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IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C1 Report, 31 October 2010
7.2 Main characteristics
SJECs have a specific operational behaviour. The comparison between the reversible
COP rev value and the real COP value of a SJEC points out this special operational behaviour,
as shown in Figure 34. For this comparison, the COP is defined as the ratio of the cold
energy produced in the evaporator Q E to the motive heat energy Q M required to operate the
steam jet ejector, see equation 1. The reversible COP rev value is calculated according to
equation 2 with the motive steam temperature T M (corresponding to the saturated steam
pressure), the evaporator temperature T E and the condenser temperature T C . The evaporator
temperature is 6 °C.
COP
COP
Q&
=
Q&
rev
=
E
M
( T −T
)
M
T
M
C
⋅
( T − T )
C
T
E
E
Equation 1
Equation 2
COP [-]
4
3
2
1
Motive steam temp.=120°C, rev
Motive steam temp.=140°C, rev
Motive steam temp.=160°C, rev
Motive steam temp.=180°C, rev
Motive steam temp.=120°C, real
Motive steam temp.=140°C, real
Motive steam temp.=160°C, real
Motive steam temp.=180°C, real
Evaporator temp. = 6°C
0
0 10 20 30 40
Condenser temp. [°C]
Figure 34: COP value of SJEC, reversible and real machine.
A higher motive steam temperature leads to a higher reversible COP rev value. Furthermore
the reversible COP rev increases with decreasing condenser temperature. As a start, the COP
value of a SJEC also increases with decreasing condenser temperature, but then remains
constant below a certain condenser temperature. The reason for this behaviour is that the
flow velocity in the steam jet ejector reaches supersonic speed and the mass flow through
the ejector cannot be increased further. The comparison shows also that the motive steam
temperature (corresponding to saturated steam pressure) can be reduced when the
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