Efficacy of Hybrid Photovoltaic–Thermal and Geothermal Heat Pump System for Greenhouse Climate Control

Abstract

This study evaluated the performance of a hybrid heat pump system integrating photovoltaic–thermal (PVT) panels with a standing column well (SCW) geothermal system in a strawberry greenhouse. The PVT panels, installed over 10% of the area of a 175 m³ greenhouse, stored excess solar heat in an aquifer to offset the reduced efficiency of the geothermal source during extended operation. The results showed that the hybrid system can supply 11,253 kWh of heat energy during the winter, maintaining the night time indoor temperature at 10 °C even when outdoor conditions dropped to −10.5 °C. The PVT system captured 11,125 kWh of solar heat during heating the off season, increasing the heat supply up to 22,378 kWh annually. Additionally, the system generated 3839 kWh of electricity, which significantly offset the 36.72% of the annual pump system electricity requirements, enhancing the system coefficient of performance (COP) of 3.38. Strawberry production increased by 4% with 78% heating cost saving compared to a kerosene boiler system. The results show that the PVT system effectively supports the geothermal system, improving heating performance and demonstrating the feasibility of hybrid renewable energy in smart farms to enhance efficiency, reduce fossil fuel use, and advance carbon neutrality.

1. Introduction

Global warming is a major concern today, threatening humanity’s sustainability not only in health but also in food production [1,2]. To overcome these extreme weather events, regulations on fossil fuels are being tightened, and the need for renewable energy is increasing [3,4,5,6,7]. Furthermore, countries around the world [8,9], including the Republic of Korea (ROK) [10,11,12], are announcing greenhouse gas roadmaps for 2030 and 2050, striving to achieve carbon neutrality. The ROK is aiming to reduce 1.6 million tons of carbon emissions in the agricultural sector by 2030 [13].

According to the 2022 Agriculture, Forestry, and Fisheries Survey, the ROK’s agricultural population has decreased by 6.5% since 2018, with 49.8% of the agricultural population aged 65 or older [14]. This aging agricultural population and extreme weather events are contributing to a decline in crop production. To address these issues, the domestic agricultural sector is transitioning to smart farms to improve productivity. However, smart farms, due to their confined conditions, require high energy input [15,16,17] and face challenges in reducing both carbon emissions and production costs simultaneously [18].

In a conventional greenhouse system, the heating system is the primary source of carbon emissions because of fossil fuel usage [19,20], and emissions increase if the energy comes from a fossil fuel-powered grid [15]. Therefore, smart farms must minimize existing fossil fuel usage and reduce energy demand to address environmental issues and production costs simultaneously. To achieve this, integrating renewable energy sources for the heating and cooling systems is necessary. Currently, the most commonly used renewable energy sources in agriculture for heating are wood pellets [21,22], natural heat [23,24,25], and industrial waste heat [23,26,27,28,29]. However, the utilization of wood pellets is currently facing challenges in its sustainable application in the country. More than 95% of the wood pellets are imported primarily from Southeast Asian countries [30], and the average import price is significantly increasing due to increasing demand worldwide [31]. As of December 2022, the average unit price of imported wood pellets and chips has increased by 35.05% year-on-year, leading to significant fluctuations in economic feasibility [32]. Therefore, searching for alternatives is necessary nowadays.

Geothermal energy is commonly used for electricity generation in the case of high temperatures and for heating in the case of medium and low temperatures [33]. Low-temperature geothermal resources are abundant and can be extracted easily through heat pumps within the premises [34,35,36,37,38]. Nowadays, geothermal heat pumps (GHPs) are increasingly used in agriculture for heating controlled-environment farms in response to sustainability concerns [39,40,41]. The GHPs can be categorized into open-loop (groundwater is discharged) and closed-loop systems (groundwater is recirculated) [42], which consist of a ground heat exchanger and emission equipment to transfer heat into the houses [43]. However, the open-loop GHPs experience a drop in heat source temperature as the geothermal heat source is consumed, necessitating continuous groundwater replenishment or the need for an auxiliary heat source [44,45]. Furthermore, regional variations in groundwater availability limit their regional adaptation. This makes it difficult to expect stable heating and cooling performance in controlled-environment farms.

Solar energy is an abundant and widely used renewable energy source in most industries, and the technologies for capturing this free energy are readily available and becoming increasingly efficient through enhancements in components [46]. Traditionally, solar energy is captured and converted into electricity by a photovoltaic (PV) system [47], and energy conversion is improved by integrating PV with a thermoelectric generator, a device that converts solar thermal energy into electricity [48,49,50]. Alternatively, solar thermal energy can be collected for other purposes. A photovoltaic–thermal (PVT) system is a combined solar technology that is able to generate electricity and collect thermal energy [51,52,53]. This combined system is more efficient than the single system and has been utilized as an auxiliary heat source for the ground-source heat pump (GSHP) [54].

The integration of hybrid PVT and GSHP systems has been investigated in greenhouses, showing improvements in overall efficiency compared to a non-hybrid system [43]. However, previous studies employed the PVT system as an auxiliary heat source for horizontal GSHPs, which are difficult to install in existing greenhouses due to the need for horizontally buried pipelines. In contrast, this study designed and developed a combined heating and cooling system that integrates PVT with an open-loop GHP, specifically a single column well (SCW) type. In our approach, the PVT acts as an auxiliary heat source alongside electricity generation to replenish the geothermal resource, thereby overcoming a key limitation of open-loop GHPs. Moreover, the SCW design makes the system more practical for installation in established greenhouse facilities. This study aimed to develop a sustainable heating system for an environmentally controlled farm, aligning with the ROK’s goal of achieving carbon neutrality. The developed system was tested in real farm conditions and compared with an existing greenhouse that uses a kerosene boiler heating system during the winter season for evaluation.

2. Materials and Methods

This study was conducted in the experimental greenhouses within the National Institute of Agricultural Sciences (NIAS) in Deokjin-gu, Jeonju-si, Jeollabuk-do, Republic of Korea. This study involves everything from system design to field application to evaluate the actual heating performance and strawberry production during the winter season (November 2021 to March 2022). For the coefficient of performance analysis, an annual data from November 2021 to November 2022 were gathered.

2.1. Overview of the Greenhouse

Two single-panel vinyl greenhouses, measuring 25.0 m × 7.0 m × 1.5–3.8 m (side and center), were used. They were equipped with a multi-layer insulation curtain and had 5 raised beds with nutrient solution delivery systems (Figure 1 and

Table 1. Specification of the greenhouse.

Contents	Value
Width	7.0 m
Side height	1.5 m
Center height	3.8 m
Length	25 m
Cover and Insulation	PO film (0.10 mm)
Multi-layer insulation curtain	5 layers
Cultivation area	5 raised beds

). The Seolhyang cultivar of strawberry was grown in greenhouses for growth comparison (Section 2.3.6). The strawberry can grow and produce optimally at daytime temperatures of 23–28 °C and nighttime temperatures of 5–10 °C [55,56]. However, it can tolerate up to daytime and nighttime temperatures of 30 °C and 3 °C, respectively [57]. In this study, the internal temperature of the greenhouses was set at 10 °C.

2.2. A Hybrid Thermal Heating and Cooling System Combining PVT Modules and Geothermal Heat

2.2.1. Standing Column Well-Type Open-Loop GHP

An open-loop SCW geothermal well was selected as the heat source for the geothermal heat pump. The open-loop systems may face water quality issues such as corrosion and scaling, whereas closed-loop systems avoid these problems but have lower heat transfer capacity. This trade-off should be considered when selecting the most suitable geothermal system. Furthermore, groundwater conditions in the ROK typically allow a single geothermal well to handle a heat capacity of 25 to 30 refrigeration tons (RT), offering advantages in terms of initial investment costs. The geothermal well used in this study had a 200 m excavation depth, a 200 mm stainless steel outer casing, and a 150 mm PVC pipe inner casing. Groundwater temperature and pumping rate were measured at 15 °C, 120 L/min, and 7200 L/h, respectively. The actual installation photo and structural diagram are shown in Figure 2.

2.2.2. Water-to-Water Heat Pump

The design indoor temperature and outdoor temperature (based on 20-year standard data) for Jeonju City, Jeollabuk-do, Republic of Korea, where the greenhouse is installed, were set at 15 °C (indoor) and −10 °C (outdoor), respectively. The maximum heating load, calculated based on the temperature conditions and greenhouse specifications, was 17,110 kcal/h. Accordingly, the heat pump capacity should be able to handle 22,243 kcal/h, considering a safety factor of 1.3 to prepare for extreme cold. Based on the US refrigeration ton, 1 RT is equivalent to 3024 kcal/h, and 7.5 RT is equivalent to 22,680 kcal/h. Therefore, the installed capacity for the heat pump system was determined to be 7.5 RT. The water-to-water heat pump used was the FS-7500H model from Four Seasons Cooling Co., Ltd, (Namyangju, Republic of Korea).

2.2.3. PVT Module (Reinforced Glass-Covered Type)

This study aimed to use a PVT module as an auxiliary heat source for a GHP. The SCW has the disadvantage of relying on electrical energy for a significant portion of the system’s energy needs, as groundwater levels drop during the winter dry season, making it difficult to secure a sufficient heat source. To address this issue, we considered PVT modules, which can simultaneously produce electricity and heat. PVT modules are ideal for complementing the space utilization limitations of existing single-system systems, such as PV or STC (Solar Thermal Collector). This study aimed to utilize PVT modules on the roofs of non-sunlit areas, such as preparation areas, machine rooms, and packaging/sorting areas, in greenhouses. The capacity of the PVT modules was calculated based on the assumption that these non-cultivation areas account for 10% of the total greenhouse area. The total area of the experimental greenhouse was 175 m², requiring a PVT module area of 17.5 m². The unit area of the PVT module is 2.08 m², and it consists of a total of nine modules. The PVT specifications are presented in

Table 2. Specification of the photovoltaic thermal module (glazed type).

Parameters		Specifications	Note
Manufacturing company		E-MAX System Co., Ltd.
Size		1040 × 2003 × 58 (mm)	Area 2.08 m2
Generation output 1		875 W/Module, 44 W/m2	Based on product specifications: 530 W/m2
Heat output	Average output 2	6.56 MJ/m2	Based on product specifications: 7.64 MJ/m2
	Heat collection 3

¹ Based on the solar thermal collector. ² The average output is the average value of the inlet temperature of the collector from 0 to 80 °C. ³ The heat collection is the value at ΔT = 30 °C.

and

Table 3. Component specifications of the photovoltaic module (glazed type).

Components	Details
Cell	Single crystal, 72 cells
Heat sink plate	Titanium-coated aluminum absorber
Permeable glass	AR-coated low-iron tempered glass
Heat collecting tube	Serpentine seamless copper tube
Manufacturing method	Cell-absorber mechanical lamination
Other functions	Natural air circulation, overheating protection device

. According to the PVT module specifications, the expected electricity and heat production are 2670 kcal/h and 7031 kcal/h, respectively.

2.2.4. Thermal Energy Supply Method

The heat energy generated using the PVT module and heat pump is transferred using the fan coil unit (FCU) and air ducts (Figure 3). A total of five FCUs, rated at 6000 kcal/h, were installed, corresponding to the number of strawberry cultivation beds installed inside the greenhouse. Air ducts were connected to the FCUs and installed along the length of the beds. To ensure even heat energy transfer through the air ducts, the spacing and sizes were varied to ensure even energy input. Additionally, six circulation fans were installed, three on each side of the greenhouse’s upper left and right, facing opposite directions, to prevent temperature fluctuations inside the greenhouse.

2.2.5. Overview of the Hybrid PVT+GHP System

An overview of the entire system and experimental equipment installed for this study is shown in Figure 4 and Figure 5, respectively. The PVT module is connected in parallel to a buffer tank along with a geothermal well to secure a heat source. This heat is then stored using a water-to-water heat pump and supplied to the greenhouse for heating. During the seasons where heating is not needed, solar thermal energy is stored in the aquifer via the buffer tank, providing seasonal storage.

2.3. Analyses

2.3.1. Heating Load Calculation

To select equipment for greenhouse heating and cooling experiments, cooling and heating loads must be calculated. These loads can be calculated using Equation (1).

$$Q_{hr}=\left[A_s\left(q_t+q_v\right)+A_f{q}_s\right]f_w$$

(1)

where $Q_{hr}$ is heating load (kcal/h), $A_s$ is the cover area of the greenhouse (m²), $A_f$ is the ground area of the greenhouse (m²), $f_w$ is a correction factor by wind speed (general area 1.0, strong wind area 1.1), $q_t$ is the overall heat loss (transmission) per unit cover area (kcal/m²·h), $q_v$ is heat loss by ventilation (kcal/m²·h), and $q_s$ is ground heat transfer per unit area (kcal/m²·h).

The heat loss per unit area $q_t$, heat loss due to ventilation $q_v$, soil heat transfer $q_s$ can be obtained through the following Equations (2), (3), and (4), respectively.

$$q_t=h_t\left(T_s-T_d\right)\left(1-f_r\right)$$

(2)

$$q_v=h_v\left(T_s-T_d\right)$$

(3)

$$q_s=h_s\left(T_s-T_d\right)$$

(4)

where $h_t$ is the overall heat loss (transmission) heat loss coefficient (kcal/m²·h·°C), $h_v$ is the ventilation heat loss coefficient (kcal/m²·h·°C), $h_s$ is the ground heat transfer coefficient (kcal/m²·h·°C), $f_r$ is the heat saving rate by type of materials (%), $T_s$ is setting the temperature for heating (°C), and $T_d$ is outdoor air temperature (°C).

The winter heating load by area of the greenhouse, calculated using Equation (1) above, is shown in

Table 4. Heating load by greenhouse in each area.

Division	10 a	20 a	30 a	50 a
Greenhouse area (m2)	990	1980	2970	4950
Greenhouse surface area (m2)	1386	2772	4158	6930
Heating load of greenhouse (kcal/h *)	99,644	199,287	298,931	498,218
Heating load of greenhouse (kW *)	116	232	348	579

Design conditions: The greenhouse is set to a heating temperature of 15 °C, and the ambient air temperature is −10 °C. * 1 kW is equal to 860 kcal/h.

below. Since the metric system yields very large values for power and energy, they were converted to the SI system. The results are presented using SI units. The outdoor temperature was set to −10 °C, and the design temperature inside the greenhouse was set to 15 °C.

2.3.2. Calculating the Capacity of the Storage and Heat Source Tank

The heat storage tank and the heat buffer tank play a crucial role in a heat pump system that uses water as the heat transfer medium. The capacities of the heat storage tank and the heat source tank, among the components of the system developed in this study, were calculated. The capacity of each tank was determined using the following Equations (5)–(7).

$$Q_{dhl}=Q_{hr}\times h{r}_h$$

(5)

$$H{C}_{hst}=Q_{dhl}\times HCF$$

(6)

$$V_{hst}={\displaystyle \frac{H{C}_{hst}}{1000\times \mathrm{\Delta }T\times \hspace{0.17em}{C}_p\times \hspace{0.17em}\mathsf{\rho }}}$$

(7)

where $Q_{dhl}$ is the maximum daily heating load (kcal), $H{C}_{hst}$ is the heat storage tank capacity (kcal), $V_{hst}$ is the heat storage tank volume (m³), $Q_{hr}$ is the maximum heating load per hour (kcal/h) $h{r}_h$ is the heating time (h), $HCF$ is the heat capacity factor, $\mathsf{\Delta }T$ is the temperature difference in the heat storage tank to be used for heating (°C), $C_p$ is the specific heat of water, $\rho$ is the water density, and $1000$ is constant to fit the ton unit.

In the ROK, when designing a heat pump heating and cooling system using a thermal storage tank for a horticultural facility, the heat pump capacity is set at 70% of the maximum heating load of the greenhouse, and the thermal storage tank capacity is set at 30% to save energy and prevent overdesign of the heat pump capacity. Therefore, the HCF, which represents the ratio of the thermal storage tank heat capacity to the maximum heating load of the greenhouse, is usually set at 30% [58]. However, in this study, for the purpose of basic factor testing, the heat pump capacity was set at 100% of the maximum heating load, and the HCF, which determines the thermal storage tank capacity, was set at 35%.

The heating load of the greenhouse was previously calculated as 22,680 kcal/h, and the calculated heat storage tank capacity was 5 tons, assuming an HCF of 35% and ΔT of 15 °C. The temperature difference between the inlet and outlet water between the heat pump and the heat source tank is 3 °C, and the flow rate is 140 L/min. The temperature difference between the inlet and outlet water and the flow rate between the buffer tank and the geothermal well are 4.8 °C and 120 L/min, respectively, indicating that geothermal heat is inflowing more. Therefore, the heat source tank can be reduced in capacity and was selected as 2 tons.

2.3.3. Coefficient of Performance (COP) of Hybrid PVT+GHP System

The efficiency of the developed system is determined through COP analysis. COP is divided into unit COP, which considers only the compressor power consumption, and system COP, which considers all power required to operate the system. Therefore, the system COP shows a relatively lower value than the unit COP. In this study, the total power consumed when using the PVT module and the geothermal heat pump was used as input energy, and the supplied heat energy discharged from the heat storage tank to the greenhouse and the electrical energy produced by the PVT module were used as output energy. In calculating the COP, the input energy factor is the power supplied through the power grid, and it is based on the power consumption required to operate the heat pump and circulate the heat medium in the PVT module. This can be expressed by the following Equation (8).

$$CO{P}_{sys}={\displaystyle \frac{\mathrm{Supplied}\mathrm{thermal}\mathrm{energy}\mathrm{to}\mathrm{greenhouse}}{\mathrm{Power}\mathrm{consumption}-\mathrm{PVT}\mathrm{products}\mathrm{power}}}$$

(8)

2.3.4. Cooling and Heating Algorithm of Hybrid PVT+GHP System

The cooling and heating experimental processes of the developed system are illustrated in Figure 6. The greenhouse heating operation method distinguishes between day and night (Figure 6a). During the day, the PVT module operates. While in operation, the PVT module generates electricity and stores heat in a heat source tank from the geothermal heat pump system. The electricity generated can be sold or later supplemented by an Energy Storage System (ESS). At night, the PVT module is shut down and determines whether the greenhouse requires heating and whether the buffer tank temperature is within the set range, determining whether to heat or supply geothermal heat through groundwater circulation. The existing system connected a kerosene boiler to a storage tank and operated to maintain the set storage temperature. Both the existing and developed systems maintained a storage temperature of 47 °C.

Figure 6b illustrates the cooling algorithm. During the summer, the PVT module operates during the daytime to primarily generate electricity. During the growing season, the heat generated by the PVT is dissipated, and during the non-growing season, heat is stored in the aquifer to prepare for the winter season. At night, the PVT module is shut down, and groundwater heat exchange and cooling operations are determined based on the cooling status inside the greenhouse and the temperatures of the heat source and storage tank. The developed system utilizes radiant cooling using shading, side windows, circulation fans, and pipes installed on both sides of the strawberry crowns, as well as air ducts for space cooling. The nutrient solution is also cooled. The cold water is supplied from the storage tank to the nutrient solution mixer through a coil heat exchanger, and then returns to the nutrient solution tank, completing the cycle. In the developed system, the discharge temperature of the heat pump was set to 10 °C.

2.3.5. Measuring Energy Production and Consumption

Since this study is a field test, the performance and greenhouse heating tests were continuously and repeatedly measured and verified based on changes in outside temperature over 100 days during winter. The design accounted for uncertainty by using TAC 5% analysis with standard meteorological data from observation points when determining the heat pump’s capacity. The outside temperature with a 20-year recurrence interval was used as the design temperature, addressing the uncertainty of key external environmental factors in the heating test.

To analyze the performance and energy balance of the PVT-coupled geothermal heat pump system, energy production and consumption were measured. The power produced by the PVT module and the power consumption required to operate the heat pump system were measured using an integrated watt-hour meter (LD1210DRM-040, LS ELECTRIC Co., Ltd., Anyang, Republic of Korea). The heat produced by the PVT module and the heat supplied to the greenhouse from the heat storage tank were also measured using an integrated calorie meter (OCM-SD20, Omni System Co., Ltd., Yeoju, Republic of Korea). An automatic control and monitoring system was established using SCADA software version 3.90 (CIMON Inc., Yongin, Republic of Korea), and temperature sensors were installed inside and outside the greenhouse. The data were measured and recorded at 10-min intervals. The energy consumption of the heating system using a kerosene boiler (KDB-735GTS, KyungDong Navien, Pyeongtaek, Republic of Korea) was measured using a cumulative oil flow meter to determine the daily amount of kerosene used. The calorific value of kerosene is 8700 kcal/L, which was then multiplied to convert the amount of energy consumed.

2.3.6. Growth Performance and Yield of Strawberry Plants

To compare the developed system with existing heating and cooling systems, strawberry growth was investigated. On 15 September 2021, 900 Seolhyang strawberry plants were transplanted into each greenhouse. The plant parameters measured were crown diameter (cm), plant height (cm), petiole length (cm), leaf length (cm), leaf width (cm), number of leaves (n), and yield (g). The growth data were collected 9 times from 16 November 2021, to 7 March 2022, and yield data were collected at 3- to 4-day intervals for a total of 17 times from 3 January to 11 March 2022. Data collection was conducted on plants grown in raised beds 2, 3, and 4 (Figure 7). The beds were divided into three zones (front, middle, and rear), and in each zone, 10 plants per bed were evaluated, totaling 90 plants per group.

2.3.7. Statistical Analysis

Only the plant data were subjected to statistical analysis using SPSS version 20. The growth indexes for each survey period were analyzed using the General Linear Model (GLM) procedure under a Randomized Complete Block Design (RCBD), with zones used as a blocking factor. Data are presented in a table. The same test was also applied to the yield data. However, because fruits did not mature synchronously across all plants, the yields varied for each harvest within zones. Consequently, the yield data were summed monthly (January and February) before analysis. The March data were not included in the analysis because harvesting was stopped in the second week of the month. However, the data were included in the total yield. The data were also visualized graphically. Statistical significance was set at p < 0.05.

3. Results and Discussion

This study evaluated the performance of a hybrid PVT+GHP system applied in strawberry greenhouse heating and cooling. The system was compared with the existing system using a kerosene boiler in terms of the heating performance, efficiency, economic analysis, and growth and yield performance of the strawberry. The findings provide insights into both the technical effectiveness and the practical applicability of hybrid renewable energy systems in smart farming environments.

3.1. Analysis of the Specifications of the Testing Instruments and Operating Temperature Ranges

The system specifications and operating temperature ranges, which serve as parameters for each component, are summarized in

Table 5. System specification and temperature range.

Components	Specification		Temperature Range	Remarks
PVT	Area 18 m3	Electricity: 3 kW Heat: 7.9 kW	0~80 °C	Electricity: 3 kW × 3.5 h = 10.5 kWh/day Heat: 7.9 kW × 3.5 h = 27.65 kWh/day
Heat Pump	7.5 RT (26.38 kW)		4~50 °C	Heat source side $∆\mathrm{T}$ = 1.7 °C, load side $∆\mathrm{T}$ = 6.5 °C
Heat Storage Tank	5 m3		40~45 °C
Buffer Tank	2 m3		4~35 °C
Geothermal Well (Groundwater)	100 L/min	Depth 200 m	10~20 °C

. The system is designed to produce both electricity and heat from solar energy within the same area. From an agricultural standpoint, the ability to generate more than twice as much heat as electricity is particularly important. However, since the PVT panel relies on solar energy, its performance is influenced by weather conditions. On cloudy days, when solar energy is less efficient, the buffer tank temperature gradually decreases and can drop below 8 °C. In such cases, geothermal water circulating through the heat exchange coil in the buffer tank provides supplemental heat, maintaining a minimum buffer tank temperature of 4 °C even when the outside temperature drops to –13 °C (Figure 8).

On clear days with about 800 W/m² of solar irradiance, the PVT panel increases the buffer tank temperature to approximately 14 °C (Figure 8). This thermal energy is then used as the heat source for the heat pump. Geothermal water only circulates when the buffer tank temperature drops below 8 °C, ensuring that solar heat is used as the primary energy source first. Once the buffer tank exceeds 8 °C, only solar heat supplies the heat pump; when it falls below 8 °C, the heat pump switches to groundwater as the heat source to raise the storage tank to 40 °C, providing warm water to the FCU. This switching between solar and geothermal heat sources is a key feature of the system.

Because the system functions as both a heating and cooling device, it is highly sensitive to outdoor temperature. When the outside temperature drops below −10 °C, which is the design limit set for 5% of the TAC (Technical Advisory Committee), maintaining the greenhouse temperature at 8–10 °C requires careful operation. Figure 9 illustrates the temperature variations in the greenhouse, groundwater, and buffer tank at −13 °C, while Figure 10 shows the temperature changes in the storage tank and across the heat pump’s source and load sides. These results confirm that the greenhouse was effectively heated and that the system continued to operate reliably even under extreme conditions of −13 °C.

3.2. Comparative Results of Greenhouse Heating Experiments

To verify the heating and cooling performance of the hybrid PVT+GHP system in the greenhouse, we compared it with the existing greenhouse using a kerosene boiler heating system. Heating experiments were carried out from 18 November 2021, to 11 March 2022, but the data shown here are from February 2022, when outdoor temperatures were the lowest. The greenhouse minimum internal temperature was set at 10 °C. As shown in Figure 11, the hybrid PVT+GHP heating system was able to meet and sustain the set minimum internal temperature, especially during nighttime. On the other hand, the kerosene boiler heating system was unable to maintain the set temperature, resulting in an internal temperature drop of more than 5 °C below the set temperature.

3.3. COP Analysis Results of the Hybrid PVT+GHP System

The annual data on heating supply, electricity production, and electricity usage were collected from 18 November 2021, to 18 November 2022 (

Table 6. Annual heat supply, electricity production, and electricity usage using the hybrid PVT+GHP system (in kWh *).

	Heat Supply		PVT Electricity Produced	Heat Stored in Aquifer Thermal Energy Storage		Electricity Usage
	PVT	Geothermal		PVT	Greenhouse	Heat Pump	Geothermal Pump
Heating mode	2568	8685	815			5160	1812
Cooling mode			448	1266	2418	1501	695
Inter-season (Spring, Fall)			2576	7441			1287
Subtotal 1	11,253		815			5160	1812
Total	11,253		3839	11,125		6661	3794

¹ Heating mode only. * 1 kWh is equal to 860 kcal.

). During heating mode, the total heat supplied by the hybrid PVT+GHP system to the greenhouse was 11,253 kWh. The PVT module supplied 2568 kWh, which is 22.82% of the total heat supply. The remaining 77.18% was by the GHP, which is equivalent to 8685 kWh. Furthermore, the PVT module produced 815 kWh of electricity, which can compensate for 11.69% of the total electricity requirements of the pumps during heating mode (5160 kWh + 1812 kWh = 6972 kWh). With this, the system can achieve a heating COP of 1.83 when relying only on the direct heat supply from PVT and geothermal without heat storage during winter (heating mode). It indicates that the system can still meet the demand for greenhouse heating.

During summer and periods when heating is not required, the solar thermal energy collected by the PVT system can be stored in a heat storage tank or aquifer via the buffer tank for later use in the succeeding winter. The total annual heat supply amounts to 11,253 kWh, with an additional 11,125 kWh stored in ATES. Thus, up to 22,378 kWh of heat can be supplied by the PVT+GHP system annually for greenhouse heating. However, the system also consumes energy, with pumps requiring a total of 10,455 kWh per year (6661 kWh for the heat pump and 3794 kWh for the geothermal pump). Based on this balance, the system COP is 2.14. Furthermore, the PVT system can generate an annual electricity of 3839 kWh, which can offset 36.72% of the pumps’ electricity consumption, effectively improving the system’s COP to 3.38. A similar study also found improvement in the system COP when PVT is added to the GHP system, enhancing heating performance, especially during sunny conditions. [43].

The recoverable heat stored in the aquifer is 60% based on our previous study [59]. This could mean that approximately 6675 kWh from the 11,125 kWh stored in the aquifer will be recovered. Groundwater temperature rises from 15 °C to 20 °C, and it takes about 40 days to cool the groundwater back down to its original temperature of 15 °C, serving as a heat source for the heat pump. This increases heat pump efficiency by 18%.

These results show that the hybrid PVT+GHP system can provide enough heat for greenhouse operations while decreasing dependence on grid electricity through integrated solar power generation. Thermal storage further enhances renewable energy benefits during colder months, maintaining consistent heating. In practice, growers can enjoy lower operational costs, greater energy independence, and fewer greenhouse gas emissions. Consequently, this system offers a promising route to more sustainable and energy-efficient greenhouse production, especially in areas with notable seasonal temperature changes.

3.4. Heating Cost Analysis and Economic Assessment

The amount of duty-free kerosene used to heat the existing greenhouse was 1177 L. The cost was calculated as KRW (Korean Won) 1,527,746, applying the average unit price of duty-free kerosene for 2022, which was KRW 1298, provided by Opinet. The cost analysis for the hybrid system was only calculated based on the heating mode heating electricity usage (Table 6). The heating energy consumption from pumping in the hybrid PVT+GHP system was 6972 kWh (5160 kWh + 1812 kWh). However, with the electricity generated from the PVT module (815 kWh), the net energy consumption was reduced to 6157 kWh. This is equivalent to KRW 332,300, which was calculated based on the unit price of electricity for agriculture (KRW 46.5/kWh) and the basic fee (KRW 1150/kW) for contracted electricity (10 kW) over four months (KRW 46,000) based on the actual amount paid. The unit price used was based on 1 October 2022 data. Overall, the heating cost of the hybrid system is cheaper than using kerosene, which could save more than 78%.

For the economic analysis, a reference greenhouse area of 990 m² (10 a) was used. The installation cost of a kerosene boiler without subsidy is estimated at 6,000,000 KRW. For the PVT+GHP hybrid heating and cooling system, it was assumed that the farmer’s share would be 20% under the subsidy program of the Ministry of Agriculture, Food and Rural Affairs (geothermal system support under the Agricultural Energy Use Efficiency Project in the ROK). With a 20% farmer contribution, the installation cost of the PVT+GHP hybrid system amounted to 34,000,000 KRW, with a service life of 10 years.

The sustainability of groundwater temperature is a concern for long-term operation. However, the system can maintain the groundwater temperature by replenishing it with heat captured by the PVT. A simulation study conducted at a high operating temperature for a residential building with a floor area of 117 m³ also found a stable groundwater temperature for over 10 years of operation using the hybrid system, with an 11.9% lower operational cost than the conventional GHP [60]. Nevertheless, a long-term sustainability and economic analysis are necessary for large-scale agricultural operations.

Loss costs were calculated by considering depreciation, fixed capital interest, repair and maintenance expenses, and heating/cooling costs. Compared to the kerosene boiler, the combined heat source system resulted in a loss cost of 6,545,000 KRW. On the benefits side, heating cost savings, additional income from increased strawberry seedling production during cooling, and revenue from photovoltaic electricity generation were considered, totaling 13,997,000 KRW. The net profit was therefore estimated at 7,452,000 KRW, indicating that the investment in the hybrid heating and cooling system could be recovered within 4.67 years.

Furthermore, if only geothermal energy is used as the heat source without utilizing solar thermal energy, the geothermal heat pump must fully compensate for the share previously supplied by solar heat. In this case, the geothermal heat pump’s share of heating demand increases from 77% to 100%, leading to higher electricity consumption. When considering the additional electricity generated by the PVT, it was found that energy savings of about 13% could be achieved compared to using only geothermal energy. In comparison with a pure PV system, since PVT also produces solar heat and reduces the operating load of the geothermal pump, an additional energy-saving effect of approximately 9% is expected.

3.5. Comparative Analysis of Strawberry Growth and Yield

Strawberry plants grown under the current system exhibited significantly higher growth indexes values up to 8 February 2022 (

Table 7. Differences in growth performance parameters per data collection period.

Collection Periods	Values	Kerosene Boiler	Hybrid PVT+GHP	SEM	p-Value
16 November 2021	Crown Diameter (cm)	15.27	16.47	0.355	0.020
	Height (cm)	26.56	25.39	0.409	0.047
	Petiole Length (cm)	12.74	11.70	0.262	0.007
	Leaf Length (cm)	12.29	11.86	0.220	0.176
	Leaf Width (cm)	10.04	10.12	0.154	0.715
	Number of Leaves (n)	nd	nd	nd	nd
13 December 2021	Crown Diameter (cm)	14.96	16.82	0.355	<0.001
	Height (cm)	26.49	25.09	0.363	0.009
	Petiole Length (cm)	12.71	11.69	0.253	0.006
	Leaf Length (cm)	12.18	11.43	0.191	0.008
	Leaf Width (cm)	10.06	9.85	0.151	0.316
	Number of Leaves (n)	5.27	5.67	0.146	0.057
28 December 2021	Crown Diameter (cm)	14.76	15.32	0.283	0.172
	Height (cm)	26.60	23.29	0.379	<0.001
	Petiole Length (cm)	12.56	11.30	0.251	0.001
	Leaf Length (cm)	11.97	10.26	0.194	<0.001
	Leaf Width (cm)	10.00	8.93	0.180	<0.001
	Number of Leaves (n)	5.90	5.83	0.147	0.750
11 January 2022	Crown Diameter (cm)	15.60	15.53	0.296	0.864
	Height (cm)	26.34	15.15	0.562	<0.001
	Petiole Length (cm)	12.57	10.76	0.225	<0.001
	Leaf Length (cm)	11.79	9.38	0.195	<0.001
	Leaf Width (cm)	9.82	8.87	0.144	<0.001
	Number of Leaves (n)	7.03	6.47	0.242	0.103
8 February 2022	Crown Diameter (cm)	14.45	13.65	0.328	0.088
	Height (cm)	20.41	19.33	0.603	0.214
	Petiole Length (cm)	10.16	9.41	0.347	0.130
	Leaf Length (cm)	8.64	8.47	0.269	0.657
	Leaf Width (cm)	7.17	6.71	0.229	0.165
	Number of Leaves (n)	8.03	6.27	0.258	<0.001
22 February 2022	Crown Diameter (cm)	14.42	13.89	0.387	0.339
	Height (cm)	15.45	18.30	0.417	<0.001
	Petiole Length (cm)	7.31	9.07	0.256	<0.001
	Leaf Length (cm)	7.08	8.07	0.195	0.001
	Leaf Width (cm)	5.86	6.46	0.180	0.022
	Number of Leaves (n)	5.83	6.67	0.299	0.054
7 March 2022	Crown Diameter (cm)	14.06	14.36	0.361	0.564
	Height (cm)	16.00	16.91	0.436	0.147
	Petiole Length (cm)	7.79	8.72	0.312	0.041
	Leaf Length (cm)	7.23	7.26	0.189	0.931
	Leaf Width (cm)	6.09	5.96	0.185	0.604
	Number of Leaves (n)	6.50	7.50	0.320	0.031
Differences 1	Crown Diameter (%)	−7.92	−12.85
	Height (%)	−39.75	−33.40
	Petiole Length (%)	−38.81	−25.51
	Leaf Length (%)	−41.14	−38.83
	Leaf Width (%)	−39.29	−41.12
	Number of Leaves (%)	23.42	32.35

¹ Percent difference in data parameters between the first and last data collections. PVT = photovoltaic–thermal collector; GHP = geothermal heat pump; SEM = standard error of the mean; nd = no data.

). However, by 22 February 2022, a sharp decline in crown diameter, plant height, petiole length, leaf length, and leaf width was recorded across all groups, with the decrease being more pronounced in the current system. The number of leaves also dropped markedly, from an average of 8.03 to 5.83, whereas leaf production remained unaffected in the Hybrid PVT+GHP system. Overall, the reductions in growth indexes between the first and last data collection points were greater in the existing system. This decline, particularly in leaf production, is likely attributed to reduced internal temperatures resulting from extremely low outdoor conditions during this period. In the current system, nighttime temperatures fell below 5 °C (Figure 11), which limited the emergence of new leaves [61].

Furthermore, the yield data showed significant differences in February, with a higher yield under the Hybrid PVT+GHP system at 2108.12 g or 57.89% higher than the existing system. Overall, the total yield was not significantly different. But the yield under the greenhouse with the hybrid PVT+GHP heating system was higher with 3140.4 g, which is more than 4% than in the greenhouse with a kerosene boiler (Figure 12). These findings highlight the benefits of the new system not only in reducing carbon emissions and costs but also in enhancing farm productivity.

3.6. Research Limitations and Recommendations

While this study demonstrated the performance of a hybrid PVT+GHP system in strawberry greenhouses under winter conditions, several limitations should be acknowledged. First, the experiment was conducted in a small-scale facility (two single-span vinyl greenhouses of 175 m² each), which may limit the applicability of the results to larger commercial operations. The findings are specific to Jeonju, South Korea, and performance could vary in different climates. The study focused on one cultivar (Seolhyang strawberry) during a single winter season, leaving responses of other cultivars, species, growth stages, or annual variability unexplored. The economic analysis relied on fixed assumptions about system lifespan, energy prices, and government subsidies, which could change and affect profitability. Lastly, although energy was monitored continuously, the system was not tested across multiple growing cycles or long-term groundwater impacts.

Future research should therefore expand research to larger-scale greenhouses and diverse crops to assess scalability and broader applicability. Long-term monitoring over multiple years would help capture seasonal and inter-annual variability, as well as confirm the reliability of the system under different operating conditions. It would also be valuable to compare alternative geothermal configurations (e.g., closed-loop vs. open-loop) and examine the potential of combining hybrid systems with energy storage or other renewable sources. Moreover, a comprehensive life cycle assessment, including carbon footprint and environmental impact, would provide deeper insights into sustainability.

4. Conclusions

In this study, a hybrid heat pump system combining photovoltaic–thermal (PVT) panels with a standing column well (SCW) geothermal system was applied in a strawberry greenhouse to evaluate its heating performance, efficiency, cost-effectiveness, and impact on crop growth and yield. The system, with PVT panels installed over 10% of the greenhouse area, stored excess solar heat in an aquifer to compensate for the reduced efficiency of the geothermal source during long-term operation. The hybrid system was compared with the existing system using a kerosene boiler.

A winter heating cost analysis revealed at least 78% savings compared to conventional heating with duty-free kerosene. These savings were achieved through a balanced utilization of solar electricity (11.69%), solar thermal (22.82%), and geothermal energy (77.18%). When relying solely on direct heat supply from PVT and geothermal sources, the system achieved a heating COP of 1.83 during the heating period. Additionally, the annual system COP of 3.38 was achieved when the heat collected from the PVT was stored during periods when no heating was required. Moreover, the electricity generated by the PVT significantly offsets the power consumption of the heat pump system. This performance allowed the greenhouse to maintain an indoor temperature of 10 °C even when outdoor temperatures dropped to −10.5 °C. Moreover, strawberry growth was enhanced in the hybrid system due to greater temperature stability, resulting in a 4% yield increase compared to the conventional system. Importantly, storing seasonal solar energy in aquifers or geological formations provided a sufficient and renewable heat source, suggesting the potential to reduce the required capacity of geothermal systems.

This study highlights the potential of hybrid renewable energy systems to reduce dependence on fossil fuels, lower energy costs, and improve agricultural sustainability. In particular, the system offers significant potential for reducing carbon emissions while maintaining economic viability for small-scale farms. However, further optimization is needed to enhance the efficiency of PVT panels and reduce initial installation costs. Overall, this work demonstrates the successful integration of renewable energy technologies into climate-controlled agriculture, providing a technical and policy foundation for advancing energy transition and carbon neutrality goals in the agricultural sector.