Experimental assessment of a standing column well performance in cold climates

Abstract

Standing column wells can provide energy savings at lower first-costs than conventional vertical ground heat exchangers while having a higher potential in dense urban areas. Unfortunately, operating these open wells with groundwater near the freezing point has limited so far their use in northern climates and studies illustrating their successful operation in heating mode are limited. The objective of this study is to provide insights on the various operating conditions affecting the performance of heat pumps linked to standing column wells and demonstrate their potential in cold climates. This work relies on a major research infrastructure designed to operate water-to-air heat pumps connected to a standing column well and its companion injection well under realistic dynamic heating and cooling conditions. During its first operating year, the laboratory was operated continously in heating mode for 26 days. Results show that combined use of a plate heat exchanger and heat pumps allows heat extraction from the ground at significant rates (between 120 W/m and 160 W/m), while keeping the groundwater temperature above 0 °C during peak heating periods. This is approximately two times more than typical values reported for conventional closed loop borehole heat exchangers. Such efficiency was possible owing to the bleed control used, which allows transferring to the injection well part of the groundwater pumped and thus promotes advective heat transport towards the standing column well. Our measurements indicate that bleed was required only 30% of the time and represented 4.6 m³ of groundwater per day on average. These results should dimiss doubts raised in the literature and demonstrate the potential usability of SCWs for cold climates.

Introduction

In cold climates, heating energy demands make buildings one of the largest energy end-use sectors. In Canada, the total energy consumption for space heating of buildings represents 61% of energy used in homes and 55% of energy used in commercial buildings [1]. This high-energy demand has sparked the need to develop and use more energy efficient technologies.

One particular technology which has proven capable of producing large reductions in energy consumption and peak demand in buildings is ground source heat pump (GSHP) [2]. GSHPs are recognized around the world for their high efficiency, low maintenance costs and low greenhouse gas emissions. These benefits are achieved through a ground heat exchanger (GHE), which takes advantage of the large thermal inertia of the ground and uses it as a heat source/sink for the system.

Among all the existing types and configurations of GHEs, the conventional vertical closed-loop GHE has attracted the most interest, both in terms of research and practical applications, owing to its simplicity and reliability [3]. Although a considerable number of studies have been carried out over the last decades to foster the use of GSHP systems worldwide, they still have not been massively adopted due to their high capital cost and long payback periods.

A less common GHE, namely the standing column well (SCW), which enacts superior heat exchange rates by recirculating groundwater in a single uncased vertical borehole, has gained research attention due to its high energy savings potential [4] and significantly lower first-costs [5]. Nonetheless, its real advantage relies on its ability to enhance the thermal transfer with the surrounding ground on command, thereby providing an increased flexibility for load side management. This is achieved by discharging part of the pumped water, which promotes advective heat transfer around the well and helps maintain the groundwater temperature within the heat pumps operational range in peak periods [4], [6], [7], [8], [9], [10]. This key feature, known as bleed, allows a significant reduction in the total length of drilling, and thereby, a more compact configuration in comparison to a conventional vertical closed-loop GHE. A performance analysis conducted by Deng O’Neill et al. [5] showed that the use of bleed could reduce the required borehole length by over 70% and the 20-year system life-cycle cost by at least 25%, in comparison with a conventional closed-loop GHE. Their more compact configuration also make SCWs ideal candidates for GSHP projects located in dense urban areas where the land available is too small for a GHE made of several vertical closed-loop boreholes [9], [11]. Indeed, the recent renovation of St. Patrick’s Cathedral in New York with 10 deep SCWs is particularly illustrative of their potential for buildings located in dense areas [12].

SCW systems, however, present more complexity than conventional closed-loop GHEs as they deal with groundwater and require more field investigations and elaborate design tools. For this reason, several authors developed models, in recent years, all with different degrees of complexity, from simple one-dimensional [6], to more complex transient coupled three-dimensional models [7], [13], [14], [15], [16], [17], [18]. Also, despite having proven their efficiency in the north-eastern part of the United States [19], where climate conditions are milder and can be cooling dominant, SCW systems are seldom used in cold climates. Several factors explain such reluctance: resistance to change, regulation over the use of groundwater, uncertainties related to groundwater quality issues and, most importantly, the lack of experimental studies on their overall performance and reliability [9].

The majority of existing experimental projects have focussed on the analysis of short-term thermal response tests for evaluating the effect of groundwater flow [20], [21], bleed [22], [23], pumping rate [24], and pumping arrangement [25], [26] on the system’s temperature evolution. Recently, Kastrinos et al. [27] have also presented the results of a field tracer test on an operating SCW.

The first experimental study investigating the feasibility of SCW operation in heating conditions published in the literature was completed by Minea [28], who presented an experimental investigation on the performance of a SCW in heating mode for a small residential building. The author performed tests in continuous and intermittent conditions and concluded that, without the use of bleed, the SCW was not able to maintain the heat pump within its operational temperature range. The seemingly poor performance was attributed to the evaporator interior heat transfer surface clogging which lowered its overall heat transfer coefficient. This situation can indeed occur when the groundwater used as heat carrier fluid contains impurities and/or has high degree of hardness. This problem, to the authors’ best knowledge, could have been avoided by properly developing the well and using an appropriate heat exchanger to isolate groundwater from the heat pump.

Recently, Beaudry et al. [29] published experimental data of a dynamic heat extraction test on a SCW in Varennes, Canada, and used it to validate their numerical model. They showed that their proposed numerical model allowed adequate representation of the thermohydraulic processes of a SCW under dynamic bleed control. However, they did not conduct any energy analysis nor did they quantify the performance of the SCW for heating conditions.

Considering their reported performance in the literature, the energy saving potential of SCWs in cold climates is substantial. Unfortunately, concerns about operating these GHEs with groundwater near the freezing point has limited so far the wide use of this efficient and cheaper solution. Also, the published studies illustrating the successful operation of a SCW in heating mode are limited. To fill that gap of knowledge, we performed a detailed performance analysis of the experimental data published by Beaudry et al. [29]. This initiative represents the first attempt at confirming the significant potential of SCWs for space heating in northern climates based on the presentation of an exhaustive experimental data set involving bleed. The objectives of this paper are to present the data gathered during a long heating period and provide insights on the various operating conditions affecting SCWs performance by evaluating (1) the effect of a bleed control strategy, (2) the heat extraction rate during the test, (3) the benefits of adding a plate heat exchanger and (4) the heat pumps’ heating seasonal performance factor (HSPF) during the test.