Analyzing the Multiscale Impacts of Implementing Energy-Efficient HVAC Improvements Through Energy Audits and Economic Input–Output Analysis

The energy crisis of the 1970s sparked great interest in reducing energy usage and greenhouse gas emissions [1]. According to researchers, the industrial sector accounts for nearly 47% of global carbon dioxide emissions and is a large contributor to other greenhouse gases [2,3]. Many studies have encouraged policymakers, industries, and national and regional governments to work together to reduce their carbon footprints. For example, the European Union targets 32.5% energy efficiency by the year 2030 [4]. Improving energy efficiency has been considered one of the pillars of achieving these goals. A plethora of research shows the energy audits’ economic and environmental benefits [5–7]. To help manufacturers find more effective ways to meet and manage their high energy demand, the US Department of Energy created Industrial Assessment Centers [8]. Currently, 31 centers across the United States provide small- to mid-sized manufacturers with no-cost energy audits that have successfully convinced facilities and average citizens to reduce unnecessary energy usage by adopting various strategies [9,10]. Audit teams analyze various energy systems for potential energy-saving opportunities during an assessment. A facility’s heating, ventilation, and air-conditioning (HVAC) system are one of the most important of these systems because it controls the indoor environment and is a large consumer of energy. HVAC systems are usually a facility’s highest consumer of energy, most of which goes toward space cooling in buildings [11,12]. As a result, inefficient design and operation of the systems can cause total energy inefficiency in a facility and lead to decreased production due to poor air quality, temperature control, and overall comfort. Recommendations for improving the energy efficiency of the HVAC systems range from simple thermostat changes to replacing entire systems with newer, more efficient ones. This article showcases some simplified engineering analysis of the HVAC energy-efficiency improvement recommendations that an energy assessment team offered to manufacturing facilities between 2008 and 2018. Then, integrates the historical facility scale data to the macroeconomic modeling framework to estimate cascading broader regional economic impacts of the industrial energy-efficient HVAC implementation. In what follows, Sec. 2 reviews the related research about different energy-efficient HVAC technologies and strategies. Section 3 then describes the research steps and methodologies taken for this study, and Sec. 4 introduces several technological case studies as examples of energy-efficient recommendations. Section 5 analyzes the cascading economic impact of these energy-efficiency investments, and the last section discusses the results and offers conclusions and recommendations.

The comfort of a space is an important aspect of an industrial facility, and nearly 26% of its energy consumption goes toward maintaining it [13]. Outdoor air is ventilated into facilities to keep the proper air quality and thermal comfort of the personnel inside. Cleanrooms are large consumers of energy due to their constant need for proper ventilation [14] and can account for 30–65% of a high-tech fabrication facility’s total energy use [15]. HVAC systems are large contributors to energy usage in manufacturing facilities because they provide comfort to personnel, help regulate the indoor environment, and maintain the quality of the air [16]. As these systems become older and more outdated, monitoring and improving their efficiency becomes more essential. Studies have found many different methods by which manufacturers can improve energy and resource efficiency [17,18].

A study by Loomans [19] showed that reducing the air change rate of a cleanroom can have potential savings of about 99.9% while still maintaining the necessary air quality requirements. The energy efficiency of industrial HVAC systems can also be improved by recovering heat that would otherwise be wasted. One study found that a heat-recovery unit that utilized finned oscillating heat pipes for heat exchange between counter-flowing airstreams was able to save commercial buildings across eight different US cities an average of 16.5% in energy savings [20]. Adding heat-recovery systems to air-handling units (AHUs) has been found to reduce the load for the AHU by 17.4% [21].

A facility’s energy usage does not stop during non-production hours. A study performed by Masoso and Grobler [22] showed that 56% of energy usage in commercial buildings came during non-working hours. The most common control strategy for buildings is to set back temperatures during unoccupied hours [23,24]. Implementing setback temperatures during unoccupied hours can reduce the load of an HVAC system and potentially save about 14% in energy usage [25,26]. Balaji et al. [27] showed that implementing an occupancy-based HVAC actuation system can offer electrical energy savings of about 17.8% in one day. A smart building energy management system has been shown to reduce electrical energy consumption by 37% and heating consumption by 10% [28]. Weather-dependent controls have also been shown to be an effective way to reduce energy usage. For example, scheduling thermostats with seasonal setback temperatures has been shown to save about 13% of gas consumption and about 2.3% of electricity consumption in residential buildings [29]. Another study [30] showed that a genetic algorithm that made predictions based on weather, occupancy, and indoor environment saved a building 25% in energy and 27% in costs due to load shifting. Industrial facilities typically have multiple zones with different numbers of occupants and require different levels of comfort, so optimizing temperature schedules in each of those areas can potentially provide energy, costs, and CO₂ savings.

Heating, ventilation, and air-conditioning systems can also lose their efficiency over time, so ensuring they are well maintained and upgraded when needed can help save energy usage and costs. A plethora of research performed to improve the efficiency of the HVAC system. A study [31] illustrated the importance of thermal transmittance and solar heat gain coefficient value of window glazing and demonstrated the energy savings potentials. Bhagwat et al. [32] concluded that upgrading and utilizing existing HVAC systems could positively affect energy efficiency. Industrial facilities that produce wasted heat can benefit from different heat-recovery systems to increase efficiency and decrease energy costs and emissions [33,34]. Ozalp [35] analyzed the effect of utilizing heat, power, and recovered waste heat for industrial processes in chemical industry. Infrared radiant heaters can consume half the energy of unit heaters [36] in spaces in which space heaters are used to replace large amounts of heat wasted through the constant opening of industrial dock doors. Upgrading hardware to newer technologies also has great potential for improvements in the overall energy efficiency of a building. Singh and Das [37] analyzed the performance of evaporation and heat wheel-based building air-conditioning systems.

Adding insulation can also decrease heat loss through surfaces and effectively reduce energy consumption [38]. Inorganic materials such as mineral wool, expanded polystyrene, polyurethane, and organic materials such as cork, cellulose, and sheep wool have been used for insulation in buildings [39] and analyzed for their differing thermal properties [40]. As a building becomes older, these thermal properties tend to weaken and make insulation more inefficient and cause energy to be wasted. According to a study, after 25 years, the thermal conductivity of vacuum insulation panels increases from 3 and 4 mW/(mK) to typically 8 mW/(mK), and even as high as 20 mW/(mK) if damaged [41]. Leakage through windows also reduces the energy efficiency of a building, and innovative window materials, such as a transparent nanocomposite material studied by Hu et al. [42], offer potential energy-saving opportunities.

Economic Input–Output (EIO) Analysis has been used to capture the impacts of energy-efficiency improvement [43–45] and rebate programs [46] to assist policy-making decisions. The basic framework of this analysis is the Leontief Input–Output model [47,48], which takes the changes in the final demand as exogenous and calculates the change in gross production required to ensure that supply equals demand for each sector.

A plethora of research has been performed to address the economic and environmental impacts of energy assessment for industries [49]. However, the research addresses the technical, economic, and environmental impacts of the industrial building energy-efficiency investment by utilizing the EIO framework is rare. Therefore, in this work, micro-level real industrial energy data are integrated into an input–output analysis framework to allow an in-depth understanding of the direct, indirect, and induced impacts of the implemented energy-efficient HVAC recommendations on the community.

Figure 1 shows the framework of this study. Industrial energy audits are useful tools for improving ways to reduce industrial energy usage. To analyze the economic and environmental impact of these improvements, we gathered data from energy assessments performed by our team in the decade between 2008 and 2018. When presenting a recommendation to a facility, the team provides estimates of both the potential savings in energy, costs, and CO₂ emissions and the costs associated with implementing the recommendation. Nine months after an assessment is completed, a survey is sent to the facility to learn which recommendations were implemented. This study focuses on the HVAC recommendations out of many industrial energy systems involved in the assessment.

Although implementing energy-saving strategies usually comes with a capital cost, those expenditures are beneficial to the larger community because they circulate money throughout the economy and provide business for many different economic sectors. This study thus employed a cascading economic and environmental approach to determine the various ways in which the regional areas are benefitted from implementing energy-saving recommendations for HVAC systems. Implementation costs are broken into equipment, labor, and material costs. Those costs are allocated as investments to the corresponding economic sectors in the economic input–output framework to analyze the regional economic impacts of implementing the HVAC recommendation. EIO data [50] were used to perform our input–output analyses. The implementation costs were allocated to different economic sectors, which are similar to those created by North American Industry Classification System (NAICS). The sectors that were directly impacted by these recommendations are identified. Those direct impacts can also increase production in other upstream industries in the supply chain, categorized as indirect effects. Both these direct and indirect effects also can create new job opportunities or increase the compensation of employees, which can then be spent on local services, education, healthcare, etc. These effects on the broader community are considered induced effects. We determined value not only from an economic outlook but from an environmental one. We then conducted three scenario analyses that present the estimated effects of the 66 HVAC-involved recommendations implemented, 125 such recommendations that were not implemented, and the two categories combined. Further, we estimated the impacts if the HVAC recommendations are adopted by all the manufacturing facilities in a state to provide some policy aspects.

This section describes a simplified version of some selected recommendations given to 116 facilities during the period of this study (2008–2018). Table 1 lists the different kinds of HVAC system recommendations, followed by five selected case studies and the calculations that we made to estimate the energy and cost savings and expected payback period. In total, there were 191 HVAC recommendations made, with 66 of them being implemented by facilities.

An assessed facility had an internal scrap transport system that allowed heated indoor air to escape as scrap from the building. Installing ductwork to pull outdoor air instead of indoor air into the collection system was recommended to reduce the ventilation air and help correct the negative pressure of the building, as shown in Figs. 2(a) and 2(b).

Natural gas usage was modeled using the ESim building energy simulation software [51]. To calibrate the model to actual natural gas use, we used daily temperatures for the same period as the natural gas data, as found in the UD/EPA Average Daily Temperature Archive. Figure 3 shows that the facility’s current natural gas usage (black line) could be reduced by 5015 MMBtu/year if they were to install the proper ductwork to reduce the wasted heat leaving through their scrap transport system.

Using the facility’s natural gas rate structure, we also calculated the estimated cost and CO₂ emission savings of installing ductwork to the scrap transport system, as shown in Table 2.

Other methods for reducing ventilation air include turning off exhaust fans when not in use, eliminating makeup air during non-production hours, and reducing the percentage of outdoor air in direct-fire unit heaters. The implementation costs of these methods can vary, but when we analyzed all of these recommendations, they produced an average of $16,184 in savings, with an average implementation cost of $14,097 and an average simple payback of about 10 months.

In another case, a facility was being kept at 70 °F through the entire winter, which was relatively warm compared to other facilities we visited around that time. We, therefore, recommended that this facility reduce its indoor temperature to 65 °F during the winter season, which would significantly reduce natural gas used for space heating without creating an environment outside the bounds of human comfort. Similar to a study performed [52], we used ISim [53] to model the cost savings from setting back the temperature. The inputs needed for ISim can be seen in Table 3.

Figure 4 shows the results of the simulation. The blue line represents the original natural gas usage during the year, while the red line shows how much natural gas would be used when reducing the temperature in the winter; the green line shows the outdoor temperature throughout the year. As these lines show, there is an inverse relationship between gas usage and outdoor air temperature, which is expected and typical for an industrial facility. ISim’s calculation of the difference between the natural gas usage at the two temperature settings showed that the total natural gas that could be saved from reducing the temperature is 8866 MMBtu/yr. Annual cost savings were calculated to be about $43,532/year. Since this recommendation required only a simple setting change on thermostats, it involved no implementation cost and provided an immediate simple payback. We calculated that the facilities that implemented this recommendation saved an average of $33,463/year.

A facility indicated they had seven HVAC units purchased 20 years ago and were nearing their end of life and very inefficient. The facility’s change-point temperature was 38 °F, so anytime the outdoor air was above 38 °F, the HVAC units would cool the building. They were also experiencing negative pressure and poor air quality. Negative pressure in the facility increases the infiltration of outdoor air through openings in the building envelope, thus increasing the cooling load on the HVAC units. Installing a properly sized rooftop unit (RTU) with an economizer would vary the fraction of outdoor air added to the air handler so that the mixed air temperature equals the required air temperature, resulting in a change-point temperature that approaches the set-point temperature. Additionally, the economizer would bring more air into the facility, correcting the negative air pressure in the building.

A common area in which makeup air units (MAUs) are used in manufacturing facilities is clean rooms, or areas that are free from dust, particles, and other contaminants to ensure the quality of the process inside the room. MAUs can bring in fresh outdoor air while exhausting any contaminated air. Figure 5 presents a schematic of how an MAU brings in outdoor air while exhausting inside air. Given that this airflow is required only during production hours, reducing the air flowrate during non-production hours can result in energy, cost, and CO₂ savings.

A facility had 20 air-handling units (AHUs) that directly served a clean room in the facility. The outdoor air being introduced into the cleanroom was conditioned by three 25,000 cfm MAUs and introduced into the mezzanine, where it mixed with return air. The MAUs had a discharge temperature of 45 °F during the cooling season and 55 °F during the heating season. Cooling and dehumidification in the MAUs were achieved by water-to-air cooling coils served by chillers. According to the HVAC service contractor, the cooling coil flowrate was being maintained at a 10% minimum flowrate during the heating season to protect against freezing. We recommended that the coils be shut off completely because freezing should not be an issue for the coils, with the heating system keeping the air temperature well above freezing.

One of the HVAC recommendations that fell into the category of “Other” included adding insulation to reduce the unnecessary heat entering and escaping the building. One of the facilities we audited had a bulk hot room that housed seven tanks, one of which needed to be maintained at 113 °F and two at 90 °F. About 70% of the room’s exterior walls were directly exposed to the outdoors. Although the room was insulated, personnel at the facility indicated that the insulation was inadequate.

At the time of our visit, the average temperature of the interior wall surface was 103 °F, and that of the exterior of the wall was 80 °F; the temperature of the indoor air was 115 °F, and that of the outdoor air was 73 °F. When assuming a standard convection coefficient of the interior wall to be 0.92 h-ft²-F/Btu, the thermal resistance of the walls was calculated to be about 3.2 h-ft²-F/Btu. We recommended that 1.5 in. of R6 insulation be added to the walls, which would increase the walls’ thermal resistance to 12.2 h-ft²-F/Btu.

Using the hourly outdoor temperature and estimating the increased thermal resistance of the surface from adding insulation, we modeled the heat loss through the wall over a whole year. Compared to the current state of the wall, we estimated that the heat loss would be reduced by 470.8 MMBtu/year. The heating for this bulk hot room was provided by the steam heaters, which were served by the facility’s steam boilers; estimating the efficiency of this system at 60%, we predicted that the natural gas savings would be about 785 MMBtu/year. This would result in natural gas cost savings of about $3648/year and a reduction of CO₂ emissions by 42 tonnes/year.

The cost of the proposed polyisocyanurate insulation material and labor was estimated to be about $2/ft². With an area of 3360 ft² that needed insulation, the project would cost $6720 and provide a simple payback of 23 months for this project. The average savings for all facilities that implemented the recommendation to add insulation was $1872/year.

This section presents the results of input–output analysis to examine the economic and environmental effects that implementing energy-efficient HVAC recommendations in industrial facilities would have on the state of Ohio. As mentioned earlier, purchasing HVAC equipment and paying for labor for its installation has direct, indirect, and induced benefits for manufacturers, suppliers, professional services, and other sectors of the economy, and all benefit from the environmental impact of such changes. This analysis examined the total impacts generated by the 116 recommended HVAC upgrades between 2008 and 2018.

Table 8 shows the estimated annual savings, implementation costs, and the simple payback for the 66 HVAC energy-efficiency recommendations that, according to the responses to our 9-month implementation survey, were implemented in 2008–2018. The implementation costs of each recommendation were divided into labor and material costs, which were then allocated to their various NAICS sectors. This analysis shows that implementing energy-efficient HVAC operation strategies allowed very high annual savings while keeping implementation costs low. All of the HVAC recommendations were found to produce simple paybacks in less than 2 years, indicating that upgrading HVAC systems proved a beneficial option for manufacturers.

Table 9 presents how these costs were allocated to each NAICS sector to perform the Economic Input–Output Analysis.

Although we identified 505 sectors that experienced some sort of economic impact as a result of the implemented improvements, the 20 selected sectors in Fig. 6 help illustrate how the percentage of direct, indirect, and induced impacts can vary across and within the sectors. For instance, six of the seven directly impacted sectors were sectors that produce equipment and materials involved in the implemented recommendations, and the seventh reflects the hiring of trained professionals to install the various systems and equipment. Other sectors experienced indirect effects from economic transactions with businesses directly impacted by providing parts and supplies. The impact on some other sectors was mostly or entirely induced, such as increased spending by employees of the impacted sectors on health care.

Figure 7 displays the economic and environmental impacts of different scenarios for energy-efficient HVAC systems. The first scenario includes the savings and impacts of the 66 recommendations that were implemented in the time period. The second scenario explores the potential savings and impacts of 125 various non-implemented recommendations in the time period. The third scenario considers the results if all 191 recommendations were implemented. Scenario 1 clearly shows that audited manufacturers had implemented recommendations that provided a short average simple back period of 5.6 months. These manufacturers invested about $943 K and were able to save just over $2 million annually. Scenario 2 shows that the expected simple payback for the non-implemented recommendations was still decent at 14.1 months. The high initial investment costs of these non-implemented recommendations often make implementing those recommendations very unattractive for manufacturers. In terms of emissions, it can be seen that the more recommendations implemented, the more carbon dioxide can be avoided. Our service (Scenario 1) avoided over 19,000 metric tons of CO₂. If all 191 recommendations would be implemented (Scenario 3), over 59,000 metric tons of CO₂ could be avoided.

To give a larger perspective of whole state, estimations for the state were extrapolated from the data discussed previously in this section. According to 2017 data, the state of Ohio has 12,371 manufacturing firms with an output of about $112 billion [55]. In terms of emissions, the industrial sector produces about 37.6 million metric tons or about 18% of the state’s carbon dioxide [56]. Suppose HVAC recommendations are implemented throughout all manufacturers in the state. In that case, it is expected that almost $656 M energy costs would be saved, and 7.8 million metric tons of carbon dioxide emissions would be avoided annually. This is about 21% of the carbon emissions produced by industries in the state. This extrapolation demonstrates the potential savings and impacts if more energy-efficient HVAC practices were implemented throughout the state.

Figure 8 illustrates the regional economy-wide impacts of each scenario explained previously. Direct impacts are generated from the increased production of equipment and labor for implementing the recommendations. Indirect impacts occur throughout the supply chain of the OEM needed to implement the recommendations. Induced impacts are captured by the increased economic activities from the backward-linked regional economic sectors, as explained previously. If all 191 recommendations are implemented in a total of 116 participating manufacturing plants, about 41 jobs could be created regionally. Scenario 2 has a much larger cascading economic impact compared to scenario 1. This is caused by the much larger direct investment from scenario 2 required to implement the recommendations.

Similar to the previous analysis, by extrapolating this number to whole manufacturers in the state, it is expected that about 4387 jobs could be created. The total economic impact would be $899 M, stemming from direct, indirect, and induced impacts.

Scientifically rigorous energy engineering-based audit recommendations made possible by the U.S. Department of Energy’s Industrial Assessment Center program helped small and medium industries to reduce energy, cost, and emissions. This article showed how industrial facilities could impact the entire state economically and environmentally by implementing HVAC energy-saving recommendations. By investing to save industrial energy, cost, and carbon savings, a facility can produce a ripple effect reaching almost every economic sector in its area directly, indirectly, and inducely. Directly investing just under $1 million created additional indirect and induced effects that totaled over $700,000 and benefitted 92% of economic sectors through its impact on supply chains. A similar analysis can be readily performed to investigate the impact of various industrial energy systems’ improvement in other geographical regions by utilizing the proposed methodology in this paper. Policymakers can use the positive connections among energy, economy, and environment as an example to encourage industrial facilities to investigate and implement such industrial energy-efficiency recommendations. Governmental policies can help manufacturers transition to energy-efficient practices. Because some facilities are hesitant to implement recommendations, they are inconvenient or their implementation cost is too high, state and federal subsidies or rebates might encourage more energy-efficient practices by manufacturers by easing those barriers.

We thank previous and current University of Dayton Industrial Assessment Center students for their contributions to this continuing effort and our industrial partners for their significant contributions.

• We would like to express our gratitude to the US Department of Energy for supporting this work through their funding of the Industrial Assessment Center program (DE-EE0009721).

There are no conflicts of interest.

The data sets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.