Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 170

to the condenser 35 ◦ C and an evaporator temperature inlet of 25 ◦ C. A maximum
cooling capacity of 2 kW could be achieved if the evaporator inlet temperature was set
to relatively high values of around 25 ◦ C, indicating insufficient heat transfer capacity
of the evaporator.

Pressure Dependence A further series of measurements were carried out at a lower
operating pressure of 12 10 5 Pa. The COP decreases with rising generator temper-

ature. A comparison of the theoretically possible and the measured cooling capacity
of the evaporator showed that, above a generator heating inlet temperature of 125 ◦ C,
the amount of ammonia produced could not be converted into cooling power by the
falling film evaporator.
Many long-term measurements of up to 11 days were carried out without shut-
ting down the machine. The results showed a COP of 0.3 to 0.5 and a continuous
evaporator cooling performance up to 1.6 kW at evaporator outlet temperatures for
air-conditioning and cooled ceilings between 22 and 15 ◦ C (see Figure 5.59). The
lowest external evaporator outlet temperatures reached were

×

5 ◦ C at an operating

−

10 5 Pa and a generator heating inlet temperature of 145 ◦ C.
The improvement in performance can bemainly attributed to the lower total pressure
of the system, which improves both the bubble formation process and the diffusion
rates in the evaporator.

pressure of 8 . 5 ×

Evaporator Performance From the measured liquid ammonia mass flow, a theoret-
ically possible evaporator cooling capacity was determined between 0.6 and 2.7 kW
for heating temperatures between 115 and 155 ◦ C. The measured cooling capacities
were only between 0.6 and 1.5 kW. The falling film evaporator could not evaporate all

0.50 10

0.45 9

COP

0.40 8

0.35 7

0.30 6

0.25 5

0.20 Heating power 4

0.15 3

Cooling power

0.10 2

0.05 1

0.00 0

115 120 125 130 135 140 145 150

Generator heating inlet temperature / °C

Figure 5.59 COP and heating and cooling power of the second prototype at a lower total pressure of
12 × 10 5 Pa