Solar-assisted Ground-coupled Heat Pump Research Articles

Modeling and investigation of the performance of a solar-assisted ground-coupled CO2 heat pump for space and water heating

The rise in the popularity of heat pumps should be accompanied by the increase in the utilization of environmentally-safe working fluids. To push for wider uptake of hybrid heat pump systems that use natural working fluids, information about their performance and operating characteristics should be made available. This study models a CO2 solar-assisted ground-coupled heat pump (SAGCHP) system and investigates its performance for space and water heating. Sensitivity analysis, parametric study, and long-term performance simulation were implemented, with the system’s seasonal performance factor (SPF), levelized cost of heating (LCOH), and ground temperature change (GTC) considered as performance indicators. It was seen that ensuring a good combination of design and operating specifications can result in an SPF (∼3.5) and LCOH (0.184 USD/kWh) that are comparable with those of SAGCHP systems that use conventional working fluids. The heat pump’s high-side pressure and output temperature exhibited notable effects on all the performance indicators. Below the optimal operating pressure, a 5% increase in the heat pump’s high-side pressure brought about a ∼7–10% improvement to the SPF, a ∼3–5% reduction to the LCOH, and a ∼6–10% increase to the GTC. Reducing the output temperature by 5% increased the SPF by ∼7–8%, decreased the LCOH by ∼3%, and increased the GTC by ∼6%. Parametric studies identified the presence of optimal heat pump discharge pressure and heat source circulation rate that should be used for operations. Long-term simulation shows that managing the GTC ensures the longevity of the system. Rather than oversizing the borehole heat exchangers (BHEs), it is more practical to reduce ground temperature decline by increasing the BHE spacing or by adding solar collectors.

An experimental work on the performance of a solar-assisted ground-coupled heat pump using a horizontal ground heat exchanger

Solar Assisted Ground Coupled Heat Pumps are thoroughly investigated today. Although the best method to extract heat from the earth is by using vertical ground heat exchangers, they are expensive. Horizontal ground heat exchangers are cheaper but the downside is that they are not so efficient. The main objective of this study is to find efficient, alternative methods to assist the horizontal ground heat exchanger in order to decrease the ground freezing period and increase its performances. For that, a Solar Assisted Ground Coupled Heat Pump system was established, using two solar assistance sub-systems (solar thermal panels and solar photovoltaic panels). A third heat source was considered, consisting in the residual heat lost from the building's heated basement. The results indicated that using auxiliary heat sources eliminated the ground freezing period. Although heat transfer into the Horizontal Ground Heat Exchanger increased the Seasonal Performance Factor by approximative 15% for the season peak, a high amount of heat was lost in the process. This study also addresses possible methods to reduce the heat loss. Overall, the results indicated better performance for the Horizontal Ground Heat Exchanger when transferring heat into it.

Performance Analysis of Solar-Assisted Ground-Coupled Heat Pump Systems with Seasonal Thermal Energy Storage to Supply Domestic Hot Water for Campus Buildings in Southern China

The supply of domestic hot water (DHW) on college and university campuses is indispensable and is also one of the main components of campus energy consumption. The density of residential patterns and similar occupancy behavior of college students make it economical to use centralized systems to cover the DHW demand, and utilization of solar energy can make the systems more economical. Seasonal thermal energy storage (STES) is a promising key technology that can minimize the imbalance between the availability of solar energy and thermal energy demand. In this paper, a solar-assisted ground-coupled heat pump (SAGCHP) system that meets the DHW demand of 960 students was investigated by means of dynamic simulation and energy-economic analysis. The simulation results in terms of the underground heat balance are compared with a standalone GCHP system and a SAGCHP system without STES. Results show that heat recharging operations during university summer and winter breaks (when there are minimal students on campus) lead to improved underground heat balance and energy performance. Finally, a sensitivity analysis on system performance was carried out by varying solar collector arrays. It was found that there exists an optimal value of solar collector area to achieve the lowest system lifecycle cost (LCC).

Effects of saturated soil on the lengths of a double U-tube borehole with two independent circuits, a parallel double U-tube borehole and on the power consumption of a GSHP

Ground in winter as a heat source can increase the coefficient of performance of the heat pump. However, cost of digging the ground is high. Thermal properties of the soil affect the length of boreholes. The saturated soil of Kerman has a unique potential for establishment of geothermal boreholes. In this paper, the effects of Kerman's saturated soil with freezing and thawing, on lengths of a double U-tube borehole with two independent circuits and a parallel double U-tube borehole have been investigated. Besides, effects of the saturated soil on the power consumption of a solar assisted ground-coupled heat pump system and a conventional ground-coupled heat pump system have been investigated numerically. The results show that the double U-tube borehole with two independent circuits in Kerman's soil comes out 28.6% shorter in length compared with a parallel double U-tube borehole in a conventional soil. Besides, the power consumption of the solar assisted ground-coupled heat pump system in Kerman's soil decreases by 6.1% in comparison with a conventional ground-coupled heat pump in the standard soil. Moreover, it has been shown that the length of the boreholes in the lower thermal conductivity of the soil can be reduced by as much as 35.4%, 40 m.

Feasibility Analysis of Solar Assisted Ground-coupled Heat Pump Indirect Heating Systems

This paper presents a novel solar assisted ground-coupled heat pump indirect heating system (SA-GCHPIHS) designed for a residential building in Beijing for space heating and supplying domestic hot water. The recharging strategy for ground heat exchanger (GHE) is analyzed and determined based on system energy performance. The impacts of operation strategies and heating load ratio are simulated and compared, and the system with heat pump and water tank installed in series (SA-GCHPIHS(s)) showed better performance than SA-GCHPIHS(r) when domestic hot water (DHW) load dominates the heating load. And more energy would be saved by SA-GCHPIHS(s) as DHW load grows larger.

Performance analysis of a proposed solar assisted ground coupled heat pump system

This paper presents the numerical simulation of a solar assisted ground coupled heat pump (SAGCHP) system which can provide both space heating and domestic hot water (DHW). The optimization process is completed on the TRNSYS based platform by simulating the influence of solar collector area on the total borehole length and system performance. Simulations are also carried out under different meteorological conditions to explore the applicability of the proposed SAGCHP system in northern China. The simulating results show that the optimized system under the specified load conditions has a collector area of 40m2 and a borehole length of 264m. The annual total heat extraction plus 75% of the hot water requirement can be provided by solar energy in the optimized design. Furthermore, the energy balance of the optimized design is confirmed with a minor difference of 0.75%, and the system is proved more efficient and economical for its application in Beijing area.

Experimental studies on a ground coupled heat pump with solar thermal collectors for space heating

This paper presents experimental studies on a solar-assisted ground coupled heat pump (SAGCHP) system for space heating. The system was installed at the Hebei Academy of Sciences in Shijiazhuang (lat. N38°03′, long. E114°26′), China. Solar collectors are in series connection with the borehole array through plate heat exchangers. Four operation modes of the system were investigated throughout the coldest period in winter (Dec 5th to Dec 27th). The heat pump performance, borehole temperature distributions and solar colleting characteristics of the SAGCHP system are analyzed and compared when the system worked in continuous or intermittent modes with or without solar-assisted heating. The SAGCHP system is proved to perform space heating with high energy efficiency and satisfactory solar fraction, which is a promising substitute for the conventional heating systems. It is also recommended to use the collected solar thermal energy as an alternative source for the heat pump instead of recharging boreholes for heat storage because of the enormous heat capacity of the earth.

Long term operation of a solar assisted ground coupled heat pump system for space heating and domestic hot water

This paper introduces a solar-assisted ground-coupled heat pump (SAGCHP) system with heat storage for space heating and domestic hot water (DHW) supply. The simulation results of the system's detailed operating performance are presented. The optimization of the system design is carried out by the TRNSYS and a numerical simulation is performed for continuous operation of 20 years under the meteorological conditions of Beijing. Different control strategies are considered and the operational characteristics of each working mode are studied. The simulating results show that the long term yearly average space heating efficiency is improved by 26.3% compared to a traditional ground coupled heat pump (GCHP) system because the solar thermal collecting system is used to elevate the thermal energy in the soil and to provide direct space heating with heat storage. At the same time, the underground heat load imbalance problem for a heating load dominated GCHP is solved by soil recharging during non-heating periods, while extra solar energy is utilized to supply DHW. The flexibility and high efficiency of the SAGCHP system could offer an alternative for space heating and DHW supply by heat pump technology and solar energy in cold winters of northern China.

Experimental study of a solar-assisted ground-coupled heat pump system with solar seasonal thermal storage in severe cold areas

This paper presents the experimental study of a solar-assisted ground-coupled heat pump system (SAGCHPS) with solar seasonal thermal storage installed in a detached house in Harbin. The solar seasonal thermal storage was conducted throughout the non-heating seasons. In summer, the soil was used as the heat sink to cool the building directly. In winter, the solar energy was used as a priority, and the building was heated by a ground-coupled heat pump (GCHP) and solar collectors alternately. The results show that the system can meet the heating–cooling energy needs of the building. In the heating mode, the heat directly supplied by solar collectors accounted for 49.7% of the total heating output, and the average coefficient of performance (COP) of the heat pump and the system were 4.29 and 6.55, respectively. In the cooling mode, the COP of the system reached 21.35, as the heat pump was not necessary to be started. After a year of operation, the heat extracted from the soil by the heat pump accounted for 75.5% of the heat stored by solar seasonal thermal storage. The excess heat raised the soil temperature to a higher level, which was favorable for increasing the COP of the heat pump.

Analysis of solar aided heat pump systems with seasonal thermal energy storage in surface tanks

Annual periodic performance of a solar assisted ground-coupled heat pump space heating system with seasonal energy storage in a hemispherical surface tank is investigated using analytical and computational methods. The system investigated employs solar energy collection and dumping into a seasonal surface tank throughout the whole year with extraction of thermal energy from the tank for space heating during the winter season. A computational model is presented in this study for the prediction of the annual periodic transient behaviour of the system under investigation. The present computational model is based on a hybrid analytical–numerical procedure which facilitates determination of the annual variation of water temperature in the surface tank, the amounts of solar thermal energy collected during each month and the annual periodic performance of the solar aided space heating system.

A Solar-Assisted Heat Pump in Use

In this paper on work which forms part of the programmes by the International Energy Agency, the authors discuss results in service of a solar-assisted ground-coupled heat pump in a house in central Switzerland north of the Alps. The solar collectors are of the unglazed low-temperature type. The system was found to work reasonably well in the local climate, although storage losses and the parasitic electrical power of the auxiliary equipment could usefully be reduced. Measured seasonal coefficient of performance, including the power losses, worked out at 2.4. Preliminary results from other systems are briefly mentioned for comparison, but no comparison with theory or economic analysis is made for the system. However, there are important inferences to be drawn on the climatic dependence of the configuration of such systems and on sensitive factors which must be considered at design. The authors work in the heat technology division of the Swiss Federal Institute for Reactor Research (EIR).