Thermal Management of Combined Photovoltaic and Geothermal Systems

Date of Award

2023

Degree Name

Ph.D. in Mechanical Engineering

Department

Department of Mechanical and Aerospace Engineering

Advisor/Chair

Andrew Chiasson

Abstract

A ground-source heat pump (GSHP) system with an array of solar photovoltaic-thermal (PVT) modules for cooling-dominated buildings is proposed and its thermal performance is analyzed in this study. As individual systems, GSHP and PVT systems have experienced slow market penetration; GSHP systems have relatively high capital cost compared to conventional heating and cooling systems, while PVT systems have only seen niche application in low-carbon new residential buildings. Coupled together, the ground heat exchanger (GHX) could be designed to optimize efficiency of the PV cells, or the PVT array could be designed for thermal management of annual ground thermal load imbalances on the ground heat exchanger (GHX), or some combination of these design approaches. Radiative coolers have seen development and various applications in recent years. Using typical PVT collectors as a nocturnal cooler in addition to their daytime multi-function leads to better space and cost utilization of the PVT system. To examine the merit of such systems, an outdoor experiment was conducted to evaluate the PVT nocturnal performance, and two models were developed to simulate its thermal performance. First, mathematical model was developed with a detailed description of the physical and environmental parameters that affect the PVT nocturnal thermal performance. The mean error between the model and observed experimental data for predicting the fluid outlet temperature was 0.76 ± 0.91 K, indicating that the model is suitable to characterize the nocturnal cooling performance of the PVT module. The nocturnal radiative cooling is influenced by the water vapor content in the atmosphere and clear sky conditions. The nocturnal cooling power was found to increase by up to 45 W/m2 under a favorable radiative cooling condition. Due to the iterative nature of the detailed model, the model is computationally intensive when integrated in iterative system simulation such as the hybrid GSHP system. The detailed model computational time for twenty years simulation of the hybrid system was around 6 hours. Thus, given the complexities and computational demands of the detailed model, a simplified regression model was developed from the experimental data that allows practitioners to simulate the PVT nighttime performance based on the basic weather conditions representative of the location. The simplified model underpredicts nocturnal cooling by up 2 °C and leads to some cumulative error in the prediction of cooling energy rejection, but serves as a quick, practical design tool. The use of PVT nocturnal cooling as a supplemental heat rejection mechanism was capable of reducing GHX size between 33% to 56% compared to the standalone GSHP systems. In addition, the PV production improved by up to 5.9% when using the ground to cool the PVT modules during the day, but with the tradeoff of additional cost of the GHX. Economically, the total cost of ownership (TCO) of the hybrid system is mostly driven by the upfront cost of the GHX. Thus, reducing the GHX size by using the PVT to reject stored thermal energy features the lowest TCO.

Keywords

Mechanical Engineering, Energy, GSHP, PVT, Photovoltaic thermal, Nocturnal cooling, Radiative Cooling

Rights Statement

Copyright © 2023, author

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