Solar Collectors
There are two common kinds of solar collectors, and they are chosen based on the specificities of the solar system requirements. The flat panels are composed by a layer of glass that allows radiation to get through with minimal reflection; A black sheet that absorbers the radiation, and it is generally made either of copper or aluminum; Copper flow pipes, in which the incoming cold water is heated up. And finally, there is an insulation layer.
The evacuated tubes are also composed of a glass layer and absorber. However, the vacuum between the layers leads to a lower heat loss, and their round shape positively affects the radiation absorption by increasing the period when the sun radiation is perpendicular to the tubes.
Evacuated Tubes
Heat Pipes: A mixture composed of water and some additives is kept inside a heat pipe, whose temperature is increased by the absorbed heat. Due to vacuum, low pressure is created, so the water evaporates in lower temperatures. Therefore, when the heat is absorbed, the water mixture evaporates and goes up to a small heat exchanger, where heat is transferred to the cold water that flows in the manifold. Then, the mixture condensates and flows down due to the gravity action.
Direct Flow: It differs from the heat pipes because of the chilled water that flows inside the tube, it is heated, and flows back to the manifold.
Solar Collector:
A solar thermal collector is a solar collector designed to collect heat by absorbing sunlight. The term is applied to solar hot water panels, but may also be used to denote more complex installations such as solar parabolic, solar trough and solar towers or simpler installations such as solar air heat. The more complex collectors are generally used in solar power plants where solar heat is used to generate electricity by heating water to produce steam which drives a turbine connected to an electrical generator. The simpler collectors are typically used for supplemental space heating in residential and commercial buildings. A collector is a device for converting the energy in solar radiation into a more usable or storable form. The energy in sunlight is in the form of electromagnetic radiation from the infrared to the ultraviolet wavelengths. The solar energy striking the Earth's surface depends on weather conditions, as well as location and orientation of the surface, but overall, it averages about 1kW/m2under clear skies with the surface directly perpendicular to the sun's rays.
Types of Solar Collectors:
There are basically two types of solar collectors:
In the non-concentrating type, the collector area (i.e. the area that intercepts the solar radiation) is the same as the absorber area (i.e., the area absorbing the radiation). In these types the whole solar panel absorbs the light.
Concentrating solar collector usually has concave reflecting surfaces to intercept and focus the sun’s beam radiation to a smaller receiving area, thereby increasing the radiation flux. This reduces heat losses and increases efficiency at high temperatures. Another advantage is that reflectors can cost substantially less per unit area than collectors. This class of collector is used for high-temperature applications such as steam production for the generation of electricity. These collectors are best suited to climates that have an abundance of clear sky days.
Types of Non-concentrating Solar Collector:
Non-concentrating solar collectors can be classified as:
Flat Plate Solar Collector:
In flat-plate collectors there is no optical concentration of sunlight and they are generally stationary . In addition to this their outlet temperature capability is below 100 °C. Temperature close to the boiling point of water can be achieved using flat plate solar collector. However to reach higher temperatures evacuated-tube collectors and focusing collectors are used. In evacuated-tube collectors they use vacuum to reduce heat lost and to protect the absorber coating from deteration.By this way they can reach temperatures up to 140 °C and they can collect both direct and diffuse solar radiation. A flat plate collector is basically a black surface that is placed at a convenient path of the sun. And a typical flat plate collector is a metal box with a glass or plastic cover (called glazing) on top and a dark-colored absorber plate on the bottom. The sides and bottom of the collector are usually insulated to minimize heat loss.[11] The main components of flat plate solar collector are:
Absorber Plate:
It is usually made of copper, steel or plastic. The surface is covered with a flat black material of high absorptance. If copper or steel is used it is possible to apply a selective coating that maximizes the absorptance of solar energy and minimizes the radiation emitted by plate.
Flow Passage:
The flow passages conduct the working fluid through the collector. If the working fluid is a liquid, the flow passage is usually a tube that is attached to or is a part of absorber plate. If the working fluid is air, the flow passage should be below the absorber plate to minimize heat losses.
Cover Plate:
To reduce convective and radiative heat losses from the absorber, one or two transparent covers are generally placed above the absorber plate. They are usually be made from glass or plastic.
Insulation:
These are some materials such as fiberglass and they are placed at the back and sides of the collector to reduce heat losses.
Enclosure:
A box that the collector is enclosed in holds the components together, protect them from weather, facilitates installation of the collector on a roof or appropriate frame [12].
Figure 3.1: Flat Plate Solar Collector (Source: US Energy Department)
Equations
The chosen collector is an evacuated tube model, which is manufactured by kingspan, model HP. The manufacturer provides the equation for the collector efficiency. This equation depends on the first heat loss coefficient, second heat loss coefficient, aperture area and also the “efficiency of the layers of the material”
Power input by the sun
(1)
Power Output
(2)
Collector efficieny(reduced equation)
(3)
Effiency (considering the incidence angle modifier)
(5)
Incidence angle modifier for an evacuated tube collector
(6)
(7)
*
are given by the manafacturer.
** The heat loss coefficients 
are determined with the steady state method.
*** Eq. (5) and (7) do not consider the effect of flow rate.
Test conditions :
Facing south, Tilt angle : 45º
1
Energy required to heat the water
3.5.8. Collector Efficiency Factor (F′)
Is the ratio of the real energy output of the collector to the energy output in the case when the total absorber area was at the average fluid temperature with the same fluid
quantity of flowing water. 3.5.9. Collector Flow Factor (F″) Is the ratio of the energy that the collector can deliver at the average temperature of the fluid to the energy that the collector can supply at the inlet collector temperature. For a
certain collector the flow factor is a function of the flowing water quantity. 3.5.10. Collector Heat Removal Factor (FR) Is the ratio of the energy collector output to the energy output of the collector in
temperature of the inlet fluid. It is temperature dependent. The thermal output factor is connected to the flow factor and to efficiency factor by the relationship: FR = F′×F″.
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