Utilization of waste hot air of medical oxygen plant in solar desalination unit employing jet impingement air heater for water production

A_c

area of glass cover (m²)

A_b

area of base plate (m²)

A_j

heat transfer area of jet plate (m²)

A_pb

back area of absorber plate (m²)

C_g

specific heat of glass cover (J/kg K)

C_a

specific heat of air (J/kg K)

D_h1

hydraulic diameter of passage below jet plate (m)

D_h2

hydraulic diameter of passage above jet plate (m)

E

annual energy output

e

rib height (m)

h

heat transfer coefficient (W/m² K)

H

enthalpy (J/kg)

I

solar irradiance (W/m²)

k

thermal conductivity (W/m K)

m_a2

mass of air in the passage below absorber (kg)

m_g

mass of glass cover (kg)

m_j

mass of jet plate (kg)

m_p

mass of absorber plate (kg)

M_a

mass flow rate of air (kg/s)

M_fw

mass flow rate of condensate water (kg/s)

M_w

mass flow rate of water (kg/s)

M_mw

mass flow rate of makeup water (kg/s)

Q

heat transfer term

Re

Reynolds number

T_a2

mean temperature of air below jet plate

T_a2

mean temperature of air below absorber plate

T_b

temperature of base plate

T_fw

temperature of condensate water

T_g

temperature of glass cover

T_j

temperature of jet plate

T_mw

temperature of makeup water

T_p

temperature of absorber plate

T_s

surrounding (ambient temperature)

T_sky

temperature of sky

T_w

temperature of water

U_b

bottom loss coefficient

V_w

wind speed (m/s)

w

specific humidity (kg/kg of dry air)

Z₁

height of upper channel (m)

α

absorptivity

ε

emissivity

τ

time (s)

τ_g

transmissivity of glass

η_p

fin effectiveness

average CO₂ emission by coal/annum

ϕ_CO2

annual CO₂ mitigation

The importance of a medical oxygen plant in a medical facility is well evident because it provides life-saving oxygen. Many pressure-swing adsorption plants have been established for oxygen production, specially during the COVID-19 period, and those plants are still in operation for fulfilling O₂ requirements. These oxygen plants release hot air as waste from a compressor unit during their functioning period. The present work aims to utilize this hot air for potable water production through a humidification–dehumidification process. The simplicity of operation of the humidification–dehumidification desalination (HDD) process always appealed to researchers because of its utilization of low temperature thermal sources. However, in the case of sole solar assistance, effective production can be achieved over limited hours and this duration is not encouraging in moderate or cold climates.

Many configurations of an HDD unit have been developed by integrating low temperature thermal energy sources (Alghassab 2024) for heating process air, water, or both, which in due course, resulted in a higher yield. The augmentation of the humidification and dehumidification process has been proposed by Elminshawy et al. (2016) by utilizing geothermal energy with a solar HDD unit. The integrated unit provided the yield of 104 L/m² with an average gained output ratio (GOR) up to 1.58. Lawal et al. (2018) used a heat pump to fulfill the need of heating and cooling in the HDD system. The proposed integration with a heat pump resulted in increasing the GOR up to 8.8. Abdullah et al. (2018) proposed a hybrid unit by integrating an HDD process with a wick-type solar still. The effect of the packing material used in the humidifier was studied and a GOR of 5.4 was found. Shojaei et al. (2020) integrated a solar humidifier in an HDD unit which was also configured with a heat pump. The results of these experiments revealed a maximum yield of 1 kg/h and a GOR of 2.36.

Chen et al. (2020) combined an HDD unit with an evaporative air-cooling unit to achieve thermal comfort and yield simultaneously. The proposed combination resulted in 25–125 L/h water and a GOR in the range 1.6–2.5. Qasem et al. (2020) coupled an HDD unit with an absorption refrigeration unit for water production in large quantities. A yield rate of 1.145 L/h was obtained by the proposed configuration with a GOR of 4.54. Jabari et al. (2020) used a solar Stirling engine for power and heat generation. It also ran an HDD system for water production. Sachdev et al. (2020) proposed to couple an HDD unit with a wind tower for simultaneous production of water and thermal comfort. The proposed configuration resulted in 4.5–5 kg yield per day. Kabeel et al. (2022) utilized cold air of an HDD unit for enhancing the performance of the air-conditioner. The proposed unit resulted in a daily yield of 36.49 L and an approximate 60.9% rise in the GOR.

Alrbai et al. (2022) utilized a fogging nozzle for augmenting the humidification process in an HDD unit which resulted in a maximum GOR of 3.4. Santosh et al. (2022) performed experiments on an HDD unit having an energy supply from waste heat at a temperature range of 45–70°C. The maximum yield of 1.398 mL/h was found due to the proposed modifications. Khass et al. (2022) analyzed an HDD system with multi stage humidification and a 151.2 kg/h yield was reported with a GOR of 1.995. Abdel Dayem & Al Zahrani (2022) studied the effect of packing materials in humidification by using a wet cooling tower as the humidifier. The study reported a GOR of 4.3 and a yield of 25 L/d. Welepe et al. (2022) presented a humidifying collector for minimizing the amount of liquid and used it with an HDD unit. The performance of the proposed system was improved by 1.3–32.2 times compared to a conventional system and a yield of 2.923 kg/h was reported.

Shaikh & Ismail (2023) utilized a multistage dehumidifier to reduce the condensing area and cost. The proposed configuration augmented the yield by 6.24% and the output ratio improved by 6.75%. Shamet et al. (2023) utilized heat of a diesel generator which was otherwise released as waste. The analysis of four different configurations of an HDD unit resulted in a maximum GOR of 3.2. Abedi et al. (2023) proposed to use a solar chimney with an HDD system which can provide 600 L of water per day. Easa et al. (2023) examined a high-speed rotary humidifier in an HDD unit. The maximum yield of about 17 kg was obtained at slot bore and spray speed of 0.001 m and 1,200 rpm, respectively. Zarei & Adibi (2023) used rushing packing in the humidifier of the HDD unit and achieved a daily yield of 312 L.

It is clear from the available literature that numerous configurations of HDD units have been developed for the augmentation of yield. In continuation, the present work proposes a layout of the HDD unit for utilizing waste hot air of medical O₂ plants. This is a novel approach as (i) waste heat of medical O₂ plants has not been utilized in HDD systems so far; (ii) in contrast to conventional solar HDD units, this approach enables the unit for nocturnal yield (or duration when solar energy is ineffective); (iii) the proposed system is configured with a solar air heater having jet plate (SAHJP) with novel ribs and a packed bed humidifier; and (iv) transient behavior of the system was analyzed. Additionally, a moderate climate (Raipur, India) was used for performance evaluation instead of a favorable hot summer climate. For comparative analysis of the proposed unit (HDD-O₂) with a conventional (HDD-C) unit, a mathematical model was developed and validated with the available experimental results. Performance parameters such as yield, GOR, humidifier dimensionless parameters, etc. have been evaluated with the help of a computer simulation program for variable inputs. Finally, economic competitiveness of the proposed system was also assessed.

In the proposed system, hot air coming from O₂ plants was divided into two routes to incorporate the additional advantage of water heating. In the first route 1-2-3, air follows the states to the humidifier and the dehumidifier for water production, while the second route 6-7 serves to preheat water before supplying to the humidifier. The flow of hot air in route 6-7 depends on the availability of hot air on the basis of the size or number of O₂ plants. At the same time, feed water from the storage tank follows the state 4-5 through the humidifier. Valves v1 and v2 regulate the flow of air and v4 ensures the necessary water flow in the humidifier through a pump. The brine at state 5 can be regulated by v8 for recirculating in the process or releasing out. A fan was also provided at the outlet of the dehumidifier for maintaining the proper flow of air in the system. The feed water availability and salinity can be ensured by attachments provided for supplying makeup water and removing the brine.

In the second situation of functioning, hot air is available from the SAHJP and the O₂ plant is not operational. This situation may represent the limited diurnal hours of a day when solar thermal energy is effectively available for heating ambient air. The O₂ plants can operate in any hours of the day as per the need; however, for water production, it is beneficial if the O₂ plants are scheduled to operate during the hours of ineffective solar energy. In this situation, the system follows a conventional process of humidification and dehumidification, and is named as HDD-C for further discussion. Here, ambient air follows the states 8-9-2-3 for yield formation and water from the storage tank follows the state 4-5. In this mode of operation, valve v5 regulates the flow of air coming from the SAHJP and valve v6 will be completely closed to avoid any diversion of air. Valves v1 and v2 will also be closed as the O₂ plant is inoperative, and thus, it does not supply hot air. A blower was provided so that ambient air can be supplied to the SAHJP for heating. Similar to HDD-O₂, brine recirculation can be regulated in HDD-C by v8.

In the third situation of operation, hot air from both O₂ plants and air SAHJP is available; however, hot air from the SAHJP will augment the performance of the proposed unit through humidification–dehumidification when its temperature is higher than the temperature of air coming from the O₂ plants. This situation may occur for a very limited number of hours, depending on the location when solar thermal energy is at its peak, and this duration will be minimum in moderate or cold climates. In these hours, air from the SAHJP will enter the humidifier for following the HDD process while hot air from the O₂ plants will serve to preheat the feed water in the storage tank. In the remaining hours, when the temperature of air supplied by the O₂ plants is higher, valves v5 and v6 will be closed to avoid any mixing with lower temperature air of the SAHJP; otherwise, moisture carrying capacity (as it depends on air temperature) of air will be affected. Thus, the system will again start to operate as in the first situation described previously. As the system mostly follows the first or second situation, except in the few hours of peak solar energy, the performance under the third situation has not been evaluated in this work.

Various correlations for calculating the heat transfer terms and thermophysical properties of process air are listed in Table 2. 

Table 2

(a) Equations involved in the mathematical model related to heat transfer and (b) equations of the thermal properties of air (Yamali & Solmus 2006)

S.N. .	Heat transfer term .	Equation .
(a)
1
2
3
4
5
6
7
8
9
10
11
.	H.T.C .	Equation .
12
13
14
15
16
17
18
19
20
21
(b)
Property .	Equation .	.
Thermal conductivity (k)
Thermal diffusivity (α)
Kinematic viscosity (υ)
Enthalpy (h)	2.82 × 10−5T4 − 0.00106T3 + 0.0615T2 + 1.32 T + 10.5
Specific humidity (w)	7.7 × 10−7T3 − 1.9510−5T2 + 0.00071 T + 0.02

S.N. .	Heat transfer term .	Equation .
(a)
1
2
3
4
5
6
7
8
9
10
11
.	H.T.C .	Equation .
12
13
14
15
16
17
18
19
20
21
(b)
Property .	Equation .	.
Thermal conductivity (k)
Thermal diffusivity (α)
Kinematic viscosity (υ)
Enthalpy (h)	2.82 × 10−5T4 − 0.00106T3 + 0.0615T2 + 1.32 T + 10.5
Specific humidity (w)	7.7 × 10−7T3 − 1.9510−5T2 + 0.00071 T + 0.02

View Large

The reason for this can be explained with yield variations, which is the highest in the air flow range of 0.03–0.04 kg/s. An approximate 91% improvement in the GOR of HDD-O₂ and 120% improvement in the GOR of HDD-C was found. HDD-O₂ resulted in 53.1% higher GOR in comparison to HDD-C when the air flow rate was increased to the 0.03–0.04 kg/s range, and overall, HDD-O₂ resulted in about 50.2% higher GOR. The increase in the mass flow rate of air initially allowed to carry higher quantity of moisture which ultimately turns to higher yield. Therefore, the GOR was also found to increase but a further higher rate does not contribute very much in favor of the GOR.

Figure 7(b) illustrates the variation in the GOR and yield of HDD-O₂ and HDD-C systems with water temperature fed to the humidifier. It is clear that with an increase in water temperature, the GOR also increased continuously. For the considered temperature range, the GOR of HDD-O₂ increased by 20.6% and this gain for HDD-C is 18.2%. At the same time, the GOR of HDD-O₂ is about 29% higher than the GOR of HDD-C at 45°C whereas, on average, it is 17% more in the considered temperature range. Due to the dependency of the GOR on yield, this variation is very similar to that of yield. The HDD-O₂ system resulted in about 49% higher yield than HDD-C. This behavior is evident as air interacting with water at higher temperatures itself gains temperature, which also increases the moisture carrying capacity of air. The air carrying more moisture can provide a higher yield and, therefore, an improvement in the GOR was achieved.

Figure 7(c) reveals the dependency of yield and GOR with the mass flow rate of feed input to the humidifier. A swift gain in the yield of HDD-O₂ and HDD-C can be observed till a mass flow rate of 0.03 kg/s, which is 95 and 83%, respectively. Thereafter, an increase in feed was found to be marginally beneficial in augmenting the yield as only about 5% gain was found for HDD-O₂ and HDD-C. On an overall basis, HDD-O₂ resulted in about 48% higher yield compared to HDD-C. This increase in yield was also responsible for the thermal energy utilized for water production and, therefore, led to an increase in the GOR in a very similar manner. The increase in GOR for HDD-O₂ was about 98% and the same for HDD-C was about 88% over the considered flow rate of 0.01–0.06 kg/s. The observed behavior is related to the amount of moisture available in process air, which increased rapidly with the amount of feed water available up to 0.03 kg/s. Since air at particular temperatures has limited moisture carrying capacity, further available feed cannot contribute very much in increasing moisture, and this is evident after the flow rate of 0.03 kg/s.

The effect of supplying more amounts of cooling water in HDD-C and HDD-O₂ for dehumidification is illustrated in Figure 7(d). It is clear that both yield and GOR upsurge with the rate of cooling water. However, this increase is mainly up to a flow rate of 0.035 kg/s as the yield of HDD-O₂ increased about 400% up to this flow rate. Thereafter, with further increase in the flow rate up to 0.05 kg/s, only 7% improvement was observed. Due to the dependency of the GOR on yield, it also followed a very similar variation. The GOR of HDD-O₂ was found to be about 53% (on average) higher in comparison to that of HDD-C. The results made clear that higher amounts of cooling water augmented the yield and GOR by carrying the heat of condensation. Figure 7(e) depicts the variation of yield and GOR with the temperature of cooling water supplied for condensing water vapor. Both the performance parameters were found to have higher values with lower temperature of water as lower temperature augments the heat transfer associated with condensation. On average, the yield of HDD-O₂ was found to be 37% higher over HDD-C for the considered temperature variation between 17 and 25°C and, at the same time, this improvement was 25% for the GOR.

Considering the recommended values of (average CO₂ emission by coal/annum) as 2 kg/kWh and enviro-economic cost (P_CO2) as 14.5 $/t, CO₂ mitigation from the HDD-O₂ unit is 3.2 t/annum, whereas the cost involved is 46.4 $/annum.

Table 5 compares the performance of the HDD-O₂ unit with the existing systems working on HDD and other methods, on the basis of novelty in method, yield, cost of water, and GOR that are measures of energy efficiency. It is clear that yield and GOR of the HDD-O₂ unit is very attractive compared to the existing units and a novel approach of the HDD-O₂ unit based on waste heat of medical O₂ plants and SAHJP is responsible for the same. Although the cost is higher in comparison to some approaches due to additional expenses for the utilization of waste heat, it is still competitive due to the achieved gain in yield and GOR. Overall, Table 5 emphasizes the suitability of HDD-O₂ as an enhancement approach for the HDD process.

The proposed HDD-O₂ system has been analyzed in this work in comparison with a conventional HDD-C unit. The performance comparison was performed with the help of mathematical simulation. The results and discussion section elaborated on the gain achieved by the HDD-O₂ unit for different operating parameters. The key findings are summarized as follows:

• Hot air from O₂ plants augmented the productivity of the HDD unit.

• The mass flow rate of air in the range 0.03–0.04 kg/s is the most beneficial with the GOR close to 1.2.

• The feeding rate of 0.03 kg/s of brackish water provided a daily yield around 8.1 kg/d.

• The cooling water rate of 0.05 kg/s can raise the yield to around 10 kg/d.

• A high temperature of feed water and a low temperature of cooling water both resulted in a continuous rise in yield and GOR.

• On average, HDD-O₂ resulted in about 43% higher yield compared to HDD-C.

• The cost of water from HDD-O₂ is 0.04 $ and the payback period is 1.73 years.

• The annual CO₂ mitigation from the HDD-O₂ unit is 3.2 t and the enviro-economic cost is 46.4 $/annum.

To further enhance the performance and viability of an HDD-O₂ unit, it is recommended that optimization of the system components can be done, modification can be included to improve the condensation-evaporation process, and CFD simulation can serve to study the flow of the working fluid though the system.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.