The environmental and socioeconomic problems caused by the widespread use of fossil fuels in the world energy mix are commonly used to justify solar energy research projects. However, Quebec relies very little on fossil fuels given that it has developed hydroelectric power, a green and renewable energy. Therefore, this project proposes a way to optimize this green energy by coupling a CO2 heat pump with hybrid photovoltaic/thermal solar collectors. This kind of system should improve efficiency in comparison to the electric baseboard heaters that are commonly used during Quebec’s long winters. Moreover, with the increasing number of summer heat waves, optimal cooling systems will be needed. CO2 has a low global warming potential (GWP) compared to other refrigerants used to replace the CFC and HCFC that were banned due to their link to ozone depletion and other environmental impacts.
For this project, an experimental setup is designed and built, involving three different solar absorber plates. The solar collectors are used as a direct expansion evaporator integrated to a transcritical CO2 heat pump. Heat is rejected in the mixed water loop of the university École de technologie supérieure, located in Montreal, and the solar collectors are installed on the university’s roof.
First, a detailed model of a solar absorber plate is proposed, detailing the thermal, optical and electrical aspects. The experimental validation showed very good agreement between the numerical and experimental results. The three different solar absorber plates are compared in real exterior weather conditions. Comparison of the experimental and numerical results indicates a maximum discrepancy of 2 [°C] for a temperature measurement taken at a specific location on the solar absorber plate and a discrepancy below 7 [%] for the electric power production over an entire day of simulation. The difference in the electrical result is largely due to one of the solar collectors being in the early morning shade of another nearby experimental set-up on the roof.
Second, a semi-transient numerical model for a stratified thermal storage tank is presented. The tank is intended to be used as a gas cooler in a transcritical CO2 heat pump system. CO2 at a supercritical state flows in a coiled heat exchanger tube immersed in the tank. Water in the tank is used to provide heat to a space-heating water loop and a second coiled heat exchanger is immersed in the tank to preheat the domestic cold water. Results include the temperature along the vertical axis of the tank, which are presented along with the evolution of the CO2 flow variables (pressure, temperature, velocity, etc.) inside the tube for different time steps. The TRNSYS Type 534 is used to validate the correct implementation of the tank’s thermal stratification. Experimental results from the literature are used to validate the CO2 flow in the tube model.
Third, the tube model flow is combined with the solar absorber plate model to study the thermal and electrical performance of the global solar evaporator. Steady-state numerical results are given for four scenarios based on the cooling of the photovoltaic module and the electrical load connected to the collector. The solar evaporator is either connected to a fixed resistive load or working at MPPT conditions. The weather conditions (ambient temperature, solar radiation, wind speed, etc.) and inlet flow parameters (mass flow, pressure and enthalpy) of the CO2 are fixed. The 2-D temperature distribution of the solar absorber plate, the CO2 flow variables (pressure, temperature, velocity, etc.) inside the tube and the electrical operating conditions are computed for each scenario. The results show a significant increase in the electrical power production, exceeding the maximum power specified under the test conditions of the IEC 60904-3 international standard. More than 1 [kW] of thermal power is recovered from the solar absorber plate leading to an average plate temperature reduction of over 25 [°C]. It also follows that the overall efficiency combining both electricity and heat production reaches a value as high as 72.3 [%] under the simulated conditions.
These three original and innovative contributions were acknowledged by the international scientific community as evidenced by the quick publications in high impact factors journal of all three papers before the defence of this thesis.
Overall, this thesis presents numerical tools to simulate the performance of a hybrid photovoltaic/thermal solar evaporator and a stratified thermal storage tank used as a gas cooler. Both components are essential to the development of a transcritical CO2 heat pump that could be used to improve residential building heating technology. At the same time, a prototype including the heat pump and the solar collectors was designed, built, equipped with sensors and commissioned.
| Date | 24 Aug 2019 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Daniel Rousse (Supervisor), Louis Lamarche (Co-supervisor) & Hakim Nesreddine (Co-supervisor) |
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Paradis, P.-L. (Author),
Rousse (Supervisor),
Lamarche (Co-supervisor) & Nesreddine (Co-supervisor),
24 Aug 2019Student thesis: Doctoral thesis › Doctorate in Engineering: Engineering