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 For the desiccant wheel the efficiency is written with the potential functions instead of temperatures for the sensible regenerator: (With F i,k where the subscript i indicates the potential and the subscript k indicates the position relatively to figure 1) ε 1 = C 1 C n 1 min F F 1,2 1,8 − F − F 1,1 1,1 ε 2 = C 2 C n 2 min F F 2,2 2,8 − F − F 2,1 2,1 To calculate the outlet conditions of the regeneration stream the conservation equations are applied to the potential functions: C C 1 n 2 n 1 ( F − F ) = C ( F − F ) 1,2 1,1 r 1,9 1,8 2 ( F − F ) = C ( F − F ) 2,2 2,1 r 2,9 2,8 Having the potential functions calculated the temperature and humidity ratio at both outlet of the wheel can thus be calculated Evaporative coolers The humidification process occurs at a constant wet-bulb temperature. During the process, the energy needed for droplet evaporation is taken from the air which yields its temperature decrease and humidity increase. The enthalpy of the moist air at the evaporative cooler inlet is the sum of the enthalpy of the dry air and that of the water vapor. At the evaporator cooler outlet; the enthalpy of the dry air decreases due to the temperature decrease while that of water vapor increases due to the increased water vapor mass but the total enthalpy of moist air is approximately constant, with a slight increase due to the addition of the enthalpy of liquid water droplets. h T, w) a, inlet h( T, w) a, outlet ( = (15) The validity of this assumption can be verified on the psychometric chart where the lines of constant wet-bulb temperature are parallel to the constant enthalpy lines, especially when the temperature and humidity ratio are, respectively, between 19 and 25°C and 7 and 15g/Kg, which is in the application domain of desiccant evaporative cooling. page 30
IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C2-A, November 9, 2009 Heat pipe vacuum tube solar collectors In an evacuated heat pipe collector (figure 4), a sealed copper pipe containing a vaporizable fluid is bonded to a copper fin plate absorber located inside a glass tube. A small copper condenser is attached from one side to the top of the heat pipe and from the other side to the storage working fluid. The heat pipe is an evaporating-condensing device. As the sun shines on the absorber, the pipe is heated and some of the liquid inside evaporates. The vapour rises toward the condenser at the top of the heat pipe and condenses on being cooled by the storage water circulating in the manifold. The liquid then returns to the heat pipe. The vacuum tube ensures minimization of the heat losses of the collector. Figure 4: Heat pipe vacuum tube collectors A model for the HPVT is proposed below, separately considering each component [12] and [13] of the tube, i.e. the glass cover, the absorber, the heat pipe fluid, the condenser and the storage fluid. The following assumptions are made regarding the model: • The properties of the materials are independent of the temperature. • The temperature gradient along the absorber and the condenser is negligible. • Due to the sufficient quantity of fluid in the heat pipe (more than 90% of the heat pipe volume), the vapour is not superheated • The liquid returning from the condenser to the heat pipe is saturated. • The glass cover is clean and completely transparent to solar radiation. • The part of radiation reflected by the absorber leaves the tube. • Due to the vacuum, convection does not occur inside the tube. • Conduction in the storage fluid direction (z) is considered to be negligible. page 31
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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 />
He<strong>at</strong> pipe vacuum tube solar collectors<br />
In an evacu<strong>at</strong>ed he<strong>at</strong> pipe collector (figure 4), a sealed copper pipe containing a vaporizable<br />
fluid is bonded to a copper fin pl<strong>at</strong>e absorber loc<strong>at</strong>ed inside a glass tube. A small copper<br />
condenser is <strong>at</strong>tached from one side to the top of the he<strong>at</strong> pipe <strong>and</strong> from the other side to the<br />
storage working fluid. The he<strong>at</strong> pipe is an evapor<strong>at</strong>ing-condensing device. As the sun shines<br />
on the absorber, the pipe is he<strong>at</strong>ed <strong>and</strong> some of the liquid inside evapor<strong>at</strong>es. The vapour<br />
rises toward the condenser <strong>at</strong> the top of the he<strong>at</strong> pipe <strong>and</strong> condenses on being cooled by the<br />
storage w<strong>at</strong>er circul<strong>at</strong>ing in the manifold. The liquid then returns to the he<strong>at</strong> pipe. The<br />
vacuum tube ensures minimiz<strong>at</strong>ion of the he<strong>at</strong> losses of the collector.<br />
Figure 4: He<strong>at</strong> pipe vacuum tube collectors<br />
A model for the HPVT is proposed below, separ<strong>at</strong>ely considering each component [12] <strong>and</strong><br />
[13] of the tube, i.e. the glass cover, the absorber, the he<strong>at</strong> pipe fluid, the condenser <strong>and</strong> the<br />
storage fluid. The following assumptions are made regarding the model:<br />
• The properties of the m<strong>at</strong>erials are independent of the temper<strong>at</strong>ure.<br />
• The temper<strong>at</strong>ure gradient along the absorber <strong>and</strong> the condenser is negligible.<br />
• Due to the sufficient quantity of fluid in the he<strong>at</strong> pipe (more than 90% of the he<strong>at</strong> pipe<br />
volume), the vapour is not superhe<strong>at</strong>ed<br />
• The liquid returning from the condenser to the he<strong>at</strong> pipe is s<strong>at</strong>ur<strong>at</strong>ed.<br />
• The glass cover is clean <strong>and</strong> completely transparent to solar radi<strong>at</strong>ion.<br />
• The part of radi<strong>at</strong>ion reflected by the absorber leaves the tube.<br />
• Due to the vacuum, convection does not occur inside the tube.<br />
• Conduction in the storage fluid direction (z) is considered to be negligible.<br />
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