Renewable Energy Sources 1:

1. Renewable Energy Sources 1: • Inexhaustible energy sources include geothermal, tidal and solar. • With the exception of geothermal and tidal, all the known renewable energy sources derive their energy from solar radiation. • Tidal energy comes from the gravitational pull of the moon and the sun on the oceans. • Geothermal energy originates in the radioactive decay of long-lived isotopes within the earth. • Tidal energy, ocean thermal and ocean currents are not currently contributing significantly towards the energy needs of the US.

2. Why Renewable Resources? • Break our dependence on the limited fossil fuel resources. • Use of most renewable energy is environment friendly. In exception of biomass it is possible to have zero emissions from the use of renewable resources. • Even with biomass, it is possible (at least theoretically) to use biomass at a rate in equilibrium with its growth (closed cycle).

3. Energy from the Sun • Sources that derive their energy from solar radiation include: ocean thermal gradients, ocean currents, biomass, wind, hydroelectric, direct use of solar radiation. • While energy renewable sources are inexhaustible, the rate at which energy may be drawn from them can be quite limited. • Energy from the sun is in the form of electromagnetic (EM) radiation. EM radiation travels through space at the speed of light, 3.0x108m/s. • Variations in solar intensity over day and night, summer and winter, as well as changing cloud cover complicate the use of solar energy.

4. Solar Energy: Definitions of terms. • Solar Constant (SC): is the power density at the top of the atmosphere on the side of the earth directly facing the sun. It’s value is 2 cal/min·cm2 (±3% for winter and summer). This number is known as the solar constant. • In the earth’s upper atmosphere, SC=0.5cal/min·cm2 • Average solar power (for a 24-hr period) on a horizontal area 1m2 at the earth’s surface is 164W/m2. For 8-hr day, the average power is estimated at 600W/m2. 600W/m2 = 190Btu/ft2·hr =1520Btu/ft2·day. • Values of the solar radiation as quoted above are often called the insolation.

5. Solar Energy. Given that the area of the United States is 3.615 x 106 square miles we can estimate the total solar energy incident on the US in a year. Using the insolation of 1520Btu/ft2·day, we find this gross amount of solar energy to be (see calculation on page 95) 5.6 x 1019 Btu/yr. Comparing this value to the 93.8 x 1015 Btu of energy consumed in the Us in 1996, we see that the sun supplies us with 600 times more energy per year than we use!!! If only we can find an efficient way of harnessing this we will be in a very good shape!

6. Solar Energy Devices. There are three general categories of solar devices – 2 common and 1 less common. The common categories are: • Those intended to heat living or working spaces, or produce hot water. • Those intended to produce electricity. The less common category is: • The powering of heat engines to drive irrigation pumps. In the design of a solar energy system the orientation of the collector is an important factor since the collector will receive most energy if it points directly at the sun.

7. Solar Energy for Space or Water Heating applications. For heating applications, two types of solar systems are common; Active System and Passive System. In an active system, a fluid (liquid or gas) is forced through a collector by an electrically driven pump or blower. Advantages of an active system are: • Compactness of components, • Flexibility in the placement of collector and storage units, • Easy control. A disadvantage is the dependence on electricity.

8. The Flat-Plate Collector. The flat-plate collector system (fig 4.5 of text) is an example of an active system. Major components of this unit can be identified as: • The Collector, receives the incoming solar radiation; • The Circulating System, (includes the pumps) carries the energy from the collector to the • Heat Exchanger, receives the heat from the circulating medium and transfers it to the • Storage Unit, stores the energy from the heat exchanger and redistributes it as needed. The collector must be a good radiation absorber and a poor emitter at the same time. The circulating fluid could be water (in warmer climates) or antifreeze (in colder climates).

9. Points to Note on Flat-Plate Collector. • When antifreeze is the circulating fluid separate heat exchangers between the collector panels and the domestic hot water system to prevent contamination in the event of a leak in the pipes containing the circulating fluid. • The storage unit must be able to store energy for use at night and on cloudy days. To accomplish this, a large hot water tank, or a box of stones (when the circulating medium is air) is used. • The efficiency of a solar heating system is less than 100% for several reasons: % of radiation transmitted depends on angle of incidence, # of glass cover sheets and composition of the glass.

10. Basic Laws of Thermal Radiation. • Stefan’s Law is related to the radiated power. In equation form, it states: P/A = εσT4; P/A is the power (in Watts) per square meter. ε is called the emissivity of the surface (0<ε<1) σ is called the Stefan-Boltzman constant. σhasvalue of 5.67x10-8 (W/m2)·K4. T is the surface temperature of the radiating surface in degrees Kelvin. • The Wien displacement Law relates the wavelength of the most intense emitted radiation to the surface temperature of the radiating surface: λmax (μm) = 2898K / T(K) For the sun whose surface temperature is 5800K, λmax (μm) = 2898K / 5800(K) = 0.5 μm.

11. Passive Solar Systems. A Passive Solar home system attempts to use energy from the sun to maintain a comfortable living temperature. It involves no burning of fuel, no pumps or blowers, no heat exchangers or other equipment and is independent of the electric utility company. In passive solar systems, the building itself functions as a solar collector and a storage unit. Three design elements are involved in a passive solar home system; Insulation, Collection and Storage.

12. Passive Solar Systems 2. • Insulation: Involves highly effective thermal insulation of external walls, roof and floors. Doors, windows and vents must be designed to minimize heat loss. • Collection: Use of large windows, or perhaps a solar collector panel on the south wall lower than the living space. • Storage:Thermal mass ( as used in passive solar discussions) is any material that becomes warm under the influence of solar energy and cools down later, giving its heat energy to the surroundings. Common thermal mass materials include water (62Btu/ft3); iron (54Btu/ft3).

13. Passive Solar Systems 3. • Storage contd: Thick outside walls can be designed to perform as passive solar storage units. The walls absorb solar radiation during the day when the sun is shining and continues to warm the house well after the sun has set. Two practical devices or schemes used for thermal mass are: • The Trombe wall (fig 4.7); and • The direct gain method (fig. 4.8).

14. Solar Thermal Electricity Generation. A schematic diagram of a solar thermal electric power generation is as follows: In order to use the heat energy from the sun to boil water to produce steam that will turn a turbine, it is necessary to focus the suns rays into a spot. Two general categories of solar-thermal electric power generation devices are in use: (1) the Power Tower, (2) Parabolic dishes and Troughs. Steam Electric Generator Sun H2O Turbine

15. Solar Thermal Electricity Generation 2 • The Power Tower: uses a large array of many reflectors to concentrate the light onto a single central receiver mounted on a tower within the array of reflectors (fig. 4.10). The central receiver is in contact with the circulating fluid. • Parabolic Dishes and Troughs: (figs. 4.12 and 4.13). Also use several reflectors but each reflector has its own collector (or receiver). All the collectors are in contact with the circulating fluid. The choice between the two is whether to use the reflected rays of the sun (The Power Tower) or a flow of heated fluid (Parabolic Dishes and Troughs) to bring the solar energy to the central site.

16. Direct Conversion of Solar Energy to Electrical Energy. Using the sun’s energy to boil water to produce steam to power a heat engine that in turn rotates an electric generator is a usable but expensive and cumbersome scheme. An alternative that is less cumbersome and perhaps less expensive is provided by the use of photovoltaic devices. These devices are capable of converting sunlight directly into electricity. Photovoltaic cells are made of semiconductors. Silicon Solar cells belong to the general class of photovoltaic cells. Solar cells have become a standard feature on space satellites to provide electric power.

17. Solar to Electrical Energy Conversion. • Photoelectric Effect: When light falls on certain metals, electrons are ejected from the surface of the metal. This phenomena is known as the photoelectric effect. • A semiconductor is said to be doped when certain atoms in the crystal structure of the semiconductor are replaced by atoms of a different element. • Doped semiconductors with an excess of electrons are called n-type semiconductors; and those with deficient electrons are called p-type semiconductors.