Photovoltaic Thermal (PVT) Heat Pump System: Where is the Efficiency Bottleneck? The Truth Behind the Performance Drop at 60Hz and 35Hz

Hello everyone, today let's talk about a pretty impressive new energy technology — the Photovoltaic Thermal (PVT) heat pump system. Some might feel overwhelmed just hearing the name, with photovoltaic, thermal, and a heat pump all combined, it sounds complicated. But actually, if we take it step by step, you'll find this technology is quite close to our daily lives and offers many benefits.
First, let's discuss why we need to study this technology. The world is currently facing an energy crisis; fossil fuels like oil and coal are running out and burning them pollutes the environment, worsening climate change. So everyone is looking for clean and inexhaustible energy sources, and solar energy is a great choice. The PVT heat pump system is a highly efficient way to utilize solar energy—it can generate electricity and produce heat simultaneously, a win-win that greatly helps reduce carbon emissions and protect the environment. However, this system is not perfect. Due to its complex structure involving many physical processes such as photovoltaic conversion, heat exchange, and fluid flow, achieving efficient and stable operation presents many challenges. Today, we'll have a thorough discussion about how this system works, its performance, and how it can be optimized.

1. PVT Collector: The "Heart" of the System When talking about the PVT heat pump system, we must first mention its core component—the PVT collector. This device is powerful, combining three functions: light collection, electricity generation, and heat production, like a super energy converter. Its structure can be complex but can also be understood simply.

From Figure 1, we can see it consists of a glass cover, EVA film, photovoltaic cells, heat-absorbing aluminum plate, TPT insulation layer, frame, and copper pipes. These parts are not randomly assembled; their positions and combinations directly affect whether the system can work properly. For example, the photovoltaic cells act like "electricity generators," converting sunlight into electricity; the heat-absorbing aluminum plate and copper pipes act as "heat collectors," capturing the sun's heat; the glass cover and insulation layer act like a "thermal jacket," reducing heat loss. Each part has its role and works in harmony to fully utilize solar energy. However, the collector's operation is complex, involving various energy conversions and transfers. Scientists have even developed formulas to describe its energy changes. For instance, an energy conservation equation acts like a "balance sheet" for the collector's energy flow: incoming solar radiation partly converts to electrical energy, partly to thermal energy collected, some is lost through convection and radiation, and the remainder is stored inside. Understanding this helps us better design the collector. For example, if the heat pump system and PVT components are placed too close, maintenance becomes difficult and efficiency may be affected; if placed too far apart, energy transfer losses occur. Therefore, design must consider climate, actual needs, and equipment conditions to maximize performance.

2. PVT Heat Pump System: Collaborative Operation of Components

Having just the collector is not enough; the entire PVT heat pump system is like a big family with many other members. From Figure 2, we see components like the photovoltaic inverter, condenser, heating system, evaporator, heating end, circulation pump, insulated water tank, compressor, underground buried pipe heat exchange system, and more. How do all these parts work together? Simply put, the electricity generated by the photovoltaic part can be fed into the grid through the inverter; the heat collected by the thermal part, with the help of the heat pump, can provide hot water or heating. It's like a coordinated team where everyone has a task and no one can be missing. The system's operation is also influenced by many external factors such as weather changes, ambient temperature, and solar radiation intensity. Just like our work is affected by weather, when the sun is strong, system efficiency is higher; on cloudy or rainy days, efficiency drops. The system's own operating parameters, like compressor frequency and water temperature, also directly affect performance. To ensure the system works well under various conditions, an intelligent control system is needed. Like driving a car where you adjust the throttle and brakes according to road conditions, the intelligent control system adjusts operating parameters in real time based on environmental changes to keep the system at its best. "Economic Analysis of Investment Returns for 'Photovoltaic + Thermal' Projects in Northwest Region.pptx"

3. System Performance Testing: How Does It Actually Perform? After all this, how does the system actually perform? Scientists have conducted many experiments and simulations to test its performance. They used tools like Matlab R2023b and LabVIEW 2023 and built a dedicated experimental platform. Let's first look at some parameters used in the experiments. From Table 1, we see that solar irradiance was measured using a solar simulator Simulator-2000, with a range of 0-1200 W/m ² ; ambient temperature was measured with a TT-1000 sensor, capable of measuring from -40°C to 85°C; the compressor frequency range was between 25-70Hz. These parameters were carefully selected to comprehensively reflect system performance under different conditions. So, what about the experimental results? Let's first look at temperature and compressor power changes. From Figure 3(a), as heating time increases, both evaporation and condensation temperatures rise; evaporation temperature increased from 33°C to 84°C, and condensation temperature from 8°C to 10°C. This is like boiling water—the longer you heat, the higher the temperature.

Looking at compressor power, Figure 3(b) shows that regardless of 35Hz or 60Hz frequency, power increases. At 35Hz, experimental values rose from 167W to 348W; at 60Hz, from 339W to 622W. This indicates that as system operation time increases, the compressor needs to work harder.

More importantly, the system's heating coefficient of performance (COP) and thermal efficiency directly reflect the system's energy-saving effect. From Figure 4(a), it can be seen that the COP value decreases linearly with heating time. At 60Hz frequency, the experimental COP drops from 7.91 to 4.24; at 35Hz, it decreases from 6.53 to 2.61. Thermal efficiency also shows a downward trend, dropping from 8.87 to 3.74 at 60Hz, and from 6.95 to 4.259 at 35Hz. However, there is no need to worry; this does not mean the system is poor. As time progresses, the water temperature that the system needs to heat increases, naturally consuming more energy, so efficiency decline is normal. Moreover, scientists have found that frequency, solar radiation, and ambient temperature greatly affect system performance. For example, when frequency decreases, COP rises, indicating improved system performance; stronger solar radiation and higher ambient temperature also increase COP, which aligns with common sense—better sunlight means a more powerful system. The comparison between experimental and simulation results is also interesting, with a maximum error ranging from 6.80% to 9.40%, indicating that the simulation results are quite reliable. In the future, system performance can be predicted through simulation without conducting complex experiments every time.

4. System Optimization: Making It More Efficient Having understood the system's performance, the next step is how to optimize it. Since frequency greatly affects performance, we can adjust the compressor frequency according to actual needs. For example, when high temperatures are not required, using a lower frequency can ensure effectiveness while saving energy. Natural factors like solar radiation and ambient temperature cannot be controlled, but the system's installation location and angle can be reasonably designed based on local climate conditions. For instance, in areas with ample sunlight, allow the collector to receive as much sunlight as possible; in colder areas, implement good insulation measures to reduce heat loss. There is also great potential for optimizing intelligent control systems. With rapid technological development, artificial intelligence and the Internet of Things can be applied to system control. For example, by analyzing big data to predict future weather conditions, system parameters can be adjusted in advance to maintain optimal operation. Additionally, improvements to the equipment itself can be made, such as developing more efficient photovoltaic cells to convert more sunlight into electricity; designing better heat-absorbing materials to improve heat collection efficiency; and improving compressor technology to make it more energy-efficient. All these can elevate the overall system performance to a higher level.

5. Summary and Outlook In summary, photovoltaic thermal (PVT) heat pump systems are a promising technology that simultaneously utilizes solar energy for power generation and heating, making them both energy-saving and environmentally friendly. Through scientists' research, we have understood its working principles, performance characteristics, and optimization directions. However, current research still has some shortcomings, such as not covering all possible operating conditions and design parameters, limiting the applicability of the results. More research is needed in the future to expand the scope, consider more influencing factors, and make system performance predictions more accurate and reliable. It is believed that with continuous technological progress, PVT heat pump systems will become more efficient and stable, entering more homes and businesses, providing clean energy for our lives, and contributing to addressing the global energy crisis and environmental issues. Perhaps in a few years, the hot water and heating in our homes will be provided by this PVT heat pump system, saving money and protecting the environment—just thinking about it is quite wonderful. This is how technology develops, gradually changing our lives and making the world a better place.