By means of the collected data, mean performances figures and energy savings system were calculated also with the aim to compare such system with a reference conventional one. The energy performance indicators used for the evaluation are presented here below.

The SFDEC is the fraction covered by the desiccant cycle to the total cooling energy delivered by the AHU and is calculated by equation (1). The monthly solar heat management efficiency describes the quantity of incident irradiation what is usefully utilized in the system and calculated by eq. (2):

QCC1 + Qcc 2

sFDEC _ 1 q (!)

QDEC

The thermal COP HC2 of the desiccant AHU indicates the ratio between the cooling energy produced by the desiccant cycle and the regeneration heat delivered by the solar heating coil HC2, whereas the thermal COP HC2+HC1 includes the global heat amount coming both from the solar and heat recovery coil.

	(3)
(4)
QH	QHC 2 + Qh

In order to estimate the energy saving obtained in comparison to a reference AHU, the primary energy consumption of both systems have been calculated with the general formula (5) taking in account for the reference unit also the necessary heating energy delivered from a gas boiler to achieve the desired temperature for the supply air. The primary energy ratio PER has been calculated by the formula (6) where it was assumed the same value of QAHU both for the desiccant and reference AHU.

PE _ Qel + Qre~heating
(5)	(6)
Vd	П fossil

Since the desiccant wheel and the additional coils cause higher pressure losses than in a conventional AHU, different electricity consumption for ventilation have considered for the two systems. In particular, in the calculation of the primary energy consumption for the reference system, the electricity consumption of the reference AHU was calculated according to the mentioned “Monitoring procedure of solar heating and cooling systems” of the IEA Task 38. More in detail, knowing the measured electricity consumption of the desiccant AHU and assuming the same efficiency of the fan motors at the same flow rates, the one of the reference AHU can be derived introducing a correction factor related to the pressure drops AP and to the volumetric flow rates :

(7)

The calculated electricity consumption for ventilation of the reference AHU amounts to 47% of the one of the desiccant cooling AHU. In the calculation of the primary energy consumption related to the cooling energy it also was assumed the same chiller performances of the one used in the DEC system.

+ + + Ч ++++^ 2146; 43% +++++* ,+++++- ■■++++■ k++++

The next figures show the cooling energy QAHu delivered to the building for ventilation and sensible cooling and the distribution of the cold production in the AHU. It can be seen that, for the considered time period, the fraction of the cooling energy covered by the desiccant cycle (QDEC) is averagely 42%. It is worth to note that, since the radiant ceiling power was not sufficient to meet the sensible load of the room, a lower supply air temperature (18-20 °C) was necessary to guarantee the indoor set point conditions. Also for this reason the second auxiliary cooling coil gives a considerable contribution (QBF2= 47%) to the cooling energy balance.

Q radiant ceiling ■ Q vent sens Я Q vent lat

Fig. 6. Distribution of the cooling energy delivered to the building (left) and produced in the AHU (right)

+++* ++++ +++++ +++++* 1363; 41% k++++ ♦

In Figure 7 shows that the energy delivered from the sensible heat exchanger and the solar heating coil HC2 are similar, whereas the contribution of the condensation heat recovery coil HC1 is 22% to the total corresponding to the half of the energy delivered by the solar heating coil. In the same figure, also the distribution of electricity consumption for the whole system is reported.

Q HX ret

Fig. 7. Distribution of regeneration heat for the desiccant cycle (left) and electricity consumptions (right)

The primary energy ratio for the desiccant unit, PER AHU is 1.03 kWhcold/kWhPE, whereas the one for the reference system amounts to 0.54 kWhcold/kWhPE. In other terms, the primary energy saving of the desiccant system is 47% for the considered time period.

In the following table some mean measured values and performance figures for this time period are summarized.

Table 2: Mean monthly performance figures — July 2008

1st International Congress on Heating, Cooling, and Buildings — 7th to 10th October, Lisbon — Portugal / Supply ventilation air flow rate 1831 [kg/h] Temperature of chilled water 12 [°C] Return ventilation air flow rate 1746 [kg/h] SFdec 42 [%] Total cooling power to building 14.5 [kW] Regeneration temperature 54 [°C] Sensible load of building 9.6 [kW] Regeneration flow rate ratio 2/3 [-] Latent load of building 3.9 [kW] Solar collector efficiency 40 [%] Total cooling power of AHU 9.1 [kW] П solar heat 26 [%] Cooling energy to building 5028 [kWh] COPth HC2 1.14 [-] Cooling energy delivered by AHU 3374 [kWh] COPth HC1+HC2 0.72 [-] Cooling power of chiller 10.6 [kW] PER ahu 1.03 [-] Electrical COP of chiller 2.73 [-] Primary Energy saving 47 [%]

3. Conclusions

The whole system is working fairly well in summer operation. First monitoring data have shown promising results. Some improvements of the solar system will be necessary in order to increase the solar heat management efficiency and consequently the dehumidification potential of the desiccant cycle. The contribution of the condensation heat coil to the energy required for the regeneration has given interesting results and will be further investigated. Some mechanical adjustments in the heat exchanger should minimize the problem of air infiltration from the return side to the supply side. Furthermore, the accuracy of the humidity measurements must be improved. Nevertheless, considerable primary energy savings have been reached.

Nomenclature

CC or BF	Cooling Coil	Qdec, ahu	Desiccant cooling Energy of AHU
HC	Heating Coil	SFdec	Cooling Energy Fraction covered by DEC cycle
DW	Desiccant Wheel	n Solar, heat	Solar heat management efficiency
HX	Sensible Heat Exchanger	COP th, HC	Thermal COP of DEC cycle
HU	Humidifier	PER ahu	Primary Energy Ratio of cooling energy from AHU
PE	Primary Energy	£ el	Conversion factor for electricity production = 0.4
^boiler	Boiler efficiency	£ fossil	Conversion factor for fossil fuel = 0.9

References

[1] Dhar P. L., Singh S. K., 2001. Studies on solid desiccant based hybrid air-conditioning systems — Applied Thermal Engineering 21, 119-134

[2] Mazzei P., Minichiello F., Palma D., 2005. HVAC dehumidification systems for thermal comfort: a critical review. Applied Thermal Engineering 25, 677-707

[3] Finocchiaro P., PhD Thesis “Analisi numerica di sistemi desiccant cooling alimentati ad energia solare per applicazioni in climi mediterranei” Dipartimento di Ricerche Energetiche ed Ambientali — Universita degli Studi di Palermo, 2007

[4] Beccali M., Finocchiaro P., Nocke B., Gioria S., “Solar desiccant cooling AHU coupled with chilled ceiling: description of a new installation at DREAM in Palermo”, Proc. of the 2° OTTI International Conference on Solar Air Conditioning, Tarragona (E), October 18th -19th., 2007, pp 389-394, ISBN 978-3-934681-61-3.