Parametric number of transfer unit analysis of a photovoltaic thermal coupled desiccant dehumidifier

Abstract

Conventionally, desiccant wheels are operated with regeneration temperatures around 70–80°C, which are difficult to achieve using photovoltaic thermal collectors. Matching the design and operation of photovoltaic thermal collectors with desiccant wheels at low regeneration temperatures is complex due to the influence of many different parameters. This study investigates a comprehensive approach to parametrically analyse a flat plate photovoltaic thermal collector and a desiccant wheel dehumidification system to find the optimal range of operating conditions to work at a low regeneration temperature of 50°C. This is achieved using the concept of “number of transfer units”, a dimensionless parameter that describes the key design and operational characteristics of the photovoltaic thermal collector as well as the desiccant wheel. This study found that for a silica gel desiccant wheel with equal volume of air flow for the supply and regeneration air, the desiccant wheel has optimal number of transfer unit values between 6 and 10 and optimal rotational speeds between 6 and 8 revolutions per hour for a regeneration temperature of 50°C. This optimal range increases to 10–14 for the number of transfer unit values and 7.5–9.5 revolutions per hour for the rotational speeds at a higher regeneration temperature of 80°C. For a glazed photovoltaic thermal collector to regenerate the desiccant wheel, the optimal range of the number of transfer unit values is found to be 0.13–0.23 for the PV/T collector to achieve low energy consumption. Using these optimal values, with a regeneration temperature of 50°C, a good dehumidification performance (3.5–5.2 g/kg) and system cooling coefficient of performance (2.5–2.7) can be achieved for inlet air temperatures from 26°C to 38°C. For temperate climates and solar irradiance greater than 800 W/m2, with a 15 m2 photovoltaic thermal collector, the desiccant cooling system can achieve 100% energy savings in comparison to a dewpoint dehumidification and cooling process. However, for humid climates where a deep dehumidification of 6–8 g/kg is required, a higher regeneration temperature of 80°C is required.