In Figure 6 temperature trends of the storage system and the average seasonal yield of the collectors varying the capturing surface and storage volume are reported. The average seasonal temperatures are always between 85°C and 100°C inclusive which correspond to average seasonal values of collector efficiency which varies from a minimum of 65% to a maximum of 75%.

In Figure 7 the trend of the average seasonal solar fraction is illustrated, defined as the seasonal refrigerant energy supplied by the absorption chiller compared to the seasonal refrigerant energy required by the building, equal to 31500 kWh cooling energy, varying the collectors surface

and the storage volume. Such a parameter grows both with the capturing surface and with the storage volume, since it is possible to obtain higher inlet temperatures at the absorption chiller generator with consequent greater refrigerant energy provided. It is important to observe that the increase marked by the solar fraction is obtained by changing from 25 m2 to 100 m2 of collectors, while for surfaces greater than 100 m2 the solar fraction hardly increases. Despite the higher temperatures supplied to the generator, the solar fraction is in fact penalised by the operative limits of the absorption machine.

Finally, in Figure 8 the average seasonal performance coefficient trend of the absorption machine varying the capturing surface and storage volume is illustrated. The average seasonal COP varies between a minimum of 0.391 to a maximum of 0.437, from the reference value provided for the absorption chiller considered and equal to 0.60. Such a value is actually attainable in nominal operative conditions, with adequate inlet temperatures to the generator obtained by means of an auxiliary heater. The performance coefficient of the machine is penalised since the plant considered uses a traditional chiller instead of an auxiliary heater for the distribution of the refrigerant load which is not distributable by the absorption chiller.

0,44

C. O.P.

0,43 0,42 0,41 0,40 0,39 0,38

3. Conclusions

The energy performance of a solar energy system which uses a simple effect absorption chiller for the production of a refrigerated water flow rate used for the cooling of an open space environment by means of a radiant ceiling was analysed. The same solar collectors are used during winter for heating the same building, by means of the same distribution terminals. Heat pipe solar collectors were simulated by virtue of the elevated temperatures required for correct functioning of the absorption chiller Control logics were proposed which regulate the inlet temperature of the radiant ceiling, which permitted the evaluation of solar fractions obtained by the plant, based on
the capturing surface area and storage volume. It was observed that during winter, due to the high temperatures reached in the storage system, the collectors operate with limited efficiency yet the system is capable of supplying solar fractions close to 100% for capturing surfaces greater than or equal to 100 m2, independently of the storage tank volume. The high temperatures reached in the tank are opportunely regulated to supply the radiant ceiling, by means of a three way valve blending system which operates for long periods of time with high recirculation flows. The total yield of the system, due to the losses of thermal energy in the tank, assumes values equal to 30% for capturing surfaces of 200 m2, independently from the storage volume.

In summer, the system yield assumes values that are variable between 75% and 13%, due to the operative limits of the absorption machine which requires the use of an auxiliary chiller. It was decided to use an electric auxiliary refrigerating machine to supply the load that was not distributable by the absorption machine in that preliminary evaluations of primary energy consumption showed convenience compared to the use of an auxiliary heater usable to raise the temperature of the generator.

The solar fractions reached increase slightly for capturing surfaces greater than 100 m2, since they exceed the operative limits of the absorption chiller, principally represented by its insufficient flexibility, which makes the use of an auxiliary system necessary. For an capturing surface of 50 m2 and a storage volume of 25 m3 an average seasonal solar fraction equal to 30.6% is attained; the maximum value is 35.7% for a plant with 200 m2 of collectors and 25 m3 of storage. The refrigerant energy supplied by the absorption chiller hardly varies for capturing surfaces greater than 100 m2; for a storage volume of 25 m3 it is 10643 kWh with a surface area of collectors equal to 100 m2 while it is equal to 11185 kWh changing to an capturing surface of 200 m2, with an increase of 5% with a doubling of the capturing surface.

Finally, it is possible to observe the slight variability of almost all the analysed parameters compared to the storage volume; storage volumes greater than 10 m3 do not bring about benefits to plant performance but it is not possible to go below such values in order to guarantee system stability.

References

[1] Arcuri, N., Bruno, R., Ruffolo, S., 2005. Prestazioni termiche di sistemi di riscaldamento a soffitto radiante alimentati da collettori solari, Proceedings from the 2nd International CLIMAMED Conference, Madrid, March

[2] Lazzarin, R., Crose, D., 2000. Il soffitto radiante nella climatizzazione ambientale, SG Editoriali, Chap. 1

[3] TRNSYS, Reference Manual, AA. VV., 2001. A transient system simulation program, Solar Energy Laboratory, Madison, Wisconsin, USA

[4] UNI 10349, 1994 — “Heating and cooling of buildings; climatic data”

[5] Gansler, R. A., KleinS. A., 1993 “Assessment of the Accuracy of generated Meteorological Data for Use in Solar Energy Simulation Studies”, Proceedings of the 1993 ASME International Solar Energy Conference, Washington D. C.

[6] Oliveti G., Arcuri N., Bruno R., De Simone M., 2007 “Energy Performances Of A Radiant Floor Heating System Supplied By Solar Collectors With Ventilation Stream Heating By An Air To Air And An Air To Water Heat Exchanger”- REHVA International Congress — Helsinki [7] Oliveti G., Arcuri N., Bruno R., Mazzuca A., 2005 “Energy performance of an absorption chiller supplied by solar collectors in Mediterranean area”, SWC 2005, Orlando, USA

[8] UNI 10339, 1995, Air-conditioning systems for thermal comfort in buildings. General, classification and requirements. Offer, order and supply specifications.

[9] Lazzarin R., Castelletti F., Busato F., 2006 Soffitti radianti e aria primaria, Condizionamento dell’aria Riscaldamento e Refrigerazione N° 6

[10] Oliveti G., Arcuri N., Bruno R., 2008, Caratterizzazione di Soffitti Radianti che Impiegano Tubi Capillari per il Riscaldamento degli Ambienti” AICARR International Convention

[11] Arcuri N., Bruno R., 2005, Prestazioni termiche di sistemi di raffrescamento a soffitto radiante e relative strategie di controllo, 60° ATI National Congress, Rome