Thermodynamic Analysis of a Hybrid System Coupled Cooling, Heating and Liquid Dehumidiﬁcation Powered by Geothermal Energy

: The utilization of geothermal energy is favorable for the improvement of energy efﬁciency. A hybrid system consisting of a seasonal heating and cooling cycle, an absorption refrigeration cycle and a liquid dehumidiﬁcation cycle is proposed to meet dehumidiﬁcation, space cooling and space heating demands. Geothermal energy is utilized effectively in a cascade approach. Six performance indicators, including humidity efﬁciency, enthalpy efﬁciency, moisture removal rate, coefﬁcient of performance, cooling capacity, and heating capacity, are developed to analyze the proposed system. The effect of key design parameters in terms of desiccant concentration, air humidity, air temperature, refrigeration temperature and segment temperature on the performance indicators are investigated. The simulation results indicated that the increase of the desiccant concentration makes the enthalpy efﬁciency, the coefﬁcient of performance, the moisture removal rate and the cooling capacity increase and makes the humidity efﬁciency decrease. With the increase of air humidity, the humidity efﬁciency and moisture removal rate for the segment temperatures from 100 to 130 ◦ C are approximately invariant. The decreasing rates of the humidity efﬁciency and the moisture removal rate with the segment temperature of 140 ◦ C increases respectively. Six indicators, except the cooling capacity and heating capacity, decrease with an increase of air temperature. The heating capacity decreases by 49.88% with the reinjection temperature increasing from 70 to 80 ◦ C. This work proposed a potential system to utilize geothermal for the dehumidiﬁcation, space cooling and space heating effectively.


Introduction
The growth of the population and the dependency of technology led to the rapid growth demand of energy [1]. In addition, the rapid growth demand of energy recognizes the challenges such as the overconsumption of fossil energy [2]. Those are disadvantageous for sustainable development. The utilization of the renewable energy and the improvement of energy utilization efficiency are two potential ways to solve the previous challenges [3]. Geothermal energy, being a renewable energy, can provide energy for the process system [4]. At the same time, the greenhouse effect intensified the requirement for air. In China, the hot summer and cold winter region is a typical climatic region. The high temperature and humidity climate makes the demands for refrigeration and dehumidification grow rapidly. The absorption refrigeration cycle (ARC) and the liquid dehumidification cycle (LDC) can be powered by the low-grade heat and are environmentally friendly [5]. Thus, aggravated energy consumption [23]. In order to reduce energy consumption, the liquid dehumidification cycle (LDC) is regarded as the alternative technology to the traditional dehumidification technology [24]. Low et al. designed a LDC driven by solar energy and found that the efficiency of the system and the minimum area of solar heating collector were 78.8% and 59.83 m 2 , respectively [25]. Guan et al. determined the optimal mass flow rate of the desiccant and indicated that the optimal efficiency of the heat exchanger was 0.56 with the optimal ratio of desiccant-to-air being 1.5 [26]. The authors also modified the analytical solutions of the dehumidifier and the deviation between the modified results and the experimental results is less than 10% [27]. Zhang et al. optimized the LDC from the viewpoint of exergy destruction. The optimal exergy efficiency and COP of the LDC were 25% and 7.4, respectively [28]. Chen et al. developed a novel LDC driven by solar energy and optimized the area of solar heating collector [29]. Su et al. combined the ARC, vapor compression cycle with the LDC, and indicated that the efficiency of the proposed system was 34.97% higher than a traditional air conditioning system [30]. The advantageous dehumidification performance and the reliability of the LDC testified the feasibility of the LDC in hot summer and cold winter regions of China.
In this work, a hybrid system composed of an absorption refrigeration cycle, a liquid dehumidification cycle and seasonal heating and cooling cycles is developed. The power of the proposed system is geothermal energy. This hybrid system can meet the dehumidification demand, the space cooling demand, and the space heating demand, concurrently. Six indicators, including the moisture removal rate (MRR), the humidity efficiency (HE), the enthalpy efficiency (EE), COP, the cooling capacity and the heating capacity are proposed to evaluate the hybrid system. This work provides an effective solution to utilize geothermal energy.

Description of System
The hybrid system is provided in Figure 1. The heat source of the proposed system is from a geothermal well. The proposed system consists of four cycles, including an absorption refrigeration cycle (ARC), a liquid dehumidification cycle (LDC) and two heat exchangers. The working pairs of ARC and LDC are lithium bromide solution (LiBr-H 2 O) and lithium chloride solution (LiCl-H 2 O), respectively. The mentioned cycles are combined to meet dehumidification, refrigeration, cooling, and heating demands.

The Geothermal Water
Geothermal water is the heat source of the proposed system. First, geothermal water is pumped from G1 with the mass flow rate of 5 kg/s and the temperature of 150 • C. The high-temperature fluid is introduced into the regenerator (G1) of LDC to provide the heat for the regeneration process of the desiccant. Then, the fluid enters the generator of the ARC to provide the heat for the separation of the vapor. Afterword, depending on the season, the mid-temperature fluid flows into the heat exchanger 3 (HX3) for heating purposes during winter. Finally, the low-temperature fluid returns to the reinjection well (G4).

The Absorption Refrigeration Cycle
The absorption refrigeration cycle (ARC) is used to produce the cooling energy for the proposed system. In this cycle, the LiBr-H 2 O is chosen as the working pair. The strong LiBr-H 2 O solution is heated by geothermal water. Due to the difference of boiling point, the vapor is produced in the generator. The vapor enters the condenser (A1). In the condenser, the cooling water of 33 • C is used to cool the vapor into saturated water. After flowing through the valve 1 (A3), the saturated water is introduced into the evaporator to produce the cooling energy. In the evaporator, the flash makes the saturated water become vapor. This process produces cooling energy. Then, depending on the season, the vapor from evaporator (A4) flows into the HX4 for the cooling purpose during summer or flows into the absorber to continue the cycle. In the absorber, the rich solution contacts with the vapor directly. The rich solution absorbs the vapor and becomes a weak solution. In the generator, the solution is divided as rich solution and vapor. The vapor is used for refrigeration and the rich solution is used to absorb the vapor from evaporator. The rich solution first flows into the HX1 (A5) to preheat the new solution and then enters the absorber (A7) after flowing through the valve 2 (A6). The weak solution from the absorber, pumped by pump 1 (A8), is introduced into the HX1 (A9) and then enters into the generator (A10) to continue the cycle. Generally, the ARC is used to produce cooling energy.

The Geothermal Water
Geothermal water is the heat source of the proposed system. First, geothermal water is pumped from G1 with the mass flow rate of 5 kg/s and the temperature of 150 °C. The high-temperature fluid is introduced into the regenerator (G1) of LDC to provide the heat for the regeneration process of the desiccant. Then, the fluid enters the generator of the ARC to provide the heat for the separation of the vapor. Afterword, depending on the season, the mid-temperature fluid flows into the heat exchanger 3 (HX3) for heating purposes during winter. Finally, the low-temperature fluid returns to the reinjection well (G4).

The Absorption Refrigeration Cycle
The absorption refrigeration cycle (ARC) is used to produce the cooling energy for the proposed system. In this cycle, the LiBr-H2O is chosen as the working pair. The strong LiBr-H2O solution is heated by geothermal water. Due to the difference of boiling point, the vapor is produced in the generator. The vapor enters the condenser (A1). In the condenser, the cooling water of 33 °C is used to cool the vapor into saturated water. After flowing through the valve 1 (A3), the saturated water is introduced into the evaporator to produce the cooling energy. In the evaporator, the flash makes the saturated water become vapor. This process produces cooling energy. Then, depending on the season, the vapor from evaporator (A4) flows into the HX4 for the cooling purpose during summer or flows into the absorber to continue the cycle. In the absorber, the rich solution contacts with the vapor directly. The rich solution absorbs the vapor and becomes a weak solution. In the generator, the solution is divided as rich solution and vapor. The vapor is used for refrigeration and the rich solution is used to absorb the vapor from evaporator. The rich solution first flows into the HX1 (A5) to preheat the new solution and then enters the absorber (A7) after flowing through the valve 2 (A6). The weak solution from the absorber,

The Liquid Dehumidification Cycle
The liquid dehumidification cycle (LDC) is applied to remove moisture in the air. This cycle selected the LiCl-H 2 O as the working pair. There are five main units for the LDC, including a dehumidifier, a regenerator, a heat exchanger and two pumps. In the dehumidifier, the outside air (1) contacts with the liquid desiccant directly. The mass and heat transformation makes the wet air become dry air. Then, the dry air enters the room (2). The solution with low concentration from the dehumidifier flows into pump 2 (D1) and then enters into HX2 for preheating (D2). In the regenerator, the weak solution (D3) contacts with the air from room (4) directly. The heat for regeneration is provided by geothermal water. In this unit, the weak solution becomes the rich solution and then flows into the HX2 (D4) to preheat the weak solution. The rich solution from D5 is pumped by pump 3 (D5) and then enters the condenser (D6). The rich solution is cooled by the refrigerant of ARC then comes into the dehumidifier to continue the cycle consequently. The LDC meets the dehumidification demand by recovering geothermal effectively.

Seasonal Cooling and Heating Cycles
The seasonal cooling and heating cycles are composed of two heat exchangers. For the cooling purpose of summer, the HX4 keeps working during summer to provide cooling energy. In addition, the air from outside is dehumidified and cooled simultaneously. The HX3 is used for the heating purpose during winter. Geothermal water flows through the HX3 to release heat during winter and then flows into the reinjection well.

The Mathematical Equations for Two Subsystems
Based on the conservation of mass and energy, the LiBr-H 2 O ARC and the LiCl-H 2 O LDC are designed and analyzed. From the Figure 1, the main units of LiBr-H 2 O ARC and LiCl-H 2 O LDC can be classified as: pumps, valves, the heat exchangers, and packed towers. The mathematical equations can be calculated as follows and the detailed equations are introduced in Appendix A.
Mass balance : where m represents the mass flow rate; h and Q are the enthalpy and heat of streams respectively; W is the power consumed by pumps.

The Performance Indicators for the Coupling System
(1) Moisture removal rate (MRR): The MRR represents the moisture removal ability of the coupling systems. It can be defined as [31]: where the ω a,in and ω a,out represent the inlet air humidity and outlet air humidity respectively.
(2) Humidity efficiency (HE): The HE is an index to evaluate the performance of the dehumidifier. It can reveal the heat and mass transformation between air and desiccant in the dehumidifier. The HE is calculated as [32][33][34]: where the ω Ts,equ is the equilibrium-humidity between the desiccant and air.
(3) COP: COP, as a usual performance index, can represent the utilization efficiency of a geothermal source. COP for the coupling system is defined as [35]: where the m a is the mass flow rate of inlet air. h a,in and h a,out represent the enthalpy of the inlet air and outlet air respectively. Q T is the heat duty consumed by the coupling system.
(4) Enthalpy efficiency (EE): The EE can evaluate the performance of the dehumidifier from the viewpoint of thermodynamics. It can be calculated as [36]: where h C is the enthalpy of the cooling pair.

Model Validations and the Design of the Coupling System
The LiBr-H 2 O ARC and the LiCl-H 2 O LDC are simulated in Aspen Plus. The ELEC-NRTL method is selected as the basic property method. In Table 1, the specification for the models is listed in detail. The generator is simulated by the Heater model and the Flash2 model. The Heater model and the ABSBR1 model are applied to simulate the regenerator. The condensation pressure, decided by the condensation temperature, is 7.38 kPa. According to the other references [14,15,37], the most optimal temperature of the generator for LiBr-H 2 O ARC is 90 • C. In this work, the generator temperature is set as 80 • C due to a temperature decrease of 10 • C. In our previous work [12,35] Tables 2 and 3. The deviations of 0.4% and 0.2% represent the reliabilities of the LiBr-H 2 O ARC and LiCl-H 2 O LDC. The design procedure is introduced in Figure 2. The first step of the design process is to determine the refrigeration temperature. After the assumption of the segment temperature, the LiBr-H 2 O ARC and LiCl-H 2 O LDC are designed, respectively. In the coupling system, geothermal source is divided into two different parts. The high-temperature part is introduced to regenerate the desiccant and the low-temperature part is used to heat the LiBr-H 2 O solution. In addition, the cooling energy produced by the LiBr-H 2 O ARC is used for the refrigeration of desiccant and room respectively. Finally, the data is recorded to evaluate the coupling system. Before the simulation of the system, the design parameters are listed in Table 4. For the accuracy of the simulation, necessary assumptions are as follows [38][39][40]: (1) The system is stable; (2) The heat loss and the drop loss of pipes are negligible; (3) The system is simulated with no kinetic energy and potential energy; (4) The units of the coupling system have no heat loss; (5) The stream and the liquid refrigeration are assumed as saturated.

Results and Discussions
Based on the simulation results, the influence of design parameters on the indicators are studied in this section. The relationship between the design parameters and the indicators are shown in vivid pictures.

The Effect of Desiccant Concentration on the Coupling System
The effect of desiccant concentration on performance indicators is provided in Figures 3-5. With an increase of the desiccant concentration, COP, ∆ω, η h and cooling capacity increase while the η ω decreases, generally.      The performance indicators with the refrigeration temperature of 10 °C are introduced in Figure 5. Figure 5a is Figure 3 is the performance indicators with the refrigeration temperature of 2 • C. For the η ω , η h and COP, the increase of segment temperature has little effect on the indicators. But the ∆ω with the segment temperature of 140 • C is lower than that of 100 to 130 • C. As the desiccant concentration increases from 30% to 40%, the η h and COP rise with the increased rate of 5.78% and 5.79%, respectively. On the contrary, the η ω decreases by 1.21% with the desiccant concentration increasing from 30% to 40%. However, the ∆ω increases by 5.06%. The last index is the cooling capacity. Figure 3c indicates that the desiccant concentration and segment temperature have serious influence on the cooling capacity. With the increase of desiccant concentration, the cooling capacity for different segment temperature rises, respectively. As the segment temperature increases, the cooling capacity for the same desiccant concentration increases concurrently. In addition, the cooling capacity with the segment temperature of 100 • C is lower than that of 110 to 140 • C. Figure 4a,c shows the performance indicators with the refrigeration temperature of 5 • C. As shown in Figure 3, the increase of desiccant concentration leads to an increase of COP, ∆ω, η h and cooling capacity and a decrease of η ω . Compared with the results with the refrigeration temperature of 2 • C, the values of COP, ∆ω, η h and η ω move increasingly closer. As the desiccant concentration rises from 30% to 40%, the η h and COP for different segment temperature increase by 7.29% and 7.28%, respectively. The ∆ω increases by 6.43% while the η ω decreases by 1.27%. For the index of cooling capacity, the effect of desiccant concentration and segment temperature is obvious. The tendency of the cooling capacity with the refrigeration temperature of 5 • C is similar to that of 2 • C. The cooling capacity with the segment temperature of 100 • C is lower than that of 110 to 140 • C. However, the cooling capacity with the segment temperature of 120 • C is close to that of 130 and 140 • C.
The performance indicators with the refrigeration temperature of 10 • C are introduced in Figure 5. Figure 5a is the η h and COP of the proposed system. Similar to the previous results, the values of COP and η h with different refrigeration temperature are almost equal. With the increase of desiccant concentration from 30% to 40%, the increase rate of η h and COP are 10.84% and 10.83%, respectively. The ∆ω decreases by 9.69%. The η ω with the refrigeration temperature of 140 • C is lower than that of 100 to 130 • C as the desiccant concentration increases from 30% to 35%. However, as the concentration increases to 40%, the η ω with different segment temperatures are increasingly close. Figure 5c shows the cooling capacity with the refrigeration temperature of 10 • C. As the desiccant concentration increases, the cooling capacity of the proposed system increases sluggishly. With the same desiccant concentration, the increase of the segment temperatures from 110 to 140 • C leads to an almost homogeneous increase for the cooling capacity.
In general, a higher desiccant concentration represents a bigger COP, η h , η ω and cooling capacity, while lowering ∆ω. As the refrigeration temperature increases from 2 to 10 • C, the ∆ω, η ω , COP and cooling capacity decreases, while the η h increases. From the viewpoint of the change of desiccant concentration, higher concentration means better performance for the hybrid system. From the viewpoint of the refrigeration temperature, lower refrigeration temperature means better performance for the hybrid system.

The Effect of Air Humidity on the Coupling System
The performance indicators are introduced with different air humidity and refrigeration temperature from Figures 6-8. Figure 6 shows the indicators with a refrigeration temperature of 2 • C. The results in Figure 6a indicate that the increase of segment temperature has a little influence on COP and η h . With the rising of the air humidity, the η h and COP increase by 24.66% and 187.37%, respectively. It means that the increase of air humidity has a great effect on COP. The increase of air humidity makes the ∆ω and η ω with the segment temperatures from 100 to 130 • C, respectively, change slightly. But the ∆ω and η ω , with a segment temperature of 140 • C, decrease by 3.54% and 3.55%, respectively. The decrease rate of ∆ω increases from 0.16% to 3.39%. The decrease rate of η ω increases from 0.15% to 3.38%. The increase of air humidity makes the cooling capacity rise. The cooling capacity with the segment temperature of 100 • C is smaller than the cooling capacities with the segment temperatures from 110 to 140 • C.   Figure 6, the η h and COP with different segment temperature are almost equal, and the segment temperature has a great effect on the ∆ω and η ω , with the segment temperature of 140 • C, and has little influence on the ∆ω and η ω with the segment temperatures from 100 to 130 • C. As the air humidity increases, the η h and COP with a refrigeration of 5 • C, increase by 20.43% and 195.07% respectively. The η h and COP with the refrigeration of 10 • C increase by 14.37% and 217.90%, respectively. The results of COP represent that the rise of refrigeration temperature has great effect on the increase rate of COP. The influence of refrigeration temperature on the increase rate of η h becomes smaller. For the refrigeration temperature of 5 • C, the decrease rate of the ∆ω and η ω with the segment temperature of 140 • C are 2.69% and 2.7%, respectively. For the refrigeration temperature of 5 • C, the decrease rate of the ∆ω and η ω with the segment temperature of 140 • C are 2.09% and 2.08%, respectively. The cooling capacities with the refrigeration temperature of 5 and 10 • C increase with the increase of air humidity, respectively.   As the refrigeration temperature increasing from 2 to 10 °C, the   ,   and COP decrease, while the h  increases. The cooling capacities with the segment temperatures from 110 to 140 °C has little change.

The Effect of Air Temperature on the Coupling System
The results of the performance indicators for different air temperature are shown in As the refrigeration temperature increasing from 2 to 10 • C, the ∆ω, η ω and COP decrease, while the η h increases. The cooling capacities with the segment temperatures from 110 to 140 • C has little change.

The Effect of Air Temperature on the Coupling System
The results of the performance indicators for different air temperature are shown in Figures 9-11. The increase of air temperature makes the ∆ω, η ω , η h and COP decrease, generally. The rise of air temperature has great effect on the indicators with different segment temperatures. The results in Figure 9 indicate that COP with the segment temperatures from 100 to 120 • C are almost equal. The η h has the same tendency. The decrease rates of η h for different segment temperatures increase from 12.18% to 25.62% and the decrease rates of COP for different segment temperatures rise from 23.41% to 35.14%. The η ω with the segment temperature of 100 • C is equal to that of 110 • C. As the air temperature increases, the decrease rate of η ω increases to 17.06%, and the increase rate of ∆ω increases to 17.06%. The cooling capacities for different segment temperatures increase with the rise of air temperature. With the rise of segment temperature, the cooling capacity with the same air temperature increases.   Figures 10 and 11 are the indicators with the refrigeration temperatures of 5 and 10 • C, respectively. For the refrigeration temperature of 5 • C, the decrease rates of η h with different segment temperatures increase from 11.65% to 25.85%. The decrease rates of η ω with different segment temperatures increase to 17.90%. For the refrigeration temperature of 10 • C, the decrease rates of η h with different segment temperatures increase from 10.64% to 23.87%. The decrease rates of η ω with different segment temperatures increase to 16.47%.
As the refrigeration temperature increases, the ∆ω and COP with same air temperature and segment temperature have little change. The η ω and η h increase with a rise of refrigeration temperature. The cooling capacities with the segment temperatures from 110 to 140 • C for different refrigeration temperatures are within the range of 200 to 280 kW. Figure 12 shows the results of the heating capacity of the proposed system. The heating capacity decreases by 49.88% with the increase of reinjection temperature. The space heating energy is collected from the reinjection water. The rise of reinjection temperature leads to the increase of constant-pressure specific heat. However, the increase rate of the constant-pressure specific heat is smaller than the increase rate of the difference for reinjection temperature. Thus, the heating capacity decreases with an increase of reinjection temperature.

The Heating Capacity of the Proposed System
Energies 2021, 14, x FOR PEER REVIEW 18 of 24 Figure 12 shows the results of the heating capacity of the proposed system. The heating capacity decreases by 49.88% with the increase of reinjection temperature. The space heating energy is collected from the reinjection water. The rise of reinjection temperature leads to the increase of constant-pressure specific heat. However, the increase rate of the constant-pressure specific heat is smaller than the increase rate of the difference for reinjection temperature. Thus, the heating capacity decreases with an increase of reinjection temperature.

Conclusions
This work proposed a hybrid system combined dehumidification, space heating and space cooling. The humidity efficiency (   ), the enthalpy efficiency ( h  ), the moisture removal rate (   ), the coefficient of performance (COP), the cooling capacity and the heating capacity for the proposed system were analyzed. The effect of design parameters, including the desiccant concentration, air temperature and air humidity, and refrigeration temperature on the performance indicators were conducted. The conclusions are summarized as follows. (2) The increase of the air humidity makes the COP, h  , and cooling capacity increase.
For the same refrigeration temperature, the increase of segment temperature from 100 to 130 °C has a small effect on   . The decrease rates of   and   for the segment temperature of 140 °C increase with the increase of air humidity.
(3) The increase of air temperature leads to a decrease of COP,  ,   and  . As

Conclusions
This work proposed a hybrid system combined dehumidification, space heating and space cooling. The humidity efficiency (η ω ), the enthalpy efficiency (η h ), the moisture removal rate (∆ω), the coefficient of performance (COP), the cooling capacity and the heating capacity for the proposed system were analyzed. The effect of design parameters, including the desiccant concentration, air temperature and air humidity, and refrigeration temperature on the performance indicators were conducted. The conclusions are summarized as follows.
(1) The increase of the desiccant concentration leads to the increase of COP, η h , ∆ω and cooling capacity. The η ω decreases with an increase of desiccant concentration. The increase of refrigeration temperature makes the η ω , COP, ∆ω and cooling capacity decrease and makes the η h increase. (2) The increase of the air humidity makes the COP, η h , and cooling capacity increase.
For the same refrigeration temperature, the increase of segment temperature from 100 to 130 • C has a small effect on η ω . The decrease rates of η ω and ∆ω for the segment temperature of 140 • C increase with the increase of air humidity.
(3) The increase of air temperature leads to a decrease of COP, η h , ∆ω and η ω . As the refrigeration temperature rises, the ∆ω with the same segment temperature and air temperature are almost equal. COP has the same tendency, but the η ω and η h increase respectively with the cooling capacities for the segment temperature from 110 to 140 • C, which are within the range of 200 and 280 kW.