solar utlization lec 7

solar utlization lec 7 - Solar Utilization Some basic...

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1 Solar Utilization Some basic flat-plate solar collector design criteria: Absorber plate coatings paint, 1 to 3 mils thick, carbon black pigment, absorptivity around 0.95 and emissivity around 0.98 selective coatings, black nickel on nickel plated steel, absorptivity around 0.96 and emissivity around 0.07, degrades in high humidity selective coatings, black chrome on nickel plated steel or copper, absorptivity around 0.95 and emissivity around 0.1, good resistance to humidity selective coatings, high temperature reactions with nickel on aluminum, absorptivity around 0.95 and emissivity around 0.3
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2 Solar Utilization Some basic flat-plate solar collector design criteria: Insulation: We use to recommend 2 to 6 inches of “good” insulation (fiberglass) underneath and on the sides of the absorber plate. R value would range from 5 to 16 h-ft 2 deg F/btu Be careful of out-gassing and heat damage when selecting insulations. Polyurethane for example will degrade with exposure to high temperature absorber plates. A layer of fiberglass insulation between the absorber plate and polyurethane. Polyurethane insulation would need to be about 5 inches thick to be equivalent to the R-16 fiberglass insulation above.
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3 Solar Utilization Some basic flat-plate solar collector design criteria: Insulation: Polyisocyanurate boards 2.5” thick would be equivalent to the fiberglass insulation above Long Term Thermal Resistance of Pentane Blown 6.0 6.2 6.4 6.6 6.8 0 20 40 60 Aging time at 73°F (Days) R-value (ft 2 .h.°F/Btu.in) Predicted R Measured R
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4 Solar Utilization Some basic flat-plate solar collector design criteria: Collector plate materials, copper tubes with a copper fin is best but expensive, copper is running about $2.98/lb Collector plate materials, copper tubes with aluminum fins, good choice also but can result in corrosion resistance between the fins and tubes Typical tube size: ½ inch copper, flow rate is generally about 0.022 gpm per sq ft of collector area, e.g. for a 28 sq ft collector, a total flow of 0.6 gpm based on the recommended flow. This collector will generally have about 10 tubes so the flow per tube is 0.06 gpm. This results in a pressure loss factor of 0.019 ft/100 ft with a velocity of 0.09fps and Reynolds number of 947, laminer flow. At this flow rate, this collector could provide a 5 degree temperature rise if it can absorb about 1600 btu/h.
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5 Solar Utilization
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6 Solar Utilization Some basic flat-plate solar collector design criteria: This particular collector has a 1 inch diameter header. The pressure drop through this collector would be extremely low. Based on conventional pipe sizing criteria, a ½ copper pipe could handle about 1.1 gpm, for 10 tubes this would mean that this collector could handle about 11 gpm or about 0.4 gpm/sq ft. At this flow rate, this collector could create a 0.3 deg F temperature rise. The Reynolds number in the pipe is 17,355 (turbulent) and the velocity is 1.62 fps. So…what do you think about this design criteria?
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7 Solar Utilization Some basic flat-plate solar collector design criteria: Cover plates
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This note was uploaded on 09/06/2011 for the course EML 4930 taught by Professor Staff during the Fall '08 term at University of Florida.

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solar utlization lec 7 - Solar Utilization Some basic...

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