Solar Collector Losses Summary: Difference between revisions
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#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 | #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 | #Reflection from collector tube glass - 4% loss | ||
(total losses before collector tube are 15%) | |||
#Radiation losses from collector tube - 7.5 kW with emissivity = .25 selective coating | #Radiation losses from collector tube - 7.5 kW with emissivity = .25 selective coating | ||
#Assume that the absorptance | #Absorptance losses at collector tube - 4 kW | ||
=Absorptance Loss= | |||
Assume that for the absorptance of 90% using [http://www.solec.org/solkotehome.htm#PRODUCT%20DESCRIPTION%20and%20FEATURES 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= | =Radiation Loss Calculation= | ||
Revision as of 18:08, 31 January 2009
Introduction
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 [1]. 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.
![[1]](http://en.wikipedia.org/wiki/File:Us_pv_annual_may2004.jpg){kind=link}
- 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
(total losses before collector tube are 15%)
- Radiation losses from collector tube - 7.5 kW with emissivity = .25 selective coating
- Absorptance losses at collector tube - 4 kW
Absorptance Loss
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