Solar Thermal System

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The solar thermal power system collects the thermal energy in solar radiation and uses at high or low temperature. The low temperature applications include water and space heating for commercial and residential buildings.1 Producing electricity using the steam-turbine-driven electrical generator is a high temperature application discussed in this chapter.

The technology of generating electrical power using the solar thermal energy has been demonstrated at commercial scale. The research and development funding have primarily come from the government, with active participation of some electric utility companies.

Figure 9-1 is a schematic of a large-scale solar thermal power station developed, designed, built, tested, and operated with the U.S. Department of Energy funding. In this plant, the solar energy is collected by thousands of sun-tracking mirrors, called heliostats, that reflect the sun's energy to a single receiver atop a centrally located tower. The enormous amount of energy focused on the receiver is used to generate high temperature to melt a salt. The hot molten salt is stored in a storage tank, and is used, when needed, to generate steam and drive the turbine generator. After generating the steam, the used molten salt at low temperature is returned to the cold salt storage tank. From here it is pumped to the receiver tower to get heated again for the next thermal cycle. The usable energy extracted during such a thermal cycle depends on the working temperatures. The maximum ther-modynamic conversion efficiency that can be theoretically achieved with the hot side temperature Thot and the cold side temperature Tcold is given by the Carnot cycle efficiency, which is as follows:

Thot n carnot where the temperatures are in absolute scale. The higher the hot side working temperature and lower the cold side exhaust temperature, the higher the plant efficiency of converting the captured solar energy into electricity. The hot side temperature Thot , however, is limited by the properties of the working medium. The cold side temperature Tcold is largely determined by the cooling method and the environment available to dissipate the exhaust heat.

Molten Salt Power Tower System Schematic

FIGURE 9-1

Solar thermal power plant schematic for generating electricity.

FIGURE 9-1

Solar thermal power plant schematic for generating electricity.

A major benefit of this scheme is that it incorporates the thermal energy storage for duration in hours with no degradation in performance, or longer with some degradation. This feature makes the technology capable of producing high-value electricity for meeting peak demands. Moreover, compared to the solar photovoltaic, the solar thermal system is economical, as it eliminates the costly semiconductor cells.

9.1 Energy Collection

The solar thermal energy is collected by concentrators. Three alternative configurations of the concentrators are shown in Figure 9-2. Their main features and applications are as follows:

9.1.1 Parabolic Trough

The parabolic trough system is by far the most commercially matured of the three technologies. It focuses the sunlight on a glass-encapsulated tube running along the focal line of the collector. The tube carries heat absorbing liquid, usually oil, which in turn, heats water to generate steam. More than 350 MW of parabolic trough capacity is operating in the California Mojave

FIGURE 9-2

Alternative thermal energy collection technologies.

FIGURE 9-2

Alternative thermal energy collection technologies.

Desert and is connected to the Southern California Edison's utility grid. This is more than 90 percent of the world's solar thermal capacity at present.

9.1.2 Central Receiver

In the central receiver system, an array of field mirrors focus the sunlight on the central receiver mounted on a tower. To focus the sun on the central receiver at all times, each heliostat is mounted on the dual-axis suntracker to seek position in the sky that is midway between the receiver and the sun. Compared to the parabolic trough, this technology produces higher concentration, and hence, higher temperature working medium, usually a salt. Consequently, it yields higher Carnot efficiency, and is well suited for utility scale power plants in tens or hundreds of megawatt capacity.

9.1.3 Parabolic Dish

The parabolic dish tracks the sun to focus heat, which drives a sterling heat engine-generator unit. This technology has applications in relatively small capacity (tens of kW) due the size of available engines and wind loads on

TABLE 9-1

Comparison of Alternative Solar Thermal Power System Technologies

TABLE 9-1

Comparison of Alternative Solar Thermal Power System Technologies

Solar Concentration

Operating Temperature

Thermodynamic

Technology

(x Suns)

on the Hot Side

Cycle Efficiency

Parabolic Trough

100

300-500°C

Low

Receiver

Central Receiver

1000

500-1000°C

Moderate

Power Tower

Dish Receiver with

3000

800-1200°C

High

Engine

the dish collectors. Because of their small size, it is more modular than other solar thermal power systems, and can be assembled in a few hundred kW to few MW capacities. This technology is particularly attractive for small stand-alone remote applications.

The three alternative solar thermal technologies are compared in Table 9-1.

9.2 Solar II Power Plant

The central receiver technology with power tower is getting new development thrust in the U.S.A. as having a higher potential of generating lower cost electricity at large scale. An experimental 10 MWe power plant using this technology has been built and commissioned in 1996 by the Department of Energy in partnership with the Solar II Consortium of private investors led by the Southern California Edison, the second largest electrical utility company in the U.S.A. It is connected to the grid, and has enough capacity to power 10,000 homes. The plant is designed to operate commercially for 25 to 30 years. Figure 9-3 is the site photograph of this plant located east of Barstow, California. It uses some components of Solar I plant, which was built and operated at the site using the central receiver power tower technology. The Solar I plant, however, generated steam directly to drive the generator without the thermal storage feature of the Solar II plant.

Solar II central receiver (Figure 9-4) was developed by the Sandia National Laboratory. It raises the salt temperature to 1,050°F. The most important feature of the Solar II design is its innovative energy collection and the storage system. It uses a salt that has excellent heat retention and heat transfer properties. The heated salt can be used immediately to generate steam and electric power. Or, it can be stored for use during cloudy periods or after the sun goes down to meet the evening load demand on the utility grid. Because of this unique energy storage feature, the power generation is decoupled from the energy collection. For electrical utility, this storage capability is crucial in that the energy is collected when available, and is used to generate high-value electricity when it is most needed. The salt selected by

Tesla Parabolic Dish Solar Steam Boiler

FIGURE 9-3

Solar II plant site view. (Source: U.S. Department of Energy.)

FIGURE 9-3

Solar II plant site view. (Source: U.S. Department of Energy.)

the Sandia laboratory for this plant is sodium and potassium nitrate which works as a single phase liquid, and is colorless and odorless. In addition to having the needed thermal properties up to the operating temperature of 1,050°F, it is inexpensive and safe.

Tables 9-2 and 9-3 give the technical design features of the experimental Solar II power plant. The operating experience to date indicates the overall plant capacity factor of 20 percent, and the overall thermal to electrical conversion efficiency of 16 percent. It is estimated that 23 percent overall efficiency can be achieved in a commercial plant design using this technology.

9.3 Synchronous Generator

The electromechanical energy conversion in the solar thermal power system is accomplished by the synchronous machine, which runs at a constant speed to produce 60 Hz electricity. This power is then directly used to meet the local loads, and/or to feed the utility grid lines.

The electromagnetic features of the synchronous machine are shown in Figure 9-5. The stator is made of conductors placed in slots of magnetic iron

Magnetic Field Make Free Energy

FIGURE 9-4

Experimental 1,050°F thermal receiver tower for Solar II power plant. (Source: DOE/Sandia National Laboratory.)

FIGURE 9-4

Experimental 1,050°F thermal receiver tower for Solar II power plant. (Source: DOE/Sandia National Laboratory.)

laminations. The stator conductors are connected in three phase coils. The rotor consists of magnetic poles created by the field coils carrying direct current. The rotor is driven by steam turbine to create a rotating magnetic field. Because of this rotation, the rotor field coils use slip rings and carbon brushes to supply DC power from a stationary source.

The stator conductors are wound in three groups, connected in three-phase configuration. Under the rotating magnetic field of the rotor, the three phase coils generate AC voltages that are 120 electrical degrees out of phase with each other. If the electromagnetic structure of the machine has p pole pairs, and it is required to generate electricity at frequency f, then the rotor must rotate at N revolution per minute given by the following:

The synchronous machine must operate at this constant speed to generate power at the specified frequency. In a stand-alone solar thermal system, small speed variations could be tolerated within the frequency tolerance band of the AC system. If the generator is connected to the grid, it must be synchronous with the grid frequency, and must operate exactly at the grid frequency at all times. Once synchronized, such a machine has inherent tendency to remain

TABLE 9-2

Solar II Design Features

Site

Thermal Storage System

• Mojave Desert in California

• 1,949 feet above sea level

• 7.5 kWh/m2-day annual average daily insolation

Tower

• Reused from Solar I plant

• 277 feet to top of the receiver

Heliostats

91 percent reflectivity

93 percent reflectivity

• 81,000 m2 total reflective surface

• Can operate in winds up to 35 mph

Receiver

New for Solar II plant

Supplier Rockwell

42.2 MW thermal power rating

Average flux 429 suns (429 kW/m2)

Peak flux 800 suns

24 panels, 32 tubes per panel

20 feet tall and 16.6 feet diameter

0.8125 inch tube OD

0.049 inch tube wall thickness

Tubes 316H stainless steel

• Supplier Pitt Des Moines

• Two new 231,000 gallon storage tanks, 38 ft ID

• Cold tank carbon steel, 25.8 ft high, 9 inch insulation

• Hot tank 304 stainless steel, 27.5 ft high,

18 inches insulation

• 3 hours of storage at rated turbine output

Nitrate salt — Chilean Nitrate

• Melting temperature 430°F

• Decomposing temperature 1,100°F

• Energy storage density two thirds of water

• Density two times that of water

• Salt inventory 3.3 million pounds

Steam Generator

• Supplier ABB Lummus

• New salt-in-shell superheater

• New slat-in-tube kettle boiler

• New salt-in-shell preheater

Turbine-Generator

• Supplier General Electric Company

• Refurbished from Solar I plant

(Source: U.S. Department of Energy and Southern California Edison Company.2)

TABLE 9-3

Solar II Operating Features

Thermodynamic Cycle

Electrical Power Generator

Hot salt temperature 1,050°F

Cold salt temperature 550°F

Steam temperature 1,000°F

Steam pressure 1,450 psi

Receiver salt flow rate 800,000 lbs/hour

Steam generator flow rate 660,000 lbs/hour

Capacity 10 MWe Capacity factor 20% Overall solar-electric efficiency 16% Cost of conversion from Solar I $40 M

(Source: U.S. Department of Energy and Southern California Edison Company.)

Magnetic Poles Synchronous Generator

FIGURE 9-5

Cross section view of the synchronous generator.

FIGURE 9-5

Cross section view of the synchronous generator.

in synchronism. However, a large sudden disturbance such as a step load can force the machine out of the synchronism, as discussed in Section 9.3.4.

9.3.1 Equivalent Electrical Circuit

The equivalent electricity circuit of the synchronous machine can be represented by a source of alternating voltage E and an internal series resistance Rs and reactance Xs representing the stator winding. The resistance, being much smaller than the reactance, can be ignored to reduce the equivalent circuit to a simple form shown in Figure 9-6. If the machine is supplying the load current I lagging the terminal voltage V by phase angle it must internally generate the voltage E, which is the phasor sum of the terminal voltage and the internal voltage drop IXs. The phase angle between the V and E is called the power angle 8. At zero power output, load current is zero and so is the IXs vector, making V and E in phase having zero power angle. Physically, the power angle represents the angle by which the rotor position lags the stator-induced rotating magnetic field. The output power can be

FIGURE 9-6

Equivalent electrical circuit and phasor diagram of synchronous machine.

FIGURE 9-6

Equivalent electrical circuit and phasor diagram of synchronous machine.

increased by increasing the power angle up to a certain limit, beyond which the rotor would no longer follow the stator field and will step out of the synchronous mode of operation. In the nonsynchronous mode, it cannot produce steady power.

9.3.2 Excitation Methods

The synchronous machine excitation system must be designed to produce the required magnetic field which is controllable to control the voltage and the reactive power of the system. In modern high power machines, Xs can be around 1.5 units of the base impedance of the machine. With reactance of this order, the phasor diagram of Figure 9-6 can show that the rotor filed excitation required at rated load (100 percent load at 0.8 lagging power factor) is more than twice that at no load with the same terminal voltage. The excitation system has the corresponding current and voltage ratings, with capability of varying the voltage over a wide range of 1 to 3 or even more without undue saturation in the magnetic circuit. The excitation power, primarily to overcome the rotor winding I2R loss, ranges from V2 to 1 percent of the generator rating. Most excitation systems operate at 200 to 1,000 Vdc.

For large machines, three types of excitation systems — DC, AC and static — are possible. In the DC system, a suitably designed DC generator supplies the main field winding excitation through conventional slip rings and brushes. Due to low reliability and high maintenance requirement, the conventional DC machine is seldom used in the synchronous machine excitation system.

Most utility scale generators use the AC excitation system shown in Figure 9-7. The main exciter is excited by a pilot exciter. The AC output of a permanent magnet pilot exciter is converted into DC by a floor standing rectifier and supplied to the main exciter through slip rings. The main exciter's AC output is converted into DC by means of phase controlled rectifiers, whose firing angle is changed in response to the terminal voltage variations. After filtering the ripples, this direct current is fed to the synchronous generator field winding.

An alternative scheme is the static excitation, as opposed to the dynamic excitation described in the preceding paragraph. In the static excitation

Excitation System Power Station

FIGURE 9-7

AC excitation system for synchronous generator.

FIGURE 9-7

AC excitation system for synchronous generator.

scheme, the controlled DC voltage is obtained from a suitable stationary AC source rectified and filtered. The DC voltage is then fed to the main field winding through slip rings. This excitation scheme has a fast dynamic response and is more reliable because it has no rotating exciters.

The excitation control system modeling for analytical studies must be carefully done as it forms a multiple feedback control system that can become unstable. The IEEE has developed industry standards for modeling the excitation systems. The model enters nonlinear ly due to magnetic saturation present in all practical designs. The stability can be improved by supplementing the main control signal by auxiliary signals such as speed and power.

9.3.3 Electrical Power Output

The electrical power output of the synchronous machine is as follows:

Using the phasor diagram of Figure 9-6, the current can be expressed as follows:

jXs jXs jXs

The real part of this current is Ireal =

This part, when multiplied with the terminal voltage V gives the output power:

v VE

Salient Pole Round Rotor Pmsm

FIGURE 9-8

Power versus power angle of round rotor and salient pole synchronous machine.

FIGURE 9-8

Power versus power angle of round rotor and salient pole synchronous machine.

The output power versus the power angle is a sine curve shown by the solid line in Figure 9-8, having the maximum value at 8 = 90°. The maximum power that can be generated by the machine is therefore:

max Xs

Some synchronous machine rotors have magnetic saliency in the pole structure. The saliency produces a small reluctance torque superimposed on the main torque, modifying the power angle curve as shown by the dotted line.

The electromechanical torque required at the shaft to produce this power is the power divided by the angular velocity of the rotor. That is as follows:

VE toXs sin 8

The torque also has the maximum limit corresponding to the maximum power limit, and is given as follows:

max rnXs

9.3.4 Transient Stability Limit

The maximum power limit just described is called the steady state stability limit. Any loading beyond this value will cause the rotor to lose synchronism, and hence, the power generation capability. The steady state limit must not

Steady State Stability

FIGURE 9-9

Load step transient and stability limit of synchronous machine.

FIGURE 9-9

Load step transient and stability limit of synchronous machine.

be exceeded under any condition, including those that can be encountered during transients. For example, if a sudden load step is applied to the machine initially operating in a steady state at load power angle (Figure 9-9), the rotor power angle would increase from to 82 corresponding to the new load that it must supply. This takes some time depending on the electromechanical inertia of the machine. No matter how short or long it takes, the rotor inertia and the electromagnetic restraining torque will set the rotor in a mass-spring type of oscillatory mode, swinging the rotor power angle beyond its new steady state value. If the power angle exceeds 90° during this swing, the machine stability and the power generation are lost. For this reason, the machine can be loaded only to the extent that even under the worst-case step load, planned or accidental, or during all possible faults, the power angle swing will remain below 90° with sufficient margin. This limit on loading the machine is called the transient stability limit.

Equation 9-5 shows that the stability limit at given voltages can be increased by designing the machine with low synchronous reactance Xs, which is largely made of the stator armature reaction component.

9.4 Commercial Power Plants

The commercial power plants using the solar thermal system are being explored in a few hundred MWe capacity. Based on the Solar II power plant operating experience, the design studies made by the National Renewable Energy Laboratory for the U.S. Department of Energy have estimated the

TABLE 9-4

Comparison of 10 MWe Solar II and 100 MWe Prototype Design

TABLE 9-4

Comparison of 10 MWe Solar II and 100 MWe Prototype Design

Performance Parameter

Solar II Plant 10 MWe

Commercial Plant 100 MWe

Mirror reflectivity

90%

94%

Field efficiency

73%

73%

Mirror cleanliness

95%

95%

Receiver efficiency

87%

87%

Storage efficiency

99%

99%

Electromechanical conversion efficiency of generator

34%

43%

Auxiliary components efficiency

90%

93%

Overall solar-to-electric conversion efficiency

16%

23%

(Source: U.S. Department of Energy and Southern California Edison Company.)

(Source: U.S. Department of Energy and Southern California Edison Company.)

performance parameters that are achievable for a 100 MWe commercial plant. Table 9-4 summarizes these estimates and compares with those achieved in the 10 MWe experimental Solar II power plant. The 100 MWe prototype design shows that the overall solar to electric efficiency of 23 percent can be achieved in a commercial plant using the existing technology. For comparison, the conventional coal thermal plants typically operate at 40 percent conversion efficiency, and the photovoltaic power systems have the overall solar-to-electricity conversion efficiency of 8 to 10 percent with amorphous silicon and 15 to 20 percent with crystalline silicon technologies.

Major conclusions of the studies to date are the following:

1. First plants as large as 100-200 MWe are possible to design and build based on the demonstrated technology to date. Future plants could be larger.

2. The plant capacity factors up to 65 percent are possible, including outage.

3. Fifteen percent annual average solar-to-electric conversion efficiency is achievable.

4. The energy storage feature of the technology makes possible to meet the peak demand on the utility lines.

5. Leveled energy cost is estimated to be 6 to 7 cents per kWh.

6. A 100 MWe plant with a capacity factor of 40 percent requires 1.5 square miles of land.

7. The capital cost of $2,000 per kWe capacity for first few commercial plants and less for future plants.

8. A comparable combined cycle gas turbine plant would cost $1,000 kWe, which includes no fuel cost.

9. Solar-fossil hybrids are the next step in development of this technology.

Compared to the pv and wind power, the solar thermal power technology is less modular. Its economical size is estimated to be in the 100 to 300 MWe range. The cost studies at the National Renewable Energy Laboratory have shown that a commercially designed utility-scale power plant using the central receiver power tower can produce electricity at a cost of 6 to 11 cents per kWh.

In other countries, the low temperature solar thermal power finds growing applications. The high temperature applications, however, have yet to develop on a large scale. An integrated combined cycle 40 MWe solar thermal with 100 MWe gas turbine power plant is proposed at Jodhpur in Rajasthan, India. According to the Indian Renewable Energy Development Agency, India has the target to install 150 MWe solar thermal and photovoltaic capacity by the year 2002, and 1,500 MWe by 2012.

References

1. Mancini, T. 1994. "Solar thermal power today and tomorrow," Mechanical Engineering, publication of the Institution of Mechanical Engineers, London, August 1994.

2. Southern California Edison Company. 1995. Technology Report No. 14, Irwindale, CA, Fall 1995.

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