Parametric study results

Supply air volume flow rate

200 m3/h

Reqen. air volume flow rate

250 m3/h

Pre-cooling air volume flow rate

250 m3/h

Sorbent Material

Zeolite

Sorbent thickness

0.25 mm

Heat exchanger plates material

Aluminium

Plates thickness

0.5 mm

Plates distance

3.5 mm

Table 1 — Structural heat exchanger characteristics

Return air temperature

26°C

Return air relative humidity

50%

Return air humidity ratio

10.5 g/kg

Ambient air temperature

35°C

Ambient air humidity ratio

20 g/kg

Heat recovery efficiency (reg.)

0.8

Adsorption Phase

1 t*

Regeneration phase

0.8 t

Pre-cooling phase

0.2 t

Parameters:

Regeneration temperature range

60-95°C

Cycle duration range

150-600 s

Table 2 — Main data used for the simulation campaign

In Figures 4 to 6 the results of a complete set of simulation calculations are represented as a function of the two considered parameters. The COP map in Figure 4 shows values significantly higher than in standard DEC cycles (typically 0.6 — 1). As expected the values are higher for low regeneration temperatures and long cycle duration, where the indirect adiabatic cooling effect is predominant on the desiccant cooling process. On the other hand for these values of the two parameters the system does not provide acceptable supply air conditions — at least in hot-humid climates — since the level of dehumidification is extremely small. Values of COP in the range of 1.2-1.4 are reachable for short cycles (i. e., below 300s) at any regeneration temperature. Figures 5 and 6 present the supply air temperature and humidity ratio maps for different values of cycle duration and regeneration temperature, respectively.

It can be seen that the level of dehumidification reachable with short cycles and regeneration temperatures higher than 80°C lies between 11 and 12.5 g/kg. These values show a dehumidification process significantly more efficient than in standard DEC systems in the same range of regeneration temperature. In the psychometric chart (Figure 7) are presented, as example, the two process paths.

Moreover the isotherm curves in Figure 5 have a different shape than the ones related to the humidity ratio for the same values of the studied parameters. From a deep analysis of the simulations data for each time-step, resulted that with high regeneration temperatures (i. e., 90-95°C) and short cycle durations (below 250s) a considerable amount of energy is still stored in the heat exchanger thermal mass at the beginning of the adsorption phase. The latter is due to the fact that the pre-cooling phase duration is fixed as percentage of the cycle
duration, and it is not optimised towards the process performance according to the regeneration temperature.

60 65 70 75 80 85 90 95

Regeneration temperature [°d

Figure 4 — COP maps as result of the parametric study versus regeneration temperature and cycle duration

60 65 70 75 80 85 90 95

Regeneration temperature [°d

Figure 5 — Supply air temperature maps as result of the parametric study versus regeneration temperature and cycle duration

60 65 70 75 80 85 90 95

Regeneration temperature [°d

Figure 6 — Supply air humidity ratio maps as result of the parametric study versus regeneration temperature and cycle duration

The data presented in Figures 4 to 6 do not take into account the possible further direct evaporative cooling (of the supply air).

If a humidifier would be operated after the heat exchanger supply air channels (see optional humidifier in Figure 3) the air could be further cooled and a full air-conditioning process could be carried out without using any conventional refrigeration machinery. Therefore another simulation run has been carried out including a direct humidifier in the supply air. For each time step the supply air conditions have been calculated implementing a direct evaporative cooling process simulation. The set supply air humidity ratio has been assumed 8.8 g/kg in order to cover the internal latent cooling loads. Figures 8 and 9 show the resulting supply air temperature and humidity ratio maps.

The direct evaporative cooling process causes a significant drop in the temperature values for regeneration temperatures higher than 80°C and cycle durations below 300s. The system manages to reach a minimum temperature of 22°C for the highest regeneration temperature (i. e. 95°C) and the shortest cycle duration (i. e.,150s).

2 CONCLUSIONS

The simulation results show a good ECOS’s performance for heat driven air­conditioning applications in the range of temperatures interesting for the use with solar thermal plants. In particular the process reaches a very efficient dehumidification with simultaneous temperature reduction. At the same time COP values are achievable which are significantly higher when compared to those of standard desiccant and evaporative cooling systems employing rotors. Therefore the ECOS process results very promising in particular for climate

Comparison Standard DEC and ECOS’s cycle paths

80

70

60

U

50

ra

40

Ф

a

30

ф

i-

20

10

6 8 10 12 14 16 18 20 22

humidity ratio [g/kg]

Figure 7 — Comparison standard DEC and ECOS’s cycle paths

24

0

4

zones with high ambient air humidity (e. g. Mediterranean and tropic areas). Nevertheless in these climatic conditions in order to ensure a proper air-conditioning operation the regeneration temperature required could be in a range not optimal for standard flat plate collectors. In these cases, evacuated tube or CPC collectors would be desirable.

Furthermore the system design does not pose limits for the realisation of low capacities (200 m3/h) units. Consequently the ECOS system results a good candidate for "split” air­conditioning applications to be connected with the heat distribution network driven by solar combi systems.

The results of the parametric study shown that a further analysis of the single phases duration is needed. Moreover an optimised choice of the employed sorbent material it would desirable, in order to achieve higher performances.

ACKNOWLEDGEMENTS

The work of M. Motta has been supported by a Marie Curie Fellowship of the European Community programme "Improving human potential and the socio-economic knowledge base” under contract number ENK6 — CT — 2002-50515

REFERENCES

[1] EC (1999): Study for the Directorate-General for Energy (DGXVII) of the Commission of the European Communities (1999): Energy Efficiency of Room Air-Conditioners

[2] Henning H. M., (2004): Hans-Martin Henning (Ed.) — Solar-Assisted Air-Conditioning in Buildings, A handbook for planners — (2004) Springer Verlag

[3] Motta M. et al. (2004): M. Motta, H. M. Henning — An original heat driven air-conditioning concept: advanced desiccant and evaporative cooling cycle numerical analysis, Proc. 44° Convegno Internazionale AICARR 2004 — Milano 3-4 Marzo 2004 Vol. II — p. 1149 — 1166