*3.3. NH3, H2S, Ultra-Fine Dust (PM2.5), and Formaldehyde Level*

As listed in Table 3, NH<sup>3</sup> and H2S emissions during the weaning, growing, and finishing periods were significantly lower (*p* < 0.05) in the AHP house compared to the control house. The average NH<sup>3</sup> emissions were reduced by 61%, and the average H2S level was decreased by 45% in the AHP house.


**Table 3.** Effect of the air heat pump system on the NH<sup>3</sup> and H2S emissions in the pig house.

a, b means that values with different superscripts within the same row are significantly different (*p* < 0.05).

Table 4 lists the ultra-fine dust concentration (PM2.5) and formaldehyde concentration due to the air heat pump system. There were no significant differences in the PM2.5 dust concentration between the two houses. On the other hand, the dust concentration tended to decrease during all periods, and the average reduction was 6.5% in the AHP house compared to the control house. During the growing and finishing periods, the formaldehyde concentration was significantly lower (*p* < 0.05) in the AHP house.


**Table 4.** Effect of the air heat pump system on ultra-fine dust (PM2.5) and formaldehyde concentration in the pig house.

a, b means that values with different superscripts within the same row are significantly different (*p* < 0.05).

*3.4. E*ff*ect of the Air Pump Heating System on the Productivity Traits of Pigs*

Table 5 lists the results of the growth performances of pigs during their weaning, growing, finishing, and average values. The body weight gain, feed intake, and feed conversion ratio (FCR) did not significantly differ (*p* > 0.05) among the control and AHP-installed house.


**Table 5.** Effect of the air pump heating system on the productivity parameters of pigs.

### *3.5. Estimation of the Installation and Annual Operational Costs*

As shown in Table 6, the initial investment for the air heat pump system was comparatively higher than for the conventional electric heating system. Nevertheless, the AHP system gained a lower annual operational cost, higher life span, and shorter payback period.


**Table 6.** Installation and operational costs of the air heat pump and conventional electric heating system.

Annual operational cost was evaluated according to annual electricity consumption (131,026 kWh) and price (0.033 USD/kWh).

### **4. Discussion**

Proper temperature maintenance inside a pig house is essential to prevent pigs from cold shock and ensure their optimal growth. In this study, the inside temperature of the AHP house was greater than in the conventional electric heating system. We speculated that the AHP system could distribute a uniform heat pattern more continuously inside the house than the conventional electric heating system due to the high COP value and lower running time period. The calculated average COP of this study was 4.07, which is lower than the values observed by Riva et al. [28] during the heating phase, but it was higher than the experiment conducted by Ji et al. [29] using an air heat pump for domestic heating purposes. In contrast, Zang et al. [30] reported that the COP value tends to increase with decreasing external temperature because the evaporator of the heat pump interacts continuously with hot air circulation during the heating phase. Nevertheless, some studies have reported that the efficiency of the AHP system tends to decrease when exposed to extreme temperature levels [31–33].

Renewable energy sources are abundant, have low cost, and are environmentally safe. In the present study, the AHP system showed lower electricity consumption, CO<sup>2</sup> emissions, and electricity cost relative to the conventional electric heating system. To the authors' knowledge, only one study has evaluated the effects of an air pump system on energy savings and housing environment in pig breeding house [28]. Riva et al. [28] reported that an AHP house could save 11% of the total energy consumption compared to a control house connected to an LPG boiler house. Similarly to the present results, Wu [34] concluded that the AHP system is a more efficient environmental safety system than conventional heating techniques, and can be introduced to minimize the depletion of energy resources. The low electricity consumption in the AHP system might be due to the high COP value, which has the potential to distribute unvarying heat inside the experimental house.

Rabczak et al. [35] reported that the emission of CO<sup>2</sup> to the atmosphere could be reduced by 40% with an air heat pump system compared to a local gas furnace system or particular heating system that is provided for specific geographical areas. Furthermore, decreased CO<sup>2</sup> emissions and energy savings have been investigated in response to the air pump heating system in buildings [36–39]. In addition to the increasing feed cost, energy prices have a huge impact on the productivity of the global pig industry, including in South Korea. In the present study, the electricity cost decreased during each growth period in the AHP-installed house compared to the conventional system.

According to the International Commission of Agricultural and Bio-Systems Engineering, CIGR (2002) [40], the recommended maximum NH<sup>3</sup> concentration is 20 ppm. In the livestock sector, pig growth was slowed by 12% to 30% in intensive swine buildings because of the elevated NH<sup>3</sup> concentration [11]. An improper ventilation system and high concentrations of NH3, H2S, and CO<sup>2</sup> lead to poor air quality inside pig houses. In the present study, the concentrations of both NH<sup>3</sup> and H2S were significantly lower in the AHP house, which is in agreement with the results of a previous study [28]. The lower noxious gas concentration may have occurred due to the increased fresh outdoor air temperature due to the compressor, as well as the subsequent dilution of NH<sup>3</sup> and H2S levels in the pig house.

Takai et al. [41] reported that the dust concentration in swine houses tends to increase in winter compared to summer. Automotive exhaust and various urea–formaldehyde products are the sources of formaldehyde formation inside houses. Exposure to 0.3 to 50 ppm will depreciate lung compliance [42]. In the present study, during the growing and finishing periods, air contamination with formaldehyde was lower, possibly due to proper air circulation inside the AHP house. On the other hand, the relationship between the dust concentration, formaldehyde level, and installation of an AHP system is unclear. Further research on dust and formaldehyde fluctuations from the utilization of renewable energy sources will be needed.

According to Riva et al. [28], the production parameters, including feed intake, weight gain, and feed conversion ratio, increased significantly in a pig house operating with the AHP system compared to one operating with an LPG gas system. The accumulation of high concentrations of fumes in an LPG house may reduce their voluntary feed intake because of the poor housing environmental conditions. Nevertheless, in our study, there were no significant differences in the productivity parameters, but the weight gain tended to increase in the AHP house during the growth stages. Therefore, further study on the productivity parameters when using the AHP system in the livestock sector will be needed.

Owing to the high COP value of the AHP system, the annual operational cost was reduced by 91% compared to the control heating system. Wu [34] reported that the air heat pump reduced electricity consumption by 46 kWh/m<sup>2</sup> . Consequently, it reduces the electricity costs. Islam et al. [43] reported that the installation cost for a renewable geothermal heat pump is considerably more expensive than for a renewable AHP system, and both systems had lower annual operating costs than the electric heating system. The payback period tends to decrease when the COP value is increased. Similarly to our result, the payback period ranges between four and five years when the COP value is 4 [44]. Owing to the high depreciation time, livestock farmers can implement an AHP-based livestock housing system to minimize their electricity costs, and it has the potential to work for a longer period.

## **5. Conclusions**

In global intensive livestock farming systems, higher electricity consumption and inadequate air quality adversely influence the environmental sustainability and slow productivity. Therefore, the implementation of innovative strategies in order to maintain production parameters while reducing energy consumption and providing proper air quality is a current issue and is worthy of being collaboratively investigated. The present study aimed to investigate an AHP system to be utilized for intensive pig farming as an efficient, eco-friendly alternative to the widely used conventional electric heating systems. According to the results of this study, the inside temperature was maintained at a significantly higher level in the AHP house. A significant decrease in average electricity consumption by 40 kWh, overall cost, and CO<sup>2</sup> emissions by 19.32 kg was observed during the experimental period in the AHP house. Furthermore, the NH<sup>3</sup> and H2S emissions were also lower in the AHP-installed house than in the house with the conventional electric heating system. Although the initial installation cost was high, the investor could obtain long-term benefits with a uniform performance for a longer period (approximately 15 years) while utilizing less electricity and causing less greenhouse gas (GHG) emissions. Therefore, the AHP system is an innovative and sustainable energy source for cost-effective and eco-friendly heating of animal houses in the livestock sector.

**Author Contributions:** Conceptualization, C.J.Y. and K.W.P.; methodology, M.G.J. and H.S.M.; writing—Original draft preparation, D.R. and M.G.J.; software, D.R. and H.S.M.; data curation, S.R.L. and M.A.D.; formal analysis, C.J.Y. and K.W.P.; investigation, C.J.Y. and H.S.M.; writing—Review and editing, M.A.D., S.R.L., and D.R.; supervision, C.J.Y. and H.S.M., project administration, C.J.Y. and K.W.P.; funding acquisition, S.R.L. and M.G.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was funded by the Industrial Technology Innovation Business (20194210100020, Development and Demonstration of Renewable Energy Mixed-Use System for the Livestock Industry) and Ministry of Trade, Industry, and Energy, Korea.

**Conflicts of Interest:** The authors declare no conflict of interest.
