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Research Papers

# Simulation and Experimental Analysis of a Solar Driven Absorption Chiller With Partially Wetted Evaporator

[+] Author and Article Information
Jan Albers

Technische Universität Berlin, Marchstraße 18, D-10587 Berlin, Germanyjan.albers@tu-berlin.de

Giovanni Nurzia

Dipartimento di Ingegneria Industriale, Università degli Studi di Bergamo, Viale Marconi, 24044 Dalmine (BG), Italy

Felix Ziegler

Technische Universität Berlin, Marchstraße 18, D-10587 Berlin, Germany

J. Sol. Energy Eng 132(1), 011016 (Jan 11, 2010) (8 pages) doi:10.1115/1.4000331 History: Received August 17, 2008; Revised July 27, 2009; Published January 11, 2010

## Abstract

The efficient operation of a solar cooling system strongly depends on the chiller behavior under part load conditions, since driving energy and cooling load are never constant. For this reason, the performance of a single stage, hot water driven 30 kW $H2O/LiBr$-absorption chiller employed in a solar cooling system with a field of $350 m2$ evacuated tube collector has been analyzed under part load conditions with both simulations and experiments. A simulation model has been developed for the whole absorption chiller (Type Yazaki WFC-10), where all internal mass and energy balances are solved. The connection to the external heat reservoirs of hot, chilled, and cooling water is done by lumped and distributed UA values for the main heat exchangers. In addition to an analytical evaporator model—which is described in detail—experimental correlations for UA values have been used for the condenser, generator, and solution heat exchanger. For the absorber, a basic model based on the Nusselt theory has been employed. The evaporator model was developed, taking into account the distribution of refrigerant on the tube bundle, as well as the change in operation from a partially dry to an overflowing evaporator. A linear model is derived to calculate the wetted area. The influence of these effects on cooling capacity and coefficient of performance (COP) is calculated for three different combinations of hot and cooling water temperature. The comparison to experimental data shows a good agreement in the various operational modes of the evaporator. The model is able to predict the transition from partially dry to an overflowing evaporator quite well. The present deviations in the domain with high refrigerant overflow can be attributed to the simple absorber model and the linear wetted area model. Nevertheless, the results of this investigation can be used to improve control strategies for new and existing solar cooling systems.

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## Figures

Figure 1

Measured cooling capacity for two groups of hot and cooling water inlet temperature (tGi and tACi)

Figure 2

Action of thermosyphon desorber and temperature difference ΔTD to determine the operating domain

Figure 3

Variation in the generator UA value with strong solution flow rate

Figure 4

Variation in the condenser UA value compared with Nusselt's theory as function of solution flow rate, which is proportional to the temperature difference (tGi−tACi)

Figure 5

Variation in solution heat exchanger UA value with strong solution flow rate

Figure 6

Measured evaporator UA values, capacities, and temperatures for tGi/tACi=85°C/29.5°C

Figure 7

Computation of wetted area for each row of the tube bundle as a function of the refrigerant flow rate

Figure 8

Model structure

Figure 9

Capacity and COP for tGi/tACi=77°C/24°C predicted by the model as function of chilled water outlet temperature tEo

Figure 10

Capacity and COP for tGi/tACi=80°C/29.5°C predicted by the model as function of chilled water outlet temperature tEo

Figure 11

Capacity and COP for tGi/tACi=85°C/29.5°C predicted by the model as function of chilled water outlet temperature tEo

Figure 12

Evaporator UA value for tGi/tACi=85°C/29°C predicted by the model as function of chilled water outlet temperature tEo

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