3.9 Components of Solar Absorption Systems
3.9.1 Solar Thermal Collectors
A thermal solar collector is a special kind of heat exchanger that transforms radiant energy into heat. It is made up of a receiver that absorbs the solar radiation and then transfers the thermal energy to a working fluid. The analysis of thermal solar collectors presents unique problems of low and variable energy fluxes and relatively large importance of radiation (51). For efficient solar collection, a solar collector should have high absorptance and low heat loss coefficient to ensure a high percentage of the incident solar irradiation is converted into useful energy. Flat plate collectors, evacuated tube collectors and compound parabolic collectors are all suitable for use in air conditioning systems (33).
3.9.1.1 Flat Plate Collectors (FPC)
The simplest form of a flat plate collector (FPC) is a metallic absorber plate exposed to solar radiation with a thermal fluid circulating within. The thermal losses for this arrangement are high due to the effects of the wind is convection losses. To improve the performance of this type of collector, a transparent cover or glazing is introduced so that it isolates the absorber plates from the wind and causes the greenhouse effect inside the collector, recuperating part of the solar radiation reflected by the absorber (52).
Figure 3.8: Schematic of flat plate solar collector schematic (53)
A flat plate collector is made up of an absorber plate of solar energy, a transparent cover (glazing) that is transparent to solar radiation and opaque to long wave, a heat transfer fluid (air, antifreeze or water) to remove heat from the absorber, and back and edge insulation to reduce heat losses by conduction. They utilize both beam and diffuse solar radiation, do not require tracking of the sun and require little maintenance. The glazing reduces radiation losses from the collector as it is transparent to short wave radiation from the sun but opaque to long-wave thermal radiation emitted by the absorber plate. It also reduces convection losses because of the stagnant air layer between the absorber plate and the cover. The heat insulating backing reduces conduction losses. Flat plate collectors outlet temperatures range between 70-90oC and up to 100oC for selectively coated absorbers (54; 55).
To conveniently describe solar collector operation, the energy balances on each component are required.
The design and performance analysis of solar collectors presents unique and unconventional problems in heat transfer, optics and material science and is concerned with obtaining least cost energy. It may be desirable to design a collector with efficiency lower than the one that is technologically possible if the cost is significantly reduced (51). By applying the conservation of energy principle on the absorber, glass cover and working fluid we come up with a set of governing differential equations. The governing equations are then solved as a system.
The useful energy collected by a collector can be obtained from the Hottel, Whillier-Bliss equation (51) as:
(3.28)
Where:
Absorptance-transmittance product of the solar collector Collector heat loss coefficient
Absorber plate temperature Ambient air temperature Solar irradiance over time,
Solar collector area
Collector heat removal factor
Collector inlet fluid temperature The efficiency is equal to:
(3.29) If the heat loss ( ) is considered as the sum of two terms, a constant factor ( ) and a second term dependent on the difference between fluid and ambient temperature [ , it can be expressed as:
(3.30)
The efficiency equation can also be computed as:
(3.31)
Where is the reduced temperature difference given as:
(3.32)
Where:
Optical collector efficiency [%]
Collector outlet temperature [oC]
Linear heat loss coefficient, Wm-2K-1 Quadratic heat loss coefficient, Wm-2K-2 3.9.1.2 Evacuated Tube Collectors (ETCs)
When higher temperatures are desired, one needs to reduce the heat loss coefficient and this can be achieved through use of evacuated tube collectors. They minimize convective heat loss by placing the absorbing surface in a vacuum and consequently they produce higher outlet temperature in the range of 80oC to 130oC (54). There are many evacuated tube collector designs on the market and all of them use selective coatings on absorbers because with a non selective absorber, radiation losses would dominate
and eliminating conduction losses alone would not be very effective. Evacuated tube collectors (ETCs) use liquid vapor phase change materials to transfer heat at high efficiency. The problem with the design of ETCs is that it is difficult to extract heat from a long thin absorber contained in a vacuum tube (56).
Figure 3.9: Evacuated tube collector (57)
A heat pipe provides the most elegant way of extracting heat from an evacuated tube (58). The need for glass to metal seals can be avoided by the Dewar or thermos–bottle design. This design is the only one that has demonstrated long life under transient outdoor conditions (56). The selective absorber coating is applied to the outside (vacuum side) of the inner tube to extract heat from the absorber surface. The collector places a feeder concentrically inside the absorber. The heat transfer fluid enters through the feeder which is open at the other end and allows the fluid to return to the annulus between feeder and absorber plate. The performance penalty due to the thermal short circuiting between incoming and outgoing fluid seems to be acceptable in this case (58). Alternatively, a copper sheet is placed inside the absorber touching the glass to obtain good heat transfer from the glass to copper sheet as shown in Figure 3.9. Hail stones damage is a serious problem for ETC designs that contain glass-to-metal seals and also there is need for a mechanism to prevent thermal shock. ETCs are excellent for operating temperatures up to 120-150oC range. They are non- tracking and many of them use some kind of reflector enhancement (58).
The losses to the environment, in this type of collector occur primarily through radiation and are described by:
(3.33)
Where:
Heat loss coefficient
Collector water inlet temperature Ambient temperature
The expression that gives the useful energy gain is then:
(3.34)
Where:
Projected area of the tube
Absorber area
Collector transmittance absorptance product Solar irradiation on the collector
3.9.1.3 Compound Parabolic Collectors (CPCs)
As shown in Figure 3.10, compound parabolic collectors are a special type of collector fabricated in the shape of two meeting parabolas. Heat losses are approximately proportional to absorber area. By concentrating the incident radiation on an aperture onto a smaller absorber, one can reduce the heat loss per collector aperture area (58). CPCs are non-imaging concentrators. They have the capability of reflecting to the absorber all of the incident radiation within wide limits. All the radiation that enters the aperture within the collector acceptance angle reaches the absorber surface because of multiple internal reflections (54). The area reflecting the solar radiation is considerably larger than the absorption area thus enhancing outlet collector temperatures and lowering heat losses to ambient by the use of vacuum
technique (52). In a CPC, the absorber can be flat, cylindrical, bifacial etc. CPCs with low concentrations and non-evacuated receivers may be economical for temperatures around the boiling point of water (58).
Figure 3.10: Schematic of a compound parabolic collector (58)