Hybrid Refrigeration System

In hybrid refrigeration system, a compressor is involved in absorption refrigeration system. The compressor works as a liaison between the low-pressure generator (LPG) and a high pressure absorber (HPA) of conventional two-stage vapor adsorption refrigeration system to enhance the absorption process.

The use of compressor in this configuration lowers the temperature required in the generator for same temperature lift and thus, permits the use of low temperature heat energy in the generator of system. Unlike single-, triple-, and double-effect vapor adsorption refrigeration system which needs heat at a high source temperature as for single-effect vapor adsorption refrigeration system, driving heat source temperature is 80-100°C while, double- and triple-effect vapor adsorption refrigeration system require driving temperatures of around 180 240°C, a hybrid refrigeration system utilizes the heat available at 60-80°C.

An absorption-compression hybrid refrigeration system using ammonia-water working fluid which was operated by mid temperature waste heat. The proposed system generated 46.7% more cooling energy as compared to an equivalent vapor adsorption refrigeration system.

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  The COP and primary energy rate are reported to be higher than 1.0 and 0.7, respectively with temperature lift of 50°C in generator.

A comparative performance study of hybrid refrigeration system of vapor compression refrigeration system and vapor adsorption refrigeration system with ammonia-water as working fluid. The results showed that hybrid system saved more energy in compressor as compared to conventional vapor compression refrigeration system.

Performance analysis

Figure 21 shows the photographs of the solar powered green building and two adsorption chillers installed. The whole system had been working since October of 2004.

Figure 21: schematic diagram for hybrid refrigeration system

The entire load of the building consists of latent load and sensible load. The latent load is treated by processed outdoor air while the sensible load is overcome partially by processed air and the rest by indoor fan coils.

The quantity of water removed from diluted solution by outdoor air in the regenerator is theoretically equal to that of moisture absorbed by strong solution in the dehumidifier. That is to say, the heat removed by outdoor air in the regenerator equals the latent load overcome by liquid desiccant in the dehumidifier if the air temperature difference between inlet and outlet of the regenerator is ignored.

Therefore, in order to effectively regenerate the liquid desiccant, the heat discharged from the condenser is at least as much as La. Suppose the efficiency of heat exchanger HE2 is h2 and that of HE3 is h3, then the heat supplied by condenser of the heat pump is

Similarly, suppose the efficiency of heat exchanger HE1 is h1, the cooling supplied from water collector WD2 in order to deal with the sensible load in the dehumidifier is

Total cooling

In addition, due to the limited cooling capacity of adsorption chillers, the cooling production of heat pump has to be increased, while the heat production is accordingly increased. If the ratio of latent load to total load,is large enough, the heat quantity required to regenerate liquid desiccant is also large while the cooling required to deal with sensible load is small.

As a result, the heat pump has to increase its heat production, and the cooing requirement from adsorption chillers maybe decreased because the cooling produced by heat pump increased accordingly. Whenis as large as a certain value, the system can operate without adsorption chillers.

Electrical coefficient of performance (ECOP), thermal coefficient of performance (TCOP) and system coefficient of performance (COPsys) can be introduced into hybrid air conditioning systems to evaluate their performance. As to this system studied here, the regeneration heat source comes from itself so it is gained freely. Therefore, the performance of the system can be evaluated only by ECOP which is regarded as equal as COPsys are as below:

Where Eh stands for the electricity power consumed by heat pump and Eother means that consumed by other devices.

Effect of evaporator temperature

This section will discussing about the effect of evaporator temperature on the COP of the system. The COPsys are introduced into the system to evaluate the performance of the designed system. The high and low the value of COP will determine how good the system is. The value of COP need to be higher enough to make sure that there is no failure occurs in the system.

For the relationship between the temperature of the evaporator and the COP of the system are as the evaporator temperature for the fixed inlet and outlet temperature of external fluid increase, then the value of COP also increase. That means when the evaporator temperature increased the performance of the system are also increase.

Other than that, the increasing of the COP will results in decreasing of the total electric consumption and reducing the requirement of heat energy in the generator respectively. This condition also can be describe as the change evaporator will affect the total irreversibility rate of the system.

The increasing of the evaporator temperature will also increase the efficiency of the system. Hence, the system with high evaporator temperature is not necessarily optimum because of high COP, low irreversibility rate, and better environmental performance. This is because the high COP, low irreversibility rate and better environmental performance does not lead to economic heat exchanger designs as the investment cost of the system.

Effect of condenser temperature

This section will be discussing about the effect of the condenser temperature on the performance of the system. The COP of the system will be decrease as the condenser temperature increase resulting the increasing of the total irreversibility rate. When the compressor power almost remain constant caused by the fixed pressure ratio through it, the total amount of low grade needed in the generator will increase. The increasing in the temperature of the condenser will increase the total irreversibility rate of the system respectively.

Other than that, when the electric consumption of the system is increase with the increasing of the condenser temperature, it will increase the amount of CO2 emission of the system that will help the global warming from happen. Thus, low condenser temperature is desirable in both the system from the energy, exergy and the operational cost points of view which also meets the environment aspects of the system.

Effect the generator temperature

This section will discuss about the effect of generator temperature on the performance of this system. The COP of this system will increase if the temperature of the generator is increase up to the certain limit.

The further increase in the temperature of the generator will decrease the COP of the system. Thus, there is a specific limit of generator temperature at which the COP of the system is at maximum. This hybrid system only need lower temperature heat source to operate at high performance. Hence, waste heat comparatively available at lower temperature can be used as a source of the energy to operate this system.

Effect of other parameters

This section will be discussing about the effect of other parameter like heat energy and electrical energy to the performance of the system. In current system, two form of energy are supplied to the system that are heat energy and electrical energy. Heat energy are supplied as an input to the generator and the electrical energy are supplied as an input to the pump, compressor and fan are supplied to achieve the desired refrigeration effect.