| 1. |
- Gehlin, Signhild, et al.
(författare)
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Backgroudn promoting TES systems in Sweden
- 2006
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Swedish nuclear plants are slowly being shut down as a result of a referendum in 1980. During the years after the referendum, four nuclear reactors were completed and taken into operation. The nuclear power then covered 50 % of the Swedish production capacity resulting in large over capacity, which meant inexpensive electricity. For many years thereafter direct electrical space heating was the only feasible option. This development continued until direct electrical heating was used for about 30 % of all space heating in Sweden. Swedish consumer prices were then about one third of that in most European countries. The much later deregulation of the electrical market and connection of the power grid to other European countries meant that Swedish consumer prices increased rapidly. Within a few years the prices are expected to be at the same level all over Europe and direct space heating is no longer a feasible alternative. This is why it is so attractive to Swedes to use heat pumps to reduce their electricity consumption. The present Swedish energy policy aims to a changeover from the use of prime energy sources to renewables. This is partly because of international agreements on the reduction of greenhouse emissions and partly to obtain a more independent energy system. Since the use of heat pumps mean that the electrical consumption is reduced by 70 % and also means less emission to the atmosphere, heat pump systems have been encouraged by subsidies. This present subsidy, which covers about 15 % of the investment, has been on and off during the last decades. Now, the new EU directive on energy efficient buildings offers a platform for the promotion of thermal energy
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| 2. |
- Gehlin, Signhild, et al.
(författare)
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Comparison of four models for thermal response test evaluation
- 2003
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Ingår i: ASHRAE Transactions. - 0001-2505. ; 109, s. 135-146
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Tidskriftsartikel (refereegranskat)abstract
- Four two-variable parameter estimation models for evaluation of thermal response test data are compared when applied on the same temperature response data. Two models are based on line-source theory, the third model is a cylinder-source-based solution, and the fourth is a numerical one-dimensional finite difference model. The data sets contain measured temperature response, heat load, and undisturbed ground temperature from three thermal response tests, together with physical data of the tested borehole heat exchangers (BHE). The models estimate ground thermal conductivity and thermal resistance of the BHE and are compared regarding test length and data interval used. For the three defined data sets, the line source approximation model shows the closest agreement with the measured temperature response. The cylinder source and numerical models show sensitivity to the inclusion of early data. A recommended minimum response test duration of 50 hours is concluded from the model comparison
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| 3. |
- Gehlin, Signhild, et al.
(författare)
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Determining undisturbed ground temperature for thermal response test
- 2003
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Ingår i: ASHRAE Transactions. - 0001-2505. ; 109:1, s. 151-156
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Tidskriftsartikel (refereegranskat)abstract
- This study treats the determination of undisturbed ground temperature in a borehole for ground heating/cooling and its effect on the accuracy of a thermal response test analysis. Three different ways of estimating temperatures were used in one groundwater-fitted borehole in crystalline rock The first method, temperature logging along the borehole, is assumed to give the correct temperature profile and results in the best estimate of the mean temperature of the ground. A good estimate is also obtained by circulating a heat carrier through the borehole heat exchanger pipes while measuring the flow temperature at a short time interval (10 seconds). The calculated temperature profile is used for deriving a mean temperature of the borehole. Heat is added to the fluid by friction heat caused by the pump work, which results in an overestimation of the borehole temperature. This influencer becomes significant after 20 minutes of pumping.
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| 4. |
- Gehlin, Signhild, et al.
(författare)
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Promoting TES Systems in Sweden
- 2006
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Ingår i: Thermal energy storage. - Pomona, NJ : Richard Stockton College of NJ.
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Konferensbidrag (refereegranskat)
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| 5. |
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| 6. |
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| 7. |
- Gehlin, Signhild
(författare)
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Thermal response test : method development and evaluation
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Since the first introduction of thermal response tests with mobile measurement devices in Sweden and USA in 1995, the method has developed and spread in North America and Europe. Thermal response tests have so far been used primarily for in situ determination of design data for BHE (borehole heat exchanger) systems, but also for evaluation of grout material, heat exchanger types and groundwater effects. A variety of analytical and numerical data analysis models have been developed. Various applications of the line source theory is the most commonly used model for evaluation of the response test data because of its simplicity and speed, and is dominant in Europe. The use of the cylinder source model and numerical models coupled with parameter-estimation techniques are common in USA. The Swedish response test apparatus TED has been used at a number of tests since 1996. The main purpose has been to determine in situ values of effective ground thermal conductivity, including the effect of groundwater flow and natural convection in the boreholes. The tests indicate that convective heat transfer may play an important role for the thermal behaviour of groundwater-filled BHE, which is the typical BHE design in Sweden. The magnitude of the induced natural convection depends on the heat transfer rate and the temperature level. The influence is small on grouted boreholes. To shed light on the influence of groundwater flow on thermal response testing, simulation models for estimating the heat transfer effect of groundwater flowing near a borehole heat exchanger were developed. The groundwater flow was represented as 1) a flow through an equivalent porous medium (continuum), 2) a flow through an impermeable medium with a porous zone, and 3) a flow through an impermeable medium with a thin vertical fracture. The three cases result in significantly different temperature field patterns around the borehole and all three cause lower borehole temperatures. The fracture flow model results in higher effective thermal conductivity than the continuum and porous zone models within a certain flow rate interval. This illustrates the efficiency of the high flow velocity in the fracture and the large temperature gradient between the borehole and the fracture flow. The effect of the flow in the fracture or porous zone decreases with the distance from the borehole, but even at distances of half a meter or more the porous zone or fracture may result in significantly enhanced heat transfer. Even a relatively narrow fracture close to a borehole may result in greater effective thermal conductivity, although estimations made with a continuum approach may indicate otherwise. A thermal response test is likely to induce a thermosiphon flow due to the temperature difference between borehole and surroundings, resulting in an enhanced effective thermal conductivity estimation. The enhancement of the effective thermal conductivity of the BHE depends on injected power rate and flow resistance in fractures. The fracture flow resistance may be quantified in terms of hydraulic condcutivity. The findings from the groundwater flow and thermosiphon simulation are encouraging for further studies, both as simulations and in field experi- ments. The author suggests further studies of the possibility to develop models for estimating and investigating the influence of groundwater from drilling data and hydraulic testing. A future aim should be to gain enough knowledge of fracture flow and thermosiphon effects that hydraulic well test and drilling data may be used in borehole thermal energy storage design.
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| 8. |
- Gehlin, Signhild
(författare)
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Thermal response test : in situ measurements of thermal properties in hard rock
- 1998
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In-situ determination of thermal properties in bedrock is important for the sizing of larger BTES systems. In-situ values of thermal conductivity may reduce required borehole length up to 30%. This thesis treats a new mobile thermal response test equipment (TED), developed in Luleå, Sweden, 1995-98. TED is set up on a small trailer, and is tried out on groundwater filled boreholes, fitted with single and double U-loop piping. It has been used at several commercial borehole direct cooling systems for telephone switching stations in Sweden, and on test-holes in a well documented closed down heat store in Luleå. The response tests show good accuracy and reliability of the measured thermal conductivity and thermal resistance provided good insulation of the equipment. The tests take into account the interaction of the bedrock with the duct piping and filling, the borehole geometry and groundwater and is a valuable tool for pre-investigations for BTES.
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| 9. |
- Gehlin, Signhild, et al.
(författare)
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Thermal response tests of boreholes : results from in situ measurements
- 1998
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Konferensbidrag (refereegranskat)abstract
- During the last year thermal response tests of boreholes in rock were carried out with a mobile test equipment (TED) in several duct stores for heating and/or cooling. Most of the tests were made for the Swedish telephone company TELIA that is constructing a great number of direct cooling systems for their telephone switching stations. The size of these duct systems in the different plants tested, varies from 4 to 60 boreholes, drilled in hard rock - mostly granite and gneiss. This paper summarises results and experience from the measurements.
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| 10. |
- Gustafsson, Anna-Maria, et al.
(författare)
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Influence of natural convection in water-filled boreholes for GCHP
- 2008
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Ingår i: ASHRAE Transactions. - 0001-2505. ; 114:1, s. 416-423
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Tidskriftsartikel (refereegranskat)abstract
- In groundwater filled borehole heat exchangers (BHE), convective flow inside the borehole water will affect the heat transfer. Since the convective flow is dependent of the temperature gradient, different injection rates and ground temperatures will result in different borehole thermal resistance. This paper describes the influence of natural convection in water-filled boreholes in impermeable bedrock for ground-coupled heat pump (GCHP) systems. An overview of groundwater-filled boreholes and the influence of groundwater movements are presented followed by numerical simulations and field measurements to further investigate the influence. The results from the simulations of the three-dimensional, steady-state model of a 9.8 ft (3 m) deep BHE are compared to evaluated results from performed thermal response test (TRT). The results show that convective flow in groundwater-filled BHE results in 5-9 times more efficient heat transfer compared to stagnant water when heat carrier temperatures are in the range of 50-86°F (10-30°C). The size of the convective flow depends on the temperature gradients in the borehole. This shows the importance of on-site investigation of thermal properties using appropriate power injection rates similar to those in the system to be built. This research is part of an on-going project to find ways to estimate the heat transfer including convective flow and to incorporate the findings into the design of GCHP systems. TRT are today a common way to determine heat transfer properties for a BHE and its surroundings. Performing TRT measurements with several injection rates is a way to evaluate the dynamic thermal response including the change in convective flow due to changes in temperature levels. If this dynamic response would be included in design tools a more thorough design of the BHE system is performed. Here, the early result of this research is presented
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