Advanced CSP Teaching Materials

Chapter 12 Operation and Maintenance

Authors Johannes Sattler1 Julian Eckstein1 Bryan O’Connell1 Reviewers Anette Anthrakidis1

Ahmed Chikouche2 Martin Eickhoff3 Mirko Meyer-Grünefeldt3 Bernhard Hoffschmidt1, 4 1Solar-Institut Jülich (SIJ), FH Aachen, Aachen University of Applied Sciences, Heinrich-Mußmann-Str. 5, 52428 Jülich, Germany 2L’Unité de Développement des Equipements Solaires (Algeria) 3German Aerospace Center (DLR), Plataforma Solar de Almería (DLR-PSA), Ctra. de Senés s/n, 04200 Tabernas, Spain 4German Aerospace Center (DLR) - Solar Research, Linder Höhe 51147 Cologne, Germany

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Table of Contents

Nomenclature ...................................................................................................................... 3 

Summary .............................................................................................................................. 3 

12  Operation and Maintenance....................................................................................... 4 

Introduction .......................................................................................................................... 5 

12.1  Operation – Potential Tasks in the Power Block.................................................. 6 

12.2  Operation – Potential Tasks in the Control Room ............................................... 7 

12.3  Maintenance – Potential Tasks in the Solar Field ............................................... 8 

12.3.1  Introduction ............................................................................................................. 8 

12.3.2  Parabolic Trough Collector Field Maintenance................................................. 9 

12.3.3  Heliostat Field Maintenance............................................................................... 22 

12.3.4  Fresnel Collector Field Maintenance ................................................................ 24 

12.3.5  Dish Collector Field Maintenance ..................................................................... 27 

12.4  Installation – Potential Tasks of Technicians on CSP Plant Construction Sites31 

List of figures ..................................................................................................................... 32 

List of tables....................................................................................................................... 32 

Reference list..................................................................................................................... 33 

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Nomenclature Symbol Meaning Unit Latin letters A azimuth angle ° DNI direct normal irradiation W/m² H elevation angle ° z zenith angle ° Acronyms CSP concentrating solar power DD day of the month DISS Direct Solar Steam MM month O&M operation and maintenance PSA Plataforma Solar de Almería PTC parabolic trough collector PTPP parabolic trough power plant HCE Heat Collector Element SCE Solar Collector Element SCA Solar Collector Assembly

Summary This chapter provides some fundamental knowledge of the installation, maintenance and power plant handling tasks that may be encountered by technicians and/or engineers who work in CSP plants. Some of the essential maintenance requirements of the main CSP technologies are discussed. These may vary for all CSP plants.

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12 Operation and Maintenance

Key questions

What are the main components in a power plant block?

Why is the tracking of the sun’s position so important?

What parts does a heliostat, a parabolic trough collector, a linear Fresnel system and Dish Stirling system consist of?

What are the methods for cleaning mirrors?

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Introduction In this chapter the potential tasks of technicians and/or engineers working in the maintenance staff of a solar thermal power plant will be described. Distinguished into three groups of working environment as listed below, the respective potential tasks will be exemplified.

Power Block

Control Room

Solar Field Additionally, the potential tasks of technicians working on CSP plant construction sites will be illustrated. Technicians working on solar thermal power plants can be categorised as mechanical, electro/mechanical or electronic technicians [1]. Each member of the particular profession may perform a defined range of tasks limited to his/her qualification. For example, an electrician without a boiler attendant certificate is not allowed to handle pressurised and high temperature components. To broaden the technicians’ field of application the operating company of a power plant may decide to send its technicians to further training courses [2]. In order to get an overview on what the differences in the above stated categories of technicians are, the following has been quoted from the website of EG&G Technical Services, Inc. [1], which provides solar thermal test support services to Sandia National Laboratories’ parabolic trough solar collector systems. Other companies may, of course, interpret and attribute tasks differently:

“Mechanical technician personnel have tasks associated with planning, fabricating, installing, collecting data, and conducting tests for the Solar Concentrator Test program. Responsibilities include fabricating and installing mechanical test equipment. The work involves welding, fitting, testing with high solar fluxes, and elevated work areas. EG&G personnel have experience in assembly, maintenance, and disassembly of large steel structures including the use of large material handling equipment; layout, fabrication, installation, and operation of cooling water loops, high pressure gas systems, high-vacuum systems, and their associated controls; installation and operation of physical measurement transducers such as flowmeters, pressure transducers, thermocouples, and heat trace systems; gas, arc, and TIG welding, flame cutting, brazing, and soldering; handling and installation of high temperature insulating materials; piping, tubing, and valves construction, assembly, and installation techniques; and working with high pressure and high temperature systems. Electro/Mechanical technician personnel have tasks associated with planning, fabricating, conducting, and reporting tests for the Solar Thermal program. Responsibilities include designing, installing, and maintaining heat trace, control, and instrumentation systems. The work involves high temperature equipment, testing with high solar fluxes, and elevated work areas. EG&G personnel have experience in fabrication, installation, troubleshooting, and maintenance techniques for industrial process control and data acquisition systems; planning and conducting high temperature experiments and maintaining detailed test records; installation, documentation, and maintenance of instrumentation; and layout, installation, documentation, and maintenance of pipe and heat tank trace systems.

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Electronic technician personnel have tasks associated with maintaining and operating the heliostat field and operating the process controller for the Central Receiver Test Facility. Responsibilities include planning and conducting maintenance of all heliostats on site. Operation of the heliostat field requires knowledge of computer-based control systems including the operating system and knowledge of the effects of high thermal fluxes on materials. EG&G personnel have experience in operating computer control systems; operation of process control systems; printed circuit board design; troubleshooting and maintenance of communication and control electronics; fabrication, installation, troubleshooting, and maintenance techniques for industrial process control and data acquisition systems; and system-level troubleshooting and repair of optical encoders, and communication and control equipment.”

12.1 Operation – Potential Tasks in the Power Block The power block of a generating station consists of several components, which include

Condensate system (incl. the condenser; condensate pump)

Feedwater system (incl. deaerator and feedwater storage tank; feedwater pump)

Steam turbine and generator

Cooling system (incl. e.g. chiller/cooling tower)

Steam generator (incl. condensate preheater, economiser, evaporator, superheaters)

Interconnecting piping and valves

Water treatment facility (for replacing discarded water in the vapour cycle)

Electrical equipment (e.g. sensors, electric control cabinets) An employee working in the power block may have several tasks such as identifying leaks and unusual machine noises or regulating hand valves and cleaning the filters of strainers1. A refilling of leaked fluids (e.g. cooling fluid) may also be required. Further tasks may be the start-up and shut-down of the steam turbine and generator or the general handling of the steam generator (qualified boiler attendants only). Electronic technicians may be responsible for power cut-offs when applicable e.g. when maintenance is required. During operation the technicians may be required to observe and handle the components in regular short-time intervals during the operation of the power block [2].

► Exercises

What are the main components in a power block of a power plant?

Which employees are allowed to handle a steam generator?

1 Strainers are devices with which debris such as scale, rust, joining compound, weld metal and other solids is removed [3, p. 12.4.2] from the water- steam and system as well as the cooling system.

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12.2 Operation – Potential Tasks in the Control Room

The most important parameters of components and their operating state such as pressure, temperature and water quality are monitored with the process control system in the control room. The employee may be involved in the monitoring and operation of the plant.

Moreover, the employee may be involved in the calibration, record-keeping of test, maintenance and operation events and operation of the heliostat field. One major issue is the calibration. It is of utmost importance that the sun position is precisely tracked. For example, a single heliostat of a solar tower power plant must have many calibrations over the day and year. The computer-based calibration process can be automatic or semi-automatic (requiring an employee to supervise the calibration process). In the calibration process of some solar towers, the heliostat image is aimed on a special calibration area beneath the receiver [2].

Figure 1 shows the calibration of a (single) heliostat of the PS10 solar tower power plant’s heliostat field. The sun’s image is seen on the white calibration area.

Figure 1: Calibration of a heliostat of the PS10 solar tower’s heliostat field [4]

The employee must have knowledge of the effects which highly concentrated solar radiation has on the material of the receiver (absorber), but also on the heat transfer fluid to correctly operate the plant in solar mode [2], [1].

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12.3 Maintenance – Potential Tasks in the Solar Field This section deals with the operation and maintenance (O&M) requirements in parabolic trough and solar tower power plants, Fresnel and Dish collector systems. Ideally, a power plant should be dispatchable2 and have only few downtimes due to maintenance or failures. In order to assure that a well-elaborated O&M Strategy must be applied. 12.3.1 Introduction Solar systems utilising direct normal irradiation (DNI) must precisely track the sun’s position in order to reflect the image onto the target. The heliostat field of a solar tower power plant and a Dish system must precisely track the sun’s position in both the azimuth and zenith angles (two axes). Large parabolic trough systems and linear Fresnel systems require precise single-axis tracking only [2].

The azimuth angle (A) is measured clockwise from true north to the point on the horizon directly below the sun, while the zenith angle (z) is measured from the vertical to the sun [5]. Additional there is the elevation angle (h) which is measured vertically from the point on the horizon up to the sun or calculated using the formula:

zh 90 (9-1)

Figure 2: Orientation of the azimuth and zenith angle [5]

Heliostats, parabolic trough mirror systems and other CSP technologies track the sun using actuators with drive motors. Moving systems require more maintenance than stationary ones.

2 A power plant is dispatchable when it can generate variable power on-demand e.g. on request of a power grid operator [6]

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12.3.2 Parabolic Trough Collector Field Maintenance

(1) Parabolic Trough Collector Parts A parabolic trough collector (PTC) consists of a receiver, facets (adapted to the shape of a parabola), a support structure, pylons, foundations as well as drive motors.

The receiver of a PTC is fixed at the focal distance of the parabolic mirror shape. The receiver is held by a support structure, which is attached to the support structure of the collector. Large parabolic trough systems track the zenith position of the sun only.

Figure 3: Parts of a parabolic trough collector, edited from [7]

► Exercises

Name all major parts of a parabolic trough collector.

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(2) Maintenance A parabolic trough power plant (PTPP) has several O&M requirements.

Unless stated otherwise, the following information is taken from the “Final Report on the Operation and Maintenance Improvement Program for Concentrating Solar Power Plants” [8]. The report describes O&M methods and the efforts taken to reduce the O&M costs of concentrating solar power plants. Several O&M methods were developed by Sandia National Laboratories over a period of six years and tested at the 150 MWe Kramer Junction solar power park.

Figure 4: Kramer Junction solar park 530 MWel, California, USA [8]

Common maintenance issues of parabolic trough power plants are described in this paper. The paper states also that the described O&M improvements are also applicable on solar power tower or dish technology concepts.

Mirror cleaning CSP technologies are usually constructed in areas that are dry and desertic. Mirror systems that are installed in semi-arid, arid or extremely arid regions, such as the Sahara desert, are often exposed to wind blowing soil grains and dirt particles through the air that inevitably also settle on the mirror surfaces. The reflectivity of soiled mirrors can reduce by 10%, which directly results in a reduction of the solar field performance. It is desired to keep the average solar field reflectivity in the 90 – 91% range (a new mirror has a reflectivity of 94%) in order to maintain cost-effectiveness at Kramer Junction. This value range was found to be realistic, achievable and a cost-effective target, based on experience rather than the result of a thorough cost/benefit analysis. The cleaning of the mirrors should be conducted in regular time intervals. The most critical factors affecting soiling rates include the time of year, frequency of rainfall, proximity of mirrors to roads or other sources

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of airborne particulates, method and frequency of washing as well as proximity to power plant equipment (e.g. mechanical draft evaporative cooling towers: these discharge moisture, which combine with airborne dust and can, depending on wind direction, settle on the mirrors).

A test involving mirrors of the SEGS plants was conducted to know more about the degradation of mirrors reflectivity with time. For this purpose, the involved mirrors in the test collector loop remained unwashed for three months. Reflectivity measurements were taken in an interval of two times per week and for each time almost 150 total readings were taken. The value of the reflectivity is given at 660 nm, which tends to correspond to the spectral reflectivity for mirrored glass. The average degradation of reflectivity is about 0.45 percentage points per day, as shown in Figure 5, but the rate is usually smaller during other periods of the year, as then airborne dust is less and the natural washing of the mirrors occurs more frequently. Problematic are times of minimal rainfall occurring when at the same time there is a high-dust or dirt level in the air. Although rare, this situation is still real and this was experienced at Kramer Junction in late July 1995 when there was light rain together with very high, gusty winds with speeds up to 35.8 km/h. This weather condition affected all of the solar fields and resulted in reflectivities dropping 12 – 19 percentage points on average to a reflectivity level in the 72 – 79% range. To get back to the normal mirror reflectivities, it was necessary to mechanically scrub the mirrors.

Figure 5: Degradation of mirror reflectivity over the summer months August-September (MM/DD) [8] While the test was undertaken, the thermal output of the collector loop was also measured. The results showed that for every change in reflectivity of 1%, there was a performance change of about 1.2% in solar field thermal delivery. “This leverage effect occurs because solar field heat losses, which are dependent on operating temperature, remain constant while the radiation input to the receiver is directly proportional to the change in reflectivity. The revenues of the plant react proportionally.” For the five SEGS plants located at Kramer Junction, every 1% reflectivity loss has an impact of approximately US$ 185,000 in annual revenues. The SEGS plants are hybridised with auxiliary natural gas boilers which aid the solar field during the summer months when the revenues are

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highest. If it were not for the hybridisation, the impact of reduced reflectivity would be much greater. Mirror cleaning therefore should be a highly important issue for any plant operating company.

Several methods of cleaning mirrors were tested and compared at Kramer Junction. Three methods are presented here. Figure 6 shows the cleaning of mirrors with the traditional method using a high-pressure rig, where one employee drives a tractor with an attached water mobile tank and two employees hold hand-held nozzles to spray demineralised water3 at high-pressure (207 bar) onto the mirrors. This is a rather slow and cost-intensive method, but the result is a reflectivity increase by three percentage points [8]. Latest scientific findings of [4] have proven that depending on the soiling and on the intensity of cleaning the reflectivity can be increased by up to 10%.

Figure 7 shows a newer cleaning system, the rotating-head rig (also called Mr. Twister), which was tested. It sprays demineralised water at high-pressure on the mirrors. “Mr. Twister” consists of a tractor pulling a wheeled tank-and-pump unit with rotating water nozzles. Only 1 driver is required who controls the cleaning process from the cab. Another half of a worker’s time is required for driving a tank truck for refilling Mr. Twister. This cleaning method is suitable for replacing the traditional high-pressure rig. The rotating-head rig manages to increase reflectivity by 3 percentage points. The water use is about the same as for the high-pressure rig. The cleaning results in reflectivities in the 93 – 94% range [8].

In the third method, the high-water-volume method (at low pressure), shown in Figure 8, a truck equipped with a water tank and spray system sprays a “deluge-type” stream of water onto two parallel rows of mirrors simultaneously. This method requires only one employee (truck driver). The advantage of the deluge-type over the high-pressure spray method is that it is four times faster and the work-time spent per collector (in hours) is considerably lowered by a factor of 12. However, the water consumption is about 20% higher than the high-pressure spray methods and the reflectivity increases only by 1 percentage point [8]. Latest scientific findings of [4] have proven that depending on the degree of soiling the increase in reflectivity can also be much higher.

3 Demineralised water is water which is free of contained impurities like minerals, dissolved gases, solids and salts.

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Figure 6: Traditional mirror cleaning method with hand-held nozzles [8]

Figure 7: Cleaning with a “Rotating-Head” rig [8]

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Figure 8: Cleaning of the mirrors with a “Deluge-Type” stream of water [8]

For good cleaning results a combination of both the high-pressure and high-volume cleaning method shows best results. If a CSP plant is located in desert environments, water use is a very important topic. At Kramer Junction, the “cost of raw water and water-treatment chemicals is a significant portion of the annual O&M budget (approximately $1 million per year)”. Hence, a study was done to asses the water use and quality so that the water consumption could be reduced. The results showed that the amount of water needed for washing mirrors is relatively low (1.4% of the total water is used) compared to the amount of water associated in the operation of the Rankine power-cycle equipment (more than 90%). The acquired knowledge was used to apply water-conservation measures which lead to a 33% reduction in water use per megawatt hour of electricity produced, as can be seen in Figure 9.

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Figure 9: Raw water usage oft he SEGS III-VII parabolic trough power plants in the time period of 1989 – 1997 [8]

Correction of receiver/mirror alignment For optimal parabolic trough power plant performance it is crucial that the sun’s rays reflected by the mirror precisely focus on the receiver, i.e. beam interception must be accurate. Therefore, it is highly important that the alignments of all modules, sun sensors and receivers as well as the tracking are checked for accuracy in the installation stage or later during maintenance work [4].

Replacement of damaged components (e.g. mirrors and tubes) Strong winds may cause the breakage of mirrors, especially of those located at the edge of the solar field where higher winds loads act on the mirrors. Newer parabolic trough plants have fortified the mirrors in these vulnerable areas. Falling mirror parts may further damage other mirrors and the receiver glass envelope4. The glass envelope may also break if the receiver is aligned poorly during installation and because of operational incidents (e.g. from washing).

In the case of glass envelope breakage of the receiver, excessive energy losses will occur in the affected trough loop, especially on windy days. After removing the broken glass envelope, a temporary replacement such as a split-glass sleeve can be fitted around the receiver tube. The sleeve is fixed with an adhesive and springs. This measure significantly reduces the working fluid’s heat losses by convection and is therefore a good option for bridging the time until a new vacuum glass sleeve can be installed.

Figure 10 shows the glass envelope of an intact parabolic trough receiver and Figure 11 shows the fitting of a temporary split-glass envelope.

4 The glass envelope around the receiver tube contains a vacuum which has the purpose to reduce the convective losses.

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Figure 10: Glass envelope of a parabolic trough receiver [7]

Figure 11: Split-glass envelopes as a temporary method of reducing heat losses [8]

If a loss of vacuum occurs within intact glass envelopes, the parabolic trough's performance will still be reasonable. The maintenance staff should therefore first repair broken glass envelopes.

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Flow loop maintenance Modern parabolic trough power plants have row lengths of approximately 300 m. Each row consists of two collectors of 150 m length, also called solar collector assembly (SCA). Each SCA has a drive mechanism in the centre and consists of 12 modules of 12 m length, which are called solar collector elements (SCE). Each module has 3 absorber tubes (heat collector element – HCE) of 4 meter length. Each collector (SCA) is connected to the neighbouring one with tubing and flexible joints, so-called ball joints. The ball joints allow the individual collectors (SCA) to be driven out of focus while the others are still tracking the sun’s elevation angle. Figure 12 shows ball-joints used in the parabolic trough experimental plant DISS, which is located on the Plataforma Solar de Almería (PSA)5, Spain.

Figure 12: Ball-joints installed at the DISS experimental plant [9]

Ball joints are available in single or dual joint design, as shown in Figure 13. Two collectors (SCA) are connected with two ball joint assemblies, where one ball joint assembly comprises a combination of 3-4 ball joints (Note: Modules (SCE) are connected by welding and not with ball joints). Ball joints are packed in order to reduce the heat losses. Modern parabolic trough plants use a combination of flex hoses and rotating joints [8], [4].

Annual thermo oil leakage amounts to approximately 1% of total inventory, requiring the O&M staff to refill the system. Oil spills constitute the greatest loss of oil and must be closed immediately. Moreover, environmental treatment is required.

5 The PSA is Europe’s largest research, development and test centre for concentrating solar tech-nologies

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Natural leakage due to the volatisation of thermo oil also occurs, which amounts to merely 0.08% of the total inventory. O&M staff at parabolic trough plants should therefore focus on preventing and mitigating oil spills.

Figure 13: Top: Packed dual ball joint; Bottom: Example of a dual ball joint design, edited from [10]

Packed piping with three single packed ball joints at the edge of a row is shown in Figure 14 below.

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Figure 14: Packed ball joints, edited from [8]

An efficient maintenance strategy requires good maintenance management as well as quality equipment. The strategy could include maintenance trucks with an on-board link to maintenance management software, as shown in Figure 15. This allows each maintenance issue to be directly logged and saved in a database for monitoring purposes and analyses at a later time. Moreover, a computer maintenance management system helps to manage the master equipment list, equipment reliability histories, work order system, purchase orders, stock issue, requests, workforce planning and spare parts management. Well functioning communication between O&M staff is very important for fast-responding and cost-effective maintenance measures [8].

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Figure 15: At Kramer Junction, maintenance data are entered from the solar field via radio transmitters [8]

Further (occasional) maintenance issues are [2]:

Replacement of damaged electrical components

Closing of oil spills

Tightening of bolts/screws

Miscellaneous issues

Measurements of collector field The employee in the solar field may be involved in the measuring of collector geometry, flux density, optical efficiency, irradiance and thermal performance. Such measurements are important to determine the effectiveness of a cleaning process or the usability of a new receiver type. Moreover, other defects of a solar collector such as the bad alignment of collector modules or the solar sensor can be detected as well as a wrong installation of mirrors [2], [4].

Figure 16: Mirrors and collector shape qualification [11]

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► Exercises

Why is it important to clean the mirrors of a solar field in regular time intervals?

What collector measurements need to be performed in a solar field of a parabolic trough power plant? Why are they so important?

What are the advantages of using a computer maintenance management system?

How much does the annual thermo oil leakage amounts to in parabolic trough power plants?

What is the problem with oil spills in parabolic trough power plants?

What is the functionality purpose of ball joints in a solar field of a parabolic trough power plant?

What problem occurs when there is a glass envelope breakage of a receiver of a parabolic trough power plant? How is the problem solved temporarily?

How do receiver glass envelopes of a parabolic trough power plant break?

If a loss of vacuum occurs within intact glass envelopes, the parabolic trough's performance will a) increase b) stay the same c) reduce but still be reasonable

In order to maintain cost-effectiveness at Kramer Junction it is desired to keep the average solar field reflectivity above a) 10% b) 38% c) 52 % d) 90%

What are the most critical factors affecting soiling rates?

Name three cleaning methods used at the Kramer Junction parabolic trough power plant.

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12.3.3 Heliostat Field Maintenance (1) Heliostat Parts Large heliostats (e.g. 100m² mirror area) consist of the components as illustrated in Figure 17. The pylon bears the weight of the entire heliostat structure. Fixed to a large horizontal beam are the mirror facets and their support structure, which includes the struts. In current heliostat fields the tracking of the sun’s position is realised with two rotary actuators (one for the azimuth and the other for the zenith angle), which are powered by drive motors.

Figure 17: Parts of a large heliostat of a solar tower’s solar field, edited from [7]

Smaller sized heliostats (e.g. 10m² mirror area) can be manufactured with rotary or linear actuators and with step motors (or other motors) to track the sun’s position.

Figure 18: Parts of a small heliostat of a solar tower’s solar field [2]

Some heliostats also use pneumatic drives [4].

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► Exercises

Name all major parts of a heliostat.

What type heliostat actuators exist? (2) Maintenance Many of the O&M measures from the parabolic trough technology also apply for the heliostat field. Figure 19 shows heliostat washing methods. The picture on the left shows a spray method and on the right the cleaning with rapidly rotating scrubbers. The soiled mirror surfaces are washed with demineralised water.

Figure 19: Heliostat washing methods [12]

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12.3.4 Fresnel Collector Field Maintenance (1) Fresnel Collector Parts A linear Fresnel plant consists of a large number of rows of slightly curved mirror facets (sometimes also planar mirror facets for small plants), a receiver (tube), secondary concentrator and a support structure. A small plant is shown in Figure 20.

The mirror rows track the sun's elevation angle and concentrate the sunlight on the receiver. A secondary concentrator is required due to the astigmatism6 of the focus resulting from the not perpendicular angle of incidence on the slightly parabolic shaped mirrors.

Figure 20: Small linear Fresnel collector, edited from [7]

Figure 21 shows part of a slightly curved primary collector row. A Fresnel collector in operation is shown in Figure 22.

6 Astigmatism means that the focal line produced by the light rays reflected by primary concentrator is distorted and a portion of the light rays misses the receiver. These rays are then reflected and concentrated on the receiver by means of using the secondary concentrator.

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Figure 21: Example of a primary collector row [15]

Figure 22: Linear Fresnel collector manufactured by Ausra [16]

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► Exercises

Name all major parts of a linear Fresnel system.

What is astigmatism?

Why is a secondary concentrator needed?

Why must a linear Fresnel collector be cheaper than a parabolic trough collector?

Name at least 2 pros and 2 cons of the linear Fresnel technology.

Is the following statement true? “A linear Fresnel collector has a higher performance than a parabolic trough collector”

(2) Maintenance The facts, that the primary collector field is located near the ground and that it is only slightly curved are making it easy to clean. According to the company Solar Power Group, the cleaning of the mirrors can be carried out during the night using an automated process. The obvious advantage of doing this at night is that optimum efficiency is guaranteed during the day. In addition the operating costs can further be reduced [17]. The automated cleaning process can be carried out by robots [4]. Novatec Solar is developing a cleaning robot which features ultra low water consumption and provides rapid cleaning with low labor cost [14].

Figure 23: Sword brush (pictures left) [14]; Cleaning robots (right) [18]

The company Solar Power Group uses a “wet cleaning system”. The gross water requirement of the automated cleaning process is roughly the same as for a manual cleaning process. At least 80% but possibly even more than 90% (tests are pending) of the water used for cleaning the mirrors is collected again, which can be recycled depending on the quality of the water. The automated cleaning process is approximately 10 times faster than the manual cleaning process [17]. The secondary collector, on the other hand, can be very difficult to be cleaned because, depending on the design, it can be located up to 10 m above the ground. Moreover, the secondary concentrator is difficult to access because the receiver is fixed directly in front of it [4].

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12.3.5 Dish Collector Field Maintenance (1) Dish Collector Parts A dish Stirling system tracks the sun’s position in the sky in both the elevation and azimuth angle similar to a heliostat of the solar tower technology. The reflective surface of the dish collector has the shape of a rotation paraboloid. Unlike the parabolic trough, linear Fresnel collector and solar tower technology, the dish technology does not use the heat energy to generate steam to power a steam turbine and generator. Instead the collector reflects and concentrates the sun’s rays onto a single spot – the power conversion unit, which is a Stirling engine. The Stirling engine converts heat energy into kinetic energy (rotational shaft motion). In a generator the rotational shaft motion is converted to electricity. Various types of dish collectors exist. The dish collector of Figure 24 consists of a pedestal (pylon), mirror facets, the facet support structure, azimuth and zenith drives, a boom and the power conversion unit (Stirling engine).

Figure 24: Dish collector [19]

A back and side view of the dish collector is shown in Figure 25.

Figure 25: Back and side view of dish collector [19]

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The below Figure 26 shows some of the 60 dish Stirling systems manufactured by Tessera Solar, which are installed in Peoria, Arizona (USA). Altogether the dish systems generate 1.5 MWe of electricity into the grid [20].

Figure 26: Dish systems in operation [20]

The peak efficiency of dish systems is stated with 30%. Moreover, the technology has a high-level annual performance [13].

According to source [21], the assembly of Dish collectors is done as described in the following 5 steps:

1) “The crankshaft is inserted into the engine block and held there with bearings. Bearings allow the crankshaft to turn within the engine block without generating excessive frictional heat. The bearings are fixed to the engine block by pressing (the outside diameter of the bearing is slightly larger than the inside diameter of the hole in the engine block). By forcing the bearing into the engine block, the bearing is firmly fixed to the block.”

2) “The pistons and connecting rods are dropped into the cylinders and attached to the crankshaft from below using high strength bolts and lockwashers. The bolts are tightened using a predetermined torque.”

3) “The regenerative heat exchanger is inserted into the conduit that flows between the main cylinder and the auxiliary cylinder and bolted in place.”

4) “The cylinder heads are bolted to the top of the engine, an access cover is bolted to the bottom of the engine. Gaskets are used between the engine block and the covers to provide an effective seal. The heat source chamber is built into the main cylinder cover.”

5) “The working fluid is pumped into the engine. The working fluid is usually pressurized helium.”

► Exercises

Name all major parts of a Dish collector.

What is the main difference of the dish to the parabolic trough, linear Fresnel collector and solar tower technology in terms of how heat energy is converted to electricity?

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(2) Maintenance Like the parabolic trough and the solar tower solar field, the dish collectors also require large machinery for maintenance.

The 60 Tessera Solar dish Stirling systems (above) are maintained by a staff of 12 workers in shifts. Most of the maintenance is carried out during the night [20].

There are two types of Stirling engine types used: kinematic engines and free-piston engines. The kinematic engine is the oldest type, it is most widely developed and the design is still improved nowadays with the aim to increase power transfer and durability. The free-piston engine on the other hand is a newer technology, which was invented in 1971, and has fewer moving parts than the kinematic engine [22]. Maintenance issues of the kinematic engine technology are [22]:

“The presence of cranks and rotating parts generate lateral forces inside the motor, and require lubrication, necessitating periodic maintenance.”

“The larger number of moving parts in this design implies a greater maintenance frequency, and potentially reduced reliability.”

Moreover, moving seals “are required between the working gas and the crankcase, and this too has implications for service frequency and durability” in the kinematic engine [22] The company Infinia Corporation (based in Kennewick,Washington, USA) has been delivering free-piston engines (and power systems) to commercial companies and government agencies since 1985. Next to high-reliability, the company claims that their products have no maintenance requirement (zero-maintenance) [23].

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General maintenance tasks of Dish/Stirling systems may include [4]:

Offset determination and correction for optimal two-axis sun tracking

Refilling of the Stirling engines’ working fluid

Repairing of Stirling engines

Figure 27: Hoisting platform for maintenance [20]

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12.4 Installation – Potential Tasks of Technicians on CSP Plant Construction Sites

There are several tasks a technician may have to accomplish on a construction site of a CSP plant (depending on qualification). Some of them are mentioned below [1], [4], [2]:

Welding

Fitting

Testing with high solar-fluxes

Working in elevated work areas

Installation of test equipment

Assembly, maintenance and disassembly of large steel structures

Installation of collectors or heliostat mirrors

Troubleshooting

Commissioning of collector drives

Commissioning of parts of the power block

Filling of cooling fluid into cooling system (commissioning phase or refill)

Filling of demineralised water into water-steam cycle (commissioning phase)

Cutting off or switching on the power supply of components (e.g. pumps, blowers)

Wiring

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List of figures Figure 1: Calibration of a heliostat of the PS10 solar tower’s heliostat field [4]....... 7 Figure 2: Orientation of the azimuth and zenith angle [5] ........................................... 8 Figure 3: Parts of a parabolic trough collector, edited from [7] .................................. 9 Figure 4: Kramer Junction solar park 530 MWel, California, USA [8] ................... 10 Figure 5: Degradation of mirror reflectivity over the summer months August-September (MM/DD) [8]................................................................................................... 11 Figure 6: Traditional mirror cleaning method with hand-held nozzles [8] ............... 13 Figure 7: Cleaning with a “Rotating-Head” rig [8] ....................................................... 13 Figure 8: Cleaning of the mirrors with a “Deluge-Type” stream of water [8] .......... 14 Figure 9: Raw water usage oft he SEGS III-VII parabolic trough power plants in the time period of 1989 – 1997 [8] ................................................................................. 15 Figure 10: Glass envelope of a parabolic trough receiver [7] ................................... 16 Figure 11: Split-glass envelopes as a temporary method of reducing heat losses [8]......................................................................................................................................... 16 Figure 12: Ball-joints installed at the DISS experimental plant [9] ........................... 17 Figure 13: Top: Packed dual ball joint; Bottom: Example of a dual ball joint design, edited from [10] ................................................................................................................. 18 Figure 14: Packed ball joints, edited from [8] .............................................................. 19 Figure 15: At Kramer Junction, maintenance data are entered from the solar field via radio transmitters [8] .................................................................................................. 20 Figure 16: Mirrors and collector shape qualification [11]........................................... 20 Figure 17: Parts of a large heliostat of a solar tower’s solar field, edited from [7] 22 Figure 18: Parts of a small heliostat of a solar tower’s solar field [2] ...................... 22 Figure 19: Heliostat washing methods [12] ................................................................. 23 Figure 20: Small linear Fresnel collector, edited from [7].......................................... 24 Figure 21: Example of a primary collector row [15].................................................... 25 Figure 22: Linear Fresnel collector manufactured by Ausra [16] ............................. 25 Figure 23: Sword brush (pictures left) [14]; Cleaning robots (right) [18] ................. 26 Figure 24: Dish collector [19] ......................................................................................... 27 Figure 25: Back and side view of dish collector [19] .................................................. 27 Figure 26: Dish systems in operation [20] ................................................................... 28 Figure 27: Hoisting platform for maintenance [20] ..................................................... 30 

List of tables

None.

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Reference list [1] EG&G: Technical support services for solar tower power plants http://www2.urscorp.com/albuquerque/TSS-Solar.htm Last updated: no information [2] Solar-Institut Jülich Heinrich-Mußmann-Str. 5 D-52428 Jülich www.sij.fh-aachen.de [3] The Steam and Condensate Loop

Publisher: Spirax-Sarco Limited, 2008 ISBN: 978-0-9550691-4-7 No author(s) stated.

[4] German Aerospace Center (DLR) [5] Azimuth and Zenith Angle http://www.srrb.noaa.gov/highlights/sunrise/azelzen.gif [6] http://www.leonardo-energy.org/taxonomy/term/725 Published 21 September 2007 [7] Prof. Dr.-Ing. B. Hoffschmidt, Dipl.-Ing. Johannes Sattler, M.Sc.

Revised lecture notes http://www.fh-aachen.de/uploads/media/RE3_Konz_Koll_Teil_1_js.pdf or via http://www.fh-aachen.de/hoffschmidt.html Aachen University of Applied Sciences, October 25, 2010

[8] Gilbert E. Cohen, David W. Kearney, Gregory J. Kolb

FINAL REPORT ON THE OPERATION AND MAINTENANCE IMPROVEMENT PROGRAM FOR CONCENTRATING SOLAR POWER PLANTS KJC Operating Company & Sandia National Laboratories, June 1999 Report Number: SAND99-1290 http://www.p2pays.org/ref/17/16933/1693301.pdf Appendix: http://www.p2pays.org/ref/17/16933/1693303.pdf

[9] Eduardo Zarza1, Loreto Valenzuela1, Javier León1

Klaus Hennecke2, Markus Eck2, Martin Eickhoff2, H.-Dieter Weyers2 The DISS project: Direct Steam Generation in parabolic troughs Operation and Maintenance experience. Update on project status Solar Forum 2001: Solar Energy 1CIEMAT-Plataforma Solar de Almería, Apartado 22, Tabernas, E-04200 Almería, Spain 2Solare Energietechnik, Deutsches Zentrum für Luft- und Raumfahrt e .V., (DLR) , Germany

[10] Hyspan: Company manufacturing ball joints http://www.hyspan.com/SolarPanelConnect.html Copyright © Hyspan Precision Products, Inc. 2005

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[11] Mirrors and Collector Shape Qualification German Aerospace Center (DLR) http://www.dlr.de/sf/desktopdefault.aspx/tabid-7236/12141_read-28813/ Copyright © 2011 Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR) [12] Riaan Meyer

Concentrated Solar Power – An Overview Stellenbosch University, South Africa http://www.docstoc.com/docs/32361158/Concentrated-Solar-Power---An-Overview/

[13] Linear Fresnel Collectors CSP Services GmbH http://www.cspservices.eu/index.php?hp=4& (Linear Fresnel Collectors) http://www.cspservices.eu/index.php?hp=6& (Dish/Stirling) http://www.cspservices.eu/index.php?hp=1& (Home) Last updated: no information [14] Linear Fresnel Systems by Novatec Solar. Novatec Solar is the new business brand

of Novatec BioSol AG http://www.rural-electrification.com/cms/upload/pdf/Presentations_Jordanian_Delegation_Visit/07_NOVATEC-BioSol_20071126.pdf http://www.novatecsolar.com/20-1-Nova-1.html The content and works provided on these Web pages are governed by the copyright laws of Germany Last updated: no information

[15] Picture of Primary Collector Row Project Proposal for a Compact Linear Fresnel Reflector Solar

Thermal Plant in the Hunter Valley *D. R. Mills, **G. L. Morrison, and ***P. Le Lièvre, *School of Physics University of Sydney Sydney, NSW 2006 AUSTRALIA **School of Mechanical & Manufacturing Engineering The University of New South Wales NSW 2052 AUSTRALIA ***Solar Heat and Power Pty. Ltd. (SHP) Exchange Square, 10 Bridge St Sydney, NSW 2000 AUSTRALIA Date of publication: no information http://www.solar1.mech.unsw.edu.au/glm/papers/Mills_projectproposal_newcastle.pdf

[16] Fresnel Collector manufactured by Ausra http://en.wikipedia.org/wiki/File:Fresnel_reflectors_ausra.jpg Last updated: 16 February 2010

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[17] Maintenance of linear Fresnel collectors http://www.solarpowergroup.com http://www.solarpowergroup.com/143.0.html Last updated: no information [18] Cleaning robots http://www.electron-economy.org Last updated: no information [19] Illustration of a Dish System http://solarpowerengineering.com/tag/solar-dish/ Last updated: 23 January 2010 [20] Photos of Dish Systems

http://www.basinandrangewatch.org/StirlingDish.html Last updated: no information

[21] Jeff Raines Stirling Cycle Engine http://www.enotes.com/how-products-encyclopedia/stirling-cycle-engine Last updated: no information [22] August Sean O'Connor BSc

The Feasibility of Grid Connected Solar Dish Stirling Generators within the South West Interconnected System of Western Australia (dissertation) Murdoch University School of Energy and Engineering, 2010 http://researchrepository.murdoch.edu.au/4126/1/O%27Connor_2010.pdf Date of publication: unknown

[23] Maintenance of Dish Systems http://www.infiniacorp.com/faqs.html Last updated: no information