Research progress on integrated solar chimney system for freshwater production

1 Introduction

Solar chimney power generation technology is one of the feasible ways to realize large-scale development and utilization of solar energy,which is mainly composed of collector,chimney and turbine.The heat storage layer under the collector absorbs the solar radiation,then in turn heats the air flowing inside the collector.As air temperature rises,the density is decreased,and thus there will form a density difference inside and outside the system.A strong updraft is created by the chimney effect,which drives the turbine at the bottom of the chimney to rotate and eventually obtain electricity output.The concept of solar chimney power technology was first proposed by Cabanyes [1]in 1903.Then Schlaich [2]put forward that solar chimney technology could be applied to a power station on large scale,that is,a solar chimney power plant (SCPP).In 1981,the world’s first SCPP was launched in Manzanares under the joint cooperation of Germany and Spain,of which the chimney is 194.6 m high,the chimney diameter is 10.3 m,the radius of the collector is 122 m with inlet and outlet height of 2 m and 8 m,respectively [3,4].The plant operated safely for approximately 15,000 hours,which verified the technological feasibility of this concept.

The structure of SCPP is simple and easy to implement,and can absorb both direct and diffuse solar radiation.It’s vividly that the entire power generation process is environmentally friendly.However,the solar energy utilization rate of solo SCPP is very low,and is greatly affected by the weather,which make the power output fluctuate obviously,resulting in the relatively lower competitive compared with other solar thermal power generation forms with regard to the large-scale engineering applications.Therefore,researchers tend to pay more attention to the coupling of solar chimney power generation technology with other technologies to achieve multiobjective production and so as to improve the system efficiency,such as photovoltaic or wind power generation,solar dryer,ventilation and seawater desalination.

In recent years,the shortage of freshwater has attracted the attention of all over the world.Desalination is one of the most common methods of obtaining large quantities of freshwater,but that process often requires high energy consumption.At present,the desalination system driven by renewable energy in an environmentally friendly way has been widely concerned by the society as an innovative method.Based on the flow characteristics and the large-size area of SCPP,scholars have continuously proposed new integrated systems for obtaining freshwater outputs using solar chimney technology.

Since there is no detailed summary of the application progress of integrated solar chimney systems for obtaining freshwater output,this paper aims to classify related researches from the perspective of freshwater acquisition methods,detailly introduces the coupling of solar chimney technology with solar distillation technology,humidification and dehumidification distillation technology respectively.Research advances in obtaining freshwater from the surrounding atmospheric environment are also involved.

2 Combined with conventional solar still

2.1 Open-type solar still

With an open seawater pool directly placed under the collector,Wang [5-7]proposed three integrated solar chimney systems for power generation and seawater desalination.As is shown in Fig.1,the three systems are namely direct condensation hydropower integrated system(a) and indirect condensation wind power integrated system(b) and the indirect condensation hydropower integrated system (c).when seawater is heated,the evaporated vapor would mix with incoming dry air to form the hot humid airflow,which then rapidly flowing upward due to the pressure difference between inside and outside the system.In the direct condensation system,the vapor would directly get condensed at certain height inside the chimney.At this time,the gravitational potential energy of condensed water would convert into kinetic energy to drive the hydroturbine installed at the bottom of the chimney to rotate.The indirect condensation system is to add heat exchange tube bundles at the base or top of the chimney to transfer heat through the partition wall surface.The heat generated during the condensation process can be used to heat the air to improve the system pressure difference,and thus results a larger power generation.They also compared the two methods of condensation through experiment in [13],and pointed out that the indirect condensation system could achieve twice freshwater yield of direct one,which also verified the technical feasibility of the system.As for the condensation system,they established the model of the condensate collection rate through theoretical analysis,and made a preliminary design for the condenser.

Fig.1 Schematic diagram of integrated systems by Wang [5-7]

Based on this,Zhou et al.[8]considered two methods of power generation at the same time,with wind turbine and hydroturbine installed in the base inside and outside the chimney,respectively,and proposed a combined solar chimney system for power generation and seawater desalination (CSCSPD),the structure of which is shown in Fig.2.They established the mathematical model to predict the system performance parameters such as power,payback period and feasibility,and carried out a comparative study.It is pointed out that although the airflow velocity,temperature rise and wind or hydraulic power generated by the integrated system CSCSPD are smaller than the SCPP of the same size,for systems with chimney height less than 445 m,the CSCSPD is more cost-effective if the water production benefit is taken into consideration.

Fig.2 Schematic diagram of CSCSPD [8]

Fig.3 Schematic diagram of WSSCPPSDIC [9]

Considering the transition section structure,Zuo et al.[9]specially designed a condenser and proposed wind supercharging solar chimney power plant combined with seawater desalination by indirect condensation freshwater production (WSSCPPSDIC),as shown in Fig.3.When the seawater layer is heated and evaporated,a large amount of water vapor enters the upper space of the collector,and the moist air flows to the center of the collector under the action of buoyancy and chimney effect and forms saturated moist and hot air flow.Then as it flows through the condenser,the saturated airflow would get condensed into liquid water on the outer surface of the condenser tube bundle.The condensation heat is expected to preheat the seawater flowing into the distillation tank inside the tube bundle.The characteristics of WSSCPPSDIC flow field were studied by three-dimensional numerical simulation,and the considerable freshwater yield was obtained.

Similarly,in Kiwan et al.[10],the open-type seawater pool is also adopted,while the humid air would directly get condensed on the inner wall of the chimney,which can obtain freshwater without special condenser equipment,as shown in Fig.4.The area under the collector is composed of ordinary heat storage layer of the outer ring and open seawater desalination area of the inner ring.The surrounding air enters the collector and is firstly heated,and then carries the water vapor when flowing through the sea pool area to form a hot and humid air stream.Heat exchange between humid airflow and the outside air occurs as it flows inside the chimney and then get condensed into freshwater on the inner surface of the chimney.The mathematical model was also established for the system,and the influence of various parameters on the system performance were also explored.It is pointed out that the comprehensive utilization rate of solar energy can be increased by more than 100 times compared with the traditional SCPP.In addition,each seawater desalination area has its corresponding optimal water depth.

Fig.4 Schematic diagram of Solar chimney power-water distillation plant [10]

2.2 Closed-type solar still

Disc solar still is one of the earliest traditional solar distillation units.With the still roof closed,the water vapor in the still could not contact with the ambient air,which also be called the greenhouse-type solar still.

With rings of closed disc solar still placed under the collector,Zuo et al.[11]proposed a solar chimney power plant combined with seawater desalination (SCPPCSD).As is shown in Fig.5,the glass cover is used to separate the seawater and the hot airflow in the collector,and to realize heat transfer between seawater and the airflow.Thus the latent heat released from the condensation of water vapor on the inner surface of glass cover can also be used to heat the airflow in the collector.Zuo [12]established a small-scale experimental model and conducted experimental research to verify the scientificity and feasibility of this concept.In addition,the unsteady energy transfer process was also analyzed,and the mathematical model of SCPPCSD was established [13].The system heat and mass transfer and flow characteristic under no-load condition were studied in Zuo et al.[14-16],the effects of structural parameters of collector and the operating parameters of solar still on the system performance were also investigated.This structure realizes the dual output of the freshwater output and electricity production,and the comprehensive utilization rate of solar energy is greatly improved.

Fig.5 Schematic diagram of SCPPCSD [11]

In fact,the pressure difference between internal and external airflow of SCPPCSD has not been improved.At the same time,due to part of energy taken away by the condensed freshwater,the temperature of the airflow in the collector is slightly decreased,which further reduces the pressure difference between inside and outside the system,resulting in a lower power generation.For that reason,Zuo et al.[17]installed a special designed wind pressure ventilator at the top of the chimney,and proposed wind supercharged solar chimney power plant combined with seawater desalination (WSSCPPCSD),which is shown in Fig.6.The ventilator blades are directly coupled to the conventional vertical axis wind wheel blades to achieve the purpose of rotating the ventilator blades simultaneously while the wind wheel is driven to rotate by high altitude wind.The wind pressure ventilator is verified could generate certain negative pressure on the top of the chimney through unsteady numerical simulation,resulting a higher pressure difference between inside and outside the system,and the power generation and freshwater production are greatly improved.In Zuo et al.[18],they also discussed the effect of different structural parameters on the system performance.

With the area partially occupied by conventional heat storage,Asayesh et al.[19]used the particle swarm optimization algorithm to optimize the system of Zuo et al.[11].In other words,the bottom of the collector is only partially desalinated,and the rest is still the common thermal storage layer.They verified the rationality of the algorithm through experiments,and calculated the optimal seawater desalination range of the solar chimney with a diameter of 250 m and a chimney height of 200 m from an economic perspective,which is between 85-125 m outward of the central axis of chimney.

Fig.6 Schematic diagram of WSSCPPCSD [17]

2.3 Integrated with solar pond

Akbarzadeh et al.[20]analyzed the power generation capacity of SCPP combined with the salinity gradient solar pond in the salt-affected areas,as shown in Fig.7.The heat stored in the bottom of the solar pool is used to heat the air inside the chimney,which act as a collector.The salt hot water is pumped to a certain height in the tower,and then being sprayed to heat the incoming cold air,and the cold brine would be collected at the bottom tank to flow back to the solar pond for recycling.The plant has the ability to generate intermittent power at any time.With a solar pond area 10% larger than the Manzanares SCPP,such system can generate 60 kW of electricity.The research results indicate that the water loss of the direct contact integrated system is large,and it is only suggested for systems that also seek freshwater output,in which extra condensing equipment is needed.

Assem [21]proposed a hydro solar-pond-chimney power scheme.The bottom of artificial solar pond is seawater,and the top is reused water.Power output,which includes hydropower from seawater and reused water and power generated by wind turbine in solar chimney,is expected to reach 480 MW.The air flowing over the lake is saturated with water vapor at 30 ℃ temperature,heat from the bottom of solar pond is extracted to heat the saturated air on the pond surface,and conveys it into the bottom of the chimney.The air then rises up under the influence of the chimney effect,and reaches saturation again in the surface level with a lower temperature.It is then transferred to chillers to obtain freshwater output.

Zhang et al.[29]proposed an integrated solar chimney system coupled with air-source heat pump.Seawater pool under the chimney absorbs solar radiation and naturally forms layers of different concentration from top to bottom,thus to some extent could act as a solar pond to storage heat and to obtain freshwater by direct desalination.Under the chimney effect,the airflow velocity on pool surface is increased,which then accelerates the evaporation of seawater.The refrigerant absorbs the heat released by saturated water vapor inside the collector as the air flows through the evaporator of the air-source heat pump,and then vapor gets condensed into water on the outer surface of evaporator,which would finally be collected in the freshwater tank.In addition,heat of the refrigerant released by the condenser can heat the seawater to further increase the air temperature inside the collector.The updraft dry airflow then pushes the turbine at the bottom of the chimney to spin,which then generates power by generator and partly provides the electricity consumed in air-source heat pump,enabling self-sufficiency.Besides,high-temperature concentrated brine in the bottom of the pool also can be used to generate electricity by means of solar pond power generation system when it is on a large scale.

Fig.7 Schematic diagram of combing solar chimney with solar pond [20]

Fig.8 Schematic diagram of solar-pond-chimney system [21]

Fig.9 Schematic diagram of solar chimney combined with air-source heat pump [22]

3 Combined with Humidifier-Dehumidifier process

Fig.10 Schematic diagram of solar chimney combined with HD process [23]

Niroomand et al.[23]equipped the SCPP with humidifier and dehumidifier installed at the entrance of the collector,and proposed a new integrated system.As shown in Fig.10,interphase heat transfer occurs between the warm droplets and dry air flowing through the humidifier,resulting in the hot and humid air flow,of which most water vapor then gets condensed on the outer surface of heat exchanger bundle as it flows through the dehumidifier.The dry air continues to flow through the system,and drive the turbine to rotate.For this system,there is no need to build large distillation tanks,and the land under the collector can still be used for farming.This type structure is suitable for offshore areas or areas that near power stations which discharge warm water.They made calculation about the main performance parameters of the system through theoretical analysis,and the influences of temperature,flow rate of inlet water in humidifier and dehumidifier tube number were also studied.The comprehensive system performance can be greatly improved by increasing the water flow,temperature and the number of dehumidifier tubes.However,increased freshwater production will lead to reduced power generation.

Kabeel et al.[24]proposed an improved system,which both solar still and HDH were involved,as shown in Fig.11.The system mainly consists of solar distillation tank,humidifier,dehumidifier and solar air-water heater.The desalination process involves two cycles of water and air,respectively.Firstly,water is pumped to the solar water heater tubes for preheating,and then the water drops fall back to the bottom of the distillation tank through the spraying device.During the air cycle,the ambient air is fed by the blower and heated by the solar-air heater.As hot air flows to the entrance of the chimney,it directly direct contact with the falling water droplets to form the moist and hot air flow.Then the moist air flows upward toward the dehumidifier under the chimney effect,and the vapor gets condensed in the dehumidifier,the dry air eventually flows out into the environment.In addition,there exists part of the seawater evaporates in the bottom solar distillation tank,and the water vapor rises and gets condensed on the inner surface of the glass cover.By setting up a smallscale experimental device,the variation characteristics of system performance and output under different operating conditions were studied,and economic analysis was made.The results showed that the output of freshwater increases with the increase of water temperature and air mass flow rate.In general,the system overall efficiency varies between 21%and 39%.

Fig.11 Schematic diagram of SS-HDH [24]

4 Freshwater harvest from atmosphere

4.1 Solar cyclone

According to the hurricane principle,Kashiwa et al.[25]proposed a solar chimney system coupled with cyclone separator to separate freshwater from ambient air,as shown in Fig.12.The air flows into the collector and is heated in the radial flow process.After flowing through the swirl blades,the hot airflow is equipped with certain rotational velocity component.As the hot air approaching the bottom of the chimney,the rotational velocity component and radial velocity component are increasing.According to the law of conservation of energy,the temperature,pressure and density of the gas are decreasing and reaching the minimum value in the contraction section.When the mixed hot air flows through the expansion cyclone separator,once the average temperature of the hot air falls below the dew point temperature,condensation begins,and the water is thrown out under the action of centrifugal force and collected on the chimney inner side.In addition,the heat released during the condensation process can further improve the system pumping force.Based on the theoretical model established for this system,the system power generation and freshwater output are calculated.The coupling system with a height of 500 m and a radius of 42 m can produce about 2 million tons of freshwater and 3 MW power generation per year,and the system output increases exponentially with the chimney height.

4.2 Aerological accelerator

Fig.12 Schematic diagram of solar chimney combined with expansion cyclone separator [25]

Fig.13 Schematic diagram of solar chimney technology combined with AeAc [26]

Ming et al.[26]proposed an improved solar chimney power station based on the high-altitude weather accelerator system (AeAc),as shown in Fig.13.The plant uses black pipes filled with water instead of collector to heat the water and air,which can not only realize power generation but also harvest freshwater from the air.The device provides a new idea for building SCPP in humid areas.The moist air is first heated by the black-painted pipe at the bottom to form the moist and hot airflow,which then flows upward to push the turbine to rotate due to the density difference between inside and outside the system.During the rising process,the airflow temperature drops continuously and the relative humidity increases,and the condensation begins when the relative humidity reaches 100%.To facilitate condensation,a porous surface is also settled at the condensation height.Then the condensed water moves down along the outside wall of the chimney to drive the hydroturbine to generate electricity.The effects of inlet air temperature,relative humidity,turbine pressure drop factor and chimney height on inlet velocity,condensation height,system output and efficiency were studied by establishing the one-dimensional compressible flow and heat transfer mathematical model.The results show that the higher the chimney,the more easily the condensation condition can be achieved.Under certain conditions,the higher the freshwater yield can be obtained by raising the height,and the power generated by hydroturbine will be greatly increased.The power generated by wind turbine is mainly affected by the temperature of the air at the chimney inlet.

4.3 Hot water-sprayer system

Fig.14 Schematic diagram of solar chimney combined with water-sprayer system [27]

Recently,Ming et al.[27]replaced bottom turbine with seawater sprayer system to obtain freshwater,as shown in Fig.14.The water is sprayed into the chimney to add moisture to the heat flow,and the evaporation of water droplets further increases the relative humidity of airflow,therefore the condensation condition can be reached more easily.This process partially replicates the natural convection of warm humid air,which can realize recovery of liquid water from the chimney internal heat flow,and greatly reduces the chimney size,leading to a lower construction investment.The three-dimensional numerical simulation was carried out by Ansys Fluent,and the relationships between the flow field distribution,the relative humidity of the updraft,the mass flow rate,the desalination efficiency and the spray quantity were analyzed.The results show that the temperature of the hot airflow would inevitably decrease after being humidified,resulting in the reduction of internal and external pressure difference of the system,which will slightly weaken the pumping force and flow performance.Under the condition of constant external environment,the greater the amount of spraying,the larger the yield and efficiency of freshwater output,and the greater the influence on the microclimate of the surrounding local environment.

5 Conclusion

In this paper,a comprehensive review of the research progress of integrated solar chimney system for simultaneous power generation and freshwater output is presented.On the one hand,the flow characteristics of the solar chimney can be used to accelerate the evaporation and condensation of water,on the other hand,the flow in the system can absorb heat released during condensation to improve the pumping force of solar chimney.At the same time,water can act as a good thermal storage layer to balance the power output of the system and reduce the impact of dynamic environmental conditions on the system.In the further exploration,more attention should be paid to the optimization the layout design from the perspective of system performance and economy,reducing the size and creating conditions for easier condensation.In addition,it has not been explored whether the systems of hot-humid air flowing and condensing inside the chimney will affect the performance of turbine blades.