In a previous paper [4], the energy and exergy performance of a solar assisted standard DEC system were studied. In this work some improvements are made to the results of the previous paper:

• The parasitic electric consumption for the operation of fans and pumps are accounted for.

• A new definition for the exergy input to the systems is considered, which accounts for the exergy content of the primary sources, namely the fuel used to feed heaters and power plants for the production of electric energy.

As far as the first point is concerned, the pressure losses Apa occurring in each component of the air — handling unit were determined by means of data provided by the manufacturers, as reported in Table 2; an additional pressure loss of 150 Pa was adopted to account for the air distribution system, which may be considered the same for all the systems. The pressure losses Apw inside the pipes for the distribution of hot and cold water were also appropriately assessed by taking into account the volumetric water flow rate (Vw), the pipe diameter and its length. The parasitic electric consumption for fans and pumps can then be assessed, assuming qf = 0.7 and np = 0.6 as the efficiency of fans and pumps, respectively, and by means of the following formulas:

As regards the definition of the exergy input to the systems, in this paper the exergy of the primary sources is accounted for, namely the exergy of the fuel burned to feed the heat generator for the production of hot water and the power plants for the production of electric energy. According to this approach, the primary exergy associated with a certain amount of electric power Pel and thermal power Qhg may be respectively assessed as:

Here the ratio between the Higher Heating Value (HHV) of the fUel and its Lower Heating Value (LHV) is used because the exergy content of the fuel is related to its HHV, while the useful energy produced during the burning process is proportional to the LHV. In this analysis, natural gas was considered as the fuel used for the heat generator (HHV / LHV = 1.1), whereas oil was adopted for the power plants (HHV / LHV = 1.06). Furthermore, pel = 0.37 was adopted as the average efficiency for the production of the electric energy and its distribution to the final user, whereas ng = 0.90 was used as the efficiency of the heat generator. In Fig. 2 the comparison between conventional HVAC and standard DEC is reported; the comparison is based on three parameters, namely:

• the Specific Primary Energy Consumption (SPEC), defined as the ratio of the Primary Energy Consumption of the system to the overall thermal building load (QL + QS);

• the Specific Irreversibility Production (SPIR), defined as the ratio of the total Irreversibility Production of the system to the overall thermal building load (QL + QS);

• the exergy efficiency Z, defined as the ratio of the useful exergy output to the overall exergy input, accounted as primary exergy (see Eqn. 3).

V abs J

In Eqn. 3 we have considered that, when solar energy is used to assist the regeneration process, only a fraction (1-F) of the thermal load Qhc on the heating coil is covered by using fuel; the remaining fraction F is covered by solar energy, whose exergy content is considered proportional to the Carnot factor associated with the absorber plate temperature Tabs. The exergy flows and the irreversibility produced by each process are determined by means of the Gouy-Stodola equation, customized for the analysis of HVAC systems and humid air streams [5, 6]; the dead state corresponds to the outdoor conditions.

The standard DEC cycle has been studied for different regeneration temperatures. Thanks to the results shown in Fig. 2, it is possible to define the minimum solar fraction F needed to attain a second law efficiency of the DEC system better than that of a conventional HVAC system. This minimum value depends on the regeneration temperature required by the desiccant wheel; as an example, when working with a regeneration temperature as high as 90°C, at least F = 0.35 is required to get a higher exergy efficiency than the conventional HVAC, whereas F = 0.15 is sufficient if the regeneration temperature is as high as 70°C. When using high regeneration temperatures, a higher thermal power is required by the heating coil, but the dehumidification potential of the desiccant wheel also gets higher, thus allowing a reduction of the size of the pre­cooling coil (EB, see Fig. 1). For this reason, if a very high solar fraction is adopted — more or less higher than 80% — it seems to be more performing to work with higher regeneration temperatures. Further aspects will be addressed later in the paper.

When looking at the first law performance, the advantage of using solar assisted DEC system is more evident, and the energy break-even point is reached for a very low solar fraction. It has also to be underlined that the contribution of the parasitic consumption is not negligible. The exergy provided to feed fans and pumps ranges from 13% of the overall exergy input in a standard DEC system to 21% in a solar-assisted system with F = 1. In the conventional HVAC system this contribution reduces to the 5%, due to the low number of components inside the AHU.