Study of horizontal geothermal heat pump systems for urban greenhouses in Quebec
Author: Xavier Léveillée-Dallaire
Year: 2023
Supervisors: Jasmin Raymond, Jónas Þór Snæbjörnsson
Abstract:
Throughout history, urban agriculture has been seen as a solution to food security and disruptions in global food supply chains, while allowing to produce vegetables locally. With climate change and global warming primarily driven by greenhouse gases, heating and cooling urban greenhouses in a sustainable manner is very important, using technologies such as geothermal heat pump systems. Horizontal geothermal heat exchangers (HGHEs) with coils are an alternative to be explored to reduce greenhouse heating and cooling costs as well as greenhouse gas emissions. Moreover, the installation cost of these systems would be more affordable than that of conventional GHE systems. The objective of this study is to evaluate the potential of HGHEs in heating and cooling urban greenhouses considering the benefits of installing the system under the greenhouse to save space and potentially increase their performance. We assume that a HGHE installed under a greenhouse would benefit from a higher soil temperature than if the system were installed next to a greenhouse, although the HGHE would not be able to cover all the heating and cooling needs.
Sizing calculations were first performed for the HGHEs of a 7.62 mx 15.24 m greenhouse located on the island of Montreal, where the annual, monthly, and hourly energy consumption was estimated from simulations of previous buildings. Three scenarios were used to perform this calculation to determine the number and length of excavations required based on the heating and cooling loads covered during peak periods; (1) 100% of the cooling and heating loads; (2) 100% and 60% of the heating and cooling loads, respectively; and (3) a percentage of variable loads based on a HGHE with similar dimensions to the greenhouse, ie, approximately 116 m2. The estimated dimensions of the excavation for cases 1 and 2 are 414 m2 and 326.4 m2. The estimated percentage of peak loads covered for case 3 is 40% and 30% of the heating and cooling loads, respectively.
Using historical atmospheric temperature data, an energy consumption profile of a greenhouse under design in La Pocatière (Quebec, Canada) and measurements of the thermal properties of the soil, finite element simulations performed with FEFLOW v. 7.5 were used to determine how much of the heating and cooling loads can be covered if the HGHEs are installed only under the greenhouse. It was at this stage that the effect of a constant temperature above the HGHEs and caused by the greenhouse could be considered. The simulations involved four scenarios compared to a base case. The study confirms that a minimum of 7.1% and 26.5% of the total heating and cooling loads of a small greenhouse are covered after one year of operation with a HGHE at a depth of 1.5 m without a greenhouse above. In contrast, installing the same system under a greenhouse with a constant temperature of 21°C increases the covered heating loads to 22.8% and the covered cooling loads to 24.2%. The sensitivity analysis indicates that the constant greenhouse temperature reduces the system's dependence on ambient temperature for heating and cooling, even though cooling is less efficient.