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ME 430ME 430 Lecture

Collection of Solar Energy (Solar Collectors)

Slides prepared by C. Cruickshankand S. Harrison

Components of Solar Thermal Systems

Standard solar water heating system with heating boiler for additional heating (S = temperature sensor)

How does a solar thermal system work?

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

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Typical Indirect North American Solar Domestic Hot Water System

SolarCollector

FixedFlow Rate

Roof Line

HeatExchanger

VariableFlowRate

StorageTank

Cold MainsInlet

Electric Pump

Hot Waterto Load

StorageTank

Positioning of Collectors Rule of ThumbFor a year-round solar system in Canada, the best performance is from collectors facing due south, tilted at an angle equal to the latitude of the location and never shaded.

For a summer-only system, such as pool-heating, the tilt angle will be at a shallow angle.

If most of the use is in the winter, the panels will be at a steeper angle to capture the sunlight at lower sun altitude angles.

Summer Biased: Latitude – 15o

Winter Biased: Latitude + 15o

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Solar Collector Designs

Different collector designs

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

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

Solar Collector Designs

Concentration of solar radiation

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

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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.

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

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.

For the energy yield, it is not the collector (gross) area that is crucial but the absorber area. The exception to this is evacuated tube collectors with reflectors.

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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 & James/Earthscan, London, UK

Geometry of Solar Collectors

Cross-section of a double evacuated tube collector (“Sydney tubes”) with description of the different surface areas

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

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Unglazed CollectorsUnglazed collectors

• no glazing or insulated collector box

• consists only of an absorber (usually made of plastic)

• high thermal losses, used only at low operating temperatures

• typically used for swimming pools

• inexpensive

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

Glazed Flat-Plate Collectors

Section through a glazed flat-plate collector

1. Frame2. Seal3. Transparent cover4. Frame – side-wall

profile5. Thermal insulation6. Full-surface

absorber7. Fluid channel8. Fixing slot9. Rear wall

Design of glazed flat-plate collectors

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

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Glazed Flat-Plate CollectorsThe absorber consists of a heat conducting metal sheet (made of copper or aluminium, as a single surface or in strips) with a dark coating. The tubes for the heat transfer medium, which are usually made of copper, are connected conductively to the absorber.

Source: http://www.sunraysolar.com/Source: http://www.daviddarling.info

Absorber fin attached to copper tube by arc welding

Absorber wrapped around copper tube

SunstripHigh performance absorber fin constructed of aluminium with a copper “water way” to prevent corrosion.

Geometry is optimized to use minimum material.

Laser WeldingAluminium absorber fin attached to copper tube by laser welding.

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Absorber FinsTemperature Profile

Arrangement of Absorbers

From “Solar Thermal Systems”, James & James, London, UK

Collector with harp absorber

Rear side of an absorber made of aluminium with enclosed, sunk-in stainless steel meander tubes

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Radiation Terms

degree of transmission

degree of absorption(absorption coefficient)

degree of reflection

τ transmitted radiation = incident radiation

ρ reflected radiation = incident radiation

α absorbed radiation = incident radiation

The variables are dependent on the material and wavelength. The sum of and is equal to 1 (100%).ρ α τ,

Glazed Flat-Plate CollectorsWhen the solar radiation hits the absorber it is mainly absorbed and partially reflected. Heat is created through the absorption and conducted in the metal sheet to the heat transfer medium tubes or channels. Through these tubes flows the liquid heat transfer medium, which absorbs the heat and transports it to the store.

α = 0.95є = 0.95

α = 0.95є = 0.10

α = 0.05є = 0.1-0.7

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

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Glazed Flat-Plate CollectorsMost spectral-selective layers have an absorption rate of 90-95%, and an emission rate of 5-15%. Commonly used selective coatings consist of black chrome or black nickel.

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

The optical losses are dependent on the transparency of the glass cover (transmission) and the absorption capacity of the absorber surface (absorption) and are described by the optical efficiency:

The thermal losses are dependent on the temperature difference between the absorber and the outside air, on the insolation, and on the construction of the collector. The influence of the construction is described by the heat loss coefficient , k (or k-value) measured in W/m2K.

Solar Collector Efficiency

Optical and thermal losses

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

Optical losses

Thermal losses

Usable thermal energyη τα=

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Glazed Flat-Plate Collector Advantages

• offers multiple mounting options

• good price/performance ratio

• cheaper than vacuum collector

Disadvantages

• lower efficiency than vacuum collectors for high temperature applications, because the heat loss coefficient is higher

• not suitable for generating high temperatures (+100oC)

• requires more roof space than vacuum collectors do

• support system is necessary for flat roof mounting

Vacuum Collectors Evacuated tube collectors

In order to suppress thermal losses through convection, the volume enclosed in the glass tubes must be evacuated to less than 10-2 bar. Additional evacuation prevents losses through thermal conduction. The radiation losses cannot be reduced by creating a vacuum. The losses are kept low by selective coatings(small ε value).

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

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Vacuum CollectorsEvacuated tube collectorsThe 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 or return lines run.

At the base, the tubes are fitted to a rail with tube holders.

Vacuum CollectorsConcentrating systems providing process heat

Schematic structure of a compound parabolic concentrator (CPC) tube collector

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

Cross-section through a “Sydney” tube with round absorber

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Vacuum Collectors Advantages

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

• supports space heating applications more effectively than do glazed flat-plate collectors

• low in weight, can be assembled at installation site

Disadvantages

• more expensive than a glazed flat-plate collector

• cannot be used for in-roof installation

• heat pipe systems need to be inclined at least 25O

Efficiencies of Collectors

Efficiency characteristics curves for different types of collector and their areas of applications (at irradiation of 1000 W/m2K)

Collector characteristics curves and applications

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

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Economics

40 – 50Unglazed

150 – 200Glazed flat plate(with a selective surface)

500 – 1000Vacuum tube

Price ( $ / m2 )Type of Collector