Alternative Heating, Ventilation, and Air Conditioning (HVAC) System Considerations for Reducing Energy Use and Emissions in Egg Industries in Temperate and Continental Climates: A Systematic Review of Current Systems, Insights, and Future Directions

Abstract

Egg production is amongst the most rapidly expanding livestock sectors worldwide. A large share of non-renewable energy use in egg production is due to the operation of heating, ventilation, and air conditioning (HVAC) systems. Reducing energy use, therefore, is essential to decreasing the environmental impacts of intensive egg production. This review identifies market-ready alternatives (such as heat pumps and earth–air heat exchangers) to traditional HVAC systems that could be applied in the industrial egg sector, specifically focusing on their use in temperate and continental climates. For this analysis, energy simulations were run to estimate the typical thermal loads of caged and free-run poultry housing systems in various Canadian locations, which were used as examples of temperate and continental climates. These estimations were then used to evaluate alternative HVAC systems for (1) their capability to meet the energy demands of egg production facilities, (2) their environmental impact mitigation potential, and (3) their relative affordability by considering the insights from a systematic review of 225 relevant papers. The results highlighted that future research should prioritize earth–air heat exchangers as a complementary system and ground source heat pumps as a stand-alone system to reduce the impacts associated with conventional HVAC system operation in egg production.

1. Introduction

The world population has reached 8 billion people and will continue to increase in the foreseeable future [1]. This increase in population is placing increasing demand on the agri-food sector. According to the FAO, the world demand for agricultural products in 2050 will be at least 50% higher than in 2013 [2]. This remarkable rise in demand challenges our collective capacity for food production and security. Moreover, since the food sector already contributes a significant share of global environmental impacts, potential sustainability interventions merit pressing attention [3,4]. One of the most environmentally impactful food sectors is livestock production [5]. Steinfeld et al. [6] estimated that livestock contributes about 18% of anthropogenic greenhouse gas emissions (GHGEs) annually. According to Goodland and Anhang [7], livestock and their byproducts’ contribution is underestimated and may represent up to 51% of annual GHGEs worldwide.

Egg production is relatively efficient compared to other forms of livestock production, but is also among the most rapidly expanding livestock sectors [8]. It is estimated that, by 2050, poultry meat and egg production will increase 2.5-fold due to growing population and dietary changes [9]. Accordingly, ameliorating sustainability outcomes in the industrial egg sector is crucial. Recent studies investigating potential environmental sustainability improvements for egg production include valorization technologies for industrial poultry waste [10], mitigation and management strategies to improve nitrogen use efficiency in egg supply chains [11], and net-zero-energy housing designs [12].

The operation of poultry houses can account for up to half of non-renewable energy consumption in egg production [13]. Most energy consumption in poultry houses is attributed to climate control, specifically to the operation of heating, ventilation, and air conditioning (HVAC) systems [12,14]. HVAC energy consumption accounts for consistently higher shares of overall energy use than other housing operations, such as lighting, feed distribution, and manure removal [15]. Hence, improving conventional HVAC systems in poultry houses represents an important sustainability intervention point to reduce non-renewable energy use and associated environmental impacts. Alternative HVAC systems, such as geothermal systems, have been recognized as potential substitutes for conventional systems in poultry farming (e.g., [12,16]).

Both active and passive systems may provide promising HVAC alternatives. Among these, systems based on geothermal, aerothermal, and hydrothermal energy can be considered. These include but are not limited to ground source heat pumps (GSHPs), water source heat pumps (WSHPs), ground source air heat pumps (GSAHPs), earth–air heat exchangers (EAHEs), and air source heat pumps (ASHPs). Heat pumps are thermal devices that can transfer heat from a heat source (e.g., ground, water, air) to a heat sink (e.g., the enclosure of the poultry house) through a thermodynamic cycle operated on a fluid heat transfer medium. Since heat pumps require a certain amount of energy as an input to operate the thermodynamic cycle, GSHPs, WSHPs, GSAHPs, and ASHPs are considered active systems. By contrast, EAHEs are considered passive systems. Geothermal systems (GSHPs, GSAHPs, and EAHEs) rely on the stable ground temperature as a heat source or sink. GSHPs are a well-researched technology that uses a closed-loop system with anti-freeze fluid as the heat transfer medium [17,18]. GSAHPs and EAHEs rely on air as the transfer medium through an open or closed-loop system [19,20]. Specifically, EAHEs have been used in livestock buildings to pre-heat or pre-cool the air supply used for ventilation [21]. Since EAHEs are passive geothermal systems, their effectiveness is more dependent on local climates compared to active systems [22]. Hydrothermal systems (WSHPs) use large bodies of water, such as lakes, ponds, or wells, as a heat source or sink. The open water source reliance of WSHPs limits the adoption of these systems geographically [23], making their application for livestock systems more difficult. Finally, ASHPs are aerothermal systems that use air as a heat source or sink to heat or cool the facility [20]. Outdoor air temperature fluctuates to a higher degree than ground temperature, which, depending on local climates, can make ASHPs less efficient compared to active geothermal systems [24].

Differences in the potential environmental sustainability implications of using alternative HVAC systems can be evaluated using methods such as life cycle assessment (LCA). LCA is a multi-criteria, systems-level decision support tool that enables understanding the magnitude and distribution of the resource/environmental impacts characteristic of product systems. It can be used to evaluate technology/management alternatives considering potential trade-offs within and across supply chain stages and impact types [25,26]. The holistic nature of LCA makes it suitable for assessing the environmental sustainability of food supply chains and other complex and interconnected systems, including potential sustainability interventions such as the implementation of alternative HVAC technologies.

To date, a variety of LCA studies have been conducted on alternative HVAC systems for residential and commercial applications. However, these have focused almost exclusively on GHGEs at the expense of other kinds of resource/environmental impacts [27]. Moreover, very few LCA studies have considered the application of such systems in intensive livestock production contexts. Those that do mainly focus on GSHPs. Studies on pig houses in Italy and Korea, for example, indicate that GSHPs reduce energy consumption and operational costs compared to conventional HVAC systems [28,29]. A comparative study of GSHPs and conventional HVAC in broiler houses in Korea found that GSHPs reduced energy costs and contributed to superior indoor air quality [30].

Reference values of energy use in livestock houses are limited in the scientific literature [14]. The energy consumption of livestock houses can be measured through audits and monitoring studies or, alternatively, energy needs can be estimated through simulation models. Such theoretical work in intensive livestock houses has become common in the last few years (e.g., [31,32,33,34]). This is because simulation models make it possible to consider the specific boundary conditions of interest, such as climate conditions, livestock farming practices, and specific HVAC systems.

The specific requirements of HVAC systems for confined poultry housing and other livestock houses may also confound the transferability of insights from studies of commercial and residential installations [12,35]. Poultry-specific requirements include maintaining an appropriate thermal-neutral range, air relative humidity, air quality, and ventilation rate inside the house [12,35,36]. Additionally, it is important to consider endogenous factors influenced by the hens themselves, such as heat and moisture production [12,14]. Many variables affect such factors, including housing type (e.g., conventional caged and free-run systems), which differ in hen density, manure management strategies, dust levels, and hen weight and age [12,14,37].

A foundational step prior to assessing alternative HVAC systems for their potential contributions to the environmental sustainability aspects of egg production is, therefore, identifying which technologies may potentially be suitable in the first place, considering the physiological requirements of laying hens and the capacity of HVAC technologies to meet these requirements in a cost-effective manner. Herein, this information gap is addressed by using dynamic energy simulations to estimate HVAC requirements for intensive egg production facilities, followed by a systematic literature review to answer the following specific questions:

• What are the typical annual energy needs and the maximum thermal loads for heating and cooling caged and free-run layer hen housing systems? This research question considers the specific physiological requirements of poultry, housing characteristics, and seasonal variations across temperate and continental climates (using several locations in Canada evincing different temperate/continental climate conditions as examples) throughout the year (RQ1).
• What insights from residential and commercial alternative HVAC systems are transferable for potential application in caged and free-run poultry housing systems in temperate and continental climates? What are the limitations? This research question considers the estimated heating and cooling loads and needs from RQ1, potential energy efficiency, and environmental impacts (RQ2).
• What subset of alternative HVAC technologies could be recommended for priority consideration for application in confined poultry housing, subject to further, detailed life cycle-based sustainability assessment in order to determine potential net benefits/impacts in the context of egg production? This research question considers technological maturity, affordability, and the findings from RQ2 (RQ3).

Several works have explored alternative HVAC technologies and other renewable energy systems for applications in livestock, such as poultry houses [32,35,38,39]. However, there needs to be more overall understanding regarding the practicality and suitability of alternative HVAC technologies in specific livestock farming systems, pointing to the need for comprehensive frameworks in the literature. The review fills this gap by using a suite of practical criteria to identify potentially suitable alternative HVAC systems for application in egg industries. Such practical considerations include the technologies’ capability to meet energy demands specific to egg production, the potential environmental impact associated with such technological scaling, and the affordability and maturity of the considered technologies. Such practical frameworks make this work valuable for stakeholders involved in the egg industry, renewable energy industries, and HVAC engineers. Moreover, the results of this work could represent a solid starting point for academia and industry in alternative HVAC system consideration solutions for egg industries and provide a practical approach for other academic researchers interested in exploring alternative HVAC technologies for application in other livestock farming sectors.

2. Methods

2.1. Simulation Methodology

RQ1 aimed at identifying the typical annual energy needs and maximum heating and cooling loads in both conventional caged and free-run laying hen housing systems. This task was performed by running dynamic energy simulations considering scenarios characterized by different housing system characteristics and seasonal variations in outdoor weather conditions. The considered weather conditions are the ones typical of different geographical locations of Canada (adopted as examples in this work) that are characterized by different temperate and continental climates. The results of the simulations are considered to represent a reliable range of thermal loads and energy needs typical of egg production in industrial facilities located in the analysed climate contexts.

2.1.1. Adopted Simulation Model

The energy simulations were performed by adopting the poultry house model previously developed and validated by Costantino et al. [31]. This model dynamically simulates the thermal behaviour of poultry houses through a thermal–electrical analogy between the analysed building and an electrical network comprising five resistances and one capacitance (5R1C), in compliance with the ISO 13790 standard [40]. This model was chosen as its reliability was demonstrated through experimental validation encompassing indoor climate conditions and energy consumption [31]. Moreover, the model was later adopted in other works for evaluating the effects of a new ventilation strategy [41] and different types of building envelopes [42] on the energy consumption of poultry houses. Tan et al. [43] used the thermal–electrical analogy component for the energy modelling of broiler houses to investigate its integration into rural energy systems. The implemented simulation model is a lumped-parameter model based on the perfect-mixing assumption (i.e., uniform indoor air temperature). This simulation approach is widely adopted for energy analyses of livestock houses, as highlighted in a specific literature review focused on the topic [44]. Additionally, the robustness of the 5R1C method [40] is also demonstrated by its adoption in other contexts, such as pig houses [45], greenhouses [46], and residential buildings [47].

The model from Costantino et al. [31] was specifically adapted to simulate laying hen houses. Here, the total thermal emission (${\mathsf{\varphi }}_{\mathrm{t}\mathrm{o}\mathrm{t}}$) due to hens is implemented in the model through the following formulation as defined in [48] and widely adopted in the literature:

$${\mathsf{\varphi }}_{\mathrm{t}\mathrm{o}\mathrm{t}}=\left(6.28·m_{\mathrm{h}\mathrm{e}\mathrm{n}}^{0.75}+25·Y_{\mathrm{e}\mathrm{g}\mathrm{g}}\right)·n_{\mathrm{h}\mathrm{e}\mathrm{n}}\left[\mathrm{W}\right]$$

(1)

where $m_{\mathrm{h}\mathrm{e}\mathrm{n}}$ is the body mass [$\mathrm{k}\mathrm{g}$] of one hen (also known as live weight), $Y_{\mathrm{e}\mathrm{g}\mathrm{g}}$ is the daily egg production [$\mathrm{k}\mathrm{g}{\mathrm{d}}^{-1}$], and $n_{\mathrm{h}\mathrm{e}\mathrm{n}}$ is the total number of hens present in the house. The values of $m_{\mathrm{h}\mathrm{e}\mathrm{n}}$ and $Y_{\mathrm{e}\mathrm{g}\mathrm{g}}$ are defined as a function of hen age using data from a commercial management guide for laying hens [49]. The sensible and latent thermal emissions are obtained from Equation (1) following the procedures defined in [48].

2.1.2. Theoretical Layer Hen House Used in the Simulations

The case study considered in this work is a laying hen house configuration that is typical in Canadian egg production. This case study was dynamically simulated through the previously presented model. As illustrated in Figure 1, the considered hen house is 117 m long and 7 m wide with a net floor area of around 820 ${\mathrm{m}}^2$. The height at the eave is 3.3 m and at the ridge is 4.3 m. The net volume is approximately 3110 ${\mathrm{m}}^3$. The main building axis has an east–west orientation with its longest dimensions having a north–south orientation.

The building envelope was assumed to be fully opaque with no glazing elements, as is common in large-scale egg production facilities. The thermophysical properties of the envelope components are reported in

Table 1. Thermophysical properties of the building envelope components of the analysed hen house.

Envelope Component	$\mathit{U}-\mathit{V}\mathit{a}\mathit{l}\mathit{u}\mathit{e}$ $[\mathbf{W}{\mathbf{m}}^{-2}{\mathbf{K}}^{-1}$]	${\mathsf{\kappa }}_{\mathbf{i}}$ $[\mathbf{k}\mathbf{J}{\mathbf{m}}^{-2}{\mathbf{K}}^{-1}$]	${\mathsf{\alpha }}_{\mathbf{s}\mathbf{o}\mathbf{l}}$ $[-$]
Walls	0.29	3.9	0.3
Ceiling	0.19	3.9	0.6
Floor	0.71	68.1	-

$U-value$: stationary thermal transmittance, ${\mathsf{\kappa }}_{\mathrm{i}}$: internal aerial heat capacity, ${\mathsf{\alpha }}_{\mathrm{s}\mathrm{o}\mathrm{l}}$: solar absorption coefficient.

. As Canadian regions were used as examples to investigate temperate and continental climates, the variables used as inputs in the model followed Canadian norms and regulations. The adopted thermal transmittances ($U-values$) of the envelope were the minimums recommended for industrial egg production in Canada [50,51]. The internal aerial heat capacities (${\mathsf{\kappa }}_{\mathrm{i}}$) of walls and ceilings were estimated based on a polyurethane sandwich panel makeup. A concrete slab was assumed for the floor. For this reason, a higher ${\mathsf{\kappa }}_{\mathrm{i}}$ was used for the calculations. The external metal sheet of the sandwich panels was assumed to be painted in a light colour for the walls and an intermediate colour for the roof. Thus, the solar absorption coefficient (${\mathsf{\alpha }}_{\mathrm{s}\mathrm{o}\mathrm{l}})$ was assumed to be equal to 0.3 and 0.6, respectively, as shown in Table 1.

The production cycle in the analysed hen house was 52 weeks (the average lay cycle length in Canada), and hens were housed from 18 to 70 weeks of age. An increase in hen body mass from around 1.5 to 2.0 kg was considered during the production cycle, as reported in [49]. The beginning of the production cycle was set for 1 January. The variation at the beginning of the production cycle was estimated to have a minor impact on the thermal loads and energy needs. Nevertheless, future works may investigate the effects of the shift of the production dates, as suggested in [33].

The air set point temperature for heating and cooling was equal to 18 °C and 20 °C, respectively, as recommended in [49]. Please note that the adopted set point temperatures were intentionally demanding to ensure that the identified HVAC solutions could maintain even the most restrictive, yet realistic, indoor climate conditions in egg facilities. However, during the operation stage, farm managers can adjust the set point temperatures to decrease energy consumption due to climate control. The impact of such variations was not assessed within the framework of this study, as it is beyond the scope of this work. More details about the impact of varying set point temperatures and other parameters on the heating needs and operational costs can be found in [52], which focuses on laying hen production in the Midwestern USA. The minimum ventilation flow rate to guarantee adequate indoor air quality is 1.1 ${\mathrm{m}}^3{\mathrm{h}}^{-1}$ per hen. This value was adopted from [53] and represented the minimum rate measured in a real hen facility during experimental monitoring. The considered ventilation flow rate was in accordance with the minimum ventilation flow rate for poultry used by Wang et al. [32] and Costantino et al. [31].

2.1.3. Definition of the Scenarios for the Simulations

The energy simulation model defined in Section 2.1.1 simulated the hen house presented in Section 2.1.2 under different scenarios. The scenarios differed in the type of housing system (conventional cage or free-run system) and outdoor weather conditions. The cage system scenarios considered a house with a total of 26,000 hens, representing the average cage stocking density [54] for a house of these configurations. In the caged scenarios, hens were raised in 4-tier stacked cages arranged inside the hen house in two rows. A space allowance of 23 hens ${\mathrm{m}}^{-2}$ per hen was considered, as is required for white laying hens, the most commonly housed hen in industrial caged egg production [55,56]. For the free-run system scenarios, a minimum space allowance of 5 hens ${\mathrm{m}}^{-2}$ was considered, which, based on the theoretical house net floor area, accommodates 4095 hens.

Climate data from four Canadian cities/towns were selected to represent a wide range of outdoor weather conditions that include both temperate and continental climates that are identified by Köppen classification as “C” and “D” letters, respectively. Those climates are considered representative of various geographical areas, especially in North America. The regions were selected based on climatic variety encompassing the Köppen classifications updated by Chen and Chen [57] and inland and coastal characteristics. The selected cities/towns and associated regions are reported in

Table 2. Climate regions according to the updated Köppen classification [53] and the respective selected reference locations considered in the scenarios. The values of heating degree days (HDD), cooling degree days (CDD), average annual outdoor air temperature (${\overline{\theta }}_{air\_out}$), and the annual total solar radiation on the horizontal plane ($E_{tot\_hor}$) are presented for each reference location.

Climate Regions	$$\begin{gathered} \mathbf{HDD}\mathbf{*} \\ \mathbf{[}\mathbf{°}\mathbf{C}\mathbf{d}\mathbf{]} \end{gathered}$$	$$\begin{gathered} \mathbf{CDD}\mathbf{*} \\ \mathbf{[}\mathbf{°}\mathbf{C}\mathbf{d}\mathbf{]} \end{gathered}$$	$$\begin{gathered} {\overline{\mathit{\theta }}}_{\mathit{a}\mathit{i}\mathit{r}\_\mathit{o}\mathit{u}\mathit{t}} \\ \mathbf{[}\mathbf{°}\mathbf{C}\mathbf{]} \end{gathered}$$	$$\begin{gathered} {\mathit{E}}_{\mathit{t}\mathit{o}\mathit{t}\_\mathit{h}\mathit{o}\mathit{r}} \\ \mathbf{\left[}\mathbf{G}\mathbf{J}{\mathbf{m}}^{\mathbf{-}\mathbf{2}}{\mathit{y}}^{\mathbf{-}\mathbf{1}}\mathbf{\right]} \end{gathered}$$
Inland–Dfc (Calgary, Alberta)	5086	37	4.0	5.0
Coastal–Dfb (Greenwood, Nova Scotia)	4188	139	7.0	4.7
Inland–Dfb (London, Ontario)	3984	233	7.3	5.0
Coastal–Cfb (Vancouver, British Columbia)	2932	41	9.7	4.4

* The baseline temperature for HDD and CDD calculation is 18.3 °C. Köppen classifications of climate regions are as follows: Dfc—snow, fully humid, cool summer; Dfb—snow, fully humid, warm summer; Cfb—mild temperate, fully humid, warm summer.

, along with heating degree days (HDD), cooling degree days (CDD), average annual outdoor air temperature (${\overline{\mathsf{\theta }}}_{\mathrm{a}\mathrm{i}\mathrm{r}\_\mathrm{o}\mathrm{u}\mathrm{t}}$), and annual total solar radiation on the horizontal plane ($E_{\mathrm{t}\mathrm{o}\mathrm{t}\_\mathrm{h}\mathrm{o}\mathrm{r}}$), to show the climatic differences between the considered locations.

The hourly meteorological data used as inputs for the simulations were the Canadian Weather for Energy Calculations (CWEC), the recognized variant of the typical meteorological year (TMY) specifically developed for Canadian locations [58]. The TMY is a meteorological dataset designed to reduce a long-term time series of meteorological variables—such as outdoor air temperature and solar radiation—into a representative year. It synthesizes the “climate normal” conditions of a specific location into a single year by concatenating 12 actual months selected from long-term data records spanning at least ten years [59]. Each month’s selection is based on statistical analyses to determine which month most closely represents the “normal” conditions in the distribution [60]. In the specific case of the CWEC, the typical meteorological year was generated using the Sandia method [61] on the hourly weather data from the Canadian Weather Energy and Engineering Datasets (CWEEDS) [62]. The dataset adopted within the framework of this study was obtained from the publicly available EnergyPlus database [63].

Eight different simulation scenarios were created by combining two different housing systems and four weather conditions. The hen house was simulated in each of them for a year with an hourly time step. The outputs of the simulations were the thermal loads and the annual energy needs for heating and cooling. A thermal load is the amount of sensible heat that has to be instantaneously provided (heat load) or removed (cooling load) from the hen house to maintain the heating and cooling set point temperatures, respectively. The integration of the heating and cooling loads over one year is the annual heating and cooling needs, respectively. The thermal loads and the energy needs are expressed in units of useful floor area [${\mathrm{m}}^2$] to enable the comparison with other works found in the literature review.

The estimated energy needs can be partially or totally met by both active (i.e., heat pumps) and passive (i.e., ventilation, cooling pads, EAHEs) systems. If active systems are used, the system efficiency has to be considered for calculating the energy consumption. Please note that the ventilation rate used in the model is representative of an average rate required for guaranteeing poultry health. Higher ventilation rates could be used to reduce the determined cooling loads or meet them entirely. Nevertheless, this analysis was considered beyond the scope of the current work.

2.2. PRISMA Methodology

In order to address RQ2 and RQ3, the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) scheme was used to review and synthesize the relevant literature [64].

2.2.1. Search Strategy and Screening Criteria

The Web of Science search engine was used to identify appropriate peer-reviewed journal articles for each review question, using relevant search queries as described in

Table 3. Summary of search queries and the number of articles reviewed for research questions (RQs) 2 and 3.

Research Questions	Search Queries	Number of Articles Reviewed over Available
RQ2	(“ground source heat pump*” or “air-source heat pump*” or “water source heat pump*” or “earth tube*” or “earth–air heat exchanger*” or “ground source air heat pump*”) and (“Life cycle assessment*” or “energy efficienc*”)	141/551
RQ3	(“ground source heat pump*” or “air-source heat pump*” or “water source heat pump*” or “earth tube*” or “EAHE*” or “ground heat exchanger” or “ground source air heat pump*”) and (“payback period*” or “payback time” or “techno-economic” or “Life cycle cost*” or “LCC” or “Life-cycle-cost*” or “Life-cycle costing”)	84/311

. The considered temporal scope covered publications from 2000 to 2023, which is suitable for identifying current alternative HVAC technologies and poultry housing practices. Relevant papers in the reference lists of selected articles were also considered within the same temporal scope. The search query for RQ2 excluded review papers. For both review questions, initial screening excluded publications not written in English and those unavailable through University of British Columbia institutional subscriptions. For the second screening, appropriate journal articles were selected based on title, abstract, and text relevance. Post-screening, 225 papers out of 862 papers originating from the initial search queries were selected for detailed review. The articles reviewed, along with their information of interest, are available in the Supplementary Materials.

2.2.2. Extraction and Synthesis of Data

The second review question aimed to identify insights and limitations underscored by previous research on alternative HVAC systems. In addition, their characteristics were analysed to evaluate whether they could be applied to confined poultry housing. Literature sources addressing assessments of alternative HVAC systems, which included GSHPs, WSHPs, ASHPs, GSAHPs, and EAHEs, were examined. The following characteristics were identified: the type of alternative HVAC technology, heating and cooling loads/needs applied, application context (residential, commercial, or livestock), useable floor area or volume of the facility, climatic region, indoor air set point temperature, outdoor air temperature, energy efficiency findings, environmental impact findings, and if technologies were stand-alone or supplementary. Indicators were used on the catalogued HVAC results to further characterize and compare technology suitability for confined caged and free-run poultry housing systems. Descriptions of indicators and their thresholds (along with supporting justification) are described in detail in

Table 4. Indicator descriptions of HVAC results’ categories.

Categories	+	~	−
Heating, cooling, and ventilation loads	The heating and cooling loads or needs of the referenced study were within 25% of those estimated in RQ1 (the selected percentage provides a general understanding that the technology could meet the loads with minor sizing modifications and that the corresponding study’s findings can be appropriately transferred to the scale of interest.)	The heating and cooling loads or needs of the referenced HVAC were within 50% of those estimated in RQ1 (the selected percentage provides a general understanding that the technology could meet the loads with moderate sizing modifications and that the corresponding study’s findings can be mostly transferred to the scale of interest.)	The heating and cooling loads or needs of the referenced HVAC were beyond 50% of those identified in RQ1 (the selected percentage provides a general understanding that the technology could meet the loads with extensive sizing modifications and that the corresponding study’s findings cannot be confidently transferred to the scale of interest.)
Useable floor area or volume of the facility	The referenced study’s useable floor area or volume is within 25% of that of the theoretical house.	The referenced study’s useable floor area or volume is within 50% of that of the theoretical house.	The referenced study’s useable floor area or volume was beyond 50% of that of the theoretical house.
Climatic region	The referenced study’s climatic zone matched the corresponding climatic zone of interest (Dfc, Cfb, or Dfb) from the updated Koppen classification model [57].	The referenced study’s climatic zone did not match the corresponding climatic zone of interest (Dfc, Cfb, or Dfb) from the updated Koppen classification model [57]	N/A
Outdoor ambient temperature	The referenced study’s outdoor ambient temperature matched within 4 °C the annual temperature average range of the region of interest [65].	The referenced study’s outdoor ambient temperature matched beyond 4 °C the annual temperature average range of the region of interest [65].	The referenced study’s outdoor ambient temperature did not overlap with the reported annual outdoor temperature average range of the region investigated [65].
Energy efficiency findings	The referenced study identified favourable energy efficiency findings with respect to an alternative HVAC technology of interest.	The referenced study identified inconsistent energy efficiency findings in terms of favourability with respect to an alternative HVAC technology of interest.	The referenced study identified unfavourable energy efficiency findings with respect to an alternative HVAC technology of interest.
Environmental impact findings	The referenced study identified favourable environmental impact findings with respect to an alternative HVAC technology of interest.	The referenced study identified inconsistent environmental impact findings in terms of favourability with respect to an alternative HVAC technology of interest.	The referenced study identified unfavourable environmental impact findings with respect to an alternative HVAC technology of interest.

N/A stands for not applicable.

.

Indicators were used to shortlist suitable alternative HVAC systems and characterize the transferability of their application and findings in egg production across temperate and continental climates. For active systems, suitability was assessed using heating and cooling loads or needs as criteria. Specifically, the reported values retrieved from the literature were compared against the results obtained from the eight simulation scenarios (RQ1). If the study’s category received only “−”, N/As, or had incomparable units, the study was not included for analysis. Climate zone and ambient outdoor temperature were used to assess passive systems’ suitability and findings’ transferability, as heating and cooling loads and needs were typically not reported in those studies. If the climate zone category was indicated as “~”and the outdoor air temperature was either “~” or N/A, the system transferability was considered unsuitable and was not included for analysis. Indicators in other categories were not used to shortlist application suitability but rather to help characterize the findings and transferability of the systems. For studies that included multiple inseparable variables in a category (i.e., different useable floor areas tested), as long as one of the variables in a category could be classified as “+”, the variable was classified as “+”.

The purpose of RQ3 was to recommend a priority subset of alternative HVAC technologies that might be applied in caged and free-run housing systems in temperate and continental climates, considering heating and cooling requirements, potential energy efficiency and environmental impacts, affordability, and technological maturity. Literature sources addressing economic assessments for alternative HVAC technologies were examined. The following economic aspects were identified and tabulated: simple payback period, location, and economic findings of the study. These elements were assessed qualitatively to compare the affordability and economic benefits or drawbacks of the alternative HVAC technologies. To better understand the possible payback period range for alternative systems in the analysed framework, a North American context was considered an appropriate affordability indicator. Although climate is essential in determining the efficiency and operational costs of HVAC systems, investment costs tend to hold a higher share of total costs [66,67,68,69]. Hence, the cost of equipment, shipment, and installation in North America was determined to be more indicative than climate-driven operational cost performance. Additionally, the technological maturity of the five considered HVAC systems was assessed using the Technological Readiness Level (TRL) assessment tool [70]. A ranking of HVAC technology recommendations was synthesized and proposed based on the information gathered from RQ2, the affordability analysis, and the technological maturity assessment.

3. Results and Discussion

3.1. Thermal Loads and Needs for Conventional Caged and Free-Run Layer Hen Housing

Dynamic energy simulations were performed to estimate the annual energy needs and thermal loads for heating and cooling in different climatic regions of Canada, as examples of temperate and continental climates. Figure 2 shows the results of the eight simulation scenarios by reporting the estimated thermal loads and annual energy needs. Figure 2a shows that the estimated heating needs range from 16 to 357 kWh m⁻² y⁻¹ for cage systems and from 19 to 116 kWh m⁻² y⁻¹ for free-run systems. The cooling needs are in the range of 1054–1430 kWh m⁻² y⁻¹ for cage systems and 588–798 kWh m⁻² y⁻¹ for free-run systems, as shown by Figure 2b. In both cases (heating and cooling needs), it is evident that the energy needs are remarkably higher for caged systems than for free-run systems—largely due to differences in hen density. The higher hen density characterized by caged systems causes higher internal heat loads that should be removed to maintain the cooling set point temperature inside the facility. Moreover, the adopted thermal envelope of the hen house was based on Canadian regulations [50,51], which recommend highly thermally insulated envelopes (as shown in Table 1). Thus, the high thermal insulation of the envelope prevents the heat from dissipating to the outdoor environment, further increasing the cooling loads.

By contrast, the difference between caged and free-run systems is lower when heating needs are considered, as shown in Figure 2a. This is explained by the high sensible heat load produced by the hens, which contributes to maintaining the heating set point temperature. Additionally, it has to be considered that the caged systems are characterized by a higher ventilation flow rate due to a higher hen number. Due to this effect, ventilation heat losses occur and must be compensated by providing supplemental heating to the facility. The estimated energy needs were calculated assuming perfect mixing of the indoor air, meaning that the vertical profile of indoor air temperature is uniform. As previously stated, this assumption was considered valid for the purpose of this work. However, special attention should be paid to the thermal stratification of indoor air temperature when implementing the identified HVAC solutions in hen houses, because the uneven distribution of heat within the enclosure negatively impacts energy efficiency and animal welfare. Moreover, significant differences may exist between the considered housing systems (caged and free run), which vary in the number of hens and their position in the enclosure. The different elevations and magnitudes of the thermal loads in the enclosure (i.e., the hens), in fact, are factors that affect the actual temperature profiles [71]. For these reasons, future studies should employ computational fluid dynamics to investigate the optimal positioning of the terminal units of the HVAC systems and the air diffusion methods to minimize uneven heat distribution within the enclosure. These studies should also consider the different buoyant plumes generated by hens in stacked cages and on the floor to better understand thermal stratification in hen houses. It should be noted that the cooling needs presented in Figure 2a,b are theoretical. Hence, they do not represent the actual energy consumption. For its estimation, an actual HVAC system and its performance should be considered, as well as potential trade-offs in consumption if passive cooling is used to partially or fully meet the cooling needs [72,73,74].

Figure 2c,d display the maximum heating and cooling loads estimated over the entire simulated year for caged and free-run systems. The estimated loads differ markedly from caged to free-run systems for the previously explained reasons. As visible from the graphs, the estimated heating and cooling loads for cage systems are around 0.3 kW m⁻². The obtained values agree with the results obtained in the work of Wang et al. [32] that estimated a maximum heating load of 0.2 kW m⁻² and cooling loads of 0.7 kW m⁻² for a commercial hen facility in China. The analysed facility is very similar to the one analysed in the framework of this review as it is characterized by a useful floor area of 990 m² and 30,000 hens are farmed. Moreover, the facility is located in Beijing, which is characterized by a similar climate (“Dwa” according to the Köppen classification).

3.2. Insights into the Suitability of Alternative HVAC Systems

To investigate the suitability of alternative HVAC technologies for layer hen housing in temperate and continental climates, 141 papers were selected for review. Out of the five alternative HVAC systems considered for this analysis, most studies investigated GSHPs, EAHEs, and ASHPs, as shown in Figure 3. The number of alternative HVAC systems considered is greater than the total number of papers reviewed, as some papers explored more than one system. Most of the reviewed papers investigated HVAC systems for residential and commercial applications (Figure 3). The majority of active systems were studied as stand-alone, with only ten papers exploring these technologies as complementary systems (Table S1 in Supplementary Materials). For EAHEs, 20 studies explored the technologies as complementary systems and 21 as stand-alone. Typically, studies for passive stand-alone systems investigated changes in indoor temperature instead of whether specific thermal loads or energy needs could be satisfied, unlike most active HVAC studies (Table S1 in Supplementary Materials). Based on the total sample reviewed, ~90 papers could not be compared or ranged beyond 50% of all estimated thermal loads and annual energy needs for heating and cooling. EAHEs and GSAHPs were the only systems with only favourable findings. Out of the 17 comparison studies between GSHPs and ASHPs, most concluded that GSHPs were better than ASHPs for efficiency, environmental impacts, or both (Table S1 in Supplementary Materials).

3.2.1. ASHPs for Caged and Free-Run Poultry Housing Applications

The findings of ASHP studies whose heating and cooling needs or loads matched those estimated in RQ1 are summarized in Table 5 for free-run systems and Table 6 for caged systems. The findings regarding energy efficiency and environmental impact from the matched ASHP studies were consistent across all climatic zones and housing systems. Differences in climatic zones did not affect those findings, and all matched ASHP studies were from residential contexts.

Regarding energy efficiency, the results show that ASHPs could provide significant energy savings in caged and free-run systems compared to conventional systems, but the energy savings from ASHPs were not as significant as those for GSHPs [20,75,76,77,78,79,80]. Additionally, studies that compared the environmental impacts of ASHPs to conventional HVAC systems concluded that the overall impacts of ASHPs were worse than those of conventional systems [79,81,82]. A couple of studies concluded that ASHPs could reduce GHG emissions and have lower environmental impacts than conventional systems and GSHPs [77,83]. However, most comparison studies between ASHPs and GSHPs or EAHEs found that ASHPs were more environmentally impactful [20,79,81,84]. The studies that reported ASHPs’ environmental unfavourability attributed this to the high electricity needs during operation. Therefore, ASHPs may be more suitable in contexts with lower indoor heating and cooling requirements than those of the simulated poultry houses and may be more beneficial for milder temperate and continental climates like Coastal–Cfb, as well as in renewable energy-based electricity grids where the environmental impacts of electricity consumption are lower. These environmental findings question the suitability of ASHPs as an environmental and resource-benefiting alternative to conventional HVAC systems despite the potential ability to meet egg production-specific heating and cooling needs. The results are slightly surprising as most ASHP studies within the total reviewed sample had favourable energy efficiency and environmental impact findings (Table S1 in Supplementary Materials). However, the shortlist of studies that matched the estimated thermal and energy loads included mostly papers reporting unfavourable outcomes (Table 5 and Table 6).

Table 5. Matched ASHP studies’ findings across temperate and continental climates for free-run systems.

Ref.	Energy Efficiency Findings	Environmental Impact Findings	Type of Finding (Favourable, Unfavourable, Inconsistent)	Inland–Dfc	Coastal–Dfb	Inland–Dfb	Coastal–Cfb
[78]	The ASHP did not meet the energy demands	N/A	Unfavourable	x
[85]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable	x
[83]	Performance was mainly driven by the climate	N/A	Inconsistent	x	x	x
[38,75,76,77]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable	x	x	x	x
[20]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable				x
[79]	The ASHP could reduce the energy supply with substantial improvements	N/A	Favourable				x
[80]	In warm climates, the GSHP saved little energy or used more energy than the ASHP, but the opposite was true in cold climates	N/A	Inconsistent				x
[86]	N/A	The environmental impact was higher than conventional and GSHP systems	Unfavourable	x
[87]	N/A	Reduced emissions were achieved compared to a conventional system	Favourable	x
[81]	N/A	The environmental impact was higher than the GSHP	Unfavourable	x	x	x
[84]	N/A	The ASHP contributed more emissions than the EAHE	Unfavourable	x	x	x	x
[77]	N/A	The environmental impact was lower than GSHPs and conventional systems	Favourable	x	x	x	x
[88]	N/A	The environmental impact was higher than conventional system	Unfavourable	x	x	x	x
[82]	N/A	The environmental impact was higher than conventional systems	Unfavourable	x	x
[20]	N/A	The ASHP contributed more emissions than a GSHP	Unfavourable			x
[79]	N/A	The ASHP could reduce emissions with substantial improvements	Favourable				x

x indicates matched studies across climates.

Table 5. Matched ASHP studies’ findings across temperate and continental climates for free-run systems.

Ref.	Energy Efficiency Findings	Environmental Impact Findings	Type of Finding (Favourable, Unfavourable, Inconsistent)	Inland–Dfc	Coastal–Dfb	Inland–Dfb	Coastal–Cfb
[78]	The ASHP did not meet the energy demands	N/A	Unfavourable	x
[85]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable	x
[83]	Performance was mainly driven by the climate	N/A	Inconsistent	x	x	x
[38,75,76,77]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable	x	x	x	x
[20]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable				x
[79]	The ASHP could reduce the energy supply with substantial improvements	N/A	Favourable				x
[80]	In warm climates, the GSHP saved little energy or used more energy than the ASHP, but the opposite was true in cold climates	N/A	Inconsistent				x
[86]	N/A	The environmental impact was higher than conventional and GSHP systems	Unfavourable	x
[87]	N/A	Reduced emissions were achieved compared to a conventional system	Favourable	x
[81]	N/A	The environmental impact was higher than the GSHP	Unfavourable	x	x	x
[84]	N/A	The ASHP contributed more emissions than the EAHE	Unfavourable	x	x	x	x
[77]	N/A	The environmental impact was lower than GSHPs and conventional systems	Favourable	x	x	x	x
[88]	N/A	The environmental impact was higher than conventional system	Unfavourable	x	x	x	x
[82]	N/A	The environmental impact was higher than conventional systems	Unfavourable	x	x
[20]	N/A	The ASHP contributed more emissions than a GSHP	Unfavourable			x
[79]	N/A	The ASHP could reduce emissions with substantial improvements	Favourable				x

x indicates matched studies across climates.

Table 6. Matched ASHP studies’ findings across temperate and continental climates for caged systems.

Ref.	Energy Efficiency Findings	Environmental Impact Findings	Type of Finding (Favourable, Unfavourable, Inconsistent)	Inland–Dfc	Coastal–Dfb	Inland–Dfb	Coastal–Cfb
[78]	The ASHP did not meet the energy demands	N/A	Unfavourable		x	x
[38]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable	x
[75]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable				x
[84]	The ASHP contributed more emissions than the EAHE	N/A	Unfavourable	x	x	x
[85]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable				x
[89]	The ASHP had higher energy consumption than a GSHP but less than conventional systems	N/A	Favourable	x	x	x
[79]	The ASHP could reduce the energy supply with substantial improvements	N/A	Favourable	x	x	x
[76]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable				x
[77]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable				x
[88]	N/A	The environmental impact was higher than with a GSHP	Unfavourable		x	x
[84]	N/A	The ASHP contributed more emissions than an EAHE	Unfavourable	x	x	x	x
[86]	N/A	The environmental impact was higher than GSHPs and conventional systems	Unfavourable		x	x
[79]	N/A	The ASHP could reduce energy consumption with substantial improvements	Favourable		x	x
[77]	N/A	The environmental impact was lower than GSHPs and conventional systems	Favourable	x	x	x
[87]	N/A	Reduced emissions were achieved compared to a conventional system	Favourable				x

Table 6. Matched ASHP studies’ findings across temperate and continental climates for caged systems.

Ref.	Energy Efficiency Findings	Environmental Impact Findings	Type of Finding (Favourable, Unfavourable, Inconsistent)	Inland–Dfc	Coastal–Dfb	Inland–Dfb	Coastal–Cfb
[78]	The ASHP did not meet the energy demands	N/A	Unfavourable		x	x
[38]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable	x
[75]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable				x
[84]	The ASHP contributed more emissions than the EAHE	N/A	Unfavourable	x	x	x
[85]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable				x
[89]	The ASHP had higher energy consumption than a GSHP but less than conventional systems	N/A	Favourable	x	x	x
[79]	The ASHP could reduce the energy supply with substantial improvements	N/A	Favourable	x	x	x
[76]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable				x
[77]	The ASHP had higher energy consumption than the GSHP	N/A	Unfavourable				x
[88]	N/A	The environmental impact was higher than with a GSHP	Unfavourable		x	x
[84]	N/A	The ASHP contributed more emissions than an EAHE	Unfavourable	x	x	x	x
[86]	N/A	The environmental impact was higher than GSHPs and conventional systems	Unfavourable		x	x
[79]	N/A	The ASHP could reduce energy consumption with substantial improvements	Favourable		x	x
[77]	N/A	The environmental impact was lower than GSHPs and conventional systems	Favourable	x	x	x
[87]	N/A	Reduced emissions were achieved compared to a conventional system	Favourable				x

3.2.2. EAHEs for Caged and Free-Run Poultry Housing Applications in Different Temperate and Continental Climates

Studies on EAHEs whose thermal loads, needs, climatic regions or reported outdoor temperatures matched those determined for egg production contexts are summarized in Table 7. The matching EAHE studies were mainly for commercial applications, with few from residential or livestock applications. Across the four temperate and continental climate zones considered (Inland–Dfc, Coastal–Dfb, Inland–Dfb, Coastal–Cfb), most EAHE studies were carried out in Coastal–Cfb climate zones or temperature ranges. Therefore, Coastal–Cfb climates had ~30% more studies that were considered suitable than the three other climatic zones.

The type of energy efficiency and environmental findings for matched EAHE studies were consistent between climatic zones and housing systems (Table 7). The additional matched studies for Coastal–Cfb climates did not affect the overall type of findings. Energy efficiency findings were favourable for EAHEs. The majority of findings favoured EAHEs due to energy need reduction potentials (e.g., [90,91,92,93]). Similarly, when investigated as stand-alone systems, findings indicated that EAHEs could passively increase and decrease indoor temperatures significantly [84,94,95]. Although some studies concluded that EAHEs could satisfy heating/cooling loads alone [96,97,98,99,100], there was little information comparable to a poultry context aside from climate region or outdoor temperature, and most studies indicated that EAHEs provided a reduction in thermal loads (e.g., [90,91,92,93]). Additionally, Boutera et al. [39], whose work investigated EAHEs in a poultry house similar to the caged scenarios applied in this paper, concluded that the technology could reduce cooling and heating loads by 38% and 45%, respectively. This indicates that EAHEs would be suitable for caged systems as complementary systems rather than stand-alone. Few matching EAHE studies investigated the environmental impacts of the system, but those that did concluded that EAHEs could reduce GHGEs [39,84].

Overall, the literature indicates that EAHEs may be favourable as a resource efficiency-benefiting solution and as a means to reduce thermal loads and needs for caged and free-run systems in the temperate and continental climates considered. These results are unsurprising given that all EAHE studies in the pool of reviewed papers had favourable findings (Table S1 in Supplementary Materials). However, it is worth mentioning that, as most EAHE studies were observational, the findings are more likely to be classified as favourable than studies of active systems tested for meeting specific thermal loads or energy needs. Furthermore, as passive and active systems’ suitability was categorized against different criteria, the ratio of matched studies should not be directly understood as more suitable for applicability as there is no common ground for comparison.

Table 7. Matched EAHE studies’ findings across temperate and continental climates for free-run and caged systems.

Ref.	Energy Efficiency Findings	Environmental Impact Findings	Type of Finding (Favourable, Unfavourable, Inconsistent)	Inland–Dfc	Coastal–Dfb	Inland–Dfb	Coastal–Cfb
[39]	N/A	The EAHE helped reduce GHGEs.	Favourable	x	x	x
[84]	N/A	The EAHE reduced annual CO2, SO2, and NOx emissions compared to the ASHP.	Favourable	x	x	x	x
[90]	The EAHE provided energy savings in the summer season.	N/A	Favourable	x	x	x	x
[96]	The EAHE effectively heated and cooled the facility.	N/A	Favourable	x	x	x	x
[101]	The EAHE could effectively reduce heating load requirements.	N/A	Favourable	x	x	x	x
[91,92]	The EAHE reduced energy consumption.	N/A	Favourable	x	x	x	x
[93,102]	The EAHE could effectively reduce energy consumption, with higher cooling potential.	N/A	Favourable	x	x	x	x
[84,94,95]	The EAHE increased average temperature by 13.5 °C, 2.7 °C, and 8 °C and decreased by 13.6 °C, 6.6 °C, and 4 °C, respectively.	N/A	Favourable	x	x	x	x
[97]	The EAHE met the cooling and heating load requirements, and efficiency did not decrease with time.	N/A	Favourable	x	x	x	x
[39,103]	The EAHE could effectively reduce heating and cooling load requirements.	N/A	Favourable	x	x	x
[104]	The EAHE reduced energy consumption.	N/A	Favourable		x	x	x
[98]	The EAHE met the cooling load requirements.	N/A	Favourable		x		x
[93]	The EAHE could effectively reduce energy consumption, with higher cooling potential.	NA	Favourable		x
[105,106,107]	The EAHE reduced energy consumption for winter and summer.	N/A	Favourable				x
[99,100]	The EAHE met the cooling and heating load requirements, and efficiency did not decrease with time.	N/A	Favourable				x

Table 7. Matched EAHE studies’ findings across temperate and continental climates for free-run and caged systems.

Ref.	Energy Efficiency Findings	Environmental Impact Findings	Type of Finding (Favourable, Unfavourable, Inconsistent)	Inland–Dfc	Coastal–Dfb	Inland–Dfb	Coastal–Cfb
[39]	N/A	The EAHE helped reduce GHGEs.	Favourable	x	x	x
[84]	N/A	The EAHE reduced annual CO2, SO2, and NOx emissions compared to the ASHP.	Favourable	x	x	x	x
[90]	The EAHE provided energy savings in the summer season.	N/A	Favourable	x	x	x	x
[96]	The EAHE effectively heated and cooled the facility.	N/A	Favourable	x	x	x	x
[101]	The EAHE could effectively reduce heating load requirements.	N/A	Favourable	x	x	x	x
[91,92]	The EAHE reduced energy consumption.	N/A	Favourable	x	x	x	x
[93,102]	The EAHE could effectively reduce energy consumption, with higher cooling potential.	N/A	Favourable	x	x	x	x
[84,94,95]	The EAHE increased average temperature by 13.5 °C, 2.7 °C, and 8 °C and decreased by 13.6 °C, 6.6 °C, and 4 °C, respectively.	N/A	Favourable	x	x	x	x
[97]	The EAHE met the cooling and heating load requirements, and efficiency did not decrease with time.	N/A	Favourable	x	x	x	x
[39,103]	The EAHE could effectively reduce heating and cooling load requirements.	N/A	Favourable	x	x	x
[104]	The EAHE reduced energy consumption.	N/A	Favourable		x	x	x
[98]	The EAHE met the cooling load requirements.	N/A	Favourable		x		x
[93]	The EAHE could effectively reduce energy consumption, with higher cooling potential.	NA	Favourable		x
[105,106,107]	The EAHE reduced energy consumption for winter and summer.	N/A	Favourable				x
[99,100]	The EAHE met the cooling and heating load requirements, and efficiency did not decrease with time.	N/A	Favourable				x

3.2.3. GSHPs for Caged and Free-Run Poultry Housing Applications in Different Temperate and Continental Climates

Studies on GSHPs whose heating and cooling loads or needs matched those estimated in RQ1 are summarized in

Table 8. Matched GSHP studies’ findings across temperate and continental climates for free-run systems.

Ref.	Energy Efficiency Findings	Environmental Impact Findings	Type of Finding (Favourable, Unfavourable, Inconsistent)	Inland–Dfc	Coastal–Dfb	Inland–Dfb	Coastal–Cfb
[108,109,112]	The GSHP was more energy-efficient than a conventional system	N/A	Favourable	x	x	x	x
[113,114]	The GSHP had lower energy consumption compared to conventional system	N/A	Favourable	x
[115]	The GSHP had lower energy consumption compared to the conventional system	N/A	Favourable			x
[116]	The GSHP saved energy consumption in heating mode compared to the conventional system	N/A	Favourable		x	x
[117]	The GSHP had lower energy consumption than the conventional system	N/A	Favourable		x
[38]	The GSHP had lower energy consumption compared to ASHP	N/A	Favourable	x	x	x	x
[75]	The GSHP was more energy-efficient than the ASHP	N/A	Favourable	x	x	x
[110]	The GSHP was more energy-efficient than conventional system	N/A	Favourable				x
[85]	The GSHP was more energy-efficient than the ASHP	N/A	Favourable	x
[118]	The GSHP was more energy-efficient than the conventional systems	NA	Favourable	x	x	x
[119]	The GSHP showed higher efficiency for cooling than heating	N/A	Favourable	x	x	x	x
[120]	The GSHP had lower energy consumption than the conventional systems	N/A	Favourable	x	x
[121]	The GSHP’s performance did not degrade	N/A	Favourable		x		x
[122]	The GSHP met the heating load requirements	NA	Favourable	x	x	x
[76,77]	The GSHP had lower energy consumption than the ASHP	N/A	Favourable	x	x	x	x
[80]	The GSHPs provided energy savings in cold climate zones, but in warmer climates, the GSHPs saved little energy or used more energy than the ASHP	N/A	Inconsistent				x
[123]	The GSHP met the cooling load requirements	N/A	Favourable	x	x	x	x
[124]	The GSHP met the heating and cooling load requirements	N/A	Favourable	x	x
[108]	N/A	The GSHP showed higher environmental impacts compared to the conventional systems	Unfavourable	x	x	x	x
[88]	N/A	The GSHP had lower environmental impacts than ASHPs	Favourable	x		x
[81]	N/A	The GSHPs showed lowest environmental impacts in most cases compared to the ASHP	Favourable	x	x	x	x
[113]	N/A	The GSHP had lower GHGEs compared to the conventional system	Favourable	x
[112]	N/A	The GSHP reduced GHGEs compared to the conventional system	Favourable	x	x	x	x
[115]	N/A	The GSHP had lower GHGEs compared to the conventional system	Favourable			x
[116]	N/A	The GSHP reduced GHGEs in heating mode	Favourable		x	x
[109]	N/A	The GSHP reduced GHGEs throughout the operational stage compared to conventional systems but showed greater overall negative environmental impact across the entire life cycle	Unfavourable	x	x	x	x
[110]	N/A	The GSHP generated higher emissions compared to the conventional heating system	Unfavourable				x
[125]	N/A	The GSHP had lower GHGEs compared to the conventional systems	Favourable	x	x	x
[126]	N/A	The GSHP had lower environmental impacts than the conventional systems	Favourable	x	x	x
[77]	N/A	The GSHP had a greater impact on all impact categories when compared to the ASHP	Unfavourable	x	x	x	x

for free-run systems and

Table 9. Matched GSHP studies’ findings across temperate and continental climates for caged systems.

Ref.	Energy Efficiency Findings	Environmental Impact Findings	Type of Finding (Favourable, Unfavourable, Inconsistent)	Inland–Dfc	Coastal–Dfb	Inland–Dfb	Coastal–Cfb
[108]	GSHPs were more energy-efficient than the conventional system	N/A	Favourable				x
[113]	The GSHP had lower energy consumption compared to the conventional systems	N/A	Favourable		x	x
[112]	The GSHP was more efficient than the conventional system	N/A	Favourable	x			x
[116]	The GSHP could save energy consumption in heating mode compared to the conventional system	N/A	Favourable	x	x	x
[127]	The GSHP reduced operational energy use compared to the conventional system	N/A	Favourable				x
[128]	The GSHP met the heating load requirements	N/A	Favourable	x	x	x	x
[38]	The GSHP had lower energy consumption compared to the ASHP	N/A	Favourable	x
[75]	The GSHP was more energy-efficient than the ASHP	N/A	Favourable				x
[129]	The GSHPs met the cooling load requirements	N/A	Favourable	x	x	x	x
[85]	The GSHP was more energy-efficient than the ASHP	N/A	Favourable				x
[118]	The GSHP was more energy-efficient than conventional systems	N/A	Favourable				x
[120]	The GSHPs had lower energy consumption than conventional systems	N/A	Favourable	x	x	x	x
[130]	The GSHP met the cooling load requirements	N/A	Favourable				x
[89]	The GSHPs used less operational energy than the conventional and ASHP systems	N/A	Favourable	x	x	x
[114]	The GSHPs used less energy than the conventional systems	N/A	Favourable		x	x
[122]	The GSHP met the heating load requirements	N/A	Favourable				x
[76]	The GSHP was more energy-efficient than the ASHP	N/A	Favourable				x
[77]	The GSHP was more energy-efficient than the ASHP	N/A	Favourable				x
[111]	During very cold periods, i.e., −20 °C, the GSHP was not able to meet the heating load requirements	N/A	Unfavourable				x
[131]	The GSHPs showed high energy efficiency	N/A	Favourable			x	x
[124]	The GSHP met the thermal load requirements	N/A	Favourable				x
[108]	N/A	The GSHP showed the most environmental impacts compared to the conventional system	Unfavourable				x
[88]	N/A	The GSHP showed lower environmental impacts compared to the ASHP	Favourable		x	x
[113]	N/A	The GSHP reduced GHGEs	Favourable		x	x
[116]	N/A	The GSHP reduced GHGEs in heating mode	Favourable	x	x	x
[89]	N/A	The GSHPs showed a higher reduction in climate, energy, and land footprints in comparison to the conventional and ASHP systems	Favourable	x	x	x
[125]	N/A	The GSHP saved GHGEs during heating compared to conventional systems	Favourable		x
[86]	N/A	The GSHPs’ environmental impacts were lower than the conventional and ASHP systems	Favourable		x	x
[77]	N/A	The GSHPs’ environmental impact was lower than conventional systems	Favourable		x	x
[112]	N/A	The GSHP reduced GHGEs	Favourable	x			x

for caged systems. Most matched GSHP studies were for commercial and residential applications, with a few for livestock applications found in free-run scenarios and caged Coastal–Cfb scenarios. The types of GSHP studies’ findings were consistent across the four climatic zones but differed slightly between housing systems. For free-run systems, the majority of the conclusions were favourable, but several environmental findings were unfavourable [77,108,109,110]. In contrast, the findings from caged systems had only one unfavourable energy and environmental finding across all climatic zones [108,111]. Regarding energy efficiency, the findings indicate that heating and cooling requirements were met by GSHP systems and that energy consumption was lower than for conventional and ASHP systems for both free-run and caged scenarios (Table 8 and Table 9). Most studies that explored the environmental impacts of GSHPs were comparative studies against conventional HVAC or ASHP systems. These generally found that GSHPs had lower environmental impacts than conventional and ASHP systems.

Overall, the literature indicates that a GSHP may be favourable as an energy-efficient system across temperate and continental climates. These results are unsurprising as most GSHP studies from the reviewed papers had favourable findings (Table S1 in Supplementary Materials). GSHPs may be more energy-efficient and environmentally favourable for egg production than ASHPs, as suggested by comparative studies (e.g., [38,75,81,88]). These conclusions can be explained by the fact that GSHPs require less electrical power to operate. This is because the heat source and sink of GSHPs is the ground, which has a relatively stable temperature over the year compared to the outdoor air temperature used in ASHPs [24,82], usually resulting in increased coefficients of performance (COPs). There were no unfavourable studies for GSHPs compared to ASHPs.

3.3. Affordability Analysis for the Application of Alternative HVAC Systems in Egg Production Systems

To investigate what subset of alternative HVAC systems could be recommended for priority consideration for egg production, affordability, technological maturity and findings from RQ2 were synthesized. Eighty-four papers were selected for review to understand the affordability of the alternative systems, which are summarized in Table S2 (Supplementary Materials). Out of the five alternative HVAC systems considered, most studies investigated ASHPs, EAHEs, and GSHPs and explored the costs of the systems as stand-alone functions. Analysis of the systems’ affordability was performed qualitatively based on the reported studies’ findings. ASHPs were determined to be always feasible along with EAHEs. GSHPs were concluded to be feasible, with the exception of a small subset of studies (e.g., [132,133,134,135]) that determined they were not feasible or only feasible with a significant reduction in initial investment cost through installation cost reduction, incorporating subsidies or dependent on installation location. However, GSHPs have been recommended as an affordable HVAC system for industrial poultry production in China [136]. Overall, comparative studies between GSHPs and ASHPs concluded that GSHPs are more costly than ASHPs (e.g., [137,138,139]). The payback period range was extensive across the reviewed studies. GSHPs had the highest payback period range of ~2–42 years, EAHEs had the second largest range of ~2–17 years, and ASHPs had a narrower range of ~3–9 years, as shown in Figure 4. The different shipping, installation, operation and maintenance costs worldwide can explain the large payback period ranges [140]. Moreover, the relative sizing differences between systems are challenging to understand as papers address this information differently, making most papers incomparable to one another and likely a contributor to the considerable payback period ranges. Lastly, differences in climatic regions and countries can affect energy consumption, causing variable operational costs for active HVAC systems.

North American-specific payback periods ranged between 8 and 20 years for GSHPs, except for one study with a 42-year payback period, as seen in Figure 4. There was only one study from North America for ASHPs, with a payback period of 4 years. There were no EAHE economic studies from North America or of relevant temperate and continental climates. However, from the original pool of review papers, it is assumed that EAHEs are more affordable than GSHPs, as even though both systems have similar installation processes, EAHEs require little operational cost due to their passive nature. Additionally, EAHEs are assumed to be less affordable than ASHPs as they require a more costly installation process than ASHPs.

3.3.1. Technological Maturity of Alternative HVAC Systems

According to the TRL assessment tool, ASHPs, EAHEs, GSHPs, and WSHPs are all “commercially available” and have high technological maturity levels, as these technologies are available in the marketplace worldwide [70]. GSAHPs fall under a technological development scale of 4 (out of 9), known as “validation of components in a laboratory environment” [70]. The GSAHP literature uses the system by having components of the system integrated “ad hoc” to make it function. This can be seen in the literature where GSAHPs are referred to as an “innovative” system [20].

3.3.2. Recommendations of Alternative HVAC Systems Based on the Synthesis of Affordability, Technological Maturity, and Results from RQ2

EAHEs are the most recommended system for further detailed assessments for potential application in egg production, as shown in

Table 10. Recommendation status of alternative HVAC systems based on energy efficiency, potential environmental impacts, affordability, and technological maturity.

Recommendation Status	Alternative HVAC Technology	Recommendation Context	Energy Efficiency	Environmental Impacts	Affordability	Technological Maturity
First priority recommendation	EAHE	As a complementary system for free-run and caged housing	Favourable	Favourable	Favourable	Mature (commercially available)
Secondary priority recommendation	GSHP	As a stand-alone system free-run and caged housing	Favourable	Mostly favourable	Unfavourable	Mature (commercially available)
Subsequent non-prioritized recommendation	ASHP	As a stand-alone system free-run and caged housing	Mostly Favourable	Mostly unfavourable	Favourable	Mature (commercially available)
Not recommended	WSHP	As a stand-alone system for free-run and caged housing in proximity to an open water source	Favourable	Favourable	Favourable	Mature (commercially available)
Not recommended	GSAHP	As a stand-alone system for free-run and caged housing	NA	NA	NA	Immature

. EAHEs are technologically mature, economically viable, and the least costly option with some potential overlap with ASHPs. They are determined to potentially be the least environmentally impactful system as they are passive and effective at reducing energy consumption. Based on the results from RQ2, an EAHE is not recommended for usage as a stand-alone system; even if passive cooling methods like ventilation can be used as a backup method for cooling a poultry house, heating the house requires an active process. Furthermore, high air temperature fluctuations in the face of increased climatic events [141] should be considered in implementing long-term heating and cooling solutions, pointing toward the need for more reliable HVAC solutions. Ensuring poultry health and productivity should be a priority, and therefore, an EAHE is recommended in combination with a backup HVAC system. An EAHE is identified as a good option to reduce energy use for all temperate and continental climatic zones and poultry housing systems. However, its impact is suspected to be more significant where heating and cooling requirements are lower, as is the case in free-run systems and Coastal–Cfb climates. When considering potential alternative HVAC technologies as complementary systems to EAHEs, GSHPs may not be a viable option based on the long payback periods indicated in this review. The insufficient energy efficiency and environmental impact findings make the consideration of WSHPs as complementary systems inconclusive. The current technological immaturity of GSAHPs hinders the prospect of them being effective complementary systems to EAHEs at present. Lastly, the relatively low payback time of ASHPs makes them a possible viable option for complementing EAHEs and may represent a way of reducing the operational phase of ASHPs, which was noted as contributing to the unfavourable environmental impact findings. Future investigations are needed to assess the suitability of alternative HVAC’s potential as a complementary system to EAHEs. Moreover, future LCA and techno-economic assessments should explore the potential economic and environmental benefits of combining EAHEs with conventional HVAC systems compared to using a GSHP or ASHP alone. This would help understand whether switching from stand-alone conventional HVAC systems to heat pump systems would be more or less advantageous than installing an EAHE with a currently used conventional system. Such comparisons may be particularly relevant for low thermal needs and loads, where EAHE are most effective, such as in free-run systems and Coastal–Cfb regions.

As summarized in Table 10, GSHPs are the second most recommended system for further assessments for application in egg production. These systems are technologically mature, determined to be energy-efficient, and potentially have lower environmental impacts than conventional and alternative active systems. Although GSHPs are high-cost, they have been recommended as affordable HVAC systems for industrial poultry production [136]. Based on the literature, GSHPs could meet the thermal loads and needs of caged and free-run systems. Studies’ findings related to energy efficiency indicated that they would be suitable for reducing operational costs, which would be particularly beneficial in contexts with high heating and cooling demands. Therefore, although these systems would benefit both poultry housing systems, they could bring a particular advantage to caged systems with higher operational demands. Notably, caged systems are expected to be entirely replaced by free-run systems in Canada by 2036 [55]. The temporal proximity of the disappearance of caged systems combined with the high investment costs of GSHPs would make their implementation only viable if a house could convert from caged to free run in the next decade.

ASHPs are determined to be a non-prioritized alternative HVAC system for application in egg production, as shown in Table 10. ASHPs are technologically mature and have shown to be energy-efficient compared to conventional HVAC technologies, but not compared to GSHPs. ASHPs remain an economical alternative to other heat pumps like GSHPs. However, the unfavourable environmental impact findings associated with thermal loads and needs, similar to those of egg production, call into question this type of system as an environmentally sustainable and resource-benefiting solution. The operational stage of ASHPs was shown to have the most environmental impacts [24,142]. This suggests that ASHPs would be more appropriate for lower heating and cooling demand contexts, milder temperate and continental climates like Coastal–Cfb, and where they are powered by renewable energy-based electricity grids [24,142]. Additional research on combining EAHEs with ASHPs to reduce the heat pumps’ electricity needs would be worth conducting as a possible way to decrease ASHPs’ environmental impacts.

As summarized in Table 10, GSAHPs and WSHPs are not recommended for further assessment for application in egg production at this time. GSAHPs are not technologically mature, and the limited literature does not indicate their suitability for egg production. WSHPs are technologically mature but, as the literature is limited, these systems’ suitability could not be determined.

4. Conclusions, Future Directions, and Limitations

Conventional HVAC systems to climate-control poultry houses have been identified as contributing a large share of non-renewable energy use in poultry house operations. Therefore, identifying environmentally preferable solutions represents new and necessary work to reduce the environmental impacts of the rapidly growing egg sector. Alternative HVAC technologies are currently mainly used for commercial and residential buildings, and the exploration of these technologies’ applicability in livestock contexts is limited. Hence, this paper explores the suitability of alternative HVAC technologies for egg production using comprehensive practical frameworks, representing a novelty in the literature. This paper aimed to identify and critically analyse information on passive and active alternative HVAC systems from the literature that could be applied in caged and free-run housing systems for egg production in temperate and continental climates. For this, dynamic simulations and a systemic review were undertaken using a set of criteria of interest to relevant stakeholders: (1) comparison of heating and cooling requirements between the reported literature and estimations from energy simulations of poultry houses in temperate and continental climates; (2) HVAC systems’ efficiency; (3) potential environmental impacts of the system; (4) affordability of the system; and (5) technological maturity. On this basis, priority recommendations of alternative systems for consideration in egg production and suggestions for future directions are as follows:

• EAHEs are the alternative HVAC technology of highest priority for future investigation as a complementary system to reduce thermal loads and needs in poultry housing. Due to their passive nature, EAHEs were determined to have the smallest costs and potential environmental impacts. Combining EAHEs with conventional systems as a potentially economical and environmentally beneficial alternative to switching from conventional to active alternative HVAC systems would be worth future exploration, particularly for low-thermal-load and -energy-needs houses such as in mild temperate climates and free-run systems.
• GSHPs are of second priority for further investigation as stand-alone systems. Despite their high installation costs, GSHPs were determined to possibly be energy-efficient and environmentally beneficial for egg production compared to other active systems due to having low operational costs. Although GSHPs would benefit both poultry housing systems, they would be particularly advantageous for caged systems due to the high thermal load and associated operational demand. Possible future work on reducing investment costs for GSHPs would be beneficial.
• ASHPs are not recommended as a priority alternative HVAC system. Despite favourable literature findings as an affordable, energy-efficient system, many environmental impact findings were unfavourable. There is no strong indication from the literature that ASHPs would be superior in terms of environmental sustainability to conventional or GSHP systems. It is worth noting that the installation of ASHPs is usually easier. Nevertheless, further environmental impact investigation is suggested before large-scale implementations of ASHPs in livestock contexts, particularly for high-thermal-load and -energy-needs applications.
• GSAHPs and WSHPs are not recommended for priority consideration at this time. WSHPs are technologically mature but, as the literature is limited, these systems’ suitability for egg production could not be determined. Moreover, as WSHPs need access to large bodies of water, their implementation can be geographically limited. GSAHPs are not technologically mature, and the limited literature also prevents the determination of these systems’ suitability. We encourage further research on WSHPs and GSAHPs as these systems are theoretically promising but require more investigation of their potential energy efficiencies, environmental impacts, and affordability to better understand their suitability across different application contexts.

EAHEs and GSHPs are recommended for further assessment to determine their potential environmental preferability compared to conventional HVAC systems for egg production in temperate and continental climates. Life cycle thinking and assessment would help understand the possible trade-offs and environmental impacts across different life cycle stages of these potential mitigation technologies. Therefore, LCAs of EAHEs and GSHPs applied to egg supply chains would help us comprehend their potential resource use and net impacts/benefits in the context of egg production compared to conventional HVAC technologies. Addressing this in future work would provide ground for the evidence-based selection of more environmentally beneficial HVAC systems for the egg sector and help induce uptake. Moreover, future works may deepen this analysis to help us understand the impact of the expected switch from caged systems to free-run systems, also from an energy point of view. Therefore, this paper, along with the future research opportunities it provides, will be valuable to egg farmers, renewable energy industries, and HVAC engineers. This review also provides a practical approach for other academic researchers interested in exploring alternative HVAC applications in other livestock farming sectors.

The study does have some limitations. First, only active HVAC studies with reported thermal needs or loads were considered. This could represent a limitation for recommending alternative HVAC systems. However, this process was required to ensure that the findings would effectively translate to the specific context of egg production. Second, the indicators used to understand the applicability of the studies’ characteristics to that of poultry housing were determined qualitatively. Therefore, the selected indicators determined the designation of studies as applicable or inapplicable, without considering actual quantitative data. This was necessary to limit the scope of the study. However, future works may start from the findings of this review and quantitatively evaluate the implications of integration of the recommended alternative HVAC solutions in hen houses. Future work may also focus on deepening the estimation of energy needs and thermal loads of poultry houses by running multiple batches of simulations using Monte Carlo methods while randomizing specific input parameters (e.g., ventilation rate and envelope thermal transmittance), in turn extending the analysis to other geographical locations. Lastly, the possible contribution of using passive cooling methods—such as increasing ventilation rate—to reduce the theoretical cooling demands was not evaluated. Hence, the estimated cooling needs may have been slightly overestimated. However, resolving this issue was beyond the scope of the present work.