Cooperation of a Horizontal Ground Heat Exchanger with a Ventilation Unit During Winter: A Case Study on Improving Building Energy Efficiency

Abstract

Renewable energy sources have a significant impact on improving a building’s energy balance. These sources are increasingly used in modern construction. One such source is a ground air heat exchanger (earth-air heat exchanger – EAHX), which is used in a building’s mechanical ventilation system. This solution allows for the initial preparation of inlet air in a ventilation unit prior to the exhaust air energy recovery process (recuperation). As a result, the solution reduces the energy demand for the heating or cooling of rooms. This article expands upon a previous study on the possibilities of improving energy efficiency of a horizontal earth-air heat exchanger (EAHX) working in cooperation with a ventilation unit. This study adds to the knowledge gained in the previous study (Romańska-Zapała et al., 2017) performed during the summer of 2016. This previous study found that, in the summer, the continuous operation of these exchangers is not optimal; therefore, there is a need for dynamic control of the ventilation unit’s fresh air source. This is due to changes in the exchanger’s operating status between heating and cooling, as a result of external temperature fluctuations. The thesis statement of this previous paper emphasized that this relationship is not only appropriate on an annual basis for the transitional seasons (spring and autumn), but also on individual days during the potentially most favorable seasons of exchanger work (summer and winter). This article presents the results of in situ measurements of the horizontal, tubular ground air heat exchanger which supports building heating by preheating the inlet air of the cooperating ventilation unit in the winter season, in southern Polish climatic conditions. In particular, this article provides details on the experimental verification of the previous paper’s thesis regarding the possibility of sub-optimal performance of an EAHX exchanger subject to continuous operation during the winter season. The experiment in this paper was performed using a ventilation unit (intake – exhaust) connected to three independent fresh air sources: a direct external air intake located on the wall, and two air intakes located outside of the building that directs air into the pipes of two earth-air heat exchangers. This article illustrates how the exchanger’s work changes the ventilation unit’s inlet air temperature. For a majority of the experiment, the exchanger – ventilation unit system was working optimally. Therefore, the system did not require intervention of the control system. However, due to several temporary increases in outdoor temperature, the occasional cooling state operation of the EAHX exchanger was also recorded. Even before the occurrence of these undesirable states, an exchanger should be switched off and a bypass should be used to directly collect external air and reduce the unnecessary load of electric fan drives. Considering the results of this study, it can be concluded that in the winter, it is favorable to automatically control the selection of a ventilation unit’s fresh air source depending on external conditions. However, in the winter, the energy effect of the control is comparatively lower than in the summer.