| 1. |
- Blomberg, P., et al.
(author)
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Calibration and Testing of Thermal Simulation Models of Air Heaters
- 2004
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In: ASHRAE Transactions. - 0001-2505. ; 110:Part II, s. 158-166
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Journal article (peer-reviewed)abstract
- Detailed measurements of the thermal characteristics of one one-row and one four-row ducted, hydronic air heating coil have been performed. The measurements were made in a carefully designed and produced laboratory setup, capable of creating almost perfect step changes of both water flow rate and supply temperature. The heater's steady-state characteristics were first modeled. The model was then calibrated with a set of measurements by means of parameter estimation. Then a couple of dynamic models, based on the calibrated steady-state models, were tested. Both the calibrations and the behavior of the dynamic models are discussed. It is shown that if the base has careful calibration of the steady-state characteristics, simple uncalibrated dynamic models can be used. The measurement files are available on CD for anyone interested in testing heater simulation models.
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| 2. |
- Claesson, Johan, 1943, et al.
(author)
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A load-aggregation method to calculate extraction temperatures of borehole heat exchangers
- 2012
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In: ASHRAE Transactions. - 0001-2505. ; 118:1, s. 530-539
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Conference paper (peer-reviewed)abstract
- Hourly simulations of extraction fluid temperatures from borehole heat exchangers tend to be very time consuming. A new load aggregation scheme to perform long-term simulations of borehole heat exchangers is presented. The starting point is the step-response function for the considered borehole heat exchanger and the corresponding long sequence of cells, each with a load and a weighting factor. On the first level, the original weighting factors are kept. On the following levels, 2, 4, 8, etc., the weighting factors are lumped together. The lumped weighting factors are obtained directly from the step-response function. The number of cells to be lumped together is chosen so that the extraction temperatures using lumped weighting factors give a sufficiently good approximation of the non-aggregated scheme. The new scheme is applied to a test case to simulate extraction fluid temperature over a 20-year time period. Comparison of the results from the new scheme with the non-aggregated setting shows that the new scheme can perform very accurate and fast simulations of borehole heat exchangers.
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| 3. |
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| 4. |
- Fahlén, Per, 1947, et al.
(author)
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Capacity Control of Air Coils for Heating and Cooling: Transfer Functions, Drive Power and System Design
- 2011
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In: ASHRAE Transactions. - 0001-2505. ; 117, s. 40-47
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Conference paper (peer-reviewed)abstract
- Liquid-to-air coils used as air heating system coils and air system cooling coils for air-conditioning, refrigeration etc. rarely use their design capacity. The capacity must therefore be reduced accordingly, traditionally by means of on-off operation or by means of control valves. Draw-backs of traditional control are excessive pressure drop and drive power to pumps due to high flows as well as the need for balancing valves and control valves with authority. There are, however, possibilities to substantially reduce the drive energy of pumps and fans for air coils, e. g. by replacing valve and damper control by direct control of decentralized pumps and fans. This may achieve better control at a lower cost while using substantially less drive energy. This paper includes basic analysis of heat transfer and control methods to study how coil design affects the transfer function of an air coil on capacity turn-down. The analysis indicates that direct flow control, using variable-speed pumps, may require only a fraction of the drive power needed by traditional valve control. Furthermore, system designs for low flow rate and pressure drop also provide opportunities for new types of laminar-flow coil designs. Results show that the transfer functions of alternative control methods for the capacity and outlet temperatures of air coils can be written as simple functions of the controlling variable (supply temperature, inlet temperature or coil liquid flow rate). The transfer functions may be tailored to specific needs by changing design parameters such as the design values of air and liquid flow rate and the heat transfer characteristics and heat transfer areas of the respective air and liquid sides. Also, the paper provides an example of alternative system design and control strategy.
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| 5. |
- Gehlin, Signhild, et al.
(author)
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Comparison of four models for thermal response test evaluation
- 2003
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In: ASHRAE Transactions. - 0001-2505. ; 109, s. 135-146
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Journal article (peer-reviewed)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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| 6. |
- Gehlin, Signhild, et al.
(author)
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Determining undisturbed ground temperature for thermal response test
- 2003
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In: ASHRAE Transactions. - 0001-2505. ; 109:1, s. 151-156
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Journal article (peer-reviewed)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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| 7. |
- Gustafsson, Anna-Maria, et al.
(author)
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Influence of natural convection in water-filled boreholes for GCHP
- 2008
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In: ASHRAE Transactions. - 0001-2505. ; 114:1, s. 416-423
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Journal article (peer-reviewed)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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| 9. |
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| 10. |
- Javed, Saqib, 1978, et al.
(author)
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A method to evaluate thermal response tests on groundwater-filled boreholes
- 2012
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In: ASHRAE Transactions. - 0001-2505. ; 118:1, s. 540-549
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Conference paper (peer-reviewed)abstract
- The design of borehole heat exchangers for ground source heat pump system applications requires thermal properties, like ground thermal conductivity and borehole thermal resistance, as inputs. These properties are often determined from an in-situ thermal response test of a pilot borehole. For groundwater-filled boreholes, the ground thermal conductivity and borehole resistance estimations are affected by the heat-injection rates used during the test. Most existing methods for evaluating thermal response tests were not originally developed to analyze tests on groundwater-filled boreholes, and these methods can sometimes give erroneous results in such situations. This paper presents a new method for the evaluation of thermal response tests on grouted and groundwater-filled boreholes. The method is based on an analytical solution, which considers the thermal capacities, thermal resistances, and thermal properties of all borehole elements. The proposed method simplifies the evaluation of thermal response tests on groundwater-filled boreholes and provides accurate estimations of ground thermal conductivity and borehole thermal resistance.
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