IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at
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IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C2-A, November 9, 2009 3. Transient model of the desiccant wheel and experimental validation Conventional models of the desiccant wheel predict a mean temperature and a mean humidity ratio at the outlet of the desiccant wheel. In reality and due to the low rotation speed of the desiccant wheel, the temperature and humidity distribution at the outlet is not uniform. The model presented in this section is a two dimensional model that gives the temperature and humidity evolution across the wheel and at its outlets. Model description The desiccant wheel scheme is given in the figure below Regeneration air z θ L z R Process air Figure 18: Schematic of the desiccant wheel In the model development the following assumption are taken: • The state properties of the air streams are spatially uniform at the desiccant wheel inlet • The interstices of the porous medium are straight and parallel • There is no leakage or carry-over of streams • The interstitial air velocity and pressure are constant • Heat and mass transfer between the air and the porous desiccant matrix is considered using lumped transfer coefficients • Diffusion and dispersion in the fluid flow direction are neglected • No radial variation of the fluid or matrix states page 46
IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C2-A, November 9, 2009 A basic element in the angular and width direction of the desiccant wheel is considered and can be presented as follows: dθ R dz Air Figure 19: Basic element of the desiccant wheel Fundamental equations of heat and mass transfer for the basic element Mass conservation equation ∂W ⎛ ∂wa ∂wa ⎞ M d + ma ⎜ + u ⎟ = 0 (27) ∂t ⎝ ∂t ∂z ⎠ Mass transfer equation M d ∂W ∂t = h m S ( w − w ) a eq Heat conservation equation ∂H ⎛ ∂ha ∂ha ⎞ M d + ma ⎜ + u ⎟ = 0 (29) ∂t ⎝ ∂t ∂z ⎠ Heat transfer equation M d ∂H ∂t = h m S ( w − w )( h + c T ) + h S( T − T ) a eq fg pv a t a d (28) (30) Variable change The time t is related to the angular position θ with the following equation t τ ro = θ π so τ ro t = θ , π Where τ ro is the rotation period of the process or regeneration and N is the angular speed of the wheel. page 47
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<strong>IEA</strong> SHC Task 38 <strong>Solar</strong> Air Conditioning <strong>and</strong> Refriger<strong>at</strong>ion Subtask C2-A, November 9, 2009<br />
3. Transient model of the desiccant wheel <strong>and</strong> experimental<br />
valid<strong>at</strong>ion<br />
Conventional models of the desiccant wheel predict a mean temper<strong>at</strong>ure <strong>and</strong> a mean<br />
humidity r<strong>at</strong>io <strong>at</strong> the outlet of the desiccant wheel. In reality <strong>and</strong> due to the low rot<strong>at</strong>ion speed<br />
of the desiccant wheel, the temper<strong>at</strong>ure <strong>and</strong> humidity distribution <strong>at</strong> the outlet is not uniform.<br />
The model presented in this section is a two dimensional model th<strong>at</strong> gives the temper<strong>at</strong>ure<br />
<strong>and</strong> humidity evolution across the wheel <strong>and</strong> <strong>at</strong> its outlets.<br />
Model description<br />
The desiccant wheel scheme is given in the figure below<br />
Regener<strong>at</strong>ion air<br />
z<br />
θ<br />
L<br />
z<br />
R<br />
Process air<br />
Figure 18: Schem<strong>at</strong>ic of the desiccant wheel<br />
In the model development the following assumption are taken:<br />
• The st<strong>at</strong>e properties of the air streams are sp<strong>at</strong>ially uniform <strong>at</strong> the desiccant wheel<br />
inlet<br />
• The interstices of the porous medium are straight <strong>and</strong> parallel<br />
• There is no leakage or carry-over of streams<br />
• The interstitial air velocity <strong>and</strong> pressure are constant<br />
• He<strong>at</strong> <strong>and</strong> mass transfer between the air <strong>and</strong> the porous desiccant m<strong>at</strong>rix is<br />
considered using lumped transfer coefficients<br />
• Diffusion <strong>and</strong> dispersion in the fluid flow direction are neglected<br />
• No radial vari<strong>at</strong>ion of the fluid or m<strong>at</strong>rix st<strong>at</strong>es<br />
page 46
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