15.07.2015   EuroSun2008-7

Simulation results

One configuration of a sorptive heat exchanger has been implemented. The simulated counter flow heat exchanger is characterized by a volume flow rate of 20m3/h per square meter of heat transfer area, is covered by a silicagel layer (Grace 127B) and discretized in ten elements in flow direction. Table 1 gives the state of air properties as applied in the simulation.

Table 1: Input air properties during different cycle stages as used in the simulation

Air flow properties: Sorption channel, position 1

Air flow properties: Coo

ing channel, position 3

m ads

Ambient

air

m des

Heated ambient air

m ads

Building return air

m l

pre — cool

Ambient

air

Tads [°C]

32.0

Tdes

85.0

Tads

26.0

Tpre-cool

32.0

^ads

[g/kg]

14.7

^des

14.7

^ads

12.5

^pre-cool

14.7

9ads [%]

50.0

9des

4.1

9ads

60.0

9des

50.0

The simulation results presented in the following show the simulated transient behaviour of process air outlet flow temperature (Fig 2) and humidity ratio (Fig 3) for the sorptive heat exchanger during one cycle. The shown data is derived from a cycle simulation characterized by independence from start values.

image083

Fig 2: Temperature distribution of outlet and inlet air during one cycle

image084

Fig 3: Humidity distribution of outlet process air during one cycle

The first cycle stage depicted in Fig 2 and Fig 3 is desorption when hot ambient air is passing the process air channels for regeneration. Temperature T2 first rises sharply due to the heating up of the heat exchanger. Concurrently, outlet humidity ratio m2 also rises sharply due to the high driving potential for desorption caused by the high desiccant material water load and rising material temperature. As desorption advances and the desiccant material is getting dryer the driving potential and consequently outlet humidity ratio m2 diminishes. At the same time outlet temperature T2 rises as less energy is used for the desorption process.

During the pre-cooling stage ambient air passes the cooling channels where it is humidified. Fig 2 shows how the outlet temperature T4 of air leaving the cooling channels decreases as the heat exchanger cools down. It reaches a temperature below ambient temperature due to the evaporative cooling. During pre-cooling, sorption channel air properties are not shown due to zero air flow rate.

During adsorption ambient air flows through the sorption channels and building exhaust air passes the cooling channels being simultaneously humidified. As given in Fig 3 dehumidication is most effective at the beginning of the adsorption phase as the sorption material water load is still low. Process air outlet temperature T2 falls as less heat of adsorption is released with decreasing dehumidification. In the counter flow arrangement simulated here the exhaust air temperature T4 at cooling channel outlet is higher than the process air outlet temperature T2.