Ground Temperature Variation Around the Horizontal Collectors of a Heat Pump

Transcription

1 Applied Mechanics and Materials Submitted: ISSN: , Vol. 659, pp Revised: doi: / Accepted: Trans Tech Publications, Switzerland Online: Ground Temperature Variation Around the Horizontal Collectors of a Heat Pump TODORAN Tudor Paul 1,a and BALAN Mugur Ciprian 1,b * 1 Technical University of Cluj-Napoca, Department of Mechanical Engineering, Bd. Muncii , Cluj-Napoca, Romania a tudor.todoran@termo.utcluj.ro, b mugur.balan@termo.utcluj.ro Keywords: Ground temperature, monitoring, heat pump, experiment, horizontal collectors. Abstract. The manuscript presents results of a 3 years experiment concerning the monitoring of temperature variation around a horizontal collector, coupled to a ground water heat pump. Ground temperature was monitored in 7 (seven) measurement points: 1 (one) in the center of the collector area and 6 (six) at 15 cm, 30 cm and 60 cm of the collector boundary. The data processing methodology, described in the manuscript, allowed corrections, comparison with natural ground temperature variation and calculation of important characteristics of temperature variation in each measurement point. The main obtained results are: minimum, maximum and average temperatures, dates when minimum, maximum and average temperatures were reached, amplitude of temperatures variations and deviations comparing to natural ground temperature variation. The study of temperature field allowed the complete characterization of the ground temperature variation, including spatial and temporal variation. Some practical conclusions of the study are also presented in the manuscript. It was highlighted that due to a correct calculation of the collectors field size and to the presence of snow in each heating season of the 3 (three) years of the experiment, the ground temperature did not decreased below 2 C and the ground temperature recovered uniformly after each non heating season. Introduction Harnessing of low temperature ground thermal potential, can be realized by the use of heat pumps. This equipment is absorbing heat from a low temperature environment and transfers it into a higher temperature environment. This heat transport is possible only with energy consumption. The ground thermal potential is different from one location to another and is influenced by soil composition and climatic parameters. Normal values of average underground temperatures are situated in the range of (2 9) C in Scandinavia, (9 11) C in Germany or (13 17) C in Italy [1]. In a previous study realized in two locations situated in the region of Transylvania, in the center of Romania, for bare clay soil temperature in Cluj-Napoca was found the average ground temperature of C and for grass covered clay soil in Reghin was found the average ground temperature of C. These temperatures are situated between the normal values corresponding to Italy and Germany. This result is in accordance with the geographical position of the sites of the study [2]. In different studies, it was adopted different experimental strategies for the study of ground thermal potential. In order to increase the heat transfer efficiency between ground and collectors, new configurations are permanently studied: in spiral [3], in inclined bores [4], etc. The most used configurations in practical applications are horizontal collectors and vertical collectors. A review of usual configurations of heat pumps with vertical collectors is presented in [5]. Heat transfer between ground and collector is another actual problem. An analytical study for the correct collector dimensioning is presented in [6]. Numerical simulation of heat transfer between ground and collectors is realized in [7]. Thermal power of a ground collector is analyzed in [8]. Monitoring system for the study of ground thermal behavior around a collector is presented in [9]. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-17/09/16,09:23:25)

2 482 Advanced Concepts in Mechanical Engineering II Experiments with ground horizontal collectors are presented in [10], [11] and [12]. Experiments with ground vertical collectors are presented in [13], [14] and [15]. Direct evaporation in ground vertical collectors is studied in [16]. Integration of ground vertical collectors in the buildings structure is presented in [17]. All these studies prove the high interest for harvesting of ground thermal potential by the use of heat pumps. The aim of this study is to present some of the results obtained after 3 years of monitoring the ground thermal behavior under the influence of a horizontal collector coupled to a heat pump. Variations of monitored parameters are presented both during operation and non-operation of the equipment. There are presented conclusions concerning the ground behavior as thermal energy accumulator. Material The study is based on a long term experiment, of more than 3 years, setup in Bistriţa, Bistriţa- Năsăud County, in the region of Transylvania, Romania. The heat pump used in the frame of the experiment to harvest the ground thermal potential is of 6 kw (thermal power of condenser) and it serve for residential heating of a 45 m 2 house and for domestic hot water (DHW) preparation. The heating system is based on low temperature heating agent fan coils. A heat accumulator was also used on the heating circuit. The location of ground mounted sensors, in the vicinity of the ground horizontal collector is presented in Fig. 1. Fig. 1. Location of ground mounted sensors. The geothermal collector is manufactured from high density polyethylene pipe of 400 m length, the outer diameter of 32 mm and pipe wall of 2 mm. The mounting depth is of 1.7 m. The step between coils is of 0.7 m. The collector is split in two identical circuits of 200 m length each, being connected to the heat pump evaporator by the use of a manifold placed in the junction area. Fig. 1 is presenting only one of the two identical circuits. The ground surface used for mounting the horizontal collector is of 254 m 2. A number of 7 ground temperature sensors are placed in the flow area (3 sensors), in the return area (3 sensors) and in the central area (1 sensor) of the collector. In the flow and return area, the distances between ground temperature sensors and the collector pipes are of 15 cm, of 30 cm and of 60 cm. The sensors mounting areas are represented and indicated on Fig. 1.

3 Applied Mechanics and Materials Vol A placement scheme of ground temperature sensors is presented on Fig. 2 together with the significance of sensors notations. Fig. 2. Placement scheme of ground temperature sensors. The first two coils of the geothermal collector are represented as "circles", on the flow line region and the last two coils are also represented as "circles", on the return line region. The temperature sensors were coded as presented in Table 1. Table 1. Sensors codification Code Code significance STC Central area temperature sensor STT15 Temperature sensor near flow line at 15 cm STT30 Temperature sensor near flow line at 30 cm STT60 Temperature sensor near flow line at 60 cm STR15 Temperature sensor near return line at 15 cm STR30 Temperature sensor near return line at 30 cm STR60 Temperature sensor near return line at 60 cm All the temperature sensors, represented as "rhombs" are mounted like the collector, at 1.7 m depth in the ground. The temperature sensors were connected by wires, together with a supplementary sensor for measuring the outside temperature (not included in this study), at two measuring stations (4 sensors at each station). The measured data were transmitted from the two stations, by radio waves (wireless) to a data logger connected to a computer. Method The ground thermal potential harvested by the heat pump using the ground horizontal collector was evaluated by temperature measurements and by comparison with natural temperature that was previously determined for two locations in the same geographical area [2]. During the 3 years of operation, all the sensors used in the experiment worked properly, with a single exception, due to a faulty connection observed for one temperature sensor. In the case of the ground temperature sensor STT30, mounted in the flow area at 30 cm by the outer collector pipe, it was necessary to correct the registered values of temperatures because in the period between the days (90 730), the temperatures recorded by this sensor were too high. For the mentioned period of incorrect operation of the sensor, the recorded values were corrected being calculated as the geometric mean between the ground temperatures values measured by the neighbor's sensors STT15 and STT60, mounted at 15 cm and at 60 cm toward the outer coil of the collector in the flow area. The geometric mean was used instead of the arithmetic mean because the distance between sensors is not equal and because the ground temperature variation is not linear. The rest of temperatures were measured correctly. In the case of sensor STT30 only the corrected temperatures are provided.

4 484 Advanced Concepts in Mechanical Engineering II Results Temperature variations are presented and studied for a period of more than 3 year between and The long temperature monitoring period allows the observation of ground behavior as thermal energy accumulator, in and between the heating seasons. The ground temperature variation in the central area of the collector (STC) together with the natural temperature variation in two different locations situated in the same geographic area: Cluj- Napoca (CJ) and Reghin (RG), is presented on Fig Temperature [ C] Time [days] CJ RG STC Fct Fig. 3. The ground temperature variation in the central area of the collector. In the lower side of Fig. 3 are indicated the periods of heat pump functioning in heating seasons (Fct). This way of presentation allows a better highlighting of the collector effect on the temperature field in the ground and it is clear that due to the heat extraction through the collector, the decrease of ground temperature is significant. In Table 2 are presented the characteristic values of temperatures and recording moments, for the central area of the collector. Table 2. Values of minimum, maximum and average temperatures in central area of the collector and the periods when it were reached Parameter CJ RG STC Minimum temperature Date / Period No. of stagnation days Average temperature Date / Period 30.06/ / / / /8.12 Maximum temperature Date / Period No. of stagnation days Amplitudine

5 Applied Mechanics and Materials Vol Similar results, as presented in Fig. 3 and in Table 1 are available for all the positions where ground temperatures were monitored: at 15 cm, at 30 cm and at 60 cm toward the collector area. Temperature variation with both time and position is presented in Fig. 4. Discussions Fig. 4. Representation of temperature variation in time and with position. From the presented results it can be also observed that the correct dimensioning of the collector did not allowed the apparition of frost that could damage the underground life, such as trees roots. On the other side it can be observed that in summer the maximum ground temperature is lower that if the ground would not be used as heat source during the winter. It can be observed that differences exist between temperatures in the central area of the collector and in the lateral areas. Even if not very large, these differences indicate that in the central area of the collector the temperature decrease in the heating season more than on lateral sides where effects of neighbors heat sources like other houses are influencing the temperature field. The amplitude of temperature variations in the presence of the geothermal collector is greater than in its absence, in the case of natural temperature variation. Conclusions The study presents original results obtained after more than 3 years of ground temperature monitoring in the area of a geothermal horizontal collector coupled with a heat pump representing the only heat source of a small residential house. The behavior of ground as thermal energy accumulator could be observed and revealed by measurements in 7 points in the same plane with the collector. The correct dimensioning of the geothermal collector determined the ground temperature to be superior of 2 C and frost did not appear. Ground temperature recovered after each heating season.

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