Heating Industrial Buildings with Heat Pump Air Systems: Is It Always the Most Advantageous Option?

Abstract

According to extant Italian legislation implementing the Renewable Energy Directive, the mandatory renewable quota for a new building is 60% referring to a single service (e.g., heating during winter) or to multiple services (e.g., heating during winter and air conditioning during summer), depending on which services are actually present. The obligation to satisfy this minimum value often leads heating and ventilation plant designers to provide heat pump systems in industrial buildings, typically air/water or direct expansion type coupled with air terminals (air heaters or ventilation units) or radiant floors. The question is: Is this always the most advantageous option for industrial buildings? A typical industrial building was modeled by Trnsys^® in two different climates. Based on the calculated thermal heating loads, the condensing radiant tubes and heat pump coupled with the air heaters systems were analyzed through dynamic simulation, evaluating their performance from an energy, environmental impact, and economic point of view. The analysis carried out revealed that a heat pump system is not always the most advantageous solution depending on the climate, the characteristics of the building (less or more thermal insulation, which corresponds to existing buildings rather than new ones), and the size of the photovoltaics system eventually installed on the roof.

1. Introduction

The rapid increase in population and urbanization have raised the demand for energy consumption with average annual growth rates around 4% [1]. Buildings are certainly among the largest users of energy, as consumption accounts for 30–40% of the total [2]. Heating, ventilation, and air-conditioning (HVAC) systems require a large quota [3]. Until recently, fossil energy has been the most important energy used for building heating and cooling in the world, but it causes the emissions of greenhouse gases to continue to rise [4]. For this reason, most industrialized nations have committed to reducing greenhouse gas emissions and to increasing the use of renewable energy technologies. Among them, the European Union (EU) has set itself a legally binding target of reducing carbon dioxide emissions by 55% by 2030 compared to 1990 levels, as well as implementing a path towards net decarbonization by 2050. These goals have been integrated into EU Regulation 2021/1119 [5] and are being implemented through the legislative package “Fit for 55”. Within this, there is revision of some very important directives:

• Energy Performance of Buildings Directive (EPBD-2024/1275), which promotes the improvement of the energy efficiency of EU buildings by 2030 [6];
• Energy Efficiency Directive (EED-2023/1791), which aims to reduce final energy consumption at the EU level by 11.7% in 2030, compared to projections made in 2020 [7];
• Renewable Energy Directive (RED-2023/2413) to increase the EU-wide target to at least 40% of renewable energy sources in the overall energy mix by 2030 [8].

It is worth citing the new Regulation (EU) 2024/1735, the “Net Zero Industry Act” [9], with the objective of satisfying at least 40% of the need for so-called green technologies with European production and that such production represents at least 15% of the global market value of such technologies. The legislative and regulatory context described was recently confirmed by Ursula Von der Leyen, who presented the political guidelines for the new mandate of the European Commission 2024–2029.

In this context, companies need to adapt their business strategies towards very ambitious objectives with which they must align through decarbonization and increasing the efficiency of energy utilization. Regarding the use of energy for heating and cooling workspaces, industrial buildings have particular characteristics, as already described by the author [10]. For example, they are typically high (up to 8 m or more), walls and ceilings can be partially covered by bridge cranes, pipes and tubes, etc. and are usually scarcely insulated, doors are large and can often be opened, floor areas are typically large, with zones occupied in different ways by workers, where the comfort conditions requested can vary [11].

For such reasons, traditional climatization plants are generally not widely used. Some authors have investigated the peculiarities of the air conditioning of industrial buildings. Stamponi et al. modeled an industrial nearly Zero Energy Building (nZEB) that houses offices and hosts workers operating on test benches connected to a thermal and electric smart grid. The control logic of the HVAC system was optimized based on a validated model [12]. In [13], a solar combined cooling, heating, and power system (hybrid photovoltaic thermal collectors (PV-T) integrated via two parallel thermal storage tanks with a reversible air-to-water heat pump) in operation in an industrial building located in Zaragoza (Spain) was modeled to estimate the annual energy output. The same authors have successively validated the model [14,15], demonstrating that the integration of the thermal and electrical generation of PV-T collectors with a high-performance rev-HP allows the solar PV-T system to be self-sufficient in satisfying the building energy demand.

The heat pump is an energy-saving and environmentally friendly system that has been known for decades but has become significantly more widespread only in recent years [16]. Although ground source heat pumps can be used [2], air source heat pumps are more extensively used as they are less expensive.

With regard to the heating plants of industrial buildings, heat pump systems have only become common in recent years. Natural gas burners firing air heater systems (ground or wall air heaters and mechanical ventilation plants) were much more widely used because of the lower installation costs and higher reliability [17]. Also high temperature radiant systems are common, mainly involving panels heated by pressurized water or steam, radiant tubes equipped with small gas burners, or electrical radiant panels [18,19]. Because of the higher level of thermal insulation in new industrial buildings, low-temperature radiant heating floor systems, mainly coupled to condensing boilers, have been installed during the last decade.

Recently, the author studied the energy performance and indoor comfort conditions of an innovative condensing radiant tubes plant [20]. Wall-mounted air heaters were supplied using hot water produced at 40 °C by recovering the condensation heat of the exhaust from the radiant tubes. This largely improved the thermal efficiency of the heating system, allowing a primary energy saving varying between 7% and 30%, depending on which heating system it was compared with (a radiant floor coupled to a condensing boiler and a traditional air-heating system, respectively). Moreover, the innovative condensing radiant tubes plant provided better thermal comfort conditions during the morning operation of the plant.

In a more recent study, the same authors presented a further development of the system: an air-water heat pump coupled to a condensing radiant tubes plant [10,21]. In this study, an optimization of the hybrid configuration of the system was performed, involving determining the values of the bivalent temperature of the external air, the nominal thermal power, and the peak power of the photovoltaic plant (if installed on the roof).

Motivations and Novelty of the Study

According to Italian Legislative Decree 28/2011 [22], Ministerial Decree 26/06/2015 [23], UNI/TS 11300-5 [24], and Legislative Decree 199/2021 [25] (implementation of the European Directive 2018/2001 on the promotion of the use of energy from renewable sources–RED II [26], recently updated by Directive 2023/2413-RED III [8]), the mandatory renewable quota for a new building is 60% referring to a single service (e.g., heating during winter) or to multiple services (e.g., heating during winter and air conditioning during summer), depending on which services are actually present.

The obligation to satisfy this minimum value often leads HVAC plant designers to provide heat pump systems (typically air/water or direct expansion type) coupled with air terminals (air heaters or ventilation units) or radiant floors. The electrical power consumption of heat pumps (hereafter HP) can be satisfied (at least in part) by installing a photovoltaic system (hereafter PV), whose electrical energy produced does not fall directly into the calculation of QR but does so only indirectly (by reducing the total primary energy delivered and/or increasing that exported).

An air/water heat pump coupled with an air heater heating system presents some critical issues when installed in an industrial building, including the following:

• The heat pump must produce water at 50–55 °C to feed the air heaters or the distribution channels. The coefficient of performance (COP) is, therefore, penalized compared to typical applications of HP in other types of building in which terminals fed by low-temperature water can be used (radiant floors, fan coils);
• If radiant floors are used, all issues related to the thermal inertia of the system (difficulty in controlling the air temperature) and the lack of flexibility in the layout of the factory (difficulty in covering the available radiant surface with equipment and production systems) should be considered;
• Given the significant heights of the rooms to be air-conditioned, air heating systems induce strong stratification of the air inside the warehouse, which is greater the less insulated it is and the colder the external climate;
• Air systems are not very suitable for partial operation, i.e., with localized operation in zones, as their operating principle is based on the mixing of the hot air introduced with that of the air-conditioned environment. In warehouses, where there is less and less presence of people due to the ever-increasing automation of production processes, this determines a level of energy consumption of the system that is difficult to reduce because of its operating principle, unlike for radiant heating.

A heat pump solution requires, in many cases, the simultaneous installation of a photovoltaic system. Depending on the installed PV power, a comparison between the two solutions can, therefore, give different results, both in terms of energy and economics. It must be considered that in winter (when the electricity produced is needed to power the HP), the PV system produces much lower electric energy compared to summer, both due to the reduced solar radiation and the need, with the air system, to bring forward the start of the system in the morning compared to the start time of the work shift (a need that does not exist or is much less with a radiant strip system).

As this study proposes as an innovative contribution, all these elements lead to the need to perform an assessment of the greater or lesser energy, economic, and environmental impacts of a condensing radiant tubes system compared to a heat pump + air heater system, as the following parameters can vary:

• Climate. The performance of both heat pumps and PVs strongly depends on the air temperature and solar radiation. This study shows the necessity of checking the effectiveness of heat pump + air heater systems before their application in a specific resort by considering, as an example, two climatic zones in Italy;
• Characteristics of the shed (less or more thermal insulation, which corresponds to existing rather than new buildings);
• Peak power of the PV system.

Using dynamic simulation software for a building/plant system (Trnsys^® rel. 18), a typical industrial building was modeled in two different climates. Based on the heating thermal loads calculated, two heating systems were modeled for comparison as follows:

• Condensing radiant tubes (CRT). This is a radiant tubes system coupled to an air heating system with terminals (air heaters) placed inside the building, the latter fed, via a decoupling storage tank, by the heat derived from the condensation of the exhaust from the radiant tubes in a dedicated heat exchanger. The air part assists the radiant part, which represents the main heating system;
• Air/water heat pump serving an air heating system with terminals placed inside the building (air heaters) (hereafter heat pump + air, HP-Air), the latter fed via a decoupling storage tank.

2. Materials and Methods

2.1. Organization of the Study

The work was divided into three phases:

• Modeling and simulation with Trnsys^® of a typical industrial building based on the overall characteristics from the literature [10,21] relating to the envelope of the building (in terms of size and exposure characteristics, windows, insulation and air exchange). The purpose of this phase was to determine the heating thermal loads of the building in two different locations (one in the Italian climate zone E and one in the climate zone F, colder than the zone E). The analysis was then extended by varying some of the parameters of the shed to simulate a more insulated building, therefore with lower heating needs;
• The satisfaction of the heating thermal loads was analyzed through dynamic simulation in Trnsys^® of the condensing radiant tubes system (data relating to the extension and positioning of the radiant tubes were provided by [20]);
• A dynamic study was also conducted with respect to the air/water heat pump coupled to air heaters placed in the building in order to quantify the possible advantages of one or the other system when some parameters vary.

In summary, having established:

• the climate;
• the size of the heat pump;
• the temperature of the hot water produced and the performance of the heat pump as a function of the external air temperature;
• the peak power of the photovoltaic system with monocrystalline silicon panels installed on the roof of the shed;

the purpose of the simulations was to compare the performance of the two systems in terms of the following:

• PE_nren,tot (annual total non-renewable primary energy consumed). This is given by the natural gas consumed by the radiant tubes for the CRT system, and by the non-renewable share of the electricity imported from the grid in the case of the HP-Air system, calculated using, respectively, the conversion factors into non-renewable primary energy f_p,nren,NG and f_p,nren,el reported in Section 2.3.
• PES_nren,tot (annual savings of total non-renewable primary energy of the CRT system compared to the HP-Air heating system). A positive value of this index indicates an actual saving of the CRT system compared to HP-Air, and vice versa, a negative value indicates a greater advantage of the heat pump system.
• Specific CO₂ emissions: annual emissions per square meter of floor area of the building emitted during the operation of the system.
• Renewable Quota (Quota Rinnovabile in Italian, QR): in the case of the HP-Air system, the share of energy from renewable sources. It is defined as the ratio between the annual quantities of primary energy as follows:

  a.

  In the numerator, the sum over all the services considered of the renewable primary energy used (renewable primary energy delivered or produced on-site), calculated using the conversion factors into renewable primary energy (f_p,ren) for each energy vector delivered/produced on-site (electricity from the grid, electricity from the photovoltaic system, thermal energy from the external environment; see Table 5);

  b.

  In the denominator, the sum over all the services considered of the total primary energy used (renewable + non-renewable) (total primary energy delivered/produced on-site, calculated using the conversion factors into total primary energy (f_p,tot = f_p,ren + f_p,nren) for each energy vector delivered/produced on-site, see Table 5).

As mentioned above, QR must be at least 60% referring to a single service (in this case, only for winter heating).

5.

NPW (Net Present Worth: an index which takes into account, given the interest rate i and the time period of the economic analysis n, both the investment costs (CAPital EXpenditure, CAPEX) and the operating costs (OPerative EXpenditure, OPEX)):

NPW = CAPEX + OPEX∙(P⁄A,i,n)

where (P⁄A,i,n) = ((1 + i)ⁿ − 1)/(i∙(1 + i)ⁿ) is the discount factor of a series of annual payments.

6.

DPP (Discounted Payback Period: an index which takes into account, given the fixed interest rate i and the time period of the economic analysis n, the time necessary for the annual cash flows to set the NPW to zero. It is the time necessary to compensate, through the possible annual savings S that the HP-Air system allows compared to the CRT system, for the initial extra investment P of the former compared to the latter. The DPP is calculated using the following formula:

DPP = (log (S/(S − P∙i)))/(log (1 + i))

2.2. Industrial Building Modeling

Thermophysical and geometrical characteristics of the transparent and opaque structures of the simulated industrial building and the climate conditions were previously described in [10,21]. For convenience, the data are also reported in

Table 1. Data for the heating according to the Italian Decree 412/93.

Climatic Zone	E	F
Type of building use	E.8 Building for industrial activity
Resort (Province–State)	Manta (Cuneo–Italy)	Agordo (Belluno–Italy)
Altitude a.s.l.	400	610
Latitude North	44°36′	46°17′
Longitude East	7°29′	12°00′
Degree days	2814	3376
Design external air temperature	−9.3 °C	−12 °C

for the two climatic zones E and F, defined by the Italian Legislative Decree 412/1993 [27] on the basis of the heating degree days (zone E: 2101–3000; zone F: >3000), and in

Table 2. Thermal transmittance of the opaque and transparent structures of the two buildings.

Parameter (Unit)	Base Building	Most Insulated Building
Thermal transmittance (W m−2 K−1)
External wall	0.389	0.136
Door	3.50	2.50
Main door	3.50	2.50
Wall facing offices	2.954	2.954
Base facing wall	3.220	3.220
Floor facing ground	0.128	0.128
Ceiling	4.086	1.754
Ceiling shed	0.208	0.062
Window	5.0	5.0
Thermal bridge wall–floor facing ground (W m−1 K−1)	0.353	0.25
Thermal bridge wall–ceiling (W m−1 K−1)	0.262	0.15

and

Table 3. Main characteristics of the building for the Trnsys^® simulation.

	Thermal Zone 1	Thermal Zone 2
Floor area (m2)	7119	716.5
Net height (m)	8.24	8.22
Indoor air temp. (°C)	18	18
Net volume (m3)	58669	5886.2
Presence of people	40	8
Operation heating plant scheduling	from 6.00 am to 6.00 pm
Presence of people and lighting scheduling + heating gain fixed at 5 W m−2	from 8.00 am to 6.00 pm
Degree of activity	2 met
Degree of clothing	1 clo
Air infiltration	0.5 vol h−1

. Figure 1 reports a schematic of the simulated industrial building.

The heating loads were calculated by dynamic simulation of the building with a time step of 0.25 h. Figure 2 reports the daily energy needs. The thermal power of the heating generators was limited to 1500 kW to be consistent with the power installed in the real building.

2.3. CRT and HP-Air Systems Modeling

The CRT and HP-Air systems modeling was described in previous works [10,20,21]; here, a brief description is reported for the convenience of the reader. In the CRT plant, types 659 and 607 were coupled and modified to simulate the dynamic operation of the system (Figure 3). When the CRT plant starts its operation, the burner turns on at maximum power. In this way, the maximum exhaust temperature is obtained. As the temperature of the indoor air increases, the proportional valve of natural gas is modulated to control the thermal power of the CRT burner to produce the heating load requested at that time step. The modulation of the natural gas flow rate decreases the exhaust temperature, while the correct minimum air excess in the burner is guaranteed by the regulation of the exhaust tab. The circuit of the hot water produced at 40 °C by the condensing heat exchanger is separated from the wall-mounted air heaters by a suitable thermal energy storage (1000 L) (Figure 4).

Type 941 is used to simulate the heat pump, whose nominal data are reported in

Table 4. Operating data (nominal data in bold) of the modeled air/water heat pump (electric power refers to the overall consumption of the HP) (T_DB = dry bulb temperature; T_WB = wet bulb temperature; T_w,out = hot water temperature).

		Tw,out = 45 °C		Tw,out = 50 °C		Tw,out = 55 °C		COP at Tw,out (°C)
TDB (°C)	TWB (°C)	kWt	kWe	kWt	kWe	kWt	kWe	45	50	55
−7	−8	484	185	495	209	515	230	2.62	2.37	2.24
−5	−6	509	186	510	210	530	232	2.74	2.43	2.28
0	−1	575	189	573	212	571	238	3.04	2.70	2.40
2	1	604	190	600	213	597	238	3.18	2.82	2.51
7	6	684	194	676	216	669	241	3.53	3.13	2.78
12	11	757	198	746	219	736	243	3.82	3.41	3.03

. It was assumed that two air/water heat pumps of the same type were installed in parallel, serving an 8000 L storage tank (modeled in Trnsys^® with type 158) through a heat exchanger (type 91) (Figure 5).

The control logic of the system provides, for each time step, the activation of the heat pump to maintain the outlet temperature in the upper part of the storage tank between 50 °C and 55 °C. The heat pump, depending on the external air temperature value, T_ext, provides the thermal power necessary for this purpose (if the yield under the given conditions is sufficient). At each time step, thermal storage provides the thermal power required by the heater units (modeled with type 670). The latter are supplied with a variable water flow rate and temperature, having set the ΔT between the inlet and outlet of the same at 10 °C.

In this way, the air flow temperature of the heater units varies as a function of the external air temperature according to the curve in Figure 6a. The air flow rate also varies as a function of the internal ambient temperature according to the curve in Figure 6b (for the latter, a minimum value of 3 vol h⁻¹ was considered sufficient to maintain an adequate degree of uniformity of air temperature within the thermal zone).

Figure 7 shows the Trnsys^® model built for the simulation of the HP-Air system, whereas further hypotheses of the study are reported in

Table 5. Main parameters for energy and economic analysis (according to Italian Legislative Decree 199/2021).

Symbol (Meaning)	Value
fp,nren,NG (Non-renewable primary energy conversion factor for natural gas)	1.05
fp,nren,el (Non-renewable primary energy conversion factor for electricity from the grid)	1.95
fp,ren,el (Renewable primary energy conversion factor for electricity from the grid)	0.47
fp,ren,PV (Renewable primary energy conversion factor for electricity from the PV field)	1
fp,ren,heat_source_HP (Renewable primary energy conversion factor for external air thermal energy)	1
QR (Minimum renewable ratio for new buildings)	60%
PV (kWp) (Peak power of the PV field)	0–250–500
PV (ηnom) (Peak efficiency of the PV field)	16.0%
PV (m2 kWp−1) (Specific area of the PV field)	6.3
Specific CO2 emission factor (kgCO2 kWh−1)
Electric energy from grid	0.26
NG	0.2
NG cost (€ Sm−3)	1.00
Electricity from the grid cost (€ kWh−1)	0.30
Electricity exported value (€ kWh−1)	0.10
CRT investment cost (k€)	200
HP-Air investment cost (k€)	550
PV investment cost (€ Wp−1)	1.0
Interest rate i	2.0%
Period of the economic analysis n (y)	15

.

3. Results and Discussion

This paragraph presents the results of the simulations carried out for the entire heating season, considering three possible cases regarding the installation of a photovoltaic system on the roof of the shed:

• 0 kW_p (no PV installed);
• 250 kW_p (occupation of approximately half of the available roof surface equal to approximately 7800 m², considering appropriate spacing between the rows of panels to avoid mutual shading)
• 500 kW_p (substantial saturation of the available roof surface)

3.1. Energy Analysis

The following figures report the results that refer to the energy performance of the CRT system in the two climate zones E and F compared to the HP-Air system. The figures present how the main energy indices described above vary in the different cases with the aim of evaluating the greater advantage of one system compared to the other:

• Figure 8: Total non-renewable primary energy consumed annually by the CRT and HP-Air systems (PE_nren,tot) in the two climate zones and for the two types of building. The main result is that the condensing radiant tubes system allows for a lower annual consumption than the system with heat pump and air heater units (case without photovoltaic system, PV = 0 kW_p). In relative terms, the advantage of the CRT system in terms of non-renewable primary energy consumption increases from climate zone E to F of the order of 3% (case without PV) to 20% (case with 500 kW_p PV), and from the least insulated building to the most insulated one.
  The reasons are to be found in the following aspects:

  –

  a greater incidence of heat losses from the building in areas with colder climates in the case of air heating systems compared to radiant ones. This is due to the higher operating temperature in the CRT case compared to HP-Air due to the operating principle of the two heating systems;

  –

  a lower COP of the heat pump in colder climates.

Only the presence of the photovoltaic system allows an advantage of the HP-Air solution. In the figures on the center and the right (respectively, PV = 250 kW_p and 500 kW_p), it is highlighted how the lower consumption of the HP-Air solution is due to the installation of the PV system; the greater the peak power of the installed photovoltaic, the lower the energy consumption of the HP-Air.

• Figure 9: Specific CO₂ emissions in the two climate zones and for the two types of buildings. In the base case (without PV), the CRT system allows for lower emissions than the HP-Air, especially in the case of more insulated buildings. The installation of a 250 kW_p photovoltaic system, and even more so of a 500 kW_p system, allows for lower emissions than the solution with a heat pump, thanks to the reduction in electricity consumption from the grid.
• Figure 10: Total non-renewable primary energy consumed monthly by the CRT and HP-Air systems in the two climatic zones, in the case of both the least and the most insulated building (Table 2). What was already highlighted in the previous point is confirmed—the presence of the photovoltaic system allows a lower consumption of non-renewable primary energy of the HP-Air system, which otherwise performs worse than the radiant tubes system. In particular, the primary energy savings allowed by the CRT are greater in the coldest months, when the HP-Air system has a more penalized performance (for the same reasons mentioned above).
• Figure 11: The figure summarizes the annual total non-renewable primary energy savings (PES) and CO₂ emission savings of the CRT system compared to the HP-Air one. The highest PES (17.9%) is in the case of zone F and a new building, while an existing building in zone E still produces significant savings in primary energy (8.9%). The savings become negative (that is, the system with the heat pump and the air heaters consumes less primary energy) when the photovoltaic system is installed. Similar considerations can be made in relation to the savings in CO₂ emissions.

Figure 11 also shows the renewable quota (QR) in the case of the HP-Air system. It can be seen that in the base case, the HP-Air solution is unable to satisfy the 60% value, with values well below 50%. The 60% quota is exceeded only if adequate photovoltaic power is installed and to a greater extent in the case of a more insulated shed.

The results must be considered to be entirely conservative, since each of the two thermal zones simulated in Trnsys^® corresponds to a single air node. That is, the stratification of the air was not taken into account. This assumes absolutely significant values in industrial buildings (which usually have heights that far exceed 8 m), especially in the case of air heating systems (of the order of one degree per meter) as highlighted by the literature, as reported in references [28,29,30,31,32,33]. In this case, in fact, there is a significant reduction in the average seasonal emission efficiency (up to 30%), with a consequent increase in heat loss through the roof and other surfaces of the shed, and therefore, an increase in consumption of the HP-Air system. The real savings achievable with the CRT system are consequently certainly greater than the values in Figure 11, up to a value that is even double [28,29,30,31,32,33].

3.2. Economic Analysis

The energy analysis was completed with an economic analysis. The graphs in Figure 12 show the discounted cash flow trends of the two plant solutions compared in the two climate zones and for the two types of buildings. The graphs report the values of the net present worth (NPW), that is, the value of the curves in the last year of the economic analysis period (15 years). Moreover, the graphs report the discounted payback period (DPP) of the investment in the HP-Air system compared to the CRT system, that is, the abscissa of the intersection point between the curve with a solid line and the dotted line for each of the four cases considered: existing building in zone E; new building with the highest insulation in zone E; existing building in zone F; new building with the highest insulation in zone F).

NPW and DPP, reported also in

Table 6. Investment costs (CAPEX), annual operating costs (OPEX), and net present worth (NPW) of the different plants compared (negative values indicate an outlay, positive values an income). Discounted payback period (DPP) of the investment in the HP-Air plant compared to the CRT plant (positive values mean a greater advantage of HP-Air compared to CRT).

		CAPEX (k€)			OPEX (k€)
		Plant	PV	Total	Electr. from the Grid	Electr. Exported	NG	Total	NPW (k€)	DPP (y)
PV = 0 kWp
Zone E	CRT	−200		−200			−85.317	−85.317	−1296	-
	HP-Air	−550	0	−550	−141.237	0		−141.237	−2365
Zone E More Insulated	CRT	−200		−200			−48.093	−48.093	−818	-
	HP-Air	−550	0	−550	−81.610	0		−81.610	−1599
Zone F	CRT	−200		−200			−-98.505	−98.505	−1466	-
	HP-Air	−550	0	−550	−167.427	0		−167.427	−2701
Zone F More Insulated	CRT	−200		−200			−55.803	−55.803	−917	-
	HP-Air	−550	0	−550	−97.289	0		−97.289	−1800
PV = 250 kWp
Zone E	CRT	−200		−200			−85.317	−85.317	−1296	>25
	HP-Air	−550	−250	−800	−90.152	17.301		−72.852	−1486
Zone E More Insulated	CRT	−200		−200			−48.093	−48.093	−818	>25
	HP-Air	−550	−250	−800	−38.149	19.842		−18.307	−785
Zone F	CRT	−200		−200			−98.505	−98.505	−1466	-
	HP-Air	−550	−250	−800	−123.958	13.010		−110.948	−1976
Zone F More Insulated	CRT	−200		−200			−55.803	−55.803	−917	>25
	HP-Air	−550	−250	−800	−57.480	14.230		−43.251	−1106
PV = 500 kWp
Zone E	CRT	−200		−200			−85.317	−85.317	−1296	14.7
	HP-Air	−550	−500	−1050	−59.396	41.377		−18.019	−782
Zone E More Insulated	CRT	−200		−200			−48.093	−48.093	−818	12.1
	HP-Air	−550	−500	−1050	−14.309	46.224		31.915	−140
Zone F	CRT	−200		−200			−98.505	−98.505	−1466	>25
	HP-Air	−550	−500	−1050	−89.856	29.142		−60.714	−1330
Zone F More Insulated	CRT	−200		−200			−55.803	−55.803	−917	19.0
	HP-Air	−550	−500	−1050	−36.380	34.696		−1.684	−572

, were calculated on the basis of the interest rate and period values of the economic analysis reported in Table 5. The reported investment cost values (CAPEX) are indicative, provided based on the author’s experience.

The most advantageous solutions are those with a higher NPW (lower in absolute value). A negative DPP value indicates a greater advantage of the CRT system compared to the HP-Air one, and vice versa, a positive DPP value indicates a greater advantage of the heat pump.

Figure 12 and Table 6 show that in the base case (without a photovoltaic system), the CRT system is significantly more advantageous even in terms of economic analysis; in fact, NPW is, in all four cases, much higher than the heat pump and air heater system (between 45% and 50%). In particular, the most advantageous case is that of the most insulated building in zone F, with a difference between the NPW values of approximately 50% in favor of the CRT system.

The installation of the photovoltaic field determines an initial outlay of the HP-Air solution that is decidedly higher than the base case (550, 800, and 1050 k€, respectively, for 0, 250, and 500 kW_p against 200 k€). Nevertheless, it allows the heat pump + air heater system to be more advantageous (thanks to the use of renewable energy sources-electrical energy from PV and aerothermal energy to the heat pump evaporator) only in the case of 500 kW_p (Figure 12. The operational savings provided by the medium-sized photovoltaic system (250 kW_p) do not allow the recovery of the higher initial investment in a short time (Figure 12).

In this sense, the investment in the HP-Air system can provide an interesting payback time (around 12 years) only in climate zone E with a new building (Table 6).

3.3. Economic Sensitivity Analysis

This section reports the results of the economic analysis varying the cost of the standard cubic meter of natural gas (1–1.5 € Sm⁻³) and the electric kilowatt hour from the grid (0.30–0.40 € kWh_el⁻¹). The lower values of the proposed pairs are those in the current phase; the higher values refer to a possible scenario of a further increase in the cost of energy. The figures refer to the three cases analyzed previously: no photovoltaic system (Figure 13), or presence of the same in the two sizes (250 kW_p in Figure 14, 500 kW_p in Figure 15), in climate zones E and F, building slightly or more insulated.

Increasing the cost of electricity from the grid makes the CRT solution even more advantageous than HP-Air (Figure 13a vs. Figure 13c). The presence of the photovoltaic system mitigates this advantage; that is, the increase in the installed peak power makes the HP-Air solution increasingly competitive compared to CRT despite the increase in the cost of electricity from the grid (compare Figure 13b with Figure 14b and Figure 15b).

Furthermore, increasing the cost of electricity from the grid makes the radiant tubes system more competitive than the heat pump system even when installing the PV plant (compare Figure 13c with Figure 14c and Figure 15c).

However, the payback time of the heat pump system is (partially) acceptable (10–12 years) only in the case of a 500 kW_p system in climate zone E and with energy prices equal to 1.5 € Sm⁻³ and 0.3 € kWh⁻¹ (Figure 15c).

In general, however, Figure 13, Figure 14 and Figure 15 highlight how the continuous line curves tend to almost never intersect the dotted line ones. This means that the payback times of the heat pump + air system are largely greater than those of the radiant tubes system.

3.4. Case of Differentiated Zones Heating

Due to increasing automatization and digitalization of industry, industrial buildings are less and less densely occupied by workers, so they can be heated with different temperature set point values depending on the use of the different zones. Approximately 30–40% of the area is used for production with a stable presence of workers and the remaining 60–70% for storage and transit areas where it is not necessary to maintain the same temperature as for production. Radiant appliances are particularly suited to meet these needs, unlike heat pumps with air heaters. In fact, the latter heat the environment based on the most homogeneous possible mixing of the heated air with the internal air.

To quantify the advantages of the radiant tubes system, we considered a case in which only 35% of the area is heated according to the set points in Figure 6 (production area), while the remaining 65% is heated to a set point of 4 °C lower. Then, simulations were carried out for the case of climate zone E, the most widespread in Italy, and for the two levels of thermal insulation of the shed.

The results are shown in Figure 16 and Figure 17 for the case without a photovoltaic system.

Given a reduction in the heating requirement of the shed of around 30% (greater in the mid-season months and lower in the colder months, Figure 16a), in partial zone heating there is a substantial advantage in the use of CRT compared to an air system powered by a heat pump (Figure 16b and Figure 17a). In particular, Figure 17b shows the annual savings in total non-renewable primary energy consumption and CO₂ emissions of the CRT system compared to the HP-Air system for climate zone E for the two types of buildings in the base case. Note that partial zone heating allows a PES of the CRT system, compared to the HP-Air system, three (in the case of a more insulated building) and up to four (building with standard insulation) times greater than in the case with a uniform set point. This allows for a consequent strong increase in emissions savings.

These results were obtained with the conservative hypothesis that, in the areas not permanently occupied by workers, the set point was reduced by 4 °C. A decrease of up to 8 °C is not uncommon, which determines a further increase in the PES achievable with the radiant tubes system.

4. Conclusions

The main scope of the study was to evaluate whether the heat pump system (a choice widely made today among system designers) is actually always justified for the heating of industrial buildings on the basis of the energy and economic indices. The study is based on dynamic simulations. Nevertheless, the results obtained are of the same order of real operating data as confirmed by the discussion of the author with HVAC plants designers and manufacturers of condensing radiant tubes. The main results of the simulations show that:

• the condensing radiant tubes system allows for a lower annual consumption of non-renewable primary energy than the heat pump and air heaters system when no photovoltaic system is installed. The advantage of the CRT system increases from climate zone E to F and from the least insulated building to the most insulated, with values between 9% and 18%. These values are surely precautionary as they can be significantly higher (even double) due to the high stratification of the air caused by air heating systems in environments of significant heights, such as industrial buildings;
• only the presence of the photovoltaic system allows for a lower consumption of non-renewable primary energy of the HP-Air, which otherwise performs worse than the CRT system;
• in the base case (without photovoltaic), the CRT system allows lower CO₂ emissions than the HP-Air, especially in the case of a more insulated building (emission savings between 5% and 28%);
• in the base case, the HP-Air solution is unable to satisfy the 60% value of the renewable quota, with values well below 50%. The 60% renewable quota is exceeded only if adequate photovoltaic power is installed and to a greater extent in the case of a more insulated building;
• the economic analysis reveals that the CRT system is significantly more advantageous in almost all the cases analyzed: the NPW is much higher than the HP-Air system. The installation of the photovoltaic field, although determining an initial outlay of the HP-Air solution that is decidedly higher than the base case, allows the heat pump + air heater system to be more advantageous only in very few of the cases considered (in particular: 250 kW_p PV system, natural gas cost 1.5 € Sm⁻³, electricity cost 0.3 kWh⁻¹). In all other cases the CRT system is more advantageous;
• the increase in the cost of electricity from the grid makes the CRT solution even more advantageous than HP-Air. The presence of the 250 kW_p PV system mitigates this advantage;
• the CRT system is much better suited, compared to HP-Air, to use with differentiated heating set points based on the real use of the building area. The savings of non-renewable primary energy and CO₂ emissions increase significantly (up to 4–5 times in the best case).

The condensing radiant tubes system allows for a higher operating temperature, allowing the air temperature inside the shed to be reduced while maintaining the same level of comfort. This determines a lower air stratification effect and, therefore, allows for an increase in the real savings of the CRT solution.

In conclusion, for the application considered here (heating of industrial buildings), the choice of preferring a priori the air/water heat pump system that feeds air terminals in the environment compared to the system with condensing radiant tubes is not justified. As a main conclusion, the most advantageous choice may vary in relation to different variables: the climate (in this study, two climatic zones of Italy were considered as an example, but the specific climate of the resort has to be considered as it determines the performance of both heat pump and PV), the thermal performance of the building considered, and the peak power of the photovoltaic system installed.

For these reasons, a careful analysis for each specific case by dynamic simulation of the building and HVAC plant is certainly advisable to avoid implementing technical solutions that do not minimize greenhouse gases emissions. These conclusions can be extended also to other types of tall buildings, like hangars and warehouses, that may have many similarities with industrial buildings.