Performance analysis of solar absorption-subcooled compression hybrid refrigeration system in subtropical city

Abstract

Solar absorption-subcooled compression hybrid refrigeration system is a new type of efficient and economical solar refrigeration device which always meets the demand of cooling load with the change of solar irradiance. The performance of the hybrid system is higher due to the improvement of evaporator temperature of absorption subsystem. But simultaneously, the variation of working process as well as performance is complicated since the absorption and compression subsystems are coupled strongly. Based on the measured meteorological data of Guangzhou, a subtropical city in south China, a corresponding parametric model has been developed for the hybrid refrigeration system, and a program written by Fortran has been used to analyze the performance of the hybrid system under different external conditions. As the condensation temperature ranges from 38°C to 50°C, the working time fraction of the absorption subsystem increases from 75% to 85%. Besides, the energy saving fraction also increases from 5.31% to 6.02%. The average COP of the absorption subsystem is improved from 0.366 to 0.407. However, when the temperature of the absorption increases from 36°C to 48°C, the average COP of hybrid system decreases from 2.703 to 2.312. Moreover, the working time fraction of the absorption subsystem decreases from 80% to 71.7%. The energy saving fraction falls from 5.67% to 5.08%. In addition, when the evaporate temperature increases from 4°C to 14°C, the average COP of the absorption subsystem decreases from 0.384 to 0.365. The work of the compressor decreases from 48.2 kW to 32.8 kW and the corresponding average COP of the absorption subsystem is improved from 2.591 to 3.082.

Keywords

solar / absorption-subcooled / compression hybrid / dynamic simulation / performance analysis

Introduction

Since the solar radiation changes consistently with the cooling load of commercial building, the application of solar refrigeration technology can effectively reduce the energy consumption of air conditioning. However, since the total area of commercial building is usually larger than its area of roof, the corresponding solar air conditioning must be equipped with auxiliary energy to solve the problem of satisfaction of cooling load of commercial building with insufficient area of collector array. When natural gas is used as the auxiliary energy, the expensive running cost makes the system uneconomical. Due to the good performance of compression refrigeration system, the solar subcooled absorption-compression hybrid refrigeration system is an economic way for air conditioning. Tang et al. [¹] proposed a new type of subcooled evaporation combined the absorption-compression refrigeration cycle to recycle the low grade energy, and used the refrigerating capacity of the absorption-subsystem to subcool the refrigerant of the compression-subsystem. He et al. [²] conducted a preliminary theoretical and experimental research based on circulation. Cao et al. [³] proposed a new kind of absorption-compression composited refrigeration cycle driven by solar energy and overcame the defect of the traditional solar air conditioning fluctuation in performance under different location and climate. Zhao and Zhang [⁴] came up with a new kind of ammonia absorption-compression refrigeration cycle driven by low temperature waste heat and made a simulation calculation for this cycle. Zhou et al. [⁵] analyzed the energy conversion process in a solar hybrid absorption-compression air conditioning system, and obtained the characteristic equation of this system. Herold et al. [⁶] presented a kind of absorption-compression mixed cycle using an internal combustion engine as the prime motor. The cycle coupled the generator of the absorption subsystem and the condenser of the compression subsystem, and the exhaust waste heat and cooling water in the internal combustion engine was used to drive the absorption-subsystem. Fukuta et al. [⁷] studied an absorption-compression composited system driven by waste heat, using propane and a kind of mineral oil as the working medium. Tarique and Siddiqui [⁸] compared and analyzed two types of the absorption-compression hybrid system using NH₃-NaSCN and NH₃-H₂O as the working substance respectively and found that the former working substance performed better at a high absorption temperature.

It can be found that the recent study of absorption-compression composited refrigeration system only concerns preliminary theoretical and experimental research. However, the working process of the solar absorption-subcooled compression hybrid refrigeration system and the change of performance under different external conditions is yet to be studied. The objective of this paper, therefore, is to simulate the working process of the hybrid refrigeration system and analyze the performance of the hybrid system under different external conditions (condensation temperature, evaporation temperature, absorption temperature) when the solar radiation and ambient temperature change dynamically. This paper is helpful to the design and engineering application of the solar absorption-subcooled compression hybrid refrigeration system.

Model of solar absorption-subcooled compression hybrid refrigeration system

The schematic of the solar absorption-subcooled compression hybrid refrigeration system is illustrated in Fig. 1. It mainly consists of the driving heat source, the absorption subsystem and the compression subsystem. The solar collector arrays and heat storage tank make up the driving heat source. The generator, the first condenser, the subcooler (evaporator of absorption subsystem), the absorber and the solution heat exchanger constitute the absorption subsystem. The compression subsystem is composed of an evaporator, a compressor, and the second condenser. The absorption subsystem and the compression subsystem are coupled by the subcooler (evaporator of absorption subsystem). When the solar radiation is strong enough to drive the absorption subsystem, the refrigerating capacity of the absorption subsystem is used to supercool the refrigerant in the compression subsystem. The low grade energy (the refrigerating capacity of the absorption subsystem) is turned into high grade energy through the second throttle valve without consuming any energy. Therefore, it can reduce the power consumption of the hybrid system. When the solar energy is insufficient, the cooling load is provided by the compression subsystem independently. Therefore, the hybrid refrigeration system always meets the demand of the cooling load despite the change of solar radiation.

Based on the conservation of mass and energy, the control equations of each component in the hybrid system are established as follows.

Heat source

Efficiency of solar collector can be expressed as [⁹]

(1)\eta =\mathrm{0.49204}-\mathrm{0.29002}\times {\displaystyle \frac{T_i-T_a}{I}}.

Temperature in heat storage tank can be expressed as

(2)T_{\mathrm{s+}}=T_s+{\displaystyle \frac{Q_u-Q_g}{\rho cV}}\times \Delta t.

Absorption subsystem

Subcooler

(3)Q_{\mathrm{abs}}=m_{\mathrm{3}}{h}_{\mathrm{3}}-m_{\mathrm{4}}{h}_{\mathrm{4}},

(4)m_{\mathrm{3}}=m_{\mathrm{4}}.

Absorber

(5)Q_a=m_{\mathrm{4}}{h}_{\mathrm{4}}+m_{\mathrm{8}}{h}_{\mathrm{8}}-m_{\mathrm{5}}{h}_{\mathrm{5}},

(6)m_{\mathrm{5}}=m_{\mathrm{4}}+m_{\mathrm{8}},

(7)m_{\mathrm{5}}{\xi }_{\mathrm{5}}=m_{\mathrm{8}}{\xi }_{\mathrm{8}}.

Solution heat exchanger

(8)m_{\mathrm{7}}{h}_{\mathrm{7}}-m_{\mathrm{8}}{h}_{\mathrm{8}}=m_{\mathrm{6}}{h}_{\mathrm{6}}-m_{\mathrm{5}}{h}_{\mathrm{5}},

(9)m_{\mathrm{7}}=m_{\mathrm{8}},

(10)m_{\mathrm{6}}=m_{\mathrm{5}}.

Generator

(11)Q_g=m_{\mathrm{1}}{h}_{\mathrm{1}}+m_{\mathrm{7}}{h}_{\mathrm{7}}-m_{\mathrm{6}}{h}_{\mathrm{6}},

(12)m_{\mathrm{6}}=m_{\mathrm{1}}+m_{\mathrm{7}},

(13)m_{\mathrm{6}}{\xi }_{\mathrm{6}}=m_{\mathrm{7}}{\xi }_{\mathrm{7}},

(14){\xi }_{\mathrm{6}}={\xi }_{\mathrm{5}},

(15){\xi }_{\mathrm{7}}={\xi }_{\mathrm{8}}.

First condenser

(16)Q_{\mathrm{cond1}}=m_{\mathrm{1}}{h}_{\mathrm{1}}-m_{\mathrm{2}}{h}_{\mathrm{2}},

(17)m_{\mathrm{1}}=m_{\mathrm{2}}.

First throttle valve

(18)h_{\mathrm{2}}=h_{\mathrm{3}},

(19)m_{\mathrm{2}}=m_{\mathrm{3}}.

Compression subsystem

Evaporator

(20)Q_e=m_{\mathrm{12}}{h}_{\mathrm{12}}-m_{\mathrm{13}}{h}_{\mathrm{13}},

(21)m_{\mathrm{12}}=m_{\mathrm{13}}.

Compressor

(22)W_c=m_{\mathrm{9}}{h}_{\mathrm{9}}-m_{\mathrm{13}}{h}_{\mathrm{13}},

(23)m_{\mathrm{9}}=m_{\mathrm{13}}.

Second condenser

(24)m_{\mathrm{11}}=m_{\mathrm{12}},

(25)h_{\mathrm{11}}=h_{\mathrm{12}}.

Since the solar absorption-subcooled compression hybrid refrigeration system is different from the traditional refrigeration system, several new indexes are defined to evaluate the performance of the hybrid system. The working time fraction of the absorption subsystem is defined as the proportion of the working time of the absorption subsystem in the total working hours of the hybrid refrigeration system. The calculating formula is

(26)\gamma ={\displaystyle \frac{t_{\mathrm{abs}}}{t_o}}.

Due to the fact that the refrigerating capacity of absorption subsystem changes with the working time fraction of absorption subsystem, the energy-saving fraction is defined as the proportion of the refrigerating capacity of the absorption subsystem in the total refrigerating capacity of the hybrid refrigeration system multiplied by the working time fraction. The calculating formula is

(27)\sigma ={\displaystyle \frac{Q_{\mathrm{abs}}}{Q_e}}\times \gamma .

In order to evaluate the performance of the absorption subsystem, the transient coefficient of performance of the absorption subsystem is defined as the fraction of the refrigerating capacity of the absorption subsystem and the heat load of generator multiplied by the efficiency of the solar collector. The calculating formula is

(28){\mathrm{COP}}_{\mathrm{abs}}={\displaystyle \frac{Q_{\mathrm{abs}}}{Q_g}}\times \eta .

In the same way, the transient coefficient of performance of the hybrid refrigeration system is defined as the fraction of the total capacity of the hybrid system and the total input power of the hybrid system. The total input power is the sum of the compressor consumption and the heating load of the absorption subsystem (neglecting the work of the solution pump).

(29){\mathrm{COP}}_{\mathrm{hyb}}={\displaystyle \frac{Q_e}{W_c+Q_g/\eta }}.

The average coefficient of performance is defined as the accumulation of transient COP from one moment to the next and then divide the total time,

(30){\mathrm{COP}}_{\mathrm{ave}}={\displaystyle \frac{\mathrm{1}}{n_{\mathrm{2}}-n_{\mathrm{1}}}}{\displaystyle \sum _{n_{\mathrm{1}}}^{n_{\mathrm{2}}}\mathrm{COP}}.

Typical meteorological data of subtropical city of Guangzhou

The performance of the solar absorption-subcooled compression hybrid refrigeration system is highly dependent on the meteorological data and the surrounding temperature. In order to accurately evaluate the system performance, a year round meteorological data of the subtropical city of Guangzhou was measured by a small wireless weather station. The parameters of the weather station is listed in Table 1. The hourly mean surrounding temperature from 8:00 am to 18:00 pm from April to October (usually the working time of air conditioning in subtropical city) is shown in Fig. 2. The lowest hourly average surrounding temperature is 22.5°C at 8:00 am (in April) while the highest one is 33.2°C at 14:00 pm (in August). The hourly mean solar irradiance is illustrated in Fig. 3. The highest hourly average solar irradiance is 650 W/m² at 12:00 pm (in September).

Results and discussion

The temperature of the generator and the subcooler, and the consumption work of the compressor is depicted in Fig. 4. The generator temperature ranges from 300 K to 370 K from 8:00 am to 18:00 pm. The subcooler temperature decreases from 315 K to 301 K and the consumption of the compressor falls down from 51.8 kW to 48.1 kW when the solar absorption-subcooled compression hybrid refrigeration system functions properly. The gas range of lithium bromide can be seen in Fig. 5. It is defined as the concentration difference of lithium bromide solution in the generator and the absorber. The highest gas range is 23% at 15:00 pm. Comparing with the traditional single-effect lithium bromide absorption refrigeration system, the hybrid refrigeration system needs lower temperature of the heat source but the gas range is much larger than the traditional one. This phenomenon is attributed to the higher subcooler temperature which elevates the pressure in the absorber. As a consequence, the generator temperature is reduced and the gas range increased.

The heat load variation of the generator and the solar collector is depicted in Fig. 6. It is found that the heat load of the generator decreases from 30 kW to 19 kW the moment the absorption subsystem starts to work at 10:14 am in August, and remains at 20 kW after the hybrid system becomes to work stably. The main reason for the rapid drop in heat load of the generator is attributed to the low temperature of the heat source at the beginning. This leads to a small gas range of the lithium bromide and a very high circulation fraction of the solution is very high, which makes the energy consumption in the generator grow accordingly. The transient COP and efficiency of the collector can be observed in Fig. 7. Both the absorption subsystem and the hybrid system transient COP decrease gradually with the increase of time. The main reason for this is that the heat load of the generator is relatively stable while the efficiency of the solar collector reduces gradually as time goes by.

The working time fraction and energy saving fraction at different condensation temperatures is demonstrated in Fig. 8. It is found that both of them increase when the condensation temperature ranges from 38°C to 50°C. This phenomenon is attributed to the fact that the temperature of the refrigerant at the outlet of the subcooler rises with the condenser temperature while the refrigerating capacity of the absorption subsystem stays the same. This makes the subcooler temperature increase (shown in Fig. 9), which increases the pressure in the absorber, and as a result, the generator temperature reduces. It can be seen in Fig. 10 that the average COP of the absorption subsystem increases with the condenser temperature while the average COP of the hybrid system changes conversely. The main reason for this is that the compressor consumption increases with the condenser temperature (shown in Fig. 11) Because of the higher subcooler temperature, the average COP of the absorption subsystem improves with the condenser temperature.

The working time and energy saving fraction at different condensation temperatures of the absorption subsystem is displayed in Fig. 12. It can be seen that both of them decreases as the condensation temperature of the absorption subsystem increases from 38°C to 50°C. The main reason for this is ascribed to the fact that the condensation pressure of the absorption subsystem grows with the condensation temperature of the absorption subsystem. This means that the temperature in the generator must be higher to activate the absorption subsystem. The average COP of the absorption subsystem and the hybrid system can be seen in Fig. 13. It is found that the average COP of the absorption subsystem decreases gradually while the average COP of the hybrid system remains unchanged as the condensation temperature of the absorption subsystem increases.

The working time fraction and energy saving fraction at different absorption temperatures is exhibited in Fig. 14. It can be seen that both of them decrease while the absorption temperature rises. The main reason for this is that the corresponding absorption pressure increases as the absorption temperature goes up. It makes the absorption effect become worse, so the concentration of lithium bromide solution at the outlet of absorber increases. Therefore, it takes a longer time to activate the absorption subsystem. The average COP at different absorption temperatures is presented in Fig. 15. Both of them decrease as the absorption temperature ranges from 36°C to 48°C.

The influence of evaporation temperature on the compressor consumption and the average COP of the system are shown in Figs. 16 and 17, respectively. The compressor consumption reduces with the increase of evaporator temperature. As a consequence, the average COP of the hybrid system improves correspondingly while the average COP of the absorption subsystem almost remain the same.

Conclusions

A corresponding parametric model was developed for the solar absorption-subcooled compression hybrid refrigeration system. According to the results of simulation calculation, when the condensation temperature ranges from 38°C to 50°C, the working time fraction of the absorption subsystem increases from 75% to 85%. The energy saving fraction also increases from 5.31% to 6.02%. Besides, the average COP of the absorption subsystem improves to 0.407 from 0.366. However, when the temperature of the absorption increases from 36°C to 48°C, the average COP of the hybrid system decreases from 2.703 to 2.312. Moreover, the working time fraction of the absorption subsystem decreases from 80% to 71.7%. The energy saving fraction falls from 5.67% to 5.08%. In addition, when the evaporation temperature increases from 4°C to 14°C, the average COP of the absorption subsystem decreases from 0.384 to 0.365. The work of compressor decreases from 48.2 kW to 32.8 kW and the corresponding average COP of the absorption subsystem improves from 2.591 to 3.082.

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