16                                       

Solar Thermal

      Projects

 

Note to Reader: This chapter is directly excerpted from ‘Solar Energy System and Design’ by W.B. Stine and R.W. Harrigan, published by John Wiley 1985.  It represents the state-of-the-art of solar thermal projects at the end of the 1970’s and has not been revised.

 

Almost 25 percent of the energy resources used by the United States today are used by industry. The need for thermal energy by industry varies all the way from low-quality (low-temperature) thermal energy used for space heating to heat at extremely high quality (temperature) required for metals processing.  An additional 30% of our energy resources are converted into electricity. This highest-quality intermediate form of energy is used for any number of tasks, including turning machinery and providing light and heat.

 

There are a number of projects in operation today that collect solar energy as heat and then apply this heat to industrial processes or convert it to electrical energy. Most of these were initiated by and received total support from the U.S.  Government as a means to advance solar energy technology.  Therefore, much information on the design, operation, and problems of these systems is available to the solar designer.  In this chapter we summarize the design parameters of these systems and provide a detailed examination of representative examples.

16.1            Industrial Demands

Process steam is used as a source of heat for many industrial processes.  Saturated steam is used because it provides large amounts of heat at a constant temperature (the saturation temperature). The temperature-pressure relationship for saturated steam is shown in Figure 16.1.  This shows, for example, that if an industrial plant had a 1.0 MPa (130 psig) steam line, the steam would have a temperature of 180ºC (356ºF).

 

Figure 16.1  Temperature of saturated steam.  Most industrial process steam is saturated and meets these conditions.

 

Industry requires process heat over a wide temperature range.  The distribution of the temperature at which this heat is used is shown in Figure 16.2.  Also shown in this figure is a second distribution that includes the amount of energy required to preheat the steam from ambient temperature.  It can be seen that one-half of the thermal energy needed by industry could be supplied by systems providing heat at temperatures less than 250ºC (482ºF). In addition to process steam, some industrial operations use process hot water (e.g., for sterilizing and degreasing) and hot air (e.g., for drying products).

 

 

Figure 16.2  Temperature requirements of industrial process heat.  The lower curve is the temperature at which heat is supplied.  The upper curve includes the energy required for preheating from ambient temperature (anonymous, 1977).

Some industrial installations have found it economically feasible to produce electrical energy on size and use the “waste” heat rejected by this process, to provide low-grade process heat for the purposes discussed above.  This concept called “total energy” or “cogeneration” is especially relevant to industrial processes where the demand for significant amounts of electrical energy and low-grade thermal energy are used in close proximity.  The solar total energy system at Shenandoah, Georgia is a prime example of this dual energy output concept, using solar energy as the energy source.

 16.2  Survey of Solar Thermal Systems

16.2.1  Process Heat Systems

Many industrial processes requiring thermal energy have been studied for their potential as economic applications of solar thermal energy systems. Many of these design studies have come to fruition in the form of operational solar energy systems, and these are summarized in Table 16.1.  The low-temperature applications (temperature below 100ºC) are mostly industrial drying operations, a number of which are associated with agriculture.

 

Table 16.1.  Systems Providing Industrial Process Heat

 

 

Most of the collectors used in the low-temperature applications are fixed-aperture, non-concentrating (i.e., flat-plate) collectors.  Exceptions include Gilroy Foods, which uses evacuated-tube collectors with slight concentration, and the La Cour Kiln project, which includes a north-facing flat reflecting surface for irradiance enhancement.  Other exceptions include York Building Products, where the SLATS concentrating collector (discussed in Chapter 9) and the Campbell Soup system, where flat-plate collectors are used as preheaters for parabolic trough concentrators.

 

Water is the collector heat-transfer fluid for most of the low-temperature systems. For applications where the demand is for hot air, two systems use air as the collector heat-transfer fluid, and the other two use water in the collector field, thereby requiring the use of a heat exchanger to produce the hot air for the demand.

 

Thermal storage is provided in many of the low-temperature systems to meet the mismatch between insolation availability and the demand. Usually this is in the form of an insulated hot-water storage tank; however, in the Lamanuzzi and Pantaleo project, heat is stored in a large bin of riverbed granite pebbles.

 

The mid-temperature projects listed here range in demand temperatures from 113ºC (235ºF) process hot water (pressurized) for tractor parts degreasing to 215ºC/2.17-MPa (419ºF/300-psig) process steam, which heats oil for potato frying.  All the collectors used in this group are concentrating collectors with single-axis tracking apertures.  The singular application of a point focus concentrator for process heat is in the Capitol Concrete Project. This is not the usual dish-type concentrator, but a group of curved reflecting slats that reflect light into a cavity receiver.

 

For these mid-temperature systems, the collector fluid chosen has been either pressurized water or a heat-transfer oil. Since the collector fluid should not boil in order to provide maximum heat transfer, systems employing water are pressurized above the saturation pressure of water at the collector's operating temperature (see Figure 16.1).  For example, the Ore-Ida collector field is maintained at a pressure of 4.25 MPa (600 psig) in order to prevent boiling at its nominal operating temperature of 247ºC (477ºF).  Systems using heat-transfer oils may be operated at a low pressure because of the low vapor pressure of the oils selected.

 

Systems using oil as the heat-transfer fluid are typically installed on the ground.  One advantage of using water as the collector heat-transfer fluid is that the system can be installed on a rooftop without posing a fire hazard. This capability is taken advantage of by three of the mid-temperature systems. Although none of the low-temperature systems use a flammable heat-transfer fluid, some chose to place the collectors on the ground for economic reasons.

Few of the mid-temperature process heat systems include a means of storing the collected thermal energy other than small “buffer storages” required for smooth system control during short-term transients. The one exception is the Johnson & Johnson system, where a large flash steam tank provides some energy storage.  Details of the design and operating characteristics of these systems may be found in reports by Harley and Stine (1983) and (1984).

 

16.2.2  Solar Thermal Power Systems

 

A number of systems have been built that convert solar energy collected as heat into mechanical or electrical energy.  These use some type of thermodynamic power conversion cycle.  Early interest in these systems was for agricultural irrigation; however, more recent systems include an industrial total energy system and systems that generate only electricity. Table 16.2 summarizes the basic design features of these solar thermal power systems.

 

 

Rankine cycles using organic working fluids were chosen for the early projects using parabolic trough concentrators. This was due to the small size of the cycle and low operating temperatures. More recent systems have capitalized on the known technology of steam Rankine cycles.

 

Because of the increase of power conversion cycle efficiency with heat-supply temperature, solar collection concepts that operate efficiently at high temperatures such as parabolic dishes and central receivers, are considered as prime candidates for use in future solar thermal power systems. This will happen if their overall cost per unit of electricity output can be made competitive with lower temperature, less efficient, but less costly schemes. One project outside the United States is also listed in Table 16.2.

 

16.2          Description of Representative Systems

Four of the systems listed in Tables 16.1 and 16.2 have been selected for an in-depth description since their designs represent typical design for that type of system.  These include the following:

 

·         Johnson & Johnson – a typical industrial process heat application using parabolic troughs.

 

·         Coolidge Irrigation Project – electrical power production using a medium-temperature parabolic trough field.

 

·         Shenandoah Solar Total Energy Project – a total energy system using medium-temperature parabolic dishes.

 

·         Solar One – electrical power production using a high-temperature central receiver system.

16.2.1    Johnson & Johnson Solar Process Heat System

The Johnson & Johnson plant located in Sherman, Texas, uses solar energy collected by a field of parabolic trough collectors to produce steam used in their gauze bleaching process. The solar portion of the system is shown in Figure 16.3.  Demand for steam is periodic, occurring four times over the 24-hour workday.

 

Figure 16.3  The Johnson & Johnson solar energy system at Sherman, Texas.  Courtesy of Sandia National Laboratories.

 

Collector Field: The collector field consists of 1070 m² (11,520 ft²) of parabolic trough collectors using water as a heat-transfer fluid.  The tracking axis orientation is northeast – southwest and was chosen because of the orientation of the existing plant facility.

 

The basic collector module is 1.83 m (6 ft) ´ 3.05 m (10 ft), and one drive string consists of eight modules. There are six drive strings connected in series to form one fluid delta-T string with the total field consisting of four delta-T strings connected in parallel. Water is pumped through these fluid loops at a pressure of 2.14 MPa (310 psia) at a constant flow rate of 14,000 kg/h (31,000 1b/h). Figure 16.4 shows the layout of the collector field.

 

Figure 16.4  Collector field layout for the Johnson & Johnson solar system showing the fluid piping.

System:  Pressurized water from the collector field is pumped through a throttling valve, reducing its pressure slightly, and then into the flash boiler-storage tank.  Water is pumped from the bottom of this tank back to the collector field, where it receives additional heating.  No attempt is made to optimize thermal stratification in the flash boiler – storage tank. 

 

When process steam is required, the pressure of the flash boiler is reduced slightly to the saturation pressure of the water in storage.  The water in the tank then boils and is passed through a throttle valve into the plant steam lines at 175ºC/0.86 MPa (345ºF/125 psia).  The overall system is depicted schematically in Figure 16.5.

Energy Flows: The thermodynamic properties of the water at different points in the system are shown in Table 16.3 (storage tank is charged to 50 percent of its capacity).  The stations at which the properties are described are noted in Figure 16.5.

Figure 16.5  Flow diagram for the Johnson & Johnson solar process steam system.

The thermodynamic processes that take place are shown in Figure 16.6 on temperature-entropy coordinates.  There are two basic processes associated with this system.  Process 1-2-3-4 is the heating of the water in storage by the solar loop, which occurs during the daytime as long as the sun is shining.  The second process, process 1-5, is the flash evaporation process which lasts only a short period of time and occurs nominally four times a day.  The flow of energy for these processes is depicted graphically in Figure 16.7.

Figure 16.6  Processes of the Johnson & Johnson solar process heat system (not to scale).

Figure 16.7  Energy flow diagram for the Johnson & Johnson solar energy system.

 

Specifications:  Table 16.4 presents detailed specifications for this system.

 

Table 16.4. Specifications of the Johnson & Johnson Solar Energy

 

System:

Operator – Johnson & Johnson, Southwestern Surgical Dressing Plant

Location – Sherman, Texas

Demand – process heat used in gauze bleaching process

Process steam 174ºC/ 0.86 MPa (345ºF/ 125 psia)

Flow rate – intermittent with maximum of 726 kg/h (1600 lb/h)

Daily energy use – 9.5 GJ (9 ´ 10⁶ Btu)

Collector Modules:

Type – parabolic trough with glass-covered tubular receiver

Manufacturer – Acurex Corp., Model 3001

Aperture – 1.83 m ´3.05 m (6 ft ´ 10 ft)

Reflective surface – Coilzak polished aluminum

Concentration ratio – 36

Collector Field:

Number of modules – 192

Total aperture area – 1070 m² (11,517 ft²)

Orientation – northeast/southwest

Modules on single-tracking drive – 8

Land use – 40% (4.6-m row spacing)

Collector Field Fluid Flow:

Heat-transfer fluid – pressurized water 2.5 M Pa (360 psia)

Delta-T string – 32 modules in series

Number of delta-T strings – 4

Flow control – constant flow rate

Maximum field outlet temperature – 215ºC (419ºF)

Storage:

Type – flash boiler/pressurized water

Medium – pressurized water

Volume – 19 m³ (5000 gal)

Thermal capacity – 29.3 GJ (27.8 ´ 10⁶ Btu)

Maximum temperature – 215ºC (419ºF)

Minimum temperature – 174ºC (345ºF)

 

16.2.2    Coolidge Solar Irrigation Facility

The system shown in Figure 16.8 is located on the Dalton Cole farm south of Coolidge, Arizona and uses solar energy collected by a field of parabolic trough collectors to produce electricity.  This electrical power is fed into the power distribution grid of Arizona Public Service Company, which is used to operate the irrigation pumps at the site.  Because it is connected to the utility power grid, the coincident availability of solar energy and the demand for irrigation water is not required.  The system was operated experimentally for 3 years by the U.S. Department of Energy to characterize performance and equipment parameters and was deactivated in 1983.  Operating experience of this system has been summarized in Larson (1983).

 

Figure 16.8 The Coolidge solar irrigation facility in Coolidge, AZ. Courtesy of Sandia National Laboratories.

Collector Field: The collector field consists of 2140 m² (23,035 ft²) of parabolic trough collectors with their tracking axis oriented in the north–south direction.  This orientation was chosen to maximize the solar power generating capability in the summer, when the greatest demand for irrigation water exists.  The layout of the collector field is shown in Figure 16.9.

Figure 16.9  Collector field layout of the Coolidge solar irrigation facility showing the fluid piping.

 

As in the Johnson & Johnson system, there are eight 1.83-m (6-ft)-wide ´ 3.05-m (10-ft)-long collector modules connected to a single-tracking motor to form a drive string.  Six of these are connected in series to form a delta-T string.  The total field consists of eight delta-T strings. 

 

The heat-transfer fluid used in this application is a low-vapor-pressure oil, Caloria HT-43, a petroleum distillate marketed by Exxon Corporation.  The flow rate of this fluid is varied so that the field outlet temperature remains at a constant 288ºC (550ºF).

 

System: The system consists of three heat-transfer loops as shown in Figure 16.10.  The first loop takes cooled Caloria HT-43 from the bottom of the storage tank and passes it through the collector field and returns it to the top of the storage tank.  The second, also a HT-43 loop, takes hot oil from the top of the storage tank, circulates it through a vaporizer heat exchanger, and returns it to the bottom of the storage tank or back to the collector field directly.

 

The third loop circulates liquid toluene through the vaporizer heat exchanger, where it is vaporized and slightly superheated to 268ºC/1.03 MPa (515ºF/150 psia).  The toluene then expands through a single-stage impulse turbine.  After leaving the turbine, the low-pressure toluene is passed through a regenerator heat exchanger before being condensed into liquid in the vapor condenser.  A pump raises the pressure of the liquid toluene before it gains heat in the regenerator and then passes into the vaporizer.  The vapor condenser is cooled by water spray over the condensing coils and a fan that provides airflow through the unit.

 

In the photograph of the system (Figure 16.8), the storage tank is the tall cylindrical tank.  The power conversion system is located to the left of that.

Energy Flows. A simplified full power case described below shows the energy flows to and from the system. The major simplifying assumptions are that the pipe pressure and heat losses are negligible and that the solar input provides just enough energy for full power operation.

 

The thermodynamic state properties of the toluene in the power conversion cycle are given in Table 16.5.  The stations at which these properties are given are shown on Figure 16.10.

 

Table 16.5  Operating Fluid Properties for the Coolidge (AZ)
Solar Irrigation Facility

 

 

 

Figure 16.10  Flow diagram for the Coolidge solar irrigation system.

 

Figure 16.11 shows these states on temperature–entropy coordinates.  The cycle diagram is superimposed on the saturation curve for toluene.  Toluene is called a "drying fluid" because the amount of superheat increases as pressure is reduced at constant entropy. Because of this characteristic, it is important to include a regenerator in the cycle. The regenerator transfers heat from the hot vapor leaving the turbine, thereby cooling it until it approaches the temperature of the condenser.

Figure 16.11  Processes of the Coolidge solar irrigation system (not to scale).

 

For a solar input of 970 W/m² normal to the collector aperture, the flow of energy through the system is shown in Figure 16.12. The condition shown is for no energy going into or from the storage. One point to note here is the high gearbox-generator loss of 40 kW or 16.4 percent of the mechanical power generated by the cycle. This is due to the small size of the cycle and the use of a single-stage impulse turbine. The turbine operates at 9300 rpm and a gearbox must be used to reduce the speed to 1800 rpm to match the speed requirements of the electrical generator.

 

Figure 16.12  Energy flow diagram for the Coolidge solar irrigation system.  Operation is at full power with no storage interaction.

 

Specifications: The system design specifications of this system are presented in Table 16.6.

System:

Operator – DOE / Arizona Public Service Co.

Location – Coolidge, Arizona

Demand – electrical power for remote site deep-well agricultural irrigation

Output – 150 kW electricity maximum

Collector Modules:

Type – parabolic trough with glass-covered tubular receiver

Manufacturer – Acurex Corp., Model 3001

Aperture – 1.83 m ´ 3.05 m (6 ft ´ 10 ft)

Reflective surface – FEK 244 (3M Corp)

Concentration ratio – 36

Collector Field:

Number of modules – 384

Total aperture area – 2140 m² (23,035 ft²)

Orientation – north/south

Modules on single-tracking drive – 8

Land use – 30% (6.5-m row spacing)

Collector Field Fluid Flow:

Heat-transfer fluid – Caloria HT-43 (Exxon Corp.)

Delta- T string – 48 modules in series

Number of delta-T strings – 8

Flow control – constant outlet temperature

Field outlet temperature – 288ºC (550ºF)

Field return temperature – 200ºC (392ºF)

Storage:

Type – thermocline tank

Medium – Caloria HT-43 oil

Volume – 114 m³ (30,000 gal)

Thermal capacity – 19.8 GJ (18.7 ´ 10⁶ Btu)

Maximum temperature – 288ºC (550ºF)

Minimum temperature – 200ºC (392ºF)

Operating time at maximum demand – 6 h

Power Conversion Cycle:

Type – Rankine cycle with superheat and regeneration

Working fluid – toluene

Turbine inlet – 268ºC/1.03 MPa (515ºF/150 psia)

Condenser – 40.5ºC/ 10 kPa (105ºF/1.46 psia)

Thermal efficiency – 20%

 

16.2.3     Shenandoah Solar Total Energy Project

This system located in Shenandoah, Georgia, uses solar energy collected by a field of parabolic dish collectors to supply process steam, electricity, and cooling.  The solar energy system shown in Figure 16.13 is adjacent to and provides energy for the Bleyle knitwear plant, which provides the demand for the system.  The electricity demand is transferred through the Georgia Power Company grid so that electrical power production in excess of plant demand may be utilized elsewhere. Operating experience gained with this system is summarized by Hunke and Leonard (1983).

Figure 16.13  The Shenandoah solar total energy project at Shenandoah, Georgia. Courtesy of Sandia National Laboratories.

Collector Field: The collector field consists of 114 7-meter-diameter parabolic dishes with cavity receivers.  The total aperture area is 4352 m² (46,845 ft²).  The dishes track about their polar and declination axes, and each dish has its own tracking motors and focal feedback system, receiving input from a central control system.  Figure 16.14 shows a layout of this field.

 

All collectors are connected in parallel to the supply and return heat-transfer fluid lines. The heat-transfer fluid chosen for this application is Syltherm 800, a silicon-based fluid, is manufactured by Dow-Corning Corporation.  The fluid can withstand the 399ºC (750ºF) maximum operating temperature of the system.  The flow rate to the field is varied and balanced so that the outlet temperature of each collector is 390ºC (734ºF).

System Description: The system is comprised of two basic flow loops; the solar field heat-transfer loop and the water-steam loop of the energy conversion cycle.  It is shown schematically in Figure 16.15.  Valves and a few minor connections have been omitted for clarity.  The solar field loop contains Syltherm 800, which, after being heated in the solar field, is passed through the steam generator heat exchangers (actually three units – one for preheat, one for boiling, and one for superheat).  In this loop is also included a small 41.6 m³ (11,000 gal) hot oil storage tank sized to provide for continuous operation during short-term insolation transients. Also included is an auxiliary heater that may be used in series with the collector field to maintain a constant temperature for operating the steam turbine.

 

Figure 16.14  Collector field layout of the Shenandoah solar total energy project showing the fluid piping.

Figure 16.15  Flow diagram for the Shenandoah solar total energy project.

The water–steam loop produces electricity using a Rankine cycle with single point extraction.  Steam leaves the turbine at a higher than usual temperature in order to supply heat to a lithium bromide absorption chiller. Steam not required by the chiller is passed through a condenser that rejects cycle heat to the surroundings.

 

Some of the steam flow is extracted between the high- and low-pressure turbines. Part of this steam is used to preheat the boiler feedwater in a deaerator–feedwater heater.  The other portion of the extraction steam goes into a de-superheater, where it is mixed with condensate in order to bring the steam to saturated vapor before being passed into the process steam supply line.

 

The absorption chiller receives heat from the turbine exhaust steam and rejects heat to the surroundings by a cooling tower.  In the chiller, heat is extracted at a temperature of 7.2ºC (45ºF) from a chilled water system going to the knitwear factory.

 

Energy Flows:  The thermodynamic states of the water-steam system at various points in the Rankine cycle for a full-demand example are shown graphically in Figure 16.16 and given in Table 16.7.  For this example it was assumed that pressure or heat loss in the interconnecting piping was negligible.

 

Figure 16.16  Processes of the Shenandoah solar total energy project (not to scale) for the full-load example.

Table 16.7. Operating Fluid Properties for the Shenandoah (GA) Solar
Total Energy System

 

Also shown in Figure 16.16 are the temperatures of the Syltherm 800 heat-transfer fluid as it passes through the steam generator heat exchangers.  Note that the pinch point temperature difference is 22.2ºC (40ºF).

 

The energy flows into and out of the system for an insolation level of 778 W/m² are shown in Figure 16.17.  The operating condition for these flows is the full-demand condition shown in the temperature-entropy diagram, where the electrical, process steam and chiller demands are a maximum.  Although thermal-to-electric conversion efficiency is only 12 percent, the conversion efficiency from solar to all forms of useful energy is 44 percent.

 

Figure 16.17  Energy flow diagram for the Shenandoah solar total energy project for the full-load example.

Specifications: The specifications of this system are presented in Table 16.8.

 

System:

 

Operator – Georgia Power Company

Location – Shenandoah, GA

Demand – electrical power, process steam, and chilled water for space cooling for knitwear manufacturing operations

Output – 400 kW electricity maximum, 626 kg/h (1380 lb/h) process steam at 0.86 M Pa/ 173ºC (125 psia/344ºF), 468 kW (133 tons) of cooling

 

Collector Modules:

 

Type – parabolic dish with cavity receiver

Manufacturer – General Electric/Solar Kinetics Inc.

Aperture – 7 m (23 ft) diameter

Reflective surface – FEK 244 (3M Corp.)

Concentration Ratio – 234

Collector Field:

Number of modules – 114

Total aperture area – 4352 m² (46,845 ft²)

Orientation – two-axis tracking

Each module has own tracking drives

Land use – 41%

Collector Field Fluid Flow:

Heat-transfer fluid – Syltherm 800 (Dow-Corning)

All modules connected in parallel

Flow control – constant outlet temperature

Field outlet temperature – 399ºC (750ºF)

Field return temperature – 260ºC (500ºF)

Storage:

Type – small buffer tank

Storage medium – Syltherm 800

Volume – 41.6 m³ (11,000 gal)

Thermal capacity – l.33 GJ (1.26 ´ 10⁶ Btu)

Maximum temperature – 399ºC (750ºF)

Minimum temperature – 363ºC (685ºF)

Operating time at maximum demand – 1 hr

Power Conversion Cycle:

Type – Rankine cycle with superheat

Working fluid – water

Turbine inlet – 382ºC/4.93 MPa (720ºF / 715 psia)

Condenser – 110ºC/145 kPa (231ºF/21 psia)

Thermal efficiency – 18%

Turbine extraction port used for process steam

Chiller:

Type – lithium bromide absorption chiller

Heat in – 102ºC (215ºF)

Heat rejection – 29ºC (85ºF)

Chilled water – 7.2ºC (45ºF)

 

16.2.4    Solar One Central Receiver Pilot Plant

Solar One is the first large-scale application of the central receiver concept. It is located just outside Barstow, California in the Mojave Desert.  The solar site, shown in Figure 16.18, is adjacent to a Southern California Edison fossil fuel power plant and produces electricity for their central power grid.

 

Figure 16.18  The Solar One 10-MW central receiver pilot plant at Barstow, CA.  Courtesy of Southern California Edison Co.

Heliostat Field: The heliostat field consists of 1818 heliostats, each with a slightly concave reflective surface area of 39.9 m² (430 ft²).  Of these, 1240 are located north of the receiver tower and 578 south of the tower as shown on Figure 16.19.

 

Figure 16.19 Heliostat field layout of the Solar One 10-MW central receiver pilot plant.

A single heliostat consists of 12 slightly concaved mirror panels fabricated by bonding a second surface glass mirror to a honeycomb core that, in turn, is bonded and sealed to a steel enclosure pan.  The panels are aligned so that each segment reflects the sun’s image on the receiver.  Each heliostat is individually tracked about the azimuth and elevation axes by two 1/8-kW (1/6-hp) motors.

 

Receiver Tower:  The receiver is located at the top of a 98.8-m (298-ft) tower.  The active receiver surface is 13.7 m (45 ft) high and 7 m (23 ft) in diameter.  This surface is formed from 1680 12.7-mm (0.5-in.)-diameter Inconel-800 tubes. The tubes are welded into 24 panels of 78 tubes each and coated with PyromarkÒ high absorptance paint.  The six south-facing panels serve as preheat panels and are connected in series with the remaining 18 panels.  Water is pumped vertically through these latter panels, with boiling taking place approximately halfway up the tubes. The steam is superheated to 516ºC (960ºF) during the remainder of its passage to the top of the receiver.

Storage: Thermal storage is provided for approximately 4 hours of operation at reduced power levels. The storage consists of a mixture of crushed rock, sand, and Caloria HT-43 heat-transfer oil (a product of Exxon).  The maximum storage temperature of 304ºC (580ºF) is limited by oil decomposition.

 

Therefore, the steam leaving the receiver must be cooled before charging the storage to its maximum temperature.  Likewise, the conversion cycle must operate at reduced output during operation on steam generated by the stored heat.

System Description: The Rankine cycle power conversion system is shown schematically in Figure 16.20. There are three flow loops shown that comprise the three primary operating modes: power production from solar-generated steam, power production from storage-generated steam, and storage charging.

Figure 16.20 Simplified flow schematic for the Solar One 10-MW central receiver pilot plant.

To produce power from the high-temperature solar-generated steam, feedwater is pumped from the condenser through a number of feedwater heaters and then out to the receiver tower and up to the receiver.  Here the water is heated by the sun’s radiation reflected onto the receiver surface. Superheated steam leaves the receiver at 516ºC/10.85 MPa (960ºF/1573 psia) and, after being piped down the tower, enters the turbine.  Some steam is bled at four extraction ports and used to heat the receiver feedwater.  Most of the steam leaves the last stage of the turbine and is condensed at 43ºC (109ºF) in the condenser, which is cooled by a wet cooling tower.

 

When more steam is available from the receiver then is needed for power production, the excess is passed through a de-superheater to reduce its temperature to the maximum storage temperature.  The steam is then passed through a heat exchanger, where it heats the cool Caloria HT-43 storage oil, which is pumped from the bottom of the storage tank.

 

The heated oil is returned to the top of the storage tank, where it transfers heat to the rock and sand.  The cooled steam is then returned to the power cycle at heater #2, where it provides feedwater heating.

 

When the receiver can no longer produce high-temperature, high-pressure steam, preheated feedwater leaving the deaerator is diverted to the storage.  Here a heat exchanger transfers heat from the hot oil at the top of the storage to the feedwater, producing 274ºC/2.65-MPa (525ºF/385-psia) steam.

 

The cooled oil is returned to the bottom of the storage tank, and the steam enters the turbine at an admission port placed a few stages downstream from the entry port for the high-pressure receiver stream.  The remainder of the flow path is similar to that used when operating on steam generated at the receiver.

Energy Flows: The thermodynamic states of the steam and oil for two operating conditions are shown graphically in Figure 16.21 and given in Table 16.9. The first condition is for full power production from solar-heated steam excess steam charging the storage.  The second condition is for power production from storage-generated steam.  These data include the pressure and heat from the interconnecting piping.

Figure 16.21 Processes of the Solar One 10-MW central receiver pilot plant (not to scale) (both solar steam generation and storage generation of steam are shown).

 

The storage charging process and the storage discharge process are shown on Figure 16.22 a and b.  Starting at station 21, the steam is first de-superheated and then passed through the oil-steam heat exchanger to heat the storage oil.  After leaving this heat exchanger, the condensate is throttled into a flash tank, where the steam produced is used to preheat receiver feedwater in the deaerator and the liquid goes to feedwater heater #2.

 

Figure 16.22  Thermodynamic process diagrams for storage charging and discharging of the Solar One 10-MW central receiver pilot plant.

The storage discharge is shown in Figure 16.22b, where the heat transfer oil is used to generate steam.  Here again the pinch point limits the oil discharge temperature and thus the amount of heat which can be extracted from storage.

 

Figure 16.23 shows the energy flows for full power output with excess energy available for storage.  The maximum solar input case shown here is for noontime at the summer solstice.

Figure 16.23  Energy flow diagram for summer solstice noontime operation for the Solar One 10-MW central receiver pilot plant.

Specifications: The specifications for this system are presented in Table 16.10.

 

Table 16.10.  Specifications of the Solar One Central Receiver Pilot Power Plant

System:

Operator – Southern California Edison

Location – Barstow, California

Demand – electrical power for utility grid

Output – 10 MW electricity maximum

Heliostat Modules:

Type – heliostats with external receiver on top of tower

Manufacturer – heliostats, Martin Marietta; receiver, Rocketdyne; system, McDonnell Douglas

Aperture – 6.9 m ´ 6.9 m (22.6 ft ´ 22.6 ft)

Reflective surface – silvered glass (second surface)

Reflective surface area – 39.9 m² (430 ft²)

Concentration ratio – 282

Heliostat Field:

Number of heliostats – 1818

Total aperture area – 72,540 m² (237,990 ft²)

Orientation – north and south of tower

Each module has own tracking drive

Land use – 25%

Receiver:

Tower height – 91 m (298 ft) (including receiver)

Receiver size – 7 m (23 ft) diameter/ 13.7 m (45 ft) high

Heat-transfer fluid – water/steam at 10.6 MPa (1537 psia)

Flow–6 south-facing panels are preheaters in series with the remaining 18 steam-generating panels (one panel forms 4% of circumference)

Flow control – constant outlet temperature

Steam outlet temperature – 516ºC (960ºF)

Return temperature – 203ºC (397ºF)

Storage:

Type – thermocline rock–oil

Medium – 1-in. crushed granite, sand, and Caloria HT 43 oil

Rock – 4.11 ´ 10⁶ kg (4532 tons)

Sand – 2.06 ´ 10⁶ kg (2266 tons)

Oil – 907 m³ (239,600 gal)

Volume – 4228 m³ (149,300 ft³)

Thermal capacity – 522 GJ (495 ´ 10⁶ Btu)

Maximum temperature – 304ºC (579ºF)

Maximum temperature – 223ºC (435ºF)

Operating time (at 7 MW) – 4h

Power Conversion Cycle–Receiver Steam Operation:

Type – Rankine cycle with superheat

Working fluid – steam

Turbine inlet – 510ºC/10.1 MPa (950ºF/1465 psia)

Condenser – 43ºC/8.4 kPa (109ºF/ 1.24 psia)

Thermal efficiency – 35%

Power Conversion Cycle – Storage Steam Operation:

Type – Rankine cycle with superheat

Working fluid – steam

1urbine inlet – 274ºC/2.65 kPa (525ºF/385 psia)

Condenser – 43ºC/8.4 kPa (109ºF/l.24 psia)

 

References

Anonymous (1977), “Analysis of the Economic Potential of Solar Thermal Energy to Provide Industrial Process Heat,” Intertechnology Corporation Report C00/2829/76/I, February.

Harley, E. I., and W. B. Stine (1983), “Solar Industrial Process Heat (IPH) Project Technical Report; October 1981-September 1982,” Sandia National Laboratories Report SAND83-2074, October.

Harley, E. I., and W. B. Stine (1984), “Solar Industrial Process Heat (IPH) Project Technical Report; October 1982-Septemher 1983” Sandia National Laboratories Report SAND84-1812, March.

Hunke, R. W., and J. A. Leonard (1983), “Solar Total Energy Project Summary Description”, Sandia National Laboratories Report SAND82-2249, March.

Larson, D.I. (1983), “Final Report of the Coolidge Solar Irrigation Project,” Sandia National Laboratories Report SAND83-7125, October.