A small-scale silica gel-water adsorption system for domestic air conditioning and water heating by the recovery of solar energy

Abstract

A small-scale silica gel-water adsorption system with modular adsorber, which utilizes solar energy to achieve the cogeneration of domestic air conditioning and water heating effect, is proposed and investigated in this paper. A heat recovery process between two adsorbers and a mass recovery process between two evaporators are adopted to improve the overall cooling and heating performance. First, the adsorption system is tested under different modes (different mass recovery, heat recovery, and cogeneration time) to determine the optimal operating conditions. Then, the cogeneration performance of domestic cooling and water heating effect is studied at different heat transfer fluid temperatures. The results show that the optimal time for cogeneration, mass recovery, and heat recovery are 600 s, 40 s, and 40 s, respectively. When the inlet temperature of hot water is around 85°C, the largest cooling power and heating power are 8.25 kW and 21.94 kW, respectively. Under the condition of cooling water temperature of 35°C, the obtained maximum COP_c, COP_h, and SCP of the system are 0.59, 1.39, and 184.5 W/kg, respectively.

Keywords

silica gel-water / heat and mass recovery / solar energy / domestic cooling and heating

Introduction

Adsorption refrigeration is known as a type of green refrigeration technology which employs natural refrigerants (such as water, ammonia and methanol) and has the adaptability for low temperature heat sources. These merits make it a promising potential for practical industrial applications [¹–³]. Silica gel-water adsorption working pair is usually used and matched with the low grade heat at the range of 60°C to 95°C [⁴,⁵]. As one of the most common adsorption refrigeration working pair, silica gel-water adsorption refrigeration systems were commercialized successfully in early 1980s [⁶]. The existing cooling systems often use vacuum check valves to achieve the switch of adsorption and desorption processes frequently. A kind of silica gel-water adsorption chiller in which two evaporators were adopted without any vacuum check valves was developed [⁷]. One of the evaporators produced the cooling effect while the other acted as the reservoir of condensed refrigerant liquid. However, there was an extra loss of heat as the evaporator need to be cooled down first, and therefore the efficiency of two-evaporator adsorption chiller was not high enough [⁸,⁹].

To overcome the disadvantages of silica gel-water adsorption chiller, including low efficiency and high cost, a number of advanced adsorption refrigeration thermodynamic cycles, such as thermal wave cycle, multi-stage cycle, heat recovery cycle, mass recovery cycle, were proposed and studied [¹]. The thermal wave cycle is still in its theoretical stage as the fulfillment of practical application seems to be difficult. The low thermal efficiency and complicated system structure of multi-stage cycle impose negative effects on the adsorption refrigeration system [¹⁰]. Heat and mass recovery cycles are featured with easy implementation and high performance on which the related researches are booming for the past decades. Rezk and Al-Dadah [¹¹] developed a lumped analytical model for a two-bed silica gel adsorption chiller combined with mass and heat recovery schemes. Akahira et al. [¹²] studied the performance of two-bed silica gel-water adsorption cooling cycle with mass recovery process. It was found that the cooling capacity of the mass recovery cycle was superior to that of the conventional cycle, especially for low regenerating temperature. There are two conventional ways to realize mass recovery cycle: one is mass recovery between adsorption and desorption beds, the other is mass recovery between two evaporators [¹³,¹⁴]. A three-bed silica gel-water system with mass recovery and new time allocation was studied by Zajaczkowski [¹⁵] and the numerical results under the condition of heat source temperature of 65°C showed that the COP was enhanced by 35% with the mass recovery process. Alam et al. [¹⁶] and Akahira et al. [¹⁷] investigated a four-bed silica gel-water system with mass recovery and the performance was verified to be excellent. Considering that the mass recovery process between two adsorbers may cause the issue of refrigerant imbalance, Li et al. [¹⁸] proposed the mass recovery-like process between two evaporators in the adsorption refrigeration chiller which could balance the pressure and refrigerant mass simultaneously. The heat recovery between two evaporators is utilized to change the pressures between two adsorbers and enhance the adsorption/desorption reactions. Wang et al. [¹⁹] studied a two-bed chiller and a four-bed chiller with heat recovery process and the obtained COPs could be improved by 38% and 25%, respectively.

A number of silica gel-water chiller systems which utilized large-scale low grade heat recovery have been successfully operated in certain practical applications, such as solar energy and waste heat from factories. Chang et al. [²⁰] designed and constructed a solar-powered two-bed silica gel-water adsorption system for heating and cooling in a golf course located in Hsinchu, Taiwan Province, China. Luo et al. [²¹] built a solar-powered adsorption air conditioning system with heat and mass recovery processes which was used to chill the headspace of a grain bin in Jiangsu Province, China. Pan et al. [²²] developed a high performance and low cost silica gel-water adsorption chiller, which was driven by the waste heat from a factory. The obtained cooling power, COP, and SCP were 42.8 kW, 0.51, and 125.0 W/kg, respectively, under typical conditions of 86/30/11°C hot water inlet/cooling water inlet/chilled water outlet temperatures. Lu et al. [²³] designed a compact silica gel chiller with mass and heat recovery. The experimental results showed that cooling capacity and COP are 17.9 kW and 0.63 when the hot water inlet temperature, cooling water inlet temperature, and chilled water outlet temperature were 79.0°C, 25.4°C, and 13.7°C, respectively.

Currently, there are few domestic applications based on the small-scale sorption system due to complex manufacturing processes and low efficiency caused by large metal thermal capacity. In this study, a small-scale silica gel-water adsorption system with a cooling capacity of 10 kW is designed and manufactured. Both the cooling/air conditioning effect by the evaporation and the water heating effect by the adsorption are considered in this system. Detailed experiments are conducted to determine the optimal cogeneration time, heat, and mass recovery time for this adsorption refrigeration system. Furthermore, the cogeneration (air conditioning and water heating) performance of this system is investigated and evaluated under different conditions, which indicates the potential of recovering solar energy to meet the cooling and heating requirements for domestic applications.

Experimental system

Application of the system for residential cooling and heating

Taking the climatic conditions of North-west China as an example, in which the humidity of air is relatively low and the solar energy is abundant. The adsorption system for residential cooling and heating applications is shown in Fig. 1. The solar collector array installed on the domestic building gathers and recoveries the solar energy for heating the water in the hot water tank, which serves as the heat source for the adsorption system. Through an adsorption refrigeration cycle, the evaporation heat in the evaporator will provide the cooling power for air conditioner, and the adsorption heat from the adsorbers will provide water heating to meet the demands of domestic use.

Small-scale silica gel-water system and its working processes

The schematic diagram of the small-scale silica gel-water adsorption chiller is depicted in Fig. 2. The adsorption chiller is composed of two independent sorption chambers, each of which consists of an adsorber, a condenser, and an evaporator. Taking the desorption process in the right chamber and the sorption process in the left chamber as an example, the half cycle is briefly introduced as follows:

1) Desorption phase of A2: hot water flows through V3, V2, A2, V4, and V3, thus A2 is heated and adsorbent (silica gel) desorbs the refrigerant vapor (water). Meanwhile, cooling water is supplied into the system with the flow path of V1, C2, A1, and V2. C2 is cooled in which the refrigerant vapor desorbed from A2 is condensed.

2) Adsorption phase of A1: A1 is cooled by the cooling water as described previously with the path of V1, C2, A1, and C2. It adsorbs refrigerant vapor from E1. The adsorption heat releases to the cooling water, which generates the heat pumping effect. Simultaneously, chilled water flows through V5 into E1 in which the liquid refrigerant evaporates due to the pressure of chamber drops under the adsorption effect of A1. The cooling effect is generated by the evaporation. The cogeneration effect is output by the chilled water and cooling water circuits, respectively.

3) Followed by the cogeneration mode, the mass recovery begins with the flow path of chilled water switching into: V7, E2, V6, E1, and V5. The circuits of hot and cooling water are the same as Steps 1) and 2). Under this condition, A1 could adsorb more refrigerant because the pressure in E1 increases, and the A2 could desorb more refrigerant because the pressure in E2 decreases. Such a process will make the cycle adsorption quantity increase significantly. After the mass recovery ends, the operation runs in heat recovery mode. In this mode, the valves for hot water circuit are closed, and cooling water flows through V1, C1, A2, V2, V3, V4, A1, and V2, while chilled water flows through V7, E2, V6, and V5. In this way, heat could be recovered from A2 to A1.

The other half of the operation process is similar to the former half with the exchange of “1” and “2” in the term of A, C, and E.

The test system includes a hot water tank, a cooling tower, a chilled water tank, a vacuum pump, regulating valves, flow meters, temperature sensors and a data logger. The system is shown in Fig. 3, whose dimension is 2400 mm in length, 1300 mm in width, and 1300 mm in height. The measuring equipment used in the experiments includes a temperature sensor with an accuracy of 0.15°C, a flow meter with an accuracy of 0.75% full scale, and a data logger with an accuracy of 0.2% full scale.

The modular adsorber used in this chiller is made of 5 sub adsorbers (Fig. 4). Each sub adsorber is packed with TYPE A granular silica gel (mass of 9 kg) in the spacing of the fins which can expand the heat transfer area to improve its heat transfer performance effectively [²⁴]. Both condensers and evaporators are a kind of shell tube type heat exchangers in which the heat transfer fluid flows inside the tube and the refrigerant flows outside. A vacuum pump is installed for periodic maintenance and a control cabinet is employed to control the automatic running of all the valves and the vacuum pump [²⁵]. Before the operation, the system is required to achieve the desired vacuum state and water (about 3 kg) is charged into the system for adsorption and desorption.

Performance calculation and experimental uncertainty

As described in Section 2, the working modes of the adsorption system make the operational parameters change periodically. To evaluate the overall performance, the main parameters are averaged within at least one complete running cycle. The input power (Q_i), cooling power (Q_c), heating power (Q_h), COP_c, COP_h and SCP of the adsorption system can be calculated by Eqs. (1)-(6).

(1)Q_i={\displaystyle \sum _{j=\mathrm{1}}^n\frac{[q_{\mathrm{hw}}{\rho }_{\mathrm{hw}}{C}_{\mathrm{hw}}(T_{\mathrm{hw},\mathrm{in}}^j-T_{\mathrm{hw},\mathrm{out}}^j)]}{n}},

(2)Q_c={\displaystyle \sum _{j=\mathrm{1}}^n\frac{[q_{\mathrm{ci}}{\rho }_{\mathrm{ci}}{C}_{\mathrm{ci}}(T_{\mathrm{ci},\mathrm{in}}^j-T_{\mathrm{ci},\mathrm{out}}^j)]}{n}},

(3)Q_h={\displaystyle \sum _{j=\mathrm{1}}^n\frac{[q_{\mathrm{cw}}{\rho }_{\mathrm{cw}}{C}_{\mathrm{cw}}(T_{\mathrm{cw},\mathrm{out}}^j-T_{\mathrm{cw},\mathrm{in}}^j)]}{n}},

(4){\mathrm{COP}}_{\mathrm{c}}={\displaystyle \frac{Q_{\mathrm{c}}}{Q_{\mathrm{i}}}},

(5){\mathrm{COP}}_{\mathrm{h}}={\displaystyle \frac{Q_{\mathrm{h}}}{Q_{\mathrm{i}}}},

(6)\mathrm{SCP}={\displaystyle \frac{Q_c}{M_{\mathrm{ad}}}},

where q_hw, r_hw and C_hw are volume flux, density, and specific heat of hot water, respectively; T_{hw, in} and T_{hw, out} are the inlet and outlet temperature of hot water; q_ci, r_ci and C_ci are volume flux, density, and specific heat of chilled water, respectively; T_{ci, in} and T_{ci, out} are the inlet and outlet temperature of chilled water; q_cw, r_cw, and C_cw are volume flux, density, and specific heat of cooling water, respectively; T_{cw, in} and T_{cw, out} are the inlet and outlet temperature of cooling water; M_ad is the mass of silica gel packed in one adsorber; and the superscript j represents the number of data and n is the amount of complete cycles.

Based on the accuracy of the measuring equipment, the total uncertainty of calculated variables can be obtained according to Eq. (7), thus the maximum standard relative errors of COP and SCP are 13.56% and 9.08%, respectively.

(7){\displaystyle \frac{\Delta x}{x}}=\sqrt{{\left({\displaystyle \frac{\partial x}{\partial q}}\right)}^{\mathrm{2}}{\left({\displaystyle \frac{\Delta q}{x}}\right)}^{\mathrm{2}}+{\left({\displaystyle \frac{\partial x}{\partial T}}\right)}^{\mathrm{2}}{\left({\displaystyle \frac{\Delta T}{x}}\right)}^{\mathrm{2}}},

where x is the calculated variable, i.e. COP or SCP; q is the value of volume flux; and T is the value of temperature.

Results and discussion

First, the silica gel-water adsorption system with heat and mass recovery processes are tested with different operational times (mass recovery time t_mr, heat recovery time t_hr, and cogeneration time t_co) in order to determine the optimal conditions for small-scale air conditioning and water heating applications. Based on the optimal operation time, the experiments are focused on different working conditions (T_hw, T_cw and T_ci) to investigate and evaluate the cogeneration performance.

Variation of heating, cooling, and chilling temperature

Figure 5 demonstrates the temperature profiles of hot water, cooling water, and chilled water in a complete working cycle. According to previous researches [¹³,²²], the cogeneration time, mass recovery time, and heat recovery time are initially set to be 600 s, 40 s, and 40 s, respectively. T_{hw, in} and T_{cw, in} are fixed at around 75°C and 32°C, respectively. When the cogeneration mode begins, both T_{hw, out} and T_{cw, out} change rapidly because the heat exchange of the two adsorbers is quite large with a large temperature difference. In the mass recovery process, T_{ci, out} varies dramatically because of the implementation of the direct connection between E1 and E2, while T_{cw, out} almost remains steady and T_{hw, out} presents the fluctuation to some extent. After that, the heat recovery process between A1 and A2 is characterized by the drop of T_{hw, out} and the rise of T_{cw, out} correspondingly. After the half working cycle is completed, T_{hw, out} reaches the minimum value suddenly and T_{cw, out} reaches the maximum value. Then, T_{hw, out} increases to a stable level slowly, keeping a certain temperature difference with T_{hw, in}, which indicates the continuous thermal energy input for the desorption reaction between silica gel and water. For cooling water, there is an obvious gap between T_{cw, in} and T_{cw, out} during the whole period of the half working cycle. This shows that the adsorption reaction lasts for the whole period and its reaction rate is much slower than that of desorption. Therefore, stable heating power is produced during this period. For chilled water, T_{ci, out} is higher than T_{ci, in} at first because the initial temperature of the evaporator is high which is almost equal to the condensation temperature. Then, T_{ci, out} decreases and soon becomes stable at about 17°C until the mass recovery mode that follows starts, and stable cooling power is yielded during this period. According to the above analysis of the operating characteristics, it can be seen that the lowest hot water outlet temperature is significantly higher than the largest cooling water outlet temperature with which the minimum temperature difference between them is about 11°C.

Optimization of cycle time

The temperature profiles of T_{ci, in} and T_{ci, out} are used to describe the operating features of the mass recovery process. Figure 6 displays the variation of inlet and outlet temperature of chilled water at different mass recovery times (20 s, 40 s and 60 s). The cogeneration time and heat recovery time are fixed at 600 s and 40 s, respectively.

As mentioned previously, the inlet and outlet of two evaporators in the mass recovery mode are exchanged with each other, and T_{ci, out} increases rapidly along with time at first and reaches the peak value. The reason for this is that the chilled water from the evaporating cabin (such as E1 in cabin 1 in Fig. 2) mixes with the residual water inside the evaporating cabin (such as E2 in cabin 2 in Fig. 2) which is heated by the condensate in the desorption phase previously. The outlet temperature of chilled water decreases with the mixing process since the adsorption effect is intensified by the increment of evaporating temperature in E1 along with the temperature increment of chilling water circuit. Such a process also makes the desorption process in A2 more thorough because the temperature in E2 decreases. When the mass recovery process is completed and the heat recovery process comes, T_{ci, out} decreases slowly once more and remains constant at the end. The trends of T_{ci, out} for different mass recovery time are illustrated in Fig. 6 and the state of its fluctuation can be described to determine the optimal mass recovery time qualitatively. The smaller fluctuation means a better performance. Compared with the 20 s mass recovery time, the fluctuation range of T_{ci, out} for 40 s and 60 s are smaller. Considering the temperature difference between T_{ci, in} and T_{ci, out}, the data for 60 s mass recovery time is not obvious compared with that for 40 s.

The effects of mass recovery time on the performance of the system are exhibited in Fig. 7. When the mass recovery time is very short or there is no mass recovery process, the cooling power recovered can be almost negligible. Once the mass recovery time increases to a particular point, the system performance is significantly improved because the cycle adsorption quantity is increased significantly. In Fig. 7, it can be seen that the cooling power for a mass recovery time of 40 s is the largest one with a smaller amount of input power. The COP_c and COP_h for 40 s are also higher than those for 20 s and 60 s.

The heat recovery effect can be determined by the temperature difference between the LOTHW and HOTCW, as shown in Fig. 8. The larger temperature difference means a better heat recovery effect. The minimum temperature difference is negative when the heat recovery time is 20 s. The minimum temperature difference is larger than 10°C when the heat recovery time reaches 40 s and 60 s. The results for a heat recovery time of 40 s and 60 s are similar, and the optimal heat recovery time is chosen as 40 s.

Cogeneration time represents the lasting time during one cogeneration mode in the operation. The results of cogeneration performance at different cogeneration times are listed in Table 1. According to Eqs. (4)-(6), COP_c and SCP are both correspondent to the variation of Q_c while COP_h is directly affected by Q_h. In Table 1, it can be found that the cogeneration time of 600 s has the maximum values of COP_c and COP_h, i.e., 0.60 and 1.48; while the cogeneration time of 700 s has a little advantage on SCP compared with that for 600 s. Taking both the COP and SCP into account, the optimal cogeneration time for cogeneration is set at 600 s in this study.

Different working conditions for cogeneration

According to the analysis and previous discussion, the optimal operational processes of the system for the following investigations are identified as: cogeneration time 600 s, mass recovery time 40 s, and heat recovery time 40 s. Based on this condition, further researches on cogeneration performance of the small-scale silica gel-water adsorption system are conducted under different working conditions.

The effect of inlet temperature of cooling water on system performance for chilling and heating are tabulated in Table 2. The inlet temperature of hot water is set around 85°C, the chilling temperature around 20°C, and the inlet temperature of cooling water at 30°C, 32°C and 35°C, respectively. For the condition of 32°C, the largest outlet temperature of cooling water for domestic water heating can be about 38°C. The largest Q_h and Q_c are 21.94 kW and 8.25 kW, respectively. Besides, the maximum SCP (183.22 W/kg) also occur under the condition of T_{cw, in} of 32°C. It seems that the COP_h and COP_c have no significant change with the variation of T_{cw, in} especially for COP_c. The range of COP_h and COP_c are from 0.97 to 1.11 and from 0.38 to 0.41, respectively.

Figure 8 shows the effect of inlet temperature of hot water under the condition of inlet temperature of cooling water at 30°C. In Fig. 9, T_{hw, in} increases from 70°C to 90°C with the trends of Q_h increases to the peak value, then keeps stable and decreases at last, and Q_c rises to the maximum value and then keeps stable with a little fluctuation. When T_{hw, in} is around 80°C, the Q_h and Q_c are 20.8 kW and 8.1 kW, respectively. The corresponding values of COP_h and COP_c can reach up to 1.20 and 0.47, respectively.

Due to the summer conditions, the cogeneration performance is also analyzed at cooling water inlet of 35°C and the experimental results are shown in Fig. 10. The maximum COP_h, COP_c, and SCP obtained of this system are 1.39, 0.59, and 184.5 W/kg, respectively, at T_{hw, in} = 88.92°C, T_{hw, out} = 86.02°C, T_{cw, in} = 37.86°C, T_{cw, out} = 41.20°C, T_{ci, in} = 24.11°C, and T_{ci, out} = 19.74°C.

Take a family of three people living in the house with an area of 100 m² as example. Generally, the air conditioning capacity required is about 15.0 kW and the heating capacity should be 19.6 kW for heating the hot water of 180 L from 30°C to 40°C within only 6.5 min. For the solar driven system, the working time each day in summer is about 8 h. Compared with the conventional compression refrigeration system and electric water heater, a cogeneration adsorption system of 15.0 kW could satisfy both the air conditioning and heating effect. For the conventional compression refrigeration system, the COP for electricity is generally about 3.0 and therefore 5.0 kW of the input power (i.e. electric consumption) is required to meet the cooling need (15 kW). Hence, the electric consumption for air conditioning per day (taking 8 h as example) is about 5.0 × 8= 40.0 kWh. For 180 L of domestic hot water heated by electric water heater, the electric consumption is about 4.2 × 0.18 × 1000 × (40 - 30) ÷ 3600= 2.1 kWh. Thus, this cogeneration adsorption system could totally save an electric consumption of 42.1 kWh and reduce a CO₂ emission of 33.0 kg (1 kWh= 0.785 kg CO₂ emission) per day.

Conclusions

A small-scale silica gel-water adsorption system with integration of heat and mass recovery processes is proposed and constructed, which is used for recovering the solar energy in summer to achieve the cogeneration of air conditioning and domestic water heating effect. The cogeneration system is investigated experimentally under different working conditions in order to evaluate its performance.

The system is tested at different operational times (mass recovery, heat recovery, and cogeneration time). The optimal conditions of these three parameters are 40 s, 40 s, and 600 s, respectively.

Q_h and Q_c are 20.8 kW and 8.1 kW, respectively, when T_{hw, in} is around 80°C, and the corresponding values of COP_h and COP_c reaches up to 1.20 and 0.47, respectively. COP_h and COP_c change little with the variation of T_{cw, in} (from 30°C to 35°C), especially for the COP_c. The range of COP_h and COP_c are obtained from 0.97 to 1.11 and from 0.38 to 0.41, respectively.

Under the condition that T_{cw, in} is more than 35°C, the maximum values of COP_h and COP_c are 1.39 and 0.59, respectively. The SCP of this system is as high as 184.5 W·kg^-1.

The small-scale silica gel-water adsorption cogeneration system with a cooling power target of 15.0 kW could be used for the domestic building with an area of 100 m², and correspondingly could save an electricity consumption of 42.1 kWh and reduce a CO₂ emission of 33.0 kg per day. But the drawback of the system is that there is no cooling power output for the night because it is driven by the solar energy. Thus, such a system is prospective for the application in the office which requires large cooling and heating power at daytime. For domestic applications, the energy storage technologies for excess chilling and water heating at daytime are essential. Given the existence of the differences between current test and solar energy condition, it will be the key direction for future work and research.

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