Numerical and experimental studies of geothermal rainwater tanks for buildings passive cooling

Lucas Striegel1^{1}, Jean-Baptiste Bouvenot1^{1}
^{\star} :
1^{1} National Institute of Applied Science (INSA), HVAC department, Strasbourg, France
Mots clés : Geo-cooling, Heat Exchanger, Rainwater collection, Thermal energy storage, Simulation, Building cooling, Measure-Simulation comparison
Résumé :

With climate changes, summer comfort and -emission reduction are two current topics that becomes more and more relevant. In this project, we are trying to developpe a low-tech system that can cool buildings without using refrigerants nor consequent energy. The basis of our solution consists of a new or existing buried rainwater tank whose initial functions are the use of rainwater as non-potable domestic water and the relief of sewage networks. A helicoidal water-to-water heat exchanger is placed in the tank to hybridize the system and create a heat storage. Thanks to an air-to-water heat exchanger connected to the ventilation, the tank delivers cooling energy to the building during summer.

Our aim is to establish a model based on the physical equations (on python), then coupling it with a classic energy building software (energy+ engine) to estimate the impacts on summer comfort. Additionally, two full-scale prototypes are running since July 2021, providing data which are used to calibrate the numerical model. After the model validation, we will be able to optimize the controls and test the solution in different condition (climate, …). It is also planned to add an adiabatic exchanger that can provide extra cooling power by water evaporation.

The literature review shows a lack of detailed studies of those systems. Some authors only treated either experimental data (Kaltz, 2010) or simulation (Upshaw, 2017), others, like Gan (2007), considered the tank as heat source for a heat pump (active system), which is different from our approach. One of the less documented scientific problems is probably the modelling of variable free-surface water storage and the mass and thermal transfers that are involved. The best lead is probably to combine physical equations from both usual hot water tank (only one fluid, pressurized) and pools (exchange between two fluids but in open space).

Moreover, with the rise of drought frequency, the applications of rainwater collection may widen, for example with the use of rainwater for laundry. This raises several new questions about the quality of the water stored in the tank. The water temperature increase may lead to microbiologic development. This problematic is not very common and deserves some investigations.

Preliminary results are very encouraging. The solution decreased the supply temperature of the ventilation by circa 13^{\circ}C, keeping indoor temperatures of the two monitored houses (around 200 m²) under 27^{\circ}C during this 2022 summer French heatwave. The cooling energy measured from mid-May to early September reached 455 kWh with an average cooling power of 365 W and pics hitting 1 kW.

Several other HVAC applications are expected. During the winter 2021-22, the prototype managed to produce about 40 kWh by pre-heating the air to protect the installation from frosting, saving the use of a 500 W resistance.

It is good to mention that this system does not aim to replace air conditioning, but it can reduce its use, especially in high performance buildings.

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