presented at the 2011 comsol conference in boston ... · october 1presented at the 2011 comsol...
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EFFICIENCY OF EVACUATED TUBULAR SOLAR THERMAL COLLECTOR Junkun Ma; Xialu Wei Southeastern Louisiana University October 1Presented at the 2011 COMSOL Conference in Boston3, 2011
Presented at the 2011 COMSOL Conference in Boston
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Introduction
Solar heating Solar thermal energy for both domestic and
commercial applications such as water heating; Composed of solar thermal collectors, storage
tank, heat exchanger, and control systems; A thermodynamic process; Passive vs. active systems; High efficiency of converting and utilizing solar energy.
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Solar Thermal Collector
Solar Thermal Collector Captures the sun’s radiation energy; Turns solar energy into thermal energy; Transfers heat to the working fluid.
Batch Collector Easy to design and install; Less energy capture; Inefficient.
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Solar Thermal Collector (continue)
Flat Plate Collector Weather proofed
box; Dark absorber plate; Flow tubes
embedded.
Evacuated Tube Collector Two concentric glass
tubes; Vacuum in between; Most efficient collector.
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Objectives
Efficiency of Single Ended Evacuated Tube Collector
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Objectives (continue)
Mounting Angle Components of gravity; Facilitating or impeding Efficiency; Range: 15o -90o.
Aspect Ratio Aspect ratio between tube diameter and length; Longer tube vs. shorter tube (same diameter); Length: 1.2m – 1.8m.
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Approaches – Geometry Modeling 2D and 3D Geometric Models
Length: 1200 mm Inner Tube Diameter: 47 mm External Tube Diameter: 52 mm
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Approaches – System Analysis
Heat Transfer
Heat Conduction in Solid:
Where, k: thermal conductivity; T: absolute temperature; Q: heat source.
QTk )(
Heat Convection in fluid Where, r : density of fluid; Cp: specific heat capacity; u: velocity of fluid.
TuCQTk p )(
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Approaches – System Analysis
Fluid Dynamics Causes: Temperature difference and density changes
Where: : density; u: velocity field; p: pressure; I: identity matrix; h: dynamic viscosity; F: volume force.
Fuuupuu ])()3/2())(([ II T hh
0)( u
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Approaches – System Analysis
Volume Force - F Acting on a unit volume of water Volume Force = Buoyancy – Gravity F = (B - G)/V =( ρ∙g ∙V' - ρ∙g ∙V)/V = ρ∙g ∙(V' - V)/V = ρ∙g ∙∆V/V
Coefficient of Thermal Expansion - α ∆V/V = α (T' - T) F = ρ ∙g∙α (T' - T)
ρ∙V = M
ρ’∙V’ = M
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Approaches – System Analysis
Simulation Assumptions Inward Heat Flux – 120 W/m2;
Inlet and Outlet
Other Assumptions Ignore thermal expansion of glass; Ambient temperature is 298.15 K; Other than natural convection, no other types of
dynamic; Ignore effects of water tank.
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Approaches – Post Processing
Energy increment at the Outlet Boundary A certain mass of water (M); Initial temperature (T0) at the inlet boundary; After being heated over a certain time (t); Final temperature (T) at the outlet boundary;
Energy Increment: ∆Q = Cp*M*( T-T0 ) = Cp*ρ*V*∆T
ρ is the density of water; V is the volume of water.
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Approaches – Post Processing
Efficiency Function ∆Q = Cp * ρ * V * ∆T
Divided by the time ---- t:
(∆Q/t) = Cp * ρ * (V/t) * ∆T
(∆Q/t) is the efficiency; (V/t) is the volume of water flows through per unit time;
Two variables: (V/t) & ∆T
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Approaches – Post Processing
Velocity Field dVvnyunxnVdVV D )**(2
dVwnzvnyunxnVdVV D )***(3
Temperature Difference dTTTT D )( 02
dTTTT D )( 03
Thus, P2D ),())(**(* 0 TVdTTvnyunxCp
),())(***(* 0 TVdTTwnzvnyunxCp P3D
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Results – 2D Finite Element Model
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Results – 3D Finite Element Model
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Results – Mounting Angles
Mounting Angle - Efficiency
130
135
140
145
150
155
160
165
170
175
15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
Mounting Angle (Deg)
Effi
cie
ncy
(W
/m)
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Results – Aspect Ratios
Aspect Ratio - Efficiency
138.6
138.8
139
139.2
139.4
139.6
139.8
140
140.2
140.4
140.6
1.20 1.25 1.30 1.35 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80
Tube Length (m)
Ad
just
ed
Eff
icie
ncy
(W
/m)
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Conclusions
Mounting Angles: Efficiency rises and reaches the maximum at 25o, then begin falling and reaches the minimum at 90o.
Aspect Ratios: As tube diameter maintains as a constant, tube with aspect ratio of (1500/47) has the highest efficiency.
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Future Work
Compare the results with experimental data
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Questions?