Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 166

Table 5.6 Boundary conditions and minimum height difference between the top level of the reservoir
and the heating zone of the bubble pump

Prototype 1 Prototype 2

Generator mean boiling temperature T b / ◦ C 130 110

NH 3 rich/weak mass concentration X Sr / X Sw 0.42/0.34 0.37/0.27

Rich/weak solution density ρ Sr / ρ Sw kg m − 3 742/783 789/837

Total pressure p /Pa 20 × 10 5 12 × 10 5

Lifting height h /m 1.0 0.5

Minimum height difference Y /m 0.12 0.03

the minimum height difference calculated is 0.12m for prototype 1 and 0.03m for
prototype 2.

Results from Prototype 1 The first pilot plant with a total height of 3.70m and
approximately 800 kg weight went into operation in November 2000. A series of
measurements were taken with an indirect, liquid heating system at generator heating
inlet temperatures of 150 to 175 ◦ C and evaporator temperatures between 25 and 0 ◦ C.
The measurements were taken with and without the dephlegmator. Maximum COPs
measured were 0.2 and the evaporator cooling capacity of the pilot plant was 0.5
to 1.5 kW, but the operation was not continuous. The evaporator capacity decreased
with time as shown in Figure 5.54, as the auxiliary gas starts to get saturated with
ammonia at an insufficient auxiliary gas circulation. This could not be attributed to
excessive pressure drops in the auxiliary gas circuit: at auxiliary gas volume flows
below 4m 3 h − 1 , the pressure drop calculated and measured after deconstruction of the
prototype was only 1 Pa and the driving force calculated from the density difference
between ammonia rich auxiliary gas and weak gas after the absorber is about 15 Pa for
a5 ◦ C evaporation temperature and a height difference of 1m between the evaporator
and absorber. The initial peak of cooling power can be explained by evaporation of
condensed ammonia within the tubes between the condenser and evaporator and the
upper section of the evaporator. For the first hour, the evaporator inlet temperatures
are very low at 8 ◦ C from start-up values of 35 ◦ C and then increase again.
A further effect explaining the performance decay with time is due to the absorber:
as the solution heat exchanger had an included reservoir with an approximate capacity
of 60 litres, the solution inlet to the absorber is initially at room temperature and only
heats up slowly. In the second prototype, the reservoir was removed from the solution
heat exchanger and reinstalled below the absorber.
Figure 5.55 shows the operating performance and the measured COPs and cooling
capacity of prototype 1 for different generator heating water inlet temperatures. The
measurements were taken with heating capacities between 10.0 and 13.5 kW at an
operating pressure of 20 10 5 Pa. The maximum evaporator cooling capacity was

1.5 kW. The evaporator could not evaporate all the available liquid ammonia into

×