Research On The Vertical Earth-to-air Heat Exchanger Systems Intergrated With Latent Energy Storage Components

Building energy consumption has become one of the three major energy users in the world.To reduce building energy consumption,geothermal energy,as one of the most common renewable energy,has been recommended as an energy-saving and environment-friendly technology.It can be used as a heat sink/source in summer/winter as the soil temperature generally remains fairly stable throughout the year below a certain depth.Earth-to-air heat exchanger?EAHE?system,as one of clean and efficient shallow geothermal energy technology,has been reported with an demonstrable effect in reducing building energy consumption.However,its disadvantages mainly include the larger land use,higher shallow soil temperature,difficulty centralized discharge of condensate water in time and larger outlet air temperature fluctuation.Thus,the EAHE system is difficult to be widely used in buildings for its practical application.Based on this,a vertical earth-to-air heat exchanger?VEAHE?system and its coupling system with different phase change energy storage units are proposed in this paper.The main contents of this paper are as follows:A VEAHE system was proposed in this study.Compared to the conventional EAHE system,it main advantage lies in the smaller land use,higher geothermal energy utilization efficiency and ease of condensate water discharge.To investigate the system's thermal feasibility,an experimental setup was established in Changsha,Hunan Province,and a series of tests were conducted in both summer and winter seasons under the system operation of 96 hours.To validate the system's economic assessment,the energy payback time,CO₂ emission mitigation potential and total carbon credits of the proposed system were calculated.Results showed that the outlet air temperature of this system ranged from 22.4? to 24.4? in summer and from 16.0? to 18.0? in winter.For the proposed syste m,as the air velocity was 1m/s,its energy payback time was calculated as 8.2 years.Meanwhile,under a given economic lifespan of 20 years,the system's CO₂,NO_X,SO₂ and PM 2.5 emission mitigation potential were calculated as 7170.57 kg,20.20 kg,5.60 kg and 1.00 kg,respectively.And the total carbon credits were calculated as$203.43.To analyze the effects of different parameters on the thermal performance of the VEAHE system,a numerical model of the VEAHE system was developed.Compared to conventional EAHE system models,the developed model considered the vertical distribution of soil temperature and thermal conductivity.The model validation against experimental data showed a good agreement,and the maximum relative deviations between them were 3.03%and 2.71%for summer and winter condition,respectively.A parametric study was then conducted to investigate the effects of tube parameters,insulation parameters,mass flow rates and soil types.Results indicate d that a system with a smaller tube diameter provides more thermal capacity with a fixed air mass flow rate,while a larger tube diameter can enhance the system's COP.An increase in tube depths causes the outlet air temperatures to decrease/increase in summer/winter,with a smaller daily oscillation.Compared to PVC and PE,stainless steel can be recommended as the most appropriate tube material,taking into account both heat exchange and soil pressure.Polyurethane,rubber or rock wool as insulation materials can be used as thermal barriers to shield the undesirable heat gains from the shallow soil with the almost same insulation effects.A thickness of30 mm and a length of from 4 m to 5 m can be recommended as the most appropriate insulation parameters for this proposed system.To provide more stable outlet air temperature and improve comfort conditions when these systems serve as ventilation systems in buildings.Two different VEAHE systems based on annular and tubular PCM components were proposed in this paper.In order to evaluate the effects of annular and tubular PCM components and their corresponding parameters on the thermal performance of VEAHE system,the numerical models of the annular and tubular PCM coupled with VEAHE systems were developed,respectively.The developped models were validated by the corresponding experimental set-up of the coupled systems.The validated results showed a good agreement between the simulated and monitored data,and the maximum absolute relative errors for the annular and tubular PCM systems are 1.59%and 1.41%,respectively.Then,the effects of annular and tubular PCM,and their corresponding parameters?including the PCM thermal conductivities,lengths and locations?on the thermal performance of VEAHE system were future studied.The results showed that the both annular and tubular PCM components could effectively reduce the outlet air temperature fluctuation and the corresponding peak temperature.For the proposed systems,the PCM thermal conductivities were certified with little effect on the system's thermal performance.And the recommended PCM lengths for the VEAHE systems integrated with annular and tubular PCM we re 7 m and 12 m,respectively.In addition,the recommended PCM locations for the VEAHE system integrated with annular PCM was located at the outlet,while it was located at the right-leg of U-tube for the tubular PCM system.Compared to the conventional EAHE system,the proposed VEAHE systems and its coupling systems with phase change energy storage units have many advantages,which can further enhance the practical application potential of geothermal energy in buildings.These systems applied in buildings can effectively reduce buildings'energy consumption,thereby alleviating the energy shortage and reducing the environmental pollution.In addition,the research of system parameters can provide the guidelines for the further optimization and practical application of these systems,so as to maximize reducing the cost of system construction,and increase the possibility of further demonstrative popularization and application of these systems.