Closed-loop Ground Source Heat Research Articles

Study on design and calculation method of borehole heat exchangers based on seasonal patterns of groundwater

The vertical closed-loop ground source heat pumps (GSHPs) utilize the ground as a heat sink. In areas where the groundwater is abundant with strong seasonal fluctuation, the flow rate and water table control the heat transfer rate of borehole heat exchangers (BHEs) and the thermal balance of the energy storage rock mass (ESRM). Current design focuses on the balance between rates of heat dissipation to and extraction from subsurface during operation seasons, but the energy carried by groundwater in other seasons is not utilized. This study proposes that thermal imbalance should be allowed during GSHP operation, which can be balanced off by appropriately utilizing the heat replenishment by groundwater in other seasons, achieving thermal balance year-round. A new approach to calculate BHE heat transfer rate, energy replenishment by groundwater, and annual energy budget of ESRM based on seasonal groundwater fluctuation was derived. The annual energy default and temperature deviation charts of ESRM were applied to predict whether and when the thermal balance would be restored. A case study of single heating condition showed that the thermal balance may be restored very soon after the heating period or in the flooding season 3–4 months later, depending on the groundwater pattern.

Assessment of installation suitability of a vertical closed loop ground source heat pump system in regional scale

Installation suitability of a vertical closed loop GSHP system in the Echigo Plain of Japan was assessed considering the regional scale groundwater flow and heat transport analysis. For evaluating and understand the variation of development potential of the system, the necessary length of ground heat exchanger (GHE) was analyzed at different points, where groundwater, geology and subsurface temperature distribution varies. The computation was done for the model case of the Japanese single-family dwelling with reference to standard for energy saving in Japan. The necessary length of GHE ranged from 67 m to 101 m within the plain and found comparatively longer at the central coastal area around the Niigata City. These results showing the variation of the necessary length of GHE in the plain scale is significant and essential for the proper design and site selection of the system in the Echigo Plain, and can also be beneficial for the development of GSHP system in Japan. Influence of the existing groundwater pumping system was also evaluated to determine if the pumping system can enhance heat exchange performance for space-heating. The effect of groundwater pumping was not so significant to improve heat exchange rates due to the natural existence of groundwater flow.

Experimental Study to Analyze Feasibility of a Novel Panelized Ground-Source Thermoelectric System for Building Space Heating and Cooling

A thermoelectric module is a device that converts electrical energy into thermal energy through a mechanism known as the Peltier effect. A Peltier device has hot and cold sides/substrates, and heat can be pumped from the cold side to the hot side under a given voltage. By applying it in buildings and attaching it to building envelope components, such as walls, as a heating and cooling device, the heating and cooling requirements can be met by reversing the voltage applied on these two sides/substrates. In this paper, we describe a novel, panelized, ground source, radiant system design for space heating and cooling in buildings by utilizing the Peltier effect. The system is equipped with water pipes that are attached to one side of the panel and connected with a ground loop to exchange heat between the cold/hot sides of the thermoelectric module and the underground region. The ground loop is inserted in boreholes, similar to those used for a vertical closed-loop Ground Source Heat Pump (GSHP) system, which could be more than a hundred meters deep. Experiments were conducted to evaluate the feasibility of the developed panel system applied in buildings. The results show that: (1) the average cooling Coefficients Of Performance (COP) of the system are low (0.6 or less) even though the ground is used as a heat sink, and thus additional studies are needed to improve it in the future, such as to arrange the thermoelectric modules in cascade and/or develop a new thermoelectric material that has a large Seebeck coefficient; and (2) the developed system using the underground region as the heat source has the potential of meeting heating loads of a building while maintaining at a higher system coefficient of performance (up to ~3.0) for space heating, compared to conventional heating devices, such as furnaces or boilers, especially in a region with mild winters and relatively warm ground.

First Italian TRT database and significance of the geological setting evaluation in borehole heat exchanger sizing

The correct sizing of the borehole field in closed-loop Ground Source Heat Pump (GSHP) systems requires accurate value of the ground thermal properties, especially the thermal conductivity. One of two methods are generally applied to evaluate this parameter: the Thermal Response Test (TRT) and databases reported in guidelines or Standards for lithologies identified in the local stratigraphy.This paper presents a database of more than 100 Thermal Response Tests performed in Italy. The equivalent thermal conductivities derived from the TRT outputs are here directly compared with the value estimated through knowledge of the local geological stratigraphy. The obtained results are analyzed in terms of geological setting. A sensitivity analysis on the borefield design has been conducted to evaluate the effects of the equivalent thermal conductivity estimation error on the calculation of the required total borehole length. The borehole length, in turn, strongly affects the initial investment costs as well as the operating conditions of the heat pump over the long term.The obtained results, in some cases significant, highlight the importance of performing the TRT not only when the thermal capacity of the GSHP is high but also in the case of strong geological uncertainty, or in particular geological settings such as high plain areas, alpine valley floors and rocky environments. In the other cases as low plain areas, this database can provide an initial estimate of the range of the expected equivalent thermal conductivity value; therefore, it can be useful for designers of both GSHP systems and other applications where the knowledge of the underground thermal behavior is necessary.

Evaluation of Suitable Areas to Introduce a Closed-Loop Ground Source Heat Pump System in the Case of a Standard Japanese Detached Residence

Evaluation of appropriate areas to introduce a closed-loop ground-source heat pump (GSHP) system in the case of a standard detached residence was conducted in Akita Plain, Japan. Depth of borehole heat exchanger (BHE) required to use a GSHP system was taken as the suitability index, which is simpler for the general public to understand and to promote the system. Heating and cooling loads of the standard size detached residence were calculated based on the 2013 Energy Conservation Standard. To estimate BHE depths and estimate a suitable area, identical 3D BHE models were constructed at 30 points in the plain. Required BHE depths were determined by performing heat exchange simulations at each location using the calculated loads and heat exchange rates. A suitability map showing distribution of the required BHE depths was prepared. BHE depths were shorter in the north-western area compared to the other parts, implying that the coastal lowlands in this area are more suitable for closed-loop system in the standard detached residences in terms of heat exchange performance as well as the cost reduction. Thickly distributed Quaternary System with higher thermal conductivity is contributing to shorter BHE depths. The suitability map is effective to adopt potential areas for the system installation in the standard detached Japanese residences.

Carbon Dioxide Emissions during Air, Ground, or Groundwater Heat Pump Performance in Białystok

The increasing global temperature has induced many states to limit carbon dioxide emissions. The European Union (EU) promotes replacing boilers with heat pumps. However, in countries where electricity is mainly supplied through fossil fuel combustion, condensing gas boilers may prove to be more ecological heat generators. Although this problem was investigated in a particular situation, an algorithm can be applied elsewhere. The running expenditures for the following different heat generators that are available in a location were estimated: water heat pump, brine heat pump, air heat pump, condensing gas boiler, condensing oil boiler, district heat network, and electrical grid. Furthermore, carbon dioxide emissions from local and distant sources were evaluated. The computations were based on hourly averaged external temperature measurements, which were performed by the Institute of Meteorology and Water Management—National Research Institute (IMGW-PIB) in a weather station in Białystok (Poland) for a ten-year period. Compared with a condensing gas boiler system, the air-to-water heat pump has higher operating costs and higher CO2 emissions. The brine heat pump (closed-loop ground-source heat pump) has lower operating costs, but higher CO2 emissions than the gas boiler system. The water heat pump (groundwater source heat pump) has the lowest operating costs and CO2 emissions of all the systems studied in this paper.

Feasibility of closed loop ground source heat pump for residential heating and cooling applications in Serbia

Households sector in Serbia presents a great chance for energy savings and introduction of RES in the future. The public policies in Serbia are currently limited, but this kind of study can influence public measures that would undeniably generate long-term social and economic benefits to the country. The aim of the present work is to assess economic feasibility of closed loop heat pump systems for heating and cooling purposes in Serbia?s residential sector. The heat pump system was compared to the most commonly used heating fuels in households. Results indicate that the implementation of ground closed loop heat pump systems for heating and cooling purposes in Serbia?s residential sector as a substitute for electric heating is economically feasible. Inadequate prices of natural gas and electricity in public supply are the main problems associated with the project?s financial benefits. The best results were obtained in the scenario with combined debt ratio (40%) and grants (~30%) for the project realization, for which equity pay-back period is approximately three years, while benefit to cost ratio is 2.52. Investigated financial metrics (equity pay-back, internal rate of return assets and net present value) indicate the same positive results considering financial viability of the project.

Operational behavior characteristics and energy saving potential of vertical closed loop ground source heat pump system combined with storage tank in an office building

A ground source heat pump (GSHP) system utilizes a relatively stable underground temperature to achieve energy-saving for heating and cooling in buildings. Among various types of GSHP systems, Vertical Closed Loop Ground Source Heat Pump (VGSHP) has received increasing attention due to a variety of advantages such as the potential to be installed in a relatively small space and improved energy efficiency. In this research, in order to improve the system performance of a VGSHP, a VGSHP system coupled with heat storage tank was evaluated. For this purpose, the detailed analysis on the operational behavior of heat storage tank and the variations of heat pump energy performance due to the connection with heat storage tank was performed, and characteristics of heat pump part load ratios (PLRs) and the corresponding energy requirements, compared to chiller and boiler based conventional system, were analyzed in detail, using dynamic energy simulation after comprehensive validation process of heat pump performance curves. The results of this study showed that the VGSHP system coupled with heat storage tank showed an energy saving effect of about 20% for cooling and about 77% for heating compared to the conventional air conditioning system, and an energy saving effect of about 2% for cooling and about 15% for heating compared to the VGSHP system without heat storage tank, mainly due to the fact that the operational pattern of heat pump such as PLR and COP characteristics was changed by the operation of heat storage tank.

3D ground-use optimisation for sustainable urban development planning: A case-study from Earls Court, London, UK

By 2050, almost 70% of the world’s population will live in towns and cities. This places increasing pressure on land to support development whilst minimising environmental impact and providing long-term sustainability. Prior knowledge of the ground at the planning and development stage is needed to assess its suitability to meet planned subsurface uses and avoid subsurface conflicts at an early stage of design.This research tested the development and application of a 3D engineering geological model and its spatial integration with 2D (hydro)geological datasets to support sustainable development decision-making at Earls Court, London, UK. The 3D engineering geological model consists of seven geological layers extending from Made Ground to the top surface of the Chalk Group. The 3D geological model, and 2D datasets derived from it, are combined with 2D geospatial datasets of urban underground space (UUS) indicators to identify potential uses of urban underground space based on its suitability to do so. This is complimented by a qualitative assessment of the potential subsurface interactions between UUS indicator uses and their implications for delivering the sustainability and energy objectives specified in the Earls Court masterplan.Infiltration sustainable drainage systems (SuDS), ground source heat potential and foundation condition potential were chosen as suitable UUS indictors against which to test the outputs of the methodology. Infiltration SuDS potential was assessed using nationally-available (hydro)geological 2D GIS datasets. Ground source heat potential was assessed using national-available 2D datasets and estimates of thermal properties of each geological layer revealed by the 3D geological model. Potentially suitable foundation conditions were assessed using density data from existing ground investigations and depth of suitable geological layers derived from the 3D geological model.The results reveal constraints for the development of rapid infiltration SuDS but opportunities for bespoke design that considers the thickness of overlying permeable sand and gravel. It identifies the susceptibility of the London Clay Formation to potential volume change and ground movement based on an assessment of its plasticity. Closed-loop ground source heat pump opportunities exist depending on site-specific thermal and hydraulic properties and heat exchange design. Opportunities for the use of the Kempton Park Gravel Member for ground source heating and cooling and combined use of thermal regulation and pile design via thermopiles are identified. A qualitative assessment of potential benefits and conflicts between UUS indicator uses reveals that the Kempton Park Gravel Member and London Clay Formation are likely to have the highest, relative, UUS value.The results demonstrate that there is potential to modify the energy and sustainability components of the Earls Court masterplan prior to invasive ground investigation and development. It is further suggested that this approach can be used to compliment research-tested, semi-qualitative means of valuing UUS indicators. The implications of the methodology for mainstreaming UUS into city masterplans is also assessed. It is concluded that opportunities now exist for the integrated 3D and 2D spatial assessment of UUS indicators into city-scale masterplans.

Ground-source heat pump systems: The effect of variable pipe separation in ground heat exchangers

Closed loop ground-source heat pump (GSHP) systems use ground heat exchangers (GHEs) to transfer heat to and from the ground and efficiently provide clean and renewable energy for heating and cooling purposes. Vertical GHEs contain pipes with circulating fluid (loops), which transfer thermal energy between the ground and the fluid. One very common assumption made in designing GSHP systems is that, when installed, these loops remain evenly separated along the length of the GHE, something that due to the nature of construction is rarely true. This can result in thermal interference not accounted for in the design, leading to a potential negative impact on the performance of the system. This paper investigates the effect of this interference, using detailed numerical simulations to compare different geometries, modelling fixed and variable pipe separations. A comprehensive parametric analysis is conducted to identify some of the most influential design parameters and the potential consequences on running and capital costs. Amongst the key findings of this study is the importance of the borehole filling material, as a highly thermally conductive material can minimise these negative effects from the thermal interference by up to 60%. Moreover, potential increases in drilling (capital) costs of up to 24% are shown, while the potential increases in running costs due to the reduced efficiency were found to be relatively minor.

Freeze damage of grouting materials for borehole heat exchanger: Experimental and analytical evaluations

The closed-loop ground source heat pump (GSHP) systems can be operated below the freezing point of water with anti-freeze mixture through the installed borehole heat exchangers (BHE) in the ground for the heating purposes of buildings. During the operations, the BHE is exposed by the thermal stresses due to heat loading. In addition if the porous grout material is immerged in water, the freezing-induced ice pressure may damage the grout. In this paper, the freezing impact on BHEs is investigated analytically and experimentally. For the theoretical approach, an analytical solution is developed by using the hollow cylinder model that accounts for both the high density polyethylene (HDPE) pipe and the grout material. Firstly, the frozen pore water pressure is incorporated in the generalized Hooke’s law equations in 2D plane stress, and secondly the model is solved for the considered boundary conditions. In order to validate the developed model, the experimental setup is conducted in agreement with the geometry of the considered analytical model and the BHE specimens are prepared with three different grout materials having large difference in the thermal and hydraulic characteristics (i.e. silica-sand based, calcite based and homemade thermally enhanced with graphite). According to the experiments for 50 h of freezing operation, the calcite based grout and the homemade grout, having lower permeability and relatively higher porosity, are fractured. In contrast, the silica-sand based grout having higher permeability did not exhibit any damage. Compared with the theoretical results, the observations from the experiments are consistent. The effective tangential stress induced by the frozen pore water pressure causes the crack development and agrees with the crack patterns. It is concluded that the porosity and the permeability play significant role on the grout failure upon freezing.

Natural convection in groundwater-filled boreholes used as ground heat exchangers

In most of the world, borehole heat exchangers used with closed-loop ground source heat pump systems are backfilled with a low permeability grout to prevent water contamination. However, in Scandinavian countries, a different approach is taken – the borehole is sealed at the top and cased down to solid bedrock. The borehole then naturally fills with groundwater in the annular space between the U-tube and the borehole wall. Compared to grouted boreholes, the groundwater filling is advantageous in that it generally results in low borehole thermal resistance due to buoyancy-driven natural convection enhancing the heat transfer. Although this phenomena has been reported in several papers since the late 1980s, no calculation models have been available for use in either design tools or simulation programs. This paper presents experimental measurements from a single well-instrumented borehole under a range of heat transfer rates and annulus temperatures. Nusselt numbers for natural convection in the annulus are correlated against modified Rayleigh number. The results are verified by comparing to borehole thermal resistances predicted with the correlations to actual measurements from a range of boreholes in Sweden and Norway.

Influence of closed loop ground source heat pumps on groundwater: a case study

Influence of closed loop ground source heat pumps on groundwater: a case study

Freeze-Thaw-Cycles on Borehole Heat Exchanger Grouts: Impact on the Hydraulic Properties

Abstract In this paper a testing device is described that measures the hydraulic conductivity of grout specimens for Borehole Heat Exchangers (BHE). During the operation of closed-loop ground source heat pumps running with antifreeze, freezing of the backfill can occur due to extensive heat extraction. This laboratory device can assess the influence of frost on the hydraulic seals of BHEs. The device is based on a triaxial flexible wall permeameter. A freely selectable number of cyclic freeze-thaw-stresses as well as a confining pressure simulating radial earth pressure (σ2 = σ3) can be applied. Specimens are composed of an annular grout body and a polyethylene pipe simulating the BHE system. The freezing direction is perpendicular to the vertical axis of the BHE from the inside to the outside. Numerical coupled modeling was applied to verify the results of the temperature distribution inside the specimens. It was observed and modeled that the propagation of the frost front and the fabric disintegration processes are correlated. Results of three different grouting materials will be presented. With its relative small dimensions the device can be easily implemented into soil mechanical laboratories and thus can contribute to quality control of grouts.

물 대 물 방식 수직 밀폐루프 지열원 히트펌프 시스템의 냉방성능에 대한 실험적 연구

A vertical closed loop ground source heat pump (GSHP) is used to produce heat from the low-grade energy source such as the outside air and ground source. It is known that a heat pump system type has better efficiency comparing to the electric heating system. This study only demonstrates that the vertical closed loop GSHP system is a feasible choice for space cooling of air conditioning. The coefficient of performance (COP) is the ratio of heat output to work supplied to the system in the form of electricity. For the vertical closed loop GSHP system in a cooling mode, the COP is the most commonly used way for judging the efficiency. For the purpose of this experiment, vertical closed loop GSHP system was installed in the laboratory and the experiment was executed. As a result, an average COP of vertical-closed loop GSHP system was 3.62 when the outside average temperature was .

Energetic and economic aspects of a heating and cooling district in a mild climate based on closed loop ground source heat pump

Over the next few years, thermal energy for heating residential buildings will be reduced, while cooling energy will be increasingly used. Therefore reversible heat pumps producing heating and cooling energy efficiently may become more popular in areas with mild climate. The Ground Source Heat Pump (GSHP) system is one of the most promising techniques which can be coupled with other solutions based on renewable energy.The present work considers a heating and cooling district based on a closed loop GSHP and its energetic and economic aspects, including the density of population in the district where the buildings are located. Economic analysis and payback times are investigated as a function of inhabitant density, adopted solutions, as well as different economical benefit scenarios.Primary energy consumption may be reduced in a range of 50–80%, compared to traditional systems. The district heating/cooling is more suitable in a medium density area and splitting a district into sub-district distribution systems may reduce the initial financial risk.Possible mechanisms for the diffusion of GSHP are here shown for district heating/cooling systems. Policy makers and stakeholders may use the results of this work for promoting possible incentives, which can be similar to the ones presented here.

주거용 직접순환식 수직밀폐형 지열원 열펌프의 연간 운전성능 평가

Vertical closed-loop ground source heat pump systems(GSHP) have been installed widely in Korea since it can extract moderate temperature level of geothermal heat in a small area. As a ground heat exchanger, a vertical closed -loop type with brine circulation is mostly preferred since it is simple and less harmful to ground environment. However, it requires a secondary heat exchange loop between the refrigerant in a heat pump and the brine. By adding a geothermal heat exchanger in the secondary heat exchange loop, circulation pumps should be attached and the temperature difference between refrigerant and ground is increased, which are important parts of performance degradation. In this paper, annual and seasonal performances of direct expansion(DX) geothermal heat pump were estimated mathematically as an alternative of classical indirect geothermal heat pump based on the annual performance evaluation. As results, DX geothermal heat pump showed 43% higher annual performance than the classical U-tube geothermal heat pump.

Initial geological considerations before installing ground source heat pump systems

Abstract The performance of an open- or closed-loop ground source heat pump system depends on local geological conditions. It is important that these are determined as accurately as possible when designing a system, to maximize efficiency and minimize installation costs. Factors that need to be considered are surface temperature, subsurface temperatures down to 100–200 m, thermal conductivities and diffusivities of the soil and rock layers, groundwater levels and flows, and aquifer properties. In addition, rock strength is a critical factor in determining the excavation or drilling method required at a site and the associated costs. The key to determining all of these factors is an accurate conceptual site-scale model of the ground conditions (soils, geology, thermogeology, engineering geology and hydrogeology). The British Geological Survey has used the modern digital geological mapping of the UK as a base onto which appropriate attributes can be assigned. As a result it is possible to generate regional maps of surface and subsurface temperatures, rock strength and depth to water. This information can be used by designers, planners and installers of ground source heat pump systems. The use of appropriate geological factors will assist in creating a system that meets the heating or cooling load of the building without unnecessary overengineering.

Modeling Approach to Design of a Ground-Source Heat Pump Bridge Deck Heating System

The approach taken to design a hydronic snow-melting system for a bridge deck on an Interstate highway in Oklahoma is described. A vertical borehole, closed-loop ground-source heat pump system is to be used as an energy-efficient means to provide the heating requirement. It is proposed to use the bridge deck as a solar collector in the summer months so that the ground can be thermally “recharged” by circulating fluid from the bridge deck to the ground. A design of this type involves the determination of a number of interdependent variables that are difficult or impossible to accurately quantify by conventional design practices. Therefore, mathematical models of a hydronically heated pavement slab, water-to-water heat pump, and a ground loop heat exchanger were developed for use in a detailed system simulation. The simulation allowed design parameters such as minimum heat pump entering fluid temperature, number of heat pumps, and flow rate to be varied, and several potential designs that kept the bridge surface temperature above freezing were selected. The hourly heating loads for candidate configurations were then used to estimate the required size of the ground loop heat exchanger. There is a trade-off between the number of boreholes that make up the ground loop heat exchanger and the number of heat pumps required. Of the configurations examined, a system with 16 heat pumps of nominal 106-kW capacity and 250 boreholes, each 76 m deep, was selected.

A Model for Simulating the Performance of a Pavement Heating System as a Supplemental Heat Rejecter With Closed-Loop Ground-Source Heat Pump Systems

The thermal loads of commercial and institutional buildings are generally cooling-dominated. When such buildings use ground source heat pump systems (GSHP) they reject more heat to the ground-loop heat exchanger than they extract over the annual cycle. In these situations, supplemental heat rejecters can be used to reduce the required size of the ground-loop heat exchanger, thereby reducing the first cost of the system. This paper describes the development, validation, and use of a design and simulation tool for modeling the performance of a hydronic pavement heating system as a supplemental heat rejecter in ground-source heat pump systems. The model uses a finite difference method to solve the transient two-dimensional heat conduction equation and has been formulated for use in component based system simulation. Full-scale experiments were conducted concurrently with the development of the model, the results of which have been used for validation purposes. An example application simulation is presented to demonstrate the use of the model as well as the viability of the use of pavement heating systems as supplemental heat rejecters in GSHP systems. [S0199-6231(00)00404-4]