Lecture 4:

Solar Thermal Heat Utilization

http://www.cs.kumamoto-u.ac.jp/epslab/APSF/

Lecturers:
Syafaruddin & Takashi Hiyama
syafa@st.eecs.kumamoto-u.ac.jp
hiyama@cs.kumamoto-u.ac.jp

Time and Venue:

Wednesdays: 10:20 – 11:50, Room No.: 208

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 Contents:

1. Principles
2. Technical Descriptions
3. Economic Analysis
4. Environmental Analysis

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 1. Principles
Solar thermal heat systems are installations converting solar radiation into heat in order
to heat swimming pools, produce domestic hot water, cover the demand for space
heating or supply other heat consumers.

Physical principles of energy conversion:

Absorption, emission, reflection and transmission

•The basic principle of solar thermal utilization is the conversion of short-wave solar
radiation into heat (photo thermal conversion process).
•If radiation incidences on material a certain part of the radiation is absorbed. A body’s
capacity to absorb radiation is called absorbing capacity or absorption α
•An ideal black body absorbs radiation at every wavelength and therefore has an
absorption coefficient equal to one
one.

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 1. Principles…cont.
•Emission ε represents the power radiated by a body.
•The relationship between absorption α and emission ε is defined by Kirchhoff’s law .
For all bodies the ratio of specific radiation and the absorption coefficient is constant
at a given temperature, and in terms of its amount, equal to the specific radiation of
the black body at this temperature.
•This ratio is exclusively a functionality of temperature and wavelength.
•Matter with a high absorption capacity within a defined wave range also has a high
emission capacity within that same wave range.range

Reflection and transmission:

•The reflection coefficient ρ describes the ratio of the reflected to the incident
radiation.
•The transmission coefficient τ defines the ratio of the radiation transmitted through a
given material to the entire radiation incident

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 1. Principles…cont.
Optical features of absorbers
Absorbers have to absorb radiation and Absorption (α) and reflection coefficient (ρ) of an ideal
(ideal) and a standard real absorber (real)
partially convert it into heat.
The absorber  for opaque for radiation (τ
= 0)
Therefore: α + ρ =1

An ideal absorber:
*no reflection any short-wave radiation (ρ =
0) and thus –
– completely absorbs solar radiation within
this wave
range (α = 1).
For long-wave radiation above a certain Within the spectrum of solar irradiance, the
boundary wavelength, the reflection coefficient ρreal is
situation is exactly the opposite. Given an close to zero, in the infrared spectrum (> 3 μm)
ideal absorber, it reflects all of the radiation close to one. The absorption coefficient
and does not absorb any at all. αreal demonstrates exactly the opposite.

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 1. Principles…cont.
Optical features of absorbers

•Absorption coefficient for various different materials, the transmission and reflection
coefficients for the solar irradiance and the infrared range of the solar radiation spectrum.
•Compared to the non-selective absorber, selective absorber surfaces show high degrees
of αs/εI
•αS is the absorption coefficient in the spectrum of solar irradiance, εI is the emission
coefficient in the infrared radiation spectrum.
•Such surfaces are thus also called α/ε-surfaces.
•Titanium oxide with 19 for example shows a particularly high αs/εI-ratio.
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 In order to reduce the convective thermal losses of the absorber to
1. Principles…cont. the environment, in many cases absorbers used in solar thermal
Optical features of systems have a transparent cover.
covers Ideal covers have a transmission coefficient of one in the range of
solar radiation, whereas reflection and absorption coefficient equal
zero in this spectrum.
In real life such conditions cannot be achieved

•Glass fulfils the required optical features within the luminous spectrum very well.
•Infrared light emitted by the collector , however, cannot pass through, but is mainly
absorbed.
•If the degree of absorption is high, the temperature of the glass cover rises and the
radiation losses to the environment are correspondingly high
•These losses can be reduced by vacuum-coating of layers that reflect infrared light.

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 1. Principles…cont.
Energy balance

General energy balance: describes the general energy balance of a medium that
absorbs radiation and converts it into heat:

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 1. Principles…cont.
Energy balance
In solar thermal systems the absorber is normally part of a collector. Other
components of the collector are frame, cover and insulation.

Stationary energy balance at the collector or the absorber 9

 1. Principles…cont.
Efficiency and solar fractional savings

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 2. Technical description
Main components: Collector
Essential component: liquid or gaseous heat transfer medium and pipes to
transport the heat transfer medium
Others:
*heat storage with none, one or several heat exchangers plus, for certain designs,
*pumps with a drive to maintain the heat carrier cycle
*sensors and control instruments

Collector consists of:

•Absorber
•Transparent
•Cover
•Frame insulation
•heat insulation
•heat carrier inlet and outlet

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 2. Technical description
Collector
Collector: converting solar radiation into heat. Part of this heat is subsequently
transported by a heat carrier flowing through the collector.

liquid-type flat plate collector 12

 2. Technical description
Absorber
Characteristics:
•Converts short-wave radiation into heat (photo-thermal conversion)
•Function of “radiation absorption “ is carried out by a type of absorber material with quite
a high absorption capacity within the luminous spectrum
•Low emission capacity, is aimed for in the thermal radiation wave spectrum
•In addition, the absorber has to enable a good heat transfer to the heat carrier and also
be temperature-resistant, as normally temperatures of up to 200 oC occur in an insulated
absorber with glass cover and selective coating.
•In concentrating collectors temperatures are generally even higher.

Materials for absorber:

• Mainly copper and aluminum.
• Market increase of solar thermal collectors is continuing, polymeric materials and
• steel could become more important in the future.
• In the simplest case, this basic material is painted black on the side receiving
radiation (maximum absorber temperature approximately 130 oC ).
• For a large number of absorbers, this side is also coated selectively (maximum
absorber temperature approximately 200 oC ).
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 2. Technical description
Absorber..cont.

Other important parts:

•The heat carrier flows through the channels inside the absorber.
•The energy proportion of the solar radiation on the absorber converted into
heat inside the absorber is partly transported to the heat carrier (by heat
transfer).
•The system of pipes in the absorber can vary in terms of pipe material, pipe
cross-section, length and pipe allocation within the collector.

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 2. Technical description
Cover
Characteristics:
*cover of collectors ought to be as transparent for solar radiation as possible and retain
the long-wave thermal reflection of the absorber.
*At the same time it has to reduce convective thermal losses to the environment.
Materials:
Suitable materials are glass sheets, synthetic plates or synthetic foils (e.g. made
of polyethylene or Teflon).
Notes for synthetic covers: The high level of material stress often leads to brittle
and tarnished synthetic materials.
Furthermore, the outer area can also be scratched very easily by atmospheric exposure. Thus
transmission values of synthetic covers are often not stable long-term.
most applications for cover :glass characterized by:
•high level of transparency and resistance to hail.
•low iron contents can reduce the absorption capacity in the short-wave spectrum  Thus it is
avoided that the glass sheet heats up.
•Convective thermal losses to the colder environment are reduced.
•Often infrared-reflecting layers are vacuum coated on the bottom side of the cover to reflect the
long-wave heat radiation from the absorber to the cover into the direction of the absorber.
•Thus losses can be reduced even further.
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 2. Technical description
Collector Box & other components
The collector box: holds the components required for radiation transmission,
absorption, heat conversion and insulation.

Materials:
aluminum, galvanized steel plate, synthetic material or wood.

Functions:
It gives the collector mechanical firmness and makes it environment-proof.
However, a low level of ventilation has to be ensured in order to reduce high or low
pressure caused by temperature fluctuations and remove possible humidity.

Other components. Thermal insulation made of standard insulation material (e.g.

polyurethane, glass fibre wool, mineral wool) belongs to the group of other
components.

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 2. Technical description
Installation
Collectors are mainly installed on pitched roofs; in this respect the integration into
the roofing or the on-roof installation, on top of the tiles, are common technical
solutions.

*Integration into the roof is less visible and cheaper than the on-roof installation. It
is preferably used for new buildings or larger collector arrays on already existing
roofs. Additionally, roofing costs are saved for the parts of the roof where the
collectors are installed.

*Installation of collectors on flat areas (e.g. on flat roofs, in gardens) facilitates

optimal adjustment and incline when compared to the installation on pitched
roofs.
Mainly standardized frames are used to integrate the collector.
Frames need to be arranged so that shading is avoided.

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 2. Technical description
Collector designs and practical applications
According to the heat carrier and the way they absorb radiation:
− Non-concentrating swimming pool liquid-type collectors
− Non-concentrating glazed flat-plate liquid-type collectors
− Non-concentrating glazed air collectors,
− Radiation-concentrating liquid-type collectors
− Radiation-concentrating air collectors

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a special type of flat-plate collector 19
 2. Technical description
Collector designs and practical applications…cont.
Non-concentrating swimming pool liquid-type collectors:
•The basic design (used most frequently in its simplest form) consists of: an absorber mat
with a corresponding system of pipes for the heat
•This collector design is often referred to as the collector type absorber
•It is preferably used for heating open-air swimming pools.
•This application needs water at a temperature around ambient temperature.
•Heat insulation to the ambient is not needed, because there is no driving force
(temperature difference) for heat losses.
•Therefore a transparent cover and an insulation on the back side of the collector are not
needed and the optical losses are only due to the reflection coefficient of the absorber
•(The absorber material is mainly EPDM (ethylene-propylen-dienmonomers) which is able
to withstand UV radiation and temperatures up to 150 C.
•This absorber type is very cheap and results highly efficient for the swimming pool
application.

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 2. Technical description
Collector designs and practical applications…cont.

Non-concentrating glazed flat-plate liquid-type collectors:

•higher temperature levels be required, glazed flat-plate collectors (many cases).
•They can be built with one or more transparent cover sheets.
•In order to further reduce the convective thermal losses from the absorber to the
cover, the space between the two can be evacuated, which turns the collector into a
vacuum flat-plate collector.
•Due to the pressure difference, the cover sheet has to be supported from the inside in
that case.
•Heat losses to the back of the collector are avoided by insulation material.
•Absorber, cover and insulation are fixed by a collector case.

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 2. Technical description
Collector designs and practical applications…cont.

Non-concentrating air collectors:

•Due to the low heat transfer coefficient between the absorber and the air, the contact
area between absorber and air flow has to be large.
•This is for example ensured by ribbed absorbers, multi-
multi-pass systems or porous
absorber structures.
•As no frost, overheating or corrosion problems can occur, air collectors have a simpler
design when compared to liquid-type collectors
•Disadvantages: the large channels and the often significant drive capacities required
for fans.
(The reason why air collectors are not widely used for the heating of buildings or the
supply of domestic hot)
•Nevertheless, they are used in individual cases, e.g. for solar food drying systems and
low-energy houses with exhaust air heat recovery that are already equipped with air
distribution and collector systems and thus do not require a water heating system.

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 Concentrating liquid-type or air collectors:
•These collector types reflect the direct share of solar radiation through mirror areas
and thus concentrate the direct radiation on the absorber area.
•The level of concentration of solar radiation is the concentration ratio or the
concentration factor C. It is defined as the ratio of the optically active collector area to
the absorber area impinged on by radiation.
•The maximum theoretical concentration ratio of 46,211 is the result of the distance
between sun and earth, and the sun radius.
•Technically, concentration factors of up to a maximum of 5,000 can be achieved at
present
the temperature that can be achieved in
the absorber depends on the
concentration factor
*The theoretical maximum absorber
temperature just equals the surface
temperature of the sun in the case of a
maximum concentration ratio
(approximately 5,000 K).
*The temperatures that can be
realistically achieved in the absorber are
significantly lower. Rotation parabolic
mirrors, for example, can achieve
absorber temperatures of a maximum of
1,600 oC

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 2. Technical description
Data and characteristic curves

Optical and thermal losses are the decisive factors for the collector efficiency
Optical losses are determined by:
The product of the cover transmission coefficient and the collector absorption
coefficient.
This loss is only dependent on the material and – approximately – radiation and
temperature-independent.
Thermal losses are described together with:
Other non-constant losses by a constant heat transition coefficient
As a first approximation, this loss is linearly dependent on the difference
between the absorber and the ambient temperature and inversely proportional
to radiation

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 2. Technical description
Data and characteristic curves…cont.
In the case of large temperature differences, assuming the linear dependency on the
temperature, an increasing deviation from the real efficiency curve is observed.
It becomes obvious that the approximation line for the efficiency is getting flatter with an
increase in radiation and thus a change in the temperature difference between absorber
and the environment has less impact.

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 2. Technical description
Data and characteristic curves…cont.

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 2. Technical description
Further system elements:

• Heat Storage
• Sensor and control systems
• Heat transfer medium
• Pipes
• Heat Exchangers
• Pumps

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2. Technical description
Energy conversion chain

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2. Technical description
Losses

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2. Technical description
System design concepts

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 2. Technical description
Applications:

• Solar heating of open air

• Swimming pools
• Small systems
• Solar-supported district heating systems

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 3. Economic Analysis
• Investment
• Operation Costs
• Heat Generation Costs

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 3. Economic Analysis
Investment
• Collector
• Storage
• Other system components
• Installation & operation
• Total investments

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 Collector
The approximate costs of the collectors currently available are between 50 and 1,200 €/m2
The decisive factor is the collector type:
*simple absorber mats: ~ 40 and 80 €/ m2
*single-glazed flat-plate collectors with black or selective absorbers: between 200 and 500
€/ m2 depending on the plant size.
*Vacuum pipe collectors, multi-covered flat-plate collectors or
collectors improved by transparent heat insulation: more than 700 €/ m2 and sometimes
above.

•Apart from the technology, the collector costs also depend on the size of the collector
•Collector modules with large areas are cheaper, relative to their size, than small
collectors; in some cases large collector modules have been offered at 220 €/ m2, or even
less for very large collector areas (i.e. below 200 €/ m2), including installation and piping

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 Storage
•The costs for the storage depend mainly on the storage volume;
•Investment costs for smaller systems with a storage content between 200
and 500 l including the heat exchanger are between 1.5 and 3 €/l storage
volume or 100 to 200 €/m2 collector area.
•Heat-insulated steel tanks of up to 200 m3 are currently the state-of-the-art
of technology.
•A 100 m3 storage costs between 300 and 400 €/ m3
•Larger reservoirs in the ground are significantly cheaper. Total costs between
75 and 80 €/m3 were estimated for a ground reservoir with a size of 12,000 m3.
This includes the labor and material costs for setting up the building site,
ground works and drainage plus steel and concrete works.
•Other sources quote costs between 50 and 80 €/ m3 for heat-insulated
ground reservoirs with metal foils for sealing and volumes between 7,000 and
40,000 m3.

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 Other system components
pipes
•
• sensors
• control instruments
• Pump
• the anti-freeze
• all installations related to security technology (e.g. security and shut-off valves, expansion tank).
For decentralized domestic hot water supply systems:
normally 20 to 30 m of pipes have to be installed
Thus, the costs for the pipes including the heat insulation are between 40 and
70 €/m2 collector area
In total, the investment costs for these components are between 60 and 90 €/
m2 collector area
For centralized solar thermal domestic hot water systems:
*the total costs of other components can vary between 65 and 130 €/ m2.
As a first estimate, this range can also be taken as being representative for larger
solar-supported district heating systems

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 Installation & operation
Solar thermal systems for domestic hot water heating for households:
•often partly or entirely self-installed.
•Costs for the potential people in charge of the system are generally very low.
•However, if the system is installed by a company, the specific installation costs are
between 70 and 250 €/m2 of collector area. These costs include:
installation of the collectors (~20 to 30 % of the overall installation costs)
mounting of pipes (accounts for the largest share of the costs)
connection to the solar storage
installation of the sensors and the control instruments and the pump
connection to the residual heating system
charging and commissioning of the system
For central solar thermal domestic hot water support and the larger solar district heating
systems:
•The specific costs for installation and commissioning of the system are often lower.
•The installation costs for larger collector arrays are approximately between 10 and 20 % of
the overall collector costs or between 30 and 50 €/m2.
•The total costs for installation and commissioning of the system are approximately
between 50 and 100 €/m2.
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 Total investments
Standard domestic hot
water systems available
in the marketplace
usually cost between
5,000 and 6,000 €.
In comparison,
Self-installation systems
exclusively designed for
domestic water heating
are significantly cheaper;
most of these systems
cost between 3,000 and
5,000 €.
Systems with a larger
collector area are
proportionally cheaper.

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 3. Economic Analysis
Operation costs
*During Normal operation, Maintenance costs only occur for the exchange of the heat
transfer medium and for small repairs (e.g. exchange of seals).
•The operation of the solar thermal system also requires auxiliary energy as the heat
transfer medium is normally pumped through the collector circuit (the operation cost)
•The related costs largely depend on the price for electricity.
•At an electricity price of 0.19 €/kWh and a demand for electricity between 0.008
and 0.03 kWh per provided kilowatt hour of thermal energy, operation costs are
around 6 to 10 €/a for decentralized solar thermal systems for domestic hot water heating
and between 18 and 25 €/a for solar combined systems.

Maintenance costs for most parts of the system are between 1 and 2 % of the overall
investment (without installation and commissioning).
Thus, the entire annual maintenance and operating costs for solar thermal domestic
water heating and the combined systems are at approximately 0.9 to 1.8 % of the
overall investment (including installation and commissioning).

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 3. Economic Analysis
Operation costs…cont.

For a larger solar-supported district heating system:

annual total costs of approximately 1 % of the overall investment costs 
for maintenance and miscellaneous costs (e.g. insurance) (excluding
installation and commissioning of the system)

Heat generation costs:

The specific energy supply costs can be derived from the absolute
investments indicated plus the costs for maintenance and operation.
The investments are amortized over the technical lifetime of the system
(20 years)

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 4. Environmental analysis

Solar energy systems are characterized by:

*noise-free operation without direct substance releases
Analysis of local environmental aspects:
*construction
*normal operation
*Malfunction
*end of operation

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 4. Environmental analysis
construction
•Only the production of the absorber is of particular environmental significance.
•In the past: galvanic coating methods were used that required a high level of energy
input and produced problematic waste.
•Recently: vacuum coating or sputtering, which is much less problematic in terms of
environmental impact during the production process, has increasingly gained
importance
The anti-reflection glasses that have recently been increasingly used to
cover the solar collectors can also be produced following environmental
criteria

Material for solar storages can be produced and processed with little
environmental impact have increasingly been used over the last few Years
•E.g: polyurethane foams (PU) that can cause environmental problems during
production and disposal, have been replaced by polypropylene (PPP)
(PPP)

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 4. Environmental analysis
construction…cont.
*During the production process: no environmental effects occur that
exceed the general average.
*If the appropriate environmental protection regulations are adhered to,
a very environmentally-friendly production is generally possible.

The rooftop installation of collectors can be dangerous.

But, the risk of dying as a result of falling from the roof during system
installation, is comparable to that of a roofer, chimney sweeper or carpenter,
and is thus considered low.

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 4. Environmental analysis
normal operation
•As the operation of solar collectors does not release any substances, they can
generally be run in a very environmentally-friendly way.
•Additionally, collectors installed on the roof are relatively similar to roofs in terms
of their absorption and reflection behavior  hardly any negative impacts
•The roof areas covered with collectors that can sometimes be seen from far away
only have a minor impact on the visual appearance of cities and villages.
•The space utilization of solar collectors is also quite low, as generally already
existing roof areas are used.
•Only if collectors are installed on free areas, a negative impact on the microclimate
might be possible. However, it is limited mainly to the shadow area and is negligibly
low.
•Evaporation during collector standstill ought to be prevented by an appropriate
system design and thus not be a health risk.
risk

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 4. Environmental analysis
malfunction
•Environmental effects caused by larger failures cannot be expected from solar
collector systems.
•Health risks for human beings or groundwater or soil contamination by a possible
leakage of the heat transfer medium containing antifreeze compound are very unlikely
due to an advanced technology.
•Such problems can also be avoided by regular inspections and the use of food-
safe heat transfer media (e.g. propylene-glycol-water-mixes).
•Fires at the collectors can only be expected if the entire building on which they are
installed is on fire.
•Possible dangers of injuries by falling collectors that have not been correctly installed
on the roof can normally be avoided by maintaining the generally valid health and
safety standards; the danger potential is at the same level as that of roof tiles.

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 4. Environmental analysis
malfunction…cont.
•Legionella can multiply significantly in domestic hot water systems and thus
become a danger for human beings if they get in contact with the infected
water.
•However, this is not a problem specific to solar systems, but this problem has
also occurred in solar systems in the past.
•As legionella die quickly at a temperature of approximately 60 oC, this
danger can be easily limited by appropriate technical measures (following
standards)
•The potential environmental impacts of solar thermal heating are also low in
case of an accident.

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 4. Environmental analysis
end of operation

•In principle, recycling main parts of solar thermal systems (e.g. solar
collector, storage) is possible.
•The producers in Germany, for example, are also committed to take the
collectors back after the end of the technical lifetime and recycle the
materials as part of the German Blue Angel Agreement.
•Thus, there are environmental effects common for certain materials
being recycled.

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