Á. Montero

Use of thermal conductivity from thermal response test for estimating steady-state temperatures in rock and stratified soil near a line source of heat

This article addresses the influence of anisotropy of the ground on steady-state temperature in the surroundings of vertical borehole and effective thermal conductivity measured by a field thermal response test. This is a key parameter in the design of ground coupled heat pumps to heat and cool buildings. First, this article provides a brief overview of the current technique of estimating thermal conductivity from data obtained in a thermal response test based on predictions for temperature from a line source of heat in an isotropic ground. Then, the analytical solutions to the isotropic model for the ground are used to obtain the solutions to the anisotropic model for stratified medium. In addition, the article provides a new analytical exact solution for temperatures around a finite line source of heat penetrating anisotropic semi-infinite medium, in which the angle between the ground surface and the sedimentary strata is arbitrary. Approximate expressions for the temperature evolution during the test and for the steady-state temperature are presented. Such approximate expressions are also given for integral mean temperature for two special orientations of the strata. The limitations of the finite line source method in stratified medium and recommendations for layout of multiple vertical or horizontal ground coupled heat exchangers or waste canisters in repository rock are discussed.

Application of a New Wireless Instrument to Characterize Thermal Properties of Ground Coupled Heat Exchangers

Abstract In this work, we report the design and laboratory test of a new instrument to measure the temperature of the heat transfer fluid along the borehole exchanger (BHE) by autonomous wireless sensor. The instrument consists of a device, which inserts and extracts miniaturized wireless sensors in the borehole with a mechanical subsystem, composed by a circulating pump and two valves. This device transmits to the sensors the acquisition configuration, and downloads the temperature data measured by the sensor along its way through the borehole heat exchanger. Each sensor is included in a sphere of 25 mm diameter and contains a transceiver, a microcontroller, a temperature sensor and a power supply. This instrument allows the collection of information about the thermal characteristics of geological structure of soil and its influence in borehole thermal behaviour, in dynamic regime, and facilitates the implementation of thermal response test (TRT) more easy and reliable.

Advanced Control Structure for Energy Management in Ground Coupled Heat Pump HVAC System

Abstract Over the last 15 years, computerized controls have become more and more common in our homes. The smart home looks at expanding the use of the computers into the difference parts of the home, creating a network that can be easily and conveniently controlled. The use of computer controls removes the need to actually flick a switch and allows elements of the home to respond automatically to the people living in it. Successful control and heating ventilating and air-conditioning (HVAC) systems is a primary concern in building project: in order to achieve the required comfort and energy efficiency goals, a lot of variables must be coordinated and kept at particular pre-designed operation points. The management control systems can be applied to try to achieve optimum settings for different operations in the complicated systems. To optimal settings must balance three aspects: (1) comfort, (2) energy efficiency, and (3) performance margins on order to be able to quickly adapt to unexpected disturbances. This work has used TRNSYS software package to model a HVAC system composed by a geothermal heat pump (GHP) and several fan-coils (FC) for a typical distribution of offices in the area of the Mediterranean Sea. In this model, a control structure has been designed using various configurations of cascade control to incorporate extra sensors and actuators in order to achieve PMV specifications and save energy.

Ground Coupled Heat Pumps in Mixed Climate Areas: Design, Characterization and Optimization

This contribution reviews the research work developed by the authors on the design, characterization and optimization of ground coupled heat pumps in mixed climates areas. The design of a ground coupled heat pump HVAC system starts with the estimation of the thermal loads that the air-conditioned area demands. The capacity of the ground source airconditioning system is determined from this thermal load estimation. With this value and a proper estimation of the ground thermal properties, the characteristics of the water to water heat pump and the required length and layout of the borehole heat exchangers are estimated. Determining ground thermal properties is crucial for an accurate design of the air conditioning system. In situ thermal response tests are carried out to have a measurement at site of ground properties. These tests are based on the Kelvin infinite line source model of heat transfer by thermal conduction. Improvements of this technique can be pursued in different ways. One approach consists in refining the model describing the borehole heat exchanger to include effects not taken into account. Finite length effects can be incorporated in the analysis procedure of test in situ outputs as shown in Bandos et al (2008a), Bandos et al (2008b) and Bandos et al (2009a). A filtering technique of the undesired effect produced in temperature measurements by the ambient temperature can be used to improve the estimating of ground thermal properties (Bandos et al (2009b), Bandos et al (2009c), Bandos et al (2009d)). Another approach to improve the in situ estimation of ground thermal properties is the development of new devices able to measure relevant quantities for the correct characterization of heat transfer between the fluid and the ground. This characterization could be done if the evolution of the fluid temperature along the heat exchanger is known. A sensor probe including a temperature sensor, an acquisition system, temporary storage and wireless communication has been developed to obtain these measurements (Martos et al (2008), Martos et al (2009), Martos et al (2010a), Martos et al (2010b), Martos et al (2010c)). With this new information it will be possible to infer some properties about the ground structure relevant for the design of the ground coupled system.