Receiver of a HighTemperature Solar Energy Thermal System

1
Receiver of a High-Temperature Solar Energy
Thermal System

• Case Study
• CHE 620

2
Introduction

• The U.S. and the Western World experienced two
  fossil fuel dislocations in the past 30 years.
• The first was in 1973-74 and the second in
  1979-1980.
• (Were probably not done with them, either.)

3
Introduction

• The result was a sudden, massive investment in
  alternative, renewable energy sources.
• The de facto demise of OPEC during the 1980s and
  90s (until, apparently, very recently) resulted
  in a steep drop in the price of oil (the maximum
  was 34 per barrel in June 1982 to a low of about
  11 a few years ago, but now back into the upper
  20s and low 30sbut lets consider real
  dollars.)

4
Introduction

• The large price drop resulted in a nosedive in
  RD into alternative energy sources.
• This is considered to be unfortunate, because
  petroleum continues to be non-renewable and
  because there is the potential for widespread use
  of alternate energy sources.
• There are currently a number of sustained efforts
  around the U.S. and around the world, but I
  believe that it will ultimately require market
  forces to substantially alter the global energy
  picture.

5
Introduction
6
Solar Energy Thermal Systems

• These fall into two broad categories
• Low temperature
• used for small scale space heating and cooling
  and water heating
• they typically use
• flat plate collectors
• water or air as the heat transfer fluid
• have a basic storage medium, such as water in
  insulated tanks
• employ a process temperature in the 45 to 100C
  range

7
Solar Energy Thermal Systems

• High temperature
• used for steam production and direct process
  heating systems
• they typically use
• concentrating collectors and receivers of the
  nontracking or active-tracking type
• steam for the process application
• employ a process temperature in the 90 to 300C
  range
• use some form of energy storage, such as organic
  oils (up to 350C) and rock beds

8
Solar Energy Thermal Systems

• In SETS, collectors are used to concentrate
  incident global solar irradiation.
• The receiver transfers this energy to a working
  fluid that is used to provide process heat.
• The heat is then used for either industrial
  applications or to drive a suitable heat engine
  to produce electricity.

9
Solar Energy Thermal Systems

• Some operational solar industrial process heat
  and electricity projects

10
Solar Energy Thermal Systems

• The receiver is a key component that determines
  the operating temperature at which power
  conversion can take place.
• Operating temperatures as high as 800C have been
  achieved, making possible highly efficient heat
  engines.

11
Receiver Design Requirements

• The receiver sits between the concentrator and
  the power conversion units.
• The receiver design requirements are thus tightly
  integrated with the specific application and
  system configuration, including
• the temperature and pressure needed by the
  thermal process or power conversion system
• the heat transfer characteristics of the working
  fluid
• the mechanical configuration of the system
• the optical properties of the concentrator

12
Receiver Design Requirements

• high tensile strength
• high fracture toughness
• high resistance to thermal shock
• good fatigue and creep strength
• stability at elevated temperatures
• high resistance to corrosion and oxidation
• easy fabricability

• available at reasonable cost
• capacity to withstand the flux nonuniformities
  created by the concentrator
• capacity to withstand stresses induced by
  nonsymmetric heating and thermal gradients
• high maximum use temperature (800C)

13
Receiver Design Requirements

• Materials temperature limits are a major design
  constraint
• stainless steels limited to about 700C
• superalloys can go to 1000C, but their
  availability is less certain
• thus, ceramics receive serious consideration, and
  they can go to 1000 to 1400C

14
Receiver Design Requirements
15
Receiver Design Requirements
16
Latent-Heat Thermal Energy Storage Materials

• Case Study
• CHE 620

17
Introduction

• Latent heat storage is an attractive means by
  which to store solar heat
• provides a high energy storage density
• stores the energy at constant temperature
• Heat can be stored in two ways
• latent heat of fusion (material melts or freezes
  at the requisite temperature)
• heat of crystallization (transformation of one
  solid phase to another)

18
Introduction

• With a temperature range of 0 to 120C, a variety
  of low-temperature, fairly constant load thermal
  applications are possible
• domestic water heating
• direct or heat-pump-assisted space heating
• greenhouse heating
• solar cooling
• Any material that melts with an accompanying high
  heat of fusion in this range may be used as a
  latent heat storage material if it meets certain
  criteria.

19
Introduction

• A latent heat storage system has three components
• a heat storage material that undergoes a
  solid-to-liquid phase transition within the
  desired operating temperature range
• a containment vessel for the storage material
• a heat exchanger for transferring the heat from
  the heat source to the heat storage material and
  from the heat storage material to the heat sink

20
Material Selection ConsiderationsDesirable
Properties

• A melting point suitably matched to the service
  temperature range of the thermal system.
• High storage capacityhigh latent heat of fusion
  per unit mass and volume, thus reducing the
  amount of material needed to store a given amount
  of energy.

21
Material Selection ConsiderationsDesirable
Properties

• High density in both solid and liquid phases,
  thereby reducing the volume of the container
  needed to hold the material.
• High thermal conductivity in both the solid and
  liquid phases, thereby reducing the temperature
  gradients required for charging and discharging
  the storage material.

22
Material Selection ConsiderationsDesirable
Properties

• Congruent meltingthe ease with which the
  material melts completely leading to identical
  compositions for the liquid and solid phases,
  thus eliminating the problem of segregation
  arising from a difference in density between the
  solid and liquid phases.

23
Material Selection ConsiderationsDesirable
Properties

• Small volumetric changes during phase transition,
  thereby enabling the use of a containment vessel
  and heat exchanger having simple geometries.
• Minimal supercooling during freezingthe melt
  should crystallize at its thermodynamic freezing
  point.

24
Material Selection ConsiderationsDesirable
Properties

• Good chemical stability.
• No chemical decomposition tendency, thus assuring
  a long system lifetime.
• Non-corrosive to containment vessel materials,
  such as stainless steel, mild steel, tin-plated
  mild steel, copper, and aluminum and its alloys.

25
Material Selection ConsiderationsDesirable
Properties

• Non-toxic, non-flammable, and non-explosive.
• Readily available in large quantities at a
  reasonable price.
• Reliabilitythe phase change process should be
  completely cyclable without undue degradation
• Low vapor pressure at operational temperatures.

26
Candidate Materials

• Possible materials can be grouped according to
  the taxonomy below

27
Paraffins

• Straight-chain hydrocarbons with little chain
  branching.
• Waxy consistency at room temperature.
• Contain one major constituent, alkanes, with
  several compositions CnH2n2
• The n-alkane content in paraffin waxes is on the
  order of 75 to 100.
• Pure paraffins consist only of alkanes for
  example, octadecane is C18H38.

28
Paraffins

• There are two solid-phase allotropes of
  paraffins
• the high-temperature form, which exists at a
  temperature that is marginally higher than the
  melting point of the substance and is soft
• the low-temperature form, which exists at a
  temperature below the melting point of the
  substance and is hard and brittle

29
Paraffins

• Advantages of paraffins
• ready availability
• large temperature range
• high heat of fusion
• freeze without supercooling
• Disadvantages
• low thermal conductivity
• large volume change on phase change
• flammable

30
Non-paraffin Organics

• Prime examples are fatty acids of general formula
  CH3(CH2)2nCOOH
• Advantages
• high heats of fusion (comparable to paraffins)
• reproducible melting and freezing points
• minimal supercooling tendency
• Disadvantages
• more costly (by 50) than paraffins
• low thermal conductivity
• varying levels of toxicity
• unstable at elevated temperatures

31
Eutectics of Organic and Inorganic Compounds

• Have the attraction of fixed melting and freezing
  points.
• But experience is limited because they are new to
  this application.

32
Inorganic Compounds

• One group of these are the salt hydrates, with
  general composition MnH2O, where M is an
  inorganic compound.
• Primary attribute is high volumetric latent
  storage density.
• Primary drawbacks include
• incongruent melting
• poor nucleation properties, resulting in
  supercooling prior to freezing

33
Inorganic Compounds
34
Properties of Candidate Materials
35
Operational System Considerations

• Corrosion
• stainless steel is compatible with most PCMs
  (phase change materials)
• mild steel is fairly resistant to most PCMs
  except for Zn(NO3)26H2O (an inorganic salt
  hydrate) and the Mg(NO3)2MgCl2 eutectic.
• copper is also fairly resistant to all PCMs
  except for Na2S2O35H2O (sodium thiosulfate
  5-hydrate)
• aluminum and its alloys generally show poor
  resistance to salt hydrates except for
  Na2S2O35H2O
• most polymeric materials are generally
  corrosion-resistant to a large number of
  inorganic salt hydrates and eutectic compounds.

36
Thermal Cycling

• Most severe condition faced by the PCM.
• If the cycle is only once a day, there will be
  over 7000 freeze/thaw cycles during a 20-year
  system life.
• Generally, laboratory tests consisting of
  thousands of cycles are needed to ascertain
  long-term stability, including measurement of
• variation of temperature within the material over
  time
• variation of heat transfer rate during charging
  and discharging of the substance over time
• variation of stored thermal energy as a function
  of the number of cycles
• Little data exists

37
Thermal Cycling

• One study of Glaubers salt (Na2SO410H2O) plus
  3 borax with and without a thickener.

38
Aperture Plate of a Solar Thermal Dish Collector

• Case Study
• CHE 620

39
Introduction

• Point-focus is one type of active tracking
  collector used in solar thermal system.
• Global irradiation is focused by a concentrator
  onto a receiver, where the radiation is absorbed
  and its energy transferred, with minimum losses,
  to heat a working fluid.

40
Introduction

• Schematic representations of reflecting
  concentrators

41
Introduction

• Another application proposed at one time
• But, alas, it was not to be!

42
Introduction
43
Introduction

• In operation, the concentrator is oriented toward
  the sun, and, through tracking devices, follows
  its motion as faithfully as possible.
• The concentrated radiation enters the receiver
  through an aperture.

44
Introduction

• Now, should a malfunction occur and the
  concentrator comes to rest, the rotation of the
  sun causes the spot of concentrated radiation to
  move away from the aperture and across various
  sections of the system.
• When this occurs, these sections are subject to
  high dosages of heat (above 300C, and severe
  damage may result.
• (The sunspot at the focus of the concentrator
  is about 0.2 mm in diameter, and the flux at the
  center of that spot is on the order of 1 to 15
  MW/m2.)

45
Materials Selection Considerations

• Obviously, the dominant consideration then is the
  ability of the material to withstand this
  sunspot walk-off without the aid of active
  cooling.
• At the earths rotational speed, it takes about
  15 minutes for the spot to traverse a distance
  equal to its diameter.
• Another consideration is the ability to withstand
  lip heating, which is the small amount of
  radiation that fails to pass through the
  aperture, but hits the aperture plate.
• This happens due to the diffuse nature of global
  irradiation, tracking errors, and warpage of the
  collector during operation.

46
Candidate Materials

• Graphite
• Silicon Carbide
• Silica
• Silicates (such as mullite and cordierite)
• Alumina

• Zirconia
• Steel
• PTFE
• Aluminum
• Copper

47
Candidate Materials
48
Candidate Materials
49
Candidate Materials

• From the data shown on the previous two pages,
  the best material is graphite grade G90.
• It is an extruded material that is impregnated
  with coal-tar pitch and then regraphitized to
  reduce its porosity.
• It is relatively expensive, however (50 per kg).

50
Candidate Materials

• Type CS graphite also performs reasonably well,
  and it costs about one-tenth as much as the G90
  grade.
• The CS material needs to be strengthened.

51
FINIS