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Solar Energy Lecture Two

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The Sun at 5800K and a hot campfire at perhaps 800 K give off radiation at a rate proportional to the 4th power of the temperature.

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Even over the vast distance, an enormous amount of energy reaches Earth from the sun.

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Solar Irradiance is solar power per unit area. Solar Irradiance is a measure of how much solar power you are getting at your location. This irradiance varies throughout the year depending on the seasons. It also varies throughout the day, depending on the position of the sun in the sky, and the weather.

Solar Insolation/Irradiation is a measure of solar irradiance over of period of time - typically over the period of a single day.

Solar irradiance and Insolation/Irradiation

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World Solar Insolation Values

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The inverse square law states that irradiance is reduced in proportion to the inverse

square of the distance from the source.

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Solar irradiation equals the total solar irradiance over time.

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Solar radiation entering Earth’s atmosphere consists of direct, diffuse, and albedo radiation.

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Albedo Radiation

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Diffuse Radiation

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Air mass is a representation of the amount of atmosphere radiation that must pass through to reach Earth’s surface.

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Peak sun hours is an equivalent measure of total solar irradiation in a day.

For example, a day with an average irradiance of 600 W/m2 over 8 hr may only reach peak sun condition for an hour or less around noon. However, the total irradiation of 4800 Wh/m2 (600 W/m2 × 8 hr = 4800 Wh/m2) is equivalent to 4.8 peak sun hours (4800 Wh/m2 ÷ 1000 W/m2 = 4.8 peak sun hr)

Solar Concentrating Collectors

Introduction• For applications such as air conditioning, central

power generation, and numerous industrial heat requirements, flat plate collectors generally cannot provide carrier fluids at temperatures sufficiently elevated to be effective.

• Alternatively, more complex and expensive concentrating collectors can be used.

• These are devices that optically reflect and focus incident solar energy onto a small receiving area.

• As a result of this concentration, the intensity of the solar energy is magnified, and the temperatures that can be achieved at the receiver (called the "target") can approach several hundred or even several thousand degrees Celsius.

• The concentrators must move to track the sun if they are to perform effectively

Concentrating collectors• Concentrating, or focusing, collectors intercept

direct radiation over a large area and focus it onto a small absorber area.

• These collectors can provide high temperatures more efficiently than flat-plate collectors, since the absorption surface area is much smaller.

• However, diffused sky radiation cannot be focused onto the absorber.

• Most concentrating collectors require mechanical equipment that constantly orients the collectors toward the sun and keeps the absorber at the point of focus.

• Therefore; there are many types of concentrating collectors

Types of concentrating collectors

Parabolic trough systemParabolic dish Power towerStationary concentrating collectors 

There are four basic types of concentrating collectors:

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Parabolic trough system Power tower Parabolic dish 

Parabolic trough system Parabolic troughs are devices that are shaped

like the letter “u”. The troughs concentrate sunlight onto a receiver tube that is positioned along the focal line of the trough.

Figure 3.1.2 Parabolic trough systemFigure 3.1.1 Crossection of parabolic trough

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Parabolic trough system 

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Parabolic troughs often use single-axis or dual-axis tracking

Figure 3.1.3 One Axis Tracking Parabolic Trough with Axis Oriented E-W

Figure 3.1.4 Two Axis Tracking ConcentratorAxis Tracking Concentrator

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Solar Thermal for Electricity

Temperatures at the receiver can reach 400 °C and produce steam for generating electricity. Multi-megawatt power plants have been built using parabolic troughs combined with gas turbines (California).

Figure 3.1.5 Parabolic trough combined with gas turbines

Parabolic dish systems A parabolic dish collector is similar in

appearance to a large satellite dish, but has mirror-like reflectors and an absorber at the focal point. It uses a dual axis sun tracker

Figure 3.2.1 Crossection of parabolic dish

A parabolic dish system uses a computer to track the sun and concentrate the sun's rays onto a receiver located at the focal point in front of the dish. Parabolic dish systems can reach 1000 °C at the receiver.

Power tower system A heliostat uses a field of dual axis sun

trackers that direct solar energy to a large absorber located on a tower. To date the only application for the heliostat collector is power generation in a system called the power tower (solar tower)

Figure 3.3.2 HeliostatsFigure 3.3.2 HeliostatsFigure 3.3.1 Power tower system

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Power tower system 

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Solar Thermal for Electricity

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Power tower system 

A heliostat (from helios, the Greek word for sun, and stat, as in stationary) is a device incorporating a mirror which moves so as to keep reflecting sunlight toward a predetermined target or receiver, despite the sun's apparent motions in the sky.

The target is stationary relative to the heliostat, so the light is reflected in a fixed direction.

Most modern heliostats are controlled by computers. The computer is given the heliostat's position on the earth (latitude and longitude) and the time and date, and uses them to calculate the direction of the sun as seen from the mirror.

A power tower has a field of large mirrors that follow the sun's path across the sky. The mirrors concentrate sunlight onto a receiver on top of a high tower. A computer keeps the mirrors aligned so the reflected rays of the sun are always aimed at the receiver, where temperatures well above 1000°C can be reached. High-pressure steam is generated to produce electricity.

Figure 3.3.3 Power tower system with heliostats

Stationary concentrating solar collectors

Stationary concentrating collectors use compound parabolic reflectors and flat reflectors for directing solar energy to an accompanying absorber or aperture through a wide acceptance angle.

The wide acceptance angle for these reflectors eliminates the need for a sun tracker.

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Stationary concentrating solar collectors

Working principles of concentrating collectorsUnlike solar (photovoltaic) cells, which use light

to produce electricity, concentrating solar power systems generate electricity with heat.

Concentrating solar collectors use mirrors and lenses to concentrate and focus sunlight onto a thermal receiver, similar to a boiler tube.

The receiver absorbs and converts sunlight into heat. The heat is then transported to a steam generator or engine where it is converted into electricity.

A concentrating solar power system that produces 350 MW of electricity displaces the energy equivalent of 2.3 million barrels of oil .