Solar Desiccant and Evaporative Cooling

New Solar Desiccant and Evaporative Cooling unit based on fixed and cooled adsorption beds and wet heat exchangers assisted by a building integrated solar PVT generator (IR)

Leader: UNIPA-DEIM Partners: EXALTO

The building integration of solar thermal collector is an important design approach for energy retrofit issues in existing buildings; nonetheless, this strategy faces many technical constrictions related to the fact itself that the building already exists. At the same time, the NZEB concept is becoming a focal point in research activities, proposing an integrated approach to energy load and demand and addressing researchers but also architects/engineers toward building design combining building architectonical and equipment requirements. Anyway, existing buildings show high complexity for integration in façade especially when window surface area prevails on the opaque one. For this reason shelter devises are generally a suitable solution but, which is its impact in real operating condition? Having a look at the overall energy balance during the year, how the heat produced by such devices could be useful during cooling season?

In the framework of the project i-next, a new prototype of a solar Desiccant and Evaporative Cooling (DEC) air handling unit coupled with a building integrated PVT solar shelter has been recently installed at DEIM.

This system is based on the innovative DEC concept called freescoo (FREE Solar Cooling) which is mainly based on the use of two fixed and cooled adsorption beds operating in a batch process and two wet heat exchangers. The proposed innovative adsorption bed is a fan and tube heat exchanger commonly used in the air conditioning sector, wherein the spaces between the fins are filled with silica gel grains.
Traditionally, in common DEC systems, desiccant rotors are normally used. However, the adsorption process realized by means of desiccant rotors presents the disadvantage of causing a temperature increase of the desiccant material. This phenomenon is caused by the release to the process air of adsorption heat due to the latent heat released into the desiccant material and by the carry-over of heat stored in the desiccant material from the regeneration section to the process section. The use of desiccant fixed cooled beds permits to overcome the mentioned thermodynamic limit.
The main feature of this component is in fact to allow the simultaneous dehumidification and cooling of air. Furthermore, since the component hosts a considerable amount of adsorption material, solar energy can be efficiently stored in the desiccant media in terms of accumulated adsorption capacity. This can potentially be used when regeneration heat is not available, strongly reducing the need for thermal storage in the solar loop.
The two fixed desiccant beds packed with silica gel are operated in a batch process. Each adsorption bed has a total volume contains about 64 kg of silica gel in grains. A system of air dumpers provides the commutation between the two adsorption beds in order to guarantee a continuous dehumidification process.
The indirect evaporative cooling process, operated downstream to the dehumidification, is realized by two wet plate heat exchangers connected in series. The evaporative cooling process can be operated at low temperature, allowing supply air temperature to the room below 20°C.
The cooling tower is used to reject the adsorption heat generated in the desiccant bed during the dehumidification process.
Total flow rate delivered to the building is 2000 mc/h, whereas the maximum cooling power is about 20 kW. Cooling power can be controlled through variable speed fans. Maximum electric power required is approximately 1.8 kW and is mainly due to the operation of two fans, two pumps and a cooling tower. Electricity demand can be covered by the PV panels integrated in the solar collectors which has a peak power of 2.7 kW and cover about 60% of the total collector surface.
The solar PVT system provides heat and electricity to the system allowing to reduce the energy demand from the grid.
The PVT collectors which constitute the shelter are arranged in a string. A common corrugated sandwich panel, on which a composite cover (60% of the area is ventilated PV module, 40% selective absorber plate) lays, is the main core of the proposed PVT shelter; A single glass shield closes the solar collector. In this configuration, 4 channels for collector are present between the cover and the corrugated panel. Air from outside is forced in the gap between the glass cover and the PV/absorber. Therefore regeneration of the silica gel can be carried out.
The PVT system has been dimensioned in order to provide high solar thermal fraction for heating and cooling the building and to fulfill the total electricity demand for the AHU auxiliaries and lighting purpose.
In addition, a small charging station for cars is connected to the energy storage
System integrates also a smart storage unit which has the role to interface the electricity generators (PV system and also a small wind turbine), the building loads (AHU, lighting, recharging station) and the grid. The main purpose of this component is to limit the stress of the grid avoiding peaks of power (Activity 8.3 leaded by CNR-ITAE).

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Licenza Creative Commons
Quest’opera è distribuita con Licenza Creative Commons Attribuzione – Non commerciale – Non opere derivate 4.0 Internazionale.

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