solar collector technology - queen's universitymy.me.queensu.ca/courses/mech4301/me 430 solar...

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Solar Collector Technology (Solar Collection 101)

Solar Calorimetry Laboratory

Dept. of Mechanical and Materials Engineering

Queen’s University

Reference Material

•“Planning and Installing Solar Thermal Systems”, James &

James/Earthscan, London, UK

•Medium Temperature Collectors, State of the Art within Task 33/IV,

Subtask C, Werner Weiss, AEE INTEC, Matthias Rommel,

Fraunhofer ISE

• “Solar Engineering of Thermal Processes”, Duffie & Beckman,

John Wiley & Sons Inc.,

•Federal Technology Alert, Parabolic-Trough Solar Water Heating,

Produced for the U.S. Department of Energy (DOE)

by the National Renewable Energy Laboratory, 2000

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Solar Collector Technology

1.Solar Resource

2.Solar Collector Basics

3.Thermal Performance

4.Solar Collector Selection

5.Conclusion

Solar Resource: Definitions Global solar

irradiance and its

components

The radiation from the sun that meets the earth without any change in direction is

called direct or beam radiation, Gdir

.

The radiation from the sun after its direction has been changed by scattering in

the atmosphere is called diffuse radiation, Gdif

.

The radiation from the sun after it is reflected on the ground is called the ground

reflected radiation, Gref

.

The sum of the beam, diffuse and reflected solar radiation on a surface is called

the global solar irradiance, GG

.

GG

= Gdir +

Gdif +

Gref

From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan, London, UK

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Solar Energy Availability

The amount of solar energy available on the earth depends on the

geographical latitude and the time of day and year at a given location.


Average Solar Energy

Monthly solar irradiation (kWh/m2 per day on a horizontal surface) around the world

The average annual global horizontal solar energy is

greater at lower latitudes, however this effect may be

reduced by tilting the receiver when at higher latitudes


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Effects of Receiving Surface

Tilt (fixed orientation)

From “Solar Engineering of Thermal Processes”, Duffie & Beckman

Tracking vs

stationary

Collector Orientation

Tracking


1.Solar Resource




5.Conclusion

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Solar Collection Basics: Collector Types

• Stationary

– Fixed racks or roof installation

– No moving mechanical components

– Radiation intensity varies over day and season

• Tracking

– Increases incident solar radiation

– Enables high concentrations/temperatures

– Usually increased mechanical complexity

• Hybrids

– “Fixed” racks can be adjusted in tilt to account

for seasonal variations

– Tracking collectors can be single or dual axis

tracking

Solar Collection Basics: Collector Types

• Stationary

– Unglazed Flat Plate (UG)

– Glazed Flat Plate (SGFP, DGFP, TGFP)

– Evacuated Tube (ETC’s)

– Stationary Compound Parabolic Concentrating (CPC)

• Tracking

– Tracking Compound Parabolic Concentrating (CPC)

– Linear Parabolic Trough

– Compound Parabolic

– Central receiver

– Fresnel concentrating

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Flat Plate Solar Collector Designs

Different collector designs

The task of a solar collector is to achieve the highest possible thermal yield.


Unglazed Swimming Pool

Collectors

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Glazed Flat-Plate Collectors Advantages

• offers multiple mounting options

• good price/performance ratio

• typically cheaper than vacuum collector

• proven performance --durable

Disadvantages

• lower efficiency for high temperature applications because the

heat loss coefficient is higher (recent work on multi-glazed is

improving high temperature performance)

• not normally used for generating high temperatures (+100oC)

• may be heavier than other options

Glazed Flat-Plate Collectors 1. Frame

2. Seal

3. Transparent cover

4. Frame – side-wall

profile

5. Thermal insulation

6. Full-surface

absorber

7. Fluid channel

8. Fixing slot

9. Rear wall From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan, London, UK

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Evacuated Tube Collectors (ETC’s) Advantages

• achieves a high efficiency even with large ΔT’s between

absorber and surroundings

• low in weight, can be assembled at installation site

• may have lower wind loading?

Disadvantages

• more expensive than a glazed flat-plate collector

• cannot be used for in-roof installation

• most heat pipe systems need to be inclined at least 25o tilt to

horizontal

Evacuated Tube Collectors Evacuated tube collectors

The absorber is installed as either flat (A) or upwards vaulted (B) metal

strips or as a coating applied to an internal glass bulb in an evacuated

glass tube.

A B

The tubes are linked at the

top by an insulated

distributor or collector box,

in which the feed and

return lines run.

At the base, the tubes are

fitted to a rail with tube

holders.

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Vacuum Collectors 1. Direct Flow

Through Evacuated

Tube Collectors

Sydney Collector

The collector tube consists of a vacuum-sealed double tube. The

inner glass bulb is provided with a selective coating of a metal

carbon compound on a copper base. Into this evacuated double

tube is plugged a thermal conducting plate in connection with a U-

tube to which heat is transferred.


Heat Pipe ETC Collectors

In this type of collector a selective coated absorber

strip, which is metallically bonded to a heat pipe, is

plugged into the evacuated glass tube.

collector with dry

connection

From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan,

London, UK

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Sun Tracking Concentrating Solar Collectors Concentration of solar radiation to increase high temperature performance

Concentration of solar radiation: single reflector

with two-axis tracking Concentration of solar radiation: multiple

reflectors with two-axis tracking From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan, London, UK

Parabolic Trough Collectors

The world's largest solar

power facility, located near

Kramer Junction, CA.

Concentrating Solar Collectors

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Other Tracking Concentrating Solar

Collector Concepts

Fresnel Concentrator

Fixed (non-tracking) Compound

Parabolic Collectors

http://www.buildinggreen.com/press/topten2004/Solargenix_CPC_x-sec.jpg

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1.Solar Resource




5.Conclusion

Solar Collector Efficiency

A

o

Q

G

Energy flows in the collector:

The efficiency, η, of a collector is

defined as the ratio of usable

thermal power to the irradiated

solar energy flux:

A o 1 2

1 2

available heat quantity(Q ) = irradiance (G ) - reflection losses (G and G )

- thermal losses (Q and Q )


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Solar Collector Thermal Performance

Energy flows in the collector:

QU = S – QL – QS

QU (or QA) is the rate of extraction of solar energy by the heat

transfer fluid circulating through the solar collector;

S is the rate of solar energy absorption;

QL is the rate of thermal loss from the collector enclosure to the

surroundings;

QS is the rate of energy storage within the solar collector

To determine the thermal

performance of a solar

collector, a heat balance must

be performed. This heat

balance must include the heat

transfer terms such as

conduction, convection and

radiation heat transfer.


Collector Efficiency

U T L abs ambL

T c T c T c

Q G ( ) - U (T -T )S - Qη = = =

G A G A G A

It is often convenient to express the performance of solar

collectors in terms of efficiency where efficiency is equal to:

In measuring the performance of a solar collector, it is easier

to estimate the absorber plate temperature as the average of

the inlet and outlet fluid temperature (flowing through the solar

collector).

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Solar Collector Efficiency


Thermal losses

Solar Collector Thermal Performance

The performance of a solar collector depends on:

• its optical and thermal performance as characterized by

the values of and UL;

• the intensity of the sunlight striking (or incident on) the

collector surface;

• the surrounding environmental temperature; and

• the temperature of the absorber plate (usually very close

to the temperature of the fluid flowing through it.

e( )

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Collector Efficiency Factor in Simplified

Performance Model The collector efficiency equation can be represented in two

ways where the fluid temperature is used to describe the

collector performance and heat losses rather than the

absorber plate temperature, Tp, e.g.:

1) based on the average temperature of the fluid flowing

through the solar collector, i.e., where Tfm=(Tfi+Tfe)/2 as is

done in Europe, or

2) Based on the temperature of the fluid entering the solar

collector only, i.e., Tfi rather than Tfm.

In both these case, a factor is required to adjust the

efficiency to compensate difference caused by making these

assumptions. These Factors are F’ (the collector efficiency

factor) or FR (the heat removal factor), respectively

The standard performance equations are thenrevised to add

thee factors.

Collector Efficiency Factors in Simplified

Performance Models The collector efficiency factors (F’ or FR) account for the fact

that the average absorber temperature is not equal to the

average or inlet fluid temperature, respectively. They are

unit-less and has a value between 0 to 1.

Therefore the efficiency can be written as

or

F’ is a weak function of flow rate but can depend on the

design of the absorber plate and the rate of heat loss, UL, of

the solar collector. FR is a stonge function of flow rate and

can depend on the design of the absorber plate and the rate

of heat loss, UL, of the solar collector.

u fm ambL

T c T

Q (T -T )η = = F' ( ) F' U

G A (G )

u fi ambR R L

T c T

Q (T -T )η = = F ( ) F U

G A (G )

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We will use the simplified “Hottel-Whillier-Bliss” representation of solar

collector performance is based on Tfi in this class. An example plot for a

fixed mass flow rate is shown below.

Values of Efficiency, η from can be represented by a linear equation of

the form y = mx + b. In such a case, y = η, m = - FRUL and b =FR .

Results are usually plotted as η vs. (Tfi-Tamb)/GT, i.e., x = (Tfi-Tamb)/GT

Solar Collector Efficiency Plot

Solar Collector Performance Plots

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

(Ti-Ta)/G, (m2 oC) / W

Eff

icie

ncy

eRF ( )afi

eR R L

T

T - Tη = F (τα) - F U

G

R LSlope = - F U

e( )

Typical Efficiencies of Collectors As can be seen, each collector has a specific performance

curve.

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Collector Flowrate Effect

As mentioned the value of FR strongly varies with flowrate as

shown in the example plot below.

Effect of Collector flowrate on FR

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5

Flowrate, L/min

He

at

Re

mo

va

l F

ac

tor,

FR

This causes the performance plot to rotate around the X intercept. An

example plot showing the effect of increasing flowrate is given below.

Collector Flowrate Effect


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9


Eff

icie

ncy

Mass flowrate

increasing

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9


Eff

icie

ncy

Typical Efficiencies of Collectors

# (W/m2oC)

1 0.5 - 0.75 1 - 2 Depends on tube spacing for ETC

2 0.65 - 0.8 3 - 8 Depends on # of covers and absorber coating

3 0.8 - 0.95 10 - 20 Depends on wind speed

3 - Unglazed Swimming

Pool Absorber

2 - Glazed Flat

Plate Collector

1 - Vacuum Tube

Collector

R eF ( ) R LF U

Solar collector efficiencies generally fall within specific ranges.

3

1

2


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9


Eff

icie

ncy

Typical Efficiencies of Collectors

# (W/m2oC)

1 0.5 - 0.75 1 - 2 Depends on tube spacing for ETC

2 0.65 - 0.8 3 - 8 Depends on # of covers and absorber coating

3 0.8 - 0.95 10 - 20 Depends on wind speed

3 - Unglazed Swimming

Pool Absorber

2 - Glazed Flat

Plate Collector

1 - Vacuum Tube

Collector

R eF ( ) R LF U

Solar collector efficiencies generally fall within specific ranges.

3

1

2

Ambient

Low Temp Medium High Temp

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Other factors

Other factors affect solar collector performance

depending on the solar collector design and the

load temperature and atmospheric conditions,

including:

• angle of incident solar radiation;

• amount of diffuse solar radiation, clouds etc.;

• tilt angle;

• wind speed and absolute air temperature.

These will not be considered further in this

course.


1.Solar Resource




5.Conclusion

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Reference Area When comparing collectors, the reference area is

important – that is, the surface area from which the

collector’s characteristics values are drawn. For North

American ratings, the reference area is equal to the gross

area.

However, solar collector test results and efficiency

characteristics, may be shown with respect to aperture,

absorber or gross area making comparisons between

products difficult.

Efficiency based on absorber or aperture will be higher

than that based on gross area although delivered energy

will be the same.

Geometry of Solar Collectors

Cross-section of a flat-plate collector

The gross surface area (collector area) is the product of the outside

dimensions, and defines for example the minimum amount of roof area that is

required for mounting.

The aperture area corresponds to the light entry area of the collector – that is

the area through with the solar radiation passes to the collector itself.

The absorber area (also called the effective collector area) corresponds to the

area of the actual absorber panel.


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Geometry of Solar Collectors

Cross-section of a heat-pipe evacuated tube collector with description of the

different surface areas

From “Planning and Installing Solar Thermal Systems”, James &


Concentrating solar collectors

• Primarily used for higher temperatures

• Work best with direct bean sunlight, i.e., no

clouds or scattering of solar radiation

• High concentration ratios require the solar

collector to track the sun as it moves across

the sky in order to focus the sunlight on its

receiver

• This increases complexity and cost but allows

high efficiency at high temperature

• Primarily used for power generation

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http://sopogy.com/blog/wp-content/uploads/2010/01/collectors.jpg

A solar field for electricity production: The 1,000

parabolic trough collectors by Hawaiian manufacturer

Sopogy, which stand in the hot Kona desert on the

Big Island of Hawaii, equal the output of a 2 MW

thermal power.

Importance of Climate

Federal Technology Alert, Parabolic-Trough Solar Water Heating,

Produced for the U.S. Department of Energy (DOE)

by the National Renewable Energy Laboratory, 2000

23

Concentrators are not good for all sunny locations as

some may have high amounts of diffuse solar

radiation that will not focus, e.g., Miami. Diffuse

radiation may not be collected.

Monthly sum of global solar irradiance (diffuse and direct) for Miami, USA

From “Planning and Installing Solar Thermal Systems”, James &


Average daily direct-beam solar radiation

(desert regions are best for concentrators)

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Solar collector selection

• Depends on end-use temperature and

ambient temperature

• Climatic conditions that affect the quantity of

beam-irradiance are important

• Durability issues such as thermal shock,

stagnation temperatures

• Installation requirements and complexity,

e.g., racking, tracking mechanisms

• Cost per unit energy at the required

temperature level

Solar collector selection

• Ultimately the choice of the best solar

collector depends on climate, location

and system aspects so a full system

simulation should be undertaken!

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Computer Modeling of Liquid Desiccant System in TRNSYS

Conclusion

• There are many solar collector options

depending on application temperature

• Selection should account for many

climatic and system aspects

• Make sure that you are comparing

products on the same performance

basis, Tfi vs Tfm

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The End

solar collector technology - queen's universitymy.me.queensu.ca/courses/mech4301/me 430 solar...

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