1. Pharos University. جامعه فاروس Faculty of Engineering
Pharos University جامعه فاروس Faculty of Engineering كلية الهندسة Petrochemical Department قسم البتروكيماويات
PE 330 ENERGY CONSERVATION LECTURE (11) Solar Energy It is available to all, it cannot be cutoff. It does not involve burning fuels or generation of pollution, it is not exhausted for it will last as long as the sun rises.

2. 1-Disadvantages of Solar energy
To be useful, it must be collected and concentrated, this means a lot of space is needed, mirrors and collectors.
It is not useful in cloudy days or at night.
To ensure an energy supply, the captured energy must be stored and supplemented with a back up system.

3. 2-Measurements of solar energy:
Solar radiation is measured in langley unit, which is referred to Samuel Langley who invented instruments for measuring solar radiation.
One Langley radiation is equivalent to one calorie falling on an area of one square centimeter.
Solar radiation flux varies from Langelys per minute.
One Langley per minute is a value expected in clear days.

4. 3-Application of solar energy
Desalination of saline water
Heating water
Production of electricity
Space Cooling

5. 3-1 Desalination of saline water
One method of producing fresh water from either salt water, contaminated water, or even liquid wastes from houses and communities is by means of a solar still.
A solar still has a basin which contains the salt water or contaminated water or waste water.
This basin is covered by inclined sheet of glass or plastic with a collecting at its bottom.
The sun shining through this glass will heat the water and vaporize some of it, then the steam coming in contact with the glass will condense on the glass and run down and collect as distilled water in a collecting container.

6. 3-1-1Types of Solar Stills:
A- Basic Solar Still B- Basin Solar Still C- Cascade Solar Still (Improvement Design) D- Wick Type Solar Still (Further Improvement in Design)

7. Basic solar still

8. Basin, Cascade, and Wick sola stills

9. 3-2 Heating Water:
Solar water heaters for domestic use, generally contains two main parts in the system:
1- The collector
2- A storage tank
The two main types of solar hot water heating systems are:
1- Closed Box Type
2- Re-circulating Type

10. 3-2-1Closed Box Type
It is very simple, consists of a metal box insulated on the sides and bottom which contains several large diameter black pipes.
The top of the box is covered by a transparent sheet of plastic.
The tops of large pipes are exposed to the sun and serve as the heat collectors, the hot water is also stored in the large pipes.
Hot water is drawn off the top ends of the pipes and replacement water is introduced at the bottom ends.
Here the large pipes act as collectors and storage tanks.

11. Advantages of Closed Box Type
No danger of freezing due to large amounts of heat is stored in the water in tubes.
Disadvantages of Closed Box Type :
May lose large amount of heat at night through the un-insulated plastic cover.
Does not produce enough hot water.

13. Closed box type

14. :3-2-2-Re-circulating Type
It consists of flat plate collector and a separate insulated storage tank. The collector consists of a copper sheet which is painted black and has water tubes embedded in the surface. Heat is trapped behind the glass sheet covering and carried off by water flowing through the tubes. Since the plate collector is located lower than the storage tank, a convection current is set up automatically (due to density differences), the warm water in collectors displaces the cooler water in the storage tank. At night the convection current ceases since the water in the solar becomes cooler than in tank, this prevents the loss of heat to night air.

15. Advantages of Re-circulating Type
Does not lose large amount of heat at night, the tank is insulated.
Produces large quantities of hot water.
Disadvantages of Re-circulating :
Danger of freezing in its flat plate collector

16. Re-circulating Type

17. Re-circulating Type

18. 3-3 Production of electricity
Photovoltaic cells are made of long crystals of silicon, made by batch process.
When sun light passes through such a crystal, some of it, is absorbed by atomic electrons, which become excited up to conduction levels. T
The electric field drives the electrons through the cell and the outer circuit, producing a flow of electrical power.
Voltage by one cell is small (0.5 volt),but large number of cells wired in series achieve high voltages.

19. Disadvantages of photovoltaic cells:
Batch production, so high cost.
Decreased performance at elevated temperatures.
Difficulty of storing electricity.

20. ِ3-4 Space Cooling (Absorption Chillers)
In an absorption cooling cycle, two fluids are used:
A refrigerant
An absorbent
The evaporated refrigerant is absorbed from the cooling coils into the secondary fluid. The resulting solution is transferred by a low power pump t a regenerator, where energy in the form of heat causes the refrigerant to be distilled out of the absorbent fluid. The refrigerant (now liquid) goes back to the evaporating coils (evaporator), vaporizing and cooling the inside of the system. The absorbent is sent back to absorber where the vaporized and heated refrigerant can be absorbed and carried way again.

21. (Absorption Chillers)

22. 4- Cooling and heating buildings
For any building there exists a balance point at which the solar radiation (Q solar) and internal heat generation rate (Q int) exactly balance the heat losses from the building.
Thus from sensible heat balance equation, at balanced condition:
(Q solar +Q int) sensible = UA (Tin-Tout)
Tout = T in –((Q solar +Q int) sensible )/A)

23. If the outdoor temperature is greater than the balanced outdoor temperature given by the above equation, i.e., when Tout > Tout,bal, then there is a need for cooling the building. On the other hand, when the outdoor temperature is less than the balanced outdoor temperature, i.e., when Tout < Tout,bal, then there is a need for heating the building. When the outdoor temperature exactly equals the balanced outdoor temperature, i.e., when Tout = Tout,bal, then there is no need for either cooling or heating the building.

24. Example (1):
A building has a U-value of 0.5 W/m2.K and a total exposed surface area of 384 m2. The building is subjected to an external load (only sensible) of 2 kW and an internal load of 1.2 kW (sensible). If the required internal temperature is 25oC, state whether a cooling system is required or a heating system is required when the external temperature is 3oC. How the results will change, if the U-value of the building is reduced to 0.36 W/m.K

25. Solution