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Traditionally, shallow geothermal energy is utilized by a ground source heat pump. In fact, the temperature of shallow geothermal energy is typically quite suitable for the cooling/heating of building envelopes and fresh air. To utilize shallow geothermal energy more efficiently, an integrated system is proposed that combines pipe-embedded walls, pipe-embedded windows, and fresh air pre-handling system with the conventional ground source heat pump system. This proposal is based on the temperature comparison among indoor air, envelopes, fresh air and undisturbed soil. A simulation model of the integrated system is developed on the platform of TRNSYS. The free-running temperature and energy consumption of the integrated system applied in an office building in Beijing are investigated. The results show that the free-running temperature of the integrated system is always below 28 °C throughout the year, and the non-air conditioning period is extended by 34% compared with the conventional GSHP system. Moreover, the integrated system can greatly reduce the peak load, and the heat pump system capacity can be reduced by as much as 30%. The annual cumulated load for the building is reduced by approximately 43%. Consequently, the energy saving rate is approximately 29% compared with that of the conventional GSHP system. The emission reduction of CO2 is more than 7 kg per square meter. Therefore, the integrated system can fully utilize shallow geothermal energy to build an energy efficient HVAC system.

Pipe-embedded windows can significantly reduce the cooling/heating load of buildings when integrated with low-grade energy. However, the studies on the load reduction effect under different climate...

The pipe-embedded double-skin facade (PDSF) is a superior approach to decrease building cooling and heating load by using natural energy. With pipes embedded in the shading, the overheating problem in the conventional double-skin facade (DSF) can be eliminated. Unlike the traditional DSF or glass windows, the thermal performance of PDSF depends on the ambient environment and water temperature in the embedded pipes. Although the complicated heat transfer process of PDSF can be numerically solved, it is difficult for engineers to obtain the whole picture of the PDSF at first glance. To easily understand the thermal property of the PDSF, the overall heat transfer coefficient (U value), solar heat gain coefficient (SHGC) and equivalent ambient temperature of the PDSF are derived based on the thermal network. The heat transfer characteristic of the PDSF at different cooling water temperatures is compared with that of the conventional DSF. The results show that the three proposed indices can clearly reflect the thermal property of the PDSF: U value and SHGC reflects the influence of insulation and the transparency respectively, and the equivalent temperature reflects the integrated influence of water temperature and outdoor temperature. The typical PDSF has a higher U value than the DSF in the same structure. However, the SHGC of the PDSF is only 60% of that of the DSF. Although the equivalent ambient temperature depends on the outdoor temperature and water temperature, the weighting of the water temperature is 0.8, which indicates that the equivalent ambient temperature for the PDSF can reach a better level with the appropriate water temperature. Moreover, relatively warm water is suggested on sunny days to reduce the solar heat gain, since the solar heat gain is dominant in the total heat gain. Correspondingly, cool water is recommended for cloudy days to decrease the conductive heat gain.

Pipe-embedded double-skin facades (PDSFs) have proved to be among the best approaches to solve the overheating problem confronted in conventional double-skin facades (DSFs). Although the heat trans...

To reduce the energy consumption for fresh air handling, a novel fresh air pre-handling system that fully exploits the shallow energy to precool and preheat fresh air is proposed in this study. The model of a typical all-air system with a pre-handling system is built on the TRNSYS platform. The energy saving potential of this fresh air pre-handling system is simulated based on the diverse climate of China. The results show that the ratios of the cumulated heat transfer capacity of the fresh air pre-handling system to the annual total of cooling and heating load range from 35% to 45% in the temperate zone and the subtropical zone, such as Shenyang, Beijing, and Shanghai, while the ratio in the tropical zone is approximately 10%. The energy saving rates are approximately 30% in most climate zones, but less than 5% in Guangzhou, the tropical climate zone. Moreover, the payback periods are within 4 years in the temperate and subtropical zones. However, the system is not economical in the tropical zone. All these results indicate that the fresh air pre-handling system with shallow geothermal energy is quite promising in most climate zones except in the hot summer and warm winter region with a tropical climate.

Utilizing natural energy directly for building cooling and heating can reduce the energy use of the HVAC systems significantly. There are various utilization forms of natural energies for building cooling and heating. The applicability of natural ventilation was evaluated in the previous studies by degree hours method which cannot be used for the evaluation of other natural energy utilizing forms. To provide guidance for selecting proper natural energy and the utilizing forms for building cooling and heating, a general method, i.e., the revised degree hours method, is proposed. Firstly, the conception and rationality of the revised degree hours method are indicated by comparing with the degree hours method. Then, the utilizations of revised degree hours method in evaluating the applicability of natural energy are discussed in terms of different climates, different natural energies and different utilizing forms. The results show that the revised degree hours can directly reflect the energy saving potential of natural energy and it is capable of fairly evaluating different utilization forms with different kinds of natural energies. According to these cases analyzed in this paper, the mechanical ventilation shows a quite promising potential in the temperate region; both the dew point evaporative cooling and the ground heat exchanger fresh air handling system are outstanding in the dry and cold region; the cooling tower combined with pipe-embedded window system works well in the hot summer and cold winter region as well as the hot summer and warm winter region. In conclusion, the revised degree hours method contributes to the low carbon development of HVAC systems by guiding the full use of natural energy for building energy efficiency.

Pipe-embedded external wall using low-grade energy has been shown to significantly reduce the cooling or heating load of buildings. However, previous studies have generally focused on the heat transfer between water pipes and pipe-embedded wall and paid less attention on the energy use of water distribution. In this study, a numerical model of serial pipe-embedded wall (SPW) considering water temperature changes in the pipeline is developed, and the load reduction and energy efficiency of the SPW are studied numerically under different climates and structures. The influence of flow velocity and flow path on the performance of SPW is also investigated. The results show the following: (1) the SPW retains a satisfactory load reduction rate, which is only slightly less than that of the parallel pipe-embedded wall (PPW); (2) the coefficient of performance (COP) of the SPW is 10 times higher than that of the PPW after considering water distribution; (3) the water temperature affects the indoor load more than the sol-air temperature does, and the SPW has load reduction potential compared with the conventional wall when the water temperature is below 30 °C in summer or above 12 °C in winter; (4) increasing the flow path or flow velocity alone will not always result in better energy efficiency for a SPW system. There exists an optimum flow velocity or flow path that will maximize the energy efficiency of a SPW system.

Traditional solar heat pump cannot utilize either solar energy efficiently or thermal energy in ambient air. A hybrid solar–air source heat pump with thermosiphon using a multi-function evaporator was proposed to take the advantages of both air thermal energy and solar energy. The novel heat pump can absorb heat from single heat source (i.e. solar energy or ambient air) or their combination. Moreover, it can work in the thermosiphon mode or the heat pump mode according to the solar intensity. A validated numerical model was established to simulate the performance of the proposed system under different working conditions, and a comparison with conventional heating systems was also made. It shows that the coefficients of performance of the proposed heat pump in the air source heat pump mode, solar-air source heat pump mode and solar heat pump mode are 3.12, 3.45 and 3.89, respectively. Besides, it's able to supply heat without energy consumption by solar thermosiphon mode when the solar energy is sufficient. The primary energy efficiency of the proposed system during a heating season are 1.14, 1.15, 1.54, and 0.97 in Beijing, Chengdu, Lhasa, and Shenyang, respectively. The energy saving rates can reach 48% and 66% compared with air source heat pump and the conventional solar heating system, respectively.

Traditional single source heat pump systems, e.g., air source heat pump (ASHP) and ground source heat pump (GSHP), cannot work efficiently in the whole year. To develop an energy-efficient heat pump system for year-round operation, temperature grades of common natural energies are compared with each other. Based on the comparison, the combination principle of hybrid sources with complementary characteristics is discussed firstly. Then three typical types of heat pumps with hybrid sources are selected as representatives to show how to take full advantages of different heat sources or sinks for energy-efficient operation and how to extend functions of hybrid source heat pumps for new solutions to the disadvantages of conventional heat pumps. Finally, the annual performances of these three selected types of hybrid source heat pump systems for three corresponding specific cases are simulated by TRNSYS. The results show that hybrid source heat pump systems can maintain nearly the same heating capacity as they do without frost during the defrosting period; maintain energy-efficient year-round cooling with no risk of freezing during winter; keep the thermal balance of soil and maintain reliable cooling and heating over long periods of operation; fully utilize solar radiation of different intensities; and achieve year-round energy-efficient performance. In contrast to a conventional ASHP and GSHP system, the energy saving rate of a typical hybrid source heat pump system is approximately 15%, and the payback period is approximately five years. Hybrid source heat pumps provide a way to match multiple heat sinks/sources for different levels of building demand in different climates.