IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at
IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at
IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C2-A, November 9, 2009 With reference to Figure 1, the conventional cycle operates as follows: outside air (1) is dehumidified in a desiccant wheel (2); it is then cooled in the sensible regenerator (3) by the return cooled air before undergoing another cooling stage by evaporation (4) and being introduced into the building. The operating sequence for the return air (5) is as follows: it is cooled to its saturation temperature by evaporative cooling (6) and then it cools the fresh air in the rotary heat exchanger (7). It is then heated in the regeneration heat exchanger by solar energy (8), it regenerates the desiccant wheel (9) by removing the humidity and exits the installation. Modelling of the components Sensible regenerator A sensible regenerator is a porous matrix which passes periodically between a hot and a cold stream. The cellular matrix of the regenerator stores heat from the hot gas stream and releases it to the cold gas stream. Heated air stream Cold air stream T 7 T 6 Hot air stream Cooled air stream T 2 T 3 Figure 2: Sensible heat regenerator principle For modelling purposes, the following assumptions are made: • Heat transfer between air and the regenerator matrix is considered using a lumped transfer coefficient or a number of transfer units (NTU) • The channels where the fluid flows are identical and parallel • No leakage occurs between the air streams The heat conservation and transfer governing equations, after Kays and London [6] and Maclaine-cross and Banks [7], are: page 26
IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C2-A, November 9, 2009 ∂Ta ∂Ta c pm ∂Tm + u + µ = 0 (1) ∂t ∂z c + w c ∂t pa a pv c pm ∂Tm µ + J t ( Ta , m − Ta ) = 0 (2) c + w c ∂t pa a pv Kays and London [6] proposed the following correlation for regenerator efficiency (RE): ⎛ ⎞ ⎜ 1 RE = η cf 1 − (3) ⎜ 93 ⎝ 9 Where: min ( ) ⎟ ⎟ * 1. Cr ⎠ M . c . N * m pm Cr = (4) C NTUcf η cf = (5) 1+ NTU cf is the efficiency of the counter-flow heat exchanger for balanced flow. The outlet temperature (T 3 ) of the sensible regenerator can thus be calculated using: RE T T 2 3 = (6) 2 − T − T 6 Desiccant wheel The wheel is divided into two sectors the first is for dehumidification of moist air while the second is for the regeneration. It is a rotating porous matrix impregnated with a desiccant material that alternates periodically between a process air stream and a hot air stream. In contact with the dry desiccant, process air is dehumidified and heated by the heat of adsorption. The saturated desiccant then enters into contact with the hot air stream and is regenerated to again dehumidify the process air, perpetuating the dehumidification-regeneration cycle. The wheel is driven by a small electrical motor, with a rotation speed that varies between 7 and 20 rph page 27
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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 />
∂Ta<br />
∂Ta<br />
c<br />
pm ∂Tm<br />
+ u + µ = 0<br />
(1)<br />
∂t<br />
∂z<br />
c + w c ∂t<br />
pa<br />
a<br />
pv<br />
c<br />
pm ∂Tm<br />
µ + J<br />
t<br />
( Ta<br />
, m<br />
− Ta<br />
) = 0<br />
(2)<br />
c + w c ∂t<br />
pa<br />
a<br />
pv<br />
Kays <strong>and</strong> London [6] proposed the following correl<strong>at</strong>ion for regener<strong>at</strong>or efficiency (RE):<br />
⎛ ⎞<br />
⎜<br />
1<br />
RE = η<br />
cf<br />
1 −<br />
(3)<br />
⎜<br />
93<br />
⎝ 9<br />
Where:<br />
min<br />
( ) ⎟ ⎟ * 1.<br />
Cr<br />
⎠<br />
M . c . N<br />
* m pm<br />
Cr = (4)<br />
C<br />
NTUcf<br />
η<br />
cf<br />
= (5)<br />
1+ NTU<br />
cf<br />
is the efficiency of the counter-flow he<strong>at</strong> exchanger for balanced flow.<br />
The outlet temper<strong>at</strong>ure (T 3 ) of the sensible regener<strong>at</strong>or can thus be calcul<strong>at</strong>ed using:<br />
RE<br />
T<br />
T<br />
2 3<br />
= (6)<br />
2<br />
− T<br />
− T<br />
6<br />
Desiccant wheel<br />
The wheel is divided into two sectors the first is for dehumidific<strong>at</strong>ion of moist air while the second is for<br />
the regener<strong>at</strong>ion. It is a rot<strong>at</strong>ing porous m<strong>at</strong>rix impregn<strong>at</strong>ed with a desiccant m<strong>at</strong>erial th<strong>at</strong> altern<strong>at</strong>es<br />
periodically between a process air stream <strong>and</strong> a hot air stream. In contact with the dry desiccant,<br />
process air is dehumidified <strong>and</strong> he<strong>at</strong>ed by the he<strong>at</strong> of adsorption. The s<strong>at</strong>ur<strong>at</strong>ed desiccant then enters<br />
into contact with the hot air stream <strong>and</strong> is regener<strong>at</strong>ed to again dehumidify the process air,<br />
perpetu<strong>at</strong>ing the dehumidific<strong>at</strong>ion-regener<strong>at</strong>ion cycle.<br />
The wheel is driven by a small electrical motor, with a rot<strong>at</strong>ion speed th<strong>at</strong> varies between 7 <strong>and</strong> 20 rph<br />
page 27
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