Effect of rotational speed on the performances of a desiccant wheel

In recent years, the boost towards the reduction of electrical loads for air conditioning and the decentralization of energy conversion devices are determining an increasing interest in small scale trigeneration systems fueled by natural gas (“gas cooling”), able to shift energy demand in summer from electricity to gas, at the same time allowing the exploitation of natural gas surplus during the warm season. A technology that meets these requirements is represented by desiccant-based dehumidification systems, in which thermal energy for regeneration can be provided by a small scale cogenerator; the main component of these systems is the desiccant wheel, whose performances, in terms of humidity reduction and process air outlet temperature, depend on several operational parameters. The rotational speed of the desiccant wheel is widely recognized as a crucial parameter: if the wheel rotates too fast, the desiccant material does not have enough time to remove the moisture, while if the wheel rotates too slowly, saturation could occur. As a result, there must exist an optimal rotational speed, depending on the operating conditions, that guarantees the best dehumidification performance. Rotational velocity of the desiccant wheel influences the process air temperature exiting the desiccant wheel too; therefore it should be chosen in order to contemporary obtain a high dehumidification performance and an enough low outlet temperature, to reduce the cooling load on the cooling device, in particular if a conventional vapor compression chiller is used, as often occurs in high humidity climates. In this paper, experimental tests on a silica gel desiccant wheel, in order to highlight the effect of rotational speed on its performance, are shown. The adsorbent material is regenerated by thermal energy up to 65°C. The experimental results were used to calculate some of the most representative performance parameters for the wheel, that are the dehumidification effectiveness, the dehumidification coefficient of performance (DCOP) and the sensible energy ratio (SER). Finally, the influence of process air inlet temperature and humidity, regeneration temperature and the ratio between the regeneration and process air flow rates on the optimal rotational velocity is discussed. It was found that, for the analyzed desiccant wheel, the velocity that optimizes the dehumidification performances varies in the range 5–10 revolutions per hour, depending on operating conditions, while SER monotonically increases with rotational velocity.