Performance of Evacuated Flat- Plate Solar Collectors Integrated with Thermal Energy Storage Systems Paul Henshall, Philip Eames CREST @ Loughborough University 1
Project Introduction Project: High Performance Vacuum Flat Plate Solar Thermal Collectors for Hot Water and Process Heat Centre for Renewable Energy Systems Technology Project funded by the Engineering and Physical Sciences Research Council (EPSRC), UK Project partners: 2
Project Introduction Existing technology: Flat plate solar thermal collector Evacuated tube solar thermal collector Flat plate solar panels, Santorini2, 23x2 CC BY-SA 3.0 Evacuated tube collector, Vakuumroehrenkollektor 01, Ra Boe CC BY-SA 2.5 http://commons.wikimedia.org/wiki/file:solar_panels,_santorini2.jpg http://commons.wikimedia.org/wiki/file:vakuumroehrenkollektor_01.jpg 3 Advantages: Large area of absorber to gross collector area Disadvantages: Lower efficiency at high operating temperatures Advantages: Good thermal insulation qualities Higher efficiency at high operating temperatures Disadvantages: Smaller area of absorber to gross collector area Disclaimer: These collectors are not the work of any of the project partners, they are presented here only for depicting existing technologies
Project Introduction Existing technology: Hermetic seal http://commons.wikimedia.org/wiki/file:flat_plate_glazed_collector.gif http://commons.wikimedia.org/wiki/file:evacuated_tube_diagram.jpg This project concerns the fabrication and evaluation of high performance vacuum flat plate solar collector. These being a hybrid of existing technologies 4
Background (1975) Eaton and Blum suggest the use of a moderate vacuum to supress convection between collector plate and glass cover (1999) Benz and Beikircher employ low pressure inert gas to suppress convection losses and investigate the range of pressures required to also suppress gas conduction losses To suppress both convection and gas conduction in a flat plate collector an enclosure pressure of less than 0.1 Pa is required 5
Configuration and Performance FP: Heat Loss Coefficient FP VFP U t (W/m 2 K) 4.6 1 U b (W/m 2 K) 0.4 1 U e (W/m 2 K) 0.4 0 U L (W/m 2 K) 5.4 2 VFP: At plate temperature of ~150 C U = U + U + U L t b e 6
Configuration and Performance 7 (EN12975-2:2006) (800 W/m2, Ambient temperature is 20 C) Efficiencies based on aperture area
Configuration and Performance Analysis suggests that a vacuum flat plate solar collector may operate with an efficiency of around 50% at average plate temperatures of ~ 140 C This would increase the versatility of a flat plate solar collector: to include applications such as: Domestic hot water/space heating Industrial process heat applications Chemical heat treatments Industrial steam washing 8
TRNSYS Modelling Variables: Season: Winter, Spring, Summer, Autumn Collector: Vacuum Flat Plate, Flat Plate, Evacuated Tube Application (Temperature): Space heating (45 C), Domestic hot water (85 C), Process heat (140 C) Other parameters: Collector area, storage volume and solar fraction 9
TRNSYS Modelling Space heating, 45 C Possible heat storage options: Thermochemical Water tank Paraffin wax (Phase change material, PCM) Solar Fraction (%) Flat Plate Collectors gross area / absorber area: ~ 1.20-1.15 VFP ET FP VFP ET FP Evacuated Tube Collectors gross area / absorber area: ~ 1.82 Various sources Absorber Area (m 2 ) Above, solar fraction with 0.3 m 3 storage volume 10
TRNSYS Modelling Domestic hot water, 85 C Possible heat storage options: Water tank Salt hydrate storage (PCM) Solar Fraction (%) VFP ET FP VFP ET FP Absorber Area (m 2 ) Above, solar fraction with 0.3 m 3 storage volume 11
TRNSYS Modelling Process heat, 140 C Possible heat storage options: Thermo-chemical Nitrate eutectics storage (PCM). Solar Fraction (%) VFP ET VFP FP ET FP Absorber Area (m 2 ) 12
Enclosure Fabrication A 400x400mm evacuated enclosure was fabricated. Two k-glass covers separated by an aluminium spacer Stainless steel support pillars Indium hermetic seal Enclosure was evacuated to 0.001 Pa (1 x 10-5 mbar). Enclosure fabricated at the University of Ulster Before evacuation U-value measured as: 2.23 W/m 2 K After evacuation U-value measured as: 0.86 W/m 2 K 13 In centre of panel Note: Here U-value refers to heat transfer from one side of enclosure to the other, this includes conduction through the support pillar array
Enclosure Fabrication A 500x500mm evacuated enclosure was fabricated. One k-glass cover bonded to a stainless steel tray. Stainless steel support pillars of varying size Indium hermetic seal Enclosure was evacuated to 1 Pa. Enclosure fabricated at the University of Loughborough 14
Enclosure Fabrication Pump down port Inlet and outlet ports Small panel fabricated to test hermetic inlet/outlet and pump down port concepts. The sample was pumped down to a pressure of 0.0075 Pa (7.5 x 10-5 mbar) 15
Summary Vacuum flat-plate collectors could offer improvements in performance over both flat-plate collectors and evacuated tube collectors. Vacuum flat-plate collectors could extend the applications and temperature range of conventional flat-plate collectors. Several vacuum flat-plate enclosures have been fabricated and tested. 16
17 Thank you for listening Questions?