Modeling analysis on solar steam generator employed in multi-effect distillation (MED) system

Abstract

Recently the porous bilayer wood solar collectors have drawn increasing attention because of their potential application in solar desalination. In this paper, a thermodynamic model has been developed to analyze the performance of the wood solar collector. A modeling analysis has also been conducted to assess the performance and operating conditions of the multiple effect desalination (MED) system integrated with the porous wood solar collector. Specifically, the effects of operating parameters, such as the motive steam temperature, seawater flow rate, input solar energy and number of effects on the energy consumption for each ton of distilled water produced have been investigated in the MED desalination system combined with the bilayer wood solar steam generator. It is found that, under a given operating condition, there exists an optimum steam generation temperature of around 145°C in the wood solar collector, so that the specific power consumption in the MED system reaches a minimum value of 24.88 kWh/t. The average temperature difference is significantly affected by the solar heating capacity. With the solar capacity increasing from 50 kW to 230 kW, the average temperature difference increases from 1.88°C to 6.27°C. This parametric simulation study will help the design of efficient bilayer wood solar steam generator as well as the MED desalination system.

Keywords

solar energy / steam generating / multi-effect desalination

Introduction

Water scarcity has become a serious issue, especially for the inland countries, the Northern Africa, and the Western Asia [¹,²]. Desalination is a process where the salt is removed from seawater to produce fresh water. The desalination process consumes energy, such as electrical, thermal, mechanical, or chemical energy [³]. Thermal desalination is more suitable in areas where electricity is insufficient or uneconomical but thermal energy, for example, solar energy, geothermal energy, or bio-thermal energy, is accessible [⁴–⁸]. Considerate work has been performed to develop and analyze thermal desalination systems. Darwish et al. have analyzed a number of feed water arrangements in multiple effect desalination (MED) systems including the forward, backward, parallel, and mixed feed cycles [⁹]. Zhang et al. have reviewed the application of solar energy in water treatment processes, where the cycle configuration and component development in thermal desalination systems are discussed [¹⁰,¹¹]. Simulations have also been conducted on the characteristics, performance, and operation strategy of parabolic trough DSG loop in recirculation mode and solar power plant [¹²,¹³]. Bataineh have investigated the annual performance of multi-effect desalination plant employing a solar steam generation one [¹⁴]. Sharaf et al. have performed a case study on multi effect distillation-vapor compression desalination system which has a capacity of distillate products of 4545 m³ per day [¹⁵]. Additionally, the MED-TVC is proved to be of a high performance ratio and a low maintenance cost, which is promising in utilization in low compression ratio MED systems and could reduce the amount of high pressure steam requirement at the same amount of distilling fresh water [¹⁶–¹⁸]. The combination of the MED cycle and the solar collector is a mainstream for economic desalination systems.

Traditional solar steam generators work for dilute salt water where the salt concentration is often less than 5%. However, the water produced from oil and gas wells has a much higher salinity (>10%). Recently, porous bilayer wood solar collectors which have natural micro-channels for capillary water transport have drawn increasing attention because of their potential application in solar desalination of water with a high salinity [¹⁹]. Experimental results have shown that bilayer solar collectors exhibit distinct properties such as a rapid water transport and evaporation, a high light absorption (≈ 99%), a low thermal conductivity to avoid thermal loss, and a long-term stability in seawater without salt accumulation [²⁰–²²]. Simulation and experimental researches have also been conducted on solar collectors employed in desalination systems [²³,²⁴]. In this paper, a thermodynamic model is developed to analyze the performance of the bilayer wood solar collector. Besides, a modeling analysis is conducted to assess the performance and operating conditions of the MED system integrated with the porous wood solar collector.

Bilayer wood solar collector

Configuration of the bilayer wood solar collector

Figure 1 shows the schematic of the bilayer wood solar collector for steam generation. The bilayer wood has interconnected channels (fiber tracheids and lumina with diameters in the tens of micrometers) within the top “light-absorbing” layer as well as the bottom “water pumping” layer. There is a perfect microstructural match at the interface between the carbonized wood for light absorption and natural wood section. As the sun heats up the black top-layer of the wood, water is drawn up through the natural channels of the wood and turns into vapor.

Figure 2 illustrates the configuration of the solar steam generator. This steam generator is composed of a seawater chamber and a steam chamber. Thermal isolation layers are required between the generator and the ambient, and between the seawater chamber and steam chamber. The porous bilayer wood solar collector is inserted between the two chambers. An optic concentration device such as lens is employed to focus solar light on the top layer of the bilayer wood evaporator. Seawater flows in through the inlet of the seawater chamber, whose pressure and temperature depends on the requirement of the steam produced. The steam can be produced through the bilayer evaporator under solar irradiation

Modeling of bilayer wood solar steam generation

As shown in Fig. 3, the steam generation process in the bilayer wood can be divided into three closely connected thermodynamic processes. At first, seawater flows in the microchannel between wood fibers, whose temperature increases gradually to the saturation temperature at the top of the solar absorption layer. In the first process, the seawater, which is at a relatively low temperature determined by the inlet temperature after the preheating, may absorb heat to increase in temperature until reaching the saturation point. Meanwhile, it flows along the fiber tubes of the wood material to reach the top of the wood layer. Then, in the second process, the seawater evaporates at the top of the wood layer. In this process, the seawater absorbs heat from the top layer and evaporates at the saturation temperature determined by the pressure inside the tank, so that steam is produced. After that and in the third process, the brine with increased salinity dissolves back to the seawater chamber, while the steam is further heated, which is similar to the overheating phenomenon, and discharged from the collector to the steam chamber. Consequently, the performance of the solar steam generator depends on its operational parameters such as, seawater and the steam outflow, seawater inlet temperature, inlet and outlet salinity, operating pressure of the seawater and steam, as well as the solar absorption capacity and efficiency.

The modeling of the bilayer solar steam generation can be modeled as follows. The process of the seawater heating can be treated as an isobaric heating process, in which the seawater is heated from the inlet seawater temperature to saturation temperature at a constant pressure. The solar energy consumption can be calculated by

(1)Q_{\mathrm{1}}=m_{\mathrm{sw,solar,in}}{c}_{\mathrm{p},\mathrm{sw}}({t_{\mathrm{sw,sat}}|}_{p_{\mathrm{chamber}}}-t_{\mathrm{sw,solar,in}}),

(2){t_{\mathrm{sw,sat}}|}_{p_{\mathrm{chamber}}}={t_{\mathrm{sat}}|}_{p_{\mathrm{chamber}}}+{BPE|}_{t_{\mathrm{sat}},X_{\mathrm{sw,solar,in}}},

where Q₁ is the energy consumption in the first process, kJ; m_sw,solar,in is the mass flow rate of seawater at the inlet, kg·s^–1; c_p,sw is the isobaric specific heating capacity of seawater, kJ·kg⁻¹·K⁻¹; {t_{\mathrm{sw,sat}}|}_{p_{\mathrm{chamber}}}is the saturation seawater temperature at the chamber pressure, K; t_sw,solar,in is the seawater temperature at the inlet, K; and {BPE|}_{t_{\mathrm{sat}},X_{\mathrm{sw,solar,in}}}is the boiling temperature elevation at the saturation temperature and salinity, K.

The evaporation process can be modeled as an isobaric process. The amount of heat supplied to evaporation depends on the salinity of the seawater, steam flow rate, and operating pressure. This process could be described as

(3)\mathrm{d}{Q}_{\mathrm{2}}=-\mathrm{d}m({h_{\mathrm{vapor,sat}}|}_{p,X}-{h_{\mathrm{liquid,sat}}|}_{p,X}),

(4){\displaystyle \frac{X}{X+\mathrm{d}X}}={\displaystyle \frac{m+\mathrm{d}m}{m}},

where Q₂ is the energy consumption in the second process, kJ; {h_{\mathrm{vapor,sat}}|}_{p,X}is the vapor saturation specific enthalpy at the given pressure and salinity, kJ·kg⁻¹; {h_{\mathrm{liquid,sat}}|}_{p,X}is the liquid saturation specific enthalpy at the given pressure and salinity, kJ·kg⁻¹.

The steam-heating process can be modeled as an isochoric process. The amount of energy consumed can be calculated with the suction pressure and discharge pressure

(5)Q_{\mathrm{3}}=m_{\mathrm{steam}}({h|}_{v,p_{\mathrm{dis}}}-{h|}_{v,p_{\mathrm{suc}}}),

where Q₃ is the energy consumption in the third process, kJ; m_steam is the mass flow rate of steam; {h|}_{v,p_{\mathrm{dis}}} is the specific enthalpy of steam at the discharge pressure and specific volume, kJ·kg⁻¹; and {h|}_{v,p_{\mathrm{suc}}} is the specific enthalpy of steam at the suction pressure and specific volume, kJ·kg⁻¹.

The total solar energy consumption of the bilayer solar steam generation is the sum of the thermal energy consumed in the three processes divided by the solar absorption efficiency at the top black layer of the bilayer wood.

(6)Q_{\mathrm{solar}}=(Q_{\mathrm{1}}+Q_{\mathrm{2}}+Q_{\mathrm{3}})/{\eta }_{\mathrm{solar}},

where Q_solar is the heating capacity of solar collector, kJ; and η_solar is the solar collecting efficiency.

Configuration of MED system

The bilayer wood solar steam generator can supply the necessary thermal energy associated with the steam to drive the MED plant, as depicted in Fig. 4.

The MED system is composed of an ejector, falling film evaporators, preheaters, heat recovery exchangers, flash tanks, and a seawater reservoir. The falling film evaporator is a core component of the MED system, in which the seawater is sprayed onto a series of tubes and evaporates. The tubes serve as a heat exchanger between the condensation of the steam inside the tubes and the evaporation of salinity water outside the tubes. The flash tank with the expansion valves generates the steam supplied to the preheater. The preheater is used to heat up the seawater. The steam ejector compresses the low-pressure vapor from the last effect back to the first effect. The motive steam of the ejector is produced by the proposed bilayer wood steam generator. The variable speed pumps control the flow rate of the outlet steam and the inlet seawater to maintain the desired temperature and pressure level of the system.

Modeling of the MED plant combined with the bilayer solar steam generator

Energy and mass balance equations

In the MED system, each component can be modeled as a control volume with fluids flowing in or out, salinity changes, as well as heat and power consumed. The basic equations describing the mass balance and energy balance are given as

(7)\sum _{\mathrm{in}}\dot{m}=\sum _{\mathrm{out}}\dot{m},

(8)\dot{Q}-\dot{W}+\sum _{\mathrm{in}}\dot{m}h-\sum _{\mathrm{out}}\dot{m}h=\mathrm{0},

(9)m_{\mathrm{f}}=m_{\mathrm{dis}}+m_{\mathrm{brine}},

(10)x_{\mathrm{f}}{m}_{\mathrm{f}}=x_{\mathrm{brine}}{m}_{\mathrm{brine}},

where m_f is the mass flow rate of fluid, kg·s^–1; m_dis is the discharged vapor mass flow rate, kg·s^–1; m_brine is the brine mass flow rate, kg·s^–1; x_f is the salinity of fluid; and x_brine is the salinity of brine.

The following assumptions are made to simplify the thermodynamic analysis of the MED system.

(1) Steady-state assumption. All components are at steady-state and dynamic characteristics are neglected.

(2) All throttling processes are viewed as isenthalpic processes.

(3) The pressure loss within heat exchangers is neglected.

(4) The heat leakage from the solar collector system is neglected.

Ejector

The main part of the steam ejector is the nozzle zone. The correlation used for the steam jet ejector is [²⁴],

(11)Ra=\mathrm{0.296}{\displaystyle \frac{{\left(P_{\mathrm{s}}\right)}^{\mathrm{1.19}}}{{\left(P_{\mathrm{ev}}\right)}^{\mathrm{1.04}}}}{\left({\displaystyle \frac{P_{\mathrm{motive}}}{P_{\mathrm{ev}}}}\right)}^{\mathrm{0.015}}\left({\displaystyle \frac{PCF}{TCF}}\right),

(12)Ra={\displaystyle \frac{m_{\mathrm{motive}}}{m_{\mathrm{ev}}}},

(13)PCF=\mathrm{3}\times {\mathrm{10}}^{-\mathrm{7}}{\left(P_{\mathrm{motive}}\right)}^{\mathrm{2}}-\mathrm{0.0009}{P}_{\mathrm{motive}}+\mathrm{1.6101},

(14)TCF=\mathrm{2}\times {\mathrm{10}}^{-\mathrm{8}}{\left(T_{\mathrm{ev}}\right)}^{\mathrm{2}}-\mathrm{0.0006}{T}_{\mathrm{ev}}+\mathrm{1.0047},

where Ra is the entrainment ratio of flow rate; P_s is the discharge pressure; P_ev is the pressure of entrained vapor, kPa; P_motive is the pressure of motive vapor, kPa; PCF is the pressure correction factor; TCF is the temperature correction factor; T_ev is the temperature of entrained vapor, °C. This correlation is valid when Ra<4, 10°C<T_ev<500°C, 100 kPa<P_motive<3500 kPa, and P_s/P_ev>1.81. This correlation is capable of determining the mass flow of the motive steam according to the required mass flow of the entrained vapor, when the motive steam pressure, the discharge pressure and the suction pressure are given.

Evaporator

The heat transfer rate in the evaporator is determined by the falling film evaporation outside the tube, the convection, or the condensation of steam inside the tube, and the heat conduction of the tube wall, which is given by

(15){\kappa }_i=\mathrm{1}/{\displaystyle \left[\right(R_{\mathrm{in}}+{\displaystyle \frac{\mathrm{1}}{{\alpha }_{\mathrm{in}}}}){\displaystyle \frac{d_{\mathrm{out}}}{d_{\mathrm{in}}}}+{\displaystyle \frac{d_{\mathrm{out}}}{\mathrm{2}\lambda }}\ln ({\displaystyle \frac{d_{\mathrm{out}}}{d_{\mathrm{in}}}})+(R_{\mathrm{out}}+{\displaystyle \frac{\mathrm{1}}{{\alpha }_{\mathrm{out}}}})},

where {\kappa }_i is the heat transfer rate, W·m⁻²·K⁻¹; R_in and R_out are the fouling resistance of inner and outer pipe wall respectively, m²·K⁻¹·W⁻¹; d_in and d_out are the inner and outer diameter of pipelines respectively, m; α_in and α_out are the inner and outer heat transfer coefficient respectively, W·m⁻²·K⁻¹; and λ is the heat conductivity coefficient, W·m⁻²·K⁻¹.

The heat transfer coefficients of each effect distiller are described by the following equations [¹⁰,²⁵].

The falling film evaporation outside the tube is given by

(16){\alpha }_{\mathrm{out}}=\mathrm{0.032}{\left({\displaystyle \frac{{\lambda }_{\mathrm{1}}^{\mathrm{3}}g}{v_{\mathrm{1}}^{\mathrm{2}}}}\right)}^{\frac{\mathrm{1}}{\mathrm{3}}}{\left({\displaystyle \frac{\mathrm{4}\mathrm{\Gamma }}{{\mu }_{\mathrm{l}}}}\right)}^{\mathrm{0.3367}}{\mathrm{Pr}}^{\mathrm{0.4629}},

(17)\mathrm{\Gamma }={\displaystyle \frac{F}{L\cdot N}},

where α_out is the outer heat transfer coefficient, W·m⁻²·K⁻¹; λ₁ is the heat conductivity coefficient of liquid, W·m⁻²·K⁻¹; v_l is the specific volume of liquid, m³·kg⁻¹; μ_l is the dynamic viscosity of liquid, Pa·s; F is the flow rate, kg·h⁻¹; L is the length of tubes, m; and N is the number of tubes.

The condensation heat transfer inside the tube is given by

(18){\alpha }_{\mathrm{in}}=\mathrm{0.555}{\{{\lambda }_{\mathrm{l}}^3{\rho }_{\mathrm{l}}({\rho }_{\mathrm{l}}-{\rho }_{\mathrm{v}})g[r+{\displaystyle \frac{\mathrm{3}}{\mathrm{8}}}{c}_{\mathrm{p,l}}(T_{\mathrm{sat}}-T_{\mathrm{w}})][{\mu }_{\mathrm{l}}{d}_{\mathrm{in}}(T_{\mathrm{sat}}-T_{\mathrm{w}})]\}}^{\frac{\mathrm{1}}{\mathrm{4}}},

where α_in is the inner heat transfer coefficient, W·m⁻²·K⁻¹; λ_l is the heat conductivity coefficient of liquid, W·m⁻²·K⁻¹; ρ_l and ρ_v are the density of liquid and vapor respectively, kg·m⁻³; r is latent heat, kJ·kg⁻¹; T_sat is saturation temperature, K; T_w is the wall temperature, K; μ₁ is the dynamic viscosity of liquid, Pa·s; and d_in is the inner diameter of pipelines, m.

The convective heat transfer coefficient of the single phase flow in tubes is given by

(19)Nu=\mathrm{0.023}{\mathrm{Re}}^{\mathrm{0.8}}{\mathrm{Pr}}^n,

where n = 0.4 for heated situation and n = 0.3 otherwise. Therefore, the heat transfer capacity of the evaporator can be calculated by

(20)Q=\kappa A\mathrm{\Delta }T.

Preheater

Heat transfer coefficient of vapor condensation in tubes is determined by [¹⁰,²⁶]

(21){\alpha }_{\mathrm{in}}=\mathrm{0.725}{\left[{\displaystyle \frac{gr{\rho }_{\mathrm{l}}^{\mathrm{2}}{\lambda }_{\mathrm{l}}^{\mathrm{3}}}{{\mu }_{\mathrm{l}}{d}_{\mathrm{in}}N(T_{\mathrm{sat}}-T_{\mathrm{w}})}}\right]}^{\frac{\mathrm{1}}{\mathrm{4}}},

Where α_in is the inner heat transfer coefficient of preheater, W·m⁻²·K⁻¹.

The average heat transfer coefficient of seawater convection could be determined with the average Nusselt number given by Chuichill as [²⁶]

(22)Nu=\mathrm{0.3}+{\displaystyle \frac{\mathrm{0.62}{\mathrm{Re}}^{\mathrm{0.5}}\mathrm{Pr}}{{[\mathrm{1}+{\left({\displaystyle \frac{\mathrm{0.4}}{\mathrm{Pr}}}\right)}^{\frac{\mathrm{2}}{\mathrm{3}}}]}^{\mathrm{0.25}}}}{[\mathrm{1}+{\left({\displaystyle \frac{\mathrm{Re}}{\mathrm{2.82}\times {\mathrm{10}}^{\mathrm{5}}}}\right)}^{\mathrm{0.625}}]}^{\mathrm{0.8}},

valid for 0.7<Pr<500 and 30<Re<10⁸

Boiling temperature elevation

The boiling temperature elevation of seawater with different salinity is given by [²⁷]

(23)\begin{array}{l}BPE=AX+B{X}^{\mathrm{2}}+C{X}^{\mathrm{3}},\hfill \\ A=\mathrm{8.325}\times {\mathrm{10}}^{-\mathrm{2}}+\mathrm{1.883}\times {\mathrm{10}}^{-\mathrm{4}}T+\mathrm{4.02}\times {\mathrm{10}}^{-\mathrm{6}}{T}^{\mathrm{2}},\hfill \\ B=-\mathrm{7.625}\times {\mathrm{10}}^{-\mathrm{4}}+\mathrm{9.02}\times {\mathrm{10}}^{-\mathrm{5}}T-\mathrm{5.2}\times {\mathrm{10}}^{-\mathrm{7}}{T}^{\mathrm{2}},\hfill \\ C=\mathrm{1.522}\times {\mathrm{10}}^{-\mathrm{4}}-\mathrm{3}\times {\mathrm{10}}^{-\mathrm{6}}T-\mathrm{3}\times {\mathrm{10}}^{-\mathrm{8}}{T}^{\mathrm{2}},\hfill \end{array}

where X is the salinity and T is the seawater temperature.

Simulation flowchart

The geometric and operating parameters for the MED system combined with the bilayer solar steam generator are listed in Table 1. Figure 5 demonstrates the flowchart for the simulation of the MED system combined with the bilayer solar steam generator. The simulation starts from assigning the steam flow rate at the outlet of last effect. With the assigned outlet salinity, the thermodynamic properties of the last effect, including the evaporation temperature, the heating capacity, and the BPE, could be calculated. With the evaporator model stated above, the calculation of the heat exchanging capacity of the last effect could be made. Step by step, a similar process could be conducted on each effect of desalination, and the inlet salinity of seawater is then determined. Afterwards, the ejector model is employed to calculate the seawater inlet temperature, thus the solar heating capacity. Finally, with the converged results on the steam flow rate, all calculation results could be calculated and output. In this investigation, Matlab R2017a is employed as the solver.

Results and discussion

Influence of brine flowrate

The brine flow rate is a critical operating parameter that determines the performance of the forward feed MED system. If the brine flow rate is too low, the salinity in those evaporators increases and then the BPE increases. As a result, the MED system will have fewer effects and therefore produce less distilled water when the inlet seawater temperature and the outlet steam temperature are held constant. On the other hand, if the brine flow rate is too large, the inlet seawater may not be heated up to the saturation temperature in the preheater and the steam generation is severely reduced. Therefore, the brine flow rate needs to be optimized to maximize the fresh water production in the MED system. The brine discharge flow rate takes both the seawater inlet flow rate and the discharge salinity control into account. In this investigation, the influence of brine outflow rate on performances is analyzed.

The influence of brine flow rate on system performance is displayed in Fig. 6. Figure 6(a) shows the distill flow rate and specific energy consumption as a function of the brine flow rate for different number of effects in the MED system with a solar absorbing capacity of 100 kW and a steam discharge temperature of 140°C. It can be seen that that the distilled water flow rate declines rapidly with the increase in the brine water flow rate in the MED system with 30 effects. On the contrary, the distilled water flow rate increases with the increase in the brine water flow rate in the MED system with 10 effects. The difference should come from the evaporation temperature, as is presented in Fig. 6(c). When the flow rate is larger, the preheating energy consumption significantly increases while the solar collector is less efficient in producing steam. However, at a lower evaporation temperature, the efficiency of desalination is influenced by the salinity of brine, which is the situation for the 10 effect system. The specific power consumption under the given operating condition can be as low as 23.61 kWh for each ton of fresh water product, but it will increase significantly with the changes in the brine flow rate and the number of effects in the MED system. Figure 6(c) exhibits the evaporation temperature in the first-effect evaporator as a function of brine flow rate for different number of effects in the MED system with a solar absorbing capacity of 100 kW and a steam discharge temperature of 140°C. It can be seen that the evaporation temperature decreases significantly with the increase in the brine flow rate. For example, the evaporation temperature decreases from 129.13°C to 69.96°C when the brine flow rate increases from 0.1 kg/s to 1 kg/s for the 20 effect.

Figures 6(b) and 6(d) show the influence of brine flow rate on system performance in the MED desalination system with a solar absorbing capacity of 30 kW. The curves of distilled water rate and specific energy consumption in Fig. 6(b) are very different from those in Fig. 6(a). It can been seen in Fig. 6(b) that the distilled fresh water flow rate increases, reaches a maximum, and then decreases, as the brine flow rate increases. Interestingly, the specific energy consumption in the MED system with the 20–30 effects and 30 kW heating capacity is lower than that for the MED system with a heating capacity of 100 kW. The difference between the 30 kW and 100 kW system may result from its sensitivity to salinity changes in the evaporation. When the capacity is larger, the salinity increases more rapidly between effects, so that the BPE increases to lower the evaporation temperature in the next effect. Therefore, a profound system should take both the capacity and seawater flow into consideration for optimal operating condition.

Influence of steam temperature

The temperature of the steam, produced by the bilayer wood solar steam generator to drive the MED system, is another critical parameter that determines the system performance. In general, increasing the steam temperature in the solar collector system will lead to more loss in thermal energy in the seawater preheating. On the other hand, when the motive steam temperature increases, the evaporation temperature of the first effect can be increased, so that more effects of evaporation can be added to the MED system. Therefore, the steam temperature can be optimized to enhance the performance of the MED system.

Figure 7 shows the changes of optimal number of effects for minimum specific power consumption with the temperature of the motive steam in the MED system. The solar absorbing capacity of the MED system is selected as 100 kW while the brine outflow rate is 0.3 kg/s. It is observed that the specific power consumption first decreases from 33.92 kWh/t to a minimum 24.88 kWh/t, and then increases to 27.06 kWh/t, when the motive steam temperature increases from 125°C to 160°C. The optimal temperature of the motive steam is found to be 145°C to minimize the specific energy consumption. The optimal number of effects increases monotonically with the increasing steam temperature.

Influence of input solar energy

Figure 8 shows the influence of input solar energy on the performance of the MED system at a constant brine outflow rate of 0.3 kg/s and a constant discharge steam temperature of 140°C. It can be seen that the fresh water production increases monotonically with the increase in the input solar energy. The specific power consumption decreases first and then reaches a constant when the input solar energy increases. The reason for such changes also comes from the influence of preheating for steam producing in solar collector and the salinity changes between effects. With the increase in solar capacity, specifically less energy is wasted on seawater heating in the tank, so that the specific energy consumption decreases. However, it does not continue to decrease because the increase in salinity may be much more significant, which leads to the increase in evaporation temperature in the first effect, and entrains less vapor in the ejector. Consequently, the optimal solar capacity should take the flow rate and heat exchange capacity into consideration.

Temperature differences between effects

Figure 9 illustrates the evaporation temperature and salinity in each effect of the MED system operating at a brine outflow rate of 0.3 and a discharge steam temperature of 140°C. The temperature difference between two adjacent effects increases from the first to the last effect of the evaporators. The temperature difference between two adjacent effects ranges from 4.48°C to 8.37°C in the MED system with a solar absorbing capacity of 200 kW, while the temperature difference ranges from 2.67°C to 4.5°C in the MED system with a solar absorbing capacity of 100 kW. The reason for this is that the salinity increases from the first effect to the last effect, which leads to an increase in BPE.

Conclusions

This paper presents a parametric study for the MED system driven by a newly discovered bilayer wood solar steam generator. A thermodynamic model has been developed to analyze the performance of the bilayer wood solar steam generator and the MED system driven by the bilayer wood steam generator. Models for falling film evaporator, preheater, ejector, and brine thermodynamic properties have been introduced to the system model. The effects of operating parameters, such as the motive steam temperature, seawater flow rate, input solar energy, and number of effects on the energy consumption for each ton of distilled water produced have been investigated in the MED desalination system combined with the bilayer wood solar steam generator. It is found that, under a given operating condition, there exists an optimum steam generation temperature of around 145°C in the wood solar collector, so that the specific power consumption in the MED system reaches a minimum value of 24.88 kWh/t. The brine flow rate is also found to have a significant effect on the specific energy consumption. The energy consumption increases rapidly with the increase in the brine water flow rate in the MED system with 30 effects, while it decreases in the MED system with 10 effects. This parametric simulation study will help the design of efficient bilayer wood solar steam generator as well as the MED desalination system.

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