Types of Direct Solar Energy

The direct absorption of energy from sunlight and its subsequent conversion to more useful forms of energy such as electricity can occur by two mechanisms:

Thermal conversion Sunlight (especially its infrared component, which accounts for half its energy content) is captured as heat energy by some absorbing material. (An everday example of such a material is a shiny metal surface, which becomes very hot when left in sunlight.) Solar energy is an excellent source of heat at temperatures near or below, the boiling point of water, a category that accounts for up to half of total solar energy usage.

Photoconversion The absorption of photons associated with the ultraviolet, visible, and near-infrared components of sunlight brings about the excitation to higher energy levels of electrons in the absorbing material. The excitation subsequently causes a physical or chemical change (rather than a simple degradation to heat).

An example of passive solar technology—systems that use no continuous active intervention or additional energy source to operate them—is the use of solar box cookers in developing countries. In temperate climates, the design of buildings to absorb and retain (by insulation) a maximum fraction of the solar energy that falls on them in winter is another example.

Solar water heaters are used extensively in Australia, Israel, the southern United States, and other hot areas that receive lots of sunshine. They are also used extensively in China, Germany, Turkey, and Japan. The water heaters represent the biggest use of active solar technologies, which are defined as those that employ an additional energy source to operate them. Solar collectors located on the rooftops of private homes and apartment buildings, as well as some commercial establishments such as car washes, contain water that is circulated around a closed system by an electrically driven pump. Sunlight is absorbed by a black flat-plate collector, which transfers the heat to the water that flows over it and is bounded on the outside by glass or a plastic window. The hot water is pumped to an insulated storage tank until it is required for bathing or laundry or to help heat swimming pool water.

In more elaborate installations, the hot water is passed through a heat exchanger, which is a system of pipes over which air is passed and thereby heated by thermal transfer. The hot air can be used immediately in winter to heat the rooms of the building. If not needed immediately, the heat can be stored in other materials such as rocks. Usually a backup system, in which water can be heated electrically or by burning fossil fuel, is incorporated to provide heat on cloudy days or in high-demand situations.

Using Thermal Conversion to Produce Electricity

By focusing the sunlight reflected by mirrors onto a receiver that contains a solid or a fluid, very high temperatures can be achieved. The hot fluid can be used to generate electricity by turning turbines.

As discussed in the next section, the fraction of thermal energy that can be extracted and converted to electricity from a mass of hot fluid at an absolute temperature Th is limited by the second law of thermodynamics to be no greater than (Th — Tc)/Th, where Tc is the final absolute temperature of the cooling water. Consequently, it is advantageous to use a gas that has been heated to the highest possible temperature to maximize the amount of energy that is transformed to electricity rather than just degraded to waste heat. Indeed, temperatures of 1500°C have been achieved in steam heated by focusing sunlight. Generally, power plants need gas heated to 1200-1350°C at a pressure of 10-30 atm to operate. Simply focusing sunlight on tubes of air cannot achieve more than 700°C and 1 atm pressure. In one promising design, sunlight is focused by mirrors onto ceramic pins, which absorb the solar energy and heat up to 1800°C. Because they have a large surface area, the pins efficiently transfer the heat to air that flows around them. The hot, pressurized air (rather than steam) is then used to turn turbines and produce electricity.

The solar thermal electricity that results from power plants of this type may become competitive in price with conventional sources. This is particularly true if the waste heat, e.g., steam near the boiling point of water, can also be used for some purpose. This technique of using the waste heat from a heat-to-electricity conversion for a constructive purpose is called the cogeneration of energy. (It is a common feature of new power plants fueled by natural gas.) Unfortunately, power plants based on steam require large amounts of cooling water to condense the steam back into the liquid state as part of the system's cycle (see Figure 8-7); in many areas that have abundant land and sunlight, there is little water available for this purpose. Scientists have also pointed out that if the absorption of sunlight by such systems occurred on a massive scale, the Earth's albedo would be altered, with consequent effects on the climate that are difficult to predict.

High-pressure boiler

Fossil fuel in air (02)

Fossil fuel in air (02)

C02 up the stack or sequestered C02 out

Steam at Th

Steam turbine t

Water condensate

Pump

Condenser

Generator

Generator

Steam at Th

Steam turbine t

Water condensate

To cooling tower or cold river water and/or

• cogeneration at Tc

Electricity out

To cooling tower or cold river water and/or

• cogeneration at Tc

■ Cooling water return flow

FIGURE 8-7 The generation of electricity from a steam turbine cycle. In solar thermal electricity generation, the water is heated by the Sun's rays rather than by burning a fossil fuel. 7"h and 7~c refer to steam and water temperatures, as discussed in the text. [Source: Modified from M. I. Hoffert et a I., "Advanced Technology Paths to Global Climate Stability: Energy for a Greenhouse Planet/' Science 298 (2002): 981.1

Another way to use the very-high-temperature heat energy is to drive a thermochemical process such as the reduction of a metal oxide to the free metal (and oxygen gas). The metal could then be used to generate electricity in batteries or to react with water to form hydrogen fuel. In either case, the product is the metal oxide, which can be recycled for reuse. An example under development is the dissociation of zinc oxide, ZnO, into metallic zinc and oxygen at temperatures above 1700°C. Alternatively, the heat could be used to produce a combination of carbon monoxide and hydrogen from carbon dioxide and methane.

Reversal of this highly endothermic reaction produces heat that can be used to generate electricity etc. without the net emission of greenhouse gases, since the methane and carbon dioxide products are collected and reused.

Limitations on the Conversion of Energy: The Second Law of Thermodynamics

In all processes that convert high-temperature heat into electricity, a portion of the original heat energy is inevitably lost as waste heat at a lower temperature. This loss is partially unavoidable as a consequence of the second law of thermodynamics and applies to the production of solar thermal electricity as well as to other energy conversion processes.

According to the second law, entropy (or disorder) must increase—or at the least be unchanged—when one type of energy is converted into another. The law tells us that for any body at absolute temperature T that possesses an amount q of heat, the entropy S is a positive quantity given by the formula

Since high-quality (low-disorder) energy such as electricity has essentially zero entropy, clearly one cannot convert 100% of the heat into electricity, since the change AS in entropy associated with the conversion would be negative. However, if some of the initial heat energy qh at the initial high temperature Th is degraded to a smaller quantity qc at a lower temperature Tc, then the entropy change AS for the process could be positive or zero:

AS = entropy of energy after conversion — entropy of energy before conversion = qc/Tc + 0 - qh/Th

For the most complete conversion to electricity possible, AS = 0; for this situation the equation can be rearranged to give the new relationship qc/Tc = q^/Th or qc = qh Tc/Th

The amount of heat converted to electricity is qh — qc> which upon substitution for qc is equal to heat converted = tjh - tjh Tc/Th = qh (1 - Tc/Th) = qh (Th - Tc)/Th

Thus the maximum fraction of the original heat that can be converted to electricity is heat converted/initial heat = (Th — TC)/Th

In other words, the maximum yield of electricity increases as the difference in temperatures between the original heat source and the waste heat increases. Thus if sunlight can be converted to heat at 1500°C (1783 K), and if the temperature of the waste heat could be held to 27°C (300 K), the fraction of energy that could be converted to electricity would be

In fact, the efficiencies calculated by the formula are somewhat overestimated when more than one physical phase is involved. For example, associated with the conversion cycle in traditional power plants is a step that condenses steam back into liquid water (Figure 8-7), a process in which entropy is decreased. Consequently, additional energy beyond that calculated must be degraded to waste heat to compensate.

PROBLEM 8-1

What is the maximum percentage of heat at 900°C that could be converted into electricity if the waste heat was produced as steam at 100°C?

PROBLEM 8-2

Electricity could be obtained by exploiting the thermal gradient between the surface and the deep waters of the ocean. The maximum gradient, about 20°, is achieved in tropical waters. What is the maximum percentage of the energy associated with this gradient that could be converted to electricity if the surface (cooling) water temperature is 25°C?

PROBLEM 8-3

In order to reach a conversion efficiency of 50%, to what minimum Celsius temperature must a heat source be raised if the waste heat has a temperature of 57°C?

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