Chapter 4 Renewable energy

Why for renewable Resource Conversion Consumption Impact The depletion of the earth's finite resources and economic considerations are all incentives to interest consumers in renewable. Reducing fossil-fuelled electricity generation reducing greenhouse gas emissions.

Renewable energy category hydroelectricity Biomass energy Geothermal energy Wind energy Solar energy

All the renewable energy sources are derived directly or indirectly from sunlight, exception of geothermal energy, tide energy .

Solar energy

What is Solar Energy? Originates with the thermonuclear fusion reactions occurring in the sun. Represents the entire electromagnetic radiation (visible light, infrared, ultraviolet, x-rays, and radio waves).

How much solar energy? The surface receives about 47% of the total solar energy that reaches the Earth. Only this amount is usable.

Advantages and Disadvantages it’s clean it’s huge and endless Advantages： All chemical and radioactive（放射性） polluting byproducts of the thermonuclear reactions remain behind on the sun, while only pure radiant energy reaches the Earth. - Energy reaching the earth is incredible. By one calculation, 30 days of sunshine striking the Earth have the energy equivalent of the total of all the planet’s fossil fuels, both used and unused! - Disadvantages： Sun does not shine consistently. Solar energy is a diffuse source. To harness it, we must concentrate it into an amount and form that we can use, such as heat and electricity. Addressed by approaching the problem through: 1) collection, 2) conversion, 3) storage. high investment

Advantages of solar energy: Solar energy is a renewable resource that is environmentally friendly. Unlike fossil fuels, solar energy is available just about everywhere on earth. And this source of energy is free, immune to rising energy prices. Solar energy can be used in many ways - to provide heat, lighting, mechanical power and electricity.

Putting Solar Energy to Use

Solar Heating Two methods of heating water: passive (no moving parts)(被动式） and active (pumps)（主动式）. In both, a flat-plate collector is used to absorb the sun’s energy to heat the water. The water circulates throughout the closed system due to convection currents. Tanks of hot water are used as storage.

Passive system-heating living spaces－被动式 The Center for Renewable Resources estimates that in almost any climate, a well-designed passive solar home can reduce energy bills by 75% with an added construction cost of only 5-10%. About 25% of energy is used for water and space heating. Solar house design Referring page 77 “ Trombe Wall”

Heating Water: Active System－主动式 Active solar energy system, in contrast to passive system, has a another source of energy required to drive the system. Active energy system --also referring page. 78 Active System uses antifreeze so that the liquid does not freeze if outside temp. drops below freezing.

Solar-Thermal Electricity- Power Towers（塔式电站） General idea is to collect the light from many reflectors spread over a large area at one central point to achieve high temperature. Example is the 10-MW solar power plant in Barstow, CA. 1900 heliostats (反射镜）, each 20 ft by 20 ft （6.1m×6.1m) a central 295 ft tower An energy storage system allows it to generate 7 MW of electric power without sunlight. Capital cost is greater than coal fired power plant, despite the no cost for fuel, ash disposal, and stack emissions. Capital costs are expected to decline as more and more power towers are built with greater technological advances. One way to reduce cost is to use the waste steam from the turbine for space heating or other industrial processes.

Power Towers （塔式） Power tower in Barstow, California. Heating to 560 °C Power tower in Barstow, California.

Parabolic Dishes and Troughs （碟状－槽式电站） Focus sunlight on a smaller receiver for each device; the heated liquid drives a steam engine to generate electricity. Parabolic dishes achieve higher temperatures than Troughs but they’re less efficient, more difficult to construct and require a precision set of tracking devices. Troughs are more popular than dishes in solar power plants because they are less expensive, easier to build and require less maintenance. --most mature one

Parabolic Dishes and Troughs Collectors in southern CA. Because they work best under direct sunlight, parabolic dishes and troughs must be steered throughout the day in the direction of the sun.

Photovoltaic Electricity !

Sunlight is converted to electricity using photovoltaic or solar cells Sunlight is converted to electricity using photovoltaic or solar cells. Photovoltaic (PV) cells are semiconductor devices, usually made of silicon, which contain no liquids, corrosive chemicals or moving parts. They produce electricity as long as light shines on them, they require little maintenance, do not pollute and they operate silently, making photovoltaic energy the cleanest and safest method of power generation.

Advantage of PV tower: Simplicity and versatility Solar cells are portable PV electricity is often cheaper than grid extensions, particularly in many areas not served by a grid.

Photovoltaic cells and modules Photovoltaic cells come in many sizes, but most are 10 cm by 10 cm and generate about 0.5v. PV cells are bundled together in modules or panels to produce higher voltages and increased power. A device known as an inverter converts DC to AC current. Inverters vary in size and in the quality of electricity they supply.

Solar Panels in Use Because of their current costs, only rural and other customers far away from power lines use solar panels because it is more cost effective than extending power lines. Note that utility companies are already purchasing, installing, and maintaining PV-home systems (Idaho Power Co.). Largest solar plant in US, sponsored by the DOE, served the Sacramento area, producing 2195 MWh of electric energy, making it cost competitive with fossil fuel plants.

Direct Conversion into Electricity PV cell Photovoltaic cells (光伏打电池） are capable of directly converting sunlight into electricity. A simple wafer of silicon with wires attached to the layers. Current is produced based on types of silicon (n- and p-types) used for the layers. Each cell=0.5 volts. Battery needed as storage No moving partsdo no wear out, but because they are exposed to the weather, their lifespan is about 20 years. Referred on page 83 fig.4.6

Principle of Solar Cells When sunlight strikes the solar cell, electrons (red circles) are knocked loose. They move toward the treated front surface (dark blue color). An electron imbalance is created between the front and back. When the two surfaces are joined by a connector, like a wire, a current of electricity occurs between the negative and positive sides.

How does the current continue Animation

Silicone lattice SEMICONDUCTORS like Silicon are neither a metals nor a nonmetals. They exist is in the twilight zone of elements. The central Silicone atom in the crystal is surrounded by an octet of electrons.

To one side of our silicon crystal lets sprinkle a little Arsenic To one side of our silicon crystal lets sprinkle a little Arsenic. Arsenic is very similar to Silicon except that it has 5 electrons in its outermost orbit. This would make it more of a nonmetal or N type material. On the other side of our Silicone crystal let's deposit a tiny amount of Aluminum. Aluminum has 3 electrons in it's outermost orbit. This makes it a metal and a P type material.

The electrons hopped the fence to find a home and form a stable octet.

When light penetrates into the P layer the negative electrons are knocked loose and  encouraged to travel through a circuit toward the positively charged N layer

Concentrator PV Systems The primary reason for using concentrators is to be able to use less solar cell material in a PV system. PV cells are the most expensive components of a PV system, on a per-area basis. A concentrator makes use of relatively inexpensive materials such as plastic lenses and metal housings to capture the solar energy shining on a fairly large area and focus that energy onto a smaller area, where the solar cell is .

Flat-Plate PV Systems The most common array design uses flat-plate PV modules or panels. These panels can either be fixed in place or allowed to track the movement of the sun. They respond to sunlight that is either direct or diffuse. Even in clear skies, the diffuse component of sunlight accounts for between 10% and 20% of the total solar radiation on a horizontal surface. On partly sunny days, up to 50% of that radiation is diffuse. And on cloudy days, 100% of the radiation is diffuse. One typical flat-plate module design uses a substrate of metal, glass, or plastic to provide structural support in the back; an encapsulant material to protect the cells; and a transparent cover of plastic or glass.

Advantage or disadvantage for concentrator and flat-flat PV? 1. fixed arrays are that they lack moving parts, there is virtually no need for extra equipment, and they are relatively lightweight. These features make them suitable for many locations, including most residential roofs. Because the panels are fixed in place, their orientation to the sun is usually at an angle that practically speaking is less than optimal. Therefore, less energy per unit area of array is collected compared with that from a tracking array. However, this drawback must be balanced against the higher cost of the tracking system. Concentrator systems increase the power output while reducing the size or number of cells needed. a solar cell's efficiency increases under concentrated light. How much that efficiency increases depends largely on the design of the solar cell and the material used to make it. a concentrator can be made of small individual cells. This is an advantage because it is harder to produce large-area, high-efficiency solar cells than it is to produce small-area cells.