Heat Transfer, Coursework Example

Identify all the energy exchange processes that influence the performance of the flat-plate solar collector.

A typical flat plate solar collector is a box made of metal whose top side is covered with a glass or plastic cover usually called glazing and the bottom is covered by a dark-colored absorber plate. This device has its sides and the bottom side insulated to minimize heat loss. This devise is constructed so that it can be used to collect /tap solar power for use in another form ( Agbo, & Okoroigwe, 2007, 25-67).

As per Sun & Wind Energy, International Issue (2007) when solar radiation passes through the transparent glazing and impinges on the observer surface which is blackened, a large amount of this solar energy is absorbed into these plates and then converted into heat energy. This heat energy is transferred to the transportation medium (water) in the fluid tubes attached to the absorber plate for storage or use. The absorber plates are usually painted black or painted with selective coating which absorb and keep heat for along time. The sides of the casing and the underside of the absorber is well insulated to prevent heat loses through conduction this ensures that all the heat energy generated is well transferred to storage or use. The cover (glazing) is transparent so that it can reduce heat losses by convection from the absorber through the control of stagnant layer of air which is in between the plate and the glass.

The cover also reduces radiation losses from the collector since transparent glass allows short wave radiation from the sun to pass through but it nearly hinders or is opaque to the long-wave thermal radiation that is emitted by the absorber plate. The major energy exchanges that influence the performance of the flat plate collector is thermal energy exchanges. The main thermal losses which occur at the front of the collector are caused by convection. Between the cover and the absorber is circulating air which transports the absorbed heat to the glazing through convection (Sarkar & Obaidullah, 2006, 89-104). .

The glazing then conducts the heat to the surface of the cover which is again lost to the environment through convection because of the air that flows around the collector. Also the absorber radiated infra-red radiations to the glazing from where it is transferred or lost to the environment. Usually the selective absorber can emit infrared of up 5%. At the back side of the collector thermal loses also occur at the insulation (Duffie, 2004, 46-78). The conduction of heat at the back side of the collector depends on the materials used and can be reduced or be kept low by using thick thermal insulation. About only 1/7 of the total heat is lost at the rear side of a single glazed flat plate collector with a selective coated absorber (Sarkar & Obaidullah, 2006, 89-104).

From the above explanation it is evident that all the processes of heat transfer are involved in performance of the flat plate collector (Charles, 2002, 68-90). Energy from the sun/ sunlight is received at the surface of the collector by radiation. In summary, the cover of the collector is made in away that it allows solar radiation to pass through to the absorbers by radiation heat exchange/transfer process. In between the absorbers and the cover is air which transfers heat by convection back to the cover, thus, the cover must be designed to hinder this kind of thermal transfer which is a major source of heat loss from the collector. The absorber is made up of a good thermal/ heat absorber material like copper or aluminum (Charles, 2002, 68-90). The absorber converts the solar radiations into heat energy which is transferred by conduction to the flowing liquid in the pipes to take it away for use or storage. The above heat exchange processes define the way the flat plate collector performs its work. To improve its performance some of the main energy losses need to be reduced.

Suggest design procedures that would increase efficiency

For any device efficiency is given priority so that the device can do the intended purpose efficiently. For the collector the major problem with its efficiency is energy losses which occur in different phases of its performance. For instance solar radiation is lost via reflection on the cover, and about 8% of it is lost. 2% energy is also lost by absorption by the glass cover, about 13 % is lost by convection after reaching at the absorber, 8% is also lost via reflection at the absorber, and about 6% is lost by backward radiation from the absorber. This allows only about 60% of the solar radiation received at the glass surface to be transferred as useful energy at the end. The fact is that those loses can be reduced significantly by having the correct designs of the materials used in the manufacturing of the collector (Charles, 2002, 68-90).

The following design suggestions can help in reducing the energy loses and improve the efficiency of the device.

Transparent cover

For the device to have high transmittance and high durability, it can be recommended that, covers of low iron, and tempered solar glass with anti-reflective coating be used. And incase two or three transparent covers are used then the y must be made of antireflective coated glass or Teflon. The aim of this design is to improve transmission and stability of temperature and reduce heat expansion.

Absorber

The absorber sheet and the absorber pipes should be made of copper for high efficiency but also aluminum can be used though its efficiency cannot be compared with that of copper. Stainless steel may also be considered incase of corrosion purposes. The absorber sheet must be coated with a selective coating that will reduce thermal loses due to infra red radiations. Another important design is that of the absorber pipes. They should be designed such that they ensure high heat transfer and because of stagnation, the hydraulic absorber should be designed such that a fluid empting behavior is allowed under stagnation conditions when steam occurs (Agyenim, Hewitt, Eames, and Smyth, 615-628, 2010)..

Insulation

The sides and the back side should be made of thick insulators so that they reduce heat loss due to high temperatures. For best results polyurethane plates can be used in between the insulation material and the back panel of the collector (Wagner 2007, 168-175).

Casing

A casing of the device should be made of aluminum, steel, wood or synthetic material to maintain stability and protect the absorber and insulation against the environmental effects.

Also to improve the efficiency of the flat panel collector air which is in-between the top glass and the absorber may be evacuated from its places so that ewe have evacuated collector. This will reduce heat loss by convection hence efficiency. Once air has been evacuated from the device the collector is now called evacuated collector (Rommel, Matthias, 2005, pg. 157-189).

Work cited

Agbo, S. N and Okoroigwe, E. C. Analysis of thermal losses in the flat-plate collector of a Thermosyphon Solar water Heater. Research Journal of Physics 1(1): 35-41. 2007. Print

Agyenim, F., Hewitt, N., Eames, P. and Smyth, M., A review of materials, heat transfer and phase change problem formulation for latent heat thermal energy storage systems (LHTESS). Renewable and Sustainable Energy Reviews, 14(2): 615-628. (2010).print

Charles E. Brown, “World Energy Resources”, Springer, USA, 2002. Print

A. Duffie, “Solar Engineering of Thermal Processes”, John Wiley & Sons, USA, 2004. Print

A. R. Sarkar and M. Obaidullah, “Solar Thermal Applications”, Short Course on Renewable Energy Technology, 17-20, BUET, Bangladesh December 2006. Print

Rommel, Matthias: Medium Temperature Collectors for Solar Process Heat up to 250°C. Estec 2005. Print

Sun & Wind Energy, International Issue, 1, Bielefeld, Germany. 2007. Print

Wagner & Co Solartechnik GmbH, 2007. Print