1. Introduction
An elegant study by Liang et.al. induces the belief that geothermal energy, if harnessed properly, can provide energy solutions in the case of textile industries needing both thermal and electrical energy. Although occasionally of low grade, it bears ample scope to be converted to high-grade green energy with the help of a ground source heat pump (GSHP), comprising a ground heat exchanger, heat pump, and the heating system, for example, a solar heater. This energy-saving technology uses working fluids that are abundantly available on Earth. Water is most suitable for this purpose. The working fluid carries the ground heat via underground tubes. The depth of the tubes is determined by the need for ground temperature, which increases by 250 °C per kilometer depth to 50 °C per kilometer beyond a depth of 3–4 km from the Earth’s surface. The tube material must be highly durable, corrosion resistant, strong, and flexible. The transfer takes place through this tube and, hence, it is an important aspect of the GHE system. There should be a good backfill material that separates the tube from the soil; one can use cement or graphite [
1].
The textile industries play an important role in a country’s economic activities due to the fact that this employment-intensive industrial sector contributes significantly to the GDPs of several countries in terms of increasing the rate of earning by exporting produce to other countries; for example, the amount of such earnings of Indian textile industries exceeds 15% of the country’s total earnings from export [
1,
2].
Due to being highly water-consuming, the textile industry is apt to generate huge quantities of wastewater [
3,
4]. As the discharged wastewater exhibits a high pH value, contains a high concentration of suspended solids, chlorides, nitrates, metals like manganese, sodium, lead, copper, chromium, and iron with alarmingly high Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) values, it poses several threats, including a risk of endangerment. Therefore, there are calls for the evolution of efficient technological means to treat the wastewater discharged by the textile industries so that the potential environmental pollution can be minimized [
4].
To mitigate the potential environmental degradation due to the release of toxic effluents, various wastewater treatment technologies are seen to have been employed by various textile industries. Adsorption technology seems to be an effective option for textile industry wastewater treatment; however, it is cost prohibitive. In fact, Chitosan (CS) may be considered a potential adsorbent material for eliminating toxic pollutants from wastewater discharged by the textile industries [
5]. This capability of Chitosan is attributed to its amino and hydroxyl groups, its physico-chemical properties, viz. chemical stability, high reactivity, and its selectivity toward pollutants containing dyes and heavy metal ions. Moreover, Chitosan is non-toxic and biodegradable, and its production cost is low [
5]. The other employable methods for the treatment of textile industry wastewater include ion exchange, membrane filtration, (Reverse Osmosis (RO), Ultrafiltration (UF), and nanofiltration (NF); again, ozonation, evaporation (multi-effect evaporation, mechanical vapor compression, and direct contact evaporation), electrochemical oxidation, flocculation phytoremediation, photochemical, and crystallization are other effective technologies being implemented at various places [
6,
7,
8,
9,
10].
Membrane Distillation (MD), used for treating textile industry wastewater, is essentially a process of thermal effluent treatment; in this process, the effluent is heated to evaporate its water content and, thus, concentrates the remaining fluid. Generally, the separation effect is based on the hydrophobicity of the polymer membrane material, which creates a barrier to the effluent in the liquid phase whilst allowing materials in the vapor phase to pass through the membrane’s pores. This non-pressure Membrane Distillation has attracted significant interest in textile wastewater treatment [
7,
8]; however, the major challenges that hinder the commercial application of MD are the fouling of membranes [
9], flux decline [
10], and high energy consumption [
11,
12]. The MD module consists of an Evaporator, Evaporation–Condensation stages, and a Condenser; the alternate evaporation and condensation stages are shown in
Figure 1. Each stage recovers the heat used to evaporate vapor from the effluent. Distillate is produced in each evaporation–condensation stage. Seemingly, the repetitive evaporation and condensation stages for yielding water of an acceptable quality necessitates the expenditure of a high amount of thermal energy [
13,
14]. The high energy requirement affects its process economy. The energy sourced by conventional fuel poses other environmental pollution threats, viz. degradation of air quality by emission of greenhouse gases and solid particulate matter of various sizes. Given such techno-economic constraints in harnessing the accruable benefit from such an efficient wastewater treatment process, it appears prudent to probe into the feasibility of using solar energy to manage the required thermal load for a membrane distillation process.
While fixing a specific technology option, it is to be kept in mind that apart from the thermal load, there is also a definite demand for electrical load and, quite often, electrical energy is spent to supply the thermal energy due to its direct physical relation with thermal energy. Hence, it appears to be worth taking stock of the present technological scenario amidst the supply need for thermal and electrical energy.
Our proposition is a case of a distributed generation system where hybrid solar thermal energy in association with MD is advocated. Moreover, the textile industries need electric power for many operations. Since electrical and thermal power are intimately linked, an impactful study on the usability of a combined heat power system is reported in the literature [
15]. Usually, a compound heat power (CHP) system comprises a gas turbine and a heat recovery boiler. Optimization of the performance of a small-scale CHP is carried out by linear modeling, where a Grasshopper optimization algorithm was used in MATLAB software R2023a [
15].
It is known that CO
2 emissions, due to the use of fossil fuels for meeting energy demands in human activities, impair public health in a great way. This directly impacts a state’s health expenditure, education expenditure, and gross domestic product (GDP). Therefore, using renewable energy sources as a replacement for fossil fuel has been a global drive. An elegant study on interlinking renewable energy sources for various human activities with health expenditure, education expenditure, emission of greenhouse gases, and GDP is reported elsewhere. Following descriptive statistics using unit root tests and FMLOS and DOL tests demonstrated that renewable energy reduces health expenditure with the concurrent enhancement of GDP in no less than five South Asian countries [
16]. Tariffs for electrical and thermal energy often affect a CHP’s size and compatibility. The integration of CHP with electrical heat pumps (EHP) is recommended for cases of thermal load exceeding the delivery capacity of a CHP. It is advocated that such a cogeneration system provides an economical solution to the problem of minimizing energy costs [
17]. In a recent study, both EHP and CHP were integrated with the understanding that both sources can meet the thermal energy demand in full and that one of higher efficiency would serve as the primary source of thermal energy, with the other remaining a backup [
18]. A cost analysis revealed that using standalone EHP as the primary source is an economically cheaper option than CHP, whereas the integrated version is more reliable on account of using two different energy sources, i.e., electricity and natural gas. Suppose the EHP fails to supply the entire thermal energy demand due to an interruption in electric supply or for some other unforeseen reason, the CHP will help supply the needed thermal energy [
18].
It may be assumed that the relationship between the heat and electricity in a CHP is nonlinear. The scheduling problem in a proposed microgrid system comprising CHP, renewable energy sources, microturbine, TES facility, and fuel cell unit is taken as a mixed integer nonlinear problem, which is solved by employing the Crow search algorithm, simulated by MATLAB [
19]. The provision for energy storage systems envisaged better energy management to meet the thermal and electrical energy requirements [
20]. It is known that the availability of solar energy is much less uncertain than other forms of renewable energy. Hence, it can be a better option for microgrid applications in any location. It is important to note that the availability of solar radiation in terms of Global Horizontal Irradiance (GHI) and Direct Normal Irradiance (DNI) varies with geographic location, seasons, and climate; however, the data of the average annual GHI or DNI can be obtained from Meteonorm software Version 8.2.0 for all locations across the globe. Although many industries use different systems to cater to the electrical and thermal needs of human activity (domestic/industry), a simultaneous supply of thermal and electrical energy from a single system using a single fuel has become an attractive proposition in recent times. A steam thermal plant where steam caters to thermal energy needs is analog but with certain practical limitations. Considering the benefits of an integrated generation system in cost and efficiency, a novel microgrid system composed of two CHP units, one wind turbine, and a microturbine unit, with a fuel cell, battery storage, and TES, is conceived [
19]. It is reported that the novel microgrid with the capacity to exchange power with the main grid is quite profitable. However, the microgrid alone is also a feasible alternative [
19].
Therefore, an attempt is made in the present investigation to study the effect of integrating solar thermal technology with a membrane distillation system on the overall energy consumption for the effective treatment of wastewater discharged by textile industries. Although process economy happens to be an important determinant of technology options for industries [
14,
15,
16,
17,
18,
19,
20,
21,
22], only limited techno-economic reports on solar thermal technology integrated with membrane distillation systems for wastewater treatment in textile industries are available in the literature. Hence, in the present investigation, an attempt is also made to study the techno-economics of wastewater treatment technology in the textile industry, which uses solar thermal energy to meet the thermal load of a typical membrane distillation system.
To invoke the plan for an integrated generation system in industrial wastewater treatment is a new concept. Attempting to obtain an alternative solution against polluting fossil fuel as a traditional process to meet the thermal load of evaporation techniques aimed to be used for industrial wastewater treatment is certainly additive to the existing technological options. Following the emerging trend of using integrated generation systems [
15], the proposed wastewater technique of a solar thermal system coupled with an MD system is novel. The novelty of the present research lies in implementing an integrated generation system that can meet the energy load of textile industries. This technology of wastewater treatment in textile industries is new in character. Most textile industries use fossil fuels to meet their energy needs. An attempt is made to study the effect of concentrating solar thermal technology in a combination membrane distillation system for the treatment of wastewater discharged from textile treatments on the overall energy consumption for the effective treatment of wastewater discharged by textile industries. Using the state-of-the-art design of a parabolic concentrator to couple with the MD technique for wastewater treatment in water-intensive textile industries is sure to add new knowledge in the industrial wastewater treatment technology domain. Establishing the techno-economic feasibility and providing the pragmatic recommendation of incentivizing the user industries to promote this hybrid technology is not only eco-conserving but also financially rewarding for any country in terms of reducing its health expenditures and enhancing gross domestic product (GDP). The advocacy of the present paper with the target of insuring socio-economic benefit is highly value additive and, hence, emerges as a completely new solution to the concerned challenges in wastewater treatment in textile industries.
3. Results and Discussion
Using the approach narrated in the preceding section, the performance of a solar thermal-based wastewater treatment is assessed in terms of useful energy delivered and the concerned measures for insurance of economic attractiveness.
In order to determine the performance of the solar operated system, it is required to calculate the area of solar collector field required to meet the energy demand of wastewater treatment in textile units. This has been estimated by using Equation (1) for the design values of insolation ranging from 500 W/m2 to 900 W/m2.
Further, the performance of the solar system (in terms of useful thermal energy delivery and solar fraction) corresponding to each value of
GHId are also estimated. These estimates are made on the basis of the hourly availability of solar radiation at the location of the selected textile unit (obtained by the use of Meteonorm 8.1 software). The results of the above estimation for the CPC-based system are presented in
Table 3.
It appears from the results in
Table 3 that the performance of the solar-based system is better for the lower regime values of design insolation. This is because a lower design value yields a higher collector area; for a specific GHI value, the higher the collector area is, the higher the magnitude of deliverable energy will be (vide Equation (1)) As such, estimation with a larger solar collector area is also tried within the selected range of
GHId (500–900 W/m
2); it is needless to say that the use of a larger collector area than is needed to fulfill the requirement of thermal energy to run the system successfully tends to involve a higher investment cost. Based on deliverable annual energy by the solar system and the corresponding values of solar fraction, it may be inferred that in order to achieve the maximum relative benefit in terms of derivable thermal energy from a solar-based system, the design value of insolation ought to be kept in the lower range. However, this assessment is not the only deciding factor for the financial attractiveness of a solar-based system.
In view of the above, a levelized cost of the thermal energy delivered for each value of the design
GHId has been evaluated and is furnished in
Figure 3. For better comprehension, the details for obtaining the levelized cost of the thermal energy are presented in
Table 4.
Corroborating the information derivable from
Figure 3, the results in
Table 4 show that the LCTE attains its lowest value for a design value of GHI as 700 W/m
2. Thus, it is proved from the calculations in the present study that the mere minimization of the
GHId value, hence securing the highest collector area, does not guarantee the maximization of economic benefit. It appears that there is an optimum value of this parameter ~700 W/m
2 which, when taken into consideration to design the solar-operated wastewater treatment system, means the accrual of maximum economic denomination is inevitable.
It is not out of context to mention that the overall aim of the study has been to analyze the feasibility of using the conceived hybrid technology in the form of a compound parabolic concentrator (CPC)-based concentrating solar thermal (CST) system with an integrated MD system as a meaningful solution to the problem of treating toxic effluent discharged from the textile industries, thereby giving a check to environment degradation. It is to be emphasized that the meaningfulness of a technological solution must be seen as employing technology which is ecologically sustainable and, at the same cost, far more rewarding compared to the contemporary options.
From the foregoing results and discussions, the proposed system is not only technologically competent but also aids in environmental protection at a high economic denomination. On this ground, the authors wish to infer that the present study has successfully found a suitable and cost-effective technology configuration for treating the wastewater discharged by fabric processing textile industries.
The values of other measures of financial performance such as the discounted payback period and internal rate of return for an investment in a solar-operated wastewater system are presented in
Table 5. The estimations have been made against each type of fuel that could be saved by installing an operated wastewater treatment system. It is clear from the results of
Table 5 that the solar-based system is an economically viable option for all the replaceable fuels except coal, due principally to its lower price in countries like India. This, however, may not be true for every country as there are countries where the cost of coal as a fuel is significantly higher, and, hence, is uncompetitive with renewable energy. It is also important to mention that the use of coal adds to the health expenditure of a country with a concurrent diminution in GDP. Moreover, environmental degradation and the cost of preventing pollution cannot be underrated. In general, fuels like FO, LDO, and natural gas are normally used in the textile sector as attractive means due to the good range of
IRR (19% to 23.5%). The result of replacing the following fuel by the use of CPC-based CST is also presented in
Table 5.