ΕΛΛΗΝΙΚΗ ΔΗΜΟΚΡΑΤΙΑ Ανώτατο Εκπαιδευτικό Ίδρυμα Πειραιά Τεχνολογικού Τομέα Ξενόγλωσση Τεχνική Ορολογία Ενότητα: Flat Plate Collectors- collecting the Heat Παναγιώτης Τσατσαρός Τμήμα Μηχανολόγων Μηχανικών ΤΕ
Άδειες Χρήσης Το παρόν εκπαιδευτικό υλικό υπόκειται σε άδειες χρήσης Creative Commons. Για εκπαιδευτικό υλικό, όπως εικόνες, που υπόκειται σε άλλου τύπου άδειας χρήσης, η άδεια χρήσης αναφέρεται ρητώς. Χρηματοδότηση Το παρόν εκπαιδευτικό υλικό έχει αναπτυχθεί στα πλαίσια του εκπαιδευτικού έργου του διδάσκοντα. Το έργο «Ανοικτά Ακαδημαϊκά Μαθήματα στο Ανώτατο Εκπαιδευτικό Ίδρυμα Πειραιά Τεχνολογικού Τομέα» έχει χρηματοδοτήσει μόνο την αναδιαμόρφωση του εκπαιδευτικού υλικού. Το έργο υλοποιείται στο πλαίσιο του Επιχειρησιακού Προγράμματος «Εκπαίδευση και Δια Βίου Μάθηση» και συγχρηματοδοτείται από την Ευρωπαϊκή Ένωση (Ευρωπαϊκό Κοινωνικό Ταμείο) και από εθνικούς πόρους.
1. Σκοποί ενότητας... 4 2. Περιεχόμενα ενότητας... 4 3. Flat Plate Collectors- Collecting the Heat... 5 3.1 Action of Materials on Light... 6 4. Practice and Exercises... 6 4.1 Exercise A: Seeking specific information... 6 4.2 Exercise B: Restructuring... 6 4.3 Exercise C: Word meanings in context... 7
1. Σκοποί ενότητας The aims of this unit are to: Provide authentic text and vocabulary specific to the needs of students of Mechanical Engineering Encourage students to combine their knowledge of English with their technical knowledge Help students understand word meanings in context Guide students into seeking specific information Teach students how to restructure ideas Teach students how to identify cause and effect Teach students how to describe processes 2. Περιεχόμενα ενότητας Contents of the unit : Flat plate collectors: how heat is collected, removal of thermal energy, convective and radiative losses, reradiation, collector insulation Transmission, reflection, absorption of light
3. Flat Plate Collectors- Collecting the Heat The flat plate collector is the simplest and most widely used means to convert, the sun's radiation into useful heat. Solar radiation passes through the transparent cover plates and strikes the blackened collector absorber surface where it is absorbed, changing to thermal or heat energy. Thermal energy is removed from the absorber by a flow of air or liquid that then delivers the heat to be used or stored. But often over half the heat absorbed is lost from the absorber surface, principally by reradiation and convection to the exterior surroundings. The losses are minimized by the use of the transparent cover plates, which act like storm windows on a house, passing visible and near-infrared radiation from the sun while reducing losses from the interior. Cover plates reduce convective losses from the absorber because of the resistance to convective heat transfer created by the air spaces above the absorber surface and between the cover plates. The outer cover plate will therefore have a lower temperature than that of the absorber surface, and convective losses will be correspondingly lower. Radiation losses are also minimized because the glass which is transparent to the sun's radiation is a good absorber of thermal energy, being nearly opaque (to the long-wave thermal radiation "shining" from the absorber to the surroundings. The cover plates are therefore "black" with respect to the reradiation from the absorber surface, and the reradiation is then absorbed rather than transmitted by the cover plates. This heats the cover plate to a temperature higher than the surroundings, but below that of the absorber. The rate at radiative heat loss from the absorber surface is thereby reduced, because it depends on the temperature of the surface to which it can radiate. The cooler outer surface of the cover plate in turn reradiate to the surroundings (or to the next cover plate), but at a reduced rate relative to the unprotected absorber. The collector must also be properly insulated to prevent conduction losses from the back and sides. lt is cheap and easy to do this but care is necessary to assure that all the possible conduction paths are blocked. The external framework and supports for the cover plates, for instance, must be insulated from the absorber plate and even from the air spaces, which are also well above the ambient temperature. Several inches of insulation are necessary under the plate (and at the sides) to reduce losses from these large areas to an economical minimum. The convective and radiative losses from a fiat plate collector tend to be large because the area for such losses is equal to the area for collection of the solar radiation. Proper design of a flat plate collector gives attention to reducing these losses by making it more difficult for the convection and radiation losses to occur. The heat transfer between the collecting fluid and the absorber surface must also.be maximized so that the absorber surface is held at as low a temperature as possible consistent with the temperature at which the heat must be utilized.
The present chapter deals with the means for reducing the losses from the collector and the ways to calculate those losses, as the outside ambient and collector surface temperatures vary, so that the collector performance can be determined. The next chapter will discuss ways to improve the heat transfer to the collecting fluid and to evaluate the effects of this heat transfer on the overall collector performance. 3.1 Action of Materials on Light When ordinary materials are exposed to light, they can transmit, reflect, or absorb it. If coefficients are defined equal to the proportion transmitted (t), reflected (r), and absorbed (a) a simple equation can be written: t + r + α = 1 This equation is valid for any wavelength of light; it holds for the radiation from the sun and for the long-wave radiation from a solar collector's absorber plate. But the coefficients are not necessarily the same at different wavelengths, and in solar collector design, materials are of special interest if their transmissivity, reflectivity, or changes favorably between the short wavelengths of visible and near-infrared light and the long wavelengths characteristic of thermal reradiation. The two groupings of wavelength do not overlap. Since wavelengths longer than two microns ('a micron is a millionth of a meter) are for the most pan absorbed by water in the earth's atmosphere, the two wavelength groupings are truly separate, with a 0.3- to 2-μ range for solar radiation, and 3 to 50 or 100 μ for long-wavelength reradiation. 4. Practice and Exercises 4.1 Exercise A: Seeking specific information Refer to the text and write questions to which the following statements are answers. 1. By means of a flat plate collector. 2. Through the transparent cover plates. 3. They act like storm windows on a house. 4. By the air spaces above the absorber surface and between the cover plates. 5. To a temperature higher than the surroundings, but below that of the absorber. 6. To prevent conduction losses from the back and sides. 7. By making it more difficult for the convection and radiation losses to occur. 8. When they are exposed to light. 4.2 Exercise B: Restructuring Change the structure of the following sentences without changing their meaning. Start with the words given.
1. Solar radiation strikes the blackened collector absorber surface changing to heat energy. The blackened collector 2. Thermal energy is removed from the absorber by a flow of air or liquid. A flow of air 3. Over half the heat absorbed is lost from the absorber surface by reradiation and convection to the exterior surroundings. Reradiation and convection cause. 4. The losses are minimized by the use of transparent cover plates. Using. 5. The outer cover plate will have a lower temperature than that of the absorber surface. The temperature of the absorber surface..... 6. Radiation losses arc minimized because the glass is a good absorber of thermal energy. The glass is a good absorber of thermal energy, which... 7. Reradiation is absorbed rather than transmitted by the cover plates. The cover plates... 8. The external framework and supports must be insulated from the absorber plates. You need... 9. The heat transfer between the collecting plate and the absorber surface must be maximized so that the absorber surface is held at a low temperature. The absorber surface can... 10. The losses from the collector must be calculated so that the collector performance can be determined. In order to.... 4.3 Exercise C: Word meanings in context Choose the item that best expresses the meaning of the word taken from the text. 1. transparent: a) apparent b) obvious c) clear 2. strikes: a) affects b) hits c) impresses 3. absorbed a) consumed b) taken in c) occupied 4. delivers a) transfers b) distributes c) emits 5. principally a) essentially
b) generally c) mainly 6. convection a) transfer b) conduction c) release 7. correspondingly a) similarly b) accordingly c) regularly 8. with respect to a) in correspondence with b) in comparison with c) in relation to 9. transmitted a) emitted b) conducted c) released 10. thereby a) therefore b) since c) however 11. ambient a) appearing b) surrounding c) resulting 12. Exposed a) subjected b) exhibited c) demonstrated