**1. Introduction**

The energy consumption of air conditioning accounts for 50% of building consumption and 15% of total electricity consumption, respectively [1,2]. Because solar energy is abundant and coincident with cooling loads, solar refrigeration systems display significant energy saving potential in air conditioning. Solar LiBr/H2O single effect absorption chillers are widespread due to simplicity, high efficiency, as well as low cost among all solar thermal cooling [3]. However, it faces an economical obstacle in high-rise buildings owing to the excessive consumption of auxiliary thermal energy [4]. Considering that the number of high-rise buildings and the corresponding consumption grow notably because of increasing urban population and land price [5], more attention should be paid to the energy saving of such buildings. Li [6] proposed the solar absorption-subcooled compression hybrid cooling system (SASCHCS) as the economically feasible solution of high-rise buildings. It was found that the financial improvement mainly lies in the high performance of the absorption subsystem and the remarkable reduction of operational cost [7]. Besides, the prototype experiment showed that compressor works

are saved by 22.2% on sunny days [8]. Therefore, the economic performance of SASCHCS rises dramatically, i.e., the payback period of hybrid systems is close to that of solar photovoltaic cooling (the most economical solution of solar refrigeration in recent years) in high-rise buildings [9]. It was shown that the area of compression subsystem evaporators and condensers as well as the size of absorption subsystems are critical for the exergoeconomic design of SASCHCS [10]. In particular, the optimal design of the above-mentioned parameters is dependent on the working condition distribution, i.e., absorption subsystems with moderate size are appropriate for the performance improvement if the hot water temperature mainly lies in the range 75–90 ◦C [11]. In addition to the system design, the performance also relies on the reasonable control of solar heat. The variable flow rate strategy in which flow rates of hot water are adjusted automatically according to the setting temperature is one of the widely-used control approaches in solar thermal systems. For the actual operation of SASCHCS, the performance su ffers from notable decrease as a result of inappropriate setting temperature, i.e., the heat loss of collectors goes up and the useful heat drops dramatically by the increased temperature in spite of the rise in the coe fficient of performance (COP) in absorption subsystems if the temperature of the generator hot water is set too high. Consequently, the relationship of SASCHCS operation and hot water set point temperature must be exact.

It was shown that the fixed outlet temperature is more appropriate than the constant inlet one for collector operation if flow rates of hot water can be controlled to make above-mentioned temperatures fixed during the entire period [12]. For solar domestic hot water facilities, it was found that the solar fraction associated with the constant temperature rise of collectors was only 2% more than that regarding fixed outlet temperatures [13]. Similar trends were also obtained by Rehman [14]. Araujo [15] compared the strategy with respect to constant and variable collector flow rates and found that the performance of solar domestic hot water systems with variable flow rate strategy was better than that with the constant one, especially for collectors with greater heat loss, i.e., the annual mean solar fraction with the variable flow rate strategy was 50% higher than that with the constant one for unglazed collectors. Furthermore, it was derived that the drop of set point temperature in collector outlet just enhanced 0.6% of heating power in solar district heating systems [16]. For solar power plants, it was concluded that the variable hot water flow rate strategy is more desirable than the constant one due to fewer parasitic loads [17]. In addition, it was shown that the setting temperature of the collector outlet relies on the tradeo ff of power cycle e fficiency and collector performance [18]. Camacho [19] found that the set point temperature of the collector outlet in winter should be 20 ◦C less than that in summer. Moreover, the operation of solar power plants becomes more flexible and performance rises remarkably when the collector setting temperature is allowed to change during the working period [20].

In addition to the solar heating and the power plant, the impact of collector setting temperature on performance of solar thermal cooling facilities has been researched as well. Qu [21] performed the comparison of solar LiBr/H2O double e ffect absorption chillers with constant and variable hot water flow rates and concluded that systems with variable hot water flow rates extend the working period by 25%. Additionally, it was recommended that lowering the set point temperature of collectors as much as possible would be beneficial for the performance of solar absorption chillers [22]. Petela [23] found that the strategy in which the collector set point temperature was shifted from 160 ◦C to 140 ◦C in morning and afternoon nearly doubled the specific cooling power from 9:00–10:00 and from 15:00–16:00 compared with the strategy with a 160 ◦C setting temperature in collector outlet. It is noteworthy that the set point temperature of the collector outlet should not be less than 140 ◦C to avoid serious performance deterioration of solar NH3/H2O absorption chillers [24]. Similarly, it was shown that the strategy in which the collector setting temperature was adjusted in terms of the cooling load enhanced the solar fraction by 11% compared to the constant hot water flow rate strategy [25].

Although some valuable criteria regarding the variable hot water flow rate for solar thermal systems have been obtained, the strategy cannot be employed to guide the setting temperature of hot water for the SASCHCS adequately or precisely. Firstly, most of the studies with respect to the influence

of collector setting temperature refer to facilities with backup heat. However, absorption subsystems of the SASCHCS are exclusively driven by solar energy, resulting in the stronger performance coupling of collectors and absorption subsystems. For instance, there are temperature rises of hot water in solar fields and generators through the heating of an auxiliary heater so that the performance of collectors/absorption chillers with generator/collector temperature is relatively weak. However, the hot water temperatures of collectors and generators are closely related to each other in the SASCHCS, i.e., the inlet temperature of generator hot water strongly relies on the collector outlet temperature, and the collector inlet temperature is mainly impacted by the stratification of storage tanks (the top layer temperature of storage tanks is equal to the inlet temperature of generator hot water) and the outlet temperature of generator hot water. Secondly, the effect of set point temperature in generator hot water has ye<sup>t</sup> to be analyzed. As mentioned above, it is inferred that the setting temperature of generator hot water not only dramatically influences the COP of absorption chillers but is relevant to the collector efficiency. Motivated by the above-mentioned knowledge gaps, the relationship of set point temperature in hot water and SASCHCS performance was investigated thoroughly. The setting temperature of collectors and generators was taken into account. The research was based on the annual cooling period of typical high-rise office buildings in subtropical Guangzhou. The off-design model of SASCHCS was developed at first. Subsequently, the impact of setting temperature in two hot water cycles was precisely analyzed. Finally, the optimal set point temperature of hot water was derived through the genetic algorithm. The novelty of this paper is the illustration regarding the effect mechanism of hot water setting temperature as well as the presentation of corresponding working guidelines for the SASCHCS. The paper is helpful for the improvement of SASCHCS operation.

#### **2. System Description: Operation Modes and Control Strategies**

The schematic of SASCHCS is shown in Figure 1. The hybrid system mainly consists of thermal driving subsystems, absorption subsystems, and compression subsystems. The thermal driving subsystem is composed of the evacuated tube collector (ETC) and the stratified storage tank. Its working fluid is pressurized water. It is noted that the ETC is employed owing to the relatively high efficiency and the low cost. The absorption subsystem is a single effect LiBr/H2O absorption chiller, and the compression subsystem is a traditional vapor compression chiller with R410a. In the case of available solar energy, the cooling demand of buildings is met together by absorption and compression subsystems so that the compressor work is saved. When the solar heat is insufficient to feed absorption subsystems, the cooling load is fully covered by compression subsystems. Accordingly, the size of absorption subsystems is designed in terms of collector area, and that of compression subsystems is designed by the peak cooling demand of high-rise buildings. Moreover, there is no doubt that the reduction of compressor work relies on the amount of cooling output in absorption subsystems (subcooling power) and the corresponding conversion to the cooling power of compression subsystems. The expression of growth in the cooling capacity and the subcooling power was derived in our previous work [26]. It was displayed that the enhancement of cooling power to subcooling power is 0.9–1.2 when the superheating is fixed by the experiment [27].

**Figure 1.** Schematic of solar absorption-subcooled compression hybrid cooling system (SASCHCS).

The control strategy of collector and generator pumps is demonstrated in Figure 2. The collector flow rate is controlled according to the setting temperature of the collector outlet, i.e., the flow rate of the collector hot water goes down automatically as the actual temperature of the collector outlet is lower than the set point one. The lowest collector flow rate is 10% of the nominal one to prevent thermal problems in the excessively low flow situation. Moreover, the collector pump is switched off when the temperature of the collector outlet is lower than the one of the storage tank bottom. It is restarted if the collector outlet temperature is 5 ◦C greater than the bottom temperature of the storage tank. In particular, the collector pump is turned off as the top layer temperature of storage tanks reaches 100 ◦C to avoid crystallization of absorption subsystems and is restarted as the temperature reaches less than 95 ◦C.

**Figure 2.** Control strategies for collector and generator pumps.

The generator pump is activated when the top layer temperature of storage tanks equals the activation temperature of absorption subsystems. The activation temperature is set to be 5 ◦C above the set point temperature of the generator hot water. Furthermore, the generator pump is turned off if the outlet temperature of the generator hot water falls to 55 ◦C because the temperature of hot water is too low to drive absorption subsystems from the experiment [8]. In another hand, the flow rate of the generator hot water is also controlled in terms of the setting temperature of the generator inlet. The minimal flow rate of the generator hot water is 30% of the rated one.
