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 A Report A2, November 2009 3.1 Solar thermal sub-system All types of solar collectors – i.e. flat plate collectors, vacuum tube collectors and concentrating collectors – are applicable. Vacuum tubes and concentrating collectors often exhibit higher collector efficiencies at elevated temperatures of the solar loop. Thus higher availability of the system is accomplished during times of limited solar irradiation. solar collector heat exchanger heat storage backup hot side flat plate collector vacuum-tube collector concentrating collector Figure 4: Solar sub-system: options for the solar collector In the solar thermal system a heat exchanger can be integrated for separation of primary and secondary loop. In conventional systems a water/glycol mixture together with a separating heat exchanger is used, assuring trouble-free operation throughout the year at the expense of a reduction of the temperature level in the secondary loop due to the heat transfer in the heat exchanger. In order to avoid the reduction of the temperature level a “direct” utilization of the solar heat may be applied. In this case either freezing of the heat carrier water has to be avoided by means of heat input from the backup-heater or an electrical heater, or a drain-back concept has to be chosen. When the later is applied, the collector loop is filled by the circulating pump only when substantial solar contribution for solar collector heat exchanger heat storage backup hot side T T with without T T T Figure 5: Solar sub-system: heat exchanger for separation of primary and secondary solar loop. page 5
IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask A Report A2, November 2009 operation of the system in heating or cooling mode is to be expected. A third option would be to drive the chiller directly with the water-glycol mixture, taking into account changing heat transfer rates in the desorber of the chiller. solar collector heat exchanger heat storage backup hot side B A storage 1 storage 2 storage 3 storage 4 B B B T A A A storage 5 storage 6 storage 7 storage 8 B B B storage 9 B PCM A A A A plus (optional) DHW 1 DHW 2 DHW 3 DHW 4 DHW (domestic hot water) 60C° T 60C° T 45C° T T 60C° Figure 6: Solar sub-system: options for the heat storage and domestic hot water preparation. In order to balance solar gain and the profile of the heat consumption, solar thermal systems comprise a hot water heat storage. For operation of the thermally driven chiller solar heat at a sufficiently high temperature level is required and any devaluation (cooling by mixing or heat exchange) of the solar heat should be avoided. Thus a direct integration of the storage without heat exchanger is favourable, eliminating a temperature drop during charging and discharging of the store. Therefore, option “storage 3” in Figure 6 is not suitable for solar cooling. In addition the active storage volume should be variable in order to achieve a quick increase of the solar loop temperature for minimum delay between onset of solar irradiation and start of the production of chilled water by the chiller. For this purpose a direct link between solar heat generation and supply of driving heat to the chiller can be chosen (“storage 1, 6, 7, and 8”). For these cases it must be assured that the store still properly serves as a hydraulic switch between heat generation and load. Therefore only minimum pressure drop between supply and return line of the solar thermal system across the store is allowed in order to avoid parasitic flows. Parasitic flows might occur when only the solar collector is charging the store (parasitic flow may occur at the auxiliary heater or the desorber) or when the store is only discharged (parasitic flow page 6
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<strong>IEA</strong> SHC Task 38 <strong>Solar</strong> Air Conditioning <strong>and</strong> Refriger<strong>at</strong>ion Subtask A Report A2, November 2009<br />
3.1 <strong>Solar</strong> thermal sub-system<br />
All types of solar collectors – i.e. fl<strong>at</strong> pl<strong>at</strong>e collectors, vacuum tube collectors <strong>and</strong><br />
concentr<strong>at</strong>ing collectors – are applicable. Vacuum tubes <strong>and</strong> concentr<strong>at</strong>ing collectors<br />
often exhibit higher collector efficiencies <strong>at</strong> elev<strong>at</strong>ed temper<strong>at</strong>ures of the solar loop. Thus<br />
higher availability of the system is accomplished during times of limited solar irradi<strong>at</strong>ion.<br />
solar collector he<strong>at</strong> exchanger he<strong>at</strong> storage backup<br />
hot side<br />
fl<strong>at</strong> pl<strong>at</strong>e collector<br />
vacuum-tube collector concentr<strong>at</strong>ing collector<br />
Figure 4:<br />
<strong>Solar</strong> sub-system: options for the solar collector<br />
In the solar thermal system a he<strong>at</strong> exchanger can be integr<strong>at</strong>ed for separ<strong>at</strong>ion of primary<br />
<strong>and</strong> secondary loop. In conventional systems a w<strong>at</strong>er/glycol mixture together with a<br />
separ<strong>at</strong>ing he<strong>at</strong> exchanger is used, assuring trouble-free oper<strong>at</strong>ion throughout the year <strong>at</strong><br />
the expense of a reduction of the temper<strong>at</strong>ure level in the secondary loop due to the he<strong>at</strong><br />
transfer in the he<strong>at</strong> exchanger. In order to avoid the reduction of the temper<strong>at</strong>ure level a<br />
“direct” utiliz<strong>at</strong>ion of the solar he<strong>at</strong> may be applied. In this case either freezing of the he<strong>at</strong><br />
carrier w<strong>at</strong>er has to be avoided by means of he<strong>at</strong> input from the backup-he<strong>at</strong>er or an<br />
electrical he<strong>at</strong>er, or a drain-back concept has to be chosen. When the l<strong>at</strong>er is applied, the<br />
collector loop is filled by the circul<strong>at</strong>ing pump only when substantial solar contribution for<br />
solar collector he<strong>at</strong> exchanger he<strong>at</strong> storage backup<br />
hot side<br />
T<br />
T<br />
with<br />
without<br />
T<br />
T<br />
T<br />
Figure 5:<br />
<strong>Solar</strong> sub-system: he<strong>at</strong> exchanger for separ<strong>at</strong>ion of primary <strong>and</strong> secondary<br />
solar loop.<br />
page 5
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