharply during the decade of the eighties and early nineties. This caused many companies with alternative energy products to go out of business, and for others to cut back on production or to change into another product or technology line. Solar thermal energy has been hit particularly hard in this respect, but solar powered photovoltaic cells have had continued growth both in space and in terrestrial applications. [3],[4]. Wind energy systems have continued to be installed throughout the world and show promise of continued growth. The burning of refuse has met with some environmental concerns and strict regulations. Recycling of some refuse materials such as paper and plastics has given an alternative to burning. Fuel cells continue to increase in popularity in a wide variety of applications including transportation, space vehicles, electric utilities and uninterruptible power supplies.
Surviving participants in the alternative energy business have in some cases continued to grow and to improve their products and their competitiveness. As some or all of the four conditions listed above change, we will see rising or falling interest on the part of the government, industry and private individuals in particular alternative energy systems. [8].
"Solar energy is free!" states a brochure intended to sell persons on the idea of buying their solar products."There's no such thing as a free lunch" should come to mind at this point. With a few exceptions, one must invest capital in a solar energy system in order to reap the benefits of this alternative energy source. In addition to the cost of the initial capital investment, one is usually faced with additional periodic or random costs due to operation and maintenance. Provided that the solar system does its expected task in a reasonably reliable manner,and presuming that the conventional energy source is available and satisfactory, the important question usually is: Did it save money compared to the conventional system? Obviously, the cost of money, the cost of conventional fuel, and the cost and performance of the solar system are all important factors. As a first step in looking at the feasibility of solar energy, we will consider its availability.
Solar energy arrives at the outer edge of the earth's atmosphere at a rate of about 428 Btu/hr ft2 (1353 W/ m2). This value is referred to as the solar constant. Part of this radiation is reflected back to space, part is absorbed by the atmosphere and re-emitted, and part is scattered by atmospheric particles. As a result, only about two-thirds of the sun's energy reaches the surface of the earth. At 40° north latitude, for example, the noontime radiation rate on a flat surface normal to the sun's rays is about 300 Btu/hr • ft2 on a clear day. This would be the approximate maximum rate at which solar energy could be collected at that latitude. A solar collector tracking the sun so as to always be normal to the sun's rays could gather approximately 3.6 x 103 Btu/ft2 • day as an absolute upper limit. To gather 1 million Btu/day, for example, would require about 278 ft2 (26 m2) of movable collectors, collecting all the sunlight that would strike them on a clear day. [5]. [6].
Since no collector is perfect and might collect only 70% of the energy striking it, and since the percent sunshine might also be about 70%, a more realistic area would be about 567 ft2 (53 m2) to provide 1 million Btu of energy per day. In the simplest terms, would the cost of constructing, operating, and maintaining a solar system consisting of 567 ft2 of tracking solar collectors justify a reduction in conventional energy usage of 1 million Btu/day? Fixed collectors might be expected to deliver approximately 250,000 Btu/yr for each square foot of surface. [7].
A most important consideration which was ignored in the discussion above was that of the system's ability to use the solar energy when it is available. A space-heating system, for example, cannot use solar energy in the summer. In industrial systems, energy demand will rarely correlate with solar energy availability.
In some cases, the energy can be stored until needed, but in most systems, there will be some available solar energy that will not be collected. Because of this factor,particular types of solar energy systems are most likely to be economically viable. Laundries, car washes, motels, and restaurants, for
ОФ "Международный научно-исследовательский центр "Endless Light in Science"
example, need large quantities of hot water almost every day of the year. A solar waterheating system seems like a natural match for such cases. On the other hand, a solar system that furnishes heat only during the winter, as for space heating, may often be a poor economic investment.
The amount of solar energy available to collect in a system depends upon whether the collectors move to follow or partially follow the sun or whether they are fixed. In the case of fixed collectors, the tilt from horizontal and the orientation of the collectors may be significant.
The remainder of this section considers the energy available to fixed solar collecting systems. Massive amounts of solar insolation data have been collected over the years by various government and private agencies. The majority of these data are hourly or daily solar insolation values on a horizontal surface, and the data vary considerably in reliability. Fixed solar collectors are usually tilted at some angle from the horizontal so as to provide a maximum amount of total solar energy collected over the year, or to provide a maximum amount during a particular season of the year. What one needs in preliminary economic studies is the rate of solar insolations on tilted surfaces. Solar Collectors
A wide variety of devices may be used to collect solar energy. A general classification of types is given in Figure 16.3. Tracking-type collectors are usually used where relatively high temperatures (above 250°F) are required. These types of collectors are discussed at the end of this section. The more common fixed, flat-plate collector will be discussed first, followed by a discussion of tube-type or mildly concentrating collectors.
The flat-plate collector is a device, usually faced to the south (in the northern parts of the globe) and usually at some fixed angle of tilt from the horizontal. Its purpose is to use the solar radiation that falls upon it to raise the temperature of some fluid to a level above the ambient conditions. That heated fluid, in turn, may be used to provide hot water or space heat, to drive an
engine or a refrigerating device, or perhaps to remove moisture from a substance. A typical glazed flat-plate solar collector of the liquid type is shown in Picture.1.
The sun's radiation has a short wavelength and easily passes through the glazing (or glazings), with only about 10 to 15% of the energy typically reflected and absorbed in each glazing. The sunlight that passes through is almost completely absorbed by the absorber surface
and raises the absorber temperature. Heat loss out the back from the absorber plate is minimized by the use of insulation. Heat loss out the front is decreased somewhat by the glazing, since air motion is restricted. The heated insulation. Heat loss out the front is decreased somewhat by the glazing, since air motion is restricted. [10].
The heated absorber plate also radiates energy back toward the sky,but this radiation is longer-wavelength radiation and most of this radiation not reflected back to the absorber by the glazing is absorbed by the glazing. The heated glazing, in turn, converts some of the absorbed energy back to the air space between it and the absorber plate.The trapping of sunlight by the glazing and the consequent heating is known as the "greenhouse effect." Energy is removed from the collector by the coolant fluid. A steady condition would be reached when the absorber temperature is such that losses to the coolant and to the surroundings equal the energy gain from the solar input. When no energy is being removed from the collectors by the coolant, the collectors are said to be at stagnation. For a well-designed solar collector, that stagnation temperature may be well above 300°F.
Picture.1. Typical double-glazed flatplate collector, liquid type, internally manifolded. (Courtesy LOF.)
This must be considered in the design of solar collectors and solar systems, since loss of coolant pumping power might be expected to occur sometime during the system lifetime.A typical coolant flow rate for flat-plate collectors is about 0.02 gpm/ft2 of collector surface. [12].
Results and discussion
The fraction of the incident sunlight that is collected by the solar collector for useful purposes is called the collector efficiency. This efficiency depends upon several variables, which might change for a fixed absorber plate design and fixed amount of back and side insulation. V These are:
1. Rate of insolation
2. Number and type of glazing
3. Ambient air temperature
4. Average (or entering) coolant fluid temperature
A typical single-glazed flat-plate solar collector efficiency curve is given in Figure 1. The measured performance can be approximated by a straight line. The left intercept is related to the product та, where т is the transmittance of the glazing and a is the absorptance of the bsorber plate. The slope of the line is related to the magnitude of the heat losses from the collector, a flatter line representing a collector with reduced heat-loss characteristics.
A comparison of collector efficiencies for unglazed, single-glazed, and double-glazed flat-plate collectors is shown in Figure 1. Because of the lack of glazing reflections, the unglazed collector has the highest efficiencies at the lower collector temperatures. This factor, combined with its lower cost, makes it useful for swimming pool heating. The single-glazed collector also performs well at lower collector temperatures, but like the unglazed collector, its efficiency drops off at higher collection temperatures because of high front losses. The double-glazed collector, although not performing too well at lower temperatures, is superior at the higher temperatures and might be used for space heating and or cooling applications. The efficiency of an evacuated tube collector is also shown in Figure 1. It can be seen that it performs very poorly at low temperatures, but because of small heat losses, does very well at higher temperatures.
A very important characteristic of a solar collector surface is its selectivity, the ratio of its absorptance as for sunlight to its emittance s for long-wavelength radiation. A collector surface with a high value of as/s is called a selective surface. Since these surfaces are usually formed
by a coating process, they are sometimes called selective coatings. The most common commercial selective coating is black chrome. The characteristics of a typical black chrome surface are shown in Figure1. where aX=sX, the monochromatic absorptance and monochromatic
Fig. 1. Efficiency of a typical liquid-type solar collector panel.
emittance of the surface. Note that at short wavelengths (~ 0.5 |i), typical of sunlight, the absorptance is high. At the longer wavelengths (~2 |i and above), where the absorber plate will emit most of its energy, the emittance is high. Selective surfaces will generally perform better than ordinary blackened surfaces. The performance of a flat black collector and a selective coating collector are compared in Figure 2. The single-glazed selective collector performance is very similar to the double-glazed nonselective collector. Economic considerations usually lead one to pick a single-glazed, selective or a doubleglazed, nonselective collector over a double-glazed, selective collector, although this decision depends heavily upon quoted or bid prices.
Air-type collectors are particularly useful where hot air is the desired end product. Air collectors have distinct advantages over liquid-type collectors:
1. Freezing is not a concern.
2. Leaks, although undesirable, are not as detrimental as in liquid systems.
3. Corrosion is less likely to occur.
3jO i-1- _________
o.s 1— \
r<
w
a
a
02 I—
_I I I 1 1 I I_I_I_t_■ I MTr
j 0,4 Q.E. 0.7 1.0 7-0 3.0 i.o 7.0 10.0
WaoiftengiTi <iO
Fig. 2. Characteristics of a typical selective (black chrome) collector surface.
Air systems may require large expenditures of fan power if the distances involved are large or if the delivery ducts are too small. Heat-transfer rates to air are typically lower than those to liquids, so care must be taken in air collectors and in air heat exchangers to pro vide sufficient heat-transfer surface. . This very often in volves the use of extended surfaces or fins on the sides of the surface, where air is to be heated or cooled. Typi cal air collector designs are shown in Figure. 4. [13].
Fig. 3. Comparison of collector efficiencies for various liquid type collectors.
Flat-plate collectors usually come in modules about 3 ft wide by 7 ft tall, although there is no standard size. Collectors may have internal manifolds or they may be manifolded externally to form collector arrays.Internally manifolded collectors are easily connected together, but only a small number can be hooked together in a single array and still have good flow distribution. Small arrays (5 to 15) are often piped together with similar arrays in various series and parallel arrangements to give the best compromise between nearly uniform flow rates in each collector, and as small a pressure drop and total temperature rise as can be attained.[14].
Fig. 4. Comparison of the efficiencies of selective and nonselective collectors
Externally manifolded collectors are easily connected in balanced arrays if the external manifold is properly designed. These types of arrays require more field connections, however, have more exposed piping to insulate, and are not as neat looking.
The overall performance of a collector array, measured in terms of the collector array efficiency, may be quite a bit less than the collector efficiency of the individual collectors. This is due primarily to unequal flow distribution between collectors, larger temperature rises in series connections than in single collectors, and heat losses from the connecting piping. A good array design will minimize these factors together with the pumping requirements for the array.
Concentrating collectors provide relatively high temperatures for applications such as air conditioning, power generation, and the furnishing of industrial or process heat above 250°F (121°C). They generally cannot use the diffuse or scattered radiation from the sky and must track so that the sun's direct rays will be concentrated on the receiver. The theory is simple. By concentrating the sun's rays on a very small surface, heat losses are reduced at the high temperature desired. An important point to make is that concentrating collectors do not increase the amount of energy above that which falls on the mirrored surfaces; the energy is merely concentrated to a smaller receiver surface.
A typical parabolic trough-type solar collector array is shown in picture.2. Here the concentrating surface or mirror is moved, to keep the sun's rays concentrated as much as possible on the receiver, in this case a tube through which the coolant flows. In some systems the tube moves and the mirrored surfaces remain fixed. This type collector can be mounted on an east-west axis and track the sun by tilting the mirror or receiver in a north-south direction. An alternative is to mount the collectors on a north-south axis and track the sun by rotating in an east-west direction. A third scheme is to use a polar mount, aligning the trough and receiver parallel to the earth's pole, or inclined at some angle to the pole, and tracking east to west. Each has its advantages and disadvantages and the selection depends upon the application. A good discussion of concentrating collectors is given in Ref. 8 Fully tracking collectors may be a parabolic disk with a "point source" or may use a field of individual nearly flat moving mirrors or heliostats, concentrating their energy on a single source, such as might be installed on a tower (a power tower). [11]. Computers usually control the heliostat motion.
Picture.2. Typical parabolic trough-type solar collector array (Suntec, Inc.).
Some trough-type collectors are also fully tracking, but this is not too common. All partial and fully tracking collectors must have some device to locate the sun in the sky, either by sensing or by prediction.Tracking motors, and in some cases flexible or movable line connections, are additional features of tracking systems. Wind loads can be a serious problem for any solar collector array that is designed to track. Ability to withstand heavy windloads is perhaps the biggest single advantage of the flat-plate, fixed collector array.
Thermal Storage Systems
Because energy demand is almost never tied to solar energy availability, a storage system is usually a part of the solar heating or cooling system. The type of storage may or may not depend upon
ОФ "Международный научно-исследовательский центр "Endless Light in Science"
the type of collectors used. With air-type collectors, however, a rock-bed type of storage is sometimes used. The rocks are usually in the size range 3/4 to 2 in. in diam eter to give the best combination of surface area and pressure drop. Air flow must be down for storing and up for removal if this type system is to perform properly. Horizontal air flow through a storage bed should normally be avoided. An air flow rate of about 2 cfm/ft2 of collector is recommended. The amount of storage required in any solar heating system is tied closely to the amount of collector surface area installed, with the optimum amount being determined by a computer calculation. As a rule of thumb, for rough estimates one should use about 75 lb of rock per square foot of air-type collectors. If the storage is too large, the system will not be able to attain sufficiently high temperatures, and in addition, heat losses will be high. If the storage is too small, the system will overheat at times and may not collect and store a large enough fraction of the energy available.
The most common solar thermal storage system is one that uses water, usually in tanks. As a rule the water storage tank should contain about 1.8 gal/ft2 of collector surface. Water has the highest thermal storage capability eter to give the best combination of surface area and pressure drop. Air flow must be down for storing and up for removal if this type system is to perform properly. Horizontal air flow through a storage bed should normally be avoided. An air flow rate of about 2 cfm/ft2 of collector is recommended. The amount of storage required in any solar heating system is tied closely to the amount of collector surface area installed, with the optimum amount being determined by a computer calculation. As a rule of thumb, for rough estimates one should use about 75 lb of rock per square foot of air-type collectors. If the storage is too large, the system will not be able to attain sufficiently high temperatures, and in addition, heat losses will be high. If the storage is too small, the system will overheat at times and may not collect and store a large enough fraction of the energy available.
The most common solar thermal storage system is one that uses water, usually in tanks. As a rule the water storage tank should contain about 1.8 gal/ft2 of collector surface. Water has the highest thermal storage capability.
Conclusion
In conclusion, the analysis of different liquid-based solar collectors and the examination of selective collector surfaces reveal significant insights into optimizing solar thermal energy capture. Our study highlights that the efficiency of these collectors is not solely dependent on their design— such as flat-plate, evacuated tube, or concentrating collectors—but also heavily influenced by the properties of the selective surfaces employed. The effectiveness of selective coatings, which enhance solar absorption while minimizing thermal losses, plays a critical role in maximizing the overall performance of liquid-based collectors. Materials like titanium nitride, black chrome, and advanced nanostructured coatings have shown promising results, demonstrating improved thermal efficiencies across various environmental conditions.
Moreover, the choice of collector design should align with specific application needs and local climate characteristics to achieve optimal performance. Future advancements in material science and nanotechnology are expected to further enhance the capabilities of selective surfaces, leading to even greater efficiencies in solar thermal systems.
REFERENCES
1. S.E.Fuller, "The Relationship between Diffuse, Total and Extraterrestrial Solar Radiation," Journal of SolarEnergy and Technology, Vol. 18, No. 3 (1976), pp.259263.
2. B.V.H. Liu and R.C. Jordan, "Availability of Solar Energy for Flat-Plate Solar Heat Collectors," Application of Solar Energy for Heating and Cooling of Buildings, ASHRAE GRP-170, American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc., New York, 1977.
3. S.A. Klein, "Calculations of Monthly Average Insolation on Tilted Surfaces," Vol. 1, Proceedings of theISES and SESC Joint Conference, Winnipeg, 1976.
4. D.W. Ruth and R.E. Chant, "The Relationship of Diffuse Radiation to Total Radiation in Canada,"Journal of Solar Energy and Technology, Vol. 18, No. 2(1976), pp. 153-154.
5. "Introduction to Solar Heating and Cooling Designing and Sizing," DOE/CS-0011 UC-59a, b, c, Aug.1978.
6. Peter J. Lunde, Solar Thermal Engineering, Wiley,New York, 1980.
7. Climatic Atlas of the United States, U.S. Government Printing Office, Washington, D.C., 1968.
8. A.W. Morgenroth, "Solid Waste as a Replacement Energy Source," ASHRAE journal, June 1978.
9. Maria Telkes, "Solar Energy Storage," ASHRAE Journal, Sept. 1974, pp. 38-44.
10. ERDA Facilities Solar Design Handbook, ERDA 77-65, Aug. 1977.
11. Taylor Moore, "Wind Power: Gaining Momentum", EPRI journal, Winter 1999, EPRI, Palo Alto, CA.
12. American Wind Energy Association, 122 C Street NW, Washington, DC, 20001, [email protected].
13. F.R. Eldridge, Wind Machines, NSF-RA-N-75-051, prepared for NSF by the Mitre Corporation, Oct. 1975, U.S. Government Printing Office, Washington, D C. (Stock No. 038000-00272-4).
14. R.F. Rolsten et al., "Solid Waste as Refuse Derived Fuel," Nuclear Technology, Vol. 36 (mid-Dec. 1977), pp. 314-327.
