Analysis of Short Time Period of Operation of Horizontal Ground Heat Exchangers

Salsuwanda Selamat, Akio Miyara, Keishi Kariya
2015 Resources  
Ground source heat pump (GSHP) systems have been proven to have higher efficiency compared to conventional air source heat pump systems for space heating and cooling applications. While vertical ground heat exchangers (GHE) are favorable in GSHP installation, this type of configuration requires higher capital costs as opposed to horizontal configuration. Numerical simulation has been used to accurately predict the thermal performance of GHE. In this paper, numerical analysis of thermal
more » ... of thermal performance for slinky horizontal GHE loops in different orientations and operation modes is discussed. It was found that the loop orientation is not so important due to the little effect it has on thermal performance. While the mean heat exchange rate of copper loop increases 48% compared to HDPE loop, the analysis supports the common claim that heat exchange rate is predominantly limited by the thermal conductivity of the ground. With the same amount of circulation work, the mean heat exchange rate increases by 83%-162% when operated in parallel loops operations. The performance in these operations can be further optimized to 10%-14% increase when spacing between adjacent loops was provided. The spacing helps to minimize interference of heat flow that would penalize the overall thermal performance. OPEN ACCESS Resources 2015, 4 508 Keywords: ground source heat pump; ground heat exchangers; numerical simulation; horizontal; slinky Introduction Ground source heat pump (GSHP) systems have been proven to have higher efficiency compared to conventional air source heat pump (ASHP) systems for space heating and cooling applications. This is due to the relatively stable subterranean temperature in which GSHP systems exploit to extract and reject heat, whereas ASHP systems are exposed to large fluctuation in ambient temperature and climate conditions. GSHP installations do not require large cooling towers, or can be coupled with one for greater efficiency. Hence, their running costs are lower than ASHP systems. Garber et al. [1] suggest that potential savings from a GSHP system largely depend on projected HVAC system efficiencies and gas and electricity prices. The risk analysis performed shows that a full-size GSHP with auxiliary back up is potentially the most economical system configuration. The thermal performance of GSHP depends on many parameters such as short-term weather variations, seasonal variations, moisture content of soil, and thermal conductivity of soil among others that would affect the temperature of ground [2] . GSHP systems extract and reject heat by means of ground heat exchangers (GHE). GHE can be generally classified into two widely installed closed-loop types, which are vertical and horizontal configurations. Vertical GHE are pipes installed vertically in boreholes typically 15-120 m deep. Meanwhile, pipes are laid and buried in trenches 1-2 m deep in horizontal ground heat exchangers (HGHE) installations. Vertical GHE, commonly installed where availability of land area is scarce, provides high and steady thermal performance as less temperature fluctuation occurs along with depth. Moister soil and possible underground water flow in deep region would also contribute to higher heat exchange rate. While vertical GHE are favorable, this type of installation may hinder small and medium enterprises and homeowners due to high capital costs involved. Furthermore, only specialized contractors are well-equipped in terms of equipment and skills for borehole drilling operations. On the other hand, the installations of HGHE are relatively cost-effective operation and rather straightforward which mainly involve excavation of shallow trenches. HGHE installations would be convenient where land area is abundant. The drawback of such shallow installations is that it is prone to unstable thermal performance due to temporal weathers and seasonal variations. Pulat et al. [3] performed economic analysis by comparing experimentally GSHP system using HGHE to conventional heating methods and it was shown that the GSHP system is more cost effective than all other conventional heating systems. Naili et al. [4] conducted in-field analysis that showed the utilization of HGHE is appropriate for cooling building in Tunisia, which is characterized by a hot climate. Numerical modeling has been used in many studies to accurately predict the thermal performance of GHE. This is useful in achieving optimum design and economic feasibility before the commissioning of GSHP systems. Nam et al. [5] developed a numerical model that combines a heat transport model with ground water flow for vertical GHE and was validated with experimental results. The heat transfer rate for an actual office building operation in Tokyo, Japan was predicted using this model.
doi:10.3390/resources4030507 fatcat:xm5m36aj5bd45cpwrzutbjbzae