Solar Collector Losses Summary
When considering the overall performance of linear slat collectors for a linear Fresnel solar concentrator system oriented in the E-W direction, several losses must be considered. For the case of a 4:1 aspect ratio array, such as a 15 foot wide array that stretches for 60 feet in the E-W direction. For this case, if we use 6″ slats, we can fit about 16 slats in this area, or a total of 480 square feet (45 sq meters) of solar intercept. We have about 5 kWhr/day per square meter in Maysville, Missouri – from . We summarize our losses. This includes air mass losses and averages over the seasons – because we are taking empirical data. We are assumining a collector tube with a single layer of insulating glass on the surface. We are considering a case where we are letting the collector tube heat up to 650K.
- Reflection losses from mirrors – 1%, assuming 99% reflective mirrors – and assuming that 4% reflection from glass is reflected back at the collector tube
- Edge Lossses – 10% – from Solar_Collector_Calculations#Losses_due_to_edge_effects_only – about if we consider the 6 hours of highest sun during the day, or from 9:30 AM – 2:30 PM
- Reflection from collector tube glass – 4% loss
- Shading losses from collector tube – 1 slat will be covered by the shadow of the collector tube – at a time, for 6% loss (1 of 16 slats shaded)
(total losses before collector tube are 21% = 9.5 kW)
- Radiation losses from collector tube – 7.5 kW with emissivity = .25 selective coating
- Absorptance losses at collector tube – 4 kW
- Conduction losses of conductor tube – 2.7 kW
We assume that all the remaining energy goes into heating the water in the collector tube.
The sum of these losses is 24 kW, with 21 kW remaining, or 53% loss.
For general purposes, we will say that the collector is slightly under 50% efficint.
assuming a 60 ft x .5ft rectangular absorber box with an open bottom (11.3 square meters, we’ll just ignore the complexity of corners). The area of 3 sides like this is 90 sq feet (8.4 sq meters).
using 3 inch of rigid fiberglass of R-value  4 (in SI units) and assuming a temperature difference of 300 degree Celsius (400C – 100C).
Power lost due to conduction = 1/R * A * (delta T) = 1/12 * 8.4 * 300 = 210 W
Add the conduction through glass – 8.4*300 = 2520W
Assume that for the absorptance of 90% using Solkote selective coating, the 10% not absorbed is lost through conduction through the glass. The incoming radiation is the 45 kW of available radiation minus 15% of the collector losses, or 38 kW available. About 4 kW is lost due to imperfect absorption.
Radiation Loss Calculation
Blackbody radiation calculator shows the following losses for a 60′ tube:
This takes a 650K optimal collector temperature, with a 2″ tube, 20m long. This collector tube area = 2.9 sq meters.
- 30 kW losses mean 2/3 of the incoming 45 kW of solar gain.
- Reduce this by selective coating of emissivity = 0.25 for Solkote, and we get 7.5 kW of radiation loss
Setting the temperature to 650K gives us