Chen C.J. Physics of Solar Energy

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11.4 Solar Thermal Power Systems  243

Figure 11.18 Accumulated installation of concentration solar power systems. Installation

of these systems up to 2009, including parabolic trough, central receiver with heliostats, paraboloidal

dish, ISCC, and CLFR systems [88].

where T

is the temperature of the cold reservoir and T

is the temperature of the

hot source. In order to improve eﬃciency, the temperature of the hot source should be

as high as possible. Since T

cannot be lower than the atmospheric temperature, on

average 300 K, in order to achieve an eﬃciency of 50%, T

should at least be 600 K,

or 327

◦

C. Without concentration, the temperature can hardly reach 150

◦

To achieve such a high temperature, concentration of solar radiation is a necessity.

Three commonly used conﬁgurations for stand-alone solar thermal power systems were

developed in the 1970s: the parabolic trough concentration system, a system of a central

receiver with heliostats, and the paraboloidal dish concentration system. Recently, the

Integrated Solar Combined Cycle (ISCC) system has attracted much attention. The

Compact Linear Fresnel Reﬂector (CLFR) system, suitable for the ISCC system, has

observed a rapid development. Figure 11.18 shows the accumulated installation of those

systems up to 2009 [88].

11.4.1 Parabolic Trough Concentrator

As shown in Fig. 11.18, to date, about two-thirds of electricity generated by concen-

trated solar thermal power plants is from parabolic trough systems. The structure of

the system is shown in Plate 11. Parabolic mirrors are mounted on an axis to track

the Sun. At the focal line of the parabolic mirrors is a linear collector, typically a

high-temperature evacuated tube. See Plate 11.

To date, the largest solar power plants are the Solar Energy Generating Systems

(SEGS) in the California Mojave Desert. The nine SEGS power plants built between

1984 and 1990 have a total capacity of 354 MW. The total area of the 232,500 parabolic

mirrors is 6.5 km

, with a total length of 370 km. The sunlight bounces oﬀ the mirrors

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244  Solar Thermal Energy

Figure 11.19 Aerial photo of SEGS system. The axes of the parabolic mirrors are north–

south, which turn from east to west every day. About 3000 mirrors are replaced each year because of

wind damage. Several damaged mirrors are shown in this photo. To avoid shadows from neighboring

mirrors, the distance between adjacent mirrors is about twice the width of the mirrors.

and is directed to a central tube ﬁlled with synthetic oil which heats to over 400

◦

The reﬂected light focused at the central tube is 71 – 80 times more intense than

ordinary sunlight. The synthetic oil transfers its heat to water, which boils and drives

the Rankine cycle steam turbine, thereby generating electricity. Synthetic oil is used to

carry the heat (instead of water) to keep the pressure within manageable parameters.

See Plate 12.

Figure 11.19 is an aerial photo of the SEGS. The axis of the parabolic mirrors

is north–south, which turns from east to west every day. The most expensive parts

are the curved mirrors. Wind damage occurs frequently, and about 3000 mirrors are

replaced each year. The mirrors are periodically cleaned by a special machine. To

avoid shadows from neighboring mirrors, the distance between mirrors is about twice

the width of the mirrors. The cost of electricity from the SEGS is still not competitive

with coal-burning power plants.

11.4.2 Central Receiver with Heliostats

To achieve high power at a centralized receiver, hundreds or thousands of heliostats,

mirrors mounted on a two-dimensional shaft to track the position of the Sun by a

centralized computer distributed on a typically circular or oval ﬁeld; see Fig. 11.20.

Because very high temperatures can be reached (e.g. 565

◦

C), superheated steam or

molten salt (e.g. 60% NaNO

+ 40% KNO

) is usually used as the working material.

Finally, it drives a standard Rankine cycle steam turbine to generate electrical power

[73].

The ﬁrst pilot project, Solar One, in the Mojave Desert, was completed in 1981,

and was operational from 1982 to 1986. It has 1818 mirrors, each 40 m

. Oil or water

was used as the working ﬂuid. An eﬃciency of 6% was demonstrated. A photo of Solar

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11.4 Solar Thermal Power Systems  245

Figure 11.20 Solar power plant with central receiver. To achieve high power at a centralized

receiver, hundreds of heliostats are used, that is, mirrors mounted on a two-dimensional shaft to follow

the position of the Sun by a centralized computer. The temperature of the central receiver can reach

500

◦

Cormore.

One is shown in Fig. 11.19(b). In 1995, Solar One was upgraded to Solar Two. Molten

salt, 60% sodium nitride and 40% potassium nitride were used as the working material.

Ten megawatts of power is demonstrated, and its eﬃciency was improved to 16%. On

November 25, 2009, the Solar Two tower was demolished.

To be used as a stand-alone power plant, an energy storage system is required.

Latent heat of molten salt is used for energy storage. However, the cost per kilowatt-

hour is increased from $0.08 – $0.15 to $0.15 – $0.20 by adding the energy storage unit

[73].

11.4.3 Paraboloidal Dish Concentrator with Stirling Engine

The third type of concentration solar power plant is the paraboloidal dish concentrator

with Stirling engine, as shown in Plate 10. According to a report released by Sandia

National Laboratory in February 2008, the Stirling engine system has shown a solar-

Figure 11.21 Stirling engine. In-

vented in 1816 by Robert Stirling,

the engine without a valve is the sim-

plest heat engine. A ﬁxed amount of

gas, typically hydrogen, is used as the

working medium. It can be driven

by a single heat source, with an eﬃ-

ciency close to the Carnot limit. It is

the ideal heat engine driven by con-

centrated sunlight.

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246  Solar Thermal Energy

Figure 11.22 Working principle of Stirling engine. (A) The gas is cold, the displacer is in the

innermost position, and the piston pushes into the cylinder. (B) The hot source heats up the gas.

position and the gas is cooled by the surrounding.

to-grid energy conversion eﬃciency of 31.25%, the highest of any solar-to-electricity

conversions, recorded on a perfectly clear and cold New Mexico winter day [48].

The Stirling engine was invented in 1816 by Robert Stirling, a priest studying me-

chanical engineering as a hobby who built the ﬁrst such engine in his home machine

shop. A schematic diagram of the Stirling engine is shown in Fig. 11.21. This engine

is diﬀerent from the two popular heat engines, the steam engine and the internal com-

bustion engine. Similar to the steam engine, it uses an external heat source. However,

instead of constantly evaporating water into steam and then discarding it, the Stirling

engine uses a ﬁxed body of gas in a closed cylinder. It is the simplest heat engine:

There are no valves. It can approach the Carnot eﬃciency. It can be operated by any

type of single heat source; thus concentrated sunlight is perfect. In Fig. 11.21, the

heat is provided by burning wood or coal. A piston is tightly ﬁt in a cylinder which

drives a ﬂywheel through a crankshaft. A gas displacer, loosely ﬁt in the cylinder, is

driven by the crankshaft with a phase shift in the motion of the piston. The working

media, the gas, is always contained in the cylinder. The details of its working cycles are

shown in Fig. 11.22. During step (A), the gas is cold, the displacer is in the innermost

position, and the piston pushes into the cylinder. Then the hot source heats up the

gas in step (B). During step (C), the expanding hot gas pushes the piston outward.

Finally, step (D), the displacer returns to the innermost position, and the gas is cooled

by the surrounding.

In order to achieve eﬀective operation, the gas must have high thermal conductivity.

The most frequently used gas is hydrogen. However, the diﬀusion coeﬃcient of hydrogen

in steel is very high. Therefore, either a special material with low diﬀusion coeﬃcient for

hydrogen is used to construct the cylinder or the hydrogen is periodically supplemented.

The Stirling engine is not suitable for vehicle applications because of its large volume

and the requirement of an eﬀective cooling mechanism.

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11.4 Solar Thermal Power Systems  247

11.4.4 Integrated Solar Combined Cycle

In previous sections, solar thermal power systems designed for stand-alone power sta-

tions were described. Because of the intermittent nature of solar energy, extensive

energy storage facilities are required, such as using sensible heat and latent heat dur-

ing phase transition as well as batteries. However, large-scale energy storage is very

expensive. Moreover, pure solar power plants often require special equipment, such as

the Stirling engine, DC–AC converters, or molten-salt heat transfer facilities.

A recent trend in the exploration of solar energy is not to compete with traditional

power stations, such as fossil-fuel-based boilers or nuclear-powered boilers, but to sup-

plement them. It is called Integrated Solar Combined Cycle ISCC. In this model, the

solar ﬁeld is an addition to a traditional power plant, as shown in Fig. 11.23. The

solar ﬁeld can take water as the input, heat it to generate superheated steam, then

supply the steam either at the maximum-temperature point (high-temperature opera-

tion), or a point with less than maximum temperature (medium- or low- temperature

operation); see Fig. 11.23.

A signiﬁcant advantage of the ISCC is that the solar energy component is built

using a traditional, well-developed power generation technology with limited additional

investment while taking full advantage of the inclusion of solar energy. It is even

possible to retroﬁt an existing fossil fuel power plant by adding a solar component

without interrupting the operation of the power plant. Thus, the ISCC is a winning

Figure 11.23 Schematics of integrated solar combined cycle. By adding a solar ﬁeld to a

traditional steam-turbine-based power station, a low-cost utilization of solar energy is created. In

a high-temperature ISCC implementation, the sunlight is utilized to generate superheated steam,

indicated by dashed lines in the Figure. (1) By feeding water to the solar ﬁeld, superheated steam is

supplied to the turbines either at a high temperature, through path (2), or at a lower than maximum

tamperature, through path (3). After a report from Bechtel [84].

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248  Solar Thermal Energy

combination for both traditional and solar plants in terms of reduced capital cost and

continuous power supply. Another advantage of the ISCC is that it produces energy

when most needed during peak times of the day and the year where air conditioners

are run at full capacity. Therefore, by adding a solar ﬁeld, the nominal capacity of the

power plant can be substantially reduced for the same service in a given area. Since

2008, eight such power stations have been constructed, notably in northern Africa. The

percentage of solar power ranges from 5 to 20%.

11.4.5 Linear Fresnel Reﬂector (LFR)

The parabolic trough concentrator uses large, individually designed mirrors to form a

huge mechanical system. The cost of the mechanical system is high, and it is vulnerable

to wind damage. Cleaning and repair are also costly. A solution to this problem is

the linear Fresnel concentrator (LFR), see Fig. 11.24. Instead of using large, custom-

designed mirrors, long and narrow ﬂat mirrors are used. These mirrors are mounted

on a one-dimensional axis, and turn individually to reﬂect the sunlight onto the small

linear concave mirror and then concentrated it onto the evacuated-tube absorber. The

heat transfer ﬂuid is usually water, to generate superheated steam up to about 365

◦

directly to run the turbine. In such an arrangement, the mirrors can be placed very

close to the ground. There are several advantages. First, there are many linear receivers

in the system. If they are close enough, then individual reﬂectors have the option of

directing reﬂected solar radiation to at least two receivers. This additional variable in

reﬂector orientation allows much more densely packed arrays and lower absorber tower

heights, because patterns of alternating reﬂector orientation can be set up such that

Figure 11.24 Linear Fresnel reﬂector system. An array of linear (narrow and long) mirrors

mounted on an axis which can be rotated to align to the sunlight individually. The sunlight is further

concentrated by a small linear concave mirror to the evacuated-tube absorber.

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11.4 Solar Thermal Power Systems  249

Figure 11.25 Linear Fresnel reﬂector system: a model. A model at the Himin Solar Energy

Museum, D´ezh¯ou, China. Photo taken by the author. Courtesy of Himin Solar Energy Group.

closely packed reﬂectors can be positioned without mutual blocking. The shadow eﬀect

can be reduced, as shown in Fig. 11.24, and the eﬃciency of land usage is improved.

The mechanical structures are close to the ground, and the cost is substantially reduced.

Furthermore, cleaning and repair become much simpler than for the parabolic trough

system. Figure 11.25 is a model of a LFR system in Himin Solar Energy Museum,

D´ezh¯ou, China.

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250  Solar Thermal Energy

Problems

11.1. Based on the algebraic deﬁnition of a parabola, y = x

/4f, prove that a parabola

is the locus of equal distance from a focus to a directrix. See Fig. 11.26.

11.2. Using the geometric deﬁnition in Problem 11.1, prove that all light rays parallel

to the y-axis would be reﬂected by a parabolic surface to the focus. See Fig. 11.26.

11.3. A vacuum solar heat collector has an internal diameter of 45 mm and a length

of 1800 mm and is ﬁlled with water of 20

◦

C. The axis of the tube is perpendicular to

the sunlight. Assuming the eﬃciency is 90%, with full sunlight, how long does it take

to boil the water inside the tube?

11.4. A solar hot water system consists of 24 vacuum tubes of outer diameter 58 mm

and inner diameter of 47 mm, and length 1800 mm. It is connected to an insulated

tank containing 200 liters of water. In a sunny day, with the sunlight perpendicular to

the plane of vacuum tubes, and the eﬃciency is 90%, how long it takes to heat up the

water by 20

◦

C? (See Figure 1.32, Figure 11.11, and Figure 11.17).

Figure 11.26 Focusing property of a parabola.

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11.5. Kyoto box is a simple solar oven used extensively in Africa; see Fig. 11.27.

Assuming a square box with four reﬂectors of L = 75 cm, on a sunny day with direct

sunlight from the zenith, what is the optimum angle θ? What is the total solar power

received by the box? If the eﬃciency is 70%, how long it will take to heat 1 gallon

(3.785 liters) of water from 25

◦

C to the boiling point?

Figure 11.27 Kyoto box. A simple solar oven used extensively in Africa.

Problems  251

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Chapter 12

Energy Storage

Because solar energy is intermittent, if it is the main supply, energy storage is a ne-

cessity. In general, there are two types of energy storage: utility-scale massive energy

storage and the application-related, distributed energy storage. For utility-scale en-

ergy storage, the most eﬀective method is using a reversible hydroelectric plant, which

stores mechanical energy as potential energy of water in a high-level reservoir. We

have discussed this in Section 1.3.1. The two most promising methods of distributed

energy storage are thermal energy storage and rechargeable batteries. Especially for

transportation (automobiles and small vessels) the rechargeable batteries will become

the dominating energy storage device. Compressed air and ﬂywheels are also important

but will not be as widespread as thermal and rechargeable batteries. In this chapter,

we will focus on thermal energy storage and rechargeable batteries.

12.1 Sensible Heat Energy Storage

Storage of energy as heat content of matter is inexpensive and easy to implement. It

can be applied to space heating and cooling as well as for power generation. Two

types of thermal energy are used: sensible thermal energy, essentially proportional to

temperature diﬀerence, and phase transition thermal energy, such as the latent heat

during freezing and melting, which could maintain a ﬁxed temperature with energy

content much greater than sensible thermal energy. Phase-change materials (PCM)

are well suited for the storage of solar energy.

Sensible heat energy storage utilizes the heat capacity and the change in tempera-

ture of the material during the process of charging or discharging — the temperature

of the storage material rises when energy is absorbed and drops when energy is with-

drawn. One of the most attractive features of sensible heat storage systems is that

charging and discharging operations can be expected to be completely reversible for an

unlimited number of cycles, that is, over the lifespan of the storage.

In sensible heat energy storage, the thermodynamic process of the material is almost

always isobaric, or under constant pressure, typically atmospheric pressure. A solid or

liquid is usually used. The speciﬁc heat of gas is too low and not practical for thermal

energy storage. The heat Q delivered by the material from initial temperature T

253

Physics of Solar Energy C. Julian Chen