Planning and design of a micro energy network for seawater desalination and regional energy interconnection

1 Introduction

Water is an indispensable natural resource that is essential to human survival as well as economic and social development,and freshwater resources are particularly important [1].At present,freshwater only accounts for 3%of global water and nearly 70% is trapped in ice sheets in the Arctic and Antarctica; thus,freshwater resources that can be directly accessed by humans only account for 1%of global water resources.With continuous economic and social development,increasing population,and ongoing changes in climate and the environment,the shortage of freshwater resources has become an increasingly important global problem that threatens the development and progress of human society.China has been identified by the United Nations as one of the 13 most water-short countries as its per capita freshwater resources are only a quarter of the world average [2].

China has a long coastline and vast sea area; hence,seawater desalination has gradually become an important freshwater resource because of its simple principle and mature technology [3],[4].However,desalination technology faces problems such as excessive energy consumption and environmental pollution,which restrict its development [5].Renewable energy resources such as solar energy and wind energy can be accessible in coastal areas;thus,full use of these resources is important to the viability of desalination technology.Micro energy networks (MENs)are important components of the energy internet [6]as they can access the end network of various distributed renewable energy sources and efficiently provide effective technical support [7].Flexible access to the end network and the configuration of various devices allow MENs to fully utilize various renewable resources for desalination and meet the comprehensive energy needs of regional cooling,heating,and electricity.

Some projects around the world have already explored desalination and MEN construction using renewable energy.For example,the renewable energy desalination pilot project that was launched in Masdar,UAE is planned to be fully powered by renewable energy networks and commercialized in the UAE,the Middle East,and North Africa by 2020 [8].Moreover,several research institutes in India are accelerating the development of renewable energy desalination systems by taking advantage of the geographical location of India; renewable energy sources - solar,geothermal,wind,and wave - are plentiful and thus should be the primary sources of energy for these desalination systems.The resultant renewable energy desalination systems could effectively prevent water shortages in areas that cannot be connected to the power grid [9].In Germany,Enercon has used its wind turbine technology to develop a wind-water desalination system.The system is equipped with an energy recovery device and water production can be adjusted so that the system can adapt to fluctuations in renewable energy generation without adverse effects [10].Furthermore,a number of demonstration projects combining seawater desalination with MENs have been launched in China; for example,a comprehensive system combining wind energy,photovoltaic energy,diesel generators,energy storage,and desalination was built on Dongfushan Island in Zhejiang Province.

Optimal energy utilization efficiency is achieved through the comprehensive combination of wind turbines,photovoltaics,desalination,and residential loads; these systems are viable water resources and multiple energy sources,and they also improve the economics and stability of the microgrid [12].In 2010,a solar desalination demonstration project was built on Dayushan Island in Zhejiang Province and the project set up a microgrid system consisting of a battery and a diesel generator to prevent instability of the energy source.This solar desalination system met the water demand of the island while also outputting power [13].The regional distribution of global desalination capacity and the locations of the projects discussed above are shown in Fig.1.

Fig.1 Proportion of seawater desalination capacity and location of desalination projects around the world [11]

The majority of the studies discussed above have focused on the development of desalination plants and new technologies.While these studies have used renewable energy as an energy source for seawater desalination,they have not considered the configuration and operation optimization of the desalination device.When renewable energy sources such as photovoltaics and wind power are connected to the seawater desalination system using various devices,a MEN can be set up to provide integrated energy output,including cooling,heating,and electricity,while also outputting freshwater.Furthermore,a MEN can also maximize the use of renewable energy to meet the diverse needs of users and maximize the overall benefits.

In this study,a MEN for seawater desalination was designed and the key equipment was modeled.The network structure and operation mode were also studied in order to develop a model for operation optimization of a MEN for seawater desalination.The optimization objective was to minimize the life cycle cost; thus,the influences of the electric cooling and heating ratios and the seawater desalination device under different combined heat and power (CHP) modes were analyzed.

2 Modeling key equipment in the MEN for seawater desalination

In order to study the operation and optimal configuration of the seawater desalination device in the MEN,models of the key equipment in the MEN,such as the photovoltaics,wind turbines,CHP,batteries,gas boilers,and the desalination device,are described in this section.

2.1 Photovoltaics

In the MEN with seawater desalination,the photovoltaic power output model can be expressed as:

Where γT is the conversion efficiency at different temperatures; Tair is the current temperature; NOCT is the normal operating temperature; Tref is the reference temperature; ηref is the reference efficiency; NPV is the number of photovoltaic panels; APV is the area of a single photovoltaic panel; and Rt is the solar radiation intensity.

2.2 Wind turbines

The fan output of wind turbines connected to the MEN is described as:

Where Cp is the performance of the wind turbine; ρ is the density of the air; AWT is the projection of the wind turbine blade sweep area in the vertical plane with the wind speed;v is the wind speed; and PrWT is the rated power of the wind turbine.Moreover,Vr is the rated wind speed,Vf is cut-off wind speed,and Vc is the cut-in wind speed.

The model also includes the relationship between wind speeds at different altitudes:

Where v' is the wind speed measured at height h'and v is the wind speed at the height of the blower tower (h); α is the conversion factor.

2.3 Battery

The storage capacity of a battery in the MEN can be expressed as:

Where SBat(t) is the storage capacity of battery at time t;α is the self-discharge rate of the battery; λcBat and λdBat are the charging and discharging efficiencies of the battery,respectively; PcBat(t) and PdBat(t) are the charging and discharging powers of the battery,respectively; and Δt is the length of time that the battery is in use.

Furthermore,the state of charge (SOC) of the battery is:

Where SrBat is the rated capacity of the battery [14].When the battery is in use,the SOC needs to be maintained within a certain range in order to maximize battery life.

2.4 Electric and gas boilers

A boiler is a device that converts energy into heat.Boilers are used as heat sources in the MEN and the boilers are described by the following equation:

Where QGB is the heat output of the boilers; ηGB is the conversion efficiency; and FGB is the energy input of the boilers.

2.5 Electric and absorption chillers

Electric chillers in the MEN provide cooling by pressurized liquefication using a compressor and absorb heat during evaporation.Absorption chillers use the waste heat recovered by the gas turbine for cooling,which can reduce energy waste and effectively improve the comprehensive utilization of energy.The cooling output of the two chiller types can be expressed as follows:

Where QEC and Qc AC are the cooling output of electric and absorption chillers,respectively.PEC is the electric power consumption of the electric chillers; QhAC is the thermal energy consumption of the absorption chillers ; and COPEC and COPAC are the coefficients of performance (COPs) of the electric and absorption chillers,respectively.

2.6 CHP

The natural gas consumption and electrical energy output of CHP are:

Where FCHP is the natural gas consumption of CHP; PCHP is the electrical energy output of CHP; QCHP is the thermal output of CHP; ηhCHP is the thermal conversion efficiency of CHP; and ηeCHP is the electrical conversion efficiency of CHP.

2.7 Desalination device

Schematics of thermal and membrane seawater desalination plants that can be incorporated in MENs are shown in Fig.2.This study mainly used thermal desalination technology,which is primarily based on consuming heat generated by CHP and gas boilers [15].

The heat consumed by a desalination unit (QD) can be calculated as:

Where WD is the water generated by the desalination unit and COPD is the COP of the desalination unit.

3 Structure and operation mode of the MEN for seawater desalination

The MEN for seawater desalination designed in this study achieved efficient and comprehensive utilization of energy by using various devices.Moreover,the system can provide heating,cooling,and electricity while desalinating seawater.Renewable energy was obtained using photovoltaic power generation equipment and wind turbines and the energy required for thermal seawater desalination was obtained from CHP and gas boilers,with gas boilers used as supplements.The MEN structure and energy flow are shown in Fig.3.

In the MEN,the CHP has two operating modes:following electrical load (FEL) mode and following thermal load (FTL) mode [16].The main difference between the two modes is that the FEL mode determines the amount of generated heat based on the power supply load and the FTL mode determines the amount of generated power based on the heat load.

When the CHP was operated in the FTL mode,the heat consumption of the absorption chiller was calculated from the cooling bus data.Once the CHP capacity met the heat consumption requirements of the system,the CHP preferentially supplied heat and the shortage was supplemented by the gas boiler.The battery was charged according to the balance state of the electric energy.Finally,the energy balance state of the network was used to determine whether the system needed to purchase power from the grid or could supply power.When the CHP was operated in the FEL mode,the electricity consumption of the electric chiller was calculated from the cooling bus data.If the system required energy in addition to the output of the renewable energy sources,the additional energy was provided by CHP and supplemented by the grid and battery.Finally,the output of the gas boiler was determined using the calculated heat balance.When the desalination device was optimized,the optimal QD value was calculated and the amount of freshwater output was converted.When the seawater desalination was not optimized,the QD value calculated according to (11) was used in the calculations.

Fig.2 Schematic diagrams of seawater desalination plants using the (a) thermal and (b) membrane methods

Fig.3 Schematic diagram of a seawater desalination plant

In this study,two optimization variables - the electric cooling ratio and the electric heating ratio - were defined and these variables can affect the operating mode of the MEN.The electric cooling ratio was determined by the output of both the electric chiller (QEC) and the absorption chiller (QcAC) whereas the electric heating ratio was determined by the output of the electric boiler (QEB) and QRH.The definitions of the electric cooling and heating ratios are as follows:

If desalination is not optimized,λ1 and λ2 are uncertain variables whereas the electric cooling and heating ratios of optimized desalination are considered to be fixed:

Thus,the output of an absorption chiller is expected to be:

Moreover,the electric power consumption of the electric chiller and the thermal energy consumption of absorption chiller are:

Where ηEC and ηAC are the performance parameters of the electric chiller and absorption chiller,respectively.

The total thermal energy provided by the gas boilers and CHP satisfies the following equation:

The natural gas consumption of the gas boilers,the natural gas consumption of CHP,and the electrical energy output of CHP are given by (6),(9) and (10),respectively,while the total natural gas consumption of the MEN is given by (20):

where Ftot is the total natural gas consumption of the MEN.

Ultimately,the power balance of the electric bus can be expressed as:

Where PL is the electricity load and Ploss is the power loss.The power loss is due to the over-production of CHP under the fixed electricity of the FTL mode and the incomplete consumption of renewable energy.

4 Operation optimization model for the MEN for seawater desalination

4.1 Optimization objective

This MEN for seawater desalination contains different equipment and energy sources.In this study,the optimization goal was to minimize the life cycle cost.The optimal configuration of the equipment in the MEN was determined by optimizing the electric cooling and heating ratios as well as the operation of the seawater desalination equipment.

Life cycle cost refers to all costs that are incurred during economically efficient use of the product.Life cycle cost analysis is an objective process for evaluating the environmental loads associated with products,processes,or actions; it assesses and quantifies environmental emissions and the impact of energy and material use,and identifies opportunities to make environmental improvements.Based on the life cycle analysis,which comprehensively considered the initial investment cost,replacement cost,annual maintenance cost,annual purchase cost,depreciation expense,and residual value of the MEN equipment,the life cycle cost of the MEN in this paper was determined to be [17]:

Where N is the number of devices in the MEN; Cc is the initial investment cost per unit capacity of equipment c; is the capacity/rated power of equipment c; Rc is the frequency of replacement for equipment c; Lc is the design life of device c; i is the interest rate; tr is the tax rate; Lp is the project design life; M is the annual maintenance cost;B is the annual energy cost; D is the annual depreciation expense; and S is the residual value.

The frequency of equipment replacement (Rc),the annual maintenance cost of the equipment (M),the annual energy cost (B),and the annual depreciation expense (D) are defined as follows:

Where floor(x) denotes the maximum integer that is not greater than x; rM is the equipment maintenance rate; Ed grid(t)is the amount of electricity purchased from the grid at time t on day d; πe(t) is the electricity price at time t; Fdtot(t) is the amount of natural gas purchased at time t on day d; πgas is the price of natural gas; and rD is the depreciation rate of the equipment.

4.2 Optimization variable

Of the key equipment in the MEN for seawater desalination,photovoltaics,wind turbines,CHP,and battery capacity substantially impact the performance of the MEN.Furthermore,this study also considered the electric cooling and heating ratios as well as the impact of desalination equipment on the planning for the MEN.Therefore,the optimization variables included photovoltaic capacity (PPr V),wind turbine capacity (PWr T),CHP capacity (PCrHP),and battery capacity ().

The electric cooling ratio (λ1),electric heating ratio(λ2),and heat consumed by desalination (QD)determined which CHP mode was used for optimization.When the electric cooling ratio and the electric heating ratio were not used for optimization,the ratios were set to 0.5,and when the desalination heat input was not used for optimization,its value was directly from the amount of freshwater produced.

4.3 Constraint conditions of optimization

The constraints on the MEN for seawater desalination included optimization of variable constraints,equipment operation constraints,and energy conservation constraints.

When optimizing the variable constraints,the physical meaning of the optimization variables and the actual situation must be considered,and the optimization variables must be maintained within a certain range.The resulting inequality constraint is:

When optimizing the equipment operation constraints,the main constraints are the equipment rated power or rated capacity limits.The constraint condition of the battery is described by the following expressions:

The operational constraints of the wind turbines,photovoltaics,CHP,electrical chiller,gas boiler,and absorption chiller are:

When optimizing the energy conservation constraints,the cooling/heat balance and electric balance are used.The electric balance and the cooling/heat balance constraints are given by the following equations:

When the electric heating and cooling ratios are not optimized,they are set to 0.5.

4.4 Optimization algorithm

This paper uses a genetic algorithm to solve the planning problem of the MEN for seawater desalination using the MATLAB platform.Genetic algorithms are a type of evolutionary algorithms that imitate the mechanisms of natural selection and genetics to determine the optimal solution.Genetic algorithms have minimal mathematical requirements for solving optimization problems as the nature of the problem does not need to be understood during the search process because of the evolutionary nature of the algorithm.For any form of objective function and constraint,whether linear or non-linear,discrete or continuous can be processed.The algorithm flow follows six steps:

Step 1 - Initialization:set the evolution algebra counter to t=0,set the maximum evolution algebra T,and randomly generate M individuals as the initial population P(0).

Step 2 - Individual evaluation:calculate the fitness of each individual in the population P(t).

Step 3 - Selection operation:apply the selection operator to the group.The purpose of selection is to directly identify the optimized individual to be included in the next generation or to generate a new individual through pairing for the next generation.The selection operation is based on the fitness assessment of the individuals in the group.

Step 4 - Crossover operation:apply the crossover operator,which plays a central role in the genetic algorithm,to the population.

Step 5 - Mutation operation:apply the mutation operator to the population.The population P(t) is selected,crossed,and mutated to obtain the next generation of the population,P(t+1).

Step 6 - Termination condition judgment:if t=T,the individual with the greatest fitness is used as the optimal solution output and the calculation is terminated.

5 Case study

5.1 Case data

When planning and designing the MEN for seawater desalination,the study chose different scenarios as examples to demonstrate the operation optimization model.The three-day dataset of solar radiation and wind speed used in this case study are shown in Fig.4.The wind speed was relatively low during the day and higher at night whereas the solar radiation was strongest during the daytime [18].The three-day cooling load,heating load,and electric load data used in this case study are shown in Fig.5.

Fig.4 Three-day dataset of solar radiation and wind speed

Fig.5 Three-day dataset of the electricity,heating,and cooling loads

In this case study,the power grids purchased electricity at TOU (time of use) power prices; the 06:00-21:00 period price was 0.1482 $/kWh,and the 22:00-05:00 period price was 0.0661 $/kWh.The price of natural gas was fixed at 0.287 $/kWh [19].The design life of the MEN is 30 years,but equipment must be replaced during that period according to the equipment life.The cost and life of the equipment are given in Table1.

Table1 Parameters of distributed power generation [20]

images/BZ_38_1284_2477_2268_2558.pngPhotovoltaics 3800 25 Wind turbines 2700 20 CHP 1200 30 Gas boilers 120 20 Electric chiller 150 10 Absorption chiller 150 15 Battery 740 5

5.2 Scenario settings

In this study,three scenarios were separately modeled and compared using the FTL and FEL models.The specific details of each scenario are given in Table2.The differences between the scenarios were mainly related to whether the optimization of the electric cooling and heating ratios and the seawater desalination device were included.

Table2 Details of each scenario

Desalination device Scenario 1 FTL / /Scenario 2FTL√/Scenario 3FTL√√Scenario 4 FEL / /Scenario 5FEL√/Scenario 6FEL√√Operating mode of CHP Electric cooling and heat ratio

5.3 Results analysis

The six scenarios used to evaluate the MEN for seawater desalination were optimized using the operation optimization model and the results were analyzed as follows.This study set minimizing the life cycle cost as the optimization goal and the full life cycle cost of each scenario was compared.

The results shown in Fig.6 demonstrate that Scenarios 3 and 6,which included the optimized electric cooling and heating ratios as well as the optimized seawater desalination device,had the minimum life cycle costs of the FTL- and FEL-mode scenarios,respectively.Scenarios 2 and 5,which optimized one variable,also had reduced life cycle costs compared to Scenarios 1 and 4.Therefore,optimization of the electric cooling and heating ratios as well as the desalination device have a positive impact on the life cycle cost of the MEN for seawater desalination.

The detailed planning scheme of the MEN for seawater desalination was analyzed using the configuration characteristics of each scenario and the characteristics of energy production and consumption.

The configuration characteristics and energy consumption characteristics of each scenario after optimizing for minimum life cycle cost are given in Tables 3 and 4,which demonstrate the planning capacity of the photovoltaics,wind turbines,battery,and CHP in the MEN under different scenarios.Moreover,characteristics of energy production and consumption of different kinds of energy are also shown.

Fig.6 Life cycle cost of the six scenarios

Table3 Configuration characteristics of FTL-mode scenarios

photovoltaics(kW)wind turbines(kW)Battery(kWh) CHP (kW)Scenario 1 358.3 387.7 148.9 1335.8 Scenario 2 403.3 236.6 207.2 1328.6 Scenario 3 635.2 466.4 108.80 1000.0

Table4 Configuration characteristics of FEL-mode scenarios

photovoltaics(kW)wind turbines(kW)Battery(kWh) CHP (kW)Scenario 4 195.0 433.6 31.1 1388.3 Scenario 5 240.5 409.0 11.3 1371.9 Scenario 6 500.0 637.6 2.7 1000.0

The comparison of the six scenarios supports the following conclusions:

(1) Scenarios 3 and 6 had the largest photovoltaic and wind turbine capacities whereas the capacities of the battery and the CHP were reduced.

(2) Optimization of the electric cooling and heating ratios and the desalination device significantly increased the consumption of renewable energy,reduced the capital investment of energy storage resources,and reduced the gas consumption of CHP.

(3) Operation optimization reduced the operating cost to the MEN for seawater desalination and substantially affected the utilization of renewable energy and environmental protection.

The photovoltaic and wind power data from the operation optimization model of the MEN are shown in Fig.7 and 8.These results show that including the optimization of the electric cooling and heating ratios and the seawater desalination device significantly increased the amount of renewable energy that was consumed,and thus can improve the excessive consumption of non-renewable energy.

Through the optimization of seawater desalination device and the flexible selection of the electric cooling and heating ratios,the MEN can use renewable energy as its primary source and increase its capacity.Furthermore,energy waste can be reduced through planning and flexible configuration of energy storage and device capacity.Finally,energy efficiency can be improved through the synergy of energy sources,equipment,and load in the MEN.

Fig.9 compares the electricity purchased in the six scenarios with different ways of regional energy consumptions.When renewable and non-renewable energy sources were consumed at the same time,less electricity was purchased than when only non-renewable energy sources were consumed,which saved the cost of purchasing electricity.Therefore,incorporating renewable energy sources in the MEN for desalination can significantly reduce power consumption and environmental costs,and thus can save the money.

Fig.7 Photovoltaic power data from the operation optimization model of the MEN for seawater desalination

Fig.8 Wind power data from the operation optimization model of the MEN for seawater desalination

Fig.9 Comparison of purchased energy consumption when a combination of renewable and non-renewable energy (yellow)and only non-renewable energy (blue) were used

6 Conclusions

This study developed a MEN model that included photovoltaics,wind turbines,batteries,CHP,gas boilers,electric cooling and heating equipment,and a seawater desalination device,and the life cycle cost of the system was calculated using life cycle analysis.Six scenarios with different combinations of FTL/FEL mode,optimized/unoptimized electric cooling and heating ratios,and optimized/unoptimized seawater desalination device conditions were studied using the model,the operation of the MEN for seawater desalination was analyzed.The simulation results demonstrated that the operation and construction costs of the MEN system can be significantly reduced by considering the operation of the seawater desalination device and optimizing the electric cooling and heating ratios.The cooling and heating modes at different renewable energy levels can be flexible selected.Moreover,the optimized MEN can improve the consumption and utilization efficiency of renewable energy sources,such as photovoltaics,and thus reduce the consumption of nonrenewable energy by seawater desalination,which also has implications for improving the operation of MENs.The model used in this study can serve as a reference for the planning and design of MENs for seawater desalination.