Kelsey+Dutta

 Kelsey Dutta and Chelsea Cottle Solar water Heaters for the Home: Solar power has been used to heat water for houses since the 1890's (Energy Quest). There are two types of solar water heater systems, active and passive (Redwood Coast..).

Active Systems : Active systems use pumps to move the water. There are two general types of active water systems. The Open loop system uses a pump to move water through solar collectors where it is heated, then moved back into the home to be used. This type of system runs a risk of freezing in the winter (Redwood Coast..). The other type of active system is the closed loop system, which moves heat transfer fluid through the solar collectors that is then pumped into a tank where it can radiate heat into water that is used inside the house. this system has the benefit of not freezing in winter (Redwood Coast..).

Passive systems : Passive systems do not use water pumps. The simplest form of passive water system is the batch heater. A batch heater is a large tank that is placed directly on the roof of the house so that it can collect sunlight directly. Gravity gets the water to where it is needed. this type of heater is very inefficient. You need a strong roof to have this system, and it freezes easily (Redwood Coast..). The other form of passive system is called the thermosiphon, which uses [|convection currents] to make sure all of the water is heated. water is heated inside the solar collectors, which will rise to the top of the tank above them, allowing cooler water to sink into the collectors and be warmed. These systems are difficult to install and also freeze (Redwood Coast..). Solar Thermal Power Plants:

Solar thermal plants are power plants that use the sun to create electricity. During sunny days, these plants use mirrors in the shape of troughs to reflect the sunlight onto receiving chambers, which contain water. The water is heated by the redirected light and gets turned into steam, which is then funneled to another chamber, where it is used to turn a turbine to generate electricity (Schott "How A...").

During the night and on cloudy days, the power plant has two different methods of creating electricity. One type of power plant is able to store heat throughout the day and use it at night. The other type, called a hybrid plant, burns coal at night and on cloudy days to heat the water (Energy Quest "Chapter 15:..").

There are several of these plants in the U.S., particularly in [|Nevada] and [|California] (Wikipedia "List of..."). Solar Tower Power Plants:

Solar tower power plants use mirrors and receivers to heat water as well, but they are set up differently. Solar tower plants have a large tower surrounded by mirrors. These mirrors, called heliostats, rotate so that they always reflect the sun onto the receivers built into the tower (Becker et. al. 6). Then, just like in the thermal plants, the receivers heat water and turn it into steam, which is then used to turn a turbine to create electricity (Schott "How A Solar..").

Several of these plants are used in [|Spain], like the [|PS10 Project] and the tower outside [|Seville] (Chapa "Seville's solar..").



Disadvantages of Solar Plants:

Solar Power Plants come with disadvantages. In order to create enough energy to be effective, you must cover a large area with these plants, so that the mirrors or troughs will reflect adequate sunlight towards the receivers. These plants can only be effective in areas with a great deal of sunlight. Also, these plants can be detrimental to the environment if not properly cared for (Energy Info. Admin. "Solar Energy:...").

-Chelsea Cottle

Photovoltaic Cells: Photovoltaic cells or solar cells are the most recognizable form of solar energy, used to power calculators and houses alike. Photovoltaic cells convert sunlight directly into electricity.



The most important port of a solar cell is the [|semiconductor], which creates an electric field, positive on one side and negative on the other(Aldous). The most common material for semiconductors is [|silicon]. Silicon has four [|valence electrons], and can bond to four other silicon atoms to form pure [|crystalline silicon]. The silicon in solar cells, however, has had some silicon atoms replaced with [|phosphorus], which has 5 valence electrons. The phosphorus still bonds to four silicons, but it has an extra electron(Stone 23). This makes it much easier for added energy(like sunlight) to [|knock off]the extra electrons, turning them into “free carriers,” unattached electrons that can carry electrical current. Silicon that has been [|doped](impurities have been added) with phosphorus is called N-type silicon (n for negative because of the extra electrons)(Aldous).



The other half of the solar cell is silicon doped with [|boron], which has only 3 valence electrons, so there’s a hole for an extra electron(Stone 23). This is P-type silicon.

The two layers of P-type and N-type silicon create an [|electric field](which create voltage), and connecting a battery to each side creates a current of electrons from one side to the other, so we get power.

Unfortunately, this solar cell can only absorb at most 25% of the sunlight energy that hits it. The main reason for this is because sunlight contains light of different wavelengths, and some of that light doesn’t have enough energy to knock an electron off. Other photons have more than enough energy, and the extra energy is wasted(Aldous). media type="youtube" key="3Zpg5yLFKnI" height="344" width="425"

Solar power is important for climate change because it is an alternative source of energy from the fossil fuels that are causing climate change. Photovoltaic cells consume only sunlight. If protected against the environment, they will never wear out, and their electrical output can be used to power anything from a wristwatch to an air conditioner.

-Kelsey Dutta

Bibliography Becker, M. et al “Solar Thermal Power Plants” prepared for EUREC Agency May 3, 2000 Mar. 10, 2009 []

Chapa, Jorge “Seville’s Solar Power Tower” May 21, 2007 Mar. 11, 2009 []

Energy Information Administration “Solar Energy: Energy from the Sun” Energy Info. Admin. Nov. 2007 Mar. 8 2009 [] Energy Quest “Chapter 15: Solar Energy” __The Energy Story__ April 22, 2002 March 8, 2009 []

Generation eXe “PS10 Solar Power Plant in Spain” Sept 17, 2007. Mar. 12, 2009 []

Schott “How Solar Thermal Power Plants Work” Mar. 8, 2009 []

Wikipedia “List of Solar Thermal Power Stations” Mar 18, 2009. Mar 18, 2009 []

 Stone, Jack L. "Photovoltaics: Unlimited Electrical Energy from the Sun." __Physics Today__ 46 (1993).

Aldous, Scott. "How Solar Cells Work." 01 April 2000. HowStuffWorks.com.  07 March 2009. Physics: A World in Motion: The Photoelectric Effect. United Learning. 1998. Discovery Education. 23 March 2009 
 * Media: **

__N-doped Si__. Digital image. __Wikibooks__. 13 Nov. 2005. 15 Mar. 2009 .

__P-doped Si__. Digital image. __Wikibooks__. 13 Nov. 2005. 15 Mar. 2009 .

__Photovoltaic Cell__. Mathetmatics Magazine, Usvat Corporation Energy Department.

__P-N Junction__. 1994. Physics Today.