Heat Exchanger activities at TWI SOLEGLASS Project Overview All glass Mid temperature Direct flow thermal solar vacuum tube Maxime Mrovcak 20 th May 2014
Project Context Solar Thermal systems are rapidly growing global market at all levels, independent of the application field or temperature levels. The mid temperature range in Thermal Solar applications, from 100 C up to 250 C, is one with the highest market potential. Solar thermal heat generation is present in various applications: - Industry processes heat - Solar Cooling and Heating, residential and service - Solar power plants Limited technologies in the mid temperature application range today because of the lack of reliable products and relatively expensive technology solutions, primarily: solar tube collectors and/or concentrators
Project Consortium 3 year project Start: September 2011 End: August 2014 RTDs SMEs associations End User
Project Objectives Scientific objectives Development and design of Mid Temperature Direct Flow Solar Vacuum Tube (SOLEGLASS) that will: Supply the heat in the temperature range 100-300 C for power generation Accommodate thermal expansion of the assembly while in service Ensure reliable vacuum retention to ensure efficiency Decrease initial material cost for the solar tube production and reduce tube assembly costs 2 Design have been explored: Taking in account all reasons mentioned in project objectives All Glass Tube will be developed in two variants: SOLEGLASS-L for lower medium temperature range (100-160 C) to be applied with related CPC (Compound Parabolic Concentrator) collector SOLEGLASS-H for higher medium temperature range (160-300 C) to be applied with related CSP (Concentrated Solar Power) collector Current Direct flow CPC Current CSP
Design of the collectors The glass tube consists of a plain inner tube welded to outer bellow tube. Glass: Borosilicate 3,3 Length: ~2m Manufacture: blown-moulded and welded
Optimisation of the glass tube Integrate a bellow shape to accommodate the thermal expansion. Various influencing parameters were investigated b d t n ABAQUS
Optimisation of the glass tube Optimising the number of waves n towards stress reduction. Kept constant d = 15mm, b = 5mm, t = 4mm The longitudinal stress tends to decrease exponentially to reach a constant value. In the final design n was chosen to be 4 due the limitation of the mould dimensions
Coating technology The efficiency of the heat transfer is also improved using an absorber coating on the inner tube 2 routes were explored for the absorber coating: Air-Sprayed (Solekote ) 92% of absorbance Plasma coated (PVD process) 98% absorbance Plasma coated tube
Vacuumation and gettering Gettering: The tubes must be evacuated to minimise heat losses. A getter is used to keep a nearly constant vacuum level. Vacuumation: The tube is evacuated using a pump and the glass tube is melted to produce a seal.
CPC Collector
Design of the collector Support system design. CPC reflector design Study the effect of the tube geometry (tube and bellow) on the absorbed irradiance. Stepped mirror configuration to accommodate the bellow. Two parabolic mirror geometries
Design of the reflectors Bellow peak Inner tube The working fluid is circulating in the inner tube. The heat exchange is maximised where the radiation intensity is highest. In this case the design is not optimised
Optimisation of the mirror shape Optimised shape found in the literature (Joshep J. O Gallagher et. al) Extended cusp V groove Which is the most suitable?
Energy gained on the absorber at 1000W/m 2 according to various incident angle 800 700 600 V_groove Extended_cusp Configuration Irradiance on the absorber over a day (KW h/m 2 ) 500 W/m 2 400 Extended cusp 10.66 V-groove 11.32 300 200 100 0 1 11 21 31 41 51 61 71 81 Incident angle
Final assembly of the reflector No applicable V-groove for r outer 2r inner V groove for the tube and Extended cusp for the bellow ends
CPC assembly Support system design. Components for 10-tubes collector How to connect the tubes together? Piping elbows
The Houskeeper seal is a standard method to create a chemical bond between glass and metal. Glass-to-metal seal A oxide layer is created on the metal and the glass subsequently melted on this oxide layer to achieve a joint. Skilled operator required Joint durability may be an issue Not suitable for CPC design Mechanical attachment of the parts was preferred.
Mechanical attachment Two deep drawn stainless steel flanges, spring elements and necessary bolts and nuts PTFE gasket Tightened to a pre-set value Commonly used in chemical process plant Quick, relatively cheap and simple to assemble
Thermal testing 1 2 3 4 7 5 6 1 - Left stainless steel end 2 - Left inner glass pipe 3 - Middle outer glass pipe 4 - Right inner glass pipe 5 - Bottom lid 6 - Right stainless steel end 7 - Tray
Temperature ( C) Thermal profile 200 Tmax:200 C 180 160 140 120 100 80 Left end metal Left inner glass pipe Right end metal Right inner glass pipe Outter glass pipe Bottom lid Tray 60 40 20 0 0 100 200 300 400 500 600 Time (min) The inflexion point on the curves is due to the use of fans No appearance of crack, failures, or leaks in the assembly or joint
Mechanical testing of glass bellows Linear variable deformation transformer (LVDT)
Mechanical testing of glass bellows (a) Failure in compression -1.91kN / -0.54mm (b) Failure in tension 4kN / 1.38mm The mechanical properties are within the required range to cope with the anticipated deformation while in service
Leak tightness The joints in the assembly were leak tested in an air furnace. The vacuum was drawn in the assembly. The joints were tested with He Air furnace 200 C Stainless cap Glass tube Stainless tube Leak tester Temperature at the joint Leak rate (mbar.l/s) Pressure (bar) 22 C 1.10-10 1.10-2 Oil Leak tight when leak rate below 1.10-5 mbar.l/s 200 C 7.10-10 9.10-3 Leak tight!
CSP Collector
CSP design Temperature range 160-300 C Double bellow glass tubes Aluminium coated reflector Sun tracking to maximise efficiency (±70 tilt) 150kg overall weight
Structural simulation Collector tilted by 0 C Stress profile due to the inertia of the weight Max: 24.7MPa Collector tilted by 70 (position with the maximal windage) Stress profile due to the inertia of the weight in addition to wind loads (600Pa ~ 112km/h) Max 125.1MPa Below the Yield strength of the reflector and steel (240-250MPa)
Progress The project ends on 31 st August 2014 The consortium is taking the following actions to complete the project: Manufacture of two full-scale 7-tube CPCs to be tested in Croatia and Macedonia Manufacture of a full-scale CSP to be tested in Croatia Acquisition of thermodynamic data Demonstration by the end of the project
Project output on completion Technology to convert solar radiation into heat/power Ready for small and industrial scale heating Manufacturing methodology Field trials Videos Final reports and PUDF Exploitation plan and full cost model
Acknowledgments The research leading to these results has received funding from the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement no 285956. Copyright Soleglass 2014. All Rights Reserved.