Publication: HVAC & R Research
Publication Date: 01-JAN-07
Delivery: Immediate Online Access
Author: Castro, Jesus ; Oliva, Assensi ; Perez-Segarra, Carlos David ; Cadafalch, Jordi
Article Excerpt
A prototype of an air-cooled absorption chiller of about 2 kW for air conditioning using [H.sub.2]O-LiBr has been developed. The unit has been conceived as a laboratory experimental test device with removable components to facilitate modifications of the initial design. Several tests have been carried out under different conditions. The experimental results have been compared with the theoretical ones based on global mass and energy balances over the different components of the system. Detailed simulation models for each heat exchanger have been developed and implemented in the numerical codes to calculate the overall heat transfer coefficients and subcooling values for the whole system simulation. The conclusions reported will lead to future design revisions and improvements to achieve better performance and reliability.
INTRODUCTION
In the last decades, a significant increase in electricity consumption has been produced due to the growth in cooling demand. In order to save energy in cooling systems, it is necessary to develop new technologies to take advantage of alternative energies, e.g., solar energy or waste heat.
In the case of solar cooling installations, the main obstacle that impedes extended use of absorption systems is the large initial investment necessary for both solar collectors and the absorption machine. For low-capacity installations (less than 15 kW), the price of the chiller is the most limiting factor. Therefore, innovative designs of both solar collectors and absorption chillers (Ziegler 2002) are necessary to reduce prices. Moreover, greater simplicity of the absorption cooling installation would reduce the cost of the investment significantly. Therefore, air cooling for the absorber and condenser can be an important issue to avoid using a cooling tower. For solar air-conditioning applications, the refrigerant-absorbent [H.sub.2]O-LiBr is clearly preferred with respect to N[H.sub.3]-[H.sub.2]O due to its higher performance, but the use of LiBr as absorbent implies the risk of crystallization, more important in air-cooled systems.
Different prototypes of air-cooled [H.sub.2]O-LiBr machines have been developed in the past decade for air-conditioning in buildings (Ohuchi et al. 1994; Tongu et al. 1993; Enjoji 1998; Ishino and Kawasaki 1998; Kawakami et al. 1998). However, those machines were gas fired. Therefore, the type of cycle used in those cases was double effect in order to get the maximum advantage of the input energy. LiBr is sometimes used as absorbent together with LiI to overcome the problem of crystallization (Tongu et al. 1993; Ishino and Kawasaki 1998). The main problem for all those machines concerns the high electrical consumption of the fans to create adequate cooling effect on the absorber and the condenser because reduced and compact designs were the main premises. Izquierdo et al. (2001) presented a suitable air-cooled absorber used for absorption systems in public transport that took advantage of the engine's waste heat.
In this paper, a prototype of a low-power absorption cooling machine with an air-cooled condenser and an absorber driven by hot water at the generator is described in detail. Its input energy is hot water below 100[degrees]C. Therefore, a single-effect cycle is suitable for this situation (Figure 1). Although the final performance of the system is slightly reduced due to the air cooling, in both the absorber and condenser the main premise of design has been to keep the electrical fan consumption in these elements within moderate limits, maintaining a reasonable performance. The prototype has a mechanical solution pump, which leads to different behavior of the machine compared to the heat-powered pumps used in the Yazaki commercial water-fired chillers (these chillers experiment with a drastic decrease of the COP due to the decrease of the circulated mass flow at low driving temperatures [Yazaki 1996]). The machine studied has a horizontal-tube, a falling film generator, and an evaporator. The air-cooled absorber and condenser consist of vertical finned tube batteries, whereas the falling film is placed inside the tubes. Different temperature, pressure, and mass flow sensors are located in the most significant points in the machine. This unit has been conceived as a laboratory experimental test device with removable components to facilitate modifications of the initial design.
This paper is divided into two main sections. The first section is devoted to the research approach and methodology. The numerical model of the whole refrigeration cycle and the detailed models used for the heat exchangers are presented. Details of the experimental device developed for validation purposes are also presented. The second section focuses on the experimental data obtained and their comparison with the numerical model.
[FIGURE 1 OMITTED]
RESEARCH APPROACH AND METHODOLOGY
In this section, the model employed for the simulation of the whole absorption cooling system and the experimental apparatus used for its validation are described. The simulation of the whole absorption cycle is based in two levels of modeling: (1) the cycle modeling level, where the main overall values of the cycle are calculated, and (2) the heat exchanger detailed modeling level, where the overall heat transfer coefficients and the subcooling values at the outlet of the heat exchangers, needed by the above-mentioned cycle modeling level, are calculated using the input data of the experimentation.
Numerical Model: Cycle Simulation
In order to study the type of air-cooled absorption system described in the previous section, a mathematical model to simulate the whole system (see Figure 1) has been developed. For each element, overall mass and energy balances are imposed. At the design mode, the nominal cooling capacity (heat exchanged at the evaporator) and the heat transfer efficiencies of the other heat exchangers are data; the overall heat transfer coefficient multiplied by the area of each heat exchanger, the heat exchanged in each element, and the main data of the absorption cycle (enthalpies, temperatures, pressures, LiBr mass fractions, and mass flows) are obtained. At the rating mode, both the area and the overall heat transfer coefficients of the heat exchangers are data. Thus, the effectiveness and heat exchanged in each element are obtained in addition to the main data of the absorption cycle. Negligible heat losses and pressure drops in the connections between elements are assumed. For each i-connection, these conditions, together with mass conservation, produce = [dot.m.sub.i] = [dot.m.sub.i'], [c.sub.i'] = [c.sub.i], [h.sub.i] = [h.sub.i'], and [p.sub.i] = [p.sub.i']. Under steady-state conditions, and neglecting both kinetic and potential energy, the equations...
NOTE: All illustrations and photos have been removed from this article.
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