A three-dimensional solar interface evaporator and its preparation method

Abstract

The present invention discloses a three-dimensional solar interface evaporator and a preparation method thereof, which relates to the technical field of solar interface distillation. The top surface of the three-dimensional solar interface evaporator is set as a wavy top surface, and the optimal wave angle is selected according to the principle of maximum evaporation rate; the optimal evaporator height is selected in combination with the influence of the capillary water lifting rate on the water lifting height and the influence of the three-dimensional solar interface evaporator height on the side evaporation rate; the substrate of the three-dimensional solar interface evaporator is prepared according to the optimal wave angle and the optimal evaporator height; a photothermal layer is prepared on the upper surface of the wavy top surface of the substrate to obtain the three-dimensional solar interface evaporator. The three-dimensional solar interface evaporator provided by the present invention has a higher clean water evaporation rate and evaporation efficiency, while ensuring the stable attachment of photothermal materials.

Description

Technical Field

[0001] The invention relates to the technical field of solar interface distillation, and more particularly to a three-dimensional solar interface evaporator and a preparation method thereof. Background Art

[0002] Solar interfacial distillation technology is an emerging water treatment technology. It uses natural clean solar energy and photothermal systems to simultaneously remove pollutants and recycle clean water. In this technology, the design of the photothermal evaporator is a key factor in determining the evaporation efficiency of the photothermal system. Three-dimensional photothermal evaporators have been widely designed and studied due to their low heat loss and high clean water evaporation rate. The three-dimensional photothermal evaporator is transmitted to the top surface through the water transport channel of the base. The water molecules are subjected to the high temperature generated by the photothermal material due to the photothermal conversion, forming interfacial evaporation. Since the top surface is usually isolated from the water surface, it is necessary to use the water transport channel and capillary force to transport water to the top surface for heating. If too much water is transported, too much heat will be used to heat the water body. Insufficient water transport will lead to insufficient utilization of heat accumulation on the photothermal surface. In this process, it is crucial to absorb sunlight to the maximum extent and ensure an appropriate water supply. Therefore, the shape of the membrane absorption surface and the height of the water transport channel need to be carefully designed. In addition, in order to avoid secondary pollution, it is equally important to ensure the stable attachment of the photothermal material on the evaporator surface, which is a problem that technicians in this field need to solve urgently. Summary of the invention

[0003] In view of this, the present invention provides a three-dimensional solar interface evaporator and a preparation method thereof.

[0004] In order to achieve the above object, the present invention provides the following technical solutions:

[0005] A three-dimensional solar interface evaporator comprises a substrate and a photothermal layer arranged on the upper surface of a wavy top surface of the substrate, wherein a water transmission channel and a water storage layer are arranged in the substrate;

[0006] One end of the water delivery channel is connected to a water source, and the other end is connected to the wavy top surface, so as to deliver water to the wavy top surface for interface evaporation;

[0007] The water storage layer is connected to the wavy top surface and is used to receive distilled water obtained by interface evaporation;

[0008] The photothermal layer is used to absorb solar energy and provide heat for interface evaporation.

[0009] Optionally, melamine sponge is used as a base film, iron-based carbon nanotubes are used as photothermal materials, and polyvinyl alcohol is used as a cross-linking agent to fix the base film and the carbon nanotubes sprayed on the wavy top surface.

[0010] A method for preparing a three-dimensional solar interface evaporator comprises the following steps:

[0011] Determine the optimal wave angle: The top surface of the three-dimensional solar interface evaporator is set as a wavy top surface, and the optimal wave angle is selected according to the principle of maximum evaporation rate;

[0012] Determine the optimal evaporator height: Combine the effect of capillary water lifting rate on water lifting height and the effect of evaporator height on lateral evaporation rate to select the optimal evaporator height;

[0013] Prepare a three-dimensional solar interface evaporator: prepare a substrate of the three-dimensional solar interface evaporator according to the optimal wave angle and the optimal evaporator height; prepare a photothermal layer on the upper surface of the wavy top surface of the substrate to obtain a three-dimensional solar interface evaporator.

[0014] Optionally, the specific steps of preparing the photothermal layer are:

[0015] MIL(Fe)-101 and melamine are mixed evenly in a ratio of 1:3-5 and then annealed at a high temperature of 800-1000°C to form an iron-based carbon nanotube photothermal material;

[0016] The iron-based carbon nanotube photothermal material is ultrasonically crushed, and the obtained suspension is evenly sprayed on the base film of the wavy top surface of the three-dimensional solar interface evaporator by a spraying method, wherein the base film is a melamine sponge soaked in a polyvinyl alcohol (PVA) crosslinking agent;

[0017] The glutaraldehyde solution is sprayed on the wavy top surface of the three-dimensional solar interface evaporator, so that the polyvinyl alcohol (PVA) on the melamine sponge and the hydroxyl groups on the iron-based carbon nanotube photothermal material are cross-linked through glutaraldehyde.

[0018] Optionally, the method for selecting the optimal wave angle according to the principle of maximum evaporation rate is:

[0019] The evaporation rate of the top surface of the two-dimensional plane under different illuminations was collected, and a linear fitting was performed to obtain the relationship between the light intensity and the evaporation rate of the top surface of the two-dimensional plane.

[0020] Determine the relationship between wave angle and receivable light intensity;

[0021] According to the relationship between the wave angle and the receivable light intensity, the receivable light intensity at each wave angle is calculated, and the relationship between the light intensity and the evaporation rate of the two-dimensional plane top surface is substituted into the relationship between the light intensity and the evaporation rate of the two-dimensional plane top surface to obtain the theoretical two-dimensional wave top surface evaporation rate corresponding to each wave angle;

[0022] According to the theoretical two-dimensional wavy top surface evaporation rate corresponding to each angle, the projected area and the actual area are corrected to obtain the actual two-dimensional wavy top surface evaporation rate and select the optimal wave angle.

[0023] A specific wave angle is designed on the evaporator surface to increase the light absorption area and refract and reflect the sunlight, which can make full use of the sunlight and achieve a higher evaporation rate.

[0024] Optionally, the relationship between the wave angle and the receivable light intensity is:

[0025] I=A[cos(x)cos(∈)+sin(∈)sin(x)cos(ε-δ)];

[0026] Where I is the acceptable sunlight intensity; A is the intensity of one sun, A = 1kW m -2 ; x is the solar altitude angle; ∈ is the inclination angle of the evaporator two-dimensional unit; ε is the solar azimuth angle; δ is the azimuth angle of the two-dimensional unit.

[0027] Optionally, the method for selecting the optimal evaporator height by combining the influence of the capillary water lifting rate on the water lifting height and the influence of the evaporator height on the side evaporation rate is:

[0028] The side evaporation rates corresponding to different evaporator heights were collected and linear fitting was performed to obtain the relationship between the evaporator height and the side evaporation rate;

[0029] Determine the total theoretical evaporation rate = evaporation rate of the wavy top surface + evaporation rate of the side surface;

[0030] Determine the relationship between the capillary lift rate and the lift height;

[0031] The fitting intersection point of the capillary water lift rate and the total theoretical evaporation rate is determined, and the height corresponding to the intersection point is the optimal evaporator height.

[0032] Optionally, the relationship between the capillary lift rate and the lift height is:

[0033]

[0034]

[0035] Among them, d is the average porosity of the basement membrane; ΔP is the pressure difference of the water transport channel; μ is the dynamic viscosity of water; h is the water rise height; σ is the surface tension between water and air; θ is the contact angle of the top water; ρ is the density of water; and g is the gravity constant.

[0036] It can be seen from the above technical solutions that the present invention provides a three-dimensional solar interface evaporator and a preparation method thereof, which has the following beneficial effects compared with the prior art:

[0037] (1) The present invention improves the photothermal surface of the evaporator into a wavy surface and selects the optimal wave angle, thereby improving the reflection and reabsorption capacity of light and increasing the surface area for receiving light, thereby achieving a higher evaporation rate and evaporation efficiency.

[0038] (2) The present invention takes into account the influence of the side area size on the evaporation rate and the influence of the capillary force water lifting rate on the water lifting height, selects the optimal evaporator height for substrate preparation, determines the optimal height of the water transfer channel, and further improves the evaporation rate and evaporation efficiency.

[0039] (3) The present invention uses melamine sponge as the base film and polyvinyl alcohol as a cross-linking agent to fix the base film and the carbon nanotube photothermal material sprayed on the wavy top surface, thereby increasing the stability of the base film and the photothermal material.

[0040] In summary, the three-dimensional solar interface evaporator provided by the present invention has a higher clean water evaporation rate and evaporation efficiency, while ensuring the stable adhesion of photothermal materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0042] Figure 1 It is a structural schematic diagram of a three-dimensional solar interface evaporator of the present invention;

[0043] Figure 2 This is a thermal image of the iron-based carbon nanotubes prepared by the present invention;

[0044] Figure 3 This is a schematic diagram of a method for preparing a three-dimensional solar interface evaporator according to the present invention;

[0045] Figure 4 A diagram of light absorption and reabsorption on the surface of a three-dimensional solar interface evaporator according to the present invention;

[0046] Among them, 1 represents the top surface, 2 represents the aquifer, and 3 represents the water transmission channel. DETAILED DESCRIPTION

[0047] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0048] The embodiment of the present invention discloses a method for preparing a three-dimensional solar interface evaporator, and a three-dimensional solar interface evaporator prepared according to the preparation method. The three-dimensional solar interface evaporator comprises a substrate and a photothermal layer arranged on the upper surface of a wavy top surface 1 of the substrate, wherein a water delivery channel 3 and a water storage layer 2 are arranged in the substrate, Figure 1 ;

[0049] One end of the water delivery channel 3 is connected to a water source, and the other end is connected to the wavy top surface 1, so as to deliver water to the wavy top surface 1 for interface evaporation;

[0050] The water storage layer 2 is connected to the wavy top surface 1 and is used to receive distilled water obtained by interface evaporation;

[0051] The photothermal layer is used to absorb solar energy and provide heat for interface evaporation.

[0052] The design improvements of the three-dimensional solar interface evaporator of the present invention mainly include three aspects: first, the shape of the top surface 1 is designed to be wavy, and on the basis of the two-dimensional photothermal layer, only the absorption light intensity of the two-dimensional photothermal layer is considered to select the optimal wave angle; second, on the basis of the three-dimensional solar interface evaporator, the influence of the side area size on the evaporation rate and the influence of the capillary lift height on the evaporation rate are considered to select the optimal evaporator height; third, a base film and photothermal layer that can be stably attached to the top surface 1 are designed.

[0053] 1. Design the wave angle of the wavy top surface 1

[0054] Step 1.1: Fit the photothermal properties of carbon materials under different illumination conditions, and select a light intensity of 0.1 kW m -2 , 0.2kW m -2 , 0.3kW m -2 , 0.4kW m -2 , 0.5kW m -2 , 0.8kW m -2 , 1kW m -2 Tests were conducted to record the evaporation rate of the photothermal layer on the top surface of the two-dimensional plane under each illumination, and then linear fitting was performed to obtain the relationship between the light intensity and the evaporation rate on the top surface of the two-dimensional plane;

[0055] Step 1.2: Determine the relationship between wave angle and receivable light intensity:

[0056] I=A[cos(x)cos(∈)+sin(∈)sin(x)cos(ε-δ)];

[0057] Where I is the acceptable sunlight intensity; A is the intensity of one sun, A = 1kW m -2 ; x is the solar altitude angle (ideally 0°); ∈ is the tilt angle of the evaporator two-dimensional unit; ε is the solar azimuth angle; δ is the azimuth angle of the two-dimensional unit;

[0058] Step 1.3, according to the relationship between the wave angle and the receivable light intensity, calculate the receivable light intensity at each wave angle, substitute it into the relationship between the light intensity and the evaporation rate of the two-dimensional plane top surface, and obtain the theoretical two-dimensional wave top surface evaporation rate corresponding to each wave angle;

[0059] Step 1.4: According to the theoretical two-dimensional wavy top surface evaporation rate corresponding to each angle, the projected area and the actual area are corrected to obtain the actual two-dimensional wavy top surface evaporation rate. By comparison, the wave angle corresponding to the largest actual two-dimensional wavy top surface evaporation rate is selected as the optimal wave angle.

[0060] 2. Design evaporator height

[0061] When a three-dimensional photothermal evaporator is used, the evaporator height factor is introduced. On the one hand, the side area used for evaporation will be increased. On the other hand, the heat loss caused by direct floating on the water surface and contact with the water body can be reduced. Since the top surface 1 is separated from the water body, it is often necessary to use capillary force to transfer water to the top surface 1. If the height is too low, it will cause excessive accumulation of water on the photothermal surface, resulting in serious heat loss. If the height is too high, it will lead to insufficient water supply and affect continuous evaporation. This embodiment combines the influence of these two aspects to design the optimal height of the evaporator. The specific method is as follows:

[0062] Step 2.1, collecting the side evaporation rates corresponding to different evaporator heights, performing linear fitting, and obtaining the relationship between the evaporator height and the side evaporation rate;

[0063] Step 2.2, determining the total theoretical evaporation rate, wherein the total theoretical evaporation rate = the evaporation rate of the wavy top surface + the evaporation rate of the side surface;

[0064] Step 2.3, determine the relationship between the capillary lift rate and the lift height:

[0065]

[0066]

[0067] Where d is the average pore size of the basement membrane; ΔP is the pressure difference of the water transport channel; μ is the dynamic viscosity of water (0.89 kPa s at 25 °C); h is the height of the water rise; σ is the surface tension between water and air (0.072 N m at 25 °C). -1 ); θ is the contact angle of water at the top (set to 0°); ρ is the density of water (997.05 kgm at 25°C). -3 ); g is the gravitational constant (9.81ms -2 );

[0068] Step 2.4, determine the fitting intersection point of the capillary water lift rate and the total theoretical evaporation rate, and the height corresponding to the intersection point is the optimal evaporator height.

[0069] 3. Design and preparation of the photothermal layer

[0070] In order to avoid secondary pollution and ensure the stable adhesion of the photothermal material on the surface of the evaporator, it is particularly important to design a photothermal layer that can be stably attached. The design and preparation steps of the photothermal layer in the embodiment of the present invention are as follows:

[0071] Step 3.1, prepare photothermal material: iron-based carbon nanotubes. The preparation method is to first use a solvothermal method to mix ferric chloride hexahydrate, terephthalic acid, and (N,N-methyleneformamide) DMF in a ratio of 4-5mmol:2-4mmol:1, and then heat at 110°C in a high-pressure reactor for 12 hours to obtain MIL(Fe)-101. Then, MIL(Fe)-101 and melamine are mixed in a mass ratio of 1:3-5 and then annealed at 800-1000°C to form an iron-based carbon nanotube photothermal material. Under the illumination of one sun, the photothermal effect of the prepared iron-based carbon nanotubes and pure carbon nanotubes was detected by an infrared thermal imager, and the results are as follows: Figure 2 As shown, the prepared iron-based carbon nanotubes have better photothermal effect;

[0072] Step 3.2: Ultrasonic crush an appropriate amount of iron-based carbon nanotubes, and spray the resulting suspension evenly on the wavy top surface of the base film of the three-dimensional solar interface evaporator by spraying, with a specific surface area of ​​5 mg / 16 cm 2 The basement membrane is a melamine sponge soaked in a polyvinyl alcohol (PVA) crosslinking agent, and the filamentous structure of the carbon nanotubes can be firmly attached to the surface of the basement membrane;

[0073] Step 3.3, spray 2% glutaraldehyde solution on the wavy top surface of the three-dimensional solar interface evaporator, so that the polyvinyl alcohol (PVA) on the melamine sponge and the hydroxyl groups on the carbon nanotubes are cross-linked through glutaraldehyde to further increase the stability.

[0074] Based on the above design, see Figure 3According to the principle shown, the evaporator base is first prepared according to the optimal wave angle and the optimal evaporator height, and then the attached photothermal layer is prepared on the wavy top surface 1 of the base to complete the design and preparation of the three-dimensional solar interface evaporator. The technical scheme and technical effects of the present invention are introduced in the following specific embodiments:

[0075] First, compared to a smooth surface where sunlight is absorbed only once and then reflected into the environment, the improvement to a wavy surface increases the ability to reflect and reabsorb light, such as Figure 4 As shown, the surface area receiving light can be increased, thereby improving the clean water evaporation rate per unit projection area of ​​the photothermal surface. The sunlight intensity received at each wave inclination angle is calculated. Taking 30°, 45°, 60°, 90°, and smooth plane (180°) as examples, the calculated received sunlight intensity is 0.259 kW m -2 , 0.383kW m -2 , 0.5kW m -2 , 0.707kW m -2 、1.00kWm -2 According to the above light intensity, the theoretical evaporation rates can be obtained as 0.59 kg m -2 , 0.78kg m -2 , 0.96kg m -2 1.27kg m -2 1.74kg m -2 Although the intensity of sunlight received decreases as the angle decreases, the light reabsorption rate and the area receiving light gradually increase as the angle decreases. Taking the surface area of ​​4×4cm as an example, the area that can receive sunlight at 30°, 45°, 60°, 90°, and 180° wave angles is 42.9cm respectively. 2 、38.4cm 2 、32.3cm 2 、22.62cm 2 、16cm 2 Therefore, the evaporation rate at each angle is corrected according to the actual area and the projected area. The actual evaporation rates at 30°, 45°, 60°, 90°, and 180° are 1.58 kg m -2 h -1 、1.87kgm -2 h -1 , 1.94kg m -2 h -1 1.77kg m -2 h -1 1.74kg m -2 h -1 It can be concluded that the film with a wave angle of 60° will have the maximum evaporation rate.

[0076] Next, the two-dimensional photothermal evaporator is limited by the theoretical evaporation rate, and the maximum evaporation rate will not exceed 1.6 kg m - 2 h -1 . When a three-dimensional photothermal evaporator is used, on the one hand, the side area used for evaporation will be increased, and on the other hand, the heat loss caused by direct floating on the water surface and contact with the water body can be reduced. Since the top surface 1 is separated from the water body, it is often necessary to use capillary force to transfer water to the top surface 1. If the height is too low, it will cause excessive accumulation of water on the photothermal surface, resulting in serious heat loss. If the height is too high, it will lead to insufficient water supply and affect continuous evaporation. Taking the melamine base film with a pore size of 200μm as an example, considering the influence of the above two aspects, the results show that 5.5cm is the optimal evaporator height.

[0077] Regarding the photothermal layer, the evaporator prepared by the cross-linking spraying method has strong stability. Carbon nanotubes are selected as the photothermal material. On the one hand, they have a high absorbance, and on the other hand, their filamentous structure can be tightly attached to the surface of the base film, and by cross-linking with PVA to form new chemical bonds, the structure is more stable.

[0078] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0079] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A three-dimensional solar interface evaporator, characterized in that: It comprises a base and a photothermal layer arranged on the upper surface of the wavy top surface of the base, wherein a water transmission channel and a water storage layer are arranged in the base; One end of the water delivery channel is connected to a water source, and the other end is connected to the wavy top surface, so as to deliver water to the wavy top surface for interface evaporation; The water storage layer is connected to the wavy top surface and stores water for interface evaporation treatment; The photothermal layer is used to absorb solar energy and provide heat for interface evaporation; A melamine sponge soaked in polyvinyl alcohol (PVA) is used as a base membrane, and iron-based carbon nanotubes are used as photothermal materials. A glutaraldehyde solution is sprayed on the wavy top surface, and the polyvinyl alcohol is cross-linked by glutaraldehyde, so that the base membrane and the carbon nanotubes sprayed on the wavy top surface are fixedly attached.

2. A method for preparing a three-dimensional solar interface evaporator using the three-dimensional solar interface evaporator according to claim 1, characterized in that: The following steps are involved: Determine the optimal wave angle: The top surface of the three-dimensional solar interface evaporator is set as a wavy top surface, and the optimal wave angle is selected according to the principle of maximum evaporation rate; Determine the optimal evaporator height: Combine the effect of capillary water lifting rate on water lifting height and the effect of evaporator height on lateral evaporation rate to select the optimal evaporator height; Prepare a three-dimensional solar interface evaporator: prepare a substrate of the three-dimensional solar interface evaporator according to the optimal wave angle and the optimal evaporator height; prepare a photothermal layer on the upper surface of the wavy top surface of the substrate to obtain a three-dimensional solar interface evaporator.

3. A method for preparing a three-dimensional solar interface evaporator according to claim 2, characterized in that: The specific steps of preparing the photothermal layer are: MIL(Fe)-101 and melamine are mixed evenly in a ratio of 1:3-5 and then annealed at a high temperature of 800-1000°C to form an iron-based carbon nanotube photothermal material; The iron-based carbon nanotube photothermal material is ultrasonically crushed, and the obtained suspension is evenly sprayed on the base film of the wavy top surface of the three-dimensional solar interface evaporator by a spraying method, wherein the base film is a melamine sponge soaked in a polyvinyl alcohol (PVA) crosslinking agent; The glutaraldehyde solution is sprayed on the wavy top surface of the three-dimensional solar interface evaporator, so that the polyvinyl alcohol on the melamine sponge and the hydroxyl groups on the iron-based carbon nanotube photothermal material are cross-linked through glutaraldehyde.

4. A method for preparing a three-dimensional solar interface evaporator according to claim 2, characterized in that: The method for selecting the optimal wave angle according to the principle of maximum evaporation rate is: The evaporation rate of the top surface of the two-dimensional plane under different illuminations was collected, and a linear fitting was performed to obtain the relationship between the light intensity and the evaporation rate of the top surface of the two-dimensional plane. Determine the relationship between wave angle and receivable light intensity; According to the relationship between the wave angle and the receivable light intensity, the receivable light intensity at each wave angle is calculated, and the relationship between the light intensity and the evaporation rate of the two-dimensional plane top surface is substituted into the relationship between the light intensity and the evaporation rate of the two-dimensional plane top surface to obtain the theoretical two-dimensional wave top surface evaporation rate corresponding to each wave angle; According to the theoretical two-dimensional wavy top surface evaporation rate corresponding to each angle, the projected area and the actual area are corrected to obtain the actual two-dimensional wavy top surface evaporation rate and select the optimal wave angle.

5. A method for preparing a three-dimensional solar interface evaporator according to claim 4, characterized in that: The relationship between the wave angle and the receivable light intensity is: I=A[cos(x)cos(∈)+sin(∈)sin(x)cos(ε-δ)]; Where I is the acceptable sunlight intensity; A is the intensity of one sun, A = 1kW m -2 ; x is the solar altitude angle; ∈ is the inclination angle of the evaporator two-dimensional unit; ε is the solar azimuth angle; δ is the azimuth angle of the two-dimensional unit.

6. The method for preparing a three-dimensional solar interface evaporator according to claim 2, characterized in that: The method for selecting the optimal evaporator height by combining the influence of the capillary water lifting rate on the water lifting height and the influence of the evaporator height on the side evaporation rate is: The side evaporation rates corresponding to different evaporator heights were collected and linear fitting was performed to obtain the relationship between the evaporator height and the side evaporation rate; Determine the total theoretical evaporation rate = evaporation rate of the wavy top surface + evaporation rate of the side surface; Determine the relationship between the capillary lift rate and the lift height; The fitting intersection point of the capillary water lift rate and the total theoretical evaporation rate is determined, and the height corresponding to the intersection point is the optimal evaporator height.

7. A method for preparing a three-dimensional solar interface evaporator according to claim 6, characterized in that: The relationship between the capillary lift rate and the lift height is: Among them, d is the average porosity of the basement membrane; ΔP is the pressure difference of the water transport channel; μ is the dynamic viscosity of water; h is the water rise height; σ is the surface tension between water and air; θ is the contact angle of the top water; ρ is the density of water; and g is the gravity constant.

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