Advanced exergy analysis of a novel air source heat pump system coupled with the liquid receiver and gas-liquid separator

In previous studies, an air source heat pump system coupled with the liquid receiver and the gas-liquid separator was proposed, which improved the defrosting efficiency according to the improvement of the refrigerant mass flow during defrosting and shortened the defrosting time. In this paper, the exergy efficiencies and exergy loss (including endogenous and exogenous exergy loss) of the new system are studied. They are compared with the original air source heat pump system by constructing the advanced exergy study model of the system. Results show that the exergy efficiencies of the compressor, evaporator, and throttle valve of the new system are increased by 0.2%, 2.3%, and 5.9%, respectively. The endogenous exergy loss of the new system accounts for 65.16% of the total exergy loss of the system, and most of the exergy loss in the new system is caused by the components themselves.


Introduction
Air source heat pump (ASHP) has a wide range of applications owing to its energy-saving, environmental protection, and high efficiency [1].However, frost accumulation on the fin surface will affect the system energy efficiency of ASHP.Current defrosting methods include the mechanical vibration method, periodic start and stop method, and heating defrosting method.A new type of air source heat pump system (new system) which combines a liquid receiver and gas-liquid separator in the original system into a component is proposed [1].
Exergy studies can provide theoretical support for system optimization from the perspective of exergy loss.Zhang et al. [2] conducted a thermodynamic analysis on an ASHP water heater.Results showed that the thermodynamic perfectibility of the evaporator is the minimum.Zhang [3] analyzed the energy consumption of single-stage and cascade air source systems.Results showed that the exergy loss of the single-stage heat pump was 9.1 kW higher than that of the cascade system.Deng et al. [4] studied that to reduce the irreversible degree of the system, a scroll-type hermetic compressor could be used, and the heat transfer temperature difference of the condenser should be reduced.It can be seen that the exergy study can accurately point out the component levels that need to be optimized for system energy efficiency improvement.However, conventional exergy studies cannot accurately distinguish between exempt exergy loss and non-exempt exergy loss of components.Chen et al. [5] used advanced exergy to improve the traditional exergy study model of a heat pump system with an ejector.After improvement, the system exergy loss was decreased by 30.2%, and the exergy efficiency was increased from 6.9% to 9.5%.Voloshchuk [6] found that 63% and 20% of the avoidable endogenous exergy loss came from the inefficient work of the evaporator and condenser by using advanced exergy ways.An advanced exergy study model of the new ASHP is constructed in this article, and the avoidable exergy loss and exergy efficiency of the components and system are studied.

System operation principle
The refrigerant path of the new system is shown in Figure 1.Valve (7) is shut down, and valve (8) is working while the system releases heat to the room (Figure .1a).Refrigerant flowing out from condenser (2) enters the outer chamber of ( 6), the refrigerant flowing out from evaporator (4) enters the inner chamber of (6).When the system absorbs heat from the room to defrost (Figure .1b), valve (8) is shut down, and valve (7) is working.
(a)heating (b) defrosting Figure 1.Operation principle of the new system [1]   3. Advanced exergy study model The cycle state points in the system are shown in Figure 2. Exergy consumption and exergy output can be calculated by the equations in Table 1.The exergy consumed by the compressor refers to the power of the compressor, and the exergy produced is the exergy increase of the flowing mass of refrigerant (R410A).For condensers, exergy consumption is the exergy reduced by the refrigerant passing, and output exergy is the exergy increased by working medium water that exchanges heat with the refrigerant.The exergy consumption and exergy output of the evaporator is the same as that of the condenser.In particular, for the throttle valve, the exergy consumed by the working fluid flowing through the throttle valve is mechanical, and the exergy produced is heat exergy.( )

Advanced exergy model
The advanced exergy study can evaluate device-to-equipment impacts and identify the potential for system improvement.For the study of this paper, the use of advanced exergy study can give specific results from the perspective of energy for the improvement points of the new.Considering the interaction of various components, the endogenous exergy loss and exogenous exergy loss of component K constitute the total exergy loss of the component., , , , , , where The above two decomposition methods are independent of each other, so the two can be combined to decompose the exergy loss., , , , , , , , ,

Calculation method of exergy loss of each part
To use the thermodynamic cycle method to determine the exergy loss of each part, three operating conditions need to be established: real conditions, limit conditions, and ideal conditions.The pinch temperature of the evaporator of the new system is set to 4°C.The outer surface of the liquid receiver and gas-liquid separator is insulated.

Conventional exergy
Parameters such as the specific enthalpy and specific entropy of R410A in the cycle, heat exchange working medium water, and air at each state point are obtained by calling Refprop in the program.Table 4 shows the steady-state operating conditions.The conventional exergy calculation results of the new system and the original system under steady-state operating conditions are shown in Table 5.The results of the conventional exergy study are shown in Figure 4.The exergy efficiency of the component of the compressor, evaporator, and throttle valve of the new system increased by 0.2%, 2.3%, and 5.9 %, compared to the system where the liquid receiver and the gas-liquid separator are two independent components.The exergy efficiency of the new system improved by 0.3%, and the exergy efficiency of the condenser decreased by 1.6%.The exergy loss coefficient of the entire system, compressor, and throttle valve decreased by 3.7%, 0.2%, 0.4%, and 4.1%; the compressor and evaporator improved by 0.3%, 1.2%.The reduction in the exergy efficiency of the condenser is due to the rise of the temperature difference between working fluid and water, which results in an increase in exergy consumption even more than exergy output.

Advanced exergy results
Each part of the new system of the exergy loss in the comprehensive decomposition method (external sources can be avoided/internal sources can be avoided/external sources cannot be avoided/internal sources cannot be avoided) are in Table 5.Firstly, the system's internal exergy loss is 6.47 kW, accounting for 65.16% of the total exergy loss of the system and each component, while exogenous exergy loss is 3.47 kW, accounting for 34.94%.The maximum exergy loss occurs by the components themselves, but there is also a certain degree of interdependence between the components.Secondly, the exergy loss of the system accounts for nearly 70% (6.67 kW) of the summary exergy loss, while the non-exergy loss accounts for 32.83%.This shows that the useful energy loss of could be greatly reduced by improving the technology, perfecting the system, and adopting other methods.From the perspective of each component, firstly, the compressor's internal exergy loss accounts for one-third of the internal exergy loss of the new system with it of 2.18 kW, and the internal exergy loss of the throttle valve accounts for a small proportion of the entire system.In the exogenous exergy loss of the whole system, the largest proportion is still the compressor, with a proportion of 44.38% (1.54 kW).All exergy loss of the evaporator is internal, and the working state of the evaporator has a significant impact on the rest of the components.Hence, the improvement of the evaporator is crucial.Second, in the avoidable exergy loss of the system, the compressor (3.22 kW, 48.27%), throttle valve (1.45 kW, 21.74%), evaporator (1.14 kW, 17.09%), condenser (0.86 kW, 12.89%), indicating the necessity of improving the compressor.The exogenous exergy loss of the throttle valve is the exogenous exergy loss exogenous source.This is because, in the actual cycle and the limit cycle, the working process of the throttle valve is regarded as an isenthalpic process, which also shows that its improvement potential is relatively small.For the traditional ASHP system, the calculation results of each part of the exergy loss in the comprehensive decomposition method (external sources can be avoided/internal sources can be avoided/external sources cannot be avoided/internal sources cannot be avoided) are shown in Table 6.The conclusions drawn from it are consistent with those discussed in Table 6.Table 6.Calculation results of advanced analysis of new system and original system (kW The proportion of exogenous exergy loss of exergy loss and the proportion of exogenous exergy loss of exergy loss are used as comparison parameters to compare the advanced exergy of two systems, as shown in Figure 5.The proportion of exogenous exergy loss of total exergy loss of the new system is 34.94%, which is 4.96 percentage points lower than the 39.91% of the original system; for each component, the compressor increased by 1.14 percentage points, and the condenser and the throttle valve were reduced by 4.28 and 9.32 percentage points respectively.In terms of the proportion of exogenous losses that can be avoided from external sources to the total exergy losses, the new system, the compressor, and the throttle valve increased by 1.98%, 1.93%, and 13.2%, respectively.One parameter decreased by 4.1%.This is consistent with the conclusion obtained from the conventional exergy.Compared with the original system, the condenser of the new system is not working well, while the performance of the compressor and the throttle valve have been improved 5. Conclusions 1) The exergy loss of the new system accounting for 55.27% (9.93 kW) of the system; among them, the compressor exergy loss is the most significant, with a proportion of 37.37% of the system exergy loss.
2) The exergy efficiencies of the compressor, evaporator, and throttle valve of the new system are increased by 0.2%, 2.3%, and 5.9% over the original system, respectively.
3) Exergy loss of the system can be greatly reduced by improving technology, with most of the exergy loss of the new system being caused by the components themselves.
4) The avoidable exergy loss of the new system compressor is the largest, so the compressor is the first choice for system optimization.

Figure 2 .
Figure 2. Status points of the new system

EE
 is the internal exergy loss/W of each component, which is caused by the component itself; K is the corner mark, representing the K component;  is the exogenous exergy loss/W of every component.Exergy loss of a certain part can be decomposed into exergy loss and others that no longer occur after improvement.
to non-avoidable endogenous exergy loss/W; exogenous exergy loss/W.The frame diagram of the comprehensive decomposition of exergy loss is shown in Figure3.

Figure 3 .
Figure 3. Comprehensive decomposition of exergy loss of advanced exergy study

Figure 4 .
Figure 4. Comparison of calculation results of conventional analysis of proposed and original system

Figure 5 .
Figure 5.Comparison of advanced calculation results of the new system and original system

Table 2 .
Table 2 lists the representative parameter values of each component under different operating conditions.Table 3 lists the advanced exergy loss calculation equations.Formatting sections, subsections and subsubsections

Table 3 .
Advanced exergy loss calculation equation

Table 5 .
Conventional exergy study results of the new and original system