Executive summary of the report
This report describes the activity regarding the development of the sorption module, that is an essential part of HYBUILD Mediterranean solution, since it allows the operation of the compression with high efficiency by storage and conversion of solar energy.
The main components that were optimised within HYBUILD project are the adsorber and the evaporator of the sorption module. The main innovation proposed in the adsorber is the integration of a porous structure, onto which zeolite is crystallised by means of a patented technique by FAHR, inside the micro-channel heat exchanger realised by AKG. To this aim, numerical and experimental activity were coupled. At first, heat transfer in different porous structures was evaluated and subsequently the most promising one were chosen for testing in small-scale adsorbers in a set-up already present at ITAE (Gravimetric Large Temperature Jump apparatus).
The different porous structures coated and evaluated are: (i) high density pressed fins produced by AKG, (ii) aluminium foams produced by AKG, (iii) aluminium foams produced by MIKR, (iv) aluminium chips produced by MIKR. The results of the activity allowed the identification of the volumetric and mass-specific cooling power that each structure can achieve and the final selection for full-scale adsorber. High- density pressed fins showed the best performance and therefore the full scale adsorber was designed and manufactured by FAHR and AKG. The results of the experimental testing were also used to implement a COMSOL model that was validated and that can be employed for the optimization of the system.
The full-scale adsorber has a fin space area of around 33 m2 with two of those heat exchangers combined in one module. This allows for achieving the required minimum of 50 m2 fin space area needed to generate a zeolite mass between 11 to 13 kg.
The second focus of the activity of T2.1 was the experimental evaluation of evaporators for sub-atmospheric evaporation conditions, typical of sorption systems. To this aim, a microchannel aluminium heat exchanger with high density aluminium fins by AKG was tested in a testing rig that was specifically built at ITAE. Two evaporation mechanisms were tested: pool boiling and thin film evaporation, since they correspond to the possible evaporation mechanisms that can be used in a sorption module if combined with a compression chiller, as in the HYBUILD solution. For the pool boiling case, different configurations were tested, i.e. 0°, 45° and 90°, as well as different filling levels of the vessels. For each case examined, the evaporation power and the heat transfer coefficient were measured/calculated. The activity was completed with the evaluation of semi- empirical models for the systematic design of such components. To this purpose, the evaporation mechanism was visualised and the possibility of applying correlations based on modified dimensionless groups commonly applied in fluid-dynamic was evaluated. The results demonstrated that it is possible to adequately correlate the conditions under which evaporation occurs (i.e. temperature and pressure in the condenser and evaporator) to the power and heat transfer coefficient of the heat exchanger. According to possible issues arising from the partial corrosion of aluminium in contact with water under vacuum conditions, it was decided to proceed with the design and manufacturing of a copper-based heat exchanger, acting as evaporator/condenser, exploiting capillary effect to operate under thin film evaporation conditions. Fahrenheit will size it according to the final capacity of the adsorbers, to properly operate the whole adsorption module.
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