Performance analysis and working fluid selection for geothermal energy-powered organic Rankine-vapor compression air conditioning

AbstractBackgroundTo utilize geothermal energy from hot springs, an organic Rankine cycle/vapor compression cycle (ORC/VCC) system was employed for air conditioning and a thermodynamic model was developed.MethodsSix working fluids, R123, R134a, R245fa, R600a, R600 and R290, were selected and compared in order to identify suitable working fluids which may yield high system efficiencies.ResultsThe calculated results show that because of high system pressure for R290 and R134a, R600a is the more suitable working fluid for ORC in terms of expander size parameter, system efficiency and system pressure. In addition, R600a is also the most appropriate working fluid for VCC in terms of pressure ratio and coefficient of performance. R600 and R600a are more suitable working fluids for ORC/VCC in terms of overall coefficient of performance, refrigerating capacity per unit mass flow rate and chilled water yield from per ton hot water.ConclusionsIn sum, R600a is the most suitable working fluid for ORC/VCC through comprehensive comparison of ORC efficiency, expander size parameter, pressure ratio, coefficient of performance, system pressure and chilled water yield from per ton hot water for six different working fluids. However, the flammability of R600a should attract enough attention

Performance of Hybrid Single Well Enhanced Geothermal System and Solar Energy for Buildings Heating

The energy reserves in hot dry rock and hydrothermal systems are abundant in China, however, the developed resources are far below the potential estimates due to immature technology of enhanced geothermal system (EGS) and scattered resources of hydrothermal systems. To circumvent these problems and reduce the thermal resistance of rocks, here a shallow depth enhanced geothermal system (SDEGS) is proposed, which can be implemented by fracturing the hydrothermal system. We find that, the service life for SDEGS is 14 years with heat output of 4521.1 kW. To extend service life, the hybrid SDEGS and solar energy heating system is proposed with 10,000 m(2) solar collectors installed to store heat into geothermal reservoir. The service life of the hybrid heating system is 35 years with geothermal heat output of 4653.78 kW. The novelty of the present work is that the hybrid heating system can solve the unstable and discontinuous problems of solar energy without building additional back-up sources or seasonal storage equipment, and the geothermal thermal output can be adjusted easily to meet the demand of building thermal loads varying with outside temperature

A novel method of acquiring geothermal energy from unconsolidated sandstone reservoir by multi-directional wells deep borehole heat exchanger

The clogging problem during injection seriously limits the development and utilization of geothermal resources in unconsolidated sandstone area. Deep borehole heat exchanger can circumvent the problem of clogging, and however, high drilling and completion costs restrict its application. To reduce the drilling and completion costs and shorten the payback period, here a multi-directional wells deep borehole heat exchanger (MDWDBHE) system is proposed by sharing the vertical well and drilling directional wells at the lower part of vertical well with high extracted thermal output per meter well depth. The research result shows that for vertical well without directional well, the dynamic payback period is 18.55 years, while for MDWDBHE having 3 and 4 directional wells with the deflection point depth of 2000 m, the dynamic payback period is respectively 7.70 and 7.51 years, indicating that MDWDBHE can be popularized on a large scale just like the shallow ground source heat pump

Simulation analysis of a single well geothermal system with open-loop structure

The relatively small heat exchange power limits the promotion and application of deep borehole heat exchanger (DBHE) with the closed loop due to the main heat transfer pattern of heat conduction through rocks with poor thermal conductivity coefficient. Furthermore, geothermal water is prohibited to extract in some places even with 100% reinjection. To solve the above problems, here a novel single-well enhanced geothermal system (SEGS) is studied with an open-loop structure having the convection heat exchange process. The heat transfer performance, mechanism and main influential factors of SEGS are studied. Result shows that the heat transfer performance of SEGS is greatly improved compared to DBHE, and the average heat transfer power of SEGS reaches 1603.6 kW and 1204.6 kW respectively with different operation modes. In addition, the study also finds that the convective heat transfer process inside the geothermal reservoir is simultaneously affected by the driving effect of the fluid in the screen and the buoyancy effect. Further, the above two effects act in the same direction when the annulus is used as the inlet, while they act in the opposite direction when the inner tube is used as the inlet

A novel enhanced deep borehole heat exchanger for building heating

Deep borehole heat exchanger (DBHE) technology can be used in various regions due to without dependence on the hot water reservoir. However, its performance can hardly be improved due to poor thermal conductivity of rocks and cement. Here a novel enhanced deep borehole heat exchanger (EDBHE) is first proposed, which applies composite filler, cement and well tube. To depress thermal resistance, graphene is used to mix with ordinary mud and cement respectively to prepare composite filler and composite cement. Especially composite filler is controlled deliberately to flow into leakage formation by adjusting its density, viscosity and back pressure in order to improve the thermal conductivity of surrounding rocks. We find that the extracted thermal output from EDBHE is 1002.34 kW, which is 2.36 times that of DBHE (424.45 kW). EDBHE can be used in low water output zone and difficult recharge zone even dry hole zone with leakage formation, which will increase geothermal heating area substantially

Thermo-economic investigation of binary flashing cycle for enhanced geothermal system

In this paper, as a promising technology, the binary flashing cycle (BFC) is proposed for enhanced geothermal systems (EGS) exploitation. The detailed thermo-economic model is developed. With the thermal efficiency, net power output, total investment cost and levelized energy cost (LEC) as evaluation criteria, the flowsheet modeling and thermo-economic analysis are conducted. It is revealed that larger geothermal well depth and geothermal brine flow rate are in favor for improving system thermal performance. However, it is not always beneficial for the economic performance. There exist the optimal depth (3300 m) and geothermal brine flow rate (57 kg/s), at which the LEC obtains the minimum (0.329 USD/kWh) and (0.328 USD/kWh), respectively. For an economic EGS operation, the geothermal brine flow rate should be no less than 50 kg/s. The influences of operating parameters, including generation temperature, dryness degree at the vapor generator outlet and flashing temperature, on the evaluation criteria are also discussed. To achieve better economic benefits, the generation temperature should be less than 110 degrees C and the dryness should be larger than 0.2. With the flashing temperature 72-82 degrees C, the EGS-BFC yields excellent thermodynamic and economic performance. The present investigation will be helpful for the geothermal well construction, resource mining and generation system operating parameters design

heatandmasstransferofammoniawaterinfallingfilmevaporator

To investigate the performance of heat and mass transfer of ammonia-water during the process of falling film evaporation in vertical tube evaporator, a mathematical model of evaporation process was presented, the solution of which that needed a coordinate transformation was based on stream function. The computational results from the mathematical model were validated with experimental data. Subsequently, a series of parameters, such as velocity, film thickness and concentration, etc., were obtained from the mathematical model. Calculated results show that the average velocity and the film thickness change dramatically at the entrance region when x < 100 mm, while they vary slightly with the tube length in the fully developed region when x > 100 mm. The average concentration of the solution reduces along the tube length because of evaporation, but the reducing tendency becomes slow. It can be concluded that there is an optimal relationship between the tube length and the electricity generated. The reason for the bigger concentration gradient in the y direction is that the smooth tube is chosen in the calculation. It is suggested that the roll-worked enhanced tube or other enhanced tube can reduce the concentration gradient in the film thickness direction and enhance the heat and mass transfer rate

Performance analysis of shallow depth hydrothermal enhanced geothermal system for electricity generation

There is a large mismatch between potential estimates and developed geothermal resources in China due to high cost and risk for enhanced geothermal system (EGS) and scattered resources and low volume flow rate for hydrothermal system. Here we propose a shallow depth enhanced geothermal system (SDEGS), which can be implemented by fracturing the shallow depth hydrothermal system to create rich fractures in order to enhance permeability and volume flow rate. The mathematical models describing the continuity, momentum and energy equations in the artificial reservoir and surrounding rock, as well as the thermoelectric conversion equations of Organic Rankine cycle (ORC) are developed. From the simulation results we find that the artificial reservoir of SDEGS can provide stable heat output and outlet water temperature of respectively 59499.97 kW and 165 degrees Cfor almost 23 years. For ORC for generating electricity, the net work output and thermal efficiency are respectively 8053.6 kW and 13.54 %. SDEGS can reduce the hydraulic fracturing risk and cost due to existing rich natural fractures in hydrothermal system, thus broadening the application ranges of EGS

Performance of geothermal single well for intermittent heating

Single well geothermal heating (SWGH) can widely be used all around the world, no matter if there are rich hot water reservoirs. However, its promotion is limited due to long payback period. To reduce the payback period, the intermittent operation mode is proposed in terms of the commercial buildings. An experimental test for continuous heating is firstly carried out, and the extracted thermal output is 448.49 kW with the static payback period of 7.17 years. A mathematical model is then developed in order to simulate the performance of intermittent operation. The simulated results show that the extracted thermal output for intermittent operation is always greater than that for continuous operation due to having the heat recovery period. The extracted thermal output is respectively 619.12 and 587.51 kW in the first and the tenth heating season at the fixed injection temperature and velocity. The extracted thermal output can keep stable if changing the injection temperature and velocity, and thus the imbalance of the extracted thermal output among different heating seasons can be adjusted. The static payback time for intermittent operation is reduced to 5.16 years compared to continuous operation, which will speed up the spreading and application of SWGH technology. (C) 2019 Elsevier Ltd. All rights reserved