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. 2021 Apr 16;145:111026. doi: 10.1016/j.rser.2021.111026

Air-source heat pump heating based water vapor compression for localized steam sterilization applications during the COVID-19 pandemic

Hongzhi Yan 1, Bin Hu 1, Ruzhu Wang 1,∗
PMCID: PMC9760490  PMID: 36569373

Abstract

Sterilization is of great importance to prevent the spread and resurgence of the COVID-19, yet sterilization methods are all energy intensive in general. Steam sterilization is easily maintained and can be applied in various scenarios with less residual pollution. However, the current steam generator (so called boiler) has brought many energy and environmental concerns. With the investigation on steam sterilization's features, this study proposed a clean and flexible steam generation system with the air source heat pump and water vapor compressor, and the system can be further simplified through the combination with district heating pipeline. The critical design parameters and simulated performance of the system are evaluated and optimized through a MATLAB simulation, and a prototype was built with experimental performance assessing. The results show the system has an average boiler efficiency of over 170% when the ambient temperature varies from 5 °C to 35 °C and the temperature of outlet steam is above 110 °C, and has the best economic performance when the operating period is above 3 years. Furthermore, the air-source heat pump boiler system is proved to effectively respond to the surging sterilization demands during the peaks of the COVID-19 pandemic and is well consistent with the UN Sustainable Development Goals.

Keywords: Air source heat pump, Water vapor compressor, Steam sterilization, COVID-19 pandemic

Nomenclature

Symbols

Q

heating capacity (kW)

P

power consumption (kW)

W

energy consumption (kW)

h

specific enthalpy (kJ·kg−1)

m

mass flow rate (kg·s−1)

Abbreviations

evap

evaporator side

cond

condenser side

L

low stage of cascade cycle

H

high stage of cascade cycle

r

refrigerant

DH

district heating

WVC

water vapor compressor

is

isentropic

i

inlet

o

outlet

HTC

high-temperature cycle

COP

coefficient of performance

FR

Flash ratio

ηis

isentropic efficiency of compressor

ηv

volumetric efficiency of compressor

V

volume of compressor(m3·min−1)

l

liquid

v

vapor

comp

compressor

d

discharge state

s

isentropic state

HP

heat pump

HTHP

high-temperature heat pump

CHX

cascaded heat exchanger

ASHPB

air-source heat pump boiler

LTC

low-temperature cycle

1. Introduction

The COVID-19 has brought severe challenges to all aspects of society and global development [1,2], and the second wave of COVID-19 has already occurs in some regions [3] and could be even worse according to some predictions [4]. It has been found that coronavirus could be infectious on inanimate surfaces for up to 9 days [5]. Therefore, daily sterilization in public places is essential to prevent the spread of the virus [6] and the resurgence in contagion [7]. In addition, a large volume of medical waste is produced that poses a significant risk to medical staff, patients and the public [8]. This medical waste must be cleaned and sterilized to prevent further spread of the virus.

The existing sterilization methods can be mainly divided into incineration treatment, chemical treatment and thermal treatment [9]. The detailed advantages and limitations are summarized in Supplementary Note 1. Compared to disinfectant [10] and infrared, high-temperature steam can evenly dissipate heat with lower secondary pollution (over-dose of ozone and other disinfectants are easy to cause secondary pollution) [11]. It was also found that the face mask sterilized by high-temperature steam can achieve similar performance to the new face mask [11]. For textiles sterilization, high-temperature steam circulation is officially specially appointed [13] and will be in high demand for hospitals, apartments, hotels and other public places [14]. Meanwhile, during the outbreak stage of the epidemic, chemical sterilization material is often in short supply while the amount of waste is large. In that case, the easily maintained high-temperature steam sterilization is considered almost the only choice [11].

However, the existing fossil-based steam generators have a negative impact on the local environment [15] and are faced with increasingly severe restrictions [16]. Meanwhile, they are extremely dependent on the supply of fossil fuels, resulting in a weak response to emergency situations.

Utilizing solar heat or electricity to generate steam is more environmentally friendly and convenient [17,18]. Recently, Lin Zhao et al. proposed a passive high-temperature steam generator for medical sterilization with a successful sterilization cycle [19], yet this technology needs further verification. Meanwhile, solar energy is unavailable on cloudy days and nights. Electricity is more stable and flexible for steam generation such as the electric boiler, but electricity is secondary energy and thus makes the operation cost of electric boilers higher than fossil fuel-fired boilers. Heat pump is considered to be a more efficient and economical electricity-to-heat conversion device [20], which can absorb the free heat from ambient air or other sources and thus has a higher efficiency (coefficient of performance or COP) over 100% [21]. So far, the heat pump systems are widely for hot-water supply (mainly <60 °C) with ambient air as the heat source, and there are even some steam-generation heat pumps [22,23]. However, the steam generation heat pumps normally employ industrial waste heat source with a temperature more than 50 °C [24], making it hard to be a localized steam generation method.

Another possible method to generate high-temperature steam is mechanical vapor compression by water vapor compressor [25,26]. However, the water vapor compression faces the following challenges and limitations such as high discharge temperature and sub-atmospheric inlet pressure [25]. The water injection can reduce the discharge temperature and the power consumption of water vapor compressor [27] and the twin-screw water vapor compressor could promote the steam temperature up to 160 °C with the isentropic efficiency about 0.6 [22].

In conclusion, considering the heating temperature of the existing air source heat pump less than 90 °C, the water vapor compressor could further upgrade the heating temperature. Based on high efficiency of cascade heat pump, which could preheat water to about 80 °C and flashing into vapor, the vapor could then be compressed into desired high pressure steam (temperature ~120–150 °C), the air-source heat pump boiler for localized steam generation was proposed as an alternative to the fossil fuel-fired boiler [28], which could be used for industrial process heating and space heating and etc.

Definitely the air-source heat pump boiler could be used for steam sterilization where the required heating capacity and temperature are much lower than that required for industry. Moreover, for those regions with assess to district heating, for which 80 °C hot water could be easily obtained, the air-source cascade heat pump could be not necessary. By flashing process to yield vapor connected with a very simple vapor compression, the steam for sterilization could be easily created localized.

2. Description and evaluation of the steam sterilization process

In essence, steam is an excellent heat carrier with a huge latent heat released in the sterilization process and then condenses into water. With the increasing demand for steam sterilization, it is necessary to analyze the application status and potential of steam sterilization.

2.1. Treatment of steam sterilization

In fact, the COVID-19 will be inactivated after the treatment at 56 °C for 30 min [29]. Therefore, the water vapor above 100 °C will have the indubitable sterilization effect if the interaction time is long enough. However, there is an enormous variation in materials and other requirements, which results in the difference of steam disinfection processes. The following table summarizes the common steam sterilization process (see Table 1 ).

Table 1.

The summary of steam sterilization treatment.

Usage Treatment Supplementary notes
Medical waste 134 °C, temperature fluctuation <3 °C [30]
120 °C(90min) for hazardous waste [31]
Combined with microwave technique, the steam temperature could be low to 93 °C [9]		 The treatment for 30min is enough, and even 15–20min is effective. For the most unfavorable situation, treatment time at 134 °C should not be less than 45min [32]
Mask ＜100 °C (10min) [33]
121 °C(15min) [12]
65 °C(30min) [34]		 The mask will deform and become sticky when it is treated at 134 °C [34]. Indirect heat exchange is suggested to avoid the reaction between the fiber and condensate water [33].
Textiles ＞100 °C, circulating steam for 15min [13,36] For items direct contact with human body, steam has absolute advantages over chemical disinfectant [37].
Tableware ＞100 °C, circulating steam for 15min [38]
Public area ＞100 °C, steam [39] 8 cycles in push-pull mode (0.2 ± 0.02 m/s)can reach 99% sterilization rate [40].
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To summarize, there are a few noteworthy features for the steam serialization:

  • (1)

    For saturated steam, there is a one-to-one relationship between the temperature and pressure value. In general, the processing time can be further shortened with the treated temperature increasing. However, excessive sterilization temperature may also cause damage to the material.

  • (2)

    The superheated steam sterilization is suitable for the some specific sterilization process such as the production of traditional Chinese medicine [41]. It is also found that the steam quality has a great influence on the germicidal efficacy [42], and it is necessary to control all links of the steam flow, such as steam source [43] and steam transmission pipeline. In order to prevent the steam from condensing in the pipeline, the generated steam should have a certain superheated degree.

Based on the above considerations, the steam generator for sterilization should be of flexible regulation on the output temperature.

2.2. The capacity and potential of steam sterilization

The treatments of steam sterilization vary in different application scenarios. For example, the medical waste sterilization is usually carried out through a sterilization pot, and the steam consumption is closely related to the number, volume, filling in the sterilization pot and the enthalpy difference before and after the sterilization steam. Another problem worthy of discussion is the capacity required by steam sterilization and the corresponding potential value. According to different usages, Table 2 summarizes the corresponding capacity and potential of steam sterilization methods.

Table 2.

The Capacity and Potential of steam sterilization.

Usage Capacity Potential
Medical waste For a typical 4.46 m³double-layer sterilization pot, a 0.5 t h−1 boiler can well satisfy its steam demand in the light of feedback based on practical application [44]. In that case, according to the density of medical waste about 100 kg m−3 [45], the 0.5 t h−1 steam generator can sterilize 446 kg medical waste at each turn. There are adequate disinfection centers to deal with the daily medical waste load. However, at the beginning of the outbreak, the medical waste tends to multiply in amount. Even in mega-cities such as Wuhan, medical waste increased from the normal level of 40 t/d to 240 t/d [2] and thus continuous operating 10 t/h boiler is needed to make up the difference.
Mask The sterilization effect of steam disinfection on masks has been verified [33,35]. However, most of the current tests are carried out in the laboratory and the specific amount of steam and the number of disinfection cycles need to be further determined. WHO and UNICEF recommend that adolescents aged 12 and above wear masks [46].As a result, the demand and energy consumption of masks will be greatly increased, as shown below:
Mask demand 129 G/month [10]
Energy consumption 4.6 PJ/month [10]
Textiles The disinfection of textiles or tableware is commonly carried out in steam disinfection cabinets as follows:8 kW = 900L = 500Sets (1bowl,1cup,1spoon,1 chopsticks) [47]
Tableware
Public area For the ground, desktop and other surface disinfection, the relationship between the small steam sterilization mop and the sterilization area is estimated as follows: 1850W/100m2 [48].Based on this energy consumption, a 0.5 t/h boiler could be used for the disinfection of about 190 households (0.5 t/h boiler≈350kw = 1.9 × 104 m2) and public area, e.g. supermarkets, schools and public transportation.
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Herein, the typical steam demands for different usages are investigated. Based on that, the energy consumption is estimated for different usages. In conclusion, in terms of the potential of steam sterilization, the most noteworthy parts are as follows:

  • (1)

    Recovery value of steam sterilization on mask: It takes up to 0.0342 kWh for the production of a mask [49]with 0.06 kg CO2 emission. More crucially, there is a supply shortage of melt-brown [50], which is the core material of the mask. And the shortage could be mitigated with the recovery of mask, let alone the energy-saving and environmental protection potential.

  • (2)

    Emergency plan of steam sterilization in hospital: High temperature steam is used as a heat source for disinfection, cooking, washing, ironing, drying, boiling water, hot water and distilled water, whose supply cannot be interrupted to maintain the medical service. In China, there are more than 2500 tertiary hospitals [51], each of which can accommodate at least 500 people, with a ward area of more than 3000 m2 [52].

  • (3)

    Steam disinfection in public areas and other items: At present, steam sterilization for such application scenarios is usually carried out by small-sized equipment such as steam mops and steam sterilization cabinets. However, it is more advantageous to use larger capacity boilers for sterilization especially in large public buildings such as airports or hotels.

3. Distributed steam generation

Based on the above analysis, it is apparent that ensuring an efficient and environmental-friendly steam supply during the pandemic is of great significance. Therefore, two electricity-based methods for distributed steam generation are proposed and discussed, which both are through water vapor compressor.

3.1. Air source heat pump boiler for steam generation

Fig. 1 sketches the air-source heat pump boiler (ASHPB) that can extract heat from the ambient air and generate high-temperature water vapor for various sterilization demands. The boiler system is mainly composed of an air source heat pump and a water vapor compressor, which are connected by a flash tank [28]. The operating parameters of a specified model of air-source heat pump boiler, with detailed introduction in Supplementary Note 2 (see Table 3 ).

Fig. 1.

Fig. 1

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Schematic diagram of the air-source heat pump boiler for steam sterilization.

Table 3.

The operating parameters of the air-source heat pump boiler.

Operating parameters Descriptions and Corresponding value
Cascade heat pump water heater LTC HTC
Refrigerant: R404A
Superheated degree: 5 °C
Sub-cooling degree: 5 °C		 Refrigerant: R245fa
Superheated degree: 5 °C
Sub-cooling degree: 5 °C
Flash tank The vessel pressure is set as saturation vapor pressure (mainly 0.0578Mpa, correspondingly the saturated temperature is 85 °C).
The flash temperature difference is set as 5 °C.
Water vapor compressor Twin-screw compressor; variable speed
The ambient temperature The ambient temperature varies from 5 °C to 35 °C
Temperature difference in the heat transfer process Air side to the evaporator:15 °C; water side to the condenser:5 °C;
In Cascade heat exchanger: 5 °C
The temperature of outlet steam Saturated water vapor; Temperature:120 °C; Pressure:0.198 MPa
The steam generation rate Set as 0.5 t h−1 (500 kg h−1) for efficiency evaluation
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3.1.1. Air-source heat pump boiler system prototype and its experimental performance

Through theoretical simulation and data sorting, the ambient temperature, intermediate temperature and flash temperature can affect the performance of ASHPB. Fig. 2 illustrates the synthetically impact of those interfering factors on ASHPB. As is shown in Fig. 2, the ambient temperature has the most remarkable influence on the boiler efficiency of ASHPB. Distinct planes stratification has been formed in the boiler efficiency according to the difference of ambient temperature. Under conditions with constant intermediate and flash temperatures, boiler efficiency increases as the ambient temperature increases. When the flash and intermediate temperatures increase, the boiler efficiency increases up to a certain point, after which it decreases. Consequently, an optimum temperature zone exists for each ambient temperature.

Fig. 2.

Fig. 2

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Boiler efficiency of air-source heat pump boiler under different ambient temperature, intermediate temperature and flash temperature.

For an ambient temperature of 15 °C, as is shown in Fig. 3 , the flash temperature and intermediate temperature have made the boiler efficiency into a convex plane and there exists an optimal region where boiler efficiency is above 1.7.

Fig. 3.

Fig. 3

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Boiler efficiency of air-source heat pump boiler under different intermediate temperature and flash temperature (Ta = 15°C).

From Fig. 4 we could see that with a flash temperature above 90 °C, the boiler efficiency hardly exceeds 1.74, which indicates the flash temperature should not be too high, otherwise it will bring a decrease in the COP value of the cascade heat pump.

Fig. 4.

Fig. 4

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Boiler efficiency of ASHPB under intermediate temperature and flash temperature.

The intermediate temperature, which reflects the internal conditions of the cascade heat pump, also has a significant impact on the overall efficiency of the system and should be well controlled. In a particular region as is shown in Fig. 4, the boiler efficiency is always above 1.74. Therefore, the design parameters of the air-source heat pump boiler should be carefully combined with the local ambient temperature conditions to optimize the performance of the equipment. Fig. 5 further shows the annual temperature distribution in some typical cities of China, and it can be seen that the heat source condition above 15 °C occupies the majority in these cities, which further implies the ASHPB has wide regional adaptability.

Fig. 5.

Fig. 5

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Annual temperature distribution in typical cities of China(Beijing, Shanghai, Guangzhou).

3.1.2. Air-source heat pump boiler system prototype and its experimental performance

Through theoretical analysis, the ASHPB has shown wide regional adaptability with high efficiency. Therefore, an ASHPB prototype was further built and tested for the brewing industry. As is shown in Fig. 6 , the prototype is mainly equipped with three cascade heat pump water heaters, a flash tank, a water vapor compressor and a water purified system. The three cascade heat pump water heaters are in parallel connection for water heating, each with heating capacity of 58.5 kW under the rated working condition (Ambient temperature: 12 °C; Outlet water temperature:85 °C), and the cascade heat pump adopted R404A and R245fa mixture as refrigerants for LTC and HTC. And the measurement method is provided in Supplementary Note 1.

Fig. 6.

Fig. 6

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The air-source heat pump boiler prototype.

Furthermore, a twin-screw compressor is adopted with variable speed for flexible output, which can supply steam up to 0.5 t/h. For brewing industry, the outlet steam temperature is around 110 °C, while the pressure is about 0.12 MPa and the mass flow rate is about 0.2 t/h. Under these boundary conditions, Fig. 7 further illustrates the theoretical and experimental boiler efficiency of the ASHPB. The solid line is the theoretical boiler obtained by the simulation model, while the two dotted lines are 105% and 95% of the theoretical boiler efficiency. The red-cross scatters show the experimental data selected from the average value with stable output over 30min according to the official standard of boiler efficiency [53]. It can be seen that the experimental results are in good agreement with the theoretical calculation with a deviation of less than 5%. The average boiler efficiency of the ASHPB is more than 180% at ambient air temperature 15 °C and it can reach over 165% when ambient air is at 7 °C.

Fig. 7.

Fig. 7

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Theoretical and experimental efficiency.

For the ASHPB, the output parameters of the steam are directly determined by the compression process in the water vapor compressor. Due to a high adiabatic index, the discharge temperature of water vapor can exceed 180 °C and water injection is adopted with two normally closed solenoid valves to cool the outlet steam. The solenoid valves will be open when the output temperature exceeds the set value and be closed when the output temperature decreases. Additionally, the output temperature can be seamlessly adjusted from 120 °C to over 160 °C to satisfy various temperature demands.

3.2. Water vapor compressor combined with the existing heating network

The hot water pipe network can transport heat to distributed urban area for long distance, which is so called district heating (DH) and has already become one of domestic heating way in many countries [54]. The heat source of district heating is usually large-scale and even with the recovery of waste heat, which makes its cost relatively lower. Previous research has studied industrial heat pump for steam generation with district heating as a heat source [55]. In fact, the system could be further simplified and the high-temperature steam can be generated through the combination of the existing hot water network and water vapor compressor as Fig. 8 shows.

Fig. 8.

Fig. 8

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Schematic diagram of WVC combined with DH for steam sterilization.

According to the investigation, the inlet temperature of district heating can reach 80–100 °C [56,57], which could be directly used for flash evaporation. Therefore, with a flash tank and a water vapor compressor, high-temperature steam could be further generated. Essentially, this method is equivalent to a simplified air source heat pump boiler but with less electricity input and less equipment investment.

4. Perspective of the localized steam generation methods

4.1. The economic analysis the ASHPB and WVC combined with DH for steam generation

Compared to the experimental performance, the economic performance of steam generator has a greater impact on customers’ willingness and thus on its application potential [58]. Therefore, a comparison work on the two proposed steam generation methods and other two typical boilers (the electric boiler and the gas-fired boiler) is carried out. Table 4 shows the working and operating conditions on ASHPB, DH combined with WVC the electric boiler and the gas-fired boiler. In terms of efficiency and the corresponding operation cost, the steam generation methods based on air-source heat pump and water vapor compressor have obvious advantages. Using 0.5 t h−1 as design capacity and 6000 h as annual operating time, due to relatively higher boiler efficiency, the ASHPB has the lowest operation cost.

Table 4.

The economic assessment on various boilers to generate steam.

Working Condition The amount of steam supply 0.5 t/h
Operation hours for one day(h) 20
Operation days for one year(day) 300
Type
 ASHPB
 DH + WVC
 Electric boiler
 Gas-fired boiler
Operation condition Boiler efficiency 170% / 95% 90%
Initial investment (10,000CNY) 70 30 6 7
Energy consumption per day 4415 kW h WVC:940 kW h
DH:7365 kW h		 7900 kW h 833.9 m³
Unit Energy Price For electricity 0.583CNY/kW·h [59];
For DH:0.31CNY/kW·h(see Table S4)		 3.82 ¥/m³ [60]
Annual Operation cost (10,000CNY) 77.2 84.9 138.2 95.6
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Fig. 9 shows the change of total operating expenses of those steam generation methods with the increase of the operating period. The ASHPB has the highest initial expense followed by DH with WVC.

Fig. 9.

Fig. 9

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Operation expenses of boilers (0.5 t h−1).

With the lowest operating cost per year, in the 1st year, the total operating expenses of electric boiler exceeds that of ASHPB. In the 4 nd year, the total operating expenses of the gas-fired boiler exceeds that of ASHPB. In the following operating period, ASHPB has the lowest total operating expenses and shows significant economic benefit, which can save CNY5, 750,000 compared to the electric boiler and CNY 1,145,000 compared to the gas-fired boiler. Additionally, the total operating expenses of DH combined with WVC is the lowest in the 3rd year and exceed that of ASHPB in the 7th year, but is still lower than that of gas-fired boiler and electric boiler. Therefore, for users with access to hot water pipeline and less budget, this method is a more suitable one for steam generation.

4.2. The application outlook of the ASHPB for steam sterilization

Except the advantages in economy (e.g. lower 10-year total operating cost), the air-source heat pump boiler is also a suitable technology for surging sterilization demands and other steam demands.

During the outbreak stage of COVID-19, the daily output of medical waste in Wuhan sharply from about 40 tons to 247 tons and far exceeded the original medical waste disposal capacity of about 50 tons per day [61]. In addition, the prices of disinfectants and masks have increased to more than 10 times the prices before the pandemic but were still unavailable [62], [63]. Material shortages and inadequate sterilization capacity will not only bring great risks to the medical staff but also endanger the health and safety of the community. In such cases, ramping up of steam sterilization capacity through heat pump could make a significant difference, because of its quick uptake speed, high flexibility and the relatively stable supply of water and electricity, which are the only two elements that are needed for ASHPB operation.

There is also a concern that the sterilization facilities may lie idle after the pandemic due to a surplus in treatment capacity [2]. In addition, the heat pump sterilization facilities that are adopted during COVID-19 can play a continuous role during the post-COVID green recovery and long-term net-zero energy transition. On one side, high-temperature steam (water vapor) is a widely used heat vector that has applications in many industrial sectors and there is always a high demand for industrial boilers as Fig. 10 shows. With retirement of coal-fired boilers in low-carbon energy system transition, the ASHPB will play a continuous role in industrial production shown as Fig. 11 . Such concept of combining the ambient heat and electricity for high-temperature steam generation in ASHPB is well consistent with the UN Sustainable Development Goals [64] which is more important during the post-COVID green recovery and long-term net-zero energy transition as described in Supplementary Note 4.

Fig. 10.

Fig. 10

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Industrial boiler output in China.

Fig. 11.

Fig. 11

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The application outlook of the ASHPB for steam sterilization and industrial utilization.

5. Conclusions

In this paper, the features and potential of steam sterilization are investigated and evaluated. Based on that, a clean and flexible steam generation system with the air source heat pump and water vapor compressor is proposed for steam sterilization, and the system can be further simplified through the combination with district heating pipeline. The air-source heat pump boiler has been studied by theoretical and experimental analysis. Through the analysis in the paper, the following conclusions can be concluded.

  • i.

    Steam sterilization has specific advantages and more extensive application possibilities.

  • ii.

    In terms of application performance, the ASHPB has a higher boiler efficiency (174% at 15 °C ambient temperature for 110 °C steam generation), and can meet most sterilization demand.

  • iii.

    In terms of economic performance, yet the ASHPB has a relatively higher initial expense, it will has the best economic performance when the operating period is above 3 years.

  • iv.

    For public health emergencies like COVID-19, the ASHPB can quickly respond to the surging sterilization demands due to its quick uptake speed, high flexibility and the relatively stable supply of water and electricity.

Credit author statement

Ruzhu Wang: Supervision, Conceptualization, Writing-Reviewing and Editing. Hongzhi Yan: Software, Validation Data curation, Writing- Original draft preparation. Bin Hu: Visualization, Investigation. Validation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This research was supported by the National Key R&D Program of China (2020YFB0606200ZL) and the Key Project of the National Natural Science Foundation of China (52036004). The authors gratefully acknowledge the financial support from the Research Council of Norway and user partners of HighEFF (Centre for an Energy Efficient and Competitive Industry for the Future, an 8-year Research Centre under the FME-scheme). The authors also thank Dr. Chuan Zhang, Dr. Bangjun Li, Dr. Baiyang Zhao, Ms. Signe Ryssdal and Mr. Martin Klevjer, who partly joined the revision work.

Footnotes

Appendix A

Supplementary data to this article can be found online at https://doi.org/10.1016/j.rser.2021.111026.

Appendix A. Supplementary data

The following is the Supplementary data to this article:

Multimedia component 1
mmc1.docx (415.7KB, docx)

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