1. Introduction
The energy consumption of heating, ventilation, and air conditioning (HVAC) systems in office buildings generally accounts for 30% of the total building energy consumption [
1]. Moreover, in developed countries or areas with extreme climates, it increases up to 50% [
2]. In order to improve energy efficiency and reduce energy consumption, passive technologies such as solar gain wall; photovoltaic (PV) green roofs and passive cooling; and HVAC technologies including radiant heating/cooling systems, variable-air-volume-based air-conditioning systems, and so on, have been proposed and developed from different aspects [
3]. A method used in these technologies involves adopting renewable energy, such as biomass [
4] and solar energy [
5], to replace fossil fuels. Solar energy is an alternative for fossil energy owing to its high reserve, wide distribution, renewal, and non-polluting capabilities.
The combination of cooling and solar energy is a suitable method to realize a better balance between solar thermal energy input and cooling output in the future market [
6]. Typically, the collection of higher solar energy is involved when a building needs increased cooling demand. Consequently, a solar cooling system is employed to reduce electricity consumption at the peak of summer and relieve electricity shortages. The main alternative routes from solar energy into the cooling system include solar thermal or electric PV, and the corresponding cooling thermodynamic cycles such as absorption and vapor compression are different [
7]. In this case, the PV electric cooling system nowadays has higher energy efficiency and better economic performance [
7]. With the development and promotion of solar heat collectors including concentrating and non-concentrating collectors, the technologies of cooling driven by solar thermal energy, such as solar adsorption cooling system [
8], solar assisted liquid desiccant dehumidifier [
9], and solar adsorption desalination-cooling system [
10], were developed and analyzed from different aspects including energetic efficiency, economic feasibility [
11], and environmental assessment [
12] by researchers from different countries and regions [
13].
Parabolic trough collector (PTC) for solar thermal energy is a relatively mature technology corresponding to a concentrating-type solar collector that ensures heat collecting efficiency at a higher temperature [
14]. The temperature of the heat source from PTC is compatible with the heat demand of a double-effect LiBr–H
2O absorption chiller that achieves a higher coefficient of performance (COP). The double-effect absorption chiller coupled with PTC can reach almost the same parity as PV-driven electric chillers, only in terms of energy efficiency [
7,
15].
Currently, several studies examined performance improvement and practical application of solar cooling system using experiments, simulations, and other methods. Tsilingiris [
16] proposed the theoretical modelling of a solar cooling system for domestic applications in homes and confirmed the economic performance of the solar cooling system at the cost of fossil fuels. Mazloumi et al. [
17] simulated the thermal performances of a solar single effect LiBr–H
2O absorption chiller system integrated with a heat storage tank at the end of PTC, and the results indicated that the optimal capacity of the thermal storage tank significantly affects systemic operation performances. Li et al. [
18] investigated the performance of a 23 kW solar powered single-effect LiBr–H
2O absorption cooling system using a PTC of 56 m
2 aperture area, and the experiment results indicated that the daily cooling COP varied from 0.11 to 0.27 in the sunny and clear sky days and the daily solar heat fraction ranged from 0.33 to 0.41. Qu et al. [
19] modeled a PTC absorption chiller system in the transient system simulation program and constructed a corresponding 52 m
2 experimental platform. The systemic performances in different wind speeds and solar radiations were analyzed and verified with the experimental data, and it was concluded that the solar thermal system could potentially supply 39% of the cooling and 20% of the heating demands for the building space. Tzivanidis and Bellos [
20] investigated the influences of design parameters such as mass flow rate and storage tank volume on the performance of a one-stage absorption chiller coupled with a PTC in the climate of Athens, and concluded that using a 54 m
2 collecting area and a storage tank of 1.35 m
3, it is possible to achieve a maximum cooling load of 12.8 kW. Besides PTC, solar cooling systems coupled with different collectors were developed and analyzed. Aman et al. [
21] proposed an ammonia–water absorption cooling system based on a flat plate solar collector for residential application and focused on energy and exergy analyses of an absorption chiller. Furthermore, Bellos et al. [
22] presented exergetic, energetic, and economic evaluations of a solar-driven absorption cooling system with four various collectors including flat plate collectors, evacuated tube collectors, compound parabolic collectors, and PTCs. The comparisons illustrated that the system with the higher solar COP is the one with PTC because of their high efficiency, and is also the one with the maximum solar exergetic efficiency.
Aiming to achieve greater energy efficiency, Delač et al. [
23] designed a solar heating and cooling system configured with condenser and absorber heat recovery, and the presentation of scenario shows recovery of up to 53% of waster condenser and absorber heat. Neyer et al. [
24] focused on the integration of a single-/half-effect NH
3/H
2O absorption chiller into parallel/serial to conventional systems and concluded that its application for solar- and combined heating and power(CHP)-driven systems are reaching non-renewable primary energy savings of 30–70% at almost equal costs compared with conventional systems. Calise et al. [
25] proposed a novel high-temperature solar assisted triple-pressure level combined cycle power plant and presented the thermoeconomic analysis of a novel solar cooling system for the combined cycle power plant. The application of solar cooling system integrated with different technologies is the current research focus, and the previous studies focused on the performances of a solar cooling system integrated with different solar collectors by simulations or experiments. However, the performances in off-design work conditions involved the whole system and the different characteristics of building cooling loads were examined in a few studies from the literature.
The specific objectives of this work are to propose a double-effect absorption chiller system driven by solar thermal energy collection through PTC, to analyze the thermodynamic performances of the solar cooling system, and to characterize its applicability in different buildings with different cooling loads using the correlation coefficient.
Section 2 proposes the integrated solar cooling system and models the equipment and system.
Section 3 analyzes the thermal performances including design and off-design work conditions, and discusses application performances in different kinds of buildings.
Section 4 summarizes the conclusions of the study.
2. System and Model
2.1. System Description
The flowchart of the proposed double-effect LiBr–H
2O absorption chiller system driven by PTC is shown in
Figure 1 The complete system consists of a solar heat collecting subsystem, a heat storage subsystem, a heat exchange subsystem, and a double-effect LiBr–H
2O absorption chiller driven by steam. Sunlight is irradiated on a parabolic reflector and is reflected on the absorber by a mirror. The heat conducting oil (HCO) into the absorption tube (state 1) is heated to a high temperature (state 2). Subsequently, the high temperature HCO (state 7) enters the heat exchanger to produce steam (state 14), and the low temperature HCO is returned to the PTC through heat release (state 8). The steam with high temperature and high pressure is used to drive the absorption chiller to produce chilled water for space cooling (states 19 and 20). The steam releasing heat becomes the condensated water (state 15) and then returns to the heat exchanger.
The double-effect LiBr–H2O absorption chiller includes a high-pressure generator (HG), a low-pressure generator (LG), a condenser, an evaporator, an absorber, a low temperature heat exchanger (LX), a high temperature exchanger (HX), two solution reducing valves, and two refrigerant expansion valves. The high temperature steam produced by the HCO flows into the HG to generate the primary steam (state r1) and a concentrated working solution (state s5). Subsequently, the two streams enter into two different circulations: solution circulation and refrigerant circulation. In the solution circulation, the weak solution (state s1) produced in the absorber is pumped through the LX and HX to recover the surplus heat of the medium concentration solution from LG (states s8 and s9) and the strong concentration solution from HG (states s5 and s6), respectively. The weak solution (state s4) is then introduced into the HG, where it is heated and concentrated into medium concentration solution (state s5) by the steam resource, and thus high pressure refrigerant steam (state r1) is produced. The medium concentration solution (state s5) then passes through the HX and enters the LG after decreasing the pressure (state s7), where the medium concentration solution is further concentrated into a strong solution (state s8). The strong solution enters the absorber after it is cooled in the LX. In the refrigerant circulation, the refrigerant steam from the HG (state r1) condenses in the LG and low-pressure refrigerant steam (state r2) is generated. Both the high-pressure refrigerant water (state r2) and low-pressure refrigerant steam (state r3) flow into the condenser. The refrigerant water (state r5) is throttled and introduced into the evaporator to generate chilled water (states 19 and 20). Finally, the refrigerant steam (state r7) is absorbed by a strong solution in the absorber.
In the solar cooling system, a heat storage tank (HST) is necessary to match the heat supply from solar PTC and the cooling demand of building, and it adopts molten phase change material (PCM) storage. The PCM HST is connected in parallel to the collector field to enable charging and discharging. There are three operation switch modes of HST. Firstly, when the heat collected by the PTC exceeds the heat demand for cooling load, the valve V1 splits HCO to two streams (states 3 and 5). One of streams through the valves V2 and V3 is sent to heat exchanger to produce steam for chiller, and another stream being the excess heat through the HCO (states 3 and 4) is stored in the HST. The return HCO from HST and heat exchanger (states 4 and 8) through the valves V4 and V5 is sent together to the PTC again. Secondly, when the heat collected by the PTC is less than the heat demand, a part of HCO (state 10) is sent to the HST to absorb the stored heat, and then, with all HCO from the PTC through controlling the valve V1, it is sent to the heat exchanger to produce steam. Thus, the heat from the HST (states 10 and 11) is removed to supplement the shortage. Finally, when the heat collected from the PTC is not available in the night or daytime with no solar radiation, the valves V1 and V5 are closed and the cycle of the collected heat of PTC (states 3 and 4) does not work. The heat demand of cooling system is supplied by the HST through controlling the valves V2 and V4. During the operation process, the heat of flows, including that of HCO, steam, and storage stream, is adjusted by changing the flow rates.
2.2. Thermodynamic Model
2.2.1. PTC
A solar PTC mainly consists of a parabolic mirror, a glass tube, and a metal tube [
26], and this is shown in
Figure 2 The parabolic mirror uses aluminum or silver to reflect solar radiation to the surface of the metal tube and is covered by a coating with high absorptivity and low emissivity; it then heats the HCO in the metal tube. The space between the glass tube and metal tube is set to vacuum to reduce heat loss.
The heat transfer process and the thermal resistance model considering one-dimensional heat transfer is shown in
Figure 3 [
27]. The solar radiation is reflected onto the glass tube through the parabolic mirror. The reflected sunlight passes through the glass tube to reach the outer surface of the metal tube. The metal pipe is heated owing to the higher density of sunlight on the outer wall of the metal tube, and the thermal energy is then transmitted to the working fluid, HCO, through heat conduction and convection, in that order. Thus, the flowing HCO is heated to collect solar heat.
The following points are assumed during the heat transfer analysis of PTC.
- (1).
The space between the metal and glass tubes does not absolutely correspond to the vacuum because of a small amount of residual air, thus the convection heat transfer exists.
- (2).
The heat conduction between the metal tube and hose is ignored.
- (3).
The heat conduction between the metal tube and bracket of the PTC is ignored.
The solar radiation energy absorbed by the metal tube,
, is expressed as follows [
28]:
where
is the solar direct normal irradiance (DNI),
is the collector area,
is the PTC’s optical efficiency,
is the solar incidence angle,
is the solar incidence angle modifier, and
is the reflectivity of glass outer wall. A part of the absorbed heat of the metal tube is transmitted to the working fluid, HCO, and another part is transferred to the inner wall of glass tube. Consequently, the heat balance is expressed as follows [
27]:
where
Q23cond is the heat conduction between the inner wall and outer wall of the metal tube;
Q34rad and
Q34conv are the radiation heat and the convection heat between metal tube and glass tube, respectively; and
Q34cond is the heat conduction of the air in the space between metal and glass tubes. The conductive heat from the outer wall of metal tube to inner wall is absorbed by the HCO through convective heat transfer, and it is assumed that they are equal at the instantaneous steady state condition.
Additionally, the heat absorbed by the HCO,
Q12conv, is calculated as follows [
29]:
where
is the convective heat transfer coefficient in the single-phase region and can be defined as follows:
and
, for which the validity is in the ranges of
,
, and
.
is the length of collector;
is the diameter inside of the metal tube;
is the Nusselt number;
is the Reynolds number;
is the Prandtl number;
is the thermal conductivity coefficient of the HCO at the temperature of
;
is the inner area of the metal tube;
and
are the average temperatures of the HCO (
) and the inner wall of metal tube, respectively; and
and
are the inlet and outlet temperatures of HCO, respectively.
The heat absorbed in the inner wall of glass tube through heat conduction, convection, and radiation transfers to its outer wall through conduction and is expressed as follows:
where
is the heat conduction between the inner and outer wall of the glass tube, and is emitted to the atmosphere through heat convection and radiation. Thus, the following expression holds:
where
and
are the convective heat and radiation heat between the outer wall of glass tube and atmosphere, respectively.
Therefore, the thermal efficiency of the collector is defined as the ratio of the solar radiation energy absorbed by the working fluid in the metal tube to the received total energy on the mirror of the PTC, and is expressed as follows [
30]:
where
is the thermal efficiency of the PTC.
In the study, the SEGS (Solar Electric Generating System) LS-2 PTC [
31] is employed to heat the HCO for the generation of steam. The design parameters [
5,
30,
31] are listed in
Table 1.
Because of the discontinuity of solar energy, the additional collection area should be considered to satisfy the heat demand for producing chilled water at night. Thus, the collected heat from the PTC covers the heat demand in the daytime and the stored heat for utilization at night. Consequently, the total collection area of the PTC is estimated according to the heat demands of cooling loads, as follows:
where
is the heat demand to match the hourly cold load when solar energy is available;
is the heat collected by the PTC to satisfy the total demand at night;
is the time when the collector begins to work;
is the last time of the collector’s work;
a,
b, and
c are the heat loss coefficients during heat storing, static storage, and heat releasing of the HST, respectively (herein, all of them are set to 0.1); and
and
are the DNI in the
i-th hour and the corresponding collection efficiency of solar thermal energy, respectively. During the computation process of the collector’s area, the thermal efficiency of PTC is obtained through simulation programs using Engineering Equation Solver (EES) after setting the cooling load of a building in a typical day.
2.2.2. Absorption Chiller
A double effect LiBr–H
2O absorption chiller driven by steam is integrated with the solar thermal collection system in which its COP is relatively high in several types of absorption chillers driven by hot water or exhausted gas. During modeling and calculation of the absorption chiller, the following assumptions are considered [
5]:
- (1).
The heat losses in each component and the pressure losses between each connection lines are ignored.
- (2).
The systemic analysis is based on the steady state, and the LiBr–H2O solution is steady during the cycle.
- (3).
The states are statured and include the outlet refrigerant steam of evaporator, the outlet refrigerant liquid of condenser, the outlet weak solution of absorber, and the outlet solution of HG and LG.
- (4).
The power consumption of solution pump is ignored.
- (5).
Counter flow heat exchanger is employed, and the logarithmic mean temperature difference is adopted in the heat transfer calculation.
During the cycle of LiBr–H
2O solution, the balances of each component including mass, energy, and solution phase equilibrium are employed to construct the thermodynamic model and the universal formulations are listed as follows [
32]:
where
m,
w, and
h represent mass, concentration, and enthalpy, respectively; and the subscripts
i and
o represent the inlet and outlet of the component, respectively.
Thus, given the pump’s power consumption, the energy balance of the absorption chiller is expressed as follows [
33]:
where
is the heat absorbed by the HG,
is the heat absorbed by the evaporator,
is the heat released by the absorber, and
is the heat released by the condenser. Its COP is defined as follows:
2.3. Evaluation Criteria
Appropriate evaluation criteria are selected to assess the performances of the absorption system based on the parabolic trough solar collector. The systemic thermal efficiency,
, is defined as the ratio of the cooling output to the total solar energy input, and is expressed as follows [
30]:
4. Conclusions
The study proposed a double-effect LiBr–H2O absorption chiller based on solar PTC, constructed the thermodynamic model, and presented the thermodynamic performance analysis on the design and off-design work conditions. The following conclusions are obtained:
The comparisons of the simulation results from the constructed thermodynamic models and the experimental data indicated that the maximum relative error of solar PTC model is approximately 1.29% and the maximum error of the absorption chiller model is only 0.056%, which satisfied the requirements of thermodynamic simulation. The influence analysis of uncertain factors on solar PTC indicated that the outlet temperature of collector will increase by approximately 2 °C when the DNI increases by 100 W/m2 in the case study. The solar irradiance has a strong effect on the collecting thermal efficiency. As for the absorption chiller, the influence of steam flow rate on its COP demonstrates that the COP increases rapidly with the increasing steam flow rate at a lower cooling load, and the increasing rate of COP then becomes slow with increases in the cooling load. In total, the combination of solar PTC and steam LiBr–H2O absorption chiller is a good method to match the energy level of medium temperature solar energy. The solar energy utilization efficiency reaches approximately 61.98% in the hotel building.
A correlation coefficient between solar irradiance and cooling load is proposed to quantify the applicability of the solar cooling system in the buildings. The performance comparison of the solar cooling system in different types of building indicates that better matching and a higher correlation coefficient between the transient solar DNI and cooling load is helpful in decreasing the heat loss and improving the systemic performance. The solar cooling system in the office building with a correlation coefficient of approximately 0.81 achieves 69.47% systemic thermal efficiency.
The thermodynamic performance of the solar cooling system on the design and off-design work conditions were only analyzed in this paper, while other performances such as economic feasibility, reliability, and availability are not considered. These aspects should be studied to promote the development of solar cooling systems in future research.