Assignment proposal:



Custom made solar collector

Swinburne Solar Hot Water Assignment

Prepared by Simon Pockley PhD with the assistance of Robert Veerman

2009-09-25

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Contents:

Rationale 2

Pattern of use 2

Orientation and spatial constraints 2

Local climate and available energy 3

Energy sources 3

Thermosyphon schematic 4

Materials 5

Comparison of materials used in available solar collectors 5

Absorber plate 5

Thermal finishes: absorptivity and emissivity 5

Insulation 7

Cost of Construction 8

Design factors 9

Exploded schematic of components 10

Construction of collector process 11

Conclusion 16

References 17

Appendix 1. Relationships between riser tube size and header tube size 18

Appendix 2. Comparison with available 180 litre solar hot water collector panel specifications 19

Rationale

Small hut in the Otways (Apollo Bay) where slow combustion stove heats a 40 gallon (180 litre) hot water storage tank in the roof. In summer, the weather can be too hot to light the stove but it would be nice to have hot water.

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Fig 1. Small hut in the Otways with north facing roof.

Pattern of use

Half the week in Melbourne and half the week in the Otways. Hot water needed for showers (usually in the morning) and for washing up. Most of the year the weather is such that the stove is used for cooking and heating water.

Orientation and spatial constraints

The north-facing roof with solar PV panel is pitched at 45 degrees. Below the level of the hot water storage tank is space for a solar hot water collector approx 2000mm x 600mm. Close to the sea where frost is not a problem – mindful of corrosion.

Fig 2. Stove + position below PV panel relative to base of storage tank

Local climate and available energy

Apollo Bay, in the Otways, is one of the most southerly inhabited areas of mainland Australia. Climatic conditions can be summarised as wet and windy.

|Average maximum temperature |21.9°C in January - 13.1°C in July |

|Average minimum temperature |14.6°C in February - 7.3°C in July |

|Average annual rainfall |1,053mm |

|Average clear days |37 per annum |

|Average rain days |167 per annum |

Table 1. Summary of local weather conditions in Apollo Bay

Such conditions are not ideal for solar energy conversion. Consequently, it is important to have realistic expectations of the capacity of the proposed system. While Apollo Bay has its own microclimate, data available for predicting the expected amount of available solar energy has been inferred from the measurements taken at Laverton (Latitude 37 degrees 51 minutes South) and those from Launceston (Latitude 41 degrees 36 minutes South).

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Fig 3. Peak sun hours deduced from Solar Radiation Handbook data tables

Energy sources

| Requirement | Source | Rationale |

| Water heating | Slow combustion (wood), solar collector | abundant firewood from re-vegetation |

| Space heating | Slow combustion stove | low cost sustainable |

| Space cooling | Orientation and passive house design | low cost sustainable |

| Refrigeration | Electric 12 volt DC | most efficient |

| Lighting | Electric 12 volt DC | most efficient |

| Cooking | Slow combustion wood stove + gas ring (LPG) | firewood + gas for short summer periods |

| Cleaning | Dustpan and broom | most efficient |

| Communications | Mobile/laptop 12 volt DC lithium re-charge | low cost option |

| Power tools | Electric 12 volt DC lithium re-charge | existing equipment - compressed air option |

| Water | Gravity feed from spring | whole property water plan |

| Waste | Composting toilet (flue extraction by updraft) | low cost and sustainable |

| Transport | Old petrol ute | Investigate biofuels |

Table 2. Energy requirements matched against source with rationale

Thermosyphon schematic

Fig 4. Dimensional schematic with hot and cold thermosyphon flows

| |Height mm |Width mm |Depth mm |

|Header tank 15 litre |300 |595 |275 |

|Storage tank 182 litre |865 |680 |680 |

|Solar collector 3.1 litre |600 |1800 |80 |

|Stove 3 litre |800 |840 |520 |

| |Tube size |DN15 - B 1/2" | |

| |Tube size |DN20 - B 3/4" | |

| |Tube size |DN20 - B 3/4" | |

Table 3. Dimensions of main system components

Materials

Comparison of materials used in available solar collectors

A table of comparison of the materials used in 180 litre collectors is available in Appendix 2.

Of primary interest was the absorber plate material, the size of header and riser tubes and the type of selective surface coating used on the absorber plate.

Unless, water ways are formed by extruded or pressed channels in the absorber plate, most flat plate collectors employ a copper, aluminium, or steel plate onto which copper tubes are bonded. In order to prevent significant reflection of infrared solar radiation the absorber plate is then given a thermal finish with a proprietary surface coating or some version of black chrome or other oxides. This finish is either painted or electroplated.

Absorber plate

Copper was chosen as the material for the tube and fin type absorber because it is one of the metals with the highest heat conductivity. It is easy to work and does not cause any corrosion problems inside the collector. In comparison to other popular materials, copper is almost 100 % recyclable.

|Material |Temp 25°C |125°C |225°C |

|Copper |401 |400 |398 |

|Aluminium |250 |255 |250 |

|Carbon Steel |54 |51 |47 |

|Stainless Steel |16 |17 |19 |

Table 4. Comparison of thermal conductivity of materials in W/mK

Data source:

Copper sheet suppliers in Melbourne had variable prices. Standard sheet size was 1800mm x 900mm. Copper sheet was purchased from George White & Co. Prices (prices ex GST) were ranked as follows:

|Company Sheet gauge |1.2 mm |1.0 mm |0.9 mm |0.7 mm |

|Fords Steel Service Centre Pty Ltd (Paul) | | |$221.00 |$187.00 |

|184 Hendersons Rd Hastings (03) 5983 8441 | | | | |

|Austral Wright Metals. | | |$230.23 |$183.51 |

|8-12 Aylward Ave Thomastown 1800 135 780 | | | | |

|James Coppell Lee |$297.00 | |$275.00 |$167.00 |

|20 Merri Concourse Campbellfield 03 93579613 | | | | |

|George White & Co. Pty Ltd. (Terry) | |$223.00 | |$156.00 |

|1 Treforest Drv Clayton (03) 9544 1100 | | | | |

Table 5. Comparison of prices for a range of plate thickness

Thermal finishes: absorptivity and emissivity

The suitability of different materials is measured by absorptivity and emissivity values. Absorptivity refers to the ability of a material to absorb radiation. Emissivity refers to the ability of a material to emit radiation.

With solar collectors, absorptivity is referenced to a true black body surface that by definition is 1.0. While emissivity refers to radiation at a certain temperature (25°C), most solar collectors operate at temperatures well above that used to measure the emissivity values. Consequently the amount of emitted radiation is probably higher than the figures suggest.

Furthermore, objects in sunlight absorb visible light and radiate infrared. An object's "colour" (emissivity) in the infrared can be very different from its colour in visible light. For example, white paint and black paint have very similar emissivities in the infrared.

Flat black paint made from lamp black mixed with spar varnish known as, ‘Parsons Black,’ was initially used on solar collectors. It is cheap to apply but is not necessarily the best material to use. Black paint has an absorptivity factor of 0.98. 2% is reflected. This means that 98% of the available solar energy is converted into heat in the collector. The collector then gets hot and heat is transferred but some of the heat is re-emitted as radiation.

Ideally, a coating with high absorptivity and low emissivity should be used. The lower the emissivity and the higher the absorptivity, the more efficiently the available heat can be used. This is why many of the solar collector manufacturers use a proprietary coating system. However, surface coatings with these properties are expensive, can be difficult to apply, or have proprietary processes unavailable to the general public.

a= solar absorptivity

e= normal emmitance

SOLAR NORMAL Ratio

SURFACE COATINGS a e a/e

----------------------------------------------------------------------

Solchrome (India) 0.94-0.98 0.10-0.14 8

Black Copper 0.98 0.63 1.56

Flat black paint 0.97-0.99 0.97-0.99 1

3M Black Velvet Paint 0.97 0.91 1.07

Carbon Black Paint 0.96 0.88 1.09

Black Chrome 0.96 0.62 1.55

Solkote (USA) 0.88-0.94 0.28-0.49 2.8-3.6

Anodize Black 0.88 0.88 1.00

Epoxy Aluminum Paint 0.77 0.81 0.95

GSFC Green 0.57 0.91 0.63

Tarnished Copper 0.55 0.04 13.75

GSFC Yellow 0.38 0.90 0.42

Chromeric Silver Paint 0.30 0.30 1.00

Copper 0.30 0.06-0.2 10.00

GSFC White 0.20 0.92 0.22

Buffed Aluminum 0.16 0.03 5.33

Table 6. Comparison of absorptivity and emissivity values for a range of coatings

Data source:

While tarnished copper has a high a/e ratio it has a relatively low absorptivity. The pragmatic choice of thermal coating was to use a matt black paint that would bond to copper. The adhesive properties of the paint rely on a primer. Wattyl Killrust Etch Primer is a fast drying, high performance epoxy primer suitable for copper.

|Ingredient |vague % |

|isopropanol |10-30 |

|toluene |10-30 |

|n-butanol |1-10 |

|naphtha petroleum, light aromatic solvent |1-10 |

|cyclohexanone |1-10 |

|anticorrosive phosphate pigment unregulated |1-10 |

|phosphoric acid |0-1 |

|bisphenol A/ epichlorohydrin resin, solid 1-10 |1-10 |

|black and grey contain carbon black |0-1 |

|grey contains titanium dioxide |1-10 |

|additives, unregulated |1-10 |

|Contains less than 0.1% benzene | |

Table 7. Percentage ingredients for Wattyle Killrust Etch Primer

Wattyl Killrust Epoxy Enamel is suitable for metals.

Wattyl could not provide any thermal performance data and its absorptivity and emissivity values are assumed to be in the range of Flat Black Paint (see table 6).

|Ingredient |vague % |

|toluene |20-40 |

|acetone |5-15 |

|aluminium powder coated | ................
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