Are shallow boreholes a suitable option for inter-seasonal ground heat storage for the small housing sector? (original) (raw)
Related papers
ANALYSES OF GROUND-SOURCE HEAT PUMPS COMBINED WITH SOLAR COLLECTORS IN DWELLINGS
In order to analyze different systems with combinations of solar collectors and ground source heat pumps, computer simulations have been carried out with the simulation program TRNSYS. The advantage of using solar heat was studied and compared for different systems with varied depths of the borehole for a single family dwelling in Sweden.
OPERATIONAL RESPONSE OF A SOIL-BOREHOLE THERMAL ENERGY STORAGE SYSTEM
This study focuses on an evaluation of the subsurface temperature distribution during operation of a Soil-Borehole Thermal Energy Storage (SBTES) system. The system consists of an array of five 9 m-deep geothermal heat exchangers, configured as a central heat exchanger surrounded by four other heat exchangers at a radial spacing of 2.5 m. In addition to monitoring the temperature of the fluid entering and exiting each heat exchanger, 5 thermistor strings are embedded in boreholes inside and outside of the array to monitor changes in ground temperature with depth. After 75 days of heat injection at a constant rate of 25 W/m, the heat loss from the system was monitored over a 4-month rest period. Although the heat injection rate is smaller than that expected in an actual SBTES systems (35-50 W/m), the average ground temperature increased by 7 °C at the end of heating. However, the average ground temperature was only 3 °C greater than the initial ground temperature at the end of the rest period due to lateral heat loss. The trends in subsurface temperatures during heat injection were consistent with the results from a simplified heat injection simulation, although they indicate that the thermal properties of the soil may be changing with time. An energy balance analysis indicates the number of boreholes in the array was too few to effectively concentrate the heat injected during the test within the array. Nonetheless, the results provide an experimental reference point between a single borehole and a larger SBTES system.
Energy, 2020
The multi-family residential building sector is the least energy efficient in the United States, thus allowing for ample opportunities for significant cost-effective energy and carbon savings. In the present study, we propose a district solar borehole thermal solar energy storage (BTES) system for both retrofit and new construction for a multi-family residence in the Midwestern United States, where the climate is moderately cold with very warm summers. Actual apartment interval power and water demand data was mined and used to estimate unit level hourly space and water heating demands, which was subsequently used to design a cost-optimal BTES system. Using a dynamic simulation model to predict the system performance over a 25-year period, a parametric study was conducted that varied the sizes of the BTES system and the solar collector array. A life-cycle cost analysis concluded that is it possible for an optimally-sized system to achieve an internal rate of return (IRR) of 11%, while reducing apartment-wide energy and carbon consumption by 46%. Both a stand-alone and solar-assisted ground-source heat pump system were designed and simulated for comparison to the BTES system, and found to be less economically favorable than the solar BTES system. Thus, the promise for district-scale adoption of BTES in multi-family residences is established, particularly for new buildings.
Ground-source heat pumps and underground thermal energy storage: energy for the future
NGU Special …, 2008
We need energy for space heating-but in most cases not where or when energy sources are available. Energy storage, which helps match energy supply and demand, has been practised for centuries, also in Norway. Energy storage systems will increase the potential of utilising renewable energy sources such as geothermal energy, solar heat and waste heat. The most frequently-used storage technology for heat and 'coolth' is Underground Thermal Energy Storage (UTES). The ground has proved to be an ideal medium for storing heat and cold in large quantities and over several seasons or years. UTES systems in the Nordic countries are mostly used in combination with Ground-Source Heat Pumps (GSHP). Several different UTES systems have been developed and tested. Two types of system, Aquifer (ATES) and Borehole (BTES) storage, have had a general commercial breakthrough in the last decades in the Nordic countries. Today, about 15,000 GSHP systems exist in Norway extracting about 1.5 TWh heat from the ground. About 280 of the Norwegian GSHP installations are medium-to large-scale systems (> 50 kW) for commercial/ public buildings and for multi-family dwellings. The two largest closed-loop GSHP systems in Europe, using boreholes as ground heat exchangers, are located in Norway.
Design of a seasonal thermal energy storage in the ground
Solar Energy, 1997
A~trac~Longterm storage of high quantities of thermal energy is one of the key problems for a widespread and successful implementation of solar district heating and for more efficient use of conventional energy sources. Seasonal storage in the ground in the temperature range of up to 90°C seems to be favourable from a technical and economical point of view. Preferably duct systems with vertical heat exchangers can be built in areas without ground water or low flow velocity compared with the geometry of the store and the storage period. The thermal performance of such systems is influenced by the heat and moisture movement in the area surrounding the heat exchangers. Thermal conductivity and heat capacity are strongly dependent on the water content. This combined heat and moisture transport was simulated on the computer for temperatures up to 90°C. This model calculates the effective heat transfer coefficient and the heat capacity of the soil depending on water content, mineral composition, dry bulk density and shape of soil components. The computer simulation was validated by a number of laboratory and field experiments. Based on this theoretical work a pilot plant was designed for seasonal storage of industrial waste heat. A heat and power cogeneration unit (174 kWth) delivers waste heat during summer to the ground storage of about 15 000 m 3 with 140 vertical heat exchangers of 30 m depth. About 418 MWh/a will be charged into the ground at a temperature level of 80c'C, about 266 MWh/a should be extracted at temperatures between 40°C and 70°C and delivered directly to the space heating system. With this design an economic calculation gave energy prices of 39 US$/MWh which is of the same order as conventional energy prices.
Renewable Energy, 2018
A hybrid installation that includes solar collectors and a ground source heat pump was developed and tested. There are many studies in the field of combined heat pump systems describing relatively large installations, designed for climatic conditions and soil thermal properties different from those in Bulgaria, where the experimental data are limited. The paper presents the construction of a small size hybrid installation containing diurnal and seasonal storages and supporting five different modes of operation with emphasis on the charging of borehole heat exchanger (BHE), heating mode with ground source heat pump (GSHP) and the followed natural relaxation. The paper also proposes a methodology for determination of different system energy efficiencies. High quality data for the different system operation modes in terms of soil and weather conditions typical of the Plovdiv region were obtained. The study proves the necessity of BHE charging with thermal energy from the sun during the summer mainly to avoid the ground thermal depletion. The comparison of the three heating modes investigated shows evident advantage of the ground source heat pump heating (GSHPH). The installation must be tested in the future for a longer time period.
Numerical Modeling of a Soil-Borehole Thermal Energy Storage System
Vadose Zone Journal, 2016
Borehole thermal energy storage (BTES) in soils combined with solar thermal energy harvesting is a renewable energy system for the heating of buildings. The first community-scale BTES system in North America was installed in 2007 at the Drake Landing Solar Community (DLSC) in Okotoks, AB, Canada, and has since supplied >90% of the thermal energy for heating 52 homes. A challenge facing BTES system technology is the relatively low efficiency of heat extraction. To better understand the fluid flow and heat transport processes in soils and to improve BTES efficiency of heat extraction for future applications, a three-dimensional transient coupled fluid flow and heat transfer model was established using TOUGH2. Measured timedependent injection temperatures and fluid circulation rates at DLSC were used as model inputs. The simulations were calibrated using measured soil temperature time series. The simulated and measured temperatures agreed well with a subsurface having an intrinsic permeability of 1.5 ´ 10 −14 m 2 , thermal conductivity of 2.0 W m −1 °C −1 , and a volumetric heat capacity of 2.3 MJ m −3 °C −1. The calibrated model served as the basis for a sensitivity analysis of soil thermal and hydrological parameters on BTES system heat extraction efficiency. Sensitivity analysis results suggest that: (i) BTES heat extraction efficiency increases with decreasing soil thermal conductivity; (ii) BTES efficiency decreases with background groundwater flow; (iii) BTES heat extraction efficiency decreases with convective heat losses associated with high soil permeability values; and (iv) unsaturated soils show higher overall heat extraction efficiency due to convection onset at higher intrinsic permeability values.
Building Simulation Conference Proceedings
Geothermal heat pumps can contribute to the transition to 100% renewable energy. Thermal depletion of the soil can be a constraint to this application for large heatingdominated buildings. A large solar assisted ground source heat pump system (SAGSHPS) is analysed for an apartment building located in Belgium. For this specific case a simulation setup of the system (SAGSHPS) is developed in the software environment TRNSYS. All the components of this system have been dimensioned by hand and then tested in the simulation setup in order to validate the sizing. No sustainable large heating system can be designed without regeneration of the soil. Geothermal storage with a volume 30% above the calculated volume still encounters thermal depletion problems. Solar collectors to inject solar heat into the ground is a proven technology to reduce depletion of the soil. A SAGSHPS containing 390 boreholes of 107 m deep combined with 100 solar collectors with an absorber area of 303m² is the sustainable setup calculated at first in this research. The last system SAGSHPS resulted with 48 boreholes of 107m deep combined with 450m² solar collectors. Furthermore, this research encloses few design guidelines. This leads to a conclusion that a SAGSHPS can be a sustainable heating system for large buildings in specific cases.
Seasonal High Temperature Heat Storage with Medium Deep Borehole Heat Exchangers
Energy Procedia, 2015
Heating of buildings requires more than 25 % of the total end energy consumption in Germany. By storing excess heat from solar panels or thermal power stations of more than 110 °C in summer, a medium deep borehole thermal energy storage (MD-BTES) can be operated on temperature levels above 45 °C. Storage depths of 500 m to 1,500 m below surface avoid conflicts with groundwater use. Groundwater flow is decreasing with depth, making conduction the dominant heat transport process. Feasibility and design criteria of a coupled geothermal-solarthermal case study in crystalline bedrock for an office building are presented and discussed.