**1. Introduction**

Increasing energy demands and crude oil depletion have forced the world to explore new energy resources, those that are not only sustainable but also have a minor environmental footprint. Shallow geothermal energy is one of the sustainable energy resources that could be applied to improve the efficiency of current and future heating and cooling systems. This renewable thermal energy is usually exploited in a geothermal heat exchanger (GHE); a system of pipes buried in the ground, in which the heat carrier fluid is circulated. GHEs are normally integrated with heat pumps and air conditioning systems. These systems use the ground as a heat source/sink during heating/cooling to

provide the thermal comfort condition of buildings' interiors in both winter and summer. Compared with the traditional air source heat pumps and air conditioning systems, the application of GHEs as a thermal energy rejecter/absorber can reduce energy consumption by 30%–60%, since the ground has a relatively stable temperature [1].

There have been two basic arrangements of GHEs: vertical and horizontal. Vertical GHEs have been widely installed in confined areas such as an urban area or an area where the earth is rocky close to the surface. The typical depth of vertical GHEs varies from 20 to 300 m [2]. Vertical GHEs have a better efficiency since the ground temperature at the deeper depth below the surface remains relatively largely constant all year around. The drawback of vertical GHEs, however, is their installation cost, which is higher than for the horizontal arrangement. In addition, the degradation in their thermal performance is relatively hard to recover because of the poor soil thermal conductivity and relatively deep depth of burring, which hinder the heat transfer rate to and from the atmosphere. Horizontal GHEs are mostly installed where a land area is available at low cost. They are buried in a horizontal trench, with a typical depth up to 2 m below the surface. The continuous thermal interaction between the ground and the atmosphere could improve the GHE's performance through an appropriate operation mode. The diverse mechanisms of the heat transfer occurring on the ground surface could also facilitate the thermal recovery in the case of a horizontal arrangement.

The technical challenges associated with GHEs are their performance degradation with an increase in the operation time, which often happens in heating or cooling seasons. This performance degradation is slow to restore, especially for vertical GHEs. Meanwhile, it can be minimised if the GHEs are run intermittently or with a smaller thermal load, which can be achieved with the proposed GHE system, which combines both vertical and horizontal arrangements. To address the degradation performance problem caused by an imbalance in the heating and cooling loads, a number of researchers have proposed hybrid GHE systems and different operation strategies [3]. For example, hybrid GHE, solar energy systems, and heat pump have been suggested by a number of researchers [4–8], to cope with performance deterioration. Dai et al. [4] carried out an experimental study on a solar assisted ground source heat pump system. In the parallel mode, the performance of the heat pump, under three different flow rate ratios of working fluid inside both solar and shallow geothermal systems, is investigated. The outcomes of this study demonstrated that the hybrid systems could recover the degradation of soil temperature much faster than happens during natural recovery. In addition, the effect of the flow rate ratio in mode 4 has a significant impact on the electricity consumption. The electricity consumption decreases with the increase in the flow rate ratio of the solar heat storage water tank. Furthermore, mode 3 is recommended for use in the coldest months.

Kjellsson et al. [5] analysed different systems with a combination of a ground source heat pump and a solar collector to provide heating for houses and domestic hot water systems. The outcomes demonstrated that the system works in an optimal regime when the solar energy is utilised to recharge the borehole during winter time and to produce the domestic hot water during summer time. The performance of a solar assisted ground source heat pump, which is used for green house heating, was investigated by Ozgener et al. [6]. The archived exergetic efficiency of the overall system was reported to be at 67.7%. Yang et al. [7] presented a theoretical and experimental study on a solar-ground source heat pump system. The experimental coefficient of performances (COPs) of modes 2, 3, and 4 were obtained 2.69, 2.65, and 2.59, respectively. While the theoretical COPs were found to be 3.67, 3.64, and 3.52, which are quite different from the theoretical values. In addition, research on small ammonia heat pumps for space and hot tap water heater with a capacity of 8.4 kW to provide space heating and hot water has been done by Aleksandrs Zajacs et al. [8]. In this study, equation calculations have been carried out using the engineering equation solver (EES) to estimate the heat pump performance. The results of the calculations are able to provide electrical energy savings of up to 75% compared to using electric heating. The volume of the tank also affects the efficiency of the heat pump and compressor usage. The optimal tank volume is 1000 L to cover 2–3 h of high electricity price peaks.

Studies on hybrid ground source heat pump systems with a cooling tower as a supplemental heat rejecter were presented by a number of researchers [9–13]. Park et al. [9] proposed a new parallel system comprising of a hybrid ground source heat pump-cooling tower. The new parallel system enables the GHE to be switched off during the recovery period of the soil thermal condition. The performance of the heat pump was investigated at different flow rates of the fluid in the primary flow loop (the heat pump), GHE, and cooling tower. The results indicated that the parallel system of the ground source heat pump-cooling tower generates 21% COP more than that produced by the conventional ground source heat pump system. Man et al. [10] provided the technical and economic analysis of a hybrid cooling tower-ground source heat pump, based on the hourly load of a two storey residential building located in Hong Kong. The hybrid system in this study not only solves the thermal degradation problem, but also reduces the operation and capital costs of the air conditioning system. A study of the operation strategy of a hybrid cooling tower-ground source heat pump system was presented by Wang et al. [11]. The operation strategy consists of a fixed cooling set point, outside air reset, wet bulb reset, and load reset. Fan et al. [13] presented a theoretical design of a hybrid cooling tower-ground source heat pump, which takes into account the effect of borehole distance, borehole depth, and thermal properties of the grout. In this study, a combined strategy of operation was introduced. The results showed that the lowest energy consumption was obtained when the control strategies of the entering water temperature and wet-bulb temperature differences are combined.

Canelli et al. [14], presented an analysis of the energy, economic, and environmental performances of three different hybrid ground source heat pump systems including (1) a hybrid boiler-chiller-ground source heat pump, (2) a hybrid boiler-chiller-ground source heat pump and fuel cell, and (3) a hybrid boiler-chiller-ground source heat pump and photovoltaic thermal system. The system was optimised to meet the heating and cooling conditions of both residential and commercial buildings, which are in a sharing load. The results indicated that the hybrid system with the fuel cells and photovoltaic thermal system has a definite advantage in terms of energy savings, operational costs, and carbon emission reductions.

A detailed numerical study was conducted by Zhu et al. [15] to investigate the performance and economic characteristics of a combined vertical ground source heat pump and phase change material cooling storage system. The optimal performance of the hybrid system was achieved by varying the ratio of the phase change material cooling storage system to the total cooling load of the system. The obtained optimal cooling storage ratio was close to 40%.

A hybrid system of the ground source electrical heat pump and ground source absorption heat pump was proposed by Wu et al. [16]. The motivation for this study was to combine the features of both heat pumps, as the ground source electrical heat pump has higher energy efficiency in the cooling mode. Conversely, the ground source absorption heat pump has higher energy efficiency in the heating mode. This study also presented the effects of supply ratios on thermal imbalance ratios, annual primary energy efficiency, and cost-efficiency characteristics.

A theoretical study on a hybrid air source heat compensator-vertical ground source heat pump system was conducted by You et al. [17], using a simulation tool, TRNSYS. Four operation strategies were analysed including an air source heat compensator for direct heat compensation, a combined air source heat compensator-ground source heat pump for heat compensation, a combined air source heat compensator-ground source heat pump for space heating, and an air source heat compensator-ground source heat pump for domestic hot water. The results showed that the hybrid air source heat compensator-ground source heat pump reduces energy consumption by 23.86% compared with the boiler-split air conditioner system. Additionally, the operational costs are reduced by 50%. Another study has done by Xianting Li et al. [18] by developing an energy-efficient heat pump system using the TRNSYS simulation tool in a combination of a hybrid source and three types of hybrid source heat pumps to calculate the efficiency of year-round operation. The results show that the hybrid heat pump system can maintain heat reliability during year-round operation including winter. This hybrid source heat pump system saves energy up to 15% and the payback period is approximately five years. In addition, Gaoyang Hou et al. [19] also conducted a simulation using the TRNSYS tool to analyse the work system of heat pumps sourced from hybrid soil with optimal control strategies. His research combines a horizontal ground loop and a liquid dry cooler with a short and long term simulation process on TRNSYS to analyse soil thermal conditions and energy variations. Gaoyang Hou et al. also performed a simulation using the TRNSYS tool to analyse the heat pump working system sourced from hybrid soil with optimal control strategies. Their research combines a horizontal ground loop and a liquid dry cooler with a short and long term simulation process on TRNSYS to analyse soil thermal conditions and energy variations. The simulation results show that the overall performance in the short-term simulation is influenced mainly by the temperature of the diverter heating set rather than by cooling. Diverter heating set was recommended about 8–10 ◦C in climatic zones similar to the Birmingham area by combining long-term coefficient of performance (COP) values and soil thermal variation.

A hybrid active air source regeneration-ground source heat pump system was studied by Allaerts et al. [20]. In this study, the borehole area is divided into two different regions, namely warm and cold regions. These two different regions were proposed to balance the extraction/rejection of heat during heating and cooling periods. In addition, a supplementary dry cooler was used to capture heat/cold during summer/winter to recover the degradation of the soil thermal condition. According to Allaerts et al. the proposed hybrid system can significantly reduce the size of the borehole area by up to 47% in the cost-optimal configuration.

The brief review of the literature [1–22] above shows that most researchers proposed a hybrid cooling tower-ground source heat pump for a cooling load dominated regime and a hybrid solar system-ground source heat pump for heating load dominated conditions. Some other studies proposed a combined system of a ground source heat pump with different additional heat rejecter/absorber systems including a boiler, chiller, fuel cell, photovoltaic, and phase change material cooling storage systems. Additionally, a ground source heat pump system with two different regions of BHEs including warm and cold was recently suggested and analysed. However, it seems a comprehensive study of a combined GHE arrangement with different operation modes has yet to be presented.

This paper is focused on the operational analysis of a combined horizontal-vertical GHE to address various demands and thermal loading conditions. The performance of the combined GHE is studied based on the results of a transient finite difference model developed in this paper. The effects of continuous and intermittent operation conditions, climate condition and fluid mass flow rate on the GHE's performance are investigated with this new model. Based on the analysis of the outcomes of numerical simulations, the recommendations for the optimum operation of combined GHEs are summarised in the conclusion.
