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 C Report, Date: 13.01.2009 T 2 Q DRV T 1 THP Q HRJ T 0 Q COL Figure 1-1: Sketch of heat flows and temperature levels for a thermally driven heat pump The “Coefficient of Performance” for cooling (COP C ) is defined as shown in Eq. 1 (if the mechanical energy can be neglected). It can be easily shown, that the amount of heat which has to be rejected (Q HRJ ) per cooling capacity (Q COL ) is directly dependent on the COP C (compare Eq. 2) Q COL COP C = Eq. 1 Q DRV Q Q HRJ COL 1 = 1+ Eq. 2 COP C For different technologies (ad- or absorption), process configurations and working pairs (e.g. H 2 O/LiBr or NH 3 /H 2 O) of thermally driven heat pumps the COP of a real application varies depending on the three temperature levels. Furthermore the temperature level of the driving heat has to be at a certain minimum level, depending on the temperature level of the cooling load and heat rejection. However, in order to estimated the relevance of the temperature level of the heat rejection system a theoretical thermodynamic approach can be used. In terms of energy conversion a thermally driven heat pump combines two cycles, a power generation and a heat pump cycle. Using the Carnot efficiency of these cycles the theoretical possible efficiency of the thermally driven heat pump can be calculated using the three temperature levels mentioned above. In Figure 1-2 the two Carnot cycles for a thermally driven heat pump are shown. The cycle on the left hand side generates the work to drive the heat pump cycle on the right hand side. page 5
IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C Report, Date: 13.01.2009 Figure 1-2: Carnot cycles of a thermally driven heat pump Acc. to Eq. 3 the theoretical COP of the Carnot process for cooling of a thermally driven heat pump can be calculated. COP th, C ( Τ2 − Τ1 ) Τ0 η PG ⋅ε = ⋅ Eq. 3 Τ ( Τ − Τ ) = HP 2 1 0 Evaluating Eq. 3 using different temperature levels for heat rejection and constant temperature levels of the driving heat T 2 = 80°C and of the cooling load T 0 = 5°C leads to a characteristic shown in Figure 1-3. The Carnot-efficiency shows, what is thermodynamically possible. A real technical solution will be far below this value. Thus a further line has been drawn in Figure 1-3 which represents 40% of the Carnot efficiency (COP th,C,40% ). The 40%-value has been chosen arbitrarily but the results represent approximately the performance of real NH3/H2O AHPapplications for a temperature level of the heat rejection of approx. 30°C. The COP th and COP th,C,40% show a strong dependency on the temperature level of the heat rejection system, e.g. the COP th,C,40% is above 0.6 for a heat rejection temperature of 30°C and decreases below 0.4 for 40°C. page 6
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- Page 614 and 615: Grundprinzip Solare Wärme thermisc
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<strong>IEA</strong> SHC Task 38 <strong>Solar</strong> Air Conditioning <strong>and</strong> Refriger<strong>at</strong>ion Subtask C Report, D<strong>at</strong>e: 13.01.2009<br />
Figure 1-2: Carnot cycles of a thermally driven he<strong>at</strong> pump<br />
Acc. to Eq. 3 the theoretical COP of the Carnot process for cooling of a thermally driven he<strong>at</strong><br />
pump can be calcul<strong>at</strong>ed.<br />
COP<br />
th,<br />
C<br />
( Τ2<br />
− Τ1<br />
) Τ0<br />
η<br />
PG<br />
⋅ε<br />
= ⋅<br />
Eq. 3<br />
Τ ( Τ − Τ )<br />
=<br />
HP<br />
2<br />
1<br />
0<br />
Evalu<strong>at</strong>ing Eq. 3 using different temper<strong>at</strong>ure levels for he<strong>at</strong> rejection <strong>and</strong> constant<br />
temper<strong>at</strong>ure levels of the driving he<strong>at</strong> T 2 = 80°C <strong>and</strong> of the cooling load T 0 = 5°C leads to a<br />
characteristic shown in Figure 1-3.<br />
The Carnot-efficiency shows, wh<strong>at</strong> is thermodynamically possible. A real technical solution<br />
will be far below this value. Thus a further line has been drawn in Figure 1-3 which<br />
represents 40% of the Carnot efficiency (COP th,C,40% ). The 40%-value has been chosen<br />
arbitrarily but the results represent approxim<strong>at</strong>ely the performance of real NH3/H2O AHPapplic<strong>at</strong>ions<br />
for a temper<strong>at</strong>ure level of the he<strong>at</strong> rejection of approx. 30°C. The COP th <strong>and</strong><br />
COP th,C,40% show a strong dependency on the temper<strong>at</strong>ure level of the he<strong>at</strong> rejection system,<br />
e.g. the COP th,C,40% is above 0.6 for a he<strong>at</strong> rejection temper<strong>at</strong>ure of 30°C <strong>and</strong> decreases<br />
below 0.4 for 40°C.<br />
page 6
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